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Meteoric water‐rock interaction in the lower plate of the Whipple Mountain metamorphic core complex, California
Author(s) -
MORRISON J.
Publication year - 1994
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1994.tb00062.x
Subject(s) - mylonite , geology , gneiss , geochemistry , metamorphic rock , igneous rock , metamorphic core complex , detachment fault , meteoric water , lithology , plagioclase , recrystallization (geology) , petrology , quartz , shear zone , seismology , paleontology , hydrothermal circulation , extensional definition , tectonics
Oxygen 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 δ 18 O 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 18 O‐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 δ 18 O qtz ‐ δ 18 O Fsp 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 δ 18 O values ∼ 7‰ and a low‐temperature ( c .350d̀C) episode involving low‐δ 18 O meteoric fluids. All the dykes that document exchange with meteoric fluids are non‐mylonitic. Coupled δ 18 O Fsp 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 δ 18 O values observed in lower plate lithologies of the Whipple Mountain metamorphic core complex.