The Oldest rocks in China
|Category||Paleontology and Stratigraphy|
|Location||International Geological Congress,oslo 2008|
|Author||Wilde, Simon۱; Liu, Dunyi۲; Wan, Yusheng۲|
|Holding Date||04 October 2008|
Recently-acquired in situ zircon U-Pb and Lu-Hf isotopic data confirm that the Anshan area in the northeastern North China Craton contains the oldest known rocks in China. Rocks with ages of ≥3.8 Ga are present in three separate trondhjemite-dominated gneiss complexes exposed immediately north of Anshan City, referred to as the Baijiafen, Dongshan and Shengoushi complexes. Baijiafen Quarry was the site where 3.8 Ga rocks (207Pb/206Pb age of 3804 Ma) were first discovered in China by Liu et al. in 1992 and is located in a discrete unit that extends for ~700 m in a NW-SE direction, in tectonic contact with 3.3 Ga granitoids. In this study, a thin trondhjemitic layer records a weighted mean 207Pb/206Pb age of 3800 Ma, identical within error to the original sample from this site. Other components mapped in the quarry include biotite schist, of possible supracrustal origin, that contains a zircon population with an age of ~3720 Ma that is cut by a 3620 Ma trondhjemitic dyke. The Lu-Hf data for these samples record a range of TDM(Hf) model ages from 3255 to 4050 Ma. The Dongshan Complex is located in Dongshan Scenic Park on the outskirts of Anshan City and extends in an E-W direction over a strike length of >1000 m. Banded trondhjemitic rocks constitute the bulk of the complex, bounded by 3.1-3.0 Ga trondhjemite and granite. A trondhjemite unit with an age of 3811 Ma was previously recorded from this location by Song et al. in 1996. However, no rocks as old as this were identified in the present study, although a ~3795 Ma quartz diorite boudin and ~3790 Ma trondhjemitic unit are present. The trondhjemitic gneiss that hosts the quartz diorite recorded a 207Pb/206Pb age of ~3680 Ma, considerably younger than the other unit. The TDM(Hf) model ages for quartz diorite zircons range from 3730 to 4020 Ma, with the majority >3.9 Ga The Shengoushi Complex is a recently-identified unit that contains 3.8 Ga rocks, located immediately east of Dongshan Scenic Park. It is ~300 m long and extends in an east-west direction, being composed of trondhjemitic gneiss, granitic gneiss and amphibolite that reveal a range of ages from 3.8 to 3.0 Ga. The oldest unit is a trondhjemitic gneiss with a 207Pb/206Pb age of 3777 Ma. The TDM(Hf) model ages range from 3760 to 4265 Ma, with a mean of 3950 Ma, notably older than zircon samples from the other two localities. This study has established a more complex sequence of events in the area than previously recognized, with the emplacement of several generations of trondhjemite and the possible development of a biotite-rich supracrustal rock prior to 3.63 Ga. All these features attest to a complex series of events in the Eoarchean of the North China Craton.