Formation of high-silica adakites and their relationship with slab breakoff:Implications for generating fertile Cu-Au-Mo porphyry systems  

作　　者：Fazilat Yousefi David R.Lentz 

机构地区：[1]Department of Earth Sciences,University of New Brunswick,Fredericton,New Brunswick E3B 5A3,Canada

出　　处：《Geoscience Frontiers》2024年第6期454-468,共15页地学前缘（英文版）

摘　　要：In recent years,the characteristics and sources of fertile adakites has received considerable attention.As well,most recently the geodynamic environment of convergent margins subducting oceanic crust aiding arc formation,evolving to slab rollback,then slab break-off after collision(i.e.late-to post-collisional slab failure(arc-like magmatism)and transpression)has gained more recognition,although their relationship to each other has yet to be explored.The geochemical characteristics imply that adakites/adakite-like,in particular high-silica adakites(HSA),can form by partial melting of subducting hydrothermally altered oceanic crust in convergent plate boundary settings during the terminal stages of subduction,lithosphere thickening,and then failure(all late to post collisional),while the melting of the mantle wedge during subduction-related dehydration creates more typical calc-alkaline basalt-andesite-dacite-rhyolite series(ADR)to form intraoceanic island arc to intracontinental margin arc systems,before the collisional stage.HSAs are characterized by high-silica(SiO2>67 wt.%),Al2O3>15 wt.%,Sr>300 ppm,Y<20 ppm,Yb<1.8 ppm,and Nb≤10 ppm,and MgO<3 wt.%,with high Sr/Y(>50),and La/Yb(>10).Some specific geochemical features,such as high Mg#(ave 0.51),Ni(ave 924 ppm),and Cr(ave 36 ppm),in HSAs are typical,in contrast to calc-alkaline arcs,although both groups display similar but less pronounced negative anomalies of Nb,Ta,and Ti in primitive mantle-normalized trace element spider diagram profiles.These unique geochemical features are likely ascribed to the involvement of garnet,hornblende,and titanite either during partial melting of hydrous MORB-like oceanic crust with only minor assimilation and fractional crystallization(AFC)within the mantle and crustal during ascent in a transpressional collisional environment.Hypotheses for origin of HSA derivative from melting in convergent margins from young,hot oceanic plates subducting into the mantle is applicable to only some adakitic systems.The difference in geochemical

关 键 词：High-silica adakites Slab break-off Altered oceanic crust Upwelling asthenosphere Fertile Cu-Au-Mo porphyry systems 

分 类 号：P597[天文地球—地球化学] P548[天文地球—地质学]