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Voluminous Neoproterozoic granitoid sheets of the Imorona–Itsindro Suite are important components of exposed basement in west-central Madagascar. Here, we report precise new zircon U–Pb ages and whole-rock geochemistry for granitoids within the Maevatanana area of Madagascar. The new laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating undertaken during this study indicates that Antanimbary granitoid and Antasakoamamy granitoid were emplaced at 747 ± 9 Ma and 729 ± 9–727 ± 8 Ma, respectively. Geochemically, the Antanimbary granitoids show poor Nb, Ta anomalies, pronounced positive Zr anomalies, and are K-rich (K2O/Na2O > 1), but the Antasakoamamy granitoids are relatively depleted in Nb, Ta, show slightly negative Zr anomalies, and are Na-rich (Na2O/K2O > 1). Both suites contain zircons with strongly negativeεHf(t), indicating participation of much older (Palaeoproterozoic and Archaean) crust. Their geochemical characteristics, along with the use of various discrimination diagrams, reveals that crustal delamination and asthenospheric upwelling resulted in crustal extension of the region before ~747 Ma, with subsequent lithospheric subduction and arc magmatism after 729–727 Ma. [ABSTRACT FROM AUTHOR]

Migmatites represent the dominant lithological facies in the Maevatanana area of Madagascar. The migmatites are composite rocks with hybrid metamorphic and magmatic features, comprising 60% melanosome bands that are interlayered and/or tectonically interleaved with 40% of centimeter-to decimeter-scale quartz–feldspar leucosome veins. The leucosome and granite veins are bordered by a series of gently dipping shear zones in the migmatites. U–Pb zircon dating shows that the leucosome veins formed at 2548.3 ± 5.3 Ma and are coeval with intrusion of the granite veins at 2552.2 ± 6.1 Ma. Furthermore, the leucosome and granite veins have uniform chondrite-normalized rare earth element (REE) patterns which are characterized by light REE enrichment relative to heavy REE, and negative or slightly positive Eu anomalies (δEu = 0.61–1.13). Primitive-mantle-normalized trace element patterns of the leucosome and granite veins are enriched in large-ion lithophile elements (Ba and K) and show pronounced depletions in high-field strength elements (Th, U, Ta, Nb, Zr, Hf, and Ti), suggesting that the leucosome and granite veins in the migmatites are syntectonic and synigneous intrusives. Primitive-mantle-normalized trace element patterns of the leucosome veins and melanosome bands in the migmatites are similar and suggest that the granite veins in the migmatites were generated by partial melting of medium-to high-K metabasalt, whereas the melanosome bands originated through metamorphism of calc-alkaline basalts. As such, the leucosome veins were likely generated by partial melting of melanosome bands in the migmatites. Calculated zircon saturation temperatures (T Zr ) of the leucosome veins are 599–685 °C, which represents the magma crystallization temperatures, and the pressures of magma formation range from 1.2 to 3.2 GPa. These P–T estimates for magma formation and crystallization are consistent with those of 605–658 °C and 1.22–1.43 GPa for metamorphism of the melanosome bands in the migmatites, suggesting that the leucosome veins in the migmatites were formed during peak metamorphism of the melanosome bands. [ABSTRACT FROM AUTHOR]

Magmatic zircon ages for the Maevatanana basalts in Madagascar indicate that Madagascar separated from Africa at 149.8 ± 2.1 Ma. Rocks produced by this basaltic magmatism associated with rifting are characterized by low SiO 2 (49.6–50.3 wt.%), high total FeO (>13.9 wt.%), TiO 2 (>3.7 wt.%), and P 2 O 5 (>0.5 wt.%), and extremely high Na 2 O/K 2 O (3.08–3.27). Geochemical variations can be ascribed to significant fractional crystallization of clinopyroxene and plagioclase. The mineralogical and geochemical characteristics of the Maevatanana basalts suggest affinities with calc-alkaline basalt. Additionally the basalts have distinct ocean-island-basalt-like geochemical features that may be related to the Marion plume. We speculate that the Maevatanana basalts are the product of the Marion mantle plume related to separation of Madagascar from Africa in the Late Jurassic. [ABSTRACT FROM AUTHOR]

Abstract: Zircon LA-ICP-MS U–Pb geochronology and geochemistry on representative bedrocks and moraine sediments in the Windmill Islands in Wilkes Land, East Antarctica were performed to characterize the age and origin of main rocks as well as the lithology and age of ice-covered basement in this sector of East Antarctica. Zircon U–Pb dating on two garnet-bearing granite gneisses and one foliated garnet-bearing granite from Bailey Peninsula yield weighted mean 207Pb/206Pb ages of 1247±13Ma, 1258±12Ma and 1242±13Ma, respectively, which are interpreted as emplacement ages of these rocks. Igneous inherited zircons in these rocks yield a weighted mean 207Pb/206Pb age of 1372±13Ma, indicating existence of a ca. 1.37Ga igneous activity in the Windmill Islands. Both garnet-bearing granite gneisses and foliated garnet-bearing granites from Bailey Peninsula are characterized by similar geochemical and isotopic compositions and were likely produced by partial melting of predominantly Paleoproterozoic ancient crustal materials at 1.24–1.25Ga. Zircon U–Pb analyses on two charnockite samples from Robinson Ridge yield weighted mean 207Pb/206Pb ages of 1196±8Ma and 1205±13Ma, respectively, indicating emplacement of the Ardery Charnockite at ca. 1.20Ga. The charnockites were likely produced by partial melting of the ancient mafic lower crust heated by underplated basaltic magma during the D2 cycle at late stage of the granulite facies metamorphism, and some lithospheric mantle materials could be involved during formation of these rocks. U–Pb analyses of detrital zircons from sand-sized moraine sediments yield concordant 207Pb/206Pb ages mainly ranging from ca. 1368Ma to ca. 1107Ma with main magmatic zircon populations at ca. 1107Ma, 1150–1160Ma, ca. 1196Ma, 1231–1255Ma, ca. 1317Ma and ca. 1368Ma, respectively. Some old zircon populations at ca. 1411Ma to ca. 2360Ma, which are considered to be derived from recycling of the Proterozoic metapsammitic gneiss rocks, have been identified. Their zircon Hf isotopic model ages () are mainly around 1.6–2.4Ga. The absence of Archean zircons in moraines suggests that the crust of Law Dome and inland areas of Wilkes Land are dominated by Palaeo-Mesoproterozoic rocks, which is consistent with the vast majority of the Nd–Hf isotopic data presented. Abundant Mesoproterzoic zircons from ca. 1107Ma to ca. 1533Ma in moraines indicate the crustal rocks in the Windmill Island and its inland areas are mainly formed in the Mesoproterozoic. The ages and Hf isotopic compositions of these moraine zircons are comparable with those of the bedrocks in the Windmill Islands, indicating that subglacial rock compositions of Law Dome and the inland areas of Wilkes Land are similar to the rock composition of the Windmill Islands area. Law Dome and the inland areas of Wilkes Land are probablly a similar Mesproterozoic high-grade metamorphic terrane as the Windmill Islands in East Antarctica. Our results also confirmed that there is no evidence for Pan-African tectonothermal events and the Windmill Islands and its inland areas are not affected by Pan-African tectonism and magmatism. [Copyright &y& Elsevier]