The Minerageny of Two Groups of Zircons from Plagioclase- Amphibolite of Mayuan Group in Northern Fujian

ACTA PETROLOGICA ET MINERALOGICA, 15(1): 73-79 (1996) 1 The Minerageny of Two Groups of Zircons from Plagioclase- Amphibolite of Mayuan Group in Northern Fujian Xuezhao Baoa1 and Gan Xiaochunb aDepartment of Earth Sciences, the University of Western Ontario, 34-534, Platt’s Lane, London, Canada, N6G 3A8 bTianjin Institute of Geology and Mineral Resources, Tianjin, China 300170 Zircons can crystallize in a wide range of physical and chemical conditions. At the same time, they have very high stability and durability. Therefore zircons can grow and survive in a variety of geological processes. In addition, the diffusivity of chemical compositions in their crystals is very low. Consequently, we can trace back the evolution history of the planetary materials containing zircon with zircon U-Th-Pb geochronology and geochemistry studies. However, this depends on our ability to decipher its genesis, namely magmatic or metamorphic origins. In this paper, magmatic and metamorphic zircons were found from plagioclase-amphibolite samples. Their geneses have been determined by zircon morphology, chemical composition zonations and geological field settings combined with their zircon U-Th-Pb ages. We have found obvious differences in micro-scale Raman spectra between these magmatic and metamorphic zircons. The magmatic zircons exhibit a high sloping background in their Raman spectra, but the metamorphic zircons exhibit a low horizontal background in their Raman spectra, which suggest that the magmatic zircons may contain a much higher concentration of fluorescent impurities than the metamorphic zircons. Moreover, reverse variation trends in Raman spectrum peak intensities from core to rim of a crystal between the magmatic and metamorphic zircons have been found. We think that this can be attributed to their reverse chemical composition zonations. These differences can be used to distinguish magmatic and metamorphic zircons. Key words: magmatic and metamorphic zircons; electron microprobe; Micro-Raman spectrum; plagioclase amphibolite, zircon genesis identification methods. 1. THE GEOLOGICAL OCCURRENCE, CRYSTAL SHAPES AND U-Pb AGE OF ZIRCONS The zircons were extracted/ separated from plagioclase amphibolite samples taken from an area close to Lvkou village, 20km west of Jianyang County in northern Fujian, China. It belongs to the lower member of the Dajinshan formation in the Mayuan Groups. These groups of rocks consist of mainly biotite - plagioclase metamorphic rock, with a small amount of plagioclase amphibolite and marble. The index metamorphic minerals include sillimanite, kyanite, garnet and diopside, which indicate that these rocks have experienced an amphibolite facies metamorphism. The geological field setting has suggested that their parent rocks have formed in the early Proterozoic era [1]. The major minerals in this plagioclase-amphibolite are amphibole and plagioclase (andesine or oligoclase), with a small amount of diopside. Two groups of zircons have been identified:

1 Corresponding author. E-mail address: xuezhaobao@hotmail.com (Xuezhao Bao) 2 Fig. 1. The microscopic photos of zircons M-y1 and M-y2 with with euhedral growth zonings. Magnification X400. M-b1 and M-b2 with rounded shape and core. Magnification X400. Zircons M-y: have rounded, long columnar crystal shapes, brown-red color, sizes of 0.3-0.5 mm. They exhibit euhedral growth zonings as shown in pictures M-y1 and M- y2 in fig.1, which is one of the features of magmatic zircons [2] Their U-Th-Pb single zircon age is 2300 million years (Ma)± (all of the U-Th-Pb single zircon ages in this paper were analyzed in the Geochronology and Isotope Geochemistry Lab, Tianjin Institute of Geology and Mineral Resources). Zircons M-b: exhibit equant rounded crystal shapes, are colorless, transparent, and have sizes of 0.1-0.25 mm, and rounded nuclei in their crystals as shown in pictures M-b1 and M-b2 in fig.1, which is consistent with the features of metamorphic or metamorphic re-crystallization zircons [3]. Their U-Pb single zircon age is 400Ma±. Since the parent rock formed in the early Proterozoic era [1], the first group of zircons crystallized from magma at 2300 Ma±, and the second group of zircons crystallized in a metamorphic event that occurred at 400 Ma±. 2. THE STRUCTURE AND COMPOSITION ZONATION FEATURES 2.1 Laboratory works 2.1.1 Electron microprobe These zircons were mounted in epoxy resin, and then grinded and polished for electron microprobe (EMP) analysis. A JEOL JXA-8600 EMP was used to analyze the ACTA PETROLOGICA ET MINERALOGICA, 15(1): 73-79 (1996) 3 chemical composition through the zircon crystals. The analysis results are listed in Table 1. 2.1.2 Raman spectrum micro analysis After EMP analysis, these samples were re-polished. A T6400 Raman spectrometer made by France’s J.Y. Company was used to analyze these zircons. Analysis conditions are: micro-Rama

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