Repeated magmatism at 34 Ma and 23-20 Ma producing high magnesian adakitic andesites and transitional basalts on southern Okushiri Island, NE Japan arc
Repeated magmatism at 34 Ma and 23-20 Ma producing high magnesian adakitic andesites and transitional basalts on southern Okushiri Island, NE Japan arc
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DOI：
10.1016/j.lithos.2014.06.008
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发表时间：
2014-09
期刊：
影响因子：
3.5
通讯作者：
M. Sato;K. Shuto;Rikako Nohara-Imanaka;E. Takazawa;Y. Osanai;N. Nakano
中科院分区：
文献类型：
--
作者：
M. Sato;K. Shuto;Rikako Nohara-Imanaka;E. Takazawa;Y. Osanai;N. Nakano
现今东北日本弧后边缘的奥尻岛南部是罕见的会聚板块边界之一，在这里，相似的岩浆类型（高镁埃达克质安山岩（HMAA）和高TiO 2玄武岩（HTB））同时喷发了不止一次。渐新世HMAA可分为两种类型：HMAA-I以高Sr/Y、低Y为特征，HMAA-II以相对低Sr/Y、高Y为特征。HMAA-I在MgO（8.5重量%）方面是原始的，Mg#（67），Ni（232 ppm）和Cr（613 ppm）的含量，和最富镁橄榄石斑晶图在地幔橄榄石阵列中的Fo和NiO。类似的Cr与Ni的关系的类型I和II HMAA表明一些相互作用的板状埃达克质熔体与地幔橄榄岩，而Ni含量高于大多数玻安岩的部分熔融地幔橄榄岩在给定的Cr含量。I型和II型HMAA的Sr和Nd同位素组成比N-MORB更富集。这些岩石的岩相学和地球化学，结合已发表的高镁安山岩（HMA）的成因结果表明，I型和II型HMAA可以通过板（N-MORB和沉积物）衍生埃达克质熔体与地幔橄榄岩的相互作用产生。HMAA和HTB的共熔作用归因于以下模型。冷却的、含水量较少的埃达克岩岩浆（球形底辟）会从俯冲板块（太平洋板块）上升，并由于与上覆的含水橄榄岩相互作用而变得含水量更大。这种水化埃达克质底辟进一步上升，并在进入上覆地幔楔时被加热。随后，在上升的埃达克质底辟和周围的地幔橄榄岩的温度和H2O梯度将被建立。HTB岩浆从周围的地幔橄榄岩区（高温和低H2O含量）在60公里或更深的分离，而埃达克质底辟（低温和高H2O含量）继续上升，其化学成分的修改，由于与周围的地幔橄榄岩的相互作用。I型HMAA，然后分离在约50公里。最有吸引力的构造岩浆模型，占埃达克质岩浆的生产在两个不同的时期在同一个冷俯冲带地区涉及亏损热软流圈上涌到下的东北日本弧后边缘的陆下岩石圈，与弧后裂谷发生在日本海开口的开始。异常的高温条件下建立的地幔楔由于亏损热软流圈上涌造成部分熔融的一个有限的一部分冷洋壳俯冲下的东北日本弧，导致埃达克质岩浆的产生。
The southern part of Okushiri Island in the present-day back-arc margin of the NE Japan arc is one of the rare convergent plate boundaries where similar magma types (high-magnesian adakitic andesite (HMAA) and high-TiO2basalt (HTB)) have been erupted concurrently at more than one time. Oligocene HMAA can be divided into two types: HMAA-I is characterized by high Sr/Y and low Y, and HMAA-II by relatively low Sr/Y and high Y. HMAA-I is primitive in terms of MgO (8.5 wt.%), Mg# (67), Ni (232 ppm) and Cr (613 ppm) contents, and the most Mg-rich olivine phenocrysts plot within the mantle olivine array in terms of Fo and NiO. The similar Cr versus Ni relations of types I and II HMAA indicate some interaction of slab-derived adakitic melts with mantle peridotite, whereas Ni contents are higher than those of most boninites derived by partial melting of mantle peridotite at a given Cr content. Types I and II HMAA have more enriched Sr and Nd isotopic compositions than N-MORB. The petrography and geochemistry of these rocks, combined with published results on the genesis of high-magnesian andesite (HMA) indicate that types I and II HMAA could be produced by interaction of slab (N-MORB and sediment)-derived adakitic melts with mantle peridotite. The comagmatism of HMAA and HTB is ascribed to the following model. A cool, less hydrous, adakite magma (spherical diapir) would rise from the subducting slab (Pacific Plate) and become more hydrous as a result of its interaction with overlying hydrous peridotite. This hydrated adakitic diapir further ascends and is heated on entering the overlying mantle wedge. Subsequently, the temperature and H2O gradients in the ascending adakitic diapir and surrounding mantle peridotite would have been established. The HTB magma segregated from the surrounding mantle peridotite region (high temperature and low H2O content) at a depth of 60 km or more, whereas the adakitic diapir (low temperature and high H2O content) continued to rise, with its chemical composition modified due to interaction with the surrounding mantle peridotite. Type I HMAA then segregated at about 50 km.The most attractive tectono-magmatic model to account for production of adakitic magma at two different periods in the same cool subduction zone region involves upwelling of depleted hot asthenosphere into the subcontinental lithosphere beneath the back-arc margin of the NE Japan arc, coincident with back-arc rifting which took place at the initiation of the Japan Sea opening. The unusually high temperature conditions established in the mantle wedge due to upwelling of depleted hot asthenosphere caused partial melting of a limited part of the cool oceanic crust subducting beneath the NE Japan arc, resulting in the generation of adakitic magma.