辽西新太古代钓鱼台花岗岩成因及演化过程:来自岩石组构的证据

辽西兴城钓鱼台地区分布一套花岗质杂岩,是新太古代"绥中花岗岩"的重要组成部分。花岗质杂岩以似斑状花岗闪长岩和石英闪长岩为主,少量细粒黑云闪长岩(包体)及脉状花岗岩,各类岩石接触关系明确,本文定义为"钓鱼台花岗岩"。锆石U-Pb同位素测试结果显示似斑状花岗闪长岩、石英闪长岩、脉状花岗岩的形成年龄分别为2538±20Ma、2476±56Ma、2470±18Ma,同为新太古代末期热事件的产物。通过该花岗岩组合的宏观-微观组构解析表明,似斑状花岗闪长岩表现为均匀块状构造,具有深熔花岗质岩浆的典型堆晶结构;细粒黑云闪长岩为细粒结构,呈小型暗色包体分布在似斑状花岗闪长岩中,包体的塑性变形、捕掳晶、淬冷边及反向脉等组构发育,具有铁镁质基性岩浆加入同深熔花岗闪长岩并快速冷却的特征;暗色的石英闪长岩主要分布在似斑状花岗闪长岩之下,接触带附近似斑状花岗闪长岩中的钾长石变斑晶明显增多,显示闪长质岩浆"底垫"侵位加热的特征;脉状花岗岩同时穿切似斑状花岗闪长岩和石英闪长岩,具有熔体富集脉体的结构特征。各类岩石中变形组构均不发育。钓鱼台花岗岩记录了新太古代末期地壳深熔和壳幔相互作用过程,岩石组构研究表明新太古代地壳再造作用是一个"静态"多期次的缓慢深熔过程,伴有同期幔源基性物质加入并混合,以及随后大规模的基性岩浆底侵。由此推断钓鱼台花岗岩形成的构造背景为幔源岩浆垂向底侵过程,可能是与俯冲带关联不明显的岩浆弧环境。

The petrogenesis and evolutionary process of Neoarchean Diaoyutai granite in western Liaoning Province: Evidences from petrofabric

There is a set of granitic complex exposed in the Diaoyutai area, western Liaoning Province, and it is an important part of "Suizhong granite". This set of granitic complex is composed mainly of porphyraceous granodiorite and quartz-bearing diorite, with a small amount of fine-grained biotite diorite (enclaves) and veinlike granite. The contacts of each kind of rock are clear, and we define it as "Diaoyutai granite" in the study. Zircon U-Pb ages show that the porphyraceous granodiorite, quartz-bearing diorite and veinlike granite are formed at 2538±20Ma, 2476±56Ma and 2470±18Ma respectively, which are all the products of the Late Neoarchean thermal event. Based on the macro-micro fabric analysis of the granitic complex, we confirm that the porphyraceous granodiorite has the massive structure and the typical accumulative texture that formed in the late stage of anataxis. The fine-grained biotite diorite is fine-grained texture and it occurs in the porphyraceous granodiorite in the form of micro mafic enclaves. There developed plastic deformation, xenocrysts, quenching boundary and reverse veins in the enclaves showing the injection of mafic magma during the period of granodiorite melting, and subsequent to the period, the magma cooled rapidly. Dark quartz-bearing diorites intrude under the porphyraceous granodiorite, and the K-feldspar metacrysts increase obviously in the vicinity of the contact zone, showing the heating characteristic of "underplating" emplacement. Meanwhile, the veinlike granite cross cut both the porphyraceous granodiorite and quartz-bearing diorite, and have the characteristic of being highly enriched in melt. Deformation fabric is not well developed in all rocks. In short, the "Diaoyutai granite" recorded the Late Neoarchean crust anatexis and the interaction between crust and mantle. The petrofabric research shows that the Neoarchean crust reconstruction is a process of static, multiple and slow crust anatexis with the cotemporaneous injection of mantle-derived magma, resulting in magma mingling and mixing and subsequent large-scale magma underplating. According to the present study, we therefore conclude that the tectonic setting is a vertical underplating environment of mantle-derived magma, and is probably the magma arc environment far from subduction zones.