DOMAINS OF EQUILIBRATION AMONG LAWSONITE, GARNET, APATITE AND TITANITE IN ECLOGITE DURING FLUID-ROCK INTERACTIONS: INSIGHTS FROM GUATEMALA

Lawsonite eclogite blocks occur in sepentinite-matrix mélanges south of the Motagua fault in Guatemala. Although overall basaltic in composition and with isotopic signatures of MORB protoliths, Guatemalan lawsonite eclogites deviate from typical MORB or even IAB average compositions and relative element enrichments, in high-field-strength-(HFSE), large-ion- lithophile-(LILE), rare-earth-(REE), and heat-producing-elements (HPE) alike. The whole-rock data are not systematic; REE patterns range from light-REE- (LREE) rich to LREE-depleted, in the range of ~15 to 50 x chondrite La and 6 to 30x Lu, some with and others lacking positive Eu anomalies; K₂O ranges from 0.02 to 2 Wt% and Ba from 30 to 1400 ppm. To further examine these chemical variations, we analyzed 8 lawsonite eclogite blocks from several localities, along with their Ca-rich minerals and garnet grains by ICP-MS and LA-ICP-MS. Microstructures and previous work indicate that REE and Sr compositions may reflect the contributions of up to 4 generations of lawsonite within the eclogite samples, each with different REE and Sr-abundances. Some domains of equilibration of lawsonite and garnet are clearly related to deposition during fluid-rock interactions, and others were probably fluid-mediated. However, the eclogites are not the sum of only the trace-element rich minerals lawsonite and garnet. They also typically contain rutile and/or titanite, apatite, epidote-group minerals and, in some cases, zircon. SEM imagery of domainal microstructures suggest compositions may be controlled by a variety of “trace-element-rich” minerals. Evolving fluid-rock partitioning may help to explain the chemical evolution of the rocks, most particularly during retrogression. Although the results are complex, HFSE, LILE, REE and HPE have all likely been mobile at some point during the P-T history of these lawsonite eclogites. Interactions between crystallization and consumption of lawsonite, titanite, REE-rich epidote, apatite and zircon during more than one episode of fluid-rock interaction best explain the whole-rock compositions.