⁴⁰Ar/³⁹Ar age constraints on the deposition and metamorphism of the Uncompahgre Group, southwestern Colorado
— K.E. Karlstrom, D.A. Gonzales, M. Heizler, and A. Zinnser

This paper presents new ⁴⁰Ar/³⁹Ar dates that constrain the age of deposition and of metamorphism of the Uncompahgre Group. This 2.5-km-thick quartzite and phyllite succession overlies granites as young as 1707 Ma and contains youngest detrital zircons of about 1709 Ma such that 1707-1709 Ma is its maximum age. A new ⁴⁰Ar/³⁹Ar date on muscovite from a (rare) granitic dike that crosscuts the Uncompahgre Group near Coal Bank Pass may approximate crystallization age and provides a minimum age for the Uncompahgre Group of 1704±5 Ma. Thus, the depositional age now appears constrained between 1707 and 1704 Ma. Samples within ~5 km of the 1435 Ga Electra Lake Gabbro give ⁴⁰Ar/³⁹Ar hornblende dates of ~1.46 Ga interpreted as either 1.46 Ga hornblende growth during Mesoproterozoic regional metamorphism or resetting of older hornblende in the aureole of the 1435 Ma pluton. Biotite dates of 1437±5 and 1438±5 Ma are interpreted to record metamorphism associated with emplacement of the 1435 Ma Electra Lake Gabbro. Samples in the northwestern Needle Mountains, at greater distance from the plutons, preserve evidence of older metamorphic events and indicate that the western and northwestern parts of the Needle Mountains were not heated above 350-500°C after ~1.6 Ga. An ⁴⁰Ar/³⁹Ar hornblende age of 1728±8 Ma in Irving Formation amphibolites is interpreted to represent a minimum age of hornblende growth; a 1607±6 Ma hornblende plateau may reflect additional alteration and resetting associated with younger (1.61 or 1.43 Ga) metamorphisms. Andalusite and sillimanite in the Uncompahgre Group define isograds of the contact aureole of the Eolus Granite and are interpreted to have grown ~1435 Ma. Andalusite in phyllites of the Uncompahgre Group overgrow two deformation fabrics and are deformed by a third, indicating polyphase deformation, with D1 and D2 pre-dating 1435 Ma pluton emplacement and D3 synchronous with pluton emplacement. The relative importance of 1.70-1.60 Ga versus 1.46-1.40 Ga deformation in forming D1 and D2 fabrics, as well as t-T history of the Uncompahgre Group from the surface to ~8 km depth by 1704 Ma and to 12-15 km depths by 1435 Ma remain unresolved. Fine grained muscovite gives ⁴⁰Ar/³⁹Ar ages as young as 1398±6 Ma suggesting slow cooling following emplacement of plutons.