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Relationship of Precambrian quartzite-schist sequence along Coal Creek to Idaho Springs Formation, Front Range, Colorado

1964; United States Government Publishing Office; Linguagem: Inglês

10.3133/pp454o

2330-7102

J. D. Wells, Douglas M. Sheridan, A. L. Albee,

Geological Modeling and Analysis

The lithology, metamorphic grade, and structural history of the quartzite-schist sequence along Coal Creek 5 miles south of Boulder, Colo., are similar to these of the high-grade metamorphic gneisses of the Idaho Springs Formation elsewhere in the Front Range.Accordingly, the quartzite-schist sequence is to be regarded as a rather localised lithologic facies of the predominantly metasedimentary Idaho Springs Formation and not as a tectonically younger stratigraphic unit of lower metamorphic grade as previously described.Four thick layers of quartzite and three of schist are preserved in the Coal Creek syncline a major fold that plunges northeast.The quartzite contains sedimentary structures, is fine to coarse, light colored, locally conglomeratic, micaceous, and andalusite bearing.The schist layers consist mainly of muscovite, biotite, and quartz and contain andalusite, cordierite, and staurolite.South of the Coal Creek syncline are gneisses typical of the Idaho Springs Formation.These include feldspar, hornblende, and biotite gneisses and mica schist in an interlayered sequence characterized by gradational boundaries.Both the quartzite-schist sequence and the gneisses of typical Idaho Springs aspect have been intruded by the Boulder Creek Granite, and by quartz monzonite, hornblende diorite, hornblendite, aplite, and pegmatite.The quartzite-schist sequence and the typical gneisses of the Idaho Springs Formation are different in general aspect, but in detail each contains layers characteristic of the other, and the two sequences intergrade.The schist in the quartzite-schist sequence is virtually identical to schist that is interlayered with and grades into gneisses of the Idaho Springs Formation.Small pods of calcium silicate rock in the two sequences are identical.The common occurrence of the critical muscovite-sillimanite suite in both the Idaho Springs gneisses and the quartzite-schist sequence, plus andalusite and cordierite in the latter, indicates metamorphism in the lower sillimanite zone.Locally the critical sillimanite-microcline suite is present in the gneisses; the presence indicates that the higher sillimanite zone was reached at least locally.The metasedimentary rocks were deformed three times during Precambrian time.During the first period of deformation, a southwestward-plunging mineral lineation formed in the metasedimentary rocks.During the second period, the Coal Creek syncline formed.Lineations associated with this fold are present in the igneous rocks as well as in the metasedimentary rocks.During the third period, cataclastic deformation occurred along the Idaho Springs-Balston shear zone south of the Coal Creek syncline, and distinctive cataclastic rock units were formed.The Boulder Creek Granite was intruded syntectonically during the second period of deformation.It contains mineral lineations and minor folds like those in the Coal Creek syncline but contains no trace of lineations of the first deformation.The quartz monzonite was intruded soon after the Boulder Creek Granite and is closely associated with it; the hornblende diorite and hornblendite were probably intruded at nearly the same time.The aplite and pegmatite were intruded late in the Precambrian after the formation of the Idaho Springs-Ralston shear zone.Three periods of metamorphism recognized in the quartziteschist sequence of the Coal Creek syncline are probably correlative with the three periods of structural deformation.The first period of metamorphism is represented by relict staurolite grains enclosed in andalusite grains.Relict microfolds, probably of the same age as the staurolite, are preserved in andalusite and cordierite grains.The second period of metamorphism is represented by the crystallization of andalusite and cordierite as well as other minerals of the high-grade rocks.This metamorphism took place during folding of the Coal Creek syncline and attendant igneous intrusion.The third period of metamorphism is represented by recrystallization during the cataclastic deformation.The cataclastic rocks contain highgrade metamorphic minerals such as sillimanite and show only slight evidence of retrograde metamorphism.