Precambrian rocks of the Zuni uplift: A summary, with new data on ductile shearing
Christopher K. Mawer and Paul W. Bauer
We present a brief synthesis of previous work on the Precambrian Zuni metaigneous complex (ZMC), a review of methods for determining protoliths of felsic schists and gneisses, new data on microscopic kinematic indicators in the ZMC and some speculation on possible relationships between the ZMC and other Precambrian terrains in central and northern New Mexico. The informally named ZMC mainly consists of metamorphosed and deformed, felsic to ultramafic schists, gneisses and foliated plutonic rocks. Regional metamorphic conditions peaked at lower amphibolite grade. U-Pb zircon isotopic ages from felsic igneous rocks indicate their crystallization at about 1655 my (Bowring and Condie, 1982). Precambrian rocks are host to several generations of abundant fluorite mineralization which is confined to narrow veins. The original fluorite source may be an unexposed pluton.
The felsic schists and gneisses are problematic in terms of both protolith and structural analysis. Because contact relationships are poorly exposed and tectonized, and three-dimensional shapes are unknown. we look to internal features for protolith determination. For protolith data, thin sections are cut perpendicular to the extension lineation; for kinematic data. sections are cut parallel to the extension lineation and parallel to the foliation. Most of the felsic schists in the ZMC appear to be metaigneous rocks, probably rhyolites and tuffs. Well-developed sense-of-shear criteria consisting of rotated porphyroclasts, asymmetric microfolds and shear bands indicate a tectonic history of early, more penetrative, north-directed ductile shearing followed by later, more discrete, south-directed normal shearing. The Precambrian igneous rocks of the Zuni uplift, along with much of the exposed Precambrian crust in central New Mexico. may have formed between 1640 and 1680 my. Subsequent tectonism perhaps involved north-verging thrusting, folding. and crustal thickening. followed by
south-verging shearing, faulting, crustal relaxation and thinning.
- Mawer, Christopher K.; Bauer, Paul W., 1989, Precambrian rocks of the Zuni uplift: A summary, with new data on ductile shearing, in: Southeastern Colorado Plateau, Anderson, Orin J.; Lucas, Spencer G.; Love, David W.; Cather, Steven M., New Mexico Geological Society, Guidebook, 40th Field Conference, pp. 143-147.