昌宁-孟连结合带弄巴地区泥盆系、石炭系的时代及沉积环境:放射虫、碎屑锆石U-Pb年龄和Hf同位素约束

Age and Tectonic Setting of Devonian and Carboniferous Fragments in Nongba Area of Changning-Menglian Suture Zone:Radiolarian,Detrital Zircon U-Pb Age and Hf Isotope Constraints

造山带中远洋深水沉积物是恢复古大洋的重要依据之一,昌宁-孟连古特提斯结合带存在大量海相沉积物,但是否存在大洋盆地相的远洋沉积还不清楚.对弄巴地区被认为最可能是洋盆相沉积的石炭系岩片和海相泥盆系岩片进行了岩石学、放射虫时代、碎屑锆石U-Pb年龄和Hf同位素研究.石炭系岩片放射虫硅质岩中鉴定出放射虫6属8种,时代为早石炭世早-中期.LA-ICP-MS锆石U-Pb定年结果显示,泥盆系岩片岩屑石英杂砂岩碎屑锆石年龄范围为387~3266 Ma,最年轻一组年龄为387~413 Ma;石炭系岩片中与放射虫硅质岩共生的基性凝灰岩碎屑锆石年龄为341~3403 Ma,最年轻一组年龄为341~354 Ma.综合锆石年龄和化石资料,限定泥盆系岩片原始沉积时代为早-中泥盆世,石炭系岩片时代为早石炭世早-中期.碎屑锆石U-Pb年龄谱特征和Hf同位素组成指示泥盆系岩片和石炭系岩片具有相似的物质源区,主要来源于亲冈瓦纳的陆壳,少量来自于古生代特提斯域新生岛弧.早-中泥盆世地层岩片原始沉积于亲冈瓦纳的大陆斜坡环境;早石炭世地层岩片原始沉积于亲冈瓦纳的大陆斜坡至古特提斯洋盆边缘环境,不是远洋深水的大洋盆地环境.寻找以远洋深水沉积物为代表的大洋盆地相沉积并开展研究是当前昌宁-孟连古特提斯研究的重要方向之一.

The pelagic abyssal sediments in orogenic belt are important basis for the restoration of the ancient ocean.The Changning-Menglian suture zone preserves a large number of marine sedimentary strata fragments,but it is not clear whether there are ocean basin facies pelagic sediments.Petrology,radiolarian age and detrital zircon U-Pb-Hf isotopes of the Carboniferous and Devonian fragments,which were thought to be the ocean basin facies sediments in the Nongba area,are studied in this paper.Early Carboniferous radiolarian fossils have been identified in the Carboniferous fragments.The detrital zircon ages of the Devonian fragment range from 3266 Ma to 387 Ma,with the youngest group of 387-413 Ma.The detrital zircon ages of the basic tuff associated with radiolarian chert in the Carboniferous fragments range from 3403 Ma to 341 Ma,and the youngest group is341-354 Ma.Based on the detrital zircon ages and fossil data,it can be concluded that the primary depositional age of the Devonian fragments is limited to the Early-Middle Devonian,and that of the Carboniferous fragment is early-middle Early Carboniferous.The U-Pb age spectrum and Hf isotopic composition of detrital zircons indicate that Devonian and Carboniferous fragments have similar provenance,mainly from the Gondwana-related continental crust,and a small amount from the PaleoTethyan island arc.The Early-Middle Devonian fragments were originally deposited in the continental slope environment of Gondwana,while the early-middle Early Carboniferous fragments were originally deposited in the Gondwana-related continental slope to the edge of Paleo-Tethys ocean basin environment.It is one of the important research directions of Paleo-Tethys to find and study the ocean basin facies sedimentary rocks in the Changning-Menglian belt.