Vol. 44 No. 6 SCIENCE IN CHINA (Series D) June 2001

Geochronology and geochemistry of the Heihe mafic pillow lavas in the Qinling Mountains, China

ZHANG Zongqing (张宗清)1, ZHANG Guowei (张国伟)2, TANG Suohan (唐索寒)1 & WANG Jinhui (王进辉)1

1. Institute of Geology, Chinese Academy of Geosciences, Beijing 100037, China; 2. Department of Geology, Northwest University, Xi’an 710069, China Correspondence should be addressed to Zhang Zongqing (email: smnd@yeah.net)

Received May 29, 2000 Abstract The Qinling Mountains in central China are the joint orogenic zone between the Sino-Korean (or North China) and the Yangtze craton blocks. The age and genesis of the Danfeng mafic volcanics in the north of the Shangzhou-Danfeng fault zone, i.e. the main suture zone in the Qinling orogenic belt, have been controverted for a long time because their age is closely related to the converged time of two blocks. The ages and the geochemical data of the Heihe pillow lavas for the Danfeng mafic volcanics in the Heihe River area in the Qinling orogen are reported in this paper. The obtained isochron age by the Sm-Nd isotopic data of the 13 whole-rock samples for the mafic pillow lavas is 963±130 (2σ) Ma, corresponding to INd = 0.51173±16 (2σ), εNd (T) = +6.6, MSWD0.57. However, the Rb-Sr isotopic analytical results for the same samples as the Sm-Nd whole-rock ones are disperse. For the Sm-Nd isotopic systems were interfered during the later geological functions, the Sm-Nd isochron age for the whole-rock sample (Q9511WR) and the min-eral phenocrystal samples: amphiboles (Hb) and plagioclases (Plag) presents the better uncer-tainty, whereas isochron ages of 930 Ma and 437 Ma are given if the WR-Plag and WR-Hb are calculated respectively, and their Rb-Sr isochron age is 268±47(2σ) Ma, Isr = 0.70475±11(2σ), MSWD0.96. The major and trace elements for the lavas show that they were formed in the quasi-N-MORB setting.

Keywords: age, Danfeng mafic volcanics, Qinling orogen.

The most geologists consider that the Danfeng mafic volcanlics in the north of the Shang-zhou-Danfeng fault zone, i.e the main suture zone between the Sino-Korean (or North China) and the Yangtze craton blocks, are a set of the rocks with ophiolite features forming in the period that the North China and the Yangtze craton blocks were converged[1—10]. However their genetic ages

have been at issue[11—16].The ages of the Heihe mafic pillow lavas for the Danfeng mafic volcanics and their major and trace elements are reported in this paper.

1 Geological setting

The Heihe mafic pillow lavas, exposed in the Heihe River area, Zhouzhi County about 80 km to southwest of the Xi’an city, are considered the middle part of the Danfeng mafic volcanic belt (fig.1). They are long striped, west and east extending, between the gneisses in the early Protero-zoic Qinling Group and the sandrocks,sandslates in the Devonian system, and are in fault contact

518 SCIENCE IN CHINA (Series D) Vol. 44

with the Devonian system. The rock belt was intruded by large area of biotite granites between it and gneissess of the Qinling Group, and by the Sifantai gabbro-diorites in the east. The volcanics mainly consist of mafic ones with less felsic ones and underwent the metamorphisms under high greeschist to low amphibolite facies. All the samples are the mafic pillow lavas from the north of the Xiaowangjian village in the Heihe River area (fig.1).

Fig. 1. Geological sketch map of the Heihe River area. 1, Carboniferous sandslates of the Caolingyi Fm; 2, Car-boniferous phyllites, sandstones; 3, Devonian shists, sandstones, slates; 4, Middle-later Proterozoic Kuanping Gr.; 5, Danfeng mafic volcanics; 6, Early Proterozoic Qinling Gr.; 7, Sifantai gabbro-diorites; 8, granites; 9, fault, geo-logical boundary. F1, Shangzhou-Danfeng fault zone; F2, Huangtai-waxiezhi fault zone; 10, sampling location.

2 Geochemical features of the mafic pillow lavas

The major and trace element composi-tions of the five samples for the mafic pillow lavas were measured and are listed in table l. For the major elements the samples fall re-spectively into the boundary zone of tholeiites and calc-alkaline basalts and the transitional zone of mid-ocean ridge and Island arc tholei- ites on the A-F-M and TiO2-MnO-P2O5 dia-grams. Their REE concentrations show the flat or LREE-depleted distribution patterns of (Ce/Yb)N 0.59—1.27 (fig. 2). The high field incompatible trace element contents are analo-

Fig. 2. REE distribution patterns for the Heihe mafic pillow lavas.

No. 6 HEIHE MAFIC PILLOW LAVAS 519

gous to MORB,for example, the samples fall into the N-type MORB and ocean-floor basalts (OFB) or low-k tholeiites (LKT) ranges on the Zr-Nb-Y and Ti-Zr-Y diagram (figs. 3 and 4), and possess the MORB normalized ratios of ~1 for elements Nb.

Table 1 Major and trace element compositions of the Heihe mafic pillow lavasa)

Sample Q9233-1 Q9233-3 Q9233-4 Q9233-6 Q9233-13

SiO2 % TiO2

Al2O3

Fe2O3

FeO MnO MgO CaO Na2O K2O P2O5

H2O+

LoI Σ

Rb ppm Ba Sr Nb Zr Y Sc La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

50.04 0.95

13.66 2.19 8.51 0.24 7.58 11.14 2.82 0.52 0.08 1.47 0.09

99.29 5 60 98 2.0 35 19 34 −

6.45 −

5.50 2.06 0.87 3.24

− 4.47

− 2.91

− 2.81 0.44

48.70 1.54 15.59 4.73 6.85 0.18 7.10 8.24 4.22 0.32 0.13 2.14 0.18 99.92

8 71 165 4.9 81 25 38

4.19 10.78

− 9.37 3.12 1.13 4.21

− 5.17

− 3.23

− 2.88 0.42

48.45 1.29 17.57 2.65 6.91 0.18 6.39 11.40 3.09 0.32 0.12 0.94 0.18 99.49

3 54 242 4.3 69 25 34

4.39 11.30 1.59 8.87 2.98 1.17 3.99 0.71 4.64 0.97 2.74 0.37 2.40 0.34

47.39 1.43 16.16 5.02 7.32 0.17 6.79 8.68 3.79 0.29 0.14 2.22 0.18 99.59

7 48 210 3.4 72 27 34

5.25 12.30 1.99 10.48 3.44 1.26 4.71 0.75 4.86 1.03 2.90 0.38 2.47 0.35

45.48 1.20 17.56 2.91 6.21 0.18 6.05 13.73 2.26 0.37 0.11 1.80 2.01 99.87

8 98 238 3.6 58 21 30

3.61 9.63

- 8.28 2.69 1.15 3.61

- 4.45

- 2.83

- 2.62 0.38

a) The major and trace element compositions were measured by XRF and ICP-MS at Institute of Rock and Mineral Analyses of Chinese Academy of Geosciences except for REE of samples Q9233-1,3,13, which were determined by the authors using the isotopic dilution method.

520 SCIENCE IN CHINA (Series D) Vol. 44

Fig.3. Zr-Nb-Y diagram of the Heihe mafic pillow lavas. A, WPA; B, P-MORB; C, VAB; D, N-MORB, VAB.

Fig. 4. Zr-Ti-Y diagram of the Heihe mafic pillow lavas.

3 Geochronological results

The isotopic analyses of samples were performed in the Isotopic Geochronological Labora-tory of the Geological Institute of Chinese Academy of Geosciences. For experimental procedure see ref. [12]. The procedural blanks were 10−9—10−10g for Rb and Sr, ~10−11g for Sm and Nd, and the 143Nd/144Nd and 87Sr/86Sr ratios were normalized to 146Nd/144Nd = 0.7219 and 86Sr/88Sr = 0.1194, 143Nd/144Nd ratios of the standard materials J.M Nd2O3 and GBW04419 were 0.511125±10(2σ) and 0.512721±12(2σ) respectively. The ages are calculated by Ludwing ISOPLOT.

3.1 Sm-Nd isotope data for whole-rock samples The Sm-Nd isotopes of the 13 whole-rock samples for the Heihe mafic pillow lavas were

analyzed and are listed in table 2. It can been seen from fig. 5 that the samples present a good isochron of T = 963±130 (2σ) Ma, corresponding to INd = 0.51173 ± 16 (2σ), εNd (T) = +6.6 MSWD 0.57.

3.2 Rb-Sr isotope data for whole-rock samples The Rb-Sr isotopes of the 11 whole-rock samples for the Heihe mafic pillow lavas were ana-

lyzed and are listed in table 2. The Rb-Sr isotopic data in the range of 0.03—0.24 for 87Rb/86Sr

and of 0.70394—0.70631 for 87Sr/86Sr are disperse, not forming any isochron line.

3.3 Sm-Nd isotope data for mineral samples The Sm-Nd isotopes for a whole-rock sample(Q9511WR) and two mineral phenocrystal

samples, hornblendes (Hb) and plagioclases (Plag), sorted from the whole-rock sample Q9511, were analyzed and are listed in table 2 and fig. 6. The plagioclases are white-lath-shaped, whereas hornblendes are the close square sections, forming from pyroxenes under retrometamor-phisms. The calculated isochron age from WR-Plag-Hb samples is 857±200 (2σ) Ma with INd =

No. 6 HEIHE MAFIC PILLOW LAVAS 521

0.51187±25 (2σ), εNd (t) = +6.1, MSWD 0.66. It is obvious that the age uncertainty is larger, be-cause the Nd isotopic systems of the mineral phenocrystal samples were possibly subjected to in-terfering in later strong metamorphsms and were not resetted fully. If WR-Plag and WR-Hb are calculated respectively, the isochron age is 930 Ma for the former and 437 Ma for the latter. Under later geological function, the Nd isotopic systems of plagioclases (Plag) changed less, but the ones of hornblendes (Hb) underwent the strong resets in retrometamorphism period that pyroxenes were transformed into hornblendes (Hb), the age value of which is almost the same with the age peak value of strong metamorphisms taken in the North Qinling Mountains[16].

Fig. 5. Sm-Nd isochron diagram for the Heihe mafic pillow lavas.

Table 2 Rb-Sr and Sm-Nd isotope analytical results for mafic lava samples from the Heihe River area

Sample Rb/ppm Sr/ppm 87Rb/86Sr 87Sr/86Sr ± 2σ Sm/ppm Nd/ppm 147Sm/144Nd 143Nd/144Nd ± 2σ Q9233-1

-3 -4 -5 -6 -7 -8

-10 -11 -13

Q94178 Q94183 Q9511

Q9511Plag Q9511Hb

6.10 3.13

17.76

4.37 10.78 9.81 8.23 7.93 6.82 4.45

16.50 11.84 11.30

191.19262.73209.02

157.28225.32348.96185.75254.31300.13231.67270.54269.25164.19

0.09220.03450.2457

0.08030.13840.08140.12830.09030.06570.05550.17660.12730.1993

0.706310.704470.70570

0.704890.704710.703940.704350.705130.705670.704880.705410.705240.70552

5 9 6 6 6 1 1 1 1 2 1 1 1

2.026 3.121 3.014 3.862 3.536 6.045 2.916 2.740 2.742 2.695 4.046 1.662 2.629 1.346 2.769

5.535 9.374 9.352 12.288 11.396 12.329 8.928 8.444 8.511 8.282 12.886 5.415 8.116 5.239 8.117

0.2214 0.2014 0.1949 0.1901 0.1877 0.1985 0.1976 0.1963 0.1949 0.1968 0.1899 0.1856 0.1959 0.1554 0.2064

0.513137 0.512995 0.512972 0.512947 0.512928 0.512983 0.512975 0.512973 0.512974 0.512990 0.512938 0.512892 0.512962 0.512715 0.512992

12 7 11 6

17 6 7 5

13 14 5

26 13 11 6

522 SCIENCE IN CHINA (Series D) Vol. 44

Fig. 6. Sm-Nd isochron diagram for mineral samples of the Heihe mafic pillow lavas.

3.4 Rb-Sr isotopic data for mineral samples The Rb-Sr isotopes for the whole-rock sam-

ple Q9511 and two mineral phenocrystal samples, hornblendes (Hb) and plagioclases (Plag), sorted from the whole-rock sample Q9511, such as the Sm-Nd mineral samples, were analyzed and the results are listed in table 2. The isotopic data of the samples present a good isochron (fig. 7) of T =268±47 (2σ) Ma, with ISr = 0.70475±11(2σ), MSWD0.96.

These geochronological data indicated that the mafic pillow lavas in the Heihe River area formed in the Jinning Period about 963 Ma and underwent the later geological functions in the Caledonian-early Variscan (∼400 Ma) and the late Variscan——Indo-China (280 Ma—200 Ma) periods.

Fig. 7. Rb-Sr isochron diagram for mineral samples of the Heihe mafic pillow lavas.

4 Discussion

The isotopic geochronological research shows that in the Qinling orogenic belt there exists a episode of the strong geological events in the Jinning period about 1000 Ma. Most geologists em-phasize that the events were the processes of crust underplated and the Yangtze and the Sino-Ko- rean (on North China) craton blocks splitted off, however, the others think that, in the Jinning pe-riod about 1000 Ma, the Sino-Korean (or North China) and the Yangtze craton blocks were con-verged in the Qinling area. The genesises of mafic volcanics for Danfeng mafic volcanic belt,

No. 6 HEIHE MAFIC PILLOW LAVAS 523

which is the west and east intermittent extending and several hundred kilometers long along the Shangzhou-Danfeng fault zone, i.e. along the main suture zone between North China and the Yangtze craton blocks, have been under discussion, that are considered neither ophiolites forming in the island arc environment[1—11] or metabasalts generating in the intra-ocean island arc setting above subduction zone[17,18]. However, the major and trace element compositions of the Heihe mafic pillow lavas in the paper are characterized as quasi N-MORB. The ages for the Danfeng mafic rocks are generally thought to form in early Palaeozoic era because the Ordovician-Rilurian radiolarian fossils were discovered in the siliceous rocks between the Danfeng mafic volcanics in the Guojiagou Village, Danfeng County, Shaanxi Province[13], and the Sm-Nd age of the lajimiao gabbros, intruded into the Danfeng mafic volcanics, is 403±17 Ma[19]. However, the new geo-chronological data in the paper indicate that the Danfeng mafic volcanics are not possibly the products in a period. The age and εNd (T) value for mafic volcalics in the Heihe River area are al-most the same as that (983±140 Ma, εNd (T) = +6.6) of the Songshugou ophiolites in the same tec-tonic zone within the error limits[20]. It is shown that, in the Jinning period, along the Shangzhou- Danfeng fault zone was a finite ocean basin[21], and there existed the plate tectonic mechanism probably, that is, in the Jinning period, the Sino-Korean (or North China) and the Yangtze craton blocks were converged along the shangzhou-Danfeng fault zone. Besides the aforesaid evidences, it is also confirmed by the following facts: (ⅰ) In the Jinning period, the Qinling Group gneisses in the north of the Shangzhou-Danfeng zone were subjected to the strong metamorphism, accom-panied by partial remelting, so that the isotopic systems of the rocks were reset completely[12,22,23]; (ⅱ) along the Shangzhou-Danfeng fault zone, there exists a granite belt, forming in the Jinning period, for example, for the Dehe granites (793 Ma)[23], the Niujiaoshan granite (959Ma)[24], the Tuwushan A-type granite (725Ma)[25], and so on, the first two granites are considered the synoro-genic ones, whereas the last granites were thought to be post-orogenic magmatism, i.e. they formed in the period in which the North China and the Yangtze craton blocks were rifted off again after they were converged; (ⅲ) in the Jinning period, the Nd isotope consisting of the mafic vol-

canics in the Qinling Mountains obviously changed, that is, their Nd isotopic εNd values are dis-tributed along the evolutionary line of MORB with εNd (0) = +10 till 1200 Ma ago, however, dur-ing the Jinning period, εNd values went quickly down[16], this change is relative to plate subduction possibly.

Acknowledgements The authors thank Prof. Zhang Qi for his helpful comments on the manuscript. This work was supported by the National Natural Science Foundation of China (Grant Nos. 49773182, 49732080).

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