Neoproterozoic low to negative δ18O volcanic and intrusive rocks in the Qinling Mountains and their geological significance
1. Jingbo LiuCorresponding (a)
2. Lingmin Zhang (a)
a. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
The Yaolinghe Group in the central part of the Qinling Mountains includes Neoproterozoic bimodal volcanic and sedimentary sequences and two units with fault contact can be divided. The lower unit consists of bimodal volcanic and sedimentary rocks intruded by a coeval plutonic complex, granitic dikes and small granitic bodies, and the upper unit is made up of greenschist with minor rhyolitic tuff. Zircon U–Pb dating yields ages of 719–790Ma for meta-rhyolitic tuff in the lower unit, 714–721Ma for the intrusive rocks, and 635Ma for meta-rhyolitic tuff in the upper unit. The δ18O values are −4 to 15‰ for whole-rock, quartz and zircon from the volcanic rocks and quartz veins in the lower unit and the intrusive rocks, and 3.4–12‰ for whole-rock and quartz veins in the upper unit. Low to negative δ18O values (−4 to 4‰) are widely present in the rocks and quartz veins of the lower unit and the intrusive rocks, suggesting that the lower unit and the intrusive rocks suffered meteoric fluids-in hydrothermal alteration. The δ18O values of whole-rock and quartz veins in the upper unit do not exhibit striking 18O depletion, indicating that meteoric fluids-in hydrothermal alteration did not occur in this unit. Low δ18O zircon grains (1–4‰) are common in some of meta-rhyolitic tuffaceous and intrusive rocks, and only two analyses have negative δ18O values. The low δ18O values of zircon were inherited from crystallization of low δ18O magma, which was formed through the remelting of previous magmatic rocks altered by meteoric water in a system of magma chamber-caldera, and weakly affected by meteoric fluid-in hydrothermal alteration. Thus the presence of low δ18O magmatic zircon is an indicator for meteoric fluid-in hydrothermal alteration. The low δ18O zircon grains found in different samples yields the ages from 720 to 790Ma, implying that meteoric fluid-in hydrothermal alterations began earlier than 790Ma, whereas the striking 18O depletion of the intrusive rocks suggest that hydrothermal alteration continued to at least 714Ma, and the lack of 18O depletion in the upper unit constrains that meteoric fluid-in hydrothermal alterations stopped before 635Ma. The long duration of meteoric fluid-in hydrothermal alteration do not support that the glacial meltwater from the Neoproterozoic glaciations in the Yangtze craton is responsible for the 18O depletion of the rocks.