The Triassic Post-collisional Magmatism For The Southern Lancangjiang Tectonic Zone, Southwestern China: Petrogenesis And Its Tectonic Implications

The post-collisional magmatism can provide a great deal of important information on the interaction of crust and mantle during subduction and collision. In particular, the natures of magma source and the tectonic settings of generation of magmatism to understand the tectonic evolution of orogenic belt and the crust-mantle interaction are very important. As a major suture of paleo-Tethyan ocean, the Triassic volcanic rocks with a depth up to 8000 m and granitic intrusion correlated with subduction and collision of the paleo-Tethyan ocean were cropped out in the Yunxian-Jinghong area, Southern Lancangjiang zone. Thus, we chose the Triassic magmatism to conduct the study on systematic lithology (lithofacies), zircon SHRIMP U-Pb and whole-rock ⁴⁰Ar/³⁹Ar geochronology, elemental and Sr-Nd-Pb isotopic geochemistry. The main objectives are: (1) to confirm patterns of distribution of different magmatism belts;(2) to investigate the petrogenesis of magma and their tectonic settings relative to their generation;(3) to probe into the geodynamic mechanism of the Triassic magmatism and their constraint on the tectonic evolution of the paleo-Tethyan Ocean;(4) to better understand the deep process and mechanism of crust-mantle interaction of typical post-collisional magmatism. The major conclusions are summarized as follow:1, The geochronological data of zircon SHRIMP U-Pb for the Triassic igneous rocks from the Southern Lancangjiang zone indicate that the intermediate-mafic volcanic rock (20SM-408) from the lower part of the Manghuai Formation and the rhyolite (02DX-95) from its uppermost part were erupted at ca. 241.0±4.9 Ma and ca. 231.0±5.0 Ma, respectively, and the main granitic intrusions in Lincang batholithwere mainly intruded at about ca. 230Ma, and the volcanic rocks from Xiaodingxi Formation and Manghuihe Formation could were generated at similar time(215-210 Ma).2 s The middle-Triassic intermediate-mafic volcanic rocks for the lower part of Manghuai Formation characterized by depletion in Nb-Ta and enrichment in LILE, LREE and initial Sr-Nd-Pb isotopic compositions ((87Sr/86Sr)r= 0.706116-0.707046, sNd(t)=-2.69^-4.02, (206Pb/204Pb)t=l 8.541-18.800, (207Pb/204Pb)t= 15.614-15.698, (208Pb/204Pb)t=3g.478₃g.820), with an affinity to basaltic andesite and andesite, show similar signatures to source of the enriched lithospheric mantle modified by the subducted pelagic sediments. Their higher Pb/Nd ratios than those of island arc volcanic rocks indicate a possible contribution of pelagic sediments. The result of simulating calculation further suggests that its origin source could be the product of the Indian MORB-type source mixed with 5-7% pelagic sediments.3 > The rhyolites for the uppermost of Manghuai Formation characterized by a relatively low initial 87Sr/86Sr (0.705717-0.707262) and high eNd(t) (-4.64^-6.34) ratios and variable initial Pb isotopic composition ((206Pb/204Pb)t==17.220-18.570), with the geochemical affinity to A-type granite, are distinct from the felsic rocks derived from partial melting of crust, but similar to Anorogenic A-type felsic rocks. The result of simulating calculation indicates that the rhyolites for the uppermost of Manghuai Formation might be originated from the underplated basic magma mixed with 5-7% lower crustal materials and 8-10% middle-upper crustal materials, and then underwent some mineral crystallization and fractionation during the ascent of magma.4^ The late-Triassic volcanic rocks for the Xiaodingxi and Manghuihe Formations possess the feature of high-Al basaltic rocks. In light of the discrepancy of their MgO contents, they are divided into two groups: group 1: high-Al and low-Mg basaltic series (Al2O3=16.00-16.85wt%, MgO=3.04-7.50wt%), and group 2: high-Al and high-Mg basaltic series (Al2O3=16.77-19.79wt%, MgO=7.74-11.00wt%). The group 1 possess Zr/Nb=9.29-15.42, Nb/La=0.26-0.66, La/Sm=4.19-6.86, ENd(t)= -2.30-3.44 (2O6Pb/2O4Pb)r=18.401-18.598. Group 2 have higher Zr/Nb (19.37-23.92) and ENd(t) (1.25-2.31) ratios and lower La/Sm (1.65-3.11) ratios. The corresponding results ofsimulating calculation implies that the mantle source of Group 1 volcanism could be product of Indian MORB-type mantle source modified by mixing with 4-7% pelagic sediments, while the Group2 rocks could be derived from the interaction of low-Mg and high-Al magma and asthenospheric components.5> Based on the previous data of the evolution of paleo-Tethyan Ocean, we reconstruct the model of evolution of paleo-Tethyan Ocean (Cangning-Menglian Ocean) in the western Yunnan Province. The Cangning-Menglian Ocean had opened before the middle-Devonian. The continent-continent or continent-arc collision had developed from the late-Permian to early-Triassic. The evolution of post-orogenic phase had occurred in the region since the middle- to -Triassic time. Subducted slab had begun to slip off or delaminate since the early-Triassic, and resulted in the generation of the middle-Triassic igneous rocks. Following the slab slip-off or delamination, the ascending asthenosphere interacted with the enriched lithospheric mantle modified by the subducted pelagic sediments and produced the late-Triassic volcanic rocks (high-Al, low-Mg and high-Al, high-Mg basaltic rocks for the Xiaodingxi and Manghuihe Formation).