PALEOPROTEROZOIC REGIONAL METAMORPHISM IN THE NEEDLE MOUNTAINS, COLORADO

Crystalline rocks of the Needle Mountains, SW Colorado can be divided into two sequences. The first sequence is composed of Paleoproterozic orthogneiss and amphibolite formed in an arc. Unconformably overlaying the gneiss is a cover sequence composed of metapelites and thick accumulations of quartzite (Uncompahgre formation). Both sequences are intruded by the 1.44 Ga Eolus granite. Prior work in the Uncompahgre formation and in the basement to the east of the Eolus granite has documented a contact aureole around the granite which ranges from greenschist facies metamorphic conditions at distance from the granite to upper amphibolite facies at the granite contact. Confining pressure is estimated to have been ~.3 GPa. We present new metamorphic data on samples from basement gneisses containing the earliest developed foliation and lineation within the gneiss sequence. Peak metamorphic assemblages in the sample considered are: garnet-biotite-hornblende-plagioclase-chlorite-quartz-epidote. Muscovite occurs as a retrograde phase. Compositional zoning in garnet determined by wavelength dispersive scanning X-ray mapping shows that the garnet likely grew in two stages. Garnet cores are higher in Ca and lower in Mg then the rims. Zoning profiles show an abrupt step in Ca from core to rim suggesting either poly-metamorphism or a change in the garnet producing reaction. The lack of inclusions in the garnet do not allow us to determine which of these two processes is responsible for the step. Garnet rim compositions were paired with compositions of matrix phases to estimate P-T conditions. Estimated conditions are 593 +/- 33 ^o C and .64 +/- .24 Gpa. If chlorite is excluded from the assemblage conditions of 577 +/- 52 ^o C and .55 +/- .16 GPa are obtained. Thus P-T estimates are consistent with burial to ~.6 GPa at temperatures over 575 ^o C. These pressure estimates are distinctly higher than those obtained from the contact aureole of the Eolus Granite. It is likely that deep burial and upper amphibolite facies metamorphism in the gneisses were accomplished by crustal thickening during arc magmatism and accretion to the North American Plate margin.