Geochemical Contrasts between Late Triassic Rb-rich and Barren Pegmatites from Ningshan Pegmatite District, South Qinling Orogen, China: Implications for Petrogenesis and Rare Metal Exploration

: The Ningshan pegmatite district in the South Qinling Orogen hosts numerous Rb-(Be) mineralized pegmatites. In this study, whole-rock geochemistry, mineral geochemistry, and zircon U – Pb isotopes of the Rb-rich and barren pegmatites were determined. The barren pegmatites consist mainly of muscovite, microcline, albite, quartz, and garnet, whereas the Rb-rich pegmatites are mainly composed of muscovite, albite, quartz, and beryl, with minor chrysoberyl, cassiterite and columbite-group mineral. The muscovite and albite are the main Rb-bearing minerals. The U – Pb zircon dating of the Rb-rich and barren pegmatites yielding an age of 212 – 203 Ma, which is similar to that of the neighboring two-mica monzogranites distributed in the Ningshan area. Compared with the two-mica monzogranites, geochemical features, such as the Zr/Hf, Rb/Sr and Nb/Ta ratios and trace element contents indicated that the Rb-rich and barren pegmatites derived from fractionation of the two-mica monzogranites. In combination, the current and previous results suggest that the fractionation of the two-mica monzogranites caused the generation of the affinitive residual melts that, finally, crystallized to form the pegmatites. Compared to those from the barren pegmatites, the apatite from the Rb-rich pegmatites have higher MnO (14.51 – 19.12 wt.%) and Cl (0.12 – 0.16 wt.%) contents and lower F/Cl rartios (20 – 29). We conclude that these differences reflect unique geochemical signatures, and the geochemical composition of the apatite can be used as exploration guidance for rare metal-rich pegmatites.


Introduction
Rare metals, such as lithium, beryllium, cesium, rubidium, tin, niobium, tantalum, and uranium, are important to the economy and defense of the country. Today, many geologists are increasingly paying attention to granitic pegmatites since they are important sources of rare metals [1]. The classification of rare-element pegmatite into LCT (rich in Li, Cs, and Ta, in addition to Rb, Be, Sn, B, P, and F), NYF (rich in Nb, Y and F, in addition to Be, REE, Sc, Ti, Zr, Th, and U), and mixed petrogenetic families [2][3][4] is the most widely applied today. LCT family is suggested to be associated usually with I-or S-type granite derived from a sedimentary source in a late-or post-orogenic setting. The NYF family is suggested to be associated with A-type granite derived from a mixed source of lower crust and mantle in a post-or anorogenic setting [3,5]. As regards pegmatite genesis, two genetic models exist. The most accepted model assumes an igneous origin, with most pegmatites originating from volatile-rich residual melts derived from the crystallization of parental granitic magmas. In this model, pegmatite is assumed to be linked genetically with fertile granite(s) exposed at the surface or buried at depth [1,3,[5][6][7][8][9]]. The other model assumes an anatectic origin (e.g., in migmatitic terranes), whereby the pegmatites form by direct partial melting of metasedimentary rocks, and have no petrogenetic relation to the neighboring granite(s) [10][11][12][13][14][15][16][17][18][19][20].
Three large granitic pegmatite regions are distributed in the Qinling Orogenic Belt in China: the Shangnan, Baoji, and Ningshan pegmatite districts. Detailed studies have been conducted on the Shangnan pegmatite district, with the classification, internal structure, mineral assemblage, mineralization characteristics, metallogenetic epoch, and genetic relationships between granites and pegmatites having been reported [21][22][23][24][25]. In contrast, the Ningshan pegmatite district, located in South Qinling Orogen, is poorly investigated. All the pegmatites in the Ningshan district are distributed in the outer contact zone on the south side of the Triassic granitoids in the Wulong-Ningshan area, but the pegmatite-granite genetic relationship remains puzzling. Notably, Rb and Be enrichment is observed in some of the pegmatites. It is therefore very important to understand the distinct geochemical patterns between the barren and Rb-rich pegmatites since it can not only enhance the knowledge about the pegmatite petrogenesis but also provides useful information for rare metal exploration regionally.
In this study, we performed mineralogical, geochemical, and zircon U-Pb isotopic data on Lizhaping, Dazengxigou, Xinjian barren pegmatites, and Xinjian Rb-rich pegmatites from the Ningshan pegmatites district. Subsequently, by undertaking a detailed geochemical comparison among the barren pegmatites, Rb-rich pegmatites, and the neighboring granites, we attempted to clarify the petrogenesis of the pegmatites. Furthermore, by comparing apatite composition of the barren pegmatites with that of Rb-rich pegmatites, we attempted to evaluate the application of apatite as an indicator mineral in rare metal exploration.

Geological Setting
The Qinling Orogenic Belt (QOB) is located in central China and is bounded to the north by the Lingbao-Lushan Fault and to the south by the Mianlue-Bashan Fault, connecting the North (NCB) and South China Block (SCB) (Figure 1a) [26][27][28]. As proposed by Meng and Zhang (1999) [26], two important sutures are located in the QOB, namely, the Shangdan suture in the north and the Mianlue suture in the south (Figure 1b). It is generally accepted that the Shangdan suture records the middle Paleozoic northward subduction of the Shangdan (Paleo-Qinling) Ocean along the southern margin of the Norh Qinling Belt (NQB) and the amalgamation of the NQB and South Qinling Belt (SQB) [27,29]. The Mianlue suture represents the late Paleozoic Mianlue (Paleo-Tethys) Ocean that closed in the early Mesozoic, followed by the final collision between the SCB and the united NCB-SQB [30][31][32][33]. The main geologic framework of the QOB was built up by the middle Paleozoic and early Mesozoic subduction-collision events of the NCB (including the NQT), SQB, and SCB at the Shangdan and Mianlue sutures [26,27,32].
Tectonically, with the Shangdan and Mianlue sutures and the Luonan-Luanchuan fault (LL), the QOB is divided into, from north to south, the southern margin of the NCB, NQB, SQB, and the northern margin of the SCB (Figure 1b) [30,34]. The SQB is characterized by south-verging thrusts and folds, showing an imbricated thrust-fold system [35]. The SQB consists of Proterozoic metamorphic sequences, late Paleozoic sedimentary sequences [35], and numerous Triassic granitoids [36][37][38][39] (Figure 1b). These Triassic granitoids intruded into the SQB as a result of the subduction and collision between the SQB and SCB along the Mianlue suture (M) [36][37][38][39]. From west to east, the granitoids have been classified into three suites, which are the Guangtoushan, Huayang-Wulong-Ningshan (including the Huayang, Wulong, Laocheng, Xichahe, and Yanzhiba plutons), and Dongjiakou suites (Figure 1b). These granitoids extends along the SQB roughly parallel to the Mianlue suture, forming a ca. 400 km long granitoid belt.  [36,37,39,40]; (c) geological sketch map of Ningshan pegmatite district (modified from [21] Three large granitic pegmatite regions are distributed in the Qinling Orogenic Belt, namely, the Shangnan, Baoji, and Ningshan pegmatite districts. The Shangnan and Baoji pegmatite districts are distributed north of the Shangdan suture. The Ningshan pegmatite district is located on the south of the Triassic granitoids in the Wulong-Ningshan area in the middle segment of SQB (Figure 1b,c) [21,22]. In terms of rare metal mineralization, mainly Nb, Ta, Be, Rb, and Li mineralization developed in the Shangnan pegmatite district; mainly Nb, U, REE mineralization developed in the Baoji pegmatite district; and mainly Be, and Rb mineralization are developed in the Ningshan pegmatite district [21,22].

Pegmatite and Sample Description
The pegmatites we studied are located 5 km NE of the township of Taishanmiao, and are distributed to the south of the Yanzhiba pluton ( Figure 1c). Previous geological surveys indicated that these pegmatites have high Rb content, with muscovite mainly being the Rb-bearing mineral. Be mineralization is also found in some Rb-rich pegmatites [41].
More than 10 representative samples from Rb-rich pegmatites along the road-cuts of the Xinjian country were collected. These pegmatites are ca. 20-30 m long and 1-3 m wide (Figure 3a). Wall rocks of the Rb-rich pegmatites are mainly the lower-middle Silurian Meiziya Formation (Figure 3a). The internal texture of the Rb-rich pegmatites is nonsignificant. The Xinjian Rb-rich pegmatites are mainly coarse-medium grained (Figure 3b), and mainly composed of muscovite, albite, quartz, and beryl ( Figure 3c,d). The beryl forms euhedral to subhedral crystals varying in size from several hundred micrometers to several millimeters ( Figure 3d). Other minerals include chrysoberyl, cassiterite, and columbite-group mineral, which commonly occur as accessory minerals (Figure 3d

Whole-Rock Compositional Analysis
Fresh whole-rock samples from Rb-rich muscovite-albite (Mus-Ab) pegmatite and inward zone of barren muscovite-microcline-albite (Mus-Mc-Ab) pegmatite were trimmed to remove weathered surfaces, cleaned with deionized water, crushed in a steel jawcrusher and then powdered to <200 mesh using an agate mill. Sample preparation and whole-rock major and trace elements were determined at the National Research Center for Geoanalysis, Chinese Academy of Geological Sciences (CAGS), Beijing, China. Major element concentrations were analyzed by X-ray fluorescence (XRF) spectrometry, with analytical uncertainties of ≤0.5 wt.%. Ferric and ferrous iron measurements were performed via wet chemical analysis (titration). The analytical precision for major oxides was based on certified standards (GSR-1, GSR-3). Trace elements were determined using ICP-MS. The analytical uncertainties are 10% when the element abundance is <10 ppm, and around 5% when the element abundance >10 ppm.

Mineral Compositions Analysis
Back-scattered electron (BSE) images and quantitative chemical analyses of minerals were performed from polished thin sections using a JEOL JXA-8100 electron probe micro-analyzer (EPMA, JEOL Ltd., Akishima, Tokyo, Japan). Silicate minerals and apatite from barren Mus-Mc-Ab pegmatite were obtained at Beijing Research Institute of Uranium Geology, China. Silicate mineral and apatite from Rb-rich Mus-Ab pegmatite at Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. The operating conditions were set at an acceleration voltage of 15 kV, a probe current of 10 nA and a beam diameter of 5 μm. Element peaks and off-peak backgrounds were measured with counting times of 10 s and 5 s, respectively, except for F, Cl (20 and 10 s) and Rb (20 and 10 s). Standards used for analyses were as follows: diopside (Mg, Si, Ca), rutile (Ti), plagioclase (Al), garnet (Fe), rhodonite (Mn), albite (Na), sanidine (K), apatite (P), pollucite (Rb), durango (F), tugtupite (Cl). Data reduction was performed using the internal ZAF matrix correction of JEOL.

Zircon CL Images and LA-ICP-MS U-Pb Dating
Zircons were separated from the three least altered samples (LZP01 and DZXG03: barren Mus-Mc-Ab pegmatite, XJ01-4: Rb-rich Mus-Ab pegmatite) by standard density and magnetic separation techniques followed by hand-picking under a binocular microscope for purification. Prior to the insitu zircon U-Pb isotope analyses, the morphology and internal structure of these zircons were studied using a binocular microscope and cathodoluminescence (CL) images, which were used as guides during the selection of the U-Pb dating spots. The zircon U-Pb dating and trace element analyses were conducted synchronously employing laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at the Institute of Mineral Resources, CAGS, in Beijing, China. Detailed information on the operating conditions for the laser ablation system, the ICP-MS instrument, and the data reduction process have been described previously by Liu et al. (2008Liu et al. ( , 2010 and Hou et al. (2009) [42][43][44].

Zircon U-Pb Ages
Zircon crystals from barren Mus-Mc-Ab pegmatite (LZP03 and DZXG01) and Rb-rich Mus-Ab pegmatite (XJ01-4) samples display variable sizes (e.g., LZP03: 100-200 μm; DZXG01: 50-120 μm; XJ01-4: 100-300 μm) (Figure 9). The U-Pb dating data are shown in Table 5. The plotted zircon U-Pb ages are shown in Figure 10. Most zircons separated from the sample LZP03 display oscillatory zoning, as shown in the cathodoluminescence (CL) images (Figure 9a), which indicates a magmatic origin. We obtained analyses of eight zircons from sample LZP03 (Figure 10a). The U concentrations of the analyzed zircons range from 586 to 2493 ppm, Th contents range from 491 to 1868 ppm, and the Th/U ratios range from 0.216 to 2.569. All eight analyses give apparent 206 Pb/ 238 U ages between 208 Ma and 831 Ma, with analyses spots #4 and #7 yielding apparent 206 Pb/ 238 U ages of 208 ± 4 and 212 ± 3 Ma, respectively (Figure 10a), which is considered the crystallization age of these pegmatites. The remaining six analyses yielded apparent 206 Pb/ 238 U ages of 472-831 Ma, which were obviously older than the analyses spots #4 and #7 (Figure 10a), suggesting that these older zircons had been inherited.
Zircons separated from sample DZXG01 show spongy and porous features on CL images (Figure 9b), suggesting that they have been subjected to metamictization or recrystallization, likely ascribable to high U concentrations or deuteric fluid [52][53][54]. These features are commonly seen in rare-element pegmatites from other places because of high fluxes and the presence of late-stage hydrothermal fluids [24,25,[55][56][57]. We obtained the analyses of nine zircons from sample DZXG01 (Figure 10b). The zircons analyzed have variable U concentrations (4882-12,941 ppm), Th contents (4179-1358 ppm), and Th/U ratios (0.014-0.258). The 206 Pb/ 238 U ratios for these nine spots yielded a lower intersection point age of 206 ± 18 Ma, with a MSWD of 9.9 (Figure 10b).
Zircons separated from the sample XJ01-4 display oscillatory zoning, as shown in the cathodoluminescence (CL) images (Figure 9c), indicating a magmatic origin. We obtained a total of seven analyses of six zircons from sample XJ01-4 ( Figure 10c). The analyzed zircons have U and Th concentrations ranging from 501 to 1674 ppm, and 295 to 1863 ppm. The zircons have high Th/U ratios ranging, from 0.589 to 1.292 (with an average of 0.89), indicative of a magmatic origin [58]. The 206 Pb/ 238 U ratios for these seven spots yielded a weighted mean age of 202.9 ± 4.2 Ma, with a mean square weighted deviation (MSWD) of 1.7 (Figure 10c).

Age of Pegmatites
Although zircon U-Pb dating for granitic pegmatites has a few problems, including huge amounts of inherited zircons, or strongly metamicted zircons, the U-Pb zircon geochronometer is one of the most important dating methods for rare metal-rich pegmatite, as the closure temperature of the zircon U-Pb system is close to the temperature of the magmatic stage of pegmatites [24,25,[55][56][57][58][59][60]. In this study, two spot ages of 208 ± 4 and 212 ± 3 Ma for the Lizhaping barren pegmatite (sample LZP03), and a lower intersection age of 206 Ma for the Dazengxigou barren pegmatite (sample DZXG01) were determined, as well as a weighted mean age of 202.9 ± 4.2 Ma for the Xinjian Rb-rich pegmatite (sample XJ01-4) (Figure 10a-c). We interpreted these ages (212-203 Ma) as the emplacement ages of the pegmatites. These results are similar to those of previous U-Pb dating studies that demonstrated the existence of Late Triassic magmatic activity in the middle segment of SQB [39,47,48,[61][62][63][64][65].
Granitic pegmatite is generally considered a product of volatile-rich (e.g., B, F, P) residual magma through the differentiation of parental granitic magma [6,7,9]. The pegmatite field surrounding a source granite is generally zoned with respect to the distribution of rare elements [1,2,4]. As emphasized by Dill (2015) [66], the temporal relation between pegmatites and neighboring granites plays a key role in distinguishing the origin of pegmatites. According to Selway et al. (2005) [67], pegmatite dikes with the most economic potential for Li-Cs-Ta deposits occur the greatest distance (up to 10 km) from the parent granite. In our study area, pegmatites are juxtaposed with the Yanzhiba pluton (Figure 1c). Some of the pegmatites intruded into the monzogranites, with a progressive evolution contact (Figure 2a). The Yanzhiba pluton can be divided into Yanzhiba granodiorites [48], Yanzhiba two-mica monzogranites [47], and Lanbandeng two-mica monzogranites [48]. Previous chronological study has shown that the Yanzhiba granodiorite formed at ~220 Ma [48], and the two-mica monzogranites in Yanzhiba and the Lanbandeng intrusion formed at ~210 Ma [47,48]. As mentioned above, the pegmatites formed in the Late Triassic (212-203 Ma), slightly younger than the two-mica monzogranites in Yanzhiba and the Lanbandeng intrusion. The spatial and temporal distribution of pegmatites and granites reveals that the Yanzhiba and Lanbandeng two-mica monzogranites are the potential parental granite of these pegmatites.
For geochemical comparison, we collected data of the Yanzhiba and Lanbandeng two-mica monzogranites published by Jiang et al. (2010) and Yang et al. (2012) [47,48]. The Zr/Hf, Rb/Sr or Nb/Ta ratios are used generally as an indicator of pegmatite evolution [8,[68][69][70][71][72][73][74][75]. The trends of the plots of La versus Zr/Hf ratios (Figure 11a) could be interpreted in terms of zircon fractional crystallization from monzogranites and two-mica monzogranites to the pegmatites. The plots of La versus Rb/Sr ratios (Figure 11b) show evolution trends that could be interpreted in terms of feldspar and mica (biotite and/or muscovite) and fractional crystallization. The trends of the plots of La versus Nb/Ta ratios could be interpreted in terms of mica (biotite and/or muscovite) fractional crystallization (Figure 11c). The reasons for these hypotheses are as follows: (1) Previous experimental studies have shown that the ratio DHf/DZr of the partition coefficients of Zr and Hf between zircon and melt are low (0.5 to 0.2) for metaluminous and peraluminous melts [72]. Consequently, zircon fractionation strongly decreases Zr/Hf in some peraluminous granites [8,70,71,74]. (2) Sr is more compatible in feldspar, whereas Rb is more compatible in mica. In response to feldspar fractionation and mica concentration, there is a marked depletion of Sr and enrichment of Rb, resulting in a high Rb/Sr ratio [68,69]. (3) Experimental studies have shown that Nb preferentially partitions into mica relative to Ta in the granitic system [75]. Thus, Ta concentrates in residual melt relative to Nb. As a result, the residual melt becomes enriched in Ta relative to Nb and yields lower Nb/Ta ratios. Accordingly, the barren and Rb-rich pegmatites in the study area are both derived from the fractionation of Yanzhiba and Lanbandeng two-mica monzogranites. This hypothesis is supported also by other geochemical signatures. In the chondrite-normalized REE pattern (Figure 5a), the barren pegmatites show a seagull-like normalized REE pattern, with a depleted LREE and a pronounced negative Eu anomaly. In contrast, Yanzhiba and Lanbandeng twomica monzogranites show a right-declined pattern with a weak Eu anomaly. Those negative Eu anomalies are more pronounced in pegmatite than in two-mica monzogranites can be interpreted as being caused by feldspar fractionation during granite emplacement because Eu prefers to partition into plagioclase. The depleted LREE pattern was caused by accessory minerals, considering that LREE are incorporated generally in the early-crystallizing accessory minerals such as monazite (80-85 wt.%) and apatite (~10 wt.%), which is virtually devoid in the last solidified pegmatites [76,77]. Rb-rich Mus-Ab pegmatite have a significantly different REE pattern, with much lower ∑REE concentrations and the lack of an Eu anomaly. Fractionation of the accessory minerals (i.e., apatite and monazite) in parental granite can cause the depletion of LREE in the residual magma, and fractionation of garnet result in the depletion HREE. As for the lack of an Eu anomaly, this phenomenon can be also observed in some other rare metal-bearing pegmatites [8,78,79]. Previous study pointed out that oxidation of Eu 2+ to Eu 3+ can make it less compatible in the Ca sites of plagioclase [80][81][82]. The REE data for plagioclase show a decrease of the positive Eu-anomaly and then a change to negative Eu-anomaly from the wall and intermediate zone to the more evolved inner zones [83,84]. Thus, the absence of an Eu anomaly can be attributed to the elevated oxidation state during the pegmatite evolution. Evidence for the oxidizing conditions during the formation of Rbrich Mus-Ab pegmatite is recorded in the abundance of cassiterite and columbite-group mineral. On the primitive mantle normalized spider diagram, compared with granite, the pegmatites show positive in Rb, U, and P anomalies, and negative anomalies of Ba, Sr, Zr, and Ti (Figure 5b). The positive in Rb, U, and P anomalies in the pegmatites is ascribed to their high incompatible properties, i.e., they do not favor crystallization of their individual (accessory) minerals at the early magmatic stage [85][86][87][88]. The depletion in Ba and Sr in the pegmatite can be ascribed to feldspar fractionation in the two-mica monzogranites [89], whereas depletion in Ti and Zr is the result of early-crystallized ferromagnesian minerals (biotite and Fe-Ti oxides) fractionation and zircon fractionation, respectively [77,90]. The trace element abundance diagrams of the Rb-rich pegmatites are characterized by pronounced enrichment in Rb, Nb, P, Sr, and Hf (Figure 5b). The highly fractionated residual magma caused the concentration of these incompatible elements. Therefore, fractional crystallization from the Yanzhiba and Lanbandeng two-mica monzogranites to the barren and Rbrich pegmatites explains the trace element distribution in the pegmatites in the study area.

Tectonic Setting and Petrogenesis of Pegmatite
As emphasized by Černý, 1991 [2], the trace element signatures of most rare-element pegmatites can be grouped into just two distinctive families: one that is enriched in lithium, cesium, and tantalum (LCT) and the other characterized by enrichment in niobium, yttrium, and fluorine (NYF). In this study, the Rb-rich pegmatites are also variably enriched in Cs, Be, Ta, Nb, and Sn (Table 1). There are abundant beryl and chrysoberyl (Figure 3c,d). Cassiterite and columbite-group mineral seem to form the most important accessory minerals (Figure 3e,f). Following the classification schemes of Černý (1991) [2] and Černý and Ercit (2005) [4], the Rb-rich pegmatites belong to the LCT family based on the rare element association, and more specifically to the rare element pegmatite class. Based on the rare element mineralogy, they are consistent with the beryl-type (beryl-columbite subtype). As discussed above, the barren and Rb-rich pegmatites are derived by fractionation from the Yanzhiba and Lanbandeng two-mica monzogranites. Therefore, we propose a petrogenetic model of the peraluminous granites and pegmatites, which resulted from decompression melting of the thickened lower crust during a post-collisional extensional tectonic regime. Notably, the post-orogenic setting is also consistent with the required tectonic setting for LCT-type rare-metal pegmatites worldwide [2,4,5].
As noted above, the granitic plutons are extensive in the Ningshan area in the middle segment of SQB (Figure 1b,c). Previous studies have shown that the intrusion of these plutons is considered the result of multi-stage granitic magmatism during the structural evolution of the Qinling Orogenic Belt. During this evolution, SQB and SCB experienced a series of subduction, collision, and postorogenic collapse events along the Mianlue suture [39,47,48,[61][62][63][64]. Combining the post-collision magmatism and granite-pegmatite genetic relationship, we summarize the petrogenesis of the pegmatites as follows: During extension of the thickened crust, the crust-derived magma moved upward though extensional fractures and along lithological boundaries and was stored in the shallow crust in a magma chamber. The process ultimately gave rise to the formation of peraluminous granites after fractional crystallization and intrusion. The fractionation of these peraluminous granites caused the generation of the affinitive more evolved felsic magma (the residual melts). They continued to intrude into the fissures of early granites and overlying sedimentary rocks and, subsequently, crystallized to form the pegmatites.

Importance of Mineral Compositions to Rare Metal Exploration
Rubidium is highly incompatible in quartz and plagioclase because of its exceptionally large ionic radius and cesium behaves similarly [87]. Quartz, feldspar, and mica are the dominant minerals that crystallize from granitic melts. The extreme fractionation resulting from the extended crystallization of quartz and plagioclase from granitic melts could generate exceptionally high concentrations of Rb and Cs in residual melts. The mineral crystallization of the residual melt caused Rb and Cs to enter micas and alkali-feldspar because their ionic radius is closer to that of K [87]. In this study, the muscovite and albite are the main Rb-bearing minerals in the Rb-bearing Mus-Ab pegmatite. The Rb-rich pegmatites are also enriched in Cs, but have low Li content. In agreement with low Li content in whole-rock geochemistry, no Li-muscovite and other Li-bearing minerals are observed. This is analogous to that displayed by similar Li-bearing mineral lacking a central intermediate zone of the best-studied LCT-family Tanco pegmatite [91]. As pointed out by Stilling et al. 2006 [91], Li, Cs, and Ta are produced from different zones at Tanco pegmatite. Li and Cs mineralizations may have had different controls [87]. In the case of only barren Mus-Mc-Ab and Rbrich Mus-Ab pegmatites having been identified, it is difficult to speculate further on the potential distribution of Li mineralization pegmatites in the area.
As emphasized by Selway et al. (2005) [67], the presence of Mn-rich fluorapatite indicates that the pegmatite has economic Li-Cs-Ta potential. The Mn contents of apatite are pointed out to be relatively high in rare metal bearing pegmatites [92,93]. In the spodumene pegmatites from the Tin-Spodumene Belt, with apatite containing up to 3.26 wt.% MnO [94], and in the Altay Koktokay no. 3 pegmatites, almost all the apatite contains more than 2.00 wt.% MnO [95]. The Mn contents in apatite appear to be related to the Fe 2+ /Fe 3+ ratios in their melt source and/or the redox conditions during apatite crystallization [92,93]. Miles et al. (2014) [96] presented an equation using Mn content in apatite as an oxybarometer in intermediate and silicic igneous rocks. Their study proposes a negative correlation between Mn in apatite and oxygen fugacity (fO2). However, the relationship of Mn in apatite as a measure of oxygen fugacity was refuted by Marks et al. (2016), who concluded that the presence of other Mn-bearing phases affecting the Mn content in the apatite [97]. In this study, compared with the MnO content (0.34-0.54 wt.%) in apatite from the Mus-Mc-Ab barren pegmatite, the apatite in our study has relatively much higher MnO content (14.51-19.12 wt.%) from Rb-rich Mus-Ab pegmatite. It can be observed that garnets are abundant in the barren Mus-Mc-Ab pegmatite (Figures 2d,f and 6a). Garnet would incorporate Fe and Mn into its structure during crystallization. Therefore, the Mn-content in apatite is mostly dependent on the presence garnet or other Fe-Mn minerals. Our results indicate that the Mn-content of apatite may be used to distinguish between barren pegmatites and Rb-rich pegmatites.
Apart from the difference in MnO-content, apatites from Rb-rich Mus-Ab pegmatites have higher Cl content and lower F/Cl ratios in comparison to those of barren Mus-Mc-Ab pegmatites (Figure 8e,f). Previous works demonstrated that halogen contents, especially fluorine and chlorine, could be evaluated using the composition of apatite in igneous rocks [98,99]. The reasons for the differences in Cl contents and F/Cl ratios between the Rb-rich Mus-Ab pegmatites and barren Mus-Mc-Ab pegmatites are most likely related to melt compositions. As mentioned above, the Rb-rich pegmatites are crystallized from the more evolved magmas than the barren pegmatites. It should be enriched in Cl due to the fractionated of the magmas. In summary, the Mn-content and Cl-contents (or F/Cl ratios) of apatite could be used as an indicator of the rare metal mineralization potential for the pegmatites in this area.

Conclusions
(1) The formation age of the barren and Rb-rich pegmatites from Ningshan pegmatite district is 212-203 Ma, which is consistent with the two-mica monzogranites in the middle segment of SQB.
(2) The Rb-rich pegmatite from Ningshan pegmatite district belongs to the LCT family. The magma of the barren and Rb-rich pegmatites originated from the thinned lower crust and formed in a post-collisional environment. The Yanzhiba and Lanbandeng two-mica monzogranites are the potential parental granite of these pegmatites.
(3) The Mn-content in apatite is mostly dependent on the presence garnet or other Fe-Mn minerals. The Rb-rich pegmatite has higher Mn and Cl content and lower F/Cl ratios than the barren pegmatite, indicating that it can be regarded as an indicator of rare metal mineralization potential.