Sardinia-Corsica block (SCB) and in the Northern sector of. Calabro Peloritain Arc (CPA), the Sila subterrane. The Ordovician magmatism of the Sardinian ...
Rend. online Soc. Geol. It., Vol. 22 (2012), pp. 174-176, 2 figg. © Società Geologica Italiana, Roma 2012 Rendiconti online Soc. Geol. It., Vol. n (2012), n-n (Stile: intestazione prima pagina)
The Ordovician Magmatism puzzle: a preliminary comparison of samples from Calabria and Sardinia volcanites PAMELA PAVANETTO (*), ANTONIO FUNEDDA (*), ALFREDO LOI (*) & MASSIMO MATTEINI (**)
ABSTRACT Magmatismo Ordoviciano: confronto tra vulcaniti della zona a Falde del Blocco Sardo Corso e dell’Arco Calabro Peloritano La comparazione tra i basamenti varisici dell'Arco calabro-Peloritano e del Blocco sardo corso viene qui approfondita studiando alcuni campioni provenienti da successioni vulcaniche che hanno età che variano tra i 458-466 Ma determinate con analisi U-Pb su singoli zirconi.
KEY WORDS: Calabria, European Variscides, Ordovician Magmatism, Sardinia, U-Pb age INTRODUCTION Different magmatism events occurred during Ordovician are now recognized in almost all the Variscan terranes scattered in the peri-Mediterranean area. The products of these events are very important archives where are stored a lot of geological and geochemical data that can help in reconstructing the complex evolution of Gondwana, and peri-gondwanian terranes at that time. This research is a contribution to unravel the pre-Variscan magmatic evolution by the geochemical whole rock data and age performed from the basement rocks. Have been taking in account the analogies between the magmatic bodies cropping out in the nappe and foreland of Sardinia-Corsica block (SCB) and in the Northern sector of Calabro Peloritain Arc (CPA), the Sila subterrane. The Ordovician magmatism of the Sardinian Corsica Block (SCB), particularly of the Nappe zone, represents an important key in understanding the geodynamic evolution of European Variscides. Even if analogies between the SCB and the Calabrian Peloritain Arc (CPA) have been yet remarked, still lack an accurate comparison between the different tectonic units of Sardinian nappes with the Calabria units. For their better exposition and literature knowledge compared to the Calabrian bodies, the Palaeozoic Sardinia formations are easier to be placed in the European Variscan scenario. There up to now three magmatic events are recognized. The older
Ordovician volcanic rocks found in Sardinia region are recently dated back to Lower Ordovician time (OGGIANO et alii, 2010). The huge amounts of volcanic rocks were emplaced during the Middle-Upper Ordovician period in the Nappe Zone of Sardinia above the so called "Sardic unconformity". As the rocks range from andesite to rhyolite and have a calcalkaline geochemical imprint, an Andean-type volcanism related to Early Ordovician oceanic subduction has been proposed to explain this event (CAPPELLI et alii, 1992, OGGIANO et alii, 2010; GAGGERO et alii, 2012). A third volcanic cycle represented by basalt and microgabbro with an alkaline to transitional signature occurred at the boundary between Ordovician and Silurian all over Sardinia basement.. SAMPLES
(*) Dipartimento di Scienze Chimiche e Geologiche, Università di Cagliari
Here have been performed geochronological and whole rock geochemical data in volcanic samples from the Sardinian Nappe, both in the Sarrabus Unit (SM200 and BS125) and in the Bithia Tectonic Unit (AF0820) and from the Sila subregion (VIV-11) in the northern sector of CPA. The samples SM-200 and BS-125 were collected in the Sarrabus volcanic complex that is composed by ca. 200 m of volcanic deposits classically called “porfidi grigi del Sarrabus” and “porfidi bianchi” from dacitic to rhyolitic in composition, generally with a porphiric texture with large phenocrysts of Kfeldspar and quartz. Sample AF08020 comes from the Bithia Tectonic Unit (PAVANETTO et alii 2012), a greenschist facies unit that contains at least three principal rock packages including: a metavolcanic sequence, a laminated sandstone and quartzite sequence, and a marble sequence. The volcanic rocks have been recently dated (PAVANETTO et alii 2012) at the MiddleUpper Ordovician (U-Pb age of 457.01 ± 0.17 Ma). The Sila volcanites sample (VIV-11) comes from a porphyroid collected in the Longobucco Unit in the northern sector of CPA, from the lowest metamorphic Bocchigliero complex. The Longobucco U. is characterized by a Variscan basement with a post-Variscan sedimentary covers not affected by Alpine metamorphism.
(**)Laboratório de Geocronología, Instituto de Geociências, Universidade de Brasília (UnB), Brasília-DF 70910-900, Brazil.
GEOCHEMICAL FEATURES
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Considering the geochemical features and REE patterns the samples show similar trends. In the Chondrite-normalized rare
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T HE O RDOVICIAN M AGMATISM PUZZLE earth patterns (SUN & MCDONOUGH, 1989) the volcanic samples show a trend with are LREE-enriched with flatdecreasing HREE and with a systematically negative Eu anomaly (Fig. 1.a) linked to the fractionating plagioclase process, related to a strong evolution of magma. In the multielements diagram primitive mantle-normalized (Fig. 1.b) the samples show an enrichment of large ion lithophile elements (LILE) with a strong positive anomaly in Cs, is evident an impoverishment of high field strength elements (HFSE). The SM-200 sample of the Sarrabus U. shows a decreasing in Ba due to an impoverishment of high field strength elements (HFSE). The strong negative anomalies have shown by Nb, Sr, and Ti. All over negative anomaly of Sr could be linked to the plagioclase fractionating. Plotting the alkali vs. silica into the TAS diagram (Fig. 1.c), the Sardinian samples fall in the dacite field and in the subalkaline field under the red line. The sample from Sila (VIV-11) falls between rhyolite and dacite. Plotting the data in the PECCERILLO & TAYLOR (1976) diagram (Fig. 1.e), the Sardinian samples fall in the High Calc-Alkaline Series, except the BS-125 that falls shoshonitic series as well as the Calabrian sample (VIV-11).
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continental crust, originated in intracontinental regimes and were contaminated by continental crust. The two samples collected from Sarrabus Unit (SM-200 and BS-125) have given an U-Pb age of 458.7 ± 2.8 Ma, and 464.6 ± 7.3 Ma (Fig. 2). The ages here obtained are concordant with the Sandbian dacites (457.01 ± 0.17 Ma) recently attributed (PAVANETTO et alii, 2012) to the Bithia Unit. metavolcanic rocks. The data carried on from Sardinia Ordovician rocks well fit with the age found for the porphyroid collected in the northern part of the CPA, in the Sila sub-region. In fact this sample (VIV-11), belonging to the Bocchigliero Unit, gives an age of 466.4 ± 6.9 Ma (Fig. 2). DISCUSSION AND CONCLUSIONS Comparing the presented data with previous ones, the samples analysed in Sardinia show an anomaly respect to what described by other authors (OGGIANO et alii, 2010; GAGGERO et alii, 2012) that considered the Middle-Upper Ordovician a
Fig. 1– Whole rocks geochemical plotting. a) and b), Trace element spiders (Sun and McDonough, 1989) a) REE patterns normalized to chondrite and b) multielement diagram normalized to primitive mantle; c) TAS classification diagram with Irvine and Baragar (1979) line (dashed line) that subdivided the subalkaline from the alkaline field; d) Cabanis and Lecolle (1989), tectonic diagram showing a within plate formation; e) Peccerillo and Taylor (1976); f) Cabanis and Lecolle (1989).
Following the tectonic classification of PEARCE et alii (1984; Fig. 1.d), the Sarrabus volcanites (samples SM-200 and BS125) have been developed in tectonic scenario of within plate (Fig. 1.d) as well as the dacite from the Bithia U. (AF08-20) (Fig. 1.d). Considering the geotectonic discrimination diagram by CABANIS & LECOLLE (1989), the VIV-11 rocks plot in the intracontinental and post-orogenic domain corresponding to the field of bodies generated in a back-arc basin context. The Sardinian samples plot in the intermediate domain, which includes the average compositions of upper and lower
typical product of a volcanic arc on continental crust, since their calcalkaline signature. Since the investigated rocks suffered a greenschist facies metamorphism, accompanied by alteration of minerals, especially feldspars, the original chemical composition inferred from the whole rock analysis could be misleading. Our analyses at this stage are not enough robust to refuse previous interpretations. The ages of the magmatic Ordovician samples studied permit to assess that the two blocks (SCB and CPA) could shared the same Middle-Upper Ordovician events. Thus, from this preliminary study major uncertainties still remain about the
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Fig. 2 –U-Pb age and inner structures images. a) 206Pb/238U of SM-200 sample gives an average age of 458.7 ± 2.8 Ma. b) CL zircon images from SM200, BS125 and VIV-11 samples with measures done in the core and in the rim; c) 206Pb/238U of BS-125 sample gives an average age of 464.6 ± 7.3 Ma; d) 206Pb/238U of VIV-11 sample gives an average age of 466.4 ± 6.9 Ma.
tectonic setting of the two blocks, that need further investigations.
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