PLAGIOCLASE PERIDOTITE AND GABBROIC ROCKS: THERMOCHEMICAL EROSION FOLLOWED BY IGNEOUS CRYSTALLIZATION IN A THICK THERMAL LITHOSPHERE

Abstract

Field, textural and mineral chemistry (EMS, LA-ICPMS) investigations of plagioclase peridotite provides one of the most direct means to study melt/rock reaction and melt impregnation in ocean continent transition zones and in the oceanic lithosphere. Over the last years we have investigated several Alpine and northern Apennine plagioclase peridotite massifs that show an overall poor development of oceanic mafic crust. We show that many of these ‘fertile’ peridotite massifs are hybrid rocks that may have been substantially modified by melt/rock reactions with MORB-like basaltic liquids producing plagioclase-bearing peridotites similar to those occurring at present-day magma-poor passive margins and along (ultra-) slow-spreading ridges. Such impregnated peridotites were locally overprinted by dunite channels and later intruded by gabbros. The vicinity of (cold) continental mantle to juvenile (ultra-) slow spreading ridges and/or the slow spreading rate might be one of the reasons for a thick and relatively cold mantle lithosphere that forces migrating magmas to stagnate and crystallize in the mantle. Major and trace element chemistry of clinopyroxene and spinel is distinctly different between relict continental mantle, residual peridotite and infiltrated mantle (for example: Na2O > 1.2 wt%, (Gd/Yb)N ²1 for cpx of spinel peridotite; Na2O 1 for cpx of plagioclase peridotite). Within the plagioclase peridotites, LREE depleted peridotites are preserved, representing either ‘primary’ refractory residues of near-fractional melting or ‘secondary’ products of melt/rock reaction at increasing melt mass (pyx + liq1 -> ol + liq2). Trace element modeling suggests that ‘true’ refractory peridotites experienced an equal degree of melting in the garnet and spinel peridotite field. The Sm-Nd isotopic signature of such depleted cpx indicates an inherited age of depletion for many of these fractional melting residues that is unrelated to the formation of the Piemonte-Ligurian ocean basin, demonstrating that old subcontinental mantle forms part of the oceanic lithosphere and might be substantially modified and refertilized by migrating magmas. Resetting of isotopic signatures approaching those of the migrating magmas might be an important consequence of refertilization. Gabbroic rocks show a wide range of composition, ranging from troctolite, olivine-gabbro, gabbronorite, and oxide gabbros. Mineral and bulk rock chemistry indicate a variety of crystallization processes ranging from predominantly fractional crystallization to solidification without fractionation. Most of the gabbros are evolved, with Mg# < 0.8, indicating crystallization at depth, on a conductive geotherm. Thus, substantial volumes of magma never reach the surface and represent a magmatic, but non-volcanic stage corresponding to ‘crust-free’ generation of oceanic lithosphere. Simple models indicate that addition of 6 to 12% melt (generated either by batch or nearfractional melting) can explain many of the trace element signatures of plagioclase peridotite. The overall proportion of gabbroic rocks compared to peridotites for several Alpine ultramafic bodies is < 10%. Impregnated peridotites plus gabbros sum to approximately 20%. A 20% of ‘gabbroic rock types’ (including true gabbro bodies and impregnated peridotites) would correspond to 4 km of oceanic crust that are dispersed in the upper 20 km of the oceanic lithosphere. Such ‘non-volcanic’ periods are followed by, or interfere with, common magmatic and volcanic stages, where melt is extracted from the asthenosphere, migrating through the lithosphere within isolated dunite channels and/or fractures and form MOR-type gabbroic intrusions at depth and/or extrude at the sea-floor (MOR-type basaltic flows).