哈拉达拉辉长岩以及其中磁铁矿-辉石脉的岩石学和地球化学研究

Petrology and geochemistry of gabbro and magnetite-pyroxene vein in the Haladala mafic complex,southwestern Tianshan Mts.,Xinjiang

西南天山哈拉达拉杂岩体主要由橄长岩、橄榄辉长岩和辉长岩组成,岩浆通过早期橄榄石和斜长石结晶分离后,残余岩浆侵位形成了辉长岩。辉长岩中单斜辉石(Cpx-I)的稀土元素含量低(REE<51×10-6),Eu负异常不明显,其他微量元素(如Sc、Cr)的含量和Mg#值(80～95)变化大(透辉石-普通辉石)。磁铁矿-辉石脉穿切辉长岩,其中单斜辉石(Cpx-II)的成分比较均一(透辉石)。与Cpx-I相比,Cpx-II的稀土元素含量明显高(REE>100×10-6),且Eu负异常非常显著。与晚期富Fe基性岩浆的快速侵位不同,辉长岩的形成经历了复杂的演化过程。Cpx-I和Cpx-II均发育出溶结构,表明晚期富Fe基性岩浆快速侵位形成磁铁矿-辉石脉之后,哈拉达拉岩体或者经历了快速抬升(或者快速降温)的过程。岩浆过程结束后,低温热液活动形成了含金方解石-磁铁矿脉。研究这种热液脉,对本地区开展金矿找矿勘探有重要意义。

The Haladala mafic pluton in the southwestern Tianshan Mountains mainly consists of troctolite, olivine gabbro and gabbro. After the fractional crystallization of olivine and plagioclase, the residual magma intruded and formed gabbro. The clinopyxene （Cpx-Ⅰ） in gabbro has lower concentrations of rare earth elements （REE 〈 51× 10^-6） with insignificant Eu negative anomalies. The concentrations of other trace elements（Sc, Cr etc. ）in Cpx-I vary largely with a wide range of Mg^#（80 -95, diopside-augite）. The magnetite-pyroxene vein intruded into gabbro. The composition of clinopyxene （ Cpx-Ⅱ ） in the vein is homogeneous （ diopside ）. Compared to Cpx-Ⅰ, Cpx-Ⅱhas higher concentrations of REE （ 〉 100 × 10^-6）with obvious Eu negative anomalies. Distinct from the rapid intrusion of the later-stage Fe-rich mafic magma which formed the magnetite-pyroxene vein, the early-stage magma went through a complex evolution before its intrusion（gabbro）. Besides, exsolution texture found in both Cpx-Ⅰ and Cpx-Ⅱ implies that the Haladala pluton underwent rapid uplift （or fast cooling） after magnetite-pyroxene vein. Gold-bearing calcite-magnetite vein, representing an episode of epithermal activity after magma process, cuts through both gabbro and magnetite-clinopyroxene vein. Further study focusing on the gold-bearing calcite-magnetite vein is of great significance to gold exploration in this region.