CALC-ALKALINE DIFFERENTIATION TREND IN THE PLUTONIC SEQUENCE OF THE WADI HAYMILIYAH SECTION, HAYLAYN MASSIF, SEMAIL OPHIOLITE, OMAN

The Wadi Haymiliyah section, in the Haylayn block (Semail ophiolite, O man) displays an unusual plutonic sequence closely similar to those of supra-subduction zone harzburgitic ophiolites (''Troodos sub-type''). It comprizes a bottom, 1000 m-thick, coarse-grained layered gabbro un it (MLGU) overlain by a 1000 m-thick, fine-grained, laminated noritic gabbro unit (MLNGU). Taken as a whole, the mineralogical and bulk-rock trends of the Haymiliah plutonic sequence are those of are-related ca lc-alkaline plutons. The MLGU layered gabbros are olivine gabbros and gabbros which differ from low-P cumulates of MORE (oceanic gabbros) by Fe3+-rich cumulus chrome spinel [Fe3+/(Fe3+ + Al + Cr) = 0.2-0.3], di opside (Mg# = 85-91) co-crystallized with highly calcic plagioclase (A n(96-80)) and intercumulus magnesian orthopyroxene (Mg# = 87-80). Plag ioclase remains highly calcic at decreasing Fo content, indicating cry stallization under high water pressure (> 500 bar). Despite an abrupt decrease in grain size, there exists modal and chemical gradations bet ween MLGU and MLNGU. In the uppermost part of the MLGU, coarse-grained , gabbroic cyclic units culminate with two-pyroxene gabbros containing up to 20 wt.% cumulus Opx. These latter are interbedded over a thickn ess of ca. 300 m with fine-grained two-pyroxene gabbros and noritic ga bbros layers which differ only by slightly higher modal opx and plagio clase contents. The bottom of the MLNGU is marked by norite layers con taing up to 80 wt.% plagioclase, cumulus Ti-magnetite and abundant int ercumulus Ti-pargasite. Unlike low-pressure differentiates of MORBs, t he MLNGU lacks pigeonite and Fe-Ti oxide layers. Cumulus titanomagneti te appears immediately after the orthopyroxene (Mg# = 72-76) in the cr ystallization order of the norites. The abundance of interstitial Ti-p oor pargasite increases at the top of the MLNGU which is brecciated by dioritic differentiates. Both features indicate increasing water pres sure and oxygen fugacity (NNO + 2 log units) symptomatic of closed-sys tem magmatic differentiation. Mg#'s of both pyroxenes (70-80) decrease moderately relative to the MLGU coarse-grained gabbros. This and the increase of plagioclase and orthopyroxene modal proportions produce in creasing SiO2-Al2O3-Na2O and Sr contents at nearly constant FeO/MgO in bulk-rock chemistry. This feature similar to the calc-alkaline differ entiation trend, is due primarily to a parental tholeiitic magma more hydrated and oxidized than MORBs (f(O2) = NNO instead of NNO-2 to NNO- 3 log unit); this trend is fully developed in the Wadi Haymiliah secti on because closure of the magma reservoir in this region allowed large r extent of magmatic differentiation than in other blocks of the Semai l ophiolite. The water-rich and oxidized nature of the parental magmas argues for the evolution of the Semail ophiolite in a marginal basin above a subduction zone rather than at a mature oceanic spreading cent er.