Magmatic Tubes, Pipes, Troughs, Diapirs, And
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Magmatic tubes, pipes, troughs, diapirs, and plumes: Late-stage convective instabilities resulting in compositional diversity and permeable networks in crystal-rich magmas of the Tuolumne batholith, Sierra Nevada, California Scott R. Paterson Department of Earth Sciences, University of Southern California, Zumberge Hall of Science, 3651 Trousdale Parkway, Los Angeles, California 90089-0740, USA ABSTRACT the case of the Tuolumne batholith involved al., 2004; Gray et al., 2008). This view has been crystals with ages ranging over ~10 m.y. A questioned by authors describing a number of A complex array of widespread, but likely solution is that crystals in subvolca- processes that occurred in largely constructed domainally developed, structures is pre- nic chambers become armored (rimmed) chambers, such as local ponding of mafi c served in the Tuolumne batholith, includ- by other crystals or exist in crystal clusters magmas, convection, mixing, and fraction- ing stationary and migrating tubes, pipes, that, in spite of changing environmental ation, which occurred in crystal mush zones troughs, diapers, and plume heads. These conditions, prevent rapid chemical commu- that approached and eventually exceeded 50% structures, all formed by local magma fl ow nication with the surrounding melts. These crystals (e.g., Wiebe, 1996; Wiebe and Collins, through crystal-mush host magmas, are structures also challenge many aspects of 1998; Marsh, 1996, 2006; Miller and Miller, often associated with the formation of schlie- the incremental chamber growth model 2002; Hersum et al., 2005; Walker et al., 2007), ren rich in accessory and mafi c minerals, and resulting in sheeted bodies championed by and is being challenged further by recent stud- are associated with fi lter pressing and accu- Glazner, Bartley, Coleman, and colleagues ies concluding that processes leading to compo- mulations of crystals with diverse magma for the Tuolumne batholith. The thousands sitional diversity occur in magmas with >50% histories and ages. Together they represent a of preserved internal structures provide crystals (e.g., Bachmann and Bergantz, 2004, network in which channelized fl ow occurred clear evidence against late annealing and 2008; Žák and Klomínský, 2007). For example, in an existing chamber of crystal-rich mag- removal of internal contacts, and are diffi - Bergantz (2000) used numerical modeling to mas, resulting in local compositional and cult to reconcile with either vertical sheeted examine rheological controls of internal magma structural diversity. or subhorizontal laccolith models; however, boundaries and concluded that if an intrusive These structures also are useful structural they are permissive of early pulsing leading unit along an internal margin is not fairly crys- tools for evaluating the internal evolution of to one or more large magma chambers. tal rich and thus stiff, its boundary is not stable magma chambers. For example, the consis- and would collapse, making it less likely that tently steep tube and pipe axes indicate that INTRODUCTION margins between crystal poor magmas are pre- neither the pluton nor features in the plu- served in chambers. A related issue comes from ton were tilted during growth, thus exclud- It has been suggested that as magmas a dramatic proposal that many of these internal ing models in which subhorizontal layers approach their solidus and become crystal contacts, including internal contacts between tilted to form the existing steep contacts. rich, their viscosities rise to such a degree that magma pulses, are entirely removed due to Although the overall direction of young- it becomes essentially impossible for them to late, thermally driven annealing (Glazner et al., ing established by geochronologic studies is convect, fractionate, and/or erupt and thus to 2008a, 2008b) a suggestion that was challenged toward the batholith center, local younging form the compositional and structural diversity by Vernon and Paterson (2008a, 2008b). Thus, directions determined from troughs cutoffs often preserved in chambers (Barriere, 1976; the timing of formation of magmatic structures indicate that outward growth occurred in Brandeis and Marsh, 1989; McBirney, 1993; and associated crystal percent will dramatically many zones. The highly variable movement Vigneresse et al., 1996; Scaillet et al., 2000; infl uence our interpretation of chamber con- directions of local diapirs and plumes require Dingwell, 2006). This has led some to conclude struction and evolution and inferences about interactions between buoyancy forces and that most preserved internal compositional and when magmas can mix, mingle, and fractionate other gradients. structural variations in plutons refl ect either in chambers and their subsequent behavior dur- The existence and characteristics of these the juxtaposition of many pulses derived from ing volcanic eruptions. structures have several other implications. the melting of a heterogeneous lower crust or These issues also are critical for evaluating Interpretations derived about crystal resi- mantle, or the processes that operated early another recent conclusion that both volcanic dence times in chambers and about crystal during ascent and chamber construction rather and plutonic rocks are mixtures of crystals mixing during eruptions need to be treated than processes operating in an already con- with distinct histories. For example, Davidson with caution, since mixed crystal popula- structed magma chamber (e.g., McNulty et al., et al. (2001, 2005, 2007) and others (Broxton tions existed well prior to eruptions, and in 1996; Coleman et al., 2004, 2005; Glazner et et al., 1989; Christensen et al., 1995; Claiborne Geosphere; December 2009; v. 5; no. 6; p. 496–527; doi: 10.1130/GES00214.1; 18 fi gures; 1 table. 496 For permission to copy, contact [email protected] © 2009 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/6/496/3338206/i1553-040X-5-6-496.pdf by guest on 26 September 2021 Magmatic structures formed in crystal-rich mushes et al., 2006; Cooper and Reid, 2003; Costa et the Tuolumne batholith, central Sierra Nevada, batholith, based on Al-in-hornblende barometry al., 2003; Barbey et al., 2008; Ramos and Reid, California. Their characteristics and relative (Ague and Brimhall, 1988; Webber et al., 2001; 2005; Wallace and Bergantz, 2005; Walker et timing indicate that their formation and pres- Gray, 2003; Anderson et al., 2007), indicate a al., 2007) have demonstrated, using isotopic ervation required development in crystal-rich depth of 6–10 km, consistent with widespread fi ngerprinting in single minerals, that crystal (e.g., >50%) magmas, and thus indicate that andalusite and local sillimanite in the surround- exchange between different melts is a com- late, local movement of magmas resulted in ing host rocks (Rose, 1957; Memeti et al., mon phenomenon and that the resulting crystal crystal accumulations (of minerals with diverse 2005a; Anderson et al., 2007). populations are often accumulated from two histories), fractionation, and formation of these The Tuolumne batholith (Fig. 1) consists of or more sources. Recent high precision U/Pb structures (Weinberg et al., 2001; Paterson et al., four nested, progressively more evolved inward, thermal ionization mass spectrometry zircon 2005; Žák and Klomínský, 2007; Vernon and intrusive units: (1) the outer Kuna Crest unit dating of multiple single grains supports this Paterson, 2008a; Ruprecht et al., 2008; Bach- to the east and its equivalents along the west- conclusion through the recognition that zir- man and Bergantz, 2008). ern and southern margins (tonalites of Glen con populations in a single sample are a mix An examination of these structures also indi- Aulin and Glacier Point, granodiorite of Gray- of xenocrysts, antecrysts, and autocrysts (e.g., cates that they are useful tools for addressing ling Lake) and inner phases, including (2) the Brown and Fletcher, 1999; Charlier et al., the internal evolution of magma chambers (e.g., Half Dome granodiorite, (3) the K-feldspar 2005; Bindeman et al., 2006; Claiborne et al., Fernandez and Gasquet, 1994; Wiebe and Col- megacrystic Cathedral Peak granodiorite, and 2006; Matzel et al., 2006b; Miller et al., 2007; lins, 1998; Pignotta et al., 2006). For example, (3) a central phase, the Johnson granite porphyry Walker et al., 2007). The resultant volcanic or tubes and pipes, but not plumes and diapirs, (Bateman, 1992; Bateman and Chappell, 1979). plutonic rock is thus a mechanical mixture of may preserve information about paleohorizon- The Kuna Crest unit is mostly fi ne- to medium- crystals, for which (1) the bulk rock geochem- tal and local fl ow directions. Troughs provide grained, dark colored, equigranular tonalite, istry may say little about equilibrium processes information about local younging (or growth) quartz diorite, and biotite-hornblende grano- of melting and crystallization; (2) the contrib- directions as well as directions of magma fl ow. diorite, typically with a strong magmatic fabric uting sources (mantle, crust, subsequent con- Diapirs record information about local gradients and abundant mafi c enclaves. The Half Dome tamination) may be obscured; and (3) the age and displacement fi elds. All of these magmatic granodiorite, consisting of an outer equigranular may be misleading or at best leaves untapped a structures record information about processes and inner porphyritic phase, is generally much great deal of useful information. by which