INFILTRATION OF REFRACTORY MELTS INTO THE LOWERMOST OCEANIC-CRUST - EVIDENCE FROM DUNITE-HOSTED AND GABBRO-HOSTED CLINOPYROXENES IN THE BAYOF ISLANDS OPHIOLITE

Up to 3 km of dunitic rocks occur below crustal gabbro in the Blow Me Down massif (Bay of Islands Ophiolite, Newfoundland). Analyses of duni te- and gabbro-hosted clinopyroxene grains (cpx) for rare earth elemen ts (REE), Zr, and Ti reveal three types of chondrite-normalized patter ns: N-group patterns are similar to cpx grains as they would form by f ractionation from a range of mid ocean ridge basalts (MORB). They are typical for a few higher level dunitic samples as well as mafic cumula tes. F-group patterns show light REE depletion, very strong middle REE fractionation and a positive Zr anomaly and occur in dunites only. R- group patterns are severely depleted in both light and heavy REEs rela tive to MORB-like cpx and two samples of the group display a positive Ti anomaly. They are also restricted to dunitic rocks. The patterns ar e explained in a two stage model in which an established dunite sequen ce, dominated by MORE-type cumulate signatures (N-group), was infiltra ted by extremely refractory melts. During infiltration of the refracto ry melt chromatographic fractionation occurred, transforming N-group d unites into F-group and R-group dunites. The F-group patterns are comp osite patterns: heavy REE, Ti +/- Zr reflect the original MORE-like cu mulate dunite host, light REEs indicate equilibrium with the infiltrat ing, refractory melts. Steep slopes in the middle REEs reflect the pos ition of the chromatographic front. For more intense percolation of re fractory melts, R-group patterns with a positive Ti anomaly will form by the same process. The rest of the R-group patterns displaying no po sitive Ti anomaly may represent either the most intensely reacted host rocks or these dunites derive directly as cumulates from refractory m elts. Only small volumes of refractory melt (a 5 m column) are require d to imprint the observed trace element pattern on the thick original dunite sequence. One of several possible origins for the refractory me lts is transformation of original MORE-type melts by way of chromatogr aphic fractionation within the highly depleted, residual uppermost man tle. In the framework of an oceanic spreading centre, the migrating, r efractory liquids are considered a late event following the main const ructive stags dominated by aggregated melts. The study demonstrates th at highly refractory melts can exist under oceanic spreading centres d ominated by a MORB-like cumulate and volcanic sequence.