An overview of adakite petrogenesis

Abstract

The term adakite was originally proposed to define silica-rich, high Sr/Y and La/Yb volcanic and plutonic rocks derived from melting of the basaltic portion of oceanic crust subducted beneath volcanic arcs. It was also initially believed that adakite only occurs in convergent margins where young and thus still hot oceanic slabs are being subducted, but later studies have proposed that it also occurs in other arc settings where unusual tectonic conditions can lower the solidus of older slabs. Currently, adakite covers a range of arc rocks ranging from pristine slab melt, to adakite-peridotite hybrid melt, to melt derived from peridotite metasomatized by slab melt. Adakite studies have generated some confusions because (1) the definition of adakite combines com- positional criteria with a genetic interpretation (melting of subducted basalt), (2) the definition is fairly broad and relies on chemistry as its distinguishing characteristic, (3) the use of high pressure melting experiment results on wet basalts as unequivocal proofs of slab melting and (4) the existence of adakitic rocks with chemical characteristics similar to adakites but are clearly unrelated to slab melting. Other studies have shown that adakitic rocks and a number of the previously reported adakites are pro- duced through melting of the mafic lower crust or ponded basaltic magma, high-pressure crystal fractionation of basaltic magma and low-pressure crystal fractionation of basaltic magma plus magma mixing processes in both arc or non-arc tectonic environments. Despite the confusing interpretations on the petrogenesis of adakite and adakitic rocks, their in- vestigations have enriched our understanding of material recycling at subduction zones, crustal evolu- tionary processes and economic mineralization.