Late Paleozoic-early Mesozoic magmatism in the Cordillera de Carabaya, Puno, southeastern Peru: Geochronology and petrochemistry

Journal of South American Earth Sciences, Vol. 3, No. 4, pp. 213-230, 1990 Printed in Great Britain

0895-9811/90 $3.00 + 0,00 Pergamon Press pk

& Earth Sciences and Resources Institute

Late Paleozoic-early Mesozoic magmatism in the Cordillera de Carabaya, Puno, southeastern Peru:

Geochronology and petrochemistry D. J . KONTAK .1, A. H. CLARK 1, E. FARRAR:, D. A. ARCHIBALDt, a n d H. BKADSGAARD 2

IDepartment of Geological Sciences, Queen's University, Kingston, Ontario, K7L 3N6, Canada; ~Department of Geology, University of Alborta, Edmonton, Alberta, T6G 2E3, Canada

( received January 1990; accepted August 1990)

Abstract- -The Inner Arc domain, the easternmost magmatic manifestation of the post-Paleozoic Central Andean orogeny in southeastern Peru and western Bolivia, comprises a remarkably diverse assemblage of plutonic and volcanic rocks, many of which would be more characteri~c of ensialic rifm or collisional mountain belts than of Andean-type convergent plate boundaries. Marked petrologic contrasts with the more homogeneous Main Arc domain, which underlies the westerly provinces of the orogen, have been maintained sines the initiation of Andean orogeny in the Late Triassic. Comdzainte on the chronology and pet rogene~ of the early stages in the protracted evolution of the Inner Arc and its Permian antecedents are provided heroin by, respectively, K-Ar and Rb-Sr geochronologic data and major and minor element ana- lymm of representative pre--Cretaceous igneous rocks ofthe Cordillera de Carabaya, southeastern Peru. Our gudies confirm the following sequence of magmatic events, which temporally overlapped with the initial stages of Andean orogeny: i) lntrumon of the gebbroic-tc-granitic San Gab~m (Corani) complez, a celt-alkaline, but crustally con-

taminated, suite that cores an ezteusive area of high--grade, low-proaure metamorphism in lower Paleosoic strata. The complex has been auigned to the mid-Paleozoic, but its age romaine poorly defined. The foliated, markedly peraluminous, two--mica granites of the smaller Limacpampa pluton may also have been emplaced during the Palsozoic, but a TriAdic age is favored on the basis of our Rb- Sr data.

ii) Eruption of alkali basaltic lavas of the Lower Permian Mitu Group along the northeastern margin of a longitudinal ensialic ri/~ that developed in response to extensional toctenism in the interval between the pre-Andean ( ' late Hercynian") and Andean orogenies.

iii) Emplacement of large granitoid plutons (Coasa, Limbani, and Aricoma centers), with I-Caledonian affinities, along the northeastern boundary ofthe Mitu rLA during the Late Triassic (ca. 225 Ma). The metaluminous to weakly peraluminons monozogranites and granodiorites comprising the greater part of the Carabaya Batholith (new term) were closely associated with mafic dikes of alkaline composition, similar in many respects to the preceding alkali basglts.

iv) Development of the Allineclipac Group or peralkaline complex (new term), an assemblage of Middle (and Lower(?) Jurassic lavas, pyroclastics, and plutous exhibiting alkaline to peralkaline affinities.

Whereas each of the above suites may be assigned to either a predominant mantle or crustal source, it is evident from the chemical and isotopic data that varied mantle and crustal environments have been involved. Thus, the distinctive chemistries of the coeval granitoid intrusions - - as ezpreased, for example, in the trace element contents of whole--rocks and biotites, the oxidation states of both rocks and biotitos, and the initial t~rontium isotope ratios - - demonstrate the contributions ofsoveral distinct protoliths. The close spatial and temporal association of mantle and crustal suites during the Permian-to-Jurasaic interval strongly implies a cause-and-effeCt relationship. In particular, the role of basaltic injection in generating large volumes of peraluminous granitoid magmas is amply supported.

Resumen---See page 230.

INTRODUCTION

THE CENTRAL ANDEAN Cordillera Oriental of southeastern Peru and northwestern Bolivia separates the ca. 3800 meter Altiplano molasse basin to the west from the Sub-Andean Range thrust and fold belt to the east (Fig. 1). Granitoid plutons and volcanic successions underlie extensive areas of this mountain belt which, in southeastern Peru, is tradi- tionally subdivided into the Cordilleras de Caraba- ya and Apolobamba and, in northwestern Bolivia, the Cordilleras Mui~ocas, Real and Quimsa Cruz. The Upper Triassic to upper Miocene igneous rocks

*Prmsnt sddro~: Nova Scotia Department of Mines and Ener- gy, Mineral Ruources Division, P.O. Box 1087, Halifax, Nova ~k~otht, B3J 8XI, Canada.

213

of this area are assigned (Clark et aI., 1983; 1984) to the Inner Arc domain of the post--Paleozoic Central Andean orogen; this entity experienced an evolution distinct from that of the entirely subduction-related "Main Arc" of the Cordillera Occidental and Alti- piano. In the Cordillera Oriental, magmas invaded variably deformed and metamorphosed Paleozoic strata (Ahlfeld and Branisa, 1960; M6gard et el . , 1971) in several widely separated episodes (Carlier et el., 1982; Kontak et al., 1984a) during Mesozoic and Cenozoic times, whereas magmatism has been quasi--continuous in the Cordillera Occidental since the Late Triassic. Furthermore, the extremely diverse magmas of the Inner Arc domain were derived from a variety of source regions in both mantle and crust (Kontak et al., 1984a), whereas the geochemi- cally and petrographically more homogeneous rocks

214 D.J. KONTAK, A H. CLARK, E. FARRAR, D, A. ARCHIBALD, and H. BAADSGAARI)

[

Cordillera Occidental

Altiplano

~ Cordillera Oriental

Sub-Andear Ranges

Foreland Basin Brazilian Shield and Cover

72 ~ 68 °

Fig. I. Major morphostructttral provinces of the Central Andes in the vicinity of the Arica Deflection. Study area is the Cordillera de Carabaya segment of the Cordillera Oriental.

of the Main Arc are predominantly, if indirectly, of mantle derivation.

We report here K-Ar and Rb--Sr dates, 8VSr/SeSr initial ratios (Sri), and selected petrochemical data for the pre-Cretaceous igneous suites of the Cor- dillera de Carabaya area (Figs. 1 and 2), in order to clarify its late Paleozoic and early Mesozoic magmatic evolution. We discuss our radiometric data in light of field observations and petrochemical studies of the rocks, and provide a modified synthesis of some of the data presented earlier (Kontak et al., 1984a; 1985). In a companion paper (Kontak et al., this issue, p. 231), we document radiometric evidence for the Eocene teetono-thermal rejuvenation of the rocks now exposed on the northeastern flank of the mountain range. Our research complements the detailed radiometric data base of McBride et al. (1983, 1987) in the Cordillera Oriental of northwestern Bolivia (Fig. 1), and provides new petrochemical data for the magmatic rocks underlying the Cordillera de Carabaya.

REGIONAL GEOLOGIC SETTING AND STRATIGRAPHY

Carlier et al. (1982) have reviewed the pre- Cretaceous magmatic history of the Peruvian sector of the Cordillera Oriental, and Laubacher (1978)

has summarized the regional geology of the Cordill-- era de Carabaya. Research on the granitoid rocks of southeastern Peru and northwestern Bolivia includes reconnaissance mapping in the Cordillera de Carabaya (Laubacher, 1978) and Cordillera Real (Martinez, 1980), and geochemical/petrological in- vestigations in southeastern Peru (Kontak, 1985; Kontak et al., 1984a,b, 1985). These studies reveal a complex history of magmatism from the late Paleo- zoic to the early Mesozoic that involved contributions from both the mantle and crust. Magmatic activity was focused along a longitudinal sedimentary trough, the "Altiplano early Paleozoic basin depocenter" of Ramos (1988), which is inferred to have separated the Lower Proterozoic Arequipa Massif from the Amazonian craton.

Two granitoid suites of possible Carboniferous age, the San Gabfin and Limacpampa massifs, in- trude lower Paleozoic, eugeosynclinal sedimentary strata (Fig. 2). However, this age assignment, based on similarities in geologic setting and petrology with the 330 Ma Amparaes orthogneiss (located about 100 km northwest of the study area in the Cordillera Vilcabamba: Carlier et ai. , 1982), is poorly constrained. The variably deformed and thermally overprinted nature of these intrusions has precluded definition of their intrusion ages by K-Ar and 4°Ar/~gAr techniques (see Kontak et al., p. 231).

Ordovician to lowermost Permian, predominantly marine, sedimentary strata in this region are unconformably overlain by a succession of red molassic clastics and intercalated volcanic flows, constituting the fill of a longitudinal (NW-trending) ensialic rift - - the Mitu Basin. By analogy with stratigraphic relationships in more northwesterly transects of the Peruvian Cordillera Oriental (Newell et al., 1953), Laubacher (1978) assigned all sedimentary and volcanic s t ra ta i n t e rven ing between the Lower Permian Copacabana Group and the Cretaceous Cotacucho Group to the Mitu Group, which elsewhere in Peru has been considered to have formed between the early Leonardian (or early Artinskian) and the Late Triassic (e.g., M6gard, 1978). In the study area, persistence of Mitu Group accumulation into the Early Jurassic was inferred on the basis of the areal association of a thick sequence of peralkaline volcanics and a pluton (Fig. 2) of peralkaline syenite (the Macusani syenite of Francis, 1956), for which Stewart et al. (1974) reported a Middle Jurassic K-Ar age of 184 Ma. The peralkaline volcanic and intrusive rocks of the Macusani area are indeed compositionally equi- valent (Kontak, 1985; also see below) and in outcrop are locally mutually gradational; moreover, we see no reason to reject the Jurassic date reported for the syenite. We would, however, prefer to restrict the term "Mitu Group" to the assemblage of red clastics and alkaline basalts exposed most widely in the vicinity of the Crucero Depression (Laubacher, 1978). We consider the peralkaline rocks to be distinct from the Mitu Group because deposition of

Late Paleozoic-early Mesozoic magmatism in the Cordillera de Carabaya, SE Peru 215

J,'-LutQIIII II

S Y E N I T E

Macul

~ *T*'*T*'NJII l l l C O A S A

*3~+~+++.+'?~[ll PLUTON ' ~ + ~ + ~ + + + + + +

~ + + + + + + + + + + + + + + + + + + +

O~* * * * * * * ÷ , ~ w * -~h,t ..R" + + + + + + +,w..~ + +~liL + + + + + + + + ~ + + . . . . . ÷**++~,k,+÷~+T%+

+ ÷ ÷ + + + L - - - - - - . - - - - ~ . ~ A R I C O M A ~

PLUTON

L I M B A N I PLUTON

Jurassic

Triassic

Permian

Carbon- iferous

Siluro- Devonian

O r d o v - i c i a n

m

SEDIMENTARY AND VOLCANIC STRATA

Al l inccdpac Group

INTRUSIVE ROCKS

Macusani Syenite Granitoid pluton

~ and associated • mafic dykes

San GabOn plutonics (?)

Mitu Group

Tarma/Copacabana Gp. Ambo Group

Ananea Group

Sandia Fm. San Josd Fm. 0 k m 2 5

] ~ 150 _

Fig. 2. Generalized pre-Cretaceous geology of the Cordillera de Carabaya region, modified from Laubacher (1978). Ages indicated for igneous rocks are from the present study. The Mitu Group of Laubacher has been subdivided (see text) into the Mitu Group proper and the Allinccapac Peralkaline Group; the boundary between these two units is arbitrarily drawn on the basis of known occurrences of peralkaline volcanic strata. The Macusani syenite of Francis (1956) is assigned to the Allincctpac Group. NB: known areas with unusual concentrations of Triassic mafic dykes are shown schematically; the uncertainty regarding the age of the San Gabbn complex is indicated by the question mark.

the alkaline and peralkaline volcanic successions was separated in time by the emplacement of major Triassic granitoid intrusions (see below). The pre- cise location and nature of the contact of the two extrusive suites remain uncertain (Fig. 2). In view of the widespread use of the name "Macusani Vol- canics," applied to the strongly peraluminous, Neo- gene, felsic pyroclastics of the area (e.g., Noble et al., 1934), we propose that the peralkaline rocks of the Macusani district, both extrusive and intrusive, be grouped as the Allincc6pac Group or peralkaline complex, named after the mountain Nevado Allinc- cApac (ca. 5850 m), which is underlain by the great- est known thickness of the volcanics (Laubacher, 1978) and probably represents a major volcanic center. It must be emphasized, however, that none

of the other alkaline-peralkaline felsic in t rus ions scattered (Kontak, 1985) throughout the western Cordillera de Carabaya have been dated, while exposures in the Ayapata sector of the Coasa pluton (see below) clearly display transitions from monzo- granitic to mafic and felsic quartz-free rocks. It is therefore possible that strongly a lkal ine m a g m a - tism occurred over a wider interval than our data indicate.

Because' of extensive intercalat ion of molasse sediments and alkali basaltic flows in the study area (Laubacher, 1978; Kontak, 1985), we cannot concur with the decision of Klinck et aL (1986) to exclude the upper Paleozoic basic volcanic rocks from the Mitu Group. Those authors, working mainly on the margins of the Altiplano in southeas te rn Peru ,

216 D. ,J. KONTAK, A. H. CLARK, ]~ FARRAR, D. A. ARCHIBALD, and H. BAADSGAARI)

introduced the term "Iscay Group" for the volcanic succession, which, in that area, unconformably overlies the clastic sediments. This new unit cannot be delimited in the Cordillera de Carabaya.

The Triassic granitoid plutons of the Coasa, Aricoma, and Limbani areas (Fig. 2) constitute the largest intrusions in the area. They form part of an extensive belt of meta- to peraluminous granitoids confined broadly to the fault-defined northeastern margin of the Mitu Basin. Although all three bodies have been termed "batholiths," their exposed extents do not justify this; we prefer to refer to them as plutons comprising a Carabaya Batholith (new term; see Fig. 2).

ANALYTICAL METHODS

Isotopic A n a l y s e s

Argon analyses followed procedures outlined by McBride et al. (1987). Potassium contents were determined in duplicate with an IL-251 atomic absorption spectrophotometer operated in the emission mode. Both K and Ar concentrations were compared to intralaboratory standards that are referenced to an interlaboratory standard, LP-6 biotite. Whole- rock powders and mineral separates were analyzed for Rb and Sr by isotope dilution with 8VRb and S4Sr spikes, respectively, using the procedures outlined by Goff et al. (1982). Sr data were normalized to STSr/~SSr = 0.1194 and Rb data to SSRb/S~Rb = 2.601. Radiometric dates in this study and dates reported from the literature have been calculated using the constants recommended by Steiger and J/iger (1977). The Rb-Sr and K-Ar data are presented in Tables I and 2, respectively, and the locations of the dated samples are shown in Fig. 3.

W h o l e - R o c k a n d Mi ne ra l C h e m i s t r y

Major and trace element abundances were obtained by X-ray fluorescence techniques, except for Cu, Pb, Zn, and Ni, which were determined by atomic absorption spectrophotometry, and ferrous iron, which was determined using a titrametric method (Wilson, 1960). REE analyses were obtained using the thin-film X-ray fluorescence method (Fryer, 1977).

PETROGRAPHY AND PETROCHEMISTRY OF THE MAGMATIC ROCKS

We summarize here salient aspects of the petrology of the magmatic suites as a basis for inter- preting our isotopic data. We emphasize petrologic features that are important for determining the source of the suites rather than their magmatic or crystallization histories.

The little known San Gab/in Complex (Fig. 2), also referred to as the Corani Batholith, comprises a

wide spectrum of rocks ranging from olivine-pyrox.-. ene-amphibole gabbro, through amphibole-biotite diorite, and granodiorite, to two-mica (leuco-) monzogranite, along with small volumes of muscovite- rich pegmatite and aplite. Petrographic studies reveal that the rocks grade from one type to another and define a continuum in magmatic evolution, but the areal relationships of the various lithologic facies are unknown. Variable amounts of small (1-3 cm) metasedimentary xenoliths occur in the intermediate and felsic rocks, and their assimilation is evident in some cases. The highly variable major element chemistry (e.g., 47-71% SIO2, <2-11% MgO, 1.5-11% CaO) reflects the range of rock types, and continuous curved trends are defined on Harker type variation diagrams for major and trace elements versus silica (Kontak, 1985). In plots of total alkalis versus silica, A1203 versus normative plagioclase (after Irvine and Baragar, 1971), and FeO/ MgO versus silica (after Miyashiro, 1974), as well as in the cation plots of Jensen (1976), the suite corres- ponds to the calc-alkaline field, although an iron- enrichment trend is noted on the AFM diagram. Contamination of the melt by alumina-rich sedimentary material is reflected in elevated A/CNK values (<1.22) and the position of analyses in the modified ACF plot (Fig. 4) of White and Chappell (1977).

The small, possibly Carboniferous, Limacpampa pluton (Fig. 2) consists of two-mica monzogranite and muscovite leucogranite. Both rock types are cut by fine-grained aplitic dykes, and gneissic fabrics are locally developed. The granites are enriched in silica (avg 73 wt.%), alumina (avg 15%), and alkalis (Na20 + K20 = ca. 8%), have 3-4 wt.% normative corundum and A]CNK values of 1.20-1.25 and are highly reduced. In most mineralogical and chemical respects, these rocks thus conform (Fig. 4b) to the S-- type granite classification of Chappell and White (1974) and White and Chappell (1977), as well as to the il menite series of Ishihara (1977).

The Mitu Group mafic volcanics in the Cor- dillera de Carabaya region are generally altered, but two relatively fresh samples have been analyzed. The presence of plagioclase (Aries) and olivine (Fo80-90) phenocrysts in a plagioclase-clinopyrox- ene-sanidine-opaques-glass-olivine matrix with a trachytic texture are features typical of alkali olivine basalts. The alkaline nature of the rocks is supported by the chemical analyses (Figs. 5a and b). The rocks correspond to the potassic series of Irvine and Baragar (1971) and the potash series of Middle- most (1975). Low MgO, Cr, Ni, and Cu contents (avg 5 wt.%, 26 ppm, 21 ppm, and 7 ppm, respectively) and mg values (0.44-0.51) indicate that the vol- canies have undergone signifmant fraetionation of olivine and pyroxene. REE data for a single sample (Fig. 5c) conform to those of other alkali basalts in both absolute abundance and pattern (Kay and Gast, 1973); the slight positive Eu anomaly suggests that plagioclase accumulation has occurred. A S7Sr/Se86 whole-rock determination (calculated to


Table la. Rb--Sr isotopic data for whole-rock and mineral separates, Permian-to-Jurassic igneous rocks of the Cordillera de Carabaya, southeastern Peru.

Ana- SSSr S~Rb Sample No. lyzed (ppm) (ppm) S~Rb / SSSr S~Sr/NSr

Mitu Group Alkali Basalts Limacpampa Pluton Gavil~n de Oro Coasa Pluton (Coasa Transect)

Aricoma Pluton

AllinccApac Group Peralkaline Volcanics

COCA-278 WR 87.07 54.99 0.6243 0.70663 LMP-2A Ms 0.446 325.2 720.7 2.7458 GDO-1 Ms 1.540 180.0 115.4 0.93011 COCA-2621 WR 4.81 114.1 23.47 0.77302 COCA-262 H WR 4.84 114.9 23.45 0.77301 COCA-2651 WR 7.40 90.7 12.12 0.74997 COCA-265 H WR 7.45 91.9 12.19 0.75014 COCA-265 Ms 0.436 236.8 536.5 1.8672 COCA-268 WR 12.53 66.70 5.264 0.72349 COCA-2681 Bt 0.532 353.0 656.2 2.5982 COCA-268 II Bt 0.362 213.8 584.5 2.4169 COCA-272 WR 13.87 65.70 4.681 0.72262 COCA-2721 Bt 0.483 291.1 595.9 2.4969 COCA-2721I Bt 0.716 487.2 627.7 2.5459 COCA-272 WR 63.40 14.66 0.2286 0.70682 BAR-13 WR 16.06 60.14 3.701 0.72037 BAR-13 Bt 1.113 230.9 205.1 1.2743 COCA-372A WR 12.44 71.23 5.662 0.73260 COCA-372 Bt 1.938 244.1 124.5 1.0886 COCA-62A I WR 15.96 7.59 0.4707 0.70927 COCA-62A H WR 16.14 7.70 0.4718 0.70922 COCA-64 WR 23.89 74.01 3.063 0.71367 COCA-64A I WR 8.73 69.52 7.874 0.71809 COCA-64A II WR 8.92 70.59 7.819 0.71813

COCA-65 WR 50.30 33.26 0.6357 0.70693 COCA-299A I WR 26.73 42.64 1.577 0.70745 COCA-299A H WR 26.56 42.18 1.570 0.70732

Key to material analysed: WR, whole rock; Ms, muscovite; Bt, biotite; Kf, K-feldspar

Table lb. Whole-rock and mineral Rb-Sr dates and s~Sr / s~Sr values for Permian-to--Jurassic igneous rocks and ores of the Cordillera de Carabaya, southeastern Peru.

Sample Nos. Age + 2o (Mater ia l Analyzed) (Ma) S~Sr / SSSr MSWD

Limacpampa Pluton Gavil~n de Oro Coasa Pluton (Coasa transect)

Aricoma Pluton Allincc~pac Group Peralkaline Volcanics

LMP-2A (Ms) ca. 199.0 ± i0 0.710 (assumed)

GDO-I (Ms) ca. 133.0 ± 7 0.710 (assumed) COCA-262I, 2651,268,271, 194.5 ± 18.7 0.71089 ± 0.00012 733 272 COCA-2621,268,271,272 188.9 ± 3.8 0.71009 ± 0.00012 30.8 COCA-268,271,272 222.1 ± 26.7 0.70741 ± 0.00045 24.8 COCA-2621,268,271,272, 204.5 ± 6.4 0.70804 ± 0.00008 745 268K Bt, 286II Bt, 272I Bt, 272II Bt COCA-268(Wr+Bt(I)) 202.5 ± 10.1 0.70833 ± 0.00010 COCA-268(WR+Bt(II)) 205.6 ± 10.3 0.70810 ± 0.00003 COCA-268(WR+Bt(I , II)) 204.1 ± 1.5 0.70821 ± 0.00015 COCA-272(WR+Bt(I)) 211.0 ± 10.1 0.70858 ± 0.00010 COCA-272(WR+Bt(II)) 192.0 ± 9.6 0.70985 ± 0.00011 COCA-272(WR+Bt(I , II)) 208.5 ± 2.6 0.70874 ± 0.00015 COCA-265(WR+Ms) 149.9 ± 7.5 0.72414 ± 0.00011 BAR-13(WR+Bt) 193.4 ± 9.6 0.71019 ± 0.00011 COCA-372A(WR+Bt) 210.7 ± 10.4 0.71563 ± 0.00011 COCA--62AII, 64,64AH, 102.4 ± 22.2 0.70711 ± 0.00010 171 65, 299A H COCA--64, 65,299A II 206.0 ± 70.8 0.70416 ± 0.70417 135

Analytical techniques and error calculation methods are as documented in Goffet al. (1982). Bt, biotite; MSWD, mean square weighted deviates value.

Key: WR, whole rock; Ms, muscovite;

218 D. J. KONTAK, A. H. CLARK, E. FARRAR, I). A. ARCHIBALD, and H. BAADSGAARI)

Table 2. K-Ar dates for Permo-Jurassic igneous rocks of the Cordillera de Carabaya in Peru and the northern Altiplano in Bolivia.

L oca t i on A n a -

S a m p l e No. Lat . S L ong . W R o c k T y p e lyzed

4°Arrad K (10 -5 c m 3 4°Aratm A g e ± 2o

(%) STP/g) (%) (Ma)

Mitu Group

CR385 16040 ' 68°36 ' basal t WR 0.645 0.759 24.3 279.9 ± 3.3

CR386 16040 ' 68°36 ' basal t G 0.443 0.451 14.8 244.9 ± 2.9

Coasa Plu ton

COCA-273" 14°02'59" 70°09'00" grani te (dyke) Ms 8.326 7.432 3.6 216.2 ± 4.3 BAR-16* 14°04'37 '' 70°10 ' g re i sen/skarn Ms 5.292 4.642 2.3 212.7 ± 4.3

BAR-17* 14°0 ̀ 70°09 . monzograni te Bt 6.700 5.824 4.6 210.9 ± 4.3

COCA-271" 14001'38 " 70007'23 .' monzograni te Bt 6.873 5.971 1.8 210.8 ± 4.2

COCA-272" 14002'59" 70°09 . monzograni te Bt 6.739 5.812 0.6 209.3 ± 4.2

COCA-272 ' 14°02'59 " 70009 , monzograni te Bt 6.739 5.695 2.3 205.3 ± 4.2

BAR-18* 14001'30 '' 70°06'30 '' monzograni te Bt 7.205 6.163 2.6 207~7 ± 4.1

BAR-13 13057'57 `' 70020 ' monzograni te Bt 6.925 5.988 4.9 210.2 ± 4.2

BAR-13 13°57'57 '' 70020 ' monzograni te Bt 6.925 5.928 5.0 207.8 ± 4.4

BAR-13 13°57'5 " 70020 ' monzograni te Bt 6.925 5.613 5.7 197.4 ± 4.0

AY-5D 13057'57 '' 70o20 . monzograni te Bt 6.494 5.278 3.7 197.9 ± 4.0

COCA-301A 13°57'57" 70°19'46" monzograni te Bt 6.550 5.068 8.5 188~9 ± 3.8

COCA-272 14°02'59 '. 70°09'00" monzograni te Kf 9.817 5.424 5.2 136.8 ± 3.0

Aricoma Pluton LB-6 14°18'49 '' 69°44'35" quar tz vein Ms 7.844 7.304 2.3 225.0 :t: 14.8

COCA--421 14°13'00 " 69°52'00" monzograni te Bt 6.362 5.696 6:3 216.8 ± 4.5

Allincc~ipac Complex MS-1 14°00 ' 70°29'30" syeni te Bt 6.771 4.826 13.5 174.7 ± 3.6

COCA-305 13°47 ' 70°25 ' syeni te Bt 6.828 4.315 7.4 155.7 ± 4.3

*Sample location coordinates t aken from 1:50,000 topographic maps . Because a discrepancy exis ts between the geologic map o Laubacher (1978) and the topographic maps , the samples have been plotted, for consistency, relat ive to the geologic m a p shown in Fig. 2. Key to mate r ia l analyzed: WR, whole rock; G, glass; Ms, muscovite; Bt, biotite; Kf, K-fe ldspar .

260 Ma) defines a Sri of 0.7043 (Table 1) - - i.e., within the range typical for alkali basalts (Powell and Bell, 1974).

The three large Triassic granitoid intrusions constituting the Carabaya Batholith (Figs. 2 and 3) differ petrologically. The Coasa pluton (1300 km 2) comprises at least two distinct suites. In the southeast, the "Coasa suite" is dominated by medium- to coarse-grained biotite-hornblende- and K-feldspar-megacrystic monzogranites, widely but incon- sistently exhibiting rapakivi textures; however, an exposure of two-mica cordierite monzogranite was observed 2 km WSW of the village of Coasa. In contrast, the "Ayapata suite" in the western area of the batholith is dominated by medium-grained biotite hornblende monzogranite-to-syenogranite, with the local development of more mafic (diorite- to--quartz diorite) and felsic (leucocratic syenogranite) facies. Inclusions of biotitic quartz diorite are common throughout the main Coasa monzogranite. Moreover, exposures close to the northeastern margin of the pluton reveal dykes of biotite- rich quartz diorite cutting, and cut by, the granite; several dykes exhibit sinuous and discontinuous forms strongly suggestive of synplutenic emplacement (e.g., Pitcher and Bussell, 1985). These relationships imply the temporal association of at least two granitoid magmas at the present level of erosion, although the mesocratic inclusions show no clear evidence of magma mingling (cf. Vernon et al., 1988).

In contrast, the Limbani pluton (200 km 2) is texturally and mineralogically homogeneous, being dominated by an equigranular, medium-grained, biotite-muscovite granodiorite. Erratics of leucocratic syenogranite were observed, but their source was not established.

The Aricoma pluton (150 km 2) is similar in lithology to the southeastern part of the Coasa pluton and consists of a medium- to coarse-grained, K-feldspar-megacrystic, biotite granodiorite-to- monzogranite, commonly with rapakivi textures. It is, however, much richer in biotite than the Coasa batholith.

Of particular relevance to our geochronologic studies is the presence of deformational textures in the eastern half of the Coasa pluton (Laubacher, 1978) and across the entire Limbani pluton: the petrofabric features are described in detail elsewhere (Kontak, 1985; Kontak et al., 1984b; and this issue, p. 231). The A1-Si ordering relationships in the alkali feldspars of these granites illustrate the extent of this tectono-thermal event. For example, in Fig. 6 it is evident that the intermediate ortho- clase structure typical of the undeformed granites has inverted to microcline in the strained equiva- lents (cf. central area and southwestern margin versus the nor theastern margin of the Coasa pluton).

Selected chemical features of the rocks of the Carabaya Batholith are summarized in Fig. 7. We note the restricted compositional range of the


70030 , 700 69°30 ' I I I

x% LB-6 225m Sample No. K-Ar date b biotite %% %,%, (203b) Rb-Sr date m muscovite

_ %%..% C2381 U-Pb date Kf K-feldspar , i ~ %%%% %%

%%% %%%%% %,~ ~ ~ ",,, t'(203b)

%..,e~*++~++l '~;J# "'... ,COCA-268 |(204b) ,,, -..... I t,,O b,

~ : , : ~ O,,achea [2S~ / . "-....

_ .. .....

AY- 5D Ig8b / / / I I ~',/~", ~ %~ COCA-301A 189b / / / / BAR-18 %0;b "~%% ~ X

OSC- 1 11--~1b)~, .A. R % . ~ \ . . . , , , , , . / / coc._O.r ,, COCA-2.73 216m / / ~¢,'J'¢'~ /."'~k \ BAR-17 "211b COCA-421 217b/ v r-~ .~.- ~j \

" "20T / . / X " ' ¢ ,~ \ / 11371~f [234] / X \ LMP-2A (200m',

COCA-272((209b) LB-6 22am \ \ / | (1 92b) \ %~/ ~,(21 lb) \ / %%

- ~X\\ LM ~ \\%\\ % % '~%%% y %%%

% Intrusive Suites: Ananea • '\\GOD- I (133m} Jurassic ~ - ~ Deformed Granite \

Triassic

Upper Devonian ? 0 25 t .J

km

3030 ,

14 °

4030 ,

I I i

Fig. 3. Map showing the locations and ages of samples of plutonic, hypabyssal, and volcanic rocks dated in the present study (with the exception of nos. CR 385 and 386: see Table 2). The dashed lines delimit the Zongo-San Gab/m tectono-thermal zone (see Farrar etal., 1988, and companion paper in this issue, p. 231 ). Key: SG, San Gaban (Corani) Complex; MS, Macusani syenite of the Allinccbpac Peralkaline Group; CO, Coasa Batholith; AR, Aricoma Batholith; LI, Limbani 8atholith; and LM, Limacpampa Pluton.

Aricoma and Limbani centers and the alkalinity of rocks in the western, Ayapata, domain of the Coasa pluten. All suites are, in addition, characterized by: 1) K20/Na20 > 1; 2) A/CNK > 1 (except for some Aya- pata rocks); 3)low Fe203/FeO (<0.25), conforming to the S--type field of Hine et aL (1978); and 4) mole % F@+-FeS+-Ti ratios corresponding to i lmenite- series granites in the classification of J in e ta l . (1981) and in conformity with the occurrence of ilmenite as the sole oxide phase (Ishihara, 1977). Trace element analyses indicate that the Ayapata suite is relatively enriched in Nb, V, Zn, Sr, and Zr. For granitoids with such similar major and trace element compositions, the REE spectra (Fig. 7) are

markedly variable. The alkaline nature of the Ayapata suite is readily apparent from both REE contents and chondrite-normalized patterns. The larger Eu anomaly and data spread for the Coasa suite, compared to those from Aricoma and Limbani, are considered to reflect more protracted crystal fractionation, probably at tended by mixing and mingling of magma fractions at depth (cf. Barbarin, 1989). The most mafic members of the Coasa suite are markedly different from the Aricoma suite, which is characterized by greater total REE enrichment, smaller Eu anomalies, and more fractionated patterns.

220 D. J, KONTAK, A. H CLARK, E. FARRAR, D. A. ARCHIBALD, and H. BAADSGAARI)

Z 0

1.5

1.0

0.5

a )

50 I

O

O

South Mountain ~ j J ~ - ~ Batholith S ~ - ' ( C l a r k e and Halliday,

60 ° " ° e ~ ' ~ j ~ ~ e 80 1980) I /LJ-~ I

~ J c_ ~ / ~ Strontian Granodiorite / ~ / j / - (Halliday eta/., 1981)

~ e l s o n Batholith (Cox, 1979)

b ) A

C

plag

musc

• . \ II B

SMB cord

I-type ~S-type ~ biot

hbl

F Fig. 4. Selected geochemical data from the plutonic recks of the San GabOn (Corani) Complex and Limacpampa pluton, a) Alumina saturation index (A~NK) us wt% SiO~, showing wide range in both parameters, and their broad correlation, in the San GabAn rocks. Key: filled circles, data from Kontak (1985);, open circles, data from Carlier et aL (1982); filled triangles, data for leucogranitas from Kontak (1985). Shown for comparison are the compositional fields for the Nelson Batholith, (Cox, 1979) and Strontian pluten (Halliday et al., 1981), both representing !-.::~ne s~ites subject to crustal contamination, and for the South Mountain Batholith {Clarke and Halliday, 1980) -- a typical crustally derived peraluminous S-type system, b) Compositions ofthe San GabOn (circles and triangles) and Limacpampa (filled squares) suites expressed in tarms of the modified ACF ~ of White and Chappell (1977). Fields for the South Mountain Batholith (SMB), Nelson Batholith (NB), Peruvian Coastal Batholith (CBP: Johan et al., 1980), and the Main Arc graniteids of northern Chile (MNC: Haynes, 1975). Mineral phases are muscovite (muse), biotite (biot), hornblende (hbl), cordierite (cord), and plagioclase (plag). Heavy dashed line separates I- and S-type granitoids (Takahashi et al., 1980).

Sr isotope ana lyses of who le - rock /mica pa i rs (Tables l a and lb) yield Sri values of 0.7156 for the Ar icoma in t rus ion , 0.7102 for the Ayapa t a su i te (sample BAR-13), and 0.7082, 0.7087, and 0.7241 for the main Coasa pluton. The most e levated Sri (0.7241) is for the cord ie r i to -bear ing facies of the l a s t -named complex (no. COCA-265: Table lb).

The chemical composit ions of biot i tes a re spec i - fic to each intrusion, especia l ly with r e B a ~ to t race e lements (Fig. 8). For a given Mg/(Mg+Fe) rat io , biotites f rom each cen te r a re m a rk ed ly enr iched in specific e lements - - e . g . , Rb and N b i n the Coasa suite (Figs. 8a and b); V, Cr, and T i a t Ayapata ; and Ba in the Limbani area. In add i t ion , t he b io t i t e composit ions lie in di f ferent fields in the Fe2*-Fe 8+-


100

6 K20* 4

2 , d y k e s 0,

45 50 SiO 2

0 0

Ill 1--"

¢3 Z 0 -I- (3

volcanic dyke o

10 b)

o ~

Ne'

O1'

Q'

c)

I I I I I I I I I La Ce Pr Nd Srn Eu Gd Dy Er Yb

Fig. 5, Mafic volcanic and hypabyssal (dyke) rocks of, respectively, Early Permian and Triassic age, plotted in terms of: a) total alkalis vs silica, with the alkaline-subalkaline dividing Jine of Irvine and Baragar (1971); b)normative OI'-N-Q'; and c) chondrite-normalized REE abundances.

of 200 Ma (see below), an Sri of 0.70597 was calculated.

The Allinccdpac Group or peralkaline complex (previously incorporated with the Mitu Group by Laubacher, 1978) consists of a group of intrusions and a volcanic carapace. The int rusive suite includes medium- to coarse-grained gabbros and diorites, and nepheline, amphibole, pyroxene syenites. The volcanic flows comprise: aphyric rocks; rhomb porphyries displaying ternary anorthoclase phenocrysts and comparable in texture and composition to the trachytic lavas of the Oslo Rift (e.g., Dons and Larsen, 1978); and amphibole-K-feldspar-phyric units. Subsolidus modification has ob- scured much of the primary, magmatic mineralogy and texture in both intrusive and volcanic rocks. The suites are enriched in total alkalis and plot within the alkaline field of Irvine and Baragar 's (1971) classification (Fig. 9a). The molecular proportions of K, Na, and A1 (Fig. 9b) correspond mainly to the plumasitic and miaskitic clans of SCrensen (1974). Harker-type variation diagrams show broadly continuous trends for most major and trace elements, suggesting a comagmatic history for the suites. REE data (Kontak, 1985) show overall enrichment but with variably fractionated patterns that reflect, in part, varying degrees of fluid-rock interaction (e.g., Taylor et al., 1981). An Sr i of 0.7042 is estimated for the volcanic suite (see Table lb), omitting apparently aberrant data.

WHOLE-ROCK Rb-Sr DATES

Mg plot (Fig. 8c) of Wones and Eugster (1965), implying that each intrusion had a distinctive in- ternal buffer (i.e.,/02).

Of restricted outcrop but of petrogenetic importance are the basaltic minor intrusions, shown schematically in Fig. 2, found: 1) as dykes along the margins of the Coasa pluten and as xenoliths in it; 2) as dykes in the northern part of the Aricoma pluton; and 3)as dykes in the Paleozoic metasedi- ments near granite contacts. In addition, lithologi- cally similar xenoliths are locally abundant in the batholiths. The rocks contain plagioclase as the dominant constituent, with lesser pyroxene (cpx > opx), amphibole, olivine, biotite, sanidine, opaque oxides, and apatite, together with secondary chlo- rite, carbonate, sphene, and epidote.

The dykes (N = 6 analyses) are all basic in chemistry (45-48 wt.% Si02, calculated to 100% an- hydrous), but the variable CaO (3.4-15%), MgO (4.3-11.2%), Ni (40-152 ppm), Cu (8-133 ppm), and Ba (50-700 ppm) contents may reflect considerable fractionation. Rare-earth (nffi3: Fig. 5c), major, and trace (i.e., elevated Ba, St, Rb, Zr, and Nb) element abundances indicate that the rocks are alkaline to marginally subalkaline (Figs. 5a and b). The more fractionated bodies are very similar in composition to the Mitu Group basic volcanics. Assuming an age

The results of our attempts to construct whole- rock Rb-Sr isochrons for the Coasa granitoid suite and for the peralkaline volcanics of the Allincc~pac complex are summarized in Table lb. The five analyzed whole-rock Coasa samples together yield an age of 194.5+ 18.7 Ma, but the large MSWD (733) indicates that this has little significance. Replicate analyses of two samples (nos. COCA 262 and 265: Table la) show that the scatter is related to geologic phenomena. We consider it likely that both samples experienced isotopic mobility during a tectono- thermal overprinting event (see below). Regression of the three remaining whole-rock samples (nos. 268, 271 and 272) yields an age of 222.1+26.7 Ma and Sri of 0.7074 with an MSWD of 24.8 (Table lb).

Samples of the Allincc~pac peralkaline volcanic rocks yield a whole-rock age of 102.4+22.2 Ma with an MSWD of 171 (Table Ib). This anomalously low age (cf. K-Ar results below) is due to two samples (62A, 64A) that have Rb-Sr values discordant to those of the rest of the suite (Kontak, 1985). Omis- sion of these samples would yield (Table lb) a re- gressed age of 206+70.8 Ma and MSWD of 139. Again, replicate analyses indicate that the scatter is geological rather than analytical in origin, and we suspect that the initial Rb and Sr isotopic values may have been modified during pest-eruptive fluid- rock interaction.

R b - S r AND K-Ar M I N E R A L DATES

Rb-Sr and K-Ar mineral dates for the Cordille- ra de Carabaya igneous rocks are presented in Tables lb and 2, respectively, and are plotted in Figure 3. Also in Fig. 3, we delimit a zone (Farrar et al., 1988; Kontak et al., 1984c, and this issue, p. 231) in which an Eocene thermal event has systematical- ly reset K-Ar muscovite and biotite dates and dis- turbed their 4°Ar/agAr age spectra.

Mitu Group

AYAPATA

The age of the alkali basalts of the upper Mitu Group (corresponding broadly to the Iscay Group of

Klinck et al., 1986) remains inadequately defined Klinck et al. reported a K-Ar age (hornblende + biotite) of 272+10 Ma for a "lava" from the Ju l i aca area. We report here (Table 2) two K-Ar whole-rock ages, informally recorded by McBride et al. (1983), for a holocrystalline basalt and a pillow-rim obsi- dian from a basaltic flow in the Serranias de Chilla near Tiwanaku, northwestern Bolivia, at the a p - parent southeastern extremity of the Mitu Group province. The ages, 280_+3.3 and 250+2.9 Ma, respectively, are similar to that of Klinck et aL and suggest this predominantly basic volcanic event was of Early Permian age throughout this region, as suggested by Kontak et a l (1985; cF Klinck et al., 1986, p. 31).

COASA

oo/ X'"-.. • /

/-'o o,- "*" / <~p#.. -..~ .. I

p e g m a t i t e ~ / .o .NE .ma.rgin ~ ~ [] neat - t rea leo • core+SW mar~in

! ! !

m

o syenogranite ~ / ' hypersolvus granite

I I I I

ARICOMA

42 7

I

LIMBANI

i

41.5 , I I I

50.5 50.9 28 204 CuKa 1

222 D.J. KONTAK, A, H CLARK, E, FARRAR, D A. ARCHIBALD, and H. BAADSGAARi;~

, , I I i 1

Fig. 6. Estimates of structural status of alkali feldapars from the Triassic batholiths, including the Ayapata domain of the Coasa Batholith. The data are plotted in the {060) uf, {204) diagram of Wright (1968). Full details of the data have been presented by Kontak (1985).


a) COASA AYAPATA ARICOMA LIMBANI

b ) K 2 0

Na20 CaO

A musc

plag . . . . . . cord ' Z'"

C F

uJ F-

C~ Z 0

v (3 0 n-

c) 100

10

~.ii!iiiii~iiiii~'.'.i~, N=5 ~iiiiiiiiiiiiiii::::i~ : : ~

"~.~: ==========================================

/ . . . . .

apli te " ~ " , , , , ~ , ,dyke ,

La Pr SmGdDy ErYb Ce Nd Eu

/ /

! | i

N=8

::iii!!" . f ' - - - - . i l ~ l b i t i t e

~ ' ~ N= 7 " ~ ~ . . . ~

i | | I J i I | |

leucogranite

d)

.. •

+

Z

I I I I

70 78 % S i 0 2

Fig. 7. Geochemical data for the Triassic batholiths, plotted in: a) the modified ACF diagram of White and Chappell (1977); b) the ternary K-Na-Ca diagram; ©) chondrite-normalized REE abundance diagrams; and d) the total alkalis vs silica diagram. In the (d) plot, the upper line denotes the alkaline/subalkaline boundary (Irvine and Barsgar, 1971), and the lower line delimits the high- alumina basalt and calc-alkaline fields (Kuno, 1960). Note that the Ayapata suite of the Coasa Batholith exhibits a progressive increase in silica content as La N values decrease and the "Eu anomalies" increase.

224 D.J. KONTAK, A. H. CLARK, E. FARRAR, D. A. ARCHIBALD, and H. BAADSGAARI)

2000

Rb

1000

1 8 0

Nb

1 0 0

a ) $

*~ % ( N a 2 0 + A * K 2 0 ) _ o

oO ~z~ • . 10

o O o

O I I

b)

• A

I I o 0.30 0 .40

Mg/(Mg + Fe 2+) I COASA *

I I AYAPATA o C / 2 5 F e 3 + i L I M B A N I A

/ 90Fe 2+ 50Mg

Fig. 8. Compoaitions of biotitee from the Triassic batholithe, plotted: a) in the Rb ue Mg/(Mg+ Fe 24) diagram; b) the Nb ue Mg/ (Mg + Fe 24) diagram; and e) in terms of Fe s + - Fe 2 + - Mg rela- tioushipe (the buffers are from Wonee and Eugster, 1965).

a

A

! •

a l ka l i ne

/

~ suba l ka l i ne

I f i 1 5 0 55

%SiO 2

/ ~ I+B

,J 60

b. (K+Na) /AI (K+Na) /1 /6 Si

p l umas i t i c < 1 < 1

m iask i t i c < 1 ~ 1

a g p a i t i c > 1 ~ !

1.2

1.0

,¢ 0.8

¢1

v 0.6

0 .4

A G P A I T I C o

I I I I I I L I l eOI K÷Na .6 1,o ° ,.4

z~

_ P L U M A S I T I C J '~ o ~ o

• M , ^ s K , , , c 1 I I

Fig. 9. Geochemical data for the AllinccApac peralkaline group, plotted in: a) total alkalis ve silica diagram of l rv ine and Baragar (1971); and b) the (K + Na)/AI us (K + Na)/1/6Si ~ g r a m of S~r- ensen (1974). Data for intrusive (open circles) and volcanic (open triangles) rocks are from Kontak (1985); closed ¢irclee are from Laubacher (1978).

Coasa Pluton

The oldest K-Ar muscovite date determined for this center is 216 Ma (COCA-273). This agrees within analytical error with the oldest Rb-Sr biotite date (209 Ma: COCA-272) and the oldest K-At biotite date (211 Ma: BAR-17). BAR-16, a hydrothermal muscovite from a greisen associated with a small skarn body at the southwestern contact of the intrusion, yields a similar K-Ar age (213 Ma: Table 2). All are younger than the 238+11 Ma U-Pb zircon date reported by Lancelot et al. (1978) for the intrusion.

The youngest Rb-Sr date (150 Ma: COCA-265) is from a sample collected from the northeastern, thermally overprinted, part of the pluton (Kontak et al., this issue, p. 231) and we suspect it to have been partially reset. A K-feldspar from another sample (COCA-272), taken outside of zone of reset mica ages, yields a K-At date of 137 Ma (Table 2), much younger than other dates reported in the area. Considering the low argon blocking temperature of

K-feldspar, it is likely that this also reflects partial resetting.

With the exception of those for samples AY--SD and COCA-301A, all of the biotite K-Ar dates from the southwestern part of the pluton are concordant within analytical precision and fall in the range 203-211 Ma. The date (189 Ma) yielded lay sample COCA-301A, taken from the northwestern part of the Coasa pluton, is significantly younger than those determined for nearby biotites but is in broad agreement with the Rb-Sr biotite date (193 Ma: BAR-13) from the same area. As replicate Rb-Sr analyses may show considerable dispersion (e.g., COCA-272, Table la), this apparent concordance may have little significance. Near the location of sample COCA-301A, the Coasa Batholith is cut by a gabbroic intrusion thought to be related to the Allincc~pac peralkaline complex, and it is possible that heat associated with intrusion of this body could have led to a reduction in the dates. However, we cannot rule out the possibility that the slight resetting of the Rb-Sr and K-At mica dates in the


northwestern quadrant of the pluton is a conse- quence of the Eocene thermal event. We consider the K-Ar biotite date of sample BAR-13 (205 Ma) to be the best estimate of the cooling age of this part of the intrusion.

Aricoma Pluton

Micas from the Aricoma pluton and its environs yield the oldest K-Ar dates (Table 2) obtained in this study. The muscovite date for sample LB-6 (225 Ma), from a hydrothermally altered zone within the metasedimentary country-rocks, and biotite da tes from samples COCA-421 (217 Ma) and COCA-372 (211 Ma), are similar to, but slightly higher than, comparable mineral dates yielded by granitoid samples from the Coasa pluton. Dalmay- r a c e t al. (1980) have reported, but without analytical details, a U-Pb zircon date of 234±9 Ma for the Aricoma pluton. As for the Coasa pluton, the U- Pb date is somewhat higher than the Rb-Sr and K- Ar mica dates (see below).

Limacpampa Pluton

An Rb-Sr analysis of muscovite (Table 1) from the Limacpampa intrusion yields a date (ca. 200 Ma) in general agreement with those from the Coasa and Aricoma plutons if a reasonable Sri (0.710) is chosen. To yield the Carboniferous age that has been assigned to this S-type pluton, an impossibly low (negative) Sr i would be required. On the basis of an assumed Sri of 0.710, a muscovite from the Gavil~n de Oro vein gold (-tungsten-tin) deposit yields an age of 133 Ma (Table 1). The deposit is within the zone of reset K-Ar mica ages (Fig. 3; Kontak et al., this issue, p. 231), and we suspect that this date has also been reset.

Allinccdpac Complex Syenite

Biotite in a sample of nepheline syenite (MS-l), collected from a southwes tern exposure of the Allincc~pac complex, yields a K-Ar date of 175 Ma (Table 2), which is in permissive agreement with the date (184 Ma) reported by Stewart et al. (1974) for the intrusion. A further sample of syenite from this intrusion (COCA 305) gave a younger K-Ar biotite date of 156 Ma (Table 2); however, this sample is distinctly foliated and the date may reflect the effects of the Eocene tectono-thermal event (Kontak etal., this issue, p. 231).

DISCUSSION

Geochronology of the Magmatic Rocks

Our new radiometric dates, in combination with previously published age determinat ions, allow limits to be placed on the time of intrusion and the subsequent cooling history of the mgior granitoid

SAES 3/~-E

bodies of the Cordillera de Carabaya. At the outset it should be emphasized that we have no geochronological data to confirm the local occurrence of late Paleozoic granitoid intrusion, such as is documented in reconnaissance fashion by Bonhomme et al. (1985) for a more northwesterly transect of the Cordillera Oriental (ca. lat. 13°27 °, long. 70°54'), where hornblende K-Ar dates for monzodior i te (331+3 Ma) and essexite (294+3 Ma) provide evidence for alkaline magmatic activity in the Car - beniferous or earlier. Our maximum K-Ar dates (216 to 225 Ma) are very similar to those presented by McBride et al. (1983) for granitoid rocks of the Cordillera Real, northwestern Bolivia. For example, the oldest K-At biotite dates reported for the Chucura (or Huayfia Potosi) and Illampu intrusions in that area are 218 and 219 Ma, respectively, and a slightly younger muscovite from the Chucura pluton yielded an undisturbed 4°Ar]~gAr age spectrum (McBride et al., 1987). For the small Ayancuma stock situated in the Cordillera Mufiecas, ca. 75 km southeast of Ananea (see Fig. 2), McBride et al. (1983) reported K-Ar biotite dates of 218 and 225 Ma and a concordant muscovite date of 221 Ma. The general agreement of the oldest K-Ar muscovite and biotite dates for these rocks was interpreted as indicating the time of post-emplacement cooling through their respective argon closure temperatures (ca. 350 and 250°C) in Late Triassic time. Further- more, the lack of any analytically significant dif- ference between the muscovite and biotite dates reported in beth studies suggests cooling through this temperature range was relatively rapid.

The overall range of K-Ar dates reported in this study is also comparable to that reported by McBride et al. (1983) for the Cordilleras Real and Mufiecas, and most of our new Rb--Sr dates lie in this range. We suspect that some of the dispersion among the dates is due to late-stage hydrothermal activity related to the numerous zones of mineral i- zation (Clark et al., 1984) in the region. Conven- tional K-Ar analyses and 4°Ar/SgAr age spectra reported in a companion paper (Kontak et al., this issue, p. 231) and in McBride et al. (1987) show that overprinting, associated with an Eocene tectono- thermal event, decreases in intensity from nor th- east to southwest across the northeastern portions of the intrusions dated here (between the dashed lines in Fig. 3). All the K-At mica dates and most of the Rb-Sr dates presented here are from samples taken southwest of this zone, and thus we consider it unlikely that the dispersion of dates is associated with this event, although the date (137 Ma) of sample COCA-272, the only K-feldspar dated in this study, probably reflects partial re-setting during the Eo- cene. Further work is required to evaluate fully the lower temperature history of the southwest portions of the intrusions.

Taken at face value, the U-Pb zircon dates for the Coasa (Lancelot et al., 1978) and Aricoma (Dal- mayrac et al., 1980) plutons m ca. 235 Ma m com- bined with our mica dates, suggest tha t 10-15

226 D.J. KONTAK, A H CLARK, E. FARRAR, D A ARCHIBALD. and H. BAADSGAARL,

million years elapsed between the time of emplacement of the intrusions and the time they cooled through the closure temperature of argon diffusion in micas. In view of the relatively shallow depth of intrusion (about 8 km: Kontak, 1985) and the concordance of muscovite and biotite dates noted above, such protracted cooling (-750°C to 250°C in 10-15 million years) seems unlikely. Therefore, we suggest that the zircons analyzed by Lancelot et al. and Dalmayrac et al. (op. cit.) may have an undetected, inherited Pb component - - a feature in conformity with the petrologic nature of the suites and the observed presence of corroded cores in the zircons. Zircons inherited from the basement during partial melting, and subsequently suffering the lead toss noted by those authors, would produce upper con- cordia intersections in excess of the true age of emplacement. In this case, the age of emplacement must be less than the youngest 2°TPb/2°6Pb date, which, we note, is 230 Ma for the Coasa Batholith. Unfortunately, our whole-rock Rb--Sr date for the Coasa pluton, although consistent with this inter- pretation, is not sufficiently constrained to resolve this dilemma. We conclude, however, that the Coa- sa and Aricoma plutons were emplaced in the Late Triassic, between 220 and 230 Ma.

Our tentative Rb--Sr mineral date (ca. 200 Ma) for the Limacpampa intrusion does not lend support to its presumed "Hercynian" age ( Laubacher, 1978; Kontak, 1985). We can only conclude that either the original age assignment is incorrect or the intrusion was totally re-set during the Triassic intrusive episode documented above. Our dates for the Allincc~- pac complex syenite support the Early Jurassic age suggested by Stewart et al. (1974).

The approximate areal coincidence of the Trias- sic granitoid batholiths with the Permo-Triassic Mitu Basin (Newell et al., 1953; Laubacher, 1978; McBride et al., 1983) is additional, albeit indirect, evidence that generation and emplacement of the felsic magmas were controlled by the tectonic fabric of this ensialic extensional domain. We have de- monstrated elsewhere (Kontak et al; 1985; Clark et al., in press) that small volumes of K-rich hornblende-biotite granodiorite of clear arc-type were emplaced in the Early Permian, contemporaneous with Mitu Group alkalic volcanism along the southwestern margin of the rift zone, but there is no clear evidence for such Permian activity in the Cordillera de Carabaya.

It is also emphasized that a general regional age progression is evident for lower Mesozoic granites of the Cordillera Oriental, such that the older (i.e., 250-260 Ma: Carlier et al., 1982) intrusions occur in the northwestern half of the Mitu Basin and the younger (i.e., 210-220 Ma: this study and that of McBride et al., 1983) in the southeastern part. Rela- tionships with the Carboniferous granitoid activity documented in outline by Bonhomme et al. (1985) are unclear. A similar but more systematic and abrupt, NW to SE age progression, from 28 to 19 Ma, is found in the Tertiary granitoid subprovince of

northwestern Bolivia (McBride et a l , op cit.). Such migration of magmatism along a structure subpar- allel to a plate boundary contrasts with the development of longitudinal belts of coeval igneous rocks that typifies arc- or subduction-zone related tectonic environments, unless convergence is strongly oblique. As pointed out by McBride et al. (1983), the relationships reported for eastern Peru and northwestern Bolivia are analogous to those displayed where continental crust has migrated over a per- sistent thermal anomaly, as documented for the granites of Nigeria (Bowden et al., 1976; van Bree- man et al., 1975).

Late Paleozoic-Early Mesozoic Magmas: Products o f Disparate Source Regions

The most striking feature of the post-Paleozoic magmatic suites of the Cordillera de Carabaya is the diversity of rock types, a characteristic of the Central Andean Inner Arc magmatic domain. How- ever, the similarities between the Permian Mitu Group alkali basalts and the Triassic (-Jurassic?) mafic dykes imply close links between the Andean and pre-Andean environments. Chemical and iso. topic data indicate both mantle and crustal sources and, moreover, heterogeneity within these sources. The San GabOn complex, the Mitu Group volcanics, the basaltic minor intrusions, and the Allincc~pac complex represent mantle-derived suites, whereas the Limacpampa intrusion and the Ca rabaya granitoid batholith are largely products of crustal fusion.

The subalkaline nature of the San Gaban complex distinguishes it from the other mantle--derived suites, which are of alkaline-peralkaline character. It has been suggested (Laubacher, 1978) that i t was intruded in a compressional environment either during, or at the close of, the "early Hercynian" orogenic event of M6gard et al. (1971). Strikingly similar petrologic and geotectonic relationships are exhibited by the Proterozoic Hepburn granitoids of the Wopmay orogen, Canada (Lalonde, 1989). In contrast, emplacement of the other mantle-derived magmas coincided with regional extension during the protracted evolution of the Mitu ensialic basin.

The petrographic and chemical similarity of the Mitu Group mafic volcanics and the younger basaltic dykes (Fig. 5) suggests a common source. Because the dykes are known to be both contemporaneous with and younger than the ca, 220-230 Ma granites, this observation has the important implication that a source of basaltic magma was accessible over a considerable period.

The crustally-derived suites may also have ori- ginated from different protoliths. The Limacpampa granites clearly represent fusion of a highly aluminous source region, probably under very reducing conditions. In contrast, the demonstrably Triassic batholith is predominantly less aluminous and, in most chemical and petrographic respects, intermediate between the S- and I-type granitoid facies

Late Paleozoic--early Mesozoic magmatism in the Cordillera de Carabaya, SE Peru 227

of Chappell and White (1974) and the ilmenite and magnetite series of Ishihara (1977). The Carabaya Batholith conforms to the "I-Caledonian" granitoid type of Pitcher (1983) with respect to its tectonic setting and biotite-dominant mafic mineralogy, but the peraluminous indices and high S r i a r e not typical of such suites. The batholith displays significant chemical variability. Differences in Sr i and biotite chemistry are considered to reflect, in part, primary heterogeneities in the source region (e.g., Strong and Hanmer, 1981). The overall chemical and mineralogical features of the batholithic granitoids are tentatively interpreted as resulting from partial melt ing of mixed meta-sedimentary and meta-igneous crustal components; fusion probably occurred under fluid-absent conditions (e.g., Viel- zeuf and Holloway, 1988), controlled mainly by biotite dehydrat ion-melt ing, but locally to some extent by hornblende breakdown.

The significance of the Allincc~pac peralkaline complex remains uncertain, but it apparently indicates a resumption of mantle-derived magmatism in the Early or Middle Jurassic, approximately coincident with the ini t ial development of the Central Andean Main Arc, now preserved along the southern Peru littoral. The radical differences in chemistry, including Sri , between this suite and the earlier alkali basalts argue against derivation from a common or similar mantle environment. Alterna- tively, it is possible that the major Late Triassic episode of crustal melting and granitoid magmatism had involved stepwise fractional melt ing, predominantly of biotite and hornblende (Presnall and Bateman, 1973), and that a further rise in lower- crustal temperatures in the Early Jurassic stimula- ted melting of residual amphibole and pyroxene, hence generating peralkaline, nepheline normative, intermediate magmas (see also Whitney, 1988). The approximate Sri value, 0.7042, defined by our work would, however, be in better agreement with a mantle source.

Interrelationships Between Mantle and Crustal Sources in Late Paleozoic-Early Mesozoic Magmatism: Local and Regional IrnpIications

The upper Paleozoic (?) and lower Mesozoic granitoid rocks have until now been treated as discrete igneous suites. However, the temporal and spatial relationships suggest a long-term unifying cause-and-effect relationship, similar to that pro- posed for the generation of basic and felsic magmas in the Permian Oslo paleorift (Ramberg and Larsen, 1978). The petrology of the Ayapata (-Coasa) granitoid suite indicates that the involvement of a mantle component in the genesis of these suites cannot be excluded. We suggest that the large granitoid batholiths represent the products of crustal fusion due to sub--crustal ponding of basic, mantle--derived, melts. Melting was probably effected by elevation of the geotherm and aided by volatiles released from basic melts. The emplacement of mantle--derived

magmas over an extended period indicates that the process that initially perturbed the mantle and caused generation of melts was also long-lived (ca. 80 Ma). The emplacement of the Allincc~pac alkaline-peralkaline suite at the apparent termina- tion of this Permian to Jurassic magmatic cycle may reflect the eventual blockage by large volumes of viscous felsic melt of the upward migration of volatiles expelled from the "ponded" basic magmas, with concomitant metasomatism of the immediately surrounding mantle. The presence of the mafic end- member component of the essentially bimodal magmatic province of southeastern Peru provides a rare opportunity to examine the variability of rock types in a province with both orogenic and anorogenic attributes. The importance of this association is realized when considering the large volumes ofcom- positionally similar granites (sensu lato) that occur in more ambiguous geological settings worldwide, such as the Trans-Labrador Batholith, Labrador (Kerr, 1987), the South Mountain Batholith, Nova Scotia (McKenzie and Clarke, 1975, Clarke and Muecke, 1985), and the anorogenic magmatic province of central North America (Anderson and Cullers, 1978; Bickford et al., 1981), where the field relationships do not provide as clear a picture as in southeastern Peru. We infer that the evidence presented here argues for a much more common crust- mantle connection in granite petrogenesis than is generally accepted.

The widespread emplacement of alkaline/peralkaline magmas in the foreland regions of sub- duc t ion- re la ted arc sys tems has been wide ly documented since the work of Kuno (1960); Munoz and Stern (1989) report such a relationship in the Plio-Quaternary Austral Andes. However, among the Permian to Jurassic units considered here, only the Allincc~pac peralkaline complex may be shown to have been contemporaneous with major Main Arc calc-alkaline volcanism and intrusion at this lati- tude (e.g., Boily et al., 1984), al though the Mitu Group alkali basal ts were con tempora ry with restricted intrusion of granodioritic rocks of sho- shonitic affinity on the southwestern and probably northeastern margins of the Mitu rift zone (Clark et al., in press), const i tut ing an unusual areal and temporal association of ensialic rift and cont in- ental-arc igneous suites.

The highly varied intrusive and extrusive rocks documented here were emplaced immediately prior to, and in the early stages of, the Andean orogenic cycle at these latitudes. Broadly similar but less varied magmatic suites have been documented from the Andean basement in northern Chile (Breitkreuz et al., 1989), where assemblages of metaluminous and peraluminous granitoids and volcanics are interpreted as reflecting a transition from plate convergence and subduction to crustal distension over mantle plumes along the western margin of Gond- wana (e.g., Kay et al., 1989). However, these events had apparently terminated by early Norian t ime (ca. 220 Ma), and were thus temporally separated

228 D.J. KONTAK, A. H. CLARK, E. FARRAR. D. A. ARCHIBALD, and H. BAADSGAARf)

f r o m t h e A n d e a n o r o g e n y proper ( M c B r i d e et al., 1976; B r e i t k r e u z et al., 1989). In c o n t r a s t , e m p l a c e - m e n t of t he C a r a b a y a B a t h o l i t h was c o n t e m p o r a r y , a t l e a s t in i t s l a t e r s t a g e s , w i t h t h e i n i t i a t i o n of A n d e a n o r o g e n y in s o u t h e r n P e r u a n d is t h e r e f o r e i n t e r p r e t e d a s r e p r e s e n t i n g the f i r s t m a j o r e v e n t in t h e e v o l u t i o n o f t he A n d e a n I n n e r Arc d o m a i n .

Clark, A. H., Kontak, D, J., and Farrar, E., 1984. A comparative study of the metallogenetic and geochronological relationships in the northern part of the Central Andean tin belt, SE Peru and NW Bolivia. In: Proceedings of the Sixth Quadrennial IAGOD Symposium, Tblisi, pp. 269-279. E. Schweizerbart'sche Verlags- buchhandlung, Stuttgart, West Germany.

Clark, A. H., Kontak, D. J., and Farrar, E., n.d. 'rhe San Judas Tadeo Wi-Mo, Au) deposit: Permian lithophile mineralization in southeastern Peru. Economic Geology, in press.

Aehnowlsdgements--Field studies were supported by grants to AHC and EF from the Natural Sciences and Engineering Re- search Council of Canada; laboratory studies by grants from NSERC to AHC, EF, and HB. Logistical assistance in the field was generously provided by Minsur, SA through the good offices of Ing. Fausto Zavaleta C. We benefited greatly from the en- couragement and advice oflng. Mario Arenas F. The manuscript was considerably improved by the constructive comments of the referees, Norman Snelling and Christoph BreitkretLz.

REFERENCES

Ahlfeld, F., and Branisa, L., 1960. Geologia de Bolivia. Iustituto Boliviano de Petr61eo, La Paz, 245 p.

Anderson, J. L., and Cullers, R. L., 1978. Geochemistry and evolution of the Wolf River batholith, a Late Precambrian rapakivi massif in north Wisconsin, USA. Preearnbrian Research 7, 287-342.

Barbarin, B., 1989. Importance des differents processus d~ybri- dation dens lee plutous granitiques du batholite de la Sierra Nevada, Californie. Schweizerische Minsralogische und Petro-- graphieche Mitteilungen 69,303-315.

Bickford, M, E., Sides, J. R., and Cullers, R. L., 1981. Chemical evolution of magmas in the Proterozic terrane of the St. Fran@ois Mountains, southeastern Missouri,l: Field, petrographic, and major element data. Journal of Geophysical Research 86 (B11), 10865-10386.

Clarke, D. B., and Halliday, A. N., 1980. Strontium isotope geology ofthe South Mountain batholith, Nova Scotia. Geochimica et Cosmochimica Acta 44, 1045-1058.

Clarke, D. B., and Muecke, G. K., 1985. Review of the petrochemistry and origin of the South Mountain Batholith and associated plutons, Nova Scotia, Canada. In: High Heat Production (HHP) Granites, Hydrothermal Circulation and Ore Genesis, pp. 41-54. Institution of Mining and Metallurgy, London, UK.

Cox, J., 1979. The Geology of the Southwest Margin of the Nelson Batholith, British Columbia. Unpublished MSc thesis, Univer- sity of Alberta, Edmonton, Alberta, Canada, 95 p.

Dalmayrac, B., Laubacher, G., and Marocco, R., 1980, ~gbz des Andes Pdruviennes. ORSTOM, Travaux et Documents 122, 501 p.

Dons, J. A., and Larsen, B. T., 1978. TheOsloPalsorifl: A Review and Guide to Excursions. Universitstaforlaget, Trondheim, Nor- way, 199 p.

Farrar, E., Clark, A. H., Kontak, D. J., and Archibald, D. A., 1988. Zongo-San Gabim Zone: Eocene foreland boundary ofthe Central Andean orogen, northwest Bolivia and southeast Peru. Geology 16, 55-58.

Francis, G. A., 1956. La Geologia de la zona entre Macmmni y Ollachea, Departamento de Puno. Bolsttn del Instituto Naeianal de lnvestiguciones y Fomento Minero, Peru 21, 5--10.

Fryer, B. J., 1977. Rare-earth evidence in iron formations for changing Precambrian oxidation states. Geochimiea et Cosmo- chirnica Acta 41,361-367.

Bolly, M., Brooks, C., and James, D. E., 1984. Geochemical char- acteristies of the late Mesozoic Andean volcanics. In: Andean Magmatism: Chemical and Isotopic Constraints (edited by R. S. Harmon and B. A. Barroiro), pp. 190-202. Shiva Publishing, Nantwich, Cheshire, UK.

Croft, S. P., Baadsgaard, H., Muehlenbachs, K., and Scarfe, C. M., 1982. Rb-Sr isochron ages, magmatic sTSrPeSr initial ratios, and oxygen isotope geochemistry ofthe Proterozoic lava flows and intrusions of the East Arm of Great Slave Lake, Northwest Terri- tories, Canada. Canadian Journal of Earth Sciences 19,343-356.

Bonhomme, M. G., Audebaud, E., and Vivier, G., 1985. Ededes K-Ar de rocas hercinicas y ne6genas de un perfll E-W en el Per0 meridional. Comunieucionss(Santiago) 35, 27-30.

Bowden, P., van Breeman, O., Hutchinson, J., and Turner, D. C., 1976. Paleozoic and Mesozoic age trends for some ring complexes in Niger and Nigeria. Nature 259, 297-299.

Breitkreuz, C., Bah/burg, H., Delakowit~, B., and Pichowiak, B., 1989. Palsozoic volcanic events in the Central Andes. Journal of South A merican Earth Sciences 2, 171-189.

Carlier, G., Grandin, G., Laubacher, G., Marocco, R., and M,~gard, F., 1982, Present knowledge of the magmatic evolution of the Eastern Cordillera of Peru. Earth Science Reviews 18, 253-283.

Chappell, F. W., and White, A. R.J., 1974. Two contrasting granite types. Pacific Geology 8, 173-174.

Clark, A. H., Farrar, E., and McNutt, 1~ H., 1983. Evolution of the Central Andean rnagnmtic arcs. EOS, Transactions of the American Geophysical Union 64,326.

Halliday, A. N., Stephen, W. E., and Harmon, R. S., 1981. Isotopic and chemical constraints on the development o f peraluminous Caledonian and Acadian granites. Canadian Miwaralogist 19, 205-216.

Haynes, S. J., 1975. Granitoid Petrochemistry, Met~llo@eny and Li~hospheric Plate Tectonics, AW~ama Province, Chile, Unpub- lished PhD thesis, Queen's University, Kingston, Ontario, Cana- da, 330 p.

Hine, R., Williams, I. S., Chappell, B. W., and White, A. J. R., 1978. Contrasts between I- and S-type granitoide of the Kcakiu- sko Batholith. Journal of the Geological Society of Austrolia 25, 219-234.

Irvine, T. N., and Baragar, W. R. A., 1971. A guide to the chemical classification of the common volcanic rocks. Canadian Jour- nal of Earth Sciences 8, 523-548.

Ishihara, S., 1977. The magnet/te--~ries and i lmenite-ser ies granitic rocks. Mining Geology (Japan) 2'/, 293-305.


Jemmn, L. S., 1976. A New Cation Plot for Classifying Sub- alhaline Volcanic Roche. Ontario Division of Mines, Miecel- laneons Paper 66, 22 p.

Jin, M. S., Kim, S. Y., and Lee, J. S., 1981. Granitic magmatism and associated mineralization in the Gyeongsang Basin, Korea. Mining Geology (Japan) 31,261-280.

Johan, Z., Le Bel, L., and McMillan, W. J., 1980. Evolution g6o- logiqus et p~trologique des complexes granitoldes fertiles. In: Min~mliention Lice aux Graniloidce (edited by Z. Johan.). M~- moire du Bureau de Recherches G4mlogiques et Mini~ree, France 8, 21-70.

Kay, R. W., and Gu t , P. W., 1973. The rare earth content and origin ofalkali-rich basalts. Journal of Geology 81,653-682.

Kay, S. M., Ramos, V. A., Mp6dozis, C., and Sruoga, P., 1989. Late Paloosoie to Jurassic silicie magmatism at the Gondwana margin: Analogy to the Middle Protorozoic in North America? Geology 17, p. 324-328.

Kerr, A , 1987. The Trans-Labrador Batholith in eastern Labra- dor: Post-orogenic magmatism related to a co. 1650 Ma event along the southern margin of the Canadian Shield. Geological Association of Canada Program with Abstracts 12, 60.

Klinck, B. A., Ellison, R. A., and Hawkins, M. P. (compilers), 1986. The Geology of the Cordillera Occidental and Altiplano West of Labs T'gicaca, Southern Peru. British Geological Survey and Institute Geol6gico, Minero y Metahlrgico, Lima, Peru, 353 p.

Kontak, D. J., 1985. The Magmatic and Metallogenetic Evolution of a Craton--Orogen Interface: The Cordillera de Carabaya, Cen- tral Andes, SE Peru. Unpublished PhD thesis, Queen's Univer- sity, Kingston, Ontario, Canada, 714 p.

Kontak, D. J., Clark, A. H., and Farrar, E., 1984a. The magmatic evolution ofthe Cordillera Oriental, southeastern Peru. In: An- dean Magmatism: Chemical and Isotopic Constraints (edited by B. A. Barreiro and R. A. Harmon), pp. 203-219. Shiva Publish- ing, Nantwich, Cheshire, UK.

Kontak, D. J., Clark, A. H., and Farrar, E., 1984b. The influences of fluid and rock compositions, and tectono-thermal processes on the AI-Si distribution in alkali feldspars in granitoid rocks, SE Peru. Bulletin de Min~ralngie 107,387-400.

Kontak, D. J., Clark, A. H., and Farrar, E., 1984c. Radiometric evidence for a major Paleogene structural break, Central Andean Eastern Cordillera. Geological Society of America Abstracts with Programs 16, 546.

Kontak, D. J., Clark, A. H., Parrar, E., and Strong, D. F., 1985. The rift-associated Permo-Triassic magmatism of the Eastern Cordillera: A precursor to the Andean orogeny. In: Magmatism at a Plate Edge: The Peruvian Andes (edited by W. S. Pitcher, M. P. Athertun, E. J. Cobbing, and R. D. Beckinsale), pp. 36-44. Blackie, Glasgow, UK.

Kontak, D. J., Farrar, E., Clark, A. H., and Archibald, D. A., 1990. Eocene tectono-thermal rejuvenation of an upper Paleosoic-lower Mesozoic terrain in the Cordillera de Carabaya, Puno, SE Peru, revealed by K-At and 4OArPSAr dating. Journal of South A merican Earth Sciences 3 (4), 231-246.

Kuno, H., 1960. High-alumina basalt. Journal of Petrology 1, 121-145.

Lalonde, A. E., 1989. Hepburn intrusive suite: Peraluminous plutonism within a closing back-arc basin, Wopmay orogen, Can- ada. Geology 17,261-264.

Lancelot, J. R., Laubacher, G., Marocco, R., and Renaud, U., 1978. U/Pb radiochronology of two granitic plutons from the Eastern Cordillera (Peru): Extent of Permian magmatic activity and consequences. GeologiecheRundschau 67,236-243.

Laubacher, G., 1978. Estudio Gedlngico de la Regidn Norte del Lago Titicoca. Institute Geologta y Miner/a, Peru, Boletln 5, 120 p.

Martinez, C., 1980. Structure et Evolution de la Chains Her- cyni~nne et de la Chains Andine clans le Nord de la Cordill#}re dee Andes de Bolioie. ORSTOM, Travaux et Documents 119, 351 p.

McBride, S. L., Caelles, J. C., Clark, A. H., and Farrar, E., 1976. Paleozoic radiometric age provinces in the Andean basement, latitudes 25-3YS. Earth and Planetary Science Letters 29, 373- 383.

McBride, S. L., Clark, A. H., Farrar, E., and Archibald, D. A., 1987. Delimitation of a cryptic Eocene tectono-thermal domain in the Eastern Cordillera of the Bolivian Andes through K-Ar dating and 4OAr-SeAr stop-heating. Journal of the Geological Society, London 144,243-255.

McBride, S. L., Robertson, R. C. R., Clark, A. H., and Farrar, E., 1983. Magmatic and metallogenetic episodes in the northern tin belt, Cordillera Real, Bolivia. Geologische Rundschau 72, 685- 713.

McKenzie, C. B., and Clarke, D. B., 1975. Petrology of the South Mountain Batholith, Nova Scotia. Canadian Journal of Earth Sciences 12,1209-1218.

M~gard. F., 1978. Etude Cdolag/que des Andes du Pdrou Central. M~moires de I'ORSTOM 86,310 p.

M6gard, F., Dalmayrac, B., Laubacher, G., Marocco, R., Martinez, C., Paredes, J., and Tomasi, P., 1971. La Chalne hercynibnne au P~rou et en Bolivie: Premiers r6sultats. Cahiers ORSTOM, S~rie G~olag/que 3, 5-44.

Middlemost, A. K., 1975. The basalt clan. Earth Science Reviews 11,337-364.

Miyashiro, A., 1974. Volcanic rock series in island ares and active continental margins. AmericanJournalofSeicnce 274,321-355.

Mm~oz, J. M., B., and Stern, C. R., 1989. Alkaline magmatism in the segment 38-39"S of the Plio-Quatornary volcanic belt of the southern South American continental margin. Journal of Geo-- phyeicalResearch 94 (B4), 4545-4560.

Newell, N. D., Chronic, J., and Roberts, T., 1953. Upper Paleozoic of Peru. Geological Society of America, Memoir 58, 276 p.

Noble, D. C., Vogel, T. A., Petorson, P. S., Landis, G. P., Grant, N. K., Jezok, P. A., and McKee, E. H., 1984. Rare--element-enriched, S-type ash-flow tufts containing phenocryste of muscovite, anda- lusite, and sillimanite, southeastern Peru. Geology 12, 35-39.

Pitcher, W. S., 1983. Granite type and tectonic environment. In: Mountain Building Processes (edited by K. Hstt), pp. 19-40. Aca- demic Press, London, UK.

Pitcher, W. S., and Bussoll, M.A., 1985. Andean dyke swarms: Andesite in synplutonic re la t ionship with tonal i te . In: Magmatiem at a Plate Edge: The Peruvian Andes (edited by W. S. Pitcher, M. P. Atherten, E. J. Cobbiug, and R. D. Beckinsale), pp. 102-107. Blackie, Glasgow, UK.

Powell, J. L., and Bell, K., 1974. Isotopic composition of strontium in alkalic rocks. In: The Alkaline Rocks (edited by H. S~r- ensen), pp. 412-420. John Wiley and Sons, London, UK.

230 D.J. KONTAK, A H. CLARK, b_i FARRAR, D. A. ARCHIBALD, and H. •AADSGAARI)

Presnall, D. C., and Batsman, P. C., 1973. Fusion relations in the system NaAISi3Os-CasAI2SisOs-KAISi3Os-H20 and the generation of granite magmas in the Sierra Nevada Batholith. Bulletin of the Geological Society of A merica 84, 3181-3202

Taylor, R. P., Strong, D. F., and Fryer, B. J., 1981. Volatiie COnr tro] of contrasting trace element distributions in peralkaline granitic and volcanic rocks. Contributions to Mineralogy and Petrology 77,267-271

Ramberg, I. B., and Larsen, B. T., 1978. Tectonomagmatic evolution. In: The Oslo Paieorifl (edited by J. A. Dons and B. T. Larsen). Norges Geologiske Undersckelse, Trondheim. Bulletin 45, 55-73

Ramos, V. A., 1988. Late Proterozoic-early Paleozoic of South America o A collisional history. Episodes 11 (3), 168-174.

Van Breeman, O., Hutchinson, J, and Bowden, P., 1975. Age and origin of the Nigerian Mesozoic granites: An Rb-Sr isotopic study Contributions to Mineralogy and Petrology 50, 157--172

Vernon, R. H., Etheridge, M. A,, and Wall, V. J., 1988. Shape and microstructure ofmicrograniteid enclaves: Indicators of magma mingling and flow. Lithos 22, 1-11.

S@rensen, H., 1974. Alkali syenites, feldspathoidal syenites and related lavas. In: The Alkaline Rocks (edited by H. Sarensen), pp. 22-52. John Wiley and Sons, London, UK.

Steiger, R. H., and Jiiger, E., 1977. Subcommission on gee- chronology: Convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters 36, 359- 362.

Stewart, J. W., Evernden, J. F., and Snelling, N. J., 1974. Age determinations from Andean Peru: A reconnaissance survey. Bulletin of the Geological Society of A merica 85, 1107-1116.

Strong, D. F., and Hanmer, S. K., 1981. The leucogranites of southern Brittany: Origin by faulting, frictional shearing, fluid flux and fractional melting. Canadian Mineralogist 19,163-176.

Vielzeuf, D., and Holloway, J. R., 1988. Experimental determination of the fluid-absent melting relations in the politic system: Consequences for crustal differentiation. Contributions to Mineralogy and Petrology 98, 257-276.

White, A. d. R., and Chappell, B. W., 1977. Ultrametamorphism and granitoid genesis. Tectonophysics 43, 7-22.

Whitney, J. A., 1988. The origin of granite: The role and source of water in the evolution of granitic magmas. Bulletin of the Geo-- logical Society of A merica 100, 1886-1897.

Wright, T. L., 1968. X-ray and optical study ofalkati feldmpar, I!: An X-ray method of determining the composition and structural state from measurement of 2 values for three reflections. Ameri- can Mineralogist 53, 88-104.

Takahashi, M., Aramaki, S., and Ishihara, S., 1980. Magnetite- series/ilmenite-series vs. I-type/S-type granitoids. In: Granitic Magmatism and Related Mineralization (edited by S. Ishihara and S. Takenouchi). Mining Geology (Japan), Special Issue 8, 13-28.

Wilson, A. D., 1960. The microdetermination of ferrous iron in silicate minerals by a volumetric and colorimetric method. Ana- lyst 85, 823-827.

Wones, D. R., and Eugster, H. P., 1965. Stability of biotite: Experiment, theory, and application. American Mineralogist 50, 1228-1272.

Resumen--E1 dominio del Arco Interno, la manifestaci6n mas oriental de la orogenia Andina Central post- Paleozoica en el sureste de Peru yen Bolivia occidental, comprende ana asociaci6n particularmente diversa de rocas plutbnicas y volc~nicas, muchas de las cuales sertan mas caracteristicas de rifts eusililicos o de cinturones orog~nicos colisionales que de un mlirgen de placa convergente de tipo andino. Marcados contrastes petrol6gicos con el dominio del Arco Principal, que es mas homog6neo, y que subyace a l a s provincias occidentales del orbgeno, se han mantenido desde la iniciaci6n de la orogenia Andina en el Tri~sico tardio. Se aportan aqut dates geocronoldgicos de K-Ar y Rb-Sr y an(disis de elementos mayores y menores de rocas lgneas representativas pre-Cret~cicas de la Cordillera de Carabaya del SE de Pertt que permiten dilucidar la cronologia y petrog6nesis de tas etapas tempranas de la prolongada evoluci6n del Arco Interno y sus antecedentes P~rmicos. Ntmstros estudios confirman la siguiente secuencia de eventos magm~ticos que se superimpusieron temporalmente con las etapas iniciales de la orogenia andina: i) Intrusi6n del complsjo gabroico a granftico San Gab~in (Corani), una suite calco-alcalina, pero con-

taminada, que intruye una extensa fLrea metam6rfica de alto grado pero de baja presi6n compuesta por estratos paleosoicos inferiores. E1 complejo ha sido asignado al Paleozoico medio, pero su edad est~ pobremente definida. Los granitos de dos micas, marcadamente peraluminosos y foliados del plutbn de Limacpampa, de menor tamaflo, puede haberse emplazado en el Paleozoico, pero nuestros dates de Rb- Sr favorecen una edad Tri~sica.

ii) Erupci6n de l a v u bamilticas alcalinaa del Grupo Mitu del P6rmico inferior a 1o largo del margen NE de an ri~ ensiAlico longitudinal, que se desarroll6 en rsepuesta a ana tectSnica extensional en el intervalo entre las orogenias pre-Andina ("Hercinica tardia') y Andina.

iii) Emplazamiento de grades plutones graniticos (centros de Coasa, Limbani y Aricoma) con afinidades 1- Caledonianas, a 1o largo del limits NE del ril~ Mitu durante el Tri~sico tardio (ca. 225 Ma). Los monsogranito8 y granitos metaluminosos a levemente peraluminosos que comprenden la mayor parts del Batolito Carabaya (t~rmino nuevo) estuvieron estrechamente asociados con diques m/ificos de com- posicibn alcalina, similares en mucbos aspectos a los basaltos alcalnos precedentes.

iv) Desarrollo del Grupo o complejo peralcalino Allincc~pac (t~rmino nuevo); una asociacibn de lavas jur~sico medias (e inferiores), piroclastitas y plutones, que exhiben afinidades alcalinas a peralcalinas.

Mientras qtm cada una de las asociaciones arriba mencionadas pueden ser asignadas ya sea a una fuente de manto o cortical, es evidente, de acusrdo a los datos quimicose isotSpicos, que han estado involucrados una variedad de ambientes de manto y corticales. Luego, las composiciones quimicas peculiares de las intru- siones granitoides cohetAneas, tal como est~n expreaadas por ejemplo en los contenidos de los elementos traza de roca-total y biotitas, yen las relaciones iniciales de is6tepos de estroncio, demuestran la con- tribuci6n de varios protolitos diferentes. La estrecha asociaci6n temporal y espacial de las aaociaciones de mante y corticales durante el intervalo P6rmico a Jur~sico, implica una relaci6n de causa-efecto. En particular, el Impel de la inyecci6n b~_ltica en la generacibn de grandes vol0menes de magmas graniticos peraluminosos ~ ampliamente seportada.