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Plagiogranite and Keratophyre in Ophiolite on Fidalgo Island, Washington

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Plagiogranite and Keratophyre in Ophiolite on Fidalgo Island, Washington Plagiogranite and keratophyre in ophiolite on Fidalgo Island, Washington E. H. BROWN I J. Y. BRADSHAW f Department of Geology, Western Washington University, Bellingham, Washington 98225 G. E. MUSTOE J ABSTRACT small (<10%; Coleman and Peterman, 1975), but this is based solely on consideration of the plagiogranites. If the keratophyres A sequence of Jurassic rocks on Fidalgo Island, Washington, is are counted also, the fraction of silicic rock in many ophiolites is on interpreted to be ophiolite. The order of rock types, from the base the order of 20% or higher. upward, is serpentinite, layered gabbro, a dike complex made up Recent studies of ophiolites have shown that the origin of mostly of plagiogranite, volcanic rocks that are dominantly keratophyres and plagiogranites is problematical. Those who have keratophyre, coarse breccia with clasts of keratophyre and plagio- interpreted ophiolites to have originated at oceanic ridge systems granite, pelagic argillite, and siltstone-sandstone turbidites. The (for example, Moores and Vine, 1971) cannot easily account for plagiogranites and keratophyres have identical chemical compo- the relative abundance of plagiogranite and keratophyre in ophio- sitions and are mutually gradational in field setting and textures, all lites as compared to the rarity of these rock types in collections of of which suggests that they are cogenetic. These rocks are distin- dredged and drilled oceanic rocks. Others (especially, Miyashiro, guished from calc-alkalic rock types by their very low content of 1973) have cited the silicic rocks as evidence of an island-arc origin K20 (where Si02 = 70%, KzO = 0.2% to 0.7%). Metasomatic for ophiolites. alteration of the rocks appears to be insignificant, judging from (1) A variety of origins for the silicic rocks have been suggested: (1) well-preserved primary igneous textures, (2) well-preserved pri- keratophyres and plagiogranites crystallized from magma as typical mary intrusive and extrusive contacts, and (3) uniformity of chemi- ocean-floor basalts and dikes and have become enriched in silica by cal composition across igneous units. metasomatism (Gass and others, 1975; Moores, 1975; Smewing An oceanic origin of the ophiolite is suggested by the capping of and others, 1975); (2) keratophyres formed by metasomatism of pelagic sediments. Their fine grain size, abundance of radiolaria, typical calc-alkalic rhyolites, dacites, and andesites (Gilluly, 1935a, and enrichment in Mn and other metals are virtually identical to 1935b; Battey, 1955; Hughes, 1973; Carmichael and others, 1974, those of modern Pacific pelagic sediments and unlike that of arc or p. 560; Miyashiro, 1975), chiefly through removal of K and addi- epicontinental sediments. This interpretation conflicts with the ap- tion of Na; (3) albite granite originated by metasomatism of more parent paucity of plagiogranite and keratophyre on the present-day mafic rocks (Gilluly, 1933); (4) plagiogranite formed by fractional sea floor. crystallization of a gabbroic melt beneath an oceanic ridge with a Field relations and chemical trends indicate that the slow spreading rate (Coleman and Peterman, 1975); (5) plagio- plagiogranite-keratophyre magma is not the product of fractiona- granite crystallized from a melt, which seems to be related in some tion of the same melt that crystallized layered gabbro. High water way to, but is not comagmatic with, the gabbros (Thayer, 1963, content of the plagiogranite-keratophyre magma is indicated by 1973); (6) plagiogranitic magmas are created by fractional melting hydrothermal alteration of the gabbro near plagiogranite intrusions of ultramafic rock in the mantle beneath oceanic ridge systems and the occurrence of hornblende instead of pyroxene in mafic va- (Ishizaka and Yanagi, 1975); and (7) keratophyric magma is rieties. We suggest that this water is from the sea and that the formed by fractional melting of hydrated mantle material beneath anomalously low K20 content of these magmas is due to exchange the abyssal sea floor (Donnelly, 1963, 1966). with sea water. Evidence relating to the origin of plagiogranite and keratophyre in ophiolites as observed in the San Juan Islands, Washington, is INTRODUCTION the subject of this report. The pre-Tertiary regional geology of northwestern Washington Occurring in most ophiolite complexes are relatively silicic rocks and adjacent British Columbia is extremely complex and far from (>52% Si02) of both volcanic and intrusive origin. The volcanic being clearly explained. For background the reader is referred to and fine-grained intrusive rocks, generally potassium deficient, are Huntting and others (1961), Raleigh (1965), Misch (1966), McKee termed keratophyre. The coarse-grained intrusive rocks comprise a (1972), Mulcahey (1974), Vance (1975), Vance and others (1975), suite of K-feldspar—free granitic types ranging from albite granite to and Brown (1977). The rocks of interest in this study make up the trondhjemite to quartz diorite and have been collectively termed Fidalgo Complex (Brown, 1977) of Jurassic age which occurs plagiogranite (Thayer, 1973; Coleman and Peterman, 1975). The sporadically throughout the San Juan Islands and on the adjacent volume of silicic rocks in ophiolites is generally estimated to be mainland. Geological Society of America Bulletin, Part I, v. 90, p. 493 -507, 15 figs., 2 tables, May 1979, Doc. no. 90511. 493 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/5/493/3418946/i0016-7606-90-5-493.pdf by guest on 28 September 2021 I.'.'.'1 Surficial deposit I- . 1 Siltstone and graywacke I I Pelagic argillite I:-'-;:i| Sedimentary breccia Keratophyre and spilite WASHINGTON PARK I \ Trondhjemite ICoarse poikilitic diorite lv:; Plagiogranite undifferentiated frwll Gabbro and pyroxenite R88&I Serpentinite X Igneous flow foliation, vertical y Igneous and sedimentary bedding y / , Fault / Contact Roads ëêâîJïïïsà Figure 1. Geology of Part of Fidalgo Island, Washington, and location map. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/5/493/3418946/i0016-7606-90-5-493.pdf by guest on 28 September 2021 PLAGIOGRANITE AND KERATOPHYRE, FIDALGO ISLAND 495 FIELD RELATIONS AND PETROGRAPHY study of Cypress Island by other workers has shown that (1) unser- pentinized parts of the mass are predominantly harzburgite with a Completed mapping on Fidalgo Island is shown in Figure 1, and tectonite fabric, but possible relict igneous textures are visible the stratigraphy is interpreted in Figure 2. This area displays a sec- (Raleigh, 1965, p. 730), and (2) the mass is a nearly horizontal tion of the Earth's crust tilted on edge, which we interpret to be an slablike body (Carlson, 1972). We interpret this peridotite to be the ophiolite. At the base is serpentinite exposed on islands in the west- basal part of an ophiolite sequence that was originally overlain by ern part of the map area. Structurally above the serpentinite and the Jurassic gabbros and plagiogranites of Fidalgo and Blakely Is- separated from it by water or glacial drift are exposures of layered lands. Although this hypothesis is difficult to prove, because of dis- gabbro. The layering strikes northwest and is graded upward to the continuity of exposures and structural dislocations, it is favored by northeast. Plagiogranite dikes intrude the gabbro approximately the following evidence: (1) the close spatial association of the ul- normal to the layering with nearly vertical and northeast-striking tramafic rock with gabbros on Blakely and Fidalgo Islands; (2) the attitudes. These dikes become increasingly abundant upward fact that on Fidalgo Island serpentinite lies downsection from the (northeast) and are the dominant lithology in a 2- to 3-km-thick gabbro, as indicated by graded bedding in the gabbro; and (3) the section of rock above the gabbro. In this upper section the gabbro occurrence of relict igneous texture, observed by Raleigh (1965), in occurs sparsely as inclusions or screens between dikes. Locally fresh harzburgite on part of the Cypress Island body, which overlying the plagiogranite is a volcanic unit of keratophyre and suggests a cumulate origin. Unfortunately, a direct, unfaulted con- spilite (Fig. 2). Elsewhere, the plagiogranite dikes are truncated at tact of peridotite with gabbro has not been seen. approximately right angles by an unconformity, directly above which is a coarse breccia unit with clasts of plagiogranite and Gabbro keratophyre. Conformably overlying the breccia is a pelagic sedimentary unit of predominantly brown argillite. Above the ar- Clean, wave-washed exposures of gabbro north of Alexander gillite is a unit of flysch-type sediments consisting of beds of Beach (Fig. 3) clearly display cumulus layering. Graded beds face to graywacke, conglomerate, and siltstone. the northeast, thus indicating the upward direction of the original Radiolaria in the pelagic sedimentary rock are Tithonian (E. A. gabbroic magma chamber. In a few places the layered gabbro is Pessagno, 1977, personal commun.), whereas the plagiogranite folded in a style indicative of plastic (soft sediment?) deformation, yields a K-Ar age of 155 ± 5 m.y. (Brown, 1977) and a U-Pb age of typical of ophiolite gabbros (compare Hopson, 1975). The folds 170 ± 10 m.y. (Whetten and others, 1976). Thus, the ophiolite are crosscut by gabbroic pegmatite dikes. Thin-section study shows section is Middle to Late Jurassic in age. two types of alteration of the gabbro. One is a low-grade regional metamorphism resulting
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