MESOPROTEROZOIC TECTONIC EVOLUTION OF TEXAS PRIOR TO THE GRENVILLE OROGENY

The southern margin of Laurentia in Texas was extensively reworked by Grenville orogenesis that included polyphase deformation and high-P, high-T regional metamorphism caused by collision of an exotic arc and southern continent. Prior to collision, the Texas Mesoproterozoic records evidence of rifting at ca. 1.4-1.3 Ga and subsequent formation of a continental margin arc and back arc basin at ca. 1.3-1.2 Ga.

Evidence for early rifting occurs near Van Horn, west Texas, where voluminous peraluminous meta-rhyolite flows and welded ash flow tuffs are interlayered with quartzites, meta-arkose, pelitic schist and meta-carbonate. One flow-banded meta-rhyolite yields a U/Pb age of 1332 +7/-3 Ma, supporting the previously reported 1380-1327 Ma age range for these rocks. These units are interpreted to be rift related based on supracrustal associations, lack of intrusive rocks, and existing geochemistry.

Evidence for an active convergent margin with continental arc magmatism and sedimentation from ~1288-1232 Ma is recorded along the entire Texas margin. In the Llano Uplift of central Texas, recent U/Pb geochronology and new mapping has confirmed a major period of magmatism from 1288 to 1232 Ma. Recent geochemistry of these granitic plutonic and volcanic rocks supports a continental arc-related origin. Volcanic rocks are interlayered with volcanoclastic, clastic and carbonate sedimentary rocks and their distribution suggests a transition from deeper water to a continental margin northward. Sedimentary rocks in the north are compatible with an arc source and fluvial, near shore, tidal flat, and probable restricted lagoon environments.

In west Texas, the association of ca. 1250 Ma carbonates, basalts and tuffs of the Allamore and Tumbledown Fms. and of ca. 1260 Ma Castner Fm. carbonates with minor basalts and probably airborne tuffs further west, all support back arc extension further inboard and to the west of the Llano arc. Other coeval rocks in west Texas include 1286 +/- 3 Ma metabasite sills that intrude the 1.33 Ga metarhyolites and show a geochemistry compatible with a back arc setting.