
Citation: | Tianyu Zhao, Xin Qian, Qinglai Feng. Geochemistry, zircon U-Pb age and Hf isotopic constraints on the petrogenesis of the Silurian rhyolites in the Loei fold belt and their tectonic implications. Journal of Earth Science, 2016, 27(3): 391-402. doi: 10.1007/s12583-016-0671-y |
The Early Paleozoic was significant for the tectonic evolution of the Gondwana, in combination with the time of the collage of various terranes/blocks and subduction zones along the margin of Gondwana (Zhu et al., 2012). Thailand and its adjacent area comprises numerous terranes/blocks, including South China, Simao, Indochina, Sibumasu, Sukhothai, etc. (Fig. 1). These terranes/blocks are separated by several volcanic belts, including Loei fold belt, Truong Son fold belt, Nan suture and Chiang Mai suture, etc. (Fig. 1) (Qian et al., 2015; Panjasawatwong et al., 2006). The tectonic settings of these volcanic belts are hotly debated. Volcanic rocks play an essential role in flourishing tectonic evolution and mineral resources. The western margin of the Indochina Block may contain multiple suture zones including the Nan suture and Loei fold belt (Fig. 1) (Chonglakmani and Helmcke, 2001). The Loei fold belt has been intensely studied regarding the controversy over its regional tectonic evolution (Vivatpinyo et al., 2014; Udchachon et al., 2011; Panjasawatwong et al., 2006; Intasopa and Dunn, 1994). The volcanic rocks in the Loei fold belt can be separated into eastern, central and western sub-belts (Boonsoong et al., 2011; Panjasawatwong et al., 2006) (Fig. 2). The volcanic rocks of the eastern sub-belt are mainly rhyolitic rocks. The central sub-belt is composed mainly of pillow basalts and hyaloclastites (Udchachon et al., 2011; Khositanont et al., 2008; Intasopa and Dunn, 1994). The western sub-belt is dominated by andesites which have been interpreted as products of arc volcanism active during the the Permo-Triassic; volcanic rocks are also found in the Phetchabun belt, which is the southern extension of the western Loei sub-belt (Kamvong et al., 2014; Salam et al., 2014; Boonsoong et al., 2011; Panjasawatwong et al., 2006; Intasopa and Dunn, 1994; Jungyusuk and Khositanont, 1992; Bunopas and Vella, 1983). Intasopa and Dunn (1994) firstly suggested that these rhyolites formed in an arc setting during the subduction of the Cathaysia Block and yielded a whole-rock Rb-Sr isochron age of 374±33 Ma (Late Devonian). Khositanont et al. (2008) reported U-Pb zircon ages of 425±7 and 433±4 Ma (Early Silurian) for the volcaniclastics in the eastern sub-belt. However, the tectonic evolution and magmatic processes of rhyolites in the eastern Loei sub-belt have been poorly studied and its northern extension is still debated.
Because of these different views and problems, we carried out a geological survey in the Loei fold belt. The purposes of this paper are to present a new LA-ICP-MS U-Pb zircon age, zircon Hf isotopic compositions and geochemical data of the rhyolites from the eastern sub-belt in northeastern (NE) Thailand. The aims of this study include: (1) to constrain the formation time and geochemical characteristics of the rhyolites; (2) to probe into the origin and petrogenesis of the rhyolites; (3) discuss their tectonic setting.
The study area is located in the northeastern portion of the folded mountain range west of the Khorate Plateau (Fig. 1). The oldest rocks are Silurian volcanic rocks, including rhyolites, andesites and tuffs, which outcrop in the eastern part of the Loei area. They are in fault contact with Carboniferous rocks. The Silurian to Devonian sedimentary rocks consist of shales, fine-grained tuffaceous, sandstones and siltstones. They are exposed along the eastern map area (Fig. 2). The Carboniferous sedimentary rocks are mainly shales and reef limestones. The volcanic rocks are divided into three sub-belts along the eastern, western and central parts of the Loei area. They are Silurian, Devonian-Carboniferous and Permo-Triassic. The volcanic rocks of western sub-belt are mainly granites and rhyolites. They are products of volcanism along an active continental margin during Permian-Triassic time (Boonsoong et al., 2011; Khositanont et al., 2008). The central sub-belt involves the Late Devonian-Early Carboniferous pillow lava, hyaloclastite, and pillow breccia (Panjasawatwongg et al., 2006). The eastern sub-belt rocks show poor relations to the other rocks, including rhyolites, andesite and volcaniclastic rocks. This paper concentrates on the rhyolites of the eastern sub-belt (Fig. 2).
Five fresh samples were selected from the eastern Loei sub-belt in NE Thailand (18°9.910′N, 102°10.508′E). They are fine-grained rhyolites with porphyritic texture, the main phenocryst phases (30 wt.%-40 wt.%) of which are quartz, sanidine and plagioclase. Crystal sizes vary from 0.5 to 4 mm. Quartz phenocrysts show dissolution textures. Plagioclases are subhedral to euhedral with sizes ranging from 0.5 to 2 mm. The groundmass is principally felsitic and microspherulitic.
Zircons from sample 13T38 were separated by standard heavy liquid and magnetic techniques, mounted in epoxy and then polished. The zircon grains CL images for observing internal structures were accomplished by JEOL JXA-8100 electron microscope. Zircon U-Pb dating was performed on LA-ICP-MS at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences, Wuhan. Detailed analytical procedure followed Liu et al. (2010). Laser sampling was conducted using a GeoLas 2005 system and ion-signal intensities were performed by Agilent 7500a ICP-MS. The spot size was 32 μm, with each analysis including a background gathering of around 20 and 50 s of data acquisition from the samples. Off-line selection and integration of background and analyte signals, and time-drift correction and quantitative calibration for U-Pb dating were performed using ICPMSDataCal (Liu et al., 2009). External standard zircon 91500 was used as primary reference material for U-Pb dating. Concordia diagrams and weighted mean calculations were made using Isoplot/ Ex_ver3 (Ludwig, 2003). The analytical results for the samples are shown in Table 1.
Spot No. | Concentration | Th/U | Isotope ratio | Calculated apparent age (Ma) | ||||||||||||||||
Th | U | 207Pb/206Pb | 207Pb/235U | 206Pb/238U | 207Pb/206Pb | 207Pb/235U | 206Pb/238U | |||||||||||||
(ppm) | Ratio | ±1σ | Ratio | ±1σ | Ratio | ±1σ | Age | ±1σ | Age | ±1σ | Age | ±1σ | ||||||||
13T38-1 | 285 | 521 | 0.55 | 0.056 3 | 0.002 5 | 0.534 6 | 0.024 2 | 0.068 4 | 0.000 8 | 465 | 98 | 435 | 16 | 427 | 5 | |||||
13T38-3 | 559 | 1 077 | 0.52 | 0.057 9 | 0.002 1 | 0.538 2 | 0.019 3 | 0.067 0 | 0.000 7 | 528 | 75 | 437 | 13 | 418 | 4 | |||||
13T38-4 | 449 | 886 | 0.51 | 0.058 0 | 0.002 2 | 0.539 6 | 0.020 0 | 0.067 2 | 0.000 8 | 528 | 82 | 438 | 13 | 419 | 5 | |||||
13T38-5 | 246 | 478 | 0.52 | 0.057 7 | 0.003 0 | 0.532 9 | 0.028 4 | 0.066 4 | 0.000 8 | 517 | 115 | 434 | 19 | 414 | 5 | |||||
13T38-6 | 868 | 1 298 | 0.67 | 0.056 5 | 0.001 9 | 0.537 5 | 0.017 9 | 0.068 3 | 0.000 7 | 478 | 74 | 437 | 12 | 426 | 4 | |||||
13T38-7 | 433 | 635 | 0.68 | 0.049 1 | 0.003 3 | 0.455 6 | 0.028 9 | 0.067 8 | 0.001 2 | 154 | 152 | 381 | 20 | 423 | 7 | |||||
13T38-8 | 469 | 1 071 | 0.44 | 0.055 3 | 0.001 9 | 0.538 0 | 0.018 7 | 0.070 0 | 0.000 7 | 433 | 76 | 437 | 12 | 436 | 4 | |||||
13T38-9 | 679 | 1 004 | 0.68 | 0.055 2 | 0.00 2 0 | 0.519 8 | 0.018 6 | 0.068 1 | 0.000 8 | 420 | 77 | 425 | 12 | 425 | 5 | |||||
13T38-10 | 271 | 585 | 0.46 | 0.055 1 | 0.002 3 | 0.518 7 | 0.021 5 | 0.068 4 | 0.000 8 | 417 | 94 | 424 | 14 | 426 | 5 | |||||
13T38-12 | 517 | 917 | 0.56 | 0.056 7 | 0.001 9 | 0.537 2 | 0.017 5 | 0.068 9 | 0.000 7 | 480 | 44 | 437 | 12 | 430 | 4 | |||||
13T38-13 | 446 | 951 | 0.47 | 0.055 8 | 0.001 8 | 0.524 8 | 0.015 9 | 0.068 7 | 0.000 8 | 443 | 75 | 428 | 11 | 428 | 5 | |||||
13T38-14 | 452 | 749 | 0.6 | 0.055 1 | 0.002 2 | 0.512 1 | 0.020 0 | 0.067 5 | 0.000 7 | 417 | 89 | 420 | 13 | 421 | 4 | |||||
13T38-15 | 235 | 464 | 0.51 | 0.059 0 | 0.002 9 | 0.556 2 | 0.028 3 | 0.067 9 | 0.000 9 | 569 | 138 | 449 | 18 | 423 | 5 | |||||
13T38-16 | 355 | 690 | 0.51 | 0.053 7 | 0.002 3 | 0.506 3 | 0.020 4 | 0.068 7 | 0.000 9 | 361 | 96 | 416 | 14 | 428 | 5 | |||||
13T38-17 | 333 | 577 | 0.58 | 0.055 1 | 0.002 2 | 0.515 4 | 0.020 2 | 0.068 3 | 0.001 0 | 417 | 93 | 422 | 14 | 426 | 6 | |||||
13T38-18 | 497 | 1 136 | 0.44 | 0.053 2 | 0.002 0 | 0.489 6 | 0.017 7 | 0.066 8 | 0.000 9 | 339 | 85 | 405 | 12 | 417 | 5 | |||||
13T38-19 | 260 | 515 | 0.5 | 0.050 1 | 0.002 5 | 0.465 1 | 0.022 5 | 0.067 6 | 0.000 9 | 198 | 115 | 388 | 16 | 422 | 5 | |||||
13T38-20 | 474 | 743 | 0.64 | 0.053 1 | 0.002 3 | 0.492 1 | 0.021 6 | 0.066 7 | 0.001 0 | 332 | 101 | 406 | 15 | 416 | 6 | |||||
13T38-22 | 597 | 1 050 | 0.57 | 0.052 5 | 0.002 4 | 0.492 6 | 0.022 3 | 0.067 9 | 0.000 8 | 306 | 107 | 407 | 15 | 423 | 5 | |||||
13T38-23 | 497 | 1 157 | 0.43 | 0.055 2 | 0.002 0 | 0.504 0 | 0.018 0 | 0.066 0 | 0.000 8 | 420 | 80 | 414 | 12 | 412 | 5 | |||||
13T38-24 | 1 315 | 2 324 | 0.57 | 0.053 7 | 0.001 6 | 0.511 5 | 0.015 0 | 0.068 9 | 0.000 7 | 367 | 73 | 419 | 10 | 429 | 4 |
Lu-Hf isotope experiments were carried out using a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Germany) in combination with a Geolas 2005 excimer ArF laser ablation system (Lambda Physik, Göttingen, Germany) that was hosted at GPMR, China University of Geosciences in Wuhan. The single spot diameter is 44 μm for all zircon grains. The 91500 zircon standard was used for calibration. Detailed operating conditions for the laser ablation system and the MC-ICP-MS instrument and analytical method followed Hu et al. (2012). Off-line selection and integration of analyte signals, and mass bias calibrations were performed using ICPMSDataCal (Liu et al., 2009). The analytical results for the samples are shown in Table 2.
Spot No. | Age (Ma) | 176Hf/77Hf | ±1σ | 176Lu/177Hf | ±1σ | 176Yb/177Hf | ±1σ | T1DM (Ma) | T2DM (Ma) | εHf(t) |
13T38-1 | 427 | 0.282 662 | 0.000 011 | 0.001 356 | 0.000 020 | 0.043 524 | 0.000 994 | 844 | 1 003 | 5.11 |
13T38-3 | 418 | 0.282 671 | 0.000 016 | 0.002 819 | 0.000 055 | 0.082 792 | 0.001 539 | 865 | 1 011 | 4.84 |
13T38-4 | 419 | 0.282 652 | 0.000 014 | 0.001 378 | 0.000 034 | 0.044 657 | 0.001 487 | 858 | 1 025 | 4.61 |
13T38-7 | 423 | 0.282 683 | 0.000 017 | 0.002 832 | 0.000 085 | 0.094 734 | 0.002 881 | 847 | 985 | 5.38 |
13T38-13 | 428 | 0.282 656 | 0.000 010 | 0.001 277 | 0.000 011 | 0.040 993 | 0.000 323 | 850 | 1 013 | 4.96 |
13T38-14 | 421 | 0.282 645 | 0.000 011 | 0.001 049 | 0.000 011 | 0.035 754 | 0.000 302 | 861 | 1 033 | 4.49 |
13T38-15 | 423 | 0.282 644 | 0.000 013 | 0.001 098 | 0.000 008 | 0.042 530 | 0.000 686 | 863 | 1 035 | 4.49 |
13T38-16 | 428 | 0.282 631 | 0.000 014 | 0.001 274 | 0.000 012 | 0.047 696 | 0.000 563 | 885 | 1 061 | 4.09 |
13T38-18 | 417 | 0.282 647 | 0.000 013 | 0.001 143 | 0.000 014 | 0.041 932 | 0.000 764 | 859 | 1 032 | 4.46 |
13T38-22 | 423 | 0.282 652 | 0.000 013 | 0.001 743 | 0.000 044 | 0.064 172 | 0.001 461 | 866 | 1 029 | 4.59 |
13T38-24 | 429 | 0.282 638 | 0.000 017 | 0.002 369 | 0.000 078 | 0.074 833 | 0.001 537 | 902 | 1 065 | 4.03 |
13T38-20 | 416 | 0.282 654 | 0.000 015 | 0.001 397 | 0.000 005 | 0.052 997 | 0.000 546 | 855 | 1 022 | 4.61 |
Major elements were analyzed by X-ray fluorescence (XRF) method at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences in Wuhan, the analysis precision is generally better than 5 wt.%. Whole rock samples were crushed in a corundum jaw crusher (to 60 meshes). About 60 g was powdered in an agate ring mill to less than 200 meshes. The samples were then digested by HF+HNO3 in Teflon bombs and analyzed with an Agilent 7500a ICP-MS at the GPMR. The detailed sample-digesting procedure for ICP-MS analyses and analytical precision and accuracy for trace elements are the same as description by (Liu et al., 2008). The analytical results for the samples are shown in Table 3.
Sample | 13T38-1 | 13T38-2 | 13T38-3 | 13T38-4 | 13T38-5 |
Major oxide (wt.%) | |||||
SiO2 | 77.00 | 76.78 | 76.47 | 76.91 | 75.61 |
TiO2 | 0.22 | 0.21 | 0.21 | 0.24 | 0.23 |
Al2O3 | 12.10 | 12.71 | 12.72 | 12.67 | 13.13 |
FeOt | 2.20 | 2.21 | 2.20 | 0.98 | 2.24 |
MnO | 0.07 | 0.07 | 0.07 | 0.03 | 0.07 |
MgO | 0.38 | 0.51 | 0.51 | 0.24 | 0.44 |
CaO | 0.50 | 0.38 | 0.37 | 0.26 | 0.61 |
Na2O | 3.64 | 3.13 | 3.27 | 3.74 | 3.63 |
K2O | 2.97 | 3.39 | 3.44 | 3.25 | 3.50 |
P2O5 | 0.04 | 0.04 | 0.05 | 0.03 | 0.06 |
LOI | 0.87 | 1.23 | 1.15 | 1.07 | 1.00 |
Total | 99.99 | 100.66 | 100.46 | 99.42 | 100.52 |
A/CNK | 1.20 | 1.34 | 1.30 | 1.25 | 1.21 |
Na2O+K2O | 6.62 | 6.52 | 6.72 | 7.00 | 7.13 |
Na2O/K2O | 0.82 | 0.87 | 0.96 | 1.05 | 1.08 |
Mg# | 26.00 | 31.00 | 31.00 | 32.00 | 28.00 |
Trace element (ppm) | |||||
La | 28.44 | 22.55 | 21.88 | 55.96 | 28.89 |
Ce | 54.17 | 45.85 | 45.52 | 90.53 | 54.50 |
Pr | 6.57 | 5.74 | 5.78 | 13.10 | 6.64 |
Nd | 24.60 | 21.37 | 21.77 | 48.60 | 24.18 |
Sm | 5.06 | 4.53 | 4.57 | 10.22 | 4.84 |
Eu | 0.91 | 0.89 | 0.86 | 1.94 | 0.87 |
Gd | 4.47 | 4.20 | 4.41 | 9.70 | 4.57 |
Tb | 0.79 | 0.78 | 0.80 | 1.73 | 0.82 |
Dy | 4.71 | 4.69 | 4.78 | 9.87 | 4.96 |
Ho | 0.98 | 1.00 | 1.02 | 1.92 | 1.03 |
Er | 3.11 | 3.11 | 3.16 | 5.31 | 3.16 |
Tm | 0.54 | 0.53 | 0.55 | 0.87 | 0.55 |
Yb | 3.23 | 3.14 | 3.20 | 4.96 | 3.26 |
Lu | 0.51 | 0.49 | 0.49 | 0.70 | 0.49 |
Y | 30.34 | 29.18 | 31.28 | 51.86 | 31.88 |
Li | 17.02 | 24.24 | 23.79 | 7.58 | 19.97 |
Be | 1.37 | 1.58 | 1.46 | 1.38 | 1.71 |
Sc | 8.91 | 7.73 | 8.74 | 8.97 | 8.62 |
Co | 69.00 | 58.01 | 58.58 | 54.50 | 72.44 |
Cu | 0.94 | 0.35 | 0.22 | 0.55 | 0.47 |
Zn | 33.91 | 49.89 | 50.53 | 27.87 | 35.33 |
Ga | 10.71 | 14.21 | 14.02 | 12.94 | 13.69 |
Rb | 68.80 | 81.97 | 82.83 | 88.77 | 84.88 |
Zr | 134.90 | 133.50 | 131.90 | 144.50 | 142.50 |
Nb | 7.67 | 7.60 | 8.95 | 10.16 | 8.47 |
Mo | 0.10 | 0.08 | 0.08 | 0.13 | 0.09 |
Cs | 2.58 | 3.64 | 3.81 | 2.74 | 2.69 |
Hf | 4.68 | 4.67 | 4.72 | 5.03 | 4.97 |
Ta | 0.82 | 0.80 | 0.79 | 0.86 | 0.91 |
Pb | 8.10 | 7.85 | 8.48 | 7.81 | 8.01 |
Th | 7.86 | 7.40 | 7.03 | 6.68 | 6.02 |
U | 2.47 | 2.55 | 2.68 | 2.53 | 2.39 |
Sn | 2.04 | 2.33 | 2.43 | 2.34 | 2.52 |
Ba | 908.99 | 1 431.28 | 1 427.61 | 1 340.43 | 1 385.20 |
Cr | 7.40 | 7.22 | 7.22 | 4.14 | 7.68 |
Ni | 2.90 | 2.63 | 2.34 | 2.15 | 3.26 |
Trace element (ppm) | |||||
Sr | 144.32 | 154.32 | 148.45 | 186.63 | 209.56 |
V | 8.50 | 8.90 | 8.50 | 7.44 | 9.12 |
∑REE | 138.08 | 118.85 | 118.79 | 255.41 | 138.76 |
(La/Yb)N | 4.60 | 4.84 | 5.94 | 5.97 | 7.61 |
δEu | 0.63 | 0.60 | 0.59 | 0.58 | 0.56 |
Tzr (℃) | 791.30 | 799.10 | 795.50 | 800.50 | 795.00 |
CIPW (%) | |||||
Q | 42.94 | 44.16 | 42.86 | 41.93 | 39.30 |
C | 2.10 | 3.33 | 3.06 | 2.65 | 2.40 |
Or | 17.58 | 20.03 | 20.35 | 19.21 | 20.69 |
Ab | 30.81 | 26.51 | 27.69 | 31.68 | 30.70 |
An | 2.14 | 1.53 | 1.50 | 0.96 | 2.66 |
Hy | 0.95 | 1.27 | 1.26 | 0.59 | 1.09 |
Il | 0.15 | 0.15 | 0.15 | 0.06 | 0.15 |
Ru | 0.14 | 0.13 | 0.13 | 0.20 | 0.15 |
Ap | 0.12 | 0.12 | 0.13 | 0.12 | 0.13 |
Sum | 96.94 | 97.23 | 97.12 | 97.39 | 97.27 |
DI | 91.33 | 90.70 | 90.90 | 92.82 | 90.69 |
FeOt represents total Fe-oxides; Mg#=molar Mg×100/(Mg+Fe); LOI. loss on ignition; A/CNK=molar Al2O3/(CaO+Na2O+K2O); DI=Q+Or+Ab. |
The zircon grains of sample are mainly euhedral and exhibit a prismatic crystal form with oscillatory magmatic zoning in CL images, ranging in length/width ratios of 3 : 1-3 : 2 (Fig. 3a). Twenty-one analyzed zircons have varying U (464 ppm-2 324 ppm) and Th (235 ppm-1 315 ppm) contents with Th/U ratios ranging from 0.43 to 0.68, in conformity with a magmatic origin (Hoskin and Schaltegger, 2003). Twenty-one analyses are concordant and yield a weighted mean 206Pb/238U age of 423.7±2.7 Ma (MSWD=1.5, n=21 (Figs. 3b, 3c).
Twelve dated zircon grains from samples 13T38 were analyzed for Lu-Hf isotope ratios (Table 2). Their 176Hf/177Hf values vary from 0.282 631 to 0.282 683. The 176Lu/177Hf ratios range from 0.001 049 to 0.002 832, indicating a low radiogenic growth of 176Hf. Samples have positive εHf(t) values of 4.03 to 5.38 (average=4.63) and two-stage model age (T2DM) range from 985 to 1 065 Ma (Table 2, Fig. 4), suggesting an origin from a juvenile crust.
The Loei rhyolites show high SiO2 (75 wt.%-77 wt.%), Al2O3 (12.10 wt.%-13.13 wt.%), K2O (2.97 wt.%-3.50 wt.%) and low MgO (0.24 wt.%-0.51 wt.%) and P2O5 (0.03 wt.%-0.06 wt.%). The K2O+Na2O values vary between 6.61 wt.% and 7.12 wt.%. The molecular A/CNK (molar Al2O3/(CaO+Na2O+K2O) ratios of the samples range from 1.19 to 1.34, classified as characteristics of strongly peraluminous (Fig. 5a) (Maniar and Piccoli, 1989). The differentiation index (DI) varies between 90.70 and 92.82, suggesting that the magma was highly evolved. Samples plot within the field of calc- alkaline series on AFM diagram (Fig. 5c) (Irvine and Baragar, 1971). In the K2O+Na2O-SiO2 and Zr/TiO2-Nb/Y diagrams (Figs. 5b, 5d) (Le Bas et al., 1986; Winchester and Floyd, 1977), all samples plot in the field of rhyolite and rhyodacite/dacite.
In the primitive mantle-normalized multielement spidergram (Fig. 6a), all samples are enriched in large ion lithophile elements (LILEs) (e.g., Ba, K, Pb) and LREEs and depleted in high field-strength elements (HFSEs) (e.g., Nb, Ta, Ti). The total rare earth elements (∑REE) of the Loei rhyolites range from 118.79 ppm to 255.40 ppm (averaging 153.98 ppm). These samples display similar chondrite-normalized REE patterns (Fig. 6b). Samples have enriched LREEs with (La/Yb)N (N herein refers to chondrite-normalized value) ratios ranging from 4.60 to 7.61. The rhyolites show obvious negative Eu-anomalies (δEu=0.58-0.63). The calculated Zr saturation temperatures of the rhyolites range from 791 to 800 ℃ (Table 3).
The emplacement age of Loei rhyolites has been confusing because of its poor contacts with other stratigraphical well-defined rocks. Intasopa and Dunn (1994) reported a whole-rock Rb-Sr isochron age of 374±33 Ma (Late Devonian). Khositanont et al. (2008) reported U-Pb zircon ages of 425±7 and 433±4 Ma (Early Silurian) for the volcaniclastics in the eastern sub-belt. Our zircon grains from the rhyolites in the eastern sub-belt of Loei fold belt yield a weighted mean 206Pb/238U age of 423.7±2.7 Ma, indicating a Middle Silurian emplacement of the rhyolites. Detrital zircons from sedimentary rocks in the Loei area record a distinct age peak at ~425 Ma (Burrett et al., 2014). Consequently, we proposed a significant magmatism event which took place in the western margin of the Indochina Block during the Middle Silurian.
Our samples are fresh samples and they have low LOI. They show Zr correlating with Rb and Ba, suggesting that their mobility of LILEs (e.g., Rb, Sr and Ba) after eruption of the rocks does not seriously change their contents (Wang Q F et al., 2014), therefore most of the data/index are available.
The high SiO2 (75 wt.%-77 wt.%), total alkali (K2O+Na2O =6.61 wt.%-7.12 wt.%), differentiation index of the Loei rhyolites samples (Table 3), suggest that the Loei rhyolites highly evolved. Although the high A/CNK values (1.20-1.34) resemble those of the S-type granites, however, different types of highly evoloved rocks (I-, S- and A-type) tend to have similar geochemical features, A/CNK will become increasingly for both I-type and S-type granitoids with sendimentary material involved or the process of fractional crystallization (Tao et al., 2013). Moreover, the positive εHf(t) of Loei rhyolites indicate that the protolith of Loei rhyolites were juvenile crust which is similar to the I-type granites. In the Rb/Sr-Rb/Ba diagram (Fig. 7) (Sylvester, 1998), all samples plot within the clay-poor area. Loei rhyolites also have high εNd(t) (-1.35-0.39) (Intasopa and Dunn, 1994), suggesting that they were from the reworking of juvenile crust rather than ancient materials.
P2O5 is an important feature to differentiate between the I-type granitoids and S-type granitoids because of apatite is soluble in peraluminous melt and P2O5 would soar with SiO2 content during magmatic differentiation in S-type melts and decrease with SiO2 in I-type (Li et al., 2007; Chappell, 1999). Samples show a decreasing trend of P2O5 contents with increasing SiO2 (Fig. 8b). On the K2O versus Na2O diagram (Liu et al., 2014) (Fig. 8a), they also show I-type trend. Furthermore, in the FeOt/MgO and (Na2O+K2O)/CaO versus Zr+Nb+Ce+Y classification diagrams (Whalen et al., 1987), all samples plot within the field of Fractionated granites (Figs. 7c, 7d).
The geochemical features of Loei rhyolites are significantly different from the A-type granite: (1) the A-type granites are rich in HFSEs and depleted in Ba and Sr (Wu et al., 2003b; Whalen et al., 1987), whereas the Loei rhyolite show lower HFSEs and higher in Ba, Sr, (2) low FeOt/MgO (3.73-5.18) are different from the A-type granite (FeOt/MgO > 10 (Whalen et al., 1987), (3) in the Nb, Zr, Na2O+K2O, K2O/MgO-Ga/Al diagrams (Fig. 9) (Whalen et al., 1987), all samples plot outside the field of the A-types, this, combined with the relatively low petrogenetic temperatures (whole rocks Zr saturation temperatures: 791-800 ℃), excludes the possibility of the A-type granitoids.
Loei rhyolites are depleted in Nb, Ta, Sr, P, Ti and Eu, therefore, primitive magma experienced fractional crystallizaiton. Depletions in Nb, Ta, and Ti commonly ascertain the fractionation of Ti-bearing phases. Apatite fractionation explains negative P concentration. Negative Eu and Sr manifest separation of plagioclase. Collectively, Loei rhyolites are highly evolved I-type rocks which have experienced extensive fractional crystallization.
Zircon Lu-Hf isotope is an effective geochemical tracer in protolith and petrogenesis (Zhao et al., 2014; Dong et al., 2013; Ji et al., 2009; Kinny and Maas, 2003). Magma derived from the mantle or juvenile crust has relatively higher 176Lu/177Hf and positive εHf(t) values, whereas low 176Lu/177Hf and negative εHf(t) values indicate derivation from ancient crust. Mixing of two end-members of juvenile crust and ancient crust, however, may produce varying εHf(t) values.
The εHf(t) values in the Loei rhyolites zircon grains are uniform and positive (4.03-5.38), which excludes the possibility of mingling of mantle-derived and crustal-derived magmas because it would produce a wide range of εHf(t) values. Besides, the hybrid melts of mantle mafic materials and crust felsic materials will result in elevated Mg# and low content of SiO2, however, which is not seen in the studied rocks. On the other hand, the fractional crystallization of basaltic magmas is another model to explicate petrogenesis of highly fractionated I-type granites (Zhang et al., 2013; Wu et al., 2003a, b). Experiments have shown that mafic magmas can produce 12 wt.%-25 wt.% of granitoid magmas by differentiation (Sisson et al., 2005), suggesting that considerable basaltic materials would have been produced in the same area. However, no mafic or ultramafic rocks so far have been found or reported in the study area. In conclusion, the Loei rhyolites may have been produced by the partial melting of juvenile crust. Further, zircon Hf T2DM (985 to 1 065 Ma) ages are older than zircon U-Pb age, suggesting the reworking of juvenile crustal sources and significant Neoproterozoic crustal growth. Neoproterozoic to Paleozoic is a main period for crustal formation of the Indochina Block (Lan et al., 2003), as well as assemblage of Gondwana (Cawood et al., 2013).
The concentration of LILEs of the Loei rhyolite is high (such as Ba, K, Pb). They are relatively depleted in HFSEs (such as Th, Nb, Ta, Zr, Ti). These are main features of magmas formed in an active continental fringe related subduction setting (Qi et al., 2014). In the La/Yb versus Th/Yb tectonic discrimination diagram (Fig. 10b) (Condie, 1989), all the Loei rhyolites plot in the field of continental margin-arc. In a series of discrimination diagrams based on trace elements, most samples fall within the 'volcanic arc granitoid' (Figs. 10a, 10c, 10d) (Harris et al., 1986; Pearce et al., 1984). Enriched LREEs combined with negative Eu patterns are also the property of subduction-related granitoids (Peter and Silver, 1983). Therefore, these rocks may have formed in a subduction-related setting.
The Truong Son fold belt, located in central Vietnam and northeastern Laos, is one of the most important tectonic and metallogenic terranes in the Indochina Block. It presents a NW-SE trending elongated fold belt, and extends from the northern Song Ma suture, which defines the boundary between the Indochina and South China blocks, to the southern Tamky-Phuoc Son-Xepon (Laos) suture, which marks the boundary with the Kontum massif (Shi et al., 2015).
The Truong Son fold belt is in contact with Kontum massif which has experienced the same thermal and deformation history. Ordovician-Silurian (ca. 462-422 Ma) peraluminous granitoids which were metamorphosed in the Triassic are also reported in Kontum massif, suggesting the presence of the Ordovician-Silurian volcanic arc magmatism in the region (Nakano et al., 2013), metamorphism and felsic magmatism occurred at 430 Ma according to U-Pb dating of zircons and monazites, suggesting a regional subduction of ocean plate along the northeastern Gondwana margin (Tran et al., 2014). Granites from the Song Chay complex in northern Vietnam have zircon U-Pb age of 428±5 Ma (Roger et al., 2000). Moreover, significant volumes of arc-related mafic rocks occurred in northern and central Vietnam, they may represent a possible heat source for the Ordovician-Silurian low-P/T metamorphism (Nakano et al., 2013). The stratigraphical features of the Loei area are in good agreement with the regional geology of northern Vietnam and the Truong Son fold belt, where Pre-Devonian low-grade metasediments and unmetamorphic Devonian and younger sediments are separated by an angular unconformity, which is called "Caledonian" in the South China Block (Roger et al., 2000; Hutchison, 1989). Therefore, the Loei fold belt and the Truong Son fold belt contain widespread Silurian arc-related igneous rocks, suggesting that the Loei fold belt was in contact with the Truong Son fold belt. However, more study is needed for better understanding the extension of the Loei fold belt in Laos.
The Indochina Block was originated from eastern Gondwana and rifted since the Early Devonian (Lehmann et al., 2013; Metcalfe, 2013). The Silurian arc-related rhyolites in the western margin of the Indochina indicate that the arc developed in response to the subduction of Proto-Tethys oceanic plate underneath the Indochina Block before the opening of the Devonian Loei ocean (ca. 361 Ma) (Intasopa and Dunn, 1994). Lehmann et al. (2013) reported Mid-Silurian dacite volcanism and associated volcanic-hosted massive sulfide (VHMS) mineralization at Dapingzhang in the Lancangjiang zone of the Yunnan Province in South China. The Dapingzhang Mid-Silurian (429 Ma) dacite gave εNd(t) values of +2 to +5, suggesting a mantle sources (Lehmann et al., 2013), which is similar to Loei rhyolites. Moreover, The Late Silurian arc-related igneous rocks are present at the Dazhonghe in the western margin of Simao Block which also have postive εNd(t) values of 3.86 to 4.89. Therefore, we argue that the Simao Block may be contiguous with the Indochina Block during the Silurian. A similar conclusion was put forward previously by (Wang Y et al., 2014) based on detrital zircon data.
(1) Zircon U-Pb ages indicate that the Loei rhyolites were erupted at 423.7±2.7 Ma.
(2) The Loei rhyolites are characterized by high SiO2 and low P2O5. They are highly fractionated I-type rhyolites which were formed in an arc environment. They are the products of the reworking of juvenile crustal sources.
(3) The Simao Block may be contiguous with the Indochina Block during the Silurian. The Loei fold belt and the Truong Son fold belt contain widespread Silurian arc-related igneous rocks, suggesting that the Loei fold belt was in contact with the Truong Son fold belt during the Early Paleozoic.
ACKNOWLEDGMENTS: Two anonymous reviewers and editors are thanked for their constructive comments. This work was supported by the NSFC (Nos. 41172202 and 41190073), the China Geological Survey (No. 1212011121256), the Ministry of Education of China (No. 20110145130001), and the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences in Wuhan (No. MSFGPMR201402). The final publication is available at Springer via http://dx.doi.org/10.1007/s12583-016-0671-y.Barr, S. M., Macdonald, A. S., Ounchanum P., et al., 2006. Age, Tectonic Setting and Regional Implications of the Chiang Khong Volcanic Suite, Northern Thailand. Journal of the Geological Society, 163(6): 1037-1046. doi: 10.1144/0016-76492005-118 |
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11. | Hidetoshi Hara, Tetsuya Tokiwa, Toshiyuki Kurihara, et al. Revisiting the tectonic evolution of the Triassic Palaeo-Tethys convergence zone in northern Thailand inferred from detrital zircon U–Pb ages. Geological Magazine, 2021, 158(5): 905. doi:10.1017/S0016756820001028 | |
12. | Quyen Minh Nguyen, Qinglai Feng, Jian-Wei Zi, et al. Cambrian intra–oceanic arc trondhjemite and tonalite in the Tam Ky–Phuoc Son Suture Zone, central Vietnam: Implications for the early Paleozoic assembly of the Indochina Block. Gondwana Research, 2019, 70: 151. doi:10.1016/j.gr.2019.01.002 | |
13. | Xin Qian, Yuejun Wang, Yuzhi Zhang, et al. Petrogenesis of Permian–Triassic felsic igneous rocks along the Truong Son zone in northern Laos and their Paleotethyan assembly. Lithos, 2019, 328-329: 101. doi:10.1016/j.lithos.2019.01.006 | |
14. | Tianyu Zhao, Thomas J. Algeo, Qinglai Feng, et al. Tracing the provenance of volcanic ash in Permian–Triassic boundary strata, South China: Constraints from inherited and syn-depositional magmatic zircons. Palaeogeography, Palaeoclimatology, Palaeoecology, 2019, 516: 190. doi:10.1016/j.palaeo.2018.12.002 | |
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Spot No. | Concentration | Th/U | Isotope ratio | Calculated apparent age (Ma) | ||||||||||||||||
Th | U | 207Pb/206Pb | 207Pb/235U | 206Pb/238U | 207Pb/206Pb | 207Pb/235U | 206Pb/238U | |||||||||||||
(ppm) | Ratio | ±1σ | Ratio | ±1σ | Ratio | ±1σ | Age | ±1σ | Age | ±1σ | Age | ±1σ | ||||||||
13T38-1 | 285 | 521 | 0.55 | 0.056 3 | 0.002 5 | 0.534 6 | 0.024 2 | 0.068 4 | 0.000 8 | 465 | 98 | 435 | 16 | 427 | 5 | |||||
13T38-3 | 559 | 1 077 | 0.52 | 0.057 9 | 0.002 1 | 0.538 2 | 0.019 3 | 0.067 0 | 0.000 7 | 528 | 75 | 437 | 13 | 418 | 4 | |||||
13T38-4 | 449 | 886 | 0.51 | 0.058 0 | 0.002 2 | 0.539 6 | 0.020 0 | 0.067 2 | 0.000 8 | 528 | 82 | 438 | 13 | 419 | 5 | |||||
13T38-5 | 246 | 478 | 0.52 | 0.057 7 | 0.003 0 | 0.532 9 | 0.028 4 | 0.066 4 | 0.000 8 | 517 | 115 | 434 | 19 | 414 | 5 | |||||
13T38-6 | 868 | 1 298 | 0.67 | 0.056 5 | 0.001 9 | 0.537 5 | 0.017 9 | 0.068 3 | 0.000 7 | 478 | 74 | 437 | 12 | 426 | 4 | |||||
13T38-7 | 433 | 635 | 0.68 | 0.049 1 | 0.003 3 | 0.455 6 | 0.028 9 | 0.067 8 | 0.001 2 | 154 | 152 | 381 | 20 | 423 | 7 | |||||
13T38-8 | 469 | 1 071 | 0.44 | 0.055 3 | 0.001 9 | 0.538 0 | 0.018 7 | 0.070 0 | 0.000 7 | 433 | 76 | 437 | 12 | 436 | 4 | |||||
13T38-9 | 679 | 1 004 | 0.68 | 0.055 2 | 0.00 2 0 | 0.519 8 | 0.018 6 | 0.068 1 | 0.000 8 | 420 | 77 | 425 | 12 | 425 | 5 | |||||
13T38-10 | 271 | 585 | 0.46 | 0.055 1 | 0.002 3 | 0.518 7 | 0.021 5 | 0.068 4 | 0.000 8 | 417 | 94 | 424 | 14 | 426 | 5 | |||||
13T38-12 | 517 | 917 | 0.56 | 0.056 7 | 0.001 9 | 0.537 2 | 0.017 5 | 0.068 9 | 0.000 7 | 480 | 44 | 437 | 12 | 430 | 4 | |||||
13T38-13 | 446 | 951 | 0.47 | 0.055 8 | 0.001 8 | 0.524 8 | 0.015 9 | 0.068 7 | 0.000 8 | 443 | 75 | 428 | 11 | 428 | 5 | |||||
13T38-14 | 452 | 749 | 0.6 | 0.055 1 | 0.002 2 | 0.512 1 | 0.020 0 | 0.067 5 | 0.000 7 | 417 | 89 | 420 | 13 | 421 | 4 | |||||
13T38-15 | 235 | 464 | 0.51 | 0.059 0 | 0.002 9 | 0.556 2 | 0.028 3 | 0.067 9 | 0.000 9 | 569 | 138 | 449 | 18 | 423 | 5 | |||||
13T38-16 | 355 | 690 | 0.51 | 0.053 7 | 0.002 3 | 0.506 3 | 0.020 4 | 0.068 7 | 0.000 9 | 361 | 96 | 416 | 14 | 428 | 5 | |||||
13T38-17 | 333 | 577 | 0.58 | 0.055 1 | 0.002 2 | 0.515 4 | 0.020 2 | 0.068 3 | 0.001 0 | 417 | 93 | 422 | 14 | 426 | 6 | |||||
13T38-18 | 497 | 1 136 | 0.44 | 0.053 2 | 0.002 0 | 0.489 6 | 0.017 7 | 0.066 8 | 0.000 9 | 339 | 85 | 405 | 12 | 417 | 5 | |||||
13T38-19 | 260 | 515 | 0.5 | 0.050 1 | 0.002 5 | 0.465 1 | 0.022 5 | 0.067 6 | 0.000 9 | 198 | 115 | 388 | 16 | 422 | 5 | |||||
13T38-20 | 474 | 743 | 0.64 | 0.053 1 | 0.002 3 | 0.492 1 | 0.021 6 | 0.066 7 | 0.001 0 | 332 | 101 | 406 | 15 | 416 | 6 | |||||
13T38-22 | 597 | 1 050 | 0.57 | 0.052 5 | 0.002 4 | 0.492 6 | 0.022 3 | 0.067 9 | 0.000 8 | 306 | 107 | 407 | 15 | 423 | 5 | |||||
13T38-23 | 497 | 1 157 | 0.43 | 0.055 2 | 0.002 0 | 0.504 0 | 0.018 0 | 0.066 0 | 0.000 8 | 420 | 80 | 414 | 12 | 412 | 5 | |||||
13T38-24 | 1 315 | 2 324 | 0.57 | 0.053 7 | 0.001 6 | 0.511 5 | 0.015 0 | 0.068 9 | 0.000 7 | 367 | 73 | 419 | 10 | 429 | 4 |
Spot No. | Age (Ma) | 176Hf/77Hf | ±1σ | 176Lu/177Hf | ±1σ | 176Yb/177Hf | ±1σ | T1DM (Ma) | T2DM (Ma) | εHf(t) |
13T38-1 | 427 | 0.282 662 | 0.000 011 | 0.001 356 | 0.000 020 | 0.043 524 | 0.000 994 | 844 | 1 003 | 5.11 |
13T38-3 | 418 | 0.282 671 | 0.000 016 | 0.002 819 | 0.000 055 | 0.082 792 | 0.001 539 | 865 | 1 011 | 4.84 |
13T38-4 | 419 | 0.282 652 | 0.000 014 | 0.001 378 | 0.000 034 | 0.044 657 | 0.001 487 | 858 | 1 025 | 4.61 |
13T38-7 | 423 | 0.282 683 | 0.000 017 | 0.002 832 | 0.000 085 | 0.094 734 | 0.002 881 | 847 | 985 | 5.38 |
13T38-13 | 428 | 0.282 656 | 0.000 010 | 0.001 277 | 0.000 011 | 0.040 993 | 0.000 323 | 850 | 1 013 | 4.96 |
13T38-14 | 421 | 0.282 645 | 0.000 011 | 0.001 049 | 0.000 011 | 0.035 754 | 0.000 302 | 861 | 1 033 | 4.49 |
13T38-15 | 423 | 0.282 644 | 0.000 013 | 0.001 098 | 0.000 008 | 0.042 530 | 0.000 686 | 863 | 1 035 | 4.49 |
13T38-16 | 428 | 0.282 631 | 0.000 014 | 0.001 274 | 0.000 012 | 0.047 696 | 0.000 563 | 885 | 1 061 | 4.09 |
13T38-18 | 417 | 0.282 647 | 0.000 013 | 0.001 143 | 0.000 014 | 0.041 932 | 0.000 764 | 859 | 1 032 | 4.46 |
13T38-22 | 423 | 0.282 652 | 0.000 013 | 0.001 743 | 0.000 044 | 0.064 172 | 0.001 461 | 866 | 1 029 | 4.59 |
13T38-24 | 429 | 0.282 638 | 0.000 017 | 0.002 369 | 0.000 078 | 0.074 833 | 0.001 537 | 902 | 1 065 | 4.03 |
13T38-20 | 416 | 0.282 654 | 0.000 015 | 0.001 397 | 0.000 005 | 0.052 997 | 0.000 546 | 855 | 1 022 | 4.61 |
Sample | 13T38-1 | 13T38-2 | 13T38-3 | 13T38-4 | 13T38-5 |
Major oxide (wt.%) | |||||
SiO2 | 77.00 | 76.78 | 76.47 | 76.91 | 75.61 |
TiO2 | 0.22 | 0.21 | 0.21 | 0.24 | 0.23 |
Al2O3 | 12.10 | 12.71 | 12.72 | 12.67 | 13.13 |
FeOt | 2.20 | 2.21 | 2.20 | 0.98 | 2.24 |
MnO | 0.07 | 0.07 | 0.07 | 0.03 | 0.07 |
MgO | 0.38 | 0.51 | 0.51 | 0.24 | 0.44 |
CaO | 0.50 | 0.38 | 0.37 | 0.26 | 0.61 |
Na2O | 3.64 | 3.13 | 3.27 | 3.74 | 3.63 |
K2O | 2.97 | 3.39 | 3.44 | 3.25 | 3.50 |
P2O5 | 0.04 | 0.04 | 0.05 | 0.03 | 0.06 |
LOI | 0.87 | 1.23 | 1.15 | 1.07 | 1.00 |
Total | 99.99 | 100.66 | 100.46 | 99.42 | 100.52 |
A/CNK | 1.20 | 1.34 | 1.30 | 1.25 | 1.21 |
Na2O+K2O | 6.62 | 6.52 | 6.72 | 7.00 | 7.13 |
Na2O/K2O | 0.82 | 0.87 | 0.96 | 1.05 | 1.08 |
Mg# | 26.00 | 31.00 | 31.00 | 32.00 | 28.00 |
Trace element (ppm) | |||||
La | 28.44 | 22.55 | 21.88 | 55.96 | 28.89 |
Ce | 54.17 | 45.85 | 45.52 | 90.53 | 54.50 |
Pr | 6.57 | 5.74 | 5.78 | 13.10 | 6.64 |
Nd | 24.60 | 21.37 | 21.77 | 48.60 | 24.18 |
Sm | 5.06 | 4.53 | 4.57 | 10.22 | 4.84 |
Eu | 0.91 | 0.89 | 0.86 | 1.94 | 0.87 |
Gd | 4.47 | 4.20 | 4.41 | 9.70 | 4.57 |
Tb | 0.79 | 0.78 | 0.80 | 1.73 | 0.82 |
Dy | 4.71 | 4.69 | 4.78 | 9.87 | 4.96 |
Ho | 0.98 | 1.00 | 1.02 | 1.92 | 1.03 |
Er | 3.11 | 3.11 | 3.16 | 5.31 | 3.16 |
Tm | 0.54 | 0.53 | 0.55 | 0.87 | 0.55 |
Yb | 3.23 | 3.14 | 3.20 | 4.96 | 3.26 |
Lu | 0.51 | 0.49 | 0.49 | 0.70 | 0.49 |
Y | 30.34 | 29.18 | 31.28 | 51.86 | 31.88 |
Li | 17.02 | 24.24 | 23.79 | 7.58 | 19.97 |
Be | 1.37 | 1.58 | 1.46 | 1.38 | 1.71 |
Sc | 8.91 | 7.73 | 8.74 | 8.97 | 8.62 |
Co | 69.00 | 58.01 | 58.58 | 54.50 | 72.44 |
Cu | 0.94 | 0.35 | 0.22 | 0.55 | 0.47 |
Zn | 33.91 | 49.89 | 50.53 | 27.87 | 35.33 |
Ga | 10.71 | 14.21 | 14.02 | 12.94 | 13.69 |
Rb | 68.80 | 81.97 | 82.83 | 88.77 | 84.88 |
Zr | 134.90 | 133.50 | 131.90 | 144.50 | 142.50 |
Nb | 7.67 | 7.60 | 8.95 | 10.16 | 8.47 |
Mo | 0.10 | 0.08 | 0.08 | 0.13 | 0.09 |
Cs | 2.58 | 3.64 | 3.81 | 2.74 | 2.69 |
Hf | 4.68 | 4.67 | 4.72 | 5.03 | 4.97 |
Ta | 0.82 | 0.80 | 0.79 | 0.86 | 0.91 |
Pb | 8.10 | 7.85 | 8.48 | 7.81 | 8.01 |
Th | 7.86 | 7.40 | 7.03 | 6.68 | 6.02 |
U | 2.47 | 2.55 | 2.68 | 2.53 | 2.39 |
Sn | 2.04 | 2.33 | 2.43 | 2.34 | 2.52 |
Ba | 908.99 | 1 431.28 | 1 427.61 | 1 340.43 | 1 385.20 |
Cr | 7.40 | 7.22 | 7.22 | 4.14 | 7.68 |
Ni | 2.90 | 2.63 | 2.34 | 2.15 | 3.26 |
Trace element (ppm) | |||||
Sr | 144.32 | 154.32 | 148.45 | 186.63 | 209.56 |
V | 8.50 | 8.90 | 8.50 | 7.44 | 9.12 |
∑REE | 138.08 | 118.85 | 118.79 | 255.41 | 138.76 |
(La/Yb)N | 4.60 | 4.84 | 5.94 | 5.97 | 7.61 |
δEu | 0.63 | 0.60 | 0.59 | 0.58 | 0.56 |
Tzr (℃) | 791.30 | 799.10 | 795.50 | 800.50 | 795.00 |
CIPW (%) | |||||
Q | 42.94 | 44.16 | 42.86 | 41.93 | 39.30 |
C | 2.10 | 3.33 | 3.06 | 2.65 | 2.40 |
Or | 17.58 | 20.03 | 20.35 | 19.21 | 20.69 |
Ab | 30.81 | 26.51 | 27.69 | 31.68 | 30.70 |
An | 2.14 | 1.53 | 1.50 | 0.96 | 2.66 |
Hy | 0.95 | 1.27 | 1.26 | 0.59 | 1.09 |
Il | 0.15 | 0.15 | 0.15 | 0.06 | 0.15 |
Ru | 0.14 | 0.13 | 0.13 | 0.20 | 0.15 |
Ap | 0.12 | 0.12 | 0.13 | 0.12 | 0.13 |
Sum | 96.94 | 97.23 | 97.12 | 97.39 | 97.27 |
DI | 91.33 | 90.70 | 90.90 | 92.82 | 90.69 |
FeOt represents total Fe-oxides; Mg#=molar Mg×100/(Mg+Fe); LOI. loss on ignition; A/CNK=molar Al2O3/(CaO+Na2O+K2O); DI=Q+Or+Ab. |
Spot No. | Concentration | Th/U | Isotope ratio | Calculated apparent age (Ma) | ||||||||||||||||
Th | U | 207Pb/206Pb | 207Pb/235U | 206Pb/238U | 207Pb/206Pb | 207Pb/235U | 206Pb/238U | |||||||||||||
(ppm) | Ratio | ±1σ | Ratio | ±1σ | Ratio | ±1σ | Age | ±1σ | Age | ±1σ | Age | ±1σ | ||||||||
13T38-1 | 285 | 521 | 0.55 | 0.056 3 | 0.002 5 | 0.534 6 | 0.024 2 | 0.068 4 | 0.000 8 | 465 | 98 | 435 | 16 | 427 | 5 | |||||
13T38-3 | 559 | 1 077 | 0.52 | 0.057 9 | 0.002 1 | 0.538 2 | 0.019 3 | 0.067 0 | 0.000 7 | 528 | 75 | 437 | 13 | 418 | 4 | |||||
13T38-4 | 449 | 886 | 0.51 | 0.058 0 | 0.002 2 | 0.539 6 | 0.020 0 | 0.067 2 | 0.000 8 | 528 | 82 | 438 | 13 | 419 | 5 | |||||
13T38-5 | 246 | 478 | 0.52 | 0.057 7 | 0.003 0 | 0.532 9 | 0.028 4 | 0.066 4 | 0.000 8 | 517 | 115 | 434 | 19 | 414 | 5 | |||||
13T38-6 | 868 | 1 298 | 0.67 | 0.056 5 | 0.001 9 | 0.537 5 | 0.017 9 | 0.068 3 | 0.000 7 | 478 | 74 | 437 | 12 | 426 | 4 | |||||
13T38-7 | 433 | 635 | 0.68 | 0.049 1 | 0.003 3 | 0.455 6 | 0.028 9 | 0.067 8 | 0.001 2 | 154 | 152 | 381 | 20 | 423 | 7 | |||||
13T38-8 | 469 | 1 071 | 0.44 | 0.055 3 | 0.001 9 | 0.538 0 | 0.018 7 | 0.070 0 | 0.000 7 | 433 | 76 | 437 | 12 | 436 | 4 | |||||
13T38-9 | 679 | 1 004 | 0.68 | 0.055 2 | 0.00 2 0 | 0.519 8 | 0.018 6 | 0.068 1 | 0.000 8 | 420 | 77 | 425 | 12 | 425 | 5 | |||||
13T38-10 | 271 | 585 | 0.46 | 0.055 1 | 0.002 3 | 0.518 7 | 0.021 5 | 0.068 4 | 0.000 8 | 417 | 94 | 424 | 14 | 426 | 5 | |||||
13T38-12 | 517 | 917 | 0.56 | 0.056 7 | 0.001 9 | 0.537 2 | 0.017 5 | 0.068 9 | 0.000 7 | 480 | 44 | 437 | 12 | 430 | 4 | |||||
13T38-13 | 446 | 951 | 0.47 | 0.055 8 | 0.001 8 | 0.524 8 | 0.015 9 | 0.068 7 | 0.000 8 | 443 | 75 | 428 | 11 | 428 | 5 | |||||
13T38-14 | 452 | 749 | 0.6 | 0.055 1 | 0.002 2 | 0.512 1 | 0.020 0 | 0.067 5 | 0.000 7 | 417 | 89 | 420 | 13 | 421 | 4 | |||||
13T38-15 | 235 | 464 | 0.51 | 0.059 0 | 0.002 9 | 0.556 2 | 0.028 3 | 0.067 9 | 0.000 9 | 569 | 138 | 449 | 18 | 423 | 5 | |||||
13T38-16 | 355 | 690 | 0.51 | 0.053 7 | 0.002 3 | 0.506 3 | 0.020 4 | 0.068 7 | 0.000 9 | 361 | 96 | 416 | 14 | 428 | 5 | |||||
13T38-17 | 333 | 577 | 0.58 | 0.055 1 | 0.002 2 | 0.515 4 | 0.020 2 | 0.068 3 | 0.001 0 | 417 | 93 | 422 | 14 | 426 | 6 | |||||
13T38-18 | 497 | 1 136 | 0.44 | 0.053 2 | 0.002 0 | 0.489 6 | 0.017 7 | 0.066 8 | 0.000 9 | 339 | 85 | 405 | 12 | 417 | 5 | |||||
13T38-19 | 260 | 515 | 0.5 | 0.050 1 | 0.002 5 | 0.465 1 | 0.022 5 | 0.067 6 | 0.000 9 | 198 | 115 | 388 | 16 | 422 | 5 | |||||
13T38-20 | 474 | 743 | 0.64 | 0.053 1 | 0.002 3 | 0.492 1 | 0.021 6 | 0.066 7 | 0.001 0 | 332 | 101 | 406 | 15 | 416 | 6 | |||||
13T38-22 | 597 | 1 050 | 0.57 | 0.052 5 | 0.002 4 | 0.492 6 | 0.022 3 | 0.067 9 | 0.000 8 | 306 | 107 | 407 | 15 | 423 | 5 | |||||
13T38-23 | 497 | 1 157 | 0.43 | 0.055 2 | 0.002 0 | 0.504 0 | 0.018 0 | 0.066 0 | 0.000 8 | 420 | 80 | 414 | 12 | 412 | 5 | |||||
13T38-24 | 1 315 | 2 324 | 0.57 | 0.053 7 | 0.001 6 | 0.511 5 | 0.015 0 | 0.068 9 | 0.000 7 | 367 | 73 | 419 | 10 | 429 | 4 |
Spot No. | Age (Ma) | 176Hf/77Hf | ±1σ | 176Lu/177Hf | ±1σ | 176Yb/177Hf | ±1σ | T1DM (Ma) | T2DM (Ma) | εHf(t) |
13T38-1 | 427 | 0.282 662 | 0.000 011 | 0.001 356 | 0.000 020 | 0.043 524 | 0.000 994 | 844 | 1 003 | 5.11 |
13T38-3 | 418 | 0.282 671 | 0.000 016 | 0.002 819 | 0.000 055 | 0.082 792 | 0.001 539 | 865 | 1 011 | 4.84 |
13T38-4 | 419 | 0.282 652 | 0.000 014 | 0.001 378 | 0.000 034 | 0.044 657 | 0.001 487 | 858 | 1 025 | 4.61 |
13T38-7 | 423 | 0.282 683 | 0.000 017 | 0.002 832 | 0.000 085 | 0.094 734 | 0.002 881 | 847 | 985 | 5.38 |
13T38-13 | 428 | 0.282 656 | 0.000 010 | 0.001 277 | 0.000 011 | 0.040 993 | 0.000 323 | 850 | 1 013 | 4.96 |
13T38-14 | 421 | 0.282 645 | 0.000 011 | 0.001 049 | 0.000 011 | 0.035 754 | 0.000 302 | 861 | 1 033 | 4.49 |
13T38-15 | 423 | 0.282 644 | 0.000 013 | 0.001 098 | 0.000 008 | 0.042 530 | 0.000 686 | 863 | 1 035 | 4.49 |
13T38-16 | 428 | 0.282 631 | 0.000 014 | 0.001 274 | 0.000 012 | 0.047 696 | 0.000 563 | 885 | 1 061 | 4.09 |
13T38-18 | 417 | 0.282 647 | 0.000 013 | 0.001 143 | 0.000 014 | 0.041 932 | 0.000 764 | 859 | 1 032 | 4.46 |
13T38-22 | 423 | 0.282 652 | 0.000 013 | 0.001 743 | 0.000 044 | 0.064 172 | 0.001 461 | 866 | 1 029 | 4.59 |
13T38-24 | 429 | 0.282 638 | 0.000 017 | 0.002 369 | 0.000 078 | 0.074 833 | 0.001 537 | 902 | 1 065 | 4.03 |
13T38-20 | 416 | 0.282 654 | 0.000 015 | 0.001 397 | 0.000 005 | 0.052 997 | 0.000 546 | 855 | 1 022 | 4.61 |
Sample | 13T38-1 | 13T38-2 | 13T38-3 | 13T38-4 | 13T38-5 |
Major oxide (wt.%) | |||||
SiO2 | 77.00 | 76.78 | 76.47 | 76.91 | 75.61 |
TiO2 | 0.22 | 0.21 | 0.21 | 0.24 | 0.23 |
Al2O3 | 12.10 | 12.71 | 12.72 | 12.67 | 13.13 |
FeOt | 2.20 | 2.21 | 2.20 | 0.98 | 2.24 |
MnO | 0.07 | 0.07 | 0.07 | 0.03 | 0.07 |
MgO | 0.38 | 0.51 | 0.51 | 0.24 | 0.44 |
CaO | 0.50 | 0.38 | 0.37 | 0.26 | 0.61 |
Na2O | 3.64 | 3.13 | 3.27 | 3.74 | 3.63 |
K2O | 2.97 | 3.39 | 3.44 | 3.25 | 3.50 |
P2O5 | 0.04 | 0.04 | 0.05 | 0.03 | 0.06 |
LOI | 0.87 | 1.23 | 1.15 | 1.07 | 1.00 |
Total | 99.99 | 100.66 | 100.46 | 99.42 | 100.52 |
A/CNK | 1.20 | 1.34 | 1.30 | 1.25 | 1.21 |
Na2O+K2O | 6.62 | 6.52 | 6.72 | 7.00 | 7.13 |
Na2O/K2O | 0.82 | 0.87 | 0.96 | 1.05 | 1.08 |
Mg# | 26.00 | 31.00 | 31.00 | 32.00 | 28.00 |
Trace element (ppm) | |||||
La | 28.44 | 22.55 | 21.88 | 55.96 | 28.89 |
Ce | 54.17 | 45.85 | 45.52 | 90.53 | 54.50 |
Pr | 6.57 | 5.74 | 5.78 | 13.10 | 6.64 |
Nd | 24.60 | 21.37 | 21.77 | 48.60 | 24.18 |
Sm | 5.06 | 4.53 | 4.57 | 10.22 | 4.84 |
Eu | 0.91 | 0.89 | 0.86 | 1.94 | 0.87 |
Gd | 4.47 | 4.20 | 4.41 | 9.70 | 4.57 |
Tb | 0.79 | 0.78 | 0.80 | 1.73 | 0.82 |
Dy | 4.71 | 4.69 | 4.78 | 9.87 | 4.96 |
Ho | 0.98 | 1.00 | 1.02 | 1.92 | 1.03 |
Er | 3.11 | 3.11 | 3.16 | 5.31 | 3.16 |
Tm | 0.54 | 0.53 | 0.55 | 0.87 | 0.55 |
Yb | 3.23 | 3.14 | 3.20 | 4.96 | 3.26 |
Lu | 0.51 | 0.49 | 0.49 | 0.70 | 0.49 |
Y | 30.34 | 29.18 | 31.28 | 51.86 | 31.88 |
Li | 17.02 | 24.24 | 23.79 | 7.58 | 19.97 |
Be | 1.37 | 1.58 | 1.46 | 1.38 | 1.71 |
Sc | 8.91 | 7.73 | 8.74 | 8.97 | 8.62 |
Co | 69.00 | 58.01 | 58.58 | 54.50 | 72.44 |
Cu | 0.94 | 0.35 | 0.22 | 0.55 | 0.47 |
Zn | 33.91 | 49.89 | 50.53 | 27.87 | 35.33 |
Ga | 10.71 | 14.21 | 14.02 | 12.94 | 13.69 |
Rb | 68.80 | 81.97 | 82.83 | 88.77 | 84.88 |
Zr | 134.90 | 133.50 | 131.90 | 144.50 | 142.50 |
Nb | 7.67 | 7.60 | 8.95 | 10.16 | 8.47 |
Mo | 0.10 | 0.08 | 0.08 | 0.13 | 0.09 |
Cs | 2.58 | 3.64 | 3.81 | 2.74 | 2.69 |
Hf | 4.68 | 4.67 | 4.72 | 5.03 | 4.97 |
Ta | 0.82 | 0.80 | 0.79 | 0.86 | 0.91 |
Pb | 8.10 | 7.85 | 8.48 | 7.81 | 8.01 |
Th | 7.86 | 7.40 | 7.03 | 6.68 | 6.02 |
U | 2.47 | 2.55 | 2.68 | 2.53 | 2.39 |
Sn | 2.04 | 2.33 | 2.43 | 2.34 | 2.52 |
Ba | 908.99 | 1 431.28 | 1 427.61 | 1 340.43 | 1 385.20 |
Cr | 7.40 | 7.22 | 7.22 | 4.14 | 7.68 |
Ni | 2.90 | 2.63 | 2.34 | 2.15 | 3.26 |
Trace element (ppm) | |||||
Sr | 144.32 | 154.32 | 148.45 | 186.63 | 209.56 |
V | 8.50 | 8.90 | 8.50 | 7.44 | 9.12 |
∑REE | 138.08 | 118.85 | 118.79 | 255.41 | 138.76 |
(La/Yb)N | 4.60 | 4.84 | 5.94 | 5.97 | 7.61 |
δEu | 0.63 | 0.60 | 0.59 | 0.58 | 0.56 |
Tzr (℃) | 791.30 | 799.10 | 795.50 | 800.50 | 795.00 |
CIPW (%) | |||||
Q | 42.94 | 44.16 | 42.86 | 41.93 | 39.30 |
C | 2.10 | 3.33 | 3.06 | 2.65 | 2.40 |
Or | 17.58 | 20.03 | 20.35 | 19.21 | 20.69 |
Ab | 30.81 | 26.51 | 27.69 | 31.68 | 30.70 |
An | 2.14 | 1.53 | 1.50 | 0.96 | 2.66 |
Hy | 0.95 | 1.27 | 1.26 | 0.59 | 1.09 |
Il | 0.15 | 0.15 | 0.15 | 0.06 | 0.15 |
Ru | 0.14 | 0.13 | 0.13 | 0.20 | 0.15 |
Ap | 0.12 | 0.12 | 0.13 | 0.12 | 0.13 |
Sum | 96.94 | 97.23 | 97.12 | 97.39 | 97.27 |
DI | 91.33 | 90.70 | 90.90 | 92.82 | 90.69 |
FeOt represents total Fe-oxides; Mg#=molar Mg×100/(Mg+Fe); LOI. loss on ignition; A/CNK=molar Al2O3/(CaO+Na2O+K2O); DI=Q+Or+Ab. |