B.4 Metabasites

B.4.1 Petrography

Metabasites appear in two main types:
(1) thick amphibolite-metagabbro sequences and
(2) thin amphibolite layers and lenses intercalated within different gneisses.

Amphibolite-Metagabbro Sequence:

From 1160 to 1610 m and from 3575 to 4000 m two sequences composed of dominating garnet amphibolites, subordinate metagabbros and small meta-ultramafic lenses were intersected. Locally, small leucocratic gneisses occur.

The main constituents of the garnet amphibolites are plagioclase, hornblende and garnet; quartz and biotite are subordinate. Their dominating fabric is massive, medium grained and streaked with quartz-plagioclase schlieren (Fig B.4.1).

Fig. B.4.1: Garnet-amphibolite with characteristic leucocratic quartz-plagioclase mobilisates.
(Core piece 328E1n, 1572.2 m, scale in cm).


The metagabbros are massive, medium to coarse grained rocks and differ from the amphibolites by their relictic, magmatic, ophitic texture with pseudomorphs after plagioclase laths included in 1 cm large clinopyroxenes (Fig. B.4.2 and B.4.3). Metagabbro with corona fabric occurs subordinately (Fig. B.4.4). Hornblende, plagioclase and garnet are the main constituents. Quartz-plagioclase schlieren are almost missing.

Typical massive metagabbro with relictic, igneous, ophitic fabric

Fig. B.4.2: Typical massive metagabbro with relictic, igneous, ophitic fabric.
(Core piece 263C10a, 1259.30 m, scale in cm).



Fig. B.4.3: Ophitic fabric of metagabbro. Large crystal of clinopyroxene (CPX) includes dark, altered laths of plagioclase (PLG). Garnet (GNT) mostly forms coronas around hornblende-plagioclase symplectite.
(Thin section 264H4nT, 1269,6 m, // nicols).


Massive metagabbro with coronitic fabric

Fig. B.4.4: Massive metagabbro with coronitic fabric. Plagioclase coronas surround roundish hornblende crystals.
(Core piece 326D10, 1559.8 m, scale in cm).


In local shear zones the amphibolites and metagabbros show a pronounced foliation.

Meta-ultramafic rocks (chlorite-amphibole fels to schist) appear as 0.1 to 6 m thick layers and lenses mainly within the metagabbros. The mineralogical composition of these rocks and their fabric is very variable. It is characterized by up to 1 cm large poikilitic porphyroclasts of brown hornblende and clinopyroxene with inclusions of pseudomorphs after olivine, now consisting of different sheet-silicates. These porphyroclasts are surrounded by a fine grained matrix of various amphiboles, chlorite, serpentine, ilmenite, brown spinel, magnetite, and sulphides.

Contacts between meta-ultramafic rocks and metabasites as well as between amphibolites and metagabbros are variable. Some boundaries appear to be intrusive with local formation of mobilisates in the contact zone. Others are cataclastic.

Some metagabbros are syntectonically altered to amphibolites. The meta-ultramafics are interpreted to be cumulates of the gabbros.


Thin amphibolites intercalated with gneisses:

Thin amphibolites with thicknesses in the range of decimeters to meters appear mainly as members of the variegated sequences from 0 to 460 m and 2470 to 2690 m. In the upper variegated sequence they form partly fine grained, banded, epidote-rich layers, partly massive garnet amphibolites and partly amphibolites with centimeter-thick calcsilicate bands of clinopyroxene + plagioclase + titanite. The amphibolites of the lower variegated sequence are similar but coarse grained garnet amphibolite boudins are especially characteristic. Down to 200 m the amphibolites show an almost complete alteration under greenschist facies conditions.

B.4.2 Metamorphic evolution

The amphibolites and metagabbros show different stages of metamorphism (Röhr et al. 1990):

(a) early high-pressure stages,
(b) a dominating amphibolite facies stage, and
(c) a late low temperature stage.

a) High-pressure stages:

A typical metagabbro with good preservation of the old high-pressure stage contains large clinopyroxenes (Jd10) with lamellar amphibole, rutile, quartz, and plagioclase (An23) inclusions. These clinopyroxenes are surrounded by vermicular clinopyroxene (Jd1-14) - plagioclase (An17-25) - quartz symplectites. Further minerals are coronitic garnet, Ti-rich, brown hornblende, zoned, granoblastic plagioclase (center: An21 -> rim: An84), rutile-ilmenite-aggregates, quartz, and biotite (Fig. B.4.5 to B.4.7).

Overview of the typical fabric of metagabbros

Fig. B.4.5: Overview of the typical fabric of metagabbros from the lower amphibolite - metagabbro sequence. Clinopyroxene with lamellar inclusions is surrounded by clinopyroxene-plagioclase symplectite. GNT = coronitic garnet, PLG = altered plagioclase laths.
(Thin section 885C3n, 3620.2 m, // nicols).



Fig. B.4.6: Metagabbro. Large clinopyroxene (CPX) with inclusions of plagioclase, lamellar quartz and rutile is surrounded by clinopyroxen-quartz-plagioclase symplectite (SYM). These are surrounded by a corona of brown hornblende (HBL).
(Thin section 885C3n, 3620.2 m, // nicols).



Fig. B.4.7: Metagabbro, detail from Fig. B.4.5. Contact between large clinopyroxene (CPX, including lamellae of quartz and plagioclase) and clinopyroxene-plagioclase-quartz symplectite (SYM). A corona of plagioclase (PLG) and hornblende (HBL) is developed between garnet (GNT) and clinopyroxene, probably due to amphibolite facies overprint.
(Thin section 885C3n, 3620.20 m, // nicols).


The composition of weakly zoned garnet in this rock is characteristically around Alm45, Pyr30, Gross25. Garnet includes kyanite, zoisite, plagioclase (An33 to An92), clinopyroxene (up to Jd30), rutile and quartz.

The pseudomorphs after magmatic plagioclase laths (Fig. B.4.5) consist of an aggregate of plagioclase (An9-32), (clino)zoisite and subordinate phengitic white mica.


b) Amphibolite facies stage

Subsequently, the high pressure rocks were adapted to amphibolite facies conditions. This happened in several steps of reaction. Some of the reactions were frozen in and various stages are preserved now side by side.

The adaption led to:

(1) the formation of hornblende-plagioclase-quartz symplectite after clinopyroxene-plagioclase-quartz symplectite and finally to a granoblastic hornblende and plagioclase matrix;

(2) the development of coronas of plagioclase +/- hornblende around garnet leading to the total replacement of garnet by pseudomorphs of plagioclase +/- hornblende +/- biotite (Fig. B.4.8);

(3) the widespread formation of green, Ti-poor hornblende;

(4) the development of coronas of titanite around ilmenite and rutile leading to the total replacement of rutile.

With depth there is some change in the extent of adaption to amphibolite facies conditions:

(1) At the surface of the ZEV, clinopyroxene and hornblende symplectites and garnet coronas are very rare. Only coronas of plagioclase +/- hornblende +/- biotite around garnet were observed by Schüssler (1987).

(2) Within the upper amphibolite-metagabbro sequence from 1160 to 1610 m of the pilot hole, high pressure relics (symplectites and garnet coronas) are rare.

(3) Within the lower amphibolite-metagabbro sequence from 3575 to 4000 m high-pressure relics are common.


Fig. B.4.8: Amphibolite. Due to adaption to amphibolite facies the high-pressure garnet reacted to form pseudomorphs (PSE) of plagioclase + hornblende + biotite + ilmenite. The clinopyroxene symplectite is transferred to poikilitic hornblende symplectite (SYM). Ilmenite (ILM) is surrounded by titanite.
(Thin section 919A1aII, 3758.9 m, // nicols).


c) Late low temperature stage:

The last metamorphic event led to local development of greenschist facies and lower-grade minerals, e.g. disintegration of plagioclase to clinozoisite and albite, growth of epidote, chlorite, prehnite, pumpellyite and development of light green rims of hornblende to actinolite around green hornblende.

B.4.3 Chemical composition

The chemical composition of the metabasites (Table B.3.1) is equivalent to subalkaline to tholeiitic basalts. Iron and garnet-rich amphibolites can be interpreted as forming the typical Fe-enriched part of tholeiitic trends. Only a few amphibolites with clinopyroxene-rich layers from 0 - 460 m may be para-amphibolites (Fig. B.4.9) and are excluded from the further discussion. Patzak et al. (1989) discriminates the different metabasites by comparison of their immobile elements with those of modern basalts of different geotectonic setting.

Compositions of metabasic rocks in the CaO-MgO-FeOtotal-diagram

Fig. B.4.9: Compositions of metabasic rocks in the CaO-MgO-FeOtotal-diagram.


Patzak et al. conclude that the amphibolites of the variegated unit from 0 to 460 m are similar to modern tholeiites of ocean islands (OIT) or anomalous segments of mid ocean ridges (E-MORB) because of their elevated content of TiO2 and P2O5 and their specific rare earth element pattern.

Composition of metabasites of the thick amphibolite-metagabbro sequence is clearly different from the amphibolites above 460 m. The metagabbros from 1160 to 1610 m have compositions equivalent to modern mid ocean ridge basalts of normal segments (N-MORB), the composition of associated garnet amphibolites is similar to E-MORB. All metabasites investigated by Patzak et al. show distinct enrichment of the mobile elements Sr, K, Rb and especially Ba.


B.5 Late Dykes

The metamorphic rocks are crosscut by numerous, partly deformed lamprophyres and few aplites. These rocks serve as important time-markers but radiometric data for the dykes are not yet available.

B.5.1. Lamprophyres

The lamprophyres are fine-grained, brownish igneous rocks that penetrated the gneisses mainly discordantly to the foliation. They are missing within the metabasites. Their thickness ranges from centimetres to several meters. Xenoliths of graphite-rich cataclastic gneiss prove the graphite-cataclastic event to be older than the lamprophyre intrusion.

Plagioclase is the prevailing matrix mineral. Variable amounts of biotite and brown hornblende appear as phenocrysts. Some dykes bear equal amounts of plagioclase and K-feldspar. Pseudomorphs after olivine up to 2 mm in size bear brown spinel inclusions and are totally replaced by calcite, quartz and chlorite. Low-temperature alteration of the lamprophyres is pervasive leading to the formation of chlorite, sericite, pyrite, carbonate and quartz. The dykes themselves are cut by numerous later veins filled with calcite, prehnite, K-feldspar etc.

Ore minerals within these dykes are dispersed. Pyrite, chalcopyrite, pyrrhotite, pentlandite, sphalerite, galena, siegenite, ilmenite anatase/rutile and spinel were identified. In places, gneissic xenoliths within lamprophyres show a marginal sulphide enrichment.

On the base of modal and chemical composition these dykes are classified as calcalkalic lamprophyres (kersantite, spessartite and vosgesite).

B.5.2 Aplites

Aplitic granites are limited to the depth interval 50 to 120 m where they form several, up to 6 m thick, cataclastically deformed dykes. Further, decimeter thick rocks of this kind appear around 2330 m and below 3575 m within the metabasites.

They consist of quartz, plagioclase (with oscillatory zoning), microcline, perthite, white mica and chloritized biotite. These rocks may be genetically related to the large Falkenberg granite outcropping north and east of the pilot hole. More or less deformed pegmatites with white mica up to cm size (Fig. B.5.1) are subordinately present.

Quartz-plagioclase pegmatoid

Fig. B.5.1: Quartz-plagioclase pegmatoid with biotite lenses, on the right contact to amphibolite.



Geological Survey of KTB Pilot Hole