Late thermal evolution of Proterozoic rocks in the northeastern Llano Uplift, central Texas

The Llano Uplift of central Texas, USA contains large exposures of polydefo rmed Grenvillian metaigneous and metasedimentary rocks that were intruded b y late- to post-orogenic granites. New U-Pb and Ar-40/Ar-39 Single mineral ages and Rb-Sr mineral isochrons document the retrograde cooling history th at followed Grenville-age metamorphism in the north-central and northeaster n Llano Uplift, and explain discrepancies between previous U-Pb crystalliza tion ages and younger determinations based on Rb-Sr and K-Ar isotopic syste ms. Dating has been carried out in the Valley Spring Gneiss (VSG) and the p ost-tectonic Lone Grove Pluton (LGP) in three areas of the Llano Uplift, fr om west to east: VSG rocks in the Babyhead area north of Llano, Texas, yiel ding hornblende, muscovite and biotite Ar-40/Ar-39 ages and Rb/Sr mineral i sochron ages from similar to 1098 to similar to 1014 Ma; a sample from the LGP at Petrick Quarry, yielding U-Pb titanite and hornblende and biotite Ar -40/Ar-39 ages from similar to 1096 to similar to 1076 Ma; and VSG samples from the Inks Lake area in the eastern Llano Uplift, yielding U-Pb titanite , hornblende and biotite Ar-40/Ar-39, and Rb-Sr mineral isochron ages from similar to 1087 to similar to 1029 Ma. Titanite from the Oatman granite to the southwest of these areas yields a U-Pb age of 1119 +/- 6 Ma. These ages , combined with closure-temperature estimates, document cooling in the Llan o Uplift from similar to 1100-1000 Ma at rates between 5 and 14 degrees C M a(-1). Although rapid uplift following early high-pressure metamorphism has important regional thermal effects, the cooling rates documented in this s tudy are interpreted to represent the relaxation of local geotherms followi ng local magmatic activity. The low-temperature history documented in this study is probably unrelated to uplift rates. This supports the suggestion t hat magmatic heating was an important factor in the late high-temperature/l ow-pressure metamorphism of the Llano region. It further suggests that the late pressure and temperature conditions recorded in the rocks were likely not synchronous across the region. (C) 1999 Elsevier Science B.V. All right s reserved.