HF ISOTOPE COMPOSITION OF LATE CENOZOIC BASALTIC ROCKS FROM NORTHWESTERN COLORADO, USA - NEW CONSTRAINTS ON MANTLE ENRICHMENT PROCESSES

Late Cenozoic basaltic lavas from northwestern Colorado and the northern Rio Grande rift have higher than expected epsilon(Hf) values relative to the epsilon(Hf)-epsilon(Nd) correlation of oceanic island basalts (OIB). In contrast, samples from the central and southern Rio Grande rift have lower than expected epsilon(Hf) values, relative to the epsilon(Hf)-epsilon(Nd) correlation of OIB. The variation in epsilon(Hf) and epsilon(Nd) values with geographic position is interpreted to be a result of sampling a predominantly asthenospheric source for lavas from the southern Rio Grande rift and adjacent Basin and Range, and a predominantly lithospheric source for lavas from northwestern Colorado. The epsilon(Hf) and epsilon(Nd) values of the depleted asthenospheric mantle are likely to have formed from ancient depletion of spinel or garnet peridotite [1], which, respectively, produced anomalously low and high Hf-176/Hf-177 relative to the OIB array. Although a variety of enrichment processes for formation of the lithospheric mantle may be envisioned, silicate metasomatism caused by basaltic liquids crystallizing in the mantle best explains the Hf-Nd relationships in Colorado; the ancient metasomatizing liquids could not have been derived from melting garnet peridotite but instead must have been derived through melting spinel peridotite to produce the anomalously high epsilon(Hf) values relative to epsilon(Nd) values. The inference that Proterozoic lithosphere was stabilized in the spinel stability field contrasts strongly with the proposed lithospheric source regions of recent magmatism. Low Lu/Hf ratios of all of the samples from the Rio Grande rift and northwestern Colorado, compared to the calculated Lu/ Hf ratio of the source based on Hf isotopic compositions, suggests that all of the lavas were derived from a garnet-bearing source. This implies that the subcontinental lithospheric mantle has stabilized garnet at the expense of spinel, perhaps through cooling of the lithosphere associated with formation of a stable craton and crustal thickening.