Geochronology and geochemistry of tuffaceous rocks from the Banxi Group: Implications for Neoproterozoic tectonic evolution of the southeastern Yangtze Block, South China

摘要

The Neoproterozoic tectonic evolution of South China is important for understanding the tectonic assembly of modern East Asia. As a key sedimentary-volcanic sequence, the Banxi Group in South China contains a wealth of information on the genesis of the Nanhua rift basin, which is linked to the assembly and breakup of the Rodinia supercontinent. However, the geochronological and tectonic signatures of the Benxi Group in the Jiangnan orogenic belt are still controversial. In this study, the tuffaceous rocks in the Banxi area were analyzed with regard to their whole-rock major- and trace-element concentrations, and zircons in the tuffaceous rocks were analyzed for LA-(MC)-ICPMS U-Pb ages, trace elements and Hf isotopic compositions. These rocks are well preserved and show high concentrations of SiO2 (61.7-72.3%), K2O (1.7-4.5%), Al2O3 (8.4-18.9%), Rb (55-195 ppm) and Th (4.3-12.4 ppm), low concentrations of TiO2 (0.24-0.95%), Sr (54-128 ppm), Nb (5.6-12.1 ppm) and Ta (0.4-0.9 ppm), LREE enrichment (La-N/Yb-N = 6.6-14.7), and negative Eu anomalies (Eu/Eu* = 0.45-0.85). They have chemical affinities with high-K calc-alkaline S-type granites and exhibit continental volcanic-arc characteristics. The zircons are magmatic in origin and yield a U-Pb age distribution with three peaks: (1) Neoproterozoic (597-949 Ma, mostly 761-788 Ma), (2) Mesoproterozoic (1122-1387 Ma), and (3) Paleoproterozoic (1952-2605 Ma). The Neoproterozoic zircons yield slightly different, but overlapping, mean ages as a function of lithology: 770.2 +/- 3.7 Ma for tuffaceous siltstones, 768.2 +/- 6.7 Ma for tuffs, and 766.4 +/- 5.2 Ma for tuffaceous slates. All zircons in the study units exhibit high U/Yb ratios (0.1-0.6) and low Hf (7106-12554 ppm) and Y (305-6617 ppm) contents, suggesting continental crustal derivation. Some of the zircons have larger LREE contents and smaller Ce anomalies relative to typical magmatic zircons, suggesting later fluid modification. The Neoproterozoic zircons have epsilon Hf(t) values ranging from -16.5 to 16.7 (mostly -8 to 10), corresponding to crustal model ages (T-DM(C)) of 0.97-2.73 Ga. These ages imply a mixture of juvenile (i.e., Neoproterozoic) arc-derived material and Paleoproterozoic heterogeneous crust as magmatic sources for the tuffaceous sequences. Based on our integrated dataset, we infer that the Banxi tuffaceous rocks were extrusive equivalents of S-type granitoids that show volcanic-arc affinities but that were nonetheless deposited in a post-collisional and subsequent rifting-related extensional setting within the newly amalgamated South China Block. We account for this apparent contradiction by proposing that, following collision of the Yangtze and Cathaysia blocks, a rapid breakup of the continental arc caused by deep mantle upwelling led to initiation of post-collisional extension and rifting in the Nanhua Basin.