Petrogenesis Of The Mesozoic Classic Adakites And A-type Granites In The Anqing Orefield:Implications For Metallogenesis Of Polymetallic Deposit

Lower Yangtze River Belt (LYRB) is one of the most important metallogenic belt in China. Deposits in the form of ore fields along the Yangtze River are set EW-NE arc distribution.There are more than 200 deposits of copper (gold), iron, molybdenum, lead, zinc, silver, tungsten, etc. All types of deposits in the Yangtze River region have related to Yanshanian intrusive rocks. The Anqing orefield is located in the northwest of Anqing, southwest of Anhui, along the Lower YangtzeMetallogenic Belt.In the Anqing Orefield, samples from the main intrusions associated with regional Fe-Cu-Au deposits are diorites with 206Pb/238U ages of 136.9 ± 1.4 Ma (Yueshan),138.1 ± 1.1 Ma (Zongpu), and 138.0 ± 0.8 Ma (Zongpu). Two A-type granite samples from the Dalongshan yielded 206Pb/238U ages at 126.8 ± 1.9 Ma and 126.7 ±2.1 Ma, respectively. Both Yueshan and Zongpu diorites have 59.89 wt.% SiO2,16.29 wt.% Al2O3,5.00 wt.% Na2O; low contents of Y (11.4 ppm) and Yb (1.08 ppm); high Sr/Y and La/Yb ratios of 132.5 and 43.7, respectively; and Sr-Nd isotopes similar to marine sediments.Geochemical and isotopic signatures suggest that both Yueshan and Zongpu diorites are classic adakites, probably formed from partial melting of subducted oceanic crust, then quickly emplaced into upper crust across overlying mantle wedge. The εHf(t) value of Yueshan adakites is-23.3 ± 0.2 (03YS16), while the ent(t) values of Zongpu adakites is -6.8 ± 0.1 (03ZPS02),-11.5 ± 0.2 (14ZP01), respectively. The occurrences of classic adakites and their zircon Hf isotope compositions imply that there exist a residual ancient oceanic crust fragment.The Dalongshan granites are A1-type granites, indicating an intraplate extensional setting. Zircon Hf isotope compositions of Al-type granites (εHf(t)= 1.5 ± 0.2 (03DLS01), εHf(t)=-9.5 ± 0.2 (03DLS03)) reveal an asthenosphere upwelling event. Therefore, the classic adakites and A1-granites in the Anqing region record a tectonic transition from a compressive to an extensional setting along the LYRB between 138 and 127 Ma. A variety of Fe-Cu-Au mineralization events are related to this tectonic transitional event.