地学前缘 ›› 2009, Vol. 16 ›› Issue (2): 90-106.

• 论文 • 上一篇    下一篇

内蒙古天和永新生代玄武岩成因及其地质意义

杨宗锋 罗照华 张华锋 章永梅 黄凡 孙晨光 戴紧根   

  1. 中国地质大学 地质过程与矿产资源国家重点实验室, 北京 100083
  • 收稿日期:2008-11-12 修回日期:2008-12-15 出版日期:2009-03-02 发布日期:2009-03-02
  • 作者简介:杨宗锋(1984—), 男, 硕士研究生, 矿物学、岩石学、矿床学专业, 岩浆活动与成矿作用研究方向。E-mail: yangzfeng2008@163.com
  • 基金资助:

    内蒙古1∶25万补力太(K49C002003)、四子王旗(K49C003003)幅区域调查修测(1212010510506); 国家自然科学基金项目(40434010)

Petrogenesis of Cenozoic basalts of Tianheyong, Inner Mongolia,  and its geological implications.

  1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China
  • Received:2008-11-12 Revised:2008-12-15 Online:2009-03-02 Published:2009-03-02

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摘要：

天和永玄武岩为碧玄岩,至少可以划分出3种矿物共生组合类型。天和永玄武岩总体具有贫硅(w(SiO2)=4397％~4568％)、富碱(w(K2O+Na2O)=591％~765％)、富钾(w(K2O)=204％~289％)、高钛(w(TiO2)=218％~237％)、高Mg值(Mg＃=68~76)的特征;稀土元素含量高(∑REE=(24662~32982)×10-6),稀土配分曲线呈右倾平滑直线,强烈富集轻稀土,轻重稀土强烈分馏((La/Yb)N>30),无明显的Eu(δEu=090~102)和Ce异常(δCe=096~100);强烈富集不相容元素,其中高场强元素(HFSE)Nb、Ta和Th出现峰值,具有近似OIB配分型式的特征;玄武岩富含相容元素Co((391~489)×10-6)、Ni((130~257)×10-6)、Cr((138~320)×10-6)。上述所有特征以及岩石结晶程度低、富含橄榄岩包体和少量捕虏晶、元素变异关系等均表明,天和永玄武岩是原生玄武岩质岩浆固结的产物。微量元素比值Ba/Rb(12~35)和碱金属的变化暗示源区可能遭受过流体的交代作用,源岩可能是富集的二辉橄榄岩。岩石成因模拟表明,形成天和永玄武岩的原生岩浆是在变压、部分熔融的条件下富集地幔源区岩石非实比熔融的产物,变压熔融柱穿切了Sp/Gt二辉橄榄岩相边界。岩浆形成于源区岩石的低度(约<5％)部分熔融,其中石榴石二辉橄榄岩部分熔融约为1％,尖晶石二辉橄榄岩部分熔融2％~5％。综合分析显示,源区部分熔融的触发机制是边际驱动的地幔对流,因而其形成深度大于东部的集宁玄武岩和汉诺坝玄武岩。

关键词: 岩石成因;天和永;玄武岩;原生岩浆;变压熔融柱;边缘对流

Abstract:

The Tianheyong basalts are basanites, which at least can be subdivided into three types of mineral assemblage. In general, the Tianheyong basalts are characterized by low silicon (w(SiO2)=4397％4568％), enriched alkali (w(K2O+Na2O)=591％765％), enriched potassium (w(K2O)=204％289％), high contents of titanium (w(TiO2)=218％237％), high magnesium value (Mg＃=6876), and high contents of REE (∑REE=(2466232982)×10-6). The chondrite normalized pattern of REE shows a right inclined smooth line, extreme enrichment of LREE, significant fractionation of LREE and HREE, without obvious anomaly of Eu(δEu=090102) and Ce(δCe=096100); extreme enrichment of incompatible elements, especially with peak value in high field strength elements (HFSE) of Nb, Ta and Th, and OIBlike distribution patterns of multielements; the Tianheyong basalts also have high content of compatible elements: Co((391489)×10-6), Ni ((130257)×10-6) and Cr((138320)×10-6). All of the above characteristics as well as poorcrystallized, abundant peridotite inclusions, few xenocrysts, and elements covariation trend indicate that the Tianheyong basalts were solidified from primitive basaltic magma. Trace elements ratio of Ba/Rb (1235) and variation of alkali metals might indicate the metasomatism of fluids. Petrogenesis simulation indicates that the Tianheyong primitive basaltic magma may have been the product of polybaric melting column with nonmodal partial melting processes of enriched lherzolite, which straddle the boundary of garnetand spinelfacies lherzolite. Magma may have been originated from low partial melting of source rocks(~<5％), which have ~1％ partial melting of garnet lherzolite and 2％5％ partial melting of spinel lherzolite. The comprehensive analysis indicates that the induced mechanism of source rock partial melting may have been the edgedriven convection, which resulted in the Tianheyong basalts having the deepest origin depth compared with that of eastern Jining and Hannuoba basalts.

Key words: petrogenesis; Tianheyong; basalt; primitive magma; polybaric melting column; edgedriven convection

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