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Journal of Asian Earth Sciences 19 (2001) 165±175
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Titanite inclusions in altered biotite from granitoids of Taiwan:
microstructures and origins
Tzen-Fu Yui a,*, Pouyan Shen b, Han-Hsing Liu b
a

b

Institute of Earth Sciences, Academic Sinica, P.O. Box 1-55, Nankang, Taipei, Taiwan
Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan
Received 22 March 2000; accepted 12 April 2000

Abstract
Biotites with three sets of titanite inclusions (i.e., sagenitic biotites) have been reported from both igneous and metamorphic rocks. Two
formation mechanisms have been postulated: the `precipitation' model for the sagenitic biotite of igneous origin (Shau, Y.H., Yang, H.Y.,
Peacor, D.R., 1991. On oriented titanite and rutile inclusions in sagenitic biotite. Am. Mineral 76, 1205±1217.) and the `percussion ®gure'
model for the sagenitic biotite of metamorphic origin (Xu, S., Ji, S., 1991. Biotite percussion ®gures in naturally deformed mylonites.
Tectonophysics 190, 373±380.) Sagenitic biotites in granitoids of the Tananao Metamorphic Complex of Taiwan were studied, especially
with regard to the variable degrees of superimposed collision-induced deformation/metamorphism. Mass balance considerations, transmission electron microscopic observations and regional geological relations exclude the simple intra-biotite precipitation model as the possible

mechanism. Instead, the formation of titanite inclusions in these igneous biotites is suggested more likely to be facilitated by inward diffusion
of Ca and outward diffusion of Ti along the basal cleavage planes and the fracture surfaces induced by shear deformation (i.e., the percussion
®gure) in biotites under greenschist-facies temperature conditions. Interdiffusion may also account for the formation of rutile particles with
varied size and varied crystallographic orientation in some titanite laths. q 2001 Elsevier Science Ltd. All rights reserved.

1. Introduction
The Widmanstatten-like titanite/rutile-biotite intergrowth
has been referred to as a sagenitic texture, which is characterized by slender, needle-like inclusions intersecting at an
angle of 608 in a matrix mineral (Gary et al., 1972). Such
titanite and/or rutile inclusions have been reported in biotite
and phlogopite (Rimsaite, 1964; Rimsaite and Lachance,
1966; von Niggli, 1965) and in chlorite produced by hydrothermal alteration of biotite (Rimsaite, 1964; Ferry, 1979;
Veblen and Ferry, 1983). Sagenitic biotites which occur in
granitoid rocks (here referred to as Y, F, T, K1 and C which
outcrop sequentially southward, Fig. 1) of the Tananao
Metamorphic Complex of Taiwan have been noted (Lo
and Wang Lee, 1981; Shau et al., 1991). At the periphery
of the Y granitoid body, homogeneously distributed titanite
and clustered rutile (needles perpendicular to titanite) intergrowths have been studied to discover their mechanism of
formation (Shau et al., 1991). Shau et al. (1991) showed that

both titanite and rutile inclusions generally, but not always,
* Corresponding author. Tel.: 1886-2-27839910x621; fax: 1886-227839871.
E-mail address: tfyui@earth.sinica.edu.tw (T.-F. Yui).

have a preferred crystallographic relationship with the hosting biotite (i.e., the {111Å} or {433Å} planes of titanite and the
{100} plane of rutile are approximately parallel to {001} of
biotite). They therefore proposed an intra-biotite precipitation process which involved the breakdown of an igneous
biotite precursor and topotaxial precipitation of titanite(/
rutile) inclusions during later metamorphism. They also
suggested that the temperature conditions for such a process
might have been comparable to those of the amphibolite
facies metamorphism.
Rather than being customarily regarded as due to exsolution
or precipitation, the titanite inclusions were attributed to
decorations within three sets of fractures intersecting at an
angle of 608 with one of them parallel to (010) plane in metamorphic biotite from mylonite by Xu and Ji (1991). They
proposed that the three sets of fractures are the percussion
®gure that would take place by rapid application of stress on
biotite (Bauer, 1869, 1874). They further suggested that stresses accumulated in a zone of intensive deformation (i.e., mylonite) may be released by slow and sudden processes
alternatively. In the case of sudden release, deformation at

high strain rates might lead to the formation of percussion
®gure in biotite. This process was postulated to be quite similar
to that of the stick-slip model (Byerlee, 1968).

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T.-F. Yui et al. / Journal of Asian Earth Sciences 19 (2001) 165±175

Fig. 1. (a) Simpli®ed map showing the present tectonic setting around the island of Taiwan after Suppe (1984). (b) Simpli®ed geological map of Taiwan
showing the principal geologic provinces after Ho (1986). The dashed rectangle outlines the studied area. (c) The distribution of Y, F, T, K1 and C granitoid
bodies in the Tananao Metamorphic Complex after Lan et al. (1990). As pointed out by Lan et al. (1990), Yui et al. (1990a) and Lo and Onstott (1995), the
collision-induced metamorphism of these granitoids increases from Y, through F, T, K1 to C granitoid body. See text for details. The circles represent sample
localities in the present study.

T.-F. Yui et al. / Journal of Asian Earth Sciences 19 (2001) 165±175

The `precipitation' and the `percussion ®gure' models for

the formation of titanite inclusions in biotite were therefore
derived from igneous and metamorphic rocks, respectively.
To differentiate these two models, optical and transmission
electron microscope (TEM), as well as electron microprobe
were employed to study the microstructures and chemical
compositions of titanite-bearing biotites in genetically
related granitoid bodies in Taiwan in the present study.
Volume fraction of titanite inclusions in one biotite is estimated and a mass balance calculation is attempted. In addition, the available information on regional variations of
biotite characteristics caused by superimposed deformation/metamorphism is also discussed to help the distinction.

2. Geological background
The Tananao Metamorphic Complex (TMC) is the basement rock of Taiwan, which is now situated at the juxtaposition of the Eurasian continental plate and the Philippine
sea plate (Fig. 1(a)). The age of the TMC probably extends
from Late Palaeozoic to Mesozoic as suggested by a few
deformed fossils, such as Permian fusulinids (Yen, 1953)
and Late Jurassic±Early Cretaceous dino¯agellates (Chen,
1989). On the basis of recent studies, it has been suggested
that the TMC has experienced at least three episodes of
subduction/accretion processes, which occurred during
Middle Jurassic, Late Cretaceous and Plio-Pleistocene to

the present, respectively (Yui et al., 1990a, 1990b). The
granitoid rocks in the present study outcrop in the northern
part of the TMC (Figs. 1(b) and 1(c)) and were formed
during Late Cretaceous (i.e., 85±90 Ma, Jahn et al., 1986;
Yui et al., 1996) as a result of the westward subduction of
the Kula plate beneath the ancient Asiatic continental
margin. These granitoid rocks are dominantly quartz-diorite
and granodiorite and consist mainly of quartz, feldspars,
biotite, muscovite, with minor amounts of garnet, amphibole, epidote, titanite, rutile, apatite, zircon, ilmenite and
pyrrhotite.
During the last major tectonic event (i.e., the Plio-Pleistocene to the present collision between the Eurasian continent and the Luzon arc), these granitoid rocks have been
overprinted by a greenschist facies dynamothermal metamorphism induced by arc-continent collision (Liou and
Ernst, 1984; Yui et al., 1990a). The integrated effects of
this superimposed deformation/metamorphism on the granitoid bodies include (1) the formation of foliation de®ned by
biotite and muscovite in the interior of the granitoid bodies,
(2) the development of mylonitic texture at the periphery of
the granitoid body, and (3) the formation of greenschist
facies metamorphic minerals at the expense of the pre-existing igneous ones, such as chlorite after biotite (i.e., chloritization), and zoisite, sericite and albite after Caplagioclase (i.e., saussuritization). The intensity of these
superimposed shearing/metamorphic effects on the granitoid rocks increased from the interior to the periphery of

167

each body, as well as increased southward geographically
(Fig. 1(c)) (Lan et al., 1990; Yui et al., 1990a; Lo and
Onstott, 1995).
3. Methods of study
Samples were collected from the less deformed interior to
the mylonitic periphery of each of the ®ve granitoid bodies
(Fig. 1(c)). Thin sections prepared from these samples were
studied by optical microscopy. Thin sections for titanitebearing biotite with different orientations were also argon
ion-milled to electron transparency for TEM studies.
Energy dispersive X-ray (EDX) analysis coupled with
scanning electron microscopy (SEM, using the JEOL
JSM35CF instrument at 25 kV) and scanning-transmission
electron-microscopy (STEM, using the JEOL 200CX instrument at 200 kV) were used for the qualitative chemical
analysis. Quantitative chemical analyses for biotite were
performed on an ARL-SEMQ instrument with wavelength-dispersive spectrometers. An accelerating potential
of 20 kV and a sample current (on brass) of 0.01 mA were
used. On-line data reduction was based on Bence and Albee
method. One sample from the T granitoid body which exhibits typical Widmanstatten-like inclusions in the biotite was

selected for TEM studies using a JEOL 200CX instrument
operating at 200 kV.
4. Results
4.1. Optical microscopy and composition
In all samples studied, biotite ¯akes are about 1±2 mm in
size. Under the microscope, those biotites with basal layers
or cleavages parallel to the incident beam (designated as
edge-on) show green to brown pleochroisms; while those
with basal planes lying nearly perpendicular to the beam
(designated as top view) show weaker pleochroism
(brown to light brown). Slender titanite inclusions ca.
0.1±2 mm in width are elongated parallel to the biotite
cleavage when viewed edge-on. In this orientation, the titanite intergrowths did not show extinction under crossed
polars because of overlapping of the individual crystals.
Three sets of titanite inclusions at 608 appeared in the top
view orientation (Fig. 2) and inclined-extinction of titanite
was observed in each set. It is also noted that one set of these
inclusions lies parallel to the (010) plane of biotite, similar
to those of the biotite percussion ®gures discussed by Xu
and Ji (1991).

The volume fraction of titanite inclusions in biotite varies
among the granitoid bodies and even within a single granitoid. In general, biotite from the northern granitoid contains
no titanite inclusions or has fewer inclusions (Fig. 3) than
biotite from the southern granitoid (Fig. 2). Within a single
granitoid body, biotite from the sheared periphery is more
densely decorated with the titanite inclusions (see Fig. 1(a)

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T.-F. Yui et al. / Journal of Asian Earth Sciences 19 (2001) 165±175

Fig. 2. Optical micrographs (plane-polarized light) of uniformly distributed
titanite inclusions in biotite (top view). Sample collected from the T granitoid body.

of Shau et al., 1991) than biotite from the weakly sheared
interior (Fig. 3). Where biotite contains only small amounts
of titanite inclusions, the latter may not be evenly distributed
but concentrate along grain boundary or fractures (Fig. 3).
The volume fraction of titanite inclusions is also correlated with the extent of the superimposed metamorphic
effect on biotite. Under the microscope, it can be seen that

chlorite replaces biotite in all the rock bodies, but more
pervasively in the sheared periphery than in the less
deformed interior and also more in the southern granitoid
bodies than in the northern ones. It is noteworthy that both
biotite and chlorite may be either free of titanite inclusions
(Fig. 4(a)) or titanite-bearing (Fig. 4(b)), indicating that
chloritization alone cannot account for the formation of
titanite inclusions. Alteration of biotite, especially in the
southern rock bodies, also led to the formation of quartz
(Fig. 5), but the titanite intergrowths remained in the same
orientation regardless of the alteration. The survival of tita-

Fig. 4. Optical micrographs (plane-polarized light) of biotite (B) partially
altered to chlorite (Ch). (a) Both phases are free of titanite inclusions.
Sample collected from the northern (Y) granitoid body. (b) Both phases
are titanite-bearing (tita). Sample collected from the southern (C) granitoid
body.

nite inclusions shows that titanite is more resistant to alteration than biotite under the prevailing conditions.
According to the present SEM-EDX, STEM-EDX and

microprobe analyses, the biotite is rich in Si, Fe, Al, Mg
and K with minor amounts of Ti and negligible Ca (see
Table 1), and the intergrown titanite crystals, when they
appear, are Ca-, Ti- and Si-rich with minor amounts of Fe
and Al. It should be noted that clusters of rutile needles
forming an asterisk pattern were also observed occasionally
in the Y granitoid body (see Shau et al., 1991).
4.2. TEM observations

Fig. 3. Optical micrographs (plane-polarized light) of biotite (top view)
from the northern (Y) granitoid body which contains fewer titanite inclusions (tita) than that from the T body shown in Fig. 2. Note that the titanite
inclusions are also not evenly distributed in biotite. Sample collected from
the interior part of the Y granitoid body.

4.2.1. Morphology of titanite
TEM images of a representative sample (from the T
granitoid body) show detailed microstructures associated
with the titanite inclusions in biotite. In the top view micrographs, titanite is seen as sets of laths with curved ends and
the laths commonly pass over or below others at different

T.-F. Yui et al. / Journal of Asian Earth Sciences 19 (2001) 165±175

169

Table 1
Representative composition of biotite (A and B) and titanite from the Y
granitoid and the calculated hypothetical biotite precursor (C) a
titanite b

biotite
Ac
SiO2
Al2O3
TiO2
FeO
MnO
MgO
CaO
K2O
Na2O
Total
Si
Al(IV)
Al(VI)
Ti
Fe
Mn
Mg
Ca
K
Na
Fe/(Fe 1 Mg)

Bc

Ca

36.20
35.37
35.22
30.53
17.16
18.00
17.52
2.04
2.65
1.50
2.61
38.48
23.24
21.60
20.98
0.96 (Fe2O3)
0.31
0.40
0.39
±
8.58
8.94
8.67
±
±
0.21
1.00
26.54
9.48
9.81
9.53
0.45
0.03
0.02
0.02
±
97.65
95.85
95.94
99.00
Numbers of ions on the basis of 22(O)
5.48
5.44
5.40
2.52
2.56
2.60
0.54
0.70
0.56
0.30
0.17
0.30
2.93
2.77
2.68
0.04
0.05
0.05
1.95
2.06
1.99
±
0.04
0.17
1.83
1.93
1.87
0.01
0.01
0.01
0.60
0.57
0.57

a
C: calculated hypothetical high-temperature igneous biotite precursor
for biotite B and its inclusions, assuming biotite C ˆ 0.97 biotite B 1 0.03
titanite. See text for details.
b
Data taken from Shau et al. (1991), analyzed by AEM.
c
Chemical compositions of biotite analyzed by electron microprobe. A:
biotite without titanite inclusions; B: biotite with 0.03 weight fraction of
titanite inclusions.

Fig. 5. Optical micrographs of biotite (B) partially altered to quartz (Q),
showing titanite crystals (tita) in quartz with the same orientation as in the
biotite host. (a) Plane-polarized light and (b) crossed polars. Sample
collected from the C granitoid body.

levels (Fig. 6, bright-®eld images, BFI). The broad interface
of titanite and biotite appears slightly warped in the topview sample as indicated in Fig. 6. This observation is similar to that for the biotite percussion ®gures from mylonites
reported by Xu and Ji (1991). Tilting of the sample foils
prepared either in top-view or edge-on orientations shows
no strain contrast or mis®t dislocations at the titanite±biotite
interface, indicating that it is incoherent. Furthermore, electron diffraction shows no de®nite crystallographic relationship between the hosting biotite and the titanite inclusions.
This observation is similar to that of sagenitic biotites
studied by Shau et al. (1991). When titanite laths intersect,
a faceted boundary is formed as exhibited in the edge-on
orientation (Fig. 7(a)), but ledges appear at the interface
when the thin foil is tilted (Fig. 7(b)). In general, the
width of an individual titanite lath can be as small as 0.1
mm when the biotite matrix is in the top-view orientation,
although the same titanite lath appears wider when the
biotite matrix was viewed in the edge-on orientation.
4.2.2. TiO2 in titanite
Within the titanite inclusions, ®ne particles or large euhedral particles were occasionally observed as shown in

Fig. 6. TEM (BFI) of Widmanstatten-like titanite in biotite (top view)
showing overlapped titanite laths 1 and 2. Note warped titanite±biotite
interface for lath 1 (arrow) and the growth front of lath 2. Sample collected
from the T granitoid body.

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T.-F. Yui et al. / Journal of Asian Earth Sciences 19 (2001) 165±175

Fig. 7. TEM of interface of two titanite intergrowths: (a) straight and ¯at
interface of titanite when viewed near edge-on, and (b) orientation tilted
nearly 308 to show ledges at titanite interface. Sample collected from the T
granitoid body and sectioned with biotite basal plane edge-on.

montage BFI (Fig. 8(a)). Dark-®eld images (DFI) (Fig. 8(b))
and a ring-SAD (selected area diffraction) pattern (Fig. 8(c))
indicated that they are rutile particles with varied crystallographic orientation.
4.2.3. Phyllosilicate intergrowths
Lattice images selected from a through-focus series,
generally recorded in the 80±120 nm range of underfocusing, indicated that stacking disturbance and interleaved
basal layers of chlorite were ubiquitous in the matrix biotite
when viewed edge-on.

5. Discussion
5.1. Feasibility of precipitation of titanite from biotite
matrix
According to Ribbe (1982), the range in lattice parameters observed for natural titanites is consistent with the

predominant, partially size-compensating substitution, (Al
1 Fe) 31 1 (F,OH) 2 ! Ti 41 1 O 22 (where rAl , rTi ,
rFe31 ˆ 0:0535 , 0:0605 , 0:0645 nm according to Shannon, 1976) with coordination number (CN) of 6. According
to Shau et al. (1991) (and references cited therein) titanian
biotite with Ti 41 substitution in the octahedral sites may also
involve substitution of other cations or anions forming one
or more of the components: K2(Mg,Fe)5TiSi4Al4O20(OH)4,
K2(Mg,Fe)5TiSi6Al2O22(OH)2, or K2(Mg,Fe)4TiSi6Al2O20(OH)4. Therefore, partitioning of the cations of Al, Fe
and Ti between titanite and biotite is possible. Compared to
these cations, Ca 21 is too large (r ˆ 0:100 nm) to be in CN
of 6 (Shannon, 1976); it probably prefers to enter the interlayer site. Biotite commonly has less than one wt% of CaO
but other micas, such as phlogopite, may contain up to 20
mol% of clintonite (Ca2(Mg4Al2)Si2Al6O20(OH)4) (Olesch,
1979). Shau et al. (1991) therefore suggested that the titanite
and rutile inclusions in the sagenitic biotite may have been
formed through the mechanism of topotaxial precipitation
during superimposed metamorphism where Ti and Ca were
exolved from a high-temperature igneous biotite precursor.
The volume fraction of the titanite inclusions in biotite
from the Y granitoid varies considerably. Table 1 shows two
representative compositions of such biotites. Biotite A, in a
sample collected from the interior, least deformed, part of
the Y granitoid body, contains no titanite inclusions,
whereas biotite B, in a sample collected from the sheared
periphery, contains about 0.027 volume fraction of titanite
inclusions. (The vol% of the titanite inclusions was estimated by simple approximation of a biotite book with
30% of each edge occupied by the titanite inclusions. The
two edge fractions parallel to the (001) plane can be properly estimated on the top view of biotite B, while the third
one is assumed to be the same based on petrographic observations on other edge-on biotite grains.) Considering the
density difference between titanite and biotite, the vol%
can then be converted to 0.03 weight fraction. Adopting
the average chemical composition of titanite (Shau et al.,
1991), the mass balance calculations yield the chemical
composition of a hypothetical precursor (biotite C in
Table 1) for biotite B and its inclusions under the intrabiotite precipitation model. Note that biotite A has a higher
Ti content and Fe/(Fe 1 Mg) ratio than biotite B, indicating
that the former is probably a high-temperature igneous
biotite (Yui and Jeng, 1990). However, both biotite A and
biotite B contain negligible amounts of calcium, not in
accord with the hypothetical precursor biotite C. This
clearly demonstrates that biotite A can provide enough Ti,
but the calcium needed for precipitating titanite inclusions
in biotite B must have an external source. This postulation
would still be valid even if biotite A is alternatively interpreted as a transient high-temperature metamorphic phase
or the minor amounts of rutile inclusions (Shau et al., 1991)
were incorporated in the calculation. Furthermore, the
varied volume fraction of the titanite inclusions in biotite
as well as the spatial distribution of the sagenitic biotite in

T.-F. Yui et al. / Journal of Asian Earth Sciences 19 (2001) 165±175

171

Fig. 8. TEM of TiO2 (rutile) polycrystals in titanite matrix: (a) ®ne (right) or large euhedral (left) crystals (BFI). (b) DFI using 110 arc of rutile in the inset SAD
pattern. (c) SAD pattern showing outer diffraction rings (e.g., 210 and 211 as labeled) of rutile polycrystals. Same specimen as Fig. 7.

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T.-F. Yui et al. / Journal of Asian Earth Sciences 19 (2001) 165±175

the Y granitoid body are also dif®cult to explain by simple
precipitation.
The above considerations demonstrate that the Widmanstatten-like titanite in a biotite matrix could not be the result
of exsolution of a one-phase solid solution. Instead, it most
likely occurred as a result of a process during which Ca was
diffusing in from the rock matrix (e.g., plagioclase, see the
following discussion) somehow. This suggestion is also
consistent with the lack of a de®nite crystallographic relationship between titanite and the hosting biotite or the
alteration products, chlorite or quartz polycrystals. In any
case, the existence of a crystallographic relationship does
not necessarily preclude diffusion of solute from outside the
biotite grain (Bonev, 1972).
A kinetic implication for the microstructure has been
reported for an oxidation reaction of ulvospinel (Buddington and Lindsley, 1964; Putnis and McConnell, 1982) and
olivine (Putnis and McConnell, 1982). In general, overgrowth around a host grain occurs under equilibrium conditions. However, under nonequilibrium conditions, diffusion
distances become shorter and the oxidized phase tends to
grow into the host grains along an energetically favorable
crystallographic plane, forming intergrowths regardless of
the lattice dissimilarity of the co-existing phases (Putnis and
McConnell, 1982). By analogy with this oxidation process,
the formation of titanite intergrowth in rather than overgrowth around biotite might indicate that the titanite formation process occurred under nonequilibrium conditions.
5.2. Mass transport through short circuits
The TEM observations show that there is no de®nite
crystallographic relationship between biotite and the titanite
inclusions, and that the titanite±biotite interface is slightly
warped and incoherent without strain contrast or mis®t
dislocations. Note that the coherency strain must have
existed if titanite or a precursor of titanite was precipitated
from the biotite lattice. The orientation of the titanite inclusions in the present study is also consistent with that of the
biotite percussion ®gure. All these observations are similar
to those reported by Xu and Ji (1991) and may therefore
suggest that the `percussion ®gure' model is preferable to
the `precipitation' model to account for the formation of
sagenitic biotite. By analogy with a fugacity-facilitated
reaction (Putnis and McConnell, 1982), diffusion of Ca
from an external source, such as plagioclase (see later
discussion), coupled with depletion of Ti from the biotite
may result in the formation of the titanite inclusions. This
process would be greatly facilitated by cleavages and fracture surfaces in the biotite. It should be noted that deformation defects, such as microcleavage, bending and kinking
have been found in phyllosilicates (Spinnler et al., 1984;
Amouric, 1987) and a dehydroxylation process in an interstrati®ed serpentine/chlorite may result in defect migration
and microcrack coalescence along the basal layer (Shen et
al., 1990). These defects could possibly facilitate mass

transport at low temperatures and the pervasive cracks
along basal layers may lead to layer openings, and hence,
account for a wide titanite lath when viewed edge-on.
5.3. Regional trends
The above discussions suggest that the titanite±biotite
intergrowths in the granitoid rocks of Taiwan most likely
have formed through biotite percussion fractures similar to
those intergrowths reported from mylonite by Xu and Ji
(1991). Such a postulation can be further substantiated by
regional geological information on the extent of the collisioninduced superimposed greenschist facies metamorphism and
shearing deformation. The intensity of this superimposed
effect on the granitoid bodies increased southward. Under
the microscope, such a trend is manifested by increasing
degrees of saussuritization of plagioclase and chloritization
of biotite (Fig. 4) in granitoids, as well as by an increasing
volume fraction of titanite inclusions in biotite (Figs. 2 and
3).
The chemical compositions of biotite in these geneticallyrelated granitoid bodies have been extensively studied (Liou
et al., 1981; Lo and Wang Lee, 1981; Ernst, 1983; Jeng and
Huang, 1984; Lan, 1982; Shau et al., 1991 and the present
study). The biotite contains negligible amounts of Ca, but
signi®cantly, its Ti content decreases systematically from
the northern toward the southern granitoid bodies (Fig. 9).
Although chemical compositions of original igneous biotite
in these granitoids may not be the same, it would be too
coincident to ascribe the systematic variation in composition shown in Fig. 9 to original chemical differences. In
addition, Lan et al. (1996) also showed that these granitoid
bodies exhibit similar TiO2 content (i.e., 0.39 ± 0.78%), as
well as similar mineral compositions. Combined with the
concomitant retrograde alteration processes, i.e., saussuritization and chloritization, it strongly indicates that the
decreasing Ti content and the increasing volume fraction
of titanite inclusions in biotite, as well as the superimposed
metamorphism, were genetically related, though the depletion of Ti in biotite must have been accompanied by other
cation substitutions to account for the charge balance (see
Dymek, 1983).
The age of the granitoids of the Tananao Metamorphic
Complex is 85±90 Ma based on U±Pb zircon dating (Jahn et
al., 1986; Yui et al., 1996). The Rb±Sr, K±Ar and 40Ar/ 39Ar
mineral (mainly biotite and muscovite) dates, however,
yield younger ages (Yen and Rosenblum, 1964; Juan et
al., 1972; Juang and Bellon, 1986; Jahn et al., 1986; Lan
et al., 1990; Lo and Onstott, 1995). Lan et al. (1990) and Lo
and Onstott (1995) also showed that the apparent K±Ar and
Rb±Sr isotopic ages are progressively younger toward the
south (Table 2). Such a phenomenon has been interpreted as
a corollary of different degrees of partial resetting of isotopic dating systems due to the southward intensi®cation of
the superimposed metamorphism (Lan et al., 1990; Yui et
al., 1990a; Lo and Onstott, 1995). From the information on

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T.-F. Yui et al. / Journal of Asian Earth Sciences 19 (2001) 165±175

Table 2
Compiled biotite apparent ages (Ma) a for the different granitoid bodies of
the Tananao Metamorphic Complex of Taiwan

Y-granitoid body b
F-granitoid body b
T-granitoid body b
C-granitoid body b

Rb±Sr

K±Ar

40

Ar/ 39Ar

35±42
11±7
8±2
12±2

62±30
11
±
9±4

46±20
13
11±8
8

a
Data taken from Yen and Rosenblum (1964), Juan et al. (1972), Juang
and Bellon (1986), Jahn et al. (1986), Lan et al. (1990) and Lo and Onstott
(1995).
b
The geographic distribution of these granitoid bodies referred to Fig. 1.

inclusions, typical sagenitic biotite in the sheared periphery
is also common (e.g., Shau et al., 1991). This ®eld occurrence clearly indicates that shear deformation may be a
more important factor than temperature in forming sagenitic
biotite. The shearing of the granitoid body not only caused
the formation of percussion ®gures in biotite providing
favorable sites for titanite nucleation through the process
as suggested by Xu and Ji (1991), but also facilitated ¯uid
in®ltration. The latter would result in saussuritization and
chloritization, which provided the necessary calcium and
silicon ions for the formation of titanite. In this respect,
temperatures at metamorphic grades above the greenschist
facies may prohibit the formation of sagenitic biotite,
because higher temperatures may stabilize Ca-rich plagioclase and hinder chloritization.
5.4. Origin of rutile particles

Fig. 9. TiO2 (wt%) vs. 100 £ Fe 11/(Fe 11 1 Mg 11) (atomic ratio) for
biotite from different granitoid bodies in the Tananao Metamorphic
Complex of Taiwan. The geographic distribution of various granitoid
bodies is shown in Fig. 1. Data taken from Liou et al. (1981), Lo and
Wang Lee (1981), Ernst (1983), Jeng and Huang (1984) and Lan (1982)
and this study.

the blocking temperature for different isotopic dating
systems, as well as petrologic data, it was concluded that
the temperature of the superimposed collision-induced
metamorphism was approximately 3008C for the Y granitoid body, increasing southward through 3508C for the F
granitoid body and reaching 4508C for the C granitoid
body (Lo and Wang Lee, 1981; Ernst, 1983; Lan et al.,
1990; Lo and Onstott, 1995).
In addition to the metamorphic temperature, the southern
granitoid bodies were also more highly sheared during the
collision-induced metamorphism. Note that the shearing
deformation was also more prominent in the peripheral
than in the interior part of each granitoid body. In this
regard, although biotite in the interior part of the northern
Y granitoid body contains no or small amounts of titanite

Rutile particles with varied size (nanometers to 0.1 mm)
and varied crystallographic orientation were occasionally
observed in titanite laths (Fig. 8(a)). The occasional occurrence, size distribution, as well as nontopotaxial characteristics of the rutile particles indicate that they were not
simply due to exsolution upon cooling. Retrograde metamorphic reaction may have caused the formation of such
rutile particles at the growth front of titanite, facilitated
somehow by diffusion along cleavages or fractures of the
percussion ®gure. This is analogous to the case of fracturesurface-facilitated nucleation and ledge-involved growth
proposed for the interphase formation in alloy steels
(Honeycombe, 1976). In such a process, the formed phase
was found to be aligned with the faceted interface, and a
crystallographic relationship may or may not occur for the
phase and the matrix, depending on the lattice mismatch and
interfacial energy which vary with supersaturation and
undercooling. The size distribution of the rutile particles
(Fig. 8(a)) can be rationalized by the time lag in such a
diffusion-controlled process.
The heating effect of electron bombardment during TEM
observation has been suggested to be responsible for homogeneous nucleation and growth of periclase particles and
formation of residual silica through dehydroxylation of

174

T.-F. Yui et al. / Journal of Asian Earth Sciences 19 (2001) 165±175

phyllosilicates (Shen et al., 1990). Although it cannot be
completely excluded that this type of artifact may produce
microcrystallites in titanite or biotite, the uneven distribution of rutile polycrystals in titanite (or rutile intergrowths in
biotite, Shau et al., 1991) is not consistent with such an
origin.
6. Conclusions
The incoherent titanite±biotite interface, crystal chemistry and mass balance considerations involving titanite and
biotite, as well as the regional geological relations for the
granitoid biotites from the Tananao Metamorphic Complex
of Taiwan, suggest that the Widmanstatten-like titanite
inclusions in igneous biotite were not formed through
simple topotaxial precipitation from a parent solid solution
phase. Rather, the formation of titanite inclusions in biotite
was more likely facilitated by interdiffusion mass transport,
especially of Ca from an external source, along a shearinduced percussion ®gure. Such a process most likely
occurred at temperature conditions of greenschist facies
metamorphism. In addition, some Widmanstatten-like titanite laths contain rutile particles with varied size and varied
crystallographic orientation. They might have also resulted
from such an interdiffusion process.
Acknowledgements
Thanks are due to Dr. C.Y. Lan for providing some thin
sections. Appreciation also goes to Profs. H.Y. Yang, Y.H.
Shau and S.T. Xu for their critical and helpful comments.
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