giant plagioclase growth during storage of basaltic magma

ORIGINAL PAPER

Giant plagioclase growth during storage of basaltic magmain Emeishan Large Igneous Province, SW China

Li-Lu Cheng • Zong-Feng Yang • Ling Zeng •

Yu Wang • Zhao-Hua Luo

Received: 3 September 2013 / Accepted: 10 January 2014 / Published online: 1 February 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract Giant plagioclase basalts (GPBs) reflect the

storage of flood basalt magma in subvolcanic magma

chambers at crustal depths. In this study of the Late

Permian Emeishan large igneous province in southwest

China, we focus on understanding the plumbing system and

ascent of large-volume basaltic magma. We report a

quantitative textural analysis and bulk-rock geochemical

composition of clustered touching crystals (CT-type) and

single isolated crystal (SI-type) GPB samples from 5- to

240-m-thick flows in the Daqiao section. Both types of

GPBs are evolved (\6 MgO wt%), but have high Ti/Y

ratios ([500) and high total FeO content (11.5–15.2 wt%).

The mineral chemistry of the two types of plagioclase

displays a small range of anorthite content (\5 mol%),

which is consistent with their unzoned characteristics. The

two types of GPBs have S-type crystal size distributions

but have quite different slopes, intercepts, and character-

istic lengths. The characteristic lengths of the five flows are

1.54, 2.99, 1.70, 3.22, and 1.86 mm, respectively. For

plagioclase growth rates of 10-11 to 10-10 mm/s, steady-

state magma chamber models with simple continuous

crystal growth suggest that CT-type plagioclase megacrysts

have the residence time of about 500–6,000 years, whereas

the residence time for SI-type plagioclase is significantly

longer, about 1,000–10,000 years. By combining field

geology, quantitative textural data with geochemistry, we

suggest that CT- and SI-type crystals grew and were

coarsened in the outer part and inner part of a magma

chamber, respectively. Magma evolution during storage is

controlled by crystallization, crystal growth, and magma

mixing, and pulsating eruptions occur in response to the

continuous supply of hot magma.

Keywords Emeishan large igneous province �Giant plagioclase basalts � Crystal size distributions �Magma residence time

Introduction

The Emeishan flood basalts are located on the western

margin of the Yangtze craton in southwestern China and

have been recognized as a large igneous province (LIP)

(Chung and Jahn 1995; Xu et al. 2001). Most previous

studies have focused on the primitive high-Mg magmas

and picrites, which have been used to constrain the magma

source (Kamenetsky et al. 2012; Li et al. 2012; Zhang et al.

2006; Zhang et al. 2008). However, low-Mg basalts are key

for understanding complex magmatic processes in the

Emeishan LIP, such as magma storage, differentiation, and

crustal contamination. ‘‘Giant plagioclase basalts’’ (GPBs;

Cox 1980; Hooper et al. 1988; Sen 2001) are characterized

by plagioclase megacrysts (up to 50 mm long) and gener-

ally have low Mg contents but high total Fe and Ti contents

and occur widely in the Emeishan LIP (Xiao et al. 2004;

Xu et al. 2001). GPBs can also be found in other conti-

nental LIPs, such as the Deccan Traps (Beane et al. 1986;

Chandrasekharam et al. 1999; Hooper et al. 1988; Sen

2001) and the Siberian Traps (Lightfoot et al. 1993). GPBs

Communicated by T. L. Grove.

Electronic supplementary material The online version of thisarticle (doi:10.1007/s00410-014-0971-0) contains supplementarymaterial, which is available to authorized users.

L.-L. Cheng � Z.-F. Yang � L. Zeng � Y. Wang � Z.-H. Luo (&)

State Key Laboratory of Geological Processes and Mineral

Resources, China University of Geosciences, Beijing 100083,

China

e-mail: [email protected]

123

Contrib Mineral Petrol (2014) 167:971

DOI 10.1007/s00410-014-0971-0

http://dx.doi.org/10.1007/s00410-014-0971-0

with plagioclase phenocrysts up to 5 cm long are exposed

in the lowest subgroup, the Kalsubai, of the Deccan Traps.

The phenocrysts have compositions that range from An61

to An64 (An = Ca/[Ca ? Na]) and are generally weakly

zoned; plagioclase megacrysts are very tabular and com-

monly form clusters of subparallel crystals (Higgins and

Chandrasekharam 2007; Sen 2001). Quantitative textural

analyses suggest that GPBs are evidence of magma storage

in subvolcanic magma chambers at crustal depths. Thus,

GPBs may hold considerable promise for understanding the

storage, evolution, and plumbing of large-volume basaltic

lavas in LIPs. However, it is difficult to decipher the

physical processes of magma storage and eruption at

crustal depths, especially crystal nucleation and growth and

magma residence time by some traditional geochemical

and qualitative petrographic studies. The theory developed

of crystal size distributions (CSDs, e.g., Cashman and

Marsh 1988; Higgins 1996, 1998; Marsh 1988, 1998) has

been used to document igneous physical processes and

provides a method for determining the magma residence

time by assuming a crystal growth rate (e.g., Cashman and

Marsh 1988; Higgins and Chandrasekharam 2007; Morgan

and Jerram 2006; Zellmer et al. 1999). Some igneous

physical processes such as textural coarsening (Higgins

1998; Higgins 2009; Magee et al. 2010), compaction and

compaction-driven recrystallization, as well as other

physical parameters such as magma ascent rates (Armienti

et al. 2013), the crystallization and cooling rate of crystals

(Cashman 1993), and the volume of the magma chamber

(Higgins and Chandrasekharam 2007) have been examined

by quantitative textural studies. In recent years, the phys-

ical and chemical processes of magma reservoirs have been

investigated by combining quantitative textural analysis

with geochemical studies such as isotopic microanalysis

(Morgan et al. 2007), in situ chemical (major and trace

element and Sr isotope) data of plagioclase (Salisbury et al.

2008), and bulk-rock composition (Yang 2012). In samples

containing small plagioclase crystals, some clusters have

radiating textures that may have developed later in a static

environment (Higgins and Chandrasekharam 2007). How-

ever, GPBs with different texture and similar chemical

compositions that occur in the same section were found in

the Emeishan LIP.

To understand the physical and chemical processes of a

magmatic system, representative GPB samples from a Late

Permian basaltic section, called the Daqiao section, were

analyzed by combining field geology, geochemical studies,

and quantitative textural analysis. Here, we investigate the

evolution of the magma during its storage and derive

inferences about the magma residence time, magma

cycling time, and magma eruption rate.

Geological background

The Emeishan LIP is located on the western margin of

the Yangtze craton in southwest China. It covers an

estimated area of [2.5 9 105 km2 (Chung et al. 1998;

Xu et al. 2001) and has a total volume of

[0.3 9 106 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 host world class Fe–Ti–V oxide deposits

and Ni–Cu–PGE sulfide deposits (Shellnutt et al. 2009,

2011; Zhou et al. 2005). The Emeishan volcanic suc-

cessions unconformably overlie the Late Middle Permian

carbonate formations (i.e., the Maokou limestone) and

are overlain by uppermost Permian units in the east and

Middle Triassic sediments in the west (Xu et al. 2001).

GPBs are present in many parts of the Emeishan LIP,

including Yongsheng County, Binchuan County, Luquan

County, and the towns of Ertan and Daqiao (Fig. 1). The

GPBs are characterized by plagioclase megacrysts

(10–50 mm long) that are substantially larger than the

other crystals in the rock (e.g., Ukstins Peate and Bryan

2008; Xiao et al. 2004). Generally, the aphyric basalts

overlie the Maokou limestone, and the megacrystic

porphyritic and aphyric basalts are interlayered later in

the Emeishan LIP (e.g., Xiao et al. 2004; Xu et al. 2001).

The Binchuan area contains a 5,384-m-thick Permian

volcanic succession that consists of six units (Xiao et al.

2004). GPBs containing groups of plagioclase pheno-

crysts up to 30 mm occur in the middle part of the

sequence (units 3 and 4). The volcanic succession in the

Ertan area, which is located approximately 400 km

northeast of Binchuan, is approximately 1,000 m thick

(Xu et al. 2001). The lower part of the Ertan lava suc-

cession consists of picrites. In the Ertan area, GPBs

containing plagioclase phenocrysts approximately

10 mm long, which is smaller than those in the Binchuan

area, dominate the middle part of the lava section. Thus,

the GPBs in the Ertan section may have formed earlier

than the GPBs in other locations. The Daqiao section,

which is located in the town of Daqiao and contains

complicated and typical giant plagioclase basalts, has

been studied to investigate pre-eruptive uplift in response

to mantle plumes in the Emeishan LIP (He et al. 2009;

Ukstins Peate and Bryan 2008). Although the GPBs of

the Daqiao section might provide information about

magma storage in Emeishan LIP, quantitative studies of

plagioclase megacrysts in the GPBs of the Daqiao sec-

tion have not been conducted.

971 of 20 Contrib Mineral Petrol (2014) 167:971

123

Field relations and petrography

The town of Daqiao is located north-northeast of Kunming

in Huidong County, Sichuan Province (Fig. 1). A complete

Permian basaltic section is exposed along the river and

records the early stages of volcanic activity from the initial

aphyric basaltic lavas that overlie the Maokou limestone to

the main stage of megacrystic (2–50 mm) plag-phyric, and

glomerophyric basalts that contain CT- and SI-type

megacrysts. The top of the uppermost lava is filled with a

tan fine-grained basaltic ash with abundant accretionary

lapilli (Ukstins Peate and Bryan 2008). The GPBs in the

Daqiao section are heterogeneous and are composed of at

least two magmatic components: One is rich in plagioclase

megacrysts, and the other contains aphyric or sparsely

phyric megacrysts (Fig. 2a). Similar heterogeneity was

described in the Deccan Trap, and the spatial arrangement

of these components and their fabric suggests that they

originated by magma mixing (Higgins and Chandrasekha-

ram 2007). By measuring the plagioclase megacrysts in the

GPBs of the Daqiao section, the GPBs were texturally

divided into two types: (1) basalts with a clustered touching

phenocrysts texture (CT-type), which contain plagioclase

megacryst aggregations on the order of 10 mm long

(Fig. 2c) and (2) basalts with a single isolated phenocrysts

texture (SI-type), which contain isolated plagioclase

megacrysts on the order of 20 mm long (Fig. 2d). An

approximately 240-m-long profile that extends from the

lowermost aphyric basaltic lava that directly contacts the

Permian Maokou limestone to the top of the GPB lava was

carefully sampled, and compositions and quantitative tex-

tural analysis were obtained in the field. The GPB flows

contain no faults. The aphyric or nearly aphyric basaltic

lavas are 23.9 m thick in the lower part of the profile. The

upper GPB lava flows were preliminarily divided in the

field into five flows based on the two types of GPBs: flow

CT1, flow SI1, flow CT2, flow SI2, and flow CT3 (Fig. 3).

Flow CT1 is 85.5 m thick, and most of the plagioclase

megacrysts are of the clustered touching type. We mea-

sured the lengths of at least six largest euhedral tabular

plagioclase megacrysts in the flow CT1 with a measuring

tape and used the average length of 12.7 mm as repre-

sentative of the largest plagioclase of flow CT1. The

lengths of the largest plagioclases of the other flows were

determined using the same technique. A weathered horizon

is present between CT1 flow and SI1 flow and indicates a

Fig. 1 The Emeishan volcanic

province. The samples are from

the towns of Daqiao and Ertan,

which are located in the inner

zone and the middle zone of the

province, respectively (modified

from He et al. 2003

Contrib Mineral Petrol (2014) 167:971 of 20 971

123

short eruption interval (Fig. 3d). Flow SI1 is 21.5 m thick.

The texture of the GPBs in this flow is significantly dif-

ferent from that of the previous flow. Most of the plagio-

clase megacrysts are isolated, and the largest plagioclase is

21.7 mm long, which is much greater than that of flow

CT1. Flow CT2 is 65.9 m thick and contains GPBs with

similar textures to CT1. The largest plagioclase in this flow

is 13.1 mm long, which is slightly greater than flow CT1.

Flow SI2 is 17.4 m thick. Most of the plagioclase mega-

crysts are isolated and are much larger than the plagioclase

megacrysts in the other flows. The largest plagioclase is

44.1 mm long, larger than any of the other flows. Flow

CT3 is 46.4 m thick, and most of the plagioclase mega-

crysts are of the clustered touching type, whereas some

occur as single and isolated crystals. The largest plagio-

clase is 17.3 mm long.

Fig. 2 Field photographs of giant plagioclase basalts. a The GPBs

are composed of two magmatic components: One is poor in

plagioclase megacrysts, and the other is rich in plagioclase mega-

crysts; b one of the largest plagioclase megacrysts in the Emeishan

LIP; c clustered touching crystals; d single isolated crystals;

e photomicrograph of CT-type basalts, plagioclases aggregation,

and clinopyroxene are observed. The red line shows how the

plagioclase crystals are touching; f photomicrograph of SI-type

basalts, isolated giant plagioclase can be observed. Pl plagioclase,

Cpx clinopyroxene


123

Sample and analytical methods

Whole-rock and mineral compositions

A total of 22 samples were collected from Daqiao section

for whole-rock analyses, and 11 samples were divided into

two parts for accuracy (Table 1). The samples were cru-

shed to a 200 mesh in an agate mill. Eleven samples for

CSD analysis were analyzed at the Beijing Research

Institute of Uranium Geology and China University of

Geosciences. Their whole-rock major and trace element

compositions were determined by XRF (PW2404) and

ICP-MS (ELEMENT XR). Whole-rock major and trace

element compositions of another 11 samples were analyzed

at the State Key Laboratory of Continental Dynamics in

Northwest University, China, using XRF (Rikagu RIX

2100) and ICP-MS (Agilent 7500a), respectively. USGS

and Chinese standards (BCR-2, GSR-1, and GSR-3) were

used to monitor the analytical precision and accuracy of the

major elements and were generally better than 5 %. For the

trace element analyses, sample powders were digested

using an HF ? HNO3 mixture in high-pressure Teflon

bombs at 190 �C for 48 h. The analytical precision was

better than 10 % for most of the trace elements.

The mineral compositions of selected samples were

measured by electron microprobe at the Beijing Research

Institute of Uranium Geology and the China University of

Geosciences. Samples YN-12-9 (SI2), YN-12-10(CT3),

and YN-12-13(CT3) were studied with an EPMA-1600 at

the China University of Geosciences Geological Labora-

tory Center with a focused electron beam (1–2 lm in

diameter), a specimen current of 20 nA, and an accelera-

tion voltage of 15 kV. EPMA-1600 made by Shimadzu

Corporation can perform highly sensitive elemental ana-

lysis of microareas of samples. The plagioclase megacrysts

were selected for detailed core-to-rim electron microprobe

analysis. Sample YN-11-191 (a CT boulder from the Da-

qiao section with clustered touching plagioclase mega-

crysts) was treated for accuracy. The microprobe analysis

was carried out using an electron probe model JXA-8100 at

Fig. 3 Textural stratigraphic

and sampling column of the

giant plagioclase basalt flows of

the Daqiao section. Five GPB

flows and the studied samples

from the volcanic section are

shown. The phenocryst contents

were estimated in the field

without quantitative statistics

and might not be accurate.

a Representative photograph of

GPBs in the flow CT3. b Single

isolated plagioclase megacrysts

show a good alignment of the

crystals in flow SI2. c GPBs

have the low crystal content and

clustered touching crystals in

flow CT2. d The contact is

curved between flow CT1 and

flow SI1, and volcanic

weathering crust can be found

along the contact. The coin for

scale is *2.5 cm in diameter


123

the Beijing Research Institute of Uranium Geology. The

probe had a focused electron beam (1–2 lm in diameter), a

specimen current of 20 nA, and an acceleration voltage of

15 kV. Five plagioclase megacrysts in these samples were

analyzed.

Crystal size distribution measurements

Eleven samples mainly from these five flows were col-

lected for quantitative textural analysis. Rock samples

10–20 cm squares were used for textural analysis following

the methods of Higgins and Chandrasekharam (2007). The

samples were slabbed normal to the foliation and polished.

The slabs were scanned using a conventional document

scanner, and the crystal margins were manually outlined

with a vector drafting program (CorelDraw). The smallest

megacrysts are typically 0.25–1.5 mm long, which is much

larger than the plagioclase in the matrix. The lower limit of

the CSD is approximately 0.25 mm, so it is not difficult to

distinguish between megacrysts and the matrix plagioclase.

For touching crystals in the CT-type samples, the complete

plagioclase megacrysts were identified by choosing the

larger or more euhedral crystal; the other touching crystals

were outlined as the incomplete megacrysts. This

Table 1 Sample locations

Sample Latitude (N) Longitude (E) Altitude (m) Location Notes

Yn-11-192(1)A 26�39.4450 102�51.0850 1,758 SI flow COR

Yn-11-192(1)B 26�39.4450 102�51.0850 1,758 SI flow COR

Yn-11-193(1)A 26�39.4420 102�51.0870 1,760 SI flow COR

Yn-11-193(1)B 26�39.4420 102�51.0870 1,760 SI flow COR

Yn-11-234A 26�39.5470 102�50.9120 1,767 CT flow COR

Yn-11-234B 26�39.5470 102�50.9120 1,767 CT flow COR

Yn-11-235A 26�39.4450 102�51.0910 1,760 SI flow COR

Yn-11-235B 26�39.4450 102�51.0910 1,760 SI flow COR

Yn-11-237A 26�39.5060 102�50.9470 1,759 CT flow COR

Yn-11-237B 26�39.5060 102�50.9470 1,759 CT flow COR

Yn-11-238A 26�39.5360 102�50.9380 1,764 SI flow COR

Yn-11-238B 26�39.5360 102�50.9380 1,764 SI flow COR

Yn-11-239A 26�39.3780 102�51.1000 1,757 CT flow COR

Yn-11-239B 26�39.3780 102�51.1000 1,757 CT flow COR

Yn-11-241A 26�39.3800 102�51.1010 1,755 CT flow COR

Yn-11-241B 26�39.3800 102�51.1010 1,755 CT flow COR

Yn-11-255A 26�39.5440 102�50.8780 1,765 CT flow COR

Yn-11-255B 26�39.5440 102�50.8780 1,765 CT flow COR

Yn-11-256(1)A 26�39.5450 102�50.9100 1,764 CT flow COR

Yn-11-256(1)B 26�39.5450 102�50.9100 1,764 CT flow COR

Yn-11-258A 26�39.5360 102�50.9400 1,762 SI flow COR

Yn-11-258B 26�39.5360 102�50.9400 1,762 SI flow COR

YN-12-4 26�39.3840 102�51.1030 1,756 CT1 flow CSD, COR

YN-12-5 26�39.4280 102�51.1050 1,760 CT1 flow CSD, COR

YN-12-6 26�39.4460 102�51.0920 1,762 SI1 flow CSD, COR

YN-12-6(1) 26�39.4460 102�51.0920 1,757 SI1 flow CSD, COR

YN-12-8 26�39.5070 102�50.9470 1,758 CT2 flow CSD, COR

YN-12-9 26�39.5380 102�50.9380 1,762 SI2 flow CSD, COR

YN-12-9(1) 26�39.5380 102�50.9380 1,765 SI2 flow CSD, COR

YN-12-10 26�39.5450 102�50.9170 1,765 CT3 flow CSD, COR

YN-12-13 26�39.5410 102�50.8950 1,770 CT3 flow COR

YN-12-15 26�39.5410 102�50.8680 1,768 CT3 flow CSD, COR

YN-12-16 26�39.5410 102�50.8680 1,770 CT3 flow CSD, COR

YN-12-18 26�39.5390 102�50.9300 1,776 SI2 flow CSD, COR

COR compositions of rocks, CSD crystal size distribution


123

procedure was used to increase the accuracy of the ana-

lysis. The crystal outlines were then filled and exported as

tiff files (Fig. 4). The grayscale image was then analyzed

using the program ImageJ, which is a Java version of the

popular program NIHImage. The CSD of the crystals was

calculated with the program CSDCorrections 1.38 (Higgins

2000). The mean crystal shape is expressed by the crystal

aspect ratio of short/intermediate/long (S/I/L). However,

the crystal aspect ratios are difficult to determine, espe-

cially the I/L, and a different I/L ratio will yield different

CSD volume (e.g., Mock and Jerram 2005). Additional

data were obtained from the field measurements. For each

flow, at least six tabular plagioclase megacrysts and more

than three crystals showing the (010) face were measured

for their I/S and I/L ratios. The I/S values vary from 9 to

13, and the L/I values are almost 1 (Table 2). The I/S

values determined in the field are larger than those esti-

mated from the mode of the distribution of intersection

widths/intersection lengths (Higgins 1994, 2000), because

the largest and most tabular crystals were only measured in

the field. We used the crystal shapes of plagioclase

megacrysts with ratios of 1:4:4, 1:5:5, and 1:6:6, which

were estimated by comparing the plagioclase volumetric

proportion determined from the total area of the intersec-

tions and the volumetric proportion determined from the

CSD for each sample with I = L (Table 2). The different

Fig. 4 Representative photomicrographs and outlined plagioclase images for CT- and SI-type samples. a, c are CT-type samples; b is an SI-type

sample. The scale bars in these images are 1 cm long


123

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ple

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(mm

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Inte

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or

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)

YN

-12

-4C

T1

1:6

:61

1.5

1.4

9.2

33

40

0.4

16

,19

45

.75

.91

7.1

6-

4.8

60

.14

-0

.54

0.0

31

.84

-0

.67

1.4

90

.12

YN

-12

-5C

T1

1:6

:61

1.5

1.4

8.4

83

88

0.4

10

,35

51

0.4

11

11

.49

-4

.27

0.1

2-

0.5

60

.03

1.7

9-

0.6

31

.60

0.1

0

YN

-12

-6S

I11

:5:5

12

.81

.18

.62

38

0.4

27

,25

79

.68

.96

9.6

97

-6

.92

0.1

5-

0.3

20

.02

3.1

6-

0.3

42

.92

0.1

8

YN

-12

-6(1

)S

I11

:5:5

12

.91

.11

2.1

12

00

.41

3,1

33

9.6

9.5

19

.69

5-

6.6

60

.22

-0

.34

0.0

32

.97

-0

.33

3.0

50

.23

YN

-12

-8C

T2

1:5

:51

0.9

1.0

6.0

31

10

0.4

32

,93

71

.81

.56

1.9

18

-6

.87

0.2

8-

0.4

80

.05

2.0

9-

0.5

91

.70

0.1

8

YN

-12

-9S

I21

:5:5

9.0

1.2

69

.59

42

00

.53

2,9

28

91

09

.13

-5

.96

0.1

-0

.40

0.0

22

.49

-0

.33

3.0

00

.19

YN

-12

-9(1

)S

I21

:5:5

9.0

1.2

51

.96

19

20

.53

2,9

28

98

7.7

32

-5

.95

0.1

-0

.40

0.0

22

.50

-0

.29

3.4

70

.13

YN

-12

-18

SI2

1:4

:49

.01

.21

4.0

72

33

0.5

26

,06

67

.96

.84

5.9

93

-6

.24

0.1

4-

0.4

30

.02

2.3

1-

0.3

13

.18

0.1

9

YN

-12

-10

CT

31

:5:5

9.6

1.2

46

.48

60

00

.52

3,1

88

12

.31

1.2

12

.06

-4

.50

.1-

0.5

40

.02

1.8

6-

0.5

61

.78

0.0

8

YN

-12

-15

CT

31

:4:4

9.7

1.2

34

.37

36

80

.51

7,2

14

14

.41

4.8

18

.42

-4

.93

0.1

6-

0.4

60

.03

2.1

7-

0.5

41

.85

0.1

1

YN

-12

-16

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values of S/I only affect the tailing corrections (Higgins

2000). The crystals were considered to have a roundness of

0.4 or 0.5.

The orientations of the crystal outlines were also mea-

sured. The alignment factor (AF) of the plagioclase

megacrysts of all samples was used to describe the degree

of agreement of the crystal orientation, which was calcu-

lated on the 40 largest grains in each sample following

Boorman et al. (2004)) and Williams et al. (2006):. The AF

has a maximum value of 100 for perfectly aligned crystals,

whereas massive rocks have an AF value of zero.

Results

Bulk-rock major and trace element composition

A total of 445 whole-rock major content measurements of

Emeishan basalts, trachyte and 95 whole-rock major

content measurements of Emeishan picrites were extracted

from the GEOROC database (http://georoc.mpch-mainz.

gwdg.de). Most of the picrites are from the western part of

the Emeishan LIP, such as the Binchuan area and the Li-

jiang area (e.g., Kamenetsky et al. 2012; ZHANG et al.

2006). The MgO contents of the Emeishan basalts range

from 2.3 to 14.2 wt%, and the MgO contents of the GPBs

in this study range from 3.9 to 5.8 wt%, suggesting that the

GPBs of Emeishan LIP belong to low-Mg basalt (Fig. 5).

The TiO2 contents vary from 3.0 to 4.6 wt%, and the Ti/Y

ratios range from 508 to 691, indicating that they are high-

Ti basalts (HT; with Ti/Y [ 500 and TiO2 [ 2.6 wt%)

(Ukstins Peate and Bryan 2008; Xiao et al. 2004). The

GPBs have Fe2O3total contents of 11.5–15.2 wt% (ESM

Table 3). Figure 5 shows that the CT-type and SI-type

samples have similar major element compositions. Plots of

Al2O3, Fe2O3total, and Na2O ? K2O versus MgO illustrate

that these elements have the largest range of all the

Emeishan basalts. The data show that these two types of

Fig. 5 Variation of SiO2, TiO2,

Al2O3, Fe2O3total, CaO, and

Na2O ? K2O versus MgO for

the Emeishan GPBs. Offwhite

and gray areas represent

compositions for all the basalts,

trachyte and picritic,

respectively. The stars and

hexagons represent

compositions for CT- and SI-

type GPBs, respectively


123

http://georoc.mpch-mainz.gwdg.de

http://georoc.mpch-mainz.gwdg.de

GPBs might not be primary mantle melts and have com-

position that is far different from those expected of melts

that are in equilibrium with mantle peridotites. They are

significantly more evolved, but they could not have been

derived from the picrites through simple fraction processes.

The total rare earth element (REE) of the CT-type GPBs

ranges from 203 to 292 ppm, and the REE of the SI-type

GPBs ranges from 151 to 339 ppm. The La/Yb ratios of the

CT-type GPBs range from 7.5 to 9.3, while the SI-type

GPBs range from 8.1 to 9.0. The Eu/Eu* of the CT-type

GPBs ranges from 0.90 to 1.01, while the SI-type GPBs

range from 0.88 to 1.02. The REE patterns and primitive

mantle-normalized spider diagrams are shown in Fig. 6.

Some low MgO basalts (\6 wt%) containing high-Ti and

low-Ti basalts were collected, which are all from Binchuan

area in the western part of the Emeishan LIP (Xiao et al.

2004). The two types of GPBs have similar trace elements,

are strongly enriched with incompatible trace elements,

and display negative Sr anomaly, which are similar to the

high-Ti low-Mg basalts, rather than low-Ti low-Mg basalts

in Emeishan LIP (Fig. 6).

Mineral composition

The GPBs contain a major population of plagioclase

megacrysts, and most are weakly zoned or unzoned. SI-

type basalts contain abundant phenocrysts composed of

plagioclase megacrysts (9–15 vol%) and clinopyroxene

(1–3 vol%). Plagioclase (An52–55Ab42–45Or2–3) commonly

occurs as coarse-grained, tabular crystals between 10 and

20 mm long. The isolated plagioclase megacrysts of the SI-

type basalts are generally euhedral without significant

resorption zones. Clinopyroxene (En46Fs18Wo36) occurs as

anhedral crystals with diameters ranging from 0.2 to

Fig. 6 a Chondrite-normalized REE patterns of the CT-type GPBs.

b Chondrite-normalized REE patterns of the SI-type GPBs. c Prim-

itive mantle-normalized trace element patterns of the CT-type GPBs.

d Primitive mantle-normalized trace element patterns of the SI-type

GPBs. Normalization values are from Sun and McDonough (1989).

Offwhite and gray areas represent compositions for all the high-Ti and

low-Mg basalts (HL basalts) and low-Ti and low-Mg basalts (LL

basalts), respectively. The low basalt data were collected from Xiao

et al. (2004)


123

1.5 mm. The CT-type basalts also contain abundant

phenocrysts composed of plagioclase megacrysts

(9–25 vol%) and clinopyroxene (1–5 vol%). However,

most of the plagioclase (An52–59Ab39–45Or2–3) crystals that

are 2 to 5 mm touch each other; the touching lines can be

found in Fig. 2e. The clinopyroxene crystals are commonly

anhedral and vary in size from 0.2 to 1.5 mm. They occur

either between touching plagioclase crystals or as isolated

crystals. The groundmass is predominantly composed of

plagioclase (50–60 %), plagioclase microlites (35–45 %),

and Ti–Fe oxides (3–5 %). The plagioclase in the matrix is

much smaller than 1 mm. The inferred crystallization

sequence based on the textures is plagioclase ?plagioclase ? clinopyroxene.

All plagioclase crystals of the Emeishan GPBs are lab-

radorite with a range in composition from An53 to An61

(ESM Table 4). Sample YN-12-10(CT3) ranges from An52

to An59, and sample YN-12-13(CT3) ranges from An53 to

An58. The compositions of plagioclase from sample 191-1L

range from An56 to An60, similar to that of sample 191-2L

(An57 to An59). Plagioclase from sample YN-12-9(SI2)

ranges from An51 to An55 (Fig. 7). The detailed major

element (mol% An) core-to-rim electron microprobe ana-

lysis of plagioclase megacrysts displays a small range of

anorthite content (\5 mol%) and thus is mainly unzoned.

CSD data

CSDs were determined in 11 samples from the two GPB

groups. The matrix plagioclase probably grew at or close to

the Earth’s surface and may not indicate the magmatic

processes in the magma chamber, so we did not measure

them. All of the samples have CSDs that are almost straight

lines on a classic CSD diagram [S-type CSD of Higgins

2006 and Marsh 1988], but lack small crystals (Fig. 8).

Samples YN-12-4(CT1) and YN-12-5(CT1) have parallel

but nonoverlapping CSDs. The mean size of the largest

interval whose population density could be measured was

13 mm. Samples YN-12-6(SI1) and YN-12-6(1) (SI1) were

extended to larger crystals of 25 mm and 20 mm, respec-

tively. Sample YN-12-8(CT2) from the flow CT2 also has

nearly straight CSD and is similar to those of sample YN-

12-4 and sample YN-12-5. The mean size of the largest

interval whose population density could be measured was

13 mm. However, samples YN-12-9(SI2), YN-12-

9(1)(SI2), and YN-12-18(SI2) continue to larger intervals

of 33 mm, which have the largest crystals and the shal-

lowest slope of any sample studied. Samples YN-12-

10(CT3), YN-12-15(CT3), and YN-12-16(CT3) have

CSDs that are similar to those of samples YN-12-4(CT1),

YH-12-5(CT1), and YN-12-8(CT2). The largest interval

whose population density could be measured was 13 mm.

CSDs of samples YN-12-15(CT3) and YN-12-16(CT3) are

different but with similar slopes. Sample YN-12-10(CT3)

has a CSD that is collinear with YN-12-15(CT3).

CSD distributions are easier to understand if the char-

acteristic lengths (CL = -1/slope) are considered rather

than the slopes; hence, we use CL in the figures. Because

all the CSDs we have 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 least-squares fit. There is a good correlation between the

characteristic length calculated using the least-squares fit

and the characteristic length calculated using CSDCorrec-

tions 1.38 except for the samples from flow SI2 (Table 2).

The characteristic lengths of the samples of flow SI2

determined by CSDCorrections 1.38 are slightly smaller

than the samples of flow SI1, which is not consistent with

the observations that most of the plagioclase megacrysts in

flow SI2 are much larger than those in the other flows.

Fig. 7 a BSE scanning electron microscope of plagioclase megacrysts in sample YN-12-9; b An, Ab, and Or of the plagioclase megacrysts.

Right y-axis is used for Or


123

Thus, the characteristic length calculated by the least-

squares fit will be used in this study.

Alignment factor

The alignment factor (AF) of all of the samples is shown in

Table 2. The AF data show a moderately well-developed

foliation of the larger crystals in most of the samples,

which may reflect magma transport and emplacement on

the Earth’s surface. However, if the crystal mush was

transported from the magma chamber by laminar flow, the

relative orientation of the crystals could be preserved. The

AF values of the CT-type and SI-type flows range from 6.0

to 46.5 and from 8.6 to 70.0, respectively. The samples

from the same flow can have significantly different align-

ment factors, such as samples YN-12-9(SI2), YN-12-

9(1)(SI2), and YN-12-18(SI2) (Fig. 9). However, sample

YN-12-8 from flow CT3 has the lowest plagioclase content

as well as the lowest alignment factor. The absence of a

significant correlation between crystal shape and AF value

suggests that the transport was turbulent. Moreover, there

are no correlations between the rare earth element (REE)

contents and the volume of plagioclase in the section (Vol

phase) or AF value, which indicates that the variation of

plagioclase volume fraction and magmatic flow does not

control the fractionation of these trace elements.

Discussion

P–T–H2O–Fo2 condition of crystallization

We assume that the composition of the melt is represented

by the bulk-rock composition of sample YN-12-8(CT2),

Fig. 8 Crystal size distributions of five flows of the Daqiao section. Most of the CSDs are nearly straight, and no crystals are smaller than

1.2 mm


123

which has the highest MgO content and the lowest pla-

gioclase content. All the mineral compositions used for

estimating the crystallization condition are shown in

Table 3. The crystallization pressures of clinopyroxene

from sample YN-12-8(CT2) were estimated using equa-

tion 31 from Putirka (2008). The crystallization pressures

of each type of GPB were also estimated using plagioclase

and equation 25a of Putirka (2008); we obtained similar

pressures of approximately 7 ± 1 kbar for the two GPBs,

and also using clinopyroxene-based and plagioclase-based

barometers, with water contents between anhydrous and

approximately 1 wt% (Fig 10a). This suggests that pla-

gioclase and clinopyroxene co-crystallized at 7 kbar and

from a magma with low water contents (\1 wt%).

The plagioclase temperatures of the two GPB types were

estimated using three different thermometers (equa-

tions 23, 24a, and 26 of Putirka (2008)). The results show

that the two GPB types give plagioclase temperatures

ranging from 1,140 to 1,190 �C with water contents of less

than approximately 1 wt% (Fig. 10b). These plagioclase

temperatures closely overlap with those derived from the

coexisting clinopyroxene and equations 32d and 33 of

Putirka (2008).

Because plagioclase composition is sensitive to vola-

tile concentration, several models have been proposed to

estimate the dissolved H2O content of a crystallizing

magma system based on the stability of plagioclase with

a hydrous liquid (e.g., Lange et al. 2009; Putirka 2005).

Lange et al. (2009) provided a new model of the effect

of dissolved H2O and temperature on plagioclase com-

position for a wide range of liquid compositions

(46–74 wt% SiO2) and indicated that changes in dis-

solved H2O and temperature produce significant changes

in the composition of the crystallizing plagioclase. Using

crystallization temperature of 1,100–1,150 �C and pres-

sures of 4,000–8,000 bar, the model shows that the H2O

content of the melt was approximately less than 0.8 wt%

(Fig. 10d).

Fig. 9 a–d CSD data plotted as a function of distance along the Daqiao section. e Alignment factor versus characteristic length. The stars and

hexagons represent compositions for CT- and SI-type GPBs, respectively


123

At the magma storage conditions obtained with the

geothermometers (H2O content of 0.4 wt%, 1,100 �C to

1,200 �C, pressure of 7 kbar), calculations using MELTS

thermodynamic algorithm (Ghiorso and Sack 1995) show

that plagioclase was the first phase to crystallize when the

oxygen fugacity was buffered at fayalite–magnetite–quartz

(FMQ). The S-type CSD samples indicate that the giant

plagioclases should be in equilibrium with the groundmass

composition, which means that the plagioclase could have

grown steadily in the magma chamber.

Crystallization and magma evolution during storage

A graph of these characteristic lengths against their loca-

tions was constructed to visually display the CSD data. The

characteristic lengths of the CT-type samples vary from

1.49 to 1.94 mm, which is much smaller than those of the

SI-type samples whose characteristic lengths vary from

2.92 to 3.47 mm (Fig. 11). The largest intersection is an

average value of 3 % of the largest crystal lengths in one

sample. There is a good correlation between the largest

Table 3 Composition of mineral used for P–T–H2O condition

Sample SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Cr2O3 Total

Clinopyroxene in YN-12-8 48.76 1.02 1.98 9.74 0.24 14.62 18.07 0.22 0.01 0.09 94.75

Average composition of plagioclase in SI-type samples 53.98 0.18 27.86 0.68 0.16 0.15 11.21 5.09 0.41 0.09 99.81

Average composition of plagioclase in CT-type samples 53.60 0.14 27.96 0.76 0.07 0.12 11.64 4.84 0.44 0.10 99.67

Matrix plagioclase in Yn-11-191 54.74 0.19 27.88 1.03 0.00 0.15 11.37 4.40 0.49 0.09 100.34

Plagioclase composition determined by MELTS 57.23 27.00 8.92 5.98 0.88

Major elements in wt%

Fig. 10 Crystallization pressure, temperature, and H2O content from

calculations performed using several models. a Pressure estimates of

clinopyroxene versus water content were calculated based on

equation 31 of Putirka (2008); b temperature estimates of plagioclase

versus water content based on equations 23, 24a, and 26 of Putirka

(2008); c temperature estimates of clinopyroxene versus water

content based on equations 32d and 33 of Putirka (2008); d water

content versus pressure based on Lange et al. (2009). 1,100, 1,120,

1,140, and 1,150 are the temperatures of plagioclase in Fig. 10d


123

intersection and the total amount of plagioclase, except for

sample YN-12-6. The CT-type samples have a much larger

range of crystal contents than the SI-type samples

(Fig. 11a). There is a good correlation between the largest

intersection and the total amount of plagioclase, which

indicates that the largest intersection of the CT-type sam-

ples ranges from 10.5 to 13.8 mm, whereas the largest

intersections of the SI-type samples range from 17.9 to

25.6 mm (Fig. 11c). The largest intersections also illustrate

the differences between CT-type GPBs and SI-type GPBs.

There is a weak positive correlation trend between the AF

value and bulk-rock major content, such as CaO and TiO2,

but there is weak negative correlation trend between the AF

value and the MgO and Na2O ? K2O content, which

indicates that the magmatic flow not only explains the

alignment of the phenocrysts, but also affects the frac-

tionation of the major elements. There is a positive corre-

lation trend between characteristic length and trace

elements. But, there is no clear correlation between volume

of plagioclase and trace elements such as Dy and Tb. These

observations suggest that magma with SI-type crystals may

reflect injections of new hot magma causing the enrichment

of REE and larger crystals. The observations of the major

elements show that the GPBs cannot evolve from the pi-

crites through simple fractionation processes. Hence,

magma mixing or source heterogeneity may give rise to

differentiation and diversification of the magmas during

storage.

Textural coarsening is clearly an important petrologic

process, but may not have received the attention it deserves

inasmuch as it does not change the chemical composition

of the rock. However, the lack of crystals smaller than

1.2 mm indicates that textural coarsening may have been

involved. Textural coarsening is also known as Ostwald

ripening, textural maturation and equilibration, and crystal

aging and annealing (e.g., Higgins 1998, 2011b; Marsh

1988). Small grains have a higher surface energy per unit

volume than larger grains, so crystals smaller than a critical

size will dissolve and feed the growth of larger crystals to

minimize the energy of the system. This process can occur

only when a crystal is held at a temperature close to its

liquidus for a long period of time. Under these conditions,

the nucleation rate is zero, but the growth rate is high for

crystals larger than the critical size. Several models have

been proposed to account for textural coarsening processes

(DeHoff 1991; Lifshitz and Slyozov 1961; Simakin and

Bindeman 2008; Wagner 1961). Among them, the Com-

municating Neighbours (CN) model was applied to plu-

tonic rocks by Higgins (1998), which is better than the

Lifshitz–Slyozov–Wagner (LSW) theory and can produce

CSDs. However, both the CN and LSW models assume a

constant temperature, which is unlikely in most magma

chambers: Complex convective overturns and injections of

new or different magmas will cause temperature cycling.

Recently, several theoretical and experimental studies on

coarsening mechanisms have shown that coarsening is

driven by temperature cycling caused by new intrusions of

hot magma (e.g., Higgins 2011a; Mills et al. 2011; Simakin

Fig. 11 a Characteristic length versus giant plagioclase content;

b characteristic length versus location; c maximum intersection size

versus location. Line 1 shows the average characteristic length of the

five flows; Line 2 shows the largest plagioclase length of each flow.

The stars and hexagons represent compositions for CT- and SI-type

GPBs, respectively


123

and Bindeman 2008). In the CN coarsening model, when

the system is closed and the volumetric proportion of the

phase is constant, the plagioclase volume in samples of

different flows changes. These observations indicate that

the magma chamber was open and that magma mixing was

an important process in the Emeishan LIP. Hence, textural

coarsening of plagioclase megacrysts was likely driven by

temperature cycling.

Residence time, eruption duration, and eruption rate

Marsh (1988) showed that the CSDs of steady-state system

are S-type and that the mean residence time of a crystal can

be calculated using the characteristic length/growth rate.

Thus, the accuracy and error of the residence time depends

on that of the characteristic length and growth rate. The

errors of the characteristic lengths in this study are all less

than 0.23 mm, which means that the influence of charac-

teristic length on the residence time is limited (Table 2).

Experimental and CSD studies indicate that the growth

rates of plagioclase in undercooling conditions in magma

chambers are consistently between 10-11 and 10-9 mm/s

(e.g., Cashman 1993; Cashman and Marsh 1988; Higgins

2006). Conditions typical of lava flows would give a

growth rate of 10-9 mm/s, and conditions typical of a

subvolcanic magma chamber or magma at greater depths

would yield growth rates of 10-11 to 10-10 mm/s. A pla-

gioclase growth rate of 10-10 mm/s was selected as the

maximum value for a subvolcanic magma chamber in the

Deccan province (Higgins and Chandrasekharam 2007).

The growth rate will depend on the magma temperature

and the cooling rate. We suggest that the two types of

GPBs in this study have different growth rates because of

the different degrees of undercooling. Thus, the entire

range of growth rates for plagioclase megacrysts (10-11 to

10-10 mm/s) was used to calculate the residence times. The

results show that the residence times are 489–4,890 years,

947–9,470 years, 539–5,390 years, 1,020–10,200 years,

and 588–5,880 years. The plagioclase megacrysts of the

CT-type samples have residence times of approximately

489–5,880 years, and those of SI-type samples are

947–10,200 years, which is similar to the magma residence

times of 500–1,500 years that were suggested by quanti-

tative studies of plagioclase megacrysts in GPBs of the

Deccan Traps using a plagioclase growth rate of

10-10 mm/s (Higgins and Chandrasekharam 2007).

The largest plagioclase length and largest intersections

provide evidence for an increase in the characteristic length

of plagioclase megacrysts over time (Fig. 11). An eruption

cycle, or the time between two eruptions from a subvol-

canic magma chamber, is equal to the time for the pla-

gioclase megacrysts to grow from a characteristic length A

(CLA) to a characteristic length B (CLB). Assuming that

plagioclase grows at the same rate, the cycle time between

the two eruptions is equal to (CLB - CLA)/growth rate,

and the cycle time should be compatible with the longest-

duration eruption of the magma that forms the middle

flows. If the errors of the characteristic lengths are not

taken into account, the longest-duration eruptions of flow

SI1, flow CT2, and flow SI2 are 50–500, 73–730, and

49–490 years, respectively. Using the thicknesses of flow

SI1, flow CT2, and flow SI2, the relevant one-dimensional

eruption rates are 0.043–0.43, 0.018–0.18, and

0.036–0.36 m/year, respectively. As mentioned above,

Binchuan area has the 5,384-m-thick Permian volcanic

succession where the complete composite section consists

of six units (Xiao et al. 2004). The aphyric basalt may have

erupted directly without much residence in a subvolcanic

system and thus may have a faster eruption rate. Thus, if

the Permian basalts of Binchuan area erupt at one time, it

may form within 299,111 years using the lowest rate of

0.018 m/year.

Possible origin of crystal clusters and pulsatory magma

eruption in Emeishan LIP

The magmatic processes that occur in the crust can be

complex. Most large-volume flood basalt magmas are

stored at crustal levels to produce aphyric to plagioclase-

dominant basaltic and basaltic andesite lavas (Bryan et al.

2010). GPBs are present in many large volcanic provinces,

such as the Emeishan LIP and the Deccan Traps. An

emplacement model for the GPBs of the Deccan Traps was

proposed by Higgins and Chandrasekharam (2007). This

new study combines quantitative textural measurements

and geochemical data and allows us to propose the fol-

lowing magma storage including physical and chemical

processes in the Emeishan LIP.

Higgins and Chandrasekharam (2007) suggest that the

clustered touching plagioclase crystals may grow in a

higher-level static environment and nucleate much later

than SI-type GPBs (Higgins and Chandrasekharam 2007).

Our results show that the crystallization pressures of the

two types of GPBs are similar, although the estimate of

pressure may only be accurate to within 3 kbar (Putirka

2008). The two types of GPBs have very similar bulk-rock

compositions and mineral compositions, which are also not

in consistent with two-level volcanic magma chambers.

Plots of MgO and Ti2O versus the largest intersection show

that the CT-type GPBs are more evolved than SI-type

GPBs, which may mean that the CT-type GPBs should not

nucleate later than the SI-type GPBs (Fig. 12). A one-

chamber model may be able to explain the textural and

chemical characteristics of the growth rates of two GPB

types that are considered to be different. After magma

accumulates in the crust, the differences between the


123

CT- and SI-type crystals could be due to the following two

scenarios. (1) If crystal clusters are related to synneusis

and/or interactions between large crystals that were already

present, the cluster may be the result of convective

movement in which crystals stick together to form the

clusters. In this case, most crystal in the cluster should be

attached by congruent crystallographic orientations (e.g.,

crystals would attach parallel to each other rather than

perpendicular). We find that most of the clusters in the CT-

type sample are oriented in radiating patterns rather than

parallel to each other. However, some clusters in the SI-

type do tend to be composed of parallel crystal aggregates,

which indicates that synneusis played a role in the forma-

tion of these clusters (Vance 1969). The lower abundance

of crystal cluster in the SI-type samples could also imply

that the SI crystals grew in a more static part of the res-

ervoir. However, an idea of the environment of crystalli-

zation may be determined by the shape of the crystals. The

most tabular plagioclase crystals must have grown in an

environment with the strongest advection, that is, shearing

or stirring (Higgins and Chandrasekharam 2007). (2) The

other possibility is that the crystal clusters in the CT-type

samples are a primary feature in the sense that they are

sites of nucleation from which different crystals grow

radiating along different directions. Crystal nucleation and

growth depend on the degree of undercooling, which is

controlled by the actual crystallization temperature, the

liquidus temperature, and how these change with time and/

or space. When a pulse of magma is stored in a magma

chamber, the undercooling in different locations is strongly

controlled by the actual crystallization temperature, which

has a close relationship with the isotherms (e.g., Zieg and

Marsh 2002). Such type of texture is commonly found in

environments that are characterized by relatively large

undercooling. Thus, the Ct-type samples could have grown

their crystal aggregates closer to the wall rock or even that

the interface between a new incoming magma and a colder

resident one. Note that although the undercooling of the

CT-type of magma versus the SI-type could have been

higher, their actual temperatures or crystal content need not

to be significantly different because the important param-

eter is the cooling rate (Fig. 13). The origin of the dis-

tinctive textures is caused by the temperature and magma

mixing where the CT-type crystals grow in a more static

area. The inner part of magma chamber may be filled with

the more new hot magma injections causing the enrichment

of trace earth elements and larger crystals. The clustered

touching plagioclase megacrysts in the outer part of magma

chamber are firstly taken to the surface forming the CT1

flow in response to magma influx. Then, the conduit to the

surface will be occupied by the magma taking the single

isolated megacrysts forming the SI1 flow. The channels

Fig. 12 Major and trace elements versus CSD data for GPB samples. The stars and hexagons represent compositions for CT- and SI-type GPBs,

respectively


123

ascending through the crust to the surface may be still open

for thousands of years. Magma eruption may have occurred

every several hundreds of years, forming the flows CT2,

SI2, and CT3.

Conclusions

The two types of giant plagioclase basalts in the Daqiao

section provide a record of the magmatic processes in the

subvolcanic magma chamber of Emeishan LIP, including

storage, crystallization, and eruption. The CT- and SI-type

plagioclases have grown in the outer part and inner part of

the magma chamber, respectively, because of different

degrees of undercooling. The crystals coarsen in response

to the continuous supply of hot magma. The evolution of

major elements in the melts was controlled by crystalli-

zation and magma mixing, and the trace elements’

enrichment in the SI-type GPBs may have been caused by

additional magma intrusions in the inner part of the magma

chamber. Quantitative studies of plagioclase megacrysts

provided information about the magma residence times in

the crust. However, the duration and rate of the eruptions

are more complicated to determine, and modeling of GPBs

has yielded imprecise results (Higgins and Chandrasekha-

ram 2007). Based on detailed fieldwork and quantitative

textural analysis, we used the magma cycling time between

two eruptions as the duration of the longest eruption of

middle flows. Although we do not account for the effects of

errors of characteristic lengths, the similar results of three

middle flows indicate that these calculations provide rea-

sonable constraints on the durations of magma eruptions

and the eruption rate of subvolcanic magma chambers that

forms GPBs flows of the Daqiao section. The results sup-

port a scenario of pulsating magma eruption of the sub-

volcanic magma chambers in response to the continuous

supply of hot magma.

Acknowledgments This study was supported by the National Basic

Research Program of China (973 Program No. 2011CB808901) and

the Geological Survey Program of China Geological Survey

(1212011220921, 1212011085468, and 201211022). We are grateful

to Yi-Gui Han, Chuan-Dong Xue, Geng-Rong Wang, and Shuai Dong

for their help in the field. Laboratory assistance and discussions with

Jiu-Long Zhou, Huan Wang, Jinhua Hao, and A-Peng Yu are

appreciated. We appreciate the constructive comments from Michael

D. Higgins and one anonymous reviewer and the editor, T.L.Grove.

The manuscript was improved by the insightful reviews from Fidel

Costa.

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giant plagioclase growth during storage of basaltic magma

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