Petrogenesis of the Early Cretaceous Aolunhua Adakitic Monzogranite Porphyries, Southern Great Xing’an Range, NE China: Implication for Geodynamic Setting of Mo Mineralization

: This paper reports on whole-rock major- and trace-elemental and Sr–Nd isotopic compositions of the Aolunhua adakitic monzogranite porphyries from the Xilamulun district in the southern Great Xing’an Range, Northeast (NE) China. The high-K calc-alkaline Aolunhua monzogranite porphyries are characterized by high Sr / Y ratios (34.59–91.02), Sr (362–809 ppm), and low Y contents (7.66–10.5 the southern Great Xing’an Range.


Introduction
Adakites with high Sr/Y ratios (≥20), Sr contents (≥400 ppm), and low Y (≤18 ppm) and Yb (≤1.9 ppm) contents were initially proposed to be melts derived from young subducted oceanic crust (≤25 Ma) by Defant and Drummond [1] and are usually interpreted to be indicators of convergent margin tectonic setting with slab subduction. Subsequent studies demonstrated that a series of crustal processes related to subduction (e.g., melting of an oceanic slab and thickened lower crust of collision zones) may have occurred since the Neoproterozoic to produce adakitic magmas [2][3][4][5]. Thus, adakitic rocks were proposed to refer to as intermediates to felsic igneous rocks (SiO 2 ≥ 56 wt. %) with geochemical characteristics similar to those of above-mentioned adakites related with young subducted oceanic crust. Recent studies have found numerous adakitic rocks that were thought to have been generated in subduction zones, rather may have formed by partial melting of basaltic rocks in thickened crust of collision zones [3,4] and even in intraplate settings without crustal thickening [2,5]. To distinguish the model for the generation of adakites, source nature and magmatic evolutional process should be fully considered. In addition, it has been reported that a large number of Cu-Mo mineralized systems are genetically related to adakitic intrusives [6,7], and that adakites possess similar geochemical signatures to tonalite-trondhjemite-granodiorite (TTG) suites. Thus, adakites have played an important role in our understanding of the growth of continental crust and associated Cu-Mo mineralization [2,[8][9][10].
The Aolunhua pluton with Mo mineralization is an excellent natural laboratory to help elucidate the petrogenesis and geodynamic setting of the Mo mineralization-related adakitic rocks within the Xilamulun metallogenic belt in the southern Great Xing'an Range, Northeast (NE) China. Contemporaneous adakitic plutons with Mo mineralization in this area (e.g., Banlashan, Haisugou, and Yangchang plutons which are listed in the Table 1) are well documented based on petrology, geochemistry, geochronology, and fluid inclusions [11][12][13][14][15]. However, the nature of the source region, magmatic evolutionary history, and geodynamic setting of these adakitic rocks are still debatable. In this contribution, whole-rock major and trace elements and Sr-Nd isotopes for the Aolunhua monzogranite porphyries were analyzed in order to decipher their source characteristics, magmatic evolution, and geodynamic setting. These results, in combination with previously published data, could help us better understand the petrogenesis and geodynamic setting of Early Cretaceous magmatism and associated Mo mineralization in the southern Great Xing'an Range.

Regional Geological Setting
The Aolunhua intrusion is a small granitic pluton occurring in the Xilamulun metallogenic belt belonging to the southern Great Xing'an Range, which is located in NE China and surrounded by the Mongol-Okhotsk suture to the north and the Solonker-Xilamulun suture to the south (see Figure 1a). The Xilamulun metallogenic belt was formed during the collision of the Great Xing'an Range and the North China Craton during the late Permian and Early Triassic and marked the final closure of the Paleo-Asian ocean [20]. In this district, there are numerous Northeast-Southwest (NE-SW) and Northwest-Southeast (NW-SE) oriented faults including the Chifeng-Kaiyuan, Solonker-Xilamulun, Nengjiang, and Hegenshan-Heihe faults (Figure 1b). The oldest formation in this region is the Xilinhot massif, which is a Precambrian high-grade metamorphic complex and composed mainly of plagioclase-bearing, biotite-bearing granitic gneiss and mica schist [21,22]. Additionally, Early Paleozoic volcanic and sedimentary rocks consisting variably of schist, sandy slate, marble, and andesite, and Mesozoic (Jurassic-Cretaceous) continental, intermediate-acidic volcanic and sedimentary rocks with weak metamorphosis also occur in this region [18]. Voluminous Mesozoic intrusions mainly composed of granite, granodiorite, monzogranite, granite porphyry, and diorite porphyry were emplaced into the sedimentary strata as mentioned above. Previous studies indicate that Cu-Mo mineralization is closely related to the generation of these Cretaceous intrusions (ca. 132-142 Ma), such as the Aolunhua, Banlashan, Haisugou, Shabutai, Yangchang, and Huanggang plutons in the Xilamulun district (see Figure 1b) [13,15,17,[23][24][25].

Whole-Rock Major and Trace Elements
Twelve samples were chosen for analysis of major and trace elements and then pulverized to less than 200 mesh. An Axios PW4400 sequential X-ray fluorescence spectrometer (XRF) with Rh-anode tube was used to measure the major element oxides at Australian Laboratory Services (ALS) Chemex (Guangzhou) Co. Ltd., following the detailed analytical methods described in the article by Liu et al. [27]. All major element oxide concentrations are reported on a volatile-free basis. Trace elements, including rare earth elements (REEs), were fused glassed dissolved in nitric acid, and then were analyzed following nitric acid dissolution and measured by inductively coupled plasma-mass spectrometry (ICP-MS) at the State Key Laboratory of Geological Processes and Mineral Resource (SKLGPMR), China University of Geosciences (Wuhan), Rh was added as an internal standard to 1% HNO 3 following the procedures of Liu et al. [27]. The international standard reference materials (e.g., BCR-2, AGV-1, and BHVO-1) were used to monitor the stability and accuracy of the instrument during analyses. Accuracy and precision of the data are better than 5% for trace elements.

Sr-Nd Isotopes
Eight samples from Aolunhua monzogranite porphyries were chosen for Sr and Nd isotopic analyses. Firstly, sample powders were dissolved in an acidic mixture of HNO 3 (1 mL), and HF (4 mL) in Teflon bombs on a hotplate at 150 • C for 48 h. The acid was then dried down to incipient dryness and the digestion process was repeated. Solutions were separated by cation and HDEHP-coated columns using conventional cation-exchange techniques. We followed the procedures described by Li et al. [28] to perform sample preparation and chemical separation. Sr-Nd isotopic measurements were performed on a Thermo Triton Thermal Ionization Mass Spectrometer (TIMS) at the State Key Laboratory of Ore Deposit Geochemistry (SKLOG), Institute of Geochemistry, Chinese Academy of Sciences (IGCAS). Mass fractionation corrections for Sr and Nd isotopic ratios are based on 86 Sr/ 88 Sr = 0.1194 and 146 Nd/ 144 Nd = 0.7219, respectively. The 87 Sr/ 86 Sr ratio of the National Institute of standard and Technology Standard Reference Materials (NIST SRM) 987 (formerly National Bureau of Standards 987) Sr standard was 0.710258 ± 7 (2σ), and the measured 143 Nd/ 144 Nd ratios of the La Jolla and JNDI-1 Nd standard solutions were 0.511841 ± 3 (2σ) and 0.512104 ± 5 (2σ), respectively [29,30].

Major and Trace Element Geochemistry
Data for major and trace elements for 12 granitic porphyry samples are listed in Table 2.    [34] and Boynton [35], respectively. Data sources of adakitic granites are the same as those in Figure 3.  [42], subducted oceanic crust-derived adakites after Wang et al. [43], thickened and delaminated lower crust-derived adakites after references [43][44][45], Low-SiO 2 adakites and high-SiO 2 adakites after Martin et al. [46]. Data sources of adakitic granites are the same as those in Figure 3.

Sr-Nd Isotope Geochemistry
Data for the eight granitic porphyry samples chosen for Sr and Nd isotopic analyses are listed in Table 3. The initial 87 Sr/ 86 Sr ratios and ε Nd (t) values were calculated at t = 132 Ma, derived from the zircon U-Pb dating result in previous literature [12,13]. In addition, we used the model proposed by DePaolo [46] to calculate the depleted mantle Nd model ages (T DM ). The results show that Sr-Nd isotopic compositions of Aolunhua monzogranite porphyries are characterized by relatively homogenous initial 87 Sr/ 86 Sr ratios of 0.705117 to 0.705793, and ε Nd (t) values of −2.98 to 0.92. The calculated values of the corresponding two-stage depleted-mantle Nd model ages (T 2DM ) are from 0.84 to 1.16 Ga as listed in Table 3.

Characteristics of the Source Region of Early Cretaceous Adakitic Rocks in the Xilamulun District
Experimental studies suggest that the composition of source rocks plays a crucial role for generating adakitic melts with different geochemical characteristics and that this can occur in <50 km thick crust [51,52]. The Aolunhua monzogranite porphyries show enrichment in Sr (362-809 ppm), depletion in Y (7.66-10.5 ppm) and Yb (0.54-0.95 ppm), with high Sr/Y ratios (34.59-91.02). These geochemical signatures indicate that they are typical of adakitic rocks. This is also supported by the depletion of HFSEs (negative Nb, Ta, and Ti anomalies in primitive mantle-normalized spider diagram, Figure 4b). Furthermore, as both Nb and Ta are immobile elements and are hardly affected by fractional crystallization or hydrothermal overprint in later geological events, intermediate Nb/Ta ratios ( Table 2) can be used to trace magma sources [53]. As shown in Figure 4b, the negative Nb and Ti concentration anomalies rule out the origin of normal mid-ocean-ridge basalt (N-MORB) or ocean island basalt (OIB)-type sources in non-subduction zone environments, with associated melts characterized by typically positive Nb and Ti anomalies [54]. The studied samples have low Mg # values (36.09-48.63), Ni (<20 ppm), and Cr (<10 ppm) contents, which is inconsistent with melts derived from classical primary mantle [5]. Instead, they mostly exhibit geochemical characteristics of high-K calc-alkaline affinities consistent with melts from partial melting of continental crust (Figure 3b). Normally, potassium concentration in melt is controlled by the presence of K-bearing phases in the residue and the degree of partial melting [55], thus the K 2 O contents of adakitic rocks can provide additional information for deciphering their source. Typical adakites derived from oceanic slab have lower K 2 O/Na 2 O and higher CaO/Al 2 O 3 than those of lower crustal-derived adakites because of K-bearing phases in the residue and small degrees of partial melting of eclogitic mid-ocean-ridge basalt (MORB) [39]. The Aolunhua samples exhibit high K 2 O/Na 2 O (0.62 to 1.83) and low CaO/Al 2 O 3 (0.12-0.18) ratios that are distinct from those of melts derived from oceanic slab, but closely resemble lower crustal derived adakitic rocks (Figure 5c). In addition, relatively low MgO contents (0.62-1.04) of the Aolunhua samples are consistent with experimental melts originated from metabasalt and eclogite at 1-4.0 GPa with involvement of mafic magmas derived from lithospheric mantle or melting of thickened lower crust (see Figure 5d) [42,55]. This result is consistent with the occurrence of mafic microgranular enclaves (MMEs) in these granitic porphyries from the Aolunhua district [13], indicating that a crust-mantle interaction process is necessary for the generation of monzogranite porphyries. Moreover, compared to classical adakites derived from young subducted oceanic slab, the samples possess slightly enriched ( 87 Sr/ 86 Sr) i ratios of 0.7051 to 0.7058 and negative ε Nd (t) values varying from −2.98 to 0.92, with young T 2DM values of 0.84 to 1.16 Ga, which are similar to adakitic rocks in this region [11][12][13][14][15][16][17][18][19][20][22][23][24][25][26]. Considering these geochemical signatures of adakitic granites in the Xilamulun district, it is suggested that they are not partial melting products of depleted mantle or oceanic slab sources because of the reasons discussed above. Instead, these granitic rocks exhibit geochemical affinity of a dominantly juvenile mafic lower crustal source. Therefore, the geochemical data combined with Sr-Nd isotopic compositions ( Figure 6) suggest that Aolunhua monzogranite porphyries and adakitic granites in the Xilamulun district are the results of partial melting of juvenile lower continental crust, with continuous magmatic underplating supplying the heat for melting.

Evolution of Adakitic Magma
Several processes have been proposed to decipher the genesis of adakitic rocks: (a) the partial melting of the mafic lower crust [2,5], (b) crystal fractionation of basaltic magma at high-pressure [57,58], and (c) crystal fractionation of basaltic magma with mixing processes at low-pressure in both arc or non-arc tectonic environments [59]. In this study, modeling of trace element ratios were carried out to better understand the melting conditions and evolutionary processes of adakitic parental magmas. Due to the low partition coefficients (<<1.0) of La and Sm [60], the La/Sm ratio is extremely insensitive to partial melting process and then can be used to distinguish fractional crystallization and partial melting processes (Figure 7a). Geochemical results show that partial melting has played a key role in the formation of adakitic rocks in the Xilamulun district. There is no obvious evidence for fractional crystallization, which is also demonstrated by the unfractionated granite fields (OGT) plotted in the diagrams of (K 2 O + Na 2 O)/CaO vs. Zr + Nb + Ce + Y and FeO T /MgO vs. Zr + Nb + Ce + Y (Figure 8a,b) and relatively constant range of Zr/Hf ratio (Figure 9a). In addition, Zr/Hf and Nb/Ta ratios are generally constant in igneous rocks without fractional crystallization, because the element pairs are similarly incompatible and have similar partition coefficients. However, the removal of certain accessory minerals such as zircons, Fe-Ti oxides, and titanites would lead to markedly decrease the Zr/Hf and Nb/Ta ratios [61,62]. In this study, the Aolunhua monzogranite porphyries have Zr/Hf (40.73-50.36) and Nb/Ta (12.27-16.54) ratios encompassing the values of lower continental crust (Zr/Hf = 41.25, Nb/Ta = 13.45, according to Wedephol [63]) and are different from the chondritic value (Zr/Hf = 34.3, Nb/Ta = 19.9, according to Münker et al. [64]), indicating that they may be derived from lower continental crust and have not experienced obvious fractional crystallization of accessory minerals. However, the slightly decreasing trend of Nb/Ta ratios exhibited in the Nb/Ta vs. SiO 2 diagram (Figure 9b) may indicate that minor fractional crystallization of Fe-Ti oxides have happened, which is also supported by the decreasing TiO 2 and Fe 2 O 3 in the Harker diagram ( Figure 10). The moderately negative Eu anomalies (δEu = 0.65-0.95) for most samples are indicative of plagioclase fractionation, which is also supported by a decreasing CaO in the Harker diagram ( Figure 10) and a slightly decreasing trend of Ba contents (Figure 9c). Thus, we suggest that the studied Aolunhua samples and adakitic granites in the Xilamulun district may have experienced minimal fractionation of plagioclase and Fe-Ti oxides. All samples are plotted in the magnesian area in the FeO T /(FeO T + MgO) vs. SiO 2 diagram (Figure 9d), indicating that they were probably derived from an oxidizing source, which are often related to magnesian granitoids [65]. The increasing FeO T /(FeO T + MgO) values with the increase of SiO 2 contents indirectly reflects a trend from strong to weak oxidation, which is also indicative of the fractionation of Fe-Ti oxides in late stage and supported by the occurrence of titanomagnetite in the Aolunhua monzogranite porphyries [13]. This result is consistent with some less fractionated Mo-bearing porphyries derived from adakitic oxidizing magma differentiation, which is a common and crucial process for the Mo-Cu mineralization associated with adakitic rocks [66][67][68].  [69] and Adam and Green [60]. Lower continental crust [70] and average N-MORB [71] were used as representative for amphibolitic lower continental crust (ALCC) and eclogitic oceanic crust (EOC) end-members, respectively. Data sources of adakitic granites are the same as those in Figure 3.   [72], indicating that these adakitic rocks have not experienced obvious fractional crystallization. Abbreviations: A-A-type granite; I-I-type granite; S-S-type granite; FG-fractionated felsic granite; OGT-unfractionated M-, I-, and S-type granite; (c) SN-source normalized data, normalized to mafic lower continental crust with Yb = 1.5 ppm and Sm/Yb = 1.87 for continental adakite [52]; (d) Ba/Th vs. Nb/La diagram for partial melting of mafic lower crust [52] showing that most samples cluster around the lower pressure line (1) (<33 km) confirming the generation of adakites by processes associated with continental crust of near normal thickness. Data sources of adakitic granites are the same as those in Figure 3. To elucidate the effect of source inheritance and melting pressure on continental adakitic rocks, a series of partial-melting models were proposed. Equilibrium melting modeling based on incompatible trace elements by using batch-melting model was an important way and employed to test the mechanism for the generation of these adakitic rocks. La/Yb and Sm/Yb ratios are indicative of the presence of garnet and amphibole in the residue and are investigated using the compositions for amphibolitic lower continental crust (ALCC) and eclogitic oceanic crustal (EOC) source compositions [69,70]. We modally calculated melting trajectories for Sm/Yb vs. La/Yb for both sources mentioned above by varying the amount of garnet present (see Figure 7b). The results indicate that Early Cretaceous adakitic rocks in the Xilamulun district are produced by partial melting of amphibolitic lower continental crust. Modal garnet for the Aolunhua was~25%-40% at 20%-40% degrees of partial melting, whereas garnet for other adakitic rocks was 5%-20% at 20%-40% degrees of partial melting. Our modelling results are consistent with the experimental studies by Qian and Hermann [51], which show that 10%-40% partial melting of hydrous meta-basaltic lower crust at 10-12.5 kbar (33-41 km depth) and 800-950 • C can produce adakitic melts (Figure 5a). In this model, three different melting depths, e.g., ≤33 km, 33-40 km and >45 km correspond to different residual mineral assemblages defined by melting experiments. Therefore, we conclude that adakitic rocks in this area originated from 20% to 40% partial melting of amphibolitic lower continental crust at low pressure, which is also supported by Dai et al. [58] who proposed that adakites with Sr/Y < 200 can be generated at different depths in the continental crust in lower pressure environments.

Geodynamic Setting for Mo Mineralization During Early Cretaceous Period
Adakitic rocks associated with Mo mineralization provide a probe to constrain the geodynamic setting for Mo deposits. As presented in recent studies, adakitic rocks formed from the partial melting of continental crust are widely used as a geodynamic indicator of lithospheric delamination, orogenic collapse, or crustal thickening [10,52]. Crustal thickening is common in a continental collision orogeny caused by object extrusion pressure, where deep-seated adakitic melts (derived from >50 km depth) occur at high pressure. However, this situation is rare in intraplate settings [52]. Generally, lower crust-derived adakitic rocks in orogenic setting have high (Sm/Yb) SN and low Yb SN because of a garnet-rich residue in the source region which decreases the Yb content in melts. In contrast, adakites derived from intraplate lower crust without thickening have lower (Sm/Yb) SN and higher Yb SN [52]. Our samples plot in the fields of intraplate adakites in an (Sm/Yb) SN vs. Yb SN diagram (Figure 8c), indicating that the Aolunhua monzogranite porphyries were not generated in a crustal thickening or orogenic collapse setting, but a common intraplate setting. This is also supported by the Ba/Th vs. Nb/La for modelled melts from partial melting of mafic normal lower crust (depth <33 km, see Figure 8d), not a thickened lower crust. Considering the rock assemblage and evolutionary history in this area, the extensional setting is also supported by the presence of widespread coeval A-type granites, calc-alkaline volcanic rocks formed by decompressional partial melting of lithospheric mantle, metamorphic core complexes, and numerous mafic and felsic dykes [73][74][75]. This result is also consistent with previous studies suggesting that NE China was in an extensional environment related to lithospheric delamination during the Early Cretaceous [11,20,73,76,77]. Thus, we favor a lithospheric mantle delamination model to interpret the occurrence of Early Cretaceous adakites in the Xilamulun district. These adakitic rocks may be generated in an extensional setting related to lithospheric delamination, as the depicted in Figure 11. The collision between North China and Siberian cratons around 160 Ma blocked the westward movement of the lithosphere which induced the subduction of the Pacific plate and led to lithospheric delamination triggered by asthenospheric upwelling and underplating. Subsequently, the heat from mantle caused partial melting of mafic lower crust, which result in Early Cretaceous magmatic activities of adakitic rocks and associated Mo mineralization in the southern Great Xing'an Range.

Conclusions
A comprehensive compilation of new and published geochemical data for adakitic rocks in the Xilamulun district reveals the following insights: (1) The Aolunhua monzogranite porphyries in the Xilamulun district are characterized by high Sr and Sr/Y, low Y and HREE abundances, without significantly negative Eu anomalies and the depletion of HFSEs (e.g., Nb, Ta, and Ti), typically characteristic of adakites. The characteristics of high ( 87 Sr/ 86 Sr) i and negative ε Nd (t) with young two-stage Nd model ages (T 2DM = 0.84-1.16 Ga) suggest that they were derived from a dominantly juvenile mafic lower crustal source with minor involvement of mantle-derived mafic magma, which is also evidenced by high K 2 O/Na 2 O, low CaO/Al 2 O 3 . (2) Positive correlation between La and La/Sm suggest that a partial melting process play an important role in the generation of adakitic rocks in the Xilamulun district. Incompatible trace element modeling shows that adakitic rocks in this area originated from 20%-40% partial melting of amphibolitic lower continental crust at low pressure. Low FeO T /(FeO T + MgO) values indirectly reflect these adakitic rocks were derived from an oxidizing source related to magnesian granitoids. The decreasing content of TiO 2 , Fe 2 O 3 , Nb/Ta ratio, and moderately negative Eu anomalies suggest that minimal fractionation of Fe-Ti oxides and plagioclase may have occurred in their evolutionary history. (3) Considering the rock assemblages and the evolutionary history in the Xilamulun district, these Early Cretaceous adakites are interpreted to be generated in an extensional setting related to lithospheric delamination.