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Article

Petrogenesis and Tectonic Implications of the Neoproterozoic Peraluminous Granitic Rocks from the Tianshui Area, Western Margin of the North Qinling Terrane, China: Evidence from Whole-Rock Geochemistry and Zircon U–Pb–Hf–O Isotopes

by
Gang Yang
,
Juan Zhang
*,
Hongfu Zhang
,
Zhian Bao
and
Abing Lin
State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China
*
Author to whom correspondence should be addressed.
Minerals 2022, 12(7), 910; https://doi.org/10.3390/min12070910
Submission received: 13 June 2022 / Revised: 13 July 2022 / Accepted: 18 July 2022 / Published: 20 July 2022
(This article belongs to the Section Mineral Geochemistry and Geochronology)
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Abstract

:
The source and petrogenesis of peraluminous granitic rocks in orogenic belts can provide insights into the evolution, architecture, and composition of continental crust. Neoproterozoic peraluminous granitic rocks are sporadically exposed in the Tianshui area of the western margin of the North Qinling Terrane (NQT), China. However, the source, petrogenesis, and tectonic setting of these rocks still remain unclear, which limits our understanding of the Precambrian tectonic and crustal evolution of the Qinling Orogenic Belt (QOB). Here, we determined the whole-rock geochemical compositions and in situ zircon U–Pb ages, trace-element contents, and Hf–O isotopic compositions of a series of peraluminous granitic mylonites and granitic gneisses in the Tianshui area at the west end of North Qinling. Zircon U–Pb dating revealed that the protoliths of the studied granitic mylonites and granitic gneisses crystallized at 936–921 Ma. The granitic rocks displayed high A/CNK values (1.12–1.34) and were enriched in large-ion lithophile elements (e.g., Rb, Ba, Th, U, and K) and light rare earth elements, and they were depleted of high-field-strength elements (e.g., Nb, Ta, and Ti). These rocks showed variable zircon εHf(t) (−12.2 / 9.7) and δ18O (3.56‰ / 11.07‰) values, suggesting that they were derived from heterogeneous crustal sources comprising predominantly supracrustal sedimentary rocks and subordinate igneous rocks. In addition, the U–Pb–Hf isotopic compositions from the core domains of inherited zircons were similar to those of detrital zircons from the Qinling Group, suggesting that the Qinling Group was an important crustal source for the granitic rocks. The lithological and geochemical features of these granitic rocks indicate that they were generated by biotite dehydration melting of heterogeneous sources at lower crustal depths. Combining our results with those of previous studies, we suggest that the NQT underwent a tectonic transition from syn-collision to post-collision at 936–874 Ma in response to the assembly and breakup of the Rodinia supercontinent.

1. Introduction

Orogenic belts with Precambrian basement comprising pre-orogenic components are important sites of crustal growth and reworking [1,2,3]. Peraluminous granites are present in many orogenic belts, providing valuable information regarding the early tectonic and crustal evolution of the orogenic belts [4,5,6,7,8,9].
The Qinling Orogenic Belt (QOB) is located between the North China and South China blocks, and it is a major component of the Central Orogenic Belt (Figure 1a). Numerous studies have shown that the QOB underwent multistage metamorphism and magmatism [10,11,12,13,14,15,16,17]. Neoproterozoic peraluminous granitic rocks in the eastern part of the North Qinling Terrane (NQT) have been recognized and widely investigated (Table 1; [18,19,20]). In contrast, the Neoproterozoic peraluminous granitic rocks in the western part of the NQT have received much less attention. The western margin of the NQT is located near the triple junction of the Yangtze Craton, North China block, and Tibetan Plateau (Figure 1a). The closure of the Paleo-Tethys Ocean, amalgamation between the North China block and the South China block, and the Tibetan Plateau uplift since the Cenozoic further complicated the geology of the region [21,22,23,24]. Precambrian magmatism in the western margin of the NQT is at a relative low degree of preservation. Therefore, the Neoproterozoic peraluminous granitic rocks in the region can provide critical constraints on the tectonic evolution of the QOB and the identity of unexposed basement.
In this study, we present new whole-rock geochemical compositions and in situ zircon U–Pb–Hf–O isotopic and trace elemental data for Neoproterozoic granitic mylonites and granitic gneisses from the Tianshui area of the NQT to constrain their source, petrogenesis, and tectonic setting. We used our results together with a compilation of U–Pb–Hf–O isotopic data for early Neoproterozoic granitic rocks from the NQT to reconstruct the early evolution of the QOB (Table 1).

2. Regional Geology and Petrography

The QOB is located in the central part of the Central Orogenic Belt and was formed by the collision between the North China block and the South China block (Figure 1a; [43]). The QOB can be subdivided by the Shangdan Fault into two mountain chains: the South Qinling Terrane and the NQT (Figure 1b; [11,12,13,43]. The NQT connects the Qilian to the west and the Dabie to the east (Figure 1b).
Precambrian basement is only rarely exposed in the western part of the NQT. The oldest exposed crystalline basement of the region is the Paleoproterozoic Qinling Group, which is composed mainly of gneisses, amphibolites, and marbles [11]. Zircon U–Pb ages of 2267–2172 Ma have been reported for the gneisses [13]. The vast exposed strata of the western part of the NQT consist primarily of Devonian–Cretaceous marine metasedimentary rocks [44] that are intruded by voluminous Mesozoic granitic plutons [45,46,47].
Neoproterozoic granitic rocks are widespread in the NQT. On the basis of mineral assemblages, textures, and geochemical characteristics, these granitic rocks are classified as strongly deformed S-type, weakly deformed I-type, and nondeformed A-type granitoids [48,49,50,51]. These three groups of rocks were formed in three stages at 980–880, 870–844, and 830–630 Ma, respectively [19,48,51]. Early Neoproterozoic peraluminous granitic gneisses (e.g., the Dehe, Fangzhuang, Niujiaoshan, and Zhaigen) in the eastern part of the NQT are interpreted to have formed in a syn-collisional tectonic regime associated with the assembly of the Rodinia supercontinent [48,49,50,51]. Early Neoproterozoic granitic rocks in the western margin of the NQT are distributed sporadically in several areas including Wushan, Yuanlong, and Xinyang [27,28,29,52]. Recent zircon U–Pb analyses of these rocks have yielded protolith ages of 910–978 Ma [26,27,28,29]. Numerous contemporary peraluminous granitic gneisses have been recognized in adjacent blocks including the Altun–Qilian–Kunlun–North Qaidam (AQKNQ) region [53,54,55,56,57,58,59,60,61].
The Neoproterozoic mafic–ultramafic rocks are lower in volume compared with the granitic rocks in the NQT. The Songshugou mafic–ultramafic complex is the largest peridotite block in the QOB, which is located on the northern side of the Shangdan suture [62]. The peridotites show Re–Os model ages of 1240~800 Ma [63,64,65], which are interpreted to be fragments of the oceanic lithosphere [62,64,66] or a mantle wedge in a forearc setting [63,67]. The meta-mafic rocks, including (retro-) eclogites and (garnet-) amphibolites, in the Guanpo–Zhaigen–Shuanghuaishu–Xixia–Songshugou areas yielded protolith ages of 840~750 Ma [68,69,70,71,72,73].
For the present study, we collected three granitic mylonite samples from the Wushan area and three granitic gneiss samples from each of the Yuanlong and Xinyang areas. The Wushan granitic mylonite is located in the ductile shear zone of the Wushan ophiolite unit in the northwestern margin of the NQT (Figure 2a). The mylonite is located adjacent to marble to the north and metabasalt to the south (Liang, 2017). The mylonite is light grey, shows a mylonitic foliation and lineation, and is composed of quartz (ca. 50 vol%), K-feldspar (ca. 30 vol%), biotite (ca. 10 vol%), and muscovite (ca. 10 vol%) (Figure 3a,d). The accessory minerals are mainly apatite, zircon, and garnet. K-feldspar grains are of an augen structure. Quartz grains display anhedral irregular undulatory extinction and are distributed along the K-feldspar grains. Kinked and elongated biotite and muscovite grains form a directional band of flaky mineral aggregate.
The Yuanlong granitic gneiss is located on the northern side of the Weihe River between the Boyang and Yuanlong areas (Figure 2b). This granitic gneiss is in contact with the Neoproterozoic Huluhe Group to the north and the early Paleozoic Caotangou Group to the south [29]. The samples were medium to coarse grained with a gneissic structure and were composed of quartz (ca. 45 vol%), plagioclase (ca. 35 vol%), biotite (ca. 10 vol%), and muscovite (ca. 10 vol%) (Figure 3b,e), with minor amounts of apatite and zircon. Plagioclase is anhedral and granular, while quartz is elongated and shows wavy extinction. Biotite together with muscovite forms a directional band of flaky mineral aggregate.
The Xinyang granitic gneiss is located in the Xiweizi ravine to the south of the Weihe River near Xinyang Town (Figure 2c). The samples were composed predominantly of quartz (ca. 40 vol%), plagioclase (ca. 30 vol%), K-feldspar (ca. 10 vol%), biotite (ca. 10 vol%), and muscovite (ca. 10 vol%) (Figure 3c,f), with accessory ilmenite, zircon, apatite, and garnet. Plagioclase and K-feldspar are subhedral–euhedral, while quartz is mostly anhedral and shows wavy extinction. Biotite together with muscovite forms a directional band of flaky mineral aggregate.

3. Analytical Methods

Whole-rock major and trace element and zircon U–Pb isotope analyses were performed at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China. In situ zircon O isotope analyses were conducted at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China. Zircon Lu–Hf isotope analyses were performed at the Xi’an Geological Survey Center of the China Geological Survey, Xi’an, China.

3.1. Whole-Rock Major and Trace Element Analyses

Whole-rock major element contents were analyzed by X-ray fluorescence (XRF) using a Rigaku RIX2100 instrument (Rigaku, japan). The Chinese national rock standard, GBW07105, and the United States Geological Survey (USGS) standard, BCR-2, were used as reference materials. The uncertainty regarding the major element contents was less than 2%. Whole-rock trace element contents were measured by inductively coupled plasma mass spectrometry (ICP–MS) using an Agilent 7500A instrument (Agilent, Santa Clara, CA, USA), with USGS reference materials (i.e., BHVO-2, AGV-2, BCR-2, and GSP-2) used as standard samples to estimate the analytical precision. The accuracy and precision of the whole-rock trace elements were better than 5% and 10%, respectively.

3.2. Preparation and Imaging of Zircons

The internal structures of zircons commonly yield valuable geological information. Zircons were observed prior to analyses of O and Lu–Hf isotopes and U–Pb dating to characterize their internal structure and to select analytical spots for isotopes and dating. Accordingly, the selection and morphology of zircons are particularly important. Rock samples were first mechanically crushed in the laboratory, following which whole zircon crystals were separated using conventional heavy liquid and magnetic techniques. Clear zircons without inclusions or cracks were handpicked under a binocular microscope. The selected zircons were mounted on a 20 mm diameter disc with epoxy resin and polished to expose the crystal interiors. An FEI Quanta 400 FEG scanning electron microscope together with an Oxford Instruments energy-dispersive spectroscopy system and a Gatan CL3+ detector were used to obtain cathodoluminescence (CL) images.

3.3. In Situ Zircon O Isotope Analyses

In situ zircon O isotopes were measured using a Cameca IMS 1280-HR secondary ion mass spectrometry (SIMS) instrument (Cameca, France) in GIG. For analyses during this study, a beam diameter of ca. 10 μm and an energy intensity of 2–3 nA were used. Penglai (δ18O = 5.31‰ ± 0.1‰; [74]) were used as a standard to correct instrument bias. A 133Cs+ ion source with a strength of ca. 2 nA was used to bombard the sample with an accelerating voltage of 10 kV, and negative secondary ions were extracted with an accelerating voltage of −10 kV. Two Faraday cups were used to receive 16O and 18O. Qinghu was also analyzed to monitor the data quality. During our session, the determined weighted mean δ18O values of the Qinghu was 5.54‰ ± 0.25‰, the same as the recommended values within error [74,75]. The detailed experimental procedures and data reduction techniques have been described by Yang et al. (2018, 2019) [76,77].

3.4. In Situ Zircon U–Pb Dating and Trace Element Analyses

In situ zircon U–Pb isotopes and trace elements were measured with a 30 μm beam diameter using an Agilent 7700a ICP–MS instrument (Agilent, Santa Clara, CA, USA) coupled to a GeoLas 2005 ArF-excimer laser (Coherent, Inc., Santa Clara, CA, USA) operating at a 193 nm wavelength and 1–20 Hz denudation frequency. Detailed procedures and operating conditions for the LA–ICP–MS U–Pb dating are described by Yuan et al. (2004) [78]. GLITTER (VER 4.0) software developed by W. L. Griffin of Macquarie University was used for data processing and age calculations. A standard zircon GJ-1 was used as the external standard for isotope ratio correction, and element contents were calculated using NIST 610 as the external standard and 29Si as the internal standard. Correction for common Pb followed Andersen (2002) [79]. Calculation of the weighted mean ages and drawing of the concordia diagrams were performed using ISOPLOT 3.0 [80]. The isotopic ratios and age errors determined during the study are reported at the 1σ level of confidence.

3.5. In Situ Zircon Lu–Hf Isotopic Analyses

After U-Pb dating, zircon grains were newly polished. In situ zircon Lu–Hf isotope analyses were carried out on the same spots or on the same zircon zones where the U–Pb age determinations were made. In situ zircon Lu–Hf isotopes were determined with a 40 μm beam diameter using an Agilent 7700x ICP–MS instrument (Agilent, Santa Clara, CA, USA) coupled to a 193 nm ultraviolet wavelength GeoLas Pro ArF-excimer laser (Coherent, Inc., Santa Clara, CA, USA). Values of 176Lu/175Lu = 0.02669 and 176Yb/172Yb = 0.5886 were applied for the interference corrections of isobars to calculate the 176Lu/77Hf and 176Hf/177Hf values of the analyzed samples [81]. During sample determinations, standard zircons GJ-1 (0.282000 ± 0.000030), MUN (0.282136 ± 0.000030), and Plesovice (0.282482 ± 0.000030) were used for calibration of the sample determinations and instrumental monitoring. The analysis results revealed that the mean values of GJ-1 (0.28199 ± 0.000019), MUN (0.282162 ± 0.000016), and Plesovice (0.282463 ± 0.000014) for the standard samples were the same as the recommended values within error. Calculation of the εHf(t) values employed the present-day chondrite ratios of 176Lu/177Hf = 0.0332 and 176Hf/177Hf = 0.282772, recommended by Francis and Blichert-Toft (1997) [82]. Calculation of the single-stage model ages (TDM1) used the present-day depleted-mantle ratios of 176Hf/177Hf = 0.28325 and 176Lu/177Hf = 0.0384 [83]. The present-day continental upper crust ratio of fLu/Hf = −0.72 and depleted-mantle ratio of fLu/Hf = 0.16 were adopted for the calculation of the two-stage model ages (TDM2) [84].

4. Results

4.1. Whole-Rock Major and Trace Element Contents

Major and trace element contents of the Wushan granitic mylonite and the Yuanlong and Xinyang granitic gneisses are presented in Table 2 and Figure 4 and Figure 5. The granitic mylonite and granitic gneiss samples had high SiO2 (68.03–75.68 wt%), K2O + Na2O (5.64–8.09 wt%), and Al2O3 (12.67–15.64 wt%) contents and low MgO contents (0.11–1.16 wt%) and Mg# values (16–37). The samples are plotted in the granodiorite and granite domains in a total alkalis–silica (TAS) diagram (Figure 4a) and in the high-K calc-alkaline series in a K2O versus SiO2 diagram (Figure 4b). The studied samples yielded aluminum saturation index (A/CNK) values of 1.12 to 1.34 and showed peraluminous characteristics according to an A/CNK–A/NK diagram (Figure 4c). All of these geochemical features indicate that the protoliths of the granitic mylonite and granitic gneiss were potassic peraluminous calc-alkaline granites.
Patterns of chondrite-normalized rare earth element (REE) contents for the nine studied samples are shown in Figure 5a. The samples show enrichment in light REEs (LREEs) and depletion of heavy REEs (HREEs) ((La/Yb) N = 3–10). The Europium anomaly (Eu/Eu*) showed strong negative values of 0.09 to 0.59. In a primitive-mantle-normalized multi-element spider diagram (Figure 5b), the samples exhibited similar trace element patterns, with enrichment in large-ion lithophile elements (LILEs; e.g., Rb, Ba, Th, U, and K) and depletion of high-field-strength elements (HFSEs; e.g., Nb, Ta, and Ti). Two of the granitic gneisses from Xinyang were characterized by low abundances of Sr and Ba (Figure 5b) and large negative Eu anomalies in the chondrite-normalized REE patterns (Figure 5a), which were similar with highly fractionated granite.

4.2. In Situ Zircon U–Pb Ages, Trace Element Contents, and Lu–Hf and O Isotope Compositions

Here, we present in situ zircon U–Pb–Hf–O isotope compositions and trace element contents for the three granitic mylonite samples (i.e., WS20-01, WS20-02, and WS20-03) from the Wushan area, the three granitic gneiss samples (i.e., YL20-01, YL20-02, and YL20-03) from the Yuanlong area, and the three granitic gneiss samples (i.e., XY20-02, XY20-05, and XY20-07) from the Xinyang area. Representative zircon CL images for the samples are shown in Figure 6. Data for the zircon U–Pb ages, Hf isotopes, O isotopes, and trace elements are given in Tables S1–S4 in the Supplementary Materials, respectively.
Zircon grains from the nine samples were mostly colorless and transparent, subhedral to euhedral, and had lengths of 90–200 µm and aspect ratios of 1:1 to 1:5 (Figure 6). Most of the zircon grains showed oscillatory zoning (Figure 6) and had Th/U ratios higher than 0.1, indicating an igneous origin [88]. A few zircon grains in the samples from Wushan and Xinyang (i.e., WS20-01, WS20-02, WS20-03, and XY20-05) displayed oscillatory zoning bounded by narrow bright metamorphic rims (Figure 6a–c,h). In addition, many zircons from the Xinyang samples had inherited cores (Figure 6g,h).

4.2.1. Zircon U–Pb Ages

The zircon U–Pb isotopic data of the studied samples are presented in Figure 7. A majority of the zircon analyses yielded concordant ages.

Wushan Granitic Mylonites

Twenty-two analyses of zircons from sample WS20-01 yielded a weighted mean 206Pb/238U age of 933 ± 7 Ma (MSWD = 0.8) (Figure 7a). Eighteen analyses of zircons from sample WS20-02 yielded a weighted mean 206Pb/238U age of 930 ± 8 Ma (MSWD = 0.7) (Figure 7b). For sample WS20-03, 19 analyzed zircons yielded a weighted mean 206Pb/238U age of 927 ± 10 Ma (MSWD = 3.2) (Figure 7c). Data for these three samples gave an overall weighted mean age of 929 ± 5 Ma (MSWD = 6.1), which is interpreted as the crystallization age of the granitic magma from which the Wushan samples were formed.

Yuanlong Granitic Gneisses

A total of 15 analyses were performed for sample YL20-01, yielding a weighted mean 206Pb/238U age of 937 ± 8 Ma (MSWD = 0.5) (Figure 7d). Nineteen analyses of zircons from sample YL20-02 gave a weighted mean 206Pb/238U age of 924 ± 9 Ma (MSWD = 1.4) (Figure 7e). Nineteen analyses of zircons from sample YL20-03 yielded a weighted mean 206Pb/238U age of 935 ± 7 Ma (MSWD = 0.8) (Figure 7f). The three samples gave an overall weighted mean age of 931 ± 4 Ma (MSWD = 1.0), which is interpreted as the crystallization age of the granitic magma from which the Yuanlong samples were formed.

Xinyang Granitic Gneisses

Fifteen analyses of cores and rims of zircons from sample XY20-02 defined a discordia line with an upper intercept at 1727 ± 140 Ma and a lower intercept at 986 ± 85 Ma (MSWD = 4.3) (Figure 7g). Seven analyses of magmatic zircon rims gave a weighted mean 206Pb/238U age of 925 ± 9 Ma (MSWD = 0.36) (Figure 7g), close to the lower intercept of the concordia diagram and likely representing the protolith age of the granitic gneiss. The remaining eight inherited cores yielded four older concordant ages (i.e., 1887, 1497, 1425, and 1280 Ma) and four discordant analyses. Twenty-three analyses of zircons from sample XY20-05 plotted a discordia line with an upper intercept at 1621 ± 280 Ma and a lower intercept at 884 ± 100 Ma (MSWD = 26; Figure 7h). Nine analyses of magmatic zircon rims gave a weighted mean age of 921 ± 9 Ma (MSWD = 0.23) (Figure 7h). The remaining analyses included 4 older concordant ages (i.e., 1244, 1111, 1070, and 1035 Ma) and 10 discordant ages. Ten analyses of zircons from sample XY20-07 yielded a weighted mean 206Pb/238U age of 936 ± 16 Ma (MSWD = 0.16) (Figure 7i). The three samples gave an overall weighted mean age of 931 ± 7 Ma (MSWD = 2.3), which is interpreted as the crystallization age of the granitic magma from which the Xinyang samples were formed.

4.2.2. Zircon Trace Elements

Zircon trace element contents were obtained from the same analytical spots from where the zircon U–Pb data were acquired. The detailed analysis results for zircon trace elements are presented in Table S2. Chondrite-normalized REE patterns for the zircons are shown in Figure 8. Most of the analyzed zircon grains displayed enrichment in HREEs and depletion of LREEs ((La/Yb)N < 0.031), and they showed positive Ce anomalies (Ce/Ce* = 1.1–137) and negative Eu anomalies (Eu/Eu* = 0.00–0.58), similar to the characteristics of typical magmatic zircons (Corfu et al., 2003; Hoskin and Black, 2010). The presence of accessory mineral inclusions, such as apatite or titanite, can explain the LREE enrichment of some zircon grains [89,90].

4.2.3. Zircon Lu–Hf Isotopes

Zircon Lu–Hf isotope data of the granitic mylonites and granitic gneisses are presented in Table S3 and Figure 9.

Wushan Granitic Mylonite

The magmatic zircon grains of the three analyzed granitic mylonite samples showed a range in initial 176Hf/177Hf from 0.281935 to 0.282289 and εHf(t) values from −9.2 to 2.1, with a weighted mean εHf(t) value of −1.5 ± 0.4 (MSWD = 7.1) (Table S3 and Figure 9a). The calculated TDM2 ages ranged from 2.10 to 1.53 Ga, with a weighted mean TDM2 age of 1.70 Ga (Table S3 and Figure 9d).

Yuanlong Granitic Gneiss

The magmatic zircon grains from the Yuanlong granitic gneiss had 176Hf/177Hf values of 0.281874 to 0.282254 and corresponding εHf(t) values of −10.6 to 1.2, with a weighted mean εHf(t) value of −2.0 ± 0.5 (MSWD = 9.6) (Table S3 and Figure 9b). The calculated TDM2 ages ranged from 2.19 to 1.56 Ga, with a weighted mean TDM2 age of 1.73 Ga (Table S3 and Figure 9e).

Xinyang Granitic Gneiss

The magmatic zircons from the three samples of the Xinyang granitic gneiss had 176Hf/177Hf values of 0.282042 to 0.282285 and corresponding εHf(t) values from −4.9 to 1.6, with a weighted mean εHf(t) value of −0.2 ± 1.0 (MSWD = 30). The calculated TDM2 ages ranged from 1.89 to 1.55 Ga, with a weighted mean TDM2 age of 1.73 Ga (Table S3 and Figure 9f). The core domains of the inherited zircons showed a wide range in initial 176Hf/177Hf values from 0.281501 and 0.282244 and εHf(t) values ranging from −12.2 to 9.7 (Table S3). The calculated TDM2 ages ranged from 2.57 to 1.57 Ga (Table S3).

4.2.4. Zircon O Isotopes

Zircon O isotope data from the nine studied samples are given in Table S4. The δ18O values calculated from the experimental data are presented in Figure 9g–i.
Magmatic zircons from the nine samples showed a wide range of δ18O values (3.56‰ to 11.07‰) (Figure 9g-i). The weighted mean zircon δ18O values were calculated as 8.88‰ ± 0.22‰ (MSWD = 67) for the Wushan granitic mylonite, 9.27‰ ± 0.34‰ (MSWD = 101) for the Yuanlong granitic gneiss, and 8.57‰ ± 0.28‰ (MSWD = 140) for the Xinyang granitic gneiss.

5. Discussion

5.1. Age of Granitic Magmatism

Numerous studies have investigated the petrogenesis and tectonic setting of Neoproterozoic granitic rocks in the eastern part of the NQT. Early Neoproterozoic granitic gneisses from the region have yielded zircon U–Pb ages ranging from 980 to 880 Ma [19,30,34,37,39,40,41,42,51,52]. In the present study, new zircon U–Pb LA–ICP–MS data for granitic mylonites and granitic gneisses from the western margin of the NQT yielded early Neoproterozoic ages ranging from 936 to 921 Ma. These ages confirm that Precambrian granitic rocks of the QOB are not restricted to the eastern part of the NQT.
A comparison of our new ages with the published data of early Neoproterozoic granitic rocks from the NQT is presented in Figure 10. The protolith ages of the granitic mylonite and granitic gneiss lie between 1000 and 880 Ma, with a peak at ca. 930 Ma [19,31,36,40,41]. In addition, syenites and syenogranites from the Fangcheng area yielded crystallization ages of 874 to 843 Ma [18,51].
As inherited zircon cores represent restite of granitic rock sources in the deep continental crust, their ages are important for determining the nature of the sources. The large contribution (Figure 7g,h) of Paleo- to Mesoproterozoic (ca. 1.9–1.0 Ga) inherited zircon ages identified in the Xinyang granitic gneiss samples indicates that their protoliths were derived from Paleo- to Mesoproterozoic basement.

5.2. Nature of the Magma Source

Peraluminous granites constitute part of the orogenic belts and are formed through the anatexis of supracrustal rocks [4,5,6,7,9] and of sedimentary rocks in particular [4,91]. However, various studies have demonstrated that the source of peraluminous granites is complex. Such granites can be derived from either pelite [92] or sandstone or greywacke [93,94]). Meta-igneous rocks have also been reported as a possible source of peraluminous granite [95,96].
The granitic mylonites and granitic gneisses from the Tianshui area of the western margin of the NQT showed similar major and trace element contents and zircon U–Pb–Hf–O isotopic compositions, indicating that the magma that formed these rocks was derived from the same source. The Hf–O isotopic compositions of zircon can be used to trace the source of granitic magma, whether derived from reworked crust, juvenile crust, or mantle-derived materials (Hawkesworth and Kemp, 2006; Kemp et al., 2010). The studied granitic mylonites and granitic gneisses from the Tianshui area showed high SiO2 (68.03–75.68 wt%) and low MgO (0.11–1.16 wt%) and Mg# (16–37). Most of the analyzed zircons were higher than the O isotope value of normal mantle (5.3‰ ± 0.6‰ (2σ); [97]. The high δ18O values and lack of relationship between the Hf and O isotopic compositions (Figure 11a) suggest that they were derived from heterogeneous crustal rocks rather than magma mixing [98].
We compiled previously published Hf–O isotope data (from the areas of Zhaigen, Fangzhuang, and Dehe) [41] for the eastern part of the NQT and compared them with our data to constrain the source of Neoproterozoic granitic magmatism in the QOB (Figure 11). Magmatic zircons from the studied granitic rocks had similar protolith ages (936–921 Ma; Figure 7), Hf isotopic compositions (εHf(t) = −10.6 to 2.1), and TDM2 ages (2.20 to 1.53 Ga) to published data for early Neoproterozoic granitic rocks from the eastern part of the NQT (Table S3 and Figure 11b). The wide range of εHf(t) values and TDM2 ages suggests that the early Neoproterozoic granitic magma of the QOB was derived from reworked Paleoproterozoic crust (2.2 to 1.6 Ga) and Mesoproterozoic juvenile crust (1.6 to 1.5 Ga) (Figure 11b).
The Hf isotopic composition from the core domains of inherited zircons is crucial in determining the composition of the source. The core domains of inherited zircons identified from the Xinyang samples showed a wide range in εHf(t) values from −12.2 to 9.7 and calculated TDM2 ages of 2.57 to 1.57 Ga (Table S3). We compared our data with a compilation of Hf isotopic data for detrital zircons from the Qinling Group [99,100] (Figure 11b,c). Most of the magmatic zircons and core domains of inherited zircons fell in the range of the Qinling Group (Figure 11b,c), implying that the Qinling Group was an important source for the studied early Neoproterozoic granitic rocks.
Zircon δ18O values can help to establish whether granitic magma originated directly from the mantle or was influenced by the involvement of supracrustal material [101,102]. Most of the analyzed zircons showed high δ18O values (Figure 9g–i and Figure 11a and Table S4) that were similar to published data for early Neoproterozoic granitic rocks from the areas of Zhaigen, Fangcheng, and Dehe in the eastern part of the NQT (Figure 11a). Moreover, these zircons showed a wide range of εHf(t) values (−12.2 to 13.6; Figure 11a). High δ18O and wide-ranging εHf(t) values are generally interpreted as indicating the involvement of intensely weathered sedimentary rocks in the magma source [97]. However, seven zircons from samples WS20-01, YL20-01, YL20-02, XY20-02, and XY20-05 had lower δ18O values of 3.56‰ to 4.73‰ and a restricted range of εHf(t) values (Figure 11a; −5.2 to 1.6), indicating the involvement of igneous rocks that had been subjected to high-temperature water–rock interaction [103,104]. The Hf–O isotopic compositions of the analyzed zircons indicate that the early Neoproterozoic granitic magma was derived from two crustal sources: predominant supracrustal sedimentary rocks and subordinate igneous rocks.
The source materials for the strongly peraluminous granitic rocks can be distinguished by the CaO/Na2O ratio [105]. Apart from two samples from the Xinyang area, data for the studied granitic rocks and previous data from the NQT show CaO/Na2O ratios of >0.3. These data indicate that the source materials of early Neoproterozoic granitic rocks in the NQT was predominantly of supracrustal psammite (Figure 11d).

5.3. Petrogenesis

Original magmatic compositions are potentially influenced by the metamorphism and alteration. Therefore, the element mobility should be evaluated prior to any petrogenetic discussions. Firstly, all samples possessed loss on ignition (LOI) values of less than 1.6 wt% (Table 2), inconsistent with the strong hydration in these rocks during metamorphism and alteration. Some major elements (e.g., Na and K) and large ion lithophile elements (e.g., Rb and Ba) were regarded to be mobile during post-magmatic metamorphism and alteration [110]. Nevertheless, most samples had coherent patterns in the chondrite-normalized REE diagram and the primitive-mantle-normalized spider diagram (Figure 5), confirming the preservation of the original signatures. Previous studies demonstrated that REE and transition metals are relatively immobile during greenschist-to amphibolite-facies metamorphism and alterations [111,112,113]. Therefore, the REE and HFSE (i.e., Th, Nb, Ta, Zr, and Hf) compositions of our samples were not affected by the regional greenschist- to amphibolite-facies metamorphism and alteration and essentially reflect the original magmatic compositions.
Although the magma source is considered to be the primary control on the geochemical composition of granitic rocks [114]), rock composition can also be influenced by magmatic processes such as partial melting, magma mixing, and fractional crystallization [115]. Previous studies have demonstrated that strongly peraluminous granitoids can be derived by mixing of mantle- and crust-derived melts [103,116,117,118]. However, as discussed above, the magma mixing model is unsuitable for explaining the genesis of the Tianshui granitic rocks.
Data of the Tianshui granitic rocks exhibited a trend of fractional crystallization in an La/Sm–Sm diagram (Figure 12a). The major elements of Fe2O3 and TiO2 showed negative relationships with Zr in Harker diagrams (Figure 12d,g), which suggests that crystal fractionation involved Ti–Fe-oxides. Although the samples showed strong negative Eu anomalies in chondrite-normalized REE patterns (Figure 5b), a weak relationship was observed between Eu/Eu* and Zr in Harker diagrams (Figure 12f), indicating that fractional crystallization of plagioclase was minor during magmatic evolution.
The geochemical composition of the granitic rocks is also affected by the melting conditions such as water content, temperature, and pressure [94,119]. Dehydration melting occurs through the breakdown of hydrous minerals [120], whereas hydration melting proceeds via the flux of free water into the lithological system [121,122,123]. The studied granitic mylonites and granitic gneisses showed high K2O contents (2.99–5.41 wt%) and Rb/Sr ratios (0.93–22.7), low Na2O/K2O ratios (0.45–0.94), and negative Eu anomalies, similar to the values expected from melt produced by dehydration melting [123].
The temperature of the primary magma can be estimated based on zircon saturation thermometry [124] and Ti-in-zircon thermometry [125,126,127]. Calculated TZr (temperature of zircon saturation) values for the studied granitic rocks were between 749 and 867 °C (mean = 823 °C) (Table 2). Schiller and Finger (2019) [128] proposed that a constant temperature correction by adding 70 °C to the calculated Ti-in-zircon temperatures can give reliable results for ilmenite-bearing granites (that is, almost all S-type and many I-type granites). According to this method, the calculated Ti-in-zircon temperatures for the studied granitic rocks ranged from 700 to 1390 °C (mean = 825 °C) (Figure 13a; [127,128]. The results for the zircon saturation and Ti-in-zircon thermometric methods gave a consistent temperature of ca. 825 °C, corresponding to the temperature for “hot granites” [129]. In addition, Miller et al. (2003) [129] suggested that the TZr values of granites that contain abundant inherited zircons represent the upper temperature limit of the magma. Therefore, our calculated TZr for the Xinyang samples, which contained numerous inherited zircons, represents a maximum magma temperature of 825 °C.
The Tianshui granitic rocks showed strong negative Eu anomalies in chondrite-normalized REE patterns, indicating that plagioclase was an important residual phase. Numerous experimental studies have shown that plagioclase dominates residues at pressures of <1.0 GPa (depth < 30 km) [130,131,132]. Stevens et al. (1997) [133] conducted experiments of biotite dehydration melting of metapelites and metagreywackes at 0.5 GPa in a temperature range of 750 to 830 °C and produced peraluminous leucogranites in equilibrium with granulite-facies residual mineral assemblages. Combining these previous results with the magma temperature calculated in the present study (825 °C), we infer that the Tianshui granitic magma was generated by biotite dehydration melting [123,134].
In summary, the early Neoproterozoic granitic rocks from the western margin of the NQT were most likely derived from biotite dehydration melting of heterogeneous sources at lower crustal depths.

5.4. Tectonic Implications

The Wushan, Yuanlong, and Xinyang granitic mylonites and granitic gneisses contained aluminum-rich minerals of muscovite and garnet and had high A/CNK values of 1.12 to 1.34. These characteristics suggest that their protolith was peraluminous granite. Early Neoproterozoic peraluminous granites have also been identified in the eastern part of the NQT and AQKNQ [20,27,31,37,38,55,56,57,60,61,136]). The combined zircon U–Pb–Hf–O isotopic compositions of the early Neoproterozoic peraluminous granites from the QOB (Figure 10 and Figure 11) confirmed that these rocks were derived predominantly from the partial melting of supracrustal sedimentary rocks.
Peraluminous granites are generally considered to form in syn-collisional settings [135,137,138]. Early Neoproterozoic peraluminous granites in the eastern part of the NQT and AQKNQ are thought to have formed during syn-collisional processes associated with the assembly of the Rodinia supercontinent [20,27,31,37,38,55,56,57,60,61,136]. Similarities in age and tectonic setting suggest a correlation of Neoproterozoic granitic magmatism between the NQT and AQKNQ. This correlation implies NQT and AQKNQ might be within a single tectonic domain in the early Neoproterozoic. Moreover, all of the Neoproterozoic peraluminous granites of the NQT fell in the syn-collision and island arc fields in an Nb versus Y tectonic discrimination diagram (Figure 13b; [135]), further indicating that the granitic rocks from the NQT were also formed in a syn-collisional setting. On the basis of our new results and those of previous studies, we propose a genetic model for the early Neoproterozoic peraluminous granites of the NQT (Figure 14a). In this model, the early Neoproterozoic peraluminous granites were formed by partial melting of predominant subducted supracrustal sedimentary rocks and subordinate igneous rocks at lower crustal depths during the syn-collision at 936–921 Ma.
Neoproterozoic alkaline rocks (ca. 874-843 Ma) have been reported from the Fangcheng area in the eastern part of the NQT and are considered to have been emplaced in a post-collisional extensional regime [18,51]. In addition, the early Neoproterozoic mafic rocks were emplaced at 840~750 Ma [68,70,71] Diwu and Long (2018) [139], also suggesting that these rocks were formed in an extensional setting related to the breakup of the Rodinia supercontinent.
During ca. 936-843 Ma, the syn-collisional (Figure 14a; 936–921 Ma) to post-collisional extensional setting (Figure 14b; at least ca. 874 Ma) resulted in the development of numerous early Neoproterozoic granitic and mafic intrusions in the QOB, in response to the assembly and breakup of the Rodinia supercontinent.

6. Conclusions

We investigated Neoproterozoic peraluminous granitic rocks from the Tianshui area, western margin of the NQT, China, using whole-rock geochemistry and zircon trace elements and U–Pb–Hf–O isotopes to determine their source nature, petrogenesis, and tectonic setting. Our main conclusions are as follows.
(1)
The early Neoproterozoic peraluminous granitic rocks from the western margin of the NQT were formed at 936–921 Ma. The ages from the core domains of inherited zircons indicated that the protolith of the granitic rocks was derived from Paleo- to Mesoproterozoic basement;
(2)
The early Neoproterozoic peraluminous granitic rocks were derived from reworked Paleoproterozoic crust (2.2 to 1.6 Ga) and Mesoproterozoic juvenile crust (1.6 to 1.5 Ga). The early Neoproterozoic granitic magma is inferred to have been derived from two crustal sources: predominant supracrustal sedimentary rocks and subordinate igneous rocks;
(3)
The early Neoproterozoic peraluminous granitic rocks were formed by biotite dehydration melting at lower crustal depths, involving the anatexis of subducted crustal materials in a syn-collisional setting.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min12070910/s1, Table S1: LA-ICPMS zircon U-Pb isotope data for the early Neoproterozoic granitic rocks in the western margin of the NQT [127]; Table S2: Zircon trace element concentrations for the early Neoproterozoic granitic rocks in the western margin of the NQT; Table S3: LA-MCICPMS zircon Lu-Hf isotope data for the early Neoproterozoic granitic rocks in the western margin of the NQT [84]; Table S4: SIMS zircon O isotope data for the early Neoproterozoic granitic rocks in the western margin of the NQT.

Author Contributions

Methodology, Z.B.; investigation, A.L.; writing—original draft preparation, G.Y.; writing—review and editing, J.Z. and H.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the National Natural Science Foundation of China (41688103) and the Science and Technology Plan Project of Shaanxi Province, China (2019JCW-19).

Data Availability Statement

Data supporting the reported results are available in this article and in supplementary materials.

Acknowledgments

We thank Mengqi Jin and Yanguang Li for their help with Hf isotopic analyses, Jianqi Wang and Ye Liu for their help with major and trace element analysis, and Xiaoping Xia and Zexian Cui for their help with SIMS zircon O isotope analysis.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Tectonic map of China (modified after [14]); (b) Simplified geological map of the western margin of the NQT (modified after [14,25]).
Figure 1. (a) Tectonic map of China (modified after [14]); (b) Simplified geological map of the western margin of the NQT (modified after [14,25]).
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Figure 2. (a) Geological map of the Wushan area (modified after [26]); (b) geological map of the Yuanlong area (modified after [52]); (c) geological map of the Xinyang area (modified after [52]).
Figure 2. (a) Geological map of the Wushan area (modified after [26]); (b) geological map of the Yuanlong area (modified after [52]); (c) geological map of the Xinyang area (modified after [52]).
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Figure 3. Field photographs (a–c) and photomicrographs (d–f) of granitic mylonites and granitic gneisses from the Wushan, Yuanlong, and Xinyang areas. Qz—quartz; Pl—plagioclase; Ms—muscovite; Kfs—K-feldspar; Bt—biotite.
Figure 3. Field photographs (a–c) and photomicrographs (d–f) of granitic mylonites and granitic gneisses from the Wushan, Yuanlong, and Xinyang areas. Qz—quartz; Pl—plagioclase; Ms—muscovite; Kfs—K-feldspar; Bt—biotite.
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Figure 4. Major element compositions of the studied granitic mylonites and granitic gneisses from the western margin of the NQT: (a) total alkalis versus silica (TAS; after [85]) classification diagram; (b) K2O versus SiO2 classification diagram [86]); (c) alumina saturation index diagram [87]. Data sources for the previous research: [19,39,40,41,42,52].
Figure 4. Major element compositions of the studied granitic mylonites and granitic gneisses from the western margin of the NQT: (a) total alkalis versus silica (TAS; after [85]) classification diagram; (b) K2O versus SiO2 classification diagram [86]); (c) alumina saturation index diagram [87]. Data sources for the previous research: [19,39,40,41,42,52].
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Figure 5. (a) Chondrite-normalized REE patterns and (b) primitive-mantle-normalized trace element spider diagrams of the studied samples. Data sources for the previous research: [19,39,40,41,42,52].
Figure 5. (a) Chondrite-normalized REE patterns and (b) primitive-mantle-normalized trace element spider diagrams of the studied samples. Data sources for the previous research: [19,39,40,41,42,52].
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Figure 6. Representative zircon CL images with ages/εHf(t)/δ18O values for the granitic mylonites from Wushan (a–c) and granitic gneisses from Yuanlong (d–f) and Xinyang (g–i) in the western margin of the NQT. The smaller circles 10 µm in diameter, the medium circles 30 µm in diameter, and the dashed circles 40 µm in diameter represent analytical sites for the O, U–Pb, and Lu–Hf isotope determinations, respectively.
Figure 6. Representative zircon CL images with ages/εHf(t)/δ18O values for the granitic mylonites from Wushan (a–c) and granitic gneisses from Yuanlong (d–f) and Xinyang (g–i) in the western margin of the NQT. The smaller circles 10 µm in diameter, the medium circles 30 µm in diameter, and the dashed circles 40 µm in diameter represent analytical sites for the O, U–Pb, and Lu–Hf isotope determinations, respectively.
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Figure 7. LA–ICP–MS zircon U–Pb concordia diagrams for the granitic mylonites from Wushan (a–c) and granitic gneisses from Yuanlong (d–f) and Xinyang (g–i) in the western margin of the NQT.
Figure 7. LA–ICP–MS zircon U–Pb concordia diagrams for the granitic mylonites from Wushan (a–c) and granitic gneisses from Yuanlong (d–f) and Xinyang (g–i) in the western margin of the NQT.
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Figure 8. Chondrite-normalized REE patterns for zircons from the granitic mylonites from Wushan (a–c) and granitic gneisses from Yuanlong (d–f) and Xinyang (g–i) in the western margin of the NQT.
Figure 8. Chondrite-normalized REE patterns for zircons from the granitic mylonites from Wushan (a–c) and granitic gneisses from Yuanlong (d–f) and Xinyang (g–i) in the western margin of the NQT.
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Figure 9. Histograms of the εHf(t), TDM2, and δ18O values of zircons from the granitic mylonites from Wushan (a–c) and granitic gneisses from Yuanlong (d–f) and Xinyang (g–i) in the western margin of the NQT.
Figure 9. Histograms of the εHf(t), TDM2, and δ18O values of zircons from the granitic mylonites from Wushan (a–c) and granitic gneisses from Yuanlong (d–f) and Xinyang (g–i) in the western margin of the NQT.
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Figure 10. U–Pb zircon age histogram with a probability density plot for Neoproterozoic granitic rocks from the NQT. Data sources for the eastern part of the NQT: [18,19,20,30,32,33,34,36,37,39,40,41,42,51,52].
Figure 10. U–Pb zircon age histogram with a probability density plot for Neoproterozoic granitic rocks from the NQT. Data sources for the eastern part of the NQT: [18,19,20,30,32,33,34,36,37,39,40,41,42,51,52].
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Figure 11. (a) Plot of δ18O versus εHf(t) for zircons from Neoproterozoic granitic rocks from the QOB. The data source for the previous research: [41]. (b) Plot of εHf(t) versus U–Pb age for zircons from the studied Neoproterozoic granitic mylonites from Wushan and granitic gneisses from Yuanlong and Xinyang in the western margin of the NQT as well as gneisses from the Qinling Group (QLG) in the NQT. Data sources: Fangcheng [51]; previous research for the biotite monzonitic gneiss and granites from Fangzhuang, Zhaigen, and Dehe [20,41] and the QLG [99,100]. (c) U–Pb zircon age histogram with a probability density plot for the QLG and comparison with ages from the core domains of inherited zircons of this study. Data sources for the QLG: [99,100,106,107,108,109]. (d) Ca/Na–Rb/Sr diagram [105] for the studied granitic mylonites from Wushan and granitic gneisses from Yuanlong and Xinyang. Data sources for the previous research: [19,39,40,41,42,52].
Figure 11. (a) Plot of δ18O versus εHf(t) for zircons from Neoproterozoic granitic rocks from the QOB. The data source for the previous research: [41]. (b) Plot of εHf(t) versus U–Pb age for zircons from the studied Neoproterozoic granitic mylonites from Wushan and granitic gneisses from Yuanlong and Xinyang in the western margin of the NQT as well as gneisses from the Qinling Group (QLG) in the NQT. Data sources: Fangcheng [51]; previous research for the biotite monzonitic gneiss and granites from Fangzhuang, Zhaigen, and Dehe [20,41] and the QLG [99,100]. (c) U–Pb zircon age histogram with a probability density plot for the QLG and comparison with ages from the core domains of inherited zircons of this study. Data sources for the QLG: [99,100,106,107,108,109]. (d) Ca/Na–Rb/Sr diagram [105] for the studied granitic mylonites from Wushan and granitic gneisses from Yuanlong and Xinyang. Data sources for the previous research: [19,39,40,41,42,52].
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Figure 12. (a) La/Sm–Sm diagram illustrating trends in the partial melting and fractional crystallization of the studied granitic rocks from the western margin of the NQT. Harker diagrams (b–i) for major and selected trace elements for the studied samples. Data source for the previous research: [52].
Figure 12. (a) La/Sm–Sm diagram illustrating trends in the partial melting and fractional crystallization of the studied granitic rocks from the western margin of the NQT. Harker diagrams (b–i) for major and selected trace elements for the studied samples. Data source for the previous research: [52].
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Figure 13. (a) Ti-in-zircon temperature [127] histogram with probability density plot for the studied granitic mylonites and granitic gneisses from the western margin of the NQT. (b) Nb–Y diagram [135] for the studied granitic mylonites from Wushan and granitic gneisses from Yuanlong and Xinyang. Data sources for the previous research: [19,39,40,41,42,52]. VAG—volcanic arc granites; ORG—ocean ridge granites; WPG—within plate granites; Syn-COLG—syn-collision granites.
Figure 13. (a) Ti-in-zircon temperature [127] histogram with probability density plot for the studied granitic mylonites and granitic gneisses from the western margin of the NQT. (b) Nb–Y diagram [135] for the studied granitic mylonites from Wushan and granitic gneisses from Yuanlong and Xinyang. Data sources for the previous research: [19,39,40,41,42,52]. VAG—volcanic arc granites; ORG—ocean ridge granites; WPG—within plate granites; Syn-COLG—syn-collision granites.
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Figure 14. (a) Schematic illustrations showing the tectonic evolution of the NQT during the Sny-collisional environment in the early Neoproterozoic. (b) Schematic illustrations showing the tectonic evolution of the NQT during the Post-collisional environment in the early Neoproterozoic.
Figure 14. (a) Schematic illustrations showing the tectonic evolution of the NQT during the Sny-collisional environment in the early Neoproterozoic. (b) Schematic illustrations showing the tectonic evolution of the NQT during the Post-collisional environment in the early Neoproterozoic.
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Table
Table 1. A summary of the localities, lithology, and protolith ages of the early Neoproterozoic granitic rocks in the QOB.
Table 1. A summary of the localities, lithology, and protolith ages of the early Neoproterozoic granitic rocks in the QOB.
LocationRock TypeProtolith Age (Ma)Analytical MethodReference
West Qinling
WushanGranitic mylonite927 ± 10–933 ± 7LA-ICPMSThis study
XinyangGranitic gneiss921 ± 9–936 ± 16LA-ICPMSThis study
YuanlongGranitic gneiss924 ± 9–937 ± 8LA-ICPMSThis study
WushanGranitic mylonite951 ± 18SHRIMP[26]
WushanGranitic mylonite910 ± 5 LA-ICPMS[27]
YuanlongBiotite monzonite granitic gneiss915 ± 8 LA-ICPMS[28]
XinyangBiotite monzonite granitic gneiss936 ± 4–979 ± 5 LA-ICPMS[28]
YuanlongGranitic gneiss924 ± 3 LA-ICPMS[29]
East Qinling
SongshugouGarnet plagioamphibolite983 ± 140 Sm-Nd [30]
DeheBiotite adamellite964 ± 5–943 ± 18 TIMS/SHRIMP[31]
CaiaoGranodiorite889 ± 10 LA-ICPMS[32]
HuangbaiyuGranodiorite670 ± 40 Rb-Sr[33]
LajimiaoDiorite973 ± 60 LA-ICPMS[34]
LianghekouMonzonite granite gneiss852 ± 2 U-Pb isochron[35]
NiujiaoshanGranitic gneiss959 ± 4 TIMS[36]
XilaoyuBiotite granite gneiss956 ± 8 TIMS[37]
NiujiaoshanDimica monzonite granite gneiss955 ± 5 TIMS[37]
GuojiapingQuartz monzonite gneiss953 ± 14 SHRIMP[37]
DeheBiotite monzonite granite gneiss943 ± 18–971 ± 10 SHRIMP/TIMS[37]
ZhaigenBiotite granite914 ± 10 SHRIMP[37]
TaibaigongTonaldiorite gneiss863 ± 17–911 ± 18 TIMS/SHRIMP[37]
HuangtuniuBiotite granite gneiss844 ± 4 TIMS[37]
NiujiaoshanGneissic granite955 ± 13 SHRIMP[38]
DeheBiotite monzonite granite gneiss948 ± 9 LA-ICPMS[19]
FangzhuangGranitic mylonite933 ± 9 SIMS[19]
ShicaogouBiotite adamellite 925 ± 11 LA-ICPMS[39]
XilaoyuGranodioritic gneiss956 ± 8 ID-TIMS[40]
NiujiaoshanTwo-mica granitic gneiss955 ± 5 ID-TIMS[40]
DeheBiotite granitic gneiss943 ± 18 SHRIMP[40]
GuanshanBiotite granitic gneiss929 ± 16 SHRIMP[40]
ZhaigenBiotite granitic gneiss914 ± 10 SHRIMP[40]
DeheBiotite monzonitic gneiss925 ± 23 LA-ICPMS[20]
ZhaigenGranite902 ± 7 LA-ICPMS[41]
FangzhuangGranite934 ± 9 LA-ICPMS[41]
DeheGranite942 ± 7 SHRIMP[41]
MashankouGranitic gneiss929 ± 7LA-ICPMS[42]
Table
Table 2. Whole-rock major and trace element data for the early Neoproterozoic granitic rocks in the western margin of the NQT. a A/CNK = Al2O3/(CaO + Na2O + K2O) molar ratio; A/NK = Al2O3/(Na2O + K2O) molar ratio; Mg# = Mg/(Mg+Fe) molar ratio; N = normalization following McDonough and Sun (1995); Eu/Eu* = Eu/(SmN * GdN)1/2.
Table 2. Whole-rock major and trace element data for the early Neoproterozoic granitic rocks in the western margin of the NQT. a A/CNK = Al2O3/(CaO + Na2O + K2O) molar ratio; A/NK = Al2O3/(Na2O + K2O) molar ratio; Mg# = Mg/(Mg+Fe) molar ratio; N = normalization following McDonough and Sun (1995); Eu/Eu* = Eu/(SmN * GdN)1/2.
SampleWS20-01WS20-02WS20-03YL20-01YL20-02YL20-03XY20-02XY20-05XY20-07
Rock Type Granitic MyloniteGranitic Gneiss
Major Oxides (%)
SiO270.4268.0369.4770.53 70.66 71.7075.6871.5975.29
TiO20.490.490.560.50 0.58 0.490.130.400.08
Al2O314.0915.6414.1614.19 14.00 13.9312.6714.0913.30
TFe2O33.833.754.303.44 4.41 3.681.333.361.11
MnO0.070.060.070.07 0.07 0.060.030.060.02
MgO1.140.961.160.95 1.14 0.920.220.730.11
CaO2.192.452.501.97 1.24 1.340.641.510.54
Na2O2.833.272.652.61 2.10 2.082.572.682.87
K2O3.013.822.993.93 4.36 4.585.414.195.22
P2O50.110.120.110.11 0.12 0.090.090.130.19
LOI1.521.271.551.38 1.25 1.111.131.261.11
TOTAL99.70 99.86 99.52 99.68 99.93 99.98 99.90100.0099.84
A/CNK a1.18 1.12 1.16 1.17 1.34 1.29 1.12 1.20 1.17
A/NK a1.78 1.64 1.86 1.66 1.71 1.66 1.25 1.57 1.28
Na2O/K2O0.94 0.86 0.89 0.66 0.48 0.45 0.48 0.64 0.55
Na2O + K2O5.84 7.09 5.64 6.54 6.46 6.66 7.98 6.87 8.09
Mg#37 34 35 35 34 33 25 30 16
TZr (°C)828 836 829 844 867 864 765 828 749
Trace Elements (ppm)
Li47.6 34.7 40.0 32.7 32.7 42.9 18.9 55.2 71.9
Be2.83 3.39 3.19 2.58 2.58 2.45 3.28 2.62 2.58
Sc9.79 10.1 11.0 9.51 9.51 9.58 2.84 6.79 2.78
V35.7 37.9 41.3 43.2 43.2 43.2 4.33 25.4 1.45
Cr18.4 19.2 21.8 21.8 21.8 24.4 2.28 12.2 0.88
Co6.13 5.85 6.87 6.94 6.94 7.46 1.24 5.31 0.32
Ni7.18 7.30 9.15 10.2 10.2 99.9 1.31 5.51 0.59
Cu5.73 5.09 8.77 8.30 8.30 16.78 4.99 6.67 1.06
Zn45.5 54.2 70.2 22.8 22.8 49.5 27.5 84.4 27.7
Ga18.4 19.2 18.4 18.6 18.6 22.0 13.8 18.3 20.0
Ge1.52 1.62 1.63 1.67 1.67 1.78 1.62 1.71 2.19
Rb121 159 155 146 146 196 259 233 430
Sr126 145 121 156 156 99.4 33.1 106 18.9
Zr196 198 208 217 217 209 75 163 59.9
Nb10.9 10.9 11.9 12.7 12.7 11.5 6.69 10.9 10.5
Cs4.99 12.0 17.9 5.16 5.16 11.9 7.50 18.6 13.4
Ba593 668 529 799 799 858 299 533 36.9
Hf5.43 5.50 5.66 5.86 5.86 5.61 2.72 4.64 2.70
Ta1.01 0.93 0.95 1.03 1.03 0.93 0.94 0.97 1.92
Pb27.2 36.9 21.1 34.6 34.6 35.6 32.7 14.3 25.3
U3.71 4.72 5.04 2.94 2.94 3.03 2.16 1.66 3.97
Th15.7 16.3 17.3 20.4 20.4 20.0 12.3 14.3 11.9
REE (ppm)
La31.6 29.5 33.1 43.0 43.0 40.9 13.9 31.4 7.87
Ce66.5 62.6 70.0 90.7 90.7 83.7 32.9 65.1 17.1
Pr7.70 7.32 8.16 10.4 10.4 9.83 3.59 7.57 2.08
Nd28.3 26.6 30.0 39.6 39.6 35.9 13.7 27.7 7.41
Sm5.85 5.41 6.22 7.76 7.76 7.04 3.97 5.77 2.54
Eu0.95 0.90 0.99 1.34 1.34 1.30 0.30 0.90 0.08
Gd5.33 4.78 5.72 6.85 6.85 6.09 4.37 5.20 2.77
Tb0.83 0.77 0.91 1.00 1.00 0.90 0.90 0.82 0.66
Dy5.02 4.80 5.52 5.84 5.84 5.27 6.06 4.83 4.11
Ho0.97 0.97 1.06 1.11 1.11 1.04 1.23 0.91 0.63
Er2.81 2.98 3.09 3.24 3.24 3.07 3.65 2.62 1.48
Tm0.41 0.45 0.44 0.46 0.46 0.45 0.55 0.38 0.19
Yb2.73 3.08 2.86 2.94 2.94 2.89 3.55 2.39 1.10
Lu0.40 0.46 0.42 0.44 0.44 0.44 0.52 0.35 0.14
Y27.9 27.2 30.7 33.0 33.0 30.9 38.5 28.2 22.7
Rb/Sr0.96 1.10 1.27 0.93 0.93 1.97 7.85 2.20 22.7
(La/Yb) N a8 7 8 10 10 10 3 9 5
Eu/Eu* a0.51 0.53 0.50 0.55 0.43 0.59 0.22 0.49 0.09
ΣREE187 178 199 248 248 230 128 184 71
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Yang, G.; Zhang, J.; Zhang, H.; Bao, Z.; Lin, A. Petrogenesis and Tectonic Implications of the Neoproterozoic Peraluminous Granitic Rocks from the Tianshui Area, Western Margin of the North Qinling Terrane, China: Evidence from Whole-Rock Geochemistry and Zircon U–Pb–Hf–O Isotopes. Minerals 2022, 12, 910. https://doi.org/10.3390/min12070910

AMA Style

Yang G, Zhang J, Zhang H, Bao Z, Lin A. Petrogenesis and Tectonic Implications of the Neoproterozoic Peraluminous Granitic Rocks from the Tianshui Area, Western Margin of the North Qinling Terrane, China: Evidence from Whole-Rock Geochemistry and Zircon U–Pb–Hf–O Isotopes. Minerals. 2022; 12(7):910. https://doi.org/10.3390/min12070910

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Yang, Gang, Juan Zhang, Hongfu Zhang, Zhian Bao, and Abing Lin. 2022. "Petrogenesis and Tectonic Implications of the Neoproterozoic Peraluminous Granitic Rocks from the Tianshui Area, Western Margin of the North Qinling Terrane, China: Evidence from Whole-Rock Geochemistry and Zircon U–Pb–Hf–O Isotopes" Minerals 12, no. 7: 910. https://doi.org/10.3390/min12070910

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