PETROGENESIS OF A MIDPROTEROZOIC ANORTHOSITE-MANGERITE-CHARNOCKITE-GRANITE (AMCG) COMPLEX - ISOTOPIC AND CHEMICAL EVIDENCE FROM THE NAIN PLUTONIC SUITE
Rf. Emslie et al., PETROGENESIS OF A MIDPROTEROZOIC ANORTHOSITE-MANGERITE-CHARNOCKITE-GRANITE (AMCG) COMPLEX - ISOTOPIC AND CHEMICAL EVIDENCE FROM THE NAIN PLUTONIC SUITE, The Journal of geology, 102(5), 1994, pp. 539-558
The Nain Plutonic Suite (NPS), about 1.30 Ga, underlies some 19,000 km
(2), intruding the boundary between Middle to Late Archean rocks in th
e east (Nain Province) and Late Archean and Early Proterozoic rocks in
the west (Churchill Province). Nd isotopic compositions in most igneo
us units of the NPS reflect their geographic position relative to the
inferred boundary between the Nain and Churchill Provinces. Rare excep
tions are local intrusions of olivine-bearing basic rocks with highest
epsilon(Nd) @ 1.3 Ga (near -3.0), present in both eastern and western
sectors of the NPS. Anorthositic rocks have variable and significant
contributions of crustal Nd but notably lower I,, than granitoid rocks
. Ferrodiorites are similar isotopically to the anorthositic group and
have chemical and mineral compositions more akin to the anorthositic
group than the granitoids. Aluminous orthopyroxene megacrysts in NPS a
northositic rocks indicate crystallization pressures of 6.2 to 11.0 kb
ar, consistent with lower crustal to upper mantle depths. The granitoi
d rocks crystallized from magmas derived in large part by crustal part
ial melting. These rocks show the influence of sources similar to the
basement gneisses, but were augmented by materials with much shorter c
rustal residence time, offering support for contemporaneous basaltic u
nderplating of the lower crust. Nd and Sr isotope data, trace element
geochemistry, and mineral compositions in major rock units of the NPS,
together with knowledge of the timing of intrusive events, permits as
sessment of the essential features of petrogenesis and development of
a paradigm that may have wider application. Separation of early granit
oid melts left depleted, hot, plagioclase-pyroxene granulite crustal r
esidues assimilated by mantle-derived basaltic magmas. That process de
veloped large volumes of anorthositic magma in upper levels of deep cr
ustal to uppermost mantle magma chambers by plagioclase flotation. Con
currently, fractionation moved residual liquids toward ferrodiorite co
mpositions. Buoyant upward movement of anorthositic magmas followed cr
ustal paths preheated by earlier passage of granitoid magmas. Only sma
ll Volumes of high density, Fe-rich, ferrodiorite melts were expelled
to higher crustal levels.