SCIENCE CHINA The geochemical characteristics ... - Springer Link

SCIENCE CHINA Earth Sciences • RESEARCH PAPER •

August 2013 Vol.56 No.8: 1318–1333 doi: 10.1007/s11430-012-4483-3

The geochemical characteristics, geochronology and tectonic significance of the Carboniferous volcanic rocks of the Santanghu area in northeastern Xinjiang, China LI Wei1*, LIU YiQun1, DONG YunPeng1, ZHOU XiaoHu1, LIU XiaoMing1, LI Hong1, FAN TingTing1, ZHOU DingWu2, XU XueYi3 & CHEN JunLu3 2

1 State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China; College of Geological Science & Engineering, Shandong University of Science and Technology, Qingdao 266510, China; 3 Xi’an Center of Geological Survey, China Geological Survey, Xi’an 710054, China

Received February 14, 2012; accepted June 20, 2012; published online September 30, 2012

The Santanghu area is located on the northeastern margin of the Junggar Basin, northern Xinjiang, Northwest China. The Carboniferous volcanic rocks in this area are widely distributed in Kaokesaiergaishan, Santanghu, Daheishan and Naomaohu districts, which are located to the north of the Kalameili Fault. These rocks, sourced from a cognate magma, consist of basic, intermediate, and acidic lavas, and pyroclastic rock. The basic volcanic rocks are enriched with large-ion lithophile elements (LILE), but are relatively depleted in high field strength elements (HFSE), and have an obvious negative Nb-Ta-Ti anomaly. They were most probably derived from a depleted mantle source, and during their ascent, these magmas were not contaminated by the crustal material as they underwent magma crystallization differentiation. Based on the Carboniferous volcanic assemblage and geochemical data, it is apparent that the early Carboniferous volcanism occurred in a subduction-related tectonic setting. New LA-ICP-MS zircon U-Pb analyses constrain the age of the andesite within the volcanic rocks as the early Carboniferous (328.9–331.3 Ma). Combined with the regional geological record, comprehensive analysis of the isotope geochronological data indicates that the subduction of the Junggar Ocean predates the early Carboniferous, and that the Santanghu island arc magmatism was induced by the subduction of the Junggar Ocean in the Carboniferous. northeastern Xinjiang, Santanghu area, active continental margins, Carboniferous volcanic rocks Citation:

Li W, Liu Y Q, Dong Y P, et al. The geochemical characteristics, geochronology and tectonic significance of the Carboniferous volcanic rocks of the Santanghu area in northeastern Xinjiang, China. Science China: Earth Sciences, 2013, 56: 1318–1333, doi: 10.1007/s11430-012-4483-3

The Central Asian Orogenic Belt (CAOB) is one of the world’s largest Phanerozoic accretionary orogenic belts. It is bounded by the Siberian Craton to the north and the Tarim-North China Craton to the south [1–6]. This orogenic belt is also the world’s most conspicuous Phanerozoic continental orogenic belt. The CAOB has a complex tectonic pattern that consists of a mosaic of many blocks separated by suture zones [7–9]. The late Paleozoic is an important *Corresponding author (email: [email protected])

© Science China Press and Springer-Verlag Berlin Heidelberg 2012

period for ocean-land tectonic evolution and thus has become a focus of research. However, in northern Xinjiang, the tectonic patterns of transformation and complicated ocean-land orogenic processes of the late Paleozoic are not yet fully understood. As a result, ocean evolution has become a bone of contention [1, 8–18]. The Santanghu area on the northeastern margin of the Junggar Basin is located in the Armantai and Karamaili suture zone at the junction of the Siberia, Junggar and Tarim plates. Because of its particular tectonic position, this area has become a key area in the study of the tectonic evoearth.scichina.com

www.springerlink.com

Li W, et al.

Sci China Earth Sci

lution of the CAOB. Additionally, the Permian-Cenozoic intracontinental Santanghu Basin is superimposed on top of the suture zones [19, 20]. A large region of the late Paleozoic volcanic rocks is exposed around the Santanghu Basin, which has become important in the study of Paleo-Asian Ocean evolution. Although previous research into the volcanics of the northern Xinjiang region has been extensive, significant disputes remain concerning the late Paleozoic tectonic setting of the northeastern margin of the Junggar Basin, especially in the Karamaili area. These disputes generally fall into one of two views: i.e. the Carboniferous volcanic rocks formed in either an intracontinental orogenic extensional setting [21–26] or a subduction-related island arc setting [27–32]. Based on a comprehensive comparison of the Carboniferous strata in northern Xinjiang, the volcanic petrology, geochronology, and geochemistry in the Santanghu area can be used to constrain the timing of the closure of the late Paleozoic Paleo-Asian ocean and the tectonic transition of the region.

1 Regional geological setting Northern Xinjiang is an important part of the CAOB in mainland China, and is important for the study of continental geology. Current research shows that the Paleozoic tectonic evolution of northern Xinjiang is typical of an accretionary orogen, which in this case consists of the subduction

Figure 1

August (2013) Vol.56 No.8

1319

and closure of the Paleo-Asian Ocean and the accretion of land masses by collision, as indicated by a series of suture zones [5, 16, 33, 34]. The Santanghu Basin, located between the Siberia and Junggar plates, contains the Permian to Cenozoic continental sedimentary facies that were superimposed on the Paleozoic orogenic belt. The basin is bounded to the north by the Hapulake and Suhaitu mountains and to the southwest by the Kelamaili and Haerlike mountains, which are all characterized by extensive late Paleozoic volcanic outcrops (Figure 1). The Beitashan-Kaokesaiergaishan area consists of the exposed late Silurian-Permian strata to the south of the Zhaheba-Armantai tectonic belt, which runs northeast to southwest across the northern part of the Santanghu Basin. To the south of Santanghu Basin, the Devonian-Carbonif- erous pyroclastic rocks, epicontinental clastic sedimentary rocks, and the abundant Hercynian granite are exposed in the Haerlikeshan area. In contrast, the Devonian-Permian strata can be only seen in the Suhaitushan-Laoyemiao area. And the late Paleozoic volcanic rocks exposed in the periphery of the Santanghu Basin provide a great deal of geological information to study the tectonic evolution of the period. From the oldest to the youngest, the lower Ordovician, lower Silurian, Devonian, Carboniferous and Permian– Mesozoic and Cenozoic units are exposed in the Santanghu area. The lower Ordovician is composed of the Wuliegai Formation (O3w), Daliugou Formation (O3d), and Miaoergou Formation. The base of the lower Ordovician is generally not seen, with the overlying strata forming an unconforma-

Simplified geological map of the Santanghu Basin and adjacent area with sample locations.

1320

Li W, et al.

Sci China Earth Sci

ble contact, and is distributed in the Banfanggou and Shimengou areas of southern Santanghu. The Wuliegai Formation is mainly composed of calcareous siltstone and sandstone, mixed with a small amount of banded marble, partially visible high-grade metamorphic schist, granulite, gneiss and migmatite, and includes brachiopods and coral fossils. The Daliugou Formation is mainly composed of intermediate-acidic volcanic rocks mixed with a few tuff and silty sandstone, with locally visible phyllite. The Miaoergou Formation is mainly composed of slightly metamorphosed clastics, and normal sedimentary clastic rocks, with trilobites, brachiopods and coral fossils. The upper Silurian Hongliugou Formation (S4D1h) has an unconformable contact with the overlying Devonian. It is mainly composed of clastics with a few tuffs, limestones and volcanic rocks, containing a wealth of corals, trilobites, and brachiopods. The Devonian sediments are mainly distributed on the north and south sides of the Santanghu Basin, the south side is mainly exposed in the Kaokesaiergaishan and the northern part of the Haerlikeshan areas. The Devonian, in a conformable contact with the underlying upper Silurian and the overlying Carboniferous strata, is composed of the Zhuomubasitao (D1z), Wulubasitao and Keankuduke Formations, a set of continuously deposited, marine fossi rich clastic rocks, with a large number of plant fossils at the top of the Keankuduke Formation. The Carboniferous is mainly distributed along on both the north and south sides of the Santanghu Basin. The Carboniferous units on the south side are mainly exposed to the NE in the Kaokesaigaishan and Daheishan areas. The Carboniferous is in a conformable contact with the underlying Devonian and unconformable contact with the overlying Permian. The Donggulubasitao and Jiangbasitao Formations of the Lower Carboniferous are mainly common clastic deposit with thin layers of tuff and bioclastic limestone, producing a large number of brachiopods, corals, crinite, animal and plant fossils that are indicative of a shallow marine depositional environment. The Carboniferous volcanic and volcaniclastic rocks unconformably overlie the sedimentary clastic rocks. These rocks can be compared with the Batamayineishan Formation in the southern part of the Santanghu Basin based on their lithological associations (which consist mainly of basalt, basaltic andesite, andesite and pyroclastic rocks, with small quantities of acidic volcanic rocks in the upper part). A Carboniferous measured section at the edge of the basin shows that the lower part is a normal clastic deposits, whereas the upper part with volcanic and volcaniclastic rocks (Figure 2). More than 30 rock samples have been collected from these volcanic rocks in the Santanghu area (Figures 1 and 2), and combined with a small number of borehole core samples from the Turpan-Hami Oilfield. Mafic volcanic samples are dark gray-green in color and exhibit a porphyritic texture under the microscope with a coarse mysterious structure, an intergranular structure or an


implicit structure. Massive basalt samples from the Dahuangshui area show a visible amygdaloidal structure. The major minerals in the basalt samples are plagioclase and clinopyroxene, with minor minerals of olivine and magnetite. The phenocrysts are mainly self-shaped tabular plagioclases. A small amount of pyroxene and plagioclase has been chloritized. Intermediate volcanic rocks are mainly basaltic andesite and andesite, with an intersertal, porphyritic texture, and an interwoven glass base structure under the microscope. The main mineral is plagioclase, and the secondary minerals are ilmenite and magnetite. Phenocrysts include plagioclase, alkali feldspar, and amphibole, with the matrix dominated by plagioclase and a small amount of biotite. Minor plagioclase and glassy component fractions have been somewhat chloritized. Acidic volcanic rocks are mainly dacite, with a porphyritic or rhyolitic structure, containing phenocrysts of plagioclase and quartz.

2 Analytical methods Rock elemental analyses and zircon U-Pb dating analyses presented here have all been performed at the State Key Laboratory of Continental Dynamics, Northwest University, in Xi’an, China. 2.1

Whole-rock major and trace element analyses

Major element oxides were determined by a RIGAKU 2100 system using Li-borate glass disks. Analyses of international rock standards (USGS) BHVO-1 and AGV-1 indicate that the precision and accuracy are both better than 5%. During the digestion of the standards and samples, 0.5 g of powder was mixed with 3.6 g Li2B4O7, 0.4 g LiF and a little LiBr, resulting in the formation of a glass bead after fusion. Trace elements were measured by an Agilent 7500a ICP-MS. Samples of about 50 mg were dissolved in a sealed high-temperature and high-pressure bomb using equal parts of super-pure HF and HNO3. Analyses of USGS rock standards BHVO-1 and AGV-1 indicate that the analytical precision is mostly better than 5%, as indicated by the relative standard deviation. For the details of the sample digestion process, see ref. [35]. 2.2

Zircon U-Pb dating

Zircons were extracted using a combined technique of heavy liquid and magnetic separation. They were then handpicked under a binocular microscope, mounted in epoxy resin, and polished until the grain centers were exposed. The polished zircons were then photographed with transmitted light, reflected light, and a cathode luminescence (CL) camera, in order to assist with the selection of

Li W, et al.

Figure 2

Sci China Earth Sci


1321

Schematic Carboniferous stratigraphic column and sample locations in the Santanghu area.

the best parts of the zircon and to explain the data. The zircons were dated on an Agilent 7500a ICP-MS instrument equipped with a 193 nm GeoLas laser-ablation system. He-

lium was used as the carrier gas to ensure that efficient aerosol was delivered to the torch. The laser ablation beam spot diameter is 30 μm with the frequency of 6 Hz. Laser

1322 Table 1

Li W, et al.

Sci China Earth Sci


Major (wt%) and trace-element (ppm) concentrations of Carboniferous rocks in the Santanghu areaa)

Samples DHS35 DHS34 Basic rocks SiO2 50.2 47.1 TiO2 0.74 0.54 Al2O3 16.9 20.0 ∑Fe2O3 11.4 10.5 MnO 0.16 0.15 MgO 4.81 6.56 CaO 6.95 7.71 Na2O 4.12 2.90 K2O 0.15 0.60 P2O5 0.13 0.12 LOI 4.73 3.99 Total 100.3 100.2 Mg# 0.50 0.59 Cr 22.7 17.8 Ni 14.6 18.7 Rb 2.90 10.6 Ba 92.0 170 Th 1.01 1.20 U 0.39 0.47 Nb 1.10 0.68 Ta 0.08 0.05 La 7.17 9.23 Ce 18.7 22.3 Pr 2.73 3.04 Sr 449 561 Nd 13.1 13.7 Zr 63.9 40.3 Hf 1.70 1.14 Sm 3.36 3.12 Eu 1.02 0.95 Gd 3.17 2.72 Tb 0.51 0.39 Dy 3.17 2.38 Y 19.3 13.7 Ho 0.72 0.53 Er 1.96 1.40 Tm 0.31 0.21 Yb 1.94 1.38 Lu 0.31 0.22 Li 10.9 20.4 Be 0.43 0.35 Sc 26.9 22.2 V 312 252 Co 33.9 33.7 Cu 158 52.4 Zn 81.1 69.7 Ga 15.8 16.7 Ge 1.13 1.30 Cs 0.08 0.19 Pb 3.20 3.02 Samples Basalt SiO2 TiO2

DHS33 M17A08 45.9 0.68 17.1 11.5 0.18 9.03 8.48 2.24 1.25 0.24 3.57 100.2 0.65 82.7 38.2 24.5 383 1.56 0.72 0.77 0.06 16.4 38.1 5.29 386 23.9 53.0 1.52 5.38 1.59 4.62 0.62 3.42 18.6 0.70 1.77 0.26 1.58 0.25 21.9 0.55 44.4 286 48.0 107 76.2 15.1 1.30 0.51 3.08

STH07 N9-10A 46.1 1.03

49.3 1.70

M25A1

ZF01

DJD01

DJD06

50.3 1.51 16.1 10.1 0.15 4.56 6.83 4.33 1.66 0.80 3.75 100.1 0.51 131 55.8 13.2 856 1.25 0.45 9.85 0.54 26.8 58.7 7.63 1035 34.6 153 3.42 6.88 2.03 6.23 0.86 4.86 26.4 0.99 2.61 0.37 2.35 0.35 18.2 1.76 23.3 169 52.3 40.2 106 20.0 1.27 0.22 8.41

51.0 1.25 17.0 8.52 0.10 4.79 6.91 4.03 0.37 0.27 6.13 100.4 0.57 241 97.9 4.43 191 2.64 0.78 5.54 0.35 15.7 34.9 4.60 440 20.3 144 3.54 4.93 1.43 4.88 0.76 4.56 25.9 0.99 2.60 0.38 2.34 0.37 25.7 1.04 21.6 165 32.5 37.0 70.0 18.2 1.16 0.72 4.53

49.6 1.31 13.8 9.12 0.13 2.94 8.01 4.30 2.82 0.52 7.25 99.8 0.43 310 164 41.4 1207 4.21 0.84 11.6 0.64 40.4 89.3 10.9 485 44.2 173 4.03 8.32 2.34 6.52 0.84 4.31 21.3 0.79 2.06 0.28 1.72 0.25 20.5 1.12 20.5 130 39.9 44.5 76.3 15.6 0.96 0.67 8.59

49.7 1.73 17.0 12.6 0.21 3.88 9.44 2.86 0.45 0.61 1.02 99.5 0.42 21.4 14.7 6.73 331 1.17 0.48 5.96 0.34 17.0 41.3 5.87 583 28.2 132 3.50 7.46 1.98 7.39 1.18 7.01 41.9 1.47 4.03 0.60 3.70 0.55 4.00 1.28 36.7 289 44.5 232 119 21.0 1.45 0.14 8.25

49.4 1.73 16.9 9.72 0.33 5.05 8.52 3.74 1.52 0.54 2.82 100.3 0.55 275 152 11.8 482 1.19 0.19 12.2 0.72 20.8 48.9 6.44 1011 28.4 196 4.36 5.77 1.78 5.53 0.80 4.63 24.7 0.94 2.50 0.36 2.23 0.32 21.3 1.87 25.5 220 55.2 69.3 86.1 20.7 1.46 0.04 5.60

ZFN03 52.0 1.16

ZFN09 ZFN10 52.0 1.27

52.3 1.25

ZFN12 51.2 1.21

Samples 09DH24 Andesite 58.9 0.40

DHA04 58.5 0.99

DJA04 STH02 STH03A STH03B STH06 47.1 1.36 16.8 9.27 0.12 7.49 7.06 3.20 0.23 0.22 7.37 100.2 0.65 39.5 52.4 3.44 692 1.72 0.50 4.30 0.28 10.5 24.7 3.46 461 16.4 117 3.14 4.34 1.44 4.82 0.80 4.87 27.5 1.05 2.85 0.42 2.65 0.39 79.0 0.87 23.2 189 54.0 30.1 76.1 18.2 1.43 0.51 2.96

47.9 1.87 17.2 12.8 0.20 3.77 9.22 3.46 0.60 0.93 1.63 99.5 0.41 6.81 10.1 6.34 461 1.13 0.45 7.29 0.38 21.9 55.6 8.02 661 39.0 163 4.35 10.3 2.53 10.2 1.61 9.53 55.3 1.97 5.42 0.81 4.97 0.73 8.36 1.61 35.5 234 43.7 402 135 22.4 1.52 0.21 10.8

48.9 1.47 15.7 11.0 0.19 4.65 9.72 2.64 0.35 0.61 4.40 99.6 0.50 219 126 4.91 685 5.36 1.15 8.93 0.47 36.9 78.4 9.49 757 40.2 159 3.90 8.46 2.19 7.37 1.06 5.97 33.5 1.20 3.22 0.48 2.96 0.43 27.4 1.32 29.3 238 44.3 158 111 21.0 1.56 0.28 7.95

49.6 1.65 16.9 12.4 0.20 3.76 9.23 2.83 1.22 0.61 1.22 99.6 0.41 20.4 13.4 20.1 619 1.13 0.53 5.67 0.32 16.9 40.5 5.77 613 27.6 129 3.46 7.17 1.96 7.25 1.15 6.85 40.4 1.43 3.95 0.59 3.68 0.55 3.82 1.18 35.1 280 50.3 261 117 21.2 1.50 0.19 9.28

44.7 1.80 14.8 10.9 0.17 4.16 11.4 2.70 0.49 0.51 7.89 99.6 0.47 122 86.2 2.46 244 0.71 0.27 6.26 0.40 16.5 40.7 5.52 606 25.2 187 3.90 5.94 1.90 5.93 0.94 5.55 31.4 1.15 3.15 0.47 2.88 0.42 14.4 1.64 23.7 196 42.9 62.1 89.8 18.2 1.41 0.34 3.83

DH03 M19A21 FIA6 DJD02 DJD03 52.4 0.74

53.4 0.94

54.0 1.19

53.2 1.42

55.4 1.41

(To be continued on the next page)

Li W, et al.

Sci China Earth Sci

1323


(Continued) Samples Basalt Al2O3 ∑Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total Mg# Cr Ni Rb Ba Th U Nb Ta La Ce Pr Sr Nd Zr Hf Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu Li Be Sc V Co Cu Zn Ga Ge Cs Pb Samples Andesite SiO2 TiO2 Al2O3 ∑Fe2O3 MnO

STH07 N9-10A 17.1 11.7 0.34 4.45 11.2 2.01 0.09 0.19 5.31 99.5 0.47 13.2 12.7 0.95 158 0.27 0.10 1.95 0.11 5.56 13.6 1.97 548 9.74 43.8 1.23 2.70 0.94 2.89 0.48 2.98 18.7 0.66 1.88 0.29 1.86 0.29 7.69 0.49 31.2 300 35.9 75.0 105 18.7 1.27 0.26 2.15

ZFN03

16.4 9.64 0.30 6.45 7.96 3.63 1.51 0.55 4.07 100.4 0.61 287 155 13.1 488 1.25 0.47 12.3 0.74 21.0 48.1 6.25 1036 28.0 195 4.29 5.58 1.71 5.27 0.76 4.35 23.7 0.90 2.37 0.34 2.16 0.32 8.88 1.79 23.8 211 78.6 49.5 85.5 18.9 1.45 0.12 5.77

ZFN09 ZFN10

15.3 9.03 0.12 5.51 7.32 4.53 2.10 0.50 2.66 100.3 0.59 639 314 27.0 956 6.66 1.44 7.88 0.46 38.6 78.3 9.61 404 39.0 164 3.69 7.44 2.02 6.04 0.76 4.14 22.6 0.83 2.09 0.31 1.86 0.30 20.0 1.49 25.2 190 52.7 34.1 72.4 17.2 1.49 0.16 9.14

19.3 8.94 0.10 2.78 6.34 4.80 1.50 0.28 2.89 100.2 0.42 25.7 52.0 20.2 507 3.30 0.72 5.06 0.32 17.7 37.9 4.83 808 20.9 145 3.34 4.75 1.54 4.58 0.68 4.06 22.7 0.86 2.20 0.33 2.04 0.32 19.5 1.16 20.1 197 40.2 35.4 64.2 18.8 1.50 0.41 5.25

18.8 9.47 0.11 3.28 8.50 3.77 1.01 0.28 0.88 99.7 0.45 34.9 61.0 18.2 399 3.28 0.79 4.93 0.33 17.7 37.4 4.78 670 20.6 141 3.25 4.70 1.52 4.53 0.69 4.08 23.3 0.87 2.24 0.33 2.04 0.32 8.97 1.14 22.6 185 47.0 35.5 71.8 19.1 1.36 0.18 5.24

ZFN12 18.8 9.22 0.12 3.53 9.18 3.69 0.98 0.28 2.00 100.3 0.47 30.5 58.7 15.8 409 3.10 0.77 4.82 0.32 16.6 35.8 4.58 573 19.7 140 3.18 4.55 1.48 4.37 0.66 3.92 22.4 0.84 2.13 0.32 1.93 0.31 9.59 1.11 21.3 187 56.9 31.0 71.6 19.0 1.28 0.25 5.06

DJA01

STA03

STA06

N910A10

M17A1

60.4 0.69 17.8 4.92 0.09

56.1 0.39 18.1 6.75 0.15

54.0 0.91 18.9 8.10 0.14

56.5 0.81 16.7 6.47 0.09

56.1 1.09 17.0 6.81 0.10

Samples 09DH24 Andesite 14.4 5.42 0.09 8.92 3.28 3.46 2.01 0.13 3.17 100.3 0.79 528 228 29.1 705 7.55 2.30 4.33 0.39 16.4 30.8 3.46 484 13.1 108 2.81 2.60 0.74 2.32 0.31 1.83 36.0 0.41 1.11 0.17 1.11 0.18 35.0 2.41 14.0 92 40.9 16.7 51.3 15.1 1.22 0.71 22.3

DHA04

DH03

17.1 7.00 0.12 2.51 3.82 4.86 2.51 0.43 2.57 100.4 0.46 12.2 7.18 42.2 1016 5.90 1.89 7.31 0.52 22.1 47.4 5.92 822 26.8 170 4.53 5.54 1.42 5.01 0.73 4.27 24.3 0.91 2.48 0.38 2.44 0.37 17.8 1.86 20.1 160 45.8 62.0 86.6 20.3 1.56 0.29 12.2

16.8 10.2 0.16 4.53 6.85 4.27 0.46 0.18 3.00 99.5 0.51 26.4 15.5 5.35 120 1.01 0.44 1.70 0.13 8.54 20.6 2.93 569 13.2 85.1 2.24 3.43 1.04 3.51 0.60 3.75 23.0 0.82 2.34 0.37 2.35 0.36 7.86 0.55 30.3 272 41.0 93.9 82.5 17.3 1.38 0.12 3.82

Samples DH94 Felsic volcanic rock 64.5 0.92 14.8 6.18 0.15

DH90 74.2 0.14 13.6 1.97 0.06

M19A21 FIA6 DJD02 DJD03

14.1 9.11 0.11 3.16 4.08 4.25 2.61 0.68 4.61 99.6 0.45 301 220 33.0 1176 4.89 0.80 12.8 0.71 40.1 90.2 11.0 319 45.4 185 4.24 8.55 2.36 6.84 0.91 4.75 24.3 0.88 2.28 0.32 1.85 0.26 25.0 1.10 20.6 102 61.6 88.2 101 18.3 0.92 0.63 5.90 DHA05 ML19 ML15 ML10 ML08 ML0 75.1 0.08 13.4 1.29 0.02

17.3 7.63 0.09 4.51 5.93 4.25 1.65 0.48 3.91 100.1 0.58 98.3 50.6 20.4 1273 2.39 0.16 7.93 0.50 27.2 56.1 6.77 1241 28.7 164 4.08 5.35 1.52 4.68 0.63 3.44 18.6 0.68 1.78 0.25 1.53 0.22 6.85 1.43 17.0 157 34.4 36.2 85.7 21.8 1.11 0.21 10.4

16.6 8.36 0.16 3.92 7.98 3.25 1.35 0.68 2.35 99.8 0.52 91.9 35.4 18.1 644 3.28 1.24 8.52 0.49 32.4 68.9 8.49 859 37.0 180 4.10 7.22 1.81 6.45 0.90 5.12 28.3 1.05 2.81 0.41 2.62 0.39 23.2 1.56 23.0 215 43.0 62.1 101 21.7 1.38 0.18 11.4

14.3 8.18 0.12 3.66 5.38 4.37 2.76 0.65 5.52 99.6 0.51 331 181 39.3 944 4.67 0.97 13.0 0.69 35.6 80.3 9.92 333 40.2 190 4.34 7.82 2.12 6.34 0.86 4.53 28.2 0.86 2.23 0.32 1.92 0.28 21.9 1.10 20.2 131 47.8 51.1 83.3 18.9 0.97 0.60 4.40

72.7 74.4 67.3 68.0 69.5 0.36 0.25 0.73 0.62 0.49 14.3 13.7 15.8 15.8 15.5 2.53 2.23 3.90 3.31 3.38 0.08 0.06 0.08 0.05 0.07 (To be continued on the next page)

1324

Samples Andesite MgO CaO Na2O K2O P2O5 LOI Total Mg# Cr Ni Rb Ba Th U Nb Ta La Ce Pr Sr Nd Zr Hf Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu Li Be Sc V Co Cu Zn Ga Ge Cs Pb

Li W, et al.

Sci China Earth Sci

DJA01

STA03

STA06

N910A10

M17A1

2.17 1.55 6.49 3.03 0.21 2.52 99.9 0.51 8.55 8.10 56.4 814 7.30 2.31 8.46 0.59 23.9 51.6 6.29 230 27.0 187 4.91 5.37 1.27 4.84 0.71 4.21 24.8 0.91 2.56 0.40 2.63 0.40 14.0 2.00 14.8 104 22.8 79.4 69.4 17.9 1.13 0.18 17.0

3.64 8.44 2.52 0.24 0.11 3.44 99.9 0.56 26.2 15.3 2.89 126 0.96 0.47 0.96 0.13 6.02 13.4 1.75 733 8.30 55.1 1.57 1.97 0.61 2.08 0.34 2.10 13.6 0.48 1.40 0.22 1.52 0.24 12.1 0.50 18.3 152 44.4 7.20 62.6 16.4 1.39 0.07 3.07

3.31 7.23 3.43 0.88 0.31 2.69 100.0 0.49 16.2 10.2 8.32 431 1.57 0.62 2.39 0.20 11.5 24.5 3.22 887 14.9 74.0 2.08 3.47 1.11 3.41 0.51 3.08 18.0 0.66 1.82 0.23 1.79 0.27 17.2 1.12 21.8 233 75.0 71.5 90.0 22.0 1.29 0.17 6.75

3.14 3.75 5.66 3.17 0.13 4.07 100.4 0.53 94.3 42.4 72.9 437 2.05 0.28 6.35 0.47 18.7 41.9 5.07 521 21.4 152 3.84 4.03 1.13 3.50 0.47 2.54 12.8 0.49 1.21 0.16 0.97 0.14 10.8 1.44 13.7 122 30.7 29.8 69.7 18.4 1.20 1.96 7.89

3.81 4.62 5.86 1.48 0.40 3.11 100.4 0.57 44.0 11.8 17.6 504 1.45 0.45 3.27 0.21 16.5 34.6 4.34 1346 18.3 76.5 2.01 3.43 1.06 2.82 0.36 1.94 10.3 0.37 0.97 0.14 0.85 0.13 5.94 1.11 16.1 191 37.8 66.6 79.7 21.7 0.91 0.10 6.51


Samples DH94 Felsic volcanic rock 2.24 3.25 4.59 1.35 0.19 2.27 100.4 0.46 26.2 12.3 23.9 423 3.80 1.30 6.25 0.40 17.3 36.3 4.29 195 17.9 167 4.10 3.98 1.10 3.77 0.57 3.58 22.7 0.80 2.23 0.35 2.39 0.39 5.95 1.30 12.4 134 40.7 19.2 80.1 17.0 1.36 0.73 16.5

DH90 0.20 0.71 4.62 3.60 0.06 0.78 100.4 0.19 19.0 12.4 70.7 1013 5.57 1.55 5.65 0.48 19.1 34.7 4.16 132 16.4 145 3.85 3.46 0.67 3.22 0.53 3.43 23.0 0.80 2.30 0.38 2.58 0.43 5.77 1.77 4.47 10.3 46.0 5.44 51.5 14.1 1.08 0.88 13.3

(Continued) DHA05 ML19 ML15 ML10 ML08 ML0 0.06 0.62 0.51 0.51 4.62 6.47 4.53 2.11 0.02 0.08 0.77 0.65 100.4 100.4 0.10 0.36 1.02 2.79 0.55 1.54 103 25.2 777 800 13.9 9.85 3.45 2.64 7.30 9.88 0.85 0.77 41.1 28.1 83.3 57.5 9.31 6.82 84.9 98.7 37.0 26.8 185 240 5.99 6.32 7.04 5.58 0.43 1.02 6.68 4.98 1.06 0.78 6.60 4.89 39.2 32.1 1.41 1.10 4.04 3.12 0.63 0.52 4.14 3.46 0.62 0.57 6.31 9.59 1.87 1.64 6.45 7.94 6.62 18.8 122 40.4 7.61 2.38 21.8 41.1 16.6 14.2 1.56 0.79 1.14 0.38 11.8 2.79

0.55 0.72 4.09 3.34 0.07 1.04 100.4 0.37 2.03 1.38 69.3 1014 5.53 1.51 5.71 0.47 19.7 39.5 4.36 204 17.0 146 3.85 3.72 0.81 3.52 0.57 3.67 24.6 0.84 2.45 0.41 2.75 0.46 7.23 1.47 5.40 13.5 40.1 41.2 43.6 15.4 1.44 1.92 12.1

0.73 0.83 0.47 1.41 1.29 0.55 5.37 4.65 7.36 3.77 3.68 2.11 0.18 0.15 0.13 1.19 1.92 0.87 100.4 100.3 100.4 0.30 0.37 0.24 4.63 5.14 2.55 2.51 3.79 1.14 77.7 104 29.2 1152 930 503 6.85 6.17 6.21 2.06 1.82 1.82 8.44 7.52 8.67 0.58 0.50 0.56 25.6 25.7 18.2 53.6 55.0 50.9 6.55 6.90 5.69 340 448 166 27.1 28.5 24.5 231 206 244 5.78 5.19 6.07 5.84 6.02 6.05 1.50 1.50 1.60 5.14 5.23 5.54 0.78 0.77 0.90 4.84 4.71 5.70 33.5 29.0 34.6 1.07 1.04 1.28 2.96 2.86 3.56 0.48 0.47 0.58 3.08 2.99 3.78 0.50 0.49 0.61 8.64 4.07 7.98 2.05 2.25 1.80 10.5 9.79 10.4 49.8 39.0 24.7 35.5 27.8 28.8 5.42 4.03 4.50 58.5 56.7 54.3 18.2 19.5 18.9 1.21 1.28 1.15 1.55 2.95 0.33 10.5 11.0 2.71

a) Mg#=molar Mg/ (Mg + Fe); LOI: Loss on ignition.

sampling uses single point ablation. For details of the analysis process, see refs. [36, 37].

3 The geochemical characteristics of volcanic rock The geochemical compositions of Carboniferous samples

from Santanghu are listed in Table 1. The total alkali-SiO2 diagram (Figure 3) shows that the bulk of the volcanic rock samples fall into the scope of the sub-alkaline series basalts. Given that the volcanic rock samples have experienced weak alteration, the active elements, K and Na, may have been bought out of or into to some degree. So, instead, we used discrimination diagrams based on the inactive element ratios Zr/TiO2-Nb/Y. The results show that all samples are

Li W, et al.

Figure 3

Sci China Earth Sci


1325

Na2O+K2O versus SiO2 [38] and of Zr/TiO2 versus Nb/Y [39] for volcanic rocks.

sub-alkaline series rocks. The basic volcanic rocks have SiO2 concentrations ranging from 44.69 wt% to 52.29 wt%, TiO2 from 0.54 wt% to 1.87 wt%, Al2O3 from 13.8 wt% to 20.03 wt%, K2O+Na2O from 2.1 wt% to 7.12 wt%, K2O from 0.09 wt% to 2.1 wt%, Na2O from 2.01 wt% to 4.8 wt%, and Na2O>K2O (Table 2). Therefore, the characteristics of the alkali-rich rocks may be the result of diagenetic alteration or fluid metasomatism. Mafic volcanic rocks have Mg# values (Mg# = Mg/(Mg + Fe2+)) that range between 0.41 and 0.65, lower than the original magma with a reference value of 0.65 to 0.75 [40]. These characteristic Mg#s show that basaltic rocks are products from the original magma that has experienced a certain degree of differentiation. Samples (e.g. 09DHS33, 09DHS34, and 09DHS35) from the Daheishan area have low TiO2 (from 0.54 wt% to 0.74 wt%), whereas other samples have high TiO2 (from 1.03 wt% to1.87 wt%). The basalts samples generally have low MgO (from 2.78wt% to 9.03 wt%) and K2O (0.09 wt% to 2.1 wt%), depleted of P2O5 (0.12 wt%–0.93 wt%) and Na2O>K2O Basalt samples have ΣREE from 47×10–6 to 232×10–6. These rocks exhibit coherent chondrite-normalized REE patterns (Figure 4), characterized by a slight LREE enrichment, HREE and LREE values that have obvious fractionation, ((La/Yb) N= 2.14–16.81; (La/Sm)N=1.33–3.35) and weakly negative or non-existent Eu anomalies (δEu=0.75–1.02). They show that magma evolution has experienced plagioclase fractional crystallization. Trace elements show the enrichment of large ion lithosphile elements (LILE: K, Cs, Sr and Ba), and the depletion of high field strength elements (HFSE: Nb, Ta, Zr, P and Hf), with obvious negative anomalies of Ta, Nb and Ti in the primitive mantle normalized diagram (Figure 4). Intermediate volcanic rocks (mainly basaltic andesite and andesite) have SiO2 ranging from 52.39 wt% to 54.2 wt%, MgO from 2.17 wt% to 8.92 wt%, Al2O3 from 14.1 wt% to 18.9 wt%, K2O+Na2O from 2.76 wt% to 9.52 wt%, K2O

from 0.24 wt% to 3.17 wt%, Na2O from 2.52 wt% to 6.49 wt% and K2O/Na2O from 0.1 to 0.63. They belong to a subalkaline volcanic rock series. Intermediate volcanic rock samples have ΣREE from 54×10–6 to 240×10–6. Chondrite-normalized REE distribution diagrams (Figure 4) show LREE-enriched REE distribution patterns similar to the mafic volcanic rock samples. HREE and LREE have obvious fractionation, ((La/Yb) N=2.61–15.55; (La/Sm) N= 1.60–4.07) and weak negative Eu anomalies or no anomalies (δEu=0.75–1.02). They show that magma evolution has experienced plagioclase fractional crystallization. Intermediate volcanics have similar trace element characteristics with mafic volcanic rocks in the primitive mantle normalized spider diagram; the difference is that they are more enriched in LILE (e.g. Cs, Ba, Th, U, K, Pb) and more depleted in Nb, Ta and Ti. Acidic volcanic rocks, mainly dacite and rhyolite, have SiO2 ranging from 64.51 wt% to 75.02 wt%, TiO2 from to 0.08 wt% to 0.92 wt%, MgO from 0.06 wt% to 2.24 wt%, Al2O3 from 13.41 wt% to 15.83 wt%, K2O+Na2O from 5.94 wt% to 9.47 wt%, K2O from 1.35 wt% to 4.53 wt%, Na2O from 4.09 wt% to 7.36 wt%, and K2O/Na2O=0.29–0.98. They also belong to a subalkaline volcanic rock series. Acidic volcanic rocks samples have a value for ΣREE of 115×10–6 to 243×10–6 Chondrite-normalized REE distribution diagrams (Figure 4) show obvious LREE-enriched REE distribution patterns. HREE and LREE have obvious fractionation, ((La/Yb)N=3.46–7.11, (La/Sm)N=1.60–4.07) and obvious negative Eu anomalies (δEu=0.75–1.02). They show that a large volume of plagioclase was crystallized in the process of magmatic fractional crystallization. Acidic volcanics have similar trace element characteristics to maficintermediate volcanic rocks in the primitive mantle normalized spider diagram; the only difference is that they are more depleted in Ti.

1326

Li W, et al.

Sci China Earth Sci

4 LA-ICP-MS zircon U-Pb dating Based on the geochemical characteristics of the Carboniferous volcanic rocks, intermediate volcanic samples DH103 (sampling location: 43°39'41"N; 94°14'14.5"E) and ML05 (sampling location: 43°44'7.2"N; 94°15'24.5"E) have been


adopted for an isotope chronological study of the Carboniferous volcanic sections located in the northern Daheishan and along the southern margin of the Malang depression. The samples come from an andesitic lava which is located in the lower part of the Middle-Upper Carboniferous (C2-3a)1) pyroclastic rock sandwich, with the andesite show-

Figure 4 Chondrite-normalized rare earth element (REE) patterns and primitive mantle-normalized trace elements spider diagrams. Symbols are the same as in Figure 2.

1) The Xinjiang Geological Survey. Naomaohu Geological Map and Instructions (1:200000) (in Chinese). 1966.

Li W, et al.

Sci China Earth Sci

ing up with a gray, green, or tan color, and a massive structure in outcrop. Feldspar phenocrysts are visible in hand specimens. Under the microscope, they show porphyritic and andesitic textures (hyalopilitic texture), with plagioclase phenocrysts chaotically distributed in the matrix, and slight sericitization.


1327

In shape and cathode luminescence images of zircon grains (Figure 5), idiomorphic crystals are observed with clear magmatic crystallization ring textures. They have Th/U ranging from 0.39 to 0.93 (Table 2), suggesting a magmatic origin [41] with integrated zircon shapes. Concordia diagrams for the samples (Figures 6 and 7) show that

Figure 5 Representative CL images. (a) Sample 09DHS103; (b) Sample 09ML05. The circles show LA-ICP-MS analysis spots with corresponding spot numbers. The 206Pb/238U ages (Ma) for each spot are labeled.

1328

Li W, et al.

Sci China Earth Sci

the sample data mostly are concordant. The weighted average age of sample DH103 is 328.9±1.9 Ma (MSWD= 1.15); another sample (ML05) is 331.3±2.3 Ma (MSWD= 0.39). The zircon ages of the two samples are very close, both showing that the Santanghu Carboniferous volcanic rocks formed at the end of the early Carboniferous.

5 Discussion 5.1 Source area The volcanic rock samples all have the same or similar REE and trace element distribution patterns (Figure 4). They show that mafic to acidic volcanic rocks have co-magmatic differentiation and evolutionary trends, with the magma originating from the same mantle source [42]. It is generally acknowledged that the La/Sm ratio increased rapidly, generally to more than 5 [43], if the magma was contaminated during the ascent by crustal material. The La/Sm ratios of Santanghu area volcanic rocks are lower

Figure 6

U-Pb concordia diagrams of zircons from sample DH103.

Figure 7

U-Pb concordia diagrams of zircons from sample ML05.


than 5, with ratios ranging from 1.33 to 3.76, suggesting no obvious crustal material contamination. In addition, Zr shows negative anomalies in primitive mantle normalized trace element spider diagrams, which also indicate that these volcanic rocks have not been contaminated by crustal material during the magma ascent. Because the Carboniferous volcanic rocks of the region are observed to have a low 87Sr/86Sr and high Nd(t) [23, 28], displaying the characteristics of isotope loss, this indicates that the Carboniferous volcanic rocks are derived from a depleted mantle source region. 5.2

Magmatic evolution

Negative P and Ti anomalies indicate that the magmatic evolution involves the crystallization fractionation of apatite and ferro-titanium oxides; negative Eu anomalies show the crystallization differentiation of plagioclase. Plots of La versus Sm (Figure 8(a)) and Yb versus Tb/Yb (Figure 8(b)) indicate that the Carboniferous volcanic rocks experienced a

Li W, et al.

Table 2

Sci China Earth Sci

1329


Zircon U-Pb analytical results for samples from the early Carboniferous in Santanghu area Ages (Ma) 206 Pb/ Pb/ 1σ 238 1σ 235 U U 367 10 331 3

DH103N.01

191

295

U-Th-Pb atomic ratios 207 206 208 207 Pb/ Pb/ Pb/ Pb/ Pb/ 1σ 1σ 1σ 1σ 1σ 206 235 238 232 206 Pb U U Th Pb 0.65 0.0599 0.0021 0.4349 0.0144 0.0526 0.0005 0.0199 0.0004 599 74

DH103N.02

117

218

0.53 0.0609 0.0017 0.4391 0.0117 0.0523 0.0005 0.0197 0.0003 634

60

370

8

329

3

394

6

DH103N.03

158

232

0.68 0.0534 0.0018 0.3910 0.0125 0.0531 0.0005 0.0182 0.0003 346

74

335

9

333

3

364

6

DH103N.04 DH103N.05

85.2 113

171 179

0.50 0.0578 0.0022 0.4222 0.0153 0.0530 0.0006 0.0195 0.0004 521 0.63 0.0540 0.0018 0.3925 0.0122 0.0527 0.0005 0.0162 0.0003 370

81 72

358 336

11 9

333 331

3 3

390 324

8 6

DH103N.06

71.6

145

0.49 0.0536 0.0022 0.4208 0.0163 0.0570 0.0006 0.0193 0.0004 352

89

357

12

357

4

385

9

DH103N.07

77.1

129

0.60 0.0553 0.0034 0.4072 0.0241 0.0534 0.0008 0.0183 0.0006 423

131

347

17

335

5

367

11

DH103N.08

70.8

130

0.55 0.0606 0.0028 0.4377 0.0192 0.0524 0.0006 0.0182 0.0005 623

95

369

14

329

4

365

9

DH103N.09 DH103N.10

107 76.5

182 125

0.59 0.0562 0.0022 0.4023 0.0147 0.0519 0.0005 0.0147 0.0004 459 0.61 0.0538 0.0029 0.3847 0.0202 0.0519 0.0007 0.0175 0.0005 361

83 117

343 331

11 15

326 326

3 4

294 351

7 9

DH103N.11

60.2

134

0.45 0.0543 0.0025 0.3935 0.0179 0.0526 0.0006 0.0210 0.0005 383

101

337

13

330

3

421

9

DH103N.12

138

259

0.53 0.0565 0.0029 0.4063 0.0200 0.0521 0.0007 0.0172 0.0005 472

110

346

14

328

4

345

10

DH103N.13

89.9

169

0.53 0.0526 0.0020 0.3545 0.0126 0.0489 0.0005 0.0159 0.0003 311

82

308

9

308

3

320

6

DH103N.14 DH103N.15 DH103N.16 DH103N.17 DH103N.18

207 86.1 62.8 107 74.2

222 170 141 200 176

0.93 0.51 0.44 0.54 0.42

412 347 598 322 287

72 89 89 99 167

322 330 368 323 328

9 11 13 12 21

310 327 332 323 333

3 3 4 4 6

289 440 391 318 329

4 9 9 8 16

DH103N.19

44.5

114

0.39 0.0599 0.0028 0.4328 0.0195 0.0524 0.0006 0.0177 0.0005 600

98

365

14

329

4

354

10

DH103N.20

71.7

165

0.44 0.0611 0.0020 0.4310 0.0131 0.0512 0.0005 0.0172 0.0003 643

68

364

9

322

3

345

6

Ml05.01

74.4

137

0.54 0.0541 0.0031 0.3927 0.0220 0.0527 0.0008 0.0178 0.0006 373

124

336

16

331

5

356

11

Ml05.02 Ml05.05 Ml05.06

43.7 89.4 109

89.8 181 129

0.48 0.0530 0.0028 0.3866 0.0197 0.0529 0.0008 0.0186 0.0005 330 0.49 0.0548 0.0026 0.3956 0.0179 0.0523 0.0007 0.0173 0.0005 406 0.85 0.0628 0.0024 0.4574 0.0168 0.0528 0.0007 0.0183 0.0003 702

114 101 79

332 338 382

14 13 12

332 329 332

5 4 4

372 346 367

11 10 7

Ml05.07

257

301

0.86 0.0540 0.0024 0.3901 0.0168 0.0524 0.0007 0.0162 0.0003 370

97

334

12

329

4

325

6

Ml05.10

78.3

197

0.40 0.1721 0.0023 10.889 0.1323 0.4587 0.0044 0.1599 0.0020 2579

22

2514

11 2434 19 2999 35

Ml05.14

283

253

1.12 0.0577 0.0017 0.4214 0.0120 0.0529 0.0006 0.0165 0.0002 519

64

357

9

333

3

331

4

Ml05.15 Ml05.16

133 60.3

221 99.7

0.60 0.0531 0.0013 0.3857 0.0091 0.0527 0.0005 0.0174 0.0002 333 0.60 0.0533 0.0033 0.3836 0.0229 0.0522 0.0009 0.0168 0.0005 341

55 132

331 330

7 17

331 328

3 5

349 336

5 11

Ml05.17

100

183

0.55 0.0549 0.0031 0.4041 0.0222 0.0534 0.0008 0.0156 0.0005 407

121

345

16

335

5

313

10

Ml05.20

98.4

192

0.51 0.0567 0.0028 0.4111 0.0200 0.0526 0.0008 0.0177 0.0005 478

108

350

14

331

5

354

9

Ml05.21

154

174

0.88 0.0579 0.0018 0.4208 0.0125 0.0527 0.0006 0.0164 0.0002 526

67

357

9

331

4

328

5

Ml05.22

54.4

122

0.45 0.0581 0.0027 0.4191 0.0189 0.0524 0.0007 0.0187 0.0005 534

99

356

13

329

4

374

10

Ml05.23

244

330

0.74 0.0560 0.0030 0.4173 0.0220 0.0541 0.0008 0.0179 0.0005 450

116

354

16

340

5

358

9

Analytical spots

Figure 8

232

238 Th U Th/ (μg g−1) (μg g−1) U

207

0.0550 0.0534 0.0598 0.0528 0.0521

0.0018 0.0022 0.0025 0.0024 0.0040

0.3731 0.3833 0.4363 0.3741 0.3808

Plots of La versus Sm (a) and Yb versus Tb/Yb (b).

0.0118 0.0149 0.0177 0.0162 0.0287

0.0492 0.0521 0.0529 0.0514 0.0531

0.0005 0.0006 0.0006 0.0006 0.0010

0.0144 0.0220 0.0195 0.0159 0.0164

0.0002 0.0004 0.0005 0.0004 0.0008

207

208

Pb/ 1σ Th 399 7

232

1330

Li W, et al.

Sci China Earth Sci

separation of magmatic evolutionary processes based on source region. 5.3

Tectonic background

By taking into account a region of volcanic rocks with the same magma source, simple analytical methods can be followed to determine the tectonic setting and environment. Normalized trace element distribution patterns for primitive mantle (Figure 4) show that the large ion lithophile elements (LILE) K, Cs, Sr and Ba are significantly enriched, whereas high field strength elements (HFSE) Nb, Ta and Ti are strongly depleted. These geochemical characteristics indicate that the volcanic rocks formed in continental crust contaminated by a plate tectonic environment [30], possibly related to the subduction of oceanic crust. The earlier discussion of source area pointed out that the Santanghu vol-


canic rocks have not been contaminated by crustal material, and thus the carboniferous volcanic rocks were likely formed in a subduction-related tectonic environment. Similarly, the Zr-Nb-Y (Figure 9(a)), La-Y-Nb (Figure 9(b)) and Cr-Y (Figure 10(a)) diagrams, which are suitable for basalt, show that samples are most likely involved in a subduction-related tectonic environment. According to the La/Yb-Sc/Ti diagram (Figure 10(b)), which is suitable for andesite, the volcanic rocks in this area were formed in an active continental margin arc environment. The Carboniferous volcanic section (Figure 2) shows that volcanic rocks are composed mainly of mafic basalt, intermediate basaltic andesite, andesite and acidic dacitic rocks, rhyolite, sandwiched pyroclastic rocks and tuffaceous material. Basalt accounts for 25% of the thickness of the volcanic section, andesite for 43%, and dacite and rhyolite for 32%. Volcanic rock assemblages are dominated by andesite,

Figure 9 Tectonic environment discrimination diagrams of the basalt. Symbols are the same as in Figure 2. (a) The Nb-Zr-Y discrimination [44]; AI, Within-plate alkali basalts; A, within-plate alkali basalts and within-plate tholeiites; B, E-type MORB, C, within-plate tholeiites and volcanic-arc basalts; D, N-type MORB and volcanic-arc basalts; (b) the La-Y-Nb discrimination diagram [45].

Figure 10 andesite.

Cr versus Y tectonic setting discrimination diagrams for mafic volcanic rocks and La/Yb versus Ti tectonic setting discrimination diagrams for

Li W, et al.

Sci China Earth Sci

dacite and rhyolite, which is indicative of an active continental margin arc environment [48]. Based on the geochemical characteristics of the Carboniferous volcanic and rock assemblages, we can infer that they formed in an active continental margin arc environment. 5.4

Tectonic evolution

Several ophiolite belts are indicative of the existence of the Paleo-Asian Ocean in northern Xinjiang. From north to south, these are distributed as the Erqis, Zhaheba-Armantai and Kelameili ophiolite belts of eastern Junggar. Controversy is ongoing regarding the subduction polarity and formation ages of these three ophiolite belts, and in particular, the time limit of the Kelameili ophiolite belt at the southern margin of the Santanghu Basin [9, 16, 49–54]. Recent studies have shown that the Armantai-Zhaheba ophiolite belt, close to the northern side of the Santanghu Basin, formed in the late Cambrian–early Ordovician [55–57], during which time the Paleo-Asian Ocean underwent intra-oceanic subduction. It then was finally closed through northward accretion to the southern edge of the Altai terrane in the late Devonian–early Carboniferous [56]. In contrast, the Kelameili ophiolite, located on the southern side of the Santanghu Basin, suggests that the ancient ocean subducted to the north, finally closing at the end of the early Carboniferous–late Carboniferous [17, 58, 59]. The active continental margin arc volcanic rocks of the early Carboniferous sufficiently demonstrate that the Kelamaili Ocean subducted northward until the end of the early Carboniferous, and finally closed in the late Carboniferous.

6

Conclusions

Based on the assemblage characteristics, geochronology, and geochemistry of the Carboniferous volcanic rocks from the Santanghu area, the following conclusions can be drawn. (1) The Carboniferous volcanic rocks of Santanghu area formed at the end of the early Carboniferous (328.9– 331.3 Ma). (2) The Carboniferous volcanic rocks consist mainly of mafic-acidic volcanic lava and pyroclastic rocks. Their geochemical characteristics indicate that the basic and intermediate-acidic volcanic rocks are co-magmatic. These samples show an enrichment of large ion lithophile elements (LILE), and relative depletions of high field-strength elements (HFSE) with significant negative anomalies of Nb-Ta-Ti. They also show a weak enrichment of light rare earth elements (LREE), no significant enrichment of heavy rare earth enrichment (HREE), and no obvious Eu anomaly. (3) The Carboniferous volcanic rocks from depleted mantle magmas have not been contaminated by crustal material during ascent. They have undergone a process of


1331

magma crystal fractionation. (4) The Carboniferous volcanic rocks from the Santanghu region formed in an active continental margin arc environment, which shows that the ancient ocean basin corresponding to the Kelameili ophiolite belt was still undergoing northward subduction at the end of the early Carboniferous. This work was supported by Land and Resources Survey Project of China (Grant No. 1212011085009), the Sinopec Project “Study and Map Compilation for Structure, Lithofacies, and Paleogeography in Northwestern China” (Grant No. YPH08103), and National Natural Science Foundation of China (Grant No. 40802051). We gratefully thank Professor Anlin Guo, Dr Liang Chen and Dr. Chunrong Diwu of the Geology Department of Northwest University, China for guidance and fruitful discussions. We are also indebted to two anonymous reviewers for their critical and constructive comments, which substantially improved an early version of the manuscript.

1 2

3 4

5

6

7

8

9

10

11

12

13

14

15

Coleman R G. Continental growth of Northwest China. Tectonics, 1989, 8: 621–635 Jahn B M, Griffin W L, Windley B. Continental growth in the Phanerozoic: Evidence from Central Asia Tectonophysics, 2000, 328: Preface, vii–x Jahn B M, Windley B F, Natal’in B, et al. Phanerozoic continental growth in Central Asia. J Asian Earth Sci, 2004, 23: 599–603 Hu A, Jahn B M, Zhang G, et al. Crustal evolution and Phanerozoic crustal growth in northern Xinjiang: Nd isotopic evidence. Part I. Isotopic characteristics of basement rocks. Tectonophysics, 2000, 328: 15–52 Xiao W J, Windley B F, Hao J, et al. Accretion leading to collision and the Permian Solonker suture, Inner Mongolia, China: Termination of the central Asian orogenic belt. Tectonics, 2003, 22: 1069 Xiao W J, Zhang L C, Qin Z, et al. Paleozoic accretionary and collisional tectonics of Eastern Tianshan (China): Implications for the continental growth of central Asia. Am J Sci, 2004, 304: 370–395 Ren J S, Wang Z X, Chen B W, et al. The Tectonics of China from a Global View (in Chinese). Beijing: Geological Publishing House, 1999. 1–50 Xiao X C, Tang Y Q, Feng Y M, et al. Tectionic Evolution of Northern Xinjiang and Its Adjacent Regions (in Chinese). Beijing: Geological Publishing House, 1992. 1—171 He G Q, Li M S, Liu D Q, et al. Paleozoic Crustal Evolution and Mineralization in Xinjiang of China (in Chinese). Urumqi: Xinjiang People’s Publishing House, 1994. 1–437 Sengör A M C, Natal’in B A, Burtman V S. Evolution of the Altaid tectonic collage and Paleozoic crustal growth in Eurasia. Nature, 1993, 364: 299–307 Sengör A M C, Natal’in B A. Paleotectonics of Asia: Fragments of a synthesis. In: Yin A, Harrison M, eds. The Tectonic Evolution of Asia. Cambridge: Cambridge University Press, 1996. 486–640 Windley F B, Kroner A, Guo J, et al. Neoproterozoic to Paleozoic geology of the Altai orogen, NW China: New zircon age data and tectonic evolution. J Geol, 2002, 110: 719–737 Gao J, Klemd R. Formation of HP-LT rocks and their tectonic implications in the western Tianshan orogen, NW China: Geochemical and age constraints. Lithos, 2003, 66: 1–22 Guo Z J, Shi H Y, Zhang Z C, et al. The tectonic evolution of the south Tianshan paleo-oceanic crust inferred from the spreading structures and Ar-Ar dating of the Hongliuhe ophiolite, NW China (in Chinese). Acta Petrol Sin, 2006, 22: 95–102 Dong, Y P, Zhang G W, Zhou D W, et al. Geology and geochemistry of the Bingdaban ophiolitic mélange in the boundary fault zone on

1332

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

Li W, et al.

Sci China Earth Sci

the northern Central Tianshan Belt, and its tectonic implications. Sci China Ser D-Earth Sci, 2007, 50: 17–24 Xiao W J, Han C M, Yuan C, et al. Middle Cambrian to Permian subduction-related accretionary orogenesis of Northern Xinjiang, NW China: Implications for the tectonic evolution of central Asia. J Asian Earth Sci, 2008, 32: 102–117 Li J Y, Yang T N, Li Y P, et al. Geological features of the karamaili faulting belt, eastern Junggar region, Xinjiang, China and its constraints on the reconstruction of late Paleozoic ocean-continental framework of the Central Asian region (in Chinese). Geol Bull Chin, 2009, 28: 1817–1826 Dong Y P, Zhang G W, Neubauer F, et al. Syn-and post-collisional granitoids in the Central Tianshan orogen: Geochemistry, geochronology and implications for tectonic evolution. Gondwana Res, 2011, 20: 568–581 Li W, Zhou D W, Liu Y Q, et al. The division of Permian tectonic sequence and the feature of residual tectonic in Santanghu Basin (in Chinese). J Northwest Univ (Nat Sci Ed), 2005, 3: 617–620 Zhou D W, Liu Y Q, Xing X J, et al. Formation of the Permian basalts and implications of geochemical tracing for paleo-tectonic setting and regional tectonic background in the Turpan-Hami and Santanghu basins, Xinjiang. Sci China Ser D-Earth Sci, 2006, 49: 584–596 Zhu Z X, Li S Z, Li G L, et al. The Chracteristic of sedimentary system-continental facies volcano in Later Carboniferous Batamayi Group, Zhi-Fang Region, East Junggar (in Chinese). Xinjiang Geol, 2005, 23: 14–18 Zhao X, Jia C Z, Zhang G Y, et a1. Geochemistry and tectonic settings of Carboniferous intermediate-basic volcanic rocks in LudongWucaiwan, Junggar basin (in Chinese). Earth Sci Front, 2008, 15: 272–279 Chen S, Zhang Y Y, Guo Z J, et al. Zircon SHRIMP U-Pb dating and its implications of post-collisional volcanic rocks in Santanghu Basin, Xinjiang (in Chinese). Acta Petrol Sin, 2009, 25: 527–538 Wu X Q, Liu D L, Wei G Q, et al. Geochemical characteristics and tectonic settings of Carboniferous volcanic rocks from LudongWucaiwan area, Junggar Basin (in Chinese). Acta Petrol Sin, 2009, 25: 55–66 Mao Z G, Zou C N, Zhu R K, et al. Geochemical characteristics and tectonic settings of Carboniferous volcanic rocks in Junggar basin (in Chinese). Acta Petrol Sin, 2010, 26: 207–216 Yang G X, Li Y J, Li Z C, et al. Genesis and tectonic settings of post collision volcanic rocks in north eastern margin of East Junggar, Xinjiang (in Chinese). Earth Sci Fron, 2010, 17: 049–060 Gong W P, Lin K X, Li Y B, et al. Geochemical characteristics of early carboniferous basalts in Santanghu Region, Xinjiang and their significance in tectonic settings (in Chinese). Oil Gas Geol, 1997, 18: 314–318 Zhao Z H, Guo S J, Han B F, et al. The geochemical characteristics and tectonic-magmatic implications of the latest-Paleozoic volcanic rocks from Santanghu basin, eastern Xinjiang, northwest China (in Chinese). Acta Petrol Sin, 2006, 22: 199–214 Zhao Z H, Wang Q, Xiong X L, et al. Two types of adakites in north Xinjiang, China (in Chinese). Acta Petrol Sin, 2006, 22: 1249– 1265 Long X P, Sun M, Yuan C, et al. Genesis of Carboniferous volcanic rocks in the eastern Junggar: Constraints on the closure of the Junggar Ocean (in Chinese). Acta Petrol Sin, 2006, 22: 31–40 Li J Z, Wu X Z, Qi X F, et al. Distribution and tectonic setting of Upper Paleozoic volcanic rock in northern Xinjiang (in Chinese). Acta Petrol Sin, 2010, 26: 195–206 Su Y P, Zheng J P, Griffin W L, et al. Geochemistry and geochronology of Carboniferous volcanic rocks in the eastern Junggar terrane, NW China: Implication for a tectonic transition. Gondwana Res, 2012, doi: 10.1016/j.gr.2012.01.004 Li J Y, He G Q, Xu X, et al. Crustal tectonic framework of northern Xinjiang and adjacent regions and its formation (in Chinese). Acta


34 35

36

37

38

39

40 41 42

43

44

45

46

47 48

49 50

51

52

53

54

Geol Sin, 2006, 80: 148–168 Li J L, Sun S, Hao J, et al. On the classification of collision orogenic belts (in Chinese). Sci Geol Sin, 1999, 34: 129–138 Liu Y, Liu X M, Hu Z C, et al. Evaluation of accuracy and longterm stability of determination of 37 trace elements in geological samples by ICP-MS (in Chinese). Acta Petrol Sin, 2007, 23: 1203– 1210 Liu X M, Gao S, Diwu C R, et al. Simultaneous in-situ determination of U-Pb age and trace elements in zircon by LA-ICP-MS in 20 μm spot size. Chin Sci Bull, 2007, 52: 1257–1264 Diwu C R, Sun Y, Yuan H L, et al. U-Pb ages and Hf isotopes for detrital zircons from quartzite in the Paleoproterozoic Songshan Group on the southwestern margin of the North China Craton. Chin Sci Bull, 2008, 53: 2828–2839 Le Maitre R W, Bateman P, Dudek A, et al. A classifiearion of Igneous Roeks and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Roeks. Oxford: Blackwell Seientific Publications, 1989. l–193 Condie K C. Geochemical changes in basalts and andsites across the Archaean-Proterozoic boundary: Identification and significance. Lithos, 1989, 23: 1–18 Deng F J. The source and identification symbol of primary basaltic magma (in Chinese). Geol Res, 1984, 18–27 Wu Y B, Zheng Y F. Genesis of zircon and its constraints on interpretation of U-Pb age. Chin Sci Bull, 2004, 49: 1589–1604 Shinjo R, Kato Y. Geochemical constraints on the origin of bimodal magmatism at the Okinawa Trough, an incipient back-ack basin. Lithos, 2000, 54: 117–137 Lassiter J C, Depaolo D J. Plumers/lithosphere interaction in the generation of continental and oceanic flood basalts: Chemical and isotope constraints. In: Mahoney J, ed. Large igneous Provinces: Continental, Oceanic, and Planetary Flood Volcanism. Geophysical Monography 100, Am Geophys Union, 1997. 335–355 Meschede M. A method of discriminating between different types of mid-ocean ridge basalts and continental tholeiites with the Nb-Zr-Y diagram. Chem Geol, 1986, 56: 207–218 Cabanis B, Lecolle M. Le diagramme La/10-Y/15-Nb/8: un outil pour la discrimination de series volcaniques et la mise en evidence des processus de melange et/cn de contamination crustal. C R Acad Sci 1989, Ser. II, 309: 2023–2029 Pearce J A. Trace element characteristics of lavas from destructive plate boundaries. In: Thorps R S, ed. Andesites. Chinchester: Wiley, 1982. 525–548 Bailey J C. Geochemical criteria for a refined tectonic dixcrimination of orogenic andesites. Chem Geol, 1981, 32: 139–159 Deng J F, Xiao Q H, Su S G, et al. Igneous petrotectonic assemblages and tectonic settings: A discussion (in Chinese). Geol J Chin Univ, 2007, 13: 392–402 Li C Y. A preliminary study of plate tectonics of China (in Chinese). Chin Acad Geol Sci Ser, 1980, 2: 11–22 Li C Y, Wang Q, Liu X Y, et al. Asian Geotectonic map (scale 1: 8000000) (in Chinese). Beijing: Cartographic Publishing House, 1982 Xiao X C, He G Q, Xu X, et al. Crustal tectonic framework and geological evolution of Xinjiang Uygur Autonomous Region of China (in Chinese). Beijing: Geological Publishing House, 2010. 1–318 He G Q, Cheng S D, Xu X, et al. An Introduction to the Explanatory Text of the Map of Tectonics Xinjiang and Its Neighbouring Areas (in Chinese). Beijing: Geological Publishing House, 2004. 1–65 Ma R S, Shu L S, Sun J Q. Tectonic Evolution and Metallogeny of Eastern Tianshan Mountains (in Chinese). Beijing: Geological Publishing House, 1997. 1–202 Long X P, Yuan C, Sun M, et al. Geochemistry and U-Pb detrital

Li W, et al.

55

56

Sci China Earth Sci

zircon dating of Paleozoic graywackes in East Junggar, NW China: Insights into subduction-accretion processes in the southern Central Asian Orogenic Belt. Gondwana Res, 2012, 21: 637–653 Jian P, Liu D Y, Zhang Q, et al. SHRIMP dating of ophiolite and leucocratic rocks within ophiolite (in Chinese). Earth Science Frontiers, 2003, 10: 439–456 Xiao W J, Windley B F, Yan Q R, et al. SHRIMP zircon age of the Aermantai ophiolite in the north Xinjiang area, China and its tectonic implications (in Chinese). Acta Geol Sin, 2006, 80: 32–37


57

58

59

1333

Zhang Y Y, Guo S J. New constraints on formation ages of ophiolites in northern Junggar and comparative study on their connection (in Chinese). Acta Petrol Sin, 26: 421–430 Wang B Y, Jiang C Y, Li Y J, et al. Geochemistry and tectonic implications of karamaili ophiolite in east Junggar of Xinjiang (in Chinese). Mineral Petrol, 2009, 29: 74–82 Xiao Y, Zhang H F, Shi J A, et al. Late Paleozoic magmatic record of East Junggar, NW China and its significance: Implication from zircon U-Pb dating and Hf isotope. Gondwana Res, 2011, 20: 532–542