Schweizerische Mineralogische und Petrographische Mitteilungen 83, 81–96, 2003 0036-7699/2003/83/81 ©2003 Schweiz. Mineral. Petrogr. Ges. Petrology and geochemistry of the Late Jurassic calc-alkaline series associated to Middle Jurassic ophiolites in the South Apuseni Mountains (Romania) Ionel Nicolae 1 and Emilio Saccani 2 Abstract In the South Apuseni Mountains (SAM), Romania, Jurassic calc-alkaline magmatic series occur in association with Jurassic ophiolites within a narrow belt that marks the boundary between the Eurasian and Adria Paleozoic conti- nental margins. This association of magmatic series has been previously reported as a single ophiolitic sequence by many authors. Calc-alkaline rocks include volcanic and intrusive rocks and, along with associated ophiolites, occur in a composite nappe system in the central and NE part of the SAM. Volcanic rocks directly overlie ophiolites and include basalts, basaltic andesites, andesites, dacites, and rhyolites, while dykes intrude both calc-alkaline volcanics and ophiolites, and are mainly represented by andesites and dacites, locally showing high-K calc-alkaline affinity. Intrusive rocks mainly include granites and granodiorites. Geochemical characteristics of the studied rocks correspond to those generated in island arc settings, with a depletion of HFSE relative to LILE, and a marked enrichment of LREE. Mineral–melt exchange equilibria, as well as bulk-rock major and trace element abundances indicate that fractional crystallization in closed systems played a major role in controlling the magmatic evolution of the SAM calc-alkaline series. Geothermometric and geobarometric estimates indicate that temperature along the fractionation trend varied from 1216 °C for the less evolved rocks down to 860 °C for the more evolved rocks, at pressures between 1.8–2.3 kbar. SAM calc-alkaline lavas were derived from a MORB-like depleted source modified by slab-derived fluids in an intra-oceanic island arc setting that developed during the closure of the Vardar oceanic basin. These island arc series were formed by both intrusion and extrusion of calc-alkaline rocks into and onto a pre-existing oceanic crust now represented by SAM ophiolites. In contrast to some previous interpretations, data presented in this paper demon- strate that no genetic link exists between the calc-alkaline series and the underlying ophiolitic rocks. Keywords: Calc-alkaline magmatism, petrology, Apuseni Mountains, Romania. Introduction The South Apuseni Mountains (SAM) repre- sent a narrow belt that marks the boundary be- tween the Eurasia plate and the Tisza (Austro- Bihorean) microplate, which represent the northernmost edge of the Adria plate (Dal Piaz et al., 1995; Dalmeyer et al., 1999). Late Jurassic calc-alkaline rocks are widespread in the SAM (Fig. 1) and are always associated with Jurassic ophiolites (Savu et al., 1981; Cioflica and Nico- lae, 1981; Hovorka, 1996). In particular, calc-al- kaline volcanics stratigraphically overlie the ophiolitic sequence, and calc-alkaline granitoids intruded the ophiolites, suggesting that the mag- matic activity developed after the emplacement of the ophiolitic sequence. The tectono-magmatic significance of the calc-alkaline series is controversial, as is the geo- dynamic significance of their association with ophiolites. Two contrasting models have been proposed. One postulates that calc-alkaline rocks were generated in an intra-oceanic arc where MOR-type ophiolites had previously been em- placed. This model assumes no genetic relation- ship between calc-alkaline and ophiolitic rocks (Savu et al, 1981; Saccani et al., 2001; Bortolotti et al., 2002). The alternative model suggests that the calc-alkaline series and the ophiolites belong to a single magmatic sequence (e.g., Radulescu and Sandulescu, 1973), with the calc-alkaline repre- senting the evolved products of intra-oceanic is- land arc activity which began with the generation of the ophiolitic suite (Nicolae, 1995; Cioflica and Nicolae, 1981). Data so far presented in the literature do not allow a definite interpretation in terms of of these tectono-magmatic models. The purpose of this pa- 1 Institutul de Geodinamica al Academiei Romane, Bucuresti, Romania. 2 Dipartimento di Scienze della Terra, Università di Ferrara, I-44100 Ferrara, Italy. <e.saccani@unife.it>