大别山燕山期亏损重稀土元素花岗岩类的成因及动力学意义

The petrogenesis and geodynamic significances of HREE depleted granitoids during Yanshan period in the Dabie Mountains.

本文通过岩石和元素地球化学研究对大别山(主要是张家咀地区)燕山期的花岗岩类进行了分类和成因研究,结果表明:(1)大别山燕山期存在两类花岗岩,一类为重稀土元素(HREE)和Y强烈亏损(Sr/Y≥40.0,La/Yb≥40.0,Yb≤1.90μg/g,Y≤18.0μg/g,Sr≥400μg/g,Rb/Sr≤0.20,Rb/Ba≤0.08)的花岗岩,另一类为重稀土元素(HREE)和Y亏损不明显(Sr/Y<40.0,La/Yb<40.0,Rb/Sr>0.20,Rb/Ba>0.08)花岗岩(包括A型和一些I型花岗岩)。重稀土元素(HREE)和Y强烈亏损的花岗岩存在两个亚类,一类富钠,与埃达克质岩石类似;另一类主要富钾,与富钾的高Ba-Sr的花岗岩类似。(2)大别山燕山期HREE和Y强烈亏损的花岗岩由增厚(>40km)的下地壳物质熔融形成,其热源可能是同期底侵的镁铁质一超镁铁质岩浆。这些花岗岩的形成可能诱导下地壳拆沉作用的发生。

The petrological and geochemical characteristics of Yanshanian granitoids in the Dabie Mountains (mainly in Zhangjiazui area) suggest that there are two kinds of rocks among Yanshanian granitoids in the Dabie Mountains. One kind is clearly depleted in HREE and Y (Sr/Y greater than or equal to 40.0, La/Yb greater than or equal to 40.0. Yb less than or equal to1.90 mug/g, Y less than or equal to 18.0 mug/g, Sr greater than or equal to 400 mug/g, Rb/Sr less than or equal to0.20, Rb/Ba less than or equal to 0.08), and the other kind shows no HREE and Y-depletion (Sr/Y < 40.0, La/Yb < 40.0, Rb/Sr > 0.20, Rb/Ba>0.08), including some A-type and I-type granites. In addition, the granitoids with the HREE and Y-depletion are further divided into two groups: Na-rich and K-rich granitoids, the former one is similar to adakitic rocks and the latter one resembles high Ba-Sr granites. The HREE and Y-depleted granites in the Dabie Mountains were most probably produced by partial melting of lower mafic rocks in the thickened crust (>40km) due to the underplating mafic magmas heating the lower crust. Generation of magma with HREE and Y depletion in the Dabie Mountains also likely resulted in delamination of some materials in the lower crust.