Generation of crystal-rich rhyodacites by fluid-induced crystal-mush rejuvenation: Perspective from the Late Triassic Nageng (sub-)volcanic complex of the East Kunlun Orogen, NW China

Abstract

Remobilization of upper crustal crystal-rich magma reservoirs (“crystal mush”) following melt extraction has been widely invoked in the formation of crystal-rich erupted products. However, the melt storage duration and conditions, and the pre-eruptive magma chamber processes remain poorly understood. Here, we present new whole-rock elemental and Sr-Nd-Pb isotope data, and zircon U-Pb-Hf isotope and trace element data of the Nageng (sub-)volcanic complex in the East Kunlun Orogen of NW China. This complex consists of the highly-evolved (high SiO2, Rb/Sr) crystal-poor rhyolite and less-evolved crystal-rich rhyodacite and dacite porphyry. These rock types are genetically linked, based on their similar chondrite-normalized REE patterns and NdHf isotopes, and their linear correlation trends of 208Pb/204Pb and 207Pb/204Pb vs. 206Pb/204Pb. Their depleted whole-rock εNd(t) (−9.74 to −7.37) and zircon εHf(t) (−7.16 to −3.48) values, together with their Proterozoic two-stage Hf model ages (1707–1474 Ma), are indicative of an ancient lower crustal magma source. The overlapping zircon ages but distinct composition (74–80 vs. 60–66 wt% SiO2) and crystallinity (~5 vs. 30–40 vol%) between the crystal-poor rhyolite and dacite porphyry can be explained by melt extraction from the crystal mush zone. The lack of resorption texture and large age interval (~8 Myr) between the zircon core (ca. 228 Ma) and rim (ca. 220 Ma) indicate that the crystal mush remained partially molten for protracted duration. The melt storage condition was constrained by Ti-in-zircon thermometry to be low-temperature, around the granitic solidus (650–700 °C). The crystal-rich nature and slightly negative Eu anomalies suggest that the dacite porphyry was formed in the roof zone (solidification front) of the crystal mush. The occurrence of resorbed zircon core (ca. 220 Ma) overgrown by much younger rim (ca. 213 Ma) for the crystal-rich rhyodacite demonstrates long-term rejuvenation of a semi-solidified crystal mush. The similar zircon core-rim Ti contents and Th/U, Yb/Gd, Zr/Hf ratios preclude that the crystal mush was rejuvenated by hotter, more mafic magmas. However, the widespread fluorite-rich magmatic-hydrothermal veining, combined with the reduced nature (indicated by the zircon Ce4+/Ce3+ drop from core to rim) of the crystal-rich rhyodacite, imply that the volatile-rich fluid influx may have been important in reactivating the crystal-mush for later crystal-rich rhyodacite eruption.