23. Layers in intrusive rocks. Dan Barker April 2009

Many intrusive igneous rocks show layering, expressed as variations in proportions or orientations of minerals, crystal sizes, and, more subtly, in mineral compositions. The causes of such variation are several, including: 1. gravitational sorting (crystals more dense than liquid sink, those less dense rise) 2. flow differentiation (crystals and liquid get separated by variations in velocity vectors) 3. injection of different magmas at different levels within the intrusive body 4. rhythmic precipitation (a magma is saturated with respect to a mineral and precipitates that until it becomes depleted, i.e., undersaturated, and becomes saturated with respect to another mineral) The following images are offered without interpretation, because the origins of layers are commonly ambiguous.

Alternating layers of nearly pure chromite and plagioclase, Bushveld complex, Dwars River, South Africa

At Dwars River again

The last image from Dwars River

Chromite-rich layers in peridotite, Addie NC

Addie NC

Vertical layers in peridotite, Rougemont, Quebec

Cross-cutting Layers in syenite. Mt Johnson, Que

Layers in syenite, Mt Johnson, Quebec

Mt Johnson, Quebec

Rhythmic layers, Kringlerne, Ilimaussaq complex, SW Greenland

Sunken block of roof rock (arrow), Kringlerne

Kringlerne from head of Kangerdluassuq fjord, Greenland

Kringlerne

Cycles of black, red, and white layers, Kringlerne. Top Base hidden below sea level

One cycle of black, red, and white layers, Kringlerne

Photomicrograph of black rock. Alkali feldspar and intense blue amphibole

Photomicrograph, red rock. Eud is eudialyte, a Na, Fe, Zr silicate that is red in hand sample. Others are alkali feldspar and amphibole. Eud

Photomicrograph, white rock. Alkali feldspar is more abundant than eudialyte and amphibole. Eud

Layers in another igneous unit of the Ilimaussaq complex, SW Greenland

Layers in syenite and granite, just under the roof of the Ilimaussaq complex, Greenland.

Layers in Nine Point Mesa intrusion, Big Bend National Park, Texas.

Top view of vertical layers in gabbro, Mount Royal, Que.

Schlieren in granite, Enchanted Rock TX

Enchanted Rock, TX

Enchanted Rock, TX

Layers in nepheline syenite, Spitskop, South Africa

Gently dipping layers in Bushveld complex, Steelpoort Valley, South Africa

Phlogopite-diopside-apatite rock, Foskor Pit, Palabora Complex, South Africa

Closeup, Foskor Pit

Phlogopite-diopside-apatite rock, Foskor Pit, Palabora

Rhythmic layering in diorite, Pleasant Bay, ME

Mafic layer in sharp contact with granite on right, gradation into intermediate rock on left. Pleasant Bay,ME

Layers in gabbro, E. edge of Port Coldwell complex, Ontario

Layered gabbro, Centre I, Port Coldwell complex, Ontario

Schlieren in ferroaugite syenite, Port Coldwell, ONT

Layers in ferroaugite syenite, Port Coldwell, Ontario. Glaciated outcrop, north shore of Lake Superior.

Layers in nepheline syenite, Port Coldwell complex, Ney Provincial Park, ONT

Nepheline syenite, Ney Provincial Park, Ontario

Layers in Centre III, Port Coldwell complex

Layered mafic and ultramafic rocks, Garbh Bheinn, Skye

"Inch-scale" layers in anorthosite, Stillwater Complex, MT

Layered gabbro, Ushuswanna Complex (2874 Ma) So Africa

G-3 granite, Soldaatenbaai, Cape Province, South Africa

G-3 granite, Soldaatenbaai. R G Cawthorn for scale.

G-3 granite, Soldaatenbaai

Layers in peridotite, Lers, French Pyrenees

Main Donegal Granite, Glen Quarry, Ireland