Adakites: some variations on a theme

Abstract

Adakites were proposed over a decade ago to be products of the melting of young subducted oceanic crust. In fact, several new localities have been discovered since the original work documented approximately ten localities in modern arcs (e.g. , southwestern Japan, Trans Mexican Volcanic Belt, etc. ). But work over the past ten years has also shown that adakites can be generated by other processes during subduction (e. g. , along the edge of tears in the subducting slab, remnant slabs left in the upper mantle, etc. ). In addition, adakites appear to be associated with a suite of rocks including high-Mg andesites resulting from either adakite interaction with the mantle (Adak-type) or melting of the mantle during adakite interaction (Piip-type), niobium enriched arc basalts (NEAB) that are believed to be derived from the partial melting of a mantle metasomatized extensively by adakites, and possibly boninites (several researchers have found an adakite component in boninites). A new rock suite, the adakite metasomatic volcanic series, has been proposed to account for the various associations. In addition, a large number of NEAB have been found to contain ultramafic mantle xenoliths with clear evidence of reaction between ultradepleted mantle and adakites. Several alternative hypotheses have been proposed for the generation of adakites primarily involving the melting of the lower crust rather than the subducting slab. One model proposes that the melting of the lower crust occurs when basaltic melts underplate the lower crust. There are many reasons that appear to rule this method out. The other model proposes that in areas where the continental crust is thick, the lower crust can become eclogitic and separate and sink into the mantle (delaminate). This delamination process will bring the lower sections of the lower crust or the upper part of the delaminated lower crust in contact with relatively hot mantle, which could initiate melting, and the production of adakites. This has led us to conclude that many of the Cretaceous adakites not associated with subduction found in East China are the result of lower crustal melting via delamination. We also emphasize that, if true, the term adakite should not be restricted to processes related only to slab melting but must include those involving the melting of the lower crust. The Archean continental crust consists of primarily trondhjemites, tonalities, and dacites (TTD). It remains problematic as to whether this continental crust was derived from slab melting or lower crustal melting. However, we believe that the higher mantle temperatures during the Archean led to more mid ocean ridges, which generated the subduction of "more" younger crust than today. Based on this, we suggest that the Archean TTD continental crust was generated primarily by slab melting. We also note that the Archean was a period of extensive gold mineralization. Several researchers have also found gold and copper mineralization associated with the adakite metasomatic volcanic series. This series may be an important indicator of ore deposits.