--> The Petrology and Tectonics of the Precambrian Panhandle Terrane in West Texas, by M. A. Barnes, G. B. Asquith, C. G. Barnes, K. Miller, and D. E. Smith; #90903 (2001)

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The Petrology and Tectonics of the Precambrian Panhandle Terrane in West Texas

M. A. Barnes1, G. B. Asquith1, C. G. Barnes1, K. Miller2, and D. E. Smith2
1Geosciences, Texas Tech University, Lubbock, TX
2Dept. of Geological Sciences, Univ. of Texas at El Paso, El Paso, TX

During the evolution of Laurentia, a Mesoproterozoic graniterhyolite belt extended from Finland through Canada to the southwestern United States. Rocks associated with this felsic magmatism decrease in age along the Laurentian margin toward the southwest. This granite-rhyolite province forms much of the west Texas and eastern New Mexico basement, is known locally as the Panhandle terrane, and underlies the Debaca terrane (a metasedimentary and metavolcanic unit intruded by gabbro). This study presents new petrographic, geochemical, isotopic, and geochronologic data for the Panhandle terrane from 41 closely spaced wells which penetrated several hundred meters into basement of the Amarillo uplift. These wells constitute the most complete set of subsurface samples for the midcontinent granite-rhyolite province and provide the first opportunity to develop a 3-D view of the basement.

U-Pb chronology supports the intrusive relations of the felsic rock types established by petrography. The oldest rock type of the Panhandle terrane is a hornblende-bearing quartz monzonite (~1380 Ma) followed by an ignimbritic rhyolite sequence and its comagmatic granite (~1360 Ma). These felsic rocks all plot in the high potassium field. Quartz syenite, which plots in the ultra-high potassium field is the final felsic phase of magmatism (~1340 Ma).

Mafic sills and dikes intrude the felsic sequence. Nd model ages for the mafic rocks imply that they are approximately contemporaneous with the felsic rocks. The mafic rocks have an alkaline, hot spot-like signature, and therefore are not subduction-related. These gabbroic rock types can be correlated with prominent, laterally continuous seismic reflections that occur within the basement of west Texas.

The most significant petrologic features for understanding the tectonic regime are 1) the bimodal nature of the suite, 2) the alkaline character of both the mafic and felsic samples, and 3) evidence from Nd isotopes that the felsic magmas were of crustal origin. These features indicate that the Panhandle felsic and mafic rocks are unrelated to subduction. Instead the data agree with previous models in which decompression melting of the mantle resulted in large volumes of mafic magmas which trigger extensive crustal melting to generate the felsic magmas.

AAPG Search and Discovery Article #90903©2001 AAPG Mid-Continent Meeting, Amarillo, Texas