Journal of Volcanology and Geothermal Research 178 (2008) 608–623
Contents lists available at ScienceDirect
Journal of Volcanology and Geothermal Research
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j vo l g e o r e s
Magma mingling in the Tungho area, Coastal Range of eastern Taiwan
Yu-Ming Lai a, Sheng-Rong Song a,⁎, Yoshiyuki Iizuka b
a
b
Department of Geosciences, National Taiwan University, Taipei, Taiwan, ROC
Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, ROC
A R T I C L E
I N F O
Article history:
Received 25 October 2007
Accepted 26 May 2008
Available online 7 June 2008
Keywords:
magma mingling
Luzon Arc
peperite
Coastal Range of eastern Taiwan
breccia
A B S T R A C T
Complex rocks, consisting of different lithologic breccias and sediments in the Tungho area of the southern
Coastal Range, eastern Taiwan, were formed by magmas and magma–sediment mingling. Based on field
occurrences, petrography, and mineral and rock compositions, three components including mafic magma,
felsic magma, and sediments can be identified. The black breccias and white breccias were consolidated from
mafic and felsic magma, respectively. Isotopic composition shows these two magmas may be from the same
source. Compared to the white breccias, the black breccias show clast-supported structures, higher An values
in plagioclase, higher contents of MgO, CaO, and Fe2O3 and lower SiO2, greater enrichment in the light rare
earth elements (LREE), and depletion in the heavy rare earth elements (HREE). The white breccias show
matrix-supported blocks and mingling with tuffaceous sediments to form peperite. Physical and chemical
evidence shows that the characteristics of these two components (mafic and felsic magmas) are still apparent
in the mingled zone. According to their petrography, mafic and felsic magmas did not have much time for
mingling. White intrusive structures and black flow structures show that mingling occurred before they
solidified. Finally, the occurrence of mingling between magmas and sediments suggests that the mingling has
taken place at the surface and not in the magma chamber.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Magma mixing and mingling are important processes in the
eruption and evolution of magmatism. When two or more magmas
encounter each other, mechanical processes and chemical diffusion
result in variously mixed or mingled final products (Donoghue et al.,
1995; Cole et al., 2001). Magma mixing and mingling are two different
kinds of processes in hybrid magmas (Vernon, 1984; Neves and
Vauchez, 1995). The term ‘mixing’ will be used in this study whether
the final product is homogeneous or not (Wilcox, 1999). Nevertheless,
when we use the term ‘mingling (commingling)’ to explain that
homogeneity had not been attained in a final rock specimen, the term
‘mixing’ is applied to homogeneous magma (Lee, 2002). Simply
speaking, in this paper, if the characteristics of participating magmas
is still apparent, there is magma mingling, if not, there is magma
mixing.
Studying magma mixing and mingling is not only helpful in
explaining volcanic-rock variety but it also has significant geological
meaning. Mixing and mingling of magmas occurs in different
geological processes, and each exhibits different behavior and signals.
When different magmas (mafic and felsic, hotter and colder etc.) come
into contact, explosions or variations in chemical compositions may
ensue. Investigating magma mixing and mingling gives us an under⁎ Corresponding author. Department of Geosciences, National Taiwan University,
Roosevelt Rd, Sec 4, No 1, Taipei 106, Taiwan, ROC. Fax: +886 2 2362 5125.
E-mail address: [email protected] (S.-R. Song).
0377-0273/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jvolgeores.2008.05.020
standing of magma's evolution, volcanic eruptions, and even allows
reconstruction of the paleoenvironment.
Peperite is a special kind of volcaniclastic rock formed by the
mingling of juvenile magma with unconsolidated sediments (Jerram
and Stollhofen, 2002). It is common in arc-related and other volcano–
sedimentary sequences, usually occurs at the bottom and frontal part
of the lava flows, and is also distributed around shallow intrusions
(Fisher, 1960; Skilling et al., 2002). Different structures of peperite
reflect the interaction processes of magma and unconsolidated
sediments, controlled by the characteristics of the paleoenvironment.
In this study, we describe a volcanic outcrop recording a magma
mingling event in the Tungho area of the Coastal Range of eastern
Taiwan. Combining field observations, petrographic textures, mineral,
and rock compositions, we characterized and quantified the mingling
processes. It is the first time that magma mingling has been observed
in this area; and it characterizes mingling as having happened during
the eruption.
2. Geological setting
Taiwan is a member of Ryuku–Taiwan–Philippine island arc chain
rimming the western border of the Pacific Ocean. It is situated on a
convergence and compressive boundary between two major plates,
the continental Eurasian plate on the west and the oceanic Philippine
Sea plate on the east. The Neogene Coastal Range volcanic rocks are
situated in eastern Taiwan, as a result of the South China Sea
subducting the Philippine Sea plate and exposing these volcanic rocks
Y.-M. Lai et al. / Journal of Volcanology and Geothermal Research 178 (2008) 608–623
after arc–continent collision (Chai, 1972; Big, 1973; Tsai, 1978; Big,
1981) (Fig. 1a). In general, the rocks accumulated depths more than
4000 m in an oceanic environment (Song and Lo, 2002). Volcanic
eruptions appeared early in the submarine environment, changing it
from a deep to shallow sea, or even occurred subaerial (Teng and Lo,
1985; Song and Lo, 1987, 2002). Due to the southward propagation of
the oblique arc–continent collision, volcanism ceased after 6–3.5 Ma
progressively from north to south (Song and Lo, 2002). Thin layers of
limestone are often found near the top of the volcaniclastic sequences
(Chang, 1967), above them are deposited sediments largely derived
from the Asian continental margin and arc characterized by quartz–
slate arenites (Teng, 1981; Chen, 1988). The volcanic sequence, named
Tuluanshan Formation (Fig. 1b), is composed of basaltic to andesitic
lavas, pyroclastic breccias, tuffs and tuffaceous sediments. Radiometric and fission track age datings show that these volcanics fall in
the range 22.2 Ma to 1.5 Ma (Ho, 1969; Richard et al., 1986; Yang et al.,
1988; Lo et al., 1994; Yang et al., 1995; Song and Lo, 2002).
3. Analytical methods
Three sample suites are used in this study. They are black breccias
(BB), white breccias (WB) and the mingled zone (MZ). A total of 30
samples including these three components were collected. The major
element compositions of selected samples were determined at the
National Taiwan University by X-ray Fluorescence spectrometry (XRF,
Rigaku RIX-2000). Detection limits and calibrated analytical uncertainty for this machine can be found in Lee et al. (1996). Trace
elements and isotopic compositions analyses were performed at
609
Guangzhou Institute of Geochemistry by Inductively Coupled Plasma
Mass spectrometry (ICP-MS, PE Elan 6000). Detection limits and
calibrated analytical uncertainty for this machine can be found in Li
(1997). Mineral composition analyses were obtained on polished thin
sections of their host rocks and were determined at the Institute of
Earth Sciences, Academia Sinica by Electron Probe Micro-Analyzer
(EPMA, JEOL EPMA JXA-8900R); the accelerating voltage and beam
current were 15 kV and 10 nA. Peak counting for each element and
both upper and lower baselines are counted for 20 s and 10 s,
respectively. Relative standard deviations (RSD) for all elements were
less than 1%.
4. Results
4.1. Field occurrences
The study area is located in the southern Coastal Range, between
Chengkong and Taitung Cities (Fig. 1b). The Mawuku creek flows
across the Taiyuan Basin and arrives at the coast in the Tungho area
(Fig. 1c). The volcanic sequences (the Tuluanshan Formation) are
distributed around the basin (Yao et al., 1989), and were exposed by
river erosion. A continuous outcrop at Highway no. 23 along the
Mawuku creek near Tungho has been investigated (Figs. 1c and 2a).
Strike is almost N–S and its total length is about 160 m. According to
field observations, magma mingling can be identified in terms of the
BB and WB appearing repeatedly and mixing together (Fig. 2b–d).
These BB and WB are spread over about 10 to 55 m and interrupt by
probably 5 m MZ.
Fig. 1. Tectonic map of Taiwan and geological map near the study area. (a) The Luzon volcanic arc system on the Philippine Sea plate collided with the Eurasian plate and formed the
Coastal Range volcanic rocks. (b) Geological map of the Coastal Range (after Teng, 1988); the square is the Tungho area. (c) Geological and location map of Tungho area. The outcrop is
along the Mawuku creek and is indicated with the rectangle.
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Fig. 1 (continued).
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611
Fig. 2. The outcrop shows the BB and WB appearing repeatedly and mixing together. (a) Sketch map showing the relationships of both breccias. The black and white colors mean the
black and white blocks, the solid, open and grey circles mean samples collected from BB, WB and MZ, respectively; (b) the outcrop is about 160 m long and predominantly comprises
of breccias; (c) The contact between BB (right side) and WB (left side); (d) the mingled zone, black and white components mingled together; (e) black block (the diameter of the coin is
about 22 mm); (f) white block; (g) the mingled sample; (h) the outcrop of MZ; and (i) peperite in the WB.
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Fig. 3. (a) Hand specimen from the BB with white intrusive structures; (b) the zoom-in of sample (a); (c) hand specimen from the WB with black flow structures; and (d) hand
specimen from the MZ with layering structures.
The volcanic rocks from this outcrop are predominantly composed of
breccias with subsidiary tuffaceous sediments (Fig. 2e and f). In the field
occurrences, we can find the BB and WB cropping out repeatedly and
mixed together (Fig. 2g and h). The BB predominantly consists of dark
gray to black blocks which are angular in grain shape and clastsupported with very few groundmasses. This may be the result of
autobrecciation of lava flow. The WB is predominantly composed of
white to light gray and matrix-supported blocks which are characterized
by subangular to subrounded shape with fiamme or flow structures. It is
worth noting that many spotted tuffaceous sands, pebbles or boulders
with/without laminated structures mingled with the WB. These
peperites likely formed when the magma encountered soft sediments
during the processes of emplacement or explosion (Fig. 2i) (Skilling et al.,
2002). These tuffaceous sediments can only be found in the MZ and WB,
but not in the BB. Samples with white intrusive structures and black flow
structures can be found near the MZ (Fig. 3a–c). Intruding structures are
indicated by white reticulate materials pinched out into the BB. Black
flow structures are characterized by parallel wave lamination with white
ones. Three components, the BB, WB and tuffaceous sediments, mingled
and formed structures resembling bedding structures (Fig. 3d).
4.2. Petrography
In the BB, total phenocryst contents are up to 61.1–75.8 vol.%
(determined by point counting, Table 1) in which the plagioclase
(73.8–80.1%, percentages of the total phenocryst assemblage) is more
abundant than orthopyroxene (8.3–19.2%), clinopyroxene (3.2–5.8%),
hornblende (0–2.2%) and some opaque (1.4–6.1%). Many minerals
show zoning texture and fracture, for example: plagioclase and
pyroxene (Fig. 4b). Plagioclases are euhedral, and may be divided into
four types: sieve-textured, sieved-textured with resorption zone,
resorpted core and oscillatory zoned (Table 2, Fig. 4a–d). The
groundmass consists of microlites of plagioclase, pyroxene, opaque
and interstitial glass.
The WB contains 42.5–53.9 vol.% phenocrysts, with plagioclase as
the dominant phase (76.0–83.1%), followed by orthopyroxene (8.5–
10.3%), clinopyroxene (3.1–3.2%), hornblende (2.1–7.8%), and some
opaque (1.5–3.2%) (Table 1). Minerals also show fractures (Fig. 4e) and
are anhedral. Two kinds of plagioclase grains are observed (Table 1):
euhedral and anhedral (Fig. 4e). Plagioclases are also divided into two
types: sieve-textured and oscillatory zoned (Table 2, Fig. 4f and g). The
groundmass consists of microlites of plagioclase, pyroxene, opaque
and interstitial glass.
Samples from the MZ were also studied. Because the two
components have mingled together, they show characteristics of both
the BB and WB.
4.3. Mineral chemistry
Electron microprobe analyses on selected plagioclase and pyroxene from the BB, WB and MZ are discussed in this study; data for
hornblende, apatite, and opaque are not shown in this paper.
4.3.1. Black breccias (BB)
Four types of textures involving plagioclase observed in the BB
(Table 2) could be identified by mineral chemistry. Plagioclase crystals
vary from An35 to An86 (Fig. 5a), and Or0 to Or3. Typical zoning
patterns of plagioclase phenocrysts in different types of the BB are
shown in Fig. 6. Fig. 6a–d shows sieved-textured with resorption zone,
Table 1
Average and ranges of modal analysis for the BB and WB
Plagioclase
Orthopyroxene
Clinopyroxene
Hornblende
Opaque
Groundmass
Plagioclase (an)b
a
BBa
Range
WB
Range
(%)
(%) (n = 5)
(%)
(%) (n = 4)
77.9
13.7
4.4
1.0
3.0
32.6
–
(73.8–80.1)
(8.3–19.2)
(3.2–6.8)
(0–2.2)
(1.4–3.1)
(24.2–38.9)
–
79.1
9.5
3.1
5.6
2.6
48.0
5.3
(76.0–83.1)
(8.5–10.3)
(3.1–3.2)
(2.1–7.8)
(1.5–3.2)
(42.3–52.9)
(3.4–8.6)
Individual mineral abundances as percentages of the total phenocryst assemblage.
(an) = anhedral, abundances as volume percentages of total rock.
b
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613
Fig. 4. Photomicrographs of plagioclase in the BB (a–d) and WB (e–g); (a) sieve-textured; (b) sieved-textured with resorption zone; (c) Resorpted core; (d) Oscillatory zoned; (e)
Fractured and anhedral minerals; (f) Sieve-textured; and (h) Oscillatory zoned.
oscillatory zoned, resorpted core, and sieve-textured plagioclases,
respectively. The cross-sections are from rim to rim except for the
sieve-textured type, which is from core to rim.
The sieved-textured plagioclase with resorption zone is the most
common texture, with core and rim compositions ranging from An36
to An42 and An69 to An80, respectively (Fig. 6a). Its analytical profile is
shown in Fig. 7a, and variation in An value from rim to rim given in Fig.
5d. Oscillatory zoned plagioclase composition jumps between An75–85
and An51–62 (Fig. 6b); its photomicrograph is shown in Fig. 7b and
variations in An values are shown in Fig. 5e. The resorpted core
plagioclase has core compositions of An75 up to An85 and rim
compositions of An55 to An65 (Fig. 6c); its photomicrograph and An
variation are shown in Figs. 7c and 5f, respectively. Finally, sieved-
texture plagioclase compositional variations are shown in Figs. 5g and
6d; note these sample analyses are from core to rim (Fig. 7d).
Orthopyroxene and clinopyroxene were analyzed from the BB, and
their compositions shown in Fig. 8a. The orthopyroxenes are bronzites
with compositions of En68–81Fs14–33Wo3–4, and most of the clinopyroxene are augites with compositions of En43–51Fs8–18Wo36–46.
4.3.2. White breccias (WB)
Plagioclase compositions of the WB vary from An37 to An56 and Or0
to Or1 (Fig. 5b). Zoning patterns of plagioclase phenocrysts from the
WB are shown in Fig. 9a and b; and the analytical profile is shown in
Fig. 7e. The ranges of An value are narrow between An47 and An38, and
are normally higher in the core than in the rim (Fig. 5h and i).
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The compositions of plagioclase and pyroxene from the MZ are
similar to the samples from the WB (Fig. 8b and c).
Table 2
Field and petrographic characteristics of the BB and WB
Outcrop character
Near boundary
Black breccias
White breccias
Dark gray to black blocks and
clast-supported matrix with
very few groundmasses
Intrusive structure
(white component in)
White to light gray and
matrix-supported blocks
Flow structure (black
component in); mingled
with sediments
62.4–63.7
42.5–53.9
Plagioclase 76.0–83.1,
orthopyroxene 8.5–10.3
Mineral fracture
54.2–55.5
61.1–75.8
Plagioclase 73.8–80.1,
orthopyroxene 8.3–19.2
Mineral zoning (plag., px.),
fracture
Petrographic variability Sieve texture, sieved texture
Sieve texture, oscillatory
of plagioclase
with resorption zone, resorption zoning
core and oscillatory zoning (few)
Small plagioclase
Euhedral
Anhedral
(b 0.5mm)
Wt.% SiO2
Modal vol.% phenocrysts
Major phenocrysts,
modal vol.%
Textural character
Orthopyroxenes are bronzites to hypersthenes with compositions
of En67–75Fs21–29Wo3–5 in the WB (Fig. 8b).
Comparing BB and WB, the plagioclase abundance in the former is
higher than in the latter. The An values of BB are An30–85 in phenocrysts
and An60–70 in microphenocrysts of the groundmass; whereas for the WB,
they are An38–54 in phenocrysts and An40–50 in microphenocrysts. Those
data show that the An content in the WB is lower than in the BB, but the
former has the same ranges for phenocrysts and microphenocrysts; and
the latter has a narrower range in microphenocrysts than phenocrysts
(Fig. 10a and b). Orthopyroxenes from the WB show higher Fs values than
those from the BB (Fig. 8a and b).
4.3.3. Mingled zone (MZ)
Plagioclase compositions in the MZ are shown in Fig. 5c, covering
the entire range of the BB and WB. In addition to the plagioclase
mentioned previously, we often found oscillatory zoning in the WB
and MZ (Figs. 7f and 9c). For both phenocryst and groundmass
microphenocrysts, their compositions are located between An37 and
An56 (Fig. 10c).
Orthopyroxenes and clinopyroxenes were also analyzed from the
MZ, and their compositions are shown in Fig. 8c. Orthopyroxenes are
bronzites with compositions of En71–74Fs24–27Wo1–2, and most of the
clinopyroxenes are augites with compositions of En46Fs7Wo47.
4.4. Whole-rock compositions
Major and trace element data are listed in Table 3. Major elements
show two kinds of breccias here, the BB are basaltic andesites and the
WB are andesites and dacites (Fig. 11).
Harker diagrams show two components. These can be divided into
mafic and felsic types (Fig. 12). Mafic black breccias contain higher
MgO (N6 wt.%), CaO (N8 wt.%), Fe2O3 (N6 wt.%) and lower SiO2
(b56 wt.%); whereas, felsic white ones are lower in MgO (b 3 wt.%),
CaO (b5.5 wt.%), Fe2O3 (b4 wt.%) and higher in SiO2 (N62 wt.%).
Samples from the MZ are also plotted on the diagrams. These samples
are located between these two components. Worth mentioning is that
the samples of white blocks, which were separated from the MZ
(T95022-W, see Table 3), are very similar to the WB (Figs. 11–14).
Two groups of breccias, divided by their major elements, can also
be observed in terms of trace elements (Table 3). The WB are more
enriched in incompatible trace elements, and depleted in compatible
trace elements than the BB (Fig. 13). Trace element concentrations
normalized to the composition of the primordial mantle show that
most of the variations in these two breccias are similar. Relative to the
primitive mantle, these elements show depletions in high field
strength elements (Ta, Nb, and Ti) and enrichment in large-ion
lithophile elements (Rb, Ba, Sr), Th, and Pb, indicating that their levels
are characteristic of the island arc suite.
The BB show little variation in rare earth elements which are about
8–13 times that of chondrite; whereas, the WB are more enriched in
LREE and depleted in HREE than are the black breccias (Fig. 14). (La)n is
similar in these two breccias, in the BB (La)n = 12.95–14.32 and in the
WB (La)n = 13.90–22.15, respectively. There is also enrichment in LREE
relative to middle REE (e.g. La/Sm = 1.71–2.27 in the WB and La/
Sm = 1.21–1.29 in the BB) and HREE (e.g. La/Yb = 3.06–4.38 in the WB
and La/Yb = 1.72–1.81 in the BB).
4.5. Isotopic compositions
Isotopic compositions hint at the sources of different components.
Plots of 143Nd/144Nd versus 87Sr/86Sr isotopes are shown in Fig. 15; the
samples from the BB, WB and the MZ all fall into the second quadrant.
By looking at their similar εNd values, it may infer that they had the
same source. εNd values range between +6.0 and +7.2 (Table 4), close
Fig. 5. Compositional variations of plagioclase. From (a) to (c) are plagioclases in the BB (T005, T018), WB (T023-1) and MZ (T003, TMIX) respectively. Samples contain phenocrysts and
microphenocrysts from the groundmass. From (d) to (g) are four kinds of plagioclase in the BB: sieved-textured with resorption zone (T005A), oscillatory zoned (T005C), resorpted core
(T005D1), and sieve-textured (T018E). They were analyzed from rim to rim with exception of the last one. (h) and (i) are plagioclase compositions from the WB (T023-1).
Y.-M. Lai et al. / Journal of Volcanology and Geothermal Research 178 (2008) 608–623
615
Fig. 6. Typical zoning patterns of selected plagioclase phenocrysts in different types of the BB. From (a) to (d) are sieved-textured with resorption zone (T005A), oscillatory zoned
(T005C), resorpted core (T005D1) and sieve-textured crystals (T018E). They were analyzed from rim to rim with exception of the last one.
to the average of the OIB (εNd = +6), and show the magma source to be
characteristic of a depleted mantle.
5. Discussion
5.1. Identification of three components
Based on the field occurrences, petrography, and mineral and rock
compositions, three components can be distinguished: they are the
BB, WB and tuffaceous sediments. The BB and WB are obviously
separated by the MZ. Meanwhile, field occurrences are also conspicuously different in both of them. The BB is characterized by clastsupported and no or scarce matrix that may have been formed by
autobrecciation of lava flow; whereas, the WB is matrix-supported
with fiamme or flow blocks. Petrographically, the phenocrysts in the
BB contain more pyroxene and have a higher total abundance than in
the WB (Table 2). Plagioclase and pyroxene compositions also show
difference between the BB and WB. The former has higher An values
(An30–85 in phenocrysts and An60–70 in microphenocrysts) than the
latter (An38–54 in phenocrysts and An40–50 in microphenocrysts).
These data suggest that the BB and WB were consolidated from
different magmas.
Their geochemical composition also supports this result. It indicates
that there were two magmas here: mafic and felsic magmas. Mafic
magma formed the BB, which contains higher MgO (N6 wt.%), CaO
(N8 wt.%), Fe2O3 (N6 wt.%) and lower SiO2 (b56 wt.%); whereas, felsic
magma formed the WB with lower MgO (b3 wt.%), CaO (b5.5 wt.%),
Fe2O3 (b4 wt.%), and higher SiO2 (N62 wt.%) (Table 3 and Fig. 12).
Compatible elements, i.e. Cr and Ni in the BB (Cr: 398–464 ppm; Ni: 126–
149 ppm) are much higher than the WB (Cr: 26–48 ppm; Ni: 20–
32 ppm). Meanwhile, the REE patterns also support different compositions in both of them; the WB exhibits greater enrichment in LREE and is
more depleted in HREE than the BB (Fig. 14).
Samples from the fringe of the MZ further support the existence of
two components. Alternating white intrusive structures and black
flow structures suggest that two liquids mingled together during the
eruption (Fig. 3a–d). Based on these lines of evidence, there is every
reason to infer that two magmas, i.e. the mafic and felsic, existed in the
study area.
According to field occurrences, we also found tuffaceous sediments
in this outcrop, which can be identified as the third component
(Fig. 2i). These tuffaceous sediments spottedly mixed in the WB,
forming peperite. This process also occurred in the MZ.
Here, mafic magma, felsic magma and tuffaceous sediments can be
identified. Although the occurrences, petrography, mineral and major
rock compositions were different, the isotopic compositions suggest
that the two magmas had the same source.
5.2. Magma source
Regarding the source of the two magmas, isotopic compositions
yield averages of εNd = +7.0 in the mafic magma and +6.4 in the felsic
magma (Fig. 15). Trace element compositions also show that most of
the variations of these two magmas are similar (Fig. 13). They both
show characteristic island arc magma affinity with depletions in high
field strength elements (Ta, Nb, Ti) and enrichment in large-ion
lithophile elements (Rb, Ba, Sr), Th and Pb. These lines of evidence
strongly suggest that the two magmas were from the same source.
However, some geochemical differences between them have been
observed in major and trace element compositions as shown above.
Such differences are possibly the result of different degrees of
fractionation of the primary magma.
5.3. Mingling of magma and sediments
When two magmas come into contact and their characteristics (i.e.
geochemical compositions of each magma) are still apparent, this is
referred to as magma mingling; if their individual characteristics are
lost, it is referred to as magma mixing (Wilcox, 1999).
The extent of magma mingling ranges from ‘net-veining’ of the
more mafic member by the more felsic member, to enclaves of the
more mafic in the more felsic member. In the MZ, mingled breccias
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Fig. 7. Back-scattered electron images of plagioclases. From (a) to (d) are sieved-textured with resorption zone (T005A), oscillatory zoned (T005C), resorpted core (T005D1), and sievetextured crystals (T018E) in the BB. (e) shows two selected plagioclase in the WB (T023-1A). (f) is from the MZ (T003). The arrowhead means analyses from rim to rim or core to rim.
show net-veined complexes (Fig. 2g), and furthermore exhibit a clear
boundary between black and white components (Fig. 2h). Mineral
abundance is very different in the black and white parts of these MZ
samples, indicating that a mingling event occurred.
The geochemical data of sample number T95022-W, i.e. the white
blocks separated from the MZ, show the same range as samples from
the WB (Table 3, Figs. 11–14). This result shows that the chemical
compositions are not in equilibrium in the MZ. Samples from it could
be separated into two groups identical to the BB and the WB. Physical
and chemical evidence show the characteristics of these two
components (mafic and felsic magmas) are still apparent in the MZ,
so that the process involving these magmas was mingling, not mixing.
Generally speaking, mafic microgranular enclaves and large Kfeldspar megacrysts have been considered to be field evidence of the
mingling and mixing type of interaction between coexisting felsic and
mafic magmas in the magma chamber (Didier and Barbarin, 1991;
Pitcher, 1993). Magma mingling usually results in melting and
reheating due to hot magma from deep in the magma chamber. The
mingled magmas stay underground for a period of time causing
minerals to show compatible textures (ex: rapakivi or antirapakivi
mantling etc) (Hibbard, 1995). In our case, we didn't find the textural
evidence outlined above, and simply identified two different
compositions of minerals (ex: plagioclase and pyroxene) between
the BB and the WB. There was no evidence of compatible textures
here. These results indicate that the mafic and felsic magmas did not
have enough time for mingling within the magma reservoir; they do,
however, appear to have mingled after eruption close to the volcanic
crater. This conclusion is also supported by the thin MZ that separates
the whole outcrop into individual rock bodies.
Another characteristic in our case is the mixing of WB and
tuffaceous sediments. Mingling in the magma chamber will not allow
for tuffaceous sediments; this suggests that magma and sediment
mingling occurred at or close to the surface during eruption.
What process occurred that allowed for these two magma
components to meet and mingle with tuffaceous sediments? The
phenocrysts in the BB are large and euhedral (Table 2, Fig. 4a–d),
suggesting the mafic magma may have cooled slowly enough to form
such phenocrysts, and possibly even formed a lava flow until being
quenched by autobrecciation to form breccias. On the contrary,
phenocrysts in the WB are smaller and there are many anhedral
Y.-M. Lai et al. / Journal of Volcanology and Geothermal Research 178 (2008) 608–623
617
Fig. 8. (a) to (c) are the compositional variations of pyroxenes in the BB (T005, T018), WB (T023-1) and MZ (T003, TMIX), respectively. Samples contain orthopyroxene and
clinopyroxene phenocrysts, and microphenocrysts from the groundmass.
phenocrysts among them with glassy groundmass (Table 2, Fig. 4e–g),
indicating that the felsic magma may have cooled rapidly. Tuffaceous
sediments mingled with breccias can be readily seen in the WB and
the MZ; thus, one may conjecture that the felsic magma intruded into
wet (unconsolidated) sediments and exploded to form peperite.
White intrusive structures and black flow structures in the MZ
(Fig. 3a–d) indicate that the two magmas mingled at liquid or viscous
liquid phases. When the two magmas met, the mafic and felsic
magmas mingled immiscibly to form intrusive and flow structures. The
An value of plagioclase shows reverse or oscillatory zoning as a result.
A process may be envisaged where the felsic magma intruded into a
shallow water encountering wet sediments, and the mafic lava flowed
along this slope at the same time.
5.4. Mingling scenario
Combining the results from this and previous studies, such as basic
volcanic sequences and regional geology (Teng and Lo, 1985; Song and
Lo, 1987, 2002), we propose a scenario for mingling occurring in the
Tungho area as follows. At first, a volcanic seamount grew to be
Fig. 9. Patterns of selected plagioclase phenocrysts from the WB (a and b, T023-1) and MZ (c, T003).
618
Y.-M. Lai et al. / Journal of Volcanology and Geothermal Research 178 (2008) 608–623
Fig. 10. Ranges of plagioclase composition in samples of (a): the BB (T005); (b): the WB (T023-1); (c): the MZ (T003). G.M: microphenocrysts in groundmass; s: small crystals, and
others are from phenocrysts.
subaerial, and large amounts of volcaniclastic and tuffaceous sediments
were deposited along the slope of this seamount (Fig. 16a). These
deposits were unconsolidated or poorly consolidated, especially the fine
tuffaceous sediments.
Then, mafic magma with less fractionation arose from the deeper
magma chamber (Fig. 16b). The mafic lava flow cooled slowly and
flowed along the slope of the seamount. It became more and more
viscous in response to cooling and crystallization. At the same time,
felsic magma intruded into wet tuffaceous sediments to explode and
form peperites (Fig. 16b). Finally, a viscous mafic lava flow made
contact with exploding felsic magma thereby allowing these two
magmas to mingle in a limited zone (Fig. 16c).
6. Conclusions
A magma mingling event was described at Tungho area in the
Coastal Range of eastern Taiwan. Three components, namely mafic
magma, felsic magma and sediments, can be identified.
Petrographic textures and whole whole-rock and mineral compositions of this area indicate that the black and white breccias were
produced from different magmas. Mafic magma and felsic magma
show differences in major elements, trace elements and mineral
compositions, but yield similar in isotopic compositions and trace
element patterns. They appear to be from the same source but have
experienced different degrees of fractionation from their primary
magma.
According to their petrography, the mafic and felsic magmas did
not have too much time for mingling. White intrusive structures and
black flow structures show the mingling event occurred just before
they solidified, moreover the occurrence of mingling between the
magmas and sediments suggests that magma and sediment mingling
might have happened at the surface and not in the magma chamber.
Acknowledgements
We are grateful to Prof. C. H. Chen, T. F. Yang and S. Tsao for their
constructive comments. We also thank two reviewers Dr. Hetu Sheth and
Dr. Dougal Jerram who made critical reviews and comments to much
improve our manuscript. This research was supported by the National
Science Council of Taiwan under grant NSC 94-2119-M-002-022.
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Y.-M. Lai et al. / Journal of Volcanology and Geothermal Research 178 (2008) 608–623
619
Table 3
Major and trace element compositions of the BB, WB and MZ
T95004
T95005
T95006
T95012
T95015-2
T95017
T95018
T95020
T95022
T95022-W a
T95023-1
BCR-1b
W
B
B
W
W
W
B
B
M
M
W
W
–
62.78
0.35
17.86
3.70
0.09
2.55
5.18
4.70
1.38
0.07
98.66
2.74
9.5
96
30
12
20
14
35
17.0
15.2
296
8.2
64
0.88
1.12
85
1.63
0.07
2.52
1.03
0.32
4.52
9.68
1.27
5.65
1.38
0.45
1.44
0.24
1.40
0.28
0.80
0.12
0.81
0.13
54.24
0.53
17.04
6.75
0.13
7.58
8.80
3.31
0.34
0.06
98.79
1.94
24.7
170
445
31
136
37
48
15.2
5.9
217
12.7
58
0.78
0.49
34
1.62
0.06
1.76
0.65
0.08
3.17
7.61
1.13
5.65
1.69
0.57
1.87
0.34
2.14
0.48
1.33
0.20
1.32
0.20
55.53
0.53
17.20
6.64
0.27
6.50
8.83
3.53
0.49
0.06
99.59
1.40
23.6
159
398
29
126
34
47
15.1
8.2
213
12.3
57
0.76
0.72
52
1.57
0.05
1.74
0.62
0.18
3.07
7.39
1.10
5.38
1.58
0.55
1.77
0.33
2.09
0.45
1.28
0.19
1.26
0.19
62.65
0.35
17.86
3.66
0.09
3.07
5.34
5.07
1.09
0.08
99.22
2.49
10.3
96
36
13
32
49
34
16.4
13.4
314
8.4
70
0.88
1.14
83
1.92
0.07
3.20
1.04
0.33
4.56
10.06
1.31
5.75
1.40
0.45
1.44
0.24
1.41
0.29
0.83
0.12
0.84
0.13
63.74
0.34
17.87
3.44
0.07
2.31
5.04
4.99
1.24
0.07
99.11
2.69
8.7
92
26
11
18
23
33
16.8
14.9
307
8.1
75
1.06
1.21
88
2.10
0.08
4.03
1.33
0.41
5.25
11.79
1.49
6.38
1.49
0.46
1.50
0.24
1.42
0.29
0.83
0.13
0.86
0.13
62.98
0.31
17.93
3.20
0.07
2.66
5.05
4.94
1.48
0.06
98.68
2.59
9.2
98
31
11
24
22
32
16.1
14.5
310
7.4
67
0.79
1.23
76
1.83
0.06
3.38
0.77
0.27
3.29
7.42
1.01
4.67
1.25
0.42
1.35
0.22
1.33
0.27
0.74
0.11
0.77
0.12
55.03
0.52
16.92
6.27
0.22
7.56
8.54
3.51
0.39
0.08
99.02
1.53
24.7
174
464
30
147
34
46
15.6
5.8
226
13.5
58
0.79
0.46
42
1.65
0.06
1.89
0.65
0.30
3.39
7.56
1.17
5.84
1.70
0.57
1.88
0.34
2.18
0.49
1.34
0.20
1.34
0.21
54.50
0.53
16.92
6.41
0.19
7.93
8.77
3.37
0.34
0.08
99.04
1.48
26.1
172
464
31
149
29
48
15.5
5.3
223
13.3
58
0.77
0.38
37
1.62
0.06
2.02
0.64
0.20
3.28
7.64
1.17
5.74
1.71
0.58
1.92
0.35
2.23
0.49
1.37
0.21
1.34
0.21
–
–
–
–
–
–
–
–
–
–
–
–
23.0
162
458
27
118
42
43
15.4
9.9
243
12.3
59
0.78
0.93
70
1.69
0.06
2.14
0.66
0.53
3.13
7.50
1.14
5.56
1.61
0.55
1.80
0.33
2.04
0.45
1.28
0.19
1.21
0.20
59.83
0.32
17.38
3.13
2.39
3.51
5.01
4.25
1.12
0.06
97.00
4.14
8.6
174
35
17
115
124
51
18.5
10.1
244
7.3
57
0.58
1.06
309
1.59
0.06
2.42
0.57
0.47
3.26
8.37
1.01
5.24
1.25
0.47
1.32
0.20
1.65
0.41
0.62
0.18
0.67
0.17
62.37
0.35
17.73
3.76
0.09
2.69
5.18
4.64
1.35
0.07
98.22
2.73
10.1
107
34
13
26
25
35
17.1
15.2
297
8.1
56
0.99
1.15
82
1.66
0.07
3.00
1.03
0.34
4.14
9.53
1.24
5.53
1.42
0.45
1.42
0.24
1.40
0.29
0.82
0.12
0.83
0.13
63.38
0.36
18.13
3.71
0.07
3.02
5.48
4.93
1.14
0.07
100.28
2.14
11.0
99
48
13
30
33
34
16.9
13.9
325
8.5
76
0.98
1.15
84
2.09
0.07
3.66
1.01
0.36
4.43
9.73
1.32
5.84
1.43
0.46
1.48
0.25
1.46
0.30
0.86
0.13
0.87
0.14
–
–
–
–
–
–
–
–
–
–
–
–
32.6
407
(16)
37
13
19
130
22.0
47.2
330
38.0
190
14.00
0.96
681
4.95
0.81
13.6
5.98
1.75
24.90
53.70
6.80
28.80
6.59
1.95
6.68
1.05
6.34
1.26
3.63
0.56
3.38
0.51
No.
T95001
W/B/Mc
SiO2
TiO2
Al2O3
ΣFe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5
Total
L.O.I.
Sc
V
Cr
Co
Ni
Cu
Zn
Ga
Rb
Sr
Y
Zr
Nb
Cs
Ba
Hf
Ta
Pb
Th
U
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
a
b
c
T95022-W is white breccias separated from the mingled zone T95022.
Calibration values for BCR-1.
W: White breccias; B: Black breccias; M: Samples from the mingled zone.
Neves, S.P., Vauchez, A., 1995. Successive mixing and mingling of magmas in a Plutonic
complex of northeast Brazil. Lithos 34, 275–299.
Pitcher, W.S., 1993. The Nature and Origin of Granite. Champman and Hall. 321pp.
Richard, M., Bellon, H., Maury, R.C., Barrier, E., Juang, W.S., 1986. Miocene to recent calcalkaline volcanism in eastern Taiwan: K–Ar ages and petrography. Tectonophysics
125, 87–102.
Skilling, I.P., White, J.D.L., McPhie, J., 2002. Peperite: a review of magma–sediment
mingling. J. Volcanol. Geotherm. Res. 114, 1–17.
Song, S.R., Lo, H.J., 1987. Volcanic rocks of the Coastal Range of Taiwan as the products of
submarine eruption—the evidences from Loho area. Acta Geol. Taiwanica 25,
97–110.
Song, S.R., Lo, H.J., 2002. Lithofacies of volcanic rocks in the central Coastal Range,
eastern Taiwan: implications for island arc evolution. J. Asian Earth Sci. 21, 23–38.
Sun, S.S., McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts:
implications for mantle composition and processes. Magmatism in ocean basins.
Geol. Soc. London. Spec. Pub. 42, 313–345.
Teng, L.S., 1981. Lithology and provenance of the Fanshuliao Formation, northern
Coastal Range, eastern Taiwan. Proc. Geol. Soc. China 23, 118–129.
Teng, L.S., 1988. Toward a comprehensive stratigraphic system of the Coastal Range,
Eastern Taiwan. Acta. Geol. Taiwanica 26, 19–35.
Teng, L.S., Lo, H.J., 1985. Sedimentary sequences in the island arc setting of the Coastal
Range, eastern Taiwan. Acta. Geol. Taiwanica 23, 77–98.
Tsai, Y.B., 1978. Plate subduction and the Plio-Pleistocene orogeny in Taiwan. Petrol.
Geol. Taiwan. 15, 1–10.
Vernon, R.H.M., 1984. Microgranitoid enclave in granites–globules of hybrid magma
quenched in a plutonic environment. Nature 309, 438–439.
Wilcox, R.E., 1999. The idea of magma mixing: history of a struggle for acceptance.
J. Geol. 107, 421–432.
Yang, T.F., Liu, T.K., Chen, C.H., 1988. Thermal event records of the Chimei igneous
complex: constraint on the ages of magma activities and the structural implication
based on fission track dating. Acta Geol. Taiwanica 26, 237–246.
Yang, T.F., Tien, J.L., Chen, C.-H., Lee, T., Punongbayan, R.S., 1995. Fission-track dating of
volcanics in the northern part of the Taiwan–Luzon Arc: eruption ages and evidence
for crustal contamination. J. SE Asian Earth Sci. 11, 81–93.
Yao, T.M., Tien, P.L., Wang, C.M., 1989. Stratigraphy and depositional environments of the
Taiyuan Basin, eastern Taiwan. Ti-Chih 9 (1), 95–112.
620
Y.-M. Lai et al. / Journal of Volcanology and Geothermal Research 178 (2008) 608–623
Fig. 11. TAS diagram (after Le Maitre et al., 1989). Solid, open and gray circles represent samples collected from the BB, WB and MZ, respectively.
Fig. 12. Major element variations of breccias plotted against SiO2 content. The symbols are the same as in Fig. 11.
Y.-M. Lai et al. / Journal of Volcanology and Geothermal Research 178 (2008) 608–623
621
Fig. 13. Spider diagrams for trace element compositions, normalized by the primitive mantle (Sun and McDonough, 1989). The symbols are the same as in Fig. 11.
Fig. 14. Chondrite normalized patterns of the rare earth elements (after Sun and
McDonough, 1989). The symbols are the same as in Fig. 11.
622
Y.-M. Lai et al. / Journal of Volcanology and Geothermal Research 178 (2008) 608–623
Fig. 15. 143Nd/144Nd versus 87Sr/86Sr isotope ratios. Vertical and horizontal lines represent εSr = 0 (87Sr/86Sr = 0.7045), and εNd = 0 (143Nd/144Nd = 0.512638), respectively. The symbols
are the same as in Fig. 11. No age-correction was performed as samples in the Tungho area are thought to be younger than 5.8 Ma (Lo et al., 2002).
Table 4
Sr and Nd isotopic ratios of breccias in the Tungho area
Sample no.
Rock type
87
143
T95005
T95012
T95018
T95020
T95022
T95022-W
T95023-1
BB
WB
BB
HZ
HZ
WB from T95022
WB
0.703802 ± 20b
0.703776 ± 13
0.703821 ± 14
0.703992 ± 35
0.704135 ± 13
0.703855 ± 12
0.703687 ± 12
0.512990 ± 11
0.512948 ± 13
0.513007 ± 13
0.512989 ± 14
0.512954 ± 08
0.512984 ± 13
0.512960 ± 11
a
b
Sr/86Sr
εNd = ((143Nd/144Nd)sample/(143Nd/144Nd)CHUR − 1) × 104.
The errors are 2-sigma.
Nd/144Nd
εNda
6.87
6.05
7.20
6.85
6.16
6.75
6.28
Y.-M. Lai et al. / Journal of Volcanology and Geothermal Research 178 (2008) 608–623
623
Fig. 16. Schematic scenario of magma mingling and volcanic evolution in the Tungho area, south Coastal Range, eastern Taiwan. The volcanic seamount grew to be subaerial and some
unconsolidated or poorly consolidated volcaniclastics were deposited along the slope of the seamount (a). Felsic magma stayed in the reservoir after fractional crystallization. Mafic
magma with less fractionation arose from deeper levels and formed a slowly cooling lava flow that flowed down the slope of the seamount. A small quantity of felsic magma intruded
into the wet sediments and formed peperites (b). The autobrecciating mafic lava flow with high viscosity met the peperite (c), and in the square marked we find the intrusive and flow
structures in the outcrops (see Fig. 3).