Geochemistry And Genetic Mechanism Of The Paleozoic-mesozoic Granites In The Western Section Of The North Qinling

The Qinling orogenic belt experienced multiple stages tectonic evolution,accompanied by corresponding granitic magmatism.The petrogenesis and the nature of source of granite are of great significance to the orogenic process and crustal evolution.The Paleozoic-Mesozoic granites in the Northern Qinling orogenic belt respond to the two Orogeny events.However,the present works mainly focus on the eastern section;fewer attentions have been paid in the western section,especially the presice isotopic chemeistry and chronology,which limit the full scope of the granites in the Qinling orogenic belt.In addition,it still remains conversial about the tectonic setting and origin of the granites in the western Qinling.This dissertation selects 8 representative plutons from the western section of the North Qinling orogenic belt?NQB?,including the Paleozoic?Sanchahe,Dangchuan,Caochuanpu,Huoyanshan and Shanmenzhen plutons?,Early Mesozoic?Guanshan and Baoji plutons?and Late Mesozoic?Taibai pluton?granites.The field investigation,petrography,geochemistry,zircon U-Pb age and Lu-Hf isotope analyses have conducted for these granites.Combined with the previous works,this dissertation constructs the geolochrological framework and spatial-temporal evolution of granites,and probe into the geochemical characteristics and genetic mechanism of granites in different stages of the NQB.This study yields the following conclusions:?1?The Paleozoic granites can be subdivided into three stages according their petrology and zircon U-Pb age.a.The first stage:⁴50Ma.The Sanchahe quartz diorite has relatively low SiO2?56.42-61.44wt.%?contents,high Na2O contents,A/CNK ratios of 0.79-0.86,high Mg#,Cr?65.7-113ppm?,Ni?32.8-54.2ppm?contents,with the zircon?_Hf?t?values of+2.24to+5.13.Its high Ba/La?22-31?and variable Ba/Th?40-162?ratios,indicating that the quartz diorite was derived from subducted oceanic fluid and sediment and melt-derived mafic magma in subduction.b.The second stage:⁴30Ma.The Dangchuan and Caochuanpu monzogranites have high SiO2 contents of 71.23-74.29wt.%,A/CNK ratios of 1.08-1.17,K2O contents of3.48-5.88wt.%,K₂O/Na₂O ratios of 1.10-2.26,low Mg#.The Dangchuan monzogranite has zircon?_Hf?t?values of-0.59 to+10.56 and Caochuanpu monzogranite has zircon?_Hf?t?values of-5.03 to+7.32,indicating a heterogeneous source region.The monzogranites were produced by mixing of mafic and granitic magma.c.The third stage:⁴10Ma.The Huoyanshan and Shanmenzhen biotite granites have SiO₂ contents of 68.73-73.07wt.%,K₂O contents of 4.40-6.23wt.%,A/CNK ratios of 1.01-1.09,K2O/Na2O ratios of 1.24-2.03,belonging to peraluminous high-K calc-alkaline series.Their high CaO/Na₂O ratios,low Mg#,suggesting that they were derived from greywackes.The Huoyanshan granite has zircon?_Hf?t?values of-9.01 to+2.65,while Shanmenzhen granite has zircon?_Hf?t?values of-26.62 to-18.86,suggesting the granites were mainly derived from partial melting of ancient crustal greywackes,indicating a post-collisional setting.From 459Ma to 410Ma,the granites display decreasing SiO₂ contents,and the?_Hf?t?values were from positive to negative,indicating the melting degree of the enriched components in the upper and middle crust increase,and the melting depth gradually becomes shallower,Showing the evolution process from subduction to collision stage.?2?The Early Mesozoic granites can be subdivided into two stages:²30Ma and²10Ma.a.The first stage:The Guanshan monzogranites have high SiO₂?66.91-75.25wt.%?and K₂O?3.44-5.30wt.%?contents,A/CNK ratios of 0.84-1.03,Mg#=36.2-55.3,variable zircon?_Hf?t?values of-21.59 to+0.19,suggesting they were derived from partial melting of ancient crust and were formed in the early stage of the transition from compressive collision to extensional mechanism.b.The second stage:The Baoji pluton has higher SiO₂,K₂O contents and lower Mg#values.The main monzogranites have zircon?_Hf?t?values of-13.57 to+6.94,T_DM2values of 1.66-2.08Ga,suggesting that they were derived from partial melting of Paleoproterozoic continental crust.Porphyritic biotite granite in the east has ziron?_Hf?t?values of-0.7 to+5.44,suggesting a Mesrproterozoic crustal source.The Baoji pluton were formed in the post-collision extension stage of the Yangtze and the North China Blocks.?3?The Late Mesozoic granitic magmatism?¹50Ma?,represented by Taibai syenogranite.The Taibai syenogranite have high SiO₂ and alkali contents,A/CNK ratios of0.97-1.05,high initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7053 to 0.7112,?_Nd?t?values of-18.6 to-0.1,zircon?_Hf?t?values of-19.7 to-21.1,suggesting mainly derived from continental crust.The syenogranite have low radiogenic Pb isotopic compositions which similar to those of basement blocks in South Qinling and Yangtze Blocks.Combined with regional geological background,it is considered that they were formed in transition stage from extrusion to extension.