Characterization of the thermally metamorphosed mantle–crust transition zone of the Neoproterozoic ophiolite at Gebel Mudarjaj, south Eastern Desert, Egypt
Characterization of the thermally metamorphosed mantle–crust transition zone of the Neoproterozoic ophiolite at Gebel Mudarjaj, south Eastern Desert, Egypt
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DOI：
10.1016/j.lithos.2012.02.014
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发表时间：
2012-06
期刊：
影响因子：
3.5
通讯作者：
A. H. Ahmed;M. Gharib;S. Arai
中科院分区：
文献类型：
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作者：
A. H. Ahmed;M. Gharib;S. Arai
在埃及东南部沙漠的Gebel Mudarjaj地区，一套地幔-地壳过渡带(Moho过渡带=MTZ)岩石出露得格外好。受较年轻的花岗岩侵入体的热变质作用，形成了具有特征变质矿物组合的镁铁质-超镁铁角岩。MTZ岩石非常薄(30-50m厚)，主要由基底部蛇纹岩化地幔方辉橄榄岩段上覆的英安岩、榴辉岩、辉长岩和辉石岩组成。基性蛇纹石化方辉橄榄岩和MTZ片麻岩的尖晶石的平均铬#分别为0.76和0.74，与弧形橄榄岩尖晶石的范围一致。与这些MTZ岩平衡的熔体在成分上类似于由高度部分熔融产生的邦尼质岩浆。基性方辉橄榄岩和MTZ二长花岗岩是由19-23%的地幔熔融形成的，其成分与超俯冲带橄榄岩相似。与辉石岩平衡的地幔熔体是在16-17%部分熔融后形成的，随后与下地壳辉长岩反应生成辉石岩。壳幔边界辉石岩团的出现，反映了中、下地壳岩浆房中高压辉石的聚集。在花岗岩侵入之前，原始的MTZ岩石在区域变质、构造破裂和侵位过程中作为解体蛇绿岩的各种碎片被部分或全部水化，形成了含反榴石的蛇纹岩化镁铁质-超镁铁质岩石。由于MTZ岩石的接触变质作用，递进变质组合随后叠加了主要特征。变质矿物组合为：(1)橄榄石+叶蜡石+透闪石+绿泥石±滑石(在基性蛇纹岩中)，(2)橄榄石+顽辉石±绿泥石(在山带英安岩中)，(3)橄榄石+铝尖晶石(蛇绿岩)+绿泥石+磁铁矿±顽辉石(在闪长岩中)。热变质作用的峰期约为650°-700°C，~lt；7kb，相当于上角闪岩相。
A suite of mantle–crust transition zone (Moho transition zone = MTZ) rocks are exceptionally well exposed in Gebel Mudarjaj area, southeastern desert of Egypt. The MTZ rocks were thermally metamorphosed by younger granitic intrusion, forming mafic–ultramafic hornfels with characteristic metamorphic mineral assemblages. The MTZ rocks are remarkably thin (30–50m thick) and are composed mainly of dunites, troctolites, gabbroic rocks and pyroxenite masses overlying a basal serpentinized mantle harzburgite section. The Cr# of spinels of the basal serpentinized harzburgites and the MTZ dunites are on average 0.76 and 0.74, respectively, which is consistent with the range for arc peridotite spinels. The melt in equilibrium with these MTZ rocks is compositionally similar to boninitic magmas produced by high degrees of partial melting. The basal harzburgites and MTZ dunites have been produced by 19–23% mantle melting, and are compositionally similar to supra-subduction zone (SSZ) peridotites. The mantle melt in equilibrium with pyroxenites was formed after 16–17% partial melting, which subsequently reacted with the lower crustal gabbroic rocks to produce pyroxenites. The occurrence of pyroxenite masses at the crust–mantle boundary suggests a medium- to high-pressure accumulation of pyroxenes in mid- to lower crustal magma chambers. The original MTZ rocks were partially or fully hydrated, prior to the granitic intrusion, during the regional metamorphism, tectonic disruption and emplacement as various fragments of a dismembered ophiolite, to form antigorite-bearing serpentinized mafic–ultramafic rocks. Progressive metamorphic assemblages then overprinted the primary features due to the contact metamorphism of the MTZ rocks. The resultant metamorphic mineral assemblages are: (1) olivine+anthophyllite+tremolite±chlorite±talc (in the basal serpentinites), (2) olivine+enstatite±chlorite (in the MTZ dunites), and (3) olivine+aluminous spinel (pleonaste)+chlorite+magnetite±enstatite (in the troctolites). The peak of thermal metamorphism was about 650°–700°C and <7kb, equivalant to the upper amphibolite facies.