扬子地台西南缘大红山群红山组的锆石U-Pb年代学研究——对其原岩形成时代和变质时代的再限定

Geochronology of zircon U-Pb from Hongshan Formation in the Dahongshan Group in the Southwest Yangtze Block for the Redefinitions of the Forming Age of the Protolith and Metamorphic Age

大红山群位于扬子地台西南缘,是该区域上变质级别相对较高、形成时代较老的地层单元。本文以侵位于大红山群的侵入岩、红山组顶部的变质火山岩样品为研究对象,运用LA-ICP-MS方法对侵入岩、变质火山岩进行锆石U-Pb定年。石榴黑云钠长片岩、变质辉长岩(DFe1406)和石英钠长石斑岩中锆石为岩浆锆石,3个样品的锆石数据点沿谐和线较集中分布,n(~207Pb)/n(~206Pb)年龄为约1.7 Ga到约1.5 Ga。变质辉长岩(DFe1454)中锆石为变质锆石,n(~207Pb)/n(~206Pb)年龄从约0.8 Ga到约0.7 Ga。结合已有同位素定年资料,可得出如下结论:(1)限定了大红山群中部红山组的成岩年龄为1673～1643 Ma;(2)获得了侵位于红山组变质辉长岩中的变质锆石n(~207Pb)/n(~206Pb)加权平均年龄为748.9±5.7 Ma,表明大红山群经历过约750 Ma前的变质,确定大红山群经历850～750Ma的变质作用,可能与Rodinia超大陆裂解期岩浆事件有关。

Objectives：The Dahongshan Group located in the western part of Yangtze Block is the stratigraphic unit with relatively high grade metamorphism and older formational age. In this paper, we regard the intrusions which intruded the Daohongshan Group and the metavolcanic rocks in the top of Hongshan Formation as study objectives,to confirm the diagenetic age of the Hongshan Formation and metamorphic age of the Dahongshan Group. Methods： LA ICP MS U Pb zircons dating. Results：The zircons from the garnet—biotite—albite schist、metagabbro（DFe1406）and quartz—albite porphyry are magmatic zircons. Analytic zircon data from three samples is concentrated distribution along the concordant curve. n（^207Pb）/ n（^206Pb） ages range from about 1.7 Ga to 1.5 Ga. However, The zircons from the metagabbro are metamorphic zircons n（^207Pb）/ n（^206Pb） ages range from about 0.8 Ga to 0.7 Ga. Conclusions：（1） constraint the diagenetic age of the Hongshan Formation in the middle of Da Hongshan Group at 1673 ~1643 Ma; （2） obtain the n（^207Pb）/ n（^206Pb） weighted mean age of 748.9 5.7 Ma of Ma metamorphic zircons selected from the metagabbro intruded the Hongshan Formation, indicating the Dahongshan Group undergone the metamorphism before about 750 Ma. we speculate that the metamorphism during 850 ~ 750 Ma of Dahongshan Group is probably related to magmatic events during the fragmentation of Rodinia supercontinent.