Granulite-facies metacumulates of the yakuno ophiolite, Japan: Evidence for unusually thick oceanic crust
Granulite-facies metacumulates of the yakuno ophiolite, Japan: Evidence for unusually thick oceanic crust
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日本 yakuno 蛇绿岩的麻粒岩相聚集体：异常厚的洋壳的证据
DOI：
10.1093/petrology/26.1.1
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发表时间：
1985
影响因子：
3.9
通讯作者：
A. Ishiwatari
中科院分区：
文献类型：
--
作者：
A. Ishiwatari
矢久野蛇绿岩中的火山岩、堆积岩和残留橄榄岩在侵位进入二叠纪沉积物之前就已完全变质。变质程度在蛇绿岩序列中向下逐渐增加，在堆晶岩中部达到麻粒岩相。相系列包括绿帘石-绿帘石-角闪岩相和绿帘石-角闪岩相，表明其地温梯度低于洋底变质作用的地温梯度。麻粒岩相变辉长岩和橄榄岩非常类似于来自地壳深部的碱性玄武岩中的捕虏体。变质作用涉及渗透性固体流动，形成粗粒晶体的优先取向和随后的局部剪切，形成细粒变裂糜棱岩。在粗粒晶体的核心中记录了750至920 °C的温度，而在变裂糜棱岩中记录了较低的温度。斜长石-橄榄石反应和不存在火成石榴石确定了变质作用的压力范围在5 ~ 10 kb之间。这些压力要求上覆堆晶火山岩序列的原始厚度为15-30公里，比正常洋壳厚得多。变火山岩和变堆积岩的岩性和化学成分表明，矢久野蛇绿岩可能是在一个小洋盆下形成的异常厚的洋壳的碎片。
The volcanics, cumulates, and residual peridotites of the Yakuno ophiolite were thoroughly metamorphosed before its emplacement into Permian sediments. Metamorphic grade increases downward through the ophiolite succession, and reaches granulite facies in the middle of the cumulates. The facies series, including prehnite-pumpellyite and epidote amphibolite facies, suggests a lower geothermal gradient than that of ocean-floor metamorphism. The granulite-facies metagabbros and peridotites closely resemble xenoliths in alkali basalts derived from deeper parts of the crust. Metamorphism involved penetrative solid flow to form preferred orientation of coarse-grained crystals and subsequent local shear to form fine-grained blastomylonites. Temperatures ranging from 750 to 920 °C are recorded in the cores of coarse-grained crystals, and lower temperatures in the blastomylonites. Plagioclase-olivine reaction and the absence of pyropic garnet define the pressure range of the metamorphism between 5 and 10 kb. These pressures require an original thickness of 15-30 km for the overlying cumulate-volcanic sequence, which is much thicker than normal oceanic crust. The lithology and chemistry of the metavolcanics and metacumulates suggests that the Yakuno ophiolite may be a fragment of unusually thick oceanic crust formed beneath a small oceanic basin.