Jd. Greenough et Jv. Owen, THE ROLE OF SUBCONTINENTAL LITHOSPHERIC MANTLE IN MASSIF-TYPE ANORTHOSITE PETROGENESIS - EVIDENCE FROM THE JOTUNITIC RED BAY PLUTON, LABRADOR, Schweizerische Mineralogische und Petrographische Mitteilungen, 75(1), 1995, pp. 1-15
The Red Bay pluton (RBP) occurs near the northeastern extremity of the
Grenville Province, which contains the world's most voluminous series
of Middle Proterozoic (c. 1.0-1.4 Ga), massif-type anorthosites. Mine
ralogically, the RBP resembles jotunites associated with anorthosites:
apatite and Fe-Ti oxides are abundant, plagioclase is sodic (An(32)-A
n(45)) given the Mg-rich nature of orthopyroxene (X(Mg) = 0.58-0.78) a
nd clinopyroxene (X(Mg) = 0.65-0.76), and biotite and hornblende are m
inor phases. Furthermore, the pluton is Fe2O3(t)-, TiO2-, and P2O5-ric
h (means of 8 samples = 11.7, 3.1, and 0.9 wt%, respectively). Non-lay
ered gabbros display high Ti/V and Ga/Al ratios that overlap jotunites
associated with massif anorthosites from several classic localities a
nd that distinguish jotunites from basalts of varied tectonic affinity
. The base of the intrusion (inferred from igneous cross-bedding and m
odal layering) is predominantly cumulus, and Mg# progressively increas
es upward into relatively unlayered gabbroic rocks. Mass balance and m
agma modelling calculations confirm that plagioclase and Fe-Ti oxide p
recipitation led to declining Ca and Fe contents but increasing Mg# (=
Mg/(Mg + 0.9 . total Fe), atomic) as the Red Bay magma evolved. Alkal
is-silica relations, elevated light REE concentrations, and high Ba co
ncentrations indicate that the magma was mildly alkaline. Positive Ba
and negative Nb and Ti anomalies (high Ba/Nb ratios) on MORB-normalize
d diagrams suggest magma derivation from subcontinental lithospheric m
antle metasomatized by subduction-derived fluids. Magmas such as shosh
onites that are associated with late-tectonic transpressional settings
characteristically have geochemical signatures indicative of such man
tle sources. Alternatively, jotunites may be subsurface analogues of e
xtension-related, orogen-parallel, mafic volcanic rocks (e.g., the Bas
in and Range province, western U.S.A.) bearing a subcontinental lithos
pheric mantle signature.