龙泉关韧性剪切带的年代学研究

A GEOCHRONOLOGICAL STUDY OF THE LONGQUAGUAN DUCTILE SHEAR ZONE

龙泉关韧性剪切带位于山西、河北两省分水岭以西的狭长地区内。本研究的考察路线为阜平县龙泉关乡至长城岭公路沿线，穿过了韧性剪切带。对龙泉关构造岩进行了石英包裹体测温和矿物对氧同位素温度计测温，该剪切带的形成温度大于４６０℃，大于黑云母的Ｋ－Ａｒ封闭温度。超糜棱岩及两侧岩石中七个黑云母等矿物的Ａｒ－Ａｒ定年和全岩Ｒｂ－Ｓｒ定年表明，该剪切带形成于１９５２—１９１４Ｍａ前后，在韧性剪切过程中Ｓｒ同位素有可能重新达到均一化。锆石的Ｕ－Ｐｂ年龄为２５０６Ｍａ，它代表原岩的形成时代。

The Longquanguan ductile shear zone is exposed as a narrow and long zone west of the division between Shanxi and Hebei Provinces. Li Jianghai et al. identified five subzones from west to east in succession: 1) the granitic gneiss subzone;2) the augen gneiss subzone; 3) felsic mylonite subzone; 4) the augen gneiss subzone, and 5) the gneiss and tectonic subzones. In the second one some of supermylonite have developed.Our investigation was carried out along the highway from Longquanguan village to Changchengling in Fuping county, Hebei Province. The profile crosses five subzones of the shear zone. Most of the samples were collected in the second subzone from Longsuan nunnery to Changchengling, within an about 10 metre range from the two sides of supermylonite.Five quartzs were separated from supermylonite and mylonite samples and their decrepitation temperatures were measured. The results show two types. The first one is that two quartzs yielded only one decrepitation event. The second type is that three quartzs yielded two decrepitation,events, of which a major event has decrepitation temperatures ranging from 350 to 460℃, offering the temperature information of postmagmatic (including the shear zone-forming stage) fluids.Oxygen isotopes of 16 minerals from six mylonititic samples were analysed. By using the equation △Qz-Mt=δQZ-δMt=5.57×106T-2, five mineral pairs give oxygen isotopic equilibrium temperatures of 399 to 486℃. In fact the oxygen isotopic temperatures of plutonic rocks are subsolidus temperatures and do not correspond to crystallization temperatures. This phenomenon is explained by the difference in the cooling rate. Plutonic rocks cool sufficiently slowly to permit minerals to re-equilibrate their oxygen isotopes as temperature decreases. In general, the oxygen isotople temperatures of plutonic rocks underestimate the crystallization temperatures of the minerals.Therefore these results indicate that the deformation temperature of the Longquanguan shear zone should be higher than 399-486℃.Seven minerals were analysed by the dating method of 40Ar/39Ar. Five biotites have undisturbed spectrums with plateau ages of 1 952-1 914 Ma. One hornblende also has an undisturbed spectrum with a plateau age of 1 972 Ma, slightly older than those for the biotites. Five whole rock samples from the mylonite fall on a Rb-Sr isochron with an age of 1 920±39Ma and with an initial 87Sr/86Sr ratio of 0.702 6.This age agrees well with the 39Ar-40Ar plateau ages of the biotites analysed.The fact that biotite, hornblende (39Ar-40Ar) and whole rock (Rb-Sr) give similar ages suggests that the Longquanguan shear zone had experienced intensive tectonic uplifting and denudation in the period of 1 952-1 914 Ma. The cooling rate was so fast that the dated ages by different methods from different samples are close each other. Therefore the shear zone formed in the period of 1 952-1 914 Ma or a little earlier.The Rb-Sr isochron of whole rock samples also suggests that the Sr isotopic compositon may completely be homogenized, although the minerals did not recrystailine completely and the porphyroblasts coexist with the fine grain recrystallization minerals during the shearing. It is possible to determine the age of the ductile shear gone by the Rb-Sr dating method from whole rock samples.Five fractions of zircons from the augen gneiss were analysed by the dating method of U-Ph. By means of the Wetherill concordia diagram, the radiogenic ratios form a straight line that intersects the concordia at two points. The upper intercept corresponds to an age of 2 506 Ma, representing the crystallization age of the zircons. The lower intercept is meaningless for geochronology. The results also suggests that the zircons in the augen gneiss preserve closure with respect of the U-Pb system during the Longquanguan shearing. That is to say, U and Ph in the zircons did not migrate under the temperature and medium conditions of the shearing. So the maximum temperature of the shearing should not be higher than close temperature of the zircon, i.e., the