EVIDENCE FOR CONTINENT-CONTINENT COLLISION: STRUCTURAL ANALYSIS AND DETRITAL ZIRCON PROVENANCE IN THE WESTERN LLANO UPLIFT, CENTRAL TEXAS

The Mesoproterozoic Valley Spring Gneiss in the western Llano Uplift, along an 8 km section of the Llano River west of Castell, TX, was deposited on Laurentia and records an uppermost amphibolite facies, polyphase deformation history consistent with continent-continent collision during a Grenville orogeny.

The Valley Spring Gneiss is characterized by both ortho- and para-gneisses. To determine the provenance, three paragneiss samples were analyzed using the Laser ICP-MS to establish ages for 80 zircons from each sample. All three samples show a major peak at 1200-1350 Ma and a smaller Paleoproterozoic peak. One sample had a small Archean peak. All data are consistent with a Laurentian origin for the sediment, with Mesoproterozoic sediment derived locally.

Five phases of synmetamorphic deformation have been documented. The mineral assemblage, including K-feldspar and sillimanite, indicates conditions above the second sillimanite isograd. Evidence for near-melt conditions throughout deformation is widespread. Leucosomes and quartz/feldspar segregations enveloping garnets are parallel to early foliations. Pegmatites and granitic dikes are associated with late stage folds and ductile shear zones. The earliest deformation is characterized by two generations of isoclinal folds and penetrative metamorphic foliations, S1 and S2, which together form the dominant, NW striking, NE dipping metamorphic layering. The third phase, F3, is characterized by tight folds that fold the S1/S2 foliation with an associated axial planar foliation. The F3 fold axes plunge moderately S65E; axial planes are dominantly NW striking. Open, late generation folds (F4 and F5) refold earlier structures on both an outcrop and map scale. F4 and F5 folds are NE plunging with NE to E striking axial planes or SE plunging with NW striking axial planes. Granites and pegmatites are both syn- and post-tectonic as evidenced by the presence or absence of the S1/S2 foliation, folding and boudinage.

The sequence, style and orientation of the structures in this area correlate with those within the Packsaddle Schist of the western uplift. Orientations differ from those in the eastern uplift, supporting the likelihood that deformation in the eastern uplift is controlled by collision of the exotic arc terrane, whereas the western uplift directly records deep-seated effects of a continent-continent collision.