摘要
The Maevatanana greenstone belt in north-central Madagascar contains widespread exposures of tonalite-trondhjemite-granodiorite （TTG） gneisses, and is important for its concentrations of various metal deposits （e.g., chromium, niekle, iron, gold）. In this paper we report on the petrography, and major and trace element compositions of the TTG gneisses within the Berere Complex of the Maevatanana area, as well as LA-ICP-MS U-Pb ages and Lu-Hf isotopic compositions of zircons from the gneisses. The gneisses consist mainly of granitoid gneiss and biotite （± hornblende） plagiogneiss, and analysis of thin sections provides evidence of crushing, recrystallization, and metasomatism related to dynamic metamorphism. Samples have large variations in their major and trace element contents, with SiO2 = 55.87-68.06 wt%, Al2O3 = 13.9-17.8 wt%, and Na2O/K2O= 0.97-2.13. Geochemically, the granitoid gneisses and biotite plagiogneisses fall on a low-Al trondhjemite to granodiorite trend, while the biotite-hornblende plagiogneisses represent a high-Al tonalite TTG assemblage. Zircon U-Pb dating shows that the Berere Complex TTG gneisses formed at 2.5-2.4 Ga. Most εHf（t） values of zircons from the biotite （q- hornblende） plagiogneisses are positive, while most εHf（t） values from the granitoid gneisses are negative, suggesting a degree of crustal contamination. Two-stage Hf model ages suggest that the age of the protolith of the TTG gneisses was ca. 3.4-2.6 Ga, representing a period of paleocontinent formation in the Mesoarchean. Geothermometries indicate the temperature of metamorphism of the TTG gneisses was 522-612℃. Based on these data, the protolith of the TTG gneisses is inferred to have formed during the development of a Mesoarchean paleocontinent that is now widely exposed as a TTG gneiss belt （mostly lower amphibolite facies） in the Maevatanana area, and which records a geological evolution related to the subduction of an ancient oceanic crust and the collision of microcontinents during the formation of the Rodinia supercontinent. The lithological similarity of Precambrian basement, the close ages of metamorphism within greenstone belts and the comparable distribution of metamorphic grade all show a pronounced Precambrian geology similarity between Madagascar and India, which can provide significative clues in understanding the possible Precambrian Supercontinent tectonics, and also important constraints on the correlation of the two continental fragments.
The Maevatanana greenstone belt in north-central Madagascar contains widespread exposures of tonalite-trondhjemite-granodiorite （TTG） gneisses, and is important for its concentrations of various metal deposits （e.g., chromium, niekle, iron, gold）. In this paper we report on the petrography, and major and trace element compositions of the TTG gneisses within the Berere Complex of the Maevatanana area, as well as LA-ICP-MS U-Pb ages and Lu-Hf isotopic compositions of zircons from the gneisses. The gneisses consist mainly of granitoid gneiss and biotite （± hornblende） plagiogneiss, and analysis of thin sections provides evidence of crushing, recrystallization, and metasomatism related to dynamic metamorphism. Samples have large variations in their major and trace element contents, with SiO2 = 55.87-68.06 wt%, Al2O3 = 13.9-17.8 wt%, and Na2O/K2O= 0.97-2.13. Geochemically, the granitoid gneisses and biotite plagiogneisses fall on a low-Al trondhjemite to granodiorite trend, while the biotite-hornblende plagiogneisses represent a high-Al tonalite TTG assemblage. Zircon U-Pb dating shows that the Berere Complex TTG gneisses formed at 2.5-2.4 Ga. Most εHf（t） values of zircons from the biotite （q- hornblende） plagiogneisses are positive, while most εHf（t） values from the granitoid gneisses are negative, suggesting a degree of crustal contamination. Two-stage Hf model ages suggest that the age of the protolith of the TTG gneisses was ca. 3.4-2.6 Ga, representing a period of paleocontinent formation in the Mesoarchean. Geothermometries indicate the temperature of metamorphism of the TTG gneisses was 522-612℃. Based on these data, the protolith of the TTG gneisses is inferred to have formed during the development of a Mesoarchean paleocontinent that is now widely exposed as a TTG gneiss belt （mostly lower amphibolite facies） in the Maevatanana area, and which records a geological evolution related to the subduction of an ancient oceanic crust and the collision of microcontinents during the formation of the Rodinia supercontinent. The lithological similarity of Precambrian basement, the close ages of metamorphism within greenstone belts and the comparable distribution of metamorphic grade all show a pronounced Precambrian geology similarity between Madagascar and India, which can provide significative clues in understanding the possible Precambrian Supercontinent tectonics, and also important constraints on the correlation of the two continental fragments.
基金
funded by Geological Survey Project grants from the China Geological Survey(grant numbers DD20160056, 121201103000150002)
