吉林蛟河地幔橄榄岩捕虏体的Sr-Nd-Hf-Os同位素特征与岩石圈地幔时代

吉林省蛟河市境内大石河新生代玄武岩中含有丰富的地幔橄榄岩包体,详细的岩石学与矿物学研究显示,这些包体的主要岩石类型为尖晶石二辉橄榄岩-方辉橄榄岩,未发现石榴石橄榄岩.岩相学及地球化学资料显示它们都是经历过熔体抽取而形成的岩石圈地幔残留.矿物平衡温度计算发现,本区的这些地幔橄榄岩包体来自地下40～60km深度,且下部以二辉橄榄岩为主,而上部以贫单斜辉石的二辉橄榄岩和方辉橄榄岩为主,显示明显的岩石圈地幔分层现象.Sr-Nd-Hf同位素资料反映这些地幔包体均表现为亏损性质,而Re-Os同位素资料确定上述岩石圈地幔形成于中元古代,明显老于上覆地壳的新元古宙时代,反映壳幔年龄上的解耦.因此我们推测,该区曾经历过华北克拉通类似的早期岩石圈地幔的整体丢失事件,然后形成于其它地区的中元古宙岩石圈地幔在本区增生.

Sr-Nd-Hf-Os isotopic characterizations of the Jiaohe peridotite xenoliths in Jilin province and constraints on the lithospheric mantle age in northeastern China

Abundant peridotitic xenoliths had been found in the Dashihe Cenozoic basalts in Jiaohe of Jilin Province, northeastern China. Petrological and mineralogical investigations indicated that these xenoliths are mostly spinel-bearing lherzolite with minor harzburgite, no garnet-bearing peridotite is found. Petrographic and geochemical data revealed that these xenoliths are fragments of the lithospheric mantle derived from the depth of 40 - 60km, constrained by equilibrium temperature. It was also indicated by the temperature calculation that the lithspheric mantle is stratigraphically layered with the lower part composed by fertile lherzolite and the upper part by refractory harzburgite and clinopyroxene-poor lherzolite. Sr-Nd-Hf isotopic data indicated that both of these lithospheric mantle are isotopically depleted, and the Os isotopic data proposed that they were formed during Mesoproterozoic, which is older than the overlying continental crust, showing geochronological decoupling between the crust and mantle. Therefore, we proposed that the studied area had undergone the whole-scale lost of the lithospheric mantle, similar to that taken place in the North China Craton, and then the Mesoproterozoic lithospheric mantle formed elsewhere accreted beneath the area.