Genesis of Guatemala jadeitite and related fluid characteristics: an insight from zircon

Zircons from one jadeitite sample, collected from north of the Motagua fault, Guatemala, were separated for SHRIMP U-Pb dating and trace element analyses. The sub- to anhedral crystal form, lack of typical magmatic oscillatory zone, the presence of fluid and albite/quartz/jadeite inclusions, and the low Th/U ratios (<0.005) indicate that these zircons are of hydrothermal origin. The U-Pb concordia intercept age of these zircons, 95.1 ± 3.6 Ma, is much older than the reported 40Ar/39Ar phengite age (i.e., 77-65 Ma) of jadeitites and related rocks from the same area. The age of 95 Ma is considered as the time of jadeitite formation and the age of 77-65 Ma, time of cooling. The formation temperature of the jadeitite would therefore be higher than the phengite 40Ar/39Ar blocking temperature and probably in the range of 400-450oC. The presence of quartz inclusions in zircons also indicates that the maximum pressure condition for jadeitite formation might be 13-14 kbar. It is further concluded that metasomatic replacement and solution precipitation are two end-member mechanisms for jadeitite genesis. Both the jadeitite sample and separated zircons have very low REE contents, ~ 1 ppm and 0.5-42 ppm, respectively. The jadeitite shows a flat and slightly concave REE pattern with Eu/Eu* = 1.24. Zircons give HREE enriched patterns and 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 responsible fluid might have evolved with time becoming more reducing and more dominated by albite 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. The fluid was most probably derived from serpentinization processes where (partially serpentinized) peridotites contain olivine as the major primary mineral (left). Other requirements for jadeitite formation include (1) albite-bearing felsic protoliths occurring as tectonic inclusions within peridotites and survived from rodingitization during initial serpentinization, and (2) fluid channels as well as pulses of in/out-flux fluid. These preconditions in subduction zones may make the jadeitite genesis scarce.