河南登封石牌河“变闪长岩体”的锆石SHRIMP年代学研究

SHRIMP U-Pb Zircon Geochronology of the Shipaihe “Metadiorite Mass” from Dengfeng County, Henan Province

河南登封石牌河“变闪长岩”包括闪长岩至英云闪长岩和花岗闪长岩两种岩类 ,它们是同一岩浆系列分异结果的产物。其结晶时代以往曾用锆石U Pb等时线测得为 2 5 2 0± 17Ma ,这次采用SHRIMP法重测 ,获得锆石U Pb年龄为 2 4 93±7Ma,比以前稍小 ,年龄更为精确。这次所测锆石均具有岩浆环带 ,锆石Th/U比为 0 .76～ 1.2 7,平均为 1.0 3,具岩浆锆石特征 ,中心和边部测年结果一致 ,说明未受到明显的变质作用影响 ,故此年龄为岩浆的结晶 /侵位年龄。根据野外地质、岩石学和地球化学等特征 ,这一岩体与山东鲁西泰山和徂徕山等地同类岩石十分相似 ,可以对比 ,其时代都属于新太古代晚期。目前已有较多资料 ,说明新太古代末 ,有广泛的闪长质 花岗质的岩浆活动和区域变质作用 ,并与构造活动密切联系 ,应代表早前寒武纪的一次重大的地质事件。

The Shipaihe “metadiorite mass” from Dengfeng County of Henan Province is composed of diorite-tonalite and granodiorite, which are fractionation products of the same magmatic series. The emplacement or crystallization age of igneous mass determined previously by zircon isotopic analysis is 2 520±17 Ma. However, recent SHRIMP U-Pb analysis of the same zircons yield a new age of 2 493±7 Ma. The Th/U ratios of zircons range from 0.76 to 1.27, with the average value being 1.03. All the zircons are characterized by distinct magmatic zoning, and their crystal centers and rims have the same age. These features indicate that the zircons are of magmatic type not subjected to metamorphism, and hence the above new SHRIMP U-Pb zircon data may represent the crystallization or emplacement age of Shipaihe “metadiorite mass”. According to characteristics of field geology, petrology and geochemistry, the Shipaihe “metadiorite mass” is very similar to the same type of rock in Taishan Complex of west Shandong Province. They are both products of late Neo-Archean in spite of their slight difference in ages. The extensive diorite-granite magmatism and regional metamorphism related to tectonism suggest the existence of an important geological-tectonic event in the late period of Neo-Archean.