The Guomangco ophiolitic melange is situated in the middle part of the Shiquanhe- Yongzhu-Jiali ophiolitic melange belt （SYJMB） and possesses all the subunits of a typical Penrose- type ophiolite pseudostratigraphy....The Guomangco ophiolitic melange is situated in the middle part of the Shiquanhe- Yongzhu-Jiali ophiolitic melange belt （SYJMB） and possesses all the subunits of a typical Penrose- type ophiolite pseudostratigraphy. The study of the Guomangco ophiolitic melange is very important for investigating the tectonic evolution of the SYJMB. The mafic rocks of this ophiolitic melange mainly include diabases, sillite dikes, and basalts. Geochemical analysis shows that these dikes mostly have E-MORB major and trace element signatures; this is the first time that this has been observed in the SYJMB. The basalts have N-MORB and IAB affinities, and the mineral chemistry of harzburgites shows a composition similar to that of SSZ peridotites, indicating that the Guomangco ophiolitic melange probably originated in a back-arc basin. The Guomangco back-arc basin opened in the Middle Jurassic, which was caused by southward subduction of the Neo-Tethys Ocean in central Tibet. The main spreading of this back-arc basin occurred during the Late Jurassic, and the basalts were formed during this time. With the development of the back-arc basin, the subducted slab gradually retreated, and new mantle convection occurred in the mantle wedge. The recycling may have caused the metasomatized mantle to undergo a high degree of partial melting and to generate E- MORBs in the Early Cretaceous. E-MORB-type dikes probably crystallized from melts produced by about 20%-30% partial melting of a spinel mantle source, which was metasomatized by melts from low-degree partial melting of the subducted slab.展开更多
The mafic-ultramafic assemblages,which thrustthrust into the Wushan-Tangzang boundary fault as some blocks and outcropped in the Yuanyangzhen,Lijiahe,Lubangou and Gaojiahe area,consist mainly of meta-peridotites,gabbr...The mafic-ultramafic assemblages,which thrustthrust into the Wushan-Tangzang boundary fault as some blocks and outcropped in the Yuanyangzhen,Lijiahe,Lubangou and Gaojiahe area,consist mainly of meta-peridotites,gabbros and basalts.The meta-peridotites are characterized by high SiO_(2) and MgO contents,lowΣREE,as well as their chondrite-normalized rare earth element patterns show some similarities to that of middle oceanic meta-peridotite.The basalts from the Yuanyangzhen,Lijiahe and Lubangou area are characterized by relatively high TiO_(2) content,low Al_(2)O_(3) content and Na_(2)O>>K_(2)O.Above all,it is the slight enrichment or flat REE distribution patterns and the unfractionated in HFS elements in the primitive-normalized trace elements distribution patterns that indicate these basalts are similar to that of the typical E-MORB.In comparison,the basalts from the Gaojiahe section are featured by depletion in Nb and Ta contents and enrichment in Th content which show that these were derived from an island-arc setting.From studies of the regional geology,petrology,geochemistry,geo-chronology and all above evidence,it can be suggested that the mafic-ultramafic rocks from the Wushan area are mainly dismembered E-MORB type ophiolite,which represent the fragments of the lithosphere of the Early-Paleozoic Qinling ocean.It is preferred that these rocks were formed in an initial mid-ocean ridge setting during the beginning stage of the oceanic basin spreading.This ophiolite together with the Gaojiahe island-arc basalts shows that there exists an ophiolitic mélange along the Wushan-Tangzang boundary fault,and marks the suture zone after the closure of the Qinling ocean in early Paleozoic.展开更多
The Dharwar Craton(DC)in India consists of three distinct Archean blocks.Previous research suggests that the Central Dharwar Block(CDB)experienced a geological history(3.3-2.5 Ga)comparable to the Western Dharwar Crat...The Dharwar Craton(DC)in India consists of three distinct Archean blocks.Previous research suggests that the Central Dharwar Block(CDB)experienced a geological history(3.3-2.5 Ga)comparable to the Western Dharwar Craton(WDC).However,3.0-2.7 Ga geological records are missed in the CDB.This study identified the 2.85 Ga gabbroic xenoliths within the Closepet batholith in the CDB.The~2.85 Ga inherited zircons and zircon Hf model ages in CDB further provide evidence for this significant magmatic event.The gabbroic xenoliths are characterized by E-MORB REE patterns and Nb,Ta,and Ti depletions,together with high radiogenic isotopic signatures,including zircon Hf(ε_(Hf)(t)_(min)=-3.2),whole-rock Nd(ε_(Nd)(t)=-0.8-0.7),and Pb isotopes(κ=4.9-7.9),indicating derivation from a slightly enriched mantle source.Additionally,they have high Nb(4.41 to 4.73 ppm),low Ti/V(23.4 to 22.5),and elevated Th/Yb(0.38 to 0.61).All these suggest a subduction-related back-arc basin setting.Coupled with widespread early Neoarchean subduction-related igneous rocks in the DC,the 2.85 Ga gabbroic magmatism signifies that the DC had transitioned into a tectonic stage dominated by lateral movement of continental blocks around 2.85 Ga,corresponding to global~2.85 Ga magmatic events.展开更多
基金funded by the National Funds of Nature Science of China (Grant No. 41272240)the Project of China Geological Survey (Grant No. 1212011121248)
文摘The Guomangco ophiolitic melange is situated in the middle part of the Shiquanhe- Yongzhu-Jiali ophiolitic melange belt （SYJMB） and possesses all the subunits of a typical Penrose- type ophiolite pseudostratigraphy. The study of the Guomangco ophiolitic melange is very important for investigating the tectonic evolution of the SYJMB. The mafic rocks of this ophiolitic melange mainly include diabases, sillite dikes, and basalts. Geochemical analysis shows that these dikes mostly have E-MORB major and trace element signatures; this is the first time that this has been observed in the SYJMB. The basalts have N-MORB and IAB affinities, and the mineral chemistry of harzburgites shows a composition similar to that of SSZ peridotites, indicating that the Guomangco ophiolitic melange probably originated in a back-arc basin. The Guomangco back-arc basin opened in the Middle Jurassic, which was caused by southward subduction of the Neo-Tethys Ocean in central Tibet. The main spreading of this back-arc basin occurred during the Late Jurassic, and the basalts were formed during this time. With the development of the back-arc basin, the subducted slab gradually retreated, and new mantle convection occurred in the mantle wedge. The recycling may have caused the metasomatized mantle to undergo a high degree of partial melting and to generate E- MORBs in the Early Cretaceous. E-MORB-type dikes probably crystallized from melts produced by about 20%-30% partial melting of a spinel mantle source, which was metasomatized by melts from low-degree partial melting of the subducted slab.
基金Supported by the National Natural Science Foundation of China(Grant Nos.40472115 and 40234041)
文摘The mafic-ultramafic assemblages,which thrustthrust into the Wushan-Tangzang boundary fault as some blocks and outcropped in the Yuanyangzhen,Lijiahe,Lubangou and Gaojiahe area,consist mainly of meta-peridotites,gabbros and basalts.The meta-peridotites are characterized by high SiO_(2) and MgO contents,lowΣREE,as well as their chondrite-normalized rare earth element patterns show some similarities to that of middle oceanic meta-peridotite.The basalts from the Yuanyangzhen,Lijiahe and Lubangou area are characterized by relatively high TiO_(2) content,low Al_(2)O_(3) content and Na_(2)O>>K_(2)O.Above all,it is the slight enrichment or flat REE distribution patterns and the unfractionated in HFS elements in the primitive-normalized trace elements distribution patterns that indicate these basalts are similar to that of the typical E-MORB.In comparison,the basalts from the Gaojiahe section are featured by depletion in Nb and Ta contents and enrichment in Th content which show that these were derived from an island-arc setting.From studies of the regional geology,petrology,geochemistry,geo-chronology and all above evidence,it can be suggested that the mafic-ultramafic rocks from the Wushan area are mainly dismembered E-MORB type ophiolite,which represent the fragments of the lithosphere of the Early-Paleozoic Qinling ocean.It is preferred that these rocks were formed in an initial mid-ocean ridge setting during the beginning stage of the oceanic basin spreading.This ophiolite together with the Gaojiahe island-arc basalts shows that there exists an ophiolitic mélange along the Wushan-Tangzang boundary fault,and marks the suture zone after the closure of the Qinling ocean in early Paleozoic.
基金financially supported by the National Natural Science Foundations of China(41890831,41421002)the MOST Special Fund from the State Key Laboratory of Continental Dynamics,Northwest University(201210133)。
文摘The Dharwar Craton(DC)in India consists of three distinct Archean blocks.Previous research suggests that the Central Dharwar Block(CDB)experienced a geological history(3.3-2.5 Ga)comparable to the Western Dharwar Craton(WDC).However,3.0-2.7 Ga geological records are missed in the CDB.This study identified the 2.85 Ga gabbroic xenoliths within the Closepet batholith in the CDB.The~2.85 Ga inherited zircons and zircon Hf model ages in CDB further provide evidence for this significant magmatic event.The gabbroic xenoliths are characterized by E-MORB REE patterns and Nb,Ta,and Ti depletions,together with high radiogenic isotopic signatures,including zircon Hf(ε_(Hf)(t)_(min)=-3.2),whole-rock Nd(ε_(Nd)(t)=-0.8-0.7),and Pb isotopes(κ=4.9-7.9),indicating derivation from a slightly enriched mantle source.Additionally,they have high Nb(4.41 to 4.73 ppm),low Ti/V(23.4 to 22.5),and elevated Th/Yb(0.38 to 0.61).All these suggest a subduction-related back-arc basin setting.Coupled with widespread early Neoarchean subduction-related igneous rocks in the DC,the 2.85 Ga gabbroic magmatism signifies that the DC had transitioned into a tectonic stage dominated by lateral movement of continental blocks around 2.85 Ga,corresponding to global~2.85 Ga magmatic events.
