SHRIMP U/Pb age of unusually large zircon in low-T jadeitite from the Osayama serpentinite melange, SW Japan

Porphyroblastic zircons up to 3 mm occur in nearly monomineralic jadeitite vein from the Osayama serpentinite melange, SW Japan. The Zrn-bearing jadeitite consists mainly of subhedral to anhedral jadeite (Jd<100), with minor amount of grossular, Zrn and rutile as primary phase, and shows variable degrees of retrogression. Anhedral Jd crystals show oscillatory zoning with Ca-rich bands (Jd87-91) at the rims, suggesting crystallization from an aqueous fluid. Zrn occurs as discrete euhedral to subhedral crystals up to 3 mm in size or as twinned crystals or aggregates in the matrix; Zrn contains tiny inclusions of Rt and rare Jd. Tiny Zrn also occurs as inclusions in a single Jd crystal. This evidence indicates synchronous growth of Zrn and Jd. Phase equilibria and the existing fluid-inclusion data constrain P-T conditions to P > 1.2 GPa at T < 350 ‡C for formation of the jadeitite. Th-U chemistry and U/Pb ages were determined using the SHRIMP-RG. The Zrn crystals show remarkable internal zoning textures including fine, euhedral growth layers with sharp contrasts in CL brightness, and are characterized by both low Th/U ratios (0.2-0.8) and low Th- and U-abundances (Th = 1-81 ppm; U = 6-149 ppm). Most U/Pb ages are concordant, with a weighted mean 206Pb/238U age of 475.4 ñ 9.8 Ma [MSWD = 5.2]. The investigated Zrns were directly crystallized from a hydrothermal alkaline fluid during the jadeitite-formation, and Zrn does not reach the closure T of the Zrn U/Pb system. Thus, it is interpreted the U/Pb ages, c. 475 Ma, as timing of crystallization of Zrn during HP-LT jadeitite-formation in serpentinites. The alkaline fluid with high Na- and Al-concentrations that resulted from subduction-zone metamorphism may have led to Zr concentration and the precipitation of Zrn in jadeitite. Consistent SHRIMP data for the blueschist-facies event suggest reliable dating of the LT Zrn in spite of its growth far below its closureT. The Zrn U/Pb age of this study provides likewise constraint for the timing of hydrothermal fluid-rock interaction related to serpentinization at HP-LT condition, establishing evidence for Ordovician subduction environments in SW Japan.