Peri-collisional extension and the formation of Oman-type ophiolites in the Banda arc and Brooks Range

Integration of data from field. geochemistry, and radiometric age invcstigations of the active Banda arc-Australia collision zonc (Banda orogen) and· the Jurassic Brooks Range ophiolite provide ncw constraints for the origin and emplacement of Oman-type ophiolites. These ophiolites arc characterized by ultramafic residuals and mafic products that arc chemically modified by "anous dcgrees of panial melting. TIle internal structure of thc igneous complcxcs documcnt acutc extcnsional strains. A high tcmperaturc metamorphic solc with some contincntal margin protoliths arc locally prcserved at the structural base of the massifs. Agc relations between the ophiolites. their metamorphic snle, and collisional mountains in which they reside indicate important temporal and spatial associations. Somc of these relations arc inconsistent witll ophiolite gcnesis in intra-oceanic settings. Formation of th~ Savu and Weber basins around the active Banda orogen provide modern analogues of peri-collision extensional processes. These extensional domains are transitional bctween zoncs of active supra-subduction zonc magmatism (ophiolitc gcnesis) and low-angle overthrusting (ophiolite emplacemcnt). Geochemical daw from the volc,lIlic basemcnt of the Savu basin arc similar to 'transitional-type' ophiolite volcanic suites. Arl Ar ages. and stratigraphic and structural data from the basin indicate volcanism and forearc extcnsion coincide in time and space with initial collision bctween promontorics of the NW Australian passive margin and the .Java trench. suggesting a link betwecn the two proccsscs. Similar age and stratigraphic relations arc also documented in the !3rooks Range and other Oman-type ophiolites. Peri-collisional extension opens small SSZ occan basins that may be obdueted during or shonly after thcy fonn by progressive convergencc at multi-plate boundaries. Thcse processes provide a self-perpetuating mechanism for ophiolite genesis and emplacement that accounts for both the similar and diversc aspccts of many uphiolitcs. An unambiguous classification for various mafic and ultram~'1lic complexes in orogenic zones (loosely cited as 'ophiolites') is needed to determine age allli tcctonic relations between ophiolites a,;d the orogenic belts in which they are found. 'Alpine-type' ultramafics and cven 'Tethyan-type' ophiolites include a diverse assortment of malic and ultramalic rocks. Somc of these complexcs are severely dismembered and even metamorphoscd: others are geochemically anomalous. Notwithstanding this diversity, most ophiolite complexes occupy a similar structural position in continental fold. thrust belts including Tethyan orogenies. At face value the dive;Sity of ophiolites argues against a single mechanism for ophiolite genesis, but structural rclations suggest a common mechanism for emplacement. The distinctive stratigraphic and strueturill nature of ophiolites (Colcman 1977) limits to some extent possible origins to submarine rift zones. These zones generally involve two major tectonic regimes: (I) continental rifts (c.g. Red Sea), which are transitional with mid-oceanic ridges. and (2) intercarc basins, which are often transitional with back arc basins (e.g. Lau Basin). Each of these regimes may also ineludc sites with a significant component of oblique spreading or transtension (c.g. Gulf of California and Andaman Sea). At convcrgent margins thesc two very different sites~ of ophiolite genesis arc usually juxtaposed and difficult to unravel. The integration of petrological, geochemical and radiomctric data from detailed lield studies of many ophiolites reveal some common fcatures that are useful for first order tcctonomagmatic discriminations. Compilations of these data by Pearce eT al. ([984) and Ishiwatari (1985) indicatc end-member compositions of MORB-type (Ligurian) anc! arc-type (Pupuan) tectonomagmatic origins. Intermediate compositions are interpreted as transitional-type