| The’Sanjiang’ Paleo-Tethys orogen lies between the Tibetan Plateau and the Yangtze Craton. It consists of Changning-Menglian belt, Jinshajiang-Ailaoshan belt and Garze-Litang belt. There are also four ancient arc belts, named Yunxian-Jinhong arc belt, Jiangda-Weixi arc belt, Yaxuanqiao arc belt and Yidun arc belt. Contrast to the Changning-Menglian belt and Jinshajiang belt, there are still many unsolved problems about the Garze-Litang belt, such as when the oceanic basin was opened, how the Yudun arc belt was formed, when the oceanic basin was closed and what was the relationship between the Garze-Litang belt and the west Jinshajiang belt. The Yidun arc belt (YAB) is loacted between the Jinshajiang belt and the Garze-Litang belt, and is the largest ancient arc belt in the study area. Because of its unique temporal-spatial distribution and complex evolutional history, the YAB has undergone a lot of polymetallic mineralizations, which has attracted widespread attentions. Thus, to study of the tectonic evolutionary history of the YAB could help us not only to better understand the evolutionary history of the northeastern part of the eastern Paleo-Tethys, but also could shed light on the tectonic background of the formation of the ore deposits. However, there are still many problems remain unsolved, such as the basement of the YAB, the tectonic models, and the relationships between Garze-Litang ocean and the Jinshajiang ocean etc.According to the unsolved problrms, this study focuses on the Late Triassic Sucuoma-Dongcuo granitic belt. Based on systematic study of the U-Pb zircon geochronology, petrography, mineralogy and geochemistry of the intermediate-acid rocks and combined with the detrital zircon geochronology and previous study, we conclude as follow:Several granitic plutons are present in the northern segment of the Yidun Arc Belt, eastern Tibetan Plateau and seven have been dated in this study. From west to east, these are:the Sucuoma (235 ±2 Ma), Ajisenduo (224± 2 Ma), Jiaduocuo (218 ±1 Ma), Cuojiaoma (219 ± 1 Ma), Maxionggou (225 ± 2 Ma), Dongcuo (222 ± 3 Ma and 222 ± 2 Ma) and Daocheng (220 ± 2 Ma) plutons. Most of the rocks (except for some mafic to intermediate rocks from the Sucuoma pluton) are granitic and contain high SiO2, Al2O3 and K2O+NaiO, but low MgO, FeO* and CaO. All granitic samples show similar trace element patterns, with depletion in high field strength elements (HFSE, e.g., Nb, Ta and Zr) and enrichment in large-ion lithophile elements (LILE, e.g., Rb, Th and U). The zircon Lu-Hf isotopic compositions of samples from the Sucuoma, Jiaduocuo and Dongcuo plutons have similar 176Hf/177Hf ratios and εHf(T) values, whereas samples from the Ajisenduo pluton exhibit lower 176Hf/177Hf ratios and more enriched εHf(T) values. The Sucuoma pluton has the lowest initial 87Sr/86Sr values but the highest εNd(T) values, whereas samples from the Ajisenduo pluton have the highest initial 87Sr/86Sr values, but the lowest εNd(t) values. Additionally, samples from the Jiaduocuo, Cuojiaoma and Dongcuo plutons show similar initial 87Sr/86Sr values and εNd(t) values. All the samples have similar high radiogenic Pb isotopic compositions with the present-day whole-rock Pb isotopic ratios varying from 18.591 to 19.392 for 206Pb/204Pb, from 15.689 to 15.765 for 207Pb/204Pb, from 39.079 to 40.442 for 208Pb/204Pb. Based on the whole-rock major and trace elements and Sr-Nd-Hf-Pb isotopic compositions, most of the granitic samples (except the samples from the Ajisenduo pluton) are high-temperature I-type granites and the magma source was a mixture of arc volcanic rocks and Triassic sediments. However samples from the Ajisenduo pluton are S-type granites that were derived from partial melting of Triassic sediments with limited mixture with arc magma. Together with a literature review, these new data indicate that there is a dominant magmatic peak at～216 Ma and that the ages grow young eastward towards the Garze-Litang suture zone. We consider that slab roll-back with subsequent slab break-off explains the origin of these granitic plutons. Also, the granitic samples show many differences with the Cordilleran-type batholiths. Therefore, we conclude that the Cuojiaoma-Dongcuo granitic belt was mainly formed under a post-extension setting rather than a subduction setting.The Ganluogou dioritic complex is located in the northern part of the Cuojiaoma-Dongcuo granitic belt. It consists of diorite, ferrodiorite and a mixing zone between them and is the largest intermediate-mafic pluton in the YAB. The diorites formed at 209±2 Ma and contain generally higher SiO2, Na2O+K2O, Th, U, Zr, and Hf contents, but lower Al2O3, MgO, CaO, Co, and Sr contents than the ferrodiorites. The ferrodiorites were emplaced at 213 ± 2 Ma and have low SiO2 and high Fe2O3* contents. In the primitive mantle-normalized trace element diagram, all the samples show depletion in Nb and Ta. However, ferrodiorites exhibit strong depletion in Zr and Hf, whereas diorites contain relatively higher Th and U contents without negative Zr and Hf anomalies. Both rock-types have similar chondrite-normalized rare earth element patterns with (La/Yb)N ratios= 4.4 to 18.2, and show weak Eu anomalies, with Eu/Eu* of 0.47 to 1. they both show narrow ranges in Sr-Nd-Hf isotopic compositions. The ferrodiorites have lower initial 87Sr/86Sr ratios and relatively higher εNd(t) values than the diorites, which record values of 0.7062-0.7066 and-5.5 to-5.7, respectively. The ferrodiorites also exhibit higher 176Hf/177Hf ratios and more depleted εHf(t) values than those of the diorites. These geochemical and isotopic features suggest that the diorites and ferrodiorites were derived from different magma sources. We propose that the diorites were derived from dehydration melting of meta-basaltic rocks from the Kangding Complex, whereas the ferrodiorites were produced by differentiation of mantle-derived tholeiitic arc magma. Both diorites and ferrodiorites were emplaced at relatively low pressure (3.3-3.9 kbar) but with quite different magma temperature (diorites:692-788℃ ferrodiorites: 821-960℃). The Ganluogou dioritic complex was therefore formed in an environment that switched from a subduction compressional setting to one of extension that was associated with the closure of Garze-Litang Ocean. Previous studies on Late Triassic magmatism in the area have shown that there was slab-break off event at-216 Ma, Combined with published detrital zircon geochronology in the area, we conclude that the final closure of the Garze-Litang segment of the Paleo-Tethys Ocean was at-216 Ma. Furthermore, after study the plagioclase megacrysts in the samples from mixing zone, we found that when the mafic magma injected into the relatively acid magma, the host magma (the acid one) did not modify the mafic one immediately. The components of the zoned plagioclase phenocrysts may record the composition variation of the surrounding magma. The magma mixing process that inferred from the Ganluogou dioritic complex may be used to interpret the forming of the micro mafic enclaves (MMEs) in the granitic plutons.This paper also reports morphology features and geochronological data of detrital zircons from the wall rocks, sedimentary xenolith of Cretaceous pluton and inherited zircons from the Late Triassic granite in the Yidun arc region, eastern Tibet Plateau. Detrital zircons from the xenolith show well roundness and the ages exhibit four prominent peaks at 2.5-2.4 Ga,1.9-1.8 Ga,480-400 Ma, and 350-300 Ma, while detrital zircons from wall rock exhibit angular shape with another four prominent peaks at 1.9Ga-1.8 Ga,850Ma-700 Ma,480 Ma,400 Ma, and 300 Ma-250 Ma. By comparative study with the age data reported by previous studies, we suggest that the sedimentary xenolith was from the Lanashan Formation and the major provenance of them were Qiangtang block, Zhongza massif and Yangtze block, while the wall rocks belong to the Lamaya Formation and the major provenance of them were Yangtze block and Yidun terrane. In addition, the ages of inherited zircons show Yangtze block affinity with the age peak at 900-700 Ma, however, the age peak that at 2.5-2.4 Ga was absent. The provenance of Lamaya Formation (Norian) was similar to the so-called Southeastern Depocenter of Spngpan-Garze basin, but was different from that of Lanashan Formation. Combining with the data reported by other researchers, we suggest that the provenances vary from the Lanashan Formation to the Lamaya Formation may indicate a record of the final closure of the southern Ganze-Litang ocean and the closure time might be before-216 Ma.After summarize all the tectonic and magmatism events during the late Paleozoic and early Mesozoic of the surrounding area, we build a new model to show the evolutionary history of the study area:at 260-250 Ma, the Jinshajiang oceanic basin began to subduct westward to form the Jiangda-weixi arc belt. At the same time, the Zhongza massif began to separate from west Yangtze Craton because of the activity of Emeishan plume. At 250-235 Ma. the Jinshajiang oceanic basin was closed and arc-continent collision was happened. Then, the Jiangda-Weixi belt was under an extension setting and formed the bimodal volcanism. During this period, the Garze-Litang Oceanic basin continued to spread. At the end of middle Triassic (-235), because of the closure of the residual sea or slab break-off, the study area was under an extension setting. The Yidun arc belt began to sink and deposited the Qugasi formation, and formed the angular unconformity between the upper Triassic sediments and middle-lower Triassic sediments. At 235-220 Ma, the Garze-Litang Oceanic basin began to subduct westward to generate the arc magma in the Yidun arc belt. However, the northern Yidun arc belt was an island arc while the southern one was a continental margin arc because of the existence of the Zhongza Massif. In the northern Yidun arc belt the oceanic slab roll-back to form the intra-arc rift and cause the magmatism growing younger to the trench, whereas the southern Yidun arc belt formed the adakite affinity volcanic rocks. At 220-216 Ma, the Garze-Litang oceanic basin was closed and subsequently slab break off and cause to the upwelling of the asthenosphere to form the Cuojiaoma-Dongcuo granitic belt, Ganluogou dioritic complex and the granites in the Songpan-Garze fold belt.At last, we also summarized the magmatism in the Paleo-Tethys orogen belt of southwestern China. Combined with the Paleomagnetic data, the evolutionary history of the study area could be summarized as follow:1. During late Cambrian to early Silurian, the continents in the study area began to separated from the Gandwana because of the opening of the Longmucuo-Shuanghu-Changning-Menglian oceanic basin, and the oceanic basin began to spread south to north at the end of Silurian.2. During late Devonian to early carboniferous, the Longmucuo-Shuanghu oceanic basin began to subduct to form the SSZ-type ophiolite. However there was no magmatism in the Langcangjiang area during this time. The Xijinwulan-Jinshajiang-Ailaoshan Oceanic basin was opened as a back arc basin, but the Ailaoshan Ocean open little earlier than the other two.3. During late carboniferous to early Permian, the Longmucuo-Shuanghu-Changning-Menglian oceanic basin continued to subduct northward, and at the end of this period, the Xijinwulan-Jinshajiang-Ailaoshan oceanic basin began to subduct westward.4. During the middle Permian to Late Triassic, the Longmucuo-Shuanghu-Changning-Menglian oceanic basin was closed between 250 Ma and 230 Ma. The Jinshajiang-Ailaoshan oceanic basin was closed at aroud 250 Ma, but the Xijinwulan-Jinshajiang oceanic basin may be closed at the end of the late Triassic. The Garze-Litang oceanic basin was opened due to the activity of the Emeishan plume and was closed at late Triassic (-216 Ma). Based on the discussion above, we can conclude that the magmatism in the eastern Jinshajiang belt and the Ailaoshan belt were comparable, they represent the same oceanic basin. Then to the western Jinshajiang belt, the magmatism in this belt was comparable to those of the eastern Jinshajiang belt before～250 Ma, but after 250 Ma it could be comparable to the Garze-Litang belt because of the existence of the Zhongza Massif. |
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