Testing the viability and significance of magma mixing in continental arcs using the Sierra Nevada batholith, California, as an example.

摘要

The occurrence of contemporaneous, chemically diverse rock types (including diorite, granodiorite and granite) in magmatic arcs suggests a genetic relationship among them. Field and geochemical studies in the Mesozoic Sierra Nevada batholith of California indicate that end member magma mixing between 52 weight percent SiO₂ basalts (chemically similar to dioritic plutons of the batholith) and 72 weight percent SiO₂ granites is an important process in generating chemical diversity in that arc. Additionally, isotopic data for Sierran arc rocks indicate that two distinct mixing processes occurred: open system (granites represent reworked pre-Mesozoic crustal rocks) and closed system (granites represent juvenile additions to the crust). The concept of closed system mixing is new to petrology and is herein termed “internal mixing”. The concept of internal mixing is hypothesized to occur between basaltic liquids and granitic liquids derived through partial melting of penecontemporaneous diorites, and is supported by chemical, isotopic and geochronologic data for some suites of Sierran rocks. Further support for this hypothesis comes from the demonstration that Sierran granites are chemically comparable to experimentally derived partial melts of Sierran diorites. Recognition of internal mixing processes dramatically increases estimates of juvenile material present in the arc and coincidentally increases estimates of crustal growth rates in the arc.; New numerical models of magma mixing that account for the recent recognition that arc basaltic magmas often contain up to 8 weight percent H₂O offer additional validity to the hypothesis that mixing can be an important process in arc magmatism. Calculation of liquidus temperatures, magma viscosities and densities for hydrous high-alumina basaltic liquids demonstrate that the addition of water reduces or eliminates recognized barriers to mixing between dry basaltic and granitic magmas. Consequently, magma mixing is presented as a viable and important mechanism for generation of intermediate composition magmas that dominate the eruptive and intrusive products of continental arcs. Furthermore, mixing is presented as an important process in the generation of Earth's continental crust.