U-PB ZIRCON DATES OF ~1.6 GA AND ~1.4 GA GRANITIC ROCKS IN THE BURRO AND FLORIDA MOUNTAINS, SOUTHWESTERN NEW MEXICO: CONSTRAINTS ON THE TIMING OF DEFORMATION

Proterozoic intrusions in the Florida and Burro Mountains of the Mazatzal province, southern New Mexico were dated using laser ablaton multi-collector ICP-MS to determine the history of magmatism and deformation in this area. A granitic orthogneiss from the western Florida Mountains had a previously reported TIMS U-Pb zircon age of 1550-1570 (minimum age, Evans and Clemons, 1988). 33 zircons yielded a concordia upper-intercept age of 1679 ± 15 Ma. (all uncertainties 2s).

A pervasively deformed granodiorite from the northern Burro Mountains yielded an age of 1626 ± 48 Ma (7 grains, no inheritance; wtd mean 207Pb/206 ages). All of the other dated samples were undeformed, but in some cases are locally deformed. A medium-grained two-mica granite from near Bullard Peak yielded an intrusive age of 1447 ± 23 Ma (12 grains) and no inheritance. 27 cores and rims were dated from a coarse-grained granodiorite from the southern Bullard Peak quadrangle. Two older core ages are inherited grains from ~1.67 Ga and ~1.63 Ga intrusions. The younger grains represent intrusion at 1425 ± 10 Ma. A fine-grained biotite granite from the Burro Mountain granite and previously dated at 1445 ± 15 Ma (TIMS, Stacey and Hedlund, 1983). One sample (19 cores and rims) from this unit yielded an intrusive age of 1431 ± 10 with inheritance at 1710 Ma and 1640 Ma. The other (21 cores and rims) intruded at 1416 ± 18 with inheritance at ~1.68 Ga. All of these samples have experienced post-Paleozoic Pb loss probably from Tertiary magmatism but no disturbance or magmatism at ~1.2 Ga was observed, despite the presence of intrusions of this age in the Red Rock area.

Deformed intrusive rocks in both areas are ~1.68 Ga and ~1.63 Ga, whereas the ~1.4 Ga rocks in the Burro Mountains are undeformed or only locally deformed. This suggests that in southern New Mexico, the main episode of Proterozoic deformation occurred sometime between 1.63-1.45 Ga. Also, our data indicate that ~1.63 Ga magmatism in the Burro Mountains was bimodal, supporting models for rift-related heating of the crust following the Mazatzal Orogeny. These conclusions contrast with studies in northern New Mexico where deformation associated with ~1.4 Ga plutonism has been reported, and is consistent with interpretations from the San Andres range where only ~1.6 Ga deformation has been reported.