PALEOCENE THOLEIITIC VOLCANISM AND OCEANIC ISLAND ARC AFFINITIES OF THE CHAGAI ARC, BALOCHISTAN, PAKISTAN

R. H. Siddiqui, M. A. Khan, M. Q. Jan, I. A. Brohi

Abstract


The main exposures of the Paleocene lava flows occur in an east-west trending subduction related magmatic belt known as Chagai Arc in the western part of Pakistan. The volcanism in this arc was initiated during the Late Cretaceous, which  intermittently  continued  up to  the  Quaternary  period. In  the regional geotectonic context this arc belongs to the Tethyan Convergence Zone and was believed to have formed due to the northward subduction of Arabian Oceanic Plate below the southern margin of Afghan Micro plate and hence considered as an Andean type arc. The Paleocene lava flows are mainly represented by amygdaloidal and nonamygdaloidal varieties of basalts (49.22-52.46 wt. % SiO2) and basaltic-andesite (52.75-53.90 wt. % SiO2). The main textures exhibited by these flows include hypocrystalline porphyritic, cumulophyric, intersertal and subpilotaxitic. Large phenocrysts of augite and diopside (< 1 - 3 mm) and smaller of plagioclase (An50-68) are embedded in a micro to crypto crystalline groundmass having the same minerals with devitrified volcanic glass. The petrochemical studies show that these are mainly medium to low K oceanic island arc tholeiites. Their low Mg-number (41-55) and higher FeO (total)/MgO ratios (1.44-2.60) indicate fractionated nature of these lava flows. The trace element patterns show enrichment in LILE and depletion in HFSE relative to N-MORB. The primordial mantle-normalized trace element patterns show marked negative Nb anomalies with positive spikes on Ba and Sr which strongly confirm their island arc signatures, which is further supported by flate-slightly LREE enriched chondrite-normalized REE patterns. The Zr/Y versus Zr, and Cr versus Y studies, lowers Mg # and lower abundance of Ni and Co suggests that the parent magma of these rock suites was generated by about 15-30 % melting of depleted sub-arc mantle source, and fractionated in an upper level magma chamber en-route to eruption. These volcanics have lower average 87Sr/86Sr ratio (0.70446), which is more consistent with a depleted mantle source and closely correlate with oceanic island arcs rather than continental margin type arcs. The average trace element ratios including Zr/Y (2.25), Ti/V (15.47), Ti/Zr (160.17), Ba/Y (13.54), Th/Y (0.04) La/YbN (1.62), Ta/Yb (0.03), Th/Yb (0.24) of these volcanics are more consistent with oceanic island arc tholeiite rather than analogous rocks of the continental margins type arcs. 


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