内蒙古西乌珠穆沁旗地区下二叠统原寿山沟组碎屑锆石LA-ICP-MS U-Pb年龄及地质意义

下二叠统寿山沟组在大兴安岭南部广泛分布,但缺少确切的时代依据.西乌旗塔宾庙林场寿山沟组以巨厚的中薄层泥岩与砂岩韵律层为标志,镜下特征显示,砂岩主体为长石岩屑砂岩.上部中粗粒长石岩屑砂岩中90颗碎屑锆石阴极发光图像显示它们多具有岩浆锆石的典型特点;锆石U-Pb定年结果显示,除8颗锆石(9％)具有元古宙和早古生代的年龄信息(2490～502Ma)外,其余样品年龄大致可分成3组:第1组3颗(3％),年龄为383Ma±3Ma;第2组6颗(7％),年龄为365Ma±4Ma;第3组73颗(81％),年龄介于280～337Ma之间,又进一步在289Ma、300Ma、309Ma、325Ma形成4个峰值,说明砂岩具有多时代物源混合的特征,但沉积下限应不老于289Ma.参考近年大石寨组测年资料推测,寿山沟组应作为大石寨组的同时异相沉积,大兴安岭南部下二叠统地层序列有待于重新认识.结合区域岩浆活动时间认为,原寿山沟组测年砂岩碎屑锆石主要来自于苏尼特左旗—锡林浩特—西乌旗一带早石炭世晚期—晚石炭世岩浆弧,部分来自乌兰浩特地区大石寨组火山岩.

U-Pb age of detrital zircons from Early Permian"Shoushangou Formation" in Xi Ujimqin Banner, Inner Mogolia

The "Shoushangou Formation", previously assigned to lower Permian, is widespread in southern Da Hinggan Mountains. The Tabinmiao profile of "Shoushangou Formation" in Xi Ujimqin Banner of Inner Mongolia is characterized by thick rhythmic interbeds comprising medium- to thin-layered mudstones and sandstones. Of them, the sandstones are mainly feldspathic lithic rocks. Zircons from sandstone in the upper sequence of the profile almost display typical structures of magmatic genesis. Eight (ca. 9% of the total 90 dated zircon grains) of the dated zircons yielded Proterozoic to early Paleozoic ages, ranging from 1490Ma to 502Ma. However, the yielded age data of the rest zircons are clustered in three groups: 383Ma±3Ma (n=3, ca. 3%), 365Ma±4Ma (n=6, ca. 7%) and between 337Ma and 280Ma (n=73). Four age peaks can be recognized between 337Ma and 280Ma, i.e., 289Ma, 300Ma, 309Ma and 325Ma. The obtained age data indicate the multi-provenance of the sandstone. Nevertheless, the youngest zircon age constrained the maximum depositional age of 289Ma. According to the isotope age data of Dashizhai Formation, the authors infer that both Shoushangou Formation of this study and Dashizhai Formation belong to different facies of the same period. On the basis of U-Pb dating, it is deduced that the studied "Shoushangou Formation" was not the previously defined Shoushangou Formation, that is, the stratigraphic sequence and its correlation with the lower Permian strata in southern Da Hinggan Mountains need to be re-evaluated. Combined with the regional geology and timing of the magmatism, it is concluded that the detrital zircons of the dated "Shoushangou Formation" mainly came from the Paleozoic granitoids and volcanic rocks in the Xilin Hot- Xi Ujimqin Banner area, with the probable addition of a small amount of volcanic rocks of the Dashizhai Formation.