胶东北部碱性超基性脉岩地球化学特征及环境和成因探讨

胶东地区脉岩属碱性超基性岩系(Na₂O+K₂O=4.67%～5.43%;SiO₂=36.70%～39.99%),岩性为单一的橄榄辉石岩。从主量元素(包括CIPW标准矿物组成)和过渡元素组成来看,该岩系近似原始岩浆组成。电子探针结果显示:橄榄石为富镁质橄榄石(贵橄榄石)(Fo=71～90),单斜辉石为透辉石(次透辉石为主)。岩石富集大离子亲石元素(K、Rb、Sr、Th和Ba),但不具有高场强元素(Nb、Ta、Zr和Hf)的亏损,表明岩石形成于大陆板内环境,为地幔橄榄岩低度部分熔融(3.4%)的产物。同时,它具有大陆边缘弧的特性,暗示其为一种滞后型弧岩浆作用的产物。稀土元素特征显示,岩石强烈富集LREE,而相对亏损HREE,暗示了源区的富集特性。Eu/Eu^*=0.89～1.00,总体不表现明显的负Eu异常,暗示斜长石不是主要的分馏矿物相。结合板内碱性岩石的矿物结晶顺序认为,本区岩浆分馏以较弱的橄榄石分馏为主。

GEOCHEMICAL FEATURES, ENVIRONMENT AND PETROGENEIS OF THE ALKALINE ULTRAMAFIC DIKES IN NORTH SHANDONG PENINSULA

The dikes of olive-pyroxene outcropping in north part of east Shandong Province belonged to alkaline ultramafic rocks(Na₂O+K₂O=4.67%～5.43%; SiO₂=36.70%～39.99%). The major elements(together with CIPW standard minerals)and transitional elements indicated that these dikes were similar to primitive magma. Electron microprobe analyses showed the olivine(Fo=71～90)being chrysolite, and clinopyroxene mostly was attributed to diopside. The dikes were formed in a tectonic setting of intraplate, with the characteristics of enrichment in LILE(K, Rb, Sr, Th, Ba, etc.), and depletion in HFSE(Nb, Ta, Zr and Hf). Meanwhile, the rocks possessed the feature of continent marginal arc, suggesting that they might be products of hysteresis(recycled)arc magmatism. The REE features demonstrated the rocks coming from the deep mantle(garnet peridotite mantle), and undergoing, before their emplacement, a low-degree partial melting(3.4%)of peridotite. During magma extraction, however, there was lack of magmatic fractional crystallization but weak fractionation of olivine, since the Eu negative anomaly(Eu/Eu^*=0.89～1.00)was not obvious, which was further supported by the crystallization order of the minerals being of intraplate rocks.