鲁西莲花山地区新太古代晚期二长花岗岩中的表壳岩包体——SHRIMP 锆石U-Pb定年和地球化学特征

为了确定鲁西莲花山地区新太古代晚期二长花岗岩中的表壳岩包体的形成时代,并探讨与相邻雁翎关地区的雁翎关岩组中的新太古代早期变质火山岩系的关系。本文对表壳岩包体及相关岩石进行了锆石年代学和地球化学研究。表壳岩包体主要由变质超基性岩和斜长角闪岩组成,另含少量(黑云)角闪变粒岩和黑云变粒岩。SHRIMP锆石U-Pb定年结果显示:角闪变粒岩的岩浆锆石年龄为2 757 Ma;侵入斜长角闪岩的奥长花岗岩脉的年龄为2 593 Ma;从变质超基性岩分选出很少锆石,它们普遍遭受强烈变质重结晶,207Pb/206Pb年龄为2 657~2 397 Ma。变质超基性岩具轻稀土亏损型或平坦型稀土模式,斜长角闪岩具平坦型稀土模式,角闪变粒岩具轻稀土略富集稀土模式,虽然大离子亲石元素相对富集,但都无明显Nb、Ta亏损。表壳岩包体的岩石组合、地球化学组成特征和形成时代可与相邻雁翎关地区的雁翎关岩组中的新太古代早期变质火山岩系对比,形成于大洋环境。新太古代早期表壳岩的原有分布范围应比现在所见到的更为广泛。

Supracrustal Enclaves in Late Neoarchean Monzogranite in Lianhuashan Area,Western Shandong: SHRIMP U-Pb Zircon Dating and Geochemical Characteristics

This study was carried out to analysis the SHRIMP U-Pb zircon dating and geochemistry of the supracrustal enclaves in the Late Neoarchean monzogranite in Lianhuashan area, western Shandong. The supracrustal enclaves are mainly composed of amphibolite, meta-ultramafic rock with some fine-grained (biotite) hornblende gneiss,and fine-grained biotite gneiss. SHRIMP zircon U-Pb dating indicates that the fine-grained hornblende gneiss has a magmatic zircon age of 2 757 Ma, whereas a trondhjemite dyke intruding in the supracrustal enclave has a magmatic zircon age of 2 593 Ma. Some zircon grains separated from meta-ultramafic rock show strong recrystallization and have a large ²⁰⁷Pb/²⁰⁶Pb age variation (2 657-2 397 Ma). The meta-ultramafic rock, amphibolite, and fine-grained hornblende gneiss show LREE-depletion-flat REE pattern, flat REE pattern, and LREE-enrichment REE pattern, respectively, with weak LILE enrichment and insignificant Nb-Ta depletion. Combined with the early work, it is concluded that the rock assemblage, geochemical composition,and formation age of the supracrustal enclaves in the Lianhuashan monzogranite are the same as those of the meta-volcanic series of the typical Early Neoarchean supracrustal rocks (Yanlingguan ‘Formation’) in the adjacent Yanlingguan area, which were formed in ocean environment. The Early Neoarchean supracrustal rocks should be widely distributed in western Shandong more than considered before.