Introduction

In the vicinities of Rizhao, Qingdao, and Weihai occur a wide range of lithologies that include volcanic, intrusive and metamorphic rocks (Ye et al. 1996; Cong 1996; Jahn et al. 1996; Zhao et al. 1997; Zhou and Lu 2000; Fan et al. 2001; Hong et al. 2003; Zheng et al. 2003; Guo et al. 2004; Huang et al. 2005; Yang et al. 2005a, b).

The intrusive rocks are represented by gabbro, granitoids, diorite, alkaline rocks, mafic dykes (Guo et al. 2004; Yang et al. 2005a, b; Liu et al. 2008a, b), as well as adakites (Guo et al. 2006), that are widely distributed throughout eastern Shandong Province. These rocks, and in particular, the alkaline rocks (Guo et al. 2005; Yang et al. 2005a, b; Liu et al. 2008a, b), contain valuable information concerning deep geodynamic processes and, as such, can be used to study the orogenic processes of continental subduction and the role of crust-mantle interaction in this part of China (Menzies and Kyle 1972; Jahn et al. 1996; Ye et al. 2000; Fan et al. 2001; Guo et al. 2004).

Alkaline rocks are generally thought to have their origins within the upper mantle (Ren 2003). These rocks are common in anorogenic, intraplate extensional, and/or rift-related tectonic settings (Currie 1970; Coulson 2003; Goodenough et al. 2003). However, alkaline rocks may also be generated during late to post-orogenic stages of magmatism (Coulson et al. 1999), such as in the Permian-Triassic Western Mediterranean Province (Bonin et al. 1987), the Pan-African Arabian Shield (Harris 1985), the Himalayas (Turner et al. 1996; Miller et al. 1999; Williams et al. 2004), Sulu belts (Yang et al. 2005a, b; Liu et al. 2008a, b), and others (Sylvester 1989; Guo et al. 2005). Felsic alkaline rocks (e.g., monzonite, syenite, and A-type granite) are also commonly intimately associated with alkaline mafic rocks (e.g., mafic dykes), especially alkali to transitional basalts (Upton et al. 2003; Yang et al. 2005a, b). As such, alkaline rocks require detailed investigation, particularly the alkaline associations within the eastern Shandong Province that are poorly understood. At present, only two alkaline associations have been reported upon in this part of China, namely, the Jiazishan and Junan-Wulian complexes, which are exposed in the eastern Shandong Province (Jiaodong) (Lin et al. 1992; Yang et al. 2005a, b; Liu et al. 2008a, b). The origin of these rocks remains controversial (i.e., they are formed as the result of slab break-off, post-orogenic extension, and foundering of lower crust) (Yang et al. 2005a, b; Xie et al. 2006; Liu et al. 2008a, b). The work of our group on ~110–120 Ma alkaline intrusions may provide further constraints in this debate, and as a result aid in determining the petrogenetic processes that occurred at a late evolutionary stage. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb geochronology, major and trace element geochemistry, as well as Sr-Nd-Pb isotope data from the younger alkaline associations of quartz-monzonite-A-type granite that formed in an extensional setting (Fig. 1) in the eastern Shandong Province are presented in this study. These data have been used to discuss the petrogenesis of the investigated alkaline associations.

Fig. 1
figure 1

a Simplified tectonic map of the Sulu Belt, eastern China (modified after Guo et al. 2004). b Geological map of the study areas showing the distributions of the alkaline intrusions (modified after BGMRS 1991)

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Geological setting and petrography

Jiaodong is generally divided into two metamorphic terrains along the east northeast-trending Wulian-Qingdao-Rongcheng Fault. The south terrain is a high-pressure, blueschist unit, and the north one is an associated unit consisting of ultra-high pressure (UHP) metamorphic granitic gneiss, granulite and subordinate eclogite, schist, amphibolite, marble, as well as quartzite (Cao et al. 1990; Zhai et al. 2000; Guo et al. 2004). Mesozoic igneous rocks are widely distributed in Jiaodong, and mainly formed between 225 Ma and 114 Ma (Zhao et al. 1997; Zhou and Lu 2000; Fan et al. 2001; Zhou et al. 2003; Guo et al. 2004, 2006, 2005; Huang et al. 2005; Meng et al. 2005; Yang et al. 2005a, b). The study area dealt within the present paper is located in the eastern section of Shandong Province near to the city of Jiaonan (Fig. 1). Alkaline associations of quartz-monzonite (JS-1 and 2, DGZ1-1, 2, and 3) and syenogranite (ZZS1, 4; DCZ-1, 2, and 4; CQY1-1 and 5; CQY2-2 and 7; CQY3-2 and 3; as well as YZS-1 and 4) from this area were investigated (Fig. 1). Some mafic dykes appear within these felsic intrusions. Each suite is described in the following subsections.

Quartz-monzonite

Quartz-monzonite intrudes into Archaean or Lower Proterozoic gneiss (Fig. 1). The light grey-coloured monzonite is medium-to coarse-grained with granular and porphyritic textures. It has a composition of 36–45 % subhedral orthoclase and 8–15 % quartz, 30–35 % euhedral andesine, as well as 8–12 % diopside and 3.0–5.0 % biotite and amphibole. Accessory minerals include apatite, zircon, magnetite, and titanite.

Syenogranite

Syenogranite also mainly intrudes into the Archaean or Lower Proterozoic gneiss (Fig. 1). It is commonly light grey to pink, with a composition of 30–35 % quartz, 25–40 % perthite, 16–20 % albite (An0-5.0), and minor muscovite. Accessory minerals include zircon, magnetite, and apatite.

Analytical procedures

U-Pb dating by the LA-ICP-MS method

Zircon was separated from six samples (JS01, DGZ01, ZZS02, DCZ01, CQY01, and YZS01) using conventional heavy liquid and magnetic techniques at the Langfang Regional Geological Survey, Hebei Province, China. Zircon separates were examined under transmitted and reflected light, as well as by cathodoluminescence petrography at the State Key Laboratory of Continental Dynamics, Northwest University, China to observe their external and internal structures. Laser-ablation techniques were employed for zircon age determinations (Table 1; Figs. 2 and 3) using an Agilent 7500a ICP-MS instrument equipped with a 193 nm excimer laser at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geoscience, Wuhan, China. Zircon # 91500 was used as a standard, and NIST 610 was used to optimise the results. A spot diameter of 24 μm was used. Prior to LA-ICP-MS zircon U-Pb dating, the surfaces of the grain mounts were washed in dilute HNO3 and pure alcohol to remove any potential lead contamination. The analytical methodology has been described in detail by Yuan et al. (2004) and Liu et al. (2010). Correction for common Pb was performed following Andersen (2002). Data were processed using the GLITTER and ISOPLOT programs (Ludwig 2003) (Table 1; Fig. 3). Errors for individual analyses by LA-ICP-MS were quoted at the 95 % (1σ) confidence level.

Table 1 LA-ICPMS U-Pb isotopic data for zircons from the alkaline rocks in Jiaodong, Shandong Province, China
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Fig. 2
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Selected zircon cathodoluminescenc CL images of alkaline rocks in the eastern Shandong Province

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Fig. 3
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LA-ICP-MS zircon U-Pb concordia diagrams for the investigated quartz monzonite, monzonite, and granite intrusions in the eastern Shandong Province

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Major elemental, trace elemental and isotopic analyses

Nineteen samples were collected to carry out major and trace element determinations as well as Sr-Nd-Pb isotopic analyses. Whole-rock samples were trimmed to remove altered surfaces, cleaned with deionised water, and then crushed and powdered using an agate mill. Major elements were analysed using a PANalytical Axios-advance (Axios PW4400) X-ray fluorescence spectrometer (XRF) at the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences. Fused glass discs were used and the analytical precision was better than 5 %, as determined based on the Chinese National standards: GSR-1 and GSR-3 (Table 2). Loss on ignition (LOI) was obtained using 1 g of powder heated to 1,100 °C for 1 h. Trace elements were analysed by plasma optical emission MS and ICP-MS at the National Research Center of Geo-analysis, Chinese Academy of Geosciences following procedures described by Qi et al. (2000). The discrepancy among triplicates was less than 5 % for all elements. Analysis results of the international standards OU-6 and GBPG-1 were in agreement with the recommended values (Table 3).

Table 2 Major oxides (wt. %) for the alkaline rocks in Jiaodong, Shandong Province, China
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Table 3 The trace elements composition (ppm) for the alkaline rocks in Jiaodong, Shandong Province, China
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For the analyses of Rb-Sr and Sm-Nd isotopes, sample powders were spiked with mixed isotope tracers, dissolved in Teflon capsules with HF+HNO3 acids, and separated by conventional cation-exchange techniques. Isotopic measurements were performed using a Finnigan Triton Ti thermal ionization mass spectrometer at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China. Procedural blanks were <200 pg for Sm and Nd, as well as <500 pg for Rb and Sr. Mass fractionation corrections for Sr and Nd isotopic ratios were based on 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively. Analyses of standards yielded the following results: NBS987 gave 87Sr/86Sr = 0.710246 ± 16 (2σ) and La Jolla gave 143Nd/144Nd = 0.511863 ± 8 (2σ). Pb was separated and purified by conventional cation-exchange technique (AG1 × 8, 200–400 resin) with diluted HBr as eluant. Procedural blanks were <50 pg for Pb. Analyses of NBS981 during the period of analysis yielded 204Pb/206Pb = 0.0896 ± 15, 207Pb/206Pb = 0.9145 ± 8, and 208Pb/206Pb = 2.162 ± 2. Total procedural Pb blanks were in the range of 0.1–0.3 ng. The analytical results for Sr-Nd-Pb isotopes are presented in Table 4.

Table 4 Sr-Nd-Pb isotopic compositions for the alkaline felsic rocks in Jiaodong, Shandong Province, China
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Results

Zircon U-Pb ages

Euhedral zircon grains in samples JS01, DGZ01, ZZS02, DCZ01, CQY01, and YZS01 are clean and prismatic, with magmatic oscillatory zoning (Fig. 3). A total of 11 grains have a weighted mean 206Pb/238U age of 121 ± 0.5 Ma (1σ) (95 % confidence interval) for JS01 (Table 1; Fig. 3a), 9 grains have a weighted mean 206Pb/238U age of 118 ± 0.4 Ma (1σ) (95 % confidence interval) for DGZ01 (Table 1; Fig. 3b), 13 grains have a weighted mean 206Pb/238U age of 114 ± 0.3 Ma (1σ) (95 % confidence interval) for ZZS02 (Table 1; Fig. 3c), 12 grains have a weighted mean 206Pb/238U age of 118 ± 0.8 Ma (1σ) (95 % confidence interval) for DCZ01 (Table 1; Fig. 3d), 9 grains have a weighted mean 206Pb/238U age of 122 ± 0.4 Ma (1σ) (95 % confidence interval) for CQY01 (Table 1; Fig. 3e), and 14 grains have a weighted mean 206Pb/238U age of 122 ± 0.5 Ma (1σ) (95 % confidence interval) for YZS01 (Table 1; Fig. 3f). These determinations are the best estimates of the crystallisation ages of the alkaline rocks. There was also no inherited zircon characteristic observed.

Major and trace elements

Geochemical data of the quartz monzonite and syenogranite intrusions in the study area are listed in Tables 2 and 3.

The quartz monzonite and granite samples have a wide range of chemical compositions, with SiO2 = 55.14–77.63 wt.%, Al2O3 = 12.13–19.78 wt.%, MgO = 0.09–4.64 wt. %, Fe2O3 = 0.56–7.61 wt. %, and CaO = 0.40–4.84 wt. %. They are relatively high in total alkalis, with K2O = 3.74–4.85 wt. % and Na2O = 3.71–4.64 wt. %, and total K2O+Na2O ranging from 8.21 to 8.99 wt. %. All felsic rocks lie in the alkaline field when plotted on the total alkali-silica (TAS) diagram (Fig. 4a). All samples also straddle the shoshonitic series in the Na2O vs. K2O plot (Fig. 4b). In a plot of the molar ratios of Al2O3/ (Na2O+K2O) versus Al2O3/ (CaO+Na2O+K2O), the rocks are mostly metaluminous, except for some samples falling along the boundary of the metaluminous and peralkaline fields (Fig. 4c). The analysed quartz monzonite and syenogranite samples display regular trends of decreasing TiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, P2O5, Sr, Zr, Ba, Cr, and Ni, increasing SiO2, as well as positive correlations between K2O, Rb, and SiO2 (Fig. 5 and the figures not shown). The 10,000 × Ga/Al ratios of the monzonite and granite samples range from 1.84 to 3.04. In the Ga/Al vs. Zr discrimination diagram of Whalen et al. (1987), the alkaline rocks are all classified as A-type granite.

Fig. 4
figure 4

Classification of the monzonite and granite intrusions from eastern Shandong Province based on three diagrams. a TAS diagram. All major elemental data have been recalculated to 100 % on a LOI-free basis (after Middlemost 1994; Le Maitre 2002). b K2O vs. Na2O diagram. The alkaline association is shown to be shoshonitic (after Middlemost 1990). c Al2O3/(Na2O+K2O) molar vs. Al2O3/(CaO+Na2O+K2O) molar plot. Most samples fall in the metaluminous field, but some samples straddle the metaluminous and peralkaline field boundary

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Fig. 5
figure 5

Selected variation diagrams of major elemental oxides vs. SiO2 plots for the alkaline felsic rocks in eastern Shandong Province

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The quartz monzonite and syenogranite intrusions are characterised by LREE enrichment and HREE depletion, with a wide range in (La/Yb) N values (6.36–43.6) and Eu/Eu* (0.2–1.4) (Table 3 and Fig. 6a). On average, quartz monzonite has a higher Eu/Eu*(1.1–1.4) than the granite (0.2–0.98). In primitive mantle-normalised trace element diagrams, quartz-monzonite and syenogranite samples show enrichment in LILEs (i.e., Rb, Pb, U, and sometimes Ba) and depletion in some Ba, Sr, and HFSEs (i.e., Nb, Ta, P, and Ti) (Fig. 6b).

Fig. 6
figure 6

Chondrite-normalised rare earth elements (REEs) diagrams and primitive mantle-normalised incompatible element distribution diagrams for the quartz monzonite, monzonite and granite intrusions in eastern Shandong Province. The normalisation values are from Sun and McDonough (1989)

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Sr-Nd and Pb isotopes

Sr-Nd and Pb isotopic data have been obtained from (nineteen) representative quartz monzonite and syenogranite samples (Table 4). The alkaline rocks show very different (87Sr/86Sr)i values ranging from 0.7066 to 0.7087, a relatively large variation in εNd (t) values from −14.1 to −17.1, and high neodymium model ages (TDM1 = 1.56–2.38Ga, TDM2 = 2.02–2.25Ga). These results suggest an enriched source region. The Sr-Nd isotopic compositions (Fig. 7) are also comparable to those of late Mesozoic volcanic rocks, alkaline rocks, granites granites and diorites, as well as adakites in Jiaodong (Zhao et al. 1997; Zhou and Lu 2000; Guo et al. 2006; Huang et al. 2005; Yang et al. 2005a, b; Liu et al. 2008a, b) (Fig. 7). The Pb isotopic ratios in the alkaline rocks are 206Pb/204Pb = 17.12–17.16, 207Pb/204Pb = 15.44–15.52 and 208Pb/204Pb = 37.55–37.72, respectively. These ratios significantly differ from those from the Yangtze lithospheric mantle (Yan et al. 2003), and are identical to those of Jiaodong alkaline rocks, Jiazishan alkaline complex and mafic rocks from the central North China Craton, as well as to the Dabie Orogen (Zhang et al. 2004; Yan et al. 2003; Xie et al. 2006; Liu et al. 2008a, b), having a clear EM-1 affinity (Zindler and Hart 1986; Fig. 8a, b).

Fig. 7
figure 7

Initial 87Sr/ 86Sr vs. ε Nd (t) diagram for the felsic rocks in eastern Shandong Province. Other igneous rocks from the Sulu Belt are also plotted for comparison: volcanic rocks from Fan et al. (2001) and Guo et al. (2004), Jiazishan alkaline Complex from Yang et al. (2005a), granites from Yang et al. (2005b), granite and diorite from Huang et al. (2005), adakites from Guo et al. (2006), I-type granitoids from Zhao et al. (1997) and Zhou and Lu (2000), as well as alkaline rocks in eastern Shandong Province from Liu et al. (2008a, b)

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Fig. 8
figure 8

208Pb/204Pb and 207Pb/204Pb vs. 206Pb/204Pb diagrams for the studied alkaline felsic rocks, compared with Early Cretaceous mafic rocks from the North China and Yangtze Craton as well as alkaline rocks in eastern Shandong Province. Fields for Indian MORB and Pacific and North Atlantic MORB, OIB, NHRL, as well as 4.55 Ga geochron are from Barry and Kent (1988), Zou et al. (2000), and Hart (1984), respectively. Data on North China Craton are from Zhang et al. 2004 and Xie et al. (2006), Yangtze mafic rocks are from Yan et al. (2003); the alkaline rocks in eastern Shandong Province are from Liu et al. (2008a, b)

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Discussion

Crustal contamination

Continued assimilation and fractional crystallisation (AFC), or magma mixing is usually postulated to explain the occurrence of co-magmatic felsic rocks (e.g., DePaolo 1981; Devey and Cox 1987; Marsh 1989; Mingram et al. 2000). AFC and magma mixing would result in a negative correlation between SiO2 and ε Nd (t) values, as well as a positive correlation between SiO2 and (87Sr/86Sr) i ratios (Fig. 9). The absence of these characteristic features in the studied Jiaodong alkaline rocks,indicates that magma evolution was not significantly affected by crustal contamination or magma mixing. Further support for this is provided in the high and consistent neodymium model ages (TDM1 = 1.56–2.38 Ga, TDM2 = 2.02–2.25 Ga) (Table 4). The geochemical and Sr-Nd-Pb isotopic signatures of the studied Jiaodong alkaline rocks are, therefore, interpreted to be mainly inherited from an enriched crusted source, as was shown in the Sr-Nd and Pb isotopic data.

Fig. 9
figure 9

Plots of: a initial 87Sr/86Sr ratio and b ε Nd (t) value versus SiO2 for the alkaline rocks from eastern Shandong Province, indicating crystal fractionation trends. FC fractional crystallisation; AFC assimilation and fractional crystallisation

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Fractional crystallisation

For the studied felsic samples, SiO2 shows a negative correlation with TiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, and P2O5 (Fig. 5a–f and h). This may relate to the fractionation of clinopyroxene, hornblende, plagioclase, Ti-bearing phases (ilmenite, titanite, etc.), and apatite. The negative Nb, Ta, and Ti anomalies exhibited in all the investigated alkaline rocks (Fig. 6a) also agree with the fractionation of Fe-Ti oxides, such as ilmenite and titanite. However, parallel rare earth elements (REEs) distribution patterns, coupled with high SiO2 contents in some of the investigated samples (e.g., ZZS-1, ZZS-4, CQY1-1, CQY1-5, CQY2-2, CQY2-7, CQY3-2, and CQY3-3) require alternative explanations. Nevertheless, the negative Ba, Sr, and Eu anomalies shown by many rocks (Fig. 6a and b) imply the fractionation of potassium feldspar and plagioclase.

Jiaodong alkaline rocks exhibit continuously decreasing Zr with increasing SiO2. This result indicates that zircon was saturated in the magma, which was also controlled by fractional crystallisation (Li et al. 2007). Zircon saturation thermometry (Watson and Harrison 1983) provides a simple and robust means of estimating magma temperatures from bulk-rock compositions. The calculated effects of fractional crystallisation are shown in the mineral vector diagrams presented as Fig. 10a and b. The alkaline rocks (the granite samples, in particular) display a combined vector of potassium feldspar and plagioclase fractionation in Fig. 10a. On the other hand, Fig. 10b shows that potassium feldspar fractionation is more important than plagioclase in controlling Ba abundance. The calculated zircon saturation temperatures (T Zr) of the alkaline rocks lie in the range 751–892 °C (Table 2), which represents the crystallisation temperature of the magma. The syenogranite samples (CQY type) show much lower T Zr values (751–794 °C) than the other rocks (819–892 °C) (Table 2).

Fig. 10
figure 10

Plots of Eu/Eu* vs.: a Sr and b Ba for the alkaline rocks. Mineral fractionation vectors were calculated using partition coefficients from Philpotts and Schnetzler (1970) and Bacon and Druit (1988). Tick marks indicate percentage of mineral phase removed in 10 % intervals; Pl-plagioclase, Kf-potassium feldspar

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Petrogenesis

Above all, the geochemical signatures of the alkaline rocks favor their derivation from silicic- rather than basaltic magmas. In other words, the studied rocks were derived from an enriched crustal source (Liu and Xu 2011). Additional support for this explanation comes from high-pressure experimental work that has demonstrated that granite and quartz monzonite cannot result directly from the partial melting of mantle peridotite (Colling 1982; Pitcher 1984).

A Proterozoic stratum in the Jiaodong peninsula is composed dominantly of biotite schist, biotite plagioclase gneiss, amphibolite, granulite, and minor slate and marble. In addition, the alkaline rocks are characterized by negative Eu anomalies and low HREE concentrations (Fig. 6a, b), which could indicate a garnet-bearing source. The Sr-Nd and Pb isotopic compositions of the alkaline rocks differ from those of the North China and Yangtze Cratons (Jahn et al. 1999; Li 2007), implying that the source of the studied rocks was neither the North China nor the Yangtze Craton alone. One possibility is that the source may have been a mixture of materials from both cratons. We use the whole-rock two-stage Nd model ages to infer the possible age of the source. The two-stage Nd model ages (TDM2 = 2.02–2.25 Ga) suggest the presence of an Early Proterozoic crustal component in the source of the studied rocks.

At present, there compete various petrogenetic models for the generation of alkaline felsic rocks (e.g., syenite and A-type granite) (Yang et al. 2005a, b; Zhong et al. 2007; Liu et al. 2008a, b), such as (1) partial melting of lower-crustal rocks under the fluxing of volatiles, (2) fractionation of mantle-derived magmas with or without crustal contamination, (3) mixing of basic and silicic melts and their differentiates, as well as, (4) partial melting of an enriched lithospheric mantle beneath an orogenic belt, due to hybridisation of melts derived from foundered lower crustal eclogite. Among them, the insignificant variations in Sr-Nd isotopes with SiO2 for the alkaline rocks (Fig. 9a, b) preclude the possibility of assimilation process in their genesis. Fractionation of mantle-derived magma without the interaction of crustal rocks, therefore, is proposed as the best model to explain the origin of the studied quartz monzonite and syenogranite intrusions. However, high-pressure experiments have demonstrated that granite cannot be formed through the partial melting of mantle peridotite. Hence, an alternative explanation must be sought for the generation of the investigated alkaline lithologies.

Field geology and petrographic observations can provide direct evidence in the recognition of magma mixing and, therefore, important clues for mantle-crust mixing (Mo et al. 2002; Wang et al. 2002; Shao et al. 2006). Generally, in the case of alkaline rocks, evidence for magma mixing includes bimodal plagioclase phenocryst populations, quenched enclaves, reverse zoning in clinopyroxene occurring within xenocrysts, gabbroic and dioritic dyke swarms, etc. These features, however, are lacking in the studied rocks. Moreover, there are no visible linear relationships identified between SiO2, K2O, Na2O, CaO, Fe2O3 and MgO, in addition, the compositional variation in MgO and FeO lie off the magma mixing trend line (not shown). Collectively, this evidence clearly demonstrates that magma mixing did not play a role in the formation of the alkaline rocks (Zorpi et al. 1989). Additional support for this is provided in the consistent Nb/Ta ratios of our studied samples. In summary, the alkaline rocks studied in this paper were not derived through the mixing of mafic and silicic melts.

In the primitive mantle normalised diagrams illustrated in Fig. 6b, all the investigated rocks show very distinctive negative anomalies for HFSEs (e.g., Nb, Ta and Ti), suggesting involvement of components from ancient continental crust (Zhang et al. 2005). This reasoning is further supported by the low ε Nd (t) values (−15.3 to −17.1) and high (87Sr/86Sr) i (0.7074–0.7088) of the studied rocks (Table 4; Fig. 7). Moreover, the fractional crystallization of minerals (principally plagioclase) suggests that the primary magma is hardly a mafic one. Hence, we still need to understand the petrogenetic process responsible for the generation of the eastern Shandong Province alkaline rocks.

Alkaline rocks are usually generated in post-collision extensional settings (Bonin et al. 1998; Yang et al. 2005a, b; Oyhantçabal et al. 2007), intra-plate rifts or deep faults (Burke et al. 2003; Ridolfi et al. 2006; Jung et al. 2007; Shellnutt and Zhou 2008), or by mantle plumes (Mchone 1996; Karmalkar et al. 2005; Srivastava et al. 2005). Based upon the discussion of source and the geological setting, we propose that the studied alkaline rocks were formed in an extensional / collapse tectonic setting.

The high-ultra high pressure metamorphic rocks of the Dabie-Sulu orogenic belt formed in response to the subduction, collision and exhumation of the Yangtze Craton relative to the North China Craton (NCC) (Wang et al. 1995; Cong 1996). In Early Triassic times (200–230 Ma), the collision of the Yangtze Craton under the NCC resulted in the formation of the Sulu orogenic. Susequent exhumation of Yangtze continental crust helped to form the Sulu Mélange zone; the resulting high- to ultra high-pressure lithotectonic assemblages (eclogite, garnet peridotite and granulite, etc.) and a deep-seated ductile deformation zone occurs right across the Jiaodong and Shandong province of China (Han 2000). After a prolonged period of sustainable and balanced stress, during the Late Jurassic, the stress field transformed into an extensional state; a piedmont depression developed as the Jiao-Lai basin with deposition of sediments (Lai-yang sediments, Han 2000). Late in the Early Cretaceous, the intensity of crustal extension increased resulting in the development of peculiar NE-trending shoshonitic dykes within the Jiaodong Peninsula, eastern Shandong Province, China. In the Late Cretaceous, as a result of continued extension of the basin (e.g., the Jiao-Lai basin), the subduction of the Pacific plate (Chen et al. 2004; Qiu et al. 2008; Yang et al. 2012) led to structural collapse of the Sulu organic belt (Zhao and Zheng 2009). As a result of this collapse, the lower part of the Sulu Mélange zone underwent partial melting, leading to the emplacement of the multiple and diverse magmas, that are represented in the study area as alkaline rocks (Han 2000).

Conclusions

Based upon geochronological, geochemical, and Sr-Nd and Pb isotopic studies, the following conclusions can be drawn:

  1. (1)

    LA-ICP-MS U-Pb zircon dating results indicates that the studied alkaline quartz monzonite and syenogranite intrusions formed between 114.3 ± 0.3 and 122.3 ± 0.4 Ma.

  2. (2)

    The investigated alkaline rocks derived from an enriched source. The parental magma originated through partial melting of an enriched crust beneath the eastern Shandong Province. The possible fractionation of potassium feldspar and plagioclase resulted in an alkaline association with negligible crustal contamination. Zircon saturation temperatures (TZr) of the felsic rocks lie in the range 751–892 °C, which approximately represents the crystallisation temperatures of the magma.

  3. (3)

    The alkaline rocks were produced due to partial melting of an enriched crust source due to the collapse of Sulu organic belt in response to the action of various processes such as extension of the Jiao-Lai basin, subduction of the Pacific plate and the exhumation of Yangtze Craton.