INTRODUCTION
Recently, the genesis of anorogenic basic magmas has been an object of increasing interest for earth scientists, in order to define the role of mantle source components (e.g. asthenospheric vs lithospheric) and their homogeneity, as well as the geodynamic setting and nature of partial melting processes [Wilson, (1989) and references therein].
Experimental studies (Basaltic Volcanism Study Project, 1981; Green et al., 1987) on the one hand, and studies of the trace element and Sr-Nd-Pb isotope geochemistry of basalts (Zindler & Hart, 1986) on the other, provide fundamental constraints for their petrogenesis. Additional constraints can be provided by investigating a compositional spectrum of nearly coeval basic magmas erupted in a restricted area. In this context, it can be generally assumed that different basic magmas ascended through nearly the same plumbing system, from mantle sources characterized by analogous metasomatic processes under identical stress regime and plate tectonic setting.
The Pliocene-Pleistocene subaerial volcanism of the Iblean area, SE Sicily, provides, in this respect, an important case study, as it is almost exclusively characterized by relatively undifferentiated basic magmas, ranging in composition from extremely silica-undersaturated alkaline magma types (nephelinites) to silica-oversaturated tholeiites, erupted during a short span of time and within a transtensional tectonic setting.
In this paper we present new major and trace element, and Sr-Nd isotope data for representative samples of subaerial lavas from the province, selected from >300 samples, covering an area of ~250 km2, mapped on a scale of 1:50 000 (Beccaluva et al., 1993). These data provide important constraints for the petrogenesis of the magmas, particularly with respect to the chemical and mineralogical characteristics of their mantle source regions and the depth and degree of partial melting.
VOLCANO-TECTONIC SETTING
The Iblean-Malta Plateau is the northeasternmost portion of the African foreland, lying to the south of Mt Etna and the Sicilian Maghrebian Chain, which represents the collisional boundary between the African and European Plates in this sector (Fig. 1). It is bounded to the east by the Malta escarpment, a zone of large vertical fault movements, to the west by a N20°E right-lateral transcurrent system, and to the northwest by the thrust belt of the Gela Nappe (Ben-Avraham & Grasso, 1990). Oblique to the main transcurrent system, there are several pull-apart grabens and two main troughs: the NE-SW trending Scordia-Lentini and Simeto grabens at the northern margin of the Iblean Plateau, and the NW-SE trending Pantelleria rift zone in the Sicily Channel.
Figure
The Iblean Plateau, reaching a few hundred metres average elevation, mainly comprises a Mesozoic to Lower-Middle Miocene carbonate sequence, discontinuously overlain by more recent sedimentary and volcanic rocks. Volcanic activity occurred in the area from the Upper Triassic to the Upper Cretaceous, and was generally of Na-alkaline affinity, as evidenced by lavas penetrated by boreholes (Rocchi et al., 1995). During the Upper Miocene and Pliocene-Lower Pleistocene eruptions occurred of both tholeiitic and alkaline magmas. The most recent and most widespread volcanism took place over an area of ~500 km2 at the northern margin of the Iblean Plateau, with subaerial lavas in the southeastern areas (~250 km2; Fig. 1) and submarine products in the northwest. K/Ar and 40Ar/39Ar dating of the subaerial lavas (Beccaluva et al., 1993; Esperança et al., 1995) gives ages between 2 and 3 Ma.
The Pliocene-Pleistocene tectonic evolution of the area was characterized by an alternation of uplift and subsidence on a regional scale (Di Grande & Raimondo, 1982), which affected the whole Iblean area. Middle Pliocene-Pleistocene volcanism, in particular, is associated with a renewed uplift phase and the development of a regional NE-SW lithospheric wrench fault system (Fig. 1), which was characterized by both extensional and transcurrent components, as a foreland reaction to the Neogene compressive events along the Calabrian-Sicilian-Tunisian compressional front (Ghisetti & Vezzani, 1981). Detailed structural studies indicate right-lateral shears on both the N20°E fault system (main shear) and the synthetic N50°E `Riedel' shears (Ghisetti & Vezzani, 1980). Volcano-tectonic trends are evidenced by >200 small subaerial eruptive centres, from which lava flows of a few kilometres maximum length were erupted. These centres are mainly aligned along the N50°E shears of the wrench fault system.
The Iblean lavas cover a wide compositional spectrum. There are no definite spatial or temporal changes in magma chemistry, although the oldest products exposed in the area are generally tholeiitic. The estimated volume of the subaerial lavas (Fig. 1) is of the order of 10-15 km3, in the following proportions: 62% tholeiites, 22% basanites, 8% alkali basalts, 4% hawaiites, 3% transitional basalts, and 1% nephelinites.
The volcano-tectonic features, such as cone alignments, concentration of eruptive centres and fractures, allow subdivision of the subaerial volcanism into three sectors (Fig. 1). The distribution of lavas in sectors 1 (SW) and 2 (NE), which represent almost 70% of the investigated volcanic area, shows a general predominance of tholeiitic over alkaline lavas, whereas in sector 3 (NW), at the southern border of the Scordia-Lentini graben, basanites are exceptionally abundant.
Compared with other occurrences of extension-related intra-plate magmatism, the Iblean volcanism is similar to that of `low-volcanicity rifts' (Barberi et al., 1982), characterized by fissure-fed volcanic activity with small eruptive volumes of relatively undifferentiated magmas, and a wide range of chemical compositions from tholeiitic to extremely alkaline. In such rifts, lithospheric thinning is generally reduced or absent, and discontinuous volcanic activity develops mainly in a transtensive tectonic regime. In particular, the Iblean volcanism has a strong affinity with `impactogenic rifts' (such as Baikal and Rhine Graben), in that it took place in a wrench fault system, apparently generated as a foreland reaction to the European-African Plate collision, along the Maghrebian compressional front of northern Sicily (Fig. 1). Geophysical data indicate a crustal thickness of ~30 km under the Iblean area (Capuano et al., 1993) and a lithosphere thickness up to 90 km (Panza & Suhadolc, 1990).
ANALYTICAL METHODS
Highly representative and fresh rocks have been selected from >300 samples from the lava cooling units outcropping in the studied area. Rock samples were cored to select the freshest chips, which were powdered in an agate mill. X-ray fluorescence (XRF) analyses were performed on powder pellets using a wavelength-dispersive automated Philips PW 1400 spectrometer (Ferrara University). Major elements were determined by a full matrix correction procedure (Franzini et al., 1975), whereas for trace elements, experimentally determined correction coefficients were used (Leoni & Saitta, 1976). Accuracy and precision for major elements are estimated better than 3% for Si, Ti, Fe, Ca and K and 7% for Mg, Na, Al, Mn and P; for trace elements (above 10 ppm), they are better than 7% for Rb, Nb, Y, Sr and V, and 15% for Zr, La, Ce, Ba, Ni, Co and Cr. Loss on ignition (LOI) was determined by a gravimetric method and FeO by titration.
Rare earth elements (REE) and Y were determined at the Centre de Recherches Pétrographiques et Géochimiques of Nancy (France) by inductively coupled plasma (ICP) emission spectrometry with an accuracy of 15% for Yb and Lu and better than 8% for the other REE [see analyses of reference standards from Roelandts & Michel, (1986)].
Sr and Nd isotopic compositions were obtained at the University of Naples after cold leaching with 2·5 N or 6·15 N (the latter only for Sr) hydrochloric acid for ~30 s. Measurements were done with a VG 354 mass spectrometer. For the reference sample NBS987, the 87Sr/86Sr ratio 0·71026 ± 2, and for La Jolla STD the 143Nd/144Nd ratio 0·51186 ± 3 were obtained. The reported errors are at the 95% confidence level.
CLASSIFICATION AND PETROGRAPHY
Major and trace element compositions and Sr-Nd isotope ratios of representative subaerial lavas are reported in Table 1.
Table 1. Major (wt %) and trace element (ppm) analyses, CIPW norms, measured 87Sr/86Sr and 143Nd/144Nd isotopic ratios for Iblean subaerial volcanics
Table 1. continued
Following the total alkali-silica (TAS) classification diagram (Fig. 2), the Iblean lavas range from silica-oversaturated tholeiitic basalts to highly undersaturated mela-nephelinites. Variations in alkalinity and degree of silica saturation are clearly reflected in CIPW norms and the saturation index (SI) proposed by Fitton et al., (1991): the tholeiites are mostly quartz (Qz)-normative (referred to as A-Tholeiites, A-Th) with an average of 21% normative hypersthene (Hy) and SI = 11; less abundant olivine (Ol)-normative tholeiites with an average of 11% normative Hy and SI = 4 are referred to as B-Tholeiites (B-Th); transitional basalts (TB) have an average of 5% normative Hy and SI = 0; alkali basalts (AB) and hawaiites (Hw) have 4% and 3% normative nepheline (Ne) and SI of -9 and -5, respectively; basanites (Bn) and minor tephrites (Tph) have an average of 9% normative Ne and SI -15; nephelinites (Neph) have an average of 15% normative Ne and SI -26. All the alkaline magma types are sodic, with Na2O/K2O ratios mostly ranging between 2·3 and 5·2.
