Source Composition Controls the Petrogenesis of Jurassic-Cretaceous Adakitic Volcanic Rocks in the Central North China Craton

Diverse origins have been proposed for continental adakites, with great emphasis on high-pressure melting of the lower crust. However, the source composition is usually ignored in interpreting the generation of the adakitic geochemical signature (e.g., high Sr/Y and La/Yb) and thus may affect our understanding of the petrogenesis and tectonic settings. Here, we present geochronological, mineralogical, geochemical, and isotopic data for the Late Jurassic to Early Cretaceous adakitic volcanic rocks from the Yangyuan area in central North China Craton (NCC) to constrain their petrogenesis and tectonic implications. These adakitic rocks are trachyandesites and dacites with an eruption age of 146–141 Ma. They are characterized by high Sr/Y (80.6–97.5) and (La/Yb)N (60.6–67.6) ratios but low MgO (0.85–1.43 wt%) contents, with negligible Eu anomalies (Eu/Eu*=0.93–1.08). Isotopically, the samples have high initial 87Sr/86Sr ratios (0.707146–0.707207) and enriched whole-rock Nd (εNd(t)=−14.0 to −13.8) and zircon Hf (εHf(t)=−19.4 to −15.8) values that are consistent with Mesozoic lower-crust-derived adakitic rocks and lower-crustal xenoliths in the NCC. Trace-element modeling results suggest that they were more likely derived by partial melting of a continental lower crust with a thickness of <40 km, although the high (Gd/Yb)N ratios (6.16–7.35) indicate the presence of residual garnet in the magma source. The geochemical characteristics, combined with published data in the central and eastern NCC, reveal that the “adakitic” signature of these rocks is mainly controlled by source rock compositions. This study suggests that continental adakitic rocks are not necessarily produced by high-pressure melting, and thus their use as an indicator of thickened or foundered lower crust should be treated with caution.