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Isotope and trace-element geochemistry of alkali basalts and associated megacrysts from the Huangyishan volcano, Kuandian, Liaoning, NE China
Chemical Geology, 97 ( 1992 ) 219-231
219
Elsevier Science Publishers B.V., A m s t e r d a m
[31
Isotope and trace-element geochemistry of alkali basalts and associated megacrysts from the Huangyishan volcano, Kuandian, Liaoning, NE China Cong-Qiang
L i u a'b'c,
Akimasa Masuda ~ and Guang-Hong Xie c
"The Institute of Physical and Chemical Research, Hirosawa 2-1, Wako-shi. Saitama. 351-01, Japan bLaborato~vfor REE Microanalysis, Department of Chemistry, Faculty of Science, The University of Tokyo, Tokyo 113, Japan ~The Institute of Geochemistry, Academia Sinica, Guiyang, Guizhou Province, People's RepuMic o#China (Received June 10, 1991 ; revised and accepted September 26, 1991 )
ABSTRACT Liu, C.-Q., Masuda, A. and Xie, G.-H., 1992. Isotope and trace-element geochemistry of alkali basalts and associated megacrysts from the Huangyishan volcano, Kuandian, Liaoning, NE China. Chem. Geol.. 97:219-231. Trace-element abundances and Sr, Nd isotopic compositions have been measured on three megacrysts of clinopyroxene, three of garnet and one of amphibole, as well as their host alkali basalts from the Huangyishan volcano, Kuandian, NE China. The Ce isotopic compositions of three host basalt samples have been also analyzed. All megacrysts studied here are shown to be cognate with the host basalts mainly based on the isotopic compositions and major-element distributions between silicate crystal and melt. The source mantle of Huangyishan basalts is isotopically and chemically heterogeneous, and must have been enriched in Rb, La and Sm relative to Sr, Ce and Nd, respectively, for a relatively long time. Highly incompatible trace-element ratios are also suggestive of enrichment of large-ion lithophile elements in the mantle compared with the mantle source of MORB and most OIB. The fine structure, i.e. the double-concave shape of REE patterns commonly found in alkali basalts, is considered to be an important clue to the origin of alkali basalts. Based on the REE partition between the megacrysts and host basalts, evaluation of several theoretical models which can account for the double-concave shape of REE patterns shows that alkali basalts with such patterns must have been in equilibrium or quasi-equilibrium with both garnet and clinopyroxene solid phases. The small positive Eu anomaly commonly observed in REE patterns in alkali basalts, in most cases, is actually the turning point of the double-curved REE pattern. Partial melting of a garnet peridotite source is the most probable process which can generate the alkali basalts under consideration.
1. Introduction In the northeastern part of the Sino-Korean platform, the Kuandian depression basin is underlain by a basement of Proterozoic metamorphic rocks (Fig. 1). In this depression, twenty Cenozoic volcanoes, among which the Huangyishan volcano is the largest one, are arC o r r e s p o n d e n c e to: Dr. Cong-Qiang Liu, T h e Institute of Physical and Chemical Research, Hirosawa 2-1, Wako-shi, Saitama, 351-01, Japan.
ranged in NE and NW directions. The lavas in the Huangyishan volcano can be divided into five flows, clearly showing five discrete eruptions. The eruption age of this volcano is 0.274 Ma (Xie et al., 1990). Each of the eruptive cycles started with pyroclastics and terminated by lava flows. Mantle xenoliths and high-pressure megacrysts are abundant, especially in the first, second and fourth cycles. The petrogenesis of the Huangyishan basalts has been studied by several authors (Lu et al., 1983; Wei, 1985; Xie et al., 1990). However,
0 0 0 9 - 2 5 4 1 / 9 2 / $ 0 5 . 0 0 © 1992 Elsevier Science Publishers B.V. All rights reserved.
220
('-Q. LIU ET AL TABLE 1
Major-element compositions (wt°/o) and trace-element abundances (ppm) of Huangyishan basalts B-I / d l ~
S"
Kuandian
~/
Faults
[-~
o
krn
Cenozoic Basalts
Fig. 1. Sketch map showing Cenozoic basalts in Kuandian and adjacent areas o f NE China. This map was modified from E et al. (1987).
several geochemical hypotheses concerning the origin of the basalts and associated megacrysts have not yet been tested by enough chemical and isotopic data. Since much of the previous work largely involving the basalts is mainly petrological and mineralogical, the present work is concentrated upon the geochemistry of both basalts and megacrysts of several kinds of minerals. The aims of this paper are to identify the geochemical features of the source mantle and to investigate the petrogenesis of the basalts. 2. Samples The major-element compositions of the Huangyishan basalts are from Wei (1985), who used a wet-chemistry method. As shown in Table 1, the lavas in the Huangyishan volcano are SiO2-undersaturated (48.00-51.5 wt%), rich in TiO2 (1.88-2.25 wt%) and alkali ( N a 2 0 + K 2 0 = 5.83-7.39 wt%). The first flow is a basanite, containing up to 13% normative nepheline. The other four flows are also nepheline-normative (2-4% normative nepheline) and fall into the alkali olivine basalt group. All lavas are of hypocrystalline to
B-2
B-3
B-4
B-5
SiO2 TiO2 AlzO3 Fe203 FeO MnO MgO CaO Na20 K20 P205
48.00 2.25 [5.04 1.90 9.30 0.18 7.60 7.76 4.89 2.50 0.58
51.49 1.90 15.72 1.76 8.58 0.16 6.90 6.37 4.90 1.8 l 0.39
51.18 1.97 15.68 1.90 9.27 0.17 6.78 6.10 4.49 2.06 0.40
50.84 1.90 15.61 1.80 8.77 0.17 7.24 7.13 4.05 2.05 0.44
48.89 1.88 15.84 2.14 10.47 0.22 6.93 7.31 3.84 1.99 0.49
ne ol
13.35 15.81
3.69 16.98
1.99 18.14
3.41 16.45
3.13 19.36
La Ce Nd Sm Eu Gd Dy Er Yb Lu Sr Ba Rb Cs Pb Mo U Th Nb Ta V Cr Ni Co
36.7 72.8 34.7 7.58 2.49 6.87 4.94 2.08 1.49 0.207 745 479 35.5 0.302 4.07 3.22 1.47 5.27 48.5 3.21 141 168 192 71.3
27.5 55.5 26.4 5.87 1.95 5.36 4.20 1.85 1.29 0.190 n.d. n.d. 44.0 0.255 3.99 1.85 1.21 4.47 34.4 2.55 149 132 157 103
26.6 54.7 26.2 5.99 2.02 5.58 4.27 1.87 1.34 0.188 579 364 34.7 0.193 3.69 2.30 1.17 4.41 35.4 2.39 145 157 140 66.9
28.3 59.6 27.7 6.20 1.99 5.70 4.32 1.96 1.44 0.189 n.d. 392 40.8 0.215 4.57 2.46 1.52 5.57 44.6 3.07 156 231 168 63.3
28.9 58.9 27.5 6.27 2.08 5.95 4.43 1.93 1.37 0.188 614 368 43.5 0.277 3.97 2.08 1.32 4.71 36.5 2.79 129 146 149 68.0
48.9 33.0 17.9 0.732 0.109 15.1 0.76
43.0 28.4 14.4 1.28 0.130 13.5 0.80
40.8 30.3 15.5 0.98 0.125 14.8 0.75
41.4 29.3 13.5 0.915 0.125 14.5 0.63
43.0 27.7 14.5 1.19 0.129 13.1 0.79
Ce/Yb Nb/U Ce/Pb Rb/Nb Th/Nb Nb/Ta La/Nb
n.d. = no data.
microporphyritic texture and the phenocrysts are euhedral (usually 0.5-1.0 mm in size),
ALKALI BASALTS AND ASSOCIATED MEGACRYSTS FROM THE HUANGYISHAN VOLCANO
composed of olivine, clinopyroxene and plagioclase. According to Xie et al. (1990), microprobe analyses show that the clinopyroxene phenocrysts are rich in TiO2 (2.04-2.58%), with Wo, En and Fs ranging between 36% and 40%, 41% and 43%, and 18% and 24%, respectively and can be classed as a Ti-augite group. Since 36 analyses of major elements of the Huangyishan lavas (Xie et al., 1990) suggest very. uniform characters for each layer, only one representative sample from each flow has been studied in this work. Megacrysts sampled mainly from the first, second and fourth basalt flows include predominantly clinopyroxene, anorthoclase, garnet, with less abundant amphibole, mica and spinel. Three megacrysts of clinopyroxene, three of garnet and one of amphibole studied here are all very large in size, ~ 2-15 cm in longest dimension. Individual crystals of these three kinds of megacrysts are extremely homogeneous in major elements. The major-element compositions are listed in Table 2, which were determined by wavelength-dispersive electron microprobe with 15-kV accelerating potential and 30-nA beam current at the University of Tokyo. The results reported are the averages of several grains crushed from a large crystal and the uncertainties are generally < _+2%. Clinopyroxene megacrysts are characterized by chemical compositions of augite with 36.0-37.5% of Wo, 50.0-52.2% of En and 11.8-13.8% of Fs. The largest clinopyroxene was analyzed for both its inner ( C p x l ) and outer (Cpxl-1) parts. No significant differences in major- and minor-element compositions have been detected between these two portions. Garnet megacrysts, up to 6 cm in size, are deep-red and glassy, with a conchoidal fracture similar to the glassy augite megacrysts. Chemical compositions of garnet megacrysts belong to the pyrope-almandine series, and are quite uniform with pyrope ranging from 61.2 to 63.8 and almandine from 23.0 to 25.3. The occurrence and chemical composition of these
221
garnet megacrysts strongly resemble those contained in Elie Ness tuff, Scotland, U.K. (Chapman, 1976 ) and in melanephelinite from Kakanui, New Zealand (Dickey, 1968), which were regarded by these authors as precipitates from their host magmas under high pressure. Only one amphibole megacryst has been studied here. The chemical compositions of amphibole megacrysts have been shown to be distinguished from that of amphibole from an associated mantle xenolith (Xie et al., 1990). The megacrysts are enriched in Ti and depleted in Cr and Mg relative to the amphibole of the xenolith.
3. Analytical procedure Isotope dilution and ICP-MS (inductively coupled plasma-mass spectrometry) measurements were carried out in the Laboratory for REE Microanalysis, Department of Chemistry, The University of Tokyo. Rare-earth element (REE), Sr and Ba abundances were determined by the single spiking isotope dilution method using a JEOL ® JMS-05RB mass spectrometer. Great care was first taken against the impurities in the reagents used to decompose the samples and separate REE from major elements, and it is confirmed that the impurities in the reagents used are negligibly small for REE. Secondly, much caution was used in reducing isobaric interference while operating the mass spectrometer. The absolute uncertainties for most REE are nearly _+ 1% or better. The absolute uncertainties for Yb and Lu are often greater than those for other REE, but are at most _+2%. Other trace elements were measured by the matrix matching method on I C P MS. In I C P - M S measurement, the JB-1 (basalt) rock was used as a standard. Uncertainties of this analytical method were < + 5%. Analytical procedures used for Sr, Nd and Ce isotope measurements were the same as those of Shimizu et al. (1988). The isotopic compositions of Sr and Nd were measured with a
222
(-Q LIIlE-[AL.
TABLE 2 Major-element compositions (wt%) and trace-element abundances (ppm) ofmegacrysts Gnt I
Gnt2
Gnt3
Amp 1
Cpx I
Cpx 1-1
Cpx2
Cpx3
SiO: TiOe A12(13 Cr203 Fc203 MnO FeO Ca() MgO Nazi) K2() Ni()
40.43 0.39 22.92 0.04
41.40 0.43 22.91 0.01
40.67 0.43 22.67 O.30
50.22 0.97 8.50 1).02
50.45 0.87 8.51 0.01
49.85 0.83 8.4/) 0.03
49.89 1.(}3 8,49 0.05
0.28 11.78 5.2t 18.02 0.01 0.02 b.d.I,
0.27 11.53 4.99 17.96 b.d.l. b.d.l, b.d.l,
0.30 12.61 5.02 17.13 0.03 b.d.l. b.d.I,
39.46 5.08 13.92 b.d.1. 11.8 0.10 2.01 10.67 11.23 2.30 2.40 b.d.[.
0.10 6.77 15.95 15.38 1.04 0.01 0.04
0.06 6.35 15.46 15.58 1.07 0.01 0.03
0.11 6.58 15.12 15.47 1.02 b.d.I. 0.05
0.13 7.14 15.78 15.20 1.45 0.01 0.04
La Ce Nd Sm Eu Gd Dy Er Yb Lu Sr Ba
0.0295 0.206 0,823 1.09 0.696 3.62 9.09 8.37 10.1 1.19 n.d. n.d.
0.0423 0.280 1.18 1.51 0.938 6.08 12.41 12.63 13.8 2.17 0.628 1.36
1.47 5.52 5.94 2.08 0.752 2.51 2.18 0.963 0.624 0.0778 58.5 n.d.
1.65 6.11 6.75 2.36 0.882 3.03 2.57 1.11 0.731 0.0953 61.2 0.732
1.25 4,68 5,66 2, 18 11,816 2,77 2.49 1.14 0.787 0.103 55.0 0.222
1,45 5,40 6.22 2.34 0.867 2.92 2.54 1.09 0.701 n.d. 60.9 0.917
0.0660 0,380 1.21 1.21 0.797 4.13 8.75 7.05 6.87 1.06 0.521 1.04
4,61 14,3 12.7 3~95 1,61 4.17 2,83 0.863 0.405 0.0427 434 n.d.
Major-element composition of amphibole is from Wei ( 1985 ). Gnt =garnet; Amp=amphibole: Cpx = clinopyroxene, b.d.I. =below detection limit: n.d. = not determined.
V G 5 4 ® sector mass spectrometer with three
Faraday collectors in the Institute of Physical and Chemical Research. The Nd isotope ratios measured were normalized to 146Nd/ ~44Nd = 0.7219. The values for the La Jolla Nd and NBS987 Sr standards were 0.51185 +0.00002 and 0.71023 +_0.00003, respectively, during the measurement period of these samples. Ce isotopic composition was analyzed on a V G 5 4 - 3 8 ® double-focusing mass spectrometer in the Laboratory for REE Microanalysis, Department of Chemistry, The University of Tokyo. The normalizing value of 136Ce/~a2ce was 0.01688. Ce isotope data (138Ce/142Ce) were reported relative to the JMC304 Ce standard of ~38Ce/ ~42Ce=0.0225762. Present values of ( 143Nd/ 144Nd)cHUR and (138Ce/142Ce)enuR used in this paper were 0.512638 (Wasserburg et al.,
1981) and 0.0225722 (Shimizu et al., 1988), respectively. 4. Results 4. I. Trace e l e m e n t s
Trace-element abundances are listed in Table 1 for basalts and in Table 2 for the megacrysts. Basalts from the five lava flows can be grouped under two main types based on their major- and trace-element compositions. REE patterns of basalts from these five layers are quite similar to each other (Fig. 2), showing the features of high enrichment of light REE (LREE), and possessing a slightly but clearly double-concave shape with a turning point around Eu. The first flow basalt (B- 1 ) shows a somewhat more differentiated REE pattern,
A.i,KALIBASALTSAND ASSOCIATEDMEGACRYSTSFROM THE HUANGYISHANVOLCANO
~ o~.~_o~..e\ . ~. \o~
100
.B-I o B2
10
w
5 100
=
~-~
~c;
223
terns of garnet megacrysts are remarkably HREE-rich, very similar to those observed for garnet megacrysts from Elie Ness (Irving and Frey, 1978). The REE pattern for the amphibole megacryst is rich in LREE and MREE and shows a small positive Eu anomaly.
[] B 3
4.2. Isotopes
50
I q)0
a'-.a
" ' ~
~m
• ~4
50
~&
• B5
10
~A..•
('e
10
5
~'LI'--A\A
la
I;°
Nd Sm Eu G,d
i))
Er
10 5
Yb L u
Fig. 2. C h o n d r i t e - n o r m a l i z e d REE abundances for host alkali basalts, Leedey chondrite REE values ( M a s u d a el al.. 1973) are employed as normalizing values in this papcr.
compared with the other four flow basalts (B2-B-5) which have very uniform REE abundances. The Ce/Yb ratio of B-1 is 48.9, while those of the other flows are somewhat lower, ranging from 40.8 to 43.0. Other trace-element features also clearly exhibit this distinction between these two groups, showing that the first flow is more enriched in incompatible elements. In Table 1 are also listed LILE/LILE and LILE/HFSE ratios, in which LILE and HFSE are short for large-ion lithophile elements and high field strength elements, respectively, and a close inspection allows us to recognize the distinctions between B-I and the other basalts. Clinopyroxene megacrysts have very uniform REE patterns, with middle REE (MREE) enhanced and heavy REE (HREE) moderately depleted (Fig. 3). The Sr abundances in clinopyroxenes are relatively invariable. The outer part (Cpx 1-1 ) of the largest clinopyroxene is relatively rich in REE, ~ 15% richer than the inner portion ( C p x l ) but no clear differentiation in REE has been found. REE pat-
Table 3 lists the isotope data for both megacrysts and host basalts. Compared with the bulk Earth, the basalts are higher in radiogenic Nd and lower in radiogenic Sr and Ce, but show enriched features relative to both mid-ocean ridge basalts (MORB) and most of oceanic island basalts (OIB) (Fig. 4 ). Like the trace-element compositions, isotopic compositions divide the basalts into two groups. The first basalt flow (B-l) has a higher '43Nd/144Nd and lower 87Sr/86Srratio than the other four flows. Differences in Ce isotopes are uncertain because of the small number of Ce isotope analyses. 87Sr/S6Sr and 143Nd/144Ndratios of megacrysts are variable with respect to mineral type but overlap those of the host basalts. Clinopyroxenes and the amphibole have a close resemblance in isotopic composition to the basalt Bl, while the garnets are apparently associated with the other four lavas. 5. Discussion 5.1. Mantle source The basalts studied were erupted in association with mantle xenoliths and megacrysts, and therefore, the basalts are believed to be minimally modified by crustal materials in their isotopic and chemical compositions, because of the rapid ascent of magma to the surface of the Earth. The similarities in isotopic compositions between megacrysts and host basalts also argue against the significant crustal contamination. As compared to the oceanic mantle source,
224
C-Q. LIU ET AL.
5
~,*,-,.// Cpxl
t~
trtrtA.'~--- o
er.oei~. ~-~"
o
g
.fJ.-
o~
-.--z....
I ""~0/0
"r" •
100
!
Garnet
Gntl
50 °~
¢-
10
O
5
e--
O 1
¢-
0.5
°n
0.1 La Ce
Nd
Sm Eu Gd
Dy
Er
Yb Lu
Fig. 3. Chondrite-normalized REE abundances for megacrysts of clinopyroxene (Cpx), garnet (Gnt) and amphibole
(,4rap).
the mantle source materials of the Huangyishan basalts show relatively enriched features, with Sr, Nd and Ce isotopic compositions close to those of the bulk Earth. Earlier isotopic studies on basalts and several mantle xenoliths from NE China indicate that the mantle under this area is isotopically and chemically very heterogeneous (Peng et al., 1986; Song and Frey, 1989; Wang et al., 1989; Xie et al., 1990). Both depleted and enriched components have been found in the mantle, but the depleted end-
member observed so far does not bear much resemblance to the MORB source mantle, having a less depleted nature, with the ecm~R(Nd) -t-6. Such features are also observed in the trace-element characteristics. As shown in Table 1, most trace-element ratios in the Huangyishan basalts are generally different from those of the oceanic basalts including both OIB and MORB (Sun, 1980). For example, N b / U and Ce/Pb ratios, which are uniform at 47 + 10 and 25 +_5, respectively, in
225
&LKALI BASALTSAND ASSOCIATED MEGACRYSTS FROM THE HUANGYISHAN VOLCANO TABLE 3 Sr, Nd, Ce isotopic compositions of megacrysts and host basalts
B- 1 B-2 B-3 B-4 B-5 Cpx l ('px 1-1 Cpx2 Cpx3 Gnt I Grit2 Gnt3 -kmp I
87Sr/86Sr
143Nd / t44Nd
13SCe / 142Ce
0.70394 0.70457 0.70452 0.70466 0.70446 0.70382 0.70389 0.70383 0.70383 0.70447 0.70437 0.70453 0.70405
0.512843 0.512740 0.512707 0.512787 0.512787 0.512870 0.512847 0.512848 0.512785 0.512800 0.512850 0.512782 0.512805
0.022569 +_0.000001 0.022567 +_0.00000 I n.d. 0.022572 + 0.000002 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
_+0.00002 + 0.00002 ± 0.00002 + 0.00001 _+0.00002 _+0.00002 ± 0.00002 _+0.00001 _+0.00002 _+0.00002 _+0.00002 _+0.00003 +_0.00001
_+0.000010 _+0.000007 + 0.000009 + 0.000010 _+0.000006 _+0.000005 ± 0.000007 +_0.000005 ± 0.000008 ± 0.000010 _+0.000009 ± 0.000010 +_0.000008
Errors are 2a, n.d. = nol determined.
•
0.5132 <4~O~x,N
host basalt
0.5132
[] clinopyroxene.
0.5131
0.5131
0.5130
~/~
[A amphibole
0.5130
0.5129
%
0.5129
0.5128 0.5127 BULK
0.5126 0.5125
0.5128
DA ~
EARTH
:
i
EARTH
0.5126
®
® i
0.5127 BULK
J
0.702 0.703 0.704
E
0.705 0.706 0.022560
87Sr / 86Sr
0.022572
0.512~q 0.022584
138Ce / 142Ce
Fig. 4. D i a g r a m A shows the Sr and Nd isotopic variations o f megacrysts and their host basalts. Diagram B shows the Ce a n d N d isotopic variations o f three basalts. The Ce and Nd isotopic compositions for M O R B and OIB in diagram B are f r o m Tanaka et al. ( 1 9 8 7 ) .
both MORB and OIB (Hofmann et al., 1986 ), are relatively constant in our basalts, too, but different from them, varying from 27.7 to 33 and from 13.5 to 17.9, and are closer to those ( N b / U = 3 0 and C e / P b = 9 ) of the primitive mantle. Several other trace-element ratios such as R b / N b and T h / N b also imply that the source mantle is enriched in LILE relative to the HFSE, as compared to the OIB and MORB. As mentioned in Sections 4.1 and 4.2, the
first basalt flow differs in both chemical and isotopic compositions from those of the other four flows. Because of the presence of abundant xenoliths in the basalts and the isotopic similarities between megacrysts and host basalts, it is impossible to consider the differences in isotopic and trace-element signature as the results of crustal contamination. Thus the mantle source of the first flow must be geochemically different from that of the other four
226
( - O . gitJ ET A t .
of clinopyroxenes are very similar (Fig. 5). Accordingly, clinopyroxene megacrysts have been interpreted as high-pressure phases of magmas. Simple comparison of AI contents with the experimental data of Thompson ( 1974 ) yields crystallization conditions of 2025 kbar for these three megacryst clinopyroxenes. The cognate relationship between the megacryst clinopyroxenes and the host basalt is supported by the following evidence. First, Sr and Nd isotopic ratios of megacryst clinopyroxenes and basalt B-1 are almost identical, and the small difference in isotopic compositions between the megacrysts and hosts could be ascribed to a small extent of crustal contamination of the magma. Second, according to the data obtained by Wei (1985) for the same samples, KD [ ( FeO / MgO ) c,-~stal/ ( FeO / MgO)host] ranges from 0.27 to 0.35, with a median value of 0.31, which resembles the corresponding value (0.29) experimentally obtained by Thompson (1974). Irving and Frey ( 1984 ) and Liotard et al. ( 1988 ) consider that pyroxenes yielding apparent KD-values between 0.2 and 0.4, may have an equilibrium relationship with their hosts under high-pressure conditions. The high-pressure origin of garnets has been
flows. The source of the first basalt has a depleted nature, with higher S m / N d and lower Rb/Sr ratios relative to those of the other four layers. The incompatible-element ratios such as T h / N b and Rb/Nb, though not indisputable, also generally support this conclusion. This finding contradicts the consideration of several authors (e.g., Wei, 1985) that all basaltic flows of Huangyishan were derived from the same source.
5.2. Origin of megacrysts Megacrysts occurring in alkali basalts are widely considered to be the precipitates of magmas under high pressures (e.g., Green and Ringwood, 1967; Binns, 1969), but they may be either cognate or foreign in origin with their host basalts (Wass, 1979). Megacryst clinopyroxenes studied here are easily distinguished from the clinopyroxene phenocrysts by the features of much higher Mg-number [Mg/ ( M g + F e 2+ + F e 3+) ], AlVa/A1TM ratios and Na20 and clearly lower TiO2 contents, and also different from the clinopyroxene in mantle xenoliths with regard to the contents of their negligible Cr203 and relatively high TiO2, though the AlV~/AITM ratios in these two kinds
• c|inop)roxent' phcnocQst • clinopyroxene mrgacryst o c|inopyroxene in xenolith
3
0.3
6F--
0.2 c) 0
1
0.1
I
I
0.1
0.2 AI 'v
**
A
0.3
60
I
I
70
80
t,~ '
90
100 Mg/(Mg+Fe}
Fig. 5. Plots of A1 vl vs. AI TM, TiO2 vs. Mg-number for clinopyroxenes in mantle xenoliths and megacryst, phenocryst clinopyroxenes. Data for clinopyroxene in the mantle xenoliths and clinopyroxene phenocrysts are from Wei ( 1985 ).
-'~LK~L1BASAL'I'SAND ASSOCIATED MEGACRYSTS FROM THE HUANGYISHAN VOLCANO
corroborated by a large number of experiments, and moreover, garnet can be coprecipitated with pyroxene from a silicate melt commonly under pressures of >20 kbar (e.g., Cohen et al., 1967; O'Hara et al., 1971; Thompson, 1974: Chapman, 1976). In this study, megacryst amphibole, clinopyroxenes and garnets are very similar to each other in 143Nd/144Ndbut apparently different in S7Sr/ a6Sr isotopic compositions. Unlike the clinopyroxenes which resemble in 878r/86Sr isotopic compositions the first basalt flow. the garnets appear to be in a close association with the other four flows. Since garnet is not expected to contain high Rb abundance and the age of this volcano is relatively young, the difference in Sr isotopic composition between clinopyroxene, amphibole and garnet could not be due to radiogenic growth. So these two kinds of minerals are probably not precipitated in strictly identical circumstances. Alternatively, it is possible that these two kinds of megacrysts were originally precipitated from a common magma source, the first flow basalt, but carried up to the surface by different magmas. The similarity in Sr compositions of megacrysts with different host magmas might be the result of high-rate diffusion of Sr elements between crystal and melt. Because the megacryst samples are relatively few, further study on more megacrysts is needed to solve this problem. In conclusion, the megacrysts under consideration have been inferred to be high-pressure phases and cognate with their host basalts, in the light of major-element compositions, traceelement characteristics and isotopic high similarity of both magmas and crystals. REE, Sr and Ba solid/liquid partition coefficients for these megacryst minerals have been computed using the B-1 basalt as a common liquid, and the results are listed in Table 4, together with those observed for megacrysts by Irving and Frey ( 1978, 1984) for the sake of comparison. Although some discrepancies are seen, the similarities between these two sets of data are remarkable.
227
TABLE 4 T r a c e - e l e m e n t a b u n d a n c e ratios for m e g a c w s t / h o s t pairs
La Ce Nd Sm Eu Gd Dy Er Yb Lu Sr Ba
Clinopyroxene
Garnet
average* L
median* 2
average* ~
median*
0.0396 0.075 0.178 0.297 0.332 0.409 0.495 0.517 0.477 0,444 0.0668 0.00136
0.058 0. 100 0.22 0.45 0.48 0.50 0.60 0.61 0,58 0,53 0.10 0.005
0.00177 O.00528 0.0408 0.219 0.402 0.826 2.36 5.00 7.64 8,71 0.000504 0.0033
0.00045 0.007 0.026 0.105 0.229 0.46 1.94 4.7 5.7 6.67 n.d. n.d.
n.d. = no data. *~This work: *2Irving and F r e t (1 984): *:~lrving a n d Frey (1978).
5.3. Petrogenesis of host hasalts The host alkali basalts are found to have REE patterns with a double-concave or doublecurved structure. This kind of REE patterns can also be found for alkali basalts from other locations not only in eastern China (see Fig. 6 ) but also in other continents (e.g., Kay and Gast, 1973; Sun and Hanson. 1975: Davidson and Wilson, 1989; Barrat et al., 1990 ). A more careful check of more published data shows that a nearly straight-line shape can also been found in LREE of some alkali basalts. The turning point of this fine structure is either mostly located around Eu or sometimes around Gd and thus a small positive Eu anomaly in most cases is visible. It is obvious that it would be necessary to determine the REE abundances of more samples worldwide with high precision for clarifying this fine structure, especially in the LREE span of alkali basalts. However, based on the REE data obtainable at present, we think this feature is intrinsic and must be of prime significance for the origin and genesis of alkali basalts. Sun and Hanson (1975) used the extent of the Eu anomaly to evaluate the Eu 2+/Eu 3+ ratios in order to draw
228
(-Q. LIU ET AL
lO00
,
-
-
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Fig. 6. Chondrite-normalized REE abundances for alkali basalts from eastern China. These unpublished data of C.-Q. Liu and A. Masuda (1990) were determined by the isotope dilution method and all show a double-concave or double-curved REE pattern for those alkali basalts. Sample locations: M.H.=Mingqi, Hujian Province; N.J.=Nanjing, Jiangsu Province; N.A.=Nushan, Anhui Province; X.S. = Xisha, Shandong Province: Q.L. = Qingyuan, Liaoning Province; W.J. = Wangqing, Jilin Province; H.H. = Hannuoba, Hebei Province.
information concerning oxygen fugacity in the mantle. Davidson and Wilson (1989) found these small positive anomalies in alkali basalts from Africa but left the origin of them unexplained. Here, we interpret this small positive Eu "anomaly" found in most alkali basalts as an appearance caused by the turning point of the double-curved structure, since: (1) this positive Eu anomaly in alkali basalts is visible in most cases but not in all cases; (2) the extent of anomaly is 2-5%, if present (Sun and Hanson, 1975 ), which is so small that very accurate determinations are required to evaluate it quantitatively; and (3) this effect can be theoretically reproduced using fractional crystallization and partial melting models as follows.
Most alkali basalts usually contain megacrysts of clinopyroxene, anorthoclase, amphibole, garnet and mica minerals. Crystallization of such megacrysts from basaltic magmas may be an important geochemical process in the petrogenesis of alkali basalts. The result of a geochemical modelling for fractional crystallization is presented in Fig. 7A, assuming a smooth REE pattern for the primary magma as shown in this diagram. Equilibrium crystallization of mineral assemblage of 15% garnet and 85% clinopyroxene to different degrees gives rise to the magmas with REE patterns very similar in fine features to those observed for alkali basalts. Moreover, the double-concave structure becomes more and more clear with the progression of solidification of magma. In a batch partial melting model, we presume that alkali basalt is generated from partial melting of a garnet peridotite source, which contains 5% garnet and 20% clinopyroxene in addition to 50% of olivine and 25% of orthopyroxene. The bulk partition coefficients were calculated with assumptions that the REE partition coefficients for olivine and orthopyroxene are negligible, and the melting mineral assemblage is the same as the initial solid. Three starting compositions, LREE-depleted, chondritic and LREE-enriched, with the concentrations of HREE two times those of chondrite, have been used in the modelling calculation and the results are illustrated in Fig. 7B-D. The shapes of REE patterns calculated for liquids at small degrees of partial melting are very similar to the B-1 basalt. For a LREE-depleted source, very small degrees ( < 0 . 5 % ) of melting are required to achieve a double-concave REE pattern. The validity of the calculated REE patterns maybe largely depends on the assumption that
Fig. 7. Comparison of REE patterns calculated by fractional crystallization of "eclogite" (diagram A) and by partial melting of a garnet peridotite source with different REE compositions (diagrams B-D) with the observed pattern for BI host alkali basalt. The average REE megacryst/host ratios obtained in this work (Table 4) for garnet and clinopyroxene were used as partition coefficients in these model calculations.
ALKALI BASALTS AND ASSOCIATED
~a
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229
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fractional crystallization of "eclogite" (Cpx. 85"/., Grit 15"/d
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230
the orthopyroxene and olivine contain no REE. However, if the REE partition coefficient pattern calculated by Sun and Hanson ( 1975 ) for a bulk residue of 15% clinopyroxene, 5% garnet, 55% olivine and 25% orthopyroxene is used, a double-concave REE pattern for liquid in the partial melting model is still obtainable, suggesting that this assumption is acceptable. According to the calculated results of a batch partial melting model, the turning point of this double-curved structure of the REE pattern should not be fixed, since the REE partition coefficients are dependent upon several chemico-physical conditions such as pressure, temperature and chemical compositions of the system. Therefore, the fact that this turning point in most cases is placed at Eu, demonstrates that the chemico-physical conditions for the generation of alkali basalts are very similar. Although fractional crystallization of megacrysts can account for the origin of the REE pattern, the highly advanced crystallizations as indicated by the calculation (Fig. 7A) seems to be in conflict with the very small fraction of megacrysts found in the host basalts. Theretore, partial melting of a garnet peridotite source is in all probability the most plausible important process to generate the alkali basalts. The slightly lower concentrations of compatible elements such as Ni and Cr could be the result of high-pressure fractional crystallization of clinopyroxene and olivine. Fractional crystallization of garnet and clinopyroxene following the partial melting of a garnet peridotite source as shown on p. 228 may make the double-concave structure of the REE pattern more conspicuous.
C-Q. LIU ET AL.
patible trace-element ratios also suggest that the source mantle is rich in large-ion elements. Moreover, the mantle source is isotopically and chemically heterogeneous. We conclude that the Huangyishan basalts are the products mainly of partial melting of a garnet peridotite source. Megacrysts of clinopyroxene, garnet and amphibole in this study are genetically associated with the host basalts. Investigation into the formation of the doubleconcave structure of the REE pattern for host alkali basalts leads to the conclusion that both garnet and clinopyroxene must have been once in equilibrium or quasi-equilibrium with the basalt magma. The double-concave REE pattern of alkali basalts can be developed only where both garnet and clinopyroxene coexist in the solid phase; such a pattern cannot be produced in the presence of only one of these mineral species. The small positive Eu anomaly found in most alkali basalts is an effect resulting from the fact that Eu happens to be the turning point of the double-concave curve, and so this Eu anomaly does not bear any information on oxygen fugacity in the mantle in this case. Acknowledgements We are grateful to the comments of Dr. S.-S Sun and the reviews of Drs. Hilary Downes, Bor-ming Jahn and Nicholas T. Arndt, which led to substantial improvements in the manuscript. This research was supported by a grantin-aid for Scientific Research from The Ministry of Education, Science and Culture, and also partly supported by the Special Researcher's Basic Science Program, Japan.
6. Conclusions References The alkali basalts containing mantle xenoliths and megacrysts from the Huangyishan volcano are inferred to be derived from a less depleted mantle with long-term features of higher Rb/Sr, L a / C e and lower S m / N d ratios relative to the MORB mantle. Highly incom-
Barrat, J.-A., Jahn, B.M., Joron, J.-L., Auvray, B. and Hamdi, H., 1990. Mantle heterogeneity in northeastern Africa: evidence from Nd isotopic compositions and hygromagmaphile element geochemistry of basaltic rocks from the Gulf of Tadjoura and southern Red Sea regions. Earth Planet. Sci. Leu., 101: 233-247.
ALKALI BASALTS AND ASSOCIATED MEGACRYSTS FROM THE HUANGY1SHAN VOLCANO
Binns, R.A., 1969. High-pressure megacrysts in basanitic lavas near Armidale, New South Wales. Am. J. Sci., 267-A: 33-49. Chapman, N.A., 1976. Inclusions and megacrysts from undersaturated tufts and basanites, East Fife, Scotland. J. Petrol., 17: 472-498. Cohen, L.H., Ito, K. and Kennedy, G.C., 1967. Melting and phase relations in an anhydrous basalt to 40 kilobars. Am. J. Sci., 265: 475-518. Davidson, J.P. and Wilson, I.R., 1989. Evolution of an alkali basalt-trachyte suite from Jebel Marra volcano, Sudan, through assimilation and fractional crystallization. Earth Planet. Sci. Lett., 95: 141-160. Dickey, J.S., 1968. Eclogitic and other inclusions in the Mineral Breccia Member of the Deborah Volcanic Formation at Kakanui, New Zealand. Am. Mineral., 53: 1304-1319. E, M.L., Lu, F.X. and Den, J.F., 1987. Cenozoic basalts and ultramafic inclusions of NE China. In: M.L. E and D.S. Zhou (Editors), Cenozoic Basalts and MantleDerived Inclusions in Eastern China. Scientific Press, Beijing, pp. 10-17 (in Chinese). Green, D.H. and Ringwood, A.E., 1967. The genesis of basaltic magmas. Contrib. Mineral. Petrol., 15: 103190. Hofmann, A.W., Jochum, K.P., Seufer, M. and While, W.M., 1986. Nb and Pb in oceanic basalts: New constraints on mantle evolution. Earth Planet. Sci. Left., 79: 33-45. Irving, N.J. and Frey, F.A., 1978. Distribution of trace elements between garnet megacrysts and host volcanic liquids of kimberlitic to rhyolitic composition. Geochim. Cosmochim. Acta, 42:771-787. Irving, ~X.J. and Frey, F.A., 1984. Trace element abundances in megacrysts and their host basalts: Constraints on partition coefficients and megacryst genesis. Geochim. Cosmochim. Acta, 42:771-787. Kay, R.W. and Gast, P.W., 1973. The rare earth content and origin of alkali-rich basalts. J. Geol., 81: 653-682. Liotard, J.M., Briot, D. and Boivin, P., 1988. Petrological and geochemical relationships between pyroxene megacrysts and associated alkali basalts from Massif Central I'France). Contrib. Mineral. Petrol., 98: 81-90. Lu. F.-X., E, M.-L. and Dun, J.-H., 1983. Ultramafic xenoliths and megacrysts in alkali basalts of Huangyishan volcano, Kuandian, northeast China. Petrol. Res., 3:77-83 (in Chinese). Masuda, A., Nakamura, N. and Tanaka, T., 1973. Fine structures of mutually normalized rare-earth patterns of chondrites. Geochim. Cosmochim. Acta, 37: 239248. O'Hara. M.J., Richardson, S.W. and Wilson, G., 1971.
231
Garnet peridotite stability and occurrence in crust and mantle. Contrib. Mineral. Petrol., 32: 48-68. Peng, Z.-C., Zartman, R.E, Futa, K. and Chem D.G., 1986. Pb-, Sr- and Nd-isotope systematics and chemi-cal characteristics of Cenozoic basalts, Eastern China. Chem. Geol. (Isot. Geosci. Sect. ), 59: 3-33. Shimizu, H., Nakai, S., Tasaki, S., Masuda, A., Bridgwater, D., Nutman, A.P. and Baadsgaard, H.. 1988. Geochemistry of Ce and Nd isotopes and REE abundances in the Amitsoq gneisses, West Greenland. Earth Planel. Sci. Lett., 91: 159-169. Song, Y. and Frey, F.A., 1989. GeochemistD' ofperidotitc xenoliths in basalt from Hannuoba, Eastern China: Implications for subcontinental mantle heterogeneity Geochim. Cosmochim. Acta, 53: 97-113. Sun, S.-S., 1980. Lead isolopic study of young volcanic rocks from mid-ocean ridges, ocean islands and island arcs. Philos. Trans. R. Soc. London, Set. A, 297: 409-445. Sun, S.-S. and Hanson, G.N., 1975. Origin of Ross Island basanitoids and limitations upon the heterogeneity of mantle source for alkali basalts and nephelinites. Contrib. Mineral. Petrol., 52: 77-106. Tanaka, T., Shimizu, H., Kawata, Y. and Masuda, A.. 1987. Combined La-Ce and Sm-Nd isotope systematics in petrogenic studies. Nature (London), 327: 113-117. Thompson, R.N., 1974. Some high-pressure pyroxenes. Mineral. Mag., 39: 768-787. Wang, J.-W., Xie, G.-H., Tatsumoto, M. and Basu, A.R.. 1989. St, Nd, Pb isotope geochemistry' and magma evolution of the potassic volcanic rocks, Wudalianchi, northeast China. Chin. J. Geochem., 8: 322-330. Wass, S.Y., 1979. Multiple origins of clinopyroxenes in alkali basaltic rocks. Lithos, 12:115-132. Wasserburg, G.J., Jacobsen, S.B., DePaolo, D.J., McCulloch, M.T. and Wen, T., 1981. Precise determination of Sm/Nd ratios, Sm and Nd isotopic abundances in standard solutions. Geochim. Cosmochim. Acta, 45: 2311-2323. Wei, K.J., 1985. Studies on geochemistry and petrology of basalts and mantle xenoliths of Huangyishan volcano, Kuandian, Liaoning, northeast China. MS Thesis, Institute of Geochemistry, Academica Sinica, Guiyang (in Chinese ). Xie, G.-H., Wang, J.-W., Wei, K.J., Liu, C.-Q., Tatsumoto, M. and Basu, A.R., 1990. REE and St, Nd, Pb isotopic geochemistry of Huangyishan basalt, Kuandian, Liaoning, northeast China. Sci. China, Ser. B, 33: 747-756.
219
Elsevier Science Publishers B.V., A m s t e r d a m
[31
Isotope and trace-element geochemistry of alkali basalts and associated megacrysts from the Huangyishan volcano, Kuandian, Liaoning, NE China Cong-Qiang
L i u a'b'c,
Akimasa Masuda ~ and Guang-Hong Xie c
"The Institute of Physical and Chemical Research, Hirosawa 2-1, Wako-shi. Saitama. 351-01, Japan bLaborato~vfor REE Microanalysis, Department of Chemistry, Faculty of Science, The University of Tokyo, Tokyo 113, Japan ~The Institute of Geochemistry, Academia Sinica, Guiyang, Guizhou Province, People's RepuMic o#China (Received June 10, 1991 ; revised and accepted September 26, 1991 )
ABSTRACT Liu, C.-Q., Masuda, A. and Xie, G.-H., 1992. Isotope and trace-element geochemistry of alkali basalts and associated megacrysts from the Huangyishan volcano, Kuandian, Liaoning, NE China. Chem. Geol.. 97:219-231. Trace-element abundances and Sr, Nd isotopic compositions have been measured on three megacrysts of clinopyroxene, three of garnet and one of amphibole, as well as their host alkali basalts from the Huangyishan volcano, Kuandian, NE China. The Ce isotopic compositions of three host basalt samples have been also analyzed. All megacrysts studied here are shown to be cognate with the host basalts mainly based on the isotopic compositions and major-element distributions between silicate crystal and melt. The source mantle of Huangyishan basalts is isotopically and chemically heterogeneous, and must have been enriched in Rb, La and Sm relative to Sr, Ce and Nd, respectively, for a relatively long time. Highly incompatible trace-element ratios are also suggestive of enrichment of large-ion lithophile elements in the mantle compared with the mantle source of MORB and most OIB. The fine structure, i.e. the double-concave shape of REE patterns commonly found in alkali basalts, is considered to be an important clue to the origin of alkali basalts. Based on the REE partition between the megacrysts and host basalts, evaluation of several theoretical models which can account for the double-concave shape of REE patterns shows that alkali basalts with such patterns must have been in equilibrium or quasi-equilibrium with both garnet and clinopyroxene solid phases. The small positive Eu anomaly commonly observed in REE patterns in alkali basalts, in most cases, is actually the turning point of the double-curved REE pattern. Partial melting of a garnet peridotite source is the most probable process which can generate the alkali basalts under consideration.
1. Introduction In the northeastern part of the Sino-Korean platform, the Kuandian depression basin is underlain by a basement of Proterozoic metamorphic rocks (Fig. 1). In this depression, twenty Cenozoic volcanoes, among which the Huangyishan volcano is the largest one, are arC o r r e s p o n d e n c e to: Dr. Cong-Qiang Liu, T h e Institute of Physical and Chemical Research, Hirosawa 2-1, Wako-shi, Saitama, 351-01, Japan.
ranged in NE and NW directions. The lavas in the Huangyishan volcano can be divided into five flows, clearly showing five discrete eruptions. The eruption age of this volcano is 0.274 Ma (Xie et al., 1990). Each of the eruptive cycles started with pyroclastics and terminated by lava flows. Mantle xenoliths and high-pressure megacrysts are abundant, especially in the first, second and fourth cycles. The petrogenesis of the Huangyishan basalts has been studied by several authors (Lu et al., 1983; Wei, 1985; Xie et al., 1990). However,
0 0 0 9 - 2 5 4 1 / 9 2 / $ 0 5 . 0 0 © 1992 Elsevier Science Publishers B.V. All rights reserved.
220
('-Q. LIU ET AL TABLE 1
Major-element compositions (wt°/o) and trace-element abundances (ppm) of Huangyishan basalts B-I / d l ~
S"
Kuandian
~/
Faults
[-~
o
krn
Cenozoic Basalts
Fig. 1. Sketch map showing Cenozoic basalts in Kuandian and adjacent areas o f NE China. This map was modified from E et al. (1987).
several geochemical hypotheses concerning the origin of the basalts and associated megacrysts have not yet been tested by enough chemical and isotopic data. Since much of the previous work largely involving the basalts is mainly petrological and mineralogical, the present work is concentrated upon the geochemistry of both basalts and megacrysts of several kinds of minerals. The aims of this paper are to identify the geochemical features of the source mantle and to investigate the petrogenesis of the basalts. 2. Samples The major-element compositions of the Huangyishan basalts are from Wei (1985), who used a wet-chemistry method. As shown in Table 1, the lavas in the Huangyishan volcano are SiO2-undersaturated (48.00-51.5 wt%), rich in TiO2 (1.88-2.25 wt%) and alkali ( N a 2 0 + K 2 0 = 5.83-7.39 wt%). The first flow is a basanite, containing up to 13% normative nepheline. The other four flows are also nepheline-normative (2-4% normative nepheline) and fall into the alkali olivine basalt group. All lavas are of hypocrystalline to
B-2
B-3
B-4
B-5
SiO2 TiO2 AlzO3 Fe203 FeO MnO MgO CaO Na20 K20 P205
48.00 2.25 [5.04 1.90 9.30 0.18 7.60 7.76 4.89 2.50 0.58
51.49 1.90 15.72 1.76 8.58 0.16 6.90 6.37 4.90 1.8 l 0.39
51.18 1.97 15.68 1.90 9.27 0.17 6.78 6.10 4.49 2.06 0.40
50.84 1.90 15.61 1.80 8.77 0.17 7.24 7.13 4.05 2.05 0.44
48.89 1.88 15.84 2.14 10.47 0.22 6.93 7.31 3.84 1.99 0.49
ne ol
13.35 15.81
3.69 16.98
1.99 18.14
3.41 16.45
3.13 19.36
La Ce Nd Sm Eu Gd Dy Er Yb Lu Sr Ba Rb Cs Pb Mo U Th Nb Ta V Cr Ni Co
36.7 72.8 34.7 7.58 2.49 6.87 4.94 2.08 1.49 0.207 745 479 35.5 0.302 4.07 3.22 1.47 5.27 48.5 3.21 141 168 192 71.3
27.5 55.5 26.4 5.87 1.95 5.36 4.20 1.85 1.29 0.190 n.d. n.d. 44.0 0.255 3.99 1.85 1.21 4.47 34.4 2.55 149 132 157 103
26.6 54.7 26.2 5.99 2.02 5.58 4.27 1.87 1.34 0.188 579 364 34.7 0.193 3.69 2.30 1.17 4.41 35.4 2.39 145 157 140 66.9
28.3 59.6 27.7 6.20 1.99 5.70 4.32 1.96 1.44 0.189 n.d. 392 40.8 0.215 4.57 2.46 1.52 5.57 44.6 3.07 156 231 168 63.3
28.9 58.9 27.5 6.27 2.08 5.95 4.43 1.93 1.37 0.188 614 368 43.5 0.277 3.97 2.08 1.32 4.71 36.5 2.79 129 146 149 68.0
48.9 33.0 17.9 0.732 0.109 15.1 0.76
43.0 28.4 14.4 1.28 0.130 13.5 0.80
40.8 30.3 15.5 0.98 0.125 14.8 0.75
41.4 29.3 13.5 0.915 0.125 14.5 0.63
43.0 27.7 14.5 1.19 0.129 13.1 0.79
Ce/Yb Nb/U Ce/Pb Rb/Nb Th/Nb Nb/Ta La/Nb
n.d. = no data.
microporphyritic texture and the phenocrysts are euhedral (usually 0.5-1.0 mm in size),
ALKALI BASALTS AND ASSOCIATED MEGACRYSTS FROM THE HUANGYISHAN VOLCANO
composed of olivine, clinopyroxene and plagioclase. According to Xie et al. (1990), microprobe analyses show that the clinopyroxene phenocrysts are rich in TiO2 (2.04-2.58%), with Wo, En and Fs ranging between 36% and 40%, 41% and 43%, and 18% and 24%, respectively and can be classed as a Ti-augite group. Since 36 analyses of major elements of the Huangyishan lavas (Xie et al., 1990) suggest very. uniform characters for each layer, only one representative sample from each flow has been studied in this work. Megacrysts sampled mainly from the first, second and fourth basalt flows include predominantly clinopyroxene, anorthoclase, garnet, with less abundant amphibole, mica and spinel. Three megacrysts of clinopyroxene, three of garnet and one of amphibole studied here are all very large in size, ~ 2-15 cm in longest dimension. Individual crystals of these three kinds of megacrysts are extremely homogeneous in major elements. The major-element compositions are listed in Table 2, which were determined by wavelength-dispersive electron microprobe with 15-kV accelerating potential and 30-nA beam current at the University of Tokyo. The results reported are the averages of several grains crushed from a large crystal and the uncertainties are generally < _+2%. Clinopyroxene megacrysts are characterized by chemical compositions of augite with 36.0-37.5% of Wo, 50.0-52.2% of En and 11.8-13.8% of Fs. The largest clinopyroxene was analyzed for both its inner ( C p x l ) and outer (Cpxl-1) parts. No significant differences in major- and minor-element compositions have been detected between these two portions. Garnet megacrysts, up to 6 cm in size, are deep-red and glassy, with a conchoidal fracture similar to the glassy augite megacrysts. Chemical compositions of garnet megacrysts belong to the pyrope-almandine series, and are quite uniform with pyrope ranging from 61.2 to 63.8 and almandine from 23.0 to 25.3. The occurrence and chemical composition of these
221
garnet megacrysts strongly resemble those contained in Elie Ness tuff, Scotland, U.K. (Chapman, 1976 ) and in melanephelinite from Kakanui, New Zealand (Dickey, 1968), which were regarded by these authors as precipitates from their host magmas under high pressure. Only one amphibole megacryst has been studied here. The chemical compositions of amphibole megacrysts have been shown to be distinguished from that of amphibole from an associated mantle xenolith (Xie et al., 1990). The megacrysts are enriched in Ti and depleted in Cr and Mg relative to the amphibole of the xenolith.
3. Analytical procedure Isotope dilution and ICP-MS (inductively coupled plasma-mass spectrometry) measurements were carried out in the Laboratory for REE Microanalysis, Department of Chemistry, The University of Tokyo. Rare-earth element (REE), Sr and Ba abundances were determined by the single spiking isotope dilution method using a JEOL ® JMS-05RB mass spectrometer. Great care was first taken against the impurities in the reagents used to decompose the samples and separate REE from major elements, and it is confirmed that the impurities in the reagents used are negligibly small for REE. Secondly, much caution was used in reducing isobaric interference while operating the mass spectrometer. The absolute uncertainties for most REE are nearly _+ 1% or better. The absolute uncertainties for Yb and Lu are often greater than those for other REE, but are at most _+2%. Other trace elements were measured by the matrix matching method on I C P MS. In I C P - M S measurement, the JB-1 (basalt) rock was used as a standard. Uncertainties of this analytical method were < + 5%. Analytical procedures used for Sr, Nd and Ce isotope measurements were the same as those of Shimizu et al. (1988). The isotopic compositions of Sr and Nd were measured with a
222
(-Q LIIlE-[AL.
TABLE 2 Major-element compositions (wt%) and trace-element abundances (ppm) ofmegacrysts Gnt I
Gnt2
Gnt3
Amp 1
Cpx I
Cpx 1-1
Cpx2
Cpx3
SiO: TiOe A12(13 Cr203 Fc203 MnO FeO Ca() MgO Nazi) K2() Ni()
40.43 0.39 22.92 0.04
41.40 0.43 22.91 0.01
40.67 0.43 22.67 O.30
50.22 0.97 8.50 1).02
50.45 0.87 8.51 0.01
49.85 0.83 8.4/) 0.03
49.89 1.(}3 8,49 0.05
0.28 11.78 5.2t 18.02 0.01 0.02 b.d.I,
0.27 11.53 4.99 17.96 b.d.l. b.d.l, b.d.l,
0.30 12.61 5.02 17.13 0.03 b.d.l. b.d.I,
39.46 5.08 13.92 b.d.1. 11.8 0.10 2.01 10.67 11.23 2.30 2.40 b.d.[.
0.10 6.77 15.95 15.38 1.04 0.01 0.04
0.06 6.35 15.46 15.58 1.07 0.01 0.03
0.11 6.58 15.12 15.47 1.02 b.d.I. 0.05
0.13 7.14 15.78 15.20 1.45 0.01 0.04
La Ce Nd Sm Eu Gd Dy Er Yb Lu Sr Ba
0.0295 0.206 0,823 1.09 0.696 3.62 9.09 8.37 10.1 1.19 n.d. n.d.
0.0423 0.280 1.18 1.51 0.938 6.08 12.41 12.63 13.8 2.17 0.628 1.36
1.47 5.52 5.94 2.08 0.752 2.51 2.18 0.963 0.624 0.0778 58.5 n.d.
1.65 6.11 6.75 2.36 0.882 3.03 2.57 1.11 0.731 0.0953 61.2 0.732
1.25 4,68 5,66 2, 18 11,816 2,77 2.49 1.14 0.787 0.103 55.0 0.222
1,45 5,40 6.22 2.34 0.867 2.92 2.54 1.09 0.701 n.d. 60.9 0.917
0.0660 0,380 1.21 1.21 0.797 4.13 8.75 7.05 6.87 1.06 0.521 1.04
4,61 14,3 12.7 3~95 1,61 4.17 2,83 0.863 0.405 0.0427 434 n.d.
Major-element composition of amphibole is from Wei ( 1985 ). Gnt =garnet; Amp=amphibole: Cpx = clinopyroxene, b.d.I. =below detection limit: n.d. = not determined.
V G 5 4 ® sector mass spectrometer with three
Faraday collectors in the Institute of Physical and Chemical Research. The Nd isotope ratios measured were normalized to 146Nd/ ~44Nd = 0.7219. The values for the La Jolla Nd and NBS987 Sr standards were 0.51185 +0.00002 and 0.71023 +_0.00003, respectively, during the measurement period of these samples. Ce isotopic composition was analyzed on a V G 5 4 - 3 8 ® double-focusing mass spectrometer in the Laboratory for REE Microanalysis, Department of Chemistry, The University of Tokyo. The normalizing value of 136Ce/~a2ce was 0.01688. Ce isotope data (138Ce/142Ce) were reported relative to the JMC304 Ce standard of ~38Ce/ ~42Ce=0.0225762. Present values of ( 143Nd/ 144Nd)cHUR and (138Ce/142Ce)enuR used in this paper were 0.512638 (Wasserburg et al.,
1981) and 0.0225722 (Shimizu et al., 1988), respectively. 4. Results 4. I. Trace e l e m e n t s
Trace-element abundances are listed in Table 1 for basalts and in Table 2 for the megacrysts. Basalts from the five lava flows can be grouped under two main types based on their major- and trace-element compositions. REE patterns of basalts from these five layers are quite similar to each other (Fig. 2), showing the features of high enrichment of light REE (LREE), and possessing a slightly but clearly double-concave shape with a turning point around Eu. The first flow basalt (B- 1 ) shows a somewhat more differentiated REE pattern,
A.i,KALIBASALTSAND ASSOCIATEDMEGACRYSTSFROM THE HUANGYISHANVOLCANO
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223
terns of garnet megacrysts are remarkably HREE-rich, very similar to those observed for garnet megacrysts from Elie Ness (Irving and Frey, 1978). The REE pattern for the amphibole megacryst is rich in LREE and MREE and shows a small positive Eu anomaly.
[] B 3
4.2. Isotopes
50
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i))
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Fig. 2. C h o n d r i t e - n o r m a l i z e d REE abundances for host alkali basalts, Leedey chondrite REE values ( M a s u d a el al.. 1973) are employed as normalizing values in this papcr.
compared with the other four flow basalts (B2-B-5) which have very uniform REE abundances. The Ce/Yb ratio of B-1 is 48.9, while those of the other flows are somewhat lower, ranging from 40.8 to 43.0. Other trace-element features also clearly exhibit this distinction between these two groups, showing that the first flow is more enriched in incompatible elements. In Table 1 are also listed LILE/LILE and LILE/HFSE ratios, in which LILE and HFSE are short for large-ion lithophile elements and high field strength elements, respectively, and a close inspection allows us to recognize the distinctions between B-I and the other basalts. Clinopyroxene megacrysts have very uniform REE patterns, with middle REE (MREE) enhanced and heavy REE (HREE) moderately depleted (Fig. 3). The Sr abundances in clinopyroxenes are relatively invariable. The outer part (Cpx 1-1 ) of the largest clinopyroxene is relatively rich in REE, ~ 15% richer than the inner portion ( C p x l ) but no clear differentiation in REE has been found. REE pat-
Table 3 lists the isotope data for both megacrysts and host basalts. Compared with the bulk Earth, the basalts are higher in radiogenic Nd and lower in radiogenic Sr and Ce, but show enriched features relative to both mid-ocean ridge basalts (MORB) and most of oceanic island basalts (OIB) (Fig. 4 ). Like the trace-element compositions, isotopic compositions divide the basalts into two groups. The first basalt flow (B-l) has a higher '43Nd/144Nd and lower 87Sr/86Srratio than the other four flows. Differences in Ce isotopes are uncertain because of the small number of Ce isotope analyses. 87Sr/S6Sr and 143Nd/144Ndratios of megacrysts are variable with respect to mineral type but overlap those of the host basalts. Clinopyroxenes and the amphibole have a close resemblance in isotopic composition to the basalt Bl, while the garnets are apparently associated with the other four lavas. 5. Discussion 5.1. Mantle source The basalts studied were erupted in association with mantle xenoliths and megacrysts, and therefore, the basalts are believed to be minimally modified by crustal materials in their isotopic and chemical compositions, because of the rapid ascent of magma to the surface of the Earth. The similarities in isotopic compositions between megacrysts and host basalts also argue against the significant crustal contamination. As compared to the oceanic mantle source,
224
C-Q. LIU ET AL.
5
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Fig. 3. Chondrite-normalized REE abundances for megacrysts of clinopyroxene (Cpx), garnet (Gnt) and amphibole
(,4rap).
the mantle source materials of the Huangyishan basalts show relatively enriched features, with Sr, Nd and Ce isotopic compositions close to those of the bulk Earth. Earlier isotopic studies on basalts and several mantle xenoliths from NE China indicate that the mantle under this area is isotopically and chemically very heterogeneous (Peng et al., 1986; Song and Frey, 1989; Wang et al., 1989; Xie et al., 1990). Both depleted and enriched components have been found in the mantle, but the depleted end-
member observed so far does not bear much resemblance to the MORB source mantle, having a less depleted nature, with the ecm~R(Nd) -t-6. Such features are also observed in the trace-element characteristics. As shown in Table 1, most trace-element ratios in the Huangyishan basalts are generally different from those of the oceanic basalts including both OIB and MORB (Sun, 1980). For example, N b / U and Ce/Pb ratios, which are uniform at 47 + 10 and 25 +_5, respectively, in
225
&LKALI BASALTSAND ASSOCIATED MEGACRYSTS FROM THE HUANGYISHAN VOLCANO TABLE 3 Sr, Nd, Ce isotopic compositions of megacrysts and host basalts
B- 1 B-2 B-3 B-4 B-5 Cpx l ('px 1-1 Cpx2 Cpx3 Gnt I Grit2 Gnt3 -kmp I
87Sr/86Sr
143Nd / t44Nd
13SCe / 142Ce
0.70394 0.70457 0.70452 0.70466 0.70446 0.70382 0.70389 0.70383 0.70383 0.70447 0.70437 0.70453 0.70405
0.512843 0.512740 0.512707 0.512787 0.512787 0.512870 0.512847 0.512848 0.512785 0.512800 0.512850 0.512782 0.512805
0.022569 +_0.000001 0.022567 +_0.00000 I n.d. 0.022572 + 0.000002 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
_+0.00002 + 0.00002 ± 0.00002 + 0.00001 _+0.00002 _+0.00002 ± 0.00002 _+0.00001 _+0.00002 _+0.00002 _+0.00002 _+0.00003 +_0.00001
_+0.000010 _+0.000007 + 0.000009 + 0.000010 _+0.000006 _+0.000005 ± 0.000007 +_0.000005 ± 0.000008 ± 0.000010 _+0.000009 ± 0.000010 +_0.000008
Errors are 2a, n.d. = nol determined.
•
0.5132 <4~O~x,N
host basalt
0.5132
[] clinopyroxene.
0.5131
0.5131
0.5130
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J
0.702 0.703 0.704
E
0.705 0.706 0.022560
87Sr / 86Sr
0.022572
0.512~q 0.022584
138Ce / 142Ce
Fig. 4. D i a g r a m A shows the Sr and Nd isotopic variations o f megacrysts and their host basalts. Diagram B shows the Ce a n d N d isotopic variations o f three basalts. The Ce and Nd isotopic compositions for M O R B and OIB in diagram B are f r o m Tanaka et al. ( 1 9 8 7 ) .
both MORB and OIB (Hofmann et al., 1986 ), are relatively constant in our basalts, too, but different from them, varying from 27.7 to 33 and from 13.5 to 17.9, and are closer to those ( N b / U = 3 0 and C e / P b = 9 ) of the primitive mantle. Several other trace-element ratios such as R b / N b and T h / N b also imply that the source mantle is enriched in LILE relative to the HFSE, as compared to the OIB and MORB. As mentioned in Sections 4.1 and 4.2, the
first basalt flow differs in both chemical and isotopic compositions from those of the other four flows. Because of the presence of abundant xenoliths in the basalts and the isotopic similarities between megacrysts and host basalts, it is impossible to consider the differences in isotopic and trace-element signature as the results of crustal contamination. Thus the mantle source of the first flow must be geochemically different from that of the other four
226
( - O . gitJ ET A t .
of clinopyroxenes are very similar (Fig. 5). Accordingly, clinopyroxene megacrysts have been interpreted as high-pressure phases of magmas. Simple comparison of AI contents with the experimental data of Thompson ( 1974 ) yields crystallization conditions of 2025 kbar for these three megacryst clinopyroxenes. The cognate relationship between the megacryst clinopyroxenes and the host basalt is supported by the following evidence. First, Sr and Nd isotopic ratios of megacryst clinopyroxenes and basalt B-1 are almost identical, and the small difference in isotopic compositions between the megacrysts and hosts could be ascribed to a small extent of crustal contamination of the magma. Second, according to the data obtained by Wei (1985) for the same samples, KD [ ( FeO / MgO ) c,-~stal/ ( FeO / MgO)host] ranges from 0.27 to 0.35, with a median value of 0.31, which resembles the corresponding value (0.29) experimentally obtained by Thompson (1974). Irving and Frey ( 1984 ) and Liotard et al. ( 1988 ) consider that pyroxenes yielding apparent KD-values between 0.2 and 0.4, may have an equilibrium relationship with their hosts under high-pressure conditions. The high-pressure origin of garnets has been
flows. The source of the first basalt has a depleted nature, with higher S m / N d and lower Rb/Sr ratios relative to those of the other four layers. The incompatible-element ratios such as T h / N b and Rb/Nb, though not indisputable, also generally support this conclusion. This finding contradicts the consideration of several authors (e.g., Wei, 1985) that all basaltic flows of Huangyishan were derived from the same source.
5.2. Origin of megacrysts Megacrysts occurring in alkali basalts are widely considered to be the precipitates of magmas under high pressures (e.g., Green and Ringwood, 1967; Binns, 1969), but they may be either cognate or foreign in origin with their host basalts (Wass, 1979). Megacryst clinopyroxenes studied here are easily distinguished from the clinopyroxene phenocrysts by the features of much higher Mg-number [Mg/ ( M g + F e 2+ + F e 3+) ], AlVa/A1TM ratios and Na20 and clearly lower TiO2 contents, and also different from the clinopyroxene in mantle xenoliths with regard to the contents of their negligible Cr203 and relatively high TiO2, though the AlV~/AITM ratios in these two kinds
• c|inop)roxent' phcnocQst • clinopyroxene mrgacryst o c|inopyroxene in xenolith
3
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1
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I
I
0.1
0.2 AI 'v
**
A
0.3
60
I
I
70
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t,~ '
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Fig. 5. Plots of A1 vl vs. AI TM, TiO2 vs. Mg-number for clinopyroxenes in mantle xenoliths and megacryst, phenocryst clinopyroxenes. Data for clinopyroxene in the mantle xenoliths and clinopyroxene phenocrysts are from Wei ( 1985 ).
-'~LK~L1BASAL'I'SAND ASSOCIATED MEGACRYSTS FROM THE HUANGYISHAN VOLCANO
corroborated by a large number of experiments, and moreover, garnet can be coprecipitated with pyroxene from a silicate melt commonly under pressures of >20 kbar (e.g., Cohen et al., 1967; O'Hara et al., 1971; Thompson, 1974: Chapman, 1976). In this study, megacryst amphibole, clinopyroxenes and garnets are very similar to each other in 143Nd/144Ndbut apparently different in S7Sr/ a6Sr isotopic compositions. Unlike the clinopyroxenes which resemble in 878r/86Sr isotopic compositions the first basalt flow. the garnets appear to be in a close association with the other four flows. Since garnet is not expected to contain high Rb abundance and the age of this volcano is relatively young, the difference in Sr isotopic composition between clinopyroxene, amphibole and garnet could not be due to radiogenic growth. So these two kinds of minerals are probably not precipitated in strictly identical circumstances. Alternatively, it is possible that these two kinds of megacrysts were originally precipitated from a common magma source, the first flow basalt, but carried up to the surface by different magmas. The similarity in Sr compositions of megacrysts with different host magmas might be the result of high-rate diffusion of Sr elements between crystal and melt. Because the megacryst samples are relatively few, further study on more megacrysts is needed to solve this problem. In conclusion, the megacrysts under consideration have been inferred to be high-pressure phases and cognate with their host basalts, in the light of major-element compositions, traceelement characteristics and isotopic high similarity of both magmas and crystals. REE, Sr and Ba solid/liquid partition coefficients for these megacryst minerals have been computed using the B-1 basalt as a common liquid, and the results are listed in Table 4, together with those observed for megacrysts by Irving and Frey ( 1978, 1984) for the sake of comparison. Although some discrepancies are seen, the similarities between these two sets of data are remarkable.
227
TABLE 4 T r a c e - e l e m e n t a b u n d a n c e ratios for m e g a c w s t / h o s t pairs
La Ce Nd Sm Eu Gd Dy Er Yb Lu Sr Ba
Clinopyroxene
Garnet
average* L
median* 2
average* ~
median*
0.0396 0.075 0.178 0.297 0.332 0.409 0.495 0.517 0.477 0,444 0.0668 0.00136
0.058 0. 100 0.22 0.45 0.48 0.50 0.60 0.61 0,58 0,53 0.10 0.005
0.00177 O.00528 0.0408 0.219 0.402 0.826 2.36 5.00 7.64 8,71 0.000504 0.0033
0.00045 0.007 0.026 0.105 0.229 0.46 1.94 4.7 5.7 6.67 n.d. n.d.
n.d. = no data. *~This work: *2Irving and F r e t (1 984): *:~lrving a n d Frey (1978).
5.3. Petrogenesis of host hasalts The host alkali basalts are found to have REE patterns with a double-concave or doublecurved structure. This kind of REE patterns can also be found for alkali basalts from other locations not only in eastern China (see Fig. 6 ) but also in other continents (e.g., Kay and Gast, 1973; Sun and Hanson. 1975: Davidson and Wilson, 1989; Barrat et al., 1990 ). A more careful check of more published data shows that a nearly straight-line shape can also been found in LREE of some alkali basalts. The turning point of this fine structure is either mostly located around Eu or sometimes around Gd and thus a small positive Eu anomaly in most cases is visible. It is obvious that it would be necessary to determine the REE abundances of more samples worldwide with high precision for clarifying this fine structure, especially in the LREE span of alkali basalts. However, based on the REE data obtainable at present, we think this feature is intrinsic and must be of prime significance for the origin and genesis of alkali basalts. Sun and Hanson (1975) used the extent of the Eu anomaly to evaluate the Eu 2+/Eu 3+ ratios in order to draw
228
(-Q. LIU ET AL
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,
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o
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*
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o
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Fig. 6. Chondrite-normalized REE abundances for alkali basalts from eastern China. These unpublished data of C.-Q. Liu and A. Masuda (1990) were determined by the isotope dilution method and all show a double-concave or double-curved REE pattern for those alkali basalts. Sample locations: M.H.=Mingqi, Hujian Province; N.J.=Nanjing, Jiangsu Province; N.A.=Nushan, Anhui Province; X.S. = Xisha, Shandong Province: Q.L. = Qingyuan, Liaoning Province; W.J. = Wangqing, Jilin Province; H.H. = Hannuoba, Hebei Province.
information concerning oxygen fugacity in the mantle. Davidson and Wilson (1989) found these small positive anomalies in alkali basalts from Africa but left the origin of them unexplained. Here, we interpret this small positive Eu "anomaly" found in most alkali basalts as an appearance caused by the turning point of the double-curved structure, since: (1) this positive Eu anomaly in alkali basalts is visible in most cases but not in all cases; (2) the extent of anomaly is 2-5%, if present (Sun and Hanson, 1975 ), which is so small that very accurate determinations are required to evaluate it quantitatively; and (3) this effect can be theoretically reproduced using fractional crystallization and partial melting models as follows.
Most alkali basalts usually contain megacrysts of clinopyroxene, anorthoclase, amphibole, garnet and mica minerals. Crystallization of such megacrysts from basaltic magmas may be an important geochemical process in the petrogenesis of alkali basalts. The result of a geochemical modelling for fractional crystallization is presented in Fig. 7A, assuming a smooth REE pattern for the primary magma as shown in this diagram. Equilibrium crystallization of mineral assemblage of 15% garnet and 85% clinopyroxene to different degrees gives rise to the magmas with REE patterns very similar in fine features to those observed for alkali basalts. Moreover, the double-concave structure becomes more and more clear with the progression of solidification of magma. In a batch partial melting model, we presume that alkali basalt is generated from partial melting of a garnet peridotite source, which contains 5% garnet and 20% clinopyroxene in addition to 50% of olivine and 25% of orthopyroxene. The bulk partition coefficients were calculated with assumptions that the REE partition coefficients for olivine and orthopyroxene are negligible, and the melting mineral assemblage is the same as the initial solid. Three starting compositions, LREE-depleted, chondritic and LREE-enriched, with the concentrations of HREE two times those of chondrite, have been used in the modelling calculation and the results are illustrated in Fig. 7B-D. The shapes of REE patterns calculated for liquids at small degrees of partial melting are very similar to the B-1 basalt. For a LREE-depleted source, very small degrees ( < 0 . 5 % ) of melting are required to achieve a double-concave REE pattern. The validity of the calculated REE patterns maybe largely depends on the assumption that
Fig. 7. Comparison of REE patterns calculated by fractional crystallization of "eclogite" (diagram A) and by partial melting of a garnet peridotite source with different REE compositions (diagrams B-D) with the observed pattern for BI host alkali basalt. The average REE megacryst/host ratios obtained in this work (Table 4) for garnet and clinopyroxene were used as partition coefficients in these model calculations.
ALKALI BASALTS AND ASSOCIATED
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the orthopyroxene and olivine contain no REE. However, if the REE partition coefficient pattern calculated by Sun and Hanson ( 1975 ) for a bulk residue of 15% clinopyroxene, 5% garnet, 55% olivine and 25% orthopyroxene is used, a double-concave REE pattern for liquid in the partial melting model is still obtainable, suggesting that this assumption is acceptable. According to the calculated results of a batch partial melting model, the turning point of this double-curved structure of the REE pattern should not be fixed, since the REE partition coefficients are dependent upon several chemico-physical conditions such as pressure, temperature and chemical compositions of the system. Therefore, the fact that this turning point in most cases is placed at Eu, demonstrates that the chemico-physical conditions for the generation of alkali basalts are very similar. Although fractional crystallization of megacrysts can account for the origin of the REE pattern, the highly advanced crystallizations as indicated by the calculation (Fig. 7A) seems to be in conflict with the very small fraction of megacrysts found in the host basalts. Theretore, partial melting of a garnet peridotite source is in all probability the most plausible important process to generate the alkali basalts. The slightly lower concentrations of compatible elements such as Ni and Cr could be the result of high-pressure fractional crystallization of clinopyroxene and olivine. Fractional crystallization of garnet and clinopyroxene following the partial melting of a garnet peridotite source as shown on p. 228 may make the double-concave structure of the REE pattern more conspicuous.
C-Q. LIU ET AL.
patible trace-element ratios also suggest that the source mantle is rich in large-ion elements. Moreover, the mantle source is isotopically and chemically heterogeneous. We conclude that the Huangyishan basalts are the products mainly of partial melting of a garnet peridotite source. Megacrysts of clinopyroxene, garnet and amphibole in this study are genetically associated with the host basalts. Investigation into the formation of the doubleconcave structure of the REE pattern for host alkali basalts leads to the conclusion that both garnet and clinopyroxene must have been once in equilibrium or quasi-equilibrium with the basalt magma. The double-concave REE pattern of alkali basalts can be developed only where both garnet and clinopyroxene coexist in the solid phase; such a pattern cannot be produced in the presence of only one of these mineral species. The small positive Eu anomaly found in most alkali basalts is an effect resulting from the fact that Eu happens to be the turning point of the double-concave curve, and so this Eu anomaly does not bear any information on oxygen fugacity in the mantle in this case. Acknowledgements We are grateful to the comments of Dr. S.-S Sun and the reviews of Drs. Hilary Downes, Bor-ming Jahn and Nicholas T. Arndt, which led to substantial improvements in the manuscript. This research was supported by a grantin-aid for Scientific Research from The Ministry of Education, Science and Culture, and also partly supported by the Special Researcher's Basic Science Program, Japan.
6. Conclusions References The alkali basalts containing mantle xenoliths and megacrysts from the Huangyishan volcano are inferred to be derived from a less depleted mantle with long-term features of higher Rb/Sr, L a / C e and lower S m / N d ratios relative to the MORB mantle. Highly incom-
Barrat, J.-A., Jahn, B.M., Joron, J.-L., Auvray, B. and Hamdi, H., 1990. Mantle heterogeneity in northeastern Africa: evidence from Nd isotopic compositions and hygromagmaphile element geochemistry of basaltic rocks from the Gulf of Tadjoura and southern Red Sea regions. Earth Planet. Sci. Leu., 101: 233-247.
ALKALI BASALTS AND ASSOCIATED MEGACRYSTS FROM THE HUANGY1SHAN VOLCANO
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