辽东南长海地区花岗质片麻岩类的地球化学特征:对其原岩性质及形成环境的制约

本文对辽东南长海地区花岗质片麻岩进行了系统的岩石学和地球化学研究,以便对其原岩性质及形成的构造环境给予制约。研究结果表明,研究区内花岗质片麻岩类可划分为富钠和富钾两类花岗质岩石,前者包括黑云二长花岗质片麻岩和花岗闪长质片麻岩,矿物组成主要包括斜长石、石英、黑云母及少量的钾长石等,后者则主要包括花岗质、二长花岗质和糜棱岩化花岗质片麻岩,其矿物组成以钾长石、斜长石、石英和次要的白云母和黑云母为主。地球化学分析结果显示,富钠花岗质片麻岩具有富硅、富钠、高铝、富集LREEs和LILEs、强烈亏损HREEs和HFSEs(Nb、Ta、P、Ti)、轻重稀土强烈分馏、并显示弱负Eu异常和Ba的相对亏损等特征;而富钾花岗质片麻岩则显示高硅、富钾、过铝质的地球化学属性、富集LREEs和LILEs、亏损HREEs和HFSEs,与富钠花岗质岩石相比,轻重稀土分馏程度相对较弱、HFSEs以及Ba相对于Rb和Th的亏损程度更强,并显示中等负Eu异常和强烈的Sr负异常等特征。上述特征表明,辽东南长海地区富钠花岗质片麻岩应起源于中酸性陆壳物质的部分熔融,并可能有玄武质物质的加入,原岩应为具有活动大陆边缘属性的花岗闪长岩,其形成应与狼林地块(或胶辽地块)东南及南部先存洋盆向陆块之下的俯冲作用相联系;而富钾花岗质片麻岩应形成于中酸性陆壳物质的部分熔融,原岩为具有碰撞型花岗岩属性的碱性花岗岩,其形成应与洋壳消减闭合、陆陆碰撞拼贴相联系。

Geochemical characteristics of the Changhai granitic gneisses in Southeast Liaoning Province, NE China: Implications for its protolith property and formed tectonic setting

In this paper, we performed systematically petrological and geochemical studies on the granitic gneisses near the Changhai Town in the southeastern Liaoning Province. Our data placed constraints on their protolith and tectonic setting. The results showed that these granitic gneisses could be divided into two types, (sodium) Na-rich and (potassium) K-rich granitic gneisses. The former are mainly composed of biotite monzogneisses and biotite granodioritic gneisses, and their mineral compositions chiefly include plagioclase, quartz, biotite and K-feldspar, etc. The latter consist of granitic, monogranitic and mylonitic granitic geisses, the mineral compositions of which are mainly composed of K-feldspar, plagioclase, quartz and minor muscovite and biotite. Geochemical analyses indicate that the Na-rich granitic gneisses are characterized by high contents of SiO₂, Na₂O and Al₂O₃, enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), strong depletion in heavy rare earth elements (HREEs) and high field-strength elements (HFSEs), relative depletion in Ba and weak negative Eu anomaly. On the other hand, the K-rich granitic gneisses are peraluminous and show geochemical features of high contents of SiO₂ and K₂O, enrichments in LREEs and LILEs, and depletion in HREEs and HFSEs. However, compared to the Na-rich ones, the K-rich granitic gneisses exhibit moderate negative Eu anomaly, relatively weaker REEs fractionation and stronger depletion in HFSEs and Ba. These features suggest that the Na-rich granitic gneisses could be derived from the partial melting of intermediate-acid crustal materials, with some basaltic material involved in the source and their protolith could be the granodiorite formed at an active continental margin The formation of these Na-rich granitic gneisses could be related to the subduction of the pre-existent oceanic basin beneath the Nangrim Massif (or Jiao-Liao Massif). On the other way, the Na-rich granitic gneisses could originate by partial melting of the crust and their protolith could be alkaline granites in a collisional tectonic setting after subtraction and closure of the previous oceanic crust.