Subduction And Closure Of The Paleotethys Ailaoshan Ocean: Constraints From Sedimentation And Magmatism

The Ailaoshan tectonic belt in central Yunnan is a key area for studying Paleothethys evolutionary history,which has preserved numbers of geological records about Paleotheys.However,the Ailoashan tectonic belt has experienced multi-stage intense tectonic deformation,such as that associated with the Cenozoic India-Eurasia collision,which has caused the stratigraphic and magmatic systems,especially those in the mélange belt,been deformed by different degrees.Although researchers have made numerous studies on the igneous rocks in the Ailaoshan tectonic belt,such as the ophiolites,subduction-and collision-related magmatic rocks,controversies still exist on the subduction processes and closure time of the Ailaoshan Ocean: Many studies have proposed that the Ailaoshan Ocean was consumed by westward subduction beneath the Indochina Block.However,some inferred that the Ailaoshan Ocean could also undergo eastward subduction beneath the South China Block.In addition,previous proposals about the closure time vary greatly from Devonian to Triassic.Permian-Triassic sedimentary sequences are well-exposed in the Ailaoshan tectonic belt and its adjacent regions.Moreover,magmatic rocks associated with the evolution of the Ailaoshan Ocean are recently found in the metamorphic complex.The significance of these sedimentary and magmatic rocks on reconstruction of the late Paleozoic and early Mesozoic tectonic evolutionary history has long been ignored.In this dissertation,we mainly focused on the Permian-Triassic magmatic and sedimentary/meta-sedimentary rocks in the Ailaoshan tectonic belt and its adjacent regions.We have conducted zircon U-Pb dating,zircon in-situ Hf-O isotope,mineralogical,whole rock major-and trace-element and Sr-Nd isotopic analyses on the Permian-Triassic magmatic rocks in the Ailaoshan metamorphic complex.We have obtained their formation age,petrogenesis and source characteristics,and discussed subduction process of the Ailaoshan Ocean.We have also distinguished the source composition and sedimentary settings of the Permian-Triassic sedimentary rocks,and constrained the closure time of the Ailaoshan Ocean.The important advances we have obtained are listed as follows:?1?The Permian-Middle Triassic sedimentary rocks from both sides of the Ailaoshan suture were probably formed in the fore-/back-arc basin,whereas the Late Triassic clastic rocks were sedimented in the foreland basin.The Permian sedimentary rocks from the eastern Indochina have relatively low SiO₂?avg.71.4 wt.%?,K₂O/Na₂O?avg.0.65?and high Fe₂O^T₃+MgO?avg.6.96 wt.%?,Al₂O₃/SiO₂?avg.0.20?and low chemical index of alteration?CIA;47–74,avg.59?and relatively high index of compositional variability?ICV;0.70–1.29,avg.1.03?.These geochemical characters are similar to greywackes from typical continental arcs,which may have been deposited in fore/back arc-basins.In contrast,the Upper Triassic sedimentary rocks have relatively high SiO₂?avg.81.9 wt.%?,K₂O/Na₂O?avg.8.48?and low Fe₂O^T₃+MgO?avg.5.26 wt.%?,Al₂O₃/SiO₂?avg.0.12?,high CIA?77–84,avg.80?and relatively low ICV?0.52–1.12,avg.0.79?.These geochemical characters are similar to greywackes from typical passive continental margin,which may have been deposited in the collisional basins,such as the foreland basin.The Middle Triassic sedimentary rocks have variable major element compositions.They are characterized by variable CIA?52–84,avg.69?and ICV?0.50–1.14,avg.0.88?.These geochemical characters indicate that those sediments may have been derived by variable sources,including both the continental arc and cratonic interior.Detrital zircon grains of the Lower-Middle Triassic sequences from western South China show single age population of 280–237 Ma?peak at 254–251 Ma?with both positive and negative ?Hf?t?values?+9.11 to-14.66?.The unimodal detrital zircon age pattern and Hf isotope features suggest that the Permian-Triassic magmatic arcs on the northern margin of Paleotethys had supplied detritus for the Lower-Middle Triassic sequences.The sedimentary setting could be the fore-arc basin.The Middle Triassic samples from eastern Indochina also contain single age population of 273–236 Ma?peak at 246–242 Ma?.These detrital zircons have both positive and negative ?Hf?t?values?-18.95 to +10.12?.The older detrital zircons?> 300 Ma?are relatively rare?³5%?.This kind of unimodal detrital zircon age patterns and Hf isotope features suggest that the sedimentary setting could be the back-arc basin,and detritus were mainly derived from the near-by Permian-Triassic Taizhong-Lixianjiang arc.The Late Triassic samples from both South China and Indochina blocks have similar polymodal detrital zircon age spectra with main age population at 290–230 Ma,500–400 Ma,850–700 Ma,1000–900 Ma,2000–1800 Ma and 2500–2400 Ma.In addition,the ?Hf?t?values of each zircon age groups of the Late Triassic South China samples are similar to their counterparts from the Indochina Block.These similarities imply that the Late Triassic sedimentary rocks from both blocks may have derived from similar source.The detrital zircon age distribution patterns indicate that the sediments were probably derived from both the Permian-Triassic arcs and cratonic interior?e.g.,early Paleozoic igneous rocks and/or recycled sedimentary sequences?.Therefore,the depositional setting of these Late Triassic samples could be the collisional basin,such as the foreland basin.?2?We have first identified the Middle Triassic gabbroic diorites and granodiorites in the Ailaoshan metamorphic complex,which could be the relict island arc of the eastwaed subduction of the Ailaoshan Ocean.The gabbroic diorites are probably derived by partial melting of MORB-like mantle wedge that had been metasomatized by subducted oceanic sediments,whilst the granodiorites were probably fractional crystallized from the contemporaneous gabbroic magmas,with degrees of crustal assimilationSIMS zircon U-Pb dating of the Ailaoshan gabbroic diorites gives the age of 235 ± 2 Ma.The gabbroic diorites are characterized by relatively low SiO₂?49.55–54.39 wt.%?contents,K₂O/Na₂O?0.31–0.40?ratios,and high Mg O?4.96–5.99 wt.%?and Mg# values?48–57?.These samples are characterized by being LILE?e.g.,Rb,Ba and U?enriched and HFSE?e.g.,Nb,Ta and Ti?depleted.The gabbroic diorites exhibit low??87?^Sr/?86?^Sr?_i?0.7046–0.7050?and zircon ?¹⁸O?5.76–6.59‰?,and high ?Nd?t??+3.63 to +4.22?and zircon ?Hf?t??+9.9 to +12.6?.The geochemical characters and isotopic modeling results indicate that the gabbroic diorites were probably derived from partial melting of the mantle wedge by involving ³%–5% subducted sediments.SIMS zircon U-Pb dating of the Ailaoshan granodiroites gives the age of 237 ± 2 Ma.The granodiorites are characterized by relatively high SiO₂?64.64–70.98 wt.%?,low Mg O?0.85–1.58 wt.%?and Mg#?36–41?values.The granodiorites are also enriched in LILEs?e.g.,Rb,Ba and U?and depleted in HFSEs?e.g.,Nb,Ta and Ti?.The granodiorites have low and varying??87?^Sr/?86?^Sr?_i values?0.7037–0.7046?and similar ?Nd?t?values?+2.90 to +4.63?to those of the gabbroic diorites.They have varying zircon ?¹⁸O?6.25–7.20‰?and ?Hf?t??+9.9 to +14.1?values.As shown in the major and trace element Harker diagrams,the granodiorites lie on an extension of the coeval calc-alkali gabbroic magma trends,signifying a potential comagmatic relationship.Our result of modeling of Rayleigh fractionation?40% amphibole + 30% clinopyroxene + 30% plagioclase?successfully reproduced the trace element compositional variation of the Ailaoshan gabbroic diorites and granodiorites.The variable geochemical and Sr-Nd-Hf-O isotopic compositions of the Middle Triassic granodiorites were probably attributed to the source heterogeneity.?3?We have first reported the Late Permian Nb-enriched basalts in the Ailaoshan metamorphic complex,whcih were possibly generated by mixing of subductionderived,mid-ocean ridge basalt?MORB?-like components and mantle plume-derived,ocean island basalt?OIB?-like enriched components within the spinel stability field.Secondary ion mass spectroscopy?SIMS?zircon U-Pb dating of the Ailaoshan Nb-enriched basalts gives the age of 261 ± 2 Ma,coeval with Emeishan plume activity.The ²61 Ma zircon grains from the basalt have limited ?¹⁸O ranges of 5.28–5.75‰?average 5.53 ± 0.15‰?,which are similar to that of typical igneous zircons in equilibrium with mantle-derived magmas.The basalts are characterized by relatively low SiO₂?46.12–48.33 wt.%?contents and?La/Yb?N?2.6–3.0?ratios,high Mg O?7.04–8.41 wt.%?,Nb?10.1–11.5 ppm?contents and Nb/La?1.2–1.3?,Nb/U?41.90–49.91?ratios.On the primitive mantle-normalized multi-elements diagram,they display positive Nb-Ta and Eu?Eu/Eu* = 1.01–1.09?anomalies.The Ailaoshan Nb-enriched basalts high and uniform ?Nd?t??+5.65 to +6.21?.Our trace elements and isotopic modeling results indicate that the Ailaoshan Late Permian Nb-enriched basalts were possibly generated by mixing of subduction-derived,MORB-like components and mantle plume-derived,ocean island basalt?OIB?-like enriched components.?4?The Permian-Triassic evolution history of Ailaoshan Paleotethys and its relationship between the Emeishan large igneous provinceThe Ailaoshan Ocean was consumed by bipolar subduction beneath the Indochina and South China blocks in Permian-Middle Triassic,and finally closed in early Late Triassic.The Ailaoshan Ocean subducted eastward beneath the South China Block,formed Permian-Triassic magmatic arc along the western margin of South China.Most of the arc rocks were probably been eroded away.The Middle Triassic?ca.235–237 Ma?gabbroic diorites and granodiorites in the Ailaoshan metamorphic complex are probobaly the relict island arc of the eastward subduction.This Permian-Triassic arc system mainly supplied the detritus for the Early-Middle Triassic fore-arc basin along the western margin of South China Block.The Permian-Triassic Taizhong-Lixianjiang arc system on the eastern Indochina was result from the westward subduction of the Ailaoshan Ocean,which mainly provided detritus for Permian-Triassic convergent basins?e.g.,fore/back-arc basins?.In Late Triassic,the sedimentary setting has been changed from the convergent basin into the collisional basin,such as the foreland basin.As a result,sedimentary basins were mainly fed by the mature sources from cratonic interior with little supplies from the Permian-Triassic magmatic arcs.Therefore,we inferred that the Ailaoshan Ocean was probably closed in early Late Triassic,following by the collision between the South China and Indochina blocks.The ocean closure and consequent Indochina–South China continent-continent collision may have connected the drainage systems of the two blocks,leading to the marked similarities in their post-Middle Triassic sedimentary provenance.In Late Permian?ca.261 Ma?,the rollback of the Ailaoshan oceanic slab could induce mantle flow,resulting in deformation and westward flow of Emeishan OIB-like plume material to the nearby subduction zone.The mixing between depleted MORB-type and enriched OIB-like Emeishan plume mantle components produced the Ailaoshan Nb-enriched basalts.