Subduction of mantle wedge peridotites: Evidence from the Higashi-akaishi ultramafic body in the Sanbagawa metamorphic belt
Subduction of mantle wedge peridotites: Evidence from the Higashi-akaishi ultramafic body in the Sanbagawa metamorphic belt
复制标题
DOI：
10.1111/j.1440-1738.2009.00696.x
复制
发表时间：
2010-03-01
期刊：
影响因子：
1.5
通讯作者：
Mizukami, Tomoyuki
中科院分区：
文献类型：
--
作者：
Hattori, Keiko;Wallis, Simon;Mizukami, Tomoyuki
东赤石含石榴子石超镁铁质岩体位于日本西南部的三坝川变质带，是一个罕见的大洋型超高压变质作用的实例。2公里x5公里的身体主要是由无水纯橄榄岩与体积较小的透镜体的单斜辉石丰富的岩石。纯橄榄岩样品在整体岩石中含有高Ir型铂族元素（PGE）和Cr，在橄榄石中含有高Mg和Ni，在尖晶石中含有高Cr。另一方面，单斜辉石丰富的岩石含有低浓度的Ir型PGE和Cr，高浓度的流体活动元素的块体岩石，和低Ni和Mg的橄榄石。单斜辉石为透辉石，Al2O3含量低。尖晶石、橄榄石和单斜辉石的岩石成分和矿物化学特征表明，以橄榄石为主的岩石是地幔橄榄岩在经历了高程度的熔融后的残留，而富含单斜辉石的岩石则是与俯冲作用有关的熔融产物的堆积。因此，东赤石超镁铁质岩体起源于地幔楔内部，很可能是弧前上地幔。然后，它被地幔流纳入三坝川俯冲通道，并经历了高压变质作用，深度超过100公里。地幔楔中如此强烈的活跃流动可能是由于板块和上覆地幔之间的界面沿着缺乏蛇纹岩，因为它太热而不适合蛇纹岩。这些异常的高温和强烈的地幔流动可能反映了俯冲作用发展的早期阶段，并可能在白垩纪三坝川带开始形成时，由亚洲大陆东北部软流圈地幔的大规模上涌和随后的向东流动所维持。
The Higashi-akaishi garnet-bearing ultramafic body in the Sanbagawa metamorphic belt, Southwest Japan, represents a rare example of oceanic-type ultrahigh-pressure metamorphism. The body of 2 km x 5 km is composed mostly of anhydrous dunite with volumetrically minor lenses of clinopyroxene-rich rocks. Dunite samples contain high Ir-type platinum group elements (PGE) and Cr in bulk rocks, high Mg and Ni in olivine, and high Cr in spinel. On the other hand, clinopyroxene-rich rocks contain low concentrations of Ir-type PGE and Cr, high concentrations of fluid-mobile elements in bulk rocks, and low Ni and Mg in olivine. Clinopyroxene is diopsidic with low Al2O3. The compositions of bulk rocks and mineral chemistry of spinel, olivine, and clinopyroxene suggest that the olivine-dominated rocks are residual mantle peridotites after high degrees of influx partial melting, and that the clinopyroxene-rich rocks are cumulates of subduction-related melts. Thus, the Higashi-akaishi ultramafic body originated from the interior of the mantle wedge, most likely the forearc upper mantle. It was then incorporated into the Sanbagawa subduction channel by a mantle flow, and underwent high pressure metamorphism to a depth greater than 100 km. Such a strong active flow in the mantle wedge is likely facilitated by the lack of serpentinites along the interface between the slab and the overlying mantle, as it was too hot for serpentine. These unusually hot conditions and strong active mantle flow may reflect conditions in the earliest stage of development of subduction, and may have been maintained by massive upwelling and subsequent eastward flow of asthenospheric mantle in the northeastern Asian continent in Cretaceous time when the Sanbagawa belt began to form.