北秦岭柳叶河盆地上三叠统物源区及其与鄂尔多斯盆地的关系——来自碎屑锆石LA-ICP-MS U-Pb年龄的证据

A tentative discussion on the source area of the Late Triassic Liuyehe basin in North Qinling Mountains and its relationship with the Ordos basin: Evidence from LA-ICP-MS U-Pb dating of detrital zircons

对陕西省周至地区北秦岭晚古生代—中生代柳叶河盆地上三叠统石英砂岩进行单颗粒锆石LA-ICP-MS U-Pb同位素分析。以岩浆锆石为主的77个测试点给出的年龄值形成7个年龄组：256～475Ma、1500～1680Ma、1750～2190Ma、2190～2310Ma、2400～2650Ma、2700～2800Ma和2850～2960Ma。其中最年轻的锆石年龄为256±4Ma,最老的锆石年龄是2954±25Ma。峰值年龄以1750～2190Ma古元古代年龄组为代表（占总测点的64%）。将柳叶河盆地上三叠统与石炭系碎屑锆石年龄结构进行对比,前者新元古代年龄结构缺失,表明晚古生代介于柳叶河与鄂尔多斯盆地间的北秦岭北部具新元古代年龄结构的古陆,晚三叠世相对沉降,成为盆地次要物源区。结合与周缘地体年龄结构对比,北秦岭南部二郎坪群、秦岭群、丹凤群、南缘的沉积楔形体刘岭群及北秦岭与加里东期活动陆缘沟-弧-盆体系相关的岩浆作用产物此时则成为盆地主要物源区。北秦岭内部北降南升。柳叶河盆地上三叠统碎屑锆石与鄂尔多斯盆地西南缘上三叠统延长组砂岩碎屑锆石较好的可对比性,以及柳叶河盆地北侧源区（北秦岭北部）的构造变动、化石等证据表明,柳叶河盆地与鄂尔多斯盆地在晚三叠世很有可能连通,柳叶河盆地可能代表鄂尔多斯盆地的南部边缘。

In this paper, the authors collected quartz sandstone samples from late Triassic Liuyehe basin of North Qinling Mountains in Zhouzhi area, Shaanxi Province, and conducted LA-ICP-MS detrital zircon U-Pb isotope analysis. The results show that the 77 analytical points of the magmatic detrital zircons yielded seven age groups： 256～475Ma, 1500～1680Ma, 1750～2190Ma, 2190～2310Ma, 2400～2650Ma, 2700～2800Ma, and 2850～2960Ma. Among the ages, the youngest age is 256 ± 4Ma, whereas the oldest age is 2954±25Ma;the major Paleoproterozoic age group （1750～2190Ma） accounts for 64%of the total measured points. The zircon age comparison between the Carboniferous Liuyehe basin and the late Triassic Liuyehe basin shows that the Neoproterzoic age is lacking in the Late Triassic Liuyehe basin, which indicates that the uplifting northern part in EW-trending North Qinling Mountains located between Liuyehe basin and Ordos basin during the Carboniferous was removed in Late Triassic and became the secondary provenance of Liuyehe basin. A comparative study of the age structure of the peripheral terranes shows that the southern part of North Qinling Mountains, including the Erlangping, Qinling, Danfeng and Liuling groups as well as the magmatic material developed in the Caledonian trench-arc-basin system along the Shangdan suture zone, became the major provenance for the basin. Combined with the paleontologic evidence, the similarity in detrital zircon age distribution pattern between the Triassic Liuyehe basin and the Yanchang formation in southwest Ordos basin strongly suggests that the Liuyehe basin was probably connected with the southern Ordos basin and represented the southern margin of the Ordos basin during the Late Triassic.