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    大别山超高压变质带燕山期花岗岩地球化学特征及成因探讨

    Geochemical characteristics and genesis of Yanshanian granite in the Dabie ultrahigh-pressure (UHP) metamorphic belt

    • 摘要: 提要:依据花岗岩类的岩性组合、产出状况、结构、侵位机制和地球化学特征等,将大别山超高压变质带中燕山期花岗岩大致可以划分为两大类:一类为高钾钙碱性系列,以大同岩体和司空山早、主期岩体为代表,岩性为花岗闪长岩—二长花岗岩,表现为SiO2>56%,Al2O3一般≥14%,MgO<3%,富集LREE,低HREE和Y、Yb(如Y<18×10-6,Yb<1.9×10-6),高Sr、Ba,高La/Yb>39和Sr/Y≥30,Sr、Eu异常不明显,与Adakite岩相似;另一类属正常钙碱性系列,以英山尖岩体为代表,岩性以二长花岗岩为主,具有高硅、低铝的特点,一般SiO2>70%,Al2O3<14%,LREE略显亏损,HREE略显富集和高Y、Yb(如Y>18×10-6,Yb>1.9×10-6),低Sr和La/Yb<10、Sr/Y<10,Sr、Eu负异常明显。高钾钙碱性花岗岩可能为增厚的下地壳部分熔融的产物,正常钙碱性花岗岩可能为正常的地壳重熔形成。

       

      Abstract: Abstract: According to the rock composition, occurrence, textures, emplacement mechanism and geochemical characteristics, the Yanshanian granitoids in the Dabie UHP metamorphic belt may be classified into two major types: the high-K calc-alkaline series and the normal calc-alkaline series. The high-K calc-alkaline series is represented by the Datong rock body and the early-stage and main-stage Sikongshan rock bodies and composed lithologically of granodiorite-monzogranite. The chemical composition is characterized by SiO2 >56%, Al2O3≥14%, MgO <3%, high Sr and Ba contents, enrichment in LREE, strong depletion in HREE, high Sr/Y and La/Yb ratios (≥30 and ≥39, respectively), low Y(<18×10-6) and Yb (<1.9×10-6) contents, and indistinct Eu and Sr negative anomalies. These characteristics are similar to those of adakite. The normal calc-alkaline series is represented by the Yingshanjian rock body, consisting dominantly of monzogranite. It is characterized by high SiO2 (>70%), low Al2O3 (<14%), low Sr and Ba contents, low Sr/Y (<10) and La/Yb (<10) ratios, high Y (>18×10-6) and Yb (>1.9×10-6) contents, depletion in LREE, enrichment in HREE, and pronounced negative Sr and Eu anomalies. The high-K calc-alkaline granites might be derived by partial melting of the thickened lower crust, while the normal calc-alkalis granites probably originated by anatexis of the normal crust.

       

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