Petrologic significance of staurolites in the Llano Uplift

Conference · Mon Feb 01 04:00:00 EST 1993 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:6003471

Carlson, W D; Reese, J F ^[1]

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1. Univ. of Texas, Austin, TX (United States). Dept. of Geological Sciences

The recent discovery of rocks with near-end-member Fe-staurolite in the SE Llano Uplift, together with the regional absence of Fe-Mg staurolite except as inclusion in garnet in a single nearby locality, confirms that the SE Uplift experienced a polybaric metamorphic history similar to that documented for the N and NW Uplift. The units containing Fe-rich staurolite are part of a unique suite of rocks within the Packsaddle Group (Rough Ridge Fm) exposed in Whites Creek. Although the predominant rock types in the Packsaddle Schist are felsic schists, felsic gneisses and amphibolites, this pelitic metasedimentary package includes interlayered andalusite-bearing rocks, quartzites, and muscovite schists. Although textural evidence is equivocal, growth of staurolite during dynamothermal metamorphism is indicated by the preferred orientation of elongate crystals parallel to the rock's dominant foliation. The unusual Fe-rich staurolites reported here are the only known occurrence of the mineral in the Uplift except for staurolite (X[sub Fe] = 0.72) described as inclusions within a large garnet from a locality in the Rough Ridge Fm [approximately]3 km to the west. The matrix surrounding this garnet is entirely devoid of staurolite. The contrast between these occurrences indicates that staurolite of ordinary Fe/Mg ratio grew during an early moderate-P to high-P dynamothermal episode, but was destroyed during later low-P static metamorphism. In contrast, near-end-member Fe-staurolite originating in the early event survived later static metamorphism. These observations are consistent with the polybaric history (dynamic metamorphism at P[>=] [approximately]6--8 kbar followed by static recrystallization at P[approximately]2--3 kbar) deduced from assemblages, textures and thermobarometry on metamafic rocks from the N and NW Llano Uplift.

• OSTI ID: 6003471
• Report Number(s): CONF-9303212--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 25:1
• Country of Publication: United States
• Language: English

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