Pre-Tertiary rocks of the Blue Mountain region of central and northeastern Oregon comprise three major sedimentary and volcanic sequences and two distinct intrusive magma series. The ages of the sedimentary-volcanic sequences are Paleozoic, Late Triassic–Late Jurassic, and middle Cretaceous (Albian to Cenomanian), respectively. The earlier intrusive magma series ranges in composition from peridotite to albite granite and was emplaced during the major orogeny in the Blue Mountain region between earlier Permian and Late Triassic time. The later intrusive magma series is related to the Idaho batholith proper, ranges from gabbro to granodiorite, and probably was emplaced during the earlier half of Cretaceous time. The emplacement of a major intrusive magma series during Permian and Triassic time suggests a much closer relationship to the northern part of the Cordillera, in Canada and Alaska, than to the southern part in southwestern Oregon and California.

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