Secular Variation of Early Cretaceous Granitoids in Kyushu, SW Japan: The Role of Mélange Rocks as a Possible Magma Source

Abstract

The Early Cretaceous volcanic-arc granitic rocks from Kyushu, SW Japan are contemporaneous with the granitic rocks of the Yanshan Orogeny (SE China) along the eastern Eurasian continental margin. The secular geochemical variations of the wholerock major elemental and Sr-Nd isotope data of the Early Cretaceous granitic rocks from Kyushu, SW Japan, as well as the zircon and apatite saturation temperatures, shows distinct changes during the Albian (∼115 to ∼100 Ma) as: (1) the mASI value of the rocks (i.e., Shiraishino granodiorites) decreases below 1, (2) the Sr-Nd isotopic data are relatively constant [ 87 Sr/ 86 Sr i = 0.70471 to 0.70573; ε Nd (t) = + 0.2 to + 1.9] within different rock types including granites, granodiorites, tonalites, and adakitic rocks (i.e., the Shiraishino granodiorites), following the increase of 87 Sr/ 86 Sr i and decrease of ε Nd (t) from Berriasian, and (3) higher maximum temperatures at ∼105 Ma. The secular changes indicate that important geodynamic changes occurred in the arc system of SW Japan as it changed from subduction-accretion during the Jurassic to continental arc during the Early Cretaceous. Thermodynamic partial melting modeling demonstrates that the Albian granitic rocks can be derived from mélange rocks, such as chloriteactinolite schists, at moderate depth and variable redox conditions. It is concluded that the genesis of the Early Cretaceous granitic rocks from Kyushu, SW Japan, may be related to upwelling of the asthenosphere and hot corner flow into the mantle wedge caused by slab rollback, which followed a shallowing of the subduction angle and subsequent flat-slab subduction during the Late Jurassic. The resultant heat induced the partial melting of the mélange rocks that formed on and were transported from the subducted plate interface.

Mg-Ba-Sr-Nd isotopic evidence for a mélange origin of early Paleozoic arc magmatism

“…However, the slab surface cannot have such high temperatures at similar pressures. In light of these issues, an increasing number of studies have proposed a mélange diapir model as an important alternative way of transferring slab components and forming arc magmas (Savov et al, 2005(Savov et al, , 2007Marschall and Schumacher, 2012;Nielsen and Marschall, 2017;Suga and Yeh, 2020). The mélange model (Marschall and Schumacher, 2012) proposes that at fore-arc depth, the slab components (sediment, AOC, and serpentinite) physically mix with the mantle wedge peridotite just above the slab in a subduction channel to form hybrid hydrated mélange rocks.…”

Early Cretaceous volcanic-arc magmatism in the Dalat-Kratie Fold Belt of eastern Cambodia: implications for the lithotectonic evolution of the Indochina terrane

“…Farther north along the SE coast of China, the Early Cretaceous period of arc-magmatism is referred to as the Late Yanshanian Orogen and is typified by I-type granite magmatism with subordinate S-type granite magmatism (Zhou et al, 2006;Li et al, 2012;Li et al, 2014;Dong et al, 2018;Shellnutt et al, 2020a;Suga and Yeh, 2020). From a geodynamic point of view, the Early Cretaceous Eurasian margin arc-magmatism is related to subduction of Paleo-Pacific oceanic crust (Maruyama et al, 1997;Yang, 2013;Wu et al, 2022).…”

Deciphering Magmatic Evolution through Zoned Magmatic Enclaves and Composite Dikes: An Example from the Late Cretaceous Taejongdae Granite in Busan, Korea

“…The subduction slab rollback represents one of the main factors controlling the stress state transition from compression to extension [36]. During the same duration, the breakdown of the subducted oceanic slab created a slab window, having a narrow gap and permitting the asthenospheric upwelling (Figure 9a) [14,37].…”