Geochemical Characteristics of Garnet-Amphibolite from the Pre-Pilot Hole of the Chinese Continental Scientific Drilling (CCSD-PP2) and Its Geological Significance
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摘要: 预先导孔(CCSD-PP2) 位于苏鲁高压-超高压变质地体南部的北苏鲁超高压变质表壳剪切叠覆岩片中, 岩心主要由榴闪岩(包括角闪岩)、花岗片麻岩、副片麻岩和榴辉岩组成.镜下鉴定和激光拉曼测试结果表明预先导孔(CCSD-PP2) 第一段榴闪岩的主要矿物为角闪石、石榴石、黑云母和钠长石, 是榴辉岩退变质作用的产物.地球化学数据显示榴闪岩的化学成分与正常型榴辉岩相对应, 稀土元素组成及配分模式与主孔榴辉岩基本相似, 但微量元素特征及在地球化学判别图解上存在一定差异, 说明该段榴闪岩与主孔榴辉岩的原岩虽然都是形成于板内环境玄武岩类, 但它们并不完全相同, 其差异可能是由原岩性质的多样性及榴辉岩退变质过程中流体组分的加入引起的.榴闪岩及其下部花岗片麻岩内锆石中柯石英、绿辉石等矿物包裹体的发现及岩石学和地球化学特征等与主孔花岗片麻岩相似的性质, 以及在接触面附近榴闪岩中的SiO2含量具有逐渐增高的趋势等, 说明榴闪岩与花岗片麻岩曾一起俯冲到地下深处, 并在折返过程中经历了强烈的退变质作用, 它们之间可能存在一定程度的物质交换.
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关键词:
- 苏鲁高压-超高压变质地体 /
- 预先导孔(CCSD-PP2) /
- 榴闪岩 /
- 退变质作用
Abstract: The pre-pilot hole of the Chinese Continental Scientific Drilling (CCSD-PP2) is located at the UHP metamorphic supracrustal slice of northern Sulu. The rock types in CCSD-PP2 are garnet-amphibolite (amphibolite), orthogneisses, paragneisses and eclogite. The main mineral assemblage of the garnet-amphibolite from the first part in CCSD-PP2 is amphibole, garnet, biotite and albite, which resulted from retrograde metamorphism. The geochemical data show the chemical compositions of the garnet-amphibolite are consistent with so-called normal eclogite, the compositions and partition pattern of REE from garnet-amphibolote are similar to eclogite from the main hole, and there are some differences in trace element and geochemical discriminating figures between the garnet-amphibolite from CCSD-PP2 and eclogite from the main hole, which demonstrates that their protoliths are within plate basalts, and the differences are caused by the characteristics of the protolith and fluid introduction during eclogite retrograde metamorphism. Coesite and omphacite inclusions in zircons were discovered both from garnet-amphibolite and orthogneiss, closely below the garnet-amphibolite. There is similar petrological and geochemical characteristics between PP2 and main hole orthogneiss. The SiO2 content in garnet-amphibolite increases gradually toward the contacts with orthogneisses. All the lines of evidence show that the garnet-amphibolite and orthogneisses experienced subduction into the upper mantle and strong retrogressive metamorphism during exhumation, and there were compositional exchanges, to a certain extent, between them. -
图 1 苏鲁高压-超高压变质地体构造图(许志琴等, 2003修编)
1.中生代花岗岩; 2.逆冲型韧性剪切带; 3.走滑断层; 4.大型走滑断层; 5.不整合; 6.CCSD-PP2位置; E.古近系; K.白垩系; O.奥陶系; ⋲.寒武系; Pt3.晚元古代; Ⅰ.南苏鲁高压-低温变质岩剪切构造叠覆岩片; Ⅱ.南苏鲁高压-中温变质剪切叠覆构造岩片; Ⅲ.北苏鲁超高压变质表壳岩构造叠覆岩片; Ⅳ.北苏鲁超高压花岗质变质岩剪切叠覆岩片; DF1-DF10.韧性剪切带编号
Fig. 1. Tectonic sketch map of the south Sulu area
图 3 榴闪岩中主要矿物特征及其转换关系
Amp.角闪石; Grt.石榴石; Bi.黑云母; Alb.钠长石; Rut.金红石: Ilm.钛铁矿; Mt.磁铁矿; Tit.榍石; Phen.多硅白云母
Fig. 3. Features of main minerals in garnet-amphibolite and its exchange relation
图 5 稀土元素球粒陨石配分模式和微量元素MORB蛛网图
a.榴闪岩稀土配分模式; b.榴闪岩微量元素蛛网图; c.花岗片麻岩稀土配分模式; d.花岗片麻岩微量元素蛛网图
Fig. 5. Chrondrite-normalized REE patterns and MORB-normalized trace element spidergrams
图 6 榴闪岩和主孔榴辉岩地球化学成分判别图解
Fig. 6. Geochemical discriminating figures of garnet-amphibolite in CCSD-PP2 and eclogite in main hole
表 1 常量元素(%)、稀土元素(10-6) 和微量元素(10-6) 化学分析结果
Table 1. Whole rock chemical compositions of the garnet-amphibolite and orthogneisses
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