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[SCG49-09] Magmatic evolution of Cretaceous north Kyushu batholiths and its role in creation of crustal structure
Keywords:North Kyushu batholiths, Cretaceous, Magmatic flare-up, Crustal structure
Understanding igneous activities in active continental margins could provide us useful information for the evolution of continental crusts. The geological and petrological data revealed that the Cretaceous granitoid batholiths in north Kyushu, the north Kyushu batholiths hereafter, were produced by partial melting of mafic lower crusts due to underplating of mantle derived mafic magmas as an acting heat source. The north Kyushu batholiths are characterized by episodic magmatism from 109 to 95 Ma, and consist mainly of hornblende-bearing granitoids with positive epsilon initial Nd values, and are locally associated with coeval high-Mg diorite. The batholiths would, therefore, play an important role to understand the growth of juvenile crust in an active continental margin due to the magmatic flare-up. We will address the geological and petrological features of the granitoids and related mafic rocks from the north Kyushu batholiths to unravel magmatic evolution and its role in the creation of crustal structure.
The host rocks of the western part of the north Kyushu batholiths are composed of the pre-Jurassic accretionary complexes with their high-P/T metamorphic facies and the Cretaceous low-P/T metamorphic rocks reaching up to granulite facies conditions. On the other hand, the Cretaceous shallow marine deposits are exposed as host rocks on the eastern part of batholiths. The magma plumbing system can be recognized in the small stocks on the eastern part of batholiths. The Shaku-dake diorite body, a member of the north Kyushu batholiths, intruding the shallow marine deposits possesses layered structures on the wall sides. The intrusive age of the diorite has been thought to be c. 106 Ma. The dioritic dikes and sills intruded the diorite body where the dikes and sills would provide the magmatic pulses to the Shaku-dake diorite body. The diorite magma was geochemically differentiated from high-Mg diorite to granodiorite through the process of assimilation and fractional crystallization. The Shikano-shima granodiorite occupied in the western part of the batholiths involves the hornblende gabbro as blocks that bears leucocratic patches or lenses geochemically resembling the Shikano-shima granodiorite. The results of melting experiment inferred that the hornblende gabbro was originally formed under the lower crust at pressure conditions more than 800 MPa during 120 to 103 Ma. The partial melting took place in the gabbro, then gave rise to the Shikano-shima granodiorite magma at the age of 104 Ma. The lower crust, 25–35 km in depth, beneath the north Kyushu batholiths is underlain by the P-wave structure with 6.5–7.0 km/s that corresponds to a hornblende-bearing gabbro. The region covering the north Kyushu batholiths locally shows positive Bouguer anomaly spots where some gabbro blocks are exposed on the surface. The areas distribution of the Shikano-shima granodiorite and the gabbro block are identical with the place of positive Bouguer anomaly. Therefore, the granodiorite magma probably ascended as a diapir from the lower crust accompanied by the source gabbro. Combined with the petrological study and the seismic structure, the upper crust was formed by the flare-up of intermediate to felsic magmas through the pathway as the dikes. In addition to this magma plumbing system, the granodiorite magmas originated from mafic lower crust would be ascended as diapirs to form the middle crust.
The host rocks of the western part of the north Kyushu batholiths are composed of the pre-Jurassic accretionary complexes with their high-P/T metamorphic facies and the Cretaceous low-P/T metamorphic rocks reaching up to granulite facies conditions. On the other hand, the Cretaceous shallow marine deposits are exposed as host rocks on the eastern part of batholiths. The magma plumbing system can be recognized in the small stocks on the eastern part of batholiths. The Shaku-dake diorite body, a member of the north Kyushu batholiths, intruding the shallow marine deposits possesses layered structures on the wall sides. The intrusive age of the diorite has been thought to be c. 106 Ma. The dioritic dikes and sills intruded the diorite body where the dikes and sills would provide the magmatic pulses to the Shaku-dake diorite body. The diorite magma was geochemically differentiated from high-Mg diorite to granodiorite through the process of assimilation and fractional crystallization. The Shikano-shima granodiorite occupied in the western part of the batholiths involves the hornblende gabbro as blocks that bears leucocratic patches or lenses geochemically resembling the Shikano-shima granodiorite. The results of melting experiment inferred that the hornblende gabbro was originally formed under the lower crust at pressure conditions more than 800 MPa during 120 to 103 Ma. The partial melting took place in the gabbro, then gave rise to the Shikano-shima granodiorite magma at the age of 104 Ma. The lower crust, 25–35 km in depth, beneath the north Kyushu batholiths is underlain by the P-wave structure with 6.5–7.0 km/s that corresponds to a hornblende-bearing gabbro. The region covering the north Kyushu batholiths locally shows positive Bouguer anomaly spots where some gabbro blocks are exposed on the surface. The areas distribution of the Shikano-shima granodiorite and the gabbro block are identical with the place of positive Bouguer anomaly. Therefore, the granodiorite magma probably ascended as a diapir from the lower crust accompanied by the source gabbro. Combined with the petrological study and the seismic structure, the upper crust was formed by the flare-up of intermediate to felsic magmas through the pathway as the dikes. In addition to this magma plumbing system, the granodiorite magmas originated from mafic lower crust would be ascended as diapirs to form the middle crust.