THE BURRO MOUNTAIN PLUTONIC COMPLEX: EVALUATING EMPLACEMENT MECHANISMS OF A 1.4 GA A-TYPE GRANITOID PLUTON IN SOUTHWEST NEW MEXICO

The Burro Mountain pluton is a ~1.4 Ga granitic pluton that cuts Paleoproterozoic rocks of the southern Mazatzal province. It is part of the vast ~1.4 Ga granite/rhyolite province that stretches across Laurentia. Mapping of the pluton has revealed significant compositional and textural variations. The intrusive body ranges from a coarse grained, biotite-hornblende granodiorite to fine-grained biotite leucogranite. The color index of the pluton decreases southwards, paralleling trends of decreasing grain size and abundance of xenoliths and feldspar megacrysts. Mapping has revealed some sharp contacts between compositional phases of the pluton. All phases of the pluton are cut by aplite and pegmatite dikes.

We studied two shear zones oriented parallel to the margins of compositional phases of the pluton, as well as neighboring metamorphic fabrics. These shear zones show varying degrees of intensity, ranging from significant grain-size reduction to weak mineral realignment. Both shear zones are located within the coarse-grained granodiorite. Shear-sense indicators indicate that the pluton was emplaced within a stress field with σ₁ oriented northwest-southeast relative to the current orientation of the pluton. In addition to the shear zones, major foliations throughout large portions of the body parallel shear zone orientation. These foliations are not pervasive throughout the pluton; they are only found within regions of the coarse granodiorite. This suggests emplacement of the more felsic zones of the pluton postdated the granodiorite regions, consistent with the presence of undeformed fine-grained granitic dikes. The Burro Mountain pluton must have been emplaced over an extended time period, outlasting deformation. Based upon these field relationships, we suggest this body formed through incremental emplacement rather than as a sudden influx of a large volume of magma into the middle crust.