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Formation and compositional variation of phlogopites in the Horoman peridotite complex, Hokkaido, northern Japan: implications for origin and fractionation of ...

Contributions to Mineralogy and Petrology

Contrib Mineral Petrol (1989) 101 : 165-175

9 Springer-Verlag1989

Formation and compositional variation of phlogopites in the Horoman peridotite complex, Hokkaido, northern Japan: implications for origin and fractionation of metasomatic fluids in the upper mantle Shoji Arai and Natsuko Takahashi Institute of Geoscience, The University of Tsukuba, Ibaraki 305, Japan

Abstract. Harzburgite and lherzolite tectonites from the Horoman peridotite complex, Hokkaido, northern Japan, contain variable amounts of secondary phlogopite and amphibole. Phlogopite-rich veinlets parallel to the foliation planes usually cut olivine-rich parts of the rocks; singlegrained interstitial phlogopites are usually associated with orthopyroxene grains. Amphiboles are disseminated in rocks or sometimes occur in the phlogopite-rich veinlets. Within individual veinlets, phlogopites show extensive inter-grain variations in K/(K + Na) ratio (0.96-0.75), generally decreasing from the central (usually the thickest) part towards the marginal parts of veinlets. In contrast, Ti contents are nearly constant in Ti-poor veins or decrease slightly with decreasing K/(K + Na) in T-rich veins. Variation of Ti in phlogopites is very large (0.1 6.8 wt%) and is inversely correlated with Mg/(Mg + Fe*) (Fe*, total iron) atomic ratios, which vary from 0.96 to 0.88. Intra-vein variation of phlogopite chemistry (especially K/(K + N a ) ratio) could be achieved by in situ fractional crystallization of trapped fluids; variation of Ti, however, cannot be explained by in situ fractionation of the fluids, indicating various Ti contents of the parent fluids. It is suggested that fluids responsible for the formation of the Horoman phlogopites and amphiboles were magmatic volatiles successively released from evolving alkali basaltic magmas. Individual fluids trapped within peridotites were fractionated, precipitating phlogopites successively poorer in K. When the fluids became rich enough in Na, amphiboles co-precipitated with phlogopites. Similar fractional crystallization of phlogopites and amphiboles is expected in the upper mantle on a larger scale if fluids move upwards. This process may control, at least partly, a lateral K / N a distribution in the upper mantle; K and Na may be concentrated in deeper and shallower parts, respectively.

Introduction Phlogopite and amphibole are the main minerals which contain LIL elements in the upper mantle. Their chemical characteristics are very important and have been studied in various ways (e.g. Delaney et al. 1980; Arai 1984, 1986; Arai and Takahashi 1987; Wilkinson and Le Maitre 1987). Wilkinson and Le Maitre (1987) concluded that veins of micas

Offprint requests. S. Arai

and amphiboles in mantle peridotites are essentially important for the source of incompatible elements in some mantle-derived magmas. However, the process of their formation in upper mantle peridotites is not well known, mainly because previous studies of mantle-derived phlogopites and amphiboles have been performed on small xenolithic fragments of peridotites in kimberlites (e.g. Delaney et al. 1980). Rare exceptions include Exley et al. (1982), Roden et al. (1984) and Bonatti et al. (1986), whose main interests were, however, in chemical or isotopic compositions of phlogopites and/or phlogopite-bearing peridotites. Detailed descriptions of the mode of occurrence of phlogopites in alpine-type peridotite complexes have not been available. Accordingly, detailed petrological investigations of the processes of phlogopite-amphibole formation in large solid intrusive peridotite complexes are required (e.g. Roden and Murthy 1985). Here we intend to describe the chemical characteristics of phlogopites in peridotites of spinel to plagioclase lherzolite facies from a solid intrusive peridotite complex, the Horoman complex, Hokkaido, northern Japan. We also describe compositional variations of phlogopites and discuss a possible fractional crystallization process involving metasomatizing fluids within mantle peridotites.

Geological background The Horoman ultramafic complex, just northwest of Cape Erimo, is emplaced as a tectonic slice at the southern end of the Hidaka metamorphic belt, central Hokkaido, northern Japan (Fig. 1). Large amounts of granitic rocks, migmatites and metamorphic rocks of the high T/low P type constitute the main part of the Hidaka belt (e.g. Komatsu et al. 1986; Komatsu 1986; Osanai et al. t 986). Radiometric ages for granitic and other igneous rocks range from 43 to 16 Ma (Shibata and Ishihara 1981) and from 23 to 16 Ma for metamorphic rocks (Shibata et al. 1984; Osanai et al. 1986). Ultramafie complexes are distributed along the western margin of the Hidaka belt, from Uenzaru (Komatsu 1975) to Horoman (Nagasaki 1966; Niida 1974, 1984) (Fig. 1). Ultrarnafic complexes in the Hidaka belt are characterized by lherzolites with or without plagioelase (Research Group of Peridotite Intrusion 1967; Niida and Katoh 1978). Petrographical and petrological descriptions of the Horoman complex were made extensively by Niida (1974, 1984), Obata and Nagahara (1987) and Takahashi (1988). The Horoman complex is a tectonic slice emplaced into metavolcanics and pelites of the Hidaka metamorphic belt. The Horoman complex (10 x 8 kin) is almost devoid of serpentinization except along shear or fault zones. There are good exposures along the Horoman River cutting a central part of the complex from

166

Petrological descriptions of the peridotites N

Textures The Horoman peridotites are deformed and foliate& The foliation planes are usually concordant with the lithological layering. Minerals are sometimes elongated on the foliation planes, giving a prominent lineation to the rocks, which is almost constant (N15~ S 15~W) throughout the complex (Niida 1984). The degree of deformation is variable within the complex; near the base the rocks are severely deformed to show mylonitic textures (Niida 1975a). Peridotites from other places show tabular equigranular to weak porphyroclastic textures. Niida (1984) concluded that the rocks were deformed mainly under upper mantle conditions. It is likely that the mylonitic textures, exclusively found at the basal part of the complex, were formed at the intrusive or crustal stage.

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Mineralogy and petrology of peridotites The Horoman complex is mainly composed of plagioclase lherzolite, spinel lherzolite and spinel harzburgite. Two kinds of cumulative peridotite suites, possibly exotic to the main peridotites, were also components of the complex. One is a dunite-wehrlite suite, enriched in chromian spinel (Cr/(Cr+A1) ratio, 0.3-0.4) (Takahashi 1988). The other is a highly refractory dunite-harzburgite suite (Fo93-9r Cr/(Cr+A1) spinel, 0.9) (Arai and Takahashi 1986). In the main Horoman peridotites, the total amount of pyroxenes and the volume ratio of clinopyroxene to total pyroxenes decrease from plagioelase lherzolite via spinel lherzolite to spinel harzburgite (Fig. 2a). Thin ( < 1 cm) Ca- and Al-rich lenses and seams are present in the plagioclase lherzolite and in the spinel lherzolite adjacent to plagioclase lherzolite layers, respectively. The lenses in the spinel lherzolite are composed of fine-grained chromian spinel, orthopyroxene and clinopyroxene. Plagioclase is added to these minerals in the seams in plagioclase lherzolite. Spinelpyroxene symplectites were exclusively found in these Ca- and A1rich lenses in the spinel lherzolite and are very rare in the seams in plagioclase lherzolite. The symplectite-bearing spinel lherzolite tends to be richer in A1 and Fe than the symplectite-free one (Fig. 2b). The symplectite is a spherical aggregate of vermicular crystals or orthopyroxene, clinopyroxene and chromian spinel. Its origin is not clear but it could be formed by the breakdown of garnet (reaction of garnet + olivine) with some chemical modification (Takahashi 1988). Some chemical characteristics of minerals are dependent on the mineral paragenesis, for example, F 9 values of olivine and Cr/(Cr + A1) atomic ratios of chromian spinel vary from 89 to 92, and from 0.05 to 0.65, respectively, from plagioclase

I

Cape E r i m o

Fig. 1. Location of the Horoman peridotite complex, Hokkaido, northern Japan (after Komatsu et al. 1986). 1 metamorphic rocks; 2 granitic rocks; 3 mafic rocks; 4 ultramafic rocks; 5 metaophiolite. 1-4 the Main Zone of the Hidaka belt; 5 the Western Zone of the Hidaka belt north to south. The Horoman complex has a rough layered structure and is mainly composed of spinel harzburgite, spinel lherzolite and plagioclase lherzolite. The complex has 4~5 layers of spinel harzburgite, which gradually change into plagioclase lherzolite through spinel lherzolite both upwards and downwards (Niida 1984); i.e. the lithology changes in an oscillatory way in the vertical direction. Gabbroic bands are conspicuous especially in the upper part of the complex. The total thickness of the complex may exceed 3000 m. Obata and Nagahara (1987) ascribed the vertical lithological change of the peridotites to mixing of variable proportions of melt and residue.

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Fig. 2. a Modal amounts of olivine (O/), orthopyroxene (Opx) and clinopyroxene (Cpx) in the Horoman peridotites; sp spinel; syrup spinel-pyroxene symplectite; pl plagioclase. Note that harzburgite and lherzolite are distinguished in terms of volume ratio of Cpx/(Opx + Cpx), < 0.1 and >0.1, respectively in this study, h Cr/(Cr + A1) atomic ratio of spinel vs F 9 value of olivine in the Horoman peridotites. Symbols are the same as in Fig. 2a

167 lherzolite to spinel harzburgite (Fig. 2b), entirely within the "olivine-spinel mantle array" (Arai 1987). Chromian spinel in the plagioclase-rich seam in plagioclase lherzolite is richer in Cr (Cr/(Cr + AI) atomic ratio, 0.25-0.30) than the coarse-grained spinel, which is plotted in Fig. 2b. Plagioclase is intermediate in composition (An6o-7o) (e.g. Niida 1984).

Geothermobarometry The minerals are homogeneous except for thin rims of spinel and clinopyroxene when in contact with olivine and vice versa. Pyroxene cores were analysed and their compositions used to calculate equilibrium temperatures; temperatures range from 750 ~ to 950 ~ C (850 ~ C on average) using the geothermometer of Wells (1977) regardless of the stratigraphical position in the complex. Other pyroxene thermometers yield similar results. The Horoman peridotites have been equilibrated within the plagioclase lherzolite facies, because the assemblage Mg-rich olivine+plagioclase is stable in peridotites when the whole-rock chemistry is suitable, i.e. when rocks are enriched in A1 and Ca (see Fig. 2). Mode of occurrence of phlogopites and amphiboles In the H o r o m a n complex, phlogopites were c o m m o n l y found in harzburgite and spinel lherzolite and tess commonly in plagioclase lherzolite. Phlogopite-bearing peridotites were mainly found in three localities along the H o r o m a n Rivet', indicating their heterogeneous distribution in the complex. The distribution of phlogopite in the complex is not correlated with the degree o f deformation of the rocks (N. Takahashi 1986).

The modal abundance of phlogopite in the H o r o m a n peridotites is extremely variable. To facilitate further description, phlogopites are classified into two types, interstitital type and vein type. The former type of phlogopite is most c o m m o n a m o n g or within orthopyroxene grains (Fig. 3 a, b) and is less c o m m o n l y associated with clinopyroxenes and spinel-pyroxene symplectites (Fig. 3 c). Interstitial-type phlogopites in pyroxene-spinel symplectites often contain small rounded spinel grains. Phlogopites associated with orthopyroxene grains frequently contain minute relics of orthopyroxene, indicating a replacement origin (Fig. 3 a, b). Relic orthopyroxene grains are irregularly penetrated by phlogopite. The vein-type phlogopites sometimes form veinlets or thin aggregates, the m a x i m u m length and thickness of which exceed 40 cm and 1 ram, respectively. These phlogopite veinlets or aggregates mainly replace olivine grains and are entirely concordant with the foliation plane of peridorites. The vein-type phlogopites sometimes have irregular or complicated grain boundaries with olivine indicating their replacement relationship (see upper centre of Fig. 4a). In some plagioclase lherzolite, phlogopite [brm coarsegrained veinlets in coarse-grained olivine-rich parts, but they form fine-grained discontinuous veinlets (Fig. 4b) in fine-grained Ca- and Al-rich seams. We confirmed that peridotites with phlogopite veinlets are discontinuously traceable for 2-3 km parallel to the foliation of the complex. Both vein-type and interstitial type phlogopites often occur in the same thin section. F o r example, a light-coloured interstitial type phlogopite crystal oc-

Fig. 3a-c. Photomicrographs of interstitial type phlogopite, a, b Interstitial-type phlogopite (P) associated with orthopyroxene (Op) in spinel harzburgite (No. 172808). Scale bar is 0.2 ram. Note a minute relic orthopyroxene grain in phlogopite, a Plane-polarized light, b Crossed-polarized light, Note a kink band in phlogopite. c Phlogopite (P) in spinel-pyroxene symplectite in spinel lherzolite (No. 172701k). Plane-polarized light. Scale bar is 0,2 mm

168

Fig. 4 a-c. Photomicrographs of vein-type phlogopite, a Phlogopite (dark) vein in olvine-rich part of spinel lherzolite (No. 5720110. Plane-polarized light. Scale bar is 0.5 mm. b Phlogopite (dark) vein in a plagioclase (F)-rieh seam of plagioclase lherzolite (No. 670311a). O olivine. Plane-polarized light. Scale bar is I mm. Note that phlogopite is relatively fine-grained and does not make a continuous veinlet (a). c Vein-type phlogopite (P) associated with amphibole (A) in spinel lherzolite (No. 57208b). O Olivine; Op orthopyroxene. Plane-polarized light. Scale bar is 0.5 mm. See Fig. 9 curs in an orthopyroxene porphyroclast near which trails of vein-type phlogopites with a deeper colour occur (see Fig. 9). Trails or veinlets of vein-type phlogopites are often curved around large orthopyroxene porphyroclasts (see Fig. 9); such veinlets are rarely continuous with trails of relics of fluid inclusions which are now composed of magnesite + brucite + serpentine (Hirai and Arai 1987). Phlogopites are often kinked (Fig. 3 b). Phlogopite veinlets in peridotites with mylonitic textures from the lowest part of the complex are intensely stretched to become phlogopite "lines". This deformation of phlogopite veinlets may be synchronous with the mylonitization of the rock. Amphibole sometimes coexists with both types of phlogopite, especially near the margin or narrow parts of phlogopite-rich veinlets (Fig. 4c), and this suggests a synchronous formation of the two minerals. Interstitial type amphibole is common, especially around the phlogopite veinlets. The pleochroic colour (Y or Z) of phlogopite in thin section is quite variable regardless of the type of the host rock. The interstitial type phlogopites are usually pleochroic (Y and Z) in pale orange, although they range from colourless to reddish brown even in a thin section. The vein-type phlogopites have little intra-vein variation but have a wide inter-vein variation, from colourless to reddish brown. The vein-type phlogopites in plagioclase lherzolite tend to be deeper in axial colour (Y or Z), from orange to reddish brown. The Horoman phlogopites, especially the vein-type ones, seem to be different both from primary and secondary phlogopites in garnet peridotite xenoliths in kimberlites

(Carswell 1975; Harte and Gurney 1975; Delaney etal. 1980) (Fig. 4). Apparent primary phlogopites are sometimes of metasomatic origin and crystallized later than primary anhydrous minerals (Aoki 1975; Harte and Gurney 1975; Erlank and Rickard 1977). On the other hand, the secondary phlogopites are evidently later products rimming garnet with kelyphite or filling cracks with serpentinite (Carswell 1975; Harte and Gurney 1975; Delaney et al. 1980). The Horoman phlogopites are not alteration products at very low temperatures because they never coexist with serpentine. In summary, phlogopites in the Horoman complex have replacement textures (Fig. 3 a, b); vein-type and interstitial type phlogopites mainly replace olivine and orthopyroxene grains, respectively. Phlogopite veinlets or trails (vein-type phlogopites) are concordant with the foliation planes of peridotites and predate some of the deformation, especially the mylonitization.

Chemical composition of phlogopites and amphiboles Minerals were analysed with a computer-controlled microprobe, JXA-50A (JEOL), at the Chemical Analysis Center, University of Tsukuba. Operating conditions were as follows; 15 kV accelerating voltage, 15 nA probe current, 2-5 gm beam diameter and 20 s counting time for each element. Data reduction was after Bence and Albee (1968).

Compositional characteristics Selected electron microprobe analyses are listed in Table 1. Phlogopites in the Horoman complex exhibit a wide range

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Fig. 5. Mg/(Mg+Fe*) atomic ratio vs number of Ti atoms for O = 22 in the Horoman phlogopites. Solid circle Vein-type phlogopite in spinel peridotites; open circle interstitial type phlogopite in spinel peridotites; solid triangle vein-type of phlogopite in plagioclase lherzolite

of chemical variation. TiO2 contents vary from 0.1 to 6.8 wt% and correlate well with the variation of pleochroic colour (Y or Z), from almost colourless to reddish brown. Ti contents of phlogopite largely change even within a thin section, e.g. a spinel lherzolite has both Ti-poor (TiO2 0.1 wt%) interstitial type phlogopite and Ti-rich (TiO2 6 wt%) vein-type one. Such a wide inter-grain heterogeneity of Ti in phlogopites has never been described in a suite of mantle-derived materials from any other locality. Primary and secondary phlogopites in garnet peridotites, are low Ti ( < 1 wt% of TiO/) and high Ti (up to 4 wt% of TiO2), respectively (Carswell 1975; Delaney et al. 1980). Primary-

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metasomatic phlogopites from Matsoku and M A R I D phlogopites are intermediate in the Ti content (1 2 wt% of TiOz) (Harte et al. 1975; Dawson and Smith 1977). Intra-grain heterogeneity of Ti contents in the Horoman phlogopites are very small. Ti contents show a negative correlation with M g / ( M g + F e * ) (Fe*, total iron) atomic ratios, which vary from 0.88 to 0.96 (Fig. 5). The interstitial-type phlogopites have a smaller range of M g / ( M g + Fe*) ratio than the vein-type phlogopites (Fig. 5). Ba could not be detected from phlogopites. The Horoman phlogopites are relatively high in A1 content and have approximately 5.5 Si atoms for 0 = 2 2 (on an anhydrous basis) (Fig. 6). A1 contents are distinctively higher than those in primary phlogopites in garnet peridotire xenoliths in kimberlites and are almost within the range of phlogopites in spinel to plagioclase peridotites (Fig. 6; Arai 1984). The relatively low Si contents of phlogopite in the Thumb garnet peridotites (Fig. 6) are due to their high equilibrium temperatures (Arai 1984). The high A1 and low Si character of the Horoman phlogopites is due to low-pressure (and possibly low-temperature) conditions of formation (Arai 1984). K/(K + Na) atomic ratios of the Horoman phlogopites vary from 0.75 to 0.96 (Fig. 7). In contrast, both primaryand secondary-textured phlogopites in garnet peridotites have high (usually > 0.95) K/(K + Na) atomic ratios (Arai 1986). Relatively low K / ( K + N a ) ratios are characteristic of phlogopites in spinel or plagioclase peridotites (Arai 1986; Arai and Takahashi 1987). In the Horoman phlogopites, the K/(K + Na) ratio tends to be higher in plagioclase lherzolite than in spinel peridotites (Fig. 7). Phlogopites, especially coarse-grained ones of interstitial type, are often zoned; the K / ( K + N a ) ratio decreases from the core to the margin (Table 1). Ti in interstitial-type phlogopite is positively correlated with the K/(K + Na) ratio. The Horoman phlogopites are high in Cr and often contain more than 1.5 wt% of Cr203 (Table I). The secondary phlogopites rimming garnet with kelyphite in garnet perido-

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170 Table 1. Selected electron microprobe analyses of phlogopite and associated amphibole in the Horoman peridotites. FeO* and Fe*, total iron as FeO and Fe respectively; Mg*, Mg/(Mg + Fe*) atomic ratio; K*, K/(K +Na) atomic ratio; 5, 9 and 13, amphiboles; others, phlogopites; 4-r and 11-r, rim of 4-c and 11-c, respectively; others are core compositions. 1, No. 812301b. 2, No. 57208b. 3-5, No. SA8407.6 and 7, No. 670311c. 8 and 9, No. 67311a2. 10-13, No. 572011f. 14, No. 172833 Vein-type (spinel lherzolite)

Vein-type (plagioclase lherzolite)

No.

1

2

3

4-c

4-r

5

6

7

8

9

SiOz TiO2 A1203 Cr203 FeO* MnO MgO NiO CaO Na20 K20

38.95 0.37 17.60 1.59 2.39 0.11 24.37 0.25 0.01 0.97 8.48

38.75 3.91 15.60 1.53 2.29 0.00 22.16 0.27 0.03 0.67 8.80

39.19 5.80 16.12 1.85 2.98 0.07 21.43 0.21 0.00 1.40 7.67

39.14 6.77 16.50 1.71 3.44 0.00 19.82 0.25 0.00 0.98 8.16

38.50 6.75 16.13 1.37 3.88 0.00 20.21 0.27 0.07 1.14 7.86

43.41 4.34 12.43 1.71 3.75 0.08 15.43 0.11 11.84 3.08 0.68

39.16 4.30 16.52 0.68 4.05 0.00 21.32 0.21 0.04 0.65 8.30

30.27 4.64 17.05 0.92 3.76 0.04 21.36 0.25 0.09 0.35 8.87

37.62 5.45 15.70 1.39 4.65 0.00 18.96 0.17 0.00 0.62 9.11

41.77 3.68 13.00 1.68 4.40 0.04 15.61 0.10 12.44 2.85 1.01

Total

95.08

94.00

96.73

96.77

96.18

96.85

95.23

96.60

93.65

96.53

Mg* K*

0.948 0.852

0.945 0.896

0.928 0.782

0.911 0.846

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rites are also high Cr (Harte and G u r n e y 1975; Carswell 1975; Delaney et al. 1980). Cr/(Cr + A1) ratios o f phlogopites are positively, t h o u g h weakly, correlated with those o f coexisting c h r o m i a n spinels. A m p h i b o l e s are pargasite to pargasitic h o r n b l e n d e according to H a w t h o r n e (1981) and are often titanian (Table 1). Their Ti contents are well correlated with those o f coexisting phlogopites (Table 1).

Compositional variation of phlogopite in individual phlogopite veinlets Phlogopite veinlets, which are sometimes traceable discontinously for a few kilometres, show variable but non-systematic inter-vein chemical variations. K / ( K + N a ) ratios o f phlogopite in individual veinlets range from 0.78 to 0.96 (Fig. 8), although the intra-grain variation is m o r e re-

stricted. In individual veinlets, phlogopites show a lengthwise decrease o f the K / ( K + N a ) ratio from the central part, where the vein is usually thickest, t o w a r d s the periphery of the veinlet, while Ti contents slightly decrease in high-Ti veins and are almost constant in low-Ti ones (Figs. 8, 9). A m p h i b o l e m o r e frequently coexists with phlogopite with lower K / ( K + N a ) ratios. F o r example, within a phlogopiterich vein amphibole tends to exist in peripheral parts where phlogopites have low K / ( K + Na) ratios. It is noteworthy that the individual veinlets are distinguished by the Ti contents o f its phlogopite, which are almost constant or only slightly variable within a veinlet (Fig. 8). This contrasts with the large Ti variation of the gross H o r o m a n phlogopites (Fig. 5). In addition to the textural difference, the vein-type phlogopites are different from the interstitial type ones in chemistry. As shown in Figs. 8 and 9, the interstitial-type

171 Table 1 (continued)

Discussion

Interstitial type (spinel lherzolite) 10 ll-c ll-r 12

13

14

38.83 0.07 18.34 0.16 2.88 0.07 23.52 0.11 0.01 1.47 7.83 93.29

43.55 1.72 13.71 1.83 3.12 0.00 16.52 0.09 11.51 3.10 0.91 96.06

38.33 4.99 16.00 1.68 3.06 0.00 21.69 0.28 0.00 0.76 8.83 95.63

0.936 0.778

39.32 0.60 17.82 0.52 2.87 0.00 23.19 0.14 0.00 1.15 8.36 93.97

39.49 0.44 18.56 0.43 2.41 0.03 24.02 0.10 0.00 1.30 8.51 95.29

0.935 0.827

38.93 2.03 17.31 1.38 2.87 0.14 22.37 0.08 0.01 1.20 7.42 93.72

0.947 0.811

0.933 0.803

0.904 0.162

Condition and time of phlogopite formation It may be difficult to estimate the P - T conditions of phlogopite formation. However, relatively high A1/Si ratios of phlogopites (Fig, 6) indicate that phlogopites could be formed in the spinel- to plagioclase-peridotite stability field of the upper mantle (Arai 1984), which is consistent with the primary mineral assemblages of the Horoman peridotites. It is clear that phlogopite was not formed at the lowtemperature crustal stage, because distributions of phlogopite veins are not controlled by low-temperature brittle deformation textures. The phlogopite veinlets are concordant with the foliation planes of peridotites, and this indicates that the injection of the metasomatic fluids may have been controlled by a structural anisotropy (i.e. strong foliation) of the Horoman peridotites under the condition of Pflula > PsoHa (Nicolas and Jackson 1982). This indicates that the main stage of the phlogopite formation was contemporaneous with or slightly after the main stage of deformation at the upper mantle and was before the mylonitization stage in the crust. The main deformation occurred at 7500-950 ~ C and within the plagioclase lherzolite facies as described above.

0.927 0.884

phlogopites tend to be low in the Ti content as compared with the vein-type ones. Chemical effect of metasomatizing fluids upon primary minerals

Fractional crystallization of metasomatic fluids

Infiltration of metasomatizing fluids seems to have only slightly altered the chemical compositions of the primary minerals of the Horoman peridotites. Fo values of olivine are almost constant in individual rock types irrespective of the amount of phlogopite and amphibole present. Chromian spinels, however, show evidence of contamination by the phlogopite-forming fluids. Chromian spinels in phlogopite-free peridotites are definitely lower in Ti than those in phlogopite-bearing ones (Fig. 10). In phlogopite-bearing peridotites, chromian spinels embedded in phlogopites are slightly but distinctly richer in Ti than those outside the phlogopite veinlets (Fig. 10).

Large systematic K / N a varations of phlogopite within individual veinlets or trails may be ascribed to the in situ differentiation of infiltrating fluids. K / ( K + N a ) ratios of the fluids may be successively lowered by precipitation of phlogopites whose K / ( K + N a ) ratios were progressively lowered within the veins or trails (Figs. 8, 9). It is likely that K is strongly partitioned into phlogopite when precipitated from aqueous fluids (e.g. Volfinger 1976), although high-pressure K~ K-Na between phlogopites and aqueous fluids have not been determined. When K/(K + Na) ratios of the fluids are sufficiently low, amphiboles coprecipitate with phlogopites (Fig. 9). If the fluid responsible for precipi-

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Fig. 8. K/(K + Na) atomic ratio vs number of Ti atoms for O = 22 of phlogopites, p Interstitial type. Other symbols, vein-type phlogopites forming veinlets or trails. One type of symbol corresponds to one phlogopite veinlet. Broken lines with an arrow indicate lengthwise variation trends from the central thickest part to the marginal part of veinlets

172

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11

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