Volcano-pluton connection: Perspectives on material and process linkages, Searchlight pluton and Highland Range volcanic sequence, Nevada, USA
The Searchlight pluton (SLP) and coeval Highland Range volcanic sequence (HRV) have been suggested to represent intruded and erupted counterparts, and the 15 km tilted crustal section in which they are exposed has been presented as an illustration of the anatomy of a large, upper crustal, intermediate to silicic magmatic system. We summarize herein three decades of published and unpublished research on this system, emphasizing new work on its final silicic products that verifies direct connections between SLP and HRV and provides insights about storage, extraction, transport, and eruption processes in the waning stages of the system. Circumstantial evidence strongly suggests that volcanic and plutonic SLP-HRV products were directly linked for one million years (∼17–16 Ma). For most of this period the system produced primarily intermediate rocks: dominantly quartz monzonite in the lower and upper SLP and trachyandesite-trachydacite lavas of the HRV (∼58–70 wt% SiO2). Over the final ∼0.2 million years of the evolution of the system, silicic magma (70–78 wt% SiO2) constructed the youngest part of the pluton (middle SLP granite) and the coeval volcanic sequence (silicic HRV: rhyolite lavas and tuffs). Throughout the system's history, subordinate mafic magma accompanied the more silicic magmas. New results reveal that the youngest silicic rocks exposed in both the middle SLP and the silicic HRV are distinctive rhyolite porphyries (RP) that are essentially identical in texture, mineral assemblage, and elemental compositions. This lithology is phenocryst-rich (40–50%), with large quartz, alkali feldspar, plagioclase, as well as biotite, amphibole, titanite, apatite, Fe-Ti oxides, zircon, and chevkinite. RP dikes emanate from the middle SLP and crosscut the upper SLP and the pluton's roof (Proterozoic basement and lower HRV); small plugs intrude the upper part of the silicic HRV, and identical RP lava is part of a composite lava flow that is the last silicic material erupted in the HRV sequence. This terminal lava also includes basaltic trachyandesite and mechanical mixtures of basaltic trachyandesite and RP. In all environments in which it is exposed, RP is extensively mingled with mafic magma. RP is also very similar in composition, mineralogy, and texture (except for its aphanitic groundmass) to the coarse low-silica granite that dominates the middle SLP. The RP connection provides strong evidence for direct connection between SLP and HRV. A second highly plausible connection between SLP and HRV is strengthened by the RP correlation. Phenocryst-poor high-silica rhyolite (HSR) dikes in the upper SLP and roof that are cut by RP dikes are very similar in mineralogy, texture, and composition to HSR lavas and tuffs that dominate the silicic HRV. The HSR dikes emanate from leucogranite that forms the upper part of the middle SLP; though differing in texture, the fine- to medium-grained leucogranite is compositionally almost identical to the HSR dikes and volcanic rocks. Though more variable and less distinctive than RP, the similarities among the HSR rocks (dikes, lavas, tuffs) and the leucogranite also suggest a direct SLP-HRV connection. Rhyolite-MELTS phase-equilibria and amphibole geobarometers indicate that crystal-poor HSR was stored directly above the crystal-rich RP, consistent with RP derivation from lower middle SLP and HSR from upper-middle SLP. Trace-element modeling suggests that the HSR and leucogranite magmas were produced through ∼10–50% fractional crystallization of low-silica rhyolite/granite melt that fed the middle SLP. Together, these results suggest that the HSR and leucogranite represent the extracted melts derived from middle SLP mush. Texture and composition of RP indicates that it was derived almost en masse from the middle SLP; its ubiquitous association with mafic magma and thermometry within mingled materials indicate that final mobilization of SLP magma was a consequence of mafic recharge. The silicic portion of the SLP-HRV system documents important processes and distinctive products of magmatic systems that are active in the upper crust: (1) production of granitic mushes (cumulates) and extraction of fractionated melt; (2) resultant “filtering:” preferential eruption of the fractionated melt, retention of the cumulate (the HRV silicic section is mostly HSR, the middle SLP mostly low-silica granite [cumulate]); (3) bulk extraction of mush (RP) facilitated by hot mafic recharging; (4) eruption of both crystal-poor, highly-evolved magmas (HSR) and crystal-rich, less evolved magma similar to “monotonous intermediates” (RP).
National Science Foundation
Crustal filter, Crystal rich, Eruptible magma, Granite, Highland Range, Melt extraction, Mush, Rhyolite, Searchlight pluton, Volcano-pluton connection
B. M. Wallrich, C. F. Miller, G. A.R. Gualda, J. S. Miller, N. H. Hinz, and J. E. Faulds. "Volcano-pluton connection: Perspectives on material and process linkages, Searchlight pluton and Highland Range volcanic sequence, Nevada, USA" Earth-Science Reviews (2023). https://doi.org/10.1016/j.earscirev.2023.104361