Miocene–Pliocene(22–5 Myr) volcanism and associated seamounts are abundant in the continent-ocean transition(COT) zone in the margin of the north South China Sea(SCS). The petrogenesis of volcanic rocks from these s...Miocene–Pliocene(22–5 Myr) volcanism and associated seamounts are abundant in the continent-ocean transition(COT) zone in the margin of the north South China Sea(SCS). The petrogenesis of volcanic rocks from these seamounts and regional tectonic evolution of COT zone are poorly known. In this paper, we obtained whole-rock major and trace element compositions and Sr-Nd-Pb isotopic data for these volcanic rocks from the Puyuan and Beipo seamounts within COT zone, in northeastern SCS. Based on the geochemical analyses, the volcanic rocks are classified as alkaline ocean island basalts(OIB) and enriched mid-ocean ridge basalts(E-MORB). The OIBs from the Puyuan seamount are alkaline trachybasalts and tephrites that show enrichment of the light rare earth elements(LREE) relative to heavy rare earth elements(HREE) and more radiogenic Sr-Nd isotopic compositions, and have significant ‘Dupal isotopic anomaly'. In contrast, the E-MORBs from the Beipo seamount are tholeiitic basalts that have less enrichment in LREE and less radiogenic Sr-Nd isotopic compositions than the counterparts from the Puyuan seamount. Petrological and geochemical differences between the OIBs and MORBs from these two seamounts can be explained by different mantle sources and tectonic evolution stages of the COT zone. Syn-spreading OIB type basalts from the Puyuan seamount were derived from an isotopically ‘enriched', and garnet facies-dominated pyroxenitic mantle transferred by the Hainan mantle plume. In contrast, post-spreading E-MORB type basalts from the Beipo seamount are considered to be derived from the melting of isotopically ‘depleted' pyroxenite mantle triggered by lithosphere bending and subsequent post-rifting at the lower continental slope of the northern margin.展开更多
The Mozambique Ridge(MOZR)is one of the basement high structures located in the Southwest Indian Ocean,parallel to the Southeast African continental margin.It was formed as a result of the tectono-magmatic evolution o...The Mozambique Ridge(MOZR)is one of the basement high structures located in the Southwest Indian Ocean,parallel to the Southeast African continental margin.It was formed as a result of the tectono-magmatic evolution of the Gondwana breakup.The origin of the MOZR has been highly debated,with models suggesting either continental or oceanic origin.With new free-air gravity anomaly and multichannel seismic(MCS)reflection data,we present results of 2D density modeling along two seismic profiles acquired by R/V Xiangyanghong 10 at the northern Mozambique Ridge(N-MOZR)between 26°S and 28°S.We observed high free-air gravity anomaly and strong positive magnetic anomaly related to the emplaced seaward dipping reflectors(SDR)and high density lower crustal body(HDLCB),and high Bouguer gravity anomaly associated with the thinning of the continental crust underneath the N-MOZR over a distance of~82 km.This suggests a thinned and intruded continental crust bound by the Mozambique Fracture Zone(MFZ)that is characterized by gravity low and negative magnetic anomaly.This fracture zone marks the continent-ocean boundary(COB)while the N-MOZR is the transform margin high,i.e.,marks the continent-ocean transition(COT)of the Southern Mozambique margin,following the definition of transform margins.We suggest that the N-MOZR was formed by continental extension and subsequent breakup of the MFZ,accompanied by massive volcanism during the southward movement of the Antarctica block.The presence of SDR,HDLCB,and relatively thick oceanic crust indicates the volcanic nature of this transform margin.展开更多
The deep crustal structure of the Northwest Sub-basin(NWSB)of the South China Sea(SCS)is of great importance for understanding the tectonic nature of the continent-ocean transition(COT)and magmatism in this oceanic ba...The deep crustal structure of the Northwest Sub-basin(NWSB)of the South China Sea(SCS)is of great importance for understanding the tectonic nature of the continent-ocean transition(COT)and magmatism in this oceanic basin.The 2-D wide-angle reflection/refraction seismic profile OBS2006-2 is almost parallel to the extinct spreading ridge(ESR)of the NWSB.In addition to the original data,we added the data of two reprocessed OBS stations,and carried out seismic phase re-picking and travel-time imaging to obtain the crustal velocity structure along this profile.Resolution tests demonstrate that the newly acquired velocity structure is more reliable than the prior interpretation.The depth of the Moho(23.5–11.8 km)and crustal thickness(20.5–6.5 km)systematically changes from continental crust of the Xisha Block to the oceanic crust within the NWSB.The COT zone has a width of^20 km and the depth of the Moho decreases from 15.0 to 11.0 km,corresponding to a^4 km decrease in crustal thickness(6–10 km).A high velocity layer(HVL,7.2–7.4 km s–1)exists at the bottom of the crust at the location where the sharp lateral transition of the continental crust to the oceanic crust occurs.Age dating shows that the Doublepeak Seamount was formed at^23 Ma,after the cessation of the NWSB seafloor spreading(~32–25 Ma).The crust beneath the Double-peak Seamount is oceanic with a thickness of 9 km.We infer that this oceanic crust was formed by magmatic upwelling and decompression melting along a pre-existing zone of weakness.展开更多
基金jointly supported by the National Key R&D Program of China (No. 2018YFC0309802)the 13th Five-Year Plan Program of the China Ocean Mineral Resour-ces Research and Development Association Research (No. DY135-S2-2-08)+2 种基金the Soft Science Project of Shandong Province Key Research and Development Plan (No. 2019 RZA02002)the China Postdoctoral Science Foundation (No. 2017M610403)the Taishan Scholar Project Funding (No. tspd20161007)。
文摘Miocene–Pliocene(22–5 Myr) volcanism and associated seamounts are abundant in the continent-ocean transition(COT) zone in the margin of the north South China Sea(SCS). The petrogenesis of volcanic rocks from these seamounts and regional tectonic evolution of COT zone are poorly known. In this paper, we obtained whole-rock major and trace element compositions and Sr-Nd-Pb isotopic data for these volcanic rocks from the Puyuan and Beipo seamounts within COT zone, in northeastern SCS. Based on the geochemical analyses, the volcanic rocks are classified as alkaline ocean island basalts(OIB) and enriched mid-ocean ridge basalts(E-MORB). The OIBs from the Puyuan seamount are alkaline trachybasalts and tephrites that show enrichment of the light rare earth elements(LREE) relative to heavy rare earth elements(HREE) and more radiogenic Sr-Nd isotopic compositions, and have significant ‘Dupal isotopic anomaly'. In contrast, the E-MORBs from the Beipo seamount are tholeiitic basalts that have less enrichment in LREE and less radiogenic Sr-Nd isotopic compositions than the counterparts from the Puyuan seamount. Petrological and geochemical differences between the OIBs and MORBs from these two seamounts can be explained by different mantle sources and tectonic evolution stages of the COT zone. Syn-spreading OIB type basalts from the Puyuan seamount were derived from an isotopically ‘enriched', and garnet facies-dominated pyroxenitic mantle transferred by the Hainan mantle plume. In contrast, post-spreading E-MORB type basalts from the Beipo seamount are considered to be derived from the melting of isotopically ‘depleted' pyroxenite mantle triggered by lithosphere bending and subsequent post-rifting at the lower continental slope of the northern margin.
基金The National Key R&D Program of China under contract No.2017YFC1405504the National Natural Science Foundation of China under contract Nos 41830537,4176113405 and 41476048.
文摘The Mozambique Ridge(MOZR)is one of the basement high structures located in the Southwest Indian Ocean,parallel to the Southeast African continental margin.It was formed as a result of the tectono-magmatic evolution of the Gondwana breakup.The origin of the MOZR has been highly debated,with models suggesting either continental or oceanic origin.With new free-air gravity anomaly and multichannel seismic(MCS)reflection data,we present results of 2D density modeling along two seismic profiles acquired by R/V Xiangyanghong 10 at the northern Mozambique Ridge(N-MOZR)between 26°S and 28°S.We observed high free-air gravity anomaly and strong positive magnetic anomaly related to the emplaced seaward dipping reflectors(SDR)and high density lower crustal body(HDLCB),and high Bouguer gravity anomaly associated with the thinning of the continental crust underneath the N-MOZR over a distance of~82 km.This suggests a thinned and intruded continental crust bound by the Mozambique Fracture Zone(MFZ)that is characterized by gravity low and negative magnetic anomaly.This fracture zone marks the continent-ocean boundary(COB)while the N-MOZR is the transform margin high,i.e.,marks the continent-ocean transition(COT)of the Southern Mozambique margin,following the definition of transform margins.We suggest that the N-MOZR was formed by continental extension and subsequent breakup of the MFZ,accompanied by massive volcanism during the southward movement of the Antarctica block.The presence of SDR,HDLCB,and relatively thick oceanic crust indicates the volcanic nature of this transform margin.
基金supported by the National Natural Science Foundation of China(Grant Nos.41730532,91958212,91858212,41606064)the Major Projects for Talent Research Team Introduction of Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou)(Grant No.GML2019ZD0204)the Guangdong Natural Science Foundation Research Team Project(Grant No.2017A030312002)。
文摘The deep crustal structure of the Northwest Sub-basin(NWSB)of the South China Sea(SCS)is of great importance for understanding the tectonic nature of the continent-ocean transition(COT)and magmatism in this oceanic basin.The 2-D wide-angle reflection/refraction seismic profile OBS2006-2 is almost parallel to the extinct spreading ridge(ESR)of the NWSB.In addition to the original data,we added the data of two reprocessed OBS stations,and carried out seismic phase re-picking and travel-time imaging to obtain the crustal velocity structure along this profile.Resolution tests demonstrate that the newly acquired velocity structure is more reliable than the prior interpretation.The depth of the Moho(23.5–11.8 km)and crustal thickness(20.5–6.5 km)systematically changes from continental crust of the Xisha Block to the oceanic crust within the NWSB.The COT zone has a width of^20 km and the depth of the Moho decreases from 15.0 to 11.0 km,corresponding to a^4 km decrease in crustal thickness(6–10 km).A high velocity layer(HVL,7.2–7.4 km s–1)exists at the bottom of the crust at the location where the sharp lateral transition of the continental crust to the oceanic crust occurs.Age dating shows that the Doublepeak Seamount was formed at^23 Ma,after the cessation of the NWSB seafloor spreading(~32–25 Ma).The crust beneath the Double-peak Seamount is oceanic with a thickness of 9 km.We infer that this oceanic crust was formed by magmatic upwelling and decompression melting along a pre-existing zone of weakness.
