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.

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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 in (1) complete replacement of Ca- Serpentinite plagioclase by a fine-grained aggregate of albite + epidote; (2) partial replacement of clinopyroxene by actinolite; and (3) partial re- The serpentinite exposed in the western part of the map area placement of orthopyroxene by chlorite. Olivine is not seen but (Fig. 1) is part of a belt of ultramafic rock (named the Fidalgo For- could be completely altered. Grain outlines of the original gabbroic mation by McClellan, 1927) that includes Cypress Island. Detailed minerals are still well preserved, and in a few places a hypidiomorphic texture, with interstitial plagioclase, can be observed. The sec- ond type of alteration is contact metamorphism caused by intrusion of plagiogranite and amphibolite dikes. The gabbro affected in this Siltstone and greywacke way shows hornblende replacing pyroxene. In the incipient stages of development, the hornblende occurs as narrow borders around Pelagic argillite pyroxene grains, and with more advanced replacement, it is found Sedimentary breccia as large poikilitic grains (1 cm in size) with relict pyroxene inclu-

Unconformity sions. Pyroxenite occurs as mafic layers in the gabbro and as intrusive Keratophyre and spilite flows, sills and dikes that are much coarser grained (grains as long as 10 flow breccias, dikes cm) than the gabbro. In the sills and dikes the dominant pyroxene is Unconformity clinopyroxene (>80%), whereas in the gabbro, orthopyroxenes and clinopyroxenes are nearly equal in abundance. Plagiogranite Dike Rocks

Layered gabbro and pyroxenite The layered gabbro, gabbroic pegmatite, and pyroxene sills and dikes are crosscut by dikes of mafic to felsic rocks which intrude roughly normal to the gabbroic layering (Fig. 3) and become increasingly abundant upward in the gabbroic section (Figs. 1, 2). The dikes comprise a suite of rock types intergradational in both texture and mineralogy, including hornblende gabbro, diorite, I I trondhjemite, albite granite, diabase, keratophyre, and basalt. Serpentinite Modes of representative samples are given in Table 1. Of all these diverse lithologic types, the diorite, containing 56% to 58% Si02, greatly predominates (~90%). Figure 2. Stratigraphie section of Fidalgo Island map area. The size, shape, and mutual relations of the dikes are mostly

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| | Basalt Layered Gabbro

/| Trondhjemite Pyroxenlte

PTWI ¡,¡¡11 Grey Foliated Porphyry 10

Figure 3. Geology of coastal area north of Alexander Beach, Fidalgo Island, Washington.

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obscured by poor exposure, except in the intertidal zone, where ex- bers of the suite of rock types studied here. However, for conveni- cellent exposures across the strike show the dikes very clearly. The ence in this report, the more mafic diorites that are part of the spec- dikes crosscut the gabbro (Figs. 3, 4) at high angles to the layering, trum are also included under this general designation. with knife-sharp contacts. Flow foliation is strongly developed in In the plagiogranite the igneous minerals present prior to low- some dikes, parallel to dike walls, and is absent in others. Xenoliths grade metamorphism were quartz, plagioclase, hornblende, Fe-Ti of gabbro occur in the dikes. In addition, some dikes show evidence oxide, and rarely minor biotite. Some dikes of albite granite occur of autobrecciation, similar to that in the Bay of Islands described by as zoned pegmatites, with quartz cores and albite margins. Plagioc- Williams and Malpas (1972). Away from the tidal zone, exposures lase occurs as euhedral grains partially to completely are generally poor, but a few dike contacts were mapped (Fig. 1) on pseudomorphed by a fine-grained mass of albite + epidote. The the basis of light versus dark rock and coarse versus fine grain size, original grain boundaries are well preserved and sharp, and relict suggesting that the zone of plagiogranite is a complex of dikes twinning and concentric zoning are commonly seen. Grain clusters rather than one or more stocklike bodies. may represent a synneusis texture. Hornblende is altered in varying Plagiogranite Dikes. The term "plagiogranite," in the sense degrees to actinolite + chlorite + epidote. In some rocks used by Thayer (1973) and Coleman and Peterman (1975), applies hornblende grains are zoned from brown cores to green rims. strictly to the quartz diorite, trondhjemite, and albite granite mem- Grains of hornblende in most specimens are euhedral to subhedral (Fig. 5), but large poikilitic anhedral hornblendes are common in some diorites. Quartz is generally anhedral and is interstitial to euhedral plagioclase and hornblende (Fig. 5). In a few specimens of albite granite, quartz grains appear to be subhedral, set in a fine matrix of altered plagioclase. In summary, relict igneous textures are clearly seen through the imprint of low-grade metamorphism. There is no indication of a metasomatic event that might have

added Si02, removed K-feldspar, or otherwise substantially changed the composition of the original igneous rock. Mafic Dikes. Dikes of fine- to medium-grained mafic rock (48%

to 56% Si02; termed "foliated hornblende gabbro" in Figs. 3 and 4) are relatively common in the cumulus gabbro but are rare higher in the section. Most of these dikes predate plagiogranite dikes and in many places are intruded by plagiogranite (Fig. 4), even to the extent of forming a type of injection lit-par-lit migmatite (Fig 6). Apparently, the fracture system that allowed intrusion of the mafic dikes was opened again for injection of plagiogranite magma. Mafic dikes predating plagiogranite now contain amphibole, plagioclase, magnetite or ilmenite, and rarely biotite, and they lack pyroxene (or olivine). Some dikes having a directionless fabric and medium grain size, except for chilled margins, could be termed diabase. Other dikes have a well-developed foliation. This foliation is parallel to the dike walls (Fig. 4), even to the extent of following apophyses of the dike into the host gabbro. The foliation that af- fects the dikes is found neither in the host nor in gabbro xenoliths in the mafic dikes. The cumulus gabbro is hydrothermally altered next to the mafic dikes such that pyroxenes are completely replaced by Ca-amphibole. The texture of the dikes appears to be metamorphic (lepidoblastic) rather than igneous (laminated). Also, the compositions of Ca-amphiboles in the dikes are typically metamorphic; the number of Allv cations in the formulas is 0.5 to 1.0, Si = 7.5 to Figure 4. Structural relations of trondhjemite, foliated 7.0. hornblende basalt, and gabbro. Location shown in Figure 3. With the above considerations, the origin of the foliated mafic

TABLE 1. MODES OF PLUTONIC IGNEOUS ROCKS

Specimen Layered Foliated Diorite Trondhj'emit e Albite granite gabbro hornblende basalt-diorite 3C 10B 2A 20A 3A 7 14C 49 15C 18C

Quartz 14 13 15 33 36 46 36 Plagioclase 61 64 46 47 52 41 59 56 50 60 Hornblende 40 50 34 44 8 3 4 Biotite 8 Clinopyroxene 22 14 Orthopyroxene 17 15 Opaque 7 3 1 1 Note: Values are percent.

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dikes poses a problem. The amphibole compositions and textures rock is obscured by faulting. This rock unit ranges widely from

within the dike suggest metamorphism; but the geometry of the mafic (48% Si02) to felsic (70% Si02) types, but the more felsic fabric and its absence from the host gabbro indicate an igneous varieties (60% to 65% Si02) predominate. origin for the amphibole and the foliation. One possible explana- In thin section the igneous minerals seen are clinopyroxene, pla- tion is the following: first, water-rich gabbroic melt crystallizing gioclase, quartz, and opaque minerals. Plagioclase and pyroxene amphibole and plagioclase intruded in a mushlike state, developing occur as phenocrysts, commonly in synneusis clumps. The a flow foliation, and subsequent recrystallization, largely mimetic, groundmass is composed of plagioclase rr.icrolites and quartz, in allowed the amphibole composition and texture to become more some rocks showing trachytic texture. In the more mafic varieties, typically metamorphic. original intermediate plagioclase is replaced by a fine-grained mass Mafic dikes postdating plagiogranite intrusions range from the of albite and epidote, and the groundmass contains abundant chlo- amphibole-bearing variety described above to fresh clinopyrox- rite. The most felsic keratophyres have fresh-looking albite pheno- ene-bearing basalts. crysts (Fig. 7). Chlorite, quartz, calcite, iron-rich epidote, prehnite, and zeolites occur in patches and veins in the rock and are inter- Volcanic Rocks preted to be the product of low-grade regional metamorphism (compare Coombs, 1974). These metamorphic minerals make up The volcanic rocks present in the section are keratophyre and generally less than 5% of the keratophyres. spilite. They occur mainly as flows and flow breccias stratigraphically above the plagiogranite (Figs. 1, 2) in the vicinity of Mt. Erie Sedimentary Rocks and also as relatively abundant dikes, some of them brecciated, in- truding the plagiogranite. The original thickness of the extrusive Breccia. Directly overlying the plagiogranite in part of the map area (Figs. 1,2) is a breccia unit with coarse fragments (as long as

Figure 5. Photomicrograph of quartz diorite, specimen 10B, showing hypidiomorphic granular texture made by euhedral Figure 6. Migmatite formed by intrusion of trondhjemite in hornblende, subhedral plagioclase (altered), and anhedral quartz. foliated hornblende gabbro.

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tional. Clasts in the breccia are highly angular. Because the breccia mite, albite granite) and the volcanic rocks (spilite and unit is itself overlain by pelagic sediment and locally has this mate- keratophyre) is essentially the same (Table 2). These two rock rial as matrix, it is interpreted to have formed as a submarine talus suites also plot in the same fields and show the same trends on dif- or slide, presumably along some type of fault scarp. Similar de- ferent types of chemical variation diagrams (Figs. 8, 9, 10, 11), all posits occurring on the present-day abyssal sea floor have been de- of which indicates consanguinity. Furthermore, their textures ap- scribed and termed edaphogenous by Murdmaa (1976). pear to be convergent; all gradations of grain size from coarse Pelagic Sedimentary Rocks. Pelagic sedimentary rocks compose plagiogranite to fine keratophyre are observable. On Fidalgo Island a unit about 300 m thick comformably overlying the breccia unit. keratophyre occurs as dikes in the upper part of the plagiogranite This rock is predominantly light- to dark-brown argillite made up complex, whereas on Cypress Island we have observed plagiogra- of more than 90% clay-size material plus radiolaria. Interbeds of nite to be intrusive into spilite-keratophyre pillow lavas. Thus, the other rock types (5 to 20 cm thick), in order of decreasing abun- field relations suggest coeval formation of these plutonic and vol- dance, are (1) green to gray tuff composed mostly of volcanic rock canic rocks. All of these relations suggest that magmas yielding this fragments and euhedral plagioclase crystals; (2) radiolarite, with range of rock types, plutonic and volcanic, mafic to felsic, were more than 80% of the rock being radiolaria; (3) highly siliceous cogenetic.

argillite, Si02 ~ 85%; (4) sandstone consisting mostly of grains of carbonate and sand-size grains of clay minerals, with minor METASOMATIC VERSUS MAGMATIC ORIGIN OF amounts of clinopyroxene, andradite garnet, and chrome spinel; THE IGNEOUS ROCKS and (5) breccia composed of grains of plagiogranite and keratophyre. Features of this rock suggesting a pelagic origin are The possibility of a metasomatic origin for spilite, keratophyre, the predominance of clay-size sediment, abundance of radiolaria, and plagiogranite has been much debated. By comparing the chem- and chemical composition (as discussed below). ical compositions of closely spaced samples in spilite and Siltstone and Graywacke. Siltstone and graywacke-type keratophyre, Battey (1955) and Smith (1968) provided good evi- sandstones make up a thick sedimentary section stratigraphically dence for chemical mobility in some examples of this rock type. above the pelagic sediment. In the map area of Figure 1 this unit is Dickinson (1963) showed that chemical changes in a particular separated from older rocks by a fault, but elsewhere on Fidalgo Is- keratophyre in Oregon can be correlated with degradation of pri- land interbeds of pelagic sediment and siltstone can be seen, and mary igneous textures. Thayer (1963) cited field and textural evi- thus the contact between these two stratigraphic units is thought to dence that certain "diorites" at Canyon Mountain are be conformable. In the lower parts of this unit the rock is nearly all metasomatized gabbro. These rocks, which he termed "epidiorite," siltstone. Higher in the section beds of sandstone and even con- occur at the margin of plagiogranite intrusions into gabbro and glomerate appear. This presumably represents a transition upward show transitional textures and mineralogy to unaltered gabbro. in the section from distal to proximal turbidite sedimentation. However, Thayer regarded the plagiogranite intrusions to be of Fragments in this rock are mostly plagioclase and quartz. Chert, magmatic origin. Hughes (1973) argued that keratophyres are

siltstone, volcanic rock, plagiogranite, and rarely epidote are other metasomatic because K20/(K20 + Na20) is unusually low in com- types of clasts seen in certain beds. Some beds are tuffaceous, con- parison to other types of igneous rock. Evidence cited in favor of a taining euhedral grains of plagioclase and clinopyroxene as well as magmatic origin of spilites and keratophyres is the preservation of fine-grained volcanic fragments and large clay grains presumably primary igneous textures (for example, Amstutz and Patwardhan, altered from glass. 1974). On Fidalgo Island the clearest example of a rock produced by Chemical Analyses metasomatism is the altered gabbro described above, formed near contacts with plagiogranite intrusive bodies. This rock is the same Whole-rock chemical analyses of 130 specimens were completed by energy dispersive X-ray fluorescence (EDAX equipment), refer- enced to a suite of 15 international standards. As a check on the

accuracy of the procedure and to improve the analyses of Na20 and MgO, 45 of these specimens were also analyzed by both atomic absorption spectrophotometry and by wave-length dispersive X-ray fluorescence. The analytical uncertainty, in terms of the percentage of the amount present of each oxide, for a 2cr variation, is judged to

be approximately as follows: Si02 2%; Al2Os 5%; Ti02 6%; FeO

4%; MgO 4% to 10%; CaO 2%; Na20 5% to 10%; K20 5%. Minor elements were measured by wavelength X-ray fluorescence and atomic absorption spectrophotometry. Uncertainties at the 2a level are about 15% of the amount present for Cr, Co, Ni, Cu, Y, Zr, Mo, and Ba and about 5% for Rb and Sr. The strontium isotopes were measured at the University of Arizona by D. E. Livingston.

RELATION OF PLAGIOGRANITE TO KERATOPHYRE

The range of chemical compositions exhibited by the suite of Figure 7. Photomicrograph of keratophyre showing glomer- plutonic dike rocks (basalt, hornblende gabbro, diorite, trondhje- oporphyritic texture.

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Oceonic Figure 8. Na20 versus FeOt/MgO in NdgO. h Gabbro * Fidalgo rocks and oceanic gabbro (data from Engel and Fisher, 1969; Miyashiro and others, 1970; Thompson, 1973). FeO, is total iron calculated as FeO.

2.0 4.0

FeO/MgO

~ " O

70.0 A

A Fidalgo Dikes and Volcanic Rocks O

/ O Trondhjemite and Figure 9. Si02 versus FeOt/MgO in 60.0 / Albite Granite Fidalgo /* • Fidalgo rocks, oceanic gabbro (Engel and /• • A / % Diorite SiOc Gabbro / ' A Keratophyre Fisher, 1969; Miyashiro and others, 1970; Pyroxenite / • Spilite Thompson, 1973), Skaergaard intrusion y X Foliated Hbld Gabbro » (Wager and Brown, 1967), and ophiolite \ x • Layered Gabbro gabbro in California (Bailey and Blake, 1974). 50.0 ^Oceanie Gabbro(l)

— • • • lAJiiiuiniu upiiiunie uuuuimo; 40.0 1 1 1 1 0.0 1.0 2.0 3.0 4.0

FeOt/MgO

TABLE 2A. CHEMICAL COMPOSITION OF

Gabbro Gabbro Gabbro Gabbro Pyroxene Gabbro Gabbro Gabbro Foliated hornblende IB mafic leucocratic 14C dike pegmatite 49 50 2B 2D 5A layer layer 14A 14D 4A 4B

Major-element oxides (wt %)

Si02 49.2 49.0 51.0 49.6 53.0 51.0 45.5 47.1 55.8 54.0 50.5 AI2O3 16.7 16.7 26.3 19.6 2.20 20.0 18.6 17.6 12.3 11.2 15.7 TÌO2 0.09 0.10 0.04 0.07 0.18 0.23 0.47 0.30 0.50 0.57 0.45 Fe as FeO 7.40 6.82 1.89 5.86 8.35 5.86 10.6 9.50 7.58 8.22 9.36 MgO 10.2 11.3 2.50 8.40 19.0 5.93 7.07 8.08 8.18 12.0 8.20 MnO 0.12 0.13 0.07 0.10 0.18 0.10 0.16 0.12 0.14 0.15 0.16 CaO 12.3 12.2 14.7 13.3 15.8 13.6 12.5 13.0 7.58 8.30 8.40 Na20 1.66 1.99 2.25 2.23 0.85 2.50 2.10 2.05 3.84 2.99 3.82 K2O 0.62 0.29 0.49 0.24 0.05 0.30 0.41 0.46 0.53 0.33 0.78 Total 98.3 98.5 99.2 99.4 99.6 99.5 97.4 99.0 96.5 97.8 97.4

Trace elements (ppm) Cr 270 160 47 110 859 70 530 800 152 Ni 63 394 229 420 65 Rb 8 1 6 2 0 0 8 2 6 Sr 305 314 506 445 12 290 210 130 197 Y 0 3 0 0 6 4 10 13 7 Zr 7 11 6 5 5 17 45 110 27

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in appearance and geologic setting as the "epidiorite" at Canyon from a typical rhyolite or dacite to produce the present Mountain. keratophyre, which is virtually free of K-feldspar. Textures provide A magmatic origin is indicated, however, for the granitic mate- evidence against such a hypothesis. Primary porphyritic and rial at Fidalgo Island. Plagiogranite dikes crosscut structures in the trachytic igneous textures are well preserved. There are no signs of gabbro and contain rotated xenoliths of gabbro. Contacts are former K-feldspar grains that have been replaced, even in the most

knife-sharp. Textures are hypidiomorphic granular, as in hypabys- felsic rock (73% Si02). sal intrusive bodies. There is no textural evidence of appreciable To provide chemical evidence on this difficult question of influx of Si02 or removal of K20. Primary igneous extrusive rela- metasomatism, samples were taken at close intervals across three tions are also well preserved in keratophyre flows: contacts be- interlayered flows and analyzed for their major-oxide constituents. tween flow units are sharp, and igneous breccia structures are unal- Of the three flows the lowermost and uppermost are spilite with,

tered. for this region, high K20 (1% to 2%). The middle flow is Phenocrysts of quartz in the keratophyre attest to the fact that keratophyre with typical low K20 (0.5%). The data points shown the siliceous nature of the rocks is a primary magmatic feature. A in Figure 12 represent two traverses 5 m apart across the flows. It is

more difficult question is whether K20 has possibly been leached assumed that each flow initially crystallized from homogeneous magma. Si, Al, Fe, Mg, and Ti are relatively uniform within each flow and show a sharp break in abundance at the boundaries between flows. This behavior suggests that the present distribution of these elements represents that of the magmas. Ca, Na, and K are more variable, and such variation indicates postmagmatic mobility of these elements. The most important question here is whether the

low K20 content of the keratophyre is due to metasomatism. It is noteworthy that K20 is diminished and Na20 concomitantly in- creased at the flow tops. Donnelly (1972) has observed the same relationship in spilite and keratophyre in the Virgin Islands. Below the flow tops, with the exception of one data point in the upper spi-

lite, the K20 values are relatively uniform within each flow and show a distinct break at the contacts. It would seem unlikely that a metasomatizing agent would leave a uniform but distinctly differ-

ent amount of K20 in each of these adjacent flows. The interpretation that seems best to us is that the keratophyre crystallized from

magma with about 0.5% K20 and the spilite with about 1.8% K20 and that metasomatism has occurred only at the flow tops due to

interaction with hot sea water, which caused exchange of Na20 for K20. This type of exchange, also postulated by Donnelly (1966) for volcanic rocks in the Virgin Islands, has been observed in experi- FeO/MgO mental studies of sea-water-basalt interaction (Bischoff and Dickson, 1975). Figure 10. Ti02 versus FeOt/MgO in Fidalgo rocks and oceanic rocks (data from Engel and Fisher, 1969; Miyashiro and others, In summary, field, texture, and chemical composition observa- 1970). Symbols and patterns as in Figure 9. tions suggest that most of the igneous rocks on Fidalgo Island

ROCKS FROM FIDALGO ISLAND

gabbro-diorite Pyroxene Diorite 15C 18C 18D basalt 3B 3C 10B 25A 28A 6K 9K

60.4 49.5 50.0 48.4 51.0 50.7 56.6 58.7 58.2 59.7 14.8 15.0 15.9 15.0 15.5 15.0 16.5 13.4 14.0 14.9 0.40 1.53 0.43 1.04 1.03 0.75 0.50 0.49 0.46 0.37 6.58 8.83 8.88 10.3 10.8 9.63 7.37 7.66 7.46 7.66 6.01 7.82 8.34 8.23 7.03 9.40 4.70 7.90 6.63 5.09 0.14 0.15 0.16 0.16 0.18 0.18 0.12 0.15 0.14 0.16 7.40 11.1 10.4 9.29 9.09 9.80 7.50 7.25 9.10 8.70 3.97 3.07 3.63 3.97 3.46 2.54 3.23 2.98 4.15 3.26 0.84 0.67 0.70 0.38 0.18 0.17 0.68 0.82 0.24 0.41 100.6 97.7 98.4 96.8 98.3 98.2 97.2 99.4 100.4 100.3

320 170 130 185 110 73 520 239 71 154 79 78 73 40 87 37 8 8 0 0 0 7 249 283 138 208 205 178 17 7 18 18 17 9 81 24 43 53 38 55 41 50 40

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Trondhjemite Albite granite Spilite Keratophyre 2A 2C 3G 31 20A 20B 3A 7 9B 3H 76 98 103G 102B 102 A 103D 99

Major-element oxides (wt %)

Si02 68.6 64.5 65.0 69.0 66.8 62.9 73.0 72.7 70.0 54.8 48.3 58.0 54.5 74.0 68.3 65.9 64.7 A120:, 13.7 13.0 13.8 14.6 14.6 15.0 13.4 13.7 14.0 14.5 14.3 15.5 15.7 12.1 14.1 14.1 13.9 TiO-2 0.29 0.30 0.25 0.25 0.30 0.38 0.19 0.32 0.36 0.51 0.55 0.68 0.81 0.30 0.39 0.47 0.34 Fe as FeO 4.11 5.92 5.36 3.53 4.14 6.21 2.80 3.33 4.28 7.94 10.1 8.86 9.33 2.90 4.22 5.23 5.77 MgO 1.12 4.44 4.12 1.61 1.36 3.30 0.73 1.21 1.22 8.40 7.76 6.31 6.00 0.82 1.50 2.25 2.80 MnO 0.07 0.18 0.14 0.05 0.07 0.12 0.04 0.02 0.09 0.13 0.18 0.15 0.17 0.03 0.10 0.13 0.07 CaO 4.60 5.80 6.00 2.19 4.40 5.50 4.17 2.64 3.80 5.40 N.3 2.58 3.40 0.84 1.04 1.63 4.10 Na20 4.05 4.11 4.07 4.83 4.08 4.35 3.60 5.57 4.26 3.04 2.97 4.45 4.64 5.09 5.56 5.72 3.73 K2O 0.61 0.78 0.19 0.40 0.70 0.67 0.29 0.22 0.69 1.90 0.33 0.38 1.52 1.72 0.11 0.61 0.26 Total 97.2 99.0 98.9 96.5 96.5 98.4 98.2 99.7 98.7 96.6 95.8 96.9 96.1 97.3 95.3 96.0 95.7

Trace elements (ppm) 220 157 70 56 100 185 270 Ni 8 63 103 18 9 Rb 7 9 7 18 Sr 166 233 260 91 172 Y 14 10 8 14 13 Zr 100 65 67 104 120 86 115 54

Note: H7Sr/K,1Sr is 0.70316 ± 0.00009 for sample 2A and 0.70426 ± 0.00015 for sample 7.

Fe Of

Figure 11. AFM plot of rocks from Fidalgo Island and trend lines for California ophiolites (after Bailey and Blake, 1974). Symbols and patterns as in Figure 9.

NdgO+y

K20Z MgO

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reflect the compositions of magmas. Metasomatic alteration can be sediments is shown in Figure 13 in comparison with Pacific pelagic identified to have occurred only in cumulus gabbro at contact sediments and ancient and modern nearshore and epicontinental metamorphic zones near intrusions of plagiogranite and at the in- sediments. With the exception of Ba, the Fidalgo pelagic shales terface of volcanic rocks with sea water. match well with the Pacific pelagic sediments. The Fidalgo turbidites are similar to nearshore sediments and probably represent de- TECTONIC SETTING bris from an eroding island arc, judging from the abundance of volcanic material. The strongest evidence for the tectonic site of origin of rocks Donnelly (1966, 1972) observed a succession of rocks in the from Fidalgo Island is found in the sediments. In the pelagic unit Virgin Islands very similar to part of the Fidalgo complex: the predominance of clay-size particles, the occurrence of beds of keratophyres are overlain by pelagic sediments, which are in turn radiolarite, and the absence of biogenous carbonate indicate an ex- overlain by rocks of island-arc affinity. He interpreted the tremely low rate of sedimentation in deep water, far from an erod- keratophyre to be of deep-water origin formed during the initial ing land mass (compare Berger, 1974). The chemical composition stages of island-arc volcanism. In the same vein, Miyashiro (1975) of this unit also provides strong evidence of an oceanic environ- suggested that pelagic sediments in the Troodos complex could ment: it has been known for some time that oceanic and nearshore have been deposited on igneous rocks of island-arc origin in a set- (or epicontinental) shales have distinctly different contents of cer- ting such as the Izu-Bonin arc that is far from a continental mass tain metals — namely, the oceanic shales are relatively richer in and has little subaerial exposure. Possibly, some arcs become ex- Mn, Co, Ni, Cu, and Ba (Wedepohl, 1960, 1971) and have higher tinct before the stage where an appreciable land mass is developed, (Fe + Mn)/Al ratios (Bostrom, 1974). Furthermore, recent studies such as the Palau ridge, so that pelagic sediments would cover (for example, Bostrom, 1974) show the metal contents to be high- submarine-arc-type volcanic rocks. However, the succession of est at or near active ridges for modern sea-floor sediments, and at strata so far recognized in such environments (Karig and Moore, the base of older oceanic sections. The cause of the metal enrich- 1975) is a thick section of volcaniclastic sediments (>1 km) cov- ment is hence attributed to processes — volcanic exhalations or ered by radiolarian-free brown clay, quite unlike the Fidalgo stra- sea-water-basalt exchange — occurring at the spreading center. On tigraphy. this basis, metalliferous sediments in ophiolites have been inter- Another possibility is that the Fidalgo section represents a mar- preted (Elderfield and others, 1972; Bonatti and others, 1976) to ginal basin. Stratigraphy in present-day marginal basins (Klein, have formed in an ocean-ridge setting. The metal content of Fidalgo 1975; Karig and Moore, 1975) indicates that the succession above volcanic rocks is terrigenous material covered by pelagic sediment. 7Q FLOW 3 | FLOW 2 £ FLOW 1 However, the opposite succession is seen on Fidalgo Island — that 60 is, terrigenous material overlies pelagic sediment. Thus, formation 50 of this ophiolite in a marginal basin is unlikely. 14 The Fidalgo succession does match that observed in the present- 12 10 day Aleutian and Peru-Chili Trenches (von Huene, 1974; Scholl, 1974). We tentatively adopt von Huene's (1974, p. 207) interpre- 8 6 tation that such stratigraphy represents a trend "from an older 4 Fe mid-ocean environment to a younger continental slope environ- 14 ment" to explain the Fidalgo sedimentary rocks. More study of 12 ancient and modern sea-floor deposits would help to clarify such 10 questions of tectonic setting. 8 The chemical composition of the igneous rocks is in part consis- o 6 x tent with the inference of an oceanic origin: (1) the low K20 con- o 4 tent is unlike that of island-arc or continental igneous suites but is 2 a® Mg similar to dredged oceanic rocks (Fig. 14), and (2) Cr is relatively BW^IS high in Fidalgo rocks, in agreement with oceanic samples (Fig. 15). 2tl i 05 Ti In other respects, however, the chemical trends of the plagiogra- $ 0 .0 nite and keratophyre are unlike the typical oceanic tholeiites. 4 Miyashiro (1973) has emphasized that volcanic rocks of the 3 Troodos ophiolite show calc-alkaline rather than tholeiitic trends, 2 and he used this as an argument for an island-arc origin of this Ca 1 ophiolite. The dikes and volcanic rocks on Fidalgo Island show the same calc-alkaline type of trend that Miyashiro found for Troodos

— that is, Si02 increases concomitantly with FeOt/MgO (Fig. 9). However, gabbro from Fidalgo Island shows a typical tholeiitic trend and is compositionally identical to dredged oceanic gabbros

(Figs. 8, 9, 10). Miyashiro (1973) noted that the K20 content in the Troodos rocks is anomalously low for calc-alkalic rocks, but he at-

tributed this to metasomatic leaching of K20 from original typical 0 1 2 3 4 5 rhyolites and dacites. We have presented evidence here, however, METERS that gives a good indication that such metasomatism has not oc- Figure 12. Variation of chemical composition across three inter- curred in the Fidalgo rocks. The plagiogranite-keratophyre suite is

layered spilite and keratophyre flows. Data points represent sam- distinguished from calc-alkalic rocks on the basis of the K20 con- ples collected on two traverses about 5 m apart. tent.

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The strongest evidence against an oceanic setting for the Fidalgo ORIGIN OF PLAGIOGRANITE AND ophiolite is the relative abundance of silicic igneous rocks there as KERATOPHYRE MAGMA compared to what has so far been found in modern oceanic crust. Only a few samples of plagiogranite and spilite, but no On Fidalgo Island the plagiogranite, basalt, keratophyre, and keratophyre, have been obtained from the sea floor (Cann, 1969; spilite show mutually gradational mineralogies, chemical compo- Aumento, 1969; Miyashiro and others, 1970; Engel and Fisher, sitions, textures, and times of emplacement, all of which imply a 1975; Ishizaka and Yanagi, 1975). This is in direct conflict with the common origin of the magmas that produced these rocks. convincing evidence for an oceanic setting provided by the Possibly, these magmas are derived by fractionation of the same sedimentary sections of ophiolites. melt as that from which the layered gabbros crystallized. However,

0.01 0.1 1.0 100 i i i i i i i . I i i i I 1 1 1—L-1 I I I I

Fe + Mn Figure 13. Compari- AI y/ to^t* son of metal content of Fidalgo sedimentary rocks with Pacific pelagic sediments (data from Goldberg and Arrhenius, 1958; Wakeel and Riley, 1961; Dymond and others, 1973; Bostrom, 1974) and nearshore or epicontinental rocks (data from Vinogradov and Ronov, 1956; De- gens and others, 1957; Weber, 1960; Tourtelot, 1962; Wedepohl, 1971; Summerhayes, 1972; and Cook, 1974).

wt.%

Figure 14. Comparison of K20 versus CaO content of rocks in continental, outer-arc, and oceanic settings and Fidalgo Island. Stars represent oceanic rocks (data from Aumento, 1969; Miyashiro and others, 1970; and Engel and Fisher, 1975). Symbols for Fidalgo rocks as in Figure 9. Data for other rocks from (1) Erikson (1969), (2) Wager and Brown (1967), (3) Isshiki (1963), and (4) Ewart and others (1973).

0.0 1.0 2.0 3.0 4.0 5.0 6.0 10.0 CaO

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several kinds of evidence argue against such a comagmatic origin of (plagiogranite-keratophyre suite) than in the cumulate (gabbro). these two suites of rock: (1) The field relations show no gradation Overlap of FeO and MgO between the two suites would occur only between the two suites. Dike rocks sharply crosscut all structures in if there are late-fractionated cumulates and early-derived melts. the gabbro, and many have chilled margins against the gabbro, Such a process requires an overlap in ages of the cumulates and implying a substantial time interval between formation of the gab- dikes. However, there is no evidence of such age overlap at Fidalgo bro and dikes. (2) Considering the proportions of igneous rocks in Island; all dikes appear to be younger than all cumulates. We con- the ophiolite as a whole, and allowing for the possibility of gabbro clude that the gabbro and plagiogranite-keratophyre are not co- in the unexposed section (Fig. 2), the plagiogranite and keratophyre magmatic. is too abundant (—30%) to be the product of fractional crystalliza- The plagiogranite-keratophyre magma was water-rich, as evi- tion of gabbro magma. (3) The chemical composition and chemical denced by the hydrothermal alteration of intruded gabbro and the trends of the two rock suites are different, as illustrated in Figures development of hornblende instead of pyroxene in the basic vari- 8, 9, 10, and 11. Separate trends for gabbro and later spilites and eties. We speculate that in the submarine environment where this keratophyres in many other ophiolites have been demonstrated also magma appears to have formed, sea water becomes incorporated in on AFM diagrams by Bailey and Blake (1974). It could be argued the magma. Support for this hypothesis is provided by the isotopic that the difference in compositions and trends results not from dif- composition of rocks in the Troodos ophiolite which indicates that ferent origin of the two suites, but from the fact that the gabbros sea water penetrates to depths of several kilometres below the sea are all cumulates and do not represent magma compositions as do floor (Spooner and others, 1974; Chapman and others, 1975).

the plagiogranites and keratophyres. This argument is difficult to The extremely low K20 content of the plagiogranite-keratophyre test. We know of no data on natural systems showing the composi- magma requires special explanation. The K20 content is highly var- tion relations between gabbroic cumulates and derivative melts. iable and does not show a uniform relationship with other compo- Noteworthy is the large overlap of FeO/MgO ratios in the gabbro nents, such as CaO (Fig. 14), as is seen in other magma series. This and plagiogranite-keratophyre suites. If the two suites originated as relationship strongly suggests that normal crystal-melt equilibria in

cumulate solid and derivative melt from the same parent magma, a fractionation series have not controlled the K20 content. Direct then FeO/MgO should be consistently higher in the melt leaching of K20 from plagiogranite-keratophyre magma into the

900

800

700

600

ä 500 Figure 15. Comparison of Cr content of Cr Fidalgo rocks with oceanic and other rocks. ppm Symbols as in Figure 9. Data from (1) Engel 400 and others (1965), (2) Wager and Brown (1967), (3) Stavert (1971), (4) Ewart and others (1973), and (5) Hotz (1971). 300 Ave. Ocean Tholeiite (D®

O 200 O Skaergaard(2) O Mt.Baker(3)>S^,///

100 Klamath Plutons(5)

•Tonga (4)

15 20 25 30

Fe0t+Mg0

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sea water is suggested by the inferred oceanic setting of the magma, Ocean, in Veevers, J. J., and Heirtzler, J. R., Initial reports of the Deep the inferred access of sea water to the magma, the variability of Sea Drilling Project, Vol. 27: Washington, D.C., U.S. Government K 0 content in the plagiogranite-keratophyre magma, and the fact Printing Office, p. 481-497. 2 Coombs, D. S., 1974, The mineral facies of spilitic rocks and their genesis, that sea water has very low concentration of K20 relative to Na20 in Amstutz, G. C., ed., Spilites and spilitic rocks: New York, and CaO. If these interpretations are correct, they raise the un- Springer-Verlag, p. 373-402. answered question of exactly how sea water gains access to the Degens, E. T., Williams, E. G., and Keith, M. L., 1957, Environmental magma. studies of carboniferous sediments, Part I: Geochemical criteria for differentiating marine from fresh-water shales: American Association In summary, we conclude that the tectonic setting in which the of Petroleum Geologists Bulletin, v. 41, p. 2427-2455. plagiogranite-keratophyre formed is an unresolved question. The Dickinson, W. R., 1963, Metasomatic quartz keratophyre in central Ore- magmas from which these rocks were derived are apparently not gon: American Journal of Science, v. 260, p. 249-266. cogenetic with the layered gabbro they intruded, and they have Donnelly, T. W., 1963, Genesis of albite in early orogenic volcanic rocks: possibly interacted with sea water beneath the ocean floor to be- American Journal of Sicence, v. 261, p. 957—972. 1966, Geology of St. Thomas and St. John, U.S. Virgin Islands, in come enriched in H20 and depleted in K20. Hess, H. H., ed., Caribbean geological investigations: Geological So- ciety of America Memoir 98, p. 85—176. ACKNOWLEDGMENTS 1972, Deep-water, shallow-water, and subaerial island-arc volcanism: An example from the Virgin Islands, in Shagam, R., and others, eds., Studies in Earth and space sciences: Geological Society of America This work has benefited greatly from our discussions in the field Memoir 132, p. 401-414. and laboratory with numerous geologists, especially R. S. Babcock, Dymond, J., Corliss, J. B., Heath, G. R., and others, 1973, Origin of metal- P. Bearth, R. G. Coleman, D. S. Coombs, B. W. Evans, W. E. liferous sediments from the Pacific Ocean: Geological Society of Glassley, E. M. Moores, and J. A. Vance. For review of the manu- America Bulletin, v. 84, p. 3355-3372. script we are indebted to R. S. Babcock, L. M. Echeverría, E. W. Elderfield, H., Gass, I. G., Hammond, A., and others, 1972, The origin of ferromanganese sediments associated with the Troodos Massif of Glassley, and T. P. Thayer. Financial support for the study was Cyprus: Sedimentology, v. 19, p. 1-19. provided in part by the Bureau of Faculty Research at Western Engel, A.E.J., Engel, C. G., and Havens, R. G., 1965, Chemical characteris- Washington University. tics of oceanic basalts and the upper mantle: Geological Society of America Bulletin, v. 76, p. 719-734. Engel, C. G., and Fisher, R. L., 1969, Lherzolite, anorthosite, gabbro, and REFERENCES CITED basalt dredged from the mid—Indian Ocean Ridge: Science, v. 166, p. 1136-1141. Amstutz, G. C., and Patwardhan, A. M., 1974, A reappraisal of the textures 1975, Granitic to ultramafic rock complexes of the Indian Ocean ridge and the composition of the spilites in the Permo-Carboniferous Ver- system, western Indian Ocean: Geological Society of America Bulletin, rucano of Glarus, Switzerland, in Amstutz, G. 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MANUSCRIPT RECEIVED BY THE SOCIETY JULY 28, 1977 Baker andesites [M.S. thesis]: Bellingham, Western Washington State REVISED MANUSCRIPT RECEIVED DECEMBER 12, 1977 College, 60 p. MANUSCRIPT ACCEPTED JANUARY 23, 1978

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