Alessandra Montanini

The association of continental crust rocks with ophiolites in the Northern Apennines (Italy): implications for the continent-ocean transition in the Western Tethys

Abstract The Late Cretaceous sedimentary melanges from the External Liguride Units of the Northern Apennines include large slide-blocks of ophiolites and lower and upper continental crust rocks representative of a continent-ocean transition between the Internal Liguride oceanic domain and the thinned continental margin of the Adria plate. The slide-blocks preserve a record of the long-lived history of rifting which led to opening of the Jurassic Western Tethys Basin. The External Liguride ophiolites consist of: (1) undepleted spinel-peridoties, partly re-equilibrated under plagioclase-facies conditions, which were interpreted as unroofed subcontinental mantle; (2) rare gabbroic rocks (mainly troctolite to olivine-bearing gabbro) probably crystallised from N-MORB magmas; and (3) basalts with N- to T-MORB affinity covered by late Callovian-early Oxfordian radiolarian cherts. Both gabbroic rocks and basalts locally intrude the mantle peridotites and postdate their re-equilibration to plagioclase-facies conditions. The slide-blocks of lower continental crust are composed of gabbro-derived mafic granulites and felsic granulites. The latter include quartzo-feldspathic granulites and rare quartz-poor to quartz-free charnockitic rocks. In both mafic and felsic granulites, granulite-facies re-equilibration was followed by a retrograde metamorphic evolution to amphibolite-, greenschist- and subgreenschist-facies conditions. Retrogression is commonly accompanied by deformations progressively changing from plastic to brittle. The upper crustal rocks occurring as slide-blocks consist of Hercynian granitoids with orogenic affinity, mainly biotite-bearing granodiorites and peraluminous two-mica leucogranites. Locally, the granitoids are intruded by basaltic dykes or capped by basaltic flows and radiolarian cherts. The granitoids underwent polyphase brittle deformations under subgreenschist-facies conditions which predated the basalt emplacement. The tectono-metamorphic evolution recorded by the slide-blocks of the External Liguride Units started in the Late Carboniferous-Early Permian (about 290 Ma), with the emplacement at deep crustal levels of the gabbroic protoliths for the mafic granulites. The associated felsic granulites likely represent the remnants of the lower continental crust intruded by the gabbro-derived granulites. Mafic and felsic granulites subsequently underwent tectonic exhumation in Permo-Triassic times, as testified by the development of granulite- to amphibolite-facies ductile shear zones. The granulites were finally exhumed to shallow levels, probably in association with the subcontinental mantle, in Late Triassic-Middle Jurassic times. The latter period was most likely characterized by extensive brittle faulting at shallow crustal levels, thus giving rise to extensional allochthons formed by stretched slices of granitoids. The Western Tethys opening is finally testified by the basalt intrusion and effusion in the Late Jurassic, followed by deep-sea pelagic sedimentation. The External Liguride crustal stratigraphy can be regarded as a fossil example of the transitional realm at the continent-ocean boundary. This reconstruction fits well with the available data on the present-day continental margins derived from passive lithosphere stretching.

Petrology of melilite-bearing rocks from the Montefiascone Volcanic Complex (Roman Magmatic Province): new insights into the ultrapotassic volcanism of Central Italy

Abstract The products of Montefiascone Volcanic Complex (MVC) encompass one of the most distinct association of potassic to ultrapotassic rocks of the Roman Magmatic Province (RMP), ranging in composition from trachybasalts to tephritic leucitites. New discovery of leucite melilitites, occurring as small lava flows, and of kalsilite–melilite pyroclastic ejecta, further expand the compositional range of the MVC products towards the extreme ultrapotassic compositions of the nearby Umbria-Latium Intra Apenninic Volcanism (IAV). Both lavas and ejecta are characterized by strong LILE and Th enrichments coupled with HFSE depletion, very radiogenic 87 Sr/ 86 Sr (0.7104–0.7106) and unradiogenic 143 Nd/ 144 Nd (0.51209–0.51213). Mineral and whole-rock chemistry indicate that the leucite melilites are transitional between ultrapotassic larnite-free, Roman-type magmas and kamafugites, whereas the ejecta (kalsilite melilitolites and clinopyroxene–kalsilite melilitolites) can be considered as intrusive kamafugites. Significant interactions with country rocks (mainly limestones and marls) have been excluded on the basis of trace elements and Sr–Nd isotopes, showing that low SiO 2 , high (Ca+Na+K)/Al ratios and relatively high CO 2 contents (leading to crystallization of interstitial carbonates in both lavas and ejecta), are primary features of the melts which yielded the melilites and melilite-bearing ejecta. Petrogenesis of the whole range of MVC products is related to melting in the deepest part of a thinned lithosphere characterized by carbonate-bearing phlogopite–clinopyroxene veins with highly radiogenic 87 Sr/ 86 Sr and unradiogenic 143 Nd/ 144 Nd; progressive dilution of a vein-derived, K-rich (kamafugitic) end-member by a basaltic melt originated from a relatively depleted mantle would be able to explain the entire compositional range of MVC magmas. The common geochemical and isotopic signatures of MVC and IAV volcanics suggest that the same petrogenetic processes were simultaneously active in both regions, involving similar mantle domains metasomatized by carbonate–silicate or carbonatitic melts.

Petrology and geochemistry of ultrapotassic rocks from the Montefiascone Volcanic Complex (Central Italy): magmatic evolution and petrogenesis

Abstract The Montefiascone Volcanic Complex belongs to the Roman Magmatic Province of Central Italy; the volcanic activity took place in an extensional, post-collisional setting during Late Pleistocene, giving rise to lava flows and pyroclastic deposits. The extrusive products consist of moderately to strongly undersaturated K-rich lavas ranging in composition from trachybasalts through leucite basanites and leucititic tephrites to tephritic leucitites. They show the typical geochemical and isotopic characters of the Roman potassic magmas, i.e., low TiO2, low K2O/Al2O3, strong enrichment in LILE, high LILE/HFSE ratios, highly radiogenic 87 Sr / 86 Sr ratios (0.71005–0.71112) and unradiogenic 143 Nd / 144 Nd (0.51209–0.51229, corresponding to eNd=−10.7 to −6.8). Large chemical variations have been recognized within the Montefiascone volcanics, resulting both from the occurrence of different primary magmas and shallow-level fractionation processes. The differentiation mainly took place by means of closed-system fractional crystallisation with local influence of crustal assimilation. The leucite basanites represent primary mantle magmas which did not yield derivative products, whereas the leucititic tephrites, tephritic leucitites and trachybasalts comprise highly differentiated rocks strongly depleted in compatible elements and enriched in LILE. Fractional crystallisation dominated respectively by clinopyroxene+leucite and clinopyroxene+plagioclase yielded the most evolved tephritic leucitites and trachybasalts. In contrast, assimilation of metamorphic basement rocks characterized by highly radiogenic 87 Sr / 86 Sr is needed to explain the moderate increase of the 87 Sr / 86 Sr ratio within the leucititic tephrites. The geochemical and isotopic signatures shown by the Montefiascone primary magmas require a clinopyroxene- and phlogopite-rich mantle source; in particular, partial melting of a veined lithospheric mantle can account for the occurrence of different primary magmas characterized by relatively constant Sr- and Nd-isotopic compositions. Depleted mantle Nd-model ages (1.1–1.5 Ga) suggest that the mantle enrichment may be a very old event, unrelated to the subduction that preceded the Roman magmatism. This hypothesis is further supported by the strong similarity in age and isotopic composition between the Montefiascone volcanics and the kamafugitic and carbonatitic rocks of the adjoining ultra-alkaline Umbria–Latium district.

The role of fractional crystallisation, crustal melting and magma mixing in the petrogenesis of rhyolites and mafic inclusion-bearing dacites from the Monte Arci volcanic complex (Sardinia, Italy)

Abstract Rhyolitic lavas and mafic inclusion-bearing dacites (MIBD) form the dominant products of the Monte Arci volcanic complex, one of the most active sites of volcanic activity in Sardinia during the Pliocene. The massif is composed of four distinct eruptive episodes (Phase 1: rhyolites; Phase 2: dacites and andesites; Phase 3: quartz-normative trachytes; Phase 4: mafic lavas ranging from subalkaline to mildly alkaline). Monte Arci magmatism has been characterized by open-system behaviour, with both mantle and crustal contributions and magma mixing. Although the mafic products are restricted to the latest stage of activity, mafic inclusions are quite common in many rhyolites and dacites. The mineral assemblages of the inclusions are dominated by plagioclase + orthopyroxene ± augite, with minor olivine, FeTi oxides and variable amounts of residual trapped liquid giving rise to a fine-grained groundmass. They represent blobs of magma entrained in a partly molten state and provide evidence of a basaltic contribution to the petrogenesis of their enclosing lavas, both as parental magmas or as a source of heat for partial melting of crustal rocks. Rhyolites are metaluminous to slightly peraluminous rocks with a wide range of SiO2 contents (67–75%). They fall into two groups: (1) Mafic inclusion-bearing rhyolites (MIBR). These have initial 87 Sr 86 Sr ratios between 0.70526 and 0.70897 and commonly show relatively high values of Ti, Mg, Fe, Ca, Sr and compatible trace elements (Ni, Cr, V); cordierite-bearing, highly restitic, crustal xenoliths are rare. (2) Inclusion-free rhyolites (IFR). They include both porphyritic lavas (IFR1) and obsidians (IFR2, IFR3), which share low values of Ti, Fe, Mg, Ca, Sr and are strongly depleted in compatible trace elements. IFR1–2 have a marked LREE/HREE fractionation and a narrow range of 87 Sr 86 Sr ratios (0.70885–0.70972), whereas IFR3 have larger Sr isotopic ratios (0.71529–0.71553), low Th, Zr, Hf, LREE contents and low LREE/HREE. Nd isotopic composition is quite uniform for all rhyolite types, with relatively low 143 Nd 144 Nd ratios (0.51216–0.51228, ϵNd ranging between −9.3 and −6.9). MIBR appear to be the product of fractionation of subalkaline (tholeiitic) magmas + assimilation of a crustal component moderately enriched in 87Sr and/or with low Sr content. IFR1–2 may be derived from partial melting of moderately 87Sr-rich crustal lithologies with retention of garnet and plagioclase (+K-feldspar) in the restite, although AFC processes (followed by effective separation from the basalts) cannot be excluded. Obsidians IFR3 need a crustal source with higher 87 Sr 86 Sr ; their peculiar geochemical features most probably reflect the complex behaviour during partial melting of accessory minerals carrying REE, Th, Zr, Hf. Among the eruptive products of Phase 2, mafic inclusion-bearing dacites (MIBD) provide evidence of basalt-rhyolite mixing: (a) coexistence of glasses with SiO2 content ranging from 64 to 72 wt.%; (b) a phenocryst assemblage including phases of basaltic and rhyolitic provenance; and (c) abundant crystal-rich inclusions related to a basaltic intrusion. The extrusion of these lavas has been preceeded by an explosive eruption of rhyolitic tuffs ( 87 Sr 86 Sr initial ratio =0.71097 ). The most satisfactory petrogenetic model for MIBD includes: (1) differentiation of subalkaline (tholeiitic) magma to a silica-poor and Ti-Fe-P rich liquid; (2) partial melting of wall-rocks yielding a rhyolitic magma segregated at the top of the basaltic reservoir; and (3) explosive eruption of the uppermost portion of the magma chamber followed by mixing of rhyolite with the underlying part of the system to form a hybrid dacite.

Evolution of recycled crust within the mantle: Constraints from the garnet pyroxenites of the External Ligurian ophiolites (northern Apennines, Italy)

The pyroxenite-peridotite sequence from the External Ligurian (northern Apennines, Italy) ophiolites is evidence of the evolution of recycled crust within the mantle. We present new major and trace element and Nd-Hf isotopic compositions of garnet clinopyroxenites and websterites from this mantle section. The garnet clinopyroxenites display clinopyroxene and bulk-rock rare earth element patterns with distinct positive Eu anomalies, which argue for the involvement of plagioclase-rich precursors in their origin. We propose that the garnet clinopyroxenites formed by crystallization of eclogite-derived melts that underwent negligible interaction with the host peridotites. The garnet websterites are interpreted to have been produced by reactions between the eclogite-derived melts and peridotites, thereby giving rise to hybrid, second-stage pyroxenites with a crustal geochemical fingerprint. In our petrogenetic scenario, a rifting-related event at ca. 220 Ma caused melting of eclogites originating from a mid-oceanic ridge basalt–type gabbroic sequence. These mafic protoliths underwent a long-lived evolution of recycling in the mantle (1.5–1.0 Ga). We show that heterogeneity of crustal protoliths, age of recycling, and interaction with the host peridotites may lead to a significant compositional and isotopic diversity of crust-derived mantle pyroxenites.

Insights into the origin of mantle graphite and sulphides in garnet pyroxenites from the External Liguride peridotites (Northern Apennine, Italy)

Abstract This paper describes a rare occurrence of graphite in non-cratonic mantle rocks. Graphite has been found in garnet clinopyroxenite layers from the External Liguride peridotites that represent slices of subcontinental lithospheric mantle exhumed at the ocean floor in Mesozoic times. The high-pressure assemblage of the pyroxenites is characterized by garnet+Al–Na-rich clinopyroxene, and testifies to an early stage of equilibration at approximately 2.8 GPa and 1100 °C. Graphite occurs as small dispersed euhedral flakes and stacks of flakes. Structural characterization by microRaman spectrometry indicates a highly ordered structure, compatible with a high-temperature mantle origin. C isotope composition of graphite has a typical mantle signature. Fe–Ni–Cu sulphides occur as accessory phases, both as blebs enclosed in silicates (E-Type) and interstitial grains (I-Type). The sulphide assemblage (Ni-free pyrrhotite, pentlandite, Cu–Fe sulphides) mainly reflects subsolidus exsolution from high-temperature Fe–Ni–Cu monosulphide solid solutions with variable Ni (up to 18 wt%) and Cu content (up to 7 wt%). The origin of E- and I-Type sulphides requires the existence of an immiscible Fe–Ni–Cu sulphide liquid, which segregated from the partial melt of the garnet pyroxenite. Graphite precipitation in the pyroxenite was presumably related to the reduction of a more oxidized carbon species interacting with the sulphide liquid as a reducing agent.

High-P tectono-metamorphic evolution of mylonites from the Variscan basement of the Northern Apennines, Italy

Strain localization within shear zones may partially erase the rock fabric and the metamorphic assemblage(s) that had developed before the mylonitic event. In poly-deformed basements, the loss of information on pre-kinematic phases of mylonites hinders large-scale correlations based on tectono-metamorphic data. In this study, devoted to a relict unit of Variscan basement reworked within the nappe stack of the Northern Apennines (Italy), we investigate the possibility to reconstruct a complete pressure (P) – temperature (T) – deformation (D) path of mylonitic micaschist and amphibolite by integrating microstructural analysis, mineral chemistry and thermodynamic modelling. The micaschist is characterized by a mylonitic fabric with fine-grained K-white mica and chlorite enveloping mica-fishes, quartz, and garnet pseudomorphs. Potassic white mica shows Mg-rich cores and Mg-poor rims. The amphibolite contains green amphibole+ plagioclase+garnet+quartz+ilmenite defining S1 with a superposed mylonitic fabric localized in decimetre- to centimetre-scale shear zones. Garnet is surrounded by an amphibole+plagioclase corona. Phase diagram calculations provide P-T constraints that are linked to the reconstructed metamorphic-deformational stages. For the first time an early high-pressure stage at > 11 kbar and 510 °C was constrained, followed by a temperature peak at 550-590 °C and 9-10 kbar and a retrograde stage (< 475 °C, < 7 kbar), during which ductile shear zones developed. The inferred clockwise P-T-D path was most likely related to crustal thickening by continent-continent collision during the Variscan orogeny. A comparison of this P-T-D path with those of other Variscan basement occurrences in the Northern Apennines revealed significant differences. Conversely, a correlation between the tectono-metamorphic evolution of the Variscan basement at Cerreto pass, NE Sardinia and Ligurian Alps was established. This article is protected by copyright. All rights reserved.

Ultra-depleted peridotites of New Caledonia: a reappraisal

Lucchi, Renata G. ... et. al.-- 87° Congresso della Societa Geologica Italiana e 90° Congresso della Societa Italiana di Mineralogia e Petrologia, The Future of the Italian Geosciences - The Italian Geosciences of the Future, 10-12 September 2014, Milan, Italy.-- 1 pageThe Montellina Spring (370 m a.s.l.) represents an example of groundwater resource in mountain region. It is a significant source of drinking water located in the right side of the Dora Baltea Valley (Northwestern Italy), SW of Quincinetto town. This spring shows a morphological location along a ridge, 400 m from the Renanchio Torrent in the lower sector of the slope. The spring was investigated using various methodologies as geological survey, supported by photo interpretation, structural reconstruction, NaCl and fluorescent tracer tests, discharge measurements. This multidisciplinary approach, necessary due to the complex geological setting, is required for the importance of the Montellina Spring. It is interesting in the hydrogeological context of Western Alps for its high discharge, relatively constant over time (average 150 l/s), and for its location outside a fluvial incision and suspended about 40 m above the Dora Baltea valley floor (Lasagna et al. 2013). According to the geological setting, the hydrogeological reconstruction of the area suggests that the large amount of groundwater in the basin is essentially favoured by a highly fractured bedrock, covered by wide and thick bodies of glacial and gravitational sediments. The emergence of the water along the slope, in the Montellina Spring, is essentially due to a change of permeability between the deep bedrock and the shallow bedrock and/or surficial sediments. The deep bedrock, showing closed fractures and/or fractures filled by glacial deposits, is slightly permeable. The shallow bedrock, strongly loosened as result of gravitational phenomena, and the local gravitational sediments are, on the contrary, highly permeable. The concentration of water at the spring is due to several reasons. a) The spring is immediately downward a detachment niche, dipping towards the spring, that essentially drains the water connected to the change of permeability in the bedrock. b) It is along an important fracture, that carries a part of the losses of the Renanchio Torrent. c) Finally, it is favored by the visible and buried morphology. Although it is located along a ridge, the spring occurs in a small depression between a moraine and a landslide body. It also can be favored by the likely concave trend of buried base of the landslide. At last, tracer tests of the Renanchio Torrent water with fluorescent tracer are performed, with a continuous monitoring in the Montellina Spring. The surveys permit to verify and quantify the spring and torrent hydrogeological relationship, suggesting that only a small fraction of stream losses feeds the spring.