Archean Podiform Chromitites and Mantle Tectonites in Ophiolitic Mélange, North China Craton: A Record of Early Oceanic Mantle Processes
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Abstract:
We report 2.5 billion-year-old oceanic mantle podiform chromitite and mantle tectonite in ophiolitic mélange in the North China craton.Tectonic blocks of peridotite, wehrlite, pyroxenite, harzburgitic tectonite, dunite, podiform chromitite, layered gabbro, sheeted dikes, and pillow lava are embedded in a strongly deformed metasedimentary and metavolcanic matrix.The blocks are traceable belt, and the 2.5-2.4Ga Qinglong foreland basin and fold-thrust belt on the Eastern block, and provides an important record of the operation of plate tectonics in the Archean.Keywords:
Tectonite
The Wiedemann Fjord mantle xenoliths from Tertiary basanitic dykes in east Greenland contain extremely depleted spinel harzburgites. The harzburgites are unusual because of their highly forsteritic olivine and low modal orthopyroxene, consistent with an origin as residues of up to 40% melting. Also present are lherzolites with olivine contents as low as 50% and with up to 25% clinopyroxene, and there is a continuous range of compositions between the most depleted harzburgites and the lherzolites. Osmium isotopic data show that the depleted harzburgites were formed by melt depletion in the Archaean. In contrast to most other Archaean xenoliths (e.g., from the Kaapvaal and Siberian cratons) the composition of the Wiedemann Fjord harzburgites is consistent with depletion by polybaric melting. The lherzolites have less refractory mineral compositions than the harzburgites (lower bulk Mg # and Cr #, higher aluminum contents in orthopyroxene (opx)) and also have more radiogenic Os isotopic compositions. The data suggest that the Wiedemann Fjord xenoliths formed as restites from high degrees of melting in the Archaean and that some of the restites were partly refertilized by metasomatism involving addition of pyroxene and reequilibration of mineral and isotopic compositions.
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Radiogenic nuclide
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The North China craton and the Yangtze craton (South China) both contain Archean rocks in eastern China. Unlike the North China craton, where Archean rocks are widespread, in the Yangtze craton the exposed Archean rocks are only known in the Kongling terrain (360 km2). Zircon U-Pb ages and Lu-Hf isotopic compositions of three granodioritic-trondhjemitic gneisses and three metasedimentary rocks from the Kongling terrain were analyzed by LA-ICP-MS and LA-MC-ICP-MS. Igneous zircons in one trondhjemitic gneiss in the north of the Kongling terrain have an age of 3302±7 (1σ) Ma. Evidence from cathodoluminescence imaging, variations in Th/U and degree of U-Pb age discordance suggest that apparently younger zircons in the same population are variably disturbed 3302 Ma grains. Thus, this trondhjemitic gneiss is the oldest known rock in South China and predates the earlier reported ∼2900 Ma granitoid magmatism by 400 Ma. Zircon cores from one granodioritic gneiss in the north of the Kongling terrain also give a concordant age group at 3200 to 3300 Ma. Regardless as inherited or not, these cores crystallized from a magma indistinguishable in age with the trondhjemite. Concordant U-Pb ages for igneous zircons in one granodioritic gneiss in the south of the Kongling terrain yielded a weighted average 206Pb/207Pb age of 2981±13 Ma (2σ, MSWD=9.7, n=21). The zircon age and initial Hf isotopic compositions are similar to those of widespread granitoid gneisses from the north of the Kongling terrain (2903-2947 Ma), and indicate that the south and north of the Kongling terrain are correlative. The results also reinforce that magmatism of the whole Kongling terrain mainly occurred at 2900 Ma. Available Hf isotopic data from the Kongling terrain show that juvenile crustal additions occurred mainly between 3150 and 3800 Ma with a significant peak at 3300 to 3500 Ma. The ∼3300 Ma zircons from the trondhjemitic gneiss have Hf crust formation ages of 3450 to 3730 Ma, some of which have nearly chondritic εHf (t). The whole-rock depleted mantle Nd model age of this rock is 3400 Ma, close to its age of magmatism and consistent with the Hf model age. Its εNd value at 3300 Ma is nearly chondritic (1.26). These lines of evidence suggest that the 3300 Ma trondhjemite represent juvenile crust additions to the pre-existing continental crust.
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Dharwar Craton
Charnockite
Indian Shield
Greenstone belt
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Metasomatism
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The Jurassic Attawapiskat kimberlites allow the study of the association of a world-class primary diamond deposit (Victor Mine) with a post-Archaean rift system, the Midcontinent Rift, which affected the southern Superior Craton at ∼1·1 Ga. Peridotite xenoliths and xenocrysts from the Attawapiskat kimberlites have been analysed to understand the processes of craton formation and modification in the Superior lithospheric mantle. Chondrite-normalized platinum group element (PGEN) signatures in olivine are complex and highly variable, but correlate with Os isotopic compositions. The existence of a depleted mantle reservoir beneath the Attawapiskat area since the Palaeoarchaean is indicated by ∼3·6 Ga TRD ages preserved in peridotitic olivine. An Mg# up to 93·6 in these olivines requires that protolith formation involved high degrees of partial melting, leading to harzburgitic and dunitic residues. Cr-rich garnets with positive slopes in depleted chondrite-normalized heavy rare earth elements (HREEN) are consistent with fractional polybaric melt extraction continuing from the garnet into the spinel stability field. TRD ages of ∼2·7 Ga in olivines with residual PGEN patterns probably reflect residual PGE alloy or refractory PGE sulphide inclusions and indicate that additional melting occurred in the mantle at the time of subduction–accretion, with hydrous melts infiltrating the overlying mantle wedge leading to iridium-group PGE (I-PGE) alloy formation. Metasomatic melts related to the Midcontinent Rift (1·1 Ga) interacted with variably depleted peridotite, leading to platinum-group PGE (P-PGE) enrichment and Mesoproterozoic TRD ages. Older depleted domains are, however, preserved (e.g. sinusoidal REEN patterns in lherzolitic garnet). After the thermal impact of the rift subsided, diamond-stable conditions were extended to shallower depths in the lithosphere via cooling, and diamonds sampled by post-rift kimberlites, such as Victor (∼180 Ma), must have formed after the Midcontinent Rift. These diamonds are likely to be of mixed parageneses: high-pressure compositions indicative of diamond stability are observed in both lherzolitic and high-Mg eclogitic to pyroxenitic garnets at Victor.
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Primitive mantle
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Chromitite
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