TY - JOUR

T1 - Late thermal evolution of Proterozoic rocks in the northeastern Llano Uplift, central Texas

AU - Rougvie, J. R.

AU - Carlson, W. D.

AU - Copeland, P.

AU - Connelly, J. N.

N1 - Funding Information: Support for this project came in part from student research grants from GSA, Sigma Xi and the Gulf Coast Association of Geological Societies. Additional funding came from NSF grant No. EAR-9219484 to W. Carlson and S. Mosher, and the Geology Foundation of the University of Texas at Austin. IL samples were collected in Inks Lake State Park under Texas Parks and Wildlife permit 16-91. The authors thank L. Land for assistance with oxygen isotopic analysis; L. Mack, T. Housh, L. Long and N. Walker for assistance with Rb–Sr analysis; K. Manser for assistance with U–Pb analyses; M. Heizler for assistance with sample irradiation; and T. Spell for assistance with analysis. They thank R. Reed, M. Helper, L. Long and R. Roback for helpful discussions. The paper benefited from reviews by M. Heizler and K. Karlstrom. Additionally, R. Reed is thanked for assistance with map figures.

PY - 1999

Y1 - 1999

N2 - The Llano Uplift of central Texas, USA contains large exposures of polydeformed Grenvillian metaigneous and metasedimentary rocks that were intruded by late- to post-orogenic granites. New U-Pb and 40Ar/39Ar single mineral ages and Rb-Sr mineral isochrons document the retrograde cooling history that followed Grenville-age metamorphism in the north-central and northeastern Llano Uplift, and explain discrepancies between previous U-Pb crystallization ages and younger determinations based on Rb-Sr and K-Ar isotopic systems. Dating has been carried out in the Valley Spring Gneiss (VSG) and the post-tectonic Lone Grove Pluton (LGP) in three areas of the Llano Uplift, from west to east: VSG rocks in the Babyhead area north of Llano, Texas, yielding hornblende, muscovite and biotite 40Ar/39Ar ages and Rb/Sr mineral isochron ages from ~1098 to ~1014 Ma; a sample from the LGP at Petrick Quarry, yielding U-Pb titanite and hornblende and biotite 40Ar/39Ar ages from ~1096 to ~1076 Ma; and VSG samples from the Inks Lake area in the eastern Llano Uplift, yielding U-Pb titanite, hornblende and biotite 40Ar/39Ar, and Rb-Sr mineral isochron ages from ~1087 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 Llano Uplift from ~1100-1000 Ma at rates between 5 and 14°C Ma-1. Although rapid uplift following early high-pressure metamorphism has important regional thermal effects, the cooling rates documented in this study are interpreted to represent the relaxation of local geotherms following local magmatic activity. The low-temperature history documented in this study is probably unrelated to uplift rates. This supports the suggestion that magmatic heating was an important factor in the late high-temperature/low-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.

AB - The Llano Uplift of central Texas, USA contains large exposures of polydeformed Grenvillian metaigneous and metasedimentary rocks that were intruded by late- to post-orogenic granites. New U-Pb and 40Ar/39Ar single mineral ages and Rb-Sr mineral isochrons document the retrograde cooling history that followed Grenville-age metamorphism in the north-central and northeastern Llano Uplift, and explain discrepancies between previous U-Pb crystallization ages and younger determinations based on Rb-Sr and K-Ar isotopic systems. Dating has been carried out in the Valley Spring Gneiss (VSG) and the post-tectonic Lone Grove Pluton (LGP) in three areas of the Llano Uplift, from west to east: VSG rocks in the Babyhead area north of Llano, Texas, yielding hornblende, muscovite and biotite 40Ar/39Ar ages and Rb/Sr mineral isochron ages from ~1098 to ~1014 Ma; a sample from the LGP at Petrick Quarry, yielding U-Pb titanite and hornblende and biotite 40Ar/39Ar ages from ~1096 to ~1076 Ma; and VSG samples from the Inks Lake area in the eastern Llano Uplift, yielding U-Pb titanite, hornblende and biotite 40Ar/39Ar, and Rb-Sr mineral isochron ages from ~1087 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 Llano Uplift from ~1100-1000 Ma at rates between 5 and 14°C Ma-1. Although rapid uplift following early high-pressure metamorphism has important regional thermal effects, the cooling rates documented in this study are interpreted to represent the relaxation of local geotherms following local magmatic activity. The low-temperature history documented in this study is probably unrelated to uplift rates. This supports the suggestion that magmatic heating was an important factor in the late high-temperature/low-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.

KW - Cooling

KW - Geochronology

KW - Llano Uplift

KW - Metamorphism

KW - Proterozoic

U2 - 10.1016/S0301-9268(98)00103-X

DO - 10.1016/S0301-9268(98)00103-X

M3 - Journal article

AN - SCOPUS:0033102606

VL - 94

SP - 49

EP - 72

JO - Precambrian Research

JF - Precambrian Research

SN - 0301-9268

IS - 1-2

ER -