Geochemistry, Zircon U-Pb Geochronology and Hf-O Isotopes of the Banzhusi Granite Porphyry from the Xiong’ershan Area, East Qinling Orogen, China: Implications for Petrogenesis and Geodynamics

: The Banzhusi granite porphyry is located in the Xiong’ershan area, East Qinling orogenic belt (EQOB). This study presents an integrated whole-rock geochemistry and zircon U-Pb-Hf-O isotope analysis of the Banzhusi granite porphyry. These rocks have metaluminous, high-K alkali-calcic and shoshonitic features and show signiﬁcant enrichment in light rare earth elements (LREEs) over heavy rare earth elements (HREEs) with negative Eu anomalies. These samples are also greatly enriched in Rb, Ba, K, Pb, Th and U and depleted in Nb, Ta, P and Ti, and they mostly overlap the ranges of the Taihua Group tonalite–trondhjemite–granodiorite (TTG) gneiss. Magmatic zircons from three samples of the Banzhusi granite porphyry yield U-Pb ages of 125.1 ± 0.97 Ma, 128.1 ± 1.2 Ma and 128.2 ± 1.3 Ma. The Hf-O isotope features of zircons from the three samples are very similar ( δ 18 O zircon = 4.84% (cid:24) to 6.51% (cid:24) , ε Hf (t) = − 26.9 to − 14.4). The co-variations of geochemical and isotopic data in these granite porphyries imply that the Banzhusi granite porphyry resulted from the mixing of the partially melted Taihua Group and mantle-derived material in a post-collisional setting from 128–125 Ma.

Zircon is a common accessory mineral that can forcefully resist the effects of later geological activities. Therefore, zircon U-Pb-Hf-O isotopes provide insights into the emplacement ages, the magma sources, the physico-chemical conditions of magma and the petrogenetic processes of granitoids [25,26]. In the present study, we measured the whole-rock geochemistry and in situ zircon U-Pb-Hf-O isotopes for the Banzhusi granite porphyry from the Xiong'ershan area in order to (1) precisely determine the emplacement ages of the granite porphyry and (2) decipher the magma source, petrogenesis and tectonic implications.
Zircon is a common accessory mineral that can forcefully resist the effects of later geological activities. Therefore, zircon U-Pb-Hf-O isotopes provide insights into the emplacement ages, the magma sources, the physico-chemical conditions of magma and the petrogenetic processes of granitoids [25,26]. In the present study, we measured the whole-rock geochemistry and in situ zircon U-Pb-Hf-O isotopes for the Banzhusi granite porphyry from the Xiong'ershan area in order to (1) precisely determine the emplacement ages of the granite porphyry and (2) decipher the magma source, petrogenesis and tectonic implications.

Whole-Rock Geochemistry
Samples were prepared by clearing away the weathered surfaces and then grinding to~200 mesh. Subsequently, lithogeochemical analyses were carried out using standard X-ray fluorescence (XRF), although the FeO content was analyzed with the potassium bichromate titrimetric method at the Analytical Laboratory of the Beijing Research Institute of Uranium Geology. The precision and accuracy for most major oxides were better than 2% (5% for MnO and P 2 O 5 ). Almost all corrected values of trace elements had uncertainties within 5% (10% for Zr, Hf, Nb and Ta). The detailed analytical method was described by [54,55].
Zircon U-Pb isotopes were analyzed on single ablation spots at the National Research Centre for Geoanalysis, Chinese Academy of Geological Sciences. A NWR193UC laser system (Elemental Scientific Lasers, Bozeman, MT, USA) operating at a wavelength of 193 nm and consisting of 35 mm spot diameters was employed for the laser ablation at a constant repetition rate of 10 Hz and a fluence of 8 J/cm 2 . Each analysis incorporated a background acquisition lasting approximately 20 s (gas blank), which was followed by data acquisition from the sample for 40 s. The ablated material was carried in He and then mixed with N and Ar before being introduced to the inductively coupled plasma (ICP) source of an Agilent 7900 quadrupole inductively coupled plasma mass spectrometry (ICP-MS, Agilent, Santa Clara, CA, USA). Standard zircons 91500 (1062.4 ± 0.4 Ma) [56] and Plesovice (337.13 ± 0.37 Ma) [57] were used as the primary reference materials for the U-Pb geochronology. GJ1 [58] was used as a secondary standard. The instrumental conditions and data acquisition were similar to those described by [54]. Concordia plots and weighted mean ages were obtained using Isoplot 4.15.

Zircon Lu-Hf Analysis
In situ zircon Hf isotope measurements were performed using a New Wave UP213 laser-ablation microprobe attached to a Neptune multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS, Thermo Finnegan, San Jose, CA, USA) at the Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China. The measurements were performed on the same zircon grains previously analyzed for U-Pb isotopes using a beam size of 55 mm, a laser pulse frequency of 8-10 Hz, and a laser beam energy density of 10 J/cm 2 . Helium was used as an ablation carrier gas with an ablation time of 60 s. The primary reference material used for monitoring accuracy and precision of internally corrected Hf isotope ratios was zircon 91500 (0.282306 ± 0.000008 using 179 Hf/ 177 Hf = 0.7325) [59]. GJ1 (0.282000 ± 0.000005) [58] was used as a secondary standard. The instrumental conditions and data acquisition were similar to those described by [54].
Based on a decay constant value of 1.865 × 10 −11 year −1 for 176 Lu [60], the present-day 176 Hf/ 177 Hf and 176 Lu/ 177 Hf of the depleted mantle are 0.28325 and 0.0384, respectively [61]. The depleted mantle Hf model age (single-stage model age, T DM1 ) was calculated in reference to the depleted-mantle source with the present-day 176 Hf/ 177 Hf ratio of 0.28325 and 176 Lu/ 177 Hf ratio of 0.0384 [61]. The "crust" Hf model age (two-stage model, T DM2 ) was calculated with respect to the average continental crust with a 176 Lu/ 177 Hf value of 0.015 [62].

Zircon O Isotope Analysis
In situ zircon O isotopes were measured using a Cameca 1280 microprobe via secondary ion mass spectrometry (SIMS) at the Centre for Microscopy, Characterisation and Analysis (CMCA) at the University of Western Australia. The beam size was 15 µm in diameter, and the 133 Cs + primary ion beam was accelerated at 10 kV, with a current intensity of −2.5 nA to −3.0 nA. Charge compensation of the Au-coated (30 µm) samples was accomplished using a normal incidence electron flood gun. The instrumental mass fractionation was corrected using the zircon reference materials Laura and Temora 2 [63]. The 91500, Penglai [64], BR266 [65] and GJ1 standards were analyzed 28 times as secondary reference materials. The analytical procedures and conditions and data acquisition were described in [66].

Whole-Rock Geochemistry
The major and trace element analytical data of eight samples from the Banzhusi granite porphyry are listed in Table 1  In situ zircon O isotopes were measured using a Cameca 1280 microprobe via secondary ion mass spectrometry (SIMS) at the Centre for Microscopy, Characterisation and Analysis (CMCA) at the University of Western Australia. The beam size was 15 µm in diameter, and the 133 Cs + primary ion beam was accelerated at 10 kV, with a current intensity of −2.5 nA to −3.0 nA. Charge compensation of the Au-coated (30 µm) samples was accomplished using a normal incidence electron flood gun. The instrumental mass fractionation was corrected using the zircon reference materials Laura and Temora 2 [63]. The 91500, Penglai [64], BR266 [65] and GJ1 standards were analyzed 28 times as secondary reference materials. The analytical procedures and conditions and data acquisition were described in [66].

Zircon U-Pb Ages
Sample BZS-1 has a translucent brown, euhedral and short prismatic shape zircon population, which have a length of 100 to 150 µm and are dominated by oscillatory zoning, indicating a magmatic origin. The results yield a weighted mean 206 Pb/ 238 U age of 125.1 ± 0.97 Ma (MSWD = 1.4, Figure 5b), which represents the crystallization age of the granite porphyry.
Sample BZS-2 also has a translucent light-brown to colorless, euhedral to subhedral zircon population with a length of 150-200 µm (Figure 5c). All zircons show oscillatory zoning or heterogeneous fractured features, suggesting a magmatic origin. All 13 zircons from sample BZS-2 are concordant with 206 Pb/ 238 U ages ranging from 126 to 130 Ma (Table 2), thus yielding a weighted mean age of 128.1 ± 1.2 Ma (MSWD = 1.7, Figure 5d). This weighted mean age is interpreted to represent the crystallization age of the granite porphyry.
The granite porphyry (Sample BZS-6) contains abundant euhedral-and long prismatic-shaped zircon population, which are translucent and colorless to brown. In the CL images, the euhedralto short prismatic-shaped grains show a length of 50 -150 µm and a length to width ratio of 3:1 to 1:1. These grains show obvious oscillatory zoning in the CL images (Figure 5e), suggesting a magmatic origin. Fifteen spots were analyzed on the representative zircon grains from BZS-6 ( Table 2). These spots have concordant ages; a weighted mean 206 Pb/ 238 U age of 128.2 ± 1.3 Ma (MSWD = 2.1) was obtained, which is regarded as the crystallization age of the granite porphyry (Figure 5f).

Petrogenesis of the Banzhusi Granite Porphyry
The Banzhusi granite porphyry is characterized by high-K alkalic-calc and metaluminous to slightly peraluminous features (A/CNK ratios = 0.93-1.17, Figure 3). All samples have negative Eu anomalies (δEu = 0.661.1; Table 1) and show significant enrichments in LREEs over HREEs on normalized diagrams ([La/Yb] N = 9.0-37.0; Figure 4a). The granite porphyry samples are enriched in Rb, Ba, K, Pb, Th and U, and moderately depleted in Nb, Ta, P and Ti compared to the primitive mantle, exhibiting distinct crustal source characteristics (Figure 4b). The data mostly overlap the ranges of metamorphic rocks from the Taihua Group but show a range and pattern distinct from those of the volcanic successions from the Xiong'er Group. The geochemical data of these samples from the Banzhusi granite porphyry suggest that they were mainly generated from partial melting of the Taihua Group.
The  (Table 3; Figures 6 and 7). The Hf isotope features suggest that the parental magma of the Banzhusi granite porphyry originated from an old crustal source. The Taihua Group, as the regional metamorphic crystalline basement, formed between 2.84 Ga and 2.19 Ga and presents T DM2 ages of 2.6-3.2 Ga [29], whereas the Paleoproterozoic Xiong'er Group volcanic rocks formed at 1.83-1.74 Ga and presents T DM2 ages mainly from 1.55 to 2.86 Ga [37][38][39]41]. Hence, we propose that the parental magma of the Banzhusi granite porphyry was dominantly derived from a crustal source in the Taihua Group based on the comparable formation ages of the Taihua Group and the T DM2 ages in this study.
Mantle zircons are generally accepted to have a narrow range of δ 18 O values averaging 5.3 ± 0.6% (2σ) [74][75][76]. The δ 18 O zircon values of less than 6.5% form from melts that contain minor to negligible sedimentary components, whereas δ 18 O zircon values higher than 6.5% signify supracrustal contributions [77][78][79][80][81][82]. However, the zircon δ 18 O zircon values of samples BZS-1, BZS-2 and BZS-6 range from 4.84% to 5.85% , 5.26% to 5.75% and 4.94% to 6.51% (Figure 8), respectively, indicating the contribution from the mantle or mantle-derived sources during zircon growth. The combination of zircon Hf-O isotopes of the Banzhusi granite porphyry indicates that the magma source was originally derived from ancient continental crust together with the nonnegligible involvement of mantle-derived magmas (Figures 6-8). Hence, our zircon O isotope data provide robust new evidence for the contribution of mantle-derived magmas in the origin of the Banzhusi granite porphyry.
The diagrams of εHf (t) versus δ 18 O zircon and the integrated oxygen isotope reservoir of the Earth (Figure 8) also suggest mixed sources from the Taihua Group and mantle-derived magmas. The low δ 18 O zircon values could be inherited from parental magmas. Relatively low δ 18 O zircon fluids or melts can be generated through metasomatism of the lower crust or mantle degassing in post-collisional extensional settings [83][84][85]. The addition of low δ 18 O zircon fluids or melts into the lower crust could accelerate the melting of the refractory Taihua Group. The required heat could be provided by underplating of hot asthenospheric mantle [86].

Relationship between the Banzhusi Granite Porphyry and Mineralization
Zircons from samples BZS-1, BZS-2 and BZS-6 of the Banzhusi granite porphyry analyzed by LA-ICP-MS in this study yielded weighted mean 206 Pb/ 238 U ages of 125.1 ± 0.97 Ma, 128.1 ± 1.2 Ma and 128.2 ± 1.3 Ma, respectively. These ages are consistent with previous zircon U-Pb ages from a recent study [18], indicating that the Banzhusi granite porphyry was formed contemporaneously with the widespread late Mesozoic magmatism in the Xiong'ershan area [21].
In this study, the zircon U-Pb data obtained from the Banzhusi granite porphyry are consistent with the regional Mo, Au, Pb, Zn, and Ag mineralization ages of 133-125 Ma [42]. Based on previous studies, three main mineralization events during the late Mesozoic have been identified in the Xiong'ershan area (i.e., 160-142 Ma, 133-125 Ma and 125-115 Ma) [21]. The formation age (128-125 Ma) of the Banzhusi granite porphyry is contemporaneous with the second episode of magmatism and metallogeny, indicating that the granite porphyry and metallogenesis were probably generated under a unified geodynamic setting.

Geodynamic Implications
To date, previous researchers have proposed four tectonic models for the late Mesozoic evolution of the EQOB. (i) During the late Mesozoic, the NCC was still subducting southward and the YC was still subducting northward beneath the Qinling orogen. Accordingly, late Mesozoic magmatism was related to the syn-collisional setting, and the subsequent post-collisional magmatism continued after the Early Cretaceous [27,93]. (ii) Li [94] and Yang et al. [95,96] proposed a second model that suggested a post-collisional evolution for the EQOB during the late Mesozoic. Therefore, late Mesozoic post-collisional granitoids were distributed in the EQOB after the Triassic collision between the NCC and YC [95][96][97]. (iii) The third model considered that the Palaeo-Pacific slab was subducted northwestward beneath East China during the late Mesozoic [4,48,98]. iv) Li et al. [1] proposed a new viewpoint that the collision between the NCC and YC occurred at circa 195-160 Ma, which led to the thickening of the lower continental crust, and the subsequent post-collisional magmatism continued until circa 125 Ma. Therefore, the main controversy in the above models is the shifted timing of the tectonic systems of the Qingling orogenic belt from a compressional regime to an extensional regime.
As shown in Figure 9a, few samples plot in the unfractionated M-, I-and S-type granite field (OGT), and most of the samples plot in the fractionated felsic granite field (FG). All samples plot in the post-collisional granite (post-COLG) field in Figure 9b. The above features suggest that the granite porphyry was emplaced in a post-collisional setting. The Banzhusi granite porphyry samples all plot in the post-COLG field and exhibit emplacement ages of 128-125 Ma, thus implying that the tectonic transition from a syn-collisional to a post-collisional setting could have accomplished at circa 128-125 Ma. In addition, the abovementioned zircon Hf-O isotope features (low εHf (t) and δ 18 O zircon values) of the Banzhusi granite porphyry imply lithospheric thinning with a significant input of upwelling mantle-derived materials, which resulted in the partial melting of the lower crust in this period. Dong et al. [99] proposed that the Qinling orogenic belt evolved to orogenic collapse during the Late Cretaceous to Palaeogene after the intense compression and denudation during the Late Jurassic to Early Cretaceous. Therefore, the QOB evolved orogenic collapse event is restricted to the Early Cretaceous by the emplacement age (128-125 Ma) of the Banzhusi granite porphyry.

Conclusions
(1) The zircon U-Pb data show that three samples of the Banzhusi granite porphyry yield emplacement ages of 125.1 ± 0.97 Ma, 128.1 ± 1.2 Ma and 128.2 ± 1.3 Ma.
(2) Whole-rock geochemistry and zircon Hf-O isotopes indicate that the parental magma of the Banzhusi granite porphyry was mainly sourced from partial melting of the Taihua Group mixed with non-negligible mantle-derived materials.

Conclusions
(1) The zircon U-Pb data show that three samples of the Banzhusi granite porphyry yield emplacement ages of 125.1 ± 0.97 Ma, 128.1 ± 1.2 Ma and 128.2 ± 1.3 Ma.
(2) Whole-rock geochemistry and zircon Hf-O isotopes indicate that the parental magma of the Banzhusi granite porphyry was mainly sourced from partial melting of the Taihua Group mixed with non-negligible mantle-derived materials.
(3) The Early Cretaceous (128-125 Ma) granitoid magmatism in the Xiong'ershan area likely occurred due to the evolved orogenic collapse event in the Qinling orogenic belt.