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Recognition of contrasting magmatic processes using SB-systematics: An example from the western Central Luzon arc, the Philippines
Chemical Geology, 67 (1988) 197-208 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
197
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RECOGNITION OF CONTRASTING MAGMATIC PROCESSES USING SB-SYSTEMATICS" AN EXAMPLE FROM THE WESTERN CENTRAL LUZON ARC, THE PHILIPPINES MARC J. DEFANT and PAUL C. RAGLAND Department of Geology, University of South Florida, Tampa, FL 33620 (U.S.A.) Department of Geology, Florida State University, Tallahassee, FL 32306 (U.S.A.) (Received November 1, 1986; revised and accepted September 17, 1987)
Abstract Defant, M.J. and l~gland, P.C., 1988. Recognition of contrasting magmatic processes using SB-systematics: An example from the western Central Luzon arc, the Philippines. Chem. Geol., 67: 197-208. The western Central Luzon volcanic system is associated with eastward subduction along the Manila Trench. Two arcs, the Bataan (BA) and the Mindoro (MA) arcs have resulted from this subduction and are separated by a NE-SW-oriented rift related zone of volcanism called the Macolod Corridor (MC). The BA consists of volcanism related to partial melting of the mantle wedge initiated by fluids from the subducted slab. The MA volcanics are also related to the subduction process but LILE and radiogenic Sr isotope data compared with the western part of the BA are significantly higher. The MC also has high LILE data compared with the western section of the BA but lower radiogenic Sr isotope values than the MA. Trace-element modelling has been used to explain SB-systematics ( St/Ca vs. Ba/Ca) observed in geochemical data from volcanoes worldwide. These trends and their sense of curvature are most probably the result of variations in plagioclase and clinopyroxene crystal fractionation. SB-systematics have been examined from four volcanoes: Mt. Natib and Mt. Mariveles within the western section of the BA; Mt. Maquiling within the MC; and the Mt. San Cristobal-Banahaw Complex within the MA. The results suggest that crystal fractionation was a dominant process during the evolution of the magma chambers below the various volcanoes. In addition, initial melt compositions suggest that the source of the MA magmas has been enriched in a crustal component. The original magma composition (s) from the MC volcano, Mt. Maquiling, appears to have been generated by smaller degrees of partial melting compared with the BA volcanoes studied.
1. I n t r o d u c t i o n
Onuma (1980)introducedtheuseofalog-log graph of Sr/Ca vs. Ba/Ca and suggested that this graph could help elucidate magma petrogenesis. He referred to chemical trends on these diagrams as "SB-systematics". The western Central Luzon volcanic system, the Philip0009-2541/88/$03.50
pines, is an excellent area in which to test and extend Onuma's (1980) concepts. The western Central Luzon volcanics have been examined in detail by Ragland et al. (1976), de Boer et al. (1980), Defant and Ragland (1981), Ragland and Defant (1983), Defant (1985), Knittel and Defant (1988) and Defant et al. (1988). Defant et al. (1988) have found that two arc systems, the Bataan arc (BA,
© 1988 Elsevier Science Publishers B.V.
198
Fig. 1 ) and the Mindoro arc ( MA, Fig. 1 ), exist in this region and are divided by a northeast-southwest rift zone, the Macolod Corridor ( MC; Fig. 1 ). Both of these arcs are apparently related to eastward subduction associated with the Manila Trench (de Boer et al., 1980). The BA is the northernmost arc in this region and consists of two subparallel west-facing volcanic lineaments (Defant et al., 1988; Fig. 1). The Western Bataan Lineament (WBL; Fig. 1 ) developed on ophiolites of the Zambales complexes and the Eastern Bataan Lineament ( EBL; Fig. 1 ) developed on the predominantly clastic sediments of the Central Valley basin. The MA is south-southeast of the BA and the MC and may also consist of two volcanic lineaments but they are not as clearly defined as those of the BA. The Western Mindoro Lineament (WML; Fig. 1 ) consists of about six volcanic centers; the Eastern Mindoro Lineament (EML; Fig. 1) of essentially four complexes, The MC represents a major zone of volcanism that cuts across Central Luzon between the BA and MA and appears to be a rift-related feature (Defant et al., 1988). Knittel and Defant (1988) have reviewed Sr isotope data from all over the Philippines, including older volcanic and plutonic assemblages (Oligocene to Miocene). They have found that the highest radiogenic Sr values observed in the Philippines occur in the MA. They suggest that this is the result of subduction of "slivers" of continental crust carried into the mantle during the collision of the North Palawan-Mindoro Terrane with the Philippine arc in the Miocene (see Hamilton, 1979; McCabe et al., 1982; Karig, 1983). Defant et al. (1988) have found that K20 and other large-ion lithophile elements ( LILE ) are lowest in the WBL compared to other regions in the western Central Luzon system (Fig. 2). They believe that the high concentrations found in the MA are the result of the crustal contamination. They suggest, however, that the high values found within the MC are the result of rifting that began at least 0.6 Ma ago. They at-
tribute these high LILE values within the MC to the generation of magmas in the mantle by smaller degrees of partial melting than below the WBL. The graph of ( K20 + Na20) vs. SiO2 (Fig. 2) displays the relationships between the various volcanic regions in western Central Luzon. Note that the MC and MA are considerably higher in K20 + Na20 at a given Si02 concentration than the WBL. The objective of this paper is to investigate further the origin of the variations in LILE with the use of SB-systematics. 2. R e s u l t s
2.1. General chemistry Selected data from the more than 350 samples that have been analyzed from this region are displayed in Table I. Only samples with Mg numbers [Mg n u m b e r = m o l a r M g × 1 0 0 / ( M g + Fe 2+ ) ] >/50 are presented in Table I because only the most mafic samples (excluding cumulates) have been used to document mantle processes. Analytical methods, precision and accuracy measurements, and additional data can be found in Defant (1985), Simon (1983), and Oles (1988). Although there is an immense geochemical data base from the region, only four of the numerous volcanoes have been sampled extensively because of the limited outcrops or inaccessibility due to rugged jungle terrane. These four volcanoes are: Mt. Natib and Mt. Mariveles within the WBL (4 and 6, Fig. 1 ) ; Mt. Maquiling within the MC (22, Fig. 1); and the Mt. San Cristobal-Banahaw complex within the MA (27 and 28, Fig. 1 ).
2.2. SB-systematics Several chemical trends (or SB-systematics) exist in volcanoes from western Central Luzon, as can be seen from Figs. 3-6. Samples from Mt. Natib form an apparently concaveupward trend from basaltic andesite through
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Fig. 1. Map of the Philippines (after Defant et al., 1988) showing major Pliocene-Quaternary volcanism: WBL=West Bataan Lineament; EBL = East Bataan Lineament; WML = West Mindoro Lineament; EML = East Mindoro Lineament. The n u m b e r s on the map correspond to the following volcanoes: 1 = Mt. Pinatubo; 2 = Mt. Balakibok; 3 = Mt. Santa Rita; 4 = Mt. Natib; 5 = Mt. Samat; 6 = Mt. Mariveles; 7 = Mt. Limay; 8 = Corregidor Complex; 9 = Mt. Palay Palay or Mt. Mataas na Gulod; 1 0 - - M t . Cariliao; 1 I = Mt. Batulao; 1 2 = Mt. Balungao; 1 3 = Mt. Amorong; 1 4 = Mt. Bangcay; 1 5 = Mt. Arayat; 16 = Mt. Taal; 17= Mt. Macolod; 18-- Mt. Panay; 19 = Maricaban West Complex; 2 0 = Mt. Casapao; 2 1 = Mt. Sembrano; 2 2 = Mt. Maquiling; 2 3 = Mt. Mapinggon; 2 4 = Mt. Bulalo; 2 5 = Mt. Nagcarlang; 2 6 = Mt. Atimbia; 2 7 - - Mt. San Cristobal; 2 8 = Mt. Banahaw; 2 9 = Mt. Malepunyo; 3 0 = Mt. Lingayen; 3 1 = Verde Island Complex; 3 2 = Mt. Macapili; 3 3 = Mt. Dumali; 3 4 = Mt. Marlanga; 3 5 = Mt. Pola; 3 6 = Mt. Maestre de Campo; 3 7 = Mt. Simara; 3 8 = Mt. Bantoi; 3 9 - - L a g u n a de Bay Complex. Several of the tectonic features are taken from de Boer et al. (1980), Hamburger et al. (1982), Karig (1983), and Wolfe and Self (1983).
andesite to dacite (Fig. 3 ). Similarly, Mt. Mariveles samples seem to form a concave-upward trend from basaltic andesite to andesite but with more curvature (Fig. 4). The data from Mt.
Maquiling ( Simon, 1983) fall along an apparently smooth concave-upward curve from basalt through andesite to dacite (Fig. 5). The samples from the Mt. San Cristobal-Banahaw
200
0
Alkaline
•
•
J •
oI
./
o
;
•
•
•
•
Subalkaline
/0
~
70
SiO 2
Fig. 2. Graph of total alkalies vs. SiO 2 (after Defant et al., 1988). The solid line in the figure represents McDonald's (1968) subdivision between alkaline and subalkaline fields. The WBL samples fall within the superimposed field. Solid triangles are from the EBL; solid circles from the MC; solid squares from the MA; open squares from Mt. San Cristobal-Banahaw Complex; and open circles from Mt. Taal.
complex ( Fig. 6; data from Defant, 1985; Oles, 1988) appear to fall along a concave-upward trend from basalt through basaltic andesite to andesite with the exception of one data point. Whether these trends in Figs. 3-6 are best fit by smooth curves or straight lines with different slopes is a significant point. As shown in the figures, either interpretation (particularly for Figs. 3 and 4) is reasonable. The importance of these two interpretations will be discussed in the next section, 3. D i s c u s s i o n
3.1. Previous work concerning SB-systematics In a series of recent papers, Onuma (1980), Onuma et al. (1981, 1983), Hirano et al. (1982a, b), Aramaki et al. (1984), Onuma and Montoya (1984) and Lopez-Escobar et al. (1985) have explored SB-systematics from volcanoes in Japan and the Andes. They report that Ca 2+
(ionic radius = 0.100 nm) is contained primarily in clinopyroxenes of per•dot•tic source rocks. On the other hand, Sr 2+ (0.177 nm) and Ba 2+ (0.136 nm) do not exist to any degree in the major phases (olivine, orthopyroxene, or clinopyroxene), nor in minor phases such as garnet or spinel, because of crystal structure control (Onuma et al., 1981 ). This observation is supported by all compilations of crystal-melt KDvalues (e.g., Henderson, 1982). They suggest that Sr and Ba are contained in the upper mantle in the two minor phases apatite and phlogopite. The possibility also exists that these elements are sufficiently incompatible such that significant amounts of each element are not in any lattice site, but rather adsorbed along grain boundaries, in crystal defects, etc. Onuma et al. (1981) point out that during partial melting of per•dot•tic sources, small degrees of partial melting enrich the magma greatly in Sr and Ba because phlogopite and apatite are early-melting phases. This enrich-
TABLE I Major- a n d selected trace-element compositions from western Central Luzon volcanic rocks No.
Si02
Ti02
A1203
Fe203*'
MgO
CaO
Na20
K20
Total
Mg number .2
Sr
Ba
0.60 0.55 0.48 0.54 0.55 0.57 0.55 0.58 0.56 0.54 0.51 0.37 0.53 0.50 0.55 0.66 0.49 0.62 0.58 0.57 0.69 0.71 0.48 0.50 0.48
18.52 18.38 18.40 19.06 18.55 18.63 18.37 18.57 18.56 17.14 17.74 17.05 17.31 17.47 17.75 18.17 16.59 17.91 18.32 18.27 17.18 17.50 17.74 17.50 17.71
7.78 7.52 7.06 7.51 7.51 7.85 8.17 7.61 8.00 7.06 6.38 6.39 6.20 5.97 6.90 7.70 4.51 7.39 7.52 7.43 8.24 8.24 5.96 6.02 5.92
4.50 4.65 4.18 4.60 4.70 4.83 5.04 4.54 4.60 4.04 2.92 3.12 3.05 2.91 3.12 3.85 2.54 3.77 3.77 3.56 3.96 3.96 3.02 2.98 2.80
7.79 8.37 7.26 7.92 8.04 8.33 7.60 7.71 8.07 6.01 5.91 6.13 5.76 6.04 5.12 5.54 5.03 8.06 6.86 7.10 7.09 7.39 6.48 7.01 6.23
3.08 3.00 3.06 3.29 3.16 3.18 2.97 3.08 2.98 3.12 3.83 3.71 3.34 3.54 3.49 3.00 4.49 3.65 3.39 3.36 3.69 3.98 3.58 3.69 3.41
0.81 0.87 0.82 0.76 0.79 0.78 0.55 0.80 0.85 0.93 0.98 0.86 0.85 1.04 0.96 0.67 1.49 1.15 0.87 0.82 0.83 0.80 1.76 1.70 1.65
97.54 98.55 98.49 99.29 98.32 100.20 98.72 98.63 99.83 98.82 99.13 97.73 97.14 97.38 98.19 99.47 98.29 100.22 99.35 100.90 99.68 100.27 100.00 99.26 97.27
56 58 57 57 58 58 58 57 56 56 50 52 52 52 50 52 55 53 52 51 51 51 53 52 51
378 362 424 387 392 378 383 389 388 258 315 282 274 285 268 241 579 352 300 308 313 336 525 525 524
235 199 306 239 252 248 276 230 230 182 175 175 198 189 166 234 485 237 167 142 172 170 500 489 499
0.56 0.49 0.52 0.42 0.53 0.61 0.51 0.57 0.49 0.56 0.64 0.63 0.61 0.71
17.40 16.43 17.38 16.87 18.31 17.95 17.87 18.10 17.20 17.71 17.98 17.68 17.47 17.26
7.92 6.15 7.70 6.30 8.05 8.39 7.49 8.24 6.40 7.06 8.53 8.70 8.85 8.62
4.27 3.49 4.70 3.70 3.59 4.21 4.18 4.51 3.16 3.74 4.00 4.21 4.60 4.13
7.34 6.07 7.59 6.35 6.24 7.49 6.27 7.67 5.80 7.42 7.58 8.09 8.41 8.10
3.16 3.00 2.91 3.37 3.08 3.08 2.96 2.82 3.24 3.63 3.31 3.23 2.97 3.39
1.16 1.51 1.06 1.35 1.16 1.93 1.44 1.02 1.21 1.48 1.69 1.62 1.46 1.01
98.65 99.39 98.52 99.13 97.49 99.44 98.58 98.28 97.09 99.86 100.92 100.04 100.54 101.15
54 56 57 56 50 52 55 55 52 54 51 52 53 51
307 268 283 295 286 500 260 303 279 620 431 446 496 302
269 289 242 310 286 445 310 248 350 469 440 438 486 194
0.69 0.93 0.99
17.88 18.50 18.64
7.67 9.60 9.63
4.03 5.81 5.39
7.22 9.74 9.77
3.91 3.19 3.27
2.06 1.06 1.03
101.13 100.41 100.32
54 57 55
942 849 858
910 556 619
0.73 0.93 0.75 0.71 0.41
16.43 18.44 17.07 16.77 15.75
5.88 9.82 6.78 6.56 3.65
2.48 4.90 3.00 3.17 1.63
5.55 9.71 6.32 6,20 3.84
3.28 2.39 3.04 2.69 3.62
2.44 0.95 2.28 2.19 2.98
96.11 99.34 100.04 97.61 97.76
55 59 56 58 56
400 486 390 382 426
550 364 499 510 638
Mr. Natib: 1 2 3 4 6 7 8 9 10 14 15 18 19 20 21 22 28 29 30 32 129 131 144 145 146
54.96 55.21 57.23 55.26 55.02 56.03 55.47 55.34 56.21 59.75 60.86 60.10 60.10 59.91 60.30 56.14 67.87 57.67 57.71 57,89 58.00 57.66 59.33 59.86 57.36
Mr. Mariveles: 88 89 90 92 93 94 95 96 97 112 113 114 115 117
56.84 59.26 56.40 60.48 56.53 55.78 57.53 55.07 59.59 58.26 57.19 55.88 55.80 57.93
Mr. S a n Cristobal: 171 174 175
57.19 51.06 51.06
Mt. Maquiling: 244 246 247 248 249
59.01 51.86 60.51 59.04 65.66
Major elements in wt.% a n d trace elements in ppm. All elements analyzed by XRF. Samples from Mt. Maquiling are from Simon (1983). T h e rest of the samples are from Defant (1985). *'Total Fe as Fe203. *2Mg n u m b e r = molar Mg × 1 0 0 / ( M g ÷ Fe 2+ ).
202 0O3
o.02
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'
0.002
l
'
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---
--
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--
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BASALTIC ANDESlTES 0.003
--
-
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I
e
I
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t i t In
0.003
i
0.01
I
0.02
0.04
Ba/ Ca
I
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003
Be / Co
Fig. 5. SB-systematics defined by volcanics with Mg number >/50 from Mt. Maquiling (MC). Data is from Simon (1983).
Fig. 3. SB-systematics defined by a set of volcanic rocks with Mg number/> 50 from Mt. Natib (WBL).
a n d his c o w o r k e r s . In fact, b e c a u s e m o s t basaltic partial melts from peridotitic sources can be
ment would be even more enhanced if signifi-
considered as eutectic-like melts (Yoder, 1976),
cant
this a s s u m p t i o n o f a h o m o g e n e o u s
amounts
of
Ba
and
Sr
were
truly
source w i t h
incompatible and did not exist in lattice sites of any mineral stable in the upper mantle. These melts would, therefore, have larger ratios of Sr/Ca and Ba/Ca than melts formed from higher degrees of partial melting. Assuming the Ca concentrations are equivalent in magmas generated from similar (with respect to major elements) mantle material and degrees of partial melting, a more enriched mantle should also produce higher Sr/Ca and Ba/Ca values (De-
respect to major elements is not even necessary. Consequently, different degrees of partial melting of the same source or the same degree of partial melting of an inhomogeneous source (primarily with respect to incompatible elements) will generate a SB-trend line with a slope of 45 ° through the mantle source, any partial melts, and the residual solids (Defant, 1985; Onuma et al., 1981 ). This assumes, of course, that Ba and Sr are behaving with ap-
fant, 1 9 8 5 ) , a p o i n t n o t addressed by O n u m a
p r o x i m a t e l y equal i n c o m p a t i b i l i t y . U n d e r ex-
0.02
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ql~
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l
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I
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~
--
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._
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II
Itic A n d e s i t e s 0.01
BASALTIC
1
I
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0.003 0.002
I
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(,,,)
,~s s
I
o
AN DESITES
Sr/Co
I
0.003
I
I
I I I I I 0.01
I 0.03
0.02
so/ca Fig. 4. SB-systematics defined by volcanics with Mg number/> 50 from Mt. Mariveles (WBL).
Ba/Ca Fig. 6. SB-systematics defined by volcanics with Mg number t> 50 from the Mt. San Cristobal-Banahaw Complex (MA). Data from this paper and Oles (1988).
203 TABLE II ~+'~/ ~o~.~.°~/ Primary Magma Cumulates - ~ ~
Partition coefficientsused for Fig. 8 ~,~+
~ o c ~
C,<3s~a~ I r "~ / / /o-~" • ~~\e~\1 ~r~/~ ^,C
/ / /
~. ~°~
/
Mantle
Clinopyroxene Plagioclase mafic rocks intermediaterocks Amphibole
2.5
0.14
0.07
2.0 2.5 1.5
1.8 4.0 0.57
0.23 0.50 0.31
See text for discussionof sources. ~Residual Solids Be/Ca
Fig. 7. Hypothetical St/Ca vs. Ba/Ca diagram displaying potential results of partial melting and subsequentcrystal fractionation (from Onumaet al., 1981). tremely low mantle pressures where plagioclase is stable in the source, this argument cannot hold because Sr is preferentially partitioned into the plagioclase. This hypothetical partial-melting trend, assuming that Sr and Ba are equally incompatible during partial melting, is shown in Fig. 7. A different SB-trend is commonly produced by crystal fractionation, as can be seen in Fig. 7 (Onuma et al., 1981). Phenocryst assemblages in these rocks consist of some combination of olivine, plagioclase, clinopyroxene (augite), orthopyroxene, amphibole and Fe-Tioxides. These minerals are assumed to be nearliquidus phases and thus, most likely candidates to control crystal fractionation. Although olivine, orthopyroxene and spinel fractionation does not affect Sr/Ca or Ba/Ca ratios, fractionation of clinopyroxene, plagioclase and amphibole does. Table II provides some critical crystal-liquid KD-values for Ba and Sr in clinopyroxene, plagioclase and amphibole, Onuma and Montoya (1984) believe that orthopyroxene may increase Sr/Ca and Ba/Ca ratios but to a negligible extent. Because Ca is a major component of clinopyroxene (augite), whereas concentrations of Ba and Sr are extremely low in this mineral, Sr/Ca and Ba/Ca
ratios increase with clinopyroxene fractionation. Concentrations of Sr and Ba in amphibole are generally higher than those in clinopyroxene, so fractionation of amphibole should also cause the ratios to increase but not in such dramatic fashion. Onuma et al. (1981) suggest that during plagioclase fractionation the Ba/Ca ratio will increase substantially, while the Sr/Ca ratio will remain fairly constant. This is reasonable because Sr is strongly partitioned into the plagioclase lattice and Ba is not (Table II).
3.2. SB-systematics resulting from crystal fractionation Before discussing the geologic significance of the SB-systematics on Figs. 3-6, consideration of the trends in these figures is necessary. As stated previously, presumably the only major minerals whose fractionation can change the slope of a Sr/Ca-Ba/Ca curve are clinopyroxene, plagioclase and amphibole. The role ofplagioclase is particularly critical in this regard because Sr is compatible in plagioclase, but relatively incompatible in the other two phases. In contrast, Ba is incompatible in all three phases. A change of slope on an SB-systematics curve theoretically can come about in three ways: a change in the proportions of these fractionating phases, the entry of an additional phase, or a change in the Sr or Ba content of one or more of these phases with fractionation. For example, a trend with an increase in slope (Figs. 3 and 4) could be explained by relatively early fractionation of plagioclase relative to clino-
204 pyroxene, while early fractionation of clinopyroxene relative to plagioclase would lead to a decrease in slope. Extremely low-pressure fractionation can lead to the first situation, whereas fractionation at around 9 kbar can cause the second (Green and Ringwood, 1967). Rather than a smooth curve, such a process would lead to a slope break when the second phase arrives on the liquidus and begins fractionating. The data in each of Figs. 3 and 4, and perhaps even those in Figs. 5 and 6, can be fit by two straight lines with different slopes as well as one smooth curve. The concomitant fractionation of olivine, orthopyroxene, or spinel would have no effect on the directions of these trends, A smooth curve can be produced in several ways, two of which are most probable: (1) As temperature decreases with increasing fractionation, KD'S will change in a systematic manner, creating a smooth curve. This process will only produce a curve if the KD'S involved are changing relative to one another in a non-proportional manner. If KD'S for both Sr and Ca change proportionally, then a straight line will be maintained. (2) Crystallization of two phases along a curved cotectic will lead to a systematic change in the proportions of the two fractionating phases. If these two phases have considerably different KD'S for Sr and Ba, such as do clinopyroxene and plagioclase, this will lead to a curved trend on a Sr/Ca-Ba/Ca graph, The trends on Figs. 3-6 can be approximated by simple modelling using the Rayleigh fractionation equation:
sonable KD'S for Ca is doubly difficult. They were estimated based upon microprobe data for coexisting phenocrysts and groundmasses from Defant (1985). KD-values for Sr and Ba were taken from averages in the summary table of Henderson (1982, table 5.2). The exception is Sr in relatively felsic rocks, for which Henderson (1982) gives a range of 1.5-8.8, with an average of 6.0. This average seems too high for the andesites and dacites in this study area, so a value of 4.0 was arbitrarily chosen as being reasonable. IfKD'S for allthree elements remain constant throughout the fractionation process, trends will always be straight lines. Fig. 8 shows fractionation trends for pure clinopyroxene (line 1 ) and pure plagioclase ( line 4 ) assuming constant KD'S from Table II. An example of plagioclase fractionation with changing KD'S is also shown on Fig. 8 (line 5). A trend for fractionation of clinopyroxene with changing KD'S is not shown because calculations have demonstrated that it almost coincides with line I in Fig. 8 and is only very slightly concave down. Although KD'S do change with fractionation for clinopyroxene, they maintain near proportionality, so the trend approximates line 1. An amphibole trend also is not plotted, but is almost parallel to the clinopyroxene trend. It is also possible to express the ( S r / C a ) / ( B a / C a ) ratio at any given point in the fractionation process (i.e. for any given Fvalue) by: (Sr/Ca) / (Ba/Ca) = Sr/Ba O s , F (Dsr - -
L=OF
(D-l)
(1)
where L is the composition of the residual liquid; 0 is the composition of the original liquid; F is weight fraction of residual liquid; and D is the bulk distribution coefficient (weighted average for KD'S of minerals in the fractionation assemblage). One main difficulty with any calculation of this type is choosing representative KD-values. Because the Ca partition obeys neither Raoult's or Henry's law, selection of rea-
1 )
--OBaF(DBa_I) = (0sr/Osa)F (Dsr-I~B"~ (2) where 0Sr/0Ba is the initial Sr/Ba ratio of the parental magma. If Dsr = DBa the elements are equally compatible (or incompatible), the ratio remains constant with increasing fractionation and the trend line has a 45 ° slope. If Dsr > DBa (i.e. Sr is more compatible than Ba) the ratio is lowered. It follows that when Ba is
205
o ~t
-I J .... • ~ ~
2
(constant KD'S ) is modelled in Fig. 8 (line 3 ). This trend is similar to those on Figs. 3-6. Line 2 represents 50% clinopyroxene and 50% plagioclase fractionation with continuously variable KD'S for plagioclase. It generates a concavedownward trend, which is rare in SBsystematics.
001
3.3. SB-systematics among data from the western Central Luzon volcanics
4
5
°°°ofoo3
I
J
]
I
I
J
J
I
o.o~
I
i
i
I
i
I
L
o.~
BQ/ Ca Fig. 8. Hypothetical fractionation trends: c u r v e 1 = clinopyroxene fractionation assuming a constant KD; c u r v e 2 = 50% clinopyroxene and 50% plagioclase fractionation with continuously variable KD'S for plagioclase; c u r v e 3 - - p l a g i o c l a s e f r a c t i o n a t i o n f o l l o w e d b y 5 0 % plagioclase and 50% clinopyroxene fractionationwith constant KD; c u r v e 4 = plagioclase fractionation assuming a constant KD; c u r v e 5=plagioclasefractionationwithchangingKD's.
more compatible than Sr (which is quite uncommon) and DB, > Dsr, the ratio will increase, Eq. 2 will determine the (Sr/Ca) / (Ba/Ca) ratio, but it will not determine the absolute location of a point on a Sr/Ca-Ba/Ca graph. The Ca partition must be included in the calculations for that. Hence, simple Rayleigh fractionation can illustrate some of the qualitative observations made above. For every mineral in Table II, Dsr is either approximately equal to or greater than DB,. Therefore, with increasing fractionation the (Sr/Ca) / (Ba/Ca) ratio will respectively not change or will increase. The ratio slightly decreases for clinopyroxene and amphibole and drastically decreases for plagioclase. Plagioclase fractionation followed by 50% plagioclase + 50% clinopyroxene fractionation
Mixing under certain circumstances could generate SB-systematic trends. We do not, however, believe that the western Central Luzon volcanics have undergone extensive assimilation that would generate the large Si02 variations among the data. This is based on Nd and Sr isotope values which fall within the mantle array ( Knittel et al., 1988). In addition, there is not evidence from petrographic observation that the volcanic samples have undergone extensive magma mixing of the kind that would generate the large SiO2 variations (Fig. 2; O'Hara and Mathews, 1981 ). Also, Sr/Ca vs. Ba/Ca linear diagrams do not show linear trends suggestive of simple mixing. We do believe, however, that the SB-systematic trend could be the result of assimilation fractional crystallization (AFC), the assimilation being a minor process. Fig. 9 is a composite plot of SB-systematics for all four of the western Central Luzon volcanoes studied. It includes the 45 ° partialmelting line of Onuma et al. {1981). This line is not based solely on theory; it is defined by primitive basalts from island arcs and the slope corresponds to variations in Ca with a constant Sr/Ba ratio ( Onuma et al., 1983 ). Any assumption that the mantle has chondritic Ba/Ca and Sr/Ca ratios seems reasonable; the mantle and chondrite values plot quite close to one another in Fig. 9. Chondritic values are from Tera et al. (1970) and mantle values are from Jagoutz et al. (1979). The continuation of each SB-trend on Figs. 3-6 has been extrapolated to the partial-melt-
206 0.1
0.01
-
-7
Average 0.001
i
Mantleo
A
-
MI.
Mariveles
B
-
Mt.
Natib
C -
Mt.
Maqulling
D -
Mt.
San C r l s t o b a l
Chondrltes
Banahaw m
o.oool
F
0.0001
j
I
I I Jllll
J
J I I IIIII
0.001
I 0.01
I
I IIIII
-
O.
Ba/Ca Fig. 9. Graph of Ba/Ca vs. Sr/Ca with the SB-systematics from Figs. 3-6 superimposed along with the hypothetical melting line of Onuma et al. (1981) included. The average mantle value is from Jagoutz et al. (1979) and the chondritic values are from Tera et al. (1970).
ing line on Fig. 9. Presumably, the most primitive {i.e. mantle-equilibrated) basaltic samples from these suites have yet to be collected; hypothetical "parental" melts are shown as points A-D. The overall trends, including the extrapolated portions, are easily explained on the basis of crystal fractionation (see the discussion on causes of apparent concave-upward trends), Each SB-systematic intersects the hypothetical partial-melting line, which in reality may be a band, indicative of small variations in relative incompatibility of Sr and Ba during par-
tial melting and thus the 45 ° slope [ see Fujii and Scarfe (1985) for a discussion of eutectic melting variations]. The most primitive samples on each SB-systematic represent near mantle-equilibrated melts ( i.e. plot on the right, or high Ba/Ca, edge of the band) and the points of intersection (A-D, Fig. 9) along the partialmelting line represent approximate initial compositions of the magma prior to crystal fractionation. The SB-systematic trends in Fig. 9 can be divided into three groups: MA, MC and WBL. The intersection of these trends with the partial-
207
melting line is an approximation of the original partial-melt compositions. The MA has the highest SB-systematic values compared with the MC and the WBL. As discussed in the introduction, the MA has had source contamination of a crustal component (Mukasa et al., 1986; Knittel et al., 1987; Defant et al., 1988; Knittel and Defant, 1988). We believe the high SB-systematic values within the MA initial magma compositions reflect this crustal component (high Sr and Ba from the granitic contaminants) and are not due primarily to smaller degrees of partial melting when compared to the other provinces ( MC and WBL). The MC SB-systematic initial magma composition values fall between the MA and the WBL (Fig. 9). The volcanics within the MC are apparently the result of crystal fractionation from a magma that was generated by smaller degrees of partial melting than those of the WBL, which supports the conclusions of Defant et al. (1988). The relatively young volcanics associated with this rift environment are higher in LILE than the WBL and the lavas plot near the alkaline field on a graph of total alkali vs. SiO2 (Fig. 2). The low radiogenic Sr values reported by Defant et al. (1988) suggests that these high LILE values are not due to intermediate to shallow-level crustal enrichment, The volcanoes of the WBL, Mt. Natib and Mt. Mariveles, have the lowest Sr/Ca and Ba/Ca values along the partial-melting line (Fig. 9). These magmas have probably been derived from partial melting of the mantle wedge and have had products from the slab incorporated within them (Defant et al., 1988; Knittel and Defant, 1988). Because these volcanoes fall along the WBL, they probably exist approximately the same distance above the subducting slab. The variations between Mt. Natib and Mt. Mariveles in t e r m s of SB-systematics (Fig. 9, points B and A, respectively) and the fact that these volcanoes are at the same distance above the Benioff zone present somewhat of enigma. The slight variations in LILE values between Mt. Natib and Mt. Mariveles
(distinctly lower values than the MC or the MA) may be due to small variations in the degrees of partial melting of the mantle wedge, the involvement of variations in the amount of material from the subducting slab that was incorporated within the mantle, or small-scale mantle heterogeneities. We conclude that the variations in the initial partial melts from the mantle below the western Central Luzon volcanics, based on SB-systematics, are the result of source contamination (MA, Fig. 9 ), smaller degrees of partial melting within the pull-apart zone ( MC, Fig. 9), or contributions from the subducting slab during partial melting of the mantle wedge (WBL, Fig. 9). These observations clearly support the conclusions of Defant et al. (1988) and Knittel and Defant (1988). In addition, based on inferences from trace-element modeling using a modification of the Rayleigh equation, it appears that the western Central Luzon SB-systematics (Figs. 3-6) are the result of crystal fractionation (probably plagioclase and clinopyroxene ). Acknowledgements We are grateful to Ebasco Services, Inc. and the Philippine Power Corporation for partially funding this project. A special thanks to L. Solebello for collecting and reviewing Sr, Ba and Ca data from ultramafic nodules. We also acknowledge R.K. Raymond for drafting and B.A. Healy for typing of the manuscript. We have benefitted from the fruitful discussions with G. DeVore and the editorial comments of J.M. Ragland, U. Knittel and D. Oles. References Aramaki, S., Onuma, N. and Portillo, F., 1984. Petrography and major element chemistry of the volcanic rocks of the Andes, southernPeru. Geochem.J., 18: 217-232. de Boer, J., Odom, A.L., Ragland, P.C., Snider, F.G. and Tilford, N.R., 1980. The Bataan orogene: Eastward subduction, tectonic rotations, and volcanism in the western Pacific (Philippines). Tectonophysics, 67: 251-282. Defant, M.J., 1985. The potential origin of the potas-
208 sium-depth relationship in the Bataan orogene, the Philippines. Ph.D. Thesis, Florida State University, Tallahassee, Fla., 622 pp. Defant, M.J. and Ragland, P.C., 1981. A numerical analysis of whole-rock data from western Luzon volcanic arcs, the Philippines. Abstr. Prog., 94th Annu. Meet., Geol. Soc. Am., 13: 438. Defant, M.J., de Boer, J.Z. and Oles, D., 1988. The geochemistry and tectonic setting of the western Central Luzon arc, the Philippines: Two arcs divided by rifting? Tectonophysics, 145: 305-317.. Fujii, T. and Scarfe, C.M., 1985. Composition of liquids coexisting with spinel lherzolite at 10 kbar and the genesis of MORB's. Contrib. Mineral. Petrol., 90." 18-28. Green, D.H. and Ringwood, A.E., 1967. The origin of highalumina basalts and their relationships to quartz tholeiites and alkali basalts. Earth Planet. Sci. Lett., 2: 41-51. Hamburger, M.W., Cardwell, R.K. and Isacks, B.L., 1982. Seismotectonics of the Northern Philippine island arc. In: D. Hayes (Editor), The Tectonics and Geological Evolution of the Southeast Asian Seas and Islands, Part 2. Am. Geophys. Union, Washington, D.C., Geophys. Monogr. Ser., 27: 1-22. Hamilton, W., 1979. Tectonics of the Indonesian region, U.S. Geol. Surv., Prof. Pap. No. 1078. Henderson, P., 1982. Inorganic Geochemistry. Pergamon, Oxford, 353 pp. Hirano, M., Hamuro, K. and Onuma, N., 1982a. Sr/Ca-Ba/Ca systematics in Higashi-Izu monogenetic volcano group, Izu Peninsula, Japan. Geochem. J., 16: 311-320. Hirano, M., Isshiki, N. and Onum~, N., 1982b. Sr/Ca-Ba/Ca systematics in Miyakejima, Onoharajima, Mikurajima and Inambajima volcanoes, the Izu Islands, Japan. Geochem. J., 16: 79-87. Jagoutz, E., Palme, H., Baddenhausen, K., Blum, M., Cendales, G., Dreibus, G., Spettel, L. V. and Wanke, H., 1979. The abundances of major, minor and trace elements in the earth's mantle as derived from primitive ultramafic nodules. Proc. 10th Lunar Planet. Sci. Conf., pp. 2031-2050. Karig, D.E., 1983. Accreted terranes in the northern part o f the Philippine archipelago. Tectonics, 2:211- 236. Knittel, U. and Defant, M.J., 1988. St-isotopic and trace element variations in Oligocene to Recent igneous rocks from the Philippine island arc: Evidence for recent enrichment in the sub-Philippine mantle. Earth Planet. Sci. Lett., 87: 87-99. Knittel, U., Defant, M.J. and Raczek, I., 1987. Recent enrichment events in the sources of Philippine arc magmas: Sr and Nd isotopic evidence. Pac. Rim Congr. '87, Brisbane, Qld., pp. 245-249. Lopez-Escobar, L., Moreno, H., Tagri, M., Notsu, K. and Onuma, N., 1985. Geochemistry and petrology of lavas
from San Jose volcano, Southern Andes (33°45'S). Geochem. J., 19: 209-222. McCabe, R., Almasco, J. and Diegor, W., 1982. Geologic and paleomagnetic evidence for a possible Miocene collision in western Panay, central Philippines. Geology, 10: 325-329. McDonald, G.A., 1968. Composition and origin of Hawaiian lavas. Geol. Soc. Am. Mere., 116: 477-522. Mukasa, S.B., McCabe, R. and Gill, J.B., 1987. Pb-isotopic compositions of volcanic rocks in the West and East Philippine island arcs: presence of the Dupal isotopic anomaly. Earth Planet. Sci. Lett., 84: 153-164. O'Hara, M.J. and Mathews, R.E., 1981. Geochemical evolution in an advancing, periodically replenished, periodically tapped, continuously fractionated magma chamber. J. Geol. Soc. London, 138: 237-277. Oles, D., 1988. Ph.D. Thesis, Rheinisch-Westfalische Technische Hochschule, Aachen (in preparation). Onuma, N., 1980. Sr-Ba systematics on meteorites, the moon and the earth. Solar System Sci. Syrup. II, Tokyo, Inst. Space Astron. Sci., pp. 1-6 {in Japanese). Onuma, N. and Montoya, M., 1984. Sr/Ca-Ba/Ca systematics of volcanic rocks from the central Andes, southern Peru, and its implication for Andean magmatism. Geochem. J., 18: 251-262. Onuma, N., Hirano, M. and Isshiki, N., 1981. Sr/Ca-Ba/Ca systematics in four volcanoes of Oshima, Izu Islands, Japan. Geochem. J., 15: 315-324. Onuma, N., Hirano, M. and Isshiki, N., 1983. Genesis of basalt magmas and their derivatives under the Izu Islands, Japan, inferred from Sr/Ca-Ba/Ca systematics. J. Volcanol. Geotherm. Res., 18: 511-529. Ragland, P.C. and Defant, M.J., 1983. Silica standardization: A discriminant technique applied to a volcanic arc system. Earth Planet. Sci. Lett., 64: 387-395. Ragland, P.C., Stirewalt, G.L. and Newcomb, W.E., 1976. A chemical model for island arc volcanism, western Luzon, the Philippines: Abstr. Prog., Geol. Soc. Am., 1976 Annu. Meet., 8: 1056. Simon, M., 1983. Petrographische und geochemische Untersuchungen an Vulkanen in Zentralluzon, Philippinen. M. Thesis, Rheinisch-Westfalische Technische Hochschule, Aachen, 83 pp. Tera, F., Eugster, 0., Burnett, D.S. and Wasserburg, G.J., 1970. Comparative study of Li, Na, K, Cs, Ca, St, and Ba abundances in achondrites and Apollo 11 lunar samples. Proc. Apollo 11 Lunar Sci. Conf., 2: 1637-1657. Wolfe, J.A. and Self, S., 1983. Structural lineaments and Neogene volcanism in southwestern Luzon. In: D.E. Hayes {Editor), The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, Part 2. Geophys. Monogr. Ser., Am. Geophys. Union, Washington, D.C., 27: 157-172. Yoder, Jr., H.S., 1976. Generation of Basaltic Magmas. Natl. Acad. Sci., Washington, D.C., 265 pp.
197
[31
RECOGNITION OF CONTRASTING MAGMATIC PROCESSES USING SB-SYSTEMATICS" AN EXAMPLE FROM THE WESTERN CENTRAL LUZON ARC, THE PHILIPPINES MARC J. DEFANT and PAUL C. RAGLAND Department of Geology, University of South Florida, Tampa, FL 33620 (U.S.A.) Department of Geology, Florida State University, Tallahassee, FL 32306 (U.S.A.) (Received November 1, 1986; revised and accepted September 17, 1987)
Abstract Defant, M.J. and l~gland, P.C., 1988. Recognition of contrasting magmatic processes using SB-systematics: An example from the western Central Luzon arc, the Philippines. Chem. Geol., 67: 197-208. The western Central Luzon volcanic system is associated with eastward subduction along the Manila Trench. Two arcs, the Bataan (BA) and the Mindoro (MA) arcs have resulted from this subduction and are separated by a NE-SW-oriented rift related zone of volcanism called the Macolod Corridor (MC). The BA consists of volcanism related to partial melting of the mantle wedge initiated by fluids from the subducted slab. The MA volcanics are also related to the subduction process but LILE and radiogenic Sr isotope data compared with the western part of the BA are significantly higher. The MC also has high LILE data compared with the western section of the BA but lower radiogenic Sr isotope values than the MA. Trace-element modelling has been used to explain SB-systematics ( St/Ca vs. Ba/Ca) observed in geochemical data from volcanoes worldwide. These trends and their sense of curvature are most probably the result of variations in plagioclase and clinopyroxene crystal fractionation. SB-systematics have been examined from four volcanoes: Mt. Natib and Mt. Mariveles within the western section of the BA; Mt. Maquiling within the MC; and the Mt. San Cristobal-Banahaw Complex within the MA. The results suggest that crystal fractionation was a dominant process during the evolution of the magma chambers below the various volcanoes. In addition, initial melt compositions suggest that the source of the MA magmas has been enriched in a crustal component. The original magma composition (s) from the MC volcano, Mt. Maquiling, appears to have been generated by smaller degrees of partial melting compared with the BA volcanoes studied.
1. I n t r o d u c t i o n
Onuma (1980)introducedtheuseofalog-log graph of Sr/Ca vs. Ba/Ca and suggested that this graph could help elucidate magma petrogenesis. He referred to chemical trends on these diagrams as "SB-systematics". The western Central Luzon volcanic system, the Philip0009-2541/88/$03.50
pines, is an excellent area in which to test and extend Onuma's (1980) concepts. The western Central Luzon volcanics have been examined in detail by Ragland et al. (1976), de Boer et al. (1980), Defant and Ragland (1981), Ragland and Defant (1983), Defant (1985), Knittel and Defant (1988) and Defant et al. (1988). Defant et al. (1988) have found that two arc systems, the Bataan arc (BA,
© 1988 Elsevier Science Publishers B.V.
198
Fig. 1 ) and the Mindoro arc ( MA, Fig. 1 ), exist in this region and are divided by a northeast-southwest rift zone, the Macolod Corridor ( MC; Fig. 1 ). Both of these arcs are apparently related to eastward subduction associated with the Manila Trench (de Boer et al., 1980). The BA is the northernmost arc in this region and consists of two subparallel west-facing volcanic lineaments (Defant et al., 1988; Fig. 1). The Western Bataan Lineament (WBL; Fig. 1 ) developed on ophiolites of the Zambales complexes and the Eastern Bataan Lineament ( EBL; Fig. 1 ) developed on the predominantly clastic sediments of the Central Valley basin. The MA is south-southeast of the BA and the MC and may also consist of two volcanic lineaments but they are not as clearly defined as those of the BA. The Western Mindoro Lineament (WML; Fig. 1 ) consists of about six volcanic centers; the Eastern Mindoro Lineament (EML; Fig. 1) of essentially four complexes, The MC represents a major zone of volcanism that cuts across Central Luzon between the BA and MA and appears to be a rift-related feature (Defant et al., 1988). Knittel and Defant (1988) have reviewed Sr isotope data from all over the Philippines, including older volcanic and plutonic assemblages (Oligocene to Miocene). They have found that the highest radiogenic Sr values observed in the Philippines occur in the MA. They suggest that this is the result of subduction of "slivers" of continental crust carried into the mantle during the collision of the North Palawan-Mindoro Terrane with the Philippine arc in the Miocene (see Hamilton, 1979; McCabe et al., 1982; Karig, 1983). Defant et al. (1988) have found that K20 and other large-ion lithophile elements ( LILE ) are lowest in the WBL compared to other regions in the western Central Luzon system (Fig. 2). They believe that the high concentrations found in the MA are the result of the crustal contamination. They suggest, however, that the high values found within the MC are the result of rifting that began at least 0.6 Ma ago. They at-
tribute these high LILE values within the MC to the generation of magmas in the mantle by smaller degrees of partial melting than below the WBL. The graph of ( K20 + Na20) vs. SiO2 (Fig. 2) displays the relationships between the various volcanic regions in western Central Luzon. Note that the MC and MA are considerably higher in K20 + Na20 at a given Si02 concentration than the WBL. The objective of this paper is to investigate further the origin of the variations in LILE with the use of SB-systematics. 2. R e s u l t s
2.1. General chemistry Selected data from the more than 350 samples that have been analyzed from this region are displayed in Table I. Only samples with Mg numbers [Mg n u m b e r = m o l a r M g × 1 0 0 / ( M g + Fe 2+ ) ] >/50 are presented in Table I because only the most mafic samples (excluding cumulates) have been used to document mantle processes. Analytical methods, precision and accuracy measurements, and additional data can be found in Defant (1985), Simon (1983), and Oles (1988). Although there is an immense geochemical data base from the region, only four of the numerous volcanoes have been sampled extensively because of the limited outcrops or inaccessibility due to rugged jungle terrane. These four volcanoes are: Mt. Natib and Mt. Mariveles within the WBL (4 and 6, Fig. 1 ) ; Mt. Maquiling within the MC (22, Fig. 1); and the Mt. San Cristobal-Banahaw complex within the MA (27 and 28, Fig. 1 ).
2.2. SB-systematics Several chemical trends (or SB-systematics) exist in volcanoes from western Central Luzon, as can be seen from Figs. 3-6. Samples from Mt. Natib form an apparently concaveupward trend from basaltic andesite through
C°rdillera~--al ~#! I
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Fig. 1. Map of the Philippines (after Defant et al., 1988) showing major Pliocene-Quaternary volcanism: WBL=West Bataan Lineament; EBL = East Bataan Lineament; WML = West Mindoro Lineament; EML = East Mindoro Lineament. The n u m b e r s on the map correspond to the following volcanoes: 1 = Mt. Pinatubo; 2 = Mt. Balakibok; 3 = Mt. Santa Rita; 4 = Mt. Natib; 5 = Mt. Samat; 6 = Mt. Mariveles; 7 = Mt. Limay; 8 = Corregidor Complex; 9 = Mt. Palay Palay or Mt. Mataas na Gulod; 1 0 - - M t . Cariliao; 1 I = Mt. Batulao; 1 2 = Mt. Balungao; 1 3 = Mt. Amorong; 1 4 = Mt. Bangcay; 1 5 = Mt. Arayat; 16 = Mt. Taal; 17= Mt. Macolod; 18-- Mt. Panay; 19 = Maricaban West Complex; 2 0 = Mt. Casapao; 2 1 = Mt. Sembrano; 2 2 = Mt. Maquiling; 2 3 = Mt. Mapinggon; 2 4 = Mt. Bulalo; 2 5 = Mt. Nagcarlang; 2 6 = Mt. Atimbia; 2 7 - - Mt. San Cristobal; 2 8 = Mt. Banahaw; 2 9 = Mt. Malepunyo; 3 0 = Mt. Lingayen; 3 1 = Verde Island Complex; 3 2 = Mt. Macapili; 3 3 = Mt. Dumali; 3 4 = Mt. Marlanga; 3 5 = Mt. Pola; 3 6 = Mt. Maestre de Campo; 3 7 = Mt. Simara; 3 8 = Mt. Bantoi; 3 9 - - L a g u n a de Bay Complex. Several of the tectonic features are taken from de Boer et al. (1980), Hamburger et al. (1982), Karig (1983), and Wolfe and Self (1983).
andesite to dacite (Fig. 3 ). Similarly, Mt. Mariveles samples seem to form a concave-upward trend from basaltic andesite to andesite but with more curvature (Fig. 4). The data from Mt.
Maquiling ( Simon, 1983) fall along an apparently smooth concave-upward curve from basalt through andesite to dacite (Fig. 5). The samples from the Mt. San Cristobal-Banahaw
200
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complex ( Fig. 6; data from Defant, 1985; Oles, 1988) appear to fall along a concave-upward trend from basalt through basaltic andesite to andesite with the exception of one data point. Whether these trends in Figs. 3-6 are best fit by smooth curves or straight lines with different slopes is a significant point. As shown in the figures, either interpretation (particularly for Figs. 3 and 4) is reasonable. The importance of these two interpretations will be discussed in the next section, 3. D i s c u s s i o n
3.1. Previous work concerning SB-systematics In a series of recent papers, Onuma (1980), Onuma et al. (1981, 1983), Hirano et al. (1982a, b), Aramaki et al. (1984), Onuma and Montoya (1984) and Lopez-Escobar et al. (1985) have explored SB-systematics from volcanoes in Japan and the Andes. They report that Ca 2+
(ionic radius = 0.100 nm) is contained primarily in clinopyroxenes of per•dot•tic source rocks. On the other hand, Sr 2+ (0.177 nm) and Ba 2+ (0.136 nm) do not exist to any degree in the major phases (olivine, orthopyroxene, or clinopyroxene), nor in minor phases such as garnet or spinel, because of crystal structure control (Onuma et al., 1981 ). This observation is supported by all compilations of crystal-melt KDvalues (e.g., Henderson, 1982). They suggest that Sr and Ba are contained in the upper mantle in the two minor phases apatite and phlogopite. The possibility also exists that these elements are sufficiently incompatible such that significant amounts of each element are not in any lattice site, but rather adsorbed along grain boundaries, in crystal defects, etc. Onuma et al. (1981) point out that during partial melting of per•dot•tic sources, small degrees of partial melting enrich the magma greatly in Sr and Ba because phlogopite and apatite are early-melting phases. This enrich-
TABLE I Major- a n d selected trace-element compositions from western Central Luzon volcanic rocks No.
Si02
Ti02
A1203
Fe203*'
MgO
CaO
Na20
K20
Total
Mg number .2
Sr
Ba
0.60 0.55 0.48 0.54 0.55 0.57 0.55 0.58 0.56 0.54 0.51 0.37 0.53 0.50 0.55 0.66 0.49 0.62 0.58 0.57 0.69 0.71 0.48 0.50 0.48
18.52 18.38 18.40 19.06 18.55 18.63 18.37 18.57 18.56 17.14 17.74 17.05 17.31 17.47 17.75 18.17 16.59 17.91 18.32 18.27 17.18 17.50 17.74 17.50 17.71
7.78 7.52 7.06 7.51 7.51 7.85 8.17 7.61 8.00 7.06 6.38 6.39 6.20 5.97 6.90 7.70 4.51 7.39 7.52 7.43 8.24 8.24 5.96 6.02 5.92
4.50 4.65 4.18 4.60 4.70 4.83 5.04 4.54 4.60 4.04 2.92 3.12 3.05 2.91 3.12 3.85 2.54 3.77 3.77 3.56 3.96 3.96 3.02 2.98 2.80
7.79 8.37 7.26 7.92 8.04 8.33 7.60 7.71 8.07 6.01 5.91 6.13 5.76 6.04 5.12 5.54 5.03 8.06 6.86 7.10 7.09 7.39 6.48 7.01 6.23
3.08 3.00 3.06 3.29 3.16 3.18 2.97 3.08 2.98 3.12 3.83 3.71 3.34 3.54 3.49 3.00 4.49 3.65 3.39 3.36 3.69 3.98 3.58 3.69 3.41
0.81 0.87 0.82 0.76 0.79 0.78 0.55 0.80 0.85 0.93 0.98 0.86 0.85 1.04 0.96 0.67 1.49 1.15 0.87 0.82 0.83 0.80 1.76 1.70 1.65
97.54 98.55 98.49 99.29 98.32 100.20 98.72 98.63 99.83 98.82 99.13 97.73 97.14 97.38 98.19 99.47 98.29 100.22 99.35 100.90 99.68 100.27 100.00 99.26 97.27
56 58 57 57 58 58 58 57 56 56 50 52 52 52 50 52 55 53 52 51 51 51 53 52 51
378 362 424 387 392 378 383 389 388 258 315 282 274 285 268 241 579 352 300 308 313 336 525 525 524
235 199 306 239 252 248 276 230 230 182 175 175 198 189 166 234 485 237 167 142 172 170 500 489 499
0.56 0.49 0.52 0.42 0.53 0.61 0.51 0.57 0.49 0.56 0.64 0.63 0.61 0.71
17.40 16.43 17.38 16.87 18.31 17.95 17.87 18.10 17.20 17.71 17.98 17.68 17.47 17.26
7.92 6.15 7.70 6.30 8.05 8.39 7.49 8.24 6.40 7.06 8.53 8.70 8.85 8.62
4.27 3.49 4.70 3.70 3.59 4.21 4.18 4.51 3.16 3.74 4.00 4.21 4.60 4.13
7.34 6.07 7.59 6.35 6.24 7.49 6.27 7.67 5.80 7.42 7.58 8.09 8.41 8.10
3.16 3.00 2.91 3.37 3.08 3.08 2.96 2.82 3.24 3.63 3.31 3.23 2.97 3.39
1.16 1.51 1.06 1.35 1.16 1.93 1.44 1.02 1.21 1.48 1.69 1.62 1.46 1.01
98.65 99.39 98.52 99.13 97.49 99.44 98.58 98.28 97.09 99.86 100.92 100.04 100.54 101.15
54 56 57 56 50 52 55 55 52 54 51 52 53 51
307 268 283 295 286 500 260 303 279 620 431 446 496 302
269 289 242 310 286 445 310 248 350 469 440 438 486 194
0.69 0.93 0.99
17.88 18.50 18.64
7.67 9.60 9.63
4.03 5.81 5.39
7.22 9.74 9.77
3.91 3.19 3.27
2.06 1.06 1.03
101.13 100.41 100.32
54 57 55
942 849 858
910 556 619
0.73 0.93 0.75 0.71 0.41
16.43 18.44 17.07 16.77 15.75
5.88 9.82 6.78 6.56 3.65
2.48 4.90 3.00 3.17 1.63
5.55 9.71 6.32 6,20 3.84
3.28 2.39 3.04 2.69 3.62
2.44 0.95 2.28 2.19 2.98
96.11 99.34 100.04 97.61 97.76
55 59 56 58 56
400 486 390 382 426
550 364 499 510 638
Mr. Natib: 1 2 3 4 6 7 8 9 10 14 15 18 19 20 21 22 28 29 30 32 129 131 144 145 146
54.96 55.21 57.23 55.26 55.02 56.03 55.47 55.34 56.21 59.75 60.86 60.10 60.10 59.91 60.30 56.14 67.87 57.67 57.71 57,89 58.00 57.66 59.33 59.86 57.36
Mr. Mariveles: 88 89 90 92 93 94 95 96 97 112 113 114 115 117
56.84 59.26 56.40 60.48 56.53 55.78 57.53 55.07 59.59 58.26 57.19 55.88 55.80 57.93
Mr. S a n Cristobal: 171 174 175
57.19 51.06 51.06
Mt. Maquiling: 244 246 247 248 249
59.01 51.86 60.51 59.04 65.66
Major elements in wt.% a n d trace elements in ppm. All elements analyzed by XRF. Samples from Mt. Maquiling are from Simon (1983). T h e rest of the samples are from Defant (1985). *'Total Fe as Fe203. *2Mg n u m b e r = molar Mg × 1 0 0 / ( M g ÷ Fe 2+ ).
202 0O3
o.02
I
I
I
I I I I
I
I
_ / D s c l t
/
f,~
0.0
1
--
~ oo,
A o S,T S
I~'.
'
0.002
l
'
I
I
---
--
-
--
o.oo4
BASALTIC ANDESlTES 0.003
--
-
o,m _ ~ /
~ L
I
e
I
I
t i t In
0.003
i
0.01
I
0.02
0.04
Ba/ Ca
I
I
I
i
0.ol
003
Be / Co
Fig. 5. SB-systematics defined by volcanics with Mg number >/50 from Mt. Maquiling (MC). Data is from Simon (1983).
Fig. 3. SB-systematics defined by a set of volcanic rocks with Mg number/> 50 from Mt. Natib (WBL).
a n d his c o w o r k e r s . In fact, b e c a u s e m o s t basaltic partial melts from peridotitic sources can be
ment would be even more enhanced if signifi-
considered as eutectic-like melts (Yoder, 1976),
cant
this a s s u m p t i o n o f a h o m o g e n e o u s
amounts
of
Ba
and
Sr
were
truly
source w i t h
incompatible and did not exist in lattice sites of any mineral stable in the upper mantle. These melts would, therefore, have larger ratios of Sr/Ca and Ba/Ca than melts formed from higher degrees of partial melting. Assuming the Ca concentrations are equivalent in magmas generated from similar (with respect to major elements) mantle material and degrees of partial melting, a more enriched mantle should also produce higher Sr/Ca and Ba/Ca values (De-
respect to major elements is not even necessary. Consequently, different degrees of partial melting of the same source or the same degree of partial melting of an inhomogeneous source (primarily with respect to incompatible elements) will generate a SB-trend line with a slope of 45 ° through the mantle source, any partial melts, and the residual solids (Defant, 1985; Onuma et al., 1981 ). This assumes, of course, that Ba and Sr are behaving with ap-
fant, 1 9 8 5 ) , a p o i n t n o t addressed by O n u m a
p r o x i m a t e l y equal i n c o m p a t i b i l i t y . U n d e r ex-
0.02
o.o3
C~OI
/
L CO
ql~
1
l
t
~ I I I 0.01
I
I
"
d e s tI e s
-
Basalts
~
--
_
._
1
o.ooe 1
II
Itic A n d e s i t e s 0.01
BASALTIC
1
I
•
ANDESITES
0.003 0.002
I
/ < : .
(,,,)
,~s s
I
o
AN DESITES
Sr/Co
I
0.003
I
I
I I I I I 0.01
I 0.03
0.02
so/ca Fig. 4. SB-systematics defined by volcanics with Mg number/> 50 from Mt. Mariveles (WBL).
Ba/Ca Fig. 6. SB-systematics defined by volcanics with Mg number t> 50 from the Mt. San Cristobal-Banahaw Complex (MA). Data from this paper and Oles (1988).
203 TABLE II ~+'~/ ~o~.~.°~/ Primary Magma Cumulates - ~ ~
Partition coefficientsused for Fig. 8 ~,~+
~ o c ~
C,<3s~a~ I r "~ / / /o-~" • ~~\e~\1 ~r~/~ ^,C
/ / /
~. ~°~
/
Mantle
Clinopyroxene Plagioclase mafic rocks intermediaterocks Amphibole
2.5
0.14
0.07
2.0 2.5 1.5
1.8 4.0 0.57
0.23 0.50 0.31
See text for discussionof sources. ~Residual Solids Be/Ca
Fig. 7. Hypothetical St/Ca vs. Ba/Ca diagram displaying potential results of partial melting and subsequentcrystal fractionation (from Onumaet al., 1981). tremely low mantle pressures where plagioclase is stable in the source, this argument cannot hold because Sr is preferentially partitioned into the plagioclase. This hypothetical partial-melting trend, assuming that Sr and Ba are equally incompatible during partial melting, is shown in Fig. 7. A different SB-trend is commonly produced by crystal fractionation, as can be seen in Fig. 7 (Onuma et al., 1981). Phenocryst assemblages in these rocks consist of some combination of olivine, plagioclase, clinopyroxene (augite), orthopyroxene, amphibole and Fe-Tioxides. These minerals are assumed to be nearliquidus phases and thus, most likely candidates to control crystal fractionation. Although olivine, orthopyroxene and spinel fractionation does not affect Sr/Ca or Ba/Ca ratios, fractionation of clinopyroxene, plagioclase and amphibole does. Table II provides some critical crystal-liquid KD-values for Ba and Sr in clinopyroxene, plagioclase and amphibole, Onuma and Montoya (1984) believe that orthopyroxene may increase Sr/Ca and Ba/Ca ratios but to a negligible extent. Because Ca is a major component of clinopyroxene (augite), whereas concentrations of Ba and Sr are extremely low in this mineral, Sr/Ca and Ba/Ca
ratios increase with clinopyroxene fractionation. Concentrations of Sr and Ba in amphibole are generally higher than those in clinopyroxene, so fractionation of amphibole should also cause the ratios to increase but not in such dramatic fashion. Onuma et al. (1981) suggest that during plagioclase fractionation the Ba/Ca ratio will increase substantially, while the Sr/Ca ratio will remain fairly constant. This is reasonable because Sr is strongly partitioned into the plagioclase lattice and Ba is not (Table II).
3.2. SB-systematics resulting from crystal fractionation Before discussing the geologic significance of the SB-systematics on Figs. 3-6, consideration of the trends in these figures is necessary. As stated previously, presumably the only major minerals whose fractionation can change the slope of a Sr/Ca-Ba/Ca curve are clinopyroxene, plagioclase and amphibole. The role ofplagioclase is particularly critical in this regard because Sr is compatible in plagioclase, but relatively incompatible in the other two phases. In contrast, Ba is incompatible in all three phases. A change of slope on an SB-systematics curve theoretically can come about in three ways: a change in the proportions of these fractionating phases, the entry of an additional phase, or a change in the Sr or Ba content of one or more of these phases with fractionation. For example, a trend with an increase in slope (Figs. 3 and 4) could be explained by relatively early fractionation of plagioclase relative to clino-
204 pyroxene, while early fractionation of clinopyroxene relative to plagioclase would lead to a decrease in slope. Extremely low-pressure fractionation can lead to the first situation, whereas fractionation at around 9 kbar can cause the second (Green and Ringwood, 1967). Rather than a smooth curve, such a process would lead to a slope break when the second phase arrives on the liquidus and begins fractionating. The data in each of Figs. 3 and 4, and perhaps even those in Figs. 5 and 6, can be fit by two straight lines with different slopes as well as one smooth curve. The concomitant fractionation of olivine, orthopyroxene, or spinel would have no effect on the directions of these trends, A smooth curve can be produced in several ways, two of which are most probable: (1) As temperature decreases with increasing fractionation, KD'S will change in a systematic manner, creating a smooth curve. This process will only produce a curve if the KD'S involved are changing relative to one another in a non-proportional manner. If KD'S for both Sr and Ca change proportionally, then a straight line will be maintained. (2) Crystallization of two phases along a curved cotectic will lead to a systematic change in the proportions of the two fractionating phases. If these two phases have considerably different KD'S for Sr and Ba, such as do clinopyroxene and plagioclase, this will lead to a curved trend on a Sr/Ca-Ba/Ca graph, The trends on Figs. 3-6 can be approximated by simple modelling using the Rayleigh fractionation equation:
sonable KD'S for Ca is doubly difficult. They were estimated based upon microprobe data for coexisting phenocrysts and groundmasses from Defant (1985). KD-values for Sr and Ba were taken from averages in the summary table of Henderson (1982, table 5.2). The exception is Sr in relatively felsic rocks, for which Henderson (1982) gives a range of 1.5-8.8, with an average of 6.0. This average seems too high for the andesites and dacites in this study area, so a value of 4.0 was arbitrarily chosen as being reasonable. IfKD'S for allthree elements remain constant throughout the fractionation process, trends will always be straight lines. Fig. 8 shows fractionation trends for pure clinopyroxene (line 1 ) and pure plagioclase ( line 4 ) assuming constant KD'S from Table II. An example of plagioclase fractionation with changing KD'S is also shown on Fig. 8 (line 5). A trend for fractionation of clinopyroxene with changing KD'S is not shown because calculations have demonstrated that it almost coincides with line I in Fig. 8 and is only very slightly concave down. Although KD'S do change with fractionation for clinopyroxene, they maintain near proportionality, so the trend approximates line 1. An amphibole trend also is not plotted, but is almost parallel to the clinopyroxene trend. It is also possible to express the ( S r / C a ) / ( B a / C a ) ratio at any given point in the fractionation process (i.e. for any given Fvalue) by: (Sr/Ca) / (Ba/Ca) = Sr/Ba O s , F (Dsr - -
L=OF
(D-l)
(1)
where L is the composition of the residual liquid; 0 is the composition of the original liquid; F is weight fraction of residual liquid; and D is the bulk distribution coefficient (weighted average for KD'S of minerals in the fractionation assemblage). One main difficulty with any calculation of this type is choosing representative KD-values. Because the Ca partition obeys neither Raoult's or Henry's law, selection of rea-
1 )
--OBaF(DBa_I) = (0sr/Osa)F (Dsr-I~B"~ (2) where 0Sr/0Ba is the initial Sr/Ba ratio of the parental magma. If Dsr = DBa the elements are equally compatible (or incompatible), the ratio remains constant with increasing fractionation and the trend line has a 45 ° slope. If Dsr > DBa (i.e. Sr is more compatible than Ba) the ratio is lowered. It follows that when Ba is
205
o ~t
-I J .... • ~ ~
2
(constant KD'S ) is modelled in Fig. 8 (line 3 ). This trend is similar to those on Figs. 3-6. Line 2 represents 50% clinopyroxene and 50% plagioclase fractionation with continuously variable KD'S for plagioclase. It generates a concavedownward trend, which is rare in SBsystematics.
001
3.3. SB-systematics among data from the western Central Luzon volcanics
4
5
°°°ofoo3
I
J
]
I
I
J
J
I
o.o~
I
i
i
I
i
I
L
o.~
BQ/ Ca Fig. 8. Hypothetical fractionation trends: c u r v e 1 = clinopyroxene fractionation assuming a constant KD; c u r v e 2 = 50% clinopyroxene and 50% plagioclase fractionation with continuously variable KD'S for plagioclase; c u r v e 3 - - p l a g i o c l a s e f r a c t i o n a t i o n f o l l o w e d b y 5 0 % plagioclase and 50% clinopyroxene fractionationwith constant KD; c u r v e 4 = plagioclase fractionation assuming a constant KD; c u r v e 5=plagioclasefractionationwithchangingKD's.
more compatible than Sr (which is quite uncommon) and DB, > Dsr, the ratio will increase, Eq. 2 will determine the (Sr/Ca) / (Ba/Ca) ratio, but it will not determine the absolute location of a point on a Sr/Ca-Ba/Ca graph. The Ca partition must be included in the calculations for that. Hence, simple Rayleigh fractionation can illustrate some of the qualitative observations made above. For every mineral in Table II, Dsr is either approximately equal to or greater than DB,. Therefore, with increasing fractionation the (Sr/Ca) / (Ba/Ca) ratio will respectively not change or will increase. The ratio slightly decreases for clinopyroxene and amphibole and drastically decreases for plagioclase. Plagioclase fractionation followed by 50% plagioclase + 50% clinopyroxene fractionation
Mixing under certain circumstances could generate SB-systematic trends. We do not, however, believe that the western Central Luzon volcanics have undergone extensive assimilation that would generate the large Si02 variations among the data. This is based on Nd and Sr isotope values which fall within the mantle array ( Knittel et al., 1988). In addition, there is not evidence from petrographic observation that the volcanic samples have undergone extensive magma mixing of the kind that would generate the large SiO2 variations (Fig. 2; O'Hara and Mathews, 1981 ). Also, Sr/Ca vs. Ba/Ca linear diagrams do not show linear trends suggestive of simple mixing. We do believe, however, that the SB-systematic trend could be the result of assimilation fractional crystallization (AFC), the assimilation being a minor process. Fig. 9 is a composite plot of SB-systematics for all four of the western Central Luzon volcanoes studied. It includes the 45 ° partialmelting line of Onuma et al. {1981). This line is not based solely on theory; it is defined by primitive basalts from island arcs and the slope corresponds to variations in Ca with a constant Sr/Ba ratio ( Onuma et al., 1983 ). Any assumption that the mantle has chondritic Ba/Ca and Sr/Ca ratios seems reasonable; the mantle and chondrite values plot quite close to one another in Fig. 9. Chondritic values are from Tera et al. (1970) and mantle values are from Jagoutz et al. (1979). The continuation of each SB-trend on Figs. 3-6 has been extrapolated to the partial-melt-
206 0.1
0.01
-
-7
Average 0.001
i
Mantleo
A
-
MI.
Mariveles
B
-
Mt.
Natib
C -
Mt.
Maqulling
D -
Mt.
San C r l s t o b a l
Chondrltes
Banahaw m
o.oool
F
0.0001
j
I
I I Jllll
J
J I I IIIII
0.001
I 0.01
I
I IIIII
-
O.
Ba/Ca Fig. 9. Graph of Ba/Ca vs. Sr/Ca with the SB-systematics from Figs. 3-6 superimposed along with the hypothetical melting line of Onuma et al. (1981) included. The average mantle value is from Jagoutz et al. (1979) and the chondritic values are from Tera et al. (1970).
ing line on Fig. 9. Presumably, the most primitive {i.e. mantle-equilibrated) basaltic samples from these suites have yet to be collected; hypothetical "parental" melts are shown as points A-D. The overall trends, including the extrapolated portions, are easily explained on the basis of crystal fractionation (see the discussion on causes of apparent concave-upward trends), Each SB-systematic intersects the hypothetical partial-melting line, which in reality may be a band, indicative of small variations in relative incompatibility of Sr and Ba during par-
tial melting and thus the 45 ° slope [ see Fujii and Scarfe (1985) for a discussion of eutectic melting variations]. The most primitive samples on each SB-systematic represent near mantle-equilibrated melts ( i.e. plot on the right, or high Ba/Ca, edge of the band) and the points of intersection (A-D, Fig. 9) along the partialmelting line represent approximate initial compositions of the magma prior to crystal fractionation. The SB-systematic trends in Fig. 9 can be divided into three groups: MA, MC and WBL. The intersection of these trends with the partial-
207
melting line is an approximation of the original partial-melt compositions. The MA has the highest SB-systematic values compared with the MC and the WBL. As discussed in the introduction, the MA has had source contamination of a crustal component (Mukasa et al., 1986; Knittel et al., 1987; Defant et al., 1988; Knittel and Defant, 1988). We believe the high SB-systematic values within the MA initial magma compositions reflect this crustal component (high Sr and Ba from the granitic contaminants) and are not due primarily to smaller degrees of partial melting when compared to the other provinces ( MC and WBL). The MC SB-systematic initial magma composition values fall between the MA and the WBL (Fig. 9). The volcanics within the MC are apparently the result of crystal fractionation from a magma that was generated by smaller degrees of partial melting than those of the WBL, which supports the conclusions of Defant et al. (1988). The relatively young volcanics associated with this rift environment are higher in LILE than the WBL and the lavas plot near the alkaline field on a graph of total alkali vs. SiO2 (Fig. 2). The low radiogenic Sr values reported by Defant et al. (1988) suggests that these high LILE values are not due to intermediate to shallow-level crustal enrichment, The volcanoes of the WBL, Mt. Natib and Mt. Mariveles, have the lowest Sr/Ca and Ba/Ca values along the partial-melting line (Fig. 9). These magmas have probably been derived from partial melting of the mantle wedge and have had products from the slab incorporated within them (Defant et al., 1988; Knittel and Defant, 1988). Because these volcanoes fall along the WBL, they probably exist approximately the same distance above the subducting slab. The variations between Mt. Natib and Mt. Mariveles in t e r m s of SB-systematics (Fig. 9, points B and A, respectively) and the fact that these volcanoes are at the same distance above the Benioff zone present somewhat of enigma. The slight variations in LILE values between Mt. Natib and Mt. Mariveles
(distinctly lower values than the MC or the MA) may be due to small variations in the degrees of partial melting of the mantle wedge, the involvement of variations in the amount of material from the subducting slab that was incorporated within the mantle, or small-scale mantle heterogeneities. We conclude that the variations in the initial partial melts from the mantle below the western Central Luzon volcanics, based on SB-systematics, are the result of source contamination (MA, Fig. 9 ), smaller degrees of partial melting within the pull-apart zone ( MC, Fig. 9), or contributions from the subducting slab during partial melting of the mantle wedge (WBL, Fig. 9). These observations clearly support the conclusions of Defant et al. (1988) and Knittel and Defant (1988). In addition, based on inferences from trace-element modeling using a modification of the Rayleigh equation, it appears that the western Central Luzon SB-systematics (Figs. 3-6) are the result of crystal fractionation (probably plagioclase and clinopyroxene ). Acknowledgements We are grateful to Ebasco Services, Inc. and the Philippine Power Corporation for partially funding this project. A special thanks to L. Solebello for collecting and reviewing Sr, Ba and Ca data from ultramafic nodules. We also acknowledge R.K. Raymond for drafting and B.A. Healy for typing of the manuscript. We have benefitted from the fruitful discussions with G. DeVore and the editorial comments of J.M. Ragland, U. Knittel and D. Oles. References Aramaki, S., Onuma, N. and Portillo, F., 1984. Petrography and major element chemistry of the volcanic rocks of the Andes, southernPeru. Geochem.J., 18: 217-232. de Boer, J., Odom, A.L., Ragland, P.C., Snider, F.G. and Tilford, N.R., 1980. The Bataan orogene: Eastward subduction, tectonic rotations, and volcanism in the western Pacific (Philippines). Tectonophysics, 67: 251-282. Defant, M.J., 1985. The potential origin of the potas-
208 sium-depth relationship in the Bataan orogene, the Philippines. Ph.D. Thesis, Florida State University, Tallahassee, Fla., 622 pp. Defant, M.J. and Ragland, P.C., 1981. A numerical analysis of whole-rock data from western Luzon volcanic arcs, the Philippines. Abstr. Prog., 94th Annu. Meet., Geol. Soc. Am., 13: 438. Defant, M.J., de Boer, J.Z. and Oles, D., 1988. The geochemistry and tectonic setting of the western Central Luzon arc, the Philippines: Two arcs divided by rifting? Tectonophysics, 145: 305-317.. Fujii, T. and Scarfe, C.M., 1985. Composition of liquids coexisting with spinel lherzolite at 10 kbar and the genesis of MORB's. Contrib. Mineral. Petrol., 90." 18-28. Green, D.H. and Ringwood, A.E., 1967. The origin of highalumina basalts and their relationships to quartz tholeiites and alkali basalts. Earth Planet. Sci. Lett., 2: 41-51. Hamburger, M.W., Cardwell, R.K. and Isacks, B.L., 1982. Seismotectonics of the Northern Philippine island arc. In: D. Hayes (Editor), The Tectonics and Geological Evolution of the Southeast Asian Seas and Islands, Part 2. Am. Geophys. Union, Washington, D.C., Geophys. Monogr. Ser., 27: 1-22. Hamilton, W., 1979. Tectonics of the Indonesian region, U.S. Geol. Surv., Prof. Pap. No. 1078. Henderson, P., 1982. Inorganic Geochemistry. Pergamon, Oxford, 353 pp. Hirano, M., Hamuro, K. and Onuma, N., 1982a. Sr/Ca-Ba/Ca systematics in Higashi-Izu monogenetic volcano group, Izu Peninsula, Japan. Geochem. J., 16: 311-320. Hirano, M., Isshiki, N. and Onum~, N., 1982b. Sr/Ca-Ba/Ca systematics in Miyakejima, Onoharajima, Mikurajima and Inambajima volcanoes, the Izu Islands, Japan. Geochem. J., 16: 79-87. Jagoutz, E., Palme, H., Baddenhausen, K., Blum, M., Cendales, G., Dreibus, G., Spettel, L. V. and Wanke, H., 1979. The abundances of major, minor and trace elements in the earth's mantle as derived from primitive ultramafic nodules. Proc. 10th Lunar Planet. Sci. Conf., pp. 2031-2050. Karig, D.E., 1983. Accreted terranes in the northern part o f the Philippine archipelago. Tectonics, 2:211- 236. Knittel, U. and Defant, M.J., 1988. St-isotopic and trace element variations in Oligocene to Recent igneous rocks from the Philippine island arc: Evidence for recent enrichment in the sub-Philippine mantle. Earth Planet. Sci. Lett., 87: 87-99. Knittel, U., Defant, M.J. and Raczek, I., 1987. Recent enrichment events in the sources of Philippine arc magmas: Sr and Nd isotopic evidence. Pac. Rim Congr. '87, Brisbane, Qld., pp. 245-249. Lopez-Escobar, L., Moreno, H., Tagri, M., Notsu, K. and Onuma, N., 1985. Geochemistry and petrology of lavas
from San Jose volcano, Southern Andes (33°45'S). Geochem. J., 19: 209-222. McCabe, R., Almasco, J. and Diegor, W., 1982. Geologic and paleomagnetic evidence for a possible Miocene collision in western Panay, central Philippines. Geology, 10: 325-329. McDonald, G.A., 1968. Composition and origin of Hawaiian lavas. Geol. Soc. Am. Mere., 116: 477-522. Mukasa, S.B., McCabe, R. and Gill, J.B., 1987. Pb-isotopic compositions of volcanic rocks in the West and East Philippine island arcs: presence of the Dupal isotopic anomaly. Earth Planet. Sci. Lett., 84: 153-164. O'Hara, M.J. and Mathews, R.E., 1981. Geochemical evolution in an advancing, periodically replenished, periodically tapped, continuously fractionated magma chamber. J. Geol. Soc. London, 138: 237-277. Oles, D., 1988. Ph.D. Thesis, Rheinisch-Westfalische Technische Hochschule, Aachen (in preparation). Onuma, N., 1980. Sr-Ba systematics on meteorites, the moon and the earth. Solar System Sci. Syrup. II, Tokyo, Inst. Space Astron. Sci., pp. 1-6 {in Japanese). Onuma, N. and Montoya, M., 1984. Sr/Ca-Ba/Ca systematics of volcanic rocks from the central Andes, southern Peru, and its implication for Andean magmatism. Geochem. J., 18: 251-262. Onuma, N., Hirano, M. and Isshiki, N., 1981. Sr/Ca-Ba/Ca systematics in four volcanoes of Oshima, Izu Islands, Japan. Geochem. J., 15: 315-324. Onuma, N., Hirano, M. and Isshiki, N., 1983. Genesis of basalt magmas and their derivatives under the Izu Islands, Japan, inferred from Sr/Ca-Ba/Ca systematics. J. Volcanol. Geotherm. Res., 18: 511-529. Ragland, P.C. and Defant, M.J., 1983. Silica standardization: A discriminant technique applied to a volcanic arc system. Earth Planet. Sci. Lett., 64: 387-395. Ragland, P.C., Stirewalt, G.L. and Newcomb, W.E., 1976. A chemical model for island arc volcanism, western Luzon, the Philippines: Abstr. Prog., Geol. Soc. Am., 1976 Annu. Meet., 8: 1056. Simon, M., 1983. Petrographische und geochemische Untersuchungen an Vulkanen in Zentralluzon, Philippinen. M. Thesis, Rheinisch-Westfalische Technische Hochschule, Aachen, 83 pp. Tera, F., Eugster, 0., Burnett, D.S. and Wasserburg, G.J., 1970. Comparative study of Li, Na, K, Cs, Ca, St, and Ba abundances in achondrites and Apollo 11 lunar samples. Proc. Apollo 11 Lunar Sci. Conf., 2: 1637-1657. Wolfe, J.A. and Self, S., 1983. Structural lineaments and Neogene volcanism in southwestern Luzon. In: D.E. Hayes {Editor), The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, Part 2. Geophys. Monogr. Ser., Am. Geophys. Union, Washington, D.C., 27: 157-172. Yoder, Jr., H.S., 1976. Generation of Basaltic Magmas. Natl. Acad. Sci., Washington, D.C., 265 pp.