CHARACTERIZING MAGMA SOURCES AND MIXING BEHAVIOR IN THE JOHN MUIR INTRUSIVE SUITE: INSIGHT FROM THE LAKE EDISON AND INCONSOLABLE PLUTONS

The John Muir Intrusive Suite (JMS) is one of the large, normally zoned, Late Cretaceous intrusive suites in the eastern Sierra Nevada Batholith. Nested plutons comprising the suite vary in age from 95 to 87 Ma (Gasching et al. 2006, GSA abstract). Prior work concluded that the JMS was largely derived from lithospheric mantle sources (Coleman et al. 1992, Science). More recently, Lackey et al. (2008, J. Petrol.) suggest that the JMS is transitional between two sources as revealed by significant shifts of δ¹⁸O of zircon (Zrc) among plutons in the suite.

In this study, we conducted detailed sampling and mapping of sections of the Lake Edison Granodiorite, previously determined to have marked variability of δ¹⁸O(Zrc) and the Inconsolable quartz monzodiorite, the mafic end member of the suite. Samples were analyzed for δ¹⁸O by laser fluorination at the University of Oregon Stable Isotope Laboratory.

The northern lobe of the Lake Edison pluton δ¹⁸O(Zrc) varies slightly (5.99–6.14‰; avg. = 6.07±0.07‰, 1S.D., n= 4) whereas the southern is subtly lower and has a similar range of δ¹⁸O: 5.78–5.91‰, avg. = 5.83±0.09‰, 1S.D., n=4. In contrast to the narrow range of δ¹⁸O in the texturally homogeneous granodiorite, diorite bodies or complexly mingled areas in marginal areas of the pluton may be higher (6.05‰ in a diorite body in the southern lobe) or lower (5.52‰ from a mingled zone in the northern lobe) in δ¹⁸O than their host. The finding of heterogeneity in these marginal zones supports the hypothesis that the Lake Edison Granodiorite formed by hybridization of two or more distinct magmas. In the Inconsolable pluton, values of δ¹⁸O(Zrc) are relatively homogeneous (5.77–5.99‰, avg. = 5.87±0.07‰, n = 5). Yet the pluton contains large (10s of m) diorite enclaves, and plagioclase phenocrysts reveal complex and truncated oscillatory growth zones, both of which suggest magma mixing.

Overall, δ¹⁸O(Zrc) in these two plutons is consistent with sources slightly above mantle values (5.3±0.3‰; Valley et al. 1998, Contrib. Min. Pet.), although higher values in the northern lobe of the Lake Edison pluton indicate a greater crustal component there. The narrower range of δ¹⁸O(Zrc) values and the relative rarity of mingling textures in the northern lobe suggest that this part of the pluton is more homogenous and more thoroughly mixed than the southern lobe.