Models of diamond generation in different geodynamic environments

Lapin A.V.* Belov S.V.**

*Institute of Mineralogy, Geochemistry and Crystal Chemistry of Rare Elements, Moscow, Russia; **Vernadsky Geological Museum of RAS, Moscow, Russia.

In recent years wide development of non-kimberlitic diamondiferous rocks of orogenic foldbelts was identified in zones of continental collision (Tjen Shan, Tibet, Gibraltar Arc, etc.) and accretion-collision zones of active continental margins (Kamchatka Peninsula, Koriak Upland, Japanese Islands, etc.)� as well as in ultrametamorphic rocks, characterized by intensive deformation with abundance of blastomylonitic and blastocataclastitic� textures (Northern Kazakhstan, the Urals, Rudnye Mountains, Rodops, Gneiss Belt of Norway and oth.)� (Kaminsky, 2007; Belov, Lapin et al., 2008).

Diamondiferous kimberlitic rocks, predominant in the area of ancient cratons don?t play any significant part in mobile zones and they are� replaced by various non-kimberlitic sources of diamonds. In orogenic foldbelts in zones of continental collision� non-kimberlitic diamondiferous rocks are presented by dikes of alkaline lamprophyres like minettes and kamptomonochiquites, pipes of alkaline basaltoids, dikes and� pipes of lamproitelike rocks, dikes of picrites, pipes and dikes of original carbonatitelike rocks of complex silicate- carbonate composition, ultrabasic rocks of intrusions, belonging to ophiolitic and platinum bearing formations. In accretion-collision zones� of active continental margins, diamondiferous non-kimberlitic rocks are established in volcanic ultrabasic� rocks of the comatiite type, basalts and melabasalts, ultramafites in massifs� of ophiolitic and platinum bearing formations, lamprophyric dikes. In ultrametamorphite belts, typical of collision fold zones, diamonds are determined in gneisses, eclogites and their metasomatised varieties.

Taking into account rather individual position of kimberlites, typical of stable ancient cratons, often attempts to explain non-kimberlitic diamond mineralisation of� igneous and metamorphic rocks with the help of model common with kimberlites are not efficient. Thus, a problem arises about possible conditions for diamonds generation, that differ from traditional kimberlitic diamond� formation.

According to the modern data, the areas of kimberlitic diamondiferous rocks abundance are limited by ancient Archean cratons, i. e. areas of early stabilization, which during the long period existed under conditions of quiet amagmatic regimes. These conditions are in agreement with low value of thermal flow and weak� penetrability of the lithosphere for abyssal fluids and melts. Kimberlites, as a rule, don?t demonstrate apparent connection with large units of break tectonics of the lithosphere ( rifts, zones of abyssal fractures, etc.)� and in respect of geologic-tectonical position they behave as most abyssal formations that are initiated by processes generated in sublithosphere zones of the mantle, perhaps, low parts of the upper mantle and the intermediate zone. Thus, the diamond in kimberlites, the stability of which is determined by significant lithostatic pressure and low thermal gradients, is accompanied by abyssal mantle paragenesis, including such typical accessory minerals as pyrope, picroilmenite, chrome diopside and others.

�������������� In contrast, occurrences of non-kimberlitic diamondiferous igneous and metamorphic rocks are, with rare exception, confined to the most geodynamically active lithosphere blocks existing under conditions of stressed deformation state. In this connection, the model of non-kimberlitic diamondiferous rocks generation should take into account characteristic features of stressed-deformation state of the lithosphere, typical of orogenic foldbelts zones of the continental collision and accretion-collision active continental margins.

According to �.V. Gzovsky (1975) the values of the highest tangential stresses can significantly increase lithostatic load and come to 1,47 GPa at�� depth up to 20 km. Taking into account that relation of the highest tangential stresses to main normal stresses,� σ₁ and σ₃ is determined by the equation τ = (σ₃-σ₁):2, the value of the highest compressing stresses will be σ₃ = 2 τ +σ₁;i.e. these stresses can be� on the order of 3,5 GPa, which corresponds to minimal pressure conditions required to diamond generation. These data conform with pressure parameters in collision orogenic zones, received with the help of various calculation models. According to P.N. Kropotkin (1996) and V.T. Filatova and the coauthors (2002) Pressures in orogenic collision zones in the crust basement usually are from 3-6 kbar to10-12 kbar and locally for short time reach 50 kbar.

�������������� Based on the data on physical-chemical geomechanics, the most favorable environment� for diamond generation and phase transition of graphite-diamond appears to be created in combination of compression and shift, when chemical reactions are accelerated and intensified. Similar environment often occurs in abyssal tectonic zones confined to ultrametamorphic belts� (Kokchetav block, the Urals, etc.) and in orogenic zones of the continental collision (Tjen Shan, Tibet and oth.), where stressed-deformation state of the lithosphere is often accompanied by magma formation, as well as in accretion- collision zones of active continental margins, where non-kimberlitic rocks can be controlled by seismofocal zones of focuses of paleoearthquake concentration.

With respect of different models of diamondiferous rocks formation in different geodynamic environments, estimation-prospecting criteria and methods , directed to kimberlitic diamondiferous rocks, are rather rarely used for diamond prospecting in mobile zones, collision folded systems, and active marginal zones of continents. Under these conditions, both composition of non-kimberlitic diamond sources and paragenesis of its accessory minerals can significantly vary depending on the composition of diamond producing substrate. Typical diamond minerals- indicators(pyrope and oth.) occur only in lamproitelike rocks and ultrahigh pressure ultrabasic massifs.� In different types of rocks� diamonds are always accompanied by moissanite and other carbides, native metals (Au, Ag, Pt, Pb, Cu and oth.) and their alloys, graphite; coesitic pseudomorphs of quartz are often present.

�Thus, one of the most important results of the new data on occurrences of non-kimberlitic diamondiferous rocks is the variety of potential diamond sources and models of diamond formation as well as their dependence on geodynamic regimes, peculiar to these or those lithosphere segments. Obviously, the above should be taken into account in estimation and prospecting for diamonds in outer craton environment.

References:

Belov S.V., Lapin A.V. and oth. Metallogeny of Platform Magmatism (carbonatites, kimberlites, trappes). Novosibirsk, Nauka, 2008. P. 580 (in Russian).

Kaminsky F.V. Non-Kimberlitic Diamondiferous Igneous Rocks: 25 Years on. // Journ. Geol. Soc. of India. V. 69. March 2007. P. 557-575.

Gzovsky M.V. The basics of Tectonophysics. M. Nauka. 1975. 535 p. (in Russan).

Kropotkin P. N. Tectonic Stresses in the Earth?s Crust // Geotectonics. �2.1996. p.3-15. (in Russian).

Filatova V.T. and oth. Tectonophysics of Intraplate Collision // Geology and Mineral Resources of the Kola Peninsula. V/1. Apatites. 2002/ p. 57-73. (in Russian)/