Two contrasting thermal evolution processes of Cretaceous granitic magma in Central Java, Indonesia, and implications for the Neo-Tethys Ocean

Abstract The late development of the easternmost Neo-Tethys Ocean was marked by two episodes of Cretaceous arc magmatism in Luk Ulo, Central Java. However, the thermal evolution process of their magma remains enigmatic. Here, we present new apatite U-Pb geochronology and geochemistry data from the Luk Ulo granitoids in Southeast Asia showing two distinct populations during the Early (112–107 Ma) and Late (72–68 Ma) Cretaceous epochs. However, the Early and Late Cretaceous granitoids record different mineral crystallization histories. The magma cooling rate for the Early Cretaceous granitoids is 56–347 °C/m.y., whereas the Late Cretaceous rate is >275 °C/m.y. The Early Cretaceous granitoids contain ilmenite, and show low zircon Ce4+/Ce3+ ratios (0.5–33.5) and ΔFMQ values (−6.4 to −1.3) with low whole-rock V/Sc ratios (0.5–4.2), which indicates low magmatic oxygen fugacity (fO2). However, the Late Cretaceous granitoids exhibit high zircon Ce4+/Ce3+ ratios (29.6–339.5) and ΔFMQ values (+0.3 to +3.0) with high whole-rock V/Sc ratios (4.8–8.1), and include magnetite, which indicates a high magmatic fO2. In addition, apatite crystals from the Late Cretaceous granitoids show lower Eu/Eu* (0.0–0.3) and Sr/Y (0.0–0.3) ratios than apatite crystals from the Early Cretaceous granitoids (Eu/Eu*: 0.1–0.7; Sr/Y: 0.1–1.9). This difference implies that more extensive plagioclase crystallization occurred in the Late Cretaceous granitoids under lower pressure conditions compared to their Early Cretaceous counterparts. Integrated with regional geological data, this indicates that Early Cretaceous Neo-Tethyan subduction along the southwestern margin of Sundaland transported reduced metasediments, while Late Cretaceous slab rollback induced extensional magmatic decompression melting, and both were accompanied by rapid cooling rates.