| Summary: | During the evolution of Laurentia, a Mesoproterozoic felsic igneous belt extended from Fennoscandia through Canada to the southwestern United States. This belt, referred to as the granite-rhyolite province in North America, forms much of the west Texas and eastern New Mexico basement. We present data from 41 closely spaced wells in west Texas that penetrated several hundred meters into the southern granite-rhyolite province and provide the first opportunity to develop a three-dimensional view of the basement from subsurface samples. The felsic rock types include hornblende-bearing quartz monzonite, ignimbritic rhyolite, and comagmatic granite with eutectic textures. These rocks plot in the high-K to ultra-high-K field. Quartz syenite, which plots exclusively in the ultra-high-K field, is the final felsic phase of magmatism. All of the felsic magmas were variably oxidizing based on biotite compositions. U-Pb chronology supports the intrusive relations established by petrography: the quartz monzonite is oldest (∼ 1380 Ma) and is followed by the granite-rhyolite sequence (∼ 1360 Ma) and the quartz syenite (∼ 1340 Ma). Nd model ages range from 1520 to 1740 Ma, indicating involvement of older Proterozoic crust. In many wells, thick mafic sills intrude the granite-rhyolite sequence. Nd model ages for the mafic rocks range from 1560 to 1440 Ma, implying that they are Mesoproterozoic in age. The mafic rocks include an alkaline, OIB-like suite, which is not supportive of a subduction origin. U-Pb ages are also reported for the Mescalero Well #1 in eastern New Mexico. In this well, metasedimentary and metavolcanic rocks of the Debaca sequence unconformably overlie a quartz syenite similar to the quartz syenite from the west Texas wells. Detrital zircons from the basal meta-arkose of the sequence are ∼ 1690 and ∼ 1320 Ma, indicating a maximum age of ∼ 1300 Ma for this sequence.
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