Petrology and geochemistry of the crust–mantle boundary in a nascent arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific
Petrology and geochemistry of the crust–mantle boundary in a nascent arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific
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DOI：
10.1093/petrology/egt030
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发表时间：
2013-09
影响因子：
3.9
通讯作者：
Cassian Pirard;J. Hermann;H. O’Neill
中科院分区：
文献类型：
--
作者：
Cassian Pirard;J. Hermann;H. O’Neill
南太平洋新喀里多尼亚的南蛇绿岩地块是地球上最大的暴露超镁质岩体之一。蛇绿岩由超贫构造岩辉锌矿的地幔段组成，上覆一大片泥质带，由过渡带与地块顶部辉长岩隔开。地幔剖面的地层剖面显示出复杂的地球化学和岩石学变化。哈尔茨伯尔岩具有高度部分熔融的特征，记录了从地幔发掘到超俯冲带环境的复杂演化过程。橄榄石和尖晶石组成表明，哈尔茨布尔土是在5-10 kbar的熔体萃取后的残余。下泥质带类似于辉石岩中的替代泥质通道，是由辉石消耗形成的。上白云岩带具有原始基性熔体结晶的积云橄榄石的化学和结构特征。上部均质岩带演化为辉石岩堆积物的过渡带，辉石岩被辉长岩侵入。结晶序列和矿物组成表明，辉石岩和辉长岩是在~ 1250°C和2-4 kbar的水氧化原始玄武岩岩浆中形成的。它们的地球化学特征表明，母熔体介于博宁质和原始弧岩浆之间，具有弧前玄武岩特征。因此，南美地块代表了一个新生弧中的地壳-地幔剖面。残余地幔岩和地壳堆积之间的隐蔽性过渡突出了用地震方法估计弧壳厚度和平均成分的困难。
The Massif du Sud ophiolite, New Caledonia, SW Pacific, is one of the largest exposed ultramafic bodies on Earth. The ophiolite consists of a mantle section of ultra-depleted tectonite harzburgite, overlain by a large dunite zone, which is separated by a transition zone from the gabbros at the top of the massif. Profiles through the stratigraphy of the mantle section show complex geochemical and petrological variations. The harzburgites are characterized by a high degree of partial melting and document a complex evolution from mantle exhumation towards a supra-subduction zone environment. Olivine and spinel compositions suggest that the harzburgites are residual after boninite-like melt extraction at 5–10 kbar. The lower dunite zone is analogous to the replacive dunite channels in the harzburgite, in that it is formed by pyroxene consumption. In contrast, the upper dunite zone has chemical and textural characteristics of cumulus olivine crystallized from primitive mafic melts. The upper dunite zone grades into a transition zone with pyroxenite cumulates that are intruded by gabbro sills. The crystallization sequence and mineral compositions indicate that pyroxenites and gabbros formed from hydrous, oxidized primitive basaltic magmas at ∼1250°C and 2–4 kbar. Their geochemistry indicates that these parental melts are transitional between boninites and primitive arc magmas and carry a fore-arc basalt signature. The Massif du Sud therefore represents a crust–mantle section in a nascent arc. The cryptic transition between residual mantle rocks and crustal cumulates highlights the difficulty of estimating the thickness and average composition of an arc crust by seismic methods.