Abstract

The Calamity Peak pluton is a satellitic granite-pegmatite intrusion located 2 km southwest of the main Harney Peak Granite dome in the Black Hills, South Dakota. Many aspects of the structural development of the Calamity Peak pluton (∼1 km2) mimic those of the larger Harney Peak dome (∼100 km2); thus the model described here for the Calamity Peak pluton provides a general basis for evaluating the evolution of Proterozoic granite-pegmatite intrusives of the Harney Peak Granite system.

The overall structure of the intrusive rocks at Calamity Peak is that of a dome, as shown by the general geometry of the pluton contacts and by a domical pattern of internal textural layering. Textural layering is defined by alternating bands of pegmatite and fine-grained granite 0.1-2.0 m thick. Fine-grained layers are in turn banded on a scale of 2-20 mm owing to modal variations of tourmaline, quartz, and feldspars. Textural differentiation of at least some of these layers resulted from segregation of initially homogeneous magmas into pegmatitic (upper) and aplitic (lower) components during crystallization.

Relationships between the pluton and regionally dipping schistose country rocks, however, do not support a simple domal structure. Schistosity conforms to the contact only along the southern margin of the pluton, the side facing with the regional dip. Country-rock schistosity is at a high angle to the contact at the apparent floor of the pluton, which is exposed in the core of the "dome," and along the northern margin, the side that opposes the regional dip.

In order to explain the structural observations, it is inferred that the initial intrusive body at Calamity Peak consisted of a flat-lying discordant sheet intruded into steeply dipping quartz-mica schist country rock; however, an arcuate sheet, concave downward, cannot be ruled out. New magma pulses (possibly several thousands) were successively emplaced along the hanging-wall contact of earlier pulses, as indicated by the upward truncation of early layers, creating a composite layered pluton. Intrusions of individual magma pulses were localized in three major composite sheets which form structurally and chemically discrete portions of the pluton. Growth of the pluton proceeded by forcible distension and plastic deformation of overlying country rocks; schistosity was molded into a conformable attitude in portions of the hanging wall, and pre-intrusive country-rock lineations were deflected around the pluton.

The layered granite-pegmatite series is crosscut by four successive stages of fracture-filling bodies. The orientations and compositions of these bodies record the late stages in the structural and fluid evolution of the plutonic system.

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