Mafic replenishment of multiple felsic reservoirs at the Mono domes and Mono Lake islands, California

摘要

The Mono Basin has been the site of frequent volcanic activity over the past 60,000 years, including the emplacement of the Mono domes and Mono Lake islands. The Mono Basin lavas are the youngest and most poorly understood products of the Long Valley Volcanic Field. We have undertaken a study of Mono Basin volcanism encompassing whole-rock major and trace element, Sr, Nd, Pb, and O isotopic, and electron microprobe glass, plagioclase, and amphibole analyses. Variations in major and trace elements suggest that fractional crystallization of feldspar (Sr, K2O), apatite (P2O5), titanomagnetite (V), zircon (Zr), and allanite (La, Ce) has influenced the evolution of the Mono Basin lavas. Field observations, petrography, and chemistry together demonstrate that injection of more mafic magma is a common process throughout the Mono Basin. Mafic enclaves of the Mono domes are stretched and rounded, with chilled margins between enclave and host rhyolite. Thin sections reveal millimeter-scale inclusions of rhyolite in the enclaves and vice versa along the host-enclave border. Paoha Island dacite has glass with 67-72 wt% SiO2 and contains microscopic clots of more mafic glasses, with SiO2 contents as low as 64 wt%. Isotopically, the June Lake and Black Point basalts and the Mono dome enclaves represent the least evolved material in the Long Valley Volcanic Field, with Sr-87/(86)Sri < 0.7056 and Nd-143/Nd-144 > 0.5126. The silicic Mono Lake lavas and Mono dome rhyolites display a significant crustal component, with Sr-87/(86)Sri > 0.7058 and Nd-143/Nd-144 < 0.5127. Oxygen and Pb isotopes throughout the sample suite also have crustal signatures, with Pb-206/Pb-204 > 19 and delta O-18 >+ 6.5%. The Mono Lake lavas generally are younger and less evolved than the Mono domes, with enrichment in trace elements including Ba and Sr accompanied by lower Nd-143/Nd-144 and higher Pb-206/Pb-204. This implies that the Mono domes and the Mono Lake lavas are derived from different magma batches, if not from separate magma chambers. There is no systematic relationship between the degree of chemical evolution and the lava ages, indicating that several magma batches have been involved in the development of the Mono domes complex. Pronounced differences in trace element composition (Nb, Y) and isotopic values between the Negit Island and Paoha Island lavas indicate that they, too, are produced by the evolution of at least two different batches of intermediatecomposition magma.