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Journal of Geological Research / 2012 / Article
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Tectonic History and Coalbed Gas Genesis

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Volume 2012 |Article ID 287962 | 11 pages | https://doi.org/10.1155/2012/287962

Geochronology and Tectonic Evolution of the Lincang Batholith in Southwestern Yunnan, China

Academic Editor: Quan-Lin Hou
Received15 Feb 2012
Accepted05 Apr 2012
Published08 Aug 2012

Abstract

Geochronological research of the Lincang Batholith is one key scientific problem to discuss the tectonic evolution of the Tethys. Two granitic specimens were selected from the Mengku-Douge area in the Lincang Batholith to perform the LA-ICPMS Zircon U-Pb dating based on thorough review of petrological, geochemical, and geochronological data by the former scientists. Rock-forming age data of biotite granite specimen from Kunsai is about 220 Ma, the Norian age. However, the west sample from Mengku shows 230 Ma, the Carnian age. The later intrusion in Kunsai area located east to the Mengku area shows directly their uneven phenomena in both space and time and may indirectly reflect the space difference of the contraction-extension transformation period of the deep seated faults. Considering the former 40Ar/39Ar data and the outcrop in Mengku, the Lincang Batholith should have experienced one tectonic exhumation and regional subsidence cycle. The regional subsidence cycle has close relations to the expansion of the Meso Tethys.

1. Introduction

The Sanjiang-Indochina region is one key area to study the evolution of the Tethys [110] (Figure 1). The Changning-Menglian zone was considered as the major faunal break between Gondwanan assemblage to the west and Cathaysian to the east [11, 12]. The subduction-related magmatism occurred along the western edge of the Lanping-Simao-Indochina terrane throughout the Triassic and the closure of Palaeo-Tethys [12, 13]. The Lincang Batholith extends ~370 km from north to south, covering the Chiangrai-Chiang Mai region of Thailand, with an area of more than 10000 km2 (Figure 2). It has been shown by many authors that there is great mineral potential [14].

The Lingcang Batholith was divided into three lithologic intervals by previous authors, the Xiaojie, Lincang, and Menghai intervals (Figure 2) [1517]. The major intrusion is porphyritic monzonitic granite, which mainly contains quartz, zonal plagioclase, K-feldspar, and biotite. In addition, geochemical studies have shown that the granite mainly has source features of the crust mixing origin [16, 18].

The Lingcang Batholith was proposed to form in a passive continental margin according to geochemical research [19]. The intrusive timing spans from late Permian to late Triassic based on whole-rock Rb-Sr, mineral 40Ar/39Ar, whole-rock 87Sr/86Sr, Rb-Sr, Sm-Nd, and some other methods (Table 1).


Lithologic sectionLithologyMeasured objects and methodsDating results, MaSample positionsTesters and the time

XiaojieGranite porphyryWhole-rock (Rb-Sr)LaomaocunZhang et al., 2006 [25]
XiaojieMonzonitic graniteWhole-rock (87Sr/86Sr)279Yun CountyChen, 1991 [26]
LincangBiotite graniteBiotite (40Ar/39Ar)Near the Lancangjiang RiverDai et al., 1986 [27]
LincangMonzonitic graniteWhole-rock (Rb-Sr)279LincangChen, 1991 [26]
LincangMedium-grained equigranular granodioriteWhole-rock (Rb-Sr)263.8Milestone along Lincang-Mengku road 327–387 kmZhang et al., 1990 [28]
LincangUnequal-sized biotite graniteWhole-rock (Rb-Sr)193Shangyun-XiaotangZhang et al., 1990 [28]
LincangMonzonitic graniteWhole-rock (Rb-Sr)LincangLiu et al., 1989 [18]
MenghaiMonzonitic graniteWhole-rock (87Sr/86Sr)279MenghaiChen, 1991 [26]
MenghaiMonzonitic graniteBiotite (Rb-Sr)256MenghaiWang, 1984 [29]

But there are also some understanding differences on the crystallization period and evolution time of the Lincang Batholith. Many predecessors discussed mineral diagenesis and tectonic evolution through traditional data [16, 20], although, not from the viewpoint of the plate tectonics. On scientific problems of the Lincang Batholith, some solutions were put forward such as single period of mixed granite [16], quasi-situ metasomatic granite [20], or Neoproterozoic type I granite complex batholith with multiperiod transforming events [19].

Therefore, the formation and evolution of the Lincang Batholith is still the key to discuss the regional tectonic evolution of the Baoshan and Lanping-Simao-Indochina block. In this study, we conducted petrographic analysis of the Lincang Batholith and provide new LA-ICP-MS U-Pb data in order to constrain the tectonic evolution of the Batholith. We discuss these results of dating in the context of regional tectonic evolution.

2. Dating Methods, Specimens, and Results

2.1. Dating Methods

The U-Pb zircon age dating of monzonitic granite samples selected from the Mengku-Douge research area in Lincang segment (Figures 2 and 3) was designed and completed in the Institute of Geology and Geophysics, Chinese Academy of Sciences. Jobs of single zircon grain micro-area U-Pb geochronological analysis were taken through Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) of 7500a-type producted by Agilient Co. Ltd., with which the laser beam diameter is 60 μm, the erosion frequency is 8 Hz, the energy density is 15~20 J·cm−2, the erosion time is about 60 s. For detailed analysis procedures, please see Xu et al. [32]. The Zircon U-Pb isotope and the U, Th data processing are finished by software Glitter4. 0 [33], and computing of U-Pb concordia diagrams, weighted average ages and graphics were completed by software Isoplot3.0 [34].

2.2. Specimens
2.2.1. Specimen from Kunsai Quarries

The biotite monzonitic granite samples were selected from the Kunsai quarries to the east of Quannei-Douge migmatite rock belt (Figures 3 and 4(a)). The north part of the rock unit is plunged into the Jurassic basin (Figure 3). Field study shows that both the rock unit and the Jurassic basin have experienced a period of ductile transformation events. The microstructure study shows brittle deformed feldspar (Figures 4(b) and 4(c)), dynamically recrystallized quartz (Figure 4(c)), microdeflected biotite (Figure 4(d)), zircon grains with relatively intact crystal, long column, some rounded output and a higher degree of porosity which is symbiotic to microcline grains (Figure 4(b)). The cathode luminescence (CL) images show that the typical magmatic zircon characteristics are colorless and transparent here with oscillatory zoning, the length of about 120 μm–420 μm and the aspect ratio of 1.2 to 4.1 (Figure 4(e)). Totally, for 22 points of 22 zircon grains of sample biotite granodiorite KS-2 from the Kunsai quarries we performed U-Pb isotope analysis in order to statistically gain a feasible dating result (Figure 4(f)). All points were selected on the edge of long- column euhedral grains where the magmatic zircon oscillatory zoning is clear.

2.2.2. Specimens from Mengku Quarries

The monzonitic granite sample MK-4 is selected from the Mengku East Quarry at the eastern side of the Mengku Town in the Shuangjiang County, which separates Jurassic red clastic rocks with one bedding bottom granitic conglomerate by the paleo-weathering crust in between (Figures 5(a), 5(b), and 5(c)). Figure 5(a) shows the position photo of the biotite monzonitic granite sample in the Mengku East Quarry. From the field outcrop, both granitic intrusion (lower left) and its weathering top can be found to be cleavaged which had been covered by the upper Jurassic red detrital sediments with terrigenous origin, namely, the Huakaizuo Formation. The MK-4 zircon grains (Figure 5(d)) change greatly and have different shapes and sizes, such as long column, fan, and irregular granular. However, size of zircon grains is uniform with length between 90 μm and 150 μm and length-width ratio in 1.1 : 1–1.8 : 1. Zircon grains develop with quartz, plagioclase in rocks. The rock sample has been affected by weathering, showing retrograde metamorphism (Figure 5(e)). Characteristic oscillatory zoning of typical magmatic zircon is rendered on the cathode CL image. The idiomorphic degree of zircon is high, but some fragmented. On the sample of MK-4, 18 zircon grains and 18 points were analyzed by ICP-MS. Edge of the long-column idiomorphic or the chipped-hypidiomorphic zircon grains, and the clear zone of magmatic zircon oscillatory were always selected to perform the micro isotope analysis (Figure 5(f)).

2.2.3. Dating Results

As shown in Figure 4(f) and Table 2, the overall harmony values of KS-2 zircon age data range from 195 Ma to 245 Ma, but some of them are below the U-Pb harmonic line and deviated from the harmony line. This is probably because the micro region selected is too close to the boundary of the zircon grain. Some zircon grains (Figure 5(f)) also have some cracks in and this may also affect the dating result. From the harmonic visible figure, zircon dating values are focused to a range from 210 Ma to 230 Ma, and the average age is about 220 Ma. This means that the crystallization age of the Kunsai quarries biotite granite is about in the late Triassic Norian period.


No. 207Pb/206Pb 207Pb/235U 206Pb/238U 208Pb/232Th 207Pb/206Pb 207Pb/235U 206Pb/238U 208Pb/232Th
RatioError (1)RatioError (1)RatioError (1)RatioError (1)RatioError (1)RatioError (1)RatioError (1)RatioError (1)

A770.057890.002310.259610.009490.032510.000890.013830.00076526382348206627815
A780.051880.002440.235620.009530.032940.000790.010350.00025280110215820952085
A790.052730.001410.243290.006040.033450.000810.011080.00055317262215212522311
A800.052130.002160.234710.008010.032650.000770.010250.0002429197214720752065
A810.052090.001310.259380.006080.03610.000860.011770.0003428925234522952377
A820.050140.001180.241920.005310.034980.000820.011130.0004420124220422252249
A830.051260.001220.242870.005410.034350.000810.011550.0006253242214218523212
A840.050270.001140.245340.005250.035390.000830.011760.0004120724223422452368
A890.05210.002370.225750.008750.031430.000750.009870.00023290107207719951995
A900.053050.001820.26490.00840.036210.000940.013760.00075331332397229627615
A910.057390.001130.239720.004410.030290.00070.009460.0003250723218419241906
A920.050760.001350.246040.006120.035150.000850.011730.0004523026223522352369
A930.050890.002790.244550.011970.034850.000860.010980.000252361272221022152215
A940.051140.001550.243770.006830.034570.000870.011670.00055247292226219523511
A950.050550.001090.241750.004880.034690.000810.011810.0004522024220422052379
A960.050180.001340.238570.005940.034480.000840.011490.00049203262175219523110
B020.051150.001190.247740.005410.035140.000840.012970.00054248252254223526011
B030.050670.002240.241650.009030.034590.000820.01090.00025226104220721952195
B040.051040.002350.241630.009530.034330.000820.010810.00025243109220821852175
B050.050790.001530.249680.006990.035670.00090.012740.00067231292266226625613
B060.050390.000950.245250.004370.035310.000820.011210.0003821325223422452258
B070.054290.002430.274630.010350.036690.000890.011470.00027383103246823262305
B080.050140.002230.243990.009130.03530.000850.011140.00026201104222722452245
B090.051410.001050.258070.004950.036430.000860.014140.00053259252334231528411

As shown in Figure 5(g) and Table 3, the overall harmony values of MK-4 zircon age data range between 190 Ma and 315 Ma. The values are somewhat scattered mainly because the granite of the Mengku quarries has been heavily weathered (Figure 5(d)). During the weathering process, chemical bonds of ZrO2 in zircon grains of our MK-4 sample would be broken. Therefore, some zircon isotope escaped. The average age of 245 Ma might indicate that the late Permian to early-middle Triassic magmatic event happened in the Mengku Quarries area of the Lincang Batholith.


Analysis 207Pb/206Pb 207Pb/235U 206Pb/238U 208Pb/232Th 207Pb/206Pb 207Pb/235U 206Pb/238U 208Pb/232Th
no.RatioError (1)RatioError (1)RatioError (1)RatioError (1)RatioError (1)RatioError (1)RatioError (1)RatioError (1)

10.051740.002440.323030.01390.045280.001080.011520.000532745628411285723211
20.091150.002882.963520.084480.235850.00530.080610.00287145025139822136528156754
30.115090.003320.509520.012250.032120.000660.021020.000681881194188204442013
40.048370.002650.220770.011080.033120.000850.011530.00063117692039210523213
50.048370.00220.272950.011450.040950.000920.012230.0006117572459259624612
60.053510.002790.297750.014150.040380.001040.013710.000783506226511255627516
70.08060.002182.274310.055850.20480.003940.059680.00155121222120417120121117230
80.08020.003412.365770.090850.21410.005770.053520.00277120237123227125131105453
90.059280.002480.299590.011320.036680.000820.009660.0004657745266923251949
100.049890.003750.237930.01630.034610.001190.009070.000721909621713219718214
110.078660.003330.346830.012790.0320.000790.012330.0006311643730210203524813
120.04060.002170.224370.011190.040110.000950.010520.00055−255722069254621211
130.06390.002990.315380.013170.035820.000890.011260.000677384827810227622613
140.074810.004410.35170.018170.034120.001080.011780.0007810635630614216723716
150.068210.002630.977450.033760.103990.002360.032330.0010687537692176381464321
160.051160.002550.266730.012270.037840.000880.010570.000522486424010239521310
170.050250.003040.251810.013950.036360.001020.010370.000642077722811230620913
180.053790.0030.270180.013690.036450.000980.009650.00063626724311231619412
190.050830.003730.267780.018020.038230.001250.009870.000552339624114242819911
200.060220.004870.29980.02170.036120.001420.011370.001136119026617229922923
210.04730.002140.209320.00870.03210.00070.008920.000386455193720441798
220.054050.003340.284420.016020.038170.001110.008580.00063737525413241717312
230.059070.00240.662120.024390.081280.001810.034380.0012157043516155041168324
240.050640.002450.214590.009440.030720.000720.007970.0004622459197819551609
250.055940.002610.263520.011130.034150.00080.009640.0004545053237921651949
260.067690.005240.322170.022120.03450.001340.010490.00098598028417219821118
270.063540.002060.755530.022050.086180.001660.020560.0008472632571135331041117
280.050140.002960.331910.018010.047970.001290.020680.00112017729114302841422
290.237820.003776.559140.084390.199840.003250.018240.0007531051220541111741736515

3. Tectonic Implications

Previous petrological and geochemical studies [18, 26, 28, 35] indicate that the Lincang granite mainly shows two types of granite with an initial strontium isotope ratio ranging from 0.71 to 0.78 and therefore it belongs to S-type granite and once may have been formed in an environment of tectonic collision. Combined with previous chronological data, we propose a three-stage model on the tectonic evolution between late Permian and Jurassic (Figure 6).

(1)  Regional Collision and Contraction before the Formation of the Lincang Batholith
Before the formation of the Lincang Batholith, the Paleo-Tethys Ocean and many intraoceanic islands distributed between the Baoshan and the Lanping-Simao-Indosinian blocks [36]. In a regional contractional environment of subduction and collision, it is extremely common to find folding, thrusting, and uplifting phenomena (Figure 7(a)).

(2)  Time Heterogeneity of In Situ Hybrid of the Lincang Batholith
The Lincang Batholith was controlled by one deep-seated fault along the Lancang Jiang River and proposed in situ hybrid origin of the Batholith in the early to middle Triassic collision between the Baoshan-Shantai and Lanping-Simao-Indochina Blocks [16]. Our study shows an ununiform law result in the aspect of mixing time of the Batholith (Figure 7(b)). The intrusion time in the east Qunsai area was later than in the Mengku area, indicating that the regional contraction in Mengku area was weakened or stopped, and triggered magma poured at ~230 Ma, however, the contraction pattern may still remain in the Qunsai area to the east.

(3)  Tectonic Denudation and Regional Exhumation Happened between Late Triassic and Middle Jurassic, Which Is Supported by ~201 Ma Biotite 40Ar/39Ar Age from the Lincang Batholith [27]
This paper research shows that the upper part of the Lincang Batholith experienced long-term denudation, resulting in the development of the weathering crust (Figure 7(a)). The upper crust is the Jurassic Huakaizuo Formation, characterized by carbonate rocks of shallow sea facies, together with some volcano clastic rocks (Figure 5(b)). Unconformity relationship is shown between the Batholith and the Huakaizuo Formation (Figures 3, 5(a), 5(b), 5(c), and 7(c)). Apparently, the Meso-Tethys expansion also happened in this era and left some effect in the Lincang Area.

Acknowledgments

The authors appreciate sincerely both the field research and inner discussion together with Professor Yong-Qing Chen from China University of Geosciences (Beijing), Research Fellow Ying-Xiang Lu from Yunnan Bureau of Geology and Mineral Resources, and Research Fellow Fang-Cheng Lin from Chengdu Institute of Geology and Mineral Resources. The authors thank Dr. Pengfei Li from University of Queensland for his perfect academic revision of the paper. The research was supported by the China Geology Survey (no. 200811008, no. 1212011121188), the Ministry of Science and Technology (no. 2006BAB01A03-3), the Chinese National Natural Foundation (no. 90814006) of the People’s Republic, and the China University of Geosciences in Beijing (no. 2-9-2001-280).

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