摘要
内蒙古四子王旗地处华北板块北缘中段,近年来在该区的早白垩世碱性玄武岩中发现了下地壳麻粒岩包体。本文研究的麻粒岩包体多数为中酸性的苏长麻粒岩,只有个别属于基性的二辉麻粒岩。麻粒岩包体的主量元素特征为SiO_2含量在49.9%到69.24%之间,MgO含量在0.92%～3.56%之间,Mg~#为43.0～62.9,Al_2O_3在13.91%～22.79%之间;稀土元素总量变化较大(∑REE在33.76×10^(-6)～184.13×10^(-6)之间),分配模式也不尽相同,反映这些包体来自于不同源区或经历了不同的演化过程,但总体特征是LREE相对HREE富集,大多数样品的Eu呈现明显的正异常;由于包体的来源不同或经历了不同的演化阶段,它们的微量元素特征也有很大的差异。本区研究的麻粒岩包体的地球化学特征与华北地台的麻粒岩包体和麻粒岩地体都有很大的不同,反映了地壳组成横向上的不均一性。计算得出二辉麻粒岩的平衡温压条件分别为940～947℃和1.2±0.1GPa;苏长麻粒岩的平衡温度范围为927～715℃。通过对麻粒岩包体的岩相学、平衡温压务件估算、锆石形态特征分析和锆石的LA-ICP-MS年代学研究,得到本区早白垩世时的下地壳组成和之前的演化过程:本区下地壳在约40km深处可分为上下两部分,上部主要由苏长麻粒岩组成,厚度至少为9km,下部可能由二辉麻粒岩组成;下地壳上部的苏长麻粒岩是晚太古代(2509±35Ma)的岩浆岩经历了早白垩世(约122Ma)的玄武质岩浆底侵,发生了麻粒岩相变质和部分熔融形成的。推测本区岩石圈减薄事件的起始时间为122Ma。
Lower crust granulite xenoliths were found from the Early Cretaceous alkaline basalt in Siziwangqi area of Inner Mongolia, which is located at the middle segment of the north margin of North China Craton. Most of the xenoliths are intermediate-acid hypersthene-feldspar granulites, while only a few ones are mafic two-pyroxene granulites. SiO2, MgO, and Al2O3 contents and Mg-# of these xenoliths vary in 49.9% similar to 69.24%, 0.92% similar to 3.56%, 13.91% similar to 22.79% and 43.0 similar to 62.9, respectively. The total REE contents vary in a large range, from 33.76 x 10(-6) to 184.13 x 10(-6). The REE patterns of the xenoliths show some variations, indicating that the xenoliths either have different source regions, or they have different evolution processes. However, the xenoliths have similar chondrite-normalized REE patterns, showing slightly enriched LREE and depleted HREE with obvious Eu positive anomaly in most of those xenoliths. Great differences in trace element compositions of the granulite xenoliths may due to their different source regions or different evolution processes. The geochemical features of these xenoliths are significantly different from those of granulite xenoliths and granulite terranes in North China Craton published before. Estimated temperature and pressure for a two-pyroxenes granulite are 940 similar to 947 degrees C and 1.2 +/- 0.1 GPa, respectively. And the estimated balance temperatures for hypersthene-feldspar granulites are between 927 degrees C and 715 degrees C. The composition and the evolution history of the lower crust of Siziwangqi area in Early Cretaceous can be inferred from the petrography and balanced P-T conditions of granulite xenoliths, and configuration features of zircons and U-Pb LA-ICP-MS dating on zircons: the boundary between upper part and lower part of the lower crust was at about 40km in depth; the upper part of lower crust was mainly composed of hypersthene-feldspar granulites, which was >= 9km in thickness; the lower part of the lower crust probably co
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2009年第5期1201-1215,共15页
Acta Petrologica Sinica
基金
国家自然科学基金项目(40672146)资助
