South East Igneous Complex at Green Island
Friday 21 April afternoon
Dierdre pointed out Green Island
to us; it was to the west of our position on the sandy beach, where knobbly
spines of rock projected through the waves and looked like the skeletons of
beached Liopleurodons and Basilosauri. Dierdre told us that Green Island is
a loess deposit, i.e. windblown dust lifted from the cold dry plains just beyond
Ice Age glaciers by gales and dropped at lower latitudes in drifts (we saw a
10m loess cliff fronting the beach at Belcroute later on). Loess forms Jersey
Royal potato growing soil ; Green Island was green with vegetation compared
to the thin soils overlying the surrounding igneous complex.
Sadly we had no geologist with us here, so I led the way towards an outcrop
to the east, not wanting to waste quality geology time. We were fascinated
by the rocks, there was so much to see - we were all scrambling back and
forth from one shout of " Oh, look at this!" to the next, puzzling
over black dykes, green dykes, pink feldspar crystals and granite veins,
clots of 1cm black rectangular crystals, pale lines and splodges, layering
in the black and white rocks ... so many questions and no-one to give answers!
Consequently this write-up is largely cribbed from 'Description of the 1:25000
Channel Islands Sheet 2' henceforth referred to as "the purple book",
chapter 5 - 'Plutonic Igneous Rocks' and chapter 6 'Minor Igneous Intrusions'
(Bishop & Bisson 1989).
These igneous rocks postdate the Precambrian Cadomian Orogeny (see 'Open
University Geological Society Journal Spring Edition 2000 Vol.21(1) "An
initial study of the Stanner-Hanter Complex...''
Section "The Tectonic
Setting" by John Jaggard briefly sets the scene).The Cadomian Orogeny folded
and metamorphosed the Jersey Shale, the oldest rock on Jersey although older
outcrops occur on the other Channel Islands. The purple book states that the
SE igneous complex was originally layered gabbro (coarse grained and basaltic
i.e. basic) which was mostly metasomatised - that is, reacted chemically with
additional fluids and dissolved gases so that the bulk rock composition is changed,
as well as the mineralogy. By contrast, during metamorphism, altered temperature
and pressure mean that different minerals are stable but the bulk rock composition
does not change (unless a gas or fluid is driven off). Thus there is always
a flux of material in metasomatism but not in metamorphism.
Granitic melts and silica-rich fluids coming off them caused the gabbros
to be metasomatised to diorites, coarse-grained intermediate rocks.The rocks
were quite pale in places, possibly quartz-diorites as illustrated in Plate
11 page 49 of the purple book. The black crystals are usually clinopyroxene
and the white ones plagioclase feldspar. The green 'dykes' contain green
amphibole and hornblende while the dark 'dykes' (melanodiorite layers) black/brown
amphiboles and pyroxenes. We saw 3cm long 'skeletal amphibole crystals'
just like Plate 12 page 50 the purple book which states that these are thin
rectangular amphiboles with pale cores of white feldspar or quartz.
The purple book Chapter 6 Page 65 Figure 17 shows a sketch
of dykes around Green Island, with swarms of thin basic and acid dykes mostly
trending WSW/ENE.
The acid dykes were porphyritic
microgranites i.e. having a finegrained pink (potassium feldspar rich) matrix
with phenocrysts of pink potassium feldspar, white plagioclase and rounded
quartz. We saw them crossing each other and branching in a much less orderly
fashion than the purple book suggests! However, the dykes that we photographed
were roughly 10 - 20cm wide whereas the purple book Page 65 describes granite
dykes 4 - 10m in width; we didn't see anything approaching that scale but
we only had about 20 minutes and so we were only able to view a small area.
This was my favorite bit of the entire trip and it was such a shame that
we didn't have longer to explore it, along with an igneous petrologist to
interrogate!
Heidi Barnes