Research On Phanerozoic Structural Deformation And Geochronology In Wuyishan Area, South China

Based on researches of ductile shear zones, Mesozoic folds and faults as well as Phanerozoic igneous rocks in Wuyishan area, we studied the timing and structural deformational styles of Phanerozoic tectono-thermal events. In addition, we discussed the tectonic evolution of the Early Paleozoic orogen in South China, Indosinian structural deformation and geodynamics, and transformation of Mesozoic tectonic regime.The Precambrian strata are widespread in the Wuyishan area, South China. This study presents four zircon La-ICPMS U-Pb ages of meta-volcano-sedimentary rocks. The new data indicate that the Huangtan and Dutan formations of the Wanquan Group were deposited at 746.0±6.2 Ma and 825±18 Ma, respectively, the Dongyan Formation of the Mamianshan Group was deposited at 818±14 Ma, whereas the Dajinshan Formation of the Mayuan Group was deposited after 879 Ma. Three episodes of Neoproterozoic deposition are identified according to lithologic stratigraphic and geochronological data. The early episode is between 879 Ma and 825 Ma, which is characterized by terrigenous sedimentary rocks in t he Dajinshan Formation of the Mayuan Group and the Longbeixi Formation of the Mamianshan Group. The middle episode consists of the Nanshan Formation of the Mayuan Group, the Dongyan and the Daling Formation of the Mamianshan Group, the Wanquan and the Longquan groups, and the Jiaoxi, Dikou and Lower Taoxi formations. These deposits are dominated by volcanic-sedimentary rocks and can be further subdivide into two sub-episodes. The early sub-episodes is between 825 Ma and 800 Ma, whereas the later sub-episodes between 751Ma and 728 Ma. The late episode is between 620Ma and 542 Ma, which is characterized by the sedimentation of the main body of t he original Tianjinping Formation, the upper Taoxi Formation, and the lower Louziba Group.Ductile shear zones have been well documented in Pre-Devonian meta-volcanic and meta-sedimentary rocks in the Wuyishan area, South China. Field investigation shows that these shear zones are discontinuously distributed in the area from Wuping County to Jiangle County, and their strikes changes from NNE-trending, to NE-trending and nearly E-W-trending. Two stages ductile deformations had been found in these shear zones. The early deformation is top-to-SE thrust shearing, whereas the late is dextral strike-slip motion. Early Paleozoic gneissic granites and migmatites developed in the Wuping ductile shear zone are the result of syn-deformation crust anatexis. Zircon La-ICPMS U-Pb ages of three syn-orogeny gneissic granites are 440.8±4.8 Ma,435.6±6.2 Ma and 431.2±3.2 Ma, respectively. Moreover, the metamorphic growth rims of zircons in migmatite developed at 433.8±6.8 Ma. These aforementioned data shows that the early thrust shearing formed between 441 Ma and 431 Ma. These top-to-SE thrusting shear zones moved again in Indosinian period, and changed into dextral strike-slip shearing.40Ar/39Ar step-heating dating of biotite and muscovite separated from two mylonites yields ages of 235.3±2.8 Ma and 238.5±2.8 Ma, respectively; and an undeformed granitic dyke intruded into the shear zone was dated,229.8±2.2 Ma, by La-ICP-MS U-Pb zircon method. These dates reveal that dextral shearing developed at the time span between 230 Ma and 239 Ma, during the Triassic Indosinian tectono-thermal event. Deformation temperature is estimated in 400～500℃and the pressure between 7 kbar and 10 kbar by syn-mylonitization mineral assemblages, mineral deformation behavior and electron microprobe analysis on muscovite in mylonitic rocks.Geochronology of zircon La-ICPMS U-Pb method suggests that Weipu grantic pluton formed between 447.1±4.7 Ma and 440.8±3.4 Ma, and Changpu magmatite developed at 445.9±3.8 Ma, which indicating that a intense thermal event (447～441 Ma) occurred in Wuyishan area. Combining with Paleozoic angular unconformity, geochronology of granites and structural deformation, we thought that Early Paleozoic orogeny in South China occurred between 465 Ma and 420 Ma, followed by extension and collapse after 420 Ma.Syenite in Tieshan, Zhenghe County, formed in extensional environment, gives an age of 169.3±1.6 Ma, as the lower bound of Early Yanshanian event. While Bachi granitic gneiss in Pingyuan, northern Guangdong Province, formed in compressional environment, yields an age of 165.4±1.2 Ma. This indicates the main timing of deformation. In contrast,40Ar/39Ar plateau age of syn-deformation muscovite in Changting, western Fujian Province, is 162±2 Ma. This is interpreted as cooling age of the thrust shear, supplying the upper bound of Early Yanshanian event. This is also supported by the Hetian granite crystallized at 152.9±1.4 Ma, which intruded the Early Yanshanian NNE-trending folds in Changting basin. These above data indicated that Early Yanshanian event formed between 169 Ma and 162 Ma. From Late Jurassic to Early Cretaceous, a lot of faulted-depression basins developed in southeast China, filled mainly by volcanic rocks in Early Cretaceous. Volcanic rocks in Wuyishan area erupted between～141 Ma and 135.8±1.1 Ma by La-ICP-MS U-Pb zircon method. Paleostress field analyses of slip vectors from normal boundary faults and syn-deposition faults indicated that NW-SE-striking extension dominated in Early Cretaceous and tilled to～130 Ma. In late Early Cretaceous, structural compression led the Lower Cretaceous to be tilted and Changle-Nan'ao fault to ductile sinistral motion. Paleostress field analyses of slip vectors from Changle-Nan'ao fault showed that NW-SE-striking compression. The youngest strata involved in the shear zone were Early Cretaceous volcanic rock. And the sinistral fault was intruded by Shanxia granitic pluton (108±2 Ma), indicating that NW-SE-striking compression occurred between 130 Ma and 108 Ma. Thereafter, intense extension dominated southeast China again, and led to Late Cretaceous volcanic action and normal faulting. Two volcanic samples erupted in 100.4±1.5 and 97.6±1.1 Ma by La-ICP-MS U-Pb zircon method. Meanwhile, alkaline granite and mafic dykes developed in southeast China. The strike of mafic dykes and slip vectors analyses of boundary normal faults of Late Cretaceous basins indicated that southeast China suffered from NW-SE-trending extension. In short, the late Mesozoic tectonic evolution of southeast China was alternated between extension and compression, maybe as the result of the slab dip angle increased from a very low angle to a median angle, the rotation of slab direction, the slab break-off and roll-back.Systematic geological map compilation shows that the axes of Indosinian folds in South China strikes E-W-trending, whereas the axes of Yanshanian folds is NNE-NE-striking. Moreover, in Wuyishan area, Indosinian NE-striking ductile shear zones are characterized by dextral motion, while Yanshanian faults are featured by top-to-SE thrusting. The Indosinian nearly E-W-trending folds and dextral ductile shear zones are the products of N-S-striking continent-continent collision, whereas the Yanshanian NE-trending folds and top-to-SE faults are resulted from the subduction of Pacific plate underneath South China. Structural deformation, magmatism and geodynamics in Early Mesozoic are different that in Late Mesozoic, indicating that South China suffered from tectonic regime transformation in Mesozoic. Comprehensive research show that tectonic regime transformation occurred in Early Jurassic.