SEDIMENT SIGNATURES IN JADEITITE AND RELATED HP/LT CRYSTALLIZATIONS ALONG THE MOTAGUA FAULT, GUATEMALA: EVIDENCE FROM N AND S GEOCHEMISTRY

Jadeitite, albitite and altered eclogite, high-P/low-T (HP/LT) rocks, occur in two adjacent serpentinite mélange belts straddling the Motagua fault zone (MFZ) in Guatemala.  South of the MFZ eclogite alteration and jadeitite record conditions of T ≤ 300 °C at P = 1.2 to ~2 GPa, while north of the MFZ jadeitite and albitite record T = 330-450 °C at P = 0.5 to 1.2 GPa.  Studies of jadeitite and albitite support an origin by precipitation from fluids coming off a subduction channel.  Guatemalan eclogites and jadeitites, in general, display large enrichments in the most fluid-mobile elements (Cs, Tl, Rb, Ba, K, Pb, Sn, Li), moderate enrichments of U, Th, Be and LREE and little to no enrichment of HFSE and HREE.  Such trace element enrichments also bear similarities to the average value of the Global Subducted Sediment compilation (Plank & Langmuir 1998) with respect to Th, Be, Ba and Li.  Thus, altered eclogites and jadeitites have some chemical resemblance to subductable sediment.  Devolatization of metasediments in subduction systems evolve water-rich C-O-H-S-N fluids, which should be recorded in these HP/LT rocks.  Thus, we report results on N content in mica and S isotopes from sulfide from some HP/LT MFZ rocks.

FT-IR measurements of NH₄ content in mica varies with mica species, rock-type, and side of the MFZ.  Paragonite from jadeitite and albitite contains nil to 100 ppm NH₄.  Phengite in albitite (N of MFZ) contains 210 to 350 ppm, phengite from jadeitite (S of MFZ) yields 890-1250 ppm, and vein phengite from altered eclogite (S of MFZ) has 600-750 ppm.  The NH₄ content varies inversely with T (= peak) and positively with P for the conditions of mica crystallization.  Determination of δ¹⁵N is in process.

Pyrrhotite in metabasite (N of MFZ) has δ³⁴S values ranging from –2.8‰ to –7.9 ‰.  These values are nearly within ± 3‰ of those for the upper mantle (δ³⁴S =0 ±3‰), which suggests that S may be partly inherited from the sulfides from mafic ocean crust with average δ³⁴S values similar to those observed in fresh oceanic basaltic glasses (0.3 ± 0.5‰).  Pyrite in eclogites and omphacite-taramite rocks yields δ³⁴S from –6.4‰ to -13.2‰, which most likely reflects a sedimentary source of S possibly mixed with that remobilized from metabasites.

N contents in micas and negative δ ³⁴S values of pyrite support sourcing of N and S from sediment devolatization fluid.