The geochemistry of mafic-ultramafic inclusions from postcollision shear zone (Ilmensky-Vishnevy mountains)

Medvedeva E.V.*, Rusin A.I.**, Baneva N.N.**

*Ilmensky State reserve of UB RAS Miass, Russia, ** Zavaritsky Institute of geology and geochemistry UB RAS Ekateriburg, Russia

Information about alkali-ultramafic association of the Ilmeny-Vishnyevye Mountains was earlier restricted by characteristic of metaultramafic rocks. The question about formational affinity of these rocks was always controversial because geological situation of the region was regarded as strongly compressed and extensive anticlinorium with gneiss core and schist frame. A new conception of its geological structure as a deep fragment of postcollisional regional shift (Voroshchuk, Rusin, 2006) and petrochemical data let us to suggest that small blocks of mafic-ultramafic rocks are the fragments of alkali-ultramafic central type intrusion (Rusin et al., 2006) separated more than 100 km long.

Granitoid blastomilonites, not Precambrian gneisses, contain the blocked bodies and are the principal rock types of the axial zone (core of anticlinorium). Petrochemical data show that intrusion is characterized by three rock associations: ultramafic-mafic, alkali-ultramafic (ijolite-jakupirangitic) and miascite-carbonatitic.

A lot of petrographic types of ultramafic (dunites, spinel olivinites, pyroxenites, websterites and others) and mafic (garnetic, fassaitic, coisitic, anortitic, spinelic and others) rocks were established in last years (Korinevsky, Korinevsky, 2007). Amphibolites are often characterized by very low SiO₂ (30-33 wt%) contents and increased Al₂O₃ (up to 28 wt%) and CaO (up to 16 wt%) contents. Obligatory presence of nepheline, olivine and vollastonite is calculated during converting to normative composition. The similar situation with ultramafic rocks is at the central type intrusion Lillebukt, North Norway (Arzamastsev et al., 2002), where deformation of the primary intrusion structure with following metamorphism of amphibolite facies is observed. The rocks of this alkali-ultramafic type haven’t analogues among other types of metamafic-ultramafic complexes of the Urals.

Results of ICP-MS analysis give us new information about geochemical specificity of the rock associations of alkali-ultramafic intrusion. Total contents and REE distribution in ultramafic (0.97-358 ppm, La/Sm=4-82, and Се/Yb= 10-99) and mafic (16-517 ppm, La/Sm =2-38, and Се/Yb =7-50) rocks (fig. 1,a) are in agreement with their possible affinity to two rock association noted above. REE contents in miascites (199-263 ppm) are lower than maximal REE contents in alkali-ultramafic association. This fact, similar trends of differentiation and isotopic data (Kononova et al., 1979) could be considered as an evidence of mantle genesis of miascites.

 Fig. 1. Variations of La/Sm and Ce/Yb ratios (a) and Ba and Sr contents (b) in the rocks of the Ilmeny-Vishnyevye Mountains. 1 - host rocks, 2 - ultramafic rocks, 3 - mafic rocks, 4 - sienites, 5 - miaskites.

Such indicated for mantle magmatism elements as Sr and Ba (Kapustin, 1983), despite significant variations, generally demonstrate the tendency to accumulation from ultramafites (Sr=90 ppm, Ва=274 ppm, Sr/Ва=0.94, n =12) through “metamafites” (Sr=426 ppm, Ва=465 ppm, Sr/Ва=2.3, n = 21), to sienites (Sr=984 ppm, Ва=1985 ppm, Sr/Ва=0.7, n = 8) and miascites (Sг=1164 ppm, Ва=1280 ppm, Sr/Ва=0.9, n =3) (fig. 1,b). Similar tendency is observed in behavior of high-charging elements (ppm): ultramafites - Nb=5.0, Та=0.3, Zr=13, Hf=0.3, “metamafites” - Nb=7.8, Та=0.8, Zr=73, Нf=1.9, sienites - Nb= 83, Та=18, Zr=65, Нf=2, miaskites - Nb=202, Та = 24, Zг=158, Нf=2.5. Indicated Zr/Hf ratio in groups of rocks changes in close limits (45, 36, 30 and 56 respectively) but Nb/Ta ratio doesn’t show clear regularities.

So, petrochemical peculiarities of these rocks show that alkali rocks of the Ilmeny-Vishnyevye Mountains are the derivatives of the deep mafic-ultramafic magmatism and are not palingenic-metasomatic rocks formed during transformation of continental crust under influence of hypothetical mantle fluids. We also have preliminary data about Vendian-Cambrian age of the rocks of alkali-ultramafic association.

Possibility of affinity of "exotic" rocks from the Ilmeny Mountains such as sandyites, firsites and oth. to alkali-ultramafic association will be regarded in the report

Researches were financially supported by RFBR (07-05-96020-Р_Ural_а), Department of Earth Sciences of RAS № 4  and № 10 and integration project between Urals and Siberian branches of RAS.

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