島弧岩漿演化研究：以西巽他與北呂宋島弧為例(Ⅱ)

The Toba eruption was the largest event in the Quaternary. Toba located in the northern part of Sumatra Island, and it is belonged to the Sunda arc system. This study focus on the volcanology, petrology, geochronology, and geochemistry to study the Toba volcanic rocks and combining the results from previous researches to provide new views of the age control, petrogenesis, and tecotonic evolution in this area. Samples were collected based on the volcanic sequences from the andesite lava flow in the bottom and four ignimbrite sequences on the top. Major and trace elements, whole rock Sr-Nd isotopes, zircon U-Pb dating and Hf-Lu isotopes of zircons were done in this study. Petrology results show that the magma stayed in the magma chamber for a long time to grow up large phenocrysts and went through the decompression and crustal contamination processes before they erupted. These volcanic rocks belong to medium- to high-K basalt to rhyolite. All samples show Eu negative anomaly, LREEs enrichment, HFSEs depletion, and LILEs enrichment. Whole rock Sr-Nd isotope data show crustal contamination results. Two old events (10.5 ± 0.2 Ma and 5.2 ± 0.6 Ma) can be identified as the Toba related magmatism according to their zircon U-Pb dating amd Hf-Lu isotope analyses. The age of four ignimbrite can be yielded of 1.43 ± 0.02 Ma (HDT), 0.99 ± 0.02 Ma (OTT), 0.55 ± 0.01 Ma (MTT), and 0.22 ± 0.04 Ma (YTT). We propose the magmatism below Toba area could be caused by crustal contamination and magma mixing between the partial melting from the lower and upper crust. The extra heat were as a result of the slab tearing and asthenosphere upwelling. This event were caused from about 15 Ma and had closely relation with Andaman Sea opening, Sumatra fault removed, and several ridge subdcuted beneath the southern Sumatra. We suggest that these events are caused by the tectonic force changed from the oblique subduction to a strike-slip movement.