Geochronology, geochemistry and isotopic study of the Plio-Pleistocene hominid sites and the Ngorongoro Volcanic Highland in northern Tanzania.

摘要

This study reports new geochronologic and geochemical results for the Plio-Pleistocene sedimentary sequences at Olduvai Gorge, Laetoli, and Natron, and for volcanic rocks within and vicinity of the Ngorongoro Volcanic Highland (NVH) in northern Tanzania. The geochronological results are based on the single crystal laser fusion {dollar}sp{lcub}40{rcub}{dollar}Ar/{dollar}sp{lcub}39{rcub}{dollar}Ar and the conventional K-Ar dating techniques, and amino acid studies of ostrich eggshells.; The dating results permit the calibration of the hominid, archaeological and geological records preserved at the three hominid sites to be determined with great precision. An age of about 1.6 to 1.5 Ma has been obtained for the first appearance of Acheulian tool in East Africa. The MSA and LSA archaeological records at Olduvai are dated to about 300 to 200 Ka, and {dollar}ge{dollar}40 Ka, respectively, and an age of {dollar}ge{dollar}200 Ka has been obtained for the Homo sapiens (L.H. 18) from Laetoli. Dates from the base of the Olduvai Beds suggest an age of 2.0 Ma for the lower boundary of the Olduvai Subchron and 2.03 to 2.01 Ma for the upper Reunion event.; Preliminary dating results from the NVH and vicinity reveal that the tectono-magmatic evolution of this part of the rift has evolved in four major phases, beginning about 5.0 Ma ago. Major and trace elements, and Sr and Nd isotopic data reveal a lithospheric mantle generation of the magmas and evolution without significant interactions with crustal rocks. Isotopic data from the NVH and vicinity reveal a general decrease in {dollar}sp{lcub}87{rcub}{dollar}Sr/{dollar}sp{lcub}86{rcub}{dollar}Sr ({dollar}sim{dollar}0.706 to {dollar}sim{dollar}0.704) ratios and an increase in {dollar}varepsilon{dollar}Nd ({dollar}sim{dollar}-15.3 to {dollar}sim{dollar}+2.3) values towards the rift axis. This has been interpreted as a reflection of decreasing magmatic injection into the ancient lithospheric mantle by an upwelling asthenospheric mantle as distance increases from the rift axis.; Major element compositions of glass shards have been obtained for several units at Olduvai Gorge and Laetoli. These data are used to evaluate the potential volcanoes within the NVH that erupted the tephra in the various units at Olduvai and Laetoli.