Timing of Anatexis within the Berere HTHP Complex Belt of Maevatanana Area, North‐Central Madgascar, and its Geological Significance

Abstract

AbstractThe Berere HTHP Complex belt in Maevatanana area of north–central Madagascar formed in the ∼ 2.5 Ga orogeny and underwent high temperature (up to 1050°C) and high pressure (up to 11.5 kbar) granulite facies metamorphism. Then a widespread anatexis took place and numerous widely distributed felsic leucosomes formed. The majority of these leucosomes are parallel to the schistosity of the complex or are present as stockworks, as thin layers, or as lenses at different scales in the host rocks. Here, we report new petrographic data, zircon LA‐ICP‐MS U‐Pb ages, and Lu–Hf isotopic data for felsic leucosomes within this complex. Anatexis, as identified by the petrological study of felsic leucosomes in the field and in thin sections, involved initial ternary feldspar exsolving to produce antiperthite and a quartz + plagioclase ± K‐feldspar + sericite mineral assemblage around feldspar grain boundaries. Dissolution is apparent along muscovite grain boundaries, and residual sericite is present around the margins of feldspar and quartz, all suggesting that anatexis was driven by reactions involving muscovite. Zircon U–Pb dating indicates that the felsic leucosomes within the complex formed at 2467–2369 Ma. The majority of samples have positive ∊Hf(t) values, although a few have negative values, suggesting their formation from magmas predominantly sourced from the depleted mantle, possibly with the involvement of minor amounts of crustal materials. Two‐stage Hf model ages and ∊Hf(t) values for these samples are consistent with those for gneisses of the basement, indicating that the felsic leucosomes were formed by the anatexis of gneisses and both of their protolith formed during the formation of continental crust in Meso‐Neoarchean (ca. 3.1–2.7 Ga). As such, the crystallization age of the felsic leucosome (∼2.4 Ga) represents the timing of regional anatexis and a change to post‐orogenic tectonism. And this anatexis is also corresponds to the thermal event in Dharwar craton in India which has a pronounced similar Precambrian geology with Madagascar, providing an important constraints on the correlation of the two continental fragments.