PROTEROZOIC CRUSTAL GROWTH IN CENTRAL NEW MEXICO – INTERPRETATIONS FROM LA-ICP-MS U-Pb GEOCHRONOLOGY AND TRACE ELEMENT GEOCHEMISTRY OF DETRITAL ZIRCON IN THE PENNSYLVANIAN SANDIA FORMATION QUARTZITE

Proterozoic crustal development of southern Laurentia is thought to have involved juvenile magmatic additions derived by convergent plate margin processes during the ca. 1.74–1.68 Ga Yavapai and 1.65–1.60 Ga Mazatzal orogenies, followed by intracrustal reworking during the ca. 1.49–1.40 Ga Picuris orogeny. We have measured the uranium-lead (U-Pb) ages and trace element compositions of detrital zircon from two samples of quartzite from the Pennsylvanian Sandia Formation exposed in the Magdalena Mountains of central New Mexico to assess the durations of these orogenies and the geochemical affinities of their associated magmatism. Zircon analysis was performed by laser ablation - inductively coupled plasma -mass spectrometry (LA-ICP-MS) at the USGS L-TRACE laboratory using an Agilent 8900 ICP-MS/MS operated in single quad mode coupled to an Applied Spectra Resolution-SE 193 nm laser ablation system with a S-155 sample chamber. The two quartzites gave similar U-Pb zircon age spectra, with ca. 85% of the grains in each sample having ages attributable to the Yavapai, Mazatzal and Picuris orogenies, albeit with one sample having far fewer grains plotting within 15% of U-Pb Concordia than the other (51/93 vs. 111/153 grains). Both samples display a continuous spectrum of ²⁰⁷Pb/²⁰⁶Pb ages for the Yavapai and Mazatzal intervals, from ca. 1.76 to 1.62 Ga, without a marked break in magmatism. ²⁰⁷Pb/²⁰⁶Pb zircon ages associated with the Picuris orogeny range from ca. 1.51 to 1.38 Ga, although contributions from distal, anorogenic granite-rhyolite sources of similar age located in the Proterozoic basement from Texas to the Midcontinent are possible. Trace element ratios of zircons used for discrimination of magma type (Nb/P, Ce/P, Sc/Yb, Ce/U, Th/U) indicate that the Yavapai and Mazatzal intervals were both dominated by I-type granitoids of infracrustal, igneous source derivation and M-type granitoids produced from juvenile, mantle sources. The data suggest that the Yavapai and Mazatzal orogenies overlapped significantly in time and magma generation processes. The youngest detrital zircon grains in the Sandia Formation have ²³⁸U/²⁰⁶Pb ages of 556, 500, 403 Ma, all formed well before the expected Pennsylvanian (ca. 320 Ma) depositional age of the unit.