島弧岩漿演化研究：以西巽他與北呂宋島弧為例(Ⅲ)

Cenozoic magmatism in Sumatra Island were formed by the Indo-Australian Plate subducted beneath the Eurasian Plate. Based on the geochemical variations and spatial distribution, volcanism here can be separated into four groups. All samples (including volcanic and plutonic rocks) were collected from Cenozoic volcanoes. A total of 1000 grains of igneous zircons from 56 samples were analyzed both U-Pb ages and Hf isotopes. We got 43 zircon U-Pb mean ages and 13 samples of them with too young ages (some of them were historical eruptions) to be identified. Our new data show the Cenozoic magmatism was most active and widespread during five stages: Paleocene (66 to 65 Ma), Early Eocene (56 to 48 Ma), Middle Eocene to Oligocene (48 to 23 Ma), Miocene (23 to 5.3 Ma), Pliocene (5.3 to 2.6 Ma), and Quaternary (2.6 Ma to recent). The magmatic gap from approximately 50 to 20 Ma might reflect the time interval between the India-Asia collision and the Indo-Australian subduction, and the same results can be supported by the detrital zircon data. Except Toba related volcanoes have large range of U concentration, other igneous rocks in Sumatra have parallel U content. Igneous rocks in Sumatra can be divided into four groups by their Hf isotopic characteristics. First, the Eastern magmatism show stable ?Hf(t) value (+9.4 to +18.0) represent the normal Sunda Arc melts. Second, Central magmatism show partial affected by crustal material and show ?Hf(t) value from +4.0 to +17.0. Third, Toba and related magmatism were mostly erupted younger than 5 Ma with obviously crust signals (?Hf(t) value = -15 to +7.3). Fourth, the Western magmatism show temporal (after 15 Ma) and spatial migration, however, their unchanged ?Hf(t) value (+6.3 to 14.0) represent magmas formed from the same materials by partial melting through time.