The Binchuan area of Yunnan is located in the western part of the Emeishan large igneous province in the western margin of the Yangtze Block. In the present study, the Wuguiqing profile in thickness of about 1440 m is...The Binchuan area of Yunnan is located in the western part of the Emeishan large igneous province in the western margin of the Yangtze Block. In the present study, the Wuguiqing profile in thickness of about 1440 m is mainly composed of high-Ti basalts, with minor picrites in the lower part and andesites, trachytes, and rhyolites in the upper part. The picrites have relatively higher platinum- group element （PGE） contents （PGE=16.3-28.2 ppb）, with high Cu/Zr and Pd/Zr ratios, and low S contents （5.03-16.9 ppm）, indicating the parental magma is S-unsaturated and generated by high degree of partial melting of the Emeishan large igneous province （ELIP） mantle source. The slightly high Cu/Pd ratios （11 000-24 000） relative to that of the primitive mantle suggest that 0.007% sulfides have been retained in the mantle source. The PGE contents of the high-Ti basalts exhibit a wider range （~PGE=0.517-30.8 ppb）. The samples in the middle and upper parts are depleted in PGE and have ~Nd （260 Ma） ratios ranging from -2.8 to -2.2, suggesting that crustal contamination of the parental magma during ascent triggered sulfur saturation and segregation of about 0.446%-0.554% sulfides, and the sulfide segregation process may also provide the ore-forming material for the magmatic Cu-Ni-PGE sulfide deposits close to the studied basalts. The samples in this area show Pt- Pd type primitive mantle-normalized PGE patterns, and the Pd/Ir ratios are higher than that of the primitive mantle （Pd/Ir=l）, indicating that the obvious differentiation between Ir-group platinum- group elements （IPGE） and Pd-group platinum-group elements （PPGE） are mainly controlled by olivine or chromites fractionation during magma evolution. The Pd/Pt ratios of most samples are higher than the average ratio of mantle （Pd/Pt=0.55）, showing that the differentiation happened between Pt and Pd. The differentiation in picrites may be relevant to Pt hosted in discrete refractory Pt-alloy phase in the mantle; whereas the differentiation in the high-Ti basalts is probably associated with the fractionation of Fe-Pt alloys, coprecipitating with Ir-Ru-Os alloys. Some high-Ti basalt samples exhibit negative Ru anomalies, possibly due to removal of laurite collected by the early crystallized chromites.展开更多
The Tengchong volcano field(TVF),situated at the southeastern margin of the Tibetan Plateau,holds crucial information regarding Cenozoic volcanic activities and geotectonic evolution of the SE Xizang.To provide new co...The Tengchong volcano field(TVF),situated at the southeastern margin of the Tibetan Plateau,holds crucial information regarding Cenozoic volcanic activities and geotectonic evolution of the SE Xizang.To provide new constraints on petrogenesis and evolution of the Tengchong volcanism,here we conducted copper(Cu)elemental and isotopic analyses on a suite of samples that document the evolution from basalts to andesites in the TVF.The basalts are Cu-depleted(29.7-36.9 ppm)and have higherδ^(65)Cu val-ues(0.19‰-0.40‰,mean=0.31‰±0.05‰;n=11)than those of mid-ocean ridge basalts(MORBs,~0.09‰)and the mantle(~0.06‰)as well as the majority of island arc lavas.Along with the low Cu/Zr ratios,these characteristics are interpreted to reflect the fractionation of isotopically light sulfides in the S-saturated systems during magma ascent,rather than source heterogeneity induced by recycled materials and redox reactions.Compared with the basalts,the andesites have slightly lower Cu contents(14.4-29.4 ppm)and lighter Cu isotopic compositions(mean=-0.14‰±0.06‰;n=13).These differences cannot be attributed to progressive sulfide fractionation of basaltic magmas but require the assimilation of lower crustal materials with lowδ^(65)Cu values during evolution of the andesitic magmas.Our results collectively suggest that Cu isotopes can provide valuable insights into magma origin and evolution.展开更多
基金supported by the National Basic Research Program of China(No.2007CB411401)the Knowledge Innovation Program of the Chinese Academy of Sciences(No.KZCX2-YW-136)the National Natural Science Formation of China(No.40873028)
文摘The Binchuan area of Yunnan is located in the western part of the Emeishan large igneous province in the western margin of the Yangtze Block. In the present study, the Wuguiqing profile in thickness of about 1440 m is mainly composed of high-Ti basalts, with minor picrites in the lower part and andesites, trachytes, and rhyolites in the upper part. The picrites have relatively higher platinum- group element （PGE） contents （PGE=16.3-28.2 ppb）, with high Cu/Zr and Pd/Zr ratios, and low S contents （5.03-16.9 ppm）, indicating the parental magma is S-unsaturated and generated by high degree of partial melting of the Emeishan large igneous province （ELIP） mantle source. The slightly high Cu/Pd ratios （11 000-24 000） relative to that of the primitive mantle suggest that 0.007% sulfides have been retained in the mantle source. The PGE contents of the high-Ti basalts exhibit a wider range （~PGE=0.517-30.8 ppb）. The samples in the middle and upper parts are depleted in PGE and have ~Nd （260 Ma） ratios ranging from -2.8 to -2.2, suggesting that crustal contamination of the parental magma during ascent triggered sulfur saturation and segregation of about 0.446%-0.554% sulfides, and the sulfide segregation process may also provide the ore-forming material for the magmatic Cu-Ni-PGE sulfide deposits close to the studied basalts. The samples in this area show Pt- Pd type primitive mantle-normalized PGE patterns, and the Pd/Ir ratios are higher than that of the primitive mantle （Pd/Ir=l）, indicating that the obvious differentiation between Ir-group platinum- group elements （IPGE） and Pd-group platinum-group elements （PPGE） are mainly controlled by olivine or chromites fractionation during magma evolution. The Pd/Pt ratios of most samples are higher than the average ratio of mantle （Pd/Pt=0.55）, showing that the differentiation happened between Pt and Pd. The differentiation in picrites may be relevant to Pt hosted in discrete refractory Pt-alloy phase in the mantle; whereas the differentiation in the high-Ti basalts is probably associated with the fractionation of Fe-Pt alloys, coprecipitating with Ir-Ru-Os alloys. Some high-Ti basalt samples exhibit negative Ru anomalies, possibly due to removal of laurite collected by the early crystallized chromites.
基金supported by the National Natural Science Foundation of China(grant 42121002)the National Key R&D Program of China(2019YFA0708400).
文摘The Tengchong volcano field(TVF),situated at the southeastern margin of the Tibetan Plateau,holds crucial information regarding Cenozoic volcanic activities and geotectonic evolution of the SE Xizang.To provide new constraints on petrogenesis and evolution of the Tengchong volcanism,here we conducted copper(Cu)elemental and isotopic analyses on a suite of samples that document the evolution from basalts to andesites in the TVF.The basalts are Cu-depleted(29.7-36.9 ppm)and have higherδ^(65)Cu val-ues(0.19‰-0.40‰,mean=0.31‰±0.05‰;n=11)than those of mid-ocean ridge basalts(MORBs,~0.09‰)and the mantle(~0.06‰)as well as the majority of island arc lavas.Along with the low Cu/Zr ratios,these characteristics are interpreted to reflect the fractionation of isotopically light sulfides in the S-saturated systems during magma ascent,rather than source heterogeneity induced by recycled materials and redox reactions.Compared with the basalts,the andesites have slightly lower Cu contents(14.4-29.4 ppm)and lighter Cu isotopic compositions(mean=-0.14‰±0.06‰;n=13).These differences cannot be attributed to progressive sulfide fractionation of basaltic magmas but require the assimilation of lower crustal materials with lowδ^(65)Cu values during evolution of the andesitic magmas.Our results collectively suggest that Cu isotopes can provide valuable insights into magma origin and evolution.
