Glimmeritic enclave in a lamprophyre from the Settupalle alkaline pluton, Eastern Ghats mobile belt

0016-7622/2010-75-6-783/$ 1.00 © GEOL. SOC. INDIA

JOURNAL GEOLOGICAL SOCIETY OF INDIAVol.75, June 2010, pp.783-790

Glimmeritic Enclave in a Lamprophyre from the SettupalleAlkaline Pluton, Eastern Ghats Mobile Belt

N. V. CHALAPATHI RAO

Centre of Advanced Study in Geology, Banaras Hindu University, Varanasi – 221 005Email: [email protected]

Abstract: A rare occurrence of glimmeritic (mica-rich) enclave – composed of abundant modal biotite, subordinateproportions of clinopyroxene and apatite, minor amounts of feldspar, carbonate and sphene – is reported from thelamprophyre of Settupalle alkaline pluton, Eastern Ghats mobile belt (EGMB), India. The enclave displays very coarsegrained equigranular texture (mica laths up to 5 mm and clinopyroxene grains up to 4 mm). In comparison, the hostlamprophyre exhibits a marked porphyritic-panidiomorphic texture comprising phenocrysts of clinopyroxene; otherphases such as biotite and potash- and plagioclase- feldspar are restricted to the groundmass. A tight closeness in mineralchemistry of the glimmerite and lamprophyre imply a possible genetic relationship between their parent magmas.Glimmeritic enclave is construed to be an autolith of the proto-lamprophyre magma, which failed to reach the surface,and lined the wall-rock along the conduit of the lamprophyric intrusion. Glimmerite enclave provides a direct evidencefor the multi-stage modification of the lithospheric mantle due to the infiltration of the potassium-rich hydrous meltssuch as lamprophyres. Mineralogy of the glimmeritic enclave is also similar to that of a vein component of the hydrous,mafic and potassic-ultrapotassic veined lithosphere in the EGMB.

Keywords: Glimmerite, Enclave, Lamprophyre, Settupalle, Eastern Ghats mobile belt.

INTRODUCTION

It is well-established from seismological and geo-chemical evidences that the Earth’s lithospheric mantle isheterogeneous on a centimetre scale and that mantlemetasomatism or mantle enrichment by the magmas and/orfluids is one of the major processes that led to suchinhomogeneities (e.g. Hoffmann, 2003; Anderson, 2008).Two main types of mantle metasomatism are recognizedfrom the study of mantle xenoliths or nodules that arebrought up by the kimberlites, lamproites, lamprophyresand alkalic basalts (e.g. Dawson, 1984; Erlank et al. 1987;Nixon, 1987; Pearson et al. 2003): (i) Patent (modal)metasomatism – when it is petrographically recognisabledue to replacement textures and development of hydrousphases and (ii) Cryptic metasomatism – where onlyincompatible element enrichment is observed without anypetrographic evidence. A variety of deep-seated potassium-rich alkaline continental magmas, such as kimberlites,lamproites and lamprophyres, are now widely inferred tobe the products of such a metasomatized mantle (e.g.Mitchell, 2006). At times, these potassic magmas ormelts similar to their composition also constitute meta-somatising agents and modify the mineralogical andchemical composition of the lithosphere (e.g. McKenzie,

1989; Foley, 1992; Kinny and Dawson, 1992; Petersonet al. 1994).

In India, a wide-variety of lower-crustal as well asmantle-derived xenoliths are recorded from the Proterozoickimberlites of the Dharwar craton and the Mesozoic alkalicbasalts and lamprophyres of Western India (see Karmalkaret al. 2009 for a recent review). Evidences for bothmodal as well as cryptic metasomatism are recorded inthese xenoliths (e.g. Nehru and Reddy, 1989; Karmalkarand Rege, 2002). The purpose of this paper is to (i) reporta rare occurrence of glimmeritic (micaceous) enclave froma lamprophyre from the Settupalle alkaline pluton,(ii) discuss its petrography, mineralogy and mineralchemistry, (iii) evaluate the relationship between theenclave and the host lamprophyre and (iv) highlightthe significance of glimmeritic enclave vis-à-vis variousmodels invoking mantle metasomatism in the genesisof alkaline magmatism in the Eastern Ghats mobilebelt.

GEOLOGICAL SET-UP OF THE SETTUPALLEALKALINE PLUTON

The Settupalle alkaline pluton (Fig.1) forms the saturated

JOUR.GEOL.SOC.INDIA, VOL.75, JUNE 2010

784 N. V. CHALAPATHI RAO

to over-saturated component of the ‘Prakasam alkalineprovince’ (Leelanandam, 1989) or the ‘Cuddapah intrusiveprovince’ (Madhavan et al. 1995) and is located to theeastern margin of the Cuddapah Basin within the EasternGhats mobile belt (EGMB). It is broadly sub-oval in shapeand covers an area of 40 sq km and is sandwiched betweenthe two nepheline syenite plutons of Elchuru (Madhavanet al., 1989) and Purimetla (Ratnakar and Leelanandam,1983). The Settupalle pluton is composed of diverse rocktypes such as fayalite syenites (fayalite+quartz andfayalite+clinopyroxene bearing varieties), hornblendesyenite, quartz syenite, negligible proportion of nephelinesyenites and lamprophyre dykes (e.g. Leelanandam et al.1989; Srinivasan and Natarajan, 1990; Srinivasan andChalapathi Rao, 1995).

Three lamprophyre dykes of the Settupalle plutonintrude the quartz syenites, and are located towards thenorthern and northwestern direction of the Settupallevillage (Fig.1). Two of the lamprophyres trend in NW-SE,whereas the other one trend NE-SW. Their strike lengthranges from 50 -75 m with a variable width of 1-2 m. White-

coloured, spherical to rounded ocelli are a conspicuousfeature of these lamprophyres and their origin is traced toliquid immiscibility between a silicate melt and a meltrelatively rich in H2O and CO2 (Leelanandam and Srinivasan,1986). The Settupalle lamprophyres come under the categoryof camptonite variety (Madhavan et al. 1998).

A mica-rich enclave (25 mm x 12.5 mm) was encounteredin a thin section from one of these lamprophyre dykes(Fig. 2). The enclave shows sharp contact with the hostlamprophyre and is mineralogically least altered, suggestingits minimal interaction with the host lamprophyremagma.

ANALYTICAL TECHNIQUES

Petrographic study of thin-sections of the micaceousenclave from Settupalle lamprophyre was carried out by theresearch polarising microscope at the Technical Universityof Clausthal, Germany. Electron Probe Micro Analyses(EPMA) was performed by the author, using a CAMECASX100 at the Institute of Mineral Resources, Technical

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Fig.1. (A) Inset map showing the location of Settupalle towards the eastern margin of the Cuddapah Basin. (B) Generalised geologicalmap of the Settupalle alkaline pluton (essentially after Srinivasan and Chalapathi Rao, 1995). Thickness of the lamprophyredykes is exaggerated. Asterick shows the lamprophyre dyke from which glimmeritic enclave of this study is recovered.


GLIMMERITIC ENCLAVE IN A LAMPROPHYRE, SETTUPALLE ALKALINE PLUTON, EASTERN GHATS MOBILE BELT 785

University of Clausthal, Germany. An acceleration voltageof 15kV, a beam current of 20nA and a beam diameter of1 micron was used. Wavelength Dispersive Spectrometersemploying TAP, PET and LLIF crystals and a PAP onlinecorrection programme were employed. Several in-house

standards were used in calibration. After repeated analysesit was found that the error on major elements is <1%.Representative EPMA data of various mineral phases in theenclave as well as in the host lamprophyre are presented inTables 1 and 2 respectively.

Lampropyhrehost

Glimmerite

1 cm

Fig.2. Thin section of the Settupalle lamprophyre depicting the glimmeritic enclave.

Table 1. Electron micro probe analyses (oxide wt%) of various minerals present in the glimmeritic enclave

Oxide Mica Clinopyroxene K-feldspar Plagioclase Apatite Sphene(wt%) feldspar

SiO2

34.91 36.24 36.06 36.67 50.76 51.20 51.13 65.79 64.780.54 30.40TiO

25.48 5.15 5.08 5.15 1.09 1.06 0.81 0.05 0.02 0.0238.12

Al2O

315.44 14.72 15.36 14.75 4.56 4.51 3.44 19.79 23.36 0.02 0.31

Cr2O

30.02 0.02 0.02 0.00 0.09 0.09 0.03 0.00 0.00 0.00 0.00

FeO 18.25 17.77 17.67 17.55 7.83 7.97 8.48 0.17 0.06 0.29 0.77MnO 0.17 0.17 0.15 0.17 0.15 0.16 0.19 0.00 0.00 0.02 0.02MgO 11.62 11.70 11.77 11.77 10.81 10.82 10.70 0.00 0.00 0.00 0.00CaO 0.05 0.00 0.03 0.00 22.35 22.25 21.81 0.16 3.69 55.83 28.92Na

2O 0.18 0.18 0.15 0.21 2.11 2.14 2.01 3.09 8.84 0.09 0.02

K2O 9.00 9.56 9.86 9.53 0.00 0.00 0.00 11.83 0.09 0.06 0.10

P2O

50.00 0.04 0.01 0.06 0.00 0.00 0.00 0.00 0.0040.56 0.03

NiO 0.00 0.04 0.02 0.04 0.00 0.00 0.01 0.00 0.00 0.00 0.03Total 95.12 95.59 96.18 95.9 99.75 100.2 98.61 100.88 100.84 97.43 98.72

Cations for 22 O 6 O 32 O 32 O 25 O 1 Si

Si 5.330 5.499 5.439 5.533 1.902 1.909 1.940 11.866 11.295 0.092 1Ti 0.629 0.588 0.576 0.584 0.031 0.030 0.023 0.007 0.003 0.001 0.983Al 2.779 2.633 2.731 2.624 0.201 0.198 0.154 4.207 4.802 0.001 0.021Cr 0.002 0.002 0.002 0.000 0.003 0.003 0.001 0.000 0.000 0.000 0.000Fe 2.330 2.255 2.229 2.214 0.245 0.249 0.269 0.026 0.009 0.041 0.042Mn 0.022 0.022 0.019 0.022 0.005 0.005 0.006 0.000 0.000 0.001 0.001Mg 2.644 2.646 2.646 2.647 0.604 0.601 0.605 0.000 0.000 0.000 0.000Ca 0.008 0.000 0.005 0.000 0.897 0.889 0.887 0.031 0.68910.167 1.023Na 0.053 0.053 0.044 0.061 0.153 0.155 0.148 1.080 2.989 0.030 0.001K 1.753 1.851 1.897 1.834 0.000 0.000 0.000 2.722 0.020 0.020 0.007P 0.000 0.002 0.001 0.003 0.000 0.000 0.000 0.000 0.00 5.836 0.001Ni 0.000 0.002 0.002 0.002 0.000 0.000 0.001 0.000 0.00 0.000 0.001Total 15.55 15.553 15.591 15.524 4.041 4.039 4.034 19.939 19.807 16.189 3.071Mg/(Mg+Fe) 0.53 0.54 0.54 0.54 0.71 0.71 0.69Or: 71.01 Or: 0.50

Ab: 28.79 Ab: 82.42An: 0.81 An: 17.08



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Fig.3. (A) Microphotograph of the glimmeritic enclave showing the equigranular texture dominated by abundant mica (M). Also presentare clinopyroxene (Cp) and feldspar (F); polarized light; uncrossed polars. (B) Microphotograph of the host lamprophyre displaysa porphyritic texture with panidiomorphic phenocrysts essentially of clinopyroxene (Cp); Mica (M) and feldspar (F) are essentiallyconfined to the groundmass. Polarized light; uncrossed polars. (C) Back-scattered electron (BSE) image showing the contact(dashed lines) between glimmerite (towards the right) and the host lamprophyre (towards the left). Difference in the texture andgrain size of various phases such as mica (M), clinopyroxene (Cp), alkali feldspar (A) and plagioclase feldspar (P) in both ofthem is quite apparent. (D) BSE showing a particularly large (> 2mm) mica lath in the glimmerite. Note that feldspar is only aminor phase in the glimmerite. (E) BSE showing abundance of apatite (Ap) occurring as radial sprays or as elongated grains inthe groundmass of the glimmerite. Bright (white) grain is the sphene (S). (F) A high magnification BSE depicting >200mmapatite (Ap) grains in the groundmass of the glimmerite.



PETROGRAPHY AND MINERAL CHEMISTRYOF THE MICA-RICH ENCLAVE AND THE

HOST LAMPROPHYRE

Petrographic studies revealed that the micaceous enclavedisplay an equigranular texture, dominated by very large(up to 5 mm) tabular laths of light brown to dark browncoloured mica, subhedral to prismatic crystals ofclinopyroxene (occasionally up to 4 mm), abundant apatite,scattered grains of alkali and plagioclase feldspar in minorproportions, and very minor amount of sphene and calcite(Fig.3). Modal percentages (vol %) of various minerals isas follows: mica (~65 %), clinopyroxene (~20 %), apatite(~5 %), feldspar (~3-5 %), sphene (~1-2 %) and calcite(< 1%). Compositionally the mica corresponds to biotite(Fig.4) and there is little variation in its chemical composition(Table 1). TiO2 content exceeds 5 wt% and imply titanian-rich nature of their parent magma. K2O contents of biotiteare up to 9.86 wt%, suggesting its pristine nature and theMg/(Mg+Fe) vary from 0.53-0.54 suggesting an evolvednature. Clinopyroxene is compositionally a salite (Table 1)and enriched in Na2O (2.01-2.11 wt%) and TiO2 (0.81-

1.09 wt%). Compositionally it is similar to that present inthe Roman type ultrapotassic rocks and their transitionalvariety (Fig.5). Orthoclase (71.01 %) is the dominantcomponent in alkali feldspar whilst plagioclase ispredominantly sodic (alibite: 82.42 %) (Table 1). Apatite isa major accessory that occurs in two parageneses in thegroundmass: (i) radiating sprays of 20-30 mm sized grains(Fig.3E) and (ii) as elongated and acicular grains up to asize of 300 mm (Figs. 3E and 3F). Titanite is a widespreadaccessory phase that occurs as ~25 mm sized grains in thegroundmass (Fig.3F). Calcite is noticed in very minoramounts.

Petrographically the Settupalle host lamprophyre showporphyritic texture where phenocrysts are chiefly composedof clinopyroxene (Fig.3B). Leelanandam and Srinivasan(1986) have reported occasional pseudomorphed olivine asphenocryst but, no such phase is observed in the presentstudy. Biotite is restricted to groundmass as small flaky lathsand is essentially <1 mm (Figs. 2 and 3B). Alkali feldsparand plagioclase are also present abundantly in thegroundmass. Apatite occurs as a rare and widely scatteredfine grained accessory mineral in the groundmass. Radiatingsprays of apatite, present in the glimmerite, are not observedin the host lamprophyre. Representative EPMA data withrespect to various mineral phases present in the lamprophyreare provided in Table 2. There is a strong uniformity in thecomposition of various phases present in enclave as well asin lamprophyre.

Table 2. Electron micro probe analyses (oxide wt%) of various mineralspresent in the host lamprophyre

Oxide Mica Clinopyroxene K- Plagioclase wt% feldspar feldspar

SiO2

35.82 35.73 50.09 50.16 65.93 65.74TiO

24.80 4.99 0.58 0.53 0.04 0.00

Al2O

314.79 14.91 4.70 4.39 19.19 23.65

Cr2O

30.02 0.00 0.01 0.01 0.00 0.00

FeO 17.52 17.45 9.84 9.70 0.06 0.06MnO 0.19 0.18 0.23 0.23 0.01 0.01MgO 11.78 11.46 10.22 10.19 0.00 0.01CaO 0.00 0.00 21.64 21.80 0.00 3.70Na

2O 0.15 0.18 2.12 2.08 0.96 7.72

K2O 9.65 9.60 0.01 0.00 15.15 0.08

NiO 0.02 0.03 0.01 0.01 0.00 0.00Total 94.74 94.53 99.45 99.1 101.34 100.97

22 6 32 32

Si 5.484 5.479 1.899 1.908 11.953 11.679Ti 0.553 0.576 0.017 0.015 0.005 0.000Al 2.669 2.696 0.210 0.197 4.102 4.826Cr 0.002 0.000 0.000 0.000 0.000 0.000Fe 2.243 2.238 0.312 0.309 0.009 0.009Mn 0.025 0.023 0.007 0.007 0.002 0.001Mg 2.688 2.619 0.577 0.578 0.000 0.003Ca 0.000 0.000 0.879 0.888 0.000 0.686Na 0.045 0.054 0.156 0.153 0.337 2.591K 1.885 1.878 0.001 0.000 3.504 0.018Ni 0.001 0.001 0.001 0.001 0.000 0.000Total 15.595 15.564 4.059 4.056 19.912 19.813

Or: 91.22 Or: 0.54Ab: 8.78 Ab: 78.64

Mg/(Mg+Fe) 0.55 0.54 0.65 0.65An: 0.00 An: 20.83

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)

Glimmerite (Settupalle)

Lamprophyre (Settupalle)

Lamprophyre (Elchuru)

Kimberlite, NK-2 (NKF)

Phlogopite

Biotite

Fig.4. Mg((Mg+Fe) vs Si (atoms per formula unit) data plot (afterRieder et al. 1998) for discrimination of mica. Note theclose compositional similarity of the biotite from theSettupalle glimmerite with that of the host lamprophyre.The compositional data of biotite from the Elchurulamprophyre (Madhavan et al. 1998) and phlogopite fromthe NK-2 kimberlite of the Narayanpet kimberlite Field(NKF; Chalapathi Rao and Dongre, 2009) are also plottedfor a comparison.



DISCUSSION

Petrographic studies demonstrate that the micaceousenclave in the Settupalle lamprophyres has an altogetherdifferent texture and modal mineralogy compared to its hostrock. However, overlapping composition of various phasessuch as biotite (Fig.4), clinopyroxene (Fig. 5), and feldspars(see Tables 1 and 2) in the enclave and the host lamprophyreimply a strong similarity in their parent magmas. Therefore,the micaceous enclave is an autolith or even a cognateinclusion but, not to be termed as a xenolith. High modalabundance of biotite in the enclave prompts the author torefer the latter as ‘glimmerite’. The name glimmerite wasfirst given by Larsen and Pardee (1929) to a biotite-richalkaline rock from Montana, U.S.A and it remained firmlyentrenched in the literature for those rocks containing highmodal biotite (> 60 %). Glimmerites widely representhydrous, veined and metasomatised lithospheric mantle (e.g.O’Brien et al. 1991; Boerner et al. 1999) and are consideredas potential precursors to potassic magmas (e.g. Petersonand Lecheminant, 1993; Carlson and Irwing, 1994),including to those which had erupted in collision zones (e.g.Becker et al. 1999).

It is pointed out here that in the Elchuru alkaline pluton,elsewhere within the Prakasam alkaline province, thelamprophyre dykes have their plutonic equivalent inshonkinite which is characterized by similar mineralogy,mineral proportions but, is essentially coarse grained andequigranular (Madhavan et al. 1990, 1992). Shonkinite occuras lensoidal masses (autholiths) within the host nepheline

syenite and represents hydrous, basic, potassic, alkalinemagma that was parental to the entire spectrum of variedrock types (nepheline syenites, malignites and lamprophyres)manifested in the Elchuru pluton (Madhavan et al. 1992).

However, in the Settupalle pluton such autoliths ofshonkinite or even glimmerite are so far not reported fromany of the syenite litho-units. A possible cumulate origin ofthe glimmerite from the host lamprophyre magma is alsonot preferred due to the strikingly similar composition oftheir major mineral phases. Therefore, the glimmeriticenclave is construed to be an autolith of the proto-lamprophyre magma that failed to reach the surface butcrystallized as lining on the wall-rock along the conduit ofthe lamprophyric intrusion. Greater abundance of biotite,apatite and titanite in the glimmerite, compared to thatpresent in the subsequently erupted lamprophyre, points outthe highly potassic, alkaline and volatile-rich nature of itsmelt. Paucity of apatite and titanite in the host lamprophyreis also likely due to the extraction of P and Ti by an earlierepisode of melt extraction associated with the petrogenesisof glimmerite.

The genesis of ultrapotassic magmas from a veinedlithospheric mantle (vein-plus-wall rock melting model ofFoley, 1992) is widely accepted upon (e.g. Tappe et al. 2006;Srivastava et al. 2009). According to this model, smallfractions of melts of ultrapotassic composition are derivedfrom the convecting (asthenospheric) mantle and crystallizein the sub-continental lithospheric mantle as veins(dominated by mica and clinopyroxene) in a wall rock ofdepleted harzburgite. These veins undergo melting upondecompression by extension and heat supplied by a mantleplume. With increase in the degree of melting the depletedcomponent of the wall rock also enters the melt. Thus, thefinal composition of the melt is determined by the (i) veincomponent, (ii) wall rock component, and (iii) componentderived from assimilation of refractory wall rock minerals.Geochemical studies on the lamprophyres from thePrakasam province infer their derivation from deeperportions of the mantle (close to spinel - garnet stability field)and their observed geochemical variation can be accountedby the vein-plus-wall rock model (e.g. Madhavan et al. 1992,1998; Ratnakar et al. 2008; Vijaya Kumar and Rathna, 2008).Predominance of mica, clinopyroxene and apatite in theglimmeritic enclave is strikingly similar to those of veinedphases envisaged by this model. However, the presence offeldspar in the glimmerite suggests the role of a proto-lamprophyre, rather than a direct asthenosphere-derivedpotassium-rich, melt in its genesis. Glimmeritic enclaveprovides direct petrographic evidence for the multi-stagemodification of the lithospheric mantle due to infiltration

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Lamproites (KLF)Glimmerite (Settupalle)Lamprophyre (Settupalle)Lamprophyre (Elchuru)

Lamproiteclinopyroxene

Roman-type(ultrapotassic) clinopyroxene

Transitionalclinopyroxene

Fig.5. Al total versus Ti total expressed as atoms per formula unit forclinopyroxene from the glimmerite and lamprophyre fromSettupalle. Also shown are the data for clinopyroxene fromthe Krishna lamproite field (KLF; Paul et al. 2007) andElchuru lamprophyres (Madhavan et al. 1998). Variousfields for clinopyroxene from the world-wide lamproite andRoman-type ultrapotassics is from Coban and Flower(2006).



by potassium-rich melts and/or fluids, such as lamprophyres,in the Eastern Ghats mobile belt.

Acknowledgements: Suggestions by an anonymousreviewer are gratefully acknowledged. Author is thankfulto the Alexander von Humboldt Foundation, Bonn, Germany,

for awarding the Fellowship during the tenure of which thispaper was finalised. Author expresses his grateful thanks tohis teacher Prof. V. Madhavan for initiating him, during theearly 1990s, to the fascinating world of the kimberlites,lamproites and lamprophyres and unabated encouragementever since.

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(Received: 9 July 2009; Revised form accepted: 10 December 2009)

Glimmeritic enclave in a lamprophyre from the Settupalle alkaline pluton, Eastern Ghats mobile belt

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