Genesis Of The Southeastern Margin Of Dabie And Xuhuai High-magnesium Adakite And Its Tectonic Significance

There are a large number of adakitic rocks in every tectonic unit of the Dabie orogenic belt,but the adakitic rocks with high and low Mg have distinct spatial and temporal distribution characteristics.Low-Mg adakitic rocks are widely distributed in the orogenic belt,but high-Mg adakitic rocks only occur in the southeastern margin of the orogenic belt,near the Tanlu fault zone,the reason is not clear.Whether the Mesozoic high-Mg adakitic rocks in Xuhuai area,southeastern margin of the North China Craton are subducted oceanic crust or subducted crust is controversial.In view of the above two scientific problems,the Ca and Mg isotopes of Mesozoic high-Mg adakitic rocks in the southeastern margin of the Dabie orogenic belt and Xuhuai are studied in this paper.The results show that the ratios of delta ^44/42Ca are 0.34±0.03‰⁰.37±0.04‰and 0.40±0.05‰⁰.41±0.04‰respectively,similar to those of mid-ocean ridge basalts(MORB,Delta ^44/42Ca=0.3±0.05‰⁰.41±0.05‰),which are slightly lighter than those of upper mantle Delta ^44/42Ca=0.47±0.04‰.In this paper,the Ca isotope fractionation of Dabie low-Mg adakitic rocks(^44/42Ca=0.23±0.03‰⁰.38±0.02‰)during the reaction between crustal melt and mantle was simulated and calculated.The results show that reaction 1?melt 1+ol melt 2+opx?can make Ca isotope composition of melt lighter gradually,which can not explain that the high-Mg adakite system is heavier than the low-Mg adakite system,and the near-MORB ⁴4/424/42 Ca.The Ca isotopic composition of high-Mg adakitic rocks can be explained by assimilation and melting of mantle peridotite?melt 1+peridotite melt 2+Opx,assimilation/crystallization rate ratio greater than 1?,indicating that melt/mantle reaction occurs at higher temperatures?e.g.¹300^oC?.The high Mg adakitic rocks in the Dabie orogenic belt only distribute in the area near the Tanlu fault zone,which may be related to the higher mantle temperature near the fault.The Delta ²⁶Mg of Xuhuai high Mg adakitic rocks is-0.29±0.05⁰.57±0.01,most of which are similar to the Mg isotope composition of MORB in the error range.The Mg-Sr picture shows a positive correlation,which can be explained by the contamination of oceanic crust melts by the lower crust.Based on previous data,it is inferred that the high-Mg adakitic rocks in Xuhuai are of oceanic crust origin.The age and spatial distribution of subduction oceanic crust-derived adakites in eastern China indicate that the mid-oceanic ridge between the Paleo-Pacific plate and the Izenaki plate is located in the middle and lower reaches of the Yangtze River at 140 Ma and moves northwest to Xuhuai area on the southeastern margin of the North China Craton at 130 Ma.