一个距今10亿年侵位的阿尔卑斯型橄榄岩体:北秦岭晚元古代板块构造体制的证据

A 1 GA B.P.ALPINE PERIDOTITE BODY EMPLACED INTO THE QINLING GROUP: EVIDENCE FOR THE EXISTENCE OF THE LATE PROTEROZOIC PLATE TECTONICS IN THE NORTH QINLING AREA

北秦岭松树沟超镁铁岩体是一个侵位于原秦岭群斜长角闪岩中的阿尔卑斯型橄榄岩。其岩体外侧有一石榴石斜长角闪岩接触变质带。利用Sm-Nd矿物等时线法获得该石榴石斜长角闪岩的年龄为983±140(2σ)Ma,它代表该岩体的侵位时代。该石榴石斜长角闪岩的ε_(Nd)(T)=+6.9±0.9,εS_r(T)=+16,f^(Sm/Nd)=-0.26,模式年龄T_(DM)=1089Ma。这些说明该斜长角闪岩是一形成时代与松树沟超镁铁岩体类似,来自亏损地幔并具有蛇绿岩Nd、Sr同位素特征的大陆边缘或岛弧玄武岩。因此,松树沟超镁铁岩及其围岩(斜长角闪岩)为一元古宙蛇绿岩,并初步证明了北秦岭在晚元古代已具有板块构造体制。考虑到该带还广泛发育有古生代蛇绿岩,因此从10亿年到3.5亿年(古生蛇绿岩侵位时代),华北陆块南缘的陆壳增生是非常有限的。

The Songshugou ultramafic massif is an Alpine peridotite body emplaced into the amphibolites of the Qinling Group in the North Qinling belt. There is an aureole zone, garnet amphibolite zone, around the massif. The authors obtained a mineral-whole rock isochron age of 983±140 (2σ) Ma for a garnet amphibolite sample from the aureole zone by the Sm-Nd isotopic dating, which represents the emplacement age of the Songshugou ultramafic massif. Other isotope characters such as ε_(Nd)(T)=+6.9±0.9,ε_(Sr)(T)=+16, f^(Sm/Nd)=-0.26 and the model age T_(DM)=1089 Ma suggest that the amphibolites as the country rocks of the Songshugou massif are island-arc or continental-margin basalts from the depleted mantle and that their Sr and Nd isotope characters are similar to those of ophiolites and their original age is similar to the emplacement age of the Songshugou massif. Therefore, the Songshugou ultramafic massif and its country rocks (amphibolites) combine to form a Proterozoic ophiolite, which provides preliminary evidence for the existence of the Late Proterozoic plate tectonics in the North Qinling area. Considering the Paleozoic ophiolites emplaced at 0.35 Ga B. P. are also developed in the same belt, the new accreted continental crust on the southern margin of the North China block was very small from 1 to 0.35 Ga B. P.