New Mexico Geological Society Annual Spring Meeting — Abstracts

Lithospheric growth of southwestern Laurentia during the Paleoproterozoic has been hypothesized to have involved addition of two dominantly juvenile crustal provinces: the 1.8-1.7 Ga Yavapai province and the 1.7-1.6 Ga Mazatzal province. Timing of assembly of these two crustal provinces has been stated to have occurred during the 1.72-1.68 Ga Yavapai orogeny and 1.65-1.60 Ga Mazatzal orogeny, with both undergoing high temperature deformation and metamorphism at middle crustal depths accompanying Mesoproterozoic intrusions from 1.45-1.35 Ga. The boundary between provinces remains poorly understood, but has been inferred to lie along the northern edge of the Jemez volcanic lineament and through the Nacimiento uplift in northern New Mexico. This basement-cored Laramide uplift exposes a 53-km-long N-S transect dominated by Paleoproterozoic and Mesoproterozoic granite, providing an ideal locality to examine age and origin of continental lithosphere across this proposed boundary on a local scale.

A quartzite metasedimentary unit from the northern Nacimiento that is intruded by ca. 1696 Ma granite was analyzed with U-Pb and Hf isotopic analyses of zircons. The detrital zircon data show a narrow unimodal age peak at 1720 Ma and a maximum depositional age of ~1.7 Ga. Detrital zircons from this sample yield initial epsilon Hf values ranging from +6.4 to +14.2 suggesting an isotopically juvenile ca. 1.7-1.8 Ga source region for the quartzites. Detrital zircon ages suggest correlation with the greater Hondo Group and equivalents of northern New Mexico.

A 1449 ±12 Ma granite from the southern Nacimiento Mountains has initial epsilon Hf values of +2.6 to +10.5 suggesting that the lower crustal melt source region contained 1.7-1.8 Ga (Yavapai) crust that was partially melted by and may have mixed with juvenile 1.4 Ga melt. Together these data suggest that >1.7 Ga (Yavapai) crustal province rocks extend in the subsurface south of the Nacimiento Mountains.

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