BH250-113
Title
Subject
Description
Minor Minerals: biotite, oxides
Major Mineral: anorthite, orthopyroxene, clinopyroxene, olivine
Minor Minerals: oxides, biotite
The Duluth Gabbro, also known as the Duluth Complex, is a large, layered mafic intrusive body located in northeastern Minnesota, USA. It is part of the Midcontinent Rift System and is one of the most studied examples of Proterozoic layered intrusions in North America. Several BH250 samples belong to this gabbro, including BH250-11, 50, 189, 192, and 193.
This gabbro is globally recognized as a classic example of a layered mafic intrusion, and the term Duluth Gabbro / Duluth Complex is widely used in both academic and geological literature. It is approximately 1.1 billion years old (Mesoproterozoic).
Mineralogy:
Major minerals (vary by zone):
Plagioclase feldspar: Predominantly labradorite to bytownite (An50–An80)
Clinopyroxene: Mainly augite
Orthopyroxene: Common in more evolved layers, typically bronzite or hypersthene
Olivine: Present in the ultramafic base, ranging from forsteritic to fayalitic
Oxide minerals: Magnetite and ilmenite, especially abundant in oxide-rich zones
Accessory/Minor minerals:
Amphibole (e.g., hornblende, actinolite), especially in altered zones
Biotite and chlorite, commonly in more evolved or hydrothermally altered portions
Serpentine, typically associated with olivine alteration
Textures and Structures:
The Duluth Gabbro displays prominent layered cumulate textures, indicating crystal settling and magmatic differentiation within a large magma chamber.
BH250-113 photomicrographs show ophitic texture where lath-shaped plagioclase crystals are enclosed within larger, anhedral crystals of pyroxene.
Rhythmic layering, adcumulates, and mesocumulates throughout the complex point to open-system magmatic processes with periodic magma recharge.
These features are visible at numerous outcrops across the complex.
Geochemistry:
Compositionally mafic to ultramafic. Enriched in Fe, Mg, Ca, and Ti; depleted in Si and alkalis. Basal ultramafic rocks contain high concentrations of Cr and Ni, indicating early-stage cumulate formation. Rare Earth Element (REE) patterns show moderate Light REE (LREE) enrichment and negative europium (Eu) anomalies in some evolved units.
Weathering affects different rock types in distinct ways, often altering their final appearance. For example, in intrusive igneous rocks (BH250-113), weathering leads to the formation of clay minerals, chlorite, oxides, and hydroxides on the surface. These weathering products obscure the original mineral composition, making it difficult to observe the rock's primary crystalline features. In contrast, weathering of carbonate rocks tends to enhance rather than obscure internal features. It often reveals sedimentary fabrics and structures that are less visible in fresh samples. This contrast highlights the important role surface processes play in either masking or revealing internal rock features, as exemplified by sample BH250-113 and BH250-127.