Evidence for UHP anatexis in the Shuanghe UHP paragneiss from inclusions in clinozoisite, garnet, and zircon
Date
2019
Authors
Liu, Qiang
Hermann, Joerg
Zheng, Shu
Zhang, Junfeng
Journal Title
Journal ISSN
Volume Title
Publisher
Blackwell Publishing Ltd
Abstract
The Shuanghe garnet-bearing paragneiss from the Dabie ultra-high–pressure (UHP)
orogen occurs as an interlayer within partially retrogressed eclogite. A first UHP
metamorphic stage at 680°C, 3.8–4.1 GPa is documented by Zr-in-rutile temperatures coupled with phengite inclusions (Si = 3.55) in clinozoisite and grossular-rich
garnet. Relic matrix phengite and phengite inclusions in zircon rims display lower
Si of 3.42. Combined with garnet compositions and Ti-in-zircon temperatures, they
provide evidence for a second UHP metamorphic stage at 800–850°C, ~3.8 GPa.
Such isobaric heating at UHP conditions has not been documented so far from the
adjacent eclogites and other rock types in the Dabie orogen and indicates proximity
to the hot, convecting mantle wedge. The dominant mineral assemblage consisting
of plagioclase, epidote, biotite, and amphibole provides evidence for widespread retrogression during the exhumation of the UHP paragneiss. Several types of polyphase mineral inclusions were identified. Phengite inclusions hosted by clinozoisite
are partially replaced by kyanite and K-feldspar, whereas inclusions in host garnet
consist of relic phengite, K-feldspar, and garnet, indicating limited sub-solidus dehydration of phengite by the reaction Ph→Kfs+Ky±Grt+fluid. Tightly intergrown
K-feldspar and quartz are preserved as inclusions with sharp boundaries and radial
cracks in garnet. Analyses of whole inclusions also show small enrichments in light
rare earth elements. These inclusions are interpreted to be derived from melting of
an inclusion assemblage consisting of Ph+Coe±Czo. A third type of polyphase inclusion consists of typical nanogranite (Ab+Kfs+Qz±Ep) inclusions in recrystallized metamorphic zircon. Ti-in-zircon thermometry and the Si content of phengite
included in these zircon domains indicate that melting occurred at 800–850°C and
3.8–4.0 GPa during isobaric heating at UHP conditions. The partial melting event led
to an equilibration of trace elements in garnet, phengite, and apatite. Using published
partition coefficients between these minerals and hydrous granitic melt, the trace
element composition of the UHP anatectic melt can be constrained. The melts are
characterized by high LILE contents and pronounced relative enrichments of U over
Th and Ta over Nb. The REE are below primitive mantle values, likely due to the
presence of residual clinozoisite and garnet during partial melting. So far, no major
granitic bodies have been found that share the same trace element pattern as the partial melts from the UHP anatexis of the Shuanghe paragneiss.
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Keywords
Dabie, partial melting, polyphase inclusion, Shuanghe, UHP metamorphism
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Source
Journal of Metamorphic Geology
Type
Journal article
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2037-12-31
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