November 2005 LIP of the Month

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Nature of the mantle source of the Tianshan Carboniferous rift-related basalts

Lin-Qi Xia, Zu-Chun Xia, Xue-Yi Xu, Xiang-Min Li, Zhong-Pin Ma, Li-She Wang
Xi’an Institute of Geology and Mineral Resources, Ministry of Land and Resources, Xi’an, Shaanxi 710054, China

Email: geologyx@pub.xaonline.com (for Lin-Qi Xia)

Abstract

The Tianshan Carboniferous rift-related volcanism in northwestern China (central Asia) represents a newly-recognized large igneous province extending over at least 343,000 km2. The volcanic successions comprise thick piles of basaltic lavas and subordinate intermediate and silicic lavas and pyroclastics, and are interpreted to result from a starting mantle plume. The isotopic and principal chemical variations within the Tianshan basalts can be accounted for by variable contamination of a plume component (87Sr/86Sr(t) ≈ 0.7045, εNd(t) ≈ + 4) by upper crust or CLM (continental lithospheric mantle).

Introduction

In this paper we report new Sr-Nd-Pb isotope data (Tables 1 and 2) of basaltic lavas from the Tianshan Carboniferous rift-related volcanic rocks in northwestern China (Xia et al., 2004), an important recently discovered large igneous province (LIP). This study focuses on the basalts, which dominate (>80 vol.%). We use these isotopic data as well as a detailed suite of major and trace element analyses (Xia et al., 2004) to evaluate the role of asthenospheric and continental lithospheric sources in the generation of the Tianshan Carboniferous LIP basalts.

Regional geological evolution

The Tianshan orogenic belt is part of a large-scale, composite orogenic belt located in northwestern China (central Asia). It was the product of accretion, subduction, and collision of various continental blocks during the formation, evolution, and disappearance of the Paleo-Asian ocean between the northern Siberia plate and the southern Tarim and North China plates (Fig. 1). By the Early Mississippian (i.e. early part of the early Carboniferous), the Paleozoic ocean basin had already closed. The consequent suture zone became an area of thickened crust characterized by complex tectonic and magmatic activity and uplift. A major regional upwelling and partial melting event led to the development of the Tianshan Carboniferous-Permian rift system and its associated large igneous province (Xia et al., 2002, 2004). The rift system comprises five parts: the Keping rift situated in the northwestern margin of the Tarim plate; and the western Tianshan rift; the Central Tianshan rift; the eastern Tianshan Bogda rift; and the eastern Tianshan Jueluotage rift (Fig. 1).

Click to see larger image.
Figure 1: Sketch map of geologic tectonic units of the Tianshan orogenic belt showing the distribution of Carboniferous volcanic rocks (modified after Xia et al., 2004). WJBS―West Jungar trench-arc-basin system (Paleozoic); EJBS―East Jungar arc-basin system (Paleozoic); BTB―Bole tectonomagmatic belt (late Paleozoic); NTOB―North Tianshan ophiolite belt (late Paleozoic); HTB―Harlike tectonomagmatic belt (late Paleozoic); BKTB―Boluokenu tectonomagmatic belt (early Paleozoic); MGOMB―Mishigou-Gangou ophiolitic mélange belt (early Paleozoic); SWTB―Southwestern Tianshan tectonomagmatic belt (late Paleozoic); STTB―South Tianshan tectonomagmatic belt (Silurian-late Paleozoic); KTB―Keping tectonomagmatic belt (Neoproterozoic-Paleozoic). Carboniferous-Permian rifts are labeled.

A typical cross section (Fig. 2) is as follows: In the Luotuogou area north of Baluntai, the Mississippian Maanqiao Formation volcanic-sedimentary formation consists of (from bottom to top) thick-bedded boulderstone, conglomerate (of which the gravels consist of basement rocks), gritstone, sandstone, sandstone and siltstone interbedded with shale and intercalated with limestone, basalts, and a few rhyolites. It has an angular unconformity or faulted contact with the Proterozoic granite-gneiss in the Baluntai micro-continental block of the central Tianshan (Fig. 2). The aforementioned Mississippian volcanic-sedimentary formation (in the central Tianshan) is characterized by a progradational sequence of transition from continental to marine facies and reflects a progressive rift extension.


Figure 2: Simplified stratigraphic columns of early Carboniferous rift-related volcanic rocks in the Tianshan region (after Xia et al., 2004).

The entire region of Carboniferous rift-related volcanic rocks are mainly exposed in a dumbbell-shaped province of ~210,000 km2 within the Tianshan orogenic belt (Fig. 1). This area represents a minimum estimate because the volcanic rocks extend west and east from China and also have a widespread distribution in Kazakstan, Kirgizstan and Mongolia, respectively. They cover approximately 343,000 km2 (Fig. 1). The thickness of the entire volcanic sequence varies from over 13,000 m in the eastern Tianshan, to several hundred meters in the central Tianshan, to over 7000 m in the western Tianshan. The magmatic province consists mainly of basaltic lavas and subordinate amounts of intermediate and silicic lavas, and pyroclastic rocks. The average lava thickness of the Tianshan LIP is estimated to be about 2000 m, thus the entire volume of the Tianshan basalts is ~ 0.7 × 106 km3. Although geochronology of the Tianshan Carboniferous rift-related volcanic rocks is rather limited (345-325 Ma―39Ar-40Ar age, Li et al., 1998; 327-306 Ma―39Ar-40Ar age, Zhao et al., 2003; 319-321 Ma―U-Pb (TIMS) zircon age, Li et al., 2004), the sedimentary piles within the volcanic series contain abundant fossils of Carboniferous age (e.g., Tournaisian stage, Visean stage, Serpukhovian stage, etc.; Che et al., 1994; He et al., 1994; Zhou et al., 1994). These fossils unequivocally constrain the eruption age of the volcanic rocks. The earliest eruptions began some time after the late Tournaisian (350–345 Ma) (calibration of stage boundaries after Gradstein et al. 2004). After the Carboniferous peak of activity, subsequent Permian volcanism was sporadic.

The massive Tianshan Carboniferous volcano-sedimentary successions were deposited on various types and ages of basement: the Proterozoic Keping continental block (in the northwestern Tarim) and the Proterozoic Yili microcraton (in the western Tianshan) that consist mainly of metamorphic rocks of Proterozoic age; the Baluntai micro-continental block of the central Tianshan that consists of metamorphic rocks of Proterozoic and Early Ordovician-Middle Ordovician age; and the eastern Tianshan mobile belt that consists mainly of Devonian island-arc volcanic rocks (Zhou et al., 1994) (Fig. 1). The nature of the basement is important in interpreting the any crustal or lithospheric mantle sources of geochemical variation.

Geochemistry of Tianshan basalts

The Tianshan basalts have been altered to various degrees after their eruption (Xia et al., 2004). This affects incompatible elements such as Rb, Ba, and K, which known to be mobile during alteration. Gain of Na2O may also have occurred during alteration. Also, 87Sr/86Sr ratios are suspect since age correction of this isotopic ratio involves Rb.. For these reasons, emphasis is placed on immobile elements such as REEs, HFSEs, Th, Y, Ti, and Mg, and εNd(t) in the following discussion.

Based on petrogeochemical data, the Tianshan basalts can be classified into two major magma types: high-Ti/Y (HT, Ti/Y > 500, Nb/Zr < 0.12, in the western Tianshan) and low-Ti/Y (LT, Ti/Y < 500) basalts (Fig. 3A). The LT lavas can be further divided into two subtypes. LT1 lavas in the central and eastern Tianshan exhibit lower Nb/Zr (< 0.12); the LT2 lavas in the northwestern Tarim have higher Nb/Zr (> 0.16) (Fig. 3A). This classification is rather preliminary and could be revised when more data are available. Most of HT lavas belong to the alkaline series; the LT2 lavas also belong to alkaline series; most of the LT1 lavas belong to tholeiitic series (Xia et al., 2004). Most of the Tianshan basalts plot in the Within-Plate Basalts (WPB) field (Fig. 3B). This is consistent with the geological evidence for an intracontinental rift setting. In the primordial-mantle-normalized spider diagrams, the central Tianshan basalts display noticeable negative Th anomalies that are associated with slightly positive Nb anomalies. A conspicuous negative Sr anomaly is also noted in samples of this area (Fig. 4B). The western and eastern Tianshan basalts are characterized by significant depletion of Nb and Ta relative to Th and by marked negative P and Y anomalies (Fig. 4F). The basalts of the Keping area (in the northwestern Tarim) and a sample (Bb-263-4) of Luotuogou area (in the central Tianshan) are characterized by slight depletion of Nb and Ta relative to Th and by prominent negative Sr and Zr anomalies (Fig. 4D).


Figure 3: Diagrams showing variation of (A) Nb/Zr vs. Ti/Y and (B) Zr/Y vs. Zr (after Pearce, 1982) for the Carboniferous and Permian basalts from the Tianshan region. Data source: Xia et al. (2004).


Figure 4: Primordial-mantle-normalized multi-element plots for basalts from various continental flood basalts (continental Large Igneous Provinces, LIPs) shown for reference (A, C, and E) and the Tianshan LIP (B, D, and F). Normalizing values from Sun and McDonough (1989). Note the variation of Th/Ta, Rb/Ta and Ba/Ta ratios (the enrichment of Rb and Ba relative to Th in the Tianshan basalts may be due to secondary alteration). Data sources: Tianshan basalts: Xia et al. (2004); Deccan (averages values for Ambenali, Madabaleshwar and Bushe): Lightfoot and Hawkesworth (1988); Greenland (Prince of Wales Mountains): Hogg et al. (1989); Madagascar: East Coast samples MAN 90-43 (plume-related), MAN 90-8 (mantle lithosphere affinity) and MAN 90-35 (crustally contaminated) from Saunders et al. (1992); Paraná Urubici (‘high-Ti’) and Gramado (‘low-Ti’) basalts are averages from Peate (1989).

The initial isotopic ratios of the Mississippian mafic basalts were corrected to 345 Ma according to the published Ar–Ar age data (Li et al., 1998) and those of Pennsylvanian basalts were corrected to 325 and 320 Ma according to the published Ar–Ar (Zhao et al., 2003) and U-Pb zircon age data (Li et al., 2004) and those of Cisuralian (lower Permian) basalts were corrected to 280 Ma according to published Ar–Ar age data (Zhao et al., 2003). Nd and Sr isotopic data are illustrated in Fig. 5A. The samples from the eastern Tianshan have a relatively limited range of compositions: εNd(t)=+5.4 to +9.6; 87Sr/86Sr(t)=0.703354–0.704411. Samples from the central Tianshan are displaced to higher 87Sr/86Sr(t) (0.703645–0.706726) and lower εNd(t) (+3.1 to +6.2). One sample from the western Tianshan (Zhaosu area) has lower εNd(t) (–1.1) , that distinguishes it from all other samples from the Tianshan LIP, but its 87Sr/86Sr(t) ratio (0.704399) is similar. Samples from the northwestern Tarim (Keping area) have highest 87Sr/86Sr(t) (0.706821–0.708080) and lowest εNd(t) (–0.98 to –2.91) (Table 1).

Table 1: Rb/Sr and Sm/Nd isotope ratios for Tianshan carboniferous and permian basalts.

 

(87Rb/86Sr)m

(87Sr/86Sr)m

(87Sr/86Sr)(t)

(147Sm/144Nd)m

(143Nd/144Nd)m

(143Nd/144Nd)(t)

εNd(t)

Zhaosu area, western Tianshan (Dahalajunshan Formation, Mississippian; t = 345 Ma)

T-98

0.0336

0.704564 ± 12

0.704399

0.1519

0.512478 ± 11

0.512134

– 1.15

Luotuogou area, central Tianshan (Maanqiao Formation, Mississippian; t = 345 Ma)

Bl-02

0.4613

0.708050 ± 60

0.705785

0.1595

0.512878 ± 5

0.512517

6.33

Bl-05

0.3786

0.707085 ± 30

0.705226

0.1684

0.512756 ± 9

0.512375

3.55

Bl-07

0.2543

0.706500 ± 50

0.705251

0.1605

0.512763 ± 6

0.512400

4.04

Bl-10

0.1471

0.707344 ± 30

0.706622

0.1522

0.512760 ± 15

0.512416

4.34

Bl-24

0.0888

0.706325 ± 40

0.705889

0.1496

0.512764 ± 7

0.512426

4.54

Bb-142-3

     

0.1742

0.512959 ± 15

0.512565

7.26

Bb-142-9

0.420

0.707104 ± 18

0.705041

0.1977

0.512961 ± 10

0.512514

6.26

Bb-255-4

0.491

0.708601 ± 15

0.706189

0.1528

0.512741 ± 15

0.512395

3.95

Bb-255-5

0.3357

0.708375 ± 18

0.706726

0.1732

0.512891 ± 15

0.512499

5.98

Bb-263-4

0.612

0.708799 ± 19

0.705793

0.1540

0.512766 ± 8

0.512418

4.38

Bb-263-5

0.296

0.707202 ± 19

0.705748

0.1544

0.512754 ± 9

0.512405

4.13

South of Tuokexun area, eastern Tianshan (Xiaorequanzi Formation, Mississippian; t = 345 Ma)

TB-33

     

0.1360

0.512950 ± 25

0.512642

8.77

Qijiaojin area, eastern Tianshan (Qijiaojin Formation, Mississippian; t = 345 Ma)

B-009

0.21818

0.70619

0.705119

0.15598

0.512878

0.512525

6.48

B-014

0.009746

0.70451

0.704462

0.1536

0.512851

0.512504

4.16

Duku highway, central Tianshan (Akeshake Formation, Mississippian; t = 325 Ma)

BT-4

0.400

0.705569 ± 19

0.703719

0.1955

0.512794 ± 10

0.512378

3.10

BT-2

     

0.1943

0.513011 ± 9

0.512597

7.38

Bb-5

     

0.1553

0.512934 ± 10

0.512603

7.50

South of Houxia area, central Tianshan (Qiergusitao Formation, Pennsylvanian; t = 320 Ma)

Bb-85-1

0.128

0.704228 ± 20

0.703645

0.1494

0.512857 ± 11

0.512544

6.21

Bb-85-3

     

0.1473

0.513079 ± 10

0.512770

10.63

Tianchi area, eastern Tianshan (Liushugou Formation, Pennsylvanian; t = 320 Ma)

Bb-111

0.210

0.705368 ± 18

0.704411

0.1539

0.512837 ± 14

0.512514

5.64

Bb112

0.284

0.705176 ± 18

0.703882

0.1411

0.512827 ± 11

0.512531

5.96

Bb-117

0.127

0.704736 ± 16

0.704157

0.1535

0.512849 ± 11

0.512527

5.89

Bb-120

0.192

0.705101 ± 16

0.704226

0.1588

0.512837 ± 7

0.512504

5.44

Bb-128

0.136

0.704680 ± 18

0.704061

0.1519

0.512831 ± 9

0.512512

5.60

Tuwu area, eastern Tianshan (Qieshan Formation, Pennsylvanian; t = 320 Ma)

Bb-158

0.077

0.704122 ± 10

0.703771

0.1392

0.512828 ± 10

0.512536

6.06

Bb-160

0.024

0.703996 ± 17

0.703886

0.1420

0.512805 ± 10

0.512507

5.50

Bb-162

0.007

0.703679 ± 20

0.703647

0.1275

0.512853 ± 12

0.512585

7.03

Bb-172

0.011

0.703404 ± 16

0.703354

0.1224

0.512977 ± 8

0.512720

9.66

Bb-185

0.052

0.704030 ± 16

0.703793

0.1493

0.512861 ± 10

0.512548

6.29

Bb-270

0.0086

0.703920 ± 15

0.703881

0.1149

0.512769 ± 10

0.512528

5.90

Bb-267

0.105

0.704800 ± 20

0.704322

0.1562

0.512966 ± 8

0.512638

8.06

Keping area, northestern Tarim (Kupukuziman Formation, Cisuralian; t = 280 Ma)

B-1

0.1664

0.708743 ± 20

0.708080

0.1309

0.512365 ± 11

0.512125

- 2.91

B-2

0.1713

0.708623 ± 11

0.707940

0.1314

0.512371 ± 10

0.512130

- 2.86

B-3

0.2608

0.708665 ± 23

0.707625

0.1309

0.512393 ± 8

0.512152

- 2.45

B-4

0.3287

0.708520 ± 18

0.707210

0.1331

0.512412 ± 9

0.512168

- 2.14

B-5

0.4587

0.708649 ± 15

0.706821

0.1320

0.512469 ± 10

0.512227

- 0.98

Note: The Sr and Nd isotopic ratios were determined using a VG354 mass spectrometer at the Institute of Geology and Geophysics, Chinese Academy of Sciences (CAS), in Beijing. The La Jolla and NBS987 standards gave 143Nd/144Nd = 0.511862±0.000007 and 87Sr/86Sr = 0.710254±0.000014, respectively.

Data sources: Luotuogou: Bl-02 to Bl-24 (Chen et al., 2001), Bb-263-4 and Bb-263-5 (Xia et al., 2004), Bb-142-3 and Bb-142-9 and Bb-255-4 and Bb-255-5 (this study); Qijiaojin: Gu et al. (2000); south of Houxia, Tianchi and Tuwu (Xia et al., 2004); Zhaosu, south of Tuokexun, Duku highway, and Keping (this study). The t (time) values indicate the ages at which the 87Sr/86Sr ratios, 143Nd/144Nd ratios and εNd values were calculated. Errors on sixth decimal place; m–measured.

Lead isotopic compositions vary more widely (Fig. 5B). The eastern Tianshan basalts have relatively low 206Pb/204Pb(t) ratios, from 17.593 to 17.966, and low 207Pb/204Pb(t) (15.384–15.473). The central Tianshan basalts have relatively high 206Pb/204Pb(t), from 17.773 to 18.142, and, with the exception of one sample (BT-4, 17.590), high 207Pb/204Pb(t) (15.478–15.619). The basalt from the western Tianshan has intermediate 207Pb/204Pb(t) (15.490) ratio and still higher 206Pb/204Pb(t) (18.134) (Table 2).

Table 2: Pb isotope ratios for Tianshan carboniferous and permian basalts.

 

(206Pb/204Pb)m

(207Pb/204Pb)m

(208Pb/204Pb)m

Pb(ppm)

Th(ppm)

U(ppm)

(206Pb/204Pb)t

(207Pb/204Pb)t

(208Pb/204Pb)t

Zhaosu area, wstern Tianshan (Dahalajunshan Formation, Mississippian; t = 345 Ma)

T-98

18.282 ± 0.019

15.498 ± 0.020

37.946 ± 0.021

5.61

0.84

0.24

18.134

15.490

37.778

Luotuogou area, central Tianshan (Maanqiao Formation, Mississippian; t = 345 Ma)

Bb-263-4

18.691 ± 0.012

15.560 ± 0.013

38.491 ± 0.012

3.5

2.5

0.77

17.920

15.518

37.681

Bb-263-5

18.33 ± 0.008

15.631 ± 0.009

38.351 ± 0.010

9.2

2.1

0.56

18.118

15.619

38.093

Bb-142-3

18.314 ± 0.013

15.605 ± 0.016

38.316 ± 0.016

6.6

1.5

0.44

18.082

15.592

38.060

Bb-142-9

18.205 ±0.011

15.583 ± 0.013

38.169 ± 0.013

11.1

0.61

0.20

18.142

15.579

38.107

Bb-255-4

18.405 ± 0.030

15.567 ± 0.030

38.253 ± 0.030

9.6

3.9

0.92

18.071

15.549

37.795

Bb-255-5

18.197 ±0.011

15.579 ± 0.011

38.135 ± 0.011

12.5

0.93

0.28

18.119

15.575

38.051

Duku highway, central Tianshan (Akeshake Formation, Mississippian; t = 325 Ma)

BT- 4

18.537 ± 0.028

15.595 ± 0.029

39.006 ± 0.030

0.42

0.39

0.12

17.590

15.545

38.009

South of Houxia area, central Tianshan (Pennsylvanian; t = 320 Ma)

Bb-85-1

18.382 ± 0.021

15.511 ± 0.023

38.129 ± 0.023

3.8

2.4

0.72

17.773

15.478

37.471

Tianchi area, eastern Tianshan (Liushugou Formation, Pennsylvanian; t = 320 Ma)

Bb-111

18.503 ± 0.026

15.477 ± 0.033

38.247 ± 0.037

4.2

2.3

0.70

17.966

15.448

37.675

Bb-112

18.297 ± 0.062

15.474 ± 0.066

38.041 ± 0.068

3.8

1.8

0.62

17.774

15.446

37.549

Bb-117

19.026 ± 0.024

15.517 ± 0.028

38.820 ± 0.032

2.5

3.7

1.07

17.626

15.443

37.252

Bb-120

18.521 ± 0.020

15.515 ± 0.021

38.441 ± 0.023

1.6

1.5

0.39

17.733

15.473

37.459

Bb-128

18.323 ± 0.013

15.479 ± 0.014

38.096 ± 0.016

2.8

1.7

0.46

17.796

15.451

37.465

Tuwu area, eastern Tianshan (Qieshan Formation, Pennsylvanian; t = 320 Ma)

Bb-158

18.054 ± 0.012

15.465 ± 0.013

37.650 ± 0.014

3.8

0.85

0.32

17.786

15.451

37.419

Bb-160

18.019 ± 0.010

15.458 ± 0.012

37.610 ± 0.012

3.7

0.74

0.28

17.779

15.445

37.404

Bb-162

18.321 ± 0.018

15.491 ± 0.020

37.925 ± 0.021

2.3

1.2

0.42

17.737

15.460

37.384

Bb-172

18.045 ± 0.034

15.471 ± 0.039

37.735 ± 0.042

2.0

0.37

0.12

17.854

15.461

37.544

Bb-185

18.033 ± 0.019

15.408 ± 0.020

37.504 ± 0.020

1.8

0.69

0.25

17.593

15.384

37.111

Bb-267

18.203 ± 0.014

15.478 ± 0.015

37.834 ± 0.016

1.0

0.57

0.19

17.597

15.446

37.245

Bb-270

17.999 ± 0.013

15.450 ± 0.013

37.645 ± 0.014

1.3

0.37

0.09

17.780

15.438

37.353

Note: Pb isotopic ratios were measured using a VG354 mass spectrometer at the Institute of Geology and Geophysics, CAS, in Beijing. During the course of this study, 6 measurements of common lead standard NBS 981 gave average values of 206Pb/204Pb = 16.948291±0.000084, 207Pb/204Pb = 15.49684±0.00017 and 208Pb/204Pb = 36.68305±0.00097 and uncertainties of <0.1% at the 95% confidence level for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb. Total procedural Pb blanks were lower than 1 ng.

Data sources: this study. The t (time) values indicate the ages at which the Pb isotopic ratios were calculated. The calculations use the measured whole-rock U, Th, and Pb contents determined by inductively coupled plasma mass spectrometry (ICP-MS) at the China University of Geosciences in Beijing and whole-rock Pb isotopic ratios. The analytical uncertainties were estimated to be generally < 5 % for the trace elements. m―measured.


Figure 5: Plots of (A) εNd (t) vs. 87Sr/86Sr (t) (after DePaolo, 1979; Zindler and Hart, 1986), (B) 207Pb/204Pb vs. 206Pb/204Pb (t), (C) εNd (t) vs. 206Pb/204Pb (t) (corrected to t = 345 Ma and 325 Ma for Mississippian lavas, to 320 Ma for Pennsylvanian lavas and to 280 Ma for Permian (Cisuralian) lavas), (D) La/Ba vs. La/Nb, (E) Mg# vs. La/Nb, (F) 87Sr/86Sr (t) vs. La/Nb andεNd (t) vs. La/Nb for the Carboniferous and Permian basalts of the Tianshan. A: UC–upper crust; LC–lower crust; EMI and EMII–enriched mantle I and II sources; HIMU–high-μ mantle source; DM–depleted mantle source. Comparative mean MORB and OIB (Sun and McDonough, 1989) compositions (corrected to 330 Ma) are shown. B and C: data sources as for Figure 4. D: Field for ocean island basalts (OIB) is from Fitton et al. (1991). F and G: Comparative mean OIB composition (Sun and McDonough, 1989) corrected to 330 Ma is shown. All other symbols and data sources as for Figs. 3 and 4.

Each region or volcanic sequence in the Tianshan LIP has, therefore, a distinct trace element and Pb-Sr-Nd isotopic composition.

Discussion

Large igneous provinces (LIPs) represent generally short duration large volume events in an intraplate setting (Coffin and Eldholm, 1994; Courtillot et al., 1999; Ernst et al. 2005). They are typically linked with mantle plumes (Morgan, 1971; Richards et al., 1989; Campbell and Griffiths, 1990; White and McKenzie, 1989; Saunders et al., 1992; Hill, 1991, 1993; Arndt et al., 1993; Ewart et al., 1998; Ernst and Buchan 2001; Macdonald et al., 2001), however, non-plume models may also be applicable in some cases (Foulger et al. 2005 and papers therein). Continental LIPs show compositional evidence for the involvement of continental lithosphere, including crust and continental lithospheric mantle (CLM), in at least part of their eruptive sequences. In cases where crustal contamination can be excluded, many studies suggest that the lithospheric mantle plays a significant role, in addition to plume materials from the deep mantle, (e.g., Ellam and Cox, 1991; Saunders et al., 1992; Gallagher and Hawkesworth, 1992; Hooper et al., 1995; Hawkesworth et al., 1995; Rogers et al., 1995; Macdonald et al., 2001; Bogaard and Wörner, 2003). However, arguments against a significant lithospheric mantle contribution (McKenzie and Bickle, 1988; Arndt and Christensen, 1992; Ewart et al., 2004) have also been advanced. . We can use the basalts of the Tianshan LIP to test models of the interaction between a plume and its lithospheric cap.

Evidence for plume involvement

Saunders et al. (1992) suggested that plume or asthenospheric components are characterized by low 87Sr/86Sr ratios, high εNd values and oceanic Pb isotopes such as those that have been reported in the Ambenali Formation of the Deccan (Fig. 5B and C), tholeiites from the East Coast of Madagascar, and the youngest parts of the Lower Lavas and the successions along the Blosseville Coast in eastern Greenland. These same units also have Th/Nb ratios of less than one (Saunders et al., 1992; Fig. 4A). In mantle-normalized plots (Fig. 4B), the majority of central (Luotuogou, Duku highway) Tianshan basalts have patterns that are broadly similar to CFBs with plume affinity (Fig. 4A). These samples from central Tianshan exhibit trace element ratios that overlap with the field of oceanic island basalts (OIB) (Fig. 5D). They are characterized by mantle-normalized Th/Nb ratios of less than one (Saunders et al., 1992; Fig. 4A and B). The pronounced negative Sr anomaly in these lavas suggests that they have been affected by extensive fractionation of plagioclase. One sample (BT-4, Duku Highway) with low trace element abundance also shows high εNd(t) (3.1) and low 87Sr/86Sr(t) (0.703719) and 206Pb/204Pb(t) (17.59) values, thus likely reflecting the isotopic signature of the least-contaminated Tianshan plume head component. The enrichment of Rb and Ba relative to Th observed in several samples may be due to alteration. The samples from Luotuogou area are pillow tholeiites (Xia et al., 2004) that show a trend of increasing 87Sr/86Sr(t) ratios with relatively constant εNd(t) values (Fig. 5A). This may suggest interaction with seawater or contamination with carbonate crust during subaqueous eruption (Yogodzinski et al., 1996).

We conclude that a plume or asthenospheric component is found in the basalts from the central Tianshan.

Evidence for lithosphere involvement

Many Tianshan lavas show negative Nb and Ta anomalies suggesting that components other than a plume must have been involved in the generation and evolution of the Tianshan basalts. The most likely components are from lithosphere. There is still vigorous debate about the way the lithosphere contributes to magma generation. Several possible processes have been proposed: contamination of plume magmas by lithosphere-derived melts (Arndt et al., 1993), whole-scale melting of the continental lithospheric mantle (Gallagher and Hawkesworth et al., 1995; Rogers et al., 1995), and melt-rock reaction resulting from the infiltration of plume-derived magmas into the lithosphere in a process related to the thermomechanical erosion of the lithosphere mantle (Macdonald et al., 2001).

It is concluded that Tianshan basalts also recorded significant lithospheric input from crustal and subcrustal sources. Thus, in addition to the plume component discussed above, two other important components can be identified. The first component, CLM, is seen in basalts of the Kupukuziman Formation of the Keping area from the northwestern Tarim, and in one sample (Bb-263-4) of the Maanqiao Formation of the Luotuogou area from the central Tianshan. In terms of trace elements, these basalts have a mantle-normalized Th/Nb ratio slightly greater than 1, which distinguishes them from the plume-related group, but this value is much lower than the Th/Nb ratio in crustally-contaminated basalts (Saunders et al., 1992; Fig. 4). Isotopically, these Tianshan basalts are highly variable, but are characterized by low to intermediate εNd(t) (- 2.91 to + 4.38), moderate to high 87Sr/86Sr(t) (0.705793 to 0.708743), and intermediate 206Pb/204Pb(t) (17.92) (Tables 1 and 2). These patterns suggest an interaction between the plume- or asthenosphere-derived magmas and the lithosphere mantle.

The second component, a crustal component, shown by the high La/Nb, high Th/Nb, and highly variable isotopic compositions is seen in basalts of the Tekes and Gouzigou areas from the western Tianshan, and the South of Tuokexun and Tianchi basalts from the eastern Tianshan (Figs 4 and 5; Tables 1 and 2). Note in particular, the distinctive trace element patterns of these basalts that are characterized by very high mantle-normalized Th/Nb ratios much greater than 1 or a pronounced negative Nb-Ta anomaly. These patterns are analogous to those of the crustally contaminated basalts of the Bushe Formation from the Deccan, a group of low-Ti basalts from the East Coast of Madagascar, and the Gramado (low-Ti) basalts from the Paraná province (Saunders et al., 1992; Fig. 4E and F). Although a high La/Nb ratio is a reliable trace element index of crustal contamination, Keppler (1996) and You et al. (1996) suggested that subduction-related fluids carry La in preference to Nb. Thus, arc rocks tend to have higher La/Nb than OIB. In Figure 5D, the majority of eastern Tianshan basalts have higher La/Nb ratios indicative of the influence of a subduction component. This may be related to the contamination of upper crust containing Devonian island-arc volcanic rocks and/or to a pre-Carboniferous subduction enrichment of the lithospheric mantle source region.

In summary, the geochemical variation of the Tianshan Carboniferous (―Permian) Basalts can be accounted for by interaction between the plume-derived magmas and the continental lithosphere. We suggested that a large mantle melting anomaly or a plume existed beneath the Tianshan (central Asia) LIP during Carboniferous (–Permian) period (Xia et al., 2004).

Composition of plume component

Negative correlation between Mg# and La/Nb for Tianshan basalts indicates that the magmas forming the lavas with lower Mg# have undergone stronger contamination of continental lithosphere (Fig. 5E). Figure 5E also shows that the Tianshan basalts form two different trend lines: one is for the basalts from the central–western Tianshan and the northwestern Tarim that have Precambrian basement, and the other is for the samples from the eastern Tianshan mobile belt with younger (Devonian) arc-crust basement. Similarly, in 87Sr/86Sr(t) versus La/Nb (Fig. 5F) and εNd(t) versus La/Nb (Fig. 5G) diagrams the plots of basalts from a relatively stable micro-continental block (or microcraton) and those from a mobile belt also distribute along two different trend lines. The place at which the trend lines intersect might locate the plume head component (87Sr/86Sr(t) ≈ 0.7045, εNd(t) ≈ + 4) for the Tianshan Carboniferous (–Permian) LIP.

Conclusion

The Tianshan Carboniferous (―Permian) rift-related volcanic rocks comprise a newly-recognized large igneous province. The isotopic and principal chemical variations within the Tianshan basalts can be accounted for by contamination of a plume component (87Sr/86Sr(t) ≈ 0.7045, εNd(t) ≈ + 4) by upper crust or CLM (continental lithospheric mantle).

Acknowledgments

This research benefited from financial supports by Land and Resources Survey Project of China (Grant 200113000022, 200313000063) and the National Natural Science Foundation of China (Grant 40472044).

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