Abstract
Abstract The southen end of the 1800 km long Coast Plutonic Complex (CPC), exposed in the Harrison Lake area of British Columbia and in the North Cascades of Washington, bears a record of great crustal thickening -20 to 40 km in localized zones during Late Cretaceous times. During this period, the CPC was positioned at the continental margin during collision/subduction of the Farallon plate. Arc magmatism and regional orogenic contraction were both active as potential crustal thickening processes. Magmatism is favored in this report as the dominant factor based on the delineation of four spatially and temporally separate loading events, the close association of the loaded areas with emplacement of large plutons, and a paucity of evidence of deep regional tectonic contraction. The timing and spatial location of crustal loading events are documented by the following: zircon ages in plutons; an early event of low pressure in pluton aureoles evidenced by andalusite, now pseudomorphed by high-pressure minerals; high pressures in country rock in pluton aureoles measured by mineral compositions in the assemblages garnet-biotite-muscovite-plagioclase and garnet-aluminum silicate-plagioclase; high pressures recorded in plutons by Al-in-hornblende barometry; and uplift ages of plutons derived from K-Ar and Ar-Ar ages of micas and hornblende in plutons.
Highlights
IntroductionThe orogen, some 1800 km long and variable in width from ~50 to 150 km, extends from southeast Alaska to northwest Washington (Figure 1)
The Coast Plutonic Complex (CPC) of western North America is a coast margin orogenic welt comprised of granitic plutons ranging in age from ~170 to 45 Ma, intruded into metamorphosed country rock (e.g., [1, 2])
From 100 to 60 Ma, great crustal thickening, 20-40 km, followed by uplift, occurred episodically in varying places across the southern Coast Plutonic Complex in the Harrison Lake area and in the Crystalline Core of the North Cascades, the two terranes being contiguous along strike prior to displacement on the Fraser River-Straight Creek fault
Summary
The orogen, some 1800 km long and variable in width from ~50 to 150 km, extends from southeast Alaska to northwest Washington (Figure 1) This belt is one of the Earth’s great mountain chains, ranking with the Alps, Himalayas, Andes, and Appalachians. The southern Insular Belt, which includes the western part of the southern Coast Plutonic Complex as well as basement rocks of Vancouver Island, is interpreted to have been displaced southward some 800 km during Late Jurassic-Early Cretaceous time based primarily on matching of age-distinctive magmatic belts [1, 8]. Country rock along the western side of the Harrison Lake area is the mostly Jurassic to mid-Cretaceous metamorphosed volcanic and sedimentary rock of the Harrison-Gambier assemblage considered to be part of the CPC magmatic suite. The andalusite in pluton aureoles is not isotopically dated but is reasonably interpreted to be close in age (less than a million years) to the dated plutons considering that andalusite occurs in narrow bands following the pluton
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