PLAGIOGRANITES OF OPHIOLITE ASSOCIATION

Yurkova R.M., Voronin B.I.

Institute of Oil and Gas Problems RAS, Moscow, Russia, bivrmyrzb@mtu-net.ru

The leucocratic intrusive rocks of ophiolites belong to the tonalite-plagiogranite series (Kolman, 1979). In some cases, they are jointed and even displaced by volcanic and subvolcanic series of quartz keratophyres (Kolman, Donato, 1983). The literary review shows that leucocratic rocks in ophiolite associations are spatially associated with the dyke complex, occurring either within it or over it, however, penetrating into the underlying gabbros and laminated series (Kolman, 1979; Malpas, 1983). Volume of acid volcanites significantly increases during formation of island-arc uplifts (Kolman, 1979, Malpas, 1983).

In the report, ophiolite associations of Sakhalin and Koryakskiy Ridge are considered. The study carried out showed the following:

1. Plagiogranites and quartz keratophyres in ophiolite complexes of Sakhalin and Koryakskiy Ridge are members of volcano-plutonic spilite-keratophyric series, represented by intrusive, subvolcanic and effusive-tuff facies.

2. Acid magma formation is suggested as a result of reconstructing the fluid-magmatic system that produced basic magmas under effect of fault-shift deformations due to transformation (metagenesis) of basic magma by fluids (Korzhinsky, Zotov, Perzev, 1984). Fluids served as a source of silica and alkali, possibly, in the form of elemental-organic combinations during acid melt formation.

3. In the late- and postmagmatic stages, fluids contributed to redistribution of petrogenic and trace elements (Si, K, Cr, Ni, V, etc.) both inside the intrusive bodies and at interaction with host rocks that resulted, particularly, in formation of hybrid rocks, metasomatic plagiogranites and albitites, albite amphibolites and propylites. Comparatively, high temperatures of rock mineral transformations could be provided by heat transfer at its emanation from a great number of intrusive bodies, and by convection at fluid flow in the permeable zone.

4. Magmatic plagiogranites and quartz keratophyres of the Koryakskiy Ridge and Sakhalin are comparable with island arc plagiogranites of Newfoundland and Fiji on ratio of petrogenic and trace elements (Fig.1).

5. Knotty-spotted, by layer oriented and breccia patterns of plagiogranitization are specific of secondary metasomatic varieties. Metasomatic plagiogranites are separated within the massif as stripes, big lenses and irregular-shaped bodies from the first metres to first hundreds meters in size both as oriented and unoriented ones along the massif borders.

Secondary plagiogranite textures are heterogeneous that is caused by segregation of dark colour minerals and quartz. They are characterized by various blastic textures: grano-nematograno and lepidograno, porphyro- and homeoblastic ones. Among dark colour minerals, blue-green amphiboles are contained in various combinations, partially or completely replaced by actinolite and chlorite; pseudomorphs of complex composition (chlorite, amphiboles) on orthopyroxenes, titanium-containing magnetites; clinopyroxenes and early hornfels, preserved from replacement; as well as stilpnomelane formed on hornfels and chlorite. Various stages of plagioclase replacement by quartz from granoblastic aggregates along grain margins with preservation of areas in the centre that are altered to completely replaced pseudomorphs in them during green schist metamorphism.

6. Analysis of various data obtained both by the author and literary ones permits to make a conclusion about formation of ophiolite plagiogranites and quartz keratophyres at mantle diaper rise in zone of transition “primitive island arc – trench” (Yurkova, Voronin, 2006).

1, 2 – plagiogranites of the Koryakskiy Ridge and Sakhalin: 1 – magmatic, 2 – metasomatic, 3 – trondhjemites of Newfoundland (Malpas 1983), 4 – “oceanic” plagiogranites (Kolman 1979); 5 – plagiogranites of Fiji (Gil, Storn 1983).

FeO* = FeO + 0.9 Fe₂O₃.

References

Gil J.B., Storn A.L. (1983) Miocene and low-potassium dacites and trondhjemites of Fiji Islands // Trondhjemites, dacites and associated rocks. M., Mir, P.456-470.

Kolman R.G. (1979) Ophiolites. M., Mir, 261p.

Kolman R.G., Donato M.M. (1983). Once again about oceanic plagiogranites // Trondhjemites, dacites and associated rocks. M., Mir, P.118-130.

Korzhinsky D.S., Zotov N.A., Perzev N.N. (1984) Transmagmatic fluids and metamagmatism // XXVII Intern. Geol. Congress. Petrology: Section S-09. Reports. M., Nauka, v.9, P.259-262.

Malpas J. (1983). Two contrasting trondhjemite associations from displaced ophiolites in West Newfoundland: First report // Trondhjemites, dacites and associated rocks. M., Mir, P.339-353.

Yurkova R.M., Voronin B.I. (2006) Rise and transformation of mantle and hydrocarbonaceous fluids in relation with ophiolite diaper formation // Genesis of hydrocarbonaceous fluids and deposits. M., GEOS, P.56-67.