| An in-depth understanding of the ocean-land transition process during the evolution of the Paleotethys Ocean is an important window for an accurate understanding of the amalgamation mechanisms between microterranes during the northward drifting of the Gondwana-derived fragments.The transition processes from subduction to collision(including the transition time frame and the transition mechanism)in the eastern Kunlun Paleotethys Ocean,located at the northernmost edge of the Paleotethys tectonic domain,has been highly controversial and has greatly constrained the in-depth understanding of the geodynamic processes between microterranes at the northern edge of Gondwana.The Late Paleozoic-Mesozoic magmatic events and associated sedimentary records,as a natural laboratory for studying the ocean-land subduction and collision processes in the East Kunlun Paleotethys Ocean,are well preserved in the accretionary complex of the Xinjiang Daofeng Mountain area on the western margin of the East Kunlun orogenic belt.In this paper,systematic field geological,petrological,zircon U-Pb,Lu-Hf isotopic and elemental geochemical studies were carried out on the Early Permian-Early Jurassic magmatic rocks and associated sedimentary rocks in the East Kunlun Daofeng Mountain area.The main research progresses are as follows:(1)Zircon U-Pb dating result of the basalt-basaltic andesite which lies in the lower part of the Maerzheng Formation is 273.1±1.1 Ma.The basalt-basaltic andesite belong to calcium-alkaline series with sodium-rich,high Mg,Mg#,(Th/Nb)Pm and low(Nb/La)Pm,strongly enrichments in large ion-lithophile elements(LILEs),depletions of high field strength elements(HFSEs).?Hf(t)and TDMC of the basalt-basaltic andesite range from+0.15 to+7.40 and from 822 to 1283Ma,respectively,suggesting that the basalt-andesite in the area mainly formed under melt metasomatism processes associated with slab melting during the Early Permian oceanic subduction stage.Zircon U-Pb dating result of the rhyolite-dacites in the upper part of the Maerzheng Group is 264.8+2.3 Ma and 266.6±2.7 Ma respectively.The rhyolite-dacite rocks belong to calcium-alkaline series with sodium-rich,low Mg and Mg#.They have characteristics of S-type granite and strongly enriched in LILEs and depleted in HFSEs.?Hf(t)and TDMC of the rhyolite-dacite range from-1.65 to+8.29 and from 764 to 1396 Ma,respectively,pointing to a characteristics of crustal-depletion mantle mixing.The comprehensive analysis suggests that the rhyolite-dacite formed as a product of the interaction between the melting of marine sediments(sandstones and mudstones)with varying proportions and the mantle wedge under a Late Permian subduction environment.(2)A newly discovered diorite vein in the southern part of the Daofeng Mountain melange zone intrudeing into the Huangyangling Formation have zircon U-Pb age of 258.2±1.9 Ma.The diorite has moderate SiO2,high Na2O,MgO,Mg#,Cr,Ni,and low FeOT/MgO,TiO2,Th,Th/Ce values,similar to the Sanukitic high-Mg andesite/diorite.Meanwhile,high Sr and Sr/Y,low Y and Yb values of this diorite are consistent with slab melt-associated adakite during the subduction process.Its ?Hf(t)ranges from-10.35 to+10.49(mean-2.64)and TDMC ranges from 612 to 1934 Ma.The comprehensive analysis suggests that it is formed by the reaction between melts from melting of slab and sediments and peridotite in the mantle wedge during subduction process in the Late Permian.(3)Magmatic rocks intruding into the Maerzheng Formation in the study area mainly consist of gabbro and granitic rocks.Two diabase rocks have zircon U-Pb ages of 206.5±4.9 Ma and 226.5±2.9 Ma respectively.They belong to calc-alkaline series with sodium-rich,high Mg,Mg#,(Th/Nb)pm,low(Nb/La)pm,strongly enriched in LILEs and depletion in HFSEs.The ?Hf(t)ranged from-6.78 to-1.82(average-3.51),showing varying degrees of enrichment in the magmatic source.Therefore,it is hypothesized that the diabases in the area were formed during a melt metasomatism process associated with partial melting of slab during the Late Triassic subduction environment,in which enriched components(e.g.sediments)of subducting slab may have been involved in this process.The monzonite granite,which exposed as large scale batholith,has a zircon U-Pb age of 209.5±1.5 Ma.It belong to high K calcium-alkaline series,relatively K-rich,low Mg and Mg#,showing typical I type granite characteristics.It shows enrichments in LILEs and depletions in HFSEs.Its high Lu/Hf ratios indicate that the marine sediments were likely brought in by subduction and involved in their formation processes;the Zr/Hf ratios deviate from the linear sequence(Zr/Hf vs.Zr),suggesting the involvement of mantle-derived components in the source area in addition to the magmatic crystallographic differentiation.In addition,its ?Hf(t)ranges from+2.15 to+8.23 and TDMC ranges from 720 Ma to 1107 Ma,showing different degrees of juvenile characteristics,which further supports the involvement of depleted mantle source components in its formation process.A synthetic analysis suggests that the monzonite granite was generated by reaction between melt of partial melting of subducted sediments(e.g.marine mudstones)and mantle wedge peridotites during the Late Triassic subduction environment.Late granitic rocks exposed as stock include mainly monzonite granite and alkali-feldspar granite have zircon U-Pb ages of 186.6±2.5 Ma and 186.1±1.8 Ma,respectively.Both belong to the calcium alkaline-high K calcium alkaline series and show high K,low Mg and Mg#,both enriched in LILEs and depleted in HFSEs.The linear sequence of Zr/Hf ratios and Zr in the rocks indicates no involvement of mantle-derived components;Y/Ho ratios show an increasing trend with increasing Nb contents and decreasing Nb/Ta ratios,indicating a correlation of fluids which formed in granitic rocks differentiation.The Jurassic granitic rocks have ?Hf(t)of+1.04?+7.23 and TDMC of 766?1162 Ma,showing consistent ?Hf(t)(+2.15 to+8.23)and Hf model ages(720 to 1107 Ma)with the early-stage Late Triassic monzonite granite,indicating that both share similar initial parental magmas.In addition,the dating results show that the zircons of the Early Jurassic granitic rock samples contain a large number of zircon clusters closely consistent with the zircon age span of Late Triassic granites(208-212 Ma).Taken together,the Early Jurassic granites are considered as the products of re-melting of Late Triassic granites or their detrital material in the context of strong extrusion during the soft collision stage without mantle-sourced magma participation.(4)Analysis of tectonic setting of the sedimentary rocks from Maerzheng Formation(P1-2m),Tuokuzidaban Formation(C1TK)and Daofengshan Formation(D3d)of the Kunnan melange Belt shows they are formed in active continental margin.Detrital zircons show characteristics of multi-peak distributions,including the sandy sparry limestone of the Maerzheng Formation(?302,?552 and?905 Ma),the feldspathic quartz sandstone of Tuokuzidaban Formation(?246 and?446 Ma)and biotite quartzite of the Daofengshan Formation(?576,?657 and?998 Ma).Two major peaks of Tuokuzidaban Formation is is closely related to the two stages of arc magmatism of the East Kunlun orogenic belt.Two main age peaks of?576 Ma and?905-998 Ma from Maerzheng and Daofengshan Formations well correspond to Pan-African orogeny and convergence-breakup of the Rodinia supercontinent respectively.The youngest age of?246Ma suggests the northward subduction of Paleotethys is not terminated at Middle Triassic time.In summary,through a systematic study of the Early Permian-Early Jurassic magmatic events in the western part of the East Kunlun orogenic belt,combined with the relevant sedimentary records in the region,this work concludes that the subduction of the Animaqing-Kunlun Paleotethys Ocean has started since Early Permian(?273Ma)and can lasted into the Late Triassic(?209 Ma),while the collision probably occurred in the Early Jurassic(?186 Ma).It underwent an ocean-land transition from ocean subduction phase to the soft collision stage of accretionary wedge-accretionary wedge,providing important timing and mechanism constraints for refining the subduction and collisional processes of the Anemaqen-Kunlun Paleotethys Ocean. |
PDF Full Text Request