New Analyses of Eocene Basalt from the Olympic Peninsula, Washington N. A. LYTTLE Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada B3H 3]5 D. B. CLARKE Department of Geology, Dalhousie University, Halifax, Nova Scotia, Canada B3H 3J5 ABSTRACT 124° 123° New chemical analyses have been made of a suite of recently col- lected samples of pillow lava, breccia, and minor intrusive rocks from the Olympic Peninsula. The rocks have been altered through addition of at least water and carbon dioxide, but the recalculated analyses show that these rocks are tholeiitic basalt rather than spi- lite as previously reported. A petrogenetic model, involving partial melting of mantle peridotite to produce a magma that underwent fractional crystallization of olivine and equilibrated near atmos- pheric pressure, accounts for the observed chemical features of the basalt. The characteristics and distribution of both the volcanic and associated sedimentary rocks suggests possible origin in an island- arc environment. Key words: igneous petrology, chemical analysis, Eocene, volcanic rocks. INTRODUCTION Geological Setting The Olympic Peninsula is in northwestern Washington (Fig. 1). The Olympic Mountains form the nothern part of the western Washington and Oregon Coast Ranges, which developed during late Tertiary time. According to Snavely and Wagner (1963), an early Tertiary geosyncline extended from the southern end of Van- couver Island; through the Olympic Mountains, Coast Ranges, and Puget-Willamette lowlands of western Washington and Oregon; to Figure 1. The Olympic Peninsula and distribution of lower Tertiary vol- the northern end of the Klamath Mountains in southern Oregon. canic rocks (shaded areas). Early in Eocene time, a thick sequence of tholeiitic lava flows and breccia was erupted from numerous centers onto the floor of the some 100 km inland (Snavely and Wagner, 1963). Weaver subsiding geosyncline. These volcanic rocks are referred to as the (1945) estimated that a minimum value for the average thickness is Crescent Formation in western Washington and as the Siletz River 900 m and that their volume is greater than that of the Columbia and Tillamook Volcanics and volcanic rocks of the Umpqua For- Plateau basalt flows. According to Snavely and Wagner (1963), the mation in western Oregon. An early Eocene age was originally as- volcanic sequence totals more than 4,500 m in thickness on the signed to these rocks by Weaver (1937). On micropaleontological Olympic Peninsula where the base is exposed. evidence Rau (1966) reported them to be of early(?) or middle Most of the lava in the Coast Ranges was apparently erupted Eocene age. The base of the volcanic rocks is exposed only on the onto the sea floor. Well-developed pillow structures are common, Olympic Peninsula, where rocks of the Crescent Formation overlie along with volcanic breccia and intercalated marine sediment a relatively thin metasedimentary sequence, the base of which is a (Park, 1946), but Weaver (1945) also reported numerous small tectonic break (Cady and others, 1972a, 1972b; Tabor and others, vents and dikes that he believed were the source of much of the vol- 1970; Tabor and others, 1972). Throughout the Olympic Penin- canic material, at least on Vancouver Island. Snavely and Wagner sula and Coast Ranges, Eocene volcanic units intertongue com- (1963) also found interflow soil zones and interbedded, locally de- plexly with fossiliferous, tuffaceous siltstone that contains graded rived mud-flow breccia and conglomerate, indicating that volcanic beds of volcanic, feldspathic, and lithic wackes. Beds of chert and islands formed in places and that a part of the volcanic series was Globigerina-bearing limy siltstone occur locally (Snavely and erupted subaerially. Wagner, 1963). Throughout the Coast Ranges, intermittent volcanic activity and The Eocene volcanic rocks are believed to have formed a vast local uplift took place until about middle Miocene time, when fold- lava field that extended from Vancouver Island southward to the ing produced a series of northwest-trending structures. Tabor Klamath Mountains and from a line west of the present coastline to (1972) obtained a K-Ar age of 29 m.y. for the time of metamor- Geological Society of America Bulletin, v. 86, p. 421-427, 6 figs., March 1975, Doc. no. 50318 421 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/3/421/3433796/i0016-7606-86-3-421.pdf by guest on 28 September 2021 422 LYTTLE AND CLARKE phism of rocks in the core of the Olympic Mountains. During late devitrification is more advanced, the glass has crystallized into Pliocene to early Pleistocene time, further folding took place spherulitic or feathery intergrowths of plagioclase and pyroxene. throughout Washington and Oregon (Weaver, 1937, 1945). Vesicles, when present, are generally filled with calcite and (or) chlorite; quartz, prehnite, and zeolites occur infrequently. Chlorite Present Work is invariably present, either in veinlets, vesicles, or as replacement for glass, olivine, pyroxene, and plagioclase. Calcite veinlets, and to The most recent petrological work on Eocene basalt in Washing- a lesser extent quartz veinlets, are common, and prehnite, with rare ton appears to be that of Glassley (1973) and this study based on a pumpellyite, is found in association with the calcite veinlets. collection of 95 samples, of which 53 were studied in thin section The mineralogy of pillows is identical to that of flows. The tex- and 24 were chemically analyzed. The objectives of this paper are tural features are those normally associated with pillows, namely, to (1) present the new chemical data on basaltic rocks from the increases in grain size away from the pillow margins and the de- Crescent Formation; (2) use the data to derive a petrogenetic model velopment of variolitic textures toward the interiors of the pillows. for the origin of the Crescent magmas; (3) use the data to re-assess the importance of spilite in the Olympic Peninsula; and (4) inter- Diabase pret the role that basalt has played in the Tertiary geological and tectonic evolution of western Washington. The generally poor outcrops of the Crescent Formation made the distinction between coarser grained interiors of thick flows and PETROGRAPHY diabase dikes difficult. For the purposes of this discussion, the medium-grained rocks with subophitic textures are termed diabase. The general petrography of the major volcanic rock types sam- The two distinct types that have been found are normal diabase pled on the Olympic Peninsula will be discussed as follows: basalt containing calcic plagioclase and albite diabase. (flows and pillows), diabase, and basaltic breccia. Sampling Several mineralogical and textural features are common to the localities are described in Appendix 1. two types of diabase. Generally fresh augite occurs in intergranular and subophitic relations with the plagioclase grains. Chlorite or Basalt (Flows and Pillows) pale-green actinolite are minor replacements of the augite along cracks and grain edges. Olivine is completely replaced by chlorite. Basalt is commonly an aphanitic, dark-green rock rich in Chlorite also occurs as abundant interstitial patches and veinlets. devitrified glass, with variolitic, intersertal, hyalopilitic, and inter- Prehnite is associated with calcite and chlorite in veins and also re- granular textures. Plagioclase occurs as microlites, as skeletal mi- places plagioclase. Epidote and pumpellyite are rare secondary crophenocrysts, or as subhedral to euhedral phenocrysts. Measured minerals. plagioclase compositions fell in the range An50 to An60. Colorless Apart from the composition of the plagioclase, the principal dif- to pale-brown clinopyroxene occurs as subhedral micropheno- ference between the two types of diabase is the degree of alteration crysts and as feathery intergrowths with plagioclase. It is generally of feldspar. Plagioclase in normal diabase is generally fresh, al- unaltered, although minor replacement by pale-green actinolite and though some kaolin and sericite are invariably present. Plagioclase chlorite occurs. Olivine is never abundant, but when present it in albite diabase is partly to completely replaced by sericite, kaolin, forms both euhedral and skeletal microphenocrysts that are invari- and in some cases, prehnite and chlorite. Also, partial replacement ably replaced by pseudomorphs of chlorite and (or) calcite. Small, by zeolite material in the albite diabase is occasionally found, and euhedral red-brown spinel is often found poikilitically enclosed laumontite was observed in one sample. within replaced olivine. Titanomagnetite is common throughout, either as subhedral or skeletal grains or as dense specks and rods in Breccia the groundmass. The groundmass may be dark-brown, fine-grained devitrified Volcanic breccia is represented mainly by two types: hyaloclas- glass, or lighter brown, partly spherulitic glass. Where tite breccia and mixed breccia. Hyaloclastite breccia is composed TABLE 1. CHEMICAL ANALYSES Sample no. 8 13 1411 16 171 23 30 34 38 40 43 47« Oxides S102 51.9 47.1 46.6 47.8 47.8 49.2 45.8 46.7 46.6 45.4 49.1 45.8 T102 2.0 2.0 2.0 1.4 2.6 1.4 1.2 1.1 2.0 1.9 1.2 2.0 A120s 15.9 16.3 15.7 17.0 12.8 13.8 14.9 15.1 13.4 14.0 14.4 15.7 Fe20, 3.1 3.9 3.3 2.5 6.4 5.4 2.7 2.9 5.4 3.3 3.1 5.1 FeO 5.9 7.8 8.5 7.2 8.8 8.2 6.3 5.1 9.2 10.8 8.5 8.8 MnO 0.18 0.18 0.19 0.15 0.25 0.21 0.18 0.14 0.22 0.21 0.22 0.27 MgO 2.8 5.2 5.8 5.9 5.0 5.9 11.5 10.3 6.0 6.2 7.5 5.7 CaO 7.9 11.3 11.1 12.2 8.7 7.3 7.1 9.4 10.0 10.0 12.7 10.0 Na20 5.9 3.1 3.0 2.8 4.5 3.8 3.3 3.0 3.6 2.4 2.3 2.6 K20 0.07 0.38 0.53 0.41 0.17 1.0 1.0 0.88 0.11 0.07 0.18 0.36 P20s 0.72 0.18 0.19 0.13 0.27 0.15 0.31 0.18 0.26 0.18 0.11 0.29 C02+ 0.47 0.28 0.22 0.30 0.20 0.32 0.07 0.19 0.30 0.05 0.06 n.d.