ECLOGITOGENIC METABASITES, LLANO UPLIFT, CENTRAL TEXAS: ASPECTS OF GEOLOGIC AND METAMORPHIC DEVELOPMENTS

Mesoproterozoic, eclogitogenic metabasites (eclogites, garnet-clinopyroxenites) in the Llano Uplift region appear to be restricted to the upper portion of the Valley Spring domain tectonostratigraphic rock sequence. They form boudin-type bodies <1m to ~500m dia., spatially enclosed by and associated with quartzofeldspathic gneisses. Their petrologic significance is based on their metamorphic and tectonic records.

First metabasite samples, from the Hussey Ranch and Whitt Ranch bodies (central Uplift), were investigated using a CAMECA SX 100 EM. HP-conditions of early regional metamorphism can be inferred from the chemical zonation of garnets. Its corresponding pattern is characterized by (1) an inner core that is Mn-rich and Mg-poor (e.g., in Whitt

#6-4, Alm57Gro23Pyr7Spe13) and (2) a prograde evolution to Mn-poorer and Mg-richer compositions towards the rim (e.g., Alm49Gro19Pyr32Spe0.5 in Whitt #6-4, outermost rim). In garnet from the Hussey Ranch locality, resorption features of the garnet rim and a small overgrowth zone with a slight enrichment of the spessartine component are observed.

A preliminary P-T-t path for the metabasite bodies examined can be reconstructed on the basis of P-T pseudosections, calculated with PERPLE_X in the system Na-Ca-K-Fe-Mn-Mg-Al-Si-Ti-H-O and contoured with isopleths for molar fractions of garnet components. The first portion of this clockwise path on an upright P-T diagram shows a pressure increase to HP/MT conditions of ~17 kbar and 550°C. Subsequently, heating of the eclogite to temperatures up to 670°C occurred at slightly falling pressures. Timing of these HP/MT peak conditions is at ~1130 Ma (Carlson et al., 2007). The aforementioned overgrowth zone of garnet in a Hussey Ranch locality sample formed at ~14 kbar and 660°C.

The derived P-T loop is interpreted to illustrate regional metamorphism and crustal thickening at the southern margin of Laurentia during the Llano orogenic, collisional HP/MT event. Host Valley Spring domain rocks and associated basaltic magmatites were likely buried to ~60 km depth and thermally relaxed at 55-60 km. Medium-grade retrogression, formation of eclogite boudins and their partial to complete amphibolitisation likely correlate with crustal uplifting and exhumation of the Llano orogenic belt rock complexes.