Genesis of Guatemala jadeitite and related fluid characteristics: Insight from zircon

Abstract

Zircons from one jadeitite sample, collected from serpentinite mélange north of the Motagua fault, Guatemala, were separated for SHRIMP-RG U–Pb dating and trace element analyses. The sub- to anhedral crystal form, lack of typical magmatic oscillatory zoning, the presence of fluid and albite/quartz/jadeite inclusions, and the low Th/U ratios (< 0.005) indicate that these zircons would have precipitated from an aqueous solution. The U–Pb concordia intercept age of these zircons is 95.1 ± 3.6 Ma, much older than the reported 40Ar/ 39Ar phengite age (i.e., 77–65 Ma) of jadeitites and related rocks from the same area. The 95 Ma age is interpreted as the time of jadeitite formation, whereas the 77–65 Ma age reflects the time of cooling after isotopic resetting due to superimposed metamorphism at ∼ 70 Ma. The superimposed metamorphic temperature would therefore be higher than the phengite 40Ar/ 39Ar blocking temperature and probably at 400–450 °C. It is further concluded that “metasomatic replacement” and “vein precipitation” are two end-member mechanisms for jadeitite genesis. Both the whole-rock jadeitite and its zircons have very low REE contents, ∼ 1 ppm and 0.5–42 ppm, respectively. The whole-rock jadeitite shows a flat and slightly concave REE pattern and a positive Eu anomaly (Eu/Eu* = 1.24). Zircons are enriched in HREE and their REE patterns can be divided into two groups: one with negative Eu anomaly and one with positive Eu anomaly. The latter tends to have smaller positive Ce anomalies. The fluid from which jadeitite formed probably evolved over time, becoming more reducing and more dominated by plagioclase decomposition reactions. Alternatively, trace element compositions of zircons simply demonstrate complicated variations of fluid chemistry during jadeitite formation. A reducing fluid with high pH values capable of mobilizing Al, Na, Zr and Hf is inferred to be the media during jadeite/zircon formation. If jadeitite formation was related to serpentinization fluid, the ultramafics must contain olivine as the major primary phase subjected to serpentinization, yielding reducing fluids with high pH values. Two additional requirements are (1) the presence of feldspar/mica-bearing protolithic blocks survived from rodingitization in peridotites to provide Al and Na, and (2) the presence of fluid channels as well as pulses of in/out-flux fluid to trigger metasomatic reactions. These preconditions in subduction zones may contribute to the rare occurrences of jadeitite in global high-P metamorphic belts.