Episodic Fluid Action in Chinese Southwestern Tianshan HP/UHP Metamorphic Belt: Evidence from U–Pb Dating of Zircon in Vein and Host Eclogite

• Published in: Minerals (Basel) 2019-12, Vol.9 (12), p.727
• Main Authors: Chen, Zhen-Yu, Zhang, Li-Fei, Lü, Zeng, Du, Jin-Xue
• Format: Article
• Language: English
• Subjects: Ablation; Amphibolite facies; Amphibolites; Coexistence; Crystallization; Crystals; Dating; Eclogite; Field strength; Fluid dynamics; Fluid flow; Garnet; Geology; Greenschist facies; Hydration; Isotopes; Lead; Magma; Metamorphic rocks; Metamorphism; Metamorphism (geology); Mineral assemblages; Mineral inclusions; Mineral resources; Morphology; Open systems; Quartz; Radiometric dating; Rare earth elements; Recrystallization; Rims; Rocks; Rutile; Subduction; Subduction (geology); Subduction zones; Trace elements; Veins (geology); Zircon; Zoning
• ISSN: 2075-163X; 2075-163X
• DOI: 10.3390/min9120727

Fluid plays a key role in metamorphism and magmatism in subduction zones. Veins in high-pressure (HP) to ultrahigh-pressure (UHP) rocks are the products of fluid–rock interactions and can thus provide important constraints on fluid processes in subduction zones. In this study, we present an integrated study of zircon in situ U–Pb dating, trace element and mineral inclusion analysis for a complex vein and its host eclogite in the southwestern Tianshan UHP terrane, aiming to decipher the episodic fluid action during slab subduction and exhumation. Both zircon in eclogite and vein have euhedral, prismatic morphology similar to those crystallized from metamorphic fluid. Zircon in eclogite shows core–rim structures with distinct bounds and mineral inclusions. Zircon in the vein shows sector zoning or weak zoning, with bright rims around most zircon grains, which suggests recrystallization of the zircon crystals after their formation and multiple evolution of the vein. Eclogite zircon rims yield a weighted mean of 311 ± 3 Ma and cores yield a range from 413 ± 4 to 2326 ± 18 Ma, respectively. Vein zircon yields four groups of age (~355 Ma, ~337 Ma, ~315 Ma, and ~283 Ma), which date four episodes of fluid flow involving zircon growth. The first two groups of age may represent prograde epidote–amphibolite facies and amphibolite/blueschist facies metamorphism stage, respectively. The third group is similar to that of eclogite zircon rims, which is thought to date the eclogitic facie metamorphism (320–305 Ma), and the fourth group dates a later retrograde metamorphism after greenschist facies. The vein-forming fluid system was supposed to be an open system indicated by trace element of vein zircon and mineral assemblage of the vein. The coexistence of rutile, zircon, and garnet in prograde vein and the heavy rare earth elements (HREE) enrichment characteristic of vein zircon suggest that the vein-forming fluid are enriched in high field strength elements (HFSE) and HREE, and such fluid could be formed under low P–T conditions.