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

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层位的定性岩石学研究表明，存在大量细粒岩屑碎片，大概率为火山成因，同时伴生较粗的石英与长石碎屑，这些粗碎屑显然源自附近的深成岩区。进一步细节分析显示，分选性差、粒序层理发育以及黏土矿物富集，指示这些层位为源自混合火山-深成岩区域的本地浊积岩层，可能还接收了部分同期火山灰降落的直接补给。不过，随着时间推移，深成岩组分相较于火山岩组分呈递增趋势，这一特征更符合该套沉积物多数组分的侵蚀成因。在数处下部碎屑层位中，富集了罕见的自形暗色黑云母；其最合理的成因解释为，源自碱性火山灰原地风化析出的微斑晶。从岩心样品516F-76-4的107~115厘米层位中分离得到的黑云母，经钾氩测年法(K-Ar method)测定，年代约为46 Ma。中始新世时期活动于里奥格兰德海隆的碱性火山，极有可能是该火山灰的源区，这与里奥格兰德海隆及特里斯坦-达库尼亚热点处的钾质碱性火山活动的其他相关证据相符。