Extraction of fractionated interstitial melt from a crystal mush system generating the Late Jurassic high-silica granites from the Qitianling composite pluton, South China: Implications for greisen-type tin mineralization

doi:10.1016/j.lithos.2020.105952

Lithos

Volumes 382–383, February 2021, 105952

https://doi.org/10.1016/j.lithos.2020.105952 Get rights and content

Highlights

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The Qitianling composite pluton comprises main-phase and high-silica granites.
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The high-silica granites are coeval with the main-phase granites.
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Extraction of interstitial melt from a crystal mush generated the highly-fractionated high-silica granites.
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The less evolved main-phase granites represent a residual crystal mush.
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The high-silica granites are responsible for the greisen-type Sn mineralization.

Abstract

High-silica granites (> 74 wt% SiO₂) in the Nanling Range are associated with important W–Sn deposits. Their petrogenesis remains controversial. The Qitianling composite pluton, one of the Mid–Late Jurassic W–Sn-related granitic plutons in the Nanling Range, comprises dominantly the main-phase medium- to coarse-grained K-feldspar megacrystic hornblende biotite monzogranites and biotite monzogranites (mostly < 70 wt% SiO₂) in the outer zone, and minor fine-grained alkali-feldspar granites (high-silica granites with ~75 wt% SiO₂) in the inner zone. SHRIMP zircon U–Pb dating indicates that the high-silica granites were emplaced at 154–155 Ma, coeval with the main-phase granites (163–153 Ma) within the analytical error uncertainty. The whole-rock Nd (ε_Nd(t) = −6.85 to −6.56) and zircon Hf–O (ε_Hf(t) = −12.2 to −2.2 and δ¹⁸O_VSMOW = 7.15‰ to 9.19‰) isotopic compositions of the high-silica granites are indistinguishable from those of the main-phase granites, implying that they are derived from the same sources. Petrographic, mineralogical and geochemical data indicate that both granite types have A2-type affinity, with the high-silica variant characterized by more evolved chemical compositions, higher Rb/Sr, lower Nb/Ta and Zr/Hf ratios, more F enrichment, and more distinct REE tetrads. Based on the geochemical data, trace element modeling and recent studies on the fractionation processes in silicic magma system, we propose that the Qitianling granites have undergone a high degree differentiation in a crystal mush system. The high-silica granites represent the highly-fractionated interstitial melt that was extracted from the crystal mush system, whereas the main-phase granites represent the residual crystal mush consisting of the ‘cumulate crystals’ and trapped interstitial melt. The extraction is inferred to have occurred when the crystallinity of the mush reached about 67–76%, at least for the most evolved sample of the high-silica granites, and the residual crystal mush might have trapped about 15–25% interstitial melt. The highly-fractionated high-silica granites with high melting temperature and low oxygen fugacity are responsible for the greisen-type tin mineralization at the Furong tin deposit.

Graphical abstract

Introduction

High-silica (> 74 wt% SiO₂) granites, and their volcanic equivalents, are enriched in incompatible elements and often are associated with W–Sn and rare metals deposits (Lee and Morton, 2015; Schaen et al., 2017, Schaen et al., 2018). South China, particular the Nanling Range, hosts a large number of Mid–Late Mesozoic (mostly 165–150 Ma) composite plutons (Fig. 1; Mao et al., 2013; Yuan et al., 2019), which comprise the main-phase granites with SiO₂ contents around 70 wt% and spatially-associated but smaller high-silica granites (Jiang et al., 2018; Wang et al., 2014a). Recently, most of the main-phase granites have been classified as A-type granites, based on their Fe-rich biotite and calcic amphibole, high whole-rock K₂O + Na₂O, Zr + Nb + Ce + Y, and Ga contents, and high Ga/Al ratios (e.g., Chen et al., 2016; Li et al., 2018; Zhao et al., 2012). Previous studies have shown that the main-phase granites in the Nanling Range mostly resulted from magma mixing processes between felsic magma derived from melting of Proterozoic basement rocks and mantle-derived mafic magma in a extensional regime related to the break-off or rollback of the subducted Paleo-Pacific (Izanagi) slab beneath the Cathaysia Block, or a slab window during the Late Mesozoic (Li et al., 2018; Mao et al., 2013; Wang et al., 2014a). However, the petrogenesis of the high-silica granites from these plutons remains controversial, including three models: (1) fractional crystallization of the main-phase granites (Chen et al., 2016; Jiang et al., 2018), (2) partial melting of crustal materials as a separate magmatic event (Chen et al., 2014; Wang et al., 2014a), and (3) fractional crystallization of crust-derived magmas coupled with strong assimilation of sedimentary rocks (Zhang et al., 2017b).

Importantly, the Nanling Range is also one of the world's largest W–Sn metallogenic provinces (Mao et al., 2013), which accounts for more than 54% of tungsten resources and major tin and rare metals resources in the world (Yuan et al., 2019). Many of these deposits are world-class, and formed during the Late Jurassic, as shown by recent cassiterite U–Pb, molybdenite Re–Os, and mica Ar–Ar dating (160–150 Ma; Mao et al., 2013; Li et al., 2018; Yuan et al., 2019). The consistent isotope ages between the W–Sn deposits and the adjacent granites indicate a genetic link between them. However, it has long been an issue of dispute regarding which specific phase of the composite plutons is genetically associated with the W–Sn mineralization. Some researchers suggested that the main-phase granites were responsible for the W–Sn mineralization because the high-silica granites have slightly younger ages than the mineralization (Wang et al., 2014b; Yuan et al., 2011), whereas others proposed that the W–Sn mineralization was related to the high-silica granites, because these granites underwent prolonged fractional crystallization that facilitated progressive enrichment of tin in residual melts, and the interaction of a silicate melt with F-rich fluid exsolved from the high-silica granitic magma might also play a crucial role in the enrichment of W and Sn (Chen et al., 2016; Jiang et al., 2018; Wang et al., 2014a).

In this paper, we present a detailed study on the petrology, mineralogy, whole-rock major and trace element and Nd isotopic compositions, and zircon U–Pb–Hf–O characteristics of the Qitianling high-silica granites within the Qitianling composite pluton in the Nanling Range. Combined with the corresponding published data for the Qitianling main-phase granites (Xie et al., 2010; Zhao et al., 2012; Zhu et al., 2009), we constrain the petrogenesis of the high-silica granites, and furthermore discuss the implications for the greisen-type Sn mineralization.

Section snippets

Geological setting

South China is composed of the Yangtze Block in the northwest and the Cathaysia Block in the southeast (Fig. 1A), which were amalgamated at about 900 Ma along the Jiangshan–Shaoxing–Pingxiang fault in the northeast and the Pingxiang–Longkang fault in the southwest (Li et al., 2009a; Shu et al., 2015). After ~900 Ma, South China was subjected to multiple orogenic, magmatic and metallogenic episodes, typically represented by the Early Paleozoic and Early Mesozoic orogenies, as well as the Late

Petrology

The main-phase granite variants mainly occur in the outer zone of the pluton, and occupy about 88% of the total exposure area of the pluton (Fig. 2), while the high-silica granites are mostly distributed in the inner zone of the pluton (Fig. 2), occupying about 12% of the total exposure area. Contacts between them are irregular wavy and gradational over several centimeters (Fig. 3A). The main-phase granites are dominated by medium- to coarse-grained potassic feldspar (K-feldspar) megacrystic

Analytical methods

Analytical methods are given in Supplementary Methods. Major and trace element data for standard materials and replicate analyses of sample FR-19-256 of the high-silica granites are shown in Supplementary Material, Supplementary Table 1, respectively.

SHRIMP zircon U–Pb ages

Two samples of the high-silica granites were selected for SHRIMP zircon U–Pb dating, and the results are listed in Supplementary Table 3.

The analyzed zircon grains are generally colorless to light yellow and transparent. Apart from some fragments produced by comminution, the zircon grains occur as subhedral to euhedral prismatic crystals, with lengths varying from 50 to 150 μm and length/width ratio in the range of 1:1 to 3:1 (Supplementary Fig. 1). Most of the zircons have concentric

Geochronology

The formation process of the Qitianling composite pluton has been subdivided into three stages according to rock types and 32 zircon U–Pb ages obtained by SHRIMP, LA–ICP–MS or conventional single zircon U–Pb methods (Zhu et al., 2009). The first stage corresponds to the hornblende-biotite monzogranites emplaced at 163–160 Ma with a peak age at about 161 Ma (Zhu et al., 2009). The second stage corresponds to the biotite monzogranites emplaced at 157–153 Ma with a peak at 157–156 Ma (Zhu et al.,

Conclusions

(1)
The Qitianling composite pluton mainly comprises the main-phase hornblende-biotite monzogranite and biotite monzogranite in the outer zone, and the high-silica alkali-feldspar granite in the inner zone. SHRIMP zircon U–Pb dating indicates that the high-silica granites were emplaced at 155–154 Ma, coeval with the main-phase granites within the analytical error.
(2)
The A2-type high-silica granites likely represent the highly fractionated interstitial melt that was extracted from the crystal mush

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We are grateful to Mr. Xu-Feng Tian for his help in sampling and field survey, and to Dr. Hu Guo, Dr. Hang-Qiang Xie, Dr. Sheng He, Dr. Ke-Jun Hou for their assistance in the EMPA, SHRIMP zircon U–Pb dating, SIMS zircon O isotope, and LA–MC–ICP–MS zircon Hf isotope analyses. We gratefully acknowledge constructive comments and helpful suggestions from Professor Xian-Hua Li and two anonymous reviewers that improved earlier versions of the manuscript. Prof. Bernd Lehmann and Prof. Xin-Fu Zhao are

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Magmatic diversity in continental rifts: A case study on the Early Tonian, plutono-volcanic Salto da Divisa Complex, Araçuaí Orogen, Eastern Brazil
2022, Lithos
Exposures from plutonic roots to volcanic roofs in deeply eroded regions may disclose the architecture and igneous processes in ancient metamagmatic edifices, like it is the Early Tonian Salto da Divisa Complex (SADICO) of Eastern Brazil. Field, petrographic, lithochemical and isotopic (in-zircon U-Pb and Lu-Hf, and whole-rock Sm-Nd) studies were conducted on the SADICO, an anorogenic plutono-volcanic complex located in the northeast Araçuaí orogen. The SADICO magmatic record comprises (with U-Pb crystallization ages, and isotopic Hf and Nd data): i) pyroxenite (ε_Nd(t): +2.2 to −5.3; Nd T_DM: 1.2–1.7 Ga); ii) mafic-intermediate dykes (ε_Nd(t):+2.1 to −5.9; Nd T_DM: 1.2–1.8 Ga) and enclaves with OIB-like signature; ii) ferroan, A-type granitoid with mafic-felsic mingling-mixing features (885 ± 9 Ma; ε_Hf(t): −5 to −7, Hf T_DM: 2.0–2.1 Ga; ε_Nd(t): −3.2 to −4.3, Nd T_DM: 1.5–1.7 Ga); iii) fluorite-bearing, metaluminous to peraluminous, ferroan A-type granites, including biotite-amphibole granite (915 to 875 Ma; ε_Nd(t): −2.8 to −5.8, Nd T_DM: 1.6–1.9 Ga), biotite granite (894 ± 10, ε_Hf(t): −4 to −11, Hf T_DM: 1.9–2.3 Ga; ε_Nd(t): −1.6 to −8.0, Nd T_DM: 1.4–2.2 Ga), and amazonite-bearing two-mica granite (ε_Nd(t): −4.0; Nd T_DM: 1.7 Ga); iv) rhyolite (905 ± 24 Ma; ε_Hf(t): −1 to −8.7, Hf T_DM: 1.8–2.2 Ga; ε_Nd(t): +3.1, Nd T_DM: 1.1 Ga) and subvolcanic silica-undersaturated trachyte (912 ± 13 Ma, ε_Hf(t): −14 to −18, Hf T_DM: 2.5–2.7 Ga; ε_Nd(t): +0.3, Nd T_DM: 1.2 Ga), also with ferroan A-type signature. Our integrated petrogenetic model envisages mantle-derived magmas (ultramafic and mafic rocks) evolved by assimilation-fractional crystallization to felsic subvolcanic and volcanic rocks, interacting with granitic magmas produced by crustal anatexis (biotite-amphibole granite) and subsequently fractionated (biotite granite) and highly fractionated (two-mica granite) with the involvement of F-rich fluids up to subvolcanic levels.

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Research articleExtraction of fractionated interstitial melt from a crystal mush system generating the Late Jurassic high-silica granites from the Qitianling composite pluton, South China: Implications for greisen-type tin mineralization

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Geological setting

Petrology

Analytical methods

SHRIMP zircon U–Pb ages

Geochronology

Conclusions

Declaration of competing interest

Acknowledgments

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Research article
Extraction of fractionated interstitial melt from a crystal mush system generating the Late Jurassic high-silica granites from the Qitianling composite pluton, South China: Implications for greisen-type tin mineralization