长江中下游地区庐(江)-枞(阳)和宁(南京)-芜(湖)盆地内与成矿有关潜火山岩体的SHRIMP锆石U-Pb年龄

庐-枞和宁-芜火山岩盆地是长江中下游地区在中生代发育的一系列断陷型火山岩盆地中规模最大的两个,以发育一套别具特色的橄榄玄粗岩系列火山岩/潜火山岩,并产有丰富的铁矿、硫铁矿及非金属等矿产资源为特征,其中又以玢岩型铁矿最著名。两个盆地内主要的成矿作用(玢岩型铁矿)都与第二旋回(分别为庐-枞盆地的砖桥旋回和宁-芜盆地的大王山旋回)的潜火山岩关系密切,其岩性在庐-枞盆地为正长斑岩,而在宁-芜盆地为辉石闪长玢岩。本文应用SHRIMP锆石U-Pb测年方法分别对庐-枞盆地内的巴家滩正长斑岩和焦冲正长斑岩以及宁-芜盆地内的阴山辉石闪长玢岩进行了精确的定年。结果表明,巴家滩正长斑岩和焦冲正长斑岩中锆石的206Pb/238U加权平均年龄分别为131.0±1.1Ma和131.5±1.6Ma,阴山辉石闪长玢岩中锆石的206Pb/238U加权平均年龄为127.8±1.8Ma。庐-枞和宁-芜盆地(乃至整个长江中下游地区)的火山岩-潜火山岩是在很短的时间内形成的,意味着"突发性的"岩石圈减薄可能是区内岩浆作用和大规模成矿的主要机制。

Zircon U-Pb SHRIMP ages of sub-volcanic bodies related with porphyritic Fe-deposits in the Luzong and Ningwu basins, Middle and Lower Yangtze River Reaches, Central China.

Luzong basin and Ningwu basin, which are two largest basins among several Mesozoic volcanic basins distributed in the Middle and Lower Yangtze River Reaches, developed a suite of unique rock series (shoshonitic series ) and abundant iron- sulfur iron- and nonmetal ore deposits, especially porphyry Fe-deposits. The dominant metallogenic bodies are sub-volcanic rocks formed in second volcanic activity cycle, syenite-porphyry for the Luzong basin and augite diorite porphyrite for Ningwu basin. In this work, we used zircon SHRIMP U-Pb technique accurately dating their emplacement ages of the Bajiatan body and the Jiaochong body in the Luzong basin and the Yinshan body in the Ningwu basin. The results are as follows: The zircon grains from these three bodies yield an emplacement ages of 131.0±1.1Ma, 131.5±1.6Ma and 127.8±1.8Ma respectively. The volcanic-subvolcanic rocks in Luzong basin and Ningwu basin (as well as all Middle and Lower Yangtze River Reaches) formed in very short time, predicate that "suddenly" lithospheric thinning was dominant mechanism of magmatism and cosmically metallogenesis in the reaches.