Tectonothermal Events And Crustal Evolution In The Eastern South Qinling Block

South Qinling belt, situated between North Qinling and Yangtze block, was generally believed to have tight affinity with Yangtze block before Mesozoic collision. So South Qinling could provide key information about the evolution about the orogen, e.g., from rifting to collision between Yangtze and South Qinling Block. We conduct detail studies concerning three different contents in South Qinling rocks, there are (1) the formation time and evolution of basement rocks Douling complex, (2) the petrology and genesis of Neoproterozoic intrusions and their tectonic significance, (3) widespread mafic dyke swarms in Wudangshan region and their geodynamic settings as well as the relationships with the orogenic process of Yangtze block and South Qinling.As the oldest basement unit of South Qinling, the Douling complex mainly consists of paragneisses and granitic orthogneisses. Zircons of magmatic origin from three orthogneisses samples give U-Pb ages of-2.5 Ga, suggesting an Archean to early Paleoproterozoic magmatic activity. Their Nd and Hf isotopic compositions imply the growth of juvenile crust at c.3.0 Ga. The magmatic activity at c.2.5 Ga points to differences in the Precambrian history between South Qinling and the Archean Kongling terrane, being the oldest basement of the Yangtze block. Three paragneiss samples contain complex detrital zircons of different ages and origins. Their U-Pb ages mainly cluster around 2.5 Ga,2.0 Ga,1.5 Ga and 800 Ma. Most zircon grains or rim domains of 2.0 Ga and 800 Ma have low Th/U ratios and lack oscillatory zoning, implying two episodes of metamorphic overprint. The Palcoproterozoic metamorphic event has been traced in the Yangtze block, confirming its close relationship with South Qinling. The record of Neoproterozoic amphibolite-facies metamorphism in detrital zircons provide constraints on the time the tectonic transition from convergent to extensional environments in the Yangtze block, corresponding to the formation and break-off of the supercontinent Rodinia.Voluminous Neoproterozoic intrusions emplaced in Douling complex without any deformation and metamorphism. Though these igneous rocks have diverse compositions, they exclusively formed at 750-720 Ma. They display large variable SiO2 content from 40.7 to 79.6 wt.%. Base on distinct geochemical features, mafic rocks define two group. Group 1 show low SiO2, MgO and Mg#, but high FeOT and TiO2 with no Nb-Ta depletions. They are alkaline and tholeiitic magma series. They have high εNd values of+2.0 to +6.2, which implied Group 1 mafic rocks originated from depleted asthenosphere mantle. Whereas, Group 2 mafic rocks are calc-alkaline gabbros and gabbroic diorites. They have typical arc-like trace element signatures, like enrichments in LREEs, LILEs but depletions in HFSEs. The εNd(t) values show lager variation from -3.0 to +6.2, interpreted as product of partial melting of modified lithospheric mantle. The felsic rocks can also divide to two groups. Group 1 felsic rocks belong to I-type granitoids, the Nd-Hf isotopic compositions imply an origin from both mantle and crust source rocks. The other group felsic rocks are S-type granites, supposed to melt from sedimentary rocks similar to Douling paragneisses. Extensive melting of both mantle and crust rocks suggest an extension setting. In combination with Ncoproterozoic tectonic events in north edge of Yangtze block, we summarize the Neoproterozoic evolution as following:Prior to 800 Ma, the north margin of Yangtze block was developed in an arc subduction setting; at-800 Ma, arc-continent collision caused amphibolite-facies metamorphism of Douling complex; thereafter, at 800-710 Ma, Yangtze block experienced long term rifting activities; and finally, at 700-620 Ma, Yangtze block was breakup from supercontinent Rodinia.Wudang and Yaolinghe Groups are transitional basement which are underlain by Douling complex and intruded by numerous mafic dyke swarms. These dykes were traditionally regarded as Neoproterozoic magmatism. We investigated these mafic dykes from southern Wudang area. The results indicate that the dykes emplaced during the Early Paleozoic (c.460 Ma) and Early Mesozoic (c.220 Ma). Two episodes of dykes share similar geochemical feature in major elements, but distinctive characteristics in trace elements and Sr-Nd isotopes. Mafic dykes of Early Paleozoic are characterized by enrichments in LREEs, LILEs and HFSEs and EMII-type isotopic feature, implying an OIB-like enriched mantle for the magma source. Early Mesozoic dykes can be subdivided into two groups. Group I show depletions in LREEs, LILEs and HFSEs with DM-type isotopic feature, indicating an origin from partial melting of asthenospheric mantle. Group II has higher Rb-, Ba-, and K-contents and EMI-type isotopic feature, probably suggesting significant contribution of lower crust to magma source owing to the subduction. These mafic dykes record the prolonged evolution of South Qinling. It is suggested that the dykes of Early Paleozoic were related to the opening of an oceanic basin separating South Qinling and the Yangtze block, while the dykes of Early Mesozoic were derived from partial melting of upwelling asthenosphere during the extension in the final amalgamation of these two blocks in Early Mesozoic. A slab breakoff model could interpret not only the petrogenesis of Mesozoic mafic dykes, but also the distinct geological features between Dabie-Sulu and South Qinling orogens. The slab breakoff occurred at great depth in Dabie-Sulu orogen, and rare magmatism was introduced. Whereas, in South Qinling, the breakoff happened at shallow depth, the asthenosphere mantle rocks could rise up to shallow depth and decompression melt, the crustal rock were heated up to product extensively granitic intrusions as a result.