Geochronology, geochemistry and metallogenic implications of the Boziguo'er rare metal-bearing peralkaline granitic intrusion in South Tianshan, NW China

摘要

We report a rare metal (Nb, Ta and rare earth elements)-bearing granitic intrusion from the Boziguo'er region in the South Tianshan Collisional Belt (STCB), which is located in the southwestern margin of the Central Asian Orogenic Belt (CAOB). LA-ICP-MS zircon dating reveals that the intrusion was emplaced during Early Permian at 290.1 ±1.4 Ma. The granitoid is composed dominantly of porphyritic albite granite unit (unit I) and epigranular, medium-grained biotite-aegirine-arfvedsonite granite unit (unit II). Although the two units display different texture and mineral assemblages, there is no distinct boundary between them. The major Nb-, Ta- and rare earth element (REE)-hosting minerals in the intrusion are pyrochlore, xenotime, bastnasite and monazite. Geo-chemically, the intrusion is characterized by high contents of SiO_2 (63.88-81.84 wt.%), Na_2O (2.51-7.66 wt.%) and total alkalis (7.42-12.35 wt.%). The trace element patterns are characterized by Rb, Nb, Ta, Zr and Hf enrichment and markedly negative Ba, Sr, P, Eu and Ti anomalies. The rocks show negative ε_(Nd)(t) values (-4.4 to - 3.1) and Mesoproterozoic two-stage Nd model ages (1.3 to 1.4 Ga). Based on petrologic and geochemical features, we infer that, although the post-magmatic alteration (albitization and greisenization) has obliterated the original mineralogical constitution of the intrusion, this process was unlikely to have played a crucial role in the development of economic concentrations of Nb, Ta and REE. Alternately, the high concentration of these elements seems to be related to magmatic process. We propose that the crustal source region had been re-fertilized by supercritical fluids released from the asthenosphere-derived magmas and was initially enriched in fluorine, alkalis, (high-strength field elements) HFSEs and REEs. The original magma was formed by the moderate-degree partial melting in a crustal source, triggered by the underplating of the asthenosphere-derived magmas. During the magmatic differentiation, fluorine was largely preserved in the melt phase, even at the late magmatic stage because of the high emplacement depth. The presence of substantial fluorine in granitic melts would not only prolong the duration of the magmatic differentiation but can also act as a complexing agent with HFSEs and REEs. These complexes escape from being involved in the early stages of fractional crystallization, resulting in a HFSE- and REE-rich residual melt. Thus, the fluorine in the magmatic system was likely the key factor contributing to the rare metal enrichment. Tectonically, the Boziguo'er A-type granitic intrusion formed in an extensional setting, and therefore the onset of regional extension in the Chinese part of the SCTB is inferred to be earlier than 290 Ma.