Mixing dry and wet magmas in the lower crust of a continental arc: new petrological insights from the Bear Valley Intrusive Suite, southern Sierra Nevada, California

Abstract

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Growth of the upper crust in intra-oceanic island arcs by intrusion of basaltic magmas: the case of the Koloula Igneous Complex, Guadalcanal, Solomon Islands (SW Pacific)

“…Chivas et al 1982;Whalen 1985;Haase et al 2014;Müntener and Ulmer 2018), re-melting of hydrothermally altered mafic crustal rocks and possibly sediments (e.g. Petford and Atherton 1996;Haraguchi et al 2003;Smith et al 2003;Saito et al 2011), lower crustal fractional crystallisation of arc basaltic magmas (Loucks 2021), polybaric fractionation (Almeev et al 2013;Hamada et al 2014;Melekhova et al 2015;Lewis et al 2021;Marxer et al 2022), or magma mixing (Eichelberger 1978;Reubi et al 2003;Reubi and Nicholls 2005;Reubi and Blundy 2009;Ichiyama et al 2020;Rezeau et al 2021;Reubi and Müntener 2022).…”

“…Schmidt and Jagoutz 2017). Intermediate to felsic magmas may form in intra-oceanic island arcs by: (1) shallow fractional crystallisation from basaltic melts (Chivas et al 1982;Whalen 1985;Müntener and Ulmer 2018); (2) melting of the hydrous lower crust of island arcs (Smith et al 2003;Saito et al 2011); (3) lower crustal fractional crystallisation of arc basaltic magmas (Loucks 2021); (4) a combination of deep and shallow (polybaric) fractionation (Almeev et al 2013;Hamada et al 2014;Melekhova et al 2015;Lewis et al 2021;Marxer et al 2022); or (5) magma mixing (Eichelberger 1978;Reubi et al 2003;Reubi and Nicholls 2005;Reubi and Blundy 2009;Ichiyama et al 2020;Rezeau et al 2021;Reubi and Müntener 2022).…”

Evidence for polybaric fractional crystallization in a continental arc: Hidden Lakes mafic complex, Sierra Nevada batholith, California

“…Although the SNB is dominated by felsic plutons that crystallized at shallow pressures (generally <0.5 GPa; Nadin et al 2016), direct evidence for deep crustal processes is present as ultramafic to mafic Cretaceous xenoliths in Miocene to Pliocene basalts in the central SNB, sourced from 1 to 3 GPa (Lockwood and Bateman 1976;Peselnick et al 1977;Domenick et al 1983;Dodge et al 1986;Dodge et al 1988;Mukhopadhyay 1991;Mukhopadhyay and Manton 1994;Ducea and Saleeby 1996;Ducea and Saleeby 1998;Lee et al 2001;Lee et al 2006). In addition, southern exposures of the batholith in the Tehachapi Mountains crystallized at pressures between 0.7 to 1.0 GPa (Ross 1985;Sams and Saleeby 1988;Pickett and Saleeby 1993;Lackey et al 2005;Saleeby et al 2008;Jagoutz 2021, Rezeau et al 2021). Orthopyroxene compositions from lower crustal Dodge et al 1986;Lee et al 2006).…”

“…We posit that the fractionation of a low-MgO basalt deeper than the exposures of much of the SNB, yet at depths sufficiently low to allow for plagioclase saturation (<0.7 GPa), could generate evolved non-peraluminous melts. Crystallization of "damp" basalt to andesitesand enhancement of orthopyroxene stability could also contribute to limiting the increase in ASI of differentiating melts at high pressures (Rezeau et al, 2021). This is supported by gabbro and gabbronorite lithologies with a maximum Mg# identical to the HLMC (Mg# = 73) in the ~0.8-0.9 GPa section of This reconciles the discrepancy between experimental and observational data, in which experiments indicate that melts produced by lower crustal fractionation rapidly become peraluminous (Cawthorn and Brown 1976;Cawthorn and O'Hara 1976;Blatter et al 2013;Nandedkar et al 2014;Müntener and Ulmer 2018;Ulmer et al 2018), yet lower crustal ultramafic cumulates are ubiquitous in arc crustal cross sections (Debari and Sleep 1991;Greene et al 2006;Otamendi et al 2012;Jagoutz 2014;Walker et al 2015;Guo et al 2020).…”

“…Thus, there are two observations that must be reconciled: (1) lower crustal arc sections dominated by ultramafic to mafic cumulates indicating the importance of deep crustal fractional crystallization and (2) experimental crystallization studies suggesting that although high-silica melts can be produced at these pressures, they will become peraluminous at relatively low SiO 2 contents (<60 wt.%), a feature rarely observed in arc batholiths. Three processes have been proposed to explain this apparent contradiction: (1) polybaric differentiation (Hamada et al 2014;Melekhova et al 2015), (2) mixing between low-Mg basaltic and granitic melts (Sisson et al 1996;Grove et al 2003;Sisson et al 2005;Blatter et al 2013;Müntener and Ulmer 2018), or (3) mixing of dry and wet magmas to produce "damp" (<2 wt.% H 2 O) basalts to andesites that then further differentiate in the lower crust (Rezeau et al 2021) (e.g., Sisson et al 2005;Müntener and Ulmer 2018). All three of these processes require the presence of evolved (low-MgO) A c c e p t e d m a n u s c r i p t ACCEPTED MANUSCRIPT basalts produced through lower crustal differentiation of more primitive, high-MgO basalts.…”