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To understand the generation of ion-adsorption REE deposits it is important to clarify the mechanism of REE enrichment in granites. We investigated Late Permian to Triassic granites from South China to investigate these issues. Ore-related granites with high REE contents (192–467 ppm) intruded in the Late Permian have high K2O and Zr + Ce + Nb + Y contents, high Ga/Al and K/Na ratios and low CaO contents, and are classified as aluminous A-type granite derived from melting of felsic igneous rocks in a high-temperature extensional setting. Ore-unrelated Triassic Pingtian granite with relatively low REE contents (81.7–127 ppm) is peraluminous with occurrence of muscovite, indicating an S-type affinity. Its high Rb/Sr and low CaO/Na2O ratios indicate magma origin from a metapelitic source related to a compressional setting owing to continental collision. In extensional settings in South China, the high-temperature conditions promoting melting of REE-rich accessory minerals and F-rich conditions improving the solubility of REE in the melts could be the reasons for the initial REE enrichment in these granites. Given that parent rocks for ion-adsorption REE deposits are all generated in extensional settings, it is considered that multiple tectonic stages and long-term extensional setting in South China contributed to generation of the specific ion-adsorption REE deposits in South China. [ABSTRACT FROM AUTHOR]

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.