Magmatically folded and faulted schlieren zones formed by magma avalanching in the Sonora Pass Intrusive Suite, Sierra Nevada, California

Abstract

The southwestern margin of the Late Cretaceous Sonora Pass Intrusive Suite, northern Sierra Nevada, California (USA), preserves a densely populated zone of magmatic structures that record dynamic magmatic layer formation and deformation (faulting and folding) within a solidifying upper-crustal magma mush. This zone consists largely of coupled melanocratic (or schlieren) and leucocratic bands hosted within the 95.6 ± 1.5 Ma Kinney Lakes granodiorite (Leopold, 2016), with orientations approximately parallel to the intrusive margin and with inward younging directions. Schlieren consist of a high modal abundance of medium-grained ferromagnesian minerals (hornblende + biotite), zircon, sphene, apatite, opaque minerals, and minor plagioclase and interstitial quartz. Leucocratic bands are dominated by coarse-grained feldspar + quartz with minor ferromagnesian and accessory minerals. Whole-rock geochemical and Sr and Nd isotopic data indicate that the schlieren are derived from the Kinney Lakes granodiorite by effective mechanical separation of mafic minerals and accessory phases. We interpret that the schlieren zone at the margin of the Kinney Lakes granodiorite formed by large-scale collapse of crystal mush by “magma avalanching,” facilitated by gravity, local convection, and possibly by host-rock stoping at the margin. This process eroded a significant portion of the solidifying margin of the chamber and resulted in the formation of magmatically deformed layered structures, which experienced further mingling, re-intrusion, magmatic erosion, and recycling processes. We envisage that magma avalanching of magma mushes in plutons can be achieved by any unstable process (e.g., tectonic, fluid-assisted, stoping, or gravity-driven) in large, long-lived magma-mush chambers.

Schlieren-Bound Magmatic Structures Record Crystal Flow-Sorting in Dynamic Upper-Crustal Magma-Mush Chambers

“…Schlieren form an array of major and trace element compositions at high-angle to the main TIC trend, which are interpreted here to represent the varying degrees of crystal accumulation of hornblende, biotite, apatite, titanite and zircon that dominate schlieren compositions. As a suitable complementary felsic fractionate is not observed (see above), we interpret that any melts resulting from schlieren accumulation were mixed back in with surrounding host magma, to form the felsic components of schlieren-bound structures (see also Alasino et al, 2019). Small shifts towards radiogenic Sr i in schlieren, and variable differences in εNd i between schlieren and host are also compatible with a flow-sorting crystal accumulation model.…”

“…In addition, mineralogy and whole-rock geochemistry suggests that schlieren are sourced from the surrounding host magma, rather than an "exotic" mafic magma. Schlieren mineral assemblages reflect, to a first order, the assemblage of the surrounding host, with different modal abundances ( Table 1, see also Reid et al, 1993;Alasino et al, 2019). Parallel REE patterns further suggest that the mixture of REE-bearing minerals in schlieren is matched in the host and indicates a common magma source, distinct from enclaves (Reid et al, 1993), which is supported by the similarity in host and schlieren isotope compositions ( Figure 11D).…”

“…Schlieren are commonly found in planar form (e.g., Bateman, 1992;Žák and Klomínský, 2007;Burgess and Miller, 2008;Pinotti et al, 2016), but also delineate curved boundaries of structures such as: channel-shaped magmatic troughs (e.g., Wahrhaftig, 1979;Barrière, 1981;Solgadi and Sawyer, 2008;Žák and Paterson, 2010;Alasino et al, 2019; see also Wager and Brown, 1968;Vukmanovic et al, 2018 for mafic systems), stationary and migrating tubes, also called ladder dikes (e.g., Reid et al, 1993;Weinberg et al, 2001;Wiebe et al, 2007;Dietl et al, 2010;Clarke et al, 2013), meter-scale diapirs and plume heads (e.g., Weinberg et al, 2001;Paterson, 2009), and mafic ellipsoids (e.g., Memeti et al, 2014). In general, studies of schlieren-bound magmatic structures aim to address two related questions: (1) under what magmatic conditions and by which process(es) do schlieren form ( Figure 1A) and (2) how do these processes influence, or contribute to, the variety of schlieren-bound structures of different geometries and characteristics ( Figure 1B)?…”

“…Processes such as oscillatory nucleation and Ostwald ripening are important in these models (lower left Figure 1A; e.g., McBirney and Noyes, 1979;Glazner, 2014). The third school of thought views schlieren as resulting from magmatic flow against a more rigid boundary, in its most dynamic form by magmatic currents or avalanching, to sort crystals by size and/or density (e.g., Irvine et al, 1998;Solgadi and Sawyer, 2008;Alasino et al, 2019; Figure 1A).…”

“…Under this framework, a range of schlieren-forming processes have been proposed that involve different combinations of these three end-members ( Figure 1A). These processes include thermal and compositional convective instabilities that could result from intrusion of new magma pulses or thermochemical gradients within a solidification front (Griffiths, 1986;Weinberg et al, 2001;Pons et al, 2006;Wiebe et al, 2007;Paterson, 2009;Rocher et al, 2018;Alasino et al, 2019), shear/hydrodynamic flow sorting (Cloos, 1936;Bhattacharji and Smith, 1964;Barrière, 1981;Pitcher, 1997;Clarke et al, 2013) together with compaction and filter pressing at rheologic boundaries to extract melts (Weinberg et al, 2001;Paterson, 2009), gravitational settling (e.g., Coats, 1936;Wager and Deer, 1939), and liquid immiscibility (e.g., Glazner et al, 2012).…”