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

摘要

To better understand the evolution of the SE-Asian continent, 235 single zircon and baddeleyite grains from large rivers were dated by the U-Pb chronometer, and 148 of these grains were simultaneously analyzed for Hf isotopes. This combination of methods is applied to sand samples from the Mekong, Irrawaddy, Salween and Red rivers, which sample the continent on the plate scale. U-Pb dates and Hf isotopic signatures of these grains resolve the age spectra of crust and they elucidate source characteristics of magmas involved in continental growth and recycling. For sufficient statistical coverage, more than 80% of zircons and baddeleyites from each rivet sample were analyzed, representing an erosion area of about 1.6 . 10(6) km(2). Our analyses reveal five different Proterozoic crustal growth events occurring at about 2.5 Ga, 2.3 Ga, 1.9 Ga, 1.1 Ga, and 0.8 Ga, however, none of these basement terrains is exposed in SE-Asia, being entirely covered by Phanerozoic sediments. The data also substantiate granitoid magmatism during the Caledonian, Indosinian, and Himalayan orogenies. Initial Hf isotope signatures (epsilon Hf-i) obtained for grains lying less than 15% discordant in the Concordia diagram range between +13.3 and -16.7, plotting in both the domains of strongly LILE depleted and enriched reservoirs, relative to a chondritic evolution of the mantle. Approximately half of the grains crystallized in magmas derived from significantly depleted mantle sources, and about 18% formed 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 seem to have produced the dominant mass of the SE-Asian continent. All younger orogenic cycles are characterized by substantial melting of continental material, which probably dominate over juvenile crust addition. An exception are the Irrawaddy terrains, where substantial mantle input characterizes post-collisional magmatism since 50 Ma. Copyright (C) 2000 Elsevier Science Ltd. [References: 51]