How the delamination and detachment of lower crust can influence basaltic magmatism
How the delamination and detachment of lower crust can influence basaltic magmatism
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DOI：
10.1016/j.earscirev.2005.03.004
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发表时间：
2005-09
影响因子：
12.1
通讯作者：
M. Lustrino
中科院分区：
文献类型：
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作者：
M. Lustrino
地球的岩石圈可以使玄武岩岩浆活动沿着预先存在的薄弱地带或不连续面集中。然而，除了对俯冲构造环境中岩浆地球化学的影响（与俯冲板块脱水有关的地幔楔交代作用），岩石圈作为板内（大洋和大陆）、大陆边缘和洋中脊岩浆活动的岩浆源的作用还没有完全了解。在许多情况下，板内岩浆活动被解释为深部热异常（地幔柱）的存在，其起源被放置在上地幔-下地幔过渡带附近（660 km不连续面）或更深处，靠近地幔-核边界（2900 km）。此外，在许多大陆溢流玄武岩省（主要起源于克拉通边缘）的地幔柱的积极作用已被调用来解释高熔体生产力。在这些情况下，没有积极的作用，熔体生产已归因于岩石圈地幔。软流圈甚至更深地幔熔体的潜在污染通常被认为是岩石圈（地壳和地幔）对玄武岩成因的唯一影响。在其他情况下，岩石圈地幔的积极作用已被提出：与地幔柱的存在有关的热异常将触发岩石圈地幔的部分熔融。目前还没有明确的地球化学示踪剂反映岩石圈和上/下地幔作为岩浆源区的相对作用。本文提出了岩石圈的另一个作用。本文提出的新模型是基于下地壳和岩石圈地幔通过拆沉和拆离再循环的作用。这一过程至少可以解释在大体积和小体积火成岩省以及大洋中脊玄武岩中发现的玄武质岩石的某些地球化学特性。陆-陆变质作用导致下陆壳的密度增加（最高可达3.8g/cm ~ 3），变质组合（玄武岩→角闪岩→石榴石单斜辉石岩/榴辉岩）中石榴石的出现，导致岩石圈龙骨（下地壳+岩石圈地幔）的重力不稳定。这可能会脱离最上层的岩石圈，沉入上地幔。因此，富SiO2的下地壳部分熔体和上升的软流圈地幔（取代体积以前所占的下沉的岩石圈地幔和下地壳）之间的交代反应导致形成的斜方辉石丰富的层与强大的地壳签名。这种交代地幔体积可能保持未开发的几个Ma之前，被重新激活的地质过程。这些来源的部分熔融将产生强烈的下地壳特征，从而产生富集地幔1型（EMI）样玄武岩浆活动。具有这种地球化学特征的玄武岩浆活动相对较少，但在某些情况下（例如，印度洋），它可以是一个地理上广泛和持久的现象。
The Earth's lithosphere can focus basaltic magmatism along pre-existing weakness zones or discontinuities. However, apart from the influence on the geochemistry of magmas emplaced in subduction tectonic settings (mantle wedge metasomatism related to dehydration of the subducting plates) the role of lithosphere as a magma source for intra-plate (both oceanic and continental), continental margin, and mid-ocean ridge magmatism is not yet fully understood. In many cases intra-plate magmatism has been explained with the existence of deep thermal anomalies (mantle plumes) whose origin has been placed near the upper–lower mantle transition zone (660 km discontinuity) or even deeper, near the mantle–core boundary (∼2900 km). Also in many continental flood basalt provinces (mostly initiated at craton margins) an active role for mantle plumes has been invoked to explain the high melt productivity. In these cases, no active role for melt production has been attributed to the lithospheric mantle. Potential contaminations of asthenospheric or even deeper mantle melts are often considered the only influence of the lithosphere (both crust and mantle) in basalt petrogenesis. In other cases, an active role of the lithospheric mantle has been proposed: the thermal anomalies related to the presence of mantle plumes would trigger partial melting in the lithospheric mantle. At present there is no unequivocal geochemical tracer that reflects the relative role of lithosphere and upper/lower mantle as magma sources. In this paper another role of the lithosphere is proposed. The new model presented here is based on the role of lower crustal and lithospheric mantle recycling by delamination and detachment. This process can explain at least some geochemical peculiarities of basaltic rocks found in large and small volume igneous provinces, as well as in mid-ocean ridge basalts. Metamorphic reactions occurring in the lower continental crust as a consequence of continent–continent can lead to a density increase (up to 3.8 g/cm3) with the appearance of garnet in the metamorphic assemblage (basalt→amphibolite→garnet clinopyroxenite/eclogite) leading to gravitative instability of the overthickened lithospheric keel (lower crust+lithospheric mantle). This may detach from the uppermost lithosphere and sink into the upper mantle. Accordingly, metasomatic reactions between SiO2-rich lower crust partial melts and the uprising asthenospheric mantle (replacing the volume formerly occupied by the sunken lithospheric mantle and the lower crust) lead to formation of orthopyroxene-rich layers with strong crustal signatures. Such metasomatized mantle volumes may remain untapped also for several Ma before being reactivated by geological processes. Partial melts of such sources would bear strong lower crustal signatures giving rise to Enriched Mantle type 1 (EMI)-like basaltic magmatism. Basaltic magmatism with such a geochemical signature is relatively scarce but in some cases (e.g., Indian Ocean) it can be a geographically widespread and long-lasting phenomenon.