Method for determining deformation time limit of ductile shear band

Abstract

The invention belongs to the crossing field of tectonic geology and isotope chronology, and particularly relates to a method for determining the deformation time limit of a ductile shear zone, which comprises the following steps: step 1, field geological survey and sample collection; step 2, sample preparation and microscopic structure research; step 3, apatite formation cause identification; step 4, micro-cell U-Pb dating test is carried out; and step 5, data processing and analysis. According to the method disclosed by the invention, the deformation age of the ductile shear zone in the granite body is rapidly and accurately obtained through apatite U-Pb micro-area analysis.

Description

Technical Field

[0001] This invention belongs to the interdisciplinary field of structural geology and isotope geography, and specifically relates to a method for determining the deformation time limit of ductile shear zones. Background Technology

[0002] Ductile shear zones are a major manifestation of strain localization in the continental lithosphere, recording key tectonic processes such as plate convergence, collisional orogeny, crustal thickening, and post-orogenic extension. In South China tectonic studies, granite bodies are widely developed in and around the Jiangnan orogenic belt. These bodies are often cut through or modified by later ductile shear zones, forming granitic mylonites. Precisely defining the activity timeline of these shear zones is of crucial scientific and economic significance for reconstructing the regional tectonic framework, understanding the superposition of multiple orogenic events (such as the Caledonian / Guangxi Movement, Indosinian, and Yanshanian Orogenies), and revealing tectonic-controlled fluid migration and mineralization (such as uranium, tin, and tungsten mineralization).

[0003] For a long time, the geological community has relied mainly on mica to determine the formation age of ductile shear zones, especially granitic mylonites. 40 Ar / 39 Ar-Ar dating and zircon U-Pb dating are both methods, but they have significant drawbacks in practical applications. The closure temperature of the Ar-Ar system for muscovite is approximately 350–400℃, and for biotite, it is approximately 300–325℃. However, the temperatures at which granites undergo ductile deformation (such as core-mantle recrystallization of feldspar and subgrain rotational recrystallization of quartz) are typically between 400℃ and 650℃. This means that mica Ar-Ar ages often record the cooling age after deformation has ended, rather than the peak age during which deformation occurred. If the geological body cools slowly, the two ages can differ by tens of millions of years. Secondly, due to the low closure temperature of the Ar-Ar system, if the shear zone later experiences magmatic thermal events such as the Yanshanian period (>300℃), the Ar-Ar clock of mica can easily be partially or completely reset, leading to mixed ages or incorrect young ages. In addition, this method usually requires crushing the sample and manually selecting high-purity mica single minerals grain by grain under a binocular microscope, which is extremely time-consuming. Furthermore, testing requires nuclear reactor irradiation (which typically takes several months for logistics and approval), resulting in a long cycle and high cost. Zircon's U-Pb system has an extremely high sealing temperature (>900℃), primarily recording magma crystallization ages. In shear deformation of greenschist to low-amphibolite facies (<650℃), zircon typically remains stable, rarely recrystallizing or forming only extremely thin metamorphic rims, making it difficult to directly record shear times unless extremely expensive SIMS or Nano-SIMS techniques are used to locate micron-scale metamorphic rims. Summary of the Invention

[0004] The purpose of this invention is to provide a method for determining the deformation time limit of ductile shear bands. This method uses apatite U-Pb micro-area analysis to quickly and accurately obtain the deformation age of ductile shear bands in granite bodies.

[0005] Technical solution to achieve the purpose of this invention:

[0006] A method for determining the deformation time limit of a ductile shear band includes:

[0007] Step 1: Field geological survey and sample collection;

[0008] Step 2: Sample preparation and microstructure study;

[0009] Step 3: Identification of the origin of apatite;

[0010] Step 4: Micro-area U-Pb dating test;

[0011] Step 5: Data processing and analysis.

[0012] Further, step 1 includes: selecting outcrops with relatively stable occurrence, plastic deformation structures, and dynamic recrystallization in the ductile shear zone of the target granite; measuring the foliation and lineation occurrence of the shear zone, and determining the shear direction using SC fabric and rotating residual porphyry; and collecting granitic mylonite with strong foliation and strong lineation as the sample.

[0013] Furthermore, in step 1, undeformed protolith is simultaneously collected as a background to determine the crystallization age of the protolith.

[0014] Further, step 2 includes: pre-crushing, grinding, and cutting the collected sample to make a probe thin section; observing the banded structure of quartz, the core-mantle structure of feldspar, and mica fish under a polarizing microscope to confirm that the rock has undergone medium- and high-temperature plastic deformation; searching for apatite associated with the deformation structure to determine the morphology, crystal structure, and relationship of apatite with the surrounding rock minerals.

[0015] Further, step 3 includes: separating apatite minerals from the sample using heavy liquid separation and magnetic separation; and using cathodoluminescence (CL) and backscattered electron microscopy (BSE) imaging techniques to analyze the internal structure of the selected apatite and identify its origin.

[0016] Furthermore, step 4 includes: performing micro-area U-Pb dating on the separated apatite using laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS).

[0017] Furthermore, in step 4, the laser ablation parameters are as follows: laser beam size 30-40 μm, laser frequency 6-8 Hz, and energy density approximately 3-4 J / cm². 2.

[0018] Furthermore, in step 4, multiple international standard apatite samples are used to calibrate the apatite age during the micro-area U-Pb dating test; ordinary lead calibration and 207Pb correction are performed.

[0019] Further, step 5 includes: processing the data obtained from the micro-region U-Pb dating test to generate a Tera-Wasserburg concordance map and calculating the 206Pb / 238U weighted average age.

[0020] Furthermore, in step 5, data processing includes subtracting common lead interference, calculating the data error range, and correcting instrument drift.

[0021] The beneficial technical effects of this invention are as follows:

[0022] 1. This invention provides a method for determining the deformation time of ductile shear zones, proposing to use apatite as the dating object and utilizing its unique physicochemical properties and LA-ICP-MS micro-area analysis technology to solve the problem of dating granite shear zones. The closure temperature of the apatite U-Pb system is approximately 350–550℃. This temperature range precisely covers the main temperature window (medium to high temperature) for the plastic rheology of feldspar and quartz to form mylonite. Therefore, apatite is more likely to record the moment when shear deformation occurs, rather than just the cooling time. Under shear stress and fluid assistance, apatite readily undergoes dynamic recrystallization or dissolution-reprecipitation, leading to the loss of radiogenic Pb and a complete reset of the isotopic clock. This allows us to determine the time of the "deformation itself." More importantly, using LA-ICP-MS technology eliminates the need for mineral processing (it can be analyzed directly on thin sections), eliminates the need for irradiation, and requires only tens of seconds for single-point testing, reducing the analysis cycle from months to days and significantly lowering costs.

[0023] 2. This invention provides a method for determining the deformation time of ductile shear zones. By utilizing apatite U-Pb micro-area dating technology, the deformation age of ductile shear zones within granite can be accurately determined. The dating results, combined with regional geological background analysis, can effectively reveal the tectonic evolution history of ductile shear zones and provide important chronological evidence for regional geological evolution and mineral deposit genesis studies.

[0024] 3. The method for determining the deformation time limit of ductile shear zones provided by this invention has wide applicability and is suitable for the study of ductile shear zones in granite areas worldwide. It greatly improves the efficiency of geological research and the accuracy of data, and provides strong support for regional geology and mineral deposit exploration. It has important promotion value and application prospects. Attached Figure Description

[0025] Figure 1This is a CL image of apatite in gneissic biotite granite within the Motianling ductile shear zone, according to an embodiment of the present invention.

[0026] Figure 2 This is a diagram illustrating the Tera-Wasserburg U-Pb harmony of apatite in gneissic biotite granite within the Motianling ductile shear zone, as described in an embodiment of the present invention. Detailed Implementation

[0027] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0028] This invention provides a method for determining the deformation time limit of a ductile shear band, specifically including the following steps:

[0029] Step 1: Field geological survey and sample collection:

[0030] In the ductile shear zone of the target granite, outcrops with relatively stable attitudes, well-developed plastic deformation structures, and dynamic recrystallization were selected. Then, the foliation and lineation attitudes of the shear zones were measured, and the shear direction (left-handed or right-handed) was determined using SC fabric and rotational residual porphyry. Subsequently, SL tectonic rocks, i.e., granitic mylonite with strong foliation and lineation, were collected as samples. These rocks have undergone intense plastic rheology and have a high probability of apatite repositioning. Simultaneously, undeformed protoliths were collected as background to determine the protolith's crystallization age (usually using zircon).

[0031] Step 2, Sample preparation and microstructure study:

[0032] The collected samples were initially crushed and ground to obtain thin sections and samples with separated apatite minerals suitable for microscopic analysis. The collected oriented rock samples were cut into probe microscopy sections (approximately 30-50 μm thick). The cut surfaces should be parallel to the foliation (X-axis) and perpendicular to the foliation (Z-axis), i.e., the XZ plane, to observe kinematic characteristics. Under a polarizing microscope, the banded structure of quartz, the core-mantle structure of feldspar, and mica were observed. The rock was confirmed to have undergone medium-to-high temperature (>400℃) plastic deformation. Apatite associated with the deformed structures was searched. The morphology, crystal structure, and relationship of the apatite to the surrounding rock minerals were determined. Metamorphic apatite typically forms during metamorphism and exhibits specific crystal morphological characteristics, such as relatively uniform grain distribution and fibrous or tabular crystal forms.

[0033] Step 3, Identification of the Origin of Apatite:

[0034] Apatite minerals were separated from pulverized 60-80 mesh samples using heavy liquid separation (using heavy liquids such as diiodomethane) and magnetic separation. During the separation process, each particle was observed and selected using a binocular stereomicroscope to ensure that the sample contained only pure, transparent apatite particles without obvious cracks or inclusions.

[0035] Cathodoluminescence (CL) and backscattered electron microscopy (BSE) imaging techniques were used to analyze the internal structure of selected apatite and identify its origin. Magmatic apatite typically shows clear oscillating zoning in CL images, with strong but uneven luminescence. Apatite that has undergone ductile shearing usually exhibits a dark, homogeneous structure without zoning in CL images, indicating complete recrystallization. It displays a complex dissolution-precipitation structure with fjord-like or spongy edges and a disordered internal structure.

[0036] Step 4: Micro-area U-Pb dating test:

[0037] Micro-area U-Pb dating of the separated apatite was performed using laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS). Specifically, the laser ablation process employed a 30-40 μm laser beam, a frequency of 6-8 Hz, and an energy density of 3-4 J / cm². Helium was used as the carrier gas during the laser ablation process to ensure the integrity of the sample surface while avoiding excessive damage, thus ensuring the accuracy and consistency of multi-point measurements. During the testing process, multiple international standard apatite samples (such as MAD2, Otter Lake, and Durango apatite) were used to calibrate the apatite age, ensuring high accuracy and repeatability of the U-Pb age data. Ordinary lead calibration was performed using the method proposed by Meffre et al. (2008), and the method of Chew et al. (2014) was used for further calibration. 207 Pb correction is performed, a step that eliminates the influence of common lead interference on U-Pb age calculation and ensures the accuracy of the measurement results.

[0038] Step 5: Data Processing and Analysis

[0039] Data obtained from micro-region U-Pb dating tests were processed using specialized software (such as ICPMSDataCal and Isoplot) to generate Tera-Wasserburg concordance maps and perform calculations. 206 Pb / 238 U-weighted average age. This process also includes subtracting common lead interference, calculating data error ranges, and correcting instrument drift, ensuring the accuracy and scientific validity of the final age data. Furthermore, CL images can help identify multiple formation stages of apatite, determining whether it underwent multiple growth events in different metamorphic events. Combining these methods, metamorphic apatite can be effectively identified, thus laying the groundwork for subsequent dating work.

[0040] Example 1

[0041] Taking the Motianling granitic mylonite as an example, this embodiment provides a method for determining the deformation time limit of the ductile shear zone, which specifically includes the following steps:

[0042] Step 1: Field geological survey and sample collection

[0043] Several representative gneissic granitic mylonite samples were collected from the ductile shear zones of the granite body in the Motianling area of ​​northern Guangxi. The protoliths of these rocks were mainly Neoproterozoic (~825 Ma) coarse- to medium-grained biotite granites, but they underwent intense ductile shear deformation under later tectonic activity. The focus was on collecting granites with fresh outcrops, unweathered surfaces, and well-developed strong penetrating foliation (S-foliation) and shear zones (C-foliation).

[0044] The collected samples were first subjected to on-site removal of the surface weathering crust, and detailed recording of occurrence elements (foliation dip / angle, lineation retraction angle). In the laboratory, the samples were further crushed into centimeter-sized particles and subjected to ultrasonic cleaning, low-temperature drying, and other treatment steps to remove surface contaminants, preparing for subsequent mineral sorting and thin section preparation.

[0045] Step 2, Preparation of thin sections and petrographic studies

[0046] After collecting samples from the field and bringing them back to the laboratory, each sample was prepared into 0.3 mm thick probe sections using a rock slicer and manual processing for microscopic observation. Detailed petrographic observation of the mylonite samples was then conducted using a polarizing microscope and a mineralogy microscope. Microscopic examination revealed intense dynamic recrystallization of quartz, primarily manifested as subgrain rotation and grain boundary migration, indicating a high deformation temperature (>400℃). Feldspar exhibited a typical core-mantle structure, with a core of residual hard and brittle feldspar phenocrysts, surrounded by fine-grained recrystallized feldspar and quartz aggregates. Biotite showed a "mica fish" oriented arrangement, indicating the direction of shear movement. Under crossed polarized light, apatite was identified as a widely distributed major accessory mineral.

[0047] Step 3, Single mineral sorting

[0048] Apatite minerals were separated from pulverized 60-80 mesh samples using heavy liquid separation (using heavy liquids such as diiodomethane) and magnetic separation. During the separation process, each particle was observed and selected using a binocular stereomicroscope to ensure that the sample contained only pure, transparent apatite particles without obvious cracks or inclusions.

[0049] Step 4, CL Image Acquisition and Cause Identification

[0050] During the separation process, a binocular microscope was used for observation and selection to ensure that the sample contained only pure apatite particles. The separated apatite particles were then fabricated into epoxy resin targets and polished to the center of the particles. Subsequently, cathodoluminescence (CL) images were captured, such as... Figure 1As shown, the apatite in the Motianling shear zone exhibits a complex internal structure under CL images. Some grains retain brightly luminescent or oscillating cores (protolith remnants), but most grains develop darker, more uniformly luminescent, broad-rimmed or fully recrystallized dark grains. This uniform dark color reflects intense dynamic recrystallization and compositional resetting under ductile shear deformation and fluid assistance. Based on this, apatite grains / regions with clearly defined recrystallization rims or complete recrystallization were selected as target sites for micro-area U-Pb dating.

[0051] Step 5, U-Pb dating of apatite micro-area

[0052] First, based on the CL image, avoid the original rock residual core and select rocks with uniform composition, no cracks, and representative of shear deformation recrystallization. Figure 1 The locations of apatite in the (medium-dark, homogeneous region) were marked. To make the test results more reliable, multiple international apatite standards, such as Madagascar (474 ​​Ma) and Durango (31.4 Ma), were used to jointly correct the apatite age during the experiment to obtain more reliable and accurate U-Pb age data. Each analysis of apatite U-Pb isotopes began with a 20-second blank gas measurement, followed by a 40-second signal acquisition and analysis when the laser was turned on. The laser beam spot was set to 30-40 μm, the laser frequency was 6-8 Hz, and the energy density was approximately 3-4 J / cm². Helium was used as the carrier gas during laser ablation. Conventional Pb calibration was performed based on the method detailed in the work of Meffre et al. (2008), combined with the Tera-Wasserburg graphical method for data correction.

[0053] Step 6, Data Processing and Result Analysis

[0054] The analyzed data were processed offline using ICPMSDataCal software, and the Tera-Wasserburg harmonic plot of apatite was plotted using Isoplot software. Data processing steps included background signal subtraction, instrument drift correction, and calculation of U-Pb isotope ratios. Since apatite typically contains common lead, it was not included in the calculations. 206 Pb / 238 Instead of using the surface age of U, the Tera-Wasserburg diagram is used ( 207 Pb / 206 Pb- 238 U / 206The lower node age of Pb is used to represent the time of geological events. In this process, multi-point U-Pb dating (n=25) was performed on the test samples to ensure data representativeness and consistency. By selecting multiple isotope grains of syntectic growth within the same sample for measurement points, a series of isotope ratios were obtained, which form a mixing line on the graph. The results obtained through the above analysis and processing show that, as Figure 2 As shown.

[0055] Figure 2 The U-Pb dating results of apatite in the granitic mylonite of the Motianling ductile shear zone in Guangxi show a good linear relationship, with a Tera-Wasserburg lower intersection age of 410.7±6.2 Ma (MSWD=0.60). The MSWD value is less than 1, indicating that the data fit is extremely high and the results are true and reliable.

[0056] Based on the apatite U-Pb dating results of the Motianling ductile shear zone in Guangxi, this invention determines that the main ductile shear deformation event occurred at approximately 410.7 ± 6.2 Ma. This age corresponds to the Early Devonian, which in geological history belongs to the late stage of the Caledonian Orogeny (Guangxi Orogeny). The age of 410.7 Ma records the deformation history of the shear zone at 350-550℃ (the apatite closure temperature range). This is highly consistent with the deformation temperature (>400℃) indicated by the feldspar core-mantle structure and dynamic recrystallization of quartz in the rock, proving that this age represents the peak or near-peak period of deformation, rather than a later low-temperature cooling period (such as the mica Ar-Ar age, which usually represents <300℃). This age confirms that although the Motianling pluton was formed in the Neoproterozoic, its currently observed strong mylonitization is mainly a product of Caledonian orogeny, strongly supporting the strong response of the Guangxi Orogeny in northern Guangxi.

[0057] In summary, compared with existing technologies, the "method for determining the time limit of ductile shear bands using U-Pb dating of apatite" proposed in this invention can more accurately reflect the timing of shear deformation, avoiding the problem of premature age caused by slow cooling or subsequent thermal disturbances. Furthermore, it eliminates the need for cumbersome single-mineral selection and can be performed directly on thin sections or with simple target preparation. More importantly, it eliminates the nuclear reactor neutron irradiation step required for the Ar-Ar method, avoiding a waiting period of several months and complex approval processes for radioactive sample transportation, thus improving efficiency several times over.

[0058] This method is not only applicable to the Motianling area, but can also be widely used in the study of ductile shear zones in orogenic granites and deeply metamorphic rock areas worldwide. Especially for complex tectonic zones with multiple phases of activity, the micro-area in-situ dating technique combined with CL images has irreplaceable analytical capabilities.

[0059] The above-described embodiments are merely preferred examples for the ductile shear zone of Motianling, Guangxi. However, the present invention is not limited to the above-described embodiments. Within the scope of knowledge possessed by those skilled in the art, various other changes and modifications can be made without departing from the spirit of the present invention. All content not described in detail in this invention can be derived from existing technologies.

Claims

1. A method for determining the deformation time limit of a ductile shear band, characterized in that, include: Step 1: Field geological survey and sample collection; Step 2: Sample preparation and microstructure study; Step 3: Identification of the origin of apatite; Step 4: Micro-area U-Pb dating test; Step 5: Data processing and analysis.

2. The method for determining the deformation time limit of a ductile shear band according to claim 1, characterized in that, Step 1 includes: selecting outcrops with relatively stable attitude, plastic deformation structures, and dynamic recrystallization in the ductile shear zone of the target granite; measuring the foliation and lineation attitude of the shear zone, and using SC fabric and rotating residual porphyry to determine the shear direction; and collecting granitic mylonite with strong foliation and strong lineation as the sample.

3. The method for determining the deformation time limit of a ductile shear band according to claim 2, characterized in that, In step 1, undeformed protoliths are simultaneously collected as a background to determine the crystallization age of the protoliths.

4. The method for determining the deformation time limit of a ductile shear band according to claim 1, characterized in that, Step 2 includes: pre-crushing, grinding, and cutting the collected samples to make probe thin sections; observing the banded structure of quartz, the core-mantle structure of feldspar, and mica fish under a polarizing microscope to confirm that the rock has undergone medium- and high-temperature plastic deformation; searching for apatite associated with the deformation structure to determine the morphology, crystal structure, and relationship of apatite with the surrounding rock minerals.

5. The method for determining the deformation time limit of a ductile shear band according to claim 1, characterized in that, Step 3 includes: separating apatite minerals from the sample using heavy liquid separation and magnetic separation; and using cathodoluminescence (CL) and backscattered electron (BSE) imaging techniques to analyze the internal structure of the selected apatite and identify its origin.

6. The method for determining the deformation time limit of a ductile shear band according to claim 1, characterized in that, Step 4 includes: performing micro-area U-Pb dating on the separated apatite using laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS).

7. The method for determining the deformation time limit of a ductile shear band according to claim 6, characterized in that, In step 4, the laser ablation parameters are as follows: laser beam size 30-40 μm, laser frequency 6-8 Hz, and energy density approximately 3-4 J / cm². 2 .

8. The method for determining the deformation time limit of a ductile shear band according to claim 6, characterized in that, In step 4, the micro-area U-Pb dating process uses multiple international standard apatite samples to calibrate the apatite age; ordinary lead calibration and... 207 Pb correction.

9. The method for determining the deformation time limit of a ductile shear band according to claim 1, characterized in that, Step 5 includes: processing the data obtained from the micro-region U-Pb dating test to generate a Tera-Wasserburg harmony map and calculate... 206 Pb / 238 U-weighted average age.

10. The method for determining the deformation time limit of a ductile shear band according to claim 9, characterized in that, In step 5, data processing includes deducting common lead interference, calculating the data error range, and correcting instrument drift.

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