The influence of extensional strain rates, crustal melting and drainage on rare-metal mineralization

Rare metals, commonly located in extensional settings, linked with highly fractionated granites and pegmatites, require immense enrichment—ranging from thousands to hundreds of thousands—owing to their low Clarke values (10^-5–10^-6). The generation of pegmatite-forming melts, crucial for the creation of rare metal ores, necessitates multi-stage silicate liquid extraction from granitic melts in a supersaturated state. At the same time, partial melting introduces a low-viscosity liquid phase, substantially weakening crustal rocks. Concomitant melt drainage, however, may counterbalance these rheological changes. The resilience of the parental anatectic rocks hinges on the melting reaction's intersection with the prograde pressure-temperature path, the volume of melt generated, and the duration of melt retention before loss. Despite the central role of crustal melting and drainage in magma fractionation and rare-metal mineralization, their influence remains under-investigated, particularly the advection of heat and mass from the lower to the upper crust, which is strain-rate dependent. Our study addresses this gap, providing a quantitative analysis of the strain-rate dependent rare-metal mineralization resulting from crust's partial melting and melt loss. We employ field-based structural analysis, two-dimensional thermo-mechanical ASPECT modeling, and mineral equilibria and mixed rheology modeling of representative anatectic rock compositions. We reveal that the positioning of highly fractionated granites and pegmatites, the particle flow paths' morphology (finite strain and kinematic), and pressure-temperature-time paths are contingent on extensional strain rates and melt fraction. During prograde metamorphism, the parental anatectic rocks of rare metal elements undergo continuous but pulsed melt production. This episodic melt removal, occurring over a specific time interval (such as ~10 m.y. for the Mufushan–Lianyunshan ore field in South China), is instrumental for the extreme fractionation of rare-element granitic pegmatites. At a lower extensional strain rate (≤ 10^-15 s^-1), lithologies may maintain minimal strength or transiently strengthen (≤ 10 MPa) post-melt loss. This results in anatectic rocks oscillating between relative weakness and strength during episodic melt loss, thereby concentrating rare metals to potentially economically viable levels. Contrarily, at a higher extensional strain rate (≥ 10^-14 s^-1), both magma fractionation and repeated melt removal cycles are suppressed, rendering the generated pegmatites unlikely to achieve economic grades. Our findings suggest that the primary exhumation driver for highly fractionated granites and pegmatites is crustal isostatic compensation, not buoyancy from partial melting. Furthermore, the extensional strain rate may modulate the intensity of crust-mantle interactions and strain localization, effectively controlling the final emplacement of rare-metal pegmatites.