The Early Archaean Geological Evolution Of The Dongshan Complex, Anshan Region: Constrained By SHRIMP Geochronology And Nd & Hf Isotopes

Anshan area is one of the very few places where≥3.8 Ga rocks have been found and the only place where consecutive geological records ranging from 3.8 Ga to 2.5 Ga are preserved. To further understand the early Archaean crustal evolution in the Anshan area, this dissertation intensively studied the Dongshan Complex, a miniature of the early Archaean crustal evolution in the Anshan area.E-W extending for over 1000 m, the Dongshan Complex is located in between the 3.1 Ga Lishan fine-grained trondhjemite and the 3.0 Ga Tiejiashan granite. The maximum width of outcrop of the Dongshan Complex along a highway is about 15 m. Based on previous work and these comprehensive studies in petrography, geochemistry, zircon and apatite SHRIMP U-Pb dating and Hf isotopic analysis, two major groups of tectono-thermal events for early Archaean crustal evolution, i.e., 3.6~3.8 Ga and 3.0~3.3 Ga, have been distinguished from the Dongshan Complex. The former can further be subdivided into three phases of 3.7 Ga, 3.8 Ga and ~3.6 Ga; the latter three phases of 3.3 Ga, 3.1 Ga and 3.0 Ga.The 3.7~3.8 Ga banded trondhjemitic rock is the principal part of the Dongshan Complex, in which 3.7 Ga age was firstly determined in this study. The protolith of the 3.7 Ga trondhjemitic rock is crustal material originally derived from depleted mantle and shortly resided in the crust. The new study suggests that the 3.8 Ga quartzdiorite originated directly from the mantle source but contaminated by the continental crustal materials. All the 3.7~3.8 Ga granitic rocks are characterized by weak fractionation between heavy and light rare earth elements, being consistent with the universal features of the early Archaean TTG rocks on this planet because of the high thermal condition in the early earth history. Part of the complex yields the age of 3.6 Ga that reveals the record of tectono-thermal event in the same period.The 3.3 Ga rocks consists of several different types of rocks, including dioritic gneiss, fine-grained trondhjemite and middle-grained trondhjemite; the biotite tremolite and amphibolite might also be formed 3.3 Ga. Some of 3.3 Ga rocks originated from the mantle source and some are the products of reworking of the continental crustal materials. The 3.3 Ga geological records are widely preserved in the crust of the Anshan area, including the 3.3 Ga Chentaigou supracrustal rock and the 3.3 Ga Chentaigou granite. Obviously there are very significant tectono-thermal events at 3.3 Ga in the Anshan area.The 3.1 Ga fine-grained trondhjemites and adamellites are consistent with the 3.1 Ga Lishan trondhjemite in age. Some of the 3.1 Ga trondhjemites are characterized by abnormal REE patterns with depleted LREE or flat REE patterns, which is rarely seen in the early earth historyin the global scope, but mainly the feature of young geologic bodies. This kind of REE patterns in the Anshan area is the result of long period of geological evolution. The 3.0 Ga age of apatite from the 3.8 Ga metamorphic quartzdiorite is consistent with the age of adjacent 3.1 Ga Tiejiashan granite, suggesting a strong magmatic and tectono-thermal event at 3.0 Ga in the Anshan area.In a word, the Dongshan Complex consists of various types of rocks ranging from 3.8 Ga to 3.0 Ga, and probably the 2.5 Ga potassic granite as well. Some of them show clear cutting relationship and genetic linkage. The Dongshan Complex, with a long history of geological evolution, provides a typical example for the study of the Anshan early Archaean crustal evolution. Based on newly obtained data, a time framework of the early Archaean crustal evolution and tectono-magmatic events for the Dongshan Complex is established.