Geochronological And Geochemical Constraints On The Tectonic Evolution Of The Luliang Complex,Trans-north China Orogen

There is still considerable controversy about the tectonic evolution of Trans-North China Orogen(TNCO)during the paleoproterozoic era,mainly about whether the North China Craton(NCC)has experienced its cratonization in an earlier period.The Luliang Complex is located in central-western Shanxi Province and exposed as the westernmost part of the TNCO.It mainly consists of Paleoproterozoic granitoid intrusions,mafic dykes and supracrustal rocks.We’ll discuss mainly on the historical background of magma mixing and the Jiehekou Group,as well as their indication on the structural evolution of the TNCO in this study.The magma mixing area in the Luliang Complex is mainly composed of amphibolite and granite gneiss Both of the amphibolite and gneiss samples are formed at～2.1Ga.Therefore,we take～2.1 Ga as the magma mixing age in Luliang Complex.Basic rocks from the magma mixing area display no pronounced fractionation between LREE and HREE.They are enriched in Cs,Rb,Ba,K,and depleted in Nb,Ta,P,Ti,while Zr,Hf have negative anomaly.They show affinity with alkali continental tholeiite.The acid samples display significant fractionation between LREE and HREE.They are A1-type granite enriched in Rb,Th,U,Pb,Zr,Hf,and depleted in Nb,Ta,P,Ti,indicating the continental-rift environment.The Jiehekou group mainly consists of quartz-mica schist and metasandstone.Three detrital samples from the Jiehekou Group show similar age distribution.They generally have two main peaks of～2.0Ga and～2.1Ga on the age histogram,as well as three inherited peaks of～2.4Ga,～2.5Ga and～2.6Ga.The youngest age peak of～2.0Ga constrains the maximum depositional age of the Jiehekou Group.This age pattern is similar to those of khondalite belt from the northern margin of the Western Block,but much different from that of the Eastern Block and the other sedimentary rocks of the TNCO.It may suggest that the Luliang khondalite share a common provenance with those khondalitic rocks in the Western Block.Hence our data are consistent with previous consideration that the Luliang khondalites were deposited on a stable continental margin of the Ordos Terrane and were thrust from the west and subsequently incorporated into the TNCO during the collision between the Eastern and Western blocks.The supracrustal rock of the Jiehekou Group has underwent metamorphism of amphibolite facies,which we haven’t found from the Pangquangou section.Both of the zircons from gneiss and amphibolite samples have significant characteristics of magmatic zircons,especially those from gneiss samples have similar form with the zircons from granites.Detrital zircons with transportation characteristics are not found in this section.The dykes were formed at～1.91Ga and～1.94Ga,and the gneiss was formed at～2.12Ga,while metamorphism happened at～1.92Ga.Therefore,we consider that intrusion took place at～1.9Ga,synchronous with metamorphism,indicating the stretching background at～1.9Ga.The TNCO has been under subduction-related compression environment after～2.1Ga until collision orogeny took place.The～1.9Ga dykes could only expain that partly extensional fractures exists in the subduction environment,which might be back-arc extension.We consider that the Jiehekou Group doedn’t exist in the Pangquangou area.