BOUNDARY FAULTS AND MAIN CHARACTERISTICS OF THE LANCANG RIVER SUTURE ZONE IN NORTHWEST YUNNAN

The eastern and western boundary faults of Lancang River suture zone are respectively Jicha Fault located on the west bank of Lancang River and Biluo Xueshan Fault along the main peak of Bilou Mt. The suture zone, consisting of the Upper Paleozoic Jidonglong and Shamu formations, is formed in the neritic shelf-shelf marginal basin environment, among which the volcanic rocks of upper member, Shamu Formation (P₂sm²) are formed in island-arc setting. The ultramafic-mafic rocks outcropped along Jicha Fault can be divided into two parts:One is Jicha ultramafic-mafic-plagiogranite with the characteristics of ophiolite, probably the fragmentary remnants of Lancangjiang oceanic crust, migrated and emplaced with the extinction of Lancangjiang Ocean (Early-Middle Triassic?); The other is Weideng mafic rocks, belonging to the Alaskan-type, intruding upward along the Jicha Fault during the subduction of the Mesozoic Lanping-Simao sedimentary basin in Cretaceous period.