Ophiolitic Chromitites from the Andriamena Greenstone Belt, Madagascar: Possible Evidence for mid-Archean Plate Tectonics

Abstract:

Determining the time of the onset of plate tectonics is critical to understanding the geodynamic processes that controlled the evolution of the early Earth. The near absence of Archean ophiolites, as defined by the presence of a residual ultramafic section, has been considered to be one of the primary arguments against Archean plate tectonics. The Andriamena greenstone belt of Madagascar contains massive chromitite bodies consisting of about 90% chromite and about 10% gangue minerals, mostly secondary (talc, green amphibole, orthopyroxene, Ca and Mg carbonate). Numerous observations argue in favor of an ophiolitic origin for these chromitites, including the high Cr# (0.67-0.74), coupled with relatively high Mg# (0.6-0.78) of the constituent chromite. In addition, these phases display extremely low TiO₂ contents (<0.25%), which are also characteristic of ophiolites and possibly suggestive of an arc environment. Though in many places the chromitite is in tectonic contact with a variety of unrelated igneous lithologies, remnants of apparently cogenetic ultramafic rock types, including dunites, harzburgites, and some pyroxenites are sometimes immediately juxtaposed with the chromitite. The very high Fo content of the olivine in the associated dunite, as high as 0.95, also attests to an ophiolitic provenance.

Platinum group element (PGE) and ¹⁸⁷Os/¹⁸⁸Os analyses were performed on several chromitite samples. Chondrite normalized PGE spectra display marked depletions in PPGE relative to IPGE, with (Pt/Ir)_N ranging from ~0 to 0.09, though Pd contents are somewhat less depleted than those of Pt. The observed PPGE depletion is another feature characteristic of ophiolitic chromitites. The IPGE enrichment is consistent with the presence of laurite microinclusions in the chromite revealed by SEM. Os isotopic compositions are tightly clustered, with ¹⁸⁷Os/¹⁸⁸Os ranging from 0.1057 to 0.1059, corresponding to T_RD model ages of ~ 3.2 Ga, assuming primitive upper mantle parameters. ¹⁸⁶Os/¹⁸⁸Os measurements are in progress.

If the ophiolitic nature of the chromitites is confirmed, our results might imply that geodynamic mechanisms similar to present-day plate tectonic processes may already have been active in the mid-Archean Earth.