天山北缘侏罗系地层热历史演化及其地质意义:磷灰石裂变径迹和镜质体反射率证据

Thermal history of the Jurassic strata in the northern Tianshan and its geological significance, revealed by apatite fission-track and vitrinite-reflectance analysis.

对天山北缘石场-玛纳斯、安集海河和四棵树河地区18个样品进行了磷灰石裂变径迹年龄测定,同时测定了中生界地层10件裂变径迹样品相应煤层的镜质体反射率。结果表明地层由老到新镜质体反射率逐渐增加,磷灰石裂变径迹中值年龄逐渐降低。石场-玛纳斯地区,下部三叠系煤层镜质体反射率 R_o 值较低,为O.56%,磷灰石裂变径迹中值年龄较大,为125.3±9.1Ma;八道湾组煤层 R_o 为0.53%～0.64%,磷灰石裂变径迹中值年龄介于81.3±4.7～87.8±5.9Ma;上部西山窑组煤层 R_o 最高,达到0.81%,磷灰石裂变径迹中年龄较低,为44.0±5.4～11.8±1.8Ma;相同层位,东部石场-玛纳斯一带 R_o比西部四棵树地区高,磷灰石裂变径迹年龄刚好相反。磷灰石裂变径迹模拟结果表明中生界三叠系、下侏罗统地层的埋藏深度相对较浅,上覆沉积持续的时间到晚侏罗世到早白垩世基本上已经结束,然后保持在基本不变的深度,直至中新世,不整合在三叠系之下的花岗岩的模拟结果也支持这样的认识。晚侏罗世—早白垩世的冷却降温事件可能是地温梯度变化和隆升作用的共同结果;中侏罗统地层埋藏增温过程持续时间较长,在玛纳斯地区直至渐新世末期。所有样品中磷灰石裂变径迹模拟都记录了10Ma 左右的快速冷却过程,近4～5km 的地壳表层物质被剥蚀,平均剥蚀速率400～500m/Ma。这一剥蚀过程应该与天山地区的快速隆升,以及向北的冲断推覆作用相对应。天山地区山前带的变形应不早于10Ma,这一认识与野外地质证据一致。

Apatite fission track （AFT） dating is carried on 18 samples collected from Shichang-Manas, Anjihai river and Sikesu area of the Northern margin of Tianshan. Vitrinite reflectance （Ro） data of 10 samples from the Triassic to Jurassic coal seam were obtained to provide maximum paleotemperatures for the stratigraphic strata experiencing. The AFT and Ro data shown that the value of Ro gradually is increased and the AFT central age decreased, along with the strata growing younger. In the section of Shichang-Manas, the Ro of Triassic is about 0.56%, and the AFT central ages is 125.3 ± 9.1Ma at the bottom of section ; the Ro of Lower Jurassic is about 0.53% ± 0.64%, and the AFT central ages are between 81.3± 4.7 and 87.8 ± 5.9Ma at the middle of the section; Ro of Middle Jurassic is about 0.81%, and the AFT central age is at range of 44.0 ±5.4 and 11.8 ± 1.8Ma at the topside of section. The Ro of Shichang-Manas, which is located in the eastern part of the study erea, is bigger than that of Sikeshu, which is laocted in the western of the area, at the same strata. The AFT central age is reversed. Modeled best-fit paleothermal histories for the Mesozoic samples, showing a Jurassic period of burial and heating. The burial depth of Triassic and the lower Jurassic strata is shallower than that of the middle Jurassic. The data suggest that maximum burial and thermal maturity of the Triassic and lower Jurassic rocks occurred at Late Jurassic to Early Cretaceous, and then retained at the same depth till Miocene. This history is consistent with the AFT modeling result of the underlying granite. The cooling of Late Jurassic to Early Cretaceous can be typically interpreted as the result of decreasing of paleothermal gradient at the Cretaceous. The burial and heating of the Middle Jurassic rocks preserved for a long time until late Oligocene. All AFT samples show evidence for Miocene rapid cooling, beginning at about 10 Ma. This cooling is typically interpreted as the result of uplift and erosion of overlying strata. The Mesozoic strata exposed on the southwestern Junggar basin margin underwent 4 - 5km of late Miocene to present, and the exhumation rate is about 400 -500m/Ma. This process was corresponding with the re uplift of Tianshan and propagation of thrusting northeast of the Tianshan boundary thrust. Miocene cooling of Mesozoic and Paleozoic granite suggests that uplift of the northern Tianshan occurred significantly later than previously proposed and is consistent with timing of intense deformation in the southwestern Junggar basin.