Upward flow of magmatic fluids from the Old Woman Granodiorite, Old Woman Mountains, Southeastern California

Isotopic compositions, mineral equilibrium, and field relations at the contact between the midcrustal Cretaceous Old Woman granodiorite and Paleozoic carbonates indicate that water-rich, silica-saturated magmatic fluids were transported upward, away from the pluton, across an impermeable 30- to 40-m thick marble which caps the granodiorite, to higher structural levels along a complex network of hydrologically induced fractures. Within the fractures, fluids reacted to form symmetrical radiating splays of wollastonite with minor amounts of diopside, vesuvianite, and quartz. In many cases, pegmatites are found in the center of these calc-silicate skarns. Cross-cutting pegmatites and wollastonite veins in the aureole indicate that during late stages of crystallization of the granodiorite there were multiple episodes of fluid expulsion. Above the marble layer at higher structural levels, magmatic fluids flowed both laterally and vertically, interacting with lithologies in a more pervasive manner. Values of δ18O for calcite in the vein skarns average 11.8 parts per thousand and pegmatite whole rock silicate δ18O values average 9.4 parts per thousand. Thus oxygen isotopic compositions are consistent with a magmatic origin for the skarn-forming fluids. Away from the vein skarns, values of δ18O for the capping marble range from 18.7 to 22.1 parts per thousand (avg. = 21 parts per thousand) and values of δ13C range from -3.8 to -3.0 parts per thousand (avg. = -3.4 parts per thousand). The high δ18O values provide evidence that the marble largely retained its premetamorphic isotopic composition, indicating that fluids from the granodiorite did not flow pervasively across the unit. Lithologies at higher structural levels show evidence of more pervasive interaction with magmatic fluids: forsterite-bearing calc-silicates have δ18O values down to 11.8 parts per thousand and coarse-grained vesuvianite- and wollastonite-bearing skarns have δ18O values of approximately 13 parts per thousand