MIDDLE PROTEROZOIC GEOLOGIC EVOLUTION OF LLANO UPLIFT, TEXAS - EVIDENCE FROM U-PB ZIRCON GEOCHRONOMETRY

The Llano uplift is a gentle structural dome that exposes Middle Prote rozoic crystalline rocks of Grenville affinity. Zircon U-Pb geochronom etry of polydeformed metaigneous rocks and post-tectonic plutons in th e uplift defines a more protracted and complex geologic evolution than previously documented. Results indicate that the uplift harbors crust at least 1,303 m.y. old, demonstrate that igneous protoliths of some metamorphic units were emplaced between 1,252 and 1,232 m.y. ago, and constrain major Grenville deformational processes to have taken place between 1,232 and 1,116 m.y. ago. This metamorphosed and deformed crus t was then perforated by a suite of coarse-grained granitic plutons th e emplacement ages of which range from 1116 to 1070 Ma. When considere d in conjunction with the present geometric organization of metamorphi c units within the uplift, the U-Pb data suggest that contacts between some of the metamorphic units are tectonic. As in the case of Grenvil le tracts elsewhere, it appears that the Llano crust is structurally i mbricated. The Middle Proterozoic temporal and geologic evolution of t he uplift is similar to that described from other Grenville-age terran es in North America, thus verifying the synchroneity of orogenic proce sses along most of the Middle Proterozoic oceanward margin of Laurenti a.