华南晚三叠世火山岩的识别: 对古太平洋洋壳俯冲启动时限的制约.

Taking the intermediate-acid volcanic rocks of the Songling Formation originally identified as the Early Jurassic in eastern Guangdong as the research object, a systematic chronology and isotopic analysis of U-Pb zircon are carried out. The result shows that the three volcanic rocks in the lower part of the Songling Formation were formed at 204-200 Ma. Therefore, the continuous Late Mesozoic volcanism in South China can bring temporally forward to Late Triassic. All three volcanic samples show depleted zircon Hf isotopic compositions, with εHf(t) values varying between +9. 2 - +5. 7, +5. 7 - +2. 2 and +10. 0 - +2. 9, respectively. It suggests that the volcanic rocks were derived by partial melting of the Paleoproterozoic crustal basement, with obvious depleted mantle-derived involvement. According to the results of previous studies, the contribution of depleted mantle-derived materials in the genesis of Late Mesozoic volcanic rocks in eastern Guangdong gradually weakened from the Late Triassic to the Middle Jurassic, but then gradually strengthened. The magmatic activities since the Late Jurassic in South China records late-stage subduction, and magmatism before Middle Jurassic is the induced result of the early-stage subduction of the paleo-Pacific plate. The Late Triassic volcanic rocks in South China identified in this research imply that the paleo-Pacific subduction was initiated in Late Triassic. [ABSTRACT FROM AUTHOR]