Highly evolved Archean basement beneath the western Cathaysia Block, South China

How has the Earth's continental crust evolved? Most of our knowledge comes from surface exposures, but zircon xenocrysts in volcanic rocks can provide samples of deeper crustal levels. The U-Pb and Hf-isotope systematics of xenocrystic zircons brought to the surface by the Cenozoic (48-49Ma) Pingnan basaltic rocks and the Mesozoic (166Ma) Pingle minettes in Guangxi Province (South China), suggest the presence of unexposed relict Archean basement beneath the western Cathaysia Block, where the oldest exposed rocks are Neoproterozoic-Phanerozoic in age. This basement has provided zircons with several distinct age populations: 3.85, 3.5 5, 3.3-3.2 and mainly 2.9-2.5Ga. These have Hf depleted-mantle model ages (TDM) of 2.5 to ≥3.9Ga. The oldest TDM (∼3.9Ga) shows the existence of Paleo- to Eoarchean components in this area. This relict basement experienced complex modification, including the addition of juvenile mantle material (with εHf up to +12.7) at ca 3.6-3.2, 2.5, 1.0 and 0.5Ga. The zircons also record thermal events that reworked (remelted) the older crustal components of the block at ca 2.0-1.8, 1.6-1.5Ga and ∼80Ma. Although these younger events have modified the Archean nature of the basement, it seems that they do not represent significant post-Archean crustal growth.14 page(s