Trace element features of hydrothermal and inherited igneous zircon grains in mantle wedge environment: A case study from the Myanmar jadeitite

摘要

Jadeitites are considered to crystallise in ultramafic rocks in the subduction channel presumably from the overlying mantle wedge, and therefore zircons from these rocks provide important insights into mantle wedge processes. Here we investigate hydrothermal zircon (Group II) formed within a subduction zone and compare these with the igneous zircon cores (Group I) from the Myanmar jadeitite. Previous U-Pb studies reported ages of Groups I and II zircons as similar to 163 Ma, and similar to 147 Ma respectively, and both show isotope signature of the depleted mantle. Group I zircons have much higher total concentrations of rare earth elements (REEs) (500-1945 ppm) than those of Group II zircon (112-307 ppm), and contains relatively higher abundance of Y, Nb, Ta, Ti, Th and U with higher (Sm/La)(N) ratios (25.3-501) and Ce-anomalies (8.04-140) but lower (Yb/Gd)N ratios (9.76-57.0) than those of the Group II ( (Sm/La)(N) ratios = 2.12-32.2, Ce-anomalies = 1.63-19.6, (Yb/Gd)N ratios = 44.8-142). Hf concentrations are broadly similar in both Groups. The Group I zircons are considered to be magmatic and crystallised from H2O-rich basaltic melt at relatively high pressure in the mantle wedge, whereas the Group II zircon overgrowth took place through recrystallisation and precipitation with distinct dissolution of the Group I zircons. Variation in the concentration of trace elements in zircons from Groups I to II in the mantle wedge is related to an intra-oceanic subduction system in the presence of Na-rich hydrothermal fluids under high-pressure and low-temperature. The Ti-in-zircon thermometer yield a mean crystallisation temperature of 742 +/- 141 degrees C for Group I zircons, whereas the Group II zircons yield 339 +/- 33 degrees C. The two groups of zircons also provide insights into the probable protolith involved in formation of the Myanmar jadeitite. (C) 2016 Elsevier B.V. All rights reserved.