Melts, mush, and more: Evidence for the state of intermediate-to-silicic arc magmatic systems

Abstract

Understanding the physical state of intermediate-to-silicic arc magmatic systems is necessary for our petrologic models of these systems. Researchers have generated a plethora of data—geophysical, geochronological, petrological, theoretical—over the past few decades. These data have changed how we view arc magmatic systems, leading to a model of crystal mush that is rejuvenated repeatedly over the lifespan of the magmatic system. However, much data are either circumstantial or incomplete. Paterson et al. (2016; October issue) use a combined set of textural, geochemical, and temporal data to demonstrate the changing physical state of the Tuolumne Intrusive Complex in California over its ~10 million year history. They offer evidence for magmatic erosion and recycling, along with the potential for a “surge growth” of the batholith that allows for significant volumes of magma to exist ephemerally under arc volcanoes. Most intermediate-to-silicic arc magmas (henceforth “arc magma”) are stored ≥3 km depth, so direct observation is impossible. Instead this question has been approached in four main ways: (1) geophysical inspections of active arcs; (2) mineral geochronology; (3) field observations of plutons and; (4) thermal and physical modeling of magmatic systems. Each of these four avenues has their own strengths and shortcomings. Many times the interpretation of these data appears to be at odds. Some overarching observations can be made about what we know of the state of active intermediate-to-silicic arc magmatic systems: Geophysical observations (such as seismic tomography) have yielded little evidence for large bodies of fully molten magma persistently underneath arc volcanoes. If anything, only small lenses of partial melt are observed at …