AGE, COMPOSITION, SOURCES AND GEODYNAMIC CONDITIONS OF EARLY PALEOZOIC GRANITOID FORMATION IN NORTHERN PART OF THE OZERNAYA ZONE,

WESTERN MONGOLIA

Rudnev S.N.*, Izokh A.E.*, Kovach V.P.**, Shelepaev R.A.*, Terentieva L.B.**

*Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia, rudnev@uiggm.nsc.ru

**Institute of Geology and Geochronology of the Precambrian RAS, St. Petersburg, Russia, v.kovach@mail.ru

Granitoids of tonalite-plagiogranite composition occur widely in Early Paleozoic batholiths of the western part of CAOB. Isotope-geochronological, petrological and geochemical studies of some typical Early Paleozoic batholiths shows that they, as a rule, have polychronic structure, and their internal structure is determined by contrast alternation of change in granitoid and gabbroid associations characterized by different composition, age, sources and geodynamic conditions of formation.

The Early Paleozoic granitoids of the Ozernaya Zone in Western Mongolia are one of such representative objects. They are represented by Cambrian Tokhtogenshilsky diorite-tonalite-plagiogranite and Ordovician diorite-tonalite-granite complexes. The Early Paleozoic granitoids occur as plutons and massifs that form extended plutonic belt within island-arc volcanogenic-sedimentary deposits of the Early Cambrian. Study of the granitoids allowed dividing them into some distinct magmatic associations, the formation of which occurred in the age interval of 540-450 Ma. The three age boundaries (540-520, 510-485 and 475-450 Ma) of intrusive magmatism differed by geodynamic conditions of formation, chemical composition and magma-generating sources, can be distinguished (Rudnev et al., 2008).

In the first stage (540-520 Ma) in island-arc setting, the high-alumina plagiogranitoids of tonalite-plagiogranite (531 ± 10 Ma, I-type) and diorite (529 ± 6 Ma) associations were formed in the Kharanur pluton; low-alumina plagiogranitoids of tonalite-plagiogranite association (519 ± 8 Ma, I-type) were formed in the Sharatologoy pluton, as well as rocks of the Khirgisnur peridotite-pyroxenite-gabbronorite complex (Kharachulu and Dzabkhan massifs). Rocks of diorite and plagiogranite associations of the Kharanur pluton are characterized by ε_Nd(T)=+7.9…+7.4, T_Nd(DM)=0.65 billion years and (⁸⁷Sr/⁸⁶Sr)₀=0.7037–0.7039, indicating the leading role of depleted juvenile sources for initial melt generation. The Sr-Nd isotopic data, coupled with geochemical characteristics suggest that the original melts of high-alumina plagiogranites of the Kharanur massif were resulted from melting the metabasites of N-MORB type in subduction zones. The close ages of high-alumina plagiogranites of the Kharanur pluton and low-titanium high-alumina gabbroids of the Khirgisnur complex (Dzabkhan massif) suggest that they were formed in the axial part of island-arc system of the Ozernaya Zone. Plagiogranites of the Sharatologoy pluton have ε_Nd(Т)=+6.5…+6.6, T_Nd(DM)=0.73–0.70 billion years and (⁸⁷Sr/⁸⁶Sr)₀=0.7038–0.7039. The Sr-Nd isotopic data, coupled with Nb-Ta and Ti negative anomalies, suggest that the main source of their melts could be metabasites located at the base of the island arc, with subordinate participation of ancient crust material. The formation of low-alumina plagiogranites of the Sharatologoy pluton in close association with simultaneous high-titanium, high-alumina gabbroids of the Khirgisnur complex (Kharachulu massif) indicates their generation in the back-arc part of the same island-arc system.

In the second stage (510-485 Ma) in collision-accretionary environments, the low-alumina plagiogranitoids of diorite-tonalite-plagiogranite association (494 ± 10 Ma, M-type) of the Sharatologoy pluton and high-alumina plagiogranitoids of diorite-tonalite-plagiogranite association (494 ± 2 Ma, I-type) of the Khirgisnur pluton, were formed. According to Sr-Nd isotopic characteristics (ε_Nd(Т)= +6.6, (⁸⁷Sr/⁸⁶Sr)₀=0.7039), tholeiite plagiogranites of the Sharatologoy pluton do not differ from the plagiogranitoids of the early association (tonalite-plagiogranite, I-type), that permits to suggest melting the sources of similar compositions.

In the third stage (475-450 Ma) in post-accretionary geodynamic setting, the rocks of diorite-granodiorite-granite association (459 ± 10 Ma, I-type) in the Kharanur pluton and the Ayrygnur massif (465 ± 11 Ma, I-type) were formed. The granitoids are characterized by ε_Nd(Т)=+5.1, T_Nd(DM)=0.74 billion years and (⁸⁷Sr/⁸⁶Sr)₀=0.7096. Taking into consideration geological setting of this rock association and big temporal gap between their formation and time of accretionary processes of island arcs and microcontinents, it can be suggested that their initial melts for them were formed as a result of partial melting the “Caledonian” juvenile crust, with the addition of more ancient crust material.

The work has been carried out with financial support of RFBR (grants N 06-05-64767, 07-05-00849, 07-05-92001); Presidium of SB RAS (integrational projects № 7.10.2 and 6.5) and grant from the President of the Russian Federation (NSH-2715.2008 .5).

References

Rudnev S.N., Izokh A.E., Kovach V.P., Shelepaev R.A., Terentieva L.B. Age, composition, sources and geodynamic conditions of granitoid formation in northern part of the Ozernaya Zone, Western Mongolia: mechanisms of Paleozoic continental crust. growth Petrology, 2008 (in press).