Formation of lower to middle crust of the Wyoming Craton, Montana (USA), using evidence from zircon Hf-O isotopic and trace element compositions

Abstract

Coupled oxygen-hafnium isotope and trace element geochemical data were obtained using thirty eight previously dated zircon grains extracted from five mafic to intermediate crustal xenoliths of the Wyoming Craton (Montana, USA). Xenoliths include mid to lower crustal (642–817 °C and 3.5–9.4 kbar) mafic granulites and amphibolites with dominantly Mesoproterozoic (1772–1874 Ma) and minor Paleoproterozoic to Late Archean (2004–2534 Ma) 207Pb/206Pb zircon ages. Zircon oxygen isotope data indicate derivation from melts in equilibrium with a mantle source that interacted with limited supracrustal material (δ18O = 4.4–5.7‰), as well as the incorporation of supracrustal fluids or melts into mantle source regions (δ18O = 6.0–8.1‰). The small within-sample isotopic variability suggests that primary zircon did not exchange with isotopically distinct fluids or melts after initial formation. Initial zircon Hf isotopic values are highly variable across all xenoliths (εHf = +3.7 to −17.6), consistent with protolith derivation from mantle sources that incorporated evolved, unradiogenic material or were modified by subduction-related fluids. Within a single granulite xenolith, two zircon types are recognized based on CL imagery, Hf isotopes and U-Pb ages (Type I and Type II). Type I magmatic zircons show dispersed ages (ca. 1700–2534 Ma) and unradiogenic initial Hf (εHf = −17.6 to −1.5, 176Hf/177Hf = 0.281074–0.281232). The spread in ages and initial εHf, but narrow range in initial 176Hf/177Hf, points to variable Pb loss in response to dissolution-recrystallization of pre-existing zircon. Type II metamorphic zircon yields a younger Proterozoic population (ca. 1700–2155 Ma) with more radiogenic initial Hf relative to Type I zircon (εHf = −7.9 to +1.4, 176Hf/177Hf = 0.281427–0.281578); this type represents newly grown metamorphic zircon that formed in the solid-state and incorporated Zr and Hf from pre-existing zircon and silicate matrix/metamorphic phases. REE patterns from all xenoliths are steep and positively sloping without discernible HREE depletion relative to LREE, implying zircon crystallization/recrystallization in the absence of garnet. Negative Eu anomalies signify simultaneous zircon and feldspar crystallization. Solid-state recrystallization may have lead to variations in LREE, Eu and Ce in certain xenoliths. Xenoliths containing magmatic zircon (1834 ± 19 Ma) with mantle-like δ18O (4.4–5.5‰) and radiogenic initial εHf (−2.3 to +3.7) likely formed through crystallization of melts derived from a mantle source that incorporated minor amounts of subducted sedimentary/supracrustal material. Proterozoic (1874 ± 8 Ma) xenoliths with elevated δ18O (6.0–7.0‰) and unradiogenic initial εHf (−8.2 to −9.6) within magmatic zircon represent melt products of subduction-induced melting and metasomatism of the overlying mantle wedge in the vicinity of the northern GFTZ. Older (ca. 2534 Ma) xenoliths containing zircons with elevated δ18O (6.4–7.2‰) and unradiogenic εHf (up to −17.6) represent crystallization of protolith magmas extracted from a mantle source metasomatized by subduction-derived fluids and melts in the Late Archean or earlier. Zircon geochronology and isotope systematics within Mesoproterozoic xenoliths support a model of ocean-closure and subsequent continental collision between the Medicine Hat Block and Wyoming Craton, resulting in the formation of subduction-related melts at ca. 1834–1874 Ma, followed by ca. 1770 Ma collision-related metamorphism thereafter.