Thermal and exhumation history of the Songnan Low Uplift, Qiongdongnan Basin: constraints from the apatite fission-track and zircon (U-Th)/He thermochronology
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Abstract:
Significant advancements have been made in the study of Mesozoic granite buried hills in the Songnan Low Uplift (SNLU) of the Qiongdongnan Basin. These findings indicate that the bedrock buried hills in this basin hold great potential for exploration. Borehole samples taken from the granite buried hills in the SNLU were analyzed using apatite fission track (AFT) and zircon (U-Th)/He data to unravel the thermal history of the basement rock. This information is crucial for understanding the processes of exhumation and alteration that occurred after its formation. Thermal modeling of a sample from the western bulge of the SNLU revealed a prolonged cooling event from the late Mesozoic to the Oligocene period (~80−23.8 Ma), followed by a heating stage from the Miocene epoch until the present (~23.8 Ma to present). In contrast, the sample from the eastern bulge experienced a more complex thermal history. It underwent two cooling stages during the late Mesozoic to late Eocene period (~80−36.4 Ma) and the late Oligocene period (~30−23.8 Ma), interspersed with two heating phases during the late Eocene to early Oligocene period (~36.4−30 Ma) and the Miocene epoch to recent times (~23.8−0 Ma), respectively. The differences in exhumation histories between the western and eastern bulges during the late Eocene to Oligocene period in the SNLU can likely be attributed to differences in fault activity. Unlike typical passive continental margin basins, the SNLU has experienced accelerated subsidence after the rifting phase, which began around 5.2 Ma ago. The possible mechanism for this abnormal post-rifting subsidence may be the decay or movement of the deep thermal source and the rapid cooling of the asthenosphere. Long-term and multi-episodic cooling and exhumation processes play a key role in the alteration of bedrock and contribute to the formation of reservoirs. On the other hand, rapid post-rifting subsidence (sedimentation) promotes the formation of cap rocks.
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Key words:
- granite buried hills /
- (U-Th)/He dating /
- fission-track dating /
- exhumation /
- Songnan Low Uplift
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Figure 1. Regional geological outline of the Qiongdongnan Basin (QDNB) (a), and the basin tectonic units (b). Fault distribution and observation points in b are from Zhou et al. (2019).
Figure 2. Comprehensive stratigraphic column of the Qiongdongnan Basin modified after Ji et al. (2021), Ren et al. (2022), and Wang et al. (2015).
Figure 3. Sample geochemical features (a) SiO2 vs. K2O+Na2O (Middlemost, 1994), (b) SiO2 vs. K2O scheme (Rickwood, 1989), (c) 10,000*Ga/Al vs. Ce plot (Whalen et al., 1987), (d) A/CNK vs. A/NK diagram (Maniar and Piccoli, 1989), (d) Rb vs. Y plot (Chappell, 1999), and (f) Y+Nb vs. Rb plot (Pearce et al., 1984).
Figure 4. Chondrite-normalized REE patterns (a) and primitive mantle normalized trace element patterns (b) of the basement samples.
Figure 5. Radial plots of apatite fission-track (left) and confined track length histograms (right). Central ages are calculated using RadialPlotter (Vermeesch, 2009). MTL-mean track length, SD-standard deviation, NL-number of spontaneous tracks.
Figure 6. Modeling results for sample Q1 (a) from the western bulge of the Songnan Low Uplift and sample Q12 (b) from the eastern bulge. Illustrated are the t-T paths on the left (a1 & b1) with the corresponding confined fission-track length frequency distribution (a2 & b2) and the ZHe diffusion profile (b3) on the right. The t-T paths on the left show different fits: green paths, acceptable fit (GOF ≥ 5%); pink paths, good fit (GOF ≥ 50%); black line, weighted mean path.
Figure 7. Comparative presentation of weighted mean paths from thermal models. The dashed line is the weighted mean thermal history for sample Q12 from the east bulge, and the solid line is the weighted mean thermal history for sample Q1 from the west bulge.
Figure 8. Activity rate of the main controlling faults in the Songnan Low Uplift during the Eocene-early Oligocene and Late Oligocene. (a)-No.2 fault, (b)-No.11 fault (Zhou et al., 2019). For locations of the observation points see Fig. 1b.
Table 1. Sample information.
Sample U-Pb Age (Ma) Burial temperature (℃) Lithology Overlying strata Q1 228.9 ± 1.0 ~ 63 Quartaz monzonite Sanya Formation Q12 270.0 ± 1.2 ~ 75 Quartaz monzonite Yacheng Formation 
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Table 2. Apatite fission-track data.
Sample Nc Ns $\rho_{\rm{s}} $ (×105cm−2) 238U (10−6) P($\chi $2)% Central Age (Ma±1$\sigma $) NL MTL (μm±1$\sigma $) SD Dpar (μm±SD) Q1 32 825 2.427 7.22 68 69.2±2.6 24 12.26±0.28 1.39 1.71±0.23 Q12 33 367 3.22 10.49 64 60.1±3.4 18 11.79±0.29 1.26 1.51±0.13 Nc: number of apatite crystals analyzed; Ns: total number of fission tracks counted; $\rho_{\rm{s}} $: spontaneous track density; P($\chi $2): chi-square probability that all single-crystal ages represent a single population of ages where degrees of freedom = Nc-1; NL: number of confined track lengths measured: MTL: Mean confined track length; SD: standard deviation; Dpar: mean track etch pit diameter parallel to the crystallographic c-axis; Apatite-Zeta NIST610=1940±50.
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Table 3. Zircon (U-Th)/He data.
Sample 238U (10−6) ±1$\sigma $ (10−6) 232Th (10−6) ±1$\sigma $ (10−6) He (ncc) ±1$\sigma $ (ncc) Unc. Age (Ma) ±1$\sigma $ (Ma) Rs (μm) FT Cor. Age (Ma) ±1$\sigma $ (Ma) Q1-1 100.5 2.4 41.7 1.0 21.4541 0.2585 368.5 8.8 46.7 0.758 486.1 11.6 Q1-2 97.7 2.3 34.0 0.9 16.1975 0.1605 205.1 4.8 51.5 0.825 248.5 5.8 Q1-3 222.4 5.0 56.0 1.2 22.4005 0.2215 83.4 1.9 57.4 0.804 103.7 2.4 Q12-1 2013.2 44.6 490.6 11.2 26.0363 0.2808 47.2 1.1 40.7 0.727 65.0 1.5 Q12-2 1711.5 36.5 487.5 12.3 12.0109 0.1328 31.2 0.7 34.5 0.689 45.3 1.0 Q12-3 715.1 11.5 174.3 2.3 12.9125 0.1308 57.3 1.0 38.5 0.775 73.9 1.3 Q12-4 334.17 5.02 79.89 0.96 11.3478 0.1149 116.3 2.01 37.0 0.762 152.7 2.64 Rs: sphere equivalent radius of hexagonal crystal; FT: alpha ejection correction factor.
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