Detailed volcanic geology of the MARNOK area, Mid-Atlantic Ridge north of Kane transform
Detailed volcanic geology of the MARNOK area, Mid-Atlantic Ridge north of Kane transform
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凯恩变换以北的大西洋中脊 MARNOK 地区的详细火山地质
DOI：
10.1144/gsl.sp.1996.118.01.05
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发表时间：
1996
期刊：
影响因子：
--
通讯作者：
P. Kempton
中科院分区：
文献类型：
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作者：
K. Lawson;R. Searle;J. Pearce;P. Browning;P. Kempton
本文根据高分辨率深拖曳声纳(TOBI)测量、近底部摄影导线和28组精确定位的挖泥船样品的地球化学分析，对中大西洋海脊中部谷底的火山地质进行了详细的研究，这些扩张段位于凯恩变换断层以北的两个相邻但截然不同的扩张段。这使得我们能够评估在缓慢扩张的山脊上小(段间)和大(变换)边界在岩浆过程中的相对重要性。我们发现，与二级分段的表现形式相比，这种转换的岩石学效应(即所谓的转换断层效应)是次要的。南部的“窄门”类型的地段有一个欠发达的新火山带，向其水深中心逐渐变小，在那里持续的断层作用会导致快速的解体。平顶海山优先位于节段的末端，特别是在非变换偏移距附近。沿这一段的长度，分馏程度有一个惊人的变化：原始熔岩在中心取样，而更多的分馏玄武岩，显示了更大范围的母体成分，在末端形成离散的火山机构。相比之下，北段有一个更宽的内谷，同岩浆断层沿着其健壮的轴向火山山脊的顶端延伸到15公里。沿段向轴向火山山脊等深峰方向增加的趋势与南段相似，但不如南段明显。由主量元素地球化学确定的熔体沿岩段的整体熔融程度没有变化，因此熔体迁移优于聚焦地幔上涌，这是地壳厚度变化的主要原因，这些变化定义了二级分段。产生放射性的同位素只显示出很小的变化，在脊段内部或之间没有出现系统的模式。同位素不规则性与不相容元素的变化无关，表明不相容元素的变化是动态熔融作用的结果，而不是长寿命源区不均质性的结果。建立了火山学、构造样式和地球化学趋势与岩浆供给和喷发的幕式之间的关系模型。
Abstract We present the results of a detailed study of the volcanic geology of the median valley floor of the Mid-Atlantic Ridge in two adjacent but contrasting spreading segments immediately north of the Kane transform fault, based on a high-resolution deep-towed sidescan sonar (TOBI) survey, near-bottom photographic traverses, and geochemical analyses from 28 precisely sited sets of dredge samples. This has allowed us to assess the relative importance of small (intersegment) and large (transform) boundaries on magmatic processes at slow spreading ridges. We find that the petrological effect of the transform (so-called ‘transform fault effect’) is minor compared to the manifestation of the second-order segmentation. The southern, ‘narrowgate’-type segment has a poorly developed neovolcanic zone, which tapers towards its bathymetric centre where continuous faulting causes rapid dismemberment. Flat-topped seamounts are preferentially located at the ends of the segment, particularly near the non-transform offset. There is a striking variation in the degree of fractionation along the length of this segment: primitive lavas were sampled at the centre, while more fractionated basalts, showing a greater range of parental compositions, form discrete volcanic edifices at the ends. In contrast, the northern segment has a wider inner valley, and syn-magmatic faults extend up to 15 km along the crest of its robust axial volcanic ridge. The along-segment trend of increasing MgO towards the bathymetric crest of the axial volcanic ridge is similar to, but less well-defined than, that for the southern segment. There is no variation in the bulk degree of melting along the segments as determined from the major element geochemistry; hence, melt migration is favoured over focused mantle upwelling as the main cause of the crustal thickness variations which define second-order segmentation. Radigenic isotopes show only very small variations with no systematic pattern emerging within or between the ridge segments. The isotopic irregularities are not related to the variations in incompatible elements, implying that the latter result from dynamic melting processes rather than from long-lived source heterogeneity. A model is developed which relates the volcanology, tectonic style and the geochemical trends to the episodicity of magma supply and eruption.