Nd isotope systematics in metamorphic rocks of the southern Russian Far East

Abstract

Sm–Nd isotopic–geochemical investigations are widely used in the study of metamorphic sequences for estimating the age of continental crust formation and the lower age limit of protoliths of metamorphic com� plexes [1]. When the protolith is of primary magmatic origin, the Nd model age reflects in the first approxi� mation the onset of the continental crust formation from the mantle source or, in other words, the time of transformation of the oceanic crust into the continen� tal crust. The Nd isotope systematics of metamorphic terrigenous sedimentary rocks offers the opportunity to estimate the average model age and likely paleogeo� graphic provenances, as well as the lower age limit of sediment accumulation. In this communication, we discuss the data of the Nd isotopic composition in metamorphosed rocks from the three largest blocks located in the southern Far East areas: Matveevka–Nakhimovka, Sergeevka, and Anyui. It should be noted that the studied com� plexes were traditionally considered as representing inliers of the Early Precambrian crystalline basement and only recently have they been regarded as Neogean complexes that were formed by accretion, collision, or postcollision processes [2, 3]. The available geochro� nological data provide no grounds for answering the following question: what is the lag between metamor� phic transformation of rocks and their geological age? The Nd isotope measurements performed for meta� morphic complexes of the southern Far East bring us nearer to assessment of the lower age limits of their formation and to determination of the isotope crust composition for the whole region. The Khanka superterrane is located in southwestern Primor’e region (Fig. 1). It is formed by several ter� ranes of various origins. The southern and northern parts of the superterrane represent fragments of the passive margin of a craton and Early Paleozoic oro� genic belt, respectively. The last of them comprises the Matveevka and Nakhimovka metamorphic terranes that differ from each other in the constituting rocks and degree of their metamorphic transformation [6]. The most intensely metamorphosed rocks of the Matveevka terrane are united into the Iman Group, which is traditionally subdivided into the Ruzhino (diopside–calcite and forsterite–calcite marbles, biotite and biotite–cordierite gneisses with intercala� tions of quartzites and marbles), Matveevka (biotite– sillimanite and biotite–garnet–cordierite gneisses with quartzite and marble intercalations), and Tur� genevka (biotite–amphibole gneisses, crystalline schists, and amphibolites) formations. In the Nakhi� movka terrane, metamorphic rocks are united into the Ussuri Group, which consists of the Nakhimovka (biotite and biotite–amphibolite gneisses with lenses of marbles and amphibolites) and Tat’yanovka (biotite, diopside, and muscovite–graphite crystalline schists) formations. The relationships between the Iman and Ussuri groups remain unknown.