THÔNG TIN TÀI LIỆU
ULTRA HIGH PRESSURE METAMORPHISM 537 continent–continent collision, but relatively few of these areas have been well explored for UHP rocks Those areas that have been examined closely are in the European Alps, the Norwegian Caledonides, the Chinese Qinling Shan–Dabie Shan–Sulu Terrane belt, the Kokchetav Massif in northern Kazakhstan, the Mid- and West-European Variscides, and, to some extent, the Himalaya European Alps UHP rocks in the Western and Central Alps occur exclusively in the Penninic unit Potential UHP rocks also exist in the Eastern Alps; these rocks are mainly eclogites, whereas in the Western and Central Alps, different metapelites, metagranitoids, and ultrabasic rocks are also involved in UHP metamorphism The lateral extent of UHP units in the Alps is in the range of several to a few tens of kilometres The thickness of these units is commonly less than km, because they are part of the nappe stack of the Alpine Orogen Age dating of the UHP event yielded values close to 35 Ma Potential UHP eclogites in the Eastern Alps were formed in the Cretaceous The protoliths of felsic UHP rocks of the Western Alps can be at least partially assigned to former Variscan metamorphic and plutonic rocks This implies that Variscan continental crust was subducted to great depths during a late Alpine continent–continent collisional event Geothermobarometric evolution has resulted in a pressure and temperature path for the majority of the UHP rocks that is characterized by a prograde branch along geothermal gradients, decreasing from 10 to 6 C km towards the pressure and temperature climax, which is settled between 30 and 40 kbar and around 750 C The retrograde path typically shows decreasing temperatures during pressure release, but the temperatures of the retrograde path are always higher than those of the prograde path at a given pressure Age dating, for instance, on zoned zircon grains has demonstrated that significant exhumation from great depths happened within a few million years Thus the uplift rates must be in the range of several centimetres per year This rate and dating to 35 Ma for UHP metamorphism were also found for ultrabasic rocks, for instance, outcropping at Alpe Arami, Central Alps within the WGR may indicate the existence of a coherent UHP terrane of up to 350 Â 150 km2 Prograde metamorphism is characterized by geotherms around 6 C km 1, reaching peak temperatures of 550 C in the south-east inland area and more than 800 C in the north-west coastal area of the WGR In the latter area, microdiamonds were found on the island of Fjørtoft Age data obtained from UHP metamorphic rocks scatter between about 440 and 390 Ma, but it has been concluded that UHP metamorphism happened close to 400 million years after closure of the Iapetus Ocean Early exhumation rates of cm or more per year are assumed The coeval garnet peridotites originated at mantle depths of about 200 km (60–65 kbar) and temperatures significantly above 1000 C However, it is possibly that they had resided there since mid-Proterozoic times, until they were uplifted by a mantle diapir in the Lower Devonian Chinese Qinling Shan - Dabie Shan - Sulu Terrane Belt In central and eastern China, the Triassic orogenic belt resulting from collision of the Yangtze Craton and the Sino-Korean Craton contains sections with UHP metamorphic rocks Although intruded by enormous volumes of Cretaceous granitoids, these areas can be traced over 100 km and more, leading to the conclusion of the existence of wide, coherent UHP terranes Under this scenario, orthogneisses with Proterozoic protolith ages would be the dominant UHP rock type there The pressure and temperature conditions of the Chinese UHP rocks resemble those of the WGR in regard to spread of peak pressure and temperature ( 600–!800 C) conditions However, a significant difference between the WGR and the Triassic orogenic belt is related to the peak temperature conditions of ultrabasic rocks Garnet peridotites of the Sulu Terrane were metamorphosed at only 800 C, and those of the Dabie Shan were metamorphosed below 800 C In spite of the relatively low temperatures, pressure conditions were estimated to be as high as 55 kbar or even more, thus yielding geothermal gradients somewhat below 5 C km It is assumed that rocks as well as other UHP rocks of the Qinling Shan–Dabie Shan–Sulu Terrane belt dates close to 220 Ma Norwegian Caledonides Felsic, basic, and ultrabasic UHP rocks are widespread in the Western Gneiss Region (WGR) of the Norwegian Caledonides Similar rocks also occur in the Caledonian Orogen further north (for instance, in the Lofoten range) and at the eastern coast of Greenland The widely scattered occurrences of UHP rocks Kokchetav Massif in Northern Kazakhstan An enormous variety of identified UHP rocks is known to occur at local sites (e.g., close to Lake Kumdy Kol) of the Kokchetav Massif Although the development of such rocks can be traced over several tens of kilometres, the recent view of a
Ngày đăng: 26/10/2022, 10:48
Xem thêm: