New Mexico Geological Society Annual Spring Meeting — Abstracts

Two parallel, linear, alkalic igneous belts have been recognized in west Texas and southwestern New Mexico. The northernmost of them measures 240 kms in length. While the southern belt extends for some 100 kms.

Zircon U-Pb ages for the northern belt range from 970 Ma to 1445 Ma from east to west. However, ages of 495-507 Ma (Clemons, 1989) along the same trend have been obtained. Ages for the southern belt range from 623 Ma to 1400 Ma from east to west.

These ages and geochemical data point toward an anorogenic magmatic event and/or events rather than an orogenic event or events. Three possible tectonic processes may have been operating during the Proterozoic and Cambrian-Ordovician; they are: (1) plutonism associated with pre-rift or pre-ridge extension, (2) plutonism associated with a mantle-derived plume, and (3) plutonism associated with shear zones.

The fact that both belts exhibit variations in ages, would seem to rule out an extensional process. Associated ring complexes would illustrate no disparity of ages. A plume model also must be rejected, in that, the northern belt exhibits an olderyoungest-oldest east-west progression. One would expect to see a younging in one of the two directions. Plutonism associated with shear zones appears to be the most viable model. Such a system may function at any time when the shear zones are active and, therefore, initiation of similar magmatism in the same shear zones separated by large time intervals can be explained.