Clinopyroxene composition of ophiolite basalts as petrogenetic indicator

Abstract

A statistical microprobe study has been carried out on augitic clinopyroxene in high-Ti to low-Ti metabasalts from various Phanerozoic ophiolites; a comparison has been made with clinopyroxenes from mid-ocean ridge basalts (MORB), oceanic intraplate basalts, island-arc tholeiites and boninites. Leaving aside the effect of cooling rate and of fractional crystallization as major factors influencing pyroxene chemistry, their composition appears to be essentially related to the type of host magma. Both the TiO 2 and the TiO 2 Al 2 O 3 in clinopyroxenes are positively correlated with the Zr/Y ratios of the host basalts, which is a diagnostic parameter insensitive to alteration increasing regularly from low-Ti to high-Ti ophiolitic basalts. Ti also shows a positive correlation and average D Ti Cpx rock partition coefficients varying from 0.3–0.6 to 1 from low-Ti to high-Ti basalts, which compare favorably with those from Tongan (southwest Pacific) basaltic andesites (0.3–0.6) and MORB (0.75), respectively. Likewise the average K Fe Mg calculated for clinopyroxene-rock pairs varies from 0.20 in low-Ti basalts to 0.26 in high-Ti basalts up to 0.33 in the highest-Ti rocks, suggesting a significant dependence of this parameter on magma composition. The compositional correspondence of clinopyroxenes from various ophiolitic basalts with those from oceanic actualistic equivalents is shown in various diagrams. Concomitantly with increasing TiO 2 and Na 2O and decreasing SiO 2 contents, clinopyroxenes from very low-Ti to high-Ti ophiolitic basalts are best equated with those from boninites, basaltic andesites and tholeiites from island arcs, and MORB, respectively. Accordingly, a parallel increase is shown by the substitutional molecules CaTiAl 2O 6 and NaTiSiAlO 6. Within each basalt type, fractional crystallization processes are reflected in the NaTiSiAlO 6 and FeTiAl 2O 6 increase and MgCrSiAlO 6 decrease from early to late crystallized clinopyroxenes.