Dating the Triassic continental rift in the southern Andes : The Potrerillos Formation, Cuyo Basin, Argentina

The Triassic successions of western Argentina commonly show thin pyroclastic levels intercalated within thick fluvial and lacustrine terrigenous deposits. The Potrerillos Formation is the thickest Triassic unit in the Cuyo Basin. It is composed of alternating cycles of gravelly- sandy- and muddy-dominated intervals, in which several laterally-continuous tuff horizons occur. U-Pb SHRIMP ages were determined on zircon grains from three tuff levels located between the lowermost and the middle sections of the Potrerillos Formation. The ages for the time of deposition of the tuffs are 239.2 ± 4.5 Ma, 239.7 ± 2.2 Ma and 230.3 ± 2.3 Ma (Middle Triassic). Chemical data indicate that these acid to intermediate volcaniclastic rocks are derived from coeval basic magmas displaying tholeiitic to slightly alkaline signatures. They are associated with the rift stage that followed the extensive post-orogenic volcanism of the Choiyoi Group, that in turn has been ascribed to slab break-off in neighbouring areas. Two of the studied samples also record a subpopulation of inherited zircon grains with crystallisation ages of 260-270 Ma. The latter are considered to be an indirect measurement for the age of the Choiyoi Group in the Cuyo basin. The rift-related Triassic event represents the culmination of the Gondwanian magmatic cycle, and is interpreted as the result of subduction cessation and anomalous heating of the upper mantle previous to the western Gondwana break-up.Centro de Investigaciones Geológica

Dating the Triassic continental rift in the southern Andes : The Potrerillos Formation, Cuyo Basin, Argentina

The Triassic successions of western Argentina commonly show thin pyroclastic levels intercalated within thick fluvial and lacustrine terrigenous deposits. The Potrerillos Formation is the thickest Triassic unit in the Cuyo Basin. It is composed of alternating cycles of gravelly- sandy- and muddy-dominated intervals, in which several laterally-continuous tuff horizons occur. U-Pb SHRIMP ages were determined on zircon grains from three tuff levels located between the lowermost and the middle sections of the Potrerillos Formation. The ages for the time of deposition of the tuffs are 239.2 ± 4.5 Ma, 239.7 ± 2.2 Ma and 230.3 ± 2.3 Ma (Middle Triassic). Chemical data indicate that these acid to intermediate volcaniclastic rocks are derived from coeval basic magmas displaying tholeiitic to slightly alkaline signatures. They are associated with the rift stage that followed the extensive post-orogenic volcanism of the Choiyoi Group, that in turn has been ascribed to slab break-off in neighbouring areas. Two of the studied samples also record a subpopulation of inherited zircon grains with crystallisation ages of 260-270 Ma. The latter are considered to be an indirect measurement for the age of the Choiyoi Group in the Cuyo basin. The rift-related Triassic event represents the culmination of the Gondwanian magmatic cycle, and is interpreted as the result of subduction cessation and anomalous heating of the upper mantle previous to the western Gondwana break-up.Centro de Investigaciones Geológica

First magmatism in the New England Batholith, Australia: Forearc and arc-back-arc components in the Bakers Creek Suite gabbros

The New England Orogen, eastern Australia, was established as an outboard extension of the Lachlan Orogen through the migration of magmatism into forearc basin and accretionary prism sediments. Widespread S-type granitic rocks of the Hillgrove and Bundarra supersuites represent the first pulse of magmatism, followed by I-A nd A-types typical of circum-Pacific extensional accretionary orogens. Associated with the former are a number of small tholeiite-gabbroic to intermediate bodies of the Bakers Creek Suite, which sample the heat source for production of granitic magmas and are potential tectonic markers indicating why magmatism moved into the forearc and accretionary complexes rather than rifting the old Lachlan Orogen arc. The Bakers Creek Suite gabbros capture an early (∼305Ma) forearc basalt-like component with low Th/Nb and with high Y/Zr and Ba/La, recording melting in the mantle wedge with little involvement of a slab flux and indicating forearc rifting. Subsequently, arc-back-arc like gabbroic magmas (305-304Ma) were emplaced, followed by compositionally diverse magmatism leading up to the main S-type granitic intrusion (∼290Ma). This trend in magmatic evolution implicates forearc and other mantle wedge melts in the heating and melting of fertile accretion complex sediments and relatively long (∼10Myr) timescales for such meltingThis research was partially supported by an Australian National University PhD Research Scholarship to Seann J. McKibbin, who is currently a postdoctoral fellow of the Research Foundation – Flanders (Fonds Wetenschapplijke Onderszoek; FWO

Proterozoic crustal evolution of central East Antarctica: Age and isotopic evidence from glacial igneous clasts, and links with Australia and Laurentia

Rock clasts entrained in glacial deposits sourced from the continental interior of Antarctica provide an innovative means to determine the age and composition of ice-covered crust. Zircon U-Pb ages from a suite of granitoid clasts collected in glacial catchments draining central East Antarctica through the Transantarctic Mountains show that crust in this region was formed by a series of magmatic events at ∼2.01, 1.88–1.85, ∼1.79, ∼1.57, 1.50–1.41, and 1.20–1.06 Ga. The dominant granitoid populations are ca. 1.85, 1.45 and 1.20–1.06 Ga. None of these igneous ages are known from limited outcrop in the region. In addition to defining a previously unrecognized geologic history, zircon O and Hf isotopic compositions from this suite have: (1) mantle-like δ18O signatures (4.0–4.5‰) and near-chondritic Hf-isotope compositions (εHf ∼ +1.5) for granitoids of ∼2.0 Ga age; (2) mostly crustal δ18O (6.0–8.5‰) and variable Hf-isotope compositions (εHf = −6 to +5) in rocks with ages of ∼1.88–1.85, ∼1.79 and ∼1.57 Ga, in which the ∼1.88–1.79 Ga granitoids require involvement of older crust; (3) mostly juvenile isotopic signatures with low, mantle-like δ18O (∼4–5‰) and radiogenic Hf-isotope signatures (εHf = +6 to +10) in rocks of 1.50–1.41 Ga age, with some showing crustal sources or evidence of alteration; and (4) mixed crustal and mantle δ18O signatures (6.0–7.5‰) and radiogenic Hf isotopes (εHf = +3 to +4) in rocks of ∼1.2 Ga age. Together, these age and isotopic data indicate the presence in cratonic East Antarctica of a large, composite igneous province that formed through a punctuated sequence of relatively juvenile Proterozoic magmatic events. Further, they provide direct support for geological correlation of crust in East Antarctica with both the Gawler Craton of present-day Australia and Proterozoic provinces in western Laurentia. Prominent clast ages of ∼2.0, 1.85, 1.57 and 1.45 Ga, together with sediment source linkages, provide evidence for the temporal and spatial association of these cratonic elements in the Columbia supercontinent. Abundant ∼1.2–1.1 Ga igneous and metamorphic clasts may sample crust underlying the Gamburtsev Subglacial Mountains, indicating the presence of a Mesoproterozoic orogenic belt in the interior of East Antarctica that formed during final assembly of Rodinia.Field and analytical portions of this project were supported by the National Science Foundation (award 0944645)

U-Pb zircon ages of the Wildhorse gneiss, Pioneer Mountains, south-central Idaho, and tectonic implications

The gneiss complex of Wildhorse Creek (Wildhorse gneiss) forms the central component of the lowest structural plate in the Pioneer metamorphic core complex of south-central Idaho. The oldest rock in the complex is a felsic ortho-gneiss, with Neoarchean U-Pb magmatic zircon ages of 2.60-2.67 Ga. The ortho-gneiss overlaps in age and is interpreted to be part of the Grouse Creek block of the Albion Mountains to the south. This Archean metagranitoid is structurally interleaved with paragneiss containing quartzite and calc-silicate rock. Structurally below the orthogneiss, some quartzites have multiple concordant populations of detrital-zircon grains as young as ca. 1700 Ma, while others have no zircon grains younger than ca. 2500 Ma. Structurally above the Archean gneiss is a heterogeneous paragneiss that contains calc-silicate and quartzitic rocks with detrital zircons as young as ca. 1460 Ma. Amphibolite in this unit contains zircons dated at ca. 1850 Ma, indicating that this rock can be no older than that and implying considerable structural complexity. The upper part of the Wildhorse gneiss contains metaquartzites bearing zircons as young as ca. 1400 Ma. The protolith of this paragneiss is interpreted as the southernmost exposures of the Lemhi subbasin of the Mesoproterozoic Belt Supergroup. The upper Wildhorse gneiss includes ca. 695 Ma intrusive orthogneiss that is coeval with Neoproterozoic rift-related volcanic or intrusive rocks near Pocatello, House Mountain, and Edwardsburg, Idaho. This Cryogenian meta-intrusive rock is the likely source of the 650-710 Ma detrital-zircon population in the Big Lost River that drains the core complex. Initial eHf values from 675 Ma zircons are between 3.4 and -2.4, suggesting the granitoids had a mixed source in both continental crust and juvenile mantle.This research was supported by National Science Foundation grants EAR 05-10980 and 08-38425 and U.S. Geological Survey grant G14AC00136. Logistical support was provided by the Idaho State University Geology field camp at Lost River Field Station

Geochemistry and geochronology of the shallow-level La Esperanza magmatic system (Permian-Triassic), Northern Patagonia

The La Esperanza plutonic-volcanic complex is the largest Late Paleozoic-Early Triassic composite magmatic system of northern Patagonia. This paper reports new SHRIMP U-Pb zircon ages and K-Ar muscovite dating as well as whole-rock geochemical data for selected units. In addition, we present some new and reprocessed whole-rock Sr isotopic compositions. On the basis of the new and published data, three compositionally and isotopically distinct high-K magnesian calc-alkaline series were distinguished. Two of these are characterized by high Ba-Sr: (i) biotite and muscovite bearing rhyolites and granites (265 ± 2 Ma; 260 ± 2 Ma) and (ii) metaluminous amphibole-biotite bearing granodiorites (273 ± 2 Ma), monzogranites (255 ± 2 Ma), dacites (253 ± 2 Ma), and slightly peraluminous granites (dated herein as 251 ± 2 Ma). There is also a low Ba-Sr series of high-silica metaluminous rocks (granites and acid dike swarms; 250 ± 2 Ma and ≈244 ± 2 Ma). Geochemistry coupled with geochronology revealed a pulsatory multi-sourced open magmatic system with mafic magma replenishment and reactivation of crystal mushes that occurred before upward migration to upper crustal levels. Mafic magmas alternated with crust-derived magmas incrementally assembled in subvolcanic levels over 30 Ma. Zircon crystallization and mica cooling ages in the granite units allowed detection of two magmatic lulls, between 270 and 265 Ma and between 260 and 255 Ma. Both episodes coincide with a period of exhumation in upper crustal levels. The new temporal and geochemical constraints allow correlation of the La Esperanza plutonic-volcanic complex with the Los Menucos Group (258-248 Ma), encompassing a volume of magmatism comparable to a moderately sized large igneous silicic province. These mid-to-late Permian to Middle Triassic rocks record the transition between subduction-related magmatism (>273 Ma) and post-orogenic extensional magmatism (<250 Ma) in the Gondwana margin. Even though this magmatism would be coeval with the proposed collision of the Patagonia terrane, no expected syn-collisional magmatism or associated deformation were found in upper crustal levels. However, the different nature and melting conditions of the inferred sources of the magmas that crystallized before 270 Ma, between 265 and 260 Ma, and from 255 to 245 Ma, suggest that the La Esperanza plutonic-volcanic complex was assembled during a 30 Ma period of major plate reorganization.Fil: Martínez Dopico, Carmen Irene. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotópica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotópica; ArgentinaFil: Lopez, Monica Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotópica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotópica; ArgentinaFil: Rapalini, Augusto Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Fanning, Christopher Mark. The Australian National University; AustraliaFil: Antonio, Paul Yves Jean. Universidade de Sao Paulo; Brasi

The Betic Ophiolites and the Mesozoic Evolution of the Western Tethys

The Betic Ophiolites consist of numerous tectonic slices, metric to kilometric in size, of eclogitized mafic and ultramafic rocks associated to oceanic metasediments, deriving from the Betic oceanic domain. The outcrop of these ophiolites is aligned along 250 km in the Mulhacen Complex of the Nevado-Filabride Domain, located at the center-eastern zone of the Betic Cordillera (SE Spain). According to petrological/geochemical inferences and SHRIMP (Sensitive High Resolution Ion Micro-Probe) dating of igneous zircons, the Betic oceanic lithosphere originated along an ultra-slow mid-ocean ridge, after rifting, thinning and breakup of the preexisting continental crust. The Betic oceanic sector, located at the westernmost end of the Tethys Ocean, developed from the Lower to Middle Jurassic (185-170 Ma), just at the beginning of the Pangaea break-up between the Iberia-European and the Africa-Adrian plates. Subsequently, the oceanic spreading migrated northeastward to form the Ligurian and Alpine Tethys oceans, from 165 to 140 Ma. Breakup and oceanization isolated continental remnants, known as the Mesomediterranean Terrane, which were deformed and affected by the Upper Cretaceous-Paleocene Eo-Alpine high-pressure metamorphic event, due to the intra-oceanic subduction of the Jurassic oceanic lithosphere and the related continental margins. This process was followed by the partial exhumation of the subducted oceanic rocks onto their continental margins, forming the Betic and Alpine Ophiolites. Subsequently, along the Upper Oligocene and Miocene, the deformed and metamorphosed Mesomediterranean Terrane was dismembered into different continental blocks collectively known as AlKaPeCa microplate (Alboran, Kabylian, Peloritan and Calabrian). In particular, the Alboran block was displaced toward the SW to occupy its current setting between the Iberian and African plates, due to the Neogene opening of the Algero-Provencal Basin. During this translation, the different domains of the Alboran microplate, forming the Internal Zones of the Betic and Rifean Cordilleras, collided with the External Zones representing the Iberian and African margins and, together with them, underwent the later alpine deformation and metamorphism, characterized by local differences of P-T (Pressure-Temperature) conditions. These Neogene metamorphic processes, known as Meso-Alpine and Neo-Alpine events, developed in the Nevado-Filabride Domain under Ab-Ep amphibolite and greenschists facies conditions, respectively, causing retrogradation and intensive deformation of the Eo-Alpine eclogites.This research was funded by Project CGL2009-12369 of the Spanish Ministry of Science and Innovation, co-financed with FEDER funds, and by Research Group RNM 333 of Junta de Andalucía (Spain)

From Source to Sink: Petrogenesis of Cretaceous Anatectic Granites from the Fosdick Migmatite-Granite Complex, West Antarctica

This is a pre-copyedited, author-produced version of an article accepted for publication in Journal of Petrology following peer review. The version of record Brown, C. R., Yakymchuk, C., Brown, M., Fanning, C. M., Korhonen, F. J., Piccoli, P. M., & Siddoway, C. S. (2016). From Source to Sink: Petrogenesis of Cretaceous Anatectic Granites from the Fosdick Migmatite–Granite Complex, West Antarctica. Journal of Petrology, 57(7), 1241–1278 is available online at: https://doi.org/10.1093/petrology/egw039Anatectic granites from the Fosdick migmatite-granite complex yield U-Pb zircon crystallization ages that range from 115 to 100Ma, with a dominant grouping at 107-100 Ma, which corresponds to the timing of dome formation during the regional oblique extension that facilitated exhumation of the complex. The occurrence of leucosome-bearing normal-sense shear zones inmigmatitic gneisses indicates that suprasolidus conditions in the crust continued into the early stages of doming and exhumation of the complex. The structure allows access to variably oriented granites in networks of dykes at deeper structural levels and subhorizontal sheeted granites at shallower structural levels within the complex. This feature allows an evaluation of the mechanisms that modify the composition of granite melts in their source and of granite magmas during their ascent and emplacement using whole-rock major, trace element and Sr and Nd isotope compositions, zircon Hf and O isotope compositions, and phase equilibria modelling of potential source rocks. Geochemical variability within the granites is attributed to source heterogeneity and blending of melts, which themselves are consistent with derivation from regional metasedimentary and metaplutonic source materials. The granites typically contain coarse blocky K-feldspar and/or plagioclase grains within interstitial quartz, and have low Rb/Sr ratios and large positive Eu anomalies. These features are inconsistent with the composition of primary crustal melts derived from metasedimentary and metaplutonic source materials, but consistent with early fractional crystallization of feldspar and subsequent drainage of the fractionated melt. Processes such as peritectic mineral entrainment and accessory mineral dissolution, entrainment and crystallization did not have any significant influence on the major and trace element composition of the granites. The granites in the networks of dykes are interpreted to represent choking of magma transport channels through the middle crust as the rate of magma flow declined during doming and exhumation, whereas the sheeted granites record collapse of subhorizontal, partially crystallized layers of magma by filter pressing and melt exfiltration during vertical shortening associated with doming and exhumation. These processes separated feldspar-rich residues from evolved melt. Based on the results of this study, caution is urged in estimating melt proportion from the volume of granite retained in migmatitic gneiss domes, as the granites may not represent liquid compositions.US National Science Foundation [ANT0944615, OPP-0338279, OPP-0944600, EAR1032156]NISPLab at the University of MarylandGeological Society of AmericaNational Science and Engineering Research Council of Canad

Dopamine transporter genotype is associated with a lateralized resistance to distraction during attention selection

Although lateral asymmetries in orienting behavior are evident across species and have been linked to interhemispheric asymmetries in dopamine signaling, the relative contribution of attentional versus motoric processes remains unclear. Here we took a cognitive genetic approach to adjudicate between roles for dopamine in attentional versus response selection. A sample of nonclinical adult humans (N = 518) performed three cognitive tasks (spatial attentional competition, spatial cueing, and flanker tasks) that varied in the degree to which they required participants to resolve attentional or response competition. All participants were genotyped for two putatively functional tandem repeat polymorphisms of the dopamine transporter gene (DAT1; SLC6A3), which are argued to influence the level of available synaptic dopamine and confer risk to disorders of inattention. DAT1 genotype modulated the task-specific effects of the various task-irrelevant stimuli across both the spatial competition and spatial cueing but not flanker tasks. Specifically, compared with individuals carrying one or two copies of the 10-repeat DAT1 allele, individuals without this allele demonstrated an immunity to distraction, such that response times were unaffected by increases in the number of distractor stimuli, particularly when these were presented predominantly in the left hemifield. All three genotype groups exhibited uniform costs of resolving leftward response selection in a standard flanker task. None of these significant effects could be explained by speed–accuracy trade-offs, suggesting that participants without the 10-repeat allele of the DAT1 tandem repeat polymorphism possess an enhanced attentional ability to suppress task-irrelevant stimuli in the left hemifield

Review of the Cambrian Pampean orogeny of Argentina; a displaced orogen formerly attached to the Saldania Belt of South Africa?

The Pampean orogeny of northern Argentina resulted from Early Cambrian oblique collision of the Paleoproterozoic-Mesoproterozoic MARA block, formerly attached to Laurentia, with the Gondwanan Kalahari and Rio de la Plata cratons. The orogen is partially preserved because it is bounded by the younger Córdoba Fault on the east and by the Los Túneles-Guacha Corral Ordovician shear zone on the west. In this review we correlate the Pampean Belt with the Saldania orogenic belt of South Africa and argue that both formed at an active continental margin fed with sediments coming mainly from the erosion of the Brasiliano–Pan-African and East African–Antarctica orogens between ca. 570 and 537 Ma (Puncoviscana Formation) and between 557 and 552 Ma (Malmesbury Group) respectively. Magmatic arcs (I-type and S-type granitoids) formed at the margin between ca. 552 and 530 Ma. Further right-lateral oblique collision of MARA between ca. 530 and 520 Ma produced a westward verging thickened belt. This involved an upper plate with high P/T metamorphism and a lower plate with high-grade intermediate to high P/T metamorphism probably resulting from crustal delamination or root foundering. The Neoproterozoic to Early Cambrian sedimentary cover of MARA that was part of the lower plate is only recognized in the high-grade domain along with a dismembered mafic–ultramafic ophiolite probably obducted in the early stages of collision. Uplift was fast in the upper plate and slower in the lower plate. Eventually the Saldania and Pampean belts detached from each other along the right-lateral Córdoba Fault, juxtaposing the Rio de la Plata craton against the internal high-grade zone of the Pampean belt.Centro de Investigaciones Geológica