Jubilee Issue: Volume 110, Number 2 (May-June 2017)
Zircon U–Pb geochronology of lower crust and quartzo-feldspathic clastic sediments from the Balagne ophiolite (Corsica)
Swiss Journal of Geosciences volume 110, pages 479–501 (2017)
The Balagne ophiolite from central-northern Corsica represents a continent-near paleogeographic domain of the Jurassic Liguria-Piedmont ophiolitic basin. Pillow and massive basalt lavas are primarily associated with Middle–Upper Jurassic pelagic sediments (mostly radiolarites at their base), continental-derived quartzo-feldspathic clastic sediments and ophiolitic breccias containing clasts of gabbros and basalts. The basalt-sedimentary succession is tectonically associated with a slice composed of an intrusive sequence overlain by basalt lavas. A “plagiogranite” from the intrusive sequence was dated by U–Pb zircon geochronology. Although affected by some uncertainty, mainly reflecting common Pb contamination, the U–Pb zircon data suggest a crystallization age of 159 ± 3 Ma (MSWD = 6.3), which is coeval with the formation of oceanic lower crust in the Schistes Lustrés units from Alpine Corsica. The predominance of quartz grains preserving typical volcanic shape, the prevalence of prismatic zircons and the arkose whole-rock composition indicate that the continental-derived quartzo-feldspathic clastic sediments have a low degree of textural maturity. U–Pb zircon geochronology carried out on two distinct levels of quartzo-feldspathic clastic sediments identified the predominance of zircons with within error U–Pb dates at ~280 Ma; minor components at ~457, ~309 and ~262 Ma were also obtained. The U–Pb date distribution is consistent with a source magmatic material mostly developed during the Variscan orogenic collapse.
The Jurassic Alpine ophiolites (Fig. 1a) are considered the lithospheric remnants of an (ultra-) slow spreading ocean called Liguria-Piedmont basin, which was located between the Europe/Corsica and Adria continental margins (e.g., Lagabrielle and Cannat 1990). Some of the ophiolitic bodies from the Alpine-Apennine belt are characterized by oceanic crust associated with continental crust material (Manatschal and Nievergelt 1997; Marroni et al. 1998; Beltrando et al. 2014) and/or mantle sequences retaining a subcontinental lithospheric origin (Rampone et al. 1995; Müntener et al. 2004; Montanini et al. 2006). These ophiolites are considered fossil analogous of modern magma-poor ocean-continent transition domains (e.g., Manatschal and Müntener 2009). The Balagne ophiolitic nappe from central-northern Corsica is characterized by the primary Jurassic association of pillow and massive basalt lavas with gabbro-basalt breccias and pelagic sediments (mostly radiolarites). The Balagne basalt lavas include quartzo-feldspathic clastic sediments of continental origin indicating proximity to a continental margin (Baud and Rollet 1973; Durand-Delga et al. 1997; Rossi and Durand-Delga 2001; Rossi et al. 2002; Marroni and Pandolfi 2003; Bracciali et al. 2007). Whether the Balagne ophiolitic nappe originated from proximity to the European continent, near the margin of the Corsica-Sardinia block, or to the Adria paleo-margin is still a matter of debate (see also Malavieille et al. 1998; Molli 2008; Li et al. 2015).
Previous geochemical studies showed that the basalts from the Balagne area have transitional mid-ocean ridge affinity, exemplified by slight enrichment of LREE relative to MREE and HREE in their REE patterns (e.g., Venturelli et al. 1979). This geochemical characteristic was attributed to crustal contamination (Durand-Delga et al. 1997) or an origin from a lithospheric mantle source (Saccani et al. 2008). The REE signature of the basalts from the Balagne ophiolite has been recently interpreted to reflect the presence of garnet in the mantle source (Saccani et al. 2015). Rossi et al. (2002) proposed an U–Pb zircon age of 169 ± 3 Ma for the formation of the lower crust in the Balagne ophiolite. This is the oldest U–Pb zircon age reported in the literature for the lower crust sequences of the Alpine-Apennine ophiolites, which mostly range between 166 and 158 Ma (Fig. 2; Bill et al. 1997; Rubatto et al. 1998; Rossi et al. 2002; Schaltegger et al. 2002; Rubatto and Scambelluri 2003; Rubatto and Hermann 2003; Rubatto et al. 2008; Kaczmarek et al. 2008; Li et al. 2013, 2015; Tribuzio et al. 2016). Indeed, relatively young U–Pb zircon dates (~156 to ~142 Ma) were obtained for few meta-gabbros and meta-plagiogranites from the ophiolites of the Western and Central Alps, which show extensive metamorphic recrystallization in response to the Alpine orogenic cycle (Liati et al. 2005; Lombardo et al. 2002; Stucki et al. 2003). These U–Pb zircon dates may be affected by radiogenic Pb loss in response to Alpine metamorphism and are therefore considered not to be robustly constrained (Li et al. 2013; Tribuzio et al. 2016).
The U–Pb zircon age obtained by Rossi et al. (2002) is associated with a high MSWD (8.4, N = 11), thereby implying age dispersion outside the analytical uncertainties, with the data involving more than one statistically valid population. No geochronological constraints are available for the source rocks of the quartzo-feldspathic sediments. Zircons from Balagne quartzo-feldspathic sediments were studied from a morphological viewpoint and were related to a continental source material mostly made up of granitoids similar to the Upper Carboniferous-Lower Permian hornblende granitoids from the Corsica batholith (Durand-Delga et al. 1997; Rossi and Durand-Delga 2001).
The main aim of this study is to provide new geochronological constraints to better assess (1) the age of the lower crust from the Balagne ophiolite, and (2) the source of the quartzo-feldspathic deposits interspersed within the Balagne basalt lavas. New field observations have been therefore coupled with geochronological investigations carried out on gabbroic rocks and quartzo-feldspathic sediments. In particular, in situ U–Pb zircon determinations were obtained for two samples of quartzo-feldspathic sediments and a “plagiogranite” from the intrusive gabbroic sequence. The data allowed us to shed new light on the formation of the Corsican segment of the Jurassic basin.
The Balagne ophiolite represents a Jurassic ophiolitic sequence and its Middle Jurassic-Upper Cretaceous sedimentary cover (Durand-Delga 1984). The ophiolite is a crustal fragment of the Liguria-Piedmont basin (Durand-Delga et al. 1997), which was unaffected by the high pressure/low temperature metamorphism typical of most tectonic units from the Schistes Lustrés units (e.g., Molli and Malavieille 2011). The Balagne ophiolite is located at the top of Alpine Corsica stack (Marroni and Pandolfi 2003).
The Balagne ophiolite (Fig. 1) mainly consists of pillow basalts locally interlayered with levels of massive basalts and deposits of ophiolitic breccias (Baud 1975; Gruppo di Lavoro sulle Ofioliti Mediterranee 1977; Durand-Delga et al. 2001; Rossi et al. 2001). Gabbros and/or serpentinites were considered to form the substrate of the basaltic sequence (see also Rossi et al. 2001), with the serpentinites suggested to derive either from plagioclase-peridotites (Baud 1975) or olivine-gabbros (Durand-Delga et al. 2001).
The ophiolitic breccias from the Balagne ophiolite contain clasts of gabbros and basalts embedded in a matrix mostly composed of gabbro and basalt fragments. The matrix locally shows internal, graded bedding and/or sedimentary structures (Marroni et al. 2007). The oxide-gabbro clasts are locally crosscut by leucocratic felsic dykes. SHRIMP U–Pb zircon geochronology (Rossi et al. 2002) was carried out for a leucocratic felsic dyke crosscutting an oxide-rich gabbro clast from the gabbro-basalt breccias. The SHRIMP analyses gave mostly discordant 206Pb/238U and 207Pb/235U dates; the 206Pb/238U dates range from 175 ± 3 (2) to 163 ± 3 Ma (2) and yield a lower intercept of 169 ± 3 Ma (2, MSWD = 8.4, N = 11). This date was interpreted as the age of crystallization of the gabbroic sequence. A slightly younger concordant zircon with a 206Pb/238U age of 156 ± 2 Ma (2) was also found and interpreted to have experienced radiogenic Pb loss. A discordant zircon with a 206Pb/238U age of 431 ± 8 Ma (2σ) and a concordant zircon with a U–Pb concordant age of 2705 ± 21 Ma (2σ) were also found and interpreted to be inherited grains.
In the lower stratigraphic portion of the Balagne ophiolitic sequence, a level of quartzitic sandstone intercalated within pillow basalts was documented near Piana di Castifao (Fig. 1; Baud 1975; Durand-Delga et al. 1997; Peybernès et al. 2001; Rossi and Durand-Delga 2001). The quartzitic sandstone has calcareous cement and shows graded-bedding indicated by grain size variation and the occurrence of mm-thick bands of mica-bearing pelites. In the proximity of the contact with the pillows, the sandstone contains fragments of micaschist and basalt. On the basis of a morphological study (Durand-Delga et al. 1997; Rossi and Durand-Delga 2001), the detrital zircons from this quartzitic sandstone level were interpreted to derive from the Upper Carboniferous hornblende granitoids from the Corsica batholith. The Balagne ophiolite was therefore interpreted to represent a sector of the Jurassic Liguria-Piedmont basin located close to the European continent (see also Marroni and Pandolfi 2003). Conversely, other authors considered the Balagne ophiolite as a relic of an intraoceanic accretionary wedge formed by offscrapping and shallow underplating of a sector of the basin originally located towards the Adria margin (Malavieille et al. 1998; Molli 2008).
At the stratigraphic top of the Balagne ophiolitic sequence, the pillow basalts are covered by a Middle–Upper Jurassic sedimentary sequence mainly made up of radiolarian cherts, which grade upward into Cretaceous Calpionella limestones and Palombini shales (Marroni and Pandolfi 2003). Radiolarian cherts are up to 10–30 m thick, but in places their thickness is reduced or even absent (Danelian et al. 2008). The Calpionella limestones are locally overlain by a sequence of massive calcarenite-breccia containing fragments of volcano-clastic material, granites and metamorphic rocks, which were interpreted to derive from the Corsica batholith (Durand-Delga et al. 1997). The oldest radiolarian cherts are Late Bathonian to Early Callovian in age (Chiari et al. 2000; Bill et al. 2001; Peybernès et al. 2001; Danelian et al. 2008).
New field data
In the southern sector of the Balagne ophiolite the pillow basalts are overthrust by a tectonic slice (see also Rossi et al. 2001) that we term Moltifao slice. This slice (~100 m in thickness) consists of a mafic sequence showing, at the base, a 5–10 m-thick level of serpentinites, which locally include domains preserving a plagioclase-rich gabbroic protolith. The serpentinites (Fig. 1) are stratigraphically overlain by decametric thick levels of evolved gabbros (gabbronorites to oxide-rich gabbros) that are in turn followed by quartz-free leucocratic rocks mostly consisting of plagioclase (hereafter referred to as albitites). The evolved gabbros and the overlying albitites exhibit a magmatic foliation locally overprinted by a tectonic foliation. The Moltifao mafic sequence is topped by pillow lava basalts. Close to the basalts, the albitites show a reddish color that is probably related to late development of hematite along grain boundaries and micro-fractures.
With the exception of the Moltifao slice, many of the exposures previously described as gabbros in the lower part of the Balagne ophiolitic sequence (Rossi et al. 2001) are considered to be ophiolitic gabbro-rich breccias. These breccias mainly occur within the pillow basalts at the base of the Balagne ophiolitic sequence. The clasts mainly consist of undeformed clinopyroxene-rich gabbros ranging from some decimeters to a few meters in size. Clasts of clinopyroxene-rich sheared gabbros, nearly undeformed gabbronorites to oxide-rich gabbros, pillow basalts and leucocratic felsic rocks are also present (overall a few decimeters in size).
Next to the Carnispola bridge, a new quartzo-feldspathic breccia level (~1 m thick) has been found. The level is mainly composed of feldspars and rounded clasts of quartz (up to 2 mm in size) and is located between a coarse-grained gabbroic breccia at the top and pillow basalts at the base (Fig. 1).
Three samples were selected for this study (Table 1): an albitite (GM1) from the Moltifao slice, a quartzo-feldspathic breccia (PC11) from a level located within the ophiolitic breccia near the Carnispola bridge and a sample (PP1) from the quartzitic sandstone level located near Piana di Castifao (cf. Durand-Delga et al. 1997; Rossi and Durand-Delga 2001).
The albitite GM1 is medium-grained and mostly composed of euhedral to subhedral Na-rich plagioclase, which is partly replaced by fine-grained aggregates of secondary albitite + epidote ± chlorite ± white mica. Subhedral to anhedral amphibole occurs in minor modal proportion and is replaced by chlorite + epidote + sphene. Accessory zircon and apatite are also present.
The quartzo-feldspathic breccia PC11 consists of clastic material within a microcrystalline siliceous matrix. The clasts are medium to coarse grained (1 to 5 mm) and mainly composed of sharp feldspars locally showing a prismatic habit (25 vol%) and quartz (20 vol%). The quartz is typically monocrystalline with engulfed shape. Irregular quartz clasts with undulatory extinction and partially recrystallized to fine-grained aggregates are also locally present. Feldspars, dark mica, zircon and apatite are locally included within quartz. Fine-grained clasts of altered dark mica also occur.
The quartzitic sandstone PP1 contains fine-grained (up to 2 mm) clasts of quartz (30 vol%), feldspars (25 vol%) and calcite (25 vol%) within a calcite-rich microcrystalline matrix. The quartz clasts typically consist of engulfed mono-crystalline grains, locally containing zircon inclusions (Fig. 3). Sharp polygonal aggregates of quartz are also present. Calcite occurs as irregular clasts and, in places, as rounded to elongated organic shell fragments. Similar fragments were described by Rossi et al. (2001) as crinoids and foraminifera. We also found rare white mica clasts.
Feldspars from both the quartzo-feldspathic breccia and the quartzitic sandstone are typically replaced by fine-grained aggregates made up of albite + white mica ± epidote ± calcite. Dark mica is replaced by fine-grained aggregates consisting of chlorite + white mica + titanite.
Whole rock major and trace element analyses of the albitite and the quartzo-feldspathic clastic sediments (Table 2) were carried out at Activation Laboratories LTD (Ancaster, Ontario) by inductively coupled plasma (ICP) optical emission spectroscopy and ICP mass spectrometry. Whole rock CO2 analysis of the quartzitic sandstone was determined at Activation Laboratories LTD (Ancaster, Ontario) by infrared cells with an ELTRA CW-800 analyzer. Details of the analytical techniques and detection limits are available from the company website. In particular, precision and accuracy of trace element analyses are commonly assessed to be within 10%, detection limit of CO2 analysis is 0.01%.
Zircons were separated from the selected samples using conventional heavy-liquid and magnetic techniques. Representative selections of these minerals, as much as possible free from inclusions and fractures, were extracted by hand-picking under a binocular microscope and mounted in epoxy resin, then polished with ¼ µm diamond paste. Cathodoluminescence (CL) and back-scattered images were used to discern the internal structure of grains, the presence of inclusions and/or altered domains, and to select specific areas for spot analysis. For U–Pb zircon investigation of the quartzo-feldspathic clastic sediments, inherited domains within single grains were not considered.
In situ U–Pb isotope analyses of zircons (Tables 3, 4, 5) were performed at the C.N.R.-Istituto di Geoscienze e Georisorse, Unità di Pavia by laser ablation-ICPMS. The used instrument couples a 193 nm ArF excimer laser (GeoLas102-Microlas) and a high resolution (HR)-ICPMS type Element I from ThermoFinnigan (Table 6). For the present work the laser was operated at a repetition rate of 5 Hz, with a spot diameter of ~20 µm and a laser fluency of 12 Jcm−2. The analytical procedures were basically the same as those described in Tiepolo (2003). Laser-induced U–Pb fractionation and mass bias for zircon analyses were corrected by external standardisation using zircon GJ1 (608.5 ± 0.4 Ma, Jackson et al. 2004). The time-resolved signal was carefully inspected to detect U–Pb isotope heterogeneities within the ablated volume; only homogeneous intervals of at least 25 s were considered. Data reduction, isotope ratio and apparent age calculations were carried out with the IOLITE software (Paton et al. 2011) utilizing the U–Pb geochronology 3 data reduction scheme (Paton et al. 2010). Concordia plot, age probability distribution and weighted average values were calculated using the ISOPLOT/EX software by Ludwig (2003). Data accuracy during analytical runs for zircon was monitored on zircon 91500 (1065 Ma, Wiedenbeck et al. 1995) that yielded results within error of the reference value.
The albitite specimen GM1 has high SiO2, Al2O3 and Na2O, and low CaO and MgO (Table 2). Its chondrite-normalized REE pattern shows LREE enrichment relative to MREE and a marked negative Eu anomaly (Fig. 4a). It also shows a significant HREE enrichment relative to MREE, which probably reflects the relatively high proportion of zircon, in agreement with the elevated concentrations of Zr, Hf, U and Th (Table 2). The Light to Middle REE pattern of the Balagne albitite resembles that of SiO2-rich plagiogranites from the Alpine Jurassic ophiolites (Borsi et al. 1996).
On the basis of the log(Fe2O3/K2O) vs. log(SiO2/Al2O3) scheme by Herron (1988) for the classification of terrigenic rocks, the selected quartzo-feldspathic sediments are arkoses. In particular, the specimen of quartzo-feldspathic breccia PC11 has lower SiO2/Al2O3 ratios and higher Na2O and K2O concentrations than the quartzitic sandstone specimen PP1 (Table 2), in agreement with the lower modal quartz/feldspar ratio (see also Table 1). The CO2 content of the quartzitic sandstone is 15 wt% and corresponds to a total modal amount of calcite of 35 mol%. The quartzo-feldspathic clastic sediments have nearly parallel REE patterns, characterized by a marked LREE enrichment and a slight HREE depletion relative to MREE, and a significant negative Eu anomaly (Fig. 4b). These patterns resemble the REE profile defined by the NASC (North American Shale Composite, Gromet et al. 1984). The quartzo-feldspathic breccia shows higher REE abundances than the quartzitic sandstone.
U–Pb zircon geochronology
The albitite from the intrusive sequence of the Moltifao tectonic slice
Twenty-eight zircon grains were separated from the albitite specimen GM1. They are 50–150 μm in size and mainly consist of pale pink fragments, typically irregularly fractured and in places showing crystal faces. The zircons have low CL emission and commonly a faint oscillatory zoning with low contrast (Fig. 5a). Rare grains with an oscillatory zoning consisting of broad bands were also found. Turbid domains, showing pores and mineral inclusions are locally present (Fig. 5b) and resemble the spongy structure observed in zircons from modern spreading ridges (e.g., Grimes et al. 2009; Schwartz et al. 2010). This structure was interpreted to reflect alteration of original magmatic zircons by a fluid-driven reaction of dissolution and re-precipitation (see also Grimes et al. 2011). No inherited zircon domains were observed.
U–Pb zircon geochronology was carried out on zircon domains as free of fractures, pores and mineral inclusions as possible. Eighteen analyses yielded concordant 206Pb/238U and 207Pb/235U dates scattering from 174 ± 8 to 149 ± 4 Ma (2σ, Table 3). The 18 concordant dates yielded a weighted mean age of 159.0 ± 3.0 Ma (95% confidence level, MSWD = 6.3, Fig. 6a). The relatively high MSWD could be influenced by zircon domains that were either damaged by U-decay, affected by radiogenic Pb loss, or where common Pb was potentially stored (von Quadt et al. 2014). The remaining 10 zircon analyses were discordant with 206Pb/238U dates ranging from 168 ± 6 to 154 ± 6 Ma (2σ, Table 3). The discordance could reflect some radiogenic Pb loss; in addition, the discordant zircons most likely experienced common Pb contamination. A regression line calculation of data on a Tera-Wasserburg concordia diagram may be used to correct for common Pb contribution (Schoene 2014). On this diagram (Fig. 6), we obtained a lower intercept age of 158.1 ± 3.2 Ma (95% confidence level, MSWD = 2.1, N = 28) considering all concordant and discordant data. This lower intercept age is within error the weighted mean age given by the concordant dates. Although the high MSWD associated with the weighted mean age left some uncertainty, we propose that the albitite GM1 crystallized at 159.0 ± 3.0. This statement implies that the event of common Pb contamination was nearly coeval with the crystallization of the albitite.
The quartzo-feldspathic breccia from the basalt-sedimentary succession
156 zircon grains were separated from the quartzo-feldspathic breccia PC11. They are dominated by a population of colorless transparent crystals; about 15% of separated zircons are opaque. The zircons are mainly prismatic; sub-prismatic to stubby grains are also present. The zircon length/width aspect ratio varies between 1 and 0.3. No systematic differences in shape and CL inner structure were found between the transparent and opaque zircon populations. CL imaging typically shows inner structure with oscillatory zoning (Fig. 5c), which is locally characterized by faintly to broad banding. Rare not zoned domains and/or inherited cores are also present.
Thirty-seven grains of zircon were dated by LA-ICPMS. Twenty-seven analyses resulted in concordant 206Pb/238U and 207Pb/235U dates (Table 4), which yielded a mean concordia age of 280.0 ± 2.4 (2σ, MSWD = 1.9, Fig. 7a). The remaining 10 analyses are discordant and did not furnish considerable geochronological information (Table 4).
The quartzitic sandstone from the basalt-sedimentary succession
Zircons separated from the quartzitic sandstone PP1 (146 grains) consist of pink transparent to dark pink opaque grains. They are mainly stubby, sub-prismatic to prismatic and sharp and have aspect ratio (length/width) between 1 and 0.3. Rounded grains are rare. The zircons have low CL emission and are not zoned, or only exhibit a weak oscillatory zoning (Fig. 5d). Dark cores and faintly broad zoned domains were locally found.
Thirty-eight grains were dated. Thirty-four analyses yielded concordant 206Pb/238U and 207Pb/235U dates (Table 5) that range from ~460 to ~210 Ma. When plotted on the age probability distribution diagram (Fig. 7b), 17 of the concordant U–Pb dates identify a main peak at 278.8 ± 2.3 Ma (2σ, MSWD = 1.3, weighted average age). Three older concordant 206Pb/238U and 207Pb/235U dates at 457 ± 15 Ma (2σ, N = 1) and 309 ± 7 Ma (2σ, weighted average age, N = 2) were also found. The remaining fourteen concordant U–Pb dates fall in two slightly younger age clusters at 262.2 ± 5.9 Ma (2σ, concordia age, MSWD = 2.3, N = 9) and 221.5 ± 6.9 Ma (2σ, concordia age, MSWD = 1.7, N = 5). The latter age is similar to the weighted mean 206Pb/238U age yielded by the remaining four discordant zircon analyses (222 ± 32 Ma, 95% confidence level), which presumably reflect variable amounts of common Pb contamination and radiogenic Pb loss. We propose that the youngest concordant dates represent mixed domains including zircon portions that experienced Pb loss and/or common Pb contamination, i.e., they are not geologically meaningful.
Age of the gabbroic crust from the Balagne ophiolite
The Balagne ophiolite preserves a primary Jurassic association of pillow and massive basalt lavas with ophiolitic breccias and continent-derived quartzo-feldspathic clastic sediments. At the top of the succession, the pillow basalts are covered by Middle–Upper Jurassic pelagic sediments mostly consisting of radiolarian cherts. The basalt-sedimentary succession is tectonically associated with the Moltifao slice, which is composed of an intrusive sequence consisting of gabbronorites, oxide-rich gabbros and albitites, overlain by pillow basalt lavas.
U–Pb zircon geochronology indicates that the crystallization of the albitite from the Moltifao intrusive sequence occurred at 159 ± 3 Ma. Ophiolitic gabbros and leucocratic felsic rocks from the Schistes Lustrés Units of Alpine Corsica have been dated by U–Pb zircon geochronology at 158 ± 3 to 160 ± 2 Ma (2σ, Li et al. 2015, see also Fig. 2) and hence yielded similar ages. Li et al. (2015) proposed that the ophiolites from the Schistes Lustrés units formed in a more ocean-ward domain of the Liguria-Piedmont basin with respect to the Balagne ophiolite that was emplaced close to a continental margin. Mostly on the basis of field relationships, however, other authors considered the ophiolites from the Schistes Lustrés units as typical of an ocean-continent transition domain (Molli 2008; Molli and Malavieille 2011; Brovarone et al. 2011).
The U–Pb zircon age of 159 ± 3 Ma proposed for the formation of the Moltifao intrusive sequence is younger than the age of 169 ± 3 Ma proposed by Rossi et al. (2002) for a felsic dyke crosscutting an oxide-rich gabbro clast in the ophiolitic breccias. Two hypotheses can be formulated to explain the younger age of the Moltifao intrusive sequence with respect to the gabbro clasts in the breccias.
A first hypothesis implies that the Moltifao tectonic slice was part of a crustal sequence originally unrelated to the main basalt-sedimentary succession of the Balagne ophiolite and located in an oceanward sector of the Liguria-Piedmont basin. The Moltifao tectonic slice could therefore be paleographically analogous to the Pineto ophiolite, which is situated at the top of the Corsica Alpine tectonic stack and is devoid of high-pressure metamorphism, similar to the ophiolites from the Balagne region. The lower crust sequence of the Pineto ophiolite displays close structural and compositional similarities to lower crustal sections at slow and ultra-slow spreading ridges (Sanfilippo and Tribuzio 2013, 2015) and is crosscut by basalt dykes chemically similar to modern N-MORB basalts (Saccani et al. 2000). It is therefore conceivable that the Pineto ophiolite represents an oceanward sector of the Liguria-Piedmont basin. According to this hypothesis, in the Balagne ophiolite, the time lapse between the formation of a continent-near and an oceanward gabbroic sequence was 10 ± 6 Ma.
A second and alternative scenario implies that the Moltifao tectonic slice was originally physically contiguous to the main basalt-sedimentary succession of the Balagne ophiolite (i.e., the Moltifao intrusive sequence was part of the original basement of the main basalt-sedimentary succession of the Balagne ophiolite). According to this hypothesis, the Moltifao intrusive sequence and the intrusive rocks from the ophiolitic breccias would have similar crystallization ages. This hypothesis is consistent with the transitional MORB geochemical affinity of the Moltifao basalt lavas (Sanfilippo, unpublished PhD thesis), similar to the basalt lavas from the main basalt-sedimentary succession of the Balagne ophiolite (Venturelli et al. 1979), and would imply that the U–Pb zircon age of 169 ± 3 Ma reported by Rossi et al. (2002) was inaccurate. This age is a lower intercept obtained from mostly discordant analyses and is associated with a high MSWD (8.4) indicating a multiple distribution of U–Pb dates. The spread of 206Pb/238U zircon dates (Fig. 8), ranging from 175 ± 3 (2) to 163 ± 3 Ma (2), could reflect the incorporation of an inherited component in the oldest analyses, in agreement with the occurrence of: (1) Silurian and Neo-Archean zircons, and (2) a concordant U–Pb zircon date at 156 ± 2 (2) overlapping within error the U–Pb zircon age obtained in this work for the albitite (Fig. 8).
The second hypothesis also implies that the lower crust from the whole ophiolitic sequences of the Balagne region crystallized at 159 ± 3 Ma, i.e., within the Oxfordian according to the International Chronostratighraphic Chart (Cohen et al. 2013). However, the oldest radiolarian cherts from the main basalt-sedimentary succession of the Balagne ophiolite were dated as Late Bathonian to Early Callovian (Chiari et al. 2000; Bill et al. 2001; Peybernès et al. 2001), corresponding to the Unitary Association Zone (UAZ) seven of Baumgartner et al. (1995). The paradox of Oxfordian gabbros, younger than the deposition of early pelagic sediments, may be explained if radiolarite deposition would have taken place during ongoing magmatism, thereby implying that the radiolarites are locally older than magmatic products. We also wish to mention that gabbroic bodies having younger radiometric dates (~161 Ma) compared to biostrathigraphic dates of older overlying pelagic sediments (Lower Bathonian) have also been reported for other ophiolites of the Alpine-Apennine belt, such as the Ligurian Jurassic ophiolites (Tribuzio et al. 2016). Therefore, we cannot exclude an incorrect calibration of the biostratigraphic time-scale for the Middle–Upper Jurassic. Indeed, the temporal boundaries between Bathonian and Callovian (Middle Jurassic), between Callovian and Oxfordian (Middle–Upper Jurassic) and between Oxfordian and Kimmeridgian (Upper Jurassic) are not firmly constrained and markedly variable in the chronostrathigraphic scales proposed in the last fifteen years (cf. Gradstein et al. 2004; Pálfy et al. 2000). In addition, Danelian et al. (2008) showed the occurrence of a biostratigraphic incompatibility in a 7 m thick radiolarite sequence overlying the basalt pillow lavas from the Balagne ophiolite. Species of UAZ 8 (Mid Callovian-Early Oxfordian) and UAZ 9 (Mid-Late Oxfordian) are present in the same assemblage with species of UAZ 7, although they are not presumed to occur together in the zonation of Baumgartner et al. (1995). This indicates some uncertainties in the definition of the radiolarian ages for the sedimentary cover of the Balagne ophiolite. We conclude that there could not be substantial age differences regarding the formation of the lower oceanic crust in the Schistes Lustrés units and in the Moltifao intrusive sequence from the Balagne ophiolite.
Provenance of the quartzo-feldspathic clastic sediments
The quartzo-feldspathic breccia and the carbonate-rich quartzitic sandstone contain abundant grains of monocrystalline quartz with engulfed shape, which is typical of quartz of volcanic origin and therefore indicates the presence of a volcanic component in the source area. The occurrence of quartz grains preserving typical volcanic shape and the prevalence of zircons with sharp, sub-prismatic to prismatic habitus suggest that these quartzo-feldspathic clastic sediments have a low textural maturity, in agreement with their arkose whole-rock compositions. The quartzo-feldspathic clastic sediments rocks also contain subordinate sharp polygonal aggregates of quartz of metamorphic origin, which in tandem with the occurrence of rare clasts of white mica are consistent with the presence of metamorphic material in the source area. The concordant zircon date at 457 ± 15 Ma found in the quartzitic sandstone could be correlated with a minor contribution from the pre-Variscan metamorphic basement (Rossi et al. 1995; Renna and Tribuzio 2009).
U–Pb zircon geochronological data identified a homogeneous population for the quartzo-feldspathic breccia, with a mean U–Pb concordia age of 280 ± 2 Ma (Fig. 7a). A heterogeneous distribution of U–Pb zircon dates was obtained for the quartzitic sandstone, where we found a main peak at 279 ± 2 Ma that is statistically indistinguishable by the mean U–Pb zircon age yielded by the quartzo-feldspathic breccia. These ages are within error with the crystallization ages of 284 ± 4 and 278 ± 2 Ma obtained for ignimbrite rhyolites from Monte Cinto-Galeria volcanic sequences of Corsica (Rossi et al. 2015). Hence, the Balagne quartzo-feldspathic clastic sediments may have been supplied by volcanic material from the Corsica-Sardinia block. However, within the Permo-Carboniferous association composing the Sesia Magmatic System (western Alps, Sinigoi et al. 2010), which constitutes a virtually complete section of the Adriatic continental crust, the volcanic event was similarly dated by U–Pb zircon geochronology at 282 ± 3 Ma (Quick et al. 2009). In addition, the minor zircon components at 309 ± 7 Ma found in the quartzitic sandstone are similar to U–Pb zircon dates found in several intrusive and volcanic rocks associations from both Corsica (Paquette et al. 2003; Rossi et al. 2015) and Adriatic continental margins (Peressini et al. 2007; Renna and Tribuzio 2009). Note also that there are not significant geochronological differences between European and Adriatic continental margins with regards to the U–Pb zircon dates coeval to the zircon component at 262 ± 6 Ma found in the quartzitic sandstone, although there is no general consensus about their geological meaning (cf. Rossi et al. 2015; Dallagiovanna et al. 2009; Renna et al. 2007; Renna and Tribuzio 2009).
In summary, the U–Pb zircon geochronology of the quartzo-feldspathic clastic sediments re-inforces the view that Balagne ophiolite formed near a continent and that its quartzo-feldspathic clastic sediments were mainly supplied by volcanic material of Permian age. On the basis of the data reported in this study, the localization of the Balagne ophiolite in proximity to Europe rather than to the Adria margin cannot be unequivocally established.
Summary and concluding remarks
The Balagne ophiolite from central-northern Corsica is a continent-near fragment of oceanic lithosphere of the Jurassic Liguria-Piedmont basin. Pillow and massive basalt lavas are primarily associated with continental-derived quartzo-feldspathic sediments, and breccias with gabbro and basalt clasts. The quartzo-feldspathic clastic sediments display a low degree of textural maturity and were derived from a source including SiO2-rich volcanics and subordinate metamorphic material. U–Pb zircon geochronology identified a predominant ~280 Ma component, corresponding to the magmatic episodes developed during the Variscan orogenic collapse.
The ophiolitic breccias document that the marginal Balagne basalt-sedimentary succession was next to a topographic high mainly composed of a gabbroic sequence, presumably located in an oceanward position with respect to the continental margin (Marroni and Pandolfi 2007). The main basalt-sedimentary succession of the Balagne ophiolite is associated with a tectonic slice composed of an intrusive sequence dated at 159 ± 3 Ma and overlain by basalt lavas. The formation of the intrusive rocks from the tectonic slice from the Balagne ophiolite is contemporaneous with the ophiolitic gabbros from the adjacent Schistes Lustrés units (Li et al. 2015). On the basis of the U–Pb zircon dates obtained in this work and reported in the literature for the gabbroic clasts in the Balagne ophiolitic breccia (Rossi et al. 2002), the intrusive sequence from the tectonic slice could represent an oceanward sector of the Liguria-Piedmont basin younger than the gabbroic crust from the main basalt-sedimentary succession of the Balagne ophiolite. This hypothesis implies that the time interval intervened from continent-near to oceanward gabbroic crust formation was 10 ± 6 Ma. An alternative hypothesis implies that the intrusive sequence from the tectonic slice was part of the original basement of the main basalt-sedimentary succession of the Balagne ophiolite and therefore had similar crystallization age to the intrusive rocks from the ophiolitic breccia.
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We are grateful to A. von Quadt, M. Marroni and editor S. Schmid for the constructive comments that considerably improved the quality of this study.This work was financially supported by Università degli Studi di Pavia.
Editorial handling: C. Sue and S. Schmid.
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Renna, M.R., Tribuzio, R., Sanfilippo, A. et al. Zircon U–Pb geochronology of lower crust and quartzo-feldspathic clastic sediments from the Balagne ophiolite (Corsica). Swiss J Geosci 110, 479–501 (2017). https://doi.org/10.1007/s00015-016-0239-y
- Alpine Corsica
- Jurassic ophiolites
- Basalt-sedimentary succession
- U–Pb zircon geochronology
- Formation of oceanic gabbroic crust