阿尔泰山南缘中—酸性斑岩的地球化学特征及其岩石成因探讨

Geochemistry of the Intermediate—Acid Porphyries on Southern Margin of the Altay Mountains and Its Implications for Petrogenesis

阿尔泰山南缘中—酸性斑岩侵入于中泥盆统北塔山组火山岩地层中,斑岩体规模较小,主要岩石类型为闪长斑岩、石英闪长斑岩、花岗闪长斑岩、石英二长斑岩等。对哈腊苏、卡拉先格尔、喀腊萨依、托斯巴斯陶和乔夏哈拉等5个与铜矿有关矿区的斑岩研究表明,其地球化学特征既有相似性,又有不同的特点,其中SiO2含量为53.4%～66.2%,A/CNK为0.98～1.27(大多数小于1),稀土元素表现为轻稀土相对富集、不具Eu异常的特点。微量元素表现为明显的Nb和Ta的负异常。但是Harker图解以及稀土元素配分曲线表明,不同地区的斑岩不存在演化关系。斑岩的Sr和Nd同位素组成基本上与与其共生的玄武岩相同,因此根据实验岩石学以及稀土模拟结果推测,它们可能是岛弧阶段底侵的基性岩石部分熔融形成的,源区相当于下地壳无水的角闪岩相,但含有少量的石榴子石,其中石榴子石和角闪石比例的差别可能是导致不同地区斑岩地球化学特征差别的主要原因。

The Subduction-related porphyries on the southern margin of the Altay Mountains intruded in the Middle Devonian volcanic rocks of the Beitashan Formation. The main rock types include dioritic porphyry, quartz dioritic porphyry, granodioritic porphyry, and quartz monzonitic porphyry, etc. Representative porphyries from five copper deposits, i. e. , Halasu, Kalaxiangeer, Kalasayi, Tuosibasitao and Qiaoxiahala, are chosen for geochemical analyses. Their geochemical data from the five deposits display somewhat similarity, and also somewhat difference. The SiO2 contents range from 53.4% to 66.2%, and A/CNK from 0.98 to 1.27, but most lower than 1. The trace element compositions are characterized by the enrichment of light rare earth elements （LREE） relative to high rare earth elements （HREE）, and absence of Eu anomalies, MORB normalized inconpatible element patterns exibit significantly negative Nb and Ta anomalies. However, the Harker diagram and chondrite-normalized REE patterns for these porphyries show that no regular variations can be recognized, which suggests that they were not derived from a common parental magma. The Sr and Nd isotopic compositions of the porphyries are close to those of the associated basalts （Beitashan Fm. ）, implying their genetic relation. Based on experimental petrology and REE modeling, it can be inferred that, they might be generated by partial melting of underplating basic rocks during arc evolution. The source regions are believed to be dehydrated amphibolite facies of lower crust, but minor garnet might be involved. The distinguished geochemical compositions of the porphyries from five copper deposits can be attributed to various ratios of garnet and amphibole in the source regions.