New Mexico Geological Society Annual Spring Meeting — Abstracts

Two main deformational events are recorded by the plutonic and supracrustal rocks of the Ladron Mountains. The dominant NE-striking foliation (S₂) of the study area, which records oblique-normal (northwest-side-up) shear, can be correlated with the dominant Proterozoic deformational fabric found throughout New Mexico. Elsewhere in New Mexico, S₂ has obliterated all but traces of the character' of the older D₁ event. In the Ladron Mountains, however, D₁ is locally recorded by an east-west vertical foliation (S₁) that preserves evidence for dextral strike-slip shear. In northern New Mexico, D₁ and D₂ were both interpreted to be part of a continuous deformational event. The above mentioned relationships suggest that they were in fact distinct in character. Post-D₂ structures are locally developed, and include pseudotachylyte, fractures, and open folds and crenulations in S₂. Metamorphic mineral assemblages and microstructural analyses suggest that both D₁ and D₂ were accompanied by amphibolite facies metamorphism. Growth of amphibole, mica, and garnet was synchronous with D₂, but peak metamorphic conditions outlasted D₂. Locally, aligned chlorite porphyroblasts and alteration of biotite, garnet and staurolite indicate minor retrogression following peak metamorphism.

U-Pb concordia dates on igneous zircon of 1660 ± 10 Ma for both a felsic metavolcanic rock and the (informally named) Bandit granophyre permit the occurrence of contemporaneous volcanism and plutonism. Intrusive and overprinting relationships suggest that the Bandit granophyre intruded pre-D₁ and the Capirote quartz monzonite intruded pre-D₂. However, both granitoids have previously been considered phases of the same pluton based on geochemical similarity. Intrusion of the Ladron quartz monzonite, whose Rb-Sr age of ~1300 Ma suggests it is probably one of the 1.4 Ga plutons found throughout the southwest, post-dated D₂. Thus, the maximum age of D₁ is 1660 ± 10 Ma and the minimum age of D₂ is 1.4 Ga.

Detailed study of the Proterozoic rocks of the Ladron Mountains has enhanced our understanding of the Proterozoic tectonic evolution of New Mexico. Specifically, it has been demonstrated that D₁ probably occurred at ca. 1.6 Ga and involved dextral strike-slip shearing, consistent with models of Early Proterozoic crustal accretion in the southwestern United States. The timing of D₂, which produced the main NE-striking fabric throughout New Mexico and Arizona, is less clear. Within the Ladron Mountains, D₂ clearly post-dated intrusion ofthe Capirote quartz monzonite but pre-dated intrusion of the Ladron quartz monzonite, which is inferred to be 1.4 Ga in age. Whether D₂ is late 1.6 Ga or early 1.4 Gain age, or somewhere in between,remains to be determined. Absolute dates on the Capirote quartz monzonite and on metaI morphic minerals that grew during D₁ and D₂ could discriminate between these two possibilities.