Abstract

The Indian Creek plutons crop out around the northern and northeastern margin of the Rosalie lobe of the Pikes Peak batholith, west of Denver, Colorado. The granite is Silver Plume type and is the youngest pre-Cambrian intrusion of the Denver Mountain Parks region.

The granite appears to have come up along the edge of the mostly consolidated Pikes Peak mass and to have spread in a series of plutons and sills, the mode of intrusion being largely governed by the rock invaded. To the west and northwest, the magma spread a tongue of granite which wedged between quartz monzonite of the Mt. Evans region and its overlying schist. The magma flowed from the south and the southeast to the north and northwest. Erosion has since stripped off most of the schist above and has cut windows through the Indian Creek granite into the quartz monzonite below.

East of this wedge-like tongue, the granite worked its way up lit-par-lit into the Idaho Springs beds, boiled up locally in a series of stocklike centers or plutons, along concentric fracture zones, and spread between beds of schist and across them at low angles. The plutons and their major sills and dikes have a strikingly constant trend from northwest to southeast and lie in four sets, roughly parallel to the northeastern margin of the Rosalie lobe of the Pikes Peak batholith. Early egress of Indian Creek magma probably followed concentric cracks developed during the earlier intrusion of the Pikes Peak granite, and later intrusions followed fractures developed by the collapse of the Rosalie lobe.

When nearly crystallized, shear zones opened, northeast to southwest, in the Indian Creek plutons and permitted a little regenerated Pikes Peak magma to fill the fractures and to crystallize there, with its characteristic pasty textures.

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