San Jacinto Intrusive Complex: 1. Geology and mineral chemistry, and a model for intermittent recharge of tonalitic magma chambers

Abstract

Geological mapping within the San Jacinto Mountains of southern California has delineated three major and numerous minor plutons of the Cretaceous Peninsular Ranges batholith. Early minor intrusives emplaced into quartz-rich metasedimentary sequences span the compositional range olivine gabbro to granite. The three large (to 250 km²) plutons span a limited compositional range between mafic tonalite (color index (CI) > 15) and K-feldspar-poor granodiorite (CI ≃ 10). All units are composed of plagioclase (An₃₀₋₄₀) (50–55%), quartz (20–30%), K-feldspar (1–8%), biotite (10–15%), hornblende (0–5%), titanite (0–2%), and accessory zircon, apatite, allanite, and ilmenite. Variations in mineral abundances are geographically systematic only within the youngest major mass (unit III), which grades from marginal mafic tonalite to central K-feldsparpoor granodiorite. Mineral foliations and banding, schlieren, and inclusion orientation within each unit usually parallel the nearest contact. Alignment of foliations and apparent flow-sorting and scour features are interpreted as reflecting flow patterns within each chamber. Mafic synplutonic dykes (of quartz diorite and tonalite) intruded the tonalites and were broken up to form extensive inclusion trains. Dyke-tonalite relations are interpreted as showing that (1) magma adjacent to the pluton wall had considerable yield strength, (2) magmatic flow adjacent to pluton walls was capable of moving material some distance (up to kilometers) to form the inclusion trains, and (3) the dykes represent conduits through which a considerable amount of liquid was added to the inflating magma chambers. Mineral compositions throughout the major plutons are relatively uniform. Mean plagioclase composition ranges from An₄₀ in the most mafic tonalites to An₃₀ in the most felsic granodiorites; the total microprobe-observed range is An₄₄ to An₂₅ (and to An₄₇ in a mafic inclusion). Mg/(Mg + Fe + Mn) of biotite and hornblende drop similarly from 0.44 to 0.36 reflecting changes in rock Mg/(Mg+Fe). The sole oxide mineral is almost pure ilmenite. The limited ranges of mineral compositions imply considerable stability of physico-chemical conditions during the crystallization of each pluton. The field, petrographic, and mineral chemical data are interpreted in terms of an intermittently recharged continuously solidifying magma chamber.

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