TIMING AND ORIGIN OF PROTEROZOIC BASEMENT ROCKS IN THE SIERRA NACIMIENTO REGION, NW NEW MEXICO: EVIDENCE FROM SHRIMP U-PB ZIRCON GEOCHRONOLOGY AND ND ISOTOPIC TRACER STUDIES

New SHRIMP ²⁰⁷Pb/²⁰⁶Pb zircon ages and whole-rock initial Nd isotopic signatures of Paleoproterozoic metaplutonic and minor supracrustal rocks, exposed in the Nacimiento Mountains during Laramide exhumation, help to delineate their ages of formation and probable petro-tectonic origins.

A metatuff produced discordant U-Pb zircon data, yielding an age of 1687 ±22 Ma (MSWD = 1.05) and εNdi = +1.9, results that are similar to those of a micaceous quartzite (1695 ±10 Ma; MSWD = 1.03; and εNdi = +2.7). These units and minor mafic metavolcanic rocks are inliers and roof pendants in a moderately to strongly foliated orthogneiss that yielded an age of 1695 ±14 Ma (MSWD = 0.88) and εNdi = +3.9, and approximately coeval with a relatively undeformed, coarse-grained monzogranite (1696 ±10 Ma; MSWD = 0.56: and εNdi = +4.0). The orthogneiss is intruded by coarse-grained granodiorite that yielded an age of 1453 ±9 Ma; MSWD = 0.48; and εNdi = +1.5, which was intruded by coarse-grained porphyritic granite with an age of 1449 ±10 Ma; MSWD = 0.49; and εNdi = +1.2). These units were intruded by the prominent Joaquin granite that yielded an age of 1424 ±9 Ma; MSWD = 0.91; εNdi = +1.3), which was in turn intruded by small, undated plutons of leucogranite and muscovite granite.

These isotopic data indicate two major plutonic episodes derived from mainly juvenile, mantle-derived material; the older between ~1700 and 1690 Ma, and the younger between ~1450-1425 Ma. Initial Nd signatures for the older plutonic units are close to model depleted mantle values at 1.7 Ga, consistent with their derivation in either juvenile island arcs or back-arc rift zones. The slightly less depleted supracrustal rocks indicate incorporation of minor amounts of older crustal material. The younger suite of granites yield initial Nd signatures consistent with their derivation from the local 1.7-Ga basement, further evidenced by a ²⁰⁷Pb/²⁰⁶Pb age of zircon inheritance at 1684 Ma. These results are consistent with previous interpretations that the Paleoproterozoic basement rocks of the Sierra Nacimiento region are part of a broad boundary zone between the southern Yavapai and Mazatzal provinces of the southwestern US.