A Geochemical Study Of Postcollisional Adakitic Granitoids In The Dabie Orogen

Adakite was originally proposed as a genetic term to define intermediate to high-silica,high Sr/Y and La/Yb volcanic and plutonic rocks derived from partial melting of the basaltic portion of oceanic crust subducted beneath volcanic arcs.lt was also initially believed that adakite only occurs in arc settings.Nevertheless,recent studies have shown that adakitic rocks can be produced in non-arc tectonic environments through partial melting of mafic lower arc crust,crystal fractionation of basaltic magma,partial melting of high Sr/Y source,partial melting or delamination of thickend lower crust.Adakitic rocks in continental setting are commonly considered to be produced by partial melting of thickened lower continental crust.However,recent studies of experimental petrology and geochemical simulation suggested that adakitic melts can be generated atnoraml lowercrust depths(30-40km),and "adakitic" feature is likely inherited from source rocks,an overthickened lower crust is thus not necessarily required.These results have been used to explain adakitic rocks occurredin the interior of craton.Postcollisional adakitic rocks commonly occur in continental collsion orogens,whether they are the products of partial melting of collision-thickened orogenic crust?A combined study of zircon U-Pb ages and trace elements,whole-rock major-trace elements and radiogenic Sr-nd-Hf isotopes was carried out for postcollisional adakitic granitoids in the Dabie orogen,relevant results are used to compared with those of accompanied normal granitoids.The results reveal that postcollisional granitoids in the Dabie orogen can be divided into three stages:Low-Mg adakitic(LMA)stage,High-Mg adakitic(HMA)stage,Normal granites(NG)stage.LMA magmatism(143-130Ma)is systematically earlier than NG magmatism(133-112Ma),and HMA magmatismocurred at about 130Ma,which testifies a ramarkable change inpartial melting depths.Postcollisional granitoids in the Dabie orogen commonly contain abundant inherited zircons of Neoproterozoic ages and Triassic metamorphic origin,which indicates their derivation from reworking of the subducted continental crust of the South China Block.They also contain small amounts of inherited zircons with Paleoproterozoic-Archean ages,and exhibit enriched whole-rock Sr-Nd-Hf isotope compositions and ancient two-stage Nd-Hf model ages(Paleoproterozoic-Archean),suggesting involvement of ancient basement crustal materials of the Yangtze block in their magma sources.Compared to normal granitoids.Dabie adakitic granitoids are remarkably depleted in HREE,do not show significant negative Eu anomalies,and exhibit high Sr/Y and La/Yb ratios.In addition,syn-mgmatic zircons from adakitic granitoids have HREE contentsand degree of negative Eu anomalies lower than normal granitoids.These results suggest that parental magmas of adakitic rocks are depleted in HREE and absent of significant nagnative Eu anomalies.Furthermore,inherited zircon core in the shows a stronger depletion in Eu and enrichment in HREE than syn-magmatic zircon rim of the same zircon grain,which indicates that "adakitic" feature of granitoids in the Dabie orogen is not inherited from their source rocks.Therefore,it is suggested that postcollisionaladakitic rocks in the Dabie orogen were derived from partial melting of thickened lower crust.