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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。
The zircon is the only crustal remnant ever discovered in the earth for the first 500 million years on earth (geology is often called “ancient ancient”) and is crucial for understanding the evolution of the Earth’s early crust. In recent years, with the widespread use of high-precision SHRIMP dating techniques, the author recently discovered two detrital zircons of about 4100 Ma in the Longquan area of the Huaxia block. One of them is the oldest zircon currently found in Asia. Its internal structure is simple and has a normal concussion zone with 207Pb / 206Pb age of (4127 ± 4) Ma. Its δ18O is similar to the oxygen isotope of mantle magma, probably from Early mantle-derived magma or deep crustal material that did not undergo surface processes. The other zircon has a nucleus structure with an age of 4100 Ma and a metamorphic edge of 4070 Ma. There are also 2 magmatic margins ranging from 3800 to 3600 Ma in the periphery, representing the 4100 Ma The zircon core of Ma experienced complex crustal evolution later, and the metamorphic edge of 4070 Ma is the most reliable global metamorphic age so far available. The above findings show the diversity of crustal and tectonic setting in the Lower Paleozoic and provide important new information for understanding the evolution of the Earth’s early continental continent.