Titanite inclusions in altered biotite from granitoids of Taiwan:
microstructures and origins
Tzen-Fu Yui
a,*, Pouyan Shen
b, Han-Hsing Liu
baInstitute of Earth Sciences, Academic Sinica, P.O. Box 1-55, Nankang, Taipei, Taiwan bInstitute 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, transmis-sion 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.q2001 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 charac-terized by slender, needle-like inclusions intersecting at an angle of 608in 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 hydro-thermal 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) inter-growths 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,
have a preferred crystallographic relationship with the host-ing 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 precipita-tion 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 608with one of them parallel to (010) plane in meta-morphic 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 stres-ses accumulated in a zone of intensive deformation (i.e., mylo-nite) 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
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 esti-mated and a mass balance calculation is attempted. In addi-tion, the available information on regional variations of biotite characteristics caused by superimposed deforma-tion/metamorphism is also discussed to help the distinction.
2. Geological background
The Tananao Metamorphic Complex (TMC) is the base-ment rock of Taiwan, which is now situated at the juxtapo-sition 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, amphi-bole, epidote, titanite, rutile, apatite, zircon, ilmenite and pyrrhotite.
During the last major tectonic event (i.e., the Plio-Pleis-tocene to the present collision between the Eurasian conti-nent and the Luzon arc), these granitoid rocks have been overprinted by a greenschist facies dynamothermal meta-morphism induced by arc-continent collision (Liou and Ernst, 1984; Yui et al., 1990a). The integrated effects of this superimposed deformation/metamorphism on the gran-itoid 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-exist-ing igneous ones, such as chlorite after biotite (i.e., chlor-itization), and zoisite, sericite and albite after Ca-plagioclase (i.e., saussuritization). The intensity of these superimposed shearing/metamorphic effects on the grani-toid rocks increased from the interior to the periphery of
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 titanite-bearing 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 instru-ment at 200 kV) were used for the qualitative chemical analysis. Quantitative chemical analyses for biotite were performed on an ARL-SEMQ instrument with wave-length-dispersive spectrometers. An accelerating potential of 20 kV and a sample current (on brass) of 0.01mA were
used. On-line data reduction was based on Bence and Albee method. One sample from the T granitoid body which exhi-bits 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 tita-nite intergrowths did not show extinction under crossed polars because of overlapping of the individual crystals. Three sets of titanite inclusions at 608appeared 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).
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 corre-lated 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-nite inclusions shows that titatita-nite is more resistant to altera-tion than biotite under the prevailing condialtera-tions.
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
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 micro-graphs, 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
Fig. 2. Optical micrographs (plane-polarized light) of uniformly distributed titanite inclusions in biotite (top view). Sample collected from the T gran-itoid body.
Fig. 3. Optical micrographs (plane-polarized light) of biotite (top view) from the northern (Y) granitoid body which contains fewer titanite inclu-sions (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.
levels (Fig. 6, bright-®eld images, BFI). The broad interface of titanite and biotite appears slightly warped in the top-view sample as indicated in Fig. 6. This observation is simi-lar 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, elec-tron diffraction shows no de®nite crystallographic relation-ship 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. TiO2in titanite
Within the titanite inclusions, ®ne particles or large euhe-dral particles were occasionally observed as shown in
T.-F. Yui et al. / Journal of Asian Earth Sciences 19 (2001) 165±175 Table 1
Representative composition of biotite (A and B) and titanite from the Y granitoid and the calculated hypothetical biotite precursor (C)a
biotite titaniteb
Ac Bc Ca
SiO2 36.20 35.37 35.22 30.53
Al2O3 17.16 18.00 17.52 2.04
TiO2 2.65 1.50 2.61 38.48
FeO 23.24 21.60 20.98 0.96 (Fe2O3)
MnO 0.31 0.40 0.39 ±
MgO 8.58 8.94 8.67 ±
CaO ± 0.21 1.00 26.54
K2O 9.48 9.81 9.53 0.45
Na2O 0.03 0.02 0.02 ±
Total 97.65 95.85 95.94 99.00
Numbers of ions on the basis of 22(O)
Si 5.48 5.44 5.40
Al(IV) 2.52 2.56 2.60
Al(VI) 0.54 0.70 0.56
Ti 0.30 0.17 0.30
Fe 2.93 2.77 2.68
Mn 0.04 0.05 0.05
Mg 1.95 2.06 1.99
Ca ± 0.04 0.17
K 1.83 1.93 1.87
Na 0.01 0.01 0.01
Fe/(Fe1Mg) 0.60 0.57 0.57
a C: calculated hypothetical high-temperature igneous biotite precursor
for biotite B and its inclusions, assuming biotite C0.97 biotite B10.03 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.
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 crystal-lographic orientation.
4.2.3. Phyllosilicate intergrowths
Lattice images selected from a through-focus series, generally recorded in the 80±120 nm range of underfocus-ing, 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 para-meters observed for natural titanites is consistent with the
predominant, partially size-compensating substitution, (Al 1Fe)311 (F,OH)2 ! Ti411O22(whererAl,rTi, rFe310:0535,0:0605,0:0645 nm according to
Shan-non, 1976) with coordination number (CN) of 6. According to Shau et al. (1991) (and references cited therein) titanian biotite with Ti41substitution 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)4TiSi6Al2-O20(OH)4. Therefore, partitioning of the cations of Al, Fe and Ti between titanite and biotite is possible. Compared to these cations, Ca21is too large (r0:100 nm) to be in CN
of 6 (Shannon, 1976); it probably prefers to enter the inter-layer 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 esti-mated 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 prop-erly estimated on the top view of biotite B, while the third one is assumed to be the same based on petrographic obser-vations 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 intra-biotite precipitation model. Note that intra-biotite A has a higher Ti content and Fe/(Fe1Mg) 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 inter-preted 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
T.-F. Yui et al. / Journal of Asian Earth Sciences 19 (2001) 165±175
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
the Y granitoid body are also dif®cult to explain by simple precipitation.
The above considerations demonstrate that the Widman-statten-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 rela-tionship 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 (Budding-ton and Lindsley, 1964; Putnis and McConnell, 1982) and olivine (Putnis and McConnell, 1982). In general, over-growth around a host grain occurs under equilibrium condi-tions. 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 over-growth around biotite might indicate that the titanite forma-tion process occurred under nonequilibrium condiforma-tions.
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 inclu-sions 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 frac-ture surfaces in the biotite. It should be noted that deforma-tion defects, such as microcleavage, bending and kinking have been found in phyllosilicates (Spinnler et al., 1984; Amouric, 1987) and a dehydroxylation process in an inter-strati®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 collision-induced 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 genetically-related 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 composi-tion shown in Fig. 9 to original chemical differences. In addition, Lan et al. (1996) also showed that these granitoid bodies exhibit similar TiO2content (i.e., 0.39 ± 0.78%), as well as similar mineral compositions. Combined with the concomitant retrograde alteration processes, i.e., saussuriti-zation and chloritisaussuriti-zation, 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 deple-tion of Ti in biotite must have been accompanied by other cation substitutions to account for the charge balance (see Dymek, 1983).
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 grani-toid 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
inclusions, typical sagenitic biotite in the sheared periphery is also common (e.g., Shau et al., 1991). This ®eld occur-rence 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 plagio-clase and hinder chloritization.
5.4. Origin of rutile particles
Rutile particles with varied size (nanometers to 0.1mm)
and varied crystallographic orientation were occasionally observed in titanite laths (Fig. 8(a)). The occasional occur-rence, size distribution, as well as nontopotaxial character-istics of the rutile particles indicate that they were not simply due to exsolution upon cooling. Retrograde meta-morphic 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 fracture-surface-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 homo-geneous nucleation and growth of periclase particles and formation of residual silica through dehydroxylation of T.-F. Yui et al. / Journal of Asian Earth Sciences 19 (2001) 165±175
Table 2
Compiled biotite apparent ages (Ma)afor the different granitoid bodies of
the Tananao Metamorphic Complex of Taiwan
Rb±Sr K±Ar 40Ar/39Ar
Y-granitoid bodyb 35±42 62±30 46±20
F-granitoid bodyb 11±7 11 13
T-granitoid bodyb 8±2 ± 11±8
C-granitoid bodyb 12±2 9±4 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.
Fig. 9. TiO2(wt%) vs. 100 £ Fe11/(Fe111Mg11) (atomic ratio) for
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 distribu-tion 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 chemis-try 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 shear-induced percussion ®gure. Such a process most likely occurred at temperature conditions of greenschist facies metamorphism. In addition, some Widmanstatten-like tita-nite 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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