Hydrogen and Oxygen Isotope Constraints on Hydrothermal Alteration of the Trinity Peridotite, Klamath Mountains, California

摘要

The Trinity peridotite represents a rare opportunity to examine a relatively fertile plagioclase peridotite that was exhumed and later subjected to intrusive events in a seailoor environment, followed by its emplacement and incorporation into a continent. Over 250 stable isolopic determinations on whole rocks and minerals elucidate the hydrolhermal evolution of the Trinity complex. All three serpentine polymorphs are present in the Trinity peridotite; these separate on the basis of their delta D values: antigorite, -46 < delta D < -82 per thousand and lizardite and chrysotile, -90 < delta D < -106 and -110 < delta D < -136 per thousand, respectively. Antigorite coexists with chlorite, talc, and treraolile in contact aureole assemblages associated with Silurian/Devonian gabbroic plutons. Lizardite and chrysolile alteration carries a meteoric signature, which suggests association with post-emplacement serpentinization, or overprinting of earlier low-temperature seafloor serpentinization. Regionally, contours of delta D values exhibit bull's-eye patterns associated with the gabbroic plutons, with delta D maxima coinciding with the blackwall alteration at the margins on the plulons. In contrast to the hydrogen isotope behavior, oxygen isotope values of the three polymorphs are indistinguishable, spanning the range 5.3 < delta ~(18)O< 7.5, and suggesting low integrated fluid fluxes and strongly ~(18)O-shifted fluids. Inferred primary delta ~(18)O values for peridotite, gabbro, and late Mesozoic granodiorile indicate a progressive ~(18)O enrichment with time for the source regions of the rocks. These isotopic signatures are consistent with the geology, petrochemistry, and geochronology of the Trinity massif, which indicate the following history: (1) lithospheric emplacement and cooling of the peridotite in an oceanic environment approx 472 Ma; (2) intrusion of gabbroic plutons into cold peridotite in an arc environment between 435 and 404 Ma; and finally (3) intrusion of felsic plutons between 171 and 127 Ma, long after the peridotite was incorporated into the continental crust.