Isotopically-diverse rhyolites coeval with the Columbia River Basalts Large Igneous Province: evidence for widespread mantle-plume driven hydrothermal alteration and remelting of the crust

Abstract:

The formation of the most recent flood basalt province on Earth, the Columbia River Flood Basalts (CRBs) of the northwestern USA, was accompanied by eruptions of several thousand km³ of rhyolite in a short time window from 16.7 to 15 Ma. These rhyolites span from low (+1‰) to high (+11‰) in δ¹⁸O values as recorded by major phenocrysts, and alteration-resistant zircons within each rhyolite commonly display diversity of up to 6‰ δ¹⁸O, indicative of batch assembly prior to eruption. Significant variation in ε_Hf also exists in zircons, ranging from -39 to 0 in rhyolites erupted through the North American cratonic crust, and from -1 to +9 in rhyolites erupted through accreted oceanic terranes to the east of the Sr⁸⁷/⁸⁶Sr = 0.706 line. This isotopic diversity cannot be accounted for by fractionation of a CRB-like parent magma, demonstrating that the syn-CRB rhyolites must have been derived from melting of the crust. Abundant low-δ¹⁸O_melt values among syn-CRB rhyolites further constrains this crustal melting to shallow depths of 5-10 km, due to the shallow depths of the necessary hydrothermal alteration of the protolith. By contrast, high-δ¹⁸O rhyolites must have been formed by remelting of sedimentary or metasedimentary rocks. Low-δ¹⁸O rhyolites are also most common in the vicinity of the crustal suture between the thick lithosphere of the Archean craton and the thin lithosphere of the accreted terranes. Thermomechanical modeling suggests that this contrast concentrates crustal heating and deformation, creating pathways for meteoric water to penetrate the crust and cause extensive hydrothermal alteration less than 1 Ma before those same rocks remelt to form low-δ¹⁸O rhyolites. Finally, we suggest that this extensive crustal hydrothermal alteration and melting may be typical of continental flood basalt provinces world wide, and particularly when there is syn-volcanic extension.