Origin of K-feldspar megacrysts in rhyolites from the Emeishan large igneous province, southwest China

Accepted Manuscript Origin of K-feldspar Megacrysts in Rhyolites from the Emeishan Large Igneous Province, Southwest China

Li-Lu Cheng, Yu Wang, Jason S. Herrin, Zhong-Yuan Ren, ZongFeng Yang PII: DOI: Reference:

S0024-4937(17)30369-9 doi:10.1016/j.lithos.2017.10.018 LITHOS 4455

To appear in: Received date: Accepted date:

14 July 2017 24 October 2017

Please cite this article as: Li-Lu Cheng, Yu Wang, Jason S. Herrin, Zhong-Yuan Ren, Zong-Feng Yang , Origin of K-feldspar Megacrysts in Rhyolites from the Emeishan Large Igneous Province, Southwest China. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Lithos(2017), doi:10.1016/j.lithos.2017.10.018

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Origin of K-feldspar Megacrysts in Rhyolites from the Emeishan Large Igneous Province, Southwest China

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Li-Lu Cheng1; Yu Wang2; Jason S. Herrin1,3; Zhong-Yuan Ren4;Zong-Feng Yang2 1, Earth Observatory of Singapore, Nanyang Technological University, 50 Nanyang Av, 639798,

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Singapore

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2, State Key Laboratory of Geological Processes and Mineral Resources, China University of

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Geosciences, Beijing 100083, China

3, Facility for Analysis Characterization Testing and Simulation, Nanyang Technological University,

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50 Nanyang Av, 639798, Singapore

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4, State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese

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Academy of Sciences, Guangzhou 510640, China

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Abstract:

Silicic rocks occur in the uppermost units of the longest volcanic succession (~5000

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m thick) in the Binchuan area of the Permian Emeishan flood basalt province of SW China. They are predominantly rhyolites and to a lesser extent trachytes, both containing potassium feldspar megacrysts as the dominant phenocryst phase up to approximately 20 mm in size. These megacrysts contain domains of albite arranged in vein-like networks, likely formed by post-magmatic alteration. Crystal size distributions (CSD) suggest that these megacrysts grew in a stable magmatic system, consistent with relatively uniform core-to-rim compositional (K2O: ~14-16 wt%) and 1

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Pb/206Pb ratios (~2.06-2.08±0.005). Both whole-rock trace

elements and Pb isotope ratios of these silicic rocks are similar to the Emeishan basalts, suggesting a common source for both mafic and felsic units and a limited role of crustal melting in genesis of the felsic units. Major and trace element models

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further indicate that these rocks could not have formed exclusively by re-melting of

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old crust or solidified basaltic rock, but must have formed through crystal

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fractionation from the flood basalts or possibly partial melting of basaltic rock followed by fractional crystallization. K-feldspar-bearing rhyolites are also observed

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in the last stages of other large igneous provinces. We suggest that they represent final

voluminous magmatic activities.

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melt fractions and their appearance in the magmatic system coincides with waning of

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Keywords: Rhyolite; Large igneous province; Basalt; Fractional crystallization;

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Emeishan

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ACCEPTED MANUSCRIPT 1. Introduction The generation and emplacement of large igneous provinces (LIPs) are anomalous transient igneous events found on all continents, and resulting in rapid and large volume accumulations of volcanic and intrusive igneous rocks (Bryan and Ernst,

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2008; Bryan et al., 2010; Kamenetsky et al., 2012). Significant occurrences of felsic

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volcanism have recently been recognized to be associated with mafic LIPs, with

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rhyolitic volcanism appearing generally late in the sequence and sometimes interbedded with basalts such as those found at Paranã-Etendeka, Karoo-Ferrar, and

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the Deccan Traps (Bryan et al., 2010; Natali et al., 2011). Both the Paranã-Etendeka

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and Karoo-Ferrar provinces are characterized by extensive rhyolite and trachydacite, whereas silicic rocks are only minor components of the Deccan Traps (Mahoney et

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al., 2008). Silicic volcanism in continental volcanic provinces can be attributed to

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some combination of: fractional crystallization (e.g., (Ayalew and Yirgu, 2003; Ewart et al., 2004; Feeley et al., 1998; Mahoney et al., 2008)), re-melting of old crustal

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rocks, remelting of solidified basalt intruded within or underplating the crust (Garland

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et al., 1995; Harris and Milner, 1997; Miller and Harris, 2007), and partial melting followed by variable fractional crystallization (e.g., (Lightfoot et al., 1987)). The Permian Emeishan LIP consists of massive volumes of flood basalts as well as numerous ultramafic/mafic intrusive rocks, andesites, trachytes, rhyolites, granites and syenites. In southwest China, the LIP is largely exposed from the western margin of the Yangtze block to the eastern margin of the Tibetan Plateau (Xu et al., 2001). Within this area, the thickest volcanic succession (~5000 m) in the Binchuan town,

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ACCEPTED MANUSCRIPT and it contains a basalt-andesite-trachyte association. The basalts of this section have been divided into high-Ti (HT) and low-Ti (LT) types based on their Ti/Y ratios (Xu et al., 2001). The presence of varied rock types in the section provides an ideal opportunity to investigate the generation of silicic rocks and the relationships between

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different rock types (Fig.1b illustrates the sequential range of rock types).

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CA-TIMS zircon U-Pb dating of these silicic rocks has yielded a mean age of

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259.1±0.5 Ma, which is interpreted as the termination age of the Emeishan flood basalts (Zhong et al., 2014). This is about one million years after the main eruption

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stage of Emeishan flood basalts at 260 Ma (He et al., 2007). The age of these rhyolites

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makes them a potential source of the widespread clay bed at the middle–upper Permian Guadalupian–Lopingian (G–L) boundary in south China, suggesting a

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possible causal link between the Emeishan eruption and the end-Guadalupian

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biological crisis. The fact that the felsic rocks occur late in the history of flood basalt volcanism may indicate that they could have been produced by partial melting of

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basalt and/or older crustal wall rock. For example, prior work has suggested that they

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are generated by the partial melting of crustal rocks based on then whole-rock geochemical data (Zhang and Wang, 2002). However, (Xu et al., 2010) argued instead that they are the products of fractional crystallization of HT basalts, an inference also based on the whole-rock geochemical data. The final composition of magmatic rocks can be influenced by a variety of complex processes involving variation in initial source composition and conditions of melt generation, mixing of different batch melts, fractional crystallization, crustal

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ACCEPTED MANUSCRIPT contamination, and post-magmatic alteration; making isolation of individual processes related to their genesis a complex endeavor. Thus, whole-rock chemical compositions taken in isolation might not always yield a unique explanation for the origin of felsic magmas. Petrological methods such as crystal size distribution (e.g., (Cashman and

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Marsh, 1988; Cheng et al., 2014b; Higgins, 2011a; Jerram and Martin, 2008; Marsh,

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1988; Mock et al., 2003; Morgan et al., 2007)) and isotopic microanalysis (Cheng et

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al., 2014a; Gagnevin et al., 2005; Morgan et al., 2007) can be used to further constrain petrogenesis, details of crystallization processes, and physical magma environments.

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The Emeishan rhyolites contain a major population of K-feldspar megacrysts up to 20

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mm long. These megacrysts potentially preserve a record of information helpful to understanding the origin of the magma. Through combined geochemical and textural

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studies, we seek to expand our understanding of the relationships between chemical

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and physical processes in the magma, and provide comprehensive insights into the petrogenesis of rhyolites in this unique, predominantly mafic, environment. In this

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study we utilize whole-rock chemical data, mineral compositions, X-ray

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compositional maps, isotopic microanalysis, and crystal size distributions of Kfeldspar megacrysts to investigate the origin of these rhyolites and trachytes and the relationship between mafic and silicic volcanism in the Emeishan LIP.

2. Emeishan Large igneous province The Emeishan LIP is located on the western margin of the Yangtze Craton in southwest China. It covers an estimated area of > 2.5 × 105 km2 (Chung et al., 1998;

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ACCEPTED MANUSCRIPT Xu et al., 2001) and has a total volume of > 0.3 × 106 km3 (Ali et al., 2005; Chung et al., 1998; Xu et al., 2001). The Emeishan volcanism occurred at the middle-late Permian boundary and was concurrent with the end-Guadalupian (~260 Ma) mass extinction (Zhou et al., 2002). The mafic-ultramafic intrusions of the Emeishan LIP

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host Ni-Cu-PGE sulfide deposits and one of the world’s largest Fe-Ti-V oxide

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deposits (Shellnutt et al., 2011; Shellnutt et al., 2009; Zhou et al., 2005). The

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Emeishan volcanic successions unconformably overlie late middle Permian carbonate formations (i.e. the Maokou limestone) and are overlain by uppermost Permian units

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in the east and middle Triassic sediments in the west (Xu et al., 2001). Recently,

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Jerram et al. (2016) demonstrated that the onset of Emeishan flood volcanism was contemporaneous with rapid deepening of the depositional environment of Maokou

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Formation sediments and the extrusion of thick sequences of pillow basalts and

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associated marine sediments. The major igneous rock type of Emeishan flood volcanism is tholeiitic basalt, which represents more than 95% of the LIP.

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Volumetrically minor rock types include alkaline basalts, mafic and ultramafic

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intrusions, and felsic rocks (Xiao et al., 2004). As mentioned in the previous section, the Emeishan basalts have been classified into two geochemical groups: high-Ti (HT) and low-Ti (LT) (Xu et al., 2001). The proposed genetic difference between LT and HT lavas is that the former underwent more extensive crustal contamination. Alternatively, Kamenetsky et al. (2012) suggested that numerous parental magma batches contributed to a diverse spectrum of more differentiated basaltic magmas within the Emeishan LIP, and that peridotite and garnet pyroxenite mantle sources

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ACCEPTED MANUSCRIPT generated the LT and HT endmembers, respectively. Song et al. (2006) concluded that the Emeishan basalts originated from enriched sub-continental lithosphere mantle (SCLM).

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3. Binchuan area and petrographical analysis The Binchuan area is located northeast of the city of Dali, in the west part of

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Emeishan LIP and the east segment of Red-River fault (Fig.1a). This location contains

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a complete flood basalt sequence, which was reported as 5336 m thick (Xu et al.,

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2001). However, this thickness was estimated without considering any tectonic thickening and therefore might be an overestimate (Xiao et al., 2004). At this location,

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the lowermost lava of this succession was deposited unconformably on the early Late

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middle Permian Maokou limestone while the uppermost lava is covered by the upper

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Permian or Lower Triassic sandstone (Xu et al., 2001). The complete composite section consists of six igneous units (Fig.1b), a general

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description of which was given by Xiao et al. (2004). The lower part of the Binchuan lava succession consists entirely of LT basalts (Unit 1 and Unit 2) that are almost

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aphyric and locally hyaloclastic. They contain minor plagioclase (<3%), olivine (13%), and augite (1-2 %) phenocrysts. The groundmass is composed of plagioclase (30–50%), basaltic glass (30–45%) and Ti–Fe oxides (5–10%). The middle section (Unit 3 and Unit 4) is also predominantly LT basalts, but many are more porphyritic than in units 1 and 2. Plagioclase phenocrysts vary from 2 to 10% modal abundance, with small amounts of olivine (1–2%) and augite (1–3%)

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ACCEPTED MANUSCRIPT phenocrysts also present. The porphyritic and aphyric basalts of Unit 3 are interlayered. Note that, in this middle section, some exhibit a glomeroporphyritic texture containing clusters of 5 mm and larger plagioclase phenocrysts. These lithologies are referred to as giant plagioclase basalts (GPB), which are thought to

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contain remobilized crystal cumulates from sub-volcanic magma chambers (Cheng et

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al., 2014b; Higgins and Chandrasekharam, 2007). Xiao et al. (2004) showed that the

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Ti/Y ratios of Units 3 and 4 are higher than those of Units 1 and 2, but slightly lower than that of Unit 5, thus suggesting that Units 3 and 4 represent an LT subgroup.

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The upper parts of Unit 5 basalts have higher phenocryst contents than those in the

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lower part. The dominant phenocryst phase is plagioclase (3–15%), while augite represents only 1–3%. The groundmass is predominantly composed of plagioclase

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(50–60%), microlitic K-feldspars (35–45%), and Ti–Fe oxides (3–5%). Within this

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unit, three thin layers (0.5 to 3 m thick) of rhyolitic tuff are intercalated. Having the highest Ti/Y ratios of all units, Unit 5 belongs to the HT basalts.

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The top of the sequence (Unit 6) is composed of trachyte-rhyolite containing K-

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feldspar phenocrysts (10–25%) with minor albite inclusions and quartz (1-5%) in an aphanitic groundmass. From the bottom to the top of the section there is no clear systematic variation in crystal size. For this study, a total of 10 K-feldspar samples were collected from the rhyolitic sequence. The exact location of each sample is recorded in Supplementary Table 1. Representative petrographic characteristics of the K-feldspar samples are shown in Figure 2. Most of the K-feldspar phenocrysts are resorbed with albite variously

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ACCEPTED MANUSCRIPT occurring at the rim (Fig.2a), core (Fig.2b), and other parts (Fig.2c) of the crystals. Albite also clearly grows along stockwork veins within K-feldspar (Fig.2d). The samples also contain quenched lithic clasts with acicular albite textures (Fig.2e). One K-feldspar megacryst is observed to be surrounded by these lithic inclusions (Fig.2f).

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Cathodoluminescence images were obtained with a CITL CL8200 Mk5-2 cold-

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cathode instrument mounted on a regular petrographic microscope. Different minerals

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may appear in different colors in cathodoluminescence maps (Fig.2g and 2h). The light colors are albite, dark red is K-feldspar and blue is quartz. In Fig.2g, albite is a

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major part of the crystal. Fig.2h shows some quartz crystals are round, probably due

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to post-magmatic alteration. The presence of post-magmatic quartz might have increased bulk rock SiO2 content compared to magmatic compositions. Further,

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amphibole breakdown products were prevalent, forming polyphase pseudomorphs

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that they occur as independent entities or they are overgrown by K-feldspar megacrysts (Fig.2g). The polyphase pseudomorphs are aggregates of quartz, high-

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SiO2 glass, plagioclase, relict amphibole, apatite, ilmenite, titanomagnetite, and small

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amounts of titanite, rutile, chromite, zircon, and allanite. Also, tiny crystals of amphibole, apatite, ilmenite, allanite, titanomagnetite, zircon, and chromite were also observed within the groundmass (Fig.2h).

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ACCEPTED MANUSCRIPT 4. Methods 4.1 Whole-rock geochemical analysis For whole-rock geochemical analysis, a portion of each of the 10 samples was crushed to a 200 mesh in an agate mill. For the trace element analyses, sample

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powders were digested using an HF+HNO3 mixture in high-pressure Teflon bombs at

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190 °C for 48 h. Major and trace elements, respectively, were analyzed by XRF using

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an AXIOS-PW4400 instrument and ICP-MS (ELEMENT) in the Institute of

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Geochemistry, Chinese Academy of Sciences, Guiyang. The analytical precision and accuracy of the major elements and were generally better than 5%. The analytical

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precision was better than 10% for most of the trace elements.

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4.2 Mineral chemical analysis and X-ray elemental maps Samples YN-12-31 and YN-12-29 were studied with a Shimadzu EPMA-1600 at

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the China University of Geosciences Geological Laboratory Center with a focused electron beam (1-2 μm in diameter), a current of 20 nA, and an accelerating voltage of

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15 kV. Results were quantified using well-characterized natural and synthetic calibration standards and a modified ZAF matrix correction procedure (Armstrong, 1988). Background intensity was calibrated based on mean atomic number and continuum absorption was corrected for (Donovan and Tingle, 1996). Oxygen was calculated by cation stoichiometry and included in the matrix correction. An interference correction was applied to Fe for interference by Mn (Donovan et al., 1993), and an absorption correction was applied for carbon coating applied to both 10

ACCEPTED MANUSCRIPT standard and sample for electrical conductivity. Analytical uncertainty at the 99% confidence level was less than 1%. As a quality check, analyses with totals outside the range 98-101 wt.% were omitted. Two samples (YN-12-28 and YN-12-31) were chosen for chemical mapping wavelength-dispersive

spectrometry

(WDS)

conducted

at

Nanyang

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using

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Technological University, with a specimen current of 20 nA and an acceleration

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maps have a resolution of 1024 × 768 pixels.

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voltage of 15 kV. Each map was collected with the dwell time of 20 ms/pixel, and the

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4.3 Isotopic microanalysis

In situ Pb isotope ratios of K-feldspar phenocrysts were determined using LA-

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MC-ICP-MS at the State Key Laboratory of Isotope Geochemistry, Guangzhou

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Institute of Geochemistry, Chinese Academy of Sciences. The procedures were developed using a 193 nm wavelength Repetition M-50-LR Excimer Laser Ablation

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System Laser Ablation system in conjunction with Neptune Plus MC-ICP-MS. Thirty

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large K-feldspar phenocrysts were separated from two crushed samples and mounted in 25 mm rings filled with epoxy. The samples were then polished and cleaned in alcohol and ultrapure water. Analytical conditions involved a pulse rate of 10 Hz, a spot diameter of 155 µm, and counting times of 60 s per peak. To externally correct for mass bias, and to evaluate the accuracy of the instrument prior to analysis, the international standards NKT-1G and BHVO-2G that have similar

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Pb intensities to

our samples were selected. Before and after five points of one sample, the internal 11

ACCEPTED MANUSCRIPT standard (BHVO-2G) was measured to monitor instrument drift. The analytical procedures for in situ Pb isotope analyses are addressed in Ren et al. (2017) and Zhang et al. (2014). All Pb isotopes were analyzed along core-to-rim transects of three large K-feldspar phenocrysts where some portions were albite. Pb isotopes between

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K-feldspar and albite are compared in section 5.4.

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4.4 Crystal size distributions (CSD) and use of large area slab samples

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Three different methods for two different kinds of samples–hand specimens (1020 cm in length) and large thin sections (7-10 cm in length)–were used to determine

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the crystal textures (Supplementary Table 1 and Fig.3). For hand samples, the method generally followed those presented by Higgins and Chandrasekharam (2007). The

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samples were cut into slabs 10-20 cm in length and polished. After the slabs were

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scanned using a conventional document scanner, crystals were outlined in the vectordrafting program (CorelDraw) using a computer mouse. Although the small K-

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feldspar crystals might actually be two-dimensional cross sections cut through the

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margins of larger K-feldspar megacrysts (Cheng et al., 2017), we used 0.25mm as the lower limit for CSD because the smallest portions of the CSD curve are not considered in this study. In addition to polished slabs, large format (7-10 cm) thin sections were analyzed for crystal size distributions in much the same way as the rock slabs to obtain a scanned image, except that a petrographic microscope was used instead to obtain sample images in a method similar to Jerram and Higgins (2007), who also combined different types of CSD data. Because each thin section was larger 12

ACCEPTED MANUSCRIPT than a single field of view, multiple photomicrographs were combined into one large mosaic using PTGui software. After the digitized mosaics were prepared, K-feldspar were identified and outlined. Crystal outlines were then filled and exported as TIFF files (Fig. 3), and the grayscale images were analyzed using the ImageJ software, a

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Java version of the popular program NIHImage. The CSD of the crystals was

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calculated using the newest CSD Corrections 1.54 software. The crystal aspect ratio

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S:I:L (short : intermediate : long) can be used to express the mean crystal shape. However, the crystal aspect ratio is difficult to determine, because random cross

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sectioning will affect two-dimensional crystal shapes (e.g., Cheng et al., 2017;

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Higgins, 1994). The (short : intermediate : long) axes can be determined for each sample using CSDslice image processing software (Morgan and Jerram, 2006). In this

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study, we used CSDslice to calculate the crystal shape of each K-feldspar megacryst.

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Most of these were approximately 1:1.2:1.8 or 1:1.2:2 (Table 1). Using these ratios to calculate CSD plots, we found that the CSD volumes were far from the true volume of

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the phase, which can be affected by the ratio of I/L (e.g., Mock and Jerram, 2005).

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Thus, we chose S:I:L as 1:1.8:1.8 based on the average S/I ratio from all samples and assuming I=L. While the CSD plots were affected only slightly, CSD volumes were in better agreement with phase volumes (Table 1). The roundness was also calculated by CSDslice based on the ratio of minor axis and major axis, with most phenocrysts having a roundness of about 0.6 (Table 1). The orientations of the crystal outlines were also measured. The alignment factor (AF) of the K-feldspar megacrysts of all samples was used to parameterize the

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ACCEPTED MANUSCRIPT alignment of crystal orientations, which was calculated using the 40 largest grains in each sample following the methodology of Boorman et al. (2004) and Williams et al. (2006). The AF has a theoretical maximum value of 100 for a hypothetical sampling of perfectly aligned crystals, whereas a purely massive rock would have a theoretical

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AF value of zero. Our AF values are presented in Table 1.

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5. Results

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5.1 Whole-rock major and trace element data

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The SiO2 content of felsic rocks analyzed in this study ranges from 68% to 76%, slightly higher than reported by other studies (e.g.,, Xu et al., 2010) and straddling the

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rhyolite-trachyte fields of the total alkali silica classification (Supplementary Table 2

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and Fig.4). There is a linear relationship between Al2O3 and SiO2, as well as P2O5 and SiO2 (Fig. 5). However, there is no clear relationship between Na2O or K2O and SiO2.

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The Na2O values range from 1 to 5 wt%, and K2O ranges from 3 to 8 wt%. These are

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the highest K2O values reported from these lithologies. The results of trace element analysis are presented in Supplementary Table 2 and also Fig.5. Primitive mantle normalized REE patterns show that the rhyolite-trachytes exhibit LREE-enriched patterns, and relatively flat patterns of HREE. There are negative Eu anomalies, suggesting fractional crystallization of plagioclase. These patterns are roughly similar to the Emeishan LIP flood basalts, shown as shaded regions in Fig 5i and Fig.5j. Primitive mantle-normalized spider diagrams display 14

ACCEPTED MANUSCRIPT obvious negative anomalies of Sr, Ti and P, a characteristic also seen in the Emeishan LIP flood basalts (Fig 5j).

5.2 Feldspar mineral chemistry and chemical mapping

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The rhyolites contain a major population of K-feldspar megacrysts. The

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compositions of these megacrysts were determined by single point EPMA analyses as

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well as several rim-to-core profiles. The results show that these K-feldspar megacrysts

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have nearly uniform compositions (Or97~Or99), but some part of crystals are replaced by albite (Ab92~Ab97) (Supplementary Table 3). Quenched lithic clasts found in the

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matrix also contain albite.

Chemical maps of three K-spar megacrysts are shown in Fig.6. The top three

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figures are backscattered electron images of these crystals (samples YN-11-28, YN-

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11-28-2, and YN-11-31). The images show that the large K-feldspar crystals contain two distinct grayscale values: a relatively light part (K-feldspar) and a relatively dark

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part (albite). The K contents are shown in Fig.6d, 6e, and 6f, revealing that most parts

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of these crystals have high K content, and some conduit-like veins have lower K content. Fig.6g, 6h, and 6i show that many of the K-poor parts of the crystals have high Na content, and the vein structures also high Na, as in Fig.6h. These high-Na parts are arranged in linear or planar veins, suggesting that the high-Na content could have been caused by exsolution and/or alteration along conduits, leading to the formation of albite (Lee et al., 1995). These veins lack the regular spacing commonly associated with perthitic exsolution in alkali feldspars. 15

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5.3 In situ Pb isotopes of K-feldspar megacrysts Crystal cumulates from earlier magma may be easily recycled when they are mobilized and entrained by later ascending magmas (Marsh, 1996). This crystal

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recycling appears to be common (Martin et al., 2010). To determine whether our

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sample crystals were recycled, isotope microanalysis was used (Martin et al., 2010;

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Morgan et al., 2007), although the uniform composition of the core-to-rim profiles

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with the K-feldspars showed no significant zoning.. The core-to-rim Pb isotope variations within four K-feldspar megacrysts are presented in Supplementary Table 4.

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The results show only a weak shift in Pb isotopic composition from core to rim 208

Pb/206Pb ratios within the K-feldspar

across all four crystals (Fig.7). The

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megacrysts have a total range of ~2.04-2.10, with most spots being around 2.06 to

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2.08 (±0.005, 1SD), increasing slightly toward crystal rims. Crystal YN-12-27(1)-1 (Fig. 7a), which was approximately 4 mm in length, has two zones: the dark part is K-

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feldspar, and the white part is albite. The

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Pb/206Pb ratios of albite are about 2.06,

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while that of K-feldspar are about 2.08. Crystal YN-12-27(1)-2 (Fig. 7b), which is similar in size and texture to YN-12-27(1)-1, has more stable

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Pb/206Pb ratios with

about the same range of 2.06 to 2.08. Crystals YN-12-30(1)-1 (Fig. 7c) and YN-1230(1)-2 (Fig. 11d) both show more variation than YN-12-27(1)-2. In all our samples, K-feldspar and albite are not clearly differentiated. The relative constancy in the Pb isotopic composition of K-feldspar indicates that they are not recycled crystals or antecrysts (Jerram and Martin, 2008). Combined with core-to-rim mineral profiles and 16

ACCEPTED MANUSCRIPT chemical maps, these Pb isotope results are consistent with the notion that albite is likely a product of secondary alteration. Comparing the Pb isotopic composition of Kfeldspar megacrysts to that of Emeishan flood basalts, we find that they are

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indistinguishable, suggesting a common origin.

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5.4 CSD data

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Crystal size distributions (CSD) can offer a statistical approach to understanding

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the crystallization history of crystal populations in magmas. CSDs were determined in six samples from the Binchuan section. The statistics for the CSD analysis are shown

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in Table 1 and Fig.8. The samples all have large crystal populations within or exceeding the 200-250 minimum range for effective statistical analysis (e.g., Mock

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and Jerram, 2005; Morgan and Jerram, 2006).

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Samples YN-12-27 (hand-sized), YN-12-27(1) (large thin section), and YN-1227(1)M (microscope images), all from the same rock sample, were evaluated for

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CSD. The same three naming protocols were followed for the other rock samples YN-

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12-28, YN-12-30 and YN-12-33. When we compare results obtained from the different sampling and imaging types, we see that the results provided by the electron and optical microscope methods show more small crystals than the rock slab method. However, there are still no sub-millimeter crystals represented among all the samples. All of the samples exhibit CSDs that form straight lines on classic CSD diagrams (S-type CSD of Higgins, 2006; Marsh, 1988) (Fig. 8). CSD distributions are easier to understand if the characteristic lengths (=-1/slope) are considered rather than the 17

ACCEPTED MANUSCRIPT slopes themselves. Hence, we use characteristic lengths (CL) in Fig. 9. Since all the CSDs measured are only slightly curved on the S-type CSD diagram (Higgins, 2006; Marsh, 1988), the slope and intercept of these CSDs can be determined using a leastsquares fit. There is a good correlation between the characteristic length calculated

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using the least-squares fit and the characteristic length calculated using

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CSDCorrections1.54. There is a weak negative correlation between crystal volume

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and characteristic length (Fig. 9a).

The alignment factor (AF) data (Fig. 9b and 9c) show only moderate foliation

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among large crystals in the samples, which may reflect magma transport and

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emplacement on the Earth’s surface. However, if a crystal mush was transported from the magma chamber by laminar flow, the relative orientation of the crystal could

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potentially also be preserved. The AF of all samples ranges from 6.55 (poor foliation)

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to 38.08 (moderate foliation) with similar crystal volume, suggesting that transport was turbulent (Fig.9). The AF and crystal length are not clearly correlated, which

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suggests the crystal orientation is not controlled by crystal size (Fig.9b). Neither is

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there a clear relationship between the absolute or relative abundances of REE and alignment factor (Fig.9c), which indicates that magmatic flow is not coincident with fractionation of trace elements.

6. Discussion 6.1 Chemical effects of alteration In using chemical data to analyze the origin of rocks and their constituent 18

ACCEPTED MANUSCRIPT minerals, the effects of alteration must be taken into consideration. Through our investigations using cathodoluminescence and X-ray mapping, we infer that some rounded quartz grains and also some albite associated with K-feldspar likely formed by alteration, and probably represent an infilling of vesicles. Bulk rock concentrations

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of major elements such as Si, Ca, Na and K are, thus, variously affected by alteration.

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These effects can be quantified by estimating the abundance of alteration minerals.

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Based on the modal abundance of alteration phases, namely round quartz grains and sodic regions in K-feldspar, we assume that the primary Si and Na contents of our

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samples are overestimated by 2 wt.% and 0.5 wt.%, respectively, in whole rock

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analyses. We can then correct the whole rock data to reverse the effects of alteration, which will be used for fractional crystallization and batch melting model in section

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6.6. Trace elements such as Rb, Ba are also easily affected by weathering. Alteration

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is not clearly reflected in loss on ignition (LOI) values, which range from 0.60 wt.%

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to 1.72 wt.%.

6.2 Origin of the K-feldspar

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As the dominant phenocryst phase, K-feldspar is potentially useful for understanding petrogenesis of the rhyolite. There are three possible scenarios for how albite within the K-feldspar megacrysts formed: the first is that the albite and Kfeldspars were cotectic; the second is that albite formed by subsolidus exsolution (perthite); and the third is that albite was produced by alteration after cessation of magmatic processes. Although some K-feldspar crystals exhibit localized enrichment

19

ACCEPTED MANUSCRIPT in Na and conversion to albite, we suggest that these different compositions are caused by alteration based on the mineral compositional profiles, chemical maps, and CSD results mentioned in previous sections. X-ray maps show that the albite grew along vein-like networks. Core-to-rim profiles and lack of zonation in chemical maps

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show that the K-feldspar grew in a stable environment, in agreement with our

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assessment of CSD. Textural coarsening is an important petrologic process, which

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may be produced by temperature cycling caused by new injections of magma (e.g., Cheng et al., 2014a; Cheng et al., 2014b; Higgins, 2011a). Our samples only lack

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crystals smaller than 0.25 mm, representing a deviation from a clear pattern of

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textural coarsening (Higgins, 2011b). This means that these silicic magmas may have experienced limited injections. The megacrysts grew in a static environment, and

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albite formed later by post-magmatic alteration. The Pb isotopic data also shows that

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these large feldspars are likely not antecrysts or xenocrysts, and they are likely petrogenetically related to the Emeishan flood basalts (Fig.10), which are clearly

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mantle-derived.

6.3 Relationship between textural and chemical characteristics As Fig.11 shows, the volume fraction of K-feldspar megacrysts ranges from 18% to 22%, but the characteristic lengths of samples ranges from 1.1 mm to 1.5 mm, with neither showing large variation. The relationship between REE patterns and Kfeldspar volume is not clear, which indicates the fractionation of REE is not governed by a process linked to K-feldspar abundance. Significant fractionation of minor and 20

ACCEPTED MANUSCRIPT trace elements is likely due to variation in the modal abundance of accessory minerals, such as apatite, zircon, and rare earth minerals (Rollinson, 1993), all of which were observed as minor or trace phases. The ratios of La/Nb, Zr/Hf also do not show clear correlations with K-feldspar abundance, suggesting that these ratios not

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fractionated by crystallization. If we assume a uniform growth rate for K-feldspar in

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all of the studied samples, the increase of crystal length indicates an increase of the K-

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feldspar crystallization time. However, there is a negative linear relationship between Zr/Hf and crystal length. This means that some element ratios change with K-feldspar

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growth. Some processes, such as accumulation fluids or mixing, will affect both rock

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textures and some element ratios (e.g., (Yang, 2012)). However, in our case, textural characteristics are fairly uniform suggesting that these K-feldspars grew in a static

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environment.

6.4 Remelting of older crustal rock

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As noted in previous sections, both extreme crystal fractionation (with or without

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assimilation of wall rock) and melting of crustal rocks (old crust and/or solidified basalt) have been proposed as contributing processes to the formation of rhyolite in basaltic continental volcanic provinces (e.g., Ayalew and Yirgu, 2003; Ewart et al., 2004; Feeley et al., 1998; Garland et al., 1995; Harris and Milner, 1997; Mahoney et al., 2008). For crustal melting, numerical modeling suggests that rhyolite could be produced by repeated basalt input over several million years, and partial melting of basalt and/or older crustal wall rock will occur simultaneously with the generation of 21

ACCEPTED MANUSCRIPT highly fractionated liquids by crystallization of basaltic magma (Annen et al., 2006; Annen and Sparks, 2002). Subsequently, mixing between crustal melts and basaltic magma, or late stage basalt differentiates, could occur. We might expect such a process could be recorded in phenocrysts textures and chemical heterogeneities.

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The major and trace element compositions of whole-rocks and the K-feldspar Pb

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isotopic data all show that these rhyolites should have the same source as the basalts.

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Thus, if they formed from remelting of old crustal rocks during ascent of the basalts, the older crustal rocks should have the similar source as the basalt in order to produce

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the observed similarities in the chemical and isotopic composition between the

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rhyolites and the basalts. As mentioned in the previous section, the Pb isotopic compositions of K-feldspar megacrysts are identical to the Emeishan flood basalts. Xu

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et al. (2010) have suggested that partial melting of lower crust made up of basic

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granulites would normally produce metaluminous and calc-alkaline magmas with low Rb/Ba ratios, but such magmas are also inconsistent with nature of the Binchuan

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samples. Thus, the sum of observations suggests that the rhyolite did not form from

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melting of older crustal rock.

6.5 Remelting of solidified basalt The felsic rocks have similar chemical characteristics as the rest of Emeishan LIP, implying that they were derived from similar parentage rather than being derived from crustal rocks. It must be considered, however, that solidified basaltic magma residing in the crust from earlier stages of the Emeishan LIP might also experience partial 22

ACCEPTED MANUSCRIPT melting and therefore represent a potential contribution to the silicic rocks studied. Giant plagioclase basalts (GPB) can be found in abundance in the middle part of the Binchuan section, suggesting at least some basaltic magmas experienced protracted histories of crystallization during crustal residence (Cheng et al., 2014b; Higgins and

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Chandrasekharam, 2007). Thus, it is also likely that much of the magmatic flux

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originating from the mantle might never have reached the surface and ultimately

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solidified within the crust. As these still-warm intrusions were subsequently intruded by further generations of mafic magmas, they might have heated to partial melting

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temperatures to produce more silicic second-generation melts.

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To test this possibility, a method developed by Peccerillo et al. (2003) was used based on compatible elements (Fig.12a and 12b). Elements with high partition

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coefficients are depleted rapidly during fractional crystallization, while they maintain

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nearly comparable concentrations between melt and rest at low to intermediate degrees of melting (Jónasson et al., 1992). Fractional crystallization is thus more

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efficient at producing compatible element depletion than producing incompatible

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element enrichment. Partial melting, meanwhile, produces liquids with moderate depletion in compatible elements and variable enrichment in incompatible elements (e.g., Peccerillo et al., 2003; Xu et al., 2010). For basalts, Sr and V behave compatibly during fractionation and melting, whereas Zr commonly behaves as an incompatible element. A high-Ti basalt sample from the Binchuan section (Sample EM-52 from Xu et al. (2001)) was used to represent the original Sr, V and Zr composition for our models. The results of batch and fractional melting models demonstrate that Sr, V and

23

ACCEPTED MANUSCRIPT Zr variations in the Emeishan rhyolite cannot be produced by batch melting of basalts alone when a bulk partition coefficient of 2.5 is used for Sr. To best match the data, a higher Dsr (>5) has to be assumed. This value is unrealistic, as it would require pure plagioclase, which has a high partition coefficient for Sr (see Supplementary Table 5),

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in the residua. Zr versus V systematics also demonstrate that partial melting of basalts,

SC

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considered in isolation, could not produce rhyolites (Fig.12b).

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6.6 Fractional crystallization

Elimination of the crustal melting and basaltic partial melting models leaves the

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alternative of fractional crystallization as the principle process for genesis of the Emeishan rhyolites, or a possible combination of both fractional crystallization and

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batch melting. This inference is consistent with the results presented in Fig.12c and

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12d, showing no clear variation in La/Nb and Th/Yb with increasing SiO2, as would be expected for a system evolving primarily by fractional crystallization with limited

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crustal contamination.

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Genesis of felsic magmas by fractional crystallization can be evaluated by leastsquares mass balance methods (Wright and Doherty, 1970). We model fractional crystallization and batch melting followed by fractional crystallization using basalt sample DY-5 (Xiao et al., 2004), an HT basalt with close spatial and temporal proximity to felsic rocks, and with comparatively high SiO2 content. Sample YN-31 was selected as representative of final melts because its composition is close to the average value of all felsic samples. Minerals used for fractionation modeling were 24

ACCEPTED MANUSCRIPT based on observed phases and their compositions are listed in the Appendix Supplementary Table 5a and obtained from Xu et al. (2010). Modeling results show that the major element compositions of these rock can be derived from sample DY-5 by crystal fractionation involving 1.84 % olivine, 24.52 %

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clinopyroxene, 31.93 % plagioclase, 15.14 % magnetite, and 0.9 % apatite, with the

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sum of squares of the residuals of 3.9, showing a reasonable match.

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Modeled trace element patterns produced by fractional crystallization and batch melting followed by fractional crystallization are shown in Fig.12e and 12f. We

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assume a single bulk fractionating assemblage and a single set of constant mineral-

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melt distribution coefficients. Published distribution coefficients for a given rock type vary considerably for some elements and those selected are presented in

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Supplementary Table 5b. Our results show that it is difficult to discern whether

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rhyolites are produced by fractional crystallization alone, or by partial melting of basalt followed by fractional crystallization. However, if we consider the physical

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conditions of LIP emplacement, it is likely that new basaltic magma might traverse

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similar conduit pathways where earlier basaltic magma was stored and solidified. Partial melting of basalt is likely, and even necessary, when the heat source provided persists over millions of years, as is the case for this and most LIP (Annen et al., 2006; Annen and Sparks, 2002).

6.7 Implications for large igneous provinces In Emeishan LIP, the most voluminous mantle-derived basalts erupted over a 25

ACCEPTED MANUSCRIPT relatively short time span of 1 million years. We can hypothesize that some fraction of the total magma flux might quickly traverse the crust while other magmas might have been stored at different levels within the crust, occupying conduits between successive pulses of magmatism. As described in section 3, GPB that occur in the

PT

middle part of Binchuan section are thought to be evidence of sub-volcanic magma

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chambers (Cheng et al.; 2014b). The GPB of Emeishan LIP belong to the high-Ti

SC

basalt group, and appear to have been stored in the shallow crust at approximately 10 km depth, based on plagioclase equilibrium (Cheng et al., 2014b). With progressive

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crystallization and increasing crustal residence time, the fate of some magma is to

MA

solidify within the crust. In the case of LIPs, early solidified magmas would be followed by large volumes of new magmas. The high heat flux of these events might

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facilitate partial melting of solidified gabbros, and that these new secondary and

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hybrid magmas might ascend and be stored in upper crustal levels, as suggested by (Annen et al., 2006; Annen and Sparks, 2002). Thus, the rhyolite could evolve from

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these secondary or hybrid magmas in a higher level chamber than the GPB, an

AC

inference supported by the heterogeneous crustal structure (Liu et al., 2001). This also indicates that the magma of the Emeishan LIP sometimes ascended more slowly and accumulated at depth during times of lower magma supply rate. Subsequent fractional crystallization further produced the giant K-feldspars and rhyolite in the waning stages of magmatism. It is difficult for these silicic magmas bearing K-feldspar to then erupt, owing to their high viscosity and crystallinity. Thus, the volume of these rocks is considerably smaller than that of the basalts. Without enough new magma

26

ACCEPTED MANUSCRIPT injection, the rhyolites became partially solidified and were only formed close to the final stages of the Emeishan flood basalts. Such K-feldspar-bearing rhyolites are also found in other large igneous provinces, such as the Ethiopian continental flood basalts (~30 Ma, > 3 × 105 km3 (Ayalew et al., 2002)), where they might have formed by

PT

similar processes. They are generally found on top of the flood basalt sequence,

RI

indicating a prevalence late in the lifespan of these provinces and an association with

SC

waning magmatism (Ayalew et al., 2002). As the driving forces of continental LIP magmatism are often poorly understood, K-feldspar rhyolites potentially preserve a

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record of the cessation of LIP magmatism that might help us to understand the process

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as a whole.

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7. Conclusions

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Evolved rocks within a large igneous province may be the result of a variety of crustal processes, such as polybaric fractional crystallization, partial melting, and

CE

magma mixing. These processes contribute to the compositional diversity inherited from mantle melting. However, these processes may also be obscured by pre-eruption

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mixing of melt batches which tends to average the compositions of the erupted magmas, thus increasing the complexity of determining the origin and diversity of different rock types. Petrological and textural analysis can lend insight, and should be used in addition to bulk rock geochemical analysis. Laser ablation MC-ICP-MS Pb isotopic analyses, EPMA, and x-ray chemical mapping of K-feldspar megacrysts presented in this study offer petrogenetic insight into the formation of these silicic

27

ACCEPTED MANUSCRIPT rocks. CSD studies reveal their crystallization history, suggesting that these rhyolites containing large K-feldspar megacrysts likely grew in a static quiescent environment. Major and trace elements models show that these silicic rocks could not be derived solely by partial melting of basaltic rocks or old crust, but instead must have formed

PT

by fractional crystallization or batch melting, followed by the crystal fractionation

RI

within the context of the larger, dominantly basaltic, Emeishan LIP. The occurrence of

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K-feldspar rhyolites coincides with waning of magmatic flux at Emeishan, and also at

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other continental LIP.

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Acknowledgments

This is part of the Ph.D. thesis of L.L., who would like to thank Zhao-Hua Luo

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for his collaboration and support during the early stages of the project. Discussions

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with Fidel Costa, Mary Reid, Jiu-Long Zhou and Le Zhang are gratefully acknowledged. We also thank Pavel Adamek for the suggestions on English writing

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on the manuscript. Reviews by M. Higgins, D. Jerram, and editorial handling and

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reviews of A. Kerr significantly improved this manuscript and are greatly appreciated. This research was supported by the National Basic Research Program of China (973 Program NO. 2011CB808901) and the Singapore Ministry of Education under the Research Centres of Excellence initiative, under the “Crystal pattern” project.

28

ACCEPTED MANUSCRIPT Caption

Figure 1(a) Location of our study area. Basalts are shown in gray, with the yellow star indicating the location of the Binchuan section. (b) Within the six units in the

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Binchuan section, Units 1 to Unit 4 belong to low-Ti (LT) basalt group, while Unit 5

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is classed as high-Ti (HT) basalt. Unit 6 is the rhyolite studied herein. Figure after

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Xu et al. (2001) and Xiao et al.,(2006).

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Figure 2 (a) K-feldspar is about 3 mm in length, with a K-feldspar core and an albite

MA

rim (Ab). (b) The core is albite, while the outer part is K-feldspar. (c) Similar to crystal seen in (a) with the central part being K-feldspar rimmed by albite. (d) Albite

is

entirely

surrounded

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K-feldspar

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(white) growth along the vein. (e) Quenched plagioclase near the K-feldspar. (f) One by

quenched

inclusion.

(g)

and

(h)

Cathodoluminescence images of rhyolite from Bianchuan section. The light cream

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color is albite (Ab), the dark red is K-feldspar (Kfs), and the blue is quartz (Qz). The

AC

dark phase with small pale crystals is amphibole with apatite.

Figure 3 (a) A cm-scale hand specimen of a giant K-feldspar rhyolite. (c) A thin section scanned image example of the giant K-feldspar rhyolite. (e) An example of an image of the giant K-feldspar rhyolite sample made by combining multiple petrographic microscope images. Figures (b), (d) and (f) show only the K-feldspar phenocrysts – colored in black – for sample images (a), (c), and (e), respectively.

29

ACCEPTED MANUSCRIPT Figure 4 Samples from the Binchuan section studied here are chemically classified as rhyolites and trachytes using the total alkali-silica systematic of Le Bas et al. (1986).

Figure 5 Relationships between trachyte and rhyolite data from our study (the red

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squares) correspond to the data from Xu et al. (2001) and Xiao et al. (2006) (the black

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circles and triangles). (a)-(h) Black solid squares show the high-Ti basalts; white

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squares are the low-Ti basalts; circles represent trachytes; and the triangles represent rhyolites. (i) and (j) Bulk rock trace element compositions of rhyolites from the

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Binchuan section normalized to primitive mantle as in Sun and McDonough (1989).

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The shaded region represents the entire range for the trace element distributions in the

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Emeishan LIP flood basalts (Xu et al., 2001; Xiao et al., 2004).

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Figure 6 (a) A backscatter electron (BSE) image of K-feldspar in YN-11-28. (b) A BSE image of a K-feldspar in YN-11-28-2 that shows a lot of veins within the crystal.

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(c) A BSE image of K-feldspar in YN-11-31. Figures (d) to (l) show EPMA WDS X-

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ray chemical maps for each crystals (a), (b), and (c), respectively; Figures (d), (e), and (f) show variations in K. Figures (g), (h), and (i) show distribution of Na. Notably, maps for the crystals shown in (a) and (c) show several dark gray areas that contain virtually no K and that are enriched in Na, representing albite.

Figure 7 The upper part of these figures shows photomicrographs of the feldspar crystals (a) YN-12-27(1)-1; (b) YN-12-27(1)-2; (c) YN-12-30(1)-1; and (d) YN-12-

30

ACCEPTED MANUSCRIPT 30(1)-2. Below each image is a figure showing the core-to-rim Pb isotopes compositions. Analysis numbers in the

208

Pb/206Pb plots representing locations are 208

shown as numbered dots on the megacryst photographs. The error of the

Pb/206Pb

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value is ±0.005.

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Figure 8 The nearly straight line in our CSDs means the crystals may have grown in a

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stable magmatic system. The green line is the distribution of hand specimen scale sample; the red line is the distribution of thin section scale sample; and the blue line is

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the distribution of mosaicing multiple petrographic microscope images sample.

Figure 9 (a) There is no clear relationship between characteristic length (CL) and

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volume of the K-feldspar. (b) There is no clear relationship between CL and

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alignment factor (AF). (c) Generally, the REE does not change with alignment factor (AF). AF has a theoretical maximum value of 100 for perfectly aligned crystals,

AC

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whereas purely massive rocks have an AF value of zero.

Figure 10 (a) The relationship between

207

Pb/204Pb and

similar to Emeishan OIB. (b) The relationship between

206

208

Pb/204Pb of our sample is

Pb/204Pb and

206

Pb/204Pb of

our sample is similar to Emeishan OIB. Blue diamonds are all the data for basalts from Zhang et al. (2006). The red squares are the data from this study.

Figure 11 There are no clear relationships between trace elements ratios and volumes

31

ACCEPTED MANUSCRIPT or between trace elements ratios and characteristic lengths (CL) for all the samples studied.

Figure 12 (a) The relationship between Zr and Sr shows our sample may have been

PT

produced by crystal fraction. (b) The relationship between Zr and V also shows they

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may have been produced by crystal fraction. (c) The relationship between La/Nb and

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SiO2 shows no clear crustal contamination. (d) The relationship of Th/Yb and SiO2 also shows no clear contamination. The red squares are rhyolites in this study; the

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black squares are HT basalts from Xu et al. (2001) and Xiao et al. (2006); and the

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black triangles are rhyolites from Xu et al. (2010). (e) Trace element patterns of our samples could be produced by the crystal fractionation model. (f) Trace element

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patterns of our samples could also have been produced by remelting followed by

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crystal fractionation model. The red line shows the model results, and the black lines

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show the range of the real data.

Reference Ali, J.R., Thompson, G.M., Zhou, M.-F. and Song, X., 2005. Emeishan large igneous province, SW China. Lithos, 79(3–4): 475-489.

32

ACCEPTED MANUSCRIPT Annen, C., Blundy, J.D. and Sparks, R.S.J., 2006. The Genesis of Intermediate and Silicic Magmas in Deep Crustal Hot Zones. Journal of Petrology, 47(3): 505-539. Annen, C. and Sparks, R.S.J., 2002. Effects of repetitive emplacement of basaltic intrusions on thermal evolution and melt generation in the crust. Earth and Planetary Science Letters, 203(3–4): 937-955. Armstrong, J., 1988. Quantitative analysis of silicate and oxide minerals: comparison of Monte Carlo, ZAF and phi-rho-z procedures. Microbeam analysis, 23: 239-246. Ayalew, D., Barbey, P., Marty, B., Reisberg, L., Yirgu, G. and Pik, R., 2002. Source, genesis, and timing of giant ignimbrite deposits associated with Ethiopian continental flood basalts. Geochimica et

PT

Cosmochimica Acta, 66(8): 1429-1448.

Ayalew, D. and Yirgu, G., 2003. Crustal contribution to the genesis of Ethiopian plateau rhyolitic

RI

ignimbrites: basalt and rhyolite geochemical provinciality. Journal of the Geological Society, 160(1): 47-56.

SC

Boorman, S., Boudreau, A. and Kruger, F.J., 2004. The Lower Zone–Critical Zone Transition of the Bushveld Complex: a Quantitative Textural Study. Journal of Petrology, 45(6): 1209-1235. Bryan, S.E. and Ernst, R.E., 2008. Revised definition of Large Igneous Provinces (LIPs). Earth-Science

NU

Reviews, 86(1–4): 175-202.

Bryan, S.E., Peate, I.U., Peate, D.W., Self, S., Jerram, D.A., Mawby, M.R., Marsh, J.S. and Miller, J.A., 2010. The largest volcanic eruptions on Earth. Earth-Science Reviews, 102(3–4): 207-229.

MA

Cashman, K.V. and Marsh, B.D., 1988. Crystal size distribution (CSD) in rocks and the kinetics and dynamics of crystallization II: Makaopuhi lava lake. Contributions to Mineralogy and Petrology, 99(3): 292-305.

Cheng, L., Costa, F. and Carniel, R., 2017. Unraveling the presence of multiple plagioclase populations

D

and identification of representative two-dimensional sections using a statistical and numerical approach. American Mineralogist, 102(9): 1894-1905.

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Cheng, L., Zeng, L., Ren, Z., Wang, Y. and Luo, Z., 2014a. Timescale of emplacement of the Panzhihua gabbroic layered intrusion recorded in giant plagioclase at Sichuan Province, SW China. Lithos, 204(0): 203-219.

Cheng, L.-L., Yang, Z.-F., Zeng, L., Wang, Y. and Luo, Z.-H., 2014b. Giant plagioclase growth during

CE

storage of basaltic magma in Emeishan Large Igneous Province, SW China. Contributions to Mineralogy and Petrology, 167(2): 1-20. Chung, S.L., Jahn, B., Genyao, W., Lo, C.H. and Bolin, C., 1998. The Emeishan flood basalt in SW China:

AC

A mantle plume initiation model and its connection with continental breakup and mass extinction at the Permian-Triassic Boundary, Mantle Dynamics and Plate Interactions in East Asia. Geodyn. Ser. AGU, Washington, DC, pp. 47-58. Donovan, J.J., Snyder, D.A. and Rivers, M.L., 1993. An improved interference correction for trace element analysis. Microbeam analysis, 2: 23-28. Donovan, J.J. and Tingle, T.N., 1996. An improved mean atomic number background correction for quantitative microanalysis. Microscopy and Microanalysis, 2(01): 1-7. Ewart, A., Marsh, J.S., Miller, S.C., Duncan, A.R., Kamber, B.S. and Armstrong, R.A., 2004. Petrology and Geochemistry of Early Cretaceous Bimodal Continental Flood Volcanism of the NW Etendeka, Namibia. Part 2: Characteristics and Petrogenesis of the High-Ti Latite and High-Ti and Low-Ti Voluminous Quartz Latite Eruptives. Journal of Petrology, 45(1): 107-138. Feeley, T., Dungan, M. and Frey, F., 1998. Geochemical constraints on the origin of mafic and silicic

33

ACCEPTED MANUSCRIPT magmas at Cordon El Guadal, Tatara-San Pedro Complex, central Chile. Contributions to Mineralogy and Petrology, 131(4): 393-411. Gagnevin, D., Daly, J.S., Poli, G. and Morgan, D., 2005. Microchemical and Sr Isotopic Investigation of Zoned K-feldspar Megacrysts: Insights into the Petrogenesis of a Granitic System and Disequilibrium Crystal Growth. Journal of Petrology, 46(8): 1689-1724. Garland, F., Hawkesworth, C.J. and Mantovani, M.S.M., 1995. Description and Petrogenesis of the Paraná Rzhyolites, Southern Brazil. Journal of Petrology, 36(5): 1193-1227. Harris, C. and Milner, S., 1997. Crustal Origin for the Paraná Rhyolites: Discussion of ‘Description and Petrogenesis of the Paraná Rhyolites, Southern Brazil’ by Garland et al. (1995). Journal of

PT

Petrology, 38(2): 299-302.

He, B., Xu, Y.-G., Huang, X.-L., Luo, Z.-Y., Shi, Y.-R., Yang, Q.-J. and Yu, S.-Y., 2007. Age and duration of

RI

the Emeishan flood volcanism, SW China: Geochemistry and SHRIMP zircon U–Pb dating of silicic ignimbrites, post-volcanic Xuanwei Formation and clay tuff at the Chaotian section.

SC

Earth and Planetary Science Letters, 255(3–4): 306-323.

Higgins, M.D., 1994. Determination of crystal morphology and size from bulk measurements on thin sections: numerical modelling. American Mineralogist, 79: 113-119.

NU

Higgins, M.D., 2000. Measurement of crystal size distributions. American Mineralogist, 85(9): 11051116.

Higgins, M.D., 2006. Verification of ideal semi-logarithmic, lognormal or fractal crystal size

MA

distributions from 2D datasets. Journal of Volcanology and Geothermal Research, 154: 8-16. Higgins, M.D., 2011a. Quantitative petrological evidence for the origin of K-feldspar megacrysts in dacites from Taapaca volcano, Chile. Contributions to Mineralogy and Petrology, 162(4): 709723.

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Higgins, M.D., 2011b. Textural coarsening in igneous rocks. International Geology Review, 53(3-4): 354376.

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Higgins, M.D. and Chandrasekharam, D., 2007. Nature of Sub-volcanic Magma Chambers, Deccan Province, India: Evidence from Quantitative Textural Analysis of Plagioclase Megacrysts in the Giant Plagioclase Basalts. Journal of Petrology, 48(5): 885-900. Jerram, D.A. and Higgins, M.D., 2007. 3D Analysis of Rock Textures: Quantifying Igneous

CE

Microstructures. Elements, 3(4): 239-245. Jerram, D.A. and Martin, V.M., 2008. Understanding crystal populations and their significance through the magma plumbing system. Geological Society, London, Special Publications, 304(1): 133-

AC

148.

Jerram, D.A., Widdowson, M., Wignall, P.B., Sun, Y., Lai, X., Bond, D.P.G. and Torsvik, T.H., 2016. Submarine palaeoenvironments during Emeishan flood basalt volcanism, SW China: Implications for plume–lithosphere interaction during the Capitanian, Middle Permian (‘end Guadalupian’) extinction event. Palaeogeography, Palaeoclimatology, Palaeoecology, 441: 6573. Jónasson, K., Holm, P.M. and Pedersen, A.K., 1992. Petrogenesis of silicic rocks from the Króksfjördur central volcano, NW Iceland. Journal of Petrology, 33(6): 1345-1369. Kamenetsky, V.S., Chung, S.-L., Kamenetsky, M.B. and Kuzmin, D.V., 2012. Picrites from the Emeishan Large Igneous Province, SW China: a Compositional Continuum in Primitive Magmas and their Respective Mantle Sources. Journal of Petrology, 53(10): 2095-2113. Le Bas, M.L., Maitre, R.L., Streckeisen, A. and Zanettin, B., 1986. A chemical classification of volcanic

34

ACCEPTED MANUSCRIPT rocks based on the total alkali-silica diagram. Journal of petrology, 27(3): 745-750. Lee, M.R., Waldron, K.A. and Parsons, I., 1995. Exsolution and alteration microtextures in alkali feldspar phenocrysts from the Shap granite. Mineralogical Magazine, 59(1): 63-78. Lightfoot, P., Hawkesworth, C. and Sethna, S., 1987. Petrogenesis of rhyolites and trachytes from the Deccan Trap: Sr, Nd and Pb isotope and trace element evidence. Contributions to Mineralogy and Petrology, 95(1): 44-54. Liu, J., Liu, F., He, J., Chen, H. and You, Q., 2001. Study of seismic tomography in Panxi paleorift area of southwestern China. Science in China Series D: Earth Sciences, 44(3): 277-288. Mahoney, J.J., Saunders, A.D., Storey, M. and Randriamanantenasoa, A., 2008. Geochemistry of the

PT

Volcan de l’ Androy Basalt–Rhyolite Complex, Madagascar Cretaceous Igneous Province. Journal of Petrology, 49(6): 1069-1096.

RI

Marsh, B.D., 1988. Crystal size distribution (CSD) in rocks and the kinetics and dynamics of crystallization. Contributions to Mineralogy and Petrology, 99(3): 277-291.

SC

Marsh, B.D., 1996. Solidification fronts and magmatic evolution. Mineralogical Magazine, 60(1): 5-40. Martin, V.M., Davidson, J., Morgan, D. and Jerram, D.A., 2010. Using the Sr isotope compositions of feldspars and glass to distinguish magma system components and dynamics. Geology, 38(6):

NU

539-542.

Miller, J.A. and Harris, C., 2007. Petrogenesis of the Swaziland and Northern Natal Rhyolites of the Lebombo Rifted Volcanic Margin, South East Africa. Journal of Petrology, 48(1): 185-218.

MA

Mock, A. and Jerram, D., 2005. Crystal size distributions (CSD) in three dimensions: insights from the 3D reconstruction of a highly porphyritic rhyolite. Journal of Petrology, 46(8): 1525-1541. Mock, A., Jerram, D.A. and Breitkreuz, C., 2003. Using Quantitative Textural Analysis to Understand the Emplacement of Shallow-Level Rhyolitic Laccoliths—a Case Study from the Halle Volcanic

D

Complex, Germany. Journal of Petrology, 44(5): 833-849. Morgan, D.J. and Jerram, D.A., 2006. On estimating crystal shape for crystal size distribution analysis.

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Journal of Volcanology and Geothermal Research, 154(1–2): 1-7. Morgan, D.J., Jerram, D.A., Chertkoff, D.G., Davidson, J.P., Pearson, D.G., Kronz, A. and Nowell, G.M., 2007. Combining CSD and isotopic microanalysis: Magma supply and mixing processes at 431.

CE

Stromboli Volcano, Aeolian Islands, Italy. Earth and Planetary Science Letters, 260(3–4): 419Natali, C., Beccaluva, L., Bianchini, G. and Siena, F., 2011. Rhyolites associated to Ethiopian CFB: Clues for initial rifting at the Afar plume axis. Earth and Planetary Science Letters, 312(1–2): 59-68.

AC

Peccerillo, A., Barberio, M.R., Yirgu, G., Ayalew, D., Barbieri, M. and Wu, T.W., 2003. Relationships between Mafic and Peralkaline Silicic Magmatism in Continental Rift Settings: a Petrological, Geochemical and Isotopic Study of the Gedemsa Volcano, Central Ethiopian Rift. Journal of Petrology, 44(11): 2003-2032. Ren, Z.-Y., Wu, Y.-D., Zhang, L., Nichols, A.R.L., Hong, L.-B., Zhang, Y.-H., Zhang, Y., Liu, J.-Q. and Xu, Y.G., 2017. Primary magmas and mantle sources of Emeishan basalts constrained from major element, trace element and Pb isotope compositions of olivine-hosted melt inclusions. Geochimica et Cosmochimica Acta, 208: 63-85. Rollinson, H.R., 1993. Using geochemical data: evaluation, presentation, interpretation. Shellnutt, J.G., Wang, K.-L., Zellmer, G.F., Iizuka, Y., Jahn, B.-M., Pang, K.-N., Qi, L. and Zhou, M.-F., 2011. Three Fe-Ti oxide ore-bearing gabbro-granitoid complexes in the Panxi region of the Permian Emeishan large igneous province, SW China. American Journal of Science, 311(9):

35

ACCEPTED MANUSCRIPT 773-812. Shellnutt, J.G., Zhou, M.-F. and Zellmer, G.F., 2009. The role of Fe–Ti oxide crystallization in the formation of A-type granitoids with implications for the Daly gap: An example from the Permian Baima igneous complex, SW China. Chemical Geology, 259(3–4): 204-217. Song, X.-Y., Zhou, M.-F., Keays, R., Cao, Z.-M., Sun, M. and Qi, L., 2006. Geochemistry of the Emeishan flood basalts at Yangliuping, Sichuan, SW China: implications for sulfide segregation. Contributions to Mineralogy and Petrology, 152(1): 53-74. Sun, S.-s. and McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. Geological Society, London, Special

PT

Publications, 42(1): 313-345.

Williams, E., Boudreau, A.E., Boorman, S. and Kruger, F.J., 2006. Textures of orthopyroxenites from the

RI

Burgersfort bulge of the eastern Bushveld Complex, Republic of South Africa. Contributions to Mineralogy and Petrology, 151(4): 480-492.

SC

Wright, T.L. and Doherty, P.C., 1970. A linear programming and least squares computer method for solving petrologic mixing problems. Geological Society of America Bulletin, 81(7): 1995-2008. Xiao, L., Xu, Y.G., Mei, H.J., Zheng, Y.F., He, B. and Pirajno, F., 2004. Distinct mantle sources of low-Ti

NU

and high-Ti basalts from the western Emeishan large igneous province, SW China: implications for plume–lithosphere interaction. Earth and Planetary Science Letters, 228(3– 4): 525-546.

MA

Xu, Y., Chung, S.-L., Jahn, B.-m. and Wu, G., 2001. Petrologic and geochemical constraints on the petrogenesis of Permian–Triassic Emeishan flood basalts in southwestern China. Lithos, 58(3– 4): 145-168.

Xu, Y.-G., Chung, S.-L., Shao, H. and He, B., 2010. Silicic magmas from the Emeishan large igneous

D

province, Southwest China: Petrogenesis and their link with the end-Guadalupian biological crisis. Lithos, 119(1–2): 47-60.

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Yang, Z.-F., 2012. Combining Quantitative Textural and Geochemical Studies to Understand the Solidification Processes of a Granite Porphyry: Shanggusi, East Qinling, China. Journal of Petrology, 53(9): 1807-1835.

Zhang, L., Ren, Z.-Y., Nichols, A.R., Zhang, Y.-H., Zhang, Y., Qian, S.-P. and Liu, J.-Q., 2014. Lead isotope

CE

analysis of melt inclusions by LA-MC-ICP-MS. Journal of Analytical Atomic Spectrometry, 29(8): 1393-1405.

Zhang, Z., Mahoney, J.J., Mao, J. and Wang, F., 2006. Geochemistry of Picritic and Associated Basalt

AC

Flows of the Western Emeishan Flood Basalt Province, China. Journal of Petrology, 47(10): 1997-2019.

Zhang, Z. and Wang, F., 2002. Geochemistry of Two Types of Basalts in the Emeishan Basaltic Province: Evidence for Mantle Plume-Lithosphere Interaction. Acta Geologica Sinica - English Edition, 76(2): 229-237. Zhong, Y.-T., He, B., Mundil, R. and Xu, Y.-G., 2014. CA-TIMS zircon U-Pb dating of felsic ignimbrite from the Binchuan section: Implications for the termination age of Emeishan large igneous province. Lithos. Zhou, M.-F., Malpas, J., Song, X.-Y., Robinson, P.T., Sun, M., Kennedy, A.K., Lesher, C.M. and Keays, R.R., 2002. A temporal link between the Emeishan large igneous province (SW China) and the endGuadalupian mass extinction. Earth and Planetary Science Letters, 196(3–4): 113-122. Zhou, M.-F., Robinson, P.T., Lesher, C.M., Keays, R.R., Zhang, C.-J. and Malpas, J., 2005. Geochemistry,

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ACCEPTED MANUSCRIPT Tabel 1 Textural parameters of the k-feldspar megacrysts rhyolite samples Sample

Altitude( m)

Numb er

Roundne ss

Area(mm 2)

749

0.59

10766

24.42

26.50

298

0.61

6363

21.30

623

0.58

5882

370

0.58

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1:1.4:2 .3 1:1.1:1 .8

A F 1 9 1 7 1 2 2 7 3 8 2 3 2 8 3 5 1 0

1:1.3:2

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S:I:L

YN-12-27

1756

1:1.2:2

YN-12-27-1

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1:1.2:2

YN-12-271M

1756

YN-12-28

1760

YN-12-28-1

1760

YN-12-281M

1760

YN-12-29

1765

YN-12-30

1761

YN-12-30-1

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YN-12-301M

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1:1.4:2 .2 1:1.2:1 .9 1:1.3:2 .1 1:1.3:2 .1 1:1.3:2

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Error(1 σ)

26.15

-2.82

0.08

-0.86

0.03

1.17

23.90

24.05

-3.54

0.13

-0.73

0.04

1.37

23.09

26.20

24.58

-2.14

0.08

-1.03

0.03

0.97

10266

21.09

22.80

24.76

-3.81

0.15

-0.68

0.04

1.47

0.61

4250

19.00

19.70

20.30

-3.78

0.20

-0.71

0.06

1.41

430

0.59

5577

23.81

25.60

23.56

-2.65

0.10

-0.92

0.03

1.09

386

0.59

10660

21.32

23.10

24.61

-3.57

0.14

-0.72

0.03

1.39

364

0.59

8940

18.72

20.60

21.83

-3.60

0.14

-0.74

0.04

1.35

211

0.60

5732

19.10

21.60

21.76

-3.87

0.17

-0.69

0.04

1.45

455

0.60

25.75

26.70

27.05

-2.78

0.11

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Vol phase(%)

CSD volume(%)

Regression volume(%)

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A M

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I R

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CSD Slope

Error(1 σ)

CSD CL

1:1.3:1 2 487 0.59 9119 20.64 21.60 21.84 -3.04 0.11 -0.85 0.03 1.18 .9 3 1:1.3:1 YN-12-33 1792 8 618 0.60 10974 21.61 23.30 23.71 -3.05 0.09 -0.83 0.03 1.20 .9 1:1.3:2 YN-12-33-1 1792 8 242 0.61 4113 20.50 23.30 23.84 -2.90 0.18 -0.86 0.06 1.16 .1 YN-12-331:1.2:1 1792 7 360 0.61 4766 23.90 23.70 24.46 -2.68 0.13 -0.90 0.04 1.11 1M .8 S,the short axes;I,the intermidate axes;L, the long axes,The values of S:I:L are calcualted by csdslice5.AF,alignment factor.Number,number of grains analyzed. Roundness,average roundness of grains analyzed. Area, area of slab analyzed.Vol phase,volume of K-feldspar megacrysts determined from the ares of K-feldspar in slab. CSD volume,regression volume,intercept,CSD slope and error are calculated using CSDCorrections 1.38. LS slope, LS CL(characteristic length) and errors. YN-12-31

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K-feldspar megacrysts-bearing rhyolites have formed through crystal fractionation form the flood basalts or possibly partial melting of basaltic rock followed by fractional crystallization. They represent final melt fractions and their appearance in the magmatic system coincides with waning of magmatic activities.

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