Abstract
Adakitic magmas in convergent margins carry vital information regarding slab dynamics, crustal thickening, and continental growth. In this study, we investigate the petrogenesis of Middle-to-Late Miocene adakitic volcanic and subvolcanic rocks from the Central and Northern Andes Cordillera through integrated whole-rock major and trace element geochemistry, in situ zircon U-Pb geochronology, and zircon Lu-Hf isotopic systematics. The studied suite comprises high-silica dacites and granodiorite porphyries characterized by high SiO2 (63.5–68.8 wt.%), elevated Al2O3 (15.2–17.4 wt.%), and high Sr (480–890 ppm), coupled with pronounced depletions in Y (7.2–12.4 ppm) and heavy rare earth elements (Yb = 0.65–1.12 ppm), yielding high Sr/Y (45–98) and (La/Yb)N (18.4–36.2) ratios without prominent negative Eu anomalies. In situ LA-ICP-MS zircon U-Pb dating yields robust crystallization ages ranging between 13.8 ± 0.3 Ma and 9.2 ± 0.2 Ma. Zircon Lu-Hf isotopic analyses show variable εHf(t) values ranging from +2.8 to +8.4, corresponding to two-stage Hf model ages (TDM2) of 0.65 to 1.05 Ga. Geochemical modeling and isotopic constraints indicate that these adakitic rocks were not generated by direct melting of the subducting oceanic slab, but rather through partial melting of juvenile, garnet-bearing basaltic lower crust in a thickened arc regime (>50 km), supplemented by MASH (Melting, Assimilation, Storage, and Homogenization) processes at the crust-mantle transition. These findings highlight the fundamental role of Miocene compressional tectonics and crustal maturation in governing the geochemical diversification of Andean arc magmatism.
