Date of Thesis

5-10-2016

Thesis Type

Honors Thesis

Degree Type

Bachelor of Science

Department

Geology

First Advisor

Christopher G. Daniel

Abstract

In-situ LA-ICPMS U-Pb monazite geochronology was used to determine the timing of regional metamorphism in the Picuris Mountains and northern Taos range, New Mexico. Monazite ages from aluminous quartzite and sillimanite-kspar bearing gneisses in Cedro Canyon, northern Taos Range, yield intercept ages of ca. 1380 Ma. Subhedral to anhedral monazite range in size between 10 µm and 50 µm, are typically aligned parallel/sub-parallel to the foliation, and generally occur along mineral grain boundaries; a few grains occur as inclusions within Fe-Ti oxides. Backscatter electron imaging revealed zoning in several monazite grains, but little to no compositional zoning is exhibited in the analyzed grains. Monazite do not appear to preserve any record of ca. 1650 Ma near-granulite facies metamorphism as previously proposed for this region.

Monazite in the Picuris Mountains, northern New Mexico, yield LA-ICPMS, U-Pb intercept growth ages from a variety of bulk compositions: a metarhyolite clast from the Marqueñas Fm. (1386 ± 11 Ma), a micaceous quartzite from the underlying Vadito Fm. (1362 ± 3 Ma), a garnet-biotite-staurolite schist from the Piedra Lumbre Fm. (1357 ± 6 Ma), and a metatuff layer from the Pilar Fm. (1359 ± 19 Ma). The majority of monazite grains are euhedral though some irregular boundaries are apparent due to the partial inclusion of matrix grains. Backscatter electron imaging showed little to no compositional zoning within the grains. Monazite in all samples are generally aligned parallel to the dominant regional foliation, but some clearly overgrow the foliation at a high angle and are interpreted as post-tectonic. Monazite record no evidence of an older, 1.65 Ga metamorphic and deformational event as proposed by previous studies. Younger, ca. 1430-1360 Ma monazite across northern New Mexico are interpreted to reflect a single regional high temperature metamorphic event and/or pervasive mid-crustal fluid flow event. All monazite growth events are associated with the Picuris Orogeny. Although some monazite in Colorado record multiple age domains, corresponding to at least two monazite growth events, northern New Mexico only records Mesoproterozoic metamorphism and deformation.

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