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A Model for the Origin of Rhyolites from South Mountain, Pennsylvania: Implications for Rhyolites Associated with Large Igneous Provinces

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A Model for the Origin of Rhyolites from South Mountain, Pennsylvania: Implications for Rhyolites Associated with Large Igneous Provinces RESEARCH A model for the origin of rhyolites from South Mountain, Pennsylvania: Implications for rhyolites associated with large igneous provinces Tyrone O. Rooney1,*, A. Krishna Sinha2, Chad Deering3, and Christian Briggs1,† 1DEPARTMENT OF GEOLOGICAL SCIENCES, MICHIGAN STATE UNIVERSITY, EAST LANSING, MICHIGAN 48824, USA 2DEPARTMENT OF GEOLOGICAL SCIENCES, VIRGINIA TECH, BLACKSBURG, VIRGINIA 24061, USA 3DEPARTMENT OF EARTH AND SPACE SCIENCES, UNIVERSITY OF WASHINGTON, SEATTLE, WASHINGTON 98195-1310, USA ABSTRACT High-silica rhyolites, ubiquitous features of continental volcanism, continue to evoke controversy as to their petrogenesis and evolution. We utilized the geochemical characteristics of late Vendian high-silica rhyolites erupted in the Catoctin Volcanic Province at South Mountain in Pennsylvania to probe the origin of the parental magmas and assess heterogeneities in the subsequent fractionation paths. We identifi ed high- and low-Ti signatures within the South Mountain rhyolites, a common feature in many large igneous provinces, and these signatures are suggestive of a genetic link between basalts and rhyolites erupted in the Catoctin Volcanic Province. Two evolutionary trends are super- imposed on the Ti-based subdivisions that refl ect variable control of plagioclase and amphibole in the fractionating assemblage of the South Mountain rhyolites. Such distinctive evolutionary trends are evident in rhyolites from other tectonic settings (e.g., arcs), where they have been interpreted in terms of cold-wet and hot-dry conditions within the differentiating magmas. We interpret the amphibole-dominated frac- tionation path of the South Mountain rhyolites as following a cold-wet fractionation path compared to the hot-dry plagioclase-dominated trends. This study, which examines the geochemical implications of cryptic amphibole fractionation, has implications for assessing the role of amphibole and volatile content in the development of rhyolites in other large igneous provinces. LITHOSPHERE; v. 2; no. 4; p. 211–220; Data Repository 2010100. doi: 10.1130/L89.1 INTRODUCTION Continental fl ood basalts are frequently used to 2000b; Riley et al., 2001; Bryan et al., 2002). assess processes associated with large igneous Large igneous province–related silicic mag- The driving mechanisms responsible for province formation but are typically not primary matism, while geographically widespread, is large igneous province formation remain a topic in composition. Continental fl ood basalts expe- particularly focused along rifts and the rift mar- of considerable debate, although a mantle plume rience signifi cant fractionation and some assim- gins dissecting large igneous provinces such origin is now widely accepted (see Campbell, ilation prior to eruption (e.g., Pik et al., 1999; as Parana/Etendeka (Marsh et al., 2001), and 2007, and many references therein). Other mod- Kieffer et al., 2004), and, in some cases, these Patagonia/Antarctic Peninsula (Pankhurst et al., els for the genesis of large igneous provinces processes may dominate the geochemical vari- 2000b; Riley et al., 2001). Continental exten- invoke processes such as interaction between ability of the erupted products (Thompson et al., sion is therefore considered to be central to the asthenosphere-derived magma and subconti- 2007). While geochemical studies of continental generation of large volumes of silicic magmas nental lithosphere (Ellam and Cox, 1991), melt- fl ood basalt provinces therefore have typically (Bryan et al., 2002). The signifi cant volumes ing of an enriched subcontinental lithosphere focused on the least differentiated samples in of silicic magmatism typically associated with (Pegram, 1990; Hergt et al., 1991; Jourdan et al., order to probe mantle processes and heteroge- mafi c large igneous provinces (>104 km3) and 2009), upper-mantle water saturation through neity (e.g., Pik et al., 1999), the record of plume- silicic large igneous provinces (>106 km3) pres- subduction (Ivanov et al., 2008), lithosphere lithosphere interaction may be most effectively ent a particular challenge in explaining how dehydration (Gallagher and Hawkesworth, preserved in rocks with more evolved composi- large volumes of silicic magma can be rapidly 1992), lateral temperature gradients associated tions. Rhyolite magmas erupted in large igneous generated (Bryan et al., 2002). with convective transport of hot mantle (Mut- provinces preserve an important record of these The origin and evolution of large volumes of ter et al., 1988; Anderson, 1994), delamination lithospheric processes, and therefore they play silicic magmas remain among the most contro- of the subcontinental lithosphere (Camp and a key role in our understanding of the develop- versial topics in modern petrology (Bachmann Hanan, 2008), mixing of melts derived from dif- ment of large igneous provinces. and Bergantz, 2004, 2008; Glazner et al., 2008; ferent mantle reservoirs (Korenaga, 2004), and Increasing awareness of the widespread Brophy, 2009). Most models emphasize the role crustal contamination of mantle-derived melts occurrence of silicic magmas in large igneous of assimilation, fractional crystallization, partial (Lightfoot et al., 1990; Brandon et al., 1993). provinces and the discovery of some domi- melting, and various combinations of these pro- nantly silicic large igneous provinces (e.g., cesses in the petrogenesis of such silicic mag- Pankhurst et al., 1998; Bryan, 2007) have high- mas. Recent advances in numerical modeling *Corresponding author e-mail: [email protected]. †Present address: Department of Geological Sci- lighted the role of these magmas in probing (Dufek and Bergantz, 2005; Annen et al., 2006), ences, University of Nevada–Reno, Reno, Nevada plume-lithosphere interaction (e.g., Garland et and trace-element characterization (Bachmann 89577, USA. al., 1995; Baker et al., 2000; Pankhurst et al., and Bergantz, 2004; Davidson et al., 2007b; LITHOSPHEREFor permission to| Volumecopy, contact 2 | Number [email protected] 4 | www.gsapubs.org | © 2010 Geological Society of America 211 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/2/4/211/3037578/211.pdf by guest on 01 October 2021 ROONEY ET AL. Bachmann and Bergantz, 2008; Glazner et al., 40.1°N Metarhyolite 2008) have illustrated the importance of frac- Metabasalt PENNSYLVANIA tionating phases in controlling trace-element pat- High-Al group expanded Low-Al group terns in high-silica rhyolites, regardless of how left the parental melt was generated (partial melting 40.0°N MARYLAND or crystal fractionation). In this contribution, we examine the trace-element characteristics South Mountain of high-silica rhyolites from South Mountain, Formation Washington 39.9°N Pennsylvania, in order to probe the conditions D.C. of magma evolution in the late Vendian (564 Ma ± 9 Ma: Aleinikoff et al., 1995) Catoctin Volca- nic Province, linked to the late Neoproterozoic 39.8°N eastern Laurentian superplume (Puffer, 2002). This study will examine parental magma hetero- geneity and evolutionary paths preserved within the South Mountain rhyolite suite. 77.5°W 77.4°W 77.3°W 77.2°W 77.1°W VIRGINIA GEOLOGIC SETTING AND BACKGROUND WEST The South Mountain rhyolites occur in VIRGINIA Lynchburg association with fl ood basalts of the Catoctin Roanoke Volcanic Province in the Central Appalachians, and they are exposed along the Blue Ridge anti- clinorium (Reed, 1955; Rankin, 1975; Badger Study and Sinha, 1988; Badger and Sinha, 2004). The Mt. Rogers Area Catoctin Volcanic Province had a signifi cant areal extent of at least 11,000 km2, and has been linked with other temporally synchronous vol- TENNESSEE NORTH CAROLINA Volcanic canic and plutonic rocks displaying an ocean- Plutonic island basalt signature along the eastern margin Basement of North America (from Newfoundland to Vir- Sediments ginia; Puffer, 2002). It has been well established that the Catoctin fl ood basalts are associated 50 km with the breakup of Rodinia and the opening of the Iapetus Ocean (Rankin, 1975; Cawood et al., Figure 1. Regional simplifi ed geological map of the Blue Ridge province between southern Pennsyl- vania and Tennessee (modifi ed after Novak and Rankin, 2004). Inset is a location map for the South 2001). The signifi cant areal extent and magni- Mountain rhyolites in Pennsylvania. The distribution of metarhyolite and metabasalt is derived tude of volcanism along this rifted margin, com- from the Pennsylvania Geological Survey digital geological map of Pennsylvania. Sample locations bined with the recognition of the role of mantle are given in Table DR1 (see text footnote one). plumes in assisting continental breakup globally (e.g., Hill, 1991), and also elsewhere in Rodinia (Li et al., 1999), prompted new models linking Mountain rhyolites) that become progressively alteration and then powdered in a ceramic Bico the rifting of Rodinia to the impingement of a less abundant southward (Reed, 1955; Badger fl at-plate grinder. The sample powders were superplume at the base of the continental litho- and Sinha, 1992; Aleinikoff et al., 1995). The fused into lithium tetraborate glass disks using sphere (Puffer, 2002; Li et al., 2008). South Mountain rhyolites are interpreted to be the procedures outlined elsewhere (Deering et Within the Blue Ridge Province, silicic vol- either metamorphosed glassy fl ows or welded al., 2008). Major elements, Zr, Sr, Rb, and Ni canism is most well developed at the southern tuffs (Fauth, 1968; Aleinikoff et al., 1995) that were analyzed by Brucker X-ray
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