RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES1006, doi:10.2205/2007ES000223, 2008

Eastern Tien Shan

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Figure 19
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Figure 20
[212]  The published records on geology of the southern Turkestan domain in the eastern Tien Shan indicate the occurrence in the area of Late Paleozoic nappes that broke a primary succession of facies zones. In the discussed region the zones of autochthon and allochthonous complexes can be outlined (Figures 19 and 20). The autochthon corresponds to the Muzduk and Kokkiya units; the allochthonous zone, to all allochthonous units in the central Tien Shan.

Autochthon

[213]  In the Kuruktag Range the Ordovician and Lower Silurian are represented by a thick, conodont and graptolite-bearing flysch sequence that was accumulated in an aulacogen in the Tarim platform [Liu et al., 1996; etc.]. The Upper Silurian is unknown in the Kuruktag Range. The Devonian section is composed of red arkosic sandstones with floral remains. The Lower Carboniferous sediments transgressively overlie the Lower Paleozoic and Precambrian rocks. The lower part of the Carboniferous section is composed of clastic sediments; the upper, of brachiopod-bearing limestones. The Permian rocks are missing [Hsu et al., 1994].

[214]  In the Haerke Range, in the Silurian, carbonate sediments yielding Wenlock-Ludlow corals, brachiopods, and trilobites were accumulated. The Lower Devonian is represented by carbonate terrigenous sediments, partly by turbidites, bearing tuff and acidic or intermediate lava beds. The Middle-Upper Devonian sections are made up of shallow limestones containing numerous organic remains and sandstone beds. In the Carboniferous the shallow carbonate terrigenous sediments yielding brachiopods, foraminifers, and conodonts, were accumulated. In the Erbeng Mountains the Middle Devonian-Lower Carboniferous section is also composed of carbonate rocks with brachiopods, corals, and foraminifers [Wang et al., 1990, 1994].

[215]  In the discussed zone the Carboniferous turbidites and olistostromes [Liu et al., 1996] that likely make up the autochthon top, are recorded. The presence of intermediate and acidic volcanites in the Devonian sediments and of Late Devonian granite intrusions dated by U-Pb method as 378 Ma old, was also reported [Bureau..., 1993].

Allochthon

[216]  In the Haerke Range the allochthonous Silurian flysch occupies a considerable area [Hsu et al., 1994]. This zone also includes cherts bearing Late Devonian-Early Carboniferous radiolarians (probable allochthon with a condensed section) and basic lavas and silicites with Early Devonian conodonts and radiolarians [Gao et al., 1998; Wang et al., 1994]. The rocks of different age are unconformably overlain by Late Carboniferous sandstones [Gao et al., 1998] that are likely referred to the neoautochthon.

Ophiolitic allochthons

[217]  The ophiolites were obducted onto the rocks of different age and transported for a distance of up to 75 km south of the Turkestan oceanic suture. Several large ophiolitic bodies are located in the Haerke Range. A complete ophiolitic section, i.e. mantle peridotites, cumulates, a dike complex, pillow tholeiitic basalts, and cherts, was described in the Serikeiailake oreade (Figure 19, 5). The Kulafu ophiolitic mélange (Figure 19, 3) includes blocks of ultrabasites, gabbro, tholeiitic basalts, cherts yielding Late Devonian-Early Carboniferous radiolarians, and of limestones bearing conodonts of the same age. The chemical composition of lavas from the Haerke Range ophiolites is similar to that of mid-oceanic ridge lavas. The rare elements ratio in the basalts indicates that the lavas were produced by depleted mantle [Gao et al., 1998]. The Ar-Ar (plateau) biotite ages of quartz-mica schists from the mélange, is 370 pm 5 and 259 pm 3 Ma old [Chen et al., 1999].

[218]  Another group of allochthonous ophiolites occurs eastwards. The Youshugou oreade (Figure 19, 6) is formed by the mélange, dunite, serpentinite, and gabbroid sheets thrusted on the pelites referred to the Devonian. A part of cumulates is metamorphosed to greenschists, others to garnet amphibolites. Geochemical properties of the basalts from dikes extruding gabbro suggest their formation in the ocean outside a mid-oceanic ridge [Allen et al., 1993; Chen et al., 1999].

[219]  The Rb-Sr isochronous age of 340 pm 4 Ma was estimated for lavas from the nonmetamorphosed ophiolitic mélange in the Liuhuang oreade (Figure 19, 4). Radiolarians from cherts of this mélange are of Early Carboniferous age as well. Gabbro from the ophiolitic mélange was dated by Ar-Ar (plateau) method as 333 Ma old [Gao et al., 1995, 1998].

[220]  The geologic map of Xinjiang shows that the youngest rocks overlain by or including ophiolites are Late Carboniferous in age [Bureau..., 1993].

Metamorphic rocks and ophiolites nearby the Turkestan suture

[221]  Similar to the more westward regions, the metamorphic rocks near the Turkestan suture are associated with ophiolites. In the west of the eastern Tien Shan a belt of eclogite-bearing blue- and greenschists and the Changawuzhi and Gulugou ophiolites (Figure 19, 1 and 2) are located near the suture. The metamorphic complex includes oceanic crust rocks (MORB basalts), argillites, sandstones, marbles, and graywackes. The greenschists were formed from volcanic and carbonate sediments yielding Late Silurian corals, trilobites, and brachiopods. The blueschists are recorded among greenschists in the form of blocks and lenses. The blueschists include gabbro and basalts metamorphosed to eclogites.

[222]  The rocks that underwent a blueschist metamorphism are characterized by heterochronous protoliths. The Sm-Nd isochronous ages of 1570, 1128, 729, and 634 Ma were determined for them. The Ar-Ar crossite and phengite ages of the blueschist metamorphism are 415 pm 2 and 420 pm 3 Ma, respectively. The age of retrograde metamorphism of these rocks was estimated by the same method at 351 pm 1 and 345 pm 7 Ma [Gao et al., 1995, 1998; Siao et al., 1994].

[223]  The Rb-Sr isochronous age of plagiogranites from the Gulugou ophiolites was defined at 358 pm 15 Ma [Gao et al., 1998]; the Ar-Ar (plateau) age of pyroxene from gabbro of the Changawuzhi ophiolites, at 439 pm 27 Ma [Chen et al., 1999; Siao et al., 1994]; that of amphibole from the ophiolitic mélange, at 430-420 Ma [Liu et al., 1996]; that of biotite from the ophiolitic mélange, at 246 Ma [Chen et al., 1999]. The latter dating likely indicates the age of movements along the thrust fault, near which the ophiolites are located.

[224]  The metamorphic rocks and the associated ophiolites presumably made up an accretionary prism at the foot of the Kazakh-Kyrgyz microcontinent. Judging from the age of blueschist and retrograde metamorphism of the rocks, the accretionary prism was formed in the Devonian and Early Carboniferous.


RJES

Citation: Burtman, V. S. (2008), Nappes of the southern Tien Shan, Russ. J. Earth Sci., 10, ES1006, doi:10.2205/2007ES000223.

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