Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Petrology and geochemistry of basic-ultrabasic forearc ophiolite banded complexes.

Raisa M.Yurkova, Boris J.Voronin

Institute of oil and gas problems RAS, bivrmyrzb@mty-net.ru

The present paper is based on the results of the study of ultrabasic-basic banded ophiolite complexes in the northwestern  segment of the active continental margin of the Pacific Ocean: Sakhalin, Kamchatka and the Koryak Ridge. A  banded rock series was formed upon polycyclic intrusion of mafic gabbronorite magma along dyke-like channels into lherzolite,  wehrlite, apodunite-harzburgite, serpentinite bands affected by the extension of the arch of an uplifting mantle diapir  in the primitive island arc-trough transition zone [3]. Magma is assumed to have crystallized dynamically to form pyroxenites  [4]. Depths varied from 20-30 to 10-12 km. These conditions were conducive to the formation of differenttemperature  bimetasomatic beds: 1) apogabbronorite beds (T=900^oC) consisting of bronzite, diopside and pargasitic hornblende;

2) apolherzolite beds (T=550-700 ^oC) composed typically of diopside, pargasitic hornblende, grossular-andradite  and herzinite. The presence of herzinite suggests high-pressure conditions. Aposerpentinite beds are formed of lizardite,  pentlandite and chrome-spinellid. Gabbronorites and pyroxenites were subjected to autometamorphic amphibolization  (T=700-800^oC) to form various combinations of edenite, edenitic, high-Mg and tschermakitic hornblende and magnesiohastingsite.  Gabbronorites, screened by serpentinites in solid-ductile condition, were recrystallized under granulite-facies metamorphic  conditions (T=830-880^oC). As a result, the following mineral associations were formed: anorthite, bronzite-hypersthene, diopsidesalite

and titaniferous magnetite. The orientation of metamorphic banding in the rocks and the arrangement of multiple twinned  bands suggest that the recrystallization of gabbronorites was induced by shear strain and sliding deformations directed along the  gabbronorite-ultrabasic rock contact. Screening is indicated by a lower degree of multiple mineral alterations of recrystallized gabbronorites  in comparison with non-recrystallized igneous types of these rocks. In addition, screening seems to have contributed to  the persistence of relatively low strontium isotope ratios in recrystallized gabbronorites (Table 1). These values exceed the upper  limit of ⁸⁷Sr/⁸⁶Sr ratios in MORB and are characteristic of rocks from most modern island arcs and active continent margins [2].  The destruction of diopside lamellae upon recrystallization of bronzite resulted in the increased role of „R„p cations in plagioclase  and Fe in newly-formed pyroxenes. These trends are more conspicuous in contact-reaction interrelations with ultrabasic rocks.

Recrystallized gabbronorites typically show a well-defined negative Eu anomaly, suggesting their non-cumulate genesis. Nonrecrystallized  gabbronorites exhibit both positive and negative Eu anomalies. The Eu-anorthite molecule ratio in plagioclase from  these rocks is noteworthy. The incipient relationship might be accidental. Data are presented in the order of collecting (see Table  1).

Table 1.  ⁸⁷Sr/Sr ⁸⁶  variations gabbronorite plagioclases. Geological Institute RAS laboratories.  Electron microprobe analysis.

Garnet amphibolites and eclogite-like rocks were formed step by step under dynamothermal metamorphic conditions in locally  elevated temperature (C=700-800^oC) and pressure (P>5 kbar) zones. These rocks occur as band-like and lens-shaped granoblastic-

textured rocks, 2.0 x 0.8 m2 in size, oriented subconcordantly with banding in websterite beds. Garnet-bearing phyllonites,  formed after granet amphibolites and eclogite-like rocks, and other types of phyllonites are characteristic of foliation zones in   banded complexes. Almandine porphyroblasts in phyllonites have a low pyrope molecule concentration (Table 2). The smaller  pyrope minal content of phyllonite garnet in comparison with that of original rocks is attributed to its recrystallization under lowertemperature  dynamometamorphic conditions. The polygenetic nature of banded complexes, in which gabbronorites are not coeval  with lherzolites and their host apodunitic-harzburgitic serpentinites, is thus revealed. Gabbronorites and lherzolites intruded serpentinites  at different depths („Q„S- conditions). In this sense, banded complexes can be interpreted as polygenic basic-hyperbasic plutons.  The composition of spinel suggests that lherzolites were crystallized at elevated pressures. The equilibrium ortho- and clinopyroxene  formation temperature (T = 950^oC), calculated with L.L. Perchuk?f geothermometer [1], and the stability limits of spinelperidotite  facies suggest that lherzolites from the banded complex were formed at a depth of 30-55 km and a pressure of 8-10 kbar  [5]. Serpentinites can exist under such conditions [3]. Gabbronorites were recrystallized at a temperature of 880-925^oC, based on  the above estimates. The stability conditions of plagioclase-pyroxene parageneses indicate that they were produced at a depth of  20-30 km and a pressure of up to 7-8 kbar. Banding is attributed to the gdyke within a dykeh intrusion of differentiated melt, which  gave rise to endocontact and high-temperature zones in the form of thin (1-1.5 cm), relatively melanocratic bands. Both hightemperature  bimetasomatic and magmatic geneses (in particular, dynamic crystallization from melt) are assumed for intermediate  rocks (wehrlites, pyroxenites) at our present level of knowledge [4]. The formation of granulitic (recrystallized) gabbronorites,  eclogite-like rocks and high-temperature garnet and plagioclase amphibolites was associated with deep local post-solidus  (C=800^oC) dynamometamorphism of gabbronorites and bimetasomatic rocks. These alterations were coeval with the autometamorphic  alteration of gabbronorites in zones unaffected by intense autometamorphic dynamic stress. Local dynamothermal metamorphism  varied from high-temperature(C=800^oC) and high-pressure (>5 kbar) to low-temperature subsurface (phyllonites) conditions.  This evidence is consistent with the concept of the protrusive-diapir evolution of ophiolite assemblages [3].  The time span from the beginning of formation to the intrusion of an ophiolite diapir, estimated at 200±10 Ma, is within  the time span of an Alpine tectonic cycle and in the more frequent magnetic inversion interval (a second geon). The lifetime of  the diapir nearly coincides with the period (212-215 Ma) of rotation of the solar system around the core of the Galaxy.  The time span from the beginning of formation to the intrusion of an ophiolite diapir, estimated at 200±10 Ma, is within  the time span of an Alpine tectonic cycle and in the more frequent magnetic inversion interval (a second geon). The lifetime of  the diapir nearly coincides with the period (212-215 Ma) of rotation of the solar system around the core of the Galaxy.

          Literature

1.     Perchuk L.L. Cotexistence  of minerals. L. Nedra  1971, 413 p. (in Russian)

2.     Sharaskin A.Y., Bogdanov N.A., Zakariadze G.S. Geochemistry   and   timing   of the   marginal basin and arc agmatism in the Philippine Sea // Plilos. Trans. Roy. Soc. London A. 1981. Vol. 300. P. 287-297.

3.     Yurkova R.M., Voronin B.J. Uplift and transformation mantle hydrocarbon fluds connetcted with ophiolite diapor formation // Genesis of hydrocarbon fluids and deposids. М.:Geos, 2006. p. 56-67.  (in Russian)

4.     Irving A. Petrology and geochemistry of composite ultramafic xenoliths in alcalic basalts and implications for magmatic processes within the mantle // Amer. J. Sci. A. 1980. Vol. 280. P. 989- 426.

5.     O'Hara M.J. Mineral paragenesis in ultrabasic 'rocks // Ultramafic and related rocks. N.Y.: Blackwall, 1967, P. 393-408.