Mesozoic collision-related magmatism and crust-mantle interaction along the Anhui segment of the Yangtze River Valley, east-central China

摘要

The middle segment of the Yangtze River Deep Fault Belt, located in the foreland of the Dabie orogen, contains widely exposed volcanic-intrusive complexes that formed during two episodes of magmatism (post-collisional and post-orogenic), reflecting crust-mantle interactions during the Late Jurassic (J_3) to Early Cretaceous (K_1). This article summarizes research on the Mesozoic igneous suites and xenolith suites in the area along the Yangtze River. 'Post-collisional magmatism' occurred during lithospheric extension at -145-130 Ma. Its beginning and end are marked by gabbroic xenoliths and pyroxene cumulates within intrusions at Tongling, and by alkali-rich magmatic rocks. The association includes peraluminous silicic rocks and metaluminous mafic-felsic igneous suites, ranging from medium-K to high-K calc-alkaline to shoshonitic compositions. Taking the Tongling region as an example, quartz monzodi-orite yields a sensitive high resolution ion microprobe (SHRIMP) zircon U-Pb age of 139.5+ 2.9 Ma, and granodiorite yields an age of 135.5 ±4.4 Ma. These intrusive rocks contain 52.79-66.46 wt.% SiO_2, 13.12-17.73 wt.% Al_2O_3, 1.37-4.62 wt.% MgO, 3.86-6.84 wt.% FeO~T, and 4.71-7.87 wt.% total alkalis (Na_2O + K_2O). ACNK values range from 0.62 to 1.20, and ANK values from 1.45 to 3.48. 'Post-orogenic magmatism' occurred during lithospheric delamination at -130-120 Ma. The start of magmatism was marked by the formation of gabbro containing spinel lherzolite xeno?liths in the Nanjing-Wuhu Basin (NWB), and its end was marked by the generation of feldspathoid phenocryst-bearing phonolite in the NWB and the Lujiang-Zongyang Basin (LZB), respectively. The association that formed during this episode ranges from alkaline to peralkaline. Taking the Niangniangshan Formation in the NWB as an example, the Nosite phonolite yields a whole-rock monomineral Rb-Sr isochron age of 120 ± 9 Ma, and contains 49.92-60.09 wt.% Si02, 17.67-20.65 wt.% Al_2O_3, 0.08-2.45 wt.% MgO, 1.32-6.62 wt.% FeO~T, and 9.24-13.92 wt.% total alkalis (Na_2O + K_2O). ACNK values range from 0.72 to 1.24, and ANK values from 1.03 to 1.35. The two magmatisms correspond to two episodes of crust-mantle interaction. The first involved intensive interaction between middle-lower crust and underplated basaltic magma derived from the upper mantle lithosphere, whereas the second involved minor interaction between the middle-lower crust and basaltic magma derived from the lower lithospheric mantle.