Layered Mafic Intrusions 


A. Def: large plutons, crystallized from gabbroic magma, distinctive layering

-All ages, but most and largest are Precambrian

-Most are lopoliths

-Most are cumulates

B. Layering

-May be defined in various ways: modal mineralogy; appearance/disappearance of phases; composition of solid-solution minerals

-Has its own terminology

-Many layers are repetitive alternations - any interpretation must account for regularities

II. Skaergaard Intrusion

A. Statistics

-East Greenland

-Eocene (~ opening of North Atlantic)

-3.2 km vertical section, ~10 km long

B. Structural divisions

-Chilled margin (1 m) - quenched original liquid? No, contaminated by wall rock

-Layered Series (2500 m) - crystallized from bottom up: lower Zone, Middle Zone, Upper Zone - subdivided by cumulate phases

-Upper Border Group (700 m) - crystallized from top down

-Marginal Border Group - crystallized from walls inward

-UBG and MBG have zones similar to Layered Series but more compressed

-Between LS and UBG is Sandwich Horizon

C. Stratigraphic column

III. Bushveld Complex

A. Statistics

-Largest LMI - rich source of Pt, Cr, others => much data available from mining operations

-South Africa


-270 km N-S by 450 km E-W, 8 km thick

B. Structural divisions

-Marginal Group = chilled zone 150 m thick

-Lower Zone (2200 m): subdivided into Basal Series and Critical Series: separated by Main Chromite Layer

-Merensky Reef (1-5 m thick, extends >300 km)

-Main Zone (3600 m) and Upper Zone (1500 m)

C. Layering of Lower Zone

-Sequences: pyroxenite - gabbro anorthosite; saw-tooth patterns of modal variations and cryptic zoning

-Repeated influxes of fresh magma?

IV. Origins of layering

A. Early workers attributed it to gravity settling

-But: this should => layers graded by crystal size - mostly not

-Skaergaard contradictions: MBG layers are vertical; also, heavy olivine near roof, light plagioclase near floor

B. In-situ crystallization

-Crystals form near wall, or floor

-But fractionation occurs mainly by liquid moving away from crystals

C. Super-saturation

-Thermal gradient advances through liquid.

-When liquid cools enough to be super-saturated in one phase, a burst of nucleation will occur

-Crystal growth depletes nearby liquid in components, creating a chemical gradient.

-When thermal gradient "catches up" with chemical, another burst of nucleation.

D. Convective effects

-Double-diffusive convection

-May arise if thermal and chemical changes have opposite effects on density

-Convection, and crystallization occur in layers

E. Density currents

-Crystal mush sliding down walls, spreading out over floor

-Some LMI have cross-bedding

F. Combinations

-Probably several processes acting at the same time

-Still an area of active research

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