Ar-Ar and UPb geochronology of Late Paleozoic basalts in western Guangxi and its constraints on the eruption age of Emeishan basalt magmatism

Science in China Series D: Earth Sciences

© 2007 Science in China Press

Springer-Verlag

www.scichina.com www.springerlink.com Sci China Ser D-Earth Sci | April 2007 | vol. 50 | no. 4 | 505-518

Ar-Ar geochronology of Late Mesozoic volcanic rocks from the Yanji area, NE China and tectonic implications

LI ChaoWen1,2, GUO Feng1,†, FAN WeiMing1 & GAO XiaoFeng1,2 1 Key Laboratory of Marginal Sea Geology, Guangzhou institute of Geochemistry, Chinese Academy of Sciences, Guangzhou

510640, China; 2 Graduate University of the Chinese Academy of Sciences, Beijing 100039, China

Ar-Ar dating results of late Mesozoic-Cenozoic volcanic rocks from the Yanji area, NE China provide a new volcano-sedimentary stratigraphic framework. The previously defined “Triassic-Jurassic” volcanic rocks (including those from Sanxianling, Tuntianying, Tianqiaoling and Jingouling Fms.) were erupted during 118―106 Ma, corresponding to Early Cretaceous. The new eruption age span is slightly younger than the main stage (130―120 Ma) of the extensive magmatism in the eastern Central Asian Orogenic Belt and its adjacent regions. Subduction-related adakites occurring in the previously defined Quanshuicun Fm. were extruded at ca. 55 Ma. Based on these new Ar-Ar ages, the late Mesozoic to Palaeocene volcano-sedimentary sequences is rebuilt as: Tuopangou Fm., Sanxianling/Tuntianying Fm. (118―115 Ma), Malugou/Tianqiaoling Fm. (K1), Huoshanyan/Jingouling Fm. (108―106 Ma), Changcai Fm. (K2), Quanshuicun Fm. (~55 Ma) and Dalazi Fm. Our results suggest that subduction of the Pa-laeo-Pacific Ocean beneath the East Asian continental margin occurred during 106 to 55 Ma, consistent with the paleomagnetic observations and magmatic records which indicated that the Izanagi-Farallon ridge subduction beneath the southwestern Japan took place during 95―65 Ma.

Ar-Ar geochronology, Late Mesozoic, volcanic rocks, tectonic implications, Yanji area

1 Introduction

The NE China fold belt is the eastern segment of the gigantic accreting continental margin of the Mongo-lia-Okhotsk orogenic belt, or is termed the Central Asian Orogenic Belt (CAOB). Since late Palaeozoic, it has undergone long-term plate subduction and continent-arc and/or microcontinent-continent collisions before the ultimate collision between the North China- Mongolian Block and the Siberian Craton 0H

[1―6]. Different from that of classical collisional orogens, such as the Alps and the Himalayas, the main difference in the CAOB lies in the paucity of extensive ancient gneiss terranes representing former continental fragments, and the molasse-filled foredeeps that generally form between them1H

[1]. Instead, it is composed mainly of subduction-accretion com-

plexes, intruded by vast positive εNd(t) plutons and cov-ered in places by their volcanic derivatives2H

[7―14]. The late Mesozoic geology was characterized by extensive vol-canism, and basin and range tectonics3H

[15―21]. In the east-ern segment of the accreting continental margin, it is supposed that the Mongolia-Okhotsk Ocean diminished and ultimately closed in Early Cretaceous, signifying the accomplishment of the accretion of the East Asian con- tinent 4H

[22,23]. On the other hand, the NE China Fold belt together

with its neighbouring areas has become a part of circum-

Received April 5, 2006; accepted July 17, 2006 doi: 10.1007/s11430-007-2046-9 †Corresponding author (email: [email protected]) Supported by the National Natural Science Foundation of China (Grant Nos. 40373018 and 40334043) and the Chinese Academy of Sciences (Grant Nos. KZCX2-104 and GIGCX-04-04)

506 LI ChaoWen et al. Sci China Ser D-Earth Sci | April 2007 | vol. 50 | no. 4 | 505-518

Pacific tectonomagmatic belt since late Mesozoic. Up to now, it is still controversial about the relationship be- tween the extensive magmatic activities in NE China (probably throughout the East China) and the Palaeo- Pacific Ocean subduction6H

[20,21,24―29]. The Yanji area is located at the junction of China, Russia and Korea, and was considered as a part of the orogenic collage between the North China Block (NCB) in the south and the Jia-musi-Khanka Massifs in the northeast. To the east is the Japan Sea back-arc basin, which is considered to have formed in response to the subduction of the Pacific Ocean during 22―15 Ma7H

[30]. Therefore, the Yanji area is the most likely candidate in which subduction-related magmatism would occur. In fact, across the Yanji are distributed voluminous volcanic rocks. Based on a few K-Ar, Rb-Sr and Ar-Ar ages, previous work considered that these volcanic sequences spanned a large eruption range from late Triassic to early Cretaceous, and attrib-uted their origin to episodic Palaeo-Pacific Ocean sub-duction events8H

[20,31]. Based on detailed field investigations in the Yanji

area, we conducted Ar-Ar dating of the typical volcanic units using a sensitive high-resolution GV-5400 Ar mass spectrometer. One of the aims of this study is to pre-cisely date the eruption ages of these lavas from differ-ent volcanic units, and to provide a reliable volcano- sedimentary framework through the Mesozoic time. Prior to this, we discovered a suite of Palaeocene (Ar-Ar age is ∼55 Ma) adakitic andesites in previously defined Quanshuicun Fm. These rocks have similar elemental and Sr-Nd-Pb isotopic characteristics to those counter-parts occurring in modern subduction zones, e.g., ex-tremely high Sr content (Sr > 1300×10−6), low Y (Y<12 ppm), and unradiogenic Sr and radiogenic Nd-Pb iso-topic compositions. Generation of these adakites is closely related with the subduction of Palaeo-Pacific Ocean (Izanagi-Farallon ridge) (our unpublished data). Therefore, the other purpose will be focused on timing of Palaeo-Pacific Ocean plate subduction beneath the East Asian continental margin.

2 Background geology and general as-pects of Mesozoic volcano-sedimentary sequences

The Yanji area is located at the junction of Khanka, Longgang and Jiamusi Massifs (Figure 1)9H

[14,32,33]. Gen-

erally, the basement consisted of Archaean Jiapigou group and late Proterozoic Zhangguangcailing group. Palaeozoic strata are widely distributed and undergone various degrees of metamorphism and deformation10H

[33], intruded by immense volumes of Phanerozoic granitic rocks11H

[14,33]. Available zircon U-Pb data indicated that emplacement of these Phanerozoic granitoids extended from late Palaeozoic (285 Ma) to early Cretaceous (116 Ma)12H

[14]. Zhang et al. (2004)13H

[14] related the multistage granitoid emplacement events to the evolution of the eastern segment of the CAOB. The Mesozoic geology was characterized by eruption of voluminous lavas and deposition of terrestrial sediments. The Mesozoic vol-canic sequences in the area include: Tuopangou, Sanxianling, Tianqiaoling, Tuntianying, Huoshanyan, Jingouling, and Quanshuicun Fms. However, the no-menclature of each volcanic unit in previous work var-ied from one site to another, which make it difficult to carry out comparative study on the Mesozoic strata. The studied samples were collected from previously defined “Upper Triassic” Sanxianling Fm., “Middle Jurassic” Tuntianying Fm., “Middle-Upper Jurassic” Huoshanyan Fm., “Upper Jurassic” Jingouling Fm., and “Lower Cre-taceous” Quanshuicun Fm. The sampling localities and eruption ages of the volcanic samples are illustrated in Figure 1. Major characteristics (e.g., distribution, lithol-ogy and contact relationships of the different strata) of the Mesozoic volcano-sedimentary sequences are intro-duced below.

Tuopangou Fm.: It was previously defined as “Up-per Triassic”, and comprises mainly intermediate tuf-faceous volcanic rocks and conglomerates, with a thick-ness about 1108 m. Tuopangou Fm. is mainly distributed in southeastern Tianqiaoling and northern Miaoling, overlying disconformably Upper Permian Kanshantun Fm. or late Palaeozoic granites and conformably under-lying the Sanxianling Fm. lavas 14H

[33]. Sanxianling Fm.: The formerly termed “Upper Tri-

assic” Sanxianling Fm. is mainly composed of interme- diate-felsic volcanic lavas with interbeds of tuffaceous sandstones and shales, with a thickness about 460 m. It is mainly distributed along the Tuopangou, Luquanzigou to Malugou areas, and exhibits a fault contact relation- ship with the Malugou Fm. 15H

[33]. Samples 20YJ-80 and 20YJ-92 were collected at Sanxianling in the Wangqing basin. 20YJ-80 is a prophyritic andesite with major phenocryst of plagioclase (10%―15%) and hornblende

LI ChaoWen et al. Sci China Ser D-Earth Sci | April 2007 | vol. 50 | no. 4 | 505-518 507

Figure 1 A simplified geological map showing the distribution of the Mesozoic-Cenozoic volcanic rocks and sampling locations in the Yanji area (after refs. 16H[14] and 17H[33]).

(5%―10%) and rare pyroxene. The matrix is mainly composed of aphanitic groundmass. Sample 20YJ-92 is a weakly porphyritic basaltic andesite with predominant phenocryst of pyroxene (5%―10%). Its matrix is com-posed of microcrystalline plagioclase, pyroxene and glass.

Malugou Fm.: It was also defined as “Upper Trias-sic”, and is composed of grey slate and intermediate volcanic rock and coal layer interbeds with a thickness of 1070 m. Malugou Fm. mainly crops out in the Lu-quanzigou-Malugou area with a conformable contact relationship with the overlying Tianqiaoling Fm.18H

[33]. Tianqiaoling Fm.: The previously called “Upper

Triassic” Tianqiaoling Fm. comprises mainly felsic lava and tuff, and is distributed in the Tianqiaoling, Qing-gouzi, Caopigou and Erchazi areas19H

[33]. These volcanic sequences overlie Tuopangou, Sanxianling, and Malugou Fms. in field.

Tuntianying Fm.: It was previously defined as “Middle Jurassic”, and comprises mainly intermedi-ate-felsic lava and tuff breccia. The volcanic sequences are distributed widely, including in the Yanji (e.g., at Antu, Laotougou and Liutingdong) and Wangqing ba-sins (e.g., at Luozigou, Diyingou, Canglin, Tianbaoshan, Tuntianying, etc.). It underlies the Jingouling Fm. in a parallel unconformity, but has no direct contact rela-

508 LI ChaoWen et al. Sci China Ser D-Earth Sci | April 2007 | vol. 50 | no. 4 | 505-518

tionship with the Sanxianling Fm. volcanic rocks in field20H

[33]. 20YJ-13 and 20YJ-47 were respectively sam-pled at Shijing in the Yanji basin and at Zhangzhicun in the Luozigou basin. Both are porphyritic andesites with predominant phenocryst of plagioclase (5%―10% in volume and 2―4 mm in length) and hornblende (about 10% in volume and 0.5―1 mm in length). The matrix is mainly composed of aphanitic plagioclase and glass and a few opaque oxides.

Huoshanyan Fm.: The formerly named “Middle- Upper Jurassic” Huoshanyan Fm. referred to a suit of dacite in 1:200000 Yanji Geological Map (αJ2-3)1). These dacitic layers disconformably overlie the Kaishantun Fm. low-grade metasediments and underlie the Changcai Fm. sediments. However, there also lacks direct evidence of the contact relationship between the Huoshanyan and Tuntianying Fms., thus the temporary order between these two volcanic units is determined by their Ar-Ar ages. Samples 05YJ-42 and 05YJ-57 were collected on a small hill adjacent to Jinfosi in the western Yanji basin. Both samples are weakly porphyritic with predominant phenocryst of plagioclase and hornblende and subordi-nate clinopyroxene. The proportion of phenocryst spans a range of 5%―10% with a length of 0.5―2 mm. The matrix is mainly composed of aphanitic plagioclase, hornblende and glass.

Jingouling Fm.: It was ever defined as “Upper Juras-sic”, referring to a suite of mafic to intermediate vol-canic lavas conformably overlain by the Changcai Fm. coal layers. The Jingouling Fm. is mainly distributed in the Diyingou, Shilingou, Jingouling, Bahaoguizi, Gaoli-cang and Jincang areas. The thickness of the volcanic rocks varies greatly with the maximum of 1268 m. Based on the lithological associations, it is divided into two subgroups: the lower segment comprises mainly sediments and intermediate volcanic breccia, whereas the upper is mainly composed of intermediate-mafic lavas21H

[33]. Sample 20YJ-71 is a basaltic andesite sampled from the Diyingou Forestry Farm in the Wangqing basin. It is weakly porphyritic with predominant phenocrysts of clinopyroxene and hornblende and a few plagioclases of 0.5―2 mm (10%―15%). The matrix is mainly com-posed of fine-grained needle-shaped plagioclase (< 0.2 mm), glass and a few opaque oxides.

Changcai Fm.: The formerly defined “Upper Juras-sic-Lower Cretaceous” Changcai Fm. comprises mainly terrestrial sediments including grey sandstone, con-glomerate, volcanic breccia, and thin coal layers22H

[33]. It crops out widely in the Antu, Yanji, Helong and Wangqing areas (e.g. Sichazi, Laotougou, Dongliang, Tuntianying, Changcai, Tushanzi, Songxiaping, Shajin-gou, Baicaogou, etc.). The thickness of Changcai Fm. spans a range of 50―500 m.

Quanshuicun Fm.: It was previously defined as “Lower Cretaceous”, and is composed of a suit of ande-site, which is distributed in the southwestern Helong basin with a maximum thickness of 300 m23H

[33]. The Quanshuicun Fm. overlies the Changcai Fm. in a paral-lel unconformity and underlies conformably the Dalazi Fm. The volcanic rocks in the Quanshuicun Fm. are adakites with MORB-like Sr-Nd-Pb isotopic composi-tions (our unpublished data). Samples 20YJ-133 and 05YJ-73 were respectively collected at Taipingcun and Banxintun in the Helong basin. Both are prophyritic with predominant hornblende (15%―20% in volume) and subordinate clinopyroxene (3%―5% in volume) phenocrysts of 2―8 mm while plagioclase phenocryst is rarely found. The matrix is mainly composed of fine- grained or aphanitic clinopyroxene, hornblende and pla-gioclase (< 0.2 mm) and glass.

Dalazi Fm.: The formerly named “Lower Creta-ceous” Dalazi Fm. referred to a suite of piedmont to pluvial conglomerates and glutenites, overlying the Quanshuicun Fm. andesites in previous literature24H

[33]. It is divided into two segments: the lower part is composed of piedmont-pluvial conglomerate and glutenite, while the upper comprises mainly terrestrial sandshale and oil shale interbeds25H

[33].

3 Analytical techniques

Argon step-heating analysis was performed at Institute of Geology and Geophysics, Chinese Academy of Sci-ences on a GV5400 mass spectrometer operating in a static mode. Samples were firstly crushed to 40―60 meshes, and fresh matrix free of phenocryst and xenocryst was handpicked under a binocular. The purity of amphibole separate in sample 20YJ-133 was over

1) The Fourth Geological Team of Jilin Bureau of Geology and Mineral Resources, The 1:200000 geological map and specification of Yanji, Jilin

Province, 1969

LI ChaoWen et al. Sci China Ser D-Earth Sci | April 2007 | vol. 50 | no. 4 | 505-518 509

99%. The detailed analytical techniques and age correc-tion method were reported in Wang et al. (2004)26H

[34]. The GA-1550 biotite standard yields an age of 98.79 ± 0.96 Ma27H

[35]. Ca and K correction factors were calculated from the CaF2 and K-glass monitors: (36Ar/37Ar)Ca = 2.6088×10−4 ± 1.1418×10−5, (39Ar/ 37Ar)Ca =7.236×10−4 ± 2.814×10−5, (40Ar/39Ar)K = 2.648×10−2 ± 2.254×10−4. Argon laser-heating analysis was done on a GV5400 mass spectrometer attached by a MIR10 CO2 laser at Guangzhou Institute of Geochemistry, Chinese Acad-emy of Sciences. The laser beam is around 3 mm in di-ameter. Detailed description of the analytical technique was reported in Qiu (2006)28H

[36]. The Ca and K correction factors are: (38Ar/36Ar)a = 0.1869, (38Ar/37Ar)Ca = 0, (36Ar/37Ar)Ca = 2.69×10−4, (39Ar/37Ar)Ca = 7.09×10−4, (40Ar/39Ar)K = 0.00165. The results were processed by an ArArCALC ver2.2c software 29H

[37]. The biotite standard ZBH2506 yields an Ar-Ar plateau age of 132.5 ± 0.20 Ma. The Ar-Ar dating results by the step heating and laser heating methods are respectively listed in Tables 1 and 2.

Major elements were analyzed at Hubei Institute of Geology and Mineral Resource, Ministry of Land and Resources (MLR), by wavelength X-ray fluorescence spectrometry (XRF) with analytical errors ≤1%. Trace element analysis was performed using an Inductively Coupled Plasma-Mass Spectrometer (ICP-MS) at Guangzhou Institute of Geochemistry, Chinese Acad-emy of Sciences. Precision for REE and HFSE is esti-mated to be 5% and about 10% for other elements. Ma-jor and trace element data are listed in Table 3.

4 Ar-Ar dating results and constraints on the Mesozoic volcano-sedimentary se-quences

Detailed description of Ar-Ar apparent age spectra and inverse isochrones ages are illustrated in Figure 2.

Sanxianling Fm.: One sample (20YJ-80) was se-lected to determine the eruption age of this group of volcanic rock. It shows flat age spectra. Seven consecu-tive steps (from 880℃ to 1190℃) over 85% 39Ar re-leased define a plateau age of 116.7 ± 0.6 Ma (2σ). During low temperature gas-release step (about 1.8% 39Ar), the spectra give an extremely low apparent age, implying loss of some radiogenic 40Ar* after eruption. The high-temperature gas-release steps give an apparent

age lower than the plateau age, suggesting that the Ca-rich phase has only a minute influence on the age of the matrix. An inverse isochronal age of 117.3 ± 1.0 Ma (n = 6, MSWD = 2.5) is almost identical to the plateau age of 116.7 ± 0.6 Ma within the analytical error. The initial 40Ar/36Ar (289.8 ± 6.6) is well consistent with that of atmosphere, precluding significant excess argon con-tamination in the sample. Thus the plateau age of 116.7 ± 0.6 Ma represents the eruption age of Sanxianling Fm. lavas.

Tuntianying Fm.: Two hornblende andesite samples (20YJ-13, 20YJ-47) were selected to determine the age of this group. Sample 20YJ-13 gives higher apparent ages during low-temperature gas-released steps (step 1-3, about 18% 39Ar), implying some atmospheric Ar trapped in mineral margin possibly as a result of surface altera-tion. In the middle-high-tem- perature gas released steps, it gives a very well apparent plateau age (116.8 ± 0.6 Ma) and the inverse isochronal age (117.5 ± 0.7 Ma, n = 10, MSWD = 1.1). The initial 40Ar/36Ar (292.6 ± 1.7) is similar to that of atmosphere. Sample 20YJ-47 also shows flat age spectra. The plateau age of 118.1 ± 0.6 Ma is well consistent with the inverse isochronal age (118.6 ± 0.8 Ma, n = 11, MSWD = 4.6). The initial 40Ar/36Ar (292.6 ± 5.0) is in good agreement with that of atmosphere. Therefore, the volcanic lavas in Tuntiany- ing Fm. erupted during 117―118 Ma.

Jingouling Fm.: One basaltic sample (20YJ-71) from Jingouling Fm. gives an early Cretaceous age. During low-temperature (700―780℃ ) gas-release step, the spectra give an extremely lofty apparent age (~134 Ma), implying some atmospheric Ar trapped in mineral mar-gin possibly as a consequence. The rest high-tempera- ture gas-release steps yield an apparent plateau age of 106.1 ± 0.6 Ma and an inverse isochronal age of 106.4 ± 0.7 Ma (n = 11, MSWD = 1.5). The initial 40Ar/36Ar of 295.6 ± 1.7 is well consistent with that of atmosphere. The Jingouling Fm. volcanic rocks extruded at ∼106 Ma.

Huoshanyan Fm.: Sample 05YJ-42 from Huoshan-yan Fm. gives gradually descendent apparent plateau spectra during low-temperature gas-release step (2.21―3.42 W), possibly caused by surface alteration that in-troduced atmospheric Ar into the mineral margins. By contrast, the lower apparent ages during high-tempera- ture gas-release steps may be caused by inhomogeneity in the mineral interior. It gives an apparent plateau age of 108.2 ± 0.9 Ma. 05YJ-57 is another dacite sample

510 LI ChaoWen et al. Sci China Ser D-Earth Sci | April 2007 | vol. 50 | no. 4 | 505-518

Table 1 Ar-Ar dating results of Mesozoic-Palaeocene lava in Yanji area by a step-heating method

Temperature (℃) Cumulative 39Ar 40Ar* (%) 36Ar/39Ar 37Ar/39Ar 38Ar/39Ar 40Ar/39Ar 40Ar/36Ar Apparent age (Ma) ±2σ

20YJ-13, mass = 4.0 mg, whole rock, J-value = 0.01003826 ± 0.00005019, integrated date = 118.4 ± 0.6 Ma

700 0.000 2.66 5.1770E-03 2.9180E-01 4.0410E-02 5.6740E+01 1.0960E+04 795.6 22.8

800 0.063 66.23 4.9120E-02 1.9270E+00 2.0750E-02 2.1720E+01 4.4220E+02 128.2 1.0

880 0.188 56.62 3.1290E-02 2.6640E+00 1.7400E-02 1.5990E+01 5.1120E+02 121.5 0.6

940 0.422 22.55 7.7000E-03 4.2160E+00 1.3130E-02 8.6740E+00 1.1270E+03 117.7 0.7

980 0.608 28.77 9.9230E-03 3.1570E+00 1.3890E-02 9.3730E+00 9.4460E+02 117.0 0.7

1020 0.687 43.26 1.7850E-02 3.0730E+00 1.4990E-02 1.1670E+01 6.5390E+02 116.1 0.8

1060 0.733 56.12 2.9920E-02 4.7180E+00 1.7620E-02 1.5130E+01 5.0570E+02 116.6 1.0

1100 0.771 59.61 3.4490E-02 6.3090E+00 1.9110E-02 1.6310E+01 4.7280E+02 115.8 1.3

1150 0.808 57.05 3.1430E-02 6.5900E+00 1.8150E-02 1.5420E+01 4.9050E+02 116.4 1.0

1200 0.844 39.93 1.6650E-02 6.5630E+00 1.6050E-02 1.1080E+01 6.6560E+02 117.0 1.1

1270 0.880 30.72 1.1330E-02 5.2290E+00 1.5070E-02 9.6120E+00 8.4850E+02 116.9 1.1

1350 0.935 30.41 1.1050E-02 5.0430E+00 1.4670E-02 9.4820E+00 8.5840E+02 115.8 0.9

1500 1.000 30.29 1.1200E-02 5.9070E+00 1.4520E-02 9.4550E+00 8.4390E+02 115.8 0.8

20YJ-47, mass = 3.9 mg, whole rock, J-value = 0.00928373 ± 0.00004642, integrated date = 121.5 ± 0.6 Ma

700 0.072 83.183 1.7300E-01 2.6130E-01 4.4680E-02 6.1460E+01 3.5530E+02 165.3 0.9

780 0.136 16.110 4.8100E-03 3.9420E-01 1.2820E-02 8.6610E+00 1.8010E+03 117.4 0.6

860 0.307 27.827 9.7090E-03 3.9990E-01 1.3450E-02 1.0230E+01 1.0530E+03 119.3 0.6

920 0.509 7.160 2.0150E-03 3.8320E-01 1.2150E-02 7.9310E+00 3.9360E+03 118.9 0.6

960 0.645 13.808 4.0600E-03 4.4330E-01 1.2780E-02 8.4670E+00 2.0860E+03 117.9 0.6

1000 0.721 27.239 9.3830E-03 6.0940E-01 1.4090E-02 1.0030E+01 1.0690E+03 118.1 0.7

1040 0.773 35.084 1.3360E-02 7.2590E-01 1.4760E-02 1.1120E+01 8.3230E+02 116.8 0.7

1080 0.816 34.655 1.3310E-02 9.2020E-01 1.4980E-02 1.1170E+01 8.3930E+02 118.1 0.7

1140 0.880 29.093 1.0400E-02 1.3490E+00 1.4510E-02 1.0240E+01 9.8390E+02 117.5 0.6

1200 0.941 25.243 8.7850E-03 2.6440E+00 1.3720E-02 9.5030E+00 1.0820E+03 115.1 0.7

1360 0.981 27.681 1.4410E-02 1.9550E+01 1.4740E-02 9.9610E+00 6.9140E+02 118.1 1.6

1500 1.000 33.820 1.3130E-02 2.2300E+00 1.5450E-02 1.0990E+01 8.3710E+02 117.8 1.3

20YJ-71, mass = 3.8 mg, whole rock, J-value = 0.00912787 ± 0.00004564, integrated date = 107.0 ± 0.6 Ma

700 0.041 85.803 1.3280E-01 3.7210E+00 3.9630E-02 4.5420E+01 3.4210E+02 103.4 3.7

780 0.077 44.030 2.3200E-02 2.8360E+00 1.5470E-02 1.5100E+01 6.5090E+02 134.1 1.1

860 0.169 65.944 4.4900E-02 4.3170E+00 1.9820E-02 1.9640E+01 4.3750E+02 107.1 0.9

920 0.285 32.181 1.1760E-02 3.5690E+00 1.3980E-02 9.9730E+00 8.4780E+02 108.1 0.8

980 0.380 56.959 3.0620E-02 3.4240E+00 1.7270E-02 1.5450E+01 5.0450E+02 106.4 0.7

980 0.518 52.700 2.5880E-02 3.4550E+00 1.6340E-02 1.4030E+01 5.4220E+02 106.2 0.8

1080 0.653 32.095 1.1750E-02 4.3140E+00 1.4310E-02 9.8090E+00 8.3480E+02 106.5 0.6

1120 0.760 28.229 1.0520E-02 6.6190E+00 1.3950E-02 9.2350E+00 8.7750E+02 106.2 0.7

1160 0.847 25.558 9.9270E-03 9.0720E+00 1.3670E-02 8.7680E+00 8.8320E+02 104.8 0.9

1240 0.905 43.714 2.2390E-02 2.2150E+01 1.6060E-02 1.1250E+01 5.0270E+02 102.8 1.7

1320 0.951 58.124 3.5170E-02 1.9260E+01 1.8230E-02 1.5350E+01 4.3650E+02 104.1 1.5

1400 0.989 74.369 6.7550E-02 1.4510E+01 2.4980E-02 2.5360E+01 3.7550E+02 105.0 2.9

1500 1.000 67.388 4.7940E-02 5.7340E+00 2.1010E-02 2.0390E+01 4.2540E+02 106.6 4.4(To be continued on the text page)

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(Continued)

Temperature (℃) Cumulative 39Ar 40Ar* (%) 36Ar/39Ar 37Ar/39Ar 38Ar/39Ar 40Ar/39Ar 40Ar/36Ar Apparent age (Ma) ±2σ

20YJ-80, mass = 4.3 mg, whole rock, J-value = 0.00921515 ± 0.00004608, integrated date = 115.2 ± 0.6 Ma

700 0.002 74.301 5.2570E-02 1.5390E+00 1.7410E-02 2.0770E+01 3.9520E+02 86.6 5.8

800 0.018 24.942 7.9820E-03 9.9640E-01 1.2930E-02 9.1760E+00 1.1490E+03 110.8 2.6

880 0.101 9.770 2.9660E-03 1.1270E+00 1.2450E-02 8.1080E+00 2.7340E+03 117.4 0.8

930 0.252 9.949 3.0320E-03 1.1760E+00 1.2380E-02 8.1200E+00 2.6780E+03 117.4 0.6

980 0.441 14.740 4.5380E-03 1.1270E+00 1.2880E-02 8.5350E+00 1.8810E+03 116.8 0.7

1030 0.625 25.266 8.5160E-03 1.0440E+00 1.3580E-02 9.6680E+00 1.1350E+03 116.1 0.8

1080 0.757 26.755 9.2410E-03 1.3470E+00 1.3980E-02 9.8450E+00 1.0650E+03 115.9 0.6

1130 0.825 32.133 1.2160E-02 1.7890E+00 1.5070E-02 1.0780E+01 8.8650E+02 117.6 0.7

1190 0.896 28.316 9.9500E-03 2.6850E+00 1.5050E-02 9.6790E+00 9.7280E+02 111.7 0.8

1250 0.955 25.760 8.6760E-03 3.5610E+00 1.4680E-02 8.9140E+00 1.0270E+03 106.8 0.7

1300 0.975 40.027 1.5750E-02 5.7720E+00 1.6080E-02 1.0540E+01 6.6940E+02 102.3 2.5

1380 0.987 48.752 2.2310E-02 4.8430E+00 1.5640E-02 1.2790E+01 5.7300E+02 105.9 1.4

1500 1.000 57.473 3.2620E-02 2.3550E+00 1.8220E-02 1.6480E+01 5.0530E+02 112.9 3.4

20YJ-92, mass = 3.4 mg, whole rock, J-value = 0.00906900 ± 0.00004535, integrated date = 115.0 ± 0.6 Ma

700 0.002 98.377 9.9970E-01 1.2050E+00 2.0530E-01 3.0020E+02 3.0030E+02 78.1 8.8

780 0.019 82.561 1.1740E-01 7.4220E-01 3.3740E-02 4.1980E+01 3.5760E+02 115.9 11.8

820 0.033 73.264 7.3580E-02 6.6750E-01 2.5790E-02 2.9640E+01 4.0270E+02 125.1 11.5

980 0.072 22.065 7.5800E-03 5.1210E-01 1.2430E-02 9.9990E+00 1.3190E+03 122.9 1.2

1020 0.129 15.948 5.1330E-03 4.7850E-01 1.2410E-02 9.3060E+00 1.8130E+03 123.3 1.1

1060 0.213 12.100 3.7490E-03 4.7460E-01 1.1990E-02 8.8800E+00 2.3690E+03 123.1 0.8

1100 0.314 11.563 3.4660E-03 4.4910E-01 1.2100E-02 8.5840E+00 2.4770E+03 119.8 0.9

1180 0.436 14.932 4.4920E-03 4.6510E-01 1.2370E-02 8.6760E+00 1.9310E+03 116.6 0.8

1180 0.570 38.183 1.5200E-02 5.5800E-01 1.4390E-02 1.1680E+01 7.6820E+02 114.2 0.8

1230 0.741 20.109 6.2260E-03 8.6470E-01 1.2830E-02 8.8440E+00 1.4200E+03 111.8 0.6

1280 0.884 14.694 4.3310E-03 1.1160E+00 1.2430E-02 8.1510E+00 1.8820E+03 110.1 0.6

1340 0.994 28.313 9.7760E-03 1.9440E+00 1.4150E-02 9.7010E+00 9.9230E+02 110.2 0.6

1400 1.000 83.267 5.0480E-02 4.5220E+00 2.6360E-02 1.7520E+01 3.4710E+02 47.4 1.3

20YJ-133, mass = 4.1 mg, hornblende, J-value = 0.00939036 ± 0.00004695, integrated date = 56.1 ± 0.4 Ma

700 0.011 81.101 9.0340E-02 5.1520E-01 2.8460E-02 3.2890E+01 3.6410E+02 102.3 3.1

800 0.018 75.127 4.4350E-02 1.5890E+00 1.9970E-02 1.7310E+01 3.9030E+02 71.5 1.9

900 0.047 87.807 8.8690E-02 2.8130E+00 2.9150E-02 2.9630E+01 3.3400E+02 60.2 0.7

960 0.067 91.359 1.2220E-01 2.6290E+00 3.5270E-02 3.9330E+01 3.2190E+02 56.7 3.5

1060 0.077 96.283 2.8030E-01 5.7970E+00 7.0940E-02 8.5600E+01 3.0530E+02 53.3 4.6

1100 0.106 85.929 7.0030E-02 8.5940E+00 3.8240E-02 2.3340E+01 3.3330E+02 55.1 5.7

1120 0.412 38.785 9.5130E-03 9.5060E+00 2.6380E-02 5.3850E+00 5.6600E+02 55.1 1.3

1140 0.635 40.927 1.0220E-02 9.5230E+00 2.6660E-02 5.6110E+00 5.4910E+02 55.4 0.7

1160 0.870 16.400 4.7510E-03 9.8000E+00 2.4230E-02 3.9800E+00 8.3770E+02 55.5 0.7

1180 0.971 30.945 7.5550E-03 9.9490E+00 2.6130E-02 4.7620E+00 6.3030E+02 55.0 0.7

1240 0.981 72.515 3.2890E-02 1.3810E+01 5.8980E-02 1.1960E+01 3.6370E+02 55.3 0.8

1300 0.991 81.538 5.3030E-02 1.4450E+01 6.9050E-02 1.7880E+01 3.3710E+02 55.6 1.5

1500 1.000 86.911 8.1250E-02 1.0070E+01 5.4950E-02 2.6760E+01 3.2930E+02 58.7 2.6

512 LI ChaoWen et al. Sci China Ser D-Earth Sci | April 2007 | vol. 50 | no. 4 | 505-518

Table 2 Ar-Ar dating results of Mesozoic-Palaeocene lava in Yanji area by a laser-heating method

Age ± 2σ Output laser power (W)

36Ar(a) 37Ar(ca) 38Ar(cl) 39Ar(k) 40Ar(r) (Ma)

40Ar(r) (%)

39Ar(k)

(%) K/Ca ± 2σ

05YJ-42, whole rock, plateau age = 108.2 ± 0.9 Ma, J-value = 0.01195200 ± 0.00001793, total fusion age = 111.2 ± 0.4 Ma

2.21 0.1294 7.6350 0.0000 18.1622 121.3022 138.6 1.5 76.0 4.12 1.332 0.078 2.64 0.1200 11.2670 0.0000 22.0178 138.5760 130.9 1.2 79.6 4.99 1.094 0.063 3.07 0.1153 14.8572 0.0000 25.4562 149.6118 122.5 1.1 81.4 5.77 0.960 0.055 3.42 0.1076 17.4036 0.0000 27.3684 151.3629 115.5 0.8 82.6 6.20 0.881 0.051 3.85 0.0762 21.5867 0.0000 31.9858 167.7449 109.7 0.8 88.1 7.25 0.830 0.048 4.20 0.0710 25.7338 0.0000 37.7463 196.3572 108.8 0.7 90.3 8.55 0.821 0.048 4.55 0.0703 27.0016 0.0000 38.7348 201.8267 109.0 0.7 90.6 8.78 0.803 0.047 4.89 0.0796 32.9316 0.0000 47.8086 249.0515 109.0 0.5 91.3 10.83 0.813 0.047 5.24 0.0534 20.9985 0.0000 35.0849 182.6180 108.9 0.5 92.0 7.95 0.936 0.054 5.65 0.0525 18.8411 0.0000 33.6215 173.7962 108.2 0.5 91.7 7.62 0.999 0.058 6.05 0.0414 14.9812 0.0004 26.9205 138.4891 107.7 0.8 91.8 6.10 1.006 0.058 6.72 0.0434 17.6641 0.0045 27.4515 139.2659 106.2 0.7 91.5 6.22 0.870 0.050 7.71 0.0239 10.6960 0.0008 15.5416 78.4008 105.6 0.7 91.6 3.52 0.814 0.047 8.99 0.0227 10.3412 0.0036 11.9944 59.5504 104.0 0.7 89.8 2.72 0.650 0.038 11.15 0.0532 23.2566 0.0143 21.0432 102.3735 102.0 0.7 86.6 4.77 0.507 0.029 13.48 0.0666 24.6396 0.0207 20.3700 96.8692 99.7 0.8 83.0 4.62 0.463 0.027

05YJ-57, whole rock, plateau age = 105.9 ± 0.6 Ma, J-value = 0.01195600 ± 0.00001793, total fusion age = 112.5 ± 0.4 Ma

2.21 0.1432 6.7387 0.0000 27.7271 183.1735 137.2 1.2 81.2 6.68 2.304 0.131 2.64 0.1312 9.6708 0.0000 28.8566 182.2668 131.3 1.3 82.4 6.95 1.671 0.095 3.07 0.1406 14.8969 0.0000 33.7578 205.4551 126.7 1.2 83.1 8.13 1.269 0.072 3.42 0.0820 17.7526 0.0000 29.8292 166.9850 116.9 1.1 87.2 7.19 0.941 0.053 3.78 0.0831 21.6943 0.0000 31.2628 166.5556 111.4 1.0 87.1 7.53 0.807 0.046 4.20 0.0648 23.2058 0.0000 30.6164 156.1229 106.8 0.8 89.0 7.38 0.739 0.042 4.62 0.0727 28.0320 0.0000 37.5227 190.1641 106.1 0.7 89.8 9.04 0.750 0.042 4.96 0.0500 22.3662 0.0000 29.7409 150.8452 106.2 0.7 91.0 7.17 0.745 0.042 5.37 0.0480 20.0235 0.0001 26.6844 135.6365 106.4 0.7 90.4 6.43 0.746 0.042 5.92 0.0486 20.3934 0.0012 27.7441 140.6220 106.1 0.6 90.6 6.69 0.762 0.043 6.52 0.0528 22.3847 0.0035 31.2214 157.6753 105.8 0.6 90.9 7.52 0.781 0.044 7.19 0.0386 16.2085 0.0010 22.7877 114.0817 104.9 0.5 90.8 5.49 0.787 0.045 8.03 0.0499 24.7472 0.0059 32.1894 160.3867 104.4 0.5 91.5 7.76 0.728 0.041 8.99 0.0189 8.8036 0.0033 12.3652 61.8514 104.8 0.7 91.6 2.98 0.787 0.045 11.15 0.0197 10.2262 0.0049 12.6672 62.0426 102.7 0.6 91.3 3.05 0.694 0.039

05YJ-73, whole rock, plateau age = 55.6 ± 0.2 Ma, J-value = 0.01197100 ± 0.00001796, total fusion age = 55.7 ± 0.3 Ma

1.11 0.0393 0.2129 0.0008 0.3003 0.9928 70.0 21.4 7.9 0.06 0.790 0.087 1.70 0.2397 0.9985 0.0000 4.3234 12.1033 59.5 7.8 14.6 0.88 2.425 0.144 2.21 0.2722 2.2662 0.0000 12.1213 31.6642 55.6 3.4 28.2 2.48 2.995 0.176 2.78 0.2774 4.1214 0.0000 21.7508 56.1676 54.9 1.8 40.6 4.44 2.955 0.169 3.28 0.2780 6.7833 0.0000 32.2779 83.7044 55.2 1.3 50.4 6.60 2.665 0.151 3.71 0.1822 6.8579 0.0000 30.6228 79.6912 55.3 1.0 59.6 6.26 2.501 0.142 4.20 0.1650 8.1744 0.0000 34.1424 89.1278 55.5 0.8 64.5 6.98 2.339 0.132 4.76 0.1233 14.7969 0.0000 53.9721 141.1702 55.6 0.5 79.3 11.03 2.043 0.115 5.98 0.0871 22.5216 0.0000 65.5679 171.8704 55.7 0.4 86.8 13.40 1.630 0.092 7.05 0.1214 38.6517 0.0196 84.8257 221.5390 55.5 0.4 85.9 17.33 1.229 0.069 7.84 0.1377 28.5618 0.0256 51.3451 134.1938 55.6 0.5 76.6 10.49 1.007 0.057 8.68 0.1424 19.8736 0.0250 28.4823 74.2143 55.4 0.7 63.7 5.82 0.803 0.045 9.93 0.1727 22.8020 0.0312 28.3802 74.1307 55.6 0.7 59.2 5.80 0.697 0.039 11.75 0.1839 24.3411 0.0349 24.5553 64.3464 55.7 0.9 54.1 5.02 0.565 0.032 14.59 0.1572 16.8436 0.0237 16.6725 43.6222 55.6 1.2 48.4 3.41 0.554 0.031

a, Air; ca, calcium; cl, chlorine; k, potassium. Argon laser-heating analysis was performed on a GV 5400 mass spectrometer attached by a MIRIO CO2 laser at Guangzhou Institute of Geochemistry, CAS.

LI ChaoWen et al. Sci China Ser D-Earth Sci | April 2007 | vol. 50 | no. 4 | 505-518 513

Table 3 Major and trace element composition of the late Mesozoic- Palaeocene lava in Yanji area

Sample 20YJ-13 20YJ-47 20YJ-71 05YJ-42 05YJ-57 20YJ-80 20YJ-92 20YJ-133 05YJ-73 Formation Tuntianying Fm. Jingouling Fm. Huoshanyan Fm. Sanxianling Fm. Quanshuicun Fm. Rock type

andesite andesite

basalt dacite dacite andesite basaltic andesite

adakite adakite Age 117 Ma 118 Ma 106 Ma 108 Ma 106 Ma 117 Ma 115 Ma 55.5 Ma 55.6 Ma SiO2 59.21 59.77 51.47 63.43 63.87 58.59 55.22 61.58 62.01 TiO2 0.63 1.03 1.12 0.65 0.63 0.88 1.25 0.73 0.77 Al2O3 18.24 16.08 17.33 17.22 16.78 16.24 17.29 14.83 15.38 Fe2O3 3.48 4.76 3.66 4.48 4.32 3.55 3.90 1.34 3.2 FeO 2.32 1.57 4.55 2.70 3.22 1.77 MnO 0.12 0.14 0.13 0.07 0.07 0.13 0.11 0.04 0.05 MgO 2.23 2.62 5.37 1.4 1.37 4.55 3.69 3.55 3.48 CaO 6.28 5.14 10.10 4.39 4.47 5.97 6.65 6.15 6.08 Na2O 3.83 4.10 3.09 4.08 3.74 3.72 4.00 3.51 3.41 K2O 2.03 2.92 0.60 2.58 2.69 1.58 1.91 2.05 2.12 P2O5 0.36 0.37 0.26 0.20 0.21 0.27 0.66 0.32 0.30 H2O+ 0.97 1.15 1.92 1.24 1.48 3.53 CO2 0.08 0.10 0.21 0.36 0.36 0.31 LOI 1.05 1.25 2.13 1.02 1.37 1.60 1.84 3.84 2.98 Total 99.78 99.75 99.81 99.52 99.51 99.78 99.74 99.71 99.78

Sc 12 13 27 10 9 14 11 7 7 Cr 12 86 99 32 39 173 78 99 100 Co 14 17 32 7 8 20 18 15 13 Ni 11 45 45 13 13 114 49 81 69 Rb 47.7 72.6 5.56 68.4 72.2 28.5 17.9 15.2 20.3 Sr 516 528 543 483 508 584 599 1378 1682 Y 23.4 20.5 19.7 14.0 13.6 18.4 26.5 8.90 8.19 Zr 221 251 132 193 201 184 305 140 131 Nb 8.0 8.7 5.7 5.9 6.2 7.1 19.2 3.2 3.1 Ba 598 609 200 570 585 1129 688 430 554 La 36.95 34.32 18.68 25.14 26.51 22.32 44.08 37.41 38.95 Ce 77.22 68.91 39.79 52.25 51.86 46.57 88.88 84.93 86.39 Pr 8.88 8.96 5.04 6.25 6.07 5.75 10.79 10.70 11.17 Nd 36.46 35.38 21.48 22.95 23.09 23.68 40.94 42.16 42.76 Sm 6.57 6.94 4.39 3.91 3.90 4.55 7.42 6.76 6.09 Eu 1.67 1.41 1.41 1.09 1.05 1.29 1.88 1.71 1.60 Gd 4.98 5.26 3.94 3.39 3.15 3.94 6.22 4.24 3.69 Tb 0.69 0.73 0.56 0.46 0.44 0.57 0.85 0.45 0.42 Dy 4.13 4.04 3.44 2.63 2.34 3.32 4.91 1.88 1.66 Ho 0.78 0.70 0.66 0.50 0.46 0.61 0.94 0.29 0.28 Er 2.43 2.16 2.15 1.35 1.30 1.77 2.58 0.89 0.76 Tm 0.34 0.30 0.28 0.19 0.19 0.25 0.37 0.09 0.09 Yb 2.27 1.79 1.85 1.30 1.27 1.56 2.41 0.60 0.56 Lu 0.39 0.28 0.29 0.21 0.21 0.26 0.40 0.08 0.09 Hf 5.26 6.40 3.00 4.22 4.62 4.28 6.36 3.54 3.46 Ta 0.56 0.64 0.36 0.40 0.43 0.47 1.01 0.20 0.19 Th 7.59 6.86 3.33 4.75 5.04 2.93 4.29 5.29 6.04 U 1.48 1.58 0.60 1.25 1.22 0.76 1.00 0.64 0.66 Pb 13.14 19.84 5.29 12.86 15.65 11.33 13.96 4.13 4.46

514 LI ChaoWen et al. Sci China Ser D-Earth Sci | April 2007 | vol. 50 | no. 4 | 505-518

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LI ChaoWen et al. Sci China Ser D-Earth Sci | April 2007 | vol. 50 | no. 4 | 505-518 515

from the Huoshanyan Fm, yielding a Cretaceous appar-ent age of 105.9 ± 0.6 Ma. Its apparent plateau spectra are similar to that of 05YJ-42. Because 39Ar/40Ar and 36Ar/40Ar ratios in both samples vary slightly, it is diffi-cult to give a reasonable inverse isochronal age and ini-tial 40Ar/36Ar. The Huoshanyan Fm. volcanic lavas erupted during 106―108 Ma.

Quanshuicun Fm.: The Ar-Ar dating results from two representative samples indicate that the Quanshui- cun Fm. adakitic lavas erupted in Palaeocene. The horn- blende separate in sample 20YJ-133 shows flat age spectra. During low-temperature gas-release steps (700―800℃), it gives a lofty apparent age (>70 Ma), implying some atmospheric Ar trapped in mineral mar- gin. The rest gas-release steps yield an apparent plateau age of 55.5 ± 0.3 Ma and an inverse isochronal age of 55.2 ± 0.7 Ma (n = 11, MSWD = 1.6), and an initial 40Ar/36Ar of 298.0 ± 0.7. Another whole rock sample (05YJ-73) gives an apparent plateau age of 55.6 ± 0.2 Ma and an inverse isochronal age of 55.7 ± 0.3 Ma (n = 13, MSWD = 0.16). Considering the analytical errors, these two Ar-Ar ages are almost identical. The eruption age of Quanshuicun Fm. adakites is about 55 Ma.

Our Ar-Ar dating results show that the Mesozoic vol-

canic rocks in the Yanji area mainly erupted in early Cretaceous (106―118 Ma), approximate to the late Mesozoic extensive magmatism in NE China and in the neighbouring regions. The adakitic rocks from the Quanshuicun Fm. extruded in Palaeocene (~55 Ma). The previously dated “Late Triassic” and “Jurassic” ages are not observed in our study. On the basis of these new Ar-Ar ages, the Mesozoic to early Cenozoic volcano- sedimentary sequences in the Yanji area is rebuilt as: Sanxianling Fm./Tuntianying Fm. (118 ― 115 Ma), Malugou Fm./Tianqiaoling Fm. (K1), Jingouling Fm./ Huoshanyan Fm. (108―106 Ma), Changcai Fm. (K2), Quanshuicun Fm. (∼55 Ma), and Dalazi Fm. (Figure 3).

5 Tectonic implications Since late Palaeozoic, the NE China Fold belt has experienced multi-staged tectonic-magmatism events30H

[8,14,20,31,32,38―42]. The Yanji area is located at the East Asian continental margin, and was widely distrib- uted in late Mesozoic volcanic rocks. The eruption age of these Mesozoic volcanic lavas (118―106 Ma) was slightly younger than the main stage (130―120 Ma) of magmatic activities in the eastern CAOB and in its ad-

Figure 3 The late Mesozoic-Palaeocene volcano-sedimentary sequences in the Yanji area (after ref. 31H[33]).

516 LI ChaoWen et al. Sci China Ser D-Earth Sci | April 2007 | vol. 50 | no. 4 | 505-518

jacent regions 32H

[14,21,24,40]. As suggested by the previous work which indicates a post-orogenic extensional re- gime during the late Mesozoic, analogue to that of the Basin and Range Province (BRP) in west USA 33H

[17,21]. Structural analysis on the early Cretaceous Basin evolu- tion of the Songliao Basin and the uplifting of the late Mesozoic metamorphic core complexes in southern Liaoning Province suggest an extensional regime in the Songliao basin and the northern NCB34H

[40,42]. The tectonic setting for the adjacent Yanji area is similar to the south- ern NCB in early Cretaceous, as suggested by the post- orogenic granitoid emplacement35H

[14]. If so, our new Ar-Ar ages of the volcanic lavas imply that the lithospheric extension during the post-orogenic stage in northeast China should go on after 106 Ma. Undoubtedly, the East Asian continental margin has interacted with the circum- Pacific tectonic domain during late Mesozoic. However, paleomagnetic evidence suggests that in the early Creta- ceous, the Palaeo-Pacific Ocean (Izanagi-Farallon plate) moved northward quickly rather than westward subduc- tion beneath the East Asian continental margin36H

[43,44]. The main effect of the Palaeo-Pacific Ocean on the tectonic evolution of the Yanji and its adjacent regions was to exert a pull-apart force, and trigger strike-slipping of lithosphere-scale faults and orogenic collapse. The ex- tension ultimately led to astheno-sphere upwelling and decompressional melting of the enriched mantle litho- sphere 37H

[21,45―47]. Comparative studies on regional geology between the

Yanji area and Hida massif in SW Japan implied that both regions experienced similar tectonic evolution his- tory and should belong to the same tectonic unit before the opening of the Japan Sea back-arc basin, which was believed to separate from the East Asian continent dur- ing 22―15 Ma38H

[30,48,49]. During the Late Cretaceous (95―65 Ma), the Izanagi-Farallon ridge subduction be- neath the SW Japan was evident, and caused the arc magmatism in SW Japan, and formed a series of accre- tionary prisms along the Japan arc system, e.g., the early Tertiary Hidaka accretionary prism in NW Japan and the late Cretaceous Shimanto accretionary prism in SW Ja- pan39H

[50,51]. The eruption age of Palaeocene (~55 Ma) Quanshuicun Fm. adakite was slightly younger than the

Late Cretaceous Izanagi-Farallon ocean ridge subduc- tion event. The petrogenesis of these adakites is consid- ered as post-subduction products (our unpublished data), analogues to the bajaites and other related calc-alkaline rocks in Baja California40H

[52]. Although the Palaeocene adakitic magma generation is different from those oc-curring in modern subduction zones, our new Ar-Ar ages of the late Mesozoic- Palaeocene volcanic rocks imply that the Izanagi- Farallon ridge subduction be-neath the NE China occurred during 106―55 Ma, roughly consistent with the palaeomagnetic observations on the relative motion of the Palaeo-Pacific ocean and its neighboring Eurasia continent, as well as the arc magmatism in SW Japan 41H

[43,44].

6 Conclusions

Based on our new Ar-Ar data, the Mesozoic-Early Ce- nozoic volcano-sedimentary sequence in the Yanji area is rebuilt as: Sanxianling/Tuntianying Fm. (118―115 Ma), Malugou/Tianqiaoling Fm. (K1), Jingouling Fm. (108―106 Ma), Changcai Fm. (K2), Quanshuicun Fm. (∼55 Ma), and Dalazi Fm. The eruption ages of the late Mesozoic Yanji volcanic lavas were slightly later than the main stage of magmatic events in the eastern CAOB and its adjacent regions. Considering the regional tec- tonic evolution, such slight temporary difference possi- bly implying that the post-orogenic lithospheric exten- sion in the Yanji area was prolonged to 106 Ma. The Quanshuicun Fm. adakites erupted at ca. 55 Ma, which was genetically related to the subduction of Palaeo-Pa- cific Ocean (e.g., the Izanagi-Farallon plate). These new ages indicate that the Izanagi-Farallon ridge subduction beneath the NE China occurred during 106―55 Ma, in agreement with the previous paleomagnetic observations on the relative motion of the Palaeo-Pacific Ocean and its neighboring Eurasia continent and contemporaneous magmatic records in SW Japan.

We would like to appreciate Dr. Li Xiaoyong for his assistance in field work. Drs. Qiu Huaning and Wang Fei are thanked for analytical help in Ar-Ar dating. Thorough and helpful reviews by Drs. Zheng Jianping and Dr. Yang Jinhui and an anonymous reviewer have significantly improved the manuscript.

1 Sëngor 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 42H[DOI]

2 Zhao X X, Coe R S, Zhou Y S, et al. New palaeomagnetic results from northern China: collision and suturing with Siberia and Kazakhstan. Tectonophysics, 1990, 181: 43―81

LI ChaoWen et al. Sci China Ser D-Earth Sci | April 2007 | vol. 50 | no. 4 | 505-518 517

3 Zhao Y, Yang Z Y, Ma X H. The important age of tectonic transformation from paleo-Asian to the Pacific tectonic domains in the East Asia. Sci Geo Sin (in Chinese), 1994, 29: 105―119

4 van der Voo R, Spakman W and Bijwaard H. Mesozoic subducted slabs under Siberia. Nature, 1999, 397: 246―249 43H[DOI]

5 Davis G A, Zheng Y D, Wang C, et al. Mesozoic tectonic evolution of the Yanshan fold and thrust belt, with emphasis on Hebei and Liaoning Provinces, northern China. In: Hendrix M S, Davis G A, eds. Paleozoic and Mesozoic Tectonic Evolution of Central Asia: From Continental Assembly to Intracontinental Deformation. Geol Soc Am Memoir, 2001, 194: 171―197

6 Li J Y. Permian geodynamic setting of Northeast China and adjacent regions: closure of the Paleo-Asian Ocean and subduction of the Paleo-Pacific Plate. J Asian Earth Sci, 2006, 26(3-4): 207―22444H[DOI]

7 Wu F Y, Jahn, B M, Wilde S A, et al. Phanerozoic crustal growth: U-Pb and Sm-Nd isotopic evidence from the granite in northeastern China. Tectonophysics, 2000, 328: 89―113 45H[DOI]

8 Wu F Y, Sun D Y, Li H M, et al. A-type granites in northeastern China: Age and geochemical constraints on their petrogenesis. Chem Geol, 2002, 187: 143―173 46H[DOI]

9 Wu F Y, Jahn B M, Wilde S A, et al. Highly fractionated I-typed granites in ME China (I): Geochronology and petrogenesis. Lithos, 2003, 66: 241―273 47H[DOI]

10 Jahn B M, Wu F Y, Capdevila R, et al. Highly evolved juvenile granites with tetrad REE patterns: the Woduhe and Baerzhe granites from the Great Xing’an Mountain in NE China. Lithos, 2001, 59: 171―198 48H[DOI]

11 Hong D W, Zhang J S, Wang T, et al. Continental crustal growth and the supercontinental cycle: evidence from the Central Asian Orogenic Belt. J Asian Earth Sci, 2004, 23: 799―813 49H[DOI]

12 Liu W, Siebel W, Li X J, et al. Petrogenesis of the Linxi granitoids, northern Inner Mongolia of China: constraints on basaltic underplating. Chem Geol, 2005, 219: 5―35 50H[DOI]

13 Sun D Y, Wu F Y, Li H M, et al. Emplacement age of the post-orogenic A-type granites in Northwestern Lesser Xing'an Ranges, and its relationship to the eastward extension of Suolushan-Hegenshan-Zhalaite collisional suture zone. Chin Sci Bull, 2001, 46(3): 427―432

14 Zhang Y B, Wu F Y, Wilde S A, et al. Zircon U-Pb ages and tectonic implications of ‘Early Paleozoic’ granitoids at Yanbian, Jilin Province, northeast China. Isl Arc, 2004, 13: 484―505 51H[DOI]

15 Li S T, Yang S G, Wu C L, et al. The late Mesozoic rifting in the northeastern China and the fault-rifting basins in East Asia. Sci China Ser B Chem, 1987, 17(2): 185―195

16 Shao J A, Zhang L Q, Mou B L. Tectono-thermal evolution of mid-dle-south section of the Da Hinggan Mountains. Sci China Ser D-Earth Sci, 1998, 41(4): 570―579

17 Guo F, Fan W M, Wang Y J, et al. Petrogenesis of the late Mesozoic bimodal volcanic rocks in the southern Da Hinggan Mts, China. Acta Petrol Sin (in Chinese), 2001, 17: 161―168

18 Ge W C, Lin Q, Sun D Y, et al. Geochemical characteristics of Mesozoic basaltic volcanic rocks in the Da Hinggan Mts: evidence for mantle-crust interaction. Acta Petrol Sin (in Chinese), 1999, 15: 396―407

19 Lin Q, Ge W C, Sun D Y, et al. Tectonic implications of Mesozoic

volcanic rocks in Northeastern China. Sci Geo (in Chinese), 1998, 33: 129―139

20 Yin C J, Peng Y Q, Qin K. Mesozoic volcanism in the eastern part of Northeast China and Transpacific plate. Region Geo China (in Chinese), 2000, 19: 303―311

21 Fan W M, Guo F, Wang Y J, et al. Calc-alkaline volcanism of post-orogenic extension in the northern Da Hinggan Mountains, northeastern China. J Volcano Geotherm Res, 2003, 121: 115―135 52H[DOI]

22 Zonenshain L P, Kuzmin M I, Natapov L M. Geology of the USSR: a plate-tectonic synthesis. In: Page B M, ed. AGU Geodynamics Series 21, 1990, 242

23 Ma X H, Yang Z Y. The collision and suturing of the three major blocks in China and the reconstruction of the Paleo-Eurasia continent. Acta Geophys Sin (in Chinese), 1993, 36: 476―488

24 Wu F Y, Lin J Q, Wilde S A, et al. Nature and significance of the Early Cretaceous giant igneous event in eastern China. Earth Planet Sci Lett, 2005, 233: 103―119 53H[DOI]

25 Zhou X M, Li W X. Origin of Late Mesozoic igneous rocks in Southeastern China: implications for lithosphere subduction and underplating of mafic magmas. Tectonophysics, 2000, 326: 269―287 54H[DOI]

26 Lapierre H, Jahn B M, Charvet J, et al. Mesozoic felsic arc magmatism and continental olivine tholeiites in Zhejiang Province and their relationship with the tectonic activity in southeastern China. Tectonophysics, 1997, 274: 321―338 55H[DOI]

27 Xu X S, Dong C W, Li W X, et al. Late Mesozoic intrusive complexes in the coastal area of Fujian, SE China: the significance of the gabbro-diorite-granite association. Lithos, 1999, 46: 299―315 56H[DOI]

28 Fan W M, Wang Y J, Guo F, et al. Mesozoic mafic magmatism in Hunan-Jiangxi provinces and the lithospheric extension. Earth Sci Front (in Chinese), 2003, 10(3): 159―169

29 Li X H. Cretaceous magmatism and lithospheric extension in Southeast China. J Asian Earth Sci, 2000, 18: 293―305 57H[DOI]

30 Nakamura E, McCulloch M T, Campbell I H. Chemical geodynamics in the back-arc region of Japan based on the trace element and Sr-Nd isotopic compositions. Tectonophysics, 1990, 174: 207―233 58H[DOI]

31 Meng Q L, Zhou Y X. The formation and evolution of magma for J2-K1 Volcanic-Intrusive complex in eastern Yanbian, Jilin province. Acta Petrol Mineral (in Chinese), 1996, 15: 30―39

32 Jia D C, Hu R Z, Lu Y, et al. Collision belt between the Khanka block and the North China block in the Yanbian Region, Northeast China. J Asian Earth Sci, 2004, 23: 211―219 59H[DOI]

33 Jilin Bureau of Geology and Mineral Resources (JBGMR). Regional Geology of Jilin Province (in Chinese). Beijing: Geological Publishing House, 1989, 1―698

34 Wang F, Luo Q H, Zhu R X, et al. A differential uplifting of Qinling orogeny belt evidences from 40Ar/39Ar thermochronology of granite. Earth Sci Front (in Chinese), 2004, 11(4): 445―459

35 Renne P R, Swisher C C, Deino A L, et al. Intercalibration of stan-dards, absolute ages and uncertainties in 40Ar/39Ar dating. Chem Geol, 1998, 145: 117―152 60H[DOI]

36 Qiu H N. Construction and development of new Ar-Ar laboratories in China：Insight from GV-5400 Ar-Ar laboratory in Guangzhou Insi-tute of Geochemistry, Chinese Academy of Sciences. Geochemica (in

518 LI ChaoWen et al. Sci China Ser D-Earth Sci | April 2007 | vol. 50 | no. 4 | 505-518

Chinese), 2006, 35(2): 133―140 37 Koppers A A P. ArArCALC - software for 40Ar/39Ar age calculations.

Comp & Geosci, 2002, 28(5): 605―619 61H[DOI] 38 Miao L C, Fan W M, Zhang F Q, et al. Zircon SHRIMP geochronol-

ogy of the Xinkailing-Kele complex in the northwestern Lesser Xing’an Range, and its geological implications. Chin Sci Bull, 2004, 49(2): 201―209

39 Tang K D, Shao J A, Li J C, et al. Nature of the Yanbian suture zone and structure of Northeast Asia. Geo Bull China (in Chinese), 2004, 23(9-10): 885―891

40 Wang P J, Liu W Z, Wang S X, et al. 40Ar/39Ar and K/Ar dating on the volcanic rocks in the Songliao basin, NE China: constraints on stratigraphy and basin dynamics. Int J Earth Sci, 2002, 91: 331―340 62H[DOI]

41 Liu J Q, Han J T, Fyfe William S. Cenozoic episodic volcanism and continental rifting in northeast China and possible link to Japan Sea development as revealed from K-Ar geochronology. Tectonophysics, 2001, 339: 385―401 63H[DOI]

42 Liu J L, Gregory A D, Lin Z Y, et al. The Liaonan metamorphic core complex, Southeastern Liaoning Province, North China: A likely contributor to Cretaceous rotation of Eastern Liaoning, Korea and contiguous areas. Tectonophysics, 2005, 407: 65―80 64H[DOI]

43 Northrup C J, Royden L H, Burchfiel B C. Motion of the Pacific plate relative to Euroasia and its potential relation to Cenozoic extrusion along the eastern margin of Eurasia. Geology, 1995, 23: 719―722 65H[DOI]

44 Engebretson D C, Cox A, Gordon R G. Relative motions between oceanic and continental plates in the Pacific basins. Geo Soc Am,

Special Paper, 1985, 206: 1―59 45 Fan W M, Guo F, Wang Y J, et al. Post-orogenic bimodal volcanism

along the Sulu Orogenic Belt in eastern China. Phys Chem Earth (A), 2001, 26: 733―746 66H[DOI]

46 Xu J, Ma G., Zhu G., et al. Formation and evolution of the Tancheng-Lujiang wrench fault system: a major shear system to the northwest of the Pacific Ocean. Tectonophysics, 1987, 134: 273―310 67H[DOI]

47 Guo F, Fan W M, Wang Y J, et al. Geochemistry of late Mesozoic mafic rocks in west Shandong Province: characterizing the lost lithospheric mantle beneath North China Block. Geochem J, 2003, 37: 63―77

48 Chang W O. A new concept on tectonic correlation between Korea, China and Japan: Histories from the late Proterozoic to Cretaceous. Gondwana Res, 2006, 9: 47―61 68H[DOI]

49 Shao J A, Tang K D. Wang C Y. The tectonic characters of Nadanhada Massif and its evolution. Sci China Ser B, 1991, 21(7): 744―751

50 Kinoshita O. Migration of igneous activities related to ridge subduc-tion in Southwest Japan and the East Asian continental margin from the Mesozoic to the Paleogene. Tectonophysics, 1995, 245: 25―35 69H[DOI]

51 Taira A. Tectonic evolution of the Japanese Island Arc system. Annu Rev Earth Planet Sci, 2001, 29: 109―134 70H[DOI]

52 Saunders A D, Rogers G, Marriner G F, et al. Geochemistry of Ce-nozoic volcanic rocks, Baja California, Mexico: implications for the petrogenesis of post-subduction magmas. J Volcano Geotherm Res, 1987, 32: 223―245 71H[DOI]