Constraining Upper Crustal Magmas in the N. Chilean Andes Chuck
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Constraining Upper Crustal Magmas in the N. Chilean Andes Chuck Lewis College of Earth, Ocean, and Atmospheric Sciences Oregon State University Senior thesis June 3rd 2020 Advisor: Professor Shanaka de Silva Abstract The 4.59-4.18 Ma Caspana ignimbrite, found within the Altiplano-Puna Volcanic Complex (APVC), is the result of an ~8km3 eruption that took place during the Neogene Ignimbrite Flare- up in the N. Chilean Andes. While most of the eruptions during this flare up are large, monogenetic eruptions, the Caspana that is characterized by a compositional gap of ~16% SiO2 between the parental andesite and daughter rhyolite. The rhyolitic plinian fallout is distinct in composition from the majority rhyolite but is also the result of fractionation from andesite. The andesitic magma in the Caspana system are shown to represent some of the most mafic magma that was recharging the upper crustal chambers in the APVC, though all the andesites seem to share a similar signature. Depth estimates and spatial locality place the chamber at the edge of the large thermal anomaly known as the Altiplano-Puna Magma Body (APMB), where cooling would allow for dense solidification during the waning stages of mantle flux. During an initial period of cooling, the system rapidly fractionated along a shallow cotectic and produced the first of two rhyolites. The system was remobilized following recharge and renewed convection produced the second of two rhyolites and fractionation models are shown for both crystallization sequences. The bulk rhyolitic pumice contains fayalite, amongst other Fe-rich minerals, that indicate low Fe3+/Fe2+. Oxidation state in the rhyolite was likely due to the crystallization of orthopyroxene in the presence of magnetite in the parental andesite. With these two phases crystallizing together without clinopyroxene, the latter of which is common in other APVC magmas, the oxidation state in the daughter can be lowered without another method of reduction. 1 1. Introduction Explosive volcanic eruptions are an intrinsic threat to nearby communities, with continental arc settings having a concentration of this geologic hazard. The assembly of granodioritic batholiths and long-lived volcanic provinces within arcs can occur by periodic high mantle flux into the upper crust, known as an ignimbrite flare-up (de Silva, 2008). Ignimbrites containing super evolved, explosive compositions tend to have a less silicic pumice overlying the basal deposits, and are commonly interpreted as an inverted magma chamber with the parental magma following the fractionate during eruption (Hildreth, 1981). The processes capable of producing rhyolitic magma have been thoroughly investigated and attributed to various causes such as incremental recharge, assimilation, rapid compositional change during fractionation, and gas percolation through dense cumulate structures (Depaolo, 1981; Grove and Donnelly-Nolan, 1986; Bachmann and Bergantz, 2006; Coleman et al., 2012). Many of the dynamics occurring in a magma chamber during evolution, inferred through phase chemistry and whole rock data alike, include the partial melting of cumulates, storage in a rigid mush, recharge, and behavior of magmatic volatiles (Eichelberger et al., 2000; Waters and Lange, 2015; Wolff et al., 2015; Shamloo and Till, 2019). Though it appears there is a non-unique solution to producing rhyolitic compositions, some of these processes certainly prevail over others for a given magmatic system. The Caspana ignimbrite, found within the Altiplano-Puna Volcanic Complex (APVC) of the North Chilean Andes, is an archetypal occurrence of bimodal compositions seen in continental arc settings (de Silva, 1991). The spatiotemporal characteristics of the APVC make the presence of bimodal compositions somewhat of an anomaly, as most of the eruptions in the region are monotonous intermediates erupted from large calderas (de Silva and Kay, 2018). This ignimbrite signifies that high silica, low fragility (explosive) magma can be produced in the upper crust even in regions where astronomically high mantle input causes the geotherm to raise, 2 promoting long term storage and generally homogenous batholiths. The compositional gap between andesite and rhyolite bears with it a phase assemblage that is not common in the APVC. While most andesites in the APVC have two pyroxenes, the andesite in the Caspana has only orthopyroxene. The bulk of the rhyolite that constitutes the ~8km3 of erupted material contains a particularly iron rich assemblage that is undocumented in the APVC: fayalite, allanite, biotite, and oxides. The purpose of this paper is therefore two-fold: 1) we will test the original hypothesis of de Silva (1991) to find if the rhyolite indeed fractionated from the andesite and 2) investigate how these phases were stabilized. Both of these questions are linked to the overarching goals of constraining development of explosive compositions that pose a threat, the construction of the continental (granodioritic) crust, and answering the geologically interesting questions of why here, why now? 2. Background 2.1 The APVC and the Rhyolites Therein The Altiplano-Puna Volcanic Complex (APVC) in the Central Volcanic Zone (CVZ) of the N. Chilean Andes is an expression of long-lived subduction zone magmatism (Figure 1). The plateau where the APVC resides is associated with the Central Andean Neogene Ignimbrite province and was constructed from approximately 10-1 Ma with four distinct pulses of high mantle flux (Folkes et al., 2011a; Kern et al., 2016). Delamination of the Nazca plate is thought to produce a MASH (Melting, Assimilation, Storage, and Homogenization) zone at the base of the crust where partial mantle melts stall due to density differences (Hildreth and Moorbath, 1988). At the base of the crust, they undergo MASH processes until their buoyancy allows them to ascend into the mid and upper crust. High flux into the region led to the construction of an arc-scale batholith beneath the APVC, as represented by a large low velocity zone (Ward et al., 2014) and was aptly named the Altiplano-Puna Magma Body (APMB). Periods of high mantle flux into the mid-crustal mash increase eruption rates (i.e. a flare up) by allowing silicic magmas 3 to rise into upper crustal chambers (de Silva et al., 2006). Conversely, periods of waning lead to more heterogeneity and generally more effusive eruptions as the thermal engine cools off and allows geotherms to relax (Burns et al., 2015). Recent work has shown that thermal erosion along the base of the crust is characterized by higher ƒO2 along the arc than the back arc, and that heat transfer through the crust is relatively efficient (Burns et al., 2020). This allows production of high-Mg andesites near the base of the crust, owing to the coupling of Si and Mg with increasing ƒO2. Additionally, it appears that clinopyroxene plays a significant role as magma leaves the lower crust to undergo further AFC processes. The subducting Nazca plate experienced changes in dip angle through time, resulting in an Eastward migration of the arc as the slab flattened. A subsequent steepening caused the arc to migrate back West, resulting in a crustal shortening that generated the thickest continental crust on Earth of ~70km (Allmendinger et al., 1997). More recent work shows that steepening of the slab followed the passing of the Juan Fernandez ridge. As the slab steepened and mantle melting increased, the delamination of the SCLM allowed for lower crustal mash and recycling of crustal material, raising the baseline isotopic compositions. These architectural changes facilitated the raising of the geotherm, promoting episodic high-mantle power input to the mid and upper crust. The mid-crust experienced substantial thermal and rheological changes as a result, promoting the formation of the Alti-Plano Puna Magma Body (APMB) (de Silva and Gosnold, 2007), which is now proposed as the mid-crustal source of the large volume dacitic eruptions (de Silva, 1989; de Silva and Francis, 1989). Amongst the large volume dacites and their parental andesites, several rhyolites have been found within the APVC (Lindsay et al., 2001b; Schmitt et al., 2001; Folkes et al., 2011b; Grocke et al., 2017b) though none seem to share an explicit petrogenetic origin. All of these rhyolites seem to be derived from within the upper crust by different manners. Large crystalline mushes that undergo periodic influx from the source and closed system fractionation, with or without large degassing events, may form some 4 of these rhyolites. Conversely, these mushes can be remobilized if recharge is substantial enough to melt the base of the mush, promoting convection and thereby renewed fractionation as the mush begins to ‘unzip’ (Burgisser and Bergantz, 2011). Tiered systems or cupolas forming a chemically distinct cap are hypothesized to occur in the APVC among other volcanic centers These rhyolites can continue to undergo internal fractionation, especially if a portion of the eruptible magma ascends during eruption but remains in the chamber. We will assess which differentiation processes can produce the observed rhyolites in the APVC and tease out which of these processes can produce the compositions and assemblages preserved in the Caspana ignimbrite. 1.2 Large Scale Beauty: Lithologies and Field Constraints of the Caspana Ignimbrite The Caspana ignimbrite (4.59-4.18 ma; de Silva, 1989) provides a unique glimpse into a highly evolved system, expressed by bimodal compositions found together in outcrop (Figure 2). The plinian deposit is located more proximal to the inferred source, which is inferred to be buried under the modern stratocones of Toconce and Cerro del Leon, along with the Chao Dacite. A co-plinian ash indicates that the eruption plume had time to settle before the larger pyroclastic density currents (PDC’s) deposited on top (Figure 2). More distal deposits are characterized by a thin reworked layer near the base. The larger outcrops in this area contain both andesitic and rhyolitic pumices within the matrix (Figure 2, 3). Additionally, some outcrops show lenses of lithics, which also provide evidence for an eruption with multiple hiatuses.