Meteoric water‐rock interaction in the lower plate of the Whipple Mountain metamorphic core complex, California

Abstract

ABSTRACTOxygen isotope ratios, whole rock major and trace element compositions, and petrological characteristics of 52 samples from nine distinct igneous lithologies in the lower plate of the Whipple Mountain metamorphic core complex of south‐eastern California indicate that both mylonitic and non‐mylonitic lithologies underwent exchange with surface‐derived meteoric waters. Broadly granodioritic lithologies are characterized by whole rock δ18O values that range from 10.6 to 2.6‰. Isotopic compositions of quartz and feldspar mineral separates indicate that quartz has largely retained original igneous compositions but that feldspar has undergone variable and often large 18O‐depletions (up to 6.5‰).Over 4 km of structural relief is exposed in lower plate gneisses below the Whipple detachment fault including non‐mylonitic lithologies at shallow structural levels above the mylonite front, and mylonitic gneisses at intermediate to deep levels below the mylonite front. Coupled δ18Oqtz ‐ δ18OFsp systematics of non‐mylonitic and mylonitic andesite to rhyolite dykes from shallow and intermediate structural levels of the lower plate document two episodes of hydrothermal alteration: a high‐temperature (>c.600d̀C) episode involving a metamorphic or magmatic fluid with δ18O values ∼ 7‰ and a low‐temperature (c.350d̀C) episode involving low‐δ18O meteoric fluids. All the dykes that document exchange with meteoric fluids are non‐mylonitic. Coupled δ18OFsp systematics of non‐mylonitic and mylonitic granodioritic gneisses from above and below the mylonite front also document low‐temperature (c. 350d̀ C) exchange with meteoric fluids. The data indicate that infiltration of meteoric fluids occurred as lower plate lithologies were juxtaposed against the base of the faulted upper plate. High‐angle normal faults in the upper plate served as the conduits for the downward circulation of surface‐derived fluids. Meteoric fluids were able to penetrate across the detachment fault into the lower plate.Uplift rates coupled with independent cooling rates indicate that surface‐derived fluids penetrated to a depth of c.4km and possibly as deep as c.8km. Penetration of surface‐derived fluid into the ductile deformation regime is not required to explain the low δ18O values observed in lower plate lithologies of the Whipple Mountain metamorphic core complex.