We've Moved!
Visit SDSU’s new digital collections website at https://digitalcollections.sdsu.edu
Description
A portion of the pre-batholithic rocks which host the Mesozoic Peninsular Ranges batholith consists of amphibolite. Amphibolite may be derived either from basaltic igneous rocks or mixtures of carbonate and clay. However, an igneous origin has generally been considered to be more likely for the majority of Peninsular Ranges amphibolites based primarily on rock associations. Petrographic and chemical analyses of amphibolites from various locations within the Peninsular Ranges indicates that they are derived from basaltic parent rocks. Samples from the western zone of the batholith display a pattern of depletion in the LREE and incompatible trace elements that is consistent with that of normal mid-ocean ridge basalt. Samples from the eastern zone are enriched to a small degree in the LREE and low field strength elements (LFSE), and depleted in the high field strength elements (HFSE) relative to N-MORB. This pattern is characteristic of volcanic arc basalts, and is consistent with enrichment of an N-MORB source with fluids derived from a subduction zone. However, volcanic arc basalts are marked by a greater degree of HFSE depletion than is observed in the amphibolites. This is reflected by a tendency for the amphibolites to plot within the ocean floor basalt fields of tectonic discrimination diagrams. Furthermore, Ba/Zr ratios for the amphibolites are lower than those found in volcanic arc basalts. Enrichment via mantle plumes, as in oceanic island basalts and enriched mid-ocean ridge basalt, would result in HFSE enrichment relative to N-MORB which is not observed in the eastern amphibolites. A back-arc basin is proposed as the tectonic environment for the formation of the amphibolites. Modern back-arc basins are known to erupt N-MORB as well as lavas which are chemically transitional between N-MORB and volcanic arc basalt, as the eastern amphibolites appear to be. The occurrence of ocean floor basalt within the Triassic-Jurassic metasedimentary rocks of the Peninsular Ranges implies that a spreading ridge was near by at the time of formation. A back-arc basin is the most likely setting for such a ridge.
