Petrogenesis Of Late Mesozoic Magmatic Rocks From The Zhangbaling Uplift In The Lower Yangtze Region

Late Mesozoic magmatic rocks of the Zhangbaling Uplift?ZBU?in eastern China can provide insights into the deep crustal processes and tectonic affinity of the massif.These rocks are dominated by granitoids?including granodiorite,quartz-monzonite,monzogranite,quartz monzodiorite and quartz monzonite porphyritic dyke?with formation ages of 132–120 Ma and a peak at 129 Ma.The Chuzhou area outcrop the Tongjingshan diorite with age of 130.6±1.2 Ma although early Cretaceous adakitic rocks is common in this region.Based on major-and trace-elements and isotopic element analysis,these intermediate–felsic rocks can be divided into two groups:low-Mg intermediate–felsic rocks with more enriched Sr–Nd isotopic signatures and low Mg#values?<50?,and high-Mg intermediate–felsic rocks with weakly enriched Sr–Nd isotopic signatures and high Mg#values?mostly>50?.These rocks show adakitic signatures of high Al₂O₃ and Sr contents,high Sr/Y,?La/Yb?_N,and?Dy/Yb?_N ratios,but low Y and Yb contents.They have enriched ⁸⁷Sr/⁸⁶Sr?t?ratios,negative?_Nd?t?and?_Hf?t?values,and relatively low Pb isotopic compositions that resemble those of coeval high Sr/Y granitoids in the Dabie Orogen.Combining their geochemical features with previous results of experimental melts,we conclude that the low-Mg intermediate–felsic rocks were derived from fluid-absent partial melting of in situ thickened lower continental crust,resulted from Triassic continental subduction and collision in this area,whereas the high-Mg intermediate–felsic rocks were derived from delaminated eclogitic lower crust followed by interaction with mantle peridotites during magma ascent.Isotopic compositions show that these intermediate–felsic rocks could have been derived from lower continental crust of the Yangtze Block including re-melting of Neoarchean and Paleoproterozoic lower continental crust.The Tongjingshanc diotie belongs to the high-K calc-alkaline series with relatively high Mg#,Mg O content,Sr/Y and?La/Yb?_N ratios but low Y and Yb contents.They are characterised by enriched LILEs but depleted HFSEs compositons without obvious Eu anomalies.Based on analysis of minor and accessory minerals,we summarize that they were formed by magma mixing between mafic magma derived from lithospheric mantle metasomated by slab fluid and felsic magma derive from thickened or delaminated eclogitic lower crust.Besides,the results of theoretical calculation reveal that the Zhangbaling intermediate–felsic rocks and the Tongjingshan diorite have relatively low formation temperature?<800??and low oxygen fugacity,suggesting that they are ore-barren.Comparing the late Mesozoic magmatism in different regions of the middle–lower Yangtze River Belt,we find that the great depth of the Tan–Lu Fault Zone enabled delamination of the thickened eclogitic mountain root beneath the ZBU,which resulted in the formation and emplacement of the high-Mg rocks followed by uplift of the Zhangbaling massif,and the late Mesozoic magmatic rocks formed in an intensely extensional environment associated with rollback of the subducting Paleo-Pacific?Izanagi?Plate,resulted in the early Cretaceous plutons were characterized by back-arc magmatism.