Ultrahigh-temperature osumilite gneisses in southern Madagascar record combined heat advection and high rates of radiogenic heat production in a long-lived high-T orogen

We report the discovery of osumilite in ultrahigh‐temperature (UHT) metapelites of the Anosyen domain, southern Madagascar. The gneisses equilibrated at ~930°C/0.6 GPa. Monazite and zircon U–Pb dates record 80 Ma of metamorphism. Monazite compositional trends reflect the transition from prograde to retrograde metamorphism at 550 Ma. Eu anomalies in monazite reflect changes in fO_2 relative to quartz–fayalite–magnetite related to the growth and breakdown of spinel. The ratio Gd/Yb in monazite records the growth and breakdown of garnet. High rates of radiogenic heat production were the primary control on metamorphic grade at the regional scale. The short duration of prograde metamorphism in the osumilite gneisses (<29 ± 8 Ma) suggests that a thin mantle lithosphere (<80 km) or advective heating may have also been important in the formation of this high‐T, low‐P terrane

Ultrahigh-temperature osumilite gneisses in southern Madagascar record combined heat advection and high rates of radiogenic heat production in a long-lived high-T orogen

We report the discovery of osumilite in ultrahigh‐temperature (UHT) metapelites of the Anosyen domain, southern Madagascar. The gneisses equilibrated at ~930°C/0.6 GPa. Monazite and zircon U–Pb dates record 80 Ma of metamorphism. Monazite compositional trends reflect the transition from prograde to retrograde metamorphism at 550 Ma. Eu anomalies in monazite reflect changes in fO_2 relative to quartz–fayalite–magnetite related to the growth and breakdown of spinel. The ratio Gd/Yb in monazite records the growth and breakdown of garnet. High rates of radiogenic heat production were the primary control on metamorphic grade at the regional scale. The short duration of prograde metamorphism in the osumilite gneisses (<29 ± 8 Ma) suggests that a thin mantle lithosphere (<80 km) or advective heating may have also been important in the formation of this high‐T, low‐P terrane

Incipient charnockitisation triggered by structurally-controlled CO2 influx in central Madagascar and magnetic implications

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Chemical and oxygen isotopic compositions, age and origin of gem corundums in Madagascar alkali basalts

Madagascar is a large producer of gem corundum recovered from continental basaltic fields. The main mining areas are sapphire-bearing palaeoplacer deposits such as Ambondromifehy and Nosy Be in Antsiranana Province, northern Madagascar; Soamiakatra-Mandrosohasina in Antananarivo Province, central Madagascar; and Vatomandry district in Toamasina Province, eastern Madagascar. In Antananarivo Province, Soamiakatra is a primary deposit where ruby is found in metagabbro and pyroxenite xenoliths, brought-up to the upper crust by the Ankaratra volcanics. Petrographic 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). In 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). They are related to syenite and anorthoclasite xenoliths in the basalts. These alkali-basalt hosts are linked with asthenosphere upwelling and E-W and N-5 lithosphere thinning during Oligocene-Quaternary times. Zircons associated with the sapphires from Mandrosohasina and Ambatomainty sapphire deposits gave U-Pb ages at 7 Ma. The sapphires have low 8180 values of 4.1 +/- 0.4%, (n = 8), within the range of sapphire in syenitic rocks. Chemical composition and mineral inclusions in sapphires, such as columbite-(Fe), tantalite-(Mn), pyrochlore group, samarksite group, uraninite and anorthoclase confirm their syenitic origin. (ii) metamorphic xenocrysts (10%) brought up by the same magma. Oxygen 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). In Toamasina Province, the sapphires of Vatomandry 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. In Antsiranana Province, there are no rubies and sapphires are either magmatic (similar to 40%) or metamorphic (similar to 60%) in origin. The 8180 = 4.5 +/- 0.5 parts per thousand (n = 11) values are similar to the mean 8180 of sapphires from other two Provinces. Two U/Pb ages on zircons gave two contrasted ages at respectively 40.6 Ma for Ambondromifehy and 0.7 Ma for Nosy Be sapphire deposits. The 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 Madagascar. On 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 Pan-African event. On the other hand, the sapphires brought up during the Eocene to Quaternary are interpreted to be either magmatic and coeval with a volcanic event involving differentiated alkaline magma, or metamorphic and extracted from the pre-existing Precambrian basement during the extrusion of the magma

New typology and origin of tsavorite based on trace-element chemistry

New electron-microprobe analyses of 'tsavorites' from the Neoproterozoic Metamorphic Mozambique Belt deposits allow the characterization of green grossular according to its trace-element chemistry (V, Cr, Mn). Five chemical types are defined: type 1, vanadian grossular with V > Cr > Mn (in atoms per formula unit); type 2, vanadian grossular with V > Mn > Cr; type 3, Mn-bearing vanadian grossular with Mn > V > Cr; type 4, Mn-bearing chromian grossular with Mn > Cr > V; and type 5, Cr- and Mn-bearing grossular with Cr > Mn > V. These types are also characterized by different absorption spectra in the ultraviolet-visible-near infrared. Type 1 tsavorite spectra show a total absorption below 430 nm due to the high vanadium contents. Type 2 tsavorite spectra present the classical absorption bands of V. Types 3 and 4 tsavorite spectra display additional shoulders at 407 and 408 nm due to Mn2+, whereas spectra of Cr-bearing types 4 and 5 tsavorite show the two additional bands of Cr3+ at 697 and 701 nm. The different absorption spectra also indicate Fe2+-Ti4+ charge transfer. We measured OH equivalent to 0.08 to 0.38 wt% eq. H2O within the structure. Concentrations of vanadium, chromium and manganese are good chemical "fingerprints" for determining the geographic provenance of economic tsavorite from Kenya, Tanzania and Madagascar

Three distinct Holocene intervals of stalagmite deposition and nondeposition revealed in NW Madagascar, and their paleoclimate implications

Petrographic features, mineralogy, and stable isotopes from two stalagmites, ANJB-2 and MAJ-5, respectively from Anjohibe and Anjokipoty caves, allow distinction of three intervals of the Holocene in NW Madagascar. The Malagasy early Holocene (between ca. 9.8 and 7.8 ka) and late Holocene (after ca. 1.6 ka) intervals (MEHI and MLHI, respectively) record evidence of stalagmite deposition. The Malagasy middle Holocene interval (MMHI, between ca. 7.8 and 1.6 ka) is marked by a depositional hiatus of ca. 6500 years. Deposition of these stalagmites indicates that the two caves were sufficiently supplied with water to allow stalagmite formation. This suggests that the MEHI and MLHI intervals may have been comparatively wet in NW Madagascar. In contrast, the long-term depositional hiatus during the MMHI implies it was relatively drier than the MEHI and the MLHI. The alternating wet–dry–wet conditions during the Holocene may have been linked to the long-term migrations of the Intertropical Convergence Zone (ITCZ). When the ITCZ's mean position is farther south, NW Madagascar experiences wetter conditions, such as during the MEHI and MLHI, and when it moves north, NW Madagascar climate becomes drier, such as during the MMHI. A similar wet–dry–wet succession during the Holocene has been reported in neighboring locations, such as southeastern Africa. Beyond these three subdivisions, the records also suggest wet conditions around the cold 8.2 ka event, suggesting a causal relationship. However, additional Southern Hemisphere high-resolution data will be needed to confirm this

Extinction Risks and the Conservation of Madagascar's Reptiles

<div><p>Background</p><p>An understanding of the conservation status of Madagascar's endemic reptile species is needed to underpin conservation planning and priority setting in this global biodiversity hotspot, and to complement existing information on the island's mammals, birds and amphibians. We report here on the first systematic assessment of the extinction risk of endemic and native non-marine Malagasy snakes, lizards, turtles and tortoises.</p><p>Methodology/Principal Findings</p><p>Species range maps from The IUCN Red List of Threatened Species were analysed to determine patterns in the distribution of threatened reptile species. These data, in addition to information on threats, were used to identify priority areas and actions for conservation. Thirty-nine percent of the data-sufficient Malagasy reptiles in our analyses are threatened with extinction. Areas in the north, west and south-east were identified as having more threatened species than expected and are therefore conservation priorities. Habitat degradation caused by wood harvesting and non-timber crops was the most pervasive threat. The direct removal of reptiles for international trade and human consumption threatened relatively few species, but were the primary threats for tortoises. Nine threatened reptile species are endemic to recently created protected areas.</p><p>Conclusions/Significance</p><p>With a few alarming exceptions, the threatened endemic reptiles of Madagascar occur within the national network of protected areas, including some taxa that are only found in new protected areas. Threats to these species, however, operate inside and outside protected area boundaries. This analysis has identified priority sites for reptile conservation and completes the conservation assessment of terrestrial vertebrates in Madagascar which will facilitate conservation planning, monitoring and wise-decision making. In sharp contrast with the amphibians, there is significant reptile diversity and regional endemism in the southern and western regions of Madagascar and this study highlights the importance of these arid regions to conserving the island's biodiversity.</p></div

Species richness, species richness of threatened species, and range-size rarity calculated separately for five major Madagascan reptile groups.

<p>See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100173#s2" target="_blank">Materials and Methods</a> for an explanation of the metrics used.</p