GEOCHEMICAL AND TECTONIC FINGERPRINTING OF OPHIOLITES

Ophiolites represent fragments of ancient oceanic lithosphere that were incorporated into continental margins during collisions, ridge-trench interactions, and/or subduction-accretionary events. They are generally found along suture zones in both collisional- and accretionary-type orogenic belts that mark major tectonic boundaries between collided plates or accreted terranes. The geological record of the Wilson cycle evolution of ancient ocean basins from rift-drift and seafloor spreading stages to subduction initiation and closure phases is commonly well preserved in most orogenic belts. Magmatism associated with each of these phases produces genetically-related, mafic-ultramafic to highly evolved rock assemblages, which occur in tectonic sheets. These units with varying internal structures, geochemical affinities and age spans become nested in collision zones, following the closure of ocean basins. Originally formed in different tectonic settings, tectonic slivers of oceanic remnants form discrete ophiolite complexes with significant diversity in their structural architecture, geochemical fingerprints, and emplacement mechanisms. These differences and regional tectonic constraints suggest that ophiolites form in a variety of geodynamic environments. We define here different ophiolite types based on their structural architecture, geochemical fingerprints, and tectonic evolutionary paths. Continental margin ophiolites form in small rift basins or embryonic oceans, have Hess-type oceanic crust pseudostratigraphy, and show N-MORB affinities; they may include exhumed subcontinental lithospheric mantle. Suprasubduction zone ophiolites (Mirdita, Oman, Troodos) form in extended arc-forearc to backarc settings, have Penrose-type architecture, and show MORB-IAT-boninitic geochemical progression of their magmas. These ophiolites signify oceanic crust generation during the closing stages of ocean basins and mark major subduction initiation events. Plume-type ophiolites form in plume-proximal ridges and oceanic plateaus, have thick plutonic and volcanic sequences, and show komatiitic, depleted (D-MORB), to enriched basalt (E-MORB) patterns. Mid-ocean ridge ophiolites form at plume-distal mid-ocean ridges or trench-distal backarc ridges, may have Penrose-type architecture, and show N-MORB, E-MORB, and/or C-MORB affinities. C-MORB ophiolites are crustally contaminated and may represent the products of ridge subduction or Andean-type backarc spreading in a continental-ensialic setting. Volcanic arc or Sierran-type ophiolites form in ensimatic arc settings, have polygenetic crustal architecture with multiply deformed, older oceanic basement and volcaniclastic cover (locally subaerial), and display calcalkaline affinities. This new classification of ophiolites helps to delineate the petrological lineage of mafic-ultramafic rock assemblages in Phanerozoic orogenic belts and Precambrian greenstone belts more effectively than the 1972 Penrose definition of an ophiolite. Fingerprinting ophiolites using their different lithological assemblages, chemical and isotopic compositions, internal structures, and regional geological characteristics is most useful to identify specific tectonic settings of ophiolite generation and to better document the processes through which these ancient oceanic rocks were incorporated into the continental margins.