长江中下游庐枞盆地火山岩的SHRIMP锆石U-Pb年龄:对扬子克拉通东部晚中生代岩石圈减薄机制的约束  

Zircon SHRIMP U-Pb Ages of Volcanic Rocks in the Luzong Basin, Middle and Lower Yangtze River Reaches: Constraints on the Model of Late Mesozoic Lithospheric Thinning of the Eastern Yangtze Craton

作　　者：薛怀民[1] 董树文[2] 马芳[3] 

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]中国地质科学院,北京100037 [3]北京大学地球与空间科学学院,北京100871

出　　处：《地质学报》2012年第10期1569-1583,共15页Acta Geologica Sinica

基　　金：深部探测技术与实验研究专项(编号SinoProbe-05-05);中国地质调查项目(编号1212010711814);地质矿产调查评价项目(编号1212011121116)资助

摘　　要：庐枞盆地位于扬子克拉通的东北缘、大别-苏鲁印支期碰撞造山带的前陆地区、郯庐断裂与长江断裂带的交会处附近,构造位置独特。盆地内的中生代火山/潜火山岩以富碱和高钾为特征,地球化学上表现为富集Rb、Th、U、K等强不相容元素和轻稀土元素,属典型的橄榄玄粗岩系列,Sr-Nd同位素组成显示它们主要形成于陆下富集型岩石圈地幔的部分熔融。盆地内的火山岩按形成的先后分为龙门院组、砖桥组、双庙组和浮山组,各组之间有明显的喷发间断。本文用SHRIMP法测得4个组火山岩的锆石U-Pb年龄分别为131±1Ma、133±1Ma、130±1Ma和127±1Ma,年龄测试结果表明整个庐枞盆地内的火山岩是在很短的时间内(≤6Ma)形成的。结合整个长江中下游地区中生代火山活动的时间非常集中,意味着火山活动具有"突发性",指示机械拆沉可能是区内岩石圈减薄的主要机制。但庐枞盆地西北缘沙溪一带的侵入岩具埃达克质(adakitic),形成于加厚地壳的部分熔融,其侵位时间(≥136Ma)略早于盆地内橄榄玄粗质岩浆的活动时间,指示区内约133～136Ma时间段构造环境和动力学机制发生了重大转变。The Luzong basin locates at northeastern margin of the Yangtze craton and the foreland area of the Dabie-Sulu orogen, and convergent section of the Tanlu fault and the Yangtze River fault, is a typical tectonic place. The volcanic-subvolcanic rocks in the basin are characterized by high-K and rich in alkali, belong to typical shoshonite series. In geochemistry, these rocks exhibit obvious depletion in high-field- strong-elements （HFSE） such as Nb and Ta, enriched in strong incompatible elements such as Rb, Th, U and K as well as light rare earth elements （LREE）. Nd and Sr isotope compositions of the rocks indicate that their parental magma mainly formed by partial melting of the enriched subcontinental lithospheric mantle. The volcanic rocks developed in the Luzong basin can divided into Longmenyuan Formation, Zhuanqiao Formation, Shuangmiao Formation and Fushan Formation respectively from early to late, with clear extrusive gap among different Formations. In this work, we used zircon SHRIMP U-Pb technique accurately dating samples from four Formations in the Luzong basin, yield emplacement ages of 131±1 Ma, 133±1 Ma, 130±1Ma and 127±1 Ma respectively, show that whole volcanic rocks in the Luzong basin formed in a very short period of time （46 Ma）. These data, combined with the time of whole volcanism in the Middle and Lower Yangtze River reaches, predicate an ＂sudden＂ volcanic event, imply delamination as the main model for lithospheric thinning of the region （and the whole lower Yangtze region）.

关 键 词：橄榄玄粗岩 庐枞盆地 中生代 扬子克拉通 岩石圈减薄 

分 类 号：P588.14[天文地球—岩石学] P597.3[天文地球—地质学]