--> ABSTRACT: Polyphase Deformation of Middle Proterozoic Coal Creek Serpentinite, Llano Uplift, Texas, by Gretchen M. Gillis; #91036 (2010)

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Polyphase Deformation of Middle Proterozoic Coal Creek Serpentinite, Llano Uplift, Texas

Gretchen M. Gillis

The middle Proterozoic Coal Creek serpentinite (1.2-1.0 billion years ago) of the southeastern Llano uplift, Gillespie County, Texas, is the only known serpentinite of Grenville age and is a critical part of tectonic models for Texas and North America during the Precambrian. The Coal Creek serpentinite is considered to be the altered remnant of the ultramafic tectonite portion of an ophiolite. Detailed mapping of the Coal Creek serpentinite and adjacent Packsaddle schist and Big Branch gneiss has demonstrated that the serpentinite and adjacent rocks have been multiply deformed synchronously with amphibolite facies metamorphism, indicating a complex post-emplacement (and perhaps emplacement) history.

Folds of the pervasive northeast-striking regional metamorphic foliation (layering) trend predominantly west-southwest and south-southeast. Near the contact with the serpentinite, boudinage with extension in approximately north-south and approximately east-west directions affects the Packsaddle schist, recording high extensional strains. Coal Creek serpentinite is massive, with two orientations of crosscutting mineralogical layering, to schistose, with up to four foliations. Chloritized Coal Creek serpentinite shows two crenulation cleavages that correspond in orientation to folds within the Packsaddle schist.

The polyphase deformation of the Coal Creek serpentinite is consistent with the complex synmetamorphic deformational history documented elsewhere in the Llano uplift. If the Coal Creek serpentinite is indeed evidence of Grenville-age subduction, its subsequent deformation history requires collision of an island-arc or continental mass with the Texas craton prior to 1 billion years ago.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.