长江中下游溧水盆地火山岩的时代、地球化学特征及其地质意义

溧水盆地位于长江中下游成矿带东部,与庐枞、宁芜等火山岩盆地相似,发育中生代火山岩。为了探讨溧水盆地不同旋回火山岩形成时代、岩浆源区及岩浆活动的大地构造背景及其与铁铜成矿关系等问题,本次研究选取溧水盆地广泛出露且与成矿关系密切的中生代龙王山旋回和大王山旋回火山岩进行了锆石精确定年和Hf同位素分析以及全岩主微量元素分析。研究表明溧水盆地岩浆活动起始时间为~135 Ma,峰期时间为135~124 Ma。龙王山、大王山旋回火山岩Hf同位素特征相似,εHf(t)值范围大多在-6~-11之间,暗示其源区可能相似。岩石地球化学特征显示龙王山—大王山旋回岩浆由中基性向中酸性过渡,并向高钾富碱方向演化,空间上从北西-南东方向演变,与盆地内铁铜金锶矿的形成有关。龙王山旋回火山岩明显Sr正异常,推测龙王山旋回可能是Sr矿的矿源层。结合岩石地球化学和同位素特征,认为岩浆源区为可能为富集地幔,在喷发早期受到了少量陆壳物质的混染。溧水盆地与长江中下游其它火山岩盆地形成时代相似,可能受同一构造背景控制。结合前人研究成果分析认为在~135 Ma为太平洋板块俯冲的转折点,即俯冲作用减弱,俯冲板块发生后撤,形成区域性拉张环境,从而形成的一系列火山岩盆地。

Geochronology and Geochemistry of Volcanic Rocks in the Lishui Basin in the Middle and Lower Reaches of Yangtze River and its Geological Implications

The Lishui Basin is located in the eastern part of the middle and lower reaches of Yangtze River metallogenic belt. It is similar to the Luzong and Ningwu basins, where Mesozoic volcanic rocks are widespread. In order to investigate the formation time, magma source and tectonic background of volcanic rocks in different cycles and their relationships with the Fe-Cu mineralization, this study presents the new zircon dating results, Hf isotopic compositions, and major and trace element analysis of widely exposed Longwangshan and Dawangshan volcanic rocks which are mostly related to mineralization of Lishui Basin. It is suggested that the magmatism began at ~135 Ma, and peaked at 135~124 Ma in the Lishui Basin. Zircons from the Longwangshan and Dawangshan volcanic rocks have similar Hf isotopic characteristics with εHf(t) values of -6~-11, suggesting similar magma sources. The geochemistry characteristics suggest that the compositions of magma range from intermediate-basic to intermediate-felsic and rich alkali, and the distribution of outcrops from NW to SE from Longwangshan Formation to Dawangshan Formation, which is related to the iron, copper, gold, and strontium deposits. The positive anomaly of Sr of the Longwangshan volcanic rocks suggests that they could be the source of the Sr deposit. Combined with the geochemical and isotopic characteristics, it is suggested that these magmatic rocks were probably derived from an enriched mantle, which was contaminated by minor crustal materials during eruption. The formation time and tectonic setting of the Lishui Basin is similar to those of the other volcanic basins along the middle and lower reaches of the Yangtze River, suggesting that they could be controlled by the same tectonic background. Based on analysis of previous studies, this study confirmed that the change of the subduction of the Pacific Plate occurred at ~135 Ma, when subduction weakened, and subsequently the subducted slab rollback occurred, and finally formed regional extensional environment, resulting in the formation of a series of volcanic basins.