New Mexico Geological Society Annual Spring Meeting — Abstracts

⁴⁰Ar/³⁹Ar step-heating analyses of K-feldspar from crystalline basement rocks located beneath the Precambrian-Paleozoic unconformity in the southern Sangre de Cristo Mountains, New Mexico, reveal complex exhumation. Thermal histories deduced from Multiple Diffusion Domain (MDD) modeling of eight igneous and metamorphic Kfeldspars collected from rocks between Santa Fe and Las Vegas display a dominant 1000 to 800 Ma cooling signature. The ~150°C temperature decrease during this time is interpreted to represent 4 to 5 km of denudation. In contrast, three K-feldspar samples collected from basement penetrating petroleum exploration well cuttings in the Las Vegas Basin, east of the range-bounding Montezuma Fault Zone, show that basement unroofing in this area occurred later, between ~700 and 600 Ma. Disparity in thermal histories between these samples suggests that the basin samples remained approximately 5 km deeper (and subsequently hotter) than the Sangre de Cristo basement samples until the basin K-feldspars closed to ⁴⁰Ar loss (I50°C) around 600 Ma. However, by Mississippian time, these rocks resided at a similar structural position below the extensive carbonate deposits that unconformably overly the Proterozoic basement across much of New Mexico. Thermochronologic and geometric constraints implicate fault initiation with east-side up motion at ca. 700 Ma to bring the Las Vegas basin samples up to the same structural level as the Sangre de Cristo samples prior to Mississipian sedimentation. Thus, the Laramide Montezuma fault that presently separates these samples represents a reactivation of this preexisting Neoproterozoic structural weakness. Ten additional step-heating analyses were conducted on metasomatic K-feldspar separated from hydrothermally altered igneous and metamorphic basement rocks in the Pecos River Valley near the Picuris-Pecos Fault Zone. Alteration is characterized by K-feldspar±epidote replacement of albite in host lithologies, and was facilitated by fluid migration along brittle structures including fractures and zones of brecciation. ⁴⁰Ar/³⁹Ar age spectra from these samples are complex and commonly exhibit intermediate age maxima. The nature of these intermediate age maxima is enigmatic but may be due to post-crystallization diagenetic or hydrothermal alteration, or mechanical fracturing of diffusion domains either by natural processes or laboratory heating procedures. Thermal histories from these samples suggest a range in the timing of mineral growth between ca. 1000 Ma and ca. 600 Ma. These data indicate two episodes of metasomatism; one prior to ~1000 Ma while the basement was regionally ~300°C, and a second that initiated at ca. 700 Ma when the basement was relatively cool at ~150°C. Additionally, basement K-feldspars from opposing sides of the Picuris-Pecos Fault do not display evidence for significant vertical offset during the Neoproterozoic, and metasomatic fluids, therefore, infiltrated fractures and zones of brecciation formed largely by strike-slip deformation. The later episode of metasomatism may be contemporaneous with the initiation of exhumation at ca. 700 Ma. Deformation and metasomatism in northern New Mexico during these times could represent inboard tectonic processes related to the rifting of Rodinia.