CHARACTERISTICS OF NRE RATIOS FOR GRANITOIDS OF DIFFERENT

GEODYNAMIC TYPES

Volodkova T.V.

Institute of Tectonics and Geophysics FEB RAS, Khabarovsk, Russia, volodkova@itig.as.khb.ru

Large-scale aerogamma-spectrometric survey data were used as an express method for petrochemical study of magmatism in Priamurye. Natural radioactive elements (NRE) are considered to be extremely inconsistent being used as elements-indicators of magmatic complexes (Volodkova, 2007, 2008). The latter are characterized by average background NRE ratios. Magmatic rocks formed due to crystallization differentiation processes in closed magma chambers (without deep fluid inflow), possess constant NRE ratios. The NRE characteristics for magmatic rocks vary sharply due to processes of plume magmatism (deep fluid inflow, mantle metasomatism), hydrothermal metasomatism changes and various geodynamic environments. The method is elaborated taking into account the influence of hydrothermal metasomatism processes; purely sub-alkali and alkali rock intrusion is associated with plume magmatism. Thus, it is possible to evaluate NRE ratio distribution for various geodynamic environments during the formation of granitoids with normal and low alkalinity.

Taking into account characteristics of NRE ratios for the crust and upper mantle layers, U/K ratio mostly varies playing the role of criterion of geodynamic type for the granitoids with low and normal alkalinity (Table 1). According to U/K criterion it has been made a rough gradation of granitoid types from literary data (the upper part of the table). Characteristics of granitoid complexes of Priamurye have been used for verification, and, on the whole, they affirm the regularity obtained; all the deviations are associated with the influence of plume processes and alkali magmatism. Thus, according to U/K ratio characteristics the Kharin and Pribrezhny IC of subalkaline type do not obey the regularity obtained.

On the whole, the sequence derived corresponds to the stages of the formation of magmatic complexes in the Siberian platform and the Pacific plate interaction zone in the Phanerozoic. As for the Paleozoic, the regimes of the initial subduction and I-type collision predominated here, being further changed by S-type collision. Geodynamics of the Sikhote-Alin and Mongolo-Okhotsk orogenic belts was determined by the collision regime up to the Cretaceous. The intraplate magmatism of A-type prevailed in the greater part of the area in the Upper Cretaceous – Cenozoic. For the Mesozoic-Cenozoic volcanic belts, the island-arc regime was also determined. The work is supported by the grant of FEB RAS 06-1-DES-113

Table 1. Characteristics of average NRE ratios for normal granitoids in various geodynamic environments

Table 1. (continued)

Granodiorites, quartz diorites of tonalite-trondhjemite series (the Urals)	[2]		I (transitional regime)		0.28		0.25		1.67		crust-mantle
High-aluminous granitoids (Australia)	[3]		S (continental collision)		0.26		0.18		1.47		crustal		1.30-1.75
Granitoids with increased alkalinity, intraplate (Australia)	[3]		А (continental rifting or oceanic spreading)		0.21		0.16		1.31		mantle		0.75-1.30
Leucogranites (Trans Baikal)	[4]		А (continental rifting or oceanic spreading)		0.12		0.16		0.76		mantle
Biotite granites (Trans Baikal)	[4]		Collisional, concordant [4]		0.18		0.26		0.71		crust-mantle		0.75-1.35
Monzonites (Trans Baikal)	[4]		А (continental rifting)		0.11		0.17		0.66		mantle?
Quartz syenites (Trans Baikal)	[4]		А (continental rifting)		0.16		0.08		0.16
Quartz monzonites (Trans Baikal)	[4]		А (continental rifting)		0.24		0.31		0.39
Felsic rocks with normal alkalinity (the Kuriles)	[1]		М (island-arc granitoids, West Pacific type)		0.20		0.30		0.65		mantle		≤0.75
Granite-metamorphic layer	[1]				0.25		0.20		1.28
Diorite layer	[1]				0.21		0.21		1.00
Basalt layer	[1]				0.33		0.50		0.66
Depleted mantle	[1]				0.40		0.53		0.80
Granitoids of Priamurye
Granites, porphyry-like monzodiorites (the Kalar Massif) PZ1	[2]	А (continental rifting)		1.05		0.80		1.15		mantle?		1.30-0.75
Granitoids, gabbroids (Middle Zeya area) PR1		I-collision?		0.18		0.27		1.45		crust-mantle?		>1.30
Upper Amur IC K1v		А (continental rifting) or S-collision		0.25		0.30		1.10		mantle		1.30-0.75
Burinda IC K1b				0.16		0.12		1.35
Pribrezhny IC P2				0.41		0.27		1.42
Bekchiul and Upper Udoma IC P1				0.25		0.31		0.77
Ioli IC K2 – P1				0.50		0.37		1.25
Bappa IC K2				0.60		0.46		1.30
Lower Amur IC K2n		I (collision)?		0.34		0.29		1.62		crust-mantle?		≥1.75
Hungari IC K1hg
Birobidzhan IC PZ1
		S (collision)		0.40		0.29		1.20		crustal?		1.30-1.75
Tyrma-Bureya IC PZ2-PZ3
Kharin IC P3-T2
		I (collision)		0.31 0.24 1.30 0.38 0.23 1.70						unclear		≥1.30
												<1.30
		S-collision or А-continental rifting?		0.29		0.13		1.70
Note: column 2 shows the reference number.

References

Volodkova T.V. Anomalies of NRE ratios and the nature of ore mineralization of Priamurye //Tikhookeanskaya geologiya. 2006. V. 25. No. 4. P.54-68. (in Russian).

Volodkova T.V. Characteristics of alkali rocks of Priamurye from spectrometric data //Abstracts. All-Russian Workshop “Alkali magmatism”. Saint Petersburg, 23-26 May, 2008 (in press). (in Russian).

Rozen O.M., Fedorovsky V.S. Collisional granitoids and layering of the crust. M.: Nauchny mir, 2001. 187p. (in Russian).

Tsygankov A.A., Matukov D.I. et al. Magma sources and stages of the formation of late Paleozoic granitoids in Western Zabaikalye // Geologiya i geofizika. 2007. V. 48. No. 1. P.156-180. (in Russian).

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