Bulletin No. 44. Geology of the Duluth Gabbro Complex near Duluth, Minnesota

Abstract

Multiple intrusions at Duluth, Minnesota, form a rock series that is here called the Duluth Gabbro Complex. Each of the principal rock types transgresses one or more of the older units. The oldest rock, a coarsegrained anorthositic gabbro that was intruded into the Keweenawan flows, makes up the upper part of the complex. It was intruded by basaltic magma of a second period of magmatic activity which formed rocks that commonly are banded, and hence called the layered series. The gabbroic rocks of the layered series as well as the older anorthositic gabbro are cut by intrusive bodies of ferrogranodiorite and granophyre and by late dikes of basalt and aplite. The lower two-thirds of the complex, the layered series. is composed chiefly of troctolite, olivine gabbro, feldspathic gabbro, and syenogabbro. About 15,000 feet of layered rocks is exposed, locally with rhythmic banding, fluxion structure, and gravity stratification demonstrating bottom accumulation by crystallization under conditions of active magma circulation. A series of samples collected from bottom to top of the layered series shows only limited development of cryptic layering. This lack of cryptic layering may be explained as the result of periodic renewal of magma in the crystallizing chamber or by multiple small intrusions. Cross-cutting relations between the different types of gabbro in the layered series show that the mass originated by multiple intrusion. The lack of chilling effects indicates that the successive intrusions were not greatly separated in time. Near the top of the series a transition exists from gabbro to syenogabbro, and a similar transition may exist from syenogabbro to ferrogranodiorite. The rock series is similar in many respects to that of the Skaergaard Intrusion, but there are important differences that can be explained by dissimilar tectonic history. The poor development of cryptic layering and the absence of ferro gabbro at Duluth, in contrast to their remarkable development in the Skaergaard Intrusion, can be explained by differing tectonic stability. The Skaergaard magma apparently crystallized in a chamber under stable conditions, whereas the Duluth magma seems to have crystallized in an environment of tectonic instability manifested by multiple intrusions of magma. The various rocks of the Duluth Gabbro Complex can be explained by crystallization-differentiation of basaltic magma, although the origin of some, such as the anorthositic gabbro and intrusive peridotite, is puzzling. The rocks of the layered series probably were derived from a basaltic magma approaching the composition of analyzed late basalt dikes. Compared with the analyses of "marginal olivine gabbro" from the Skaergaard Intrusion, the Duluth parent magma seems to have been notably richer in K2O, TiO2, MnO, and P2O5. The Duluth Gabbro Complex is an immense sill-like mass that extends for 150 miles northeast from Duluth, and relations in other parts may differ from those at Duluth.