Researching On Paleozoic Plate Tectonics Of Karamaili Area, East Junggar, Xinjiang

Karamaili area, located in northeastern margins ofJunggar basin, Xinjiang, is part of Paleozoic oroge-nic belt of East Junggar.This thesis investigates Paleozoic plate tectonicsof the area in basis of stratigraphy, petrology, andgeological structure. According to a lot of new geo-logical data, Paleozoic tectonic evolution of thearea is divided into two big stages. During the earlyPaleozoic big stage the area was part of the margi-nal sea-basin of the paleo-Siberian plate, and inthe late Paleozoic, paleoplate units may be dividedinto as follows from north to south: (1) wildhorsespring volcanomagmatic arcs, (2) Karamaili limitedoceanic basin (suture), and (3) paleo-Junggar block.Early Paleozoic foldbelt, late Paleozoic ophioliticmelanges, volcanomagmatic arcs, sediment formationsdeposited in the intercontinental remnant sea-basin,post-collisional magmatic activities, and structuraldeformation are stated in the thesis. At the same time,the five stage model of late Paleozoic tectonic evolu-lion of the area has been summed.1. Early Paleozoic FoldbeltEarly Paleozoic foldbelt is sparsely exposed on thearea as faulted-blocks and consists of Sorbastaugroup and Kubusu group which are in NWW-extension. Early Paleozoic calcalkaline granodiorites intrudedthe Sorbastau, which is composionally similar to Cale-donianâ… -type granite (Pitcher, 1983). They are over-lain by sediments of the late Silurian-the early ofthe early Devonian unconformablly.The Sorbastau group is composed of two parts, about1400m thick, and metamorphosed into greenschists. Thelower consists mainly of grey-green flysches and theupper, dark-green metabasalts interbedded with iron-siliceous pods and terrigenous clastics, slmilar tooceanic crust. The Kubusu is constituted by grey finegrain classics interbedded with arkoses, siliceouspods, andesites, and andesitic turfs which might besimilar-to arc-type volcanosedimental formations.The group is about 3800m thick. Middle Silurian micro-fossils such as Acritarchs and Chitinozoa have beenfirst found in the group.Sum up, the foldbelt might be formed in the end ofthe early Paleozoic due to collision of paleo-Junggarblock with paleo-Siberina plate. Afterwards, the areahad turnt the marginal seabasin into the continentand became part of the paleo-Siberian.2. Ophiolitic MelangesThe Karamaili ophiolitic melanges are exposed on areasnorth to the Karamaili fault, in NWW extension, empla-ced structurally into the Middle Devonian forearc sedi-ments, and overlain by Visean-Namurian intercontinentalremnant sea-basin sediments.Ophiolitic melanges are mainly composed of fragmentsof metamorphosed peridotites, gabbroic cumulates, pla- giogranites, diabase dikes, metabasalts, and radiolariasiliceous rocks.Metaperidotites include serpentinized harzburgites,dunites, and plagiolherzolites, which contain chromitepodes. Their chemical composition is poor in SiO₂, Al₂O₃,Na₂O+K₂O, rich in MgO and FeO^*. Oontents of their NiOand Cr₂O₃ are similar to Alps peridotites. Harzburgitesand dunites have lower REE content than plagiolherzolitesand U-shaped chondrite normalized REE paterns. Plagio-lherzolites have REE content similar to that of chon-drite and thus possiblly primary pyrolites.Cumulates include troctolite, olive-gabbro, and bandedgabbro. They may be subdivided into two types. One isexposed in Hongliugou and Nanmingshui and characterizedby coarser grain, light colour and deep alteration,compositionally, LREE-depletion and strong Eu-positiveanomaly. The other is located in Qingshui and chara-cterized by finer grain, dark colour, weak alteration,LREE moderate enrichment and weak Eu-positive. TiO₂content of cumulates is lower and reveals that theophiolites are similar to the low-Ti type of ophiolites.Plagiogranites have granophyric and graphic texturesand consist mainly of oligoclase and quartz. They arecompositionally characterized by poverty in K₂O,richness in SiO₂, similar to M-type granite. TheirREE contents are similar-o MORB but moderate rich inLREE.Diabase dikes intruded either into harzburgites, pla-giogranites and metabesalts, or are associated withfragments of cumulates and metaperidotites as blocksof the sheet swarm. Their petrochemical composition is similar to N-type MORB.Basalts in the melanges have three types, i. e. (1) thosesimilar to N-type MORB, (2) similar to T-type MORB, and(3) similar to ocean-island alkaline lavas.Siliceous rocks associated closely with basalts havethree genetic types, that is, (1) thermoexhalation,(2)biochemical sedimentation, and (3) volcanosedimentation.They contain terrigenous deposts more or less, thisimplies that the Devonian Karamaili ocean-basin is asmall limited basin. Early Devonian microfossils suchas Radiolaria etc. have been found.According to regional geology, 388-392 Ma (K/Ar) ofQingshui cumulates, and Early Devonian microfossils,the author suggests that the ophiolitic melanges arefragments of the Karamaili limited ocean-basin crustwhich formed in the early Devonian.3. Wildhorse Spring Paleovolcanomagmatic Arc RegionThe field and indoor study has revealed that the Lao-yaquan-Huangyangshan granite batholith intruded bothinto the Middle Devonian forearc sediments and theophiolitic melanges,and obviously was formed in thepost-collisional stage. A real paleovolcanomagmaticarcs relative to subduction of paleoplate are, in fact,located near the Wildhorse Spring and consist of D₁³-C₁¹acid—intermediate lavas, volcanic clastics and theirequivalent plutons. The arc region may be subdividedinto as follows from north to south, (1) Jingerdabackarc sedimentational belt, (2) Wildhorse Springvolcanomagmatic belt and (3) Huangyangshan-Laoyaquanforearc sedimentational belt. This shows from another side that the Karamaili oceanic basin had been conver-ged in the end of the Early Devonian.4. Intercontinental Remnant Sea-basinThe Karamaili ocean had been changed into the inter-continental remnant sea-basin whose floor was thetransitional crust after the paleo-Junggar blockcollided with the Wildhorse Spring arcs in the earlyof the early Carboniferous. In the basin, the Visean-Namurian sediments had been deposited. The sedimentsin the backarc region are named as the Sandul ashiformation, on the Karamaili suture, as the Nanming-shui formation, and on the northern margins of paleo-Junggar block, as the Tamugang formation. The lowestof the basin sequences is molasse, upwards, similar toflysch, and the uppest, gradually changing into thecoarse clastics similar to molasse. This shows thetectonic setting and the evolutionary history of thebasin.5. Post-collisional ActivitiesPost-collisional activities including not only erup-tion and intrusion of the calcalkaline magmas but alsothe formation of the alkaline granite batholith arewell-developed in the area. The origin of these magmasis relative to the orogeny which took place in thelate Carboniferous, but the intrusion and eruptionof them might start in the late or the post-orogeny.6. Structural DeformationBased on researching on the structural deformation ofthe area, three deformation megaperiods have beenrecognized,that is, (1) subducting and colliding deformation mega-period, (2) welting-orogenic deforma-tion mega-period, (3) intraplate thrusting and nappedeformation mega-period. At the same time, paleotetonicstress field is discussed.7. Late Paleozoic Tectonic EvolutionThe late Paleozoic evolution of plate tectonics in thearea has been divided into five stages as follows:(1) stage of continental spreading and forming of ocean-basin (D₁^1-2), (2) stage of converging and closing ofocean basin (D₁³-C₁¹),(3) stage of intercontinentalremnant sea-basin(C₁²-C₂¹), (4) stage of welding of con-tinental blocks and orogeny (C₂²), and (5) stage ofintraplate evolution (C₂³ or? P-Mz).At last, the thesis sums up the mineralization in thedifferent stages of plate evolution in the area, anddiscusses how to search for mineral resources.