In Cameroon, the Ngazi-Tina region belongs to the Adamawa-Yade domain of the Pan-African Central African Fold Belt (CAFB). It is composed of two petrographic types: quartz-monzonites (majority) and nepheline syenites....In Cameroon, the Ngazi-Tina region belongs to the Adamawa-Yade domain of the Pan-African Central African Fold Belt (CAFB). It is composed of two petrographic types: quartz-monzonites (majority) and nepheline syenites. Two morphological types, prismatic and pyramidal, were recognized in the zircon grains samples. These zircon types display internal structure</span><span style="font-family:Verdana;">s</span><span style="font-family:""><span style="font-family:Verdana;"> typical of magmatic zircons. Zircons separated from the Ngazi-Tina samples contain higher abundances of Hf (close to 8000 ppm) and moderate trace elements (Y, Th, U, Nb, Ta) and REE contents, suggesting a variable degree of magmatic evolution. The chondrite-normalized REE patterns of zircons are characterized by LREE depletion relative to HREE with positive Ce and negative Eu anomalies, typical of magmatic zircons. The high Hf content together with high Ce/Ce*, Th/U, Zr/Hf ratios suggest magma crystallization under variable oxidation and oxygen fugacity. The application of Ti-in-zircon thermometer reveals crystallization temperatures ranging from 678</span><span style="color:#4F4F4F;font-family:Verdana;">°</span><span style="font-family:Verdana;">C to 811</span><span style="color:#4F4F4F;font-family:Verdana;">°</span><span style="font-family:Verdana;">C and 658</span><span style="color:#4F4F4F;font-family:Verdana;">°</span><span style="font-family:Verdana;">C to 768</span><span style="color:#4F4F4F;font-family:Verdana;">°</span><span style="font-family:Verdana;"></span><span style="font-family:Verdana;">C for quartz monzonites and nepheline syenites respectively. These features indicate probably a partial melting of continental crust as the source of these zircons grains and emplacement in</span></span><span style="font-family:Verdana;"> the</span><span style="font-family:Verdana;"> magmatic-arc setting.展开更多
This paper investigates the geochemistry of bulk rock and infers the petrogenesis of ultramafic rocks in the Boali Precambrian terrane in Mbi Valley,in the North of the Central African Republic(CAR).The studied rocks ...This paper investigates the geochemistry of bulk rock and infers the petrogenesis of ultramafic rocks in the Boali Precambrian terrane in Mbi Valley,in the North of the Central African Republic(CAR).The studied rocks comprise coarse primary olivine and orthopyroxene relics(dominant phase),magnesio-hornblende,magnetite,and antigorite.Whole-rock analysis indicates low SiO_(2)(average of 43.14 wt%)and high MgO(19.84–26.98 wt%)contents and their Mg number(Mg#)ranges from 74 to 82.TheydisplayhighNi(526–865 ppm),Cr(1500–3680 ppm)contents.AFM(Na_(2)O-K_(2)O)-FeO-MgO)and ACM(Al_(2)O_(3)-CaO-MgO)ternary diagrams have revealed that the studied samples correspond to arc-related ultramafic cumulates.Chondrite-normalized REE plots display an increasing trend from La to Sm(CeN/SmN:0.74–1.81),weak negative to no Eu(Eu/Eu*=0.72–1.05)and strong negative Ce(Ce/Ce*=0.33–0.98)anomalies.Primitive mantle normalized of multi-element diagrams exhibit LREE enrichment and large ion lithophile elements(LILE)relative to high field strength elements(HFSE),and notable negative anomalies in Nb.This suggests the generation of the parent melt by slab dehydration and wedge melting processes.In addition,incompatible trace element composition and ratios assumed that the source magma had an enhanced mantle source associated with a prominent influence of continental crust.Metasomatism of mantle wedge by plate-dehydrated,LILE-rich fluids and the incorporation of sediments derived from subduction explain the enhancement of the source.Integrated major and trace element compositions jointly with the tectonic reconstruction of this region and LA-ICP-MS U–Pb data on zircon constrain the emplacement age at ca.2099 Ma in a continental margin arc setting involving subduction of an oceanic plate beneath the continental lithosphere,dehydration of the slab and mantle wedge melting.This result intimate that the remnants of Paleoproterozoic oceanic crust or subduction event and subsequent basins closure extended from the Congo craton in Cameroon to CAR and NE Brazil.展开更多
文摘In Cameroon, the Ngazi-Tina region belongs to the Adamawa-Yade domain of the Pan-African Central African Fold Belt (CAFB). It is composed of two petrographic types: quartz-monzonites (majority) and nepheline syenites. Two morphological types, prismatic and pyramidal, were recognized in the zircon grains samples. These zircon types display internal structure</span><span style="font-family:Verdana;">s</span><span style="font-family:""><span style="font-family:Verdana;"> typical of magmatic zircons. Zircons separated from the Ngazi-Tina samples contain higher abundances of Hf (close to 8000 ppm) and moderate trace elements (Y, Th, U, Nb, Ta) and REE contents, suggesting a variable degree of magmatic evolution. The chondrite-normalized REE patterns of zircons are characterized by LREE depletion relative to HREE with positive Ce and negative Eu anomalies, typical of magmatic zircons. The high Hf content together with high Ce/Ce*, Th/U, Zr/Hf ratios suggest magma crystallization under variable oxidation and oxygen fugacity. The application of Ti-in-zircon thermometer reveals crystallization temperatures ranging from 678</span><span style="color:#4F4F4F;font-family:Verdana;">°</span><span style="font-family:Verdana;">C to 811</span><span style="color:#4F4F4F;font-family:Verdana;">°</span><span style="font-family:Verdana;">C and 658</span><span style="color:#4F4F4F;font-family:Verdana;">°</span><span style="font-family:Verdana;">C to 768</span><span style="color:#4F4F4F;font-family:Verdana;">°</span><span style="font-family:Verdana;"></span><span style="font-family:Verdana;">C for quartz monzonites and nepheline syenites respectively. These features indicate probably a partial melting of continental crust as the source of these zircons grains and emplacement in</span></span><span style="font-family:Verdana;"> the</span><span style="font-family:Verdana;"> magmatic-arc setting.
基金financial support of SCAC (Service de la Coopération et d’Action Culturelle) Bangui。
文摘This paper investigates the geochemistry of bulk rock and infers the petrogenesis of ultramafic rocks in the Boali Precambrian terrane in Mbi Valley,in the North of the Central African Republic(CAR).The studied rocks comprise coarse primary olivine and orthopyroxene relics(dominant phase),magnesio-hornblende,magnetite,and antigorite.Whole-rock analysis indicates low SiO_(2)(average of 43.14 wt%)and high MgO(19.84–26.98 wt%)contents and their Mg number(Mg#)ranges from 74 to 82.TheydisplayhighNi(526–865 ppm),Cr(1500–3680 ppm)contents.AFM(Na_(2)O-K_(2)O)-FeO-MgO)and ACM(Al_(2)O_(3)-CaO-MgO)ternary diagrams have revealed that the studied samples correspond to arc-related ultramafic cumulates.Chondrite-normalized REE plots display an increasing trend from La to Sm(CeN/SmN:0.74–1.81),weak negative to no Eu(Eu/Eu*=0.72–1.05)and strong negative Ce(Ce/Ce*=0.33–0.98)anomalies.Primitive mantle normalized of multi-element diagrams exhibit LREE enrichment and large ion lithophile elements(LILE)relative to high field strength elements(HFSE),and notable negative anomalies in Nb.This suggests the generation of the parent melt by slab dehydration and wedge melting processes.In addition,incompatible trace element composition and ratios assumed that the source magma had an enhanced mantle source associated with a prominent influence of continental crust.Metasomatism of mantle wedge by plate-dehydrated,LILE-rich fluids and the incorporation of sediments derived from subduction explain the enhancement of the source.Integrated major and trace element compositions jointly with the tectonic reconstruction of this region and LA-ICP-MS U–Pb data on zircon constrain the emplacement age at ca.2099 Ma in a continental margin arc setting involving subduction of an oceanic plate beneath the continental lithosphere,dehydration of the slab and mantle wedge melting.This result intimate that the remnants of Paleoproterozoic oceanic crust or subduction event and subsequent basins closure extended from the Congo craton in Cameroon to CAR and NE Brazil.
