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

Proterozoic basement domains beneath Phanerozoic cover in the midcontinent have been defined mainly from petrologic and geochemical data (including stable and radiogenic isotopes) using samples from scattered outcrops and drill-holes and from regional geophysics. Our understanding of the nature and distribution of such domains is best controlled by petrologic and geochemical data. In many instances these domain boundaries have very little, if any, geophysical expression. Furthermore, there are significant geophysical features that do not correspond with boundaries inferred from petrologic and geochemical data and are discordant to regional trends defined by the latter. This lack of correlation between petrologic and geophysical data makes it difficult to extrapolate domain boundaries in regions with limited subsurface or outcrop control. In recent years significant advances in understanding deeper crustal domain distributions have been been possible through use of Sm-Nd isotopic studies that define average crustal residence ages of magma sources for orogenic, post-orogenic, and "anorogenic" plutons. These studies have helped define (a) the southern limits of Archean cratonic basement, (b) eastern and southern limits of Paleoproterozoic orogenic provinces, (c) age transitions within the Paleoproterozoic provinces, and (d) continental growth history of southern Laurentia. A major conclusion is that Proterozoic juvenile crust generally becomes younger southward; this is shown particularly well by crustal residence ages from the 1.47 and 1.37 Ga Mesoproterozoic volcanic-plutonic suites, which define domains with older crust and those that represent juvenile crust at the times of intrusion. A recent attempt to extend studies on the ages of source rocks for 1.37 Ga plutons using SHRIMP analyses of zircon cores from several plutons did not succeed. We analysed possible cores from more than 60 zircons from several plutons in a north-south profile and having Nd crustal residence ages ranging from 1.5 to 1.9 Ga. We found no old cores, implying that the corresponding magmas were not saturated in zircon.