MESOPROTEROZOIC GEOLOGY OF THE INKS LAKE AND SOUTHEAST LAKE BUCHANAN REGION, NORTHEASTERN LLANO UPLIFT, BURNET AND LLANO COUNTIES, CENTRAL TEXAS

The Llano Uplift of central Texas presents an erosional window view of the Texas basement of Mesoproterozoic age (~1.366 to ~1.006 Ga), recording the assemblage of the megacontinent Rodinia along the southern margin of Laurentia. In the study area, granitic magmatites and regional metamorphites represent the Grenvillian Llano mobile belt (LMB) development and were previously dated at ~1.253 to ~1.070 Ga. The LMB rocks have been subdivided into three domains, of which two are present in the northeastern Llano Uplift and in the study area. These are the continental Valley Spring Domain (VSD) and the marine basinal Packsaddle Domain (PD). Study results, based on geologic mapping (scale of 1:10,000) will be integrated with previous studies in the eastern half of the Llano Uplift.

A ~40 km² mapping area is in extension of the study by Morris (2006) who described a ~4.9 km thick tectonically stacked sequence of heterogeneous mapping units. Some of the felsic VSD-gneisses and minor PD schists/gneisses were traced towards the west and southwest into the Inks Lake and southeast Lake Buchanan areas. They are bordered by the post-deformational Kingsland (1.07 Ga) and Lone Grove (1.09 Ga) granite plutons. Polydeformational fabric characteristics are compatible with the previous findings (five deformational phases, D1-D5). Strike of dominant regional foliation S3 changes from east-west to northeast-southwest and defines a southeast plunging antiform. This foliation and associated shear indicators indicate transport movement to the northwest. Fine to medium grained granitic orthogneisses are abundant.

Multistage metamorphic mineral assemblages indicate a range from relict eclogite facies to amphibolite facies conditions, and retrogression of crustal P-T conditions along a clockwise P-T-t path during the Llano orogenic event. A small body of the Sheridan olivine basalt dike (1.006 Ga K/Ar) may indicate a change from contractional to extensional tectonism.