Magma Recharge Induces Contrasting Rejuvenation of Two Distinct Magmatic Reservoirs Beneath Xiaoxiong Caldera, South China

Abstract Caldera-forming eruptions commonly involve the simultaneous eruption of multiple silicic magmas with heterogeneous compositions. The physical conditions under which these different silicic magmas are generated and stored and what triggers their eruption are essential questions in understanding the ability of such magma reservoirs to become rejuvenated and erupted. In our study, we investigate the petrogenetic relationships between plutonic and volcanic rocks from the Xiaoxiong Caldera in Southern China. Our results show that two compositionally distinct magma reservoirs were emplaced contemporaneously at 87.3 ± 0.3 Ma beneath the caldera but resided at different crustal levels. Low-silica rhyolite, trachyte, and porphyritic quartz monzonite were derived from a deeper reservoir located at ~19–30 km, whereas high-silica rhyolite and porphyritic granite were extracted from a shallower reservoir at ~7–9 km depth. Magma recharge induced thermal rejuvenation of these two pre-existing, cold, and non-eruptive reservoirs, promoting crystal–melt segregation and ultimately triggering the eruption of compositionally diverse silicic magmas. When the parental magmas of low-silica rhyolite, trachyte, and porphyritic quartz monzonite are extracted from a deep reservoir in the middle to lower crust, the system remains undersaturated in both water and zircon. Upon ascent to the shallow crust, these magmas subsequently reach saturation with respect to water and zircon. In contrast, prior to the extraction of high-silica rhyolite from the upper crustal reservoir, both the high-silica rhyolite and porphyritic granite had reached saturation in zircon and water. Following extraction, the high-silica rhyolite experienced significant degassing. Our results reveal that melts stored at different depths beneath the same volcanic caldera can remain in a cold, quiescent state for extended periods. Subsequent late-stage reheating rejuvenates these reservoirs through distinct mechanisms, leading to the generation of magmas with divergent geochemical features and evolutionary histories prior to eruption.