Geochemistry is a powerful tool to help characterize the tectonic setting of igneous rocks associations.However,when continental mafic dykes and flood basalts are the target most of the proposed geochemical discrimina...Geochemistry is a powerful tool to help characterize the tectonic setting of igneous rocks associations.However,when continental mafic dykes and flood basalts are the target most of the proposed geochemical discrimination diagrams fail to correctly classify them,i.e.many mafic展开更多
Mineralogical,geochemical and isotopic(Sr and Nd) studies on the recently reported ca. 124 Ma'anorogenic lamproite' dyke from the Palanpur area, Kutch seismogenic rift zone, northwestern India, are presented. ...Mineralogical,geochemical and isotopic(Sr and Nd) studies on the recently reported ca. 124 Ma'anorogenic lamproite' dyke from the Palanpur area, Kutch seismogenic rift zone, northwestern India, are presented. We propose a new classification for the dyke as a damtjernite(ultramafic lamprophyre; UML)based on its porphyritic-panidiomorphic texture, abundance of phlogopite, presence of nepheline in the groundmass, and the composition of liquidus phases such as olivine, phlogopite, magnetite, and clinopyroxene(diopside). The Palanpur UML is primitive(Mg# =74-77),silica-undersaturated(SiO_2<40 wt.%),potassic to slightly sodic in nature, and is strikingly similar to the ～69 Ma UML dykes and sills of the Tethyan Indus suture zone, which are considered as the earliest yet known manifestations of the Deccan Large Igneous Province(LIP). Bulk-rock(^(87)Sr/^(86)Sr)_i(0.70460-0.70461) and ε_(N)d(t)(+2.56 to-0.69) of the Palanpur UML signify derivation from a slightly depleted mantle source similar to that of asthenospheric magmas such as OIB. This is further attested to by the high incompatible trace element ratios(viz., La/Ba, Nb/U, Nb/La and Ta/Yb) that are typical of plume-type magmas. However, the Neoproterozoic T_(DM) depleted mantle Nd model ages( ～655-919 Ma) also necessitate some involvement of a lithospheric mantle component in its genesis. High bulk-rock Fe_2 O_3~t and TiO_2 contents require the involvement of a fertile peridotitic mantle source, whereas high La/Yb(60-80) implies a control by residual garnet. Higher Rb/Sr and lower Ba/Rb suggest phlogopite as a residual phase and high Nb and lower La/Sm favour carbonatite, rather than silicate melt as metasomatising agent. Low degrees of partial melting of a primitive garnet lherzolite mantle can account for the observed REE patterns in the Palanpur UML. The Palanpur UML shares a temporal similarity to the Kerguelen plume-derived Rajmahal basalts and associated alkaline rocks from the eastern India. The tectonomagmatic significance of its emplacement during the mid-Cretaceous vis-a-vis various models involving the timing of eruption of the Deccan and the Rahmahal Traps and the rifting in the Kutch basin induced by far-field plate reorganization is evaluated.展开更多
This study focuses on Paleoproterozoic plutonic masses and their host rocks located within the Mako belt and associated basins.The objective is to identify their implications for the lithological and tectonic structur...This study focuses on Paleoproterozoic plutonic masses and their host rocks located within the Mako belt and associated basins.The objective is to identify their implications for the lithological and tectonic structure of the KedougouKenieba Inlier through an approach integrating geological mapping,petrography,structural analysis,geochemistry,geophysics and geochronology.Indeed,the lithostructural and geochemical characterization of the plutonic masses shows that Dioudioukounkou is a peralkaline calc-alkaline granite from a volcanic arc located in an oceanic context of an active compressive margin.The post-orogenic granite of Dioudioukounkou is 385 ppm enriched in light rare earth elements.The metaluminous Koulountou pluton and the peraluminous Tiguida pluton represent island arc granites.Tiguida is syncollisional and associated with a low percentage of light rare earth elements with a LaN/SmN ratio below the detection limit and a Na_(2)O+K_(2)O content of 6.89%,which is the highest alkaline content in Sesam.Koulountou is a post-collisional granite marked by an active compressive margin environment.It is enriched in light rare earth elements with a LaN/SmN ratio of 2.55.Tinkoto and Diabba are hosted in tholeiitic and calc-alkaline volcanics intersected by calc-alkaline plutonites.Dioudioukounkou shows a concordant U-Pb age of zircon of 2156.1 Ma±5.6 Ma.This age appears to correspond to the phase of major magmatic activity associated with early Eburnean deformation D1 comparable to CiscoW.Tinkoto and Koulountou formed at 2109.3 Ma±6.0 Ma and 2106.7 Ma±8.1 Ma,respectively.Tinkoto is older and linked to the main CiscoM shear corresponding to D2.Koulountou is linked to the second order structures of CiscoM.The geophysical signatures of high to medium magnetic intensities highlight a succession of tectonovolcanic arcs:the arc in context of an oceanic volcanic arc associated with CiscoW,the arc in context of an island arc associated with CiscoM,the arc in context of a back-arc basin associated with CiscoE or D3,and the arc in context of an intracontinental volcanic arc associated with the CiscoF shear contact of Faleme or D4.The basins are defined in low to medium magnetic zones and correspond to the Diale back-arc basin and the Kofi intracontinental basin.These formations are intersected by the volcanic arc granite of Dioudioukounkou,the island arc granites of Koulountou and Tiguida,the back-arc basin granites of Tinkoto and Saraya,and finally the pluton of the intracontinental basin of Gamaye.The latest lithostructural architecture of the Kedougou-Kenieba inlier highlights the tectonomagmatic complexes of Maco,Sesam,Diabba and Kofi,which constitute the regional substrate enclosing these plutonites.The geodynamic evolution of these complexes is linked to a single magmatic event associated with tectonomagmatic episodes.Diabba,which represents the back-arc basin,shows an overturned funnelshaped geosynclinal architecture,while the intracontinental Kofi basin reveals an hemigraben configuration opened at the end of late orogenic phase behind the intracontinental Faleme collision arc.展开更多
基金The Brazilian Sao Paulo State Research Foundation(FAPESP)partially supported this research(grants 2012/15824-6 and 2012/07243-3)
文摘Geochemistry is a powerful tool to help characterize the tectonic setting of igneous rocks associations.However,when continental mafic dykes and flood basalts are the target most of the proposed geochemical discrimination diagrams fail to correctly classify them,i.e.many mafic
文摘Mineralogical,geochemical and isotopic(Sr and Nd) studies on the recently reported ca. 124 Ma'anorogenic lamproite' dyke from the Palanpur area, Kutch seismogenic rift zone, northwestern India, are presented. We propose a new classification for the dyke as a damtjernite(ultramafic lamprophyre; UML)based on its porphyritic-panidiomorphic texture, abundance of phlogopite, presence of nepheline in the groundmass, and the composition of liquidus phases such as olivine, phlogopite, magnetite, and clinopyroxene(diopside). The Palanpur UML is primitive(Mg# =74-77),silica-undersaturated(SiO_2<40 wt.%),potassic to slightly sodic in nature, and is strikingly similar to the ～69 Ma UML dykes and sills of the Tethyan Indus suture zone, which are considered as the earliest yet known manifestations of the Deccan Large Igneous Province(LIP). Bulk-rock(^(87)Sr/^(86)Sr)_i(0.70460-0.70461) and ε_(N)d(t)(+2.56 to-0.69) of the Palanpur UML signify derivation from a slightly depleted mantle source similar to that of asthenospheric magmas such as OIB. This is further attested to by the high incompatible trace element ratios(viz., La/Ba, Nb/U, Nb/La and Ta/Yb) that are typical of plume-type magmas. However, the Neoproterozoic T_(DM) depleted mantle Nd model ages( ～655-919 Ma) also necessitate some involvement of a lithospheric mantle component in its genesis. High bulk-rock Fe_2 O_3~t and TiO_2 contents require the involvement of a fertile peridotitic mantle source, whereas high La/Yb(60-80) implies a control by residual garnet. Higher Rb/Sr and lower Ba/Rb suggest phlogopite as a residual phase and high Nb and lower La/Sm favour carbonatite, rather than silicate melt as metasomatising agent. Low degrees of partial melting of a primitive garnet lherzolite mantle can account for the observed REE patterns in the Palanpur UML. The Palanpur UML shares a temporal similarity to the Kerguelen plume-derived Rajmahal basalts and associated alkaline rocks from the eastern India. The tectonomagmatic significance of its emplacement during the mid-Cretaceous vis-a-vis various models involving the timing of eruption of the Deccan and the Rahmahal Traps and the rifting in the Kutch basin induced by far-field plate reorganization is evaluated.
文摘This study focuses on Paleoproterozoic plutonic masses and their host rocks located within the Mako belt and associated basins.The objective is to identify their implications for the lithological and tectonic structure of the KedougouKenieba Inlier through an approach integrating geological mapping,petrography,structural analysis,geochemistry,geophysics and geochronology.Indeed,the lithostructural and geochemical characterization of the plutonic masses shows that Dioudioukounkou is a peralkaline calc-alkaline granite from a volcanic arc located in an oceanic context of an active compressive margin.The post-orogenic granite of Dioudioukounkou is 385 ppm enriched in light rare earth elements.The metaluminous Koulountou pluton and the peraluminous Tiguida pluton represent island arc granites.Tiguida is syncollisional and associated with a low percentage of light rare earth elements with a LaN/SmN ratio below the detection limit and a Na_(2)O+K_(2)O content of 6.89%,which is the highest alkaline content in Sesam.Koulountou is a post-collisional granite marked by an active compressive margin environment.It is enriched in light rare earth elements with a LaN/SmN ratio of 2.55.Tinkoto and Diabba are hosted in tholeiitic and calc-alkaline volcanics intersected by calc-alkaline plutonites.Dioudioukounkou shows a concordant U-Pb age of zircon of 2156.1 Ma±5.6 Ma.This age appears to correspond to the phase of major magmatic activity associated with early Eburnean deformation D1 comparable to CiscoW.Tinkoto and Koulountou formed at 2109.3 Ma±6.0 Ma and 2106.7 Ma±8.1 Ma,respectively.Tinkoto is older and linked to the main CiscoM shear corresponding to D2.Koulountou is linked to the second order structures of CiscoM.The geophysical signatures of high to medium magnetic intensities highlight a succession of tectonovolcanic arcs:the arc in context of an oceanic volcanic arc associated with CiscoW,the arc in context of an island arc associated with CiscoM,the arc in context of a back-arc basin associated with CiscoE or D3,and the arc in context of an intracontinental volcanic arc associated with the CiscoF shear contact of Faleme or D4.The basins are defined in low to medium magnetic zones and correspond to the Diale back-arc basin and the Kofi intracontinental basin.These formations are intersected by the volcanic arc granite of Dioudioukounkou,the island arc granites of Koulountou and Tiguida,the back-arc basin granites of Tinkoto and Saraya,and finally the pluton of the intracontinental basin of Gamaye.The latest lithostructural architecture of the Kedougou-Kenieba inlier highlights the tectonomagmatic complexes of Maco,Sesam,Diabba and Kofi,which constitute the regional substrate enclosing these plutonites.The geodynamic evolution of these complexes is linked to a single magmatic event associated with tectonomagmatic episodes.Diabba,which represents the back-arc basin,shows an overturned funnelshaped geosynclinal architecture,while the intracontinental Kofi basin reveals an hemigraben configuration opened at the end of late orogenic phase behind the intracontinental Faleme collision arc.
