Intrusions and intrusive complexes in an ophiolite near San Luis Obispo; a chemical and petrologic study

Abstract

The San Luis Obispo ophiolite is a sequence of mafic and ultramafic rocks that lie as a thrust-like slice on top of Franciscan melange in the southern California Coast Range. The ophiolite comprises the classic Steinmann Trinity of basal serpentinized ultramafic rocks overlain sequentially by spilitized pillow lavas and cherts. The ophiolite is intruded by numerous diabasic dikes and several large intrusive complexes that contain peridotite, gabbro, diabase and felsic rocks. Field relations indicate that this ophiolite originated as a sequence of submarine volcanic flows and breccia units extruded onto an ultramafic basement. Later the volcanic and ultramafic rocks were intruded along their contact by mafic magma that crystallized as a stratiform complex of cumulate peridotite and gabbro. Next, this stratiform complex was intruded by diorite and albite-granite (trondhjemite) sills, which may have been produced by partial melting of gabbros in the upper levels of the stratiform complex. Finally all earlier units were intruded by diabasic sills and dikes that probably were late feeders for the pre-existing volcanic unit. All the ophiolite rocks underwent subsequent highly localized brecciation, under greenschist-facies conditions, in which felsic rocks were incipiently melted by frictional heating. The host rocks and intrusions were metamorphosed at low temperature and the mafic rocks were altered by Na- and Si-metasomatism. Despite this pervasive alteration, evidences of the formational stages of the ophiolite are preserved in disequilibrium mineral assemblages, primarily within mafic and ultramafic rocks of the intrusive complexes. Whole-rock analyses of the ophiolite suite indicate that alteration has obscured but not obliterated the original chemical character of the mafic rocks. The ophiolite diabases and basalts contain up to 6 weight percent Na 2 0 and 58 percent Si0 2 , in contrast to normal Si0 2 contents of 50 percent or less and Na 2 0 contents of less than 3 percent for unaltered oceanic and continental tholeiite basalts. However, the low K 2 0 contents (less than 0.8 percent) of all units in the ophiolite suite indicate that these rocks are not part of an alkalic kindred, but rather are altered tholeiites. Compositions of relict amphibole and plagioclase in the mafic rocks indicate that the ophiolite formed in a high-temperature environment and the deep seated rocks were metamorphosed under amphibolite-facies conditions. A later change in the environmental conditions allowed subsequent low-temperature (greenschist facies) metamorphism and metasomatism. The structure and chemistry of the San Luis Obispo ophiolite support the hypothesis that this body may represent a fragment of oceanic crust and mantle. The volcanic rocks and the intrusive complexes probably formed at a mid-ocean ridge, where conditions of high heat-flow provided the requisite high-temperature environment and promoted the production of mafic magmas. The change to low-temperature conditions probably was due to inception or acceleration of crustal spreading at that portion of the ridge where this ophiolite formed. The nature of brecciation in the ophiolite indicates that the low-temperature environment also was one of greater tectonic disturbance, relative to conditions at the ridge. This tectonically active environment probably corresponds to an island arc where the ophiolite-bearing plate was subducted in an adjacent trench. The absence of a high-pressure mineral assemblage in the San Luis Obispo ophiolite suggests that this segment of ocean-crust and upper mantle was not buried in a subduction zone, but rather was rafted on top of subduction melange (Franciscan Formation) into its present position.