从青藏高原探讨挤压板块的热历史

THERMAL HISTORY OF COMPRESSING PLATES,WITH SPECIAL REFERENCE TO THE XIZANG PLATEAU

Xizang plateau is situated on the convergent margin between the Indian and Eurasian plates. The inter-collision of the two continents caused the increase in crustal thickness and radioactive energy, thus leading to the great changes in heat flow and temperature profile. On the basis of heat flow data regarding global plates and radioactive elements distribution in various geospberes and of the relative moving rate between the two continents, a thermal conductive equation has been derived and the temperature-depth correlation curves calculated with respect to the interior of the Xizang Plate. With the change in temperature profile, the source region of magmatism shifts gradually from the upper mantle towards the upper crust. It is suggested that remolten granites could net be generated in the crusts of normal continental plates. Instead, their occurrence would only be expected when the crust has been thickened as a result of collision or subduction on the convergent margin. This is supported by the geochemical data on Eu anomalies of REE, Al2O3/TiO2 and TiO2/P2O3,and lead isotope in granitic rocks of different ages in this region. Based on the temperature-depth curves and the K-feldspar and biotite cooling ages,the average uplifting rates have been worked out with respect to different localities in the Xizang plateau. It is indicated that the uplifting rate in the Himalayan region is as high as 0.60---0.84 mm/yr, against a range of 0.36 to 0.48 mm/yr for the vast areas of the plateau. The relationship between plate thickness and crustal thickness has also been deduced from the thermal conductive equation. It indicates that the plate thickness increases with crustal thickening from the oceanic plate to the normal continental plate, but the plate becomes thinner as crustal thickness increases in the collision margin. For the Xizang plateau, the calculated plate thickness is 74--85 km. Based on this value the uplifting elevation of the plateau under the condition of isostatie presure equilibrium should be 4.6-5.7 km, which is consistent with the actual observation.

Xizang plateau is situated on the convergent margin between the Indian and Eurasian plates. The inter-collision of the two continents caused the increase in crus-tal thickness and radioactive energy, thus leading to the great changes in heat flow and temperature profile. On the basis of heat flow data regarding global plates and radioactive elements distribution in various geospheres and of the relative moving rate between the two continents, a thermal conductive equation has been derived and the temperature-depth correlation curves calculated with respect to the interior of the Xizang Plate. With the change in temperature profile, the source region of magma-tism shifts gradually from the upper mantle towards the upper crust. It is suggested that remolten granites could not be generated in the crusts of normal continental plates. Instead, their occurrence would only be expected when the crust has been thickened as a result of collision or subduction on the convergent margin. This is supported by the geochemical data on Eu anomalies of EEE, Al2O3/TiO2 and TiO2/P2O5, and lead isotope in granitic rocks of different ages in this region.Based on the temperature-depth curves and the K-feldspar and biotite cooling ages, the average uplifting rates have been worked out with respect to different localities in the Xizang plateau. It is indicated that the uplifting rate in the Himalayan region is as high as 0.60-0.84 mm/yr, against a range of 0.36 to 0.48 mm/yr for the vast areas of the plateau. The relationship between plate thickness and crustal thickness has also been deduced from the thermal conductive equation. It indicates that the plate thickness increases with crustal thickening from the oceanic plate to the normal continental plate, but the plate becomes thinner as crustal thickness increases in the collision margin. For the Xizang plateau, the calculated plate thickness is 74-85 km. Based on this value the uplifting elevation of the plateau under the condition of isostatic pressure equilibrium should be 4.5-5.7 km, which is consistent with the actual observation.