OPS混杂岩概念及其填图方法:以日本Mino和Chichibu侏罗纪增生杂岩为例

Concept of OPS mélange and its field mapping technique for ancient accretionary complexes:Using some examples mainly from Jurassic accretionary complexes of the Mino and Chichibu belts in Japan

OPS混杂岩是大洋板片地层的岩石经历破碎和混杂之后的产物,是造山带中一种标志性的混杂岩类型。日本列岛保留了自晚古生代以来的俯冲—增生记录,包括石炭纪、二叠纪、侏罗纪、白垩纪—古近纪和新近纪—第四纪未变质或轻微变质的增生杂岩。这些增生杂岩产出有典型的OPS混杂岩,包括砂岩、硅质泥岩、硅质岩、灰岩、玄武岩等岩块,以及泥岩基质。尽管OPS混杂岩中的岩石可能有不同年龄,但所有增生杂岩中OPS混杂岩的岩石组合总是相似的。OPS混杂岩可形成于沉积、构造、底辟等多种过程中,多产在汇聚型板块边缘的、经历多期次变形的增生杂岩中。为了理解碰撞造山带中强烈变形或变质的混杂岩,有必要对日本地区出露的弱变形和未变质的OPS混杂岩成因进行研究。在野外调研OPS混杂岩时,有一些关键点值得注意:首先,在OPS混杂岩野外地质填图时,需要想象不同的岩石在OPS地层层序中所处的位置。尽管OPS岩石被肢解为碎片,仍需观测这些岩块或大型岩片中的原始沉积顶底,因为多数OPS混杂带经历了多期次的构造变形,在不同期次的构造过程中都有可能形成伴生有细粒碎块的大型岩片和岩块。其次,沉积岩的年龄和火山岩的地球化学分析对理解OPS混杂带原岩的性质非常有帮助。最后,野外工作中,需要时刻考虑风化作用造成一些基质被剥蚀掉、无法在露头上得以保存(在建立模型时需将它们补齐),此外要紧跟新的研究成果及认识,对露头进行反复观察。

OPS mélange is a mélange which is the product of the fragmentation and mixing of Ocean Plate Stratigraphy(OPS).It is a very typical mélange in subduction‑accretion orogenic belts.The Japanese Islands yield a record of subduction‑accretion tectonics since the late Palaeozoic.Non‑or weakly metamorphosed accretionary complexes in Japan are of Carboniferous,Permian,Jurassic,Cretaceous‑Palaeogene,and Neogene‑Quaternary ages.These accretionary complexes include typical OPS mélanges.The OPS mélanges contain clasts of sandstone,siliceous mudstone,chert,limestone and basalt within a mudstone matrix.Although the age of each lithology is different,the lithology of every OPS mélange is the same in all of the accretionary complexes.OPS mélange is formed by various processes such as sedimentary,tectonic,and diapiric processes.However,most are products of multiple deformational processes during the development of accretionary wedges along a convergent margin.In order to investigate the highly deformed or highly metamorphosed mélanges in the collision orogenic belts,it is necessary to know the nature of OPS mélange in the weakly deformed and unmetamorphosed accretionary complexes of Japan.There are several key points for investigating OPS mélange in the field.For geological mapping of OPS mélange,we need to imagine the stratigraphic position of the rocks on the outcrop in the OPS stratigraphy.Even though the OPS were disrupted into fragments,we need to check the stratigraphic top within the blocks and slabs in the mélange.As most OPS mélanges were formed in multi‑deformational processes,large slabs and blocks may be formed in different tectonic processes with small clasts.Age determination of sedimentary rocks and chemical analysis of igneous rocks are helpful in understanding the nature of the protolith of the OPS mélange.In the field,we need to consider the existence of an unobservable matrix susceptible to weathering,and to observe the outcrop repeatedly with new knowledge from new research works in the literature.