Constraints of Reservoir Pressure and H2O on Pre–Eruptive Melt Accumulation and Migration under Water–Rich Systems Based on the Volcanic–Plutonic Connection in the East Kunlun Orogen, Northern Tibet Plateau

Abstract

In recent years, the volcanic–plutonic relationship has been a contentious topic among researchers. Based on this issue, they have delved deeper into the constraints of magma reservoir pressure and H2O on pre–eruptive melt accumulation and migration. We selected granodiorite, tonalite, and rhyolite in the Dehailonggang volcanic–plutonic complex to investigate the volcanic–plutonic connection and constraints of reservoir pressure and H2O on the pre–eruptive melt accumulation and migration in water–rich systems. Granodiorite, tonalite, and rhyolite exhibit temporal–spatial similarities (247 Ma) and the same magmatic origin consisting of ca. 75 ~ 80% enriched mantle materials mixed with ca. 20 ~ 25% lower crustal materials. TIMA shows that both granodiorite and tonalite display a typical cumulate texture. The bulk–rock compositional complementary of the granodiorite, tonalite, and rhyolite, coupled with in situ geochemical signatures of feldspars and zircons, feldspar CSD, and rhyolite–MELTS modeling, indicate that 1) the granodiorite represents the crystal cumulate formed after crystal–melt phase separation of the original mush in the magma reservoir; 2) the interstitial melt of the original mush was extracted, migrated, and ultimately erupted as the rhyolite; 3) the tonalite serves as an intermediate product resulting from the phase separation from the original mush to the rhyolite. Rhyolite–MELTS modeling reveals that in water–rich environments, an increase in reservoir pressure (prior to reaching overpressure threshold) can lead to a reduction in melt viscosity. This, in turn, accelerates mechanical compaction and phase separation processes, ultimately shortening the pre–eruptive melt aggregation timescale. In contrast, it is noteworthy that H2O has a relatively minor influence on phase separation in such water–rich systems (> 4 wt. %). This study demonstrates the volcanic–plutonic genetic coupling and highlights the significance of reservoir pressure in controlling the dynamics of pre–eruptive melt within water–rich systems.