Abstract

The Bald Mountain batholith in the Elkhorn Mountains of northeastern Oregon has intrusive relations to a heterogeneous assortment of Permian sediments and a Triassic metagabbro-serpentine sequence. The batholith is a composite intrusive composed of at least eight distinct rock types which range from norite to quartz monzonite. The different units were emplaced in a mafic to felsic sequence that commenced with norite, continued with tonalite and granodiorite, then leucogranodiorites, and ended with the emplacement of leucocratic quartz monzonite. The tonalite and granodiorite are gradational facies of the major intrusion and represent about 97 per cent of the 171 square miles of exposed batholithic rocks.

Field and petrographic evidence support the conclusion that each unit in the batholith crystallized from a discrete volume of magma. Direct evidence for stoping is limited, and the field facts imply emplacement by forceful intrusion. Areas of granitization are of restricted occurrence and regarded as by-products of magmatic activity.

Crystal fractionation of a noritic parental magma is regarded an inadequate explanation for the intrusive sequence. After the noritic magmas had crystallized, additional magmatic pulses produced the major intrusive mass and the three small leucocratic bodies. These later intrusives are regarded as originating from distinct magmas that were not sequential differentiates of noritic magma. Each later intrusion represents a disconnected magma that crystallized individually. After emplacement of the major intrusion and before emplacement of the leucocratic units, the major intrusion differentiated in place and produced a marginal tonalite that grades into a core of granodiorite.

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