E. H. Brown

A magma-loading model for Barrovian metamorphism in the southeast Coast Plutonic Complex,British Columbia and Washington

Barrovian metamorphism linked to crustal thickening in the southeast Coast Plutonic Complex of British Columbia and Washington is proposed in this paper to be the product of magmatic loading. Relevant observations and arguments are the following. (1) Isotopic ages coupled with structural and fabric relations document that throughout the region Barrovian metamorphism and plutonism are broadly coeval. (2) Baric patterns in country rock indicate steep-sided crustal loads originating within the high-grade parts of the orogen.(3) Patterns of subsidence and uplift are localized and diachronous, precluding a single-event regional thickening mechanism. (4) Country-rock fabrics are dominated by steep, orogen-parallel foliations and shallow, strike-parallel lineations, features not easily reconciled with tectonic thickening mechanisms. (5) Abroad zone of schists flanking the large (25 x 100 km) Scuzzy batholith in British Columbia bears metamorphic isograds and foliations that track the border of the pluton across regional structural trends, implicating pluton emplacement as the cause of metamorphism. (6) The crustal-loading event in the vicinity of the Scuzzy pluton is bracketed by mineral and textural features as being coeval with emplacement of the pluton, which occurred over a period of ∼7 m.y. (7) The regionally developed Chiwaukum Schist in Washington has petrologic and structural features and a metamorphic age that suggest it was formed by, and represents the floor of, an eroded extension of the Scuzzy pluton. The proposed magma-loading model invokes the diapir and ballooning concepts of previous workers in which rising plutons displace country rock downward. This mechanism results in a type of convective crustal overturn, and at least partially solves the space problem related to batholithic emplacement. As a result of this process, relatively broad tracts of country rock acquire Barrovian mineralogy. Following plutonism, magmatically thickened crust is eroded and isostatically up-lifted to expose the plutonic roots and high-grade country-rock basement.

Structural geology and accretionary history of the Northwest Cascades system, Washington and British Columbia

Pre-Tertiary rock units on the western flank of the Cascades in Washington and southern British Columbia constitute the Northwest Cascades system (NWCS). This litho-tectonic package is characterized by predominantly oceanic lithologies subjected to high P/T metamorphism and structurally mixed to form a regional melange. Although the focus of this report is on rocks of the NWCS as originally defined, survey of the regional geology suggests broadening of the limits of this system to include the San Juan Islands, the Pacific Rim complex, and pre-Tertiary rocks on the western flank of the central and southern Washington Cascades. The Shuksan Metamorphic Suite, a relatively coherent and broadly exposed unit of the NWCS, contains a regionally consistent stretching lineation which provides evidence of east-northeast–directed underthrusting (subduction) during the Early Cretaceous blue-schist metamorphism. Metamorphic F2 folds are approximately normal to the stretching lineation and appear to have developed late in the same deformational event. The melange structure postdates the high-pressure regional metamorphism. It is characterized by a tectonic assemblage of diverse and unrelated rock units occurring as fragments as much as 10 or more kilometres in breadth and separated by a network of low- to high-angle north-northwest–striking faults. Pre-fault restoration is impossible. Argillaceous or serpentinitic material commonly occurs in deformation zones between more competent lithologies. Mylonite zones as much as several metres thick are widely developed and display stretched clast lineations, indicating a north-northwest–south-southeast transport direction for both low- and high-angle faults. The high-angle faults are therefore strike-slip in nature. Shear-sense indicators are mixed in thrust fault zones, but many indicate dextral movement for the strike-slip faults. K/Ar whole-rock ages of mylonites range from 20–127 Ma, but two Late Cretaceous ages of 87 and 91 ± 3 Ma are considered as best representing the time of faulting. The inferred shift in direction of tectonic transport from east-northeast–west-southwest during Early Cretaceous metamorphism to north-northwest–south-southeast during Late Cretaceous faulting and melange formation is correlated with a mid-Cretaceous change in plate interaction between the North American and Farallon/Kula plates from high- to low-angle convergence. The NWCS melange is interpreted to have formed during large dextral transform motion along the continental margin and to have reached final emplacement by thrusting against the southern end of Wrangellia and the Coast Plutonic Complex.

Petrologic, structural, and age relations of serpentinite, amphibolite, and blueschist in the Shuksan Suite of the Iron Mountain–Gee Point area, North Cascades, Washington

Part of the Shuksan blueschist terrane, near Iron Mountain and Gee Point, North Cascades, Washington, has associated serpentinite, amphibolite, barroisite schist, blueschist, rare eclogite, and blackwall-type metasomatic rock. Field, petrographic, and microprobe observations indicate that the amphibolite and barroisite schists were metamorphosed in contact with peridotite and suggest that the peridotite may have been a heat source. The serpentinite and associated rocks are structurally concordant with the regional blueschists and have been overprinted by blueschist metamorphism. Isotopic dating gives metamorphic ages of 148 ± 5 to 164 ± 6 m.y. for the amphibolite and barroisite schist and 129 ± 5 m.y. for nearby regional Shuksan blueschists. The origin of the serpentinite + amphibolite + blueschist assemblage is interpreted to be the result of sequential events in a subduction zone. As subduction began, oceanic crustal materials underwent high-temperature metamorphism along the hot ultramafic hanging wall and were converted to amphibolites; materials that were subducted later came in contact with a cooler hanging wall and recrystallized as blueschist. This hypothesis may be applicable to the origin of similar rock associations in the Franciscan terrane and other orogenic belts.

Pluton emplacement by sheeting and vertical ballooning in part of the southeast Coast Plutonic Complex, British Columbia

Batholiths of the Coast Plutonic Complex, in the area of Harrison Lake, British Columbia, are interpreted to have formed by horizontal sheeting and vertical inflation. Zoned ovoid batholiths exhibit sheeted margins as thick as 3 km. Shallow floors are indicated by moderately to gently inward dipping magmatic foliations, the observed trace of floor contacts across topography, and a published deep seismic reflection transect. Mineralogic aureoles under the floors preserve a paragenetic record of increasing pressure during aureole crystallization. Initial pressures of

Intra-arc crustal loading and its tectonic implications, North Cascades crystalline core, Washington and British Columbia

Widespread high-pressure Cretaceous metamorphism in the Coast Plutonic Complex of southeast Alaska and British Columbia and its southeast extension, the Cascades crystalline core, is commonly attributed to thrust loading during superterrane collision. However, terrane boundaries within the Cascades core are intruded by relatively shallow mid-Cretaceous plutons, and crustal loading of ∼2-5 kbar postdates these plutons. These observations are not consistent with proposed collisional models, and we suggest that loading occurred by structural and/or plutonic processes operating within a magmatic arc. Such loading may be an important process in arc tectonics, although it has not been widely reported.

Plagiogranite and keratophyre in ophiolite on Fidalgo Island, Washington

A sequence of Jurassic rocks on Fidalgo Island, Washington, is interpreted to be ophiolite. The order of rock types, from the base upward, is serpentinite, layered gabbro, a dike complex made up mostly of plagiogranite, volcanic rocks that are dominantly keratophyre, coarse breccia with clasts of keratophyre and plagiogranite, pelagic argillite, and siltstone-sandstone turbidites. The plagiogranites and keratophyres have identical chemical compositions and are mutually gradational in field setting and textures, all of which suggests that they are cogenetic. These rocks are distinguished from calc-alkalic rock types by their very low content of K 2 O (where SiO 2 = 70%, K 2 O = 0.2% to 0.7%). Metasomatic alteration of the rocks appears to be insignificant, judging from (1) well-preserved primary igneous textures, (2) well-preserved primary intrusive and extrusive contacts, and (3) uniformity of chemical composition across igneous units. An oceanic origin of the ophiolite is suggested by the capping of pelagic sediments. Their fine grain size, abundance of radiolaria, and enrichment in Mn and other metals are virtually identical to those of modern Pacific pelagic sediments and unlike that of arc or epicontinental sediments. This interpretation conflicts with the apparent paucity of plagiogranite and keratophyre on the present-day sea floor. Field relations and chemical trends indicate that the plagiogranite-keratophyre magma is not the product of fractionation of the same melt that crystallized layered gabbro. High water content of the plagiogranite-keratophyre magma is indicated by hydrothermal alteration of the gabbro near plagiogranite intrusions and the occurrence of hornblende instead of pyroxene in mafic varieties. We suggest that this water is from the sea and that the anomalously low K 2 O content of these magmas is due to exchange with sea water.

Is the southeast Coast Plutonic Complex the consequence of accretion of the Insular superterrane? Evidence from U-Pb zircon geochronometry in the northern Washington Cascades

Zircon U-Pb geochronometry of orthogneisses and plutons in the southwestern crystalline core of the North Cascades, coupled with fabric and textural studies of the orthogneisses, plutons, and their metamorphic host rocks, indicates extensive synmetamorphic plutonism at 89-96 Ma. Metamorphic mineral assemblages define a culmination composed of an axial kyanite-sillimanite zone rimmed by lower grade zones. High-grade index minerals are typically syntectonic to posttectonic. Metamorphic fabrics are characterized by an orogen-parallel, northwest-striking, steep foliation that contains a subhorizontal stretching and mineral lineation interpreted to be the product of ductile strike-slip deformation. This fabric is crosscut by 96-92 Ma plutons yet is imprinted on 92-89 Ma orthogneisses, suggesting spatially diachronous fabric development during orogeny. Documentation of the spatial and temporal coincidence of magmatism with the peak of orogeny, together with the kinematic significance of the metamorphic fabric, precludes generation of the metamorphic fabric and plutons in response to thrust loading. The authors suggest that this part of the Coast Plutonic Complex evolved as a transpressional magmatic arc.

A new structural and tectonic interpretation of the western part of the Shuksan blueschist terrane, northwestern Washington

The Shuksan Metamorphic Suite (SMS) is a blueschist terrane exposed in the northwest Cascades of Washington. Along its eastern flank, the SMS is known from previous studies to consist of meta-MORB and pelitic and metalliferous metasedimentary rocks that resemble ocean-floor deposits. In the western part of the SMS, however, the metasedimentary rocks lack metalliferous enrichment and contain interbeds of mafic to felsic lithic: tuff and volcanic-derived sandstone. Relatively small, isolated bodies of meta-igneous rock in this area comprise a variable suite of arc-related mafic to felsic plutonic and volcanic rocks. These have been assigned previously to units other than the SMS, postmetamorphic faulting being invoked to explain their emplacement. In the South Chuckanut, Lyman, and Bowman Mountain areas, however, the SMS metasediments contain clasts derived from the nearby meta-igneous units, metamorphic structures have a similar orientation in both the metasedimentary and metaigneous rocks, and the metamorphic facies in the metasedimentary and meta-igneous units are the same. These observations require (1) reassignment of the meta-igneous units to the Shuksan Suite, (2) rejection of previous interpretations of the contact between the metasedimentary and meta-igneous rocks as being an expression of either the Shuksan thrust or Haystack thrust, and (3) broadening of the tectonic setting of deposition of Shuksan protolith rocks to include an arc component. A maximum protolith age for the western part of the SMS of 163 ± 2 m.y. is established by a U/Pb zircon age of metaquartz diorite from Bowman Mountain. We infer deposition of the SMS protolith in a Late Jurassic marginal basin behind a west-facing arc.

Metamorphic facies and tectonics in part of the Cascade Range and Puget Lowland of northwestern Washington

Metamorphic assemblages in pre-Tertiary rocks of northwestern Washington can be grouped into eight facies types which represent recrystallization under widely diverse P-T conditions. Consideration of these assemblages, together with their regional distribution and age, allows speculation concerning some aspects of the regional metamorphic and tectonic history: (1) high-pressure assemblages indicate that much of the metamorphosed rock has been affected by subduction; (2) two ages of subduction are represented, Carboniferous to Permian and Early Cretaceous; (3) during the Cretaceous event, rock masses of local to regional extent gained mutually distinctive metamorphic assemblages, indicating that the rock units acted as separate tectonic elements during the subduction process; (4) sea-floor metamorphism may be indicated by the occurrence of low-pressure assemblages in ophiolitic rocks; (5) in some rock units, low-pressure assemblages are overprinted by high-pressure minerals, suggesting a history of sea floor followed by subduction metamorphism; (6) some rock units contain only low-pressure assemblages and thus may have escaped subduction; (7) diverse origins of rock units and profound movement on unit-bounding faults are suggested by the disparity of ages and facies type.

Geochemistry and tectonic interpretation of some metavolcanic rock units of the western North Cascades, Washington

Metavolcanic rocks of the western North Cascades of Washington, including the Chilliwack Group (CG), the Elbow Lake Formation (ELFm), and the Yellow Aster Complex (YAC), have been analyzed for clinopyroxene composition and whole-rock major, trace, and rare-earth elements (REE9s) to determine their original chemistry and tectonic setting. Covariant trends, elemental abundances, and ratios of high field-strength elements (HFSE9s) and REE9s indicate that the previously correlated CG and ELFm, and possibly equivalent basalts of the YAC, are separate rock units which evolved in distinct tectonic settings. Volcanic flows within the upper part of the upper Paleozoic CG are composed of tholeiitic basalt with lesser basaltic andesite and minor dacite. The CG originated as an intraoceanic island arc. Volcanic rocks of the ELFm are largely alkalic basalt, distinguished from the CG by HFSE enrichment and low Y/Nb and Zr/Nb. The ELFm is interpreted to represent structurally dismembered oceanic islands of Pennsylvanian to Jurassic age. An affinity with the Cache Creek terrane is suggested by correlation with the Orcas Chert of the San Juan Islands, which bears a Tethyan fauna. Basalt dikes of the YAC are of continental affinity, perhaps produced during an episode of continental rifting. High Cr and Ni contents, large-ion lithophile element enrichment, and transitional REE patterns distinguish YAC dikes from the CG and ELFm.