Magmatism At 550~450 Ma Within The South Qiangtang-Baoshan Plate Of The Tibetan Plateau

Previous research on the Tibetan Plateau,located on the eastern margin of the Himalaya–Alpine orogenic belt,has focused on the multi-stage evolution of the Tethyan Ocean,including the genesis of ophiolites and high-pressure metamorphic belts and magmatism.However,the early stages of the formation and evolution of the plateau remain relatively understudied.The South Qiangtang–Baoshan Plate is an important tectonic unit located within the hinterland of the plateau,forming part of the Tethyan tectonic domain.The plate is bounded by the Longmu Co–Shuanghu–Lancangjiang suture zone to the north and the Bangong–Nujiang suture zone to the south.Few studies have examined the South Qiangtang–Baoshan Plate,primarily as a result of the extreme climate,the logistical challenges posed by working in this area,and the complex tectonic evolutionary history of the Tibetan Plateau.In particular,the late Neoproterozoic to early Paleozoic evolution of this area remains unclear,with many significant geological problems needing to be addressed.One of these problems is the influence of the Pan-African orogenic event on the evolution of the South Qiangtang–Baoshan Plate,including the nature and timing of formation of the late Neoproterozoic to early Paleozoic basement of this plate and whether this basement material records Pan-African orogenesis.All of these unknowns impede our understanding of the early stages of the formation and evolution of the Tibetan Plateau.The composition and timing of formation of basement material provides insights into the evolution of the basement of the northern Gondwana margin.There exists substantial early Paleozoic magmatism and associated widely distributed unconformities within the Tibetan Plateau;however,the tectono-thermal event that formed these features remains debated.This is exemplified by the two main models of the early Paleozoic tectonic setting of the northern Gondwana margin:(1)a passive continental margin setting where early Paleozoic magmatism occurred in a post-collision extensional setting after the assembly of Gondwana,or(2)early Paleozoic magmatism was the result of an Andean-type orogenic event related to the southward subduction of proto-Tethyan oceanic crust beneath the northern Gondwana margin.Magmatic rocks provide insights into the geodynamics of plate movements and tectonic activities as well as information on the formation and tectonic evolution of basement sequences.In particular,mafic and basic rocks can provide useful information on ancient tectonic settings and the evolution of tectonism in a given region,especially if these rocks form part of a bimodal magmatic sequence.However,the complex evolution of tectonism and magmatism,and the changing sources of magma in a region means that understanding the evolution of the basement needs to include regional geological,petrological,geochemical,sedimentological,and geodynamic data.This paper builds on previous research and focuses on previously reported and newly discovered magmatic and sedimentary rocks within the central South Qiangtang–Baoshan Plate,a sequence that includes granitoid,andesite,basalt,gabbro,bimodal volcanic,and rift-type sedimentary units.This study presents the results of detailed fieldwork and newly obtained systematic zircon U–Pb geochronological,Lu–Hf isotopic,and whole-rock geochemical data for late Neoproterozoic to Late Ordovician rock units.These data provide insights into the contemporaneous magmatism and sedimentation within the southern Tibetan Plateau and adjacent areas,as well as enabling spatial and temporal comparisons between this area and typical Pan-African orogenic belts,and furthering our understanding of the tectonic evolution of the northern Gondwana margin.All of these data are incorporated into a model of the early tectonic evolution of the Tibetan Plateau within the northern Gondwana margin.The oldest rocks identified to date in the central South Qiangtang–Baoshan Plate are the late Neoproterozoic(~550 Ma)Dabure clastic sediments and interlayered basalts.These clastic units are compositionally and texturally immature turbidite sequences.Laser ablation–inductively coupled plasma–mass spectrometry(LA–ICP–MS)U–Pb dating of magmatic zircons from the associated Dabure basalts yields late Neoproterozoic ages(~550 Ma).The basalts are alkaline,contain elevated concentrations of Ti and Fe,and have chondrite-normalized rare earth element(REE)patterns that are enriched in the light REE(LREE)and are free of Eu anomalies.These basalts have primitive-mantle-normalized multi-element variation patterns that contain negative Nb and Ta anomalies,all of which are indicative of within-plate basaltic magmatism.The zircons within the basalts show heterogeneous Hf isotopic compositions that are indicative of derivation from a region of enriched mantle,with the resulting magmas assimilating continental crustal material prior to emplacement.This suggests that the Dabure basalt formed during the initial breakup of Precambrian Gondwana,related to the initial period of extension within the Longmu Co–Shuanghu–Lancangjiang Ocean.Our new data for Late Cambrian granite gneiss and gabbro units in the Duguer area of the central South Qiangtang–Baoshan Plate include zircon LA–ICP–MS U–Pb dating of three granitic gneiss samples,which yield concordant ages of 492.0 ± 1.8,500.7 ± 1.5,and 501.8 ± 1.6 Ma.These ages provide insights into the possible existence of ancient basement rocks within the South Qiangtang–Baoshan plate.The granitic gneisses are geochemically similar to high-K and calc-alkaline S-type granites.Zircons within these samples have Hf isotopic compositions that yield negative ?Hf(t)values(–7.4 to –1.1)and old two-stage zircon Hf model ages(TDM2 = 1757–1406 Ma).These data are similar to those obtained from contemporaneous granitic gneisses in the Bensong Co area,suggesting these units were derived from magmas generated from a similar source region.All of the magmas that formed these granitic gneisses were generated by the partial melting of ancient pelitic rocks,with the resulting melts contaminated by mixing with a small amount of mantle-derived material.The gabbro in this area yields a concordant LA–ICP–MS zircon U–Pb age of 490 ± 4 Ma,and this unit is a middle-K and calc-alkaline gabbro that is enriched in the LREE,is free of Eu,Nb,and Ta anomalies,and is geochemically similar to typical enriched mid-ocean ridge basalt(E-MORB).The zircons within this gabbro yield a wide range of ?Hf(t)values(–9.2 to 1.2),suggesting that this unit was derived from magmas generated by the partial melting of an enriched region of the mantle that subsequently assimilated minor amounts of crustal material prior to emplacement.Early Ordovician magmatic rocks within the central South Qiangtang–Baoshan Plate crop out in the Duguer,Gemuri,Bensong Co,and Ejiumai areas.These units include a wide variety of lithologies,including basalts,andesites,and granites.This study is the first to analyze the basalts and andesites in the Duguer area.The basalt yielded a weighted mean zircon LA–ICP–MS U–Pb age of 486 ± 10 Ma and is a low-K and tholeiitic basalt that is enriched in the LREE,has slightly negative Eu anomalies in a chondrite-normalized REE diagram,is free of Nb and Ta depletions,and is geochemically similar to typical E-MORB.The zircons within this basalt show a wide range of ?Hf(t)values(–6.9 to –0.04),suggesting that the basalt formed from a magma generated by the partial melting of an enriched region of the mantle,with the resulting magmas assimilating minor amounts of crustal material prior to emplacement.Three andesite samples from this area yielded zircon LA–ICP–MS U–Pb ages of 473 ± 15,475 ± 4,and 477 ± 3 Ma.These samples are generally low-K and tholeiitic,are enriched in the LREE,have slightly negative Eu,Nb,and Ta anomalies,and are geochemically similar to typical ocean island basalt(OIB).The zircons within these samples show a wide range of ?Hf(t)values(–6.6 to 2.2),and these andesites were derived from a magma generated by mixing between mantle-derived mafic and silicic magmas which latter generated by the partial melting of ancient crustal material.Finally,the granites within the Gemuri,Bensong Co,and Ejiumai areas yield ages that cluster between 486 and 477 Ma.These granites are all high-K,calc-alkaline,and peraluminous S-type granites that contain zircons with a wide range of generally negative ?Hf(t)values(–18.7 to 1.7).These granites formed from magmas generated by the partial melting of ancient crustal material,with the resulting magmas assimilating a small amount of mantle-derived material.These data suggest the granites have a similar petrogenetic history to the Late Cambrian Duguer granitic gneisses,although the two did not form contemporaneously.Combining our new data with the results of previous research suggests that the Late Cambrian to Early Ordovician magmatic rocks within the central part of the South Qiangtang–Baoshan Plate developed in a post-collisional tectonic setting of Pan-African orogenesis.The magmatism was associated with post-orogenic delamination.The Middle to Late Ordovician geology of the central South Qiangtang–Baoshan Plate is dominated by the granitic gneisses of the Duguer area,the genesis of Dawashan rift-type deposits,and the formation of bimodal volcanic sequences.The granitic gneisses in the Duguer area have not previously been reported,and one sample of this unit yielded a concordant zircon LA–ICP–MS age of 455 ± 3 Ma.These granitic gneisses are high-K,calc-alkaline,and peraluminous S-type granites that are enriched in the LREE and show negative Eu anomalies in chondrite-normalized REE diagrams.These samples have primitive-mantle-normalized multi-element variation patterns that contain positive Pb,U and large ion lithophile element(LILE)anomalies,and negative Ba,Sr,and high field strength element(HFSE)anomalies.The zircons within these granitic gneisses also yield negative ?Hf(t)values(–7.1 to –3.8),indicating that this unit formed from magmas generated by the partial melting of ancient crustal material,with the resulting magmas assimilating a small amount of mantle-derived material.The Middle to Upper Ordovician Dawashan Formation is dominated by flysch sediments that include metamorphosed sandstone,greywacke,and phyllite units as well as bimodal rhyolitic and basaltic volcanics.This formation formed in a rift-type abyssal–bathyal marine setting at the northern margin of Gondwana.The magmatism in this region was associated with post-Pan-African extensional and collapse-related tectonism,representing the end of the Pan-African orogenic event within the Tibetan Plateau.Comparing and contrasting the geological history of the study area with the geology of the wider region indicates that the late Neoproterozoic to Late Ordovician magmatism within the central South Qiangtang–Baoshan Plate is contemporaneous with widespread magmatism across the Tibetan Plateau.These magmatic events can be divided into two phases.The first is a 550~520 Ma period of magmatism that is sporadically recorded at the northern margin of Gondwana and is contemporaneous with the peak metamorphism during the Pan-African orogenic.The second period of magmatism at 520~450 Ma was larger than the first and generated voluminous magmatic rocks as well as minor amounts of contemporary metamorphism,all of which is concentrated within the Tibetan Plateau and adjacent regions and is 30~50 Myr younger than typical Gondwanan Pan-African orogenic belts.The Pan-African collisional orogeny represents the final assembly of eastern and western Gondwana,an event that caused extension parallel to the orogenic belt as well as lithospheric extension and thinning(i.e.,delamination).This suggests that the first period of magmatism outlined above was associated with local extension at the northern margin of Gondwana that was the result of the ongoing intense compressive orogeny within the interior.The second period of magmatism was generated by lithospheric extension and thinning that occurred during either the later stages of,or post-dated the Pan-African orogenic event.The presence of a Precambrian/Cambrian–Ordovician unconformity records extension in a rift basin caused by lithospheric thinning and the collapse of orogenic belts,suggesting that lithospheric delamination led to the breakup of the northern Gondwana margin.