The Llano Uplift Coal Creek serpentinite is one of few exposed Grenville-age ultramafic rocks and is an important element in reconstructing the tectonic history of the Grenville orogeny in Texas, USA. During Grenville-age collisional orogenesis, the ultramafic harzburgite protolith underwent dynamothermal prograde metamorphism, which resulted in partial to complete serpentinization during emplacement, deserpentinization during subsequent regional high-temperature, medium-pressure metamorphism, and continued post-kinematic metamorphism. An unrelated late (post-Grenville) serpentinization occurred when the ultramafic rocks were exposed near or at the surface.
Disequilibrium assemblages and structural fabrics allow the pressure−temperature−time path to be estimated with peak conditions ranging between 685 °C and 810 °C and pressures of 0.8−1 GPa. Although most of the serpentinite has been replaced by lizardite, layers of incompletely replaced olivine and enstatite have metamorphic chemistries and are truncated by randomly oriented anthophyllite. Foliated metasomatic blackwall zones of tremolite, Mg chlorite, talc, and magnetite have a continuous foliation with the adjacent serpentinite.
Tectonic emplacement of the serpentinite with a northward vergence occurred early in the deformational history of the Llano Uplift. The serpentinite was structurally imbricated with an exotic ensimatic arc and most likely represents the mantle portion of an obducted ophiolite. The serpentinite and ensimatic arc underwent deformation and regional amphibolite facies metamorphism together as a package, necessitating the overriding of some southern continental block. This polyphase deformational and metamorphic history is similar to the rest of the southeastern uplift and consistent with continent-arc-continent collision. The Texas Grenville-aged tectonic history provides an unambiguous record of Mesoproterozoic orogenic events not obscured by any older or younger orogeny.
