Southern Laurentia Research Articles

The Suwannee suture: Significance for Gondwana-Laurentia terrane transfer and formation of Pangaea

In southeastern Laurentia the Suwannee suture separates the undeformed part of the Gondwanan Suwannee terrane (SWT) from crust of Laurentia and previously accreted Gondwanan terranes that were deformed during the Alleghanian orogeny. Undeformed early Paleozoic sedimentary rocks of the SWT are considered of Gondwanan-African derivation on the basis of various considerations, including a lack of Mesoproterozoic (Grenville) detrital zircons. The cratonic heritage of Suwannee lithosphere is complicated, however, because Sm–Nd depleted mantle model ages from both Neoproterozoic and Alleghanian (Carboniferous–Permian) granites north and south of the suture zone are Mesoproterozoic. Mesoproterozoic lithosphere has not been reported in conjugate West Africa, but is present in Amazonia, suggesting that the SWT: 1) may be a composite terrane, containing both African upper crust and Amazonian lower crust/lithosphere, 2) was separated from Gondwana during Atlantic Ocean rifting along a Mesoproterozoic–Neoproterozoic suture, and/or 3) contains Laurentian Grenville-age lower crust and lithosphere that must also underlie other accreted proximal Gondwanan terranes (e.g., Uchee terrane). In addition, the ~300Ma, syn- to post-tectonic granitoids emplaced across the Suwannee suture zone are the product of crustal anatexis, not supra-subduction magmatism, on the basis of Mesoproterozoic Sm–Nd Tdm and zircon xenocrysts. The lack of documented ~300Ma, supra-subduction magmatism in both Laurentian and Gondwanan components in southern Laurentia (e.g., southernmost Appalachian and Ouachita orogens) strongly suggests that accretion of the SWT and related Gondwanan terranes occurred during dextral transpressional convergence between Laurentia and a combined African-Amazonian craton, a characteristic shared with conjugate crust in southwestern Europe. In this interpretation, intervening oceanic lithosphere and contemporary volcanic arcs (if any) were largely removed by disaggregation and dispersion along the transpressional boundary possibly combined with highly oblique overthrusting and/or partly subducted at the leading edge(s) of other blocks of Gondwanan lithosphere that were accreted to southern Laurentia in the late Paleozoic (e.g., Wiggins, Sabine, Coahuila, Maya, etc.). Mesozoic opening of the Gulf of Mexico has subsequently and variably altered the original configuration of these terranes.

U-Pb zircon age constraints on two episodes of Paleoproterozoic magmatism and development of the Grizzly Creek shear zone, White River Uplift, western Colorado, U.S.A.

New U-Pb zircon ages from Precambrian exposures in the White River Uplift of western Colorado reveal at least two episodes of Proterozoic granitic magmatism and help to constrain the age of the 1-km-thick Grizzly Creek shear zone. Gneissic granite exposed along Mitchell Canyon northwest of Glenwood Springs, crystallized at 1765±9 Ma and is the oldest igneous unit recognized in the area. The gneissic foliation is defined by alternating layers of biotite and elongated pink K-feldspar up to 5 cm in size, and the fabric strikes west-northwest and dips moderately to steeply north. Cross-cutting relationships with gneissic country rocks and other igneous units exposed in adjacent drainages were not observed. However, deformation and metamorphism are inferred to be between the age of intrusion (ca. 1765 Ma) and the age of younger, unfoliated, coarse-grained to K-feldspar megacrystic granite exposed near the mouth of No Name Canyon, which crystallized at 1743±8 Ma. This younger granite is cut by the Grizzly Creek shear zone to the north and only contains a locally developed magmatic foliation south of the shear zone. Foliated to mylonitic, fine-grained biotite granite exposed in the hanging wall of the shear zone along No Name Canyon crystallized at 1745±10 Ma, suggesting that it might be related to coarse-grained granite exposed in the shear zone footwall. These new ages define two granitic magmatic events in this area at ca. 1765 and 1745 Ma and provide a maximum age of deformation in the Grizzly Creek shear zone of 1743 Ma. Similarities in the orientation, structural style, and kinematics between the Grizzly Creek shear zone and other well-documented structures in the region raise the possibility that the shear zone records multiple episodes of both Paleoproterozoic and Mesoproterozoic (ca. 1.4 Ga) deformation, in which case the younger events would have occurred in the absence of local magmatism. Thus, the Grizzly Creek shear zone might represent a kinematic link between major crustal shear zones to the north and south throughout crustal assembly and stabilization in southern Laurentia, but details of the timing and kinematic relationships between these structures remain uncertain.

Correlating Proterozoic synorogenic metasedimentary successions in southwestern Laurentia: New insights from detrital zircon U-Pb geochronology of Paleoproterozoic quartzite and metaconglomerate in central and northern Colorado, U.S.A.

Detrital zircon ages from quartzite and metaconglomerate exposed in the Park Range, Front Range, and Sawatch Range of northern and central Colorado reveal new information about age and provenance trends within the Yavapai province of southwestern Laurentia and provide a critical test of regional correlations. Six samples from the three exposure areas—the Lester Mountain area in the Park Range, Coal Creek area in the Front Range, and Collegiate Peaks area in the Sawatch Range—are dominated by Paleoproterozoic detrital zircon that define a relatively narrow range of peak ages between 1772 and 1701 Ma. The unimodal character of most age spectra and the age of the dominant age-probability peaks indicate that the metasedimentary successions were derived largely from local sources within the ca. 1.78–1.70-Ga Yavapai province. Age-probability peaks between 1830 Ma and 2716 Ma are attributed to older source regions to the north and northeast such as the Trans-Hudson orogen, Black Hills, or Wyoming province. However, the percentage of older grains is relatively minor and decreases to almost zero from north to south. Maximum depositional ages constrained by the youngest age populations are 1763 Ma, 1717 Ma, and 1701 Ma for the respective localities from north to south. All of the metasedimentary successions were deformed and metamorphosed after deposition, and quartzite at one locality is intruded by ca. 1672-Ma plutons. On the basis of similarities between outcrop characteristics, structural style, and comparison of detrital zircon age spectra, we correlate the two southern localities with a regional suite of quartzite successions deposited ca. 1.70 Ga following the culmination of the Yavapai orogeny. The northern succession represents an earlier cycle of sedimentation predating ca. 1750-Ma deformation in the northern Yavapai province. The southward progression of ages is consistent with regional tectonic models for the evolution of the Paleoproterozoic accretionary provinces in southern Laurentia, and the multiple cycles of sedimentation suggest that basin formation and collapse were important processes in the tectonic evolution of the continent.

Basin formation near the end of the 1.60–1.45 Ga tectonic gap in southern Laurentia: Mesoproterozoic Hess Canyon Group of Arizona and implications for ca. 1.5 Ga supercontinent configurations

Detrital zircon data from the upper parts of the Proterozoic Hess Canyon Group of southern Arizona reveal abundant 1600–1488 Ma detrital zircons, which represent ages essentially unknown from southern Laurentia. This basinal succession concordantly overlies a >2-km-thick-section of 1657 ± 3 Ma rhyolite of the Redmond Formation. The rhyolite is intercalated with and hence contemporaneous with the lower parts of the overlying White Ledges Formation, a 300-m-thick orthoquartzite unit at the base of the Hess Canyon Group. These quartzites contain a unimodal detrital zircon age probability distribution with peak ages of 1778, 1767, and 1726 Ma, supporting regional correlation with other ca. 1.65 Ga quartzite exposures in southwestern Laurentia. However, the ∼900-m-thick argillaceous Yankee Joe and minimum 600-m-thick quartzite-rich Blackjack Formations contain younger detrital zircons, with peak ages ranging from 1666 to 1494 Ma and a maximum depositional age of 1488 ± 9 Ma. Prominent age peaks at 1582–1515 Ma and 1499–1488 Ma represent detritus that is exotic and not derived from known southern Laurentian sources. The Blackjack Formation is cut by the 1436 ± 2 Ma Ruin Granite, indicating that deposition, deformation, and intrusion occurred between 1488 and 1436 Ma. This basin likely developed before or in the early stages of the 1.45–1.35 Ga intracontinental tectonism in southwestern Laurentia. Our findings necessitate the presence of an ∼170 m.y. disconformity within the Hess Canyon Group and document a previously unrecognized episode of Mesoproterozoic basin sedimentation (>1.5 km of section) between 1488 and 1436 Ma in southern Laurentia. This new record helps to fill the 1.60–1.45 Ga magmatic gap in southern Laurentia and supports hypotheses for a long-lived Proterozoic tectonic margin along southern Laurentia from 1.8 to 1.0 Ga. The 1.6–1.5 Ga detrital zircon ages offer important new constraints for ca. 1.5 Ga Nuna reconstructions and for the paleogeography of contemporaneous basins such as the Belt Basin in western Laurentia.

Coats Land crustal block, East Antarctica: A tectonic tracer for Laurentia?

Undeformed rhyolite and granophyre in the Coats Land crustal block of East Antarctica, dated as 1112 ± 4 Ma, are identical in age to both the Umkondo large igneous province (LIP) of the Kalahari craton (southern Africa) and the early Keweenawan LIP of Laurentia (North America). Although marine and satellite data demonstrate that Coats Land was close to Kalahari within the Gondwana supercontinent, the Coats Land rocks yield Pb isotope compositions strikingly distinct from those of the Umkondo province, yet indistinguishable from rocks of the Keweenawan province. The anorogenic Red Bluff granitic suite, along the present-day southern Laurentian margin in the Franklin Mountains (Texas, USA), is of comparable age, general rock type, and Pb isotope composition to rocks of Coats Land and may provide a piercing point for a Coats Land–Laurentia link. Paleomagnetic poles permit the Coats Land block to be close to this part of Laurentia ca. 1100 Ma and allow juxtaposition of Kalahari and southern Laurentia ca. 1000 Ma. The Coats Land crustal block may therefore be a critical tectonic tracer for placing Laurentia within late Mesoproterozoic and Neoproterozoic paleogeographic reconstructions. If this hypothesis is correct, Laurentia collided with the Kalahari craton along Antarctica's Maud orogen, which would represent a continuation of the ca. 1000 Ma Grenville orogen of eastern and southern Laurentia.

Composition of the mantle lithosphere beneath south-central Laurentia: Evidence from peridotite xenoliths, Knippa, Texas

Mantle xenoliths in ∼83 Ma basanites from south-central Texas provide a rare opportunity to examine the lithospheric mantle beneath southern Laurentia. These peridotites represent lithosphere at the boundary between Mesoproterozoic continental lithosphere and transitional Gulf of Mexico passive margin. Here we report petrographic, mineral, and major element data for 29 spinel peridotite xenoliths from Knippa and use these to characterize the lithospheric mantle beneath south central Texas. The xenoliths comprise spinel-bearing lherzolites and harzburgites with coarse, equigranular textures. Some peridotites contain veins of lizardite. There are no pyroxenites or eclogites. The peridotites contain olivine (Fo89-92), orthopyroxene (En89-92), clinopyroxene (Wo40-45En45-49Fs3-5), and spinel. Spinel Cr# (Cr/(Cr+Al)) distinguishes lherzolites (Cr# = 0.14–0.21) and harzburgites (Cr# = 0.25–0.36). Mineral and major element compositions indicate that the lherzolites are residues after <10% melt extraction from primitive upper mantle and the harzburgites formed by <15% melt extraction. Calculated oxygen fugacities indicate equilibration of the harzburgites at –1 to +0.61 and lherzolites at 0 to –2.6 log units with respect to fayalite-magnetite-quartz (FMQ) buffer, similar to lightly metasomatized spinel peridotites elsewhere. The degree of melt depletion and oxidation of the Knippa peridotites are consistent with present data sets for slightly metasomatized lithospheric mantle and/or backarc samples rather than forearc settings. Equilibration temperatures range from 824 to 1058 °C (mean= 916 °C), calculated at reference pressure of 2.0 GPa. Calculated mean seismic velocities Vs = 4.44 km/sec and Vp =7.87 km/sec show no systematic difference between lherzolites and harzburgites, and agree with present geophysical measurements of upper mantle velocity beneath Texas. The seismic velocities calculated for these samples will provide important constraints for interpretation of EarthScope and other geophysical data sets.

Isotopic and geochemical constraints on the Paleoproterozoic Hutchison Group, southern Australia: Implications for Paleoproterozoic continental reconstructions

Geochemical, Sm–Nd isotopic, and U–Pb–Lu–Hf zircon analyses reveal the sedimentary succession formerly recognised as the Hutchison Group consists of three temporally and isotopically distinct groups, divided by a crustal-scale shear zone (Kalinjala Mylonite Zone). The BIF endowed, newly recognised Middleback Group lies to the east of the Kalinjala Mylonite Zone and has a provenance consistent with uniquely Archean source rocks, including highly evolved ɛ Nd(1790 Ma) values, positive Eu anomalies, flat REE patterns and no detrital zircon younger than 2570 Ma. The depositional age of this Group is likely to be c. 2500 Ma and is a possible temporal equivalent to Late Archean successions in the Gawler Craton and the BIF-endowed Hamersley Group in the Pilbara Craton, Western Australia. Two groups are recognised to the west of the Kalinjala Mylonite Zone. The older of the metasedimentary successions, here termed the Darke Peak Group, consists of felsic volcanic rocks, quartzite, pelite and carbonate rocks deposited at c. 1865 Ma. The volcanic rocks have a geochemical signature of enriched, alkaline, intracontinental magmatism with A-type affinities, and mark the initiation of Paleoproterozoic rifting. Sources to the metasedimentary component of this Group were dominated by highly evolved, intra-crustally reworked, c. 2500 Ma and c. 2000 Ma felsic rocks, consistent with a provenance dominated by local basement complexes of the Late Archean to Early Proterozoic Gawler Craton. A succession of dominantly psammitic and pelitic units, here termed the Cleve Group, disconformably overlies the Darke Peak Group and was deposited between 1780 Ma and 1730 Ma. Provenance to the Cleve Group included abundant 1850 Ma and 1790 Ma zircons, and had a geochemical composition consistent with felsic, upper crustal sources which were significantly less evolved than the Darke Peak Group. The more radiogenic initial ɛ Nd values recorded in the Cleve Group require the input of juvenile felsic material not easily reconcilable with sources within the Southern Australian Proterozoic. Provenance data strengthen Proterozoic assembly models which suggest that the Southern Australian Proterozoic was contiguous with components of either the North Australian Craton or the southern Pilbara Craton by the time of deposition of the Cleve Succession ( c. 1780 Ma). The data are also internally consistent with reconstructions of Nuna which place southern Laurentia proximal to the southeast margin of Proterozoic Australia at c. 1800 Ma.

The Iapetan rifted margin of southern Laurentia

The Iapetan rifted margin of southern Laurentia includes the northeast-striking Blue Ridge, Ouachita, and Marathon rifts, which are offset by the northwest-striking Alabama-Oklahoma and Texas transform faults, framing the Alabama and Texas promontories and the Ouachita and Marathon embayments of the continental margin. Interpretations of the original trace, structural style, and age of the rifted margin rest on identification of synrift rocks and structures, as well as continental-shelf and off-shelf sedimentary deposits on the passive margin. Both late Paleozoic Ouachita-Appalachian allochthons and post-orogenic Atlantic-Gulf passive-margin deposits cover the Iapetan rift margin, necessitating the use of data from deep wells and geophysical surveys along with geologic maps of the exposed Ouachita-Appalachian thrust belts to characterize the synrift and post-rift rocks and structures. The continental margin and passive-margin shelf strata are primarily in the footwall of the Ouachita allochthon; however, some Ouachita thrust faults displaced shelf-margin basement and cover. Appalachian thrust faults imbricate synrift fill of the intracratonic Birmingham graben and the passive-margin shelf. Palinspastic restoration of thrust-belt structures uses balanced cross sections to locate the original trace of the Iapetan margin. Thickness and subsidence history of the passive-margin successions, as well as a general lack of preserved synrift deposits, indicate an upper-plate structure along the Blue Ridge rift on the Alabama promontory and along the Ouachita rift on the Texas promontory. The upper plate on the Texas promontory is conjugate to a lower-plate rift structure on the Argentine Precordillera. Although data are limited, the evolution of the passive margin along the Marathon rift in the Marathon embayment suggests a lower-plate structure. Geophysical modeling supports a steep continental margin along the Alabama-Oklahoma transform, and a similar structure can be inferred for the Texas transform. The Blue Ridge rift north of the Alabama promontory is dated by synrift volcanic rocks as young as 564 Ma, and passive-margin transgression beginning in earliest Cambrian is documented along the Alabama promontory and farther north. The age of the Ouachita rift is documented by the 530–539 Ma synrift volcanics of the transform-parallel intracratonic Southern Oklahoma fault system, by Early Cambrian synrift sediment along the conjugate rift margin in the Argentine Precordillera, and by late synrift graben-fill of Early to early Late Cambrian age in the rift-parallel intracratonic Mississippi Valley and Birmingham graben systems, as well as by subsidence history of the passive margin on the Texas promontory. The diachroniety of rifting reflects an inboard shift from the Blue Ridge rift to the Ouachita rift along the Alabama-Oklahoma transform and rifting of the Argentine Precordillera from the Ouachita embayment.

The geochemistry and petrogenesis of the Paleoproterozoic Green Mountain arc: A composite(?), bimodal, oceanic, fringing arc

The inferred subduction affinity of the ∼1780-Ma Green Mountain arc, a dominantly bimodal igneous terrane (together with immature marine and volcaniclastic sedimentary rocks) accreted to the southern margin of the Wyoming province, is integral to arc-accretion models of the Paleoproterozoic growth of southern Laurentia. Conversely, the dominantly bimodal nature of many putative arc-related igneous suites throughout southern Laurentia, including the Green Mountain arc, has also been used to support models of growth by extension of pre-existing crust. We report new geochemical and isotopic data from ∼1780-Ma gabbroic and granodioritic to tonalitic rocks of the Big Creek Gneiss, interpreted as consanguineous with previously studied metavolcanic rocks of the Green Mountain Formation.The ∼1780-Ma Big Creek Gneiss mafic rocks show clear geochemical signatures of a subduction origin and provide no supporting evidence for extensional tectonism. The ∼1780-Ma Big Creek Gneiss felsic rocks are attributed to partial melting of mafic and/or mixed lower-crustal material. The bimodal nature of the suite results from the combination of arc basalts and felsic crustal melts. The lack of andesite is consistent with the observed tholeiitic differentiation trend of the mafic magmas. The lower ɛNd(1780Ma) values for the felsic rocks vs. the mafic rocks suggest that the unexposed lower crust of the arc may be older than the arc and that Trans-Hudson- or Penokean-aged rocks possibly form the substratum of the arc. Our results reinforce previous interpretations that arc-related magmatism played a key role in the Paleoproterozoic crustal growth of southern Laurentia, but also support the possibility of unexposed older crust as basement to the arcs.

Revised regional correlations and tectonic implications of Paleoproterozoic and Mesoproterozoic metasedimentary rocks in northern New Mexico, USA: New findings from detrital zircon studies of the Hondo Group, Vadito Group, and Marqueñas Formation

Detrital zircon ages from quartzite and metaconglomerate in the Tusas and Picuris Mountains in northern New Mexico reveal new information about age and provenance trends within a >1000 km 2 Proterozoic sedimentary basin and provide a critical test of regional correlations. Samples from the Paleoproterozoic Vadito and Hondo groups are dominated by a single detrital zircon population with age probability peaks that range from 1765 to 1704 Ma. Minor Archean and ca. 1850 Ma age probability peaks were also recognized in some samples. Close correspondence between detrital zircon ages and the age of surrounding basement rocks indicates predominately local sources, and we interpret systematic shifts in peak ages with stratigraphic position to represent changes in local sources through time. Similarities of age spectra support previous correlation of stratigraphic units between discontinuous exposures of the Hondo Group. We interpret that these supracrustal rocks were deposited in a single basin that we refer to as the Pilar basin. Two samples of the Marquenas Formation, a pebble to boulder conglomerate previously correlated with the ca. 1700 Ma Vadito Group, are dominated by Paleoproterozoic detrital zircon with age probability peaks at 1707 and 1715 Ma in the middle and upper units, respectively. Unlike the Vadito and Hondo group samples, the Marquenas Formation also contains abundant ca. 1700–1600 Ma zircon derived from Mazatzal-aged sources to the south and Mesoproterozoic zircon with age probability peaks at 1479 and 1457 Ma. Weighted averages of 1477 ± 13 Ma and 1453 ± 10 Ma for the youngest detrital zircon in the middle and upper Marquenas Formation provide new maximum depositional age constraints, indicating that it is not part of the Vadito Group. The minimum age is not well constrained but is interpreted to be ca. 1435 Ma on the basis of the timing of regional metamorphism and deformation previously documented in the Picuris Mountains. These data represent the first evidence of sedimentation directly associated with ca. 1.4 Ga regional metamorphism, plutonism, and deformation in the southwestern United States and provide an important new constraint on the tectonic evolution of southern Laurentia during this time.

Tectonic evolution of western Amazonia from the assembly of Rodinia to its break-up

Plate tectonic reconstructions of the late Mesoproterozoic–Neoproterozoic supercontinent of Rodinia juxtapose the western margin of Amazonia against eastern Laurentia based on palaeomagnetic, isotopic, and geological evidence. Mesoproterozoic (‘Grenvillian’) orogenic belts are of crucial importance to these reconstructions as they act as key tectonic tracers for Amazonia–Laurentia interactions. They include orogenic belts sited on Amazonia (such as the Sunsás Orogen), para- autochthonous elements such as the Mesoproterozoic metamorphic basement inliers within the Andean Belt (e.g. in the Colombian Andes), exotic terranes accreted to Amazonia during Rodinia assembly (such as the Arequipa Massif), and orphaned fragments of Amazonian basement in Central and North America. A review of the timing of Sunsás orogenesis demonstrates that it occurred from about 1.2–1 Ga in eastern Bolivia and the western Amazon region of Brazil. This is significantly older than the timing of metamorphism in Mesoproterozoic basement inliers of the Colombian Andes, which record a late metamorphic event between 0.9 and 1.0 Ga. Orphaned fragments of Amazonian basement in Laurentia (such as the Blue Ridge/Mars Hill terrane) suggest collision between southeastern Laurentia and Amazonia at ca. 1.15 Ga. The Arequipa Massif (and Antofalla Basement) most likely represents an exotic basement terrane that was caught up in the collision of southeastern Laurentia with western Amazonia. Recent palaeomagnetic data suggest that Amazonia moved northeastwards along the eastern Laurentian margin during Grenvillian collision. Amazonia evidently collided with southern Laurentia at ca. 1200 Ma and, as a result of progressive dextral transcurrent movement, it encountered the Labrador–Greenland sector of eastern Laurentia (and possibly Baltica) by 980 Ma. The timing of the Rodinia break-up in western Amazonia is poorly constrained. Evidence exists supporting a Neoproterozoic western Amazonian active margin, which would imply its at least partial separation from the conjugate rift margin of eastern Laurentia (i.e. formation of a proto-Iapetus Ocean) prior to ca. 650 Ma. This rifting event may be linked to A-type magmatism at ca. 770–690 Ma which is documented in both southeast Laurentia and western Amazonia. Final separation must have been completed by Early Cambrian times based on the unequivocal evidence for drift-related sedimentation on the Laurentian margin of the Iapetus Ocean.

Geochemistry of Neoproterozoic-Cambrian metasedimentary rocks of the Caucete Group, Sierra de Pie de Palo, Argentina: Implications for their provenance

Abstract The Caucete Group is a low-grade metasedimentary sequence of probable late Neoproterozoic-Cambrian age. It is exposed on the western flank of the Sierra de Pie de Palo, a basement block of the Cuyania terrane from western Argentina. The geochemical composition (major, trace and rare-earth elements, and Sm–Nd isotopes) of the El Quemado, La Paz, El Desecho, and Angacos formations (Caucete Group) was used to characterize the sedimentary provenance. We recognized two types of sources: A felsic source (crustal signature) and another mafic source with juvenile signature. Provenance is interpreted to be the mixing between Paleoproterozoic felsic rocks and Mesoproterozoic–Neoproterozoic juvenile rocks sources. This is supported by previous geochronological studies. The Cuyania basement is interpreted to be the Mesoproterozoic and Neoproterozoic juvenile component defined in the Caucete Group (e.g.: ophiolite assemblage of Sierra de Pie de Palo). The Cuyania basement is also considered to be the felsic source (paragneisses and meta-greywackes of the Pie de Palo Complex). We do not consider Gondwana as a provenance source based on the discrepancies in the Sm–Nd isotopic signatures and the detrital zircon provenance ages between the Caucete Group and the surrounding units that are considered to be autochthonous to the southern margin of Gondwana. The Caucete Group Laurentian Sm–Nd isotopic signature suggests that this sector could have been part of the source area. Hence an allochthonous origin for the Caucete Group (Cuyania terrane), derived from southern Laurentia, is supported for the geochemical results presented in this contribution.

U-Pb geochronology of Proterozoic granites in the Sawatch Range, central Colorado, U.S.A.

We report new U-Pb zircon ages for four previously undated Proterozoic granitoid intrusions exposed in the southern Sawatch Range, central Colorado. Coarse-grained to K-feldspar megacrystic granite (Henry Mountain granite) exposed along Taylor Canyon 20 km north of Gunnison, Colorado, crystallized at 1697 ± 7 Ma. It cuts across high-temperature deformational fabrics in metavolcanic and metasedimentary country rocks, thus bracketing at least one Paleoproterozoic tectonic event locally. This granite also contains a well-developed northeaststriking, subvertical foliation that postdates emplacement. The three other intrusions all yielded Mesoproterozoic ages. The Monarch Pass pluton, comprising coarse- to medium-grained granodiorite exposed 50 km east of Gunnison, crystallized at 1447 ± 9 Ma. It cuts across well-developed fabrics in metavolcanic host rocks and contains a widespread biotite foliation. Coarse-grained to K-feldspar megacrystic granite (Horsethief granite) exposed 5–10 km northwest of Taylor Park Reservoir was emplaced at 1437 ± 5 Ma, and it is locally deformed. Fine-grained, muscovite-biotite granite (Taylor River granite) that cuts across the southwestern part of the Henry Mountain pluton crystallized at 1428 ± 23 Ma. A subvertical, northeast-striking biotite foliation cuts across the contact between these two intrusions, suggesting that northwest-directed subhorizontal shortening occurred locally during the Mesoproterozoic. These ages provide new opportunities to constrain the age of tectonism in central Colorado and to further understand the Proterozoic tectonic evolution of southern Laurentia.

Fluid-metasomatized mantle beneath the Ouachita belt of southern Laurentia: Fate of lithospheric mantle in a continental orogenic belt

The nature and history of the cratonic lithosphere beneath the southern margin of Laurentia are poorly understood due to the paucity of basement exposure in the Gulf Coast region. One place where we are afforded the opportunity to study the deep lithosphere of southern Laurentia is within the Balcones igneous province of Texas. Mantle xenoliths occur in Late Cretaceous alkali basaltic magmas erupted through the remnants of the Appalachian-Ouachita structural belt. Geochemical signatures in the form of enrichments in fluid-mobile trace elements (e.g., La) relative to fluid-immobile trace elements (e.g., Nb) are preserved in these mantle xenoliths. We interpret these signatures to represent metasomatism by subduction-related fluids, suggesting that the mantle xenoliths represent fragments of continental lithospheric mantle that served as the upper plate during a convergent episode. These xenoliths may thus represent fragments of Laurentian lithospheric mantle that were stabilized by collisional processes during the continent-continent collisions associated with the Grenville orogeny and subsequent Paleozoic orogenies. These observations suggest that some of the original continental lithosphere was preserved beneath the orogenic belt during continent-continent collision and did not undergo wholesale removal or delamination. Alternatively, the fluid signatures may have been caused by Farallon flat-slab hydration of the overriding lithosphere, but such a scenario seems unlikely because flat subduction is thought to have culminated in the early Cenozoic and hence after the eruption of these xenoliths.

The Grenville orogenic cycle of southern Laurentia: Unraveling sutures, rifts, and shear zones as potential piercing points for Amazonia

Abstract The magnetic anomaly map of North America serves as a useful base from which to attempt palinspastic reconstruction of terranes accreted during the Elzevirian orogeny (1250–1200 Ma); the Shawinigan (1200–1150 Ma), Ottawan (1080–1020 Ma), and Rigolet (1020–1000 Ma) phases of the Grenvillian orogeny; and post-Grenvillian magmatism (760–600 Ma) and deformation prior to Iapetan rifting at 565 Ma. Accreted terranes had unique histories prior to amalgamation and share common tectonic events afterwards. Comparisons with magnetic signatures of the Paleozoic craton–craton suture, sutures of accreted terranes, and the Jurassic rifted-margin for the southern-central Appalachians provide a basis for discriminating among alternative Grenvillian sutures beneath the Appalachian orogen. The Elzevirian suture is partially preserved beneath the Appalachians where it separates the Reading Prong terrane from Laurentia (i.e., Adirondacks and composite-arc terrane and Canadian Grenville Province). The Shawinigan suture is partially preserved in the Llano area (Texas), but separated the now-fragmented and allochthonous Amazonian (as indicated from Pb-isotope data) blocks of the outboard Blue Ridge terrane from the Reading Prong terrane in the Appalachians. Isolated blocks of the Sauratown Mountains terrane are interpreted as outboard of the Blue Ridge terrane, but were also accreted during the Shawinigan phase. Within present-day Laurentia, the only fragment of a terrane believed to have been accreted during the main Ottawan phase is the Mars Hill terrane (North Carolina–Tennessee). This suggests that the outboard Ottawan suture may have served as the locus of Iapetan rifting along much of Laurentia. The Rigolet phase (1020–1000 Ma) is characterized by widespread “Basin and Range” type extension (NW–SE) associated with sinistral or dextral movement on the NY-AL lineament, mobilization of core-complexes (Adirondack Highlands), and AMCG magmatism along the outboard flank of the extensional region. Following the Rigolet phase, the Appalachian region continued to be characterized by NW–SE extension during the passage of a possible hotspot along a NE-track (760–600 Ma) across the Blue Ridge and other terranes, and during initial Iapetan rifting (565 Ma). The palinspastic rifted-margin of Laurentia crosses many of these terranes and sutures as well as the possible region of Rigolet extension and the possible hotspot track, thus providing many potential piercing points within the Grenville orogen for comparison with Paleozoic terranes like the Precordillera in South America.

Was Cathaysia part of Proterozoic Laurentia? – new data from Hainan Island, south China

AbstractWe report here new field and analytical data from Precambrian rocks on Hainan Island of the Cathaysia Block, south China, and examine its probable connection to Laurentia. Granitoids and newly discovered felsic volcanic rocks dated at 1433 ± 6 Ma and 1439 ± 9 Ma (SHRIMP U‐Pb zircon) on Hainan Island are coeval with, and isotopically similar to the 1500–1350 Ma trans‐continental granite‐rhyolite province in southern Laurentia. Quartzites unconformably overlying the ca. 1430 Ma volcaniclastic rocks on Hainan Island are interpreted as locally‐sourced Grenvillian foreland basin deposits that can be correlated with the Deer Trail Group of south‐western Laurentia. The detrital provenance of the quartzites contains age populations comparable to the 1610–1490 Ma, westerly‐sourced non‐Laurentian detrital grains reported in the Belt Basin of south‐western Laurentia. Our new data thus make Cathaysia the most likely continental block next to western Laurentia before and during the late Mesoproterozoic assembly of Rodinia.

Crustal evolution of southern Laurentia during the Paleoproterozoic: Insights from zircon Hf isotopic studies of ca. 1.75 Ga rocks in central Colorado

Lu-Hf depleted mantle model (TDM) ages, obtained by analysis of zircons previously dated by U-Pb methods, demonstrate that ca. 1.75 Ga bimodal Paleoproterozoic rocks in the Gunnison-Salida region of central Colorado, and by extension in much of the southwestern U.S., were formed by partial melting of preexisting crustal rocks, the ages of which mostly ages greater than 2.0 Ga indicate that range from 1.83 to 1.87 Ga. Some calculated Lu-Hf TDM even older crust was probably involved, consistent with the limited presence of ca. 2.5 Ga xenocrystic zircons in some rocks. These results suggest that rocks related to the Trans-Hudson and Penokean orogens are cryptically present much farther to the south than previously believed. Coupled with the bimodality of the volcanic suite in central Colorado, and indeed in much of the southwestern U.S., these results indicate that, in contrast to current juvenile arc-accretion models, melting of older crust related to extensional tectonics played an important role in the genesis of many magmatic rocks between 1.6 and 1.8 Ga.

Cambrian–Ordovician trilobite family Missisquoiidae Hupé, 1955: Systematic revision and palaeogeographical considerations based on cladistic analysis

The missisquoiids are among the most important trilobites in the Cambrian–Ordovician boundary interval of Laurentia and Gondwana. This study deals with a systematic review of the family Missisquoiidae based on a cladistic analysis and explores their palaeobiogeographical history. A total of 22 missisquoiid species were selected for cladistic analysis. The cladistic results demonstrate that the family Missisquoiidae includes Parakoldinioidia, Pseudokoldinioidia, Tangshanaspis, and Tasmanocephalus; Lunacrania and Hardyia are included with reservation. The well-known genus Missisquoia is treated as a junior synonym of Parakoldinioidia. Based on the distribution of the missisquoiids, six palaeogeographical areas are recognised: Sino-Korea, Yangtze, Australia, southern Laurentia, northwestern Laurentia, and northeastern Laurentia. Palaeogeographical analyses show that the missisquoiids originated in Gondwana and continued to expand their geographical range within the continent and eventually expanded into the Laurentia. Optimisation results of geographical area transitions demonstrate that there were transitions between the two continents, and evolution of the Laurentian missisquoiids appears to have been strongly influenced by the Gondwanan counterparts. We suggest that dispersal of non-fossilised egg stage probably transported by oceanic currents may have been responsible for these inter-continental transitions.

Research Focus: Crust formation in the western United States

Between about 1.8–1.6 Ga, an accretionary belt ~1300 km wide was added to the southern margin of the Archean Wyoming craton. In the prevailing arc-accretion model (Condie, 1982; Karlstrom and Bowring, 1988; 1993), most of this material is considered to be mantle-derived juvenile crust, and therefore represents newly formed crust (at 1.8–1.6 Ga). The interesting and highly provocative paper by Bickford and Hill in this issue of Geology (p. 167–170) challenges the view that crustal growth in southern Laurentia primarily involved accretion of juvenile arc rocks to an Archean core. Although the Bickford and Hill paper focuses specifi cally on Paleoproterozoic crustal evolution of southern Laurentia, it has relevance to mechanisms and rates of crustal formation and growth throughout geologic time. Bickford and Hill present an alternative interpretation for Paleoprotero zoic crustal evolution in which they suggest that signifi cant volumes of older (ca. 1.85 Ga Trans-Hudson/Penokean) crust may be present in southern Laurentia, and that rifting of this crust produced the widespread bimodal volcanic sequences present in the southwestern United States (see also Hill and Bickford, 2001). The principal lines of evidence cited by Bickford and Hill to support their interpretations are the presence of pre-1800 Ma inherited zircons and Nd and Pb isotopic signatures suggestive of older crust, and the abundant rhyolite-dominated bimodal volcanic sequences present in southern Laurentia. In their model, rifting of older crust resulted in basaltic magmatism that provided a heat source for the partial melting of Trans-Hudson/Penokean crust to form the rhyolites. The basaltic component of these bimodal sequences would represent juvenile additions to the crust. The rifting and arc-accretion models are not mutually exclusive.

Does the arc-accretion model adequately explain the Paleoproterozoic evolution of southern Laurentia: An expanded interpretation: COMMENT AND REPLY: COMMENT

Assembly of lithosphere in southern Laurentia is best interpreted to have taken place along a long-lived (1.8–1.0 Ga) active margin via subduction-accretion processes broadly analogous to present-day convergence between Australia and Indonesia ([Karlstrom and Bowring, 1988][1]; [Karlstrom et al.,