玄武岩浆起源和演化的一些基本概念以及对中国东部中 -新生代基性火山岩成因的新思路(英文)

Generation and Evolution of Basaltic Magmas: Some Basic Concepts and a New View on the Origin of Mesozoic-Cenozoic Basaltic Volcanism in Eastern China

以全球大地构造为背景讨论了玄武岩浆起源和演化的一些基本概念。这些概念的正确理解有助于合理解释各种环境中火成岩的形成机制,也有助于依据野外岩石组合来判别古构造环境。在此基础上结合已有资料和观察,对中国东部中生代岩石圈减薄及中-新生代基性火山岩成因提出了一些新解释。这些解释与地质观察相吻合,且符合基本的物理学原理。虽然中国东部基性火山活动可称为“板内”火山活动,但它实际上是板块构造的特殊产物。中国东部中生代岩石圈减薄是其下部被改造为软流层的缘故。这种改造是加水“软化”所致。水则源于中国东部地幔过渡带(410～6 6 0km)内古太平洋(或其前身)俯冲板块脱水作用。其将岩石圈底部改造为软流层的过程,实际上就是岩石圈减薄的过程。因为软流层是地幔对流的重要部分,而大陆岩石圈则不直接参与地幔对流。中生代玄武岩具有εNd<0的特征,说明其源于新近改造而成的软流层,亦即原古老岩石圈之底部。中国大陆北北东-南南西向的海拔梯度突变界线与东-西部重力异常,陆壳厚度变化,以及地幔地震波速变化梯度吻合。因此可将北北东-南南西向梯度线称为“东-西梯度界”。该界东-西海拔高差(西部高原与东部丘陵平原) ,陆壳厚度差异(西部厚而东部薄)和10 0～15

Some basic concepts of basaltic magma generation and evolution are discussed in the context of global tectonics. These concepts need better understanding before invoking elusive possibilities in igneous petrogenesis on all scales and in all tectonic environments. A hypothesis for the Mesozoic lithosphere thinning and Mesozoic-Cenozoic basaltic volcanism in eastern China is presented. This hypothesis is consistent with observations and complies with basic physics. While the eastern China volcanism can be defined as “intra-plate' volcanism, it is in fact a special consequence of plate tectonics. The Mesozoic lithosphere thinning in eastern China is best explained by a process that “transformed' the deep portion of the lithosphere into convective asthenosphere by hydration. The water that did so may come from dehydration of subducted Pacific (or predecessor) oceanic lithosphere that is presently lying horizontally in the transition zone beneath eastern Chinese continent as detected by seismic tomographic models. The Mesozoic volcanism may be genetically associated with the lithospheric thinning because the basaltic source is ancient isotopically enriched (ε_(Nd) <0) lithosphere—being converted to the asthenosphere. The NNE-SSW Great Gradient Line (GGL) marked by the sharp altitude, gravity anomaly, crustal thickness, and mantle seismic velocity changes from the plateau west to the hilly plains of eastern China is an expression of variation in lithospheric thickness from probably >150～200 km beneath the plateaus in the west to the thin, probably <80 km, beneath eastern China. The “remote' western Pacific subduction systems induce asthenospheric flow from beneath eastern China towards the subduction zones, which, in turn, requires asthenospheric material replenishment from beneath the plateaus to the eastern China. As a result, such eastward asthenospheric flow experiences an upwelling and decompression (from beneath thickened to thinned lithosphere), which causes the flowing asthenosphere (e.g., ε_(Nd) >0) to partially melt and produce Cenozoic eastern China basaltic volcanism. Such volcanism may have actually begun at the end of the Mesozoic lithosphere thinning in the late Cretaceous. This simplistic concept is currently being substantiated with detailed petrologic and geochemical data.