湖北大冶与矽卡岩型铁矿床有关的铁山岩体中黑云母、角闪石40Ar-39Ar同位素年龄及其地质意义

摘要

长江中下游是中国东部中生代大规模成矿的重要地区之一,是中国东部一个重要的Cu-Fe-Au-Mo多金属成矿区带.鄂东南地区铜铁多金属矿床是长江中下游地区金属成矿带的重要组成部分,但关于岩体年龄及岩体热年代学的研究比较少,本文以湖北大冶与矽卡岩型铁矿床有关的铁山岩体中的黑云母与角闪石为研究对象,利用40Ar-39Ar阶段加热同位素定年方法对该岩体进行了成岩时代的研究,获得了黑云母、角闪石的40Ar-39Ar坪年龄分别为(133.0±1.0)Ma、(137.0±2.0)Ma,等时线年龄分别为(133.4±1.5)Ma、(137.3± 1.6) Ma,结合前人的资料,认为黑云母、角闪石的40Ar-39Ar坪年龄代表了铁山岩体的成岩时代,结果与鄂东南地区和长江中下游地区成矿岩体年龄一致.与铜陵地区主要成矿岩体年龄对比,利用岩体冷速率与成矿关系以及小岩体成大矿的原理,铁山岩体冷速率偏高,成矿规模一般与冷速率呈反相关,但可能由于其有着特殊的生长结构,浅部小侵入体可以得到一定深度岩浆房的成矿流体和热能补充,以致会出现“小岩体成大矿”的现象.同时,在结合已有的岩石学、地球化学以及矿床同位素年龄资料基础上,可以推断出鄂东南地区早白垩世矽卡岩铁矿床形成于岩石圈伸展-减薄环境.%The Middle-Lower Yangtze River belt is one of the large-scale Mesozoic metallogenic areas in eastern China and also an important Cu-Fe-Au-Mo metallogenic belt in eastern China. The polymetallic Cu～Fe deposits in southeast Hubei area constitute an important part of the Yangtze River be nevertheless, studies of rock body chronology and thermal chronology remain very insufficient. Biotite and hornblende of the Tieshan rock body related to the skarn-type Fe deposits in Daye of Hubei Province yield 40Ar-39Ar plateau ages of 133.0± 1.0 Ma and 137.0±2.0 Ma and isochronal ages of 133.411.5 Ma and 137.3±1.6 Ma respectively. An integrated study of available petrological, geochemical and rock body U-Pb ages led the authors to believe that the rock body ages of southeast Hubei area are the same as the ages of the Yangtze River belt, and the skarn-type Fe mineralization in Daye of Hubei Province was associated with lithospheric extension and thinning during the Late Mesozoic in eastern China. Meanwhile, utilizing the rock body cooling rate theory (the mineralization scale retro-correlation to the rock body cooling rate), the authors found that, although the cooling rate of the Tieshan rock body is rather higher in comparison with the cooling rates of the main mineralization rock bodies in Tongling, small shallow plutons could be complemented by ore-forming fluids and heat energy of deep magma chamber.