New Mexico Geological Society Annual Spring Meeting — Abstracts

The Burro Mountains expose a Proterozoic intrusive suite of ~1.4 Ga granitic plutons, cutting 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 muscovite leucogranite. New mapping has revealed both gradational and sharp contacts between compositional phases of the pluton, redefining previous surficial mapping into four intrusive units: the Jack Creek Granodiorite, Burro Mountain Granite, Langford Mountains Granite and Separ Granite. The Jack Creek Granodiorite, combining previous mapping of the Jack Creek Granite and the granodiorite unit, is a coarse-grained, megacrystic granodiorite. It is the only unit with deformation, ranging from mild foliation to mylonitic grain reduction and migmatite production. These deformational fabrics are concentrated to two orthogonal shear zones, indicative of an extensional tectonic regime coeval to the emplacement of the Jack Creek Granodiorite. This tectonic regime confirms recent research suggesting ~1.4 Ga extension through rapid cooling thermochronology.

What was formerly mapped as the Burro Mountain Granite is now broken down into three units. The Burro Mountain Granite is defined as moderate-grained biotite granite. The Langford Mountains Granite and the Separ Granite are luecogranites separated by textural differences. All phases of the intrusive suite are cut by aplite and pegmatite dikes. Cross-cutting relationships and the presence of xenoliths have established relative ages for the units, in order of oldest to youngest: Jack Creek Granodiorite, Burro Mountain Granite, Langford Mountains Granite and the Separ Granite. An additional unit of ~1.4 Ga magmatism was also identified through field studies, the Spring Creek Diorite. The Spring Creek Diorite is a fine grained mafic rock, marked by megacrystic feldspars. Initially, it appeared to be a younger unit cutting both the Jack Creek Granodiorite and Separ Granite; however it is found foliated within regions of unfoliated Jack Creek Granodiorite, demonstrating that it is the oldest unit. It is not mappable through this project, as it does not have any large scale outcrops.

Division into these units is confirmed through major element geochemistry, specifically the addition of the Langford Mountains Granite and the Separ Granite, as the two units plot in a unique region. The Jack Creek Granodiorite spans a broad geochemical range, matching other large-scale granodiorite such as the Half Dome Granodiorite of the Tuolumne Intrusive Suite.

Field relations indicate that the Burro Mountain Intrusive Suite was emplaced through two distinct periods of different emplacement mechanisms: (1) antitaxial crack-seal emplacement of the Jack Creek Granodiorite and initial emplacement of the Burro Mountain Granite (2) syntaxial crack-seal emplacement of the continued emplacement of Burro Mountain Granite, and the subsequent Langford Mountains Granite and Separ Granite. These were determined through the presence of xenoliths, patterns in textural variations and gradational contacts.