U-PB AGES AND SR-ISOTOPE VALUES OF CAVE SPAR FROM THE GUADALUPE MOUNTAINS, WEST TEXAS AND NEW MEXICO: IMPLICATIONS FOR LANDSCAPE EVOLUTION

The interplay between tectonics and landscape evolution is of broad interest to earth scientists. Here we show geochemical and chronological data from cave spar that constrains the timing and tectonic setting for uplift of the Guadalupe Mountains. Several scenarios have been suggested for uplift of this region, including uplift related to Laramide compression, and Basin and Range and Rio Grande rift extension. Cave spar consists of large euhedral calcite crystals that precipitated deep in the phreatic zone (500-700 meters,) in small geode-like caves. Depositional environment and age of these crystals puts an upper bound on significant uplift at 28 Ma. U-Pb dating of spar crystals shows they formed during three relatively brief periods between 60 and 28 Ma. In addition to U-Pb dating, we measured ⁸⁷Sr/⁸⁶Sr and δ⁸⁸Sr of the spar to determine if the water that precipitated the calcite was in equilibrium with the host rock or with another deeper source rock, and to determine the temperature of calcite precipitation. Our results show that the spar has a much higher ⁸⁷Sr/⁸⁶Sr (0.710 - 0.716) than the host Permian limestone (0.706 - 0.709). This indicates the spar formed from waters that are mixed with, or formed entirely from, a source other than the surrounding bedrock. In addition we did highly precise measurements of the variation in ⁸⁸Sr/⁸⁶Sr, expressed as δ⁸⁸Sr, which has previously been shown to depend on temperature of precipitation (Fietzke and Eisenhauer, (2006). Our δ⁸⁸Sr results from the spar calcite are consistent with formation at 50 to 70° C. Together this data suggest that the spar was precipitated from deeply sourced hydrothermal waters. Our U-Pb results show that the spar was precipitated during the beginning of Basin and Range tectonism in two late Eocene- Oligocene episodes, one between 36 and 33 Ma, and a second between 32 and 28 Ma, and during an older event between 60 and 55 Ma.. Formation of cave spar deep below the water table is evidence that supports an interpretation that this region was low during the Laramide, and remained so until at least 28 Ma when the area was uplifted by extensional processes.

Fietzke, J., and Eisenhauer, A., 2006, Determination of temperature-dependent stable strontium isotope (88Sr/86Sr) fractionation via bracketing standard MC-ICP-MS: Geochemistry Geophysics Geosystems, v. 7, no. 8, p. 6.