A crustal–upper mantle model for southeastern Sicily (Italy) from the integration of petrologic and geophysical data

Journal of Geodynamics 66 (2013) 92–102

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A crustal–upper mantle model for southeastern Sicily (Italy) from the integration of petrologic and geophysical data Fabio Carmelo Manuella ∗ , Alfonso Brancato, Serafina Carbone, Stefano Gresta Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Sezione di Scienze della Terra, Università di Catania, Corso Italia 57, 95129, Italy

a r t i c l e

i n f o

Article history: Received 22 October 2012 Received in revised form 12 February 2013 Accepted 12 February 2013 Available online 26 February 2013 Keywords: Hyblean Plateau Sicily Ionian basin Serpentinization Moho

a b s t r a c t An interdisciplinary approach is proposed to investigate the structure and composition of the PermoTriassic basement of the Hyblean Plateau and Sicily Channel. Comparisons of published data on peridotites and spinels from different geodynamic settings, and new data on Hyblean spinels, reveal the affinity of the Hyblean basement with an ultra-slow spreading oceanic lithosphere, rather than with the Africa continental plate. Similar results derive from volcanic rocks of the studied area, whose Nb/Yb vs. Th/Yb ratio hints at their affinity with the MORB–OIB array, even excluding any possible contamination with continental crust lithologies, unlike North Africa lavas. The comparison of He isotopic ratios from Hyblean Plateau and Sicily Channel highlights their similarity with values measured in fluids emitted from the Rainbow and Logatchev hydrothermal fields in Mid-Atlantic Ridge. Based on petrologic and geochemical evidence for the oceanic nature of the Permo-Triassic basement in southeastern Sicily, and the occurrence of serpentinized harzburgite xenoliths in Hyblean diatremes, the P-wave velocity model proposed for the investigated area is used to estimate lithospheric pressure, density, degree of serpentinization and magnetic susceptibility also considering both abyssal and ophiolitic serpentinites. The resulting values suggest the presence of peridotites affected by different degrees of serpentinization (35–100 vol.%) ranging to a depth of 8–19 km. As a whole, combined seismic, gravimetric and magnetic data indicate the presence of a marked anomaly at a depth of about 19 km. As a consequence, we consider the Moho discontinuity as a serpentinization front, by fixing the relative top at a depth of 19 km. Our results suggest that the oceanic lithospheric model for southeastern Sicily could be broadened to the Sicily Channel, which is possibly correlated to the adjacent Ionian oceanic basin, inferred as belonging to the Oman–Iraq–Levantine–Sicily seaway. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction The Hyblean Plateau (southeastern Sicily, Italy) consists of a thick, Mesozoic to Quaternary, platform-type carbonatic sequence with several levels of basic volcanic rocks (Fig. 1). It is traditionally considered an uplifted, emerged portion of the Pelagian Block, which belongs to the Africa continental plate (Burollet et al., 1978; Catalano et al., 2000; Lentini et al., 1996; Scarascia et al., 2000), extending from Tunisia to Sicily. This lithospheric model merely derives from the interpretation of deep seismic soundings, although direct evidence for possible outcrops of the Permo-Triassic crystalline basement in the Hyblean Plateau, or in deep oil wells, are lacking (Bianchi et al., 1987; Finetti et al., 2005). Commercial boreholes reached platform-type carbonates with basic igneous rocks and gabbros, Upper Triassic in age

∗ Corresponding author. Tel.: +39 095 359136; fax: +39 095 7195712. E-mail addresses: [email protected] (F.C. Manuella), [email protected] (A. Brancato), [email protected] (S. Carbone), [email protected] (S. Gresta). 0264-3707/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jog.2013.02.006

(Bianchi et al., 1987 and references therein), at a depth of about 6 km. On the other hand, a long-lasting study of the Hyblean xenoliths induced Scribano et al. (2006a,b) to suggest that the lithospheric basement is made up of a fossil oceanic core-complex, which hosted an abyssal-type hydrothermal system. Indeed, numerous deepseated xenoliths were found in the volcanoclastic levels (Scribano, 1986) occurring in Miocene alkaline-mafic diatremes (Fig. 1; Valle Guffari and Cozzo Molino; Carbone and Lentini, 1981). These xenoliths are represented by dominant mantle lithologies (spinel-facies harzburgites and pyroxenites), minor gabbroic rocks, in addition to a number of sedimentary and volcanic rocks from the MesoCenozoic succession. Minor amounts of small xenoliths (diameter ranging from 1 up to 5 cm; Sapienza and Scribano, 2000) are included in Lower Pleistocene basanite lavas (Fig. 1; Sigona Grande; Scribano, 1987). These xenoliths consist of ultramafic rocks bearing evidence for interaction with the host lavas, which are represented by blebs, isolated or joined with the host lava by channel-like fracture (Scribano, 1986). Crustal xenoliths are very rare in the Quaternary lava (<2 vol.% with respect to ultramafic xenoliths), as they

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Fig. 1. Stratigraphic and structural scheme of the Hyblean Plateau (modified after Finetti et al., 2005), showing the main xenoliths occurrences in southeastern Sicily.

are strongly reabsorbed by the host lava (Sapienza and Scribano, 2000). Most of the Hyblean xenolith suite, occurring in the Miocene volcanoclastic deposits, are 5–30 cm in diameter, and they kept their original features due to the rapid ascension in the eruptive column of the diatremes (Sapienza and Scribano, 2000). In this regard, Hyblean mantle and crustal xenoliths host mineral assemblages [serpentine polytypes, Ni-rich sulfides, Na-rich sylvite, saponite, apatite, aragonite (Manuella, 2011); Na-rich alkali feldspar, chlorite/smectite and/or smectite/illite mixed layer (Scribano et al., 2006a); respectively], in addition to calcite and hydrocarbons (Ciliberto et al., 2009; Scirè et al., 2011), which demonstrate beyond controversy that clues for serpentinization and the associated hydrothermal fluid circulation in peridotites and gabbros, during Permo-Triassic age (Scribano et al., 2006a and references therein) were preserved from any possible contact with the host lava. In the present work, we propose the integration of published petrologic and geochemical data from mafic and ultramafic xenoliths, and volcanic rocks from the Hyblean area and the Sicily Channel, with recent geophysical dataset for the Hyblean Plateau, with particular regard to its seismic velocity values (Brancato, 2005; Giampiccolo et al., 2003), and gravimetric and magnetic anomalies (Ciminale and Wasowski, 1989). This interdisciplinary approach aims at investigating for the structure and composition of the Permo-Triassic basement of the Hyblean sector and the Sicily Channel, in comparison with continental and oceanic geodynamic settings.

2. Geological setting Southeastern Sicily represents a carbonatic platform, the Hyblean Plateau (Fig. 1), which is cross-cut by two main fault systems, one extensional, trending NE–SW produced by the collapse of the Plateau beneath the Neogene Appenine–Maghrebide thrust belt, and a NNW–SSE trending oblique fault belt (the Hyblean–Malta Escarpment), which separates the Plateau from the Ionian abyssal plain (about 3000 m). Roughly N–S structures constitute the western margin of the plateau, separating it from a depressed sector of the foreland underplating allochthonous units. To the west of this lineament the Miocene Hyblean successions have subsided to a depth of about 3000 m and thus about 4000 m lower than their counterparts outcropping on the plateau (Cogan et al., 1989). This major collapse of the Hyblean successions is accompanied by a significant southwestward areal extension of the Gela foredeep, within which a distinct advance of the allochthonous units occurred, forming a vast, and largely submerged, arcuate front (Argnani, 1989; Finetti et al., 1996; Lentini et al., 1994; Torelli et al., 1998). On a more general level, the major flexure of the foreland is reflected in the orogenic areas to the north, with the presence of a wide axial depression within the chain units, known in the literature by the inappropriate name of “Caltanissetta Basin”, in which thick Messinian evaporites occur, deposited in the front of the thrusts prograding towards external areas.

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The northward extent of the foreland below the main thrust wedge in Sicily is constrained, based on geophysical data and indirect geological reconstructions (Finetti et al., 2005 and references therein). Strongly deformed carbonate bodies, connected to the successions of the Hyblean Plateau, below the allochthonous of the orogenic belt, to as far as the northern slope of the Mt. Etna were assumed by Cristofolini et al. (1979), Lentini (1982), and later by Grasso et al. (1990), Ben-Avraham and Grasso (1991) to extend for at least 33 km north of the present thrust front in the off-shore Peri-Thyrrenian Sea. The sedimentary and volcanic cover, about 10 km thick below the central area of the Hyblean Plateau, was detected by commercial boreholes down to the depth of about 6 km, where Middle Triassic layers have been found. However, there is no information about the Permo-Triassic interval, as inferred from the seismic lines (Bianchi et al., 1987). The exposed sedimentary rocks in the Hyblean region are mostly of Tertiary age. The succession is divided into a western and an eastern facies association (Grasso and Lentini, 1982). In the western part of the Hyblean Plateau, well-exposed Late Oligocene–Miocene limestones and marly limestones, the Ragusa Formation, grading up to the marls of the Tellaro Formation, were deposited on a carbonate ramp under neritic to pelagic conditions. Pliocene and Quaternary pelagic carbonates passing basinward into clays are present, in places intercalated with mafic volcanics. The eastern Tertiary succession consists of Miocene carbonates, belonging to the Climiti and Palazzolo Formations, and overlying reefal to lagoonal limestones with intercalated volcanoclastic rocks (Carlentini and Monte Carrubba Formations; Grasso et al., 1982). These rest upon rudist-bearing carbonates of Upper Cretaceous age overlying volcanic seamounts. The volcanic activity in the Hyblean region can be distinguished into four main cycles: (1) Upper Triassic–Hettangian, (2) Middle-Upper Jurassic (Buccheri Formation; Patacca et al., 1979), (3) Upper Cretaceous–Paleocene, (4) Upper Miocene–Lower Pleistocene (Carbone et al., 1987). The outcropping volcanic rocks cover a surface of about 350 km2 of the Plateau: Cretaceous volcanites occur along north–south alignment of the Ionian coast (Carbone et al., 1984), and the Upper Miocene and Pliocene–Lower Pleistocene volcanites extend in the north-eastern side of the Plateau. During the Upper Miocene, the eastern part of the Hyblean area was affected by an explosive activity that produced diatremic tuffbreccia deposits (Carlentini Formation; Carbone and Lentini, 1981), outcropping along a NE–SW trending belt, which contain juvenile clasts of nephelinitic lavas and deep-seated xenoliths.

3. The structure of the Hyblean crustal basement 3.1. The xenolith viewpoint 3.1.1. Essential petrologic information on the Hyblean xenoliths The Hyblean xenoliths are mainly represented by spinelharzburgites, exhibiting both protogranular and porphyroclastic texture (Punturo et al., 2000), in which olivine has a Mg# [Mg/(Mg + Fe2+ )] ratio of 0.91 ± 0.01 on average, with NiO = 0.2–0.5 wt.%; orthopyroxene shows a Mg# of 0.89 ± 0.02 on average, with CaO <0.9 wt.%; clinopyroxene (Cr-diopside) exhibits a narrow compositional range (En52 Wo47 Fs1 –En50 Wo43 Fs7 ); spinel has a Cr# [Cr/(Cr + Al)] ratio corresponding to 0.27 ± 0.02 and a Mg# from 0.67 to 0.80, on average (e.g., Scribano et al., 2009). A number of pyroxenites completes the mantle xenoliths, consisting of: (1) Al-spinel bearing websterites, (2) opx-bearing spinel clinopyroxenites, (3) opx-bearing garnet clinopyroxenites, (4) Cr-diopside clinopyroxenites (Scribano et al., 2009 and references therein). Crust-derived xenoliths are mostly formed by feldspar-bearing gabbros, whose modal composition is characterized by the

Fig. 2. MgO (wt.%) vs. Al2 O3 (wt.%) ratio for peridotites from Hyblean Plateau (HP; Punturo, 1999; Sapienza et al., 2005; Tonarini et al., 1996), abyssal peridotites from Mid-Atlantic Ridge (MAR, hereafter) and Southwestern Indian Ridge (SWIR, hereafter) (AP; GEOROC database; http://georoc.mpch-mainz.gwdg.de/georoc/Start.asp), North Africa (NA; Beccaluva et al., 2008; GEOROC database), continental lithospheric mantle and primordial mantle (CLM, PM; McDonough, 1990). The diagram shows a significant separation between peridotites from the Hyblean Plateau and modern spreading ocean centers (MAR and SWIR), with respect to North Africa peridotites which are akin to the composition of continental lithospheric mantle and primordial mantle.

occurrence of Fe–Ti oxide minerals, plagioclases showing a compositional range from An40 Ab58 Or1 to An82 Ab18 , and clinopyroxene having Mg# = 73–78 and CaO = 20–21 wt.%. 3.1.2. The nature and composition of the Hyblean crustal basement Detailed information on the nature and composition of the crustal basement of southeastern Sicily are derived from petrologic and geochemical investigations of the crustal and mantle xenoliths. In this regard, the comparison (Fig. 2) of whole-rock MgO (wt.%) vs. Al2 O3 (wt.%) in peridotites from different geodynamic settings shows a good affinity of Hyblean harzburgites and abyssal peridotites, which have lower Al2 O3 amounts than North African xenoliths, reflecting a different degree of the refractory nature of the upper mantle in continental lithospheric mantle with respect to abyssal peridotites (Bodinier and Godard, 2003; Bonatti and Michael, 1989). Similar results derive from the Mg# vs. Cr# ratio in spinels, which represents an useful petrogenetic indicator in abyssal and orogenic peridotites (Bonatti and Michael, 1989; Dick and Bullen, 1984; Ghazi et al., 2010; Le Mée et al., 2004; Voigt and von der Handt, 2011). Based on this ratio (Fig. 3), spinels in Hyblean harzburgites, including new data reported in Table 1 as retrieved from EDS (energy dispersive X-ray spectroscopy) microanalyses, plot in the field of abyssal peridotites (MAR, SWIR, Gakkel Ridge) and Oman ophiolites, instead North African Table 1 Microprobe analyses of Hyblean harzburgite-hosted spinels. Samples

HS 1

HS 2

HS 3

HS 4

HS 5

HS 6

HS 7

MgO FeO Al2 O3 Cr2 O3

18.37 16.46 41.75 23.42

19.85 14.19 44.88 21.08

19.4 14.87 43.07 22.66

18.32 15.41 32.12 34.15

17.36 15.36 31.02 35.5

18.37 16.46 41.75 23.42

19.85 14.19 44.88 21.08

0.27 0.75

0.24 0.80

0.26 0.79

0.28 0.71

0.27 0.75

0.24 0.80

0.26 0.79

Cr# Mg#

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Fig. 3. Mg# (Mg/Mg + Fe2+ ) vs. Cr# (Cr/Cr + Al) ratio in spinels of peridotites from Hyblean Plateau (Table 1; Punturo, 1999; Scribano, 1986, 1987), Oman ophiolites (Le Mée et al., 2004), abyssal harzburgites from MAR and SWIR (Moll et al., 2007; Petdb database, http://www.petdb.org), North Africa (Beccaluva et al., 2008; GEOROC database), Garrett transform fault (Constantin, 1999), Hess Deep (Allan and Dick, 1996; Arai and Matsukage, 1996; Edwards and Malpas, 1996), Gakkel Ridge (Hellebrand et al., 2002). The diagram shows that peridotite-hosted spinels from the Hyblean Plateau fall within the field of abyssal peridotites, unlike those from North Africa, in association with spinels from two ultra-slow spreading ridges, like SWIR and Gakkel. Spinels from fast-spreading ridges (Garrett and Hess Deep) plot in the upper portion of the fields of abyssal peridotites, confirming that the Cr#/Mg# is a robust parameter to discriminate peridotites from different petrotectonic settings, in agreement with previous works (e.g., Ghazi et al., 2010; Voigt and von der Handt, 2011).

peridotites form a separate cluster with the lowest Cr# (<0.2) that is typical of the undepleted subcontinental mantle (Bonatti and Michael, 1989). Therefore, Hyblean peridotites are akin to abyssal peridotites rather than to continental mantle rocks from North Africa, in agreement with the model proposed by Scribano et al. (2006a,b) of a fossil oceanic lithosphere. In this regard, the Mg#/Cr# in spinels denotes the affinity of Hyblean harzburgites with those from ultra-slow spreading ridges (<20 mm y−1 ; Palandzhyan, 2007), represented by SWIR and Gakkel. Following the dependence of Cr# in spinel on the spreading rate (hereafter, SR) of oceanic ridges as observed by Hellebrand et al. (2002), the paleospreading rate of the Permo-Triassic oceanic basement in southeastern Sicily was estimated by using Cr# values in spinels from Hyblean harzburgites. An analytical relation between Cr# and SR was retrieved from plotting data reported by Hellebrand et al. (2002; Fig. 4), SR = exp[(Cr# + 0.0355)/0.1158] (R2 = 0.910). The average value of paleospreading rate for the Hyblean oceanic basement (Fig. 4) corresponds to 14 ± 3 mm y−1 , which is comparable to the supposed slow spreading rate of the oceanic Ionian basin, reported by Catalano et al. (2001), located in the eastern side of the Hyblean Plateau. In addition to the aforementioned evidence for the oceanic nature of the Hyblean Permo-Triassic basement, felsic igneous rocks (e.g., granites, felsic metaigneous and metasedimentary rocks), which could be attributed to a hypothetical Africa continental crust, are missing in the entire xenolith population found in the

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Fig. 4. Cr# vs. spreading rates (SR) of oceanic ridges (SWIR, Gakkel, MAR, CIR – Central Indian Ridge, EPR – East Pacific Rise) as reported by Hellebrand et al. (2002), with the estimated average value of the paleospreading rate of the Hyblean oceanic basement, corresponding to 14 ± 3 mm y−1 , which is comparable to the supposed slow spreading rate of the oceanic Ionian basin, reported by Catalano et al. (2001), located in the eastern side of the Hyblean Plateau. Also reported the regression equation and the relative confidence correlation.

Hyblean diatremes (Sapienza and Scribano, 2000), and any possible crustal contamination of the Hyblean volcanic rocks was ruled out by Bianchini et al. (1999) and Trua et al. (1998). Indeed, values of the Nb/Yb vs. Th/Yb ratio for volcanic rocks from North Africa plot in the field indicating a magma–continental crust interaction (Fig. 5), according to Pearce (2008), unlike Hyblean tholeiites and alkaline basic lavas that fall within the MORB–OIB array, in the E-MORB and OIB fields (Scribano et al., 2009), respectively, consistent with their oceanic affinity (Trua et al., 1998). This inference is also confirmed by the compatibility of cumulate gabbroic xenoliths (Scribano et al., 2006b) with an E-MORB melt from which these rocks originated, based on the Nb/Yb vs. Th/Yb ratio (Fig. 5). Similarly, lavas from the

Fig. 5. Nb/Yb vs. Th/Yb ratio for volcanic rocks from Hyblean Plateau, Sicily Channel, Pantelleria, Linosa, North Africa (GEOROC database), and Hyblean gabbro xenoliths (Scribano et al., 2006b), with the MORB–OIB array proposed by Pearce (2008). The diagram shows that volcanic rocks from the Hyblean Plateau and Sicily Channel plot within the MORB–OIB array, thus excluding any possible continental crust contamination, unlike lavas from North Africa.

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Fig. 6. He isotopic ratios from Libyan lherzolite xenoliths (Beccaluva et al., 2008), Hyblean peridotite xenoliths (Correale et al., 2012; Sapienza et al., 2005), Sicily Channel (GEOROC database), Pantelleria (Parello et al., 2000), Linosa (Fourré et al., 2012), Rainbow, Logatchev, Snake Pit, TAG, Broken Spur, Lucky Strike, and Menez Gwen hydrothermal fields in MAR (Jean-Baptiste et al., 2004 and references therein). The diagram exhibits the similarity of He isotopic values for Hyblean peridotites and volcanic rocks from the Sicily Channel with those measured in fluid from serpentinite-hosted hydrothermal systems in MAR, as Rainbow and Logatchev, unlike values detected in Libyan lherzolite xenoliths and volcanic-hosted hydrothermal systems (i.e., Snake Pit, TAG, Broken Spur, Lucky Strike, and Menez Gwen).

Sicily Channel, including Pantelleria and Linosa Islands, also cluster in the OIB field (Fig. 5), suggesting a common source history for the volcanic rocks erupted on the Hyblean–Malta platform, as previously inferred from He isotopic ratios (Sapienza et al., 2005). We estimated mean values, as well as standard deviations, of He isotopic ratios (Fig. 6) from Libyan lherzolites (6.0 ± 0.5R/Ra , where Ra = air 3 He/4 He; Beccaluva et al., 2008), Hyblean peridotites (7.3 ± 0.3R/Ra ; Sapienza et al., 2005), volcanic rocks in the Sicily Channel (7.0 ± 0.7R/Ra ; Martelli et al., 2008) and in Linosa Island (7.3 ± 0.3R/Ra ; Fourré et al., 2012), and hydrothermal fluids sampled in Pantelleria (7.0 ± 0.1R/Ra ; Parello et al., 2000). Comparing these values with those obtained from fluids discharged from some hydrothermal systems occurring in the MAR (8.65 ± 0.08R/Ra , Menez Gwen – 37◦ 50 N; 8.13 ± 0.05R/Ra , Lucky Strike – 37◦ 17 N; 7.51 ± 0.05R/Ra , Rainbow – 36◦ 14 N; 8.9R/Ra , Broken Spur – 29◦ N; 7.9 ± 0.5R/Ra , TAG – 26◦ N; 8.2 ± 0.4R/Ra , Snake Pit – 23◦ N; 7.2R/Ra , Logatchev – 14◦ 27 N; Jean-Baptiste et al., 2004 and references therein), a remarkable difference occurs between Libyan lherzolite xenoliths with respect to the Hyblean Plateau, Linosa, Pantelleria and Sicily Channel, as a whole. A similarity is evident in Fig. 5 for these latter localities with respect to the Rainbow and Logatchev hydrothermal fields, two serpentinite-hosted hydrothermal systems (Douville et al., 2002) whose values of He isotopic ratios range between the N-MORB values (8.0 ± 1.0R/Ra ; Graham, 2002) and those from a metasomatized MORB mantle (EMORB = 5.95 ± 1.34R/Ra ; Füri et al., 2011). Instead, He isotopic ratios from volcanic-dominated hydrothermal systems of Menez Gwen, Lucky Strike, Broken Spur and Snake Pit (Charlou et al., 2000) clearly differ from those collected in the Hyblean Plateau and neighboring localities. Evidence for mantle metasomatism were found in some ultramafic xenoliths from the Hyblean area (Scribano et al., 2009), including spinel-harzburgite veined by phlogopite-bearing clinopyroxenite, amphibole-bearing clinopyroxenite, and Al-spinel websterite. These ultramafic xenoliths hint at an upper mantle heterogeneity resulting from the mixing of a peridotite layer with a High U/Pb (hereafter, HIMU) signature, due to the enrichment of fluid mobile elements from alkaline hydrothermal fluids (Scribano

et al., 2006a), and the intrusion of metasomatizing silicate melts of deep-seated origin, which produced pyroxenites with a Depleted Mantle (hereafter, DM) affinity (Correale et al., 2012). Indeed, Scribano et al. (2009) invoked the partial melting of veins and pods of clinopyroxenite hosted in a depleted lithospheric mantle beneath the Hyblean Plateau as a possible source of different alkaline primary magmas, from nephelinite to alkaline basalt. The same authors supposed that a higher degree of partial melting could generate tholeiite melts, diluting the chemical contribution of the pyroxenite veins. The origin of basaltoid magma from the partial melting of the metasomatized ultramafic basement of the Hyblean Plateau was also inferred from the presence of a Ba–Ti-rich oxymica (oxykinoshitalite) in an olivine nephelinite (Manuella et al., 2012b). Our results seem in accordance with the above-mentioned evidence. Moreover, the occurrence of a mineral paragenesys of hydrothermal origin in some gabbroic xenoliths prompted Scribano et al. (2006a) to note the existence of a serpentinite-hosted hydrothermal system, whose Lower Triassic age was deduced from in situ U–Pb analyses on hydrothermal zircons (246 ± 10 Ma; Sapienza et al., 2007). Mineralogical investigations into partially serpentinized harzburgite xenoliths (Manuella, 2011), bear evidence for serpentinization in the Hyblean upper mantle. Numerous secondary minerals (godlevskite, heazlewoodite, millerite, Na–S-rich fluorapatite, aragonite) occur in serpentine veins, along with Na-rich sylvite dendritic aggregates (mole NaCl = 0.03–0.21), and high amounts of chlorine in serpentine (0.05–0.09 wt.%) as well in abyssal serpentinites (e.g., Sharp and Barnes, 2004). These evidence suggest the interaction of seawater-derived hydrothermal fluids and peridotites developed in the Hyblean upper mantle up to a temperature of about 350–400 ◦ C and at a pressure below 0.2 GPa (Manuella, 2011), similarly to abyssal serpentinites from MAR (Alt and Shanks, 2003; Bach et al., 2004 and references therein; Mével, 2003). In addition, abundant aliphatic–aromatic solid hydrocarbons were detected in some highly serpentinized and carbonated mantle xenoliths (Scirè et al., 2011), and in metasomatized gabbroic xenoliths (Ciliberto et al., 2009), whose possible origin was attributed to an abiogenic synthesis via Fisher–Tropsch-type (FTT) reaction. Moreover, C isotopic values (ı13 C = −17.3 ± 0.2‰) measured in hydrothermal methane fluxes at Pantelleria (D’Alessandro et al., 2009) approach the C isotopic composition (ı13 C = −17.7‰) of methane emitted at the Rainbow hydrothermal field (Konn et al., 2009), which are consistent with a FTT synthesis (Taran et al., 2007). Hydrocarbon-bearing saponite-rich hydrothermal clays (Manuella et al., 2012a), diapirically upraised from the fossil oceanic basement of the Hyblean Plateau, were found in the northeastern margin of the Hyblean region. These clays are characterized by the abundance of some incompatible trace elements, including fluid-mobile elements (hereafter, FME; >10× primordial mantle), which are indicative of the mixing of hot Si-rich hydrothermal fluids (350–400 ◦ C) and cold seawater. 3.2. The geophysical viewpoint 3.2.1. Seismic anomalies The aforementioned geochemical data on Hyblean xenoliths and volcanic rocks support the model for the crustal basement of the Hyblean Plateau, as proposed by Scribano et al. (2006a,b), suggesting the presence of a fossil oceanic core-complex mainly constituted of peridotites affected by a different degree of serpentinization (S% = serpentine vol.%). Indeed, Hyblean mantle xenoliths show evidence for serpentinization and carbonation (Manuella, 2011; Scirè et al., 2011; Scribano et al., 2009), which is also proven by high values of loss on ignition (hereafter, LOI = 4–11 wt.%).

F.C. Manuella et al. / Journal of Geodynamics 66 (2013) 92–102 Table 2 Correlation of the degree of serpentinization (S%) from VP (km s−1 ) values, at different pressure values (MPa).a Pressure (MPa)

q

m

R

100 200 300 400 500 600 800

7.82 7.85 8.00 7.97 8.03 8.03 8.13

0.03 0.03 0.03 0.03 0.03 0.03 0.03

0.96 0.97 0.91 0.97 0.92 0.97 0.99

a Linear regression equations (S% = (q − VP )/m, where q stands for intercept with VP -axis, and m stands for the slope of the corresponding equation) and relative R values, were calculated from data published by Christensen (1966), Horen et al. (1996), and Miller and Christensen (1997).

The inferred oceanic Hyblean basement agrees with the model proposed by Escartín et al. (1997) for the segment end at a slowspreading ridge, in which dramatic changes in rheology, magnetic and gravimetric anomalies, and seismic structure can be induced by serpentinization (Bach et al., 2006). Particularly, laboratory measurements of compressional (P-) and shear (S-) wave velocities on abyssal and ophiolitic serpentinized peridotites (e.g., Horen et al., 1996; Miller and Christensen, 1997) exhibit a strong negative correlation between S% and density. Considering published data (Christensen, 1966; Horen et al., 1996; Miller and Christensen, 1997), linear regression equations of the P-wave velocities vs. S% values were estimated at different pressure (Table 2). Relative density values, considered as independent of the pressure, were calculated by using the equation  = 3298.74 − (8.18 × S%) (R = 0.99), where values of 3298.74 and 8.18 derive from a linear best-fit procedure by using the published data (Christensen, 1966; Horen et al., 1996; Miller and Christensen, 1997). The 3D P-wave velocity model for the Hyblean lithosphere, as retrieved by Brancato (2005) and Brancato et al. (2009) (third column in Table 3), was considered to reconstruct a geophysical model for the crustal basement of the Hyblean Plateau, both in pressure (MPa) and density (kg m−3 ). The pressure values are calculated according to the Stevin’s law (second column in Table 3). The relative values of P-wave velocities vs. depth were used to estimate the variations of S% starting from a depth of 8 km downward, for a fixed lithospheric pressure of 200 MPa at 7 km, corresponding to the Hyblean upper crust, as estimated by a density value of 2900 kg m−3 (Finetti, personal communication).

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Table 3 Variations of the degree of serpentinization (S%) and density (), with a referred standard deviation of 0.25 to VP , at increasing depths and pressures in the Hyblean crustal basement.a Depth (km)

Pressure (MPa)

VP (km s−1 )

q

m

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

200 224 249 275 301 327 354 381 409 438 467 496 526 556 586 616

4.85 5.15 5.35 5.55 5.75 5.95 6.15 6.35 6.55 6.75 6.95 7.15 7.15 7.15 7.15 7.15

7.82 7.89 7.92 7.96 8.00 7.99 7.98 7.98 7.98 7.99 8.01 8.03 8.03 8.03 8.03 8.03

0.03 100 0.03 91 0.03 86 0.03 80 0.03 75 0.03 68 0.03 61 0.03 54 0.03 48 0.03 41 0.03 35 0.03 29 0.03 29 0.03 29 0.03 29 0.03 29

S%

 (kg m−3 ) 2481 2553 2597 2641 2685 2742 2799 2867 2910 2960 3010 3059 3059 3059 3059 3059

a Linear regression equations (S% = (q − VP )/m; q stands for intercept with VP -axis, and was calculated by using an approximation around the pressure value indicated in column 2; m stands for the slope of the relative equation). VP values (column 3) reported by Brancato (2005).

The S% values plotted in Fig. 7 were obtained by the linear regression equations reported in Table 3. As a first, S% and VP values, starting from a depth of 19 km downward, show a constant estimate, thus indicating that rheological properties of deeper lithologies do not show a significant variation. Then, for both variables, the above observations allow us to constrain our subsequent analyses for a depth ranging from 8 up to 19 km. A strong correlation is observed, for the considered depth range, as a dependence on S% and VP , with a R = 0.99 in both cases (Fig. 7). On the ground of the estimated strong correlations among S%, VP and  for both the published data on serpentinized peridotites from MAR and ophiolites, and peridotides from the Hyblean oceanic basement, we could suggest an affinity for these geodynamics settings. In particular, the comparison of variation of the VP values in the Hyblean basement with the published data on the slow/ultraslow-spreading centers in MAR and SWIR (SouthWest Indian Ridge) (e.g., Minshull et al., 1998; Muller et al., 1997; Schroeder et al., 2002) suggests that 19 km could be interpreted as the Moho, thus representing the boundary between serpentinized and unaltered upper mantle, i.e., a serpentinization front (Minshull et al., 1998).

Fig. 7. Variation of depth vs. degree of serpentinization (S%) and P-wave velocities (VP ) in the Hyblean crustal basement (data from Table 3). Dashed lines represent best-fit linear regression for values relative to depths ranging from 8 to 19 km. Depth values downward 19 km up to 23 km are plotted only for comparison. The change in the gradient of the dashed line, as from the depth of 19 km, marks the Moho discontinuity (front of serpentinization) in the Hyblean lithosphere.

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3.2.2. Gravimetric anomalies In southeastern Sicily a strong positive gravimetric anomaly (>100 mGal), relating to the Bouguer anomalies calculated with a calculation density of 2400 kg m−3 , was observed (Bianchi et al., 1987; Bernardelli et al., 2005). Particularly, Bianchi et al. (1987) noticed a correspondence between the gravimetric profile and the trend of the Moho discontinuity beneath the Hyblean Plateau. This evidence matches with calculated density values in Table 3, thus indicating that a difference occurs between the upper crust, supposed to have an average density of 2900 kg m−3 , and the Hyblean crustal basement, in which altered mantle rocks have a calculated density lower than 2900 kg m−3 up to a depth of 15 km. In addition, referring to the onshore density value of 2400 kg m−3 (Bernardelli et al., 2005), the depth range is bounded to 8–10 km, in which serpentinized peridotites have a density ranging from 2481 to 2597 kg m−3 . Latter values confirm the presence of low density rocks above the Moho discontinuity, as previously indicated at a depth of 19 km.

3.2.3. Magnetic anomalies The crustal basement of the Hyblean Plateau is characterized by magnetic anomalies, associated to high susceptibility values, ranging from 1800 up to 4500 (×10−5 SI), N–S trending (Bianchi et al., 1987), as well as high intensity values of the Earth’s magnetic field (>400 nT; Chiappini et al., 2000). Furthermore, a dome-shaped magnetic basement beneath southeastern Sicily is located at a depth of 6–9 km (Cassano et al., 1986). The latter was considered as a primary source of magnetic anomaly in the Hyblean lithosphere with respect to the volcanic horizons, which, indeed, could represent a secondary contribution (Ciminale and Wasowski, 1989). The geophysical model for the Hyblean lithosphere, based on seismological and gravimetric features of the lower crust and upper mantle, seems to support the occurrence of serpentinized peridotites underlying the sedimentary and volcanic sequence. These rocks could be considered as the primary source of the measured magnetic anomalies, as supported by previous petrologic investigations on mantle xenoliths (Manuella, 2011; Scirè et al., 2011). The magnetic susceptibility K (×10−5 SI) for the inferred peridotites, occurring below the depth of 7 km, was calculated by using the best-fit relation by Oufi et al. (2002), log10 (K) = 9.80 − 4.25 × , whose relative estimates are reported in Table 4.

Table 4 Variations of the degree of serpentinization (S%) and density () at increasing depths and pressures in the Hyblean crustal basement.a Depth (km) 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

S% 100 91 86 80 75 68 61 54 48 41 35 29 29 29 29

 (kg m−3 )

K (×10−5 SI)

2481 2553 2597 2641 2685 2742 2799 2867 2910 2960 3010 3059 3059 3059 3059

18,066 9027 5890 3742 2442 1406 805 462 271 166 102 63 63 63 63

a Magnetic susceptibility (K) values were calculated on the basis of the best-fit regression equation between K and density (g cm−3 ) reported by Oufi et al. (2002).

Fig. 8. Variation of magnetic susceptibility K (black diamonds) vs. depth and density in the Hyblean crustal basement, as retrieved from data in Table 3. A marked decrease in the magnetic susceptibility starts from 13 km downward, corresponding to a progressively lower degree of serpentinization (red and green triangles) up to 19 km (Moho). In particular, green triangles mark significant values of the degree of serpentinization.

Fig. 9. Comparison of VP (a) and density (b) values vs. depth from the Hyblean Plateau, Malta Plateau and Pantelleria Island, as retrieved from Table 2, and Chironi et al. (2000). A general match occurs for VP values measured near Pantelleria Island and the Malta Plateau vs. the VP model retrieved for the Hyblean Plateau (Brancato, 2005). Differently, the correspondent calculated values of density, in 8–15 km depth range, are below the constant assumed value of 2900 kg m−3 .

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Fig. 10. Schematic map of the oceanic model as proposed in the present work, with a schematic geological profile along the section A–B. Simplified tectonic lineation after Lentini et al. (2006). The figure shows a possible affinity of the Permo-Triassic oceanic basement below the Hyblean Plateau and the Sicily Channel with the adjacent Ionian-Tethys oceanic basin.

Relative plots of depth and density values, as retrieved from Table 3, vs. magnetic susceptibility are shown in Fig. 8. Values of K higher than 2000 (×10−5 SI) are observed for possible serpentinized peridotites, whose relative depths are confined in the range of 8–12 km, being very low (approximating to a zero value) starting from 19 km (Fig. 8). The latter value seems to confirm the top of the Moho discontinuity, as previously observed considering both seismic and gravimetric data. High magnetic susceptibility values could correspond to highly serpentinized peridotites, characterized by relative low densities (2400–2600 kg m−3 ; Fig. 8), thus suggesting the presence of the observed gravimetric anomalies. 4. Discussion and conclusions This paper represents the first interdisciplinary approach to reconstruct the structure and composition of the Hyblean PermoTriassic lithosphere. The only information coming from geological surveys referred to the upper crust of the studied area. Differently, the aforementioned petrologic and geochemical considerations lead us to conclude that: (1) a strong similarity occurs between Hyblean peridotite xenoliths and abyssal peridotites, particularly with those from ultra-slow spreading ridges such as SWIR and Gakkel, on the basis of whole-rock geochemical data and mineral chemistry of peridotite-hosted spinels (Figs. 2 and 3); (2) a low paleospreading rate is estimated for the Hyblean PermoTriassic ocean basin, corresponding to 14 ± 3 mm y−1 (Fig. 4); (3) the absence of any possible contamination by continental crust lithologies of volcanic rocks from southeastern Sicily and the Sicily Channel is supported by whole-rock geochemical data (Fig. 5); (4) the oceanic affinity of Hyblean mantle xenoliths and volcanic rocks from southeastern Sicily and the Sicily Channel with modern

serpentinite-hosted hydrothermal systems in MAR (i.e., Rainbow and Logatchev) is supported by He isotopic ratios. Therefore, the above cited petrologic and geochemical evidence demonstrate the affinity of Hyblean Permo-Triassic basement with an ultra-slow spreading ridge, rather than a passive continental margin (North Africa). Other possible origins for serpentinization in the Hyblean basement (e.g., fault bending) can also be excluded. In this regard, fault bending could be related to the roll back of the subducting Ionian plate below the Maghrebian thrust belt in the northeastern margin of Sicily and the Calabrian arc, which started with the collision between the African and European plates, during the Alpine orogeny, since Late Cretaceous, which brought to the progressive closure of the Ionian basin still active. Differently, the rifting of the Ionian-Tethys oceanic basin developed during Permo-Triassic period. This time discrepancy (about 200 Ma) would suggest us that serpentinization is strictly associated with the spreading of the Ionian-Tethys ocean. In addition, geophysical data further substantiate the model of a Permo-Triassic oceanic basement in the Hyblean Plateau, in agreement with Scribano et al. (2006a,b), as well as the presence of serpentinized ultramafic lithologies below the Meso-Cenozoic sedimentary and volcanic sequence up to a depth of 19 km, corresponding to the Moho discontinuity, which could be interpreted as a serpentinization front. The latter assumption derives from evidence for seismic profiles of the oceanic lithosphere in MAR and SWIR, in which peridotites occurring below the Moho discontinuity show a low degree of serpentinization (≤35 vol.%; Minshull et al., 1998; Muller et al., 1997; Schroeder et al., 2002). Furthermore, a moderate lateral VP values reduction below the depth of 19 km was observed by Brancato et al. (2009), who suggested that a magmatic intrusion could be the possible source for VP anomaly. Based on our investigations, the observed VP anomaly

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could be better linked to the presence of peridotites affected by a variable degree of serpentinization. This possible heterogeneity seems in accordance with the high attenuation values, both QP and QS , calculated by Giampiccolo et al. (2003). The agreement of geophysical data on the Hyblean lithosphere with those from slow-spreading oceanic environments could confirm the petrologic model as proposed by Scribano et al. (2006a,b). Geochemical and isotopic comparisons of volcanic rocks from the Hyblean Plateau, as well as from the Sicily Channel, show a clustering in the OIB field suggesting a common source (Figs. 2 and 4). Similarly, the VP values measured along transects close to the Pantelleria Island and the Malta Plateau (Chironi et al., 2000) hint at a general match with the VP model retrieved for the Hyblean Plateau (Brancato, 2005). In particular, the accordance seems to fit better starting from the depth of 13 km, with a less variation ranging from about 8 to 10% for the Hyblean Plateau estimates (Fig. 9). Again, starting from the depth of 19 km, the considered VP patterns show a marked step which could correspond to the above reported Moho discontinuity (Fig. 9). Conversely, the calculated density values for the Hyblean Plateau and along the above reported transects do not show a similar correspondence, probably due to the different lithospheric pressures as induced by the upper crust lithologies. Hence, further investigation is needed, which is beyond the scope of the present work, and therefore this comparison will not be treated here. The occurrence of serpentinized peridotites in the Hyblean crustal basement could have produced some structural effects as a result of volume increase (∼40%) in altered mantle rocks (Iyer et al., 2010). Serpentinization can induce vertical movements in the oceanic lithosphere, corresponding to an uplift of 100–1000 m at a rate of mm to cm per year (Skelton and Jakobsson, 2007). Accordingly, cataclastic to mylonitic textures were observed in Hyblean gabbroic xenoliths (Scribano et al., 2006a), resembling deformed mafic rocks from some oceanic core complex in MAR (Boschi et al., 2006; Hirose and Hayman, 2008), which are preferentially confined to detachment fault zones. In addition, some transparent conical shaped bodies (Catalano et al., 2000; Catalano and Sulli, 2006), characterized by diffraction hyperbolae, which interrupt the continuity of the sedimentary sequence were observed in two seismic profiles along the Hyblean–Malta Escarpment (CROP M-3 and M-21), as well in the Ionian oceanic crust (CROP M-22), which were previously interpreted as igneous intrusions. Conversely, we suppose that these bodies in the Hyblean upper crust may be diapiric intrusions of serpentinites, primarily based on a comparison with piercement structures in the Central Indian Ocean and in the Arctic Ocean Ridge (Krishna et al., 2002; Rajan et al., 2012). Finally, we suggest that the Permo-Triassic oceanic basement below the Hyblean Plateau can be extended from the Ionian basin westward, probably to the Sciacca Platform, crossing the Sicily Channel (Fig. 10). This inference is also supported by previous geological models as proposed by Finetti (2005) and Vai (2003), who hypothesized the existence of a slowspreading oceanic basin, located between southeastern Sicily and southwestern Puglia, as a possible remnant of the Mesozoic Ionian-Tethys ocean, belonging to an Oman–Iraq–Levantine-Sicily seaway. However, further interdisciplinary investigations will be necessary to better define the boundaries of the proposed Sicily–Ionian lithospheric model, as well for searching the distribution of hydrocarbon reservoirs that could be hosted in the serpentinized ultramafic basement, in agreement with Scirè et al. (2011), who suggested that serpentinites would also represent source rocks for exploitable petroleum reserves.

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