SHRIMP Age-Constraints for the Calc-Alkaline Volcanism in the Olivenza-Monesterio Antiform (Ossa Morena, SW Spain)
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MINERALOGICAL MAGAZINE, VOLUME 62A SHRIMP age-constraints for the calc-alkaline volcanism in the Olivenza-Monesterio Antiform (Ossa Morena, SW Spain) B. Ord6fiez-Casado Department of Earth Sciences, ETH Zttrich, Sonneggstrasse 5, D. Gebauer CH-8092 Ztirich, Switzerland L. Eguiluz Departamento de Geodin~mica, Universidad del Pais Vasco, C.P. 644, E-48080 Bilbao, Spain Cathodoluminescence (CL) based ion-microprobe The zircon population from an adjacent porphyric ages are reported for zircons from a reworked tuff rhyolite sample reveal the presence of zircon within the Malcocinado formation as well as two domains that suffered complete Pb loss by leaching porphyric rhyolites and a rhyolite porphyry of the without any reorientation of the crystal lattice that Bodonal-Cala complex. Both volcanic successions, give ages of 324__ 6 Ma. This Carboniferous age is the Malcocinado and the Bodonal-Cala formation interpreted as representing a metasomatic process overly discordantly the Precambrian Tentudia sedi- during low to very low grade metamorphism, ments (uppermost part of the Serie Negra). Although demonstrated also by the presence of ore fluids. these successions are located in independent areas to The zircons of the Cala porphyry reveal, under CL, the N and S of the Olivenza-Monesterio antiform, the presence of rounded cores overgrown by dark, they can be correlated mainly in terms of lithostrati- trace element rich rims with or without oscillatory graphic and geochemical affinities. They were patterns. The isotopic analyses in these euhedrally considered to be of Upper Riphean to Vendian age terminated rims yielded an age of 512_+8 Ma, (Eguiluz, 1987). interpreted as the age of formation of the porphyry. The youngest detrital grain found in the This age is not in agreement with the Early Malcocinado formation indicates a maximum age Ordovician age reported by U/Pb multi-grain zircon of deposition of the sedimentary precursor at 522 _+ 8 data (lower intercept age, Sch~ifer, 1990), revealing Ma (2~ age, 95% c.1. as all other errors given the combined effects of inherited cores and post- below, unless stated otherwise). As the nearby magmatic Pb loss in the conventional multi-grain Monesterio anatectic gneiss dome was dated at data. 524_+7 Ma (Orddfiez et al., 1997) followed by rapid exhumation, the provenance of the zircons of the Malcocinado formation may be close to the Discussion and conclusions depositional sites. The volcanic sequences studied overly the Tentudia The presence of Pan-African detrital zircons succession, the youngest detrital zircons of which are around 600-500 Ma indicates the derivation from 564___ 15 Ma (Sch~ifer et al., 1993), thus representing Gondwana sources. Concordant ages were also found a maximum age of deposition of the Tentudia at 2-2.3 Ga and one grain yielded an age of sediments. On the other hand, pebbles of the 3442_+ 10 Ma (2~176 age, 1~). All these ages Ahillones granite (552___ 10 Ma, Orddfiez in prep.) are also reported in other parts of the European are embedded in the Malcocinado formation. The Hercynides (e.g. Gebauer, 1993). deposition of the sedimentary precursor of the The zircons of the porphyric rhyolite of the Malcocinado formation must be at the most Lower Bodonal-Cala complex show, under CL, inherited Cambrian or younger as the age of the youngest cores overgrown by euhedral rims with oscillatory detrital zircon grain is 522 -4-_8 Ma. On the other hand, growth zoning, indicating new crystallization during the porphyric rhyolites with similar geochemical and a magmatic process. The age of magmatic formation lithostratigraphic affinities indicate volcanism around of these rims is at 514_+9 Ma. Previous multi-grain 514_+9 Ma, in tune with the age of the subvolcanic analyses of the same zircon sample (Sch~ifer, 1990) Cala-porphyry 512_+8 Ma that cuts the rhyolite demonstrated the presence of a very heterogeneous layering. This volcanism was interpreted as synoro- zircon population containing a mixture of differently genic calcalkaline, representing the volcanic products old zircon components. at an active continental margin above a SW directed 1112 GOLDSCHMIDT CONFERENCE TOULOUSE 1998 subduction zone (Eguiluz, 1987). In the absence of SHRIMP-data the dated volcanics are Middle to further geochemical data, the dated volcanics are Upper Cambrian in age. interpreted here to be rather late- to post-collisional, and coeval with the formation of anatectic gneiss References domes around 520 Ma (Ord6fiez et al., 1997). Metasomatic overprinting of the volcanics, prob- Eguiluz, L. (1987) Ph.D. Thesis Univ. Bilbao, 666 p. ably related to low grade metamorphism, occurred Gebauer, D. (1993) In: Pre-Mesozoic Geology in the 324 Ma ago. Alps. Springer-Verlarg Berlin, 93-117. The dated Malcocinado volcanic series are over- Lififin, E., Palacios, T. and Perej6n A. (1984) Geol. lain unconformably by the Lower Palaeozoic Mag., 121, 221-8. Torre~irboles formation. Its age is palaeontologically Ord6fiez Casado, B., Gebauer, D. and Eguiluz, L. (1997) constrained to be Lower Cambrian (Lifi~n et al., XIV Reuni8o de geologia do Oeste Peninsular, Vila 1984). Our data rather favour a Middle Cambrian or Real, Abstracts, 161-4. younger age. Thus, more work is necessary to Sch~ifer, H-J., Gebauer, D., N~igler, T.F. and Eguiluz, L., unambiguously define the stratigraphy of the Lower (1993) Contrib. Mineral Petrol., 113, 289-99. Palaeozoic in the Ossa Morena Zone. Based on the Sch~ifer, H.-J., (1990) Ph.D. Thesis ETH Ziirich, 153 p. 1113 .