GeoScienceWorld Lithosphere Volume 2020, Article ID 8856566, 17 pages https://doi.org/10.2113/2020/8856566 Review Article Magma Loading in the Southern Coast Plutonic Complex, British Columbia and Washington E. H. Brown Department of Geology, Western Washington University, USA Correspondence should be addressed to E. H. Brown; [email protected] Received 2 May 2020; Accepted 22 September 2020; Published 10 November 2020 Academic Editor: Tamer S. Abu-Alam Copyright © 2020 E. H. Brown. Exclusive Licensee GeoScienceWorld. Distributed under a Creative Commons Attribution License (CC BY 4.0). 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. 1. Introduction addressed by Brown and McClelland [5] and Brown et al. [6]. This report is an update, based on field and laboratory 1.1. Setting and Statement of the Problem. The Coast Plutonic data carried out by numerous workers since publication of Complex (CPC) of western North America is a coast margin the previous papers. The earlier work is summarized here, orogenic welt comprised of granitic plutons ranging in age but the reader looking for more background detail is directed from ~170 to 45 Ma, intruded into metamorphosed country to these studies. rock (e.g., [1, 2]). The orogen, some 1800 km long and vari- The geologic history of the southern segment of the CPC able in width from ~50 to 150 km, extends from southeast potentially applies to the 1700 km northerly part of this great Alaska to northwest Washington (Figure 1). This belt is one orogenic belt and may be a significant crustal thickening of the Earth’s great mountain chains, ranking with the Alps, process of magmatic arcs elsewhere on Earth. An example Himalayas, Andes, and Appalachians. How did it form? is the northern Andean orogen for which both tectonic and The growth of this mountain belt is related to the conver- magmatic processes have been invoked as the major crustal gence and subduction of the Farallon oceanic plate against growth mechanism, but the relative significance of these the western margin of the Laurentian-North American plate. two processes is debated ([7], and references within). The CPC is a magmatic arc, but its history involves great Good access and intriguing geology of the CPC in the crustal thickening, up to 40 km, suggesting a thrust loading Pacific Northwest have attracted many geologists in recent component during its development as many geologists have years. Besides mapping and other outcrop study, this work concluded (e.g., [3]). However, for the southern CPC, a case in the past 20 years has yielded a considerable number of can be made that magmatism is by far the dominant loading radiometric ages and pressure-temperature values providing process, proposed by Brown and Walker [4] and further further constraints on the evolution of the orogen. Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/doi/10.2113/2020/8856566/5293340/8856566.pdf by guest on 29 September 2021 2 Lithosphere Settler QN Cogburn CPC USA IN CAN Harrison- Gambier Insular Coast 100 km Belt Hozomeen Fm Ocean Intermontane Harrison Methow Lake area Quesnellia Fig. 3 Plutonic B.C WA Group Complex Belt Pacifc Chelan Skagit 200 km 49° Gneiss Fig. 2 Nason terrance Mtns Figure 1: Regional setting of the Harrison Lake-North Cascades study area (defined by box) at the southern end of the Coast Plutonic Complex, offset by the Straight Creek-Fraser River fault. Adapted from Monger and Journeay [58]. Cascades Fraser R. Crystalline Straight N Core In the Pacific Northwest, the CPC is split by the Fraser Creek Fig. 6 fault River-Straight Creek strike-slip fault active at ~45 Ma (Figures 1 and 2). Thus, we have two domains to work on: 47°30′ the “Crystalline Core” of the North Cascades in Washington Mt. Stuart 20.0 km Batholith and the Harrison Lake area in British Columbia. The CPC 121° geology is mid- to Late Cretaceous in age and was mutually Figure 2: Map of related metamorphic/plutonic components in the continuous in the two regions prior to displacement along Harrison Lake area and North Cascades. Prior to Tertiary dextral the fault. strike-slip displacement on the Fraser River-Straight Creek fault, The orogen during the time frame of plutonism and these belts of rock were contiguous. Inset: IN = Insular terrane; country rock metamorphism (Figures 3–6) was outboard CPC = Coast Plutonic Complex; QN = Quesnellia; SEA = Seattle. from the western edge of the Quesnellia terrane and inboard of the accreted Insular Belt [2]. The southern Insular Belt, which includes the western part of the southern Coast Plu- senting ocean crust and overlying continent-derived tonic Complex as well as basement rocks of Vancouver sediment, respectively [9]. In the Cascades, mafic schists of Island, is interpreted to have been displaced southward some ocean floor origin together with ocean arc deposits, named 800 km during Late Jurassic-Early Cretaceous time based pri- the Chelan Mountains terrane, correlate with the Cogburn marily on matching of age-distinctive magmatic belts [1, 8]. schists. A structurally overlying unit of pelitic schists is the The precise location of this fault zone in the southern Coast Nason terrane in the Cascades correlative with the Settler ter- Plutonic Complex is not identified but lies somewhere in rane in British Columbia (Figure 2). Equivalents of the the region west oHarrison Lake where relatively older pluton Harrison-Gambier unit are not found in the Cascades, nor ages (>100 Ma) are found. is the 70-45 Ma Skagit Gneiss of the Cascades found in the Plutons ranging up to 600 km2 in an aerial extent occur in Harrison Lake area. clusters and as separate masses encased in country rock. The Great depth of burial and subsequent exhumation of the plutons being more resistant to erosion than country rock pluton-country rock complex are documented by mineralogy stand out in alpine exposures. Country rock along the west- in pluton aureoles. A shallow initial contact metamorphism ern side of the Harrison Lake area is the mostly Jurassic to caused by plutons, followed by burial, is evidenced by relict mid-Cretaceous metamorphosed volcanic and sedimentary andalusite+biotite assemblages in pluton aureoles that are rock of the Harrison-Gambier assemblage considered to be overprinted with high-pressure assemblages including kya- part of the CPC magmatic suite. Country rock units along nite. The andalusite in pluton aureoles is not isotopically the eastern side of Harrison Lake include the Harrison- dated but is reasonably interpreted to be close in age (less Gambier assemblage and also the Late Jurassic-Early Creta- than a million years) to the dated plutons considering that ceous Cogburn mafic schists and Settler pelitic schists repre- andalusite occurs in narrow bands following the pluton Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/doi/10.2113/2020/8856566/5293340/8856566.pdf by guest on 29 September 2021 Lithosphere 3 A 96g 6.1 A Mt. Mason 5.8 pluton SE Ascent Creek 49° 45′ 5.5 pluton 6.3 6.5 92e B79b Scuzzy Lillooet 5.0 pluton A′ pluton 86p 91h 7.5 107m A 50°00′ A 96 d S S A 7.7 B74a Breakenridge 5.7 pluton 102m 6.2 N 8 103p A 5.6 84d 122° 00′ Hb88 CC 4.5 8.5 M82r H 5.3 6.1 plutons HG 102d 6.1 = pressure, u A 91j kbars BCF 98p 49° 30′ d 6.3 = unaltered A 3.8 B80b Undated andalusite 6.2 SE Urquhart A = pseudomorphed 97 d 95f 3.9 pluton andalusite CG 5 A 5.2 A Hb88h = hornblende Harrison Lake 92c M82h = muscovite A 5.9 A B74b = biotite 3 4.6 96f K/Ar, Ar-Ar age, with key to author 84d = zircon age of pluton A wiht d key to source A HG 4 6.0 BCF = Butter Creek fault SS = Snowshoe pluton Spuzzum Country rock A-A′ cross section Fig. 5 pluton terranes HG = Harrison-Gambier CG = Cogburn Fig. 15 10 km SE = Settler Views: Fig. 11 Figure 3: Geologic map of the Harrison Lake area plutons, country rock, isotopic ages, and pressures of metamorphism. Map sources: Roddick [59], Reamsbottom [60, 61], Lapen [29], Brown and McClelland [5], Mitrovic [24], and Gibson and Monger [22]. H = Hut Creek pluton; CC = Clear Creek pluton. Key to sources of isotopic ages: (a) Wanless et al. [62], (b) Monger [63], (c) Walker and Brown [30], (d) Parrish and Monger [26], (e) Friedman et al.