赣南南迳盆地加里东期火山岩锆石U-Pb年龄及其地质意义

赣南南迳盆地位于华南加里东褶皱带中西部,盆地内火山岩组合常因风化强烈,出露不好,长期被认为属于中生代火山岩系的一部分。该火山岩组合与下伏晚奥陶世地层呈整合接触,与上覆中泥盆世地层呈角度不整合,应为早古生代火山岩组合,但缺乏精确的年代学和岩石地球化学制约。对南迳盆地火山岩进行了锆石U-Pb年龄、Sr-Nd-O同位素及地球化学研究。安山岩和英安岩SHRIPM锆石U-Pb年龄分别为442.1±3.9Ma和439.9±3.7Ma,表明一次晚奥陶世末期—早志留世初期的岩浆事件。微量元素分析显示,Rb、K和轻稀土元素富集,Nb、Ta、Zr、Hf、Ti元素亏损,Nb/U、Nb/Th、Ti/Yb值较低,轻、重稀土元素分馏明显,负Eu异常,主量元素贫钛、富铝。研究认为,南迳盆地火山岩是早古生代时期,随着南华洋关闭、扬子地块与华夏地块发生碰撞,地壳加厚、岩石圈地幔拆沉、软流圈上涌,导致EMI型富集地幔与下地壳发生部分熔融的结果。

Zircon U-Pb age of the Caledonian volcanic rocks in Nanjing basin of southern Jiangxi Province and its geological significance

Nanjing basin is located in the central and west part of the Caledonian fold belt in South China, and the volcanic rocks in the basin are often characterized by strong weathering and poor exposure, therefore long considered to be part of the Mesozoic rocks. The volcanic rocks exhibit conformable contact with the underlying Late Ordovician strata, and unconformable contact with the overlying Middle Devonian strata. These rocks are supposed to be Early Paleozoic volcanic rocks, but without precise chronologic and rock lithogeochemical constraints. This paper reports zircon U-Pb chronology, Sr-Nd-O isotopes and geochemistry of volcanic rocks in Nanjing basin. The zircon SHRIPM U-Pb ages of andesite and dacite are 442.1±3.9Ma and 439.9±3.7Ma respectively,indicating a late Late Ordovician-Early Silurian magmatic event. Trace element analyses show enrichment of Rb,K, LREE, and depletion of Nb, Ta, Zr, Hf, Ti,low ratios of Nb/U, Nb/Th,Ti/Yb and strong fractionated REE(LREE/HREE=7~9) and negative Eu anomalies (0.6~0.76), with the characteristics of depletion of Ti(0.67%~0.79%), richness of Al(14.3%~16.1%). It is thus concluded that the crustal thickening, lithospheric mantle delamination and upwelling of the asthenosphere resulted from the partial melting of the EMI-type mantle and the lower crust with the closure of the South China Ocean and the collision of the Yangtze block with the Huaxia block during the Early Paleozoic.