http://www.sciencedirect.com/science/article/pii/S1674987114000401

In this paper,a new discrimination diagram using absolute measures of Th and Nb is applied to postArchean ophiolites to best discriminate a large number of different ophiolitic basalts.This diagram was obtained using2000 known ophiolitic basalts and was tested using w560 modern rocks from known tectonic settings.Ten different basaltic varieties from worldwide ophiolitic complexes have been examined.They include two basaltic types that have never been considered before,which are:(1)medium-Ti basalts(MTB)generated at nascent forearc settings;(2)a type of mid-ocean ridge basalts showing garnet signature(G-MORB)that characterizes Alpine-type(i.e.,non volcanic)rifted margins and ocean-continent transition zones(OCTZ).In the Th-Nb diagram,basalts generated in oceanic subductionunrelated settings,rifted margins,and OCTZ can be distinguished from subduction-related basalts with a misclassi fi cation rate1%.This diagram highlights the chemical variation of oceanic,rifted margin,and OCTZ basalts from depleted compositions to progressively more enriched compositions re fl ecting,in turn,the variance of source composition and degree of melting within the MORB-OIB array.It also highlights the chemical contributions of enriched(OIB-type)components to mantle sources.Enrichment of Th relative to Nb is particularly effective for highlighting crustal input via subduction or crustal contamination.Basalts formed at continental margin arcs and island arc with a complex polygenetic crust can be distinguished from those generated in intra-oceanic arcs in supra-subduction zones(SSZ)with a misclassi fi cation rate1%.Within the SSZ group,two sub-settings can be recognized with a misclassi fi cation rate0.5%.They are:(1)SSZ in fl uenced by chemical contribution from subductionderived components(forearc and intra-arc sub-settings)characterized by island arc tholeiitic(IAT)and boninitic basalts;(2)SSZ with no contribution from subduction-derived components(nascent forearc sub-settings)characterized by MTBs and depleted-MORBs.Two additional discrimination diagrams are proposed:(1)a Dy-Yb diagram is used for discriminating boninite and IAT basalts;(2)a Ce/Yb-Dy/Yb diagram is used for discriminating G-MORBs and normal MORBs.The proposed method may effectively assist in recovering the tectonic af fi nity of ancient ophiolites,which is fundamental for establishing the geodynamic evolution of ancient oceanic and continental domains,as well as orogenic belts.