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

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Abstract

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.

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