CRYPTIC SODIC METABASITES (BLACK JADE) ASSOCIATED WITH JADEITITES FROM GUATEMALA: EVIDENCE FOR FLUID INTERACTIONS DRIVEN BY SUBDUCTION

The Guatemala Suture Zone (now a NOAM-CARIB boundary zone) is known for a pair of subduction-related serpentinite-matrix mélanges containing blocks of eclogites, jadeitites, and related rocks that record both high-P/low-T conditions and fluid-transport crystallization products. Among these rocks from the North Motagua Mélange (NMM) but not the South Motagua Mélange (SMM; in relation to the Motagua Fault) is a tough black lithology found with jadeitite, locally referred to as black jade. Varying from very fine-grained (~100 µm grain size) to coarse-grained, these rocks consist principally of an intergrowth of omphacite and ferro-taramite (or taramite) with lesser titanite ± epidote and late albite ± grossular. The mafic character of this rock suggests a basaltic or metabasaltic protolith; however, there is no textural evidence for pseudomorphism or relict protolith phases. In an attempt to evaluate possible protolith and chemical modification (metasomatism?) of black jade, whole-rock major and trace-element compositions have been measured for 10 samples from 7 localities, which are compared with data for 9 NMM eclogites, 6 NMM grt-amphibolites, 19 SMM eclogites and ranges for N-MORB and OIB.

Plots of ratios of trace elements with different mobilities have been made to compare possible basaltic protoliths with black jade. In a Th/Yb versus Ta/Yb plot, black jades largely plot in the calc-alkaline field, whereas eclogite from NMM largely plot in or closer to the tholeiitic or MORB-OIB array. In a comparison of Nb/La versus (La/Nb)_N, black jades plot toward the fertilized zone of continental arcs, compared to the other metabasites that scatter between MORB and more primitive arc compositions. Among major element plots, a Na₂O versus MgO is the most revealing; black jades form a group streaming away from MORB toward high Na₂O (to 12 wt%) and low MgO (to 2 wt%), with the metabasites lying closer to but not in the MORB field. The trend of fertilization in all the NMM metabasites is similar to that found in andesites; however, all were produced by fluid infiltration rather than melting. Thus, black jades probably formed by fluid metasomatism of a MORB-like metabasite by a fluid like that which crystallized jadeitite, but the texture may indicate direct crystallization from a fluid.