PRECAMBRIAN BASEMENT OF WEST TEXAS AND EASTERN NEW MEXICO, THE STORY FROM CORE AND CUTTINGS

Seismic interpretations, U-Pb zircon ages, and geochemical and isotopic analysis from the last decade combined with earlier information, provides a geologic framework for the Precambrian basement of west Texas and eastern New Mexico. Much of the crystalline basement documents voluminous, episodic magmatism that is a key component in the development of southern Laurentia. This complex history involves at least four major magmatic events and at least two periods of basin development from 1.49 to 1.07 Ga. The Texas Panhandle Precambrian basement consists of quartz monzonite, granite, rhyolite, and quartz syenite emplaced in separate periods of magmatism at ~1.47 and ~1.37 Ga. This activity may extend as far south as the Carrizo Mountain Group near Van Horn (1.33 Ga). Sm-Nd isotopic signatures indicate that the source regions of these magmas young to the south.

From 1.28 to 1.22 Ga, a broad, carbonate-dominated shelf, known locally as the Debaca sequence, extended throughout the region from the Llano Uplift in the southeast to the western Grand Canyon regions. Well log data coupled with seismic data and deep crustal reflection data image large basins with shallow, cross-cutting dikes. Outcrop and well data support the presence of associated mafic magmatism, along with episodic rhyolite ash falls which may be sourced in the Burro Mountains of southwestern New Mexico.

Renewed bimodal magmatism began at about 1150 Ma, after the deposition of quartz-rich sandstone of the Lanoria Formation, and continued until ~1070 Ma. This magmatic episode was characterized by tholeiitic basalt and alkaline (“A-type”) rhyolite/granite, all typical of extensional settings. U-Pb (zircon) dating provides evidence of widespread, Grenville-age (1070-1110 Ma) plutonic rocks in the Texas/New Mexico basement. Dated samples include alkali feldspar granite of the Abilene gravity minimum, monzonitic xenoliths from Potrillo maar west of El Paso, a differentiated sill from the Texas Panhandle and 40Ar/39Ar age from a gabbroic sill in the basement of eastern New Mexico. In addition, evidence of the northwest directed Grenville collisional event is apparent in the deformed nature of rocks located in the southernmost part of the Laurentian continent.