The Levanto-Bracco ophiolitic units as a Core Complex of a Tethyan Jurassic slow spreading ridge

In the Levanto-Bracco area (Eastern Liguria, Northern Apennine) a beautifully exposed portion of Jurassic gabbroperidotitic oceanic lithosphere crops out. It forms two northeast vergent subunits belonging to the Vara Unit. One is the Velva subunit, consisting of lherzolitic peridotites covered by ophicalcites, then by Jurassic cherts and Cretaceous pelagites. South westwards (Framura-Levanto), the lherzolitic peridotites pass to gabbros (Bonassola) through a Jurassic tectonic contact, both covered by a composite thick volcanosedimentary succession. Eastwards the underlying Mt. San Nicolao (Bracco) Subunit include all Bracco gabbroic Massif, one among the best preserved magma chamber of the Jurassic Western Tethys. The gabbros, well exposed both in Bonassola and the Bracco ridge show evident magmatic stratification marked both by compositional and granulometric banding, cut by veins with hornblende and oligoclase/albite neoblastesis. The Bracco gabbroic Massif (800-1000 m thick) lies upon lherzolitic mantle peridotites and consists of a layered alternation of melatroctolites, troctolites, olivine gabbros, gabbros and plagioclasites. The ultramafic lithotypes are concentrated towards the stratigraphic base of the gabbro. At the top, the gabbroic complex makes transition to the Jurassic sedimentary cover through some intensely fractured and hematitized gabbros, rarely actual gabbro breccias, similar to the ophicalcites that are found at the top of the lherzolites. According to the different thickness of the volcanic sedimentary cover, the eastern Velva and Mt. San Nicolao subunits have been interpreted as oceanic mantle/crust topographic highs. On the contrary the western (FramuraLevanto) Velva Subunit ophiolitic succession, which shows several hundred meters of basalt flows and pelagic cherts on top, should have formed in a basin low. In the gabbros (both Bonassola and Bracco) the magmatic banding is characterised by iso-orientation and flattening of minerals, especially plagioclase, with also folding, indicating synmagmatic deformation of the crystalmush. During cooling, (around 900°-1000°C) the gabbroic massif was affected by HT-LP oceanic metamorphism producing shearing with flaser structures (mylonites) locally associated to a brown hornblende, pyroxene and high-Ca plagioclase paragenesis. Similar structures can be found in the ophicalcitized mantle lherzolites. These structures are cut by few basalt dykes, with cm to m thickness. These dykes, seem to be contemporaneous to the hornblende and oligoclase veins (formed at 600-500°C), as they crosscut each other. They mark the inception of a brittle regime of deformation in the oceanic crust. In the last phases, the environment became colder (< 400°-300°), and more fractures, cataclastic zones and finally ophicalcites formed. This tectonic history, from mylonites to ophicalcites, documented in the gabbroic and peridotitic masses has already been interpreted as recording the extensional exhumation of a poorly depleted mantle slice, with rare gabbroic bodies. The interpretations differ from subcontinental lithospheric detachment to various kinds of oceanic exhumation with gravity discharge. The coeval ages of the Bracco gabbros (164 Ma) and the pelagic sediments beneath the massive basalt flows of Levanto (Bayocian-Early Bathonian:169-164 Ma), together with the lack of continental "extensional allochthonous" fragments associated to the lherzolites, exclude, in our view, that this exhumation occurred in a sub-continental lithospheric environment. As supporting evidence, in the last 20 years, several modern analogues have been found along slow-spreading oceanic ridges (e.g., Atlantis Bank -Southwest Indian Ridge, Atlantis Massif - Mid-Atlantic Ridge), characterized by the exposure of "oceanic core complexes", and along very slow spreading ones (e.g. Gakkel Ridge). The structures and stratigraphy of the Bracco gabbro Massif and the eastern Velva Unit peridotites fit well the models of exhumation along extensional lithospheric oceanic detachments of modern oceanic core complexes. In particular, they could represent an "Inner Corner" (intersection of the inner side of a ridge and the active termination of a transform fault) of the Western Tethys mid oceanic ridge. The Framura-Levanto "basinal" topography, instead, may have developed in an "Outer Corner" or in a relative depression near the ridge axis in which basalt lavas flowed. We think that the major lithospheric extensional The Levanto-Bracco ophiolitic units as a Core Complex of a Tethyan Jurassic slow spreading ridge GIANFRANCO PRINCIPI (*), BENEDETTA TREVES (**) & FRANCESCO MENNA(*) _________________________ (*) Dept. of Earth Sc., University of Florence, Italy. (**) Inst. of Geosciences and Georesources, CNR, Florence section, Italy. 86° CONGRESSO SOCIETÀ GEOLOGICA ITALIANA 18-20 SETTEMBRE 2012, ARCAVACATA DI RENDE (CS) 312 detachment surface in the eastern Ligurian ophiolites is represented by the mylonitic zones and by the ophicalcitic 'carapace' that characterize the upper portions of both peridotites and gabbros, whereas the mylonites deeper inside the gabbroic bodies represent minor shears that accommodate the crust-mantle exhumation.