郯庐断裂带中南段新生代玄武岩源区地幔特征及其演化

FEATURES AND EVOLUTIONS OF THE MANTLE SOURCE AREA OF CENOZOIC BASALTS IN THE MIDDLE-SOUTHERN SEGMENT OF THE TAN-LU FAULT ZONE

郯庐断裂带中南段新生代玄武岩随时代逐渐变新 ,碱性逐渐增强 ,轻、重稀土元素分离程度及不相容元素的富集程度也逐渐增强 ,Sr、Nd同位素组成越来越亏损。古地温线特征表明 ,断裂带之下的上地幔具有高的大地热流值。老第三纪拉斑玄武岩地幔源区主要以轻微富集的EMⅠ组分为主 ,新第三纪以来的碱性玄武岩来源于亏损地幔。研究区地幔源区随时间的演化受老第三纪软流圈上涌的影响 ,大量的深源流体对上部岩石圈地幔进行广泛的交代 ,从而使郯庐断裂带下面的岩石圈地幔出现不相容元素和LREE富集 ,且由轻微富集地幔转变为亏损地幔。

With becoming young of the Cenozoic basalts from the middle southern segment of the Tan Lu fault zone, their alkali, the fractional degree of LREE/HREE and incompatible elements enrichment increase. The evolutions of Sr,Nd isotopic compositions with time become depleted. Paleogeotherm indicates high thermal flow of the upper mantle below the Tan Lu fault zone. According to these features, the origin of the basalts magma and the evolutions of the mantle source areas are discussed in this paper. Paleogene basalts originated mainly from EMⅠcomponents. Neogene and Quaternary basalts were derived from the depleted mantle. The upwelling of asthenosphere brought a lot of free fluids. The initial free fluid widely replaced lithospheric mantle. The lithospheric mantle beneath the Tan Lu fault zone appeared enrichments of incompatible elements and LREE, resulting in the mantle evolution from the enrichment to depletion.