COAL CREEK SERPENTINITE: A COMPLEX RECORD OF ULTRAMAFIC SERPENTINIZATION AND GRENVILLE-AGED TECTONISM ALONG THE SOUTHERN MARGIN OF LAURENTIA

The Coal Creek Serpentinite in the Llano Uplift of central Texas records a complex history of Grenville-aged serpentinization during emplacement, deserpentinization during prograde dynamothermal metamorphism, and a much later post-Grenville reserpentinization when the ultramafic body was exposed at the surface. The large (6 km by 2.3 km) tabular harzburgite protolith was structurally imbricated with an exotic ensimatic arc and likely represents the mantle portion of an obducted ophiolite. The arc was thrust northward over other Llano uplift rocks along the southern margin of Laurentia.

Evidence of concurrent serpentinization during emplacement includes tremolite and magnetite slickenlines within the thrust zone between the serpentinite and arc related rocks and a continuous foliation between mafic dikes, blackwall zones, and serpentinite. The ultramafic and arc plutonic complex underwent polyphase folding and foliation formation under amphibolite facies conditions. Deserpentinization during dynamic metamorphism is indicated by layers of relic aligned olivine alternating with enstatite, both with metamorphic chemistries, that parallel the predominant fabric. At peak temperatures, randomly oriented anthophyllite and tremolite overgrew the foliations; as temperatures and pressures decreased, talc partially replaced tremolite and anthophyllite. The disequilibrium assemblages and structural fabrics allow a P-T-t path to be estimated; peak conditions ranged between 685°C and 810°C at pressures of 0.8-1GPa, indicating that the arc was overridden by a continental block during the Grenville-aged orogeny. Mesh-texture Al-poor lizardite subsequently replaced most of the serpentinite; d¹⁸O values indicate formation by interaction with meteoric waters. The metamorphosed ultramafic was subaerially exposed by Late Cambrian, again during Late Pennsylvanian, and after Miocene uplift.