Crustal recycling during subduction at the Eocene Cordilleran margin of North America: a petrogenetic study from the southwestern Yukon

2003 ◽  
Vol 40 (12) ◽  
pp. 1805-1821 ◽  
Author(s):  
George A Morris ◽  
Robert A Creaser
Keyword(s):  
Partial Melting ◽  
Continental Margin ◽  
Pyroclastic Flows ◽  
Canadian Cordillera ◽  
Early Eocene ◽  
Crustal Recycling ◽  
Rock Types ◽  
Plutonic Complex ◽  
Coast Plutonic Complex ◽  
Cordilleran Margin

The early Eocene (57.3–55.4 Ma) Bennett Lake and Mount Skukum Volcanic Complexes lie on the Coast Plutonic Complex and Intermontane Belt boundary of the Canadian Cordillera at the British Columbia – Yukon border, some 200 km east of the current and Eocene continental margin. Both complexes contain rock types from basaltic andesite to rhyolite in a series of lava and pyroclastic flows. The location relative to the continental margin, the rock types, and the presence of an enhanced LILE/HFSE (large-ion lithophile / high field strength element) signatures in all samples imply that contemporaneous subduction was the controlling factor in the formation of these complexes. The majority of samples, however, return unusually low compatible element concentrations for given rock types. We interpret this data to show that partial melting of the crust was the major source of erupted magmas. One formation of andesites at Mount Skukum and one late dyke at Bennett Lake do show higher concentrations of compatible trace elements, suggesting the presence of primitive magmas in the crust at the time of eruption, which contaminated and were erupted with the crustal melts. Sr–Nd isotopic data at both complexes are consistently primitive regardless of rock type and compatible element content, requiring a primitive crustal source for these magmas. We propose that the complexes were formed as a result of early Eocene subduction of the Kula Plate beneath the Canadian Cordillera. Intrusion of hot primitive melts caused partial melting of young crust to produce the majority of lavas observed. Contamination of these melts by primitive magmas is observed at both Mount Skukum and Bennett Lake.

Structure of the Canadian Cordillera from Seismic Refraction and Other Data

1975 ◽  
Vol 12 (2) ◽  
pp. 182-208 ◽  
Author(s):  
M. J. Berry ◽  
D. A. Forsyth
Keyword(s):  
Seismic Data ◽  
Seismic Refraction ◽  
Vancouver Island ◽  
Geological Structure ◽  
Canadian Cordillera ◽  
Low Velocity ◽  
Model Calculations ◽  
Plutonic Complex ◽  
Coast Plutonic Complex ◽  
Crustal Density

A synthesis of Canadian Cordilleran refraction data recorded prior to 1971 with other geophysical data shows major features which correlate well with the regional geological structure. The wavelength of M topography decreases from about 200 km at 54°N to about 110 km at 52°N and culminates in a major lithospheric discontinuity east of Vancouver Island. The seismic data indicate the region of the Fraser River at Quesnel, the region immediately east of and parallel to the Coast Plutonic Complex and possibly the western edge of the Hazelton Mountains are sites of significant changes in lithospheric structure.Lateral variations in the average crustal density are necessary to reconcile both gravity and seismic data. The crust beneath the central, intermontane region is characterized by a mass deficiency, whereas the density of the crust beneath Vancouver Island appears greater than average.Calculation of synthetic record sections shows that the Pn phase may propagate considerably beyond crossover in a layer a few wavelengths thick. A model for the Canadian Cordillera which includes a low velocity layer as little as 8 km below the M appears plausible. Model calculations suggest that the M approximates a discontinuity beneath the Coast Plutonic Complex, but is better modelled as a transition zone beneath the Omineca Crystalline belt.

Interpretation of isotopic ages and 87Sr/86Sr initial ratios for plutonic rocks in the Whitehorse map area, Yukon

1979 ◽  
Vol 16 (10) ◽  
pp. 1988-1997 ◽  
Author(s):  
Gregg W. Morrison ◽  
Colin I. Godwin ◽  
Richard L. Armstrong
Keyword(s):  
British Columbia ◽  
Oceanic Crust ◽  
Late Cretaceous ◽  
Canadian Cordillera ◽  
Initial Ratio ◽  
Eastern Margin ◽  
The North ◽  
Plutonic Complex ◽  
Coast Plutonic Complex ◽  
North American Craton

Sixteen new K–Ar dates and four new Rb–Sr isochrons help define four plutonic suites in the Whitehorse map area, Yukon. The Triassic(?) suite, defined on stratigraphic evidence, is the southern extension of the Yukon Crystalline Terrane and is correlative with plutonic suites in the Intermontane Belt in British Columbia. The mid-Cretaceous (~100 Ma) suite in the Intermontane Belt in the Whitehorse map area is time equivalent to plutonic suites in the Omineca Crystalline Belt to the east. Late Cretaceous (~70 Ma) and Eocene (~55 Ma) suites include volcanic and subvolcanic as well as plutonic phases and are correlative with continental volcano–plutonic suites near the eastern margin of the Coast Plutonic Complex. The predominance of the mid-Cretaceous suite in the Intermontane Belt in Whitehorse and adjacent map areas in Yukon and northern British Columbia suggests that this area has undergone posttectonic magmatism more characteristic of the Omineca Crystalline Belt than of the Intermontane Belt elsewhere in the Canadian Cordillera.87Sr/86Sr initial ratio determinations suggest that the southern extension of the Yukon Crystalline Terrane in the western part of the Whitehorse map area and in northern British Columbia includes Precambrian crust separated from the North American craton by Paleozoic oceanic crust of the Intermontane Belt.

Paleomagnetic evidence for displacement from the south of the Coast Plutonic Complex, British Columbia

1985 ◽  
Vol 22 (4) ◽  
pp. 584-598 ◽  
Author(s):  
E. Irving ◽  
G. J. Woodsworth ◽  
P. J. Wynne ◽  
A. Morrison
Keyword(s):  
British Columbia ◽  
Sierra Nevada ◽  
Lateral Displacement ◽  
Late Triassic ◽  
Time Range ◽  
Low Grade ◽  
Canadian Cordillera ◽  
Western Cordillera ◽  
Plutonic Complex ◽  
Coast Plutonic Complex

The mid-Cretaceous Spuzzum and Porteau plutons of the Coast Plutonic Complex of British Columbia have two magnetizations, A and B. The A magnetization (eight sites, 83 specimens, D = 30.3°, I = 56.7°, α95 = 4.9°, paleolatitude = 37 ± 5°N, paleopole 65.0°N, 14.9°W, A95 = 6.2°) is considered to have been acquired in the age range 105–90 Ma. This result differs from the field established for cratonic North America in this time range. The difference could be caused either by previously undetected tilting about a horizontal axis of the plutons, or by their rotation about a vertical axis and lateral displacement relative to the craton. Previously observed mid-Cretaceous magnetizations from other rock units from the western Canadian Cordillera and the Cascades of Washington, United States, are similarly discordant with respect to the craton. This similarity over such a large area indicates that, although local undetected tilting could be partly responsible, it is unlikely to be the prime cause, and we argue therefore that lateral displacement and rotation have occurred. It would seem that much of the western part of the Canadian Cordillera has moved north by about 2400 km and rotated clockwise since the mid-Cretaceous. The paleolatitude of the southern Coast Plutonic Complex of British Columbia is statistically identical to that recently observed (39 ± 3°N) for three plutons from the Central Sierra Nevada of California, which raises the possibility that the two complexes were much closer together at the time of their emplacement than at present. The second magnetization called B (four sites, 27 specimens, D = 5.1°, I = 67.6°, α95 = 4.7°, paleopole 86.5°N, 51.2°W) is parallel to the mid-Tertiary field, as previously determined from nearby intrusions, and is considered to be an overprint acquired during regional heating and low-grade metasomatism. Some earlier paleomagnetic studies of mid-Cretaceous rocks from the Coast Plutonic Complex indicated either an absence of displacement or uncertain evidence for it, and we attribute this to the nonrecognition, in this earlier work, of similar magnetically stable overprints of Tertiary age. Overprints in several Triassic rock units in the western Cordillera are parallel to the A magnetization, indicating that the mid-Cretaceous and the mid-Tertiary probably were periods of severe magnetic overprinting in British Columbia. Mid-Cretaceous and Late Triassic results from the western Cordillera of British Columbia are systematically different, indicating that movements relative to the craton occurred between these times.

Geochemistry of Archean meta-igneous rocks, Lake Despair area, Wabigoon Subprovince, northwestern Ontario

1980 ◽  
Vol 17 (8) ◽  
pp. 1046-1063 ◽  
Author(s):  
Fred J. Longstaffe ◽  
Robert H. McNutt ◽  
Henry P. Schwarcz
Keyword(s):  
Trace Element ◽  
Partial Melting ◽  
Metamorphic Grade ◽  
Igneous Rocks ◽  
Rock Types ◽  
Northwestern Ontario ◽  
Metavolcanic Rocks ◽  
Plutonic Complex ◽  
Tectonic Settings ◽  
Amphibole Fractionation

The Lake Despair area, northwestern Ontario, is underlain by a series of igneous and meta-igneous rocks including mafic and felsic metavolcanic rocks, gneisses (Footprint gneiss), and plutonic granitoids (Jackfish Lake Plutonic Complex; Northwest Bay Complex). The metavolcanic sequence consists of a bimodal suite of interlayered metabasalts of tholeiitic character and metadacites with metamorphic grade ranging from greenschist to amphibolite facies. Trace element studies of the metabasalt, while indicating an oceanic provenance, do not uniquely distinguish between possible oceanic tectonic settings. The metadacites, the Footprint gneiss, and the Northwest Bay Complex are chemically similar to the high Al tonalitic rocks common in other Archean terrains. The Footprint gneiss, which forms part of the gneissic core of the Rainy Lake batholith, was derived from an igneous parent, and was quasi-isochemically metamorphosed.The youngest major intrusion, the Na-alkalic Jackfish Plutonic Complex, is composed of hornblende diorite and monzodiorite with minor leucodiorite, quartz monzodiorite, granodiorite, and Na-syenite. The diorite and monzodiorite crystallized from an intermediate magma formed by partial melting of a garnet and (or) amphibole-rich basaltic parent. Remaining residual liquids, enriched in alkalis and silica by amphibole fractionation, crystallized to form the more leucocratic rock types. Perthitic microcline megacrysts, characteristic of the quartz monzodiorite and granodiorite, formed near the end of magmatic processes. The Na-syenite was emplaced as a separate pulse of magma or possibly was produced by the interaction of alkalis and the upper portions of the solidified rim of the magma chamber.

Geochemistry of syntectonic, crustal fluid regimes along the Lithoprobe Southern Canadian Cordillera Transect

1995 ◽  
Vol 32 (10) ◽  
pp. 1699-1719 ◽  
Author(s):  
Bruce E. Nesbitt ◽  
Karlis Muehlenbachs
Keyword(s):  
Deep Convection ◽  
Canadian Cordillera ◽  
Depth Of Penetration ◽  
Crustal Fluid ◽  
Quartz Veins ◽  
Economic Significance ◽  
Rock Types ◽  
Brittle Ductile Transition ◽  
Fluid Convection ◽  
Convection Model

In conjunction with the Lithoprobe southern Canadian Cordillera program, an extensive examination of geochemical indicators of origins, movement, chemical evolution, and economic significance of paleocrustal fluids was conducted. The study area covers approximately 360 000 km2from the Canadian Rockies to Vancouver Island. Research incorporated petrological, mineralogical, fluid-inclusion, δ18O, δD, δ13C, and Rb/Sr studies of samples of quartz ± carbonate veins and other rock types. The results of the study document a variety of pre-, syn-, and postorogenic, crustal fluid events. In the Rockies, a major pre-Laramide hydrothermal event was identified, which was comprised of a west to east migration of warm, saline brines. This was followed by a major circulation of meteoric water in the Rockies during Laramide uplift. In the southern Omineca extensional zone, convecting surface fluids penetrated to the brittle–ductile transition at 350–450 °C and locally into the underlying more ductile rocks. A principal conclusion of the study is that most quartz ± carbonate veins in metamorphic rocks in the southern Canadian Cordillera precipitated from deeply converted surface fluids. This conclusion supports a surface fluid convection model for the genesis of mesothermal Au–quartz veins, common in greenschist-facies rocks worldwide. The combination of our geochemical results with the results of other Lithoprobe studies indicates that widespread and deep convection of surface fluids in rocks undergoing active metamorphism is a commonplace phenomena in extensional settings, while in compressional-thrust settings the depth of penetration of surface fluids is more limited.

scholarly journals Crustal deformation and regional metamorphism across a terrane boundary, Coast Plutonic Complex, British Columbia

Tectonics ◽  
1987 ◽  
Vol 6 (3) ◽  
pp. 343-361 ◽  
Author(s):  
M. L. Crawford ◽  
L. S. Hollister ◽  
G. J. Woodsworth
Keyword(s):  
British Columbia ◽  
Crustal Deformation ◽  
Regional Metamorphism ◽  
Plutonic Complex ◽  
Coast Plutonic Complex

scholarly journals Evidence of mingling between contrasting magmas in a deep plutonic environment: the example of Várzea Alegre, in the Ribeira Mobile Belt, Espírito Santo, Brazil

2001 ◽  
Vol 73 (1) ◽  
pp. 99-119 ◽  
Author(s):  
SILVIA R. MEDEIROS ◽  
CRISTINA M. WIEDEMANN-LEONARDOS ◽  
SIMON VRIEND
Keyword(s):  
Partial Melting ◽  
Granitic Magma ◽  
Geochemical Data ◽  
Mobile Belt ◽  
Tholeiitic Magma ◽  
Rock Types ◽  
Upper Proterozoic ◽  
Incompatible Elements ◽  
Wide Range ◽  
Intermediate Rock

At the end of the geotectonic cycle that shaped the northern segment of the Ribeira Mobile Belt (Upper Proterozoic to Paleozoic age), a late to post-collisional set of plutonic complexes, consisting of a wide range of lithotypes, intruded all metamorphic units. The Várzea Alegre Intrusive Complex is a post-collisional complex. The younger intrusion consists of an inversely zoned multistage structure envolved by a large early emplaced ring of megaporphyritic charnoenderbitic rocks. The combination of field, petrographic and geochemical data reveals the presence of at least two different series of igneous rocks. The first originated from the partial melting of the mantle. This was previously enriched in incompatible elements, low and intermediate REE and some HFS-elements. A second enrichment in LREE and incompatible elements in this series was due to the mingling with a crustal granitic magma. This mingling process changed the composition of the original tholeiitic magma towards a medium-K calc-alkalic magma to produce a suite of basic to intermediate rock types. The granitic magma from the second high-K, calc-alkalic suite originated from the partial melting of the continental crust, but with strong influence of mantle-derived melts.

scholarly journals Igneous Rock Associations 26. Lamproites, Exotic Potassic Alkaline Rocks: A Review of their Nomenclature, Characterization and Origins

2020 ◽  
Vol 47 (3) ◽  
pp. 119-142
Author(s):  
Roger H. Mitchell
Keyword(s):  
Partial Melting ◽  
Lithospheric Mantle ◽  
Tectonic Setting ◽  
Alkaline Rock ◽  
Mantle Metasomatism ◽  
Geochemical Evolution ◽  
Alkaline Rocks ◽  
Isotopic Compositions ◽  
Economic Significance ◽  
Rock Types

Lamproite is a rare ultrapotassic alkaline rock of petrological importance as it is considered to be derived from metasomatized lithospheric mantle, and of economic significance, being the host of major diamond deposits. A review of the nomenclature of lamproite results in the recommendation that members of the lamproite petrological clan be named using mineralogical-genetic classifications to distinguish them from other genetically unrelated potassic alkaline rocks, kimberlite, and diverse lamprophyres. The names “Group 2 kimberlite” and “orangeite” must be abandoned as these rock types are varieties of bona fide lamproite restricted to the Kaapvaal Craton. Lamproites exhibit extreme diversity in their mineralogy which ranges from olivine phlogopite lamproite, through phlogopite leucite lamproite and potassic titanian richterite-diopside lamproite, to leucite sanidine lamproite. Diamondiferous olivine lamproites are hybrid rocks extensively contaminated by mantle-derived xenocrystic olivine. Currently, lamproites are divided into cratonic (e.g. Leucite Hills, USA; Baifen, China) and orogenic (Mediterranean) varieties (e.g. Murcia-Almeria, Spain; Afyon, Turkey; Xungba, Tibet). Each cratonic and orogenic lamproite province differs significantly in tectonic setting and Sr–Nd–Pb–Hf isotopic compositions. Isotopic compositions indicate derivation from enriched mantle sources, having long-term low Sm/Nd and high Rb/Sr ratios, relative to bulk earth and depleted asthenospheric mantle. All lamproites are considered, on the basis of their geochemistry, to be derived from ancient mineralogically complex K–Ti–Ba–REE-rich veins, or metasomes, in the lithospheric mantle with, or without, subsequent contributions from recent asthenospheric or subducted components at the time of genesis. Lamproite primary magmas are considered to be relatively silica-rich (~50–60 wt.% SiO2), MgO-poor (3–12 wt.%), and ultrapotassic (~8–12 wt.% K2O) as exemplified by hyalo-phlogopite lamproites from the Leucite Hills (Wyoming) or Smoky Butte (Montana). Brief descriptions are given of the most important phreatomagmatic diamondiferous lamproite vents. The tectonic processes which lead to partial melting of metasomes, and/or initiation of magmatism, are described for examples of cratonic and orogenic lamproites. As each lamproite province differs with respect to its mineralogy, geochemical evolution, and tectonic setting there is no simple or common petrogenetic model for their genesis. Each province must be considered as the unique expression of the times and vagaries of ancient mantle metasomatism, coupled with diverse and complex partial melting processes, together with mixing of younger asthenospheric and lithospheric material, and, in the case of many orogenic lamproites, with Paleogene to Recent subducted material.

Geochemistry of Late Proterozoic amphibolites and ultramafic rocks, southeastern Canadian Cordillera

1988 ◽  
Vol 25 (8) ◽  
pp. 1323-1337 ◽  
Author(s):  
James H. Sevigny
Keyword(s):  
Partial Melting ◽  
Major And Trace Elements ◽  
Ultramafic Rocks ◽  
Canadian Cordillera ◽  
Elemental Abundances ◽  
Late Proterozoic ◽  
Group I ◽  
Heterogeneous Mantle ◽  
High Degree ◽  
Ree Patterns

Late Proterozoic amphibolites and ultramafic rocks from the southeastern Canadian Cordillera have been analysed for major and trace elements in order to determine the nature and origin of the protoliths. Geologic relations indicate that these rocks were produced during an episode of continental rifting in the Precambrian. Based on rare-earth-element (REE) patterns, immobile-incompatible-element ratios, and characteristic elemental abundances, amphibolites are subdivided into alkaline and tholeiitic metabasalts. Alkaline basalts are recognized by their steep REE patterns, high Zr/Y, high TiO2 and P2O5 abundances, and low Y/Nb and Ti/Zr. Tholeiitic basalts are subdivided into three groups: (I) high-Mg#, high-field-strength-element (HFSE)-depleted, light-REE (LREE)-enriched tholeiites with flat heavy REE (HREE) patterns; (II) LREE-enriched tholeiites depleted in HREE; and (III) low-Mg# tholeiites with flat REE patterns. Ultramafic rocks occur as boudins of partially recrystallized Cr-spinel-bearing harzburgite or therzolite, enriched in LREE (Ce/Sm = 1.7–1.9), HFSE, CaO, Al2O3, and TiO2 relative to depleted mantle.Geochemical data suggest that the basalts were derived from a heterogeneous mantle source that underwent different degrees of partial melting with variable amounts of subsequent crystal fractionation of the melts. High Mg#, high Cr and Ni abundances, low HFSE abundances, and high olivine saturation temperatures suggest that group I tholeiites are primary mantle melts produced by a relatively high degree of partial melting of a LREE-enriched, HFSE-depleted source. Group II and III basalts have undergone moderate olivine and pryoxene and limited plagioclase fractionation. Mass-balance calculations suggest that the ultramafic rocks represent a crustally contaminated primary-mantle-derived melt.Les éléments majeurs et traces des amphibolites et des ultramafites, d'âge protérozoïque tardif, du sud-est de la Cordillère canadienne ont été analysés dans le but de déterminer la nature et l'origine des protolithes. Les relations géologiques indiquent que ces roches se sont formées durant un épisode de rifting continental dans le Précambrien. Les diagrammes des terres rares, les rapports des éléments immobiles et incompatibles et les compositions chimiques caractéristiques ont permis de subdiviser les amphibolites en métabasaltes tholéiitiques et alcalins. Les basaltes alcalins sont reconnus par les courbes abruptes dans les diagrammes des terres rares, les rapports Zr/Y élevés et les fortes teneurs en TiO2 et P2O5 et les rapports Y/Nb et Ti/Zr faibles. Les basaltes tholéiitiques sont subdivisés en trois groupes : (I) avec Mg# élevé, appauvrissement en éléments de force de champ élevée, tholéiites enrichies en terres rares légères avec courbe horizontale des terres rares lourdes; (II) tholéiites enrichies en terres rares légères et appauvries en terres rares lourdes; et (III) tholéiites avec Mg# faible et avec courbe horizontale des variations des terres rares. Les ultramafites se présentent en boudins formés d'harzburgite incluant un spinelle chromifère partiellement recristallisé ou de therzolite qui sont enrichies en terres rares légères (Ce/Sm = 1,7–1,9), en éléments à force de champ élevée, en CaO, Al2O3 et TiO2, comparativement à un manteau appauvri.

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