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
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
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2013
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通讯作者：
Z. Jian-xin
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作者：
Z. Jian-xin
龙首山东部花石口井地区出露的一套浅变质沉积岩原被认为是古元古代的“龙首山群”，两套浅变质沉积岩的碎屑锆石年龄为0.52 ~ 3.56Ga，与邻近的寒武系大黄山群岩石的年龄谱相似，因此，通过对花石口井地区的2个砂岩样品和大黄山群的1个砂岩样品的分析，共获得129个一致的数据，年龄主要在0.7 ~ 1.2Ga之间（47%，峰值在0.8Ga、0.94Ga、1.0Ga附近）和2.5 - 2.8Ga之间（31%，峰值在2.5Ga、2.7Ga附近）。两个较小的年龄组在0.5Ga至0.6Ga之间（7%，峰值在0.56 Ga附近）和1.4 - 1.8 Ga（10%，峰值在1.5Ga左右），其余锆石的年龄为1.8 ~ 2.4Ga，物源分析表明，大黄山群碎屑物质主要来源于祁连地块，晚新元古代-早古生代颗粒可能来源于北祁连造山带的相关火成岩，新元古代的颗粒来源于祁连地块广泛分布的岩浆岩，中元古代至太古代的颗粒可能来源于祁连地块的再生变质基底岩。可能沉积于祁连陆块的陆缘，属祁连造山带而非阿拉善地体。
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.