MESOPROTEROZOIC GEOLOGY OF NORTHEASTERN LLANO UPLIFT, CENTRAL TEXAS

Mesoproterozoic, regional-dynamothermal metamorphites and mostly post-deformational granitic plutons of the inner Llano mobile belt are exposed in the Llano Uplift region of central Texas. In the area E and SE of Lake Buchanan, Llano and Burnet Counties, a field mapping based study within a 92 km2 area reveals the exceptional character of the northeastern Llano Uplift and contributes to the geological history of the region. Lithological mapping units, rock textures and recognition of established rock packages suggest a >6km thick, tectonically stacked sequence of heterogeneous para- and orthogneisses and schists. Most abundant are pink and pink-gray, quartzo-feldspathic gneisses of the Valley Spring domain (VSD). Less frequent are dark colored, schistose gneisses, marble and schists with pyroxene, hornblende, sillimanite, and graphite grouped to represent Packsaddle domain (PD) rocks. Domain rocks are in contact with the eastern portion of the Lone Grove granite pluton (1.09 Ga) in the W and the Kingsland granite pluton (1.07 Ga) in the S of the area mapped. Youngest are two small dike bodies of the Sheridan olivine basalt (1.006 Ga).

VSD and PD metamorphites are polymetamorphic (Me to Ma) and polydeformed. (D1 – D5). A P-T-t path can be derived taken data from the Hussey and Whitt metabasites W of the area mapped. Comparable eclogite facies conditions (Me) of P max. of 2.2 [GPa] / 631 ^oC (Whitt) and P max. of 2.01 [GPa] / 570 ^oC (Hussey) can be assumed to have also been reached about 1130 Ma (Oxford metabasite) ago in the northeastern Llano Uplift.

A change to retrograde amphibolite conditions followed associated with tectonic transport and emplacement (D3), formation of dominant shear foliation S3, aqueous fluids access, possible overturning and stacking (?) of lithotectonic sequences, and folding effects during D4 and D5, and exhumation. In all, the collisional Llano orogenic events may have spanned a duration of <50 Ma during the assembling of Rodinia.