4.1 Whole-rock geochemistry
Whole-rock major and trace element compositions of monzogranite (XC19-1) and mafic enclaves (XC19-2) samples are plotted in Fig. 3, 4 and listed in supplementary table 1.
4.1.1 The host monzogranite
The monzogranite samples have high SiO2 (70.06 wt.%), TiO2 (0.30 wt.%), K2O (4.29 wt.%) and low Na2O (4.14 wt.%,), CaO (1.63 wt.%), TFe2O3 (1.71 wt.%) with Mg# values of 24.17, indicating a peraluminous high-K calc-alkaline composition (Fig. 3). The total amount of REE in the monzogranite is ∑REE = 161ppm. The pattern of HREE in chondrite-normalized REE patterns is relatively flat ((Gd/Yb)N=1.92), Fig. 4b) with enrichment of LREE and depletion of HREE ((La/Yb)N=16.6, Fig. 4b).The monzogranite samples show slightly weak Eu anomaly, Eu/Eu*=0.83, Ce/Ce*=1.00. They are enriched in LILEs, (including Rb, Ba, Tu and U) and depleted in HFSEs (including Nb and Ta, Fig. 4a).
4.1.2 The mafic enclaves
Compared to monzogranite, the mafic enclaves exhibit high TFe2O3 (4.67 wt.%). CaO (2.98 wt.%), Na2O (4.36 wt.%) and low SiO2 (65.64 wt.%), TiO2 (0.94 wt.%), K2O (3.11 wt.%) with similar Al2O3 (14.8 wt.%), Mg# values (24.32), indicating a metaluminous high-K calc-alkaline composition (Fig. 3). The total amount of REE in the monzogranite is ∑REE = 101ppm. Chondrite-normalized REE patterns and primitive mantle-normalized trace element spiderdiagrams of mafic enclaves are consistent with the monzogranite. The pattern of HREE in chondrite-normalized REE patterns is relatively flat ((Gd/Yb)N=2.37), Fig. 4b) with enrichment of LREE and depletion of HREE ((La/Yb)N=16.8, Fig. 4b).The mafic enclaves samples show slightly weak Eu anomaly, Eu/Eu*=0.89, Ce/Ce*=0.96. They are enriched in LILEs, (including Rb, Ba, Tu and U) and depleted in HFSEs (including Nb and Ta, Fig. 4a).
4.2 Mineral chemistry
EPMA analyses the plagioclase, amphibole biotite and apatite that are dominant mineral phases in both monzogranite and mafic enclaves are listed in supplementary table 2, 3, 4, 5.
4.2.1 Plagioclase
The plagioclase in monzogranite is oligoclase, and the An value ranging from 10 to 27 (Fig. 5a), the variations of An value along the core-mantle-edge trend indicating a slightly turbulent magmatic crystallization process. Plagioclase in mafic enclaves have higher An values that varied from 15 to 45 (Fig. 5a).
4.2.2 Amphibole
Amphibole from monzogranite and mafic enclaves can be categorized as crust-mantle source magnesio-amphibole (Fig. 5b, c). The results show that the MgO contents and Mg# of amphibole in the monzogranite (10.95–12.29 wt.%, 53–60) are quite similar with those of mafic enclaves (11.75–13.30 wt.%, 56–62). Amphibole in the monzogranite crystallized at a pressure and temperature of 0.80 to 2.65 ± 0.6kbar (mean 1.82kbar, Schmidt et al., 1992) and 718 to 748 ± 30℃ (mean 733℃, Putirka et al., 2016). Amphibole in the mafic enclaves was formed at a slightly higher pressure of 2.08 to 3.13kbar ± 0.6kbar (mean 2.65kbar), and at a higher temperature of 742 to 769 ± 30℃ (mean 757℃, Fig. 6). According to estimated lithostatic pressure conditions at 33km/GPa, the monzogranite has shallower emplacement depth (2.6 to 8.8km) than those of the mafic enclaves (6.9 to 10.3 km).
Calculating oxygen fugacity using the valence state of Fe in amphibole and biotite or the relative content of Fe and Mg elements is the most popular method. In this paper, the amphibole and biotite are used to estimate the oxygen fugacity of monzogranite and mafic enclaves. The amphibole in the monzogranite and mafic enclaves have high Oxygen fugacity (Fig. 7a,).
4.2.3 Biotite
Biotite in monzogranite and mafic enclaves have similar compositions, with high MgO (10.83–12.04 wt.% and 11.14-11.67wt.%), TiO2 (3.09–3.80 wt.% and 3.25–3.78 wt.%) and FeOT (16.76–19.41 wt.% and 18.54–19.16 wt.%) contents, and low Al2O3 (13.19-14.21wt.% and 13.20-13.54 wt.%) contents, and similar Mg# values (56–59 and 54–59). Biotite from monzogranite and mafic enclaves can be categorized as crust-mantle source magnesian biotite (Fig. 5d, e). Biotite in the monzogranite crystallized at a pressure and temperature of 0.86 to 1.31 ± 0.33kbar (mean 1.08kbar, Uchida et al., 2007) and 595 to 614 ± 24℃ (mean 599℃,Henry et al., 2005). Biotite of the mafic enclaves was formed at a similar pressure of 0.80 to 1.19kbar (mean 0.95kbar), and at a similar temperature of 544 to 602 ± 24℃ (mean 587℃, Fig. 10). The biotite falls near the NNO buffer line, close to the FQM, showing relatively high oxygen fugacity (Fig. 7b, Wones, 1989).
4.2.4 Apatite
Apatite in monzogranite and mafic enclaves have similar contents of SO3 (0.03–1.27 wt.% and 0.07–0.60 wt.%), F (2.61–4.32 wt.% and 2.43–3.55 wt.%) and Cl (0-0.02 wt.% and 0-0.04 wt.%). According to the calculation (the negative values are excluded), the OH content of apatite in monzogranite is 0.08–0.29 apfu, with an average value of 0.19 apfu. The Cl content was 0.000-0.003 apfu, with an average value of 0.001 apfu. The OH content of apatite in the mafic enclaves is 0.05–0.36 apfu, with an average value of 0.20 apfu. The content of Cl is 0.000-0.005 apfu, with an average value of 0.002 apfu. As shown in the F-Cl-OH ternary disgram (Fig. 5f), apatite in the monzogranite display slightly lower volatile contents (0.29–1.03 wt.%, mean 0.66 wt.%) than those of the apatite in the mafic enclaves (0.18–1.29 wt.%, mean 0.72 wt.%).
4.3 Zircon U-Pb ages and trace element
The geochronology of zircon U-Pb in monzogranite and mafic enclaves from the Zhashui pluton was investigated. Zircon cathodoluminescence (CL) image, zircon U-Pb concordant ages and trace element are shown in Fig. 8, 9, 10. Zircon U-Pb isotope and trace element date are listed in supplementary table 6.
Zircon grains from monzogranite (XC19-1) and mafic enclaves (XC19-2) are generally colorless to grayish black, euhedral-subhedral, and ranging in size from 50 to 300µm with aspect ratios of 1:1 to 5:1. As revealed by cathodoluminescence (CL) images (Fig. 6), most grains have well-developed oscillatory zoning. The U content of zircons in the monzogranite and mafic enclaves are 208-1283ppm and 305-3488ppm, Th content are 98-1404ppm and 157-3655ppm, Th/U ratio are 0.30–1.10 and 0.3–2.1, which is basically consistent with the characteristics of magmatic zircons Th/U > 0.4, so these zircons are judged to be magmatic zircons. The grains of monzogranite and mafic enclaves have 206Pb/238U ages of 206 ± 3Ma to 224 ± 3Ma and 211 ± 4Ma to 224 ± 5Ma, yielding the weighted mean ages of 214 ± 1.2Ma (n = 16, MSWD = 1.9) and 219 ± 3.7Ma (n = 23, MSWD = 0.92, Fig. 8), respectively. Considering the analysis errors, we propose that the mafic enclaves are contemporary with host monzogranite.
The total REE in zircons from the monzogranite and mafic enclaves varies widely, ranging from 602-5747ppm (mean 1682ppm) and 668-7226ppm (mean 1756ppm), respectively. Both of them are depleted in LREE and enriched in HREE, with similar negative Eu anomalies (monzogranite: Eu/Eu*=0.13–0.63, mafic enclaves: Eu/Eu*=0.13–0.51), positive Ce anomaly (monzogranite: Ce/Ce*=2.05-114.25, mafic enclaves: Ce/Ce*=1.92-147.33). The host monzogranite have very similar chondrite-normalized REE patterns to mafic enclaves. Because the rock was granite, the αSiO2 = 1. The sample contained sphene instead of rutile, so the αTiO2 = 0.8. Temperatures calculated by Ti-in-zircon (Ferry and Watson, 2007) in monzogranite range from 589°C to 677°C (mean 637°C), while those in mafic enclaves range from 596°C to 895°C (mean 692°C, Fig. 6).
4.4 Zircon Lu-Hf isotopes
Zircon Lu-Hf isotopes from monzogranite and mafic enclaves are plotted in Fig. 11 and listed in supplementary table 7.
The monzogranite (XC19-3-2) and mafic enclaves (XC19-1-4) were analyzed, and the 176Lu/177Hf ratio of zircons is less than 0.002. The monzogranite of εHf(t) values are − 0.99 to + 1.81 with 3 negative values and 13 positive values, and the mafic enclaves of εHf(t) values are − 1.02 to + 3.26 with 3 negative values and 23 positive values (Fig. 6). The fLu/Hf(s) values of zircons from monzogranite and mafic enclaves range from − 0.98 to -0.96 and − 0.98 to -0.88, respectively, which are significantly lower than the fLu/Hf(s)= -0.34 of the continental mafic crust (Ames et al.,1996), so the two-stage model age can more accurately reflect the extraction time of materials from the depleted mantle in the source region. The two-stage model ages of zircons are 1102 to 1280Ma and 1018 to 1290Ma, respectively.