Permian And Triassic Magmatic Activities In The Chinese Altai And Their Geodynamic Implications

Located in the central part of the largest accretionary orogeny in the world,the Central Asian Orogenic Belt,the Altai orogeny is a Paleozoic orogeny which experienced its final closure during late Paleozoic.A large amount of magmatic rocks,both mafic and felsic,recorded the tectonic development of this region during-and after the final collision.Research of those rocks can shed light on the process of continental formation and history of tectonic transition during the accretionary orogeny.With systematic petrological,geochemical,isotopic and chronological studies on the Permian Ashele basalt and Permian–Triassic granitoids?e.g.SW-Fuyun,Kalasu,Halasu,Alaer,Shangkelan,Xibuodu granites?,this thesis is focused on the petrogenesis and geodynamic implications of the post-collisional magmatism.In combination of geochemical data reported in the literature,the patterns of magmatic activities and tectonic development of Chinese Altai during the late-Palezoic and early-Mesozoic will be discussed.The Ashele basalt,located in the Habahe area of the southern Chinese Altai,is a terrestrial basalt erupted in the late Triassic?225 Ma?.It displays a mixture of geochemical features of MORB and arc magmatism,as evidenced by its LREE-depleted patterns?La/Sm=0.77–0.82?,LILE-enriched?Ba?Sr?Pb?Th?U?and HSFE-depleted?Nb?Ta?Zr?Hf?characteristics.In comparison with typical MORB rocks,the Ashele basalts possess reltively high initial Sr isotope composions((⁸⁷Sr/⁸⁶Sr)_i=0.7048–0.7052),less radiogenic?_Nd?t??+5.1⁺5.5?and?_Hf?t??14.4–16.4?values as well as low Nb/Y?0.024–0.027?and Zr/Y?1.17–1.31?ratios,indiacting a highly depleted mantle source.Its high LILE,Th/Yb?0.45–1.2?and V/Sc?8.8–9.3?ratios reflect strong metasomatism of subduction-related dehydration fluids.Its eruption is coeval with the early Mesozoic movement of the Erqis fault,indicative of decompression melting in the mantle.The Ashele basalt may represent a juvenile continental lithospheric mantle formed by accretion of highly depleted oceanic-arc lithospheric mantle during the development of the accretionary orogeny in the Chinese Altai.Perno-Triassic granitoids,such as SW-Fuyun,Kalasu,Halasu,Alaer,Shangkelan,Xibuodu granites,were emplaced in the Chinese Altai during Permian?298–279 Ma?and Triassic?216–212 Ma?times.They are largely high-K calc-alkaline granites with high Si?SiO₂>70 wt.%?,K,Na and Al.They all possess peraluminous composition with ASI values in the range of 1.02–1.45.With the exception of peralumious and Rb-rich S-type Shangkelan granite,all othe granites display S-and I-hybrid type features,such as low Ga/Al and FeO/MgO ratios,slight peraluminous as well as negative correlations between P an Si,representing contribution from both meta-igneous and meta-sedimentary rocks.The Permian rocks experienced remarkable fractional crystallization of K-feldspar while Triassic Shangkelan and Xibuodo granites experienced only limted fractional crystallization.The granites are characterized by low Mg#?10.2–37.9?values,and low Cr?<15 ppm?and Ni?<10 ppm?contents,which preclude direct contribution from the mantle.Both the Permian and Triassic granite are of high silica content?SiO2>70 wt.%?,and are thus classified as high-Si granites.The Permian granites may experience intensive fractional crystallization while the Triassic graintes were likely formed though low-pressure melting at relatively shallow crustal levels.The granite rocks all display decoupled Nd and Hf isotopic systems.The Permian SW-Fuyun and Halasu have more depleted Nd composition relative to Hf isotope,which may probably be inherited from arc-derived rocks formed by partial melting of sub-arc mantle metasomatized by slab-derived melt,while others have more depleted Hf ratios relative to Nd isotope,which was probably caused by hybride crustal source with juvenile meta-igneous rocks and terrestrial sediments.This reflects a reduction in melting depth and contribution from subduction-related rocks during the tectonic transition from post-collision in Permian to non-orogenic in Triassic.A dataset from literatures and newly acquired data show systematic variations between 320 Ma and 200 Ma within the Chinese Altai.After 300 Ma,concentrations of K and Na rises in both felsic and mafic rocks,while Sr isotopic ratios increase towards more enriched compositions,reflecting an reduction of subduction input after the termination of subduction and accretion.After 300 Ma,the Permian rocks display high V/Sc ratios,Ba enrichment and Th depletion while Triassic magmatism show the opposite variation tendency,corresponding to a reduction of subduction input.The Permian granitic rocks are also characterized by low Rb,Y and high Ca concentrations relative to their Triassic counterparts,indicative of juvenile crustal material,while Triassic granites display the opposite characteristics corresponding to high degrees of mature continental sediment involvement.Such variance in magmatic activity suggests that the Chinese Altai experienced post-collisional tectonics during Permian,which was replaced by a non-collisional intraplate tectonic regim in the Triassic.The dominance of meta-igneous rocks as the source of felsic magmatism in the former was replaced by more mature continental sedimentary sources in the latter.The transition to intraplate tectonics in Triassic led to fade-out of subduction input and a reduction of melting depth as well as maturation of the newly formed continent with intensive high-silica magmatism.