Ductile thrusting recorded by the garnet isograd from blueschist-facies metapelites of the Ile de Groix, Armorican Massif, France

被引:63
作者
Bosse, V
Ballevre, M
Vidal, O
机构
[1] Univ Nice Sophia Antipolis, CNRS, UMR 6526, Lab Geochronol Geochim, F-06102 Nice, France
[2] Univ Rennes 1, CNRS, UMR 6118, F-35042 Rennes, France
[3] Univ Grenoble 1, CNRS, UMR 5025, Lab Geodynam Chaines Alpines, F-38041 Grenoble, France
关键词
Armorican Massif; blueschist facies; Ile de Groix; metapelites; P-T path; garnet isograd;
D O I
10.1093/petrology/43.3.485
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
Mineral assemblages in the blueschist-facies metapelites from the Ile de Groix (Amorican Massif, France) permit the distinction of two main units. The Upper Unit is characterized by: (1) high modal proportions of garnet; (2) larger grain size; (3) the rarity of graphite-bearing layers; (4) a single, although composite, foliation S-1. A Lower Unit is defined by: (1) low modal proportions of garnet; (2) smaller grain size; (3) an abundance of graphite-bearing layers; (4) a pervasive crenulation cleavage S-2. In the Upper Unit, coexisting garnet and chloritoid are more magnesian and less manganiferous than in the Lower Unit. The differences in modal Proportions and chemistry of coexist g minerals reflect different P-T conditions. The P-T history of the blueschist-facies metapelites estimated using a simplified petrogenetic grid in the NFMASH system and thermodynamic calculations, which suggest peak P-T conditions tit about P = 16=18 kbar, T = 450-500degreesC and P = 14-16 kbar, T = 400-450degreesC in the Upper and Lower Units, respectively. Peak P-T conditions were followed by a nearly isothermal decompression for both units at slightly different temperatures (of the order of 50degreesC). The contact between the two units, i.e. the garnet isograd, is interpreted as a greenschist-facies ductile thrust. Thrusting of the higher-grade unit, i.e. the Upper Unit, over the Lower Unit occurred after the high-pressure event, i.e. during the exhumation of both units. The observed superposition of higher-grade rocks over lower-grade rocks argues against models where the exhumation history is entirely controlled by crustal-scale vertical shortening (i.e. extension).
引用
收藏
页码:485 / 510
页数:26
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