宁镇地区中生代安基山中酸性侵入岩的地球化学:亏损重稀土和钇的岩浆产生的限制

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

江苏省宁镇地区的中生代安基山中酸性侵入岩以亏损重稀土和钇及富集锶为成分特征,它们与消减过程的板片熔融无关,也不是基性岩浆分离结晶和地壳物质混染的产物,很有可能是相对较厚的地壳下部的镁铁质物质部分熔融产生的。相比,与安基山岩体邻近的同时代的苏州-无锡地区的和江浙地区的中酸性侵入岩和火山岩不具有亏损重稀土和钇特征,它们可能是中上地壳物质部分熔融的产物。研究表明,中生代时宁镇地区可能存在一个相对较厚的地壳(>40km)。由于宁镇地区现在地壳的厚度仅30km左右,因此该地区中生代或之后可能经历了一个地壳的减薄过程。

Anjishan intrusive body in early Cretaceous in Ningzhen area of Jiangsu province, east China mainly consists of intermediate-acid rocks (quartz-diorite and granodiorite). These intermediate-acid intrusive rocks are characteristic 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 high La/Yb and Sr/Y ratios. These rocks lie in the adakite and high-Al TTD field on Sr/Y - Y diagram. Although Anjishan intermediate-acid intrusive rocks show compositional characteristics similar to adakite, our results indicate that they are not related to subduction, therefore they may not be the product of slab melting. On the other hand, nor is it likely for a crystal fractionation 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 basalts, and evidence of assimilation by crustal material has not been found. Thus, we conclude that Anjishan intermediate-acid intrusive rocks were most likely derived from partial melting of mafic materials in the lower part of a thick crust. In comparison, the contemporaneous intermediate-acid intrusive 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 compositional characters of Anjishan intermediete-acid intrusive rocks suggest that their crustal source had garnet and pyroxene/amphibole residues but without plagioclase, therefore, the Anjishan intermediate-acid magmas were most likely derived from lower part of a thick continental crust, whose thickness in early Cretaceous was over 40 km. It must be noted that the present thickness 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.