阿拉善地块南缘龙首山东段“龙首山岩群”的再厘定——来自碎屑锆石U-Pb定年的证据

龙首山东段滑石口井地区存在一套浅变质的沉积岩系,原被认为属于古元古代“龙首山岩群”.本文取自这套变沉积岩系的2件样品的碎屑锆石年龄范围介于0.52～3.56 Ga之间,与相邻的寒武系大黄山群碎屑锆石年龄谱相似,其时代可能为中、晚寒武世.取自原“龙首山岩群”和寒武系大黄山群的3件变沉积岩样品中,共获得129个谐和的碎屑锆石年龄,主要分布在0.7～1.2 Ga(约占47％,峰值～0.8、～0.94、～1.0 Ga)和2.5～2.8 Ga(约占31％,峰值～2.5、～2.7 Ga),相对较小的年龄群集中在0.5～0.6 Ga(约占7％,峰值～0.56 Ga)和1.4～1.8 Ga(约占10％,峰值～1.5 Ga),其余年龄零星分布于1.8～2.4 Ga,少量锆石年龄＞3.0 Ga.碎屑物源分析认为,大黄山群的碎屑物质主要来自祁连地块,其中新元古代末一早古生代初期的碎屑物质来自北祁连造山带相关的火成岩,新元古代碎屑物质来自祁连地块广泛分布的新元古代岩浆岩,而中元古代一太古宙碎屑物质可能来自祁连地块再循环的变质基底岩石.结合区域地质资料认为,龙首山东段的浅变质沉积岩系和寒武系大黄山群可能沉积于祁连地块北侧的大陆边缘,在构造背景上属于祁连造山带,而不属于阿拉善地块.

Redefinition of the "Longshoushan Group" outcropped in the eastern segment of Longshoushan on the southern margin of Alxa Block: Evidence from detrital zircon U-Pb dating results

A suite of low-grade metamorphic sedimentary rocks, exposed in Huashikoujing area of eastern Longshoushan, was originally regarded as the Paleoproterozoic "Longshoushan Group". Detrital zircons from two low-grade metamorphic sedimentary rocks yielded ages ranging from 0.52 Ga to 3.56 Ga, similar to the age spectrum of the rock from neighboring Cambrian Dahuangshan Group. Thus, the metamorphic sedimentary rocks were probably deposited in the middle-late Cambrian period. A total of 129 concordant data were obtained from two sandstone samples from Huashikoujing area and a sandstone sample from Dahuangshan Group, whose ages predominately range between 0.7 Ga and 1.2 Ga (47%, peaks at around 0.8 Ga, 0.94 Ga, 1.0 Ga) and between 2.5 Ga and 2.8 Ga (31%, peaks at around 2.5 Ga, 2.7 Ga). Two smaller age groups range from 0.5 Ga to 0.6 Ga (7%, peaks at around 0.56 Ga) and from 1.4 Ga to 1.8 Ga (10%, peaks at around 1.5 Ga). The other zircons have sporadic ages ranging from 1.8 Ga to 2.4 Ga, with a few zircons older than 3.0 Ga. Provenance analysis shows that detrital matters of Dahuangshan Group were mainly derived from Qilian Block. Late-Neoproterozoic to early-Paleozoic grains were probably derived from related igneous rock in north Qilian orogen, Neoproterozoic grains were derived from magmatic rock distributed widely in Qilian Block, and Mesoproterozoic to Archean grains were probably derived from recycled meta-basement rocks in Qilian Block. Based on regional geological information, the authors consider that the low-grade metamorphic sedimentary rocks and Cambrian Dahuangshan Group, which were probably deposited in continental margin of Qilian Block, belong to the Qilian orogen rather than the Alxa Terrane.