The project co-ordinator: Professor A.H.F. Robertson (Edinburgh University)
Participants (teamleader): Edinburgh University (Prof. A.H.F. Robertson), University of Bretagne Occidental (Prof. Y. Lagabrielle), Geological Institute Russian Academy of Science (Dc. S.D.Sokolov), Institute of Geochemistry and Analytical Chemistry Russian Academy of Science (Dc. S.A.Silantyev), All Russian Geological Research Institute (Dc. A.K.Khudoley), Institute of Lithospere of Marginal Seas ( Prof. S.A.Palandzhyan).
The research has been carried out by a very experienced, well-balanced international team of Earth scientists using a multidisciplinary approach (structural and regional geology, stratigraphy, lithology, geochemistry, and palaeomagnetology)..
The main object of the work was study of the Late Mesozoic (Western Koryak) thrust-fold belt of North-East Russia (Fig. 1).
The main scientific results are the following:
The accretionary prism of the Taigonos Peninsula (Fig. 6) was studied in the field. The main results of the fieldwork are the following: (1) Detailed map of the Cape Povorotny area has been compiled (Fig. 7); (2) The structure of the Cape Povorotny area is represented by tectonic slices composed of rocks that were formed in different tectonic settings. Previously these rocks were considered as a unique coherent sequence; (3) The following tectono-stratigraphic units have been identified in the accretionary complex: (a) oceanic basalt-chert assemblages (Fig. 8, Fig, 9); (b) serpentinite melange with ophiolite fragments (Fig. 10, Fig. 10a); (c) island arc volcanics (Fig.11), clastic rocks and olistostromes of Lagerny unit (Fig. 11a); (d) turbidite and olistostrome seguences (Fig. 12); (e) metamorphic rocks of subduction origin; (f) tuff and terrigenous rock association of forearc origin; (4) Volcanics and cherts have been studied and sampled for chemical research and palaeomagnetic study (Table 1, Table 2, Table 3, Table 3a, Table 3b); (5) Cherts have been sampled to extract radiolaria and are shown a wide range of age (Middle Triassic to Early Cretaceous); (6) Several serpentinite melange units from different slices have been sampled for comparison of related ophiolites
Oceanic assemblages from the accretionary prism of Cape Povorotny (Taigonos key area) are represented by basalts and pillow-lavas associated with cherts, radiolarites and pelagic limestones. Study of more than radiolarite samples points to Middle Triassic and Early Cretaceous ages of host rocks. Early Cretaceous cherts differs in chemical composition from typical oceanic sediments and contain terrigenous material. Among accreted basalts three types are recognised (Table 3; Fig.13): (1) Moderate- to high-titanium N-MORB tholeiite and ferrotholeiite with (La/Yb)n=0.66-0.85; (2) Enriched intraplate alkaline basalts with Zr/Y=6.55 and (La/Yb)n=5.35-12.3; (3) Depleted island-arc tholeiite and andesite-basalts intruded fragments of oceanic crust of the first type. Moreover, boninites (TiO2=0.23-0.4%, MgO up to 11.4%) with V-shaped distribution of REE, and suprasubduction high-titanium basalts (TiO2=2.55, (La/Yb)n=1.35-3.34) with relative Ta-Nb minima are associated with clastic rocks.
Metamorphic rocks are represented by amphibolites and greenschists, found in the accretionary complex of the Cape Povorotny (Taigonos Peninsula) in tectonic slices of Kingeveem rock assemblage, serpentinite melange and in olistostromes and turbidite complexes. Different mineralogical types of amphibolites from accretionary structure of the Taigonos Peninsula, Northeastern Russia, were examined to identify their metamorphic conditions and protholith origin. It has been shown that metamorphic conditions of formation of garnet-free amphibolites andgarnet amphibolites, determined by amphibole-plagioclase and garnet-amphibole pairs, correspond to 450 - 500oC and 8 kb. The formation of low alumna actinolite in amphibolites examined occurred at T=350-380oC and P ú2 kb. Clinopyroxene-garnet amphibolites from Taigonos Peninsula were formed at T=760-820oC and P=8 kb. Geochemical data (major elements as well REE patterns (Tabls. 4, Fig. 14a, Fig. 14b, Fig. 14c) indicate that amphibolites were formed mainly after different members of MORB magmatic suite at temperature 450-820oC and 8 kb. Such P/T conditions of metamorphism may have occurred during subduction of relatively young, warm oceanic crust. The P/T metamorphic conditions of the rock assemblages indicate that the Povorotny Cape rock complexes were affected by two main metamorphic events. The first corresponded to high-pressure/medium-temperature conditions and was unique to the high-grade amphibolite. The second occurred in the form of low-temperature recrystallisation at greenschist-facies conditions and is recognised in all igneous rocks of Povorotny Cape and in part of the high-grade amphibolites.
In the Penzhina key area (Fig. 15) there are rootless ultramafic rock bodies with terrigenous units of Hauterivian age as a host rocks (Fig. 16). Within ultramafic bodies, massive ultramafic rocks alternate with sedimentary serpentinite units. The largest ultramafic body of Mount Dlinnaya (Fig. 17) consists of ultramafic rocks separated by breccia, conglomerate-breccia and sandstone (Fig. 18, Fig. 19, Fig. 20). Sedimentary serpentinites are composed of homogenous detritus of ultramafic rocks or heterogeneous detritus of peridotite, gabbro, metavolcanics, diabase, spilite, amphibolite, chert, siltstone, mica-quartz schist and marble composition. Current ideas on their origin (volcanic, tectonic and olistostrome) can not easily explain peculiarities of structure, composition and formation of ultramafic rock bodies. Study of related samples carried out by Geological Institute (Moscow), Bretagne Occidentale University (Brest), and Geochemical Institute (Moscow) teams shows that ultramafic rock bodies were formed in the front of Uda-Murgal island arc as the result of serpentinite diapiric intrusion and erosion. Modern analogues are seamounts of Mariana and Izu-Bonin forearcs (DSDP, Leg 25).
Palaeomagnetic study of Middle-Late Jurassic cherts from Kingeveem-3 slice (Taigonos key area) show that deposition occurred at 35█ N latitude [[Table 5, Fig. 21]. Kinematic restoration of the Kingeveem-3 slice transport implies that it was a part of Izanagi plate (Fig. 22). These data are very important for palinspastic reconstructions as previously it was accepted that accreted terranes were transported with the Kula (Zonenshain et al., 1990) or Farallon plates (Sokolov et al., 1997). Trajectories of transport of the Izanagi plate were estimated from study of cherts from the Taigonos Peninsula and compared with those from synchronous terranes of North America and Farallon plate [Bazhenov et al., 1999]. This work has constrained the kinematic motion history of the plate whichis now restored to a new location, that of the Farallon- Izanagi boundary in Late Jurassic to Early Cretaceous (Fig. 22). This conclusion is unexpected and needs verification by additional palaeomagnetic, palaeontological and palaeobiological studies.
The chemical composition and lithology of Middle Jurassic to Lower Cretaceous
cherts and mudstones of accretionary structure in the Pekulney Ridge (Ust-Belaya
key area) were studied (Table 6,
Fig. 23, Fig.
24). La/Ce and Zr/Y systematics was used to compare the cherts and sedimentary
rocks from ODP wells and circum-Pacific fold belts. The main types of cherts
are: (a) chert formed as the result of hydrothermal activity in the spreading
zone, (b) silica-rich metalliferous and radiolarian chert deposited on abyssal
plain, (c) chert-argillaceous sediments deposited in the upwelling zone, (d)
siliceous tuffite deposited in forearc basin. Cherts of a, b and c types were
deposited in oceanic environments (Mesopacific) far from sources of terrigenous
and volcanic rocks.