Campanian and Early eocene stages of granite formation IN the

southERN sredinny range (KAMCHATKA): composition of granitoids and their geodynamic setting

Luchitskaya M.V.*, Soloviev A.V.*, Hourigan J.K.**

Intrusive complexes of granite composition are widespread in Sredinny Range, where they are confined to the metamorphic sequences of Malka Rise. According to works of last years the structure of Malka Rise is considered as fold-and-thrust (Kirmasov et al, 2004; Richter, 1995; Soloviev, 2008). Autochthon includes complexes of Kolpakovka group, cut by gneissosed Krutogorova granites, deposits of Kamchatka group, Kheivan and Khozgon formations. Allochthon is composed of Andrianovka, Iruney, Khimka and Kirganik formations deposits. Lower Eocene deposits of Baraba formation, unconformably overlapping both metamorphic complexes and Cretaceous deposits of Iruney formation (Soloviev, 2008), are referred to neoautochthon.

Granitoids are represented by two types: gneissosed and equigranular ones. Gneissosed granitoids correspond to granites of Krutogorova complex, cutting Kolpakovka group formations. Equigranular granites cut both Kolpakovka group formations and Kamchatka group rocks. They also cut the thrust zone between schists of Kheivan formation (autochthon) and metabasites of Andrianovka formation (allochthon) thus being “stitching” intrusives.

U/Pb SHRIMP datings show that two stages of granite formation are distinguished, Campanian (~78–80 m.a.) and Early Eocene (~522 m.a.). Granitoids of first stage are undergone metamorphism and were gneissosed; Early Eocene granitoids formed synchronously with metamorphism peak (Soloviev, 2008).

Granitoids of both types are referred to rocks of normal row and partly subalkaline and correspond to granites and granodiorites; they belong to medium- and high-potassium calc-alkaline series; they are characterized by high ASI=0.95–1.3, thus being peraluminous granites. Petrochemical features of both types granitoids indicate their similarity to S-granites of collisional orogens. Most part of granitoids fall in the field of S-type granites, compiled by P.Sylvester according to indicator parameters Al₂O₃/TiO₂ and CaO/Na₂O (Sylvester, 1996).

REE patterns allow to distinguish two groups among gneissosed and equigranular granitoids. First group is characterized by fractionated spectra (La_N/Yb_N=14.30–71.37) and the lack or weakly positive Eu-anomaly; second group, by higher contents of TREE (La_N/Yb_N=2,68–5.59) and distinct negative Eu-anomaly (Eu/Eu*=0.41–0.46). REE patterns of the second group granitoids a similar to host gneisses and collisional S-granites, which formation is related to partial melting of metapelites.

Petrography and petrochemical features of Malka Rise granites show their similarity to S-type granites. The latter are considered as a result of anatexis of metasedimentary crustal protolith as a result of increased radioactive decay and heating during formation of anomalously thickned crust of collisional systems, or as a result of lithosphere delamination and input of hot astenosphere mantle at the crust base in postcollisional conditions (Rosen, Fedorovsky, 2001; Patino, Harris, 1998). REE features allow that both gneissosed and equigranular granitoids may be formed as a result of partial melting of different sources, composed of 1) magmatic rocks of basic or sedimentary rocks of greywacke composition, metamorphosed in amphibolite to granulate facies. 2) metapelites.

Data (Soloviev, 2008; Khanchuk, 1985) show that formations of Kolpakovka group are metamorphosed deposits of accretionary prism and carried out datings of terrigenous protolith indicate its Cretaceous age (Soloviev, 2008). First stage of granite magmatism, formation of 78–80 m.a. gneissosed granites is probably related to accretional setting at the Kamchatka margin of Eurasia. The reasons of granitoid magmatism manifestation in accretionary prisms both on the example of Kamchatka and other regions of Pacific margin are still unclear. Heating at the base of accretionary prism as a result of mafic underplating during mantle wedge melting above subduction zone or subduction of oceanic ridge and formation of mantle window (Maeda, Kagami, 1996; Stein et. al., 1994) may be suggested.

Second stage of granite magmatism, formation of equigranular granites, coincides in time with collision between Achayvayam-Valaginsky ensimatic island arc and Kamchatka margin of Eurasia. Nearly 60 m.y. ago island arc approach to Kamchatka margin. Terrigenous sedimentation continued in relic basin between arc and margin till ~55 m.y.ago. Then during the collision rapid thrusting of marginal sea and island arc slices on heterogeneous formations of the margin; intensive and rapid transformations of the structure, including deep submergence, quick (maximum 3–5 m.a.) heating of the crust took place. It result in metamorphism of high temperatures (550–650єC) and moderate pressures, and also to granite melting 522 m.y. ago.

Work is supported by RFBR (project 07-05-00255), FTsNTP leading scientific schools NSh-3172.2008.5, MD-2721.2008.5, Programmes of Earth Sciences Department of Russian Academy of Sciences №6, №8, №14 and Fund of Assistance to Home Science.

References

Kirmasov A.B, Soloviev A.V., Hourigan J.K. Collisional and postcollisional structural evolution of Andrianovsky suture (Sredinny Range, Kamchatka) // Geotectonics. 2004. N4. P.64–90 (in Russian).

Richter A.V. Structure of metamorphic complex of Sredinno-Kamchatsky massif // Geotectonics. 1995. N1. P.71–78 (in Russian).

Soloviev A.V. The study of tectonic processes in the areas of lithospheric plates convergence: methods of track dating and structural analysis. M.: Nauka, 2008. 319p. (Proceedings of GIN RAS; Is. 577) (in Russian).

Khanchuk A.I. Evolution of ancient sialic crust in island arc systems of East Asia. Vladivostok: DVNTs AN SSSR, 1985. 138p. (in Russian)

Rosen O.M., Fedorovsky V.S. Collisional granitoids and layering of the earth’s crust. M.: Scientific World. 188p. (Proceedings of GIN RAS; Is. 545) (in Russian).

Patino Douce A.E., Harris N. Experimental constraints on Himalayan anatexis // J. Petrology, 1998. 39. P.689–710.

Sylvester P.J. Post-collisional strongly peraluminous granites // Lithos, 1998. 45. P.29–44.

Maeda J., Kagami H. Interaction of a spreading ridge and an accretionary prism: implications from MORB magmatism in the Hidaka magmatic zone, Hokkaido // Geology, 1996. 24. P.31–34.

Stein G., Lapiere H., Charvet J, Fabbri O. Geodynamic setting of volcano-plutonic rocks in so-called “paleoaccretionary prisms”: fore-arc activity or postcollisional magmatism? the Shimanto belt as a case study // Lithos, 1994. 33. P.85–107.