Ocean–Continent Conversion in Beishan Orogenic Belt: Evidence from Geochemical and Zircon U-Pb-Hf Isotopic Data of Luotuoquan A-Type Granite

: Devonian magmatism is one of the most important tectonothermal events in the Central Asian Orogenic Belt (CAOB). However, li tt le is known regarding the petrogenesis and geodynamic se tt ing of the widely distributed Devonian granitoids in the eastern Southern Beishan Orogenic Belt (SBOB). Early-Devonian granitic magmatism has been recognized from the Luotuoquan area

Herein, we present comprehensive whole-rock geochemical, in situ zircon U-Pb geochronology, and Hf isotopic data of A-type granites from the Luotuoquan complex.In addition to previously published findings, we discuss the genetic mechanism and tectonic settings of the early Paleozoic granitoids of the SBOB, elucidating their significant implications for understanding the tectonic evolution during this period in the BOB while also providing valuable insights into Beishan Ocean closure.[5]).(b) Simplified tectonic map of the Beishan orogenic collage and its adjacent area showing the tectonic subdivisions (modified after [5,13]).The zircon U-Pb age data of granitoids in (b) are from previous studies [4,6,10,[30][31][32][33][34][35].

Geological Setting
The BOB comprises a complex assemblage of blocks, magmatic arcs, and ophiolitic mélanges that were formed through the subduction-accretion process of the Paleo-Asian Ocean [5].Based on the spatial and temporal distributions of the ophiolitic mélanges and rock associations, the BOB is divided into several arcs [5], comprising (from north to south) the Queershan, Heiyingshan, Hanshan, Mazongshan, Shuangyingshan, Huaniushan, and Shibanshan arcs, which are separated by the Hongshishan, Shibanjing, Hongliuhe, and Liuyuan ophiolitic mélanges, respectively (Figure 1b).
Herein, we present comprehensive whole-rock geochemical, in situ zircon U-Pb geochronology, and Hf isotopic data of A-type granites from the Luotuoquan complex.In addition to previously published findings, we discuss the genetic mechanism and tectonic settings of the early Paleozoic granitoids of the SBOB, elucidating their significant implications for understanding the tectonic evolution during this period in the BOB while also providing valuable insights into Beishan Ocean closure.

Geological Setting
The BOB comprises a complex assemblage of blocks, magmatic arcs, and ophiolitic mélanges that were formed through the subduction-accretion process of the Paleo-Asian Ocean [5].Based on the spatial and temporal distributions of the ophiolitic mélanges and rock associations, the BOB is divided into several arcs [5], comprising (from north to south) the Queershan, Heiyingshan, Hanshan, Mazongshan, Shuangyingshan, Huaniushan, and Shibanshan arcs, which are separated by the Hongshishan, Shibanjing, Hongliuhe, and Liuyuan ophiolitic mélanges, respectively (Figure 1b).

Zircon Dating and CL Imaging
Zircons were separated from the granite samples PM06-34 and PM12-20 of the Luotuoquan complex for laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb dating.Zircon grains were extracted using standard density and magnetic separation techniques.The selected zircon grains were handpicked under a stereoscopic microscope and mounted in epoxy resin before being polished to dissect the crystals in half for analysis.Cathodoluminescence (CL) and reflected-light photomicrographic analysis of the prepared sample targets were utilized to image the morphology and internal structure of the zircons to aid in selecting zircon grains for U-Pb dating.Zircon U-Pb dating analyses were conducted on a quadrupole inductively coupled plasma mass spectrometer (ICP-MS) (THERMO-ICAPRQ) coupled to a 193-nm ArF Excimer laser (Resolution-LR, Applied Spectra, West Sacramento, CA, USA) at Hebei Key Laboratory of Strategic Critical Mineral Resources.The laser spot size was set to 29 µm, the laser energy density was 3 J/cm 2, and the repetition rate was 8 Hz.Each analysis comprised a 10 s blank, a 40 s sampling ablation, and a 20 s sample-chamber flushing after the ablation.The ablated material was carried into the ICP-MS by the high-purity helium gas stream with a flux of 0.4 L/min.The whole laser path was fluxed with argon (0.9 L/min) to increase energy stability.A zircon 91,500 standard was used for external age calibration, and a zircon GJ-1 standard was used as a secondary standard to supervise the deviation of age calculation.Calibrations for trace element concentration were carried out using NIST SRM610 as an external standard and Si as the internal standard.ICPMSDataCal (Ver.4.6) [43] and Isoplot 3.0 [44] programs were used for data reduction.

In Situ Lu-Hf Isotopes
In situ zircon Lu-Hf isotopic analyses were performed using a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Brunswick, Germany) equipped with a Geolas 2005 excimer ArF laser ablation system (LambdaPhysik, Göttingen, Germany).All data on zircon were acquired in a single-spot ablation mode at a spot size of 44 µm.The energy density of laser ablation used in this study was ~7.0 J/cm −2 .Each measurement consisted of a 20-s acquisition of the background signal, followed by a 50-s acquisition of ablation signals.Instrumental conditions and data acquisition were as described by Wu et al. (2006) [45].Zircon 91,500 was used as the reference standard.The chondritic ratios of 176 Hf/ 177 Hf = 0.282772 and 176 Lu/ 177 Hf = 0.0332 were used in our calculation of ε Hf (t) values [46].Single-stage model ages (T DM1 ) were calculated by reference to depleted mantle with a present day 176 Hf/ 177 Hf ratio of 0.28325 and 176 Lu/ 177 Hf ratio of 0.0384 [47].The two-stage Hf model age (T DM2 ), also interpreted as crust formation age, was calculated by projecting the zircon 176 Hf/ 177 Hf (t) back to the depleted mantle model growth curve, assuming a mean crustal value for Lu/Hf ( 176 Lu/ 177 Hf = 0.015) [48].

Whole-Rock Geochemical Analysis
Whole-rock geochemical analyses were performed at the Institute of Regional Geology Survey of Hebei Province.Fresh chips of whole-rock samples were powdered to 200 mesh using a tungsten carbide ball mill.Major and trace elements were analyzed by X-ray fluorescence (Axios X; PANalytical B.V.) and inductively coupled plasma mass spectrometry (XSeries II; Thermo Fisher Scientific), respectively.The analytical precision is generally better than 2% for major elements.For trace element analyses, sample powders were digested using HF + HNO 3 mixture in high-pressure Teflon bombs at 190 • C for 48 h or longer.The analytical precision is generally better than 5% for trace elements.
The analytical results are presented in Table 2, including εHf(t) values and model ages calculated using 206 Pb/ 238 U ages.The 32 spots from the Luotuoquan granites exhibit initial 176 Hf/ 177 Hf ratios ranging from 0.282205 to 0.282415 and positive εHf(t) values of +0.90-+5.19.Furthermore, the TDM2 model ages range from 1.05 to 1.34 Ga.The analytical results are presented in Table 2, including ε Hf (t) values and model ages calculated using 206 Pb/ 238 U ages.The 32 spots from the Luotuoquan granites exhibit initial 176 Hf/ 177 Hf ratios ranging from 0.282205 to 0.282415 and positive ε Hf (t) values of +0.90-+5.19.Furthermore, the T DM2 model ages range from 1.05 to 1.34 Ga.

Major and Trace Elements
The whole-rock major and trace element compositions of the five monzogranite samples and three syenogranite samples were analyzed, and the results are presented in Table 3.

Trace Elements
In the chondrite-normalized rare earth element patterns, the samples exhibit relative enrichment of light rare earth elements (LREE)([La/Yb] N = 2.63-16.87)and significant negative Eu anomalies (Eu/Eu* = 0.23-0.45),while the REE abundances range from 161.2 to 459.7 ppm (Figure 6a).All samples are depleted in Ba, U, Sr, P, and Ti and enriched in Rb, Th, Nd, Zr, and Hf relative to the primitive mantle (Figure 6b).

Trace Elements
In the chondrite-normalized rare earth element patterns, the samples exhibit relative enrichment of light rare earth elements (LREE)([La/Yb]N = 2.63-16.87)and significant negative Eu anomalies (Eu/Eu* = 0.23-0.45),while the REE abundances range from 161.2 to 459.7 ppm (Figure 6a).All samples are depleted in Ba, U, Sr, P, and Ti and enriched in Rb, Th, Nd, Zr, and Hf relative to the primitive mantle (Figure 6b).

Petrogenesis and Magma Sources
The syenogranite samples in this study have undergone mylonitization, as evid by petrographic observation (Figure 2d).Therefore, it is imperative to evaluate the of alteration on both major and trace elements before discussing their petrogenes Our samples demonstrate differentiated correlations between "immobile alteratio ments (e.g., Zr) and the other trace elements (Figure 7), where Nb, Ta, Th, and Hf d a stronger correlation with Zr than Rb and Ba, suggesting that this alteration has les on these elements.Consequently, the subsequent discussion will primarily focus o immobile elements such as Nb, Ta, Zr, and REEs.
The Luotuoquan monzogranite and syenogranite exhibit similar geochemica acteristics and are discussed together.They have high SiO2 (71.04-76.00wt.%) and Na2O (7.04-8.62 wt.%) but low Al2O3 (mean of 13.36 wt.%), MgO (mean of 0.03 wt.% CaO (mean of 1.64 wt.%), indicating A-type granite geochemical characteristics.theless, highly fractionated I-type and S-type granites also share similarities with granites.Highly fractionated S-type granites tend to exhibit higher P2O5 (mean wt.%) and lower Na2O (mean of 2.81 wt.%) than A-type granites [56].The monzog syenogranite displays low P2O5 (mean of 0.11 wt.%) and high Na2O (mean of 3.09 indicating it does not belong to the highly fractionated S-type granite.Compare highly fractionated I-type granite, A-type granite shows iron enrichment and magn depletion with higher Fe2O3 T / MgO ratios.The Fe2O3 T / MgO value of Luotuoquan ranges from 2.95 to 24.28 (mean of 6.95), which is significantly greater than that of fractionated I-type granite (2.27) and S-type granite (2.38) [57,58].Additionally, typ type granites are enriched in trace elements such as Th, Nb, Ta, Zr, Hf, Ga, and Y being depleted in Sr, Ti, P, Cr, Co, Ni, V, etc., with obvious negative Eu anomaly [ All samples exhibit high Th, Zr, K, Ga, Y, and Yb but low Sr, P, Eu, and Ti, whi implies that Luotuoquan granite had A-type geochemical features.Previous studi gested that A-type granitoids share similar geochemistry characteristics of high 1 Ga/Al values (>2.6) and Zr + Nb + Ce + Y (>350 ppm) [57].The discrimination dia

Petrogenesis and Magma Sources
The syenogranite samples in this study have undergone mylonitization, as evidenced by petrographic observation (Figure 2d).Therefore, it is imperative to evaluate the impact of alteration on both major and trace elements before discussing their petrogenesis [55].Our samples demonstrate differentiated correlations between "immobile alteration" elements (e.g., Zr) and the other trace elements (Figure 7), where Nb, Ta, Th, and Hf display a stronger correlation with Zr than Rb and Ba, suggesting that this alteration has less effect on these elements.Consequently, the subsequent discussion will primarily focus on more immobile elements such as Nb, Ta, Zr, and REEs.
The Luotuoquan monzogranite and syenogranite exhibit similar geochemical characteristics and are discussed together.They have high SiO 2 (71.04-76.00wt.%) and K 2 O + Na 2 O (7.04-8.62 wt.%) but low Al 2 O 3 (mean of 13.36 wt.%), MgO (mean of 0.03 wt.%), and CaO (mean of 1.64 wt.%), indicating A-type granite geochemical characteristics.Nevertheless, highly fractionated I-type and S-type granites also share similarities with A-type granites.Highly fractionated S-type granites tend to exhibit higher P 2 O 5 (mean of 0.14 wt.%) and lower Na 2 O (mean of 2.81 wt.%) than A-type granites [56].The monzogranite-syenogranite displays low P 2 O 5 (mean of 0.11 wt.%) and high Na 2 O (mean of 3.09 wt.%), indicating it does not belong to the highly fractionated S-type granite.Compared with highly fractionated I-type granite, A-type granite shows iron enrichment and magnesium depletion with higher Fe 2 O 3 T /MgO ratios.The Fe 2 O 3 T /MgO value of Luotuoquan granite ranges from 2.95 to 24.28 (mean of 6.95), which is significantly greater than that of highly fractionated I-type granite (2.27) and S-type granite (2.38) [57,58].Additionally, typical A-type granites are enriched in trace elements such as Th, Nb, Ta, Zr, Hf, Ga, and Y while being depleted in Sr, Ti, P, Cr, Co, Ni, V, etc., with obvious negative Eu anomaly [57,59].All samples exhibit high Th, Zr, K, Ga, Y, and Yb but low Sr, P, Eu, and Ti, which also implies that Luotuoquan granite had A-type geochemical features.Previous studies suggested that A-type granitoids share similar geochemistry characteristics of high 10,000× Ga/Al values (>2.6) and Zr + Nb + Ce + Y (>350 ppm) [57].The discrimination diagrams (Figure 8) demonstrate that the majority, if not all, of the samples are plotted within the A-type granite field.We, therefore, conclude that the Luotuoquan monzogranite-syenogranite pluton are typical A-type intrusions.
Minerals 2023, 13, 1411 10 of 18 (Figure 8) demonstrate that the majority, if not all, of the samples are plotted within the A-type granite field.We, therefore, conclude that the Luotuoquan monzogranite-syenogranite pluton are typical A-type intrusions.56,59,68], and partial melting of calc-alkaline tonalite-granodiorite [69]; (4) the melting of lower crustal rocks due to heating by mantle magma underplating [58,70,71].Mafic rocks contemporaneous with A-type granite are rare in the study area, and no mafic microgranular enclaves are found in the granites, indicating little crust-mantle mixing.Experimental petrology shows residual granulite facies in the lower crust are low in K, Si and high in Ca, Al, and Mg [69], which cannot explain the production of Luotuoquan A-type granite rich in Si, alkali, low in Al and Mg by partial melting.The CaO/Na2O ratios distinguish between pelite-derived melts (CaO/Na2O < 0.5) melts derived from greywackes or igneous sources (CaO/Na2O = 0.3-1.5)[72].High-t perature melts generally exhibit lower Al2O3/TiO2 ratios than low-temperature melts [ The monzogranite-syenogranite is characterized by high CaO/Na2O ratios (mean of 0 and low Al2O3/TiO2 ratios (mean of 63.73), suggesting a source of metagreywackes or m amorphic igneous rock.The samples are all plotted within the field of partial melts rived from metagreywackes in the molar Al2O3/(MgO + FeO T ) vs. molar CaO/(Mg FeO T ) and molar K2O/Na2O vs. molar CaO/(MgO + FeO T ) diagrams (Figure 9).Theref the granitic magma source is likely related to partial melts from metagreywackes.
The Hf isotope analysis of U-Pb dated zircon grains can trace the original mag sources and distinguish between the reworking of continental crust and the remeltin juvenile crust [74,75].In this study, the early Devonian Luotuoquan monzogranitenogranite has considerably positive zircon εHf(t) of +0.9-+5.2(Figure 10) and slig young two-stage Hf model ages of 1.05-1.34Ga (Figure 10).The SBOB is thought to h developed abundant Mesoproterozoic to Neoproterozoic basement complex, which suite of metamorphosed clastic rock [17,18,20,37].The significantly positive εHf(t) va and relatively young two-stage Hf model ages suggest that the granitic rocks origina from either the depleted mantle or through partial melting of recently accreted juve crustal material within the depleted mantle.Relevant mafic rocks are rarely contemp neous, and the absence of mafic microgranular enclaves suggests limited direct invo ment of newly derived mantle magma in granite formation.Therefore, the early Devon granitoids from the SBOB primarily originated from partial melting of the overlying M oproterozoic crust facilitated by the underplating of mantle-derived magma.56,59,68], and partial melting of calc-alkaline tonalite-granodiorite [69]; (4) the melting of lower crustal rocks due to heating by mantle magma underplating [58,70,71].Mafic rocks contemporaneous with A-type granite are rare in the study area, and no mafic microgranular enclaves are found in the granites, indicating little crust-mantle mixing.Experimental petrology shows residual granulite facies in the lower crust are low in K, Si and high in Ca, Al, and Mg [69], which cannot explain the production of Luotuoquan A-type granite rich in Si, alkali, low in Al and Mg by partial melting.
The Hf isotope analysis of U-Pb dated zircon grains can trace the original magma sources and distinguish between the reworking of continental crust and the remelting of juvenile crust [74,75].In this study, the early Devonian Luotuoquan monzogranitesyenogranite has considerably positive zircon ε Hf (t) of +0.9-+5.2(Figure 10) and slightly young two-stage Hf model ages of 1.05-1.34Ga (Figure 10).The SBOB is thought to have developed abundant Mesoproterozoic to Neoproterozoic basement complex, which is a suite of metamorphosed clastic rock [17,18,20,37].The significantly positive ε Hf (t) values and relatively young two-stage Hf model ages suggest that the granitic rocks originated from either the depleted mantle or through partial melting of recently accreted juvenile crustal material within the depleted mantle.Relevant mafic rocks are rarely contemporaneous, and the absence of mafic microgranular enclaves suggests limited direct involvement of newly derived mantle magma in granite formation.Therefore, the early Devonian granitoids from the SBOB primarily originated from partial melting of the overlying Mesoproterozoic crust facilitated by the underplating of mantle-derived magma.[75]).Published data of granitoids from [6,42].The data on Tarim, North China, and Yangtze blocks were from [78].
The A-type granite has garnered significant attention due to its distinctive tectonic background.The Luotuoquan A-type granites fall within the volcanic arc and within-plate granites on tectonic discrimination diagrams (Figure 11a,b) rather than pertaining to ocean ridge granites.The studied granites are generally plotted within the A2 field (Figure 11c,d).The A2-type granitoids represent magmas sourced from the underplated crust or continental crust that has experienced a cycle of island-arc magmatism or continent-continent collision [86].We propose that their potential origins were associated with islandarc magmatism.The monzogranite in the Hongliuhe ophiolite has a weighted age of 412.4  Published data of granitoids from [6,42].The data on Tarim, North China, and Yangtze blocks were from [78].
The A-type granite has garnered significant attention due to its distinctive tectonic background.The Luotuoquan A-type granites fall within the volcanic arc and within-plate granites on tectonic discrimination diagrams (Figure 11a,b) rather than pertaining to ocean ridge granites.The studied granites are generally plotted within the A2 field (Figure 11c,d).The A2-type granitoids represent magmas sourced from the underplated crust or continental crust that has experienced a cycle of island-arc magmatism or continent-continent collision [86].We propose that their potential origins were associated with islandarc magmatism.The monzogranite in the Hongliuhe ophiolite has a weighted age of 412.4 Figure 10.Zircon ε Hf (t) vs. Age (Ma) diagrams of granitoids from the SBOB (base map after [75]).Published data of granitoids from [6,42].The data on Tarim, North China, and Yangtze blocks were from [78].
The A-type granite has garnered significant attention due to its distinctive tectonic background.The Luotuoquan A-type granites fall within the volcanic arc and withinplate granites on tectonic discrimination diagrams (Figure 11a,b) rather than pertaining to ocean ridge granites.The studied granites are generally plotted within the A2 field (Figure 11c,d).The A2-type granitoids represent magmas sourced from the underplated crust or continental crust that has experienced a cycle of island-arc magmatism or continentcontinent collision [86].We propose that their potential origins were associated with Minerals 2023, 13, 1411 13 of 18 island-arc magmatism.The monzogranite in the Hongliuhe ophiolite has a weighted age of 412.4 ± 2.9 Ma, indicating that the closure of the oceanic basin was completed prior to this event [41].The granites from the Liuyuan area, with ages ranging from 436-423 Ma [40], 415 Ma [42], and 397 Ma [4,40], represent post-collision background products that are possibly associated with subduction plate detachment.Moreover, recent discoveries have revealed the presence of early Devonian post-collision granites in the middle of BOB.U-Pb dating has determined that these granites range in age from 402 to 387 Ma [15,31,35,52].The aforementioned evidence suggests that the formation of Luotuoquan A-type granites can be attributed to a post-collision tectonic setting.
Minerals 2023, 13, 1411 13 of 18 ± 2.9 Ma, indicating that the closure of the oceanic basin was completed prior to this event [41].The granites from the Liuyuan area, with ages ranging from 436-423 Ma [40], 415 Ma [42], and 397 Ma [4,40], represent post-collision background products that are possibly associated with subduction plate detachment.Moreover, recent discoveries have revealed the presence of early Devonian post-collision granites in the middle of BOB.U-Pb dating has determined that these granites range in age from 402 to 387 Ma [15,31,35,52].The aforementioned evidence suggests that the formation of Luotuoquan A-type granites can be attributed to a post-collision tectonic setting.[89,90].During the early Paleozoic, a significant increase in crustal growth occurred due to the melting of subducting oceanic crust, resulting in the emplacement of large amounts of granitoids in the SBOB from 452 to 424 Ma [6].This process was accompanied by a gradual decrease in zircon saturation temperatures and an increase in crustal thickness (Figure 11).The A-type granites, which were formed between 415 and 397 Ma [4,42], exhibit a higher Zr saturation temperature range of 755-831 °C and a thinner crust thickness ranging from 32 to 28 km (Figure 12).These findings suggest that the SBOB had already transitioned into an extensional setting during the early Devonian.[89,90].During the early Paleozoic, a significant increase in crustal growth occurred due to the melting of subducting oceanic crust, resulting in the emplacement of large amounts of granitoids in the SBOB from 452 to 424 Ma [6].This process was accompanied by a gradual decrease in zircon saturation temperatures and an increase in crustal thickness (Figure 11).The A-type granites, which were formed between 415 and 397 Ma [4,42], exhibit a higher Zr saturation temperature range of 755-831 • C and a thinner crust thickness ranging from 32 to 28 km (Figure 12).These findings suggest that the SBOB had already transitioned into an extensional setting during the early Devonian.In summary, the Luotuoquan A-type granites yield U-Pb ages of approximately 404-399 Ma, indicating post-collision extensional setting during the early Devonian.This evidence indicates a post-collision extension setting in the early Devonian and implies the closure of the Beishan Ocean prior to this time.
(2) The petrographic and geochemical signatures of the Luotuoquan monzogranite and syenogranite indicate they are A-type granites and were emplaced in a post-collision extensional setting.Furthermore, these granites are the result of partial melting primarily from Mesoproterozoic crusts composed mainly of metagreywackes.
(3) The occurrence of early Devonian granitoids suggests that SBOB had already undergone extensional tectonics following the closure of the Beishan Ocean during this period.
In summary, the Luotuoquan A-type granites yield U-Pb ages of approximately 404-399 Ma, indicating post-collision extensional setting during the early Devonian.This evidence indicates a post-collision extension setting in the early Devonian and implies the closure of the Beishan Ocean prior to this time.
(2) The petrographic and geochemical signatures of the Luotuoquan monzogranite and syenogranite indicate they are A-type granites and were emplaced in a post-collision extensional setting.Furthermore, these granites are the result of partial melting primarily from Mesoproterozoic crusts composed mainly of metagreywackes.
(3) The occurrence of early Devonian granitoids suggests that SBOB had already undergone extensional tectonics following the closure of the Beishan Ocean during this period.

Figure 2 .
Figure 2. Geological map of the mafic-ultramafic intrusions and granite plutons in the Luotuoquan area.Figure 2. Geological map of the mafic-ultramafic intrusions and granite plutons in the Luotuoquan area.

Figure 2 .
Figure 2. Geological map of the mafic-ultramafic intrusions and granite plutons in the Luotuoquan area.Figure 2. Geological map of the mafic-ultramafic intrusions and granite plutons in the Luotuoquan area.

Figure 2 .
Figure 2. Geological map of the mafic-ultramafic intrusions and granite plutons in the Luotuoquan area.

Figure 4 .
Figure 4. U-Pb concordia diagrams of zircons from Luotuoquan monzogranite (a) and syenogranite (b).The yellow line circle represents the spot of LA-ICP-MS analysis for U-Pb dating.The red dashed line circle represents the spot of LA-MC-ICP-MS analysis for Lu-Hf isotope compositions.

Figure 4 .
Figure 4. U-Pb concordia diagrams of zircons from Luotuoquan monzogranite (a) and syenogranite (b).The yellow line circle represents the spot of LA-ICP-MS analysis for U-Pb dating.The red dashed line circle represents the spot of LA-MC-ICP-MS analysis for Lu-Hf isotope compositions.

Figure 6 .
Figure 6.Chondrite-normalized REE element patterns (a) and primitive mantle-normalize element spider diagrams (b) for the Luotuoquan granites.Normalization values are from [5

Figure 7 .
Figure 7. Trace elements vs. Zr for Luotuoquan A-type granites.Various petrogenetic models have been proposed for A-type granites: (1) high crystallization differentiation of mantle-derived basaltic magma [60-63]; (2) mixing of mantlederived and crustal materials [64-67]; (3) Partial melting of lower crustal material: partial melting of granulite facies remnants after granitic magma extraction [55,56,59,68], and partial melting of calc-alkaline tonalite-granodiorite [69]; (4) the melting of lower crustal rocks due to heating by mantle magma underplating [58,70,71].Mafic rocks contemporaneous with A-type granite are rare in the study area, and no mafic microgranular enclaves are found in the granites, indicating little crust-mantle mixing.Experimental petrology shows residual granulite facies in the lower crust are low in K, Si and high in Ca, Al, and Mg [69], which cannot explain the production of Luotuoquan A-type granite rich in Si, alkali, low in Al and Mg by partial melting.

Figure 8 .
Figure 8. Discrimination diagrams for early Devonian granitoids from BOB. Zr (a) and Nb (b) vs. 10,000 * Ga/Al and (K 2 O + Na 2 O)/CaO (c) and FeO T /MgO (d) vs. Ce + Nb + Zr + Y diagrams[57].Published data are from the same references as in Figure5.Various petrogenetic models have been proposed for A-type granites: (1) high crystallization differentiation of mantle-derived basaltic magma[60][61][62][63]; (2) mixing of mantlederived and crustal materials[64][65][66][67]; (3) Partial melting of lower crustal material: partial melting of granulite facies remnants after granitic magma extraction [55,56,59,68], and partial melting of calc-alkaline tonalite-granodiorite[69]; (4) the melting of lower crustal rocks due to heating by mantle magma underplating[58,70,71].Mafic rocks contemporaneous with A-type granite are rare in the study area, and no mafic microgranular enclaves are found in the granites, indicating little crust-mantle mixing.Experimental petrology shows residual granulite facies in the lower crust are low in K, Si and high in Ca, Al, and Mg[69], which cannot explain the production of Luotuoquan A-type granite rich in Si, alkali, low in Al and Mg by partial melting.The CaO/Na 2 O ratios distinguish between pelite-derived melts (CaO/Na 2 O < 0.5) and melts derived from greywackes or igneous sources (CaO/Na 2 O = 0.3-1.5)[72].Hightemperature melts generally exhibit lower Al 2 O 3 /TiO 2 ratios than low-temperature melts[73].The monzogranite-syenogranite is characterized by high CaO/Na 2 O ratios (mean of 0.53) and low Al 2 O 3 /TiO 2 ratios (mean of 63.73), suggesting a source of metagreywackes or metamorphic igneous rock.The samples are all plotted within the field of partial melts derived from metagreywackes in the molar Al 2 O 3 /(MgO + FeO T ) vs. molar CaO/(MgO + FeO T ) and molar K 2 O/Na 2 O vs. molar CaO/(MgO + FeO T ) diagrams (Figure9).Therefore, the granitic magma source is likely related to partial melts from metagreywackes.The Hf isotope analysis of U-Pb dated zircon grains can trace the original magma sources and distinguish between the reworking of continental crust and the remelting of juvenile crust[74,75].In this study, the early Devonian Luotuoquan monzogranitesyenogranite has considerably positive zircon ε Hf (t) of +0.9-+5.2(Figure10) and slightly young two-stage Hf model ages of 1.05-1.34Ga (Figure10).The SBOB is thought to have developed abundant Mesoproterozoic to Neoproterozoic basement complex, which is a suite of metamorphosed clastic rock[17,18,20,37].The significantly positive ε Hf (t) values and relatively young two-stage Hf model ages suggest that the granitic rocks originated from either the depleted mantle or through partial melting of recently accreted juvenile crustal material within the depleted mantle.Relevant mafic rocks are rarely contemporaneous, and the absence of mafic microgranular enclaves suggests limited direct involvement of newly derived mantle magma in granite formation.Therefore, the early Devonian granitoids from the SBOB primarily originated from partial melting of the overlying Mesoproterozoic crust facilitated by the underplating of mantle-derived magma.

Table 1 .
LA-ICP-MS zircon U-Pb dating results of the Luotuoquan granites.

Table 1 .
LA-ICP-MS zircon U-Pb dating results of the Luotuoquan granites.

Table 2 .
In situ zircon Hf isotopic results of the Luotuoquan granites.

Table 3 .
Major and trace element compositions of the Luotuoquan granites.