Geochemistry, Petrogenesis and Tectonic Environment of Circum-Superior Belt Basalts, Canada

Abstract

S U M M A RY: The Circum-Superior Belt of Proterozoic basalts and sedimentary rocks surrounds, and in some places, unconformably overlies the Archaean Superior Craton in northern Quebec, Canada. Four main groups of volcanic rocks are recognized. The oldest unit, the Eskimo Formation on the Belcher Islands, consists of subaerial massive continental tholeiites with 4-7% MgO, relatively high SiO 2 and K20, enriched LREE and other incompatible elements, negative Nb and Ta anomalies, and eNd (T = 1.8 Ga) of -7 to -8. The overlying Flaherty Formation contains largely subaqueous, massive or pillowed, mildly alkaline (transitional) basalts with relatively low SiO2, high alkalis and TiO2, enriched incompatible trace elements (including Nb and Ta), and eNa of + 3. Stratigraphically higher units (on the Ottawa Islands and the Chukotat Formation, Cape Smith Belt) contain subaqueous, massive or pillowed flows, and layered gabbro-peridotite flows with compositions ranging from MgO-rich ol-phyric basalt and evolved cpx-plag-phyric basalt to ol-cpx cumulate. Maximum MgO in noncumulate samples reaches 17 wt%, and trace element characteristics are MORB-like: LREE and other incompatible elements are depleted, and end is + 4 to + 5. The highest units, exposed on the Ottawa Islands, consist of komatiitic basalts and include unusual spinifex-textured layered flows. Although maximum noncumulate MgO contents are about 16 wt%, olivine xenocrysts in cumulate layers have the composition Fo92, and provide evidence of a komatiite (MgO = ,,-22%) precursor magma. Komatiitic basalts are less depleted than the underlying Mg-basalts, with flat to enriched LREE, negative Nb and Ta anomalies, and eNd = 0.The chemical features of Eskimo Formation basalts and spinifex basalts can be explained by crustal contamination of parental magmas derived from depleted mantle with end = + 5 : the continental tholeiites by 20-30% contamination of parental basalt and the spinifex basalts by about 7% contamination of parental komatiite. The Mg-basalt series results from partial melting of depleted mantle followed by low pressure fractional crystallization, whileFrom PHARAOH, T.

Petrology and geochemistry of a boninite dyke from the western Bastar craton of central India

“…Pearce et al (1992) suggested the genesis of boninites by melting of depleted residual metasomatic MORB mantle where metasomatism is due to fluids and melts are derived from the dehydrated subducted slab. Earlier studies (e.g., Arndt et al 1987; reported the role of assimilationfractional crystallisation (AFC) of komatiites for the formation of boninites. The present dyke represents a Whundo type of magmatism and is not consistent with a komatiite AFC model in its genesis because of no record of komatiite in the vicinity.…”

Geochemistry and petrogenesis of Neoarchaean high-Mg low-Ti mafic igneous rocks in an intracratonic setting, Central India craton: Evidence for boninite magmatism

“…It may be (i) Archaean equivalent of modern boninites (Redman and Keys, 1985), (ii) volcanic equivalent of Late Archaean high-Mg norites (Sun et al, 1989;Hall and Hughes, 1990a), (iii) derived from a refractory mantle source (Sun and Nesbitt, 1978), and (iv) believed to be derived from komatiite through assimilation-fractional crystallisation (AFC) processes (Arndt et al, 1987;Sun et al, 1989). Here it is important to mention that high-Mg norites and some SHMB are not found to associate with komattite occurrences Hughes, 1987, 1990b).…”

Neoarchean siliceous high-Mg basalt (SHMB) from the Taishan granite–greenstone terrane, Eastern North China Craton: Petrogenesis and tectonic implications

“…Nevertheless, it is evident that this type of SHMB generally has spinifex texture similar to typical komatiites (Sun and Nesbitt, 1978;Zhou et al, 2002;Polat et al, 2006b), and is commonly intercalated with the coeval komatiite (Arndt et al, 1987;. For the Taishan SHMBs, notwithstanding that some late Archean komatiites occur in the TSGT, the following lines of evidence do not support the AFC model, plus crustal contamination, of the Taishan komatiites.…”