珠江口盆地白云凹陷中新统珠江组物源及其研究意义

Provenance and Its Geological Implications of Miocene Zhujiang Formation in Baiyun Sag, Pearl River Mouth Basin

南海北部陆坡深水区3Ma以来碎屑物质来源具多样性、复杂性和变化性。利用砂岩和异源生屑灰岩岩石学及泥岩稀土元素地球化学特征研究了珠江口盆地白云凹陷中新统珠江组深水扇的物源。结果表明,珠江组砂岩岩屑主要为变质岩屑和火山岩屑,并含较多深成岩屑。阴极射线下,石英以发蓝紫色—褐色光为主,长石主要发亮蓝色光。异源生屑灰岩为陆坡钙屑碎屑流沉积,来自于神狐暗沙隆起中新统碳酸盐台地。泥岩∑REE与燕山褶皱带燕山期中酸性火山岩较接近,分配模式与上地壳基本一致,并与燕山中酸性火山岩和花岗岩较接近。渐新世末,南海运动使南海海盆第二次海底扩张,并使陆架坡折带由白云凹陷南侧跳跃到凹陷北侧。该构造事件在青藏高原邻近盆地和中国东部盆地都有反映,反映可能发生了青藏高原隆升事件,使珠江组物源向西拓展。珠江期虽然存在活跃的深水牵引流,但珠江组原始物质仍应源自古珠江携带的华南沿海燕山褶皱带的中酸性火山岩、花岗岩和内陆古老的沉积—变质岩。

The clastic source for the deep-water slope in the northern South China Sea are characterized by diversity, complexity and variability since 3Ma. The possible source area of the Miocene Zhujiang Formation in the Baiyun Sag, Pearl River Mouth Basin was studied by using petrological analysis of sandstones and heterologous biological limestones and rare earth elements of mudstones. It was shown that the fragments of the Zhujiang Formation sandstones were mainly consisted of metamorphic rock debris and volcanic rock debris, with many plutonic rock debris. Under cathode rays, the quartz were blue and purple - brown, and the feldspars were bright blue. Heterologous bioclastic limestones were slope calcarenaceous debris flow deposits, and might come from the Miocene carbonate platforms in the Shenhu Shoal uplift. The average EREE of the mudstones is close to that of Mesozoic intermediate--felsic volcanic rocks in the Yanshanian fold belt, t^Eu had obviously negative anomaly, the distribution model of LREE enrichment and HREE loss was consistent with the upper crust and the acid volcanic rocks and granites in the Yanshanian fold belt. In the latest Oligocene, the South China Sea movement made the second sea-floor spreading of the South China Sea basin and the jumping of the shelf break from the south side of the sag to the north in the Baiyun Sag. This tectonic event occurred in the adjacent basins around the Qinghai-- Xizang（Tibetan） Plateau and also occurred in the eastern China basins, reflecting the occurrence of the Qinghai-- Xizang（Tibet） Plateau uplift event, which made the provenance during the Zhujiang Period expand westward. Although active traction currents existed since the Zhujiang Period, the provenance was still the intermediate-felsic volcanic rocks and granites in the Yanshanian Fold Belt and ancient sedimentary--metamorphic rocks in the South China inlands, which were carried by the ancient Pearl River.