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Journal of Petrology 45(6) © Oxford University Press 2004; all rights reserved
Origin of the Gabbro–Peridotite Association from the Northern Apennine Ophiolites (Italy)
1 DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITÀ DI PAVIA, VIA FERRATA 1, 27100 PAVIA, ITALY
2 ISTITUTO DI GEOSCIENZE E GEORISORSE, CNR, SEZIONE DI PAVIA, VIA FERRATA 1, 27100 PAVIA, ITALY
3 DEPARTMENT OF GEOLOGY, ROYAL HOLLOWAY, UNIVERSITY OF LONDON, EGHAM TW20 0EX, UK
RECEIVED MARCH 4, 2002; ACCEPTED NOVEMBER 18, 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL FRAMEWORK AND...ANALYTICAL TECHNIQUESGEOCHEMICAL RESULTSDISCUSSIONGEODYNAMIC IMPLICATIONS AND...REFERENCES |
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The Northern Apennine ophiolites are remnants of the Middle Jurassic–Early Cretaceous lithosphere from the Ligurian Tethys. New trace element and Nd–Sr isotope investigations were performed on: (1) the rare gabbros associated with the subcontinental mantle rocks from the External Liguride ophiolites; (2) the gabbro–peridotite association from the poorly known ophiolitic bodies from Cecina valley (Southern Tuscany). Clinopyroxenes from the External Liguride and Cecina valley gabbros have similar trace element compositions, which are consistent with formation from normal mid-ocean ridge basalt (N-MORB) magmas. Sm–Nd mineral isochron ages are 179 ± 9 Ma for an External Liguride gabbro and 170 ± 13 Ma and 173·5 ± 4·8 Ma for two different gabbroic bodies from the Cecina valley ophiolites. These ages are interpreted to date the igneous crystallization of the gabbros and are slightly older than the oldest pelagic sediments of the Ligurian Tethys. Initial
Nd (+8·5 to +8·9) and 87Sr/86Sr of clinopyroxene are consistent with the interpretation that the studied gabbros were derived from N-MORB magmas. The least serpentinized mantle rocks from the Cecina valley ophiolites are porphyroclastic spinel lherzolites displaying a residual geochemical signature. They are similar to the least depleted residual peridotites from modern oceans. Nd and Sr isotopic ratios for separated mantle clinopyroxene are respectively higher (e.g. Nd = +11) and lower than those of clinopyroxene from associated gabbros at the time of the gabbro intrusion. The gabbro–peridotite associations from the Northern Apennine ophiolites record the progression of the rifting process that led to opening of the Ligurian Tethys. KEY WORDS: gabbro; peridotite; ophiolite; Northern Apennine; oceanic lithosphere
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL FRAMEWORK AND...ANALYTICAL TECHNIQUESGEOCHEMICAL RESULTSDISCUSSIONGEODYNAMIC IMPLICATIONS AND...REFERENCES |
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The Middle Jurassic–Early Cretaceous Ligurian Tethys is considered to have developed in conjunction with the opening of the Central Atlantic Ocean (Lemoine et al., 1987
; Bill et al., 2001). Lithospheric remnants of this basin are represented by ophiolites from the Northern Apennines, Western Alps and Corsica (Fig. 1). These ophiolites are characterized by a substrate of serpentinized mantle rocks and gabbroic plutons, which is discontinuously overlain by pillow lavas, sedimentary breccias or pelagic sediments (Abbate et al., 1980; Cortesogno et al., 1987). Similar lithostratigraphic features were reported for the Mid-Atlantic Ridge, for instance at the intersection with the Kane Fracture Zone (i.e. the MARK area) and from the 15°N region (Cannat et al., 1997a, 1997b). The resemblance between the Ligurian Tethys ophiolites and the MARK area lithosphere is supported by the petrological and geochemical similarities of the gabbroic sequences (Tribuzio et al., 2000a).
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Rampone et al. (1998)
showed that crustal and mantle rocks of the Ligurian Tethys ophiolites locally have contrasting Nd isotope values. In particular, gabbros and basaltic rocks from the so-called Internal Liguride ophiolites (Northern Apennine) display typical normal mid-ocean ridge basalt (N-MORB) Nd isotope values, whereas associated mantle lherzolites are characterized by an extremely depleted Nd isotopic signature (see also Rampone et al., 1996). On the basis of such isotopic contrast, it was argued that: (1) a melt-residue genetic relation does not exist for this crust–mantle association; (2) the lherzolites differ from the mantle rocks of modern oceans, despite the mineralogical, major and trace element similarities (Rampone et al., 1996, 1998). The Ligurian Tethys was thus considered as an embryonic oceanic basin similar to the Red Sea, or as an ocean–continent transition similar to the non-volcanic continental margin of Western Iberia (see also Rampone & Piccardo, 2000; Piccardo et al., 2002).
To unravel the nature of the crust–mantle link in the Ligurian Tethys, we have carried out new trace element and Nd–Sr isotope analyses of the gabbro–peridotite association from the ophiolitic bodies cropping out along the Cecina valley in Southern Tuscany (Northern Apennines). In addition, to provide a comprehensive overview of the gabbro–peridotite association from the Northern Apennine ophiolites, we have analysed the rare gabbros from the so-called External Liguride ophiolites, which are characterized by mantle material of subcontinental nature (Beccaluva et al., 1984; Ottonello et al., 1984; Rampone et al., 1995). Combining the data presented with literature information on the Northern Apennine ophiolites and Atlantic Ocean lithosphere, we conclude that the gabbro–peridotite associations of the Ligurian Tethys record the progression of a rifting process that resulted in a magma-poor slow-spreading centre.
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GEOLOGICAL FRAMEWORK AND SELECTED SAMPLES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL FRAMEWORK AND...ANALYTICAL TECHNIQUESGEOCHEMICAL RESULTSDISCUSSIONGEODYNAMIC IMPLICATIONS AND...REFERENCES |
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The ophiolitic bodies from the Northern Apennines may be assembled into three groups: (1) Internal Ligurides; (2) External Ligurides; (3) Southern Tuscany (Fig. 1). The Internal Liguride ophiolites are stratigraphically associated with a turbiditic sequence of Late Cretaceous to Early Paleocene age, which evolved over time from distal to proximal (Marroni et al., 1992
). These ophiolites preserve close lithostratigraphic similarities to slow-spreading centres from the Mid-Atlantic Ridge and are thus considered to represent an intra-oceanic domain of the Ligurian Tethys (Barrett & Spooner, 1977; Cortesogno et al., 1987; Tribuzio et al., 2000a; Williams et al., 2002).
The External Liguride ophiolites occur as huge olistoliths within Late Cretaceous sedimentary mélanges, together with sparse bodies of continental origin (Marroni et al., 1998). The continental crust rocks are mainly granulite-facies metagabbros and peraluminous granitoids, which both have Late Carboniferous–Early Permian age (see also Montanini & Tribuzio, 2001). The association of the External Liguride ophiolites with continental crust rocks was related to an ocean–continent transition, which fits well with non-volcanic continental margins (Marroni et al., 1998).
The ophiolitic bodies from Southern Tuscany crop out as huge olistoliths in Late Cretaceous to Eocene flyschoid sequences and are not associated with continental crust material (Abbate et al., 1980). The palaeogeography pertaining to these ophiolites has not been accurately established yet. The selected ophiolitic bodies (Cecina valley) are related to a heterogeneous flyschoid sequence of Middle Paleocene–Early Eocene age (Mazzanti, 1966).
The External Liguride ophiolites
The mantle ultramafics from the External Liguride ophiolites are mainly fertile spinel lherzolites containing sporadic pyroxenite layers and disseminated titanian pargasite (Beccaluva et al., 1984; Ottonello et al., 1984). Temperature estimates for the spinel-facies equilibration (1000–1050°C) and Nd–Sr isotope compositions have been related to a lithospheric subcontinental origin (see also Rampone et al., 1995). The lherzolites commonly show late growth of plagioclase, which is locally associated with the development of mylonite fabric (Beccaluva et al., 1984). The plagioclase growth has been ascribed to subsolidus decompression in Middle to Late Jurassic times (Rampone et al., 1993, 1995).
The gabbros are volumetrically subordinate to the mantle peridotites and basaltic rocks (Terranova & Zanzucchi, 1982). They crop out as dykes and metre-scale lenticular bodies within the mantle ultramafics and, rarely, as huge olistoliths (up to hundreds of metres in size). The selected samples for this study were collected from an olistolith [see Marroni et al. (1998) for location] made of coarse-grained, subophitic olivine-bearing gabbro and layered troctolite. Preliminary geochemical data on olivine-bearing gabbros selected for this study (GEL3 and GEL9) indicate that they are cumulates derived from MORB-type melts (Marroni et al., 1998; Tribuzio et al., 2000a). New petrographic investigations have revealed that the igneous clinopyroxene from sample GEL3 is locally overgrown by secondary clinopyroxene (almost pure diopside) and tremolite.
The Cecina valley ophiolites
The petrological and geochemical features of the Cecina valley mantle rocks are poorly known. On the other hand, there are many studies of the associated gabbros, which are coarse-grained subophitic olivine-bearing gabbros to minor layered troctolites, both of cumulus origin (Serri, 1980; Hébert et al., 1989; Tiepolo et al., 1997). The samples selected for this study are olivine-bearing gabbros collected from the ophiolitic bodies of Riparbella and Montecastelli [see Tiepolo et al. (1997) for location]. The Riparbella samples (CBC3, CB17 and CB16) were collected in a breccia (
100 m x 30 m in size) that stratigraphically covers a large mass of mantle-derived serpentinites. Tribuzio et al. (1999) showed that the outermost rims of clinopyroxene and plagioclase from these rocks are enriched in incompatible trace elements relative to the cores. These chemical variations were related to a process of post-cumulus migration of a highly evolved trondhjemitic liquid in the gabbroic crystal mush (see also Tribuzio et al., 2000b).
The Montecastelli gabbros (MF1 and MF2) were sampled from a kilometre-scale pluton that mainly consists of olivine-bearing gabbro (Tiepolo et al., 1997). The associated kilometre-scale mantle peridotite (Mazzanti, 1966) is made of variably serpentinized rocks, which locally include small dykes (thickness <0·2 m) of rodingitized gabbro. The least serpentinized peridotites are spinel lherzolites displaying porphyroclastic texture and rather constant primary modal compositions. In the selected samples (TP1, TP2 and TP9), the original grains of olivine and orthopyroxene are 0·5 cm in size and late serpentine makes up 45% of the rock volume. Clinopyroxene and spinel occur in minor modal amounts (5–10% and <5%, respectively) and their grain size is smaller than that of olivine and orthopyroxene. Spinel is generally interstitial to olivine and orthopyroxene. Commonly, orthopyroxene displays thin exsolution lamellae of clinopyroxene, and vice versa. The orthopyroxene exsolution lamellae in clinopyroxene are locally replaced by serpentine. Both orthopyroxene and clinopyroxene porphyroclasts are in places bent and rimmed by fine-grained aggregates of neoblastic pyroxene, associated with minor spinel and, locally, olivine.
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ANALYTICAL TECHNIQUES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL FRAMEWORK AND...ANALYTICAL TECHNIQUESGEOCHEMICAL RESULTSDISCUSSIONGEODYNAMIC IMPLICATIONS AND...REFERENCES |
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Clinopyroxene cores were analysed for rare earth elements (REE) and selected trace elements (Table 1) by secondary ion mass spectrometry at Istituto di Geoscienze e Georisorse, Sezione di Pavia, according to the method described by Bottazzi et al. (1994)
. The energy filtering technique (Shimizu et al., 1978) was applied to remove molecular ion interferences by applying –100 V voltage offset. In addition, 45Sc and 52Cr signals were corrected for the presence of 29Si16O+ and 24Mg28Si+ interferences, respectively. Precision and accuracy are estimated to be better than ±10% above 1 ppm concentration. Below 1 ppm concentration, precision is constrained by counting statistics to be in the range 10–30%.
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Major element analyses of minerals from the Cecina valley mantle peridotites were carried out using a JEOL JXA-840A electron microprobe, located at Centro Grandi Strumenti (Università di Pavia). Three JEOL wavelength-dispersive spectrometers were employed. Operating conditions were 20 kV and 20 nA, and minerals were used as standards. X-ray intensities were corrected to oxide percent concentration by the procedure of Bence & Albee (1968)
, on the basis of semi-empirical 
- and ß-factors (Albee & Ray, 1970). Representative electron microprobe analyses are reported in Table 2. The formula units of olivine and spinel were calculated assuming three cations and four oxygens. The formula units of pyroxenes were obtained by the method of Cawthorn & Collerson (1974), on the basis of six oxygens.
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Mineral concentrates were extracted using a magnetic separator, and purified by careful handpicking under a binocular microscope. Clinopyroxene separates were leached at 150°C for
40 min in 6 M HCl. Nd and Sr were separated for isotope analyses (Table 3) using conventional ion exchange technique after dissolution in Savillex capsules and FEP beakers. Nd (as NdO+) and Sr isotope compositions were determined on a five-collector VG354 mass spectrometer at Royal Holloway using multidynamic modes (Thirlwall, 1991a, 1991b) and normalized to 146Nd/144Nd = 0·7219 and 86Sr/88Sr = 0·1194. 143Nd/144Nd and 87Sr/86Sr analyses were carried out in two standardization periods. The means of the laboratory 143Nd/144Nd standard for these periods were 0·511422 ± 8 (2 SD, n = 69) and 0·511413 ± 7 (2 SD, n = 54). The latter analyses were adjusted to the usual standard value of 0·511418, equivalent to La Jolla 143Nd/144Nd = 0·511860. The means for laboratory 87Sr/86Sr standard SRM987 were 0·710246 ± 21 (2 SD, n = 58) and 0·710220 ± 15 (2 SD, n > 100). The 87Sr/86Sr analyses performed in the second standardization period were corrected to the long-term standard mean of 0·710248. Typical blank levels were 250 pg Nd and 700 pg Sr.
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Sm and Nd concentrations were determined by isotope dilution. Reproducibility of isotope dilution Sm/Nd ratios is
0·1%. Sm and Nd contents in clinopyroxene are consistent with the ion microprobe data, with differences <20%. The Sm/Nd values obtained by the two methods vary within 14%. Rb and Sr concentrations of clinopyroxenes were not determined by isotope dilution, because the low Rb/Sr values of clinopyroxenes do not require an age correction for Sr isotope compositions. |
GEOCHEMICAL RESULTS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL FRAMEWORK AND...ANALYTICAL TECHNIQUESGEOCHEMICAL RESULTSDISCUSSIONGEODYNAMIC IMPLICATIONS AND...REFERENCES |
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The gabbros
Clinopyroxene trace element compositions
Clinopyroxenes from the selected gabbros of External Liguride and Cecina valley ophiolites have similar chondrite-normalized, incompatible element patterns (Fig. 2), which resemble those of clinopyroxenes from Internal Liguride gabbros (Tribuzio et al., 1995
). They are characterized by depletion in light rare earth elements (LREE, CeN/SmN = 0·2–0·4) and nearly flat middle to heavy rare earth element patterns (MREE and HREE, respectively) at 8–16 times chondrites. Sr, Zr and Ti are depleted relative to neighbouring REE. The clinopyroxenes with the lowest REE abundances commonly have the highest Cr and Sr contents (Table 1). The concentrations of Sc and V cover a narrow range (95–133 ppm and 309–392 ppm, respectively).
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Isotope compositions and geochronology
To derive Sm–Nd mineral isochron ages for External Liguride and Cecina valley gabbros, we carried out Sm–Nd analyses of whole rocks and separates of clinopyroxene and plagioclase (Table 3). Isochron ages were calculated using a York-type regression (York, 1969
) and are interpreted to date the igneous crystallization event. This is consistent with the high closure temperature for the Sm–Nd exchange in clinopyroxene and plagioclase from igneous mafic rocks (Van Orman et al., 2001). Errors quoted for the isochron ages are based on 2
analytical uncertainties multiplied by the square root of the mean squares weighted deviates (MSWD).
The clinopyroxene–plagioclase–whole-rock isochron (MSWD = 0·97) of gabbro GEL3 from the External Liguride ophiolites yields an age of 179 ± 9 Ma and an initial Nd of +8·69 ± 0·22 (Fig. 3). The gabbro MF1 from the Montecastelli pluton (Cecina valley ophiolites) gives a clinopyroxene–plagioclase–whole-rock isochron (MSWD = 2·03) of 169 ± 18 Ma and initial Nd of +8·54 ± 0·53. We have also analysed the whole-rock Sm–Nd compositions of sample MF2, which has nearly the same initial Nd isotope ratio as MF1. The four Montecastelli data points yield an isochron of 170 ± 13 Ma (MSWD = 1·21, initial Nd = +8·52 ± 0·36).
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Plagioclase, clinopyroxene and whole-rock compositions of olivine-bearing gabbro CB17 from the Riparbella body (Cecina valley ophiolites) give an isochron age of 173·7 ± 4·8 Ma (MSWD = 0·84, initial
Nd = +8·93 ± 0·14). Plagioclase and whole-rock Sm–Nd compositions of Riparbella sample CB16 were also determined. The five Riparbella data points define an isochron (MSWD = 0·84), corresponding to an age of 173·5 ± 4·8 Ma and an initial Nd of 8·94 ± 0·14. It is noteworthy that, although the gabbros from the Montecastelli and Riparbella bodies of the Cecina valley ophiolites give almost coincident ages, the calculation of a nine-point regression yields a high MSWD (>20). This suggests that the Montecastelli and Riparbella plutons formed from parental liquids with slightly different Nd isotope compositions. The clinopyroxene separates from the External Liguride and Cecina valley gabbros were also analysed for Sr isotope compositions. The clinopyroxene from the External Liguride sample GEL3 is characterized by high 87Sr/86Sr (0·7045). The clinopyroxenes from Montecastelli (MF1) and Riparbella (CB17) samples gave 87Sr/86Sr values of 0·7028 and 0·7025, respectively.
The Cecina valley mantle lherzolites
Major element mineral compositions
Olivine has 90–91 mol % of forsterite component. The porphyroclastic orthopyroxene has an Mg/(Mg + Fetot) value between 0·90 and 0·91, and Al and Ca contents of 0·20–0·24 and 0·03–0·09 atoms per formula unit (a.p.f.u.), respectively. The associated clinopyroxene displays Mg/(Mg + Fetot) values of 0·90–0·92, and Al and Na concentrations of 0·28 a.p.f.u. and 0·02–0·03 a.p.f.u., respectively (Fig. 4). Interstitial spinel has high Mg/(Mg + Fe2+) and low Cr/(Cr + Al) and Ti values (Fig. 5).
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Two-pyroxene geothermometry on the basis of the Wells' method (1977) yields temperature estimates of 1060 ± 60°C for the porphyroclastic assemblage. These values are consistent with those obtained (T = 1080 ± 60°C) by the two-pyroxene geothermometer of Brey & Kohler (1990)
assuming P = 1·5 GPa. However, the local finding of high Ca contents in orthopyroxene indicates higher crystallization temperatures (up to 1310°C) according to the single orthopyroxene method of Brey & Kohler (1990). This suggests that the estimates obtained on the basis of the two-pyroxene geothermometry suffer from pyroxene exsolution, i.e. they probably document the cessation of the exsolution process recorded by porphyroclastic pyroxenes.
Neoblastic pyroxenes are slightly Al and Cr depleted relative to porphyroclasts. The Al and Cr decrease in neoblastic orthopyroxene and clinopyroxene is probably due to their development in association with newly formed spinel, which incorporates a high amount of these elements. Neoblastic orthopyroxene has invariably low Ca concentrations (0·02–0·04 a.p.f.u.), which imply crystallization temperature of 890–1040°C assuming P = 1·5 GPa (Brey & Kohler, 1990). Two-pyroxene geothermometry (Wells, 1977; Brey & Kohler, 1990) applied to neoblastic pyroxene grains gives consistent estimates of 960 ± 60°C.
Trace element and Nd–Sr isotope compositions of porphyroclastic clinopyroxene
Porphyroclastic clinopyroxene displays a severe depletion of LREE relative to MREE (CeN/SmN 0·04) and HREE, which are nearly flat at 8 times chondrite (Fig. 6). Normalization to chondrite also reveals that Sr, Zr and Ti are depleted relative to neighbouring REE. The concentrations of Cr, V and Sc are 6110–7800 ppm, 235–280 ppm and 48–54 ppm, respectively.
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Two clinopyroxene separates were analysed for Nd and Sr isotope compositions (Table 3). The 143Nd/144Nd and 147Sm/144Nd ratios are rather high and yield
Nd values of +10·9 and +11·1 at the time of the gabbro intrusion (Fig. 7). The 87Sr/86Sr values are remarkably variable (0·7020–0·7025). Wide variations in Sr isotope composition are typically observed for clinopyroxene separates from ophiolitic and abyssal peridotites, and attributed to seawater-related alteration (e.g. Bodinier et al. 1991; Snow et al., 1994). The 87Sr/86Sr variations observed for the Cecina valley lherzolites may therefore be due to the local replacement of orthopyroxene exsolution lamellae by serpentine that was not completely removed by the leaching procedure. This implies that the analysed clinopyroxenes provide an upper limit on their primary Sr isotope ratio.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL FRAMEWORK AND...ANALYTICAL TECHNIQUESGEOCHEMICAL RESULTSDISCUSSIONGEODYNAMIC IMPLICATIONS AND...REFERENCES |
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The geochemical data reported here concerning the gabbros from the Northern Apennine ophiolites allow us to determine the compositions of their parental liquids and provide constraints on the duration of plutonic crystallization in the Ligurian Tethys lithosphere. The nature and evolution of the Cecina valley mantle lherzolites are also discussed on the basis of their petrological and geochemical features. A comparison between the Cecina valley and Internal Liguride (Rampone et al., 1996
) mantle lherzolites has been carried out, as they are both characterized by a refractory signature, similar to the abyssal peridotites from modern oceans, but show significant geochemical differences. We finally consider the origin of the gabbro–peridotite association from the External Liguride units, in conjunction with literature data for the peridotites (Rampone et al., 1995). In particular, recent geochronological and geochemical studies of gabbros and peridotites from the Western Iberian margin (Charpentier et al., 1999; Schärer et al., 2000; Abe, 2001; Beard et al., 2002) allow us to verify the inferred analogy between the External Liguride units and non-volcanic continental margins (Marroni et al., 1998). The implications for the rifting process that led to the opening of the Ligurian Tethys are reported in the conclusions section.
Evidence for crystallization of gabbros from N-MORB parental magmas
The trace element compositions of the liquids in equilibrium with clinopyroxene cores from the least evolved gabbros of the External and Cecina valley ophiolites are reported in Fig. 8. A calculated liquid composition for a Mg-rich gabbro of the Internal Liguride ophiolites (data from Tribuzio et al., 1995) is also shown. The comparison supports the idea that the different gabbroic plutons from the Northern Apennine ophiolites formed from similar primitive N-MORB-type liquids, which followed similar igneous differentiation processes (Tribuzio et al., 2000a). The N-MORB origin is consistent with the initial Nd (+8·5 to +8·9) of Sm–Nd isochrons reported in this study for the Cecina valley and External Liguride gabbros, and by Rampone et al. (1998) for the Internal Liguride gabbros.
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The low 87Sr/86Sr ratios of clinopyroxene from Cecina valley gabbros (0·7025–0·7028) are consistent with an N-MORB origin. The clinopyroxene from the External Liguride sample has a higher 87Sr/86Sr value (0·7045), which approaches the Sr isotope composition of seawater in Middle to Late Jurassic times (Veizer, 1997
). Relatively high 87Sr/86Sr values were also found for the clinopyroxenes from the Internal Liguride gabbros (0·7031–0·7034 relative to 0·7024–0·7030 for the enclosing rocks) and ascribed to contamination by tremolite crystallized during low-temperature interaction with seawater-derived fluids (Rampone et al., 1998). We conclude that the high 87Sr/86Sr ratio of the External Liguride gabbro is related to subsolidus contamination in the separate by secondary clinopyroxene and tremolite that developed in response to ocean-floor alteration.
Timing of gabbro crystallization in the Ligurian Tethys
The uncertainties on the Sm–Nd isochrons reported in this study for the External Liguride and Cecina valley gabbros, and by Rampone et al. (1998) for the Internal Liguride gabbros (164 ± 14 Ma), do not allow us to distinguish any significant age differences among the plutonic bodies of the Northern Apennine ophiolites. However, the U–Pb zircon age of 153 ± 1 Ma reported for a late leucocratic dyke from the Internal Liguride ophiolites (Borsi et al., 1996) suggests a younger age for the Internal Liguride gabbros relative to the External Liguride and Cecina valley counterparts. An older age of the External Liguride and Cecina valley gabbros is also indicated by comparison with U–Pb zircon ages (161 ± 1 to 166 ± 2 Ma) of gabbros from the Western Alps ophiolites (Bill et al., 1997; Rubatto et al., 1998; Schaltegger et al., 2002; Rubatto & Hermann, 2003).
The Sm–Nd isochron ages of the External Liguride and Riparbella (Cecina valley) gabbros are also older than the oldest pelagic sediments (radiolarites) associated with the Ligurian Tethys ophiolites, which are so far dated as Bathonian (Bill et al., 2001). The base of the Bathonian corresponds to 164·0 ± 2 Ma and 166·0 +3·8/–5·6 Ma, according to the recent revisions of the Middle Jurassic time scale by Odin (1994) and Pálfy et al. (2000), respectively. Hence, the External Liguride and Riparbella (Cecina valley) gabbros formed before the onset of radiolarite sedimentation, which is inferred to be temporally associated with the outpouring of basalt flows (e.g. Abbate et al., 1980; Cortesogno et al., 1987; Molli, 1996). We conclude that formation of rifting-related gabbroic bodies pre-dated continental break-up, similar to what is inferred for the ocean–continent transition of the Western Iberian margin (Schärer et al., 2000; Beard et al., 2002). In particular, the ‘syn-rift’ gabbros of the Ligurian Tethys are coeval with pelagic sedimentation over the continental margin (see Bill et al., 2001), which followed the disruption of shallow-water carbonate platforms of Early Jurassic age.
The Cecina valley mantle lherzolite: evidence for a refractory origin
Clinopyroxene from the Cecina valley lherzolites shows a marked depletion of LREE, Sr and Zr relative to HREE, similar to residual abyssal peridotites (Fig. 6). We have calculated the extent to which clinopyroxene has been depleted using the non-modal fractional melting model of Johnson et al. (1990), assuming a slightly depleted spinel lherzolite source. Mineral proportions and trace element compositions of clinopyroxene in such a mantle source are those reported by Yang et al. (1998). Calculations were carried out using the melting stoichiometry for spinel lherzolites and trace element partition coefficients reported by Johnson et al. (1990) and Ionov et al. (2002), respectively.
The model shows that the trace element compositions of clinopyroxene may be related to 6% fractional melting of a slightly depleted spinel lherzolite source (Fig. 9). The Ce concentrations are slightly higher in the Cecina valley clinopyroxene than in computed clinopyroxene resulting from 6% fractional melting, possibly in relation to a small fraction of aggregated melt retained in the porous residue (Johnson & Dick, 1992). It is noteworthy, however, that a slight increase of adopted Cpx/LiqD value for Ce (i.e. from 0·086 to 0·100) yields a Ce concentration in calculated clinopyroxene that is consistent with the observed composition.
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The refractory origin of the Cecina valley lherzolites is supported by the spinel compositions. In particular, the Cr/(Cr + Al) value of spinel and the REE compositions of clinopyroxene plot at the least depleted end of the trend defined by residual abyssal peridotites (Fig. 10). According to the fractional melting model of Hellebrand et al. (2001)
based on spinel compositions, the Cecina valley lherzolites are related to 6–8% partial melting, in good agreement with the modelling results obtained from clinopyroxene trace element compositions.
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Comparison between the Cecina valley and Internal Liguride lherzolites
Although the REE and high field strength element (HFSE) compositions of clinopyroxene from the Cecina valley and Internal Liguride lherzolites point to a similar refractory origin (Fig. 6), these rocks display significant geochemical differences. In particular, the spinels from the Internal Liguride lherzolites are poorer in Al and Mg and richer in Ti than spinels from the Cecina valley lherzolites (Fig. 5). According to the peridotite partial melting model of Hellebrand et al. (2001)
, the high Cr/(Cr + Al) value of spinel from the Internal Liguride lherzolites (Fig. 10) corresponds to a high degree of fractional melting (14–16%), which seems inconsistent with the trace element chemistry of the associated clinopyroxene. Because these lherzolites were subjected to late low melt-fraction percolation associated with partial dissolution of spinel + clinopyroxene and crystallization of plagioclase + orthopyroxene (Rampone et al., 1997), we propose that the Internal Liguride spinels record a process of chemical modification (Al and Mg depletion and Ti enrichment). Similar relations have been described for impregnated peridotites from modern oceans and ascribed to reaction between original spinel and melt to produce plagioclase and secondary spinel (e.g. Dick & Bullen, 1984; Cannat & Casey, 1995).
Close inspection of porphyroclastic clinopyroxene compositions shows that most clinopyroxenes from the Internal Liguride lherzolites, even considering core portions of grains, have slightly lower Al, Na and Sr contents than those from the Cecina valley lherzolites (Figs 4 and 6). Remarkably, spinel and porphyroclastic clinopyroxene from External Liguride lherzolites show large chemical variations, attributed to late development of plagioclase (Rampone et al., 1993, 1995), which are qualitatively similar to those observed between the Cecina valley and Internal Liguride counterparts. In particular, porphyroclastic clinopyroxenes from External Liguride lherzolites have lower Al, Na and Sr, and slightly higher REE and HFSE concentrations in plagioclase-rich than in plagioclase-poor samples (see also Fig. 6). We thus suggest that the low modal proportion of newly formed plagioclase (<5%, Rampone et al., 1997) in Internal Liguride lherzolites resulted in a slight depletion of Al, Na and Sr concentrations in porphyroclastic clinopyroxenes, with little modification of REE and HFSE abundances.
To test whether the chemical variations between Cecina valley and Internal Liguride clinopyroxenes can be related to late development of plagioclase, we have carried out a series of mass balance calculations. These calculations have been performed for Na and Sr, because the abundances of these elements in the other peridotite minerals are negligible. The average compositions of Cecina valley clinopyroxenes (Na2O 0·4 wt %, Sr 2·4 ppm) are assumed to represent original mantle clinopyroxene. The average compositions of Internal Liguride clinopyroxenes (Na2O 0·2 wt %, Sr 0·7 ppm) and plagioclases (Na2O 0·9 wt %, Sr 4·3 ppm) are considered as the products of the impregnation process (data after Beccaluva et al., 1984; Rampone et al., 1996, 1997). The possible whole-rock changes of Na and Sr concentrations in response to melt impregnation have been neglected. The variations in Na and Sr contents of porphyroclastic clinopyroxene can be reproduced by assuming that the initial proportion of original clinopyroxene was 10 wt % and that the melt impregnation process gave rise to 3 wt % plagioclase crystallization and 2 wt % clinopyroxene dissolution, in accordance with petrographic observations (Rampone et al., 1997).
We therefore conclude that the Cecina valley and Internal Liguride lherzolites had a similar spinel-facies refractory signature. After having undergone fractional melting, the Cecina valley lherzolites were locally deformed under spinel-facies conditions, as indicated by recrystallization of pyroxene porphyroclasts, probably in relation to uplift towards shallower levels. On the other hand, the Internal Liguride lherzolites were subjected to late-stage melt impregnation under plagioclase-facies conditions (Rampone et al., 1997), which resulted in significant chemical modifications of spinel and, to a lesser extent, porphyroclastic clinopyroxene. Both Cecina valley and Internal Liguride lherzolites were subsequently intruded by N-MORB-type gabbros.
Origin of the refractory signature of the Cecina valley and Internal Liguride lherzolites
The initial Nd isotope ratios of Cecina valley gabbros are lower than the Nd isotope values of associated mantle lherzolites at the time of intrusion (Fig. 7). 87Sr/86Sr ratios seem to confirm the isotopic depletion of lherzolites relative to gabbros, in spite of the uncertainties associated with potential seawater-related contamination. The 87Sr/86Sr value of a Cecina valley lherzolite (sample TP1) is markedly lower than the lowest 87Sr/86Sr value of associated gabbros (0·7020 and 0·7025, respectively) and is inconsistent with typical N-MORB values. A similar isotopic contrast between mantle and crustal sections is observed for Internal Liguride ophiolites (Rampone et al., 1996, 1998), as the mantle lherzolites have higher Nd values and lower 87Sr/86Sr ratios (+11·6 to +14·6 at 170 Ma, and 0·7022) than associated gabbros.
The average Nd isotope composition of clinopyroxenes from Internal Liguride lherzolites yields an Early Permian model age (t = 274 Ma), assuming a depleted mantle source (Rampone et al., 1996). This age has been interpreted as the time of the depletion event recorded by the Internal Liguride lherzolites, and has been related to asthenosphere accretion to continental lithosphere during the post-collisional phase (270–300 Ma) of the Variscan orogeny (see also Rampone et al., 1998). This interpretation is supported by the fact that the Nd isotope ratios of Internal Liguride lherzolites at post-Variscan times cluster near the initial Nd isotope values of some post-Variscan gabbros that at present crop out in the Alpine belt (Thöni & Jagoutz, 1992; Montanini & Tribuzio, 2001). However, the genetic link between Internal Liguride lherzolites and post-Variscan basic magmatism (Rampone et al., 1996; Montanini & Tribuzio, 2001) seems inconsistent with the high initial 87Sr/86Sr values of the gabbros (>0·7025).
The same depleted mantle source used by Rampone et al. (1996) yields a Nd model age of 234 Ma for the Cecina valley lherzolites, which is inconsistent with a post-Variscan depletion event. We have also calculated the Sr model age of Cecina valley lherzolite TP1, assuming a depleted mantle with 87Sr/86Sr = 0·7026 and 87Rb/86Sr = 0·0509 (Ito et al., 1987). The result obtained (t = 777 Ma) suggests an ancient evolution for the Cecina valley mantle section. Remarkably, low 87Sr/86Sr values (0·7017–0·7018) were also found for a few lherzolites of the External Liguride and Lanzo (Western Alps) ophiolites, and ascribed to MORB-source material that was isolated from the convective mantle in the Proterozoic (Bodinier et al., 1991; Rampone et al., 1995). Similarly, the Sr model age resulting from Cecina valley lherzolite TP1 could be considered as a minimum age of differentiation from the asthenosphere.
However, Salters & Dick (2002) have recently shown that abyssal peridotites from the Southwest Indian Ridge are characterized by extreme Nd isotope heterogeneity. In particular, present-day Nd values for clinopyroxenes from abyssal peridotites of the Atlantis II fracture zone and 10–16°E range from +8·6 to +15·0 and from +7·7 to +13·1, respectively. At both locations, the range of 143Nd/144Nd ratios of mantle clinopyroxenes extends to higher values than those of associated basaltic rocks, thus implying that the isotope compositions of the crustal rocks require the presence of a component not observed in the mantle sequences at the ridge. This component could be represented by pyroxenite or eclogite, as these rock types have a lower solidus than peridotite and would thus be exhausted before melting of the peridotite stops (Salters & Dick, 2002).
Mantle isotope heterogeneity in modern slow-spreading ridges is also indicated by melt inclusions within olivine from basalts of the FAMOUS area. Pb isotope microanalyses of these inclusions show an extremely wide Pb isotope compositional range; they are thus interpreted to be incompletely aggregated mantle melts (Shimizu, 1998; Shimizu & Layne, 1998). This indicates that the relatively homogeneous isotope compositions of basalts and gabbros result from shallow-level homogenization among melts generated from a compositionally heterogeneous mantle (Shimizu & Layne, 1998). A similar process of melt aggregation was also suggested by Kempton & Hunter (1997) and Coogan et al. (2000) for the gabbros of the MARK area, on the basis of minor isotopic disequilibrium between clinopyroxene and plagioclase, and trace element zoning in plagioclase, respectively.
Both the Cecina valley and Internal Liguride lherzolites are isotopically depleted relative to associated crustal rocks, similar to what is observed for modern slow-spreading ridges (Salters & Dick, 2002). This confirms the petrological and trace element resemblances between the gabbro–peridotite associations from modern slow-spreading centres and those from both Cecina valley and Internal Liguride ophiolites (see also Tribuzio et al., 2000a). In addition, the isotopic heterogeneity of abyssal peridotites (Salters & Dick, 2002) suggests that isotope model ages of Cecina valley and Internal Liguride lherzolites have little geochronological meaning. If we assume that the residual signature of these lherzolites developed in conjunction with the formation of the Ligurian Tethys, in the Middle Jurassic, then: (1) the Cecina valley and Internal Liguride mantle sections originally contained Nd- and Sr-enriched material; (2) the history of Cecina valley and Internal Liguride lherzolite protoliths is mostly unknown.
The gabbro–peridotite associations from the External Liguride ophiolites and Western Iberian margin: are they analogues?
The non-volcanic continental margin of Western Iberia is characterized by large bodies of mantle peridotites, emplaced from the deep continental lithosphere during the Early Cretaceous, in response to the extensional processes that led to the opening of the North Atlantic Ocean (Whitmarsh & Wallace, 2001, and references therein). The peridotites are spinel and plagioclase bearing, and most have a fertile geochemical signature (Kornprobst & Tabit, 1988; Charpentier et al., 1999; Abe, 2001; Hébert et al., 2001). They locally contain pyroxenite layers and small bodies of gabbro and dolerite, and are covered by basalt flows, post-rift sediments or continental material (Boillot et al., 1995; Fuegenschuh et al., 1998). These lithostratigraphic and mantle petrological features have been recognized in the External Liguride units, which could be considered as fossil remnants of an ocean–continent transition (see also Marroni et al., 1998; Rampone & Piccardo, 2000).
The correspondence between External Liguride units and the Western Iberian margin is confirmed by the fact that in both sequences gabbro crystallization began before continental break-up [see Schärer et al. (2000) and Beard et al. (2002), for the Western Iberian margin]. The gabbros and associated basaltic rocks from the Western Iberian margin have incompatible trace element concentrations indicating normal- to transitional-MORB affinity and initial Nd values ranging from +8·8 to +2·2 (Kornprobst et al., 1988; Seifert et al., 1997; Charpentier et al., 1998). The initial Nd of External Liguride gabbros (this study) and basalts (Rampone et al., 1998) are within the range reported for the igneous mafic rocks of the Western Iberian margin, although mafic rocks with a slightly enriched geochemical signature have not been found in the External Liguride ophiolites (see also Marroni et al., 1998).
The inferred analogy between External Liguride units and non-volcanic continental margins is nevertheless reinforced by recent geochemical studies of the mantle peridotites from the Western Iberian margin (Charpentier et al., 1999; Abe, 2001; Chazot et al., in preparation). The Western Iberian lherzolites are isotopically heterogeneous, with Nd values at the time of the gabbro intrusion ranging from +3·8 to +13·2, and have clinopyroxenes (Fig. 6) with trace element compositions showing a fertile geochemical signature. Overall, these features are consistent with the isotope and trace element signatures of External Liguride lherzolites (Rampone et al., 1995). In particular, lherzolites with disseminated Ti-rich amphibole have been collected at the Galicia Bank (Charpentier et al., 1999; Chazot et al., in preparation) and these rocks have petrological and trace element features (see also Fig. 6) that are closely similar to those of the External Liguride lherzolites (Rampone et al., 1995). Remarkably, mantle clinopyroxenes with a highly depleted trace element signature, similar to that of Cecina valley and Internal Liguride mantle clinopyroxenes, have never been found in the Western Iberian margin (Charpentier et al., 1999; Abe, 2001; Chazot et al., in preparation).
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GEODYNAMIC IMPLICATIONS AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL FRAMEWORK AND...ANALYTICAL TECHNIQUESGEOCHEMICAL RESULTSDISCUSSIONGEODYNAMIC IMPLICATIONS AND...REFERENCES |
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The gabbro–peridotite association of the Northern Apennine ophiolites formed by intrusion of N-MORB-type melts into a heterogeneous mantle. The External Liguride gabbros crystallized within a subcontinental mantle section (Beccaluva et al., 1984
; Ottonello et al., 1984; Rampone et al., 1995) at 179 ± 9 Ma, thus giving rise to a gabbro–peridotite association similar to that of non-volcanic continental margins (i.e. the Western Iberian margin). The External Liguride gabbros testify to mantle melting prior to continental break-up and probably record the onset of the magmatism related to opening of the Ligurian Tethys.
The Cecina valley and Internal Liguride (Rampone et al., 1996) lherzolites are similar to residual abyssal peridotites. In particular, although the Cecina valley and Internal Liguride lherzolites were subjected to different processes after the partial melting event, they can be modelled as having formed by the same degree of fractional melting (6% of a slightly depleted spinel lherzolite source). In addition, both Cecina valley and Internal Liguride (Rampone et al., 1996) lherzolites are isotopically depleted relative to associated crustal rocks, similar to what is observed for the modern oceanic lithosphere (Salters & Dick, 2002).
The Cecina valley lherzolites reached shallow levels prior to oceanization, as indicated by the Sm–Nd mineral isochron of associated Riparbella gabbros (173·5 ± 4·8 Ma). The development of the Cecina valley gabbro–peridotite association is interpreted as an intermediate stage of the rifting process that led to the opening of the Ligurian Tethys. Such a rifting process probably resulted in a magma-poor slow-spreading centre, whose remnants are represented by the Internal Liguride ophiolites (see also Tribuzio et al., 2000a).
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ACKNOWLEDGEMENTS |
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M. Tiepolo is thanked for his precious help and the useful suggestions. A. Montanini, G. B. Piccardo, E. Rampone and A. Zanetti are acknowledged for friendly and stimulating discussions. Thorough reviews by L. A. Coogan, P. D. Kempton, O. Müntener and J. Shervais have been greatly appreciated. This work was supported by FAR, PRIN and CNR funds.
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FOOTNOTES |
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* Corresponding author. Telephone: 0382 505874. Fax: 0382 505890. E-mail: tribuzio{at}crystal.unipv.it
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REFERENCES |
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