HR: 09:15h
AN: V21A-04 INVITED     [Abstracts]
TI: Constraints on the source and evolution of silicic magmas from diverse tectonic environments
AU: * Vogel, T A
EM: vogel@msu.edu
AF: Michigan State University, Department of Geological Sciences, East Lansing, MI 48824 United States
AB: Calc-alkaline silicic magmas (>65 wt. % SiO₂) occur in a wide range of tectonic environments and diverse models have been proposed for their origin. They are common in continental convergent zones and most models for the origin of silicic magmas in these areas involve partial melting of continental crustal rocks. In these models the compositions of the silicic magmas vary according to the relative contribution of evolved continental crust and mantle-derived melts. In contrast, silicic magmatism has been considered to be minor in intra-oceanic arcs, where evolved crust is absent. However recent studies have shown that silicic magmatism can be a significant component in intra-oceanic arcs. These studies propose that the generation of silicic magmas in ocean arc systems involve the partial melting of recently emplaced, mantle derived, stalled (crystallized) calc-alkaline magmas. Abundant silicic magmas can be produced both with and without the presence of evolved continental crust. This paper evaluates the source and evolution silicic ignimbrites and related deposits in three areas: 1.)Southwest Nevada, USA, which is associated with rifting; 2.)In Central America associated with the active volcanic front; 3.)In the Macolod Corridor, Luzon, Philippines, which is associated with subduction and rifting. Fractional crystallization from primitive basaltic or primitive andesitic magma can be rejected as the process that produced the silicic magmas in these areas. This conclusion is based on oxygen isotopes, Sr and Nd isotopes, and very high K₂O/Na₂O values in the silicic magmas. In the southwest Nevada individual, large-volume, ash-flow sheets contain two or more silicic magma batches along with a more mafic magma batch, all of which are independently generated. In Central America, the along arc variation of the silicic deposits with respect to selected trace element ratios (Ba/La, U/Th, Ce/Pb), oxygen isotopes, Nd and Sr ratios mimic the along arc variation in the basaltic lavas from the active arc. Some ignimbrite units have as many as seven distinct and independent magma types. In the Macolod Corridor, Philippines, at least three distinct silicic compositions are recognized. A common factor in all of these silicic deposits is their high K₂O/Na₂O values, which cannot be produced by fractional crystallization of primitive magmas. Data from melting experiments of primitive basalts and andesites demonstrate that it is impossible to produce high K₂O/Na₂O silicic magmas by fractional crystallization or partial melting of a low- K₂O/Na₂O source. In areas with evolved continental crust, such as southwest Nevada, this is not a problem. However, in oceanic arcs an enriched K₂O/Na₂O source is required. Partial melting or extreme fractionation of evolved K-rich basalts can produce the high K₂O/Na₂O silicic magmas observed. In Central America and the Macolod Corridor, our data are consistent with the model that large-volume silicic magmas result from melting of ponded K-rich basaltic plutons, or melt extraction from these partially crystallized plutons. In the case of southwest Nevada the diverse silicic magmas are produced by melting of different crustal sources with little mixing with a mantle-derived source. Extension may be a fundamental control for producing large-volume silicic ignimbrites because space for large magma bodies is produced in releasing bends or pull-apart structures.
DE: 1010 Chemical evolution
DE: 1020 Composition of the crust
DE: 3640 Igneous petrology
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly