蒙古-鄂霍茨克洋南向俯冲:小兴安岭西北部落马湖群中三叠世岩浆记录

Southward subduction of the Mongo-Okhotsk Ocean:Middle Triassic magmatic records of the “Luomahu Group” in northwest of Lesser Khingan Mountains

小兴安岭西北部出露的落马湖群被认为是兴安地块新元古代变质基底的组成部分。通过对卧都河乡附近的落马湖群黑云角闪斜长片麻岩进行LA-ICP-MS锆石U-Pb测年,获得246±1 Ma形成年龄,认为落马湖群是一套包含不同时代地质体的构造杂岩。黑云角闪斜长片麻岩的岩相学及岩石地球化学研究表明,其原岩为基性岩浆岩,具有较高的MgO、TFeO含量及Mg^(#)值相对富Na_(2)O,低K_(2)O,中等的Cr、Ni、Co含量,与下地壳平均成分类似;富集大离子亲石元素Rb、Ba、Sr、Th,亏损高场强元素Nb、Ta、Ti、P、Hf;锆石原位εHf(t)值介于+9.55~+14.43之间,Hf同位素单阶段模式年龄(t_(DM1))和二阶段模式年龄(t_(DM2))分别介于319~520 Ma和350~666 Ma之间,Hf同位素组成与中亚造山带兴安岛弧显生宙火成岩中锆石Hf同位素组成类似;岩浆源自"水化"古生代多宝山岛弧带增生岩石圈地幔的部分熔融并有陆壳物质混染,与蒙古-鄂霍茨克洋板块南向俯冲有关,可能受到北北东向陆内转换构造带控制。

Luomahu Group outcropped in the northwest of Lesser Xing ’ an Range is considered to be part of the Neoproterozoic metamorphic basement of the Xing ’an Massif. Based on zircon U-Pb dating of the biotite-hornblende-plagioclase gneiss from the Luomahu Group outcropped near Woduhe Township, the results show that the age of protolith is 246±1 Ma.Combined with the related regional research data, it is suggested that the Luomahu Group is a set of tectonic complex containing geological bodies of different ages.The petrography and geochemistry of the biotite-hornblende-plagioclase gneiss indicate that the protolith is a basic magmatic rock,rich in MgO, TFeO and Mg^(#) value, relatively rich in Na_(2)O, low in K_(2)O, and mediate contents of Cr, Ni and Co, similar to the average composition of the lower crust.In addition, large ion lithophile elements Rb, Ba, Sr, Th are enriched, and high field strength elements Nb, Ta, Ti, P, Hf are depleted.The in-situ εHf( t) values of zircons range from + 9.55 to + 14.43, and the Hf isotope single-stage model ages( t_(DM1)) and two-stage model ages( t_(DM2)) are 319 ~ 520 Ma and 350 ~ 666 Ma,respectively. The Hf isotopic composition is similar to the Phanerozoic igneous rocks of Xing ’ an island arc in Central Asia Orogenic Belt. It is suggested that the magma was originated from the partial melting of the " hydrated" Paleozoic hyperplasia lithospheric mantle and was mixed with continental crust materials in the Duobaoshan island arc zone, related to the southward subduction of the Mongolia-Okhotsk Ocean plate and controlled by the NNE trending intracontinental transform tectonic belt.