New Mexico Geological Society Annual Spring Meeting — Abstracts

Because geologic processes such as intrusion of plutonic bodies, movement of hydrothermal fluids, metamorphism, tectonism and sedimentation and erosion involve the transfer of heat, thermal history information has wide applicability towards solving diverse geologic problems. From the inception of the ⁴⁰Ar/³⁹Ar dating technique, it has been known that there is a relatively orderly hierarchy for argon closure temperatures and mineral type. Commonly, 500°C, 350°C and 300°C closure temperatures are assigned to hornblende, muscovite and biotite, respectively. K-feldspars have variable closure temperatures and generally provide thermoehronological information over a temperature range of about 150 to 350°C. Through analysis of cogenetic minerals from single samples, apparent argon ages can be plotted versus closure temperature and a portion of the thermal history can be deduced. Discontinuities in a thermal history, related to changes in the cooling and/or heating rate, can generally be used to gain insight related to tectonic events (e.g.uplift, erosion, faulting) and timing of thermal perturbations.

New Mexico potentially holds the key to understanding several regional aspects of the geologic history of the southwestern U.S. Despite this potential, not a single cogenetic mineral pair has been analyzed by the Ar/Ar technique, and less than thirty ⁴⁰Ar/³⁹Ar ages have been published from metamorphic and plutonic rocks. The tectono-thermal evolution of the Middle Proterozoic; the uplift and erosion history of the Late Proterozoic; the burial history of the basement during the Phanerozoic; the timing of emplacement and subsequent cooling history of plutonic rocks; the temporal, thermal and spatial relationship between fluid flow, mineralization and plutonism and the thermal evolution of flank uplifts associated with Rio Grande rift are just some examples of areas lacking thermochronological information. Generally, dating a few samples from various structural packages within complex metamorphic and/or igneous terranes does not provide an adequate data base by which to unravel the geologic evolution. Without characterization of the local and regional cooling rates, analysis of a mineral here and there from a complicated study area does little to advance (and may setback) our comprehension of a geologic problem.

The Cimarron and Manzano Mountains are two Proterozoic terranes where detailed thennochronological studies are underway (Grambling and Dalmeyer, 1993; RaIser and Heizler, 1993). Preliminary ⁴⁰Ar/³⁹Ar results on hornblende, muscovite and K-feldspar have clearly documented a complex distribution of ages, but at this point only idle interpretations can be put forth. Analysis of muscovite from an undeformed pegmatite near the Priest Pluton in the Manzano Mountains, suggests that the terrane cooled below about 350°C by ca. 1395 Ma. Muscovite from low-temperature shear zones in this area reveal vastly different argon systematics compared to their undeformed counterparts, and with further study in and around these discrete shear zones, the temperature conditions at which they formed, as well as, the timing of movement may be delineated. K-feldspar analysis indicate formation of the shear zones between 250 to 350°C, record a Phanerozoic overprint and a relatively rapid cooling event at ~850 Ma.

A collaborative study (Campbell et al., 1994) involving fluid inclusion and adularia dating is in progress in the Capitan Mountains of central New Mexico. Conventional K/Ar dating is common in and around mineralized areas as it is generally assumed that the very high level (thus cool) crustal conditions in which many deposits occur, facilitate rapid cooling and thus closure of the argon system. As is often the case, once ⁴⁰Ar/³⁹Ar analyses are undertaken, the assumptions which are necessary to interpret K/Ar results are shown to be invalid. ⁴⁰Ar/³⁹Ar age spectra for several adularia from the Capitans reveal significant age gradients resultant from plutonic cooling and potentially a low temperature reheating event Without the ⁴⁰Ar concentration distribution insight afforded by ⁴⁰Ar/³⁹Ar technique, the timing of vein formation and its relationship to plutonism would be incorrectly concluded and/or left ambiguous.