Geochemistry of Anjishan Intermediate-acid intrusive rocks in Ningzhen area: Constraint to origin of the magma with HREE and Y depletion.

Anjishan intrusive body in early Cretaceous in Ningzhen area of Jiangsu pro vince, east China mainly consists of intermediate-acid rocks (quartz-diorit e and granodiorite). These intermediate-acid intrusive rocks are characteri stic of HREE and Y depletion, similar to those of adakite. They have low Yb and Y concentration (Yb 0.78 similar to 1.16 mug/g; Y 7.4 similar to 12.5 mug/g), but high Sr concentration (550-1579 mug/g), therefore possessing hi gh La/Yb and Sr/Y ratios. These rocks lie in the adakite and high-Al TTD fi eld on Sr/Y - Y diagram. Although Anjishan intermediate-acid intrusive rock s show compositional characteristics similar to adakite, our results indica te that they are not related to subduction, therefore they may not be the p roduct of slab melting. On the other hand, nor is it likely for a crystal f ractionation or AFC process of the basaltic magma to generate these rocks, since Anjishan intermediate-acid intrusive rocks don't exhibit a successive compositional variational trend with the associated mafic rocks and basalt s, and evidence of assimilation by crustal material has not been found. Thu s, we conclude that Anjishan intermediate-acid intrusive rocks were most li kely derived from partial melting of mafic materials in the lower part of a thick crust. In comparison, the contemporaneous intermediate-acid intrusiv e and volcanic rocks in the Suzhou-Wushi and Jiangsu-zhejiang areas, which are located near Ningzhen area, don't show HREE and Y depletion, therefore the rocks are thought to be most probably the product of partial melting by materials of a middle-upper crust level. Moreover, the adakitic compositio nal characters of Anjishan intermediete-acid intrusive rocks suggest that t heir crustal source had garnet and pyroxene/amphibole residues but without plagioclase, therefore, the Anjishan intermediate-acid magmas were most lik ely derived from lower part of a thick continental crust, whose thickness i n early Cretaceous was over 40 km. It must be noted that the present thickn ess of the crust in the Ningzheng area is only about 30 km. Thus, our study suggests that the thick crust in the area could be thinned after the early Cretaceous.