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时间而变化。这些变化可以用两个不同的模型来解释；(1)化学变化仅仅反映了地幔源的变化（原生岩浆不受岩石圈相互作用的影响）；或者(2)这些变化是由于原始岩浆与它们所经过的岩石圈相互作用而发生的变质所致。在中安第斯山脉，喷发的安山岩与典型大陆地壳的成分相似，这表明它们反映了直接的地幔-地壳通量，并支持了俯冲带岩浆添加地壳生长的模型，中安第斯山脉地壳厚度达到~70km。而且很难想象原始岩浆在到达地表的过程中不会被改变。另一种解释是，安第斯安山岩在化学上与大陆地壳相似，因为它们被地壳广泛污染。本项目将通过研究玻利维亚的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化最小的熔岩，其87SR/86SR与岛弧岩浆的来源相似。因此，它们代表了最接近安第斯山脉中部原始岩浆的例子，似乎是研究安第斯山脉下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.