THE SOURCE OF THE ESPINHAÇO DIAMONDS: EVIDENCES FROM SHRIMP U-PB ZIRCON AGES OF THE SOPA CONGLOMERATE AND PB-PB ZIRCON EVAPORATION AGES OF METAVOLCANIC ROCKS

Abstract

We present here the results of the SHRIMP U/Pb zircon ages obtained from the diamond-bearing Sopa Conglomerate and also Pb-Pb zircon evaporation age of the associated metavolcanic rocks, outcropping in the Diamantina region of the Espinhaco Range. The Sopa Conglomerate, a clast-supported rock that appear in the lower part of the Espinhaco Supergroup, was deposited mainly in alluvial fan, braided river and fan delta systems. The apparent 238 U- 206 Pb and 207 Pb- 2()6 Pb zircon ages varied from 1442 Ma to 3683 Ma and from 1726 Ma to 3599 Ma peaks, respectively. Different generations of zircon populations showed by the radiometric ages surely demonstrate the age of cratonic rocks, however it is still unknown if one of them could be correlated with a primary source rock of the diamonds. The series of K- and Fe-rich metavolcanics is coeval with the Sopa Conglomerate. The results of a single date of these rocks yielded ages of 1710±12 Ma to the crystallization of the volcanic protolith. Detrital zircons of the Sopa Conglomerate matrix are older than the primary zircons of the metavolcanic rocks outcropping in the lower Espinhaco Supergroup. Therefore, geochronological data do not provide additional evidence for the idea that diamonds are originated from these metavolcanic rocks, as supposed by some authors after geologic regional studies.