Neotethyan Jurassic supra-subduction ophiolitic complex with Triassic subducted sole: Mineral chemistry, sole P–T estimates, and U/Pb geochronology of an intra-oceanic domain (Central Dinarides, Bosnia and Herzegovina)

Abstract

The ophiolitic blocks in the Cretaceous mélanges of the Central Dinaridic Ozren and Borja–Mahnjača massifs revealed two different evolutionary periods of the Neotethys Ocean: (1) A pre-subduction Middle Triassic to Early Jurassic spreading, and (2) Late Early Jurassic (Toarcian) intra-oceanic subduction and the formation of late Early to Middle Jurassic supra-subduction ophiolitic complex. The goal of this paper is to report the rebuilding of the upper oceanic plate mid-ocean ridge (MOR) abyssal to supra-subduction zone (SSZ) ophiolites. This process is indicated by an increase of Cr# in spinel (Spl) from ~0.1 to 0.6, exceptionally to 0.75 in peridotite, the Mg# decrease in orthopyroxene1 (Opx1) from ~89–91.5 in abyssal to ~86–88 in SSZ types of peridotites, as well as with overall Al and Ti decrease in pyroxene1. However, refertilization was rarely detected in newly formed Cpx, Opx, and Spl (2, 3) generations. A relatively thin, amphibole (Amp)-rich gabbro-dolerite layer of this Jurassic Ozren–Borja–Mahnjača ophiolitic complex may have formed in a nascent fore-arc, slow-spreading ridge. Remnants of Middle Triassic oceanic crust was dated at 242 ± 1 Ma from a relic zircon population in a trondhjemite (remelted plagiogranitic) dyke of the sole eclogite by LA–ICP–MS U–Pb method, whereas its main zircon population of 176 ± 2 Ma constrains the metamorphic-anatectic recrystallization age of the dyke in eclogite. Another trondhjemitic dyke gave a magmatic crystallization zircon age of 216 ± 6 Ma with rare inherited Middle Triassic (240–230 Ma) zircon. The clinopyroxene (Cpx)–garnet (Grt)–rutile (Rt) eclogites indicate the intra-oceanic subduction of the Triassic oceanic crust to about 50 km, which was estimated from Perple_X modelling of 1.5–1.6 GPa and 860–870 °C. However, a sole skarn achieved 950 °C at 0.5 GPa. Metamorphic zircon of a sole eclogite yielded 173 ± 2 Ma (D1 subduction event at ~180–175 Ma). Partial melting of the subducted slab and the mantle wedge initiated the transition of MOR to SSZ type ophiolites. The late Early Jurassic lower oceanic crust was dated on a gabbro (178 ± 1 Ma, zircon) and plagiogranite (177 ± 1 Ma, zircon). The Spl lherzolite, harzburgite, and dunite are crosscut by early Middle Jurassic Cpx–plagioclase (Pl) and Amp–Cpx–Pl gabbro, gabbro-pegmatite, leuco-gabbro (174 ± 1 Ma, zircon), and dolerite (174 ± 5 Ma, apatite) dykes, all suggesting an advanced evolutional stage and a shallower level of ophiolites due to extension and the deeper mantle melting in the SSZ setting. Inferred slab roll-back enhanced the sole exhumation (D2) between ~175–171 Ma followed by obduction of the upper plate, hot mantle rocks over the exhuming sole (the zircon age of 171 ± 0.5 Ma at D3 ~171–168 Ma from the sole Grt–pargasite–ilmenite skarn). The outboard-directed compression led to the formation of the rear fore-arc basin upper oceanic crust basalt–radiolarite section (~170–164 Ma) sited on the upper plate lower oceanic crust. Rutile and apatite from an eclogite gave exhumation (cooling) ages of 164 ± 3 Ma and 167 ± 8 Ma, respectively (D4 at ~168–160 Ma). An accretionary wedge ophiolitic breccia, with fragments of the late Early to Middle Jurassic oceanic crust and Triassic and Middle Jurassic radiolarites, indicates the closing of the Middle Jurassic Neotethys after ~164 Ma.