U-Pb geochronology of detrital zircon in quartzites of the Sawtooth Metamorphic Complex, Sawtooth Range, Idaho, USA

Abstract

In southwest Laurentia, the Paleoproterozoic Selway terrane has been proposed to extend as basement from the Wyoming Province to the Proterozoic rifted margin of Laurentia primarily based on limited exposures. The Sawtooth Metamorphic Complex (SMC), Idaho, a series of high-grade metasupracrustal rocks lies within the proposed region of the Selway terrane and may represent remnants of this terrane. Alternatively, it may represent another basement terrane of cratonic North America. To test the hypothesis that the SMC belongs to the Selway terrane, detrital zircon U/Pb ages of five quartzite samples were analyzed by LA-ICP-MS to establish age, possible provenance, and age distribution patterns. Qtz1 represented by two samples has detrital zircon ages from 1068 ± 6 Ma to 2832 ± 6 Ma with peak age frequencies of 1050-1100, 1350-1450, and 1750-1800 Ma (n = 52). Qtz3 yields distinctly older ages that range from 1704 ± 108 Ma to 2977 ± 4 Ma (n = 58). Qtz4 has zircon ages from 492 ± 13 to 2709 ± 19 Ma (n = 82) and has similar peak age frequencies from 1050-1150, 1400-1450, and 1750-1800 Ma. Concordant rim ages of zircon from quartzite and both rim and core ages of zircon in four quartzofeldspathic gneiss samples (n > 110) yielded Mesozoic ages (90 – 170 Ma) and one age of 1813 ±12 Ma. Based on this limited dataset, age distributions are similar to those of Belt – Purcell Supergroup (1400 – 1470 Ma) and/or Windermere Supergroup (570 – 750 Ma) sourcing the Wyoming Province and other surrounding Archean – Paleoproterozoic terranes. Detrital zircon ages of qtz3 (1704 – 2977 Ma) pre-date Belt – Purcell Supergroup deposition and are consistent with zircon ages used to infer the Selway terrane. Zircon ages (<1000 >Ma), abundant Grenville-aged (1000 - 1150 Ma) zircon grains, and εHf signatures in these zircons support an additional possible provenance for SMC quartzites similar to Windermere – Cordilleran material. Detrital zircon ages suggest that qtz1 and qtz4 of the SMC are most closely correlated with the Neoproterozoic Windermere Formation basement and qtz3 may represent the Paleoproterozoic Selway basement. To test the hypothesis that the SMC belongs to the Selway terrane, detrital zircon U/Pb ages of five quartzite samples were analyzed by LA-ICP-MS to establish age, possible provenance, and age distribution patterns. An alkali-feldspar-biotite quartzite (qtz1) represented by two samples has detrital zircon ages from 1068 ± 6 Ma to 2832 ± 6 Ma