Meta-igneous (non-gneissic) tonalites and quartz-diorites from an extensive ca. 3800 Ma terrain south of the Isua supracrustal belt, southern West Greenland: constraints on early crust formation |
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| Authors: | Allen P Nutman Vickie C Bennett Clark R L Friend Marc D Norman |
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| Institution: | (1) Research School of Earth Sciences, Australian National University Canberra, ACT 0200, Australia, AU;(2) Department of Earth and Planetary System Sciences, Hiroshima University, 3-2 Kagamiyama 1-Chome, Higashi-Hiroshima, Japan 739, JP;(3) Present Address: “Melbury”, Hensleigh Drive, St. Leonards, Exeter, EX2 4NZ, UK e-mail: allen.shiher@btinternet.com, GB;(4) Department of Geology, Oxford Brookes University, Headington, Oxford, OX3 0BP, UK, GB;(5) GEMOC, School of Earth Sciences, Macquarie University, North Ryde NSW 2109 Australia, AU |
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| Abstract: | In the Itsaq Gneiss Complex south of the Isua supracrustal belt (West Greenland) some areas of early Archaean tonalite and
quartz-diorite are non-gneissic, free of pegmatite veins, and in rarer cases are undeformed with relict igneous textures and
hence were little modified by heterogeneous ductile deformation under amphibolite facies conditions in several Archaean events.
Such well-preserved early Archaean rocks are extremely rare. Tonalites are high Al, and have bulk compositions close to experimental
liquids. Trace element abundances and modelling suggest that they probably originated as melts derived from basaltic compositions
at sufficiently high pressures to require residual garnet + amphibolites ± clinopyroxene in the source. The major element
characteristics of the quartz-diorites suggest these were derived from more mafic magmas than the tonalites, and underwent
either igneous differentiation or mixing with crustal material. As in modern arc magmas, high relative abundances of Sr, Ba,
Pb, and alkali elements cannot be generated simply from a basaltic source formed by large degrees of melting of a depleted
mantle. This may indicate an important role for fluids interacting with mafic rocks in generating the earliest preserved continental
crust. The high Ba/Th, Ba/Nb, La/Nb and low Nb/Th, Ce/Pb, and Rb/Cs ratios of these tonalites are also observed in modern
arc magmas. SHRIMP U-Pb zircon geochronology was undertaken on seven tonalites, one quartz-diorite, a thin pegmatitic vein
and a thin diorite dyke. Cathodoluminescence images show the zircon populations of the quartz-diorite and tonalites are dominated
by single-component oscillatory-zoned prismatic grains, which gave ages of 3806 ± 5 to 3818 ± 8 Ma (2σ) (quartz-diorite and
5 tonalites) and 3795 ± 3 Ma (1 tonalite). Dating of recrystallised domains cutting oscillatory-zoned zircon indicates disturbance
as early as 3800–3780 Ma. There are rare ca. 3600 Ma and 3800–3780 Ma (very high U and low Th/U) ≤ 20 μm wide partial overgrowths
on the prismatic grains. Given likely Zr-undersaturation of precursor melts and evidence of zircon recrystallisation and metamorphic
regrowth as early as 3800–3780 Ma, the age determinations on the prismatic oscillatory-zoned zircon populations give the igneous
crystallisation age of the tonalite and quartz-diorite protoliths. When the coherency of the geochemistry is considered, these
samples represent the best preserved suites of ca. 3800 Ma felsic igneous rocks yet documented.
Received: 1 December 1998 / Accepted: 23 July 1999 |
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