Aureole structure of 1.4 Ga plutons in southern Colorado and their tectonic implications.

摘要

This study focused on the structural and metamorphic evolution of two 1.4 Ga plutons intruded along the Mazatzal-Yavapai crustal boundary in Colorado. The goal of this study was to evaluate the role 1.4 Ga plutonism played in the tectonic development of southern Laurentia. The 1.44 Ga Oak Creek pluton intrudes upper amphibolite facies gneisses of the Wet Mountains in central Colorado. Quantitative metamorphic petrology and mineral assemblages indicate the pluton intruded at ∼900 MPa and ∼800°C during regional metamorphism, corresponding to paleo-depths of ∼25 km. The Eolus Granite of the Needle Mountains of southwest Colorado intrudes a cover sequence deposited during Mazatzal orogenesis and a sequence of amphibolite facies gneisses, which form the basement. Mineral textures and quantitative pressure-temperature estimates record a metamorphic field gradient adjacent to the pluton from ∼450°C at distance from the pluton, to ∼675°C at the pluton's contact at 300 MPa confining pressures corresponding to paleo-depths of ∼8 kilometers. The Oak Creek Pluton cooled at rates of ∼1°C/Ma following emplacement, whereas the aureole of the Eolus granite cooled at ∼8°C/Ma. Structures within and adjacent to both plutons are consistent with NW-SE shortening and NE-SW extension. These shortening and extension directions are most consistent with a distant transform or transpressive plate boundary along the margin of Laurentia at 1.4 Ga. The contrasting depth of burial of the two plutons and their distinct cooling histories requires major differential exhumation along the Mazatzal Front after 1.4 Ga.