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莫尔道嘎地处额尔古纳地块北段,该地区大量侵入岩和侵入-变质杂岩以往被认为属新元古代,类型有风水山片麻杂岩、中基性杂岩、巨斑状钾长花岗岩、二长花岗岩岩。对该地区新元古代侵入岩和侵入-变质杂岩进行系统采样,采用LA-ICP-MS锆石U-Pb测年方法进行了测年研究。结果显示:研究区除个别岩体(混合岩化钾长花岗岩)为新元古代形成外,绝大部分以往认为的新元古代侵入岩或变质侵入杂岩为早中生代形成,揭示了研究区有强烈的早中生代构造岩浆作用。结合区域上同时代早中生代岩石的分布和年代学资料,额尔古纳和大兴安岭北段的早中生代侵入岩和变质杂岩可以分为243～246、210～229、200～205Ma 3个时期,其中200～205Ma一组最为发育。243～246Ma一组主要为莫尔道嘎地区中基性杂岩,形成背景与二叠纪末期兴蒙造山之后伸展作用有关。而210～229Ma的花岗岩和变质杂岩组合具有非常典型的大陆碰撞岩石组合特征:钙碱性系列过铝质、S型花岗岩,岩石副矿物以黑云母+角闪石+白云母为主,混合岩和淡色花岗岩(白云母花岗岩),深熔岩(莫尔道嘎巨斑状钾长花岗岩),缺乏同时期火山岩等;其形成与早中生代时期蒙古—鄂霍茨克洋封闭引起的陆陆碰撞有关。研究区早中生代花岗质侵入岩、变质杂岩特征及其同位素测年结果显示:蒙古—鄂霍茨克洋中段可能在中三叠世末期—晚三叠世早期封闭,这时已经开始发生陆陆碰撞,碰撞高峰时期可能在晚三叠世末期,早于以往认为的早侏罗世。
Mordaga is located in the northern section of the Erguna massif. In the past, a large number of intrusive rocks and intrusive metamorphic complexes were previously considered to be Neoproterozoic, of which type is Fengfengsian gneiss, Porphyry potassium long granite, two long granite rock. The Neoproterozoic intrusions and intrusive metamorphic complexes were systematically sampled and analyzed by LA-ICP-MS zircon U-Pb dating. The results show that most of the previously thought Neoproterozoic intrusions or metamorphic intrusive rocks were formed in the early Mesozoic except that some individual rocks (mixed kava-grained granite) were formed during the Neoproterozoic. It is revealed that there are Strong Early Mesozoic tectonic magmatism. According to the distribution and geochronology data of the early Mesozoic rocks in the same period of the region, the Early Mesozoic intrusive rocks and metamorphic complexes in the northern section of the Ergun and the Greater Hinggan Mountains can be divided into three periods of 243 ~ 246,210 ~ 229,200 ~ 205Ma, One of the most developed group of 200 ~ 205Ma. A group of 243 ~ 246Ma is mainly composed of mid-mafic complex in the Molwadag area, and the formation background is related to extension after the Late Permian Xingmeng orogen. The assemblage of granite and metamorphic complex from 210 to 229 Ma has very typical characteristics of continental collision rocks: calc-alkaline series of peraluminous and S-type granites, and biotite-bearing rocks are dominated by biotite + hornblende + muscovite Rock and light-colored granite (muscovite), deep-lava (molorda giant plaque potassium-bearing granite), and lack of contemporaneous volcanic rocks. The formation was related to the continental-continental collision caused by the closure of the Mongolia-Okhotsk Ocean during the Early Mesozoic. The characteristics of the Early Mesozoic granitic intrusions and metamorphic rocks in the study area and their isotopic dating show that the Middle Mongolia-Okhotsk Ocean may be closed during the middle Triassic to early Triassic, Land collision, the peak of the collision may be late Late Triassic, earlier than previously thought in the early Jurassic.