Coast Plutonic Complex Research Articles

Geotectonics of Cretaceous and Eocene plutons in British Columbia: a paleomagnetic fold test

A total of 295 cores (590 specimens) were collected at 59 sites in the Coast plutonic complex along an E–W section southwest of Kitimat, British Columbia. The sites represent the Ponder, Alastair Lake, and Quottoon plutons in the 40–50 Ma eastern K–Ar age zone and the Ecstall and Butedale plutons in the 64–80 Ma central age zone. After af demagnetization a stable remanent magnetization was isolated at 32 sites and these data were combined with available data from the Skeena River section about 100 km to the north. The remanence directions in sites from the NNW-trending north and south limbs of the Hawkesbury Warp provide a positive fold test when compared to the WNW-trending centre limb directions.In the Eocene eastern age zone the NNW limbs give a concordant pole position relative to the cratonic North American pole whereas the centre limb has undergone ≈ 50° of the counter-clockwise rotation and ≈ 10° of upward tilt of its western end to give a discordant pole. In the late Upper Cretaceous central age zone, the Ecstall–Butedale pluton was tilted 15° to the west on all limbs before the Eocene intrusion and Hawkesbury Warp deformation events to give a NNW-trend pole and WNW-trend pole diverging in opposite directions from the cratonic reference pole.The geologic field evidence from structural trends, from fault, fold, contact, and foliation attitudes, and from distribution of plutonic phases is consistent with the structural model. The regional geotectonic events are related to possible Cenozoic plate interactions on the western margin of the North American plate. This combination of concordant and discordant poles cannot be explained in terms of an excursion of the geomagnetic paleopole during intrusion, a large scale northward translation of the western Cordillera during the Cenozoic, or a combination of clockwise rotations and northward translations on the margin of the advancing North American plate. The fold test and polarity reversal pattern indicate that all plutons acquired a primary thermoremanent magnetization (TRM) during cooling and probably within ≈ 1 Ma after emplacement.

Metal distribution patterns across the eastern flank of the Coast plutonic complex, South-central British Columbia

Machine-plotted maps of metal distributions, combined with field work, indicate the presence of a previously unrecognized regional zoning pattern in the Taseko Lakes and Pemberton map-areas. Mineral deposits occur in well-defined camps, the most productive of which has been the Bridge River gold camp. A distinctive asymmetric metal and mineral zoning pattern occurs in the Bridge River district. Two elongate, northwesterly trending centers of gold-bearing vein deposits lie within a larger area characterized by antimony minerals. The antimony zone is succeeded to the northeast by a mercury zone. This pattern is explained by mineral deposition under a regional thermal gradient decreasing outward from the eastern margin of the Coast Plutonic Complex, implying that mineralization occurred during or shortly after final cooling of the eastern margin of the Coast Plutonic Complex about 50 m.y. ago.Porphyry deposits occur in geographically separate areas from most vein deposits. Porphyry mineralization occurred during the Late Cretaceous, early Tertiary, and Miocene. Copper-molybdenum-gold porphyry deposits characterize the first two events, whereas porphyry molybdenum deposits formed during the Miocene. Principal movements along the Yalakom fault predated mineral deposition in the Bridge River camp. Late Tertiary porphyry molybdenum deposits appear genetically related to Cascade volcanism rather than to plutonism of the Coast Plutonic Complex.

Rb/Sr Ages and a profile of initial Sr87/Sr86 ratios for plutonic rocks across the Yukon Crystalline Terrane

Nine Rb/Sr apparent ages are reported for igneous rocks of the Yukon Crystalline Terrane. The oldest age (144 m.y.) is from the Triassic? Klotassin quartz diorite and is thought to be a hybrid age that probably reflects the effects of younger intrusives on rocks at least 190 m.y. old. Five ages of about 100 m.y. presumably reflect the cooling of the Coffee Creek quartz monzonite. K/Ar ages for this event are slightly younger than the Rb/Sr ages, suggesting slow cooling. Rb/Sr ages of 53 and 67 m.y. were obtained for the Ruby Range batholith and an age of 61–67 m.y. for the Nisling Range alaskite. The Rb/Sr ages obtained generally confirm recently determined K/Ar ages. There is a regional decrease in initial Sr87/Sr86 ratios, southwestward across the Yukon Crystalline Terrane. This may mean that Precambrian rocks extend under the Yukon Crystalline Terrane, but are absent under the adjoining Coast Plutonic Complex.

Granitic rocks of the southern Coast Plutonic Complex and northern Cascades of British Columbia

The mapped area, about 170 km east of Vancouver, British Columbia, lies at the intersection of the northwest-trending Mesozoic—early Cenozoic Coast Plutonic Complex and the north-trending late Cenozoic Cascade belt. The Late Cretaceous mesozonal Spuzzum intrusions (90 to 80 m.y. old) of the Coast Crystalline Complex are made up of a central diorite complex and a marginal tonalite. Modal variation in the diorite, which is pyroxenic in the central parts and hornblendic in the marginal, was controlled by P H 2 O in the magma. The Yale intrusions (59 to 35 m.y. old), of tonalite, granodiorite, and quartz monzonite, are stocks and sills that may represent the latest intrusions of the Coast Plutonic Complex. The Chilliwack composite batholith (40 to 16 m.y. old) is represented by the Chilliwack batholith, the Mount Barr batholith, and the Silver Creek stock; these epizonal batholiths consist largely of tonalite, granodiorite, and quartz monzonite. Variation in the Chilliwack composite batholith is due mainly to differentiation at depth, followed by minor evolution both as the various phases rose and also after they were emplaced. The Fraser River—Straight Creek fault zone may have controlled the emplacement of many of the late Cenozoic plutons. During the past 40 m.y., intrusion and volcanism may have been nearly continuous in southwestern British Columbia and Washington.

The Yukon Crystalline Terrane: Enigma in the Canadian Cordillera

The Yukon Crystalline Terrane, an area of Proterozoic and Paleozoic metamorphic rocks intruded by Mesozoic and Tertiary plutons and covered by Tertiary volcanic strata, has been one of the least understood tectonic elements in the Canadian Cordillera. It incorporates characteristics of the Omineca Crystalline Belt, Intermontane Belt, and Coast Plutonic Complex. The metamorphic rocks are not part of a single time-stratigraphic assemblage as the term “Yukon Group” has implied. They can be divided into gross lithologic assemblages which are correlated with Proterozoic and Paleozoic strata of the Omineca Crystalline Belt, and the metamorphic rocks of the Yukon Crystalline Terrane are therefore the continuation of the Omineca Crystalline Belt. Four plutonic suites are recognized in the Yukon Crystalline Terrane. The two oldest (Late Triassic and mid-Jurassic) are related to similar plutons in the Intermontane Belt and mark its continuation through the central part of the Yukon Crystalline Terrane. The mid-Cretaceous suite in the northern part of the Yukon Crystalline Terrane is a westward continuation of plutonic rocks of the Omineca Crystalline Belt. Early Eocene alaskite marks the superposition of Coast Plutonic Complex igneous rocks on the southern part of the Yukon Crystalline Terrane. Tertiary tholeiitic basalts, which cover part of the Yukon Crystalline Terrane, lie on an eroded plateau whose relief and physiography approximate the present terrane.

Structure of the Canadian Cordillera from Seismic Refraction and Other Data

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.

Age and tectonic implications of paleomagnetic results from plutons near Prince Rupert, British Columbia

A total of 25 sites (125 cores, 250 specimens) were selected from four plutons along the Skeena River cross-section of the Coast plutonic complex. After af demagnetization a stable remanence direction was isolated for the diorite of the Kasiks pluton, the gneissic diorite and quartz diorite of the Quottoon pluton, and the diorite and quartz diorite of the Ecstall pluton but not for the granodiorite phase of the Ponder pluton. The Kasiks-Quottoon pole (144° W, 76°N (P0.05:7°, 7°)) is concordant with other Eocene poles from cratonic North America, and thus no postemplacement tectonic rotation or translation is indicated for these plutons. The Ecstall pole (47°W, 68°N (P0.05:13°, 16°)) in Greenland is discordant with other Upper Cretaceous poles. This pluton has probably undergone a westerly tilt of ≃20°. The Kasiks and Quottoon plutons most likely cooled quickly within the time span of one normal polarity interval (<½ m.y.) and the Ecstall pluton within the time span of one normal and one reversed interval (<1 m.y.). This result supports the hypothesis that the pole positions and K-Ar ages of 43 ± 5, 48 ± 7, and 75 ± 12 m.y. for the plutons date the age of their emplacement rather than the age of their unroofing or of their thermal metamorphism, which would give different paleomagnetic results. Acceptance of the K-Ar dates as the age of emplacement suggests that the Coast plutonic complex evolved at the western margin of the North American plate as three prolonged episodes of plutonic emplacement to form the western (84–140 m.y.), central (64–80 m.y.), and eastern (40–50 m.y.) K-Ar date zones.