40ar-39ar thermochronology of two ductile shear zones from yiwulüshan, west liaoning region: age...

40Ar-

39Ar thermochronology

of two ductile shear zones

from Yiwulüshan, West

Liaoning Region: Age

constraints on the Mesozoic

tectonic events

ZHANG Xiaohui1, LI Tiesheng

1 & PU Zhiping

2

1. Research Center of Mineral Resources Exploration, Institute of Geo-

logy and Geophysics, Chinese Academy of Sciences, Beijing 100101,

China;

2. Guangzhou Institute of Geochemistry, Chinese Academy of Sciences,

Guangzhou 510640, China

Correspondence should be addressed to Zhang Xiaohui (e-mail:

[email protected])

Abstract Two ductile shear zones trending EW and NNE

respectively not only controlled the tectonic framework of

the northern North China, but also constrained the geody-

namic background for gold mineralization in this region.

Field observations and microstructural analyses reveal that

the EW trending ductile shear zones are mainly contributed

to dextral compressional deformation resulting from top-to-

the-southeast oblique thrust shearing, whereas the NNE

trending ones are genetically related to sinistral strike-slip

and extensional faulting. One sample from the former

yielded an 40

Ar-39

Ar plateau age of (219 4) Ma (Bi) and two

samples from the latter gave 40

Ar-39

Ar plateau ages of (116

2) Ma (Bi) and (127 3) Ma (Bi). These ages provide con-

straints on the top-to-the-southeast oblique thrusting event

occurring in Late Triassic and the sinistral extensional and

strike-slip faulting event which occurred in Early Creta-

ceous.

Keywords: 40

Ar-39

Ar age, biotite, ductile shear zone, West Liaoning

Region.

The tectonic framework along the northern and east-ern margins of West Liaoning Region, North China, is characterized by two kinds of ductile shear zones trending EW and NNE respectively

[1], which also constrained the

geodynamic background for gold mineralization at this region. For example, the well-known Paishanlou gold deposit is just situated at the crosscutting section of these two kinds of ductile shear zones

[2]. Field observations and

microstructural analyses[2 6]

reveal that the EW trending ductile shear zones are mainly contributed to dextral compressional deformation having resulted from top-to- the-southeast oblique thrust shearing, whereas the NNE trending ones are genetically related to sinistral strike-slip and extensional faulting. On the other hand, both zones are commonly superposed by multi-phase later deforma-tion of generally brittle regime

[1]. Owing to such over-

printing effects, precise determination for the time of duc-

tile deformation in these ductile shear zones has been

lacking, which further hampered the understanding for

their kinematic effects and the Mesozoic tectonic event

sequences in the eastern areas of North China. Therefore,

we present 40

Ar-39

Ar ages on the representative deformed

and mylonitized samples collected from localities with

well-developed characteristic macro and micro indicators

of shear sense, to provide constraints on the time of duc-

tile deformation in these two ductile shear zones for the

first time and further on the Mesozoic tectonic evolution

of West Liaoning Region.

1 Geological setting and sampling

The study area is situated along the areas of Jinzhou

and Fuxin cities, in the western Liaoning Pronvice. Its

tectonic framework consists of massives, which are com-

posed mainly of Archean crystalline basement and mid-

dle-late Proterozoic sedimentary covers, as well as the

Mesozoic rift basins. Recent studies reveal that the com-

plex tectonic framework in West Liaoning Region is

characterized by three different structural styles: (1) the

late Archean TTG gneiss-dominated oval structure; (2) the

Hercynian-Indosinian EW trending middle-upper crustal

linear structure superposed on (1); and (3) the late Meso-

zoic upper crustal range and basin structure superposed on

(1) and (2)[2]

. Regional geological surveys show[2 5]

that

major EW trending ductile shear zones in the study area

are mainly situated along the areas of Haertao, Guanshan

and Paishanlou-Baichangmen, which not only developed

along the Archean TTG gneisses, metamorphosed su-

pracrustal rock enclaves, but also crosscut Hercynian-

Indosinian intrusions. The rocks of different protolith re-

sulted in varied mylonites with different deformational

characteristics. Structural foliations of these mylonites

include those defined by mylonitic minerals and those

defined by shear surfaces. The shear foliations mostly

strike nearly EW(70° 85°) and dip north about 30°

50°. Stretching lineations defined by biotite, actinolite,

quartz and feldspar consistently plunge northwest (320°)

with a dip of 20° 30°. ‘A’ type folds, S-C fabrics,

asymmetrically rotated porphyroclasts and boudins uni-

versally indicate a top-to-the-southeast oblique thrusting

sense of shear. Well-developed NNE trending ductile

shear zone extends along the areas of Haertao-Guanshan-

Paishanlou-Jinzhou. They mainly developed along the

Archean deepseated intrusive complexes, metamorphosed

supracrustal rock enclaves and the middle-late Proterozoic

Changcheng System, and in part transected the Jurassic

terrestrial conglomerate-sandstones and pyroclastics and

the Yanshannian granitoids. They typically strike 20°N-

40°E and dip 30° 50° to the northwest. Stretching linea-

tions defined by tiny sericite or strongly elongated quartz

commonly plunges 220° 250° with a dip of 10° 20°.

S-C fabrics and asymmetrical porphyroclasts at different

localities indicate a consistent sinistral strike-slip and ex-

Chinese Science Bulletin Vol. 47 No. 13 July 2002 1113

NOTES

tensional sense of shear. Field observations also reveal

that NNE trending ductile shear zone intersects

EW-trending ductile shear zone. The relationship between

them is that foliations of the latter curved to accommodate

those of the former. The best method of

40Ar-

39Ar dating of mylonites is

using single K-bearing minerals such as hornblende, biotite and muscovite

[6], which has been proved by many

successful examples from China and abroad[7 9]

. With regard to West Liaoning Region, the intersection of structural patterns formed during different geological times and under different tectonic regimes made it difficult for ap- propriate samples to have been acquired and separated, so convincing isotopic geochronological data have been lacking. As a result, the precise determination of the main Mesozoic tectonic event sequences has been constricted. In view of this, on the basis of the detailed field geological investigations, we chose the Paishanlou-Baichangmen section of the EW trending ductile shear zone and the Chefang-Paishanlou section of the NNE trending ductile shear zone (fig. 1), where typical macro and micro indicators of shear sense are well developed, and sampled representative deformed and mylonitized rocks for 40

Ar-39

Ar dating (table 1). Among them, sample PS2000-40 was collected from the middle section of Paishanlou-Baichangmen EW trending ductile shear zone near Xiaowutaigou, and samples NN2000-29 and PS2000-35 were respectively collected from NNE trending ductile shear zone near Chefang of Yixian

Fig. 1. Sketch geological map of Yiwulushan, West Liaoning Region

(modified after ref. [2]). 1, Archean crystalline basement rocks; 2, Pro-

terozoic Changcheng System; 3, Hercynian-Indosinian granitoid; 4,

Yanshannian granitoid; 5, granodiorite and diorite; 6, Mesozoic and

Cenozoic volcanic and terrestrial sedimentary covers; 7, ductile shear

zones; 8, brittle faults; 9, sampling localities.

and near the Paishanlou gold deposit. Details of the sam-

ples are shown in table 1.

2 Analytical method and results

Pure mineral separates were obtained through stan-

dard crushing techniques, magnetic and dense separation.

The biotite separates were handpicked under a binocular

microscope to an estimated purity of 99%. The sepa-

rates were washed in water, acetone, and ethanol prior to

packaging in aluminum foil for irradiation. The samples

were irradiated in the Swimming Pool reactor at the Chi-

nese Academy of Atomic Energy Sciences, along with

Chinese Standard Sample ZBH-2506 (Bi). Biotite samples

weighed an average value of 200 mg. The irradiation pa-

rameter J was 0.0123, irradiation time was 55 h and inte-

gral neutron flux was 9 1012

n/cm2

s. Gas was ex-

tracted from the samples using a double vacuum resis-

tance furnace and analyzed on an MM1200 mass spec-

trometer at the Guangzhou Institute of Geochemistry, the

Chinese Academy of Sciences. Additional analytical de-

tails relevant to the present study are summarized in refs.

[10] and [11]. The detailed analytical data are listed in

table 2. Age spectra and isochrons for each of the samples

are plotted in fig. 2.

3 Discussion

The age spectra determined by the 40

Ar-39

Ar step- heating method for three mineral separates yield relatively good plateau ages for a substantial portion of the gas re-leased. The plateau age of biotite from sample PS2000-40 is (219±4) Ma corresponding to 68.62%

39Ark released

during steps 5 10 (fig. 2(a)), and the isochron age cor-responding to that of the plateau is (219±4) Ma with a good correlation of 0.9983 (fig. 2(b)). The plateau age of biotite from sample NN2000-29-2 is (116±2) Ma with 76.82%

39Ark released during steps 2 8 (fig. 2(c)), and an

isochron plot for this sample also suggests an age of (116±2) Ma with a good correlation of 0.9991 (fig. 2(d)). The plateau age for sample PS2000-35 is (127±3) Ma corresponding to 94% gas release during steps 2 to 9 (fig. 2(e)), and the correspondent isochron age is (129±3) Ma with a good correlation of 0.9990 (fig. 2(f)). The above results show that the isochron ages corresponding to the plateau steps for three mineral separates are substantially concordant with their respective plateau ages. This sug-gests that the plateau ages of three mineral separates are reliable. The initial

40Ar/

36Ar ratios of PS2000-40,

NN2000-29-2, and PS2000-35 are 289.2, 273.9 and 281.5 respectively, relatively close to but a little lower than that of the present-day atmosphere (295.5±5), indicating that there exists a slight

40Ar loss. Such losses may have

something to do with the later brittle faulting reactivation overprinted on the prior ductile shear zone. For example, the section, where the sample NN2000-29 with an initial 40

Ar/36

Ar ratio of 273.9 was collected in the NNE-trending

1114 Chinese Science Bulletin Vol. 47 No. 13 July 2002

Table 1 Description of samples for 40

Ar-39

Ar datinga)

Sample No. Sampling locality Rock type Mineral assemblage Protolith

PS2000-40 41 46 89.8

121 42 87.0protomylonite Pl+Bi+Ms+Ep+Qz+Hb biotite-plagioclase gneiss

NN2000-29 41 35 71.6

121 26 40.5mylonite Bi+Pl+Qz+Ep+Hb hornblende-biotite-plagioclase gneiss

PS2000-35 41 51 63.1

121 46 00.6mylonite Pl+Kp+Qz+Bi+Ms arkose-quartzite

a) Mineral abbreviations are as follows: Pl, plagioclase; Bi, biotite; Kp, keldspar; Ms, muscovite; Qz, quartz; Ep, epidote; Hb, hornblende.

Table 2 40

Ar/39

Ar analytical data

Step T/39

Ar%40

Ar*%

39Ar/

36Ar

40Ar/

36Ar

37Ar/

39Ar

Apparent age

t 1 /MaNote

1 520 0.38 5.78 0.70 313.62 82.61 498.93±91.47

2 600 0.60 2.08 294.71 64.21

3 680 0.61 0.68 1.80 297.52 40.46 24.56±20.51

4 750 29.79 14.82 6.32 346.91 0.74 170.98±3.24

5 810 27.20 51.23 29.11 606.14 1.12 221.19±0.45

6 870 14.09 63.66 48.88 813.77 1.32 219.89±0.81

7 930 5.84 65.46 56.08 856.28 3.57 208.06±1.86

8 980 10.56 58.07 39.69 705.17 1.81 214.34±1.05

9 1070 7.97 60.98 43.55 757.84 2.37 220.11±1.11

10 1340 2.96 49.60 27.47 586.52 5.01 219.69±3.14

Biotite (PS2000-40)

tf = (210±4) Ma

tp = (219±4) Ma

(steps 5 10)

Isochron age:

tBi =(219±4) Ma

R = 0.9983

1 440 22.59 14.97 12.71 347.56 0.20 88.42±0.57

2 520 27.76 69.44 123.52 968.90 0.18 116.75±0.21

3 600 14.96 85.63 317.71 2069.57 0.17 119.49±0.39

4 680 6.12 79.71 223.71 1462.95 0.60 111.91±0.72

5 750 7.32 81.83 254.94 1634.81 1.18 112.64±0.84

6 810 9.73 84.59 307.27 1929.39 0.78 113.97±0.45

7 870 8.67 86.56 350.57 2215.37 0.62 117.27±0.57

8 930 2.26 79.89 220.23 1475.85 3.26 114.85±2.30

9 1340 0.59 32.16 44.83 435.73 19.90 67.92±24.57

Biotite

(NN2000-29)

tf = (110±2) Ma

tp = (116±2) Ma

(steps 2 8)

Isochron age:

tBi = (116±2) Ma

R = 0.9991

1 360 0.49 13.42 7.97 341.30 28.61 118.12±182.15

2 520 6.86 33.18 23.59 442.31 5.55 127.53±9.89

3 600 15.91 53.87 58.16 641.10 1.47 121.96±4.00

4 680 21.79 82.16 215.24 1663.27 2.84 130.12±2.49

5 750 15.62 86.82 320.05 2257.70 1.12 125.69±3.90

6 810 8.48 91.52 505.24 3523.74 0.11 130.81±5.66

7 870 3.80 81.35 219.39 1591.40 0.28 121.25±11.88

8 980 6.45 78.89 181.78 1404.90 5.77 125.14±9.81

9 1070 15.10 74.70 148.03 1171.17 2.14 121.42±3.31

10 1340 5.51 67.76 90.87 917.86 1.70 139.85±11.09

Biotite

(PS2000-35)

tf = (127±3) Ma

tp = (127±3) Ma

(steps 2 9)

Isochron age:

tBi = (129±3) Ma

R = 0.9990


NOTES

Fig. 2. 40

Ar-39

Ar age spectra and isochron plots for biotite samples.

ductile shear zone, was affected by the later overprinted

brittle activity of the eastern marginal normal fault of

Fuxin basin (Sunjiawan-Shaohuyingzi normal fault). And

the slight loss also indicates that the later superposed de-

formation dominated by brittle reactivation has negligible

influence on the 40

Ar-39

Ar dating for the earlier ductile

shear zone.

The relatively flat and undisturbed age spectra

showed by three samples suggest that each of these pla-

teau ages should document the time of one tectonothermal

event, that is, the formation time of the ductile shear zone.

The principle of 40

Ar-39

Ar dating method[6]

states that, the

plateau age is recorded when the 40

Ar-39

Ar systems of the

mineral separates are frozen under their respective closure

temperatures. In this sense, these plateau ages represent

the frozen time of the tectonothermal event. On the other

hand, in view of the petrological characteristics of the

dated samples, the protoliths of epidote-two-mica-plagio-

clase protomylonite (PS2000-40), biotite-plagioclase my-

lonite (NN2000-29) and quartzofeldspathic mylonite

(PS2000-35) are biotite-plagioclase gneiss from formerly

Archean Dayingzi Formation (scale 1 200000 geological

mapping) and present Xiaoqianmaling gneiss of the Ar-

chean basement (scale 1 50000 geological mapping),

hornblende-biotite-plagioclase gneiss from formerly Ar-

chean Waziyu Formation (scale 1 200000 geological

mapping) and present Xiaoqianmaling gneiss of the Ar-

chean basement (scale 1 50000 geological mapping),

and arkose-quartzite from mid-Proterozoic Changcheng

System respectively. The micas are microscopically


highly oriented. The widespread ductile deformation of

quartz and the brittle deformation of feldspar in mylonites

from the shear zone indicate that mylonitization occurred

under greenschist facies condition, at temperatures of

about 300 350[9]

. Such coexistence phenomenon of

both progressive and retrogressive metamorphisms has

once been called as metamorphic double-effects by some

geologists[4,12]

. This range of temperature is consistent

with the 40

Ar-39

Ar closure temperature of biotite. This

gives strong convincing geological lines of evidence that

the plateau ages reported here record the main time of

ductile deformation and mylonitization of the EW- and

NNE-trending ductile shear zones. In conclusion, the for-

mation time of the EW-trending ductile shear zone is

about 219 Ma, whereas the formation time of the

NNE-trending ductile shear zone is in the range of 116

127 Ma.

The tectonic framework of West Liaoning Region is

characterized by the intersection and superposition of the

structural patterns formed during different geological

times and under different tectonic regimes. Our 40

Ar-39

Ar

dating results present strong evidence for the time of duc-

tile deformation associated with the Hercynian-Indosinian

top-to-the-southeast thrusting and the late Mesozoic sinis-

tral extensional and strike-slip faulting[13 16]

. The results

indicate that from-northwest-to-southeast oblique thrust-

ing occurred in Late Triassic and the sinistral extensional

and strike-slip faulting occurred in Early Cretaceous.

The tectonic evolution of the eastern North China,

where the study area is situated, has been changed and

transfered from earlier Paleo-Asian domain to later Circum-

Pacific domain, and the founding and developing of the

Circum-Pacific tectonic regime during Mesozoic and Ce-

nozoic[17]

. Late Permian to Early Triassic(?) amalgamation

of the Mongolian belt with the North China Archean cra-

ton along the Suolun-Linxi suture resulted in the Meso-

zoic North China-Mongolian plate[16,18,19]

. Southward

subduction of the Mongolian belt beneath the Archean

craton is supported by the widespread occurrence in the

south of the suture of numerous plutons ranging in age

from 285 to 217 Ma[20]

, which resulted in profound struc-

tural deformation of the affected areas with the develop-

ment of fold and thrusts, nappes and ductile shear

zones[21]

. Recent studies towards clarifying major Meso-

zoic contractional events in the Yanshan orogenic belt

revealed[16,19]

that, compared with the nappe formation

and thrust faulting events of Late Jurassic (161 148 Ma)

north-directed (for example, Chengde thrust fault and

thrust faults in the western Liaoning Province) and Early

Cretaceous (pre-143 Ma to 127 Ma) well constrained by a

wealth of geochronological data, pre-Middle Jurassic

(pre-180 Ma) south-vergent thrust faulting (for example,

Pingquan-Gubeikou fault) was perhaps the most intense

and, to date, least understood[13]

phase of Mesozoic de-

formation in the Yanshan belt. This phase of thrusting

could either have been a consequence of the collisional

suturing of Paleozoic Mongolian arcs against an An-

dean-style continental arc along the northern margin of

the North China plate or an expression of a backarc, fore-

land fold and thrust belt of U.S.Cordilleran type formed

during southward subduction beneath the North China

Archean craton[19]

. Our age determination indicates that

the EW trending ductile shear zone in the study area

should result from this phase of thrusting event. After that,

the domination for the tectonic evolution of the study area

has come to transfer from Paleo-Asian domain to the

Circum-Pacific domain[17]

. As for the NNE-trending duc-

tile shear zone, which is not only parallel with but also of

the same movement nature of sinistral strike-slip as its

eastern neighboring Tan-Lu fault zone, our geochro-

nological results are in good agreement with the conclu-

sion that the large scale left-lateral displacement in the

Tan-Lu fault zone took place in the Early Cretaceous re-

cently made by Zhu et al.[22]

based on the 40

Ar-39

Ar dating.

Such coincidence suggests that the NNE-trending ductile

shear zone in the study area is developed mainly due to

oblique, high-speed subduction of the Izanagi beneath the

East Asian continent in the Early Cretaceous[23]

, and

therefore belongs to circum-Pacific tectonics. As the latest

developed and well-preserved structural style in the study

area, the NNE-trending ductile shear zone not only has

profound influence on the tectonic evolution of the basin

and range areas in West Liaoning Region[24]

, but also car-

ries important implications for the gold mineralization in

eastern China, as indicated by the consistence of its range

of activity time determined by our study with the age of

the gold mineralization in Paishanlou gold deposit[25]

situ-

ated at the crosscutting section of these two kinds of duc-

tile shear zones and other regions as Jiaodong[26 28]

in the

eastern North China. In addition, it is worthwhile pointing

out that the age of (127±3) Ma for the NNE-trending duc-

tile shear zone well corresponds with the ages (U-Pb age

of zircon: (125.2±0.9) Ma[29]

; magnetic polarity ages: M1r

chron zone, 124 123.6[30]

;40

Ar-39

Ar age: (125.0±0.18)

Ma[31]

) of the fossil-bearing strata at the Sihetun section of

the Lower Yixian Formation, implying that volcanic erup-

tions represented by Yixian Formation which caused dra-

matic environmental changes[30]

may be genetically re-

lated with the sinistral strike-slip and extensional tectonic

settings which led to the formation the NNE-trending duc-

tile shear zone.

Acknowledgements This work was jointly supported by the Chinese


NOTES

Academy of Sciences (Grant Nos. KZCX1-07 and KZCX1-Y-03-01-05)

and the National Key Basic Research Project (Grant No. G1999043302).

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(Received February 25, 2002)


40ar-39ar thermochronology of two ductile shear zones from yiwulüshan, west liaoning region: age...

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