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首页> 外文期刊>Contributions to Mineralogy and Petrology >Slab and sediment melting during subduction initiation: granitoid dykes from the mantle section of the Oman ophiolite
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Slab and sediment melting during subduction initiation: granitoid dykes from the mantle section of the Oman ophiolite

机译：俯冲开始时的平板和沉积物融化：阿曼蛇绿岩地幔部分的花岗岩状堤

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摘要

New geochemical data are presented for a suite of tonalites, granodiorites, trondhjemites and granites intrusive into depleted mantle harzburgites of the Oman-UAE ophiolite. A detailed field, petrological and geochemical examination suggests that these 'mantle granitoids' are the product of three processes: (a) the mixing of melts derived from both mafic and metasedimentary sources, (b) interaction with the mantle harzburgite host and (c) the fractional crystallisation of plagioclase, hornblende +/- accessory phases. Geochemical data are used to characterise the identity of the protolith(s) by first screening the data for those samples which have experienced fractional crystallisation during emplacement. The resultant 'reduced' data set has moderately fractionated REE, with small negative Eu anomalies and fractionated primitive mantle-normalised trace element patterns with high concentrations of fluid mobile elements and lower concentrations of HFS elements and with positive peaks for Rb and Pb and negative troughs for Ba, Nb, Sr and Ti. The character of the protolith was quantified using a melting model based upon a MORB-type basalt similar in composition to the Oman Geotimes lavas and a model using the MUQ (MUd from Queensland) global sediment composition (Kamber et al. Geochim Cosmochim Acta 69:1041-1058, 2005) both with an amphibolite/granulite facies mineralogy. The two compositions bracket the mantle granitoid data set with partial melts of the MORB source yielding trace element compositions lower than the granitoids, whereas melts of the MUQ source yield melts with compositions higher than the granitoids. Mixing of the calculated melt compositions indicates that the measured granitoid compositions represent between 10 and 30 % mixing of a metasedimentary melt into the melt of a mafic source. Current petrological, structural and geochronological data suggest a model for the origin of the Oman ophiolite in which it is formed by spreading above a subduction zone. The results of this study support this model in three ways: (a) trace element data show that the granitoids have interacted with the mantle, implying that they originated below the mantle section of the ophiolite and were emplaced into the mantle wedge from below; (b) a most probable setting for the melting of a sediment basalt mix is in the upper part of a subducting slab; (c) the high temperature/shallow melting of the sediment and basaltic slab mix can only have been driven by the high temperatures of the overlying mantle wedge; this is most likely during subduction initiation. Thus, sediment melting during subduction initiation represents an important means of refertilising what is otherwise highly depleted mantle.
机译：提出了新的地球化学数据，涉及一组侵入阿曼-阿联酋蛇绿岩的贫化地幔哈氏岩的同色岩，花岗闪长岩，长生辉石和花岗岩。详细的现场，岩石学和地球化学检查表明，这些“地幔花岗岩”是三个过程的产物：（a）混合来自镁铁质和准沉积物来源的熔体;（b）与地幔Harzburgite主体的相互作用;以及（c）斜长石，角闪石+/-辅助相的分步结晶。通过首先筛选那些在放置过程中经历分步结晶的样品的数据，可使用地球化学数据来表征原石的特性。所得的“减少”数据集具有中等程度的REE，具有较小的Eu负负异常和原始的地幔归一化痕量元素模式，具有较高浓度的流体移动元素和较低浓度的HFS元素，且Rb和Pb的正峰和负槽包括钡，铌，锶和钛。使用基于类似于阿曼Geotimes熔岩成分的MORB型玄武岩的熔融模型和使用MUQ（昆士兰州的MUd）全球沉积物组成的模型对原生岩的特征进行定量（Kamber等人，Geochim Cosmochim Acta 69： 1041-1058，2005）均具有闪石/粒岩相矿物学。这两种成分将地幔花岗石数据集与MORB源的部分熔体合并在一起，从而使痕量元素的成分低于粒体，而MUQ源的熔体所产生的熔体成分高于该类花岗岩。所计算的熔体成分的混合表明，所测量的类花岗岩成分代表了准沉积物熔体与铁镁质源熔体之间的10％至30％的混合。当前的岩石学，结构学和年代学数据为阿曼蛇绿岩的起源提供了一个模型，该模型是通过在俯冲带上方扩散而形成的。这项研究的结果以三种方式支持该模型：（a）痕量元素数据表明，类花岗岩与地幔相互作用，这意味着它们起源于蛇绿岩的地幔剖面之下，并从下方进入到地幔楔中; （b）沉积玄武岩混合物最可能熔化的位置是俯冲板的上部; （c）沉积物和玄武岩板状混合物的高温/浅层融化只能由上覆地幔楔的高温驱动;这很可能是在俯冲过程中开始的。因此，俯冲开始时的沉积物融化代表了使原本高度贫瘠的地幔转轨的重要手段。

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