Geochemical Characterization of Schirmacher Orthogneiss, Schirmacher Oasis, CDML, East Antarctica

Transcription

1 Twentieth Indian Expedition to Antarctica, Scientific Report, 2006 Ministry of Earth Sciences, Technical Publication No. 18, pp Geochemical Characterization of Schirmacher Orthogneiss, Schirmacher Oasis, CDML, East Antarctica Amit Dharwadkar, M. J. D'Souza, and D. Jayapaul Antarctica Division, Geological Survey of India, Faridabad Abstract The Schirmacher Oasis forms the northernmost outcrop of the polydeformedpolymetamorphosed central Dronning Maud Land, East Antarctica. The most dominant lithounit is quartzo-felspathic gneisse, which is showing metamorphism up to upper amphibolite facies, has apparently undergone at least 4 phases of deformation. Geochemical characterization of the gneisses reveals their metaluminous to peraluminous nature with enrichment in Fe, Ti, Mg and Ca. They are depleted in Si as compared to average granite composition. These orthogneisses are part of the supracrustals and represent intrusives associated with the 1000 ma crust forming orogeny recognized in Central Dronning Maud Land. These gneisses have been extensively reworked during the later 500 ma Pan-African orogeny. Keywords: Schirmacher oasis, geochemistry, quartzo-feldspathic gneiss, orthogneiss, orogeny, metamorphism and deformation. Introduction Schirmacher Oasis in the central Dronning Maud Land (CDML), East Antarctica is a low lying coastal nunatak covering an area of 35sqkm between longitudes 11 20'E and ll 0 55'E along latitude 70 45'S. Several workers have carried out geological and structural studies in this area (Ravich and Kamenev, 1975; Kaul et al., 1991; Sengupta, 1988; Ravikant and Kundu,). In this publication an attempt is being made to evaluate geological set up of Schirmacher Oasis on the bases of geochemical studies of orthogneiss mainly the quartzo-feldspathic gneiss and charnockite. Attempt is also made to integrate these results with our understanding of the geological and tectonic evolution of this region. This study is based on field studies carried out during our wintering in Antarctica in year

2 148 Amit Dharwadkar, M. J. D'Souza, and D. Jayapaul Geological Setup The Schirmacher Oasis marks the northernmost outcrop of CDML. The geological setup of CDML is complex as it has suffered metamorphism and deformation during two prominent tectono-thermal events. Ongoing studies in CDML has indicated that the crust was formed initially during 1000 Ma Grenvillian event which has been extensively reworked during the subsequent Pan African orogeny that caused amalgamation of east and west Gondwanaland. The Schirmacher Oasis is an important piece of land which may have occupied a key place in the amalgamating land masses. It exposes polydeformed and metamorphosed para and orthogneiss. The dominant grade of metamorphism exhibited by the lithounits is upper amphibolite to granulite facies with widespread indication of retrogression. The peak Ultra High temperature metamorphism has been reported from Schirmacher Oasis. The Orthogneiss is the most dominant lithounit exposed. The orthogneissic rock is dominantly quartzo-feldspathic granitic gneiss, augen gneiss, migmatite, charnockite and enderbites. The charnockite in patches is seen to be retrograding to quartzofeldspathic gneiss. These patches are formed on either side of thin joint planes. Besides the orthogneisses of granitic affinity Schirmacher Oasis also expose in the supracrustals sequence igneous ultramafic and mafic intrusives like granulite, metagabbro, metaultramafiites, metanorite etc. his study has been restricted to studying the orthogneiss of granitic affinity. The rocks of the Schirmacher Oasis have been subjected to at least 4 major deformational episodes accompanied by associated metamorphism. This metamorphism is overprinted on the dominant foliation plane identified as S 1 plane the pervasive regional foliation (ENE-WSW) which was developed during the D2 deformation. A foliation plane earlier to this is indicated by the enclaves. However the D2 deformation event and the associated metamorphism has as such obliterated much of earlier imprint of deformation or metamorphism. Methodology Samples of various orthogneisses exposed in the Schirmacher Oasis were collected from different areas (Fig. 1). Total 48 samples were collected during the course of field work. Geochemical analysis for major and few trace elements was carried out at the PPOD Lab., AMSE, GSI, Bangalore, using the XRF technique. The Rare Earths were analysed at the NAA Lab., GSI, Pune. The results of major element analyses are

3

4 150 Amit Dharwadkar, M. J. D'Souza, and D. Jayapaul 06 OR 10 1? 14 1fi Ifi 20 A/CNK Fig. 2: Most of the gneisses plot in the peraluminous field but some plot in the metaluminous field (after Shand, 1985) presented in Table la. Table lb presents the REE data while Table lc gives the CIPW normative data. Discussion The average composition of the orthogneisses from Schirmacher Oasis plots in the granite-syenogranite field. The gneisses, in general show depletion in Si0 2, and alkalis and enrichment in FeO T, Ti0 9, MgO and CaO as compared to average granite composition. The gneisses are mostly peraluminous to metaluminous (Fig. 2) resulting in the presence of corundum (average 1.4) in the CIPW norms (Table lc). In the A-F-M diagram (Fig. 3) the Schirmacher gneisses show a calc-alkaline evolutionary trend and appear to be cogenetic in nature. The Schirmacher gneisses show an even spread in the high-k series with a few showing affinity towards shoshonitic series (Fig. 4).. Simple correlation plots of Harker type (Fig. 5) reveal that Silica has strong to moderate negative correlation with Ti0 2, A , FeO, CaO and MgO while FeO has a positive correlation with A , Ti6 2, CaO and MgO. A negative correlation of FeO with K 9 0 is observed. There is no mutual correlation between the alkalis (Na 9 0 and K 2 0) with respect to each other.

5 Table 1: Table showing major oxide, trace element and REE chemical analysis of Schirmacher orthogneiss

6

7

8

9

10

11

12

13

14

15 Geochemical Characterization of Schirmacher Orthogneiss

16 162 Amit Dharwadkar, M. J. D 'Souza, and D. Jayapaul Fig. 7: Major element tectonic discrimination plot after Maniar & Piccoli, 1984, showing post orogenie signature. The R1-R2 multicationic discriminant plot by Batchelor and Bowden, 1985, suggests a syn to late tectonic origin for the Schirmacher gneisses (Fig. 6). This observation is also supported by independent field studies and metamorphic history of these gneisses. In the Maniar and Piccoli (1984) major element discrimination diagram, the Schirmacher gneisses plot in the RRG+CEUG+CCG field and the post orogenic field indicating that these gneisses have a mixed origin with substantial contribution from lower crust (Fig. 7). Since the Schirmacher orthogneisses have been emplaced over a long temporal as well as spatial regime, it is quite likely that they will exhibit mixed chemical signatures. The REE data shows that these orthogneisses has low La/Lu ratio indicating their less evolved nature. The gneisses show a medium negative Eu anomaly (Fig. 8) and show affinity somewhere between upper and lower crust. The geochemical characteristics of the Schirmacher gneisses indicate that they have been sigiiificanlly contributed by reworking of the older crust. They have been emplaced in a syn to late tectonic setup at moderate depths.

17 Geochemical Characterization of Schirmacher Orthogneiss Fig. 8: REE spidergram of Schirmacher gneisses (chondrite normalized). Acknowledgement The present work is the result of studies carried out during the course of 20th Indian Antarctic Expedition, The authors are indebted to Mr. Rasik Ravindra Deputy Director General, GSI, who was then, Director, Antarctica Division, GSI, Faridabad for his help and guidance throughout the expedition. The authors extend there sincere thanks to NCAOR/DOD, Goa for providing logistic support during the above studies. We are thankful to Dr. S. Mukerji, Director and other colleagues from the Antarctica Division for their encouragement and technical support. References Sengupta, S., (1988): History of deformation in relation to metamorphism-migmititic events in the Schirmacher Hills, Queen Maud Land, east Antarctica. Jr.Geol.Soc.Ind., V.32, pp Tingey, R.J., (1991): The regional geology of Archean and Proterozoic rocks in Antarctica. In: The Geology of Antarctica, R.J.Tingey(Ed.), Clarendon, Oxford, pp l-73.yoshida, M.(1995): Assembly of East Gondwanaland during the Meso-Proterozoic and its

18 164 Amit Dharwadkar, M. J. D'Souza, andd. Jayapaul Rejuvenation during the Pan-African Period. In India and Antarctica during the Precambrian, Geol. Soc. Ind., Mem 34, pp Ravich, M.G. and Kamenev, E.V. (1975): Crystalline basement of Antarctic platform. In John Wiley and Sons, Newyork, Israel Program, for sceintific translations, Jerusalem, pp 582. Kaul, M.K., Singh, R.K., Srivastava,D., Jayaram,S. and Mukerji,S.(1991): Petrographic and structural characteristics of a part of the East Antarctic Craton, Queen Maud Land, Antarctica. In: Geological Evolution of Antarctica, M.R.A.Thomson, J.A.Crame and J.W. Thomson (Eds), Cambridge Univ.Press, pp Maniar P. D. and Piccoli P.M., (1989): Tectonic discrimination of granitoids. Geol. Soc. Amer. Bull., V.101, pp Pearce, J.A., Harris,N.B.W. and Tindle, A.G. (1984): Trace element discrimination diagrams for thetectonic interpretation of granitic rocks. Jr. Petrol., V.25, pp