The Precambrian And Late Paleozoic Continental Crust Evolution In The South Margin Of The Dunhuang Block

Dunhuang Block is located in the eastern Tarim Craton.As one of the most important parts of the Tarim Craton,the Precambrian basement rocks of this bolck have been studied for many years.The Aketashitage and its adjacent area in the south margin of the Dunhuang Block have well preserved series of Archean and Paleoprotozoic basement rocks,but Neoprotozoic and Neopaleozoic magmatic rocks rarely occur.In addition,the Paleoprotozoic evolution of continental crust in this area is still poorly constrained.In this paper,new whole-rock geochemistry,zircon U-Pb ages and Hf isotopic data has been achieved for not only the Precambrian amphibolites,dioritic porphyrites,and diabase dikes,but also the Late Paleozoic adakitic rocks,which provide new constraints on the Precambrian and Late Paleozoic continental crust evolution at the south margin of Dunhuang Block.Our Zircon U–Pb dating of the Paleoproterozoic basement rcoks in the Aketashitage area yielded a metamorphic age at 1862±10 Ma for the amphibolites and a crystallization at 1810±11 Ma for the dioritic porphyrites,repectively.The amphibolites have SiO2 ranging from 44.71 wt.% to 48.86 wt.%,with higher Na2O?2.87-4.15 wt.%?relative to K₂O?1.23-2.38 wt.%?,and show paraluminous affinities.They show relatively low REE contents?59.59-75.55 ppm?,with LREE enriched and HREE depleted patterns,similar to those of OIB.The amphibolites are highly depleted in HFSE,with weakly positive Eu anomalies?Eu/Eu*=1.08-1.13?,showing geochemical features of magmas formed in a subduction zone.Their TiO2 and Nb contents and Zr/Y ratios indicate that the protolith of the amphibolites were produced in a island arc setting.The dioritic porphyrites haveuniform SiO2 contents?58.53-60.02 wt.%?and rather high K₂ O contents?3.45-4.42 wt.%?,with low Na2O/K₂ O mostly lower than 1.They have low Al2O3 and MgO contents,and low Mg# values?12.38-15.00?,belonging to paraluminous?ASI=0.77-0.80?.They are LREE and LILE enriched and HFSE?Nb?Ta?Ti?depleted,with weakly negative Eu anomalies?Eu/Eu*=0.95-0.99?.Their mostly negative ?Hf?t?values and old model ages?2464-2920 Ma?both indicate that the dioritic porphyrites were likely produced by partial melting of the Archean lower basic crust.The LA-ICP-MS zircon U-Pb dating of the Precambrain diabase dikes yielded a crystallization age at 884.6±4.2 Ma.They have low K₂O?0.36-0.70 wt.%?and Na2O?1.9-2.31 wt.%?contents,with SiO2 varing from 46.89 wt.% to 48.49 wt.%.These rocks are LREE enriched and show flat HREE patterns,with?La/Yb?N at 2.5-2.8 similar to E-MORB.Moreover,the diabase dikes have weakly negative Eu anomalies?Eu/Eu*? 1.0-1.1?,which might be related to the cumulation of plagioclase.They are also enriched in LILEand depleted in HFSE,with profound negative Ta and Nb anomalies.The low Zr/Nb ratios for the diabase dikes indicate that they are originated from the transitional or enriched mantle.Zircon U–Pb dating of the three plutons,including gneissic granites,mylonized granites and augen gneisses,yielded crystallization ages at 358.2 ± 4.0 Ma,382.1 ± 4.4 Ma and 332 ± 2.4 Ma,respectively.They show geochemical features of high-K calc-alkaline igneous rocks and are all LREE-enriched and show relatively flat HREE patterns,with pronounced depletions in Nb,Ba,Ta and Ti and negative Eu anomalies to different degree.Moreover,these rocks are also characterized by high Sr?330.7–656 ppm?,low Yb?0.51–1.52 ppm?and Y?5.20–13.73 ppm?,and thus high Sr/Y ratios?32.21–122.14?,showing geochemical affinities of adakitic rocks.Their low MgO contents and negative ?Hf?t?values indicate an origin of partial melting of thickened continental lower crust.Based on the variations of the negative Eu anomalies and?La/Yb?N ratios of these granitic rocks,we infer that the continental crust of the southern Dunhuang Block was progressively thickened from Late Devonian to Early Carboniferous,which is most likely triggered by continental collision between the Dunhuang Block and the Central Altyn massif.