Mesozoic granitoids of northeast Asia
Most of the plutonio masses in northeastern Asia were emplaced in the late Mesozoic. They exhibit a wide range of compositions and, on the basis of ranges, the batholiths form two groups, one acidic with a narrow range from granodiorite to granite, and the other with a wide range extending from gabbro through diorite and tonalite to granodiorite and granite. The Verkhoyansk-Chukchi region, with its Preriphean crust, is characterized by the acidic series, as are miogeosynclinal regions in general. The gabbro-granite series is typical of eugeosynclines such as the northeast-trending Okhotsk-Chukchi volcanogenic belt.
The acidic series shows no close relationship with volcanic processes and is suggestive of low-temperature melts. The gabbro-granite series is closely related in space and time with volcanism (mainly andesitic) and is thought to be co-magmatic with the volcanic rocks. They imply derivation from relatively dry, high temperature magmas. Although the gabbro-granite series has 87Sr/86Sr ratios below 0.707, mantle sources are considered unlikely; they are accounted for by proposing a lower crust, granulite-eclogite source.
The Okhotsk-Chukchi volcanogenic belt is divided into an inner (continent side) zone and an outer (ocean side) zone. Plutonic rocks in the inner zone are potassic, whereas those in the outer zone are sodic. The boundary between the zones is thought also to mark the junction between sialic continental crust and crust transitional to the ocean basin.