Petrogenesis of adakitic rocks unrelated to slab melting and adakitic porphyries associated with Cu mineralization

Abstract

It is generally believed that adakites are produced by slab melting under young and hot condition in plate convergent margins. However, other rocks that are not associated with slab melting but display the geochemical characteristics of an adakite, are termed “adakitic” or “adakite-like” rocks in present paper. Although formation mechanism of adakitic rocks is different from that of adakites, they are all generated by the crystallization of melts (magmas), which are termed “adakitic melts”. The unique geochemical features of adakitic melts arise from mafic rock melting in the specific environments including 800–1000 °C, 1.5–3.0 GPa, and 1.5–6.0 wt% H2O, with the superfluous garnet and/or rutile and minor plagioclase within the source. Adakitic rocks can be formed in continental-collision zones and intraplate settings, and thus have no specific indicative significance for tectonic setting. This paper divides adakitic rocks into four genetic types formed by (1) direct partial melting of thickened lower rocks, (2) partial melting of delaminated lower crust, (3) differentiation of mafic magma under high-pressure conditions, and (4) magma mixing. The adakitic rocks may coexist with coeval ultrapotassic-potassic volcanic rocks, Nb-enriched basalts or shoshonitic igneous rocks, which hints that the adakitic rocks have a genetic relationship to the mantle-derived rocks when the both form a magmatic association.Most host rocks of porphyry Cu–Mo–Au deposits show adakitic feature geochemically in both continental collision zones and subduction zones, but a few of the host rocks from subduction zone are not adakitic. This paper firstly identifies that the adakitic porphyries in collision zones show trace-element contents (e.g., Sc, Th, and Y) and ratios (e.g., Th/Yb, Sr/Y, and La/Yb) clearly different from those in subduction zone. The magmatic sources of adakitic porphyries from a continental crust have higher Th and lower Sc contents, and more garnet and rutile remains than those of adakites from a subduction zone. The higher Th/Yb, Sr/Y, and La/Yb ratios indicate that the source depth of porphyric rocks from collision zones is probably deeper; whereas the depth of porphyric rocks from subduction settings is variational and relatively shallow, therefore a few of them don't display adakitic signature. Further investigations into the genetic link between adakitic melts and porphyry Cu–Mo–Au deposits are required.