EXHUMATION HISTORY OF A GARNET PYROXENITE-BEARING MANTLE SECTION FROM A CONTINENT-OCEAN TRANSITION (NORTHERN APENNINE OPHIOLITES, ITALY)

Abstract

Garnet clinopyroxenite and garnet websterite layers occur locally within mantle peridotite bodies from the External Liguride Jurassic ophiolites (Northern Apennines, Italy). These ophiolites were derived from an ocean-continent transition similar to the present-day western Iberian margin. The garnet clinopyroxenites are mafic rocks with a primary mineral assemblage of pyrope-rich garnet + sodic Al-augite (Na2O ~ 2.5 wt %, Al2O3 ~1 2.5 wt %), with accessory graphite, Fe-Ni sulphides and rutile. Decompression caused Na-rich plagioclase (An50-45) exsolution in clinopyroxene porphyroclasts and extensive development of symplectites composed of secondary orthopyroxene + plagioclase (An85-72) + Al-spinel ± clinopyroxene ± ilmenite at the interface between garnet and primary clinopyroxene. Further decompression is recorded by the development of an olivine + plagioclase-bearing assemblage, locally under syn-kinematic conditions, at the expense of two-pyroxenes + Al-spinel. Mg-rich garnet has been also found in the websterite layers, which are commonly characterized by the occurrence of symplectites made of orthopyroxene + Al-spinel ± clinopyroxene. The enclosing peridotites are Ti-amphibole-bearing lherzolites with a fertile geochemical signature and a widespread plagioclase-facies mylonitic foliation, which preserve in places a spinel tectonite fabric. Lu-Hf and Sm-Nd mineral isochrons (220 ± 13 Ma and 186.0 ± 1.8 Ma, respectively) have been obtained from a garnet clinopyroxenite layer and interpreted as cooling ages. Geothermobarometric estimates for the high-pressure equilibration have yielded T ~1100° C and P ~2.8 GPa. The early decompression was associated with moderate cooling, corresponding to T ~950°, and development of a spinel tectonite fabric in the lherzolites. Further decompression associated with plagioclase-olivine growth in both peridotites and pyroxenites was nearly isothermal. The shallow evolution occurred under a brittle regime and led to the superposition of hornblende to serpentine veining stages. The garnet pyroxenite-bearing mantle from the External Liguride ophiolites represents a rare tectonic sampling of deep levels of subcontinental lithosphere exhumed in an oceanic setting. The exhumation was probably accomplished through a two-step process that started during Late Palaeozoic continental extension. The low-pressure portion of the exhumation path, probably including also the plagioclase mylonitic shear zones, was related to the Mesozoic (Triassic to Jurassic) rifting that led to continental break-up. In Jurassic times, the studied mantle sequence became involved in an extensional detachment process that resulted in sea-floor denudation. © 2006 Oxford University Press.