Geochemical and Nd isotopic compositions of 3.8 Ga meta-quartz dioritic and trondhjemitic rocks from the Anshan area and their geological significance

The 3.8 Ga meta-quartz dioritic rocks discovered recently in the Anshan area show relatively high ∑REE contents and low (La/Yb) _n ratios, with t_DM of 3.50-4.16 Ga and ɛ_Nd (3.8 Ga) of -1.24 to 4.51. Slightly older 3.8 Ga trondhjemitic rocks have been found in two locations in the Anshan area. One is the Baijiafen gneiss, which crops out near Baijiafen village and is characterized by relatively high SiO ₂ and Na ₂O, low MgO, ∑FeO and CaO, rather low ∑REE contents and low (La/Yb) _n ratios. The other is the Dongshan banded gneiss located in the Dongshan Scenic Park in Anshan City, which has a similar chemical composition to the Baijiafen gneiss, except for slightly higher MgO, ∑FeO, CaO and ∑REE concentrations. All the 3.8 Ga trondhjemitic rocks exhibit large variations in Nd isotopic compositions, with t_DM and ɛ_Nd (3.8 Ga) of 3.07-3.87 Ga and 0.82-12.23 for the Baijiafen gneiss, and 2.82 to 3.40 Ga and 7.37-12.44 for the Dongshan banded gneiss, respectively. It is suggested that the 3.8 Ga trondhjemitic rocks from the Anshan area were formed by partial melting of intermediate to acidic continental material, and not basaltic material. TTG rocks of early Archean age are generally low in ∑REE contents and (La/Yb) _n ratios compared with those of late Archean age. The regular variations of REE patterns with time may reflect thermal evolution of the primitive Earth. In the early stages, the Earth had higher geothermal gradients (>30 °C/km). After about 3.5-3.6 Ga, its geothermal gradient dropped into the garnet stability field.