Late Neoproterozoic, Ordovician and Carboniferous events recorded in monazites from southern-central Madagascar

被引:34
作者
Berger, A
Gnos, E
Schreurs, G
Fernandez, A
Rakotondrazafy, M
机构
[1] Univ Bern, Inst Geol, CH-3012 Bern, Switzerland
[2] Univ Lyon 1, Lab Dynam Lithosphere, F-69622 Villeurbanne, France
[3] Univ Antananarivo, Dept Sci Terre, Antananarivo, Madagascar
关键词
monazite dating; granulites; Gondwana; Madagascar; Ordovician; Carboniferous;
D O I
10.1016/j.precamres.2005.11.010
中图分类号
P [天文学、地球科学];
学科分类号
07 ;
摘要
The evolution of the basement of southern Madagascar north and south of the Ranotsara shear zone was investigated using (U+Th)/Pb electron probe monazite age dating in combination with petrographic constraints. Several monazite grains show a stepwise progression of younger ages towards the rim indicating partial and complete resetting during tectonic, metamorphic and/or fluid events. The oldest ages, ranging from 630-2400 Ma, occur relatively rare in relic cores. A first, clear age-population is dated at 550-560 Ma. Most ages fall in two populations at 420-460 and 490-500 Ma, which in some samples overlap in error. We interprete these ages as dating low-pressure and high-temperature metamorphism. We have also clear evidence for Carboniferous (300-310 Ma) monazite overgrowth rims, which can not directly be related to macroscopic structures or metamorphic parageneses. In combination with literature data, we propose that the observed monazite age populations are related to Gondwana amalgamation and subsequent rifting events during the break up of Gondwana. Our study confirms that only the electron or ion microprobe yields sufficient spatial resolution to date individual shells of multiple zoned monazites in the polymetamorphic basement of Madagascar. (c) 2005 Elsevier B.V. All rights reserved.
引用
收藏
页码:278 / 296
页数:19
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