Late‐stage magma flow in a shallow felsic reservoir: Merging the anisotropy of magnetic susceptibility record with numerical simulations in La Gloria Pluton, central Chile

Abstract

[1] La Gloria Pluton is a 10 Myr old epizonal intrusion located in the southern Andes. We present anisotropy of magnetic susceptibility data that indicate a magnetic fabric that is mainly oblate. We find that lineations are weak and have a N-NW trend with a nearly horizontal dip, while foliations are more pronounced, have NW trends, and have dips that vary from vertical at the walls of the intrusion to horizontal at the center and under the roof of the chamber. To interpret these magmatic fabrics, we developed a time-dependent 2-D magmatic fluid dynamic numerical simulation. Our model is calibrated with MELTS and accounts for the coupled processes of cooling, crystallization, and degassing of a magma chamber. Simulations indicate that the resulting convective flow pattern in the crystallizing reservoir is consistent with the magnetic fabric, which is largely produced in the shear zone between the convecting liquid-dominated core and the growing solidification fronts adjacent to the walls. The magnetic fabric records the last increment of strain induced by convective magmatic flow in the cooling reservoir during crystallization at the rheological magma locking point along solidification fronts. Despite the small size of the pluton, the core of the chamber remains thermally insulated from the colder host rocks, surviving up to 20 kyr above the solidus, which allows enough time for the extraction of residual leucogranitic melt and partial late magmatic reactive recrystallization. The results of the simulations are also consistent with the previously determined compositional and mineralogical zonation patterns in the pluton.

Protracted Multipulse Emplacement of a Postresurgent Pluton: The Case of Platoro Caldera Complex (Southern Rocky Mountain Volcanic Field, Colorado)

“…We explain the steep magnetic foliation preserved at the present-day erosional level as developed through convective flow during emplacement of the pulses against each other. Furthermore, we hypothesize that the "onion-skin" architecture has a 3-D shape as envisioned in Figure 14a (e.g., Gutiérrez et al, 2013;Payacán et al, 2014;Sant'Ovaia et al, 2000). The sparsity of structural data prevents a comparable evaluation of the relatively younger Alum Creek porphyry (Pulse 4), but the five AMS stations also show steep foliations and thus imply a small-scale vertical, perhaps stock-like intrusion (Figure 14a).…”

“…These microphenocrysts (now embedded within the matrix) were able to rotate within the interstitial space between larger plagioclase phenocrysts and align in response to magma flow (e.g., Benn, 1994;Paterson et al, 2019). The magnetic fabrics thus represent proxies for magmatic strain during late-stage flow and emplacement that were likely captured instantaneously along inward propagating crystallization fronts (e.g., Gutiérrez et al, 2013;Marsh, 1996;Paterson et al, 1998Paterson et al, , 2019. In addition, we infer a simple AMS-to-strain relation in which the principal AMS axes (k 1 , k 2 , and k 3 ) correspond to the X i , Y i , and Z i axes of the instantaneous strain ellipsoid (e.g., Borradaile, 1991;Borradaile & Henry, 1997;Cogné & Perroud, 1988;Hrouda, 1993;Ježek & Hrouda, 2002).…”

“…Circumferential magmatic to submagmatic fabrics in shallow plutons elsewhere have been interpreted to record concentric expansion of plutons, diapiric magma ascent, laccolithic growth, or a helical magma flow in a posttectonic stress regime (e.g., Machek et al, 2019;McCarthy et al, 2015;Paterson & Vernon, 1995;Trubač et al, 2009;Žák et al, 2011). The domainal nature of the Alamosa River monzonite, however, best corresponds to a convective flow pattern (Gutiérrez et al, 2013;Payacán et al, 2014). Such a pattern also explains the occurrence of sparse, shallowly dipping AMS foliations.…”

Ignimbrites to batholiths: Integrating perspectives from geological, geophysical, and geochronological data

“…Extensive upward (and downward?) flow late during pluton emplacement (or by convection in the reservoir; Gutierrez et al, 2013), as recorded by widespread steep mineral foliations in eroded Cordilleran plutons, would have disrupted and largely obliterated the early history of tabular magma assembly. Large individual plutons and composite batholith-size bodies, even if incrementally constructed by magma batches having lenticular aspect ratios, aggregate on scales that eventually occupy much of the crust.…”

On the kinematics and dynamics of crystal‐rich systems

“…Conflicting geological, geochemical, and geophysical evidence suggest that magmas and migmatites are either short-lived or can persist in a crystalrich state for time spans up to 10 6 years [Barboni et al, 2016;de Silva and Gregg, 2014;Kaiser et al, 2017;Paterson et al, 2016;Pritchard and Gregg, 2016;Stelten et al, 2017;Ward et al, 2014]. In the plutonic record this is often expressed as complex time-transgressive mineral and magnetic fabrics with evidence for both fluid and granular (crystal-crystal) interactions [Collins et al, 2006;Gutiérrez et al, 2013;Holness et al, 2007;Jerram et al, 2003;Nicolas, 1992;Paterson, 2009;Sawyer, 2008] and in the volcanic record by the eruption of crystal-rich magmas with a complex crystal cargo [Bachmann et al, 2002;Charlier et al, 2007;Klemetti and Clynne, 2014;Lindsay et al, 2001;Thomson and Maclennan, 2013] (see Figure 1). Persistent crystal-rich conditions in basaltic magma bodies have also been inferred based on seismic evidence from ocean islands and mid-ocean ridges [Lin et al, 2014;Sinton and Detrick, 1992] and by the occurrence of picrites [Rhodes, 1995].…”