Petrography and whole-rock geochemistry of the oligocene-miocene Khojak formation Khojak-Pishin belt,Pakistan: Implications on provenance and source area weathering

The oligocene and early miocene khojak formation represent a deltaic to sub-marine fan succession in the khojak-pishin flysch belt within the katawaz basin of pakistan. the sandstone within this succession is dominated by sedimentary and metamorphic clasts and has been derived from recycled orogeny. the higher sio2/al2o3 ratios (4.0) show moderately mature nature of the sandstone. the weathering indices such as chemical index of alteration (cia; 76.69),chemical index of weathering (ciw; 86.79),chemical proxy of alteration (cpa; 92.33) and index of chemical variability (icv; 16.83) suggest moderate to intense weathering at the source terrain. the high th/u values (5.25) as compared to upper continental crust (ucc; 3.82) also support the enhanced weathering of the source area. trace elements such as zr,nb,y,th and u are slightly enriched compared to ucc suggest the dominantly felsic sources for the formation. high enrichment of cr,ni and v,and moderate ratios of cr/ni (1.72) and cr/v (1.83) indicate substantial contributions from ultramafic sources while fairly high percentages of fe2o3 (7.66) and mgo (3.90) as compared to ucc (5.03% and 2.20%,respectively) hints toward mafic sources. the tectonic setting for the khojak sandstone is considered as continental arc to active continental margin. this study supports the notion that the western himalayan orogenic belt was shedding largely felsic detritus to katawaz basin through proto-indus river,while mafic and ultramafic detritus were being fed by distant kohistan island arc and en-route waziristan,zhob and muslim bagh ophiolite and associated mélanges. the sediment dispersal towards south-southwest was controlled by chaman-nushki transform fault system. © 2016,national centre of excellence in geology. all rights resereved.