COMPLEX MAGMA MIXING IN THE LOWER CRUST RECORDED BY MAJOR AND TRACE ELEMENT ZONING IN HIGH-PRESSURE AMPHIBOLE: KLAMATH MOUNTAIN PROVINCE, NORTHERN CALIFORNIA

Jon Davidson’s seminal work on isotopic zoning in feldspars was crucial in development of modern methods to study the complex nature of crystal loads and the influence of mixing and assimilation in arc magmas. Trace element zoning in minerals such as clinopyroxene and calcic amphibole (Amp) can provide similar information at even finer scales of resolution. Here we report on zoning in aluminous Amp phenocrysts (tschermakite and ferrotschermakite) from a suite of Jurassic garnet-bearing andesitic dikes from the southern Klamath Mountains. Garnet-rutile and garnet-amphibole assemblages indicate crystallization from magmas with >8wt% H₂O at pressures >1.0 GPa; that is, at or near the base of the crust. Calculated temperatures (Putirka, 2016) range from 990–850°C. Amp displays prominent zoning reversals: both major and trace elements show reversals in the highest-T crystals and trace elements show reversals in lower-T ones. In contrast, many elements and ratios (e.g., Sr, Ba, Eu/Eu*) vary regularly with temperature, but some (e.g., Ti, Ni, Sc) show distinct populations at T > 950°C. Plagioclase (Pl) is oscillatory-zoned, with An contents ranging from 68 to 38. The zoning in Amp and Pl demonstrate that the garnet andesite magmas experienced repeated magma mixing events during residence in the deep crust. Mixing involved melts (calculated from Amp compositions; Zhang et al., 2017) that ranged from basalt (9.3% MgO) to rhyodacite (1.5% MgO). However, mixing was not in a single magma system: within the 10x35 km region sampled, Amp zoning is distinct from one dike to another: no single trend is present. This lack of uniform compositional trends indicates evolution of the magmas in multiple magma chambers, each undergoing mixing. However, zones in Amp crystals from one dike can be linked to zones in other dikes. These similarities suggest that mixing involved addition of mantle-derived magmas, as well as evolved magmas from adjacent small chambers. If so, we suggest that the dikes provide a snapshot of initial development of a hot zone in the Jurassic lower crust of the Klamath province.

Putirka, K. (2016) Am. Min. 101, 841–858. Zhang et al. (2017) Am. Min. 201, 1353–1367.