塔里木盆地东南缘安南坝地区镁铁质麻粒岩的成因——来自地球化学及Sr-Nd-Pb同位素的制约

甘肃阿克塞县安南坝地区镁铁质麻粒岩呈脉状、透镜状赋存于新太古代米兰岩群和TTG片麻岩中。岩石主要由斜长石(Pl)+斜方辉石(Opx)+单斜辉石(Cpx)+角闪石(Amp)+磁铁矿(Mt)等组成。安南坝镁铁质麻粒岩中Ti、P、Nb、Ta、Th、Hf、Sr及REE等元素与Zr相关性较好,表明其在变质作用过程中保持基本稳定。地球化学数据显示其原岩属于拉斑玄武质岩系列,Si O_2、Ti O_2、Al_2O_3、P_2O_5含量相对较低,Ca O、Mg O含量相对较高。Mg~#值为41.52~43.09,低于原生玄武质岩石的Mg~#值,Fe_2O_3~T、Mg O、Ca O与Si O_2含量呈负相关性,指示原岩岩浆演化过程中可能发生了辉石、角闪石等镁铁质矿物的分异结晶作用。镁铁质麻粒岩∑REE较低,稀土元素配分模式为轻稀土元素弱富集、重稀土元素相对平坦的右倾型,Eu异常不明显(Eu/Eu~*=0.91~1.01)。岩石富集Rb、Ba、Sr等大离子亲石元素,亏损Nb、Ta、Zr、Ti等高场强元素,具有显生宙典型岛弧玄武质岩石的地球化学特征。Sr、Nd、Pb同位素组成显示镁铁质麻粒岩原岩源自富集地幔,并受到一定程度的地壳物质混染。构造环境分析表明安南坝镁铁质麻粒岩原岩形成于与俯冲有关的岛弧环境。在俯冲作用机制下,俯冲板片流体交代使地幔楔发生富集,形成富集地幔,随着(弧后)伸展作用的加强,进一步诱发富集地幔的部分熔融形成镁铁质岩浆,最终岩浆就位形成辉长岩或辉绿岩脉,后期在麻粒岩相变质作用条件下变质为镁铁质麻粒岩。

Petrogenesis of mafic granulites from Annanba area in southeastern Tarim: Evidence form geochemistry and Sr-Nd-Pb isotopes

Mafic granulites as veins or lens are commonly identified within Milan Rock Group and TTG gneiss in Annanba area of Aksay County, Gansu Province. The granulite is composed of plagioclase (Pl), orthopyroxene (Opx), clinopyroxene (Cpx), amphibole (Amp) and magnetite (Mt), which is characteristic of typical medium-low pressure granulite facies rocks. Ti, P, Nb, Ta, Th, Hf, Sr and REE are well related to Zr of mafic granulites from Annan area, which shows that these elements remained basically unchanged and kept stable during the metamorphic process. Geochemical analysis shows that the mafic granulites belong to tholeiite series rocks, depleted in SiO₂, TiO₂, Al₂O₃ and P₂O₅, and enriched in CaO, MgO, Mg^# (41.52~43.09). The values of Fe₂O₃^T, MgO and CaO are negatively correlated with SiO₂, suggesting separation crystallization of pyroxene and amphibole during the evolution of the magma. The chondrite-normalized REE patterns show right-inclined feature with no obvious Eu(Eu/Eu^*=0.91~1.01) anomalies, whereas the primitive mantle normalized trace element spidergrams are characterized by enriched LILE (such as Rb, Ba and S) and depleted HFSE (such as Nb, Ta, Zr and Ti), similar to the typical feature of Phanerozoic arc basalt. Sr, Nd, Pb isotopic compositions suggest that the protolith of mafic granulites was derived from enriched mantle, and underwent crustal contamination. Combined with the tectonic evolution of southeastern Tarim, the authors hold that the protolith of mafic granulites was formed in island arc, and related to a subduction environment. The formation process of mafic magma may be as follows:the mantle wedge was metasomatized by the fluid and simultaneously formed enriched mantle; back-arc extension took place under the mechanism of subduction, which triggered partial melting of enriched mantle, and then the mafic magma moved upward to the upper crust along the fissures to form gabbro or diabase veins; later, they were metamorphized into mafic granulite under the granulite facies metamorphism.