Melt extraction and melt refertilization in mantle peridotite of the Coast Range ophiolite: an LA–ICP–MS study

The middle Jurassic Coast Range Ophiolite (CRO) is one of the most important tectonic elements in western California, cropping out as tectonically dismembered elements that extend 700 km from south to north. The volcanic and plutonic sections are commonly interpreted to represent a supra-subduction zone (SSZ) ophiolite, but models specifying a mid-ocean ridge origin have also been proposed. These contrasting interpretations have distinctly different implications for the tectonic evolution of the western Cordillera in the Jurassic. If an SSZ origin is confirmed, we can use the underlying mantle peridotites to elucidate melt processes in the mantle wedge above the subduction zone. This study uses laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) to study pyroxenes in peridotites from four mantle sections in the CRO. Trace element signatures of these pyroxenes record magmatic processes characteristic of both mid-ocean ridge and supra-subduction zone settings. Group A clinopyroxene display enriched REE concentrations [e.g., Gd (0.938–1.663 ppm), Dy (1.79–3.24 ppm), Yb (1.216–2.047 ppm), and Lu (0.168–0.290 ppm)], compared to Group B and C clinopyroxenes [e.g., Gd (0.048–0.055 ppm), Dy (0.114–0.225 ppm), Yb (0.128–0.340 ppm), and Lu (0.022–0.05 ppm)]. These patterns are also evident in orthopyroxene. The differences between these geochemical signatures could be a result of a heterogeneous upper mantle or different degrees of partial melting of the upper mantle. It will be shown that CRO peridotites were generated through fractional melting. The shapes of REE patterns are consistent with variable degrees of melting initiated within the garnet stability field. Models call for 3% dry partial melting of MORB-source asthenosphere in the garnet lherzolite field for abyssal peridotites and 15–20% further partial melting in the spinel lherzolite field, possibly by hydrous melting for SSZ peridotites. These geochemical variations and occurrence of both styles of melting regimes within close spatial and temporal association suggest that certain segments of the CRO may represent oceanic lithosphere, attached to a large-offset transform fault and that east-dipping, proto-Franciscan subduction may have been initiated along this transform.     

Mantle heterogeneity beneath the Antarctic–Phoenix Ridge off Antarctic Peninsula

Abstract   We determined the Sr, Nd and Pb isotopic compositions of basalts recovered from the Antarctic–Phoenix Ridge (APR), a fossil spreading center in the Drake Passage, Antarctic Ocean, in order to understand the nature of the subridge mantle source. There are no known hotspots in close proximity to the site. We observe that small-scale isotopic heterogeneity exists at a shallow level in the subaxial mantle of the APR. Enriched (E-type) mid-ocean ridge basalts (MORB) coexist with normal (N-type) MORB in this region. The E-type basalts are: (i) relatively young compared to the N-type samples; (ii) were erupted after the extinction of the APR; and (iii) have been generated by low-degree partial melting of an enriched mantle source. Extinction of the APR likely caused the extent of partial melting in this region to decrease. We interpret that the geochemically enriched materials dispersed in the ambient depleted mantle were the first fraction to melt to form the E-type MORB.