Stable Isotope Evidence for Large-Scale Infiltration of Metamorphic Fluids Generated during Shallow Subduction into the Eastern Peninsular Ranges Mylonite Zone (EPRMZ), Southern California

Abstract

New δ¹⁸O and δD data confirm and amplify the conclusion by Holk et al. (1991) that, during the late Cretaceous or earliest Tertiary, there was large-scale infiltration of metamorphic water at depths of at least 15 km into the lower plate of the EPRMZ, a major crustal shear zone that transects a 15 x 15 km tonalite pluton west of Borrego Springs, California. Whole-rock δ¹⁸O values in undeformed tonalite are +11.2 ± 0.4 (n = 9) and in mylonitized tonalite are +12.3 ± 0.3 (n = 21), indicating a 1 per mil increase in ¹⁸O during open-system water-rock exchange. Quartz, feldspar, and biotite δ¹⁸O values in undeformed tonalite are +12.9 ± 0.4 (n = 8), +11.0 ± 0.6 (n = 8), and +7.7 ± 0.7 (n = 8), compared with +13.5 ± 0.2 (n = 8), +11.8 ± 0.3 (n = 9), and +9.2 ± 0.3 (n = 10) in mylonitized tonalite. The extremely uniform quartz-feldspar and quartz-biotite ¹⁸O fractionations in the mylonitized tonalite indicate equilibration at 500-600°C. Biotite and hornblende δD values from undeformed tonalite are -73 ± 3 (n = 12), compared with -52 to -68 (n = 21) in mylonitic tonalite, even though the biotite and hornblende have the same Fe/Mg (≈1.25; Anderson, 1983) in both kinds of tonalite. This implies that the EPRMZ was infiltrated by large amounts of high-D water (δD ≈ -10 to -30) at 500-600°C, probably derived at depth by dehydration of subducted oceanic crust or underplated volcanogenic sediments. Syndeformational tonalitic pegmatites have quartz and feldspar δ¹⁸O values of +13.5 ± 0.3 (n = 4) and +12.2 ± 0.3 (n = 6), and biotite and muscovite δD values of -64 ± 1 (n = 2) and -42 (n = 1), suggesting a genetic relationship between the pegmatites and these deep-seated fluids. Higher whole-rock δ¹⁸O values (+13.2 to +16.1, n = 3) and much lower δD values (-72 to -114, n = 3) occur where the EPRMZ cuts upper-plate amphibolite-facies metasediments. The shift to lower δD values in these finer grained rocks probably occurred during interactions with heated meteoric groundwaters during Pliocene detachment faulting associated with the opening of the Gulf of California.

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