长江中下游地区怀宁盆地火山岩锆石LA-ICPMS定年

Zircon LA-ICPMS Dating of the Volcanic Rocks from Huaining Basin in the Middle——Lower Yangtze Valley

怀宁盆地为长江中下游地区众多中生代火山岩盆地之一,自下而上发育彭家口组和江镇组火山岩,岩性分别以粗面质凝灰熔岩和粗面安山岩、流纹岩为主。彭家口组样品的SiO2含量分别为61.56%和70.61%,江镇组样品的SiO2含量为61.91%～63.77%,均属于橄榄安粗岩系列,彭家口组样品更加富钾。运用LA-ICPMS对彭家口组和江镇组火山岩进行了锆石U-Pb定年,获得了206Pb/204Pb加权平均谐和年龄分别为130.0±1.7 Ma(MSWD=0.73)和130.5±1.7 Ma(MSWD=0.78),指示怀宁盆地火山岩为早白垩世的产物,喷发持续时间较短。该年龄与长江中下游地区其他盆地火山岩的年龄一致,表明长江中下游地区中生代火山岩的喷发时代集中在135～127 Ma之间,是在强烈拉张的背景下,岩石圈地幔部分熔融的产物。

The Huaining basin is one of the many Mesozoic volcanic basins in the Middle-Lower Yangtze Valley, where from the bottom to up the volcanic rocks of the Pengjiakou and the Jiangzhen Formations distributed. The rock types are dominantly trachyte tufflava, trachyandesite and rhyolite. SiO2 contents of samples from the Pengjiakou Formation are 61.56% and 70.61%, respectively and that from the Jiangzhen Formation are from 61.91% to 63.77%. All the samples are shoshonite serials with the samples of the Pengjiakou Formation being more potassium enrichment. Zircon LA-ICPMS U-Pb isotopic dating on the volcanic rocks from the Pengjiakou and Jiangzhen Formations gives weighted mean ^206pb/^235U ages of 130.0 ± 1.7 Ma （MSWD =0.73） and 130.5 ± 1.7 Ma （MSWD=0.78 ）, respectively, which indicates they erupted in the Early Cretaceous within a briefly active duration. The ages of volcanic rocks in the Huaining basin are consistent with that of other basins distributed in the Middle-Lower Yangtze Valley, suggesting that erupted ages of the Mesozoic volcanic rocks in the Middle--Lower Yangtze Valley concentrated in a period of 135 to 127 Ma and the volcanic rocks were derived from partial melting of lithospheric mantle under a intense extended setting.