GEOCHEMISTRY, PETROGENESIS AND CRUSTAL CONTAMINATION OF HOTSPOT RELATED VOLCANISM ON THE NORTH WESTERN MARGIN OF INDIAN CONTINENT AND ITS IMPLICATIONS FOR PALEOSEDIMENTARY ENVIRONMENTS

R. H. Siddiqui, I. A. Brohi, N. Haidar

Abstract


The Bibai volcanics of a proposed hotspot origin occur in the Suleman and Kirthar Ranges of Pakistan and are intermittently exposed near Waziristan, Zhob, Loralai, Muslim Bagh, Quetta, Khuzdar, Bela and Karachi areas in a zone more than 1200 km long and up to 2 km wide. The volcanics are mainly alkali basalts  but  occasionally picro-basalt (MgO 17.57 wt %), trachybasalt, basanite, tephrite, hawaiite and trachyandesite are also found. Major and trace element chemistry confirm their alkaline affinity and within plate (hotspot) signatures. Major elements show enrichment in K2O, TiO2 and Al2O3 and depletion in CaO and MgO as compared to N-MORB. Their LILE and LREE enriched patterns and marked positive Nb and Ba anomalies are consistent with an enriched mantle source. The lower Mg # (41-61), REE, Zr - Zr/Y, La-Cr and Cr-Yb- study show that the parent magma for the Bibai volcanics was generated by 15%, partially melted garnet-lherzolite source and was fractionated in an upper level magma chamber. The source diagonastic ratios including K/Ba, P/Zr, and La/Ce for the Bibai and 0-2 Ma Reunion alkali basalts are almost similar, but the Bibai alkali basalts show higher K/Y, Sr/Y, Ba/Y and La/Yb ratios which indicate incorporation of crustal material into the Bibai parent magma. The Bibai alkali basalts also show remarkable enrichment in LILE and LREE relative to average 0-2 Ma Reunion and oceanic island alkali basalt, suggesting contamination of the parent magma by partial melts of Indian continental crust en-route to eruption.


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