THE GRAVEYARD POINT INTRUSION: A STRONGLY DIFFERENTIATED MAFIC PLUTON IN EASTERN MALHEUR COUNTY, OREGON

The Graveyard Point intrusion is a tholeiitic mafic complex consisting of one or more 20 to 150- m-thick sheet-like plutons exposed within a 3 by 7 km area in eastern Malheur County, Oregon. Analyses of the chilled borders indicate the initial magma was an olivine tholeiite that was transitional in composition between the MORB-like high-alumina olivine tholeiite (HAOT) erupted along the northern margin of the Basin and Range province and the more evolved tholeiitic basalts of the Snake River Plain. The complex is late Miocene in age and was emplaced into middle Miocene sediments and silicic tuffs during the early development of the western Snake River Plain. Rocks in the thicker parts of the intrusion are strongly differentiated and range in composition from olivine gabbro to silicic granophyre. Within the intrusion, distinctive sub-horizontal zones of gabbroic cumulates, pegmatoidal ferrogabbro and granophyre can be traced for up to 2 km along strike.

More than 90 specimens from the intrusion were chemically analyzed by XRF. The chemical compositions of some rocks are consistent with small amounts of crystal accumulation; however, trace-element and Sr-isotopic data indicate that wall-rock assimilation had little or no role in the evolution of the magmas. Instead, all evidence indicates that the broad range of rock compositions within the intrusion can be attributed to various degrees of fractional crystallization of a basaltic magma having the composition of the chilled border. Because rocks in the intrusion are clearly related in space, time and origin, they may provide the best documented example of a shallow-level crystal fractionation trend for eastern Oregon olivine tholeiites. The chemical trends of the Graveyard Point intrusion differ substantially from the trends of eastern Oregon volcanic suites of similar age, suggesting that the volcanic suites did not evolve by shallow level crystal fractionation.