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Exhumation history of a garnet pyroxenite-bearing mantle section from a continent-ocean transition (Northern Apennine ophiolites, Italy)
No full textGarnet 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 ca. 2.5 wt %, Al2O3 ca.12.5 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) þ 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 þ 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 þ 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 ca.1100°C and P ca. 2.8 GPa. The early decompression was associated with moderate cooling, corresponding to T ca. 950°C, 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 denudatio
Refertilization of subcontinental mantle recorded by the lherzolite-websteritehorneblendite association from St. Lucia (Corsica)
No full textThe St. Lucia nappe from Alpine Corsica consists of late-Variscan lower crust exhumed along the European rifted
margin of the Ligurian Tethys and Mesozoic sediments (Beltrando et al., 2013). The basement includes a High Grade
Mafic Complex and a Granitoid Complex of Early Permian age. The base of the Mafic Complex is associated with
mantle slices consisting of mylonitic spinel-bearing lherzolites and mm- to cm-thick websterite and hornblendite layers
concordant with the foliation of the host rocks. The mylonite microstructure in the peridotites is characterized by
aligned porphyroclasts of pyroxene (± spinel) in a fine-grained polyphase matrix composed of olivine + pyroxenes +
spinel. Large exsolved orthopyroxene (Opx) porphyroclasts mantled by neoblastic Cpx + Opx + spinel testify a lowstrain
spinel tectonite stage predating the mylonite deformation. Major element compositions of olivine, Cpx and spinel
porphyroclasts of the lherzolites indicate a fertile geochemical signature. The websterites are Opx-poor rocks containing
Al-Ti-rich Cpx porphyroclasts, Cr-poor spinel, disseminated kaersutite and accessory Fe-Ni sulphides. Spinel-facies
mylonite recrystallization in both lherzolites and websterites occurred at ~850-900°C. The hornblendites are composed
of K2O-rich kaersutite + Ti-rich phlogopite + ilmenite. Cpx porphyroclasts from the lherzolites have heterogeneous
trace element compositions pointing to four different geochemical signatures. Type 1 Cpx is strongly LREE-depleted
(CeN/SmN down to 0.05) with nearly flat MREE-HREE at 8-10 times chondrite. Type 2 Cpx is moderately LREEdepleted
(CeN/SmN = 0.31-0.49), whereas Type 3 has nearly flat LREE. Type 4 Cpxs are weakly LREE depleted, peak
at MREE and have variable HREE (YbN = 7-12). The websterite Cpxs display convex-upward REE patterns and higher
concentrations of incompatible trace element. The kaersutite from the hornblendites are enriched in Rb, Ba, U, Nb, Ta,
LREE and depleted in HREE with respect to the websterite ones. The peridotite protholiths are consistent with residual
rocks after low degrees of near-fractional melting of spinel facies DM. Type 4 Cpx from the lherzolites likely reflect
equilibrium with percolating MORB-type melts. Putative liquids in equilibrium with Cpx and kaersutite from the
websterites and the hornblendites have transitional to alkaline affinity. Our study has evidenced that the St. Lucia
mantle slices underwent injections of melts with MORB to alkaline affinity forming different kinds of magmatic layers,
which were associated with metasomatic haloes in the ambient peridotite.
Beltrando M., Zibra I., Montanini A. & Tribuzio R. 2013. Crustal thinning and exhumation along a fossil magma-poor
distal margin preserved in Corsica: a hot rift to drift transition, Lithos, 168-169, 99-112
Dolerite to peralkaline rhyolite dykes from the post-Variscan Tenda crystalline massif (northern Corsica): evidence for a cogenetic origin.
No full textGabbro-derived granulites from the External Liguride Units (Northern Apennine, Italy)
No full textIn Santonian-Early Campanian sedimentary melanges of the External Liguride units (northern Apennine), slide blocks of subcontinental mantle and MOR basalts are associated with lithologies derived from the continental crust. One of these sedimentary melanges, the Mt. Ragola complex, is characterized by the close association of mantle ultramafic, mafic and quartzo-feldspathic granulites. Mafic granulites show a wide compositional range. They generally display a marked metamorphic layering, but undeformed rocks which preserve a gabbroic fabric are found locally. The most frequent lithologies are Al-spinel gabbronorites, generally containing minor olivine, and Fe-Ti oxide-bearing gabbronorites. Troctolites, olivine gabbronorites and anorthosites were also recovered. Relics of primary textures as well as mineral and bulk-rock compositional variations indicate a comagmatic intrusive origin for the protoliths of the mafic granulites. This intrusive mafic complex underwent a subsolidus reequilibration under granulite facies conditions, at 0.6-0.9 GPa and 810-920°C, and was derived from crystallization at intermediate levels of tholeiite-derived liquids, possibly affected by crustal contamination. Its primary features are similar to those of the upper zone of the Ivrea layered complex. The gabbroic protolith for the granulites of External Liguride units were probably crystallized into the extending Adria lithosphere in relation to the initial stages of the opening of the western Tethys
Magmatic activity during continental breakup and ocean opening recorded by the External Liguride Ophiolites (Northern Apennines, Italy): implications for a mixed peridotite-pyroxenite mantle source
No full textThe association mantle ultramafics - granulites - granitoids from External Liguride Units (Northern Apennine, Italy)
No full textRocce in facies granulitica nel Complesso di M.Ragola (Unità Liguri Esterne, Appennino Settentrionale).
No full textA SUBCONTINENTAL MANTLE-LOWER CRUSTAL ASSOCIATION FROM CENTRAL CORSICA (FRANCE): PRELIMINARY DATA ON THE ST. LUCIA NAPPE MANTLE SEQUENCE
No full textAn undeformed pyroxenite-peridotite sequence from the External Ligurian ophiolites records multiple events of melt-rock interactions.
No full textThe External Ligurian mantle sequences represent deep subcontinental lithosphere exhumed in response to Mesozoic lithospheric thinning and opening of the Jurassic Alpine Tethys. They mainly consist of spinel-plagioclase lherzolites, with tectonite to mylonitic structures, and diffuse pyroxenite layering. The deformed pyroxenites have been related to recycling of old crustal material in Lower Palaeozoic to Triassic times (e.g. Montanini et al., 2015; Borghini et al., 2016). The lherzolites of the present study (Monte Gavi) are undeformed and show evidence of melt infiltration and crystallization of plagioclase (Pl) + orthopyroxene (Cpx) at the expense of spinel (Spl) and clinopyroxene (Cpx). The lherzolites include a pyroxenite body with a thickness of 6-10 m and a length of ~ 50 m. The primary assemblage of the pyroxenite consists of Cpx and Al-Spl. Cpx is resorbed and variably replaced by Opx + Pl aggregates. Spl is extensively transformed into Ca-rich Pl + Fe-rich olivine + Cr-Spl ± ilmenite. Clinopyroxene has low Mg# (81-83) and up to 10 wt% Al2O3. Close to the main pyroxenite body, the lherzolite includes up to 10 cm-thick spinel pyroxenite layers containing Mg-rich Cpx (Mg# = 89-90) and, locally, Mg-rich olivine incorporated from the host lherzolite. REE compositions of melts in equilibrium with the preserved primary Cpx display a slight LREE enrichment and a negative HREE fractionation requiring a garnet-bearing source. The Fe-rich pyroxenite body has “melt-like” patterns of highly siderophile elements (HSE), whereas the Mg-rich pyroxenites are enriched in Os and Ir. Bulk rock 187Os/188Os ratios recalculated at the age of the Alpine Tethys opening (165 Ma) show increasingly radiogenic composition from Mg- to Fe-rich pyroxenites (187Os/188Os = 0.185-0.518). We propose that the pyroxenites formed by crystallization of Al-Fe-rich melts derived from aged pyroxenite/eclogite-rich sources. Whereas the thick pyroxenite body represents a melt-dominated system, the thin pyroxenite layers formed by melt/peridotite hybridization. Extensive replacement of the primary assemblage was most likely produced by reactive migration of depleted MORB-type melts under plagioclase facies conditions (P ~0.6 GPa). The pyroxenites preserve high T (1200-1250 °C) recorded by slowly diffusing elements like REE, presumably in response to the melt infiltration event, followed by a rapid subsolidus cooling until 900 °C during rifting-related exhumation of the mantle sequence.
REFERENCES
Montanini, A., Tribuzio R. (2015): Evolution of recycled crust within the mantle: Constraints from the garnet pyroxenites of the External Ligurian ophiolites (northern Apennines, Italy). Geology, 43, 911-914,
Borghini, G., Rampone, E., Zanetti, A., Class, C., Cipriani, A., Hofmann, A.W., Goldstein, S.L. (2016): Pyroxenite Layers in the Northern Apennines’ Upper Mantle (Italy)—Generation by Pyroxenite Melting and Melt Infiltration. J.Petrol., 57, 625-653
Igneous to metamorphic evolution of mafic rocks from a fossile ocean-continent transition (External Liguride ophiolites, Italy)
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