Talc-chlorite-amphibole felses of the KTB pilot hole, Oberpfalz, Bavaria: Protolith characteristics and phase relationships

Mineralogy and Petrology (1995) 52:25-59 Mineralogy a l l a

Petrology © Springer-Verlag 1995 Printed in Austria

Talc-chlorite-amphibole felses of the KTB pilot hole, Oberpfalz, Bavaria: protolith characteristics and phase relationships

S. Matthes% M. Okrusch ~, Ch. R6hr 2' 3 U. Schiissler 1, P. Richter a, and K. von Gehlen 2

1 Mineralogisches Institut, Universitfit Wiirzburg, 2 Institut ftir Geochemie, Petrologic und Lagerst/ittenkunde, Universit~it Frankfurt M., and 3 KTB-Feldlabor, Windischeschenbach, Federal Republic of Germany

With 13Figures

Received August 7, 1992; accepted November 5, 1993

Summary The metagabbro-amphibolite sequences in the KTB pilot hole contain intercalations of talc-chlorite-amphibole felses (or "h6sbachites"), which show transitional contacts to the adjacent metagabbros. The h6sbachites are characterized by relics of a primary igneous texture and still contain igneous minerals like clinopyroxene, biotite and pseudomorphs after olivine, while brown Ca-amphibole was presumably formed in a late-magmatic stage. The geological, textural, mineralogical and geochemical evidence indicates that the h6sbachites were derived from ultramafic cumulates, differentiated from a basaltic magma, either in the inner parts of dolerite sills or in small gabbro intrusions.

A pervasive metamorphic overprint under medium-pressure, amphibolite-facies conditions which was accompanied by penetrative deformation led to assemblages with

green Ca-amphibole + anthophyllite + cummingtonite

___ tremolite/actinolite + clinochlore + talc + olivine

+ ilmenite ___ Cr-bearing spinel + sulfides.

Phase relationships are consistent with a prograde P-T path leading to the formation of anthophyllite from olivine + talc at peak metamorphic temperatures of 640-700 °C, at assumed pressures of 8-10 kbar, similar to those derived from mineral assemblages in the adjacent metabasites and metasediments. High-pressure relics locally present in coronitic metagabbros and retrograded eclogites of the KTB pilot hole were not recognized in the h6sbachites. A retrograde overprint under greenschist-facies conditions led to the total replacement of igneous or metamorphic olivine by aggregates of

26 S. Matthes et al.

antigorite + magnetite, chloritization of biotite and the formation of late tremolite/ actinolite.

Zusammenfassung

Talk-Chlorit-Amphibol-Felse der K TB- Vorbohrun 9, Oberpfalz: Eduktcharakteristik und Phasenbeziehungen

Die Metagabbro-Amphibolit-Folge in der KTB-Vorbohrung enth/ilt Einschaltungen yon Talk-Chlorit-Amphibol-Felsen ("H6sbachite"), die graduelle Uberg/inge zu den benachbarten Metagabbros aufweisen. Die H6sbachite sind durch Relikte yon prim/iren magmatischen Gefiigen gekennzeichnet und fiihren noch magmatische Mineralrelikte wie Klinopyroxen, Biotit und Pseudomorphosen nach Olivin, wfihrend brauner Ca- Amphibol wahrscheinlich spfitmagmatisch gebildet wurde. Verbandsverh~ltnisse, Re- liktgefiige und Reliktminerale sowie Haupt- und Spurenelement-Geochemie sprechen dafiir, dab die H6sbachite auf ultramafische Kumulate zuriickgehen, die aus einem basischen Magma differenziert wurden, und zwar entweder im Innern von doleritischen Lagergfingen oder in kleinen Gabbro-Intrusionen.

Eine durchgreifende metamorphe Uberpr/igung unter Bedingungen der Mitteldruck- Amphibolitfazies, die yon einer penetrativen Deformation begleitet war, ffihrte zu Mine- ralparagenesen mit

griinem Ca-Amphibol + Anthophyllit _+ Cummingtonit _+ Tremolit/Aktinolith

+ Klinochlor + Talk + Olivin + Ilmenit _+ Cr-haltigem Spinell + Sulfiden.

Die Phasenbeziehungen weisen darauf hin, dab sich im Zuge eines prograden P-T- Pfades Anthophyllit aus der Paragenese Olivin + Talk bildete. Als P-T-Bedingungen beim H6hepunkt der Metamorphose k6nnen Temperaturen yon 640-700 °C in einem angenommenen Druckbereich von 8-10 kbar abgesch/itzt werden,/ihnlich wie sie auch aus den Mineralparagenesen in den angrenzenden Metabasiten und Metasedimenten der KTB-Vorbohrung ableitbar sind. Hochdruckrelikte, die gelegentlich in koroniti- schen Metagabbros und retrograd iiberpr/igten Eklogiten der KTB-Vorbohrung auftre- ten, wurden in den H6sbachiten nicht gefunden. Eine retrograde Uberpr/igung unter griinschieferfaziellen Bedingungen fiihrte zu einer vollstfindigen Verdr/ingung yon mag- matischem und metamorphem Olivin durch Aggregate von Antigorit + Magnetit, zur Chloritisierung yon Biotit und zur Bildung einer sp/iten Generation von Tremolit/ Aktinolith.

Introduction

The pilot hole of the Continental Deep Drilling Program of the Federal Republic of Germany (KTB), situated in the tectonic unit of Erbendorf-Vohenstrauss (Fig. 1), has penetrated small ultramafic bodies, intercalated within metagabbros. The main type is essentially composed of various amphiboles + Mg-chlorite + talc and shows a typical heteroblastic texture.

The same rock type has been described as "h6sbachites" from a number of crystalline complexes of the Variscides (Fig. 1), such as the Vorspessart (Matthes and Kriimer, 1955; Matthes and Okrusch, 1965; Matthes and Schubert, 1967; Nasir, 1986, 1990), the Bergstr~sser Odenwald (Schubert, 1969) and the B611stein Odenwald (Knauer et al., 1974). In the Erbendorf-Vohenstrauss Zone similar rocks occur (Voll, 1960; Schiissler, 1987), however, in places with contact-metamorphic overprint. Serpentinites are present in surface outcrops in the vicinity of the drill-site, but were not found in the KTB pilot hole.

Talc-chlorite-amphibole felses 27

Schiefer- Gebirge ]

~inwald

h . ~ / / ~" Kyffh~user @Y' C j

Spessart MQnchberg Klippe ~ ::i:[:... KT8 ::::~iii[::[! ':ii drilling site / / .

~ ~ -V ,....-i;i :iii!iliiii

/ b 5'0 km

ZE~,

I] 2 Fig. 1. Occurrences of h6sbachite (stars) in the crystalline complexes of the Odenwald, the Spessart and the Erbendorf-Vohenstrauss Zone (ZEV). Geology modified after Franke (1989b). 1 Variscan granites, largely post-tectonic. 2 Rhenohercynian Realm: Devonian and Carboniferous sediments and volcanites with Variscan metamorphism of very low grade. 3 Northern Phyllite Zone: Pre-Devonian and Devonian sediments and volcanites metamorphosed in Variscan times under greenschist-facies conditions. 4 Mid-German Crystalline Rise: Precambrian to Silurian sediments, volcanites and plutonites metamorphosed in Variscan times under amphibolite-facies conditions. Saxothuringian Realm: 5 Devonian to Carboniferous sediments and volcanites, affected by Variscan metamorphism of very low grade. 6 Precambrian to Silurian sediments, volcanites and plutonites, metamorphosed in Variscan time under greenschist to amphibolite-facies conditions, in part polymetamorphic. Moldanubian Realm: 7 Moldanubian s. str., mostly polymetamorphic gneisses and migmatites with rare eclogitic relics, last Variscan overprint under low-pressure, upper amphibolite-facies conditions about 325 Ma ago. 8 Units regarded as allochthonous (Miinchberg Gneiss Complex, ZEV, Tepla-Barrandian) mostly polymetamorphic mica- schists, gneisses and metabasites, in part with eclogite relics, last metamorphic overprint under medium-pressure, amphibolite-facies conditions about 380 Ma ago

Interestingly h6sbachite, presumably from the Spessart occurrence, was used to manufacture moulds for casting bronze axes and knifes by the prehistoric Urnfield Culture at various places in Franconia and Thuringia (Okrusch and Schubert, 1986).

It is the aim of the present paper to describe textural and geochemical evidence testifying to the protolith character of the talc-chlorite-hornblende felses


(h6sbachites) recorded in the KTB pilot hole and to reveal the phase relationships in these ultramafics in order to evaluate their metamorphic evolution. Our results will be compared with the P-T paths recently obtained from phase relationships in the associated metabasites (O'Brien et al., 1992) and metasediments (Reinhardt et al., 1989; Reinhardt, 1990).

Geological position

The KTB location is situated in the Zone of Erbendorf-Vohenstrauss (ZEV), a tectonic unit currently regarded as allochthonous (e.g. Weber and Vollbrecht, 1989; Franke, 1989a; Weber, 1992; see Fig. 1). It is dominated by interlayered paragneisses and metabasites with minor intercalations of meta-ultramafics, calc-silicate rocks, graphite quartzites, graphite schists and minor orthogneisses. This succession underwent a main event of penetrative deformation and medium-pressure, amphibolite facies metamorphism, radiometrically dated at around 380 Ma ago, i. e. in the Middle Devonian (e.g. Hansen et al., 1989; Kreuzer et al., 1989). However, the metamorphic evolution of the ZEV is clearly polyphase as demonstrated by relics ofeclogite and high-pressure granulite assemblages and various retrograde reaction textures (Voll, 1960; Busch, 1970; Bliimel, 1986; Schiissler, 1987, 1990; Okrusch et al., 1991; Kleemann, 1991; Patzak et al., 1991; Schalkwijk, 1991; O 'Brien et al., 1992).

The KTB pilot hole encountered a variegated succession of various amphibolites of tholeiitic composition, of metagabbros and of meta-sediments, represented mainly by kyanite/sillimanite-garnet-biotite gneisses. As shown in Fig. 2, the meta-

I nves t iga ted s a m p l e s

~ ^ ~ ^N^

~ ^~ 36o .o0 (E} - - ^ ~

N N

1329 .75 (01 ~ 1381 .49 (A-1t N N 1381 ,64 (A - l } 1382 .36 (O) 1384 .75 (A-3) A A ^ 1384 .80 (A-31 ~ A 1410 .80 (C ) ~ ~ 1449 .29 (A-2} ~ A 1449 .88 (A -2~ 1450.11 (A-21 N N 1583 ,65 (61 1583 .70 (El

N ~

2509 .64 (El - -A~_N~

la ~A

3246 .03 (C) - -

3716 ,78 (A -2 ) - - A ~ 3717 .20 (A -2 ) 3718 .60 (BI 3719 .511B) A A A .......... / o

depth (m )

0

,°° S

1160

1610

2470

2690

3

3575

4000

b i o - ( hb l . ) gneisses, '~ arnphibolite ±talc-silicate layers

met agabbr o-amphib olite r ~ succession

[ ~ mono tonous paragnelsses

[ ~ amphibolite

Fig. 2. Lithological section and struc- tural interpretation of the KTB pilot hole (after Kohl et al., 1989) with posi- tions of the investigated ultramafics


gabbros and related h6sbachites are concentrated in two successions at depths of 1160-1610 and 3575-4000 m. For a metagabbro at 1265 m, an Early Ordovician intrusion age of 494 + 3 Ma was recorded by the U - P b method (yon Quadt, 1990). On the other hand, paragneisses at 176, 679-946, 2522-2997 and 3532 m yielded spores, acritarchs and fusinites testifying to a considerably younger, Lower Devonian age, whereas a paragneiss at 469 m revealed acritarchs of Early Paleozoic, possibly Cambrian age (Pflu9 and Pr6ssl, 1991).

Based on critical mineral assemblages in the metabasites of the KTB pilot hole, the P-T conditions at the peak of the amphibolite-facies metamorphism were estimated at about 650-720 °C and 8-10 kbar (Patzak et al., 1991; Schalkwijk, 1991; O'Brien et al., 1992). From assemblages in the associated metasediments, Reinhardt (1990) deduced a prograde P-T path leading through the sillimanite into the kyanite stability field and culminating at 660-720 °C and 6-8 kbar, while the retrograde path led back into the stability field of sillimanite with P-T conditions of 650 °C/4 kbar (see also Reinhardt et al., 1989).

In the metagabbros of the KTB pilot hole, rare but clear indications of an earlier high-pressure event under eclogite-facies conditions were recognized: inclusions of omphacite (Jd26), kyanite and zoisite in garnet, symplectites of plagioclase + clinopyroxene after former omphacite and higher pyrope contents in garnets, as compared to the common garnet amphibolites. A subsequent granulite-facies overprint led to the symplectitic breakdown of omphacite and to an increase in the Mg/Fe ratios of garnets (Patzak et al., 1991; O'Brien et al., 1992). The same evidence was found in retrograded eclogite lenses, which were detected in the KTB pilot hole at a depth of 3875 m and the nearby VSP 1 hole (O'Brien et al., 1992).

Contact relationships and general petrography of the talc-chlorite-amphibole felses

The talc-chlorite-amphibole felses, here shortly designated as "h6sbachites", form layers and lenses up to 6 m thick, intercalated with metagabbros (Fig. 3) or, more rarely, amphibolites. The metagabbros are massive, medium to coarse-grained rocks with conspicuous relics of igneous, ophitic to subophitic textures (for details see Patzak, 1991; Schalkwijk, 1991; O'Brien et al., 1992). Contact relationships between the h6sbachites and the hosting metabasites are extremely variable. Both gradual transitions and sharp contacts with straight or curved borders occur (Fig. 3). In some places contacts are accentuated by leucocratic veins or are tectonically disturbed (Keyssner et al., 1988; Massalsky et al., 1988; yon Gehlen et al., 1989, 1990, 1991; Sigmund et al., 1990).

According to textural features and modal compositions (Table 1), several rock types can be distinguished. The position of investigated samples in the drill cores is indicated in Fig. 2.

Massive h6sbachite (type A-l)

This rock type forms the thickest ultramafic body in the borehole, penetrated at a depth of about 1380-1385 m; it is homogeneous over the total length of core (with a recovery of nearly 100%). The h6sbachite is a massive, blastomylonitic rock formed by porphyroclasts of brown Ca-amphibole, more rarely of clinopyroxene (3-10 mm in diameter), in a greyish-green, fine-grained patchy matrix (Fig. 4). A conspicuous


Fig. 3. Contact between h6sbachite and metagabbro in a depth of 1412 m

Fig. 4. Microphotograph of massive hSsbachite type A-l, sample 1382.36 m. Porphyroclast of brown hornblende (dark) with poikilitic inclusions of fine-grained serpentine + talc +_ chlorite (pseudomorph after olivine) in a matrix of pale-green amphibole + talc + chlorite + opaques. Plane polarized light

Talc-chlorite-amphibole felses

Table 1. Modal composition of hOsbachites from the KTB pilot hole (estimated vol.-%o)

31

Type A1 A2 B B C C D E Sample 1381.64 1449.29 3718.60 3719.51 1382.36 1410.80 1329.75 1583.65

Amphibole 57 83 45 50 53 42 75 85 Chlorite 25 9 12 30 19" 29* 10 11 Talc 11 3 15 5 20 25 - - - - Serpentine - - + 25 10 5 + - - - - Clinopyroxene 3 - - - - 2 . . . . Plagioclase . . . . . . 10 Apatite + + - - + + + + - - Calcite 1 - - 1 + + - - - - - - Rutile . . . . + + - - - - Titanite - - - - - - + - - - - - - + Epidote - - + + - - - - - - + 1 Prehnite . . . . . . . 1 Allanite + . . . . . . . Opaques 2 3 2 3 2 3 4 1

I

* including relics of biotite

feature, typical to all types of h6sbachite, are in tergrowths of innumerable , t iny ilmenite-platelets dust ing the pyroxenes and b rown Ca-amphiboles in a pa tchy m a n n e r (Fig. !4). The matr ix is composed of green, rarely b rown pargasit ic hornblende, colourless tremolite-actinoli te, chlorite and talc with subordina te magnet i te and calcite. Postcrystal l ine deformat ion affected both, the large amphibole crystals and the mat r ix minerals which are often bent or kinked.

Schistose hSsbachite (type A-2)

Thin h6sbachi te layers and lenses (of cent imeter size) within metagabbros (e.g. at abou t 1450 and 3717 m) are more t ho rough ly deformed. The amphibole porphyroclasts recrystallized into fine-grained, mosaic aggregates, while the matr ix minerals show a coarser grain size and a higher degree of a l ignment (Fig. 5). The amphiboles are green pargasit ic to edenitic hornblende, t remoli te-act inoli te and anthophyl l i te .

Cataclastic hOsbachite (type A-3 )

In late, cataclastic shear zones (a few centimeters thick) the h6sbachites underwent brittle de format ion with no signs of recrystal l izat ion (e.g. at abou t 1385 m). The breccias con ta in fragments of b rown Ca-amphibole in a braided, fine-grained matr ix of chlorite an d green hornblende.

HSsbachite rich in serpentine (type B)

In a 2 m thick body of textural ly and moda l ly heterogeneous h6sbachi te (3718-3720 m), por t ions character ized by str iking igneous textures occur (Fig. 6). Indented, poikilitic c l inopyroxenes conta in pseudomorphs after olivine, consist ing of anti-


Fig. 5. Microphotograph of schistose h6sbachite type A-2, sample 1449.29 m. Sheared porphyroclast of brown hornblende (upper left) is recrystallized to a fine-grained aggregate of hornblende and opaques. The matrix minerals hornblende, chlorite and the talc aggregates show subparallel alignment. Compared to the massive h6sbachite shown in Fig. 4, the grain size relation of the porphyroclast domain and the matrix is reversed. Plane polarized light

Fig. 6. Microphotograph of serpentine-rich h6sbachite type B, sample 3719.51 m. Porphyro- clast of brown hornblende (dark) with poikilitic inclusions of olivine pseudomorphs in a matrix consisting (i) of symplectitic intergrowths of pargasitic hornblende and clinochlor, and (ii) of serpentine + magnetite. Plane polarized light


gorite and a pale olive-green sheet silicate (see below). Porphyroclasts of brown or green Ca-amphibole are subordinate. The matrix contains a high amount of antigorite besides colourless to green pargasitic hornblende, chlorite, talc, magnetite and calcite.

H6sbachite with relics of biotite (type C)

In this relatively massive variety, recovered at depths of about 1382 and 1411 m, composite flakes of chlorite intergrown with a pale brown, biotite-like sheet silicate (see below) constitute up to 25 volvo of the matrix. They form also poikilitic inclusions in porphyroclasts of brown to pale-green pargasitic hornblende, which are overgrown by pale-green pargasitic hornblende or cummingtonite. Other matrix minerals are chlorite, talc, serpentine, pale-green pargasitic hornblende, and fine, lamellar intergrowths of cummingtonite and anthophyllite (Fig. 7).

At a depth of 3246 m, the only ultramafic inclusion not related to metabasites occurs as a small lens (10 cm x 4 cm in size) surrounded by biotite-plagioclase gneiss. The fine-grained, patchy, rock consists of green actinolite, in part dusted with

Fig. 7. Microphotograph of biotite-bearing h6sbachite type C, sample 1382.36 m. Lamellar intergrowths of anthophyllite (A) and cummingtonite (C) in assemblage with clinochlore (Chl) and Talc (To). Crossed polarizers


ilmenite platelets, cummingtonite, talc, Mg-biotite as well as minor epidote and titanite.

Plagioclase-bearin9 h6sbachite (type D)

This rock type, occurring at about 1330 and 3720 m, is transitional to the metagabbros as it contains granoblastic aggregates of plagioclase (An24-2s, partly sericitized). These fill the interstices between poikilitic porphyroclasts of brown, titanian pargasitic hornblende, which show the typical ilmenite dust. The fine- grained matrix consists of pale-green hornblende, tremolite-actinolite and talc.

Homeoblastic chlorite-amphibole felses and schists (type E)

This ultramafic rock type, which occurs as frequent interlayers, lenses and schlieren (a few centimeters in size) within the metagabbros (e.g. at about 350 m, 1584 m and 2510 m), is devoid of the characteristic textural features of h6sbachite, such as hornblende porphyroclasts. The homeoblastic, locally schistose rock consists of aggregates of prismatic, green magnesio-hornblende with margins of Fe-poor actinolite, which are surrounded by braided aggregates of chlorite. The cleavage planes of individual chlorite flakes are often decorated by tiny grains of epidote and titanite, and by fine-grained aggregates of prehnite. Chlorite-amphibole felses with minor plagioclase and biotite are transitional to the adjacent metagabbros.

Late fractures crosscutting the chlorite-amphibole layers and the metagabbros are often filled with prehnite.

React ion textures and mineral chemis try

Minerals were analyzed using the CAMECA SX 50 microprobes at the Mineralo- gical Institute, University of Wfirzburg and the Bayerisches Geoinstitut, University of Bayreuth. Instrument conditions were 15 kV and about 10 nA sample current. Pure elements, oxides and silicates were used for reference. Correction procedures were carried out using the CAMECA PAP program.

Pseudomorphs after olivine

Although olivine was not detected in the investigated h6sbachites, oval-shaped poikilitic inclusions, pseudomorph after primary olivine, are often recorded. They are formed by aggregates of

- - antigorite + alumo-serpentine + diabantite (in h6sbachite type B), - - talc and/or serpentine (in type C), - - talc, in part rimmed by serpentine (in type A-l), - - Mg-chlorite _+ tremolite _+ anthophyllite _+ calcite (in type A-l).

The presence of former olivine is also indicated by high amounts of antigorite together with talc and tiny magnetite grains in the matrix of type B. The same holds true for type A-1, where the matrix contains patchy aggregates of Mg-chlorite with a fine network of magnetite testifying to former mesh-textured serpentine derived from olivine.


Clinopyroxene

Clinopyroxene is interpreted as an igneous relic. Its occurrence is restricted to the massive (type A-l) and the serpentine-rich h6sbachites (type B) where it forms porphyroclasts or, more frequently, the inner parts of porphyroclastic brown hornblende. Besides the typical ilmenite dust, clinopyroxene in the serpentine-rich h6sbachite (type B) may contain minute grains of exsolved Cr-spinel (1/~m).

Microprobe analyses of pyroxenes from two different samples (Table 2) yielded a chromian diopside to augite with 87-95 mol.~o of quadrilateral pyroxene components. With respect to the presumed ultramafic protolith, the high Cr contents of the clinopyroxene relics are noteworthy. Similar chromian augites have been reported from a Bushveld gabbro (Hess, 1949), the Takashima, Japan, chrome peridotite (Ishibashi, 1970) and from peridotites related to the Frankenstein gabbro, Odenwald, Germany (Kreher, 1992). The Ti contents of the pyroxenes, generally ranging from 0.3 to 0.6 wt.-~ TiO2, may have been initially higher, prior to the unmixing of the ilmenite platelets during metamorphic overprint. The highest TiO2

Table 2. Chemical composition of selected clinopyroxenes from h6sbachites

type A-I B

sample 1381.49 3719.51

code 1 2 3 4

sio 2 53 TiO 2 0 AI203 1 Cr203 0 MgO 16 CaO 23 Mno 0 FeD* 4 Na20 0 K20 0 Total 99

47 52.55 52.59 52

26 0.36 0.60 0

01 1.27 3.35 2

37 0.42 0.61 0

06 16.26 15.63 16

64 23.65 21.93 22

16 0.ii 0.19 0

i0 4.60 3.97 3

48 0.51 1.08 0

00 0.01 0.00 0

55 99.74 99.95 99

97 52.72

53 0.39

91 1.99

48 0.34

i0 15.20

20 22.38

15 0.15

69 5.14

92 0.66

01 0.01 96 98.98

Si 1.964

Ti 0.007

A1 0.044

Cr 0.011 Fe 3+ 0.038

Mg 0.879

Ca 0.930

Mn 0.O05 F e 2 + 0.088

Na 0.034

K 0.000

Total 4.000

1 926

0 010

0 055

0 012

0 099

0 888

0 929

0 003

0 042

0 036

0 000

4 000

1.915 1.927 1.953

0.016 0.015 0.011

0.144 0.125 0.087

0.018 0.014 0.010

0.052 0.044 0.023

0.848 0.873 0.839

0.856 0.865 0.888

0.006 0.005 0.005

0.069 0.068 0.136

0.076 0.065 0.047

0.000 0.000 0.001

4.000 4.001 4.000

XM. ~ 0.91 0.95 0.92 0.93 0.86

* total Fe = FeO; ** XMg = Mg/(Mg + Fe2+); cations were calculated by charge balance (Neumann, 1976). 1-4 porphyroclasts; 5 clinopyroxene intergrown with edenitic hornblende, at the rim of amphibole-serpentine symplectite


content, which was found in the unexsolved part of a clouded clinopyroxene is 3.5 wt.-%.

Amphiboles

The amphiboles in the various h6sbachite types display a striking textural and optical variability and cover a wide compositional range (Table 3a, b, c, Fig. 8a, b, c).

The porphyroclasts of brown Ca-amphibole, often surrounding igneous clinopyroxenes and displaying the typical ilmenite dust, are interpreted as relics of late igneous origin. According to the IMA nomenclature (Leake, 1978), their chemical composition varies in the range of magnesio-hastingsite, pargasite and pargasitic hornblende (Table 3a- # 4, 5, 3b- # 4, 3c- # 7, 8; Fig. 8b). The brown Ca-amphiboles are characterized by relatively high TiO2 contents (average 1.3, maximum 3.2 wt.~) which, of course, would be increased by re-integrating the exsolved ilmenite platelets. By contrast, green cores of zoned amphibole porphyroclasts, ranging in composition from pargasite to magnesio-hornblende, exhibit lower Ti contents of <0.5 wt.-~ (e.g. Table 3b- # 2a).

Due to the metamorphic overprint, the porphyroclasts of clinopyroxene and brown amphibole are partially replaced by light green, pargasitic hornblende (Table 3a-# 6) which, in turn, grades into colourless tremolite-actinolite. In some cases, amphibole porphyroclasts are overgrown by sheaf-like aggregates of anthophyllite +_ tremolite (in type A-l) or by cummingtonite + pargasitic hornblende (in type B). Replacement textures are best developed where the porphyroclasts are strongly deformed.

In the patchy matrix of the h6sbachites, various amphiboles (partly replaced by talc) occur together with criss-cross oriented Mg-chlorite, scaly aggregates of talc and patches of calcite. The Ca-amphibole composition is highly variable (Fig. 8a, b), even within single grains, ranging from pargasite and magnesio-hastingsite (Table 3b-# 3), via pargasitic (Table 3a-#7, 8), magnesio-hastingsitic, edenitic, tschermakitic and magnesio-hornblende to tremolite-actinolite (Table 3a-# 2, 3; 3b-# 5, 6). Most of these matrix amphiboles are green to light-green and display generally lower TiO2 contents (average 0.73 wt.~o) than the rare brown matrix amphiboles. The colourless rims of zoned hornblendes vary from magnesio- hornblende to tremolite.

In the course of the prograde metamorphic evolution, the deformed pale-green tschermakitic to magnesio-hornblendes of the plagioclase-bearing h6sbachite sample 3719.93 react with ilmenite to form green pargasitic hornblende (Fig. 8a, b).

In the serpentine-rich h6sbachite (type B, sample 3719.51), amphiboles partici- pate in the following reaction textures:

- - coronas of radiating actinolite (Table 3a-# 3) surround aggregates of serpentine + talc + magnetite,

- - symplectites of amphibole [titanian tschermakite (Table 3a- # 1) to pargasitic hornblende] + serpentine + magnetite, with a reaction rim of tremolite (Table 3a-# 2) between hornblende and serpentine.

Anthophyllite analyzed in several samples reveals XMg values of 0.67 -0.81, up to 0.18 Ca and up to 0.33 A1 p.f.u. (Table 3b-# 1; Fig. 8c). Cummingtonite recognized

Talc-chlorite-amphibole felses

Table 3a. Chemical composition of selected amphiboles from 3719.51 (type B)

37

serpentine-rich hfsbachite

code 1 2 3 4 5 6 7 8

SiO 2 43.38 57.63 57.53

TiO 2 3.16 0.03 0.00

AI203 12.22 0.07 0.17

Fe203 1.51 0.00 0.00 Cr203 1.13 0.13 0.00 MgO 16.10 22.17 21.59

CaO 11.61 12.15 11.83

MnO 0.04 0.19 0.21

FeO 4.83 4.20 5.57

Na20 2.37 0.02 0.00

K20 0.25 0.00 0.00 Total 96.60 96.59 96.90

si 6.270 8.017 8.017 A1 1.730 0.000 0.000

Total 8.000 8.017 8.017

A1 0.351 0.011 0.028

cr 0.129 0.014 0.000 Fe 3+ 0.165 0.000 0.000

Ti 0.344 0.003 0.000

Mg 3.469 4.597 4.485

Fe 2+ 0.542 0.374 0.487

Mn 0.000 0.000 0.000

Total 5.000 5.000 5.000

Mg 0.000 0.000 0.000

Fe 2+ 0.041 0.115 0.162

Mn 0.005 0.022 0.025

Ca 1.798 1.811 1.766

Na 0.156 0.005 0.000

Total 2.000 1.954 1.953

Na 0.508 0.000 0.000

K 0.046 0.000 0.000

Total 0.554 0.000 0.000

Total 15.554 14.970 14.969

X~ 0.856 0.904 0.874

43

2

ii

0 1

15 93

12 94

0 08

6 19

3.33

0.ii

97.34

83 43.64 43.57 45.17 43.20

53 2.49 0.38 0.34 0.09

39 12.18 15.90 13.55 16.29

00 0.00 3.26 3.57 4.01

01 0.91 0.09 0.07 0.05 16.19 16.41 17.32 16.94

12.07 10.70 10.17 9.72

0.08 0.08 0.08 0.08

6.40 2.93 3.21 3.61

3.39 3.25 2.97 2.91

0.04 0.15 0.i0 0.09

97.34 96.67 96.50 96.94

6.342 6.297 6.215 6.441 6.152

1.658 1.703 1.785 1.559 1.848

8.000 8.000 8.000 8.000 8.000

0.284 0.368 0.887 0.719 0.886

0.116 0.104 0.010 0.008 0.006

0.000 0.000 0.349 0.383 0.430

0.275 0.270 0.041 0.036 0.010 3.436 3.483 3.489 3.682 3.596

0.749 0.772 0.223 0.172 0.073

0.010 0.003 0.000 0.000 0.000

4.870 5.000 5.000 5.000 5.000

0.000 0.000 0.000 0.000 0.000

0.000 0.000 0.127 0.211 0.357

0.000 0.001 0.004 0.004 0.004

2.006 1.866 1.635 1.554 1.483

0.000 0.133 0.235 0.232 0.156

2.006 2.000 2.000 2.000 2.000

0.934 0.816 0.664 0.590 0.647

0.020 0.007 0.027 0.018 0.016

0.955 0.823 0.691 0.608 0.664

15.831 15.823 15.691 15.608 15.664

0.821 0.818 0.909 0.906 0.893

* Xug --- Mg/(Mg + Fe2+), cations were calculated for O = 23; Fe a+ was estimated by the midpoint method (Papike et al., 1974) for calcic amphiboles and by normalization to 15 excluding Na and K (Leake, 1978) for Mg-Fe amphiboles, using the AMPHIBOL-program of Franz and Hiiussinger (1990). 1 tschermakite from amphibole-serpentine symplectite 2 tremolite from amphibole-serpentine-symplectite, rim around 1 3 actinolite from rim of magnetite-talc-serpentine aggregate 4 pargasitic hornblende lamellae in clinopyroxene porphyroclast 5 brown pargasitic hornblende at the rim of clinopyroxene porphyroclast 6 light pargasitic hornblende displacing clinopyroxene porphyroclast 7 larger light green pargasitic hornblende from the matrix 8 light green pargasite intimately intergrown with chlorite

in the biotite-bearing h6sbachite 3246.03 yielded XMg = 0.74, 0.03 A1 and 0.13 Ca p.f.u. (Table 3b-# 7; Fig. 8c).


Table 3b. Chemical composition of selected amphiboles from h6sbachite 1381.49 and talc- biotite-amphibole fels 3246.03

sample 1381.49 (type A-I) 3246.03 (type C)

code 1 2a 2b 3 4 5 6 7

SiO 2

TiO 2

AI203 Fe203 Cr203 MgO CaO MnO FeO Na20 K20 Total

55

0

0

0

0

22 0

0 34

16 94

0 i0

0 00

98 01

97 43.04 55.90 42.05 42.66

05 0.46 0.00 1.44 0.96

42 14.73 0.43 13.27 13.69

68 3.48 0.00 4.48 4.38

08 0.i0 0.06 0.13 0.39

76 14.26 22.55 14.86 16.30 67 11.20 0.64 11.09 11.34

0.14 0.45 0.07 0.07 7.32 17.28 6.05 3.95

3.24 0.12 2.82 2.89

0.12 0.01 0.18 0.35

98.09 97.44 96.45 96.98

Si 7.907

A1 0.070 Cr 0.009 Fe 3+ 0.014

Total 8.000

A1 0.000

Cr 0.000 Fe 3+ 0.058

Ti O.005

Mg 4.793 Fe 2+ 0.143

Mn 0.000

Total 5.000

57 25

0 01

0 29

0 00

0 ii

20 71

9.24

0.05

9.23

0.05

0.00

96.94

56.17 56.93

0.04 0.00

0.97 0.16

1.56 0.00

0.01 0.01

19.84 23.79 12.97 0.87

0.20 0.42 6.40 14.90

0.07 0.01

0 , 0 0 0 . 0 2 98°23 97.11

6.203 7.941 6.162 6.163 8.042 7.832 8.030

1.797 0.059 1.838 1.837 0.000 0.159 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.000

0.000 0,000 0.000 0.000 0.000 0.007 0.000

8.000 8,000 8.000 8.000 8.042 8.000 8.030

0.706 0,013 0.454 0.494 0.048 0.000

0.011 0,007 0.015 0.045 0.012 0.000

0.378 0.000 0.495 0.477 0.000 0.156

0.050 0,000 0.159 0.104 0.001 0.004

3.064 4.776 3.246 3.510 4.337 4,124

0.791 0.204 0.631 0.371 0.602 0.715

0.000 0.000 0.000 0.000 0.000 0.000

5.000 5.000 5.000 5.000 5.000 5.000

0.000 0.000

0.iii 0.106

0.009 0.009

1.741 1.755

0.139 0.130

2.000 2.000

Mg 0.000 0.000 Fe 2+ 1.858 0.090

Mm 0.041 0.017

Ca 0.i01 1.730

Na 0.000 0.163

Total 2.000 2.000

0 000

0 482

0 006

1 391 0 014

1 892

0 000

1 848

0 054

0 097

0 000

2 000

0 027

0 001

0 000

0 000

4 972

0 000

0 000

5 000

0.000 0.030

0.031 1.758

0.024 0.050

1.938 0.131

0.008 0.003

2.000 1.972

Na 0.027 0.743 0.033 0.662 0.679 0.000 0.011 0.000

K 0.000 0.022 0.002 0.034 0.065 0.000 0.000 0.004

Total 0.027 0.765 0.035 0.696 0.744 0.000 0.011 0.004

Total 15.027 15.765 15.035 15.696 15.744 14.934 15.011 15.006

XMg 0.706 0.777 0.699 0.814 0.880 0.800 0.847 0.740

For calculations see Table 3a. 1 anthophyllite-needle from the matrix 2a magnesio- hastingsite, core composition 2b anthophyllite, rim composition 3 brown magnesio- hastingsite from the matrix 4 nearly opaque magnesio-hastingsite 5 actinolite-needle from the matrix 6 actinolite-needle 7 cummingtonite-needle

Fine-grained, lamellar intergrowths of anthophyllite and cummingtonite (Fig. 7) were microscopically recorded in the biotite-bearing h6sbachite (type C, sample 1382.36) and confirmed by high-resolution X-ray diffraction. Microprobe analyses of several anthophyllite/cummingtonite intergrowths yielded no systematic compositional differences between the two F e - M g amphibole species, except for minor differences in Ca (Table 3c cf. # 1-3 and # 4-6; Fig. 8b). The following variations were recorded:


Table 3c. Chemical composition of selected amphiboles from hfsbachite 1382.36 (type C)

code 1 2 3 4 5 6 7 8

SiO 2 56.82

TiO 2 0.01

AI203 0.80

Fe203 0.00 Cr203 0.00 MgO 25.04

CaO 0.46

MnO 0.35

FeO 14.35

Na20 0.23

K20 0.00 Total 98.07

56

0

0

0

0

23

0 0 40

15 94

0 i0 0 07

97 56

39 56.13 56.99 56.72 56.44 42.55 43.78

01 0.02 0.00 0.04 0.04 0.52 0.60

34 0.17 0.31 0.55 0.64 12.73 13.07

09 2.19 0.17 0.00 0.41 6.45 4.36

00 0.01 0.02 0.03 0.00 0.24 0.09

84 25.00 24.50 24.36 24.57 16.62 15.98

37 0.43 0.57 0.63 0.70 11.54 11.75

0.49 0.43 0.33 0.36 0.i0 0.17

13.21 14.98 14.88 14.42 3.06 5.31

0.09 0.ii 0.17 0.22 2.96 2.95

0.00 0.00 0.02 0.02 0.23 0.17

97.75 98.08 97.73 97.80 96.99 98.23

si 7.899 7.945

A1 0.101 0.055

Cr 0.000 0.000 Fe 3+ 0.000 0.000

Total 8.000 8.000

0 030

0 000

0 000

0 001

4 968

0 000

0 000

5 000

0 222

1 668

0 041

0 069

0 000

2 000

A1 Cr Fe 3+ Ti

Mg Fe 2+ Mn Total

0.001 0.000

0.009

0.001 4.989

0.000

0.000

5.000

0.019

1.878

0.048

0.056

0.000

2.000

Mg Fe 2+ Mn Ca Na Total

7.854 7.950 7.942 7.892 6.165 6.271

0.028 0.050 0.058 0.105 1.835 1.729

0.001 0.000 0.000 0.000 0.000 0.000

0.117 0.000 0.000 0.002 0.000 0.000

8.000 8.000 8.000 8.000 8.000 8.000

0 000

0 000

0 114 0 002

4 884

0 000

0 000

5 000

0 331

1 545

0 059

0 065

0 000

2 000

0.001 0.032 0.000 0.339 0.478

0.002 0.003 0.000 0.027 0.011

0.018 0.000 0.041 0.703 0.470 0.000 0.004 0.004 0.057 0.064

4.979 4.961 4.955 3.590 3.411

0.000 0.000 0.000 0.285 0.565

0.000 0.000 0.000 0.000 0.000

5.000 5.000 5.000 5.000 5.000

0.116 0.123 0.167 0.000 0.000

1.748 1.743 1.686 0.085 0.071 0.051 0.039 0.042 0.012 0.020

0.085 0.095 0.105 1.792 1.803

0.000 0.000 0.000 0.Iii 0.105

2.000 2.000 2.000 2.000 2.000

Na 0.063 0.028 0.025 0.030 0.046 0.059 0.722 0.715

K 0.000 0.013 0.000 0.000 0.004 0.003 0.042 0.031

Total 0.063 0.030 0.026 0.031 0.050 0.062 0.764 0.746

Total 15.063 15.030 15.026 15.031 15.050 15.062 15.764 15.746

XMg 0.76 0.73 0.77 0.74 0.74 0.75 0.91 0.84

For calculations see Table 3a. 1-3 anthophyUite-needles from the matrix, intergrown with 4-6 cummingtonite-needles from the matrix 7, 8 pargasitic to magnesio-hastingsitic hornblende porphyroclasts

anthophyllite (18 analyses)

A1 0.11 (0.02-0.19) Ca 0.06 (0.06-0.07) XMg 0.76 (0.72--0.80)

cummingtonite (15 analyses)

0.08 (0.03-0.24) 0.12 (0.08-0.23) 0.75 (0.72-0.77)

Cummingtonite (7 analyses) replacing light-green magnesiohastingsitic hornblende in sample 1382.36 rn revealed 0.06-0.37 A1, 0.08-0.41 Ca and XMg 0.74--0.82.


o I T r e m o l i t e g

o.9

~ 0.7 Actinolite

} a) 0.5 i

8.00 7.75

t r e m o l i t i c H b .

x

mt inol i t ic Hbl.

~x

7.50 7.25

I

X

hA I • • tschermakO I HbL A ×

x zxz~ x X v

M g - Hbl .

I I

7.00 6.75

Si p. f. u.

I

T s c h e r m a k i t e

D 1381.64/A-1 1382,36/C

- I - 1410.80/C /k 1450,11 /A -2 "~ 1449,88/A- 2 • 1583,65/E • 3246,03/C O 3719.51 /B X 3719,93/D

I

6 .50 6 .25 6 .00 5 .75

0.9 cq

LL + O) 0.8

O)

0.7

I _ I I

1 3 8 2 . 3 6 / 0

V A n t h o p h y [ l i t e

• C u m r n i n g t o n i t e

C u m i n . r e p l a c i n g h o r n b l e n d e

V V ~¢v'+ []

~D eDA

S i p . f. u.

I m I I [

8.0 7.9 7.8 7.7 7.6 6 .75 6 .50 6 .25 6 .00 5 .75

0.9

0.7

0.5

I e d e n i t i c ~ r g a s i t . P a r g a s i t e

I Hbl. ~/~i. #1 o~ <> o * * , ~ " 5 ~ , ~ u _

t~ [ ] 1% [] u L ~ x X I x [ ]

b)

Si p . f. u.

Fig. 8. Classification of the analyzed amphiboles after Leake (1978). a Ca-amphiboles with (Na + K)A < 0.5. b Ca-amphiboles with (Na + K)A > 0.5 and Ti < 0.5. e Fe-Mg amphiboles. Rock types and sample numbers are listed in the boxes. For explanation see text

In the replaced Ca-amphiboles (9 analyses), the Mg/(Mg + Fe t°t) ratios display an extremely small variation of 0.76-0.78 while, presumably due to the uncertain Fe3+/Fe 2+ estimate, XMg covers a wider range from 0.85 to 0.91.

Biotite and biotite/chlorite intergrowths

The h6sbachite type C is defined by the presence of composite flakes of sheet silicates, forming inclusions in porphyroclasts of Ca-amphibole or constituents of the matrix. One of the sheet silicate phases is chlorite (Table 5- # 6), the other resembles biotite: absorption in Y, Z ranges from colourless to pale brown, birefringence is up to 0.03, optical character (-). However, only in the biotite-talc-amphibole fels (sample 3246.03), intercalated within gneisses, a proper biotite composition was obtained with the microprobe, namely an A1, Fe-poor meroxene (Table 4). In all other samples, the brownish parts of the composite "biotite"-chlorite flakes yielded a chlorite composition (Fig. 9), however, with up to 4.73 wt.-% K z O and up to 0.44 wt.-% Na20 (Table 5-#4, 5, 7). We therefore assume that the brown parts are not homogeneous, but represent submicroscopic intergrowths with variable propor- tions of biotite and chlorite. The intergrowths also contain sagenite-type exsolutions of futile indicating that they are derived from primary Mg-Ti-rich biotite, probably of igneous origin.


Table 4. Chemical composition of bio- tires from biotite-talc-amphibole fels at 3246.03 m (type C)

SiO 2 39.11 TiO 2 1.23

AI203 14.70 CE203 0.40 MgO 20.01 CaO 0.00 MnO 0.04 FeO* 9.21 BaO 0.00 Na20 0.07 K20 9.67 Total 94.44

39 i0 1 13

14 76 0 ii

19 89 0 01 0 07

i0 51

0.04 0.08 9.32

95.02

38.36 i.ii

15.16 0.07

18.52 0.06 0.17

13.36 0.03

0.02 8.16

95.02

Si 5.723 5.709 5.645 A1 2.277 2.291 2.355 Total 8.000 8.000 8.000

A1 0.259 0.249 0.274 Cr 0.046 0.013 0.008

Ti 0.135 0.124 0.123 Ye 1.127 1.283 1.644 Mm 0.005 0.009 0.021 Mg 4.365 4.329 4.063 Total 5.938 6.007 6.133

Ba 0.000 0.002 0.002 Ca 0.000 0.002 0.009 Na 0.020 0.023 0.006 K 1.805 1.736 1.532 Total 1.825 1.763 1.549

Total 15.763 15.770 15.682

XMg 0.79 0.77 0.71

* total F e = F e O ; ** XMg=Mg/ (Mg + Fe); cations were calculated for O = 22

0.5

+ + 0 . 3

I J_ v &

u. 0.1

K 0

i, 0~1

lii!inl J!i! 0.2

J ! o - ] M g - .

0.3 0.4 0.5

AI[4] / (AI [4] + Si)

Fig. 9. Classification of the analyzed chlorites from various h6sbachites (open circles) and from the biotite-bearing h6sbachite sample 1382.36 (open diamonds) using the nomenclature of Trochim (in Tr6ger, 1982). Chemical composition of sheet silicates derived from igneous biotite (sample 1410.80): Filled squares: >4.5 wt.~o K20, filled triangles: 2.0-0.7 wt.% K20, filled circle: <0.5 wt.% K20


Table 5. Chemical compositions of selected chlorites

type A-I B

sample 1381.49 3719.51

code 1 2 3 4

C

1410.80

5

C

3246.03

7

SiO 2 28.73 35.36 30.99 30.40

TiO 2 0.04 0.02 0.00 0.41

A1203 19.39 12.38 18.18 17.65

Fe203 0.64 0.00 0.00 0.00 MgO 27.25 20.22 29.26 21.94

CaO 0.07 0.54 0.09 0.21

MnO 0.04 0.i0 0.12 0.16

FeO 10.05 19.28 7.87 15.39

Na20 0.04 0.00 0.01 0.07

K20 0.06 0.02 0.02 1.46

Total 86.31 87.92 86.54 87.69

27.62

0.12

20.62

1 18

25 85

0 00

0 05

ii 28

0 00

0 47

87 19

29.55 29.02

0.14 0.12

20.52 19.17

0.00 0.00

27.19 23.53

0.06 0.ii

0.00 0.21

9.82 13.76

O.O5 O.07

0.03 0.64

87.36 86.63

Si 5.685 7.093 6.003 6.099 5.482 5.727 5.826

Al 2.315 0.907 1.997 1.901 2.518 2.273 2.174 Total 8.000 8.000 8.000 8.000 8.000 8.000 8.000

A1 2.207 2.020 2..154 2.273 2.306 2.415 2.362

Fe 3+ 0.096 0.000 0.000 0.000 0.177 0.000 0.000

Ti 0.006 0.003 0.000 0.062 0.018 0.020 0.018

Fe 2+ 1.673 3.234 1.275 2.582 1.889 1.592 2.310

Mm 0.007 0.017 0.020 0.027 0.008 0.000 0.036

Mg 8.039 6.047 8.450 6.562 7.649 7.856 7.042

Na 0.015 0.000 0.004 0.027 0.000 0.019 0.027

K 0.015 0.005 0.005 0.374 0.119 0.007 0.164

Total 12.058 11.327 11.907 11.907 12.166 11.909 11.960

Total 20.058 19.327 19.907 19.907 20.166 19.909 19.960

XM~ ~ 0.83 0.65 0.87 0.72 0.80 0.83 0.75

* XMg = Mg/(Mg + Fe2+); cations were calculated for O = 28 with Fe 3+ estimated after Laird and Albee (1981). 1 common chlorite 2 grass green phyllosilicate pseudomorphing olivine, poikilitic inclusion in clinopyroxene porphyroclast (very low AI iv) 3 intimately intergrown with pargasite (dominating part of the matrix) 4 brownish chlorite, alteration product of biotite 5 colourless chlorite from the rim of brownish chlorite 6 large chlorite 7 chlorite, alteration product of biotite

Chlorite

Chlorites, forming important constituents of the matrix and in one type of olivine pseudomorphs, are colourless to pale green in thin section and optically (+ ) with normal interference colours. Microprobe analyses yielded mainly clinochlore to Mg-pycnochlorite compositions (Table 5 -#1 , 3, 6) using the nomenclature of Trochim (in Tr69er, 1982; see Fig. 9). Clinochlore in the serpentine-rich h6sbachite sample 3719.51 revealed 0.14 wt .~ NiO ( # 3), while a light-olive green diabantite, forming part of olivine pseudomorphs in the same sample, contains even 0.33 wt.~o NiO and 0.54 wt.~o Cr203 ( # 2 )

The colourless parts of the composite chlorite-"biotite" flakes in type C are chlorites of Mg-rhipidolite composition (Table 5 - # 5; Fig. 9).


Table 6. Chemical compositions of talc from hfsbachite sample 3719.51 (type B)

code 1 2 3

SiO 2 59.44

A1203 1.85 MgO 29.33

MnO 0.03

FeO* 4.12

Total 94.77

56 15

3 08

29 52

0 02

4 08

92 85

57.50

2.07

29.43

0.ii

4.23

93.34

Si 7.720 7.475 7.610

A1 0.280 0.483 0.323

Total 8.000 7.958 7.933

A1 0.003 0.000 0.000

Fe 0.447 0.454 0.468

Mm 0.003 0.002 0.012

Mg 5.679 5.859 5.807

Total 6.133 6.315 6.287

Total 14.133 14.273 14.220

.e XMg 0.93 0.93 0.93

* total Fe = FeO; ** Xug = Mg/(Mg + Fe); Cations were calculated for O = 22 1 from amphibole-chlorite matrix 2, 3 from magnetite-tremolite-talc-serpentine aggregate

Talc

Talc is a common matrix phase in all h6sbachites and is a frequent constituent of various symplectites pseudomorphing olivine. Moreover, talc may occur, together with minor chlorite, as replacement product of brown and green amphiboles. Microprobe analyses of matrix talc in the serpentine-rich h6sbachite (type B, sample 3719.5) revealed elevated A1 and Fe contents (Table 6) and up to 0.2 wt. ~ NiO.

Serpentines

Microprobe analyses of antigorites from sample 3719.51 (Table 7 - # 4 - 8 ) yielded non-ideal mineral compositions: Based on 14 anhydrous oxygen atoms, Si is consistently >4, whereas the sum of octahedral cations is always <6. The Fet°t/(Fe t°t + Mg) ratios of 0.08-0.11 are at the upper end of the range known from common serpentines (cf. Deer et al., 1962; Page, 1968; Whittacker and Wicks, 1970; Peacock, 1987). The A1 contents are variable and may reach 0.2 p.f.u.

Much higher A1 contents of 0.8-1.0 p.f.u, were recorded in a pale olive-green alumo-serpentine with relatively high birefringence, forming a constituent of olivine pseudomorphs (Table 7 -# 1-3). One of the analysed flakes ( # 1) revealed 0.3 wt.~ Cr 2 0 3 and 0.2 wt.~ NiO.


Table 7. Chemical composition of serpentines from h6sbachite sample 3719.51 (type B)

code 1 2 3 4 5 6 7 8

sio 2 AI203 MgO CaO MnO FeO* Total

si

A1 Mg Ca Mn Fe Total

XMg**

44

8

23

0

0

14 90

70 41.71 46

42 8.90 7

47 22.85 29

28 0.50 0

07 0.i0 0

Ol 14.62 5

95 88.68 90

92 44.27 48.23 44.38 45.05 45.60 98 0.59 0.77 O.ll 1.77 0.44

83 36.06 34.45 37.34 34.37 35.76 04 0.02 0.04 0.06 0.09 0.04

09 0.27 0.ii 0.12 0.17 0.23

63 7.07 5.40 5.16 7.27 6.21 49 88.28 89.00 87.17 88.72 88.28

4.135 3.998 4.177 4.146 4.389 4.167

0.918 1.005 0.837 0.065 0.083 0.012

3.236 3.265 3.959 5.034 4.674 5.227

0.028 0.051 0.004 0.002 0.004 0.006 0.005 0°008 0.007 0.021 0.008 0.010

1.084 1.172 0.419 0.554 0.411 0.405

9.406 9.499 9.404 9.822 9.569 9.827

4 183

0 194 4 757

0 009

0 013

0 564

9 720

4 236

0 048

4 952

0 004 0 018

0 482

9 740

0.75 0.74 0.90 0.90 0.92 0.93 0.89 0.91

* total Fe = FeO; ** XMg = Mg/(Mg + Fe); cations were calculated for O = 14. 1 green alumo-serpentine pseudomorphing olivine in clinopyroxene porphyroclast 2 green alumo- serpentine pseudomorphing olivine 3 alumo-serpentine from tremolite-magnetite-talc-serpentine aggregate 4 antigorite pseudomorphing olivine in clinopyroxene porphyroclast 5 antigorite from the amphibole-serpentine symplectite 6 antigorite surrounding green alumo- serpentine (# 2 see above) 7, 8 antigorite from tremolite-magnetite-talc-serpentine aggregate

Accessory minerals

In the various textural and modal varieties of h6sbachites and the homeoblastic chlorite-hornblende felses the following accessories were recognized: apatite, ilmenite, magnetite, Cr-bearing spinel, rutile, titanite, epidote, allanite, prehnite, pyrrhotite (in part Ni-bearing), chalcopyrite, pentlandite, cobalt-pentlandite, cobaltite and rare graphite (Keyssner et al., 1988 and our results).

Microprobe analyses on accessory minerals were carried out in the serpentine- rich h6sbachite sample 3719.51. The tiny magnetite grains forming mesh-shaped aggregates in olivine pseudomorphs are virtually pure (Table 8- # 2); the same holds true for coarser magnetite crystals in the matrix ( # 1), which are intergrown with graphite (Keyssner, pets. comm.). The sample contains a number of zoned spinel grains, about 50 #m in size. Since these are not included in clinopyroxene and amphibole porphyroclasts, they are possibly metamorphic phases rather than igneous relics. The cores of these spinels are translucent, brown chrome-pleonaste, surrounded by a zone of chromian magnetite which, in turn, is rimmed by nearly pure magnetite (Table 8 -# 3-7). The Cr-bearing spinels contain rare inclusions of pargasitic hornblende with 1.5 wt.-~ Cr203. Grains of chrome-pleonaste (Table 8- # 7, 8) are also included in larger, virtually unzoned plates of ilmenite containing up to 10 mole-~ MnTiO 3 and 1 mole-~ MgTiO3 (Table 8 -# 9).

A microprobe analysis of a titanite from the neighbourhood of a pseudomorph after olivine in sample 3719.51 yielded relatively high Fe and unusually high Mg contents (cf. Das et al., 1988) which are explained by the bulk rock chemistry.


Table 8. Chemical compositions of maonetite, brown spinel, ilmenite and titanite from serpentine-rich h6sbachite 3719.51 (type B)

code 1 2 3 4 5 6 7 8 9 i0

SiO 2 . . . . . . . . TiO 2 0.03 0.04 0.01 0.38 0..18 0.07 0.42 0.77 54.22

A1203 0.02 0.02 43.30 0.78 0.23 0.03 41.73 42.66 0.01

CE203 0.02 0.00 20.75 15.85 11.08 1.48 20.71 18.87 0.09

Fe203 69.12 68.83 2.24 51.25 56.11 67.00 2.07 1.97 0.00 MgO 0.01 0.05 9.94 0.16 0.09 0.05 10.13 9.12 0.28

CaO . . . . . . . . . MnO 0.02 0.02 0.00 1.07 0.45 0.00 0.00 0.00 4.75

FeO 31.03 30.72 23.21 28.94 29.60 30.72 21.81 23.23 40.42

NiO 0.07 0.18 0.i0 0.06 0.14 0.09 0.12 0.12

ZnO 0.00 0.00 0.35 0.87 0.43 0.02 0.42 0.84

Total 100.32 99.86 99.90 99.35 98.31 99.46 97.41 97.58 99.77

Mg 0.001 0.003 0.429 0.009 0.005 0.003 0.447 0.403 0.021

Ca . . . . . . Mn 0.001 0.001 0.000 0.035 0.015 0.000 0.000 0.000 0.201

Fe 2+ 0.997 0.991 0.562 0.925 0.963 0.994 0.540 0.576 1.691

Zm 0.000 0.000 0.007 0.025 0.012 0.001 0.009 0.018

Ni 0.002 0.006 0.002 0.002 0.004 0.003 0.003 0.003

Total 1.000 1.000 1.000 0.995 1.000 1.000 1.000 1.000 1.913

Si . . . . . . . .

Ti 0.030 0.040 0.010 0.380 0.180 0.070 0.420 0.770 2.040

A1 0.001 0.001 1.476 0.035 0.011 0.001 1.457 1.491 0.001

Cr 0.020 0.000 20.750 15.850 11.080 1.480 20.710 18.870 0.004

Fe 3+ 1.997 1.998 0.049 1.474 1.642 1.951 0.046 0.044 0.000

Total 2.000 2.000 2.000 2.000 1.998 1.999 1.997 1.994 2.044

Total 3.000 3.000 3.000 2.995 2.998 2.999 2.997 2.994 3.958

31 55 32 39

1 71

0 04

3 86

3 72 24 49

0 00

97.76

0.184

1.045 0.000

0.869

0.806

0.067

0.001

0.096

3.068

Cations of spinels were calculated for O = 4, of ilmenite for O = 6; Fe 3+ was estimated under consideration of an ideal formula occupation; titanite cations were calculated by charge balance. 1 magnetite together with graphite 2 net-like magnetite together with serpentine 3 centre of brown spinel (pleonaste) 4 magnetite rim around brown spinel, near to brown spinel 5 magnetite between 4 and 6 6 magnetite rim around brown spinel, near to surrounding matrix 7 centre of brown spinel, included in ilmenite 8 brown spinel near surrounding ilmenite 9 Surrounding ilmenite 10 Mg-rich titanite from the neighbourhood of a pseudomorph after olivine (near serpentines 2 and 6 from Table 7)

Geochemistry

Analytical methods

The major elements Si, Ti, A1, Fetot, Mn, Ca, K, and the trace elements S, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, La, Ba and Ce were analyzed by standard XRF methods using lithium tetraborate fusion discs and powder pellets, respectively. Mg and Na were determined by standard AAS methods, Li and Yb by flameless AAS after decomposing the samples in HF-H2SO4. Fe(II) and P were analyzed spectrophotometrically, CO2 volumetrically and "H2 O+'' by loss on ignition.

Major element chemistry

Recalculating the bulk rock analyses (Table 9) on a H 2 O, CO2-free basis, the various h6sbachites and the homeoblastic chlorite-hornblende felses cover a compositional


range of 43-48 wt.% SiO2, 8-15 Wt.-~ AlzO 3 and 13-26 wt.% MgO, and are thus not strictly ultrabasic in their geochemical character.

Ab + An

Cpx + Opx 50 OI

Fig. 10. Triangular plot (Ab + An)--(Cpx + Opx)--O1 showing the composition of different types of ultramafics (hatched area) and metagabbros (crosshatched area; data from Patzak, 1991) in the KTB pilot hole. Nomenclature after Streckeisen (1976). Closed circles, high-Cr, Ni group of h6sbachites; closed squares, low-Cr, Ni group of h6sbachites; open squares, homeoblastic chlorite-amphibole felses

ppm Cr

2 5 0 0 I \ \ I \

Odenwald.-~ \ 2000 ~ ~1

", ) 1500 ~ _ M

1000 Spessart

50O

I ~ [ I I I I

0.50 0.60 0.70 0.80 0.90 1.00 mg

ppm Ni

1000

8 0 0

6 0 0

4-00

2 0 0

I R i i i 0.50 0.60 0.70 0.80 0 .90 1.00

mg

m O i

Fig. 11. Variation diagram Niggli-mg vs. Cr and vs. Ni, respectively. Symbols as in Fig. 10. For comparison, the fields of h6sbachites from the Odenwald and the Spessart Crystalline Complexes are delineated (data sources see text)


Consequently, the CIPW norm displays always some normative plagioclase (An75_96), generally ranging between 21-32 wt.-%, besides 21-33 wt.-% O1, 17-43 wt.-% Hy and 5-16 wt.-% Di. According to the IUGS nomenclature (Streckeisen, 1976), most of the ultramafics would plot into the field of mela-olivine gabbronorites (Fig. 10). Only the plagioclase-bearing h6sbachites (type D) have higher normative plagioclase contents of 37 and 45 wt.-% (An76 , An63), and one of the cataclastic h6sbachites (A-3, sample 1384.75 m), with a modal amount of about 90 vol.% hornblende, displays even 50 wt.% P1 (An54) and the highest N a 2 0 and K 2 0 contents recorded in the ultramafics. These samples plot in the olivine gabbronorite field and are transitional, in their chemical composition, to the adjacent metagabbros of the KTB pilot hole, investigated by Patzak (1991; see Fig. 10).

Trace element chemistry

Judging from their trace element contents, two different groups of ultramafics can be distinguished (Table 9; Fig. 11; 12b, c, d):

Rock MORB

10

1

0.5

0.1 ®

Rb I~a I~ I~b l laOe Sr P Zr ][i "Y Yb ~/ Cr Ni

Rock Mo.B

0.5 ~

0.1 Rb Ba K Nb La Ce Sr P Zr Ti Y Yb V Cr Ni

Rock MORB

10

1

0.5

0.1

¢ o

© , , 4 , , , ~ , , ~ + , , ,

Rb Ba Nb LaCe r P Zr i Yb V Cr Ni

Rock

10

5

1

0.5

0.1

, , , , , , , , , , , , , , ,

b Ba K Nb LaCe Sr r Ti Y Yb V Cr i

Fig. 12. MORB-normalized trace-element patterns of a Metagabbros of the KTB pilot hole (10 samples, light hatching; data from Patzak, 1991); b Cr-, Ni-rich h6sbachites from the depths 1449.29 to 1449.88 (2 samples, dark hatching) and below 3716 m (3 samples, light hatching); c Cr-Ni-poor h6sbachites (7 samples, light hatching; open circles indicate deviating values); ti homeoblastic chlorite-hornblende felses (2 samples). Normalization after Pearce (1982)


~J

~J

C~

~J

o~

. o

@

, ° . ° ° , , ° ° ~ ° , ° ° ~ 0 ~ 0 0 ~ 0 ~ 0 ~ 0 0

. , , . ~ i , ® , . . , , ,

. , . ° ° . . , , , . , ° °

. . , , . . . . , , , . . ,

• • • . , , . , . ° , • • °

r~q 0 ~ t'~ 0 0 ,.4 U'~ 0 0 0 I~ 0 0

~ o o o

, , , , ° , , ~ ° ° , , , , ~ 0 ~ 0 ~ 0 0 0 ~ 0 0

° ° . . ° . , , . * * ~ , ,

~ 0 ~ 0 ~ 0 0 0 0 0 0 . , , . ° . ° , . , , . . .

~ 0 ~ 0 0 ~ 0 0 0 ~ 0 0 ~ ~ V

, , , . , . . . . , , . . ,

~ 0 ~ 0 ~ 0 0 0 ~ 0 0

~ o ~ . ° . . , ~ . . ° . ° ° . .

o m , , , ° ° , ° ° ° ° , ° . ,

° ° ~ , . ° , , ° * * , , °

~ 0 0 ~ o 0 0 ~ o 0

~ o o ~, . . . . . ~ ® ~ o o ; o ~ ~

l

V ~ ~ ~ 0 ~ ~ V

~ V

,-4

I'~I ~-~ t ~" ~ V

V ' ~ l , , - t

CO

0 CX) e,~ i ~ C'~I C"1 ' ~ ( ~ ,'") I " I " Lr~ 0 ,--I e,,1 0 0 " ~ (:0 , ~ i'%1 i.,~ r ~ ,,--I v

i'-,. v ,-.I

,,-',i

, '4

o O ~ o ~ o ~ o , o ~ , , ~ (X~ r "~ tO V

( " Q O

V O (~,1 ,,-4 r " - (~1 ,-,I , - I

• .-,I ',~[3 , -4 ',.~)

~ 0 . . ' ~ 3 r " r~ ~ I.d r~ ~ ~ Gj r~ ,r.


The schistose h6sbachites (A-2) and the serpentine-bearing h6sbachite (B) form a group with distinctly ultrabasie tendency, characterized by high contents of Cr (1230-1300 ppm), Ni (685-785 ppm), in some cases also of Cu, and Cr/Ni ratios of > 1.6 (Fig. 11). Conformably, the incompatible elements are low. Compared to the adjacent metagabbros, which show a geochemical character transitional between N-MORB and E-MORB (Patzak, 1991; Patzak et al., 1991; see Fig. 12a), this 'h6sbachite group is distinctly depleted in K and Sr (Fig. 12b).

The other types ofh6sbachite and the homeoblastic chlorite-amphibole felses (E) conform to a second group with lower contents of Cr (530-870 ppm) and Ni (410-665 ppm) and with Cr/Ni ratios generally < 1.2. The lowest values of 350 ppm Cr and 57 ppm Ni, which are in the range of the metagabbros, were recorded in the cataclastic h6sbachite 1384.75. The incompatible elements are generally higher than in the high-Cr, Ni group of h6sbachites and, except for the higher contents in Cr and Ni and the low V and K values, the trace element patterns of these ultramafics resemble that of the metagabbros (Fig. 12c vs. 12a).

The homeoblastic chlorite-hornblende felses (type E) are characterized by high contents of the incompatible elements Li, Rb, Ba, K, Nb, La, Ce and P leading to a trace element pattern similar to that of the metagabbros (Fig. 12d vs. 12a). On the other hand, Cr and Ni are distinctly higher and, like in the h6sbachites with ultrabasic tendency, Sr is markedly lower.

D i s c u s s i o n

Protolith

The close spatial association and preserved primary contacts between the ultramafic talc-chlorite amphibole felses (h6sbachites) and the metagabbros (Fig. 3) document a common history for these two rock types. Relics of igneous textures and magmatic minerals as well as the major and trace element geochemistry leave no doubt that the metagabbros and related h6sbachites were derived from marie and ultramafic igneous protoliths.

The metagabbros reveal ophitic to sub-ophitic textures with Ti-bearing augite, Ti-bearing amphibole, intermediate to calcic plagioclase, ilmenite and rare biotite as igneous relics (Patzak et al., 1989, 1991; Patzak, 1991; Schalkwijk, 1991; O'Brien et al., 1992). Similar textures occur in metagabbros from surface outcrops of the Erbendorf-Vohenstrauss Zone (Voll, 1960; Schiissler, 1987, 1990) and the Erbendorf Greenschist Zone (Matthes and Olesch, 1989).

Relics of the igneous to late-igneous stage in the h6sbachites are

- - porphyroclasts of Ti-augite with poikilitic inclusions of olivine pseudomorphs; - - porphyroclasts of brown Ti-bearing Ca-amphibole, often surrounding Ti-augite,

both with poikilitic inclusions of olivine pseudomorphs, i. e. aggregates of serpentine and/or chlorite and/or talc (Fig. 4, 6);

- - mesh serpentine with typical strings of magnetite after olivine (Fig. 6); - - biotite widely replaced by chlorite.

As already stated for the Spessart and Odenwald occurrences (Matthes and Kr~mer, 1955; Matthes and Schubert, 1967; Schubert, 1969), similar textures are typically developed in hornblende peridotites and their (sub-)volcanic equivalents,


i.e. hornblende picrites (e.g. Johannsen, 1931, Fig. 17; 1938, Fig. 158; yon Horstig, 1957; Nesbor, 1988).

However, the textural relationships in the h6sbachites as well as the associated metagabbros of the KTB pilot hole (O'Brien et al., 1992) indicate that the Ti-bearing Ca-amphibole is a late-magmatic phase, which partially replaced and overgrew, rather than crystallized in equilibrium with, Ti-augite. The same reaction textures were recorded in some Norwegian gabbros (Otten, 1984) and the Frankenstein gabbro, Odenwald (Kreher, 1992). In accordance with these authors, we assume that this replacement took place under subsolidus conditions and was caused by water influx during cooling. In analogy to the metagabbros (O'Brien et al., 1992), a late-magmatic, amphibole-forming reaction could be formulated as

117 clinopyroxene (Di88) + 153 olivine (Fo82) + 113 plagioclase (An75)

+ 13 ilmenite + 56 [Na] + 100 H20 + 43 [O] = 100 brown amphibole,

using the analyzed clinopyroxene and amphibole in sample 3719.51 and pure ilmenite, and estimating the chemical compositions of olivine and plagioclase from the CIPW norm.

A (late-) igneous origin of the brown hornblende is clearly indicated by its high titanium content ranging up to 0.34 Ti p.f.u. Applying the Ti thermo/-neter of Otten (1984), based on the experimental data of Helz (1973), a temperature of about 960 °C is estimated, provided the oxygen fugacity was close to the QFM buffer.

Because of the intense metamorphic overprint, no further indications of possible late- to post-magmatic, pre-metamorphic alteration processes have survived. Partial or total replacement of olivine by serpentine and/or iddingsite at this stage may have occurred. The antigorite present in the h6sbachites must be of metamorphic origin.

Geochemically the h6sbachites and the homeoblastic chlorite-amphibole felses cover a wide compositional field, intermediate between typical ultrabasites and the metagabbros (Fig. 10). Using the relative amounts of CIPW norm minerals as a guide, the protoliths of the h6sbachites would classify as mela-olivine gabbronorites to olivine gabbronorites (Streekeisen, 1976; see Fig. 10), whereas most of the metagabbros would classify as olivine gabbros (Patzak, 1991). This fact supports the textural evidence that the h6sbachites were initially formed as cumulates, enriched in mafic minerals, by differentiation of a gabbroic (or basaltic) magma.

Differences in the trace-element geochemistry of the h6sbachites, i.e. in the contents of Ni, Cr (Fig. 11), and incompatible trace elements, commonly regarded as immobile (Fig. 12a-d), indicate that this differentiation process proceeded to different degrees in the various occurrences. Since the igneous mineralogy of the h6sbachite protolith is not entirely clear, the type of differentiation is difficult to assess. Nevertheless, the enrichment in Ni and Cr leaves no doubt that the cumulate contained higher amounts of olivine and magnetite, while the Sr depletion (Fig. 12b, d) indicates that the cumulate was impoverished in plagioclase. It must be kept in mind that trace elements like Rb, Ba and K possibly behaved mobile during metamorphism (e.g. Sehiissler et al., 1989). They are thus no suitable tracers for igneous fractionation processes between the respective protoliths of the metagabbros and the h6sbachites.


Contact relationships between the metagabbros (in part with intercalated h6sbachites) and the surrounding garnet amphibolites seem to indicate a primary intrusive contact between the protoliths of the two rock types, in a few places. On the other hand, there are clear indications that gabbroic portions in metabasite suites, penetrated by the KTB pilot and main holes, were tectonized and metamorphosed, together with former basalts, to form garnet amphibolites (Biihn and Okrusch, 1993). We therefore assume that the metabasite sequence is derived either from basaltic flows, subsequently intruded by gabbro/dolerite dikes or sills or, alternatively, from thick, structurally differentiated basaltic sills with doleritic portions in their interior zones. At any rate, slower cooling rates in these parts enabled crystal settling leading to the formation of ultramafic cumulates, the protoliths of the h6sbachite. Water enrichment during this process initiated the formation of late-magmatic amphibole, both in the dolerites and the associated cumulates. An unmetamorphosed example of such a situation was described, for instance, by Hentschel (1956), in the drill hole Weyer I, Lahn area, where a 111 m thick, differentiated sill with a picrite layer at its bottom and a Ti-enriched basalt at its top was penetrated.

Phase relationships and estimate of metamorphic conditions

Inspite of the complicated textural relationships in the h6sbachites, involving igneous and late-igneous relics as well as various retrogressive reaction textures, mineral assemblages formed during the peak of pervasive amphibolite facies metamorphism can be established, which affected the Zone of Erbendorf-Vohenstrauss about 380 Ma ago (Kreuzer et al., 1989). It is interesting to note that relics of the eclogite- and high-pressure granulite facies stage, well preserved in some of the associated metagabbros (O'Brien et al., 1992), were not recorded in the meta-ultramafics.

During the prograde amphibolite-facies event green amphiboles (in the compositional range pargasite--pargasitic hornblende--magnesio-hornblende), Mg-chlorite and talc were formed in the h6sbachites. In all probability, metamorphic olivine also crystallized during this stage, but was totally replaced, together with possible relics of igneous olivine, by aggregates of serpentine_ talc + magnetite during the later retrogression. Judging from the textural evidence, anthophyllite, cummingtonite and tremolite/actinolite (to magnesio-hornblende) form a second, post-deformational generation of amphiboles, presumably formed at the culmination of the amphibolite-facies event.

Thus the following mineral assemblages reflect near-peak or peak-metamorphic conditions (+ ilmenite + magnetite):

la. Green amphiboles + chlorite + talc + anthophyllite + cummingtonite, lb. green amphiboles + chlorite (? + olivine) + anthophyllite + cummingtonite,

2. green amphiboles + chlorite + talc (+ olivine) + tremolite/actinolite 3. green amphiboles + chlorite + talc (+ olivine), 4. green amphiboles + chlorite (+ olivine) + brown spinel, 5. green amphiboles + chlorite + tremolite/actinolite, 6. green amphiboles + chlorite + plagioclase, 7. chlorite + talc + tremolite/actinolite + cummingtonite.


The observed mineral assemblages are typical for the amphibolite facies. The presence of anthophyllite and cummingtonite and the absence of orthopyroxene restrict the peak-metamorphic conditions in a relatively narrow zone between the forsterite-anthophyllite-cummingtonite and the forsterite-enstatite isograds (Trommsdorff and Evans, 1974; Evans, 1977).

In the pure system C a O - M g O - S i O 2 - H 2 0 , the assemblage-talc-olivine- tremolite and the assemblages anthophyllite/cummingtonite + talc + tremolite or anthophyllite/cummingtonite + forsterite + tremolite mutually exclude each other, as anthophyllite and cummingtonite are formed from forsterite + talc according to the univariant, prograde reactions

talc + forsterite~,-~-cummingtonite + H 2 0 (la)

talc + forsterite~,-~-anthophyllite + H 2 0 (lb)

(Evans, 1977, Fig. 1; see Fig. 13). Although no clear textural indication was recog-

PH20 (kbar)

14 /~

r~ / /÷J

12

10

8

6

I /

I I /

t I I I ( ~

T C

I ~,__.i FO

"l

Qz

TR~TC DI" - ~ k ANT

FO

4

2 / CaO (Mg, Fe) O

400 500 600 700 800

Temperature (°C) Fig. 13. P-T diagram showing equilibrium curves in part of the system MgO-SiOz-H20 (full lines, smaller letters) and their displacement by the presence of Fe in amounts typical of ultramafic metamorphic rocks (broken lines, bold letters) as calculated by Evans and Guooenheirn (1985). The relevant phase relationships in the CM(F)SH system are also shown. Abbrevations are: ANT anthophyllite, ATG antigorite, CC calcite, DI diopside, EN enstatite, FO forsterite, QZ quartz, TC talc, TR tremolite/actinolite, V water vapour. Schematically displayed are (i) the prograde and retrograde P-T path deduced from critical assemblages in the adjacent metasediments (Reinhardt, 1990) and the h6sbachites, and (ii) the P-T conditions of the eclogite (E) and the high pressure granulite (G) stage as estimated from mineral relics in meta-eclogites and metagabbros (O'Brien et al., 1992)


nized that reactions (la) and (lb) took place in the h6sbachites, we assume a prograde formation of anthophyllite and cummingtonite at the peak of the amphibolite-facies metamorphism. No indication was found for a formation of anthophyllite from enstatite according to the reversed reaction

anthophyllite + forsterite ~--enstatite + H2 O, (2)

or the reaction

talc + enstatite ~ anthophyllite. (3)

The equilibrium curve of reaction (la) has been experimentally determined by reversals at lower pressures; according to the data of Greenwood (1963) and Chernovsky (1976) the equilibrium temperature at P(HzO) = 4 kbar is about 700 °C. However, the extrapolation of this curve to higher pressures and the position of the invariant point(s) formed by the intersection with the equilibrium curves for reactions (2), (3) and

talc + forsterite < = = > enstatite + H 20 (4)

is still disputed. A review of the various possibilities, based on available experimental and thermodynamic data, is given by Day and Halbach (1979). For instance, the "minimum deviation" model of these authors would lead to an invariant point at about 650 °C/12.6 kbar, whereas Evans and Guggenheim (1985) calculated about 680 °C/6.2 kbar (Fig. 13). These authors showed that, in the Fe-bearing system, the invariant point is shifted to markedly higher pressures and somewhat lower temperatures (Fig. 13).

Moreover, the incorporation of Fe in the coexisting phases talc (XFe = 0.08), anthophyllite (XFe: 0.3-0.2) and forsterite makes reaction (1 a) divariant and allows a stable coexistence of the three minerals (Hinrichsen, 1967, Fig. 4; Poppet al., 1977, Fig. 2) in a limited temperature range.

Using the estimate of Evans and Guggenheim (1985) as a guide and assuming a pressure range of about 8-10 kbar as derived from critical assemblages in the associated metabasites (Patzak et al., 1991; O'Brien et al., 1992) and metasediments (Reinhardt, 1990), peak metamorphic temperatures roughly between 640 and 700 °C can be estimated, similar to those obtained for the metabasites and the metasediments (Fig. 13; see below).

The stable occurrence of tremolite in coexistence with forsterite (Evans, 1977), of talc (Day and Halbach, 1979; Evans and Guggenheim, 1985) and of clinochlore (in the absence of quartz: Fawcett and Yoder, 1966; StaudigeI and Schreyer, 1977; Jenkins and Chernovsky, 1986) are consistent with these peak metamorphic conditions, even allowing for incorporation of some Fe in the Mg-silicates (e.g. Evans and Guggenheim, 1985; McOnie et al., 1975).

An attempt to estimate peak-metamorphic conditions more precisely by applying the approach of Holland and Powell (1990) to sample 1382.36 proved to be unsuccessful, due to the high variance of the assemblage.

During uplift, retrogression under greenschist-facies conditions led to the replacement of olivine by aggregates of antigorite + magnetite, according to the reversed reactions

antigorite ~,-~- forsterite + talc + H : O (4a)


(Johannes, 1975; Evans et al., 1976; see Fig. 13), and

antigorite + magnetite < = = > olivine + talc + H20 + Oz. (4b)

During this retrograde event, biotite was altered to chlorite, the various amphiboles were replaced by talc, and green hornblende, in contact with serpentine, partly reacted to form secondary tremolite/actinolite + chlorite. Thus talc, chlorite and tremolite/actinolite are present in at least two different generations, formed under amphibolite- and greenschist-facies conditions, respectively, although their textural distinction is not always possible.

Like other rock types penetrated by the KTB pilot hole, the h6sbachites are often affected by late cataclastic deformation; the fissures are filled with prehnite, epidote and calcite.

Conclusions and Discussion

The close spatial relationships between metagabbros and associated ultramafics in the KTB pilot hole indicate that both rock types shared the same geological history, comprising an igneous and late igneous stage as well as a polyphase metamorphic evolution.

Judging from textural, mineralogical and geochemical evidence, the talc-chlorite- hornblende felses (h6sbachites) intercalated within metagabbros and amphibolites are derived from cumulates enriched in mafic minerals like clinopyroxene, olivine and magnetite. These were formed by crystal settling in a differentiating basaltic dike or sill or, alternatively, in small gabbroic bodies intruding basaltic flows. During a late-magmatic stage, brown amphibole was crystallized in these cumulates leading to ultramafics of hornblende peridotite or hornblende picrite composition.

In contrast to the closely associated metagabbros, no mineral relics testifying to earlier high-pressure stages under eclogite and granulite-facies conditions were recognized in the h6sbachites. Presumably, the reaction kinetics in the coarse- grained igneous protoliths of the h6sbachites were too slow to facilitate eclogite and granulite-forming reactions.

Mineral assemblages in the h6sbachites testify to the dominant amphibolite- facies metamorphism with peak conditions in the stability field of the assemblage common hornblende _+ tremolite/actinolite + clinochlore + talc + olivine. During the metamorphic evolution, a formation of anthophyllite and cummingtonite took place, which we regard as prograde. Thus peak metamorphic temperatures are defined by the stable coexistence of anthophyllite + olivine, and the absence of orthopyroxene. At an assumed pressure range of 8-10 kbar, temperatures between 640 and 700 °C can be estimated, well conforming to the range derived from mineral assemblages in the adjacent metabasites and metasediments.

A retrograde metamorphic overprint under greenschist-facies conditions, which affected all rock types in the KTB pilot hole at variable degrees (e.g. Patzak et al., 1989, 1991) led to the replacement of relictic igneous and metamorphic olivine by antigorite + magnetite, chloritization of biotite and to the formation of a second generation of tremolite/actinolite.

Combining the textural and petrological evidence in the h6sbachites and the associated metagabbros, a complicated P-T evolution can be deduced. A first clockwise P-T loop--well documented in the metagabbros and the retrograded


eclogite lenses of the VSP 1 and the KTB pilot hole-- leads to an eclogite stage with a pressure maximum of about 14 kbar and temperatures of roughly 650 °C. The subsequent granulite stage took place under slightly lower pressures, but higher temperatures (O'Brien et al., 1992). The second prograde P-T evolution, which happened about 380 Ma ago, culminated in the dominant amphibolite facies metamorphism at temperatures of 640-700°C and pressures of 8-10 kbar and was followed, during uplift and exhumation, by a greenschist-facies retrogression.

In the associated metasediments, the prograde and retrograde branch of this second P-T loop are well documented (Reinhardt, 1990; see Fig. 13), whereas relics of the high-pressure stages were not recognized so far. Therefore, O'Brien et al. (1992) argued that the metasedimentary units were tectonically juxtaposed with the metabasite units containing the metagabbros and related h6sbachites. According to these authors, this tectonic coupling took place after the high-pressure events and prior to the amphibolite-facies metamorphism culminating at about 380 Ma (Kreuzer et al., 1989). Such a model- - implying far-reaching consequences for the geological interpretation of the Zone of Erbendorf-Vohenstrauss--would explain the discrep- ancies in the protolith ages of the metasediments, palynologically dated as Lower Devonian (Pflu9 and Prfssl, 1991), and the metagabbro at 1265 m depth, radiometrically dated as Lower Ordovician (494 + 3 Ma: yon Quadt, 1990).

Acknowledgements

We want to thank the team of the KTB field laboratory for their assistance during sampling, E. Schm~dicke and T. Will for their help with the thermodynamic calculations, D. Krausse for his advice at the Bayreuth microprobe, D. Waasmaier for the high-resolution XRD analysis and K.-P. Kelber for the photographs and line-drawings. Thanks are due to B. Biihn, P. Mirwald, W. Schubert, Z Will and an anonymous reviewer for their critical remarks which improved the manuscript materially. The financial support of Deutsche Forschungsgemein- schaft (grant Ge 120/69-1) is gratefully acknowledged.

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Authors' addresses: S. Matthes, M. Okrusch, U. Schiissler, P. Richter, Mineralogisches Institut, Universit/it Wiirzburg, Am Hubland, D-97074 W/irzburg; K. yon Gehlen, Institut fiir Geochemie, Petrologie und Lagerst~ittenkunde, Universit~it Frankfurt am Main, Senckenberganlage 28, D-60325 Frankfurt am Main; and Ch. R6hr, Riihl AG Umwelt- technik, Usinger Strasse 31, D-61169 Friedberg-Ockstadt, Federal Republic of Germany

Documents

Talc-chlorite-amphibole felses of the KTB pilot hole, Oberpfalz, Bavaria: Protolith characteristics and phase relationships