河北阳原新生代玄武岩中两类辉石岩包体的矿物学和地球化学特征

MINERAL CHEMISTRY AND GEOCHEMISTRY OF THE TWO SUITES OF PYROXENITE XENOLITHS IN CENOZOIC BASALTS FROM YANGYUAN, HEBEI

位于大兴安岭-太行山重力梯度带西侧的阳原新生代玄武岩中含有大量橄榄岩和辉石岩包体。辉石岩包体含有绿色的Cr辉石岩和黑色的Al辉石岩包体两种类型,均为二辉岩,且多数样品中单斜辉石含量高于斜方辉石。岩石结构和主微量元素组成说明阳原辉石岩是高压堆晶体。其EMI型Sr-Nd同位素特征,明显不同于寄主玄武岩,暗示两者没有成因联系,而可能代表古老岩浆事件的产物。综合文献资料发现重力梯度带西侧的辉石岩包体大多具有富集同位素特征,而东侧的样品以亏损同位素组成为主。这种空间上的成分差异暗示重力梯度带分隔两个不同的地幔域,也可能与重力梯度带两侧岩石圈地幔形成时代不同有关。

Cenozoic basalts in Yangyuan, Hebei, located in the west of the Daxing＇anling -Taihangshan gravity lineament （DTGL） , contain abundant peridotite and pyroxenite xenoliths. Two types of pyroxenite xenoliths are noted, namely, green Cr-and black Al-pyroxenites. All pyroxenites are websterites in which clinopyroxene is more abundant than orthopyroxene. Texture, major and trace element compositions suggest that the Yangyuan pyroxenites represent magmatic cumulates at high pressure. Their EMI-type Sr - Nd isotopic composition is different from that of the host basalts, ruling out the genetic relationship between them. It instead may have resulted from ancient magmatic events. Compilation of literature data reveals that the pyroxenite xenoliths collected from the west of DTGL are mostly characterized by enriched isotopic composition, whereas those from the east of DTGL show predominantly depleted isotopic composition. Such a spatial variation in pyroxenite composition, which supports the notion that the DTGL separates two different mantle domains, is likely related to the different formation ages of the lithospheric mantle beneath either side of the DTGL.