巴拉望中南部始新世–中新世地层沉积记录及其对南海东南缘构造演化的意义

The Palawan Microcontinental Block on the southeastern margin of the South China Sea was rifted from the mainland Asia since the seafloor spreading of the South China Sea in the Early Oligocene, and finally collided with the Cagayan Ridge–Philippine Mobile Belt in the Early–Middle Miocene. Therefore, the evolution of the southeastern margin of the South China Sea is of great importance for understanding the paleogeographic environment and spreading history of the South China Sea. Due to the arc-continent collision, Cenozoic strata on the southern margin of the Palawan Microcontinental Block were thrust, uplifted and cropped out on the central and south Palawan. Therefore, the central and south Palawan provides one of the best geological windows to look into the tectonic evolution of the southeastern margin of the South China Sea. Field survey and micropaleontological analysis on the Eocene–Miocene sediments will be conducted to establish a high resolution biostratigraphic framework and also constrain the sedimentary environment. We will also investigate sandstone petrography, mudstone geochemistry and as well as detrital zircon U-Pb chronology and Hf isotope to trace the provenance. Particularly, this study aims to (1) reconstruct the paleogeographical environment of the southern margin of the Palawan microcontinental block in the Eocene–Early Oligocene before the spreading of the South China Sea and (2) constrain the uplift and erosion history of the different tectonic units on the Palawan Island during the arc-continent collision. This study will improve the understanding of the paleogeographical environment of the South China continental margin, and also contribute to the knowledge of the temporal-spatial evolution of the arc-continent collision between the Palawan Microcontinental Block and the Cagayan Ridge–Philippine Mobile Belt and its relationship with termination of the spreading of the South China Sea.

位于南海东南缘的巴拉望微陆块自早渐新世以来伴随着南海的扩张而从华南大陆裂离出来，并于早–中中新世与卡加延海脊–菲律宾活动带发生弧陆碰撞。因此，南海东南缘的构造演化对于了解南海古地理变迁和扩张过程具有重要意义。由于弧陆碰撞，巴拉望微陆块南缘的新生代地层遭受逆冲抬升并出露于巴拉望中南部，成为了解南海东南缘构造演化的重要窗口。本项目拟对巴拉望中南部始新世–中新世地层进行详细的野外地质调查和微体化石分析，建立高分辨率的生物地层框架，并了解其沉积环境。利用碎屑组成、地球化学和碎屑锆石U-Pb年龄及Hf同位素等多种手段示踪沉积物的源区变化。本项目旨在（1）恢复南海扩张以前（始新世–早渐新世）巴拉望微陆块南缘的沉积环境和物源，（2）重建巴拉望岛不同构造单元在弧陆碰撞中的隆升剥蚀历史。这不仅可以拓宽对华南陆缘古地理格局的认识，也可以进一步认识南海东南缘弧陆碰撞的时空演化及其与南海停止扩张之间的成因联系。

位于南海东南缘的巴拉望微陆块自早渐新世伴随着南海的扩张而从华南大陆裂离出来，并于中新世与卡加延海脊–菲律宾活动带发生碰撞。因此，南海东南缘的构造演化对于了解南海古地理变迁和扩张过程，以及古南海俯冲-关闭历史具有重要意义。由于弧陆碰撞，巴拉望微陆块南缘的新生代地层抬升出露于巴拉望中南部，成为了解南海东南缘构造演化的重要窗口。本研究对巴拉望中南部的始新世–中新世地层进行野外地质调查和微体化石分析，借此建立生物地层框架并恢复古水深，同时利用泥岩微量元素和Nd同位素、砂岩重矿物组成及碎屑锆石U-Pb年龄示踪地层沉积物的源区。由此进一步恢复南海扩张以前华南陆缘的古地理格局，获取巴拉望微陆块与卡加延海脊之间弧陆碰撞的地质记录。.浮游有孔虫和钙质超微化石分析显示，裂陷期层序Panas-Pandian组沉积于中始新世–早渐新世（47.7–32.9 Ma），由此将巴拉望中南部的破裂不整合限定为33–32 Ma。该时间接近珠江口盆地（~30 Ma）和IODP U1435站位（~34 Ma）的破裂不整合，说明巴拉望微陆块为珠江口盆地的共轭陆缘。遗迹化石Helminthopsis和底栖有孔虫组合指示半深海–深海环境，说明巴拉望中南部在中始新世–早渐新世位于华南陆缘南侧、古南海北侧的陆坡上。物源分析结果显示Panas-Pandian组的源区主要为华夏地块，并具有两个可能的源区：近源的盆地间隆起和远源的珠江东北支流流域（华夏地块内部）。这暗示了类似于现今珠江东北支流的河流在中始新世（47.7–42.1 Ma）就已存在，而且在当时的华南陆缘上不可能存在大规模的深海盆。中新世Isugod盆地包括Isugod组和Alfonso XIII组，微体化石定年结果显示这两套地层沉积于晚中新世(11.9-9.8 Ma和9.8-5.6 Ma)。底栖有孔虫组合表明，可能由于斜向碰撞，在盆地底部（~12 Ma）发生了超过500 m的构造下陷。物源分析结果显示，Isugod组浊积岩和Alfonso XIII组泥岩具有两个主要的源区：巴拉望微陆块属性的Panas-Pandian组和卡加延海脊火山弧–弧前基底属性的巴拉望蛇绿岩套，说明在Isugod组沉积时，巴拉望微陆块和卡加延海脊已经拼贴在一起，为弧陆碰撞造山的开始提供了良好的时间约束（~12 Ma）。