Late Triassic Magmatism And Tectonic Evolution Of Foping Dome Area In South Qinling

Qinling orogenic belt is an important part of China central orogenic belt and also the convergence belt between North China Block and South China Block.The available researches show that the Qinling orogenic belt has experienced multi-stage orogenic cycles and the final assemblage was accomplished with the collision between North China Block and Yangtze Block in Indosinian.The assemblage between North China Block and Yangtze Blcok not only generated a large scale UHP metamorphic belt in Dabie-Sulu area,but also a large area of granite intrusions in South Qinling area related to this collided and collage event.While the Triassic granite intrusions become an important entry point for the study of collisions between North China Block and Yangtze Block in Qinling area as lack of record of UHP metamorphism in South Qinling area.And the obtained understanding about specific time of collision between the north and south blocks and deep dynamic mechanism are not the same.This paper selects migmatite from Foping dome and surrounding Wulong pluton in South Qinling as the study object for this controversy.We explor the intrinsic coupling relationship of crust partial melting with orogenic belt evolution,granite pluton formation and geochemical properties through recovering partial melting time of orogen crust,protolith features,melting condition of South Qinling orogen belt and study the diagenetic ages,geochemical properties differences,temporal and spatial distribution and deep dynamics background of source area.Combined with the lithofacies paleographic features of South Qinling,metamorphism and the diagenetic characteristic of magmatic rocks in South Qinling,we reveal the tectonic evolutions and deep dynamic mechanism of South Qinling in Indosinian.The obtained understanding of this paper is as following:(1)The migmatites from Foping dome of South Qinling record multiple partial melt events.Zircon U-Pb age features of migmatite and leucosome from Longcaoping secondary dome indicate the partial melting event mainly occurred at late Triassic(214-209Ma).The protoliths of migmatite can be divided into two types.The first type of migmatite records the late Triassic chronological characteristics and the second one keeps the Neoproterozoic and late Triassic chronological features.Zircon in suit Hf analysis of migmatite show that the εHf(t)and model ages of first type migmatite are similar to those of late Triassic granitic intrutions from South Qinling and the εHf(t)values of Neoproterozoic zircons of the second type migamtite are analogous to Neoproterozoic basement and intrusions of South Qinling,which indicate the protoliths of two type migmatites are late Triassic granitoid and Neoproterozoic metavolcanics respectively.In addition,Ti saturation thermometer of migmatites show the first type migmatites record the relative higher temperature clustering at 750-850℃.Zircon 176Hf/177Hf ratios of core domains are simiar with those of rim domains,but the total amount of REE and Th/U ratios from core domains are slightly higher than rim domains,which indicate that this migmatite was generated at deep crust under the relatively anhydrous condition.Temperatures of the second type migmatite mainly cluster at 650-700℃ and the 176Hf/177Hf ratio features of Neoproterozoic zircons are obviously different from that of Triassic,which suggest that the second type migmatites are generated by partial melting at shallow depth under the fluid prsesnt condition.The differences of protoliths and melting characteristic of this two type migmatite suggest the solidus of crust partial melting from Longcaoping area migrated from deep to shollow crust in a short time.The mineral assemblage characteristics of migmatite near the Foping county show a large amount of aluminous minerals(muscovite and garnet)and zircons U-Pb age records are complex(2907-187Ma).The core domains of zircon keep some ancient records,including some Archean and Neoproterozoic chronogical records,and the rim domains records vary from late Triassic to early Jurassic(224-187Ma).Rare earth element distribution patterns of some zircon rim domains show flattened distribution of heavy rare earths and average age of these analyzes points is 196±5 Ma(MSWD=2.6,n =7).This age is analogous to the average age of leucosome,indicating the partial melt time of migmatite from Foping county is maily early Jurassic and the protolith is metagreywacke.Zircon Ti saturation thermometer indicates the temperature mainly cluster at 650-700℃,showing the shollow crust melting characterisitcs.Combined with the characteristics of granites and metamorphic rocks in South Qinling,it is believed that the partial melting of migmatite is generated during the rapid decompression stage of thickened crust in the orogenic belt.(2)Zircon U-Pb age chronological features of Wulong pluton granitoid indicate the crystallized ages are 219-192Ma and the geochemical characteristics of granite are quite different.The low Mg#granitoids(Mg#<50)can be divided into two groups according to the geochemical features.The first group has high Sr/Y ratios(Sr/Y>40)features and the second group shows normal Sr/Y ratios(Sr/Y<40)characteristics.Granitoids of the first group have relative higher Na2O and Sr content relative to the second group.In the trace elements and rare earth elements normalized spider diagram,the first group shows characteristics of significant Sr positive anomaly,deficit of heavy rare earth elements and no obvious Eu negative anomaly compared to the second group.Granitoid samples of the first group exhibit significant trend of garnet fractionation in correlation diagram of trace element ratios.In the initial Sr and Nd correlation diagram,the Wulong granitoids show analogous geochemical features with Neoproterozoic magmatic rocks.Detailed geochemical chatacterisitic analysis indicates the Wulong pluton granitoids of the first and second groups originate from the melting of thicken and normal crust respectively.High Mg#granitoids have relatively high MgO,Cr and Ni content compared with low Mg#granitoids,and most of the samples exhibit high Sr/Y ratio features.Initial 87Sr/86Sr and 143Nd/144Nd ratio characteristics of these samples are analogous to Neoproterozoic igneous rocks.Petrochemical and isotopic geochemical features suggest that most high Mg#granitoids are generated by mixing between delaminated crust melt and metasomatic mantle.Minor granitoids with high MgO content are result of melting of metasomatic mantle.Temporal and spatial distribution of Wulong pluton granitoid shows that the low Mg#granitoids are distributed radially around Foping dome.Specifically,the granitoids near the dome are simultaneous with migmatites(cryatallized age of granitoids near Longcaoping secondary dome is 214Ma and those near Foping secondary dome are 196-192Ma),and these granitoids originate from normal crust melting.While granitoids away from dome are older than migmatite(cryatallized ages of granitoids away from Longcaoping secondary dome are 219-217Ma and those away from Foping secondary dome are 214-212Ma.)and these granitoids show petrogenesis of thicken crust melting.It is suggested that the low Mg#granitoids are related to decompression partial melting of Foping crust and migamtite migration.Crust decompression partial melting accelerates the collapse of South Qinling orogen crust and changes the orogenic crust thickness.A large amount of heat carried by rapid migration upward of partial melting crust promotes the melting of shallow crust.However,partial melting effects on reworking crust thickness and heating may be localized.High Mg#granitoids of Wulong pluton do not exhibit regular spatial and temporal diatribution,indicating that high Mg#granitoids mainly show exotic emplacement mode.(3)By comparing the diagenetic period and genesis features of granitoid in South Qinling orogen.lithofacies paleographic characterisitics,magmatism and metamorphism of Mianlue suture,it is considered that the orogenic evolution process of South Qinling can be divided into following several stages:Before early Triassic,it was subduction stage of Mianlue oceanic crust subducted to South Qinling orogen,which result in ophiolitic melange and some island arc volcanic rocks in Mianlue suture.It was the transitional stage from Mianlue oceanic crust subduction to collision between Yangtze Block and South Qinling Block during middle Triassic.In this stage,the foreland basin and backarc basin depositional systems were developed on north and south sides of Mianlue suture.And low magnesium granite intrusion with high Sr/Y ratios was formed in north of Mianlue suture.Tectonic adjustment stage after continental crust collision occurred at late Triassic(～220-210Ma).The stress state of orogenic belt was transitioned from continuous extrusion to extension affected by the slab break-off after collision,which result in large-scale decompression partial melting of South Qinling crust and large area granitoid intrusions in late Triassic.Longcaoping migmatite and Wulong pluton granitoid were mainly generated during tectonic system transition of South Qinling orogen after collision.Crust melting stage after orogen collapse continued from late Triassic to early Jurassic.The magmatic activity was decreased with the end of thicken crust collapse and weaken mantle activity in this stage,which represent by shallow crust melting.Migmatite from Foping secondary dome and some granitoids from Wulong pluton record melting event of this period.