O- and Sr-isotope data are reported for cordierite bearing dacites and andesites forming part of the Neogene volcanic province of SE Spain. The almandine-bearing biotite-cordierite-labradorite dacite from the Cerro del Hoyazo with its numerous inclusions of metamorphic and igneous rocks has been studied in some detail. A syngenetic derivation of the Hoyazo dacite magma and part of its inclusions (interpreted as restite) by means of anatexis of (semi-)pelitic rocks has previously been proposed.
δ18O values and 87Sr/86Sr ratios,
thought to be closely representative of the original magmas, vary from +12.2 to
+ 15.6 and 0.7095 to 0.7171, respectively. The metamorphic rock inclusions have
δ18O values (+ 13.0- + 16.2) comparable to the range for the
volcanics. These results support an anateetic origin for the entire suite of
cordierite-bearing volcanics. The inclusions of basic igneous rocks in the
Hoyazo dacite have, in comparison with the dacite, lower δ18O values (+ 11.1-+ 13.1) and equal to lower 87Sr/86Sr ratios (0.7081 0.7112), confirming an independent origin.
The six analysed samples of the Hoyazo dacite show a strong linear correlation of 87Sr/86Sr versus 87Rb/86Sr which, if interpreted as an isochron, yields an age of 210±17 Ma. Similar linear arrays for samples from the Mazarron and Mar Menor areas yield nearly concordant ages. Samples for the Vera area define a 535±22 Ma line. These linear correlations may be interpreted either as (1) mixing lines, in which case the indicated ages have no geological meaning, or (2) true isochrons dating pre-eruptional events of the parent (meta-)sediments.