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Geochemistry of Early Cretaceous adakitic rocks in Tongling region of Anhui Province:Constraints for rock-and ore-forming

2012; Science Press; Linguagem: Inglês

2095-8927

Jiancheng Xie,

Geological and Geochemical Analysis

Tongling region is one of the most important non-ferrous metal producers where the formation of Cu-Au deposits has closely genetic relations with Early Cretaceous adakitic intrusions.However,petrogenesis of adakitic intrusions has been hotly debated in the past decade.Here we report new elemental and Sr-Nd-Pb isotopic data in Tongling region.The Tongling adakitic rock(SiO2 = 57.6% ～ 64.2%) have high Al2 O3( 15.3%) and relatively high MgO concentrations,high Ba( 640 × 10-6) and Sr concentrations( 483 × 10-6,average 900 × 10-6),high Sr /Y( 41.3) and(La /Yb) N( 12.1) ratios,low Y and Yb contents,low K2 O /Na2 O(0.54 ～ 0.83) and Th /U ratios(2.62 ～ 5.75),indicating that they are genetically related to slab melting and subsequent interaction with the enriched mantle.The relatively low eNd(t) values(-11.3 to-13.7) and high(87 Sr /86 Sr) i ratios(0.7068 to 0.7092) of the Tongling adakitic rocks may be plausibly explained by contamination of enriched mantle materials and/or continental crust.The Tongling adakitic rocks are characterized by high radiogenic Pb isotopic compositions with(206 Pb /204 Pb) i = 17.93 ～ 18.64,(207 Pb /204 Pb) i = 15.46 ～ 15.59,and(208 Pb /204 Pb) i = 37.88 ～ 38.51,which are mainly plotted in the field of MORB,near the intersection of EM1 and EM2,and are obviously different from those of the upper and lower continental crust and Dabie adakites.These exclude major contamination from the continental crust.Given that eastern China has both EM1 and EM2 types of enriched mantle,whereas the Tongling region is located near the transition from EM1 to EM2,these isotopic characteristics of Tongling adakitic rocks are best explained by slab melts with assimilation of enriched mantle components.The Tongling adakitic rocks have a high temperature,low pressure regime,possibly resulted from slab subduction.These distinctly geochemistry characteristics indicate the adakitic rocks and the large-scale Cu-Au mineralization of Tongling and the Lower Yangtze River belt result from partial melting of the subducting oceanic crust.