COMMON LEAD, SM-ND, AND U-PB CONSTRAINTS ON PETROGENESIS, CRUSTAL ARCHITECTURE, AND TECTONIC SETTING OF THE PENOKEAN OROGENY (PALEOPROTEROZOIC) IN WISCONSIN

Citation
N. Vanwyck et Cm. Johnson, COMMON LEAD, SM-ND, AND U-PB CONSTRAINTS ON PETROGENESIS, CRUSTAL ARCHITECTURE, AND TECTONIC SETTING OF THE PENOKEAN OROGENY (PALEOPROTEROZOIC) IN WISCONSIN, Geological Society of America bulletin, 109(7), 1997, pp. 799-808
Citations number
40
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Geosciences, Interdisciplinary
ISSN journal
0016-7606
Volume
109
Issue
7
Year of publication
1997
Pages
799 - 808
Database
ISI
SICI code
0016-7606(1997)109:7<799:CLSAUC>2.0.ZU;2-6
Abstract
Nd and Pb isotopes indicate significant direct involvement of older co ntinental crust during the generation of the Penokean synorogenic intr usive suite in Wisconsin, Initial values for plutons show a clear tren d of increasing epsilon(Nd), from -6 to +4 southward from the Superior Province to the suture between the Pembine-Wausau and Marshfield terr anes, indicating decreasing amounts of contamination by older crust, C rossing into the Archean Marshfield terrane, there is a rapid shift ba ck down to lower epsilon(Nd) values (-1 to -7), Feldspar common Pb fro m synorogenic plutons generally defines a mixing array between Archean crust and primitive are crust, which has Pb isotope compositions that are similar to those of the Proterozoic mantle. A mixing model based on Nd and Pb concentrations and isotopic ratios indicates that synorog enic intrusive rocks acquired their isotopic ratios through interactio n with Archean crust, rather than through contamination of a mantle so urce region by sediment subduction, In addition, direct dating of expo sed basement in the Pembine-Wausau terrane indicates, for the first ti me, that Archean crust is present here (U/Pb zircon age of 2607 +/- 22 Ma). The combined Nd and Pb isotope data demonstrate the importance o f distinguishing between contamination of the mantle and remelting cru st basement for the origin of old Nd model ages in Precambrian orogens , Such distinctions are critical for evaluating crustal growth estimat es in Precambrian orogenic terranes. The preferred tectonic model for the Penokean orogeny involves back-are rifting of the southern Superio r Province during a major period of Penokean orogenesis, followed by c ollision of the Marshfield terrane to the south, which was also intrud ed by Penokean-age magmas, In addition to the isotopic and geochronolo gy data presented here, this model is supported by gravity data which indicate the presence of low-density material (probably Archean crust) south of the Niagara fault; this feature is interpreted to be a burie d extension of the southern margin of the Superior Province beneath th e Pembine-Wausau terrane, During the Penokean orogeny, synorogenic int rusive rocks acquired their isotope compositions through direct intera ction with Archean crust.