西藏班公湖-怒江缝合带中段A-型花岗岩的岩浆源区与板片断离

被引:28
作者
曲晓明
辛洪波
杜德道
陈华
机构
[1] 中国地质科学院矿产资源研究所
关键词
A-型花岗岩; Nd、Sr、Pb同位素; 岩浆源区; 板片断离; 班公湖-怒江缝合带; 青藏高原;
D O I
暂无
中图分类号
P588.121 [];
学科分类号
070901 ;
摘要
近年来地质调查在班公湖-怒江缝合带中段发现了一批A-型花岗岩,岩体呈大小几百平方米的岩株产出,岩性上分为黑云母花岗岩和花岗闪长斑岩两种,侵入白垩系地层中。全岩元素地球化学分析显示这些A-型花岗岩相对富硅,SiO2含量在68.62%~75.36%之间,全碱(K2O+Na2O=8.03%~9.37%)和全铁(Fe2O3T=0.86%~5.39%)含量偏高,Al2O3(12.76%~15.54%)偏低,显示弱过铝质和亚铝质特征。微量元素中大离子亲石元素(LILE)Rb、Th、U、K、Pb明显富集,Ba、Sr明显亏损;同样在高场强元素(HFSE)中Nb、Ta、Ti亏损明显,但Zr、Hf相对富集,这些与岛弧型花岗岩都是明显不同的,显示出了A-型花岗岩的特征。岩体稀土元素含量总体较高(∑REE=122.37×10-6~291.19×10-6,平均为201.31×10-6),相对富集轻稀土元素(LREE/HREE=4.89~9.58,平均为5.93),负Eu异常明显(δEu=0.14~0.54,平均为0.34),造成分配曲线呈向右缓倾的V-型分布型式。Nd、Sr、Pb同位素分析显示班公湖-怒江缝合带中段的这些A-型花岗岩富集放射成因同位素,其中87Sr/86Sr、87Rb/86Sr及Isr比值分别在0.719842~0.786395、6.7171~48.8063和0.706900~0.710378之间;143Nb/144Nb和147Sm/144Nd比值分别在0.512123~0.512392和0.0853~0.2847之间,εNd为较大的负值(在-3.37~-10.34之间),显示出地壳组分的重要影响。206Pb/204Pb、207Pb/204Pb和208Pb/204Pb比值分别在18.703~19.070、15.680~15.732、和39.121~39.576之间,在Zartman等(1981)的Pb构造模式图上位于上地壳线和造山带演化线之上,显示出异常高的富集组分。分析表明班公湖-怒江缝合带中段的A-型花岗岩是岩石圈地幔在洋壳俯冲过程中受到沉积物熔体的交代富集,之后早白垩世晚期(110Ma±)在碰撞后伸展阶段由于俯冲的洋壳板层断离形成了"板片窗",软流圈物质借此上涌,引起富集的岩石圈地幔(接近EMⅡ)发生部分熔融形成的。
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页码:759 / 772
页数:14
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