Mesozoic Granitoids In Eastern Songpan-Garzê: Geochemistry, Petrogenesis And Tectonic Implications

Songpan-Garze, located in the eastern part of the Tibetan plateau, underwent two phases of orogenesis: Paleotethys and Neotethys. Two key issues are hotly debated for this region, in terms of the nature of the Songpan-Garze basement and the tectonic transformation since the Triassic orogen. Mesozoic granitic intrusions are widely distributed in the Songpan-Garze, and provide important constraints on the crust-mantle interaction and tectonic evolution. The source and petrogenesis of these granitic rocks are important clues for understanding the tectonic evolution of this orogen. We studied nine granitic plutons in the hinterland of the Songpan-Garze area, i.e. Niuxingou, Menggu, Taiyanghe, Laojungou, Siguniangshan, Mengtonggou, Manai and Keryin, and tried to put constrains on the basement nature and tectonic evolution of the Songpan- Garze terrane.Zircon U-Pb dating for Niuxingou, Menggu, Taiyanghe and Laojungou plutons in Songpan-Garze using LA-ICP-MS has obtained ages of 234±3 Ma, 243±4 Ma, 229±3 Ma and 224±5 Ma, respectively. Zircon grains with ages of 285 666 Ma were also recognized Menggu and Laojungou plutons, and were explained to be xenocrysts captured from the country rock. In addition, four old zircons in the Menggu pluton formed a disconcordant line. The lower intersection age (251±21 Ma) was consistent with that of the pluton (243±4 Ma) in error, and the upper intersection age (2459±41 Ma) reflected the existence of old basement, probably the extension of the Yangzi craton. This indicates that the Songpan-Garze area is not underlain with relic oceanic crust of Tethys as previously thought. The Niuxingou and Manai plutons are mainly monzonite, with shoshonitic to high-K calc-alkaline and metaluminous characteristics. They have intermediate SiO₂ (53.7～63.6 wt%), high Rb (26.1～202 ppm) , Sr (1010～1660 ppm) , Ba (1280～2590 ppm ) , Cr (37.6～193 ppm ) contents and high Mg^# (47～63) values, which are quite similar to that of high Ba-Sr granitoids and imply a genetic relation with an enriched mantle source. The occurrence of MME (mafic microgranular enclave) within the two plutons and the Sr-Nd-Hf isotope compositions suggest a magma mixing process for their genesis. The Taiyanghe pluton also has high Ba, Sr but low Mg, Cr and V contents with high LILEs and HFSEs contents. The low SiO₂ content (～53 wt%) does not support a normal crust origin, and its lowε_NdT (-7.3～-6.5) andε_HfT (-3.2～+3.9, mostly less than +0.2) values exclude a pure mantle source. It was probably derived from partial melting of a pyroxenite or gabbro cumulates in the crust-mantle transitional zone.The Siguniangshan K-feldspar granite has high total alkali (7.22～9.38 wt%), LILEs and HFSEs but relatively low Ba, Sr contents, with high 10000*Ga/Al ratios (2.34～3.03), indicating a strong affinity of A-type granite. Its distinctly fractionated REE, flat HREE and negative Eu anomaly suggest the pluton has experienced substantial fractionation crystallization. The relatively low I_Sr (0.7055～0.7069) and near-zeroε_NdT (-3.69—2.60) values indicate the involvement of mantle-derived material during its genesis.The Laojungou and Menggu plutons are classified as granite and granodiorite, respectively, with high-K calc-alkaline and metaluminous to weakly peraluminous characteristics. They have relatively high Al₂O₃(14.2～16.0 wt%), Sr (307～697 ppm), low HREE (Y< 16.4 ppm) contents and intermediate to high Sr/Y ratios (22.4～54.8), exhibiting a strong affinity to adakitic rocks. Their significant REE fractionation, relatively low MgO (0.38～1.98 wt%) content,ε_NdT(-6.9～-3.1) andε_HfT(-4.7～+1.2) values suggest that they were probably derived from partial melting of lower crust in garnet stability field.The Mengtonggou and Xilizhai plutons are high-K calc-alkaline, metaluminous to weakly peraluminous quartz monzonite and granite, respectively. In comparison with the Laqjungou and Menggu, they have relatively high SiO₂ (> 70 wt%), low Al₂O₃ (13.6～15.2 wt%), Sr (110～397 ppm) contents. The two plutons also contain relatively low HREE (Y= 9.49 -21.1 ppm, mostly less than 17 ppm) contents, but their low Sr/Y (5.22～32.6) ratios make them different from adakitic rocks. Their relatively high initial Sr (⁸⁷Sr/⁸⁶Sri =0.7060～0.7075) and low Nd (ε_NdT=-6.0～-3.6) isotope compositions imply that they probably originated from a source similar to that of the Laojungou and Menggu plutons. Their high Al₂O₃ (13.6 -16.1%), Sr (110 - 697 ppm), low HREE contents and high Sr/Y ratios (5.22～54.8) show adakitic characteristics, and subsequent fractionation of plagioclase resulted in the decrease of Al₂O₃, Sr contents and variably negative Eu anomalies (δEu=0.32 - 0.95).The Keryin pluton has high A/CNK ratios (1.27～1.33), showing characteristics of strongly peraluminous leucogranites. Its high Rb/Sr, Rb/Ba ratios, low Nd (ε_NdT = -9.9 - -9.3) and high Sr (⁸⁷Sr/⁸⁶Sri= 0.7181 - 0.7192) values indicated that the source of this pluton was dominated by crustal materials, probably metapelites. The magma temperature (768 - 773℃) estimated using Zr saturation thermometer is significantly higher than that of the Tibetan counterpart (< 700℃). This suggests that the Keryin pluton did not result from crustal thickening, and an extra heat source, most likely provided by mantle, may trigger the partial melting.These granitoids, including high-Ba-Sr, A-type, adakitic and strongly peraluminous leucogranite, occurred in a relatively limited time span. Though coming from different sources, they all reflect a strong mantle-underplating event in the early Mesozoic. In addition, their variable composition and genesis indicate a substantial change of crust thickness. From adakitic Menggu (243 Ma) to S-type Keryin (188 Ma) and A-type Siguniangshan (185 Ma) plutons, there was a gradually shallowing trend for partial melting, which was probably related to a mantle upwelling scheme and reflect the tectonic transformation during orogen collapse.