3.5 Ga old amphibolites from eastern Hebei Province, China: Field occurrence, petrography, Sm-Nd isochron age and REE geochemistry
Journal
Precambrian Research
Journal Volume
34
Journal Issue
3-4
Pages
311-346
Date Issued
1987
Author(s)
Abstract
Amphibolite blocks frequently occur in association with grey gneisses within a small area near the villages of Caozhuang and Huangbaiyu in eastern Hebei. They are interpreted as remnants of an earlier bimodal suite now engulfed in late Archaean K-rich granitic magma and are further cut by pegmatite dykes. They are considered together as giant enclaves within late Archaean granite plutons (U-Pb zircon ages = 2.5-2.6 Ga). The amphibolites have been dated by the Sm-Nd isochron method. The results yielded: T = 3470 ± 107 Ma, INd = 0.50827 ± 11, and ε{lunate}Nd (T) = +2.7 ± 0.6. This age represents the time of protolithic basalt eruption and possibly suggests a period of formation of primitive mafic crust. This is the oldest reliable age so far obtained for the Archaean rocks of China. The Sm-Nd results of grey gneiss samples yielded model ages (TDM) of about 3.24 Ga. It is not clear whether this 'younger' age in comparison with that of the amphibolites indicates the intrusive nature of these rocks or is a result of contamination by later granite about 2.5 Ga ago. The Rb-Sr and common Pb isotopic systems have been highly disturbed, and no isochron ages could be established. The common Pb data show retarded growth of radiogenic Pb and hence imply reduction of U-Pb ratios in the past. This is consistent with the petrographic study, which suggests that the amphibolites have undergone an earlier period of granulite facies metamorphism and were subsequently retrogressed to the present amphibolite facies mineral assemblages. The positive ε{lunate}Nd (T) value of +2.7 for the amphibolites implies that the protolithic basalt magmas were generated from mantle sources which had undergone long-term depletion in light rare earth elements (LREE). The depletion event must have taken place prior to 4 Ga ago if their source had a chemical nature similar to the modern oceanic mantle; it could have occurred as early as 4.5 Ga ago if their source was less impoverished in LREE. If the mantle depletion was related to extraction of continental or sialic material, the Sm-Nd data of the Caozhuang amphibolites lend support to the existence of sialic crust, if not of a continental size, before 4 Ga ago. © 1987.
Type
journal article