Petrogenesis And Tectonic Setting Of The Late Mesozoic Magmatic Rocks In The Dabie Orogen

The late Mesozoic magmatic rocks are widely spread in the Dabie orogen(DBO).Numerous studies on these rocks have been most concerned on the geodynamic evolution of the orogen,while weakly constrained on the magmatism metallogenesis.By the way,there are still controversies on the geodynamic of these rocks.The research contents of this work include:(i)The determination of the ore-bearing magmatic rocks from Shapinggou(SPG)Mo deposit and Gongdongchong(GDC)Pb-Zn deposit in the Beihuaiyang zone;(ii)the origin of the Mesozoic ore-bearing rocks in Qinling-Dabie orogen were discussed,with a combination of ore-bearing and-barren magmatites in the DBO;(iii)petrogenesis of the Sikongshan pluton in central Dabie unit and quartz monzodiorite in Yuexi area,north Dabie unit,which is helpful for discussing the tectonic background of the Mesozoic magmatism;(iv)comparison of the Mesozoic magmatic rocks from Dabie orogen and Middle-Lower Yangtze River Belt.Major conclusions in this work are summarized as follow:Both the ore-bearing granite porphyry of SPG(115.5±1.8 Ma~116.3±2.1 Ma)and ore-related syenite quartz porphyry of GDC(126.8±1.3 Ma~126.3±1.4 Ma)belong to the third stage Mesozoic magmatic rocks of Dabie orogen.Granite porphyry from SPG yield zircon ?Hf(t)values of-18.1 to-14.5;quartz syenite porphyry from GDC yield whole-rock ?Nd(t)values of-18.1 to-14.5 and zircon ?Hf(t)values of-18.1 to-14.5,as well as weakly enrichment of radioactive Pb isotope.Also taking into account data from previous studies of ore-bearing,we found that the Sr-Nd-Pb-Hf isotopes of the ore-bearing rocks similar to the upper crust,while those of the ore-barren rocks to the middle-lower crust of the Dabie orogen.Sikongshan pluton in the central Dabie unit composed of quartz diorite(136.2± 1.6 Ma)and monzogranite(131.9±2.5Ma to 134.6±1.7 Ma).The quartz diorite was formed by partial melting of thickened lower crust,while the monzogranite was generated from FC of mainly plagioclase and perhaps minor amphibolite from the adakitic Tiantangzhai granitoid.Mafic microgranular enclaves(MMEs,131.8±1.5 Ma to 130.4±1.7 Ma)were common in quartz monzodiorite at Yuexi area,north Dabie unit.The MMEs show typical igneous textures,and both the host rock and MMEs contain abundant acicular apatite,suggest that the role of magma mixing in generating quartz monzodiorite of Yuexi area.The felsic end-member should be the early stage adakitic melt of Dabie orogen.The adakitic rocks in Dabie orogen were started at the western part of the DBO(143-134 Ma),and then widely spread in the whole orogen(134-130 Ma).Therefore,the change of subduction angle and roll-back model of the Paleo Pacific Plate was recommended to explain the late Mesozoic magmatic rocks in DBO.Compared with adakitic rocks in MLYRB,adakitic rocks in DBO show features as follow:(i)adakitic rocks in DBO composed of intermediate-acidic rocks,with no mafic rocks;while those in MLYRB of mainly intermediate rocks and minor mantle-derived rocks.(ii)Lower Y and higher Sr trends,high(Gd/Yb)N and(Dy/Yb)N and positive relationship between them.(iii)High Ba,low Rb/Ba,which is similar to the lower crust rocks.(iv)Low U and high Pb,and high Th/U(higher than normal lower crust).(v)Rather enriched Sr-Nd-Hf isotope and low radioactive Pb isotope of DBO adakitic rocks,while less enrichment of Sr-Nd-Hf and high radioactive Pb isotope for adakite from MLYRB.Adakitic rocks in DBO were generated by thickened and ancient lower crustal sources which ever experienced subduction and dehydration,while those in MLYRB originated from mixing of lower crustal melt and enriched lithospheric mantle-derived melt.The geochemical features of the second stage mantle-derived volcanic of DBO and MLYRB indicated that crustal materials were input into both of their mantle sources.Those volcanics from Beihuaiyang(BHY)were featured by low K2O/Na2 O,high Sr,Ba and high Rb,low Rb/Sr and Rb/Ba,low Pb,Th and U,relative high Th/U,with Sr-Nd-Pb-Hf isotope similar to the first stage adakitic rocks of DBO,similar to lower continental crust,indicate input of the subducted SCB lower crust.While those volcanics from MLYRB had opposite geochemical features to BHY volcanics,similar to oceanic crust,which indicate input of subducted Paleo Pacific Plate material.Both the third stage rocks of DBO and MLYRB had shallow magmatic sources.The third stage rocks in DBO were reflected by the similarity of Sr-Nd-Pb-Hf isotopes to the upper crust of DBO.While those in MLYRB were indicated by high zircon ?18O,similar to S-type granites.We proposed a model that MLYRB as wide continental margin arc and DBO as back-arc of the western subduction of Paleo Pacific Plate system.The Mesozoic magmatism can be explained by changing of the subduction angle and slab roll-back of this system.