PALEOMAGNETISM AND 40AR/39AR GEOCHRONOLOGY OF THE LATEST CRETACEOUS WHITEHORN GRANODIORITE AND HOST STRATA, SOUTHERN MOSQUITO RANGE, COLORADO

Exposed at the southern end of the Mosquito Range,near Salida,Colorado,the Whitehorn Granodiorite and related dikes intrude predominantly Pennsylvanian sedimentary rocks and, locally,Precambrian rocks. The pluton is about 8 km in east-west and about 25 km in north-south dimension. The pluton has yielded K-Ar mineral (hornblende and biotite) dates ranging from 73 to 66 Myr; emplacement is interpreted to have postdated the onset of Laramide style contractional deformation in the area. Geobarometry on contact zone mineral assemblages yields calculated pressures from about 3.4 to 4.1 Kb and it is plausible that the area has experienced some 10 km of exhumation since the latest Cretaceous. We have collected a total of 32 sites in the Whitehorn Granodiorite and related rocks (23 sites in granodiorite, 3 sites in dikes, 6 sites in Pennsylvanian strata in contact with the pluton). For granodiorite sites yielding essentially a single component of magnetization in either AF or thermal demagnetization, NRM intensities range from about 1.2 to about 0.6 A/m. Magnetizations characteristic of most sites in the granodiorite, related dikes, and remagnetized host rocks are exclusively of normal polarity and are of northwest declination and moderate positive inclination (the grand mean of 16 sites demagnetized, including two sites in remagnetized host rocks, is Declination=334.3^o,Inclination=57.5^o, a 95=4.2^o,k=78.8). Biotite separates from two fresh samples obtained from different quarries yield weighted mean ages, over heating steps that released more than 90 percent of cumulative 39Ar, of 68.36+/-0.21 (WH-2) and 68.85+/-0.21 Ma (WH-26). Hornblende separate from WH-2 yields an isochron age of 68.2+/-0.6 Ma; that from WH-26 yields a weighted mean age of 69.06+/-0.43 Ma. This insitu magnetization, probably acquired during Chron 31n, differs from an expected latest Cretaceous(about 70 to 66 My) direction(about 340/62 to 347/61). The discordance is tentatively interpreted as a slight (less than 15^o) down to the east tilting of the Whitehorn Granodiorite and host rocks, located on the eastern limb of the broad Sawatch anticline that developed during Laramide style deformation in central Colorado and is consistent with available fission track data that define an east-tilted geometry for the Precambrian of this area.