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 TiO~2~ 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 TiO~2~ and P~2~O~5~ 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 ε~Nd~(t) values (0.82 to 5.28), but the Group 2 tholeiitic basalts show relatively depleted signatures (most ε~Nd~(t) \>2.5) when compared to their Group 1 counterparts (ε~Nd~(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.
Palaeocene (c. 55–58 Ma) adakitic andesites from the Yanji area, NE China, are typically clinopyroxene-bearing sodic andesites containing 60· 9–62· 2% SiO2 and 4· 02–4· 36% MgO, with high Mg-number [100 Mg/(Mg + ΣFe) atomic ratio] from 65· 5 to 70· 1. Whole-rock geochemical features include high Cr (128–161 ppm) and Ni (86–117 ppm) concentrations, extremely high Sr (2013–2282 ppm), low Y (10–11 ppm) and heavy rare earth elements (HREE; e.g. Yb = 0· 79–1· 01 ppm), and mid-ocean ridge basalt (MORB)-like Sr–Nd–Pb isotopic compositions [e.g. 87Sr/ 86Sr(i) = 0· 70298–0· 70316, εNd(t) = +3· 8 to +6· 3 and 206Pb/ 204Pb = 17· 98 – 18· 06], analogous to high-Mg adakites occurring in modern subduction zones. However, mineralogical evidence from clinopyroxene phenocrysts and microcrystalline plagioclase clearly points to magma mixing during magma evolution. Iron-rich clinopyroxene (augite) cores with low Sr, high Y and heavy REE contents, slightly fractionated REE patterns and large negative Eu anomalies probably crystallized along with low-Ca plagioclase from a lower crustal felsic magma. In contrast, high Mg-number clinopyroxene (diopside and endiopside) mantles and rims have higher Sr and lower HREE and Y concentrations, highly fractionated REE patterns (high La/Yb) and negligible Eu anomalies, similar to those found in adakites from subduction zones. The Yanji adakitic andesites can be interpreted as a mixture between a crust-derived magma having low Mg-number and Sr, and high Y and HREE, and a mantle-derived high Mg-number adakite having high Sr and low Y and HREE concentrations. During storage and/or ascent, the mixed magma experienced further crustal contamination to capture zircons, of a range of ages, from the wall rocks. The absence of coeval arc magmatism and an extensional tectonic regime in the Yanji area and surrounding regions suggest that these Palaeocene adakitic andesites were formed during post-subduction extension that followed the late Cretaceous Izanagi–Farallon ridge subduction. Generation of these adakitic andesites does not require contemporaneous subduction of a young, hot oceanic ridge or delamination of eclogitic lower crust as suggested by previous models.
In this paper,taking Shenzhen Relief Map as an example,the basic theory and process of automated relief shading are introduced,and its merits are also discussed.
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.
Abstract Magmatic processes that occur during the transition from oceanic to continental subduction and collision in orogens are critical and still poorly resolved. Oceanic slab detachment in particular is hypothesized to mark a fundamental change in magmatism and deformation within an orogen. Here, we report on two Quaternary volcanic centers of Myanmar that may help us better understand the process of slab detachment. The Monywa volcanic rocks are composed of low‐K tholeiitic, medium‐K calk‐alkaline, and high‐K to shoshonitic basalts with arc signatures, while the Singu volcanic rocks show geochemical characteristics similar to asthenosphere‐derived magmas. These volcanic rocks have low Os concentrations but extremely high 187 Os/ 186 Os i ratios (0.1498 to 0.3824) due to minor (<4%) crustal contamination. The Monywa arc‐like rocks were generated by small degrees of partial melting of subduction‐modified asthenospheric mantle at variable depths from the spinel to garnet stability fields. Distinct from the Monywa arc‐like rocks ( 87 Sr/ 86 Sr i = 0.7043 to 0.7047; ε Nd i = +2.3 to +4.7), the Singu OIB‐like rocks exhibit higher 87 Sr/ 86 Sr i (0.7056 to 0.7064) and lower ε Nd i (+0.8 to +1.6) values. These isotopic characteristics indicate a large contribution of an isotopically enriched asthenosphere layer beneath the Burmese microplate, which possibly flowed from SE Tibet. We interpret that this short‐lived, small‐scale, and low‐degree melting Quaternary volcanism in Myanmar was triggered by its position above a slab window resulting from the tearing of the oceanic lithosphere from buoyant continental lithosphere of the Indian plate.
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