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| The Chaijiazhuang pluton occurs in the western Qinling orogen, Tianshui area, central China. The pluton contain some magmatic mafic microgranular enclaves, which are fine-grained and contain acicular apatite and plagioclase megacrysts,. The mafic microgranular enclaves have lower SiO2 (60.27% - 60.38%) and higher Mg#=54-55 and Nb/Ta(14.8-16.6), this indicates that they may result from partial melting of the sub-continental lithosphere mantle(SCLM). Geochemically, the host monzonitic granite show some adakitic characteristics with high Sr, Ba, low Y and HREE, high Sr/Y(88-98) and Y/Yb(13-15) ratios, but tend to have a higher concentration of K2O（4.01%～4.53%）. The high (La/Yb)N ratios, concave-unward shapes of the HREE and a lack of significant Eu anomalies, high Ba and Sr as well as low aboundances of Y and HREE suggest a feldspar-poor, garnet ± amohibole-rich fractionation mineral assemblage. Coupled with previous studies we propose that the Chaijiazhuang monzonitic granite may be result from partial melting of thickened lower crust, the latter was triggered by underplating mafic magma which was caused by subducting slab breakoff during the late orogenic stage in Western Qinling Orogen. |
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| Keywords:Petrology; adakitie; magma mixing; Qinling orogenic belt; slab break-off |
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