Abstracts of International conference Geochemistry of magmatic rocks-2009 . School ซGeochemistry of Alkaline rocksป

Sheet dike complex of the forearc ophiolite

Raisa M.Yurkova,š B.I.Voronin

Institute of oil and gas problems RAS, bivrmyrzb@mtu-net.run

This research was conducted in the northwest Pacific active continental margin: Sakhalin, Kamchatka, Koryaks range, Karagin Island, Shirshova range in Bering Sea. The formation of ophiolite is related to the development of magmatic-metamorphic diapir in the lithospheric wedge above Benioff zone [Yurkova, Voronin, 2006]. Magmatism evolution is traced from abyssal facies (lherzolite, gabbro-norite) to hypabyssal (dike series of diabase and gabbro-diabase, plagiogranite intrusions) and further to effusive underwater facies (spilite, quartz keratophyre). Ophiolite associations are noted for the following.

The early pseudomorphic harzbugite serpentinization has been shown as a result from the effect ofš reducing fluids with the formation of structural serpentine water and methane in the presence of nickelšššššš catalyst accordingš to thereactionš 2Mg₂Si0₄+Mg₂Si₂0₆+4CO+12H₂→Mg₆Si₄0₁₀(OH)₈+4CH₄. This yielded antigorite- serpentine (with a lattice constant a=35.5Å) and taenite (a natural alloy of Fe-Ni composition) scattered in antigorite as tiny (2-5 mk) inclusions. Experimental, thermodynamic (T=450-600œC, P=13-16 kbar), and balance calculations have confirmed that antigorite has formed in the mantle, at thešš depth of 40-50 km.

The bimetasomatic contact-reactional units emerged at diffrent stages of formation of ophiolite suite both in high (T=900œC) and low (T-160œC) temperature conditions realised respectively during the 30-40 km deep magmatic rock intrusions into serpentinite and during the contacts of ultramafic and brecciated volcanic-sedimentary rocks. Thus the following different-temperature bimetasomatic layers are formed: 1. apogabbronorite (T=900œC) composed with bronzite diopside and the pargasitic hornblende;šš 2. apolherzolite (T=550-700œC) typically comprising diopside, pargasitic hornblende, grossular, andradite and hertzinite; 3. aposerpentinite, including antigorite, lizardite, pentlandite, chromespinellide.

There have been synchromsn of seirpentinite rise and dike packets injection. Dike series serve the joining of a heterogenous ophiolite complexes. They are presented link packets and swarms of dikes and semidikes and locally single dikes of diabase and gabbro-diabase. They cut across all ophiolite complexes starting with dunite and harzburgite and in this case are significant indicators of the conditions of ophiolite association formation in the single magmatic-metamorphic geothermal system. On the assumption of petrologic and geochemical similarity of dike rocks and rocks of boninite and intermediate tholeiitic magmatic series.

The dike complex formation is reconstructed in the base of the frontal part of the primitive island arc in the conditions of the mantle diapir upriase in the third and second seismic layers. The interrelations between the dike complex and the rest of the members of ophiolite association do not suggest significant horizontal movements of ophiolite before semidikes packets intrusion. The concentration of semidikes packets is observed in the central inherited magmatogene part where bipyroxene anorthite gabbro crystallized. Thin packets of semidikes cross the serpentinous ultrabasite of the diapir banks. In dike complexes "dike-in-dike", two series (generations) of dikesš are combined. One of them consists of subvertical or highly dipping weaklyš windingš semidikes leaning against enclosing gabbroid blocks in magma chilling zone. They are not always parallel to each other. In a number of cases they are slightly displaced relative to each other by subvertical dislocations. In a number of cases the dykes of this series thin out at different levels inside amphibolite blocks. The other series of packets consists of arching curved or straight parallel
semidikes crossing semidikes of the former series at acute angle. Semidike rocks of the former
series have a more melanocratie composition (see the table 1.). Packets of a thickness from 3 to 10 meters containš from 2 to 5 semidikes.š Chill zones from 5 to 15 cm are distinguished and successive inequigranular layers from edge to the center that is fine- and coarse-grained layers (0.3-0.5 m) in the inner parts of semidikes are noted.

Glomeroporphyritic structures are characteristic of dike diabase and gabbro-diabase. In porphyritic growths plagioclase, clinopyroxene (Wo₄₀En₅₀Fs₁₀), and amphibole-chlorite pseudomorphism by orthopyroxene (Wo₃En₆₇Fs₃₀) are combined. Autometamorphic minerals for the major part are represented by the titanicš hornblende and sphene. Apogabbro and apodiabase thermal and dynamiothermal amphibolite and the major part of rodingite formed at the expense of heat and fluids related to the injection of dike magmatic series (T=550-700œC, P>5kbar). Zonal structure of semidikes is inherited with rodingite formation.

At the expense of rocks of the dyke series formed large group of low- and medium-temperature (500-550œC) rodingites. Noteworthy is that Ca - and other chemical components necessary to form rodingites were derived from the magmatic rocks and magmatism-related fluids. The Ca-enriched hydrothemal solutions circulating through contact zones form early plagioclase albitization occurring in diabases. The formation of rodingites at the expense of effusive-tuff rocks of the spilite-keratophyre complex lying above the dyke series was related to serpentiniteš protrusion in early post-volcanic environments.

There as an injection of ophiolite blocks in marginal volcanic sedimentary complexes formed by autokinetic lows. At the contact of serpentinites with turbidites and hemipelagites in forearc flyschoid island complexes associated with ophiolites there emerged bimetasomatuc layers are comprising either or xonotlite (T=160œC) and chlorite.