Zircon ages and Hf isotopic compositions of plutonic rocks from the Central Tianshan (Xinjiang, northwest China) and their significance for early to mid-Palaeozoic crustal evolution
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We present new zircon ages and Hf-in-zircon isotopic data for plutonic rocks and review the crustal evolution of the Chinese Central Tianshan (Xinjiang, northwest China) in the early to mid-Palaeozoic. The Early Ordovician (ca. 475–473 Ma) granitoid rocks have zircon εHf(t) values either positive (+0.3 to +9.5) or negative (−6.0 to −12.9). This suggests significant addition of juvenile material to, and coeval crustal reworking of, the pre-existing continental crust that is fingerprinted by numerous Precambrian zircon xenocrysts. The Late Ordovician–Silurian (ca. 458–425 Ma) rocks can be assigned to two sub-episodes of magmatism: zircon from rocks of an earlier event (ca. 458–442 Ma) has negative zircon εHf(t) values (−6.3 to −13.1), indicating a predominantly crustal source; zircon from later events (ca. 434–425 Ma) has positive zircon εHf(t) values (+2.6 to +8.9) that reveal a predominantly juvenile magma source. The Early Devonian (ca. 410–404 Ma) rocks have near-zero zircon εHf(t) values, either slightly negative or positive (−1.4 to +3.5), whereas the Mid-Devonian rocks (ca. 393 Ma) have negative values (−11.2 to −14.8). The Late Devonian (ca. 368–361 Ma) granites are undeformed and are chemically similar to adakite but have relatively low negative whole-rock εNd(t)values (−2.4 to −5.3). We interpret the Early Ordovician to Mid-Devonian magmatic event to reflect combined juvenile crustal growth and crustal reworking processes via episodic mafic underplating and mantle–crust interaction. The Late Devonian episode may signify delamination of the over-thickened Chinese Central Tianshan crust.Keywords:
Devonian
Underplating
Devonian
Late Devonian extinction
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标明日期的高精确锆石 U-Pb 显示那主要恼人的身体(Tongan， Niumiao， Huashan， Lisong ) ，并且在 Huashan-Guposhan 建筑群的一块 mafic microgranular 被外国领地包围的土地在 160163 妈被形成。从 Huashan 花岗石的锆石的 ? Hf (t) 价值变化从 ? 2.8 变化到从 Lisong 花岗石的 +0.3 和那些从 ? 2.3 到 +0.3，它与从 Lisong 花岗石的 mafic 被外国领地包围的土地的那些价值(到 +7.4 的 +2.6 ) 显然是不同的。这些 Hf 同位素的数据显示 mafic 被外国领地包围的土地和主机花岗石从岩浆的不同来源被使结晶，为 mafic-felsic 岩浆混合提供证据处理。最高 ? 从 mafic 被外国领地包围的土地的锆石的 Hf (t) 价值直到 +7.4，显示 mafic 岩浆从相对弄空的披风来源被发源。在这个区域的地区性的地质学和同时期的 mafic 和碱的岩石上的研究显示 mafic 岩浆没从玄武岩的少年外壳重做，但是从披风的部分融化被发源。然而， mafic 岩浆是否从岩流圈或 lithospheric 披风的部分融化被导出，尚待被决定。Huashan 花岗石和 Lisong 花岗石从导出披风、导出外壳的岩浆的混合岩浆被形成，并且 mafic 被外国领地包围的土地在混合过程期间被看作导出披风的岩浆的遗体。从 Niumiao 闪长岩的锆石的 ? Hf (t) 价值变化从 ? 1.1 变化到从 Tongan 石英二长岩的 +2.1，和那些从 ? 1.7 到 +1.7。这些价值是比从 mafic 被外国领地包围的土地的那些低的，建议闪长岩和二长岩从有 mafic 被外国领地包围的土地的不同导出来源的岩浆被形成。为 Niumiao 闪长岩和 Tongan 二长岩的 ? Hf (t) 价值仅仅比为 Hushan 花岗石和 Lisong 花岗石的那些稍微高。丰富的 mafic 被外国领地包围的土地也发生在 Niumiao 闪长岩并且在 Tongan 二长岩。因此，我们建议 Niumiao 闪长岩和 Tongan 二长岩也可能从象花岗石的一样的混合岩浆被形成，但是通过部分结晶化和外壳的污�
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(1971). Paleozoogeography in Ordovician, Silurian, and Early Devonian on basis of tabulatomorph corals and the boundaries of the Silurian system. International Geology Review: Vol. 13, No. 3, pp. 427-434.
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Precambrian time is the whole of geologic time from the very beginning of the earth history until the earliest fossiliferous Cambrian beds were deposited. Precambrian time covers almost 90% of the total length of time that has passed since the formation of the earth. Until recently, however, this long period of geologic time was among the least known segments of the geologic record.The actual absence of fossils in Precambrian rocks makes it very difficult to correlate rocks of one locality to those of the others or to identify the age of geological formations from different localities. By introducing the dating methods based on radioactive decay, reliable age data on minerals and rocks have been accumulated, especially since 1950. Precambrian is now outstanding in availability of a very notable number of exact ages, among which the oldest ones are estimated at 3, 500 million years. The Precambrian rocks are exclusively found in the vast shield areas of the world. In the African continent, they occupy 57% of the whole continent in its areal distribution.In recent years, researches on the Precambrian rocks in the African continent have made a remarkable progress, especially on the following five points:(1) Since the improvement and spread of radiometric dating techniques, various Precambrian orogenic belts and early Palaeozoic one have been dated. Stratigraphic successions of the Precambrian system were greatly revised. Fairly well correlation of Precambrian rocks from one region to another is done throughout the African continent.A. Holmes (1963) wrote vividly this situation of drastic revision of the stratigraphy on Precambrian system in Africa as follows: “For me, probably the most dramatic and unexpected surprise of a decade packed with surprises was the announcement of the great age of the Bushveld Complex, about 2, 000 million years, and the consequent realization that the Transvaal Group of strata must be older still. Until 1901 the Transvaal ‘System’ was correlated on lithological grounds with the Palaeozoic Cape ‘System’. Then for over half a century the Transvaal ‘System’ was confidently thought to be of late Precambrian age and, lithologically, a typical representative of the Algonkian. Yet it has turned out to be immensely older than such characteristically Archean rock sequences as the Grenville of the Canadian shield and Svecofennian of the Baltic shield.”(2) The almost all Precambrian rocks of the African continent have been hitherto considered to represent the Precambrian Craton (Shield). Recently, time, character, and areal distribution of the Precambrian orogenic cycles in Africa have been confirmed, and the following five orogenic cycles have been recognized.a. Upper Luanyi Cycle (more than 3, 000m. y. ago)b. Shamvaian Cycle (2, 700-230m. y. ago)c. Limpopo Cycle (2, 150-1, 650m. y. ago)d. Kibaran Cycle (1, 290-850m. y. ago)e. Katangan Cycle (620-485m. y. ago)The Katangan belt of orogenesis, late Precambrian to early Palaeozoic in age, is shown to be extensively developed throughout Africa. Of recent years, awareness of the significance of this Katangan Cycle has been growing. From an important but essentially local feature, it has grown to the status of a “Pan-African thermo-tectonic episode.”The Kibaran belt of east and central Africa is also probably extended to the Orange River belt and to Natal in South Africa.These Katangan and Kibaran belts represent a distinctive regime of younger orogens consisting of mobile zones which have suffered orogenic deformation from time to time during the past ca. 1, 200m. y., and this younger tructural regime is readily differentiated from older cratons which have remained stable over the past ca. 1, 500m. y.a large part
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"‘Upper Precambrian Correlations’ and ‘Upper Precambrian-Cambrian Boundary’: Activities in Sweden." Geologiska Föreningen i Stockholm Förhandlingar, 101(1), pp. 75–76
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