PROTEROZOIC OROGENIC AND MAGMATIC DOMAINS IN THE MIDCONTINENT REGION, USA: GEOCHEMICAL AND GEOPHYSICAL EVIDENCE

Over the past 50, and especially the last 25, years, Proterozoic basement domains in the Midcontinent have been defined from (i) petrologic and geochemical data (including stable and radiogenic isotopes) on samples from scattered outcrops and drill-hole penetrations and (ii) regional geophysical data (gravity, aeromagnetic, and reflection seismic). These have helped to define (a) the southern limits of Archean cratonic basement, (b) possible age transitions within Paleoproterozoic provinces (Penokean-Trans Hudson, Yavapai, Mazatzal), (c) eastern and southern limits of Paleoproterozoic basement, (f) Mesoproterozoic crustal provinces dominated by felsic plutonic and volcanic rocks (Eastern and Southern Granite-Rhyolite Provinces), and (f) the 1100 Ma Midcontinent Rift System and associated mafic rocks. We conclude that Proterozoic juvenile crust in Laurentia generally becomes younger southward and eastward, consistent with post-1900 Ma growth for 800 m.y. by accretion of various terranes and older microcontinent fragments. This growth ended with Grenvillian collisions about 1080 Ma.

In spite of recent progress, several major questions remain unanswered (or undiscovered). These include: (a) How did the southern margin of pre-1900 Ma Laurentia (southern Wyoming to northern Great Lakes) form? (b) What is the nature of the transition from the Penokean-Trans Hudson provinces (1.9-1.8 Ga) to the Yavapai Province (1.8-1.7 Ga) to the Mazatzal Province (1.7-1.6 Ga) crust? (c) What is the detailed nature of the 1.6-1.5 Ga margin of southern and eastern Paleoproterozoic crust, and how did it form (rifting or accretionary hiatus)? (d) How did the 1.47 Ga (Eastern) and 1.37 Ga (Southern) granite-rhyolite provinces form; what is their internal structure and stratigraphy? We will examine these and other questions in the light of recent or new isotopic and geophysical data and offer speculations on the answers. The bottom line is that many questions will remain unresolved unless future investigators can have access to (a) detailed high-resolution geophysical coverage of the upper crust (aeromagnetic, reflection seismology, and gravity), and (b) many more high-quality subsurface (drill-hole) samples for detailed petrologic and geochemical studies.