秦岭岩群副变质岩碎屑锆石年龄谱及其地质意义探讨

报道了北秦岭地块中秦岭岩群夕线黑云石英片岩61个碎屑锆石测点的207Pb/206Pb年龄测试结果,其中15个测点在SHRIMPⅡ上进行,46个测点来自LA-ICPMS的测定结果。61个年龄值显示锆石主要来源为1·5~1·9Ga(共51个测点,占所有测点的83·6%)的古元古代末至中元古代早期的大陆地壳,1粒碎屑锆石来自新太古代岩石,5粒来自中元古代晚期的蚀源区,4个测点为0·9~1·0Ga。由于秦岭岩群被新元古代早期960~900Ma的花岗岩体侵入,新的碎屑锆石测年结果表明秦岭岩群副变质岩的沉积时代至少介于1500~960Ma之间,亦可能为中元古代末期。秦岭岩群在早古生代造山过程中受到强烈改造,但本次测定未从夕线黑云石英片岩中获得这次变质作用的年代学信息。

U-Pb ages of detrital zircons from the para-metamorphic rocks of the Qinling Group and their geological significance

U-Pb isotopic ages of 61 detrital zircon grains from the sillimanite-biotite-quartz schist of the Qinling Group in the North Qinling metamorphic terrane are reported in this paper. Of the 61 zircon grains, 15 were measured by SHRIMP Ⅱ, and 46 by LA-ICPMS. The results indicate that 51 zircon grains were derived predominantly from late-Palaeoproterozoic to early Mesoproterozoic continental crust, with ages ranging from 1.5 to 1.9 Ga. One zircon grain yielded a Neoarchean age, 5 grains derived from the erosion of the late-Mesoproterozoic rocks, and 4 grains yielded ages of about 0.9-1.0 Ga. It is noteworthy that the Qinling Group was intruded by the early Neoproterozoic granitoid rocks at about 960-900Ma, so it is reasonable to assume that the protolith of the para-metamorphic rocks of the Qinling Group formed between 1 500 and 960 Ma, (Late Mesoproterozoic to Neoproterozoic). The Qinling Group is also strongly overprinted by early Paleozoic metamorphic process, whereas it is not detected by the dating result of zircon grains from the metamorphic rock samples we have studied in this paper.