华北克拉通麻粒岩的地球化学特征及其对太古宙下地壳组成的指示意义

GEOCHEMISTRY OF GRANULITES IN NORTH CHINA CRATON:IMPLICATIONS FOR THE COMPOSITION OF ARCHEAN LOWER CRUST

华北克拉通麻粒岩的SiO2 含量分布具有明显的双峰式分布特征 ,两个峰值区间分别为 4 0 %～ 57% (峰值为 51 % )和 57%～ 78% (峰值为 67% ) ,但Mg#较集中 ,主体分布范围为 4 0 %～ 52 % ,Mg#和SiO2 之间的相关性较差。各类麻粒岩地球化学组成具有强烈的U、Th亏损和不同程度的Ba、Sr富集特征。另外 ,中、酸性麻粒岩Nb、Ta强烈亏损并发生明显的分异作用 ,Nb/Ta平均比值高达 3 1 .71～ 3 6.56,基性麻粒岩则表现出Rb亏损而Ta富集的特征 ,Nb/Ta平均比值仅为 1 7.2 0。各类麻粒岩的Eu/Eu (Tb) 与Mg#无关 ,主体大于 1。与世界范围内的同类太古宙麻粒岩相比 ,华北克拉通麻粒岩产热元素U、Th平均含量远远低于世界平均值 ,稀土元素平均含量普遍较低 ,但具有较高的Sm/Nd比值 ,而且酸性麻粒岩的SiO2

The contents of SiO\-2 in granulites in the North China Craton show a notable bimodel distribution, with the two main distribution ranges being 40%-57% and 57%-78%, respectively. The Mg\+# is concentrated in a small range from 40% to 52%. The Mg\+# and SiO\-2 in all the granulites are weakly correlated. All the granulites are strongly depleted in U and Th, and enriched in Ba and Sr. At the same time, strong Nb and Ta depletion and notable differentiation are recognized in the felsic granulites, with an average ratio of Nb/Ta ranging from 32.66 to 35.89. On the contrary, mafic granulites are depleted in Rb and enriched in Ta, with an average ratio of Nb/Ta being only 17.42. There are no relations between Eu/Eu\+* and Mg\+# in all the granulites, and the Eu/Eu\+* is mainly greater than 1. In comparison with the same type of Archean granulites over the world, the average contents of most REE and the heat\|producing elements U and Th in the North China Craton, as well as the average content of SiO\-2 in felsic granulites, are far lower than the world mean value.\;