Geochemical complexities preserved in the volcanic rocks of the Zambales Ophiolite, Philippines

Abstract

The geochemical characteristics of the volcanic rocks from the Zambales Ophiolite, Luzon, Philippines, indicate that the oceanic crust now preserved in the ophiolite must have been generated in an oceanic subduction zone environment. The volcanic rocks and dikes range in composition from magnesian basalt (including mid‐ocean ridge basalt (MORB), island arc tholeiite, and boninite‐like lavas) to andesite and dacite and have trace element abundances which suggest derivation from a mantle source which was heterogeneous on both regional and local scales. the sources are mixtures of components variably depleted with respect to a MORB source. Differences exist between respective exposures of the ophiolite. From north to south the following geochemical patterns in the basaltic rocks emerge: Sm/Nd decreases with corresponding decreases in 143Nd/144Nd. Rare earth element (REE) patterns range from extremely light REE (LREE) depleted with generally low abundances of all REE (1–5X chondritic) to patterns which are slightly LREE depleted, similar to MORB patterns. Throughout the range, Sr isotopic abundances are relatively high (87Sr/86Sr = 0.7035–0.7040). Perhaps more significant than the regional geochemical trends, however, is that all volcanic provinces display ranges of rock compositions, and the spatial distribution of basalts with such variable geochemical affinities indicate superposition, interfingering, and possible mixing of different lavas. We suggest that the array of geochemical data from the Zambales ophiolite can be explained in terms of processes observed in present‐day convergent plate margins, such as the Marianas or Lau Basin in the western Pacific. Complicated plate boundaries which have existed for long periods of time, including closely opposing and changing subduction systems, the rifting of arcs, and the formation of backarc basins may result in the superposition of one lava type on another or may produce many small domains in the upper mantle sources for subduction‐related lavas, some of which become extremely depleted or secondarily enriched. Magmas derived from such a heterogeneous mantle will display ranges in geochemical characteristics, possibly similar to those observed in the Zambales ophiolite. Indeed, recent studies from present‐day backarc systems suggest that young arc volcanism is superimposed on MORB‐like crust and that basalt compositions of certain backarc basins are zoned inward from arc‐like to MORB‐like. These studies indicate that relationships between arc and MORB‐like crust in young subduction complexes is extremely complex (Hawkins and Melchior, 1985; Volpe et al. 1987, 1990). We extrapolate arguments from studies of zoned backarc systems to the Zambales Ophiolite and, in turn, use data from the Zambales Ophiolite as a ruler for defining the scale of variation in these complex supra‐subduction zone systems.