The External Ligurian mantle sequences are interpreted as deep subcontinental lithosphere exhumed to the ocean floor in response to Mesozoic lithospheric thinning and opening of the Jurassic Ligurian-Piedmontese basin. The sequences consist of spinel-plagioclase lherzolites with diffuse pyroxenite layers which have been related to recycling of crustal material (Montanini et al., 2012) or to eclogite-bearing peridotite sources (Borghini et al., 2016). The mantle lherzolite body considered in the present study (Monte Gavi) includes an undeformed, irregularly shaped body of spinel pyroxenites. This body has a thickness of 6-10 m, a length of about 50 m and encloses several meter-sized lherzolite lenses. Close to the main pyroxenite body, the host lherzolite frequently includes up to 10 cm thick spinel pyroxenite layers. The pyroxenites are coarse-grained and consist of clinopyroxene- and Al-spinel-rich domains. Clinopyroxene is resorbed, variably replaced by orthopyroxene + plagioclase aggregates, and locally rimmed by titanian pargasite. Spinel-rich domains are largely transformed into Ca-rich plagioclase + Fe-rich olivine + Cr-spinel ± ilmenite. Clinopyroxene locally has relatively low Mg# (≈ 83) and Cr2O3 (≤ 0.3 wt%), and up to 10 wt% Al2O3 and 1.8 wt% Na2O. The thin pyroxenite layers are characterized by Mg-rich clinopyroxene (Mg# = 0.89-0.90) and in places include forsterite-rich deformed olivine, which is interpreted to be a relic of the host lherzolite. The Fe-rich pyroxenites have basaltic, “melt-like” patterns of highly siderophile elements (HSE), whereas the Mg-rich pyroxenites are significantly enriched in Os and Ir. Bulk rock 187Os/188Os ratios recalculated for the age of the Ligurian-Piedmontese basin opening (165 Ma) vary from slightly to moderately radiogenic (0.185-0.518). We propose that the pyroxenites formed by crystallization of Al-rich melts derived by an aged pyroxenite/eclogite-rich source. In this view, the thick pyroxenite body represents a melt-dominated system, whereas the thin pyroxenite layers are hybrid rocks derived from melt/peridotite reactions. Resorption and extensive replacement of the primary clinopyroxene-spinel assemblage was most likely related to reactive migration of ultra-depleted melts under plagioclase facies conditions, during exhumation of the mantle sequence. The host lherzolites also show textural, mineralogical and geochemical evidence of melt infiltration and crystallization of plagioclase + orthopyroxene at the expense of spinel and clinopyroxene.

Multiple events of melt-rock interaction recorded by undeformed spinel pyroxenites from the External Ligurian ophiolites / Montanini, Alessandra; Tribuzio, Riccardo; Van, Acken; D., Luguet; Ambre,. - ELETTRONICO. - (2017), pp. 274-274. (Intervento presentato al convegno CONGRESSO SIMP-SGI-SOGEI-AIV 2017 tenutosi a Pisa nel 7-9 settembre).

Multiple events of melt-rock interaction recorded by undeformed spinel pyroxenites from the External Ligurian ophiolites

MONTANINI, Alessandra;
2017-01-01

Abstract

The External Ligurian mantle sequences are interpreted as deep subcontinental lithosphere exhumed to the ocean floor in response to Mesozoic lithospheric thinning and opening of the Jurassic Ligurian-Piedmontese basin. The sequences consist of spinel-plagioclase lherzolites with diffuse pyroxenite layers which have been related to recycling of crustal material (Montanini et al., 2012) or to eclogite-bearing peridotite sources (Borghini et al., 2016). The mantle lherzolite body considered in the present study (Monte Gavi) includes an undeformed, irregularly shaped body of spinel pyroxenites. This body has a thickness of 6-10 m, a length of about 50 m and encloses several meter-sized lherzolite lenses. Close to the main pyroxenite body, the host lherzolite frequently includes up to 10 cm thick spinel pyroxenite layers. The pyroxenites are coarse-grained and consist of clinopyroxene- and Al-spinel-rich domains. Clinopyroxene is resorbed, variably replaced by orthopyroxene + plagioclase aggregates, and locally rimmed by titanian pargasite. Spinel-rich domains are largely transformed into Ca-rich plagioclase + Fe-rich olivine + Cr-spinel ± ilmenite. Clinopyroxene locally has relatively low Mg# (≈ 83) and Cr2O3 (≤ 0.3 wt%), and up to 10 wt% Al2O3 and 1.8 wt% Na2O. The thin pyroxenite layers are characterized by Mg-rich clinopyroxene (Mg# = 0.89-0.90) and in places include forsterite-rich deformed olivine, which is interpreted to be a relic of the host lherzolite. The Fe-rich pyroxenites have basaltic, “melt-like” patterns of highly siderophile elements (HSE), whereas the Mg-rich pyroxenites are significantly enriched in Os and Ir. Bulk rock 187Os/188Os ratios recalculated for the age of the Ligurian-Piedmontese basin opening (165 Ma) vary from slightly to moderately radiogenic (0.185-0.518). We propose that the pyroxenites formed by crystallization of Al-rich melts derived by an aged pyroxenite/eclogite-rich source. In this view, the thick pyroxenite body represents a melt-dominated system, whereas the thin pyroxenite layers are hybrid rocks derived from melt/peridotite reactions. Resorption and extensive replacement of the primary clinopyroxene-spinel assemblage was most likely related to reactive migration of ultra-depleted melts under plagioclase facies conditions, during exhumation of the mantle sequence. The host lherzolites also show textural, mineralogical and geochemical evidence of melt infiltration and crystallization of plagioclase + orthopyroxene at the expense of spinel and clinopyroxene.
2017
Multiple events of melt-rock interaction recorded by undeformed spinel pyroxenites from the External Ligurian ophiolites / Montanini, Alessandra; Tribuzio, Riccardo; Van, Acken; D., Luguet; Ambre,. - ELETTRONICO. - (2017), pp. 274-274. (Intervento presentato al convegno CONGRESSO SIMP-SGI-SOGEI-AIV 2017 tenutosi a Pisa nel 7-9 settembre).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11381/2832803
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