High δ¹⁸O and possible pre-eruptional Rb-Sr isochrons in cordierite-bearing Neogene volcanics from SE Spain

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.