Geochemistry , Zircon U – Pb Geochronology , and Lu – Hf Isotopes of the Chishan Alkaline Complex , Western Shandong , China

Mass alkaline magmatic activities in Western Shandong during the late Mesozoic controlled the mineralization processes of gold and rare earth element (REE) polymetallic deposits in the region. The Chishan alkaline complex is closely associated with the mineralization of the Chishan REE deposit, which, as the third largest light REE deposit in China following the Baiyenebo (Inner Mongolia) and Mianning (Sichuan) deposits, is considered a typical example of alkaline rock mineralization throughout the North China Craton. To determine how the Chishan alkaline complex and REE deposit interact with each other, a systematic study was conducted on the petrology, rock geochemistry, zircon U–Pb geochronology, Lu–Hf isotopes of the quartz syenite, and alkali granite contained in the Chishan alkaline complex. The results reveal that the deposits feature similar geochemical characteristics typical of an alkaline rock series—both are rich in alkali, high in potassium, metaluminous, and poor in Ti, Fe, Mg, and Mn. In terms of REEs, the deposits are strongly rich in light REEs but poor in heavy REEs, with weak negative Eu anomalies. In terms of trace elements, they are rich in large ion lithophile elements Ba, Sr, and Rb but poor in high field-strength elements Nb, Ta, and Hf. Zircon LA-ICP-MS U–Pb dating indicated that the quartz syenite and alkali granite formed in Early Cretaceous at 125.8 ± 1.2 Ma and 127.3 ± 1.0 Ma, respectively; their εHf(t) values are −22.67 to −13.19, with depleted model ages (TDM) ranging from 1296 Ma to 1675 Ma and crustal model ages (TDM) of 2036–2617 Ma. The Chishan alkaline complex originated from partial of the EM I-type (enriched mantle I) lithospheric mantle with assimilation of ancient crustal materials. The complex is of the same origin as the REE deposit, and developed in an extensional setting that resulted from plate subduction and lithospheric thinning and upwelling in the eastern area of the North China Craton.

The Chishan alkaline complex in Western Shandong is closely associated with the mineralization of the Chishan REE deposit which, as the third largest light REE (LREE) deposit in China following the Baiyenebo (Inner Mongolia) and Mianning (Sichuan) deposits, is considered as a typical example of alkaline rock mineralization throughout the North China Craton [25], and valued by scientific research institutes and mining enterprises [25,27,28,[31][32][33][34][35].In particular, the recent discovery of the REE mineralization bodies from the neighboring Shagou (Xuecheng) and Longbaoshan (Zaozhuang) alkaline complexes has triggered a considerable interest in this alkaline magmatic rock series.However, despite the understandings on the formation age [25,35] and geochemistry [28,32,33] of this complex, the lack of systematic research still limits further identification of the local tectonic magma evolution and REE accumulation processes of the area.
In this study, we attempted to discuss the magmatic origin, evolution, and tectonic setting of these alkaline magmas through a systematic study on the petrology, geochemistry, zircon U-Pb geochronology, and Lu-Hf isotopes of the Chishan alkaline complex, with a view to further gain insights into the mechanisms behind the lithospheric thinning of the North China Craton and the genetic connections between alkaline rocks and polymetallic deposits.

Regional Geology
The Chishan alkaline complex lies near the Chishan village about 18 km southeast of Weishan County of Zaozhuang City, Shandong Province.Tectonically, this complex belongs to the Luxi Block at the southeastern margin of the North China Craton and the west side of the Tanlu fault zone (Figure 1a) [28,29].The Luxi Block is bounded by the Tanlu fault zone to the east, the Liaocheng-Lankao fault zone to the west, the south of the Qihe-Guangrao fault to the north, and the Fengpei fault to the south (Figure 1b) [36][37][38][39][40][41][42][43][44][45].The exposed formations in the study area include the Neoarchean Taishan rock group, Paleozoic Cambrian to Permian carbonate and clastic rocks, Mesozoic Triassic to Cretaceous clastic and volcanic rocks, and Cenozoic clastic rocks (Figure 1b), with the Neoarchean granodiorite constituting the crystalline basement.In this area, the Yanshanian intermediate-to-alkaline complexes consisting of diorite porphyrite, syenite, syenite porphyrite, diabase, and lamprophyre mostly occur in forms of batholiths, stocks, or dykes.The representative rock bodies include the Shagou, Tongshi, Chishan, and Longbaoshan volcanic-intrusive complexes composed of monzonitic-orthofelsic rocks.The principal part of the Chishan alkaline complex comprises quartz syenite, aegirine quartz syenite, and alkali granite.The complex stretches in the NE-SE orientation, intruding into the early granodiorite and interfacing with the country rock like irregular branches.Varying degrees of alkaline metasomatism can be observed along most parts of the contact.The outcrop of this complex, occupying an area of an area of 0.5 km 2 , is almost fully covered by Quaternary except for sporadic bedrock outcrops at the Chishan hilltop.The Chishan alkaline complex is primarily controlled by tectonic faults.Tectonic fissures in the rocks are often filled up by rare earth veins and occur in the NW orientation as single veins, spiderwebs, or disseminations (Figure 2  The principal part of the Chishan alkaline complex comprises quartz syenite, aegirine quartz syenite, and alkali granite.The complex stretches in the NE-SE orientation, intruding into the early granodiorite and interfacing with the country rock like irregular branches.Varying degrees of alkaline metasomatism can be observed along most parts of the contact.The outcrop of this complex, occupying an area of an area of 0.5 km 2 , is almost fully covered by Quaternary except for sporadic bedrock outcrops at the Chishan hilltop.The Chishan alkaline complex is primarily controlled by tectonic faults.Tectonic fissures in the rocks are often filled up by rare earth veins and occur in the NW orientation as single veins, spiderwebs, or disseminations (Figure 2

Petrology and Petrography
The quartz syenite is grayish white or light flesh pink in color (Figure 3a), porphyroid or porphyritic in texture, and massive in structure.The mineral components mainly include K-feldspar, plagioclase, quartz, with portions of hornblende.Microscopy revealed a fine-grained porphyritic texture.The phenocrysts primarily include K-feldspar (±10%) and a small amount of hornblende (±1%).The matrix, which is primarily composed of K-feldspar (±53%), plagioclase (±10%), quartz (±10%), and hornblende (±5%), exhibits a xenomorphic platy, columnar-granular, or irregular granular shape, and 0.01-1.0mm grain size.The K-feldspar phenocrysts, which mainly consists of orthoclase, features a platy or irregular granular texture and 1.8-2.5 mm grain size, with detectable Carlsbad twinning (Figure 3c) and zonal structures, and are mostly clayized.The hornblende (±1%) phenocrysts are columnar in shape, approximate 1.2 mm in grain size, green to olive green in color (Figure 3c,e), and have been partly replaced by epidote and chlorite.The quartz occurs interstitially between feldspar grains (Figure 3e).

Petrology and Petrography
The quartz syenite is grayish white or light flesh pink in color (Figure 3a), porphyroid or porphyritic in texture, and massive in structure.The mineral components mainly include K-feldspar, plagioclase, quartz, with portions of hornblende.Microscopy revealed a fine-grained porphyritic texture.The phenocrysts primarily include K-feldspar (±10%) and a small amount of hornblende (±1%).The matrix, which is primarily composed of K-feldspar (±53%), plagioclase (±10%), quartz (±10%), and hornblende (±5%), exhibits a xenomorphic platy, columnar-granular, or irregular granular shape, and 0.01-1.0mm grain size.The K-feldspar phenocrysts, which mainly consists of orthoclase, features a platy or irregular granular texture and 1.8-2.5 mm grain size, with detectable Carlsbad twinning (Figure 3c) and zonal structures, and are mostly clayized.The hornblende (±1%) phenocrysts are columnar in shape, approximate 1.2 mm in grain size, green to olive green in color (Figure 3c,e), and have been partly replaced by epidote and chlorite.The quartz occurs interstitially between feldspar grains (Figure 3e).The alkali granite features fine-grained granite texture and massive structure (Figure 3b).Microscopy revealed a fine-grained, subhedral granular texture.The main mineral components include orthoclase (±65%), microcline (±10%), quartz (±20%), and small fractions of biotite (±3%) and epidote (±1%).Accessory minerals include sphene and magnetite.The orthoclase is subhedral columnar or granular in shape with Carlsbad twinning (Figure 3d) and evident perthitic texture.The microcline is subhedral columnar in shape and 1-5 mm in grain size with crosshatch twinning (Figure 3d).The quartz is xenomorphic granular in shape and approximates 0.02-1.2mm in grain size; a portion of the quartz occurs interstitially between other minerals in fine granular forms (Figure 3f).The alkali granite features fine-grained granite texture and massive structure (Figure 3b).Microscopy revealed a fine-grained, subhedral granular texture.The main mineral components include orthoclase (±65%), microcline (±10%), quartz (±20%), and small fractions of biotite (±3%) and epidote (±1%).Accessory minerals include sphene and magnetite.The orthoclase is subhedral columnar or granular in shape with Carlsbad twinning (Figure 3d) and evident perthitic texture.The microcline is subhedral columnar in shape and 1-5 mm in grain size with crosshatch twinning (Figure 3d).The quartz is xenomorphic granular in shape and approximates 0.02-1.2mm in grain size; a portion of the quartz occurs interstitially between other minerals in fine granular forms (Figure 3f).The biotite is layered with about 0.1-1 mm by 0.02-0.2mm in grain size, and notably pleochroism; the rock is reddish brown to sepia and appears as sandy beige or olive green to green.Some of the biotite is associated with epidote.

Samples and Analysis
All 12 samples used for our study originated from the fresh alkaline rocks of the Chishan REE deposit.Six samples were quartz syenite, and the other six were alkali granite.Ten samples were collected from the −160 m level of the deposit, and two were obtained from the surface outcrop.Figure 2 shows the exact sampling locations.Except for samples 18CS-01 (quartz syenite) and 18CS-27 (alkali granite) on which zircon dating analysis was performed, the total rock major and trace element analyses were performed on all other 10 samples.

Total Rock Major and Trace Element Analysis
The total rock major and trace element analyses were performed by Beijing Createch Testing Technology Co., Ltd.(Beijing, China).Ten fresh quartz syenite and alkali granite samples were crushed to smaller than 200 mesh.For the major elements, the lithium borate plus lithium nitrate melting method and X-ray fluorescence spectrometry (XRF-1800) with an accuracy higher than 1% were used.For the trace elements, the lithium borate melting method and quantitative inductively-coupled plasma (ICP) spectrometry (Agilent 7500ce) with an accuracy better than 5% were used.For REEs, the ICP-MS (Agilent 7500ce ICP-MS) with an accuracy of better than 5% was used.Table 1 presents the analysis results.

Zircon U-Pb Dating
Monomineral separation for zircon dating was completed by the Fengze Source Rock and Mineral Test Technology Co., Ltd.(Langfang, China); target fabrication, transmittance and reflectance measurement, and cathode luminescence (CL) photography and testing were all completed by Beijing Createch Testing Technology Co., Ltd.(Beijing, China).Further details about the target fabrication and testing are described in the work of Song et al. [47].All of the zircons were studied with micrographs and cathode luminescence (CL) images to illustrate their microstructures.The zircon U-Pb compositions were determined using a sensitive high-resolution laser ablation multi-collector inductively-coupled plasma source mass spectrometer (AnalytikJena PlasmaQuant MS Elite ICP-MS) composed of an ESI NWR 193 nm FX laser and a Neptune mass spectrometer.Helium was used as the gas carrier for denudation material.The analytical spot sizes were 35 µm, but each spot was rastered over 120 µm for three minutes to remove common Pb on the zircon surfaces.Five consecutive scans were performed for each zircon spot.The analysis process is described in the work of Yuan et al. [48].The final test data were processed with ICP-MS DataCal [49].Finally, the U-Pb age concordia plot was developed, and the weighted average ages were calculated using ISOPLOT 3.70 [50].Table 2 presents the analysis results.

Zircon Lu-Hf Isotopes
Zircon Hf isotopes were measured with a Neptune system in or near the same position used for U-Pb dating analysis.The laser and mass spectrometers used were the same as previously described for zircon U-Pb dating test.The analysis conditions, models of instruments, and analysis processes are described in the work of Geng et al. [51].During the analysis, the εHf(t) values were calculated by relying on the zircon U-Pb ages at the measuring points.For the purpose of this study, the 176 Lu decay constant of 1.867 × 10 -11 year −1 [52], chondrite 176 Lu/ 177 Hf ratio of 0.0332, and 176 Hf/ 177 Hf ratio of 0.282772 were used [53].The depleted mantle model age (T DM ) was calculated at the present 176 Lu/ 177 Hf ratio of 0.0384 and 176 Hf/ 177 Hf ratio of 0.28325 of the depleted mantle [54].The formulas for calculating are shown below [55].In the formulas, f cc, f s and f DM respectively represent the f Lu/Hf of the large continental crust, sample, and depleted mantle, and T is the crystallization age of zircon.Table 3 provides the analysis results.

Analysis Results of Total Rock Major and Trace Elements
Major element analysis (Table 1) indicated that the quartz syenite features a SiO 2 content of 69.02-71.72%,a Na 2 O + K 2 O content of 8.87-10.94%,and MgO, CaO, and Fe 2 O 3 contents of 0.15-0.49%,0.73-1.59%,and 0.48-1.92%,respectively.The rock is relatively high in SiO 2 , rich in alkali, low in Ti (with a TiO 2 content of 0.06-0.17%),and rich in Al (with an Al 2 O 3 content of 13.51-16.12%).On the total alkali versus silica (TAS) diagram, the quartz syenite falls within either the quartz monzonite or granite zone (Figure 4a).The aluminum saturation index (A/CNK) of 0.80-1.00suggests that the quartz syenite belongs to the metaluminous series.On the A/NK-A/CNK diagram, all the samples fall within the metaluminous zone (Figure 4b).The quartz syenite features an alkalinity rate (AR) of 2.58-9.02.On the AR-SiO 2 diagram (Figure 4c), the rock samples were projected into the alkaline rock zone, signifying that the quartz syenite belongs to the alkaline rock series.
The alkali granite features a similar major and trace element geochemistry as the quartz syenite, with a SiO 2 content of 64.07-71.66%,a Na 2 O + K 2 O content of 7.62-8.35%,and MgO, CaO, and Fe 2 O 3 contents of 0.62-1.55%,2.32-4.14%,and 0.73-2.16%,respectively.The rock is relatively high in SiO 2 , rich in alkali, low in Ti (with a TiO 2 content of 0.19-0.40%),and rich in Al (with an Al 2 O 3 content of 14.07-16.75%).On the TAS diagram, the alkali granite typically falls within either the quartz syenite or granite zone (Figure 4a).The A/CNK value mostly falls within 0.75-1.00.On the A/NK-A/CNK diagram, the peralkaline contents are mostly found in the metaluminous zone (Figure 4b).The alkali granite exhibits an AR of 2.58-9.02.On the AR-SiO 2 diagram (Figure 4c), the rock samples were projected into the alkaline rock zone, except for a few that fall within the strongly alkaline rock zone.Hence, the rocks in this complex are alkaline rocks.
The trace element analysis of the Chishan alkaline complex indicated that the rocks are rich in large ion lithophile elements (LILEs) Ba, Sr, and Rb but poor in high field-strength elements (HFSEs) Nb, Ta, and Hf.The primitive mantle-normalized spidergram (Figure 5b) indicates a similarly incompatible element distribution pattern between the quartz syenite and alkaline granite of the Chishan alkaline complex.Both minerals exhibit remarkable positive k anomalies, strong negative Sr and Ti anomalies, and positive Ba and Zr anomalies.
The Chishan alkaline complex features a high total REE ( REE) value, with the quartz syenite featuring an average REE value of 692.18 × 10 -6 and the alkali granite yielding an average REE of 247.61 × 10 −6 .The complex is extremely rich in LREEs.The LREE/HREE ratio ranges from 11.00 to 51.17 with an average of 29.88.The (La/Yb) N value is 13.70-231.51;the δEu value is 0.84-1.24,exhibiting a negative Eu anomaly.The δCe value is 1.01-1.23,exhibiting an unremarkable Ce anomaly.On the chondrite-normalized REE diagram (Figure 5a), the REE curve displays a visible right-inclined separation, showing a strong LREE enrichment.The rocks are poor in HREEs (heavy rare earth elements) and display appreciable LREE/HREE fractionation.This finding suggests that the rocks have experienced strong LREE/HREE fractionation and are highly rich in LREEs.

Zircon U-Pb Dating
In the quartz syenite (18CS-01, Figure 6a), zircons exist as automorphic columns or stumps with a complete crystal form and smooth surface.Most of the zircons contain zonal texture, suggesting a magmatic origin.The zircon grains in the sample are large, with clear internal texture, well-arranged zones, and an aspect ratio of 1:1.5-1:5, which agrees with the description of a magmatic zircon.In the alkali granite (18CS-27, Figure 6b), the zircons are well automorphic, mostly columnar in shape with a complete crystal form and smooth surface, and vary in size from 30 µm up to 100 µm.The zircons contain a clear visible zonal texture, whereas some zircon samples showed inherited zircon cores.The Th/U values are unexceptionally greater than 0.4.Almost all the zircon samples displayed the features typical of an alkaline magmatic zircon.Some of the zircons displayed a core-mantle texture, suggesting inherited growths.In the sample, the zircons feature relatively large grain sizes, clear internal texture, well-arranged zones, and an aspect ratio of 1:1.

Zircon U-Pb Dating
In the quartz syenite (18CS-01, Figure 6a), zircons exist as automorphic columns or stumps with a complete crystal form and smooth surface.Most of the zircons contain zonal texture, suggesting a magmatic origin.The zircon grains in the sample are large, with clear internal texture, well-arranged zones, and an aspect ratio of 1:1.5-1:5, which agrees with the description of a magmatic zircon.In the alkali granite (18CS-27, Figure 6b), the zircons are well automorphic, mostly columnar in shape with a complete crystal form and smooth surface, and vary in size from 30 µm up to 100 µm.The zircons contain a clear visible zonal texture, whereas some zircon samples showed inherited zircon cores.The Th/U values are unexceptionally greater than 0.4.Almost all the zircon samples displayed the features typical of an alkaline magmatic zircon.Some of the zircons displayed a core-mantle texture, suggesting inherited growths.In the sample, the zircons feature relatively large grain sizes, clear internal texture, well-arranged zones, and an aspect ratio of 1:1.5-1:5, which agree with the description of a magmatic zircon.The measured U and Th contents spanned broadly from 249 × 10 −6 to 2338 × 10 −6 and from 20 × 10 −6 to 133 × 10 −6 , respectively; the measured Th/U values ranged narrowly from 0.01 to 0.06.

Zircon Lu-Hf Dating
In situ Hf isotope analysis was conducted on five already-dated zircon samples from the quartz syenite and from the alkali granite, respectively, at roughly the same points as those used for zircon U-Pb dating.Table 3 presents the results.Then, the initial ratios and εHf(t) values of zircon Hf isotopes were calculated with the measured ages.From Table 3, the 176 Lu/ 177 Hf ratios were smaller than 0.002 at all points, suggesting that the zircons have gained no considerable radiogenic Hf after the complex formation.Consequently, the zircon 176 Hf/ 177 Hf ratios can be used to discuss the genetic information of the complex [55].As the zircons in the samples show low Lu/Hf ratios (-0.95--0.98), the one-stage model ages of the zircon Lu-Hf isotopes can represent the time at which the source materials separated from the mantle [60].Zircons from the quartz syenite (18CS-01) have variable 176 Hf/ 177 Hf ratios (0.28225-0.

Zircon Lu-Hf Dating
In situ Hf isotope analysis was conducted on five already-dated zircon samples from the quartz syenite and from the alkali granite, respectively, at roughly the same points as those used for zircon U-Pb dating.Table 3 presents the results.Then, the initial ratios and εHf(t) values of zircon Hf isotopes were calculated with the measured ages.From Table 3, the 176 Lu/ 177 Hf ratios were smaller than 0.002 at all points, suggesting that the zircons have gained no considerable radiogenic Hf after the complex formation.Consequently, the zircon 176 Hf/ 177 Hf ratios can be used to discuss the genetic information of the complex [55].As the zircons in the samples show low f Lu/Hf ratios (−0.95-−0.98), the one-stage model ages of the zircon Lu-Hf isotopes can represent the time at which the source materials separated from the mantle [60].Zircons from the quartz syenite (18CS-01) have variable 176 Hf/ 177 Hf ratios (0.28225-0.

Rock Ages
Zircon U-Pb dating analysis demonstrated that in the Chishan alkaline complex, the quartz syenite formed at 125.8 ± 1.2 Ma, and the alkali granite formed at 127.3 ± 1.0 Ma, which concurs with the ages of the quartz syenite (122.4 ± 2.0 Ma) and aegirine quartz syenite porphyrite (130.1 ± 1.4 Ma) yielded by Liang et al. [35] through LA-MC-ICPMS zircon U-Pb measurement.This finding suggests that the rock complex formed in Early Cretaceous, which concurs with the ages of the late Mesozoic magmatic rocks extensively distributed in Eastern China [61].According to Lan et al. [32], the dolomite Rb-Sr age of the Chishan REE deposit ores is 119 ± 1.4 Ma, which is a bit later than the emplacement of this alkaline rock complex, suggesting a close genetic connection between this alkaline rock system and mineralization of the REE ores in this area.

Magmatic Sources and Evolution
The Mesozoic Era witnessed three major geodynamic events in the North China lithosphere: collisional orogeny, tectonic regime transition, and mass lithospheric thinning [11].Numerous alkaline intrusions emerged in the North China Craton [23], giving rise to various types of metal deposits.As the Chishan REE deposit represents a typical example of alkaline rock mineralization throughout the North China Craton [25], studying the material sources of alkaline rocks bears significance in the studies of both geodynamic mechanisms and metallogenic relationships.
The quartz syenite and alkali granite in the Chishan alkaline complex are rich in alkali (with a Na 2 O + K 2 O content of 7.62-10.94%),high in potassium (with a Na 2 O/K 2 O content of 0.28-0.89),metaluminous (with an aluminum saturation index (A/CNK) of 0.75-1.00),and poor in Ti, Fe, Mg, and Mn, which are typical features of an alkaline rock series.The rocks feature strong LREE/HREE fractionation; the REEs display a right-inclined distribution pattern with weak positive and negative Eu anomalies (the δEu is 1.16-4.15).For the similar compositions and close spatial relationship, we regard that quartz syenite and alkali granite are the members of one intrusion (Chishan alkaline complex).
On the Ta/Yb-Th/Yb diagram (Figure 8) [62], the absolute majority of the rock samples from the Chishan alkaline complex fall within or near the enriched mantle zone, suggesting a close connection between the material sources of the Chishan alkaline rocks and enriched mantle.As reported by Yan et al. [27], the aegirine syenite features a Rb content of 170.46 × 10 −6 -4550.67 × 10 −6 , a 87 Rb/ 86 Sr ratio of 2.6800-0.0500,and a 87 Sr/ 86 Sr ratio of 0.71176-0.70780.These findings agree with the geochemistry of mantle-derived magmas and are close to the features of an EMI (enriched mantle I)-type deposit.The rocks must have originated from highly enriched mantle-derived materials.
Zircons are well representative of the isotopic composition of magma sources as their high stability has guarded their Lu-Hf isotopes from magmatic differentiation and late weathering processes.For this reason, zircon Lu-Hf isotopes are often used for tracing magma sources and magmatic evolution [55].Zircon εHf(t) reflects the composition of a magma source.A positive εHf(t) represents a depleted mantle or a young, new crust overgrowing a depleted mantle and indicates remelting from the new crust.The negative εHf(t) values of the studied samples might indicate Chishan alkaline complex was originated from an enriched mantle source or thickened lower crust or a mixed source region [55,63,64].Given that the quartz syenite and alkali granite of the Chishan Alkaline Complex share the same zircon Hf isotopic composition with relatively low εHf(t) values (−17.60-−13.19), the Hf isotopic compositions fall above the 2.5 Ga continental crust evolutional line on the diagram for T-εHf(T) (Figure 9), and represent a mixing of mantle sources with remobilized old continental crust.Also, the results are consistent with those of the contemporary Longbaoshan alkaline complex (εHf(t) = −19.20 to −14.00) [65], which is believed to be produced by partial melting of the enriched lithospheric mantle (EMI) [66][67][68][69].Further, there showed some inherited zircons in the alkali granite.Specially, the ages and Hf isotopes composition of inherited zircon cores could be an important and critical clue to identify the mantle source of rocks.Although, the ages and Hf isotopes of the inherited zircons cores in Chishan complex were not measured this time, the late Archean ages (2.51-2.64Ga) with positive εHf(t) values (−0.2 to −6.2) derived from inherited zircon cores of the Longbaoshan complex [65] suggest that the ancient crust of the North China Craton was also involved in the formation of those alkaline complex.Thus, it is reasonable to deduce that the Chishan alkaline complex originated from partial of the EMI-type lithospheric mantle with assimilation of ancient crustal materials.
isotopes of the inherited zircons cores in Chishan complex were not measured this time, the late Archean ages (2.51-2.64Ga) with positive εHf(t) values (-0.2 to -6.2) derived from inherited zircon cores of the Longbaoshan complex [65] suggest that the ancient crust of the North China Craton was also involved in the formation of those alkaline complex.Thus, it is reasonable to deduce that the Chishan alkaline complex originated from partial of the EMI-type lithospheric mantle with assimilation of ancient crustal materials.Furthermore, in terms of Nd isotopic composition, the εNd(t) value of the Chishan alkaline rocks is -8.7 to -8.11 [70], the εNd(t) value of the Neoarchean gneiss basement is -25 [70], and the εNd(t) of the Early Cretaceous (125-127 Ma) basic magmatic rocks (e.g., the Fangcheng basalt and Yinan gabbro) in Western Shandong, which directly originate from the partial melting of an enriched mantle, ranges from -15.4 to -12.6 [67,69].Such a condition indicates that the Chishan alkaline rocks could never have derived from the crust.Rather, these rocks must have been associated with an enriched mantle.The εNd(t) value of the perovskite and monazite in the Chishan REE ore veins is −10.1to−8.2 [32], which agrees with the Nd isotopic composition of the alkaline rocks, confirming that the alkaline rocks are of the same origin as the REE ores.isotopes of the inherited zircons cores in Chishan complex were not measured this time, the late Archean ages (2.51-2.64Ga) with positive εHf(t) values (-0.2 to -6.2) derived from inherited zircon cores of the Longbaoshan complex [65] suggest that the ancient crust of the North China Craton was also involved in the formation of those alkaline complex.Thus, it is reasonable to deduce that the Chishan alkaline complex originated from partial of the EMI-type lithospheric mantle with assimilation of ancient crustal materials.Furthermore, in terms of Nd isotopic composition, the εNd(t) value of the Chishan alkaline rocks is -8.7 to -8.11 [70], the εNd(t) value of the Neoarchean gneiss basement is -25 [70], and the εNd(t) of the Early Cretaceous (125-127 Ma) basic magmatic rocks (e.g., the Fangcheng basalt and Yinan gabbro) in Western Shandong, which directly originate from the partial melting of an enriched mantle, ranges from -15.4 to -12.6 [67,69].Such a condition indicates that the Chishan alkaline rocks could never have derived from the crust.Rather, these rocks must have been associated with an enriched mantle.The εNd(t) value of the perovskite and monazite in the Chishan REE ore veins is −10.1to−8.2 [32], which agrees with the Nd isotopic composition of the alkaline rocks, confirming that the alkaline rocks are of the same origin as the REE ores.Furthermore, in terms of Nd isotopic composition, the εNd(t) value of the Chishan alkaline rocks is −8.7 to −8.11 [70], the εNd(t) value of the Neoarchean gneiss basement is −25 [70], and the εNd(t) of the Early Cretaceous (125-127 Ma) basic magmatic rocks (e.g., the Fangcheng basalt and Yinan gabbro) in Western Shandong, which directly originate from the partial melting of an enriched mantle, ranges from −15.4 to −12.6 [67,69].Such a condition indicates that the Chishan alkaline rocks could never have derived from the crust.Rather, these rocks must have been associated with an enriched mantle.The εNd(t) value of the perovskite and monazite in the Chishan REE ore veins is −10.1 to −8.2 [32], which agrees with the Nd isotopic composition of the alkaline rocks, confirming that the alkaline rocks are of the same origin as the REE ores.

Tectonic Setting
Intense tectonic activities, including three major geodynamic events (collisional orogeny, mass lithospheric thinning, and tectonic regime transition within the North China Craton since the Mesozoic), have resulted in extensive magmatic and metallogenic processes [11,67].During the Triassic, the collision between the North China and Yangtze Plates gave rise to the Dabie-Sulu ultrahigh-pressure metamorphic zone.In the Early Jurassic (180-130 Ma), the lithosphere started thinning.The Yangtze Plate underplated the North China Plate after a deep subduction.Mantle-derived magmas reached the Tanlu fault zone to the crust-mantle boundary.Crustal thickening and remelting triggered a string of magmatic processes, giving rise to high K calc-alkaline to high K alkaline rocks.The lithospheric mantle changed to an EMII-type deposit [67].In the Early Cretaceous (130-90 Ma), when Western Shandong was in a backarc extensional environment caused by the subduction of the Yangtze Plate toward the North China Plate, the lithosphere thinned enormously, and extensive enriched mantle melting occurred.The tectonic stress regime also changed from extrusion toward extension.Mass uptrusion of asthenospheric mantle materials, along with mass diagenetic processes, caused the formation of late Mesozoic alkaline rocks [11,65,[71][72][73][74].
From the lg(CaO/Na 2 O + K 2 O) − SiO 2 discrimination diagram (Figure 10a) [75], the rock projection points of the Chishan alkaline complex mostly fall within the extensional alkaline-alkalilime rock zone, demonstrating that the rocks formed in an extensional tectonic setting.On the ω(Rb) − ω(Y + Nb) discrimination diagram (Figure 10b) [76], all the samples fall within the post-collisional granite zone, and the diagenetic background is an active continental margin.These findings also confirm that the Chishan alkaline complex formed in an extensional tectonic setting.The source region of the rocks is closely associated with the tectonic setting of their origin.The source region of post-collisional granite normally results from the mantle contaminated by a part of the crustal materials.The negative Nb, Ta, and Ti anomalies in the rocks are comparative to those of an island arc tectonic setting [77], concurring with the mass lithospheric detachment and thinning background in Eastern China during the Mesozoic [78][79][80][81].This condition suggests that the formation of the Chishan alkaline complex was controlled by the lithospheric thinning of the North China Craton; it must have originated from a backarc extensional setting caused by plate subduction.

Tectonic Setting
Intense tectonic activities, including three major geodynamic events (collisional orogeny, mass lithospheric thinning, and tectonic regime transition within the North China Craton since the Mesozoic), have resulted in extensive magmatic and metallogenic processes [11,67].During the Triassic, the collision between the North China and Yangtze Plates gave rise to the Dabie-Sulu ultrahigh-pressure metamorphic zone.In the Early Jurassic (180-130 Ma), the lithosphere started thinning.The Yangtze Plate underplated the North China Plate after a deep subduction.Mantlederived magmas reached the Tanlu fault zone to the crust-mantle boundary.Crustal thickening and remelting triggered a string of magmatic processes, giving rise to high K calc-alkaline to high K alkaline rocks.The lithospheric mantle changed to an EMII-type deposit [67].In the Early Cretaceous (130-90 Ma), when Western Shandong was in a backarc extensional environment caused by the subduction of the Yangtze Plate toward the North China Plate, the lithosphere thinned enormously, and extensive enriched mantle melting occurred.The tectonic stress regime also changed from extrusion toward extension.Mass uptrusion of asthenospheric mantle materials, along with mass diagenetic processes, caused the formation of late Mesozoic alkaline rocks [11,65,[71][72][73][74].
From the lg(CaO/Na2O + K2O) − SiO2 discrimination diagram (Figure 10a) [75], the rock projection points of the Chishan alkaline complex mostly fall within the extensional alkalinealkalilime rock zone, demonstrating that the rocks formed in an extensional tectonic setting.On the ω(Rb) − ω(Y + Nb) discrimination diagram (Figure 10b) [76], all the samples fall within the postcollisional granite zone, and the diagenetic background is an active continental margin.These findings also confirm that the Chishan alkaline complex formed in an extensional tectonic setting.The source region of the rocks is closely associated with the tectonic setting of their origin.The source region of post-collisional granite normally results from the mantle contaminated by a part of the crustal materials.The negative Nb, Ta, and Ti anomalies in the rocks are comparative to those of an island arc tectonic setting [77], concurring with the mass lithospheric detachment and thinning background in Eastern China during the Mesozoic [78][79][80][81].This condition suggests that the formation of the Chishan alkaline complex was controlled by the lithospheric thinning of the North China Craton; it must have originated from a backarc extensional setting caused by plate subduction.

Conclusions
(1) The Chishan alkaline complex formed in the Early Cretaceous.The quartz syenite and alkali granite formed at 125.8 ± 1.2 and 127.3 ± 1.0 Ma, respectively, and are closely associated with the mineralization of the Weishan REE deposit.
(2) The Chishan alkaline complex shows the geochemistry of mantle-derived magmas features and originated from partial of the EMI-type lithospheric mantle with assimilation of ancient crustal materials.The complex must be of the same origin as the REE deposit.
(3) The formation of the Chishan alkaline complex developed in an extensional setting that resulted from subduction of the North China and Yangtze Plates, and lithospheric thinning and upwelling in the eastern the North China Craton.

Figure 1 .
Figure 1.Simplified geological map showing the major tectonic units in China (a) and regional geological sketch of Western Shandong (b).(a) Geologic and tectonic map of China and the location of Luxi Block, modified after [40].(b) Geological map of the Luxi Block, modified after [37].

Figure 1 .
Figure 1.Simplified geological map showing the major tectonic units in China (a) and regional geological sketch of Western Shandong (b).(a) Geologic and tectonic map of China and the location of Luxi Block, modified after [40].(b) Geological map of the Luxi Block, modified after [37].

Figure 3 .
Figure 3. Petro-mineralogical diagram of the Chishan alkaline complex: (a) Quartz syenite; (b) alkali granite; (c) Carlsbad twins of the syenite; (d) crosshatch twins of the microcline; (e) hornblende occuring in columnar form; (f) quartz occurs interstitially between the feldspar grains.The (c) and (e) photos under the microscope refer to the sample present in (a); the (d) and (f) photos under the microscope refer to the sample present in (b).Qz-quartz; Or-orthoclase; Pl-plagioclase; Ampamphibole; Mc-microcline.

Figure 3 .
Figure 3. Petro-mineralogical diagram of the Chishan alkaline complex: (a) Quartz syenite; (b) alkali granite; (c) Carlsbad twins of the syenite; (d) crosshatch twins of the microcline; (e) hornblende occuring in columnar form; (f) quartz occurs interstitially between the feldspar grains.The (c) and (e) photos under the microscope refer to the sample present in (a); the (d) and (f) photos under the microscope refer to the sample present in (b).Qz-quartz; Or-orthoclase; Pl-plagioclase; Amp-amphibole; Mc-microcline.

Figure 5 .
Figure 5. Chondrite-normalized REE patterns and primitive mantle (PM) normalized trace element spider diagram for quartz syenites (a,b) and alkali granites (c,d) from the Chishan alkalic complex (chondrite and PM values are from [59]).

Table 1 .
Analysis results of the major (wt %) and trace (ppm) elements of the Chishan alkaline complex.

Table 2 .
Zircon U-Pb dating data of the Chishan alkaline complex.