文摘
Ultramafic–mafic complexes that intruded into Archaean ortho- and paragneisses in the Andriamena region of north-central Madagascar fall within the domain of a 900-km-long belt consisting of gabbroic and granitoid plutons that has been interpreted as the roots of a Neoproterozoic (700–800 Ma) continental arc. This Andean-type arc is reported to extend from north-central Madagascar through the Seychelles into India. The ultramafic bodies ( 2 km × 1 km) are crudely zoned, with dunite forming the cores surrounded by harzburgite, lherzolite, pyroxenite, pegmatoidal pyroxenite and fine-grained gabbroic rocks on the margins of the intrusions. One sampled contact between an ultramafic entity and the basement is chilled and sharp, indicating an intrusive rather than a sheared contact; this argues against allochthonous emplacement as in ophiolite suites. The composition of this chilled basaltic margin is similar to volcanic lithologies from well-known continental Andean-type arcs. PGE data from dunites, harzburgites, lherzolites and websterites of the Lavatrafo body show enrichment in the Pd-group PGEs with significant Pt-enrichment and prominent negative Ru anomalies. [Ru/Ru*]N ranges from 0.037 to 0.31 with average [Ru/Ir]N = 0.17. These features are characteristic of zoned Alaskan–Uralian-type complexes ([Ru/Ru*]N = 0.029–0.15), which generally form the roots of volcanic arcs. Dunites, forming the cores of some intrusions, show enrichment in the LREE, are depleted with respect to chondrites (0.1× chondrite) and display negative Eu anomalies possibly caused by orthopyroxene fractionation. The Andriamena complexes are suggested to represent the roots or feeder conduits of a continental Andean-type arc and provide new evidence, in the form of ultramafic intrusions, for a convergent margin on the western edge of the Rodinia supercontinent. This tectonic setting is consistent with published data suggesting eastward subduction, on the eastern edge of the Mozambique Ocean, underneath the continental terrains of Madagascar, the Seychelles and NW India.
