“Alpine” mafic and ultramafic rocks — the ophiolite suite: A contribution to the discussion of the paper “the origin of ultramafic and ultrabasic rocks” by P.J. Wyllie

Abstract

Abstract Ophiolites, though largely ignored in recent discussions of “Alpine” ultramafic rocks, characterize the Tethyan Belt from the Mediterranean east to the Himalayas and perhaps far beyond. The sequence consists fundamentally of a basal ultramafic layer overlain by diabasic rocks which in turn grade upward into extrusive breccias or pillow lavas. A layer of coarse gabbro may occur between ultramafics and diabase. Radiolarian cherts typically are associated with the lavas, and dense, siliceous pelagic limestones may intergrade with the cherts. Equivalent chert-limestone sequences are widespread among non-volcanic Tethyan sedimentary rocks. The Tethyan ophiolites perhaps resulted from diapiric up-welling and partial melting of mantle material during an early, possibly tensile, stage of the Alpine orogenic cycle. This occurred near the end of the Jurassic, when flysch troughs and associated ridges first appeared; lavas were extruded into the deep troughs and residual ultramafic rocks, perhaps lubricated by intergranular melt, moved plastically beneath the volcanic carapace. Gabbro was intruded later between ultramafics and the diabase-lava layer. The ophiolite complex was thus a composite subsea laccolith. Garnetiferous and pyroxenic ultramafic intrusives are known from the Rif and Betic chains. These may represent undifferentiated mantle material similar to that which gave rise to the highly differentiated ophiolite sequence. Ophiolites are now found primarily as disrupted blocks and slabs in more or less chaotic flysch, apparently displaced from the parent flysch trough during post-emplacement uplift and gravity sliding. Field data and geophysical measurements suggest that a large majority of ultramafic masses of “Alpine” type are rootless and allochthonous.