A STRONTIUM AND NEODYMIUM ISOTOPIC INVESTIGATION OF THE LARAMIE ANORTHOSITES, WYOMING, USA - IMPLICATIONS FOR MAGMA CHAMBER PROCESSES AND THE EVOLUTION OF MAGMA CONDUITS IN PROTEROZOIC ANORTHOSITES
Js. Scoates et Cd. Frost, A STRONTIUM AND NEODYMIUM ISOTOPIC INVESTIGATION OF THE LARAMIE ANORTHOSITES, WYOMING, USA - IMPLICATIONS FOR MAGMA CHAMBER PROCESSES AND THE EVOLUTION OF MAGMA CONDUITS IN PROTEROZOIC ANORTHOSITES, Geochimica et cosmochimica acta, 60(1), 1996, pp. 95-107
A strontium and neodymium isotopic investigation of plagioclase-rich c
umulate rocks in the 1.43 Ga Laramie anorthosite complex (LAG) provide
s new insights into the evolution of magma chambers and underlying mag
ma plumbing systems during the crystallization of Proterozoic anorthos
ites. The 725 km(2) LAC intruded the boundary between Archean rocks of
the Wyoming Province in the north and Proterozoic island are terranes
in the south, the Cheyenne belt, at pressures of 3-4 kilobars. Initia
l strontium and epsilon(Nd) isotopic ratios from the three large compo
site anorthositic intrusions (Poe Mountain, Chugwater, and Snow Creek)
and late troctolitic intrusions range from 0.7034-0.7055 and +0.9 to
-4.9, respectively. The isotopic ratios in the cumulates vary as a res
ult of (1) the isotopic composition of the parental mantle-derived hig
h-Al gabbroic magmas, (2) the amount of contamination by Archean rocks
during ascent through the crust, and (3) mixing of replenishing magma
s and resident magmas with different isotopic compositions in the magm
a chamber at the final level of emplacement. Strontium isotopic and no
rmative An variations across the 5-7 km thick stratigraphic section of
layered plagioclase-rich cumulates in the 200 km(2) Poe Mountain anor
thosite indicate that multiple inputs of magma are needed to construct
even relatively small intrusions in Proterozoic anorthosites. The ear
liest and strati graphically lowest intrusion was emplaced as a plagio
clase-rich magma that crystallized large volumes of compositionally ho
mogeneous anorthosite. At least three subsequent chamber-wide episodes
of magma replenishment, followed by mixing, are indicated by abrupt s
hifts in I-Sr and/or normative An in the stratigraphically higher leve
ls of the Poe Mountain anorthosite. Strontium and neodymium isotopic d
isequilibrium between a high-Al clinopyroxene megacryst and the layere
d cumulates is consistent with a high-pressure origin for Al-rich pyro
xene megacrysts in Proterozoic anorthosites. The megacryst crystallize
d in a magma chamber at pressures of 10-12 kilobars from relatively un
contaminated basaltic parent magma (high-Al gabbro) and was transporte
d through the crust in progressively contaminated anorthositic magma.
The range of observed isotopic compositions in the anorthositic rocks
is nearly identical to that of high-Al gabbroic dikes in the LAG, supp
orting the proposition that high-Al gabbros are parental to anorthosit
e (Mitchell et al., 1995). The isotopic data require that the anorthos
itic parental magmas were contaminated during ascent through the crust
by Archean orthogneisses and/or metapelitic rocks. Decreasing Is, and
increasing epsilon(Nd) With a relative decrease in age of the intrusi
ons (Snow Creek --> Poe Mountain/Chugwater --> troctolites) indicates
that the magma conduit became increasingly insulated from crustal cont
amination over time. This study indicates that the strontium and neody
mium isotopic systems can be used to distinguish between processes tha
t occurred at lower crustal/upper mantle pressures, during the ascent
of magma diapirs, and within individual magma chambers in Proterozoic
anorthosites, and underscores the need for stratigraphic control when
addressing the origin of plagioclase-rich cumulates.