(Table 1) K-Ar age determinations on biotite from coarse sand within DSDP Sample 72-516F-76-4,107-115

Qualitative petrographic study of selected clastic horizons within the Eocene section of Hole 516F has revealed the presence of abundant fine-grained lithic fragments, probably volcanic, along with coarser fragments of quartz and feldspar apparently derived from a nearby plutonic terrain. In detail, poor sorting, presence of graded bedding, and an abundance of clay suggest these are turbidite horizons locally derived from a mixed volcanic/plutonic terrain, possibly with some direct contribution from contemporary volcanic ash falls. A progressive increase in plutonic versus volcanic components with time is, however, more consistent with an erosional origin for most of this material. Unusual euhedral dark biotite is abundant in several of the lower clastic horizons; it is most easily interpreted as microphenocrysts weathered in situ out of alkalic volcanic ash. Biotite separated from Sample 516F-76-4,107-115 cm, has been dated by the K-Ar method at about 46 Ma. Alkaline volcanoes active on the Rio Grande Rise in the middle Eocene would be the most probable source of this ash and would be consistent with other evidence for potassic, alkaline volcanism along the Rio Grande Rise and at the Tristan da Cunha hot spot.

对516F钻孔始新统地层中选定碎屑岩层开展的定性岩相学研究显示，其中赋存大量细粒岩屑碎片（大概率为火山成因），同时伴有较粗的石英与长石碎屑，这些碎屑显然源自附近的深成岩区。详细分析表明，沉积物分选差、发育粒序层理且黏土含量丰富，指示这些岩层属于源自混合火山-深成岩区的原地浊积岩层，可能还接收了部分同期火山灰降落的直接补给。不过，随时间演化，深成岩组分相较于火山岩组分的占比逐步升高，这一特征更契合该套沉积物大部分由侵蚀作用形成的成因模式。数个下部碎屑岩层中产出大量罕见的自形暗色黑云母，其最合理的成因解释为从碱性火山灰中原地风化析出的斑晶微晶。对516F-76-4样品107~115厘米段中分离得到的黑云母开展钾氩（K-Ar）定年，结果显示其年龄约为46 Ma。中始新世时期活跃于里奥格兰德海隆的碱性火山，是这套火山灰最有可能的物源区，这一结论与里奥格兰德海隆及特里斯坦-达库尼亚热点沿线存在钾质碱性火山作用的其他证据相互吻合。