@article{fdi:010062240,
  title = {{C}hemical and oxygen isotopic compositions, age and origin of gem corundums in {M}adagascar alkali basalts},
  author = {{R}akotosamizanany, {S}. and {G}iuliani, {G}aston and {O}hnenstetter, {D}. and {R}akotondrazafy, {A}. {F}. {M}. and {F}allick, {A}. {E}. and {P}aquette, {J}. {L}. and {T}iepolo, {M}.},
  editor = {},
  language = {{ENG}},
  abstract = {{M}adagascar is a large producer of gem corundum recovered from continental basaltic fields. {T}he main mining areas are sapphire-bearing palaeoplacer deposits such as {A}mbondromifehy and {N}osy {B}e in {A}ntsiranana {P}rovince, northern {M}adagascar; {S}oamiakatra-{M}androsohasina in {A}ntananarivo {P}rovince, central {M}adagascar; and {V}atomandry district in {T}oamasina {P}rovince, eastern {M}adagascar. {I}n {A}ntananarivo {P}rovince, {S}oamiakatra is a primary deposit where ruby is found in metagabbro and pyroxenite xenoliths, brought-up to the upper crust by the {A}nkaratra volcanics. {P}etrographic studies indicate two different conditions of ruby formation, at the boundary of the eclogite domain ({T} 1100 {C}, {P} 20 kb) and granulite facies ({T} 1100 {C}, {P} < 15 kb). {I}n contrast, most of the sapphires in placer and paleoplacer have two origins: (i) differentiation of alkaline magma in chambers at the lower continental crust-mantle boundary (90% of the sapphires). {T}hey are related to syenite and anorthoclasite xenoliths in the basalts. {T}hese alkali-basalt hosts are linked with asthenosphere upwelling and {E}-{W} and {N}-5 lithosphere thinning during {O}ligocene-{Q}uaternary times. {Z}ircons associated with the sapphires from {M}androsohasina and {A}mbatomainty sapphire deposits gave {U}-{P}b ages at 7 {M}a. {T}he sapphires have low 8180 values of 4.1 +/- 0.4%, (n = 8), within the range of sapphire in syenitic rocks. {C}hemical composition and mineral inclusions in sapphires, such as columbite-({F}e), tantalite-({M}n), pyrochlore group, samarksite group, uraninite and anorthoclase confirm their syenitic origin. (ii) metamorphic xenocrysts (10%) brought up by the same magma. {O}xygen isotopic compositions of rubies from placer deposits, 8180 = 3.1 +/- 1.1%0(n = 6) are typical of ruby in mafic and ultramafic rocks and 'plumasite' in mafic rocks (1.25 < {SO} < 7.5%0, n = 35). {I}n {T}oamasina {P}rovince, the sapphires of {V}atomandry are mainly of metamorphic origin (similar to 85%) and their 8180 = 4.1 0.4 parts per thousand (n = 9) are low and overlaps the range defined for metasomatic sapphires linked to 'plumasites' and biotite schists in shear zones; 15% of the sapphires are magmatic in origin with a low delta {O}-18-isotopic range in the sapphire-bearing syenites field. {I}n {A}ntsiranana {P}rovince, there are no rubies and sapphires are either magmatic (similar to 40%) or metamorphic (similar to 60%) in origin. {T}he 8180 = 4.5 +/- 0.5 parts per thousand (n = 11) values are similar to the mean 8180 of sapphires from other two {P}rovinces. {T}wo {U}/{P}b ages on zircons gave two contrasted ages at respectively 40.6 {M}a for {A}mbondromifehy and 0.7 {M}a for {N}osy {B}e sapphire deposits. {T}he characteristics of the corundum, their isotopic compositions as well as their ages demonstrate the existence of two distinct sources of corundum associated with alkali-basalts in {M}adagascar. {O}n one hand, the rubies associated with metagabbros and garnet-bearing pyroxenites are linked to mafic and ultramafic complexes of eclogite facies at the boundary between lower crust and upper mantle, retrograded to granulite fades during the {P}an-{A}frican event. {O}n the other hand, the sapphires brought up during the {E}ocene to {Q}uaternary are interpreted to be either magmatic and coeval with a volcanic event involving differentiated alkaline magma, or metamorphic and extracted from the pre-existing {P}recambrian basement during the extrusion of the magma.},
  keywords = {{M}adagascar ; {P}lacer in alkali-basalt ; {G}em corundum deposits ; {C}hemical ; composition ; {O}xygen isotopes ; {G}enetic model ; {MADAGASCAR}},
  booktitle = {},
  journal = {{J}ournal of {A}frican {E}arth {S}ciences},
  volume = {94},
  numero = {{SI}},
  pages = {156--170},
  ISSN = {1464-343{X}},
  year = {2014},
  URL = {https://www.documentation.ird.fr/hor/fdi:010062240},
}