| The Dunhuang Block(DHB),located between the Tarim and North China cratons,was considered a microcontinental block with a Precambrian metamorphic crystalline basement in the southern Central Asian Orogenic Belt(CAOB).Recent studies suggest that the DHB underwent multi-stages of tectonic evolution processes,and finally formed voluminous complicated magmatic-metamorphic complexes in the Paleozoic.The formation of these rocks has been interpreted as being associated with the subduction and closure of the southern Paleo-Asian Ocean(PAO).Also,the genesis and geodynamic mechanism of granitoids are of great geological significance for understanding the continental crustal evolution of the DHB and tectonic evolution of the southern margin of the CAOB.Based on previous research,according to field geological works,this thesis mainly chose the Paleozoic magmatic rocks in the northern DHB as the research object.We conduct a detailed study on the petrography,petrology,zircon U-Pb geochronology,whole-rock major and trace elements geochemistry,minerals chemistry,and Sr-Nd-Pb-Hf isotopic compositions for the Paleozoic magmatic rocks in the DHB.Our new results will(1)reveal the spatial-temporal distribution characteristics of Paleozoic magmatism,(2)trace the source region,petrogenesis,tectonic implications of Paleozoic typical magmatic rock,and(3)further discuss the crustal evolution process of the DHB and the southern margin of the CAOB.The progress and preliminary conclusions have been obtained as follows:(1)The Cambrian granites have been identified in the Lianghu-Xiaowan-Dapokouzi area in the northern DHB.Zircon U-Pb dating reveals that these plutons formed at ca.510±2 Ma,representing the oldest pluton reported of the DHB in the Paleozoic.The geochemical characteristics show that they belong to the metaluminous and medium-K calc-alkaline series,and display positive?Hf(t)(+11.0–+14.7)and?Nd(t)(+2.3–+5.6)values,with Hf and Nd model ages of 754–520 Ma and 970–740 Ma,respectively,as well as low initial Sr-Nd isotopic compositions and high radiogenic Pb isotopic features,suggesting that the magmas originated from the remelting of juvenile lower crust with addition of minor mantle-derived materials.The Dapokouzi granites display a geochemical affinity of adakitic rock such as high Sr(730–733 ppm),low Y(1.84–1.93 ppm),and Yb(?0.21 ppm)contents,yielding high Sr/Y(380–398)ratios.These characteristics suggest that the Dapokouzi adakitic granites belong to the product of melting of the thickened juvenile lower crust(at least>40 km crustal depth).In combination with regional geological data,we conclude that the Cambrian plutons in the northeastern DHB formed in a continental arc setting,which is the product of magmatic events related to the early subduction of the southern PAO.More importantly,the subduction of the southern PAO probably began in the early Cambrian.(2)The Ordovician and Silurian magmatism events in the DHB are mainly distributed in the northern Guazhou area,which represented by the Lianghu quartz diorites and Shigong monzogranites,and zircon U-Pb dating reveals that they were emplaced at ca.455±3 Ma and431±3 Ma,respectively.The geochemical characteristics show that these plutons belong to metaluminous and calc-alkaline series I-type granitoids,which exhibit high Sr and low Y concentrations and high Sr/Y ratios,signifying geochemical affinities of adakitic rocks.The quartz diorites have negative?Nd(t)(-1.3–-3.2)and positive?Hf(t)(+3.8–+8.0)values,with Nd and Hf model ages of 1120–1090 Ma and 1165–906 Ma,respectively,as well as high Mg O-Cr-Ni contents and Ba/La and La/Sm ratios,suggesting that magma were derived from the partial melting of the subducted oceanic slab(sediments+fluids)and subsequent interaction with peridotite in the mantle wedge,with limited assimilation of crustal materials during magma ascent,perhaps formed a subduction-relation setting.The monzogranites display negative?Nd(t)values of-3.6,positive to negative?Hf(t)values of-2.5 to+3.0,with Nd and Hf model ages of 1320 Ma and 1197–547 Ma,respectively,as well as low Mg O-Cr-Ni contents and low Mg#values,indicating that magma originated from the melting of heterogeneous crustal sources,or through the mixing of a thickened juvenile crust with Mesoproterozoic crust materials,with the addition of minor mantle-derived materials.The monzogranites likely belong to the product of the syn-collisional tectonic setting.Based on the preliminary estimation of crustal thickness,we propose that the thickening of the continental crust of the DHB began roughly in the late Ordovician(?35 km)and probably culminated in the early Silurian(50?55 km).Thickening processes resulted from the underplating of mantle-derived magmas during the subduction of the southern Paleo-Asian Ocean,and the subsequent likely collided with the Shibanshan terrane(a Paleozoic continental arc)in southern Beishan orogen at ca.440–430 Ma.(3)The Devonian granitoids are mainly distributed in the Sanweishan-Dongshuigou-Mogutai areas.The Dongshuigou pluton is composite,primarily consisting of quartz diorite,granodiorite,and biotite granite,which was emplaced in the middle Devonian(ca.390–380 Ma).The dacite porphyry intruded into the granodiorite in the late Devonian(ca.367±4 Ma).The quartz diorite,granodiorite,and dacite porphyry show typically arc-like geochemical signatures such as relatively high Sr contents and low Sr/Y and(La/Yb)N values,which belong to middle-high K,calc-alkaline,and metaluminous I-type granitoids.The quartz diorites and granodiorites have nearly positive?Nd(t)(-0.73 to+0.38)and?Hf(t)(+3.12 to+10.7),with Nd and Hf model ages of 1.12–1.04 Ga and 1.15–0.83 Ga,respectively,together with low initial Pb isotopes similar to those of typical lower crust-derived rocks,suggesting that these arc-related magmatic rocks were derived from partial melting of the juvenile lower crust induced by the underplating of mantle-derived magmas,with the addition of minor mantle materials.The biotite granites exhibit high Sr and low Y contents and high Sr/Y ratios,showing geochemical affinities of adakitic rocks.Also,they have high Mg#values and Cr-Ni contents,positive?Nd(t)values(+3.0),negative to positive?Hf(t)values(-3.67 to+12.2,mostly>0),with model ages of 1.34–0.57 Ga and 0.82Ga,respectively,and low initial Pb isotopic compositions,suggesting that the biotite granites were likely derived from partial melting of subducted oceanic crust,which was modified by interactions with mantle wedge,with the addition of minor Mesoproterozoic materials during magma ascent.(4)This study has identified ca.450–440 Ma gneissic tonalites(Second and Third formation)and 368±3 Ma acid volcanic rocks(Four formation)from the Dunhuang Group in the Sanweishan-Huangshuigou areas of DHB.The gneissic tonalites exhibit the geochemical affinities of island arc magma such as relative enrichment in large ion lithophile elements(e.g.,Rb,Ba,U,and Pb),and depletion in high field strength elements(e.g.,Nb,Ta,and Ti),and display a wide range of?Hf(t)values(-9.7 to+10.4),suggesting that magmas originated from the partial melting of juvenile crust and old crustal materials.The Laojuntang acid volcanic rocks in the Sanweishan area show prominently negative?Hf(t)values of-5.6 to-1.9 with the Paleoproterozoic model ages(1640 Ma average age),indicating that they were derived from the melting of ancient lower crustal materials.Combined with previous studies,we propose that the Dunhuang Group may have formed at the least in the late Devonian(?368 Ma)rather than at ca.1.95–1.83 Ga as previously thought.(5)Combined with previously published data and comparison with adjacent tectonic units,this study suggests that the DHB should belong to a microcontinent in the southern CAOB,with Archean-Paleoproterozoic metamorphic crystalline basement.After the formation of the early Cambrian sedimentary cover,the DHB was strongly modified and reworked by the subduction-accretionary orogeny process of the southern PAO in the Paleozoic and Mesozoic,resulting in the activation of the crust and formed widespread Cambrian-Permian magmatic rocks and Late Ordovician-Devonian metamorphic rocks in the whole DHB.Additionally,the Paleozoic magmatism of DHB could be roughly divided into six periods:Middle Cambrian(?510 Ma),Late Ordovician-Early Silurian(?440 Ma),Early Devonian(?410 Ma),Late Devonian(390?360 Ma),Middle Carboniferous(?335 Ma),Middle-Late Permian(?250–280 Ma).Most of the magmatic rocks exposed in the Paleozoic belong to sodium-rich,calc-alkaline,metaluminous to weakly peraluminous I-type granitoids.These tectono-thermal events indicate that the DHB experienced the early Paleozoic subduction-collision orogenic processes and the late Paleozoic subduction-collision-extension tectonic evolution process.Two crustal thickening events occurred in Silurian and Carboniferous,and crustal thickness is up to 50?55 km.In addition,the Xiaowan and Sanweishan areas of the Dunhuang block likely belong to magmatic arcs of the Cambrian and Devonian,respectively.The geochemical and Sr-Nd-Pb-Hf isotopic characteristics indicate that arc magmatism events may have played an essential role in the Paleozoic crustal growth in the northern DHB.In contrast,the late Silurian-Carboniferous magmatism events in the southern DHB were mainly the reworking process of ancient crust. |
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