# Zinc isotopic composition of the lower continental crust estimated from lower crustal xenoliths and granulite terrains

Ganglan Zhang, Yongsheng Liu, Frédéric Moynier, Yangtao Zhu, Zaicong Wang, Zhaochu Hu, La Zhang, Ming Li, Haihong Chen

► To cite this version:

Ganglan Zhang, Yongsheng Liu, Frédéric Moynier, Yangtao Zhu, Zaicong Wang, et al.. Zinc isotopic composition of the lower continental crust estimated from lower crustal xenoliths and granulite terrains. Geochimica et Cosmochimica Acta, 2020, 276, pp.92-108. 10.1016/j.gca.2020.02.030. insu-02916287

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# Zinc isotopic composition of the lower continental crust estimated from lower crustal xenoliths and granulite terrains

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Ganglan Zhang¹, Yongsheng Liu¹*, Frédéric Moynier¹, ², Yangtao Zhu¹, Zaicong Wang¹, Zhaochu Hu¹, La Zhang¹, Ming Li¹, Haihong Chen¹

¹ State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China

² Institut de Physique du Globe de Paris, Université Paris Diderot, Université Sorbonne Paris Cité, CNRS UMR 7154, 75238 Paris Cedex 05, France

*Corresponding author: yshliu@cug.edu.cn or yshliu@hotmail.com (Yongsheng Liu).

## Abstract

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This study presents high precision Zn stable isotope analyses for lower crustal rocks (9 granulites from Archean terrains and 30 lower crustal xenoliths) from the north margin of the North China Craton (NCC) to understand the behavior of Zn isotopes during deep crustal processes and the Zn isotopic composition of the lower continental crust (LCC). The lower crustal xenoliths range in composition from mafic to felsic with MgO contents of between 0.4 to 20.2 wt.%. The δ⁶⁶Zn (the permil deviation of the ⁶⁶Zn/⁶⁴Zn ratio from the JMC-Lyon standard) values of lower crustal xenoliths range from -0.17‰ to 0.38‰. The lack of clear correlation of δ⁶⁶Zn with geochemical indicators (e.g., Al₂O₃, Cr, Ba/La, ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd) indicates that assimilation of Precambrian lower crust, fluid metasomatism, and accumulation of pyroxene and plagioclase had a limited effect on Zn isotopic compositions of these
26 lower crustal xenoliths. The $\delta^{66}\text{Zn}$ values (0.18‰ to 0.34‰) of garnet-bearing mafic granulites decrease with increasing $\text{FeO(T)}$ and V contents, which are likely results of the accumulation of Fe-Ti oxides.

29 The average $\delta^{66}\text{Zn}$ of the lower continental crust is estimated to be 0.29 ± 0.02‰ (95% SE) using lower crustal xenoliths from the Neogene Hannuoba basalts. This $\delta^{66}\text{Zn}$ value is similar to the estimated value (0.28 ± 0.04‰, 95% SE) obtained from the granulites from Archean terrains, suggesting that there is no significant difference in the Zn isotopic composition between the Archean and present-day lower continental crust. Combining the $\delta^{66}\text{Zn}$ data of the lower crustal xenoliths and granulite terrains and different weighting methods, the Zn isotopic composition of the lower continental crust is estimated to be 0.28 ± 0.04‰ (95% SE).

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38 Keywords: Zn isotopes, lower continental crust, xenoliths, Fe-Ti oxides.

39 **1. Introduction**

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Zinc is a first group transitional element and has five natural stable isotopes ($^{64}\text{Zn}$, $^{66}\text{Zn}$, $^{67}\text{Zn}$, $^{68}\text{Zn}$ and $^{70}\text{Zn}$). Over the past twenty years, zinc isotopes have been widely applied to trace processes in a variety of environments (see Moynier et al. (2017) for a recent review). One aspect of Zn isotopes that remains to be better understood is the compositions of different reservoirs on the Earth. Chen et al. (2013) through the study of volcanic rocks from Kilauea Iki and Hekla, suggested that Bulk Silicate Earth (BSE) has $\delta^{66}\text{Zn}$ (permil deviation of the $^{66}\text{Zn}/^{64}\text{Zn}$ ratio from the JMC-Lyon standard) of 0.28 ± 0.05‰ (2SD). However, some recent studies on non-metasomatized peridotites, komatiites, and picrites have estimated values for the BSE between 0.16‰ to 0.20‰ (Wang et al., 2017; McCoy-West et al., 2018; Sossi et al., 2018). Compared to the relatively homogenous composition of the mantle, the continental crust exhibits larger Zn isotopic variation (-0.03‰ to 0.88‰; Telus et al., 2012; Xia et al., 2017; Doucet et al., 2018; Xu et al., 2019). A recent study on Archean crustal granitoids from the northern Kaapvaal Craton suggests that partial
melting of the early felsic crust does fractionate Zn isotopes by up to 0.15‰ (Doucet et al., 2018). Furthermore, fluid exsolution during the later stages of magmatic differentiation in the continental crust could form pegmatites with heavy Zn isotopic compositions (δ⁶⁶Zn from 0.53‰ to 0.88‰; Telus et al., 2012). Zinc isotopic compositions of the leucosome and melanosome phases in migmatites from the Dabei orogenic belt also indicate that the Zn isotopic compositions of leucosomes can be modified by fluid exsolution (Xu et al., 2019). Additionally, Little et al. (2016) suggest that the average isotopic composition of lithogenic Zn (δ⁶⁶Zn) is 0.27 ± 0.07‰ (1SD).

Although many studies have focused on mantle peridotites, mantle-derived melts and shallow crustal rocks, the Zn isotopic composition of the lower continental crust remains poorly constrained. To better understand the behavior of Zn isotopes during lower crustal processes and to constrain the Zn isotopic composition of the lower continental crust, it is necessary to conduct a detailed investigation of lower crustal rocks. Granulite-facies xenoliths brought to the Earth's surface by fast-rising magmas and exposed granulite terrains can provide valuable insight into the lower continental crust (Rudnick and Taylor, 1987; Rudnick and Fountain, 1995; Villaseca et al., 1999; Hacker et al., 2011; Rudnick and Gao, 2014 and references therein). However, major differences exist between these two types of lower crustal rocks. Granulite xenoliths are mostly found in Phanerozoic basalts or kimberlites and are dominated by mafic compositions, while exposed granulite terrains are mainly formed in the Archean and are richer in silica (e.g., Rudnick and Taylor, 1987; Rudnick and Fountain, 1995; Gao et al., 2000). In order to estimate the Zn isotopic composition of the lower continental crust and its possible evolution from the Archean to the Phanerozoic, it is necessary to study both types of lower crustal rocks.

Lower crustal xenoliths from the Neogene Hannuoba basalts and the nearby Archean granulite terrains from the north margin of the North China Craton provide an excellent opportunity to study the Zn isotopic composition of the lower continental crust. First, lower crustal xenoliths and Archean granulite terrains coexist in the same area (Gao et al., 2000; Zhai et al., 2001), which can be used to compare the Zn
isotopic composition of the present-day and Archean lower continental crust. Second, the different types of lower crustal xenoliths derive from different depths of the lower crust (Liu et al., 2001). Thus, the Zn isotopic composition of the lower continental crust can be estimated according to the proportions of the different xenoliths from different crustal depths. Finally, the lower crustal xenoliths range in composition from mafic to felsic and have been previously well characterized (major and trace elements, Sr-Nd-Pb-Li-Mg isotopes, and U-Pb zircon ages; Gao et al., 2000; Chen et al., 2001; Liu et al., 2001, 2004, 2010; Zhou et al., 2002; Wilde et al., 2003; Teng et al., 2008; Zheng et al., 2009; Jiang et al., 2010; Wei et al., 2015; Yang et al., 2016). Here, 30 lower crustal xenoliths from the Neogene Hannuoba basalts and 9 granulites from Archean terrains were analyzed to determine the behavior of Zn isotopes during lower crustal processes and estimate the average Zn isotopic composition of the lower continental crust.

## 2. Geological setting and sample descriptions

### 2.1 Geological setting

The North China Craton (NCC), with the remnants of 3.8 Ga crust (Liu et al., 1992), is one of the oldest continental blocks in the world. The NCC is bounded to the north by the Central Asian Orogenic Belt, the southwest by the Qinling-Dabie Belt, and the southeast by the Sulu Belt (Wang and Mo 1995) (Fig. 1). Based on ages, lithological assemblages, tectonic evolution, and P-T-t paths, the NCC can be divided into the Eastern Block, the Trans-North China Orogen, and the Western Block (Zhao et al., 2001). The cratonization of NCC was achieved by the amalgamation of the Eastern Block and the Western Block along the Trans-North China Orogen in the late Paleoproterozoic (Zhao et al., 2001). From the Paleozoic to early Triassic, the subduction of the Paleo-Asian Ocean Plate in the north and the Paleo-Tethys Ocean Plate in the south resulted in the NCC forming part of the eastern Asian continent (Wang and Mo 1995). During the Mesozoic to Cenozoic, the eastern part of the NCC
111 was strongly affected by subduction of the Pacific Ocean Plate and experienced
112 widespread tectonothermal reactivation, reflected by voluminous Mesozoic igneous
113 intrusions, Cenozoic intraplate basalts and metamorphic core complexes in eastern
114 China (Wu et al., 2019 and references therein). The modern geophysical data indicate
115 that the eastern part of the NCC has undergone large-scale lithospheric thinning and
116 that ~100 km of the lithospheric root has been removed (Wu et al., 2019 and
117 references therein).

118 The Huai'an complex is located in the northern segment of the NCC and
119 represents the first locality where Precambrian high-pressure mafic granulite terrains
120 in the NCC were discovered. The Huai'an complex is mainly composed of
121 mafic-intermediate granulites, pelitic granulites, and tonalitic gneisses (Gao et al.,
122 2000). The zircon U-Pb ages for the Huai'an complex indicate that the protoliths of
123 igneous granulites and gneisses were formed in the Neoarchean (2.55-2.50 Ga) and
124 underwent granulite-facies metamorphism at ~1.85 Ga (Guo et al., 2005; Zhao et al.,
125 2008; Liu et al., 2012). The ~2.5 Ga juvenile crust-forming event was likely related to
126 a continental collision process (Zhai et al., 2001). The metamorphic age of ~1.85 Ga
127 was caused by the final amalgamation between the Eastern and Western Block (Zhao
128 et al., 2008).

129 The Neogene Hannuoba basalts are widely distributed along the northern
130 boundary of the NCC (Fig. 1) and comprise alternating layers of alkaline and
131 tholeiitic basalts (Zhi et al., 1990; Qian et al., 2015). The volcanism was related to the
132 widespread Cenozoic rifting in the NCC (Gao et al., 2000). A substantial number and
133 variety of upper mantle and lower crust xenoliths are included within the alkaline
134 basalts, including peridotites, pyroxenites, and granulites (Song and Frey, 1989;
135 Tatsumoto et al., 1992; Chen et al., 2001; Liu et al., 2001, 2004, 2010; Xu, 2002;
136 Rudnick et al., 2004).

137 **2.2 Sample descriptions**

138 All the lower crustal xenoliths were collected from two locations: Damaping and
Zhouba (Fig. 1). Xenoliths are 4 to 20 cm in diameter. The lower crustal xenoliths show a broad range of chemical compositions from mafic to felsic with MgO contents of 0.4 to 20.2 wt.% (Liu et al., 2001; Fig. 2a). The lower crustal xenoliths include pyroxenites, two-pyroxene mafic granulites, garnet-bearing mafic granulites, plagioclase-rich mafic granulites, intermediate granulites, felsic granulites, and metasediments (Liu et al., 2001). The pyroxenites and two-pyroxene mafic granulites are composed of orthopyroxene (0-29%), clinopyroxene (49-86%) and plagioclase (0-44%), and are free of Fe-Ti oxides. The garnet-bearing mafic granulites comprise orthopyroxene (0-10%), clinopyroxene (20-40%), plagioclase (0-15%), garnet (30-60%) and Fe-Ti oxides (0-15%; Fig 3). The plagioclase-rich mafic, intermediate and felsic granulites have high modal plagioclase contents (50-80%), and minor clinopyroxene, orthopyroxene, K-feldspar, apatite, quartz, and zircon. Two xenoliths (ZB19 and ZB22) represent granulite-facies metasediments, supported by the occurrence of graphite and sillimanite (Liu et al., 2001). Detailed mineral assemblages of the lower crustal xenoliths can be found in Table S1.

The continuous layers of alternating plagioclase and pyroxene (cumulate textures) preserved in some xenoliths point to their origin as products of fractional crystallization in a magma chamber (Liu et al., 2001). The progressive decrease in MgO contents and increase in Al₂O₃ contents from the mafic to intermediate xenoliths indicate that these samples transition from pyroxene-dominated to plagioclase-dominated (Fig. 2a, b) (Liu et al., 2001). The garnet-bearing mafic granulites show LREE-enriched REE patterns and high modal garnet contents (30-60%), indicating that the garnets were formed during high-pressure metamorphism rather than crystal accumulation (Liu et al., 2010). Furthermore, their extremely high Sr contents (400 to 1108 ppm) and positive Eu anomalies suggest that they were high-pressure metamorphic products of plagioclase-rich cumulates (Liu et al., 2010). Also, the accumulation of Fe-Ti oxides results in higher TiO₂ (0.55 to 2.66 wt.%) and FeO(T) (8.9 to 15.8 wt.%) contents in the garnet-bearing mafic granulites relative to other lower crustal rocks without Fe-Ti oxides (Fig. 2c). The lower crustal xenoliths with Fe-Ti oxides are labeled in Table 1.
The zircon U-Pb ages and Sr-Nd isotopic compositions of the lower crustal xenoliths indicate that their protoliths formed in three episodes: ~2.5 Ga, 1.8-1.9 Ga, and 97-158 Ma (Liu et al., 2004). Some intermediate (DMP27 and DMP70) and felsic granulites (ZB20 and ZB21) have ages of ~2.5 Ga and 1.8-1.9 Ga, respectively (Liu et al., 2004). The ~2.5 Ga intermediate and 1.8-1.9 Ga felsic granulites are characterized by radiogenic Sr and unradiogenic Nd isotopic compositions ($^{87}$Sr/$^{86}$Sr: 0.707340-0.746240; $^{143}$Nd/$^{144}$Nd: 0.511125-0.511463) (Liu et al., 2004). These geochemical characteristics indicate that the ~2.5 Ga intermediate and 1.8-1.9 Ga felsic granulites represent the Precambrian lower crustal components preserved in the present-day lower crust of the NCC (Liu et al., 2004). The zircons from the other mafic and intermediate xenoliths contain two age populations of 160-140 Ma and 140-80 Ma, corresponding to the older cores and new metamorphic rims of zircons, respectively (Liu et al., 2004). Conversely, the Mesozoic xenoliths show relatively unradiogenic Sr (0.705614 to 0.709210) and radiogenic Nd (0.511651 to 0.512173) isotopic compositions, which are interpreted as cumulates that crystallized from the mixing of Mesozoic mantle-derived mafic melts with pre-existing lower crust (Liu et al., 2004, 2010) (Fig. 2d). These Precambrian protoliths and Mesozoic cumulates underwent 80-150 Ma granulite-facies metamorphism because of heating from mantle-derived basaltic melt underplating (Liu et al., 2004).

The derivation depths of the lower crustal xenoliths can be roughly estimated by correlation of seismic velocities determined for the xenoliths with regional seismic refraction data (Table S1) (Liu et al., 2001). The pyroxenites and garnet-bearing mafic granulites have P-wave velocities ($V_p$; 7.19-7.82 km/s) between the lowermost crust and upper mantle in the Hannuoba area. Calculated $V_p$ (6.64-7.39 km/s) of the two-pyroxene and plagioclase-rich mafic granulites are similar to the observed $V_p$ of the lowermost crust. The intermediate and felsic granulites have $V_p$ of 6.47-6.75 km/s and 6.36-6.52 km/s, corresponding to the middle and upper part of the lower crust, respectively. In general, these lower crustal xenoliths from the Neogene Hannuoba basalts represent the present-day lower crust of the NCC.

The rock sequence from the Huai'an complex represents an exposed lower crust
section (Zhai, 1996). Nine samples that were collected from the Manjinggou-Wayaokou lower crust section include mafic, intermediate, and felsic granulites as well as metasediments (Gao et al., 2000) (Fig. 1). The granulites from the Manjinggou area are dominated by mafic compositions, while intermediate-felsic granulites are the dominant rock types in the Wayaokou area. In addition to pyroxene and plagioclase, the mineral assemblages of the granulites from exposed terrains also contain H₂O-bearing minerals (amphibole and/or biotite) (Table S1). The granulite samples from the Manjinggou-Wayaokou section show a systematic decrease in pressure from 12-14 kbar to ~9 kbar, corresponding to crustal depths of 44-52 km to ~32 km (Zhai et al., 2001). The formation of the Archean lower crust in this area was related to a Neoarchean continental collision process and uplifting to surface in the late Paleoproterozoic (Zhai et al., 2001).

## 3. Analytical methods

### 3.1 In-situ major and trace elements

The in-situ major and trace element (V, Cr, Ni, and Zn) compositions of oxide minerals in the garnet-bearing mafic granulites were analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS; GeoLas 2005 + Agilent 7500a) with a spot size of 44µm at the China University of Geosciences (Wuhan). The element contents were calibrated against BCR-2G, BHVO-2G and BIR-1G (USGS reference glasses) without applying internal standardization (Liu et al., 2008a). The time-drift correction and quantitative calibration were processed using ICPMSDataCal software (Liu et al., 2008a). The results for the MPI-DING reference glasses (GOR132-G, ML3B-G, and KL2-G) generally agree with recommended values within 5% (relative deviation, RD) for major elements and 8% (RD) for trace elements (V, Cr, Ni, and Zn) (Table S2).
224 **3.2 Whole-rock trace elements**

The trace element concentrations of whole rocks were analyzed with an Agilent 7700 ICP-MS at the China University of Geosciences (Wuhan). The detailed sample-digestion procedures were modified from Liu et al. (2008b). About 50 mg of sample powders were weighted into Teflon bombs, and 1 ml of concentrated $HNO_3$ and 1 ml of concentrated HF were added. The sealed bombs were heated at 190°C in the oven for 48h. After cooling, the bombs were opened and evaporated to dryness at 120°C. Then, 1ml of concentrated $HNO_3$ was added to the samples and evaporated to dryness again. The samples were re-dissolved by adding 3ml of 30% $HNO_3$. Next, the bombs were resealed and heated at 190°C in the oven for 12h. The final solutions were diluted with ~100g of 2% $HNO_3$ for ICP-MS analysis. Replicate analyses give a precision better than 7% (RSD) (Table S3). The measured values for the reference materials (BHVO-2, BCR-2, AGV-2) are consistent with the recommended values within 6% (RD) for nearly all elements in Table S4, but the relative differences for Ce and Pr are 5.5-7.8% (Table S4).

239 **3.3 Whole-rock Zn isotopes**

For Zn isotopic analysis, around 20 to 40 mg of sample powders were dissolved in Savillex beakers using double-distilled concentrated HF + $HNO_3$ (1:1), HCl + $HNO_3$ (3:1) by heating at 120°C on a hotplate for 24 and 48 hours, respectively. All main minerals (e.g., pyroxene, plagioclase, garnet, and Fe-Ti oxide) in these sample powders were completely digested, and clear solutions were obtained. After evaporation, the samples were re-dissolved in 1ml of 8 mol/L HCl + 0.001% $H_2O_2$ in preparation for column chemistry. Then, efficient separation of Zn was achieved by anion exchange chromatography with 2 ml of pre-cleaned AG-MP-1M anion resin. First 38 ml of 8 mol/L HCl + 0.001% $H_2O_2$ and 20 ml of 0.5 mol/L HCl were added to elute the matrix elements (e.g., Na, Mg, Al, Fe, Ti, Ni, Cu, Ba) before collecting the Zn in the last 10 ml of 0.5 mol/L $HNO_3$. The procedures of column chemistry were following the protocols established by Liu et al. (2014, 2016), which were modified
after Maréchal et al. (1999). The Zn yields, based on analyses of Zn contents in all the elution collected before and after the Zn cut, are >99%. The total blank of the procedure was less than 3 ng, which is negligible compared with the ~1.2 µg Zn in the loaded samples.

Zinc isotope ratios were measured using a Nu Plasma 1700 multi-collector (MC)-ICP-MS at the China University of Geosciences (Wuhan). The instrument is equipped with Cetac ASX-112FR automatic sampler, a Glass Expansion nebulizer system, and standard and skimmer cones. Copper was added and used in conjunction with sample-standard bracketing for mass bias correction (Maréchal et al., 1999), and a standard solution prepared from NIST SRM 683 was used for this purpose ($\delta^{66}\text{Zn} = 0.12 \pm 0.04\‰$ (N=295) at the 95% confidence level; Chen et al., 2016; Yang et al., 2018). The Zn concentrations of the samples were checked and adjusted to match the SRM 683 solution within 10%. Zinc solutions were introduced to the Ar plasma using a nebulizer with an uptake rate of ~100 µl min$^{-1}$.

In total, seven masses ($^{60}\text{Ni}$, $^{63}\text{Cu}$, $^{64}\text{Zn}$, $^{65}\text{Cu}$, $^{66}\text{Zn}$, $^{67}\text{Zn}$ and $^{68}\text{Zn}$) were collected using Faraday cups L6, L4, L3, L2, Ax (C), H2 and H4, respectively, and all Faraday amplifiers used $10^{11} \Omega$ resistors. Among them, $^{60}\text{Ni}$ was used to monitor the interference of $^{64}\text{Ni}$ on $^{64}\text{Zn}$, and the results show that $^{64}\text{Ni}$ has a limited effect on $^{64}\text{Zn}$ ($^{60}\text{Ni}/^{64}\text{Zn} < 5 \times 10^{-4}$). The samples were analyzed in low-resolution mode at a Zn concentration of 300 ppb, which yielded a signal intensity of ~4V for $^{64}\text{Zn}$. Each analysis consisted of the integration of 25 cycles of 8 seconds. Finally, zinc isotope ratios are reported in standard $\delta$-notation in permil relative to JMC Lyon 3-0749L: $\delta^{\text{X}}\text{Zn}(\text{‰}) = [({^{\text{X}}\text{Zn}}/{^{64}\text{Zn}})_{\text{sample}}/({^{\text{X}}\text{Zn}}/{^{64}\text{Zn}})_{\text{JMC Lyon 3-0749L}} - 1] \times 1000$, where X = 66 and 68.

International standards and replicate analyses were used to test the accuracy and precision of the method. The long-term reproducibility is better than 0.05‰ (2SD) for $\delta^{66}\text{Zn}$ data based on the BHVO-2 analyses (n=16) over 12 months (Table S5). The results for standard samples (BHVO-2, AGV-2, BIR-1, and W-2) are consistent with previous studies and recommended values (Moynier et al., 2017) within uncertainty (see Table S5). All replicates of samples (n=8), digested from different aliquots of
sample powder, show identical δ⁶⁶Zn values within the long-term reproducibility (Table 1). The δ⁶⁶Zn and δ⁶⁸Zn values of all samples analyzed here fall onto a mass-dependent fractionation line with a slope of 1.99 (Fig. S1).

## 4. Results

### 4.1 Major and trace elements

Whole-rock trace element concentrations are provided in Table S3. The Zn contents (51.7 to 91.2 ppm) of garnet-bearing mafic granulites show strong positive correlations with FeO(T) (8.9 to 15.4 wt.%) and V contents (135 to 563 ppm) (Fig 4a, b).

The results for oxide minerals from garnet-bearing mafic granulites indicate that they are mainly composed of iron and titanium, and the TiO₂ and FeO(T) contents show extensive ranges of variation with 21.6 wt.% to 52.4 wt.% and 38.4 wt.% to 68.6 wt.% (Table S6), respectively. All of the Fe-Ti oxides have significantly higher Zn (144 to 754 ppm) and V (1692 to 4651 ppm) concentrations compared with the compositions of the respective whole rocks (Zn: 69.0 to 91.2 ppm; V: 375 to 563 ppm).

### 4.2 Zn isotopes

The δ⁶⁶Zn values of all xenoliths from the Hannuoba basalts range from -0.17‰ to 0.38‰, but the range is much narrower (0.18‰ to 0.38‰) for 28 out of 30 samples analyzed (Table 1; Fig. 5). The two outliers include a garnet-bearing mafic granulite (DMP133) and an intermediate granulite (DMP01), both having low δ⁶⁶Zn values of -0.17‰ and 0.06‰, respectively. The pyroxenites and two-pyroxene mafic granulites have δ⁶⁶Zn from 0.21‰ to 0.30‰. The δ⁶⁶Zn values of garnet-bearing mafic granulites (except for DMP133) vary from 0.18‰ to 0.34‰ and show negative correlations with FeO(T) (8.9 to 15.4 wt.%) and V contents (135 to 563 ppm) (Fig. 4c, d). The Zn isotopic compositions of plagioclase-rich mafic granulites, intermediate
308 granulites (except for DMP01), and felsic granulites cluster with an average δ<sup>66</sup>Zn of
309 0.30 ± 0.05‰ (2SD, n=9). Two metasediment xenoliths display indistinguishable
310 δ<sup>66</sup>Zn values (0.23‰ and 0.38‰) with metaigneous samples (Table 1; Fig. 5).

311 Nine granulites from Archean terrains have δ<sup>66</sup>Zn values from 0.13‰ to 0.36‰
312 (Table 1), and their range overlaps with the lower crustal xenoliths (Fig. 5). Three
313 metasediments from Archean terrains have δ<sup>66</sup>Zn values of 0.13‰, 0.30‰, and
314 0.31‰ (Table 1).

315 **5. Discussion**

316 The formation and evolution of the lower continental crust involved many
317 geological processes, including, for example, ancient crustal assimilation, fluid
318 metasomatism, addition of supracrustal sediments, magmatic differentiation and
319 mineral accumulation. These geological processes may affect the Zn isotopic
320 composition of the lower continental crust. Here, the Zn isotopic composition of the
321 lower continental crust is estimated using lower crustal xenoliths and Archean
322 granulite terrains. Finally, the differences in Zn isotopic compositions between the
323 lower continental crust and shallow continental crust and lithospheric mantle are
324 discussed.

325 **5.1 Behavior of Zn isotopes during lower crustal processes**

326 **5.1.1 Assimilation of Precambrian lower crust**

327 The assimilation of exogenous crustal materials during magma differentiation
328 could modify the stable isotopic composition of the magma system (Schoenberg and
329 von Blanckenburg, 2006). The large variations in Nd and Sr isotopic compositions of
330 the Mesozoic lower crustal xenoliths (Fig. 2d) were considered to reflect mixing
331 between mantle-derived melts with the Precambrian lower crust of the NCC (Liu et al.,
332 2004). Also, the Precambrian lower crust should have unradiogenic Nd and radiogenic
333 Sr isotopic compositions, such as sample DMP27 (¹⁴³Nd/¹⁴⁴Nd: 0.511125; ⁸⁷Sr/⁸⁶Sr:
0.709050; Liu et al., 2004). However, no correlation between δ⁶⁶Zn and ¹⁴³Nd/¹⁴⁴Nd or ⁸⁷Sr/⁸⁶Sr in the lower crustal xenoliths is observed (Fig. 6a, b). The Sr and Nd isotopic compositions of two samples (DMP01 and DMP133) with low δ⁶⁶Zn values (0.06‰ and -0.17‰) are not different from other lower crustal xenoliths (Fig. 6). This evidence suggests that assimilation of Precambrian lower crust had a limited effect on the Zn isotopic compositions of the lower crustal xenoliths.

## 5.1.2 Fluid metasomatism

Fluids play a vital role in the composition and evolution of the lower continental crust, including nonpolar gases, salts, H₂O, and rock-derived solutes (Manning, 2018). For example, light Mg isotopic compositions of some lower crustal samples were interpreted as reflecting interaction with fluids (Wang et al., 2016; Yang et al., 2016). The loss-on-ignition (LOI) can give the relative quantity of volatile materials in rocks. Furthermore, the impact of fluid metasomatism on the lower crustal rocks can be traced using Ba/La ratios, as Ba is more mobile than La in fluids, but they have similar compatibilities during magma differentiation (Kogiso et al., 1997). The lower crustal xenoliths with light Mg isotopic compositions (-0.74‰ to -0.56‰; Fig. 7a) and high LOI (2.9 wt.% to 4.8 wt.%; Fig. 7b) were attributed to interactions with isotopically light fluids (Yang et al., 2016). However, their δ⁶⁶Zn values are indistinguishable from some samples with lower LOI and higher δ²⁶Mg values (Fig. 7a, b). There is also no trend between δ⁶⁶Zn and Ba/La ratios (Fig. 7c). In general, no substantial impact from fluid metasomatism on δ⁶⁶Zn in these lower crustal xenoliths was observed, even among lower crustal xenoliths with relatively high LOI or low δ²⁶Mg values.

## 5.1.3 Magmatic differentiation and mineral accumulation

### 5.1.3.1 Pyroxene and plagioclase

The cumulate textures preserved in some xenoliths are likely to be the result of
fractional crystallization in a magma chamber (Liu et al., 2001). The progressive
decrease in MgO and Cr contents and increase in Al₂O₃ contents from the mafic to
intermediate granulites indicate that these samples transition from
pyroxene-dominated to plagioclase-dominated (Liu et al., 2001). The pyroxenites and
two-pyroxene mafic granulites are composed of orthopyroxene (Opx), clinopyroxene
(Cpx) and plagioclase (Pl), and are free of Fe-Ti oxides (Table S1). The
well-constrained partition coefficients for Zn between minerals and silicate melt
(DOpx-melt=0.451, DCpx-melt=0.333, and DPl-melt=0.13; Paster et al., 1974; Davis et al.,
2013) indicate that Opx, Cpx, and Pl can be the major Zn-bearing phases in the
pyroxenites and two-pyroxene mafic granulites, depending on the modal abundances
of these minerals. The Eu²⁺ can substitute for Ca²⁺ and lead to its fractionation from
Sm and Gd in plagioclase (Drake and Weill, 1975), and Cr is a strongly compatible
element in pyroxene (Dunn and Sen, 1994). Therefore, the effect of fractional
crystallization of pyroxene and plagioclase on the Zn isotopic compositions of the
lower crustal rocks can be evaluated by comparing δ⁶⁶Zn with Cr and Eu/Eu*.
Pyroxenites and two-pyroxene mafic granulites, however, do not show any correlation
of δ⁶⁶Zn with magmatic differentiation indexes (Al₂O₃, Cr and Eu/Eu*) (Fig. 8),
suggesting that limited Zn isotopic fractionation occurs during fractional
crystallization and accumulation of pyroxene and plagioclase in the lower continental
crust.

380 5.1.3.2 Iron-titanium oxides

381 The garnet-bearing mafic granulites comprise orthopyroxene, clinopyroxene,
382 plagioclase, garnet and Fe-Ti oxide (Fig. 3), and are free of olivine (Table S1). These
383 rocks are high-pressure metamorphic products of low-pressure plagioclase-rich
384 cumulates, and the garnets were formed during high-pressure metamorphism rather
385 than crystal accumulation (Liu et al., 2010). The garnet-bearing mafic granulites have
386 high and varying TiO₂ and FeO(T) contents (Fig. 2c), consistent with the different
387 modal contents of Fe-Ti oxides (0-15%) in these samples. The chemical compositions
and mineral modal contents indicate that the garnet-bearing mafic granulites have
388
undergone different degrees of Fe-Ti oxide accumulation. Thus, the Zn isotope studies
389
of these samples can help to evaluate the effect of Fe-Ti oxides on the Zn isotopic
390
composition of the lower crust.
391

As FeO(T) and V contents increase in the garnet-bearing mafic granulites, Zn
392
contents increase and δ66Zn values decrease gradually (except for DMP133) (Fig. 4).
393
Considering that the Fe-Ti oxides have high FeO(T), V, and Zn contents relative to the
394
whole rocks (Table S6), precipitation of Fe-Ti oxides may be responsible for driving
395
the isotopic fractionation observed within these granulites (Fig. 4c, d). Two
396
end-member mixing calculations (see Supplementary materials) show that both Zn
397
concentrations and Zn isotopic compositions of the garnet-bearing mafic granulites
398
can be reproduced by 0-16% accumulation of Fe-Ti oxides with αoxide-melt of 0.9998
399
(Fig. 4c, d), and suggest that the effect of the accumulation of Fe-Ti oxides on the
400
δ66Zn values of lower crustal cumulates will not exceed 0.2‰. Collectively, Fe-Ti
401
oxide accumulation can modify the Zn isotopic compositions of cumulates towards
402
lower δ66Zn values relative to primitive melt, as previously suggested in Chen et al.
403
(2013).
404

### 5.1.4 Addition of upper crustal sediments into the lower crust

Upper crustal sediments can be transported to lower crustal depths in a variety of
406
ways (see Fig. 10 in Chin et al. (2013)), mainly including continental underthrusting
407
(DeCelles et al., 2009), viscous return flow of wallrock during pluton ascent (Saleeby
408
et al., 2003), and underplating by slab-derived sediment diapirs (Behn et al., 2011;
409
Hacker et al., 2011). Sediments show a wide range of δ66Zn from -0.51‰ to 1.56‰
410
(Pichat et al., 2003; Andersen et al., 2011; Hendry and Andersen, 2013; Little et al.,
411
2016) (Fig. 5). Regardless of what mechanism transported the sediments into the LCC,
412
the fact that they are present at lower crustal depths suggests that the Zn isotopic
413
composition of the LCC may be affected by metasediments.
414

The metasedimentary samples from the Hannuoba basalts and granulite terrains
415
have δ⁶⁶Zn values from 0.13‰ to 0.38‰, and their range overlaps with the metaigneous granulites (Fig. 9). Partial melts from metasedimentary rocks, albeit in small proportions, may also influence the Zn isotopic compositions of the lower crustal rocks. Compared to most of the lower crustal xenoliths (with δ⁶⁶Zn of 0.18‰ to 0.38‰), one intermediate granulite (DMP01) and a garnet-bearing mafic granulite (DMP133) have distinct lower δ⁶⁶Zn values with 0.06‰ and -0.17‰, respectively. As discussed above, the effect of fluid metasomatism and fractional crystallization of silicate minerals (pyroxene and plagioclase) do not cause such anomalous δ⁶⁶Zn values. Considering that sample DMP01 and DMP133 are free of Fe-Ti oxides, the distinct Zn isotopic compositions of DMP01 and DMP133 may be due to protolith heterogeneity, such as the assimilation of partial melts from lower crustal metasediments. Collectively, the addition of upper crustal sediments into the lower crust may cause Zn isotopic heterogeneity in the lower continental crust, but more research is needed on lower crustal rocks to confirm this viewpoint.

## 5.2 Zinc isotopic composition of the lower continental crust

In general, granulite-facies xenoliths brought to the Earth's surface by fast-rising magmas and uplifted granulite terrains are employed to unravel the composition of the lower continental crust (Rudnick and Taylor, 1987; Rudnick and Fountain, 1995; Villaseca et al., 1999; Rudnick and Gao, 2014 and references therein). The lower continental crust consists of mixed felsic to mafic granulites and exhibits an increase in the proportion of mafic rocks with increasing depth (Rudnick and Fountain, 1995; Rudnick and Gao, 2014 and references therein). The different types of lower crustal rocks from different depths can be used to estimate the average Zn isotopic composition of the lower continental crust. The derivation depths of the lower crustal xenoliths from the Hannuoba basalts can be roughly estimated by correlation of seismic velocities determined for the xenoliths with regional seismic refraction data (Liu et al., 2001). The pyroxenites and garnet-bearing mafic granulites are likely to be derived from the crust-mantle boundary. The two-pyroxene and plagioclase-rich mafic
granulites are from the lowermost crust. The intermediate and felsic granulites represent the middle and upper part of the lower crust in the Hannuoba area, respectively (Liu et al., 2001).

The average Zn isotopic compositions of the different layers of the lower crust and bulk lower crust were calculated by weighting the proportions of the different lithologies and their average δ⁶⁶Zn values and Zn concentrations (excluding DMP01 and DMP133; Table 2, S7). A standard deviation estimates the variability among sample measurement; instead, the standard error of the mean provides an estimate of the uncertainty around the population mean (Altman and Bland, 2005). Therefore, the 95% standard error (95% SE) was used to reflect the uncertainty of the average δ⁶⁶Zn value of the lower crust at the 95% confidence level (Table 2, S7). The estimated values of δ⁶⁶Zn for the lowermost crust, middle lower crust and upper lower crust are 0.26 ± 0.03‰ (95% SE), 0.28 ± 0.02‰ (95% SE) and 0.32 ± 0.09‰ (95% SE), respectively (Table S7; Fig. 10). As the depth decrease, the different layers of the lower crust display limited Zn isotopic variation within uncertainties (Fig. 10). Using the approach described above, a δ⁶⁶Zn value for the present-day lower crust of 0.29 ± 0.02‰ (95% SE) is estimated by using lower crustal xenoliths from the Neogene Hannuoba basalts (Table S7). Furthermore, a similar δ⁶⁶Zn value for the Archean lower crust (0.28 ± 0.04‰, 95% SE) is obtained from granulites from Archean terrains (Table S7), suggesting that there is no significant difference in the Zn isotopic composition between the present-day and Archean lower continental crust of the NCC.

Since the lithological composition of the lower continental crust can vary considerably between different tectonic settings (e.g., shields and extensional regions) (Rudnick and Fountain, 1995), different models were used to calculate the average Zn isotopic composition of the bulk lower crust (Table 2). The average δ⁶⁶Zn values for the mafic, intermediate, felsic and sedimentary components of the lower continental crust were taken from the lower crustal xenoliths and granulite terrains (Table 2). The lower crustal types with different lithological proportions show limited variation in Zn isotopic composition (δ⁶⁶Zn ranging from 0.28 ± 0.02‰ (95% SE) to 0.27 ± 0.03‰)
(95% SE); Table 2). The consistency of these estimated δ<sup>66</sup>Zn values gives confidence that the average δ<sup>66</sup>Zn value (0.28 ± 0.04‰, 95% SE) calculated using various methods is a reasonable estimate of the Zn isotopic composition of the lower continental crust. The δ<sup>66</sup>Zn value (0.28 ± 0.04‰, 95% SE) of the lower continental crust is statistically higher than results for lithospheric peridotites in the NCC (0.18 ± 0.04‰, 95% SE; Wang et al., 2017) and other localities (Doucet et al., 2016; Huang et al., 2018a; Sossi et al., 2018), but similar to the Zn isotopic compositions of oceanic basalts (Herzog et al., 2009; Chen et al., 2013; Wang et al., 2017; Huang et al., 2018b) (Fig. 9).

A non-modal melting model shows that the magnitude of Zn isotopic
fractionation between melt and its complementary residue (Δ<sup>66</sup>Zn<sub>melt-mantle</sub>) is 0.042‰
for 25% partial melting in the garnet stability at 1400°C (McCoy-West et al., 2018)
and 0.08‰ to 0.06‰ for 0.5% to 30% partial melting in the spinel stability at 1300°C
(Sossi et al., 2018). The discrepancy of δ<sup>66</sup>Zn between oceanic basalts and
lithospheric peridotites can hence be attributed to isotopic fractionation during mantle
partial melting (Wang et al., 2017; Huang et al., 2018b; Sossi et al., 2018). Since
mantle-derived basaltic melt underplating is an important means by which the crust
grows (Bohlen and Mezger, 1989), the heavy Zn isotopic composition of the lower
continental crust also appears to be caused by partial melting of the peridotitic mantle.

As a whole, Zn isotopic compositions of lower crustal rocks are homogeneous
relative to granitic rocks from the shallow crust (-0.03‰ to 0.88‰; Telus et al., 2012;
Xia et al., 2017; Doucet et al., 2018; Xu et al., 2019; Fig. 5, 9), although the addition
of upper crustal sediments may cause localized heterogeneity. Doucet et al. (2018)
suggest that crustal melting can fractionate Zn isotopes (0.15‰) due to
disequilibrium/kinetic fractionation, but the magnitude of Zn isotopic fractionation is
much smaller than the overall variation observed in granitic rocks. Hence, partial
melting of a homogeneous granulite-facies source in the lower crust would generate
granitic melts with limited Zn isotopic variation. In contrast, fluid exsolution during
late-stage magma evolution could be responsible for the Zn isotopic variations of
granitic rocks (Telus et al., 2012; Xu et al., 2019; Fig 9). Therefore, the significant Zn
504 isotopic variations in the upper continental crust was most likely caused by late-stage
505 magmatic differentiation, rather than variability in the source regions of granitic melts.
506 Thus, Zn isotopes may be a potential tool for tracing magma evolution in the shallow
507 continental crust.

508 6. Conclusions

509 The main conclusions of this study on the Zn isotopic compositions of different
510 types of lower crustal rocks are:

511 1. The δ⁶⁶Zn values of lower crustal rocks from the north margin of the NCC range from 0.13 ‰ to 0.38‰ (37 out of 39 samples), indicating that the lower continental crust has a limited Zn isotopic variation relative to the shallow crust. Combining the δ⁶⁶Zn values of the lower crustal xenoliths and granulite terrains and different weighting methods, a weighted average δ⁶⁶Zn of 0.28 ± 0.04‰ (95% SE) is estimated for the LCC.
512
513
514
515
516

517 2. Assimilation of Precambrian lower crust, fluid metasomatism, and
518 accumulation of pyroxene and plagioclase had a limited effect on Zn
519 isotopic compositions of these lower crustal xenoliths.

520 3. During magmatic differentiation, fractional crystallization of Fe-Ti oxides
521 can modify the Zn isotopic compositions of cumulates toward lighter δ⁶⁶Zn
522 values relative to the primitive melt.

523 4. The δ⁶⁶Zn values of the lower crustal xenoliths from the Neogene Hannuoba
524 basalts and granulites from Archean terrains suggest that the Zn isotopic
525 composition of present-day LCC is similar to Archean LCC.

526 Acknowledgments

527 We thank Dr. ShengAo Liu for kind support during the development of the
528 analytical method for Zn isotopes, and Dr. Fang Huang for providing NIST SRM 683
529 solution. Dr. Mark Rehkämper, Dr. Alex McCoy-West, Dr. Luc S. Doucet and four
530 anonymous reviewers are thanked for thorough reviews that have greatly improved
the quality of the manuscript. We thank Dr. Edward Inglis and Dr. Michael Antonelli
for their comments and language editing. This research is co-supported by the
National Natural Science Foundation of China (41530211 and 41927803), the 111
Project (BP0719022), MOST Special Funds of the State Key Laboratory of
Geological Processes and Mineral Resources (MSFGPMR01), the National
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737

738

739
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741

Figure captions

742 **Figure. 1.** Simplified geological map of the Hannuoba area with sample locations marked by red stars (modified from 1:200000 geological map). The inset shows the location of the study area and tectonic subdivision of the Chinese continent (Wang and Mo, 1995).
743
744

745

746 **Figure. 2.** Plots of MgO versus SiO₂ (a) and Al₂O₃ (b); FeO(T) versus TiO₂ (c); and ⁸⁷Sr/⁸⁶Sr versus ¹⁴³Nd/¹⁴⁴Nd (d) for the lower crustal xenoliths. The major element and Sr-Nd isotopic compositions of the lower crustal xenoliths are from Liu et al. (2001, 2004, 2010). The Sr and Nd isotopic compositions of mantle-derived melts are from Tatsumoto et al. (1992). The arrows in (c) and (d) represent the trends expected for Fe-Ti oxide accumulation and ancient crustal assimilation, respectively. Two-Pyroxene MG: two-pyroxene mafic granulite; Garnet MG: garnet-bearing mafic granulite; Plagioclase MG: plagioclase-rich mafic granulite.
747
748
749
750
751
752
753

753

754 **Figure. 3.** (a) Photomicrographs of Fe-Ti oxides in the garnet-bearing mafic granulites. Intergranular Fe-Ti oxides and some sulfides are visible on the right. (b) Round and white Fe-Ti oxides host in garnet from sample DMP157 and showing needle-like exsolution. (c, d) Different phases of gray and white Fe-Ti oxides coexist in sample DMP157 and DMP467.
755
756
757
758

758

759 **Figure. 4.** Correlations of δ⁶⁶Zn and Zn concentrations with FeO(T) (a, c) and V (b, d) contents for the garnet-bearing mafic granulites (except for DMP133), which can be modeled by two end-member mixing calculations. Three different curves are given for three different α_oxide-melt values in (c) and (d). An empirically derived α_oxide-melt of 0.9998 can best account for the garnet-bearing mafic granulites data presented here. The ticks in (a), (b), (c) and (d) show 1% increments of Fe-Ti oxide accumulation. Modeling details are given in the Supplementary Materials. The FeO(T) contents are from Liu et al. (2001, 2010). Error bars represent long-term reproducibility (0.05‰, 2SD).
760
761
762
763
764
765
766
767

767
**Figure. 5.** Zinc isotopic compositions of the lower crustal samples of this study (top part) compared to the δ<sup>66</sup>Zn values of various ultramafic to felsic samples from previous studies (bottom part), including migmatites and granitoids (Telus et al., 2012; Xu et al., 2019); Archean crustal rocks (Doucet et al., 2018); andesites and rhyolites (Xia et al., 2017); arc volcanic rocks (Huang et al., 2018b); mid-ocean ridge basalts (MORBs; Wang et al., 2017; Huang et al., 2018b); ocean island basalts (OIBs; Herzog et al., 2009; Chen et al., 2013; Wang et al., 2017); picrites (McCoy-West et al., 2018); komatiites (Sossi et al., 2018); non-metasomatized and metasomatized peridotites (Wang et al., 2017). The brown area represents the range of the δ<sup>66</sup>Zn values of sediments (Pichat et al., 2003; Andersen et al., 2011; Hendry and Andersen, 2013; Little et al., 2016). The light blue bar represents the average δ<sup>66</sup>Zn value (0.18 ± 0.06‰) of the NCC lithospheric mantle (Wang et al., 2017). Error bars of the samples from previous studies represent two standard deviations (2SD). Error bars of the lower crustal rocks represent long-term reproducibility (0.05‰, 2SD).

781

**Figure. 6.** Whole-rock variations of δ<sup>66</sup>Zn versus <sup>143</sup>Nd/<sup>144</sup>Nd (a), <sup>87</sup>Sr/<sup>86</sup>Sr (b) for the lower crustal xenoliths. Strontium and Nd isotopic compositions are from Liu et al. (2004, 2010). Error bars represent long-term reproducibility (0.05‰, 2SD).

783

784

785

**Figure. 7.** Whole-rock variations of δ<sup>66</sup>Zn versus δ<sup>26</sup>Mg (a), LOI (b), Ba/La (c) for the lower crustal xenoliths. δ<sup>26</sup>Mg values are from Yang et al. (2016); LOI values are from Liu et al. (2001, 2010). Error bars represent long-term reproducibility (0.05‰, 2SD).

787

788

789

**Figure. 8.** Whole-rock variations of δ<sup>66</sup>Zn versus Al<sub>2</sub>O<sub>3</sub> (a), Cr (b), and Eu/Eu<sup>*</sup> (c) for the lower crustal xenoliths. Error bars represent long-term reproducibility (0.05‰; 2SD). Eu/Eu<sup>*</sup> = Eu<sub>N</sub>/(Sm+Gd)<sub>N</sub>/0.5, where the subscript N indicates normalization to chondritic values from Taylor and McLennan (1985).

790

791

792

793

794

**Figure. 9.** Histogram of δ<sup>66</sup>Zn values of the lithospheric mantle peridotites, oceanic basalts, lower crustal rocks, and migmatites and granitoids. The δ<sup>66</sup>Zn data of the lithospheric mantle peridotites are from Doucet et al. (2016), Wang et al. (2017), Huang et al. (2018a), and Sossi et al.

795

796

797
798 (2018); oceanic basalts are from Herzog et al. (2009), Chen et al. (2013), Wang et al. (2017), and
799 Huang et al. (2018b); lower crustal rocks are from this study; migmatites and granitoids are from
800 Telus et al. (2012), Doucet et al. (2018), and Xu et al. (2019).

801

802 **Figure. 10.** The variation of δ<sup>66</sup>Zn with depth for the upper lower crust, middle lower crust, lowermost crust, and lithospheric mantle and schematic plot of the vertical lower crustal structure in the Hannuoba area. The lithological composition of the lower continental crust was modified from Liu et al. (2001). The δ<sup>66</sup>Zn value of the NCC lithospheric mantle is from Wang et al. (2017).
803
804
805
806 Error bars represent 95% SE.
<figure><img src="image_2.png" /><figcaption>Figure 1</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Figure 2</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 3</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 4</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 5</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 6</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Figure 7</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Figure 8</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Figure 9</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Figure 10</figcaption></figure>
<table><caption>Table 1. Zinc isotopes and selected major and trace element compositions of the lower crustal rocks</caption><thead><tr><th>Sample</th><th>Location</th><th>TiO<sub>2</sub><sup>a</sup><br>wt.%</th><th>MgO<sup>a</sup><br>wt.%</th><th>FeO<sub>(T)</sub><sup>a</sup><br>wt.%</th><th>Al<sub>2</sub>O<sub>3</sub><sup>a</sup><br>wt.%</th><th>V<br>ppm</th><th>Cr<br>ppm</th><th>Zn<br>ppm</th><th>Ba/La</th><th>δ<sup>66</sup>Zn<br>‰</th><th>2SD<sup>b</sup><br>‰</th><th>δ<sup>68</sup>Zn<br>‰</th><th>2SD<sup>b</sup><br>‰</th><th>N<sup>c</sup></th></tr></thead><tbody><tr><td colspan="14"><strong>Lower crustal xenoliths</strong></td></tr><tr><td colspan="14">Pyroxenite (average δ<sup>66</sup> Zn: 0.26 ± 0.07‰ (2SD))</td></tr><tr><td>DMP10</td><td>Damaping</td><td>0.69</td><td>18.6</td><td>10.5</td><td>5.8</td><td>253</td><td>604</td><td>68.9</td><td>2.8</td><td>0.23</td><td>0.05</td><td>0.47</td><td>0.08</td><td>3</td></tr><tr><td>DMP11</td><td>Damaping</td><td>0.52</td><td>20.2</td><td>10.8</td><td>4.7</td><td>214</td><td>1609</td><td>82.4</td><td>5.5</td><td>0.27</td><td>0.05</td><td>0.53</td><td>0.08</td><td>3</td></tr><tr><td>DMP37</td><td>Damaping</td><td>0.59</td><td>16.2</td><td>8.4</td><td>8.1</td><td>204</td><td>959</td><td>56.8</td><td>6.4</td><td>0.26</td><td>0.05</td><td>0.58</td><td>0.08</td><td>3</td></tr><tr><td>DMP49</td><td>Damaping</td><td>0.57</td><td>16.4</td><td>9.4</td><td>7.6</td><td>229</td><td>642</td><td>69.3</td><td>16.4</td><td>0.27</td><td>0.05</td><td>0.53</td><td>0.08</td><td>3</td></tr><tr><td colspan="14">Two-Pyroxene mafic granulite (average δ<sup>66</sup> Zn: 0.27 ± 0.10‰ (2SD))</td></tr><tr><td>DMP03</td><td>Damaping</td><td>0.48</td><td>14.0</td><td>6.7</td><td>13.8</td><td>125</td><td>370</td><td>46.8</td><td>55.0</td><td>0.30</td><td>0.05</td><td>0.62</td><td>0.08</td><td>3</td></tr><tr><td>DMP45</td><td>Damaping</td><td>0.36</td><td>15.1</td><td>9.4</td><td>11.6</td><td>156</td><td>314</td><td>80.0</td><td>53.1</td><td>0.29</td><td>0.05</td><td>0.58</td><td>0.08</td><td>3</td></tr><tr><td>DMP66</td><td>Damaping</td><td>0.42</td><td>13.6</td><td>7.0</td><td>12.1</td><td>168</td><td>404</td><td>56.9</td><td>43.1</td><td>0.21</td><td>0.05</td><td>0.46</td><td>0.08</td><td>3</td></tr><tr><td>DMP71</td><td>Damaping</td><td>0.6</td><td>14.0</td><td>10.0</td><td>9.6</td><td>225</td><td>458</td><td>81.9</td><td>26.0</td><td>0.30</td><td>0.05</td><td>0.52</td><td>0.08</td><td>3</td></tr><tr><td>DMP28</td><td>Damaping</td><td>0.64</td><td>13.9</td><td>10.6</td><td>9.6</td><td>231</td><td>410</td><td>91.9</td><td>22.4</td><td>0.23</td><td>0.05</td><td>0.47</td><td>0.08</td><td>3</td></tr><tr><td>Replicate</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.24</td><td>0.05</td><td>0.51</td><td>0.08</td><td>3</td></tr><tr><td>Average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.23</td><td>0.05</td><td>0.49</td><td>0.08</td><td>3</td></tr><tr><td colspan="14">Garnet-bearing mafic granulite (average δ<sup>66</sup> Zn: 0.25 ± 0.12‰ (2SD); except for DMP133)</td></tr><tr><td>DMP15<sup>d</sup></td><td>Damaping</td><td>0.79</td><td>14.7</td><td>11.2</td><td>13.8</td><td>220</td><td>178</td><td>65.4</td><td>8.4</td><td>0.25</td><td>0.05</td><td>0.52</td><td>0.08</td><td>3</td></tr><tr><td>DMP133</td><td>Damaping</td><td>1.59</td><td>17.5</td><td>15.8</td><td>10.8</td><td>529</td><td>198</td><td>166</td><td>13.5</td><td>-0.18</td><td>0.05</td><td>-0.37</td><td>0.08</td><td>3</td></tr><tr><td>Replicate</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>-0.16</td><td>0.05</td><td>-0.34</td><td>0.08</td><td>3</td></tr><tr><td>Average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>-0.17</td><td>0.05</td><td>-0.36</td><td>0.08</td><td>3</td></tr><tr><td>DMP140<sup>d</sup></td><td>Damaping</td><td>0.71</td><td>15.8</td><td>10.1</td><td>12.5</td><td>215</td><td>281</td><td>55.4</td><td>37.5</td><td>0.29</td><td>0.05</td><td>0.56</td><td>0.08</td><td>3</td></tr><tr><td>DMP467<sup>d</sup></td><td>Damaping</td><td>2.66</td><td>8.5</td><td>15.4</td><td>17.9</td><td>563</td><td>2</td><td>91.2</td><td>18.1</td><td>0.18</td><td>0.05</td><td>0.41</td><td>0.08</td><td>3</td></tr><tr><td>Replicate</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.18</td><td>0.05</td><td>0.38</td><td>0.08</td><td>3</td></tr><tr><td>Average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.18</td><td>0.05</td><td>0.40</td><td>0.08</td><td>3</td></tr><tr><td>DMP08</td><td>Damaping</td><td>0.55</td><td>12.4</td><td>8.9</td><td>17.2</td><td>135</td><td>134</td><td>51.7</td><td>52.4</td><td>0.30</td><td>0.05</td><td>0.59</td><td>0.08</td><td>3</td></tr><tr><td>DMP137</td><td>Damaping</td><td>0.65</td><td>13.1</td><td>9.6</td><td>15.4</td><td>161</td><td>161</td><td>57.2</td><td>24.1</td><td>0.34</td><td>0.05</td><td>0.68</td><td>0.08</td><td>3</td></tr><tr><td>DMP141<sup>d</sup></td><td>Damaping</td><td>1.65</td><td>10.5</td><td>12.2</td><td>15.0</td><td>447</td><td>219</td><td>82.7</td><td>24.2</td><td>0.20</td><td>0.05</td><td>0.42</td><td>0.08</td><td>3</td></tr><tr><td>DMP157<sup>d</sup></td><td>Damaping</td><td>1.67</td><td>11.5</td><td>13.4</td><td>14.8</td><td>407</td><td>76</td><td>91.0</td><td>20.7</td><td>0.23</td><td>0.05</td><td>0.51</td><td>0.08</td><td>3</td></tr><tr><td>Replicate</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.25</td><td>0.05</td><td>0.49</td><td>0.08</td><td>3</td></tr><tr><td>Average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.24</td><td>0.05</td><td>0.50</td><td>0.08</td><td>3</td></tr><tr><td>DMP468<sup>d</sup></td><td>Damaping</td><td>1.47</td><td>10.5</td><td>11.5</td><td>16.3</td><td>375</td><td>230</td><td>69.0</td><td>25.4</td><td>0.24</td><td>0.05</td><td>0.50</td><td>0.08</td><td>3</td></tr><tr><td colspan="14">Plagioclase-rich mafic granulite (average δ<sup>66</sup> Zn: 0.31 ± 0.06‰ (2SD))</td></tr><tr><td>DMP06</td><td>Damaping</td><td>0.96</td><td>7.0</td><td>6.9</td><td>18.1</td><td>146</td><td>11</td><td>112.0</td><td>35.6</td><td>0.30</td><td>0.05</td><td>0.60</td><td>0.08</td><td>3</td></tr><tr><td>DMP07</td><td>Damaping</td><td>0.62</td><td>5.1</td><td>5.8</td><td>24.4</td><td>1801</td><td>26</td><td>69.6</td><td>83.5</td><td>0.31</td><td>0.05</td><td>0.65</td><td>0.08</td><td>3</td></tr><tr><td>DMP62</td><td>Damaping</td><td>0.2</td><td>5.8</td><td>4.0</td><td>19.2</td><td>61</td><td>34</td><td>81.6</td><td>60.1</td><td>0.33</td><td>0.05</td><td>0.62</td><td>0.08</td><td>3</td></tr><tr><td colspan="14">Intermediate granulite (average δ<sup>66</sup> Zn: 0.28 ± 0.07‰ (2SD); except for DMP01)</td></tr><tr><td>DMP27</td><td>Damaping</td><td>1.32</td><td>3.7</td><td>7.5</td><td>16.9</td><td>124</td><td>73</td><td>112.9</td><td>32.5</td><td>0.28</td><td>0.05</td><td>0.56</td><td>0.08</td><td>3</td></tr><tr><td>Replicate</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.30</td><td>0.05</td><td>0.53</td><td>0.08</td><td>3</td></tr><tr><td>Average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.29</td><td>0.05</td><td>0.55</td><td>0.08</td><td>3</td></tr><tr><td>DMP70</td><td>Damaping</td><td>0.55</td><td>5.3</td><td>3.6</td><td>15.2</td><td>89</td><td>98</td><td>79.6</td><td>86.8</td><td>0.28</td><td>0.05</td><td>0.61</td><td>0.08</td><td>3</td></tr><tr><td>DMP01</td><td>Damaping</td><td>0.27</td><td>5.0</td><td>3.6</td><td>16.6</td><td>56</td><td>91</td><td>50.5</td><td>137.4</td><td>0.06</td><td>0.05</td><td>0.12</td><td>0.08</td><td>3</td></tr><tr><td>Replicate</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.06</td><td>0.05</td><td>0.14</td><td>0.08</td><td>3</td></tr><tr><td>Average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.06</td><td>0.05</td><td>0.13</td><td>0.08</td><td>3</td></tr><tr><td>DMP512</td><td>Damaping</td><td>0.90</td><td>5.5</td><td>4.7</td><td>18.0</td><td>108</td><td>49</td><td>102.0</td><td>53.8</td><td>0.27</td><td>0.05</td><td>0.50</td><td>0.08</td><td>3</td></tr><tr><td>Replicate</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.25</td><td>0.05</td><td>0.49</td><td>0.08</td><td>3</td></tr><tr><td>Average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.26</td><td>0.05</td><td>0.50</td><td>0.08</td><td>3</td></tr><tr><td>DMP508</td><td>Damaping</td><td>0.92</td><td>7.0</td><td>7.1</td><td>16.9</td><td>155</td><td>41</td><td>94.1</td><td>37.4</td><td>0.30</td><td>0.05</td><td>0.59</td><td>0.08</td><td>3</td></tr><tr><td colspan="14">Felsic granulite (average δ<sup>66</sup> Zn: 0.33 ± 0.05‰ (2SD))</td></tr><tr><td>ZB20</td><td>Zhouba</td><td>0.95</td><td>0.4</td><td>0.9</td><td>12.8</td><td>47</td><td>2</td><td>13.3</td><td>169.0</td><td>0.33</td><td>0.05</td><td>0.71</td><td>0.08</td><td>2</td></tr><tr><td>ZB21</td><td>Zhouba</td><td>0.37</td><td>1.0</td><td>3.2</td><td>18.7</td><td>42</td><td>2</td><td>55.6</td><td>99.4</td><td>0.33</td><td>0.05</td><td>0.66</td><td>0.08</td><td>3</td></tr><tr><td colspan="14">Granulite-facies metasidement (average δ<sup>66</sup> Zn: 0.31 ± 0.22‰ (2SD))</td></tr><tr><td>ZB19</td><td>Zhouba</td><td>0.86</td><td>2.8</td><td>6.1</td><td>17.2</td><td>125</td><td>136</td><td>77.5</td><td>12.9</td><td>0.23</td><td>0.05</td><td>0.49</td><td>0.08</td><td>3</td></tr><tr><td>ZB22</td><td>Zhouba</td><td>0.77</td><td>3.3</td><td>7.3</td><td>15.6</td><td>116</td><td>153</td><td>83.6</td><td>35.1</td><td>0.38</td><td>0.05</td><td>0.76</td><td>0.08</td><td>3</td></tr><tr><td colspan="14"><strong>Granulites from Archean terrains</strong></td></tr><tr><td colspan="14">Mafic granulite (average δ<sup>66</sup> Zn: 0.27 ± 0.06‰ (2SD))</td></tr><tr><td>MQG15</td><td>Manjinggou</td><td>1.43</td><td>8.6</td><td>14.4</td><td></td><td></td><td></td><td></td><td></td><td>0.28</td><td>0.05</td><td>0.54</td><td>0.08</td><td>3</td></tr><tr><td>MQG02</td><td>Manjinggou</td><td>0.99</td><td>9.4</td><td>14.7</td><td></td><td></td><td></td><td></td><td></td><td>0.27</td><td>0.05</td><td>0.53</td><td>0.08</td><td>3</td></tr><tr><td>WYK09</td><td>Wayaokou</td><td>1.43</td><td>6.1</td><td>14.4</td><td></td><td></td><td></td><td></td><td></td><td>0.25</td><td>0.05</td><td>0.47</td><td>0.08</td><td>3</td></tr><tr><td>Replicate</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.25</td><td>0.05</td><td>0.53</td><td>0.08</td><td>3</td></tr><tr><td>Average</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.25</td><td>0.05</td><td>0.50</td><td>0.08</td><td>3</td></tr><tr><td colspan="14">Intermediate granulite (average δ<sup>66</sup> Zn: 0.28 ± 0.15‰ (2SD))</td></tr><tr><td>MQG01</td><td>Manjinggou</td><td>0.71</td><td>3.5</td><td>8.2</td><td></td><td></td><td></td><td></td><td></td><td>0.33</td><td>0.05</td><td>0.62</td><td>0.08</td><td>3</td></tr><tr><td>MQG20</td><td>Manjinggou</td><td>1.46</td><td>4.4</td><td>13.9</td><td></td><td></td><td></td><td></td><td></td><td>0.23</td><td>0.05</td><td>0.42</td><td>0.08</td><td>3</td></tr><tr><td colspan="14">Felsic granulite (average δ<sup>66</sup> Zn: 0.36 ± 0.05‰ (2SD))</td></tr><tr><td>WYK06</td><td>Wayaokou</td><td>0.58</td><td>1.8</td><td>4.2</td><td></td><td></td><td></td><td></td><td></td><td>0.36</td><td>0.05</td><td>0.68</td><td>0.08</td><td>3</td></tr><tr><td colspan="14">Granulite-facies metasidement (average δ<sup>66</sup> Zn: 0.25 ± 0.21‰ (2SD))</td></tr><tr><td>MQG06</td><td>Manjinggou</td><td>2.40</td><td>7.1</td><td>19.9</td><td></td><td></td><td></td><td></td><td></td><td>0.30</td><td>0.05</td><td>0.57</td><td>0.08</td><td>3</td></tr><tr><td>MQG08</td><td>Manjinggou</td><td>0.96</td><td>6.0</td><td>8.5</td><td></td><td></td><td></td><td></td><td></td><td>0.13</td><td>0.05</td><td>0.27</td><td>0.08</td><td>3</td></tr><tr><td>MQG05</td><td>Manjinggou</td><td>0.82</td><td>2.9</td><td>8.4</td><td></td><td></td><td></td><td></td><td></td><td>0.31</td><td>0.05</td><td>0.58</td><td>0.08</td><td>3</td></tr></tbody></table>

<footnote>a Major elements data are from Gao et al. (2000) and Liu et al. (2001, 2010).</footnote>

<footnote>b Two times the standard deviation.</footnote>

<footnote>c N represents the times of repeat measurements of the same purified solution by MC-ICP-MS.</footnote>

<footnote>d Samples with Fe-Ti oxides.</footnote>
<table><caption>Table 2. Model parameters for the calculation of Zn isotopic composition of the bulk lower continental crust</caption><thead><tr><th>Lithology</th><th>Peridotite</th><th>Pyroxenite</th><th>Mafic granulite</th><th>Intermediate granulite</th><th>Felsic granulite</th><th>Metasediment</th><th>δ<sup>66</sup>Zn (‰)</th><th>95% SE<sup>d</sup></th><th>n<sup>c</sup></th></tr></thead><tbody><tr><td>Zn content (ppm)</td><td>37<sup>c</sup></td><td>69</td><td>74</td><td>97</td><td>34</td><td>81</td><td></td><td></td><td></td></tr><tr><td>Average δ<sup>66</sup>Zn (‰)</td><td>0.19<sup>c</sup></td><td>0.26</td><td>0.27</td><td>0.28</td><td>0.34</td><td>0.27</td><td></td><td></td><td></td></tr><tr><td>2SD</td><td>0.06</td><td>0.07</td><td>0.11</td><td>0.09</td><td>0.07</td><td>0.20</td><td></td><td></td><td></td></tr><tr><td>Hannuoba area<sup>a</sup></td><td>4</td><td>4</td><td>20</td><td>25</td><td>35</td><td>12</td><td><strong>0.28</strong></td><td><strong>0.02</strong></td><td>37</td></tr><tr><td>Archean shields<sup>b</sup></td><td>0</td><td>0</td><td>65</td><td>15</td><td>15</td><td>5</td><td><strong>0.27</strong></td><td><strong>0.02</strong></td><td>33</td></tr><tr><td>Extensional regions<sup>b</sup></td><td>0</td><td>0</td><td>40</td><td>25</td><td>25</td><td>10</td><td><strong>0.28</strong></td><td><strong>0.02</strong></td><td>33</td></tr><tr><td>Shields/platforms<sup>b</sup></td><td>0</td><td>0</td><td>90</td><td>0</td><td>0</td><td>10</td><td><strong>0.27</strong></td><td><strong>0.03</strong></td><td>24</td></tr></tbody></table>

<footnote><sup>a</sup> The proportions of different lithologies are calculated by the simplified compositional model of the lower crust is modified from Liu et al. (2001).</footnote>

<footnote><sup>b</sup> The proportions of different lithologies are inferred by Rudnick and Fountain (1995).</footnote>

<footnote><sup>c</sup> The average Zn content and δ<sup>66</sup>Zn of the Hannuoba peridotite are from Wang et al. (2017).</footnote>

<footnote><sup>d</sup> 95% standard errors (95% SE = t×SD/(n)^(0.5), where t = inverse survival function of the Student's t-distribution at the 95% confidence level and (n-1) degrees of freedom).</footnote>

<footnote><sup>e</sup> Numbers of samples for calculation.</footnote>