Rapid maturation and stabilisation of middle Archaean continental crust: the Akia terrane, southern West Greenland
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from the Akia terrane, southern West Greenland, supported by Sm-Nd isotope geochemistry, document its middle Archaean accretional history and provide new evidence about the location of its northern boundary. Zircon populations in grey gneiss and inherited zircons in granite show that magmatic accretion of new continental crust, dominated by intrusion of tonalite sheets in a convergent island arc setting, occurred between c. 3050 and 3000 Ma, around and within a c. 3220 Ma continental core. In the central part of the terrane, tonalite sheets were intercalated with older supracrustal rocks of oceanic affinity by intrusion, thrusting and folding during the Midterhøj and Smalledal deformation phases of Berthelsen (1960). Continued tonalite injection led to a thermal maximum with granulite facies conditions at c. 2980 Ma, dated by metamorphic zircons in grey gneiss. The metamorphic maximum was contemporaneous with upright, angular folds of the Pâkitsoq deformation phase. Within a few million years followed high-grade retrogression and intrusion of two large dome-shaped tonalite-granodiorite complexes, granites s.l. derived from remobilisation of grey gneiss, and post-kinematic diorite plugs. Whereas the relative chronology of these events is firmly established from field observations, zircons from the post-granulite facies intrusions all yielded statistically indistinguishable emplacement ages of c. 2975 Ma. These results show that crustal growth occurred in several short-lived events starting at c. 3220 Ma, and that final maturation and stabilisation of new, thick continental crust took place rapidly (within c. 20 Ma) at c. 2975 Ma.Keywords:
Continental arc
Continental Margin
<p>The Lewisian Gneiss Complex (LGC) in NW Scotland, a classic example of Archean lower crust, is mostly composed of deformed and metamorphosed tonalite&#8211;trondhjemite&#8211;granodiorite (TTG) gneisses, gneissose granite sheets, and subordinate mafic, ultramafic, and metasedimentary lithologies. It has been traditionally subdivided into three regions that are interpreted to record discrete ages and metamorphic histories, and which are separated by crustal-scale shear zones. A smear of concordant U&#8211;Pb zircon ages from the granulite-facies central region has been interpreted to record metamorphic resetting of earlier magmatic and granulite facies metamorphic ages during a subsequent high-temperature metamorphic event. Here, we present U&#8211;Pb and Hf isotope data collected via laser-ablation split-stream (LASS) analyses of zircon cores from twenty-seven felsic meta-igneous rocks from the northern, southern, and central regions of the LGC, as well as U&#8211;Pb data from zircon rims within most of those samples.</p><p>In samples from the northern and southern regions, the crystallization age (i.e., from zircon cores) was calculated from the upper-intercept age, yielding age range of 2.82-2.63 Ga for the northern, and 3.11&#8211;2.63 Ga for the southern region. Zircons in these samples generally have thin or no rims, suggesting an absence of a prolonged high-grade (granulite facies) metamorphic event in those regions. In the central region, zircon cores yield U&#8211;Pb crystallization ages between ca. 3.0 Ga and 2.7 Ga, while zircon rims define a continuous spread of ages from ca. 2.8 to 2.4 Ga. Overall, the central region exhibits a continuous and overlapping smear of zircon core and rim ages, suggesting a protracted thermal event in which high-ultrahigh temperature conditions were maintained for >200 m.y., and that discrete magmatic and metamorphic &#8216;events&#8217; are difficult to identify. Nevertheless, an estimation of the crystallization age of each sample is crucial for interpreting their Lu&#8211;Hf isotopic signature. Zircon cores from the tonalite&#8211;trondhjemite gneisses have broadly chondritic compositions with a range of calculated mean initial &#949;Hf of +2.5 to &#8211;1.2, potentially reflecting a mixture of juvenile material and reworked crust, with one outlier at &#949;Hf<sub>i</sub> = +4.5 perhaps indicating a renewed influx of juvenile magma. Granite gneisses also have near-chondritic values, although the range is larger and the two youngest granite gneisses have slightly sub-chondritic &#949;Hf<sub>i</sub> (&#8211;1.5 and &#8211;2.5), which indicates that pre-existing crust was involved in their formation. Since there is no significant difference in the Hf isotopic composition between rocks from the three regions, or between the TTG and granite gneisses, we suggest that the broadly chondritic &#949;Hf<sub>i</sub> in most of our samples reflects mixing of both depleted mantle and evolved crust during their generation. Despite the similarity of the U-Pb and &#949;Hf data from the three regions, the data do not allow to unambiguously discriminate whether the LGC is composed of different levels of a once continuous Archean continent or discrete microcontinents that were amalgamated in the late Archean to Paleoproterozoic.</p>
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The Hardwood Gneiss is an areally small unit of Precambrian granulite-grade rocks exposed in the Archean gneiss terrane of the southern Lake Superior region. The rocks are located in the southwestern portion of the Upper Peninsula of Michigan and consist of a structurally conformable package of quartzitic, metapelitic, amphibolitic, and metabasic units. Three texturally distinct garnet types are present in the metabasites and are interpreted to represent two metamorphic events. Geothermobarometry indicates conditions of ~8.2-11.6 kbar and ~770°C for Ml, and conditions of ~6.0-10.1 kbar and ~610°-740°C for M2. It is proposed that Ml was Archean and contemporaneous with a high-grade metamorphic event recorded in the Minnesota River Valley. The M2 event was probably Early Proterozoic and pre-Penokean, with metamorphic conditions more intense than those generally ascribed to the Penokean Orogeny in Michigan, but similar to the conditions reported for the Kapuskasing zone of Ontario. The high paleopres-sures and temperatures of the Ml event make the Hardwood Gneiss distinct from any rocks previously described in the southern Lake Superior reigon, and suggest intense tectonic activity during the Archean.
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Rounded zircons included within igneous zircons from the Uivak I gneisses, northern Labrador, have ages of up to 3863 ± 12 Ma (2σ), showing the (former) existence of rocks that predate the Uivak gneisses by more than 100 Ma. The igneous zircons themselves have experienced varying amounts of early Pb loss, but age estimates based on maximum 207 Pb/ 206 Pb from three separate gneiss samples agree to within error at 3732 ± 6 Ma (2σ), which is taken as the age of emplacement of the igneous precursor to the dominant component of the Uivak gneisses. A new generation of zircons was formed during migmatization at ca. 3620 Ma. The volumetrically less important Lister gneiss was emplaced at 3235 ± 8 Ma (2σ). In the late Archaean, different parts of the gneiss complex were subjected to different grades of metamorphic overprinting. The zircon chronology of gneisses affected by granulite facies metamorphism suggests a rapid sequence of events, with simultaneous recrystallization of old zircons and growth of new U-poor zircons at 2766 ± 17 Ma (2σ), followed by partial melting at 2744 ± 4 Ma (2σ), and further recrystallization and growth of U-poor zircons afterwards. No late Archaean zircon growth has been recorded from orthogneisses unaffected by granulite fades metamorphism.
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The Huai9an Complex is situated in the northern segment of the Trans-North China Orogen (TNCO), a continent-continent collisional belt along which the discrete Archean Eastern and Western Blocks amalgamated to form the basement of the North China Craton. The complex consists of six distinct lithologic units: the Huai9an TTG gneisses, the Manjinggou high-pressure mafic granulites, the Khondalite Series, the Dongjiagou granitic gneiss, the Huai9an charnockite, and the Dapinggou K-feldspar granite. SHRIMP U--Pb geochronology, combined with Th and U data and cathodoluminescence (CL) imaging of zircons, enables resolution of magmatic and metamorphic events that can be directed towards understanding the late Archean to Paleoproterozoic history of the TNCO. CL images reveal the coexistence of magmatic and metamorphic zircons in most lithologies of the Huai9an Complex, of which the metamorphic zircons occur as either single grains or overgrowth rims surrounding and truncating oscillatory-zoned magmatic zircon cores. SHRIMP U--Pb analyses on magmatic zircons reveal that the tonalitic, trondhjemitic and granodioritic protoliths of the Huai9an TTG gneisses were emplaced at 2515 ± 20 Ma, 2499 ± 19 Ma and 2440 ± 26 Ma, respectively, much earlier than the emplacement of the Dongjiagou granitic gneiss dated at 2036 ± 16 Ma. However, their metamorphic zircons yield similar concordant ^207^Pb/^206^Pb ages of 1847 ± 17 Ma, 1842 ± 10 Ma and 1847 ± 11 Ma for the tonalitic, trondhjemitic and granodioritic gneisses, respectively, and 1839 ± 46 Ma for the Dongjia granitic gneiss. These ages demonstrate that the Huai9an Complex underwent a regional metamorphic event at ∼1850 Ma, which is further supported by a mean ^207^Pb/^206^Pb age of 1848 ± 19 Ma for metamorphic zircons in the Manjinggou high-pressure mafic granulite and 1849 ± 10 Ma and 1850 ± 17 Ma for igneous zircons in the anatectic Huai9an charnockite and Dapinggou garnet-bearing S-type granite, respectively. The timing of late Archean to Paleoproterozoic magmatism and regional metamorphism in the Huai9an Complex is in general agreement with recent SHRIMP zircon data for other metamorphic complexes in the TNCO. These data prove that the high-grade gneiss complexes were not the basement to the low-grade granite-greenstone terranes in the TNCO. Furthermore, the lithologies of the orogen are considered to have developed as a long-lived magmatic arc that was subsequently tectonically disrupted and juxtaposed during the collision of the Eastern and Western Blocks at ∼1.85 Ga, leading to final assembly of the North China Craton.
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