Highly refractory harzburgites from the Moa-Baracoa Ophiolitic Massif, Eastern Cuba: Insights into forearc mantle melt-rock interactions

Abstract

Melt-rock interactions are important for understanding the long-term evolution of the Earth and for addressing various mantle compositions. However, details of the mechanisms and evolution of interactions in supra-subduction zones remain poorly understood. This study presents petrographic and geochemical analyses of the harzburgites from the Moa-Baracoa Ophiolitic Massif, eastern Cuba, to constrain the melt-rock interaction within the lithospheric mantle. The harzburgites preserve pristine microstructures by recording three-stage petrogenetic processes: incongruent melting of orthopyroxene (Opx) in stage I; crystallization of interstitial clinopyroxene (Cpx), spinel (Sp), and base-metal sulfides (BMS) at the expense of Opx in stage II; and re-equilibration characterized by Sp-Cpx symplectite in stage III. The harzburgite compositions are highly refractory with Al2O3 (0.21–0.81 wt%) and TiO2 (~0.04 wt%) and show “U”-shaped rare earth element (REE) patterns and significant LREE and large ion lithophile element (LILE) enrichments. The olivines have an E-type fabric and a narrow range of compositions (Fo = 90.6–91.6). Interstitial Cpx has moderate Mg# (92.1–94.9) and Al2O3 (1.06–2.88 wt%) and extremely low TiO2 contents (<0.03 wt%). Opx and interstitial Cpx have low ∑REE contents (0.09–0.14 ppm and 0.24–0.49 ppm, respectively) and are depleted in LREE and variably enriched in LILE. Spinels possess similar Cr# values (56.7–64.8) but variable Mg# values (37.2–58.4). BMS is primarily dominated by pentlandite, which is partly or completely replaced by magnetite and/or heazlewoodite. Modeling of whole-rock HREE variations suggests that the harzburgites experienced >25% partial melting. Widespread interstitial Cpx and BMS, elevated whole-rock Cu and Pt, and variable LILE enrichment in Opx and Cpx indicate the interaction of the refractory harzburgites with migrating sulfur-saturated, low-silica melts. Melts equilibrated with interstitial Cpx appear to have an affinity for FAB. Mineralogical, chemical, and olivine fabric evidence suggests that the Moa-Baracoa harzburgites originated in a nascent forearc mantle. The harzburgites experienced partial melting facilitated by migrating low-silica melts in stage I. Subsequent interactions with FAB melts at relatively high temperatures precipitated Sp, Cpx, and BMS in stage II. Finally, the re-equilibration of high-temperature/pressure pyroxenes produced Sp-Cpx symplectite as the harzburgites were rapidly emplaced into the lithospheric mantle during subduction initiation.