Precambrian geology of La Madera quadrangle, Rio Arriba County, New Mexico

Abstract

Precambrian rocks have been deformed by three episodes of regional dislocation. The earliest recognizable deformation was an integral part of the regional metamorphism. Pervasive shearing and flowage of materials during the first deformation resulted in the formation of isoclinal shear folds with axial-plane flow and fracture cleavage that contains prominent textural and mineralogic lineations parallel to the fold axes. A second period of deformation, in which the first deformation axial-plane cleavage is folded, produced nearly isoclinal similar folds overturned to the northeast with gently doubly plunging, northwest-trending axes. First deformation fold axes and b-lineation were transposed by the second period of folding and now trend south-southwest and plunge about 30 deg. Second deformation axial-plane flow, fracture, and slip cleavage transect the first deformation axial-plane cleavage and form a prominent lineation parallel to second deformation fold axes. A third regional dislocation was primarily cataclastic and produced wide-spread step folds with nearly vertical west-trending axial-plane cleavage. Small-scale third deformation folds trend west and plunge from 30 to 60 degrees. Fracture cleavage and slip cleavage are the usual types of axial-plane cleavage in these folds, but locally, slip cleavage grades into axial-plane flow cleavage.Quartzite, muscovite quartzite, feldspathic schist, granitic gneiss, hornblende-chlorite schist, and phyllite are the major metamorphic rocks. Granitic pegmatite dikes and sills intrude the metamorphic complex. The first regional metamorphism resulted in the formation of quartzite, schist, and gneiss, from an original assemblage of quartz sandstone, shale, basic flow rock, granite or granitized rock, and silicic volcanic rock. First deformation equilibrium mineral assemblages are representative of the greenschist and almandine amphibolite facies of regional metamorphism. Thermal and/or hydrothermal metamorphism that followed the second period of regional folding induced regrowth of kyanite, hornblende, muscovite, and to a lesser degree, feldspar in the form of porphyroblasts which include and cut across metamorphic textures formed during regional metamorphism. This stage of activity also locally converted kyanite to pyrophyllite and kaolinite plus muscovite to andalusite within the large quartzite masses. The circulation of hydrothermal fluid was controlled by pre-existing shear zones and rock cleavage. Cataclastic structures associated with regional dislocation cut across minerals formed during the thermal and/or hydrothermal metamorphism. Pegmatites are intrusive along the axial-plane cleavage of third deformation folds. Pegmatitic alkali metasomatism has converted some hornblende-chlorite schist to biotite schist and resulted in the growth of feldspar and muscovite in aureoles around some pegmatite bodies.