Evolution of the SE-Asian continent from U-Pb and Hf isotopes in single grains of zircon and baddeleyite from large rivers

To better understand the evolution of the SE-Asian continent, 235 single zi rcon and baddeleyite grains from large rivers were dated by the U-Pb chrono meter, and 148 of these grains were simultaneously analyzed for Hf isotopes . This combination of methods is applied to sand samples from the Mekong, I rrawaddy, Salween and Red rivers, which sample the continent on the plate s cale. U-Pb dates and Hf isotopic signatures of these grains resolve the age spectra of crust and they elucidate source characteristics of magmas invol ved in continental growth and recycling. For sufficient statistical coverag e, more than 80% of zircons and baddeleyites from each rivet sample were an alyzed, representing an erosion area of about 1.6 . 10(6) km(2). Our analys es reveal five different Proterozoic crustal growth events occurring at abo ut 2.5 Ga, 2.3 Ga, 1.9 Ga, 1.1 Ga, and 0.8 Ga, however, none of these basem ent terrains is exposed in SE-Asia, being entirely covered by Phanerozoic s ediments. The data also substantiate granitoid magmatism during the Caledon ian, Indosinian, and Himalayan orogenies. Initial Hf isotope signatures (ep silon Hf-i) obtained for grains lying less than 15% discordant in the Conco rdia diagram range between +13.3 and -16.7, plotting in both the domains of strongly LILE depleted and enriched reservoirs, relative to a chondritic e volution of the mantle. Approximately half of the grains crystallized in ma gmas derived from significantly depleted mantle sources, and about 18% form ed in magmas showing nearly chondritic signatures (epsilon Hf-i: -1.37 to 1.59; 27 grains). The remaining grains yield strongly negative epsilon Hf-i documenting the incorporation of important amounts of melts generated from crust formed during the Paleoproterozoic events at 2.3 and 1.9 Ga, which s eem to have produced the dominant mass of the SE-Asian continent. All young er orogenic cycles are characterized by substantial melting of continental material, which probably dominate over juvenile crust addition. An exceptio n are the Irrawaddy terrains, where substantial mantle input characterizes post-collisional magmatism since 50 Ma. Copyright (C) 2000 Elsevier Science Ltd.