Abstract

Field and petrographic study of the Precambrian Rough Ridge Formation of the Packsaddle Group documents a more complex tectonic history for the Llano Uplift than has previously been reported. The Rough Ridge Formation was affected by at least two metamorphic events and five phases of deformation that may be part of a single progressive event. The predominant mineral assemblages in pelitic and mafic rocks are characteristic of the amphibolite facies at relatively low pressure. Ubiquitous granoblastic polygonal textures overprinting multiple foliations indicate that post-tectonic recrystallization followed dynamothermal metamorphism, a probable consequence of regional reheating during later granite intrusion. The most intense deformation produced isoclinal folds and pervasive axial planar foliation (S2), resulting in complete transposition of original bedding. This foliation cuts an earlier, local foliation (S1) and is folded by small F3 folds and a crenulation cleavage, S3. Both S2 and S3 are cut by a differentiated crenulation cleavage, S4. The most prominent folds in the area and the associated pervasive axial planar cleavage, S5, overprint all earlier structures. In one pelitic unit, oriented inclusions show that garnet formed after S3 and cordierite after S5. Growth and recrystallization of micas, quartz, and feldspars span all phases of deformation. The timing of tectonic and metamorphic events is bracketed by the pretectonic intrusion of the 1,167 ± 15-m.y.- old Red Mountain Gneiss and the post-tectonic intrusion of a 1,080 ± 30-m.y. old melarhyolite dike. Structural and petrologic characteristics of the Rough Ridge Formation indicate that the Llano Uplift underwent a complex tectonic and metamorphic development similar to that described for the Grenville orogeny elsewhere in North America.

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