Full text of DOI:10.1007/BF02900606

are dark red-brown to orange yellow, mostly 3—4 cm in
size, and a few bigger than 8 cm.
Almandine megacrysts from
Yingfengling Cenozoic basalt
in Leizhou Peninsula and
their parental magma origin
2
Garnet megacryst compositions
All analyses of the megacryst compositions were
carried out at National Key Center of GEMOC in Mac-
quarie University, Australia. Major element compositions
were determined with a Cameca-SX50 electron micro-
probe. Trace element analysis was carried out using the
laser ablation microprobe inductively coupled plasma
mass spectrometer (LAM-ICPMS). The analytical tech-
nique, conditions and precision are the same as those de-
YU Jinhai1 & S. Y. O’Reilly2
1. National Key Laboratory of Metallogenic Mechanism of Endogenetic
Deposits, Department of Earth Sciences, Nanjing University, Nanjing
210093, China;
2. National Key Center of GEMOC, Department of Earth and Planetary
Sciences, Macquarie University, North Ryde, NSW 2109, Australia
scribed by Norman et al.[11]
.
Microprobe analyses of more than seven points for
each sample show that garnet composition is very homo-
geneous in a megacryst grain or different grains in an ag-
gregate. However, the remarked composition variation is
observed among different, discrete megacryst grains. As
the Mg# of garnet decreases, Fe and Mn contents consid-
erably increase, and Mg, Al and Ti gradually decrease
Abstract
The garnet megacrysts from Yingfengling ba-
salts are characterized by high FeO (>20%), CaO (7.02%
—8.16%) and low MgO (5.88%—10.87%). Significant com-
position variations are observed in these megacrysts, of
which Ni, V, Sc, Co, and HREE are positively correlated with
their Mg#, and Zr, Hf, Ga, Y, Sr, Nb, Zn and LREE-MREE
are negatively correlated with Mg#. Megacryst parent
magma is a highly evolved residual melt with strongl deple-
tion in Ti, Sr, Hf, Nb and HREE. This parental magma was
generated by more than 60% of crystallization fractionation
of clinopyroxene, garnet, plagioclase and ilmenite from
quartz tholeiitic magma. It has not erupted to the surface,
but stayed at the upper mantle and formed the megacrystic
cumulate. Megacrysts and their host basalt are in disequilib-
rium.
(table 1). Ca seems not closely related to Mg# change[10]
.
In comparison with those garnet megacrysts from other
locations, Yingfengling garnets have significantly high
FeO and CaO content (fig. 1). They contain grossular
component of 40%—54%, pyrope 23%—41% and andra-
dite 19%—23%. So these garnets can be described as Mg,
Ca-rich almandine.
Keywords: almandine megacryst, geochemistry, LAM-ICPMS, Lei-
zhou Peninsula.
Cenozoic basalts frequently contain varied mega-
crystic minerals, which have been used to reveal the geo-
chemical behavior and evolution of the basaltic liquid in
the deep mantle[1—4]. Garnet megacryst is important and
relatively sparse. Its occurrence reflects the megacrysts
forming in deeper mantle (>2.0 GPa)[1]. Garnet mega-
crysts have been also found at some Cenozoic basalts in
eastern China[5—8]. However, these garnets are of pyrope
as those in kimberlites. This note will report the alman-
dine megacrysts found in Yingfengling volcano, study
their geochemical characteristics using the LAM-ICPMS
technique, and speculate on the origin of the megacryst
parental melt.
Fig. 1. The plot of Mg# against CaO content of the garnet megacrysts
from Leizhou and other localities in eastern China. All data, except Le-
izhou, are from refs. [5—8].
3
Trace element features
Trace element compositions of almandine mega-
1
Fundamental geology and megacryst petrography
crysts are also homogeneous, like their major elements.
Compared with the pyrope megacryst from alkaline basalt
in Tsavarin-Tsharam of NW Mongolia, Yingfengling al-
mandine megacrysts have lower V, Ni, Sc, Zr, Hf and Nb
concentrations, and same Y, Sr, Ga and LREE contents. Of
them V, Ni and Sc contents clearly decrease with Mg# de-
crease (table 2), while Y, Sr, Ga and LREE increase as
Mg# decrease (table 2). The most peculiar geochemical
characteristic of Yingfengling almandine megacryst is
their arch REE patterns with intense depletion of LREE
Yingfengling volcano is located in the center of
Leizhou Peninsula, South China (21.5°N, 110.2°E), which
contains abundant deep-seated xenoliths and mega-
crysts[9,10]. The megacrysts in Yingfengling basalt include
garnet, clinopyroxene, plagioclase, ilmenite and apatite.
Most megacrysts occur as discrete ones, some as compos-
ite megacrysts or megacryst aggregates. Aggregate
megacryst is smaller in grain size, however, identical with
the discrete megacryst in composition. Garnet megacrysts
Chinese Science Bulletin Vol. 46 No. 14 July 2001
1215

NOTES
Table 1 Compositions of garnet megacrysts and host basalt from Yingfengling
Sample
Lzgt-19 Lzgt-24 Lzgt-11 Lzgt-22 Lzgt-17
Lz-54*
16
Lz-17* Lzgt-14* Lz-24*
Lz-27*
16
Mongol
LZB
3
Average No.
8
9
12
10
8
7
9
3
SiO2
TiO2
39.14
0.74
39.20
0.70
38.95
0.63
38.21
0.37
38.34
0.37
39.52
0.59
38.66
0.58
38.33
0.61
38.50
0.55
37.89
0.43
40.37
0.62
53.23
1.47
Al2O3
Cr2O3
FeOt
21.54
0.02
21.55
0.02
21.36
0.42
9.46
7.68
0.05
21.80
0.02
22.83
0.41
8.12
8.05
0.06
20.90
0.00
25.71
0.44
6.45
7.05
0.03
20.76
0.02
26.46
0.52
6.28
7.32
0.05
21.76
0.01
21.28
0.01
22.73
0.46
8.52
7.67
0.04
21.01
0.01
24.03
0.46
7.00
8.16
0.07
21.26
0.02
24.43
0.46
6.84
8.02
0.04
20.66
0.02
26.50
0.46
5.88
7.59
0.07
22.35
0.02
15.10
1.82*
7.83
0.14
7.13
8.80
2.94
20.95
0.41
20.07
0.39
15.87
0.41
MnO
MgO
CaO
10.00
7.18
10.87
6.92
15.09
5.52
Na2O
0.07
0.04
0.05
Si
2.972
1.928
2.973
1.927
2.965
1.956
2.997
1.932
2.991
1.910
2.979
1.933
2.970
1.927
2.978
1.924
2.979
1.939
2.982
1.916
2.967
1.936
Al
Ti
0.042
0.001
1.330
0.026
1.132
0.584
0.010
0.040
0.001
1.355
0.027
1.070
0.624
0.007
0.036
0.001
1.453
0.026
0.922
0.657
0.008
0.022
0.000
1.686
0.029
0.754
0.593
0.005
0.022
0.001
1.727
0.034
0.730
0.612
0.007
0.033
0.001
1.265
0.025
1.221
0.559
0.006
0.033
0.000
1.461
0.030
0.976
0.631
0.006
0.035
0.000
1.561
0.030
0.810
0.679
0.011
0.032
0.001
1.581
0.030
0.788
0.665
0.006
0.025
0.001
1.744
0.031
0.690
0.640
0.010
0.034
0.001
0.975
0.026
1.653
0.435
0.007
Cr
Fe
Mn
Mg
Ca
Na
Mg#(gt)
Alm
0.46
41.8
0.44
42.6
0.39
46.2
0.31
54.3
0.30
54.1
0.49
39.8
0.40
45.2
0.34
49.2
0.33
50.5
0.28
54.4
0.63
29.2
Spss
0.9
37.8
19.5
0.9
35.7
20.8
0.9
30.9
22.0
1.0
25.1
19.7
1.1
24.3
20.4
0.8
40.7
18.7
0.62
0.27
1.0
32.6
21.1
0.57
0.22
1.0
27.1
22.7
0.53
0.19
1.0
26.3
22.2
0.53
0.19
1.0
23.1
21.4
0.48
0.16
0.9
55.4
14.6
Pyrope
Gros
Mg#(cpx)
Mg#(melt)
0.57
The samples with * label are megacryst aggregate. Mongol is a pyrope from Mongolia, its EMP data were given by Prof. Griffin. LZB is host
quartz tholeiite, its K2O is 0.63% and datum in Cr2O3 row is Fe2O3 content. All iron of garnet megacryst is expressed as FeOt. Structural formula is
calculated based on 12-oxygen ion. Fe3+ obtained in the light of charge balance is about 5.7% of all iron, therefore neglecting Fe3+ does not significantly
affect the conclusion in this text. Mg#=Mg/(Mg+Fe) (mol). Mg# (cpx) is for clinopyroxene in related aggregate; Mg# (melt) is that of the calculated
parental melt or host basalt.
with Mg#, and HREE (Tm, Yb, Lu) are positively corre-
lated with Mg#. Er contents nearly maintain constant.
Relative to those pyrope megacrysts from Huangyishan,
NE China[3] and Tsavarin-Tsharam, NW Mongolia, Ying-
fengling garnet megacrysts have similar LREE, higher
MREE and strongly depleted HREE (Er-Lu) (fig. 2). In
the primitive mantle normalized spidergram (not shown
here), almandine megacrysts show clearly Ti, Sr, Nb, Zr
and Hf negative anomalies.
4
Characteristics of megacryst parent melt
Mg# of Yingfengling garnets is low ranging from
0.28 to 0.49, and that of the clinopyroxenes in megacryst
aggregates ranges from 0.48 to 0.62 (table 1). In the
megacryst aggregate, the Mg# of garnet and clinopyroxene
shows a good relationship and trace element partition co-
efficients between them have a little and regular change,
Fig. 2. REE patterns of Yingfengling megacrystic garnets.
and HREE and enrichment of MREE (fig. 2). Fig. 2 shows
that LREE and MREE (La-Ho) are negatively correlated
1216
Chinese Science Bulletin Vol. 46 No. 14 July 2001

Table 2 Trace element compositions of Yingfengling garnet megacrysts and their host basalt (Pg/g)
Lzgt-19 Lzgt-24 Lzgt-11 Lzgt-22 Lzgt-17 Lz-54* Lz-17* Lzgt-14* Lz-24* Lz-27* Mongol
13
44.11
Sample No.
Average No.
LZB
3
5
5
9
9
5
2
3
5
2
3
Sc
Ti
18.06
3823
14.22
3921
9.49
5.01
5.28
19.42
3087
11.74
3371
6.42
7.35
4.74
20.68
3690
105.3
73.17
3.10
2183
38.4
2141
28.8
3138
71.8
3029
71.9
2401
38.8
3486.3
154.7
72.23
27.71
42.64
9.20
8960
140.8
43.68
188.57
89.36
17.79
293.28
17.41
89.14
9.35
V
143.4
81.56
10.72
73.42
10.76
0.40
128.0
76.04
6.36
116.1
78.50
14.06
72.27
9.84
99.4
Co
Ni
Zn
Ga
Sr
66.99
4.11
66.00
2.97
76.35
9.46
61.99
2.13
69.00
3.00
62.82
2.12
69.70
11.02
0.50
69.96
10.99
0.53
116.69
14.98
0.49
136.91
15.64
0.50
82.79
11.45
0.61
92.58
13.24
0.60
93.30
13.50
0.68
129.99
15.96
0.54
0.35
0.47
Y
60.30
45.64
0.045
0.040
0.25
69.20
58.62
0.040
0.040
0.31
61.87
51.03
<0.082
0.065
0.35
71.87
57.78
0.070
0.058
0.60
72.32
52.37
0.046
0.050
0.58
53.86
33.46
0.041
<0.013
0.19
66.53
45.60
<0.090
<0.081
0.56
71.81
56.59
0.043
0.038
0.46
68.15
48.50
0.050
0.045
0.48
72.41
50.30
0.040
0.048
0.61
59.35
61.40
0.06
Zr
Nb
La
Ce
Pr
0.06
8.31
0.28
17.07
2.44
0.12
0.14
0.17
0.29
0.28
0.08
0.15
0.22
0.20
0.28
0.11
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
1.50
1.94
2.16
4.00
4.07
1.28
2.34
2.94
2.95
4.04
1.48
10.83
3.21
2.20
2.88
3.22
6.13
6.02
1.82
3.12
4.26
4.35
5.90
1.83
1.41
1.76
2.08
3.83
3.76
1.26
2.13
2.64
2.65
3.52
1.14
1.26
6.70
8.32
9.60
16.74
3.56
15.51
3.45
5.54
9.27
11.34
2.69
11.95
2.75
15.55
3.37
4.82
3.59
1.63
2.06
2.20
1.43
2.10
1.27
0.56
12.10
2.52
14.06
2.88
14.32
2.60
20.41
3.03
20.15
3.13
10.58
2.20
14.99
2.84
16.36
2.79
17.05
3.00
20.28
3.13
9.81
3.18
2.33
0.63
5.52
6.14
4.98
4.52
5.00
5.26
5.84
4.97
5.45
5.13
6.49
1.63
Tm
Yb
Lu
Hf
Th
U
0.60
0.63
0.46
0.33
0.38
0.57
0.60
0.42
0.46
0.40
0.87
3.04
2.96
1.79
1.16
1.41
3.02
2.65
1.70
1.81
1.46
5.42
1.30
0.18
2.22
1.50
0.36
0.31
0.27
0.17
0.08
0.10
0.30
0.21
0.13
0.16
0.10
0.71
0.83
1.05
0.84
0.87
0.78
0.60
0.84
0.88
0.83
0.79
1.14
0.019
<0.020
0.012
0.017
<0.022
0.016
0.016
0.020
0.007
0.008
<0.012
0.006
<0.019
0.017
0.008
0.007
<0.009
0.010
<0.007
<0.006
0.010
<0.018
Data for Mongolian pyrope megacryst are analyzed with LAM-ICPMS in this study, LZB is analyzed with the solution ICP-MS method.
suggesting that garnet and clinopyroxene are in equilib-
rium. Temperature and pressure calculations also indicate
that crystallization temperature is positively correlated
with Mg# [10], implying that the megacryst aggregate is not
an occasional assembly of the minerals with different ori-
gins. Thus the Mg# of garnet parent melt can be estimated
based on the composition of the coeval clinopyroxene in
tion results show that the parent melt of Yingfengling
megacrysts is rich in REE, ranging from 107 to 308 Pg/g
(those rare earth elements of unavailable partition coeffi-
cients are estimated with interpolation). Their REEs are
significantly differentiated with LREE enrichment and
intense HREE depletion, even Lu lower than HREE of
chondrite (fig. 3).
megacrystic aggregate and Wood’s calculation equation[12]
Calculations indicate that Mg# of their parental liquids is
0.27 — 0.16 from Mg-rich endmember to Fe-rich one.
This means that the megacryst parental melt will contain
much low MgO (about 2.5%—3.5%), even if assuming it
has high FeOt (12%—17%). Due to very high Mg# of host
basalt (0.57, table 1), it is suggested that host melt is not
megacryst parental liquid.
.
In the spidergram of the primitive mantle normalized
trace elements abundance (fig. 4), the megacryst parent
melts show the remarked Nb, Sr, Zr, Hf, Ti and HREE
(Yb, Lu) anomalies. The relative depletion of these ele-
ments is closely related with fractional crystallization of
garnet, ilmenite, plagioclase and apatite, because these
minerals are most important carriers of above- mentioned
elements and they have been found in the Yingfengling
volcano. It is suggested that the parental melt had under-
gone fractional crystallization of garnet, clinopyroxene,
ilmenite and plagioclase without apatite before Lz-54
By using the partition coefficients between garnet
and melt [13], the trace element characteristics of the parent
melt can be brought to light (figs. 3 and 4). The calcula-
Chinese Science Bulletin Vol. 46 No. 14 July 2001
1217

NOTES
megacryst aggregate formed.
Rayleigh fractionation was used here, where C0 is the
weight concentration of a trace element in the parental
liquid, CL is the weight concentration of a trace element in
the liquid, F is the fraction of melt remaining, and D is
bulk partition coefficient of the fractionating assemblage
during crystal fractionation. The partition coefficients for
garnet and clinopyroxene are from the experimental data
of Johnson et al.[13]. Partition coefficients of the trace ele-
ments between oligoclase and melt are obtained according
to the partition of plagioclase and garnet in megacryst
aggregate (Dpl/gt) and the given Dgt/melt. REE partition co-
efficients for ilmenite are from Paster’s data[18]. Bulk par-
tition coefficient is calculated by using the proportion of
garnet/clinopyroxene/plagioclase/ilmenite of 5Ή8Ή6Ή1.
This proportion is determined based on the field statistics
of varied megacrysts occurred in Yingfengling volcanic
rock, the proportion of minerals in aggregate and in con-
sideration of trace element variation tendency of the
megacryst minerals. Because the proportion of the crystal-
lizing minerals during fractional crystallization is proba-
bly varied, the modeling in this study is average. Model-
ing indicates that the primary magma of megacryst parent
melt is probably a quartz tholeiitic one compositionally
similar to Yingfengling host basalt (table 1, fig. 5). The
parental melt of Lz-54 megacryst, which is the richest
magnesium one within all Yingfengling megacrysts, can
be produced from quartz tholeiitic magma through about
60%—70% (F = 0.3—0.4) crystal fractionation (fig. 5).
Fig. 3. The REE patterns of the parent melts of garnet megacrysts.
Legends are the same as in fig. 2.
Fig. 4. Primitive mantle normalized trace element abundance of the
parent melts of Yingfengling garnet megacrysts. Legends are the same as
in fig. 2.
5
The origin of parental magma
Coeval crystallization of garnet, clinopyroxene, pla-
gioclase and ilmenite indicate their parental melt of basic
or intermediate-basic one. However, in Leizhou Peninsula
and Hainan Island (Lei-Qiong area) such low Mg#
(0.27—0.16) and depleted HREE volcanic rocks have not
been found[14—16], implying that this melt did not erupt to
the surface, but detained in upper mantle and solidified.
P-T estimations indicate that the crystallization takes
Fig. 5. REE pattern comparison of the parent melt of Lz-54 megacryst
with those melts generated by fractional crystallization modeling of
quartz tholeiitic magma.
place at 1270—1130ć and >1.5 GPa[10]
.
Above-mentioned geochemical characteristics indi-
cate that the formation of megacryst parent melt must ex-
perience fractional crystallization. The fact that the vari-
ous degrees of partial melting of the mantle peridotites
with different compositions under any experiment condi-
tions cannot directly produce the melt with FeO of >12%
and MgO of <5% also imply that such low Mg# melt re-
sult from strong crystallization differentiation [17]. Then,
which kind of magma is the primary melt of megacryst
parent rock? Several basalts found in the Lei-Qiong area
have been tested as primary magma to model the frac-
tional crystallization process. Equation CL= C0×F(Dꢀ1) for
Olivine tholeiite, alkali olivine basalt and basanite
have higher HREE than the parent melt of the Lz-54 gar-
net megacryst, and higher or similar LREE concentrations.
Because the crystallization fractionation of the garnet and
so on will lead to LREE content increase as well as HREE
content decrease, these magmas cannot be the primary
liquid of Yingfengling garnet.
6
Discussion and conclusion
Yingfengling garnet megacryst is a rare almandine
featured by high FeO and CaO and low MgO. They have
large composition variation in major and trace elements,
1218
Chinese Science Bulletin Vol. 46 No. 14 July 2001

associated xenoliths: evidence for migration of geochemically en-
riched melts in the upper mantle beneath Scotland, J. Petrol., 1999,
40: 935.
reflecting a long crystallization process. With the de-
creasing of the magnesium number of garnets, Ni, V, Sc,
Ti, Co and HREE (Tm, Yb, Lu) are decreasing, and Zr, Hf,
Ga, Y, Sr, Nb, Th, U, Zn, and LREE-MREE (La-Ho) are
increasing. The composition variations of the garnet
megacrysts depend on the composition variation of their
parent melt, and the composition change of the parent
melt results evidently from the crystallization fractiona-
tion. It is required that the parent melt is a residual liquid
with small volume, so that some amount of crystal frac-
tionation can lead to the remarked change of trace element
concentration in the magma remaining. On the other hand,
fractional crystallization modeling has confirmed that the
parental melt of Lz-54 megacryst was generated from
quartz tholeiitic magma by 60%—70% fractionation, and
the formation of parental melt of Lz-27 megacryst (lowest
Mg# endmember) needs 80%—90% crystal fractionation.
All these evidence that Yingfengling megacrysts were
crystallized from a highly evolved, residual magma. The
host is not their parent magma, i.e. the host and
megacrysts are in disequilibrium. Hence the trace element
concentrations of megacryst and host basalt cannot be
utilized to calculate the partition coefficients. This
evolved parent magma does not erupt to the surface, and
appears to be hidden in upper mantle and crystallize to
megacrystic cumulate or dyke. The host was formed later
than the primary magma of megacrysts, and captured the
crystallized megacrysts during its uplifting.
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megacrysts from the Kurile Island arc: a window to across-arc
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Province, Chinese Science Bulletin, 1998, 43(23): 2013.
10. Yu, J. H., Luo, S., Megacrysts from Cenozoic Yingfengling basalt
in Leizhou Peninsula, South China Mineral Acta (in Chinese with
English abstract), 2000, 20(2): 191.
11. Norman, M. D., Melting and metasomatism in the continental
lithosphere: laser ablation ICPMS analysis of minerals in spinel
lherzolites from eastern Australia, Contrib. Mineral. Petrol., 1998,
130: 240.
Because more primitive and Mg-richer megacrysts
or aggregates have not been found, early fractional
crystallization of original magma probably took place at
deeper and different magma chambers from late crystalli-
zation environment of residual melt.
If the primary magma of Yingfengling megacrysts is
more depleted N-MORB more crystal fractionation is
needed to form the parental melt of Lz-54 megacryst. The
early crystallization of N-MORB probably involved oli-
vine. Its fractionation will give rise to Mg# decrease and
REE and other incompatible element increase of magma
remaining. As the evolving magma becomes silica satura-
tion, clinopyroxene, garnet and plagioclase are growing
principal crystallization phases.
12. Wood, B. J., Blundy, J. D., A predictive model for rare earth ele-
ment partioning between clinopyroxene and anhydrous silicate
melt, Contrib. Mineral. Petrol., 1997, 129: 166.
13. Johnson, K. T. M., Experimental determination of partition coeffi-
cients for rare earth and high-field-strength elements between cli-
nopyroxene, garnet and basaltic melt at high pressure, Contrib.
Mineral. Petrol., 1998, 133: 60.
14. Zhu, B., Wang, H., Nd-Sr-Pb isotopic and chemical evidence for
the volcanism with MORB-OIB source characteristics in the
Leiqiong area, China, Geochimica (in Chinese with English ab-
stract), 1989, 18(3): 193.
15. Liu, C., Xie, G., Masuda, A., Geochemistry of Cenozoic basalts
from eastern Chinaüüĉ. Major element and trace element
compositions: petrogenesis and characteristics of mantle source,
Geochimica (in Chinese with English abstract), 1995, 24(3): 1.
16. Flower, M. F. J., Zhang, M., Chen, C. Y. et al., Magmatism in the
South China Basin 2. Post-spreading Quaternary basalts from
Hainan Island, South China, Chemical Geology, 1992, 97: 65.
17. Kushiro, I., Compositions of partial melts formed in mantle peri-
dotites at high pressures and their relation to those of primitive
MORB, Phys. Earth. Planet. Inter., 1998, 107: 103.
Acknowledgements We are indebted to Prof. Zhou, X. for his criti-
cism of the manuscript. We also thank Dr. N. Pearson and A. Sharma for
their assistance with regard to electron microprobe and LAM ICP-MS
analyses. We are still grateful to Prof. W. L. Griffin and Luo, S. for pro-
viding some samples. This work was supported by the National Natural
Science Foundation of China (Grant No. 49773191), an AusAID Austra-
lia-China Institutional Links Program and the State Key Basic Research
Program (Grant No. 1999043202)
18. Paster, T. P., Schauwecker, D. S., Haskin, L. A., The behavior of
some trace elements during solidification of the Skaergaard Lay-
ered Series, Geo. Cos. Avta, 1974, 38: 1549.
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(Received January 11, 2001)
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