| 其他摘要 | The Altay orogenic belt is an important part of the Central Asian Orogenic Belt. Many important understandings on the history of tectonic evolution and crustal growth of the belt have been achieved through previous studies on magmatic rocks in the belt. There is lack of works on those widely outcropped Paleozoic strata in the region among previous studies. Although there are more than ten thousand of pegmatite dikes in the Altay orogenic belt, only a small amount of them are rare metal-bearing pegmatite dikes which are important sources of rare metal in our country. No consistent understanding on the source of rare metals in pegmatite dikes has been obtained. Therefore, after collecting a large amount of representative samples of the Paleozoic strata in the Altay orogenic belt, a systematical study on the petro-geochemical characteristics of those strata in various ages has been undertaken, the Paleozoic sedimentary tectonic environment of the belt has been constrained, the source of rare metals in mineralized pegmatite of the belt has been discussed after analyzing the contents of rare metals in various strata in the region. There are Precambrian, Paleozoic, and Mesozoic strata, which are relative widely distributed, in the Altay orogenic belt. The Paleozoic strata, which are most widely distributed in the belt, mainly include the Lower Ordovician Habahe Group, Middle Silurian Kulumuti Group, Lower Devonian Kangbutiebao Formation, Middle Devonian Altay Formation, Upper Devonian-Lower Carboniferous Kumasu Formation, and Lower Carboniferous Hongshanzui Formation, with major metasedimentary rocks, such as mica schist, quartz schist, leptynite, and meta-clastic rocks. Some strata had experienced metamorphism from greenschist to amphibolite. The geochemical study of lowly metamorphosed clastic rocks from various Palaeozoic strata in the Altay orogenic belt shows that they were mainly originated from mud and sandy sedimentary rocks, containing a small amount of the weathering product of volcanic rocks. The characteristics of their chemical indexes of alteration (CIA) show that the Habahe Group (CIA of 50-70), Kulumuti Group (CIA of 57-68), Kangbutiebao Formation (CIA of 50-67), Altay Formation (CIA of 52-60) and Hongshanzui Formation (CIA of 66-68) were sourced from weak chemically weathered rocks under the relatively cold, dry condition, whereas the Kumasu Formation (CIA of 71-79) was sourced from medium to strong chemically weathered rocks under the warm-hot, humid - wet conditions. Their indexes of compositional variability (ICV) are generally higher than 1.0 with variation mainly from 1.04 to 1.61, indicating a poor maturity for clastic rocks which were directly deposited in active tectonic environment. Their compositions are plotted into fields of acidic islane arc in diagrams of K2O versus Rb, La/Sc versus Sc/Th, and La/Th versus Hf, indicating that their clastic materials could be sourced from acidic island arc. Their compositions of major and trace elements show that the Habahe and Kulumuti groups, and Kangbutiebao and Altay formations were deposited in the environment of continental island arc, whereas the Kumasu and Hongshanzui formations were deposited in the junction between the continental island arc and active continental margin. The contents of Be, Nb and Ta of strata in the Altay orogenic belt are similar to those of the average of the upper crust, with the concentration coefficients of about 1, while the Li contents of strata, especially the Precambrian Kemuqi and Supute groups, are obviously higher than the average of the upper crust, with the concentration coefficients varying from 1.68 to 3.41. This indicates that the Precambrian strata could partly be source of the rare metal-bearing pegmatite. In general, the detrital materials of Paleozoic strata in the Altay orogenic belt could be sourced from weak chemically weathered rocks of the acidic island arc, deposited in the continental island arc basin related to a |
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