Geochemistry and SHRIMP geochronology of alkaline rocks of the Zijinshan massif in the eastern Ordos basin, China

Russian Geology and Geophysics 50 (2009) 751–762 www.elsevier.com/locate/rgg

Geochemistry and SHRIMP geochronology of alkaline rocks of the Zijinshan massif in the eastern Ordos basin, China Yang Xingke a,*, Chao Huixia a, N.I. Volkova b, Zheng Menglin c, Yao Weihua d a

Key Laboratory of Western China’s Mineral Resources and Geological Engineering, Ministry of Education, Chang’an University, 126 Yanta Road, Xi’an, 710054, China Sobolev Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia c Research Institute of Prospecting and Development, SINOPEC, 20 Xueyuan, Beijing, 100083, China d Research Institute of Exploration and Development, Changqing Oilfield Company of CNPC, 151 Weiyang Road, Xi’an, 710021, China b

Received 23 June 2008; received in revised form 26 January 2009; accepted 26 February 2009

Abstract Characteristics of geology, petrology, and geochemistry of the Zijinshan massif were studied in the eastern part of the Ordos basin. Geochemical analysis shows that the massif is characterized by high alkali, relatively high Fe, and low Mg and Ca contents. The rocks are undersaturated in SiO2, rich in REE (with no Eu anomaly) and belong to the alkaline-peralkaline series. The geologic history of the Zijinshan massif consists of several stages of magmatism. The obtained isotope-geochronological (U-Pb SHRIMP) data show that the magmatic activity climaxed in the interval 150–110 Ma, while the age of 16 zircon grains fitted a narrower interval, 132–125 Ma, i.e., the Early Cretaceous. The younger age corresponds to the middle and late stages of the evolution of the Ordos basin and agrees with a large Early Cretaceous tectonothermal event in North China. This event led to the large-scale uplift of the eastern flank of the Ordos basin, rise of the Lüliang asthenosphere, and to the formation of a large west-sloping monocline. The U-Pb SHRIMP studies have also revealed magmatic zircons of Carboniferous–Permian age, which evidences the multistage character of the thermal process. © 2009, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Ordos basin; magmatic activity; tectonic thermal event; Early Cretaceous; Zijinshan; SHRIMP chronology

Introduction In the study of the dynamics of basin formation the greatest challenges are the thermal process and thermal structure of the basin (He and Li, 1995; Liu, 2005; Ma et al., 2003). Chinese authors believe that thermal structures of basins can be classified in detail on the basis of their present-day structure, geomorphological type, age of deposits, and depth of thermal (magmatic) effect. According to the depth of thermal (magmatic) action, five types of tectonic structures have recently been recognized (Yang et al., 2005): (1) faulting zones with related ancient geothermal anomalies; (2) zones of high hydrothermal-volcanic activity (with manifestations of subsurface effusive magmatism, hydrothermal activity and hypabyssal porphyry intrusions); (3) thermal dome; (4) structure with medium-depth intrusions (including intracrustal thermal plume anomalies); (5) mantle-derived thermal plumes.

* Corresponding author. E-mail address: [email protected] (Yang Xingke)

The Ordos basin is not only among the basins where first oil fields were found but also it is the second largest oil basin in the world. It lies in the west of the North China craton (Fig. 1, inset right above) in the zone of junction of two tectonic regions (eastern and western) which underwent multiple expansion and compression. It is a complicate intracratonic basin (Ingersoll and Busby, 1995) resulting from multiple superposition of various structures upon the primary Ordos basin. Seismic and magmatic activities were earlier studied on the southeastern margin of the Ordos basin (Fen–Wei rift). The Shanxi flexural foldbelt is situated in the eastern part of the basin (Fig. 1, inset left below). It is a giant monoclinal structure dipping from east to west (Yang, 2002), complicated by a series of open flat meridional subfolds, by the Wubu sublatitudinal fault (F1), and by the large Lishi submeridional fault (F2). The folds and faults are mostly semiconcealed. Along with the problems of tectonics, mineralization, and oil accumulation in this basin, worthy of discussion is a possible thermal effect of a deep-seated magma source in the eastern part of the basin and on its periphery.

1068-7971/$ - see front matter D 2009, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2009.08.002

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Fig. 1. Schematic geological structure of the Zijinshan massif (modified after: Wu, 1966). 1, Quaternary deposits; 2, sandstones and shales of the Ermaying Group of Middle Triassic age (T2er); 3, trachyte porphyries and trachyte volcanic breccias; 4, pseudoleucite phonolites and phonolite volcanic breccias; 5, melanite-aegirineaugite nepheline syenite; 6, aegirine-augite nepheline syenite; 7, biotite-melanite nepheline syenite; 8, malignite; 9, nepheline titanaugite; 10, aegirine-augite syenite; 11, monzonite; 12, flow directions; 13, sampling localities and sample numbers; 14, zone numbers; 15, faults (F1 is the Wubu fault and F2 is the Lishi fault); 16, area under study. The upper right inset shows the location of the study area on the structural-geological scheme of Northeastern China: I, Great Hingayn–Mongolian orogenic belt; II1, North China craton, II2, Alashan block; III, orogenic belt of the Qinling–Dabieshan; IV, Yangtze craton; Or, Ordos basin; Od, large Ordos basin; Q, Qinshui basin; B, Fen–Wei rift.

Analysis of the isotope-geochronological data on intrusive bodies of the eastern part of the basin obtained earlier by the Rb-Sr method (Huang, 1991; Luo et al., 1999; Tang et al., 1992; Zhou et al., 1996) and K-Ar method (Shanxi..., 1989) and recent zircon U-Pb age of 127 Ma (Ying et al., 2007) allows the estimated ages of magmatism to be divided into five intervals: 94–91 Ma (K2), 132–125 Ma (K1), 154–141 Ma (J3), 293–287 Ma (P1), and 343 Ma (C1). The earlier data are widely dispersed and rather uncertain. Based on the field works in the Zijinshan alkaline massif, analysis of the samples, their petrogeochemical characteristics, and U-Pb SHRIMP ages, this paper describes stages, sources, and evolution of magmatism. The available data suggest a

medium-depth source of the magma intruded on the eastern flank of the Ordos basin. According to the U-Pb SHRIMP dating, the main stage of magmatism dates from the Early Cretaceous. Hence, a succession of magmatic events has been established, which is very important for reconstructing tectonothermal events in this region.

Geological characteristics of the Zijinshan alkaline massif The Zijinshan alkaline massif localized in the northwestern Lin County of the Shanxi Province is composed of Mt. Zijin,

Yang Xingke et al. / Russian Geology and Geophysics 50 (2009) 751–762

Mt. Dadu, and Mt. Jiaopai and forms a stock and a sill of 23 km2 in total area. The wall rocks are grayish-green feldspar sandstones with interbeds of red clays 386–430 m thick, which belong to the Ermaying Group of Middle Triassic age (T2er), overlain by Quaternary sediments (Fig. 1). The massif has a zonal-circular structure, with its center at a height of 1767 m asl within the ridge of Shanshen Temple, north of Mt. Zijin. There are five zones recognized in the radial direction (inward): zone I is composed of gray monzonites; zone II, of black-gray and green aegirine-augite syenites; zone III, of black-gray malignite or, less frequently, melanite-biotite-titanaugite nepheline syenites, aegirine-augite nepheline syenites, and melanite-aegirine-augite nepheline syenites; zone IV, of gray-green and reddish-brown phonolite volcanic breccias; and zone V, of yellow-brown trachyte volcanic breccias. All these zones are crescent in shape and belong to the alkaline massif (Fig. 1). The mantle-derived alkaline magmas were intruded into the Ordovician limestones (beyond the region under study) or into the Triassic rocks of the Ermaying Group (Yang et al., 1988). The Zijinshan massif lies at the joint of a concealed fault at 38° N (like the Wubu fault F1) and a submeridional fault (like Lishi fault F2), which favored the magma intrusion into higher horizons.

Petrographic and petrogeochemical characteristics Sampling and analysis. Of the samples examined in the field and in laboratory, three samples were chosen for petrochemical analysis, determination of contents of trace and rare-earth elements, Sr-Nd isotope and zircon U-Pb isotope dating. Sample Z1 is aegirine-augite syenite, Z4 is trachyte porphyry, and Z5 is trachyandesite. Sampling localities are shown in Fig. 1. The geographical coordinates of Z1 are 110°52′ E and 38°07′ N, while Z4 and Z5 were sampled 150 m west and 710 m northwest of Z1, respectively. Analysis of the samples for petrogenetic and trace elements was carried out in the Laboratory of Element Analysis, Institute of Geology and Geophysics of the Chinese Academy of Sciences. The contents of petrogenetic elements were determined with the use of XRF1500 and IRIS plasma emission spectrometers, as well as AA-6200 and Z-8000

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atom-absorption spectrophotometers. Trace and rare-earth elements were analyzed by the ICP-MS method. Standard samples (GSR-1, GSR-2, and GSR-3) were used for control. The analytical error was less than 5%. Isotopes of Sm, Nd, Rb, and Sr were tested in the Laboratory of Solid Isotope Geochemistry, Institute of Geology and Geophysics of the Chinese Academy of Sciences. The samples were disintegrated under the action of HF + HClO4 in a sealed Teflon container at low temperature for a week. To isolate pure Sm and Nd, we used an AG50W×8(H+) cation-exchange column and P507 solvent-saturated resin. Isotope analysis was carried out on a VG354 mass spectrometer. The Nd isotope ratios were normalized according to 146 Nd/144Nd = 0.7219; the intralaboratory blank was about 5 × 10−11 g. The age was calculated by Isoplot program; decay constant λ147Sm = 6.54 × 10−12 a–1 (Mu et al., 2001). Petrography. Petrographical characteristics of the three analyzed samples (Fig. 2) are as follows: Aegirine-augite syenite (Z1, Fig. 2, a): gray, of hypidiomorphic granular to xenomorphous texture and massive structure. The rock consists mainly of orthoclase (47 vol.%), aegirine-augite (27 vol.%), and nepheline (11 vol.%); less abundant are alkaline amphibole (8 vol.%) and leucite (3 vol.%). Accessory minerals are represented by opaque ore mineral (2 vol.%), sphene (1.5 vol.%), and apatite (0.5 vol.%). Orthoclase (K-feldspar) contains albite ingrowths forming the perthite texture. Aegirine-augite demonstrates distinct pleiochroism and zoning: The cores of crystals are pale yellowgreen (diopside), surrounded by a blue-green rim (aegirine-augite). Xenomorphous grains of nepheline are colorless, with distinct cracks. Nearly isometric grains of leucite occur among grains of orthoclase. Alkaline amphibole is pale yellow and distinctly pleiochroic. Trachyte porphyry (Z4, Fig. 2, b): gray, of porphyry texture and massive structure, composed mainly of phenocrysts (27 vol.%) and groundmass (68 vol.%). Phenocrysts are represented by plagioclase (12 vol.%), orthoclase (8 vol.%), amphibole (3 vol.%), and aegirine-augite (4 vol.%). Phenocrysts of plagioclase are hypidiomorphic-idiomorphic zonal crystals, often twinned according to the albite law. Subhedral laths of orthoclase are colorless, with Karlsbad twins. Zonal irregular phenocrysts of aegirine-augite are characterized by

Fig. 2. Photomicrographs of thin sections of alkaline rocks of the Zijinshan massif. Pl, plagioclase; Or, orthoclase; Aeg-Aug, aegirine-augite; Hb, hornblende; Ri, riebeckite; Sph, sphene; Lc, leucite; Sc, scapolite. a is sample Z1, hypidiomorphous-grained aegirine-augite syenite, plane-polarized light; b is sample Z4, trachyte porphyry, crossed nicols; c is sample Z5, porphyry trachyandesite, plane-polarized light.

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Table 1 Chemical composition of alkaline rocks of the Zijinshan massif (wt.%) Sample

SiO2

TiO2

Al2O3

Fe2O3

FeO

MnO

MgO

CaO

Na2O

K2O

P2O5

Total

1 2 3 4 5 6 7 8 9 10 11 12 13

58.72 58.51 52.51 51.02 51.67 49.92 52.44 48.44 51.94 54.01 54.06 61.81 57.74

0.68 0.59 1.06 2.03 1.03 1.03 0.69 0.59 0.97 0.38 0.89 1.27 1.10

17.20 17.93 15.11 15.38 14.06 14.50 18.04 19.65 19.01 19.50 18.78 18.85 16.58

3.46 3.02 5.97 5.18 6.54 3.36 5.81 3.44 4.17 5.14 5.22 3.49 4.27

2.16 1.95 4.12 3.69 3.02 5.03 1.32 1.91 1.46 1.27 1.40 2.29 2.19

0.11 0.12 0.14 0.15 0.16 0.15 0.10 0.10 0.11 0.14 0.13 0.12 0.08

1.58 1.65 2.75 2.57 3.09 3.3 0.41 0.69 0.70 0.26 0.40 1.02 0.83

5.52 5.86 7.59 7.06 7.56 7.33 2.83 3.88 3.73 1.31 1.76 0.01 3.42

4.60 5.08 3.58 3.60 2.82 3.05 3.16 3.07 3.53 4.39 3.38 5.07 4.89

4.42 3.99 5.75 5.16 6.93 7.55 12.46 14.34 11.93 11.39 11.99 3.23 4.92

0.31 0.26 0.49 0.93 0.86 0.64 0.50 0.13 0.13 0.05 0.45 0.70 0.58

98.76 98.96 99.07 96.77 97.74 95.86 97.76 96.24 97.68 97.84 98.46 97.86 96.60

Note. Sample 4, 12, and 13 (4 = Z1, 12 = Z4, and 13 = Z5) have been analyzed at the Institute of Geology and Geophysics of the Chinese Academy of Sciences (analyst Li He). The rest are taken from the literature: 1, 3, 5, 7, 9, and 10 from (Shanxi..., 1989); 2, 6, and 11, from (Wu, 1966), and 8, from (Zhou and Zhao, 1994). 1 and 2, monzonite; 3 and 4, aegirine-augite syenite; 5 and 6, malignite; 7, biotite-melanite nepheline syenite; 8, nepheline syenite; 9, melanite-aegirine-augite nepheline syenite; 10, phonolite; 11, phonolite volcanic breccia; 12, trachyte porphyry; 13, trachyte volcanic breccia.

color changing from grayish-blue in the core to light yellowgreen in the rim. They sporadically occur in the groundmass, which is composed chiefly of aegirine-augite, plagioclase, and orthoclase. Accessory minerals are represented by opaque ore mineral (3.8 vol.%), sphene (1 vol.%), and apatite (0.2 vol.%). Trachyandesite (Z5, Fig. 2, c): pale yellow, of porphyry texture and massive structure, composed of phenocrysts (28.5 vol.%) and groundmass (68 vol.%). Phenocrysts are often plagioclase (17 vol.%), less frequently orthoclase (5 vol.%), riebeckite (3.5 vol.%), and scapolite (3 vol.%). Subhedral laths of colorless plagioclase are characterized by zoning. Riebeckite is gray-blue to light yellow-green, occurs

Fig. 3. Alkaline rocks of the Zijinshan massif on the TAS diagram. F, foidite; Pc, picrobasalt; B, basalt; O1, basaltic andesite; O2, andesite; O3, dacite; S1, trachybasalt; S2, basaltic trachyandesite; S3, trachyandesite; U1, tephrite; U2, phonotephrite; U3, tephriphonolite; Pn, phonolite; R, rhyolite; T, trachyte, trachydacite; 1–13 are specimen numbers (see Table 1).

Fig. 4. Alkaline rocks of the Zijinshan massif on the Q–A–P–F diagram for igneous rocks (Strekeisen, 1978). Q, quartz; A, alkali feldspar; P, plagioclase; F, feldspathoid. II, alkali-feldspar rhyolite; IIIa, rhyolite; IIIb, rhyodacite; IV and V, dacite; VI*, quartz-alkali-feldspar trachyte; VI, alkali-feldspar trachyte; VI′, foid-bearing alkali-feldspar trachyte; VII*, quartz trachyte; VII, trachyte; VII′, foid-bearing trachyte; VIII*, quartz latite; VIII, latite; VIII′, foid-bearing latite; IX and X, basalt and andesite; XI, phonolite; XII, tephrite phonolite; XIII, phonolite basanite, phonolite tephrite; XIV, basanite, tephrite; XVa, phonolite foidite; XVb, tephrite foidite; XVc, foidite.

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Fig. 5. The Zijinshan rocks plotted in the SiO2–A.R. diagram (Wright, 1969). A.R. = (Al2O3 + CaO + K2O + Na2O)/(Al2O3 + CaO – K2O – Na2O).

Fig. 6. The Zijinshan rocks in the Al2O3–SiO2 diagram (Zhang and Cong, 1976). Rocks: A, high alumina; B, alumina; C, low alumina; D, extremely low alumina.

In general, the rocks of this alkali massif have the following petrochemical characteristics: (1) they are undersaturated or strongly undersaturated with SiO2 (48.44 to 61.81 wt.%); (2) as inferred from the diagram SiO2–A.R. (Fig. 5), they belong to the peralkaline series; (3) rocks are characterized by high contents of alkalies, relatively high content of Fe, and low contents of Mg and Ca. On the variation diagram SiO2–Al2O3 they plot in the field of low-alumina rocks (Fig. 6). Geochemistry of rare-earth elements. The main features of REE distribution in the Zijinshan rocks (Table 2, Fig. 7) are as follows: (1) high concentration of REE, reaching 45 to 270 ppm, i.e., 13.7 to 81.4 chondrite norms; (2) the rocks are enriched in LREE (42 to 249 ppm, i.e., 19.7 to 117.7 chondrite norms); (3) the ΣLREE/ΣHREE varies from 3.9 to 19.7, which

as prismatic crystals demonstrating amphibole cleavage. Scapolite grains often have cracks. The groundmass is composed of plagioclase, orthoclase, and riebeckite. Accessory minerals are represented by opaque ore mineral (3 vol.%) and sphene (0.5 vol.%). Petrochemical characteristics. Results of chemical analysis of our samples as well as data on Zijinshan samples from other authors (Huang, 1991; Yan et al., 1988; Zhou and Zhao, 1994) are reported in Table 1, and their petrochemical types are shown in Figs. 3 and 4. On the TAS diagram (Fig. 3) the rocks lie in the field of trachyandesites, phonotephrites, and phonolites. With the use of the Q–A–P–F diagram (Strekeisen, 1978) (Fig. 4) they can be classified as trachytes, latites, phonolites, and foidites. Table 2 Contents of REE in alkaline rocks of the Zijinshan massif (ppm) Sample La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

LREE/ (La/Yb)N Eu/Eu* HREE

1

44.00

70.20

24.50

66.40

7.33

2.02

6.05

0.57

3.23

0.56

1.52

0.23

1.24

0.27

15.69

20.11

1.01

2

37.40

77.60

8.70

34.30

5.80

2.10

5.80

0.90

4.40

0.70

2.20

0.30

1.80

0.30

10.12

11.77

1.24

3

38.92

76.13

10.80

42.31

9.03

2.66

8.02

1.17

5.59

1.01

2.56

0.40

2.17

0.33

8.46

10.16

1.06

4

38.94

74.88

9.31

39.22

7.53

2.27

6.61

0.95

4.88

0.90

2.35

0.34

2.12

0.30

9.33

10.43

1.09

6

41.40

46.70

33.70

53.40

9.11

2.90

8.84

1.18

5.53

1.03

2.62

0.36

1.85

0.29

8.63

12.68

1.1

7

13.50

24.90

2.60

11.80

2.80

0.90

2.50

0.10

2.70

0.40

1.80

0.30

1.80

0.30

5.71

4.25

1.15

8

17.71

40.52

6.37

29.48

8.30

2.79

8.58

1.43

7.58

1.42

3.67

0.55

3.09

0.40

3.94

3.25

1.14

12

64.37

111.00 13.02

48.68

7.74

2.39

6.61

0.88

4.68

0.90

2.35

0.35

2.20

0.34

13.50

16.62

1.12

13

59.78

109.40 12.16

46.37

7.52

2.41

6.26

0.86

4.78

0.93

2.42

0.35

2.22

0.34

13.09

15.28

1.18

14

30.90

63.10

7.90

32.50

7.50

2.40

6.70

1.00

5.40

0.60

3.00

0.40

2.90

0.30

7.11

6.04

1.14

15

73.30

88.10

39.20

37.70

9.12

2.50

8.02

1.18

4.55

0.81

2.45

0.41

2.47

0.39

12.32

16.82

0.99

16

13.50

11.90

9.29

5.70

1.19

0.36

1.45

0.09

1.14

0.24

0.42

0.04

0.14

0.04

11.78

55.04

0.96

17

29.30

46.40

4.30

12.40

1.70

0.40

1.10

0.20

1.60

0.20

0.90

0.20

0.50

0.10

19.69

33.21

0.94

Note. Samples 4, 12, and 13 (Z1, Z4, and Z5, respectively) have been analyzed at the Institute of Geology and Geophysics of the Chinese Academy of Sciences (analyst Jin Xindi). The rest are taken from literature: 1, 6, 15, and 16 after (Huang, 1991); 2, 7, 14, and 17 after (Yan et al., 1988); 3 and 8, after (Zhao and Zhou, 1994). 1 and 2, monzonite; 3, 4, and 14, aegirine-augite syenite; 6, malignite; 7, 8, and 15, nepheline syenite; 12 and 13, trachyte porphyry; 16 and 17, phonolite.

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Fig. 7. REE spectra of the Zijinshan rocks. a, Monzonite; b, aegirine-augite syenite; c, malignite; d, nepheline syenite; e, phonolite; f, trachyte porphyry. Sample numbers follow Table 2.

indicates the intense fractionation of rare earths, demonstrating the enriched type of distribution; (4) curves of REE distribution have a slight negative slope, with (La/Yb)N reaching 3.25–55.04; (5) (La/Sm)N and (Gd/Yb)N equal 1.49–12.06 and 0.42–0.80, respectively, which suggests significant differences in the fractionation of LREE and HREE; (6) Eu/Eu* = 0.94–1.27, which indicates that there is no evident Eu minimum. Isotope characteristics of magma source. On the basis of oxygen isotopy of the Zijinshan alkaline rocks (Yan et al., 1988), it has been established that the range of δ18O variablity at each stage is very narrow: from 7.9 to 9.2, averaging 8.4‰. It is believed that δ18O for mantle-derived ultramafic and mafic rocks varies from 5.5 to 9.0‰, and for alkaline rocks it is somewhat higher (Li et al., 2003). On the basis of analytical data on Rb, Sr, Sm, and Nd isotopes (Table 3) and using the formula (87Sr/86Sr)0 = 87 86 Sr/ Sr – 87Rb/86Sr(eλ t – 1), we have calculated that 87 86 ( Sr/ Sr)0 is equal to 0.70422–0.70512, which implies that ISr varies from 0.704528 to 0.706399. It is known that the

primary isotope ratios of Sr in mantle rocks range from 0.702 to 0.706. Thus, the Zijinshan alkaline rocks have a mantle source. Since the present-day (87Sr/86Sr)0 for EM1 of the lithosphere mantle is 0.7050–0.7055 (Ren et al., 2004), the majority of our data fit the values for EM1 lithosphere mantle. As inferred from the obtained Sm-Nd isotope data, the primary values of 143Nd/144Nd vary from 0.512457 to 0.512516, and εNd(t), from –5.93 to –10.90, which corresponds to the isotope characteristics for a homogeneous chondrite reservoir (the present-day 143Nd/144Nd = 0.512638). Thus, we can hypothesize a deep-seated mantle source for the Zijinshan rocks.

U-Pb isotope-geochronological (SHRIMP) data and their geological significance Several hundred grains of zircon have been extracted from the three samples, Z1, Z4, and Z5, selected in the field and laboratory. First they were sorted by a standard procedure, then representative well-shaped zircon grains were chosen

Table 3 Isotope composition of Sm, Nd, Sr, and Rb in alkaline rocks of the Zijinshan massif Sample

Sm, ppm

Nd, ppm

147

Sm/144Nd

143

Nd/144Nd

Sr, ppm

Rb, ppm

87

Rb/86Sr

87

Z1

7.650

36.51

0.1268

0.512270

1716.4

146.42

0.2481

0.705023

Z4

7.589

43.80

0.1049

0.512033

2052.8

75.54

0.1058

0.705261

Z5

6.796

39.87

0.1032

0.512021

1932.1

136.41

0.2034

0.706674

Note. Analyses were made by Chen Fugen and Chu Zhuyin at the Institute of Geology and Geophysics, Chinese Academy of Sciences.

Sr/86Sr

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757

Fig. 8. Cathode luminescent images of separate grains of zircons from the Zijinshan rocks. In samples Z1, Z4, and Z5, 37 zircon grains have been analyzed, of which nine are shown here.

under a binocular to make and polish target specimens, after which zircons were photographed in the visible and cathodoluminescent light (CL) to examine their internal structure and choose crystal sites appropriate for dating. Cathodoluminescent images of some zircon grains from Z1, Z4, and Z5 are shown in Fig. 8. The U-Pb isotope analysis of zircons was carried out by a SHRIMP-II in the Beijing Ion Microprobe Center of the Chinese Academy of Sciences. A standard zircon of the Geological Survey of Australia TEM was additionally put on the targets with zircon grains Z1, Z4, and Z5. Each analysis yields an average of 7-fold scanning. Isotope data for the Australian standard zircon (417 Ma after: Black et al., 2003) were used to make a correction for interelement fractionation. The correction formula is Pb/U = A(UO/U)2 (Claoue-Long et al., 1995). To calibrate contents of U, Th, and Pb, we used another standard zircon SL13 (572 Ma, with U = 238⋅10−6) placed onto the test target. The data were processed by means of the PRAWN program of the National University of Australia (Williams et al., 1996). The content of common lead was corrected from the actually determined 204Pb. Table 4 reports the obtained U-Pb SHRIMP data, with an error for each point of 1σ. Fifteen points were chosen in sample Z1, whose analytical data are given in Fig. 9, a and in Table 4; of them, eleven points are close in age, averaging 343.0±5.4 Ma (MSWD = 1.6). These zircons are long and short prismatic, deeply colored, often rounded. The other four points show ages of 303 to 286 Ma (293.0±6.9 Ma on average, MSWD = 1.3). These zircons are long prismatic and well-shaped. Ten points were analyzed in sample Z4 (Fig. 9, b and Table 4). Most zircon grains from this sample are represented by long plates, few grains are deeply colored crystals with

distinct zonal structure. The obtained ages vary from 139 to 119 Ma, averaging 132.0±2.1 Ma, MSWD = 1.6. Twelve points were analyzed in sample Z5 (Fig. 9, c and Table 4). Zircons from this sample are represented by deeply colored well-shaped crystals. The obtained ages are clustered in two intervals: the age of six points averages 125.0±6.7 Ma (MSWD = 0.079), and the average age of other five points is 293±17 Ma (MSWD = 1.7). The age of the point Z5-7 (343.0±7.7 Ma) coincides with the age of nine points at sample Z1. Statistics shows that the age of crystallization of most zircons from aegirine-augite syenite (Z1) is 343.0±5.4 Ma; the zircons with an age of 293.0±6.9 Ma are scarcer. Trachyte porphyry (Z4) that formed as a result of emplacement of a hypabyssal intrusion or effusive volcanism has an age of crystallization of 132.0±2.1 Ma. The age of trachyandesite crystallization (Z5) produced by volcanic eruption is 125.0 ±6.7 Ma. But this rock also contains entrapped magmatic zircons aged 343 and 293 Ma. The age of 16 points of zircons from samples Z4 and Z5 (10 in Z4 and 6 in Z5) is 132–125 Ma (Table 4), which corresponds to the Early Cretaceous, when the asthenosphere uplifted in the Lüliang Mountain area in the eastern part of the Ordos basin, which reflects the deep-seated magmatic activity of the transformation of orogenic depression in this region. It should be noted that this process is in agreement with considerable tectonic transformation, thinning of the lithosphere, underplating of mantle magmas and tectonothermal events that occurred in North China at that time (Ren et al., 2006). In addition, the revealed magmatic zircons with an average age of 343 Ma (11 grains from Z1) and about 293 Ma (4 grains from Z1) suggest that the magmatic activity in this region also occurred at earlier stages: in the Carboniferous and Early Permian. Comparison with magmatism manifestations in adjacent areas shows that they

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Yang Xingke et al. / Russian Geology and Geophysics 50 (2009) 751–762

Table 4 Results of U-Pb isotope dating (SHRIMP) of zircons from alkaline rocks of the Zijinshan massif Sample and point

U

Th

206

Pb, %

207

Pb/206Pb

207

Pb/206Pb* Error, %

232

Th/238U

208

Pb/232Th

206

Pb/238U

Age, Ma (1σ) Pb/232Th

ppm

208

206

Pb/238U

Z4-2.1

586.1831

1112.523

0.5453

0.0499

0.0455

2.5973

1.961048

0.006385

0.020979

128.6547

133.8423

Z4-3.1

367.0821

468.4909

0.7748

0.0526

0.0462

3.0546

1.318714

0.006364

0.021518

128.2154

137.2418

Z4-6.1

391.5421

579.0011

0.5832

0.0499

0.0451

3.1430

1.527966

0.006489

0.021611

130.7397

137.8322

Z4-7.1

425.3

637.6085

0.0001

0.0505

0.0505

3.7868

1.549071

0.006637

0.022027

133.6987

140.4526

Z4-8.1

514.0486

756.4036

0.6482

0.0560

0.0507

2.7179

1.520415

0.006255

0.020481

126.041

130.6927

Z4-9.1

818.95

1859.215

1.2013

0.0509

0.0410

2.2406

2.345767

0.006293

0.020351

126.8034

129.8762

Z4-10.1

368.8153

532.4763

1.1021

0.0484

0.0393

3.3397

1.491777

0.005587

0.02136

112.6054

136.2444

Z4-11.1

321.2227

438.8622

1.4830

0.0470

0.0347

6.8380

1.411674

0.004954

0.021463

Z4-13.1

443.3363

666.1746

0.4751

0.0504

0.0465

3.1202

1.552628

0.005954

0.020025

Z4-14.1

291.3991

365.6551

0.6185

0.0284

0.0232

4.9125

1.29657

0.003013

0.020773

Z5-2.1

523.063

379.8387

0.0618

0.0532

0.0527

1.8803

0.75034

0.013592

0.045437

Z5-3.1

448.905

650.6309

0.1655

0.0491

0.0478

3.2236

1.49759

0.005916

0.019666

119.2154

125.5433

Z5-4.1

377.5402

301.5385

0.0001

0.0511

0.0511

2.4785

0.825263

0.014328

0.047521

287.5398

299.2867

Z5-5.1

701.3893

692.2076

1.0979

0.0624

0.0535

1.5118

1.019741

0.01305

0.04409

262.0731

278.1368

Z5-6.1

452.0999

302.1854

0.3871

0.0532

0.0501

2.0256

0.69064

0.013211

0.045018

265.2795

283.8637

Z5-7.1

363.2919

323.6752

0.2611

0.0538

0.0517

2.1148

0.92059

0.016394

0.054732

328.6761

343.506

Z5-8.1

356.8634

515.9435

0.4497

0.0503

0.0467

3.8361

1.49387

0.005551

0.01971

111.8873

125.8257

99.88184 119.9832 60.81042 272.8825

136.8944 127.8163 132.5415 286.4487

Z5-9.1

293.749

293.0755

0.5261

0.0531

0.0488

2.4488

1.030898

0.0144

0.046707

288.9864

294.2743

Z5-10.1

357.4214

526.7273

0.8974

0.0498

0.0424

3.3699

1.522712

0.005989

0.02022

120.6996

129.0466

Z5-11.1

450.7453

758.2168

0.3110

0.0493

0.0467

3.0648

1.7381

0.006086

0.019749

122.643

126.0692

Z5-12.1

394.3582

524.7796

0.4982

0.0486

0.0445

3.4616

1.374987

0.00565

0.019669

113.8733

125.5649

Z5-13.1

227.5574

302.0451

1.9254

0.0430

0.0269

13.1306

1.371493

0.003897

0.022215

78.62245

141.6406

Z1-1.1

646.7168

449.7228

0.1163

0.0533

0.0524

1.5052

0.718528

0.017063

0.056442

341.9694

353.9538

Z1-2.1

1103.027

567.6129

0.1771

0.0551

0.0537

1.8320

0.531715

0.01677

0.052356

336.1439

328.9671

Z1-4.1

260.4532

256.7456

0.8332

0.0524

0.0456

3.0650

1.018559

0.013005

0.048188

261.1594

303.3849

Z1-5.1

180.9595

158.591

0.5527

0.0522

0.0477

3.1479

0.905545

0.014315

0.04823

287.2802

303.6482

Z1-6.1

632.3457

742.7388

0.1552

0.0575

0.0562

1.9209

1.213652

0.017998

0.056324

360.5448

353.2284

Z1-7.1

413.0455

281.0428

0.5761

0.0563

0.0516

1.9320

0.703052

0.016046

0.055995

321.7579

351.2251

Z1-8.1

845.5278

543.581

1.4015

0.0625

0.0511

1.9347

0.664277

0.014404

0.053906

289.0533

338.4604

Z1-9.1

131.8207

84.26622

1.2340

0.0573

0.0472

3.3923

0.660515

0.013455

0.056734

270.1502

355.7308

Z1-10.1

355.1528

228.6931

0.5751

0.0513

0.0466

2.3091

0.66535

0.010285

0.046477

206.8233

292.8556

Z1-11.1

425.813

459.6549

0.1802

0.0527

0.0512

2.1215

1.115387

0.013121

0.046468

263.4864

292.8019

Z1-12.1

676.6412

573.7876

0.1528

0.0541

0.0529

1.9473

0.876204

0.016387

0.053511

328.5287

336.0426

Z1-14.1

259.7152

276.7719

0.0001

0.0542

0.0542

3.8983

1.101127

0.015964

0.05605

320.1242

351.5584

Z1-15.1

318.2518

191.987

0.3399

0.0518

0.0490

3.1780

0.623323

0.015812

0.052139

317.0855

327.6389

Z1-18.1

777.9820

515.3577

0.0342

0.0565

0.0562

1.1626

0.6845

0.0237

0.0729

473.9379

453.6960

Z1-19.1

545.1548

332.4405

0.0374

0.0584

0.0581

1.4132

0.6301

0.0228

0.0653

455.8619

407.6835

Note. Analyses were made by Yang Xingke, Chao Huixia, and Miao Laicheng at the Beijing Ion Microprobe Center.

are coeval with the Mesozoic belt of Mt. Taihang on the eastern margin of the basin.

Discussion In the Paleozoic and Triassic the North China block was relatively stable, but the situation changed in the Middle–Late

Mesozoic when the Yanshanian movement began. In the Jurassic and Cretaceous, under the conditions of active magmatism various magmas intruded along the faults inside the Ordos basin and on its eastern margin. For example, K-Ar age of alkali syenites of Mt. Huyan on the western site of the Xishan coal deposit near Taiyuan, Shanxi Province, is about 139–125 Ma, and the isotope age of granodiorite-porphyry stock exposed by drilling on the Qingjiao area in the southern

Yang Xingke et al. / Russian Geology and Geophysics 50 (2009) 751–762

759

Fig. 9. Concordant U-Pb SHRIMP ages of zircons from samples Z1 (a), Z4 (b), and Z5 (c).

part of this deposit is 105 Ma (Yang et al., 1988). Thus, this Late Jurassic-Early Cretaceous stage of magmatic activity, which is documented in the Ordos basin, belongs to the same large and intensive tectonothermal event that affected the

whole North China block in the Middle–Late Mesozoic. The dating of fission tracks on the northern flank of Mt. Lüliang (70–62 Ma) suggests that the period of uplifting and cooling of mountain blocks coincided with or occurred somewhat later

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Yang Xingke et al. / Russian Geology and Geophysics 50 (2009) 751–762

than the stage of cooling after a powerful tectonothermal event in the North China craton, which indicates the general uplift of this territory at that time. Thermoluminescent study of fission tracks (Sun and Li, 1997) showed that since the end of the Paleozoic the Ordos basin experienced at least three tectonothermal events: at 215, 135, and 72 Ma. The tectonothermal event that occurred in the Early Cretaceous (135– 125 Ma) led to a tectonic inversion at the late stage of the evolution of the Ordos basin. The time of uplifting of this region coincides in time with its tectonic inversion and is in agreement with synchronous tectonothermal events in other regions of North China. The determined paleogeothermal gradients and heat flows in the eastern part of the Ordos basin and in the adjacent Qinshui basin in the Late Jurassic–Early Cretaceous suggest that a single large tectonic event occurred in the Late Mesozoic. The isotope age of the rocks of the Qinshui basin and magmatism age in the surrounding belt supports the Late Jurassic–Early Cretaceous age of this tectonothermal event (Ren, 1997; Ren et al., 1999). The region of development of the Zijinshan massif is in the zone of steep contact of the eastern flank of the rigid block Ordos and weaker block Lüliang. In Yanshanian time the deep-seated structure of the region was characterized by an asthenosphere plume (weak block) rising into the thickened lithosphere (rigid block). At the point of upwelling of the hot asthenosphere matter, the following events took place: rupture of the lithosphere block, ascending and underplating of the hot magma into the lower horizons of the Earth’s crust, melting of the surrounding rocks, then the further upwelling to the surface, where a number of alkaline and subalkaline complexes formed at a shallow depth (Xing et al., 2006). It is these processes that led to the formation of the Zijinshan alkaline rocks. Late Jurassic–Early Cretaceous time corresponded to the stage of intense tectonic events within the North China craton, in which the Ordos basin is located. As a result, intrusive magmatism and volcanic activity were widely expressed in the eastern part of the Ordos basin, in particular near Mt. Taihang, which led to the formation of the Zijinshan massif. Another example is the Zhangjiakou Formation whose rhyolites formed by mixing of the Archean crustal material and matter from the mantle source consequent on considerable thinning of the lithosphere of the North China craton in the Late Mesozoic are dated at 126±1 Ma (Yang et al., 2006). Mantle-derived intrusive rocks are also rather frequent in the cover of the basin’s interior. Thus, the thermal process was of regional scale at that time and specific conditions existed in the deep-seated parts of the Ordos basin, which favored the progress of this tectonothermal event. The same process led to the uplift of the Lüliang dome, to the change of the inclination of the faulting zone and large-scale upheaval of the eastern flank of the basin. The tectonomagmatic evolution of the area under study is as follows: (1) In the Early Paleozoic this territory, together with the North China block, was uplifted; as a consequence there are no Late Ordovician–Early Carboniferous sediments.

(2) In Variscinian time deep-seated intrusions of monzonites and aegirine-augite syenites were emplaced. In the eastern part of the basin, deep-seated magmatism probably occurred around Mt. Taihang. Our SHRIMP U-Pb isotope data show that the magmatic zircons have ages of 343 and 293–287 Ma, which suggests several stages of magmatic events that occurred on the periphery of the basin or in depth. (3) In the Triassic-Jurassic, continuous sedimentation occurred in the eastern part of the basin and near Mt. Lüliang. (4) The Yanshanian tectonic activity in the Ordos basin and its vicinities was very powerful; considerable deformations of sediments and intense tectonic motions occurred mostly in the Late Jurassic. (5) In the Cretaceous, the tectonic stress and character of deformations of the Earth’s crust in North China drastically changed; the dominating motions were extension of the lithosphere with a slight compression, the lithosphere became increasingly thinner, whereas the crust continued to suffer the extension deformation. This led to the formation of a rift basin and a rift valley with widespread magmatism and effusive volcanism (154–141 and 132–125 Ma). It was the period when massifs of alkaline rocks (trachyte porphyries, trachyandesites, trachyte and phonolite volcanic breccias, less frequently malignites and nepheline syenites) were formed, which are now exposed to the day or are concealed at a shallow depth and are objects of numerous isotope-geochronological studies. Since Himalayan time this region has been in the state of persistent rise and denudation.

Conclusions The Ordos basin is a zone of junction of tectonic deformations of the eastern and western regions of China. Its thermal structure remains to be studied. Therefore, the systematic study and accumulation of geochemical and isotope-geochemical (SHRIMP) data on magmatism in the eastern part of the basin is very important for reconstructing the tectonothermal evolution of the basin. The Zijinshan alkaline massif was emplaced into the Middle Triassic sediments of the eastern part of the Ordos basin as a complex semicircular intrusion. The main rocks of the Zijinshan massif are monzonites, syenites, trachytes, latites, trachyandesites, as well as phonolite and trachyte volcanic breccias. The Zijinshan rocks are undersaturated or drastically undersaturated with SiO2. They belong to the alkaline-peralkaline series, are rich in alkalies, enriched in Fe and depleted in Mg, Ca, and Al. They are all characterized by a high content of rare-earth elements with a predominance of LREE over HREE and no negative Eu anomaly. Initial values of 87Sr/86Sr and 143Nd/144Nd fall into the interval of EM1 lithosphere mantle values, which indicates a deep-seated upper-mantle source. On the basis of isotope data obtained by the zircon SHRIMP U-Pb method, we can argue that the Zijinshan alkaline rocks were produced by multistage intrusive magmatism and effusive volcanism. At the early stages (343 and

Yang Xingke et al. / Russian Geology and Geophysics 50 (2009) 751–762

293–287 Ma), syenites were intruded, and later, in the Early Cretaceous (132–125 Ma), hypabyssal intrusions of porphyries were emplaced, accompanied by effusive volcanism, which agrees with the history of tectonothermal events in the North China craton. The Early Cretaceous magmatism in the eastern part of the Ordos basin belongs to the type of thermal structures, when the magma intruded at intermediate depths. It was characterized by the asthenosphere upwelling near Mt. Lüliang in the eastern part of the Ordos basin, which triggered the deep magmatic activity during the development of the orogenic basin on this territory, which complies with numerous tectonic transformations, lithosphere thinning, underplating of mantle magma, and tectonothermal events within the North China craton as a whole. The complex tectonic evolution of the area under study was characterized by a multistage history of tectonomagmatic activity. It is quite possible that the magmatism also occurred in the Carboniferous–Early Permian, as shown by the study of tectonothermal events in the eastern Ordos. This is very important for the study of processes of oil and gas accumulation, formation of coal and uranium deposits, as well as for prospecting research. The authors are grateful to professors Liu Chiyang, Li Youzhu, Jiang Changyi, and Liu Dunyi, associate professors Yang Minghui and Su Chunqian as well as to engineers Yang Zhiqing, Tao Hua, and Jin Xindi for their guidance, help in paper writing, sampling in the fieldwork, and test and analysis, which strongly improved the manuscript. This work was supported by grant No. 2003CB214601 from the National Grand Fundamental Research Program 973 of China.

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Editorial responsibility: V.A. Vernikovsky