Ages of the Steens and Columbia River Flood Basalts and Their
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Ages of the Steens and Columbia River ¯ood basalts and their relationship to extension-related calc-alkalic volcanism in eastern Oregon P.R. Hooper* G.B. Binger Department of Geology, Washington State University, Pullman, Washington 99164, USA K.R. Lees Department of Earth Sciences, Open University, Milton Keynes MK7 6AA, UK ABSTRACT INTRODUCTION But the causal relationship between extension and the Columbia River ¯ood-basalt eruption Stratigraphic and chemical correlations The unusually large volumes of tholeiitic is more dif®cult to establish. The problem is of Tertiary volcanic units in eastern Oregon basalt, erupted within short time spans, that complicated by uncertainty about the relation- con®rm that the Steens Basalt represents characterize continental ¯ood-basalt provinces ships between the eruption of Steens Basalt the earliest eruptions of the Columbia Riv- are typically associated with lithospheric ex- (Carlson and Hart, 1987) over the impinging er ¯ood-basalt province. Field correlations tension (White and McKenzie, 1989). In some Yellowstone hotspot in southeast Oregon and are supported by major and trace element large-scale ¯ood-basalt provinces, extension is the larger eruption of the Columbia River Ba- analyses and con®rmed by 40Ar/39Ar dates. accompanied by lithospheric thinning and an- salt Group almost 300 km farther north on the Within the basalt of Malheur Gorge, situ- ticipates the separation of major tectonic Columbia Plateau. Similarity in age and com- position has led many authors to assume that ated between Steens Mountain and the plates. More controversial is whether exten- the Steens Basalt was the earliest eruption of southernmost extent of the previously sion was the cause or consequence of the mas- the Columbia River ¯ood-basalt province mapped Columbia River Basalt Group, the sive ¯ood-basalt eruptions (White and Mc- (Brandon and Goles, 1988; Geist and Rich- lowest unit correlates with the Steens Ba- Kenzie, 1989; Richards et al., 1989; Hooper, ards, 1993; Camp, 1995; Hooper, 1997; Ta- salt, and the conformably overlying middle 1990). kahashi et al., 1998, among others), but the and upper units correlate with the Imnaha The prevalent view of the origin of the Co- stratigraphic relationships between the two and Grande Ronde Basalt Formations of lumbia River and other continental ¯ood ba- tholeiitic eruptions have never been estab- the Columbia River Basalt Group. New salts is that the large magma volumes that dis- lished. Furthermore, the radiometric ages sug- dates indicate that Imnaha and Grande tinguish ¯ood basalts require a hotspot in the gested that the Imnaha Basalt, the lowest for- Ronde Basalt Formations on the Columbia upper mantle. Many workers have ascribed mation of the Columbia River Basalt Group, Plateau (.90% of the Columbia River Ba- this hotspot to a plume arising from deep was older (17.2 Ma 6 0.5 Ma; McKee et al., salt Group) erupted between 16.1 and 15.0 within the mantle (Campbell and Grif®ths, 1990), with or without the additional process- 1981) than more recent and precise 40Ar/39Ar Ma. These were immediately preceded by dates of the Steens Basalt (16.60 Ma 6 0.02 the Steens Basalt, a plagioclase-phyric tho- es of signi®cant lithospheric extension and thinning (White and McKenzie, 1989; Rich- Ma; Swisher et al., 1990). leiite that erupted above the calculated po- ards et al., 1989; Hooper, 1990). In contrast, Subsequent mapping of the Tertiary volca- sition of the Yellowstone hotspot at 16.6 Carlson and Hart (1987) have ascribed the Co- nic rocks in part of east-central Oregon (Ferns Ma. In eastern Oregon, the ¯ood-basalt lumbia River basalt eruption to extension as- et al., 1993a, 1993b; Cummings et al., 2000), tholeiites of Steens Mountain and Malheur sociated with backarc spreading. Anderson together with analyses for major and trace el- Gorge form a voluminous but brief inter- 40 39 1 (1994) and his coauthors have argued that ements and new Ar/ Ar dates, has clari®ed lude (16.6±15.3 Ma) superimposed on the continental ¯ood basalts tend to occur along the stratigraphic relationship between the low-volume, calc-alkalic to mildly alkalic, boundaries between lithospheric plates of con- Steens Basalt and the Columbia River Basalt volcanism associated with continuing Eo- trasting thickness, which would allow creation Group, allowing them both to be linked to im- cene to present east-west extension. of convection systems adequate to account for pingement of the Yellowstone hotspot beneath the large magma volumes. Keywords: calc-alkalic volcanism, Colum- East-west extensional stress was prevalent 1 GSA Data Repository item 2002014, major and bia River Basalt, extension, hotspot, Steens during the eruption of the Columbia River Ba- trace element analyses, details of 40Ar/39Ar dates, Basalt. and petrographic descriptions of the volcanic units salt Group, as graphically illustrated by the of east-central Oregon, is available on the Web at consistent north-northwest±east-southeast ori- http://www.geosociety.org/pubs/ft2002.htm. Re- *E-mail: [email protected]. entation of the feeder dikes (Hooper, 1997). quests may also be sent to [email protected]. GSA Bulletin; January 2002; v. 114; no. 1; p. 43±50; 5 ®gures; 1 table; Data Repository item 2002014. For permission to copy, contact [email protected] q 2002 Geological Society of America 43 Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/114/1/43/3387249/i0016-7606-114-1-43.pdf by University of Nevada Reno user on 02 March 2020 HOOPER et al. TABLE 1. STRATIGRAPHY AND 40Ar/39Ar DATES FOR VOLCANIC UNITS eastern Oregon at 16.6 Ma (Engebretson et al., OF EAST-CENTRAL OREGON 1985; Pierce and Morgan, 1992). This study provides improved dates for the initial erup- tions of the whole ¯ood-basalt province and emphasizes the distinction between the calc- alkalic to mildly alkalic magmatism associat- ed with east-west lithospheric extension and the tholeiitic ¯ood basalts associated with the Yellowstone hotspotÐa distinction with im- portant implications for interpreting the origin of ¯ood basalts. VOLCANIC STRATIGRAPHY IN EAST- CENTRAL OREGON Many of the volcanic units in east-central Oregon have been given formal names by ear- lier workers (e.g., Kittleman et al., 1965, 1967). Lees (1994) grouped these and other units into informal sequences of similar age (Table 1), a convenient nomenclature that is followed here. Basalt of Malheur Gorge The oldest Tertiary volcanic sequence in the area studied, between Farewell Bend, Vale, and Juntura (Fig. 1), is the basalt of Malheur Gorge (Evans, 1990a, 1990b). It is divided into three distinct stratigraphic and composi- tional units (Table 1; Lees, 1994; Binger, 1997). At the base, the Lower Pole Creek unit is composed of dark, coarsely plagioclase- phyric basalt. Both chemically and petro- graphically, the Lower Pole Creek ¯ows are similar to ¯ows with reversed magnetic polar- ity in the lower part of the Steens Basalt sec- tion at Steens Mountain, 100 km to the south- west (Fig. 2; Binger, 1997; Johnson et al., 1998a). Analogous, coarsely plagioclase-phyr- ic basalts with reversed magnetic polarity form the basal ¯ow unit of the Columbia Riv- ¯ows are overlain by the aphyric basaltic an- Birch Creek unit have a mean of 15.7 Ma 6 er Basalt Group at Squaw Butte, a few kilo- desites of the Birch Creek unit, which are pet- 0.1 Ma (Table 1). Two ¯ows from original meters northwest of Boise, Idaho (Figs. 1 and rographic and chemical equivalents of the sections of Imnaha basalt on the Columbia 2; Fitzgerald, 1984; Martin, 1984), and also Grande Ronde Basalt Formation of the Co- Plateau (Hooper et al., 1984) provide new form the magnetically reversed ¯ows at the lumbia River Basalt Group (Fig. 2; Lees, ages of 15.4 Ma 6 0.2 Ma and 15.5 Ma 6 base of the otherwise magnetically normal Im- 1994). Only a few Birch Creek ¯ows are pre- 0.3 Ma, determined by using the same 40Ar/ naha Basalt along the southern margin of the sent at Malheur Gorge, and they have not been 39Ar techniques in the laboratory used to date Wallowa Mountains in northeast Oregon (e.g., recognized farther south. The number of the Grande Ronde Basalt on the Columbia sample C1 of Carlson, 1984; Hooper, 1997). Grande Ronde type ¯ows and their total thick- Plateau at between 16 and 15 Ma (Long and The Lower Pole Creek ¯ows are succeeded ness increase progressively north to the Co- Duncan, 1982; R.A. Duncan, Oregon State upward, without apparent break, by the mod- lumbia Plateau (Lees, 1994; P.R. Hooper, un- University, 1998, personal commun.). Dates erately plagioclase-phyric Upper Pole Creek published mapping, 1981). of the Imnaha and Grande Ronde Basalt ¯ows tholeiitic ¯ows with higher concentrations of Flows of Steens Basalt on Steens Mountain show no discernible correlation with strati- SiO2, Al2O3, K2O, Ba, Rb, and Zr than are are dated at 16.60 Ma 6 0.02 Ma (base) and graphic position (Long and Duncan, 1982). found in the Lower Pole Creek unit (Fig. 2). 16.59 Ma 6 0.02 Ma (top) (Swisher et al., More recent 40Ar/39Ar dates for Grande Ronde The compositional range of the Upper Pole 1990). Two new dates for the Lower Pole Basalt by Baksi (1989) have a similar range Creek ¯ows is similar to the main magneti- Creek ¯ows (basalt of Malheur Gorge) have a (15.5 Ma to 16.1 Ma). The often quoted older cally normal section of the Imnaha Basalt of weighted mean of 16.9 6 0.8 Ma. Four new K/Ar dates for ¯ows of the Imnaha Basalt For- the Columbia River Basalt Group (Fig. 2; dates for Upper Pole Creek ¯ows have a mean mation fall outside this range at 17.2 Ma 6 Hooper et al., 1984).