WEHRLITIC MAGMA SERIES AND UPPER MANTLE TRANSITION ZONE, CALIFORNIA COAST RANGE OPHIOLITE: OCEAN RIDGE OFF-AXIS MAGMATISM

Point Sal CRO remnant is composed of dual magmatic sequences: 1) a primary crustal and upper mantle sequence composed of the submarine basaltic lower lava, sheeted sill/dike complex, isotropic gabbro/diorite, and cumulus gabbro (basaltic magma series) plus restitic mantle harzburgite, and 2) a late-stage intrusive/extrusive sequence (wehrlitic magma series, WMS) rooted in a dunite/wehrlite/clinopyroxenite complex (upper mantle transition zone, UMTZ) atop the mantle harzburgite. Dunite derived from harzburgite tectonite (by dissolution of enstatite) grades up through cpx-impregnated dunite into wehrlite and ol clinopyroxenite (+ or - pl) with disrupted dunite. UMTZ rocks intrude overlying cumulus gabbro. Dikes of pl wehrlite, ol gabbro, gabbronorite, and microdiorite invade the UMTZ and gabbro cumulates. These came from parental mushy "wehrlitic magma" (composed of ol+cpx+ankaramitic melt, partially extracted from the UMTZ) and its differentiates. The upper pillow lavas are an extrusive member of the WMS. Primitive cpx-ol phyric ankaramite (the so-called "boninite" in CRO remnants) in basal upper lavas correlate chemically with pl-ol clinopyroxenite in UMTZ, and basaltic upper lavas match the WMS mafic dike rocks.

The primary crustal sequence formed from spreading-ridge axial MORB magma. The UMTZ then formed where the nearly depleted deep-mantle axial diapir brought restitic harzburgitic and its residual interstitial melt up to subcrustal depth, changing high-P melt 1+ol+en to low-P melt 2+ol+cpx and concentrating melt 2 (ankaramitic) upward by dynamic compaction. The upper UMTZ, mushy with interstitial melt, intruded the newly formed axial crust spreading above it, and some extracted ankaramitic and fractionated basaltic melt reached the seafloor, erupting as small off-axis seamounts similar to those along the EPR on ocean crust <0.2 m.y. old (White et al., 1998).