Petrogenesis Of Taxkorgan Alkaline Complex Belt And Its Tectonic Significance

The Tibetan Plateau, caused by India-Asia continental collision, has attracted many researchers all over the world. However, there is still not a convincing mechanism to explain how the plateau formed. It is the India-Asia continental collision that gave rise to the large lithospheric shortening of the Tibetan Plateau and kept its high-elevation. Among many scientific problems of Tibetan Plateau, one of the best attractive problems is the process and mechanism of lithospheric shortening. Pamir syntax is an outstanding example of lithospheric extreme thickening during continental collision and thus would be a perfect place to unveil dynamics of lithospheric shortening. The Taxkorgan alkaline complex belt right in this area implies and records the process of lithospheric shortening.Taxkorgan alkaline complex belt, paralleling the Karakorum fault, is located in the middle-east Pamir syntax and along the boundary between China and Tajikistan. This belt is composed of several intrusions among which Kuzigan, Karibasheng and Zankan are typical ones. Taxkorgan alkaline belt is made up of alkaline syenites, alkaline syenitoids and alkaline to sub-alkaline granites, with aegirine-augite syenite, diopside syenitoid and biotite adamellite as their representative rocks. By using SHRIMP U-Pb zircon, we obtain accurate ages of Kuzigan and Karibasheng intrusions, and both of them are ~11Ma. Based on field observation, petrology, mineralogy, chronology of Ar-Ar and SHRIMP U-Pb zircon and former studies, we find that there was a strongly tectonic-magmatic event at ~11Ma and a tectonically thermal event at ~8Ma in Pamir area. Being rich in potassium, alkali and calcium, this belt is a series of potassic and alkalic granitoids and belongs to A-type granites. However, this kind of A-type granitoids was not formed at the end of orogeny. It is a magmatic product induced by the regional extension (e.g. large scaled strike-slip movement of the Karakorum) during the orogenic process. After researches on petrology, mineralogy, geochemistry and phase equilibrium, the three alkaline intrusions came from sources that had no plagioclase (>60km) but had garnet and rutile (>50km) in their melting residue. Therefore, the magmas were the product of low partial melting of mafic eclogite at the base of the thickened crust, and their magma sources were affected by fluids. These results suggest that the crust of the Pamir syntax had been thickened at -11 Ma, with a curst thickness over 50km. But, because crustal thickening is a cooling process, there must have some heat contribution of deep process (e.g. mantle magmatism) to Taxkorgan alkaline magmatism. A reasonable mechanism is delamination that had happened at ~11Ma in Pamir syntax. Hence, except convergent thickening and block extrusion, delamination of lithosphere is also a very important mechanism to accommodate lithospheric shortening.During continental collision, convergence induced the lithospheric shortening and thickening of the Tibetan Plateau. In this process, the crust will be fractionated: the moving down mafic rocks will be eclogitization gradually and thus have a higher density to result in regionally gravitational instability. When this instability comes to a certain limit, delamination would happen. Consequently, the lithosphere become thinning, the asthenosphere is upwelling, magmatism is playing and the mechanical properties is weakening. Then the thinning lithosphere could be thickened again by asthenosphere top descending (caused by heat elimination), magma underplating, compressing and so on. Delamination to thinning tothickening, a cycle of deep process, will happen again and again in a long period, and induce crustal thickening and uplifting in Tibetan Plateau.