BH250-189
Title
Subject
Description
Minor Minerals: biotite, chlorite, oxides
Mineralogy Comments: myrmekite
BH250-189 — Troctolite (Duluth Complex, Minnesota)
BH250-189 is best classified as a troctolite from the Duluth Complex, one of the most famous layered mafic intrusions in North America and a major component of the 1.1-billion-year-old Midcontinent Rift System. Troctolite is a coarse-grained intrusive igneous rock composed primarily of plagioclase and olivine, with only minor pyroxene. This distinguishes it from more typical gabbros, where pyroxene is a major constituent. In the Duluth Complex, troctolites commonly occur alongside anorthosite, gabbro, and ultramafic rocks, reflecting crystal accumulation and mineral segregation within a large evolving magma chamber. BH250-189 is particularly valuable because it demonstrates how a single intrusive complex can generate multiple rock types depending on which minerals crystallize, accumulate, and later react with evolving residual melts.
In hand sample, BH250-189 displays a coarse-grained phaneritic texture, indicating slow cooling deep within the crust. The rock consists of abundant light-colored plagioclase intergrown with darker olivine and minor pyroxene, producing a mottled black-and-white appearance typical of troctolite. Some darker minerals show alteration to biotite, chlorite, and oxide phases, suggesting late-stage fluid interaction after primary crystallization.
Under the microscope, large plagioclase crystals dominate much of the thin section and display well-developed polysynthetic albite twinning, forming the structural framework of the rock. These plagioclase grains likely represent early crystal accumulation on the floor of the magma chamber. Olivine occurs as fractured anhedral to subhedral grains and commonly shows reaction rims that record changing magmatic conditions during cooling.
Phase Equilibrium and Reaction Textures: Why BH250-189 is a Powerful Teaching Sample
One of the most important features in BH250-189 is the development of orthopyroxene coronas around olivine. These coronas indicate that early-formed olivine became unstable as the residual melt evolved and became increasingly silica-rich during fractional crystallization. A simplified reaction illustrating this process is:
Olivine (Mg₂SiO₄) + SiO₂ → Orthopyroxene (Mg₂Si₂O₆) + Spinel/Magnetite
This provides an excellent real-world example of a peritectic-style reaction, where an earlier mineral reacts with remaining melt to produce a new mineral assemblage during cooling. The reaction rims preserved in BH250-189 make it an outstanding teaching sample for connecting thin-section observations to phase equilibrium diagrams.
In photomicrographs BH250-189c and BH250-189d, the reactions appear even more complex. Some olivine grains seem to be partially consumed and replaced not only by orthopyroxene, but also by fine-grained plagioclase, oxide minerals, and locally biotite or chlorite. These textures suggest that the trapped intercumulus melt evolved beyond a simple olivine–silica system and became enriched in Al, Ca, Na, Fe, volatiles, and residual fluids. Under these conditions, additional reactions may have occurred:
Olivine + Si-rich + Al-Ca-rich melt → Orthopyroxene + Plagioclase + Oxides
or more generally:
Olivine + evolved intercumulus melt → Orthopyroxene + Plagioclase ± Amphibole ± Oxides
These textures reflect late-stage melt migration, intercumulus crystallization, and subsolidus re-equilibration within a crystal mush.
Another fascinating feature in BH250-189 is the presence of unusual myrmekite-like vermicular textures visible in photomicrographs BH250-189a and BH250-189b. At first glance these resemble classic myrmekite, which typically consists of quartz vermicules in plagioclase adjacent to potassium feldspar in granitic rocks. However, this sample is a mafic cumulate rock, and these textures appear to involve fine vermicular intergrowths of mafic minerals and possible Fe-Ti oxides rather than quartz. They are better interpreted as symplectitic reaction textures formed during late-stage magmatic or subsolidus alteration. These features likely developed when residual melts or deuteric fluids reacted with earlier plagioclase, olivine, and pyroxene along grain boundaries.
The rock also preserves classic cumulate textures characteristic of layered intrusions. Early plagioclase and olivine crystals accumulated while residual melt migrated through the crystal framework. Intercumulus melt later crystallized pyroxene, oxides, and hydrous minerals in the spaces between earlier grains. These textures provide evidence for crystal settling, magma recharge, and repeated chemical evolution within the Duluth magma chamber.
Geochemically, BH250-189 strongly supports a mafic cumulate interpretation. The sample contains approximately 50.04 wt.% SiO₂, 22.55 wt.% Al₂O₃, 10.37 wt.% CaO, and 5.38 wt.% MgO. Elevated aluminum and calcium reflect abundant plagioclase, while magnesium reflects olivine and mafic silicates. Total iron (7.48 wt.% Fe₂O₃(T)) is consistent with mafic minerals and oxide phases. Low K₂O and P₂O₅ values confirm that this is part of a mafic intrusive system rather than an evolved felsic rock.
The REE data are equally instructive. BH250-189 contains relatively low total REE abundances, which is typical of cumulate rocks because plagioclase and olivine do not strongly concentrate rare earth elements. The sample shows modest LREE enrichment relative to HREEs, suggesting minor influence from evolved intercumulus melt. The positive Eu anomaly is particularly important because Eu²⁺ readily substitutes for Ca²⁺ in plagioclase, reflecting significant plagioclase accumulation. This creates an excellent connection between thin-section observations and geochemical signatures.
BH250-189 beautifully demonstrates that layered intrusions are far more dynamic than simple crystal-settling models often suggest. This single sample records crystal accumulation, reaction coronas, symplectitic textures, evolving melt chemistry, fluid interaction, and phase-equilibrium reactions. Together with BH250-50 and BH250-113, it helps students understand that the Duluth Complex is not a single uniform gabbro body, but rather a chemically and texturally diverse magmatic system.
Coverage
Date
Creator
Source
Contributor
George Hudak and Dean