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TitleArc tempos of the Gangdese batholith, southern Tibet
AuthorMa, X; Attia, S; Cawood, T KORCID logo; Cao, W; Xu, Z; Li, H
SourceJournal of Geodynamics vol. 149, 101897, 2022 p. 1-20,
Alt SeriesNatural Resources Canada, Contribution Series 20230117
Mediapaper; digital; on-line
File formatpdf
AreaTibet; China
Lat/Long WENS 75.0000 100.0000 35.0000 30.0000
Subjectsgeneral geology; magmatic arcs; magmatic rocks
Illustrationslocation maps; graphs; cross-sections
Released2022 01 12
AbstractThe character of arcs varies over time with significant temporal fluctuations in the quantity and spatiotemporal patterns of magmatism. However, the driving mechanisms for this episodic behavior of arcs need more constraints. This paper analyzed the published data along with our new zircon U-Pb dating and Hf isotopic and whole-rock geochemical data of plutonic rocks in the Gangdese belt in southern Tibet to explore the features, potential drivers, and tectonic implications of episodic arc activity in the Gangdese arc. A comprehensive compilation of U-Pb ages and Lu-Hf isotopic analyses of zircon grains from igneous rocks in the Gangdese belt, sedimentary rocks in trench fill sequences, forearc basins and foreland basins, and sands from modern river reveals that: 1) Gangdese arc activity was episodic during Late Cretaceous to Middle Eocene, displaying two magmatic flare-ups (ca. 100-80 and 65-45 Ma) and one magmatic lull (ca. 80-65 Ma), and 2) both flare-up magmas show relatively positive eHf(t) values (+5 ~ +15) indicative of juvenile sources suggesting these magmas are dominated by contributions from the depleted mantle. In contrast, the magmatic lull between these two magmatic flare-ups could be caused by flat subduction of the Neotethyan slab beneath the southern margin of the Lhasa terrane. These flare-ups likely contributed greatly to the crustal thickening of the Gangdese belt. Constraints from paleo-elevation and geochemical proxies for crustal thickness showed that the ~100-80 Ma flare-up was accompanied by the formation of a thick arc root while the ~65-45 Ma flare-up likely developed in a thinner crust without an arc root.
Summary(Plain Language Summary, not published)
Magmatic arcs are belts of magmatic activity developed above subducting oceanic plates. Magmatic activity in the arcs is not constant, but varies in intensity over time, with abundant magmatism during "flare-ups" and little to no activity during "lulls". This paper documents the variations in magmatic activity in the Gangdese arc of southern Tibet, and investigates their potential causes. The age of magmatic rocks can be calculated by analysing the amount of uranium and lead within zircon crystals in the rocks. By compiling new and existing zircon ages from across the Gangdese arc, we show that: 1) magmatic activity was episodic, with two major flare-ups (at ~100-80 and ~65-45 million years ago), separated by a lull; and 2) the chemistry of the zircons and magmatic rocks indicates that, during the flare-ups, magma was predominantly formed by melting mantle material deep within the Earth (and not the crust). The lull in magmatic activity may have been caused by a decrease in the subduction angle of the subducting oceanic plate, which temporarily shut off magmatism. Chemical analyses also suggest that the older flare-up was associated with thick crust and a high mountain belt, but that the later flare-up affected thinner crust with a lower elevation.

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