Abstract We present mineral chemistry data and petrological evidence from the Yarlung Zangbo suture zone ophiolites (Southern Tibet) suggesting that they represent a collage of heterogeneous massifs. Mantle sections in these ophiolites consist of harzburgite and lherzolite cut by several generations of gabbroic to diabasic intrusions, all affected by high-temperature deformation. Pyroxenitic bands are parallel to the mantle foliation. Crustal plutonic sections, consisting of dunite, wehrlite and gabbro, are thin or absent and have been observed only in the Dazhuqu massif. Plagioclase is an additional phase associated with crustal peridotites. The mineral chemistry of silicate minerals and spinel in the mantle and crustal rocks varies widely and is believed to reflect complex melt percolation and reaction. The massifs record polybaric exhumation steps from at least 50 km depth to the plagioclase stability field. Pyroxene has re-equilibrated compositions from 1200 °C down to medium-grade metamorphic conditions. The mantle peridotites are interpreted as the residues of 10–40% partial melting of a fertile lherzolitic source. High Cr number, low TiO 2 content and relatively high Fe 3+ number of spinels suggest that the ophiolitic massifs were generated in a suprasubduction zone (arc or back-arc) environment.

Abstract Ultramafic and mafic rocks recovered at Holes 1068A and 1070A were drilled during Leg 173 of the Ocean Drilling Program (ODP) in the ocean-continent transition zone of the Iberia Abyssal Plain. Peridotites show contrasting petrographic characteristics. Hole 1068A peridotites are fine grained and show a well-defined high-temperature foliation marked by elongated pyroxene as well as aligned spinels. Hole 1068A peridotites are strongly serpentinized. Hole 1070A peridotites are coarse grained and show little evidence of high-temperature foliation. The degree of serpentinization is lower and relicts of silicate minerals are preserved. In both sets of recovered material, spinels show a wide range of composition and suggest a complex magmatic evolution. Gabbros dykes, which are found only in Hole 1070A, are very coarse grained and are locally sheared and/or crushed. Magmatic amphiboles are kaersutites and Ti-rich tschermakites that are partially replaced by hornblende and actinolite, and are associated with plagioclase of intermediate composition. Peridotites and pyroxenite have low TiO 2 , A1 2 O 3 and CaO contents in carbonate-free samples. Ni and Cr contents fall into the upper-mantle array. On the other hand, gabbros have relatively high TiO 2 and V contents reflecting modal ilmenite, and suggesting that they are relatively differentiated. This paper presents the very first geochemical data on platinum group elements (PGE) of peridotites and gabbros from passive margins. Peridotites and gabbros show low PGE (25.83 ppb and 1.44 ppb), Pd (2.75 ppb and 0.15 ppb), Pd/Ir ratios (1.45 and 1.3) and mafic index. Pyroxenite has the highest PGE (27.97 ppb), Pd/Ir (19.87) and Pt/Ir (10.25). Interelemental correlation and observation of PGE-bearing sulphide phases suggest that the PGE are hosted by single sulphide phases. From a PGE point of view, extraction of magmas involved very PGE-depleted liquids similar to gabbroic veins cutting the peridotites at Hole 1070A. Partial melting is interpreted as occurring just before oceanic accretion. Geochemical attributes suggest that the peridotites belong to the Ronda and Beni Bousera peridotitic depleted end-member clan. Thus they are believed to be of subcontinental origin. Deformation and retrograde metamorphism of peridotites and gabbros are consistent with exhumation in a rift environment post-dating the 120 Ma magmatic stage.

Full article available in PDF version.