Zircon Inheritance Reveals Exceptionally Fast Crustal Magma Generation Processes in Central

Zircon Inheritance Reveals Exceptionally FastCrustal Magma Generation Processes in CentralIberia during the Cambro-Ordovician

F BEA1 P MONTERO1 F GONZAcurren LEZ-LODEIRO2 ANDC TALAVERA1

1DEPARTMENT OF MINERALOGY AND PETROLOGY CAMPUS FUENTENUEVA UNIVERSITY OF GRANADA

18002 GRANADA SPAIN2DEPARTMENT OF GEODYNAMICS CAMPUS FUENTENUEVA UNIVERSITY OF GRANADA 18002 GRANADA SPAIN

RECEIVEDJANUARY 16 2007 ACCEPTED SEPTEMBER 24 2007ADVANCE ACCESS PUBLICATION OCTOBER 18 2007

The Variscan basement of the Central Iberian Zone contains abun-

dant Cambro-Ordovician calc-alkaline to peraluminous metagran-

ites and metavolcanic rocks with two notable features first they

were apparently produced with no connection to any major tectonic

or metamorphic event second they have an unusually high zircon

inheritance U^Pb dating combined with cathodoluminescence

imaging reveals that about 70^80 in some samples nearer

100 of the zircon grains contain inherited pre-magmatic cores

despite the temperature reached by the magmas (about 9008C calcu-lated using the Ti-in-zircon thermometer) being high enough to

dissolve all the available zircon (from the rockrsquos zircon saturation

temperature 770^8608C) The fact that the dissolution of zircon

was so incomplete can only be attributed to the kinetics of heat trans-

fer to and from the magmasThree-dimensional modeling of zircon

dissolution behavior in melts with a composition similar to the

Iberian Cambro-Ordovician magmas indicates that the survival of

zircons from the suggested late Pan-African protolith would be possi-

ble only if melt production was rapid specifically less than 104 years

and probably about 2103 years from the beginning of melting

(7008C) to the thermal peak (9008C) Melt production was

followed by fast magma transfer to upper crustal levels resulting

either in surface eruption or in the emplacement of small (5400 m

thick) sills or laccoliths We suggest that these elevated rates of

crustal melting could only have been caused by intrusion of mantle-

derived mafic magmas most probably at the base of the crustThis

scenario is consistent with a rifting regime in which crust and mantle

were mechanically decoupled this would explain the scarcity of

contemporaneous crustal deformation Furthermore fast melting

rates in the lower crust followed by fast melt transportation to the

upper crust could also explain the lack of contemporaneous

metamorphismThe speed of the partial melting process resulted in

the production of felsic magmas that inherited the geochemical char-

acteristics of their granitoid crustal protolith This explains the

apparent contradiction between the calc-alkaline to peraluminous

geochemical characteristics of the magmas and the inferred exten-

sional (ie rift-related) tectonic setting Our model is compatible

with the hypothesis of fragmentation and dispersal of terranes from

the northern margin of Gondwana that led to the opening of the

Rheic and Galicia^South Brittany oceans and ultimately caused

the detachment of the Iberian microplate from Armorica and

Gondwana during the early Paleozoic

KEY WORDS igneous petrology migmatite granite geochemistry

crustal contamination ICP-MS laser ablation

I NTRODUCTIONThe pre-Variscan basement of the Central Iberian Zone(Fig 1) contains numerous small bodies of Cambro-Ordovician granitoids and felsic volcanic rocks whichwere strongly deformed and variably metamorphosedduring theVariscan These rocks have two notable charac-teristics First they are not obviously connected with anymajor tectonic or metamorphic event (Gutiecurren rrez-Marcoet al 2002) Second they have an unusually high propor-tion of inherited zircon in the 18 massifs we have studiedso far no less than 70^80 and in some samples nearer

Corresponding author E-mail fbeaugres

The Author 2007 Published by Oxford University Press Allrights reserved For Permissions please e-mail journalspermissionsoxfordjournalsorg

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100 of zircon grains contain pre-magmatic cores(Figs 2^4 see also Solacurren et al 2006) This high degree ofinheritance greatly exceeding what is to be expected forcommon felsic volcanic rocks and granites (eg Milleret al 2003) seems to be a common feature of the Cambro-Ordovician rocks of western Europe (eg Laumonier et al2004 Teipel et al 2004 Helbing amp Tiepolo 2005) whichmust undoubtedly reflect some petrogenetic peculiarityof their evolutionPre-magmatic zircons survive when the magma

temperature is not high enough to dissolve all the availablezircon or when kinetic effects hinder its dissolutionIn Iberia the former can easily be excluded because thepeak magmatic temperature recorded by the Cambro-Ordovician rocks exceeds their zircon saturation tempera-tures (Figs 5 and 6) Among the kinetic factors capable ofpreventing or delaying zircon dissolution those associated

with shielding by major phases (Bea 1996) or with limitedvolume melt-reservoirs (Watson 1996) can also bediscarded because these magmas were highly mobile asindicated by their upper crustal emplacement and lack ofrestitic material Most probably therefore the elevatedzircon inheritance was caused by fast heat transfer tothe protolith and fast cooling of the resulting magmasUnderstanding how this occurred will lead to a betterunderstanding of the petrogenesis and geodynamic signifi-cance of the Cambro-Ordovician magmatism of CentralIberia and by inference of western EuropeThe principal objective of this study is to determine the

minimum heating and cooling rates that might havecaused such a high degree of zircon inheritance To dothis we used the 3D instant dissolution rate model forspherical zircons in felsic melts developed by Watson(1996 equation 17) This permits calculation of the change

CZ

WALZ

CIZ

OS-D

CXG-D

OMZ

SPZ

GTOMZ

Lisboa

Madrid

1

2

3

4

5

8

6

9

7

0 100 200 km

0ordm

38ordm38ordm

minus9ordm43ordm

0ordm

43ordm

-9ordm

Fig 1 Geological map of the Iberian peninsula showing the location of the Iberian massif in grey and the Cambro-Ordovician rocks ofthe Central Iberian Zone (CIZ) in black Other peralkaline to peraluminous Cambro-Ordovician rocks occur in the the Ossa Morena Zone(OMZ) and the allochthonous terranes of the GaliciaTras-Os-Montes-Zone (GTOMZ) CZ Cantabrian ZoneWALZWestern Asturias^LeonZone SPZ South Portuguese Zone OS-D Ollo de Sapo Domain CXG-D Schist^Graywacke Complex Domain The massifs enclosed by agrey oval are those studied here (see Fig 4) 1 El Barquero and Manlsaquo ocurren n 2 Puebla deTrives 3 Sanabria andViana^Covelo 4Villadepera andMiranda do Douro 5 Hiendelaencina and Antonlsaquo ita 6 San Pelayo 7 Bercimuelle and Castellanos 8 Mochares and Pollacurren n 9 indicates theCarrascal^Portalegre granitoids and the Urra Formation studied by Solacurren et al (2005 2006)

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in radius of zircon crystals suspended in a melt as afunction of heating andor cooling rate To apply Watsonrsquosequation using realistic assumptions we first identified theprobable protolith from the ages recorded in the inheritedzircon cores This allowed us to estimate at least approxi-mately the bulk composition of the source and the modalabundance shape and grain-size distribution of pre-magmatic zircons Then to estimate a lower limit forthe maximum temperature reached by the Cambro-Ordovician magmas we determined the concentration ofTi and the U^Pb age of zircons by laser ablation induc-tively coupled plasma mass spectrometry (LA-ICPMS)This permitted us to apply the Ti-in-zircon thermometer(Watson amp Harrison 2005 Watson et al 2006) and tocalculate the crystallization temperature of both neo-formed and inherited zircons The results of applyingWatsonrsquos equation indicate that the generation of crustalmagma in Central Iberia during Cambro-Ordoviciantimes occurred through repeated fast pulses each ofwhich lasted no more than a few thousand years Suchfelsic magmatism can be explained by a process of mafic

magmas underplating along linear arrays of lithosphericfractures that originated during the separation of theIberian microplate from Armorica and Gondwana

GEOLOGICAL SETT ING ANDPETROGRAPHYThe pre-Variscan basement of the Central Iberian Zonecontains three main belts of Cambro-Ordovician igneousrocks (Fig 1) the metavolcanic rocks and metagranitesof the Ollo de Sapo Formation (Parga-Pondal et al1964 see a recent overview by D|currenaz Montes et al 2004)the metagranites of the northernmost zone of theSchist^Graywacke Domain (eg Vialette et al 1987Valverde-Vaquero amp Dunning 2000 Bea et al 2003) andfurther south near the boundary with the Ossa MorenaZone the Carrascal^Portalegre granitoids and themetavolcanic rocks of the Urra Formation (eg Solacurren et al2006) What follows is a short petrographic description ofthese rocks largely based on the lithologies of the Ollo de

Fig 2 Representative examples of zircons from the Cambrian^Ordovician metagranites and metavolcanic rocksThe numbers indicate the age(in Ma) obtained using either ion-microprobe (ellipses) or LA-ICPMS (circles) The ages of the cores are mainly Ediacaran and the rimsCambro-Ordovician In some cases there are also narrow discontinuous overgrowths withVariscan ages (eg Z1)

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Sapo Formation With minor modifications this is alsoapplicable to the other two beltsThe metavolcanic rocks originally consisted of dacitic to

rhyolitic ignimbrites and tuffs (Navidad et al 1992) Theyare currently represented by augen-gneisses interbeddedwith micaceous schists sandstones and quartzitesOverlying them is a siliciclastic series of Ordovician toEarly Devonian age and underlying them is a mainlymetapelitic sequence of probable Early Cambrian agePetrographically they are distinctive because of the pre-sence of huge (410 cm) K-feldspar megacrysts locallywith rapakivi structure euhedral oligoclase phenocrysts(up to 3 cm) and rounded and frequently embayedphenocrysts of quartz (up to 15 cm) which when themetamorphic grade is low sometimes have a noticeableblue color resulting from inclusions of sagenitic rutile Thephenocrysts are surrounded by a fine-grained groundmassof quartz K-feldspar muscovite biotite and rare albiteAccessory minerals include apatite zircon ilmenite mag-netite monazite rare xenotime and irregularly distributedFe^Cu sulfidesThe metagranites crop out as small laccoliths or sill-like

bodies with a thickness of 300^400m emplaced withinthe underlying metasedimentary sequence They consist of

0 20 40 60 80 100Percentage of zircon grains with a discordant core

met

avol

cani

c ro

cks

met

agra

nite

s

Villadepera

Sanabria U

Sanabria M

Sanabria L

Puebla de Trives

Mantildeoacuten

Hiendelaencina U

Hiendelaencina L

El Barquero FG

El Barquero CG

Viana-Covelo

San Pelayo

Pollaacuten

Mochares

Miranda

Castellanos

Bercimuelle

Antontildeita

Fig 4 Percentage of zircon grains with pre-magmatic cores in 18Cambro-Ordovician massifs of Central Iberia calculated from aminimum of 50 zircon grains from each massif and a total of about1800 zircon grains The location of the massifs is shown in Fig 1 CGcoarse grained FG fine grained L lower M middle U upperThis figure indicates the stratigraphic position of the samples in thethicker sequences of the Ollo de Sapo Formation

0

10

20

30

Abu

ndan

ce (

)

750 800 850 900zircon saturation temperature (degC)

Fig 5 Zircon saturation temperatures calculated using the expres-sion of Watson amp Harrison (1983) for representative samples of themetavolcanic rocks (22) and the metagranites (19) The two groupshave an identical average and therefore are presented together Datasources Montero et al (2007) and our unpublished data All Zr dataare XRFdeterminations on fused discs The average is 8268C similarto the lsquohotrsquogranites of Miller et al (2003)

000

005

010

015F

ract

ion

500 1000 1500 2000 2500 3000

Age (Ma)

n= 523 ion microprobe and LA-ICPMS data

Inherited zircon

Fig 3 Histogram of zircon U^Pb concordant or subconcordantion-microprobe and LA-ICPMS age data for zircon from the CentralIberian Cambro-Ordovician rocks Most grains were analyzed at thecore and the rim Variscan ages found occasionally in narrow over-growths of zircons from migmatized samples (eg Fig 2 z1) areexcluded The abundance of Precambrian ages and among these thevast dominance of Ediacaran (605^615Ma) ages should be notedData sources Electronic Appendix 1 C Talavera (unpublished)

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coarse-grained augen-gneisses locally with abundantaplo-pegmatitic dikes and rare metasedimentary xenolithsThe major mineralogy consists of large crystals ofK-feldspar (up to 5^7 cm) frequently with abundant inclu-sions of oligoclase and biotite within a coarse-grainedgroundmass formed of quartz oligoclase K-feldsparbiotite muscovite and occasional tourmaline cordieriteand garnet The accessory minerals consist of apatiteilmenite minor magnetite zircon monazite and rarexenotime and huttonite The metagranites frequently cropout insideVariscan thermal domes and are therefore oftennoticeably migmatized (Bea et al 2003)When the migma-tization was metatexitic this caused the development ofnarrow discontinuous rims of Variscan age over theCambro-Ordovician zircons (Fig 2 Bea et al 2006b)When the migmatization was diatexitic theVariscan rimsbecame thicker and newly formed Variscan zircons mayappear (Bea et al 2003)The crystallization age of these rocks obtained by U^Pb

ion microprobe and LA-ICPMS and 207Pb206Pb stepwisesequential evaporation ranges from 496 to 483Ma forthe metavolcanic rocks and from 488 to 474Ma for themetagranites (Solacurren et al 2005 2006 Bea et al 2006b

Montero et al 2007 F Bea unpublished data) Both rocktypes have chemical composition similar to felsic peralu-minous to calc-alkaline igneous rocks with K2O4Na2Oand Fe(FethornMg) 049^06 The metagranites are gran-odioritic to granitic with aluminium saturation index(ASI) 107^129 87Sr86Sr 07048^07112 and Nd(t)2 to 4 and the metavolcanic rocks are rhyodacitic todacitic with ASI121^149 87Sr86Sr 07069^07118 andNd(t) 35 to 5 (Montero et al 2007)

Z IRCON INHER ITANCE SATURAT ION TEMPERATURESAND TITANIUM THERMOMETRYWe separated zircon from 18 massifs in the two northern-most belts of Cambro-Ordovician rocks in Central Iberiathree metagranites and 10 metavolcanic rocks from theOllo de Sapo Domain and five metagranites from theSchist^Graywacke domain (Fig 1) These samples havebeen previously studied by cathodoluminescence (CL)imaging and dated with the U^Pb (ion microprobe andLA-ICPMS) and the 207Pb206Pb stepwise evaporationmethods A complete description of the procedures

0002

0004

0006

0008

0010

0004

0006

0008

0010

700 750 800 850 900 700 750 800 850 900

neoformed Inherited

kern

el d

ensi

ty

Ti-in-zircon temperature (degC)

Fig 6 Kernel density distribution plot of Ti-in-zircon temperatures (Watson amp Harrison 2005Watson et al 2006) of neoformed and inheritedzircons of the Miranda do Douro orthogneiss This sample was used because it contains large and inclusion-free zircon grains capable of beinganalyzed with a 60 mm diameter laser beam The neoformed grains (22 determinations) peak at 8208C and reach a maximum of 8908CThe inherited grains (nine determinations) peak at 7538C and do not exceed 7958CThe quasi-Gaussian distribution and the absence of outliersindicate that noTi-rich inclusions were ablated during analysis

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employed and the precision attained has been given by Beaet al (2006b) and Montero et al (2007) Solacurren et al (2006)carried out similar studies on the metavolcanic rocks ofthe Urra^Portalegre FormationThe above investigations have revealed an unusually

large proportion of inherited zircon crystals If weaccept that discordant cores such as those shown in Fig 2are restitic (which was confirmed wherever U^Pb spotanalyses are available see also Fig 3 and ElectronicAppendix 1 available for downloading from httpwwwpetrologyoxfordjournalsorg) the examination ofsome 1800 zircons by CL imaging has shown that theproportion of grains with inherited cores varies from 75to 95 in the metagranites and from 87 to 95 in themetavolcanic rocks (Fig 4)To the authorsrsquo knowledge sucha high degree of inheritance is common in low melt-fraction migmatites (eg Montero et al 2004) or low-Tpegmatoid mobilizates (eg Gilotti amp McClelland 2005)but is extremely rare in high-level granites or rhyodaciticvolcanic rocksMiller et al (2003) have demonstrated that zircon inheri-

tance and zircon saturation temperatures (TZr) are anti-thetic and divided North American granites into lsquohotrsquo(average TZrfrac14 8378C little or no inheritance) and lsquocoldrsquo(averageTZrfrac14 7668C high zircon inheritance most oftenclustering around 50) types With minor modificationsthis two-fold categorization seems applicable worldwideirrespective of granite age and typology implying thatthe inverse relation between inheritance and TZr is prob-ably a reflection of the conditions that very often occurduring granite petrogenesis Remarkably the CentralIberian Cambro-Ordovician igneous rocks do not followthis rule because they have simultaneously an averageTZr of 8268C (Fig 5) characteristic of lsquohotrsquogranites and azircon inheritance that not only matches but nearly dupli-cates that of lsquocoldrsquo granites This unusual combination sug-gests disequilibrium processes that do not normally occurin granite magmasTo understand the atypical processes involved in Iberian

Cambro-Ordovician magma generation we must considerthe following Zircon solubility in common crustal meltsdepends on melt composition and temperature but it isalmost independent of pressure and water content(Watson amp Harrison 1983) Because the variations to beexpected in the melt bulk-composition have less influencethan the variations to be expected in temperature(eg melts with 68wt and 72wt SiO2 dissolve 148and 131ppm Zr at 8008C but 253 and 223 ppm Zr at8508C) the first hypothesis that might explain the abnor-mally elevated zircon inheritance of the Cambro-Ordovician magmas is that they were never sufficientlyhot to dissolve all the zircon grains entrained from thesource these being exceptionally abundant for someunspecified reason

This hypothesis can be evaluated as follows Under equi-librium conditions the temperature for total zircon disso-lution in a magma roughly corresponds toTZr which inthe present case averages 8268C and does not surpass8708C (Fig 5) The minimum temperature attained by themagma on the other hand can be estimated by applyingthe Ti-in-zircon thermometer (Watson amp Harrison 2005Watson et al 2006) to the Cambro-Ordovician rims of thezircon grains To this end we analyzed the low-abundance(55) but interference-free 49Ti isotope plus 238U 207Pband 206Pb (to ascertain the age) and 92Zr and 29Si(as internal standards) with a LA-ICPMS system ablating60 mm diameter spots Details of the analytical procedurehave been given by Bea et al (2006a) The study wascarried out on a metagranite the Miranda do Douroorthogneiss (Bea et al 2006b) which has the largest andmost inclusion-free zircon crystals of all the studiedbodies In all other samples zircon grains were either toosmall or too inclusion-rich for reliable Ti analysis Theresults of the Miranda do Douro study (Fig 6) reveal thatwhereas the crystallizationtemperatures of the inherited zir-cons peak at about 7708Cand never surpass 8008C the neo-formedCambro-Ordovician grains peak at 8208Cand somevalues approach 9008C which would then represent theminimum temperature reached by the magmas If weaccept these results (see discussion) we must conclude thatthe temperature attained by the Cambro-Ordovicianmagmas was certainly high enough to dissolve all entrainedzircons The reasons why so many of them survived shouldbe therefore related to the zircon dissolution kineticsIn amelting protolith themain factors delaying the disso-

lution of zircon are shielding by major phases small andisolated melt reservoirs and fast heating and cooling rates(Watson 1996) As the Iberian Cambro-Ordovicianmagmas were highly mobile capable of eruption at theEarthrsquos surface the melt fraction should have been highenough to ensure total interconnectivity of the melt pores(ie the system behaved as an infinite melt reservoir)Similarly the temperatures recorded by zircon indicate thatbiotitewhich is themajormineralwiththegreatest tendencyto include accessories (Bea 1996) was involved in meltingreactions so that the fraction of zircon shielded from themelt must have been low Consequently the only acceptableexplanation for the observed high zircon inheritance isincomplete dissolution owing to the short life-span of themagmatic pulses Before trying to determine the duration ofthese withWatsonrsquos equation it is necessary to have an ideaabout the nature andcomposition of the protolith

THE NATURE OF THEPROTOL ITHThe distribution of 523 concordant or nearly con-cordant ion-microprobe and LA-ICPMS ages for the

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Cambro-Ordovician rocks of Central Iberia (Fig 3) showsa polymodal distribution with a mode at c 490Ma that weinterpret as the age of crystallization a second mode atc 610Ma that we interpret as the age of the predominantprotolith and some minor modes at older ages Themarked dominance among the inherited components ofthe c 610Ma age as well as the chemical and Sr and Ndisotope bulk-rock composition led Montero et al (2007) tosuggest that the protolith of the Cambro-Ordovicianmagmas of Iberia mostly consisted of intermediate tofelsic calc-alkaline igneous rocks of late Pan-African age(or younger immature sediments derived from them)As a result of vigorous Variscan crustal reworking theserocks are poorly exposed in Iberia except for some smalldioritic to granitic massifs in the Merida region (Bandrecurren set al 2004) Calc-alkaline plutonism at 615Ma howeverwas one of the most important magmatic events of theneighboring Anti-Atlas region of Morocco (Gasquet et al2005) a region attached to Iberia during the Ediacaran(eg Ennih amp Liegeois 2001 2003) that was subsequentlylittle affected by the Variscan orogeny Merida andMorocco granitoids can therefore give an idea at leastapproximately of the grain-size distribution of zircon theZr concentration and the bulk-rock composition of the pro-tolith which are needed for application of WatsonrsquosequationThe data of Bandrecurren s et al (2004) indicate that the Pan-

African granitoids of Merida contain zircon grains withmaximum dimensions of 270100100 mm (a volumeequivalent to a 86 mm radius sphere) and have an averageconcentration of Zr of 125 ppm This value however isprobably underestimated because the samples were ana-lyzed after acid digestion our X-ray fluorescence (XRF)data for the same rocks reveal concentrations between 150and 270 ppm Zr Additionally the c 600Ma granites ofMorocco with average XRF Zr concentrations of227 ppm contain zircon grains with dimensions mostfrequently around 1507050 mm (volume equivalent toa 56 mm radius sphere) and only the largest ones reach300120 90 mm (volume equivalent to a 92 mm radiussphere) (unpublished data of the authors) Neither thezircon grain size nor the Zr concentration of these rocks isexceptional but instead both are close to what one wouldexpect for common granodiorites and granites

EST IMATION OF L IMITS FORHEAT ING AND COOLING RATESFROM WATSON rsquoS EQUAT IONWatsonrsquos equation (1996 equation 17) for calculating theinstant dissolution rate of spherical zircon crystals is

ethdr=dtTHORN 1017 frac14 U 1 25 1010=r

exp 28380=Teth THORN

thorn 7 24 108 expeth23280=T THORN

where drdt is the instant dissolution rate (cms) r is theradius of a spherical zircon crystal (cm)T is the absolutetemperature (K) and U is the difference between thecurrent Zr concentration of the melt and the concentrationrequired for zircon saturation according to the experimen-tal model of Watson amp Harrison (1983) In partially moltensystems calculating U requires knowledge of the Zrconcentration and bulk-rock composition of the protolithand the volume of the melt reservoirsThe equation can be used stepwise to calculate the

variations of the zircon radius as a function of time for agiven heating (or cooling) gradient This requires input-ting the newT the new zircon radius that resulted fromthe previous step and the new U value calculated consider-ing the amount of zircon dissolved in the previous step andthe change in zircon solubility caused by the variation inTand melt composition

Heating ratesWe applied the equation to a melting protolith similar incomposition to the Pan-African rocks described in the pre-vious section using the following initial conditions

(1) beginning of melting occurs at 7008C(2) maximum temperature reached by the magma is

9008C(3) volume of melt reservoirs is infinite(4) major element composition of the melt (to calculate

zircon solubility) is calculated using the equations ofWinther (1995) for a granodioritic protolith at 8 kbarand 2 H2O

(5) Zr concentration in the protolith (residing only inzircon) is 225 ppm

Figure 7 shows the calculated time^temperature coordi-nates at which spherical zircons with radius of 25 50 75100 150 and 200 mm will dissolve totally in the melt as afunction of the heating rate The following features standout Zircons with a spherical radius of 50 mm (ie with avolume similar to that of the population most commonlyfound in the protolith) would survive only if heatingoccurred at a rate of 018Cyear or higher Zircons with aradius of 100 mm (ie with a volume larger than the largestzircons of the protolith) would survive only if the heatingrate was 00258Cyear or higher In summary the survivalof protolith zircons requires less than 10 000 years prob-ably around 2000 years from the beginning of melting(7008C) to the thermal peak (9008C)

Cooling ratesThe above calculations represent only half of the historyzircon grains that were partially dissolved during heatingwill continue to dissolve during cooling as long as themelt does not become zircon saturated Therefore wecalculated again the time^temperature coordinatesat which spherical zircon survivors with radius of

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20 40 60 and 80 mm will totally dissolve in the melt as afunction of the cooling rate To estimate the Zr concentra-tion in the melt at 9008C we considered that all protolithzircon grains had a spherical radius of 100 mm and thatduring heating the radius of the dissolving crystal wascoupled to the Zr concentration of the melt so that thetotal concentration of Zr in the system (melt plus crystals)was always equal to that of the protolith 225 ppm Theresults are shown in Fig 8 from which it follows that thesurvival of 20 40 60 and 80 mm zircons requires linearcooling rates from 9008C to 8308C faster than 04 01005 and 00258Cyear respectively As the cooling ratesof erupted felsic magmas are often much faster than these(eg Harris et al 2002) we can conclude that most survi-vors would have a chance of cooling with little size reduc-tion if the magma crystallized quickly by being rapidlytransported from the melting region to Earthrsquos surfaceThe situation however is different if the magmas crys-

tallized at depth such as in the case of the metagranitesAccording to Gonzacurren lez Lodeiro (1981) Iglesias Ponce deLeocurren n amp Ribeiro (1981) and Lancelot et al (1985) theserocks occur as high-level sills or laccoliths about300^400m thick To assess the behavior of zircon underthese conditions we calculated the 1D cooling paths of

granitic sills with the same initialTof 9008C and differentthickness (300 450 and 600m) which were emplaced atdifferent depths (500 1000 2500 5000 and 10 000m) andcompared them with the curves of zircon disappearanceduring cooling as calculated withWatsonrsquos equationThe results of this calculation are shown in Fig 9 from

which the following features stand out During cooling ofa 300m thick body independent of the depth of intrusion(to 10 km) all zircons with a radius larger than 40 mm willsurvive If the sill thickness increases to 450m only zirconswith a radius larger than 70 mm would survive If the sillthickness increases to 600m even the zircons with aradius of 80 mm will disappear unless the sill is emplacedat a depth less than 2 km It seems therefore that the cri-tical parameter governing zircon survival in granitemagmas emplaced in the upper crust is the thickness ofthe magmatic body with the depth of intrusion seeminglyplaying a secondary role

DISCUSS IONThe applicability of the above calculations to geologicalsystems depends on the validity of the numerical model ofzircon dissolution the proper choice of the initial condi-tions and model parameters and the deviations caused by(1) the residence of Zr in minerals other than zircon and(2) the variable grain size and non-spherical shapeof zircon crystals These circumstances can be evaluatedas follows

750

775

800

825

850

875

900

01 1 10 100

200150100755025

050

˚C y

minus1

025

˚C y

minus1

010

˚C y

minus1

002

5˚C

yminus1

000

5˚C

yminus1

T (degC)

Time (103 years)

zircon spherical radius (micrometers)

005

˚C y

minus1

001

0˚C

yminus1

Fig 7 Disappearance curves (bold continuous lines) of sphericalzircon grains as a function of the heating rate (fine continuous lines)calculated withWatsonrsquos equation (See text for the calculation para-meters) It should be noted that heating rates are represented as curvesbecause the horizontal coordinate (time) is logarithmicThe interceptof a heating rate curve with the disappearance curve of zircon with agiven radius marks the point at which that zircon will be totally dis-solved into the melt For example zircons with a spherical radius of50 mm would not dissolve totally if the heating rate is 0108Cyearbut would dissolve if the heating rate is 0058Cyear or lessRemarkably zircons with a radius of 100 mm which is larger thanthe largest found in the probable protolith of the Cambro-Ordovician magmas of Central Iberia will dissolve if the heatingrate is slower than 00258Cyear that is if heating from the beginningof melting (7008C) to the thermal peak (9008C) occurred in about 104

years (See text for discussion)

040 degC y minus1

20 microm40 microm

60 microm

80 microm

020 degC y minus1

010 degC y minus1

006 degC y minus1

004 degC y minus1

0025 degC y minus1

001 degC yminus1

750

775

800

825

850

875

900

T (degC)

Time (103 years)

0 1 2 3 4 5

Fig 8 Disappearance curves (bold continuous lines) of sphericalzircon survivors as a function of the cooling rate (fine continuouslines) calculated withWatsonrsquos equation (See text for the calculationparameters) As before the intercept of a given cooling rate curve withthe disappearance curve of zircon with a given radius marks the pointat which that zircon will be totally dissolved into the melt For exam-ple zircons with a spherical radius of 80 mmwould not dissolve totallyif the cooling rate is 00258Cyear or faster In the rapid coolingregime of volcanic conditions even the smallest zircon grains wouldnot dissolve during cooling

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Watsonrsquos (1996) equation relies on two factors (1) zirconsolubility in silicic melts which for non-peralkaline liquidsdepends primarily on the temperature and the melt major-element composition (Watson amp Harrison 1983) (2) Zrdiffusion in the melt which depends additionally on theH2O content of the melt The equation treats temperatureand melt composition as independent variables (the latterfor calculating zircon solubility) but it assumes a constant3 H2O for the melt In principle this assumption mightbe a serious limitation In practice however it does notcritically affect the model first because crustal magmasrarely have less than 2^3 H2O (eg Clemens 1984Carrington amp Harley 1996) and second because theeffects on Zr diffusion mostly occur in the first 2^3dissolved H2O (Harrison amp Watson 1983)It should also be considered that Watsonrsquos equation is a

simplification According to Watson however it deviates510 from the results of more rigorous moving boundaryfinite-difference methods (Watson 1996) Therefore as thisdeviation is tolerable for our purposes and nothing indi-cates that the Cambro-Ordovician magmas of CentralIberia were exceptionally H2O-poor we can accept thenumerical foundation for calculating the curves of zircondisappearance plotted in Figs 8 and 9The choice of a Pan-African protolith is justified by the

dominance of inherited 600^620Ma ages (Fig 3)Moreover this selection is not critical because the zircongrain-size distribution and the Zr concentration estimatedfor this protolith are typical for common granitoids Onlyif the protolith had zircons with a spherical radius largerthan 150 mm or a bulk-rock concentration of Zr greaterthan 400^500 ppm would we expect significant

departures from the model Because such features aremostly limited to peralkaline rocks and these areunknown among the 600^620Ma magmatism of NWGondwana (eg Gasquet et al 2005) we can safely excludethis possibility Neither is the major-element composition ofthe melt critical because it was necessarily silicic and var-iations of less than 5 SiO2 have little effectIn contrast the determination of the peak temperature

reached by the melts is crucial especially if it is overesti-mated The Ti-in-zircon thermometer requires TiO2 activ-ity equal to one (Watson et al 2006) In the present casethe presence of primary ilmenite and rutile inclusionsindicates that such a condition is satisfied If not itwould have caused underestimation which would notinvalidate our conclusions but instead indicate evenfaster heat-transfer rates More important perhaps is thatTi-in-zircon temperatures can be easily overestimated ifthere are minute inclusions of Ti-bearing minerals or glasswithin the analyzed volume The problem is especiallyserious when Ti is determined using a LA-ICPMS systemsuch as the one used here which to obtain reasonable 49Tisensitivity requires ablation of craters with a diameter of60 mm and a depth of about 40 mm Nevertheless thecareful selection under the microscope of the areas to beanalyzed and especially the nearly Gaussian distributionof the results notably exempt of outliers (see Fig 5)indicates that inclusions have caused little trouble in thepresent case Consequently we can assume that the maxi-mum temperatures recorded by the Ti-in-zircon thermo-meter (9008C) represent a minimum estimate of themagmarsquos thermal peak an assumption totally consistentwith a large body of experimental data indicating thattemperatures of this order are required for generatinglarge volumes of silicic crustal magma in vapor-absentconditions (eg Clemens 2003 and references therein)A final consideration is that Watsonrsquos equation assumes

that all zircon grains are spheres of the same size andthat all Zr resides in zircon Real rocks however havenon-spherical zircon crystals and these are of differentshapes and sizes Real rocks also have a variable fractionof Zr residing in minerals other than zircon such astitanite amphibole or garnet (eg Bea et al 2007)Certainly all these differences may affect the zircon disso-lution rate Shapes other than a sphere would increase itas a sphere represents the smallest surfacevolume ratioNevertheless the existence of a large variety of zirconsizes could delay the dissolution of the largest grainsbecause the Zr concentration in the melt would increaserapidly owing to the fast dissolution of the smallest grainsZircon dissolution would also be delayed if phases otherthan zircon release Zr to the melt but on the other handit would be accelerated if a Zr-bearing mineral such asgarnet appears as a product of melting reactions andextracts Zr from the melt However on balance none of

60 microm

500 m1000 m

2500 m

5000 m

10000 m

40 microm20 microm

80 microm

750

775

800

825

850

875

900

T (degC)

Time (103 years)

0 1 2 3 4 5

500 m1000 m

2500 m5000 m10000 m

500 m1000 m

2500 m

10000 m5000 m

600 m thick

450 m thick

300 m thick

Fig 9 Disappearance curves (bold continuous lines) of sphericalzircon survivors as a function of the cooling rate calculated as inFig 8 compared with the 1D cooling curves of granitic sills of differ-ent thickness emplaced at depths from 500 to 10 000m It should benoted that even 40 mm zircons can survive if the sill thickness is300m In 600m thick sills 80 mm zircons can survive if the sill wasemplaced at a depth of 2000m or less

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these effects are likely be of great importance and what ismore they tend to mutually cancel so we can accept thatthe results of Watsonrsquos equation acceptably match the realsituation that is that the magmatic pulses that generatedthe Central Iberian Cambro-Ordovician magmas werevery fast probably taking around 2000 years from thebeginning of melting until their eruption or emplacementas thin sills or laccoliths at upper crustal levels

GEODYNAMIC IMPL ICAT IONSRapid melt generation and crystallization of the Cambro-Ordovician magmas constrains the possible geodynamicsetting in which they were formed a highly controversialmatter not only in Iberia but throughout the EuropeanVariscides (eg Crowley et al 2000) In Iberia apart fromthe peraluminous to calc-alkaline rocks described herewhich mostly occur in the Central Iberian Zone thereare several small massifs of peralkaline granitoids and afew gabbros that are restricted to the Ossa Morena Zoneand the allochthonous complexes of the Galicia Tras-os-Montes Zone Whereas there is a general agreementthat the peralkaline rocks and associated gabbrosoriginated in a rifting environment (eg Ribeiro 1987Ribeiro amp Floor 1987 Santos Zalduegui et al 1995Montero et al 1998 Montero amp Floor 2004) theperaluminous to calc-alkaline rocks of Central Iberiasolely by virtue of their chemical signature have beeninterpreted by several workers as evidence of an activemargin setting (eg Gebauer et al 1993 Valverde-Vaqueroamp Dunning 2000 von Raumer et al 2003)However the link between the geochemical signature

and geodynamic setting is not definitive and may beequally explained as a legacy from their protoliths as pro-posed for the Cambro-Ordovician rocks of the northernBohemian Massif by Klimas-August (1990) and Floydet al (2000) In the present case the fast melting andmagma-transport rates inferred from the elevated zirconinheritance are enough to cause that effect first becausethe short duration of the whole process would surely havenegatively affected the efficiency of melt^restite segrega-tion especially if there were no syn-magmatic deformation(eg Bea et al 2005) second because the fast melting ratescause the effective partition coefficients to converge to onedespite their equilibrium values (Bea1996) In these condi-tions is not surprising that both the chemical and isotopicsignature of the resulting magmas would be close to that oftheir late Pan-African protoliths and therefore useless forgeodynamic discrimination purposesThe geodynamic environment proposed for the

Cambro-Ordovician magmas must be compatible withthe generation of crustal melts at the elevated ratesinferred here and at the same time account for theabsence of any perceptible orogenic eventWith respect tothe first point it should be considered that the only heating

mechanism capable of melting crustal materials at therequired rate is the advection of heat by mafic magmasas revealed by the numerical analysis of Huppert ampSparks (1989) Other crustal-heating mechanisms havemuch larger time constants from 105^106 years for thedisplacement of isotherms caused by tectonics burial orerosion (eg Chapman amp Furlong 1992 Zen 1995 Huertaet al 1998) to (1^3) 107 years for radiogenic heating(eg Vanderhaeghe amp Teyssier 2001 Bea et al 2003) Theanalysis of Huppert amp Sparks (1989) also predicts thatfelsic magmas generated following the intrusion of maficmagmas would have peak temperatures of 9008C abun-dant pre-magmatic crystals and a highly porphyritic char-acter a set of features found in the Central IberiaCambro-Ordovician rocks that are difficult to explain byany other mechanism This gives additional support to theidea that heat for crustal melting was supplied by mantle-derived mafic magmas Additionally the imperceptiblehybridization between these and the felsic magmas(eg Montero et al 2007) suggests that the meltable crustalmaterial just overlay the mafic intrusions where the den-sity difference and the quick solidification of the maficmagma at the contact would make mixing unlikely(Huppert amp Sparks1989)This locates the mafic intrusionsat the crust^mantle interface Lastly the fast melt trans-port to upper crustal levels points to extensional ratherthan compressional forces All these reasons thereforestrongly suggest that the Central Iberian Cambro-Ordovician magmas were generated during the rifting ofcontinental crust caused by an upwelling mantle plumewhich probably occurred during the early Paleozoic frag-mentation and dispersal of terranes from the northernmargin of west Gondwana (Crowley et al 2000 Matte2001) and ultimately led to the formation of the IberianmicroplateIn this scenario crustal deformation depends to a signifi-

cant extent on the mechanical coupling between mantleand crust (Burov amp Guillou-Frottier 2005) if the couplingis weak the concentration of plume-related extension inthe mantle lithosphere has little effect on the crust Thismight explain the scarcity of contemporaneous deforma-tion The lack of any Cambro-Ordovician metamorphicimprint in all exposed midcrustal sections of CentralIberia may also be explained by the swiftness of thegeneration and emplacement of the crustal magmasMetamorphism involves conductive heat transfer a processinherently slow that requires much more than a fewthousand years to be perceptible at a crustal scale Herethe heat advected to the lower crust by mantle magmaswas first consumed by melting reactions and then quicklytransported to the uppermost crustal sections by theso-produced magmas thus causing a negligible thermalimpact on most of the crustal section above the meltingzone except perhaps the lowermost 1000^2000m

JOURNAL OF PETROLOGY VOLUME 48 NUMBER 12 DECEMBER 2007

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CONCLUSIONSThe most important conclusions of this paper can besummarized as followsThe Cambro-Ordovician igneous rocks of Central

Iberia dacites to rhyolites and high-level granites containabout 70^80 and in some samples nearer 100 ofzircon grains with inherited pre-magmatic cores Theelevated zircon survival occurred despite the fact thatpeak temperature of the magmas estimated with theTi-in-zircon thermometer at 9008C or higher surpassedthe rockrsquos zircon saturation temperature This wasthe result of the swiftness of the magmatic pulsesModeling the dissolution of zircon suspended in a melt asa function of heating and cooling rates indicates thatthe pulses lasted only a few thousand years probablyabout 2000 years from the beginning of melting to finalemplacementConsidering the time constants involved these rates of

crustal recycling can only be achieved by anatexis inducedby the intrusion of hot mantle-derived magmas into thecrust This mechanism also explains why the crustalmagmas are highly porphyritic and have reached peaktemperatures of 9008C or higher The imperceptiblehybridization between mafic and felsic magmas indicatesthat the locus of the mafic intrusions was at the crust^mantle interface The fast melt transport to upper crustallevels points to extensional rather than compressionalforces and explains the negligible metamorphic imprintof this event on mid-crustal sectionsThe calc-alkaline to peraluminous signature of the

Cambro-Ordovician magmas which has been consideredas proof of a subduction environment was inherited fromtheir late Pan-African protolith owing to the swiftness ofthe melt-generation process First the short time involvedled to a low efficiency of felsic melt^restite segregationespecially in the absence of syn-magmatic deformationSecond the fast melting rates led the effective partitioncoefficients to depart from their equilibrium values andconverge to one Accordingly the geochemical signatureof the resulting magmas cannot be invoked as a proof of asubduction settingThe most probable setting for the generation of the

Central Iberian Cambro-Ordovician magmas is a conti-nental rifting environment in which crust and mantlewere mechanically decoupled as indicated by the scarcityof contemporaneous deformation Crustal melting wastriggered by the intermittent arrival of batches of maficmagmas at the mantle^crust interface along linear arraysof lithospheric fractures The heat advected to the lowercrust was first consumed by melting reactions and thenquickly transported to the uppermost crustal sections bythe so-produced magmas causing negligible metamor-phism of the crustal section above the melting zone exceptin the first 1000^2000m

Our interpretation is in good agreement with the idea offragmentation and dispersal of terranes from the northernmargin of west Gondwana during the early Paleozoiccaused by among other factors an upwelling mantleplume (Crowley et al 2000) which led to the opening ofthe Rheic Ocean and Galicia^South Brittany oceans(Matte 2001) and ultimately detached Iberia fromGondwana and Armorica

ACKNOWLEDGEMENTSWe are indebted to M Wilson Ron Frost and ElenaBelousova whose suggestions and comments greatly con-tributed to improving the original manuscript and toJ H Scarrow for her assistance with the English Thiswork was financially supported by the Spanish grantCLG2005-05863BTE and the Andalucian grantRNM1595

SUPPLEMENTARY DATASupplementary data for this paper are available at Journalof Petrology online

REFERENCESBandrecurren s A Egu|curren luz L Pin C Paquette J L Ordocurren nlsaquo ez B Le

Fecurren vre B Ortega L A amp Gil Ibarguchi I (2004) The northernOssa^Morena Cadomian batholith (Iberian Massif) magmaticarc origin and early evolution International Journal of Earth Sciences93 860^885

Bea F (1996) Controls on the trace element composition of crustalmelts Transaction of the Royal Society of Edinburgh Earth Sciences 8733^42

Bea F Montero P amp Zinger T (2003) The nature and origin of thegranite source layer of Central Iberia evidence from trace elementSr and Nd isotopes and zircon age patterns Journal of Geology 111579^595

Bea F Fershtater G B Montero P SmirnovV N amp Molina J F(2005) Deformation-driven differentiation of granitic magma theStepninsk pluton of the Uralides Russia Lithos 81 209^233

Bea F Montero P Gonzacurren lez-Lodeiro FTalavera C Molina J FScarrow J H Whitehouse M J amp Zinger T F (2006a) Zirconthermometry and U^Pb ion-microprobe dating of the gabbros andassociated migmatites of the Variscan Toledo Anatectic ComplexCentral Iberia Journal of the Geological Society London 163 847^855

Bea F Montero P Talavera C amp Zinger T (2006b) A revisedOrdovician age for the oldest magmatism of Central Iberia U^Pbion microprobe and LA-ICPMS dating of the Miranda do Douroorthogneiss Geologica Acta 4 395^401

Bea F Montero P amp Ortega M (2007) A LA-ICPMS evaluation ofZr reservoirs in common crustal rocks implications for Zr and Hfgeochemistry and zircon-forming processes Canadian Mineralogist

44 693^714Burov E amp Guillou-Frottier L (2005) The plume head^continental

lithosphere interaction using a tectomically realistic formulationfor the lithosphere GeophysicalJournal International 161 469^490

Carrington D P amp Harley S L (1996) Cordierite as a monitor offluid and melt H2O contents in the lower crust An experimentalcalibration Geology 24 647^650

BEA et al MAGMAGENERATION IN CENTRAL IBERIA

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ovember 2018

Chapman D S amp Furlong K P (1992) Thermal state of the conti-nental lower crust In Fountain D M Arculus R amp Kay RW(eds) Continental Lower Crust Amsterdam Elsevier pp 179^199

Clemens J D (1984) Water contents of silicic to intermediatemagmas Lithos 17 272^287

Clemens J D (2003) S-type granitic magmasccedilpetrogenetic issuesmodels and evidence Earth-Science Reviews 61 1^18

Crowley Q G Floyd P A Winchester J A Franke W ampHolland J G (2000) Early Paleozoic rift-related magmatism inVariscan Europe fragmentation of the Armorican TerraneAssemblageTerra Nova 12 171^180

Diez Montes A Navidad M Gonzacurren lez-Lodeiro F amp Mart|currennezCatalacurren n JR (2004) El Ollo de Sapo In Vera JA (ed) Geolog|curren ade Espanlsaquo a Madrid SGE-IGME 69^72

Ennih N amp Liegeois J P (2001) The Morocan Anti-Atlas theWestAfrica craton passive margin with limited Pan-African activityImplications for the northern limit of the craton Precambrian

Research 112 289^302Ennih N amp Liegeois J P (2003) The Morocan Anti-Atlas theWest

Africa craton passive margin with limited Pan-African activityImplications for the northern limit of the craton reply to com-ments by E H Bouougri Precambrian Research 120 185^189

Floyd P A Winchester J A Seston R Kryza R amp Crowley QG (2000) Review of geochemical variation in Lower Palaeozoicmetabasites from the NE Bohemian Masif intracratonic riftingand plume^ridge interaction In FrankeW HaakV Oncken Oamp Tanner D (eds) Orogenic Processes Quantification and Modelling in

the Variscan Belt Geological Society London Special Publictions 179155^174

Gasquet D Levresse G Cheillez A Azizi-Samir MR ampMouttaqi A (2005) Contribution to a geodynamic reconstructionof the Anti-Atlas Morocco) during Pan-African times with theemphasis on inversion tectonics and metallogenic activity at thePrecambrian^Cambrian transition Precambrian Research 140157^182

Gebauer D Mart|currennez-Garc|currena E amp Hepburn J C (1993)Geodynamic significance age and origin of the Ollo de SapoAugengneiss (NW Iberian Massif Spain) Paper presented at theGeological Society of America 1993 Annual Meeting BostonGSA Annual Metting Abstracts with programs 342

Gilotti J A amp McClellandW C (2005) Leucogranites and the timeof extension in the East Greenland Caledonides Journal of Geology113 399^417

Gonzacurren lez Lodeiro F (1981) La estructura del anticlinorio del lsquoOllo deSaporsquoen la regiocurren n de Hiendelaencina (extremo oriental del SistemaCentral Espanlsaquo ol) Cuadernos Geolog|curren a Ibecurren rica 7 535^545

Gutiecurren rrez-Marco J C Robardet M Racurren bano I Sarmiento G NSan Josecurren Lancha M A Herranz P amp Pieren Pidal A P (2002)Ordovician In Gibbons W amp Moreno T (eds) The Geology of

Spaim London Geological Society pp 31^49Harris A J L Flynn L P Matias O amp Rose W I (2002) The

thermal stealth flows of Santiaguito dome GuatemalaImplications for the cooling and emplacement of dacitic block-lavaflow Geological Society of America Bulletin 114 553^546

HarrisonT M amp Watson E B (1983) Kinetics of zircon dissolutionand zirconium diffusion in granitic melts of variable water contentContributions to Mineralogy and Petrology 84 67^72

Helbing H amp Tiepolo M (2005) Age determination of Ordovicianmagmatism in NE Sardinia and its bearing onVariscan basementevolution Journal of the Geological Society London 162 689^700

Huerta A D Royden L H amp Hodges K V (1998) The thermalstructure of collisional orogens as a response to accretion erosion

and radiogenic heating Journal of Geophysical Research Solid Earth

103 15287^15302Huppert H E amp Sparks S J (1989) The generation of granitic

magmas by intrusion of basalt into continental crust Journal ofPetrology 29 599^624

Iglesias Ponce de Leocurren n M amp Ribeiro A (1981) Position stratigraphi-que de la formation Ollo de Sapo dans la recurren gion de Zamora(Espagne)^Miranda do Douro (Portugal) Comunicacoes Servicio

Geologico de Portugal 67 141^146Klimas-August K (1990) Genesis of gneisses and granites from

the eastern part of the Izera metamorphic complex in the light ofstudies on zircons from selected geological profiles Geologia Sudetica24 1^71

Lancelot J R Allegret A amp Iglesias Ponce de Leocurren n M (1985)Outline of Upper Precambrian and Lower Paleozoic evolution ofthe Iberian Peninsula according to U^Pb dating of zircons Earthand Planetary Science Letters 74 325^337

Laumonier B Autran A Barbey P Cheilletz A Baudin TCocherie A amp Guerrot C (2004) On the non-existence of aCadomian basement in southern France (Pyrenees MontagneNoire) implications for the significance of the pre-Variscan(pre-Upper Ordovician) series Bulletin de la Sociecurren tecurren Gecurren ologique de

France 175 643^655Matte P (2001) The Variscan collage and orogeny (480^290Ma)

and the tectonic definition of the Armorica microplate a reviewTerra Nova 13 122^128

Miller C F McDowell S M amp Mapes RW (2003) Hot and coldgranites Implications of zircon saturation temperatures and pre-servation of inheritance Geology 31 529^532

Montero M P amp Floor P (2004) Los complejos alcalinos prevaris-cos In magmatismo del Palezoico Inferior en las unidadesbasales) (Vera J A (ed) Geolog|curren a de Espanlsaquo a MadridGSE^IGME pp 149^150

Montero P Floor P amp Corretge G (1998) The accumulation ofrare-earth and high-field-strength elements in peralkaline graniticrocks The Galineiro orthogneissic complex northwestern SpainCanadian Mineralogist 36 683^700

Montero P Bea F Zinger T F Scarrow J H Molina J F ampWhitehouse M J (2004) 55 million years of continuous anatexisin central Iberia single zircon dating of the Penlsaquo a Negra ComplexJournal of the Geological Society London 161 255^264

Montero P Bea F Gonzacurren lez-Lodeiro F Talavera C ampWhitehouse M (2007) Zircon crystallization age and protolithhistory of the metavolcanic rocks and metagranites of the Ollo deSapo Domain in central Spain Implications for the Neoproterozoicto Early-Paleozoic evolution of Iberia Geological Magazine 144 doi101017S0016756807003858

Navidad M Peinado M amp Casillas R (1992) El magmatismo pre-Herc|currennico del Centro Peninsular Sistema Central Espanlsaquo ol) In(Gutiecurren rrez-Marco J C Saavedra J amp Racurren bano I (eds) PaleozoicoInferior de Iberoamacurren rica Badajoz University of Extremadurapp 485^494

Parga-Pondal I Matte P amp Capdevila R (1964) Introduction a lagecurren ologie de lsquolrsquoOllo de Saporsquo Formation porphyrode antesiluriennedu nord ouest de lrsquoEspagne Notas y Comunicaciones del Instituto

Geolocurren gico y Minero de Espanlsaquo a 76 119^153Ribeiro M L (1987) Petrogenesis of early Paleozoic peralkaline

ryolites from the Macedo de Cavaleiros region (NW de Portugal)Geologische Rundschau 76 147^168

Ribeiro M L amp Floor P (1987) Magmatismo peralcalino no MacizoHesperico Sua distribuicao e significado geodinamico In Bea FCarnicero A Gonzalo J C Locurren pez Plaza M amp Rodr|currenguez

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ovember 2018

Alonso M D (eds) Geolog|curren a de los granitoides y rocas asociadas del

Macizo Hespecurren rico Madrid Rueda pp 211^221Santos Zalduegui J F Schalaquo rer U amp Gil Ibarguchi L (1995) Isotope

constraints on the age and origin of magmatism and metamor-phism in the Malpica-Tuy allochthon Galicia NW SpainChemical Geology 121 91^103

Solacurren A R Montero P L R M Neiva A M R Zinger T ampBea F (2005) PbPb age of the Carrascal Massif centralPortugal Geochimica et Cosmochimica Acta 69 A856^A856

Solacurren A R Pereira M F Ribeiro M L Neiva A M RWilliamsI S Montero P Bea F amp ZingerT (2006) The Urra FormationAge and Precambrian inherited recordVII Congresso Nacional deGeologia Univ Evora (Portugal) Libro dos Resumos 1 29^32

Teipel U Eichhorn R Loth G Rohrmuller J Holl R ampKennedy A (2004) U^Pb SHRIMP and Nd isotopic data fromthe western Bohemian Massif (Bayerischer Wald Germany)Implications for Upper Vendian and Lower Ordovician magma-tism InternationalJournal of Earth Sciences 93 782^801

Valverde-Vaquero P amp Dunning G R (2000) New U^Pb ages forEarly Ordovician magmatism in Central Spain Journal of the

Geological Society London 157 15^26Vanderhaeghe O amp Teyssier C (2001) Crustal-scale rheological

transitions during late-orogenic collapseTectonophysics 335 211^288Vialette Y Casquet C Fucurren ster J M Ibarrola E Navidad M

Peinado M amp Villaseca C (1987) Geochronological study of

orthogneisses from the Sierra de Guadarrama (SpanishCentral System) Neues Jahrbuch fulaquo r Mineralogie Monatshefte 10465^479

von Raumer J F Stampfli G M amp Bussy F (2003) Gondwana-derived microcontinentsccedilthe constituents of the Variscan andAlpine collisional orogensTectonophysics 365 7^22

Watson E B (1996) Dissolution growth and survival of zirconsduring crustal fusion Kinetic principles geological models andimplications for isotopic inheritanceTransactions of the Royal Societyof Edinburgh Earth Sciences 87 43^56

Watson E B amp Harrison T M (1983) Zircon saturation revisitedtemperature and composition effects in a variety of crustal magmatypes Earth and Planetary Science Letters 64 295^304

Watson E B amp Harrison T M (2005) Zircon thermometer revealsminimum melting conditions on earliest Earth Science 308841^844

Watson E BWark D amp Thomas J (2006) Crystallization thermo-meters for zircon and rutile Contributions to Mineralogy and Petrology

151 413^433Winther K T (1995) A model for estimating the composition of par-

tial melts Mineralogy and Petrology 53 189^195Zen E A (1995) Crustal magma generation and low-pressure high-

temperature regional metamorphism in an extensional environ-ment Possible application to the Lachlan Belt Australia AmericanJournal of Science 295 851^874

BEA et al MAGMAGENERATION IN CENTRAL IBERIA

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ovember 2018