南部北上帯の先シルル紀基盤

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The pre-Cretaceous "Hikami granite" in the Southern Kitakami Belt has been repeatedly studied in connection with a problem of pre-Silurian basement in the belt. Also in the same belt there have been known several other granitic masses of much smaller scale compared to the large Hikami mass. Here they are collectively termed as the Hikami granitic rocks which generally show cataclastic feature and range in modal composition from melanocratic quartz diorite to granite. In many instances they are accompanied with Siluro-Devonian fomations. Our research was directed to the above stated small masses of the Hikami granitic rocks in order to clarify field relationship between these masses and the Siluro-Devonian formations (Fig. 1). Among them basal arkose sandstonte of the Silurian Kawauchi Formation is observed to overlie granitic rock of the Yokamachi mass with a blinded unconformity. Basal pelitic rock of the Silurian Okuhinotsuchi Fomation, having the character of residual clay in chemical composition, unconformably overlies the Okuhinotsuchi mass. While, a peculiar tectonic contact associated with cataclasis is observed between granitic rocks of the Komatsu-toge mass and Siluro-Devonian rocks. Thus these small masses of the Hikami granitic rocks may be concluded to be pre-Silurian in age. Furthermore, clasts of weakly metamorphosed sedimentary rocks are found in coarse-grained elastic rocks of Siluro-Devonian formations in this belt. Source rocks of these clasts, together with the Hikami granitic rocks, suggest the characteristic features of pre-Silurian basement (Lower Paleozoic and/or Upper Proterozoic?) in the Southern Kitakami Belt.
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南部北上帯の先シルル紀基盤

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