拉萨地块中部晚白垩世火山岩Ar-Ar年代学和古地磁研究结果及其构造意义

摘要

为进一步确定拉萨地块白垩纪-古近纪的古地理位置,我们对青藏高原拉萨地块措勤地区林子宗火山岩18个采点进行了古地磁研究.结果表明高温(高场)特征剩磁分量主要为亚铁磁性的磁铁矿所携带,特征剩磁分量在95％置信水平下通过了褶皱检验.倾斜校正后采点平均的特征剩磁方向为D/I=16.2°/17.7°,α95=5.6°,对应古地磁极位置为63.1°N,224.6°E,A95=5.1°.另一方面,Ar-Ar年代学结果表明采样剖面的林子宗火山岩形成年龄为～99-93Ma,与拉萨地块林周盆地的林子宗群火山岩的形成年龄存在较大差异.由此我们得到晚白垩世拉萨地块中部措勤地区的古纬度为8.5°±6.9°N,与林周盆地古近纪林子宗群典中组和年波组所揭示出的古纬度相当,进一步表明亚洲大陆最南缘的拉萨地块在晚白垩世-古近世期间位于北半球～10°N的低纬度地区.结合最新的特提斯海相地层古地磁结果,晚白垩世-古近世拉萨地块的古地理位置限定了印度与欧亚大陆的初始碰撞时间不晚于60.5 Ma;～93 Ma以来,拉萨地块和单一刚性欧亚大陆之间存在～1900 km的构造缩短.%In order to better understand Cretaceous to Paleogene paleogeography of the Lhasa terrane, 180 drill core samples at 18 sites were collected from Linzizong volcanic rocks near the Coqen town in the middle Lhasa terrane for paleomagnetic studies. Detailed rock magnetic analyses reveal fine-grained ferrimagnetic magnetite as remanence carrier, whilst stepwise thermal and alternating field demagnetizations isolate a high temperature or high field characteristic remanent magnetization (ChRM) from the majority of the sampled rocks. We interpret this characteristic remanence as primary one in the light of a positive fold test result. The ChRM has a tilt-corrected site-mean direction of D=16. 2°, 7=17. 7° with α95=5. 6% and a corresponding paleopole at 63. 1°N, 224. 6°E with A95 =5. 1°. On the other hand, the Ar-Ar dating on three rock samples show that the collected Linzizong volcanic rocks have a geochronological age ranging from ～99 to 93 Ma, which is much older than those outcropped in the Linzhou Basin, eastern part of the Lhasa terrane. Then both geochronological and paleomagnetic results reported here indicate a paleolatitude of 8. 5° + 6. 9° N in the Northern Hemisphere for the middle part of the Lhasa terrane during the Late Cretaceous, which is well compatible with those estimated from the Early Paleogene Dianzhong and Nianbo formations in the Linzhou Basin, further indicating that the southernmost margin of the Eurasian continent might have a low paleolatitude of ～10°N in the Northern Hemisphere during the Late Cretaceous and Paleocene times. Together with recently published paleomagnetic results for the Paleocene Tethyan Himalayan marine sediments, the low paleolatitudes for Late Cretaceous and Paleocene Lhasa terrane support the view that the initial contact between India and Asia should be no later than ～60. 5 Ma. In addition, our results indicate also that at least -1900 km collision-induced crustal shortening should have occurred between the Lhasa terrene and Siberia given that the Eurasian continent behaves as a single rigid plate during the Cenozoic.