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| 1 | Ophiolites of the Central Asian Orogenic Belt:Geochemical and petrological characterization and tectonic settings显示文摘We present a compilation of published data(field,petrography,ages and geochemistry)from 73 ophiolitic complexes of the Central Asian Orogenic Belt.The ophiolitic complexes,ranging in age from Neoproterozoic to Triassic.have been geochemically classified as subduction-related and subductionunrelated categories applying recent,well-established discrimination diagrams.The subductionunrelated category is further subdivided into Mid-Ocean Ridge type(MOR),a common rift-drift stage and Plume type,and the subduction-related category is subdivided into Backarc(BA),Forearc(FA).Backarc to Forearc(BA-FA)and Volcanic Arc(VA)types.The four subduction-related types define highly different geochemical features,with the BA and FA types defining end members showing subduction influence of 10%-100%and 90%-100%subduction influence,respectively,and the two other types(BAFA and VA)define values between the two end members.The subduction-related category comprises79%of the examined ophiolites,of which the BA type ophiolites is by far the dominant group,followed by the BA-FA type,and with FA and VA types as subordinate groups.The Neoproterozoic and Ordovician complexes exhibit the highest,whereas those of Silurian age exhibit the lowest subduction-influence.Of the remaining 21%subduction-unrelated ophiolites,the MOR type dominates.Both the subductionrelated and subduction-unrelated types,in particular the latter,are commonly associated with alkaline basalts taken to represent ocean island magmatism.Harzburgite,dunite,gabbro and basalt are the common lithologies in all ophiolite types,whereas the BA-FA,FA and VA types generally contain intermediate to felsic rocks,and in the FA type boninites occur.The subduction-related ophiolites types generally show low metamorphic grade,whereas greenschist.amphibolite and blueschist grades occur in the subduction-unrelated and BA types.The highly different subduction contribution(from 0 to 100%in the MOR and FA,respectively),attest to variable dips of the subducting slab,as well as variable flux of subduction-related elements into the mantle above subducting slabs,from where the ophiolite magmas got their geochemical fingerprints.As most MOR ophiolites get subducted to the deep mantle,the subduction-related ophiolites have become a dominant ophiolitic type on Earth’s surface through all times supporting the idea about the early start of Plate Tectonics. | Harald Furnes Inna Safonova | 2019 | Geoscience Frontiers2019，10，4： | 15 |
| 2 | Four billion years of ophiolites reveal secular trends in oceanic crust formation显示文摘We combine a geological,geochemical and tectonic dataset from 118 ophioiite complexes of the major global Phanerozoic orogenic belts with similar datasets of ophiolites from 111 Precambrian greenstone belts to construct an overview of oceanic crust generation over 4 billion years.Geochemical discrimination systematics built on immobile trace elements reveal that the basaltic units of the Phanerozoic ophiolites are dominantly subduction-related(75%),linked to backarc processes and characterized by a strong MORB component,similar to ophiolites in Precambrian greenstone sequences(85%).The remaining 25%Phanerozoic subduction-unrelated ophiolites are mainly(74%) of Mid-Ocean-Ridge type(MORB type),in contrast to the equal proportion of Rift/Continental Margin,Plume,and MORB type ophiolites in the Precambrian greenstone belts.Throughout the Phanerozoic there are large geochemical variations in major and trace elements,but for average element values calculated in 5 bins of 100 million year intervals there are no obvious secular trends.By contrast,basaltic units in the ophiolites of the Precambrian greenstones(calculated in 12 bins of 250 million years intervals),starting in late Paleo- to early Mesoproterozoic(ca.2.0-1.8 Ga),exhibit an apparent decrease in the average values of incompatible elements such as Ti,P,Zr,Y and Nb,and an increase in the compatible elements Ni and Cr with deeper time to the end of the Archean and into the Hadean.These changes can be attributed to decreasing degrees of partial melting of the upper mantle from Hadean/Archean to Present.The onset of geochemical changes coincide with the timing of detectible changes in the structural architecture of the ophiolites such as greater volumes of gabbro and more common sheeted dyke complexes,and lesser occurrences of ocelli(varioles) in the pillow lavas in ophiolites younger than 2 Ga.The global data from the Precambrian ophiolites,representative of nearly 50%of all known worldwide greenstone belts provide significant clues for the operation of plate tectonic processes in the Archean. | Harald Furnes Maarten de Wit Yildirim Dilek | 2014 | Geoscience Frontiers2014，5，4： | 13 |
| 3 | Paleoarchean bedrock lithologies across the Makhonjwa Mountains of South Africa and Swaziland linked to geochemical,magnetic and tectonic data reveal early plate tectonic genes flanking subduction margins显示文摘The Makhonjwa Mountains, traditionally referred to as the Barberton Greenstone Belt, retain an iconic Paleoarchean archive against which numerical models of early earth geodynamics can be tested. We present new geologic and structural maps, geochemical plots, geo- and thermo-chronology, and geophysical data from seven silicic, mafic to ultramafic complexes separated by major shear systems across the southern Makhonjwa Mountains. All reveal signs of modern oceanic back-arc crust and subductionrelated processes. We compare the rates of processes determined from this data and balance these against plate tectonic and plume related models. Robust rates of both horizontal and vertical tectonic processes derived from the Makhonjwa Mountain complexes are similar, well within an order of magnitude, to those encountered across modern oceanic and orogenic terrains flanking Western Pacific-like subduction zones. We conclude that plate tectonics and linked plate-boundary processes were well established by 3.2-3.6 Ga. Our work provides new constraints for modellers with rates of a 'basket' of processes against which to test Paleoarchean geodynamic models over a time period close to the length of the Phanerozoic. | Maarten de Wit Harald Furnes Scott MacLennan Moctar Doucouré Blair Schoene Ute Weckmann Uma Martinez Sam Bowring | 2018 | Geoscience Frontiers2018，9，3： | 4 |
| 4 | Geochemical Characterization of Intermediate to Silicic Rocks in the Global Ophiolite Record显示文摘Ophiolites consist predominantly of mafic-ultramafic rocks but also contain in subordinate amounts intermediate to silicic intrusive–extrusive rocks in them.These rocks,although not in large volumes in comparison to mafic–ultramafic units,reveal significant clues about melt evolution,magmatic accretion processes,and partial melting of pre-existing oceanic crust during ophiolite evolution.They also yield ample accessory minerals,such as zircon and monazite,which are widely used to date the timing of oceanic crust generation.We present a global synthesis of the occurrence of such leucocratic,intrusive and volcanic rocks from 150 Phanerozoic to Archean ophiolites,and evaluate models for their genesis during the development of oceanic crust in different tectonic environments.In the ophiolite complexes we have investigated,intrusive and extrusive rocks show a wide range in composition.Intermediate rocks(with Si O2 between 52and 63 wt.%)include andesite and diorite,whereas the silicic rocks(with Si O2 more than 63 wt.%)include dacite,trondhjemite,tonalite and granite.For subduction-relatedandsubduction-unrelated MORB-type ophiolites in the Phanerozoic orogenic belts,the most commonly reported intermediate to silicic intrusive rocks,independent of their original tectonic setting,are trondhjemite,plagiogranite and tonalite,and less commonly quartz-diorite and diorite.These rock types have been documented from 80 percent of the 104ophiolites included in this study.Intermediate to silicic lavas and/or pyroclastic rocks(basaltic andesite,andesite,dacite and rhyolite)are less abundant,and have been reported from only 35 percent of the 104 ophiolites.Trachyte occurs in some of the Rift/Continental Margintype,subduction-unrelated ophiolitesEvolvedrocksinsubduction-unrelated,Rift/Continental Margin ophiolites are predominantly basaltic andesite and andesite,whereas MOR type(mid-ocean ridge)ophiolites exhibit nearly equal proportions of basaltic andesite/andesite and rhyodacite and Plume/MOR type ophiolites are characterized by rhyolites.Intermediate to silicic volcanic uints in the Backarc sub-group of subduction-related ophiolites are characterized by similar amounts of basaltic andesite/andesite and rhyodacite,whereas in the Backarc to Forearc,Forearc,and Volcanic Arc sub-groups they are mainly basaltic andesite/andesite.Intermediate to silicic rocks in Rift/Continental Margin and Plume/MOR type ophiolites are generally LREE-enriched,whereas those in the MOR type vary from LREE-depleted to LREE-enriched.The Backarc and Backarc to Forearc types are similar to the MOR type;silicic rocks of the Forearc and Volcanic Arc types are generally LREE-enriched.The main process in the formation of the majority of the intermediate to silicic rocks in both subduction-unrelated and subduction-related ophiolites is partial melting of basaltic and/or gabbroic rocks beneath the spreading centers,whereas a minor volume in subduction-related ophiolites are adakites that were produced from partial melting of a subducting slab.Silcic to intermediate rocks in Plume/MOR type ophiolites are generated by fractional crystallization of basaltic melt.The incompatible,non-conservative elements,such as Ba and Th,are weakly to strongly enriched in subduction-related ophiolites as a result of shallow to deep enrichment associated with subduction zone processes.The field occurrence and the geochemical character of leucocratic rocks in ophiolites show considerable variations,providing additional constraints on the petrogenesis of ophiolites in different tectonic settings.Fractional crystallization appears to have been an important process in the formation of the intermediate to silicic rocks in Rift/CM and P/MOR ophiolites in the subduction-unrelated class,as well as VA ophiolites in the subduction-related class.In these types of ophiolites magma chambers might have operated as closed-systems for a sufficient amount of time for fractionation and compositional zoning of the magma to take place(Fig.1C1).Fractional crystallization was of subordinate importance during the igneous construction of the MOR,BA,FA,and BA-FA ophiolites.Instead,partial melting of hydrothermally altered crust was the most common process resulting in the majority of the intermediate to felsic rocks of the Rift/CM(estimated to 60%),MOR(estimated to 95%),and the BA,FA and BA-FA(estimated to 75%)ophiolites,as well as a high proportion of the VA ophiolites(~40%)(Fig.1C2).Slab-generated melts that result in the production of intermediate to silicic rocks of adakitic character represent a relatively small fraction(~15%in Phanerozoic ophiolites)of subduction-related ophiolites(Fig.1C3).Some adakites are very low in incompatible elements(e.g.,La),suggesting that these melts were generated from highly depleted MORB.Precambrian ophiolites contain a higher percentage of adakites(ca.35%),indicating that physical conditions were more favorable for slab melting during the Precambrian than in the Phanerozoic.This phenomenon may have resulted from higher geothermal gradient along relatively shallow-dippingsubductionzonesduringthe Precambrian. | Yildirim DILEK Harald FURNES | 2017 | Acta Geologica Sinica(English Edition)2017，91，S1： | 0 |
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