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

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.


Introduction
The Sanjiang-Indochina region is one key area to study the evolution of the Tethys [1][2][3][4][5][6][7][8][9][10] (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 km 2 (Figure 2).It has been shown by many authors that there is great mineral potential [14].
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 40 Ar/ 39 Ar, whole-rock 87 Sr/ 86 Sr, Rb-Sr, Sm-Nd, and some other methods (Table 1).
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.

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 The distribution of metal deposits also indicates that there is an important boundary along the Lincang Granite Batholith.Stereographic maps of lower hemisphere projection indicating the initial occurrence of Lincang granite Batholith are also plotted in the regional map.L, S, Q refer to joints parallel to the initial foliation, parallel to the flow direction but upright to the initial foliation, upright to both the flow direction and initial foliation.Orientation of shear fractures can be generated by local stress system of the Batholith with emplacements [24].
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].

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     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.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)).

Dating Results.
As shown in Figure 4(f) and      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.
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 ZrO 2 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.

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 40 Ar/ 39 Ar 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)).  .Apparently, the Meso-Tethys expansion also happened in this era and left some effect in the Lincang Area.

FigFigure 2 :
Fig.3 (a)).The north part of the rock unit is plunged into the Jurassic basin (Figure3).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 (Figures4(b) and 4(c)), dynamically recrystallized quartz (Figure 4(c)), microdeflected biotite (Figure 4(d)), zircon grains with relatively intact crystal, long column, some Fig.5 Fig.4
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)).Figure5(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.

Figure 4 :
Figure 4: Geochronological research of granite sample KS-2.(a) Position photo of the biotite monzonitic granite sample in the field of Kunsai Quarry.(b) The zircon grows together with the microcline CPL.(c) The microstructure of complete recrystallized quartz indicates the rock once experienced high-greenschist facies of deformation, CPL.(d) One biotite crystal is folded, PPL.(e) Magmatic zircon is shown by the SEM cathodoluminescence (CL) image.The circled area is deduced to form during the crystallization of granite and is just the measured area by laser-ICPMS (f) Zircon La-ICPMs U-Pb dating figure, and the average age is about 220 Ma.

J2 U n cFigure 5 :
Figure 5: Geochronological research of the granite sample MK-4.(a) Position photo of the biotite monzonitic granite sample in a field Quarry of east Mengku.(b) Granitic intrusion (lower left) and its fossil weathering were cleavaged.(c) The red detrital material of terrigenous origin, Huakaizuo Formation in Jurassic, was normal gradational.(d) Granitic intrusions are selected as sample.(e) The micro characteristics of MK-4 and the zircon setting are shown.The zircon grains are associated with quartz and plagioclase and have been degraded by some weathering process, CPL, (f) Magmatic zircon is shown by the SEM cathodoluminescence (CL) image.The circled area is deduced to form during the crystallization of granite and is just the measured area by laser-ICPMS.(g) Zircon La-ICPMs U-Pb dating figure, and the average age is about 245 Ma.

Figure 6 :
Figure 6: Isotopic age histogram showing the major tectonic event in the area east to the Tibet Peateau (after [8]).

Figure 7 :
Figure 7: Formation and tectonic evolution model for the Lincang Batholith.

Table 1 :
Isotopic age data for the Lincang Batholite in the earlier stage.

Table 2
, the overall harmony values of KS-2 zircon age data range

Table 2 :
U-Pb isotopic dating for the single grain Zircon from granite of Kunsai.

Table 3 :
U-Pb isotopic dating for the single grain Zircon from granite of Mengku.