Abstract:

The Jianshan pluton was emplaced in the late Early Cretaceous, which can be divided into three rock units.The analytical results of electron-microprobe show that the alkali feldspars are mainly Na-orthoclase and anorthoclase, with minor orthoclase, the amphiboles of the Jiashan pluton are ferro-edenite with minor ferrohornblende and ferro-actinolite, and the clinopyroxene includes hedenbergite and augite. Geochemically the Jiashan syenite is rich in Si, alkalis, Fe, REE, Th, Ga, Nb, Zr and Hf, poor in Mg, Ba, Sr and Ti, and depleted in transition elements such as Cr, Co, Ni, and V, and is enriched in light REE with a moderate Eu negative anomaly, which show that the Jiashan syenite belong to an A-type granite.The ⁸⁷Sr/⁸⁶Sr initial ratios of the rock ranging from 0.701 41 to 0.707 41 and the ε_Nd(t) values of -2.27 to -5.58 indicate that the magma was derived from partial melting of an enriched subcontinental lithospheric mantle. Delamination or lithospheric thinning probably caused the partial melting of the enriched subcontinental lithospheric mantle. Nd depleted mantle model age implys that mantle enrichment beneath Yanshan area may occur in the late Mesoproterozoic.

Key words: syenite, Sr-Nd isotopic composition, geochemistry, petrogenesis, Jiashan, northern Hebei