SEDIMENTARY STRUCTURES, PALEO-WEATHERING, AND PROTOLITHS OF METAMORPHIC ROCKS, GRENVILLIAN LLANO UPLIFT, CENTRAL TEXAS
Date: Tuesday, March 12, 1996
Time: 9:40 AM
Place: GEO 100, Geology Building, University of Texas at Austin
Robert M. Reed (e-mail), Rachel A. Eustice, James R. Rougvie, Dept. of Geological Sciences University of Texas, Austin, TX 78712;
and Joseph F. Reese, Dept. of Geology, Idaho State University, Pocatello, ID 83209
The Llano Uplift of central Texas consists of Middle Proterozoic rocks poly-deformed and poly-metamorphosed during Grenville orogenesis. Three major litho-tectonic packages (domains) are defined for metamorphic rocks: the Valley Spring domain (VSD), the Packsaddle domain (PD), and the Coal Creek domain. While intense deformation has commonly obscured the protoliths, rocks with sedimentary or volcanic protoliths have been distinguished and probably make up significant proportions of both the VSD and the PD.
The VSD is composed primarily of felsic gneisses, with subsidiary mafic gneisses, amphibolites, and calc-silicate marbles. Well-layered, laterally extensive felsic gneisses may have volcanic and/or sedimentary protoliths. delta18O values of quartz separates from some of these gneisses are as high as 17-18 per mil and suggest that the protoliths or the source rocks have undergone a weathering cycle prior to metamorphism. A calc-silicate rock from the north-central Uplift preserves peritidal carbonate textures. Mudcracks, algal structures and possible evaporate relicts have been replaced by a garnet-bearing calc-silicate assemblage. Previously described strataform Mn-silicate lenses could be metamorphosed sedimentary Mn deposits.
The PD consists of felsic and mica schists, amphibolites, and many minor lithologies. A PD subarkosic quartzite shows cross-bedding and scour structures in outcrop, and in thin section the outlines of original sedimentary grains are discernible. Pelitic interlayers in this rock contain sillimanite. Quartzites are present elsewhere, although deformation has destroyed primary textures. Extensive marble units in the PD are commonly interlayered with graphitic schists. Both require subaqueous but not necessarily marine conditions. Some pelites are chemically unusual, with highly aluminous layers consisting almost entirely of andalusite, and other layers enriched in Fe (Fe / (Fe+Mg+Ca+Mn) > 0.90 for mafic minerals). These layers are interpreted as laterites formed from intense weathering of exposed argillaceous sediments.
The metamorphosed peritidal carbonate in the VSD requires surface exposure probably at low to mid latitudes during deposition. Laterites in the PD indicate extensive premetamorphic weathering probably at low latitudes. Preservation of the delicate carbonate textures within the VSD calc-silicate rock requires growth of a highly competent assemblage prior to pervasive deformation. Shallow marine carbonates, intense weathering, and a pre-deformation metamorphism are further additions to the increasingly complicated tectonic history of the Llano Uplift.
Hard to believe that peritidal carbonate textures could be preserved in high-grade preCambrian rocks? Then skip this link, because it will introduce you to some ideas about Texas pC rocks that are even harder to believe.