J. Cent. South Univ. (2018) 25: 1489-1500

DOI: https://doi.org/10.1007/s11771-018-3842-4

Petrogenesis of granite from Xiaofan Mo deposit, Dabie Orogen

LIU Qing-quan(刘清泉)^{1, 2}, SHAO Yong-jun(邵拥军)², ZHOU Ke-ping(周科平)¹, LI Yong-feng(李永峰)³

1. School of Resources and Safety Engineering, Central South University, Changsha 410083, China;

2. Key Laboratory of Metallogenic Prediction of Non-ferrous Metals and Geological Environment

Monitoring, Ministry of Education (School of Geosciences and Info-Physics, Central SouthUniversity), Changsha 410083, China;

3. Henan Provincial Non-Ferrous Metals Geological and Mineral Resources Bureau,Zhengzhou 450016, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract: The Mesozoic granitoids in the Dabie Orogen are of particular geological interest as indicators for Mesozoic lithospheric evolution because of their close association with porphyry Mo mineralization. Here, we present a study using petrogeochemistry data to constrain the petrogenesis of the Xiaofan granites in the Dabie Mo mineralization belt (DMB), Henan Province, China. Field investigations show that the Xiaofan pluton mainly consists of porphyritic granite. The Xiaofan granites have high SiO₂ contents of 74.29 wt%–76.07 wt% (average: 75.18 wt%), Al₂O₃ contents of 11.66 wt%–12.83 wt% (average: 12.13 wt%), and K₂O contents of 5.37 wt%–7.90 wt% (average: 6.86 wt%) and low MgO (0.06 wt%–0.16 wt%), TiO₂ (0.09 wt%–0.10 wt%), and P₂O₅ (0.047 wt%–0.103 wt%) contents. They are enriched in Rb, U, K and Hf but depleted in Ba, Nb, Ta, Sr and Ti. By geochemical and mineralogical features, we propose that the Xiaofan granites belong to A-type type granite and dominantly sourced from the crust. The granites from the Xiaofan Mo deposit may have formed in a post-collision extensional setting.

Key words: granite; petrogenesis; Xiaofan Mo deposit; Dabie Orogen

Cite this article as: LIU Qing-quan, SHAO Yong-jun, ZHOU Ke-ping, LI Yong-feng. Petrogenesis of granite from the Xiaofan Mo deposit, Dabie orogen [J]. Journal of Central South University, 2018, 25(6): 1489–1500. DOI: https://doi.org/10.1007/s11771-018-3842-4.

1 Introduction

Granitoid magmas have been widely used as a natural probe for tracing the crustal evolution process [1–3]. Mesozoic granitoids are widespread in the Dabie molybdenum belt (DMB) [4]. Comprehensive investigations of these granitoids represent a promising opportunity to obtain deeper insight into the petrogenesis, mechanism of magma formation, geodynamic evolution and metallogeny of the associated mineral deposits. The DMB contains tens of granitic complexes, which occur either as individual plutons with an area extent of tens to hundreds of km² or as small stocks (generally of <1 km²) [5–8]. Several deposits have also been found in the Dabie Orogenic belts. The molybdenum deposits occur along the tectonic lineament, which has a nearly east–west trend, and are mostly concentrated in a zone stretching from the Lingshan area of Xinyang in Henan Province to the Shapinggou area of Jinzhai in western Anhui province [9]. More than 10 superlarge, large and intermediate molybdenum polymetallic ore deposits are located in the belt, including the Shapinggou, Tangjiaping, Dayinjian, Yaochong, Qian’echong, Xiaofan, Mushan, Doupo and Tianmugou deposits (Figure 1). All of these Mo deposits are genetically associated with granitic magmatism. Therefore, petrogenesis, especially the source of ore-related granites in the DMB, is of crucial importance.

The Xiaofan molybdenum deposit is located in Luoshan County, Henan Province, China, which is a porphyry molybdenum deposit formed in Early Cretaceous in the Dabie Mo belt. Molybdenum mineralization is associated with the Xiaofan granites. YANG et al [10] reported the granite zircon U–Pb age of 139.3 ± 0.64 Ma. Thus, the magmatic evolution and petrogenesis of the Xiaofan granite remain poorly known. In this work, we report new geochemistry data to define the the genetic type, origin of the magmas, and geodynamic setting.

2 Regional geology

The Dabie Orogen lies at a collision zone between the North China craton (NCC) and Yangtze craton (YC), which is part of the Qinling–Dabie Orogen, as shown in Figure 1. It is divided by the Shangcheng–Macheng Fault into eastern and western parts.

The eastern Dabie Orogen is further divided into the North Huaiyang zone (NHZ), the Northern Dabie Complex zone (NDCZ), the Southern Dabie Complex zone (SDCZ) and the Susong Complex zone (SCZ) [12]. The NHZ is bounded by the Triassic suture and Xiaotian–Mozitan faults to the north and south and comprises the Paleoproterozoic Qinling Group, Upper proterozoic–Paleozoic Erlangping Group, Xinyang Group and Xiaojiamiao Formation. The Qinling Group is composed of granulite, biotite-plagioclase gneiss, plagioclase- hornblende gneiss, sericite-quartz schist and marble [13]. The Erlangping Group includes low-grade metamorphic rocks and sedimentary rocks [13, 14]. The Xinyang Group is subdivided into the Guishan and Nanwan Formations [15], and the Guishan Formation is composed of muscovite-quartz schist, two-mica-quartz schist, amphibolite and plagioclase-hornblende schist. The Nanwan Formation consists of muscovite-quartz schist, two- mica-quartz schist and epidote-mica-quartz schist [16]. The Xiaojiamiao Formation is composed of the two-mica schist and epidote-plagioclase amphibolites. The NDCZ is located between the Xiaotian–Mozitan fault to the north and the Wuhe– Shuihou fault to the south. It mainly includes Cretaceous granite, high-grade metamorphosed Neoproterozoic tonalite-trondhjemite-granodiorite gneiss, high-temperature Triassic eclogites and minor metasedimentary rocks [17–19]. The SDCZ is bounded by the Wuhe–Shuihou and Taihu– Mamiao faults to the north and south, respectively, and mainly comprises Cretaceous granite, Neoproterozoic tonalite-trondhjemite-granodiorite gneiss, eclogites, garnet-bearing peridotites, marble, garnet-mica schists and two-mica gneisse [18, 20, 21]. The SCZ is separated from the SDCZ by the Taihu–Mamiao fault and mainly consists of Cretaceous granite, low- to middle-metamorphic- grade volcanic-sedimentary rocks and Sinian marbles [22]. The western part of the Dabie Orogen is bounded by the Shang–Ma Fault in the east, similar to the tectonic units in the eastern part of the Dabie Orogen, but it lacks a middle unit, similar to the NDZ.

Figure 1 Generalized geological map of Dabie Orogen (modified from Ref. [11]), CAOB, Central Asian Orogenic Belt; CCOB and Central China Orogenic Belt

Intrusions are widely distributed in the eastern Dabie Orogen along the NW- and NNE-trending faults. The intrusive rocks are composed of granitoid plutons and minor mafic-ultramafic rocks. The Yanshannian granitoids consist of deep-seated granite batholiths, such as the Shangcheng, Xinxian and Lingshan intrusions, and other small stocks, such as those at Shapinggou, Tangjiaping, Qian’echong, Yaochong, Dayinjian, Mushan and Xiaofan [5]. The large range of chronometric ages indicates that those stocks can be divided into an early stage (143–130 Ma) and a late stage (130– 113 Ma) [23–29]. The early granitoid rocks generally have high Sr/Y ratios, and the late stage rocks have low Sr/Y ratios [25, 29]. Some are genetically associated with porphyry and porphyry- skarn Mo deposits and form the DMB [31].

3 Geology of granite

The Xiaofan pluton extends in the N–S direction and has an outcrop area of 0.039 km². It has intruded into the Nanwan Formation two-mica-quartz schist and consists mainly of granite (Figure 2). The granite shows a porphyroid texture that contains phenocrysts consisting of plagioclase, K-feldspar, quartz and biotite. Its fine-grained matrix is composed of similar minerals as phenocrysts. The accessory minerals are zircon,titanite, apatite and magnetite with lesser allanite. The plagioclases have subhedral granular shapes with polysynthetic twinning, whereas the K-feldspars are mainly subhedral shapes with carlsbad twinning. The quartzes have an irregular granular texture. The biotites are the most important dark minerals and are commonly distributed among the felsic minerals with a scale-like structure (Figure 3).

Figure 2 Generalized geological map of Xiaofan area (modified from Ref. [32])

4 Sampling and analytical methods

Six samples from the Xiaofan pluton were selected for this study and are shown in Figure 2. We conducted whole-rock major and trace elements analyses.

Major and trace element geochemical analyses were undertaken at the ALS Mineral/ALS Chemex (Guangzhou) Co. Ltd. (Guangzhou, China). Major oxide concentrations were measured with an X-ray fluorescence (XRF) spectrometer. Fused glass disks with lithium borate were used, and the analytical precisions were better than ±0.01%, as estimated based on repeated analyses of the standards GSR-2 and GSR-3. The trace element concentrations were determined by ICP–MS. Analyses of United States Geological Survey (USGS) rock standards (basalt, Columbia River 2 [BCR-2]; basalt, Hawaiian Volcanic Observatory 1 [BHVO-1]; and andesite [AGV-1]) indicate that the precision and accuracy were better than ±5% for the trace elements studied. The detailed analytical methods and procedures are described in Refs. [33, 34].

5 Results

The major and trace elements of the samples are listed in Table 1. On the total alkali-silica (TAS) diagram (Figure 4(a)), the Xiaofan samples plot in the granite fields. On the quartz, alkali-feldspar and plagioclase (QAP) triangle (Figure 4(b)), the most samples are classified as granite with one as monzogranite.

Xiaofan granites have high SiO₂ contents of 74.29 wt%–76.07 wt% (average: 75.18 wt%), and K₂O+Na₂O contents of 8.04 wt%–9.86 wt% (average: 9.03 wt%) with Na₂O/K₂O ratios of 0.26–0.36 (average of 0.32), indicating that they are high-K calc-alkaline series or shoshonitic series (Figure 5(a)). The granites have Al₂O₃ contents of 11.66 wt%–12.83 wt% (average: 12.13 wt%), with aluminum index (A/CNK) values of 1.05–1.10,corresponding to metaluminous composition (Figure 5(b)). The granite contains low MgO(0.06 wt%–0.16 wt%), TiO₂ (0.09 wt%–0.10 wt%) and P₂O₅ (0.047 wt%–0.103 wt%) and has rare earth element (REE) contents ranging from 35.81×10^–6 to 75.05×10^–6. The light REE (LREE) and heavy REE (HREE) contents are significantly fractionated with (La/Yb)_N values of 14.70–20.60 and δEu values of 0.68–0.90, indicating negative Eu anomalies (Figure 6(a)). They are enriched in Rb, U, K and Hf but depleted in Ba, Nb, Ta, Sr and Ti (Figure 6(b)).

Figure 3 Photographs of granites from Xiaofan deposit:

Table 1 Major (wt%) and trace (×10^–6) elements composition of Xiaofan granite

Continued

Figure 4 Total alkali silica (TAS) diagram (a) [37] and QAP modal classification diagram (b) [38] for granites in Xiaofan

6 Discussion

6.1 Genetic type and magma source

The Xiaofan granite has high SiO₂, and low Mg^#, Ni, and V contents, suggesting that they were originated from crustal materials. Thus, the Xiaofan granite might be I-, S-, or A-type granite. According to the following evidence, we propose that the Xiaofan granite is the A-type granite. Generally, A-type granites have high K₂O + Na₂O, Ga/Al, and low CaO, Ba, Sr, and are enriched in Zr, Zn, Nb, and REE [40, 41]. As shown in the diagrams of 10000×Ga/Al vs K₂O+Na₂O, TFeO/MgO, Nb, and Zr (Figure 7), the Xiaofan granite samples plot in the A-type granite field, indicating that the granites belong to A-type granite. FROST et al [42] argued that A-type granites are geochemically enriched in Fe element. On the SiO₂ vs TFeO/(TFeO+MgO) diagram (Figure 8), the Xiaofan granite exhibiting the characteristics of the A-type granites [42], showing A-type affinity. In addition, it is generally accepted that A-type granites are originated from relative high-temperature magmas compared with I- and S-type granites [43, 44]. The calculated T_Zr of the Xiaofan granite samples vary from 821 to 847 °C at an average temperature of 832 °C (Table 1), which is similar to those of typical A-type granites [45]. As discussed above, we conclude that the Xiaofan granite belongs to A-type granite.

Figure 5 Diagrams for granite:(A=Al₂O₃, C=CaO, N=Na₂O, and K=K₂O (molar proportion))

Figure 6 Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element patterns (b) of granites (Data for chondrite were obtained from Ref. [35])

The Xiaofan granite has relatively high HREE values and flat HREE patterns (Figure 5(a)), indicating garnet as the main residual phase [46]. Additionally, the samples have high Rb/Ba ratios and low Sr contents, consistent with middle to upper continental crust [47]. Therefore, the source region of the Xiaofan granites might be relatively shallow (probably<30 km). The Xiaofan A-type granite is significantly characterized by metaluminous (A/CNK=1.05–1.10), which are similar to those of A-type granitic melts formed by dehydration melting of diorite on the condition of 0.4–0.8 GPa (A/CNK=0.98–1.08) [43]. Experiment indicate that partial melting of calc-alkaline tonalite to granodiorite can produce metaluminous A-type granitic melts on the condition of P=0.4 GPa, T=950 °C, which would lose A-type geochemical features as pressure increased up to >0.8 GPa [43]. Therefore, we propose that the primitive magma of the Xiaofan A-type granite was originated from intermediate felsic rocks under the condition of 0.4–0.8 GPa pressure and about 14–28 km depths [48].

6.2 Geodynamic setting and its implications

Zircons U–Pb age confirms that the Xiaofan granite was formed in the Early Cretaceous. The Dabie Orogen is a Triassic collision zone between the North China Craton and Yangtze Block, the coesite and diamond inclusions in eclogites suggest that the continental crust subducted into mantle depthes [49, 50]. Geophysical data show that the Dabie Orogen is average crust, rather than the thickened crust [51], which indicates that it has a significant thinning of the lithosphere. The Izanagi Plate (or Paleo-Pacific) started to subduct beneath the Eurasian continent in a NWW direction in the late Jurassic-Early Cretaceous period [52], which led to the transformation of tectonic regime in the Dabie Orogen. The geodynamic setting of mineralization in this period has been a NE-SW direction extension. After this period, the Izanagi Plate underwent NE-directed subduction parallel to the Eurasian continental margin, which led to large-scale continental extension [31, 53]. Therefore, the Early Cretaceous granites and related Mo deposits in the Dabie Orogen may have formed in a extensional setting triggered by the oblique subduction of the Izanagi Plate (31, 54–57). In the (Y + Nb) vs Rb and SiO₂ vs Al₂O₃ diagrams (Figure 9), the Xiaofan granites are primarily plotted in the post-collisional granite field.

Figure 7 10000 Ga/Al vs (K₂O+Na₂O), TFeO/MgO, Zr and Nb diagrams of Xiaofan granite [41]

Figure 8 SiO₂ vs TFeO/(TFeO+MgO) diagram of Xiaofan granite [42]

It is generally accepted that A-type granites formed in extensional environments [60, 61]. We infer that the Xiaofan granite was formed in the post-collision extension setting combination with the regional tectonic setting and geochemical data. The thinning of the lithosphere and upwelling of asthenosphere had taken place during this extension (Figure 10). The direct heating effect of the asthenosphere caused the partial melting of the crust by thermal and chemical erosion. Therefore, the formation of the Xiaofan A-type granite formed in the extensional setting in the Dabie orogen.

Figure 9 Tectonic discrimination diagrams of Xiaofan granites:

Figure 10 Schematic tectono-metallogenic model for Xiaofan granites

7 Conclusions

1) The granites from the Xiaofan Mo deposit have relatively high SiO₂, K₂O and Al₂O₃ contents, low CaO, MgO and P₂O₅ contents, high LREE contents, negative Eu anomalies, high-K calc- alkaline series or shoshonitic characteristics, and metaluminous affinity. They are enriched in Rb, U, K and Hf but depleted in Ba, Nb, Ta, Sr and Ti.

2) The geochemical and mineralogical features confirm that the Xiaofan granites belong to A-type granite and originated from crustal materials.

3) The delamination, thinning of the lithosphere and upwelling of asthenosphere may have occurred in the Dabie Orogen during the Early Cretaceous. We suggest that the Xiaofan granites formed in a post-collision extensional setting.

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(Edited by FANG Jing-hua)

中文导读

大别造山带肖畈钼矿床花岗岩岩石成因

摘要：大别造山带中生代花岗岩类岩石不仅能够提供中生代岩石圈演化信息，而且该类岩石往往与斑岩型钼矿床的形成有着密切的联系。本文选择大别钼成矿带上肖畈钼矿床花岗岩，开展岩石地球化学特征研究，岩相学特征表明肖畈花岗岩体属斑岩花岗岩。岩石地球化学结果显示，花岗岩具有高SiO₂含量（74.29 wt%~76.07wt%，平均值75.18 wt%)，高Al₂O₃含量（11.66 wt%~12.83 wt%，平均值12.13 wt%）和高K₂O含量（5.37 wt%~7.90 wt%，平均值6.86 wt%），以及低 MgO含量 （0.06 wt%~ 0.16 wt%），低TiO₂含量（0.09 wt%~0.10 wt%）和低P₂O₅含量（0.047 wt%~0.103 wt%），岩石富集Rb，U，K和Hf，亏损Ba，Nb，Ta，Sr和Ti。结合岩石产出地质背景和矿物学特征，认为肖畈花岗岩属于源于下地壳的A型花岗岩，形成于后碰撞伸展环境。

关键词：花岗岩；岩石成因；肖畈钼矿床；大别造山带

Foundation item: Project(2017M622596) supported by the Postdoctoral Science Foundation of China; Project(2015CX008) supported by the Innovation Driven Project of Central South University, China; Project(12120114052201) supported by the Geological Scientific Research Project of Land and Resources of Hunan Province, China

Received date: 2017-01-13; Accepted date: 2018-03-10

Corresponding author: SHAO Yong-jun, PhD, Professor; Tel: +86–13973149482; E-mail: shaoyongjun@126.com