| The Altyn orogenic belt, locating at the border of four provinces of Tibet, Sinkiang, Qinghai and Gansu, is the natural northwest margin of the Qinghai-Tibet plateau and separates Qaidam basin from Tarim basin. The latest progresses in the regional geology in this area show that the belt is not only a Mesozoic-Cenozoic strike-slip system, but also a complicated orogenic residue composed of blocks formed at different levels and tectonic setting. The discoveries and further studies of the different HP-UHP metamorphic rocks in Altyn mountains have attracted many attention of geologists all over the world.Based on the existing studies of HP-UHP rocks in this region, the author focus his studies mainly on the minerageny and chronology of zircons from granitoid gneiss and basic-intermediate gneiss, which are the direct country rocks of garnet Iherzolite.(1) Cathode-luminescence investigations reveal complicated internal structures of zircons from UHP granitoid gneiss, mainly consisted of core-mantle-rim frames. The cores show the oscillatory bands of magmatic feature.Zircons in UHP basic-intermediate gneiss and garnet Iherzolite have core-mantle-rim structures or mantle-rim structures. The cores and mantles of zircons have the sector or planar zone.(2) Ranman microprobe analyses for zircons in granitoid gneiss show that the metamictization extent of zircons is so little that there is Pb enough for dating. Numerous mineral inclusions, such as omphacite, rutile, apatite, quartz and feldspar, have been found in the zircons. The feldspar occurs mainly in the core, and the omphacite and rutile in the mantle. Combined with the Cathode-luminescence images, we argue that the mineral assemblages of omphacite + rutile + apatite + quartz and apatite + quartz -t-feldspar are formed under the metamorphic and magmatic conditions, respectively.(3) Characteristics of trace element for zircons from the UHP granitoid gneiss can be divided into three different groups. 1) The core part, ΣREE=1224.95~1891.82ppm, EHREE=1177.29 ~1849.70ppm and Th/U=0.661~ 0.801, show the characteristic of magmatic zircon. 2) The mantle region, ΣREE=297.14ppm, Σ HREE=277.44ppm and Th/U=0.013, show the characteristic of metamorphic zircon. 3) values of ?REE, Σ HREE and Th/U between that of in group 1) and 2) may resulted from the mix of the core and mantle of zircon.Characteristics of trace element for zircons from the UHP basic-intermediate gneiss can be divided into two different groups. The first group have the high abnormal value of Ce (17.766-49.292) and the high Th/U(>0.679); the ratios of (Yb/Gd)cN and Nbrta are 1.435-3.109 and 131.786~343.043 respectively; the second group have the low abnormal value of Ce(0.202-2.250) and the low Th/U(<0.1), and (Yb/Gd)CN =7.284-14.17, NbHa= 44.379- 111.772, , showing the characteristics of typical metamorphic zircon.(4) In-situ U-Pb dating was realized by LA-ICP-MS and SHRIMP respectively. LA-ICP-MS dating give the peak metamorphic age of graniteod gneisss is 495?.5Ma, which is close to the result of 487±10Ma gained by SHRIMP dating for the same rock. The peak metamorphic ages of basic-intermediate gneiss and garnet Iherzolite dating by LA-ICP-MS, assumed to be 501 ±31 Ma and 509±12Ma respectively, which is consist with the age of graniteod gneiss within the error, suggest that they are the products of the same UHP metamorphism. Combined with the occurrence of these three rocks interbeds in the outcrops , the three UHP rocks in this region all underwent the UHP metamorphism, and the peak metamorphic age is 487-509 Ma.(5) The Sm-Nd isochronal age and single-zircon U-Pb age of eclogites in Jianggasayi, western Altyn, is 500±10Ma and 503±5.3Ma respectively, which was considered as the peak metamorphic age of HP metamorphism. Some evidences of UHPM have been found recently in these eclogites (Zhang Jianxin et al., 2002; Chen D L et al, 2003). The determination of peak metamorphic age (487-501 Ma) of the UHP metamorphic rocks in Yingelisayi, give further evid... |
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