| The Sanjiang Orogenic Belt,as an important eastern element of the huge Himalaya-Alps Tethyan orogenic system,records the complete Paleo-Tethyan history,which has been greatly affected by the Cenozoic India-Eurasia continental collision.The Sanjiang Orogenic Belt can be divided into three segments:the northern segment is located in the regions north of Weixi,the southern segment in the regions south of Yunxian,while the middle segment is in the regions between Weixi and Yunxian.Numerous studies have revealed the spatial distribution patterns of tectonic facies for the Northern and Southern segments of this belt.The Longmu Co-Shuanghu high-to ultrahigh-pressure metamorphic belt and the Changning-Menglian ophiolitic belt for the northern and southern segments,respectively,which are defined the locations of major sutures.However,the precise location of the main suture in the middle segment is still elusive due to an extremely high degree modification caused by the indent of the northeastern comer of the Indian plate.This thesis focuses on tectonic evolution of the middle segment of the Sanjiang Orogenic Belt in Weixi and Liuku regions in term of tectonic facies analysis.Our major purposes are(1)to identify the location of the main suture and/then(2)to discuss the Paleo-Tethyan tectonic evolution of the middle segment by multiple disciplines joint studies including structural geology,geochemistry,radioactive isotopic geochronology.A low-graded metamorphic Carboniferous to Permian sequence exposes along the two banks of the Nujiang River in Liuku-Fugong region.This sequence is made of quartz sandstone,muddy quartz sandstone,shale,and limestone,and basalt in upper portion.The sedimentary rocks are of high textural maturity.LA-ICP-MS detrital zircon U-Pb analyses reveal an age-spectrum of high comparability with that of the Upper Paleozoic strata of the Lhasa terrane.New zircon U-Pb dating demonstrates that the basalt erupted during Early Permian(285-281 Ma),thus is the equivalent of the Woniushi Formation in the Baoshan continental block.Available geochemical data have shown that these basalts are of rift-aff-inity.Synthesizing sedimentary,geochemical,and geochronological data,we suggest that the Carboniferous to Permian sedimentary and igneous rocks in Liuku-Fugong region formed in a passive continent marginal basin.The Chongshan-Biluoxueshan Complex exposes in the east of the passive continent marginal sedimentary rocks.The boundary between them is a fault zone along Bijiang-Fugong-Wayao,being sub-parallel to the Nujiang River.This Complex is high-graded metamorphosed and strongly deformed,consisting of metamorphic Upper Permian greywacke and tuff-bearing siltstone,shale,and limestone.Numerous Cretaceous and Cenozoic leucogranites intruded the deformed sedimentary rocks.Widespread volcanic elastics of the sedimentary rocks suggest that they likely deposited in a fore-arc or intra-arc basin.Along the western margin of the Lanping basin,the folded Upper Permian sediments were covered by the Upper Triassic conglomerate in lower and N-MORB-affinity basaltic andesite in upper.Its bottom is an angular unconformity.Further study is required to reveal the tectonic implications of this basaltic andesite.In the regions east and northeast of the Lanping basin,a huge pile of dacite,rhyolite,and minor basalt developed upon the Permian fore-arc basin sediments.Our field observations and measurement revealed the volcanic pile has a>1.6km thickness.Numerous granitic batholiths intruded the volcanics.Detailed zircon U-Pb analyses indicate that the huge volcanic pile accumulated during a short period<7Ma(251?244Ma).Besides,the ages of the granites are undifferentiated from the volcanics due to the limitation in precision of analytical techniques.Thus,all of the igneous rocks define a magmatic flare-up event.New bulk geochemistry including Sr-Nd isotopics demonstrate that(1)the magmas of dacite,rhyolite,and granite were all derived from ancient crustal mass,(2)the minor basalts are of arc-affinity,and(3)the granitic magma of the intrusion had formed under a higher-pressure condition than those of volcanics.Our structural studies further suggest a crustal shortening and thickening during the arc-like magmatism and volcanism.Accordingly,we suggest that the main Paleo-Tethyan suture of the middle segment of the Sanjiang Orogenic Belt is the Bijiang-Fugong-Wayao fault zone,which connects to the Longmu Co-Shuanghu suture in north and the Changning-Menglian suture in south,respectively.This fault zone separates a passive continental margin in the west from a fore-arc basin in the east.A continental margin arc developed upon the fore-arc basin sediments in its east.Such a tectonic facies pattern suggests a same subduction pole of the Paleo-Tethys as the northern and southern segments of the Sanjiang Orogenic Belt.It is interesting that the middle segment exhibits a quite different magmatic history in contrast with its northern and southern counterparts although they have the same subduction history.Such a spatial difference may be interpreted in term of variable paleogeography pattern before subduction:in the middle segment,the Paleo-Tethys subducted beneath the huge Yangtze continent,while in the northern and southern segments,it subducted under smaller eastern Qiangtang and Simao continental blocks,respectively.Behind these small continents were two closed oceanic basins,the Southern Jinshajiang and the Ailaoshan Oceans,respectively,which tend to backwards during the subduction of Paleo-Tethys.The middle segment of Sanjiang orogenic belt,the crust of the upper plate was successively shortened and thickened and may have prevented volcanic eruption,leading to accumulation of basaltic magmas within the crust that gradually heated,softened,and subsequently melted the crust,forming felsic magmas.When the crust was weakened to a critical threshold,crust-derived felsic magmas rose up to surface with minor basalts,resulting in a magmatic "flare-up".This model improves our understanding of the heterogeneity of magmatic activities in the arc belt along the suture zone in southern Tibet and in other continental arc systems in general. |
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