5:15 PM - 6:45 PM
[SMP22-P01] Cretaceous Crustal Evolution and Arc Building in NE Japan: Evidence from Abukuma and Tsukuba Regions
Keywords:Granitoids, Zircon U-Pb geochronology, Zircon Hf isotope, Crustal Evolution
The Earth differs from the other planets in our Solar System in having a high-standing continental crust, which has an andesitic to dacitic bulk composition. Based on trace element similarities between bulk continental crust and arc magmas, it is widely accepted that the continental crust, at least since the Phanerozoic, has been formed through arc magmatism. Examining the petrology and geochemistry of arc crusts therefore is critical for understanding the formation and evolution of continental crust.
The igneous intrusions in the NE Japan documented episodic accretions during the Cretaceous, providing a natural laboratory for studying arc building processes. The igneous intrusion bodies exposed in large volume include two regions: the Kitakami-Abukuma Mountains and the Tsukuba Mountains from the north to the south. Zircon U-Pb dating combined with Hf isotope values reveals that igneous rocks in the Tsukuba Mountain forming at the Late Cretaceous (~78 to ~65 Ma) with extremely unradiogenic Hf isotope values (εHf(t)=-4.0 to -21) are distinctive from the those in the Kitakami-Abukuma Mountains, which were formed in the Early Cretaceous (~115 to ~100 Ma) with juvenile Hf isotopic signature (εHf(t)=1.5 to 8.8). Thus, the magmatism in the Tsukuba Mountains, despite locating in the NE Japan, are more likely belong to the extension of magmatism in the Ryoke or San’yo belt in SW Japan. The relative uniform high SiO2 (70-75 wt%) and large variation of Hf isotope (-21.0 to -4.0) indicates the granitoids formed by lower crustal melting. The Hf model ages (2479-1392 Ma) of granitoids are significantly older than those of coeval granitoids in SW Japan which show Nd model ages at 1435-748 Ma. This indicates that the arc magmatism in the Tsukuba area involved older lower crust linked to a possible micro-continent derived from the Southern Korean Peninsula.
Within the Kitakami-Abukuma Mountains, the gabbroic and granitic rocks recorded two magmatic peak fluxes: ~122 Ma in the Kitakami Mountain with adakitic signatures and ~108 Ma in the Abukuma Mountains with typical global arc features. The Cretaceous ages and HFSE (e.g., Nb, Ta, Zr, Hf) depletion of the Kitakami and Abukuma magmatic rocks indicate these rocks are related to subduction of Izanagi plate. Magmatic rocks in the Kitakami Mountains shows more depleted mantle-like Hf signatures (εHf(t)=7.9-10.6) than those in the Abukuma Mountains (εHf(t)=1.5-8.8). Hf isotopic signatures are consistent with whole rock Sr-Nd isotopic compositions in literature. Based on the geochemical features of the magmatic rocks in NE Japan, A modified geodynamic evolution model including slab-break off, mantle upwelling and partial melting is proposed to explain the formation of ~122 Ma adakitic rocks in the Kitakami region. Subsequent slab subduction was reestablished, leading to the formation of normal island arc rocks in the Abukuma area at around 108 Ma.The re-established slab might be colder, preventing the slab from melting so that not generating adakitic magma but incorporation more old crustal materials.
The igneous intrusions in the NE Japan documented episodic accretions during the Cretaceous, providing a natural laboratory for studying arc building processes. The igneous intrusion bodies exposed in large volume include two regions: the Kitakami-Abukuma Mountains and the Tsukuba Mountains from the north to the south. Zircon U-Pb dating combined with Hf isotope values reveals that igneous rocks in the Tsukuba Mountain forming at the Late Cretaceous (~78 to ~65 Ma) with extremely unradiogenic Hf isotope values (εHf(t)=-4.0 to -21) are distinctive from the those in the Kitakami-Abukuma Mountains, which were formed in the Early Cretaceous (~115 to ~100 Ma) with juvenile Hf isotopic signature (εHf(t)=1.5 to 8.8). Thus, the magmatism in the Tsukuba Mountains, despite locating in the NE Japan, are more likely belong to the extension of magmatism in the Ryoke or San’yo belt in SW Japan. The relative uniform high SiO2 (70-75 wt%) and large variation of Hf isotope (-21.0 to -4.0) indicates the granitoids formed by lower crustal melting. The Hf model ages (2479-1392 Ma) of granitoids are significantly older than those of coeval granitoids in SW Japan which show Nd model ages at 1435-748 Ma. This indicates that the arc magmatism in the Tsukuba area involved older lower crust linked to a possible micro-continent derived from the Southern Korean Peninsula.
Within the Kitakami-Abukuma Mountains, the gabbroic and granitic rocks recorded two magmatic peak fluxes: ~122 Ma in the Kitakami Mountain with adakitic signatures and ~108 Ma in the Abukuma Mountains with typical global arc features. The Cretaceous ages and HFSE (e.g., Nb, Ta, Zr, Hf) depletion of the Kitakami and Abukuma magmatic rocks indicate these rocks are related to subduction of Izanagi plate. Magmatic rocks in the Kitakami Mountains shows more depleted mantle-like Hf signatures (εHf(t)=7.9-10.6) than those in the Abukuma Mountains (εHf(t)=1.5-8.8). Hf isotopic signatures are consistent with whole rock Sr-Nd isotopic compositions in literature. Based on the geochemical features of the magmatic rocks in NE Japan, A modified geodynamic evolution model including slab-break off, mantle upwelling and partial melting is proposed to explain the formation of ~122 Ma adakitic rocks in the Kitakami region. Subsequent slab subduction was reestablished, leading to the formation of normal island arc rocks in the Abukuma area at around 108 Ma.The re-established slab might be colder, preventing the slab from melting so that not generating adakitic magma but incorporation more old crustal materials.