Early Cretaceous igneous rocks in the Kitakami Mountains consist of volcanic rocks, dike rocks, and plutonic rocks, from older to younger. Plutonic rocks are composed mainly of adakitic granites in central part of zoned plutonic bodies surrounded by adakitic to non-adakitic granites in marginal part. These adakitic plutons is divided into E and W zones based on the ages and geochemistry. Zircon U-Pb ages were determined with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for 22 samples from 13 rock bodies including the Early Cretaceous adakitic granites in the Kitakami Mountains. Zircons from the adakitic granites of E zone give older ages (127-117 Ma) compared with those of W zone (119-113 Ma). Zircon ages of the calc-alkaline to shoshonitic rocks and dike rocks range from 128-124 Ma, which are similar to the oldest rocks of the E zone (127-125 Ma). Zircon ages become younger from the northern Hashikami pluton and marginal facies of the Tanohata pluton (127-125 Ma) to southern Takase granites (118-117 Ma), in the E zone adakitic granites. Petrochemical differences between the E zone and W zone rocks indicate that the adakitic melt of E zone rocks are considered to be derived from vapor-absent melting condition, while those of W zone rocks are from higher pressure and vapor-present condition. Taking all these data into consideration, the Early Cretaceous magmatisms in Kitakami can be explained by the differential subduction model of the Farallon-Izanagi plates or slab rollback model accompanied with asthenospheric upwelling.
We report new Sr-Nd-Hf isotope data from 48 Cretaceous-Palaeogene granitoids from the Northeastern (NE) Japan. The spatial variation of Sr-Nd isotopic compositions in those granitoids is known to be characterised by increasing isotopic source enrichment from the northeast to the southwest. The NE Japan has therefore been divided into five geochemical zones with distinct initial isotopic compositions: the Kitakami, North, Sado, South, and Transitional Zones. There are two main models for the origin of these spatial variations: (1) the granitoids were derived from the lithospheric mantle or lower crust, therefore the spatial variation in their Sr-Nd initial isotopic composition reflects the spatial heterogeneity of the source materials; and (2) differences supply rate of subducted sediments to the source mantle beneath the subduction zone. New Sr-Nd isotope data are consistent with previously reported values, and new Hf isotopic data indicate a coupling with Nd isotopic compositions. We suggest a new diagram for discriminating isotopic zones based on the Nd-Hf isotopic characteristics of the granitoids. The Nd-Hf isotopic discrimination diagram divides the region into the Kitakami, North, South, and Transitional zones, in line with previous works based on the Sr-Nd isotopic characteristics, but the Sado zone could not be identified. The mixing model of subducted sediments and source mantle cannot explain the variation of Hf isotopic composition of the granitoids found here, and therefore the results lend support to the model of an isotopically heterogeneous mantle-lower crust source for the Cretaceous-Paleogene granitoids; these heterogeneities result directly in the spatial isotopic variation of the granitoids.
Completely serpentinized ultramafic rocks occur in Cretaceous sedimentary rocks in the North Shimanto belt, southwest Japan. The serpentinites are divided into bastite serpentinite and massive serpentinite. Chromian spinels in the bastite serpentinites are characterized by Cr#[= Cr/(Cr + Al)] and XMg[= Mg/(Mg + Fe2+)] corresponding to those of residual harzburgites from forearc regions. On the other hand, chromian spinels in the massive serpentinite show high YFe[= Fe3+/(Cr + Al + Fe3+)] and low TiO2 content, indicating that it was a cumulate formed by island arc magmatism. Petrological characteristics of the serpentinites from the North Shimanto belt are similar to those of the Mineoka-Kobotoke belts of the Circum-Izu area, the South Shimanto belt. The serpentinites from the North Shimanto belt were possibly fragments of the lower crust to upper mantle materials beneath an island arc, which intruded into sedimentary rocks after the accretion and emplacement of the North Shimanto belt.
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April 11, 2015
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