Ophiolitic Series Research Articles

Extensional to compressive Mesozoic magmatism at the SE Eurasia margin as recorded from the Meratus ophiolite (SE Borneo, Indonesia)

The Meratus ophiolitic series (SE Borneo) present a specific assemblage that have recorded (1) a continental extensional episode mostly seen within the peridotites and 2) later subduction-related magmatic events marked by the emplacement of calc-alkaline magmas. These events relate the magmatic activity and geodynamic evolution of the SE Eurasia margin in Mesozoic times. The ophiolitic series comprise ultramafic rocks with minor metavolcanic rocks. The ultra-mafic rocks include dominant lherzolites and pyroxemtes with rather scarce harzburgites and dunites. Spinel peridotite, mineral chemistry data and bulk rock Rare Earth Element (REE) abundances show that most rocks underwent a low degree of partial melting. However, a few samples display significant depletions in Light REE (LREE), which are inter preted as the result of fractional melting under shallow conditions. Plagioclase-bearing peridotites are characterized by high REE abundances which also point to a very low degree of melting followed by reequilibration in the plagioclase facies, as seen from phase chemistry data. These peridotites are locally crosscut by dikelets containing high-temperature K- and Cr-rich amphiboles. Lavas closely associated with the Meratus peridotites have REE compositions ranging rom the ones typical of enriched MORB (E-MORB) to norma (N-MORB) types. We believe that the Meratus peridotites represent a fragment of subcontinental lithospheric mantle tha locally suffered a low degree of fractional melting during the last stages of a continental rifting phase, in agreement with presence of metamorphic K- and Cr-rich amphiboles in the peridotites. The E-MORB basalts might result from the melting of an enriched subcontinental lithosphere thermally eroded during the rifting phase by rising asthenosphere might have produced N-MORB volcanic rocks. Back-arc basin basalts (BABB) now associated with E-MORB and N-MORB have also been found in the metamorphic soles of the peridotites. These rocks would have formed in a back-arc basin now accreted to the eastern margin of Eurasia. The latter are partly covered by calc-alkaline magmatism (Alino Formation). The ophiolitic series was later crosscut by calc-alkaline melts (Manunggul Formation). The Meratus ophiolitic series hence displays a dual origin. They witness 1) a continental episode mostly seen within the peridotites, and 2) later subduction- related events marked by the emplacement of calc-alkaline magmas. © Elsevier, Paris

Extensional to compressive Mesozoic magmatism at the SE Eurasia margin as recorded from the Meratus ophiolite (SE Borneo, Indonesia)

The Meratus ophiolitic series (SE Borneo) present a specific assemblage that have recorded (1) a continental extensional episode mostly seen within the peridotites and 2) later subduction-related magmatic events marked by the emplacement of calc-alkaline magmas. These events relate the magmatic activity and geodynamic evolution of the SE Eurasia margin in Mesozoic times. The ophiolitic series comprise ultramafic rocks with minor metavolcanic rocks. The ultramafic rocks include dominant lherzolites and pyroxenites with rather scarce harzburgites and dunites. Spinel peridotite, mineral chemistry data and bulk rock Rare Earth Element (REE) abundances show that most rocks underwent a low degree of partial melting. However, a few samples display significant depletions in Light REE (LREE), which are interpreted as the result of fractional melting under shallow conditions. Plagioclase-bearing peridotites are characterized by high REE abundances which also point to a very low degree of melting followed by reequilibration in the plagioclase facies, as seen from phase chemistry data. These peridotites are locally crosscut by dikelets containing high-temperature K-and Cr-rich amphiboles. Lavas closely associated with the Meratus peridotites have REE compositions ranging from the ones typical of enriched MORB (E-MORB) to normal MORB (N-MORB) types. We believe that the Meratus peridotites represent a fragment of subcontinental lithospheric mantle that locally suffered a low degree of fractional melting during the last stages of a continental rifting phase, in agreement with the presence of metamorphic K- and Cr-rich amphiboles in the peridotites. The E-MORB basalts might result from the melting of an enriched subcontinental lithosphere thermally eroded during the rifting phase by rising asthenosphere which might have produced N-MORB volcanic rocks. Back-arc basin basalts (BABB) now associated with E-MORB and N-MORB have also been found in the metamorphic soles of the peridotites. These rocks would have formed in a back-arc basin now accreted to the eastern margin of Eurasia. The latter are partly covered by calc-alkaline magmatism (Alino Formation). The ophiolitic series was later crosscut by calc-alkaline melts (Manunggul Formation). The Meratus ophiolitic series hence displays a dual origin. They witness 1) a continental episode mostly seen within the peridotites. and 2) later subductionrelated events marked by the emplacement of calc-alkaline magmas.

Petrology and geochemistry of the Cyclops ophiolites (Irian Jaya, East Indonesia): Consequences for the Cenozoic evolution of the north Australian margin

The Cyclops massif (Irian Jaya - Western Indonesia) displays all components of an ophiolitic sequence including residual mantle peridotites (harzburgites and dunites), cumulate gabbros, dolerites, normal mid-oceanic ridge basalts (N-MORB) and minor amounts of boninitic lavas. This ophiolitic series tectonically overlies high temperature (HT)-high pressure (HP) mafic rocks metamorphosed during the Miocene.

Arguments en faveur de la position, au Jurassique, des ophiolites de Balagne (Haute-Corse, France) au voisinage de la marge continentale européenne

The allochthonous ophiolitic series of Balagne (Corsica) belonged to the Jurassic Ligurian-Piedmont ocean and was situated close to the Western European continent. Sedimentological and geochemical data constrain such a palaeoposition: (a) latest Malm limestones lying on the ophiolites rework different components of the continental basement; (b) the observation of interbedded zircon-bearing sandstones within pillow-lavas, and the nature of the zircons identical to those of the neighbouring granitic Variscan batholith of Western Corsica; (c) the E-MORB composition of these oceanic basalts, created during the beginning of oceanization. During Upper Eocene, the Balagne ophiolitic nappe had to be obducted onto the European crust, whereas the ligurian ‘schistes lustrés’ acquired their metamorphism by subduction, to the west, under the same continental basement.

Mg-metasomatism and formation conditions of Mg-chlorite-muscovite-quartzphyllites (leucophyllites) of the Eastern Alps (W. Hungary) and their relations to Alpine whiteschists

The lower Austroalpine orthogneiss-micachist complex of the Sopron-Fertorakos area of W. Hungary contains Mg-chlorite-muscovite-quartzphyllites (leuco- phyllite) and Mg-chlorite-bearing kyanite quartzites whose chemical compositions differ greatly from their surrounding rocks. Formation of leucophyllites took place in shear zones and was associated with depletion in alkalies and iron and enrichment of magnesium and H2O. Mg-zonation of relict igneous muscovites of leucophyllites and changes in the whole rock chemical compositions suggest Mg-metasomatism. Material gains and losses have been assessed using the composition-volume relationship approach. Proceeding from metagranite through transition rocks to leucophyllites, MgO, H2O, FeO, and alkalies show continuously increasing dispersion in isocon plots with Mg-enrichment even in sheared gneiss not in contact with leucophyllite. The metasomatic processes that formed the Mg-rich rocks may be similar to those responsible for the formation of high pressure whiteschists in the Central and Western Alps. The geochemical characteristics of the Dora Maira whiteschists (Italy) and their country gneisses are very similar to those of the Sopron leucophyllites, supporting the theory that Mg-metasomatism produced the whiteschist chemistry. On the basis of oxygen isotope compositions of relict igneous muscovites, the precursor granitic rock had a δ18O value around 13‰ proving its crustal anatectic origin. The leucophyllites have whole rock oxygen isotope compositions around 8.5‰ which is in conflict with the theory of an Mg-rich sedimentary protolith. Rather, the low δ18O values reflect fluid/rock interaction with a low δ18O fluid. Quartz-mineral oxygen isotope fractionations yield a metamorphic temperature of 560 ± 30 °C which agrees with earlier estimates from mineral stabilities. Silicon contents of phengites correspond to a metamorphic pressure of ∼13 GPa at this temperature indicating eclogite facies metamorphism. The fluids in equilibrium with leucophyllites had oxygen isotope compositions around 7.9‰, similar to those calculated for the ultrahigh pressure Dora Maira whiteschists (7.6‰), further supporting the genetic link between the leucophyllites and whiteschists. Hydrogen isotope compositions of mixed white mica + chlorite samples from leucophyllites range from −40 to −35‰, correlating with chlorite contents. The calculated endmember chlorite and white mica have δD values of −30 and −40‰, respectively. The similar δD values of the white micas in leucophyllites, gneisses and metagranites suggest an overall equilibration with respect to H isotopes. The calculated δD value of the fluid is approximately 0‰, suggesting a seawater origin. This conclusion was also reached for the Dora Maira whiteschists. A possible fluid source that satisfies both metasomatic and isotopic data is dehydration of hydrothermally altered oceanic crust. The mafic–ultramafic complex of the Alpine Penninic unit underlying the Austroalpine nappes is a likely candidate. The subduction and subsequent dehydration of the ophiolite series would supply the Mg-rich fluids whose migration brought about the metasomatic alteration of the overlying gneiss-micaschist complexes.

Origin and evolution of the Paleozoic Cabo Ortegal ultramafic-mafic complex (NW Spain): UPb, RbSr and PbPb isotope data

UPb and RbSr dating of various minerals from the mafic-ultramafic Cabo Ortegal complex reveal the occurrence of high-grade metamorphism and mantle melting between 406 and 383 Ma. This event reflects deep subduction of oceanic lithosphere underneath a continental plate margin, including melting of detrital material and carbonates. Magmatism lead to the injection of locally garnet-bearing pyroxenite layers into the ultramafic rocks. It also caused the emplacement of carbonate-rich garnet-clinopyroxene rocks and pegmatites into peridotites and mafic granulites. The upper time limit is events is defined by the crystallization of 406 ± 4 (2 σ) Ma zircon in carbonate-rich garnet pyroxenites, and the lower limit is given by 383 ± 1 Ma old rutile from a garnet pyroxenite, and by 383 ± Ma titanite that crystallized in a carbonate-rich layer. All other zircon, monazite and titanite analyses lie between these two age limits defining an average age of 388 ± 1 Ma. These small age differences observed between the different minerals substantiate that the UPb chronometer in zircon, monazite and titanite behaved as a closed system in the garnet-clinopyroxene stability field (∼800°C and 1.35–1.65 GPa). The new geochronological data neither confirm the occurrence of a 480 or 420 Ma high-grade metamorphic event, as suggested earlier. In consequence, the Cabo Ortegal complex most likely was produced by a single subduction event (406–383 Ma), which was immediately followed by obduction, and incorporation of the complex into the orogenic belt of NW Iberia. Zircon dating of a plagiogranite in the Ophiolitic Series that tectonically underlies the complex substantiates basaltic crust formation at 472 ± 3 Ma, in association with melting of Precambrian detrital material. This Ordovician magmatic event probably occurred coevally with formation of the protoliths from which the mafic granulites and eclogites of the Cabo Ortegal complex were produced by subduction ∼70 m.y. later. On the other hand, emplacement of the exceptionally well-developed pyroxenite layers in the Cabo Ortegal complex appears to be exclusively a result of peridotite re-melting during earliest Devonian subduction at 406–383 Ma.

Peraluminous potassium-rich granitoids in the Semail Ophiolite

Potassic granites are found as dikes and small stocks in the uppermost part of the mantle sequence of the Semail ophiolite, predominantly in the northern and less frequently in the central and southern part. New chemical analyses show that the biotite granites lie on the same evolutionary trend as the quartz diorites and tonalites, occurring as late intrusives in the gabboro and mantle sequence. Based on mineral chemistry, the P-T conditions of andalusite-cordierite bearing pegmatoidal granites were calculated, giving pressures of 1.8–2.2 kbar and a solidus within the stability field of andalusite. Hydrothermal experiments on five natural granite and tonalite compositions gave minimum melting between 650 and 675° C at 2 kbar. At 700° C, excess H2O, there is already 70–80% melt from the granitic starting compositions. Potassic granites in the ophiolite can be the result of extreme fractional crystallization in the late intrusive series. Granitic melts can also be produced by a two-stage process, in which fluids escaping from the underlying metamorphic sole trigger partial melting of the more felsic members in the ophiolite series. Two-mica granites and pegmatites in the underlying metamorphic sole, possibly melting products of amphibolites and biotite gneisses, are contemporaneous. These escaping fluids could also explain the Pb and Sr isotopic compositions of the leucocratic dikes, indicating variable crustal contamination.

496 My age of plagiogranites in the Chamrousse ophiolite complex (external crystalline massifs in the French Alps): evidence of a Lower Paleozoic oceanization

Genuine ophiolitic series are extremely rare in the Variscan orogenic belt and the Chamrousse formation appears as an exception. The main lithological and geochemical characters of the ophiolite are summarized and a special attention is paid to the field relationships and the magmatic filiation of the leucocratic rocks. The time evolution of the mafic magmas is considered to express the transition of attenuated continental lithosphere to oceanic lithosphere. The silicic rocks represents the final products from a multisequence fractional crystallization of a tholeiitic LIL-enriched magma and their compositional diversity results from the local increase of the H 2O partial pressure during syn-accretion tectonics. Dating the plagiogranites, using theUP/b method on zircons, also give the date of the oceanization. The resulting age of 496 ± 6Ma is closely comparable (1) to the accretion age of ophiolites from peri-Atlantic Paleozoic belts, and (2) to the protolith crystallization age of the “leptyno-amphibolitic groups” (LAGs) from Western Europe. But these relics (ophiolites and LAGs) of the Cambro-Ordovician distensional event later undergo very different tectonometamorphic evolution. The distinct post-magmatic evolutions may be related to their rather distinct original geodynamic significance and to their primitive location with respect to the subsequent zonation of the Hercynian belt.

La Desirade Island (Lesser Antilles) revisited: Lower Cretaceous radiolarian cherts and arguments against an ophiolitic origin for the basal complex

La Desirade Island in the Guadeloupe archipelago is a keystone of the structural framework of the Lesser Antilles island arc. Hypothetical ophiolitic series and wide discrepancy in the radiometric datings of its igneous basement have raised considerable debate concerning the structural significance of this island and the geodynamic constraints for the Caribbean plate. Recent detailed field work leads to a reconsideration of the problem. The volcanic basement of La Desirade can be subdivided into three independent units. The cherts interbedded in the unit 2 basalts contain Hauterivian-Barremian radiolarians that represent the oldest biostratigraphic assemblage found in the Lesser Antilles. This affirms the Jurassic radiometric age of the unit 1 trondhjemite outcrop. Observations and analyses allow us to discard the concept of an ophiolitic origin for La Desirade, whose basement was generated, like the northern half of the Lesser Antilles arc, by island-arc processes during the Mesozoic.

Chromite deposits in the ophiolite complex of Northern Oman

In the ophiolite series of N. Oman numerous small and large elongated podiform chromite deposits occur within the peridotite complex at a certain level of 100–200 meters below the layered zone, which grades into the overlying gabbro complex. The chromite grains were corroded after their deposition; their composition shows a large variation in Cr/Al ratio and a small variation in Cr/Fe ratio. The chromium is thought to have been liberated from clino-pyroxenes during partial melting of lherzolitic upper mantle material. Precipitation of large quantities of chromite at the specific level below the gabbro complex is related to the plagioclase- /spinel-lherzolite transition, but the influence of a change in oxygen fugacity is also considered.

Pre-Permian Global Tectonics: A Tectonic Test

Post-Permian sea-floor spreading coupled with fragmentation and widespread drift (> 1,000 km) of large segments (>107 km2) of continental crust appear established. However, studies of the nature and patterns of orogenic belts formed from the early Archean to the Recent suggest the post-Permian, large-scale dispersive movements of major continental fragments are uncommon, possibly unique in the geological record. Widespread drift and agglomeration of arcs, inter-arc basins, seas, and protocontinental segments also are indicated in conjunction with numerous subparallel spreading centers and subduction zones in the relict Archean rock complexes formed between 2.5 and 3.5 b.y. (b.y. = 109 yrs). But the Archean-type global tectonics were characterized by both spreading and accretive movements of evolving arcs, intervening basins, and protocontinental segments. Accretive motions dominated throughout the final 500 m.y. of Archean time, culminating in perhaps one, or, at most, three major protocontinents at about 2.5 aeons ago. The ensialic nature of most Proterozoic orogenic belts and their lithic components, as well as their interrelations to Archean terranes, indicate that they evolved in large part on and between the closely spaced Archean protocontinental clusters. These underwent deformation, refractionation, and thickening as the Proterozoic orogens evolved, but without the major fragmentation, widespread dispersion, and recollision of continents typical of the post-Permian. Late Proterozoic and early Paleozoic orogenic patterns and features, including ophi-olitic series within some orogens, indicate rifting, rotation, subordinate drift, and collision of large fragments of the evolving megacontinental masses. But the dominating continental component in associated orogenic belts indicates the scale of drift was limited to “second-order” movements commonly involving much less than 1,000-km drift, and no more than 20° rotation of these fragments. The rifting and collisions indicated in the late Proterozoic and early Paleozoic appear to be the incipient phase of the global dispersal of major continents in the post-Permian. Structural evidence for these conclusions appears in the accordant tectonic patterns that result when the Americas, Africa, India, and Australia are reclustered into the pre-Triassic Gondwana and western Laurasia defined by many independent lines of evidence. In this megacontinent, especially in Gondwana, fold belts of Paleozoic, Proterozoic, and Archean ages are juxtaposed into subparallel, coherent entities. In general the fold belts decrease in age from north to south. These first-order features seem to require a consistent interrelation and orientation of southern Gondwana with remarkably uniform regional force fields in the mantle, under, and seaward beyond Gondwana throughout at least 3.5 b.y. of geologic time. The further accordance, along great circles or parallels, of the Archean fold belts of Permian North America with those in Permian Gondwana suggests either a single mega-Archean orogenic system, or two complementary subparallel systems which have undergone little differential reorientation throughout the entire Proterozoic and Paleozoic. Because the motions necessary to generate the subparallel mega-Archean fold systems of the world did not deviate widely from the contemporary meridians of latitude, they seem largely of internal (radiogenic?) origin or only indirectly related to obvious extraterrestrial forces, such as changes in rates of rotation of the earth.

Les lambeaux allochtones du revers septentrional du massif des Psiloriti (Crete moyenne, Grece)

Abstract Mainly Gavrovo Series, two allochthonous series, Mesozoic Pindic and ophiolitic series, paleogeography

A propos de l'age de la serie ophiolitique dans les Dinarides yougoslaves; la coupe de Mihajlovici aux confins de la Serbie et du Montenegro (region de Plevlja, Yougoslavie)

Abstract The Mihajlovici area in southwestern Yugoslavia is a mosaic of fault blocks produced by two sets of large faults, trending NW and NE. The horsts are lower Jurassic limestone, and the grabens are occupied by diabase-radiolarite, known as the ophiolitic series, of lower Malm age (upper Jurassic). Abundant ammonites and microfauna identify several horizons, notably the Toarcian (upper Lias, i.e., lower Jurassic) and the uppermost bed of the lower Malm, thus fixing the Malm age of the ophiolitic series.