Searching for the source of the Footprint Tuff (Laetoli, Tanzania): a petrological and indicator-mineral perspective
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Date
2019-08
Authors
Gao, Song
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Abstract
The East African Rift System represents the world's quintessential example of an active intracontinental rift system. Within the Rift, a complexly stratified 152-m thick tuff sequence at Laetoli (northern Tanzania) is a well-preserved paleoanthropological site. This sequence comprises fossil- and artifact-rich aeolian, water-infiltrated and air-fall tuff beds, the best-known of which is the so-called Footprint Tuff, which hosts the earliest record of hominin bipedalism. This stratified carbonate-rich air-fall tuff unit is located approximately 30 km southwest of the Sadiman volcano. It has been a matter of debate for years whether the Sadiman volcanism could be the source of the Footprint Tuff. Based on the petrographic, mineralogical, geochemical and geochronological data obtained in the present work, volcanic activity at Sadiman can no longer be ruled out as one of the sources of the Laetolil tuffs because indicator minerals from these two localities (including melilite, garnet, perovskite, titanite and apatite) share many compositional similarities in the distribution and abundance of trace elements. The Sr-Nd isotopic characteristics of nephelinites from Sadiman suggest that their parental magmas originated by partial melting of a sub-continental lithospheric mantle source affected by plume activity. An Ar-Ar phlogopite age of 4.3 ± 0.1 Ma, obtained in the present work, provides further constraints on the timing of this magmatism. The possible evolutionary paths of parental magmas were reconstructed for the first time on the basis of petrographic observations and trace-element fractionation models.
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Keywords
Footprint Tuff, Indicator-mineral, Geochemistry, Mineralogy, Sadiman volcano, Petrology, Geochronology, Trace element