Geochemical heterogeneity of ophiolites from the Severnaya, Zelenaya and Barkhatnaya mountains (Kuznetsky Alatau)
Dugarova N.A.
Tomsk State University, Tomsk, Russia
nadyadugarova@mail.ru
The Kuznetsky Alatau ridge represents a NW segment of the Altai-Sayan folded belt (ASFB). Due to its specific composition, the Kuznetsky Alatau terrain is considered as a Caledonian collisional system consists of three paleotectonic assemblages (from the top to the bottom): a) fragments of the Late Proterozoic suboceanic crust; b) Early – Middle Cambrian island arc complexes; c) Late Cambrian – Early Ordovician continental marginal and accompanying Early Paleozoic riftogenic or intraplate magmatic formations [4]. A specific signature of this area is spatially varied development of carbonate and terrigenous and volcanic rocks of Late Proterozoic and Cambrian time actively dislocated, as well as of subcontinental Middle Paleozoic volcanic sediments [1]. Tectonics of region was accompanied by dynamic magmatic activity and forming high alkali complexes. In general, “mosaic” (or block-type) structural style conditioned by developing tectonic deformations of the NW and submeridianal trend is typical for the region [2].
Fragments of suboceanic crust formed by the set of petrographic varieties, which are typical for the ophiolitic section, are mostly localized in the axial part of the Kuznetskiy Alatau Ridge. These fragments form an apparent belt. According to today’s conceptions, there are two age levels of ophiolitic genesis in the region: Late Riphean – Early Vendian; and Late Vendian – Early Cambrian. At the second level, forming the biggest ultramafic-mafic paragenesises with the most full petrographic compositions took place. These are mountain peaks: Stanovoi Ridge, Chemodan, Barkhatnaya, Zayachia, Severnaya and Zelenaya, which are traditionally considered as an ophiolitic association of the Kuznetsky Alatau. They are the most possible primary source for platinum group elements (PGE) from the placer of the Kia river basin [4]. However, the recent isotopic geochemical data indicate more ancient and probably simultaneous forming the oceanic crust products in this region.
The signature of gabbroids from the ophiolitic association is the presence of regional metamorphism signs at the level of amphibolite facies. A basite component of ophiolites from the Severnaya and Zelenaya mountain peaks is mostly represented by rocks of sodium tholeitic series. At the same time, the presence of subalkaline rocks is typical. Their origin is probably connected with developing later dikes from local zones of postorogenic activation, or with developing albitization processes under conditions of regional metamorphism. A criterion for discriminating different natures of high alkali rocks is their detailed petrographic diagnosis. Particularly, the presence of mineral paragenesis with signs of metamorphic transformations at the level of amphibolite facies completely rejects a postorogenic nature of basite veins.
Petrochemical parameters of the studied rocks vary within wide limits (10-20% Al2O3; 0,5-2,5% TiO2; 3-17% MgO), that indicates their possible belonging to the “cumulative” or “layered” complex. According to the K2O/Na2O ratio, these rocks refer to formations of sodium and subsodium series. At the same time, majority of normal alkaline basites corresponds to the field of the MOR-type ultrasodium basalts (K2O/Na2O<0.1). Nevertheless, some of these basites correspond to the products of calc-alkaline series by FeO/MgO ratio. In general, according to petrochemical characteristics, rocks from the Barkhatnaya mountain peak correspond to middle- and high-titanium low-magnesium basites widely spread in the upper parts of gabbroitic section and in the parallel dike complex. The specialization of differentiated suites from cumulative and layered complexes of ophiolitic section is more typical for rocks from the Zelenaya and Severnaya Mountains. Observed heterogeneity of basites shows a possible vertical sequence of magmatic complexes in the ophiolitic section [3, 5].
V
aried
differentiation degree of the matter composition of ultramafic-mafic
suites from ophiolitic paragenesises is also planned in the rare
earth elements (REE) patterns. Particularly, a significant REE
differentiation from plutonic to hypabyssal formations is not found
in some mafic varieties developed on the Barkhatnaya, the Severnaya,
and the Zelenaya mountains [4]. We should note that the europium
maximum occurs when plagioclase appears in these rocks. In general,
REE distribution of the most rocks corresponds to the MORB-type. In
the other varieties, we should take into consideration the prevalence
of light REE over heavy REE at the insignificant europium minimum.
The shape of REE patterns for these rocks is close to patterns for
island arc systems (Fig.).
Based on the whole complex petrochemical parameters, observed rocks of ophiolitic suite from the Severnaya, Zelenaya, and Barkhatnaya mountain peaks can be referred to the products of sodium tholeitic petrochemical series, which are typical for the MORB magmatism. However, complication of the apparent tholeitic specialization of basites is always observed with signs of calc-alkali derivatives. These derivatives are similar to the formations of island arc systems, back-arc spreading areas, or SSZ-type ophiolites. That assumes tectonic association of different type fragments of oceanic crust from ancient sea basins in the folded belt structures.
References:
1. Alabin L.V. Structural-formational and metallogenic zonality of the Kuznetsky Alatau / Novosibirsk: Nauka, 1983. 102p.
2. Krasnova T.S. Petrology of ultramafic massifs of the Severnaya-Zelenaya and Barkhatnaya mountain peaks (Kuznetsky Alatau) / Ph. Doctoral thesis. Tomsk: TSU, 2005. 20p.
3. Krasnova T.S., Gertner I.F. Olivine fabrics in chromite-bearing ultramafites of the Barkhatnaya mountain peak (Kuznetsky Alatau) / Dynamic metamorphism and the fabric evolution in associations of mafic-ultramafic rocks: Proceedings of scientific seminar. Tomsk: TSU, 1996. P. 68-71.
4. Krasnova T.S., Gertner I.F., Utkin Yu.V. Perspectives of platinum mineralization of the Kuznetsky Alatau ophiolites / Petrology of magmatic and metamorphic complexes. V.2: Proceedings of the 2d scientific meeting. Tomsk: CSTI, 2001. P. 229-235.
5. Sun S.S., McDonough W.F. Cmecical and isotopic systematics of oceanic basalts: implications for mantle composition and processes //Magmatism in oceanic basins. Eds. Saunders A.D. & Norry M.J.. 1989. Geol. Soc. Spec. Publ. No 42. P. 313-345.