Research Of Mesozoic Magmatism And Tectonic Setting In Southeast Zhejiang Province

South China Block, under conformed by the North China Plate, Indochina plate and the Pacific plate, which is features by large-scale Mesozoic tectonic deformation, metal mineralization as well as a large area igneous rocks, has stimulated the attention of many geologists. However, dynamics background of large-scale magmatism has been controversial. To better understand the causes and tectonic setting of Mesozoic magmatism, detailed geochronology and geochemistry studies of Triassic, Late Jurassic-Early Cretaceous, early Late-Cretaceous magmatism have been conducted in SE Zhejiang. Combined with Mesozoic tectonic-sedimentary-metamorphic event studies, a new interpretation on the Mesozoic tectonic evolution is conducted.The late Permian-Triassic granites in southeastern China have important tectonic significance for the evolution of East Asia. A detailed study utilizing zircon U-Pb dating, major and trace element geochemistry, and zircon Hf isotope geochemistry has been carried out for the Dashuang granite, Jingju granite, Hucun dyke and Kengkou dyke in Zhejiang Province, South China. LA-ICP-MS zircon U-Pb analyses yielded age of monzonite and Qutarz-monzonite is241±2.1-229.6±2.5Ma,231±2.7Ma from Dashuang pluton, and LA-ICP-MS zircon U-Pb ages of Qutarz-monzonite and moyite are245.7±1.8Ma,241.2±2.7Ma from Jingju pluton, which indicates that the magmatism took place during from early Triassic to Mid-Triassic. The two granites show high contents of total alkalis (Na2O+K2O=7.73-10.24%), high field strength elements (e.g. Zr=215-471ppm, Y=25.8-36.5ppm, Nb=15-28ppm and Zr+Nb+Ce+Y=293-849ppm) and rare earth elements (total REE=299-701ppm) as well as high Ga/Al ratios (10000×Ga/Al=2.44-2.9). The lowest magmatic temperatures estimated from zircon saturation thermometer were800-837℃for the two granites, which suggested that they have the petrographic and geochemical characteristics of A-type granites. In-situ Hf isotopic analyses indicated that two granites have εHf(t) values ranging from-20to-6and two-stage depleted mantle Hf model ages from1.6Ga-2.6Ga, which was inferred that the two granites magma formed by partial melting of Paleoproterozoic metaigneous in Cathaysia Block. A series of A-type granites distribution along the NE-trending fracture show extensional tectonic setting of southeastern China from early Triassic to Mid-Triassic, which may be caused by strike-slip conversion stretching after collision between SCB (South China block) and surrounding block. Granitic dykes were commonly developed in Chencai Group of Zhejiang provinces, which were important symbol of post-magmatism activity. LA-ICP-MS zircon U-Pb analyses yielded age about242.1±1.5Ma and232.2±3.6Ma for two samples from Hucun and Kengkou granitic dykes, which were products of early Indosinian magmatism activity. The two dykes were enriched in K, Al, LREE and Rb, Th, U, Pb, and were depleted in Ba, Nb, Ta, Sr, P, Ti. Their REE patterns were highly fractionated, with (La/Yb).v ratios of28.46-38.07and weakly negative Eu anomalies (Eu/Eu*=0.65-0.73). In-situ Hf isotopic analyses indicated that Hucun dykes have εHf(t) values ranging from-13.9to-6.4and two-stage depleted mantle Hf model ages from1.68Ga-2.15Ga, which was inferred that the Hucun dyke magma formed by partial melting of Paleoproterozoic metasedimentary in Cathaysia Block. Kengkou dykes have εHf(t) values ranging from-40.7-31.5and two-stage depleted mantle Hf model ages from0.99Ga-2.49Ga, indicated its source were complicated. The two Indosinian dykes (242Ma and232Ma), together with discovery of Indosinian A-type granites in the province of Zhejiang and Fujian, difined important extensional events in Triassic. We suggested that Indosinian dykes and A-type granites in the coastal reigion of the SCB probably formed under an extensional regime related to collision between SCB and surrounding block, but not with oblique subduction of the paleo-Pacific plate.It is commonly accepted that tectonic regimes underwent a tremendous change, during the Late Jurassic-Early Cretaceous period. The feature of magmatism can provide a critical constraint to the changes in the period. Zircon LA-ICPMS U-Pb geochronology studies have shown that granite porphyry and K-feldspar granite of Late Jurassic yielded age of144±1.8-150.6±1.9Ma, and early Cretaceous granitic-rocks are implaced at133.1±1.4-134.5±1.6Ma. Geochemical characteristics show that the two-stage granitic-rocks have high contents of metaluminous, potassium and sub-alkaline, and are enrichment of Rb, Th, Ba, Zr, Hf, depleted in Sr, P, Ti, Nd, Ta. The total REE is ranging from95-216ppm, and have high LREE (LREE/HREE=3.0-14.8), and (La/Yb) N changes between2.1to14, and negative Eu anomaly (δEu=0.1-0.7). Late Jurassic granitic rocks have εHf(t) values ranging from-3.2--11.4, and the two-stage depleted mantle Hf model age of1.37-1.92Ga, while Early Cretaceous granitic-rocks have εHf (t) ranging from-13.2to4.3, and the two-stage depleted mantle Hf model ages ranging from0.91-2.02Ga. The result suggests that mantle-drived magma injection have significantly occured in early Cretaceous than that in Late Jurassic. Combined with regional stratigraphic and magmatic characteristics, this study suggests that lithospheric thinning occurred under the local extension environment background in Late Jurassic. To the early Cretaceous period, large-scale lithospheric extension occurred and triggered a massive magmatism. The change is most likely to cause by roll-back of subducting Paleo-Pacific plate.Liangnong pluton, lithologically consisting mainly of monzogranite and granodiorite suited, is a typical late Mesozoic complex in the coastal area of Zhejiang province. Zircon LA-ICP-MS dating results imply that granodiorite was emplaced at106.3±1.1Ma, and monzogranites were emplaced at103.9±1.6Ma,105±1.8Ma, indicating that they were generated during early Late-Cretaceous. The major rock is metaluminous with A/CNK range from0.91to1.09. The rock shows relatively high SiO2, K2O and is enriched in Rb Th, Ba, Zr, Hf, depleted in Sr, P, Ti, Nb, Ta, which also have high REE, LREE and moderate negative Eu anomalies (Eu*/Eu=0.6～0.8). The pluton is general developed spotted rock structure with dark dioritic enclaves visible. LA-ICP-MS analyses yield age of104.67±0.82Ma for dioritic enclaves. Dioritic enclaves geochemical characteristics show that it’s aluminum-rich, iron-rich, high-magnesium, high titanium, and relative enrichment of Ba, U, Sr, Ti, loss of Th, Nd, Zr. It has lower∑REE ratio and higher Eu*/Eu than that of major-rock, indicating the extent of its differentiation is lower. Zircon Hf isotope composition of major-rock and dioritic enclaves is different, εHf (t)(-10.8～4.9) and Zircon Hf two-stage model ages (TDMC=1.3～1.6Ga), suggest that they are mainly generated from partial melting of Mesoproterozoic crust and have took place crust-mantle interaction obviously. Combined with the characteristics of igneous-rocks in the region, we infer that a widespread mantle-drived magma injection have happened in SE coastal area in early Late-Cretaceous, lithospheric extension caused by roll-back of subducting Paleo-Pacific plate plays a critical role.Comprehensive analysis of early and late Mesozoic magma-metamorphic-tectonic deformation characteristics in South China, a new interpretation of tectonic evolution of South China was conducted. It is suggested that tectonic evolution of South China in early Mesozoic mainly exhibit intraplate orogenic feature, constrainted by collision between SCB and surrounding blocks. The main cause of of early Mesozoic tectonic thermal event is lithospheric detachment after plates collision. Late Mesozoic tectonic evolution is mainly affected by the subduction of the Pacific plate. It may just be under rift environment influenced by stress transfer of plate subduction in early-middle Jurassic. Paleo-Pacific plate begin subduct in Middle-Late Jurassic. Roll-back of plate have happened, subsequently, which trigger a massive lithospheric extension, and cause mantle-derived magma underplating involved in the formation of granitic rocks from late Jurrasic to early Cretaceous. Massive volcanic-intrusive rocks were formed by the double impact of subduction and lithospheric extension in Late-Cretaceous.