First report of felsic whiteschist in the ultrahigh-pressure metamorphic belt of Dabie Shan, China
Rolfo, Franco; Compagnoni, Roberto; Xu, Shutong; Jiang, Laili
European Journal of Mineralogy Volume 12 Number 4 (2000), p. 883 - 898
published: Jul 17, 2000
manuscript accepted: Mar 13, 2000
manuscript received: Sep 28, 1998
ArtNo. ESP147051204015, Price: 29.00 €
Abstract We report the first occurrence of a talc-kyanite assemblage typical of whiteschists in felsic ultrahigh-pressure metamorphic rocks of Dabie Shan, China. The whiteschist assemblage occurs in a leucocratic layer, less than half a meter thick, which crosscuts a coesite-bearing eclogite. Both its geologic setting and geochemistry suggest that the protolith was a felsic dyke, which suffered loss of some elements, in particular alkalies. The whiteschist consists of quartz, minor epidote, kyanite, talc and omphacite. Common accessories are rutile, apatite and zircon. Epidote is zoned, and the core includes quartz pseudomorphs after coesite and contains up to 5 wt% REE. Kyanite includes fresh omphacite with the highest jadeite content (XJd ≤ 0.60). The host eclogite consists of omphacite (XJd = 0.44 to 0.37 from core to rim), garnet, quartz, kyanite, epidote, phengite (Si ≤ 3.47 a.p.f.u.) and amphibole, and accessory rutile, apatite, zircon and ilmenite. Fresh coesite occurs in epidote, and its polycrystalline quartz pseudomorphs are included in both omphacite and kyanite. The whiteschist and the host eclogite share the same tectonometamorphic evolution, and show three metamorphic stages: (I) coesite-eclogite-facies stage at P ≥ 2.6-2.7 GPa and T = 710 ± 20°C; (II) early decompression stage at P ≥ 1.5-2.0 GPa and T = 650 ± 30°C; (III) late adiabatic decompression to P = 0.7 GPa and T = 670 ± 40°C.