The latest Neoarchean–Paleoproterozoic evolution of the Dunhuang block, eastern Tarim craton, northwestern China: Evidence from zircon U–Pb dating and Hf isotopic analyses
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Similarities between Proterozoic (~ 1.8-2.5 Gyr) and Archean (> 2.5 Gyr) banded iron-formations are probably more significant than their differences. The contrasts largely reflect differences in the tectonic settings of Proterozoic and Archean terrains. Archean banded iron-formations are not as thick nor laterally as extensive as the major Proterozoic iron-formations. Nevertheless, some Archean iron-formations have strike lengths of over 150-200 km and may have been quite extensive prior to the deformation that has affected most Archean terrains. Stratigraphic sequences in which iron-formations occur are highly variable and indicate that iron-formations formed in many depositional environments. Sedimentary textures in the iron-formations are dominated either by granules and oolites or laminations (including microbanding) reflecting differences in their physical conditions of deposition. Granular and oolitic textures are abundant in only three Proterozoic depositional basins and most Precambrian iron-formations are laminated. Despite differences in associated lithologies and sedimentary textures Precambrian iron-formations have similar bulk compositions and mineral assemblages, implying that the chemical conditions of iron-formation deposition were similar through much of the Precambrian. The formation of banded iron-formation appears not to have reached a maximum around 1.8-2.0 Gyr but to have been an important process over a long period in the Precambrian.
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Abstract High‐pressure granulites are generally characterized by the absence of orthopyroxene. However, orthopyroxene is reported in a few high‐pressure, felsic–metapelitic granulites, such as the Huangtuling felsic high‐pressure granulite in the North Dabie metamorphic core complex in east‐central China, which rarely preserves the high‐pressure granulite facies assemblage of garnet + orthopyroxene + biotite + plagioclase + K‐feldspar + quartz. To investigate the effects of bulk‐rock composition on the stability of orthopyroxene‐bearing, high‐pressure granulite facies assemblages in the NCKFMASHTO (Na 2 O–CaO–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 –Fe 2 O 3 ) system, a series of P – T – X pseudosections based on the melt‐reintegrated composition of the Huangtuling felsic high‐pressure granulite were constructed. Calculations demonstrate that the orthopyroxene‐bearing, high‐pressure granulite facies assemblages are restricted to low X Al [Al 2 O 3 /(Na 2 O + CaO + K 2 O + FeO + MgO + Al 2 O 3 ) < 0.35, mole proportion] or high X Mg [MgO/(MgO + FeO) > 0.85] felsic–metapelitic rock types. This study also reveals that the X Al values in the residual felsic–metapelitic, high‐pressure granulites could be significantly reduced by a high proportion of melt loss. We suggest that orthopyroxene‐bearing, high‐pressure granulites occur in residual overthickened crustal basement under continental subduction–collision zones and arc–continent collision belts.
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