Abstract:

In this paper, we introduced petrographical, petrogeochemical and geochronological characteristics of the Erzhigou rock mass in the Boketu area of Eastern Greater Khingan Mountains. We studied its formation age, petrogenesis and tectonic setting. Based on LA-ICP-MS U-Pb dating, we found that the emplacement age of the Erzhigou rock mass ranged from 317.2±2.2 to 319.2±2.3 Ma, consistent with a Late Carboniferous magmatism event in the study area. The lithology of Erzhigou rock mass included granodiorite, with petrogeochemical characteristics of high silicon content, rich alkaline and quasi-aluminous to peraluminous and low MgO, CaO. The 10000 Ga/Al series diagram, as well as P₂O₅-SiO₂correlation, showed that the Erzhigou rock mass was high-K calc-alkaline I-type granite. Some results were obtained as follows: Ba/Nb=70.44-98.20; Ba/La=17.85-28.12; δEu appeared to be negative anomaly, implying crustal source region, where a small amount of mantle substances mixed with strongly oxidative fluids. Besides, since the contents of SiO₂ exceeded 56%, with MgO lower than 3%, these rocks belonged to high-K calc-alkaline series, with Al₂O₃ content higher than 14%. For trace REE, Sr concentration was higher than 400×10^-6, with Y and Yb concentrations lower than 18×10^-6and 1.9×10^-6, respectively. These rocks were rich in large ion lithophile elements (LILE) (e.g., Rb, K, Ba, Th), and relatively depleted in high field-strength elements (HFSE) (e.g., Ta, Nb, P, Hf, Zr). δEu appeared to be negative anomaly. LREE/HREE=4.14-6.19 and (La/Yb)_N=12.05-18.03, showing enrichment of LREE and depletion of HREE. These rocks had the characteristics of adakite, but they were much richer in potassium than typical adakite, indicating C-type adakite characteristics of the Erzhigou rock mass. The Rb and Ba contents, and the ratios Rb/Yb (51.49-148.93) and Ba/Yb (430.17-698.83) in these rocks, were obviously higher than that in MORB, but similar with those in adakite of the Hailar Basin and Kailas in Tibet, suggesting a petrogenesis mechanism of granite generation through fusion of lower basaltic crust generated and thickened by underplating. Late Carboniferous magmas in the study area were formed in the compression environment during the orogenic stage. The tectonic setting of the Honkirnaur-Xing'an and Songnen massifs in the Late Carboniferous period (younger than 317.2±2.2 Ma), is that compression and crustal thickening in the late collision orogenic stage is transformed to extension-collapse environment in the post orogenic stage, followed by more stable intraplate tectonic environment through evolution.

Key words: Xing'an massif, adakite, Carboniferous, U-Pb chronology, tectonic significance