Fluid flow in subduction zones: evidence from Nd- and Sr-isotope variations in metabasalts of the Franciscan complex, California

Abstract

Greenstone, blueschist and eclogite metabasaltic blocks from the Franciscan complex of California preserve extensive petrographic and chemical evidence for interaction with hydrous fluids at high-P, low-T metamorphic conditions. The Nd and Sr isotope variations within and among the blocks constrain the origin of the basaltic protoliths, the nature of the fluid metasomatism that occurred within the upper levels (15–45 km) of the paleosubduction zonc, and the character and provenance of the rock that generated the hydrous fluids within the paleosubduction zone. Samples with little or no petrographic evidence of retrograde alteration and unaltered garnet separates have ɛNd. With increasing degrees of retrograde alteration, Nd isotope compositions are consistently lower, ranging down to ɛNd(160)=5. Actinolitic alteration rinds which are present on some blocks have the least radiogenic compositions with ɛNd=1.6 to 6.1. While Nd isotope compositions of unaltered blockes are in the range expected for basalt derived from normal depleted mantle, the Sr isotope compositions are more radiogenic ranging from ɛSr(160)=−5 to +11. Compositions of unaltered eclogite and blue-schist blocks are consistent with a protolith origin in normal oceanic crust derived from depleted mantle. The Sr isotopy systematics indicate that the protoliths were modified by seawater alteration in an ocean-floor hydrothermal system. Isotopic compositions of samples from parts of blocks that have a retrograde metamorphic overprint show a strong correlation between less radiogenic Nd compositions and the extent of retrograde metamorphism. Maximum Nd isotope ratios of the metasomatizing fluid are provided by analyses of actinolitic rinds, and range from ɛNd(160)=1.6 to 6.1. A possible source for fluids of this composition is subducted sediment that was derived from a continental craton. Because rind formation occurred while the basaltic blocks were within an ultramafic matrix, the fluids must have migrated from sediments in the accretionary wedge into an overlying wedge of mantle material imbricated with blocks of oceanic crust. This suggests possibly km-scale movement of fluids that carry an amount of the rare-earth elements sufficient to significantly modify the trace-element budget of subducted basalt.