Geochemical characteristics of basaltic volcanism within back-arc basins
Geochemical characteristics of basaltic volcanism within back-arc basins
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DOI：
10.1144/gsl.sp.1984.016.01.05
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发表时间：
1984
期刊：
影响因子：
--
通讯作者：
A. Saunders;J. Tarney
中科院分区：
文献类型：
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作者：
A. Saunders;J. Tarney
总结而言，弧后盆地是由类似于大洋中脊的伸展过程形成的。然而，尽管沿主要洋脊喷发的岩浆主要是LIL元素、Ta和Nb亏损的N型MORB，但许多弧后盆地是由N型MORB和岛弧之间的玄武岩或甚至钙碱性玄武岩(即。LIL元素(K、Rb、Ba、Th)相对于HFS元素(Nb、Ta、Zr、Hf、Ti)的富集性。在更大的范围内，玄武岩的成分、盆地的构造背景和邻近俯冲带的成熟度是可能的。因此，形成于马里亚纳俯冲系统早期阶段的帕雷斯韦拉盆地，被与N型MORB难以区分的玄武岩所覆盖，而马里亚纳海槽后期喷发的是N型MORB和具有钙碱性特征的玄武岩，通常在空间上非常接近。钙碱性成分在狭窄的敏感盆地中最发育，如布兰斯菲尔德海峡，那里的延伸部分毗邻成熟的大陆岩浆弧。从N型MORB到钙碱性玄武岩的这种成分范围，只有通过引用提供弧后盆地地壳的地幔物质组成的化学变化才能令人满意地解释。可以认为有两个主要过程：(I)富含LIL的水合流体选择性地污染地幔楔体，可能与来自不断下降的脱水大洋岩石圈的沉积物一起；以及(Ii)玄武岩形成过程中反复熔融(和不相容元素)提取。前者将使弧后玄武岩的地幔源区富含LIL元素，后者将亏损所有不相容元素的源区，但两者的净作用是增加源区的LIL/HFS元素比值。因此，随着俯冲带的成熟，连续的弧后玄武岩的LIL/HFS元素比值将从最初的N型MORB‘背景’值增加到更典型的岛弧玄武岩的比值。该模型对弧后地区的地幔动力学有影响，因为从俯冲板块转移的物质可能会破坏上覆地幔的稳定，当构造条件允许伸展时，可能会导致底辟上升。
Summary Back-arc basins are formed by extensional processes similar to those occurring at mid-ocean ridges. However, whereas the magmas erupted along the major ocean ridges are predominantly LIL element-, Ta- and Nb-depleted N-type MORB, many back-arc basins are floored by basalts transitional between N-type MORB and island arc or even calc-alkaline basalts (viz. enrichment of LIL elements (K, Rb, Ba, Th) relative to HFS elements (Nb, Ta, Zr, Hf, Ti)). On a broad scale, it is possible to relate basalt composition, tectonic setting of the basin, and maturity of the adjacent subduction zone. Thus, the Parece Vela Basin, formed during the earliest stages of the Mariana subduction system, is floored by basalts indistinguishable from N-type MORB, whereas the later Mariana Trough is erupting N-type MORB and basalts with calc-alkaline characteristics, commonly in close spatial proximity. The calc-alkaline component is best developed in narrow, ensialic basins such as Bransfield Strait, where the extension is adjacent to mature, continent-based magmatic arcs. This range of compositions, from N-type MORB to calc-alkaline basalt, can be satisfactorily explained only by invoking chemical variations in the composition of the mantle material supplying the back-arc basin crust. Two major processes may be suggested: (i) selective contamination of the mantle wedge by LIL-enriched hydrous fluids, perhaps together with sediments, derived from the descending, dehydrating oceanic lithosphere; and (ii) repeated melt (and incompatible element) extraction during basalt genesis. The former process will enrich the mantle source of back-arc basalts with LIL elements; the latter will deplete the source in all incompatible elements, but the net effect of both processes is to increase the LIL/HFS element ratio of the source regions. Consequently, as the subduction zone matures, the LIL/HFS element ratio of successive back-arc basalts will be expected to increase, from initial N-type MORB ‘background’ values, to ratios more typical of island-arc basalts. The model has implications for mantle dynamics in back-arc regions, because transfer of material from the subducted slab may destabilize the overlying mantle, potentially leading to diapiric uprise when tectonic conditions permit extension.