DOI: 10.6084/m9.figshare.25425232
title: <b>Formation of LCT-type rare metal granites-pegmatites by</b><b>multi-stage </b><b>crystal-melt separation</b><b>from depleted lower crustal sources:</b><b>Insights from the Triassic Bili</b><b>utai plut</b><b>on, NE China</b>

description: Rare metals are strategic resources with important reserves in LCT (Li-Cs-Ta) family rare metal granites-pegmatites (RMGPs). However, the formation processes of LCT-type RMGPs remain enigmatic, although most previous researches invoke anatectic models with preenrichment metasedimentary sources to account for their specific geochemical features and rare metal mineralization. Here we present an integrated investigation for the newly-identified Biliutai rare metal granitic pluton in the Southern Great Xing’an Range Metallogenic Belt (SGXRMB), NE China. This pluton preserves various sheet-like rock units ranging from ore-barren I-type main body granitoids (i.e., biotite/two-mica granites and granodiorites with relatively depleted Sr-Nd-Hf-O isotopes) to LCT-type RMGPs (i.e., Be-Nb-Ta-richgarnet-muscovite granites-pegmatiteswith variable to enriched Sr-Nd-Hf-O isotopes). Zircon-columbite-group minerals (CGMs)-garnet U-Pb dating results show that both the LCT-type RMGPsand the main body granitoids were emplaced simultaneously at 243-233 Ma, indicating apossibleevolutionary sequence from a long-lived trans-crustal magmatic system. Detailed compositional/textural variationsin both whole rocks and minerals define complex magmatic differentiationprocesses involved two-stages of crystal-melt separationthat can account for the incremental growthand heterogeneity of the Biliutai pluton. The first-stage crystal-melt separationoccurred at a deep granitic mush reservoirinduced by the recharge of a hotter intermediate magma, producing the granodiorites and the biotite/two-mica granites represented as successive melt batches that recorded distinct degree of magma mixing/mingling. The second-stage crystal-melt separationand in situ fractional crystallization took place at the shallow emplacement level through diking infltrationcaused by magma degassing, generating the RMGP dikes composed of incompatible elements-richpore melts, plagioclase cumulatesand magmatic fluids, leaving the main body granitoids asa congealed crystal mush consisting of ‘cumulate crystals’ and trapped interstitial melts. The Biliutai pluton represents typical I-type granitic magmas that were derived from a rare metal-depleted sourcecomposed of pre-existing juvenile lower crust(i.e., Na-rich metabasaltic rocks) with subordinate (10-25%) ancient metamorphic basement. An intense interaction occurred between the Linxi Formation slates and the F-enriched magmatic fluids during the magmatic-hydrothermal processesof the RMGPs. We invoke that the post-source fluid-rock interaction is a neglected criticalpetrological process,which can induce not only the dramatic geochemical variations from I-type granitoids to LCT-type RMGPs, but also the enrichment of rare metalsin the metasedimentary rocks. Our results highlight that some of the LCT family RMGPsworldwide might not be of anatectic originand were unlikely derived from S-type granitic magmas, and the so-called sedimentary source preenrichment may be the result of rapid diffusion of rare metals rather than the cause leading formations of the RMGPs. We propose a petrogenetic model for the Biliutai RMGPs, in which a trans-crustal magmatic system experiences at least two stages of crystal-melt separationand fractional crystallization, ultimately generating rare metal-rich melts without involving a preenrichment source. This mechanism can well explain not only the generation of unanatectic-origin RMGPs but also why LCT-type pegmatites in many regions are nearly coeval with their inferred I-type source granitoids.