U-Pb Geochronology of Detrital and Inherited Zircons in the Yidun Arc Belt, Eastern Tibet Plateau and Its Tectonic Implications

This paper reports geochronological data of detrital zircons from the country rock and sedimentary xenoliths of the Cilincuo pluton（79±0.7 Ma） in the southern Yidun arc belt and the inherited zircons from the Late Triassic granites in the eastern Yidun arc belt, eastern Tibet Plateau. Detrital zircons ages from the sedimentary xenoliths have four prominent peaks at 2.5–2.4 Ga, 1.9–1.8 Ga, 480–400 Ma, and 350–300 Ma, whereas those from the country rock exhibit another four prominent peaks at 1.9–1.8 Ga, 850–700 Ma, 480–400 Ma, and 300–250 Ma. Based on comparison with age data from previous studies, we suggest that the sedimentary xenoliths are from the Lanashan Formation and the major provenance of them is Qiangtang Block, Zhongza massif and South China Block, whereas the country rock belongs to the Lamaya Formation and the major provenance of them is similar to those of the neighbouring Songpan-Garzê terrane. In addition, the inherited zircons from the Late Triassic granites in the eastern Yidun arc belts have a prominent Neoproterozoic age population（900–700 Ma）, which suggests that there is an old basement with west Yangtze Craton affinity beneath the Triassic sediments. Combining with previous studies, we propose that the provenances of the formations vary from the Lanashan Formation to the Lamaya Formation which may indicate a record of the final closure of the Garzê-Litang Ocean.

Heterogenous zircons from Mesozoic igneous rocks in the North China Craton and their tectonic implications

Based on a synthesis of zircon inheritance from Mesozoic igneous rocks in the eastern portion of the North China Craton （NCC） , it is proposed that inherited zircons with Neoproterozoie ages identified from these rocks are of a heterogenous derivation from the Yangtze/South China block, rather than from the NCC itself. The mechanism that introduces these zircons incorporated into the NCC is likely by tectonic underplating during the Triassic continental subduction of the Yangtze block beneath the NCC. Tectonic addition of abundant crustal materials represented by the heterogenous zircons into the NCC, probably along Moho or weak interfaces within the NCC＇s crust, led to the crustal thickening in the NCC. These heterogenous materials, either as （partial） source rocks or as contaminants of the magmas generated during an extension environment following the crustal thickening, were reworked and therefore have significant contribution to petrogenesis of the Mesozoic igneous rocks. The crustal thickening resulted from the tectonic underplating, as indicated by the distribution extent of the heterogenous zircons, is spatially similar to that of the lithospheric thinning, with both mainly occurring in the eastern segment of the NCC. This is probably suggestive of an intrinsic relation between the thickening and thinning events during the Mesozoic evolution of the NCC.

Southwest Boundary of Yangtze and Cathaysia Blocks:Constraints from the Luojiashan Gabbro in Yingyangguan Region,Northeastern Guangxi

The Luojiashan gabbro is a newly discovered mafic pluton emplaced in the Neoproterozoic Yingyangguan Formation in Yingyangguan region of northeastern Guangxi,South China.Comprehensive whole-rock geochemical and zircon geochronological and Hf isotopic analyses are performed on the gabbro and comparisons are made with the coeval mafic-ultramafic sills and dikes located in Longsheng region of northern Guangxi in order to understand the magmatic origin,evolution,and tectonic setting of the Luojiashan gabbro and to address the location of the suture zone of the Southwestern Jiangnan Orogen(SJO).LA-ICP-MS zircon U-Pb ages obtained from the Luojiashan gabbro show that it was emplaced at~770 Ma.The gabbro contains abundant inherited zircons aged at 0.9-1.3 Ga,consistent with age spectrum of the Cathaysia Block.Chondrite-normalized REE pattern,primitive mantle-normalized trace element spider diagram,incompatible element ratios of Nb/Ta,Zr/Hf,La/Nb,Ba/Th,Th/La,and Ba/La,and Th/Yb-Ta/Yb discrimination diagram of the gabbro are indicative of OIB-like geochemical characteristics and of derivation from partially melted garnet peridotite of the asthenospheric mantle.Tectonic discrimination based on the trace and rare earth elements also indicate that the Luojiashan gabbro was emplaced in a within-plate extensional rift setting,probably as a result of Rodinia supercontinent dismantling,lithospheric thinning,and underplating and upwelling of the asthenospheric mantle.Based on zircon age,Hf isotopic data and comparison between northern and northeastern Guangxi,it is suggested that the Yingyangguan region was tectonically situated in a different tectonic locale from the Longsheng region of northern Guangxi at about 770 Ma during the post-orogenic mafic-ultramafic magmatic event,with the former within the Cathaysia Block and the latter along the southeast margin of Yangtze Block.The suture between the Yangtze and Cathaysia blocks must be located between Yingyangguan of northeastern Guangxi and Longsheng of northern Guangxi.