Magma Sources and Petrogenetic Processes in Continental Arcs: The Southern Volcanic Zone of the Andes

大陆弧岩浆作用反映了板块构造的一个主要过程，即大洋和大陆板块的会聚。大洋板块俯冲到大陆板块之下，产生新的大陆地壳，并将地壳物质并入对流地幔。这些都是重要的过程，需要详细了解才能了解地球的演化。安第斯火山反映了大洋纳斯卡板块在南美板块之下的活跃俯冲。安第斯山脉南缘火山岩带(SVZ)在南纬33o-42oS之间存在着明显的沿走向地质变化，从北向南俯冲地壳年龄和陆壳厚度减小。同时，沿着科迪勒拉主干走向和穿过主科迪勒拉走向，熔岩的成分(常量元素和微量元素丰度、放射性同位素比率)有系统地变化。对于这些地球化学趋势，有几种不同的解释。P.I.S前人和建议的研究都是为了了解本区火山活动的起源和演化；具体地说，确定俯冲地壳、软流圈、岩石圈和大陆地壳成分对弧形岩浆的相对重要性，并确定熔化度变化、随后的熔融-固体分离、岩浆混合和地壳同化的重要性。为了做到这一点，将使用斑晶相组成加上全岩主要和微量元素丰度和同位素比率(0，锶，钕，铅)。迄今为止，重点一直放在SVZ火山锋面上或附近的层状火山上。该项目将侧重于火山前锋以东50至200公里处最近喷发的玄武岩熔岩。由于这些弧后(回溯)熔岩的演化程度低于火山前缘熔岩，它们的地球化学特征更直接地反映了源岩成分(特别是确定大陆地幔岩石圈的地球化学特征的可能性)和地幔过程。此外，沿着单个层状火山(Villarrica)附近的火山前锋，从层状火山喷发的熔岩与周围几个(卫星)小玄武岩锥之间存在显著的地球化学差异。这些差异可以解释为反映了卫星锥形熔岩中的地壳污染要少得多，或者反映了源区不均质性的影响，这些影响在层状火山下的长期岩浆室中被平均消除。这些都是重要的、非常不同的另类解释。为了检验维拉里卡层状火山附近结果的一般性，并区分这些解释，将研究卫星锥形熔岩与SVZ内其他层状火山有关的情况。*//

Continental arc magmatism reflects a major process in plate tectonics, the convergence of oceanic and continental plates. The subduction of an oceanic plate beneath a continental plate results in creation of new continental crust and incorporation of crustal material into the convecting mantle. These are important processes that need to be understood in detail in order to understand the evolution of the earth. Andean volcanoes reflect the active subduction of the oceanic Nazca plate beneath the South American plate. In the Southern Volcanic Zone (SVZ) of the Andes, from 33o-42oS, there are major along-strike geologic changes; e.g., from north to south there are decreases in the age of the subducted crust and continental crust thickness. Concurrently, along and across strike of the Main Cordillera the lavas vary systematically in composition (major and trace element abundance, radiogenic isotope ratios). There are several alternative explanations of these geochemical trends. The P.I.'s previous and proposed studies have the objective of understanding the origin and evolution of volcanism in this region; specifically, to define the relative importance of components from the subducted crust, asthenosphere, lithosphere and continental crust to the arc magmas, and to determine the importance of variations in degree of melting, subsequent melt- solid segregation, magma mixing and crustal assimilation. To do this phenocryst phase compositions plus whole-rock major and trace element abundances and isotopic ratios (0, Sr, Nd, Pb) will be used. To date emphasis has been on stratovolcanoes on or near the volcanic front of the SVZ. This project will focus on recent basaltic lavas erupted 50 to 200 km east of the volcanic front. Because these back (retro) arc lavas are less evolved than volcanic front lavas, their geochemical characteristics more directly reflect source compositions (especially, important is the possibility of determining geochemical characteristics of continental mantle lithosphere) and mantle processes. In addition, along the volcanic front near a single stratovolcano (Villarrica), significant geochemical differences between lavas erupted from the stratovolcano and several surrounding (satellite), small basaltic cones have been found. These differences can be interpreted as reflecting much less crustal contamination in the satellite cone lavas, or the influence of source heterogeneities which are averaged out in long-lived magma chambers beneath the stratovolcano. These are important and very different alternative interpretations. To test the generality of results near the Villarrica stratovolcano and to distinguish between these interpretations, satellite cone lavas associated with other stratovolcanoes in the SVZ will be studied. ***//