CRETACEOUS EPOCHS OF GRANITE FORMATION IN EAST ASIA:

AGE AND GEODYNAMIC ORIGIN, REGIMES OF FORMATION

Stepashko A.A.

Yu.A.Kosygin Institute of Tectonics and Geophysics FEB RAS, Khabarovsk, Russia, srepashko@itig.as.khb.ru

Since the Jurassic, effect of the Pacific plates on continental margin is still the most important factor of geotectonic evolution, including magmatic one, in East Asia. Larson and Pitman (1972) were the first to suggest that an increase in the spreading rate in MidCretaceous was to cause the larger subduction and increase in pressure on the Pacific borders and, therefore, to be followed by epoch of granite formation within its continental rim. New estimates of crust's growth rate variations in the North Pacific centre of spreading (Stepashko, 2006, 2008) allow to confirm and specify role of oceanic dynamics in genesis of Cretaceous granitoids playing a particular role in the eastern margin of Asia. The growth of oceanic crust since the Jurassic had a cyclic character, when systematic acceleration of spreading was changed by the same regular deceleration. In accordance with hypothesis of Larson-Pitman, the spreading acceleration resulted in larger compression in East Asia. Glaucophane-schist metamorphism of high pressures, genetically associated with processes of subduction is the best indicator of such regime on the continent-ocean border. Its absolute age that is well documented for Sakhalin and Khokaido (Bogdanov, Khain et al., 2002) confirms all intervals of compression strengthening in marginal Asia that were caused by the Pacific spreading acceleration. Two cycles have been established for the Cretaceous. The first one, Early Cretaceous is in the interval 140-120 Ma with maximal rate of spreading on boundary of 130 Ma. The second cycle, Late Cretaceous is of age 120-65 Ma with maximum on the boundary 87-88 Ma.

Numerous determinations of absolute age obtained in the recent decades confirmed previous hypotheses about existence of peculiar epochs of granite formation in East Asia (Larson and Pitman, 1972; Zagruzina, 1980). In East China (Wu Fu-Yuan et al., 2005), the main epoch of granite formation is confined to Early Cretaceous (140-110 Ma) with maximum of introduction in interval 132-120 Ma. In the Pre-Amur area and southern margin of Siberian craton, granite-granodiorite batholiths of the Upper Amur (140-134 Ma) and Tungiro-Oleokma complexes (137-136 Ma), granodiorite-granite complexes, i.e. Amudzhikanskiy and Amananskiy-2 (1320128 Ma), bimicaceous granites of Taksakandinskiy sequence (134 Ma), intrusions of Burundinskiy monzodiorite-granodiorite (130-127 Ma) and Tynda-Bakaranskiy (127 Ma) complexes formed in that period. In East China, the large scale gold mineralization (Wu Fu-Yuan et al., 2005) is associated with granitoids of Early Cretaceous epoch. The deposits of the Pre-Amur gold-bearing province are of the same age (Moiseenko, Stepanov, Sheergina, 1999). By isotope Rb/Sr data, they formed: Kirovskoe – 128, Bamskoe – 129, Pokrovskoe – 131, Beresitovoe – 130 and Tokurskoe – 133-112 Ma. Early Cretaceous epoch of granite formation with obvious gold specialization fixes maximal compression of Asian margin that resulted from goteriv-barremskoe acceleration of the Pacific spreading (Stepashko, 2008) with maximal rate on the boundary of 130 Ma.

The second interval of oceanic crust rapid growth is in the Cenomanian-Companian (100-80 Ma) with the spreading maximal rate on the boundary 87-88 Ma. In that period, the intrusions of the Dukchinskiy (99 ± 3 Ma), Omsukchanskiy (97 ± 4 Ma), Leurvaamskiy (91± 4 Ma) and Neorchanskiy (84 ± 3 Ma) granitoid complexes formed in North-East Asia. An outburst of granite formation in the Korean peninsula is associated with the same interval 90-80 Na in the south, by geochronological data (Tsusue, 1984). Gold usually precedes tin in the Far East of Russia. The time review of tin-bearing granitoid formation from Chukotka to Sikhote-Alin (Rodionov, 2000) showed that culmination of granite formation, when intrusions formed simultaneously in all tin-bearing provinces of East Asia, is associated with interval 90 ± 5 Ma (Stepashko, 2006). Both an existence of Late Cretaceous epoch of granite formation in East Asia and its synchronism with the Konjakskiy maximum of the Pacific spreading rate are of no doubt.

Due to the data by Rodionov (2000), culmination in formation of more acid granodiorite-granite plutons (96089 Ma) in East Russia was notably more ancient of maximal introduction (88085 Ma) of diorite-granodiorite type intrusions. The age boundary that separates granitoids of two types corresponds to the spreading maximal rate (87-88 Ma). It should be suggested that formation and introduction of magmatic melts gets more complicated in the narrow age interval, when compression is particularly intensive at the spreading culmination. As a result, a pair of granitoid complexes form: the first complex – when compression exceeds the middle level and continue to strengthen during the spreading acceleration, and the second one – when the spreading rate decreases, and compression respectively decreases, but it still exceeds the middle level. In East China, The Early Cretaceous granites of the Dabi orogen (Haijin Xu et al., 2007) are bright confirmation of pair complex formation model. By geochemical data and U-Pb age determinations, two stages of magmatism are distinguished here. The granitoids of the early stage (~132 Ma) are represented by the deformed and schistose quartz monzonites and monzogranites. Undeformed granitoids of the second stage with age (~128 Ma) that are represented by monzogranites and granite-porphyries are geochemically sharply differed from them. The suggested pairness is also observed for the Early Cretaceous granitoids of the Siberian craton southern rim. For example, in the Selenga-Stanovoi superterrane, batholiths of the Tungiro-Oleokma complex formed in the first stage (137-136 Ma) and massives of the Amananskiy-2 complex – in the second one (132-128 Ma). Their introduction was separated by phase of folding and regional metamorphism (Larin et al., 2007).

An existence of three granite formation global boundaries in the Pacific rim, i.e. 150 ± 15 Ma, 100 ± 10 Ma and 70 ± 10 Ma was suggested for the Late Mesozoic in classical paper by I.A.Zagruzina (1980). Geochronological studies of the late XX – early XXI centuries indicate an existence of only two epochs of granite formation for the Cretaceous, i.e. 130 ± 5 Ma and 87 ± 10 Ma. Early- and Late Cretaceous periods of the Pacific spreading high rate that caused an increase in compression in East Asia were a genetic reason of each one.

References

Stepashko A.A. (2006) Peculiarities of the Pacific plate Cretaceous dynamics, and stages of magmatic activity in north-eastern Asia // Geotectonics. N3. P.70-82.

Stepashko A.A. (2008) Cycles of the Pacific spreading // Oceanology. N3. P.436-444.