Elsevier

Lithos

Volumes 382–383, February 2021, 105950
Lithos

Research article
Source rocks control the geochemical diversity of granite: The Lincang pluton in the western Yunnan Tethyan belt, SW China

https://doi.org/10.1016/j.lithos.2020.105950Get rights and content

Highlights

  • The Lincang pluton has a transitional I-S-type granitic affinity.

  • Compositional diversity of the granite is controlled by igneous–sedimentary source.

  • Large-volume pluton formed by long-term assembly of individual magma batches.

Abstract

The present study of the Lincang pluton in the western Yunnan Tethyan belt reveals that its compositional variation is largely inherited from a mixed igneous–sedimentary source. Geochemical composition analyses show that this pluton has a wide range of normative corundum values (0.5–3.1 %). With increasing mafic character [molar (FeO + MgO)/100g], the A/CNK values decrease at mafic character < 0.09; and they start to increase at mafic character > 0.09. As silica increases, P2O5 contents become less scattered and decrease at SiO2 > 69 wt.%. These characteristics indicate the transitional I-S-type affinity of this pluton, which is further supported by zircon trace element and oxygen isotopic features. Inherited zircons from the Lincang pluton display similar age–εHf(t) distributions to those of zircons from metavolcanic and metasedimentary rocks of the Lancang Group, suggesting that the Lancang Group served as source rock for the granite. Variable melting proportions of metavolcanic rocks, meta-arkoses, and mica schists of the Lancang Group can explain the positive correlation between 1000/Sr and initial 87Sr/86Sr ratios for the Lincang pluton. The melting of such mixed igneous–sedimentary source rocks is also in line with the negative correlation between CaO/Na2O and initial 87Sr/86Sr ratios as well as with the transitional I-S-type affinity of the Lincang pluton. Modeling of Sr–Nd ternary mixing shows that generation of the Lincang pluton requires the admixture of ~30–70 % metavolcanic rocks and variable proportions of meta-arkose and mica schist. The protracted emplacement period (ca. 30 million years) and heterogeneous initial 87Sr/86Sr ratios of the Lincang pluton suggest an incremental assembly of magmas. It is likely that individual magma batches were derived from a mixed igneous–sedimentary source, emphasizing the importance of source lithology on the compositional diversity of the granite.

Introduction

The I- and S-type classification scheme proposed by Chappell and White (1974) defined I- and S-type granites derived from igneous and sedimentary rocks, respectively (Chappell and White, 1974, Chappell and White, 1984). Subsequently, a line of criteria were proposed to distinguish these two types of granites, including chemical composition of bulk rock (Chappell et al., 2012; Chappell and White, 2001; Clemens et al., 2011; Stevens et al., 2007), mineral paragenesis (Chappell and White, 2001), and trace element and oxygen isotope compositions of zircon (Harris et al., 1997; Gao et al., 2016; Burnham and Berry, 2017; see Table 1). Further studies showed that significant contributions of sediments were involved in parental magmas of some I-type granites, as revealed by in-situ mineral Nd–Hf–O isotope compositions (Hammerli et al., 2018; Kemp et al., 2007). Moreover, some S-type granitic magmas with substantial input of mantle materials or magmas partly derived from partial melting of igneous rocks have been recognized (Collins, 1996; Flood and Shaw, 1977; Gray, 1984; Healy et al., 2004).

These insights have substantially challenged the I-S-type dichotomy. In practice, I-type granites commonly possess some characteristics of S-type granite and vice versa; as a consequence, some terms, such as transitional I–S type granite and fractionated or peraluminous I-type granite (Chappell et al., 2012; Clemens, 2018; Clemens and Benn, 2010; Ghani et al., 2013; Grosse et al., 2011; Wu et al., 2003), have been proposed to illustrate the petrogenesis of these transitional granites. Although some granites with transitional I-S-type affinities have been reported based on geochemical features (Clemens, 2018; Clemens and Benn, 2010; Ghani et al., 2013; Grosse et al., 2011), cases that have closely mixed igneous–sedimentary source feature and exhibit characteristics of both I- and S-type granites have been seldom documented. Zircons, particularly those containing inheritance, have been used to unravel the complex sources of granites (e.g., Farina et al., 2014; Hopkinson et al., 2017; Jeon and Williams, 2018; Villaros et al., 2012). Zircon data can be combined with in-situ Hf isotopic and whole-rock geochemical and Sr–Nd isotopic compositions to further constrain the impact of the sources on the compositional diversity of granites.

Here, we provide the example of the Lincang pluton in southwestern China to demonstrate that its transitional I-S-type affinity is largely inherited from a mixed igneous–sedimentary magma source. Although abundant zircon U–Pb ages as well as geochemical and isotopic data have been presented for the Lincang pluton, its petrogenesis remains controversial. Most investigators considered the Lincang pluton to be S-type granite according to its geochemical characteristics, such as its peraluminous characteristic, enriched Sr–Nd–Hf isotopic composition, and high K2O/Na2O ratio (Dong et al., 2013; Hennig et al., 2009; Kong et al., 2012; Peng et al., 2013; Wang et al., 2014); however, this is at odds with the presence of amphibole, thus leading to a conclusion of transitional I-S-type affinity (Cong et al., 2020). In addition, the nature of the source material of the Lincang pluton remains in dispute. Proterozoic crust (Deng et al., 2018; Hennig et al., 2009; Kong et al., 2012), Paleozoic sediments (Peng et al., 2013), and the nearby Lancang Group (Dong et al., 2013) have been suggested as source rocks.

In the present study, new zircon U–Pb ages and Hf isotopic compositions, in particular for inherited zircons, and whole-rock elemental and Sr–Nd isotopic data are presented, with the aim of constraining the source components and the cause of the compositional variation of the Lincang pluton. In addition, some implications for granite petrogenesis are discussed.

Section snippets

Geological setting

The Sanjiang Tethyan Orogen, located in the southeastern Tibet Plateau and western Yunnan Province, was amalgamated by several tectonic segments affiliated to the Gondwana continent (Metcalfe, 2013; Zhong, 1998). In western Yunnan, the Sanjiang Tethyan Orogen can be divided into three terranes separated by two ophiolite belts: the Changning–Menglian ophiolite belt (CMOB) in the west and the Jinshajiang–Ailaoshan ophiolite belt (JAOB) in the east (Deng et al., 2014; Jian et al., 2009a, Jian et

Lincang pluton

The Lincang pluton extends ~350 km with a north–south orientation and has an average width of 30 km (Fig. 1b). It intrudes into the Lancang Group and the Manghuai Formation (Li, 1996). Remnants of the Damenglong Group and a Permian granodiorite are preserved in the inner part of the pluton (Li, 1996; Yu et al., 2003). The Lincang pluton mainly consists of amphibole-free biotite monzogranite and granodiorite. Minor amphibole can be found in some outcrops. Alkali-feldspar granite and two-mica

Analytical methods and results

Analytical methods are presented in Table S1. The results from zircon U–Pb dating and trace element analyses are given in Table S2, Table S3. Analytical data of zircon Lu–Hf isotopes, whole-rock major and trace elements, and whole-rock Sr–Nd isotopes are presented in Table 2, Table 3, Table 4.

Pre-magmatic zircons and implications for the magma source

The newly obtained zircon U–Pb data indicate that the Lincang pluton was emplaced at 232–218 Ma, consistent with previous results based on sensitive high-resolution ion microprobe (SHRIMP) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb age investigations (237–203 Ma; Peng et al., 2006, Peng et al., 2013; Jian et al., 2009b; Dong et al., 2013; Deng et al., 2018). This comprehensive data set implies an incremental (multi-batch) assembly of the pluton over

Conclusions

According to the similarities in the age patterns and Hf isotopic compositions of zircons between the Lincang pluton and the Lancang Group, it can be concluded that the Lancang Group served as a magma source for the Lincang pluton.

Melting of mixed igneous–sedimentary rocks of the Lancang Group resulted in the transitional I-S-type characteristics of the Lincang pluton, which can be reconciled with the positive correlation between initial 87Sr/86Sr ratios and 1000/Sr, the negative correlation

Declaration of Competing Interest

None.

Acknowledgments

This study was financially supported by the National Key R&D Program of China (grant 2016YFC0600404) and the National Natural Science Foundation of China (grant 41872049). The authors appreciate two anonymous reviewers for their diligent and constructive comments. We are indebted to Y. Wang and Y.-Z. Yang for field assistance and to P. Xiao, Z.-H. Hou, and Q. Chen for the measurements.

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