Abstract
Adakites are Y- and Yb-depleted, SiO2- and Sr-enriched rocks with elevated Sr/Y and La/Yb ratios originally thought to represent partial melts of subducted metabasalt, based on their association with the subduction of young (<25 Ma) and hot oceanic crust. Later, adakites were found in arc segments associated with oblique, slow and flat subduction, arc–transform intersections, collision zones and post-collisional extensional environments. New models of adakite petrogenesis include the melting of thickened and delaminated mafic lower crust, basalt underplating of the continental crust and high-pressure fractionation (amphibole ± garnet) of mantle-derived, hydrous mafic melts. In some cases, adakites are associated with Nb-enriched (10 ppm < Nb < 20 ppm) and high-Nb (Nb > 20 ppm) arc basalts in ancient and modern subduction zones (HNBs). Two types of HNBs are recognized on the basis of their geochemistry. Type I HNBs (Kamchatka, Honduras) share N-MORB-like isotopic and OIB-like trace element characteristics and most probably originate from adakite-contaminated mantle sources. Type II HNBs (Sulu arc, Jamaica) display high-field strength element enrichments in respect to island-arc basalts coupled with enriched, OIB-like isotopic signatures, suggesting derivation from asthenospheric mantle sources in arcs. Adakites and, to a lesser extent, HNBs are associated with Cu–Au porphyry and epithermal deposits in Cenozoic magmatic arcs (Kamchatka, Phlippines, Indonesia, Andean margin) and Paleozoic-Mesozoic (Central Asian and Tethyan) collisional orogens. This association is believed to be not just temporal and structural but also genetic due to the hydrous (common presence of amphibole and biotite), highly oxidized (>ΔFMQ > +2) and S-rich (anhydrite in modern Pinatubo and El Chichon adakite eruptions) nature of adakite magmas. Cretaceous adakites from the Stanovoy Suture Zone in Far East Russia contain Cu–Ag–Au and Cu–Zn–Mo–Ag alloys, native Au and Pt, cupriferous Ag in association witn barite and Ag-chloride. Stanovoy adakites also have systematically higher Au contents in comparison with volcanic arc magmas, suggesting that ore-forming hydrothermal fluids responsible for Cu–Au(Mo–Ag) porphyry and epithermal mineralization in upper crustal environments could have been exsolved from metal-saturated, H2O–S–Cl-rich adakite magmas. The interaction between depleted mantle peridotites and metal-rich adakites appears to be capable of producing (under a certain set of conditions) fertile sources for HNB melts connected with some epithermal Au (Porgera) and porphyry Cu–Au–Mo (Tibet, Iran) mineralized systems in modern and ancient subduction zones.
Highlights
The origin of intermediate to felsic magmas has been the subject of intense scientific debate for several decades
We present in this paper an overview of main occurrences of Cu–Au mineralization associated with adakites and high-Nb basalts along with a discussion of existing ore-forming models in subduction zones with reference to slab and lower crustal melting, adakite production and adakite–mantle wedge hybridization
Most Mesozoic to Cenozoic copper-gold deposits and showings within the Indonesian arcs (Sunda, Banda, Sangihe, Halmahera) are associated with typical normal and high-K calc-alkaline and shoshonitic magmas [189], some large, world-class porphyry and epithermal mineralized systems in Indonesia are clearly hosted in adakites and high-Nb basalts (Figure 12 and Table 8)
Summary
The origin of intermediate to felsic magmas has been the subject of intense scientific debate for several decades. Adakites display peculiar geochemical characteristics such as high Sr/Y and La/Yb ratios (in comparison with “normal” calc-alkaline rocks) consistent with the model involving dehydration melting of hot basaltic crust during amphibolite to eclogite transition in the subducting slab with garnet- and amphibole-dominated solid residue controlling the chemistry of adakitic magmas [33,37,39] These papers triggered a flurry of publications on slab melting and adakites over the decade that examined tectonic setting and conditions for adakite generation in subduction zones ranging from the Aleutians, Kamchatka and Japan to the Philippines and the Aegean Islands on to Central and Southern America [40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55]. We present in this paper an overview of main occurrences of Cu–Au mineralization (both porphyry and epithermal) associated with adakites and high-Nb basalts along with a discussion of existing ore-forming models in subduction zones with reference to slab and lower crustal melting, adakite production and adakite–mantle wedge hybridization
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