RAPID CONTINENTAL-CRUST FORMATION AT 1.7 GA FROM A RESERVOIR WITH CHONDRITIC ISOTOPE SIGNATURES, EASTERN LABRADOR

The origin of the 1.71-1.64 Ga tonalitic to granitic continental crust in eastern Labrador was studied by major-, trace-, and rare earth-element analyses, as well as by Rb-Sr and Sm-Nd geochronology and Pb, Nd, and Sr isotope systematics. Apart from a few exceptions, U-Pb ages of the rocks are identical to: (1) their isochron-ages of 1.712 +/- 0.066 Ga (2-sigma-error) for Sm-Nd, and 1.617 +/- 0.021 Ga for Rb-Sr; (2) T(CHUR)Nd of the individual rocks lying between 1.68 and 1.63 Ga, and (3) T(CHUR)Nd of the isochron, yielding 1.693 +/- 0.040 Ga. All T(DM)Nd are too old by 200-350 Ma, relative to T(CHUR)Nd and U-Pb, Sm-Nd and Rb-Sr-ages. epsilon-i-Nd lie between -0.13 and + 0.42 for individual rocks, and at + 0.48 +/- 0.96 for the isochron, overlapping the chondrite zero-value. epsilon-i-Sr of most rocks range between -1.29 and + 3.63 (I(Sr): 0.70259-0.70277), and epsilon-i-Sr of the isochron lies at + 2.72 +/- 2.03 (I(Sr): 0.70277 +/- 0.00022). Pb isotopes, measured in leached K-feldspar from the same samples define a distinct field for initial isotopic compositions (alpha: 15.85-16.14, beta: 15.26-15.32 and gamma: 35.34-35.61) which is in agreement with crust formation from a mantle source with time-integrated mu-values of 9.11-9.36, omega-values of 36.0-36.74, and Th/U of 3.86-4.01. Relative to model-compositions of lower-, bulk-, and upper-continental crust, the Labradorian rocks show distinct overall enrichment in K, Ba, Fe/Mg, and La/Y, but normal patterns for trace elements such as Th, Ta, Zr, Ti, Sc and Co. The data are consistent with rapid juvenile crust formation from a mantle reservoir having chondritic Nd, Sr, and Th/U at 1.7 Ga, whereas Pb isotopes are more radiogenic than single-stage evolution from primordial Pb. It appears that these characteristics are primary signatures of the mantle segment sampled, and not an effect of sediment mixing or melting of older arc material. The isotope data, and the good agreement between U-Pb and Sm-Nd ages show that partial melting of the mantle, and subsequent melting of the newly formed protolith, to produce granitoid rocks, occurred in less than 70 Ma. Field observations suggest the new crust to be formed in relation to subduction of oceanic lithosphere underneath the Archaean shield. However, coeval emplacement of rift-related monzonite-anorthosite intrusives, generated from a different mantle reservoir, strengthens the view that continental growth includes complex and highly episodic processes leading to heterogeneous temporal and spatial depletion of the mantle.