The Semail ophiolite of Oman and the United Arab Emirates （UAE） provides the best preserved large slice of oceanic lithosphere exposed on the continental crust, and offers unique opportunities to study processes of ...The Semail ophiolite of Oman and the United Arab Emirates （UAE） provides the best preserved large slice of oceanic lithosphere exposed on the continental crust, and offers unique opportunities to study processes of ocean crust formation, subduction initiation and obduction. Metamorphic rocks exposed in the eastern UAE have traditionally been interpreted as a metamorphic sole to the Semail ophiolite. However, there has been some debate over the possibility that the exposures contain components of older Arabian continental crust. To help answer this question, presented here are new zircon and futile U-Pb geochronological data from various units of the metamorphic rocks. Zircon was absent in most samples. Those that yielded zircon and futile provide dominant single age populations that are 95-93 Ma, partially overlapping with the known age of oceanic crust formation （96.5-94.5 Ma）, and partially overlapping with cooling ages of the metamorphic rocks （95 90 Ma）. The data are interpreted as dating high-grade metamorphism during subduction burial of the sediments into hot mantle lithosphere, and rapid cooling during their subsequent exhumation. A few discordant zircon ages, interpreted as late Neoproterozoic and younger, represent minor detrital input from the continent. No evidence is found in favour of the existence of older Arabian continental crust within the metamorphic rocks of the UAE.展开更多
The welded metamorphic sole at the base of the Bay of Islands Ophiolite Complex(BOIC)in the Northern Appalachians of Newfoundland shows a typical inverted pressure-temperature(P-T)metamorphic gradient from HT-MP granu...The welded metamorphic sole at the base of the Bay of Islands Ophiolite Complex(BOIC)in the Northern Appalachians of Newfoundland shows a typical inverted pressure-temperature(P-T)metamorphic gradient from HT-MP granulite to LT-LP greenschist facies.It incorporates mafic volcanic/plutonic protoliths mixed with pelagic,hemi-pelagic and coarser epiclastic sedimentary protoliths.New LA-ICP-MS U–Pb concordia ages,trace elements,and Ti-in-zircon geothermometry for -250 zircon analyses from three metabasites of the upper HT sole amphibolites with N-MORB-like protoliths are reported.Two samples collected within meters of the ophiolite peridotite-sole contact of the Blow Me Down Mountain and North Arm Mountain massifs yielded the oldest comparable concordia ages of 487.7±2.6 Ma and 489.1±3.1 Ma,respectively,that are both within error of the igneous age of 488.3±1.5 Ma of the directly overlying BOIC ophiolite,which formed at a supra-subduction zone(SSZ)forearc spreading center.A third slightly younger age of 484.2±2.4 Ma was obtained for an upper HT amphibolite sample with similar phase assemblages but collected30 m below the peridotite contact of the Table Mountain massif.Zircon crystals analyzed have similar size and morphologies,subparallel rare earth element(REE)variation patterns,and steep heavy REE-enrichments((Lu/Gd)_(cn)>20),significant positive Ce anomalies(dominantly>5)and slight positive to dominantly negative Eu anomalies(1.2–0.4).Zircon shows Th/U mean values of 0.37–0.48 with little to no rim to core variation.Minimum Ti-in-zircon mean crystallization temperatures range from764–787℃.These neocrystallized zircon crystals appear to be derived from thin leucosomes within the three amphibolites.Two other samples also from the upper HT sole show evidence of inherited detrital zircon with core dates spanning the Cambrian Notre Dame Arc through older Laurentian-like basement and rift age ranges.Subcretion of the sole took place below a hot forearc asthenospheric wedge,that is,a consequence of the newly-formed BOIC forearc spreading center extending from the back arc to a triple junction along the westward-(or paleo-northward)verging trench of the Notre Dame arc.The early HT sole formation age at ca.489–488 Ma is long prior to initiation of obduction at ca.470 Ma and long after initiation of subduction beneath the paleo-northward verging Notre Dame peri-Laurentian arc at ca.514 Ma.This indicates Newfoundland sole ages of the BOIC and St.Anthony Complex are correlated with the age of SSZ spreading,but not necessarily subduction initiation because previously existing and self-sustaining subduction was ongoing.Sole ages are then not correlated with the younger age of obduction-related orogenic events(e.g.,proposed Taconic I and II)in the Newfoundland Appalachians.展开更多
Former studies show that the Muztag ophiolite, outcropped in the East Kunlun area of Xinjiang, formed in a supra-subduction zone environment. This study is to gain more information about the type of subduetion zone. T...Former studies show that the Muztag ophiolite, outcropped in the East Kunlun area of Xinjiang, formed in a supra-subduction zone environment. This study is to gain more information about the type of subduetion zone. Through field work, thin section observation and microprobe analysis, petrological and mineralogical characteristics of the metamorphic peridotites of this ophiolite are obtained. Although the olivines of metamorphic peridotites appear in three textures of metamorphic relict, metamorphic recrystallizations and orthopyroxene-melting crystallizations by thin-section observations, they have stable and low Fo range of 87.8- 89.5 by microprobe analysis. The orthopyroxenes show metamorphic relict and melting relict textures, with a low En of 88-90 and a wide range of Al2O3 content, from 2.90 wt% to 5. 13 wt%. The spinels develop anhedral-subhedral textures, with Cr^# （=Cr/（Cr＋AI）） focusing on two ranges of 0. 508-0. 723 and 0. 100-0. 118, respectively. Based on these petrological and mineralogical observations, and combined with the era and tectonic setting for the Muztag ophiolite, it can be concluded that the ophiolite formed in a supra-subduction zone where the oceanic crust subducted down to the continental are with a thick continental crust, and resulted from ocean-continent subducion within the Paleotethyan arehiopelagic ocean in the East Kunlun area of Xinjiang.展开更多
The metamophic peridotite of ophiolite belt in Mt. Ailao is composed of lherzolite and harzburgite. The former shows the charateristics of primary pyrolite and the latter shows those of deleted (relict)pyrolite. By pa...The metamophic peridotite of ophiolite belt in Mt. Ailao is composed of lherzolite and harzburgite. The former shows the charateristics of primary pyrolite and the latter shows those of deleted (relict)pyrolite. By partial melting of lherzolite, two primary magmas: tholeiitic magma and picrite-basalt magma are formed. The former evoluted into gabbro-diabase-pyroxenic basalt rock series and show the characteristics of MORB; while the latter evoluted into gabbro-diorite-albite basalt-picrite basalt one, and show the characteristics of para-MORB.展开更多
The Tauride ophiolites lie on the northern and southern flanks of an E-W-trending Tanride carbonate platform. They mainly consist of three tectonic units namely in ascending order, ophiolitic melange, sub-ophiolitic m...The Tauride ophiolites lie on the northern and southern flanks of an E-W-trending Tanride carbonate platform. They mainly consist of three tectonic units namely in ascending order, ophiolitic melange, sub-ophiolitic metamorphic sole and oceanic lithospheric remnants. They were generated above intra-oceanic subduction zones and emplaced over the Tauride carbonate platform from different Neotethyan oceanic basins in the Late Cretaceous. Tauride ophiolites from west to east are described and reviewed. All are underlain by well-preserved dynamothermal metamorphic soles of varied structural thicknesses up to 500 m that have a constant structural position between ophiolitic melange below and harzburgitic mantle tectonites above and display typical inverted metamorphic sequences from amphibolite facies above to greenschist facies below. The metamorphic soles are shown to have evolved during the initiation of subduction and emplacement processes. In the PozantlKarsantl area the contact between the metamorphic sole and the overlying serpentinized harzburgites is characterized by a 1.5-2-m-thick zone of sheared serpentinized harzburgitic mantle intercalated with amphibolites and cut by thick mafic dykes （7-8 m） which postdate intraoceanic metamorphism and high-temperature ductile deformation. This contact is interpreted as an intra-oceanic decoupling surface along which volcanics from the upper levels of the down-going plate were metamorphosed to amphibolite facies and accreted to the base of the hanging wall plate. The metamorphic soles and overlying ophiolitic rocks were intruded by numerous isolated post-metamorphic diabase dykes filled by island arc tholeiitic magma. Subduction initiation and roll-back processes best explain the structural and petrological relationships of Late Cretaceous ophiolite genesis, metamorphic sole formation and subsequent dyke emplacement of the Tauride ophiolites.展开更多
The 150-km-long Feather River ultramafic belt(FRB) of the northern Sierra Nevada,California,consists of serpentinized ultramafic rocks,gabbroic rocks,with lesser amounts of amphibolitic rocks. The gabbroic rocks conta...The 150-km-long Feather River ultramafic belt(FRB) of the northern Sierra Nevada,California,consists of serpentinized ultramafic rocks,gabbroic rocks,with lesser amounts of amphibolitic rocks. The gabbroic rocks contain metagabbro and dikes within it. Based on the electron microprobe analyses,the metagabbro consists of typical igneous composition for clinopyroxene and early hornblende,indicating that they bear low grade metamorphism. The gabbroic rocks show Nb and Ta depletion in primitive-mantle normalized plots that is similar to typical island arc rocks. We present that the metagabbro within the ultramafic rocks is forming in the suprasubduction zone setting. Tectonic models are presented for the metagabbro and amphibolitic rocks in FRB. The low P burial metamorphism of the upper plate metagabbro and the high grade amphibolitic rocks are typical of relationships of upper plate ophiolites to metamorphic soles. The amphibolitic rocks record as metamorphic sole forming during subduction initiation beneath the ultramafic rocks and metagabbro. The unusual and protracted tectonic history of the FRB is similar to other ophiolite belts of the Cordillera of California but differs in the lack of crustal rocks.展开更多
文摘The Semail ophiolite of Oman and the United Arab Emirates （UAE） provides the best preserved large slice of oceanic lithosphere exposed on the continental crust, and offers unique opportunities to study processes of ocean crust formation, subduction initiation and obduction. Metamorphic rocks exposed in the eastern UAE have traditionally been interpreted as a metamorphic sole to the Semail ophiolite. However, there has been some debate over the possibility that the exposures contain components of older Arabian continental crust. To help answer this question, presented here are new zircon and futile U-Pb geochronological data from various units of the metamorphic rocks. Zircon was absent in most samples. Those that yielded zircon and futile provide dominant single age populations that are 95-93 Ma, partially overlapping with the known age of oceanic crust formation （96.5-94.5 Ma）, and partially overlapping with cooling ages of the metamorphic rocks （95 90 Ma）. The data are interpreted as dating high-grade metamorphism during subduction burial of the sediments into hot mantle lithosphere, and rapid cooling during their subsequent exhumation. A few discordant zircon ages, interpreted as late Neoproterozoic and younger, represent minor detrital input from the continent. No evidence is found in favour of the existence of older Arabian continental crust within the metamorphic rocks of the UAE.
基金Funding to J.F.Casey for mapping,sample collections,and geochemical work in the Bay of Islands region were derived from U.S.National Science Foundation grants EAR80-26445,EAR-83-09535,EAR-88-04756a University of Houston Departmental of Earth and Atmospheric Sciences grant in 2017.
文摘The welded metamorphic sole at the base of the Bay of Islands Ophiolite Complex(BOIC)in the Northern Appalachians of Newfoundland shows a typical inverted pressure-temperature(P-T)metamorphic gradient from HT-MP granulite to LT-LP greenschist facies.It incorporates mafic volcanic/plutonic protoliths mixed with pelagic,hemi-pelagic and coarser epiclastic sedimentary protoliths.New LA-ICP-MS U–Pb concordia ages,trace elements,and Ti-in-zircon geothermometry for -250 zircon analyses from three metabasites of the upper HT sole amphibolites with N-MORB-like protoliths are reported.Two samples collected within meters of the ophiolite peridotite-sole contact of the Blow Me Down Mountain and North Arm Mountain massifs yielded the oldest comparable concordia ages of 487.7±2.6 Ma and 489.1±3.1 Ma,respectively,that are both within error of the igneous age of 488.3±1.5 Ma of the directly overlying BOIC ophiolite,which formed at a supra-subduction zone(SSZ)forearc spreading center.A third slightly younger age of 484.2±2.4 Ma was obtained for an upper HT amphibolite sample with similar phase assemblages but collected30 m below the peridotite contact of the Table Mountain massif.Zircon crystals analyzed have similar size and morphologies,subparallel rare earth element(REE)variation patterns,and steep heavy REE-enrichments((Lu/Gd)_(cn)>20),significant positive Ce anomalies(dominantly>5)and slight positive to dominantly negative Eu anomalies(1.2–0.4).Zircon shows Th/U mean values of 0.37–0.48 with little to no rim to core variation.Minimum Ti-in-zircon mean crystallization temperatures range from764–787℃.These neocrystallized zircon crystals appear to be derived from thin leucosomes within the three amphibolites.Two other samples also from the upper HT sole show evidence of inherited detrital zircon with core dates spanning the Cambrian Notre Dame Arc through older Laurentian-like basement and rift age ranges.Subcretion of the sole took place below a hot forearc asthenospheric wedge,that is,a consequence of the newly-formed BOIC forearc spreading center extending from the back arc to a triple junction along the westward-(or paleo-northward)verging trench of the Notre Dame arc.The early HT sole formation age at ca.489–488 Ma is long prior to initiation of obduction at ca.470 Ma and long after initiation of subduction beneath the paleo-northward verging Notre Dame peri-Laurentian arc at ca.514 Ma.This indicates Newfoundland sole ages of the BOIC and St.Anthony Complex are correlated with the age of SSZ spreading,but not necessarily subduction initiation because previously existing and self-sustaining subduction was ongoing.Sole ages are then not correlated with the younger age of obduction-related orogenic events(e.g.,proposed Taconic I and II)in the Newfoundland Appalachians.
文摘Former studies show that the Muztag ophiolite, outcropped in the East Kunlun area of Xinjiang, formed in a supra-subduction zone environment. This study is to gain more information about the type of subduetion zone. Through field work, thin section observation and microprobe analysis, petrological and mineralogical characteristics of the metamorphic peridotites of this ophiolite are obtained. Although the olivines of metamorphic peridotites appear in three textures of metamorphic relict, metamorphic recrystallizations and orthopyroxene-melting crystallizations by thin-section observations, they have stable and low Fo range of 87.8- 89.5 by microprobe analysis. The orthopyroxenes show metamorphic relict and melting relict textures, with a low En of 88-90 and a wide range of Al2O3 content, from 2.90 wt% to 5. 13 wt%. The spinels develop anhedral-subhedral textures, with Cr^# （=Cr/（Cr＋AI）） focusing on two ranges of 0. 508-0. 723 and 0. 100-0. 118, respectively. Based on these petrological and mineralogical observations, and combined with the era and tectonic setting for the Muztag ophiolite, it can be concluded that the ophiolite formed in a supra-subduction zone where the oceanic crust subducted down to the continental are with a thick continental crust, and resulted from ocean-continent subducion within the Paleotethyan arehiopelagic ocean in the East Kunlun area of Xinjiang.
文摘The metamophic peridotite of ophiolite belt in Mt. Ailao is composed of lherzolite and harzburgite. The former shows the charateristics of primary pyrolite and the latter shows those of deleted (relict)pyrolite. By partial melting of lherzolite, two primary magmas: tholeiitic magma and picrite-basalt magma are formed. The former evoluted into gabbro-diabase-pyroxenic basalt rock series and show the characteristics of MORB; while the latter evoluted into gabbro-diorite-albite basalt-picrite basalt one, and show the characteristics of para-MORB.
基金Cukurova University Research Foundation (Nos.MMF99.22,MMF2000.22,MMF2001.13,MMF2001.33,MMF2002BAP41,MMF2003BAP16,MMF2011BAP13,and MMF2011BAP26)Turkish Academy of Sciences (No.TüBA-GEBIP/2003-111)TüBITAK (Nos.199Y011,102Y041,106Y231,and 113Y412)
文摘The Tauride ophiolites lie on the northern and southern flanks of an E-W-trending Tanride carbonate platform. They mainly consist of three tectonic units namely in ascending order, ophiolitic melange, sub-ophiolitic metamorphic sole and oceanic lithospheric remnants. They were generated above intra-oceanic subduction zones and emplaced over the Tauride carbonate platform from different Neotethyan oceanic basins in the Late Cretaceous. Tauride ophiolites from west to east are described and reviewed. All are underlain by well-preserved dynamothermal metamorphic soles of varied structural thicknesses up to 500 m that have a constant structural position between ophiolitic melange below and harzburgitic mantle tectonites above and display typical inverted metamorphic sequences from amphibolite facies above to greenschist facies below. The metamorphic soles are shown to have evolved during the initiation of subduction and emplacement processes. In the PozantlKarsantl area the contact between the metamorphic sole and the overlying serpentinized harzburgites is characterized by a 1.5-2-m-thick zone of sheared serpentinized harzburgitic mantle intercalated with amphibolites and cut by thick mafic dykes （7-8 m） which postdate intraoceanic metamorphism and high-temperature ductile deformation. This contact is interpreted as an intra-oceanic decoupling surface along which volcanics from the upper levels of the down-going plate were metamorphosed to amphibolite facies and accreted to the base of the hanging wall plate. The metamorphic soles and overlying ophiolitic rocks were intruded by numerous isolated post-metamorphic diabase dykes filled by island arc tholeiitic magma. Subduction initiation and roll-back processes best explain the structural and petrological relationships of Late Cretaceous ophiolite genesis, metamorphic sole formation and subsequent dyke emplacement of the Tauride ophiolites.
基金financially supported by the National Natural Science Foundation of China (Nos. 40739904, 41072093, and 41302117)SINOPEC
文摘The 150-km-long Feather River ultramafic belt(FRB) of the northern Sierra Nevada,California,consists of serpentinized ultramafic rocks,gabbroic rocks,with lesser amounts of amphibolitic rocks. The gabbroic rocks contain metagabbro and dikes within it. Based on the electron microprobe analyses,the metagabbro consists of typical igneous composition for clinopyroxene and early hornblende,indicating that they bear low grade metamorphism. The gabbroic rocks show Nb and Ta depletion in primitive-mantle normalized plots that is similar to typical island arc rocks. We present that the metagabbro within the ultramafic rocks is forming in the suprasubduction zone setting. Tectonic models are presented for the metagabbro and amphibolitic rocks in FRB. The low P burial metamorphism of the upper plate metagabbro and the high grade amphibolitic rocks are typical of relationships of upper plate ophiolites to metamorphic soles. The amphibolitic rocks record as metamorphic sole forming during subduction initiation beneath the ultramafic rocks and metagabbro. The unusual and protracted tectonic history of the FRB is similar to other ophiolite belts of the Cordillera of California but differs in the lack of crustal rocks.
