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Article

Magmatic Garnet and Magma Evolution in Cuonadong Leucogranites: Constraints from Petrology and Mineral Geochemistry

by
Haibo Yan
1,2,
Deshui Yu
3,*,
Shoujing Wang
3 and
Chi Ma
3
1
State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
2
CAS Center for Excellence in Deep Earth Science, Guangzhou 510640, China
3
Zhengzhou Institute of Multipurpose Utilization of Mineral Resources, Chinese Academy of Geological Sciences, Zhengzhou 450006, China
*
Author to whom correspondence should be addressed.
Minerals 2022, 12(10), 1275; https://doi.org/10.3390/min12101275
Submission received: 20 August 2022 / Revised: 6 October 2022 / Accepted: 7 October 2022 / Published: 9 October 2022
(This article belongs to the Special Issue Mineral/Ore Growth: From the Ions to the Macrocrystals)
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Abstract

:
Located at the eastern segment of the Tethyan Himalayan tectonic unit, the Cuonadong leucogranites (muscovite granite and pegmatite) have a mineral assemblage of quartz, plagioclase, and alkali feldspar, as well as muscovite and garnet. Garnets in both muscovite granite and pegmatite belong to the almandine–spessartine solid solution, with minor andradite, grossular, and pyrope, and show a specific Mn zoning of a relatively rim-ward Mn depletion trend. However, a few garnets in pegmatite show grossular enrichment towards the rim. All the analyzed garnets are characterized by HREE enrichment and LREE depletion with obvious Eu anomalies. The difference is that garnets from the muscovite granite show distinct left-declined or flat HREE patterns, while those from the pegmatite are featured by flat or right-declined HREE patterns. Moreover, garnets from the pegmatite show relatively more distinct HREE- and Y-rich cores compared with those from the muscovite granite. The MnO concentration, spessartine content, and MnO/(MnO + FeO) ratio of the garnets from the Cuonadong dome increase from the muscovite granite to the pegmatite, suggesting that the pegmatite likely formed from a more evolved environment. Elevated grossular and CaO contents of the garnet rim in the pegmatite may reflect an influence of fluids in their composition. The major and trace element compositions and zoning textures of garnets from the Cuonadong leucogranites suggest a magmatic origin and a formation at moderately low temperatures and relatively low-pressure conditions. From the muscovite granite to the pegmatite, the system entered a fluid-rich environment and the garnets from the pegmatite likely crystallized from a lower-temperature fluid.

1. Introduction

The Himalayan orogenic belt is the product of the Cenozoic India–Eurasia collision. In the middle and late period of the collision (Late Eocene–Miocene), the crustal material underwent intense anatexis [1,2], and formed two granite belts extending thousands of kilometers, namely, the Tethyan Himalayan leucogranite belt and the High Himalayan leucogranite belt [3]. The Tethyan Himalayan leucogranite belt, located in the northern part of the Himalayan orogenic belt, has potential rare metal resources related to granite, such as Li, Be, Nb, Ta, W, and Sn [3,4,5,6,7,8,9,10,]. The Cuonadong dome, located in the eastern segment of the Tethyan Himalayan tectonic unit, is the first dome in the Himalayan orogenic belt to be explored and delineated for rare polymetallic ore bodies [11]. The leucogranite of Cuonadong is distributed in the core of the dome.
Granitic melts not only experience solidification and crystallization but also magmatic–hydrothermal processes, even at the late stage of magmatic evolution [12,13]. The magmatic–hydrothermal process at the late stage of magmatic evolution has been recorded especially in highly fractionated granite [12,14]. Minerals usually record the evolution history of granitic magmas [15,16,17,18], such as garnet, apatite, and tourmaline. Garnet, as a common accessory mineral in leucogranite, is widely distributed in the Cuonadong dome, and its texture and chemical changes record important information about the process of granitic magma evolution [19,20]. On the one hand, the different components of garnet can reflect the formation environment and genesis of granitic magma. On the other hand, garnet is usually enriched with trace elements such as REE, Zr, Hf, and Y, and the geochemical behavior of these elements can indicate the evolution of granitic magma [21,22,23,24]. Volatile and magmatic fluid phases are separated from the melt at the late stage during the crystallization of highly fractionated granite [16,18,25,26]. Thus, garnet is widely used to indicate the degree of granitic magma evolution and forming conditions [27,28,29,30].
A great deal of research has been done on Cuonadong leucogranite, mainly focusing on geochronology, geochemistry, mineralogy, and petrogenesis [7,8,9,11,31,32,33,34]. However, there are still few studies on the elemental evolution characteristics of minerals between different types of leucogranite, the physicochemical conditions for the mineral formation, and the magmatic evolution process.
In this paper, the garnets in the muscovite granite and pegmatite from the Cuonadong dome were selected as the research object, and the major and trace element geochemical characteristics of garnet were analyzed by electron microprobe analysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS). The main aims are to (1) find out the origin of garnet in the Cuonadong leucogranite; and (2) reveal the magma evolution process of the Cuonadong leucogranites through the geochemical characteristics of garnet.

2. Geological Setting

From north to south, the Himalayan terrane is mainly composed of four tectonic units: Tethyan Himalayan Sequence (THS), High Himalayan Crystalline Sequence (HHCS), Lesser Himalayan Sequence (LHS), and Sub-Himalayan Sequence (SHS) (Figure 1), and their structural boundaries are the Indus-Yarlung Zangbo Suture Zone (IYZSZ), South Tibetan Detachment System (STDS), Main Central Thrust (MCT), Main Boundary Thrust (MBT), and Main Front Thrust (MFT) [35,36,37].
Located between the IYZSZ and the STDS (Figure 1), the THS is mainly a set of sedimentary rocks, including sandstone, siltstone, mud shale, and low-grade metamorphic slate and phyllite [38,39,40,41]. The HHCS lies between the STDS and the MCT and is composed of a series of high-grade metamorphic sedimentary rocks [32,42].
The Himalayan leucogranites are mainly distributed in the E–W direction (Figure 1). The northern belt is the Tethyan Himalayan leucogranite belt. The Tethyan Himalayan leucogranites are mostly produced in the core of the Tethys Himalayan gneiss dome [43] or intruded into the THS, with representative leucogranites from east to west, including Yalhashangbo, Cuonadong, Rampa, Kangmar, Lhagai Kangri, Malashan, and others [3,44,45]. The southern belt is the High Himalayan leucogranite belt, which is distributed around the STDS and intruded the HHCS as sheets with various scales, and is represented by Khula Kangri, Dinggye, Jilong, and other leucogranites from east to west [46,47].
A large number of Au–Sb, Pb–Zn–Sb, and Pb–Zn polymetallic deposits are produced in the THS, forming an Au–Sb–Pb–Zn Tethyan Himalayan metallogenic belt with a 600 km E–W extension, which is an important Pb–Zn–Au–Sb resource base on the Qinghai-Tibet Plateau [48,49]. These Au–Sb–Pb–Zn deposits are mainly distributed along the Himalayan dome, showing the spatial distribution characteristics of zoning, sheeting, and clustering, and showing the characteristics of an annular mineralization zoning centered on the dome [50,51]. Be–Rb–Nb–Ta–W–Sn and other rare metal deposits discovered in Cuonadong, Lalong, Cuore, and other places reveal the metallogenic potential of Himalayan rare metals, which are also mainly developed in the dome [4,5,6].
Minerals 12 01275 g001 550
Figure 1. Simplified geological map showing the distribution of the Himalayan leucogranite, gneiss dome, and tectonic zones (modified after [3,10,52]).
Figure 1. Simplified geological map showing the distribution of the Himalayan leucogranite, gneiss dome, and tectonic zones (modified after [3,10,52]).
Minerals 12 01275 g001

3. Geology of the Cuonadong Dome

The Cuonadong gneiss dome with an area of ca. 100 km2 is located in the eastern segment of the Tethyan Himalayan tectonic unit, adjacent to the South Tibetan Detachment System (STDS) (Figure 1). The Cuonadong gneiss dome can be divided into three parts from outside to inside: cap layer (rim), detachment system (mantle), and core (Figure 2, [4,39,53]). The cap layer is mainly Jurassic Ridang Formation and Triassic Nieru Formation low-grade metamorphic rocks, and the lithology is mainly argillaceous silty slate, schist, and phyllite [54,55]. Between the upper overburden and the dome core is the detachment system, which is bounded by the upper detachment fault and the lower detachment fault, respectively [56]. The detachment system is mainly composed of a garnet–kyanite–staurolite–biotite schist with medium-thin layers of skarnized marble, followed by a sillimanite-bearing granitic gneiss, tourmaline-bearing granitic gneiss, and garnet-bearing muscovite gneiss, and the degree of metamorphism gradually deepens from top to bottom [33,54]. Furthermore, a large number of pegmatite dikes are developed in a core and a mantle [4,54]. The core mainly consists of a Neoproterozoic–Early Paleozoic granitic gneiss and Cenozoic leucogranites (mainly two-mica granite, muscovite granite, tourmaline granite, garnet granite, and pegmatite) [33,57,58]. Previous studies suggest that the Cuonadong leucogranites are highly evolved granites that have undergone high degree of crystallization and fractionation during the ascending emplacement of magma [3,7,8,11].
Since the Cenozoic, the magmatism developed in the Cuonadong dome includes (1) Oligocene (ca. 34–26 Ma) deformed two–mica granite and pegmatite [53,59]; (2) Early Miocene (ca. 24–17 Ma) weakly deformed two-mica granite [8,57,60]; and (3) Middle Late Miocene (ca. 16–14 Ma) undeformed garnet-, tourmaline-, and muscovite-bearing granite and beryl-bearing pegmatite [7,9,61]. However, the ages of the Cuonadong leucogranites published in large numbers are around 22–14 Ma.
In recent years, a large number of deposits have been discovered in the Cuonadong dome, including skarn-type and/or hydrothermal vein-type Be–W–Sn deposits and granite–pegmatite-type Be–W–Sn–Rb deposits [4,5,6,8,9,10,56].

4. Samples and Analytical Methods

4.1. Sample Description

A total of three samples were used for the chemical analysis of garnet in this paper. The muscovite granite (CND–03) was collected from the western part of the Cuonadong dome, and the pegmatite samples, CND–18 and CND–38, were collected from the southern and central parts of the Cuonadong dome, respectively. The sampling locations are shown in Figure 2. Muscovite granite is gray-white, with a medium- and fine-grained granite texture and massive structure. Major mineral compositions include quartz (~35–40 wt%), alkali feldspar (~15–20 wt%), plagioclase (~30–35 wt%), muscovite (~7–10 wt%), garnet (~2–3 wt%), and biotite (~1–2 wt%). Garnet grains are brownish and euhedral-subhedral in hand specimens and are commonly cracked with inclusions of quartz and feldspar (Figure 3a–c). Pegmatite is white or yellowish-white, with a pegmatitic texture and massive structure, mainly composed of quartz (~35–42 wt%), alkali feldspar (~10–15 wt%), plagioclase (~35–40 wt%), muscovite (~3–5 wt%), garnet (~3–8 wt%), and biotite (~0.5–1 wt%). Garnets in the pegmatite have a reddish-brown color and show euhedral–anhedral crystals. In the investigated garnets, we found only a few inclusions (Figure 3d–i).

4.2. Electron Microprobe Major Element Analysis

All the samples were placed in a vacuum chamber and coated with carbon before electron microprobe analysis (EPMA). Major elements and back-scattered electron (BSE) images of the garnet were carried out by a JEOL JXA-8230 electron microprobe at the Testing Center, Shandong Bureau of China Metallurgical Geology Bureau. The voltage was 15 kV, the current 10 nA, and the beam diameter 10 μm during the analysis. The chemical formula of garnet was calculated based on 12 oxygens.

4.3. LA–ICP–MS Trace Element Analysis

The trace elements analysis of garnet was conducted on polished thin sections using a laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) equipped with an Agilent 7900 ICP–MS and a Resonetics RESOlution 193 nm laser at the Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. The laser energy was 80 mJ, with a frequency of 8 Hz and a spot diameter of 60 μm. BHVO–2G, BIR–1G, BCR–2G, and SRM 610 were selected as external standard samples in this study, while GOR132-G was used as a monitoring standard sample. Data processing was performed by ICPMS DataCal [62].

5. Results

5.1. Major Element Compositions

Major element compositions are presented in Table S1.
The garnets of the muscovite granite (CND-03) contain relatively similar contents of SiO2 (34.8–36.4 wt%), Al2O3 (19.4–20.6 wt%), FeO (26.8–33.1 wt%), MnO (10.7–16.1 wt%), MgO (0.062–0.173 wt%), and CaO (0.862–1.14 wt%). Furthermore, there is little difference in the content of major elements between different garnets. Garnets of the muscovite granite predominantly belong to the almandine–spessartine with a chemical formula of Alm58.13–71.81Sps24.81–38.01Adr1.82–3.24Prp0.26–0.72Grs0.00–1.41 (Figure 4a,b), which accounts for about 95% of the total molecular composition.
Compared with garnets from the muscovite granite, garnets from the pegmatite (CND-18 and CND-38) have a higher components of MnO (13.2–22.7 wt%) and spessartine (31.2–53.4 mol%) but lower FeO (18.0–30.0 wt%) and almandine (39.8–65.4 mol%) components, while the SiO2 (35.5–36.6 wt%), Al2O3 (19.6–20.7 wt%), and MgO (0.030–0.583 wt%) contents are roughly similar. Similarly, garnets from the pegmatite also belong to the almandine–spessartine solid solution with a formula of Alm39.81–65.39Sps31.23–53.43Adr0.00–3.50Prp0.12–2.39Grs0.00–18.60 (Figure 4c–f). However, it is worth noting that the CaO (1.32–7.21 wt%) and grossular (1.48–18.6 mol%) components of garnets in CND-18 are higher than those in CND-38 and CND-03.

5.2. Trace and Rare Earth Element Concentrations

The analytical results of the trace and rare earth elements are summarized in Table S2.
All the analyzed garnets are depleted in large ion lithophile elements with negative Nb and Ti and positive Ta anomalies (Figure 5). Garnets in both muscovite granite and pegmatite display low contents of Be (≤0.958 ppm), Nb (≤20.8 ppm, most <5 ppm), Ta (≤5.562), Hf (vast majority < 2), U (vast majority < 5), and Pb (vast majority < 5) and higher abundances of Y (391–3050 ppm), Ti (36.5–820 ppm), Li (67.1–286 ppm), Sn (2.06–164 ppm), and Zn (88.3–311 ppm) (Figure 6 and Figure 7).
Rare earth element data demonstrate that garnets from the muscovite granite and pegmatite are rich in HREE (70.7–2258 ppm) and Y (391–3050 ppm) but are depleted in LREE (0.114–3.92 ppm). On the chondrite-normalized REE patterns diagrams (Figure 8), all the analyzed garnets are characterized by HREE enrichment and LREE depletion with pronounced Eu anomalies. The difference is that garnets from the muscovite granite show distinct left-declined or flat HREE patterns with HREE/LREE of 988–7926 and (Yb/Dy)N of 1.31–7.51, while garnets from the pegmatite are featured by flat or right-declined HREE patterns with HREE/LREE of 26.5–2310 and (Yb/Dy)N of 0.134–3.40.

5.3. Zoning Textures

Overall, the garnets from the muscovite granite and pegmatite exhibit zoning with rim-ward spessartine depletion and almandine enrichment trends, except for sample CND–18, which shows a complex and/or irregular variation pattern (Figure 4). However, it is important to note that sample CND–18 shows grossular enrichment sharply towards the rim (Figure 4c,d). Element mapping suggests that the grain CND–03–a has higher Mn and lower Fe contents in the extremely narrow rim compared to the mantle (Figure 9). In addition, garnets from the pegmatite show enrichment of HREE and Y in the core (Figure 5c–f, Figure 7, and Figure 8c–f). In contrast, the HREE and Y zonings in garnets from the muscovite granite are inconspicuous (Figure 6 and Figure 8a,b). The zonation in all the analyzed garnets is less prominent for MREE, Li, and Zn (Figure 6 and Figure 7). Diagrams show that the rim-ward depletion trends of Zr, Hf, Nb, Ta, and U are similar to those of Nd and Sn (Figure 5, Figure 6 and Figure 7).

6. Discussion

6.1. Chemical Zoning in Garnet

The normal zoning, which is Mn-rich in the core and Fe-rich in the rim, is the dominant type, indicating primary growth [64]. Such a garnet is generally thought to be formed at conditions of low nucleation density, high growth rate, and low cation diffusion rate [65].
Crystal growth is usually controlled by various factors, including magma conditions and dynamic processes, while the compositional zoning or chemical gradient is commonly controlled by chemical diffusion [21,65,66]. The possibility is that the compositional zoning affected by diffusion can be tested using the diffusivity data for Mn, Fe, Mg, and Ca in garnet [67]. The garnet is assumed to be a spherical body of defined size, with neither defects nor cracks, in an infinite reservoir of relevant chemical components. The time (t) necessary for an element to diffuse through a distance (X) in a sphere of radius (r) can be determined applying the equation of [68], t = X2 × t′/D, where D is the diffusion coefficient (m2/s), t′ = 0.4 for a sphere (X = r), and t′ = 0.03 for a hollow sphere (X < r). The diameter of the garnets in this study roughly varies between ~0.5 × 10−3 and ~1.5 × 10−3 m. By using the diffusivities of [67], the time for the Mn-enriched core to re-equilibrate with the rim compositions ranges from 8 × 106 to 7 × 107 yr. Previous studies suggested that the timescale of magmatic processes producing gr·anitic magmas, from melting to crystallization, rarely exceeds 105 years [22,69,70]. Therefore, it seems that chemical diffusion had little or no effect on the compositional zoning documented in the Cuonadong garnets.

6.2. Origin of Garnet

Garnets in granitic rocks have been studied by many scholars and their origin has been explained by various models: (1) the refractory restite phase [71]; (2) the peritectic entrainment phase from the zone of partial melting [22,72,73]; (3) xenocrysts sourced from assimilated country rocks [24,74,75]; (4) crystals formed at higher pressures (P ≥ 7 kb) [70,76,77,78,79]; and (5) crystallization from strongly peraluminous melts at low pressure or from post-magmatic fluids or highly fractionated and Mn-enriched magma [80,81,82,83]. Firstly, neither garnet-bearing xenoliths in the leucogranites (muscovite granite and pegmatite) nor metamorphic mineral inclusions and reaction rims in the garnet were found. Furthermore, minerals in the leucogranites are typically igneous. Secondly, previous studies have shown that the leucogranites were derived from crustal melts [34,57,84,85] and garnets in this study were formed at low pressure (see Section 6.3). Thirdly, the garnets are euhedral to subhedral and equilibrate with other minerals, lacking resorption processes. Lastly, the garnets are predominately presented by an almandine–spessartine solid solution with a relatively high percentage of spessartine molecules and MnO content (Figure 4), which is consistent with garnets from pegmatites or highly fractionated granites [54,81]. In addition, all the analyzed garnets of the leucogranites fall in the range of magmatic compositions (Figure 10 and Figure 11) and exhibit characteristics of HREE enrichment and significantly negative Eu anomalies (Figure 8), resembling that of garnet crystallizing from a highly evolved melt [21,29,70,86]. Therefore, the garnets in this study are typical magmatic garnets.

6.3. P-T Conditions of Garnet Crystallization

It has been generally accepted that magmatic garnets in granitic rocks formed at above ~700 °C display inverse bell-shaped profiles with respect to spessartine (rim-ward Mn enrichment) or are generally spessartine unzoned. However, garnet showing a bell-shaped profile concerning Mn may be magmatic and formed in highly evolved magmas (SiO2 = 73–76wt%) crystallizing at low temperatures (<~700 °C) or are metamorphic (xenocrystic garnet) [21,88]. While garnets in sample CND–18 show fluctuating Mn profiles, the garnets in the muscovite granite and pegmatite are commonly characterized by bell-shaped spessartine profiles (Figure 4), indicating that garnet crystallization occurred below 700 °C. This is roughly consistent with the zircon saturation temperatures (662–711 °C, [31,57]) of the leucogranites from the Cuonadong region and other northern Himalayan leucogranites [11,31,34,57].
Previous studies have shown that Mn-rich garnets with 20–25 mol% spessartine may crystallize at shallow levels (below 5 kbar) and possibly to shallower depths (1–3 kbar) with higher Mn contents [21,69,89]. The garnets in the muscovite granite and pegmatite have high MnO (>10 wt%) and spessartine (>25 mol%) compositions, indicating a low-pressure condition. In general, the CaO contents are relatively low in garnets from the muscovite granite and pegmatite, although garnets in the sample CND–18 of pegmatite show rim-enriched grossular contents (Figure 4c,d and Figure 11). In addition, the occurrence of muscovite in the leucogranites indicates that the magma crystallizes at a maximum temperature of ~710 °C and a maximum pressure of 3.8 kbar [90]. Hence, in any case, although no accurate geothermobarometer is available, the geological, compositional, and textural features demonstrate that the garnets from the leucogranites formed at moderately low temperatures and relatively low-pressure conditions.

6.4. Implications for Evolution Process of Magma

The crystal texture and chemical composition of garnets usually record the evolution process of magma and magmatic–hydrothermal fluids and are important indicators to trace the compositions, properties, and environmental changes of fluids [18,20,81,82,83]. During the process of magma crystallization, generally represented by a Mn-rich almandine–spessartine solid solution or spessartine, the garnet is usually a mineral crystallized in the late stage of magma crystallization and fractionation. With an increasing degree of fractionation, the MnO and spessartine contents and/or MnO/(MnO + FeO) ratio of garnet gradually increase [64,81,91,92]. The MnO concentration, spessartine content, and MnO/(MnO + FeO) ratio of garnets from the Cuonadong dome all increase from the muscovite granite (10.7–16.1 wt%; 24.8–38.0 mol%; 0.243–0.375) to the pegmatite (13.2–22.7 wt%; 31.2–53.4 mol%; 0.305–0.545) (Table S1), indicating that the degree of magma fractionation increases from the muscovite granite to the pegmatite.
Garnet is enriched with trace elements such as REE, Hf, Zn, and Y, which can indicate the formation environment of the pegmatite magma and the evolution process of the granitic magma [18,20]. The aqueous medium-acidic silic-aluminous magma system usually goes through three stages from formation, migration, and cooling to finally complete solidification: (1) the magmatic stage—the system mainly exists in the melt phase and crystal phase; (2) the magmatic–hydrothermal transition stage—the system is characterized by the coexistence of a melt phase, crystal phase, and fluid phase; and (3) the hydrothermal stage—the system is dominated by a fluid phase and crystal phase, and the melt phase gradually disappears [93,94,95].
HREE and Y are compatible elements in garnets. When garnet is combined with minor or no HREE- and Y-bearing minerals (e.g., xenotime, thorite, and zircon), the contents of HREE and Y in garnet gradually increase with the enhancement of the evolution degree [18,96,97]. However, the contents of Y and HREE in garnets gradually decrease from the muscovite granite to the pegmatite with the evolution of magma (Figure 6, Figure 7 and Figure 8), and garnets in the pegmatite display rim-ward HREE and Y depletion trends. Moreover, these HREE- and Y-bearing minerals are scarce in these pegmatites according to our observation. The distribution coefficients of Y and REE in garnet are negatively correlated with the content of fluid and the degree of magma evolution [28,98]. Thus, the lower HREE and Y contents in garnets from the pegmatite and the depletion of HREE and Y at the garnet rim in the pegmatite suggest a fluid-rich melt at the late stage of evolution. After the magmatic evolution to pegmatite, the system entered a fluid-rich environment, which led to the decrease of Y and HREE in garnet. Therefore, as the degree of magma evolution increased, the garnets from the pegmatite likely crystallized from a lower-temperature fluid. As a result, the contents of Y and HREE in garnet and/or the garnet rim in the pegmatite gradually decrease.
In addition, the garnets in sample CND–18 of the pegmatite show rim-ward enrichment of grossular and CaO contents (Figure 4c,d). This phenomenon is indicative of the interaction of magmatic fluids with previously crystallized minerals or country rocks and incorporation of Ca into a highly fractionated granitic body [83,99]. Thus, the increasing grossular and CaO contents of the garnet rim in the pegmatite may reflect an influence of fluids in their composition.

7. Conclusions

(1)
The degree of fractionation from the muscovite granite to the pegmatite gradually increases in the Cuonadong dome. Garnets in the muscovite granite and in the pegmatite are dominated by an almandine–spessartine solid solution, and these garnets exhibit Mn, HREE, and Y zonings and are magmatic in origin.
(2)
Garnets from the Cuonadong leucogranites formed at moderately low temperatures and relatively low-pressure conditions.
(3)
Garnets from the Cuonadong leucogranites are characterized by HREE enrichment and LREE depletion, with significant Eu anomalies. From the muscovite granite to the pegmatite, the contents of HREE and Y in garnets gradually decrease, which indicates the crystallization from a lower-temperature fluid of garnets from the pegmatite. Elevated grossular and CaO contents of the garnet rim from the pegmatite may reflect an influence of fluids in their composition.
(4)
Garnet can be used as an indicator of magma evolution in these leucogranites.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min12101275/s1, Table S1. Major element compositions (wt%) and structural formula of garnets from the muscovite granite and pegmatite; Table S2. Trace and rare earth element concentrations (ppm) of garnets from the muscovite granite and pegmatite. Notes: ‘-’ represents the content below detection limit.

Author Contributions

Conceptualization, H.Y. and D.Y.; methodology, H.Y.; software, D.Y.; validation, H.Y.; formal analysis, D.Y.; investigation, C.M. and S.W.; resources, H.Y.; data curation, S.W. and C.M.; writing—original draft preparation, D.Y.; writing—review and editing, H.Y. and D.Y.; visualization, H.Y.; supervision, D.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Guangdong Major Project of Basic and Applied Basic Research, grant number (2019B030302013).

Data Availability Statement

Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 2. Geological map of the Cuonadong gneiss dome (modified after [8,58]).
Figure 2. Geological map of the Cuonadong gneiss dome (modified after [8,58]).
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Figure 3. Representative hand specimens and back-scattered electron (BSE) images of the Cuonadong muscovite granite and pegmatite: (a) garnet-bearing muscovite granite; (b,c) BSE images of garnets in the muscovite granite, showing quartz and plagioclase inclusions; (d) garnet-bearing pegmatite; (e,f) BSE images of garnets in the pegmatite; (g) garnet-bearing pegmatite; (h,i) BSE images of garnets in the pegmatite showing few inclusions. Note: The red circles represent the EPMA positions, and the yellow circles represent the LA–ICP–MS analysis positions. Abbreviations: Grt = garnet; Mus = muscovite; Qtz = quartz; Pl = plagioclase; Bt = biotite.
Figure 3. Representative hand specimens and back-scattered electron (BSE) images of the Cuonadong muscovite granite and pegmatite: (a) garnet-bearing muscovite granite; (b,c) BSE images of garnets in the muscovite granite, showing quartz and plagioclase inclusions; (d) garnet-bearing pegmatite; (e,f) BSE images of garnets in the pegmatite; (g) garnet-bearing pegmatite; (h,i) BSE images of garnets in the pegmatite showing few inclusions. Note: The red circles represent the EPMA positions, and the yellow circles represent the LA–ICP–MS analysis positions. Abbreviations: Grt = garnet; Mus = muscovite; Qtz = quartz; Pl = plagioclase; Bt = biotite.
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Figure 4. The core–rim chemical profiles of garnets: (a,b) garnets from the muscovite granite; (c–f) garnet grains from the pegmatite.
Figure 4. The core–rim chemical profiles of garnets: (a,b) garnets from the muscovite granite; (c–f) garnet grains from the pegmatite.
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Figure 5. Primitive mantle-normalized trace element spider diagrams of garnets. (a,b) garnets from the muscovite granite; (c–f) garnet grains from the pegmatite (primitive mantle normalization data from [63]).
Figure 5. Primitive mantle-normalized trace element spider diagrams of garnets. (a,b) garnets from the muscovite granite; (c–f) garnet grains from the pegmatite (primitive mantle normalization data from [63]).
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Figure 6. (a–d) Rare earth element and trace element core–rim profiles of garnet from the muscovite granite.
Figure 6. (a–d) Rare earth element and trace element core–rim profiles of garnet from the muscovite granite.
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Figure 7. (a–h) Rare earth element and trace element core–rim profiles of garnet from the pegmatite.
Figure 7. (a–h) Rare earth element and trace element core–rim profiles of garnet from the pegmatite.
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Figure 8. Chondrite-normalized REE patterns of garnets: (a,b) garnets from the muscovite granite; (c–f) garnets from the pegmatite (chondrite normalization data from [63]).
Figure 8. Chondrite-normalized REE patterns of garnets: (a,b) garnets from the muscovite granite; (c–f) garnets from the pegmatite (chondrite normalization data from [63]).
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Figure 9. Element maps of Al (a), Fe (b), Ca (c), and Mn (d) for the garnet CND–03–a from the muscovite granite.
Figure 9. Element maps of Al (a), Fe (b), Ca (c), and Mn (d) for the garnet CND–03–a from the muscovite granite.
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Figure 10. Mn–Mg–Fe ternary diagram. The pink area represents the magmatic garnet field from S-type granite [21], and the light blue area represents the magmatic garnet field from [87]. Garnets from the muscovite granite and pegmatite fall in the field of magmatic garnet reported by [87].
Figure 10. Mn–Mg–Fe ternary diagram. The pink area represents the magmatic garnet field from S-type granite [21], and the light blue area represents the magmatic garnet field from [87]. Garnets from the muscovite granite and pegmatite fall in the field of magmatic garnet reported by [87].
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Figure 11. CaO versus MnO (wt%) diagram (modified after [70,77,81,82]). The compositions of garnets from the muscovite granite and pegmatite dominantly lie in the magmatic garnet field (aplite-pegmatite).
Figure 11. CaO versus MnO (wt%) diagram (modified after [70,77,81,82]). The compositions of garnets from the muscovite granite and pegmatite dominantly lie in the magmatic garnet field (aplite-pegmatite).
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Yan, H.; Yu, D.; Wang, S.; Ma, C. Magmatic Garnet and Magma Evolution in Cuonadong Leucogranites: Constraints from Petrology and Mineral Geochemistry. Minerals 2022, 12, 1275. https://doi.org/10.3390/min12101275

AMA Style

Yan H, Yu D, Wang S, Ma C. Magmatic Garnet and Magma Evolution in Cuonadong Leucogranites: Constraints from Petrology and Mineral Geochemistry. Minerals. 2022; 12(10):1275. https://doi.org/10.3390/min12101275

Chicago/Turabian Style

Yan, Haibo, Deshui Yu, Shoujing Wang, and Chi Ma. 2022. "Magmatic Garnet and Magma Evolution in Cuonadong Leucogranites: Constraints from Petrology and Mineral Geochemistry" Minerals 12, no. 10: 1275. https://doi.org/10.3390/min12101275

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