Magma generation in the upper mantle, field evidence from ophiolite suites, and application to the generation of oceanic lithosphere

Abstract

The Bay of Islands ophiolite may be divided into a lower series of ultramafic tectonites representing mantle material and a higher series of cumulate and extrusive rocks and sediments which may be correlated with oceanic crust. The tectonite series consists of a lower spinel—lherzolite member overlain by harzburgites. Both are cut by numerous olivine-pyroxene veins which represent early crystallization products from a picritic tholeiite magma derived at 18-22 kbar by approximately 23 % partial melting of the spinel lherzolite. The remainder of this magma crystallized as differentiated cumulate and extrusive rocks under crustal conditions. The petrogenetic model derived implies a rising diapiric body beneath an accreting centre and allows for the production of tholeiitic, transitional and mildly alkaline basalts from a single parent. The nature of the basalt erupted depends upon the rate of upwelling and crystallization in the diapir. The model suggests that crystallization takes place in the diapir at depths of less than 60 km.

Boninitic and tholeiitic dykes in the Lewis Hills mantle section of the Bay of Islands ophiolite: implications for magmatism adjacent to a fracture zone in a back-arc spreading environment

“…Secondly, assuming these crystallization products were not transported upward great distances during asthenospheric flow, the field relations in the Springers Hill area suggest these products formed in the uppermost mantle in the refractory harzburgite -dunite mixture just below the mantlecrust transition zone. Based on these assumptions, and using the idealized vertical logs of the relatively complete ophiolite section in North Arm Mountain (Malpas 1978) and of a typical harzburgite-type ophiolite (Nicolas 1989), the crystallization products formed at approximately 4 and 10 krn depth, respectively. Even allowing for the assumptions, these depths fall well within the shallow depths (< 30 km) predicted for the generation of refractory magmas of the type which must have passed through the Springers Hill harzburgite-dunite mixture.…”

Variations in structure and petrology in the Coastal Complex, Newfoundland: anatomy of an oceanic fracture zone

“…The BOIC has a stratiform igneous structure conforming to the Penrose Ophiolite Conference (1972) definition (Williams 1973;Malpas 1978;Casey et al 1981;Casey & Karson 1981). From base to top this includes harzburgite tectonites, ultramafic cumulates, layered and isotropic gabbroic rocks, sheeted diabase-dykes, and basaltic pillow lavas.…”

Petrology of the İspendere (Malatya) ophiolite from the Southeast Anatolia: implications for the Late Mesozoic evolution of the southern Neotethyan Ocean

“…This indicates a Ti-poor magma source for the ophiolites that formed in a supra-subduction zone setting, whereas MORBtype ophiolites are characterized by relatively high Ti contents. Clinopyroxenes with low Ti contents have ben reported from many Eastern Mediterranean ophiolitic basalts and plutonic rocks, including the Pindos, Troodos (Capedri & Venturelli 1979), Oman (Pallister & Hopson 1981), Bay of Islands (Malpas 1978), Kızıldag (Bagcı et al 2005), Tekirova -Antalya (Bagcı et al 2006), Pozantı-Karsantı (Parlak et al 2000(Parlak et al , 2002 and Sarıkaraman (Yalınız & Göncüoglu 1999) ophiolites. Comparison of Si relative to Al IV in the clinopyroxenes (Table 6) indicates derivation from a tholeiitic melt based on the divisions of Kushiro (1960).…”