The Petrology and Structure of the Wet Mountain Metamorphics and the San Isabel Batholith Margin Along South Hardscrabble Creek, Custer County, Colorado

Precambrian metamorphic and igneous rocks, Cambrian dikes, and Quaternary alluvium are exposed along South Hardscrabble Creek, Custer County, Colorado. The Precambrian metamorphic rocks can be mapped as two separate lithodemic units. Lithodeme 1 consists of biotite gneisses and schists interlayered with hornblende gneisses and schists, and amphibolite. The biotite gneisses and schists have a sedimentary protlith of shale, greywacke, and quartz wacke, whereas the amphibolites represent rocks of theoeiitic basalt composition. Some of the hornblende gneisses were fromed when later granitic material invaded the amphibolites to form lit-par-lit or injection gneisses. Lithodeme 2 is a granite gnesiss. A granite protolith for this unit is apparent from xenoliths of lithodeme 1 incorporated in it. The granite was intruded during latter stages of deformation and is probably the source of granite material present in the injection ogneisses of lithodeme 1. These metamorphic rocks are as old as 1,700 Ma. A rhyodacite, dated at 1441+/- 14 Ma, intrudes the metamorphic country rock as small sills. The youngest intrusive event, the San Isabel batholith, was emplaced 1360 +/- 5 Ma. Two textural facies, a coarse- and medium-grained, of the batholith are present. As the batholith cooled, mafic (lamprophyric), aplitic, and pegmatitic dikes were intruded throughout the study area. Some of these melts were coincidently intruded together and formed composite dikes. Cambrian dikes of basaltic composition show signs of deformation from the Laramide Orogeny. The Broadway Alluvium is related to the Pinedale Glaciation and is the youngest unit in the study area. Major structural elements trend northwest. Four prominent joint sets are present in the area. The major joint set trends N63 degrees W, 70 degrees SW, while a minor set trends N48 degrees E and dips steeply to the northwest or southeast. These joints developed due to compressive forces in an east-west direction. This set is interpreted as tensional fractures that developed during cooling and contraction of the San Isabel batholith. The fourth set is nearly horizontal and resulted from unloading during the late Tertiary. Numerous faults of a possible Precambrian origin are common throughout the study area. Many of these faults were reactivated during the Laramide Orogeny.