Abstract
The size, longevity, and mobility of upper-crustal magma mushes, and thus their ability to mix and interact with newly arriving batches, are key factors determining the evolution of magma reservoirs. Magmatic structures in plutons represent local sites of structural and compositional diversity and provide an opportunity to test the extent of physical and chemical processes that operated through time. Regional compilation of compositionally-defined magmatic structures, specifically those involving schlieren, in the Tuolumne Intrusive Complex (TIC), yields a synthesis of ~1500 schlieren-bound structure measurements. Field observations, petrography, and whole-rock geochemistry were integrated to test schlieren formation mechanisms. At a local scale (1 mm – 1 m), we find that schlieren-bound structures formed from the surrounding host magma during dynamic magmatic processes such as crystal flow-sorting, magmatic faulting, and folding. Fluidization of the magma mush, interpreted from 1 m – 1 km wide domains of clustered schlieren-bound structures, appears to have operated within a hydrogranular medium, or ‘crystal slurry’ (Bergantz et al., 2017). At the regional scale (10’s km), outward younging patterns of troughs, migrating tubes, and plumes indicate that the mush convected, driven by intrusion of new pulses. Troughs and planar schlieren are weakly oriented parallel to nearby major unit contacts, which could be related to internal mush convection or effects of high thermochemical gradients at internal unit boundaries. We hypothesize that these younging patterns and orientations have the potential to constrain the size of mobile magma mixing regions, that in the TIC extended to a minimum of 150 km2 (~1500 km3) and were long-lived (>1 m.y). These require the generation of extensive melt-present reservoirs that could flow magmatically, formed from the amalgamation of intruding magma pulses, and precludes dike, sill, or laccolith emplacement models. We conclude that schlieren-bound structures are faithful recorders of the multi-scale, hypersolidus evolution of upper-crustal magma bodies, and represent useful tools for studying plutonic systems.
Highlights
On the path to solidification, magma reservoirs reside in the crust as a crystal-rich mush (Marsh, 1981)
Other studies have considered tectonism as a mechanism to mobilize crystal-mushes and re-orient earlier-formed magmatic structures, including fabrics (e.g., Žák and Paterson, 2010; Alasino et al, 2019). To reconcile these findings in the context of schlieren-bound structures in the TIC, we suggest that the strain rates producing regional fabrics are slower, but longer-lived than strain rates producing local schlieren-bound structure fabrics
Schlieren layers are locally sourced from nearby magmas and provide evidence for local magma flow, mineral-melt separation, and compositional differentiation
Summary
On the path to solidification, magma reservoirs reside in the crust as a crystal-rich mush (Marsh, 1981). The primary focus has been on describing magmatic systems from the mechanical perspective of hydrogranular, dense, crystal-rich slurries, with or without exsolved volatiles (e.g., Schleicher et al, 2016; Bergantz et al, 2017; Degruyter et al, 2019; McIntire et al, 2019; Petford et al, 2020) This perspective is relevant for magma mushes as it incorporates the effects of particle-particle interactions and deformation on the state of the mush, which are both welldocumented in field-based studies of magmatic structures in plutons (e.g., Paterson et al, 2018). Fewer studies in silicic magmatic systems, the focus of this study, are of great interest as they demonstrate that compositionally defined magmatic structures may form over a wide compositional and rheological range (e.g., Wiebe and Collins, 1998; Weinberg et al, 2001; Paterson, 2009; Clemens et al, 2020)
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