The Northern Samail Ophiolite: An Oxygen isotope, microprobe, and field study
The Northern Samail Ophiolite: An Oxygen isotope, microprobe, and field study
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DOI：
10.1029/91jb02743
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发表时间：
1992-05
影响因子：
--
通讯作者：
D. Stakes;H. Taylor
中科院分区：
文献类型：
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作者：
D. Stakes;H. Taylor
介绍了阿曼北部萨迈勒蛇绿岩100×20千米范围内采集的228个全岩和矿物样品的地质、岩石学和氧同位素数据。这些样品大多来自三个详细的剖面，通过枕状熔岩、席状岩脉和白垩纪洋壳横向不均匀碎片的层状辉长岩，一直延伸到岩石莫霍面和穿过岩石莫霍面。这些剖面包括一系列的岩石学和构造样式，每个剖面相对于彼此和大洋中脊玄武岩表现出独特的^(18)/^(16)O变化，这是蛇绿岩中普遍存在的海水-热液相互作用的结果，这种作用的强度沿蛇绿岩的走向而变化。总体而言，层状辉长岩中的^(18)O亏损，大部分席状岩脉和枕状熔岩中的^(18)O富集，这与以前在蛇绿岩南部(伊布拉地区)观察到的结果相似，在那里，辉长岩剖面中的^(18)O亏损与洋壳较浅部分的^(18)O富集量相平衡。瓦迪希尔蒂剖面被选为相对完整的地壳的一个例子，它与伊布拉的不同之处在于辉长岩中的δ^(18)O更均匀，略高(+5.4%至+6.3%)，以及含有更多含水蚀变矿物(角闪石、绿帘石、绿泥石和斜长石)。Wadi Kanut-Shafan和Wadi Rajmi剖面的剖面要复杂得多，揭示了离轴侵入和地壳深部剪切的影响。沙凡-卡努特地区斜长花岗岩-辉绿岩侵入岩在蛇绿岩上叠加了局部的热液光环带，明显表现在~(18)O高度亏损的岩脉、石英硫化物脉体、浅层岩石中丰富的绿帘石、方铅矿和绿泥石，以及深部辉长岩的低温水合蚀变；后者使全岩δ^(18)O(+6.2~+6.9)总体增加。这种晚期侵入体遍布萨迈尔蛇绿岩的北半部。Wadi Rajmi地区可能是一个化石转换或扩张的裂谷，代表了三个剖面中最复杂的；它还包含最丰富的高度变形和热液蚀变的岩石，以及迄今在任何蛇绿岩中发现的最深和最大的^(18)O贫化带(局部δ^(18)O<；+2.0)。大量高温热液流体的通道是由现在被低^(18)O辉长岩和低^(18)O岩脉占据的裂缝提供的。区域样品和穿过蛇绿岩的不同断面的物质平衡估计给出了地壳整体δ^(18)O平均值(+5.9至+6.3)，如果将地壳的垂直和横向不均质性合并到一个三维模型中，这些平均值在采样误差范围内与平均MORB值约为+5.8几乎相同。这支持并放大了早期工作人员的结论，即只要考虑到所有海底岩浆/热液作用的累积效应(空间和时间)，海水的δ^(18)O就受到与洋壳的热液相互作用的缓冲和控制。整体地壳平均δ^(18)O相对于MORb的轻微不平衡富集度，可由基底辉长岩中普遍存在的晚期低温蚀变事件的矿物学和同位素证据解释；这些影响在岩石莫霍面附近显著，并可能表明在遮挡板片拆离期间或之后与低温水流体的交换。
Geological, petrological, and oxygen isotopic data are presented for 228 whole rock and mineral samples collected from a 100×20 km area of the northern Samail ophiolite in Oman. Most of these samples are from three detailed profiles through the pillow lavas, sheeted dikes, and layered gabbros of this laterally heterogeneous fragment of Cretaceous oceanic crust, down to and across the petrologic Moho. The profiles encompass a range of petrologic and tectonic styles, and each profile exhibits distinctive ^(18)/^(16)O variations compared to one another and to mid-ocean ridge basalts, as a result of pervasive seawater-hydrothermal interaction that varied in intensity along strike in the ophiolite. In general, ^(18)O depletions are observed in the layered gabbros and ^(18)O enrichments in most of the sheeted dikes and pillow lavas, similar to results previously observed in the southern part of the ophiolite (Ibra area), where ^(18)O depletions within the gabbroic section are quantitatively balanced by ^(18)O enrichments in the shallower parts of the oceanic crust. The Wadi Hilti profile, selected as an example of relatively intact crust, differs from Ibra in having more uniform and slightly higher δ^(18)O in the gabbros (+5.4 to +6.3), as well as in containing more hydrous alteration minerals (amphibole, epidote, chlorite, and prehnite). The profiles in the Wadi Kanut-Shafan and Wadi Rajmi sections are much more complex and reveal the impact of off-axis intrusions and deep crustal shearing. Plagiogranite-wehrlite intrusions in the Shafan-Kanut area superimposed a local hydrothermal aureole on the ophiolite, evident in dikes highly depleted in ^(18)O, quartz-sulfide veins, abundant epidote, thullite, and chlorite in shallower rocks, and low-temperature hydrous alteration of deeper gabbroic rocks; the latter produced an overall increase in whole rock δ^(18)O (+6.2 to +6.9). Such late stage intrusions are found throughout the northern half of the Samail ophiolite. The Wadi Rajmi area, which is a possible fossil transform or propagating rift, represents the most complex of the three profiles; it also contains the most abundant highly deformed and hydrothermally altered rocks, together with the deepest and largest zone of ^(18)O depletion yet found in any ophiolite (locally δ^(18)O < +2.0). Conduits for large volumes of high-temperature hydrothermal fluids were provided by fractures now occupied by low-^(18)O gabbro pegmatites and low-^(18)O dikes. Material balance estimates for the regional samples and from the various transects through the ophiolite give crustal bulk δ^(18)O averages (+5.9 to +6.3) that are, within sampling error, almost identical to the average MORB basalt value of about +5.8, if both vertical and lateral crustal heterogeneities are integrated into a three-dimensional model. This supports and amplifies the conclusion of earlier workers that the δ^(18)O of seawater is buffered and controlled by hydrothermal interaction with oceanic crust, as long as the cumulative effects (both spatial and temporal) of all seafloor magmatic/hydrothermal processes are considered. The very slight out-of-balance enrichment of the integrated crustal average δ^(18)O compared to MORB may be explained by the ubiquitous mineralogical and isotopic evidence for a late, low-temperature alteration event in the basal gabbros; these effects are prominent in the vicinity of the petrologic Moho and may indicate exchange with low-temperature aqueous fluids during or after detachment of the obducted slab.