Constraints of REE and trace elements of high-pressure-veins and host rocks in western Tianshan on origin of deep fluids in paleosubduction zones

1　INTRODUCTIONFluids in the deep subduction zones play animportant role in the process of crust mantle inter action. This has been proved by a large number ofstudies on the geochemistry of island arc volcanicrocks[1 4]. Compared with mid ocean ridge basalts(MORB), island arc basalts (IAB) are enriched inlarge ion lithophile elements (LILE), such as Ba,Rb, Cs, and depleted in high field strength ele ments, such as Nb, Ta, Ti, and Zr. Generally,this is att…     

Fluid flow on centimeter-scale in deep paleosubduction zones in western Tianshan, China: Evidence from high-pressure veins

High-pressure（HP） veins were extensively developed in western Tianshan high-pressure（HP） metamorphic belt. The HP vein and host-rocks were analyzed by electronic microprobe to trace the origin of vein-forming fluids. Analytical data show that the immediately adjacent host-rocks of the studied HP vein are eclogites and gradually turned into blueschist as the distance from the veins increases, which indicates that the vein-forming fluid was derived from adjacent host-rocks; the boundaries between the vein and the host-rocks are sharp, which indicates that the fracture of the host-rocks is brittle during the vein-forming process. It is suggested that this type of HP veins is precipitated from the liquid formed by the dehydration of the host-rocks during the prograde metamorphism from blueschist to eclogite facies, which results in hydrofracturing of the rocks and provides the space for the vein to precipitate. The width of the eclogite-facies host-rocks is usually 1-2 cm, which provides the direct evidence that the fluid flow is on centimeter-scale.     

Geochemical characteristics and genesis of volcanic and sub-volcanic rocks from porphyrite-type iron deposits in Ningwu metallogenic province,eastern China： Constraints from elements

Ningwu porphyrite-type iron deposits are located in Ningwu Mesozoic volcanic basin,which belongs to the middle and lower reaches of the Yangtze River metallogenic province.The volcanic rocks can be divided into Longwangshan,Dawangshan,Gushan and Niangniangshan Formations from early to late.All these volcanic rocks are rich in alkali,and show the similar patterns in rare earth element（REE） distribution.However,some differences can be found in the trace elements and REE patterns.The study of petrology and REE geochemical characteristics shows that these rocks are derived from the underplating of the lithospheric mantle and are contaminated by crustal materials,undergo AFC process during the magmatic evolution.