MAJOR AND TRACE ELEMENT METASOMATISM BY SUBDUCTION-RELATED FLUIDS ALONG THE EASTERN PENINSULAR RANGES MYLONITE ZONE

Stable isotope studies (Holk et al., 2006) demonstrated infiltration of subduction-related metamorphic water at depths of at least 15-20 km during early Tertiary thrust faulting of a 15X15 km tonalite pluton west of Borrego Springs, CA. New geochemical data reveal this pluton experienced a complex history of metasomatism and alteration as these deep fluids ascended through the EPRMZ. Protomylonites are enriched in SiO₂ and Na₂O and depleted in K₂O relative to undeformed tonalite. Transition zone tonalites are depleted in K₂O, MgO, and total Fe and enriched in CaO relative to undeformed tonalite. The increase in Na₂O and decrease in K₂O is likely the result of albitization of plagioclase as Na-rich subduction fluids leached Ca and K from feldspar, with the Ca taken up by epidote. Most trace elements are depleted in protomylonite and in the transition zone relative to undeformed tonalite. Th, Nb, and Y are enriched in protomylonite relative to undeformed tonalite. Sr, Hf, and Zr are enriched in the transition zone. Elements unaffected by deformation include Ba, Be, Sc, Sr, Ga, Hf, and Zr in the protomylonite, and Ba, Ga, and Ag in the transition zone. The large ion lithophile and transition metals display the greatest degree of depletion in the deformed rocks. The transition zone tonalites experienced a greater degree of trace element depletion than the protomylonites, suggesting this zone as the locus of hydrothermal fluid circulation. Protomylonites are enriched in REE relative to undeformed tonalite, with all these rocks displaying a negative Eu anomaly. Transition zone tonalites have much lower REE concentrations than undeformed tonalite and a positive Eu anomaly. One undeformed tonalite and one protomylonite each contain allanite and display very high Th and L-REE concentrations. A comparison of δD values between the EPRMZ, Catalina Schist, and the Orocopia Schist indicate that the fluid source was dehydration of seawater-saturated accretionary wedge sediments. The transition zone is intruded by 0.5-to-10-m-thick pegmatite dikes and cut by 1-10 cm-thick mylonite-to-ultramylonite zones. These features are also present in the protomylonite zone, but pervasive deformation obscured this evidence. Pegmatites may have served as an effective fluid transport medium with the shear zones acting as transport avenues.