A Geochemical Study Of Eclogites In The East Kunlun Orogenic Belt

Subduction zone is an important site for material cycling on Earth.Eclogite is formed by crustal subduction in subduction zone,and thus is an important target for understanding subduction-zone processes.The subduction of oceanic and continental plates produces eclogites with different geochemical characteristics.There are tectonic transition from oceanic subduction to continental collision in continental collisional belt,so that both oceanic and continental eclogites may be exposed in the same orogenic belt.The East Kunlun orogenic belt,located in the northern part of the Tibetan Plateau,is an ideal laboratory for studying the transition from oceanic subduction to continental collision in convergent plate margins,where both oceanic and continental eclogites are exposed.In this thesis,we carried out petrological and geochemical studies on eclogites and garnet amphibolites(retrograde eclogite)from Gazhima,Wenquan and Xiarihamu in the East Kunlun orogenic belt.The results are used to trace the protolith nature of the eclogites,to constrain the P-T-t trajectory of the East Kunlun eclogites,and finally to provide constraints on the tectonic evolution of the East Kunlun orogenic belt.Geochemical and zirconological studies of the eclogites show that both oceanic and continental eclogites are exposed in the East Kunlun orogenic belt.All the eclogites in the three areas have depleted whole-rock Nd-Hf isotopic compositions.The eclogites at Gazhima have trace element compositions similar to those of oceanic island basalt(OIB)and enriched midoceanic ridge basalt(E-MORB).Garnets in OIB-type eclogites have higher δ18O values than normal mantle,and zircons have older Hf model age of 700-1100 Ma.These observations sugegst that OIB-type eclogites in Gazhima are continental eclogites.E-MORB type eclogites have younger Hf model age of 500-750 Ma with very depleted Hf isotope compositions,suggesting that they are oceanic eclogites.The trace element compositions of the studied Wenquan eclogites are similar to that of MORB and IAB,with extremely depleted zircon Hf isotopic compositions and δ18O values of the garnets higher or lower than the normal mantle.These observations suggest that they are oceanic eclogites.The garnet amphibolites at Xiarihamu have variable trace element compositions,ranging from OIB-type to MORB-type.The δ18O values of garnet and zircon in Xiarihamu samples are higher than normal mantle,suggesting the low-temperature alteration of their host rock before subduction-zone metamorphism.The T-MORB type eclogites of Xiarihamu has a higher content of Al2O3、LREE and Th,lower content of Fe2O3 and TiO2 but more enriched Sr-Nd isotopic composition relative to E-MORB type eclogites.This compositional feature is related to the incorporation of more crustal-derived material in the mantle source of their protolith.The OIB-type eclogites are continental origin with Neoproterozoic protolith ages and Hf model ages of 700-1000 Ma,whereas the MORB-type eclogites are oceanic origin with Hf model ages of 500-700 Ma.Zircons in the East Kunlun eclogites are dominated by eclogite-facies metamorphic zircons with inclusions of rutile,omphacite,garnet and quartz.The eclogite-facies zircons are unzoned in CL images and exhibit flat HREE patterns and lack of negative Eu anomalies.Their LA-ICPMS and SIMS U-Pb dating yield consistent ages of 430-410 Ma,recording the metamorphic age of the eclogite-facies mtamorphism.The zircons from the Gazhima area exhibit core-rim structures.The cores exhibit spongy structure in CL image,high Lu/Hf ratios(>0.005)and Th/U ratios(0.07-0.6),steep HREE patterns and markedly negative Eu anomalies.Theses observations suggest that they are relict of protolith magmatic zircons.They have apparent 206Pb/238U ages of 460-447 Ma,representing the later reworking on protolith zircon.The age of～800 Ma found in the SIMS dating results from the Gazhima and Xiarihamu areas represent the protolithic age of the continental eclogites.Combined with previous studies,the protolith age of oceanic eclogite is Cambrian,and that of continental eclogite is Neoproterozoic.The metamorphic P-T conditions of eclogite-facies metamorphism are constrained to be 14-20 kbar and 580-730℃ by garnet-clinopyroxene thermometer and jadeite component in omphacite banometer,whereas those of amphibolite-facies metamorphism are constrained to be 552651℃ and 8-11 kbar by amphibole-plagioclase thermometer and amphibole banometer.These observations indicate that the East Kunlun eclogites have experienced a clockwise P-T path.In combination with the previous studies,we construct a tectonic evolution model of the East Kunlun orogenic belt as follows:from 550 to 480 Ma,the Proto-Tethys Ocean crust(Kunlun Ocean crust)began to subduct northward,and with the continuing subduction,the ocean crust partially melted to form basic and acidic magmatic intrusions in the middle Kunlun belt;around 480 Ma,the Qimantagh back-arc basin began to expand and the oceanic crust of the back-arc basin began to subduct northward;around 430-410 Ma,the continuously subducted Kunlun oceanic crust collided with the Qaidam massif and formed co-collisional granitoids.Subsequently,the continental crust subducted to mantle depths;during 410-370 Ma,the delamination of overthickended lithosphere took place to cause the Buchan-type metamorphism and associated magmatism.The exposure of both oceanic and continental eclogites in the same area in the East Kunlun orogenic belt indicate the transformation from oceanic subduction to continental subduction in the East Kunlun orogenic belt.