INTEGRATED OXYGEN ISOTOPE PERSPECTIVES ON CONTINENTAL ARC MAGMATISM

Increasingly precise geochronologic constraints of magma fluxes in magmatic arcs are derived from crystal populations (e.g., zircon) in mid-crustal plutons. The heterogeneity of these same crystal populations attests to mixing and recycling of magma sources, begging the question of how sources and conduit networks dynamically regulate the construction of arc crust. Whereas oxygen isotopes (δ¹⁸O) provide one of the clearest measures of the budget of magma sources, when used in conjunction with other isotopes and elemental tracers, few arc sections are well-characterized for δ¹⁸O. The Mesozoic Sierra Nevada batholith, however, has broad characterization of δ¹⁸O, from upper mantle sections sampled by xenoliths, through exhumed sections of the lower, middle, and upper crust, as well as complementary volcanic cover and pre-existing arc wallrock. Area-adjusted mass balance calculations of δ¹⁸O (WR) show that: 1) pulses of high (>9‰) and low (<7.0‰) δ¹⁸O values are bound to specific periods within the Cretaceous section of the batholith; 2) small-volume mafic and felsic magmas, migmatites, and xenolith capture original δ¹⁸O heterogeneities and reveal vectors of hybridization; 3) silicic volcanic sections of the arc have lower δ¹⁸O than coeval plutonic sections. These patterns indicate that during periods of static growth of the tonalitic to granodiorite portions of intrusive suites, magmas from multiple sources are strongly hybridized in plumbing systems. Departures from δ¹⁸O equilibrium are wrought by tectonically triggered suppression and re-establishment of magma sources in the arc, including insertion of altered volcanic/volcaniclastic sediments and older crustal rocks and platform sediments from the fore- and retro-arc regions, respectively. Thus, arc systems may alternately sample younger and older rock causing shifts δ¹⁸O that are decoupled from radiogenic isotope systems. In addition, caldera systems remobilize volcanic cover rocks that are hydrothermally altered by surface waters, a process that causes a persistent decoupling of plutonic and volcanic δ¹⁸O values. In aggregate, the Sierran system is an ideal window to examine δ¹⁸O tracking of arc magma system evolution and provides a guide for investigations of other arcs.