松花江早更新世水系演化:来自TIMA矿物和地球化学的证据

水系演化研究是揭示流域地貌—构造—气候演化之间相互作用的重要途径。松花江水系演化研究目前还相对薄弱,尤其是第四纪松花江中上游是否发生流向反转存在争议。自动定量矿物分析系统TIMA(TESCAN Integrated Mineral Analyzer)在源区识别和古地理重建方面有极大的应用潜力。为此,本文利用TIMA技术对位于松花江T2阶地的哈尔滨荒山岩心沉积物进行重矿物及全岩矿物地球化学组成分析。结果表明,以深度62.3 m为界,岩心上、下地层沉积物的重矿物(例如,锆石、磷灰石、金红石、榍石、石榴石、钛铁矿、铁磁矿物和硅铁矿)及全岩矿物地球化学组成均存在明显差异。62.3 m以上地层沉积物的重矿物组合是闪石类+帘石类+榍石+铁磁矿物,硅铁锂钠石在上段地层中出现,全岩矿物地球化学元素较为稳定,波动幅度较小;62.3 m以下地层沉积物的重矿物组合是闪石类+帘石类+钛铁矿+榍石,方解石、铬铁矿、蛇纹石、黄铁矿和磁黄铁矿仅在下段地层中出现,全岩矿物地球化学元素波动幅度较大。TIMA重矿物和全岩矿物地球化学组成反映了岩心沉积物的物源发生明显变化,进而指示了松花江的水系演化。结合在依兰发现的河湖相地层,我们提出了松花江水系演化的新模式。早更新世时期,佳依(佳木斯—依兰)分水岭将松嫩水系和三江平原水系分隔开,作为松花江上游的牡丹江向东流经依兰—通河—哈尔滨,最终注入松嫩古湖。在0.94 Ma B. P.之后,松辽分水岭局部隆升,古松花江发生反转,从西向东流至通河—依兰地区形成古大湖。湖泊水位不断升高致使湖水溢流切穿佳依分水岭,形成现代松花江水系的基本格局。这挑战了以前的向源侵蚀导致佳依分水岭被切穿的水系演化模式。

Drainage evolution of the Songhua River system in the Early Pleistocene: Evidence from TIMA minerals and geochemistry

The study of drainage system evolution is an important way to reveal the interaction between geomorphological-tectonic-climatic evolution in watershed. At present, the research on evolution of Songhua River system is still relatively weak, especially whether the flow direction reversed in the middle and upper reaches of Songhua River in Quaternary is controversial. The automated quantitative mineral analysis system TIMA(TESCAN Integrated Mineral Analyzer)has great application potential in source area identification and paleogeographic reconstruction. Therefore,in this paper, TIMA technique was used to analyze the geochemical composition of heavy minerals and whole rock minerals in the core sediments of Harbin Huangshan located in T2 terrace of Songhua River. The results showed that there were significant differences in the geochemical composition of heavy minerals and whole rock minerals in the upper and lower strata of the core at a depth of 62.3 m, and the contents of zircon, apatite, tourmaline, pyroxene, ilmenite, ferromagnetic minerals and ferrosilicon in the upper and lower strata were very different. The heavy mineral assemblage of the sediments above 62.3 m was amphibole + epidote + sphene + ferromagnetic minerals. The sugilite occurred in the upper strata, and the geochemical elements of the whole rock minerals were relatively stable with small fluctuation range. The heavy mineral assemblage of the sediments below 62.3 m was amphibole + epidote + ilmenite + sphene. Calcite, chromite, serpentine,pyrite and pyrrhotite occurred uniquely in the lower strata, and the whole rock mineral geochemical elements fluctuate greatly. The geochemical composition of heavy minerals and whole rock minerals in TIMA reflected the obvious change of provenance of core sediments, which indicated the drainage evolution of Songhua River. Based on the fluvial and lacustrine strata discovered in Yilan, we propose a new model of the evolution of Songhua river drainage system. During the Early Pleistocene, the Jiayi(Jiamusi-Yilan)watershed separated the Songnen river system from the Sanjiang Plain system, and Mudan River, as the upper reaches of the Songhua River, flowed eastward through Yilan, Tonghe and Harbin, and finally flowed into the ancient Songnen Lake.After 0.94 Ma B. P., the Songliao watershed partially uplifted, and the ancient Songhua River reversed and flowed from west to east to form the ancient great lake in Tonghe-Yilan area. The rising water level of the lake caused the overflow of the lake to cut through the Jiayi watershed, forming the basic pattern of the modern Songhua River drainage system. This challenges the previous model of drainage evolution in which headward erosion caused the Jiayi watershed to be cut through.