THE ROLE OF SUBCONTINENTAL LITHOSPHERIC MANTLE IN MASSIF-TYPE ANORTHOSITE PETROGENESIS - EVIDENCE FROM THE JOTUNITIC RED BAY PLUTON, LABRADOR

Citation
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
Citations number
60
Categorie Soggetti
Mineralogy,Geology
ISSN journal
00367699
Volume
75
Issue
1
Year of publication
1995
Pages
1 - 15
Database
ISI
SICI code
0036-7699(1995)75:1<1:TROSLM>2.0.ZU;2-3
Abstract
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.