JADEITITES FROM THE PAIRED HIGH-PRESSURE BELTS OF THE MOTAGUA FAULT ZONE, GUATEMALA: IMPLICATIONS FOR DIFFERENT P-T CONDITIONS

Jadeitites are rare, with only ~12 locations world-wide, associated with serpentinites and other HP-LT lithologies preserved in suture zones.  Textures, phase assemblages and tectonic contexts suggest they crystallize from hydrous subduction-related fluids in fractured serpentinizing peridotite.  They represent a litho-tectonic and fluid-migration record of their hosting collisional belts.  In Guatemala jadeitites are preserved from two distinct events juxtaposed across the Motagua Fault (MF), boundary between the Maya (North American plate) and Chortís blocks (Caribbean plate).  Ar-Ar geochronology on mica in jadeitites yields 113–125 Ma ages south of the fault and 65–77 Ma to the north.  A study of ~8 jadeitite-bearing areas north of the fault, over a ~65 km W-E distance, and ~3 to the south, spanning ~15 km, shows northern and the southern jadeitites manifest differences in the conditions that produced the two belts.

While jadeitites are variable and potentially cryptic, the majority contain jadeite crystals with clusters of inclusions that are characteristic to their origin (i.e., their belt and formation age).  Jadeite from the north typically contains analcime (Anl), albite, omphacite, and abundant two-phase fluid core-cluster inclusions.  Mica (paragonite, phengite, and preiswerkite), zoisite, and zircon inclusions are common.  Jadeitites from the south usually contain quartz and omphacite and fewer fluid inclusions.  Pectolite, phengite, apatite, titanite overgrowths on rutile and zircon are also observed.  Northern jadeitites record maximum P below the reaction Ab=Jd + Qtz (<10.5 kbar at 300°C) and into the Anl stability field (<6.6 kbar) plus later fluid-influenced alteration.  In the North, eclogites are rare and primarily recognized by omphacite in garnet of grt-zoisite amphibolites.  Southern jadeitites record P above that reaction with less alteration which can be in the Jd+Qtz field.  Lawsonite eclogites (20–25 kb at 400–550 °C) abound in a southern area and are reasonably well preserved.

The northern jadeitites document lower pressure formation and considerable aqueous alteration of higher P rocks, whereas the southern ones records higher-P formation with less alteration.  Moreover, the inclusions can be used to distinguish between jadeitites (or jade) from north and south of the MF.