Characteristics And Petrogenesis Of The Mesozoic Different Stage Granites From Eastern Segment Of The Jiangnan Orogenic Belt

Yanshanian magmatic rocks wildly distribute in the southern Anhui Province and can be subdivided into two stages: the early stage of 152–137 Ma,and the late stage of 136–122 Ma.Based on investigation,the 140 Ma Dengjiawu granite and the 130 Ma Xinxikou-Takeng granites that located in the Qinshan-Changai belt generally have a close genetic relation with molybdenite deposit.This is clearly incompatible with the existing knowledge that the ore-forming rocks in the southern Anhui Province are mainly early stage granodiorites.By now,the representative late stage Huangshan granite has not been found to be related to mineralization.LA-ICPMS zircon U-Pb dating results show that the Dengjiawu granite in Qinshan-Changai belt was formed at 140.2±3.5 Ma,which belongs to early stage magmatism,the Xinxikou-Takeng granites was formed between 127.0±2.3 Ma and 130.5±1.7 Ma,and the Fencun granite in northeastern Jiangxi Province was formed at 130.9±1.8 Ma,all of which belong to late stage magmatic activities.The Xinxikou-Takeng granites are characterized by rich in Si?SiO2=74.85%-76.93%?,K?K2O/Na2O=1.33-2.38?and poor in Mg?MgO=0.03%-0.05%?.Al₂O₃ contents range from 11.4% to 12.43% with A/NK values of 1.07 to 1.14 and A/CNK values of 0.96 to 1.03,indicating these rocks belong to high potassium calc-alkaline series and subaluminous to metaluminous A-type granite.While Fencun granite is characterized by rich in Si?SiO₂=4.04%-75.13%?,K?K₂O/Na₂O=1.55-1.65?and Mg?MgO=0.10%-0.19%?.Al₂O₃ contents range from 13.38% to 13.48% with A/NK=1.28-1.32,and A/CNK=1.18-1.19,belonging to high potassium calc-alkaline series and S-type granite.Xinxikou-Takeng granites have comparatively special Nd-Hf isotopic characteristics,namely depleted zircon Hf isotopic conponents??Hf?t?=-1.8 — 3.3,average 0.6?and enriched whole rock Nd isotopic conponents??Nd?t?=-4.55—-3.89,average-4.13?.It can be interpreted as that zircons have already crystallized before depleted magma mixing with the enriched crustal material,then mixed magma formed the granite with enrichment Nd isotopic components.Nd isotopic characteristics of the Xinxikou-Takeng granites is similar to Huangshan granite,little more depleted than that of granites in Qinshan-Changgai belt?87Sr/86Sr?t?=0.7120-0.7125,?Nd?t?=-7.24—-4.38?.Depleted characteristics of zircon Hf isotope in Xinxikou-Takeng granites is similar to the Fencun granite??Hf?t?=-0.5-9.6,average 3.7?.The early stage ore-bearing granodiorites form southern Anhui Province,early stage ore-bearing granites in Qinshan-Changgai belt and late stage ore-barren Huangshan granite have the consistent Sr-Nd-Hf isotopic features.Summarizing results of present geochemical studies,it is indicated that the early stage ore-bearing granodioritederived from partial melting of thickened Meso-Neoproterozoic lower crust,while the Qinshan-Changai belt ganites was envoluted by this kinds of granodiorite magma,which underwent a process of plagioclase + hornblende + Shangxi Group AFC.Huangshan granite was also originated from anatexis of Meso-Neoproterozoic lower crust,but had experienced early stage magma extraction of I type acid-intermediate magma.Xinxikou-Taken granites might aslo originated from the melting of magma extraction residue,just somewhat same as Huangshan pluton,with the differences lying in that the mantle material may existed in the former Fencun granite in Northeastern Jiangxi Province was also a result of partial melting of Meso-Neoproterozoic lower crust and it is likely to be a Himalaya-type granite which originated from the argillaceous rocks source area under high pressure condition,and mantle derived materials may be existed in Fencun granite.By contrastive studies on the two stages granites,it is suggested that the early stage granites was formed in an environment of relatively thick lower crust and lower temperature,while the late stage A-tpte granite was originated from a rather shallow source area with a higher temperature.These indicate that the tectonic setting of magmatisms has turned form post-orogenic to anorogenic,from early to late.The late magmatic stage corresponds to a back-arc extension environment which formed in the background of the increasing subducted angle of the paleo-Pacific plate.