华北克拉通中部古元古代高Mg低Ti-P镁铁质岩墙的富集地幔源区——锆石Hf同位素的制约  

Enriched mantle source for Palcoproterozoic high Mg and low Ti-P mafic dykes in central part of the North China craton:constraints from zircon Hf isotopic compositions

作　　者：韩宝福[1] 张磊[1] 王亚妹[1] 宋彪[2] 

机构地区：[1]北京大学地球与空间科学学院造山带与地壳演化教育部重点实验室,北京100871 [2]北京离子探针中心

出　　处：《岩石学报》2007年第2期277-284,共8页Acta Petrologica Sinica

基　　金：教育部"跨世纪人才计划"资助项目

摘　　要：华北克拉通中部是古元古代岩墙群最为发育的地区。以镁铁质成分为主的岩墙群可以分为变质的和不变质的两类。不变质岩墙群是在1780～1760Ma之间大约20Ma时间内形成的,是华北克拉通古元古代最晚期铗铁质岩浆活动的产物。不变质岩墙又可以细分成高Mg-Ti低P、低Mg高Ti-P和高Mg低Ti-P等3组。本文报道了采自丰镇附近高Mg低Ti-P岩墙样品的锆石U-Pb年龄和Hf同位素组成分析结果。新获得的SHRIMP锆石U-Pb年龄为1769±4Ma,与其它不变质岩墙的锆石、斜锆石U-Pb年龄和^(40)At/^(39)Ar年龄的范围相同。高Mg低Ti-P岩墙的锆石ε_(Hf)(t)值的变化范围在-6.4～+0.4之间,平均值为-2.2,略高于时代相近的花岗质岩石。由于镁铁质岩浆在上升、侵位和结晶过程中几乎没有受到地壳物质混染,因而锆石ε_(Hf)(t)值可以代表镁铁质岩浆的富集的岩石圈地幔源区的特征。文献资料显示,高Mg低Ti-P岩墙的岩石圈地幔源区的^(87)Sr/^(86)Sr初始比值为0.7040～0.7050,平均值为0.7046,ε_(Nd)(t)值为-5.6～-2.8,平均值为-4.4。Paleoproterozoic mafic dyke swarms in central part of the North China craton can be divided into two types： metamorphosed and unmetamorphosed. The unmetamorphosed dykes formed in a short duration of~ 20Ma from 1780 to 1760Ma and were produced by the latest Paleoproterozoic mafic mgmatism in the North China craton. They may be subdivided into following three groups： high Mg-Ti and low P, low Mg and high Ti-P, and high Mg and low Ti-P, and individual groups have different geochemical features. A new zircon U-Pb age obtained for the high Mg and low Ti-P mafic dykes to the northwest of Fengzhen city is 1769±4Ma, in good agreement with previous data such as ^40Ar/^39 Ar and conventional and SHRIMP zircon and baddeleyite U-Pb ages for the unmetamorphosed dykes. The БHt（t） values of the dated zircons at 1770Ma vary from -6.4to＋0.4, with an average of - 2.2, and are slightly higher than those of the granitic rocks formed at similar time, indicating that the magmas, with little crustal contamination during their ascent, emplacement and crystallization, were derived from an enriched source from the subcontinental lithospheric mantle induced by subducted sediments. In addition, the enriched mantle source for the high Mg and low Ti-P mafic dykes had the БNd （t） values of -5.6 to-2.8 （ average -4.4） and the initial ^87Sr/^86Sr ratios of 0.7040 to 0.7050 （ average 0.7046）, as shown by the data available from the literature.

关 键 词：镁铁质岩墙群 古元古代 华北克拉通 锆石U-PB年龄 HF同位素 富集地幔 

分 类 号：P588.125[天文地球—岩石学] P597.3[天文地球—地质学]