THE MéLANGE – OPHIOLITE CONUNDRUM: INSIGHTS FROM IAPETAN AND TETHYAN OPHIOLITES, CANADA AND ALBANIA

The association of mélanges with ophiolites has long been recognized from orogenic belts worldwide. The Albanian Mirdita zone and the southern Québec ophiolite belt present such an association, and are here used to study orogenic processes leading to mélange formation. The Mirdita ophiolite is located in the Dinaro-Hellenic fold belt and forms a flat-lying, 4000km2 nappe comprising Tethyan oceanic lithosphere that has been obducted onto Adria (Schmid et al., 2008) and/or a Pelagonian microcontinent (Dilek et al., 2007). The southern Québec ophiolites are rather discontinuously exposed along a linear belt extending for 100’s of km in the Québec Appalachians (Tremblay & Castonguay, 2002), and form a composite slab of Iapetan oceanic lithosphere typically represented by the Thetford-Mines ophiolite (TMO).

The age of the TMO is ca. 480Ma (U/Pb on zircon), and its metamorphic sole rocks yield amphibole and muscovite, high-temperature ⁴⁰Ar-³⁹Ar ages clustering between 471 and 466Ma. The sedimentary sequence overlying the ophiolite belongs to the Saint-Daniel Mélange, and shows an apparent maximum thickness of 3 km. The Saint-Daniel Mélange unconformably overlies crustal rocks and mantle peridotite of the TMO, and locally, the infraophiolitic metamorphic sole as well. The base of the Saint-Daniel is therefore interpreted as an erosional unconformity, which is marked by a ca. 500m-thick discontinuous unit made up of mudstone, chert, sedimentary breccias, debris flows and mélanges. The breccias and mélanges contain fragments of ophiolitic lithologies and micaschists dispersed in an argillaceous to sandy matrix. Well-bedded to strongly disrupted horizons of pebbly mudstone, quartz-rich sandstone and green-to-black argilite constitute the upper part of the Mélange. The lower age limit of the Saint-Daniel is constrained by muscovite ⁴⁰Ar-³⁹Ar ages of 464-463Ma from micaschist fragments.

U/Pb zircon ages on the Mirdita ophiolite vary between 160 and 165Ma, whereas its metamorphic sole rocks, which are part of the Rubik-Peripheral complex, yield ⁴⁰Ar-³⁹Ar ages of 171 to 162Ma (Dilek et al, 2007). The sedimentary cover of the ophiolite is distributed in a series of small basins (ca. 20 km²) that commonly overlie the basaltic volcanic rocks. The base of the sedimentary sequence is locally marked by Late Triassic to Middle Jurassic radiolarian cherts, that are overlain by Middle to Late Jurassic conglomerates and mélanges grading into calcareous-siliciclastic Late Jurassic – Early Cretaceous turbiditic flysch (Bortolotti et al., 2005 & Gawlick et al., 2007). The mélanges and conglomerates comprise various types of ophiolitic and sedimentary rock fragments that consist of ophiolitic gabbro, basalt, peridotite and serpentinite, as well as continent-derived sandstone, chert, volcanics, carbonates and metamorphic sole-like rocks which can be reconciled with those exposed in the Rubik-Peripheral complex. In the Blinisht area, the mélange consists of limestone, sandstone, chert and ophiolite-derived fragments dispersed in a crudly stratified, scaly argillaceous matrix. The mélange grades into a broken formation that consists of turbiditic cherty mudstone and argilite showing slump fold structures, which indicate sliding on an east-facing slope, away from the inferred obduction front and towards the center of the basin. A small basin (1km-wide) located south of Rreshen was also investigated; the base of the supraophiolitic cover sequence is there marked by conglomerate made up of ophiolitic debris embedded in a mafic matrix. This conglomerate grades into mafic lithic greywacke and quartz-rich turbiditic flysch.

In summary, the supraophiolitic cover sequence of the Mirdita ophiolite is characterized, from base to top, by a transition from sedimentary mélanges and conglomerates, to broken formations and well-bedded flysch that were obviously formed in tectonically-active basins. As in southern Québec, Albanian mélanges appear to be derived from the erosion of the underlying ophiolitic and infraophiolitic nappes. It is suggested that the uplifting of a syncollisional (accretionnary) ridge during obduction and final emplacement of the ophiolites, led to the unroofing of ophiolitic and underlying continental rocks, and to the formation of mélanges and broken formations due to mass wasting and synsedimentary sliding onto the advancing ophiolite nappe. The increasing input of calcareous/siliciclastic material in both sedimentary sequences suggests the progressive stabilization of the sedimentary basins as obduction proceeded. Supraophiolitic mélanges of southern Québec and Albania are thus interpreted as syncollisional piggy-back basins that formed within ca. 5 to 10Ma during obduction.

REFERENCES. Bortolotti, V. et al., 2005. Island Arc 14, 471-493. Dilek, Y. et al., 2007. Gondwana Res. 11, 453-475. Gawlick, H.-J. et al., 2007. Int. J. Earth Sci. 97, 865-881. Schmid, S.M. et al., 2008. Swiss J. Geosc. doi:10.1007/s00015-008-1247-3. 48 pp. Tremblay, A. & Castonguay, S., 2002. Geology 30, 79-82.