Partial Melting of Lithospheric Mantle and Formation of the Early Cretaceous Alkaline Rocks in the Guandimiao REE Deposit, Luxi Terrane, Eastern China

: The Luxi Terrane (eastern China) exposes widespread Early Cretaceous alkaline rocks, whereas their petrogenesis remains controversial, including fractional crystallization, partial melting and crustal contamination regime. Here, we present petrology, geochemistry, sphene U-Pb geochronology and trace element data from the syenogranite, quartz syenite and quartz monzonite of the Guandimiao alkaline complex rocks to investigate their petrogenesis. Geochemical data suggest that these alkaline rocks show alkalic and peralkaline characters, and high Ga/Al ratios, SiO 2 , light rare-earth element (LREE), Zr and Nb, and low MgO, CaO, Eu contents, corresponding to A-type granites. Sphene trace elements in syenogranite and quartz monzonite show obvious fractionation between LREE and heavy rare-earth element (HREE) and high Th/U ratios, indicating a magmatic origin. They yield U-Pb lower intercept ages of 128 ± 2.3 Ma and 127 ± 1.3 Ma, representing the crystallization ages of these alkaline rocks. The negative correlations between CaO, Fe 2 O 3 (Total), MgO, P 2 O 5 , TiO 2 , MnO and the pronounced depletion in Nb, Ta and Ti suggest that the alkaline rocks were formed by fractional crystallization. Additionally, the positive correlation between La/Hf and La, Th and Th/V, Ce/Yb and K 2 O, and Tb/Yb and Yb suggest that the alkaline melts are generated by partial melting. Such high Rb/Nb, (Th/Nb) N and Nb/Th ratios indicate crustal contamination during the magma emplacement. We, therefore, propose the magma source of the alkaline rocks in the Guandimiao complex originated by partial melting of lithospheric mantle, which experienced fractional crystallization and crustal contamination processes during its emplacement. Such complex alkaline rocks were probably formed in an extensional back-arc setting induced by the retreat of the subducting Izanagi plate.


Introduction
The Yanshanian orogenesis, correlated with the subduction of the Izanagi plate, resulted in widespread magmatism and ore formation in the Luxi Terrane [1][2][3], and formed a series of alkaline intrusive rocks and rare-earth element (REE) deposits in eastern China. Such REE deposits comprise the Weishan, Longbaoshan and Guandimiao REE deposits, which are spatially hosted by NW-and NWW-oriented faults [1][2][3][4][5]. Previous studies on the Early Cretaceous alkaline rocks in the Luxi Terrane indicate that the petrogenesis of the alkaline rocks remains controversial [6][7][8][9]. Previous studies proposed that decreases in TiO 2

Whole-Rock Geochemistry
Fresh samples were selected, crushed, and powdered to less than 200 mesh in an agate mill for whole-rock analysis. Geochemical data (major and trace elements) were obtained at the testing center of the Shandong Provincial Lunan Geology and Exploration Institute, China. Major elements were analyzed by X-ray fluorescence using a Rigaku RIX 2100 spectrometer with analytical uncertainties of 1-5%. Trace elements were determined using a PEE lan 6000 ICP-MS instrument with analytical uncertainties of 1-3%. Details of the analytical techniques are described in [25]. Analyses of basalt and andesite standard (BHVO-1, BCR-2, and AGV-1) indicated that the analytical precision and accuracy were better than 5% for major elements and 10% for trace elements and REEs [26].

Sphene LA-ICP-MS U-Pb Dating
The syenogranite and quartz monzonite samples were crushed to 40-60 mesh (250-380 μm) and sphene crystals were separated through standard magnetic and density separation techniques. Sphene grains were carefully handpicked under a binocular microscope, mounted in epoxy, polished down to near half-sections to expose internal structures, and then cleaned in an ultrasonic washer containing a 5% HNO3 bath. Prior to analysis, polished sections of sphene were carbon coated for Back-Scattered-Electron

Whole-Rock Geochemistry
Fresh samples were selected, crushed, and powdered to less than 200 mesh in an agate mill for whole-rock analysis. Geochemical data (major and trace elements) were obtained at the testing center of the Shandong Provincial Lunan Geology and Exploration Institute, China. Major elements were analyzed by X-ray fluorescence using a Rigaku RIX 2100 spectrometer with analytical uncertainties of 1-5%. Trace elements were determined using a PEE lan 6000 ICP-MS instrument with analytical uncertainties of 1-3%. Details of the analytical techniques are described in [25]. Analyses of basalt and andesite standard (BHVO-1, BCR-2, and AGV-1) indicated that the analytical precision and accuracy were better than 5% for major elements and 10% for trace elements and REEs [26].

Sphene LA-ICP-MS U-Pb Dating
The syenogranite and quartz monzonite samples were crushed to 40-60 mesh (250-380 µm) and sphene crystals were separated through standard magnetic and density separation techniques. Sphene grains were carefully handpicked under a binocular microscope, mounted in epoxy, polished down to near half-sections to expose internal structures, and then cleaned in an ultrasonic washer containing a 5% HNO 3 bath. Prior to analysis, polished sections of sphene were carbon coated for Back-Scattered-Electron (BSE, Langfang Regional Geological Survey, Hebei, China) analyses using a JXA-880 electron microscope. Image analysis software was used under operating conditions of 20 kV and  20 nA, at the Langfang Regional Geological Survey, Hebei Province, China, to identify the internal structure and texture of all sphene crystals. Sphene samples were checked carefully using the microscope and BSE images for fluid inclusions and cracks. LA-ICP-MS analyses were conducted at the Isotopic Laboratory, Tianjin Center, China Geological Survey. Details of the analytical procedures are given in [27,28].
Laser sampling was performed using a Neptune double-focusing multiple-collector ICP-MS attached to a NEW WAVE 193 nm-FX ArF Excimer laser-ablation system. All analyses were conducted with a beam diameter of 35 µm, an 8 Hz repetition rate, and energy density of 11 J/cm 2 . GJ-1 was used as an internal standard for U-Pb dating analyses. NIST SRM 610 glass was used as an external standard to calculate U, Th, and Pb concentrations of sphene crystals. Every eight analyses were followed by two analyses of the standard zircon GJ-1. Isotopic ratios were calculated using ICPMSDataCal 8.4, China University of Geosciences, Wuhan, China [29] and were plotted using Isoplot version 3.0 software [30]. Common-Pb corrections were made following the methodology of [31].

Petrography
The quartz syenites (21XC03, 21XC05) show light gray color, granular texture and massive structure (Figure 2A,B). The paragenesis of the quartz syenites are dominated by orthoclase (~60-61%), plagioclase (~24%), quartz (~6-9%), hornblende (~6-9%) and biotite (~1%) ( Figure 3A-D). Accessory minerals include zircon, apatite and sphene. Orthoclase is subhedral to anhedral with crystal size of 180-3000 µm and typically show carlsbad twinning. Plagioclase is subhedral with crystal size of 200-2500 µm, shows polysynthetic twinning, and is partly altered sericite. (BSE, Langfang Regional Geological Survey, Hebei, China) analyses using a JXA-880 electron microscope. Image analysis software was used under operating conditions of 20 kV and 20 nA, at the Langfang Regional Geological Survey, Hebei Province, China, to identify the internal structure and texture of all sphene crystals. Sphene samples were checked carefully using the microscope and BSE images for fluid inclusions and cracks. LA-ICP-MS analyses were conducted at the Isotopic Laboratory, Tianjin Center, China Geological Survey. Details of the analytical procedures are given in [27,28]. Laser sampling was performed using a Neptune double-focusing multiple-collector ICP-MS attached to a NEW WAVE 193 nm-FX ArF Excimer laser-ablation system. All analyses were conducted with a beam diameter of 35 μm, an 8 Hz repetition rate, and energy density of 11 J/cm 2 . GJ-1 was used as an internal standard for U-Pb dating analyses. NIST SRM 610 glass was used as an external standard to calculate U, Th, and Pb concentrations of sphene crystals. Every eight analyses were followed by two analyses of the standard zircon GJ-1. Isotopic ratios were calculated using ICPMSDataCal 8.4, China University of Geosciences, Wuhan, China [29] and were plotted using Isoplot version 3.0 software [30]. Common-Pb corrections were made following the methodology of [31].

Sphene U-Pb Geochronology
Two samples were dated for sphene U-Pb geochronology, syenogranite and quartz monzonite, with LA-ICP-MS data given in Table 2, the morphology shown in Figure 6 and data plotted in Figure 7.    (Table 2).
Sphene grains in quartz monzonite (21XC06) are subhedral in shape, with a len of 100-200 μm and aspect ratio of 1:1 to 2:1 ( Figure 6). Twenty-eight spots were analy on 28 grains. The analyzed spots yield lower intercept dates of 127 ± 1.3 Ma on a Te Wasserburg diagram (2σ, n = 28, MSWD = 3.8) ( Figure 7B). They show Th and U conte of from 244 to 462 ppm and 48 to 298 ppm. The Th/U ratios range from 1.2 to 6.9 (Table

Trace Element Geochemistry of Sphene
Sphene REE data from the syenogranite (21XC01) shows the high total REE, resp tively ranging from 26,594 to 44,233 ppm and 25,538 to 42,146 ppm, and low HREE c tent of 953 to 2267 ppm (Table 3). All REE data from syenogranite exhibit similar ch drite-normalized REE patterns with pronounced LREE enrichment and HREE deplet     (Table 2).
Sphene grains in quartz monzonite (21XC06) are subhedral in shape, with a len of 100-200 μm and aspect ratio of 1:1 to 2:1 ( Figure 6). Twenty-eight spots were analy on 28 grains. The analyzed spots yield lower intercept dates of 127 ± 1.3 Ma on a Te Wasserburg diagram (2σ, n = 28, MSWD = 3.8) ( Figure 7B). They show Th and U cont of from 244 to 462 ppm and 48 to 298 ppm. The Th/U ratios range from 1.2 to 6.9 (Tabl

Trace Element Geochemistry of Sphene
Sphene REE data from the syenogranite (21XC01) shows the high total REE, res tively ranging from 26,594 to 44,233 ppm and 25,538 to 42,146 ppm, and low HREE c tent of 953 to 2267 ppm (Table 3). All REE data from syenogranite exhibit similar ch drite-normalized REE patterns with pronounced LREE enrichment and HREE deplet Sphene grains from syenogranite (21XC01) are euhedral-subhedral and range from 180 to 300 µm in size, with length-to-width ratios of from 1:1 to 2:1 ( Figure 6). Twentyeight spots were analyzed on 28 grains. The 28 analyses yield lower intercept dates of 128 ± 2.3 Ma (2σ, n = 28, MSWD = 1.6) ( Figure 7A). The Th contents range from 163 to 505 ppm and U contents range from 22 to 110 ppm. They show Th/U ratios of 4.4 to 10.3 ( Table 2).

Early Cretaceous Alkaline Magmatism in the Luxi Terrane
Sphene grains from syenogranite (21XC01) and quartz monzonite (21XC06) are eu hedral-subhedral, with lengths of 100-300 μm (Figure 6), and the chondrite-normalize REE patterns show pronounced LREE enrichment, and HREE depletion, implying magmatic origin (Figure 8) [38][39][40][41]. The Th/U ratios of the sphenes from the syenogranit and quartz monzonite are relatively high (>1) [39][40][41][42], supporting the magmatic origin The sphene trace element data mostly fall in the magmatic field (Figure 9), which furthe implies a magmatic origin. Therefore, the lower intercept age of 127-128 Ma from th sphenes in syenogranite and quartz monzoite represent the crystallization age [43]. Liu e al. [7] reported LA-ICP-MS zircon U-Pb age of 128 Ma from pyroxene syenite i Guandimiao alkaline complex, which is consistent with the calculated sphene ages in th study. Therefore, the crystallization ages of alkaline rocks in the Guandimiao comple are bracketed in the range of 127-128 Ma. with variable enrichment comparable to the syenogranite.

Early Cretaceous Alkaline Magmatism in the Luxi Terrane
Sphene grains from syenogranite (21XC01) and quartz monzonite (21XC06) are euhedral-subhedral, with lengths of 100-300 µm (Figure 6), and the chondrite-normalized REE patterns show pronounced LREE enrichment, and HREE depletion, implying a magmatic origin (Figure 8) [38][39][40][41]. The Th/U ratios of the sphenes from the syenogranite and quartz monzonite are relatively high (>1) [39][40][41][42], supporting the magmatic origin. The sphene trace element data mostly fall in the magmatic field (Figure 9), which further implies a magmatic origin. Therefore, the lower intercept age of 127-128 Ma from the sphenes in syenogranite and quartz monzoite represent the crystallization age [43]. Liu et al. [7] reported LA-ICP-MS zircon U-Pb age of 128 Ma from pyroxene syenite in Guandimiao alkaline complex, which is consistent with the calculated sphene ages in this study. Therefore, the crystallization ages of alkaline rocks in the Guandimiao complex are bracketed in the range of 127-128 Ma. Liang et al. [6] reported zircon U-Pb ages of 122-130 Ma for the quartz syenite aegirine-augite syenite from the Weishan complex. Zircon grains from the quartz sye and alkaline granites of the Weishan complex were dated by LA-ICP-MS at 125-127 [9], and crystallization ages of 129-131 Ma were given for the Longbaoshan alkaline r by Lan et al. [8]. Hence, the Early Cretaceous alkaline magmatism in the Luxi Ter occurred from 122 to 131 Ma, and formed a series of alkaline rocks. Liang et al. [6] reported zircon U-Pb ages of 122-130 Ma for the quartz syenite and aegirine-augite syenite from the Weishan complex. Zircon grains from the quartz syenite and alkaline granites of the Weishan complex were dated by LA-ICP-MS at 125-127 Ma [9], and crystallization ages of 129-131 Ma were given for the Longbaoshan alkaline rocks by Lan et al. [8]. Hence, the Early Cretaceous alkaline magmatism in the Luxi Terrane occurred from 122 to 131 Ma, and formed a series of alkaline rocks.

Petrogenesis and Magma Origin
The Guandimiao complex consists of pyroxenites, hornblendites, dioritic porphyries (128 Ma), pyroxene syenites (128 Ma), quartz syenites, syenogranites (128 Ma) and quartz monzonites (127 Ma) with different compositions varying from ultramafic to acidic rocks [7,10], which may suggest the fractional crystallization model. To address the fractional crystallization process, we discussed the previously published hornblendite and the alkaline rocks in this study [7]. The negative correlations between major elements and SiO 2 suggest that the alkaline rocks are likely the result of fractional crystallization during magmatic evolution ( Figure 10) [6,44]. The decreases in CaO, Fe 2 O 3 (Total) and MgO with increasing of SiO 2 were probably caused by the fractional crystallization of hornblende and biotite ( Figure 10A-C), which is consistent with the occurrence of hornblende and biotite in the Guandimiao complex. The negative correlation between SiO 2 and P 2 O 5 implies the crystallization of apatite ( Figure 10D), which is consistent with the occurrence of apatite. The differences in the HREE patterns indicate that the magma source is highly evolved, and silica and titanium are unsaturated in the quartz monzonite ( Figure 8). The negative anomalies of the Nb, Ta and Ti are accounted for in the crystallization rutile and sphene ( Figure 5B). The alkaline rocks contain abundant sphene and no rutile. Furthermore, the TiO 2 contents decrease with increases in SiO 2 , implying that ilmenite and rutile may form as restite during the early fractional crystallization, while sphene formed during the later stage ( Figure 10E). In summary, the major and trace-element geochemistry indicates that fractional crystallization played an important role in the formation of the alkaline rocks.
Since the La/Hf, Th/V, Ce/Yb and Tb/Yb ratios are sensitive to magmatic processes, they can be used to determine the different magmatic processes, including the partial melting and fractional crystallization [18,21,45]. On the La/Hf-La, Th-Th/V, Ce/Yb-K 2 O and Tb/Yb-Yb discrimination diagrams (Figure 11), the alkaline rocks under examination exhibit a positive correlation between La/Hf and La, Th and Th/V, Ce/Yb and K 2 O, and Tb/Yb and Yb, which are consistent with the partial melting trend, demonstrating the role of partial melting in generating the alkaline melts.
The alkaline rocks represent Rb/Nb ratios between 2.00 and 9.06 (average 5.07), which are close to the crust ratios (5.36-6.55) and significantly higher than the mantle ratios (0.24-0.89). Therefore, the Rb/Nb ratios of the alkaline rocks support crustal contamination to some degree. The alkaline rocks show (Th/Nb) N ratios 1.69 to 11.16, implying that the alkaline rocks have assimilated crustal material ((Th/Nb) N > 1) [46]. In addition, the Nb/Th ratio is considered a critical indicator for crustal contamination processes [36]. The alkaline rocks with Nb/Th ratios between 0.75 and 4.95 are further consistent with those observed in the crust (~1.1), which substantiate the involvement of crustal material. The crustal contamination is further supported by the presence of inherited zircon grains (2553-2178 Ma) in the Guandimiao pyroxene syenite [7].
The Nb/Ta ratios (18.1-46.9) of the alkaline rocks are higher than the ratios of average crust (12)(13) [47], which is consistent with the mantle source (15.5-19.5) [46], implying a mantle origin. Seven samples show Y/Nb ratios of lower than 1.2, which is also consistent with a mantle source (<1.2) [48][49][50], implying that the primary alkaline magma derived from the mantle. The La/Nb and La/Ta ratios (4.1-11.8, and 87-355 respectively) are close to the lithospheric mantle (La/Nb>1), and significantly higher than the ratios of the asthenosperic mantle (La/Nb =~0.7, and La/Ta =~10 respectively), implying lithospheric mantle origin [51,52]. These alkaline rocks fall within or near the enriched mantle field (Figure 12), which further implies the lithospheric mantle origin [53]. Three samples of the alkaline rocks with Y/Nb ratios higher than 1.2 are consistent with a crustal source (>1.2) [49,50], which may be the result of crustal assimilation during magma emplacement. Zircon grains in the Weishan alkaline rocks display negative εHf(t) values ranging from −22.67 to −13.19 and yield a T DMC of 2036-2617 Ma, suggesting that the Weishan alkaline rocks originated from the lithospheric mantle with the assimilation of crustal material [6,9]. The Guandimiao and Weishan alkaline rocks show a close spatial relationship and crystallization ages, suggesting the magma may have derived from the same source [6,9]. The magma source of the alkaline rocks in the Guandimiao complex originated from the partial melting of lithospheric mantle, which experienced fractional crystallization and crustal contamination processes during its emplacement.  [8]. Major elements contents of hornblendite taken from [7].
Since the La/Hf, Th/V, Ce/Yb and Tb/Yb ratios are sensitive to magmatic proces they can be used to determine the different magmatic processes, including the par melting and fractional crystallization [18,21,45]. On the La/Hf-La, Th-Th/V, Ce/Yb-K and Tb/Yb-Yb discrimination diagrams (Figure 11), the alkaline rocks under examinat exhibit a positive correlation between La/Hf and La, Th and Th/V, Ce/Yb and K2O, a Tb/Yb and Yb, which are consistent with the partial melting trend, demonstrating the r of partial melting in generating the alkaline melts.  The alkaline rocks represent Rb/Nb ratios between 2.00 and 9.06 (average 5.07), which are close to the crust ratios (5.36-6.55) and significantly higher than the mantle ratios (0.24-0.89). Therefore, the Rb/Nb ratios of the alkaline rocks support crustal contamination to some degree. The alkaline rocks show (Th/Nb)N ratios 1.69 to 11.16, implying that the alkaline rocks have assimilated crustal material ((Th/Nb)N > 1) [46]. In addition, the Nb/Th ratio is considered a critical indicator for crustal contamination processes [36]. The alkaline rocks with Nb/Th ratios between 0.75 and 4.95 are further consistent with those observed in the crust (~1.1), which substantiate the involvement of crustal material. The crustal contamination is further supported by the presence of inherited zircon grains (2553-2178 Ma) in the Guandimiao pyroxene syenite [7].
The Nb/Ta ratios (18.1-46.9) of the alkaline rocks are higher than the ratios of average crust (12)(13) [47], which is consistent with the mantle source (15.5-19.5) [46], implying a mantle origin. Seven samples show Y/Nb ratios of lower than 1.2, which is also consistent with a mantle source (<1.2) [48][49][50], implying that the primary alkaline magma derived from the mantle. The La/Nb and La/Ta ratios (4.1-11.8, and 87-355 respectively) are close to the lithospheric mantle (La/Nb>1), and significantly higher than the ratios of the asthenosperic mantle (La/Nb = ~0.7, and La/Ta = ~10 respectively), implying lithospheric mantle origin [51,52]. These alkaline rocks fall within or near the enriched mantle field (Figure 12), which further implies the lithospheric mantle origin [53]. Three samples of the alkaline rocks with Y/Nb ratios higher than 1.2 are consistent with a crustal source (>1.2) [49,50], which may be the result of crustal assimilation during magma emplacement. Zircon grains in the Weishan alkaline rocks display negative εHf(t) values ranging from −22.67 to −13.19 and yield a TDMC of 2036-2617 Ma, suggesting that the Weishan alkaline rocks originated from the lithospheric mantle with the assimilation of crustal material [6,9]. The Guandimiao and Weishan alkaline rocks show a close spatial relationship and crystallization ages, suggesting the magma may have derived from the same source [6,9]. The magma source of the alkaline rocks in the Guandimiao complex originated from the partial melting of lithospheric mantle, which experienced fractional crystallization and crustal contamination processes during its emplacement.

Tectonic Implications
The crystallization ages of the alkaline rocks from the Luxi Terrane are in the range of 122-131 Ma, corresponding to the peak time of lithospheric destruction of the NCB (120-130 Ma) [54,55]. In addition, the lithospheric destruction mainly occurred in eastern NCB due to the subduction of the Izanagi plate during the Early Cretaceous [42,[56][57][58][59][60][61][62][63][64][65][66][67][68]. Hence, the subducting Izanagi plate during the Late Mesozoic may have played an important role in the lithospheric destruction of the NCB. The Izanagi plate subducted beneath the NCB during the Jurassic, resulting in the thickening of the lower crust [60][61][62]. Asthenosphere upwelling and partial melting of the lithospheric mantle were caused by
Hence, the subducting Izanagi plate during the Late Mesozoic may have played an important role in the lithospheric destruction of the NCB. The Izanagi plate subducted beneath the NCB during the Jurassic, resulting in the thickening of the lower crust [60][61][62]. Asthenosphere upwelling and partial melting of the lithospheric mantle were caused by the rollback of the subducting Izanagi plate during the Early Cretaceous [59,[62][63][64][65][66].
The alkaline rocks show enrichment in LREE and LILEs (Rb and Ba), depletion in HREE and HFSEs (Nb, Ta, Ti) characteristics that correspond to volcanic arc granites [42,[67][68][69]. In the tectonic discrimination diagram, the alkaline rocks are mainly plotted in the volcanic arc granites field ( Figure 13A-C), which further implies that they were formed in a volcanic arc setting. The alkaline rocks in the Guandimiao complex mainly include syenogranites, quartz syenites and quartz monzonites, which are consistent with active continental margin granites [42]. Furthermore, in the Th/Yb-Yb diagram, the alkaline rocks are mainly plotted in the active continental margin field ( Figure 13D) [42]. Hence, the alkaline rocks were probably formed in or near the active continental margin. The active continental margin affinity may be related to the subduction of the Yangtze or Izanagi plate beneath the NCB. The alkaline rocks in the Guandimiao complex are alkali and peralkaline, and show high Ga/Al ratios, SiO2, LREE, Zr and Nb, and low MgO, CaO, Eu contents, which, together with the high abundance of alkali feldspar, are similar to typical A-type granites [42,48,49]. In the tectonic discrimination diagrams (Figure 14), all the alkaline rocks are plotted in the A-type granite field, which further supports that the alkaline rocks are A-type granites. Therefore, the parent magma of the alkaline rocks from the Guandimiao complex is anhydrous, and may have been formed in an anorogenic setting [68]. The alkaline rocks are A-type granites, indicating an extensional decompression process during the magma emplacement [70][71][72][73][74]. Previous studies show that the Izanagi plate subduction might have played an important role in the magmatic activity of the eastern NCB during the Early Cretaceous [69]. Therefore, the alkaline rocks in the Guandimiao complex were probably formed in an extensional back-arc setting induced by the retreat of the subducting Izanagi plate [69,75]. The alkaline rocks in the Guandimiao complex are alkali and peralkaline, and show high Ga/Al ratios, SiO 2 , LREE, Zr and Nb, and low MgO, CaO, Eu contents, which, together with the high abundance of alkali feldspar, are similar to typical A-type granites [42,48,49]. In the tectonic discrimination diagrams (Figure 14), all the alkaline rocks are plotted in the A-type granite field, which further supports that the alkaline rocks are A-type granites. Therefore, the parent magma of the alkaline rocks from the Guandimiao complex is anhydrous, and may have been formed in an anorogenic setting [68]. The alkaline rocks are A-type granites, indicating an extensional decompression process during the magma emplacement [70][71][72][73][74]. Previous studies show that the Izanagi plate subduction might have played an important role in the magmatic activity of the eastern NCB during the Early Cretaceous [69]. Therefore, the alkaline rocks in the Guandimiao complex were probably formed in an extensional back-arc setting induced by the retreat of the subducting Izanagi plate [69,75].
complex is anhydrous, and may have been formed in an anorogenic setting [68]. The alkaline rocks are A-type granites, indicating an extensional decompression process during the magma emplacement [70][71][72][73][74]. Previous studies show that the Izanagi plate subduction might have played an important role in the magmatic activity of the eastern NCB during the Early Cretaceous [69]. Therefore, the alkaline rocks in the Guandimiao complex were probably formed in an extensional back-arc setting induced by the retreat of the subducting Izanagi plate [69,75]. Figure 14. (A-C) Rock-type discrimination diagrams [46].
In summary, the rollback of the subducting Izanagi plate during the Early Cretaceous triggered the asthenosphere upwelling and lithospheric thinning and partial melting of the lithospheric mantle [76][77][78]. The lithospheric mantle-derived magma was emplaced at a shallower crustal depth and contaminated by crustal material during as- Figure 14. (A-C) Rock-type discrimination diagrams [46].
In summary, the rollback of the subducting Izanagi plate during the Early Cretaceous triggered the asthenosphere upwelling and lithospheric thinning and partial melting of the lithospheric mantle [76][77][78]. The lithospheric mantle-derived magma was emplaced at a shallower crustal depth and contaminated by crustal material during ascending, and underwent a fractional crystallization process to form the Guandimiao alkaline rocks ( Figure 15). cending, and underwent a fractional crystallization process to form the Guandimiao al kaline rocks ( Figure 15).

Conclusions
(1) The sphenes from syenogranite and quartz monzonite show an euhedral-subhedra morphology, which, together with the obvious fractionation between LREE and HREE and high Th/U ratios, imply a magmatic origin.