PRECAMBRIAN BASEMENT MAP OF COLORADO--A GEOLOGIC INTERPRETATION OF AN AEROMAGNETIC ANOMALY MAP

Precambrian basement rocks of Colorado primarily compose the Colorado province, a >500-km-wide belt of Proterozoic arc-related volcanic-plutonic rocks that was accreted to the Archean Wyoming province along the Cheyenne belt at 1.78-1.74 Ga. The arc rocks were deformed and metamorphosed by an older province-wide, episodic thermotectonic episode resulting from collision and subsequent subduction along the Cheyenne belt during the interval 1.78-1.65 Ga. Following a stable interval of ca. 200 m.y., the rocks were deformed again by a province-wide, intracratonic episode of coeval ductile shearing and folding on northeast axes, metamorphism, and intrusion of voluminous alkali-calcic granitic plutons (~1.45-1.42 Ga).

The aeromagnetic map of Colorado provides a means to improve knowledge of the extent of the Proterozoic lithologic units and structures in the subsurface. In general, the Paleoproterozoic metavolcanic and metasedimentary rocks are essentially non-magnetic, whereas the abundant coeval granitic intrusions are mainly moderately magnetic. The Mesoproterozoic intrusive rocks are in part magnetic and in part non-magnetic, depending on their iron-oxide mineralogy. Ilmenite-bearing plutons, such as the Log Cabin and Sherman batholiths—north of the Skin Gulch shear zone are non-magnetic; whereas, magnetite-bearing plutons, such as the Vernal Mesa and Eolus batholiths in southwestern Colorado, are moderately magnetic. The third type of 1.4 Ga plutons—peraluminous, two-mica granitic intrusions —which dominate plutons of this age in central Colorado, are mainly moderately magnetic, although some bodies are not uniformly magnetized. The only known 1.0 Ga pluton, the Pikes Peak batholith in the southern Front Range, causes a pronounced negative anomaly, which extends eastward under a cover of Phanerozoic strata. Prominent northeast-trending negative anomalies mainly reflect 1.4 Ga ductile shear zones. In covered areas, many northeast-trending lows are likewise interpreted as representing 1.4 Ga shear zones. These shears not only were important structure elements of the Colorado province but, through activation, controlled localization of the Laramide Colorado mineral belt as well as other younger plutons and tectonic and topographic features.