Petrogenetic Processes and Sources of Volcanic Rocks in the Central Andes: Constraints from Mafic Monogenetic Centers inBolivia (Collaborative Research)

对安第斯山脉火山产物的研究表明，沿走向和跨走向的化学成分随着时间的推移而变化。这些变化可以用两种截然不同的模型来解释：(1)化学变化只是反映了地幔来源的变化(原生岩浆不会因与岩石圈的相互作用而改变)；或(2)这些变化是由于与所经过的岩石圈相互作用而引起的原生岩浆的改变。在中央安第斯山脉，喷发的安山岩的成分与典型的陆壳相似，这导致人们认为这些安山岩反映了直接地幔凝结通量，并支持在俯冲带通过岩浆叠加的地壳生长模式。在中央安第斯山脉，地壳厚度达到~70公里，很难想象原生岩浆在到达地表的过程中不会发生变质。另一种解释是，安第斯安山岩在化学上类似于大陆地壳，因为它们已经被地壳广泛污染。该项目将通过研究玻利维亚的镁铁单基因中心来解决这一关键问题。现有资料表明，这些熔岩是安第斯山脉中部分异程度最低的熔岩，具有与岛弧岩浆来源相似的87SR/86SR。因此，它们是最接近中央安第斯山脉原生岩浆的例子，似乎是了解次安第斯地幔组成的重要窗口。

Studies of volcanic products along the Andes show a variation in chemistry along strike and across strike with time. These variations can be explained by two contrasting models; (1) that the chemical variations simply reflect variations in the mantle source (primary magmas are not modified by interaction with the lithosphere); or (2) that these variations are due to modification of primary magmas due to interaction with the lithosphere through which they pass. In the Central Andes, the similarity of the compositions of the erupted andesites to typical continental crust, has led to the suggestion that these reflect the direct mantle to curst flux, and support a model for crustal growth through magmatic addition at the subduction zone Crustal thicknesses in the Central Andes reach~70km, and it is difficult to envisage that primary magmas would not be modified en route to the surface. The alternative interpretation is that Andean andesites are chemically similar to continental crust because they have been extensively contaminated by the crust. This project will address this key issue by studying mafic monogenetic centres in Bolivia. Available data suggests that these are the least differentiated lavas in the Central Andes and have 87SR/86SR similar to the source of island arc magmas. They therefore represent the closest examples to primary magmas in the Central Andes, and appear to be a vital window into the composition of the sub-Andean mantle.