Anatomy of a frozen axial melt lens from a fast-spreading paleo-ridge (Wadi Gideah, Oman ophiolite)
Anatomy of a frozen axial melt lens from a fast-spreading paleo-ridge (Wadi Gideah, Oman ophiolite)
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来自快速扩张的古山脊的冻结轴向熔融透镜体的解剖（Wadi Gideah，阿曼蛇绿岩）
DOI：
10.1016/j.lithos.2016.11.022
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发表时间：
2017
期刊：
影响因子：
3.5
通讯作者：
Dietrich
中科院分区：
文献类型：
--
作者：
Mueller;Koepke;Garbe-Schönberg;Dietrich
在快速扩张的洋中脊，夹在席状岩墙部分和最上面的辉长岩之间的轴向熔融透镜体（AML）被认为是地壳形成的主要岩浆源。在这里，我们提出了我们的研究结果，从一个单一的露头的冻结AML在阿曼苏丹国的Samail蛇绿岩，它提出了一整套不同的岩性和复杂的切割关系：varitextured辉长岩与文物的原始嵌晶单斜辉石是侵入大量的石英闪长岩和英云闪长岩承载文物同化席状岩脉，这反过来又被切断的奥长花岗岩岩脉。整个区域被边缘冰冷的玄武岩岩墙切割。变结构辉长岩的地球化学演化趋势，包括一些石英闪长岩和英云闪长岩，可以最好地模拟实验MORB母体熔体成分的分步结晶，水斑片状变构辉长岩包含域的原始嵌晶单斜辉石和演变的粒状网络代表原位结晶的记录。一些石英闪长岩，往往与席状岩脉的捕虏体和异常高的Al 2 O3含量，显示散装微量元素模式更符合雅阁实验部分熔融岩脉材料产生的熔体。高度演化的横切奥长岩脉显示出特征性的微量元素模式，这意味着在较低的水活动下，席状岩脉部分熔融形成，这由相对较低的Al 2 O3含量表明。与冷却边缘横切所有其他岩性的晚期玄武岩岩墙显示出相对亏损的地球化学特征，具有明显的负Nb-Ta异常，这意味着与岩浆阿曼古脊活动（V2）的第二阶段的成因关系。结合岩石学/地球化学趋势的字段关系揭示了多个序列的MORB型岩浆冷却（导致结晶分离）和再加热（产生部分熔融）在这个特殊的地平线的形成过程中，这些都是最好的解释垂直AML迁移交替循环。由于调查的露头显示出许多独特的岩性和岩相学特征，是众所周知的，从最上面的辉长岩钻在1256站点的综合大洋钻探计划（IODP）在赤道东太平洋，我们的结果的基础上，在现场的三维观测有助于阐明从1-D钻芯获得的地质观测。
At fast-spreading mid-ocean ridges, axial melt lenses (AMLs) sandwiched between the sheeted dyke section and the uppermost gabbros are assumed to be the major magma source of crust formation. Here, we present our results from a field study based on a single outcrop of a frozen AML in the Samail ophiolite in the Sultanate of Oman which presents a whole suite of different lithologies and complex cutting relationships: varitextured gabbro with relics of primitive poikilitic clinopyroxene is intruded by massive quartz diorites and tonalites bearing relics of assimilated sheeted dykes, which in turn are cut by trondhjemite dykes. The whole is cut by basaltic dykes with chilled margins. The geochemical evolutionary trend of the varitextured gabbros, including some of the quartz diorites and tonalites, can be best modelled by fractional crystallisation of an experimental MORB parental melt composition containing 0.4 to 0.8 wt.% H2O. Patchy varitextured gabbros containing domains of primitive poikilitic clinopyroxene and evolved granular networks represent the record of in situ crystallisation. Some quartz diorites, often with xenoliths of sheeted dykes and exceptionally high Al2O3contents, show a bulk trace element pattern more in accord with melts generated by experimental partial melting of dyke material. Highly evolved, crosscutting trondhjemite dykes show characteristic trace element patterns implying a formation by partial melting of sheeted dykes under lower water activity which is indicated by relatively low Al2O3contents. The late basaltic dykes with chilled margins crosscutting all other lithologies show a relatively depleted geochemical character with pronounced negative Nb-Ta anomalies implying a genetic relationship to the second phase of magmatic Oman paleo-ridge activity (V2). The field relationships in combination with the petrological/geochemical trends reveal multiple sequences of MORB-type magma cooling (resulting in fractional crystallisation) and re-heating (producing partial melting) during the formation of this special horizon; these are best explained by alternating cycles of vertical AML migration. Since the investigated outcrop shows many characteristic lithological and petrographic features that are well-known from the uppermost gabbros drilled at Site 1256 by the Integrated Ocean Drilling Program (IODP) in the equatorial Eastern Pacific, our results based on 3-D observation in the field help to elucidate the geological observations obtained from the 1-D drill core.
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