Westernmost Exposures Research Articles

The Cretaceous-Paleogene contact in the Tornillo Group of Big Bend National Park, West Texas, USA

Abstract The Cretaceous-Paleogene (K-Pg) contact interval is constrained by vertebrate fossil sites at seven sites in the Tornillo Group and lies within an 80–100-m stratigraphic section between the top of the Javelina Formation and the base of the “log jam sandstone” marker bed in the Black Peaks Formation. In western exposures of this interval, the highest occurrence of in situ dinosaur specimens and the lowest occurrence of Paleocene mammal specimens are separated by an unusual conglomerate bed. This thin conglomerate bed coincides with the contact between Cretaceous and Paleogene strata and contains reworked Cretaceous fossils. It is superficially similar to conglomerate beds elsewhere attributed to the effects of tsunamis generated by the Chicxulub impact; however, the maximum depositional age of ca. 63 Ma based on detrital zircons indicates that the conglomerate was deposited about three million years after the K-Pg boundary event. Paleocene mammalian fossils from immediately above the conglomerate bed represent a fauna that can be no older than the middle Torrejonian (To2 interval zone). The contact between Cretaceous and Paleocene strata is therefore disconformal and represents a hiatus of at least three million years. A condensed section occurs at the westernmost exposure of the K-Pg contact, where at least 80 m of strata are absent below the conglomerate bed; these strata are present in exposures farther east. This condensed section likely records an erosional event resulting from uplift and deformation of the nearby Terlingua monocline. Although the 80 m of strata below the conglomerate bed are poorly fossiliferous, several clearly in situ dinosaur specimens indicate that this entire interval is Late Cretaceous in age. There is no compelling evidence for preservation of the K-Pg boundary event horizon at any of the seven sites in the Tornillo Group, and so the hiatus represented at the Cretaceous/Paleocene contact here likely also includes some part of latest Cretaceous time. Mammalian specimens from sites in the “log jam sandstone,” ~40 m above the middle Torrejonian sites, represent an early Tiffanian fauna (Ti1 interval zone). Latest Torrejonian (To3) sites have not been recognized, and therefore a second disconformity likely coincides with the base of the “log jam sandstone” marker horizon in the Black Peaks Formation.

Detrital zircon U–Pb and Hf signatures of Paleo-Mesoproterozoic strata in the Priest River region, northwestern USA: A record of Laurentia assembly and Nuna tenure

Rocks of the Gold Cup Quartzite, Belt Supergroup and Deer Trail Group crop out in the Priest River region of northeastern Washington and northern Idaho (USA). As these sequences represent the westernmost exposures of Paleoproterozoic–Mesoproterozoic strata between New Mexico (USA) and the northern Yukon (Canada), they are key to understanding the assembly of Laurentia and evaluating the duration and configuration of the supercontinent Nuna/Columbia. Here, we present detrital zircon U–Pb and Lu–Hf isotope data from these sequences. The results indicate that the <1.73 Ga Gold Cup Quartzite contains similar detrital zircon components to other widespread late Paleoproterozoic units that record final assembly of the main Archean blocks of western Laurentia; as such, they do not require any non-Laurentian source. Prominent ca. 1.7–1.5 Ga, isotopically juvenile zircon grains form the main component of the overlying ca. 1.47 Ga Lower Belt Supergroup. Some of these detrital zircon components lack a Laurentian source and are instead consistent with derivation from the Gawler Craton in southeastern Australia. The >1.38 Ga upper Belt Supergroup strata are structurally overlain by <1.3 Ga Deer Trail Group rocks that contain detrital zircon components consistent with mixed Laurentian sources. The paucity of any syndepositional volcanic detrital zircon in Deer Trail Group strata along with its fine siliciclastic and carbonate nature suggest deposition within a tectonically-quiescent basin coeval with breakup of Nuna.

Between the supercontinents: Mesoproterozoic Deer Trail Group, an intermediate age unit between the Mesoproterozoic Belt–Purcell Supergroup and the Neoproterozoic Windermere Supergroup in northeastern Washington, USA

Mesoproterozoic and Neoproterozoic basins in western North America record the evolving position of the Laurentian craton within two supercontinents during their growth and dismemberment: Columbia (Nuna) and Rodinia. The western-most exposures of the Columbia rift-related Belt–Purcell Supergroup are preserved in northeastern Washington, structurally overlain by the Deer Trail Group and depositionally overlying the Neoproterozoic Windermere Supergroup. It has been disputed whether the Deer Trail Group is correlative with the Belt–Purcell Supergroup, or younger. To help resolve the uncertain correlation of these units and their bearing on supercontinent evolution, we characterized the detrital zircon age populations of units from the Deer Trail Group, the Windermere Supergroup, and the Belt–Purcell Supergroup in northeastern Washington. These data show that the western part of the Columbia supercontinent (now located in Australia and eastern Antarctica) remained attached to western Laurentia and continued to supply 1600–1500 Ma detrital zircon grains to the Belt–Purcell Supergroup until after ca. 1391 Ma. The Deer Trail Group is younger than the Belt–Purcell strata, with the basal unit younger than ca. 1362 Ma and a middle unit younger than ca. 1300 Ma. The Deer Trail Group has a pre-Grenville-age provenance from the southwestern USA and possibly east Antarctica. The Buffalo Hump Formation is younger than the Deer Trail Group, with Grenville-age (ca. 1112 Ma) detrital zircon grains and a detrital zircon signature like that of the overlying Neoproterozoic Windermere Supergroup. We interpret the Deer Trail Group to have been deposited during the rift-demise of supercontinent Columbia and before the Grenville-age assembly of the supercontinent Rodinia.

Pre-Cenozoic evolution of the Aghil Range (western Tibetan Plateau): A missing piece of the Tibet-Pamir-Karakorum geopuzzle

The Tibetan Plateau, largely derived from the accretion of several Gondwana microplates to the southern margin of Asia since the late Palaeozoic, is the highest and largest topographic relief on Earth. Although the first order geodynamic processes responsible for its pre-Cenozoic evolution are quite well-known, many issues are still debated, among which is the timing of collision of each terrane with the southern margin of Asia. Even more uncertain is the pre-Palaeozoic history of these terranes, due to the lack of basement exposures. As a contribution to understanding the pre-Cenozoic evolution of the Tibetan Plateau, this paper focuses on the Aghil Range, a remote and poorly investigated area close to the Karakorum Fault between Kunlun and Karakorum (Xinjiang, China) in western Tibet. The tectono-metamorphic and magmatic evolution of the Aghil Range is investigated using a multidisciplinary approach that combines field mapping, petrology and geochronology (UPb on titanite, zircon, monazite and xenotime using SHRIMP-RG). We demonstrate that the Aghil Range preserves a coherent slice of Neoproterozoic crystalline basement with a late Palaeozoic sedimentary cover deposited on a passive continental margin during the Gondwana break-up. This represents the westernmost exposure of Precambrian crystalline basement known so far in the Tibetan Plateau. Furthermore, petrological and geochronological results allow reconstructing the Mesozoic poly-metamorphic evolution of this sector of the Tibetan Plateau, which records the evidence of Middle Jurassic (ca. 170 Ma) and Late Cretaceous (66 Ma) collisional events, as well as of the Late Jurassic (ca. 150 Ma) early subduction of an accretionary complex developed on its southern margin. Evidence of Late Cretaceous subduction-related magmatism preceding the last collisional event is also recorded. These results allow tentative correlation of the different terranes of Central Tibet with those of the Pamir-Karakorum Range on both sides of the Karakorum fault.

Ichnology of the Devonian (Emsian) Campbellton Formation, New Brunswick, Canada

The Campbellton Formation has long been known to yield a fossil assemblage of Devonian (Emsian) fish and eurypterids at its westernmost exposure near Campbellton and Atholville, and a well described flora and early land animal fauna toward its easternmost exposure near Dalhousie Junction. Although the body fossil assemblage (paleobotany, vertebrate and invertebrate paleontology), paleoecology and paleoenvironmental context of the formation have been extensively studied, ichnofossils are rare and have not been described previously. Fossils from the vertebrate and eurypterid bearing ‘Atholville Beds’ contain a low diversity ichnofossil assemblage represented by three ichnotaxa:Monomorphichnus, ?Taenidium and Helminthoidichnites. Monomorphichnus is proposed here as being produced by the produced by the activity of the eurypterid Pterygotus anglicus.

The tectono-magmatic framework to gold mineralisation in the Sadiola-Yatela gold camp and implications for the paleotectonic setting of the Kédougou-Kénieba inlier, West Africa

The Kédougou-Kénieba inlier is the westernmost exposure of Birimian crust in the West African Craton. The inlier is host to several multi-million ounce gold deposits (e.g., Sadiola, Yatela, Loulo, Massawa, and Gounkoto) and a large iron ore resource (i.e. Falémé). Although the inlier has received increasing academic attention over the last decade, only limited data exists for its north-eastern part, which contains the world-class Sadiola-Yatela gold camp. New field data, U-Pb magmatic zircon ages, and whole rock geochemistry data from the north-eastern Kédougou-Kénieba inlier are presented and integrated with published data in order to generate a revised tectono-magmatic framework to gold mineralisation for the westernmost Birimian terranes.The granitoids from the Sadiola-Yatela region display a temporal evolution from ca. 2140 to 2080Ma calc-alkaline metaluminous plutons (e.g., diorite, hornblende-biotite-granodiorite) to ca. 2080–2060Ma high-K calc-alkaline, metaluminous to slightly peraluminous granites (e.g., biotite-monzogranite). The least-evolved rocks are associated with flat REE patterns and correspond to plutons of the ca. 2140Ma Finman complex and the ca. 2115Ma Sadiola quartz-feldspar porphyry. The vast majority of the Eburnean (ca. 2115–2060Ma) granitoids in the Sadiola-Yatela region are characterised by negative Nb-Ta anomalies, marked Pb positive anomalies, and high LILE/HFSE ratios. These Eburnean granitoids share a close affinity with relatively hydrous, oxidised magmas and source regions. The polycyclic deformation recorded in sedimentary and magmatic rocks of the region includes a period of early convergence (D1s), followed by a period of fold-and-thrust tectonics (D2s), and later transcurrent tectonics (D3s-D4s). The bulk of the gold mineralisation in the region occurred during D3s, with sinistral displacement and hydrothermal fluid circulation along the Senegal-Mali Shear Zone and steep NNE-trending shear zones connected to structural traps in the Kofi basin.The results of this study, when integrated with published data, indicate an evolution from primitive island arc magmas to increasingly evolved high-K calc-alkaline granitic melts as a result of crustal thickening during the Eburnean orogeny combined with fractionated crystallisation and melting-assimilation-storage-homogenisation processes. The late Eburnean tectono-thermal event between ca. 2080 and 2060Ma is proposed as the geodynamic engine that empowered gold mineralisation in the Kédougou-Kénieba inlier.

Retrogressed lawsonite blueschists from the NW Iberian Massif: P–T–t constraints from thermodynamic modelling and 40Ar/39Ar geochronology

Blueschist facies terranes in the Variscan Ibero-Armorican Arc are restricted to scarce and relatively small areas. One of these examples is the Ceán Unit, which is the westernmost exposure of the middle allochthonous sheet of the Variscan belt in the Malpica–Tui Complex (NW Iberian Massif). The Ceán Unit is a highly condensed metamorphic succession with a lower part in the blueschist facies and an upper part without HP relicts. It comprises variable proportions of glaucophane–chloritoid-bearing metapelites and mafic rocks with abundant well-preserved pseudomorphs after euhedral lawsonite. Both lithologies show systematic changes in texture and mineral composition that are spatially related depending on deformation. The metamorphic evolution of the metabasic rocks has been constrained in the P–T space through pseudosection approach and is characterised by H2O-undersaturated prograde evolution induced by the crystallisation of lawsonite. Peak conditions in the blueschist/LT-eclogite facies have been constrained at ca. 2.2 GPa and 560 °C. Exhumation-related metamorphism is characterised by a nearly isothermal decompression from the lawsonite-bearing fields to fields with stable albite at P ≈ 1 GPa. This lead to the pseudomorphism of lawsonite in the early-decompression stages, and a subsequent amphibolite–greenschist facies overprint at P < 0.8 GPa and T ≈ 440–480 °C. The preservation of the lawsonite crystal shape despite complete retrogression indicates that pseudomorphism occurred as a static process and that particular levels of the blueschist host rock were not affected by penetrative deformation during exhumation. 40Ar/39Ar step heating of phengitic muscovite from the pelitic schists interbedded with the lawsonite pseudomorph-bearing metabasic rocks yield plateau ages of ca. 363 ± 2 and 354 ± 1 Ma. The older age is interpreted as the age of the peak blueschist facies metamorphism. The age of 355 Ma is interpreted as a cooling age and is inferred to represent some point relatively close to peak conditions at the onset of the isothermal decompression. 40Ar/39Ar dating of muscovite from the quartzo-feldspathic mylonites of the Bembibre–Ceán detachment, at the base of the Ceán Unit, yields an age of ca. 337 ± 3 Ma, interpreted as the age of the post-nappe extensional tectonics. Similar data obtained from the blueschists of Ile de Groix (Armorican Massif; Bosse et al. in Chem Geol 220:21–45, 2005) support the equivalence of the Ceán Unit and the Upper Unit of Ile de Groix along the Ibero-Armorican Arc and suggest that these units share a blueschist facies event constrained at ca. 360–370 Ma, that is inferred to represent the Late Devonian–Early Carboniferous subduction of the northern margin of Gonwana beneath Laurussia.

Refining the age of magmatism in the Altos Cuchumatanes, western Guatemala, by LA–ICPMS, and tectonic implications

The Altos Cuchumatanes Range is made up of a core of igneous and metamorphic rocks, surrounded by lower Palaeozoic and Mesozoic sedimentary strata. These units constitute the westernmost exposure of basement rocks in Guatemala and represent some of the most important crustal units in the Maya Block. New laser ablation–inductively coupled plasma mass spectrometry U-Pb zircon geochronology allows better definition of their igneous ages, inheritance and petrologic evolution. The Altos Cuchumatanes magmatism occurred during the Middle Ordovician (461 Ma) and lower Pennsylvanian (312–317 Ma), replicating similar age trends present in southern Mexico (Acatlán Complex) and the Maya Block, from Chiapas to central Guatemala (Rabinal-Salamá area) and Belize (Maya Mountains). The U-Pb inheritance from cores of the studied zircons makes it possible to decipher the pre-magmatic history of the area. During the Late Ordovician to Permo-Carboniferous, the Altos Cuchumatanes and Maya Block were located adjacent to northeastern Mexico, near the Mixteco terrane, where Ordovician megacrystic granites intruded a passive-margin sedimentary sequence. The Ordovician granites present at the southern limit of the Maya Block, in the Altos Cuchumatanes, in central Guatemala and in Belize, are the result of partial crustal melting during the initial opening of the Rheic Ocean, when both Maya and Mixteco terranes would have lain close to NW Gondwana until the closure of that ocean. The crystallization of the early Pennsylvanian granites seems to be the result of an E-dipping subduction zone that accommodated convergence between Laurentia and Gondwana.

Petrogenesis of the Eocene and Mio–Pliocene alkaline basaltic magmatism in Meseta Chile Chico, southern Patagonia, Chile: Evidence for the participation of two slab windows

The Meseta Chile Chico (MCC, 46.4°S) is the westernmost exposure of Eocene (lower basaltic sequence, LBS; 55–40 Ma, K–Ar ages) and Mio–Pliocene (upper basaltic sequence, UBS; 16–4 Ma, K–Ar ages) flood basalt volcanism in Patagonia. The MCC is located south of the Lago General Carrera-Buenos Aires (LGCBA), southeast from the present day Chile Triple Junction (CTJ), east of the actual volcanic gap between Southern South Volcanic Zone and Austral Volcanic Zone (SSVZ and AVZ, respectively) and just above the inferred location of the South Chile Ridge segment subducted at ∼6 Ma (SCR-1). Erupted products consist of mainly ne-normative olivine basalt with minor hy-normative tholeiites basalt, trachybasalt and basanite. MCC lavas are alkaline (42.7–53.1 wt.% SiO 2, 3–8 wt.% Na 2O+K 2O) and relatively primitive (Ni: 133–360 ppm, Cr: 161–193 ppm, Co: 35–72 ppm, 4–16.5 MgO wt.%). They have a marked OIB-like signature, as shown by their isotopic compositions ( 87Sr/ 86Sr o=0.70311–0.70414 and εNd=+4.7–+5.1) and their incompatible trace elements ratios (Ba/La=10–20, La/Nb=0.46–1.09, Ce/Pb=15.52–27.5, Sr/La<25), reflecting deep mantle origin. UBS-primitive lavas have characteristics similar to those of the Eocene LBS basalts, while UBS-intermediate lavas show geochemical imprints (La/Nb>1, Sr/La>25, low Ce/Pb, Nb/U) compatible with contamination by arc/slab-derived and/or crustal components. We propose that the genesis and extrusion of magmas is related to the opening of two slab windows due to the subduction of two active ridge segments beneath Patagonia during Eocene and Mio–Pliocene.

Thermochronology of the Grenville Orogeny in west Texas

Abstract The Carrizo Mountain Group (CMG) comprises the westernmost exposures of the metamorphic core of the Grenville Orogen in Texas. Unlike the correlative Llano uplift, 500 km to the east, the CMG underwent one phase of medium P/T metamorphism; peak T was attained during D2 in all CMG exposures. The peak metamorphic paleotemperatures decrease to the northwest, in the overall direction of tectonic transport, from staurolite to biotite grade. Garnet in the southeast Carrizo Mountains grew isothermally and probably during decompression (i.e. part of a clockwise P–T loop) at ca. 520 °C, in between biotite-grade D1 and D3 events. Hornblende in this region cooled below closure T, during D2, at 1057±6 Ma, within error of muscovite closure in the northwest Carrizo Mountains at 1059±2 Ma. Hornblende in higher grade exposures to the southeast cooled through closure T as late as 1002±7 Ma. Muscovite in the garnet-grade and higher exposures closed to Ar diffusion at 992±4 Ma to 981±3 Ma. Biotite and (partially recrystallized) hornblende from the northwest Carrizo Mountains cooled through closure in this same time period. We interpret these data as signifying: (1) Ar loss due to peak metamorphism in hornblende (southeast Carrizo Mountains) and muscovite (northwest Carrizo Mountains) at ca. 1059 Ma; 2) post-Tmax cooling through 500 °C in the higher grade outcrops until ca. 1000 Ma; and (3) cooling of all CMG exposures through 300 °C, due to uplift along the Streeruwitz thrust and erosion, from 1000 to 980 Ma. These ages significantly post-date known tectonism in the Llano uplift and extend the duration of Grenville orogenesis in Texas by ca. 115 million years. The 1000–980 Ma time period is a time of thrusting in frontal regions of the Grenville Province of Canada and the northeastern US, and shearing in the Natal Province, South Africa, suggesting ultimate closure of the Grenville Orogen and supporting recent Rodinia reconstructions.

Devonian Antler fold and thrust belt and foreland basin development in the southern Canadian Cordillera: implications for the Western Canada Sedimentary Basin

Research Article| March 01, 2001 Devonian Antler fold and thrust belt and foreland basin development in the southern Canadian Cordillera: implications for the Western Canada Sedimentary Basin Kevin G. Root Kevin G. Root Alberta Energy Company Ltd., 3900, 421 - 7 Avenue SW, Calgary, AB, T2P 4K9 Search for other works by this author on: GSW Google Scholar Author and Article Information Kevin G. Root Alberta Energy Company Ltd., 3900, 421 - 7 Avenue SW, Calgary, AB, T2P 4K9 Publisher: Canadian Society of Petroleum Geologists Received: 19 Jan 2001 Accepted: 12 Mar 2001 First Online: 02 Mar 2017 Online ISSN: 2368-0261 Print ISSN: 0007-4802 © The Society of Canadian Petroleum Geologists Bulletin of Canadian Petroleum Geology (2001) 49 (1): 7–36. https://doi.org/10.2113/49.1.7 Article history Received: 19 Jan 2001 Accepted: 12 Mar 2001 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Kevin G. Root; Devonian Antler fold and thrust belt and foreland basin development in the southern Canadian Cordillera: implications for the Western Canada Sedimentary Basin. Bulletin of Canadian Petroleum Geology 2001;; 49 (1): 7–36. doi: https://doi.org/10.2113/49.1.7 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of Canadian Petroleum Geology Search Advanced Search Abstract This contribution describes evidence for Middle Devonian contractional deformation and foreland basin development in the Purcell and Rocky mountains of southern Canada. These are interpreted as early manifestations of the Antler Orogeny, which is well documented in the western United States. Middle Devonian foreland basin strata include the Mount Forster Formation and correlative strata, which change laterally from > 500 m of lithic sandstones, conglomerates and volcanic flows in westernmost exposures to predominantly shale and gypsum in exposures to the east, and the Harrogate Formation, which comprises shallow water carbonates and minor mudstone. Significant local lateral thickness and facies changes, and angular unconformities that truncate folds, were created within the Middle Devonian foreland basin strata in the Delphine Creek region of southeastern British Columbia in response to syndepositional folding within an interpreted wedge-top basin. A normal fault, formed during Neoproterozoic rifting, was inverted during the Devonian contraction. Middle Devonian foreland basin strata pinch out to the northeast against the West Alberta Ridge, which was a Middle Devonian paleohigh interpreted to have been a peripheral bulge.Integration of the interpretations of Middle Devonian fold and thrust belt and foreland basin development with published structural and stratigraphic evidence from elsewhere in western Canada leads to the conclusion that Antler fold and thrust belt and foreland basin development may have occurred along the length of the Canadian portion of the ancient Cordilleran continental margin during the Devonian and Mississippian. The Western Canada Sedimentary Basin is interpreted to have been a distal foreland basin during the Devonian and Mississippian. Important changes in subsidence and uplift patterns within the basin, and an increase in the general rate of subsidence, may have been influenced by a combination of flexural loading by thrust sheets and foreland basin strata and horizontal in-plane stresses associated with Antler convergent tectonism. Faults that influenced deposition of mid-Paleozoic sediments, and created permeable pathways for the flow of hot fluids, may have formed as the result of horizontal in-plane stresses. Hot fluids interpreted to have significantly influenced diagenetic processes in mid-Paleozoic carbonate reservoir rocks may have been tectonically expelled in response to Antler thrust belt development. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The gold belts of western Asturias and Galicia (NW Spain)

Northwestern Iberia was recognized as a substantial gold province by the Romans some 2000years ago. The gold belts occur in the westernmost exposure of the European Hercynides where the largest exposure of pre-Permian rocks within the Iberian Peninsula are known as the Iberian Hercynian Massif. It is a collisional Orogen in which the NW Iberian Massif mainly represents the footwall of the suture. The four gold belts identified in western Asturias and Galicia (NW Spain) are formed by multiple gold mineralizing events representing several styles of mineralization in a structurally complex, but favorable terrain of Paleozoic carbonate and clastic host rocks. With an historical production estimated in excess of 3million oz of gold and an existing global resource of 4.2million oz of gold, the region has rewarded the persistent in search of gold for nearly four millenia.

Preliminary mineralogical and petrological study of the Ortosa Au–Bi–Te ore deposit: a reduced gold skarn in the northern part of the Rio Narcea Gold Belt, Asturias, Spain

The Ortosa deposit (NW Spain) in the northern part of the Rio Narcea Gold Belt (RNGB) is located in the Cantabrian Zone of the Iberian Massif. This zone corresponds to the westernmost exposure of the European Hercynides. The deposit is hosted by marine shales, siltstones, calcareous siltstones and interbedded sandy limestones of the upper part of the Silurian Furada Formation. These rocks are intruded by a main stock and numerous sills and dikes consisting of a reduced, ilmenite-bearing quartz-monzodiorite (Ortosa intrusion). Skarn metasomatism and associated gold mineralization overprinted these sedimentary and igneous rocks, forming endo- and exoskarns.The earliest stage of alteration involved potassium metasomatism from which metasomatic biotite developed in the hornfels around the intrusion. In the endoskarn, the first metasomatic mineral to form is actinolite. Subsequently, quartz, pyroxene (Hd30–45), and sulfides (mainly arsenopyrite and pyrrhotite) formed, followed by a second generation of amphibole (ferroactinolite and ferrohornblende). The exoskarn is a pyroxene-garnet skarn, which is often banded. The prograde minerals are pyroxene (Hd10–30) and grossular garnet. The retrograde mineralogy consists of hedenbergite-rich pyroxene (Hd50–87), amphibole (ferroactinolite–ferrohornblende), and the metallic minerals with minor fluorapatite, K-feldspar, albite, epidote–clinozoisite, vesuvianite and calcite. A final stage of retrograde alteration is characterized by calcite, quartz, and chlorite.Pyrrhotite and arsenopyrite are the more abundant metallic minerals, and löllingite, chalcopyrite, pyrite and sphalerite are present in smaller amounts. The gold occurs as native gold and maldonite, and is accompanied by hedleyite, native bismuth, and bismuthinite. These Au–Bi–Te mineral assemblages occupy cavities and fractures in the arsenopyrite or in the pyrrhotite.Estimated physiochemical conditions of formation based on the composition and stability fields of major calc-silicate and sulfide minerals indicate that the hedenbergite-rich pyroxene and the earliest sulfides (löllingite–pyrrhotite–arsenopyrite) crystallized at temperatures between 470 and 535°C at low logfS2 between −10 and −6.5 and low logfO2 of −22. The Ortosa skarns can be included in the reduced gold skarn subtype defined by Meinert (Mineralogical Association of Canada, Quebec city, Que., Canada, 1998, 26,359–414 ).

U-Pb geochronologic constraints on Paleoproterozoic tectonism in the Monashee complex, Canadian Cordillera: Elucidating an overprinted geologic history

U-Pb isotopic data and field relationships are used to outline the Paleoproterozoic geologic history of the Monashee complex, the largest of three Paleoproterozoic exposures in the Canadian Cordillera and one of the westernmost exposures of crystalline basement in North America. The effects of Paleoproterozoic tectonism are best preserved in deep structural levels of the complex, where overprinting related to high-grade Cordilleran (early Tertiary) metamorphism and deformation was incomplete. Determination of precise crystallization ages is hindered by the U-Pb age discordance in the zircon, monazite, and titanite; the discordance is attributed to inherited Pb in some of the grains and to Pb loss, overgrowth, and recrystallization that occurred during one or more thermal overprints. Igneous crystallization ages are interpreted from the upper intercepts of linear arrays that are defined by three or more analyses. Varying degrees of confidence are attributed to the ages based on the probability of inheritance. Large bodies of augen orthogneiss and granodioritic orthogneiss yield precise igneous crystallization ages of 2077 ± 2 and 1862 ± 1 Ma, respectively. Crystallization ages of about 2.27 and 2.10 Ga are interpreted with less certainty from dioritic orthogneiss and granitic orthogneiss, respectively. Deformation associated with a migmatitic gneissosity occurred after intrusion of the 2077 ± 2 Ma augen gneiss, the youngest dated rock that contains the fabric, and before intrusion of a 1848 ± 3 Ma granite, the oldest confidently dated rock that postdates the fabric. Postdeformational pegmatite dikes are dated as 1845 ± 3 and 1836 ± 2 Ma. Metamorphism is interpreted as occurring during monazite growth at 2060 ± 1 Ma in pelitic schist and during titanite growth at ca. 1.85 Ga in amphibolitic gneiss. The gneisses are basement to an unconformably overlying cover sequence, the lower part of which was deposited prior to intrusion of a 1852 ± 4 Ma pegmatite. There is sufficient age resolution on the intrusive rocks to conclude that Monashee basement has affinities with other Paleoproterozoic exposures in the Cordillera and the western Canadian shield. The knowledge that a Paleoproterozoic geologic history can be elucidated through the Cordilleran overprint suggests that older histories may be preserved in the other Paleoproterozoic exposures in the Cordillera.

Roberts Mountains allochthon and the western margin of the Cordilleran miogeocline in the Northern Ritter Range pendant, eastern Sierra Nevada, California

The truncated southwestern edge of the Roberts Mountains allochthon is exposed in the Northern Ritter Range pendant in the eastern Sierra Nevada, structurally overlying parautochthonous rocks of the Cordilleran miogeocline. The Northern Ritter Range pendant exposes units that have the same stratigraphic affinities and structural relationships as rocks of the Antler orogenic belt in Nevada. Paleozoic metasedimentary rocks exposed in the pendant consist of two major units: (1) a structurally complex, disrupted chert and argillite unit interpreted to be correlative with the Roberts Mountains allochthon; and (2) a stratigraphically coherent siliceous and calcareous unit, the Rush Creek sequence, that is interpreted to be part of a transitional outer shelf and slope assemblage of the lower Paleozoic Cordilleran miogeocline. In the Northern Ritter Range pendant, the Roberts Mountains allochthon structurally overlies the Rush Creek sequence along a north-striking, steeply dipping fault zone that may be a preserved remnant of the Roberts Mountains thrust, which in north-central Nevada emplaced the allochthon over outer shelf and slope strata of the Cordilleran miogeocline during the Late Devonian–Early Mississippian Antler orogeny. These stratigraphic and structural belts are truncated on the southwest side of the pendant by the Gem Lake shear zone, a northwest-trending dextral strike-slip fault associated with the Cretaceous Sierra Nevada batholith. The Northern Ritter Range pendant thus defines both the southwestern limit of the Antler orogenic belt and the westernmost exposures of parautochthonous miogeoclinal rocks in the central Cordillera.

Geochronology of the mid-German crystalline rise west of the River Rhine

The mid-German crystalline rise has its westernmost exposures at the western margin of the Rhine graben in the southern Pfalz and the northern Alsace. The outcrops are made up of granitoid rocks and minor volcano-sedimentary sequences. Radiometric ages obtained by U/Pb, Pb/Pb, Sm/Nd and Rb/Sr analyses of the igneous rocks from this area range from ∼433 to ∼325 Ma thus covering a time span from the Silurian to the end of the lower Carboniferous. Because the investigated rocks are — according to their chemical composition — largely related to subduction zone environments, the following three geodynamical scenarios are postulated, always taking subduction of oceanic crust beneath the mid-German crystalline rise into account: (a) subduction of the Rheic ocean during the Silurian from the north; (b) subduction of the Rhenohercynian ocean during the late Devonian (∼369 Ma) from the north; (c) subduction of the Saxothuringian ocean during the lower Carboniferous (∼334 Ma) from the south.

Enigmatic Uppermost Permian-Lowermost Triassic Stratigraphic Relations in the Northern Bighorn Basin of Wyoming and Montana

Abstract Nineteen measured sections in the northern Bighorn Basin of Wyoming and Montana provide the basis for an analysis of Permian-Triassic stratigraphic relations. The Permian-Triassic unconformity is well-defined in north-central Wyoming where gray, yellowish-weathering, calcareous siltstones of the Lower Triassic Dinwoody Formation disconformably overlie carbonates of the Upper Permian Ervay Member of the Park City Formation with little physical evidence of a significant hiatus. The Dinwoody is gradationally overlain by red beds of the Lower Triassic Red Peak Formation of the Chugwater Group. The Dinwoody depositionally thins to zero near the Wyoming-Montana state line. Northward, the erathem boundary is enigmatic because fossils are absent, Permian rocks change facies to resemble the Dinwoody, and there is no record of a Permian-Triassic unconformity. Discontinuous exposures and the restrictive access policies of the Crow Indian Reservation also contribute to the problem. Some previous workers assigned these rocks to the Permian, while others considered them Triassic in age. Up to 12 m (37 feet) of Dinwoody-like Permian rocks unconformably overlie the Pennsylvanian Tensleep Sandstone and grade into the Lower Triassic Red Peak Formation in the westernmost surface exposures along the eastern flank of the Bighorn Basin in Montana. South and east of this area, but north of the Dinwoody termination, Ervay carbonate is overlain by up to 24 m (74 feet) of red siltstone capped by.25 (0.8 feet) to 0.5 m (1.6 feet) of laminated dolomitic limestone. This interval is assigned to the basal Red Peak. Thin Lower Triassic dolomitic mudstones within the Dinwoody and basal Red Peak are commonly laminated, with associated peloidal micrite and coarse quartz silt. These carbonates are also poorly fenestral, and may contain small quantities (1% or less) of small peloidal rip-ups and very fine-grained quartz sand. Carbonates within the Dinwoody-like Permian are similar, but contain variable quantities of sand-size quartz and little to no coarse quartz silt.

Is the central Piedmont suture a low-angle normal fault?

In the crystalline southern Appalachians, the Carolina are terrane is in fault contact with the Piedmont terrane along a seismically reflective surface dipping toward the hinterland and called the central Piedmont suture. The central Piedmont suture may be interpreted as a thrust, but existing data also support a Silurian-Devonian, normal-slip origin: (1) There are lower-grade rocks in the hanging wall than in the footwall. (2) A normal-fault solution allows simultaneous metamorphism of the Piedmont terrane and Carolina terrane, prior to their juxtaposition along the central Piedmont suture. (3) Mineral ages in the Piedmont terrane are older in the west than in the east, consistent with an eastward-progressive unroofing. (4) Along the western edge of the Carolina terrane, a linear belt of Devonian subalkalic to alkalic granitoids and gabbro-norites with low initial 87Sr/86Sr ratios may represent mantle-derived magmas along the axis of rifting that are contemporary with major crustal extension. The westernmost Piedmont terrane includes the Chauga belt. The Chauga belt comprises metavolcanic and metaplutonic units similar in rock type and age to those of the western Carolina terrane. Chauga belt rocks are interpreted to be the westernmost exposures of the Carolina terrane, translated west on the lower plate by extension. The Piedmont and Carolina terranes may thus compose a single lithotectonic element. The Piedmont terrane would represent the basement on which the arc was constructed; the terrane was uplifted during extension along a major low-angle normal fault, recognized today as the central Piedmont suture.

Formula omitted] mineral age record of a polyorogenic evolution within the Seve and Köli nappes, Trøndelag, Norway

In the central Scandinavian Caledonides, eugeoclinal terranes represented in the Köli Nappes were thrust > 500 km onto the Baltoscandian platform during the early to middle Paleozoic. Immediately underlying thrust sheets of the Seve Nappe Complex are suspected to have been derived from the outer margin of continent Baltica. Hornblende within amphibolite at one locality within westernmost exposures of Seve Nappe units in Trandelag, Norway, displays an internally discordant 40Ar 39Ar age spectrum which suggests a polyphase thermal evolution including: 1. (1) initial post-metamorphic cooling through argon retention temperatures at c. 475–480 Ma; 2. (2) partial rejuvenation during a Silurian (c. 400–425 Ma) thermal overprint. Muscovite within the exposure was more extensively rejuvenated during the Silurian overprint. At twelve other Trandelag sample locations, hornblende, muscovite and biotite within both the Seve and Köli Nappe Complexes record post-metamorphic cooling ages between 420 and 435 Ma and display no direct evidence of a pre-Silurian thermal record. However, the widespread occurrence of intracrystalline extraneous argon components and textural complexities clearly point toward a pre-Silurian tectonothermal history. Comparison with 40Ar 39Ar mineral ages reported elsewhere in the central Scandinavian Caledonides suggests a polyorogenic evolution consisting of: 1. (1) variable high-pressure metamorphism of distal Baltoscandian miogeoclinal sequences during entrainment in an accretionary wedge which developed in the Late Cambrian-Early Ordovician over a west-dipping subduction zone; related arc volcanic activity occurred in more outboard, western sectors; 2. (2) ductile imbrication of high- and intermediate-pressure sequences within the accretionary wedge (diachronous cooling and/or chronologically distinct tectonic episodes are indicated throughout the Early and Middle Ordovician); 3. (3) ultimate accretion of a variety of eugeoclinal terranes along the Baltica continental margin; 4. (4) imbrication and metamorphism of the eugeoclinal terranes and previously deformed portions of the miogeocline during eastward translation onto the Baltoscandian platform in the Silurian. Crystalline Baltica basement and overlying structural cover units were depressed to significant crustal depths during eastward nappe transport with resultant formation of high-temperature eclogues. Previously reported crystallization ages of c. 425 Ma suggest that the eclogites formed at approximately the same time as minerals within the superencumbent nappe units cooled through argon closure temperatures. Following eastward nappe transport, the basement experienced rapid uplift and resultant adiabatic decompression. This was followed by rapid cooling through argon retention temperatures at 390–400 Ma.

Nd Isotopes and the Origin of Grenville-Age Rocks in Texas: Implications for Proterozoic Evolution of the United States Mid-Continent Region

Nd isotopic data were obtained for Precambrian Grenville-belt rocks in Texas. The samples represent most components of the crust of the Llano, Van Horn, and Franklin Mountains exposed terranes. Almost all Precambrian igneous, metamorphic, and sedimentary rocks from the three regions document addition to North America of mantle-derived crustal materials in the 1.6-1.0 Ga interval. The exception is a quartzite from the westernmost (Franklin Mountains) exposure, which was derived from ~1.8 Ga crust of the southwestern United States. The initial values of all rocks except the quartzite lie in the +1 to +6 range for igneous/metamorphic ages of 1.37 to 1.06 Ga. These results can be interpreted in two ways: (1) as documenting 0-20% additions of older crustal material to mantle-derived products 1.4-1.0 Ga ago; or (2) as documenting derivation of the Grenville exposures by recycling of older crustal protoliths separated from the mantle 1.6-1.3 Ga ago. The Nd data in isolation do not resolve these two interpretations. The model mantle separation ages () of the rocks are very similar to published values of granulites in Mexico and Virginia: all these regions of the United States and Mexico show a strong peak of around 1.4 Ga ago. If the ages represent older crustal protoliths, then they would have formed coevally with the 1.5-1.3 Ga Granite-Rhyolite Terranes of the continental interior USA. This would imply that the Granite-Rhyolite Terranes were formed during orogenic/accretionary processes in the adjoining Grenville Belt, and are not anorogenic in association, as conventionally assumed.