New Mexico Geological Society Annual Spring Meeting — Abstracts

Detailed mapping (1:12,000) of the Bartolo Canyon area, N. Manzano Mountains, in central New Mexico provides new information on the Proterozoic deformational history. The area consists of Early Proterozoic greenschist grade supracrustal rocks as well as an Early Proterozoic pluton -the 1656 Ma Monte Largo Granodiorite. These rocks record a complex polyphase tectonic history.

The supracrustal rocks are subdivided into four mappable units which are tectonically interlayered and attenuated. The stratigraphic sequence based on cross bedding within the quartzite, is 1) quartzite, 2) quartz-mica schist and phyllite, 3) banded and massive metarhyolite which is interlayered with 4) amphibolite and chlorite schist. The metarhyolite in this area yields a U-Pb age of 1680 Ma and we interpret these units to be part of a regionally-extensive arc related (ca 1.65 Ga) volcano-sedimentary sequence.

The Proterozoic deformation was intense and involved at least three major generations of ductile folding and fabric development. F1 isoclinal folds (and possible layer parallel thrusts) and associated transposition of bedding into S1 resulted in the dominant compositional layering, intense tectonic interlayering, and repetition of units across strike. S1 was folded into a broad upright, shallowly-plunging F2 antiform cored by the Monte Largo Granodiorite. S2 is developed in both the pluton the country rock as a crenulation cleavage. S1/S2 forms a composite NE-striking subvertical foliation that is the dominant fabric of the Manzano Mountains. F3/S3 are shallowly SE-dipping shear bands with associated kinks and crenulation cleavage. The shear bands show top to the NW movement which we interpret to be related to the Monte Largo thrust to the south.

Metamorphism in the Monte Largo pluton associated with S1 and S2 is greenschist grade as suggested by pervasive reaction-softening and retrogression of plagioclase to sericite and biotite to chlorite. Pegmatite veins in the metarhyolite may attest to the existence of a pre-S1 higher temperature metamorphism in the country rock that was pervasively retrograded during F1 and F2. Temperatures of deformation associated with S3 shear bands were lower than earlier fabrics as shown by low temperature quartz microstructures.