Identification of ca. 850â€¯Ma high-temperature strongly peraluminous granitoids in southeastern Guizhou Province, South China: A result of early extension along the southern margin of the Yangtze Block
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Â© 2018 Elsevier B.V. Strongly peraluminous granites are characterized by high A/CNK values ( > 1.1) and are important for understanding the regional tectonic regime. Here, we present integrated studies of zircon Uâ€“Pb ages, trace elements and Luâ€“Hf isotopes, whole-rock major and trace elements, and Nd isotope compositions for the newly discovered Nage granite porphyry in southeastern Guizhou Province. The porphyry has a crystallization age of 852 Â± 5 Ma and shows wide ranges in SiO 2 (64.5â€“72.51 wt%) and Al 2 O 3 (13.08â€“19.34 wt%) contents, with low TiO 2 (0.41â€“0.84 wt%) and P 2 O 5 ( < 0.15 wt%) contents. All the samples are strongly peraluminous with A/CNK > 1.1, and they have similar trace element patterns with depletion in high field strength elements (HFSE, e.g., Nb and Ta) and enrichment in large-ion lithophile elements (LILE, e.g., Rb, Th and U). Initial zircon Hf isotopes of the porphyry vary from Îµ Hf (t) = âˆ’7.6 to âˆ’0.6, whereas whole-rock Nd isotopes are relatively uniform with Îµ Nd (t) = âˆ’6.0 to âˆ’6.7. They record high zircon saturation temperatures (T zr ) (786â€“842 Â°C, mostly > 810 Â°C), but low oxygen fugacity (log fO 2 = âˆ’16 to âˆ’25, corresponding to Î”FMQ âˆ’3 to Î”FMQ âˆ’7) and emplacement pressure of < 0.5 GPa. Major and trace elements, 10,000 Ga/Al ratios and Zr + Nb + Ce + Y values, show good correlations with T zr , indicating that fractional crystallization may have played an important role. Although these granites show low P 2 O 5 contents that decrease with increasing Rb, the high A/CNK ( > 1.1), K 2 O/Na 2 O (0.7â€“2.6; average 1.4) and CaO/Na 2 O (0.2â€“0.8; mostly > 0.5) ratios suggest that clay-poor metasedimentary rock were dominant in the source. Combined with the Ndâ€“Hf isotopes, we suggest that the Nage granite porphyry formed by anatexis of an immature arkose and greywacke source, which was a mixture of weathered juvenile igneous materials and minor recycled ancient crustal components. The ca. 850 Ma strongly peraluminous granites, along with coeval extension-related magmatism along the southwestern margin of the Yangtze Block, indicate an extensional environment started as early as ca. 850 Ma.
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