Melt migration and intrusion in the Erro-Tobbio peridotites (Ligurian Alps, Italy): Insights on magmatic processes in extending lithospheric mantle

Abstract

The Alpine/Apennine ophiolites are largely thought to represent lithosphere sectors mostly formed at an ocean/continent transition and allow the direct observation of petrologic and geodynamic processes in extensional systems evolving from continental rifting to ultraslow spreading. The Alpine/Apennine peridotites experienced multiple melt/rock interaction and melt intrusion events occurred at different lithospheric depths, thus providing insights on mantle dynamics and lithosphere-asthenosphere interactions during progressive lithosphere extension. Here we present an overview of this multi-stage melt migration and intrusion history, as recorded in the Erro-Tobbio (ET) peridotites (Ligurian Alps, Italy). In the ET spinel peridotites, the oldest intrusion event is documented by the diffuse occurrence of cm-scale folded pyroxenite bands. They display variably fractionated REE spectra, marked by LREE depletion and absent Eu N anomaly. Unusual trace element signature (high Sc,V contents and low MREE/HREE ratios) in clinopyroxenes from one pyroxenite layer is witness of a precursor garnet-bearing magmatic assemblage. Spinel pyroxenites likely originated as high- P (> 15–20 kbar) intrusions that preceded the extension-related peridotite exhumation. In the spinel peridotites, field, textural and chemical evidence ( e.g . olivine embayment replacing pyroxene porphyroclasts, increasing modal olivine at constant bulk Mg values), points that they experienced open-system melt migration by reactive porous flow, subsequent to pyroxenite intrusion and folding. Melt/rock interaction (causing olivine crystallization and pyroxene dissolution) occurred at high melt volumes at deep lithospheric levels. At shallower lithospheric depths, the ET peridotites were impregnated by melts causing significant plagioclase enrichment and crystallization of poikilitic orthopyroxene replacing mantle olivine and clinopyroxene. Reacted clinopyroxenes preserve strong LREE depletion, indicating that impregnating melts originated as depleted melt fractions. After impregnation, peridotites underwent multiple gabbroic intrusion events. Structural and geochemical features of melt impregnation and melt intrusion products point to a progressive change in melt composition and dynamics. Peridotite impregnation was caused by diffuse migration of opx-saturated depleted melts, and is consistent with cooling and crystallization of migrating melts when the peridotites, due to lithosphere extension and thinning, became part of shallower and colder lithospheric environments. The subsequent intrusion events originated by MORB-type aggregated magmas that had not experienced significant compositional modifications during ascent. The transition from porous flow melt migration to emplacement of magmas in fractures reflects progressive change of the lithospheric mantle rheology, across the ductile to brittle transition, during extension-related uplift and cooling of the ET mantle.