Significance of Allanite-(Ce) in Granitic Gneisses from the Ultrahigh-Pressure Metamorphic Terrane, Dabie Shan, Central China

Xiaochun Liu1, Shuwen Dong1, Huaimin Xue2 and Jianxiong Zhou3
1 Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081,
2 Nanjing Institute of Geology and Mineral Resources, Chinese Academy of Geological Sciences, Nanjing 210016
3 Institute of Mineral Deposits, Chinese Academy of Geological Sciences, Beijing 100037, China

Abstract: A recent study of granitic gneisses in the ultrahigh-pressure (UHP) metamorphic terrane from Dabie Shan, central China reveals that allanite-(Ce) is a widespread REE-bearing accessory phase. The microprobe analyses show that allanites-(Ce) have nearly uniform chemical compositions, with total REE contents ranging from 23 to 28 wt.% and a La/Ce ratio of 0.57–0.73. All allanites-(Ce) are replaced or rimmed by epidotes and commonly form a relatively REE-poor rim near the interfaces. The textural relations and mineral chemistry suggest that allanite-(Ce) crystallized prior to metamorphism and therefore was of igneous origin, whereas the surrounding epidote was the product of metamorphism. Granitic gneiss, composed chiefly of sodic plagioclase, microcline and quartz with minor amounts of Mn-rich garnet, biotite, low Si phengitic mica, epidote, chlorite and sometimes amphibole, was formed at 495°C and ∼5 kbar, corresponding to the retrograde metamorphism of UHP rocks. Accordingly, it is believed that granitic gneisses have not experienced UHP metamorphism. This implies that a large scale tectonic juxtaposition developed before the epidote-amphibolite facies retrograde metamorphism of UHP rocks during the late continental collision in Dabie Shan.

Keywords: allanite-(Ce) • granitic gneisses • UHP rocks • tectonic juxtaposition • Dahie Shan

Mineralogical Magazine; August 1999 v. 63; no. 4; p. 579-586; DOI: 10.1180/002646199548628
© 1999, The Mineralogical Society
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