OPHIOLITIC MAGMATISM IN THE LIGURIAN TETHYS - AN ION MICROPROBE STUDYOF BASALTIC CLINOPYROXENES

Ion microprobe data (REE, Na, Sc, Ti, V, Cr, Sr, Zr) of unaltered clin opyroxenes in the ophiolitic basalts from the Northern Apennines have been used in a cpx-based geochemical modelling of MORB magmatism from both External (EL) and Internal (IL) sectors of the Ligurian Tethys (i .e. Jurassic Ligure-Piemontese basin), alternative to the more common whole-rock approach. Clinopyroxenes from EL basalts display slightly f ractionated LREE (Ce(N)/Sm(N) approximately 0.5) and HREE (Gd(N)/Yb(N) approximately 1.5) patterns and large variations in the REE compositi on (up to 6 times from microphenocryst cores to interstitial clinopyro xenes). Interstitial clinopyroxenes in IL basalts are similar to the m icrophenocrysts from the most primitive EL basalts. By contrast, IL mi crophenocrysts are characterized by greater LREE (Ce(N)/Sm(N) approxim ately 0.3) and lesser HREE (Gd(N)/Yb(N) < 1.2) fractionation. The comp arison of trace element variations in whole-rocks and clinopyroxenes c learly shows that the olivine and plagioclase portion of the fractiona tion sequence is poorly represented by the EL and IL basalts. In fact, ophiolitic basalts mainly consist of a minor interstitial glass (now deeply altered) associated with a prevailing plagioclase-clinopyroxene assemblage crystallized from liquids significantly evolved along the olivine-plagioclase-clinopyroxene saturation boundary. Thus, bulk rock chemistry is largely governed by clinopyroxene composition. This, in addition to alteration, indicates that the bulk rock chemistry does no t provide reliable chemical information to constrain the composition a nd the generation of the parental magmas. Unfortunately, most clinopyr oxenes are characterized by complex zoning, probably caused by disequi librium partitioning during crystal growth as a result of kinetic fact ors. On this ground, estimation of melt chemistry and inferences about the origins of these basalts are only allowed by the core composition s of microphenocrystic clinopyroxenes. Modelling. of (Nd/Yb)N and Ti/Z r in the parental magmas, as deduced from the clinopyroxene compositio ns, indicates thata EL and IL basalts do not represent products of dif ferent mantle source compositions. Rather, they were generated by vary ing degrees of fractional melting in the spinel stability field, lower for the EL (a few percent) relative to IL, totalling no more than 10% of an asthenospheric MORB source, and leaving in the residua clinopyr oxene with REE patterns similar to those shown by IL suboceanic type p eridotites. Accordingly, these latter are interpreted as refractory re sidua after MORB-generating fractional melting occurred during rifting and opening of the Ligure-Piemontese basin. By contrast, residual cli nopyroxenes from the EL subcontinental type peridotites are not consis tent with low degrees of fractional melting, in agreement with the cur rent interpretation that EL peridotites are unrelated to the MORB magm atism in the Ligure-Piemontese basin and represent lithospheric mantle material already emplaced towards the surface by a tectonic denudatio n mechanism during the early stages of oceanic rifting.