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On the geology of the Tertiary Wrangell lavas in the St. Clare province, St. Elias Mountains, Yukon
The Wrangell lavas in the St. Clare province of southwestern Yukon are part of the larger Wrangell volcanic belt that has been active throughout the Late Cenozoic. These lavas have erupted in a transitional tectonic environment that reflects regional transpression along the Queen Charlotte transform-Fairweather-Totschunda Fault System and subduction of the Farallon Plate beneath North America. The volcanic province is composed of subalkaline basalt (31%), basaltic andesite (30%), andesite (21%), dacite (2%) and nepheline normative basalt (16%). The hypersthene normative basalt is (in order of appearance) spinel-olivine-plagioclase ± Fe-Ti oxide ± clinopyroxene phyric, whereas andesite contains plagioclase, Fe-Ti oxide, clinopyroxene, ± orthopyroxene phenocrysts, and dacite and intrusive latite contain phenocrysts of plagioclase, ± clinopyroxene, hornblende, ± biotite, ± sanidine. The nepheline normative rocks, where porphyritic, contain phenocrysts of olivine, plagioclase and hornblende. In the central part of the map area, the lowermost flows are nepheline normative basalt that are interbeddded with clastic sediments and overlain by basaltic andesite, andesite and volcanic conglomerate. This succession is overlain by basalt interbedded with clastic sedimentary rocks and pyroclastic rocks. In the southern part of the map area, alkaline basalt occurs at this stratigraphic level. The uppermost Wrangell lavas are andesitic with minor interbedded volcaniclastic rocks. The hypersthene normative lavas of the St. Clare province are transitional in terms of their Na2O + K2O/SiO2 ratios between alkaline and subalkaline magma series and in terms of their FeO/MgO versus SiO2 ratios between tholeiitic and calc-alkaline series. Chemical composition of these rocks reflects the unique tectonic setting within which they are found.
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Flood basalts of the Wrangellia Terrane, southwest Yukon: Implications for the formation of oceanic plateaus, continental crust and Ni-Cu-PGE mineralization
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The Wrangellia Terrane along the northwest margin of North America is an extensive accreted oceanic plateau. These volcanic sequences erupted onto an extinct island arc in less than 5 million years at ca. 230 Ma. Triassic Wrangellia basalts and intrusions form a 1 to 10 km-wide linear belt of mafic and ultramafic rocks extending 300 km across southwest Yukon. A total of 85 samples were collected for geochemical and isotopic analysis from 10 widespread areas along the entire length of the linear belt. Field observations during the summer of 2004, and a synthesis of previous research for the Yukon portion of Wrangellia, are part of a larger research project involving Wrangellia basalts extending from Vancouver Island to central Alaska. The Wrangellia volcanic sequences represent one of the finest examples of an accreted oceanic plateau worldwide. They provide an excellent opportunity to gain a better understanding of the mantle source of oceanic plateaus and to assess the role of accretion of oceanic plateaus in continental growth.
Late Devonian magmatism and clastic deposition in the upper Earn Group (central Yukon, Canada) mark the transition from passive to active margin along western Laurentia
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for a copy of this paper please contact the Yukon Geological Survey; geology@yukon.ca.
The Early Tertiary Sifton Range volcanic complex, southwestern Yukon
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The early Tertiary magmatic episode in the northern Canadian Cordillera is linked to the restructuring of the Kula-North American plate system from orthogonal to oblique convergence. Resultant volcanism was widespread, and remnant successions outcrop along the eastern margin of the Coast Plutonic Complex (CPC). The Sifton Range volcanic complex of southwestern Yukon is a member of the Paleogene Sloko-Skukum Group, and comprises a 900-m thick, shallow-dipping, volcanic succession dominated by intermediate to evolved lava and pyroclastic rocks deposited in a northwesterly trending half-graben. Locally, the volcanic sequence is intruded by alkali-feldspar granites of the CPCs Nisling Plutonic Suite dated at 57.5 Ma. Felsite sills radiate from the main intrusive body, and together with numerous basaltic to dacitic dykes traverse the volcanic package. Both the felsic volcanic rocks and epizonal granitoids exhibit anomalous enrichments in large-ion lithophile elements indicating crustal contributions during the late-stage petrogenesis of the complex. In addition, the Sifton Range intrusive rocks exhibit modal mineralogy reflective of lower ambient pressures relative to the compositionally similar Annie Ned granites along the Alaska Highway between Stony Creek and Mendenhall, 20 km south of the complex. The amount of post-Eocene uplift (ca. 30 m/Ma) that exposed the contact between the intrusive and corresponding volcanic rocks is constrained by the presence of a calc-silicate bed at an elevation of 1830 m within the upper volcanic stratigraphy.
Preliminary report on the bedrock geology of the Rackla River area, southern Wernecke Mountains, Yukon (parts of NTS 106C/4, 5 and 106D/1, 8)
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The Rackla River area is underlain by normal faulted and gently folded sedimentary strata of the Paleoproterozoic Wernecke Supergroup, Mesoproterozoic Pinguicula Group, Neoproterozoic Hematite Creek Group and Windermere Supergroup, and Paleozoic Bouvette Formation. Gabbro dikes and sills that are likely age equivalent to the ca. 1380 Ma Hart River Sills cut the Wernecke Supergroup rocks. The presence of a mafic volcaniclastic horizon within the Bouvette allows its informal subdivision into a lower and upper member. These volcaniclastic rocks may the distal equivalent to volcanic rocks near the Tiger deposit, located ~20 km to the southwest. Three major angular unconformities are documented in the map area: at the base of the Rapitan Group, the base of the lower Bouvette, and the base of the upper Bouvette Formation.
Geology of mid-Cretaceous volcanic rocks at Mount Nansen, central Yukon, and their relationship to the Dawson Range batholith
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Preliminary observations on the geology of the Rackla belt, Mount Ferrell map area (NTS 106C/3), central Yukon
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Bedrock Geology, Morris Lake (105B/5), southern Yukon (1:50,000 scale)
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Between the Cretaceous granitic rocks (Hake Batholith on the west; Cassiar Batholith to the east) are three belts of metamorphic rocks, collectively part of Yukon-Tanana terrane. These are remnants of oceanic and continental volcanic arcs, and marginal basin sediments of Early to mid-Paleozoic age. At the head of Borden Creek are thick carbonate and andesitic volcanic rocks correlated with Klinkit Group. The Ram Creek fault and Hidden Lake fault are not exposed but deduced to be steeply dipping brittle structures with northeastward thrust or transpressional offset, based upon more complete exposure to the southeast in 105B/3 map area. The former is likely of Cretaceous age; the latter was active between mid-Permian and Early Jurassic time.
Resedimentation of the late Holocene White River tephra, Yukon Territory and Alaska
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The Wrangell region of eastern Alaska represents a zone of extensive volcanism marked by intermittent pyroclastic activity during the late Holocene. The most recent and widely dispersed pyroclastic deposit in this area is the White River tephra, a distinct tephra-fall deposit covering 540 000 km 2 in Alaska, Yukon, and the Northwest Territories. This deposit is the product of two Plinian eruptions from Mount Churchill, preserved in two distinct lobes, created ca. 1887 years B.P. (northern lobe) and 1147 years B.P. (eastern lobe). The tephra consists of distal primary air-fall deposits and proximal, locally resedimented volcaniclastic deposits. Distinctive layers such as the White River tephra provide important chronostratigraphic control and can be used to interpret the cultural and environmental impact of ancient large magnitude eruptions. The resedimentation of White River tephra has resulted in large-scale terraces, which flank the margins of Klutlan Glacier. Preliminary analysis of resedimented deposits demonstrates that the volcanic stratigraphy within individual terraces is complex and unique.
Geochemistry of Devono–Mississippian volcanic and intrusive rocks of the Finlayson Lake district, Yukon-Tanana terrane, Yukon
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The Finlayson Lake district in southeastern Yukon is a remnant of a Late Paleozoic arc–back-arc system that consists of metamorphosed volcanic, plutonic, and sedimentary rocks of the Yukon-Tanana and Slide Mountain terranes. These rocks host more than 40 Mt of polymetallic resources in numerous occurrences and styles of volcanogenic massive sulphide (VMS) mineralization. Geochemical data from these rocks support previous interpretations that volcanism and plutonism occurred in arc–marginal arc (e.g., Fire Lake formation) and continental back-arc basin environments (e.g., Kudz Ze Kayah formation, Wind Lake formation, and Wolverine Lake group) where felsic magmatism formed from varying mixtures of crust and mantle-derived material. The rocks have elevated high field strength element (HFSE) and rare earth element (REE) concentrations in VMS-proximal stratigraphy relative to VMS-barren assemblages, suggesting that the petrogenetic conditions that generated felsic rocks likely played a role in the localization of VMS mineralization. Future work aims to constrain magmatic processes and outline prospectivity criteria for delineating productive VMS assemblages within the district, and in similar geodynamic settings globally.
Surficial Geology of the Howard’s Pass Area (NTS 105I/11 and parts of 105I/10, 6 and 7), Yukon and Northwest Territories
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