Geochemistry of Mesozoic Mafic Rocks Adjacent to the Chenzhou-Linwu fault, South China: Implications for the Lithospheric Boundary between the Yangtze and Cathaysia Blocks

摘要

To constrain the Mesozoic tectonic evolution and the lithospheric boundary between the Yangtze and Cathaysia blocks in South China, we present geochronological and geochemical data for Mesozoic basaltic lavas and related mafic dikes west (Group 1) and east (Group 2) of the Chenzhou-Linwu fault. Three episodes of mafic magmatism around the Chenzhou-Linwu fault were identified: ca.175 Ma, 125-150 Ma, and 80-95 Ma, respectively. Group 1 rocks (alkaline basanite and trachybasalt), with ages of >125 Ma, have a wide range of ~87Sr/~86Sr(t) values (0.7035-0.7069), and epsilon_(ND(t)) values (-3.75 to +6.10). In contrast, Group 2 rocks (subalkaline basalt and basaltic andesite), with ages of > 125 Ma, exhibit ~87Sr/~86Sr(t) values of 0.7075-0.7087 and epsilon_(Nd(t)) values of -2.04 to +1.05. Both groups are strongly enriched in incompatible elements, with variable negative Nb-Ta anomalies. However, Group 1 rocks commonly have higher LREE and Ba/Nb, Rb/Nb, Ba/Th, and Ba/La ratios and lower Th/Nb, Th/La, and Zr/Nb ratios than Group 2 rocks. Rocks with ages of 80-95 Ma from both groups have very similar elemental and isotopic compositions (~87Sr/~86Sr(t) = 0.7033-0.7052, epsilon_(Nd(t)) = +3.99 to +8.00), consistent with those of OIB. Strong coupling between incompatible elemental ratios and isotopes suggests that Group 1 rocks might have been derived from an EMI-like continental lithospheric mantle with an OIB source. In contrast, Group 2 rocks come from an EMII-like mantle source contaminated by an OIB component. We conclude that Mesozoic mafic rocks with ages of >125 Ma originated chiefly from an enriched lithospheric mantle heated by ascending asthenosphere, whereas the mafic rocks with ages of ca. 80-95 Ma were derived from upwelling asthenospheric mantle in response to intra-continental lithospheric extension in the South China interior The spatial variations of EMI- and EMII-like source signatures for Mesozoic mafic rocks around the Chenzhou-Linwu fault suggest that the fault represents the Mesozoic lithospheric boundary between the Yangtze and Cathaysia blocks. The Jinxian-Anhua fault was only a near-surface boundary between the sutured blocks. The crust of the Cathaysia block might have been thrust westward over the Yangtze block with a displacement of >400 km at a time no later than ca. 175 Ma. A model for crustal detachment collision (>ca. 175 Ma) and subsequent intra-continental lithospheric extension (175-80 Ma) is proposed for the Mesozoic tectonic evolution of South China.