The Early Cretaceous Volcanic Rocks In North Part Of Central Segment Of The Gangdese In The Tibet And Its Tectonic Implications

The Qinghai-Tibet Plateau belongs to Tethyan tectonic domain, known as natural laboratory for continental dynamics study. Composite arc-basin systems separated by the Tethyan ocean in the Paleozoic—Mesozoic has constituted the material foundation of the Qinghai-Tibet Plateau, forming three major orogenic systems. India-Asian collision yielded the largest and highest plateau in the earth. The Gandese belt (Lhasa block/terrane) is located in the central part of the Tibet, it preserved a wealth of records about Tethyan evolution, for a long time, many studies focus on these records. The significant magmatic rocks in the belt are the media, which are explored the formation and evolution of the Qinghai-Tibet plateau, but most people concentrate on the studies about the magmatic activity and tectonic evolution during the India-Asia collision process, while it is insufficient to study the Pre-Cenozoic magmatic activity and tectonic setting. Previous studies concentrated in the Mesozoic-Cenozoic magmatic activity of the southern Gangdese. The huge volcanic rocks in the northern part have not been studied extensively and insightfully based on geochronology and geochemistry, which put some limitations on the understanding of Mesozoic evolution history of the belt. In order to enrich geological evidences of the Mesozoic tectonic evolution of the Gangdese, so the study of the Mesozoic volcanic rocks in the northern part is essential. Based on field investigations, three suits of volcanic rocks related to in northern part of Gangdese were selected for comprehensive studies on petrology, geochemistry, and zircon U-Pb SHRIMP & LA-ICPMS dating in this thesis, with the aim of revealing their age and petrogenesis and tectonic implication. These studies also have significant implications in understanding the Mesozoic subduction history of the northern part of Gangdese. Some results are gained as following:1. A large area of volcano-sedimentary strata exposed in northern part of central Gangdese, the strata are the Early Cretaceous mainly, the lower and upper part are mainly sedimentary rocks and volcanic rocks, respectively, the volcano-sedimentary strata distributed symmetrically in both sides of the Yongzhu-Namco ophiolitic melange belt.2. The volcanic rocks from the Zenong Group mainly contain basalt, andesite and dacite based on the TAS diagram. From the SiO2-K2O diagram they mainly are composed of high-K calc-alkaline series, they show a similar trace element characteristics, e.g., the enrichment of LILE (Rb,Ba,Th,U) and depletion of HFSE (Nb,Ta,Ti), similar to those of an island arc volcanic rock. The similar spider diagram and REE models imply that theses volcanic rocks were probably derived from a same magmatic source. Zircons from the Zenong volcanic rocks were dated by SHRIMP and LA-ICPMS, obtaining an age of 116.7-110.9Ma. Thus, this thesis suggest that the Zenong volcanic rocks were resulted from the southward subduction of the Yongzhu—Namco ocean in the Early Cretaceous.3. Duoni Formation volcanic rocks mainly contain basalt, basaltic andesite, dacite and rhyolite. The intermediate-basic rocks show geochemical characteristics similar to the volcanic rocks in arcs or active continental margins, such as LILE's enrichment and HFSE's depletion, which thought to be the melting products of continental marginal mantle wedge due to northward subduction of the Yongzhu-Namco ocean. The acid rocks have relatively high SiO2, K2O, enrich in Rb, Ba, Th, U and LREE, high Rb/Sr, and spidergram patterns similar to the acid rocks of the Linzizong Group derived from a crust source. Thus, the acid rocks in Duoni Formation were come from a magmatic source different from those of the intermediate-basic rocks. Zircons from two acid rocks in Duoni Formation were dated by SHRIMP and LA-ICPMS, obtaining age of 116Ma and 114Ma, respectively. So, the formation tectonic setting of Duoni Formation is related to the arc-arc collision.4. Qushenla Formation volcanic rocks include basalt and andesite, which show a similar trace element characteristics, e.g., the enrichment of LILE(Rb,Ba,Th,U) and depletion of HFSE (Nb,Ta,Ti), similar to those of an island arc volcanic rock. However, they originate complex sources, they were probably resulted from the southward subduction of the Bangong-Nujiang ocean in the Early Cretaceous.5. According to tectonic facies, based on comprehensive magmatic activity, sedimentary records, structural geology reseach, the tectonic history was proposed between the Yongzhu-Namco arc-arc collision belt in detail. The belt rifted in the extensional setting since the Middle-Late Triassic, spread in the Jurassic-Early Cretaceous, the Yongzhu-Namco ocean dually subducted in-130Ma, the initial arc-arc collision begin from 116 to 110Ma, the collision process end in 110.6. On the basis of composite arc-basin system and accretionary orogenesis, an accretionary orogenic belt is proposed in the northern part of Gangdese, based on comprehensive research of regional tectonics. Since the Middle-Late Triassic, the subdution zone of the Bangong-Nujiang Ocean retreated, the process control the evolution of the northern part of Gangdese and result in the magmatic front migrating toward the trench, forearc accretion and back-arc extension, the composite arc-basin systems formed in the Jurassic-Early Cretaceous.7. From the viewpoint of comparative tectonics, by comparative analysising the Yongzhu-Namco arc-arc collision zone in the Gangdese and the ongoing Molucca Sea arc-arc collision zone in the southeast Asia, we realized that the composite arc-basin system is a practical key to understand the evolution of orogenic belt.