摘要:
锆石这一矿物是迄今为止所发现的地球最初500个百万年间(地质学界通常称为“冥古代”)的唯一地壳物质残留,对于了解地球早期地壳演化过程至关重要。近年来,随着高精度 SHRIMP 测年技术的广泛运用,笔者最近在华夏地块龙泉地区发现了2颗约4100 Ma的碎屑锆石。其中一颗为目前发现亚洲最古老的锆石,其内部结构简单,具有正常的震荡环带,207Pb/206Pb年龄为(4127±4) Ma;其δ18O与地幔岩浆的氧同位素类似,可能来自于早期的幔源岩浆或者未经历地表过程的深部地壳物质的熔融。另外一颗锆石具有明显的核边结构,其核部结晶年龄约为4100 Ma,变质边年龄约为4070 Ma,外围还有2层岩浆增生边年龄介于3800~3600 Ma,代表了该4100 Ma的锆石核部在后期经历了复杂的地壳演化过程,4070 Ma的变质边是目前已获得最为可靠的全球变质年龄。上述发现表明冥古宙时地壳性质和构造环境存在多样性,为认识地球早期大陆演化过程提供了重要新信息。
关键词:
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锆石
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冥古宙
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龙泉
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华夏陆块
Abstract:
At present, zircons are the only residual material of the Hadean crust, and constitute a crucial clue to understanding the early crustal evolution. Rencently, we found the two ~4100 Ma detrital zircons from a Paleozoic quartzite in the Longquan area of the Cathaysia Block, through SHRIMP zircon data. One zircon (207Pb/206Pb age of 4127±4 Ma) shows normal oscillatory zonation and constant mantle-like oxygen isotope ratios (δ18O = 5.8‰ to 6.0‰), and might have crystallized in ancient mantle-derived magmas or partial melts of primitive crust. Obviously, this is the oldest age in the reported zircon ages of Asia. The other zircon grain has a ~4100 Ma magmatic core surrounded by a ~4070 Ma metamorphic mantle. The 4070 Ma indicates the most reliable metamorphic age of the earth. Outside the mantle, two overgrowth bands are 3800~3600 Ma, implying that the magmatic core (crust) underwent complex evolutions after ~4100Ma. These data demonstate the diversity of the Earth’s earliest crustal nature and tectonic setting, and provide crucial evidence for us to understand the earliest crustal evolution.
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