Directory UMM :Data Elmu:jurnal:J-a:Journal of Asian Earth Science:Vol18.Issue5.2000:

Melting experiment of a Wannienta basalt in the Kuanyinshan

area, northern Taiwan, at pressures up to 2.0 GPa

T.C. Liu

a,

*, B.R. Chen

a

, C.H. Chen

b

a

Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan, Republic of China

b

Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, Republic of China

Received in revised form 19 November 1999; accepted 6 December 1999

Abstract

Melting experiments involving ®fteen runs were performed at pressures between 1.0 and 2.0 GPa in order to locate the

liquidus temperatures, the solidus temperatures, and the melting intervals of the Wannienta basaltic magma, northern Taiwan.

The experimental results showed that the liquidus and solidus temperatures were raised by 60 GPa and 40 GPa respectively. The

liquidus mineral at 1.0 GPa is orthopyroxene, whereas the liquidus mineral is clinopyroxene at 1.5 and 2.0 GPa. The crystallized

phases are clinopyroxene and plagioclase at temperatures between 1220 and 1270

8

C and pressures between 1.0 and 2.0 GPa.

Garnet appears at 2.0 GPa near the solidus. The geochemical evolution of the residual magma with decreasing temperature

show the following trends: At 1.0 GPa, Al, Na, and K are progressively enriched while depletions occur in Mg. At 2.0 GPa, Si,

Fe and K are progressively enriched with decreasing temperature while depletions occur in Mg, Ca, and Na. The fractionation

trend of the Kuanyinshan volcanic series is similar to the trend observed in residual magmas at pressures between one

atmosphere and 1.0 GPa. These results indicate that the depth for fractional crystallization of the Wannienta basaltic magma to

produce andesites could be modeled at low pressure. The fractionates involved included iron-titanium oxides, olivine,

plagioclase, and clinopyroxene.

7

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1. Introduction

Taiwan is situated at the junction of the Ryukyu arc

and the Luzon arc in southeastern Asia (Fig. 1a)

within the collision zone between the Philippine Sea

Plate and the Eurasia Plate. The Philippine Sea Plate

is subducted along an E±W hinge line located at about

latitude 24

8

N and dips northward at an angle of 45

8

±

50

8

to a depth of about 300 km (e.g. Kao and Wu,

1996). On the other hand, the western edge of the

Phi-lippine Sea Plate is obducted onto the Eurasia Plate in

eastern Taiwan.

The volcanic provinces in northern Taiwan were

interpreted as the westward extension of the Ryukyu

Arc in previous studies (Yen, 1958; Yen et al., 1981).

The incompatible trace element patterns of

Kuanyin-shan basalts are similar to island arc shoshonitic

volca-nics (Chen, 1982; Juang and Chen, 1989; Liu and

Chen, 1991). Chen (1989), however, proposed that the

volcanic activity in northern Taiwan could be related

to the splitting of the Okinawa Trough based on Nd±

Sr±O isotopic data. The Kuanyinshan area of this

study is located in northern Taiwan and covers an

area of about 30 km

2

(Fig. 1b). The depth to the

Beni-o€ Zone at Kuanyinshan is about 150 km (Tsai et al.,

1977).

The Kuanyinshan is a composite volcano mainly

composed of three successive lava ¯ows and

agglomer-ates. Based on the proportion of ma®c minerals in the

rocks, Ichimura (1950) classi®ed the volcanics in this

area into one type of basalt and ®ve categories of

andesites (two pyroxene andesite, hornblende bearing

two

pyroxene

andesite,

hypersthene

andesite,

hypersthene

bearing

hornblende

andesite,

and

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hypersthene bearing biotite hornblende andesite). The

basalt is less abundant. The voluminous dacites occur

in the Chinkuashih gold±copper district, about 45 km

east of the Kuanyinshan volcano. The sequence of

lava ¯ows consist of a clinopyroxene andesite lava

¯ow comprising layer 1, a two-pyroxene andesite lava

¯ow in layer 2, and a hypersthene hornblende andesite

lava ¯ow in layer 3 (Fig. 1b) (Wang, 1958; Chen and

Hwang, 1982; Hwang and Lo, 1986).

Extensive descriptions of the petrography and

geo-chemistry of the Kuanyinshan volcanics were

pub-lished by Chen (1982). Most of the volcanics are

porphyritic with phenocrysts consisting of zoned

plagi-oclase, olivine, augite, hypersthene, amphibole, and

biotite. Yen (1958) suggested that Kuanyinshan

volca-nic activity started in the Plio-Pleistocene and ended in

the early or middle Pleistocene based on strata

corre-lation. The volcanic activity in this area was dated

between 0.63 and 0.20 Ma by Juang and Chen (1989)

using the K±Ar method. Wang (1989) traced the

ear-liest volcanic activity in this area back to 1.1 Ma

based on ®ssion track dating.

Fractional crystallization is one of the main

mechan-isms by which andesitic magmas are derived from

basaltic magmas. Fractionation involves the separation

of magnetite (e.g. Osborn, 1969), olivine (e.g. Nicholls,

1974), amphibole (e.g. Allen and Boettcher, 1978), or

an assemblage of these or other mineral phases (e.g.

Sarkar et al., 1989). Both hydrous (Kay et al., 1982)

and anhydrous (Gill, 1981) fractionation have been

proposed. Several authors (e.g. Singer et al., 1992)

have employed multiple di€erentiation trends in

deriv-ing andesites from basalts.

Based on a geochemical study of the Kuanyinshan

shoshonitic series, Chen (1982) proposed that the

Kua-nyinshan andesites were likely derived from basalts

through separation of an

amphibole±plagioclase±mag-netite assemblage. Hwang and Lo (1986) suggested

that there are three di€erentiation trends with di€erent

fractionates consisting of amphibole, plagioclase or

magnetite. This fractionation mechanism was

con-®rmed by the trace elements distributions described by

Chen (1990).

The Wannienta basalt for this study is present in the

Kuanyinshan volcanic province (Fig.1b). The distance

between the Wannienta basalt and Kuanyinshan

ande-sites is less than 3 km. The Wannienta basalt is

con-sidered as the most probable parental magma for the

Kuanyinshan andesites based on the major elements,

trace elements, and isotopes in previous studies. In this

study, the crystallization sequences of a Wannienta

basalt were investigated at pressures between 1.0 and

2.0 GPa in order to estimate the depth of

fraction-ation. The di€erentiation trends of the basaltic magma

were determined by analyzing the composition of

glasses and coexisting phenocrysts. The observed

crys-tallization trends at various pressures were then used

to estimate the depth for fractionation of basaltic

magma that di€erentiated to form the Kuanyinshan

andesites.

2. Experimental method

2.1. Starting material

In order to model the fractionation of the basaltic

magmas in the Kuanyinshan group, the rock with the

highest Mg number (64; de®ned as molar Mg/

(Mg+Fe)

100) in this area was chosen. The

Wan-nienta basalt is ®ne-grained and grayish-black in color

with ®ne vesicles. The phenocrysts are olivine, augite,

and

plagioclase.

The

detailed

petrography

was

described by Chen (1982) and Liu et al. (1998). The

forsterite content of olivine phenocrysts range from

Fo85

in the cores to Fo72

along the rims (Chen, 1982;

Liu et al., 1998). The liquidus temperature at

atmos-pheric pressure is high (1270

8

C) (Liu et al., 1998). All

evidence shows that the composition of the basalt is

near primitive (Draper and Johnston, 1992).

The sample powder of Wannienta basalt used in this

study is the same one used in the previous paper (Liu

et al., 1998). The IGPET computer program was

employed to calculate the CIPW norm of the basalt

and to plot the Di-Ol-Sil pseudoternary diagram from

the projection of plagioclase.

2.2. Apparatus and procedures

Melting experiments were performed in a

piston-cylinder apparatus (Boyd and England, 1960) at

National Taiwan Normal University. The experimental

techniques for high pressure runs are similar to that

described by Liu and Presnall (1990). The pressure-cell

assembly is the same as that described by Liu et al.

(1997). Platinum tubes were used as sample capsules.

All experiments were of the piston-out type (Presnall

et al., 1978) with no pressure correction. In all cases,

W5Re±W26Re thermocouples were used with no

pressure correction applied to the emf values.

Tem-peratures were corrected to the International Practical

Temperature Scale of 1968 (Anonymous, 1969). The

duration of the experiments ranged from 3.5 to

6 hours. Reported pressures are nominal and no

cor-rections were incorporated for friction.

2.3. Identi®cation and analysis of phases

Experimental charges were mounted in epoxy and

polished in longitudinal section. Phases in the run

pro-ducts were ®rst identi®ed microscopically in re¯ected

light. Characteristic relief, re¯ectivity, and crystal habit

were used for phase identi®cation, along with electron

microprobe analysis and back-scattered electron

ima-ging in questionable cases. The compositions of

plagio-clase, clinopyroxene, orthopyroxene, garnet, and glass

were determined using the automated JEOL

JXA-8900R electron microprobe at the Institute of Earth

Sciences, Academia Sinica.

Analyses were obtained using an accelerating voltage

of 15 kV. A beam current of 10 nA with a beam

diam-eter of about 1

m

m was employed for all elements. A

synthetic spinel was used as a standard to analyze Al

and Mg elements. For the other elements, a synthetic

glass was applied as a standard. Grains of plagioclase,

clinopyroxene,

orthopyroxene,

and

garnet

in

the

quenched products chosen for analysis are usually

lar-ger than 10

m

m in diameter and the diameter of

ana-lyzed glass pools is usually larger than 30

m

m. Matrix

corrections were made using a ZAF procedure.

3. Results and discussion

3.1. Crystallization sequence and melting properties of

the magma

Fifteen runs were performed in order to locate the

liquidus temperatures, the solidus temperatures, and

the melting intervals of the basaltic magma at

press-ures between 1.0 and 2.0 GPa. Results of the

quench-ing

experiments

are

listed

in

Table

1.

The

temperature±pressure diagram of the Wannienta basalt

(Fig. 2) was constructed based on the data in Table 1

and the data at atmospheric pressure presented by Liu

et al. (1998). It should be emphasized that there is a

small di€erence between the data at atmospheric

press-ure and the data at high presspress-ure. The experiments by

Liu et al. (1998) were performed at atmospheric

press-ure under anhydrous conditions. The experiments at

high pressures were, however, performed with rock

powders in which the loss on ignition is about 2%.

The two sets of data were used together to plot the

temperature±pressure diagram of the basalt.

The liquidus temperature of the basaltic magma at

1.0 GPa is determined to be 1280

8

C on the basis of

the quenching experiments (Table 1 and Fig. 2). With

decreasing temperature, the number of crystallized

phases increase beginning with orthopyroxene as the

near-liquidus mineral. Clinopyroxene and plagioclase

crystallize within the lower temperature range of 1250±

1220

8

C during which time orthopyroxene is consumed.

Orthopyroxene re-appears at about 1220

8

C.

Back-scat-tered electron imaging indicates that the run at 1180

8

C

has only a trace amount of glass. The solidus

ture of the basaltic magma at 1.0 GPa is therefore

taken to be 1180

8

C, indicating a melting interval of

about 100

8

C. The crystallization sequence at 1.0 GPa

is therefore orthopyroxene, clinopyroxene, and

plagio-clase.

The liquidus and solidus temperatures at 1.5 GPa

were estimated as 1305 and 1185

8

C, respectively

(Table 1). The crystallization sequence at 1.5 GPa is

clinopyroxene±plagioclase. At 2.0 GPa, the liquidus

temperature is raised to approximately 1333

8

C whereas

the solidus temperature drops below 1220

8

C. The

crys-tallization sequence at 2.0 GPa is clinopyroxene,

plagi-oclase, and ®nally garnet.

The crystallization sequences of the Wannienta

basaltic magma are similar to those of high magnesian

basalt in previous studies (e.g. Gust and Per®t, 1987;

Draper and Johnston, 1992). Iron-titanium oxide is the

liquidus phase and is joined by olivine, plagioclase,

and two pyroxenes at progressively lower temperature

down to 1080

8

C under atmospheric pressure. Above

1.0 GPa, the near liquidus mineral is clinopyroxene.

Garnet appears only at 2.0 GPa in run no. KYBP15

(1220

8

C) whereas garnet is present above 1.5 GPa in

Draper and Johnston's study (1992). Elthon and Scarfe

(1984) only synthesized garnet above 2.5 GPa. Garnet

is absent at lower pressures in their study in which the

data at lower temperatures and pressures are not

available. At successively higher pressures, plagioclase,

clinopyroxene, and garnet are the liquidus phases in

both anhydrous (Johnston, 1986) and hydrous (Baker

and Eggler, 1983, 1987) experiments.

3.2. Mineral chemistry of synthetic phases

Clinopyroxenes in the quenching products were

ana-lyzed with an electron microprobe and the results are

presented in Table 2 and also plotted in Fig. 3. The

Wo (CaSiO3) component in the clinopyroxenes range

between 34 and 43% and are therefore classi®ed as

augites

following

the

classi®cation

of

Morimoto

(1988). The Fs (FeSiO3) component ranges from 11 to

19%.

The orthopyroxenes were synthesized only at 1.0

GPa in this study. Microprobe analyses of

orthopyrox-Table 1

Quenching experiments

Run no. P(GPa) Temperature (8C) Duration (h:min) Phase(s)a

KYBP4 1.0 1300 4:00 Gl

KYBP1 1.0 1280 4:05 Gl

KYBP6 1.0 1260 6:00 Gl + Opx

KYBP2 1.0 1240 3:30 Gl + Cpx + Pl

KYBP3 1.0 1200 5:00 Gl + Opx + Cpx + Pl

KYBP5 1.0 1180 6:00 Gl + Opx + Cpx + Pl

KYBP10 1.5 1340 4:00 Gl

KYBP8 1.5 1300 4:00 Gl + Cpx

KYBP11 1.5 1240 4:30 Gl + Cpx

KYBP12 1.5 1200 5:00 Gl + Cpx + Pl

KYBP13 2.0 1340 3:30 Gl

KYBP9 2.0 1325 4:00 Gl + Cpx

KYBP14 2.0 1280 4:30 Gl + Cpx

KYBP16 2.0 1260 6:00 Gl + Cpx + Pl

KYBP15 2.0 1220 5:00 Gl + Cpx + Pl + Ga

a

Cpx: Clinopyroxene; Ga: Garnet; Gl: Glass; Opx: Orthopyroxene; Pl: Plagioclase.

enes are listed in Table 3 and also plotted in Fig. 3.

The En (MgSiO3) component of the orthopyroxenes in

this

study

decreases with decreasing

temperature

whereas the Fs component increases. The results are

consistent with the fractionation trends of pyroxenes

within the Skaergaard and Bushveld complexes (Deer

et al., 1992).

Plagioclase is abundant at pressures between 1 atm

and 2.0 GPa. The compositions of plagioclase in this

study are listed in Table 4 and plotted in Fig. 4. They

range from labradorite to andesine in composition.

The An (CaAl2Si2O8) component of the synthesized

plagioclases in this study all decrease with decreasing

temperature in both the 1.0 and 2.0 GPa experiments

which is consistent with the results of previous studies

(Bowen, 1913; Schairer, 1957; Yoder et al., 1957; and

Johannes, 1978). The synthesized plagioclases at high

pressures are more calcic than those formed at low

pressures (Table 4 and Fig. 4). The compositions of

plagioclase phenocrysts in the Wannienta basalt

clus-tered around An83

as determined by Chen (1982) and

around An80

in study by Liu et al. (1998). The

plagio-clases in this study having An contents (An77

to An79)

close to those values only appeared at pressures of 1.5

and 2.0 GPa. The composition of the synthesized

pla-gioclase (An61) at 1087

8

C and atmospheric pressure,

however, is similar to the composition of plagioclase

(An63) in the groundmass of the Wannienta basalt

(Liu et al., 1998). This indicates that the clinopyroxene

and plagioclase phenocrysts in the Wannienta basalt

were formed at high pressures whereas minerals in the

groundmass were formed at pressures between 1 atm

and 1.0 GPa. Garnet is only present in run no.

KYBP15 at 1220

8

C and 2.0 GPa and has a

compo-sition of Py61Alm34Spes5

(Table 5).

3.3. Evolution of the basaltic magma

The glasses in the quenched products at 1.0, 1.5,

and 2.0 Gpa were analyzed by microprobe and are

listed in Tables 6±8. Most of the glass analyses totaled

between 98 and 101%. They were normalized to 100%

to be plotted in the variation diagrams for comparison.

The compositions of glasses at each speci®c

tempera-ture and pressure are analogous to the compositions of

the residual magmas under these conditions. Several

Table 2

Clinopyroxene compositions in the runs

Run No. KYBP2 KYBP3 KYBP5 KYBP8 KYBP11 KYBP12 KYBP9 KYBP14 KYBP16 KYBP15

P(GPa) 1 1 1 1.5 1.5 1.5 2 2 2 2

T (8C) 1240 1200 1180 1300 1240 1200 1325 1280 1260 1220

Average of 4 3 1 3 3 3 5 5 4 4

Wt(%)

SiO2 48.69 (0.95)a 48.87 (0.92) 47.38 49.66 (1.02) 50.03 (1.24) 49.06 (1.21) 49.06 (1.61) 50.08 (1.25) 51.07 (1.66) 50.95 (1.29)

TiO2 0.62 (0.58) 0.69 (0.21) 0.91 0.05 (0.01) 0.06 (0.25) 0.57 (0.34) 0.62 (0.29) 0.35 (0.20) 0.58 (0.33) 0.86 (0.32)

Al2O3 8.74 (1.12) 6.68 (1.46) 7.89 6.22 (1.31) 7.81 (0.33) 6.13 (2.55) 7.90 (1.25) 7.58 (1.29) 6.37 (1.45) 4.81 (0.49)

Cr2O3 0.11 (0.01) 0.13 (0.16) 0.00 0.32 (0.01) 0.03 (0.01) 0.17 (0.04) 0.27 (0.19) 0.31 (0.05) 0.41 (0.21) 0.21 (0.20)

tFeO 9.15 (1.25) 9.42 (2.01) 11.69 8.96 (1.45) 9.19 (1.44) 9.58 (1.55) 8.31 (1.05) 6.70 (1.40) 7.21 (1.54) 6.83 (0.77) MnO 0.31 (0.24) 0.24 (0.11) 0.24 0.24 (0.02) 0.25 (0.02) 0.03 (0.01) 0.30 (0.99) 0.26 (0.10) 0.23 (0.02) 0.19 (0.04) MgO 16.47 (0.67) 15.46 (1.81) 14.76 17.07 (0.05) 15.32 (1.15) 16.36 (1.14) 14.99 (0.99) 15.66 (0.52) 15.01 (1.21) 14.73 (0.61) CaO 15.03 (0.06) 17.94 (1.07) 15.99 17.01 (1.55) 17.66 (1.56) 17.88 (1.24) 17.92 (2.40) 18.55 (1.20) 18.66 (2.18) 20.63 (1.43) Na2O 0.73 (0.02) 0.62 (0.23) 0.89 0.20 (0.01) 0.24 (1.44) 0.58 (0.24) 0.82 (1.23) 0.92 (0.50) 0.65 (0.23) 0.32 (0.03)

K2O 0.04 (0.01) 0.05 (0.04) 0.04 0.05 (0.02) 0.06 (1.48) 0.05 (0.11) 0.04 (0.02) 0.03 (0.03) 0.03 (0.03) 0.03 (0.03)

Total 99.88 100.11 99.74 99.78 100.65 100.38 100.23 100.44 100.22 99.55 Cations per 6 Oxygens

Si 1.790 1.813 1.776 1.835 1.831 1.815 1.807 1.828 1.868 1.884 Ti 0.017 0.019 0.026 0.001 0.002 0.016 0.017 0.010 0.016 0.024 Al 0.210 0.187 0.224 0.271 0.327 0.267 0.343 0.326 0.276 0.210 Cr 0.003 0.004 0.000 0.009 0.001 0.005 0.008 0.009 0.012 0.006 Fe 0.281 0.292 0.336 0.276 0.282 0.296 0.256 0.204 0.221 0.211 Mn 0.010 0.008 0.006 0.008 0.008 0.001 0.009 0.008 0.007 0.006 Mg 0.903 0.855 0.825 0.940 0.836 0.902 0.823 0.852 0.819 0.812 Ca 0.892 0.713 0.642 0.673 0.693 0.709 0.707 0.725 0.731 0.817 Na 0.052 0.045 0.065 0.014 0.017 0.042 0.059 0.060 0.046 0.023 K 0.002 0.002 0.002 0.002 0.003 0.002 0.002 0.001 0.001 0.001 Total 4.160 3.940 3.940 4.029 3.990 4.055 4.031 4.032 3.997 3.994

Table 3

Orthopyroxene compositions in the runs

Run no. KYBP6 KYBP3 KYBP5

P(GPa) 1 1 1

T(8C) 1260 1200 1180

Average of 4 1 3 Wt(%)

SiO2 53.15 (0.50)a 53.13 49.66 (1.19)

TiO2 0.22 (0.02) 0.31 0.36 (0.10)

Al2O3 5.59 (0.73) 7.43 9.21 (2.43)

Cr2O3 0.27 (0.09) 0.30 0.08 (0.07)

tFeO 8.48 (0.37) 9.56 13.04 (1.21) MnO 0.31 (0.03) 0.30 0.34 (0.05) MgO 28.36 (0.43) 27.17 24.70 (1.29) CaO 2.69 (0.31) 2.55 2.47 (0.65) Na2O 0.12 (0.02) 0.14 0.17 (0.05)

K2O 0.01 (0.01) 0.02 0.04 (0.06)

Total 101.2 (0.06) 100.91 100.06 (0.06) Cations per 6 Oxygens

Si 1.908 1.855 1.783 Ti 0.006 0.008 0.010 Al 0.136 0.306 0.217 Cr 0.007 0.008 0.002 Fe 0.245 0.279 0.329 Mn 0.009 0.009 0.010 Mg 1.462 1.414 1.322 Ca 0.100 0.095 0.095 Na 0.008 0.009 0.012 K 0.000 0.001 0.002 Total 3.881 3.984 3.782

Wo 06 05 05

Fs 14 16 22

En 80 79 73

a_{Standard deviation in parentheses.}

Table 4

Plagioclase compositions in the runs

Run no. KYBP2 KYBP3 KYBP5 KYBP12 KYBP16 KYBP15

P(GPa) 1 1 1 1.5 2 2

T(8C) 1240 1200 1180 1200 1260 1220

Average of 4 4 4 3 3 3

Wt(%)

SiO2 49.70 (1.60) a

52.38 (0.90) 55.53 (0.53) 52.48 (1.54) 48.23 (1.31) 48.27 (1.21) Al2O3 30.77 (1.48) 28.80 (0.01) 26.62 (0.10) 29.51 (3.12) 30.11 (2.45) 32.24 (1.09)

tFeO 1.05 (0.06) 1.08 (0.62) 1.31 (0.05) 2.01 (0.11) 1.15 (0.15) 1.20 (0.11) CaO 14.62 (1.55) 12.50 (0.26) 9.44 (0.09) 13.41 (3.31) 18.24 (2.45) 16.13 (1.44) Na2O 3.28 (0.68) 4.20 (0.16) 4.74 (0.22) 2.20 (1.25) 2.45 (1.13) 2.30 (1.22)

K2O 0.42 (0.23) 0.69 (0.01) 2.07 (0.09) 0.60 (0.33) 0.32 (0.30) 0.42 (0.34)

Total 99.84 (0.23) 99.65 (0.23) 99.71 (0.23) 99.74 (0.23) 99.5 (0.23) 100.55 (0.23) Cations of 8 oxygens

Si 2.283 2.400 2.517 2.389 2.241 2.206

Al 1.666 1.555 1.422 1.583 1.649 1.750

Fe 0.040 0.041 0.050 0.077 0.006 0.042

Ca 0.719 0.613 0.458 0.654 0.908 0.800

Na 0.292 0.373 0.417 0.194 0.221 0.201

K 0.025 0.040 0.120 0.003 0.019 0.019

An 69 60 46 77 79 78

Ab 28 36 41 22 19 20

Or 3 4 12 1 2 2

a

Standard deviation in parentheses.

workers have pointed out that the glass composition

can be signi®cantly altered by the formation of quench

crystals in the experiments (e.g. Jaques and Green,

1979, 1980). In this study, some quench crystals were

found. In another study (Liu and Presnall, 2000), we

had found that the glass compositions only changed

within a few microns of the quenched crystals. The

results estimated from the glass composition

deter-mined by microprobe are consistent with the results of

the quenching experiments in that study. Therefore,

the glass compositions are believed to represent the

compositions of the melt coexisting with the

crystal-lized assemblage in that run. All the spots for glass

analyses in this study are at least 10

m

m away from

crystallized phases.

The di€erentiation trends at each pressure are

dis-cussed below.

3.4. 1.0 GPa

The compositions of glasses change irregularly

between Ol-normative and Qz-normative with

decreas-ing temperature (Table 6). The compositions of glasses

at 1.0 GPa are plotted versus temperature in Fig. 5.

With decreasing temperature (read from right to left in

Fig. 5), glasses become progressively enriched in

Al

2

O

3

, Na

2

O, and K

2

O and depleted in MgO, while

total iron contents change irregularly. The SiO2

con-tent ¯uctuates within the range of 52 and 55%. The

Fig. 5. Variations of SiO2, Al2O3, total Fe as FeO, MgO, CaO,

Na2O, and K2O of residual glasses versus temperature at 1.0 GPa.

The KYBP]are the run numbers listed in Table 1.

Fig. 6. The di€erentiation trend of residual liquids in Harker's dia-gram at 1.0 GPa. Symbols: solid dots: the glass compositions at 1.0 GPa. open circles: average compositions of each rock type in Kua-nyinshan volcanic group from Chen (1990): 1: augite olivine basalt; 2: augite basalt; 3: biotite hornblende andesite; 4: augite andesite; 5: hornblende bearing two pyroxene andesite; 6: hypersthene horn-blende andesite.

Table 5

Garnet composition in this study

Run no. KYBP15

P(GPa) 2

T(8C) 1220

Average of 3

Wt(%)

SiO2 39.67 (0.26)a

TiO2 0.14 (0.02)

Al2O3 21.92 (0.09)

Cr2O3 0.05 (0.08)

tFeO 15.56 (0.17)

MnO 0.67 (0.12)

MgO 15.48 (0.12)

CaO 5.72 (0.04)

Na2O 0.11 (0.03)

K2O 0.01 (0.01)

Total 100.32 (0.01) Cations per 24 Oxygens

Si 5.585

Al 3.811

Ti 0.127

Mg 3.402

Fe 1.919

Ca 1.903

Na 0.031

K 0.001

Cr 0.005

Mn 0.288

Total 16.111

Pyrope 61

Almandine 34

Spessartine 05

a

compositions of glasses at 1.0 GPa were also plotted

in Harker's diagram (Fig. 6) and compared with the

compositions of the Kuanyinshan volcanics. The

extensive fractionation is shown by the wide

compo-sitional spread of the Kuanyinshan natural volcanics

whereas the glasses at 1.0 Gpa cluster within a small

Table 6

Glass compositions in the runs at 1.0 GPa

Run no. KYBP1 KYBP6 KYBP2 KYBP3 KYBP5

T(8C) 1280 1260 1240 1200 1180

No. of analyses 2 2 2 3 5

Wt(%)

SiO2 51.54 (0.39)a 53.98 (0.23) 52.76 (0.20) 53.37 (0.26) 54.75 (0.75)

TiO2 0.80 (0.06) 0.91 (0.08) 0.99 (0.05) 1.29 (0.06) 0.35 (0.20)

Al2O3 17.58 (0.20) 17.72 (0.17) 17.65 (0.16) 17.06 (0.34) 24.01 (1.51)

Cr2O3 0.06 (0.08) 0.00 (0.00) 0.02 (0.02) 0.03 (0.05) 0.03 (0.03)

tFeO 6.90 (0.04) 3.36 (0.05) 6.61 (0.06) 8.08 (0.27) 2.65 (0.97) MnO 0.21 (0.06) 0.14 (0.08) 0.22 (0.04) 0.10 (0.05) 0.07 (0.04) MgO 7.69 (0.06) 8.03 (0.01) 6.73 (0.01) 4.62 (0.21) 1.41 (1.17) CaO 9.38 (0.02) 10.01 (0.05) 9.58 (0.07) 7.50 (0.05) 10.10 (0.87) Na2O 2.74 (0.04) 2.78 (0.07) 2.90 (0.09) 2.83 (0.12) 4.52 (0.26)

K2O 1.58 (0.01) 1.54 (0.04) 1.69 (0.07) 2.93 (0.06) 1.99 (0.23)

Total 98.48 98.47 99.15 97.81 100.48

CIPW Norm

Il 1.52 1.73 1.88 2.54 0.66

Or 9.34 9.10 9.99 17.31 11.76

Ab 23.19 23.52 24.54 23.95 34.56

An 31.00 31.32 30.15 25.19 39.35

Di 12.65 14.60 14.18 9.98 8.97

Hy 10.07 17.98 12.08 17.40 0.00

Q 0.00 0.21 0.00 0.00 0.00

Ol 10.63 0.00 6.31 1.49 2.54

Mg] 57.21 57.11 55.13 51.27 53.24

a_{Standard deviation in parentheses.}

Table 7

Glass compositions in the runs at 1.5 GPa

Run no. KYBP10 KYBP8 KYBP11 KYBP12

T(8C) 1340 1300 1240 1200

No. of analyses 4 4 4 4 Wt(%)

SiO2 52.33 (0.71)a 53.57 (0.18) 53.35 (0.65) 52.86 (1.18)

TiO2 0.88 (0.00) 0.91 (0.06) 0.99 (0.04) 0.85 (0.10)

Al2O3 18.45 (0.19) 18.53 (0.11) 17.69 (0.21) 17.60 (0.16)

Cr2O3 0.00 (0.00) 0.06 (0.02) 0.00 (0.00) 0.01 (0.02)

tFeO 6.92 (0.10) 6.01 (0.07) 4.99 (2.35) 5.63 (1.94) MnO 0.14 (0.19) 0.22 (0.08) 0.22 (0.04) 0.17 (0.09) MgO 6.66 (0.00) 7.02 (0.18) 7.38 (0.93) 7.50 (0.67) CaO 8.20 (0.06) 8.68 (0.14) 9.75 (0.31) 9.68 (0.34) Na2O 3.29 (0.01) 3.93 (0.10) 2.90 (0.09) 2.78 (0.05)

K2O 1.84 (0.06) 1.88 (0.07) 1.65 (0.13) 1.57 (0.07)

Total 98.71 100.81 98.92 99.65 CIPW Norm

Il 1.67 1.73 1.88 1.61 Or 10.87 11.11 9.75 9.28 Ab 27.84 31.35 24.54 23.52 An 30.14 27.37 30.38 30.91 Di 8.62 12.75 14.51 13.85 Hy 7.90 0.00 14.08 14.85 Ol 11.67 15.39 3.79 4.61 Mg] 51.98 46.61 55.32 56.78

a_{Standard deviation in parentheses.}

Fig. 7. Variations of SiO2, Al2O3, total Fe as FeO, MgO, CaO,

Na2O, and K2O of residual glasses versus temperature at 1.5 GPa.

range. This implies that fractionation of the

Kuanyin-shan volcanics cannot be modeled at 1.0 GPa.

3.5. 1.5 GPa

The residual magmas at 1.5 GPa become enriched in

SiO2, MgO, and CaO and depleted in Al2O3, FeO,

Na2O, and K2O as temperature decreases (Table 7 and

Fig. 7). The Ol-norm in glasses decreases with

decreas-ing temperature. In Fig. 8, the liquid lines of descent

are compared to the Kuanyinshan fractionation trend.

Table 8

Glass compositions in the runs at 2.0 GPa

Run no. KYBP13 KYBP9 KYBP14 KYBP16 KYBP15

T(8C) 1340 1325 1280 1260 1220

No. of analyses 4 4 3 4 3

Wt(%)

SiO2 52.74 (0.31)a 53.51 (0.18) 52.46 (0.56) 55.11 (0.33) 56.45 (2.67)

TiO2 0.79 (0.01) 0.92 (0.05) 0.90 (0.03) 0.80 (0.12) 1.51 (0.03)

Al2O3 18.47 (0.25) 18.50 (0.12) 18.52 (0.18) 18.40 (0.15) 18.21 (0.29)

Cr2O3 0.03 (0.01) 0.06 (0.02) 0.03 (0.03) 0.01 (0.04) 0.04 (0.08)

tFeO 6.02 (1.06) 6.00 (0.06) 6.91 (0.08) 7.34 (0.04) 7.35 (0.79) MnO 0.16 (0.00) 0.19 (0.10) 0.27 (0.02) 0.21 (0.03) 0.19 (0.17) MgO 7.88 (0.22) 7.06 (0.16) 6.69 (0.05) 5.81 (0.05) 4.07 (1.21) CaO 8.23 (0.65) 8.72 (0.14) 8.18 (0.05) 7.34 (0.05) 4.73 (0.15) Na2O 3.55 (0.15) 3.93 (0.08) 3.25 (0.06) 3.12 (0.03) 3.00 (0.29)

K2O 1.64 (0.24) 1.86 (0.06) 1.83 (0.05) 2.22 (0.03) 3.76 (0.55)

Total 99.51 100.75 99.04 100.44 99.31

CIPW Norm

Il 1.50 1.75 1.71 1.52 2.87

Or 9.69 10.99 10.81 13.12 22.22

Ab 30.04 31.23 27.50 26.40 25.39

An 29.62 27.34 30.54 29.86 23.47

Di 9.08 1.10 8.21 5.39 0.00

Hy 4.49 12.93 9.13 23.53 21.46

Q 0.00 0.00 0.00 0.00 3.25

Ol 15.05 15.33 11.09 0.60 0.00

Mg] 49.65 46.68 52.62 53.08 48.04

a_{Standard deviation in parentheses.}

Fig. 8. The di€erentiation trend of the residual liquids in Harker's diagram at 1.5 GPa. Symbols: solid dots: the glass compositions at 1.5 GPa; open circles: the same as in Fig. 6.

Fig. 9. Variations of SiO2, Al2O3, total Fe as FeO, MgO, CaO,

Na2O, and K2O of residual glasses versus temperature at 2.0 GPa.

The glasses at 1.5 GPa also cluster between 53% and

about 54% SiO2.

3.6. 2.0 GPa

The compositions of residual liquids at 2.0 GPa are

plotted versus temperature in Fig. 9. As temperature

decreases, the residual liquids become enriched in

SiO2, FeO, and K2O and depleted in MgO, CaO, and

Na2O while Al2O3

changes very little (Table 8 and

Fig. 9). In the Harker's diagrams, the di€erentiation

trend of the residual liquids does not adequately

dupli-cate that of the Kuanyinshan volcanics (Fig. 10).

3.7. AFM diagram

The compositions of Kuanyinshan volcanics and

ex-perimental liquids at each pressure are compared in

the Na2O+K2O-FeO+Fe2O3-MgO (AFM) diagram in

Fig. 11. The fractionation trend at atmospheric

press-Fig. 10. The di€erentiation trend of the residual liquids in Harker's diagram at 2.0 GPa. Symbols: solid dots: the glass compositions at 2.0 GPa; open circles: the same as in Fig. 6.

Fig. 11. Na2O+K2OÿFeO+Fe2O3ÿMgO (AFM) diagrams (Wagner and Deer, 1939) illustrating the variation of Kuanyinshan volcanics (Chen,

ure is most similar to the di€erentiation trends of the

Kuanyinshan volcanics whereas fractionation trends at

higher pressures deviate signi®cantly. This is consistent

with the trends found in the Harker's diagrams of

Figs. 6, 8, and 10.

In the pseudoternary Di-Ol-Sil diagram (Fig. 12),

the compositions of Kuanyinshan volcanics and

exper-imental liquids at pressures from 1 atm to 2.0 GPa

were all plotted for comparison. With increasing

press-ure, the liquid lines of descent shift toward the Ol-apex

which is consistent with previous studies (e.g. Presnall

et al., 1978; Elthon and Scarfe, 1984; Liu and Presnall,

1990). Comparatively speaking, the fractionation trend

of experimental liquids at atmospheric pressure most

closely follows the di€erentiation trend of the

Kua-nyinshan volcanics.

3.8. Historical evolution of magmas in Kuanyinshan

The biotite hornblende andesite, augite andesite, and

two-pyroxene andesite were dated as 0.63, 0.53 and

0.43 Ma respectively by Juang and Chen (1989) based

on the K±Ar method. Using the ages of the rocks and

experimental results, the history of magmatic evolution

in the Kuanyinshan volcanic group can be interpreted

as follows:

At 0.63 Ma, the basaltic magma intruded the crust

at pressures between 1 atm and 1.0 GPa and

fractio-nated into an andesitic magma compositionally similar

to the biotite hornblende andesite which occurs as a

dyke in Kuanyinshan. At 0.53 Ma, another batch of

basaltic magma moved upward into the crust at

press-ures between 1 atm and 1.0 GPa. This basaltic magma

fractionated into an andesitic magma and erupted to

form the clinopyroxene andesite of Layer 1. At 0.43

Ma, another batch of basaltic magma moved into the

crust and fractionated into an andesitic magma, which

then erupted to form a two-pyroxene andesite as Layer

2.

The composition of the hypersthene hornblende

andesite in Layer 3 is beyond the range of

fraction-ation in this study. It is proposed that the basaltic

magma could evolve into a hypersthene hornblende

andesite with additional fractionation.

Since the compositions of pyroxene and plagioclase

phenocrysts in the Wannienta basalt are similar to

those synthesized at high pressures, it is suggested that

the basaltic magma had crystallized at high pressure.

At 0.20 Ma, the basaltic magma containing

high-press-ure phenocrysts invaded the crust and erupted to form

the Wannienta basalt.

4. Conclusions

The experimental results show that the liquidus and

solidus temperatures increase by 60

8

C/GPa and 40

8

C/

GPa, respectively. The liquidus mineral at 1.0 GPa is

orthopyroxene whereas the liquidus mineral is

clino-pyroxene at 1.5 and 2.0 GPa. At lower temperatures

and pressures between 1.0 and 2.0 GPa, the

crystal-lized phases are clinopyroxene and plagioclase. Garnet

appears at 2.0 GPa near the solidus.

The evolution of the residual magma shows the

fol-lowing geochemical trend with decreasing temperature:

enrichment in aluminum, sodium, and potassium and

depletion in magnesium at 1.0 GPa; enrichment in

sili-con, iron, and potassium and depletion in magnesium,

calcium, and sodium at 2.0 GPa. The fractionation

trend of the Kuanyinshan volcanic series is similar to

that

exhibited

by

residual

magmas

at

pressures

between 1 atm and 1.0 GPa. This implies that the

depth of fractional crystallization of the Wannienta

basaltic magma to produce andesites could be modeled

at low pressure. The fractionates involved in the

frac-tionation included iron-titanium oxides, olivine,

plagio-clase, and clinopyroxene.

Acknowledgements

We would like to thank Dr. Jennifer Lytwyn,

Uni-versity of Houston, for her revision to signi®cantly

improve the manuscript. Professor Cheng-Hong Chen

of National Taiwan University generously allowed us

access to his graphite-evaporator for carbon coating

on polished sections. This research was supported by

the National Science Council of the Republic of China

under grant NSC86-2116-M-003-007 to TCL.

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