Detailed geochronological, geochemical, and Sr-Nd-Hf isotopic data are presented for early Paleozoic volcanic rocks in the Karadaban area from the northern Altyn region, NW China, with the aim to constrain their petrogenesis and tectonic implications. The Karadaban volcanic rocks show a bimodal distribution in composition, with rhyolite and basalt. The LA-ICP-MS zircon U-Pb age indicates that the volcanic rocks were erupted at 512 Ma. The mafic rocks are calc-alkaline, enriched in light rare earth elements (LREE) and large-ion lithophile elements (LILE; Ba and U) and depleted in high-field strength elements (HFSE; Nb and Ta). These features together with their depleted isotopic signature (initial
Bimodal volcanism typically characterizes an extensional environment, which can occur within various tectonic settings, including continental rifts [
The Altyn Tagh Belt (ATB) marks the northern margin of the Qinghai-Tibet Plateau [
The Altyn Mountain is located at the juncture of the Tarim Basin, the Qaidam Basin, and the Qilian orogen in Western China [
(a) Tectonic outline of Western China; (b) sketch map of the Altyn region; (c) tectonic map of the northern Altyn Tagh region (after [
The tectonic units of the ATB have been divided into four different schemes (Figure
Geological map of the Karadaban area (after [
In this study, 12 samples within the Karadaban bimodal volcanic rocks were collected for geochemical analysis, 8 of them were used for Sr-Nd isotopic analysis, and analyses of Hf isotopes were made on zircon grains from samples KB-26 and KB-81 that were previously dated by laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-ICP-MS). Lithology of these samples are basalt and rhyolite (Figure
Photographs of outcrops and photomicrographs showing the geological and mineralogical characteristics of the Kaladaban bimodal volcanic rocks: (a) outcrop of bimodal volcanic rocks; (b, c) outcrop of and hand specimen of rhyolite; (d) photographs of rhyolite; (e, f) outcrop of and hand specimen of basalt; (g) photographs of basalt. Pl: plagioclase; Px: pyroxene; Q: quartz; Ms: muscovite.
The rhyolite samples are gray to light yellow in color, massive, and porphyritic and rhyolitic in texture, which are interbedded with the basalt (Figure
The Karadaban igneous samples were collected from adits and outcrops (Figure
Major and trace elements were analyzed in the Analytical Laboratory of Research Institute of Uranium Geology, Beijing (BRIUG). Major element compositions were measured by wavelength dispersive X-ray fluorescence spectrometry (XRF) on fused glass beads using a Philips PW2404 spectrometer; analytical uncertainties are less than ±1% for major elements. FeO content was determined by conventional wet chemical titration technique. Both trace element and REE abundance were analyzed using ICP-MS (Finnigan MAT Ltd.); uncertainties are less than ±5% for
Cathodoluminescence (CL) images were obtained by using a scanning electron microscope (SEM, Leo 1450VP, Germany) at the Chinese Academy of Geological Sciences before in situ U-Pb and Hf isotopes analyses. Zircon LA-ICP-MS U-Pb dating was performed using a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) at the MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences (CAGS), Beijing. The laser spot diameter and frequency were 40
In situ zircon Hf isotopic analyses were carried out at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, using a Neptune MC-ICP-MS with an ArF excimer laser ablation system. During analyses, the spot sizes of 32 and 63
Samples for isotopic analysis were dissolved in Teflon bombs after being spiked with 84Sr, 87Rb, 150Nd, and147Sm tracers prior to HF+HNO3 (with a ratio of 2 : 1) dissolution. Rubidium, Sr, Sm, and Nd were separated using conventional ion exchange procedures and measured using multicollector mass spectrometer (IsoProbe-T) at the Analytical Laboratory of BRIUG [
Twelve whole-rock major and trace element data are presented in Table
Major (%) and trace element (ppm) data for the volcanic rocks in the northern Altyn Tagh region.
Rock type | Basalt | Rhyolite | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Sample | KB-19 | KB-23 | KB-70 | KB-58 | KB-26 | KB-33 | KB-46 | KB-76 | KB-81 | KB-82 | KB-132 | KB-138 |
SiO2 | 45.43 | 46.65 | 44.80 | 44.74 | 76.14 | 68.06 | 75.64 | 71.25 | 72.60 | 68.78 | 68.70 | 71.41 |
TiO2 | 1.79 | 3.75 | 1.57 | 2.98 | 0.19 | 0.41 | 0.26 | 0.20 | 0.32 | 0.65 | 0.52 | 0.23 |
Al2O3 | 17.42 | 14.46 | 15.48 | 16.93 | 12.79 | 11.59 | 10.31 | 9.60 | 14.60 | 13.79 | 12.73 | 14.57 |
TFe2O3 | 12.19 | 17.12 | 11.49 | 13.90 | 2.07 | 8.30 | 4.91 | 6.86 | 2.64 | 4.54 | 7.50 | 3.06 |
MnO | 0.48 | 0.50 | 0.66 | 0.64 | 0.06 | 0.17 | 0.05 | 0.28 | 0.05 | 0.17 | 0.22 | 0.01 |
MgO | 8.74 | 4.23 | 8.63 | 7.67 | 0.57 | 5.21 | 2.76 | 6.12 | 0.86 | 1.33 | 2.05 | 1.33 |
CaO | 6.61 | 7.85 | 10.59 | 1.29 | 0.30 | 0.13 | 0.27 | 0.11 | 0.17 | 1.08 | 0.54 | 0.35 |
Na2O | 3.10 | 2.80 | 1.77 | 3.11 | 5.79 | 1.28 | 0.19 | 0.05 | 2.46 | 4.27 | 0.17 | 3.99 |
K2O | 0.80 | 0.18 | 1.44 | 0.67 | 1.27 | 1.30 | 2.11 | 1.22 | 3.80 | 4.08 | 4.56 | 2.81 |
P2O5 | 0.24 | 0.53 | 0.21 | 0.65 | 0.03 | 0.07 | 0.03 | 0.04 | 0.04 | 0.10 | 0.05 | 0.03 |
LOI | 3.11 | 1.92 | 3.35 | 6.87 | 0.74 | 3.42 | 3.41 | 3.68 | 1.87 | 1.20 | 1.47 | 1.68 |
Total | 96.80 | 98.07 | 96.64 | 92.57 | 99.21 | 96.52 | 96.54 | 95.72 | 97.54 | 98.79 | 97.03 | 97.79 |
A/CNK | 0.97 | 0.76 | 0.65 | 2.07 | 1.12 | 3.09 | 3.33 | 6.03 | 1.72 | 1.03 | 2.06 | 1.42 |
A/NK | 2.92 | 3.01 | 3.46 | 2.90 | 1.17 | 3.30 | 3.96 | 6.85 | 1.79 | 1.21 | 2.44 | 1.52 |
Na2O/K2O | 3.89 | 15.56 | 1.23 | 4.68 | 4.56 | 0.98 | 0.09 | 0.04 | 0.65 | 1.05 | 0.04 | 1.42 |
Rb | 31.00 | 4.95 | 60.50 | 46.70 | 38.70 | 71.40 | 101.00 | 23.80 | 143.00 | 117.00 | 115.00 | 92.70 |
Ba | 1012.00 | 194.00 | 981.00 | 888.00 | 618.00 | 214.00 | 396.00 | 458.00 | 935.00 | 862.00 | 14744.00 | 635.00 |
Th | 4.87 | 5.08 | 4.88 | 11.90 | 24.90 | 15.80 | 13.50 | 11.80 | 28.50 | 30.40 | 22.70 | 24.70 |
U | 1.26 | 1.94 | 1.39 | 4.41 | 8.24 | 4.59 | 3.65 | 3.12 | 7.60 | 6.88 | 6.84 | 6.40 |
Nb | 11.50 | 11.70 | 10.90 | 13.00 | 19.40 | 18.50 | 18.30 | 22.60 | 26.50 | 16.10 | 25.30 | 24.70 |
Ta | 0.64 | 0.70 | 0.56 | 0.82 | 1.65 | 1.43 | 1.34 | 1.53 | 1.91 | 1.54 | 1.92 | 2.02 |
K | 6607.48 | 1494.15 | 11953.23 | 5520.07 | 10542.08 | 10791.11 | 17514.80 | 10127.04 | 31543.24 | 33867.48 | 37851.89 | 23325.40 |
Cr | 188.00 | 174.70 | 197.00 | 215.30 | 4.13 | 18.00 | 14.00 | 2.46 | 4.26 | 10.00 | 4.04 | 3.40 |
Ni | 98.80 | 112.00 | 93.60 | 118.46 | 2.04 | 9.56 | 6.74 | 1.07 | 1.93 | 4.81 | 2.17 | 2.51 |
V | 297.00 | 264.00 | 275.00 | 315.00 | 5.31 | 25.10 | 20.30 | 8.51 | 6.51 | 75.40 | 3.08 | 4.97 |
Co | 50.20 | 35.30 | 53.90 | 67.90 | 0.43 | 2.57 | 1.28 | 0.61 | 1.24 | 9.66 | 0.85 | 1.70 |
La | 24.70 | 25.60 | 27.60 | 43.70 | 61.20 | 51.00 | 45.20 | 47.20 | 56.90 | 46.00 | 59.70 | 66.80 |
Ce | 50.10 | 61.50 | 56.70 | 83.60 | 107.00 | 110.00 | 85.10 | 79.80 | 93.10 | 86.20 | 102.00 | 131.00 |
Pb | 36.10 | 44.80 | 44.70 | 283.00 | 8.48 | 4.06 | 5.99 | 18.40 | 7.30 | 94.50 | 91.00 | 6.18 |
Pr | 6.22 | 10.00 | 7.93 | 12.90 | 15.30 | 13.20 | 10.30 | 9.02 | 13.80 | 10.20 | 15.60 | 15.50 |
Sr | 370.00 | 394.00 | 460.00 | 144.00 | 107.00 | 32.10 | 26.60 | 22.60 | 75.80 | 80.70 | 322.00 | 104.00 |
Nd | 26.70 | 48.50 | 36.50 | 57.00 | 58.10 | 52.40 | 39.00 | 33.00 | 50.40 | 39.70 | 66.50 | 61.40 |
P | 1038.70 | 2308.71 | 929.59 | 2841.15 | 109.11 | 314.23 | 139.66 | 157.11 | 174.57 | 440.79 | 213.85 | 139.66 |
Sm | 5.92 | 12.40 | 8.14 | 13.70 | 11.80 | 10.50 | 7.91 | 7.17 | 10.50 | 8.16 | 14.80 | 12.30 |
Zr | 116.00 | 295.00 | 107.00 | 289.00 | 372.00 | 363.00 | 261.00 | 439.00 | 442.00 | 225.00 | 527.00 | 334.00 |
Hf | 3.23 | 8.12 | 3.03 | 7.40 | 11.50 | 10.50 | 7.65 | 11.20 | 12.60 | 7.87 | 15.70 | 10.30 |
Eu | 1.67 | 3.50 | 2.29 | 4.03 | 1.25 | 1.28 | 0.91 | 1.05 | 1.31 | 1.09 | 2.22 | 1.38 |
Ti | 10728.35 | 22475.58 | 9409.78 | 17860.60 | 1144.76 | 2427.36 | 1582.28 | 1192.70 | 1905.93 | 3883.78 | 3086.65 | 1402.48 |
Gd | 5.87 | 11.90 | 8.10 | 13.00 | 10.50 | 9.77 | 8.08 | 7.60 | 10.00 | 8.37 | 14.70 | 11.70 |
Tb | 1.08 | 2.29 | 1.44 | 2.42 | 1.93 | 1.97 | 1.77 | 1.63 | 1.93 | 1.61 | 2.78 | 2.22 |
Dy | 6.09 | 13.10 | 8.23 | 14.00 | 10.90 | 11.70 | 11.20 | 10.40 | 12.00 | 9.25 | 16.20 | 13.00 |
Y | 33.20 | 69.70 | 39.00 | 74.50 | 53.90 | 60.40 | 62.30 | 61.10 | 73.70 | 56.40 | 85.00 | 69.40 |
Ho | 1.21 | 2.62 | 1.50 | 2.65 | 2.20 | 2.38 | 2.29 | 2.17 | 2.51 | 1.88 | 3.22 | 2.56 |
Er | 3.58 | 7.76 | 4.15 | 7.37 | 6.91 | 6.62 | 6.54 | 6.69 | 7.87 | 5.94 | 10.00 | 7.76 |
Tm | 0.52 | 1.18 | 0.59 | 1.10 | 1.11 | 1.02 | 0.96 | 1.06 | 1.27 | 0.92 | 1.55 | 1.18 |
Yb | 3.23 | 7.31 | 3.51 | 7.13 | 7.57 | 6.13 | 6.12 | 6.91 | 8.17 | 5.83 | 9.95 | 7.51 |
Lu | 0.49 | 1.13 | 0.54 | 1.03 | 1.13 | 0.88 | 0.84 | 1.08 | 1.17 | 0.87 | 1.52 | 1.14 |
137.37 | 208.79 | 167.22 | 263.63 | 296.90 | 278.85 | 226.22 | 214.78 | 270.93 | 226.02 | 320.74 | 335.45 | |
LREE | 115.31 | 161.50 | 139.16 | 214.93 | 254.65 | 238.38 | 188.42 | 177.24 | 226.01 | 191.35 | 260.82 | 288.38 |
HREE | 22.06 | 47.29 | 28.06 | 48.70 | 42.25 | 40.47 | 37.80 | 37.54 | 44.92 | 34.67 | 59.92 | 47.07 |
LREE/HREE | 5.23 | 3.42 | 4.96 | 4.41 | 6.03 | 5.89 | 4.99 | 4.72 | 5.03 | 5.52 | 4.35 | 6.13 |
La/Nb | 2.15 | 2.19 | 2.53 | 3.36 | 3.15 | 2.76 | 2.47 | 2.09 | 2.15 | 2.86 | 2.36 | 2.70 |
La/Ta | 38.53 | 36.57 | 49.37 | 53.49 | 37.09 | 35.66 | 33.73 | 30.85 | 29.79 | 29.87 | 31.09 | 33.07 |
LaN/YbN | 5.49 | 2.51 | 5.64 | 4.40 | 5.80 | 5.97 | 5.30 | 4.90 | 5.00 | 5.66 | 4.30 | 6.38 |
0.86 | 0.87 | 0.85 | 0.91 | 0.34 | 0.38 | 0.35 | 0.43 | 0.39 | 0.40 | 0.45 | 0.35 | |
0.96 | 0.94 | 0.93 | 0.85 | 0.83 | 1.02 | 0.93 | 0.89 | 0.79 | 0.93 | 0.80 | 0.96 |
Note: major elements were obtained by X-ray fluorescence spectrometer (XRF) at the Analytical Laboratory of BRIUG, Beijing, using fused lithium-tetraborate glass pellets. The analytical precision as determined on the Chinese national standard GSR-3 was generally around 1–5%. Trace elements were analyzed using a PerkinElmer ELAN-DRC-e ICP-MS at the Analytical Laboratory of BRIUG, Beijing. The powdered samples (50 mg) were dissolved in high-pressure Teflon bombs using a HF+HNO3 mixture for 48 h, at ~195°C. Rh was used as an internal standard to monitor signal drift during counting. The international standards GBPG-1 and OU-6 and the Chinese national standard GSR-3 were used for analytical quality control, with the precision generally better than 5%.
The SiO2 content of the volcanic rocks lie within a range from 44.74 to 76.14 wt.% and with a compositional gap at 46–68 wt.% SiO2, which shows the bimodal feature (Table
Nb/Y vs. Zr/TiO2 (a) and TiO2 vs. Zr/P2O5 (b) diagrams (after [
Chondrite-normalized REE element pattern and primitive-mantle normalized trace element spider grams for the bimodal volcanic rocks. The normalized values of chondrite are from [
The rhyolite exhibits a wide range of TFe2O3 content (2.07–8.30 wt.%), with low TiO2 (0.19–0.65 wt.%) and P2O5 (0.03–0.10 wt.%) content, and high Al2O3 (9.60–14.57 wt.%). The A/NK and A/CNK values are 1.17–6.85 and 1.03–6.03, respectively, which classify the felsic samples as peraluminous. The felsic rock samples show
Two samples were collected for LA-ICP-MS U-Pb zircon dating. The results are listed in Table
LA-ICP-MS zircon U-Pb dating data for Karadaban rhyolite (KB-26, KB-81) from the northern Altyn Tagh region.
Spot no. | Isotopic ratios | Ages (Ma) | Pb (ppm) | Th (ppm) | U (ppm) | Th/U | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
207Pb/206Pb | 207Pb/235U | 206Pb/238U | 207Pb/206Pb | 207Pb/235U | 206Pb/238U | |||||||||||
Ratios | 1 |
Ratios | 1 |
Ratios | 1 |
Age | 1 |
Age | 1 |
Age | 1 | |||||
KB-26-1 | 0.05943 | 0.00173 | 0.66147 | 0.01805 | 0.08121 | 0.00092 | 583.4 | 69.43 | 515.5 | 11.03 | 503.4 | 5.50 | 179.22 | 370.82 | 399.64 | 0.93 |
KB-26-2 | 0.05904 | 0.00197 | 0.66388 | 0.02115 | 0.08200 | 0.00102 | 568.6 | 72.21 | 517.0 | 12.91 | 508.0 | 6.07 | 123.32 | 251.45 | 301.98 | 0.83 |
KB-26-3 | 0.06160 | 0.00230 | 0.69672 | 0.02517 | 0.08246 | 0.00106 | 661.1 | 74.99 | 536.8 | 15.06 | 510.8 | 6.29 | 99.43 | 236.65 | 248.95 | 0.95 |
KB-26-4 | 0.06051 | 0.00196 | 0.67705 | 0.02272 | 0.08114 | 0.00115 | 620.4 | 74.99 | 525.0 | 13.76 | 502.9 | 6.83 | 154.70 | 317.22 | 392.62 | 0.81 |
KB-26-5 | 0.06212 | 0.00171 | 0.71062 | 0.02002 | 0.08284 | 0.00080 | 679.6 | 63.88 | 545.1 | 11.88 | 513.0 | 4.77 | 166.46 | 348.84 | 435.81 | 0.80 |
KB-26-6 | 0.05778 | 0.00277 | 0.64371 | 0.02960 | 0.08131 | 0.00153 | 520.4 | 110.17 | 504.6 | 18.29 | 503.9 | 9.13 | 119.30 | 233.79 | 362.94 | 0.64 |
KB-26-7 | 0.06759 | 0.00998 | 0.74943 | 0.09941 | 0.08220 | 0.00236 | 857.4 | 313.87 | 567.9 | 57.76 | 509.3 | 14.03 | 72.24 | 126.11 | 209.43 | 0.60 |
KB-26-8 | 0.06709 | 0.00357 | 0.75325 | 0.04424 | 0.08096 | 0.00171 | 840.4 | 111.88 | 570.1 | 25.63 | 501.8 | 10.17 | 152.07 | 321.51 | 385.92 | 0.83 |
KB-26-9 | 0.06001 | 0.00196 | 0.68555 | 0.02180 | 0.08313 | 0.00096 | 605.6 | 70.36 | 530.1 | 13.13 | 514.8 | 5.72 | 138.37 | 270.60 | 366.74 | 0.74 |
KB-26-10 | 0.06166 | 0.00188 | 0.71332 | 0.02235 | 0.08387 | 0.00096 | 661.1 | 66.66 | 546.7 | 13.24 | 519.2 | 5.69 | 155.19 | 331.54 | 381.97 | 0.87 |
KB-26-11 | 0.06045 | 0.00200 | 0.69004 | 0.02203 | 0.08306 | 0.00076 | 620.4 | 76.84 | 532.8 | 13.24 | 514.4 | 4.53 | 185.01 | 370.24 | 446.68 | 0.83 |
KB-26-12 | 0.06368 | 0.00253 | 0.71492 | 0.02687 | 0.08180 | 0.00084 | 731.5 | 78.70 | 547.7 | 15.91 | 506.9 | 5.01 | 138.00 | 285.23 | 355.98 | 0.80 |
KB-26-13 | 0.05910 | 0.00198 | 0.68358 | 0.02316 | 0.08365 | 0.00076 | 572.3 | 74.06 | 528.9 | 13.97 | 517.8 | 4.54 | 158.96 | 331.23 | 379.46 | 0.87 |
KB-26-14 | 0.05821 | 0.00232 | 0.66206 | 0.02606 | 0.08279 | 0.00095 | 538.9 | 88.88 | 515.9 | 15.92 | 512.8 | 5.68 | 113.33 | 243.40 | 306.63 | 0.79 |
KB-26-16 | 0.05920 | 0.00201 | 0.70101 | 0.02393 | 0.08602 | 0.00125 | 576.0 | 78.69 | 539.4 | 14.28 | 531.9 | 7.40 | 190.04 | 361.89 | 432.30 | 0.84 |
KB-26-17 | 0.05913 | 0.00270 | 0.67400 | 0.03558 | 0.08217 | 0.00146 | 572.3 | 127.76 | 523.1 | 21.58 | 509.1 | 8.70 | 165.05 | 378.47 | 221.56 | 1.71 |
KB-26-18 | 0.05868 | 0.00262 | 0.67151 | 0.03046 | 0.08362 | 0.00180 | 553.7 | 98.13 | 521.6 | 18.50 | 517.7 | 10.72 | 198.49 | 415.08 | 424.27 | 0.98 |
KB-26-19 | 0.05990 | 0.00199 | 0.68925 | 0.02301 | 0.08357 | 0.00121 | 611.1 | 72.21 | 532.4 | 13.83 | 517.4 | 7.17 | 173.72 | 337.42 | 377.19 | 0.89 |
KB-26-20 | 0.05808 | 0.00174 | 0.64937 | 0.02015 | 0.08131 | 0.00126 | 531.5 | 66.66 | 508.1 | 12.40 | 504.0 | 7.51 | 318.15 | 645.51 | 558.34 | 1.16 |
KB-26-21 | 0.05859 | 0.00189 | 0.68120 | 0.02204 | 0.08464 | 0.00116 | 553.7 | 70.36 | 527.5 | 13.31 | 523.8 | 6.92 | 195.70 | 338.88 | 417.41 | 0.81 |
KB-26-22 | 0.05928 | 0.00358 | 0.68206 | 0.03900 | 0.08349 | 0.00326 | 576.0 | 132.23 | 528.0 | 23.55 | 516.9 | 19.38 | 159.41 | 200.48 | 402.64 | 0.50 |
KB-26-23 | 0.06142 | 0.00196 | 0.68436 | 0.01974 | 0.08144 | 0.00093 | 653.7 | 68.51 | 529.4 | 11.90 | 504.7 | 5.52 | 193.52 | 293.47 | 459.68 | 0.64 |
KB-26-24 | 0.06116 | 0.00484 | 0.67772 | 0.04259 | 0.08179 | 0.00295 | 655.6 | 170.35 | 525.4 | 25.78 | 506.8 | 17.56 | 165.11 | 212.71 | 383.31 | 0.55 |
KB-81-1 | 0.06004 | 0.00213 | 0.66399 | 0.02230 | 0.08091 | 0.00107 | 605.6 | 75.91 | 517.1 | 13.61 | 501.6 | 6.39 | 26.49 | 187.64 | 330.97 | 0.57 |
KB-81-2 | 0.05821 | 0.00203 | 0.65346 | 0.02105 | 0.08228 | 0.00114 | 538.9 | 77.77 | 510.6 | 12.93 | 509.7 | 6.77 | 40.88 | 352.60 | 490.77 | 0.72 |
KB-81-3 | 0.06149 | 0.00381 | 0.68379 | 0.03845 | 0.08167 | 0.00213 | 657.4 | 133.32 | 529.1 | 23.19 | 506.1 | 12.70 | 27.98 | 190.92 | 345.59 | 0.55 |
KB-81-4 | 0.06088 | 0.00233 | 0.68519 | 0.02497 | 0.08238 | 0.00118 | 635.2 | 83.32 | 529.9 | 15.05 | 510.3 | 7.03 | 35.97 | 246.22 | 435.34 | 0.57 |
KB-81-5 | 0.05833 | 0.00143 | 0.66421 | 0.01685 | 0.08261 | 0.00082 | 542.6 | 53.70 | 517.2 | 10.28 | 511.7 | 4.88 | 48.21 | 329.28 | 585.66 | 0.56 |
KB-81-6 | 0.06265 | 0.00597 | 0.71734 | 0.07026 | 0.08308 | 0.00219 | 696.0 | 203.68 | 549.1 | 41.56 | 514.5 | 13.03 | 36.85 | 394.07 | 406.53 | 0.97 |
KB-81-7 | 0.06094 | 0.00240 | 0.69022 | 0.02642 | 0.08248 | 0.00106 | 636.7 | 85.17 | 532.9 | 15.87 | 510.9 | 6.34 | 27.12 | 218.68 | 320.83 | 0.68 |
KB-81-8 | 0.05847 | 0.00244 | 0.65658 | 0.02750 | 0.08163 | 0.00098 | 546.3 | 90.73 | 512.5 | 16.86 | 505.8 | 5.85 | 21.62 | 130.54 | 270.59 | 0.48 |
KB-81-9 | 0.05860 | 0.00201 | 0.66130 | 0.02126 | 0.08232 | 0.00093 | 553.7 | 78.69 | 515.4 | 12.99 | 509.9 | 5.51 | 47.42 | 382.00 | 561.75 | 0.68 |
KB-81-10 | 0.05723 | 0.00204 | 0.65293 | 0.02387 | 0.08296 | 0.00129 | 501.9 | 77.77 | 510.3 | 14.67 | 513.7 | 7.66 | 41.15 | 295.92 | 494.37 | 0.60 |
KB-81-11 | 0.05881 | 0.00225 | 0.65222 | 0.02397 | 0.08123 | 0.00140 | 561.1 | 83.32 | 509.8 | 14.73 | 503.4 | 8.37 | 39.12 | 298.38 | 478.59 | 0.62 |
KB-81-12 | 0.06282 | 0.00263 | 0.70654 | 0.02906 | 0.08236 | 0.00136 | 701.9 | 89.65 | 542.7 | 17.29 | 510.2 | 8.12 | 30.47 | 185.74 | 375.81 | 0.49 |
KB-81-13 | 0.05858 | 0.00304 | 0.66136 | 0.03487 | 0.08226 | 0.00154 | 550.0 | 112.95 | 515.4 | 21.32 | 509.6 | 9.14 | 34.35 | 204.96 | 421.38 | 0.49 |
KB-81-14 | 0.05774 | 0.00222 | 0.65190 | 0.02464 | 0.08241 | 0.00126 | 520.4 | 87.95 | 509.6 | 15.15 | 510.5 | 7.53 | 43.03 | 280.42 | 531.47 | 0.53 |
KB-81-15 | 0.06107 | 0.00232 | 0.68192 | 0.02667 | 0.08146 | 0.00133 | 642.6 | 76.84 | 527.9 | 16.10 | 504.8 | 7.91 | 39.98 | 273.58 | 493.02 | 0.55 |
KB-81-16 | 0.06055 | 0.00176 | 0.69754 | 0.02070 | 0.08384 | 0.00119 | 633.4 | 61.10 | 537.3 | 12.38 | 519.0 | 7.07 | 84.00 | 430.41 | 1047.75 | 0.41 |
KB-81-17 | 0.05988 | 0.00341 | 0.67154 | 0.03465 | 0.08291 | 0.00142 | 598.2 | 122.21 | 521.6 | 21.05 | 513.5 | 8.43 | 18.15 | 95.22 | 225.51 | 0.42 |
KB-81-18 | 0.06111 | 0.00288 | 0.70427 | 0.03351 | 0.08431 | 0.00127 | 642.6 | 101.84 | 541.3 | 19.97 | 521.8 | 7.56 | 27.92 | 147.84 | 342.84 | 0.43 |
KB-81-19 | 0.05864 | 0.00216 | 0.67410 | 0.02493 | 0.08355 | 0.00124 | 553.7 | 79.62 | 523.2 | 15.12 | 517.3 | 7.38 | 77.50 | 604.90 | 919.21 | 0.66 |
KB-81-20 | 0.05860 | 0.00236 | 0.67375 | 0.03269 | 0.08375 | 0.00275 | 553.7 | 88.88 | 523.0 | 19.84 | 518.5 | 16.36 | 74.02 | 694.08 | 839.68 | 0.83 |
KB-81-21 | 0.06190 | 0.00196 | 0.73603 | 0.02451 | 0.08623 | 0.00126 | 672.2 | 62.80 | 560.1 | 14.34 | 533.2 | 7.48 | 49.82 | 345.96 | 562.44 | 0.62 |
KB-81-22 | 0.06049 | 0.00282 | 0.70626 | 0.03306 | 0.08539 | 0.00125 | 620.4 | 101.84 | 542.5 | 19.67 | 528.2 | 7.44 | 23.17 | 103.25 | 282.14 | 0.37 |
KB-81-23 | 0.05967 | 0.00188 | 0.67760 | 0.01948 | 0.08299 | 0.00103 | 590.8 | 68.51 | 525.3 | 11.79 | 514.0 | 6.11 | 54.79 | 360.50 | 643.54 | 0.56 |
KB-81-24 | 0.05925 | 0.00185 | 0.68230 | 0.02168 | 0.08385 | 0.00130 | 576.0 | 68.51 | 528.2 | 13.09 | 519.1 | 7.72 | 48.62 | 343.90 | 562.80 | 0.61 |
KB-81-25 | 0.05836 | 0.00160 | 0.65881 | 0.01870 | 0.08181 | 0.00099 | 542.6 | 63.88 | 513.9 | 11.45 | 506.9 | 5.90 | 55.56 | 436.41 | 655.44 | 0.67 |
KB-81-26 | 0.05794 | 0.00136 | 0.65695 | 0.01507 | 0.08249 | 0.00107 | 527.8 | 56.47 | 512.7 | 9.24 | 511.0 | 6.37 | 98.72 | 850.43 | 1118.25 | 0.76 |
U-Pb concordia diagrams for zircons from rhyolite (KB-26, KB-81). Data-point error crosses are 2
For sample KB-26, 206Pb/238U ages determined from 24 analytical spots ranged from 502 to 523 Ma with a weighted mean age of
Zircon Lu-Hf isotopic data are listed in Table
Zircon Lu-Hf isotopic data for rhyolite (KB-26, KB-81) from the northern Altyn Tagh region.
Sample no. | 176Yb/177Hf | 176Lu/177Hf | 176Hf/177Hf | 2 |
|||||||
---|---|---|---|---|---|---|---|---|---|---|---|
KB-26-1 | 503 | 0.251434 | 0.003659 | 0.282705 | 0.000027 | 0.282670 | -2.4 | 7.5 | 834 | 995 | -0.89 |
KB-26-2 | 508 | 0.169238 | 0.002433 | 0.282607 | 0.000026 | 0.282584 | -5.8 | 4.5 | 949 | 1186 | -0.93 |
KB-26-3 | 510 | 0.217950 | 0.003014 | 0.282726 | 0.000032 | 0.282697 | -1.6 | 8.6 | 788 | 930 | -0.91 |
KB-26-4 | 502 | 0.204649 | 0.003001 | 0.282644 | 0.000032 | 0.282616 | -4.5 | 5.5 | 909 | 1117 | -0.91 |
KB-26-5 | 513 | 0.169703 | 0.002366 | 0.282637 | 0.000032 | 0.282614 | -4.8 | 5.7 | 904 | 1115 | -0.93 |
KB-26-6 | 503 | 0.172736 | 0.002428 | 0.282691 | 0.000031 | 0.282668 | -2.9 | 7.4 | 825 | 999 | -0.93 |
KB-26-7 | 509 | 0.173820 | 0.002490 | 0.282637 | 0.000033 | 0.282613 | -4.8 | 5.6 | 907 | 1119 | -0.92 |
KB-26-8 | 501 | 0.234971 | 0.003456 | 0.282859 | 0.000034 | 0.282827 | 3.1 | 13.0 | 596 | 642 | -0.90 |
KB-26-9 | 514 | 0.263981 | 0.003888 | 0.282780 | 0.000035 | 0.282743 | 0.3 | 10.3 | 724 | 824 | -0.88 |
KB-26-10 | 519 | 0.161216 | 0.002365 | 0.282724 | 0.000034 | 0.282701 | -1.7 | 8.9 | 776 | 915 | -0.93 |
KB-26-11 | 514 | 0.193726 | 0.002916 | 0.282776 | 0.000041 | 0.282748 | 0.2 | 10.5 | 710 | 811 | -0.91 |
KB-26-12 | 506 | 0.065240 | 0.000959 | 0.282643 | 0.000039 | 0.282634 | -4.6 | 6.3 | 861 | 1074 | -0.97 |
KB-26-13 | 517 | 0.215409 | 0.003017 | 0.282837 | 0.000038 | 0.282808 | 2.3 | 12.7 | 622 | 675 | -0.91 |
KB-26-14 | 512 | 0.207593 | 0.003169 | 0.282819 | 0.000039 | 0.282789 | 1.7 | 11.9 | 651 | 722 | -0.90 |
KB-81-01 | 501 | 0.193927 | 0.002814 | 0.282829 | 0.000043 | 0.282803 | 2.0 | 12.1 | 630 | 697 | -0.92 |
KB-81-02 | 510 | 0.165903 | 0.002392 | 0.282632 | 0.000030 | 0.282609 | -5.0 | 5.5 | 912 | 1129 | -0.93 |
KB-81-03 | 506 | 0.286633 | 0.004118 | 0.282808 | 0.000031 | 0.282769 | 1.3 | 11.0 | 687 | 771 | -0.88 |
KB-81-04 | 510 | 0.241194 | 0.003556 | 0.282895 | 0.000033 | 0.282861 | 4.4 | 14.4 | 543 | 559 | -0.89 |
KB-81-05 | 511 | 0.281597 | 0.004062 | 0.282903 | 0.000033 | 0.282864 | 4.6 | 14.5 | 538 | 551 | -0.88 |
KB-81-06 | 514 | 0.171756 | 0.002528 | 0.282724 | 0.000028 | 0.282699 | -1.7 | 8.8 | 780 | 922 | -0.92 |
KB-81-07 | 511 | 0.301216 | 0.004433 | 0.282912 | 0.000035 | 0.282870 | 5.0 | 14.7 | 530 | 538 | -0.87 |
KB-81-08 | 506 | 0.184141 | 0.002788 | 0.282874 | 0.000032 | 0.282847 | 3.6 | 13.8 | 563 | 593 | -0.92 |
KB-81-09 | 510 | 0.213336 | 0.003195 | 0.282675 | 0.000033 | 0.282645 | -3.4 | 6.7 | 867 | 1047 | -0.90 |
KB-81-10 | 514 | 0.224146 | 0.003340 | 0.282708 | 0.000031 | 0.282676 | -2.3 | 7.9 | 822 | 975 | -0.90 |
KB-81-11 | 503 | 0.215673 | 0.003251 | 0.282701 | 0.000035 | 0.282670 | -2.5 | 7.5 | 830 | 995 | -0.90 |
KB-81-13 | 510 | 0.192282 | 0.002833 | 0.282762 | 0.000037 | 0.282735 | -0.4 | 9.9 | 730 | 844 | -0.91 |
KB-81-14 | 511 | 0.160352 | 0.002419 | 0.282766 | 0.000028 | 0.282743 | -0.2 | 10.2 | 716 | 826 | -0.93 |
KB-81-15 | 505 | 0.167696 | 0.002422 | 0.282716 | 0.000032 | 0.282693 | -2.0 | 8.3 | 789 | 942 | -0.93 |
Fourteen Hf isotopic spot analyses were conducted on zircons from the rhyolite with radiogenic 176Hf/177Hf(
Fourteen in a total of fifteen Hf isotopic spot analyses were conducted on zircons from the rhyolite with radiogenic 176Hf/177Hf(t) ratios varying from 0.282432 to 0.282870, with an average of 0.282728. Their
The results of Sr-Nd analysis for the rhyolite samples are presented in Table
Rb-Sr and Sm-Nd isotopic compositions for the Early Cambrian bimodal volcanic rock in the North Altyn Tagh tectonic belt.
Sample | Rock | Age (Ma) | 2 |
2 |
||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
KB-26 | Rhyolite | 512 | 1.072 | 0.71677 | 0.70895 | 3 | 0.1225 | 0.51207 | 0.51166 | 2 | -6.3 | 1.81 |
KB-33 | Rhyolite | 512 | 0.1004 | 0.71380 | 0.71307 | 3 | 0.1260 | 0.51217 | 0.51175 | 2 | -4.5 | 1.70 |
KB-46 | Rhyolite | 512 | 0.9701 | 0.71785 | 0.71077 | 3 | 0.1362 | 0.51231 | 0.51185 | 2 | -2.5 | 1.66 |
KB-76 | Rhyolite | 512 | 3.2086 | 0.73287 | 0.70946 | 3 | 0.1276 | 0.51223 | 0.51180 | 1 | -3.5 | 1.63 |
KB-132 | Rhyolite | 512 | 0.0103 | 0.71140 | 0.71133 | 2 | 0.1440 | 0.51226 | 0.51178 | 1 | -4.0 | 1.95 |
KB-19 | Basalt | 512 | 0.0717 | 0.70656 | 0.70604 | 2 | 0.1806 | 0.51277 | 0.51217 | 4 | 3.6 | 1.75 |
KB-23 | Basalt | 512 | 0.0532 | 0.70811 | 0.70772 | 6 | 0.1698 | 0.51269 | 0.51212 | 6 | 2.8 | 1.59 |
KB-70 | Basalt | 512 | 0.0645 | 0.70864 | 0.70817 | 3 | 0.1764 | 0.51271 | 0.51212 | 3 | 2.7 | 1.79 |
The North Altyn Tagh subduction-collision complex is one of the most important tectonic units in Western China. The study on Karadaban bimodal volcanic rocks provides new evidences for the early Paleozoic tectonic evolution. In this study, we obtained the zircon LA-ICP-MS U-Pb age conducted on the rhyolite in the northern Altyn region, and the age of volcanic rocks was
Previous studies show that the magmatic activity in the Altyn area generated a series of magma evolution from basic to felsic rocks, which mainly occurred from 520~480 Ma [
Bimodal volcanic rocks occur within a variety of tectonic settings, such as continental rifts, oceanic islands, zones of continental break-up, back-arc basins, orogenic belts, intraoceanic island arcs, and mature island arcs/active margins [
Samples from the northern Altyn region have experienced less alteration. The loss of ignition (LOI) values for the mafic and felsic volcanic rocks in the study area are 1.92–6.87 and 0.74–3.68 wt.%, respectively. This alteration is a result of low-temperature metamorphism related to intense deformation and fluid infiltration, whose effects are principally evident in the various types of abundance of the mobile elements such as K2O, Rb, Ba, and Cs. However, the Fe, Al, Ca, Mg, REE, and HFSE elements are considered to be relatively immobile during the low-temperature alteration [
The low silica content (
Plots of Nb/La vs. Nb/Th (a) and (Th/Ta)PM vs. (La/Ta)PM (b) for the basalt of Kaladaban bimodal volcanic rocks (after [
Excluding the garnet residue in the source, the Nb/Yb and Ta/Yb ratios are effective in constraining the relative depletion of the mantle source when the degree of partial melting is even lower [
Nb/Yb vs. Th/Yb diagram for the basalt of the North Altyn Tagh bimodal volcanic rocks. The field of MORB-OIB is from [
Felsic volcanic rocks are characterized by high SiO2, K2O+Na2O, and Fe2O3T content; low Al2O3, CaO, and MgO content; depletion in HFSEs (e.g., Nb, Ta, P, Ti, and Hf); and strong negative Eu anomalies. These compositional features suggest that the samples in the present study originated from a source that lacked garnet but contained plagioclase in the residue, which indicates partial melting under low-pressure conditions [
Chemical classification diagrams for the felsic volcanic rocks in the North Altyn Tagh region. (a) Ga/Al vs. Zr diagram (after [
Plots of the volcanic rocks from the North Altyn Tagh region in Nb-Y-3Ga and Nb-Y-Ce (after [
Two types of process have been proposed for the origin of the silicic end-member in the bimodal magmatic suite, including (1) extensive fractional crystallization from a common mantle-derived magma parental to the mafic end-member, coupled with crustal contamination [
Summary of zircon U-Pb ages for igneous rocks in the Altyn Tagh region.
No. | Locality | Age (Ma) | Method | Lithology | Tectonic setting | Reference |
---|---|---|---|---|---|---|
1 | Hongliutan | U-Pb LA-ICP-MS | Plagiogranite | Subduction zone | [ | |
2 | Lapeiquan | U-Pb LA-ICP-MS | Plagiogranite | Subduction zone | [ | |
3 | North Highland 4337 | Zircon SHRIMP | Granodiorite | Island arc | [ | |
4 | Hongliugou | U-Pb LA-ICP-MS | Granite | Subduction zone | [ | |
5 | Highland 4337 | Zircon SHRIMP | Granodiorite | Active continental margin | [ | |
6 | Kaladawan | 488~477 | Zircon SHRIMP | Dacite | Subduction zone | [ |
7 | North Altyn region | U-Pb LA-ICP-MS | Rhyolite | Island arc | [ | |
8 | South Kaladawan | Zircon SHRIMP | Monzonite granite | Active continental margin | [ | |
9 | North Bashikaogong | U-Pb LA-ICP-MS | Quartz diorite | Subduction zone | [ |
Plot of
Based on the discussion above, we conclude that the felsic volcanic rocks produced mainly by partial melting of the juvenile crustal and the origin melt source itself likely resulted from mantle-derived magmatic upwelling in an extensional setting, probably near the subduction-related active continental margin.
The north Altyn region is a part of the Proto-Tethys tectonic domain. The Cambrian-Ordovician basic rocks and calc-alkaline granite in this region are interpreted to be island arc volcanic rocks [
On the one hand, according to the feature of trace element, the Karadaban volcanic suite displays some geochemical features of subduction-related melts, such as enrichment of LILEs but depletion in HFSEs and Sr-Nd-Hf isotope compositions of magmas. This suggests that the mixing of juvenile crust and mantle-derived magma, as well as the Karadaban bimodal volcanic suite, was formed in a subduction-related environment. The conclusion is further supported by a series of tectonomagmatic discrimination diagrams. Based on the Hf-Th-Nb and Zr-Ti-Y ternary diagrams, the Karadaban rocks are classified as volcanic arc calc-alkaline basalts (Figure
Hf-Th-Nb and Zr-Ti-Y tectonic discriminant diagram of Karadaban basalt. Data for the Lapeiquan diabase are from [
The Altyn region is of great significance to realize the evolution of the Tibetan Plateau and the tectonic pattern of Western China. It developed as a unified back-arc basin system between the northern Altyn Complex and MJB before the formation of the Altyn Tagh, which is the northern boundary of the Proto-Tethys in China [
The following conclusions can be drawn from this study.
New LA-ICP-MS zircon U-Pb dating indicates that the newly discovered bimodal volcanic rocks in the northern Altyn Tagh area formed at ca. 512 Ma Whole-rock geochemical and Sr-Nd-Hf isotopic compositions suggest that the mafic rocks were derived mainly from a depleted mantle source but mixed with crustal components. The felsic rocks were probably generated by partial melting of the juvenile crustal as a result of basic magma upwelling The generation of the Karadaban bimodal volcanic rocks was not only related to an active continental margin setting but also associated with the subduction-related back-arc extension environment
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This study was financially supported by the Development and Research Centre of the China Geological Survey, Beijing (Grant No. 12120113090000).
Supplementary Table 1: major (%) and trace element (ppm) data for the volcanic rocks in the northern Altyn Tagh region. Supplementary Table 2: LA-ICP-MS zircon U-Pb dating data for Karadaban rhyolite (KB-26, KB-81) from the northern Altyn Tagh region. Supplementary Table 3: zircon Lu-Hf isotopic data for rhyolite (KB-26, KB-81) from the northern Altyn Tagh region. Supplementary Table 4: Rb-Sr and Sm-Nd isotopic compositions for the volcanic rocks in the northern Altyn Tagh region.