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

Home , Fauth (crater), Large igneous province, Mount Rogers

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 fractionation 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;

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 heterogeneity 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 fl uorescence and northern terminations, with more mafi c are phenocryst poor (10%–20% of the rock (XRF), and the balance of the trace elements volcanism dominant in the central region volume), with feldspar (albite), quartz, biotite, were obtained by laser ablation using Cetac (Fig. 1). The Mount Rogers rhyolites (ca. 759 ilmenite, and specular hematite (after magne- LSX-200 and Micromass Platform inductively Ma: Aleinikoff et al., 1995) are thought to rep- tite) as phenocryst phases (Fauth, 1968; Mitra coupled plasma–mass spectrometry (ICP-MS). resent an initial (unsuccessful) rifting episode and Sinha, 2004a, 2004b). Trace-element reproducibility based on stan- that was followed by the development of the dard analyses was typically better than 5%. Neoproterozoic Laurentian continental margin METHODS XRF analyses are presented in Table 1, and at ca. 577–550 Ma, coincident with the erup- results of ICP-MS trace-element analyses are tion of the Catoctin Volcanic Province (Bad- We selected 25 samples of the least-altered presented in Table 2. ger and Sinha, 1988; Rankin, 1994; Aleinikoff metarhyolites from 255 km2 of metarhyolites et al., 1995; Novak and Rankin, 2004; Tollo et exposed in the South Mountain region of south- 1GSA Data Repository Item 2010100, Table DR1, loca- al., 2004). We focus on the northern portion of ern Pennsylvania (Fig. 1; Table DR11). Samples tion information for South Mountain samples, is available at www.geosociety.org/pubs/ft2010.htm, or on the Catoctin Volcanic Province in Pennsylva- were carefully trimmed, jaw crushed, and hand- request from [email protected], Documents Sec- nia, which is dominated by silicic lavas (South picked under a binocular microscope to avoid retary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.

212 www.gsapubs.org | Volume 2 | Number 4 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/2/4/211/3037578/211.pdf by guest on 01 October 2021 A model for the origin of rhyolites from South Mountain, Pennsylvania | RESEARCH

TABLE 1. MAJOR AND TRACE ELEMENT DATA FOR THE RHYOLITES OF THE SOUTH MOUNTAIN REGION OF PENNSYLVANIA

Sample SiO2 TiO2 Al2O3 Fe2O3T MnO MgO CaO Na2O K2O P2O5 LOI Totals Ni Rb Sr Zr (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (ppm) (ppm) (ppm) (ppm) AM-0603-14 75.72 0.14 11.55 2.81 0.06 0.04 0.09 4.30 3.85 0.01 1.27 98.57 12 78 12 744 AM-0603-17 75.06 0.32 11.83 2.98 0.04 0.12 0.00 2.19 5.51 0.02 1.74 98.07 12 110 25 719 AM-0603-19 78.71 0.14 10.38 2.12 0.02 0.08 0.00 2.75 4.39 0.01 1.30 98.6 11 71 41 447 AM-0603-22 78.19 0.14 10.73 2.10 0.03 0.07 0.00 1.88 4.96 0.01 1.79 98.11 10 117 16 452 AM-0603-26A 76.08 0.24 11.30 2.95 0.07 0.03 0.24 3.32 4.96 0.00 0.66 99.19 10 116 8 989 AM-0603-31 75.48 0.30 12.07 2.93 0.05 0.10 0.06 2.83 4.75 0.02 1.23 98.59 10 93 38 720 AM-0603-32 74.16 0.32 12.31 3.07 0.05 0.13 0.09 2.83 4.61 0.05 2.22 97.62 10 94 40 739 AM-0603-34A 73.15 0.34 12.43 3.11 0.01 0.05 0.20 3.41 5.55 0.03 1.52 98.28 9 84 56 744 AM-0603-35A 76.94 0.21 10.94 3.11 0.01 0.04 0.00 3.72 4.05 0.00 0.83 99.02 11 89 10 1099 AM-0603-40 74.97 0.20 11.37 3.03 0.06 0.09 0.58 3.20 5.16 0.01 1.16 98.67 11 121 31 1124 AM-0603-41 75.90 0.20 11.39 3.17 0.08 0.06 0.29 3.53 4.68 0.01 0.53 99.31 12 111 25 1074 AM-0603-45 72.69 0.38 12.39 4.09 0.05 0.21 0.00 3.68 4.70 0.04 1.56 98.23 10 92 46 999 AM-0603-46 76.96 0.20 11.01 2.68 0.01 0.06 0.00 2.76 5.23 0.01 0.95 98.92 11 121 7 785 AM-0603-47A 78.82 0.16 9.58 3.09 0.05 0.22 0.00 2.83 3.79 0.01 1.30 98.55 13 66 18 783 AM-0603-52 75.44 0.29 11.05 3.48 0.06 0.06 0.27 3.09 5.00 0.01 1.07 98.75 13 108 23 1138 AM-0603-53A 76.32 0.29 10.63 3.39 0.01 0.03 0.17 3.52 4.10 0.01 1.37 98.47 11 82 12 1141 AM-0603-54 76.36 0.28 10.80 2.95 0.04 0.28 0.22 2.77 5.20 0.00 0.94 98.9 11 107 11 1063 AM-0603-57 76.26 0.28 11.05 3.51 0.07 0.04 0.30 3.82 3.98 0.01 0.50 99.32 13 84 27 1159 AM-0603-58 75.15 0.29 10.95 3.32 0.07 0.07 0.30 3.35 4.70 0.00 1.63 98.2 11 104 21 1162 AM-0603-61 75.46 0.28 11.17 3.10 0.13 0.06 0.31 2.81 5.54 0.01 0.97 98.87 12 124 28 1157 AM-0603-65* 71.83 0.31 14.42 3.29 0.07 0.08 0.19 6.18 3.22 0.52 99.59 47 39 592 AM-0603-65B 71.58 0.31 14.22 3.19 0.07 0.10 0.12 5.99 3.29 0.03 0.91 98.9 12 49 41 738 AM-0603-69A 77.75 0.14 11.40 1.51 0.02 0.08 0.04 3.65 4.20 0.01 1.10 98.8 9 84 30 436 AM-0603-7A 74.67 0.22 13.29 1.85 0.00 0.15 0.05 4.34 4.11 0.00 1.17 98.68 10 72 34 834

Note: X-ray ﬂ uorescence analyses for samples in this study yielded all major elements as wt.% plus Zr, Rb, and Sr as ppm. Fe is reported as total Fe2O3. Analytical details are outlined in the Methods. LOI—loss on ignition. *Analysis undertaken by Mitra and Sinha (2004a, 2004b).

TABLE 2. TRACE ELEMENT DATA FOR THE RHYOLITES OF THE SOUTH MOUNTAIN REGION OF PENNSYLVANIA Sample Ba La Ce Pr Nd Sm Eu Gd Tb Y Dy Ho Er Yb Lu Cr Nb Hf Ta Pb Th U AM-0603-14 281 145 210 44 172 36.3 3.9 33.0 5.4 199 32.8 5.8 16.3 13.3 2.1 12.2 156 20.6 9.5 37.7 10.6 9.1 AM-0603-17 693 78 185 22 80 16.0 2.4 17.0 2.7 93 14.8 2.7 8.3 8.1 1.4 4.9 95 18.0 10.0 7.5 10.8 4.6 AM-0603-19 306 60 150 17 63 12.7 0.8 13.8 2.2 77 13.1 2.4 7.5 7.5 1.3 1.9 122 14.2 9.3 10.8 13.6 2.9 AM-0603-22 191 74 182 22 80 17.7 1.2 18.3 2.8 90 15.9 2.8 8.6 8.7 1.5 7.2 117 14.2 9.2 16.7 13.7 3.1 AM-0603-26A 155 84 197 24 90 19.1 1.6 19.0 3.1 116 18.4 3.3 10.3 9.7 1.6 2.0 110 23.5 6.8 15.9 11.5 3.5 AM-0603-31 624 30 150 12 49 11.1 1.7 11.9 2.0 73 11.3 2.1 6.9 7.2 1.3 2.2 87 17.5 7.7 7.9 10.5 2.2 AM-0603-32 616 50 167 16 60 12.0 1.8 13.2 2.2 82 12.4 2.3 7.4 7.1 1.2 2.4 96 18.1 8.1 9.3 11.2 2.6 AM-0603-34A 849 119 260 32 116 20.4 2.7 20.2 3.0 102 15.7 2.9 9.0 8.5 1.4 2.2 105 17.8 8.5 8.8 10.7 2.4 AM-0603-35A 86 20 31 7 29 10.3 0.7 11.6 2.5 93 16.6 3.0 9.1 8.9 1.6 2.9 138 26.3 8.2 9.8 11.8 2.5 AM-0603-40 90 130 260 37 146 31.6 1.9 30.7 4.9 177 29.0 5.2 15.3 13.9 2.2 2.1 150 27.2 8.4 9.1 13.1 5.3 AM-0603-41 51 128 261 34 130 26.8 1.7 26.7 4.2 160 24.2 4.4 13.0 12.1 2.0 11.4 140 26.5 8.7 9.3 12.5 4.7 AM-0603-45 821 82 186 24 86 16.9 2.9 17.9 2.9 116 16.5 3.0 9.4 8.7 1.5 7.6 101 21.3 8.9 14.1 8.5 3.7 AM-0603-46 176 55 121 15 57 12.1 1.1 13.9 2.5 111 15.7 3.0 9.7 9.2 1.6 3.7 102 20.0 8.0 14.0 10.9 3.9 AM-0603-47A 210 87 125 25 95 18.8 1.7 16.9 2.7 97 15.5 2.9 9.1 9.2 1.5 1.9 118 20.5 8.0 8.0 9.6 3.1 AM-0603-52 48 213 266 53 199 37.8 1.5 34.9 5.2 155 28.3 4.6 12.3 11.0 1.8 15.0 140 24.8 7.9 13.7 12.1 2.9 AM-0603-53A 53 82 155 21 77 15.9 0.7 16.6 3.0 132 18.8 3.5 10.7 10.1 1.7 1.9 140 25.0 7.2 8.5 11.5 3.5 AM-0603-54 102 91 172 24 93 23.1 1.1 26.1 4.5 175 28.4 4.9 13.0 10.7 1.8 2.3 141 23.8 7.5 7.0 11.2 2.1 AM-0603-57 46 118 287 32 115 21.6 0.8 20.7 3.2 118 17.7 3.3 10.3 10.6 1.8 27.4 162 26.0 8.6 13.7 13.6 4.3 AM-0603-58 45 120 287 33 119 22.9 0.9 23.2 3.6 132 20.0 3.7 11.2 11.0 1.9 2.4 167 25.4 8.0 15.5 12.6 3.9 AM-0603-61 59 120 283 33 121 23.3 0.9 23.4 3.7 135 20.0 3.7 11.3 11.2 1.9 5.3 150 25.5 8.5 19.4 12.4 4.3 AM-0603-65* 829 37 71 45 14.0 2.5 15.8 3.2 107 19.1 3.5 11.2 8.8 1.3 56 16.3 5.4 10.8 4.4 AM-0603-65B 857 34 73 11 45 13.4 2.5 14.3 2.7 128 18.4 3.4 10.3 9.1 1.5 14.0 98 18.7 9.4 29.3 11.4 6.7 AM-0603-69A 628 53 133 16 61 12.5 1.7 12.7 1.9 68 10.8 1.9 6.4 7.5 1.3 2.3 106 15.3 8.5 8.7 9.4 3.9 AM-0603-7A 386 76 119 21 79 15.5 1.6 14.6 2.5 102 15.1 2.9 9.2 9.3 1.6 3.5 120 22.1 8.8 22.9 13.1 5.3 Notes: Data derived by laser ablation inductively coupled plasma–mass spectrometry and reported as ppm. Analytical details are outlined in the Methods. *Analysis from Mitra and Sinha (2004a, 2004b).

LITHOSPHERE | Volume 2 | Number 4 | www.gsapubs.org 213

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.

RESULTS differences: REE concentrations decrease with for a given Zr. This variation is typical in many

increasing SiO2 more rapidly within the high- large igneous provinces, where rhyolites may The South Mountain rhyolites are typically Al group in comparison to the low-Al group, be broadly divided into high-titanium (HT) and

peraluminous, high in silica (>72% SiO2), and excluding Eu (Fig. 3). For most trace elements, low-titanium (LT) groups on the basis of HFSE deﬁ ne two groupings based on heterogeneity such as Y, Nb, Yb, and Zr (excluding Sr, Ba, and heterogeneity (Garland et al., 1995; Ayalew et in major- and trace-element concentrations and Eu—not shown), the concentrations within the al., 2002). It is important to note that this LT/HT distinctive trace-element differentiation trends. high-Al group are lower than that of the low-Al division does not correlate with the previously

These two groups are well deﬁ ned in terms of group at similar SiO2 contents (Fig. 3). Neither deﬁ ned variations in Al2O3; both HT and LT

variable concentrations of Al2O3, Sr, and Ba the high- or low-Al group clusters consistently varieties occur within the low-Al group. While

(Fig. 2). The low-Al group (<12 wt% Al2O3) has with samples from the Mount Rogers forma- the data are limited, the Mount Rogers rhyo- Sr (7–41 ppm) and Ba (50–628 ppm) concen- tion, but there is broad overlap in many element lites typically occur at very low levels of Nb/Y, trations that are lower than the high-Al group concentrations between the Mount Rogers and consistent with their relative depletion in Nb in

(>12 wt% Al2O3), where Sr (25–56 ppm) and Ba South Mountain, with the exception of Nb and comparison to South Mountain. (386–857 ppm) are more enriched. Most sam- Yb (Fig. 3). Chondrite-normalized REE pat- The observed trace-element variations are ples in the low-Al group have higher values of terns reveal further differences between the two best explained as reﬂ ecting primary magmatic

SiO2 (75–78 wt%) in comparison to the high-Al groups: (1) the most pronounced Eu anomalies processes rather than secondary alteration.

group (SiO2 ~72–75 wt%). Major-element vari- occur within the low-Al group, and (2) the middle For example, a comparison among Y and Nb ation of the South Mountain rhyolites is simi- rare earth elements (MREE) are more depleted and La shows trends that are consistent with lar to that observed in the high-silica rhyolites within the high-Al group (Fig. 4). Primitive magmatic processes. Y is not decoupled from erupted at Mount Rogers, with the exception mantle–normalized trace-element patterns are Nb and La (r2 = 0.34, r2 = 0.36, respectively),

of TiO2, where some overlap occurs but Mount broadly similar, though variations in Sr, P, and which are thought to be less mobile during sec- Rogers samples have lower values (Fig. 2; Ba are particularly apparent (Fig. 5). Within the ondary alteration processes (Price et al., 1991; Novak and Rankin, 2004). Rare earth element low-Al group, further heterogeneity is observed Cotten et al., 1995). This inference is further

(REE)–SiO2 differentiation trends (in particu- in terms of high ﬁ eld strength elements (HFSE): supported by the lack of strong negative Ce lar Y, Dy, and Yb) exhibit the most distinctive A subset of the low-Al group plots at higher Nb anomalies in all samples (Fig. 4), which can be caused by the formation of Ce4+ under oxi- dizing conditions of surface environments, while other REEs remain in the trivalent state

17 0.5 (Class and le Roex, 2008), resulting in pref- Mt. Rogers erential mobilization of Ce. Samples that do 0.45 16 High-Al group exhibit some Ce mobility (Fig. 4) do not, how- 0.4 15 Low-Al group ever, defi ne any of the trends and groupings 0.35 described here (Briggs et al., 2008). 14 0.3 (wt. %) (wt. %) 3 2 O 2 13 0.25 DISCUSSION TiO Al High Al 12 0.2 Low Al 0.15 Origin of Rhyolites in Mafi c and Silicic 11 Large Igneous Provinces 0.1 10 0.05 The ultimate origin of rhyolite magmas 9 0 72 74 76 78 80 72 74 76 78 80 in mafi c and silicic large igneous provinces 0.09 1.2 SiO2 (wt. %) SiO2 (wt. %) remains a controversial topic, and there are 0.08 three dominant hypotheses: (1) melting of 1 existing crust, (2) melting of fl ood basalts or 0.07 underplated material, and (3) open-system

0.8 (wt. %) 0.06 fractional crystallization. 3 O 2 0.05 (1) The large-scale anatexis of hydrated 0.6 lower crust along accretionary continental CaO (wt. %) CaO 0.04

CaO/Al margins is suggested to be a signifi cant process 0.4 0.03 capable of generating large-volume rhyolites 0.02 erupted in silicic large igneous provinces (e.g., 0.2 Pankhurst and Rapela, 1995; Pankhurst et al., 0.01 1998; Riley et al., 2001; Bryan et al., 2002; 0 0.00 Bryan, 2007). However, recent advances in 72 74 76 78 80 72 74 76 78 80 SiO (wt. %) numerical modeling have highlighted the dif- SiO2 (wt. %) 2 fi culty in generating rhyolites solely through Figure 2. Major-element variation in the South Mountain rhyolites. High-Si rhyolites (<72% SiO ) 2 intrusion-driven anatexis and require signifi - from the ca. 750 Ma Mount Rogers suite in Virginia (Novak and Rankin, 2004) are also shown for comparison. Data are presented in Table 1. Two groups of South Mountain rhyolites are evident, cant mass contributions from the intruding basalts (Annen and Sparks, 2002; Dufek and divided at ~12% Al2O3. CaO/Al2O3 versus SiO2 illustrates the relative importance of plagioclase fractionation in the low-Al group and Mount Rogers samples in comparison to the high-Al group. Bergantz, 2005; Annen et al., 2006).

214 www.gsapubs.org | Volume 2 | Number 4 | LITHOSPHERE

250 200 180 200 160 140 150 120 Nb (ppm)

Y (ppm) 100 100 80 60 50 Mt. Rogers 40 High-Al group 20 Low-Al group 0 0 Figure 3. Trace-element variation in the South 72 74 76 78 80 72 74 76 78 80 Mountain rhyolites. High-Si rhyolites (<72% SiO (wt. %) SiO2 (wt. %) 2 1000 SiO ) from the ca. 750 Ma Mount Rogers suite 1400 2 900 in Virginia (Novak and Rankin, 2004) are also 1200 800 shown for comparison. Data are presented in Table 2. As with the major elements, trace- 700 1000 element variation identiﬁ es two groups of 600 800 South Mountain rhyolites that correspond Ba (ppm) Zr (ppm) 500 to the low- and high-Al groups. For most ele- 600 400 ments, the high-Al group has lower concentrations of trace elements at similar values of SiO . 300 2 400 Notable exceptions are Sr and Ba, where the 200 200 high-Al samples are much more enriched than 100 the low-Al group and Mount Rogers samples. 0 0 Overall, the low- and high-Al groups deﬁ ne dis- 72 74 76 78 80 72 74 76 78 80 tinct fractionation trends that indicate hetero- SiO (wt. %) SiO (wt. %) 2 2 geneity in the magmatic evolution of these two 20 60 groups in terms of variable plagioclase, alkali 18 feldspar, and amphibole. 50 16

14 40 12 30 10 (ppm) Sr Yb (ppm) 8 20 6

4 10 2 0 0 72 74 76 78 80 72 74 76 78 80 SiO (wt. %) SiO2 (wt. %) 2

1000

Figure 4. Chondrite-normalized (Boynton, 1984) rare earth element (REE) plot of the South Mountain rhyolites. Some REE mobility is likely 100 given the evidence of some Ce anomalies; however, these samples do not deﬁ ne or anchor the

high- and low-Al trends. High-Al2O3 samples (black) have a much less pronounced Eu anom-

aly than the low-Al2O3 variety (gray). Low-Al2O3 10 samples extend to higher heavy (H) REE con-

centrations, while the high-Al2O3 variety have a Low-Al group mild U-shaped middle (M) REE depletion that is High-Al group typical of amphibole removal.

1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu

LITHOSPHERE | Volume 2 | Number 4 | www.gsapubs.org 215

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.

1000 2.0 Mt. Rogers High-Al group 1.8 Low-Al group 1.6 High-Nb group 100 1.4

1.2

1.0 10 Nb/Y 0.8

0.6

1 0.4

Low-Al group 0.2 High-Al group 0.0 0.1 24681012 Cs Rb Ba Th U Nb Ta K La Ce Pb Pr Sr P Nd Sm Zr Hf Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Zr/Y

Figure 5. Primitive mantle–normalized (Sun and McDonough, 1989) spidergram of the South Moun- Figure 6. Trace-element ratio plot that is typi-

tain rhyolites. High-Al2O3 samples (black) have a less pronounced negative Sr-P anomaly in com- cally used to distinguish between low-Ti and

parison to the low-Al2O3 variety (gray). Low-Al2O3 samples also exhibit a strong depletion in Ba that high-Ti (HT) magma series in large igneous is absent from the high-Al variety. provinces. The low- and high-Al groups both broadly plot in the same region; however, four samples from the low-Al group plot at higher (2) Regional studies of rhyolites erupted through preserved heterogeneity in the paren- Nb for a given Zr value and are indicative of a within mafi c large igneous provinces have tal magmas and geochemical diversity/litho- genetic connection with the HT magma series. This geochemical distinction also has a geo- identifi ed geochemical provinciality that mir- spheric signatures through crustal assimilation graphical component in that the four HT-type rors the composition of the fl ood basalts with and crystal fractionation. rhyolites are the only low-Al samples north which they are associated (Garland et al., of 40°N. This northern shift toward HT variety 1995; Marsh et al., 2001; Ayalew and Yirgu, Origin of Major- and Trace-Element magmas is consistent with regional studies of 2003; Miller and Harris, 2007). Magmatism Variations at South Mountain the Catoctin fl ood basalts (and plutonic equiva- within many continental fl ood basalt prov- lents) that have suggested a Ti maximum for this large igneous province in the Sutton Moun- inces is divided into dominantly HT and LT We interpret the geochemical variations tain region of Quebec (Puffer, 2002). magma suites, refl ecting heterogeneity in man- evident within the South Mountain rhyolites in tle sources of large igneous province magmas terms of open-system crystal fractionation, con- (e.g., Peate et al., 1999; Pik et al., 1999; Greene sistent with most existing models for rhyolite et al., 2009). Partial melting of HT/LT under- origin in other dominantly mafi c large igneous Crystal fractionation is considered to be plated or intruded basaltic material would provinces. The parental magmas of the South the dominant control of trace-element varia- allow for the retention of the spatial relation- Mountain rhyolites are inferred to have a broad tions in high-silica rhyolites (Bachmann and ship between HT and LT rhyolites and associ- subdivision into HT and LT varieties based Bergantz, 2008), and variations in the relative ated erupted basalt (e.g., Lightfoot et al., 1987; upon the preserved HT and LT rhyolites trends proportions of plagioclase and clinopyroxene Miller and Harris, 2007). However, these mod- evident in the South Mountain suite. Samples and/or amphibole in fractionating assemblages els have similar diffi culties to crustal anatexis, defi ned as HT (higher Nb at a given Zr concen- of magmas evolving in similar environments is because insuffi cient heat may be available to tration; Fig. 6) are restricted to the region north frequently invoked to account for differences

melt the maﬁ c intrusions (Dufek and Bergantz, of 40°N, though LT varieties occur throughout in the concentration of Eu, Al2O3, and Sr (e.g.,

2005; Annen et al., 2006). Furthermore, in the study area. The origin of LT and HT mag- Ayalew et al., 2002). Variability in CaO, Al2O3,

anorogenic environments, migmatitic terranes mas within ﬂ ood basalt provinces remains unre- Ba, and Na between the high- and low-Al2O3 have not shown leucosomes of A-type afﬁ n- solved but may be explained in terms of a broad groups at South Mountain points to alkali- ity (Bonin, 2007), and experimental studies on heterogeneous upwelling (Kieffer et al., 2004), feldspar and plagioclase control, respectively. partial melts of lower-crustal rocks have failed zoned mantle plumes (Pik et al., 1999), or con- Decreasing Dy/Yb with increasing fraction-

to produce liquids of an appropriate high-silica tributions from both an upwelling plume and ation index (e.g., SiO2; Fig. 7) is observed composition (Beard et al., 1994). the lithospheric mantle (Greene et al., 2009). within the high-Al South Mountain rhyolites (3) Models that involve open-system crys- Deducing the precise origin of LT and HT mag- and is commonly attributed to crystal frac- tal fractionation, with extraction of melt from matism within the South Mountain magma suite tionation involving amphibole (+titanite) over a residual crystal mush to form large crystal- requires further isotopic constraints and lies pyroxene (Macpherson et al., 2006; Davidson poor silicic magmas (Bachmann and Bergantz, beyond the scope of this study. The observed et al., 2007a; Brophy, 2009). On the basis of our 2004; Hildreth, 2004), appear to be the most division of the South Mountain rhyolites into observations, it is clear that the high-Al sam- feasible mechanism by which large-volume HT and LT parental magmas does not account ples from South Mountain may be attributed rhyolite magmas in large igneous provinces for the dominant geochemical heterogeneity of to more amphibole and less feldspar removal may be generated (e.g., Garland et al., 1995; high- and low Al groups observed within the than the low-Al samples. Our interpretation Ewart et al., 1998; Baker et al., 2000; Ayalew South Mountain rhyolites, which instead must that the South Mountain rhyolites were derived et al., 2002; Peccerillo et al., 2003). These have developed during open-system crystal from open-system fractional crystallization is models account for LT and HT provinciality fractionation processes. consistent with similar interpretations made for

216 www.gsapubs.org | Volume 2 | Number 4 | LITHOSPHERE

1.8 tain rhyolites (Fig. 9) has implications for the High-Al group magmatic conditions during rhyolite magma 1.7 Low-Al group evolution in large igneous provinces. The dis- 1.6 tinctive REE patterns deﬁ ned by the low- and

1.5 high-Al2O3 groups from South Mountain mirror Figure 7. Variation of Dy/Yb those of rhyolites from rifted arcs (Bachmann 1.4 (chondrite normalized) with and Bergantz, 2008; Deering et al., 2008). In SiO shows a negative cor- 1.3 2 such environments, the evolution of the REE relation for the high-Al2O3

(Dy/Yb)N chemistry is determined by whether the magma 1.2 group (dotted outline), sug- is either related to a hot-dry-reduced or cold- 1.1 gestive of amphibole fractionation (see text for fur- wet-oxidized source (Bachmann and Bergantz, 1 ther discussion). 2008; Christiansen and McCurry, 2008; Deer-

0.9 ing et al., 2008). Hot-dry-reduced magmas fractionate olivine, plagioclase, and pyroxene, 0.8 72 73 74 75 76 77 78 79 80 producing deep Eu anomalies, while cold-wet- oxidized magmas are dominated by amphibole SiO2 (wt. %) and titanite fractionation, producing less signiﬁ - cant Eu anomalies but a depletion in the MREEs the Mount Rogers high-silica rhyolites (Novak brites on the Ethiopian Plateau are associated (Davidson et al., 2007b; Bachmann and Ber- and Rankin, 2004). with the eruption of the African-Arabian ﬂ ood gantz, 2008; Glazner et al., 2008). The transition basalt province and broadly form two groups between these magma types has been observed Amphibole Fractionation in Other Large divisible in terms of Sr enrichment (dividing at within the same magmatic center in the Taupo Igneous Provinces ~50 ppm). The majority of the variation between volcanic zone (New Zealand) and is attributed

the low- and high-Sr groups (which mirror to changing P-T-fO2-fH2O conditions in the Amphibole has been observed in both peralu- the high- and low-Al groups of South Moun- intermediate mush (Deering et al., 2008). The minous and peralkaline large igneous province tain) is explained in terms of variable ratios implication of these similarities in geochemical magmas (Riley et al., 2001; Ayalew et al., 2002; of feldspar:clinopyroxene in the fractionating variations is that some peraluminous magmas Peate et al., 2005), though the role of amphibole assemblage (Ayalew et al., 2002); however, the at South Mountain had sufﬁ ciently elevated

as a fractionating phase is frequently overlooked strong correlation in terms of Dy/Yb versus SiO2 water content (greater than ~4%) to allow the because it is generally only a minor constituent in the high-Sr samples (Fig. 8) suggests that in stabilization of amphibole. While large igneous of the modal assemblage. Increasingly, however, addition to the plagioclase and pyroxene, some provinces and associated continental rifts are the role of cryptic fractionation has been recog- amphibole fractionation has occurred. frequently thought of as ostensibly dry environ- nized in the geochemical evolution of arc mag- ments, some basalts from large igneous prov-

mas, where the correlation of Dy/Yb with SiO2 Amphibole and the Implications for the inces may contain magmatic amphibole (Kieffer is strong evidence for amphibole fractionation Volatile Content of Silicic Large Igneous et al., 2004), and erupted silicic magmas may

(Davidson et al., 2007a). Application of these Province Magmas contain up to 5% H2O (Webster, 1992). The ori- geochemical tools to rhyolites erupted in large gin of the hypothesized elevated water content igneous provinces may identify other instances The interpreted presence of amphibole in the high-Al group remains unclear; however, of amphibole fractionation. The Wegel Tena, fractionation from the peralkaline Ethiopian it may relate to either heterogeneity in the vola- Jima, Lima Limo, and Debre Birhan ignim- ignimbrites and the peraluminous South Moun- tile content of the parental basaltic magma or

1.8

B 1.7 A 2 1.6 Metaluminous Peraluminous 1.5 1.5 N 1.4

1.3 1 (Dy/Yb) 1.2 Peralkaline Al/(Na+K) 1.1

0.5 1.0 Ethiopia low Sr Ethiopia high Sr 0.9

0 0.8 0.5 0.7 0.9 1.1 1.3 1.5 64 69 74 79 SiO (wt. %) Al(Ca+Na+K) 2 Figure 8. Role of amphibole in generating the geochemical diversity of silicic magmas from Ethiopian large igneous provinces. (A) Ethio- pian silicic magmas (Ayalew et al., 2002) are broadly peralkaline and can be broadly divided into low- and high-Sr varieties (division set at

~50 ppm Sr; Ayalew et al., 2002). (B) High-Sr Ethiopian silicic rocks follow a negative correlation between (Dy/Yb)N and SiO2, suggesting some amphibole fractionation.

LITHOSPHERE | Volume 2 | Number 4 | www.gsapubs.org 217

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.

2 litic magmatism in large igneous provinces is Metaluminous Peraluminous generally attributed to the commencement of rifting (Pankhurst et al., 2000a; Marsh et al., 1.5 2001; Peate et al., 2005), and the abundant silicic volcanism associated with the Catoctin 1 Volcanic Province along the paleo–continental Peralkaline Al/(Na+K) margin is consistent with the successful rifting Mt. Rogers 0.5 of Laurentia at this time. High-Al group Low-Al group The broad division of the South Mountain rhyolites into low- and high-Ti varieties mirrors 0 0.50.70.91.11.31.5 similar classifi cations noted in other younger fl ood basalt provinces (e.g., Paraná; Garland et Al(Ca+Na+K) al., 1995). We suggest that this primary division Figure 9. Alkali-alumina discrimination diagram showing the peraluminous nature of the rhyolites is related to distinct parental of the high-Al group South Mountain rhyolites, though postemplacement Na loss basaltic magmas derived from either a hetero- is common in silicic magmas and in particular for low-level metamorphic rocks geneous eastern Laurentian superplume, or from such as South Mountain (e.g., Mount Rogers formation; Novak and Rankin, 2004). However, the lack of correlation between loss on ignition and Na would sug- the interaction of these plume magmas with the gest that such a loss is not signifi cant and that the South Mountain samples are continental lithosphere. These basaltic magmas unlikely to have been peralkaline. The low-Al rhyolites and Mount Rogers Forma- subsequently evolved toward rhyolitic compo- tion samples have a similar range and are typically peraluminous; however, some sitions through open-system fractionation. We Mount Rogers samples are thought to extend into the mildly peralkaline fi eld. have recognized two distinctive fractionation paths that are independent of this Ti subdivision, revealing signifi cant variation in terms of

assimilation of hydrothermally altered crustal with low fO2 may exhibit less pronounced Eu the residual mineral assemblages. The dominant rocks. Oxygen isotope studies of large igneous anomalies (e.g., Ethiopia; Ayalew et al., 2002). South Mountain rhyolite group exhibits major- provinces (e.g., Bindeman et al., 2008), conti- The consequences for amphibole fractionation and trace-element characteristics typical of sub- nental extensional environments (Bindeman in these systems is less clear, while peralkaline stantial plagioclase removal. However, a smaller and Valley, 2003), and island arcs (Bindeman Ethiopian ignimbrites indicate a progressive subset of the South Mountain rhyolites follows et al., 2001) have all shown that silicic magmas decline of Dy/Yb with fractionation (Fig. 8), an alternate path that is consistent with the erupted in these settings can assimilate signifi - the currently limited partition coeffi cient data presence of amphibole in the source mush. We cant volumes of hydrothermally altered rock. for amphibole in peralkaline rocks suggest that interpret these heterogeneous fractionation paths it may not fractionate Dy from Yb (Marshall et as proxies for the volatile content of the evolv- Amphibole and Volatiles in Peralkaline al., 2009). Further trace-element investigation ing magmas, and suggest that such techniques Magmas of these systems is necessary to examine this applied to rhyolites from other large igneous discrepancy. Importantly, the appearance of provinces may yield new constraints on the vola- The existence of amphibole in large igne- amphibole in peralkaline systems is indicative tile content of large igneous province magmas. ous province or rift environments is frequently of low water content in these magmas (Scail- linked to peralkaline magmas (MacDonald et al., let and MacDonald, 2001; Ayalew et al., 2002). ACKNOWLEDGMENTS 2008; Marshall et al., 2009). Variation in terms These observations suggest that for high-silica of amphibole stability and volatile content in rhyolites erupted at South Mountain (and in We thank Tom Vogel for reading and commenting peralkaline magmas differs markedly from per- other large igneous provinces), the volatile on an earlier version of this manuscript, Sheldon aluminous and metaluminous magmas. Fluorine content and peralkalinity may have signifi cant Turner for his help with drafting Figure 1, and has long been recognized as a key volatile in control over the fractionating assemblages and Robert Smith for discussions about the Catoctin controlling the stability of amphibole in water- resultant trace-element patterns. Consequently, in Pennsylvania. The manuscript benefi ted from poor magmatic systems (e.g., Grigoriev and the presence of amphibole does not necessarily the comments of an anonymous reviewer and the

Iskull, 1937; Wones and Gilbert, 1982) and is require a high aH2O value, as discussed previ- editorial handling of R. Russo. This project was an important volatile in large igneous provinces ously. However, without available exposures in part funded by a grant to Briggs by the Michi- (e.g., Yirgu et al., 1999). For mildly peralkaline of the intermediate progenitor containing the gan State University Center for Undergraduate magmas, F-rich amphibole replaces clinopy- amphibole, determination of the amphibole Research in Earth System Science (CURESS).

roxene at low fH2O, contrary to phase relations composition and, hence, the F contents, remains observed in peraluminous and metaluminous an unresolved aspect of our model. REFERENCES CITED magmas (Scaillet and MacDonald, 2001). Per- alkaline magmas also exhibit an inhibition of CONCLUSIONS Aleinikoff, J.N., Zartman, R.E., Walter, M., Rankin, D.W., Lyttle, P.T., and Burton, W.C., 1995, U-Pb Ages of meta- plagioclase crystallization in favor of alkali rhyolites of the Catoctin and Mount Rogers Forma- feldspar, regardless of CaO or volatile content Rhyolites from the South Mountain region tions, Central and Southern Appalachians—Evidence of the magma (Scaillet and MacDonald, 2001). of Pennsylvania record processes active in the for two pulses of Iapetan rifting: American Journal of Science, v. 295, p. 428–454. These phase relations have signiﬁ cant conse- lithosphere during the eruption of the late Ven- Anderson, D.L., 1994, The sublithospheric mantle as the quences for the trace-element characteristics of dian Catoctin Volcanic Province, part of the source of continental ﬂ ood basalts—The case against the continental lithosphere and plume head reser- the magmas erupted in large igneous province eastern Laurentian superplume (e.g., Puffer, voirs: Earth and Planetary Science Letters, v. 123, and rift settings; mildly peralkaline magmas 2002). The presence of large volumes of rhyo- p. 269–280, doi: 10.1016/0012-821X(94)90273-9.

218 www.gsapubs.org | Volume 2 | Number 4 | LITHOSPHERE

Annen, C., and Sparks, R.S.J., 2002, Effects of repetitive Contributions to Mineralogy and Petrology, v. 114, Garland, F., Hawkesworth, C.J., and Mantovani, M.S.M., emplacement of basaltic intrusions on thermal evolu- p. 452–464, doi: 10.1007/BF00321750. 1995, Description and petrogenesis of the Parana tion and melt generation in the crust: Earth and Plan- Briggs, C., Rooney, T., and Sinha, A.K., 2008, Silicic volcanics rhyolites, southern Brazil: Journal of Petrology, v. 36, etary Science Letters, v. 203, p. 937–955, doi: 10.1016/ in the South Mountain region: A volcanic center with p. 1193–1227. S0012-821X(02)00929-9. the breakup of Rodinia: Eos (Transactions, American Glazner, A.F., Coleman, D.S., and Bartley, J.M., 2008, The Annen, C., Blundy, J.D., and Sparks, R.S.J., 2006, The gen- Geophysical Union), v. 89, Fall Meeting Supplement, tenuous connection between high-silica rhyolites and esis of intermediate and silicic magmas in deep crustal abs. V31C-2158. granodiorite plutons: Geology, v. 36, p. 183–186, doi:

hot zones: Journal of Petrology, v. 47, p. 505–539, doi: Brophy, J.G., 2009, La-SiO2 and Yb-SiO2 systematics in mid- 10.1130/G24496A.1. 10.1093/petrology/egi084. ocean ridge magmas: Implications for the origin of Greene, A.R., Scoates, J.S., Weis, D., and Isreal, S., 2009, Ayalew, D., and Yirgu, G., 2003, Crustal contribution to the oceanic plagiogranite: Contributions to Mineralogy and Geochemistry of Triassic fl ood basalts from the Yukon genesis of Ethiopian Plateau rhyolitic ignimbrites; Petrology, v. 158, p. 99–111, doi: 10.1007/s00410-008 (Canada) segment of the accreted Wrangellia plateau: basalt and rhyolite geochemical provinciality: Jour- -0372-3. Lithos, v. 110, p. 1–19, doi: 10.1016/j.lithos.2008.11.010. nal of the Geological Society, v. 160, p. 47–56, doi: Bryan, S.E., 2007, Silicic large igneous provinces: Episodes, Grigoriev, D.P., and Iskull, E.W., 1937, The regeneration of 10.1144/0016-764901-169. v. 30, p. 1–12. amphiboles from their melts at normal pressure: The Ayalew, D., Barbey, P., Marty, B., Reisberg, L., Yirgu, G., and Bryan, S.E., Riley, T.R., Jerram, D.A., Stephens, C.J., and American Mineralogist, v. 22, p. 169–177. Pik, R., 2002, Source, genesis, and timing of giant Leat, P.T., 2002, Silicic volcanism: An undervalued com- Hergt, J.M., Peate, D.W., and Hawkesworth, C.J., 1991, The ignimbrite deposits associated with Ethiopian continen- ponent of large igneous provinces and rifted volcanic petrogenesis of Mesozoic Gondwana low-Ti fl ood tal fl ood basalts: Geochimica et Cosmochimica Acta, margins, in Menzies, M., Klemper, S.L., Ebinger, C., basalts: Earth and Planetary Science Letters, v. 105, v. 66, p. 1429–1448, doi: 10.1016/S0016-7037(01)00834-1. and Baker, J., eds., Rifted Volcanic Margins: Geological p. 134–148, doi: 10.1016/0012-821X(91)90126-3. Bachmann, O., and Bergantz, G.W., 2004, On the origin of Society of America Special Paper 362, p. 97–118, doi: Hildreth, W., 2004, Volcanological perspectives on Long Val- crystal-poor rhyolites: Extracted from batholithic crys- 10.1130/0-8137-2362-0.97. ley, Mammoth Mountain, and Mono Craters: Several tal mushes: Journal of Petrology, v. 45, p. 1565–1582, Camp, V.E., and Hanan, B.B., 2008, Derivation of the Columbia contiguous but discrete systems: Journal of Volcanol- doi: 10.1093/petrology/egh019. River Flood Basalts by plume emplacement and delami- ogy and Geothermal Research, v. 136, p. 169–198, doi: Bachmann, O., and Bergantz, G.W., 2008, Rhyolites and their nation against the cratonic margin of North America: 10.1016/j.jvolgeores.2004.05.019. source mushes across tectonic settings: Journal of Geochimica et Cosmochimica Acta, v. 72, p. A132. Hill, R.I., 1991, Starting plumes and continental break-up: Petrology, v. 49, p. 2277–2285, doi: 10.1093/petrology/ Campbell, I.H., 2007, Testing the plume theory: Chemical Earth and Planetary Science Letters, v. 104, p. 398–416, egn068. Geology, v. 241, p. 153–176, doi: 10.1016/j.chemgeo doi: 10.1016/0012-821X(91)90218-7. Badger, R.L., and Sinha, A.K., 1988, Age and Sr isotopic sig- .2007.01.024. Ivanov, A.V., Demonterova, E.I., Rasskazov, S.V., and Yasny- nature of the Catoctin Volcanic Province: Implications Cawood, P.A., McCausland, P.J.A., and Dunning, G.R., gina, T.A., 2008, Low-Ti melts from the southeastern for subcrustal mantle evolution: Geology, v. 16, p. 692– 2001, Opening Iapetus: Constraints from the Lauren- Siberian Traps large igneous province: Evidence for 695, doi: 10.1130/0091-7613(1988)016<0692:AASISO>2.3 tian margin in Newfoundland: Geological Society a water-rich mantle source?: Journal of Earth System .CO;2. of America Bulletin, v. 113, p. 443–453, doi: 10.1130/ Science, v. 117, p. 1–21, doi: 10.1007/s12040-008-0008-z. Badger, R.L., and Sinha, A.K., 1992, Stratigraphic charac- 0016-7606(2001)113<0443:OICFTL>2.0.CO;2. Jourdan, F., Bertrand, H., Feraud, G., Le Gall, B., and Wat- terization and correlation of volcanic fl ows within the Christiansen, E.H., and McCurry, M., 2008, Contrasting ori- keys, M.K., 2009, Lithospheric mantle evolution moni- Catoctin Formation, Central Appalachians: Southeast- gins of Cenozoic silicic volcanic rocks from the western tored by overlapping large igneous provinces: Case ern Geology, v. 32, p. 175–195. Cordillera of the United States: Bulletin of Volcanol- study in southern Africa: Lithos, v. 107, p. 257–268, doi: Badger, R.L., and Sinha, A.K., 2004, Chemical stratigraphy ogy, v. 70, p. 251–267, doi: 10.1007/s00445-007-0138-1. 10.1016/j.lithos.2008.10.011. and petrogenesis of the Catoctin Volcanic Province, Class, C., and le Roex, A.P., 2008, Ce anomalies in Gough Kieffer, B., Arndt, N., Lapierre, H., Bastien, F., Bosch, D., Central Appalachians, in Tollo, R.P., McLelland, J., Cor- Island lavas—Trace element characteristics of a recycled Pecher, A., Yirgu, G., Ayalew, D., Weis, D., Jerram, riveau, L., and Bartholomew, M.J., eds., Proterozoic sediment component: Earth and Planetary Science Let- D.A., Keller, F., and Meugniot, C., 2004, Flood and Tectonic Evolution of the Grenville Orogen in North ters, v. 265, p. 475–486, doi: 10.1016/j.epsl.2007.10.030. shield basalts from Ethiopia: Magmas from the Afri- America: Geological Society of America Memoir 197, Cotten, J., Ledez, A., Bau, M., Caroff, M., Maury, R.C., Dul- can superswell: Journal of Petrology, v. 45, p. 793–834, p. 435–458, doi: 10.1130/0-8137-1197-5.435. ski, P., Fourcade, S., Bohn, M., and Brousse, R., 1995, doi: 10.1093/petrology/egg112. Baker, J.A., MacPherson, C.G., Menzies, M.A., Thirlwall, Origin of anomalous rare-earth element and yttrium Korenaga, J., 2004, Mantle mixing and continental breakup M.F., Al-Kadasi, M., and Mattey, D.P., 2000, Resolving enrichments in subaerially exposed basalts—Evidence magmatism: Earth and Planetary Science Letters, crustal and mantle contributions to continental fl ood from French-Polynesia: Chemical Geology, v. 119, v. 218, p. 463–473, doi: 10.1016/S0012-821X(03)00674-5. volcanism, Yemen; constraints from mineral oxygen p. 115–138, doi: 10.1016/0009-2541(94)00102-E. Li, Z.X., Li, X.H., Kinny, P.D., and Wang, J., 1999, The breakup isotope data: Journal of Petrology, v. 41, p. 1805–1820, Davidson, J., Turner, S., Handley, H., Macpherson, C., and of Rodinia: Did it start with a mantle plume beneath doi: 10.1093/petrology/41.12.1805. Dosseto, A., 2007a, Amphibole “sponge” in arc crust?: South China?: Earth and Planetary Science Letters, Beard, J.S., Lofgren, G.E., Sinha, A.K., and Tollo, R.P., 1994, Geology, v. 35, p. 787–790, doi: 10.1130/G23637A.1. v. 173, p. 171–181, doi: 10.1016/S0012-821X(99)00240-X. Partial melting of apatite-bearing charnockite, granu- Davidson, J., Morgan, D.J., Charlier, B.L.A., Harlou, R., Li, Z.X., Bogdanova, S.V., Collins, A.S., Davidson, A., De lite, and diorite—Melt compositions, restite mineral- and Hora, J.M., 2007b, Microsampling and isotopic Waele, B., Ernst, R.E., Fitzsimons, I.C.W., Fuck, R.A., ogy, and petrologic implications: Journal of Geophysi- analysis of igneous rocks: Implications for the study of Gladkochub, D.P., Jacobs, J., Karlstrom, K.E., Lu, S., cal Research–Solid Earth, v. 99, p. 21,591–21,603, doi: magmatic systems: Annual Review of Earth and Plan- Natapov, L.M., Pease, V., Pisarevsky, S.A., Thrane, K., 10.1029/94JB02060. etary Sciences, v. 35, p. 273–311, doi: 10.1146/annurev. and Vernikovsky, V., 2008, Assembly, confi guration, and Bindeman, I.N., and Valley, J.W., 2003, Rapid generation of earth.35.031306.140211. break-up history of Rodinia: A synthesis: Precambrian both high- and low-δ18O, large-volume silicic magmas Deering, C.D., Cole, J.W., and Vogel, T.A., 2008, A rhyolite Research, v. 160, p. 179–210, doi: 10.1016/j.precamres at the Timber Mountain/Oasis Valley caldera com- compositional continuum governed by lower crustal .2007.04.021. plex, Nevada: Geological Society of America Bulletin, source conditions in the Taupo volcanic zone, New Lightfoot, P.C., Hawkesworth, C.J., and Sethna, S.F., 1987, v. 115, p. 581–595, doi: 10.1130/0016-7606(2003)115< Zealand: Journal of Petrology, v. 49, p. 2245–2276, doi: Petrogenesis of rhyolites and trachytes from the Dec- 0581:RGOBHA>2.0.CO;2. 10.1093/petrology/egn067. can Trap—Sr, Nd and Pb isotope and trace-element Bindeman, I.N., Fournelle, J.H., and Valley, J.W., 2001, Low- Dufek, J., and Bergantz, G.W., 2005, Lower crustal magma evidence: Contributions to Mineralogy and Petrology, δ18O tephra from a compositionally zoned magma genesis and preservation: A stochastic framework for the v. 95, p. 44–54, doi: 10.1007/BF00518029. body: Fisher Caldera, Unimak Island, Aleutians: Jour- evaluation of basalt-crust interaction: Journal of Petrol- Lightfoot, P.C., Naldrett, A.J., Gorbachev, N.S., Doherty, W., nal of Volcanology and Geothermal Research, v. 111, ogy, v. 46, p. 2167–2195, doi: 10.1093/petrology/egi049. and Fedorenko, V.A., 1990, Geochemistry of the Siberian p. 35–53, doi: 10.1016/S0377-0273(01)00219-0. Ellam, R.M., and Cox, K.G., 1991, An interpretation of Karoo Trap of the Norilsk area, USSR, with implications for Bindeman, I.N., Fu, B., Kita, N.T., and Valley, J.W., 2008, picrite basalts in terms of interaction between asthe- the relative contributions of crust and mantle to fl ood- Origin and evolution of silicic magmatism at Yellow- nospheric magmas and the mantle lithosphere: Earth basalt magmatism: Contributions to Mineralogy and stone based on ion microprobe analysis of isotopically and Planetary Science Letters, v. 105, p. 330–342, doi: Petrology, v. 104, p. 631–644, doi: 10.1007/BF01167284. zoned zircons: Journal of Petrology, v. 49, p. 163–193, 10.1016/0012-821X(91)90141-4. MacDonald, R., Belkin, H.E., Fitton, J.G., Rogers, N.W., doi: 10.1093/petrology/egm075. Ewart, A., Milner, S.C., Armstrong, R.A., and Duncan, A.R., Nejbert, K., Tindle, A.G., and Marshall, A.S., 2008, The Bonin, B., 2007, A-type granites and related rocks: Evolution 1998, Etendeka volcanism of the Goboboseb Mountains roles of fractional crystallization, magma mixing, crys- of a concept, problems and prospects: Lithos, v. 97, and Messum igneous complex, Namibia. Part II: Volu- tal mush remobilization and volatile-melt interactions p. 1–29, doi: 10.1016/j.lithos.2006.12.007. minous quartz latite volcanism of the Awahab magma in the genesis of a young basalt–peralkaline rhyolite Boynton, W.V., 1984, Cosmochemistry of the rare earth ele- system: Journal of Petrology, v. 39, p. 227–253, doi: suite, the Greater Olkaria volcanic complex, Kenya rift ments: Meteorite studies, in Henderson, P., ed., Rare 10.1093/petrology/39.2.227. valley: Journal of Petrology, v. 49, p. 1515–1547, doi: Earth Element Geochemistry: New York, Elsevier, Fauth, J.L., 1968, Geology of the Caledonia Park Quadran- 10.1093/petrology/egn036. p. 63–114. gle Area, South Mountain, Pennsylvania: Harrisburg, Macpherson, C.G., Dreher, S.T., and Thirlwall, M.F., 2006, Brandon, A.D., Hooper, P.R., Goles, G.G., and Lambert, R.S., Pennsylvania Geological Survey, 133 p. Adakites without slab melting: High pressure differ- 1993, Evaluating crustal contamination in continen- Gallagher, K., and Hawkesworth, C., 1992, Dehydration melt- entiation of island arc magma, Mindanao, the Philip- tal basalts—The isotopic composition of the Picture ing and the generation of continental fl ood basalts: pines: Earth and Planetary Science Letters, v. 243, Gorge Basalt of the Columbia River Basalt Group: Nature, v. 358, p. 57–59, doi: 10.1038/358057a0. p. 581–593, doi: 10.1016/j.epsl.2005.12.034.

LITHOSPHERE | Volume 2 | Number 4 | www.gsapubs.org 219

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.

Marsh, J.S., Ewart, A., Milner, S.C., Duncan, A.R., and Miller, Antarctic Peninsula: Chronology of magmatism associ- Riley, T.R., Leat, P.T., Pankhurst, R.J., and Harris, C., 2001, R.M., 2001, The Etendeka Igneous Province: Magma ated with the break-up of Gondwana: Journal of Petrol- Origins of large volume rhyolitic volcanism in the types and their stratigraphic distribution with implica- ogy, v. 41, p. 605–625, doi: 10.1093/petrology/41.5.605. Antarctic Peninsula and Patagonia by crustal melting: tions for the evolution of the Parana-Etendeka fl ood Peate, D.W., Hawkesworth, C.J., Mantovani, M.M.S., Rog- Journal of Petrology, v. 42, p. 1043–1065, doi: 10.1093/ basalt province: Bulletin of Volcanology, v. 62, p. 464– ers, N.W., and Turner, S.P., 1999, Petrogenesis and petrology/42.6.1043. 486, doi: 10.1007/s004450000115. stratigraphy of the high-Ti/Y Urubici magma type in Scaillet, B., and MacDonald, R., 2001, Phase relations of per- Marshall, A.S., MacDonald, R., Rogers, N.W., Fitton, J.G., the Parana fl ood basalt province and implications for alkaline silicic magmas and petrogenetic implications: Tindle, A.G., Nejbert, K., and Hinton, R.W., 2009, Frac- the nature of “Dupal”-type mantle in the South Atlan- Journal of Petrology, v. 42, p. 825–845, doi: 10.1093/ tionation of peralkaline silicic magmas: The Greater tic region: Journal of Petrology, v. 40, p. 451–473, doi: petrology/42.4.825. Olkaria volcanic complex, Kenya Rift Valley: Journal 10.1093/petrology/40.3.451. Sun, S.-s., and McDonough, W.F., 1989, Chemical and iso- of Petrology, v. 50, p. 323–359, doi: 10.1093/petrology/ Peate, I.U., Baker, J.A., Al-Kadasi, M., Al-Subbary, A., Knight, topic systematics of oceanic basalts: Implications for egp001. K.B., Riisager, P., Thirlwall, M.F., Peate, D.W., Renne, mantle composition and processes, in Saunders, A.D., Miller, J.A., and Harris, C., 2007, Petrogenesis of the Swazi- P.R., and Menzies, M.A., 2005, Volcanic stratigraphy ed., Magmatism in the Ocean Basins: Geological Soci- land and northern Natal rhyolites of the Lebombo rifted of large-volume silicic pyroclastic eruptions during ety, London, Special Publication 42, p. 313–345, doi: volcanic margin, south east Africa: Journal of Petrology, Oligocene Afro-Arabian fl ood volcanism in Yemen: 10.1144/GSL.SP.1989.042.01.19. v. 48, p. 185–218, doi: 10.1093/petrology/egl061. Bulletin of Volcanology, v. 68, p. 135–156, doi: 10.1007/ Thompson, R.N., Riches, A.J.V., Antoshechkina, P.M., Pear- Mitra, A., and Sinha, A.K., 2004a, Felsic magmatism in s00445-005-0428-4. son, D.G., Nowell, G.M., Ottley, C.J., Dickin, A.P., Hards, fl ood basalt provinces: A study of the South Mountain Peccerillo, A., Barberio, M.R., Yirgu, G., Ayalew, D., Barbieri, V.L., Nguno, A.K., and Niku-Paavola, V., 2007, Origin (Catoctin) high-silica rhyolites: Geological Society of M., and Wu, T.W., 2003, Relationships between mafi c of CFB magmatism: Multi-tiered intracrustal picrite- America Abstracts with Programs, v. 36, no. 2, p. 76. and peralkaline silicic magmatism in continental rift rhyolite magmatic plumbing at Spitzkoppe, western Mitra, A., and Sinha, A.K., 2004b, Geochemical comparison settings: A petrological, geochemical and isotopic Namibia, during Early Cretaceous Etendeka magma- of two rhyolite provinces in the Central Appalachians study of the Gedemsa volcano, central Ethiopian rift: tism: Journal of Petrology, v. 48, p. 1119–1154, doi: associated with rifting: Geological Society of America Journal of Petrology, v. 44, p. 2003–2032, doi: 10.1093/ 10.1093/petrology/egm012. Abstracts with Programs, v. 36, p. 503. petrology/egg068. Tollo, R.P., Corriveau, L., McLelland, J.M., and Bartholomew, Mutter, J.C., Buck, W.R., and Zehnder, C.M., 1988, Con- Pegram, W.J., 1990, Development of continental lithospheric M.J., 2004, Proterozoic tectonic evolution of the vective partial melting: 1. A model for the formation mantle as refl ected in the chemistry of the Mesozoic Grenville orogen in North America: An introduction, of thick basaltic sequences during the initiation of Appalachian tholeiites, USA: Earth and Planetary Sci- in Tollo, R.P., Corriveau, L., McLelland, J.M., and Bar- spreading: Journal of Geophysical Research–Solid ence Letters, v. 97, p. 316–331, doi: 10.1016/0012-821X tholomew, M.J., eds., Proterozoic Tectonic Evolution Earth and Planets, v. 93, p. 1031–1048, doi: 10.1029/ (90)90049-4. of the Grenville Orogen in North America: Geological JB093iB02p01031. Pik, R., Deniel, C., Coulon, C., Yirgu, G., and Marty, B., 1999, Society of America Memoir 197, p. 1–18, doi: 10.1130/0 Novak, S.W., and Rankin, D.W., 2004, Compositional zoning Isotopic and trace element signatures of Ethiopian -8137-1197-5.1. of a Neoproterozoic ash-fl ow sheet of the Mount Rog- fl ood basalts; evidence for plume-lithosphere inter- Webster, J.D., 1992, Water solubility and chlorine partition- ers Formation, southwestern Virginia Blue Ridge, and actions: Geochimica et Cosmochimica Acta, v. 63, ing in Cl-rich granitic systems—Effects of melt com- the aborted rifting of Laurentia, in Tollo, R.P., Corriveau, p. 2263–2279, doi: 10.1016/S0016-7037(99)00141-6. position at 2 kbar and 800°C: Geochimica et Cosmo- L., McLelland, J.M., and Bartholomew, M.J., eds., Pro- Price, R.C., Gray, C.M., Wilson, R.E., Frey, F.A., and Taylor, chimica Acta, v. 56, p. 679–687, doi: 10.1016/0016-7037 terozoic Tectonic Evolution of the Grenville Orogen in S.R., 1991, The effects of weathering on rare-earth ele- (92)90089-2. North America: Geological Society of America Memoir ment, Y and Ba abundances in Tertiary basalts from Wones, D.R., and Gilbert, M.C., 1982, Amphiboles in the 197, p. 571–600, doi: 10.1130/0-8137-1197-5.571. southeastern Australia: Chemical Geology, v. 93, igneous environment, in Veblen, D.R., and Ribbe, P.H., Pankhurst, R.J., and Rapela, C.R., 1995, Production of Juras- p. 245–265, doi: 10.1016/0009-2541(91)90117-A. eds., Amphiboles: Petrology and Experimental Phase sic rhyolite by anatexis of the lower crust of Patagonia: Puffer, J.H., 2002, A late Neoproterozoic eastern Laurentian Relations: Mineralogical Society of America, Reviews Earth and Planetary Science Letters, v. 134, p. 23–36, superplume: Location, size, chemical composition, in Mineralogy, v. 9B, p. 355–385. doi: 10.1016/0012-821X(95)00103-J. and environmental impact: American Journal of Sci- Yirgu, G., Dereje, A., Peccerillo, A., Barberio, M.R., Donati, Pankhurst, R.J., Leat, P.T., Sruoga, P., Rapela, C.W., Marquez, ence, v. 302, p. 1–27, doi: 10.2475/ajs.302.1.1. C., and Donato, P., 1999, Fluorine and chlorine distribu- M., Storey, B.C., and Riley, T.R., 1998, The Chon Aike Rankin, D.W., 1975, Continental margin of eastern North tion in the volcanic rocks from Gedemsa volcano, Ethi- province of Patagonia and related rocks in West Ant- America in Southern Appalachians—Opening and opian rift valley: Acta Vulcanologica, v. 11, p. 169–176. arctica: A silicic large igneous province: Journal of Vol- closing of proto–Atlantic Ocean: American Journal of canology and Geothermal Research, v. 81, p. 113–136, Science, v. A275, p. 298. doi: 10.1016/S0377-0273(97)00070-X. Rankin, D.W., 1994, Continental margin of the eastern Pankhurst, R.J., Rapela, C.W., and Fanning, C.M., 2000a, United States: Past and present, in Speed, R.C., ed., Age and origin of coeval TTG, I- and S-type granites in Phanerozoic Evolution of North American Continent- the Famatinian belt of NW Argentina: Transactions of Ocean Transitions: Geological Society of America, MANUSCRIPT RECEIVED 22 NOVEMBER 2009 the Royal Society of Edinburgh–Earth Sciences, v. 91, Decade of North American Geology, p. 129–218. REVISED MANUSCRIPT RECEIVED 26 MARCH 2010 p. 151–168. Reed, J.C., 1955, Catoctin Formation near Luray, Virginia: Geo- MANUSCRIPT ACCEPTED 19 APRIL 2010 Pankhurst, R.J., Riley, T.R., Fanning, C.M., and Kelley, S.P., logical Society of America Bulletin, v. 66, p. 871–896, 2000b, Episodic silicic volcanism in Patagonia and the doi: 10.1130/0016-7606(1955)66[871:CFNLV]2.0.CO;2. Printed in the USA

220 www.gsapubs.org | Volume 2 | Number 4 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/2/4/211/3037578/211.pdf by guest on 01 October 2021