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
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
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作者：
Han Bao
华北克拉通中部古元古代镁铁质岩脉群可分为变质岩脉和非变质岩脉两种类型。非变质岩脉形成时间为1780～1760Ma，时间较短，约为20Ma，是华北时代最新的古元古代镁铁质岩浆作用产生的，可细分为以下三种：丰镇市西北高镁低钛玛里岩脉新获得的锆石U-Pb年龄为1769±4Ma，与~(40)Ar/~(39)Ar、常规锆石和SHRIMP锆石等资料吻合较好。 1770Ma测年锆石的e_(Hf)(t)值在-6.4~+0.4之间,平均为-2.2,略高于同期形成的花岗质岩,表明岩浆在上升、侵位和结晶过程中几乎没有受到地壳污染,来自于未变质岩脉。此外，高镁低钛镁铁质岩脉富集地幔源区的e_(Nd)(t)值为- 5.6～-2.8（平均-4.4），初始~(87)St/~(S6)Sr比值为0.7040～0.7050（平均） 0.7046），如文献中的数据所示。
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 eraton.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 marie dykes to the northwest of Fengzhen city is 1769±4Ma, in good agreement with previous data such as ~(40)Ar/~(39)Ar and conventional and SHRIMP zircon and baddeleyite U-Pb ages for the unmetamorphosed dykes.Thee_(Hf)(t)values of the dated zircons at 1770Ma vary from-6.4 to+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 thee_(Nd)(t) values of - 5.6 to -2.8(average -4.4)and the initial ~(87)St/~(S6)Sr ratios of 0.7040 to 0.7050(average 0.7046),as shown by the data available from the literature.