U–Pb dating of gem-quality vanadium-bearing grossular garnet (var. tsavorite) from north-eastern Tanzania

Abstract

The direct dating of gem-quality garnet mineralization has been extremely limited, even though it has been known for decades that garnet may be dated by the U–Pb method. Here, we demonstrate the application of in situ laser U–Pb geochronology on gem-quality tsavorite to determine the timing of mineralization from two localities in the Mozambique Belt, Tanzania.

U–Pb dating of tsavorite from Merelani and Umba provides ages of 569.5 ± 6.8 Ma and 540.0 ± 5.8 Ma, respectively. Due to the high closure temperature of the U–Pb system in garnet, we argue that these ages correspond to tsavorite mineralization. These ages postdate the East African Orogeny (650–620 Ma), the most significant metamorphic episode recorded in the Mozambique Belt, which had been previously considered to be the main tsavorite mineralization event. Instead, these dates correspond to the later Kuungan orogenic episode (570–530 Ma), associated with the final amalgamation of Gondwana. The mineralization of tsavorite tens of millions of years after the East African Orogeny in the Mozambique Belt illustrates the benefits of direct dating of gem-quality garnet to determine mineralization timing and style.