Geochemistry of Amphibolites from the Nellore Schist Belt, Andhra Pradesh, India: An Example of Back-Arc Basin Low-K Tholeiitic Magmatism

Geochemistry of ortho-amphibolites from the Degapudi-Kalichedu area of the Nellore Schist Belt (NSB), an Archaean greenstone belt in southeastern India, has indicated their affinity to a low-K tholeiitic magma derived from an LIL-enriched, apatite-bearing mantle by about 5-20% partial melting, followed by fractional crystallization. Amphibolites range from high-Mg-type early differentiates (massive amphibolite and actinolite schist) to low-Mg-type middle-stage differentiates (schistose amphibolite, hornblende schist and related gametiferous hornblende schist). During and subsequent to the generation of their parental magma, the early crystallizing mineral phases are oli vine-clinopyroxene-plagioclase-magnetite-apatite. A back-arc or marginal basin palaeotectonic setting is indicated for NSB by geochemistry of these amphibolites, with precursors of high-Mg-type emplaced in a narrow basin associated with mature subduction zone system and of low-Mg-type in a wider basin associated with youthful subduction zone system.