Abstract

The Precambrian Coal Creek serpentinite, Llano Uplift, Texas, occurs within the upper part of the Packsaddle Schist, which appears to represent a thick sequence of shelf-edge volcaniclastic arc-flank metasediments. The serpentinite has a foliation defined by lizardite pseudomorphs after original foliated harzburgite tectonite, which deviates as much as 15° from the regional country-rock foliation. A few samples contain the relict assemblage olivine + orthopyroxene + anthophyllite reflecting re-equilibration to regional metamorphic conditions near 710 °C at ∼3.5 kb. Oxygen and deuterium isotopic data indicate the lizardite could have formed in equilibrium with magmatic water at about 300 to 400 °C during regional uplift.

A likely petrogenetic model involves the olistostromal emplacement of a fragment of ophiolitic material, the Coal Creek ultramafic body, into the volcaniclastic arc-flank sediments. The island-arc model suggested for the Llano Uplift and the existence of ophiolitic material imply that brittle plate collisions were locally important 1,200 m.y. ago and that the orogenic event that affected the Llano terrane (that is, the Grenvillian orogeny) involved island-arc-continent interactions. The Llano Uplift contains the only exposed ophiolitic material along the entire “Grenvillian” orogenic belt.

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