Geochemistry of Polyphase Gneisses from Schirmacher Oasis, East Antarctica

Gamet-biotite gneisses (GBG), augen gneisses (AG) and quartzo-feldspathic gneisses (QFG) constitute the main litho-units in the granulite terrain of Schirmacher Oasis, East Antarctica. The GBG and AG show chemical and mineralogical similarities. The QFG differs from GBG and AG both in major and trace elements. Overall composition of these different polyphase gneisses indicate increase in silica and alkalies from GBG to QFG and decrease in CaO, FeO and MgO. In trace elements Rb increases from GBG to QFG, while Sr and Ba increase in AG and decrease in QFG. The REE distribution patterns in GBG. AG and QFG differ in that, the total REE in GBG is 160.70, in AG 148.32 and the QFG 119.30. The LREE/HREE and Ce_n/Tb_n ratios of GBG and QFG are similar while in AG these differ slightly (GBG = 5.16 and 3.18; AG = 6.93 and 5.10; QFG = 5.62 and 3.90 respectively). Majority of the gneisses exhibit negative Eu anomaly (GBG = 0.55; AG = 0.35; QFG = 0.43) except two from AG having no anomaly. Overall major, trace and REE abundances in the garnet-biotite and augen gneisses suggest their origin by progressive metamorphism of a pelitic source. Partial melting of GBG locally appears to have formed quartzo-feldspathic gneisses. These gneisses resemble the metapelites of the Eastern Ghat granulite terrain and Kerala khondalite belt of India in their overall compositions.