The dredge hauls 20-DR and 21-DR at the lower and upper slope of the Gibbs Deep in the eastern basin of the Bransfield Strait revealed a sequence of pristine and altered magmatic rocks with affinities to porphyry systems as weil as their probable, low-grade metamorphic host rocks.
Four different types of rocks including magmatites and paragenic host rocks can be classified:
1. Relatively pristine and weakly altered dolerites
These fine- to medium grained, mainly blue-grey rocks display mm-sized, euhedral plagioclase crystals and minor mafic minerals (pyroxene/hornblende). Magnetite, which is often replaced by hematite, is accessory. Oxidation processes affected part of the sam pies resulting in an overall brownish staining and hematite-filled microfractures. Furthermore, calcite- and quartz-filled veins appear in the fresh as weil as in the alte red rocks.
The following sam pies were taken:
21-DR-1, 1 a, 2a and 3b are pristine, dark bluish dolerites with a fine grained, equigranular texture and rare, quartz-filled veins.
21-DR-1b and 2b are weakly and moderately oxidized dolerites with fine- to medium-grained textures and inhomogeneously distributed grainsizes. They commonly reveal .. hematite- and calcite-filled microfractures.
2. Dolerites with distinct, very low- to low-grade metamorphie overprint
These fine grained, bluish to greenish rocks display different stages of secondary alteration. They show equigranular and more often porphyric textures with plagioclase and clinopyroxene in a dense matrix. Alteration is mainly recognizable by the chloritisation of the pyroxenes and green staining of the matrix (i.e. probable chloritisation and epidotisation) .
The following subgroups of sam pies were classified:
20-DR-2, 3 and 7 are micro-porphyric, green dolerites with an olive-green matrix and patchy, cm-sized quartz mobilisates. Quartz-veins incorporate mm-wide druses filled
with euhedral chalkopyrite and sphalerite crystals. A distinct impregnation of sulfides is made up with round and angular pyrite aggregates (up to 5 mm in diameter) in the matrix and small chalcopyrite-filled veins «1 mm).
- 21-DR-4 is a porphyric dolerite with numerous, 2-4 mm long plagioclase tablets and minor, anhedral mafic minerals. The matrix and part of the mafic minerals are affected by a transfomation to chlorite and epidote. Sulfide impregnation and veining are absent.
21-DR-6 closely resembles the foregoing sam pie with the exception of a cm-wide impregnation zone of tiny, disseminated pyrite crystals and rare pyrite aggregates. In these zones, oxidation has started to decompose the sulfide leading to a reddish rock colour.
21-DR-9 yields the highest degree of seconday alteration with completely chloritised mafic minerals, saussuritized plagioclase and a dense, green matrix. Remarkable are cm-sized calcite patches, which are in part aligned along fractures.
3. Magmatic rocks with breccia-textures
These are hypabysal rocks, wh ich are characterized by fragments of granitoid rocks (up to 7 cm in diameter) incorporated within a medium grained dolerite of intermediate geochemical character. All sam pies display moderate to strong secondary alteration including chloritisation, epidotisation, patchy quartz segregation and veining as weil as sulfide impregnation. Especially within the weakly altered rocks of this suite it becomes evident that the granitoid fragments are xenoliths, which were incorporated by a later pulse of magma. In the strongly alte red sampies, tectonic deformation is evident by disruption of the primary texture, turning the rock into a tectonic breccia.
The following sampies were chosen for investigation:
21-DR-5 is a moderately alte red dole rite which bears several xenoliths of granitoid material as weil as K-feldspar xenocrysts. The magmatic matrix is characterized by chloritisation and epidotisation, which also affected the rim sections of the xenoliths.
Rare calcite veins are present in the host dolerite.
21-DR-7a, b, c, d, e, and f are distinctly altered, xenolith bearing, hypabysal rocks.
They show a strong, penetrative epidotisation and chloritisation of both matrix and xenoliths, often leading to halos of epidote and chlorite around the xenoliths. Veining is absent in these rocks and there is either no sulfide impregnation. Remarkable is a
Cruise Report 80-155 (HVDROARC)
discrete section of the rock, which probably displays K-feldspar metasomatism, weil visible by its yellow-greyish colour (sampie 21-DR-7e). The largest sam pie (21-DR-7f) was selected for zircon separation. '
21-DR-8 shows the strongest metasomatic overprint of all sampies. Matrix and xenoliths are tectonically disrupted to porphyroclasts displaying cataclastic features.
They are penetratively transformed to chlorite and epidote, while yellow-brownish blasts are probable K-feldspar crystals, which formed by K metasomatism. The space in between the porphyroclasts as weil as the fractures in the rock are filled by patchy quartz aggregates and disseminated sulfide.
4. Host rocks of the porphyry-type magmatites
Graphite-phyllites, marly schists, gneisses and meta-greywackes were dredged together with the porphyry-type magmatites. These rocks are probably not dropstones, wh ich is evident by their angular shape and their relatively fresh appearance. Textures and mineral assemblages of these rocks give evidence for lower greenschist-facies metamorphic conditions. A penetrative metamorphic foliation with isoclinal, intafolial microfolds is developed in the schists and gneisses.
The following sam pies were taken:
20-DR-15 is a fine-grained, dark grey graphite-phyllite with a penetrative foliation and microfolding. The rock is mainly composed of white mica, quartz and minor amounts of graphite.
\ 20-DR-16 is a marly schist, which shows compostional layering of fine-grained and slightly coarser grained sections. The greenish colour testifies to a high modal amount of chlorite and epidote while small mafic, anhedral minerals could be amphibole.
20-DR-17 is a paragneiss, which shows several mm-thick, quartz/feldspar- and mica-rich layers and intensive isoclinal folding.
20-DR-18 and 21-DR-10 are pale green metagreywackes, which are characterised by their compact, unfoliated texture. Quartz, feldspar and a large amount of chlorite show a homogeneous distribution in the rocks