Genesis And Source Lithology Of Late Mesozoic Mafic Dikes In Southeastern China

Late Mesozoic magmatism in southeastern China was controlled by the subduction of paleo-Pacific plate,and the mantle beneath this area should also have been affected by such process.Late Mesozoic mafic dikes are widely distributed in southeastern China,especially in the coastal areas of Fujian province,which provide an ideal setting for studying the mantle evolution and subduction process of paleo-Pacific plate.In this study,source lithologies of late Mesozoic mafic dikes in southeastern China are identified.On this basis,we present major element,trace element and Sr-Nd-Hf isotopic compositions for the late Mesozoic mafic dikes in Fujian province,to discuss the source properties and further,constrain the subduction model of paleo-Pacific plate.Here it summarizes the chronological and geochemical data of mafic dikes in Hunan,Jiangxi,Zhejiang,Fujian and Guangdong provinces.Source lithologies of these rocks are identified by the compositions of fractionation-corrected primary magmas.The study indicates that the mantle lithologies of these mafic dikes in southeastern China do not show obvious difference in space,but significant changes with time.The source lithologies of mafic dikes during the period of 150–110 Ma are composed of Si O₂-rich pyroxenite and Si O₂-poor pyroxenite,whereas the source lithologies during110–64 Ma are primarily composed of Si O₂-poor pyroxenite,with minor peridotite.Therefore,it is suggested that the lithological variation of late Mesozoic mantle beneath southeastern China is controlled by the subduction of paleo-Pacific plate via the variation of the subduction angle.Mafic dikes are widely distributed in southeastern China,most of which exhibit arc-like geochemical features.However,it remains controversial whether the mantle source of these mafic rocks is the lithospheric mantle or the asthenosphere.Here we present a comprehensive study on mafic dikes from Fujian province in southeastern China,with the aim to understand their source properties.Two types of mafic rocks have been recognized based on their diverse trace-element features.Type-I rocks show arc-like trace-elemental characteristics,especially marked by the enriched large ion lithophile elements?LILEs?and light rare earth elements?LREEs?and relative depleted high field strength elements?HFSEs?.While those type-II rocks are distinguished by their relatively flat patterns in primitive-mantle-normalized trace element diagram.Despite of the trace elemental differences between two types of rocks,all of the mafic dikes show two trends in the plots of ⁸⁷Sr/⁸⁶Sr_?i?versus La/Nb,and they can be explained by the influences of crustal contamination and melt-lithospheric mantle interaction,respectively.Most type-II rocks show lower La/Nb?1.0–1.5?and slightly higher Sr isotopic compositions?0.706–0.709?,which coincide with those of sub-continental lithospheric mantle?SCLM?.More importantly,⁸⁷Sr/⁸⁶Sr_?i?,La/Nb,Sr/Y and Zr/Y of type-I rocks are generally correlated to the thickness of the underlying lithosphere,and the signals of lithosphere is more clear with increasing lithospheric thickness.This again highlights the important influences of melt-lithosphere interaction during their formation.Such observations also indicate that these mafic rocks are more likely to originate from the asthenosphere rather than the lithospheric mantle.