Indosinian magmatism and rare metal mineralization in East Tianshan orogenic belt: An example study of Jingerquan Li-Be-Nb-Ta pegmatite deposit

摘要

The East Tianshan orogenic belt, a part of the Central Asian Orogenic Belt like Altai orogenic belt, has shown great potential for rare metal mineralization. Jingerquan Li-Be-Nb-Ta deposit is the largest pegmatite deposit in this region. Muscovite granite, quartz monzonite and gabbro have developed in the Jingerquan Li-Be-Nb-Ta pegmatite deposit in the eastern part of East Tianshan orogenic belt. Based on the content of elemental Li and the presence of Li-bearing minerals, we recognize two sets of intrusive assemblages, namely, class I, including muscovite granite and quartz monzonite, which are rich in Li with presence of spodumene and/or lepidolite, and class H mainly gabbro, which are poor in Li and absent of Li-bearing minerals. Class I rocks are rich in Si, Al, Na, K, talc-alkaline to high-K talc-alkaline series, and poor in Ca, P, Fe, and Mg, with A/CNK >= 1.1, which are similar to S-type peraluminous granites. Class I rocks are characterized by Rb, Ta, Nb, and Hf enrichments and significant Ti, Ba, and Sr depletions. The chondrite-normalized rare earth element (REE) patterns exhibit significant Eu anomalies (delta Eu = 0.01-0.20), "tetrad effect," and increased fractionation of rare earth elements with relatively low FREE. Class H rocks are calc-alkaline rocks that are rich in Ca, Al, and P and poor in Si, K, with A/CNK < 1; they are characterized by Ba and Sr enrichments and Th, Nb, and Ta depletions. The chondritenormalized REE patterns exhibit no significant Eu anomalies (delta Eu = 0.97-1.40) and a smooth curve that slopes to the right. The monazite SIMS U-Th-Pb ages of class I (246-252.9 Ma) and the zircon LA-ICP-MS U-Pb ages of class II (247-250.7 Ma) indicate that both types of intrusions were formed in intraplate environment in Indosinian period. The epsilon Hf-(t) values (12.0-15.2) and the depleted mantle model ages T-DM (279-411 Ma) of class II rocks indicate that the magmas were mainly derived from partial melting of the depleted mantle. In the intraplate extensional setting, the mantle magmas represented by class II basaltic magmas intraplated into the crust and induced partial melting of the middle and upper crust to produce class I S-type granitic magmas. After that, the granitic magmas underwent continuous differential evolution and volatile enrichment, and finally form Li-Be-Nb-Ta mineralized granitic pegmatite. The geological setting, diagenetic and metallogenic chronology, and mineral assemblages of the East Tianshan orogenic belt and the Altai orogenic belt can be compared, and rare metal mineralization of both belts were controlled by the Indosinian tectonic-magmatic event.