Detrital and authigenic feldspars in Permian and early Triassic sandstones, Eastern Alps (Austria)

Sedimentary Geology, 62 (1989) 59-77 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

59

Detrital and authigenic feldspars in Permian and early Triassic sandstones, Eastern Alps (Austria) KARL

KRAINER

1 and CHRISTOPH

SPOTL

2

1 Universitiit Innsbruck, InstitutJ'~r Geologie und Paliiontologie, Innrain 52, 6020 Innsbruck (Austria) 2 Universitiit Bern, lnstitutfiir Geologie, BaltzerstraBe 1, 3012 Bern (Switzerland) Received June 21, 1988; revised version accepted October 10, 1988

Abstract Krainer, K. and SpiStl, C., 1989. Detrital and authigenic feldspars in Permian and early Triassic sandstones, Eastern Alps (Austria). Sediment. Geol., 62: 59-77. Detrital and authigenic feldspars of Permian and early Triassic (Scythian) sandstones of the Eastern Alps (Austria) have been investigated using thin section microscopy, cathodohiminenscence (CL), X-ray diffraction and electron microprobe. Sandstones of Permian alluvial sequences are low in detrital feldspars due to strong weathering and diagenetic alteration. Within early Permian sandstones detrital feldspar type is strongly dependent on the source rock. Late Permian sandstones of all sections investigated are very tow in detrital feldspars, most of which are alkali varieties. Early Triassic sandstones are characterized by a better degree of sorting and rounding and higher feldspar contents, especially in the marine environment (subarkoses to arkoses). Alkali-feldspar dominates by far in all sections of the Alpine Buntsandstein Formation (Drau Range, Gurktal Nappe and Northern Calcareous Alps). In the Werfen Formation of the Drau Range and Gurktal Nappe alkali-feldspars also dominate, whereas albites are the most common detrital feldspars in the Northern Calcareous Alps due to different source rocks. Despite the fact that late Permian (Gr'rden Formation) and early Triassic sandstones (Alpine Buntsandstein and Werfen Formations) are derived from reworking of intra-Permian rhyolitic volcanics to a great extent containing sanidines, it is evident from X-ray diffraction and CL investigations that detrital alkali-feldspars within these units are not of volcanic but of metamorphic origin, being microchnes with bright blue CL. Sanidines of intra-Permian volcanics, as being very unstable due to their structural state, appear to have been completely destroyed during weathering and diagenesis. Authigenic feldspar overgrowths (almost pure potassium feldspars) frequently occur in Scythian sandstones of the Drau Range and Gurktal Nappe, particularly in the marine environment. Compared to the detrital cores anthigenic feldspar overgrowths are characterized by different optical orientation, chemical differences in minor and trace elements and absence of CL. Dissolution of volcanic sanidine, volcanic rock fragments, plagioclases and other unstable silicate minerals increased the concentration of pore waters for A1, Si and K, leading to the precipitation of diagenetically formed K-feldspar overgrowths in sandstones of marine origin.

Introduction Only

rocks. By means of XRD, EMPA, CL and SEM feldspar grains and cements were analysed with

over about

the last two

decades

sedimentary

petrologists

mineralogical

investigations of feldspars, one of

the

most

important

0037-0738/89/$03.50

undertaken

have

constituents

in

detailed siliciclastic

© 1989 Elsevier Science Publishers B.V.

regard to chemical composition, weathering

features,

diagenetic alteration (1985)

briefly

host and

summarized

rock

structural state, provenance

modification. the

and

Helmold

relatively

small

60 amount of published data, compared with the vast literature on igneous and metamorphic feldspars (cf. Ribbe, 1983; Brown, 1984). The purpose of this paper is to report the occurrence of detrital and authigenic feldspars from Permian and Scythian sandstones of the Eastern Alps, particulary of some U p p e r Austroalpine units (Northern Calcareous Alps, Drau Range and Gurktal Nappe) and to characterize them with regard to their textural, chemical and structural features. Preliminary results are scattered throughout the literature (Krainer, 1985, 1987a, b; Poscher, 1985; Sp~tl, 1987, 1988a); a comprehensive review however, is still missing.

Analytical procedure Feldspar grains of medium and fine sand grain size (1.0-3.0 ~) were analysed in thin sections under the polarizing microscope, in cathodoluminescence (CL) images (Technosyn cold cathode luminescence model 8200 MK II) and with the electron microprobe (ARL-SEMQ with detached energy-dispersive detection system KEVEX). Measurements were performed at operating conditions of 15 kV, 20 nA sample current and 200/~A emission current for Na, K, Ca, Mg, Fe, Mn, Si, Ti and in some cases Ba and matrix effects were corrected according to Bence and Albee (1968). Results between 97% and 101% were recalculated so that Or + A b + A n ( + C e ) totalled 100% by weight. Additionally sandstone samples were crushed, sieved and separated by heavy liquids (tetrabromethane and D M F in a 83:17 mixture) in order to obtain a K-feldspar concentrate (Plymate and Suttner, 1983). The ground powder was examined by means of a Philips X-ray diffractometer at the following operating conditions: C u K , radiation, Ni-filter, 40 kV, 20 mA, 1 / 4 degree per minute chart speed.

Permian and early Triassic stratigraphy of the Eastern Aips In most parts of Europe post-Variscian sedimentation commenced during the Late Carboniferous. Molasse sedimentation in the Eastern Alps

(Upper Austroalpine units) of ?Westphalian to Stephanian age has been reported only from a few places, whereas bulk sedimentation did not start before the earliest Permian. Major progress in stratigraphy and depositional history has been achieved by extensive investigations in recent years (Niedermayr and ScheriauNiedermayr, 1982; Stingl, 1983, 1984, 1987; Mostler and Rossner, 1984; Krainer, 1985, 1987a, b; Niedermayr, 1985; Poscher, 1985; Krainer and Stingl, 1986, SpiStl, 1987, 1988b). Fig. 1 gives an overall view of the tectonic setting and topographic position of the sections studied. Proximal to distal alluvial fan sediments (redcolored breccias, conglomerates, immature sandstones) laterally grading into fine-grained sandflat-playa complexes characterize the early Permian throughout all tectonic units. Plant remains are known from the base (Van Amerom et al., 1982; Fritz and Boersma, t987). In the Drau Range, Gurktal Nappe and the westernmost part of the Northern Calcareous Alps rhyolitic volcanics (ignimbrites and pyroclastic flows) are wide-spread on top of the early Permian sequence. Late Permian siliciclastic sediments of ephemeral braided rivers and playas overlie early Permian intramontane basin filling and mark a sudden and significant change in sedimentation (Fig. 2). In the central and eastern realm of the Northern Calcareous Alps a few hundred meters of halite-bearing marine evaporites were laid down in an approximately E-W-trending late Permian rift, now represented by an strongly deformed d6collement horizon at the base of the Calcareous Alps (SpiStl, 1988b). In the southern part of the Eastern Alps (Drau Range, Gurktal Nappe) fluvial to shallow-marine sedimentation prevailed during Scythian time. The transition between late Permian and Scythian is documented by an abrupt change in the depositional environment, caused by a climatic shift, presumably corresponding to the P e r m i a n / Triassic boundary (Brandner et al., 1986). Based on transgressional and regressional events, the Scythian of the Drau Range can be subdivided into three megasequences (Lower and Upper Alpine Buntsandstein Formations and Werfen Formation), which can be event-correlated

61

_..,--- m s

77qe i ,

ilnl

Fq ,

7;q13

Fig. 1. Geologic sketch m a p of the Eastern Alps (after Fliigel and Faupl, 1987) with locations (dots) of investigated Permian a n d / o r early Triassic (Scythian) sections. 1 = sub-alpine molasse; 2 = Helvetic and Klippen zone; 3 = Rhenodanubian flysch; 4 = metasedimentary rocks of the Penninic zone; 5 = crystalline basement of the Penninic zone; 6 = Permo-Mesozoic in North Alpine facies; 7 = Paleozoic and 8 = Permo-Mesozoic in Central Alpine facies; 9 = crystalline basement ("Altkristallin"); 10= PermoMesozoic of the Southern Alps; 11 = Paleozoic of the Southern Alps; 12 = peri-Adriatic intrusive masses; 13 = Neogene volcanics.

.ITHOLOGY i

[

I

NORTHERN WEST

[

I

m~m~mm ~

Alpin i

Werfen

CALC. ALPS I EAST •

Mu schelkalk

Alpine

Werfen Ab

Fm.

' " " " " ' Buntsdst. . . . • ,Lower Fm. •

•

O O

Or

GURKTAL NAPPE

STAGES

Formationgroup Werfen Fm. Or + Ab +OG

Fm.

Ab

",e.ee"e =."Uppe r

DRAU RANGE

ANISlAN

Werfen Fm. Or + Ab +OG

Upper _ _

Alpine Alpine Buntsdst. Buntsandstein Lower Fm. Fm. Or + - Ab÷OG

SCYTHIAN

Or+OG

Alpine Haeelgebirge

io

Gr~den

oooOoOoO

Fm.

Or

0 0 0 0

Fm. Ab÷Or+OG Priibichl

G r ~ d e n Fm. Or+A b

G r~den Or

Fm.

LATE PERMIAN

Fm.

EARLY PERMIAN

Fm.

Or O

8

0

~

= ~ ' ~ %" ~ ~ " , B a s a l b r e c c i a ,

I~ IJ IJ Il l Ir I I I I ~ ~ ~I I I

Paleozoic Rocks (weekly metamorphosed)

L a a s Fm. Or~Ab Metamorphic Rocks

Werchzirm

Ab Paleozoic Rocks

PRE-PERMIAI~

Fig. 2. Simplified Permian/early Triassic stratigraphic nomenclature of the Eastern Alps, U p p e r Austroalpine units. Abbreviations in parentheses refer to the dominant feldspar types (OG stands for authigenic overgrowths).

62 with the fossiliferous marine South Alpine Tethys stratigraphy (Krainer, 1987a). Lower and Upper Alpine Buntsandstein Formations are built up of fining-upward megasequences, starting with quartz-rich conglomerates (gravelly braided rivers) and grading upward into fining-upward deposits of sandy braided streams. In some of the sections studied a clastic shallow-marine tidal facies caps the sequence. The Werfen beds were deposited on a shallow-marine epicontinental shelf under tidally influenced conditions. In the Northern Calcareous Alps a comparable evolution took place during Scythian time. In the middle and eastern part Werfen beds comprise the whole Scythian, interfingering with fluviatile to marginal marine transgressive sheets of the Alpine Buntsandstein Formation in the west. Almost the same transgressional and regressional events can be inferred within the North Alpine early Triassic. This allows a detailed intra-Scythian correlation with the South Alpine Tethys stratigraphy herein recognized for the first time (Brandner et al., 1984, 1986). Sedimentation of Upper Austroalpine units continued until the early Cretaceous resulting in a total maximum thickness of 7 km for the Drau Range and the Gurktal Nappe and 10 km for the Northern Calcareous Alps, respectively. These values represent maximum burial depths for the Permo-Scythian sediments discussed in this paper.

Sandstone petrography For detailed petrographic descriptions of the sandstones studied the reader is referred to Krainer (1982, 1985, 1987a, b), Niedermayr and ScheriauNiedermayr (1982), Stingl (1983, 1984), Hess (1985), Poscher (1985), Krainer and Stingl (1986) and SpiStl (1987). In the present study only a brief summary can be given (Table 1). Permian sandstones are uniformly very poorly to moderately sorted angular lithic arenites (Figs. 3, 4), whilst early Triassic equivalents vary widely in composition and maturity. In most cases, fluvial sandstones were deposited as moderately sorted, subrounded lithic arenites to arkosic arenites. Marine sandstones are subrounded to wellrounded subarkoses, arkoses and sublitharenites

(Figs. 5, 6). Metamorphic rock fragments plus mono- and polycrystalline quartz constitute the main components of early Permian sandstones (e.g. Laas Formation). Lithic fragments in sandstones of the same age as the Gurktal Nappe are derived from Paleozoic metasiltites, metagraywackes, phyllites and reworked sedimentary rocks due to intra-Permian tectonics ("Saalian movements"). Late Permian sandstones (Gr~Sden Formation) do not display any difference in composition between Drau Range and Gurktal Nappe. They are characterized by high amounts of volcanic-derived rock fragments (intra-Permian rhyolitic volcanics), and low percentages of metamorphic rock fragments. Scythian sandstones (Alpine Buntsandstein Formation, Werfen Formation) differ from late Permian equivalents in that they show a significantly lower content of volcanic rock fragments and higher percentages of mono- and polycrystalline quartz, as well as an increased amount of detrital feldspars. The matrix content of Permian sandstones is typically high, in many sections exceeding 15% (wackes). Illite-crystallinity indices together with the occurrence of pyrophyllite, paragonite and paragonite/muscovite-mixed layer (Schramm et al., 1982; Niedermayr et al., 1984) indicate anchizonal metamorphic overprint of Alpidic (Cretaceous) age. In contrast to generally uncemented Permian sandstones, Scythian sandstones in most cases are well cemented by carbonates and authigenic overgrowths of quartz and feldspar. As far as the heavy mineral composition is concerned, the early Permian sandstones are dominated by tourmaline-zircon and minor percentages of garnet and apatite. Late Permian heavy mineral suites are by far more uniform and laterally constant than their early Permian precursors. They contain mostly zircon and tourmaline, plus small amounts of rutile and rare apatite. Unlike the Permian sandstones, the Scythian heavy mineral association is characterized by large amounts of apatite (plus minor percentages of tourmaline, zircon, rutile and garnet), except in the westernmost part of the Northern Calcareous

63 TABLE 1 Sandstone petrography (typical ranges) and average detrital feldspar composition Formation

Sandstone type a

Source rock

Feldspar composition b

arkoses, subarkoses

volcanic,

western part:

Qm20_4oF23_3sLt30_50

metamorphic

eastern part:

fluvial environment: lithic arkoses and arkosic arenites

volcanic, metamorphic

southern part: northern part:

Drau Range Werfen Fm.

Alpine Buntsandstein Fm.

Or~. 6Abs. 3An 0.1 Or89.7Abg. 2An 0.4Ce0.7 and Or 1.5Ab94.1An a.3Ceo.l Or95.6Ab4.3 An 0.1 OrsT.0Ab 12.7An 0.3 and Or2.TAb92.sAn4. 5

Qm 20- 50F4_16Lt 41-60 marine environment: sublitharenites, subarkoses, arkoses GrSden Fm. Laas Fm.

Qm26-60F15- 24Lt 23_ 5 lithic arenites Qml 1- 36Fo- 5Lt 66- 93 lithic arehites, lithic wackes

volcanic, metamorphic metamorphic

Or93Ab6.9Ano.1 and Oro.s Ab99.TAn o.5 Or93.1Ab6.5 An 0.4 and Orl.8Ab96.9Anl. 3

Qml_ 20 Fo_ 20Lt63_ 99

Gurktal Nappe Werfen Fm. Alpine Buntsandstein Fm.

arkoses, subarkoses

volcanic

Or92AAbT.3Ano.3Ceo.3

Qm 35_ 40F30_,lo Lt 20_ 30 fluvial environment: iithic arenites, sublitharenites

metamorphic volcanic, metamorphic

Or93.6 Abs.a An 0.2Ce0.4

Qm43- 60 F8_ 26Lt 25_ 48 lithic arenites

volcanic,

no data available

Qm 18- 35Fo- 5Lt 62- 78 lithic arenites, lithic wackes

metamorphic low-grade metamorphic

OrHAb97.5 Anl.a

Qms2-s2F1_t3Ltls_38 marine environment: sublitharenites, subarkoses GrSden Fm. Werchzirm Fm.

Q m l - 23FI_14Lt 71_ 99

Northern Calcareous Alps Werfen Fm.

subarkoses, arkoses

low-grade

Or2.7Ab94.9 An 2.4

Alpine Buntsandstein Fm.

Qm 50- 70 1:75 - 30Lt 5- 37 lithic and sublithic arenites, subarkoses

metamorphic volcanic, metamorphic

Or96 Ab3.2 An 0.8

GrSden Fm./Pr~ibichl Fm.

Qm51-6oF4_TLts_ 2o lithic graywackes, arkosic wackes

volcanic, metamorphic

Or95 Ab4.5An o.5

Qms1-sqF4_ 7Lt6_lo arkoses, lithic arkoses

low-grade

Qm 32- 39 F3o - 40Lt 6- ls

metamorphic

Or1 AAb95.4 An 3.5 and Orgo Ab9. 2An o.s

Alpine Haselgebirge Fm.

a Q m F L t formula includes monocrystalline quartz, feldspar and total aphanitic lithic fragments (lithics plus polycrystalline quartz) according to Dickinson (1982) b Ce = celsian.

64

Fig. 3. Poorly sorted, angular--subangular early Permian sandstone (lithic arenite, Laas Fm., Drau Range) composed of metamorphic rock fragments, mono- and polycrystallinequartz and some detrital feldspars. Plane light, bar scale = 0.5 mm.

Alps where non-marine sedimentation prevailed. The abundance of apatite in marine sandstones is consistent with the relative stability of this mineral in pore waters of weakly alkaline chemistry (cf. Nickel, 1973).

Detrital feldspars of Permian age Drau Range and Gurktal Nappe Early Permian Most feldspars are found in the coarse silt/fine sand fraction. There is a marked difference in the absolute feldspar content between the northern part and the southern part of the Laas Format i o n / D r a u Range (up to 18% compared to values below 5%). Detrital feldspars display a highly altered appearance and are sometimes replaced by carbonates (calcite dominates over dolomite). Alkalifeldspars, mostly untwinned a n d microperthitic grain types, are about nine times as abundant as plagioclase. Polysynthetic twins (A-twins after Gorai, 1951) are typical of albites. Microprobe analyses are given in Tables 1 and 2, as well as in Fig. 7. Comparable measurements on feldspars from metamorphic rock fragments

(granitic gneisses) yielded virtually identical results for both alkali-feldspar (Or94.4Abs.sAn0A) and albites (OrHAb97.TAnl.2), indicating that the bulk of detrital feldspars is derived from feldsparrich granitic gneisses. Some albites may also have been derived from low-grade metamorphic schists. Early Permian sandstones of the Gurktal N a p p e (Werchzirm Formation) in their turn contain up to 14% detrital feldspars, which are weakly to highly altered to phyllosilicates. Albite is the only feldspar species, and up to 38% of the grains are twinned (mostly A-twins, some C-twins, according to Gorai, 1951). Chemical traverses across single grains permit a differentiation between a population of very uniform composition (Or0.2_1 5 Ab97.5- 99.8An 0.2- 2.2 ) and another characterized by broader fluctuations (Oro. 2_ 7.3Ab91.5- 99.3An 0.4-1.9 )"

Late Permian Total feldspar content in sandstones of the Drau Range and Gurktal N a p p e is less than 5% in most samples and often near zero. Because of intensive alteration and replacement by clay minerals (Fig. 9) only a few analyses could be obtained. The feldspars are typically untwinned apart from the minor occurrence of some crosshatch twins (microclines). Only in one section a

65

Fig. 4. Moderately sorted subangular late Permian sandstone (lithic arenite, Grt~den Fm., Drau Range) composed of volcanic rock fragments and mono- and polycrystalline quartz. Plane light, bar scale = 0.5 mm.

few albites were detected. I n o r d e r to e s t i m a t e the source rock of these species, feldspars f r o m intraP e r m i a n volcanics, volcanic a n d m e t a m o r p h i c r o c k f r a g m e n t s were analysed. R h y o l i t i c pyroclastics of the G u r k t a l N a p p e

exhibit two d i f f e r e n t types of alkali-feldspars: Or99.1Abo.9An 0 a n d Or77AAb2x.2Ano.7. I n some s a m p l e s f e l d s p a r s are entirely albitized (Or1. 6 Ab97.1Anl.3), p r e s u m a b l y c a u s e d b y h y d r o t h e r m a l processes. If c o m p a r e d to the results o b t a i n e d

Fig. 5. Well sorted and well rounded early Triassic sandstone (arkose, Alpine Buntsandstein, Drau Range) composed mainly of quartz and detrital feldspars, cemented by authigenic quartz and feldspar overgrowths. Plane light, bar scale = 0.5 ram.

66

Fig. 6. Same as Fig. 5, but crossed nicols.

from

volcanic

these

two

types

pebbles could

in

conglomerates,

be recognized:

again

Or97.vAb2. 0

Metamorphic kali-feldspars

rock

fragments

(Or91.2 Ab8.8 A n o) a n d

mb94.3An4.4). Therefore,

A n o . 3 a n d O r T o A b 2 9 . 2 A n o . 8.

contain

we assume

both

albites

al-

( O r t.3

that most

al-

TABLE 2 Representative electron microprobe analyses of detrital and authigenic feldspars A

B

C

D

E

FeO MnO BaO MgO CaO NazO K20

64.02 0.00 19.24 0.00 0.20 0.05 n.d. 0.55 0.01 0.55 15.43

64.59 0.00 18.80 0.00 0.00 0.00 n.d. 0.00 0.00 0.83 15.46

69.07 0.00 20.54 0.02 0.04 0.03 n.d. 0.14 0.04 10.06 0.02

64.42 0.00 19.38 0.00 0.05 0.03 n.d. 0.00 0.16 3.27 12.57

63.67 0.00 18.77 0.06 0.00 0.03 n.d. 0.00 0.00 0.04 17.46

64.37 0.00 19.32 0.00 0.24 0.10 0.14 0.14 0.09 0.42 15.47

Total

100.05

99.68

99.96

99.88

100.03

94.84 5.10 0.06

92.44 7.56 0.00

0.14 99.64 0.22

71.09 28.13 0.78

99.66 0.34 0.00

SiO2 TiO 2 AI203

Cr203

Or Ab An Ce

F

G

H

I

J

65.53 0.00 19.75 0.00 0.01 0.00 0.00 0.02 0.00 0.01 14.90

68.27 0.02 20.82 0.00 0.00 0.03 n.d. 0.00 0.05 11.40 0.03

67.75 0.02 20.15 0.00 0.00 0.00 n.d. 0.00 0.27 11.89 0.06

64.97 0.00 22.43 0.00 0.07 0.05 n.d. 0.03 2.23 9.76 0.22

100.29

100.22

100.63

100.14

99.76

95.34 3.93 0.47 0.26

99.90 0.10 0.00 0.1)13

0,19 99.57 0.24

0.30 98.48 1.22

1.32 87.64 11.04

'

FeO = FeO + F e 2 0 3 . C e = celsian, n.d. means not determined. Samples: A = detrital microcline (Laas Fm., Drau Range); B = microcline inside a metamorphic rock fragment (Laas Fm., Drau Range); C = detrital albite (Werchzirm Fm., Gurktal Nappe); D = Alkali-feldspar inside a volcanic rock fragment (Gr6den Fm., Drau Range); E = authigenic K-feldspar (Alpine Buntsandstein Fm., Drau Range); F = detrital K-feldspar (Werfen Fm., Gurktal Nappe); G = authigenic K-feldspar tim (Werfen Fro., Gurktal Nappe); H = detrital albite (Alpine Haselgebirge Fro., Northern Calcareous Alps); I = detrital aibite (Werfen Fm., Northern Calcareous Alps); J = detrital oligoclase (Werfen Fm., Northern Calcareous Alps).

67

Or

90.

O1"

°\

60

50

I!

9

•

o 40 a¢

30

~.J '~a~ e°I°

•

6o

An

~o

•

*

~

"

..fo

h

X,~ o'o An

Fig. 7. Chemical composition of detrital and volcanic feldspar in Permo-Scythian sediments and volcanics of the Drau Range and Gurktal Nappe as determined by electron microprobe. (a) Werchzirm Fm.; (b) Laas Fm.; (c) intra-Permian volcanics; (d) Gr6den Fm. and Alpine Buntsandstein Fro.; (e) Werfen Fm.

kali-feldspars in late Permian sandstones were derived from volcanic as well as metamorphic source rocks, whereas detrital albites probably were completely derived from low-grade metamorphic rocks. It should be pointed out that One type of volcanic a l k a l i - f e l d s p a r s (Or70_ 77) has not been observed in sandstones of late Permian age, indicating that such highly unstable phases underwent almost complete weathering and subsequent diagenetic destruction. Northern Calcareous Alps Late Permian continental red-beds (alluvial fan, alluvial plain, flood plain and inland sabkha facies)

are usually low in feldspar (4-7%). Most grains are diagenetically altered a n d replaced by sericite-chlorite-carbonate. Alkali-feldspars have an average chemical composition of Or95Ab4.sAn0.5 (Fig. 8). Isolated sandstone intercalations and lenses inside the marine evaporitic complex (Alpine Haselgebirge Formation) are arkoses and lithic arkoses (Tables I and 2). Preliminary results (Spt~tl, 1987, 1988a) suggest a regional change in composition from almost pure albite-bearing types, which probably underwent diagenetic albitization, to alkali-feldspar-rich types. However, deciphering their spatial distribution is greatly impeded by halokinetic as well as alpidic nappe tectonics.

68 Or 90

Or

60

An

Ab

~

~

~

~

.~

&

~

~o

~o

~n

Fig. 8. Chemical composition of detrital feldspar in Permo-Scythian sediments of the Northern Calcareous Alps as determined by electron microprobe. (a) Gr6den Fm.; (b) Alpine Haselgebirge Fm.; (c) Alpine Buntsandstein Fm.; (d) Werfen Fm.

Fig. 9. Late Permian sandstone (Gr~den Fm., Drau Range) with some detrital feldspars, almost completely altered to clay minerals (C), sometimes with small feldspar relics (F). Crossed nicols, bar scale = 0.1 mm.

69

Detrital feldspars of early Triassic age Drau Range and Gurktal Nappe Feldspar content of the Alpine Buntsandstein Formation is greater than 20% in the marine sandstones versus less than 20% in the fluvial sandstones (Table 1). This is mainly due to the fact that detrital feldspars in fluvial sandstones underwent intensive alteration during early diagenesis, probably in the phreatic regime. In most sections only alkali-feldspars occur, being slightly altered in the fluvial environment and relatively fresh in sandstones of marine origin. Most individuals are untwinned. Angular grains predominate in the fluvial facies, whereas a large number of rounded grains occur in sandstones deposited under marine influences (Fig. 10A). The average chemical composition can be obtained from Table 1. Chemical profiles across individual crystals indicate the same range of c o m p o s i t i o n (Or88_97) , but no distinct internal zoning. In the northern part of the Drau Range alkalifeldspars are frequently microperthitic and display a wider range in chemical composition (Orss_99) compared to the southern part. The large amount of microperthites indicates that most detrital

feldspars stem from granitic gneisses (supported by bright blue CL-colors and X R D data). Microprobe measurements of feldspars from metamorphic rock components yielded an average composition of Or94.1Abs.gAn0 . Detrital feldspars from the Alpine Buntsandstein Formation of the Gurktal Nappe are almost of the same chemical composition as those from the southern Drau Range. Analyses of 126 grains indicate an average of Or93.6Abs.TAn0.2fe0.4, the Or molecule ranging from 88 to 98 mole %. Plagioclase grains were not identified. Sandstones of the marine Werfen Formation are slightly richer in detrital feldspars compared with time-equivalent sandstones of the continental Alpine Buntsandstein Formation (mean 20-30%). Alkali-feldspars predominate in all sections studied, and are associated with small amounts of albite. They appear fresh, and are untwinned in most cases (Fig. 10B), whereas albites frequently exhibit polysynthetic twinning. Their average chemical composition is given in Table 1.

Northern Calcareous Alps The fluvial facies of the Alpine Buntsandstein Formation displays a slightly higher feldspar content than their late Permian precursors (Gr~Sden

Fig. 10. A. Well sorted and rounded early Triassic sandstone (arkose, Alpine Buntsandstein, Drau Range) rich in detrital alkali-feldspars showing authigenic overgrowths with rhombic outline. B. Early Triassic marine sandstone (arkose, Werfen Fro.) with detrital alkali-feldspars showing authigenic overgrowths. Plane light, bar scale = 0.1 ram.

70 and Pr~bichl Formations). Microprobe analyses are hampered by almost complete diagenetic replacement by clay minerals (Fig. 8). Marine epicontinental sandstones of the Werfen Formation are uniformly arkoses or subarkoses. Albites dominate by far over alkali-feldspars, which is in contrast to the above mentioned results from the Drau Range and Gurktal Nappe and can be attributed to different source rocks in the hinterland.

Feldspar overgrowths Occurrence Authigenic K-feldspar overgrowths have only been observed in Scythian sandstones (marginal marine environment of the Alpine Buntsandstein and Werfen Formations) of the Drau Range and Gurktal Nappe (Figs. 10A, B, 11, 12), as well as in late Permian sandstones of the evaporitic Alpine Haselgebirge Formation. The percentage of authigenic feldspar relative to the total feldspar content amounts 5-10% in the Werfen Formation and marine Buntsandstein Formation (exceptionally up to 35%). The shape of the overgrowths mainly is a function of the preexisting pore space, and euhedral overgrowths of rhombic outline (adular habitus) dominate over saw-toothed rims. In all cases overgrowths are clearly visible because they appear fresh and transparent, whilst the well-rounded cores are characterized by their cloudy appearance. Only in few examples do the overgrowths display evidence of slight alteration (Figs. 10-12). The bulk of the samples studied shows a prominent difference in optical orientation between cores and rims (due to different chemical composition and structural state), though in a few grains both seem to be in optical continuity. All overgrowths are untwinned, even if the detrital core exhibits twinning (microcline crosshatch twinning). The boundary between cores and overgrowths is optically and chemically sharp (Figs. 11, 12). Whereas detrital feldspar cores are characterized by bright blue CL, overgrowths do not

show any sign of luminescence (cf. Kastner, 1971; Nickel, 1978; Kastner and Siever, 1979; Ruppert, 1987). Due to textural relationships it is evident that K-feldspar overgrowths formed prior to quartz overgrowths and carbonate cements. Euhedral feldspar overgrowths protrude into the originally open pore space which was subsequently cemented by anhedral quartz. In some samples both quartz and feldspar rims are replaced by carbonates. The above mentioned features have been reported by many authors (Baskin, 1956; Fiichtbauer, 1967; Stablein and Dapples, 1977; Waugh, 1978; Ali and Turner, 1982), regarding K-feldspar precipitation as a shallow burial process.

Chemist~ About 35 selected samples were subjected to electron microprobe analysis. The results indicate that overgrowths are virtually pure KA1Si 308, the Or molecule ranging from 99.08 to 99.92 mole % (average mean 99.6%--see Figs. 11 and 12). Internal zoning, as described by Ali and Turner (1982), was not observed. Authigenic rims of K-feldspar are significantly depleted in minor and trace elements compared to the detrital cores. Fe and Mg were detected in 60% of the rims examined (average mean 0.05% FeO and 0.04% MgO). According to our observations these concentrations are not caused by minute carbonate inclusions, but probably by very small clay flakes delimitating the boundary of the detrital core. Up to 0.13% BaO was detected in only 25% of all measurements, whereas in only 20% the Ancomponent was above the detection limit (DL) for Ca, reaching concentrations up to 0.1% CaO. Mn, Ti and Cr lay well below the D L for the electron microprobe (cf. Table 2).

Formation of diagenetic ooergrowths Before discussing details of authigenic rim formation the following should be emphasized: - - Only overgrowths of K-feldspar on alkalifeldspar cores are present and they are - - widespread in sandstones of marine origin, but

71

Fig. 11. Thin section photomicrographs of detrital feldspars with authigenic overgrowths from early Triassic sandstones (Werfen Fm.) of the Gurktal Nappe (A, C) and Drau Range (B, D). Points analysed by electron microprobe are marked by dots. Crossed nicois, bar scale = 0.1 ram. A. I (core): Or92.27Ab7.73Ano.o; 2 (rim): Or99.52Abo.17Ano.31, B. 1 (core): Or91.76AbT.ssAno.36; 2 (rim): Or99.4sAbo.~2Ano.o. C. 1 (core): Or95.soAb4.1~Ano.05; 2 (core): Or95.11Ab4.53Ano.~; 3 (core): Or94.76Abs.04Ano.20; 4 (rim): Or99.9sAbo.02Ano.o; 5 (rim): Or99.5~Abo.43Ano.o; 1 (core): Or93.69Ab6.10Ano.21; 2 (rim): Or99.73Abo.27Ano.o. D. 1 (core): Or96.36Ab3.64Ano.o; 2 (core): Or94.96Ab4.s9Ano.15; 3 (core): Or93.sTAb6.03Ano.lo; 4 (core): Or94.94Abs.01Ano.05; .5 (core): Or94.66Ab5.1sAno.16; 6 (rim): Or99.37Abo.63Ano.o, 7 (rim): Or99.83Abo.17Ano.o.

73 very rare to completely absent in alluvial facies. In an excellent paper, Walker et al. (1978) demonstrated that ancient first-cycle alluvium probably rarely--if ever--has the same mineralogy, texture and chemical bulk composition as the initial sediment, due to extensive diagenetic alteration. This is valid for late Permian sandstones of the Eastern Alps too, which are almost free of detrital feldspars, especially plagioclase, though the source rocks contain large amounts of both, alkali-feldspars and plagioclase. The former show strong dissolution features and were partially replaced by clay minerals. Other unstable mineral grains, like apatite, biotite and volcanic glass fragments underwent strong alteration, too. With the exception of authigenic clay minerals --modified by alpidic very low-grade metamorphism (cf. Schramm et al., 1982; Niedermayr et al., 1984)--and some carbonate cement no minerals formed diagenetically within the late Permian immature siliciclastics, probably indicating that a considerable amount of ions has been removed. Modern seawater is undersaturated with respect to microcline and plagioclase; therefore precipitation of authigenic K-feldspar requires high aK+/aH+ ratios and pore fluids enriched in Si and A1. Dissolution of alkali-feldspar and plagioclase in turn is believed to act as an important process for release of K, A1 and Si. Additional sources for necessary ions are volcanic rock fragments and biotite flakes which were subjected to intensive leaching yielding Si, A1, K, Mg and Fe. CL investigations (non-luminescent) prove that alkali-feldspars of volcanic origin (sanidines), due to their structural state, have been almost entirely -

-

dissolved and replaced in continental red-beds, except in small tuffaceous layers. Alkali-feldspars of volcanic source rocks and plagioclase derived from metamorphic areas are virtually completely absent, too. Only some alkali-feldspars of metamorphic origin, characterized by their bright blue CL (Nickel, 1978) were observed. This observation could be confirmed by plotting feldspar compositional data on the diagram of Trevena and Nash (1981, fig. 1). The entire range of Permo-Scythian sandstones, apart from intra-Permian pyroclastic rocks, cluster within the field of plutonic/ metamorphic feldspars. This is why we believe that dissolution of sanidines and volcanic rock fragments in particular increased the electrolyte concentration of interstitial pore waters until the solubility product for microcline was exceeded, resulting in authigenic precipitation of overgrowths on relatively stable metamorphic feldspar cores. Diagenetic evolution proceeded and subsequently authigenic quartz rims were formed, followed by calcite cementation. Finally it is worth mentioning that during diagenesis apatite displays the same behavior as feldspar: apatite is almost completely absent in continental red-beds and appears in sandstones of marine origin in great amounts (Alpine Haselgebirge Formation, marginal marine part of the Alpine Buntsandstein Formation, Werfen Formation), sometimes showing authigenic overgrowths, too. This fact can be attributed to early diagenetic apatite instability and replacement under probably slightly acidic conditions in the phreatic alluvial environment, but relative apatite and feldspar stability in marine pore fluids of pH >/7 (cf. Nickel, 1973).

Fig. 12. Thin section photomicrographs of detrital feldspars with authigenic overgrowths from early Triassic sandstones (A, B, C, E: Alpine Buntsandstein; D, F: Werfen Fm.) of the Drau Range and Gurktal Nappe. Points analysed by electron microprobe are marked by dots. Crossed nicols, bar scale=0.1 mm. A. 1 (core): Or97.91Ab2.09Ano.o; 2 (core); Or9a.ssAbs.15Ano.o; 3 (core): Or96.99Ab2.93Ano.o8; 4 (core): Or9s.7oAbl.21Ano.o; 5 (rim): Or99.08Abo.gEAno.o; 6 (rim): Orgo.oTAbo.03Ano. o. B. 1 (core): Orso.60Abg.31Ano.09; 2 (rim): Or99.36Abo.64Ano.o; 3 (rim): Or99.78Abo.22Ano. 0. C. 1 (core): Or97.19Ab2.slAno.o; 2 (rim): Or99.12Abo.ssAno.o; 3 (rim): Or99.1sAbo.26Ano.56 . D. I (core): Or94.60Abs.30Ano.o; 2 (core): Or93.17Ab6.s3Ano.o; 3 (core): Or92.05Ab7.95Ano.o; 4 (core): Or93.14Ab6.86Ano.o; 5 (rim): Or99.22Abo.78Ano.o; 6 (rim): Or99.9sAbo.02Ano.o; 7 (rim): Or99.65Abo.35Ano.o; 8 (rim): Or99.32Abo.6sAno.o; 9 (rim): Or99.66Abo.34Ano.o . E. 1 (core): Oros.ooAbE.ooAno.o; 2 (core): Or9s.90Abl.loAno.o; 3 (rim): Or99.97Abo.03Ano.o; 4 (rim): Or99.31Abo.69Ano. o. F. 1 (core): Orgo.59Ab7.soAno.o; 2 (rim): Or99.96Ab0.o4 An o.o.

74

Structural state of K-feldspar cores and authigenic rims

exhibit diagenetic overgrowths. Diffractograms typically display one single (131) peak with slight, but distinctive signs of splitting, interpreted as intermediate microcline according to Fig. 13 and Blasi (1984). Some samples already exhibit a small (131) reflection at approximately 30.3 ° . On the basis of XRD data, a few samples could be misinterpreted in terms of a monoclinic sanidine phase. The fact that they entirely plot within the field of intermediate microclines (Fig. 13) and that the cores appear bright blue in CL images, however, points to a metamorphic/plutomc source rock for the bulk of detrital alkali-feldspar cores. Because separation of cores and diagenetically formed rims was impossible, interpretation of the structural state of the latter appears rather problematic. Published data are ambiguous. Triclinic authigenic K-feldspar crystals (Baskin, 1956; Kastner, 1971; Stablein and Dapples, 1977) as well as monoclinic rims have been reported (Baskin, 1956; Surdam and Parker, 1972; Woodard, 1972; Sheppard and Gude, 1973: Waugh, 1978). The former represent virtually pure

X-ray diffractograms, calibrated against quartz, were evaluated according to the (131)-(131) method (Goldsmith and Laves, 1954) and the (060)-(204) method (Wright, 1968, modified by Suttner and Basu, 1977). Despite the fact that our results represent average values (no single grain measurement was performed, due to the small grain size), some remarkable trends were found. Feldspars derived from metamorphic hinterland (in most cases perthitic types) are characterized by a broad, hardly differentiated peak between the (131) and (131) reflections. These feldspars plot between maximum microcline and orthoclase (intermediate microclines)--cf. Fig. 13. Volcanic feldspars (from pyroclastic and ignimbritic layers) show a sharp (131) peak without a clear triclinic (131) reflection, thus unequivocally pointing to a monoclinic structural state (Fig. 13). Interpretation becomes rather difficult when looking at feldspars from various origins which

4~..00

\

21.5",

\

.gO

\

%0~ 21.3\ •

'1' 2 1 . 2 ~ 21.1~ 21.0"-~ \

.BO

\\

\

21.4~ \

/

\

\ ~

\

Maximum • microcline

/,/

\\ \\

\

J

\

0

8 •

.70 orthoclase

.6O High • sanidine

50.40

.SO

.80

.70 2e

.a[3

.90

~1.00

2 0 4 CuK,~

Fig. 13. Structural states of alkali feldspars examined (diagram after Wright, 1968, fig. 3). SymboLs: ~ = metamorphic feldspars without authigenic overgrowths; * = volcanic feldspars without authigenic overgrowths; ~ = feldspars of metamorphic origin with attached authigenic rims.

75

K-feldspar endmembers and are extremely deficient in trace elements. No internal zoning can be identified (Stablein and Dapples, 1977; Hearn and Sutter, 1985; Hearn et al., 1986). The second group sometimes exhibits clear K, Na, and Ba fluctuations when microprobe traverses are made across the rim (Ali and Turner, 1982). Additionally, many of these overgrowths are not in optical continuity with the detrital cores. Although no detailed investigations exist concerning the various reasons for low temperature formation of metastable monoclinic K-feldspar, Kastner and Siever (1979) pointed out that their seems to be a genetic relationship between rock type and authigenic feldspar mineralogy. Whereas carbonates typically contain authigenic microclines, diagenetic feldspar rims in sandstones and shales display a wider range of ordering (ordered maximum microcline to disordered sanidine). The latter represents an important constituent in diagenetically altered tuffaceous beds (Desborough, 1975; Broxton et al., 1987). However, the fact that metamorphic alkali-feldspars display a wide range of structural states (Tilling, 1968; Hiss, 1978; Bambauer and Bernotat, 1982) implies inferences must be made with caution. Conclusions

Investigations of Permo-Scythian sections of the Northern Calcareous Alps, Gurktal Nappe and Drau Range (Upper Austroalpine units, Eastern Alps) revealed an interesting distribution pattern of detrital and authigenic feldspars. Siliciclastic sediments of alluvial fans and playas are characterized by low feldspar content caused by extensive weathering and diagenetic dissolution of plagioclases and partial alteration of alkalifeldspars. The amount of volcanic rock fragments derived from intra-Permian ignimbrites and pyroclastics gradually decreases upward. Diagenetic precipitation processes are limited to authigenesis of clay minerals and small amounts of carbonate. This model is valid with the exception of late Permian evaporites in the middle and eastern realm of the Northern Calcareous Alps which possess small horizons of arkosic sandstones, rich in authigenic rim formation.

The Permian/Triassic boundary, recently documented to be of great influence on the sedimentary environment because of a sudden climatic change (Brandner et al., 1984, 1986), marks the Tethys-wide transgression of the shallow-marine Werfen Sea interfingering with Scythian continental red-beds (Alpine Buntsandstein Formation). When comparing North Alpine and Drau Range/Gurktal Nappe sections of the Werfen Formation, at first a clear-cut distinction can be made on grounds of different source rocks in the hinterland: the former are Ab-rich arkoses (probably derived from low-grade mica schists and phyllites), whereas alkali-feldspar arkoses characterize the latter (mainly stemming from granitic gneisses). The fact that albites of the Northern Calcareous Alps (Werfen Formation) are remarkably fresh and almost free of authigenic overgrowths is in contrast to albites of Werfen beds in the southern realm (Drau Range, Gurktal Nappe): there alkali-feldspar are slightly corroded and display well developed authigenic rims. The amount of diagenetically formed K-feldspars decreases rapidly to zero beyond the paleo-shoreline towards the hinterland. In summary, we conclude that the post-depositional history includes partial feldspar corrosion and dissolution of volcanic derived sanidines, early diagenetic illite formation and K-feldspar precipitation on detrital microclines, followed by subsequent syntaxial quartz overgrowths and carbonate cementation. The ions necessary for authigenesis of Kfeldspars in marine Scythian sediments of the Drau Range and Gurktal Nappe were released by destruction and dissolution of volcanic sanidines and rock fragments, plagioclases of diverse origin and biotites. Whilst these processes led to minor clay authigenesis in continental red-beds of Permo-Scythian age, interstitial pore waters of the Werfen Formation and the marginal marine Alpine Buntsandstein Formation were enriched in these ions during burial so that authigenic microcline (and quartz) could develop. Small-scale fluid transport from underlying Permian immature clastics into the overlying marine sediments, though not proved, might

76 account for an unknown minerals.

Fluid

carbonates

was

percentage of authigenic

supply probably

derived only

from of

Anisian

subordinate

i m p o r t a n c e , b e c a u s e o f t h e s e a l i n g s h a l e s a n d siltstones at their base and missing evidence for later intensive carbonate cementation in the underlying sandstones.

Acknowledgements T h e a u t h o r s a r e i n d e b t e d t o H. M o s t l e r ( U n i versity of Innsbruck) and G. Hoinkes (University of Graz) for their critical comments

and discus-

s i o n s . B. P e d e v i l l a is g r a t e f u l l y a c k n o w l e d g e d

for

h e r k i n d l y i m p r o v i n g t h e E n g l i s h . W e w o u l d like to thank A.W.

L.J.

Crook

Suttner

(Indiana

(University

viewed the manuscript

University)

of Canberra)

who

and re-

and made helpful sugges-

tions.

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