Prolonged (∼140 Myr) Evolution of a Crystal Mush Reservoir in the Chinese Altai Mantle Slope Zone: Insights from the Jiangjunshan Blue-Green Microcline-Bearing Granite

Abstract

The petrogenesis of highly evolved, rare-metal-rich granites is globally contested between models of extreme fractional crystallization of a single magma and multi-stage crustal reworking. One key criterion for distinguishing between these models is the temporal structure of the magmatic record: the former is expected to produce a relatively restricted crystallization-age range, whereas the latter may generate multiple, temporally separated age populations spanning a much longer interval. The emplacement age of the Jiangjunshan blue-green microcline-bearing granite has long been disputed, with previously reported Permian and Jurassic ages differing by more than 100 Myr, making it a valuable case study for testing these contrasting models.

Integrated zircon and monazite U–Pb geochronology reveals three temporally separated magmatic episodes at 290–273 Ma (Permian), 231–200 Ma (Triassic), and ∼150 Ma (Jurassic), spanning approximately 140 Myr. Despite its limited exposure, the Jiangjunshan pluton preserves a nearly complete record of Permian–Jurassic magmatism in the Chinese Altai, providing a remarkably condensed archive of regional magmatic evolution. This protracted, multi-stage geochronological record indicates episodic magma generation and reactivation of the magmatic system. To reconcile this protracted long-duration record with thermomechanical principles, we propose a novel, generalizable petrogenetic framework. We interpret the Jiangjunshan body not as a series of discrete intrusions, but as the product of a long-lived, crystal mush reservoir that was rooted in a lithospheric-scale mantle slope zone. This deep-seated tectonic discontinuity provided long-lived favourable structural conditions, enabling the periodic influx of heat and fluids from mantle/crustal sources during regional tectonic events. These influxes repeatedly reactivated the near-solidus crystal mush, driving localized melt extraction, pervasive fluid-rock interaction, and episodic mineral re-equilibration—processes recorded by the complex zircon textures and age domains.

Zircon Hf isotopes (εHf(t) = –19.6 to +11.0) demonstrate pulsed open-system behavior, with each reactivation episode incorporating variable mixtures of juvenile material and ancient crust, including remobilized components from the mush itself. The Jiangjunshan granites exhibit extreme geochemical signatures, including pronounced M-type REE tetrad effects, exceptionally low K/Rb (36.7–97.6) and Zr/Hf (5.6–6.3) ratios, and high F concentrations. These whole-rock features indicate intense fractionation and fluid-mediated exchange within this stagnant, volatile-rich reservoir.

Our mantle slope zone-hosted crystal mush model resolves the thermal paradox of prolonged magmatic histories and provides a universal mechanism to explain extreme geochemical fractionation and rare-metal enrichment in highly evolved granites worldwide. This work underscores the fundamental role of long-lived lithospheric discontinuities in localizing, sustaining, and chemically refining continental magmatic systems.