BH250-189
Title
Subject
Description
Minor Minerals: biotite, chlorite, oxides
Mineralogy Comments: myrmekite
Petrology of Duluth Complex Troctolite:
The Duluth Complex is a large Mesoproterozoic layered mafic intrusion in northeastern Minnesota, composed of ultramafic to mafic rocks, including peridotite, troctolite (e.g., BH250-189), anorthosite (BH250-46), and gabbro (BH250-50).
Troctolite: Mineralogy and Texture:
Troctolite is a mafic cumulate rock dominated by plagioclase and olivine, with <5% pyroxene or hornblende. In the Duluth Complex, troctolite is a feldspathic cumulate marked by euhedral to subhedral plagioclase (An₅₇–₆₀) and olivine, with minor pyroxenes and interstitial oxides.
Plagioclase occurs as zoned grains and can form nearly monomineralic layers.
Olivine is commonly rimmed by orthopyroxene or biotite.
Clinopyroxene (augite) appears as interstitial subophitic grains or thin oikocrysts.
Orthopyroxene (hypersthene) forms reaction coronas around olivine, often as skeletal or symplectitic (kelephitic) intergrowths, reflecting late crystallization from residual melt.
Oxides like magnetite and ilmenite are minor but significant, occurring interstitially.
Myrmekite:
Myrmekite, vermicular quartz intergrowths in plagioclase, forms during subsolidus alteration involving alkali-rich fluids. Its abundance in Duluth troctolites suggests late-magmatic or deuteric metasomatism, driven by alkali and silica-rich fluids reacting with plagioclase.
Orthopyroxene Rims on Olivine: Reaction Textures:
In both troctolites and gabbros, olivine is often mantled by orthopyroxene, sometimes transitioning outward to amphibole or biotite. These coronitic textures are not primary but reflect late-stage silica enrichment and hydration during cooling.
Reaction Example:
Olivine (Mg₂SiO₄) + SiO₂ → Orthopyroxene (Mg₂Si₂O₆) + Spinel/Magnetite
This reaction is driven by increasing silica activity and can be enhanced by plagioclase breakdown or melt evolution. Some coronas show distinct layers: inner orthopyroxene rims followed by outer hydrous phases, indicating interaction with late-stage magmatic or metamorphic fluids.
Cooling History and Re-equilibration:
Corona textures and symplectites (e.g., orthopyroxene-plagioclase intergrowths) indicate slow cooling and subsolidus diffusion, allowing re-equilibration of earlier-formed cumulates. These features likely formed after primary crystallization, during late magmatic stages or during reheating from later magma injections.
Petrogenetic Significance:
The orthopyroxene coronas and associated reaction textures are key indicators of silica enrichment, oxidation, and fluid interaction during the final stages of crystallization. They reflect the complex magma evolution, cooling history, and role of interstitial melts and fluids in large, layered intrusions like the Duluth Complex.
Coverage
Date
Creator
Source
Contributor
George Hudak and Dean