Petrogenesis of the Early Cretaceous Zhouguan Granodiorite in Jiaodong Peninsula: Evidence from Mineralogy, Geochemistry, Geochronology, and Sr-Nd Isotopes

: The Jiaodong Peninsula is the most important gold mineralization area in China, and the formation of gold deposits is closely related to granitoids. The isotopic ages of the Early Cretaceous granodiorites in the northwestern Jiaodong Peninsula are concentrated in the range of 111~123 Ma, and are coeval with the formation of the gold deposits in the area. However, the studies on the geotectonic settings of the granodiorites, especially their petrogenesis and relationship with gold deposits in the northwestern Jiaodong Peninsula, are scarce. Based on ﬁeld and petrographic observations, geochemistry, EPMA analysis, zircon U-Pb chronology, and Sr-Nd isotopes of the Early Cretaceous Zhouguan granodiorite in the Jiaodong area, the formation age of Zhouguan granodiorite is determined as 115 Ma ± 0.77 Ma; the analysis of EPMA shows that biotite is mainly composed of Fe-biotite and Mg-biotite, with its MgO content ranging from 9.797% to 11.635%. The crystallization temperature of biotite is in the range of 500 ◦ C~625 ◦ C and the emplacement depth of the rock mass is 3.98~8.71 km. The amphibole in the mass mainly includes magnesiohornblende, pargasite, and magnesiosadanagaite; among them, the former two are of crustal origin, while magnesiosadanagaite is of mantle origin. The crystallization pressure and depth of the former two are in the range of 0.75~3.02 kbar and 2.81~11.4 km, respectively, while the crystallization pressure and depth for the latter is 4.64 kbar and 17.53 km, respectively. The ( 87 Sr/ 86 Sr) values range from 0.710424 to 0.711074 and the ( 143 Nd/ 144 Nd) values range from 0.511530 to 0.511808. The parental magma of the Zhouguan granodiorite is highly oxidized with high-water content that is favorable for Au enrichment. Combined with the Nb-Y and Yb-Ta diagrams, a model describing the formation of Zhouguan granodiorite is proposed.


Introduction
The Jiaodong area is the largest gold producer in China, with more than 5000 t of proven gold resources [1][2][3][4]. Some large gold deposits in China such as the Sanshandao, the Jiaojia, and the Linglong gold deposits are all distributed here. The metallogenic age of the Jiaodong gold deposits is the late of the Early Cretaceous, and they have a spatial and temporal relationship with widely distributed intermediate-acid magmatic rock. The origin  [43]) and (c) The study area.
Magmatic rocks are widely distributed in the northwest Jiaodong Peninsula. They mainly include the Late Jurassic Linglong biotite two-feldspar granite with an age of 160 Ma-150 Ma, which belongs to the calc-alkaline granite formed by in situ crust remelting, the Early Cretaceous Guojialing granodiorite with an age of 130 Ma-126 Ma, which belongs to high-K calc-alkaline granodiorite of mixed crustal-mantle origin and the Early Cretaceous Weideshan granodiorite with an age of 120 Ma-100 Ma, i.e., the high-K calc-alkaline granodiorite of mixed crustal-mantle origin [44,45]. The Guojialing and the Weideshan granodiorites intruded into the Linglong granite in the NEE direction mainly along the Lingbei fault and the Yidao fault in the study area. The intrusions of the Weideshan stage in the study area mainly include the Zhouguan, the Nansu, and the Dazeshan stocks ( Figure 1b). In the north and east of the Zhouguan granodiorite, a lot of gold deposits are distributed around. Compared to Guojialing granite, Nansu and Dazeshan granodiorite are more spatially and temporally significant, and the systematic study of Zhouguan granodiorite is more significant for studying the relationship between magmatism and gold mineralization. So in this study, we focused on the Zhouguan granodiorite (Figure 1c), which is located near Dongzhouguan Village, Yidao Town of Laizhou City. The rock ranges from light grey to grey-white in color, with a texture ranging from semi-hedral to medium-grained and a massive structure ( Figure 2a); and is mainly composed of plagioclase (30%~40%), K-feldspar (15%~20%), quartz (25%~30%), biotite (5%~10%), amphibole (3%~4%), and opaque minerals (1%), with accessory minerals of zircon, apatite, sphene, and magnetite (Figure 2b-d). The plagioclase occurs as euhedral-to-semihedral long platy crystals with zoning. Biotite exhibits from light brown to brown pleochroism and can be replaced by chlorite or epidote, or replaced by sericite in its edge. The K-feldspar occurs as regular platy phenocrysts with zoning and a streaky structure.

Geochemical Analysis
Samples (ZG0721-b1, -b3, -b5, -b8, and -b10) were collected from Zhouguan granodiorite for geochemical analysis. The geochemical analysis was carried out in the Jinan Mineral Resources Supervision and Testing Center of the Ministry of Land and Resources. The trace elements and REE analysis were performed on the plasma emission spectrometer (ICP-AES, IRISIntrepid II), atomic absorption spectrometer (AASPE400, PE600), atomic fluorescence spectrometer (AFS-820), and plasma mass spectrometer (ICP-MS, XSERIES2) using the national standards GB/T14506-2010 "Chemical Analysis Method of Silicate Rock" for control. The operating temperature and relative humidity are 20 • C and 30%, respectively. The error of major element analysis is within 1%, the accuracy of trace element content is better than 3% when the content of trace elements is greater than 10 × 10 −6 , while the accuracy is better than 10% when the content of trace elements is less than 10 × 10 −6 [46][47][48]. The data are given in Tables 1 and 2.

Zircon U-Pb Dating
The granodiorite sample (ZG0721-b8) was selected for Zircon U-Pb dating. Zircon crystals for U-Pb dating were separated from the Zhouguan granodiorite, whose target making, cathodoluminescence (CL) photography was performed by Langfang Chengxin Geological Service Co., Ltd. The analysis for U-Pb dating was conducted using the instrument LA-MC-ICP-MS at Jinan Mineral Resources Supervision and Testing Center of the Ministry of Land and Resources. More detailed analytical conditions and procedures are described by Song et al. (2010) [49] and Wang et al. (2012) [50].
In this study a total of 20 U-Pb dating points in the samples were analyzed. The results were corrected by the U-Th-Pb isotope content and age of the reference sample, and the common lead was corrected according to the measured 204 Pb. The individual data point error is 1σ, and the weighted average age error is 96% confident. Data processing was performed by the laboratory staff using Squid and Isoplot programs.

EPMA Analysis of Biotite and Amphibole
Representative biotite and amphibole from Zhouguan granodiorite were used for EPMA analysis. The EPMA analysis was completed in the Key Laboratory of Gold Mineralization Process and Resource Utilization, Ministry of Natural Resources, Shandong Institute of Geological Sciences. The instrument model was JXA-8230, the acceleration voltage for spectral analysis was 15 kV, the current was 1 × 10 −8 A, and the beam spot diameter was 1~10 µm. All the standards were Canadian Astimex standards. The fresh and unaltered biotite and amphibole minerals were separated (picked up under binocular microscope) from the Zhouguan granodiorite and used for EPMA analyses, the EPMA data of biotite were calculated in units of 22 oxygen atoms and related parameters. The -Fe 2+ and Fe 3+ values were obtained with the calculation method of Lin Wenwei and Peng Lijun (1994) [51]. The amphibole EPMA data were calculated using Geokit software.

Sr-Nd Isotopes
Four samples of granodiorite were selected for Sr and Nd isotope ratio analysis. The whole-rock Sr-Nd isotopes were determined by thermal ionization mass spectrometry (ISOPROBE-T) at Langfang Chengxin Geological Service Co., Ltd. (Langfang, China ). The 100 mg sample powders were dissolved in a HF + HNO 3 + HClO 4 mixture, separated using the conventional cation exchange technique with HCl as an eluent for Sr and Nd, and tested by a Thermal ionization mass spectrometer (TIMS).
The 87 Sr/ 86 Sr ratios were normalized to 88 Sr/ 86 Sr standard = 0.1194, while the 143 Nd/ 144 Nd ratios were normalized with 146 Nd/ 144 Nd standard = 0.7219 as the internal standard. The NBS987 Sr standard yielded a 87 Sr/ 86 Sr value of 0.710250 ± 7, and the JMC Nd standard gave 143 Nd/ 144 Nd value of 0.512109 ± 3.

Characteristics of Whole-Rock Major and Trace Elements of Zhouguan Granodiorite
As shown in Table 1, the SiO 2 contents of the granodiorite range from 68.07% to 68.72%, with the Na 2 O/K 2 O ratios of 1.09~1.37. The total alkali contents (Na 2 O + K 2 O) are 7.23%~7.88%; the samples in the study area fall into the granodiorite field in the Na 2 O + K 2 O vs. SiO 2 (TAS) diagram (Figure 3a), and belong to high-K calc-alkaline series ( Figure 3b). The differentiation index (DI) of the rock ranges from 78.31 to 80.5, while the aluminium index (A/CNK) is 1.43~1.59 which shows that the Zhouguan granodiorite belongs to peraluminous granite on the A/CNK-A/NK diagram (Figure 3c).
In the chondrite-normalized REE diagram (Figure 4a), it is shown that the ΣREE contents of the Zhouguan granodiorite are in the range of 138.01~175.72 ppm, and also shows light rare earth element (LREE) enrichment and heavy rare earth element (HREE) depletion. The (La/Yb) N ratios are in the range of 36.05~53.53 and the LREE/HREE ratios are 17.33~20.59, suggesting that a significant fractionation of light and heavy rare earth elements occurred. The granodiorite has weak to moderately negative Eu (0.74~0.78) and δCe 0.78~0.81, with higher Sr and La contents and lower Y and Yb contents.   Sun and McDonough, 1989 [52]).
In the primitive mantle-normalized spider diagram (Figure 4b), the granodiorite shows the enrichment of large ion lithophile elements (LILE) such as Ba, Sr and K, and the depletion of some high field-strength elements (HFSE), such as Nb, Ta, and P, with Sr/Y ratios of 90.18~133.93.

Zircon U-Pb Age
Through LA-MC-ICP-MS zircon U-Pb dating of the Zhouguan granodiorite sample, the zircon CL images show that the zircon grains are relatively large, with its aspect ratio being 1:3~1:2. Most zircon grains show core-rim textures, with well-defined oscillatory zoning in the rim and a bright homogeneous core, which reflects its inherited growth feature and conforms to the characteristics of magmatic zircon ( Figure 5).
The analysis data of U-Pb dating are shown in Table 3. The U and Th contents are in the range of 55.38~778.64 ppm and 23.14~451.5 ppm, respectively. The Th/U ratios vary from 0.92 to 1.41. Some zircon grains were spotted in both the core and rim. The age of the core varies from Precambrian through the Jurassic to Early Cretaceous, suggesting the inherited origin of some zircon grains.

Biotite Analysis Results
The biotite formula is calculated on a 22 oxygen atom basis. The main characteristics of biotite in the Zhouguan granodiorite are as follows: The SiO 2 content in biotite is in the range of 34.28%~36.727% while MgO content is 9.797%~11.635% (Table 4), which means that biotite is significantly rich in Mg; the Al VI + Fe 3+ + Ti and the Fe 2+ + Mn values are 0.316~0.556 and 1.07~1.27, respectively, with the Fe 2+ /(Fe 2+ + Mg) ratio being 0.26~0.52. This indicates that the biotite was not subjected to late fluid alteration and belongs to primary biotite [53], which is consistent with the observation of biotite under a microscope. On the composition classification diagram of biotite (Figure 6a), both Mg-biotite and Febiotite are the main sub-type. The Al 2 O 3 content of biotite is in the range of 13.09%~15.25% and the cation number of Al VI per formula is 0.01~0.114. The high Ti and low Al VI in the biotite formula indicate that the biotite was formed under the conditions of relatively high temperature and high oxygen fugacity [54,55]. The crystallization temperature of biotite is calculated as 500 • C~625 • C based on the temperature calculation formula of Ti and Mg/(Mg + Fe) contents in the biotite (Figure 6b). Moreover, there is a positive correlation between the total Al content of granodiorite and its consolidation pressure [56]; the formation pressure of biotite is calculated as 105.43~230.45 MPa (166.75 MPa in average) with the formula: P (kbar) = 3.03 × T Al−6.53 (±0.33), where T Al refers to the total number of Al cation in the biotite formula as calculated on a 22-oxygen-atom basis. The pressure then is converted to the emplacement depth by the formula: P = ρgD, where ρ = 2700 kg/m 3 , g = 9.8 m/s 2 , and the calculated crystallization depth of biotite was in the range of 3.98~8.71 km (6.3 km in average).

Amphibole Analysis Results
The main characteristics of amphibole in the Zhouguan granodiorite are as follows: The SiO 2 content is in the range of 42.27%~46.38%, and the MgO content is 10.44%~13.35%, which is obviously Mg-enriched ( Table 5). The Al 2 O 3 content is 5.79%~10.93%, FeO T content is 14.46%~17.76%, and CaO content is 10.74%~111.51%. Therefore, the amphibole belongs to Ca-amphibole (Ca a > 1.50), which can be further subdivided into magnesiohornblende, pargasite, and magnesiosadanagaite [59] (Figure 7). The chemical composition of the amphibole is not only affected by the parental magma, but also related to the crystallization conditions (temperature, pressure, oxygen fugacity, and water content), so the composition of amphibole can be used to estimate the temperature and pressure conditions during crystallization in calc-alkaline magma. In this study, the crystallization pressures of the amphibole is calculated, using the formula [60][61][62][63]: P(kbar) = (−3.46 + 4.23Al T ); the formation pressure of amphibole is calculated to be 0.75~3.02 kbar and 4.64 kbar. These pressures can be converted to the crystallization depth with the formula: P = ρgD, where ρ = 2700 kg/m 3 , g = 9.8 m/s 2 , and the calculated crystallization depth of amphibole is in the range of 2.81~11.4 km and 17.53 km.
The water content of magma has a positive correlation with oxygen fugacity [63]. According to the molecular formula of amphibole to calculate the water content of magma during crystallization, as corrected by Ridolfi (2009) [42], the calculated water content of the magma is in the range of 4.6%~5.6%, with an average of 5.1%, indicating that during this period, the magma has high water content.

Whole-Rock Sr-Nd Isotopes
The whole-rock Sr-Nd isotopic compositions are listed in Table 6. The results show that the ( 87 Sr/ 86 Sr) values range from 0.710424 to 0.711074, below the continental crustal average (0.717), and higher than the mantle average (0.709) [64], showing that it has the characteristics of A-type granite, which is similar to the magmatic rock in the basalt source area. The ( 143 Nd/ 144 Nd) values range from 0.511530 to 0.511808. The initial Sr-Nd isotopic ratios and the ε Nd (t) were calculated using the average crystallization age of 115 Ma. The Zhouguan granodiorite exhibit relatively homogeneous isotopic compositions, with ( 87 Sr/ 86 Sr) i = 0.709370-0.710619, ( 143 Nd/ 144 Nd) i = 0.511461-0.511726, and ε Nd (t) = −14.78-−19.95.

Age of Zhouguan Granodiorite
In recent years, with the advance and application of high-precision isotope dating technology, a lot of isotopic chronology studies on the Early Cretaceous granites in Jiaodong were performed [6,[66][67][68][69][70]; however, the data on the ages of Zhouguan granodiorite are very few [71]. In this study, a systematic LA-MC-ICP-MS dating of the Zhouguan granodiorite was carried out and its age was determined as 115 Ma, which represents the emplacement time of the Zhouguan granodiorite.
Geologists adopted different isotope dating approaches to date the metallogenic age of the gold deposits in the area. These methods include the Ar-Ar method regarding the alterations in the sericite [65,72,73], the Ar-Ar method for muscovite and feldspar minerals associated with gold veins [72], the Ar-Ar and Rb-Sr methods for gold-bearing pyrite [74][75][76][77], and the SHRIMP U-Pb method [78][79][80][81][82] for hydrothermal zircon in goldbearing quartz veins. The results show that the metallogenic ages of most gold deposits fall in the range of 115-122.5 Ma (Figure 8), although the types of deposits, their distribution area, and the geologic environments are slightly different. Therefore, most geologists generally believe that the large-scale gold mineralization event in the northwest Jiaodong area mainly occurred in 115 Ma ± 5 Ma, i.e., the late of the Early Cretaceous. The magmatic activity that produced the Zhouguan granodiorite during the same period may have an important influence on the formation of the gold deposits.  [72][73][74][75][76][77][78]83,84]).

Geochemical Features and Petrogenesis
The TAS and A/CNK-A/NK diagrams constructed from the results of whole-rock geochemical analysis show that the Zhouguan granodiorite belongs to the high-K calcalkaline series and metaluminous to weakly peraluminous granitoids. The δEu value is in the range of 0.68~0.77, the moderately negative Eu anomaly indicates that plagioclase remained in the source area during the magmatic evolution, or indicates that it may be the result of fractional crystallization. Meanwhile, the low contents of HREE and Y suggest that amphibole/garnet may be present in the source area. The positive abnormalities in elements Nd and Hf demonstrate the typical geochemical characteristics of the high Ba-Sr granite, whereas the depletion of high-field strength elements (HFSE), such as Nb, Ta, and P shows its magmatic arc affinity. The Nb/Ta ratio is in the range of 16.6-21.9 (average 19.9), which is higher than the average ratio of the continental crust. The granodiorite has higher Sr and La contents but lower Y and Yb contents. In the Sr/Y-Y diagram (Figure 12c (Figure 10a), indicating their origination of the continental crust [81,82]. The samples plot in the vicinity of Early-Middle Proterozoic crust and Archaean crust in the ε Nd (t) vs. T diagram, suggesting the source of Early-Middle Proterozoic and Archaean crusts (Figure 10b). The Sr and Nd isotopic compositions show that magmas originate from the lower crustal basement metamorphic rocks and lithospheric mantle melting [85].
Based on the synthesis of the whole-rock chemical analysis, single mineral EMPA analysis and the concept of the "trans-crustal magmatic system" [86], the Zhouguan granodiorite is considered to be a typical crust-mantle mixed origin granitoid in a continental magmatic arc environment. The forming process was that the magma of the mantle rose and mixed with the acid magma formed by the partial melting of the lower continental crustal rocks, and the mix magma crystallized in the upper crust to form Zhouguan granodiorite.

Significance of Amphibole and Biotite to Mineralization
A high oxygen fugacity of magma is required for the dissolution and migration of the Au element. Under a low oxygen fugacity environment, sulfur exists in the form of S 2+ . During the crystallization process of magma, metal elements, such as Fe, Cu, Zn, and Au elements, etc, will form sulfides with S 2+ to precipitate in magmatic rocks. Therefore, the Au element is dispersed in the rock mass, which is not conducive to the enrichment and mineralization of gold [34]. Under a high oxygen fugacity environment, the S element exists in the form of SO 4 2− , which is beneficial to the migration and enrichment of the Au element in magma or fluid [42]. The high water content of magma, which has a positive correlation with oxygen fugacity, is another important factor when controlling the migration and enrichment of the Au element in gold deposits [33,64]. Water-rich magma can maximize the content of the Au element in the fluid, which is conducive for the Au element to migrate in the fluid [89].
In the biotite Fe 3+ -Fe 2+ -Mg diagram of the Zhouguan granodiorite (Figure 11a), all the data points are located between the two buffer lines of Ni-NiO and Fe 3 O 4 -Fe 2 O 3 , which indicates that the biotite was formed under a high oxygen fugacity condition; the amphibole Al IV -Fe 2+ /(Mg + Fe 2+ ) diagram shows that the amphibole was also formed under high oxygen fugacity conditions (Figure 11b), whose oxygen fugacity log f (O 2 ) is −11.82~−13.96. The water content ω (H 2 O) of the magma is 4.62~5.43, with an average of 5.1%, indicating that the magma has a high water content. In summary, the parent magma forming the Zhouguan granodiorite has the characteristics of high oxygen fugacity and high water content, which is favorable for mineralization.

Analysis of Petrogenetic Tectonic Environment in Relation to Metallogenic Model
The elements Rb, Y, and Nb in granitoids can be effectively used to distinguish between the granites formed in different tectonic environments, including volcanic arc granites, syn-collisional granites, intraplate granites, and ridge granites, etc. In the tectonic setting diagram of Y vs. Nb and Ta vs. Yb of granites (Figure 12a,b), the samples of Zhouguan granodiorite fall into the fields of volcanic arc granite and syn-collision granite. Combined with the analysis of the general plate movement environment, the Zhouguan granodiorite was formed in the extensional tectonic setting during the retreat of the Pacific plate, which belongs to the lithospheric extensional environment or the tectonic environment related to the gravitational collapse after collision orogeny. Based on the previous studies, and combined with the whole-rock and detailed single mineral analyses of the Zhouguan granodiorite, a model describing the formation of Zhouguan granodiorite is proposed. In the Early Cretaceous (125-135 Ma), the NWtrending compression caused by the subduction of the Pacific plate resulted in the destruction of the North China Craton [93][94][95][96][97]. A large-scale left-lateral strike-slip movement continued along the Tan-Lu fault zone. At the same time, a series of inherited secondary faults, which were formed by the same mechanism, were widely developed in the geological bodies on both sides of the Tan-Lu fault zone. These inherited secondary left-lateral strike-slip faults are characterized by small displacement, near vertical, and cutting Moho surfaces, whose name is short displacement cut-crust strike-slip fault [98].
During this period, the tectonomagmatic activity in the northwestern Jiaodong area was relatively weak, and the crust-mantle interaction was weak, too. A small amount of mantle-derived basic magma rose along the short displacement cut-crust strike-slip faults and formed basic dyke rocks at the shallow upper crust. The other partly mantlederived basic magma was mixed with crust-derived magma to form intermediate acid magma, which then ascended and emplaced, forming the Guojialing granodiorites [86]. At this time, under the geodynamic background of intermedium acid magma intrusion and compression tensional movement of the plate, a series of shovel-type faults, such as the Jiaojia, the Sanshandao and the Zhaoping faults in NNE-trending faults occurred in the relatively weak lithological contact zones of the upper crust (0-5 km). Meanwhile, under the mechanical action of short displacement cut-crust strike-slip faults, a series of near-vertical faults, such as the Qixia, the Fengyi, the Yidao, and the Hulu Xian faults were formed; they are also the NNE-trending faults ( Figure 13).
In the late of the Early Cretaceous (110-125 Ma), the tectonic environment changed from a compressional tectonic regime to an extensional tectonic system [98], and the front part of the Pacific subducting plate retreated, which resulted in the stretching, thinning, and damaging of the North China Craton. During the extensional thinning process of the North China Craton, the short displacement cut-crust strike-slip faults, which formed in the early stage, expanded and provided channels for the basic magma to rise. The rupture of the Moho caused a partial melting of the lithospheric mantle, producing basic magma. Part of the basic magma formed a series of dyke rocks during the intrusion of magma along near-vertical faults, such as lamprophyre, diorite porphyrite, etc. When the basic magma moved up to the lower crust, another part of the basic magma mixed with the magma from the partial melting of the lower crust, and formed the intermediate acid magma with high water content and high oxygenfugacity. The Zhouguan granodiorite was formed in the tectonic-magmatic system of this period ( Figure 14).

Conclusions
(1) Through LA-MC-ICP-MS zircon U-Pb dating, the emplacement age of the Zhouguan granodiorite is determined as 115 Ma ± 0.77 Ma, indicating that the granodiorite occurred in the late of the Early Cretaceous. (2) According to the petro-geochemical analysis, the Zhouguan granodiorite belongs to a high-K calc-alkaline series, metaluminous to weakly peraluminous granite, i.e., a typical crust-mantle mixed granitoids formed in a continental arc environment. (3) According to an EPMA analysis of biotite and amphibole, the parent magma, which formed the Zhouguan granodiorite, is highly oxidized and hydrous, which is favorable for mineralization. (4) Based on predecessors' research, and considering the development of short displacement cut-crust strike-slip faults and the trans-crustal magmatic system in the Jiaodong area, the petrogenesis of the formation model of the Zhouguan granodiorite in the area is proposed in this study.

Data Availability Statement:
The data presented in this study are available in this paper.