PALEOMAGNETISM OF MESOPROTEROZOIC IGNEOUS ROCKS, WEST-TEXAS BORE CORE

Subsurface information shows that the crystalline basement of central and western Texas and eastern New Mexico consists of granitic and rhyolitic rocks, cut by younger mafic intrusions that range in dimensions from m-scale to giant layered intrusions. Isotopic age data show an abundance of 1340 to 1380 Ma quartz monzonite, quartz syenite, and rhyolite, representing the southern parts of the granite-rhyolite province, with no U-Pb age data older than 1380 Ma. Recent U-Pb (zircon) dating has identified additional, widespread, Grenville-age (1070-1110 Ma) plutons. The apparent polar wander path for North America during the time period between ca. 1.4 and 1.3 Ga is poorly determined. Utilizing the extensive geochronologic data base for the subsurface of this part of the North American craton, we have begun study of the paleomagnetism of core retrieved from deep drilling in the region. From the Pioneer Bivins 99-R deep core (Potter County, Texas), drilled to 4174 feet, we have collected over 40 core segments from depths ranging from 2606 to about 3753 feet of core. In this general area, the subsurface consists of several hundred meters of the southern granite-rhyolite province including hornblende-bearing quartz monzonite, ignimbritic rhyolite and comagmatic granite with eutectic textures. The rhyolite-granite sequence (ca. 1360 Ma) is cross cut by mafic sills and dikes; chilled margins in the gabbro against granite or rhyolite imply that the mafic sills and dikes intrude the felsic sequence. A U-Pb age determination of about 1080 Ma has been recently obtained from the main mafic intrusion in this core. Core segments sampled are 1/3 splits of the full core, with a maximum thickness of about 1.4 cm. All samples from the thick gabbroic intrusion (sill?) yield exceptionally well defined magnetization response in progressive alternating field and thermal demagnetization, with magnetizations, residing in magnetite, of positive inclinations ranging from about 20^o to 35^o characteristic of these rocks. The inclination is consistent with a late Mesoproterozoic age, based on data from igneous rocks of similar age. Samples from rhyolite reveal a magnetization carried by both magnetite and hematite of positive inclination that is notably steeper (45 to 60^o) than that of the gabbro, possibly implying the preservation of a primary magnetization, of ca. 1360 Ma, in these rocks.