Characteristics And Tectonic Significance Of The Litang Ophiolitic Melange

The Litang ophiolitic melange along the Ganze-Litang suture separates the Songpan-Ganze orogenic belt to the east,and the Yidun-Saruli island arc to the west.This ophiolitic melange is the key to understand nature and tectonic evolution of the Songpan-Ganze orogenic within the eastern Tethys domain.This paper focuses on geological investigations and petrography,petrogeochemistry and chronology of the Cunge,Caganglongwa,Dasenlongwa and Douwuba rock formations of the Litang Ophiolitic melange.Tectonic nature,implication and new model are further proposed based the new data presented in this paper.The Cunge Formation with basic rocks has right-dip curve in standardized diagram of rare earth element(REE)chondrites.Contents of light REE are medium to high,while Heavy REE are moderately differentiated.This formation has TiO2 content of>1%,(La/Sm)N>1.3,and relative richer of Nb than La,revealing characteristics of OIB.in the TAS diagram,Most of the basic rocks of the Caganglongwa are in the basalt area.Using Si02-K20 diagram to further map subalkaline rock series,Most of them fall into the low-potassium plaque series,and some points migrate to the calc-alkaline series,which has the characteristics of typical mid-ocean ridge basalt(N-MORB).The zircon U-Pb crystalline age of basalt measured in Dasenlongwa is 238.8�5 Ma(n=12).The total amount of rare earth elements in basic rock samples is higher,and the fractionation of light and heavy rare earth elements is obvious,(La/Yb)N=3.27?11.70,And no obvious Eu anomaly(?Eu=0.86?1.17).The right-inclined distribution curve of light rare earth enrichment is consistent with the intraplate basalt(WPB).In the structure discrimination diagram,Most samples fall into the basalt range in the WPB.The latest U-Pb age of metamorphic quartz sandstone detrital U-Pb of Dowuba is 484 � 5 Ma,It indicates that the Dowuba at least accepted the material deposition from the turn of the Early Paleozoic Cambrian-Ordovician.According to previous studies,During the Early Paleozoic,the Yangtze block was adjacent to the Lhasa block and the Qiangtang block in Tibet,separated by the Shallow sea.During the Ordovician period,the oldest stratum Douwuba is deposited.The Ganzi-Litang Ocean began to crack from the western margin of the Yangtze Block in the early Devonian,and stretched into a deep water basin in the Mid-Late Devonian,the oceanic crust developed in the Early Carboniferous,and entered the stage of ocean basin expansion in the Permian-Early Triassic.The rock of different tectonic environments in the Litang ophiolite melange in the investigation area gradually formed during this period.During the expansion of the Ganzi-Litang ocean basin in the Early Middle Triassic,another rift occurred on the western margin of the Yangtze block,but the rift did not expand to form oceanic crust.Instead,it died with the subduction of the Ganzi-Litang ocean basin in the Late Triassic.