NEOPROTEROZOIC COLLISIONAL AND INTRAPLATE GRANITOID MAGMATISM

OF THE YENISEI RIDGE

Nozhkin A.D.

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

From the point of view of geodynamics, the Yenisei Ridge is one of the most interesting structures in the Late Proterozoic folded framing of the Siberian craton. Widespread in the Angara region, the terrigenous sequences of the Teya and Sukhoi Pit Groups, whose thickness totals 7 to 9 km, underwent metamorphism and granitization bearing on the Grenvillian orogeny (1.0-0.95 Ga). Two long belts of syncollisional granite-gneiss domes and conjugate areas of low-pressure rocks regionally metamorphosed from greenschist to amphibolite facies formed during this epoch. The relation of these processes to the Grenvillian orogeny has been supported by new 40Ar/39Ar isotope data. The hornblende of metabasites from the outer aureole of the Upper Kutukas granite-gneiss dome has an age of 955+-10 Ma. This metamorphic event dates from the Grenvillian orogeny, which is also expressed in other lithospheric blocks of the Asian continent (Ernst et al., 2008; Yarmolyuk et al., 2005).

The granite-gneiss domes up to 2000 km2 in total area (the Teya dome) are composed of gray biotite gneisses, porphyroblastic microcline gneisses and granite-gneisses, with subordinate Na-K and high-Na gneiss-granites. They are accompanied by abundant swarms of pegmatite veins of zonal type. The late collisional stage is characterized by the formation of granitoid plutons with distinct intrusive contacts, composed of plagiogranites, granodiorites, low-alkali K-Na granites, and quartz diorites. The Eruda, Kalamin, and Mid-Tyrada plutons are of this kind. The same epoch records rheomorphism and further growth of granite-gneiss domes represented by an association of pinky-red porphyroblastic gneisses, granite-gneisses, and high-K gneiss-granites, less frequently, by gneissoid leucogranites as well as veined granite-aplites and pegmatoid granites. The U-Pb zircon age of the above granitoid plutons and gneiss-granite domes (Uvolga, Gusyanka) is the same, 860-880 Ma (Nozhkin et al., 1999; Vernikovsky and Vernikovskaya, 2006; new unpublished data on the Mid-Tyrada and Gusyanka plutons). The granitoid plutons are obviously of magmatic origin whereas the lead process at the first and second stages of dome formation is metasomatic granitization in the form of high-temperature silica-alkaline metasomatism passing into melting. The surrounding rocks were transformed at the expense of both fluid solution and intergranular melt whose appearance provided the fluidity of mineral matter, growth of order II domes made up of gneiss-granites. In the succession of rocks from porphyroblastic gneisses through granite-gneisses to gneiss-granites (i.e., as the metasomatic granitization enhances and passes into melting) the contents of K and silica grow, Rb, U, Th drastically increase, with Zr, Sn and LREE also augmenting, while Ti, Al, Ca, Mg, and IGE decrease. As compared with the source rocks, in the gneiss-granites of the first and second stages of granitization, the U concentration increases by a factor 1.5 and 3, Th — 3 and 5, and K — 1.5 and 1.8, respectively (Nozhkin et al., 1983).

In the postcollisional epoch, several rift troughs form and intraplate magmatism occurs. The earlier troughs (Upper Vorogovka and Glushikha) are filled with volcanosedimentary complexes of the Neoproterozoic Upper Vorogovka Group, which formed after a gap and erosion of the underlying rocks. The later troughs (Teya-Chapa and Uvolga) are composed of sedimentary and volcanosedimentary sequences corresponding to the Chingasan level of the Late Neoproterozoic. The troughs of both types are dominated by subaerial coarse-clastic variegated deposits at the bottom and by terrigene or terrigene-carbonate deposits at the top. At present, the deposits of early and late troughs occur in grabens and graben-synclines, as a rule with conglomerates at the base, discordantly overlying metamorphic sequences of different Precambrian stratigraphic levels and granitoids.

The intracontinental rift magmatism was the most intensive within the zones of influence of the Ishimba and Tatarka faults looking like overthrusts in the present-day structure. According to geological, petrologo-geochemical, and isotope-geochronological data, three epochs in the formation of intraplate magmatism-related rift structures have been recognized: at 750, 700, and about 670 Ma (Nozhkin et al., 2007; 2008). The products of volcanism of these epochs are represented by metarhyolite-basalt (750 Ma), trachybasalt-trachyte (700 Ma), and alkali-ultrabasic (alkali-picritic) (670 Ma) associations (Diner et al., 2000; Nozhkin et al., 2008). Volcanism and accompanying intrusive magmatism (dikes and stocks of quartz porphyries, gabbro-dolerites, alkaline syenite porphyries, camptonites, alkaline picrites, etc.) manifested themselves simultaneously with the accumulation of terrigenous deposits of the Upper Vorogovka, Chingasan, and Chapa Groups (Nozhkin et al., 2008). The volcanosedimentary complexes of these levels were formed in narrow fault-related grabens with evident signs of rift structures. Within the uplifted framing composed of Paleo- and Mesoproterozoic metamorphic sequences, the rifting and intraplate magmatism processes paralleled the formation of granitoid intrusions of the Ayakhta (760-750 Ma) (Vernikovsky and Vernikovskaya, 2006), Kutukas (690-700 Ma) (Nozhkin et al., 2008), and alkaline intrusions of the Mid-Tatarka (about 700 Ma) (Sveshnikova et al., 1976; Vernikovsky et al., 2008) complexes. The granitoids are Na-K granites, subalkalic granites and leucogranites, and, less frequently, syenites. Their geochemistry corresponds to A-granites (Nozhkin et al., 2001, 2008; Vernikovsky and Vernikovskaya, 2006). They formed approximately 120 and 170 Myr after the collisional event. Coeval to rifting and intraplate magmatism, these granitoids are likely to relate to the extension environments. The alkaline-ultrabasic rocks of the Chapa complex as well as carbonatites and alkaline metasomatites of linear type of the central part of the Angara region formed at about 650-670 Ma. Thus, rift-related intraplate granitoid and alkaline magmatism manifested itself within the Yenisei Ridge intensively and recurrently in the Late Neoproterozoic (about 750-650 Ma). In geochemical characteristics, the subalkalic basaltoids and alkaline rocks are similar to volcanic rocks of oceanic islands and continental rift zones whose relationship with deep-seated mantle sources and mantle plumes has the most solid support. It is supposed that the Neoproterozoic rifting and intraplate magmatism are linked in a certain way to the plume activity responsible for the breakup of the supercontinent Rodinia. This is in agreement with the time of manifestation of rifting and intraplate processes in the Sayan region, Olokit graben, Aldan shield, and other continental blocks of Rodinia-Laurentia, South China, India, and Australia (e.g., Ernst et al., 2008; Heuman et al., 1992; Li et al., 2008; Park et al., 1995; Rytsk et al., 2002; Sklyarov et al., 2003; Torsvik et al., 2001; Xu et al., 2005; Yarmolyuk et al., 2005). In the late Mesozoic and in the Neoproterozoic, these lithospheric blocks might be interrelated as separate parts of this supercontinent (Metelkin et al., 2007).

This work was supported by grants 04-05-64301 and 08-05-00521 from the Russian Foundation for Basic Research and by integration project 6.7.1 from the Presidium of the Siberian Branch of the RAS.