Picture Gorge Basalt: Internal stratigraphy, eruptive patterns, and its importance for understanding Columbia River Basalt Group magmatism

Abstract

The Picture Gorge Basalt (PGB) of the Columbia River Basalt Group (CRBG) has been previously thought to be limited in its eruptive volume (<3000 km3) and thought to not extend far from its type locality. At present, PGB represents only 1.1 vol% of the CRBG with a relatively limited spatial distribution of ~10,000 km2. New age data illustrate that the PGB is the earliest and longest eruptive unit compared to other main-phase CRBG formations and that some dated basaltic flows reach far (~100 km) beyond the previously mapped extent. This study focuses on extensive outcrops of basaltic lavas and dikes south of the type locality at Picture Gorge, in order to reassess the spatial distribution and eruptive volume of the PGB. Field observations coupled with geochemical data indicate that PGB lava flows and mafic dikes covered a significantly greater area than shown on the published geologic maps. We find that additional mafic dikes located farther south of the original mapped distribution have geochemical compositions and northwest-trending orientations comparable to the dikes of the Monument dike swarm. We also identify new lava flows that can be correlated where stratigraphic control is well defined toward the original mapped PGB distribution. Our analyses and correlations are facilitated by comparison of 20 major- and trace-element abundances via a principal component analysis. This statistical comparison provides a new detailed distribution of PGB with stratigraphic significance that more than doubles the total distribution of PGB lavas and dikes and brings the eruptive volume to a new minimum of at least ~4200 km3. Geochemically correlated basaltic lavas and dikes in the extended distribution of PGB represent the earlier and later sections of the internal PGB stratigraphy. This is an intriguing observation as new geochronological data suggest an eruptive hiatus of ~400 k.y. during PGB volcanic activity, which occurred from 17.23 Ma to 15.76 Ma. The geochemical identifiers used to differentiate PGB from other main-phase CRBG formations include lower TiO2 (<2 wt%) concentrations, lower incompatible trace-element (i.e., La, Th, and Y) abundances, and a more pronounced enrichment in large-ion-lithophile elements (LILEs) on a primitive mantle–normalized trace-element diagram (Sun and McDonough, 1989). Geochemical characteristics of PGB are interpreted to represent a magmatic source component distinct from the other main-phase CRBG units, possibly a localized backarc-sourced mantle melt. However, this source cannot be spatially restricted as there are observed PGB lava flows and dikes extending as far east as Lake Owyhee and as far south as Hart Mountain, covering at least 15,000 km2. In context with the existing stratigraphy and the new extent of PGB lavas and dikes, these ages and coupled geochemical signatures demonstrate this mantle component was not spatially localized but rather tapped across a wide region.

Columbia River Rhyolites: Age-Distribution Patterns and Their Implications for Arrival, Location, and Dispersion of Continental Flood Basalt Magmas in the Crust

“…This phase correlates well with the later PGB (cf. [98]) and particularly well with the voluminous Grande Ronde Basalt, which is ~16.3 to 15.9 Ma. Thus, magma reservoirs hosting these mafic magmas must have been widespread in the crust below those rhyolite centers (Figure 12 and Section 4.2).…”

“…Direct stratigraphic evidence for contemporaneity of CRBG volcanism and rhyolites is within areas where CRBG units are intercalated with rhyolites. This area includes (1) the greater Malheur River Gorge area (Figure 3), where Grande Ronde Basalt is intercalated with rhyolites overlying older CRBG units (e.g., [16,70,98], and (2) peripheral areas in which units of the Dinner Creek Tuff are intercalated into Grande Ronde Basalt at Rattlesnake Road [18], north of Baker City, and along the Oregon/Idaho state border east of Baker City (Figure 3). Additionally, several other ignimbrite units emplaced farther south are intercalated with Steens Basalt [103].…”

Province-Wide Tapping of a Shallow, Variably Depleted, and Metasomatized Mantle to Generate Earliest Flood Basalt Magmas of the Columbia River Basalt, Northwestern USA

“…Recent work by [2] extended the PGB formation to the east (Figure 1) and yielded 40 Ar/ 39 Ar ages that indicate that eruptive activity of the Picture Gorge Basalt started significantly earlier around 17.25 Ma, making the PGB older than or, as a minimum, as old as the earliest CRBG lavas anywhere. The updated timeframe now makes it probable that the lower [2,12,24,25]; red line for ages of AB1&2 flows of this study; 'prior PGB' = stratigraphic position prior to [2], upward arrow on PGB to indicate youngest age of 15.75 Ma [24] (see text for discussion). (B) Flow stratigraphy of Imnaha Basalt after [26]; AB = American Bar, RC = Rock Creek, FC = Fall Creek, and LC = Log Creek subunits.…”

“…Its volume is 3300 km 3 and is relatively small, accounting for only 1.1% of the CRBG as compared to the other major units like the Imnaha Basalt and Grande Ronde Basalt, which comprise 15% and 72% of the CRBG, respectively [1]. However, recent work by [24] has revealed that the extent of PGB lavas is likely larger by ~14,000 km 2 with a total distribution of ~25,000 km 2 (Figure 1), yielding an additional volume ranging from ~3000-8000 km 3 based on using different thickness estimates.…”

“…(1) the exact driving force of this magmatism, mantle plume or plate tectonics (e.g., [5][6][7][8][9]), (2) the precise timing of CRBG magmatism [2,[10][11][12][13][14], (3) the nature of crustal storage sites prior to eruption [5,[15][16][17]; and (4) chemical variations among lavas and what these imply about mantle sources [18][19][20][21][22][23]. [2,12,24,25]; red line for ages of AB1&2 flows of this study; 'prior PGB' = stratigraphic position prior to [2], upward arrow on PGB to indicate youngest age of 15.75 Ma [24] (see text for discussion). (B) Flow stratigraphy of Imnaha Basalt after [26]; AB = American Bar, RC = Rock Creek, FC = Fall Creek, and LC = Log Creek subunits.…”