From Middle Cretaceous To Early Paleogene, The Provenance Analysis Of Xigaze Forearc Basin

Xigaze Forearc Basin(XFB) located in the middle of Yarlung Zangbo SutureZone(YZSZ), was sedimented in middle Cretaceous to early Paleogene. On the northof it is Gangdese Magmatic Arc, on the south of it are Ophiolite and Melange Belt.These tectonic units in the YZSZ are closely connected with each other. Because ofthe tectonic evolution, the Ophiotite and the Melange Belt are poorly preserved, it is averyhard work to reveal formation of the YZSZ and the evolution of the Tethys fromthat mixed record. It becomes very important to pay attention to the XFB, which iswell preserved in the YZSZ.XFB consists of Xigaze Group which was deposited in Middle Cretaceous to LateCretaceous and Cuojiangding Group which was deposited in Late Cretaceous to EarlyPaleogene. Both of them are low in quartz content of the detrital components, so doesthe feldspar content. Most of the lithic fragmentsfrom the XFB are andesite and dacite.The characteristics of the overall components of the detrital rocks from the bottom tothe top show a delivery variability.From the Dickinson triangular diagram, which is used to discuss the provenancedistrict, most of the sediments are coming from the Arc Orogen Sources, and show thedelivery variability from the Undissected Arc associated with Transitional Arc toTransitional Arc associated with the Dissected Arc, very few of them are coming fromthe Recycled Orogenic Belt.Combined with the paleocurrent directions and the triangular diagram analysis,and the nearby geologic setting, Gangdese Magmatic Arc located on the north of itcan be the main provenance district of the XFB. Detrital Zircon date from theXigaze Group and Cuojiagding Group, each can be divided into two main peak belts.One belt of the Xigaze Group detrital zircon data is concentrated from175Ma to150Ma and the other is concentrated from130Ma to100Ma. This belts just match thedata from Yeba Formation and Sangri Group from the Gangdese Magmatic Arc. Andone belt of the Cuojiangding Group detrital zircon data is concentrated from125Ma to100Ma, the other is concentrated from70Ma to50Ma. They just match the data fromSangrin Group and Linzizong Group. More over, not only about the age of those datafrom isopotic dating but also the rock type mainly composed of andesite and daciteare match the sediment fragments from XFB very well.In the process, the lack of older belt between170Ma and150Ma fromCuojiangding Group may causedby the Yeba Formation which has been completelyeroded and formed a lower topography that allowed rives passing through from the farmore north to the south to supply sediments. That makes the Zenong Group one of themain provenance members during the beginning of deposition of Cuojiangding Group.After that, the intrusion and eruption of Linzizhong Group began in65Ma may causea new high topography on the south of the Gangdese Magmatic Arc, which becomesthe obstacle of the south migration of Zenong Group. Thus Linzizhong Group becamethe new main provenance of the XFB.