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