The Graveyard Point Intrusion: an Example of Extreme Differentiation of Snake River Plain Basalt in a Shallow Crustal Pluton

JOURNAL OF PETROLOGY VOLUME 48 NUMBER 2 PAGES 303^325 2007 doi:10.1093/petrology/egl062 The Graveyard Point Intrusion: an Example of Extreme Differentiation of Snake River Plain Basalt in a Shallow Crustal Pluton CRAIG M. WHITE* GEOSCIENCES DEPARTMENT, BOISE STATE UNIVERSITY, BOISE, ID 83725 USA RECEIVED AUGUST 29, 2005; ACCEPTED SEPTEMBER 28, 2006; ADVANCE ACCESS PUBLICATION NOVEMBER 8, 2006 The Graveyard Point intrusion is the only known example of a well-exposed differentiated mafic pluton associated with the late Miocene^Pleistocene magmatism of the western Snake River Plain (SRP). It is exposed in a 6 km by 4 km area adjacent to the Oregon^ Idaho border, and exposures range in thickness from 20 to 160 m.The thicker parts of the intrusion are strongly differentiated and contain a 25^60 m thick section of well-laminated cumulus-textured gabbros that grade upward into pegmatoidal ferrogabbro. Evolved liquids formed sheets of Fe-rich siliceous granophyre. At least two injections of magma are indicated by abrupt discontinuities in the rock and mineral compositions, and by the lack of mass balance between the bulk intrusion and its chilled borders. The laminated gabbros are interpreted to have formed from a tongue of augite and plagioclase crystals that were carried in with the second pulse of magma. Following the final emplacement of the intrusion, in situ differentiation proceeded through a two-stage process: the ferrogabbros are explained as interstitial liquids forced out of the crystal mush by compaction, and the siliceous granophyres are interpreted to be residual liquids that migrated out of the partly crystallized ferrogabbros in response to the exsolution of volatiles. Because the geochemical trend inferred for the mafic to intermediate composition liquids in the Graveyard Point intrusion is similar to the trend for many western Snake River Plain lavas, the pluton may be a good model for shallow sub-volcanic magma chambers elsewhere in the SRP. However, some western SRP lavas contain anomalously high concentrations of P2O5 , which are best explained by mixing within the active crustal mush column or with partial melts of previously formed differentiated mafic intrusions. I N T RO D U C T I O N granophyre Recent papers by Marsh (2000, 2004) emphasized that magmas evolve in response to combined physical and chemical processes that occur over a wide range of scales in spatially integrated intrusive complexes referred to as ‘magmatic mush columns’. However, in most young volcanic provinces the intrusive parts of this system are hidden and their petrological features can be only inferred from petrographic, chemical and isotopic studies of lavas and tephra. The Snake River Plain (SRP) of Idaho and eastern Oregon is one such young province where extensive complexes of crustal level, possibly interconnected, mafic intrusions have been indicated by geophysical evidence, but remain largely unknown because they have not been exposed by erosion (Mabey, 1982; Sparlin et al., 1982; Peng & Humphreys, 1998). The SRP contains a compositionally diverse suite of mainly tholeiitic lavas, many of which appear to have evolved by fractional crystallization over a range of crustal depths (e.g. Leeman & Vitaliano, 1976; Leeman, 1982; Reid, 1995; Geist et al., 2002). For this reason, the concept of a magmatic mush column may be particularly applicable to this province, and knowing the nature of the plutonic system becomes a critical part of understanding the magmatic system. To that end, this paper presents results of the first detailed study of a western SRP intrusive complex and suggests a physical model for the evolution of magmas in the upper part of the mush column beneath this well-known volcanic province. *Corresponding author. Telephone: 208-426-3633. E-mail: ß The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@ oxfordjournals.org KEY WORDS: Snake River Plain; mafic intrusions; tholeiitic; sill; JOURNAL OF PETROLOGY VOLUME 48 GEOLOGIC A L S ET T I NG The western SRP is a NW-trending intracontinental rift basin about 70 km wide and 300 km long (Fig. 1a). The boundary faults are parallel to major structural trends in the Pacific Northwest, but perpendicular to the assumed NE-trending track of the Yellowstone hotspot (Pierce & Morgan, 1992; Smith & Braile, 1994). Although there is no consensus on the origin of the western SRP, many workers have suggested that the 17^14 Ma episode of voluminous magmatism attributed to the Yellowstone hotspot either directly or indirectly initiated the formation of the western SRP (e.g. Geist & Richards, 1993; Glen & Ponce, 2002; Shervais et al., 2002). The rifting and subsidence that appears to have begun about 10 Ma is commonly attributed to regional extension that occurred throughout the northern Basin and Range Province (e.g. Hooper et al., 2002). Basaltic magmatism in the western SRP began with the onset of extension and has produced a diverse suite of tholeiitic lavas, many of which are strongly enriched in iron (FeO
414%) (Bonnichsen & Godchaux, 2002). More than 2 km of sediments fill the deepest parts of the western SRP basin in southwestern Idaho (Wood, 1994), but the basin shallows rapidly to the west and dies out in eastern Oregon where it abuts the older, north^ south-trending Oregon^Idaho graben (Cummings et al., 2000). Erosion has exposed the Graveyard Point intrusion because it was emplaced near the western margin of the plain where the least amount of subsidence has occurred. G E O L O G Y O F T H E I N T RU S I O N Age and field relations Mafic intrusive rocks forming the Graveyard Point complex are exposed in a series of discontinuous outcrops within an area of c. 6 km by 4 km. The magmas were emplaced into middle Miocene fluvial and lacustrine sediments and silicic pyroclastic rocks originally mapped by Kittleman et al. (1967) as part of the Sucker Creek Formation. Ferns (1989) cited a K^Ar whole-rock date of 67  04 Ma for diabase from the lower part of the main intrusion, a value that is only slightly younger than 40 Ar^39Ar ages of basalts of similar composition erupted along the southern margin of the western SRP in Idaho (White et al., 2002). Mafic rocks of the Graveyard Point complex occur in three types of exposures: (1) 1^10 m wide dikes of olivine diabase; (2) 20^30 m thick sills, also composed of olivine diabase; (3) 100^160 m thick, irregularly shaped but generally sheet-like intrusions composed of olivine diabase, gabbroic cumulates, ferrogabbro and granophyre. Chilled borders in all of the floored intrusions are chemically and petrographically similar to one another and to the fine-grained diabase in the dikes. The field relations are consistent with a structural model in which all exposures of the sill-like bodies are NUMBER 2 FEBRUARY 2007 part of a single, wedge-shaped intrusion that was offset by normal faults. (...truncated)