Deformation of the Subcontinental Lithospheric Mantle in NE China: Constraints from Rheological and Fabric Study of Mantle Peridotite Xenoliths from Jiaohe, Jilin Province

0. INTRODUCTION

Mantle-derived peridotite xenoliths in Cenozoic basaltic rocks provide a unique opportunity to directly study the nature of the subcontinental lithospheric mantle (SCLM) that is still poorly understood (Li et al., 2020; Pan et al., 2015; Zou et al., 2014; Huang and Xu, 2010; Xiao et al., 2010; Zheng et al., 2006). In Jiaohe City, mantle peridotite xenoliths are well studied in terms of geochemistry. Previous geochemical studies of peridotite xenoliths in Jiaohe City suggest that the underneath subcontinental lithospheric mantle is expected to be chemically heterogeneous (Hao et al., 2016; Yu et al., 2009; Zhang et al., 2007). An accreted Mesoproterozoic lithospheric mantle could have possibly replaced the lost original lithospheric mantle revealed by Re-Os isotopic study on the Jiaohe peridotite xenoliths (Zhou et al., 2007). The remnants of subducted oceanic crust in subcontinental lithospheric mantle from Jiaohe, identified by trace element and Sr-Nd-O isotope studies were considered to be of great importance to understanding the mantle heterogeneity (Yu et al., 2010). And mantle xenoliths could be affected by the enrichment components or the mantle fluids as indicated by fertile peridotites Lu/Hf isotopic data (Yu et al., 2009) and the low value of helium isotope (³He/⁴He ratios) studies (Li et al., 2002). However, relatively little is known about rheological properties and deformation history of this subcontinental lithospheric mantle.

Field observations and experimental studies indicate that the development of different lattice preferred orientations (LPOs) pattern (i.e., fabric) of olivine could be closely related to their geodynamic settings (Wang, 2010; Karato et al., 2008; Jung et al., 2006; Jung and Karato, 2001). In particularly, olivine is the most abundant but the weakest mineral controlling deformation rate of the upper mantle (Jin et al., 1989). Therefore, studying fabric of olivine in peridotite xenoliths provides a primary insight into the condition of mantle deformation. In the past decades, the pattern of olivine LPOs has been well studied and is considered to be mainly controlled by temperature, stress, pressure and water content (Bernard et al., 2019; Behr and Hirth, 2014; Karato et al., 2008; Jung et al., 2006; Katayama et al., 2004) and also it could be affected by other parameters such as partial melting and pre-existing LPO (Kumamoto et al., 2019; Qi et al., 2018; Boneh and Skemer, 2014; Karato, 2008; Vauchez and Garrido, 2001). Here we make the first comprehensive investigation on the rheological property using peridotite xenoliths collected in the northern section along Tan-Lu faults close to the Jiaohe City. This paper focuses on analyzing the xenolith microstructures, major element compositions of the constituent minerals, T and stress estimates, olivine and pyroxene LPOs. The results are further used to explain the deformation mechanisms, determine the rheological state and constrain the geodynamic setting of the SCLM beneath Jiaohe.

1. GEOLOGICAL SETTING AND SAMPLES

Peridotite xenoliths hosted in the alkali basalts are widely distributed in northeastern China and most of them are located along the Tan-Lu fault zone (Fig. 1). Northeastern China is located in the eastern part of Paleozoic Central Asian Organic Belt and the Tan-Lu fault is a continental-scale strike-slip fault, which is considered to extend deeply into the lithospheric mantle (Zhu et al., 2018; Xu and Zhu, 1994). The development of the Tan-Lu fault could have influenced lithospheric mantle evolution in the region (Xiao et al., 2010). The Jiaohe volcanic field in Jilin Province contains abundant mantle xenoliths, which are erupted with Late Cenozoic basanite lavas aged at 2–24 Ma (Wang, 1996) near the northen part of Tan-Lu fault (Huang and Xu, 2010). Therefore, the sampled xenoliths likely record conditions similar to the current SCLM beneath Jiaohe. The sampling location, Dashihe in Jiaohe volcanic filed, is also known as one of the largest olivine mining areas in China.

Figure  1.  Simplified tectonic framework of northeastern China and mantle xenolith locations. The xenoliths in this study were collected from Jiaohe (modified after Hao et al., 2016; Yu et al., 2010; Zhang et al., 2007).

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Mantle xenoliths in Jiaohe volcanic filed are mainly composed of spinel-lherzolites, with few harzburgites and garnet pyroxenite (Hao et al., 2016; Yu et al., 2010, 2009; Li et al., 2002). Most of the peridotite xenoliths are fresh and show no visible or weak foliation. And, all the studied peridotite xenoliths in this research were collected form Dashihe area, Jiaohe City. As lherzolite is the possible main mineral assemblages in the upper mantle and Jiaohe volcanic filed, it is likely to record the deformation of SLCM in Jiaohe area. In this study, we selected several fresh medium- to large-grained spinel lherzolite xenoliths, 5–15 cm in diameter, from the Dashihe area for rheological and fabric analysis. The mineral assemblages are 50%–65% olivine (Ol), 5%–15% clinopyroxene (Cpx), 10%–25% orthopyroxene (Opx) and 1%–5% spinel (Sp). Olivine and orthopyroxene crystals have grain sizes commonly varying from ~1 to 5 mm. Different types of fluid inclusions are found in these lherzolite xenoliths (Fig. 2a), and they are interpreted as "initial" volatiles in the SCLM that are trapped during the SCLM evolution (Zhang et al., 2007).

Figure  2.  Detailed petrological and microstructural characteristics of the peridotite xenoliths from Jiaohe. (a) Fluid inclusion trail in a deformed olivine crystal in plane-polarized light. (b)–(d) the typical microstructures in the Jiaohe peridotite xenoliths with cross-polarized light. Subgrain boundaries (SGB) in olivine crystals and 1 200 triple junctions between crystals. (e)–(i) sub-microstructural characteristics of decorated olivine crystals from Jiaohe in plane-polarized light. Free dislocations, dislocation walls with different dislocation array inside, subgrain boundaries (SGB) and dislocation loops (some are helix formation), respectively.

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2. RHEOLOGICAL PROPERTIES OF JIAOHE XENOLITHS

2.1 Microstructures

The lherzolite xenoliths display primarily granuloblastic to porphyroclastic textures. Granuloblastic textures are characterized by triple junctions and straight grain boundaries (Fig. 2b); while porphyroclastic textures are characterized by porphyroclasts (> 1 mm) surrounded by small neoblasts (Fig. 2c). Brown spinel is observed as an isolated phase with frequent embayed grain boundaries (e.g., Fig. 2d). Olivine crystals commonly display undulatory extinction and kink band with curved or straight grain boundaries. Neoblastic olivine and orthopyroxene are polygonal with straight grain boundaries and triple junctions. A few samples show exsolution of clinopyroxene lamellae in orthopyroxene grains (Fig. 2d). Exsolutions of ferruginous lamellae in two perpendicular directions are also observed in olivine. Locally, pyroxenes form aggregates show xenomorphic texture. Some of the largest crystals are elongated in shape, with aspect ratios in the range of 2 : 1–4 : 1.

The oxidation decoration technique was adopted in this study to further study the sub-microstructures (dislocations) of xenoliths. The oxidation decoration technique is a simple but powerful method that is widely used to study the overall dislocation distribution of deformed olivines (Gan et al., 2011; Jin et al., 1989; Karato, 1987; Kohlstedt et al., 1976). Oxidation of iron-rich olivine was achieved by heating polished chips up to 900 ℃ in air for 1 h to render visible of dislocation substructures under an optical microscope. More details on the operating conditions used in this research have been described in Gan et al. (2011). Various dislocations structures are observed from our samples, including "free" dislocations that are less organized, dislocation walls with free dislocations inside (Fig. 2e), dislocation walls with elongated straight dislocation arrays (a series of high density dislocation lines are parallel to each other) inside (Fig. 2f), dislocation walls with bending dislocation arrays inside (Fig. 2g), subgrain boundaries constrained by two straight boundaries in vertical directions (Fig. 2h) and dislocation loops indicating multiple cross-slip to overcome barriers (Fig. 2i). The existence of dislocation arrays and dislocation walls indicates the decrease of free energy and the deformation by dislocation climb and recovery. The finding of dislocation loops in the samples suggests a relatively high temperature condition.

2.2 Deformation Conditions

2.2.1   Mineral chemistry and temperature estimates

The mineral chemistry data (Table 1) were obtained using a JEOL JXA-8100 type electron probe micro-analyzer (EPMA) at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan). In the lherzolites, neither olivine nor pyroxene shows significant compositional zoning. Major element compositions of olivines are relatively homogeneous with forsterite contents (Fo = 100 × Mg/(Mg + Fe)) ranging from 89 to 91. Orthopyroxenes and clinopyroxenes are classified as enstatite and diopside, respectively. The observed Cr-values (Cr^# = 100 × Cr/(Cr + Al)) of spinel grains range from 8 to 34.

Table  1.  Electron microprobe analyses (wt.%) of minerals of the Jiaohe peridotite xenoliths.

Sample	JH-0 (transitional)					JH-1 (granuloblastic)					JH-2 (transitional)					JH-3 (granuloblastic)
	Ol	Cpx	Opx	Sp		Ol	Cpx	Opx	Sp		Ol	Cpx	Opx	Sp		Ol	Cpx	Opx	Sp
SiO2	40.21	52.31	55.24	0.03		40.12	51.23	54.08	0.01		40.23	51.95	54.42	0.04		40.58	52.85	55.67	0.07
TiO2	0.02	0.28	0.09	0.16		0.02	0.49	0.10	0.06		0.03	0.29	0.09	0.14		0.02	0.17	0.06	0.14
Al2O3	0.05	4.98	3.08	44.73		0.04	6.71	4.09	59.75		0.04	5.03	3.74	50.37		0.03	3.45	2.55	39.54
Cr2O3	0.00	1.33	0.41	23.31		0.01	0.70	0.26	8.09		0.00	1.00	0.47	18.26		0.01	1.22	0.47	30.15
FeO	9.45	2.31	5.84	12.06		10.58	2.37	6.60	9.93		9.35	2.30	5.66	10.11		8.73	1.86	5.02	10.35
MnO	0.10	0.06	0.13	0.12		0.12	0.06	0.13	0.10		0.10	0.07	0.11	0.13		0.10	0.05	0.11	0.17
MgO	49.51	15.65	34.01	18.87		48.78	14.92	33.50	21.32		49.60	16.35	34.01	20.25		49.95	16.99	34.84	18.73
CaO	0.04	20.91	0.49	0.00		0.02	21.00	0.41	0.00		0.05	21.02	0.71	0.00		0.04	21.73	0.59	0.01
Na2O	0.00	1.51	0.05	0.00		0.00	1.66	0.04	0.01		0.00	1.20	0.06	0.01		0.00	0.89	0.04	0.00
K2O	0.00	0.01	0.01	0.00		0.00	0.00	0.00	0.00		0.01	0.01	0.00	0.00		0.00	0.01	0.01	0.01
Total	99.38	99.33	99.34	99.30		99.69	99.14	99.23	99.27		99.42	99.20	99.26	99.30		99.46	99.21	99.37	99.16
Mg#	90	92	91	74		89	92	90	79		90	93	91	78		91	94	93	76
Cr#				26					8					20					34
Sample	JH-4 (protogranular)					JH-5 (porphyroclastic)					JH-6 (porphyroclastic)					JH-7 (granuloblastic)
	Ol	Cpx	Opx	Sp		Ol	Cpx	Opx	Sp		Ol	Cpx	Opx	Sp		Ol	Cpx	Opx	Sp
SiO2	40.28	52.70	55.19	0.03		40.14	52.66	54.94	0.03		40.17	52.07	54.67	0.05		39.44	50.84	53.79	0.04
TiO2	0.02	0.26	0.08	0.13		0.01	0.10	0.05	0.08		0.03	0.44	0.11	0.11		0.03	0.65	0.14	0.15
Al2O3	0.03	4.93	3.12	44.51		0.04	3.51	2.96	45.56		0.04	5.71	3.91	54.93		0.15	6.83	4.46	58.91
Cr2O3	0.00	1.36	0.42	23.12		0.00	0.91	0.38	23.08		0.01	0.89	0.33	12.88		0.00	0.71	0.28	8.88
FeO	9.33	2.30	5.77	11.97		9.78	2.19	5.92	11.63		9.64	2.49	6.00	10.60		10.89	2.66	6.51	10.29
MnO	0.11	0.07	0.11	0.16		0.12	0.06	0.13	0.13		0.12	0.06	0.13	0.12		0.13	0.07	0.14	0.11
MgO	49.75	15.50	34.11	19.15		49.31	16.85	34.22	18.58		49.49	15.63	33.50	20.70		48.60	15.18	33.23	20.90
CaO	0.03	20.63	0.49	0.00		0.03	22.34	0.54	0.01		0.04	20.57	0.55	0.00		0.06	20.05	0.61	0.00
Na2O	0.00	1.56	0.06	0.01		0.00	0.62	0.02	0.02		0.00	1.42	0.05	0.00		0.01	1.79	0.10	0.01
K2O	0.00	0.00	0.00	0.00		0.00	0.01	0.00	0.01		0.01	0.00	0.01	0.00		0.01	0.00	0.00	0.00
Total	99.54	99.32	99.34	99.06		99.44	99.24	99.16	99.11		99.54	99.28	99.25	99.39		99.31	98.77	99.25	99.27
Mg#	90	92	91	74		90	93	91	74		90	92	91	78		89	91	90	78
Cr#				26					25					14					9
Ol. Olivine; Opx. orthopyroxene; Cpx. clinopyroxene; Sp. spinel. Mg# = 100 × Mg/(Mg + Fe); Cr# = 100 × Cr/(Cr + Al).

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The equilibrium temperatures for eight samples were calculated using several published two-pyroxene geo-thermometers (Bertrand and Mercier, 1985; Wells, 1977; Lindsley and Dixon, 1976; Wood and Banno, 1973) (Table 2). Two-pyroxene geo-thermometers are mainly based on Ca and Mg exchange between orthopyroxene and clinopyroxene pairs in peridotite xenoliths. Calculated equilibrium temperatures for spinel lherzolites in Jiaohe fall within a wide range of 754 to 1 073 ℃. Equilibrium temperatures vary little between samples when using the same thermometer (Table 2). The average equilibrium temperature is ~891–993 ℃ for all the samples, which is ~100 ℃ higher than that reported in Yu et al. (2009). The equilibration pressures were not estimated for Jiaohe spinel peridotite xenoliths, because geobarometers may be highly dependent on the temperature variations (Kil and Wendlandt, 2004). Thus, neither geobarometers for spinel peridotite nor the xenolith-based geotherm is constructed in this study. Nevertheless, we can still suggest that spinel lherzolites at Jiaohe originated from depths from 40 to 60 km based on the previous studies and stability field for spinel and garnet (Huang and Xu, 2010; Zhou et al., 2007).

Table  2.  Estimated equilibration temperatures (℃) in the spinel stability field for studied xenolith.

Sample	Fo	Wood and Banno (1973)	Lindsley and Dixon (1976)	Wells (1977)	Bertrand and Mercier (1985)	Average
JH-0	90	1 027	819	916	997	940
JH-1	89	983	754	874	952	891
JH-2	90	1 068	895	962	1 048	993
JH-3	91	1 073	889	957	1 016	984
JH-4	90	1 036	836	926	1 018	954
JH-5	90	1 045	854	937	989	956
JH-6	90	1 060	886	958	1 057	990
JH-7	89	1 036	851	936	1 048	968
Fo = 100 × Mg/(Mg + Fe).

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2.2.2   Stress estimation

The applied stress is considered to be related with density of dislocations in the deformation of crystal. Dislocation density, dynamically recrystallized grain size and subgrain size in peridotite xenoliths are three common ways to estimate the steady-state stress that is responsible for the deformation (Jin et al., 1989; Karato et al., 1980; Kohlstedt and Goetze, 1974). To estimate stress magnitude, the statistical subgrain size method was used in this study (Jin et al., 1989). This method not only takes advantage of oxidation decoration of the thin section, but also avoids/reduces the influence of annealing, static recovery, multiple deformation and lack of recrystallized grain. Subgrain wall spacings, (100) walls instead of (001) walls, were measured using intuitive distance tools in pdf from the photomicrographs of well-visible decorated olivine grains in plane-polarized light (e.g., Fig. 2h).

A total of ~134 measurements were obtained from selected xenoliths for (100) subgrain wall spacings, which show a log-normal distribution with geometric mean of 118 μm and arithmetic mean of 144 μm. The mean values correspond to the calculated stress for spinel lherzolites in Jiaohe fall within the range of 2.7–8.5 MPa (Table 3). The stress levels in Jiaohe are smaller than the typical values from other areas in the northeastern China, which are about 10–50 MPa (Jin et al., 1989).

Table  3.  Differential stresses estimated from subgrain wall spacings for studied xenolith using different paleopiezometer.

	Subgrain wall spacings (μm)	Stress (MPa)
		σ = 1 000/d	σ = (d/280)-1/0.67	σ = 100 × (d/15)-1/0.69
Arithmetic mean	117.8	8.5	3.6	5.0
Geometric mean	143.5	7.0	2.7	3.8
Paleopiezometer		Durham et al. (1977)	Karato et al. (1980)	Ross et al. (1980)
Where d is the subgrain boundary spacing in μm and σ is the stress in MPa.

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2.2.3   Water contents

Water content of minerals in the Jiaohe peridotite xenoliths has been measured and analyzed by Hao et al. (2016). The whole-rock water content in Jiaohe varies from 10 ppm–65 ppm. Whereas the water contents of Opx and Cpx vary from 14 ppm–72 ppm and 37 ppm–234 ppm, respectively. And, most of the coexisting olivines have no detectable OH peak indicating the average water content is ~0 ppm (Hao et al., 2016). Compared with ~0–2 300 ppm water content of olivine in the upper-mantle xenoliths (Wang, 2010; Ingrin and Skogby, 2000), the Jiaohe peridotite is considered to be deformed under a relatively dry condition.

2.3 Lattice Preferred Orientations

The lattice preferred orientation (LPO) data of olivines and pyroxenes were collected by an electron back-scattered diffraction (EBSD) detector attached to a Quanta 2000-type field emission scanning electron microscope (FE-SEM) at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences, Wuhan. The EBSD patterns are generated by the interaction of a vertical incident electron beam with a carefully polished sample set at an angle of 70° with respect to the incident electron beam. Diffraction patterns are processed and indexed using the CHANNEL 5.0 software distributed by HKL Technology. Olivine LPOs were measured on a grain-by-grain basis and in operator-controlled indexing mode in order to avoid pseudo-symmetry errors for olivine (e.g., Yamamoto et al., 2017; Vauchez et al., 2005).

We measured the LPOs of the samples regardless of thin section orientation and rotated the olivine LPOs relying on Cpx or rotated the maximum concentrations of olivine [100] axes parallel to X (E-W direction, lineation) for those without enough Cpx data (e.g., Yang et al., 2019; Behr and Hirth, 2014; Zaffarana et al., 2014; Soustelle and Tommasi, 2010). The rotation of datasets is based on the adjustment of orthogonal XYZ structural framework, with clustering of [001] and [010] of clinopyroxene in the X and Z direction respectively. The LPOs of olivine for the analyzed lherzolite with enough grains are shown in the pole figure and it shows predominantly A-type fabric (Jung et al., 2006) except sample JH-6 (Fig. 3). The type-A fabric is associated with plastic creep of olivine under low to moderate stress, low water contents and a wide range of temperature (e.g., Behr and Hirth, 2014). Specifically, the type-A fabric in this study is characterized by concentration of the [100] axes subparallel to the lineation (X), [010] axes subnormal to the foliation and generally more dispersed [001] axes (Fig. 3). However, JH-6 only has strong concentration of [100] axes. Although some previous studies have reported similar fabrics (e.g., Yamamoto et al., 2017), it has not yet been identified as a specific fabric. The olivine fabric strength, representing the intensity of the LPO, is also reported in this study and is calculated by Bunge's J-index using the program SuperJctfPC.exe courtesy of D. Mainprice. The J-index is equal to 1 for a random distribution and infinite for a single crystal. In our research, J indexes range between 2.1 and 5.1. And for the statistics of natural peridotites, J indexes range between 2 and 20, with a peak of ~8 (Soustelle et al., 2010; Ismaïl and Mainprice, 1998).

Figure  3.  LPOs pattern of olivine grains for lherzolite in Jiaohe. Pole figures are presented in the lower hemisphere by equal area projection of [100], [010], and [001] axes. N represents the number of analyzed grains. Numbers below the scale bar are the maximum concentration % of the axes and J is the fabric strength.

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3. DISCUSSION

3.1 Dislocation Microstructure and LPO

Understanding the relationship between LPO and deformation conditions is important to inferring geologically or geophysically significant conclusions (Wang Z H et al., 2020; Wang Y F et al., 2013; Karato, 2008). In general, the pattern of olivine LPOs is considered to be mainly controlled by deformation conditions such as temperature, pressure, stress and water concentration (Karato et al., 2008; Jung et al., 2006; Katayama et al., 2004). Meanwhile, the deformation conditions could also be recorded and revealed by studying microstructures of deformed minerals. In this study, we measured the olivine LPOs and also studied the microstructures of olivine related to deformation conditions which ensures a systematic understanding of the LPOs of olivine in Jiaohe.

The type-A olivine fabric is the predominant fabric observed in the Jiaohe lherzolites (Fig. 3) and also most common in natural samples. For the microstructures of olivine, the decorated method indicated a large variety of dislocation structures. The existing dislocation loops suggest activation of cross slip. Calculated stress from the subgrain wall spacings and computed temperature from the major elements are also consistent with the condition inferred from the measured A-type olivine fabric in this study. For the water concentration, although A-type olivine fabric can be developed in water-rich deep condition, it still can be taken as a dry fabric for shallow upper mantle (depths < 200 km) (Ohuchi and Irifune, 2013; Katayama et al., 2004), which is the case for the Jiaohe xenoliths. Due to the high H diffusion rate for olivine, the IR measured water content of olivine is relatively lower and may not represent the deformation condition (Park and Jung, 2015). However, pyroxenes in Jiaohe peridotite also indicate a low water concentration (Hao et al., 2016) suggesting water loss through olivine crystals may be not significant during the ascending-process. Regarding the deformation mechanism of olivine, deformation map is a commonly-used method that deformation mechanisms are plotted as a function of stress and grain size (e.g., Ohuchi et al., 2015; Wang et al., 2013; Wang, 2010). As grain sizes commonly vary from ~1 to 5 mm for the sampled olivine crystals, dislocation creep is considered as the dominant deformation mechanism for dry olivine when simply using the similar deformation map described in Chatzaras et al. (2015) (Fig. 4). Combined with the observations of dislocation structures from the samples, we can conclude that dislocation creep is the dominant deformation in olivine.

Figure  4.  Deformation mechanism map for dry olivine as a function of stress and grain size (map calculation after Chatzaras et al., 2015; the values of the flow law parameters for dry olivine from Hirth and Kohlstedt, 2003). Red solid lines represent the boundaries between two deformation mechanisms and the yellow hatched area represents the conditions indicated in this study.

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On the other hand, the relationship between different olivine LPO patterns and deformation conditions is still in debate. This is not only because new parameters such as melt content, pre-existing LPOs, confining pressure were found to be related to the olivine fabric (Kumamoto et al., 2019; Qi et al., 2018; Boneh and Skemer, 2014; Karato, 2008; Karato et al., 2008; Vauchez and Garrido, 2001), but also due to some new research progresses. For example, pervious experimental and theoretical studies indicated olivine LPOs is a function of water content and stress and a water-induced transition can occur between A-type fabric to E-type fabric (Jung et al., 2006; Katayama et al., 2004; Jung and Karato, 2001). However, most recent studies argued that water content may not clearly related to olivine fabric types (Bernard et al., 2019; Kumamoto et al., 2019; Bernard and Behr, 2017). Apart from that, there are various mechanisms to be responsible for olivine LPOs, such as deformation-induced recrystallization (Karato, 2008). These studies suggested that the different fabrics for JH-6 in our peridotite samples may likely be formed due to some unexpected physical parameters caused by a heterogeneous SCLM. However, further studies are needed for a more specific result. In a short summary, the observed results from dislocation microstructures and the olivine fabric data are consistent with the measured water content in Jiaohe, suggesting both methods are effective to constrain deformation conditions.

3.2 Deformation under Jiaohe SCLM

In general, the observed microstructure of dislocations change according to the latest deformation and the measured LPOs likely preserve signatures from the largest deformation stage and hard to be modified by subsequent annealing for deformed xenoliths. Both of them recorded the deformation conditions and allow us partially to reconstruct their deformation process. In this study, the observed microstructures are consistent with the LPOs features, indicating they could be resulted from the latest significant deformation event that affected the continental lithosphere. Therefore, we conclude that the major deformation prevailing at the base of the lithosphere beneath Jiaohe could have taken place at a low stress, medium temperature and dry conditions.

Furthermore, our observations provide more details for understanding the geodynamic evolution in the studied area. Previous studies suggest that mantle diapir and active shear zone related to Tan-Lu faults, may have occurred in the region based on various constraints including microstructural, geochemistry and P-T characteristics of peridotite xenoliths (Kuandian, Yitong, Liuhe, Beiyan) and P-wave tomography along Tan-Lu faults (Lei et al., 2020; Xiao and Zhang, 2011; Lin et al., 1992). Compared with the published xenoliths data in the northeastern China (Yu et al., 2010; Jin et al., 1989), our samples reveal a relatively higher temperature and lower stress conditions. Besides, the observation of variable amount of fluid inclusions and fine aggregates of orthopyroxene in the peridotite samples provide evidence for fluid-rock interaction and an evolution of the fluid transport process (Fig. 2a) (Zhang et al., 2007, 2004). These results may indicate that Tan-Lu fault could have altered the xenoliths and provided heat and fluids for the lithosphere deformation in Jiaohe. Further, observed A-type olivine LPO was considered to be everywhere in the upper mantle and parallel to the horizontal flow. And Tan-Lu fault is considered as lithospheric weak zone that is likely to distort the stress field locally. Thus, the observed low stress signature and A-type olivine fabric in the studied sample can also be explained by Tan-Lu fault zone.

4. CONCLUSIONS

We characterize and report the rheological properties of a suite of spinel lherzolite xenoliths from the subcontinental lithospheric mantle underneath Jiaohe area. Geochemistry studies and microstructural observations indicate the xenoliths have a relatively high equilibrium temperatures of ~891 to 993 ℃ and low stress condition of ~2.7–8.5 MPa. Dislocation microstructures on the olivine crystals are characterized by free dislocation, dislocation walls, dislocation loops and subgrains indicating a dominantly dislocation creep deformation, and are consistent with the observed type-A fabric of olivine. With the comprehensive analyses of the microstructures, LPOs, and the deformed temperature and stress, we conclude that the major deformation of SCLM has taken place at a low stress, medium temperature and dry condition in Jiaohe. We are of an option that the Tan-Lu fault might be responsible for the observed rheological and fabric properties of SLMC in Jiaohe.