Magmatic accretion and thermal convection at the sheeted dike complex-gabbro boundary in superfast spreading crust, ODP Hole 1256D
Magmatic accretion and thermal convection at the sheeted dike complex-gabbro boundary in superfast spreading crust, ODP Hole 1256D
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超快扩张地壳片状岩脉杂岩-辉长岩边界处的岩浆增生和热对流，ODP 孔 1256D
DOI：
10.1016/j.tecto.2015.08.023
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发表时间：
2015
期刊：
影响因子：
2.9
通讯作者：
Trela J
中科院分区：
文献类型：
--
作者：
Trela J
在快速扩张的中心形成新的海洋深成地壳的岩浆过程仍不清楚。位于东太平洋隆起东侧的海洋钻探计划孔 1256D 的 312 号勘探队几乎没有钻穿最上面的辉长岩，但却提供了无与伦比的机会来限制深层岩浆过程和辉长岩层的构造模式。辉长岩 1 和辉长岩 2 分别厚 52 米和 24 米，侵入蚀变和烘烤的片状岩脉复合体。这两种辉长岩都缺乏宏观结构，迄今为止都被认为是各向同性的。在这里，我们对 33 个非方位角定向样品中的斜长石斑晶进行数字 3D 结构分析。与之前的观察结果相反，这两种辉长岩表现出明显且一致的形状偏好方向。两种辉长岩均表现出岩浆、平衡的微结构，以及少量的岩浆下微结构。计算得出的瑞利数范围在1015到1018之间，证明辉长岩岩浆最初经历了湍流岩浆流。岩浆叶理倾角适度，辉长岩 1 的平均倾角为 48°，辉长岩 2 的平均倾角为 61°。辉长岩 1 的岩浆线倾角在 1 至 44°（平均 28°）之间，而辉长岩 2 的倾角范围为 6 至 69°（平均 41°）。这些倾斜的岩浆结构，再加上辉长岩与其主岩之间的倾斜岩浆接触，表明这两个辉长岩体不是水平的岩床，而更可能是倾斜地切穿片状岩脉复合体的倾斜侵入体。1256D 孔周围的地震反射数据显示，倾斜的反射体与被动边缘的辉长岩碟形侵入体产生的反射体类似。根据倾斜组构、倾斜接触面和区域地震资料，我们认为快速扩张中心最上面的辉长岩侵入体呈碟状。这种新的入侵形式以前从未在海洋环境中报道过，可能构成海洋增生的重要组成部分。
The magmatic processes responsible for accretion of new oceanic plutonic crust at fast-spreading centers remain unclear. Expedition 312, at Ocean Drilling Program Hole 1256D, on the eastern flank of the East Pacific Rise, barely drilled through the uppermost gabbros but offers unparalleled opportunities to constrain deep magmatic processes and the mode of construction of the gabbroic layer.Gabbro 1 and gabbro 2, respectively 52 and 24 m-thick, were intruded in an altered and baked sheeted dike complex. Both gabbros lack a macroscopic fabric and were until now considered isotropic. Here, we use digital 3-D fabric analysis of plagioclase phenocrysts in 33 non-azimuthally oriented samples. In contrast with previous observations, the two gabbros display a distinct and consistent shape-preferred orientation. Both gabbros exhibit magmatic, equilibrated microstructures, with minor submagmatic microstructures. The calculated Rayleigh numbers range between 1015and 1018, proving that the gabbroic magma was initially undergoing turbulent magmatic flow. Magmatic foliations dip moderately, with a mean of 48° in gabbro 1 and 61° in gabbro 2. Magmatic lineations plunge in gabbro 1 between 1 and 44° (mean 28°), while in Gabbro 2, plunges range from 6 to 69° (mean 41°). These inclined magmatic structures, combined with the inclined magmatic contacts between the gabbros and their host-rock, show that these two gabbro bodies, instead of being horizontal sills, more likely are inclined intrusions that cut obliquely through the sheeted dike complex.Seismic reflection data around Hole 1256D shows inclined reflectors similar to those produced by gabbroic saucer-shaped intrusions in passive margins. On the basis of the inclined fabrics, inclined contacts and the regional seismic data, we propose that the uppermost gabbroic intrusions at fast-spreading centers form with a saucer-shape. This new shape of intrusion, never reported before in the oceanic environment, may constitute a significant component of oceanic accretion.