GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 196-11
Presentation Time: 10:45 AM


HARLOW, George E., Department of Earth and Planetary Sciences, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024, MARTIN, Celine, Lamont Doherty Earth Observatory, Columbia University, New York, NY 10027; Earth and Planetary Sciences, American Museum of Natural History, Central park West@ 79st Street, New York, NY 10024 and SORENSEN, Sorena S., Dept of Mineral Sciences, Smithsonian Institution, National Museum of Natural History, PO Box 37012, MRC 119, Washington, DC 20013-7012,

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.