川东北元坝地区中生代构造与动态热演化史——磷灰石、锆石(U-Th)/He定年分析

摘要

In Yuanba area of the northeastern Sichuan Basin, vitrinite reflectance and (U-Th)/He dating analyses of apatite and zircon in chip samples from the Xujiahe ( T3x) -Jiading ( Kj) Formations were carried out. A dynamic evolution model based on He age and depth/temperature was set up, by means of which we concluded that the He closure temperature of apatite was about 95 °C. Most of the Mesozoic strata ( T3x-KJ) in the study area experienced the He closure temperature of apatite (95 t) while no sample experienced the He closure temperature of zircon, and some samples from T3#2-J,z underwent the maxium palaeogeothermal of about 170 X.. From Paleogene to Neogene (0. 2-36.4 Ma), the Mesozoic strata in Yuanba area went through a severe cooling uplift and denudation with the denudation rate of 109. 9 m/Ma. In Kj and the overlying strata, the maximum denudation thickness was about 4 000 m. The dynamic thermal evolution history in the study area was illustrated systematically as followed. In Mesozoic, the maximum palaeogeothermal of strata was 170-190 t. As strata upraise, palaeogeothermal decreased. From 36 to 176 Ma, palaeogeothermal was 95-170 t. From 0 to 36 Ma, the present geotemperature is below 95 t.%通过对川东北元坝地区须家河组(T3x)—嘉定组(K1j)钻井岩屑样品镜质体反射率和锆石、磷灰石(U-Th)/He定年分析,建立了该区He年龄—深度/温度动态演化模式,推断出元坝地区磷灰石He封闭温度为95℃左右.元坝地区T3x-K1j中生代地层基本都经历了磷灰石He封闭温度(95℃)；所有样品未经历锆石He封闭温度,T3x2 -J1z地层部分样品可能经历了约170℃的最高古地温.元坝地区中生代地层在古近纪—新近纪(0.2～36.4 Ma)发生重大冷却抬升剥蚀,剥蚀速率约为109.9 m/Ma,K1j及以上地层最大抬升剥蚀厚度约为4000m.系统揭示了该区动态热演化历史,中生代地层最高古地温接近于170 ～ 190℃,随后地层发生抬升,古地温下降；在36 ～176 Ma之间时,古地温在95 ～170℃之间；在0～ 36 Ma时,现今地温小于95℃.