A volcano-sedimentary sequence with albitite layers in the Variscan basement of NE Sardinia: A petrographical and geochemical study

摘要

A metamorphosed volcanoclastic-sedimentary succession has been discovered near the village of Lula, in NE Sardinia with the following main lithotypes from bottom to top: 1) metavolcanics; 2) yellowish metasandstones; 3) greenish to grey metapsammopelites. The metavolcanics, of dacitic/andesitic composition, and with variable modal amounts (10-40%) of albite phenocrysts, include two albitite layers. The yellowish metasandstones form sequences with basal microconglomerates, passing upwards with a wavy erosional surface to the overlying metasiltite-metapelite sequence. The greenish to grey metapsammopelites mainly consist of alternating albite-rich phyllites and dark phyllites. The Lula metavolcanics show REE patterns similar to those of the Ordovician metavolcanics from Gerrei, Sarcidano and Sarrabus. NMORB normalised trace element patterns with negative Nb and Ta anomalies demonstrate the calcalkaline affinity of the Lula metavolcanics. The albitite layers consist of up to 90-95 modal percent albite and up to 9 wt.% Na _2O. Rocks with this unusual composition could have been generated only by hydrothermal fluids associated with the Ordovician volcanism or to the final cooling stages of a calcalkaline pre-Variscan intrusive body. The K _2O, Na _2O, SiO _2 and Al _2O _3 contents reveal that the yellowish metasandstones plot in the fields of greywackes or pelitic greywackes, the greenish to grey metapsammopelites in the fields of pelites or pelitic greywackes. Compared to the metapsammopelites, the metasandstones reveal slightly higher SiO _2, Zr and Hf contents, significant Na _2O enrichment, slightly lower Al _2O _3, Fe _2O _3, Ba, Rb and Cs contents and strong K _2O depletion. The greenish to grey metapsammopelites show chondrite normalised REE patterns almost identical to those of the Ordovician metavolcanics and similar to those of North American Shale Composite and Post Archaean Australian Shale. The shallow metavolcanic succession is probably made up of primary volcanic products and/or deposits of their reworked detrital materials. Thin, discontinuous dark metapelitic layers in the metavolcanic succession are actually sedimentary layers marking short quiescence periods in volcanic activity. The protoliths of the metasedimentary rocks were shallow marine sediments. The protoliths of the yellowish metasandstones may be attributed to mid- to high-energy environments while those of the metapsammopelites indicate alternating mid- to low-energy environments. In the yellowish metasandstones 45 microsequences have been identified. They show thickening-to-thinning-upwards sedimentary trends that may be interpreted as a backward to forward migration of the sequences with respect to the shoreline resulting in an increase or decrease of depositional energy, respectively. All the rocks were metamorphosed and multideformed during the Variscan orogeny. The P-T conditions of a biotite-bearing metasandstone have been estimated at T = 430-470 °C, P = 0.65-0.95 GPa by P-T pseudosection modelling.