Magmatic evidence for Neogene lithospheric evolution of the central Andean “flat-slab” between 30°S and 32°S
Magmatic evidence for Neogene lithospheric evolution of the central Andean “flat-slab” between 30°S and 32°S
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DOI：
10.1016/0040-1951(96)00032-7
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发表时间：
1996-06
期刊：
影响因子：
2.9
通讯作者：
S. Kay;J. M. Abbruzzi
中科院分区：
文献类型：
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作者：
S. Kay;J. M. Abbruzzi
从安第斯山脉中新世岩浆喷发的地球化学数据，通过现在的火山不活跃的“平板”之间的30°S至32°S，加上地质和地球物理数据，阐明了岩浆和大陆岩石圈演化的细节逐渐变浅俯冲带。Pb、Sr、Nd同位素和微量元素数据表明，主Cordillusters、Precordillusters和Pampillusters岩浆在地幔和地壳中都受到了混染，而且混染的性质在时间和空间上都有变化，反映了构造事件。污染物包括“富集”（高LIL元素，高Sr和Pb，和低Nd同位素比值）和“贫化”（低LILE元素，低Sr和Pb，和高Nd同位素比值）类型。在西部地区，主山脉早中新世熔岩的污染物与较晚中新世熔岩少“丰富”的签名。逐渐“富集”的原因是：（a）越来越多的沉积物和构造侵蚀地壳俯冲到地幔楔中;（B）主科迪勒拉山脉下地壳增厚，其成分逐渐“富集”。这种“富集”是通过地壳内混合过程增加上地壳而发生的，这种混合过程是由一个传播的楔形尖端驱动的，与向西楔入、加热和地壳变形有关，这些都是从缩短的前科迪勒拉冲断带下面向东进行的。再往东，通过弧后地壳喷发的岩浆有更多的“枯竭”特征。那些在中部通过不断发展的前心脏冲断带爆发的火山被一个更老、更薄的格伦维尔（约1100 Ma）基底所污染，其“枯竭”特征在安第斯山脉中部是独一无二的。晚中新世Pocho熔岩喷发进一步东与隆起的Sierras Pampeanas显示“富集”通过时间。可以说，这些岩浆可能含有一种成分，它是从变薄的大陆岩石圈底部向西机械地移走，并逐渐并入对流的软流圈。一阶质量平衡计算比较了20 Ma时的岩石圈几何形状和现在的岩石圈几何形状，结果表明，20 Ma时海沟周围600 km范围内的岩石圈已经损失了约60%。这种岩石圈的损失是需要的，以适应同时变浅的海洋板块，增厚和缩短的地壳。大陆岩石圈再循环进入软流圈可能是洋岛玄武岩中“富集”的EM 1和EM 2地幔组分的来源。
Geochemical data from Andean Miocene magmas erupted through the now volcanically-inactive “flat-slab” between 30°S to 32°S, coupled with geological and geophysical data, illuminate details on magmatic and continental lithospheric evolution over a progressively shallowing subduction zone. Pb, Sr and Nd isotopic and trace-element data show that Main Cordilleran, Precordilleran and Pampean magmas were contaminated in both the mantle and crust, and that the nature of the contaminants varied in time and space, reflecting tectonic events. Contaminants included both “enriched” (high LIL-element, high Sr and Pb, and low Nd isotopic ratios) and “depleted” (low LILE-element, low Sr and Pb, and high Nd isotopic ratios) types. In the western region, Main Cordilleran earlier Miocene lavas had contaminants with less “enriched” signatures than later Miocene lavas. Progressive “enrichment” is attributed to: (a) increasing amounts of sediment and tectonically eroded crust being subducted into the mantle wedge; and (b) contamination in a thickening Main Cordilleran lower crust whose composition was progressively “enriched”. This “enrichment” occurred through addition of upper crust by an intracrustal mixing process driven by a propagating wedge tip associated with westward wedging, heating and deformation of crust from beneath the shortening Precordillera thrust belt to the east. Further east, magmas erupted through back-arc crust have more “depleted” signatures. Those erupted in the central part through the evolving Precordilleran thrust belt were contaminated by an older, thinner Grenville (∼ 1100 Ma) basement whose “depleted” signature is unique among Central Andean terranes. Late Miocene Pocho lavas erupted further east in conjunction with uplift of the Sierras Pampeanas show “enrichment” through time. Arguably, these magmas could contain a component mechanically removed from the base of the thinning continental lithosphere to the west, and progressively incorporated into the convecting asthenosphere. First-order mass balance calculations comparing the lithospheric geometry proposed for 20 Ma with that today suggests that ∼ 60% of the lithosphere within 600 km of the trench at 20 Ma has been lost. This lithospheric loss is required to accommodate simultaneous shallowing of the oceanic slab, and thickening and shortening of the crust. Continental lithosphere recycled into the asthenosphere could contribute to the sources of the “enriched” EM1 and EM2 mantle components in oceanic island basalts.