U-Pb dating results of volcanic lava can determine the diagenetic age of volcanic lava, and define the geological age of the volcanic lava in sedimentary strata. However, with the three parameters of the harmonic age (consistent age), weighted average age and minimum age obtained from the zircon U-Pb dating results, which age can represent the diagenetic age of volcanic lava?
Understanding the magmatic diapiring process and time scale in the deep lithosphere is of great significance for rationally explaining the persistent and widely varying zircon ages in volcanic lava and judging the diagenetic age of volcanic lava bodies (Liu et al. 2019). The crystal growth in the magma chamber is a very long geological process. When the temperature of the magma chamber deep in the mantle is lower than the initial crystallization temperature of zircon, zircon crystals (ZrSiO3) begin to crystallize and grow. Because the Zr element in the zircon is the same as the Ti element and can be substituted for each other, and the Ti element is extremely sensitive to temperature, the influence of other factors is not considered, and the calculation is based on the content of the Ti element in the zircon in the area ablated by the laser, The calculated results can roughly represent the crystallization temperature of zircon in the magma chamber (Gao and Zhen. 2011; Yuan et al. 2018). In this paper, the zircon Ti thermometer formula corrected by (Watson et al. 2006) is used for calculation, and the results obtained are: in the dacite of Xiaopinggou section, the crystallization temperature of zircon is roughly 669℃~788℃, and the average temperature is 710 °C, this result is consistent with the calculated temperature of zircon Ti content in pillow lava from 621 °C to 790 °C (average 695 °C). Therefore, the calculation results show that the pillow lava and the dacite were formed in the same high temperature environment (Fig. 6), and the zircon U-Pb age in the dacite can also indicate the geological age of the pillow lava. As the magma slowly rises and cools, the zircon crystals continue to grow, and new zircon is born. However, for the volcanic lava that erupted from the surface, when the temperature dropped rapidly from the magma chamber at an extremely fast rate, it was too late to grow enough zircon crystals to meet the experimental conditions. Therefore, the youngest zircon age is the age closest to the consolidation diagenesis after the magma erupted to the surface, and this result can be used to define the oldest geological age in which the volcanic lava occurred in the sedimentary strata.
However, only using the age of one youngest zircon to define the diagenetic age of volcanic rocks is accidental, and only when the number of samples is greater than or equal to 3, it conforms to the general mathematical statistical principle (Dickinson W. R and Gehrels GE 2009). The minimum age combination composed of four zircon points with a similar minimum age represents the formation age of the volcanic rock. The ages of these four zircon are 315±2.51Ma, 316±2.85Ma, 316.9±3.01Ma, 318±2.96Ma respectively, and the age of these four zircon is calculated to be 315.4±1.3Ma (such as Figure 7), this result implies that the depositional age of this formation should be no earlier than the early Late Carboniferous (late Bashkirian).
In this paper, the U-Pb age of dacite measured in the Xiaopinggou section of Mount Bogda is 315.4±1.3 Ma, which is close to (Wei et al.2016) the U-Pb lava of Baiyanggou in the western section of Mount Bogda. U-Pb age (311±2.8Ma), slightly larger than (Lei et al.2016) U-Pb age (308±3.3Ma) of the gabbro in the Sujishan section of the eastern part of Bogda, and also similar to (Gao et al.2014; Wang et al.2015b) The U-Pb ages (314.9±1.2Ma, 312±1Ma) of the quartz horn porphyries in the Liushugou Formation section in the Sepikou area of the eastern part of Bogda and the rhyolite in the Jijitaizi Liushugou Formation section (314.9±1.2Ma, 312±1Ma) are very close(Liu et al.2020). The geological age determined by the paleontology contained in the Liushugou Formation in the Ganhezi area is consistent with the (Zhong et al.2018) U-Pb age of the Wudong pillow lava, 283±8Ma, which is the early Permian The world's conclusions are inconsistent.