Dizhi lixue xuebao (Feb 2024)
Quartz deformation characteristics, deformation temperature, and their constraints on pegmatites of the 509 Daobanxi lithium deposit in the West Kunlun area, Xinjiang
Abstract
Objective The 509 Daobanxi deposit in the West Kunlun orogenic belt is a newly discovered large pegmatite-type lithium-polymetallic deposit in northwestern China. As a typical granite pegmatite lithium deposit in the region, the metallogenic characteristics and pegmatite evolution of the 509 Daobanxi deposits are of great significance for understanding the entire lithium-polymetallic mineralization process of the West Kunlun metallogenic belt. The granite pegmatites contain assemblages of plagioclase, spodumene, quartz, muscovite, etc., exhibiting strong mylonization and forming typical ductile deformation characteristics in the 509 Daobanxi deposit. Quartz, an essential mineral in granite pegmatite, is ideal for tracking pegmatite's mineralization process and studying the deformation behavior of continental rocks in long-term geological history. Methods To study the late-stage emplacement process of pegmatite evolution, comprehensive analyses were conducted on the quartz deformation structures measurements, fluid inclusion temperature, and quartz trace elements for the 509 Daobanxi granite pegmatites. Cathodoluminescence (CL) analysis of quartz in deformed granite pegmatite samples was performed to reveal the compositional zoning of Ti in quartz. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to analyze 64 points from samples Zk2707-9 and Zk1107-2. Results The minerals of spodumene and plagioclase in deformed pegmatites primarily show brittle fracturing characteristics, with the features of rigid body deformation and the muscovite presence of mica-fish. Meanwhile, the conspicuous feature is that quartz grains mainly develop dynamic recrystallization and contain subgranis. According to the microstructural characteristics of spodumene, plagioclase, and quartz, the deformation temperature of mylonitized granite pegmatite is 300~400℃. The CL images of quartz bands in the granite pegmatite samples have no apparent zoning, indicating that the Ti content reaches a relative equilibrium state in the quartz deformation stage. The LA-ICP-MS analysis shows quartz from the 509 Daobanxi granite pegmatites contains a lower concentration of Ti (1.03 ×10−6 to 7.67×10−6 and 1.04 ×10−6 to 6.75×10−6), suggesting relatively lower deformation temperatures. The Ti-in-quartz thermobarometry indicates quartz deformation temperatures ranging from 371 to 398°C and 351 to 377°C, respectively. The thermometric measurement shows that homogenization temperatures of the quartz fluid inclusions in pegmatite varied from 260℃ to 283℃, likely recording the temperature of the late stage of pegmatite evolution. Conclusion Comprehensive analysis shows that the 509 Daobanxi granite pegmatites underwent a period of intense ductile deformation during the emplacement process, with low temperature and high strain rate. The emplacement of pegmatite is a product of the rapid cooling process, and the grain size reduction caused by dynamic recrystallization (GBM) under high-stress and low-temperature conditions profoundly changed the rheological properties of pegmatite. The supercooling process from ~400℃ to ~260℃ (ΔT=140℃±), resulting in less rapid mineral crystalline new nuclei in pegmatites, is more conducive to the formation of coarse quartz and other mineral particles, forming the significant characteristics of granite pegmatites. [ Significance ]In fact, the emplacement process of granitic pegmatites remains a puzzle, and high-quality, accurate systematic work is needed to understand the evolution process and behavior of granite pegmatite. By studying the 509 Daobanxi granite pegmatites, we proposed that the pegmatite emplacement was a product of the rapid cooling process, and supercooling plays an essential role in pegmatite emplacement. Similar deformation characteristics are widely developed in the Tugeman lithium deposit in the Altyn Tagh area and the Jiajika lithium deposit in the western Sichuan. Although the current work is preliminary, our study provides some clues for exploring the emplacement process of granitic pegmatites.
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