Mineralogical Constraints On The Origin Of The Neoproterozoic Felsic Intrusions In The Northwestern Margin Of The Yangtze Block, South China

The Neoproterozoic is an important period of crustal growth and reworking in the Yangtze Block, South China. There are voluminous igneous rocks distributed along the margins of the Yangtze Block. Four models, including "Plume model","Arc model","Rifting model" and "Subduction-back-arc basin model", have been proposed to explain their heat sources and tectonic settings. There are numerous felsic plutons in the Hannan region, at the northwestern margin of the Yangtze Block. They mainly comprise the～730Ma Erliba and Wudumen intrusions and the～760Ma Xixiang and Tianpinghe intrusions, which are important for studying secular evolution of the Yangtze Block.The two generations of granitoids have different lithologies, mineral assemblages and compositions. Rocks from Erliba and Wudumen intrusions consist of granodiorite and tonalite with adakitic chemical compositions. They are composed of biotite, plagioclase and quartz, associated with minor amphibole, epidote and K-feldspar. Amphibole and biotite in the Wudumen intrusion have higher MgO and lower FeO than those from the Erliba intrusion. Rocks form the Xixiang intrusion are monzodiorite which are composed of amphibole, plagioclase, K-feldspar, quartz with minor epidote, clinopyroxene and biotite. Clinopyroxene occurs as a residual mineral within amphibole. Rocks from the Tianpinghe intrusion are monzonitic granite, which consists of biotite, plagioclase, K-feldspar and quartz with minor epidote and amphibole.Epidote in rocks from these four felsic intrusions is magmatic origin. It can be divided into three types:1) subhedral to anhedral epidote with embayed rims and allanite-rich core;2) euhedral epidote preserving well-developed twin textures;3) anhedral epidote with embayed margins in contact with quartz and feldspar. The epidote has pistacite values (100*Fe3+/[Fe3++Al3+]) ranging from27to32and TiO2less than0.2wt.%. The occurrence of epidote in rocks from the Hannan felsic intrusions suggests that they underwent a rapid emplacement. Rocks from the Erliba, Wudumen and Xixiang intrusions have similar Zr-saturated (756-766℃) and P-saturated temperatures (818-848℃). Rocks from the Tianpinghe intrusion have relatively high Zr-saturated (798℃) and P-saturated temperatures (876℃). In addition, the Erliba and Wudumen intrusions have similar emplacement pressures (2.96-3.11kbar) and temperatures (787-789℃). The Tianpinghe intrusion crystallized at shallow depth (ca.2.00kbar) and relatively low temperature (ca.747℃), whereas the Xixiang intrusion crystallized under high pressure (ca.3.54kbar) and low temperature (ca.676℃). The four intrusions have similar oxygen fugacity near the nickel-nickel oxide (NNO) buffer, suggesting oxidized parental magmas. The Erliba and Wudumen intrusions are estimated to have been generated under pressures higher than12kbar, the Xixiang intrusion under pressure of>10kbar, and the Tianpinghe intrusion under pressure of>6kbar.The four felsic intrusions were produced by melting of the newly formed mafic lower crust. Their petrology and geochemical differences may have resulted from variations of melting degree and melting pressure, as well as magma differentiation. Rocks from the Erliba intrusion have low MgO and FeO and high SiO2, corresponding to experimental melts formed by9-12%partial melting of amphibolite under pressures of15kbar, whereas rocks from the Wudumen intrusion have relatively high MgO and FeO and low SiO2, similar to melts generated by19-22%melting of amphibolite. Rocks from the Xixiang intrusion have high MgO (2.3-3.1wt.%) and CaO (3.1-6.2wt.%), suggesting that they were produced by>20%melting. The most primitive rock samples of the Tianpinghe intrusion have whole rock compositions which are similar to those of experimental melts formed by12-15%partial melting of the high-Al basalt under pressures of8kbar.Mineral compositions could be used to constrain tectonic setting at the time when the magma generated. Biotites in rocks from the Hannan felsic intrusions are calc-alkalic, and plot in the field of active continental margin, indicating that these intrusions were formed in an active continental margin setting.