Eastern Laurentia Research Articles

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.

Radiometric and stratigraphic constraints on terminal Ediacaran (post-Gaskiers) glaciation and metazoan evolution

Radiometric constraints on mid-Ediacaran Period glaciation (Gaskiers) in Newfoundland narrowed the known temporal gap between widespread ice ages and the evolution of complex metazoans to several million years. To further evaluate this claim we studied an Ediacaran glacial diamictite at the base of the Fauquier Formation of northern Virginia, and discovered a conformable relationship between the post-glacial cap carbonate and overlying volcanic rocks of the Catoctin Formation. U/Pb zircon age constraints for the rift-related volcanic flows suggest initial emplacement around 571 million years ago. Application of the Catoctin age to the Fauquier succession indicates the occurrence of an ice age about 10 million years younger than the 582 Ma Gaskiers event, supporting the view of multiple Ediacaran Period glaciations. Furthermore, the age constraint from eastern Laurentia falls within radiometric uncertainty of fossiliferous strata in Avalonia, indicating that the Fauquier glaciation was coincident with early metazoan evolution.

Phylogenetic and biogeographic analysis of deiphonine trilobites

Cladistic parsimony analysis of the subfamily Deiphoninae Raymond, 1913 was conducted to produce a hypothesis of relationship for the group. The genera Deiphon Barrande, 1850 and Onycopyge Woodward, 1880 are found to be monophyletic, while the genus Sphaerocoryphe Angelin, 1854, as it was previously defined, is paraphyletic. A modified Brooks Parsimony Analysis using the phylogenetic hypothesis reveals patterns of biogeography, in particular, vicariance and geodispersal, during the Ordovician-Silurian. The analysis yields three major conclusions about deiphonine biogeography: Eastern Laurentia and Baltica were close enough during the late Ordovician to exchange taxa via sea level rise and fall; chance dispersal occurred between Northwestern Laurentia and Australia; and deiphonine trilobites likely originated in Baltica or Eastern Laurentia.

Detrital zircon analysis from the Neoproterozoic–Cambrian sedimentary cover (Cuyania terrane), Sierra de Pie de Palo, Argentina: Evidence of a rift and passive margin system?

Fil: Naipauer, Maximiliano. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Baltica, Amazonia and the SAMBA connection—1000 million years of neighbourhood during the Proterozoic?

Possible large-scale geological correlation between Baltica (East European Craton) and Amazonia (central South America) is discussed using compilations of recent data and regional geological models. A fit with northwest Amazonia attached to southwest Baltica – the SAMBA ( South America– Baltica) connection – produces a closely matching pattern of westward younging Proterozoic growth zones and is suggested to have existed from at least 1.8 Ga to at least 0.8 Ga. West Africa was probably attached to northeast Amazonia and southeast Baltica during the same time period. As in most plate tectonic reconstructions, northwest Baltica was attached to eastern Laurentia (Greenland) from 1.9 Ga to at least 1.3 Ga, and western Amazonia thus formed a continuation of the southeast Laurentia–western Baltica active margin, being characterized by intermittent crustal growth and orogenic deformation during the late Palaeoproterozoic and Mesoproterozoic, as well as periods with more inboard intraplate magmatism. In order to reach the “standard” Rodinia configuration, Baltica must have separated from Laurentia and, together with Amazonia and West Africa, rotated c. 75° clockwise relative to Laurentia. During the 1.1–0.9 Ga Grenvillian – Sveconorwegian – Sunsas orogeny, the western margin of Baltica and Amazonia collided with the southeast margin of Laurentia, as part of the formation of the Neoproterozoic supercontinent Rodinia. Final break-up of Rodinia occurred around 0.6 Ga, although separation of Amazonia and West Africa from Baltica may have occurred earlier. With ocean opening along the southwest (Tornquist) and northwest (Iapetus) margins of Baltica, and the formation of the Timanian orogen along the leading northeastern edge at 0.6–0.55 Ga, Baltica moved to the (present-day) northeast, into a 200 million year period of solitude, the only period during its history when Baltica existed as a continent of its own, before reuniting with Laurentia during the Caledonian orogeny and then merging with the rest of Eurasia.

Large igneous provinces (LIPs) and carbonatites

There is increasing evidence that many carbonatites are linked both spatially and temporally with large igneous provinces (LIPs), i.e. high volume, short duration, intraplate-type, magmatic events consisting mainly of flood basalts and their plumbing systems (of dykes, sills and layered intrusions). Examples of LIP-carbonatite associations include: i. the 66 Ma Deccan flood basalt province associated with the Amba Dongar, Sarnu-Dandali (Barmer), and Mundwara carbonatites and associated alkali rocks, ii. the 130 Ma Parana-Etendeka (e.g. Jacupiranga, Messum); iii. the 250 Ma Siberian LIP that includes a major alkaline province, Maimecha-Kotui with numerous carbonatites, iv. the ca. 370 Ma Kola Alkaline Province coeval with basaltic magmatism widespread in parts of the East European craton, and v. the 615–555 Ma CIMP (Central Iapetus Magmatic Province) of eastern Laurentia and western Baltica. In the Superior craton, Canada, a number of carbonatites are associated with the 1114–1085 Ma Keweenawan LIP and some are coeval with the pan-Superior 1880 Ma mafic-ultramafic magmatism. In addition, the Phalaborwa and Shiel carbonatites are associated with the 2055 Ma Bushveld event of the Kaapvaal craton. The frequency of this LIP-carbonatite association suggests that LIPs and carbonatites might be considered as different evolutionary ‘pathways’ in a single magmatic process/system. The isotopic mantle components FOZO, HIMU, EM1 but not DMM, along with primitive noble gas signatures in some carbonatites, suggest a sub-lithospheric mantle source for carbonatites, consistent with a plume/asthenospheric upwelling origin proposed for many LIPs.

Grenvillian remnants in the Northern Andes: Rodinian and Phanerozoic paleogeographic perspectives

Abstract Grenvillian crust is encountered in several basement inliers in the northern Andes of Colombia, Ecuador and Peru and is also represented as a major detrital or inherited component within Neoproterozoic to Paleozoic sedimentary and magmatic rocks. This review of the tectonic and geochronological record of the Grenvillian belt in the northern Andes suggests that these crustal segments probably formed on an active continental margin in which associated arc and back-arc magmatism evolved from ca. 1.25 to 1.16 Ga, possibly extending to as young as 1.08 Ga. The lithostratigraphic and tectonic history of the Grenvillian belt in the northern Andes differs from that of the Sunsas belt on the southwest Amazonian Craton and from the Grenvillian belt of Eastern Laurentia. It is considered that this belt, along with similar terranes of Grenvillian age in Middle America and Mexico define a separate composite orogen which formed on the northwestern margin of the Amazonian Craton. Microcontinent accretion and interaction with the Sveconorwegian province on Baltica is a feasible tectonic scenario, in line with recent paleogeographic reconstructions of the Rodinian supercontinent. Although Phanerozoic tectonics may have redistributed some of these terranes, they are still viewed as para-autocthonous domains that remained in proximity to the margin of Amazonia. Paleogeographic data derived from Phanerozoic rocks suggest that some of the Colombian Grenvillian fragments were connected to northernmost Peru and Ecuador until the Mesozoic, whereas the Mexican terranes where attached to the Colombian margin until Pangea fragmentation in Late Triassic times.

Late Proterozoic–Paleozoic evolution of the Arctic Alaska–Chukotka terrane based on U-Pb igneous and detrital zircon ages: Implications for Neoproterozoic paleogeographic reconstructions

The Seward Peninsula of northwestern Alaska is part of the Arctic Alaska–Chukotka terrane, a crustal fragment exotic to western Laurentia with an uncertain origin and pre-Mesozoic evolution. U-Pb zircon geochronology on deformed igneous rocks reveals a previously unknown intermediate-felsic volcanic event at 870 Ma, coeval with rift-related magmatism associated with early breakup of eastern Rodinia. Orthogneiss bodies on Seward Peninsula yielded numerous 680 Ma U-Pb ages. The Arctic Alaska–Chukotka terrane has pre-Neoproterozoic basement based on Mesoproterozoic Nd model ages from both 870 Ma and 680 Ma igneous rocks, and detrital zircon ages between 2.0 and 1.0 Ga in overlying cover rocks. Small-volume magmatism occurred in Devonian time, based on U-Pb dating of granitic rocks. U-Pb dating of detrital zircons in 12 samples of metamorphosed Paleozoic siliciclastic cover rocks to this basement indicates that the dominant zircon age populations in the 934 zircons analyzed are found in the range 700–540 Ma, with prominent peaks at 720–660 Ma, 620–590 Ma, 560–510 Ma, 485 Ma, and 440–400 Ma. Devonian- and Pennsylvanian-age peaks are present in the samples with the youngest detrital zircons. These data show that the Seward Peninsula is exotic to western Laurentia because of the abundance of Neoproterozoic detrital zircons, which are rare or absent in Lower Paleozoic Cordilleran continental shelf rocks. Maximum depositional ages inferred from the youngest detrital age peaks include latest Proterozoic–Early Cambrian, Cambrian, Ordovician, Silurian, Devonian, and Pennsylvanian. These maximum depositional ages overlap with conodont ages reported from fossiliferous carbonate rocks on Seward Peninsula. The distinctive features of the Arctic Alaska–Chukotka terrane include Neoproterozoic felsic magmatic rocks intruding 2.0–1.1 Ga crust overlain by Paleozoic carbonate rocks and Paleozoic siliciclastic rocks with Neoproterozoic detrital zircons. The Neoproterozoic ages are similar to those in the peri-Gondwanan Avalonian-Cadomian arc system, the Timanide orogen of Baltica, and other circum-Arctic terranes that were proximal to Arctic Alaska prior to the opening of the Amerasian basin in the Early Cretaceous. Our Neoproterozoic reconstruction places the Arctic Alaska–Chukotka terrane in a position near Baltica, northeast of Laurentia, in an arc system along strike with the Avalonian-Cadomian arc terranes. Previously published faunal data indicate that Seward Peninsula had Siberian and Laurentian links by Early Ordovician time. The geologic links between the Arctic Alaska–Chukotka terrane and eastern Laurentia, Baltica, peri-Gondwanan arc terranes, and Siberia from the Paleoproterozoic to the Paleozoic help to constrain paleogeographic models from the Neoproterozoic history of Rodinia to the Mesozoic opening of the Arctic basin.

1.3–0.9Ga Oaxaquia (Mexico): Remnant of an arc/backarc on the northern margin of Amazonia

Abstract Rocks with ages of ca. 1 Ga occur in central and southern Mexico as inliers surrounded by ubiquitous Mesozoic and Cenozoic rocks. They appear to share a common history consisting of: (i) ca. 1300–1200 Ma arc magmatism and deposition of sediments including evaporites; (ii) ca.1160–1100 Ma intrusion of syenite, granite and anorthosite, the later part of which is synchronous with migmatization; (iii) intrusion of a ca. 1035–1010 Ma anorthosite–gabbro–charnockite–granite (AMCG) suite; (iv) a 1000–980 Ma granulite facies tectonothermal event with a stretching axis parallel to the long axis of Oaxaquia; (v) gradual exhumation at 750 and/or 545 Ma; and (vi) 517 Ma intrusion of an isolated calcalkaline granitoid pluton. The common Precambrian geological record of these outcrops suggests that they belonged to a single terrane (Oaxaquia) and formed a juvenile arc/backarc bordering a continent that underwent collision with, and overthrusting of, the Avalonian arc at 1000–980 Ma. This buried Oaxaquia to 25–30 km and was followed by further supra-subduction zone magmatism at ca. 917 Ma. These Precambrian rocks are unconformably overlain by uppermost Cambrian and Silurian platform rocks containing Gondwanan fauna and ca. 1 detrital zircons of Oaxacan provenance. The neighbouring Mixteca terrane includes lower Paleozoic, rift-passive margin sedimentary rocks that also contain 900–750 Ma detrital zircons probably derived from the Goias arc in eastern Amazonia. The arc-backarc tectonic setting inferred for the 1300–900 Ma rocks also suggests that Oaxaquia lay on an active periphery of Amazonia until ca. 900 Ma, well after the amalgamation of Rodinia. This precludes a location for Oaxaquia off southern and western Amazonia that are inferred to have been juxtaposed against eastern Laurentia; contiguity with eastern Amazonia is also unlikely given the absence of the 900–750 Ma convergent tectonics in the Goias arc. This leaves northern Amazonia as the most likely position, a location that requires the least relative displacement between Oaxaquia and Amazonia. The inferred 750 and 545 Ma exhumation episodes of Oaxaquia correspond to two proposed breakup stages of Rodinia.

A palaeomagnetic and40Ar/39Ar study of late precambrian sills in the SW part of the Amazonian craton: Amazonia in the Rodinia reconstruction

A new key palaeomagnetic pole (Plat. = 64.3 degrees S, Plon. = 271.0 degrees E, N = 14, A(95) = 9.2 degrees; Q = 5) is calculated from a primary magnetization isolated in early Neoproterozoic Aguapei basic sills and dykes hosted by 1.3-1.0 Ga sedimentary rocks from the southwestern part of the Amazon craton. The characteristic remanence carried by stable, pseudo-single domain titanomagnetite shows two antipodal polarities that pass a reversals test. Magnetic anisotropy for most sites shows fabric orientations that are typical of sills, with horizontal magnetic foliations concordant to the flat-lying bedding of the host sedimentary rocks. Ar-40/Ar-39 analyses for one of the sills reveal a well-defined plateau age at 981 +/- 2 Myr. A tectonic reconstruction for Amazonia relative Laurentia based on this new pole 'is consistent with' a position of the present northwestern part of Amazonia attached with eastern Laurentia close to Greenland at ca. 981 Ma. On basis of palaeomagnetic and geological data, we propose a scenario where Amazonia moved northeastwards along the present southeast coast of Laurentia from ca. 1200 to 980 Ma. By 980 Ma, Amazonia is placed alongside Laurentia and Baltica, in a position similar to other reconstructions of Rodinia but with a significantly different orientation. (Less)

Stratigraphy and structure of the Laurentian rifted margin in the northern Appalachians: A low-angle detachment rift system

The Neoproterozoic–Early Cambrian rifted margin of eastern Laurentia is framed by promontories and embayments defined by northeast-trending rifts offset by northwest-trending transforms. A regional first-order synthesis of the available stratigraphic data in the northern Appalachians reveals significant along-strike variations in the thickness, composition, age, and facies of synrift and early postrift stratigraphy. These variations are consistent with models for low-angle detachment rift systems and allow for resolution of regional structures specific to low-angle detachments, including upper-plate margins, lower-plate margins, and transform faults that bound zones of oppositely dipping low-angle detachments.

Indentor tectonism in the eastern Grenville Province

The easternmost Grenville Province behaved in a structurally distinct manner from the remainder of the eastern Grenville Province (EGP) during Grenvillian orogenesis (1085–985 Ma). Geochronological, structural and geophysical data are used to demonstrate that, in contrast to frontal-thrust ramp tectonics in western and central Labrador, Grenvillian orogenesis in eastern Labrador mostly involved a dextral strike-slip, lateral-ramp regime. It is also argued that allochthonous terranes in the western part of the EGP were displaced 150 km northwest, relative to their eastern counterparts during the late Grenvillian. Whereas central and western Labrador and adjacent Quebec can be successfully modeled in terms of frontal, collisional tectonism, utilizing channel-flow models that have been applied to the Himalayan–Tibetan tectonic system, eastern Labrador cannot. The point where frontal-thrust ramp tectonics gives way to a lateral-ramp regime is interpreted as an indentor corner. In contrast to the Himalayan situation where India is being subducted under Asia, the indenting crust in the EGP was thrust over pre-Grenvillian Laurentia. The lateral-ramp concept has important implications with respect to Rodinia reconstructions between eastern Laurentia, western Baltica and northern Amazonia.

Detrital zircon fingerprint of the Proto-Andes: Evidence for a Neoproterozoic active margin?

Neoproterozoic Palaeogeographic reconstructions of Rodinia conventionally place the western (Proto-Andean) margin of Amazonia against the eastern (Appalachian) margin of Laurentia. Separation and formation of the Iapetus Ocean is generally considered to have occurred later at ∼550 Ma. We examine the U–Pb detrital zircon “fingerprint” of autochthonous rocks from the northern and central segments of the Proto-Andean margin, which formed part of the western margin of Amazonia during the Late Neoproterozoic–Phanerozoic. The Proto-Andean margin is clearly the most feasible source region for most of the zircon grains, except for a 550–650 Ma sub-population, broadly age-equivalent to the Brasiliano/Pan-African Orogeny in eastern Amazonia. No obvious source for this detritus is known in the northern and central Andes. Derivation from eastern Amazonia is considered unlikely due to the stark paucity of detritus derived from the core of the Amazonian craton. Instead, we propose that a Late Neoproterozoic magmatic belt is buried beneath the present-day Andean belt or Amazon Basin, and was probably covered during the Eocene–Oligocene. If this inferred Neoproterozoic belt was an active margin, it would record the initiation of Proto-Andean subduction and imply at least partial separation of West Gondwana from its conjugate rift margin of eastern Laurentia prior to ca. 650 Ma. This separation may be linked to the ca. 770–680 Ma A-type magmatism found on eastern Laurentia in the southern Appalachians, and on the Proto-Andean margin in the Sierra Pampeanas and the Eastern Cordillera of Peru.

Fritzolenellus lapworthi (Peach and Horne, 1892) from the lower Cambrian(Cambrian Series 2) Bastion Formation of North-East Greenland.

Fossil collections from the lower Cambrian (Cambrian Series 2) Bastion Formation at Albert Heim Bjerge in North-East Greenland acquired during 1950 and 1951 contain some 60 specimens of olenellid trilobites. Although morphologically variable, the material can be attributed to a single species, Fritzolenellus lapworthi (Peach and Horne, 1892), previously known from the ‘Fucoid’ Beds of north-west Scotland. Immature specimens allow a first, limited, description of the ontogeny of Fritzolenellus. The presence of F. lapworthi in North-East Greenland offers an important tie point for correlation of Olenellus-bearing strata (Cambrian Series 2, Stage 4) across eastern Laurentia.

Neoproterozoic basin evolution in Fennoscandia, East Greenland and Svalbard

Neoproterozoic successions of Fennoscandia, East Greenland and Svalbard are related to crustal extension and formation of sedimentary basins along the margins of northern Baltica (Fennoscandia) and eastern Laurentia (East Greenland and Svalbard), preceding final break-up of Rodinia. The early rift stage (late Tonian-Cryogenian) is characterized by up to 16 km thick sedimentary successions of deep-marine sandstones and conglomerates linked to rift and strike-slip basins. Pericratonic basins expanded during Cryogenian-Cambrian coastal onlap. Cryogenian tropical climate is reflected by carbonate and evaporitic formations, most of which predate Cryogenian-Ediacaran glaciations. Glacial units, collectively referred to the Varanger Ice Age, may be equivalent to the Marinoan (c. 630 Ma) and the Gaskiers (c. 580 Ma) glacial periods. The final stage in break-up of Rodinia commenced with the emplacement of dolerite dyke swarms along the Baltoscandian margin at c. 600 Ma and the opening of the Iapetus Ocean and other sea ways. No such dyke swarms have been recorded along the East Greenland segment of the Laurentian margin. Several Tonian-Cambrian tectonic and magmatic events recorded within the Kalak Nappe Complex in northern Finnmark make this unit an exotic terrane relative to the autochthonous Baltoscandian platform.

Facies development of a Late Ordovician mixed carbonate-siliciclastic ramp proximal to the developing Taconic orogen: Lourdes Formation, Newfoundland, Canada

The Upper Ordovician (late Whiterockian to Mohawkian) Lourdes Formation represents a narrow (tens of kilometers), short-lived [∼5–7 million years (my)], open-ocean (high-energy) mixed siliciclastic-carbonate ramp that onlapped allochthonous strata along the orogen side of the local Taconic foreland basin. Platform development followed a 6–8 my hiatus during which weathering had concentrated chemically mature siliciclastics that were admixed with initial carbonate sediments. A cross-platform facies gradient contains paleokarst and peritidal carbonates and sandstones, shallow-ramp carbonate bioherms and skeletal shoals, and deeper ramp calcareous shales. Transgressive systems tracts are marked by ramp-wide sheets and shoals of skeletal grainstone and low accumulation rates, and highstand systems tracts are marked by significant admixture and interbedding of siliciclastics with cross-ramp carbonate facies. Platform demise coincides with increased siliciclastic input, which is likely tectonically influenced. The Lourdes platform is equivalent to epicontinental foreland ramps along eastern Laurentia, but its narrower width precluded formation of oceanographically restricted platform-interior facies.

Persistent depositional sequences and bioevents in the Eifelian (early Middle Devonian) of eastern Laurentia: North American evidence of the Kačák Events?

Abstract The late Eifelian–earliest Givetian interval (Middle Devonian) represents a time of significant faunal turnover in the eastern Laurentia and globally. A synthesis of biostratigraphic, K-bentonite and sequence stratigraphic data indicates that physical and biotic events in the Appalachian foreland basin sections in New York are coeval with the predominantly carbonate platform sections of southern Ontario and Ohio. The upper Eifelian ( australis to ensensis conodont zones) Marcellus Subgroup in New York comprises two large-scale (3rd-order) composite depositional sequences dominated by black shale, which are here assigned to the Union Springs and Oatka Creek Formations. The succession includes portions of three distinctive benthic faunas or ecological–evolutionary sub-units (EESUs): ‘Onondaga’, ‘Stony Hollow’ and ‘Hamilton’. In the northern Appalachian Basin in New York, the boundaries of these bioevents show evidence of abrupt, widespread extinctions, immigration and ecological restructuring. In the Niagara Peninsula of Ontario and from central to northern Ohio, the same sequence stratigraphic pattern and bioevents are recognized in coeval, carbonate-dominated facies. The correlations underscore a relatively simple pattern of two major sequences and four subsequences that can be recognized throughout much of eastern Laurentia. Moreover, the biotic changes appear to be synchronous across the foreland basin and adjacent cratonic platform. However, the degree of change differs substantially, being less pronounced in carbonatedominated mid-continent sections. Finally, we make the case that the two major faunal changes align with regional sequence stratigraphic patterns as well as with the global Kačák bioevents.

Rift-related basalts in the 1.2–1.3 Ga granulites of the northern Oaxacan Complex, southern Mexico: evidence for a rifted arc on the northwestern margin of Amazonia

Meta-igneous granulites with estimated c. 1.3−1.2 Ga protolith ages in the northern Oaxacan Complex of southern Mexico have bimodal protoliths: rift-related basalts (probably continental tholeiites) and felsic volcanic rocks that could also have been arkosic sediments. Negative Nb, Ta and Ti anomalies in the mafic rocks are interpreted to reflect derivation from a mantle source previously enriched by subduction processes. In contrast, contemporaneous igneous country rocks in the southern Oaxacan Complex have arc affinities and the simplest interpretation suggests a south-to-north transition from arc to back-arc or intra-arc rift. Various locations in 1.2−1.0 Ga Rodinia reconstructions have been proposed for these and correlative rocks in Middle America: an autochthonous location; and as an allochthonous terrane originating either off northwestern or southern Amazonia or the Arequipa-Antofalla terrane. The collision between southern Amazonia and southern Laurentia at 1.2 Ga followed by relative northward travel (present-day coordinates) of Amazonia along eastern Laurentia appear to eliminate all but a northwestern Amazonian location for Middle America. The development of an arc complex on the northwestern margin of Amazonia would be consistent with the relative northward motion of Amazonia.

Grenville-age A-type and related magmatism in southern Laurentia, Texas and New Mexico, U.S.A.

Large Grenville-age plutons are known to be present in the 1340–1370 Ma basement in Texas and eastern New Mexico, but their distribution and compositional ranges are poorly known. New U–Pb zircon dates (SHRIMP-RG, Stanford University) on subsurface samples have identified widespread presence of compositionally diverse Grenville-age (1070–1110 Ma) plutonic rocks. In the Texas Panhandle, core from a > 175 m thick, sill-like gabbroic intrusion yielded a date of 1081 ± 8.3 Ma. In situ differentiation of this tholeiitic magma led to a ∼ 7-fold increase in incompatible element concentrations as REE patterns remained essentially flat. Trace element data suggest an E-MORB-like source. To the south, alkali–feldspar granite with A-type affinities was recovered from drill core from the Abilene gravity minimum near Albany, Texas. It yielded a date of 1078 ± 23 Ma; similar to undeformed (post-orogenic) granites in the Llano uplift. Further west, an anorthosite xenolith from the Eocene Three Sisters intrusion in El Paso, Texas yielded a date of 1068 ± 30 Ma which is within error of the 1110 ± 19 Ma age determined for the main stage of the nearby A-type Red Bluff granitic suite. Xenoliths from Potrillo maar volcano in the center of the southern Rio Grande rift include monzonitic xenoliths with granulitic texture which yielded dates of ∼ 1072 Ma. In contrast, Potrillo maar xenoliths with igneous textures yielded U–Pb dates of ∼ 27 Ma. Development of the granulitic texture is interpreted to reflect granulite facies metamorphism during Tertiary time (26–27 Ma) on the basis of age zoning and U concentrations in the zircon crystals. Our results show that Grenville-age magmatism in the Texas and New Mexico subsurface was widespread and was coeval with syn- and post-deformation granites in the Llano uplift of central Texas. The compositions of dated samples suggest that mafic magmas were broadly tholeiitic in character and that granitic magmas were “A-type”. These magmatic affinities are thought to be associated with crustal extension rather than with subduction. Moreover, the presence of anorthosite xenoliths in the El Paso-Kilbourne Hole region, the broadly tholeiitic character of mafic rocks, and the A-type nature of the granitic rocks are broadly similar to AMCG (Anorthosite–Mangerite–Charnockite–Granite) suites present in the Grenville province of eastern Laurentia.

Neoproterozoic A‐type magmatism in the Western Sierras Pampeanas (Argentina): evidence for Rodinia break‐up along a proto‐Iapetus rift?

AbstractA‐type orthogneisses of mid Neoproterozoic age (774 ± 6 Ma, U‐Pb SHRIMP zircon age), are reported for the first time from the Grenvillian basement of the Western Sierras Pampeanas in Argentina. These anorogenic meta‐igneous rocks represent the latest event of Rodinia break‐up so far recognized in Grenvillian basement exposures across Andean South America. Moreover, they compare well with A‐type granitoids and volcanic rocks along the Appalachian margin of Laurentia (Blue Ridge), thus adding to former evidence that the Western Sierras Pampeanas Grenvillian basement was left on the conjugate rifted margin of eastern Laurentia during Rodinia break‐up and the consequent opening of the Iapetus ocean.