Abstract
Nevado de Longaví volcano (NLV), in the Southern-Central Chilean Andes, has erupted during the Holocene magmas with compositions that are in several ways atypical for the region. These characteristics include elevated La/Yb ratios in evolved magmas, in an area of only moderately thick crust, coupled with low concentrations of K, Th, and other incompatible elements and elevated ratios of fluid-mobile (B, Cs, Li, Sb) to fluid-immobile elements. Samples have an unusual mafic mineralogy dominated by amphibole. The petrology of the Holocene products of NLV have been related to the influence of an oceanic transform fault (Mocha Fracture Zone; MFZ) that supplies the mantle wedge with unusually high amounts of fluids via dehydration of serpentinite bodies hosted by the subducted oceanic lithosphere. Because the trace of this transform fault is oblique to the convergence vector, its position along the arc has varied through time, as has the magnitude of its influence on the nature of the magmas erupted at NLV. The whole-rock and mineral chemistry of volcanic products from NLV, tied to a simplified stratigraphy, documents the secular changes in the magmatic system as the oceanic fault approached its current position. Magmas erupted ∼1–0.6 Ma are relatively low in water (as inferred from mineralogy and chemical proxies) and reduced (NNO-1 to NNO+0.5), and are similar to compositions found in neighboring volcanoes. From 0.25 Ma to the present, magmas are water-rich and oxidized (NNO-0.5 to NNO+1.7). In the intervening 0.6–0.25 Ma, mafic magmatism acquired a transient crustal component, which we identify as subducted sediment melts, on the basis of radiogenic isotopes and Pb, Th, and U abundances. Fluids released from serpentinite in the fracture zone were rich in Li, B, Sb, Cs and Ba, but not in K, Th, U and Sr. The fluid addition led to enhanced melting, particularly hydrous magmas that stabilized amphibole early during fractionation, higher oxygen fugacities, and distinctive chemical compositions.
Highlights
Subduction-related magmatism involves multiple geological reservoirs, which successively add elemental and isotopic contributions to the diverse geochemical signatures of magmas that reach the Earth’s surface
In this contribution we examine the case of Nevado de Longaví volcano (NLV) in the Southern Volcanic Zone of the Andes (SVZ), which is currently located above the projection of the oceanic Mocha Fracture Zone (MFZ)
Underlying Pleistocene volcanic rocks (Unit 0) are characterized by early Fe and Ti enrichments, and strong MgO, Al2O3, and CaO decreases from mafic to intermediate compositions, whereas K2O and Na2O contents increase sharply with increasing silica (Figure 3). These differentiation trends are typical of the arc tholeiite affinity displayed by Quaternary SVZ volcanoes south of NLV
Summary
Subduction-related magmatism involves multiple geological reservoirs, which successively add elemental and isotopic contributions to the diverse geochemical signatures of magmas that reach the Earth’s surface. Subducted oceanic lithosphere, composed of altered oceanic crust, variably hydrated mantle, and sediments, releases water-rich fluids during prograde metamorphic reactions and may partially melt with increasing pressure and temperature. One potential water reservoir is serpentinite formed by hydration of oceanic mantle lithosphere, through permeability pathways in the crust such as fracture zones and normal faults developed during bending of the plate at the trench (Ranero et al, 2003; Moscoso and ContrerasReyes, 2012; Manea et al, 2014). For the Aleutian arc, Singer et al (1996), Jicha et al (2004), and Singer et al (2007) documented the trace element and isotopic characteristics of Seguam volcano, which is located above the Amlia fracture zone. In the Mexican volcanic belt, Manea and Manea (2008) have shown that the geophysical signature of a serpentinized fracture zone (the Tehuantepec FZ) can be traced beneath the continental plate and to project beneath El Chichón volcano, another strange adakitic center. Whalen et al (2003) speculated that the Th-depleted signature of a ~3 Ga arc-related intrusive complex was a consequence of a localized source of serpentine-derived fluids, probably related to a subducting fracture zone
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