Touping Peng

Geochemistry of Mesozoic mafic rocks adjacent to the Chenzhou-Linwu fault, South China : implications for the lithospheric boundary between the Yangtze and Cathaysia Blocks

To constrain the Mesozoic tectonic evolution and the lithospheric boundary between the Yangtze and Cathaysia blocks in South China, we present geochronological and geochemical data for Mesozoic basaltic lavas and related mafic dikes west (Group 1) and east (Group 2) of the Chenzhou-Linwu fault. Three episodes of mafic magmatism around the Chenzhou-Linwu fault were identified: ca.175 Ma, 125-150 Ma, and 80-95 Ma, respectively. Group 1 rocks (alkaline basanite and trachybasalt), with ages of >125 Ma, have a wide range of 87Sr/86Sr(t) values (0.7035-0.7069), and εNd(t) values (-3.75 to + 6.10). In contrast, Group 2 rocks (subalkaline basalt and basaltic andesite), with ages of > 125 Ma, exhibit 87Sr/86Sr(t) values of 0.7075-0.7087 and εNd(t) values of -2.04 to + 1.05. Both groups are strongly enriched in incompatible elements, with variable negative Nb-Ta anomalies. However, Group 1 rocks commonly have higher LREE and Ba/Nb, Rb/Nb, Ba/Th, and Ba/La ratios and lower Th/Nb, Th/La, and Zr/Nb ratios than Group 2 rocks. Rocks with ages of 80-95 Ma from both groups have very similar elemental and isotopic compositions (87Sr/86Sr(t) = 0.7033-0.7052, εNd(t) = +3.99 to + 8.00), consistent with those of OIB. Strong coupling between incompatible elemental ratios and isotopes suggests that Group 1 rocks might have been derived from an EMI-like continental lithospheric mantle with an OIB source. In contrast, Group 2 rocks come from an EMII-like mantle source contaminated by an OIB component. We conclude that Mesozoic mafic rocks with ages of >125 Ma originated chiefly from an enriched lithospheric mantle heated by ascending asthenosphere, whereas the mafic rocks with ages of ca. 80-95 Ma were derived from upwelling asthenospheric mantle in response to intra-continental lithospheric extension in the South China interior. The spatial variations of EMI-and EMII-like source signatures for Mesozoic mafic rocks around the Chenzhou-Linwu fault suggest that the fault represents the Mesozoic lithospheric boundary between the Yangtze and Cathaysia blocks. The Jinxian-Anhua fault was only a near-surface boundary between the sutured blocks. The crust of the Cathaysia block might have been thrust westward over the Yangtze block with a displacement of >400 km at a time no later than ca. 175 Ma. A model for crustal detachment collision (>ca. 175 Ma) and subsequent intra-continental lithospheric extension (175-80 Ma) is proposed for the Mesozoic tectonic evolution of South China.

Late Permian-Triassic magmatic evolution in the Jinshajiang orogenic belt, SW China and implications for orogenic processes following closure of the Paleo-Tethys

Growth of continental crust involves the complex interplay of subduction zone magmatism, to generate the crust, followed by stabilization through crustal thickening, magmatism and ultimately isolation from the active plate margin. The Jinshajiang orogenic belt, SW China, provides an exceptional record of continental development as the result of closure of the Paleo-Tethys seaway and ensuing collision. A compilation of U-Pb age and geochemical data for the plutonic and volcanic rocks within the southern part of the Jinshajiang orogenic belt, including new high-precision ages for the Ludian granitoid batholith of 231 to 220 Ma, enables us to explore the interaction between magmatism and orogeny in the context of the Paleo-Tethys closure and continental amalgamation. These age and geochemical constraints, in conjunction with other geologic evidence, suggest that subduction of the Paleo-Tethys ocean dominated local tectonics prior to the Triassic, creating a volcano–plutonic arc along the eastern margin of the Qamdo-Simao terrane. Following consumption of the ocean, collision zone magmatism, dated at 247 to 237 Ma, was manifested by eruption of voluminous volcanic rocks in a suture-parallel zone. Crustal anatexis was contemporaneous with the earliest phases of collision, producing high-silica rhyolites of Early Triassic age (ca. 247-246 Ma). Between 245 and 237 Ma, the local tectonic regime switched from compression to extension, probably due to strain partitioning caused by oblique convergence, which led to the development of rift-basins and extensive syn-tectonic bimodal volcanism associated with deep-water sediments. From 234 Ma to 214 Ma, the emplacement of high-K, calc-alkaline granodiorites-monzogranites occurred prior to, or during, isostatic uplift and extension, probably caused by breakoff of the subducted slab. The resultant exhumation brought deep-seated granitoid batholiths to the surface, and was contemporaneous with intrusion of ultramafic-mafic melts. Ophiolitic mélange (ca. 362-294 Ma) and collision-related magmatic suites (247-214 Ma) are unconformably overlain by a Late Triassic (229-217 Ma) conglomerate-rich sequence that represents an overlap assemblage, across the Qamdo-Simao terrane (Indochina) and Yangtze Block of South China.

U-Pb geochronology and geochemistry of the Dashibao Basalts in the Songpan-Ganzi Terrane, SW China, with implications for the age of Emeishan volcanism

Permian marine basalts (the Dashibao Formation) in the Songpan-Ganzi Terrane to the west of the Yangtze Block, SW China, yield a SHRIMP zircon U-Pb weighted mean age of 263 ± 2 Ma. The Dashibao basalts are characterized by high TiO2 contents (1.73-4.65 wt. %) and Ti/Y ratios with a mean of 577, and OIB-like rare earth element (REE) and incompatible element patterns. Geochemical variation within the basalt succession allows division into two groups; Group 1 with an alkaline composition is distinguished by higher TiO2 and P2O5 contents, along with higher Ti/Y and Sm/Yb ratios than the underlying Group 2 that consists predominantly of tholeiitic lavas. Both groups possess weakly to moderately positive εNd(t) values (0.82 to 5.28), but the Group 2 tholeiitic basalts show relatively depleted signatures (most εNd(t) >2.5) when compared to their Group 1 counterparts (εNd(t) <2.5). REE modeling is consistent with variable degrees of melting of primitive mantle within the garnet stability field, and reveals that the Group 1 alkaline basalts could have been generated by lower degrees of melting (5-11%) than the Group 2 tholeiites (up to 19%). The initial Nd isotope discrepancy is interpreted in terms of depleted asthenospheric involvement in the early stage Group 2 tholeiitic magma. Combined geochronology, petrography and geochemistry for the Dashibao Formation confirms that it was temporally and genetically associated with the Emeishan basalts, and is therefore an integral part of the Emeishan large igneous province. The new zircon U-Pb dating supports the view that the Emeishan volcanism could be a boundary event occurring at or around the Middle-Late Permian (the Guadalupian-Lopingian) transition, and thereby confirms the validity of a causal connection with the end-Guadalupian mass extinction.

SHRIMP zircon U-Pb geochronology of Indosinian granites in Hunan Province and its petrogenetic implications

The SHRIMP zircon U-Pb geochronology of three typically Indosinian granitic plutons with peraluminous and potassium-rich affinities (Tangshi ultraunit for Weishan and Baimashan, and Longtan ultraunit for Guandimiao) is presented in Hunan Province, South China. The analyses of zircons from biotite monozonite granites for Weishan, Baimashan and Guandimiao plutons show the single and tight clusters on the concordia, and yield the weighted mean 206Pb/238U ages of 244 ± 4, 243 ± 3 and 239 ± 3 Ma, respectively, representing the crystallized ages of these Indosinian granites. These data suggest that the Indosinian granitic plutons as previously thought formed at a narrow age span. In combination with other data, it is inferred that the Indosinian granites within the South China Block probably distributed in Hunan, Jiangxi, Guangxi and Guangdong provinces as planar shape, and were the derivation of the crustal materials in the intracontinental thickening setting. These precisely geochronological data provide important constraints for better understanding the spatiotemporal pattern of the Indosinian peraluminous granites and early Mesozoic tectonic evolution of the South China Block.