Cambrian basaltic magmatism of different geodynamic settings recorded in the Katun accretionary complex - a transitional zone between the Paleo-Asian Ocean and Siberian continent

Safonova I.Yu.*, Simonov V.I.*, Komiya T.**, Kurganskaya E.V.*

* - Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia; ** - Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan

             The Paleo-Asian Ocean (PAO) was located between Siberia and East Gondwana from 750 Ma until the Carboniferous (e.g. Zonenshain et al., 1990). Its western part included a chain of seamounts (Safonova, 2008) and Kuznetsk-Altai island arc (Buslov et al., 2001), which existed in the transitional zone between the ocean and the Siberian continent. The Cambrian oceanic subduction resulted in accretion of the seamounts to the island arc, and later those units together were accreted to the SE margin of the Siberian continent during the closure of the PAO. The accretion formed an accretionary complex (AC), which is presently hosted by the Katun zone of Gorny Altai in SW Siberia. The Katun zone comprises two groups of basalts of different ages: 1) Early Cambrian oceanic basalts of the Manzherok and Eskonga Fms. incorporated in the Katun AC, and 2) Middle Cambrian basalts of the Ust’-Syoma Fm., which dikes and lavas cut and discordantly overlap the accretionary units. The Early Cambrian basalts are associated with oceanic plate stratigraphy (OPS) sediments: reefal limestone and slope facies sediments (Manzherok Fm.) and siliceous shale and chert (Eskonga Fm.; Dobretsov et al., 2004; Safonova, 2008). Evidence for their Early Cambrian age comes from the occurrence of abundant remnants of microphytoliths and calcareous algae in carbonates and sponge spicules in siliceous shale (Terleev, 1991). The AC units are unconformly overlapped by Middle Cambrian rocks, including the Ust-Syoma volcanogenic sedimentary formation. The basaltic dikes up to 10 m thick cut the AC over a distance of 100 km suggesting an extension tectonic regime.

The Early Cambrian basalts of the Katun AC plot in the fields of subalkaline and alkaline basalts in the Nb/Y vs SiO₂ diagram and in the field of high-Fe tholeiites in the Al₂O₃-TiO₂+FeO*-MgO classification triangle. According to the concentrations of TiO₂, LREE and Nb the samples can be divided into two groups: depleted and enriched (Figs. 1,3,4). In the depleted group SiO₂=45.1-52.6; Fe₂O₃=11.2-12.1; TiO₂=0.72-1.52; Al₂O₃=13.8-16.8; P₂O₅=0.14-0.66 wt.%; Mg#=33-64 (Figs. 1,2); Ba/Rb=3-300; Zr/Nb=19-32. Low La_Nav (12.6), La/Yb_N (0.5-2.4) and Th (0.2-0.6 ppm), and Nb depletions (Nb/La_pm=0.16-0.96; Nb/Th_pm=0.24-0.69) make the depleted varieties similar to N-MORB (Fig. 3,4). For the enriched varieties SiO₂ spans 45.4-48.9 wt.%; Fe₂O₃=11.6-15.1; TiO₂=2.0-4.2; P₂O₅=0.18-1,56; Al₂O₃=13.4-18.1 wt.%; Mg#=35.4-53.3 (Fig. 1); Ba/Rb=4-170; Zr/Nb=3-15. High LREE (La_Nav=52.8; La/Sm_N=1.3-3.65; Gd/Yb_N=1.4-3.4) and Nb (Nb/La_pm=1.23-2.87; Nb/Th_pm=1.85-4.75) suggest their mantle plume origin (OIB-type). The observed variations in incompatible elements can be explained by the variable degrees of melting: high degrees within the stability field of spinel for the MORB-type depleted basalts (Gd/Yb_n=0.8-1.4; Mg#_av=53) and low degrees within the garnet stability field (Gd/Yb_n>2; Mg#_av=41) for OIB-type enriched basalts (Figs. 1,2).

The Middle Cambrian samples are subalkaline and transitional basalts having variable MgO (5.75 and 14.77) and Al₂O₃ (7.51 and 20.61), respectively; SiO₂=45.0-54.2; Fe₂O₃=8.5-12.3; TiO₂=0.58-0.89; P₂O₅=0.1-0.28 wt.% (Figs. 1,2). The samples are characterized by zero to weakly enriched LREE (La/Sm_N=0.85-1.7) and depleted Nb (Figs. 3,4; Nb/La_pm=0.3-0.8; Nb/Th_pm=0.2-0.99). Similarly to subrasubduction basalts a part of samples have Th/Nb_pm>La/Nb_pm. The others are closer to N-MORB, however possessing lower HFSE (Ti, Nb, Zr, Y) and higher Al₂O₃ and MgO (Figs. 1,2). Generally, the major and trace element composition of the Ust-Syoma basalts is close to island-arc tholeiites (IAT-type), but have lower REE. The Ust-Syoma basalts have characteristic porphyric texture with diopside and plagioclase phenocrysts up to 5 mm size that makes them similar to lamprophyres. All the samples are characterized by wide variation of alkalis and LILE (Na₂O=0.8-3.8; K₂O=0.2-3.34; Rb=1.1-69; Sr=3.5-817) possibly due to their mobility during post-magmatic alteration.

                The chemical composition of clinopyroxene phenocrysts from the Manzherok (OIB-type) and Ust-Syoma (IAT-type) samples corresponds to diopside-augite and diopside-salite, respectively. Clinopyroxene monomineral thermometry shows that the Manzherok basalts were formed at higher temperatures compared to the Ust-Syoma samples (averages from several thermometers):  1130-1220^оС and 1075-1120 ^оС, respectively.

             According to Tatsumi (1991) high-Mg island-arc tholeiites can be melted near the volcanic front at mantle depths close to the mantle-crust boundary. Therefore we suggest that the high-Mg Ust-Syoma basalts (Gd/Yb_n<1.4; Mg#=61.5-63; Figs. 1,2) possibly formed in mantle wedge, i.e. suprasubduction, conditions, as a result of the melting of the subducting oceanic slab in the presence of fluid phases. Moreover, the signatures of extension regime (dikes), similar to MORB low-Ti and high-Mg composition and occurrence of adakite-like rocks and granites of Cambrian age in the frontal area of subduction (Shokalsky et al., 2000)  allows us to suggest oceanic ridge subduction, which resulted in formation of a slab window.

Conclusively, the available geological, mineralogical and geochemical data show that the Early Cambrian depleted basalts formed in an oceanic spreading setting. Evidence for this comes from their association with siliceous sediments, low to medium TiO₂, and flat REE patterns (Figs. 1,3).  The enriched basalts formed in an oceanic island setting; they are associated with siliceous-carbonate slope facies sediments and characterized by high TiO₂, LREE and Nb/La_pm (>1) suggesting an enriched mantle source. The OIB-type basalts were formed at depths of the spinel (Gd/Yb_n<1.5) and garnet (Gd/Yb_n>2) facies (Fig. 2).

The Ust-Syoma basalts (Middle Cambrian) are compositionally close to MORB and IAT. However, low TiO₂ and Nb, flat REE patterns at medium to high #Mg suggest their formation during ridge subduction. The typical IAT chemical features (low Ti, La/Nb_pm<Th/Nb_pm, La/Sm_n=1.3; Figs. 3,4) are thought to be inherited from the mantle wedge, which the basaltic melts were ascending through. Such a hypothesis requires more support by performing more detailed geological, geochemical isotope and petrological studies.

The study was financially supported by RFBR-JSPS grant no. 07-05-91211.

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