Comparative field and petrogenetic study of arc magmatism in the lower crust; exposed examples from a continental margin and an intraoceanic setting
Stanford University, Palo Alto, CA
186 p., Illus.
The Tonsina-Nelchina assemblage of south-central Alaska and the Sierra de Fiambala of northwestern Argentina are unique exposures of magmatic arc crust that record the evolutionary history of mantle-derived melts during their passage through the crust. The Tonsina-Nelchina assemblage is a cross-sectional exposure through the Jurassic Talkeetna island arc from upper mantle through upper crust. The arc is compositionally stratified, with ultramafic to gabbroic cumulates in the lower crust, gabbroic to tonalitic intrusive rocks in the midcrust, and dominantly andesitic volcanic rocks in the upper crust. This arc was formed in a purely oceanic setting with no input from continental crust. The bulk composition of the arc was calculated by mass balance to be high-Mg basalt (11.3 wt.% MgO, 15 wt.% Al$\sb2$O$\sb3$) with a flat rare-earth element pattern. Fractionation of voluminous Al-poor ultramafic cumulates (dunites, wehrlites, and pyroxenites) from a high-Mg primary magma in the lower crust of the arc produced high-alumina basalts. This suggest that although the most common rocks in volcanic arcs are high-alumina basalts, they are not primary magma compositions. The volcanic rocks of the upper crust display chemical trends typical of calc-alkaline volcanics found in other island-arc suites. In contrast to the Tonsina-Nelchina assemblage, the Sierra de Fiambala is an uplifted block of lower crust from a continental magmatic arc. It consists of Ordovician gabbroic to intermediate plutonic rocks intruded into metasedimentary host rocks during regional high-grade metamorphism, including a large (30 km$\sp2$ in map view) concordant gabbroic intrusion. The crystallizing assemblage in the gabbronorites is the same as that from Tonsina-Nelchina: olivine $\pm$ pyroxene followed by pyroxene + plagioclase $\pm$ amphibole. The most primitive of the gabbronorites that represent liquid compositions are Mg-rich (11 wt.% MgO) and have flat to slightly light rare-earth-enriched patterns on rare-earth element diagrams. They are very similar in composition and mineralogy to the most primitive of the Tonsina-Nelchina gabbronorites. Differentiation proceeded with progressive assimilation of metasedimentary host rocks, producing derivative compositions that are significantly more enriched in incompatible elements than the Tonsina-Nelchina gabbronorites at the same MgO content. This enrichment in incompatible trace elements relative to island arc magmas is characteristic of continental arc volcanic lavas worldwide. Previous workers have suggested that this enrichment in continental arc magmas is a characteristic of the source mantle, an enriched lithosphere beneath continents. Documentation of the differentiation history of the Fiambala gabbronorite during its residence in the lower crust clearly shows that there is no need to call on enriched sub-continental lithosphere, the enrichment can occur during chemical evolution in the lower crust.
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