Trace Elements Geochemistry,geochronology And Hf Isotopic Evolution Of Kohistan Island Arc And Indian Plate,Northern Pakistan

Kohistan island arc(KIA) and Indian Plate in the northern Pakistan provide a rare opportunity to thoroughly study subduction,collision and crustal growth processes in detail.This collision zone is part of western Himalayas and widely explored for geochemical,geochronological and petrological studies.Kohistan island arc is a mature arc and most of the petrological and geochronological events are now well understood but still the timing of collision of Indian plate and KIA is debatable.Less research work is carried out in KIA and Indian plate in context of trace elements geochemistry and Hf isotopes from zircons and current research work is mainly focused on this aspect.Metaplutonic and Chilas complexes of KIA are explored for correlation of trace elements(Sc,Ti,Y,Zr,Nb,Hf,U,Pb,Th,REE)in zircons.Swat orthogneisses and Alpurai group rocks from Indian plate are interpreted for trace elements and hafnium isotopes from zircons.Based on zircon trace element geochemistry,metaplutonic and Chilas complexes are discriminated into two magmatic bodies.The geological span of time determined in this research work for the Indus valley metaplutonic rocks vary from 96-115Ma with ?Hf(t)ranging from(+10.6 to+15.6),while the rocks from the Swat valley metaplutonic and Chilas complexes have ranging from(-6.8 to+12).Most of the zircons from KIA are plotted in the fields defined for arc setting.The whole rock geochemistry of the dated rocks elucidate that the Swat valley rocks are more enriched in large ion lithophile elements(LILE)and light rare earth elements(LREE)as compared to the Indus valley metaplutonic complex rocks.The metaplutonic complex is cooler zone compared to the Chilas complex as interpreted from titanium concentration in zircons.Swat orthogneisses are late Paleozoic and Hf isotope in these rocks ranging from-11.1 to+1.4,while the Alpurai group rocks display ?Hf(t)ranging from-35 to+11.5.The yielded ages for Swat orthogneisses are 256±1.9 Ma and are plotted in the zones defined for continental setting with few zircons deviated towards mid oceanic ridge and oceanic arc setting.The temperature calculated for Swat orthogneisses based on Ti content is ranging from 679-942?.Zircon U-Pb and Hf isotope study of the detrital zircon grains from the Alpurai Group rocks display Archean to Paleozoic succession.The prominent negative ?Hf values indicate dominantly crustal reworking together with juvenile crust.The detrital zircon grains reveal mainly Indian craton source,both from southern and central Indian provenance.It is suggested in this research study that the Archean age rocks are present in the basement of Besham Complex which are hidden now because of tectonic processes.Geochemical variation trends are also interpreted in the Jijal complex of KIA based on major oxide data of clinopyroxenes.