SOURCES OF MESOZOIC MAGMATISM IN SOUTHERN RIM OF THE

MONGOLO-OKHOTSK FOLD BELT

Sorokin A.A.*, Kotov A.B.**, Sorokin A.P.*, Mel'nikova O.V.*

*Institute of Geology and Nature Use FEB RAS, Blagoveshensk, Russia, sorokin@ascnet.ru

**Institute of Pre-Cambrian Geology and Geochronology RAS, St.Petersburg, Russia, akotov@peterlink.ru

The Mesozoic epoch played an important role in structure formation of East and Central Asia. At that time, a collision of big continental massives, i.e. Siberian and Sino-Korean continents, and Amur superterrane followed by closure of Solonkerskiy and Mongolo-Okhotsk paleooceanic basins occurred. In addition, the convergence border started to form along the Pacific margin. High magmatic activity appeared one of the consequences of these tectonic events. Four groups of the Mesozoic magmatic associations being various in ages can be distinguished within the Mamynskiy, eastern Argunskiy terranes being southern rim of the Mongolo-Okhotsk Fold Belt eastern segment (Table 1).

Table 1.

	Association	Age, mln. years	87Sr/86Sr(0)	εNd(T)	TNd(DM)	TNd(DM-2st)
Group IV	Basalt andesites, basalt trachyandesites	105-94	0.7053- 0.7065	-0.7..+0.3	840-815	975-896
	Trachybasalts, basalt andesites, rhyolites, subalkaline granitoids	120-115	0.7053-0.7098	-3.7...-0.1	1025-880	1236-944
Group III	Andesites, dacites	126-124	0.7043-0.7062	-4.7..-0.9	1145-965	1331-1016
	Monzodiorites, granodiorites	130-127	0.7065-0.7080	-4.3..-4.6	1199-1055	1335-1321
	Granites, granodiorites	140-134	0.7063-0.7069	-3.9..-3.4	1311-1017	1262-1221
Group II	Alkaline granites*	130-122*	0.703*	0....+2*	-	960-720*
	Trachyrhyolites	136-133	0.7572-0.7604	+3.1..+4.8	707-583	691-547
Group I	Granites, leucogranites	219-185	0.7055- 0.7073	-3.9..-1.0	1583- 981	1322- 1080

Note: * – by (B-M. Jahn et al. 2001), the rest – by (Sorokin et al. 2004) and unpublished data of the authors.

The above data allow concluding that:

1) The youngest Nd-model ages (T_Nd (DM) = 0.7-0.7 Ga, T_Nd (DM – 2st) = 0.501.0 GA) and positive ages of ε_Nd(T) parameter = 0 + 4.8 are specific for Neocomian trachyrhyolites and alkaline granites (Group II). Together with geochemical data (in particular, high contents of LREE, HFSE), it can be suggested that the enriched mantle source (or mixture of mantle sources) made a great contribution to formation of primary melts for these rocks.

2) Early Cretaceous granitoids and andesites being close to them by age (Group III) are characterized by relatively old Nd-model ages (T_Nd (DM) = T_Nd (DM – 2st) = 1.0-1.3 Ga), negative ε_Nd (T) = -0.9…-4.7, moderate values of initial ratios ⁸⁷Sr/⁸⁶Sr₍₀₎ = 0.7043-0.7080. The geochemical peculiarities of these rocks (particularly, moderate LILE contents; deficit of Nb, Ta; some excess of Sr) allow to suggest that both juvenile mantle and ancient crustal sources participated in their formation. The Early Mesozoic granites and leucogranites (Group I) have similar isotope-geochemical peculiarities.

3) The later volcanites of bimodal trachybasalt-rhyolite complex as well as basalt andesites, basalt trachyandesites of the late Early – early Late Cretaceous (Group IV) are characterized by somewhat younger Nd model ages (T_Nd (DM) = 0.8-1.0 Ga, T_Nd (DM – 2st) = 1.2-0.9 Ga), and slightly higher ε_Nd (T) = -3.7… + 3.3 at ⁸⁷Sr/⁸⁶Sr₍₀₎ = 0.7098-0.7053 compared to the earlier granitoids and andesites. Taking into consideration enrichness of these rocks in such elements as K, Rb, Th, U, REE, the participation of enriched mantle in magma formation together with crustal sources should not be ruled out.

The studies have been carried out with financial support of Russian Foundation for Basic Research (grants N06-05-36014-GFEN, 08-05-10044, 08-05-00643, 07-05-00147), Presidium of Far East Branch RAS (06-1-ONZ-115).

References

Bor-ming Jahn, Fuyuan Wu, R.Capdevila, F.Martineau, Zhenhua Zhao, Yixian Wang. Highly evolved juvenile granites with tetrad REE patterns: the Woduhe and Baerzhe granites from the Great Xing'an Mountains in the NE China // Lithos. 2001. V59. P.171-198.

Sorokin A.A., Ponomarchuk V.A., Sorokin A.P., Kozyrev S.K. (2004) Geochronology and correlation of Mesozoic magma formations in the northern margin of the Amur superterrane // Stratigraphy. Geological correlation. V.12, N6, P.38-54.