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Exploitable or potentially exploitable deposits of critical metals, such as rare-earth (REE) and high-field-strength elements (HFSE), are commonly associated with alkaline or peralkaline igneous rocks. However, the origin, transport and concentration of these metals in peralkaline systems remains poorly understood. This study presents the results of a mineralogical and geochemical investigation of the Na-metasomatism of alkali amphiboles from a barren peralkaline granite pluton in NE China, to assess the remobilization and redistribution of REE and HFSE during magmatic-hydrothermal evolution. Alkali amphiboles from the peralkaline granites show evolutionary trends from calcic through sodic-calcic to sodic compositions, with increasing REE and HFSE concentrations as a function of increasing Na-index (Na#, defined as molar Na/(Na+Ca) ratios). The Na-amphiboles (i.e., arfvedsonite) can be subsequently altered, or breakdown, to form Na-clinopyroxene (i.e., aegirine) during late- or post-magmatic alteration. Representative compositions analyzed by in-situ LA-ICPMS show that the alkali amphiboles have high and variable REE (1295–2218 ppm) and HFSE (4194–16,862 ppm) contents, suggesting that these critical metals can be scavenged by alkali amphiboles. Compared to amphiboles, the early replacement aegirine (Aeg-I, Na# = 0.91–0.94) has notably lower REE (577–797) and HFSE (4351–5621) contents. In contrast, the late hydrothermal aegirine (Aeg-II, Na# = 0.92–0.96) has significantly lower REE (127–205 ppm) and HFSE (6.43–72.2 ppm) contents. Given that the increasing Na#from alkali amphibole to aegirine likely resulted from Na-metasomatism, a scavenging-release model can explain the remobilization of REE and HFSE in peralkaline granitic systems. The scavenging and release of REE and HFSE by alkali amphiboles during Na-metasomatism provides key insights into the genesis of globally significant REE and HFSE deposits. The Na-index of alkali amphibole-aegirine might be useful as a geochemical indicator in the exploration for these critical-metals.

The composition of Kuerti mafic rocks in the Altay Mountains in northwest China ranges from highly geochemically depleted, with very low La, Ta and Nb and high ɛ Nd(t) values, to slightly enriched, arc lava-like composition. They display flat to light rare earth element (REE)-depleted patterns and have variable depletions in high field-strength elements (HFSE). These mafic rocks were most probably derived from a variably depleted mantle source containing a subduction component beneath an ancient intra-oceanic backarc basin. Together with the slightly older arc volcanic rocks in the Altay region, the Kuerti mafic rocks display generally positive correlations of their key elemental ratios (e.g., Th/Nb, La/Yb and Th/Yb). These indicate that the more mid-ocean ridge basalt (MORB) component was contained in these magmas, the less arc component was present in their mantle source. Therefore, we propose a two-stage melting evolution model to interpret the compositional evolution of the Kuerti mafic rocks and associated arc volcanic rocks. First, arc basaltic melts were extracted from the hydrated arc mantle wedge beneath Kuerti, leaving behind a mantle source that is variably depleted in incompatible trace elements. Then, mafic rocks were erupted during seafloor spreading in the Kuerti backarc basin from the upwelling asthenospheric mantle. The variably depleted mantle source produced mafic rocks with composition ranging from arc lava-like to more geochemically depleted than MORB. The recognition of Kuerti mafic rocks as backarc basin basalts (BABB) is consistent with the proposed tectonic model that an active backarc basin-island arc system along the paleo-Asian ocean margin was formed in the Altay region during Devonian-Early Carboniferous. New data further indicate that the final orogenic event in the Altay Mountains, i.e. the collision of the north and south continental plates in the region, most probably took place in Late Carboniferous and Permian.