| PDF: | |
| Time: | 2020 |
| Journal: | Earth Science |
| Volumn: | 45 |
| Issue: | 11 |
| Pages: | 4091-4117 |
| Type: | EI |
| Author: | Siwei Li,Pujun Wang,Lin Ding,Jiayuan Du |
Abstract: | There are different understandings on the types, genesis and geodynamic mechanism of the volcanic rocks formed in the Early Cenozoic in the northern margin of the South China Sea. In this paper, it presents zircon U-Pb data and geochemical data for the trachytic agglomerate or breccia lava, trachyte, rhyolite and trachydacite revealed by drilling in Huizhou sag of the Pearl River Mouth basin. The results show that the Cenozoic volcanic rocks in the study area were formed at 57.53-59.15 Ma, near the boundary between Paleocene and Eocene, and fell into the Thanetian stage. These rocks are characterized by high SiO2(64.60%-73.22%), Al2O3(11.86%-16.57%), low MgO(1.16%-1.80%), high Sr(305.20×10-6-465.50×10-6, 371.15×10-6 on average), low Y(7.47×10-6-9.84×10-6), Yb(0.90×10-6-1.43×10-6) and high Sr/Y ratios (35.19-54.55). They are enriched in LREE and depleted in HREE, together with enrichment in large-ion lithophile elements (LILE) Ba, K and depletion in high field strength elements (HFSE) Nb, Ta, Ti and P, as well as small positive Eu anomalies, similar to those of adakite. These rocks are calc-alkaline, high-K calc-alkaline and shoshonite metalumious (A/CNK=0.85-1.04). The Early Paleogene adakites are interpreted as the products of partial melting of newly underplated mafic lower crust under garnet-amphibolite facies conditions. It is concluded that the Cenozoic delamination in the northern margin of the South China Sea plays an important role in the formation and evolution of magma. |