ESS 439: Layered Mafic Intrusions (LMI) What are they and why are they important? •Represent basaltic magma that has crystallized at depth in large magma chambers •Show the effects of slow cooling and strong fractionation •Natural laboratories to study differentiation in all its aspects: complementary to volcanics in the sense that they preserve the crystalline products of fractionation. •Repositories of the some of the largest ore bodies on earth Cu, Ni, PGE, Cr, V… Some well-studied LMIs (not an exhaustive list) Name Age Location Area (km 2 ) Bushveld Precambrian S. Africa 66,000 Dufek Jurassic Antarctica 50,000 Duluth Precambrian Minnesota, USA 4,700 Stillwater Precambrian Montana, USA 4,400 Muskox Precambrian NW Terr. Canada 3,500 Great Dike Precambrian Zimbabwe 3,300 Kiglapait Precambrian Labrador 560 Skaergård Eocene East Greenland 100 Development of cumulate textures typical of layered mafic intrusions.
a. Crystals accumulate by crystal settling or b. Orthocumulate: intercumulus liquid simply form in place near the margins of the crystallizes to form additional plagioclase magma chamber (known as in situ rims plus other phases in the interstitial crystallization). In this case plagioclase volume (colored). There is little or no crystals (white) accumulate in loose mutual exchange between the intercumulus liquid contact, and an intercumulus liquid (red) fills and the main chamber.. the interstices.
After Wager and Brown (1967), Layered Igneous Rocks. © Freeman. San Francisco Example of orthocumulate texture. In this case, the cumulus mineral is orthopyroxene and it is surrounded by later-forming intercumulus plagioclase (showing albite twinning). Stillwater complex, Montana. Field width 5 mm. Development of cumulate textures (cont.)
Adcumulates: open-system exchange Heteradcumulates: The texture illustrated between the intercumulus liquid and the above is common in the larger layered main chamber (plus compaction of the intrusions. In the example illustrated, pyroxene cumulate pile) allows components that nucleates from the intercumulus liquid and would otherwise create additional grows to form giant poikilitic crystals (grapefruit intercumulus minerals to escape, and size) enclosing numerous cumulus plagioclase plagioclase fills most of the available crystals. During growth of the oikocrysts, space. Some liquid is trapped and forms communication between the intercumulus the small intercumulus pyroxenes, etc. liquid and main magma is maintained. Example of adcumulus texture. Anorthosite from Bushveld Complex, SA Layering Layer: any sheet-like cumulate unit distinguished by its compositional and/or texture Uniform layering. Layer is mineralogically and texturally homogeneous. Modal layering (aka. mineral graded layering): Smooth and gradual variations in the modal abundances of the cumulus minerals which are commonly plagioclase and pyroxene (norites and gabbronorites) or plagioclase and olivine (troctolites)
Size graded layering: Smooth and gradual variations in the grain sizes of cumulus minerals. This type of layering is not common, except in the Duke Island Intrusion, AK. Phase layering: the appearance or disappearance of cumulus minerals in the crystallization sequence, e.g., appearance of plagioclase at contact between Ultramafic Series and Banded Series in the St6illwater Complex Cryptic Layering (not obvious to the eye): Systematic variation in the chemical composition of cumulus minerals (pyroxenes, olivines, plagioclase feldspars) with stratigraphic height in a layered sequence Rhythmic layering: Layering pattern is repeated. Macrorhythmic layering: individual layers are several meters thick. Microrhythmic layering, layers are a few cm thick
Intermittent layering: Irregular patterns: A common type consists of rhythmically graded layers interlayered with uniform layers. Uniform Layering Uniform chromite layers alternating with plagioclase-rich layers, Bushveld Complex, S. Africa. Graded Layering Modal layering: This common type is defined Size graded layering: This type defined by by smooth and gradual variations in the modal smooth size variations in cumulus minerals (not abundances, commonly plagioclase and common except in the Duke Island Intrusion) pyroxene or plagioclase and olivine
From McBirney and Noyes (1979) J. Petrol., 20, 487-554 Other layering types
Inch-scale layering in norite in the Stillwater Modally graded layers in the Skaergaard Complex, Montana. This type of layering is intrusion, East Greenland. Commonly referred to not formed by crystal settling processes. as rhythmic layering. Graded layers are Note the doublet units (hammer for scale). separated by uniform layers (hammer for scale). Cross-bedding in size-graded and modally-graded cumulate layers in the Duke Island Intrusion, Alaska. The main cumulus minerals are olivine and orthopyroxene
Cross-bedding in modally-graded gabbroic cumulate layers. Skaergård Intrusion, E. Greenland. Muskox Intrusion, NWT, Canada (funnel shape with a well-defined feeder dike) The Bushveld Complex, South Africa (2.1 Ga)
The biggest LMI: 300-400 km x 9 km The Red Granite intruded 50-100 Ma afterwards The Bushveld is the source of most of the world’s Pt, Pd, Cr & V
Simplified geologic map and cross section of the Bushveld Complex. After Willemse (1964), Wager and Brown (1968), and Irvine et al. (1983). The Skaergård Intrusion in E. Greenland
This intensively studied body of mafic igneous rock 34 95 is a classic example of extreme closed- system fractional crystallization, e.g., 68 olivine ranges from
Fo70 at bottom of 67 LZ to Fo at the top 0 18 of UZ 53 27 We will continue discussion of the 4 102 75 Skaergaard Intrusion in the next class 83 89 Numbers refer to thin sections studied in lab
Map from Stewart and DePaolo (1990) Contrib. After Stewart and DePaolo Mineral. Petrol., 104, 125 (1990) Contrib. Min. Pet., 104, 125 Skaergaard Intrusion, East Greenland
Pegmatite patch in layered gabbro Skaergaard Intrusion, East Greenland Wager peak
Triple group
Bearskin Glacier The Stillwater Complex, Montana (2.7 Ga)
East Boulder Pd-Pt mine
Stillwater Pd-Pt mine
Mountain View area View of Beartooth plateau looking South from Chrome Mountain Macrorhythmic layers in Banded series, Stillwater Complex Irregular layering in gabbronorite (GN III), Banded series, Stillwater Complex, MT. In addition to uniform layers, the darker units are rich in pyroxene and the lighter units are rich in plagioclase. This Pyroxene wispy type of layering is most snowball likely due to high temperature deformation of pre-existing more regular layers. UW Field camp at Stillwater Complex, 2004 Stratigraphy of Stillwater Complex •Basal Series: a 50-150 m thick complex unit consisting of a sill/dike complex, norites, gabbros, and o-pyroxenite. The series also contains xenoliths of country rock (pyroxene cordierite hornfels) and is the site of massive sulfides (Cu-Ni ores)
•Ultramafic Series: The base of this series is marked by first appearance of cumulus olivine (phase layering). The UM series consists of two zones:
:Lower Peridotite Zone: >20 cycles (each ranging from 20-150 m thick) of macrorhythmic layering with a distinctive sequence of lithologies. The lowermost unit is an olivine plus chromite cumulate with intercumulus opx, plag and minor cpx, overlain by harzburgite (olivine + opx cumulate with intercumulus cpx and plag) in turn overlain by orthopyroxenite (opx cumulate with intercumulus cpx and plag)
:Upper Orthopyroxenite Zone: single, thick (up to 1070 m), fairly uniform sequence of orthopyroxenite (cumulus opx with intercumulus cpx and plag) and rare thin layers of olivine-rich harzburgite.
•Banded series: Complex group of cumulates all of which contain cumulus plagioclase (norites, gabbros, gabbronorites, anorthosites, troctolites). The main Pt-Pd reef occurs ~ 400 m above the contact between the Ultramafic series and the Banded series Modal stratigraphy of Stillwater complex after McCallum et al. (1980) and Raedeke and McCallum (1984) Schematic representation of processes associated with the formation of layered mafic complexes
Venting of fractionated Assimilation of magma to surface volcano country rock Country rock Heat loss
Density plume “Fountain”
Convection
Density currents Compaction and crystallization of Layered sequence accumulated along floor intercumulus melt
Chilled margin Country rock with Multiple influxes of primary magma thermal aureole from deeper source (recharge)