Palynology of metamorphic rocks (methodological study)

Review of Palaeobotany and Palynology, 48 (1986): 347--356 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

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PALYNOLOGY OF METAMORPHIC ROCKS (METHODOLOGICAL STUDY)

BLANKA PACLTOV.~ Charles University, Department of Paleontology, Prague (Czechoslovakia) (Received November 28, 1985)

ABSTRACT

Pacltov~, B., 1986. Palynology of metamorphic rocks (methodological study). Rev. Palaeobot. Palynol., 48: 347--356. The paper deals with the methodical approach to the micropaleontological--palynological investigation of metamorphic rocks. It has been found that remains of organicwalled microfossils (OWM) could have been preserved even in high-grade metamorphosed rocks, in various modes and states of fossilization. For obtaining them, a simple careful non-destructive technique has been proposed; its description is given. It has been stated that microfossils in metamorphic rocks have been preserved as: (a) strongly degraded transparent remains of OWM; (b) dark-brown to black coalified to graphitized remains of OWM; (c) secondarily mineralized OWM; (d) inorganic tests of microfossils; (e) inorganic casts of microfossils (inorganic fillings of OWM) as shown in Plates I--V. Using an evolutionary approach and results of comparative morphological studies of Precambrian cryptarchs and Palaeozoic acritarchs, the proposed technique was applied to the investigation of the Bohemian Massif. This leads to changes in opinions on the ages of the rocks studied. INTRODUCTION T h e a u t h o r ' s studies o f m i c r o f o s s i l s f r o m m e t a m o r p h i c r o c k s w e r e b a s e d o n t h e m o r p h o l o g y and e v o l u t i o n o f P r e c a m b r i a n c r y p t a r c h s and E a r l y P a l a e o z o i c acritarchs. H o w e v e r , p a l y n o l o g i c a l investigation o f r a t h e r highgrade m e t a m o r p h i c r o c k s is usually r e g a r d e d as i n e f f e c t i v e b e c a u s e o f high t e m p e r a t u r e s a n d p r e s s u r e s d e s t r o y i n g organic walled m i c r o f o s s i l s (OWMs), b u t a c c o r d i n g t o p u b l i s h e d d a t a , t h e s e m i c r o f o s s i l s can essentially resist m e t a m o r p h i s m w i t h i n a c e r t a i n t e m p e r a t u r e range. I n a d d i t i o n t o p u b l i s h e d d a t a , I used rich c o m p a r a t i v e m a t e r i a l derived f r o m t h e P r e c a m b r i a n a n d E a r l y P a l a e o z o i c r o c k s o f t h e B a r r a n d i a n region ( C z e c h o s l o v a k i a ) . I was e n g a g e d in t h e p r o b l e m s o f fossilization, c o n t a m i n a t i o n , i n f l u e n c e o f t e m p e r a t u r e a n d p r e s s u r e u p o n microfossils. A c c o r d i n g t o S m i t h and S a u n d e r s ( 1 9 7 0 ) , OWM m a y resist m e t a m o r p h i s m o n l y u p to t h e g r e e n s c h i s t facies, t h o u g h a t t e m p t s h a v e also b e e n m a d e t o o b t a i n i n g t h e m f r o m high-grade m e t a m o r p h i c r o c k s ( A n d r u s o v a n d C o r n a , 1 9 7 6 ;

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349 Gunia, 1978; Kalvacheva, 1978, 1979, 1982; Pacltov~, 1980, 1981, 1984a, b; Harazim et al., 1981; Krause and Pacltov~, 1984). Thanks to the intense evolution of OWM during the Early Palaeozoic, the Palaeozoic morphologically more complicated acritarchs can easily be distinguished from the Proterozoic morphologically more primitive cryptarchs (Diver and Peat, 1979). A detailed stratigraphy of the Early Palaeozoic depends on the preservation and identification of the index acritarchs or (if possible) of the whole thanatocoenosis. This is a rather difficult problem in investigating metamorphic rocks where undisturbed microfossils are encountered rarely, and mostly only their fragments are found. It is therefore important to pay attention to the morphological diversity of these micro fossil remains, as this fact itself indicates the evolutionary progress of the microfossil assemblage. This automatically leads to biostratigraphic conclusions. These principles were in some cases applied to the crystalline rocks of the Bohemian Massif. Rock samples of higher-grade metamorphic rocks, such as crystalline limestones, graphitic crystalline limestones, graphitic quartzites, erlans, paragneisses, micaschist-rock etc., were studied from many boreholes and exposures in the Moldanubicum, Moravicum, Lugicum, Silesicum, the metamorphic basement of the Bohemian Cretaceous Table and some metamorphic islands of the Bohemian Massif. Most of these metamorphic rocks have so far been regarded as unfossiliferous and were assigned to the Precambrian. However, in all of them, the remains of Early Paleozoic microfossils, such as acritarchs, chitinozoans, various tubes and cuticles, as well as inorganic tests of microfossils, have been found using a special careful technique. For example, studying rock samples from the varied series of the Moldanubicum (borehole NV-1, 1500 m), the Silurian index microfossils (Deunffia Downie, Domasia Downie, Leiofusa Eisenack etc.) were encountered throughout the profile. In the samples from the surface layers of this series, the Devonian index fossil Polyedryxium Deunff (Plate I, 2, 3) has been found. In these studies it was necessary (1) to verify the possibility of contamination by recent and fossil microorganisms, and their distinction from

PLATEI Colourless transparent remains of organic-walled microfossils (OWM). 1. Cf. Veryhachium sp. Graphitic crystalline limestone. Zechovice locality. Varied series of the Moldanubicum. 2, 3. Polyedryxiurn sp., two optical sections. Graphitic crystalline limestone. ~esk~ Krumlov U V~penky locality. Varied series of the Moldanubicum. 4. Remains of two species of OWM, the left black acritarch covered with graphite; on the second fragment, acanthomorphous acritarch, detailed morphology of process is visible. Derived from the same sample as the fossil given above under 2 and 3. 5. Cymatiosphaerasp. Graphitic phyllite. Hamr North locality, crystalline bedrock of the Bohemian Cretaceous Table.

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351 synsedimentary microfossils, (2) to provide a sufficient comparative material from non-metamorphic or slightly metamorphic rocks of k n o w n geological ages, especially Proterozoic and Early Palaeozoic, (3) to seek suitable techniques and methods of investigation, (4) to learn to identify microfossils in a poor state of preservation and (5) to continue the investigation on the basis of the so far obtained knowledge of evolution and of the detailed morphology of microfossils, which is unavoidable for the application to biostratigraphy. Thus, at first, studies of contamination by recent microorganisms were carried o u t (Pacltov~, 1984b; Pacltov~and Sajverov~, 1984). Especially spores of Fungi and Hepaticeae were recognized to contaminate the presentday surface rocks. The author thinks t h a t it will be necessary to revise many Precambrian finds of spherical microfossils. Similarly, stratigraphic gaps should be investigated in boreholes. When the rock had been exposed in the geological past, it could have been contaminated at t h a t time. Furthermore, the author was devoted to work out a suitable simple scraping laboratory technique and to examine the capacities of non-traditional investigation. The author applied this technique (in addition to classical laboratory techniques, such as thin section, polished section, maceration) to metamorphic rocks. I used a light microscope, applying all possibilities, as well as a scanning microscope (SEM). The above-mentioned new simple technique the author has termed "scraping" technique. It proved to be useful. DESCRIPTION OF THE SCRAPING TECHNIQUE The well-cleaned surface of a sample immediately broken (most suitable are samples derived from drill cores) is scratched with a sharp instrument; the scraped out powder falls directly on the slide, and an inert medium is added. Then the slide is covered. For scraping rather soft rocks, such as graphite, a preparation needle is sufficient. For hard rocks, such as paragneisses, micaschist, marble, erlan or quartzite, a diamond point is most convenient. In preparing the slide, contamination must be prevented. For rapid information, an uncovered slide may be observed under a light microscope. When the rock is to be observed under a SEM, it should be scraped onto the slide which is then put onto the stub of the microscope, or it may be scraped immediately onto the holder.

PLATE II Mineralized OWM. Scanning electron micrographs. Biotitic paragneiss. Varied series of the Moldanubicum. 1. Borehole NV-1, depth 231.5 m. Triangular fragment of an OWM;in the corners openings are perceptible. 2. Borehole NV-1, depth 1073 m. Fragment of acanthomorphic acritarch with a process.

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The advantage of this m e t h o d is that the microfossils do not continue to be further disturbed, and that their state of preservation depends only on the degree of metamorphism, the chemical composition of the rock before metamorphism and on the character of the original sedimentation environment. Using the scraping technique, the author observed the following states of microfossils preservation: (a) Strongly degraded, white to yellowish-white transparent remains of OWM which under a light microscope are better visible through various filters (Plate I). The microfossils are sometimes covered with graphite, carbonized matter or metal (e.g. Fe203 etc.) c o m p o u n d s (Plate III, 1, 2). (b) Dark-brown carbonized or graphitized remains of OWM microfossils (Plate I, 4, Plate III, 5). (c) Secondarily mineralized OWM (Plate II). (d) Inorganic tests of microfossils. (e) Inorganic casts of microfossils (secondary fillings of the vesicles either w i t h o u t or with organic remnants on them (Plate III, 3, 4, 6). Another advantage of the scraping m e t h o d is that the inorganic test of a microfossil remains preserved. The higher the degree of metamorphism, the fewer organic traces are present. However, the inorganic filling of the test (the cast) maintains its shape even when the mineral filling recrystallized. Thus, there is no reason w h y remains of microfossils should not be sought in metamorphic sediments (on the assumption that the rock was fossiliferous before the metamorphism). At any rate it may be stated that the microfossils which were metamorphosed together with the rock (in some cases rock-forming elements may be involved) can with experience be distinguished from the contaminating matter younger than the sediment. But there is also the question of the possibility of redeposition of microfossils derived from rocks older than the rock containing them. Due to the resistance of microfossils this case is relatively frequent. However, the redeposition is n o t prejudical and, in contrast, this may be applied to reconstruction of the paleogeographic conditions of the region during the sedimentation time span. PLATE III 1. Tube with spiral winding. Colourless transparent OWM partly covered with carbonized matter. Borehole NV-1, depth 1178.3 m. Biotitic paragneiss to erlan gneiss. Varied series o f the Moldanubicum. 2. Fragment of an OWM covered with carbonized matter. ~esk~ Krumlov U V~penky locality. Graphitic crystalline limestone. 3. Cast of an OWM partly covered with graphite. Biotitic paragneiss slightly graphitic. Borehole NV-1, depth 1073.6 m. 4. Scanning electron micrograph of an OWM fragment where inorganic filling is visible. Biotitic paragneiss to erlan gneiss. Borehole NV-1, 1178.3 m. 5. Graphitized fragment of an OWM cf. Dornasia elongata Downie. Borehole NV-1, depth 1208.4 m. Fine-grained erlan. 6. Cast of an OWM partly covered with graphite, cf. Leiofusa sp. Derived from the same sample as in Fig. 5.

354 PLATE IV

1, 2. Scanning electron micrograph of mineralized cuticle. 1, detail of 2. Borehole NV-1, depth 498.3 m.

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PLATE V

1, 2. Scanning electron micrograph of an OWM cast. Light-coloured crystalline limestone dissolved in 10% acetic acid. Z~lesf v~penka -- Quarry locality, the R y c h l e b y Mountains. Lugicum. 2, detail o f 1.

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Although micropaleontological investigation of metamorphic rocks is demanding and requires new approaches as to techniques, methodology and taxonomic classification of metamorphosed microfossil remains, it is promising. It undoubtedly will cause revolutionary changes in the opinions on age determination of large masses of metamorphic rocks and on the tectonics of extensive regions. This naturally also affects economic geology. By using paleopalynology for studies of metamorphic rocks, new fields of activity open for paleopalynology. REFERENCES Andrusov, D. and ~orna, O., 1976. ~ber das Alter des Moldanubicums nach mikrofloristischen Forschungen. Geol. Pr. Sprhvy, 85: 81--89. Diver, W.L. and Peat, C.J., 1979. On the interpretation and classification of Precambrian organic-walled microfossils. Geology, 7 : 401--404. Gunia, T., 1978. N e w sitesof fauna and rnicroflora in metamorphic rocks of the Sudetes. Scr. Fac. Sci. Nat. Univ. Purkynianae Brun. Geol., 2(8): 81--84. Harazim, J. Pacltov~, B. and Pouba, Z., 1981. Organic remains in the crystalline complexes of the Silesicum and eastern Lugicum (Problem of the age of metamorphosed sediments). In: Z. Pouba (Editor), Correlation of the Proterozoic and Palaeozoic Stratiform Deposits (VI). I U G S U N E S C O Projects 22 and 91. Inst. Geol. Sci.,Charles Univ., Prague, pp. 224--247 (in Czech). Kalvacheva, R., 1978. Acritarch stratigraphy of Lower Paleozoic formations in the West Balkan mountains. Palinol., I: 303--310. Kalvacheva, R., 1979. Palynological evidence (Acritarchs) for the age of the Lower Palaeozoic rocks in the Vakarel Hill,Bulgaria. C.R. Acad. Bulg. Sci.,~2(10): 1398--1400. Kalvacheva, R., 1982. Planktonni organizmi -- indikatori na vzrastta. Nauka Technika, Bulgarian Telegraph Agency, 18(34): 6--9 (in Bulgarian). Krause, J. and Pacltov~i, B., 1984. Results of the micropaleontological study of the Travn~--Ladek unit in Rychleby Mts. In: Z. Pouba (Editor), Correlation of the Proterozoic and Palaeozoic Stratiform Deposits (VII). I U G S U N E S C O Project 91. Geol. Surv and Inst. Geol. Sci., Charles Univ., Prague, pp. 257--274 (in Czech). Pacltovfi, B., 1980. Further micropaleontological data for the Paleozoic age of Moldanubian carbonate rocks. ~as. Mineral. Geol., 25(3): 275--279. Pacltovfi, B., 1981. Significance of micropaleobotany and paleopalynology for investigations of crystalline rocks (Methodology and problems). Zemnf plyn nafta, 26(4): 555--560 (in Czech). Pacltov~i, B., 1984a. The methodology and new technique of micropaleontological study of metamorphic rocks. In: Z. Pouba (Editor), Correlation of the Proterozoic and Palaeozoic Stratiform Deposits (VII). I U G S U N E S C O Project 91. Geol. Surv. and Inst. Geol. Sci., Charles Univ., Prague, pp. 275--306 (in Czech). Pacltov~, B., 1984b. The significance of paleopalynology for biostratigraphy of the metamorphic rocks. Geol. Pr. Spr~ivy, 81; 9--14 (in Czech). Pacltov~i, B. and Sajverov~, E., 1984. Spherical Precambrian microfossils and the problems of contamination. In: Z. Pouba (Editor), Correlation of the Proterozoic and Paleozoic Stratiform Deposits (VII). IUGS UNESCO Project 91. Geol. Surv. and Inst. Geol. Sci., Charles Univ., Prague, pp. 245--256 (in Czech). Smith, N.D. and Saunders, R.S., 1970. Paleoenvironments and their control of acritarch distribution: Silurian of East-central Pennsylvania. J. Sediment Petrol., 40(1 ): 324--333.