The Mesozoic Tectonic Transformation From Compression To Extension In The Northeast Of South China

South China is locted at the convergence of the Paleo-Tethys and the Paleo-Pacific tectonic domains, suffering complicated Mesozoic tectonic activities, like the collision between South China and North China, the collision between South China and the Indochina and the later subduction of the Pacific. The South China experienced important tectonic conversion in Mesozoic. The Indosinian tectonic event presented as unconformity and thrust structure under a compression environment, also with a little magmatic activity and ductile deformation. The Yanshan tectonic event was marked by a large scale of magmatism and the forming of NE-NEE direction rift basins. In this paper, we focus on the Indosinian uplifting and the Yanshanian extension, by researching the Chencai Group metamorphic rocks and Mesozoic dykes with thermal geochronology, geochemistry and zircon SHRIMP geochronology to reveal the conversion process of the tectonic evolution in northeastern South China. The main conclusions of this thesis are as follows:(1) The Mesozoic compressional deformation characteristics in Zhejiang and Jiangxi, the 240Ma mylonite,240-220Ma metamorphic rocks and syn-orogenic granite all indicate that the Indosinian deformation began in ca. 240Ma and propagated form southeast to northwest. Compression propagated to Shangrao in Jiangxi in mid to late Early Jurassic and produced intense deformation of pre-Middle Jurassic strata, and may propagated to Pingxiang and Yichun until Middle Jurassic.(2) It is suggested that the Mesozoic rifting basins developed from inland to the coastal area in Zhejiang and Fujian by the analysis of the Mesozoic basins in Zhejiang and Jiangxi. The Yiyang basin and Jinqu basin formed in 140-135Ma with 15% extension rate, while the Hangjiahu basin developed in 100Ma with 30% extension rate, which indicates the extension in Late Cretaceous is more intense than Early Cretaceous. The Mesozoic rifting basins underwent tectonic inversion at the end of Cretaceous which caused deformation, uplifting and erosion.(3) With the zircon and apatite fission track and thermal history modeling of the Chencai Group and Lipu diorite, combined with associated sedimentologic investigation in the Late Triassic Wuzao Formation, all these results indicate at least three discrete evolving episodes for the old rocks exhumation:the late Triassic thrusting, the early Cretaceous (134-105Ma) relative uplift in the extensional setting and the late Cretaceous-Paleogene (66-41Ma) regional uplifting. We also established the tectonic thermal evolution model.(4) The neutral dyke in Shangrao, Jiangxi is andesitic porphyrite, forming in the middle Jurassic (160±3Ma). It is Aluminum rich sub-alkaline rock, with enrichment of light rare earth elements, large-ion lithophile elements (Th, U) and low radiogenic element (Pb), depletion of high field strength elements (Nb, Ti). Its chondrite normalized cure incline to the right, with slight Eu negative anomaly, showing the magma source had been suffering fluid metasomatism caused by the dehydration effect of the Pacific plate subduction. The Wangjiazhai bimodal intrusive rocks consisted of dolerite and granite porphyry formed in the early Cretaceous (127 Ma). The dolerite plots in the field of alkalic and shoshonite series. It is characterized by enrichment in light rare earth elements (LREE) relative to heavy rare earth element (HREE), weak to negligible Eu anomalies (8Eu=0.99-1.09), positive anomalies of Rb, Ba, Pb and La and negative anomalies of Nb, Ta and Ti; The LREE of rhyolite is more than the dolerite and the HREE has little difference. The granite porphyry has high contents of larger-ion lithophile elements (LILE) (Rb, Ba, Th, Pb and Zr) and low contents of Nb, Ta, Sr and Ti, also shows the characteristic of A-type granite. The Wangjiazhai bimodal dolerite and (A-type) rhyolite represent the lithospheric thinning, magma upwelling and tectonic extension. The granite porphyry of Lishui Fengping formed in the early Cretaceous (130±1Ma), with rich in Si and Al, medium alkali content, poor in Mn, Mg, Ti and P. The chondrite normalized curve incline to the right with strong Eu negative anomaly, which is considered as a typical crust remelting granite. The trace element characteristics indicate that the granite porphyry is originated from the crust or materials related to the subduction, formed by partial melting of the rich sandy and argillaceous rocks.(5) According to the characteristics of deformation, basin and dykes, analyses on magmatism, metamorphism and rock exhumation, we propose that the Mesozoic tectonic transformation from compression to extension in the northeast of South China. The Indosinian compressional deformation began in ca.240Ma and propagated form southeast to northwest and may propagated to Pingxiang and Yichun until Middle Jurassic. The tectonic stress changed from compression to extension in ca. 160Ma. The transformation firstly happened in Ji’An-Shangrao, forming Ji’An alkalic olivine basalt and Shangrao andesitic porphyrite which related to extension. The study area was totally in an extensional setting until 140Ma with rifting basin, A-type granites or alkaline intrusive rocks, metamorphic core complex, bimodal volcanic rocks and mafic dykes. The large-scale extension transferred from Ji’An-Shangrao to the coastal area in Zhejiang and Fujian. The extension ages in the coastal area concentrated in the 110-90Ma. The tectonic inversion at the end of Cretaceous marked the end of the extensional event.In this paper, we propose a time node and dynamic evolution of the Mesozoic tectonic transformation from compression to extension in the northeast of South China, which is an important geological significance to understand the tectonic transformation of the region.