March 2006 LIP of the Month

Cretaceous-Paleogene basalts of the Tian Shan

Aleksander V. Mikolaichuk
Institute of geology NAS KR, Erkindik ave., 30, Bishkek, 720481, Kyrgyzstan
mikolaichuk@mail.ru

Vladimir A Simonov
Institute of Geology and Mineralogy SB RAS, Academician Koptyug ave., 3, Novosibirsk, 630090, Russia
simonov@uiggm.nsc.ru

Introduction

Basaltic extrusives, stocks and dikes of Mesozoic-Cenozoic age occur throughout the Tian Shan. They cover an area of 285 000 sq. km, extending from the mountains bordering the Fergana basin in the west to the Junggar Alatau in the east (Fig. 1). Because of their poor exposure, they have received only limited study. An early observation was that they are mainly concentrated in the Central Tian Shan (Fig. 2), where they outline a large lithospheric block, located in the north-east flank of the Talas-Fergana fault and reflected by deep geophysical anomalies (Knauf et al., 1980). Further work by Vinnik (1998) and Roecker (2001) showed that the crust and the mantle of the Central Tian Shan have geophysical characteristics, that can be explained by the presence of mantle flow related to an ascending mantle plume. This evidence for a mantle plume underlying the region of young Tian Shan basaltic rocks has caused revision of models for Cenozoic-Mesozoic geodynamics of this region.

Geological setting and age of the basalts

Mesozoic-Cenozoic basaltic rocks occur in Paleozoic complexes in the form of dikes and stocks. Flows and sills of basalts occur in the Neotectonic basin, within continental sedimentary units of the Suluterek (or Kokturpak) suite (Dodonova, 1972; Chediya et al., 1973; Dmitrieva and Nesmeyanov, 1982; Fortuna et al., 1994). In the Tuyon region (China) and also in the Fergana basin (Fig. 2, Tashkumyr region), basalt extrusives and sills are found among Cretaceous-Paleogene marine sedimentary rocks (Verzilin, 1976; Sobel and Arnaud, 2000; Huang et al., 2005). The primary problem is an estimation of the age of this basaltic magmatism. Tuyon extrusives and sills yield stepwise Ar/Ar dates ranging from 113-46 Ma. This age range let the authors to conclude, that the mantle plume has undergone multiple phases of activation (Sobel and Arnaud, 2000). In the Kyrgyz Tian Shan there are numerous K/Ar dates on basalts ranging from 55-45 Ma (Dobretsov and Zagruzina, 1977; Grachev, 1999), that would be consistent with an Eocene age of petrification found in strata of the Suluterek suite (Dmitrieva and Nesmeyanov, 1982). Since, the time of Shul’ts (1948) the proposed Cretaceous age of the base of the Suluterek suite has remained unproven, although the presence of Upper Cretaceous spore-pollen complex in its underlying units has been established (Chediya et al., 1973; Fortuna et al., 1994). Available Cretaceous ages for basalts and alkaline gabbro were explained by excessive argon owing to secondary alteration of the rocks (Grachev, 1999).

In order to resolve the age uncertainties we have carried out Ar/Ar dating of basalts, for which geology suggest an Upper Cretaceous age or for which a Cretaceous age is suggested by K/Ar techniques – in the regions of Tekelik, Suluterek and Baiduly. Earlier Ar/Ar dating was done only on the Toraigyr basalts (Sobel and Arnaud, 2000). Location of these regions can be found in Fig. 2, and their detailed geological descriptions are given below.

In the Tekelik river valley, cutting the south side of Aksai basin, there is a section of the Suluterek suite, composed of five basaltic flows with a total thickness up to 80 m (Dodonova, 1972). The lower flow (thickness 6 m) overlies the Paleozoic sedimentary rocks and fills local depressions. It is composed of black basalts. The overlying flows are olivine basalts (6-10 m). They are separated by bright red clays of residual soil, with relicts of amydaloidal texture. The thickness of residual soil does not exceed 0.5-1 m and it is only below the upper basalt flow that the horizon of red clays and breccias with thickness up to 30 m is developed. Some K/Ar ages from this section are available, the oldest age being 74 Ma (Dobretsov and Zagruzina, 1977). In spite of testing all sections, only samples from flow-numbers 1, 2 and 4 were suitable for analytical studies and they show comparable dating results in the interval 71-76 Ma (Fig. 3 and Table 1).

In the Baiduly Ridge the Suluterek suite occurs as terrigenous-carbonaceous and lava facies. The lava-flows of olivine basalt are distributed over a width up to 3 km traced along a south ridge slope for a distance of 19 km. On the north side, the basalt extrusives are bounded by South Sonkul fault (Nikolaev Line) and from east, south and west sides, the basalts are surrounded by terrigenous-carbonaceous deposits, which contain Upper Cretaceous – Eocene spore-pollen complexes (Fortuna et al., 1994). From our data the age of basalts at the base is 61 Ma (Fig. 4 and Table 1, sample 465). The section consists of two lava flows (thickness 35 and 25 m), separated by a layer (0.5-1 m) of marls and limestones with a considerable component of volcanic material.

A third region with Mesozoic-Cenozoic basalts is situated on the left side of the Boom canyon near the mouth of the Suluterek river. Together with Oligocene-Pliocene red beds covering the Suluterek suite the units define an asymmetrical syncline, with width about 0.7-1 km. From the south and north this fold structure is complicated by upthrows, and only on the eastern end of syncline do we observe discordant Suluterek suites overlapping onto the Late Devonian sediments (Fig. 5). The subsurface of the suite consists of red ungraded sandstones and clays with lenticles and streaks of marls and chemogenic limestones (7-30 m). Two massive flows of amygdaloidal-textured olivine basalts with thickness of 20 and 10 m lie above. Basalts are covered by carbonaceous breccias. At the same stratum level the streaks of basaltic tuffs are noted (Chediya et al., 1973; Dodonova, 1972). The top of the suite (50-70 m) consists of red sandstones and clays.

The lower (below the basalts) horizon of the section is characterized by Upper Cretaceous spore-pollen complexes (Chediya et al., 1973) and in the south part of Boom canyon, near the red Cenozoic sediments, a deposit of Upper Cretaceous dinosaur bone fragments (Efremov, 1944) is known. These data do not contradict an age of 84 Ma obtained for basalts of Suluterek section (Dobretsov and Zagruzina, 1977), but the intensive alteration of the basalts does not eliminate the possibility of excess argon which increased the age, as it was interpreted by A.F Grachev (1999). Therefore during field work special attention was given to sampling the most unaltered rocks from both flows. The results show an age of 61 Ma (see Table 1) which replaces the older estimate of 84 Ma.

One of the most studied Central Tian Shan Cenozoic sections is located along the valley of the Toraigyr river on the north side of Issyk Kul basin (Fig. 6). Paleozoic formations composing the base of Kungei Alatoo are cut by granites of the Silurian age. The light limestones and marls (2 m) lie on the eroded surface of granites. The carbonate rocks include a terrigene component, nonuniformly distributed throughout the unit. They are overlain by strata (5 m) of red clay and light arkose sandstones with conglomerates. There are lenticles of limestones at the top of the layer. Grey amygdaloidal-textured massive basalts lie on them, comprising two strata (with thicknesses 10-12 m) separated by red clays. The Ar/Ar age of basalts is 53±1 Ma (Sobel and Arnaud, 2000). Among many-coloured (red and green) clays and argillites of the upper part of the Suluterek suite bone fragments of Eocene mammals were collected (Dmitrieva and Nesmeyanov, 1982). The top of the Cenozoic section consists predominantly of Oligocene – Miocene coarse sedimentary rocks. The described complex of units composes a piedmont of Kungei Alatoo in northwest and Pliocene - Pleistocene sediments of Issyk Kul basin in the southeast (Mikolaichuk, 2000).

All our new data (Simonov et al., 2005) on the Ar/Ar age of the Tian Shan Cretaceous-Paleogene basalts are presented in Table 1 and Fig. 9.

Petrochemical and Geochemical data

Cretaceous-Paleogene basalts of the Tian Shan have a homogeneous composition. Olivine and plagioclase basalts predominate. Xenoliths of ultramafic rocks are found in basalts from Bailamtal, Kastek, Tuyon and Uchkuduk regions (Dobretsov et al., 1979; Grachev, 1999; Sobel and Arnaud, 2000).

On the plot of total alkalis vs SiO₂ nearly all Cretaceous-Paleogene basalts of the Tian Shan fall into subalkaline and alkaline series. The high contents of titanium suggest involvement of plume sources such as the OIB (Oceanic Island Basalt) type. As a group, the rocks of Tian Shan are uniform in composition and have major similarities to alkaline olivine basalts from continental rift areas. The pattern of rare earth elements on the spider diagrams is most similar to that for basalts of typical mantle plumes (Grachev, 1999; Sobel & Arnaud, 2000).

During processing of basalt samples from the Tekelik region, melt inclusions in minerals were also studied (Simonov et al., 2005). The experiments with melt inclusions at high temperature were done on the basis of the procedure described by Simonov (1993) and Sobolev & Danyushevsky (1994). The quenched homogenized inclusions were analysed by electron probe Camebax-Micro in the Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia. Contents of major and trace element and water in melt inclusions are determined by an ionic microanalyzer IMS-4f in the Institute of Microelectronics RAS (Yaroslavl, Russia) under the method described in A. Sobolev (1996). The most representative information on melt inclusions was obtained from samples TK-2. As a result of investigation of glasses of quenched homogenized melt inclusions more than 35 analyses of their compositions were made. Primary melt inclusions (sizes 3-30-80 microns) form rectilinear zones of growth along edges of plagioclase phenocrysts. Inclusions also are situated in the center of crystals. The forms of inclusions are mainly rectangular and round-rectangular plates. Inclusions have a multiphase composition: the ground mass is microgranulated and dark, and on rims of inclusions a light glass border is visible.

It was established, that the contents of inclusions become homogeneous in the interval 1175-1215° C. The calculation of liquidus temperatures with the help of program PETROLOG (Danyushevsky, 2001) on the basis of data on melt inclusions compositions has shown, that the majority of values of calculated temperatures are higher than homogenization temperatures. This indicates higher water content in the melt (Sobolev, 1997), approximately 1 wt. %. But data, determined by an ionic microanalyzer, indicate, as a whole, low-levels of H₂O (0.02-0.06 wt. %) in the glasses of melt inclusions. At the same time, the leakage of volatile components is quite possible during heating inclusions, therefore it is difficult to judge on this analysis the water content of the melts.

The sum of alkalis (from 4 up to 8 wt. %) from inclusions matches with data on the studied basaltic rocks, and belongs to subalkaline and alkaline series. On Harker diagrams (Fig. 10) the data from inclusions data plot within the distribution of compositions from Cretaceous-Paleogene basalts of the Tian Shan fields. The main group of inclusions contains 1.8-3 wt. % TiO₂, and by this fact, correspond to basalts. At the same time, there are inclusions with higher titanium content (up to 3.44 wt. %). Practically all the samples with such high values of TiO₂ are located in close association with samples showing a composition corresponding to the OIB (Oceanic Island Basalt) type. Inclusions are more sodium-rich (K/Na = 0.26-0.6) in comparison to OIB. Based on the petrochemical characteristics (e.g. SiO₂ - 45-50 wt. %, Al₂O₃ - mainly 15-20 wt. %) the melt inclusions coincide with basaltic compositions, and suggests crystallization of plagioclase at a low-pressures, and also that the data are reliable. As a whole, melt inclusions and basalts of Tian Shan are compositionally similar to OIB (Fig. 10), which is a direct indication that the Cretaceous-Paleogene basalts of the Tian Shan formed as a result of mantle plume evolution.

Studies of melt inclusions with the help of an ionic microanalyzer have allowed determination of major and trace element and water contents in the Cretaceous-Paleogene magmas of the Tian Shan. On the consideration of rare elements, which are not affected by secondary processes (Zr, Y, Nb), melt inclusions indicate a plume source for the considered magmatic systems which compositionally lie in the OIB (basalts of oceanic islands such as Hawaiian) field.

The rare earth elements patterns (Fig. 11) for melt inclusions in plagioclases of the Cretaceous-Paleogene basalts of the Tian Shan plot with data on rocks which are transitional in composition between tholeiitic and alkaline series of the Hawaiian islands.

Discussion

Our studies outline the main characteristics of the Cretaceous-Paleogene basalts of the Tian Shan, and allow us to consider the role of magmatic systems in geodynamic processes of Central Asia at this time.

Petrochemical and geochemical features of the reviewed basalts testify to participation of plume magmatic systems such as OIB (Oceanic Island Basalt) type. Melt inclusions in minerals also are similar in composition to OIB. For this reason we favor a model in which Tian Shan Cretaceous-Paleogene basalts are generated from a mantle plume source.

The total area of the Tian Shan Cretaceous-Paleogene basalts distribution is 285 000 sq. km, a number that is comparable to other Large Igneous Provinces (Ernst and Buchan, 2001), e.g. Emeishan traps (China) - 250 000 sq. km.

Our investigations demonstrate, that the most reliable ages of young basaltic rocks of the Tian Shan range from 60-76 Ma.

Thus, data on the Cretaceous-Paleogene basalts of the Tian Shan testify that they were formed as a result of a large plume during a rather short time. Practically synchronously with them (about 66 Ma, Baksi, 1994) the Deccan traps were emplaced. As demonstrated by detailed paleotectonic reconstructions (Anczkiewicz et al., 2000) the vast oceanic basin, which existed 80 Ma, was closed by Eocene (50 Ma) time, as a result of India and Eurasia collision. It follows that at 66 Ma these continental blocks were only a short distance apart. It allow us to suggest a hypothesis, that Deccan traps and same-aged basaltic rocks of the Tian Shan represent a plume cluster (Ernst and Buchan, 2002), having originated from the same deep mantle source in the form of a superplume rising from the core – mantle boundary.

References