| The comparative study of the southern and northern continental margin basins of the South China Sea(SCS) is an important way to understand the process of spreading of the SCS and the pattern of paleogeography. Due to historical reasons, the study to the tectonic and sedimentary evolution at the southern margin basins in SCS is still very weak at present. It greatly limits the overall understanding to basic geological problems such as the spreading of the SCS and the changes of sea and land. Palawan-Mindoro micro-continental block locates in the southeast margin of South China Sea. Du to the subduction of oceanic crust in South China Sea, there is a collision orogeny between Palawan-Mindoro micro-continental block and Philippine mobile belt. The Eocene-Miocene marine sequence in Palawan-Mindoro micro-continental block has been completely exposed. It does be an only onshore region to study the sedimentary sequence in the southern margin of the South China Sea Basin. This paper integrates the research progress of sedimentary strata and sedimentary environment in the SCS and its adjacent basins, in order to make a comprehensive analysis of spreading process of the SCS and the evolution of paleogeographic pattern. Through the study on break-up unconformity, the detrital zircons, Hf isotope in Mindoro Cenozoic strata and field geological observation, we achieved a new understanding of the spreading time and way in the SCS. Through the contrast between Taiwan and Mindoro, the provenances of Mindoro were investigated.According to the unconformity, the southern part of the SCS exiss the diachronism like the northern part. The break-up unconformity become younger gradually from the northeast to the southwest, suggesting that the spreading process may be from east to west. East subbasin of seafloor spreading time is Early Oligocene(~33Ma). Taiwan Cenozoic strata break-up unconformity is in 33-39 Ma, Mindoro Cenozoic strata break-up unconformity is in 33-28 Ma, suggesting that the formation time of oceanic crust in SCS is 33-39 Ma. Some of the older oceanic crust may have been lost in the East subduction. The paleogeographic pattern of the SCS is the marine environment in the south and the terrestrial environment in the north in early stage, now with its opposite. Transgressions gradually extend from east to west. It’s forming present morphology from a narrow bay.We collected 5 samples in Mindoro. They are P-06 in Late Eocene(Lasala Formation), S-03-2 in Late Eocene – Early Miocene, S-04 in Miocene. The most of the zircons have Th/U>0.3, combine with zircon morphology, we infer that they are magma zircons. Zircon grains age are mainly concentrated in the Yanshan period, followed by Paleoproterozoic. There are 5 peak ages like 115 Ma, 161 Ma, 88 Ma, 115 Ma, 165 Ma. The most of εHf(t) value are negative with wide ranges(-39.0~﹢9.3). Generally speaking, the samples in Miocene strata have more positive εHf(t) value than Late Eocene – Early Miocene. Especially in Yanshanian, followed by The Indosinian and Caledonian, but the increased positive values are not obvious. Proterozoic has the similar εHf(t) value. Through the above characteristics, we suggest that the Mindoro island is the mainland property, and is a part of Palawan micro continental block.At the same time, we put the Mindoro Cenozoic strata in comparison with Taiwan. Obvious changes occurred in the northern basins of the SCS at ~25Ma, which in the early stage the sedimentary deposition is mainly from the Southern China. After ~25Ma, the sediment from the Yangtze block was gradually increased. The sources at the southern basins of the SCS have the same material sources with northern basins before ~25Ma, which is mainly the near source erosion deposition of Southern China. Due to the spreading of the SCS after ~25Ma, oceanic basins of the SCS resist the Yangtze block conveying sediment sources. The Mesozoic granites and volcanic rocks in the interior of the continental block are the main sources of the southern basins of SCS. |
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