The Hall Valley area, in northwestern Park County, Colorado, contains foliated high-grade Precambrian metamorphic rocks resulting from recrystallization of a complexly folded and faulted clastic sedimentary sequence containing two series of strata separated by an angular unconformity of regional extent. Some of the metamorphic rocks were further modified by migmatization and granitization, which seem to have followed an interval of extensive faulting and dislocation of the previously metamorphosed rocks. Field and laboratory data for rocks outside areas of extensive migmatization and granitization indicate the fabric, composition, and probable manner of deposition of the sedimentary layers that were present before metamorphism and provide an argument against continued use of the terms Idaho Springs formation and Swandyke gneiss for the meta-sedimentary rocks of the Hall Valley area.

The history of the foliated metamorphic rocks and the later migmatites and granitic rocks is interpreted as follows: (1) Deposition of a thick series of quartzose sandstones, arkoses, shales or mudstones, and limestones was terminated by uplift and gentle folding and the development of an extensive erosion surface. (2) Sedimentation resumed, and a thick, lenticular sequence of clastic quartzo-feldspathic sedimentary rocks intercalated with abundant clastic materials of probable basaltic composition was deposited over the unconformity. (3) Mafic dikes and sills were intruded into the sedimentary rocks during an interval that may have overlapped the time interval of deposition of the sedimentary rocks above the unconformity. (4) Deformation produced northwest-trending folds and was accompanied in its later stages by elevation of temperature and recrystallization of the sedimentary and igneous rocks into high-grade metamorphic rocks. (5) After a time interval of unknown duration the rocks were again dislocated along a fault of considerable displacement and northeast strike, almost at right angles to the previously formed northwest structures. (6) Migmatization and granitization later than the fault resulted from pervasive metasomatism southeast of the fault and to a lesser extent northwest of the fault. Metamorphic rocks outside the areas of migmatization show mild retrogressive changes, possibly caused by widespread penetration of solutions generated during migmatization and granitization. (7) A body of Boulder Creek granodiorite was intruded into the northeast portion of the Hall Valley area. Although the granodiorite appears to cut across the fabric of the migmatites and granitic rocks, it may be genetically related to them. (8) The area was intruded by several dikes and irregular bodies of felsic rocks that probably are related to similar rocks of Tertiary age elsewhere in the Front Range.

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