PARADIGM SHIFTS IN THE OPHIOLITE CONCEPT DURING THE LAST 50 YEARS, FOLLOWING MIYASHIRO’S 1973 CLAIM THAT THE TROODOS (CYPRUS) COMPLEX HAD FORMED IN AN ISLAND ARC

Miyashiro’s 1973 paper on the origin of the Troodos ophiolite in an island arc (IA) marked a major paradigm shift in the ophiolite concept and followed the 1972 Penrose definition, whereby the ophiolite–ocean crust analogy was confirmed as a manifestation of seafloor spreading at mid–ocean ridges (MORs). Miyashiro showed that glassy volcanic rocks in the Troodos extrusive sequence with high MgO & SiO₂ contents were unlike basaltic lavas at MORs, but rather characteristic of IAs. Discovery of ophiolitic rocks in modern convergent margins in the 1980s indicated that extension & magmatism in a forearc region during the early stages of subduction produced oceanic crust with arc chemistry & seafloor spreading structures. These findings led to the ’suprasubduction zone (SSZ)’ concept (Pearce et al., 1984), which explained the IA affinity & spreading structures in a unifying model. Stern & Bloomer (1992) and Bloomer et al. (1995) elaborated on the SSZ model in the context of the development of the IBM forearc crust due to subduction initiation magmatism during trench rollback. Moores et al. (2000) introduced a ‘historical contingency’ model to resolve the “ophiolite conundrum” (= apparent conflict between the MOR vs SSZ origin of ophiolites). They ascribed the flux melting influence in the magmatic evolution of SSZ ophiolites to metasomatism of the Earth’s mantle during former subduction events. They further defended (2021) their historical contingency model by pointing to SSZ–type lavas of the SE Indian Ridge oceanic crust, the formation of which may have involved partial melting of previously subducted mantle. But multiple lines of evidence from SSZ ophiolites require active subduction & associated hydrous melting of a mantle wedge, and fractionation of wet basalts. Whattam & Stern (2011) coined the ‘subduction initiation rule’ (SIR) & proposed that MORB & SSZ ophiolites represent end members of the SIR spectrum, progressing from less to more HFSE depleted & LILE enriched compositions due to increased subduction influence with time. However, the SIR does not apply to all ophiolites. Dilek & Furnes 2011) introduced a new classification of ophiolites & discussed how ophiolites show diverse evidence for their magmatic evolution in different tectonic settings. The corollary to this classification is that there is no need to invoke specific rules to explain the diversity of ophiolites.