Abstract

The Llano uplift is a gentle structural dome that exposes Middle Proterozoic crystalline rocks of Grenville affinity. Zircon U-Pb geochronometry of polydeformed metaigneous rocks and post-tectonic plutons in the 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 crust was then perforated by a suite of coarse-grained granitic plutons the emplacement ages of which range from 1116 to 1070 Ma.

When considered in conjunction with the present geometric organization of metamorphic units within the uplift, the U-Pb data suggest that contacts between some of the metamorphic units are tectonic. As in the case of Grenville tracts elsewhere, it appears that the Llano crust is structurally imbricated. The Middle Proterozoic temporal and geologic evolution of the uplift is similar to that described from other Grenville-age terranes in North America, thus verifying the synchroneity of orogenic processes along most of the Middle Proterozoic oceanward margin of Laurentia.

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