Early Cretaceous Sr‐rich silicic magmatism by slab melting in the Kitakami Mountains, northeast Japan
Early Cretaceous Sr‐rich silicic magmatism by slab melting in the Kitakami Mountains, northeast Japan
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日本东北部北上山脉的早白垩世富锶硅质岩浆作用，由板片熔化引起
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发表时间：
1994
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影响因子：
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通讯作者：
S. Kanisawa
中科院分区：
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作者：
Nobutaka Tsuchiya;S. Kanisawa
日本东北部北上山脉早白垩世深成岩侵入侏罗纪增生杂岩和微陆块，与东亚大陆东缘沿着向西俯冲有关。这些深成岩可分为A型和B型。前者可进一步划分为高Sr系列、低Sr系列和中Sr系列。高锶系列岩石出现在北北上带的三个分带岩体的中心部分;命名为桥上岩体、田端岩体和宫古岩体。与低Sr系列岩石相比，高Sr系列岩石具有明显的化学特征。特别地，高Sr系列的特征在于SiO2大于61wt%，较高的Na 2 O和较低的K2 O浓度，以及略高的Al 2 O3和较低的总FeO和MgO浓度。在微量元素化学方面，高Sr系列岩石富Sr、Ga，贫Rb、Pb、Y。这些特征类似于那些高Al 2 O3奥长岩，英云闪长岩，英安岩（高铝TTD）。质量平衡计算表明，高Sr系列岩浆可能是由洋中脊玄武岩（MORB）、5%蚀变MORB和5%沉积物部分熔融，留下石榴子石、单斜辉石、石英、金红石和磷灰石等残岩形成的。根据拉斑玄武岩的相关系和俯冲板片的地温梯度，认为板片熔融发生在2.0 ~ 2.2GPa（约70 ~ 80 km）的深度，俯冲洋壳的年龄可能小于10 ~ 25 Ma。俯冲板片岩浆与地幔楔岩浆共存是北上山早白垩世岩浆活动的一个特别重要的特征。这表明白垩纪前弧下的地幔比正常的前弧要温暖。Farallon-Izanagi海岭的俯冲可能导致了温暖的地幔。
Early Cretaceous plutonic rocks in the Kitakami Mountains, northeast Japan, related to the westward subduction along the eastern margin of the East Asian continent, intruded the Jurassic accretionary complex and a microcontinental block. These plutonic rocks are classified into types A and B. The former is further subdivided into high-Sr, low-Sr, and intermediate series. High-Sr series rocks occur as the central parts of three zoned plutons in the North Kitakami belt; named the Hashigami, Tanohata, and Miyako plutons. The high-Sr series rocks show distinct chemical characteristics compared with those of the low-Sr series. In particular, the high-Sr series is characterized by SiO2 greater than 61 wt %, higher Na2O and lower K2O concentrations, and slightly higher Al2O3 and lower total FeO and MgO concentrations. In trace element chemistry, the high-Sr series rocks are richer in Sr and Ga, and poorer in Rb, Pb, and Y. These characteristics resemble those of the high-Al2O3 trondhjemite, tonalite, and dacite (high-Al TTD). Mass balance calculation indicates that the high-Sr series magma can be derived by partial melting of hypothetical oceanic crust (mid-oceanic ridge basalt (MORB), plus 5% altered MORB, plus 5% sediments) leaving garnet, clinopyroxene, quartz, rutile, and apatite as restite. On the basis of the phase relations for tholeiitic basalt and the estimated geothermal gradient of the subducted slab, slab melting is considered to have occurred at a depth of 2.0 to 2.2 GPa (about 70 to 80 km), and the age of subducted oceanic crust may be younger than 10 to 25 Ma. The coexistence of the magma derived from the subducted slab with that derived from the mantle wedge is a particularly important characteristic of the Early Cretaceous magmatism in the Kitakami Mountains. This suggests that the mantle beneath the Cretaceous forearc was warmer than for normal forearcs. Subduction of the Farallon-Izanagi ridge may have caused the warm mantle.