Geochemical characterization and petrogenesis of Niat-Jal amphibolites, southeast Kohistan, Pakistan

摘要

The Kamila Amphibolite Unit (KAU) of southeast Kohistan, Pakistan, represents a thick sequence of meta-volcanics which is underlined by the mafic-ultramafic rocks of the Spat Complex along the MMT (Main Mantle Thrust). The meta-volcanics of KAU are intruded by diorites, granitoids and tronjhemites of the Thak intusive complex. KAU is subdivided into four distinct units; Babusar, Niat and Jal amphibolites from south to north with a thin slice of Sumal amphibolites within Niat-Jal amphibolites. Niat and Jal amphibolites generally show strong foliation and alternating bands of mafic and felsic compositions. Niat amphibolites are generally fine to medium grained and composed of amphibole (hornblende and actinolite) and substantial quartz with subordinate plagioclase, epidote and chlorite. Magnetite, sphene, muscovite, and apatite occur as accessory minerals. The Jal amphibolites are fine grained and melanocratic and generally comprised of hornblende and plagioclase with subordinate quartz and sericite. Epidote and sphene occur as accessory minerals. Twenty six samples from both groups have been analyzed for whole rock geochemistry. The studied amphibolites fall in tholeiite group and show enrichment in HFS elements and depletion in LIL elements. Spider diagrams and MgO versus Zr plot show heterogeneous compositional patterns, with three distinct patterns: primitive, less evolved and more evolved. Average concentration of TiO2 (1.99 wt% for Niat amphibolites and 1.56 wt% for Jal amphibolites) and K2O (0.14 wt% for Niat amphibolites and 0.20 wt% for Jal amphibolites) exhibit MORB like composition whereas average Y/Nb ratio (8.9 for Niat amphibolites and 8.4 for Jal amphibolites) are close to N-MORB. Zr/Y vs Zr plot characterized the studied rocks as MORB and they fall in the ocean floor basalt (OFB) field on the triangular Ti/100,Zr,Y*3 diagram.