New Mexico Geological Society Annual Spring Meeting — Abstracts

Proterozoic intrusions in the Burro Mountains of the Mazatzal province, southern New Mexico were dated using laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS). A pervasively deformed granodiorite from the northern Burro Mountains yielded an age of 1625 ± 53 Ma. An undeformed two-mica granite from the Bullard Peak quadrangle yielded an intrusive age of 1441 ± 41 Ma (17 grains). 27 cores and rims were dated from an undeformed coarse-grained granodiorite. Two older core ages are inherited grains from a ?1.67 Ga intrusion. The younger grains represent intrusion at 1427 ± 23 Ma. Two samples of undeformed fine-grained biotite granite mapped as the Burro Mountain granite were dated. One sample (25 cores and rims) from this unit yielded an intrusive age of 1431 ± 24 and inheritance at 1650 ± 33 Ma. The other (21 cored and rims) intruded at 1416 ± 18 with inheritance at?1.67 Ga.

Monazite dating of pervasively deformed ?1.67 Ga metasedimentary rock of the Bullard Peak Series of the Burro Mountain region has placed the main events of metamorphism of these units at 1470 ± 16 Ma and 1415 ± 14 Ma (Sanders, 2003). An Ar⁴⁰/Ar³⁹ date on an amphibolite in this region records metamorphism of this unit at 1419 ± 6 Ma. None of the?1.4 Ga plutons of the region are deformed, suggesting emplacement of these units produced a thermal event only. This suggests that in southern New Mexico, the main episode of Proterozoic deformation occurred sometime between 1.63 and 1.47 Ga.