摘要
鄂北大洪山地区最新野外地质调查研究发现,董云鹏沿着土门—周家湾—小阜—园潭一带识别的火山弧实际上是玄武岩、安山岩、英安岩、流纹岩、砂岩、板岩等多种岩石的组合,采集三件酸性岩样品从地球化学方面对火山岩岛弧环境性质进行了验证,并且采用LA-ICP-MS锆石U-Pb测年法测得土门英安岩的形成时间为(841±12)Ma(MSWD=8.6)。关口垭、六里冲、杨家棚、绿林和厂河等地广泛出露辉绿岩、辉长岩、玄武岩、枕状熔岩、远洋硅质岩和泥质岩组合,这些岩石组合多以岩片的形式混杂于一套弧前碎屑岩(基质)中,具有结合带增生杂岩的特点。其中镁铁质岩具有低钾、高镁的特点,为一套拉斑玄武岩系列岩石组合,"平坦"到LREE亏损的稀土配分模式和不明显的负铕异常,整体亏损大离子亲石元素,且Nb、Ta未见明显亏损,这与洋中脊蛇绿岩基性岩端元地球化学特点相符。在绿林辉绿岩中获得(820±17)Ma(MSWD=5.5)的锆石U-Pb测年数据,结合前人已经获得的一些同位素年代学证据,这可能代表晚期蛇绿岩的形成时间。这次调查大体确定了大洪山地区岛弧火山岩和增生杂岩的形成时代及分布范围,晋宁期岛弧火山岩和增生杂岩的厘定为鄂北大洪山地区存在晋宁期结合带的观点提供了强有力的证据,大洪山地区晋宁期结合带的厘定说明该区可能存在一条区别于勉略带(晚古生代—早中生代)的另一结合带。
During the latest field geological survey in the Dahongshan region, northern Hubei, the authors found that the island arc along the route Tumen- Zhoujiawan- Xiaofu- Yuantan defined by Dong Yunpeng actually were combination of basalt, andesite, dacite, rhyolite, sandstone, mudstone and so on. Three samples of acidic rock have been collected to verify the environmental properties of the volcano rock island arc by the properties of geochemistry. The Tumen dacite has been LA-ICP-MS U-Pb zircon dated, and the results indicate that the weighted average age of the 23 analysis points is （841 ± 12） Ma （MSWD = 8.6）, which means the forming time of the dacite. But extensively exposed rock associa- tion of diabase, gabbro, basalt, pillow lava, deep-sea siliceous rocks and argillaceous rocks in the Guankouya, Liuli- chong, Yangjiapeng, Lulin, Changhe, and these rocks are accumulated in a series of fore-arc clastic rocks in the form of tectonic slices which has typical characteristics to accretionary complexes. Whole-rock major and trace element analyses of mafic rock samples from the Dahongshan region show tholiitic basaltic series features with low-K （potassium）, high- Mg （magnesium） characteristics, and flat to LREE-depletion REE patterns with unobviously negative Eu anomaly; these rocks also correspond to basic rock endmembe~ of Mid-ocean ridge ophiolite in geochemistry with overall LREE depletion and without significantly negative Nb, Ta anomalies. U-Pb zircon dating by LA-ICP-MS show weighted mean age of （820 ±17 ） Ma （ MSWD = 5.5 ） for diabase from Lulin, which could represent the formation of later stage of ophiolite. Formation time and distribution of the island-arc volcanic rocks and accretionary complexes in the Dahongshan region were generally determined during this geological survey. The documetation of Jingningian island-arc volcanic rocks and accretionary complexes provided powerful evidence for the existence of Jingningian suture zone in the Dahongshan region, which has indicated that there is another suture distinguish from late Paleozoic to early Mesozoic Mianlue.
出处
《资源环境与工程》
2015年第6期757-766,共10页
Resources Environment & Engineering
基金
中国地质调查局项目(12120113012800)资助
关键词
岛弧火山岩
增生杂岩
晋宁期
鄂北大洪山
island-arc volcanic rocks
accretionary complexes
Jingningian
Dahongshan region in northern Hubei
