DISCRIMINATING BETWEEN XENOCRYSTS AND PHENOCRYSTS IN SEDIMENT-DERIVED DACITES

Understanding the petrogenesis of peraluminous magmas, particularly the compositional diversity within these rocks, remains a significant challenge. We investigate a garnet and cordierite-bearing dacite from the El Hoyazo volcanic deposits in southern Spain, utilizing a combination of mineral chemistry and δ18O and δ13C isotopic analyses to unravel the processes contributing to their formation. Our research focuses on discriminating between xenocrystic and phenocrystic mineral phases to clarify the genesis of these complex volcanic deposits.

Comprehensive petrographic, electron microprobe, and laser ablation analyses reveal the existence of multiple generations of garnet, likely representing both xenocrystic and phenocrystic phases. The δ18O values are notably high, suggesting a predominant contribution from sediment melts. These elevated δ18O signatures are inconsistent with those expected from purely mantle-derived magmas, implying significant melting and assimilation of crustal material. Additionally, δ13C analyses reveal isotopic compositions that point to contributions from organic-rich sediments, reinforcing the role of sediment melts in the magmatic evolution of the dacites.

Our findings suggest that the El Hoyazo dacite results from a complex mixing process involving mantle-derived magmas and sediment melts. While sediment melts dominate the isotopic signatures, as evidenced by high δ18O values, additional xenocrystic minerals such as pargasite indicate a mantle-derived component. This hybrid model provides a coherent explanation for the mineralogical and isotopic characteristics observed in the El Hoyazo deposits and offers a broader understanding of peraluminous magma petrogenesis.

By integrating mineral chemistry with isotopic data, this study provides a comprehensive framework for deciphering the intricate processes involved in magma formation. Our research highlights the critical role of distinguishing between xenocrystic and phenocrystic phases in understanding the genesis of garnet and cordierite-bearing felsic magmas.