Proterozoic tectonic evolution of southern Laurentia new constraints from field studies and geochronology in southern Colorado and northern New Mexico, U.S.A. /

by 1975- Jones, James V.

Field studies and U-Pb geochronology from the Sangre de Cristo Mountains, southern Colorado, provide new constraints on the Proterozoic tectonic evolution of southern Laurentia. Early deformation and metamorphism was broadly coeval with arc-related magmatism between ca. 1750 – 1720 Ma and produced gneissic fabrics that are subvertical and strike NW. These NW-striking fabrics are interpreted to have formed during early, NE – SW convergent tectonism, perhaps in outboard arcs, prior to northward accretion to the southern margin of Laurentia. Whereas early deformation was penetrative in character, subsequent Proterozoic deformation was strongly localized throughout the range. Emplacement of granitic and tonalitic plutons occurred between ca. 1695 – 1682 Ma and was coeval with widespread deposition of thick sequences of supermature quartzite. Magmatism and quartzite sedimentation are interpreted to result from contemporaneous crustal responses to collapse or relaxation of the Yavapai accretionary orogen between ca. 1700 – 1660 Ma. Deformation related to the Mazatzal Orogeny occurred at 1637±6 Ma locally, involving dextral shear in NEtrending shear zones and producing subhorizontal, NW – SE shortening features such as subvertical, NE-striking foliation and large scale, upright folding of older layering. Quartzite is inferred to have been deformed during this time and buried 4 to granitic emplacement depths (10 – 15 km). Two newly-dated Mesoproterozoic granitic intrusions were emplaced at 1434±2 Ma and 1407±6 Ma, respectively, accompanied by development of a NE-striking, subvertical tectonic foliation and localized shear zones between ca. 1420 – 1412 Ma. The map-scale geometry of these intrusions and coeval deformation suggest that ca. 1.4 Ga granites were emplaced into a broadly convergent strain field, and the inhomogeneous character of deformation is consistent with models for NW – SE shortening deformation throughout the region.