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 format...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 characteristics of adakites compared to ADR,such as relative higher MgO,Cr,Cu,and Ni,are due to their slab source,as well as interaction of the slab-derived adakitic melts with overlying hot lithospheric mantle;altered oceanic slabs are also relatively rich in siderophile and other chalcophile elements,as well as sulfates and sulfides.HSA magmas related to slab failure have special geochemical properties,such as Sr/Y>20,Nb/Y>0.4,Ta/Yb>0.3,La/Yb>10,Gd/Yb>2,and Sm/Yb>2.5.Slightly higher Nb+Ta is due to high T melting of rutile.Varieties of Nb/Ta compared to silica are also significant in HSA as a result of slab failure(roll back to break-off).High T-P partial melting of the hydrothermally altered oceanic slab produces HSA with quite high activities of H2O,SO2,HCl,with chalcophile metals that remain incompatible at higher fO2(low fH2);these situations happen in late-to post-collisional settings where the subducting oceanic crust experienced slab failure,resulting in advective heat addition to the system from upwelling asthenosphere.In such a slab failure setting,transpression and transtension play a significant role in the rapid emplacement of a high amount of fertile adakitic magmas through the subduction-modified lithosphere and crust into the upper crust.When oxidized slab melts interact with the subduction-modified lithospheric mantle,the resulting magmas stay oxidized,potentially contributing to the special conditions conducive to formation of porphyry Cu-Au mineralization.展开更多
Eocene intermediate to felsic subvolcanic rocks of the Torud-Ahmad Abad magmatic belt(TAMB),in the northern part of the Central Iran zone,are exposed between the Torud and Ahmad Abad regions in South-Southeast Shahroo...Eocene intermediate to felsic subvolcanic rocks of the Torud-Ahmad Abad magmatic belt(TAMB),in the northern part of the Central Iran zone,are exposed between the Torud and Ahmad Abad regions in South-Southeast Shahrood.These igneous rocks include hypabyssal dacite,trachyte,andesite,trachy-andesite,and basaltic andesite;they are mainly composed of phenocrysts and microcrystalline groundmass of pyroxene,amphibole,and plagioclase,with minor biotite and titanomagnetite;they form domal structures(plugs and stocks),dikes,and sills that intruded into Neoproterozoic to cogenetic Eocene volcano-sedimentary sequences.Based on isotopic analysis of these intermediate to acidic rocks,initial ratios of^(143)Nd/^(144)Nd range from 0.512775 to 0.512893 and initial ratios of^(87)Sr/^(86)Sr range from 0.703746 to 0.705314,with quite positiveε_(Nd(i))values of+3.69 to+6.00.They are enriched in light rare earth elements and large ion lithophile elements and depleted in heavy rare earth elements and high-field strength elements,the SiO_(2) content is(52-62)wt.%,and Na_(2)O content>3 wt.%,Al_(2)O_(3) content>16 wt.%,Yb<1.8 ppm,and Y<18 ppm.These geological,geochemical,and Sr and Nd isotopic data are consistent with adakitic signatures originating by partial melting of the subducted Neo-Tethys oceanic slab(Sabzevar branch)and lithospheric suprasubduction zone mantle.The mantle signatures typifying the rapidly emplaced adakitic rocks(slab(high-silica adakite)and suprasubduction zone(low-silica adakite)melts)together with their locally voluminous extent are evidences that support a locally extensional geodynamic setting;and the evidence is consistent with an evolution to local transpression in the Late Eocene in this convergent margin are environment to rifting(basalts to adakites)towards submarine conditions in the Neogene.展开更多
文摘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 characteristics of adakites compared to ADR,such as relative higher MgO,Cr,Cu,and Ni,are due to their slab source,as well as interaction of the slab-derived adakitic melts with overlying hot lithospheric mantle;altered oceanic slabs are also relatively rich in siderophile and other chalcophile elements,as well as sulfates and sulfides.HSA magmas related to slab failure have special geochemical properties,such as Sr/Y>20,Nb/Y>0.4,Ta/Yb>0.3,La/Yb>10,Gd/Yb>2,and Sm/Yb>2.5.Slightly higher Nb+Ta is due to high T melting of rutile.Varieties of Nb/Ta compared to silica are also significant in HSA as a result of slab failure(roll back to break-off).High T-P partial melting of the hydrothermally altered oceanic slab produces HSA with quite high activities of H2O,SO2,HCl,with chalcophile metals that remain incompatible at higher fO2(low fH2);these situations happen in late-to post-collisional settings where the subducting oceanic crust experienced slab failure,resulting in advective heat addition to the system from upwelling asthenosphere.In such a slab failure setting,transpression and transtension play a significant role in the rapid emplacement of a high amount of fertile adakitic magmas through the subduction-modified lithosphere and crust into the upper crust.When oxidized slab melts interact with the subduction-modified lithospheric mantle,the resulting magmas stay oxidized,potentially contributing to the special conditions conducive to formation of porphyry Cu-Au mineralization.
基金supported by the NSERC Discovery to David R.Lentz。
文摘Eocene intermediate to felsic subvolcanic rocks of the Torud-Ahmad Abad magmatic belt(TAMB),in the northern part of the Central Iran zone,are exposed between the Torud and Ahmad Abad regions in South-Southeast Shahrood.These igneous rocks include hypabyssal dacite,trachyte,andesite,trachy-andesite,and basaltic andesite;they are mainly composed of phenocrysts and microcrystalline groundmass of pyroxene,amphibole,and plagioclase,with minor biotite and titanomagnetite;they form domal structures(plugs and stocks),dikes,and sills that intruded into Neoproterozoic to cogenetic Eocene volcano-sedimentary sequences.Based on isotopic analysis of these intermediate to acidic rocks,initial ratios of^(143)Nd/^(144)Nd range from 0.512775 to 0.512893 and initial ratios of^(87)Sr/^(86)Sr range from 0.703746 to 0.705314,with quite positiveε_(Nd(i))values of+3.69 to+6.00.They are enriched in light rare earth elements and large ion lithophile elements and depleted in heavy rare earth elements and high-field strength elements,the SiO_(2) content is(52-62)wt.%,and Na_(2)O content>3 wt.%,Al_(2)O_(3) content>16 wt.%,Yb<1.8 ppm,and Y<18 ppm.These geological,geochemical,and Sr and Nd isotopic data are consistent with adakitic signatures originating by partial melting of the subducted Neo-Tethys oceanic slab(Sabzevar branch)and lithospheric suprasubduction zone mantle.The mantle signatures typifying the rapidly emplaced adakitic rocks(slab(high-silica adakite)and suprasubduction zone(low-silica adakite)melts)together with their locally voluminous extent are evidences that support a locally extensional geodynamic setting;and the evidence is consistent with an evolution to local transpression in the Late Eocene in this convergent margin are environment to rifting(basalts to adakites)towards submarine conditions in the Neogene.
