Date of Thesis

2015

Thesis Type

Honors Thesis

Degree Type

Bachelor of Science

First Advisor

Christopher G. Daniel

Keywords

New Mexico, Geochronology, Detrital zircon, Paleoproterozoic, Geology

Abstract

In Paleoproterozoic metasedimentary rocks from the Manzano Mountains in the Mazatzal crustal province (ca. 1.68-1.60 Ga), Mazatzal age detrital zircon are intermixed with Yavapai age (1.78-1.70 Ga) and older detrital zircon (Whitmeyer and Karlstrom, 2007). LA-ICP-MS 207Pb/206U ages for igneous and detrital zircon constrain the age of rhyolitic magmatism and provide some insight into the provenance of the metasedimentary succession. Maximum depositional ages were determined in all samples by looking at the youngest, significant peak age. Detrital zircon from the oldest metasedimentary unit, a lithic arenite, show a narrow unimodal age distribution ranging from 2810 Ma to 1645 Ma with a peak age of 1696 Ma (n=116). This formation is crosscut by the Ojito pluton (1659 ± 5 Ma) bracketing the minimum age of deposition. The overlying Sevilleta metarhyolite yields a crystallization age of 1658 ± 2 Ma. Continuing up section, two samples of the White Ridge quartzite yielded similar unimodal age distributions between 1600 Ma to 1875 Ma with peak ages of 1676 and 1653 Ma (n=237). Two samples from the Sais formation quartzite show a narrow bimodal age distribution ranging from 2638 Ma to 1645 Ma with peak ages of 1721 Ma and 1672 Ma (n=209). Near the top of the section, one sample of the Blue Springs quartzite gives a bimodal age distribution that ranges from 3108 Ma to 1700 Ma with peak ages of 1795 Ma and 1785 Ma (n=107) and a second sample shows a unimodal age distribution ranging from 3200 Ma to 1660 Ma with a peak age of 1784 Ma (n=94). The youngest formation sampled is the Blue Springs metarhyolite that yields a crystallization age of 1620 ± 3 Ma. In general, detrital zircon peak ages become older moving up section into younger metasedimentary units. Zircon grains from all metasedimentary and metavolcanic samples show low U/Th ratios, as well as growth zoning patterns under cathodoluminescence consistent with igneous origin. Typically, the younger the formation, the more rounded are the zircon. Detrital zircon with Mazatzal ages (1.68-1.60 Ga) are inferred to be locally derived from surrounding exposures of the Mazatzal crustal province. Older 1.78-1.70 Ga aged detritus is inferred to be sourced from the Yavapai crustal province ~110 km to the north. The older Archean age (ca. 2.5 to 3.8 Ga) detrital zircon may represent reworked detritus from Paleoproterozoic metasedimentary rocks in the Mojave and or Yavapai province. The absence of local Mazatzal age (ca. 1.68-1.63 Ga) detritus at the top of the section suggests the loss of local topographic relief, possibly allowing for an influx of more mature, and older detritus from the west or north to infiltrate into the Mazatzal Province. It may also reflect a maturing river system with headwatd erosion extending into the older crustal provinces. The lack of Mazatzal age detritus at the top of the Manzano Group is inconsistent with deposition during a major orogenic event such as the ca. 1.65-1.60 Ga Mazatzal Orogeny.

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