Magmatism Of The Western Chinese Altai Orogen: Geochronology, Petrogenesis And Tectonic Implications

It is well known that the Central Asian Orogenic Belt (CAOB) is the largest and most typical accretionary orogenic belt in the world. The Chinese Altai Orogenic Belt, situated between Sayan and Mongolian Orogen, is the key part of the CAOB. There are a large of granitoids which occupy about forty percent of the exposed rock area in Chinese Altai, and these magmatic rocks in this orogenic belt can provide valuable information on its formation and history of evolution. In order to understand the time of magmatism, petrogenesis and evolution of the western Chinese Altai Orogenic Belt, some representative plutons, dikes and metamafic rockes ( i.e. Kuoketasi Pluton, Habahe Pluton , Hemu-Kanas Pluton, Tielieke mafic dike, Tarlang gabbro and Tarlang amphibolite) have been studied, involving field geology, petrography, major, trace, Sr-Nd isotopic geochemistry and zircon U-Pb or hornblende ⁴⁰Ar-³⁹Ar dating. The major conclusions are summarized as follow:(1) The geochronological data of zircon SHRIMP U-Pb for granitic plutons in the western Altai Orogenic Belt indicate that the Hemu-Kanasi and Tielieke plutons from the Habahe Group in the northwest of Chinese Altai were intruded at about ca. 393±7 Ma and 410±6 Ma, respectively, and Habahe pluton from the Altay Formation in the south of Chinese Altai were generated at about ca. 390±5 Ma. These data indicate that Caledonian magmatism was dominant in the western Altai Orogenic Belt.(2) The two-mica granite of Hemu-Kanasi from the northwest of Chinese Altai is characterized by high Al₂O₃ (13.98-15.58 wt%) contents and K₂O/Na₂O ratios (1.09 -1.89), indicting the pluton is peraluminous S-type (ASI =1.05-1.4). Hemu-Kanasi pluton is enriched in Rb (115.5- 220.2 ppm), Cs (4.31-13.43 ppm), Sr (94.20 -268.50 ppm) contents and light rare earth element concentrations (La/Yb_N= 3.7 -7.3) and exhibits high heavy rare earth element concentrations (Yb_N= 19-46) and weak HREE fractionation (Gd/Yb_N =1.06-1.62), with obviously europium anomalies (δEu =0.49-0.62), indicating the pluton probably formed by dehydration melting of mica. Habahe pluton is characterized by low K₂O (0.22-4.74 wt%) and Rb, Cs contents, with light rare earth element concentrations (La/Yb_N=2.3-7.4) and moderate to high heavy rare earth element concentrations (YbN=14-23) and weak HREE fractionation (Gd/Yb_N =0.70 -1.15), with obviously europium anomalies (δEu =0.50-0.68). These characteristics are consistent with an origin by dehydration melting of hornblende-bearing, middle crustal source dominated by mafic to intermediate rocks. Kuoketasi granitic pluton is relatively small which occurs in the southwestern Chinese Altai orogen. The pluton is characterized by relatively high MgO (1.68-1.97 wt%) and Sr (215-330 ppm) contents, with low K₂O/Na₂O (0.41-0.48) ratios and mantle-like Sr-Nd isotopic compositions (ε_Nd(t)=2.6-4.2; (⁸⁷Sr/⁸⁶Sr)_i= 0.7046-0.7048), indicating Kuoketasi pluton probably formed by fractional crystallization of the mantle-Derived magma, with limited assimilation of old crustal material.(3) Nd-Sr isotopic compositions of Hemu-Kanas (ε_Nd(t)=-1 .0-0.9; (⁸⁷Sr/⁸⁶Sr)_i =0.7079-0.7107) and Habahe (ζs_Nd(t)=-1.2-0.6; (⁸⁷Sr/⁸⁶Sr)_i =0.7064-0.7071) plutons indicate that their sources are similar, with significance of juvenile materials. The source of granitoids from the western Chinese Altai is mainly greywackes and mafic to intermediate rocks. The character of source and juvenile Nd isotopic composition indicate that the western Chinese Altai probably was an accretionary wedge with absence of old crustal material.(4) Tielieke mafic dike is characterized by very low SiO₂ (45.18-49.48 wt%), with wide MgO (3.33-8.64 wt%) and low K₂O (0.02-0.10 wt%) contents, belonging to sub-alkaline. The samples display flat to light rare earth elements (LREE)-depleted patterns and are depleted in high field-strength elements (HFSE). These features, SHRIMP U-Pb 372.3±7.4 Ma and Nd-Sr isotopic compositions are similar to Kuerti mafic rocks, indicating a variably depleted mantle source and a subduction component. However, Tielieke mafic dike probably formed in an island arc with the extensional continental margin setting.(5) Tarlang gabbro formed in the mid-late Permian, with the ⁴⁰Ar-³⁹Ar plateau age of 266.9±4.2 Ma. The gabbro exhibits subduction-releted characteristics and was coeval with transtensional movement in the Chinese Altai, probably reflecting a tectonic transition in the Permian.(6) Tarlang amphibolite occurs in granitic plutons (>400 Ma) or Devonian stratum either as xenoliths or lens, indicating it formed before 400 Ma, whereas 297 Ma probably represents the age of metamorphism. Its occurrence and high TiO₂ (0.82 -3.32 wt%) contents indicate amphibolite was an accretionary seamount fragment along active continental margin when oceanic crust subducted.(7) Devonian to mid-late Permian magmatism existed in the western Chinese Altai. The seamount fragment and subduction-related granites and mafic rocks indicate that the south Altai was an active continental margin but not rift in stabilized continental margin between the Devonian and Permian at least. There are source materials provided for granite formation and mafic magma upwelling in active continental margin, whereas mafic magma provides heat for granite formation. However, compared with previously magmatism, Permian magmamtic area is small and sporadic, indicating that even subduction existed, tectonic mechanism and magmatism had been changed, gabbro with subduction-releted characteristics is a reflection of the tectonic transition.