RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES1006, doi:10.2205/2007ES000223, 2008
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Figure 19 |
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Figure 20 |
[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].
[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
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].
[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
2 and 420
3 Ma, respectively. The age of retrograde
metamorphism of these rocks was estimated by the same method at 351
1 and 345
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
15 Ma
[Gao et al., 1998];
the Ar-Ar (plateau) age of pyroxene from gabbro of the Changawuzhi ophiolites, at 439
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.
Citation: 2008), Nappes of the southern Tien Shan, Russ. J. Earth Sci., 10, ES1006, doi:10.2205/2007ES000223.
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