Geochronology And Geochemistry Of The Baimashan Pluton And Hengshan Pluton In Central Hunan And Its Relationship With Tectonic Evolution In The Region

Baimashan pluton is located in the central part of Hunan Province.It is a Caledonian,Indosinian and Yanshanian pluton.Baimashan pluton is located in the eastern part of the north of the Jiangnan anticline belt.Its lithology is biotite granite.The LA-ICP-MS zircon U-Pb ages of the Baimashan Caledonian intrusion are 416±6 Ma,418±8,455±7 Ma and 462±7 Ma.It is a series of granites of weak peraluminous-strong peraluminous(A/CNK mean=1.11),magnesia and high potassium calc-alkaline.The content of trace elements is low(REE=72x10-6?446.5×10-6),light rare earth enrichment,Ba,Nb,P and Ti relative depletion,Th,U and Pb relative enrichment,showing a significant negative Eu anomaly(delta Eu is 0.5-1.02).The values of ?Hf(t)of the rock mass are between-7.37?3.97,-5.47?3.02 and-6.76?-4.1,respectively,indicating that the source area is crustal.The Caledonian intrusion in Baimashan is of two magmatic stages,and the type of granite is type I.It is speculated that the lower crust metamorphic igneous rocks were formed by partial melting.The early Caledonian rock mass was formed in compressive environment,while the late Caledonian rock mass was formed in extensional environment.The regional tectonic setting was controlled by the Paleo-Asian tectonic domain.The LA-ICP-MS zircon U-Pb ages of the Baimashan Indosinian pluton are 232±2 Ma and 233±3 Ma.It is a series of granites of weak peraluminous(A/CNK ranging from 1 to 1.1),magnesia and high potassium calc-alkaline.The content of trace elements is low(REE=102.9×10-6?154.8×10-6),light rare earth enrichment,Ba,Nb,P and Ti relative depletion,Th,U and Pb relative enrichment,showing obvious negative Eu anomaly(delta Eu is 0.26?0.62).The values of ?Hf(t)of the rock mass are between-8.28?4.81 and-8.78?-4.22,respectively,indicating that the source area is crustal.The type of granite is type I granite.It is presumed that the lower crust metamorphic igneous rocks were formed by partial melting.The rock mass was formed in a compressive environment,and the regional tectonic setting was influenced by the Tethys-Paleo-Pacific tectonic domain.Hengshan pluton is located in the Middle-East of Hunan Province.It is a Caledonian and Indosinian pluton.Hengshan pluton is located in the Caledonian fold system of South China.The Yanshanian pluton is of dimica granite.The U-Pb ages of LA-ICP-MS zircons are 157±2 Ma and 151±2 Ma.It is strongly peraluminous(A/CNK>1.1),magnesian,high potassium calc-alkaline series granite.The content of trace elements is low(REE=50.4×10-6-197.6×10-6),the enrichment of light rare earth elements,the relative depletion of Ba,Nb,Sr and Ti,and the relative enrichment of U and Pb show obvious negative Eu anomalies(delta Eu is 0.32?0.62).The values of ?Hf(t)of the rock mass are between-13.14?-11.06 and-18.76?-11,indicating that the source area is crustal.The granite type belongs to S-type granite,which may be formed by partial melting of metamorphic argillaceous rocks in the middle crust.Baishifeng granite was formed in a collisional environment,and its regional tectonic setting was influenced by the collision between the Asian continent and the western Pacific continent.