2009-04-20_GENERAL DOCUMENTS - M2009076 (6)

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FOLIATION AND LINEATION 0 All the gneissic Precambrian rocks and some of the granitic rocks are characterized by well-developed foliation and lineation. Foliation in the gneissic rocks is expressed by compositional layering in the rock and by preferred planar orientation of platy and tabular minerals. With few exceptions both features are parallel, and they are parallel to the major lithologic layers shown on plate 2. This type of foliation probably represents bedding in the original sediments. Where the rocks were granulated by the younger Precambrian deformation, a cataclastic type of foliation formed. This foliation is characterized by a subparallel mesh of close-spaced fractures that are healed mainly by quartz. The cataclastic foliation typically is parallel to the older foliation described above, but locally it breaks across it. The granitic rocks commonly have a foliation that is termed a primary flow structure. In the granodiorite, primary flow structure is expressed as an alignment of elongate inclusions parallel to discordant contacts (Harrison and Wells, 1959, p. 12). In the biotite-muscovite granite, tabular feldspar crystals commonly show a similar parallel alignment that has resulted from the flowage of partly crystallized magma. Granodiorite and quartz diorite also show a secondary metamorphic foliation that is similar in character to the foliation in the gneissic rocks. In the Chicago Creek area the secondary foliation in granodiorite is locally superposed on the primary foliation and is continuous with the foliation in the gneissic rocks (Harrison and Wells, 1959, p. 12). The gneissic rocks of the district are characterized by many kinds of lineation, but five categories are recognized: (1) the axes of small folds and crinkles; (2) elongate minerals, such as sillimanite and hornblende, and elongate mineral aggregates, such as small pods of granitic material; (3) the axes of boudinage, or "pinch" structures that formed by stretching; (4) rodding (rod-shaped features that result from the rolling or shearing • between layers); and (5) slickenside striae. The lineations have systematic orientations that can be related to each of the two Precambrian fold systems. Small folds, crinkles, and mineral alignments parallel the major folds of the older fold system throughout the district. Also, small warps, crinkles, boudinage, and sparse mineral alignments are oriented approximately at right angles to the major folds and probably formed at a late stage in the older deformation. Within the zone of younger Precambrian folding, small folds, crinkles, and sparse mineral alignments parallel the younger folds, abundant rodding and slickenside striae are oriented about at right angles to the trends of the younger folds; they formed by the shearing that accompanied the younger folding. FOLDS The gneissic rocks were deformed twice during Precambrian time. The first deformation, which took place at considerable depth in the earth's crust, was pervasive and resulted in major folds that trend sinuously north-northeast. These folds define the structural framework of the district. The second deformation, which took place at somewhat shallower depth, folded the incompetent biotite gneisses and associated rocks along axes that trend N. 55° E. and sheared the more competent microcline gneiss. Folds and shears of the younger deformation, which are superposed on the older Precambrian folds, are largely restricted to the southeast half of the Idaho Springs district (pl. 2). These effects of the younger Precambrian deformation represent part of the Idaho Springs-Ralston shear zone (Tweto and Sims, 1963, p. 998). The major folds of the older deformation are wide and largely open. Their axes trend sinuously nearly north to about N. 50° E. (pl. 2). The Idaho Springs anticline is the dominant fold in the southeastern part of the district. This anticline is one of the major folds in this part of the Front Range, for it is an asymmetric feature that marks the boundary between a large area of rocks that strike mainly northeast on the northwest side of the axis, and a large area of rocks that strike west to northwest on the southeast side of the axis. Tweto and Sims • (1963) interpreted this anticline as an early manifestation of the Idaho Springs-Ralston shear zone, The anticlinal axis trends about N. 60° E. in the southern part of the area and turns to N. 45° E. in the central part; it 11