海南岛陆缘扩张带形成及新生代岩石圈动力学机制:来自幔源包体的地球化学证据

发布时间：2018-04-18 22:38

本文选题：幔源包体 + 单斜辉石　；参考：《大地构造与成矿学》2017年01期

【摘要】：海南岛陆缘扩张带蓬莱地区新生代玄武岩中捕获大量尖晶石二辉橄榄岩和方辉橄榄岩幔源包体。激光剥蚀等离子体质谱(LA-ICP-MS)分析结果表明,蓬莱地幔橄榄岩含有三种不同地球化学特征的单斜辉石(Cpx):(1)a类单斜辉石Mg~#=92.3~93.4,来自富集Cpx的二辉橄榄岩,具有极低的LREE和不相容元素含量,HREE平坦,Th、U、La、Sr正异常,经历了7%~10%的尖晶石相部分熔融,仅受到极低程度强不相容元素(Th、U、La、Sr)初期富集交代作用;(2)b类单斜辉石Mg~#=89.9~90.3,来自较富集Cpx的二辉橄榄岩,具有中等的LREE和LILE含量,HREE平坦,微量元素蛛网图上显示Th、U正异常,Rb、Ba、Nb、Ta、Sr、Ti负异常,经历4%~5%的尖晶石相部分熔融,可能受到了含LREE和Th、U等不相容元素的硅酸盐熔体交代;(3)c类单斜辉石Mg~#=91.4~92.8,来自贫Cpx的二辉橄榄岩和方辉橄榄岩,具有富集的LREE和LILE含量,HREE弱分异,微量元素蛛网图上显示Th、U正异常及强烈的Nb、Ta、Ti负异常,经历了8%~20%的尖晶石相部分熔融,其交代熔体可能是来自源区有石榴子石残留的碳酸盐熔体。全岩主、微量元素及模拟计算结果表明,这些幔源包体的主量元素主要受部分熔融程度影响,并且方辉橄榄岩经历的部分熔融程度大于二辉橄榄岩。地幔橄榄岩的Sr-Nd同位素组成表明该区具有MORB-OIB型亏损地幔特征。此外,蓬莱部分地幔橄榄岩包体显示正斜率的HREE分异特征((Gd/Yb)_N=0.4~0.7),暗示该区地幔经历了源自石榴子石稳定区的变压熔融,总体熔融程度为18%以上,指示了较高的地幔潜能温度。综合前人对海南岛新生代玄武岩最新研究成果,我们认为海南地幔柱可能为该区软流圈地幔置换古老岩石圈地幔提供了热源,导致了区域岩石圈地幔的破坏,从而引起包括地幔柱本身、软流圈和富集岩石圈的熔融。岩石圈地幔性质的改变和不均一性可能是海南岛陆缘扩张带新生代岩石圈减薄的主要动力学机制。
[Abstract]:The Cenozoic basalts in the Penglai area, the continental margin of Hainan Island, capture a large number of spinel lherzolite and plagioclase mantle xenoliths.The results of laser denudation plasma mass spectrometry (LA-ICP-MS) show that Penglai mantle peridotite contains three kinds of clinopyroxene with different geochemical characteristics, such as clinopyroxene, Mg#923 / 934, derived from Cpx enriched lherzolite.It has a very low LREE and incompatible element content and a positive anomaly of hree flat thin La-Li Sr, which has undergone 710% partial melting of spinel phase.It was only enriched and metasomatism in the early stage by very low degree strongly incompatible element (Th) and strongly incompatible element (Th). The initial metasomatism of Th 2b type clinopyroxene was 89. 9 ~ 90. 3. It was from the erpyrotite enriched with Cpx. It had moderate LREE and LILE contents and flat hree.After 4% partial melting of spinel phases, it may be that silicate melts containing incompatible elements such as LREE and Thu have been metasomatized by silicate melts, such as monoclinopyroxene type Mg1. 1. 4 + 9. 8. They are derived from erpyrozolite and zodizolite which are poor in Cpx, and have rich LREE and LILE contents and weak differentiation of hree.The trace element cobwebs show that the positive and negative anomalies of Thou U and NbTa-Ta-Ti have experienced 8% or 20% partial melting of spinel phase, and the metasomatic melt may have originated from carbonate melt with garnet residue in the source area.The main elements of the mantle xenoliths are mainly affected by the degree of partial melting, and the partial melting degree of the peridotite is greater than that of the second pyroxenite.The Sr-Nd isotopic composition of mantle peridotite indicates that the region is characterized by MORB-OIB type depleted mantle.In addition, some mantle peridotite inclusions in Penglai show a positive slope of HREE differentiation, indicating that the mantle in Penglai has experienced a pressurized melting originating from the pomegranate stable region, and the total melting degree is over 18%, indicating a higher mantle potential temperature.Based on the recent research on the Cenozoic basalt in Hainan Island, we believe that the Hainan mantle plume may provide a heat source for the replacement of the ancient lithospheric mantle by the asthenospheric mantle in this area, resulting in the destruction of the regional lithospheric mantle.This results in melting of the mantle plume itself, the asthenosphere and the enriched lithosphere.The change and heterogeneity of lithospheric mantle may be the main dynamic mechanism of Cenozoic lithospheric thinning in the continental margin of Hainan Island.
【作者单位】：中国科学院广州地球化学研究所同位素地球化学国家重点实验室;中国科学院大学;桂林理工大学地球科学学院;
【基金】：国家科技重大专项(2011ZX5023-004-011) 国家自然科学基金项目(40872080,41072081) 国土资源部公益科研专项(201211024-03)联合资助
【分类号】：P736

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