Geochemistry and economic significance of the pindos ophiolite mantle sequence.

Abstract

In the Pindos ophiolite complex of northwestern Greece, mantle sequence peridotites comprise more than 90% of the exposed lithologies. At central Pindos, the complex is divided into two parts, northern and southern. Harzburgites, with little or no free clinopyroxene, are dominant in the latter, while clinopyroxene-bearing harzburgites are present in the former. The Moho is exposed in the southeast part of the southern block where cumulate rocks are in contact with mantle peridotite. The Moho has a transitional character and is characterized by increased abundance of discordant dunites, extensive emplacement of dykes of variable mineralogy, and the pervasive impregnation of the depleted harzburgite by basaltic melt. The cumulate rocks near this transition are dominated by dunite with subordinate troctolite and gabbro and minor wehrlite. There is no evidence of a well-developed magma chamber, with cumulates mostly appearing as sill-like bodies in a series of intrusions. Away from the Moho, cumulate olivine and two-pyroxene gabbros become abundant. Serpentinization has variably affected the peridotites but has not changed their major- element composition appreciably except for the addition of H(_2)O. A small depletion in the MgO wt.% content of the rocks is observed with increasing alteration, as well as mobilization of sulphur and re-distribution of sulphides. The complex has experienced two stages of re-equilibration at low pressures (<10 kb), the first between 850-950ºC, recorded by mineral equilibria with high-blocking temperatures, and the second at -750ºC, recorded by olivine-spinel pairs. The harzburgites show slightly elevated oxygen fugacities between QFM and QFM+1.5 (log units).The chemistry of the Pindos peridotites is quite variable. Comparison with peridotites from various geotectonic settings shows that the northern part of the complex has fairly uniform composition and experienced smaller degrees of partial melting in a mid-ocean ridge environment. The southern part is more depleted and resembles peridotites dredged from present-day intra-oceanic subduction zones. This is also supported by modelling of residues of partial melting of spinel Iherzolite using temperature- and pressure-dependent major- element distribution coefficients, showing degrees of melt extraction of the order of -12-15% and ~20%-40% for the northern and southern part respectively. The PGE content of the harzburgites is fairly uniform and similar to that of mantle- derived rocks world-wide. The abundance of the PGE is controlled by residual sulphides, while a small depletion in Pd is consistent with the presence of residual alloys. Chromitites have more variable PGE abundances and show both positive and gentle negative patterns. Oxygen fugacity and sulphide saturation are the controlling parameters of the abundances and patterns of the PGE.