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[SGL23-03] The Middle Miocene tectonics in the north-central Hokkaido inferred from the deposition of serpentinite-bearing conglomerate
Keywords:detrital serpentinite, the Middle Miocene
Exposure of ultramafic rocks, which constitute the mantle, suggests the existence of large-scale tectonics and is important for considering the geotectonic history in the exposed areas. Since serpentinites formed by regional metamorphism or alteration of ultramafic rocks are generally considered to be physically and chemically fragile and not transported over long distances, the preservation of clastic serpentinite particles requires that the deposition site be near the serpentinite body and that the clastic material be buried immediately after deposition [1]. The presence of clastic serpentinite particles in sedimentary rocks may provide important evidence for the timing of exposure of the serpentinite body. This study examines the Middle Miocene tectonics of north-central Hokkaido based on the depositional environment and petrological characteristics of clastics in the coarse-grained clastic rocks containing large amounts of serpentinite grains in the Middle Miocene Sanjissenzawa fossiliferous sandstone member in the Chikubetsu Formation [2] in the Horokanai area, north-central Hokkaido, Japan.
The coarse-grained clastic rocks are composed of subaquaeous debris flow deposits, and channels and lobes are considered as possible depositional sites. The high degree of roundness of the gravels in the conglomerate indicates that the gravels were polished at the primary accumulation site and then transported and deposited. The gravels are mostly serpentinite and diorite, and the source of clastics is monogenic. The presence of serpulids and bryozoan fossils on the gravel surface indicates that the primary accumulation site of the gravel was a gravelly reef.
Most of the serpentinite gravels are very strongly serpentinized, but a few remnant minerals remain. The mesh-structure of serpentine with Opx and Opx-altering bastite, suggesting that the serpentinite source rock is dunite and/or harzburgite. However, some of the rocks contain euhedral chromite showing characteristics of saturated mantle peridotite. XRD of serpentinite gravels indicate that the serpentine in the serpentinite is mainly lizardite, but some serpentinites contain antigorite and brucite, which form at high temperatures. Since the temperature conditions of forming of antigorite and brucite are similar to those of the metamorphism of the green schist facies, it is possible that the serpentinization of the serpentinite is contemporaneous with the metamorphism of the diorite.
Subhedral amphibole as a reaction margin of Cpx and chlorite in the diorite suggest metamorphism of green schist facies. The diorite is penetrated by serpentinite veins, suggesting that the it behaved as a tectonic block in serpentinite.
The combination of serpentinite from depleted mantle peridotite and diorite metamorphosed into green schist facies is very similar to the lower part of the Horokanai ophiolite. On the other hand, saturated mantle peridotite material, which is absent in the present-day Horokanai ophiolite, is found in the sedimentary rocks, suggesting that the source of the clastic material was an ultramafic body with slightly different characteristics than the present-day Horokanai ophiolite. [1] Arai, 1992. Memoirs of the geological society of Japan. [2] Hashimoto et al., 1965. Hokkaido Development Agency.
The coarse-grained clastic rocks are composed of subaquaeous debris flow deposits, and channels and lobes are considered as possible depositional sites. The high degree of roundness of the gravels in the conglomerate indicates that the gravels were polished at the primary accumulation site and then transported and deposited. The gravels are mostly serpentinite and diorite, and the source of clastics is monogenic. The presence of serpulids and bryozoan fossils on the gravel surface indicates that the primary accumulation site of the gravel was a gravelly reef.
Most of the serpentinite gravels are very strongly serpentinized, but a few remnant minerals remain. The mesh-structure of serpentine with Opx and Opx-altering bastite, suggesting that the serpentinite source rock is dunite and/or harzburgite. However, some of the rocks contain euhedral chromite showing characteristics of saturated mantle peridotite. XRD of serpentinite gravels indicate that the serpentine in the serpentinite is mainly lizardite, but some serpentinites contain antigorite and brucite, which form at high temperatures. Since the temperature conditions of forming of antigorite and brucite are similar to those of the metamorphism of the green schist facies, it is possible that the serpentinization of the serpentinite is contemporaneous with the metamorphism of the diorite.
Subhedral amphibole as a reaction margin of Cpx and chlorite in the diorite suggest metamorphism of green schist facies. The diorite is penetrated by serpentinite veins, suggesting that the it behaved as a tectonic block in serpentinite.
The combination of serpentinite from depleted mantle peridotite and diorite metamorphosed into green schist facies is very similar to the lower part of the Horokanai ophiolite. On the other hand, saturated mantle peridotite material, which is absent in the present-day Horokanai ophiolite, is found in the sedimentary rocks, suggesting that the source of the clastic material was an ultramafic body with slightly different characteristics than the present-day Horokanai ophiolite. [1] Arai, 1992. Memoirs of the geological society of Japan. [2] Hashimoto et al., 1965. Hokkaido Development Agency.