Journal of Earth Science  2018, Vol. 29 Issue (4): 920-938   PDF    
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U-Pb Geochronology, Elemental and Sr-Nd Isotopic Geochemistry of the Houyaoyu Granite Porphyries:Implication for the Genesis of Early Cretaceous Felsic Intrusions in East Qinling
Xiaohu He1, Hong Zhong2,3, Zhifang Zhao1, Shucheng Tan1, Weiguang Zhu2, Siqi Yang4, Wenjun Hu2, Zhong Tang5, Congfa Bao5    
1. Department of Geology, School of Resource Environment and Earth Science, Yunnan University, Kunming 650091, China;
2. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China;
3. University of Chinese Academy of Sciences, Beijing 100049, China;
4. Kunming University of Science and Technology, Kunming 650093, China;
5. Institute of Yunnan Geology Survey, Kunming 650051, China
ABSTRACT: The Early Cretaceous Houyaoyu granite porphyries are located in the south margin of the North China Craton. Field observations, petrography, geochronology, major and trace elemental and Sr-Nd isotopic compositions are reported to elucidate the genesis of the Houyaoyu granite porphyries. SIMS zircon U-Pb analyses for the Houyaoyu granite porphyries yield two concordant ages of 133.2±2.3 (2σ) and 131±1.1 (2σ) Ma, respectively. Major and trace elemental compositions indicate that these porphyries are high-K I-type granites with high contents of SiO2, K2O, Rb, U, Pb, low Nb, Ta, Ti, and P. Initial 87Sr/86Sr ratios range from 0.708 3 to 0.709 7, and εNd(t) values range from -9.13 to -12.3, with corresponding two-stage depleted-mantle Nd model ages (T2DM) varying from 1.57 to 1.91 Ga. This suggests that the Houyaoyu granite porphyries were predominantly derived from ancient lower conti-nental crust, with minor involvement of mantle-derived components. On the basis of the tectonic evolu-tion of the Qinling Orogen and geochemical characteristics of the Houyaoyu granite porphyries, it is proposed that they were formed in an extensional tectonic setting related to lithospheric destruction of the North China Craton, and produced Mo and Pb-Zn mineralization in East Qinling Orogen.
KEY WORDS: East Qinling    granite porphyries    ancient lower continental crust    destruction of North China Craton    

0 INTRODUCTION

The Qinling Orogen is well known as a product of the continent-continent collision between the North China Craton (NCC) and the South China Block (SCB) (Xu and Zhang, 2018; Dong et al., 2012). In the East Qinling, numerous Late Mesozoic igneous rocks (ca. 158–108 Ma) are closely related to molybdenum and Pb-Zn mineralization. Therefore, elucidating the genesis of these intrusions could provide crucial constraints on the formation of the Mo metallogenic belt and Pb-Zn deposits (Zhang Z W et al., 2011, 2007, 2001), such as the Jinduicheng Mo Deposit (mineralization age at 141 Ma, Li H Y et al., 2012), Shiyaogou Mo Deposit (mineralization age at 131 Ma, Gao et al., 2010), Yinjiagou Ag-Pb-Zn Deposit (mineralization age at 143 Ma, Wu and Zheng., 2013a, b). Late Mesozoic intrusions in East Qinling also provide a window on the evolution of the south continental margin of the NCC. Numerous studies on the petrology, geochronology and geochemistry of these intrusions (Fig. 1) have previously been conducted to reveal their genesis (Zeng et al., 2013a, b; Li N et al., 2012; Qi et al., 2012; Xiao et al., 2012; Hu et al., 2011; Mao et al., 2011, 2010; Wang et al., 2011; Gao et al., 2010; Dai et al., 2009; Guo et al., 2009; Ye et al., 2008a; Chen et al., 2003), but a debate about the genesis of these intrusions continues. For example, Zhao et al. (2012) suggested that the intrusions originated from partial melting of the mafic lower continental crust via heating from underlying basaltic melts. Zeng et al.(2013a, b) studied the Babaoshan granitic porphyries adjacent to the Houyaoyu intrusions, and proposed that they were the products of partial melting of the SCB continental crust subducted under the NCC. Chen et al. (2000) and Li N et al. (2007) suggested the intrusions formed from the re-melting of continental crust in a continent-continent collisional setting. In fact, most researchers have suggested that these intrusions were the products of crust-mantle interaction, and that they were mainly derived from the ancient lower continental crust of the NCC involved with a small portion of mantle materials (Zeng et al., 2013a, b; Li N et al., 2012; Qi et al., 2012; Hu et al., 2011; Mao et al., 2011, 2010; Wang et al., 2011; Gao et al., 2010; Dai et al., 2009; Guo et al., 2009; Ye et al., 2008a).

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Figure 1. Regional geological map showing the study area within East Qinling orogenic belt (revised after Mao et al., 2010).

However, the mixing proportions of crust and mantle are still unknown. Previous studies have shown that these small intrusions were formed between ~158 and ~108 Ma, which corresponds to Late Jurassic–Early Cretaceous magmatic activity (Gao et al., 2014; Li N et al., 2012; Hu et al., 2011; Ding et al., 2010; Mao et al., 2010), and during this period the tectonic mechanism of the southern NCC margin was transformed from compression to extension (Xu et al., 2013; Zhai et al., 2004, 2003). Mao et al. (2010) declareed that intensive magmatism was resulted from a lithospheric thinning processes, which may have been induced by either thermal erosion and metasomatism of the subcontinental lithospheric mantle beneath eastern China (Griffin et al., 1998; Menzies and Xu, 1998), or by lithospheric delamination (Deng et al., 2007, 2004; Wu et al., 2005; Gao et al., 2004, 2002).

In this paper, the Houyaoyu granite porphyries in East Qinling are examined on the basis of geochronology, major- trace elemental and Sr-Nd isotopic compositions. It is suggested that the parental magma of the Houyaoyu granite porphyries was mostly derived from partial melting of an over-thickened lower continental crust that was induced by the transformation of the structural system from extrusion to extension together with the involvement of a small amount of mantle material. This study also reveals the discovery of enclaves existing in the Houyaoyu granite porphyries. Based on Sr-Nd isotopic compositions, these enclaves are considered to be representative of the lower NCC crust, which dominantly generated the granitic porphyries. Thus, the present study probably provides common petrogenetic evidence for the magmatic intrusions that formed during the Early Cretaceous in East Qinling.

1 GEOLOGICAL SETTING

The Qinling Orogen is a complex orogenic belt, which recorded the development of plate tectonics from oceanic subduction and arc-type magmatism to arc-continent and continent- continent collision and witnessed major episodes of accretion and collision between discrete continental blocks, such as the NCC, North Qinling Block and the SCB (Chen et al., 2018; Dong and Santosh, 2016; Wu and Zheng, 2013a, b). According to available geology, geochemistry and geochronology materials, Dong and Santosh (2016) suggested that the tectonic history of the Qinling Orogen at least has gone through five stages: (1) The southward subduction of Mesoproterozoic ocean between the North Qinling terrane and NCC led to the collage of the North Qinling terrane and the NCC at ca. 1.0 Ga and remained the Kuanping suture. (2) The Neoproterozoic accretion as represented by the widely distributed terranes and volcanic- sedimentary rocks. (3) Paleozoic two-stage subduction including Early Paleozoic ocean-continent subduction constrained by the ophiolitic mélange, island-arc related volcanics, intrusions and (ultra)high-pressure and (ultra)high-temperature metamorphic events in the North Qinling belt, and Late Paleozoic continent-continent subduction (Bader et al., 2013; Wu and Zheng, 2013a, b; Dong et al., 2011; Meng and Zhang, 2000). (4) The Triassic collisional orogeny occurred between the South Qinling Block and SCB along the Mianlue suture (Wang et al., 2018; Dong et al., 2011; Meng and Zhang, 2000). (5) Mesozoic intra-continental orogeny, including Early Jurassic differential tectonics, Late Jurassic to Early Cretaceous compression and thrusting, and Late Cretaceous to Paleogene orogen collapse and depression (Li N et al., 2015b; Li Y F et al., 2004).

The Houyaoyu granite porphyries are located in the East Qinling Orogen, and belong to part of the southern NCC margin (Fig. 1). The exposed strata are mainly composed of the Upper Taihua Group formed in earlier Paleoproterozoic (medium- to high-grade metamorphic rocks including hornblende schist, amphibolite gneiss) (Li N et al., 2015b; Lu et al., 2015; Xu et al., 2009; Diwu et al., 2007; Chen and Zhao, 1997; Hu et al., 1988), Middle Proterozoic Xiong'er Group (mainly consisting of mafic-intermediate-acid volcanic lava and fluvial to lacustrine facies sedimentary rocks), the Neoproterozoic Guandaokou Group (mainly consisting of carbonate sedimentary formation composed of dolomite), and Quaternary sediments(mainly consisting of loose sand and clay minerals) (Li C Y et al., 2012; Ye et al., 2008a, 2006; Zhang et al., 2011, 1997, 1996). The crystalline basement is the Taihua Group, which consists of intermediate- high grade metamorphic rocks, and the Xiong'er Group, Luanchuan Group, Guandaokou Group, and Quaternary sediments then overlap in this order to form the cover.

Magmatism was fairly frequent and extensive throughout geological history in the East Qinling district, particularly during the Yanshanian Stage. Numerous small intrusions with ages from ~158 to ~108 Ma are exposed (Mao et al., 2010) in this area, which are mainly composed of syenite granite porphyry, (biotite) monzonite granite porphyry, quartz diorite, K-feldspar granite porphyry (Qi et al., 2012; Hu et al., 2011; Wang et al., 2011; Gao et al., 2010; Mao et al., 2010; Dai et al., 2009; Guo et al., 2009; Ye et al., 2008b; Zhang et al., 2006). These small intrusions supplied plenty of ore-forming materials for Mo (W) and Pb-Zn polymetallic mineralization during the Yanshanian Stage, and generated a large-scale molybdenum belt and Pb-Zn polymetallic deposits in eastern Qinling. Furthermore, small amounts of mafic rocks are exposed in this area in the form of batholiths, dikes, veins, and branches (Lu, 1998).

The Houyaoyu granite porphyries were emplaced into the Longjiayuan Formation of the Late Proterozoic Guandaokou Group consisting of dolomite (Fig. 2). The Fe-Pb-Zn ore bodies are located in the contact belt between country rocks and the intrusions, and there are clear boundaries between the intrusions and the Fe-Pb-Zn ore bodies (Fig. 3). Thus, the emplacement of the Houyaoyu granite porphyries is considered to cause the formation of the Houyaoyu Fe-Pb-Zn Deposit.

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Figure 2. Simplified geological map of the Houyaoyu region.
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Figure 3. Field photographs of the Houyaoyu granite porphyries and Fe-Pb-Zn ore bodies. (a) Fe-Pb-Zn ore bodies in tunnel; (b) the contact relation between intrusion and ore bodies; (c) the field outcrop of granite; (d) hand specimen of ore; (e) and (f) enclaves in porphyry granite.
2 SAMPLES AND ANALYTICAL METHODS 2.1 Samples

Thirteen rock samples with moderate alteration were collected from different areas of the Houyaoyu granite porphyries, and two enclaves (YY-22, YY-56) in the granite intrusion (see Figs. 3e, 3f) were collected. The Houyaoyu granite porphyries and enclaves exhibit massive structures, and porphyritic textures in hand specimen. These samples were systematically identified via microscope and classic porphyritic textures were identified. The Houyaoyu granite porphyries are composed of quartz (20 vol.%–25 vol.%, often showing round and harbor shape by alteration), K-feldspar (40 vol.%–45 vol.%, often exhibiting kaolinization or argillization), biotite (15 vol.%, often exhibiting chloritization), and other minor silicate minerals (e.g., apatite, sphene) and opaque minerals (e.g., pyrite, magnetite). The phenocrysts consist of quartz, K-feldspar and biotite (Figs. 4c, 4d). The two enclaves are significantly different from the Houyaoyu granite porphyries, because they consist of plagioclase with kaolinization, quartz, and show more severe alteration (Figs. 4e, 4f).

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Figure 4. Hand specimen and microscope photos of the Houyaoyu granite porphyries. (a), (b) are hand specimen showing that they are massive structure and porphyritic texture; (c), (d) are photomicrograph showing that the Houyaoyu granite porphyries with classic porphyritic texture consist of quartz (Qz), K-feldspar (Kfs), biotite (Bt), apatite, sphene, pyrite (Py), magnetite. Furthermore, phenocrysts consist of quartz, K-feldspar and biotite, and the same is true for matrix composition; (e), (f) are photomicrograph from YY-22 and YY-56.

To determine the emplacement age of the Houyaoyu granite porphyries, we collected two samples (YY-02 and YY-52) and performed SIMS U-Pb dating on zircons. The petrological characteristics of the two samples are described as follows. Both YY-02 and YY-52 are slightly altered K-feldspar granitic porphyries with porphyritic textures and matrixes consisting of K-feldspar, plagioclase, quartz, and biotite, as well as accessory minerals including zircon, apatite, and iron-titanium oxides. The phenocrysts mainly comprises of quartz, altered K-feldspar, and biotite (Fig. 4). As no zircon grains were obtained from the enclaves, we are unable to determine the age of the enclaves.

2.2 Geochronology

Two samples (YY-02 and YY-52) were firstly grounded into powder. The zircons were separated using heavy liquid method before being handpicked out under a binocular microscope for mounting in epoxy resin. To identify the internal structure and choose potential target sites for the U-Pb analysis of zircon, cathodeluminescence (CL) images were obtained (Fig. 5) using a scanning electron microanalyzer at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), Beijing.

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Figure 5. Representative cathodoluminescene (CL) images SIMS zircon U-Pb concordia diagram of selected zircons from the Houyaoyu granite porphyries.

Measurements of U, Th, and Pb isotopes were conducted using the Cameca IMS-1280 secondary ion mass spectrometer (SIMS) at the IGGCAS. U-Th-Pb ratios and absolute abundances were determined according to standard zircon 91500 (Wiedenbeck et al., 1995). The detailed analytical methods are described in Li X H et al. (2009). A long-term uncertainty of 1.5% (1 RSD) for 206Pb/238U measurements of standard zircons was propagated to the unknown zircons (Li et al., 2010; Sláma et al., 2008; Black et al., 2004), even though the measured 206Pb/238U error in a specific session was generally around 1% (1 RSD) or less. Measured compositions were corrected for common Pb using non-radiogenic 204Pb, but corrections were found to be sufficiently small to be insensitive to the choice of common Pb composition, and an average of present-day crustal composition (Stacey and Kramers, 1975) was used for common Pb (assuming that common Pb is largely related to surface contamination introduced during sample preparation). Uncertainties related to individual analysis are reported at 1σ level, and mean ages for pooled U/Pb (and Pb/Pb) analyses are quoted with a 95% confidence interval. Data reduction was conducted using the Isoplot/Ex v. 2.49 program (Ludwig, 2001).

2.3 Major and Trace Element

All of the studied rock samples were collected from the Houyaoyu granite porphyries. Major oxides of the samples were measured using an Axios PW4400 X-ray fluorescence spectrometer (XRF) on fused glass beads at ALS Chemex (Guangzhou) Co. Ltd, and trace elements were analyzed using inductively coupled plasma mass spectrometry (ICP-MS) at the State Key Laboratory of Ore Deposit Geochemistry (SKLODG), Chinese Academy of Sciences (CAS), following the procedures of Qi et al. (2000). Instrumental drift was corrected running a reference standard solution after every five samples. Results show that analytical precisions and accuracies for most of the trace elements measured were generally better than 5%.

2.4 Sr-Nd Isotope

The chemical separation and isotopic measurement procedures were described in Zhang G W et al. (2001).Whole-rock Sr-Nd isotopic analyses were performed using a VG AXIOM multi collector-ICP-MS (MC-ICP-MS) at the Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, Peking University.

Mass fractionation corrections for Sr and Nd isotopic ratios were based on the 86Sr/88Sr ratio of 0.119 4 and 146Nd/144Nd ratio of 0.721 9, respectively. The 87Sr/86Sr ratio of the Standard NBS987 and 143Nd/144Nd ratio of the Standard SHINESTU determined in this study were 0.710 250±0.000 007 (2σ) and 0.512 118±0.000 003 (2σ), respectively.

3 RESULTS 3.1 Zircon U-Pb Geochronology

In the CL images, all zircon grains show typical oscillatory zoning (Fig. 5), which are typical characteristics of magmatic zircon. Most of the Th/U ratios range between 0.2 and 1.17 (Table 1), greater than 0.1, it suggests a magmatic origin (Wu and Zheng, 2004; Belousova et al., 2002). The crystals from YY-02 and YY-52 are brown and translucent in relation to radioactive damage from their high-uranium contents (Li N et al., 2007).

Table 1 Cameca SIMS zircon U-Pb isotopic compositions of samples YY-02 and YY-52 from the Houyaoyu granite porphyries

All U-Pb data for the zircons of two samples collected from the Houyaoyu granite porphyries are presented in Table 1. Based on 9 and 13 analytical results of YY-02 and YY-52, respectively, two weighted mean average 206Pb/238U ages of 133.2± 2.3 Ma (2σ, MSWD=1.2) and 131±1.1 Ma (2σ, MSWD=0.002 9) were obtained for the Houyaoyu granite porphyries (Fig. 5). In addition, four older 207Pb/206U ages (1 791.7±10.6, 1 925.5±7.4, 2 500.8±5.8, 2 498.5±5.1 Ma) were obtained for Sample YY-02, which are representative of the ages of inherited zircon grains from ancient strata of the NCC.

3.2 Major and Trace Element Geochemistry

Major and trace elemental data of 15 samples for the Houyaoyu granite porphyries are listed in Table 2. Most of the rock samples had been subjected to moderate alteration and had high LOI values ranging from 2.27 wt.% to 7.33 wt.% (mostly between 2.27 wt.% and 4.11 wt.%). Their major oxide contents (recalculated to 100 wt.% in a volatile-free as discussed below) were slightly variable, with SiO2 contents ranging from 68.28 wt.% to 74.06 wt.%, Al2O3 from 12.89 wt.% to 15.97 wt.%, MgO from 0.21 wt.% to 1.66 wt.%, CaO from 0.39 wt.% to 3.26 wt.%, and Fe2O3 from 0.86 wt.% to 3.55 wt.%, respectively. In the TAS diagram, most samples are plotted in the granite area and only a few are plotted into the granodiorite and quartz monzonite area (Fig. 6a). In addition, most samples are plotted into the high-K area in the K2O-SiO2 diagram (Fig. 6b). It is considered that the relatively high Na2O+K2O values (3.58 wt.%–7.34 wt.%) and the highly variable K2O/Na2O ratios (1.43–24.46) could result from late alteration, which would produce a moderate degree of deviation in the K2O-SiO2 and TAS diagrams. The Rittmann indexes (σ) ranged between 0.46 and 2.59 (average 2.0), indicating that these rocks belong to the shoshonitic and high-K alkaline series.

Table 2 Major (wt.%) and trace element (ppm) compositions of the Houyaoyu granite porphyries and enclaves
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Figure 6. (a) SiO2 versus total alkali (Na2O+K2O) (TAS) (after Middlemost, 1994); (b) K2O versus SiO2 diagram of the Houyaoyu granite porphyries (solid line after Peccerillo and Taylor, 1976; dashed line after Middlemost, 1985).

Although the trace element contents of the Houyaoyu granite porphyries are largely variable, their primitive mantle- normalized trace element patterns are mostly similar to each other (Fig. 7a), indicating that they could have shared a common parental magma, or were most likely derived from a similar source. They are enriched in LILEs (such as Ba, Th, U, K), Pb, and depleted in Nb, Ta, P and Sr, similar to those of other intrusions formed during the Mesozoic in East Qinling (Li D et al., 2012; Qi et al., 2012; Zhao et al., 2012). The ratios of Nb/Ta, Zr/Hf of all samples are similar, 12.88–18.42 (average 15.74) and 37.89–41.04 (average 39.46), respectively. Some of the geochemical characteristics of these granites are similar to those of the adakites defaned by Defant and Drummond (1990), with respect to be depleted in Y (< 18 ppm) and Yb (< 1.9 ppm), and enriched in Sr (rarely < 400 ppm) with high Sr/Y and La/Yb ratios. They are also characterized by strong LREE enrichment on the chondrite-normalized REE diagram, with (La/Yb)N values varying from 40 to 82, and δEu varying from 0.69 to 0.98 (Table 2).

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Figure 7. Plots of primitive mantle-normalized incompatible trace-element and chondrite-normalized rare earth element for the Houyaoyu granite porphyries. Primitive mantle data are from Sun and McDonough (1989), and chondrite data from Boynton (1984).

However, the two enclaves (YY-22, YY-56) have obviously different geochemical characteristics, showing higher contents of K2O, MgO, CaO, TiO2, Y, and lower of Al2O3, Sr.

3.3 Sr-Nd Isotopic Compositions

In this study, eight samples from the Houyaoyu granite porphyries were analyzed to obtain their whole-rock Rb, Sr, Sm, Nd concentrations and Sr-Nd isotopic compositions (Table 3). Using results of SIMS zircon U-Pb dating for the Houyaoyu granite porphyries, the initial 87Sr/86Sr and εNd(t) values were calculated at t=131 Ma. In addition, depleted mantle Nd model ages (TDM) were calculated using the model of DePaolo (1991). Most of the Sr-Nd isotopic compositions of the Houyaoyu granite porphyries are characterized by relatively homogenous initial 87Sr/86Sr ratios ranging from 0.707 7 to 0.709 7, and εNd(t) values ranging from -9.13 to -12.32 (see Table 1 and Fig. 8), with corresponding two-stage depleted-mantle Nd model ages (T2DM) ranging from 1.57 to 1.91 Ga (with the exception of two samples YY-22, YY-56). Notably, two exceptional samples YY-22 and YY-56 have fairly high ISr (0.734 5 and 0.747 5) and negative εNd(t) values (-24.26 and -26.04).

Table 3 Whole-rock Sr-Nd isotopic compositions for the Houyaoyu granite porphyries and enclaves
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Figure 8. εNd(t) vs. ISr diagram of the Houyaoyu granite porphyries. Data of other intrusions in East Qinling include the Wenyu (Zhao et al., 2012), Niangniangshan (Zhao et al., 2012), Huashan (Ni et al., 2009), Laoniushan (Ni et al., 2009; Zhang et al., 2006), Heyu (Zhang et al., 2006), Lantian intrusions (Wang et al., 2011; Zhang et al., 2006), Xiong'er Group (Cui et al., 2011; He et al., 2010). The Taihua Group (Ni et al., 2009; Xu et al., 2009) is used to represent lower continental crust of the NCC, and MORB (Jacobsen and Wasserburg, 1979) is used to represent primitive mantle.
4 DISCUSSION 4.1 Petrogenesis of the Houyaoyu Granite Porphyries

The results show that major oxides (Fe2O3, CaO, Al2O3, P2O5) and Sr concentrations decrease with increasing values of SiO2, indicating fractionation of Fe-Ti oxides, plagioclase, and apatite (Fig. 9). It is considered that apatite may have played an important role in parental magmatic evolution, as shown by the obvious depletion of P in the spider diagram (Fig. 7b). In addition, the negative Nb and Ta anomalies (Fig. 7a) may be related to the following two occurrences: (1) separation of Ti containing minerals (e.g., titanite and rutile) from magma; (2) parental magma derived from a source depleted in Nb and Ta. However, as the TiO2 contents show no relationship with the SiO2 contents (Fig. 6b), this suggests that the Houyaoyu granite porphyries were derived from a crustal source slightly depleted in Nb and Ta.

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Figure 9. Major and trace elemental compositions versus SiO2 diagrams.

According to the nature of their protolith, granitic rocks have commonly been divided into I-type, S-type and M-type (Chappell and White, 1974). Loiselle and Wones (1979) later introduced A-type granite, where A stands for mildly alkaline, anorogenic, and anhydrous, according to the chemical compositions. In the Zr vs. TiO2 diagram (Fig. 10a), the Houyaoyu granite porphyries are plotted into the I-type granite field, and this is also supported by the P2O5 vs. SiO2 diagram (Fig. 9f). It should be noted, however, that due to moderate alternation, the Na2O contents decrease with increasing SiO2 (Fig. 6b), thereby causing a distortion of the A/CNK values. Therefore, the A/CNK values cannot be used to distinguish I-type and S-type granite. In contrast, Zr and Ti are immobile elements and were hardly affected by later alteration, thus the Zr vs. TiO2 diagram is reliable for classification (Hastie et al., 2007). In addition, the P2O5 contents slightly decrease with increasing SiO2 (Fig. 9f), thereby showing a good trend and indicating the characteristics of I-type granite (Li et al., 2007; Chappell and White, 1992). As diagnostic peraluminous minerals such as muscovite, cordierite, and garnet in S-type granites (Barbarin, 1999) were not found in the Houyaoyu granite porphyries, we therefore classify the Houyaoyu granite porphyries as I-type granites.

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Figure 10. Plots of Zr versus TiO2 and La versus La/Sm diagram of the Houyaoyu granite porphyries.

Rocks from the Houyaoyu granite porphyries have high Sr (100 ppm–712 ppm, mostly > 400 ppm), low Y (6.09 ppm–15.7 ppm, < 18 ppm) and Yb (0.493 ppm–1.07 ppm, < 1.8 ppm), similar to the geochemical characteristics of adakitic rocks (Castillo, 2012; Richards and Kerrich, 2007; Martin et al., 2005; Defant and Drummond, 1990). Previous studies have revealed that most Late Jurassic–Early Cretaceous granites in East China and in the south of the NCC have characteristics of high Sr and low Y (Li H Y et al., 2012; Li N et al., 2012; Qi et al., 2012; Wang et al., 2011; Gao et al., 2010; Dai et al., 2009; Ye et al., 2008a; Zhang et al., 2006; Zhai, 2004).

Generally, adakitic rocks are formed via the following three mechanisms: (1) partial melting of oceanic slab in a subduction setting (He et al., 2014; Gutscher et al., 2000; Yogodzinski and Kelemen, 1998; Defant and Drummond, 1990); (2) basaltic magmas experiencing complex fractional crystallization processes (Dessimoz et al., 2012; Chiaradia, 2009; Li J W et al., 2009; Richards and Kerrich, 2007; Macpherson et al., 2006); (3) partial melting of the lower continental crust in relation to delamination or crustal thickening (Yuan et al., 2010; Huang et al., 2008; Wang et al., 2007; Gao et al., 2004; Hou et al., 2004; Muir et al., 1995; Atherton and Petford, 1993; Kay and Kay, 1993). Previous studies suggested that the Qinling Orogen experienced a number of continent-continent collision events from the Middle Paleozoic to the Late Triassic, which was accomplished at 220–240 Ma due to a collision between the SCB and NCC (Wu et al., 2013a; Zhao et al., 2013; Dong et al., 2011; Meng and Zhang, 2000, 1999; Zhang et al., 1997, 1996; Li S G et al., 1993). However, as the formation of the Houyaoyu granite porphyries is ca. 131 Ma, there is no relationship between their formation and the collision events for the Qinling Orogen. The generation of the Houyaoyu granite porphyries is therefore not associated with partial melting of oceanic slab in a subduction setting, and this is also supported by the lack of contemporaneous mafic rocks (e.g., basalts, lamprophyres, diabases, gabbros) in East Qinling. In addition, there is no positive correlation between the SiO2 and Sr/Y and La/Yb ratios (Fig. 11), and the diagram of La-La/Sm (Fig. 10b) shows that the parental magma of the Houyaoyu granite porphyries was generated by a partial melting process. Therefore, the Houyaoyu granite porphyries could not have resulted from the fractional crystallization of basaltic magma (Wang et al., 2014). Generally, adakitic rocks formed by crustal delamination have high MgO (Mg# > 50), Cr (> 30 ppm), and Ni (> 20 ppm) (Richards and Kerrich, 2007), thus the Houyaoyu granite porphyries were not formed by a delaminated lower continental crust. Furthermore, the Houyaoyu granite porphyries are characterized by depletion of HREEs, Y, and negative εNd(t) values (-9.13– -12.3), which could be well explained by the partial melting of an over-thickened lower continental crust (generally having a depth of > 50 km, ~15 kbar) (Wang et al., 2014; Hou et al., 2013, 2004). Finally, the South China Block was fully connected to the North China Plate by the Late Triassic, implying that there is no direct relationship between the formation of these small intrusions and the closure of the North Qinling Ocean.

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Figure 11. SiO2 versus trace elements diagram of the Houyaoyu granite porphyries.

On the chondrite-normalized REE diagram (Fig. 7a), the Houyaoyu granite porphyries are characterized by no (or weakly negative) Eu anomalies, with δEu varying from 0.69 to 0.98, which suggests that almost no plagioclase fractionation occurred in the evolutional process of parental magma, or no plagioclase served as a residual component during intracrustal partial melting. In addition, high (La/Yb)N values indicate that parental magma experienced a long evolutionary time, or stand for the geochemical composition of the protolith. Figure 7b shows the negative anomalies for P, suggesting that apatite played an important role in crystal fractionation, and this is also evident from the negative correlation between P2O5 and SiO2. Because there are no significant trends between TiO2, Nb, Ta, and SiO2, it is suggested that the negative anomalies for Nb, Ta, and Ti (Fig. 7b) are inherited from a crustal source, which is also supported by the strongly positive Pb anomalies.

The Sr-Nd isotopic compositions of the Houyaoyu granite porphyries are divided into two obvious groups (Table 3). One group with medium initial 87Sr/86Sr ratios (0.707 7–0.709 7) and negative εNd(t) values (-9.13– -12.3) represents the composition of the Houyaoyu granite porphyries and implies that they originated from the lower crust. However, the Sr-Nd isotopic characteristics are different from those of the lower crust of the NCC, indicating that mantle materials were involved in the generation of parental magma. The two enclave samples have high initial 87Sr/86Sr ratios (0.734 5 and 0.747 5) and more negative εNd(t) values (-24.26 and -26.04), similar to those of the ancient lower crust of the NCC.

We summarized Sr-Nd isotopic data for another six intrusions in East Qinling, and plotted their εNd(t) and initial 87Sr/86Sr ratios (ISr) values in Fig. 8. In the εNd(t) vs. ISr diagram, most of the data for the intrusions (except YY-22 and YY-56) in East Qinling are plotted in an area between MORB (representing primitive mantle) and the Xiong'er Group and Taihua Group (representing lower continental crust of NCC), suggesting a mixing source of mantle and crust. In addition, low MgO, Ni, and V concentrations also indicate the presence of minor mantle materials in the source. The ancient two-stage depleted- mantle Nd model ages (T2DM) from 1.57 to 1.91 Ga, overlapping the ages of two inherited zircons in this study, suggest that the Houyaoyu granite porphyries might be derived from old lower continental crust.

In summary, we suggest that the Houyaoyu granite porphyries originated from a crust-mantle interaction, mainly in relation to an ancient continental crust, with minor involvement of mantle-derived components. This interpretation is also be supported by the discovery of enclaves from the lower crust and the old U-Pb zircon ages.

4.2 Timing and Tectonic Setting of Magmatic Events in East Qinling

Our SIMS zircon U-Pb dating results show that the Houyaoyu granite porphyries formed at about 131 Ma (in the Early Cretaceous stage in geologic time scale); the Early Cretaceous Age is consistent with previous results from LA-ICP-MS zircon U-Pb dating (Hu et al., 2010), indicating that Pb-Zn polymetallic mineralization began at that time. As the combining of the North China Craton (NCC) and the South China Block (SCB) was completed before 200 Ma (Li N et al., 2015a; Wu et al., 2013a; Zhao et al., 2013; Dong et al., 2012, 2011; Liu et al., 2011; Meng and Zhang, 2000, 1999; Zhang et al., 1997, 1996), the formation of the Houyaoyu granite porphyries, therefore, has no relationship with the collision of the Qinling Orogen. The Qinling Orogen was subsequently involved in a transition from compression to extension, which is evidenced by many Mesozoic extensional structures (including metamorphic core complexes such as the Xiaoqinling, Xiaoshan, Xiong'ershan, and Lishan), and detachment faults developed during 135–123 Ma in the internal NCC (Lin et al., 2008; Yang et al., 2007; Liu et al., 2005; Zhang J J et al., 2003; Zhang and Li, 1998; Shi et al., 1993). In fact, the drifting Izanagi Plate turned to a roughly parallel direction to the Eurasian eastern continental margin after 135 Ma, rather than subducting beneath the continent (Goldfarb et al., 2007; Maruyama et al., 1997). This means that the Izanagi Plate had a reduced tectonic impact on the NCC after 135 Ma, and this is the main cause of the transition in the tectonic regime from compression to extension in the East Qinling district. Mantle materials began to upwell in an extensional tectonic environment and heated the over-thickened lower continental crust of the NCC, which led to partial melting of the crust. Lithospheric destruction of the North China Craton was simultaneously initiated; this caused numerous magmatic events that are confirmed by the large number of Mesozoic felsic intrusions in East Qinling.

On the basis of the above demonstration, we assumed that the Houyaoyu granite porphyries were formed in an extensional tectonic environment related to the lithospheric destruction of the North China Craton. The partial melting of the ancient lower continental crust of the NCC and the involvement of minor mantle materials generated the parental magma during mantle under plating, and crust-mantle interaction took place during the above processes. It is thus considered that Mo and Pb-Zn mineralization occurred in the Early Cretaceous in the south of NCC simultaneously with the destruction of the North China Craton (Bao et al., 2014; Mao et al., 2011).

4.3 Relationship between Magmatic Events with Mineralization in East Qinling

Numerous previous studies of the petrology, geochemistry, and geochronology for the Yanshanian intrusions in East Qin- ling have revealed two main magmatic events that occurred in the Late Jurassic–Early Cretaceous (158–141 Ma) and the Early Cretaceous (135–108 Ma), respectively (Li N et al., 2012; Qi et al., 2012; Hu et al., 2011; Wang et al., 2011; Gao et al., 2010; Mao et al., 2010; Dai et al., 2009; Guo et al., 2009; Ye et al., 2006; Zhang et al., 2006). The magmatism supplied such a large quantity of materials that formed a large-scale Mo mineralization belt and several Pb-Zn polymetallic deposits; all the Mo ore deposits and most of the Pb-Zn polymetallic deposits are related to the Yanshanian felsic intrusions. This proposal is also supported by the stable isotope characteristics (C, H, O, S) of vein minerals and Pb, Mo isotopes of ore minerals (Qi et al., 2012; Gao et al., 2010; Dai et al., 2009; Guo et al., 2009; Ye et al., 2006). The Houyaoyu intrusion is one of these intrusions in East Qinling, and it is closely related to the formation of Pb-Zn polymetallic deposits. Thus, its genesis is an important implication for the tectonic setting of the Early Cretaceous magmatic events and the relationship between magmatic events with mineralization in East Qinling District. Mao et al. (2011) summarized the characteristics and tectonic settings of Mesozoic molybdenum deposits in the East Qinling-Dabie orogenic belt, and found that these Mo deposits are genetically, spatially and temporally associated with Mesozoic intrusions. In fact, many vein type Pb-Zn-Ag deposits are located in the surrounding porphyry or porphyry-skarn Mo deposits, suggesting that they belong to the same ore system. Therefore, porphyry stock works and Pb-Zn polymetallic veins can be used as vectors for further prospecting in East Qinling Orogen.

5 CONCLUSIONS

(1) The Houyaoyu granite porphyries are I-type granites and have adakitic characteristics. The SIMS zircon U-Pb ages of the Houyaoyu granite porphyries are ~131 and ~133 Ma, suggesting that intense magmatic activities occurred during the Early Cretaceous in the south of the NCC.

(2) Trace elemental and Sr-Nd isotopic compositions indicate that the Houyaoyu granite porphyries were mainly derived from ancient lower continental crust, with minor involvement of mantle-derived components. The resultant crust- mantle interaction provided large amounts of ore-forming materials for Mo, Pb, and Zn mineralization in East Qinling.

(3) It is thus proposed that the Houyaoyu granite porphyries formed in an extensional tectonic setting related to the lithospheric destruction of the North China Craton.

ACKNOWLEDGMENTS

This research was jointly supported by the National Key R & D Program of China (No. 2016YFC0600405) and the National Natural Foundation of China (Nos. 41425011, 41262004). The authors are grateful to Dr. Aimin Fang and his colleagues from Lushi Mining Co., Ltd for providing invaluable assistance during our field investigation, and also grateful to two anonymous reviewers and Dr. Jianbo Chen, Dr. Min Tang for their constructive reviews, which considerably improved the paper. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0788-2


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