Abstract

Birch Creek pluton is a small body of quartz monzonite and granodiorite of Cretaceous age that was emplaced within Precambrian argillite and dolomite comprising part of the steep eastern limb of the White Mountain anticline, a major south-plunging fold. The granitic magma was intruded initially along a nearly vertical fault parallel to and within the eastern limb of the anticline. The dolomitic rocks were heated and decarbonated during intrusion of the granite. We suggest that the liberated CO2 entered the vapor phase of the magma, causing isothermal crystallization of the magma over and around most of the pluton. Moreover, the viscosity of the magma increased in the upper part of the magma column so that a relatively rigid shell of granite and metamorphosed wall rocks was formed which prevented continued upward and eastward intrusion. Yielding to the pressure of rising magma beneath, the viscous magma bulged laterally in a northwest direction where structural interpretations suggest the dolomite shell was very thin or absent. The forcible nature of this stage of emplacement is clearly shown in the core of the anticline by the curvilinear traces of strata, crestal axes of folds, and pre-pluton faults, which wrap concordantly around the northwest corner of the pluton.

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