The Spanish Central System(SCS)contains several suites of Palaeozoic mafic igneous intrusions with contrasting geochemical affinity:Ordovician tholeiitic metabasites,Variscan calc-alkaline gabbros(Gb1)and microdiorite...The Spanish Central System(SCS)contains several suites of Palaeozoic mafic igneous intrusions with contrasting geochemical affinity:Ordovician tholeiitic metabasites,Variscan calc-alkaline gabbros(Gb1)and microdiorites(Gb2),shoshonitic monzogabbros(Gb3)and alkaline diabases and lamprophyres(Gb4).Not all of these rocks are accurately dated,and several aspects of their genesis are still poorly understood.We present new whole-rock geochemical data(major and trace elements,and Sr-Nd isotopes),U-Pb and Lu-Hf isotopic ratios on magmatic zircons and 40 Ar/39Ar amphibole geochronology results in order to establish a precise chronology for the successive events of magmatism in the SCS,and discuss the nature of their mantle sources.Accurate ages have been determined for the Variscan gabbros(305-294 Ma),the microdiorites(299 Ma)and the accompanying felsic porphyries(292 Ma),the shoshonitic monzogabbros(285 Ma),and the alkaline diabases(274 Ma)and monzosyenites(271-264 Ma).According to this information,the Variscan mafic magmatism would be mainly concentrated in the range of 305-294 Ma,with a final manifestation represented by the minor shoshonitic dykes.The alkaline magmatism proved to be slightly older than previously thought and yielded at least two distinct pulses:diabases and lamprophyres-monzosyenites.Zircon Hf isotopes evidence the involvement of depleted and slightly enriched mantle sources.The bulk of the eHf values are in the broad range of-8 to+11,indicative of melting both depleted and enriched mantle regions.The high within-sample Hf isotope variation(up to-11 epsilon units)shown by samples from the Variscan series(gabbros,microdiorites and monzogabbros)could be explained mainly by hybridisation of magmas derived from heterogeneous lithospheric mantle sources.Pressure estimates indicate that the Variscan mafic magmas were extracted from the lithosphere.The Nd-Hf isotopic composition of these suites of rocks suggests the recycling of pelitic sediments during the Cadomian orogeny.Deeper(asthenospheric)mantle levels were involved in the generation of the alkaline suite,whose anomalous negative eHf values(moderately decoupled with respect to radiogenic Nd)could be associated with subducted oceanic components raised by mantle upwelling associated with lithosphere thinning and extension during the Permian.展开更多
So far,the nature and evolution of the lower crust under central Spain have been constrained mainly on the basis of a heterogeneous suite of granulite xenoliths from the Spanish Central System(SCS).In recent years,ult...So far,the nature and evolution of the lower crust under central Spain have been constrained mainly on the basis of a heterogeneous suite of granulite xenoliths from the Spanish Central System(SCS).In recent years,ultramafic volcanics from the Calatrava Volcanic Field(CVF)have also provided deep-seated crustal xenoliths which have not been studied in detail.Our data,combining mineral,whole-rock and isotopic geochemistry with U–Pb–Hf isotope ratios in zircons from the CVF and SCS xenoliths,highlight the felsic composition of the lower crust under central Iberia.A number of the Calatrava xenoliths represents Variscan igneous protoliths,which are a minor population in the SCS,and were likely formed by crystallisation of intermediate and felsic melts in the lower crust during the Variscan orogeny(leucodiorite protolith age of 314±3 Ma and leucogranite protolith age of 308±2.5 Ma).U–Pb data of metamorphic zircons show that granulite-facies metamorphism mainly occurred from 299 to 285 Ma in both areas.These ages are slightly younger than those of granitic intrusions that could be genetically related to the granulitic residue,which points to a main role of U–Pb isotope resetting in lower crustal zircons during HT or UHT conditions.The zircon U–Pb–Hf isotopic ratios support the idea that the lower crust in central Iberia consists mainly of Ordovician–Neoproterozoic metaigneous and metasedimentary rocks associated with the Cadomian continental arc of northern Gondwana.These rocks provide evidence of mixing between juvenile magmas and an enriched crustal component,ultimately extracted from an Eburnean crust.Other more evolved components present in detrital zircons are likely related to recycling of Archean crust derived from North Africa cratonic terranes.展开更多
基金supported by the CGL2016-78796 project of the Ministerio de Economiay Competitividad of Spain and the UCM Research Group 2018/19 n°910492。
文摘The Spanish Central System(SCS)contains several suites of Palaeozoic mafic igneous intrusions with contrasting geochemical affinity:Ordovician tholeiitic metabasites,Variscan calc-alkaline gabbros(Gb1)and microdiorites(Gb2),shoshonitic monzogabbros(Gb3)and alkaline diabases and lamprophyres(Gb4).Not all of these rocks are accurately dated,and several aspects of their genesis are still poorly understood.We present new whole-rock geochemical data(major and trace elements,and Sr-Nd isotopes),U-Pb and Lu-Hf isotopic ratios on magmatic zircons and 40 Ar/39Ar amphibole geochronology results in order to establish a precise chronology for the successive events of magmatism in the SCS,and discuss the nature of their mantle sources.Accurate ages have been determined for the Variscan gabbros(305-294 Ma),the microdiorites(299 Ma)and the accompanying felsic porphyries(292 Ma),the shoshonitic monzogabbros(285 Ma),and the alkaline diabases(274 Ma)and monzosyenites(271-264 Ma).According to this information,the Variscan mafic magmatism would be mainly concentrated in the range of 305-294 Ma,with a final manifestation represented by the minor shoshonitic dykes.The alkaline magmatism proved to be slightly older than previously thought and yielded at least two distinct pulses:diabases and lamprophyres-monzosyenites.Zircon Hf isotopes evidence the involvement of depleted and slightly enriched mantle sources.The bulk of the eHf values are in the broad range of-8 to+11,indicative of melting both depleted and enriched mantle regions.The high within-sample Hf isotope variation(up to-11 epsilon units)shown by samples from the Variscan series(gabbros,microdiorites and monzogabbros)could be explained mainly by hybridisation of magmas derived from heterogeneous lithospheric mantle sources.Pressure estimates indicate that the Variscan mafic magmas were extracted from the lithosphere.The Nd-Hf isotopic composition of these suites of rocks suggests the recycling of pelitic sediments during the Cadomian orogeny.Deeper(asthenospheric)mantle levels were involved in the generation of the alkaline suite,whose anomalous negative eHf values(moderately decoupled with respect to radiogenic Nd)could be associated with subducted oceanic components raised by mantle upwelling associated with lithosphere thinning and extension during the Permian.
基金The LA-ICPMS laboratory at the University of Johannesburg was funded by NRF-NEP grant#93208supported by PPM and the CoE DSI-NRF CIMERAsupported by,the PID2020-115980 GB-I00 research project of the Ministerio de Ciencia e Innovación of Spain.
文摘So far,the nature and evolution of the lower crust under central Spain have been constrained mainly on the basis of a heterogeneous suite of granulite xenoliths from the Spanish Central System(SCS).In recent years,ultramafic volcanics from the Calatrava Volcanic Field(CVF)have also provided deep-seated crustal xenoliths which have not been studied in detail.Our data,combining mineral,whole-rock and isotopic geochemistry with U–Pb–Hf isotope ratios in zircons from the CVF and SCS xenoliths,highlight the felsic composition of the lower crust under central Iberia.A number of the Calatrava xenoliths represents Variscan igneous protoliths,which are a minor population in the SCS,and were likely formed by crystallisation of intermediate and felsic melts in the lower crust during the Variscan orogeny(leucodiorite protolith age of 314±3 Ma and leucogranite protolith age of 308±2.5 Ma).U–Pb data of metamorphic zircons show that granulite-facies metamorphism mainly occurred from 299 to 285 Ma in both areas.These ages are slightly younger than those of granitic intrusions that could be genetically related to the granulitic residue,which points to a main role of U–Pb isotope resetting in lower crustal zircons during HT or UHT conditions.The zircon U–Pb–Hf isotopic ratios support the idea that the lower crust in central Iberia consists mainly of Ordovician–Neoproterozoic metaigneous and metasedimentary rocks associated with the Cadomian continental arc of northern Gondwana.These rocks provide evidence of mixing between juvenile magmas and an enriched crustal component,ultimately extracted from an Eburnean crust.Other more evolved components present in detrital zircons are likely related to recycling of Archean crust derived from North Africa cratonic terranes.
