Chemical composition and age of magmatic rocks from the underwater Hokkaido Rise

Geological and geophysical investigations carried out within the Hokkaido Rise showed that intrusives composing outcrops of the crystalline basement on the ocean floor form a continuous series from monzonites and diorite-monzonites to granites with prevalence of granodiorites with stable mineralogical association: biotite - hornblende - K-feldspar. Acidic volcanic rocks are characterized by a similar mineralogical association with almost complete absence of plagioclase-pyroxene species. It seems that the Hokkaido Rise, as well as the marginal oceanic Zenkevich swell as a whole are not primary oceanic structural formations and have undergone a complex and long history of geological development with intense orogenic movements that occurred in Middle Cretaceous and preceded subalkaline basalt outpouring during postorogenic subsidence of the Earth crust.

针对北海道海隆（Hokkaido Rise）开展的地质与地球物理调查显示，构成洋底结晶基底（crystalline basement）露头的侵入岩形成了连续的岩性序列，从二长岩、闪长二长岩直至花岗岩，其中以花岗闪长岩占主导，且具有稳定的矿物组合：黑云母-角闪石-钾长石。酸性火山岩具有相似的矿物组合，几乎完全不含斜长石-辉石类矿物。据此推测，北海道海隆连同整体的边缘大洋曾克维奇海隆（Zenkevich Swell）均非原生大洋构造单元，二者均经历了复杂且漫长的地质演化历程：白垩纪中期发生强烈造山运动，随后地壳进入造山后沉降阶段，此期间喷发出亚碱性玄武岩。