New geochronologic data from basement rocks support the interpretation that the Argentine Precordillera (Cuyania) terrane was rifted from the Ouachita embayment of the Iapetan margin of Laurentia. New data from the Ozark dome show a range of ages in two groups at 1466 ± 3 to 1462 ± 1 Ma and 1323 ± 2 to 1317 ± 2 Ma, consistent with existing data for the Eastern Granite-Rhyolite province and Southern Granite-Rhyolite province, respectively. Similarly, a newly determined age of 1364 ± 2 Ma for the Tishomingo Granite in the Arbuckle Mountains confirms previously published analyses for this part of the Southern Granite-Rhyolite province. Along with previously reported ages from basement olistoliths in Ordovician slope deposits in the Ouachita embayment, the data for basement ages support the interpretation that rocks of the Southern Granite-Rhyolite province form the margin of Laurentian crust around the corner of the Ouachita embayment, which is bounded by the Ouachita rift and Alabama-Oklahoma transform fault. In contrast, both west and east of the corner of the Ouachita embayment, Grenville-Llano basement (approximately 1325–1000 Ma) forms the rifted margin of Laurentia.
New U/Pb zircon data from basement rocks in the southern part of the Argentine Precordillera indicate crystallization ages of 1205 ± 1 Ma and 1204 ± 2 Ma, consistent with previously reported ages (approximately 1250–1000 Ma) of basement rocks from other parts of the Precordillera. These data document multiple events within the same time span as multiple events in the Grenville orogeny in eastern Laurentia, and are consistent with Grenville-age rocks along the conjugate margins of the Precordillera and Laurentia. Ages from one newly analyzed collection, however, are older than those from other basement rocks in the Precordillera. These ages, from granodioritic-granitic basement clasts in a conglomerate olistolith in Ordovician slope deposits, are 1370 ± 2 Ma and 1367 ± 5 Ma. These older ages from the Precordillera are consistent with indications that the Iapetan margin in the Ouachita embayment of Laurentia truncated the Grenville front and left older rocks of the Southern Granite-Rhyolite province (1390–1320 Ma) at the rifted margin.
Chronostratigraphic correlations of synrift and post-rift sedimentary deposits on the Precordillera and on the Texas promontory of Laurentia document initial rifting in the Early Cambrian. Previously published data from synrift plutonic and volcanic rocks in the Wichita and Arbuckle Mountains along the transform-parallel intracratonic Southern Oklahoma fault system inboard from the Ouachita embayment document crystallization ages of 539–530 Ma. New data from synrift volcanic rocks in the Arbuckle Mountains in the eastern part of the Southern Oklahoma fault system yield ages of 539 ± 5 Ma and 536 ± 5 Ma, confirming the age of synrift volcanism.
The Precordillera in the Andean frontal thrust belt in northwestern Argentina is an exotic terrane, which is interpreted to have been rifted from the Ouachita embayment of southern Laurentia in Cambrian time and accreted to the western margin of Gondwana during Ordovician time (Fig. 1) (e.g., Thomas and Astini, 1996, 1999, 2003; Astini and Thomas, 1999; Ramos, 2004). The Laurentian origin of the Precordillera terrane is based on a variety of data from Precambrian basement rocks, Early Cambrian synrift deposits, and a Cambrian–Ordovician passive-margin carbonate succession (e.g., Astini et al., 1995; Astini, 1998; Astini and Thomas 1999; Thomas and Astini, 1999, 2003; Ramos, 2004). Proposed alternative interpretations include different times of rifting, different times of accretion, different palinspastic sites along the Laurentian margin, and a Gondwanan rather than Laurentian origin (e.g., Dalla Salda et al., 1992a, 1992b; Dalziel, 1997; Keller et al., 1998; Pankhurst et al., 1998; Rapela et al., 1998; Keller, 1999; Cawood et al., 2001; Aceñolaza et al., 2002; Finney, 2007). Ultimately, resolution of the tectonic history of the Precordillera is significant to interpretation of the structural style, trace, and age of the margins of Iapetus, as well as to global plate reconstructions. The purpose of this article is to integrate new data for resolution of the ages of basement and synrift rocks on both sides of the Iapetan rift.
Rifting of the Argentine Precordillera from the Ouachita embayment of Laurentia defines the conjugate margins of the Ouachita rift as the eastern rift margin of the Texas promontory of Laurentia and the western rift margin of the Precordillera (directions are present day with Precordillera palinspastically restored) (Fig. 1). Similarly, the Alabama-Oklahoma transform fault separates the northwest-trending edge of the Ouachita embayment from the Precordillera. This article has two objectives: (1) to review existing data and present new U/Pb isotopic ages of Precambrian basement rocks on opposite sides of the rift-and-transform boundary of the Precordillera and the Ouachita embayment of Laurentia, and (2) to review existing data and present new U/Pb isotopic ages that bear on the age of rifting. These data provide further validation of the Laurentian origin of the Precordillera terrane and the palinspastic restoration of the Precordillera in the Ouachita embayment of Laurentia.
Along the Iapetan rifted margin of southeastern Laurentia, a large-scale transform offset of the rift outlines the Ouachita embayment between the Texas and Alabama promontories (Figs. 1 and 2). The rift margin has been covered by passive-margin strata of Cambrian–Ordovician age, by allochthons and foreland-basin sediment of the late Paleozoic Appalachian-Ouachita orogen, and by synrift and post-rift strata associated with Mesozoic rifting and opening of the Gulf of Mexico and Atlantic Ocean (Fig. 2). The Iapetan rifted margin can be palinspastically restored, however, using data from deep wells and geophysical surveys to extend the geometry of geologic structures from the outcrops (summary in Thomas, 2011). Inboard from the rift and transform margins of the Ouachita embayment, pre-Iapetan Precambrian crystalline basement rocks, as well as the base of the passive-margin cover, are exposed around the Llano uplift and Ozark dome, and in late Paleozoic fault blocks in the Arbuckle Mountains (Fig. 2). Along both the Ouachita rift and Alabama-Oklahoma transform margins of the Ouachita embayment, in the late Paleozoic Ouachita thrust belt, passive-margin Cambrian–Ordovician off-shelf deep-water mud-dominated facies were thrust over the coeval shelf-carbonate facies on Laurentian continental crust; the Precambrian crystalline basement rocks and post-rift passive-margin shelf-carbonate cover remained in the Ouachita footwall and were only mildly shortened by late Paleozoic Ouachita thrusting (summary in Thomas, 2011). The Southern Oklahoma fault system encompasses Cambrian synrift volcanic and plutonic rocks, which are exposed in late Paleozoic high-amplitude fault-bounded uplifts of the Arbuckle and Wichita Mountains (Fig. 2). In addition to the basement outcrops in the Llano, Arbuckle, and Ozark uplifts, basement data have been obtained from deep wells (Van Schmus et al., 1993; Rohs and Van Schmus, 2007). The off-shelf passive-margin succession is exposed in internal uplifts within the Ouachita thrust belt; part of the off-shelf passive-margin succession (Ordovician Blakely Sandstone) locally contains boulders of basement rocks (Fig. 2) (Stone and Haley, 1977; Bowring, 1984), providing another source of data for the age of basement rocks. Outcrops around the basement uplifts, in addition to deep wells, provide data to constrain the lithostratigraphy and chronostratigraphy of the passive-margin carbonate-shelf succession (Johnson et al., 1988).
In the Argentine Precordillera (Cuyania) microcontinent terrane (Fig. 3), Paleozoic sedimentary rocks are imbricated by frontal Andean thrust faults, including both thin-skinned east-directed thrust faults and basement-rooted west-directed Pampean (broken foreland) thrust faults (Ramos et al., 1986; Ramos, 2004, 2010). Cambrian–Ordovician passive-margin carbonate-shelf rocks and rare Cambrian synrift rocks are imbricated in the frontal Andean thrust sheets (Fig. 3), which are detached locally within the synrift succession and more commonly within the lower part of the passive-margin carbonate succession (Astini et al., 1995; Astini and Vaccari, 1996). No basement rocks are exposed in the sedimentary thrust belt of the Precordillera. The Precordillera microcontinent terrane includes the sedimentary succession in the frontal thrust sheets of the Andes (strictly the Precordillera geologic/physiographic province), the basement rocks and metasedimentary cover in the Western Sierras Pampeanas (Sierra de Pie de Palo, Sierra de Umango, Sierra de Maz) east of the sedimentary thrust sheets, and isolated exposures of basement and sedimentary cover rocks farther south at Ponón Trehue (San Rafael block) and Cerro San Jorge (Las Matras block) (Fig. 3); this broad definition corresponds to the Cuyania terrane (Ramos et al., 1998; Ramos, 2004, 2010). In addition, Tertiary plutons within the Precordillera contain xenoliths of crystalline rocks, providing samples of the basement directly beneath the Precordillera (Kay et al., 1996).
PRECAMBRIAN BASEMENT ROCKS AROUND THE OUACHITA EMBAYMENT OF LAURENTIA
Basement rocks around the Ouachita embayment of Laurentia are assigned to either the Granite-Rhyolite province (1500–1320 Ma) or the Grenville-Llano province (1325–1000 Ma), which are separated by the Grenville-Llano front (summary in Van Schmus et al., 1993). From outcrops in southern Canada, the Grenville province has been traced beneath the Paleozoic sedimentary cover, using geophysical surveys and samples from deep wells, southward through the eastern interior part of Laurentia (Figs. 1 and 2) (Lidiak and Heinze, inRankin et al., 1993). On the basis of subsurface data (age determinations from basement rocks penetrated in wells), the Grenville front at the western margin of the Grenville province is traced with reasonable confidence as far south as northern Tennessee (Figs. 1 and 2) (Lidiak and Heinze, inRankin et al., 1993; Reed, 1993). South and west from central Tennessee, few wells have penetrated Precambrian basement rocks beneath the Paleozoic cover, or farther south beneath the Mesozoic–Cenozoic Gulf Coastal Plain (summary in Thomas, 1988). The distribution and resolution of the sparse data do not clearly define the extent of the Grenville province or front; however, the Grenville front is generally inferred to intersect the Alabama-Oklahoma transform margin of Laurentian crust approximately in Mississippi (Figs. 1 and 2) (Rohs and Van Schmus, 2007). If that inference is generally correct, the southeastern part of the trace of the Alabama-Oklahoma transform fault cuts the Grenville province; whereas the northwestern part, extending into the corner of the Ouachita embayment, cuts the Granite-Rhyolite province (Figs. 1 and 2).
Rocks of the Granite-Rhyolite province are separated into two subdivisions on the basis of age differences (summary in Rohs and Van Schmus, 2007). Rocks of the Eastern Granite-Rhyolite province have ages of 1480–1460 Ma and are exposed in the St. Francois Mountains at the crest of the Ozark dome, an intracratonic uplift inboard from the Alabama-Oklahoma transform margin of Laurentia (Fig. 2). Rocks of the Southern Granite-Rhyolite province have ages of 1390–1320 Ma and are exposed principally in the Arbuckle Mountains along the eastern part of the Southern Oklahoma transform-parallel intracratonic fault system (Fig. 2). As we demonstrate later herein, these two age provinces overlap in the St. Francois Mountains of southeastern Missouri, where the older volcanic strata of the Eastern Granite-Rhyolite province are intruded by alkaline granite and syenite complexes of the Southern Granite-Rhyolite province. In the St. Francois Mountains, rocks of the younger Southern Granite-Rhyolite province also include a significant amount of gabbro and diabase (“Skrainka diabase”).
In Texas, northwest of the outcrops of the Llano (Grenville) province in the Llano uplift, subsurface data define a northeast-trending trace of the Grenville-Llano front across the Texas promontory, extending northeastward to an intersection with the intracratonic Southern Oklahoma fault system near the Ouachita rift boundary of the Ouachita embayment (Figs. 1 and 2) (Reed, 1993; summary in Van Schmus et al., 1993; Rohs and Van Schmus, 2007). The Southern Granite-Rhyolite province, with crystallization ages of approximately 1400–1340 Ma (Van Schmus et al., 1993), borders the Llano province on the northwest. South of the intersection of the Ouachita rift with the Southern Oklahoma fault system, the rift bounds the Llano (Grenville) province; north of the intersection, the rift bounds the Granite-Rhyolite province (Figs. 1 and 2).
Summary of Available Age Data
Ages of 1350–1000 Ma characterize rocks of the Grenville province in eastern Laurentia, although the province includes older protoliths and detrital zircons in metasedimentary rocks (e.g., Rankin et al., 1993; McLelland et al., 1996). The Grenville province extends along the southeast side of the easternmost Canadian Shield in eastern Canada (e.g., Cawood et al., 2001; Bartholomew and Hatcher, 2010) and is identified in outcrops in the Adirondack dome and along basement massifs of the Appalachian Mountains (e.g., Blue Ridge, Green Mountains) (Rankin et al., 1989). The initial use of the name Grenville designated the geologic-mapping province on the basis of rock types and degree of metamorphism. Since the advent of radiometric dating, the term Grenville has taken on an age connotation, generally with reference to a regional metamorphic event with an age of approximately 1000 Ma. With the growing recognition of a range of ages within rocks of the Grenville province, multiple events are considered to constitute the Grenville orogeny, including four thermal events at 1250–1200 Ma, 1200–1150 Ma, 1080–1020 Ma, and 1020–1000 Ma, along with accretion of older terranes (summary in Bartholomew and Hatcher, 2010). From northern Laurentia southward to the Alabama promontory, Grenville rocks form the eastern part of the Laurentian craton, and the Iapetan rifted margin of Laurentia is entirely within rocks of the Grenville province (Fig. 1) (e.g., Thomas, 2006).
Exposures of the Eastern Granite-Rhyolite province in the St. Francois Mountains on the Ozark dome include rhyolitic to dacitic tuff, rhyolitic flows, granitic plutons, and basaltic dikes, as well as minor inclusions of metamorphic rocks (summary in Bickford and Anderson, inVan Schmus et al., 1993; Rohs and Van Schmus, 2007). The U/Pb zircon ages document a primary magmatic episode at 1470 ± 30 Ma, characteristic of the Eastern Granite-Rhyolite province. Data from deep wells document the extent of the Eastern Granite-Rhyolite province north and east of the outcrops in the St. Francois Mountains (Lidiak, Bickford, and Kisvarsanyi, inVan Schmus et al., 1993). Other intrusive rocks exposed in the St. Francois Mountains, however, yield ages of 1380–1350 Ma, characteristic of the Southern Granite-Rhyolite province, indicating overlap of the two subdivisions of the Granite-Rhyolite province (Bickford and Anderson, inVan Schmus et al., 1993; Van Schmus et al., 1996; Rohs and Van Schmus, 2007; Lowell et al., 2010). In the area around the St. Francois Mountains, separate outcrops and drill samples of basement rocks yield ages of 1480 ± 42 Ma, 1470 ± 3 Ma, 1462 ± 6 Ma, and 1323 ± 6 Ma, further indicating both the Eastern and Southern Granite-Rhyolite provinces (Van Schmus, Bickford, Sims, Anderson, Shearer, and Treves, inVan Schmus et al., 1993; Harrison et al., 2000, 2002). In the subsurface, data from drill holes indicate the continuity of the Southern Granite-Rhyolite province across the region between the outcrops in the St. Francois Mountains and in the Arbuckle Mountains (Fig. 2) (Van Schmus et al., 1993; Rohs and Van Schmus, 2007).
Southern Granite-Rhyolite province rocks are exposed in the Arbuckle basement uplift, which extends in the subsurface southeastward to the Ouachita rifted margin of Laurentia, near the northwestern corner of the Ouachita embayment (Figs. 1 and 2) (e.g., Ham et al., 1964; Thomas et al., 1989). Mesozonal plutons of the Southern Granite-Rhyolite province exposed in the eastern Arbuckle Mountains include granite and granitic gneiss (Denison, 1973). U/Pb zircon ages of 1374 ± 15 Ma for the Tishomingo Granite, 1399 ± 95 Ma for the Troy Granite, and 1396 ± 40 Ma for the Blue River Gneiss were reported by Bickford and Lewis (1979), and 1397 ± 7 Ma for the Burch Granodiorite by Thomas et al. (1984). More recently, Rohs and Van Schmus (2007) determined ages of 1363 ± 8 Ma for the Tishomingo Granite, 1368 ± 3 Ma for the Troy Granite, 1389 ± 10 Ma for the Blue River Gneiss, and 1390 ± 7 Ma for the Burch Granodiorite (Fig. 2). In the subsurface in northeastern Oklahoma (north and east of the outcrops in the Arbuckle Mountains), drill samples are mostly rhyolite and epizonal granite plutons (Denison, 1981). The 1400–1360 Ma ages of these rocks are consistent with those characteristic of the Southern Granite-Rhyolite province (Thomas et al., 1984; Van Schmus et al., 1996). Precambrian magmatic rocks in the subsurface southwest of the Arbuckle Mountains and northwest of the Grenville-Llano front have ages of 1380–1340 Ma (Barnes et al., 2002), further indicating the original extent of the Southern Granite-Rhyolite province.
Rocks exposed in the Llano uplift, an intracratonic uplift inboard from the Ouachita rift margin on the Texas promontory of Laurentia (Fig. 2), have a tectonic history similar to that of the Grenville province of the eastern United States and Canada (Mosher, 1998). Multiply deformed igneous and sedimentary rocks indicate a collision orogen coeval with the Grenville orogeny. Accreted exotic arc-terrane rocks have U/Pb zircon ages of 1326–1275 Ma (Mosher, 1998; Mosher et al., 2008). Plutonic and supracrustal rocks have U/Pb zircon ages of 1288–1232 Ma, but include an older gneiss dated at 1366 ± 3 Ma (Reese et al., 2000; Mosher et al., 2008). Collision-related deformation and metamorphism are dated between 1147 Ma and 1128 Ma, and late syntectonic and post-tectonic plutons range in age from 1119 to 1070 Ma (Mosher, 1998; Reese et al., 2000; Mosher et al., 2008). The range of ages in the Llano uplift generally distinguishes the Grenville province from the Granite-Rhyolite province; however, the 1366 ± 3 Ma protolith age suggests a pre-Grenville basement of Laurentia, coeval with parts of the Southern Granite-Rhyolite province (Reese et al., 2000).
In the Ouachita thrust belt in Arkansas and Oklahoma, disharmonically thrust-imbricated and ductilely deformed shale-dominated off-shelf Cambrian–Mississippian passive-margin facies are thrust over the Laurentian continental margin and shelf facies (Fig. 2) (summary in Arbenz, 2008; Thomas, 2011). The Cambrian–Lower Ordovician off-shelf passive-margin succession is characterized by black shale, and includes sandstone, calcareous mudstone, chert, and carbonate-clast conglomerates (summary in Arbenz, 1989; Viele and Thomas, 1989). One sandstone unit (Middle Ordovician Blakely Sandstone) contains scattered boulders of granite and meta-arkose, indicating a supply of clasts from steep scarps that exposed basement rocks along the Laurentian continental margin (Stone and Haley, 1977). Separate granite boulders have U/Pb zircon ages of 1407 ± 13 Ma, 1350 ± 30 Ma, and 1284 ± 12 Ma; detrital zircons from arkosic sandstone in the Blakely Sandstone have ages of 1350–1300 Ma (Bowring, 1984). The ages of the granite boulders suggest that rocks of the Southern Granite-Rhyolite province formed this segment of the Laurentian margin along the Alabama-Oklahoma transform (e.g., Bickford and Anderson, inVan Schmus et al., 1993). Although rocks of the Grenville orogen (1350–1000 Ma) extend along most of the Laurentian rifted margin both northeast and southwest of the Ouachita embayment, the ages of these boulders show that, locally at least, the Alabama-Oklahoma transform fault cut across the trace of the Grenville front into the Southern Granite-Rhyolite province (Fig. 2).
In this article, we report previously unpublished U/Pb zircon and baddeleyite data from igneous basement rocks in the St. Francois Mountains (Ozark dome) and in the Arbuckle Mountains (Southern Oklahoma fault system). Analytical methods are described in the Appendix, and results are listed in Table 1.
New isotopic ages from the St. Francois Mountains (Fig. 2) further refine the dominant magmatic event at 1470–1460 Ma and a later event at 1325–1315 Ma, characteristic of the Eastern Granite-Rhyolite province and Southern Granite-Rhyolite province, respectively. Our new isotopic ages include three U/Pb zircon ages from the well-known but previously undated volcanic succession of the St. Francois Mountains exposed in the East Fork of the Black River at Johnson’s Shut-Ins State Park, Missouri. These are, in descending stratigraphic order (Fig. 4): 1462 ± 1 Ma for the upper ash-flow tuff of the Cope Hollow Rhyolite, 1464 ± 3 Ma for the lower ash-flow tuff of the Johnson’s Shut-Ins Rhyolite, and 1466 ± 3 Ma for the upper ash-flow tuff of the Taum Sauk Rhyolite, the lowest of the volcanic formations exposed in the East Fork of the Black River (Kisvarsanyi et al., 1981). The isotopic ages for the three volcanic rocks agree well with the relative stratigraphic positions; thus, the entire volcanic succession, which is ∼650 m thick, was erupted in ∼4–8 m.y. (at 95% confidence limits). Coeval with the pyroclastic eruptives of the western St. Francois Mountains, intrusive igneous rocks were emplaced into the Butler Hill volcanic-plutonic complex of eastern St. Francois County and western Ste. Genevieve County (Bickford and Mose, 1975; Lowell, 1976). Well-known among these plutonic rocks are the small isolated intrusions of biotite granite porphyry with xenoliths of weakly foliated granodiorite gneiss along Pickle Creek in Hawn State Park (Lowell, 1976). Our sample of the granodiorite gneiss xenolith yielded a crystallization age of 1462 ± 1 Ma (Fig. 4), which is in close agreement with the U/Pb determination of 1500 ± 30 Ma for the same rock by Bickford and Mose (1975).
The younger magmatic event of the St. Francois Mountains is documented by new U/Pb ages for two small granitic stocks of the central and western St. Francois Mountains, as well as a microgabbro dike of the “Skrainka diabase” (Kisvarsanyi et al., 1981). Our new isotopic ages (Fig. 5) include a U/Pb baddeleyite age for the Devil’s Tollgate Gabbro (1323 ± 1 Ma; “Skrainka diabase”), and U/Pb zircon ages for the Graniteville Granite (1317 ± 2 Ma) and the Munger Granite Porphyry (1323 ± 2 Ma), the latter of which intrudes the aforementioned volcanic succession at Johnson’s Shut-Ins State Park. The isotopic ages of this younger magmatic event also overlap the range of ages of basement boulders in the Blakely Sandstone in the Ouachita Mountains (Bowring, 1984), further indicating the probable source of the boulders from rocks of the Southern Granite-Rhyolite province.
A newly dated sample of the Tishomingo Granite in the Arbuckle Mountains (Fig. 2) has a U/Pb zircon age of 1364 ± 2 Ma (Fig. 6), which is in good agreement with the previously published ages of 1374 ± 15 Ma (Bickford and Lewis, 1979) and 1363 ± 8 Ma (Rohs and Van Schmus, 2007). These ages and rock types are consistent with those of the Southern Granite-Rhyolite province.
BASEMENT ROCKS IN AND AROUND THE ARGENTINE PRECORDILLERA TERRANE
Characterization of basement rocks in the Precordillera comes from basement massifs in the Western Sierras Pampeanas around the eastern side of the belt of sedimentary thrust sheets, isolated exposures of basement and Precordillera cover strata south of the primary outcrops of the sedimentary thrust sheets, and xenoliths of basement rocks in Tertiary plutons within the Paleozoic strata of the Precordillera (Fig. 3). In addition, olistoliths in slope facies in the western Precordillera include basement rocks.
Summary of Available Age Data
Basement rocks are exposed along an alignment of massifs north and east of the Precordillera sedimentary outcrops (Sierra de Maz, Sierra de Umango, and Sierra de Pie de Palo, Fig. 3). Ages of rocks from these massifs have led to alternative interpretations of the tectonic history and affinities of the basement rocks.
In the Sierra de Maz (Fig. 3), a complex history of zircon overgrowths in metasedimentary garnet schist gives U/Pb ages (sensitive high-resolution ion microprobe [SHRIMP]) of 1880–1700 Ma for zircon cores, and a range of overgrowth ages of 1230–1180 Ma (weighted mean 1208 ± 28 Ma) (Casquet et al., 2006). These ages are interpreted to reflect a Paleoproterozoic provenance of the metasedimentary rocks, and high-grade metamorphism during an orogenic event coeval with the Grenville orogeny (Casquet et al., 2006). Other orthogneisses in the Sierra de Maz have crystallization ages of 1092 ± 6 to 1086 ± 10 Ma (Rapela et al., 2010). For complex zircons from orthogneiss, ages (SHRIMP) of 1330–1260 Ma are interpreted to represent crystallization of the igneous protolith, and ages of 1175–1095 Ma are interpreted to record metamorphism (Rapela et al., 2010). A granitic orthogneiss in the nearby Sierra de Umango (Fig. 3) has a U/Pb zircon magmatic-crystallization age of 1108 ± 13 Ma (Varela et al., 2003).
In a regional context, the Sierras de Maz and Umango have been interpreted to be part of a continental crustal block, extending northwest to Arequipa-Antofalla and perhaps southeast to the Sierra de Pie de Palo, and representing a Mesoproterozoic mobile belt autochthonous to Gondwana (Casquet et al., 2006, 2010). The Arequipa-Antofalla terrane was accreted to Amazonia during the Sunsas orogeny at 1200–1000 Ma, but it includes orthogneisses with ages of 1900–1800 Ma (Loewy et al., 2004), similar to the ages of zircon cores in the Sierra de Maz (Casquet et al., 2006, 2010). Comparable ages of basement rocks in the Sierras de Maz, Umango, and Pie de Palo, as well as Arequipa-Antofalla, are consistent with a common history of Grenville-age continental accretion and assembly of Rodinia. The Arequipa-Antofalla and related terranes have been interpreted to have formed a ribbon continent, first rifted from Amazonia to open the Puncoviscana Ocean in latest Precambrian time, then re-accreted by subduction beneath the Pampean arc, and finally rifted from Laurentia to open the Iapetus Ocean (Escayola et al., 2011). Such a tectonic scenario indicates that the Mesoproterozoic basement terrane was accreted before the Precordillera arrived at the western Gondwanan margin, and that the basement rocks represent the continental margin to which the Precordillera was accreted.
The Sierra de Pie de Palo (Fig. 3), east of the Precordillera sedimentary cover, includes the Pie de Palo Complex of orthogneisses, high-grade metasedimentary rocks, and a mafic-ultramafic arc complex (Vujovich and Kay, 1998; Vujovich et al., 2004; Chernicoff et al., 2009). The earliest reported Pb/Pb zircon ages from the gneisses of 1091–938 Ma (McDonough et al., 1993) are similar to more recently reported U/Pb zircon ages of 1224–1032 Ma (Casquet et al., 2001) and 1204 ± 4 Ma, 1196 ± 8 Ma, 1169 ± 8 Ma, 1166 ± 15 Ma, 1110 ± 10 Ma, 1092 ± 21 Ma, and 1025 ± 10 Ma (Vujovich et al., 2004; Rapela et al., 2010; Mulcahy et al., 2011). Zircon ages of 1224–938 Ma for orogenic events recorded in the Pie de Palo basement are consistent with the ages of basement rocks in the Sierras de Maz and Umango, as well as with the Grenville orogeny of Laurentia.
Along the western side of the Sierra de Pie de Palo, the west-directed Las Pirquitas fault (Fig. 3) separates the higher grade gneisses of the Pie de Palo Complex on the east from metasedimentary rocks of the Caucete Group on the west. Detrital-zircon populations in metasedimentary quartzite (part of Caucete Group) in the footwall of the Las Pirquitas fault on the western side of the Pie de Palo are similar to detrital-zircon populations in the Lower Cambrian synrift Cerro Totora Formation, the oldest part of the Precordillera sedimentary succession (Naipauer et al., 2005, 2010). These relationships suggest that the Caucete Group in the Las Pirquitas footwall was originally part of the Cambrian sedimentary cover of the Precordillera terrane and was near the leading edge at the time of collision during the Ordovician (Naipauer et al., 2005, 2010; Chernicoff et al., 2009; van Staal et al., 2011).
Although the Caucete Group is documented to be part of the Precordillera (Cuyania) terrane, alternative interpretations consider the Pie de Palo Complex either to represent the basement of the Precordillera terrane (e.g., Ramos et al., 1998; Vujovich and Kay, 1998) or to be a separate microcontinental fragment. The mafic-ultramafic belt in the Pie de Palo may represent a Mesoproterozoic suture between the orthogneisses in the sierra and a separate basement terrane (the Precordillera) on the west; the Grenville-age suture may have been reactivated as the Las Pirquitas fault during the Ordovician collision of the Precordillera terrane (Chernicoff et al., 2009). Mylonites (ca. 515 Ma) in the hanging wall of the Las Pirquitas fault have been interpreted to indicate that the Pie de Palo Complex was within an active convergent margin along western Gondwana before the arrival of the separate Precordillera terrane (Mulcahy et al., 2007, 2011). In contrast, comprehensive structural models show that the mylonites could be a result of extension during opening of Iapetus along the margin of a Precordillera terrane that included the Pie de Palo Complex (van Staal et al., 2011).
Along the Rio Bonete, the northwest-trending Jagüé shear zone, which is the probable northern boundary of the Precordillera terrane (Martina and Astini, 2009), includes mylonitic granite and marble (Fig. 3). The granite is interpreted to be part of the Precordillera basement, and the associated calcitic and dolomitic marbles suggest protoliths of the Precordillera carbonate cover (Martina et al., 2005; Martina and Astini, 2009). The association of the sheared granite and the marble is consistent with a passive-margin transgression similar to that documented for the characteristic succession in the Precordillera (Astini et al., 1995). The structural position of the basement rocks at Rio Bonete suggests a location along the original margin of the Precordillera at the Alabama-Oklahoma transform fault (Fig. 1). Twenty zircons from the mylonitic granite yielded a weighted mean age (U/Pb laser ablation–inductively coupled plasma–mass spectrometry [LA-ICP-MS]) of 1118 ± 17 Ma (Martina et al., 2005).
More than 150 km south of the primary outcrops of the Precordillera platform rocks and basement, isolated exposures in the San Rafael block (Fig. 3) are surrounded by Cenozoic outwash from the Andes and include basement granite and Paleozoic sedimentary cover at Ponón Trehue (Bordonaro et al., 1996; Astini, 2002, 2003; Cingolani et al., 2003). The granite has a previously reported Rb-Sr isochron age of 1063 ± 106 Ma (Cingolani and Varela, 1999).
Basement rocks and metasedimentary carbonates exposed locally farther south in the Cerro San Jorge and Limay Mahuida area, the Las Matras block (Fig. 3), are evidently part of the Precordillera terrane (Astini et al., 1995; Ramos, 2004, 2010; Sato et al., 2004). The basement rocks have a Rb-Sr age of 1212 ± 47 Ma and a Sm-Nd age of 1178 ± 47 Ma (Sato et al., 1998, 1999, 2004). More recently, a U/Pb zircon age of 1244 ± 42 Ma has been reported (Sato et al., 2004).
In part of the Precordillera, Tertiary dacitic to andesitic plutons intrude the sedimentary succession, rising from the basement beneath the sedimentary thrust sheets. Basement xenoliths in Tertiary plutons at Ullún (Fig. 3) yield zircon ages of 1102 ± 6 Ma (U/Pb) and 1099 ± 3 Ma (Pb/Pb) for mafic gneisses and 1096 ± 50 Ma and 1118 ± 54 Ma for acidic gneisses (Kay et al., 1996). Additional U/Pb zircon data yielded a crystallization age of ca. 1165 Ma and a metamorphic overprint at ca. 1060 Ma (Rapela et al., 2010). Common Pb ratios indicate geochemical similarity of these rocks to the basement rocks of the Llano uplift on the Texas promontory of Laurentia (Kay et al., 1996).
In summary, previously reported U/Pb zircon ages of basement rocks from basement massifs around the Argentine Precordillera, as well as from xenoliths, are mostly within the range of 1244 ± 42 to 1025 ± 10 Ma. Some, or all, of the basement rocks north and east of the Precordillera sedimentary thrust belt (in the Sierras de Maz, Umango, and Pie de Palo, Fig. 3) may represent a separate terrane or terranes, but alternatively may be part of a greater Precordillera (Cuyania) terrane (e.g., Ramos et al., 1986; Ramos, 2004, 2010). Regardless of whether these massifs represent the basement of a singular Precordillera terrane or a constellation of separate small terranes along the leading edge of a composite Precordillera, the counterparts on the Laurentian conjugate margins are the Grenville and Llano provinces, which have similar ages, as well as a history of multiple events. The basement massifs south of the Precordillera have cover stratigraphy correlative with the Precordillera sedimentary succession and, thus, represent the Precordillera basement, as do the xenoliths from Tertiary plutons. Omitting the ages of the debatable basement massifs, the range of securely Precordillera basement ages is 1244 ± 42 to 1099 ± 3 Ma, within the age range of the Grenville orogeny of eastern Laurentia.
In this article, we report previously unpublished isotopic ages for basement rocks at Ponón Trehue in the San Rafael block south of the primary outcrops of the Precordillera and for basement clasts in a conglomerate olistolith in slope deposits of the western Precordillera (Fig. 3). Analytical results are listed in Table 1.
Basement granite at Ponón Trehue underlies a structurally and stratigraphically complex succession of basal conglomerate and overlying carbonate (Astini et al., 1995; Bordonaro et al., 1996; Cingolani and Varela, 1999; Cingolani et al., 2003). We report new U/Pb zircon ages for two granite samples (Fig. 7): one, BT-128-2, has a weighted mean 207Pb/206Pb age of 1205 ± 1 Ma, and the other, BT-130-3, has a weighted mean 207Pb/206Pb age of 1204 ± 2 Ma. The dates record the age of crystallization of the granites, and both are similar to previously reported ages of basement in other parts of the Precordillera (Fig. 3). These new data provide further evidence that the age of Precordillera basement is coeval with the Grenville orogeny of eastern Laurentia.
Unique “samples” of Precordillera basement were collected as clasts from a conglomerate olistolith in the Middle Ordovician Los Sombreros Formation along the Quebrada de Los Ratones, a tributary of the Rio San Juan, in the western Precordillera (Fig. 3). The Los Sombreros Formation, a deep-water and mud-dominated succession, records deposition on the continental slope and rise west of the Precordillera carbonate platform (Astini et al., 1995; Thomas and Astini, 2003). The western shelf edge of the Precordillera carbonate platform, now concealed beneath Andean thrust sheets, evidently marks the western rifted margin of Precordillera continental crust. The Los Sombreros muddy slope facies contains olistoliths of rocks from the platform, slope, and basement (Astini et al., 1995; Keller, 1999). The olistoliths include (1) Middle Cambrian to Lower Ordovician platform-carbonate rocks, Lower to Middle Cambrian limestone-shale strata, and Ordovician shales, which represent parts of the exposed stratigraphy of the Precordillera platform; (2) Middle Cambrian deep-water carbonates, which define the minimum age of the western Precordillera slope; (3) lithic-clast conglomerate, quartz-pebble conglomerate, arkosic sandstone, and red beds, interpreted to represent the basal part of the platform succession and older synrift deposits (Astini and Thomas, 1999); and (4) basement granites, indicating that the source of the olistoliths cut down through the entire cover succession. The composition and position of the conglomerate olistoliths in the muddy slope deposits suggest slope-collapse faults and/or submarine canyons that fragmented the outer edge of the carbonate platform along with the underlying synrift deposits and basement (Fig. 8). Ultimately, the conglomerate olistolith within a muddy turbidite succession (Los Sombreros Formation) on the western slope of the Precordillera was imbricated by Andean thrusting and exposed by erosion on the present surface (Thomas and Astini, 2003).
In exposures of the Los Sombreros Formation along the Quebrada de Los Ratones, one large (several meters across) olistolith of polymictic conglomerate contains rounded clasts, ∼25 cm in diameter, of granodiorite, granitic gneiss, and granodioritic gneiss in a coarse sandstone matrix (Fig. 9). Some of the rocks are highly sheared and recrystallized, but a primary igneous texture is largely intact. The lithology of the clasts suggests derivation from a variably deformed mesozonal pluton. Some alteration evidently occurred during surface weathering and transportation. The lithology and texture of the conglomerate suggest synrift deposition in alluvial fans along fault scarps in an extensional graben system on rifted continental crust, consistent with synrift graben-fill deposits that underlie the passive-margin platform-carbonate succession of the Precordillera (Thomas and Astini, 1999, 2003). The provenance of the large clasts in the conglomerate must have been the local continental basement exposed on proximal fault scarps.
We report two new U/Pb zircon ages of 1370 ± 2 Ma and 1367 ± 5 Ma (Fig. 10) for separate basement clasts within the single conglomerate olistolith. Both ages are best interpreted as the date of emplacement and crystallization of the plutonic protoliths. Both ages are significantly older than the oldest known basement rocks in the Precordillera, although they are near the ages of an early event (1330–1260 Ma) in the Sierra de Maz (Rapela et al., 2010). Moreover, the ages of these unusual samples show that this part of the Precordillera contains basement rocks that are similar in age to the Southern Granite-Rhyolite province of southern Laurentia but different from most of the Grenville province near the Laurentian margin (Figs. 1 and 2) (e.g., Van Schmus et al., 1993; Mosher, 1998), and also similar to the 1366 ± 3 Ma gneiss in the Llano uplift (Reese et al., 2000). Although the Granite-Rhyolite province generally lacks granodioritic phases, basement rocks exposed in the Arbuckle uplift include the Burch Granodiorite and granodioritic gneisses in the Blue River Gneiss (Lidiak and Denison, 1999); drill penetrations of subsurface basement rocks in the Arbuckle region include local diorites (Ham et al., 1964). The granodiorites in the Arbuckle uplift are near the rifted conjugate margin of Laurentia and the Precordillera, suggesting lithologic and temporal counterparts for the granodiorite and granodioritic gneiss clasts in the olistolith along the rifted margin of the Precordillera, and further supporting the interpretation of rifting of the Precordillera from the Ouachita embayment of Laurentia.
SYNRIFT ROCKS AND AGE OF RIFTING
The age of rifting and opening of the Ouachita segment of the Iapetus Ocean between the Precordillera terrane and Laurentia has been based primarily on the synrift and passive-margin successions on the conjugate margins of the Texas promontory of Laurentia and the Precordillera (Figs. 1 and 2) (e.g., Thomas and Astini, 1999). A bimodal igneous suite along the Southern Oklahoma fault system (Ham et al., 1964; Gilbert, 1983; Hogan and Gilbert, 1998) provides geochronological constraints for the transform-parallel synrift intracratonic fault system.
Sedimentary Record of Rift History
The passive-margin succession exposed around the Llano uplift and drilled in the subsurface on the Texas promontory of Laurentia has a basal sandstone of latest Middle Cambrian age, and an overlying carbonate succession generally less than 1000 m thick (summary in Thomas and Astini, 1999). The basal sandstone overlaps a paleotopographic surface with more than 200 m of relief on the Precambrian basement (Barnes et al., 1972). The age of thermal subsidence and passive-margin transgression onto the Laurentian margin along the Ouachita rift is consistent with rifting during the Early Cambrian (Thomas and Astini, 1999).
Paleozoic rocks in the Precordillera are in the hanging walls of Andean thrust faults, and the contact with basement rocks is not exposed. The oldest Paleozoic rocks exposed in the Precordillera are a synrift succession of red beds, evaporites, and carbonates of the Lower Cambrian Cerro Totora Formation (Astini and Vaccari, 1996). The Cerro Totora Formation has an olenellid and Salterella fauna, indicating an Early Cambrian age (Astini et al., 1995, 2004), and strontium isotopes from Cerro Totora evaporites are consistent with that age (Thomas et al., 2001). The Cerro Totora Formation is similar lithologically, stratigraphically, paleontologically, and isotopically to the coeval Rome Formation in a basement graben on the Alabama promontory of Laurentia, indicating a similar tectonic framework of sedimentation on opposite sides of the Alabama-Oklahoma transform (Fig. 1) (Thomas et al., 2001). A passive-margin carbonate succession above the Cerro Totora clastic-evaporite facies ranges in age from Early Cambrian through Early Ordovician (Astini et al., 1995). The Precordillera carbonate succession is much thicker than the partly coeval carbonate-shelf succession on the Texas promontory of Laurentia, where no synrift deposits are preserved; the base of the carbonate succession in the Precordillera is older than that on the Texas promontory (Thomas and Astini, 1999). Stratigraphic comparisons show that passive-margin subsidence and transgression began earlier on the Precordillera than on the Texas promontory, and that the magnitude of subsidence was greater on the Precordillera. This seeming paradox is explained by observations of complementary asymmetry of post-rift thermal subsidence on the conjugate rift margins of a simple-shear low-angle-detachment rift system (Thomas and Astini, 1999, fig. 5). The lack of synrift sedimentary accumulations, as well as the paleotopographic relief beneath the basal transgressive sandstone, indicates an upper-plate margin in Texas; whereas the preserved synrift graben-fill sediment, as well as earlier thermal subsidence and post-rift transgression, indicates a lower-plate margin on the Precordillera (Thomas and Astini, 1999). The ages of synrift rocks in the Precordillera and the diachronous ages of initial passive-margin deposition are consistent with rifting of the Precordillera from Laurentia in the Early Cambrian, the age of the Cerro Totora synrift red beds and evaporites (Astini et al., 1995; Thomas and Astini, 1996, 1999). Initial rifting in the earliest Cambrian led to opening of an Ouachita ocean floor and migration of the Precordillera microcontinent along the Alabama-Oklahoma transform fault (Thomas, 1991; Thomas and Astini, 1996). Movement along the transform fault is consistent with episodic reactivation of the basement faults of the Mississippi Valley and Birmingham grabens until the Ouachita mid-ocean ridge migrated past the corner of Laurentian crust on the Alabama promontory (Thomas, 1991). The end of active extension on the basement faults marks the time of separation of the Precordillera microplate from Laurentia in the Late Cambrian.
Synrift Igneous Rocks along the Southern Oklahoma Fault System
In the context of Iapetan rifting, the Southern Oklahoma fault system is parallel with transform faults and extends >500 km northwesterly into the Laurentian continental crust from the Ouachita embayment in the rifted margin (Fig. 1); however, the Southern Oklahoma fault system intersects the Ouachita rift margin ∼150 km south of the corner of the embayment at the intersection of the rift with the Alabama-Oklahoma transform fault (summaries in Thomas, 2010, 2011). The Southern Oklahoma fault system encompasses a suite of bimodal plutonic and volcanic rocks, including gabbro, basalt, granite, and rhyolite (Hogan and Gilbert, 1998), the composition of which indicates deep sources in the upper mantle. High-amplitude, short-wavelength gravity and magnetic anomalies indicate dense mafic rocks with steep boundaries in the shallow continental crust (Keller and Stephenson, 2007). Identity of specific synrift basement faults is obscured by the large volume of igneous rocks, as well as by overprint of late Paleozoic large-magnitude basement faults (Denison, inJohnson et al., 1988). Faults within volcanic rocks, as well as angular discordances between different units within the layered complex, suggest rift-bounding faults with >1 km of vertical separation and cumulative extension across the system of 17–21 km (McConnell and Gilbert, 1986). The geometry and composition of the igneous rocks indicate crust-penetrating near-vertical fractures as magma conduits, consistent with a leaky transform fault.
The entire bimodal suite of igneous rocks is extensively exposed in the Wichita Mountains, where the oldest exposed unit is layered gabbro and anorthositic rocks of the Glen Mountains Layered Complex. Only the younger rhyolite is exposed in the Arbuckle Mountains. Diabase dikes cut all of the other igneous rocks and mark the end of igneous activity. Geochronologic analyses, using a variety of techniques, have documented ages that center at 540–530 Ma (Tilton et al., 1962; Ham et al., 1964; Bowring and Hoppe, 1982; Lambert and Unruh, 1986; Lambert et al., 1988; Wright et al., 1996; Hogan and Gilbert, 1998); the ages of the younger rhyolites are within error of the ages of the older gabbros, even though they are clearly separated by a time of uplift and erosion (Ham et al., 1964). For example, for the Mount Sheridan Gabbro, 40Ar/39Ar analyses of amphibole yielded ages of 535 ± 8 Ma and 533 ± 2 Ma, and of biotite an age of 533 ± 4 Ma; amphibole yielded an age of 539 ± 2 Ma for the Mount Scott Granite sheet (Hames et al., 1998). U/Pb zircon ages of ca. 534 Ma for rhyolite in the Wichita Mountains confirm the limited age range (Hanson et al., 2009). Taken together, the available ages indicate a short time span for emplacement of the entire igneous complex.
A transgressive passive-margin succession of basal sandstone and overlying shallow-marine carbonates overlaps the Early Cambrian igneous rocks, and the age of the base of the transgressive succession is middle Late Cambrian (Denison, inJohnson et al., 1988). Above the basal sandstone along the Southern Oklahoma fault system, the overlying carbonate succession is exceptionally thick, consistent with large-magnitude synrift thermal uplift followed by post-rift cooling and deep subsidence of the shallow igneous rocks (Thomas and Astini, 1999).
Rift History of the Eastern Precordillera and Western Gondwana
An A-type granite orthogneiss within the Pie de Palo Complex in the Sierra de Pie de Palo has a U/Pb zircon age of 774 ± 6 Ma and is interpreted to represent an early phase of Iapetan rifting (Baldo et al., 2006). By comparison, ages of synrift igneous rocks along the Blue Ridge rift in eastern Laurentia include older phases at 765–680 Ma (Tollo et al., 2004). The times of continental breakup and opening of Iapetus along the Blue Ridge rift are indicated by U/Pb ages of synrift igneous rocks at 572 ± 5 to 564 ± 9 Ma (Aleinikoff et al., 1995). The similarity of ages of synrift rocks suggests that the Blue Ridge rift may have extended southward across the eastern margin of the Precordillera terrane before it was rifted from the Ouachita embayment some 30 m.y. later (Fig. 2) (Thomas, 1991). This setting places the basement rocks of the Sierra de Pie de Palo as the basement of the leading part of the Precordillera.
East of the alignment of basement massifs (Sierras de Maz, Umango, and Pie de Palo), the Ordovician Famatina volcanic arc marks the suture of the accreted Precordillera (Cuyania) terrane with Gondwana (e.g., Astini et al., 1995; Thomas and Astini, 2003; Ramos, 2004). Famatina is a continental-margin arc on the western margin of Gondwana, beneath which the Precordillera was subducted (Astini and Dávila, 2004). The rocks of the western part of Gondwana, in the vicinity of Famatina, record a tectonic history quite distinct from those of Mesoproterozoic mobile belts west of Famatina, as well as of the Grenville province of eastern Laurentia. A comprehensive interpretation of the setting of Famatina on the western margin of Gondwana yields the following: 700–640 Ma, Brasiliano orogeny imprinted on Mesoproterozoic basement; 640–600 Ma, rifting and opening of the Puncoviscana basin, contemporaneous with opening of Iapetus; 550–520 Ma, Pampean orogeny, including deformation of Puncoviscana sedimentary rocks; and 500–460 Ma, Famatina subduction of the Precordillera terrane and initiation of the Ocloyic orogeny (Collo et al., 2009). This succession of events, in contrast to the rift and passive-margin history of eastern Laurentia, further suggests that the Precordillera terrane had been separated from western Gondwana, beginning with early rifting and opening of Iapetus no later than approximately 600 Ma. The Mesoproterozoic basement of the Precordillera, as well as the Sierras de Maz, Umango, and Pie de Palo (whether or not the latter elements remained part of the Precordillera terrane), indicates a history similar to that of the Grenville-Llano province of eastern Laurentia, including multiple accretionary events in the time span of approximately 1250–1000 Ma, as well as incorporation of some older accreted terranes.
New Data from Synrift Igneous Rocks along the Southern Oklahoma Fault System
We present two new U/Pb zircon ages for the Colbert Rhyolite, a synrift volcanic unit in the Arbuckle Mountains (Fig. 2). Our results were previously reported in an abstract (Thomas et al., 2000), and the data are fully documented here (Fig. 11; Table 1). Two samples of fine-grained quartz-phyric rhyolite yielded a small amount of short-prismatic, equant zircon. Two small fractions of zircon from one sample, BT-98-107 (rhyolite dike in Tishomingo Granite), yielded one concordant and one discordant analysis with a weighted mean 207Pb/206Pb age of 536 ± 5 Ma (Fig. 11). We interpret this date to be the crystallization age of the rhyolite. Three small fractions of zircon from another rhyolite sample, BT-98-112B (hypabyssal rhyolite intrusion), yielded two concordant analyses and one discordant analysis. The three analyses have a weighted mean 207Pb/206Pb age of 539 ± 5 Ma (Fig. 11), which we also interpret as the crystallization age of the rhyolite. The U/Pb zircon ages overlap at 95% confidence limits, and both are similar to previously reported ages for other components of the Southern Oklahoma igneous suite (Fig. 2). Thus, we consider the age of the synrift volcanism in this part of the Arbuckle Mountains to be well established. Combining all of the recent data, an age range of 539 to 530 Ma spans the synrift igneous rocks along the Southern Oklahoma fault system and is consistent with the Early Cambrian age of rifting of the Precordillera from Laurentia as indicated by the stratigraphically documented synrift and post-rift subsidence history.
A growing body of geochronological data has documented a succession of accretionary events that together constitute what has come to be called the Grenville orogeny. Similar multicomponent metamorphic-plutonic events of Mesoproterozoic age are recognized in both western Gondwana-Amazonia and eastern Laurentia. Events associated with the Sunsas belt along southwestern Amazonia are paralleled by events in Arequipa-Antofalla, Sierra de Umango, Sierra de Maz, and Sierra de Pie de Palo. Together, these events span the time of 1250–1000 Ma. Similarly, along eastern Laurentia, successive components of the Grenville orogen span a range of 1250–1000 Ma. New data from the San Rafael block at Ponón Trehue in the Precordillera yield ages of 1205 ± 1 Ma and 1204 ± 2 Ma, which are within the age range of other Mesoproterozoic events. In contrast, ages of granodioritic-granitic basement clasts in a conglomerate olistolith in the northern western Precordillera are 1370 ± 2 Ma and 1367 ± 5 Ma, which are distinctly older than other basement rocks of the Precordillera. These samples indicate the inclusion in the Precordillera microplate of rocks older than the Grenville orogeny as defined in Laurentia. Although data are limited for the subsurface in eastern Laurentia, mapping the Grenville front (the western boundary of the Grenville orogen) suggests that the rift and transform outline of the Ouachita embayment of the Iapetan margin truncated the Grenville front, leaving older rocks of the Southern Granite-Rhyolite province at the Laurentian Iapetan margin in the corner of the embayment. New data from the Arbuckle and St. Francois Mountains confirm ages of 1364 ± 2 to 1317 ± 2 Ma for the Southern Granite-Rhyolite province, in contrast to ages of 1466 ± 3 to 1462 ± 1 Ma for the Eastern Granite-Rhyolite province. The palinspastic location of the Precordillera in the Ouachita embayment is consistent with rocks of the Southern Granite-Rhyolite province along the rifted Laurentian margin as temporal and lithologic counterparts of the part of the Precordillera terrane that includes anomalously old basement rocks.
The age of rifting of the Precordillera from Laurentia has been documented by the approximately 520 Ma age of Early Cambrian synrift sedimentary rocks (red beds and evaporites) in the Precordillera, the approximately 510 Ma age of the transition from synrift to post-rift passive-margin carbonates in late Early to early Middle Cambrian in the Precordillera, and complementary asymmetry of upper-plate subsidence on the Texas promontory with respect to lower-plate subsidence on the Precordillera. Synrift igneous rocks along the transform-parallel intracratonic Southern Oklahoma fault system have ages of 539 to 530 Ma, consistent with synrift magmatism and volcanism in the early stage of rifting along the Ouachita rift and Alabama-Oklahoma transform at the boundaries of the Precordillera microplate in the Ouachita embayment of Laurentia. New data from the Arbuckle Mountains in the eastern part of the Southern Oklahoma fault system yield ages of 539 ± 5 Ma and 536 ± 5 Ma, confirming the age of synrift volcanism.
The range of predominantly Mesoproterozoic ages (1250–1000 Ma) between the Sunsas orogen along the southwest margin of Amazonia and the front of the Grenville orogen in eastern Laurentia suggests a protracted process of terrane accretion and mobilization during the assembly of Rodinia. A similarly protracted process of supercontinent breakup and opening of Iapetus began with initial rifting and synrift magmatism at approximately 780 Ma, culminated in opening of Iapetus at approximately 565 Ma, and concluded with late-stage rifting of microcontinents such as the Precordillera until approximately 530 Ma. After the beginning of Cambrian time, as biostratigraphically dated (541 Ma), much of the Laurentian margin of Iapetus remained as a passive margin, from which microcontinents were shed until approximately 530 Ma. Meanwhile, a complex interplay of microplates extended through a diachronous succession of extension and contraction of Brasiliano, Puncoviscana, and Pampean elements along the Amazonian (Gondwanan) margin of Iapetus prior to accretion of the Laurentian Precordillera terrane by subduction beneath western Gondwana and magmatism of the Famatina continental-margin arc during the Ocloyic orogeny in the Ordovician.
APPENDIX: SAMPLES, ANALYTICAL METHODS, AND DATA TABULATION
Our samples include rocks from the St. Francois Mountains of Missouri and the Arbuckle Mountains of Oklahoma inboard from the Ouachita embayment of southern Laurentia, and from outcrops of basement rocks at Ponón Trehue and from basement clasts in an olistolith at Quebrada de Los Ratones in the Argentine Precordillera (Table 1). The samples from the St. Francois Mountains include three ash-flow tuffs from the well-known and previously undated section of volcanic strata at Johnson’s Shut-Ins State Park, three granitoid igneous rocks (Graniteville Granite, Munger Granite Porphyry, and a granodiorite gneiss xenolith at Hawn State Park), and a microgabbro of the “Skrainka diabase” suite (Devil’s Tollgate Gabbro). The samples from the Arbuckle Mountains include basement granite (Tishomingo Granite) and synrift rhyolite (Colbert Rhyolite). From the Argentine Precordillera, the Ponón Trehue Granite is part of the basement of the San Rafael block, and the clasts are from a conglomerate olistolith in the western slope deposits in the Ordovician Los Sombreros Formation. All whole-rock samples ranged in mass from 0.5 to 10 kg, and zircon and baddeleyite concentrates were obtained by standard techniques of crushing, Wilfley table concentration, and separation by heavy liquid and magnetic susceptibility methods. Following concentration of zircon, and in one sample baddeleyite, individual grains or small fractions of these minerals were picked, abraded, and cleaned, as is standard treatment for isotope dilution analysis. Table 1 lists the age for each sample and provides clarifying remarks.
All samples were analyzed by isotope-dilution thermal ionization mass spectrometry (ID-TIMS) in 1999 using the procedures outlined by Krogh (1973, 1982) with slight modifications described by Tucker et al. (2001). Our mass spectrometry procedures were as follows. Lead fractions smaller than 1 ng and all uranium fractions were analyzed in a VG Sector 54 mass spectrometer using a single-collector procedure with a Daly-type photomultiplier detector operating in pulse-counting mode. Daly bias and nonlinearity were periodically monitored with NIST (National Institute of Standards and Technology) and CBNM (Central Bureau for Nuclear Measurements) isotopic reference materials, and correction factors for Daly gain were used in data reduction. Mass dependent fractionation of Pb and U was monitored regularly since 1994, and a discrimination factor for Pb (0.094 ± 0.04 percent amu−1; 2σ) and U (0.111 ± 0.04 percent amu−1; 2σ) was applied to the measured ratios. Lead fractions larger than 1 ng were analyzed using a two-step, quasi-static procedure with a collector assembly of three Faraday cups and the Daly photomultiplier detector. The 204Pb was measured in the Daly channel only, whereas the 205Pb was measured in the H1 Faraday channel at position 1 and in the Daly channel at field position 2. Other lead isotopes were measured in Faraday channels H1 to H3. Isotope ratios involving 205Pb, 206Pb, 207Pb, and 208Pb were calculated using a static multicollector algorithm. This procedure allowed for internal gain calibration of the Daly channel relative to the H1 Faraday channel in each scan and eliminated biases related to Daly gain and ion beam instability.
Total-procedure blanks over the period of analysis averaged between 2 pg for Pb and 0.5 pg for U; total common-Pb contents for each analysis are reported in Table 1. Common-Pb corrections were made by first correcting the measured ratio for mass-dependent fractionation and introduced spike, and then subtracting Pb equal in amount and composition to the laboratory blank. Any remaining 204Pb is assumed to represent a model lead composition given by Stacey and Kramers (1975) at the estimated age of the rock. For all of our samples, the uncertainty in the amount and composition of common-Pb represents an insignificant contribution to the uncertainty of the isotopic ages (Table 1). Error propagation is similar to that described by Ludwig (2001, 2003), and analytical reproducibility of the mineral fractions confirms that the parameters used in data reduction and the errors have been evaluated correctly.
Because of the slight discordance of some analyses, and the apparent absence of inherited zircon or ancient Pb-loss in any sample, our cited ages are based on the average 207Pb/206Pb age weighted according to the inverse variance of the individual analyses (Ludwig, 2001, 2003). The validity of this approach is confirmed by the reproducibility of the 207Pb/206Pb ages, which is portrayed graphically in Figures 4–7 and 10–11 and quantified by the mean square of the weighted deviates (MSWD). For all samples, the MSWD is less than 2.28, indicating that errors assigned to individual 207Pb/206Pb ages (commonly less than ±2 m.y.) may be somewhat overestimated. Age uncertainties are quoted (Table 1) and shown in concordia diagrams (Figs. 4–7 and 10–11) at 95% confidence limits.
Parts of this research were supported by a grant from the U.S. National Science Foundation (EAR-9706735) to Thomas and a grant from the National Geographic Society (6090-97) to Thomas and Astini. This research was initiated while Thomas was on sabbatical leave from the University of Kentucky and Astini held a Fulbright Research Award, supporting our work together as visitors at the Geological Survey of Alabama. Astini acknowledges research support from Consejo Nacional de Investigaciones Cientificas y Técnicas (112-200801-03265) and from Agencia Nacional de Promoción Científica y Tecnológica (07-33630) (Argentina). We acknowledge constructive comments during the preparation of this article from Rich Harrison, Sharon Mosher, Ed Osborne, and Doug Rankin. Reviews of the manuscript for Geosphere by Richard Hanson and Bob Hatcher are gratefully acknowledged. We especially thank Bill Dickinson for the invitation to prepare this paper for the special issue of Geosphere, as well as for his comments on the manuscript.