天山北緣安集海河組湖相灰巖氧碳同位素變化的古環境意義

李騫+田孝茹+胡瀚文+郭召杰

摘要：漸新統安集海河組代表了天山新生代快速隆升之前北緣的湖相沉積，其頂部至上覆沙灣組沉積相變反映了天山隆升對周緣地區環境的影響。對安集海河剖面進行了實地測量，并對安集海河組湖相灰巖夾層進行系統采樣，通過沉積相分析、氧碳同位素測定，還原安集海河組時期湖泊環境特征及末期的演變。結果表明：安集海河組整體屬于典型的深湖和半深湖亞相沉積，頂部為濱湖沉積；該組δ18O值與δ13C值整體較高，δ18O值在中部偏高，下部和上部相似而較中部稍低，頂部顯著偏負，δ13C值在中部明顯偏負，下部變化較大但比中部稍高，上部顯著偏正；區域氣候資料與大氣環流模擬表明，該時期氧碳同位素變化不受降水來源和季風等因素的影響。綜合沉積相分析與氧碳同位素試驗結果，安集海河組大部分時期氣候以溫暖濕潤為主，末期接近沙灣組時期氣溫出現明顯下降，氣候變為干旱寒冷；全球平均氣溫在安集海河組末期未出現顯著變化，因此，這一區域氣候變化可能更多地與印度—亞歐板塊碰撞遠程效應導致的天山活化隆升有關。

關鍵詞：古環境變化；安集海河組；新生代；湖相灰鹽；氧碳同位素；沉積相；構造隆升；天山

中圖分類號：P66；X141 文獻標志碼：A

Abstract： Oligocene Anjihaihe Formation in northern Tianshan represents lacustrine sedimentary before the rapid uplift of Tianshan in Cenozoic. The variation of sedimentary facies at the top of Anjihaihe Formation and overlying Shawan Formation indicates the influence of the uplift of Tianshan. Field measurement was carried out in Anjihaihe section， and lacustrine limestone was sampled from Anjihaihe Formation for sedimentary facies analysis and oxygen and carbon isotopes analysis. The lacustrine environment during Oligocene was reconstructed. Results reveal that Anjihaihe Formation is mostly formed in semi-deep to deep lacustrine environment and in shore environment at the top. δ18O values and δ13C values in Anjihaihe Formation are relatively higher than normal lakes； δ18O values of the middle part are slightly higher than those of lower and upper parts； at the top of Anjihaihe Formation， δ18O values drift negatively significantly. On the other hand， δ13C values are much higher in the upper and top than those in the middle and lower parts with few exceptions. Regional climatic data and simulation of atmospheric circulation indicate that the variation of isotope in Oligocene is not the result of air mass change and monsoons. Given the data of oxygen and carbon isotopes， along with sedimentary facies analysis， it is concluded that Anjihaihe Formation is characterized by a worm and humid environment while the subsequent top part witnesses an obvious fall in temperature and the climate turns cool and dry. Considering that global climate showes no distinct change at the end of Anjihaihe Formation， this regional change may be to a large degree relate to the active uplift of Tianshan caused by the far field effect of India-Eurasia Plate collision during Cenozoic.

Key words： paleoenvironment change； Anjihaihe Formation； Cenozoic； lacustrine limestone； oxygen and carbon isotopes； sedimentary facies； tectonic uplifting； Tianshan

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