Incremental Emplacement of the Late Jurassic Midcrustal, Lopolith-Like Qitianling Pluton, South China, Revealed by AMS and Bouguer Gravity Data

International audienceModes of magma emplacement may provide key information on geodynamic-tectonic settings of magmatism, when the threshold of magma volume and duration of emplacement process is crossed, brittle/ductile crustal deformation behavior, and volumes and rates of magma transfer in the crust. The majority of upper-crustal granitic intrusions are incrementally emplaced as laccoliths or sills, while midcrustal or deeper-level granitic intrusions appear to be predominantly emplaced as sills or lopoliths, but relatively few examples have been studied. We present a case study on the Jurassic Qitianling pluton from South China, which was constructed in the upper part of the middle crust (at a depth of ~10-15 km). To characterize its emplacement mechanism and evolution, we have combined field and microscopic observations, Anisotropy of Magnetic Susceptibility measurements and Bouguer gravity anomaly modeling. The main findings are (1) the generally undeformed granite-country rock contacts with a narrow thermal aureole, (2) irregularly wavy and gradational contacts between the main granite facies, (3) dominant subhorizontal magnetic fabrics in the granites with a scattered magnetic foliation, (4) rare occurrence of dykes, and (5) a subcircular surface and lopolith-like pluton geometry. Accordingly, we suggest that the Qitianling pluton forms an ~25-km wide and ~5-km thick lopolith that was incrementally emplaced during a period of tectonic quiescence, most likely by under-accretion of multiple magma sheets deflected from magma pulses that ascended through pipe-like channels. The mineralogical and compositional variation of the pluton may be the result of in situ fractionation, fractionation at deeper crustal level, or variation in source-derived melt compositions

Cretaceous Episodic Extension in the South China Block, East Asia: Evidence From the Yuechengling Massif of Central South China

International audienceIn the Cretaceous, the subduction of the Izanagi/Paleo-Pacific plates beneath the South China Block (SCB) created a wide back-arc domain characterized by numerous extensional basins coeval with voluminous magmatism. The SCB witnessed the whole evolution by records of widespread extensional structures to accommodate the lithospheric stretching. In the interior of the SCB, the Yuechengling (YCL) Massif preserves a large, low-angle detachment fault, the Ziyuan Detachment (ZYD) at the western margin, and a high-angle ductile normal fault, the Tianhu Fault (THF), in the middle of the massif. Both faults display ductile shearing with top-to-the west kinematics but play different roles in two stages of extension. In the early stage at 140-120 Ma, the THF deformed the eastern YCL pluton at a temperature of 350 °C, but the ZYD shows limited movement at this time. On the contrary, the later stage (100-85 Ma) is characterized by pervasive middle- to high-temperature deformation ( 400-500 °C) and rapid exhumation along the ZYD, but the THF only underwent a near-surface brittle overprint. Across the SCB, the two-phase extension is widely recorded in other extensional structures and coincides with magmatic flare-ups at its eastern margin, suggesting episodic changes in the subduction dip. Combined with two compressional events that took place between the intervals of extension, the SCB experienced two cycles of compression-extension at 155-120 and 120-85 Ma. This periodicity is tentatively interpreted as a combined effect from the Izanagi/Paleo-Pacific subduction angle change and a thickening-foundering process in the arc region

Tectonostratigraphic history of the Ediacaran-Silurian Nanhua foreland basin in South China

This paper presents the tectonostratigraphic evolution of the Ediacaran-Silurian Nanhua Basin in South China and explores the relationship between clastic sedimentation in the basin and evolution of the adjacent Wuyi-Yunkai orogen. Sedimentary facies in the basin comprises, in an ascending order, turbiditic marine, shallow marine, and fluvial-dominated deltaic facies, featuring a lateral migration from southeast to northwest. We interpret the Ediacaran-Silurian Nanhua Basin as a foreland basin with a three-stage evolution history. Stage 1: the Ediacaran-Cambrian stage, recording the start of tectonic subsidence with turbiditic marine siliciclastic deposition, fed by exotic orogens outboard South China; Stage 2: the Ordovician to earliest-Silurian stage, characterized by a migrating depocenter with dominant shallow marine and deltaic siliciclastic deposition, fed by the local and northwestward propagating Wuyi-Yunkai orogen; Stage 3: the Silurian stage, showing the arrival of depocenter in the Yangtze Block during the waning stage of the orogeny with deltaic deposition in the remanent foreland basin. The Wuyi-Yunkai orogen remained the dominant sedimentary source region during Stage 3. Stage 1 was likely related to the collision of the South China Block toward northern India during the assembly of Gondwana, whereas Stages 2 and 3 recorded sedimentation during the northwestward propagation and subsequent orogenic root delamination/collapse of the Wuyi-Yunkai orogen, respectively. The Wuyi-Yunkai orogeny in South China is interpreted to have resulted from the far-field stress of the collision between South China and Indian Gondwana

Mid-Neoproterozoic angular unconformity in the Yangtze Block revisited: Insights from detrital zircon U-Pb age and Hf-O isotopes

Middle Neoproterozoic sedimentary rocks are widespread in the South China Block, especially around the periphery of the Yangtze Block. They are separated into the Sibao and Danzhou groups (and their equivalents) by a “mid-Neoproterozoic angular unconformity” in the south margin of the Yangtze Block. This unconformity, previously interpreted as the evidence of continental collision between the Yangtze and Cathaysia blocks, is crucial for understanding the early-middle Neoproterozoic tectonic regime in the South China Block. We report integrated in situ U–Pb, Hf and O isotopes analyses of detrital zircon grains from sandstones across this unconformity, and use these data to decipher their sedimentary provenances and shed new light on the basin evolution and the tectonic significance of this mid-Neoproterozoic unconformity.A total of 324 concordant U–Pb zircon analyses are obtained from three sandstone samples, including two from the Yuxi Formation of the Sibao Group and one from the Baizhu Formation of the Danzhou Group that are located below and above the unconformity, respectively. Detrital zircon grains from the lower part of the Yuxi Formation are dominated by a single age population peaked at ca. 830 Ma, providing a maximum depositional age for the formation. Zircon grains from the middle part of the Yuxi Formation and the lowest part of the Baizhu Formation have very similar, “multi-peaked” age patterns, with five distinctive peaks at ca. 840 Ma, ca. 940 Ma, ca. 1620 Ma, ca. 1850 Ma and ca. 2500 Ma, and zircon grains within the youngest age population are of the similar Hf–O isotopic features in the two samples, indicating that sediments across the mid-Neoproterozoic unconformity share similar sedimentary provenances and deposited within the similar tectonic setting. Appearance of abundant low-δ18O magmatic zircon grains from these sediments is coincident with regional intraplate anorogenic magmatism at ca. 850 Ma, suggesting that both the Sibao and Danzhou groups were most likely deposited in a continental rift basin. Thus, this study strongly argues against the orogenic origin of the regional mid-Neoproterozoic unconformity between them. Our new data suggest that the mid-Neoproterozoic unconformity is likely attributed to a rapid regional crust uplifting (associated with widespread mafic and granitic magmatism), unroofing and basin subsidence related to the proposed mantle plume activity at ca. 825 Ma

Detrital provenance evolution of the Ediacaran–Silurian Nanhua foreland basin, South China

We report here in-situ U–Pb and Hf isotopic results of detrital zircons from sixteen Cambrian–Silurian siliciclastic samples across the Nanhua foreland basin, South China. Together with published data from Ediacaran–Silurian sandstones in the region, we establish the temporal and spatial provenance evolution across the basin. Except for samples from northeast Yangtze, all other Ediacaran–Silurian samples exhibit a prominent population of 1100–900 Ma, moderate populations of 850–700 Ma and 650–490 Ma, and minor populations of 2500 Ma and 2000–1300 Ma, grossly matching that of crystalline and sedimentary rocks in northern India. Zircon Hf isotopes further reveal four episodes of juvenile crustal growth at 2.5 Ga, 1.8 Ga, 1.4 Ga and 1.0 Ga in the source regions. Utilizing the basin history and late Neoproterozoic to early Paleozoic paleogeography of South China, we conclude that the Ediacaran–Cambrian sediments in the Nanhua foreland basin were mainly sourced from northern India and adjacent orogens, and the Ordovician–Silurian sediments were derived from both locally recycled Ediacaran–Cambrian rocks and eroded Cathaysian basement. The Wuyi–Yunkai late-orogenic magmatic rocks also contributed to the Silurian sediments in the basin. The upper-Ordovician to Silurian samples in northeast Yangtze received higher proportions of local Cryogenian (850–700 Ma) magmatic rocks which were uplifted during late-Ordovician to Silurian time. We speculate that there was an Ediacaran–Cambrian collisional orogen between South China and northern India, shedding sediments to the early Nanhua foreland basin. Far-field stress during the late stage of this collisional orogeny triggered the Ordovician–Silurian intraplate Wuyi–Yunkai orogeny in South China, and erosion of the local Wuyi–Yunkai orogen further provided detritus to the late Nanhua foreland basin