Neoproterozoic low to negative δ18O volcanic and intrusive rocks in the Qinling Mountains and their geological significance

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 719-790 Ma for meta-rhyolitic tuff in the lower unit, 714-721 Ma for the intrusive rocks, and 635 Ma for meta-rhyolitic tuff in the upper unit. The delta O-18 values are -4 to 15 parts per thousand 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 parts per thousand for whole-rock and quartz veins in the upper unit. Low to negative delta O-18 values (-4 to 4 parts per thousand) 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 delta O-18 values of whole-rock and quartz veins in the upper unit do not exhibit striking O-18 depletion, indicating that meteoric fluids-in hydrothermal alteration did not occur in this unit. Low delta O-18 zircon grains (1-4 parts per thousand) are common in some of meta-rhyolitic tuffaceous and intrusive rocks, and only two analyses have negative delta O-18 values. The low delta O-18 values of zircon were inherited from crystallization of low delta O-18 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 delta O-18 magmatic zircon is an indicator for meteoric fluid-in hydrothermal alteration. The low delta O-18 zircon grains found in different samples yields the ages from 720 to 790 Ma, implying that meteoric fluid-in hydrothermal alterations began earlier than 790 Ma, whereas the striking O-18 depletion of the intrusive rocks suggest that hydrothermal alteration continued to at least 714 Ma, and the lack of O-18 depletion in the upper unit constrains that meteoric fluid-in hydrothermal alterations stopped before 635 Ma. 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 O-18 depletion of the rocks. (C) 2013 Elsevier B.V. All rights reserved.