| For a long time,the tectono-magmatic evolution and geodynamic mechanisms of the Proto-Tethys Ocean have been hot issues in fundamental geological research.The Southwestern Yunnan region is located in the eastern segment of the Proto-Tethys tectonic domain,mainly consisting of a series of micro-continents with different tectonic affinities and ophiolite mélange belts.These geological units have experienced and recorded the long-term subduction-accretion process of the southward branch of the Proto-Tethys Ocean.The Early Paleozoic igneous rocks of various stages and types were widely developed in Southwestern Yunnan and displayed complicated spatio-temporal variation characteristics,providing a key window for revealing the tectono-magmatic evolution of the Proto-Tethys Ocean.This paper focuses on the widespread Early Paleozoic intrusive rocks and volcanic rocks in Southwestern Yunnan and conducts comprehensive studies on detailed petrography,LA-ICP-MS zircon U-Pb dating,whole-rock major and trace elements,whole-rock Nd isotopes,in-situ zircon Hf isotopes,and in-situ apatite Nd isotopes,in order to investigate the geochronological framework,constrain their spatiotemporal variations,and decipher the source nature and magmatic processes of the Early Paleozoic magmatic rocks.Synthesizing the characteristics of Early Paleozoic magmatism in Southwestern Yunnan,the records of ophiolite mélanges,and the tectonic affinities of different micro-continents,this paper reveals the deep geodynamic processes during the Early Paleozoic,and further reconstructs the tectono-magmatic evolution of the eastern segment of the Proto-Tethys tectonic domain.The achievements are as follows:1.The spatio-temporal framework of Early Paleozoic igneous rocks in Southwestern Yunnan has been establishedBased on previous research results,this paper has carried out LA-ICP-MS zircon UPb dating of the Early Paleozoic igneous rocks in Southwestern Yunnan to constrain their accurate formation ages and established an accurate geochronological framework,including the Cambrian(537–496 Ma),Late Cambrian–Middle Ordovician(496–462 Ma),Middle Ordovician–Early Silurian(462–442 Ma)and Early Silurian–early Early Devonian(442–413 Ma).The Early Paleozoic magmatic rocks are characterized by zonal spatial distribution,with the age of magmatic activity showing a trend of becoming progressively younger from the Tengchong-Baoshan Block to the Simao Block.Among them,the Early Paleozoic magmatic rocks in the Tengchong-Baoshan Block have the widest distribution and largest outcrop scale,while the Early Paleozoic magmatic rocks in other geological units are smaller in scale,characterized by isolated distributions.2.The whole-rock and in-situ mineral geochemistry reveal the petrogenesis and tectonic setting of Early Paleozoic igneous rocks in different geological units of Southwestern Yunnan(1)The Middle–Late Ordovician(465.9–454.9 Ma)granitic rocks in the Pingda area of the Baoshan Block have high Si O2 and Al2O3 contents,low Mg O contents,as well as high A/CNK ratios.These granitic rocks belong to weakly peraluminous to strongly peraluminous series,indicating a S-type granite affinity.In addition,the granitic rocks have low Al2O3/Ti O2 ratios and varying Ca O/Na2 O,Rb/Sr,and Rb/Ba ratios,indicating that these granitic rocks represent partial melting products of heterogeneous sources composed of variable proportions of pelite and psammite.Their enriched zircon Hf isotopic and apatite Nd isotopic compositions also indicate that the magma source of these S-type granites was dominant by Paleoproterozoic meta-sedimentary rocks.Based on regional research results,this study suggests that the Middle–Late Ordovician S-type granites in the Pingda area were formed during continuous subduction of the Proto-Tethys oceanic plate beneath the Baoshan Block.(2)The Late Ordovician(456.7–444.3 Ma)hornblende gabbros and tonalites are developed in the Nantinghe area of the Changning-Menglian suture zone.The hornblende gabbros exhibit geochemical characteristics of calc-alkaline rocks,with enrichments in light rare earth elements and large ion lithophile elements and depletions in heavy rare earth and high field strength elements,as well as the development of negative Nb-Ta anomalies.These geochemical features are similar to typical geochemical affinities of arcrelated magmatic rocks.These hornblende gabbros have depleted Hf isotopic compositions and originate from a depleted mantle wedge metasomatized by subductionrelated fluids.The primary magma was formed by approximately 10 % partial melting of mantle wedge peridotites at the spinel to garnet lherzolite transitional zone.It confirms the presence of dehydration processes of subducting oceanic slab and interactions between subduction-related fluids and mantle wedge in the Nantinghe area during the Late Ordovician.The tonalites occur as rootless bands within the hornblende gabbros,together with their consistent zircon crystallization ages and Hf isotopic compositions compared with hornblende gabbros,indicating that the tonalites and the hornblende gabbros might represent products of the evolution of same parental magma.Compared with the hornblende gabbros,the tonalites have a significantly higher plagioclase contents(60–70 %)and exhibit dramatically positive Eu,Ba,and Sr anomalies.It is suggested that the tonalites are regarded as products of late-stage crystallization differentiation and cumulate processes of mafic magma.Both the tonalites and hornblende gabbros originate from the same or a similar mantle source region.(3)The Late Ordovician(455.4–447.2 Ma)high-Mg andesites and A-type granites are developed in the Lancang Terrane.The geochemical affinities of the high-Mg andesites from the Huimin Formation of the Lancang Group resemble those of the sanukite in the Neogene Setouchi volcanic arc in Southwest Japan.They are enriched in large ion lithophile elements and depleted in high field strength elements,with a slightly right-leaning rare earth element distribution pattern,exhibiting geochemical attributes of arc-type andesites.They originated from a mantle wedge source region which was metasomatized by hydrous melts derived from subduction-related terrigenous sediments.The A-type granites in the Bulangshan area are characterized by high(K2O + Na2O)/Ca O,K2O/Mg O ratios,and 10000Ga/Al values.They originated from partial melting of calcalkaline rocks in the Paleo-to Meso-Proterozoic crust under high-temperature and lowpressure conditions,indicating an extension-related tectonic setting.(4)The Late Ordovician to Early Silurian(448.9–443.4 Ma)meta-gabbros in the Yunxian-Menghai tectonic zone display an MORB-type rare earth element distribution pattern,homogenous and significantly depleted isotopic compositions,and trace element characteristics of arc-style magmatic rocks,with enrichments in large ion lithophile elements and depletions in high field strength elements.These geochemical characteristics of the meta-gabbros are similar to typical intra-oceanic arc basalts developed in the IzuBonin-Mariana Intra-oceanic Arc,indicating the influence of subduction-related fluids during their formation.Combined with contemporaneous intra-oceanic arc sedimentaryvolcanic rock assemblages in the Yunxian-Menghai tectonic zone,it is suggested that there developed a relatively complete intra-oceanic subduction-related rock sequence during the Late Ordovician to Early Silurian,indicating the presence of intra-oceanic subduction within the Yunxian-Menghai Ocean.(5)The Early–Middle Ordovician(470.6–459.7 Ma)granitic rocks in the Lincang granitic batholith,including the Shuangjiang monzogranite and Longtang granodiorite,have geochemical affinities of S-type granites.Their parental magmas originated from Paleoproterozoic meta-greywackes which were plagioclase-rich and clay-poor,through a muscovite dehydration melting process.These granitic rocks were formed in an active continental margin setting associated with northward(currently eastward)subduction of the Proto-Tethys Ocean.3.Geodynamical model of Early Paleozoic tectono-magmatic evolution in Southwestern YunnanBased on above data and previously reported results,we established the Early Paleozoic tectono-magmatic evolution model in Southwestern Yunnan of the eastern segment of the Proto-Tethys tectonic domain:(1)537–496 Ma: the Cambrian continental arc-type intrusive rocks in the Baoshan Block were formed in an active continental margin setting,related to southward(currently westward)subduction of the Proto-Tethys(Yunxian-Menghai)oceanic crust beneath the Baoshan Block;(2)496–462 Ma: in response to southward(currently westward)subduction of the Yunxian-Menghai Ocean,the fluids released by the dehydration of subducting oceanic crust interacted with the mantle wedge,resulting in continental arc-type calc-alkaline basalts and andesites in the margin of the Baoshan Block.The continuous underplating of mantle-derived magmas induced large-scale partial melting of crustal materials,which led to the formation of the largest Early Paleozoic Pinghe-Pingda granitic pluton in the Baoshan Block.At the same time,the northward(currently eastward)subduction of the Yunxian-Menghai oceanic slab began.The subducting Yunxian-Menghai oceanic slab broke off and resulted in upwelling of asthenospheric materials,which triggered partial melting of crustal materials at different depths and gave rise to granitic rocks and mafic enclaves at the margin of the Simao Block;(3)462–442 Ma: bidirectional subduction of the Yunxian-Menghai oceanic slab continued to develop,forming the Lancang magmatic arc(Lancang Terrane)and the Lincang magmatic arc on either side of the main ocean basin.On the southern side(currently the western side)of the main ocean basin,the partial melting of mantle wedge metasomatized by subduction-related fluids/melts resulted in continental arc-type calcalkaline gabbros and high-Mg andesites.The A-type granites were formed in the extensional environment related to the back-arc basin.On the northern side(currently the eastern side)of the main ocean basin,the development of meta-gabbros with geochemical characteristics similar to intra-oceanic arc basalts in the Mayidui area of the Lincang magmatic arc,indicated the occurrence of an intra-oceanic subduction event within the Proto-Tethys Ocean;(4)442–413 Ma: the Lancang Terrane accreted to the margin of the Baoshan Block in response to the termination of the southward(currently westward)subduction of the Yunxian-Menghai oceanic slab.The Yunxian-Menghai Ocean transformed into a remnant ocean basin,but the northward(currently eastward)subduction and the arc-basin system are still developed.The subduction of the YunxianMenghai Ocean continued until the early period of the Early Devonian and resulted in the island arc volcanic rocks in the Dazhonghe area,indicating an active continental margin environment. |
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