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The petrogenesis of subduction-related subalkaline intermediate and silicic igneous rocks: a study case from Sierras de Valle Fertil – La Huerta, San Juan, Argentina

Giuliano César Camiletti

Brown University

The average composition of the Earth’s continental crust is andesitic, this composition also dominates in subduction zones, so the generation of an intermediate-composition magma has been the subject of discussion for a long time. Understanding how intermediate magmas form, is understanding how continental crust was made. Much of our knowledge of petrologic processes in the generation of intermediate subalkaline magmas came from chemical variations found in volcanic and shallow-crust plutonic rocks. Hence, a better understanding of the petrologic mechanisms that operate in the magmatic arcs, which generate intermediate/silicic magmas, requires developing investigations in geologic sequences exposing lower crustal levels of “ancient” magmatic arcs. In this regard, it is relevant exploring which mechanism dominates to make intermediate magmas. The main aim of the project is to find which processes are involved to produce intermediate/silicic magmas in a lower- to the middle-crust section of the Early Ordovician active margin of West Gondwana currently Sierras de Valle Fértil – La Huerta, San Juan, Argentina. Our hypothesis is that in-situ fractionation rather than source-controlled partial melting is the mechanism that governed large-scale geochemical signals. Based on field geology and through geochemical modeling examined with experiments and natural experiments we will help to work out which are the governing processes driving the generation of continental crust. Furthermore, we will figure out how much of crustal component is involved on the generation of intermediate-magmas. Our results and ongoing research will add a new natural example to the developing trans-crustal magmatic column model. This perspective allows linking deep processes with shallow-level plutonism and volcanism. Unraveling a full understanding of the differentiation pathways of igneous subduction-related magmas remains a challenge, this project will provide some insights into this undertaking that is now of world-wide interest. Finally, we will achieve a model of generation of an intermediate-composition magma, that framed in the trans-crustal magmatism model, will provide a feasible way to generate a thick middle crustal arc section.

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