Genetic relationship between granite and pegmatite in Kaluan hard- rock- type lithium deposit in Xinjiang: results from zircon U- Pb dating, Hf- O isotopes and whole- rock geochemistry
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    Abstract:

    The Kalu’an lithium deposit is located in the Altay orogenic belt in Xinjiang, which is one of the main rare metal deposits in China. Due to less research work in the Kalu’an lithium deposit, its petrogenesis and metallogenic mechanisms need to be systematically studied. In this paper, results of major elements, trace elements, rare earth elements (REEs) and Pb- Sr- Nd isotopes analyses of spodumene- bearing pegmatites, spodumene- barren pegmatites, outer granites, outer pegmatites, hornstones, schists and slates, and zircon U- Pb and Hf- O isotopic data of outer granites, spodumene- bearing pegmatites and outer pegmatites from the hard- rock- typelithium deposit in Kalu’an, Xinjiang are used to determine the formation age, magmatic origin and evolutionary process of granite and pegmatite, and to further investigate their genetic relationship. Geochronological data indicate that the SIMS U- Pb ages of zircons from the outer biotite granites, spodumene- bearing pegmatites and outer pegmatites are 407. 9±2. 3 Ma ( n =25, MSWD=1. 4), 205. 0±12. 0 Ma ( n =6, MSWD=2. 2), and 205. 0 Ma respectively. The outer biotite granites from Kalu’an are characterized by high SiO2 (71. 16%~75. 39%), CaO (1. 15%~1. 85%), low Fe2O3/FeO (<0. 4). They are also peraluminous (A/CNK=1. 04~1. 09) and contain alkaline dark minerals (>8%). In terms of trace and REE compositions, outer biotite granites have weakly differentiated Zr/Hf (25~55) and Nb/Ta (5. 4~8. 8) values, are relatively enriched in Rb, Th, U, Pb, Hf and LREEs, and depleted in Ba, Nb, Sr, P and Ti. These geochemical features indicate that Kalu’an biotite granites belong to typical weakly differentiated S- type granites, whose source rocks may be a set of metamorphic sedimentary rocks. Spodumene- bearing pegmatites and outer pegmatites have consistent Hf- O isotopic compositions with outer biotite granites: ε Hf( t )=-0. 2~2. 0 and δ 18O=7. 88‰~10. 87‰ for spodumene- bearing pegmatites, ε Hf( t )=-0. 7~8. 1 and δ 18O=6. 87‰~9. 48‰ for outer pegmatites, and ε Hf( t )=1. 3~7. 4 and δ 18O=8. 62‰~10. 08‰ for biotite granites, suggesting that they may have the same source region. Outer granites from Kalu’an have similar REE and trace element patterns, Pb- Sr- Nd isotope composition, and Nd isotope pattern age with those of schists and slates, implying that they were generated by partial melting of schists and slates.Meanwhile, spodumene- bearing pegmatites and spodumene- barren pegmatites also show similar REE and trace element patterns, but their ΣREE contents are significantly lower than those of outer biotite granites, indicating that these pegmatites may have a genetic relationship with outer biotite granites. However, the formation ages of outer biotite granites are nearly 200 Ma earlier than those of pegmatites, suggesting that the pegmatites are not the products of crystalline differentiation of outer biotite granites.It is noteworthy that the two spodumene- bearing pegmatites have similar REE and trace element patterns, Pb- Sr- Nd isotope composition, and Nd isotope pattern age with those of schists and slates, implying that pegmatites are related to partial melting of schists and slates. The CaO/Na2O ratio values (0. 01~0. 19) of outer pegmatites, spodumene- bearing pegmatites and spodumene- barren pegmatites indicate that their source region is dominated by argillaceous rock; while the CaO/Na2O ratio values (0. 36~0. 51) of biotite granites indicate that their source region is dominated by arenite. Based on the above- mentioned research results, we suggest that biotite granites and pegmatitesare the products of different lithologies (clay- poor clastic rocks versus clay- rich argillaceous rocks) of the same source rock through different tectonic evolution in different ages, and their source rocks could be S2- 3 KL schists and slates with close genetic relationship.

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LIU Tao, TIAN Shihong, WANG Denghong, ZHANG Yujie, LI Xianfang, HOU Kejun, JIEKEN·Kalimuhan, ZHANG Zhongli, WANG Yongqiang, ZHAO Yue, QIN Yan.2020. Genetic relationship between granite and pegmatite in Kaluan hard- rock- type lithium deposit in Xinjiang: results from zircon U- Pb dating, Hf- O isotopes and whole- rock geochemistry[J]. Acta Geologica Sinica,94(11):3293-3320

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History
  • Received:May 20,2020
  • Revised:August 27,2020
  • Adopted:August 30,2020
  • Online: October 14,2020
  • Published: