| Popis: |
The Archaean gneiss complex of the Saglek-Hebron area in northern Labrador is dominated by polyphase similar to 3.7 Ga old tonalitic-trondhjemitic-granodioritic (TTG) Uivak I Gneiss, that exhibit 3.8-3.9 Ga inheritance. These gneisses are interleaved with a heterogeneous group of supracrustal rocks known as the Nulliak supracrustal assemblage. Early Archaean ultramafic rocks occur either as tectonically-emplaced slivers of lithospheric mantle along structural contacts within the Uivak I Gneiss, or as layers of metakomatiite in Nulliak supracrustal units. The metakomatiites yield a Pb-Pb isochron age of 3845 +/- 160 Ma, reflecting a significant change from an initial Archaean mantle-like mu(1)-value of 7.9 to a range of mu(2)-values of 2.0-4.2. Members of the tectonically-emplaced lithospheric mantle suite have less well correlated Pb-206/Pb-204 and Pb-207/Pb-204 compositions. Both ultramafic suites were collected in an area identified as transitional between granulite and amphibolite facies. However, model calculations show that the data scatter exhibited by the lithospheric mantle cannot be accommodated by post-3.85 Ga U-depletion. Evidence is provided that the lithospheric mantle suite reflects variable degrees of mixing between a depleted mantle and a high-mu reservoir suggesting that the two ultramafic rock suites had different evolutionary histories. Timing of late Archaean reworking during medium- to high-grade metamorphism was determined by mineral whole-rock dating of a garnet pyroxenite from an area dominated by Kiyuktok Gneiss (reworked similar to 3.75 Ga Uivak Gneiss). Sm-Nd mineral whole-rock data for this gneiss yielded an isochron equivalent to an age of 2570 +/- 17 Ma, significantly younger than its Pb-Pb isochron age of 2735 +/- 43 Ma. The negative epsilon(Nd)(t) = - 7.9 reflects a significant crustal residence time, indicating that the Sm-Nd system of the whole rock has been reset. The internal Pb mineral isochron is interpreted to reflect the maximum age of peak metamorphism. The age discrepancy of ca. 165 Ma between the Sm-Nd and the Pb-Pb isochron ages suggests slow cooling at a minimum rate of 1.3 degrees C Ma(-1). This indicates that the Saglek-Hebron segment of the North Atlantic Craton experienced gradual uplift during late Archaean exhumation. (C) 1999 Elsevier Science B.V. All rights reserved. |
Full Text from ScienceDirect