2010

Paleozoic basite-ultrabasite assemblages of the Chersky Range

(Verkhoyansk-Kolyma folded region)

Trunilina V.F., Roev S.P., Orlov Yu.S.

Diamond and Precious Metal Geology Institute, SB RAS

v.a.trunilina@diamond.ysn.ru

One of the most hotly debated aspects of the geological structure of the territory is the nature of the basement on which it developed. Most researchers are inclined to believe that the basement includes portions of oceanic and transition-type crust, while others adhere to the idea of typical continental crust. A major evidence for the existence of ancient ocean basins on the territory of the Verhoyansk-Kolyma folded region is the presence of fragments and blocks of metamorphosed ultrabasic and basic rocks widely developed within the river basin of Uyandina and the Chersky Range. The rocks are generally believed to be ophiolitic in nature. Their origin is thought to be due to the obdu63.ction of oceanic crust during the amalgamation of terranes and emplacement of the Kolyma-Omolon microcontinent (Parfenov, 1995; Oxman, 2000).

However, the results of the field and laboratory studies of the basic and ultrabasic magmatic rocks from the river basin of Uyandina, the Chersky Range and adjacent areas of the Verkhoyansk-Kolyma folded region carried out by the authors make this now prevailing point of view doubtful.

The oldest ultrabasite-basite assemblages (723±15, 602±5 Ma, Rb-Sr method) from the river basin of Uyandina and its tributaries are close to the derivatives of continental magmatism with regard to typomorphic features of their minerals such as high iron and manganese content of olivines (f up to 32%, MnO up to 0.4%), prevalence of a monoclinic modification of pyroxenes, their low Cr₂O₃ content (avg. 0.1%), the presence of pargasite, kaersutite, garnet of the pyrope series (up to 72% py) and the absence of chrome-spinellides. The chemical composition and concentration of trace elements in the basites and ultrabasites are intermediate between those of oceanic and continental rocks. Compositionally they range from picrites and normal gabbroids (basaltoids) to diorites (andesites), and from alkaline picrites and subalkaline gabbros  (trachybasalts) to benmoreites (hawaiites) and monzonites (mugearites). More than a half of the rocks are olivine-diopside-normative in composition, about 30% - nepheline-olivine-diopside-normative, while the amount of hypersthene-diopside varieties characteristic of ophiolitic assemblages is as low as 15%. Petrochemically, most of the compositions correspond to intra-plate rocks; those corresponding to mid-ocean ridges are less common.  As compared to the latter, the rocks under study are enriched in Rb, Li, Ba, Th and Nb. The REE trends, like in ophiolites, are subhorizontal or show the reduction of their chondrite-normalized values from HREE to LREE, but with higher (avg. an order of magnitude) contents of both of them. These features as well as the combination of olivine-diopside-normative rocks of normal alkalinity and nepheline-olivine-diopside-normative subalkaline rocks are inherent in magmatic rocks from narrow interior ocean basins of which the beds are formed from thinned continental crust (Saunders, Tarney, 1987). This is evidenced, in the case of the Uyandina block, by the nearby exposures of Neoproterozoic ultrametamorphic gneiss-granites and rapakivi-granites (Trunilina, Roev, 2005), which are known to form only in the continental environment. The calculated pressure in the magma generation chambers and the depth to the seismofocal zone are 12-15 kbar and 137-151 km, respectively, for the rocks of normal alkalinity and 18-20 kbar and 188-208 km for the subalkaline varieties. These values are considerably higher than those given in literature for oceanic ophiolites.

The Early Ordovician rock unit is found to consist of volcanic sheets of basaltoids dominated by subalkaline and alkaline nepheline-normative varieties with compositions varying from trachybasalt to mugearite. Petro- and geochemically they belong to intra-plate continental formations. The calculated depth to the seismofocal zone is 135-189 km for the subalkaline varieties and 180-232 km for the nepheline-normative rocks. This implies they were derived from different mantle sources at different times. As compared to oceanic rocks, they are enriched in REE with their normalized values decreasing from LREE to HREE. The normalized La/Sm and La/Yb ratios are 3-4 and 10.6-11.7, respectively. In terms of trace elements contents, the rocks correspond to the geochemical type of Na-alkaline basalts, with deviations to shoshonite, on the one hand, and to tholeiitic continental basalts, on the other hand. Both are characteristic of intra-plate mamatism of the rear continental parts of Benioff-Zavaritsky zones and, on this basis, are assigned to rift-related continental or continental-marginal rocks.

The Middle Ordovician ultrabasite-basite assemblage includes metaperidotites, amphibolites, olivine-clinopyroxene and, more rarely, two-pyroxene gabbro-amphibolites, metabasalts, and trachyandesite-basalts occurring amidst terrigenous-carbonate rocks. Basaltic eruptions took place prior and subsequent to the emplacement of gabbroids, containing their numerous xenoliths, which led to the formation of a thick cover unconformably overlying serpentinites and gabbro-amphibolites. The gabbroid massifs are shaped as stocks and sheets. In thick sheets, transitions from gabbro to monzonite are evident. The rocks of the assemblage are not comparable to ophiolitic ones. This is evidenced by the geological structure obviously indicating a time interval between the formation of ultrabasites-basites and that of basalt sheets, by sharply prevalent monoclinic varieties of clinopyroxene, and by enrichment in alkalies, Ti and REE. Compositional parameters of the least altered samples are inconsistent with those of the rocks of ophiolitic assemblages. For metaperidotites the parameters are: AL₂O₃ = 4.3-14.5%, TiO₂ = 0.16-0.47%, K₂O = 0.08-0.78%, Na₂O = 0.29-0.9%, CaO = 0.54-11.1%, MgO/(MgO+FeO) = 0.77-0.86. Normative hypersthene, which is typical of ophiolitic rocks, is absent. The calculated depth to the seismological zone is 96-121 km for the ultrabasic rocks, 141-156 km for the basic rocks of normal alkalinity, and up to 308 km for the subalkaline ones. These values are much higher then those obtained for ophiolites, which implies different sources of their parent melts. The rocks are rich in REE, particularly LREE (two orders of magnitide higher than in chondrite), which is characteristic of the derivatives of enriched rather then depleted mantle. The normalized La/Yb ratios are in all cases greater than 1, REE trends are subhorizontal or with a clear decrease from LREE to HREE (Fig. 8). Cr-Y and Zr/Y-Zr ratios are consistent with those of continental derivatives. In general, the Middle Ordovician basite-ultrabasite assemblage of the Chersky Range is most close in its compositional parameters to magmatic assemblages of large fault zones in active continental margins or at lithospheric plate boundaries.

Magmatic rocks of Devonian age are represented by basalt sheets and abundant sills and dikes of picrites, alkaline picrites, dolerites, gabbro, olivine gabbro, gabbro-norites, and trachyandesite-basalts occurring amidst coaly marls, siltstones, and coaly pelitomorphic and organogenic limestones. Rb-Sr isochron age of the basic rocks is 382±15 Ma. Regarding typomorphic features of minerals (moderately ferruginous olivine with f = 49.4-49.7; prevalence of a monoclinic modification of pyroxenes compositionally corresponding to those of the derivatives of basite-hypebasite and alkaline continental rocks; pargasite and kaersutite, titanomagnetite, sphene; chromites; chrome-spinellides of the dunite-peridotite and dunite-pyroxenite-gabbro association) and intense K, Rb, Ba and LREE enrichment, the rocks are referred to as intra-plate rift-related rocks of the geochemical type of Na- alkaline continental basalts. The chemical composition of the basic rocks is olivine-diopside-normative and nepheline-olivine-diopside-normative. In rare cases, along with normative nepheline normative leucite is present. The calculated depth to the seismofocal zone is, on the average, 241, 203 and 185 km for the rocks with 40%, 40-45% and over 45% SiO₂, respectively. The estimated pressures in the magma generation chambers are 30, 23 and 17 kbar. The rocks with the lowest SiO₂ content contain maximum amounts of alkalies, which implies migration of parent magmatic chambers either into less depleted or intensely metasomatically altered mantle horizons.

Magmatic rocks of Carboniferous age are known from the adjacent area within the Selennyakh anticlinorium. They include basalt and hawaiite sheets as well as gabbro and syenite-gabbro stocks occurring amidst Carboniferous limestones. Most common are olivine-diopside-normative varieties, with nepheline-olivine-diopside-normative ones being subordinate. The average composition of the rocks corresponds to tholeiitic basalts of the geochemical type of continental tholeiites, with deviations to Na-alkaline basalts in terms of Na₂O (avg. 3.14%), Cr (280 g/t) and Ni (176 g/t) contents. Y/Nb ratios (0.9-1.9, avg. 1.3), as well as average contents of TiO₂ (1.58%), P₂O₅ (0.17%) and Zr (112 g/t) are characteristic of continental tholeiite series (Floyd, Winchester, 1975). The REE trends exhibit considerable enrichment in LREE and the absence of Eu-minimum. In terms of most indicator petro- and geochemical coefficients, the rocks are classified as intra-plate continental rift-related formations. The least differentiated rock varieties are calcareous-alkaline basalts, with an estimated depth to the seismofocal zone of 151 km and a pressure of 18 kbar in a magma chamber.

Thus, among the rocks of the ultrabasic-basic assemblages in the river basin of Uyandina, the Chersky Range and adjacent areas we have identified Neoproterozoic rocks of the paraocean basin, which was initiated on the thinned continental crust; Early Ordovician continental intra-plate rocks, Middle Ordovician transform fault zones; and Devonian and Carboniferous continental rift-related rocks. The presence of typical ophiolitic assemblages was not confirmed.

References

Parfenov, L.M., 1991. Tectonics of the Verkhoyansk-Kolyma Mesozoides in the context of plate-tectonics // Tectonophysics, v. 139, p. 319-342 (in Russian).

Parfenov, L.M., 1995. Terranes and the history of formation of Mesozoic orogenic belts in East Yakutia. Tikhookeanskaya Geologiya, v. 14, № 6, p. 32-43 (in Russian).

Oxman, V.S., 2000.  Tectonics of the Chersky collisional belt. M.: Geos., 268 p. (in Russian).

Saunders A.S. and Tarney, J., 1987. Geochemical features of basaltic volcanism in backarc basins // Geology of marginal basins. M.: Mir, p. 102-133 (in Russian).

Floyd P.A., Winchester, J.A., 1975. Magma type and tectonic setting discrimination using immobile elements // Earth and Planet. Sci. Lett. V. 27, p. 21-218.

Trunilina, V.A., Roev, S.P., 2005. Composition and ore-bearing potential of A-type granites of the Kolyma tectonic block (northeast Yakutia) // Mineral Deposits: Meeting the Global Challenge. Berlin: Springer, v. 1, p. 469-472.