Petrogenesis of plagiogranites from the Troodos Ophiolite Complex, Cyprus
Petrogenesis of plagiogranites from the Troodos Ophiolite Complex, Cyprus
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DOI：
10.1007/s00410-019-1569-3
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发表时间：
2019-04
影响因子：
3.5
通讯作者：
C. Marien;J. Hoffmann;C. Garbe-Schönberg;C. Münker
中科院分区：
文献类型：
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作者：
C. Marien;J. Hoffmann;C. Garbe-Schönberg;C. Münker
少量的长英质熔体，通常被称为大洋斜长花岗岩，在辉长岩剖面和洋壳席状岩墙根部区域内以熔体袋或岩墙的形式出现。塞浦路斯特罗多斯蛇绿岩杂岩中的斜长花岗岩是长英质岩石的最佳出露体之一，它们嵌在仰冲大洋岩石圈的完整剖面中。然而，其确切的岩石成因仍然是一个有争议的问题，主要是由于有限的高质量的微量元素和放射性同位素数据。以前提出的特罗多斯斜长花岗岩模型包括低压脱水熔融和分离结晶在更深层次的洋壳。为了评估这两种模式，海洋斜长花岗岩从Troodos蛇绿岩复杂的分析，在这项研究中，他们的主要和微量元素，并为第一次也为Hf-Nd-Sr同位素组成。微量元素测量还首次包括高场强元素（HFSE）丰度的高精度测量，现在允许更好地解开特罗多斯斜长花岗岩的岩石成因及其可能的地幔来源。在一般情况下，Troodos斜长花岗岩表现出一个狭窄的Nb/Ta和Zr/Hf的洋中脊玄武岩（MORB）的组成重叠的范围。与早期的研究一致，可以确定三个组成组：两组由分离结晶或结合分离结晶和围岩同化形成，一组来自轻微蚀变的洋壳的部分熔融。大多数的Troodos斜长花岗岩（主组）的产品广泛的分离结晶环境弧拉斑玄武岩镁铁质熔体。第二组斜长花岗岩（Spilia群）是玻安岩前体熔体的分离结晶和弧拉斑玄武岩地壳物质的同化作用产生的。变量HFSE浓度和诊断Hf-Nd同位素的签名，是唯一的两套房允许区分两个父母的熔体和分馏过程。一小群斜长花岗岩（Zoopigi组）被解释为来自活动岩浆房透镜体（AML）导电层的部分熔融。Nb/Ta、Zr/Hf和轻稀土元素（LREE）的高富集支持了这一模式。总的来说，我们的数据表明，低压分步结晶（也与围岩同化相结合）可能是控制塞浦路斯长英质岩石形成的主要过程，而脱水熔融似乎不太重要。如果比较太古代英云闪长岩-奥长花岗闪长岩套（TTG），斜长花岗岩的成分从塞浦路斯镜浅层次的过程在薄洋壳，这是说明了他们的窄，MORB的范围内的HFSE比和重稀土元素（HREE）的明显富集，区分他们从太古代TTG。
Small volumes of felsic melt, commonly known as oceanic plagiogranites, appear as melt pockets or dikes within the gabbroic section and the sheeted dikes root zone of the oceanic crust. Plagiogranites from the Troodos Ophiolite Complex on Cyprus are among the best exposures of felsic rocks that are embedded in a complete section of obducted oceanic lithosphere. Nevertheless, their exact petrogenesis is still a matter of debate, largely due to limited high-quality trace element and radiogenic isotope data. Previously proposed models for Troodos plagiogranites have included both low-pressure dehydration melting and fractional crystallisation at deeper levels of the oceanic crust. To evaluate both models, oceanic plagiogranites from the Troodos Ophiolite Complex were analysed in this study for their major and trace elements, and for the first time also for Hf–Nd–Sr isotope compositions. The trace element measurements also include for the first time high-precision measurements of high-field-strength element (HFSE) abundances that now permit to better unravel the petrogenesis of the Troodos plagiogranites and their possible mantle sources. In general, the Troodos plagiogranites exhibit a narrow range of Nb/Ta and Zr/Hf that overlap the compositions of mid-ocean ridge basalts (MORB). In line with earlier studies, three compositional groups can be identified: two groups formed by either fractional crystallisation or combined fractional crystallisation and wall rock assimilation, and one group derived from partial melting of slightly altered oceanic crust. The majority of the Troodos plagiogranites (Main Group) are the product of extensive fractional crystallisation of ambient arc-tholeiitic mafic melts. A second group of plagiogranites (Spilia Group) is generated by fractional crystallisation of boninitic precursor melts and the assimilation of arc-tholeiitic crustal material. Variable HFSE concentrations and diagnostic Hf–Nd isotope signatures that are unique to both suites allow discriminating between the two parental melts and fractionation processes. A small group of plagiogranites (Zoopigi Group) is interpreted to derive from partial melting in the conductive layer of active magma chamber lenses (AML). Elevated Nb/Ta, Zr/Hf, and light rare-earth element (LREE) enrichments in these rocks are in support of this model. Collectively, our data suggest that low-pressure fractional crystallisation (also in combination with assimilation of wall rocks) might be the predominant process controlling the formation of felsic rocks on Cyprus, whereas dehydration melting appears to be less important. If compared to Archean tonalitic–trondhjemitic–granodioritic suites (TTGs), compositions of plagiogranites from Cyprus mirror shallow-level processes in thin oceanic crust, which is illustrated by their narrow, MORB-like range of HFSE ratios and their distinct enrichment in heavy rare-earth elements (HREE) that distinguishes them from the Archean TTGs.