Abstract
AbstractProtracted magma storage in the deep crust is a key stage in the formation of evolved, hydrous arc magmas that can result in explosive volcanism and the formation of economically valuable magmatic-hydrothermal ore deposits. High magmatic water content in the deep crust results in extensive amphibole ± garnet fractionation and the suppression of plagioclase crystallization as recorded by elevated Sr/Y ratios and high Eu (high Eu/Eu*) in the melt. Here, we use a novel approach to track the petrogenesis of arc magmas using apatite trace element chemistry in volcanic formations from the Cenozoic arc of central Chile. These rocks formed in a magmatic cycle that culminated in high-Sr/Y magmatism and porphyry ore deposit formation in the Miocene. We use Sr/Y, Eu/Eu*, and Mg in apatite to track discrete stages of arc magma evolution. We apply fractional crystallization modeling to show that early-crystallizing apatite can inherit a high-Sr/Y and high-Eu/Eu* melt chemistry signature that is predetermined by amphibole-dominated fractional crystallization in the lower crust. Our modeling shows that crystallization of the in situ host-rock mineral assemblage in the shallow crust causes competition for trace elements in the melt that leads to apatite compositions diverging from bulk-magma chemistry. Understanding this decoupling behavior is important for the use of apatite as an indicator of metallogenic fertility in arcs and for interpretation of provenance in detrital studies.
Highlights
The chemical diversity observed in the rock record of volcanic arcs is determined by a multitude of processes operating between the magma source region and the surface
We present a novel approach for understanding arc magma evolution by combining the trace element compositions of apatites from volcanic rocks with fractional crystallization modeling
We show that apatite records both deep crustal fractionation and shallow crustal crystallization processes, and tracks parameters valuable to understanding the generation of hydrous arc magmas and metallogenically fertile arcs
Summary
The chemical diversity observed in the rock record of volcanic arcs is determined by a multitude of processes operating between the magma source region and the surface. STUDY AREA The Andes of central Chile are an ideal region for a study of regional arc magma evolution and metallogeny because volcanic rock sequences (Fig. 1) record an extended period of subduction-related magmatism that culminated in high-Sr/Y magmatism (Sr/Y > 50) and the genesis of three major porphyry Cu deposits between 12.3 and 4.3 Ma (Perelló et al, 2012; Toro et al, 2012; Spencer et al, 2015): Los Pelambres, Rio Blanco–Los Bronces, and El Teniente (Kay and Mpodozis, 2001) This transition in magma chemistry has been attributed to shallowing of the subducting slab, linked to subduction of the Juan Fernández Ridge from ca.
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