Ridge collision and in situ greenstones in accretionary complexes: An example from the Late Cretaceous Ryukyu Islands and southwest Japan margin
Ridge collision and in situ greenstones in accretionary complexes: An example from the Late Cretaceous Ryukyu Islands and southwest Japan margin
复制标题
增生杂岩中的山脊碰撞和原位绿岩：以晚白垩世琉球群岛和日本西南边缘为例
DOI：
10.1111/j.1440-1738.1994.tb00098.x
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发表时间：
1994
期刊：
影响因子：
1.5
通讯作者：
K. Kawabata
中科院分区：
文献类型：
--
作者：
K. Kiminami;S. Miyashita;K. Kawabata
含绿岩的混杂岩单元在琉球群岛和日本西南部的新世-中新世四万十超群中很常见。增生杂岩中的绿岩大多起源于大洋扩张脊和海山，形成于远离会聚边缘的地方。然而，超群中的一些混杂岩样单元含有被挤压并侵入到未固结细粒陆源碎屑沉积物中的绿岩。推测原地绿岩的喷发是海沟区岩浆活动的结果。地球化学特征表明，绿岩原岩类似于扩张脊产生的镁铁质熔岩。从琉球群岛的奄美大岛（Cenomanian-Turonian）向东到日本中部（晚Maestrichtian-最早古新世）1300 km，含原地绿岩的地层的化石年龄逐渐变年轻，这意味着海沟地区的一个火成岩活动场所沿着琉球群岛和日本西南缘向东迁移。西北太平洋的板块重建表明，在晚白垩世至早第三纪日本附近存在库拉-太平洋海脊。在这种情况下，建议形成的绿岩响应库拉太平洋脊弧前碰撞。
最好的识别古脊弧前碰撞的存在洋中脊玄武岩（MORB）挤压的沉积物推断已积累在海沟区。造山带中与这些MORB有关的地层的穿时产状对于记录洋脊碰撞是有用的。
Abstract Melange units containing greenstones are common throughout the Cretaceous-Miocene Shimanto Supergroup in the Ryukyu Is and southwest Japan. Most greenstones in the accretionary complex originated in oceanic spreading ridges and seamounts, and they formed far from the convergent margin. Some melange-like units in the supergroup, however, contain greenstones that were extruded upon and intruded into unconsolidated fine-grained terrigenous clastic sediments. It is inferred that eruption of the in situ greenstones resulted from igneous activity in the trench area. Geochemical signatures indicate that the greenstone protoliths were similar to mafic lavas generated at spreading ridges. Fossil ages of the strata containing in situ greenstones become younger over a distance of 1300 km eastward from Amami-Oshima (Cenomanian-Turonian) in the Ryukyu Is to central Japan (Late Maestrichtian-earliest Paleocene), implying that a site of igneous activity in the trench area migrated eastward along the Ryukyu Is and southwest Japan margin. Plate reconstructions of the northwest Pacific Ocean suggest the presence of the Kula-Pacific ridge near Late Cretaceous to early Paleogene Japan. In this context, it is suggested that the greenstones formed in response to Kula-Pacific ridge-forearc collision.
Ancient ridge-forearc collisions are best recognized by the presence of mid-ocean ridge basalt (MORB) extruded on sediments inferred to have accumulated in the trench area. Diachronous occurrences of the strata associated with these MORB in an orogenic belt are useful for documenting the ridge collision through time.