Research Progress on Cenozoic Volcano Genesis and Fluid Action in Northeast China

The tectonic evolution of northeast China is closely related to the subduction of the Pacific plate. The dehydration of the slab subduction process produces metasomatic agents that have important effects on the physical and chemical properties of the mantle wedge, including the decrease of seismic wave velocity and the increase of Poisson’s ratio and electrical conductivity. In order to investigate the tectonic evolution and fluid action of northeast China, this paper compares the previous seismic and electromagnetic imaging results of northeast China and explores the relationship between the genesis of Cenozoic volcanoes and fluid action in northeast China through rheological analysis. The results show that the western Pacific plate subducted into the mantle transition zone beneath northeast China, and sustained dehydration occurred. The upward migration of these released water caused partial melting at the base of the upper mantle. Some of the upwelling streams pierced the weak tectonic boundary under the buoyancy effect, which finally formed the large-scale Cenozoic volcanic events in northeast China.

Detrital zircon geochronology of Late Cretaceous successions in the Ganzhou basin, South China: Evidence of a major tectonic transition

Situated within the southern segment of the South China Block (SCB), the Ganzhou Basin formed due to subduction of the paleo-Pacific plate beneath to the SCB. Late Cretaceous successions in this basin consist of fluvial and lacustrine facies red beds hosting abundant dinosaur and dinosaur egg fossils. This study reports detrital zircon geochronological data from a crystallized tuff and four sandstones found in the Late Cretaceous Ganzhou Group of the Ganzhou Basin. Age distributions included four major age subpopulations of predominantly Triassic, Devonian-Ordovician, Neoproterozoic and Paleoproterozoic ages. These indicate source material derived from Yanshanian and Triassic granitoids as well as from Kwangsian and Jiangnan orogens. Age signatures generally resemble those recorded in the adjacent Nanxiong Basin but also include distinctive features. Provenance signatures from successive units indicate a tectonic transition from intracontinental extension at ∼120 Ma to compression near the Cretaceous/Paleogene boundary. This tectonic transition was probably driven by continent-continent collision between the Indian and Eurasian plates, as well as by a shift in the subduction direction of the paleo-Pacific plate beneath the Eurasian plate.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5776518

THE INTRAPLATE VOLCANO-TECTONIC ACTIVITY IN NORTH-EASTERN AND SOUTH SECTORS OF THE PACIFIC LITHOSPHERIC PLATES WITH THE CONNECTION OF THE CHANGE OF ITS RELATIVE MOTION

The analysis of altimetric data in combination with bathymetry and gravimetry materials in the north-eastern and southern sectors of the Pacific Ocean, as well as detailed data on the underwater relief, the structure of the sedimentary cover, the composition and absolute age of basalts obtained within the area of domestic geological exploration for ferromanganese nodules (the Clarion-Clipperton zone) is carried out. Structural trends formed by local cone-shaped local structures of presumably volcanic nature, grouped along transform faults belonging to various stages of the kinematics of the Pacific Plate, have been traced in the structure of the oceanic lithosphere at various scale levels. The first trend corresponds to the extension of the fault system corresponding to the spreading system on the crest of the East Pacific rise before the restructuring of its planned geometry in the Paleocene-Eocene, the second coincides with their extension after the change in the relative movement of the Pacific Plate. The trends are characterized by planned disagreement, and an increase in the number of seamounts is observed in the areas of their intersection. Within the area of detailed studies, obvious signs of volcanic-tectonic activity were revealed: high dissection of the underwater relief, hills of different heights with steep slopes, whose volcanic nature is confirmed by differentiated basalts raised from their slopes, the absolute age of which indicates the multistage outpourings that occurred in an intraplate environment. The angular velocity of rotation of the spreading axis and the linear velocity of its advance with changes in the kinematics of the Pacific plate are estimated and possible reasons for changes in its relative motion are considered. An improved scheme of adaptation of the spreading zone to a change in the direction of relative plate movement is proposed, acc0ording to which an essential factor of intraplate volcanic-tectonic activity is the relaxation of stresses in the plate caused by external influence on it.

Late Cretaceous granitoids of the Sikhote–Alin orogenic belt, southeastern Russia: implications for the Mesozoic geodynamic history of the eastern Asian continental margin

We present new U–Pb age data for granitoids in the Central Sikhote–Alin orogenic belt in SE Russia, which refute the established opinion about the absence of the Late Cretaceous magmatism at the eastern margin of the Paleo-Asian continent. It was previously thought that a period of magmatic quiescence occurred from 88 to 50 Ma, related to subduction of the Paleo-Pacific Plate under the eastern margin of the Paleo-Asian continent, although this is inconsistent with evidence from the Sikhote–Alin, Sakhalin, and Japan regions. Three suites of plutonic rocks with different ages were identified in this study. The first suite has ages of 105–92 Ma and formed in a syn-orogenic setting. The second (86–83 Ma) and third (ca. 73 Ma) suites formed during the post-orogenic stage of the Sikhote–Alin orogenic belt. The second and third suites were coeval with Late Cretaceous granitoids that formed in a suprasubduction continental arc known as the Eastern Sikhote–Alin volcanic–plutonic belt (ESAVPB). However, the studied rocks are located far inland from the ESAVPB. The ages of the studied granitoids coincide with the timing of a change in the angle of convergence between the Paleo-Pacific Plate and eastern margin of the Paleo-Asian continent. This change in motion of the oceanic plate with respect to the continental plate was probably caused by a rupture in the subducted slab (i.e., a slab tear), followed by asthenospheric upwelling and partial melting of the overlying crust, which ultimately generated post-orogenic intrusive magmatism.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5738616

The Volcano‐Tectonic Evolution of the Macquarie Ridge Complex, Australia‐Pacific Plate Boundary South of New Zealand

<p>The Macquarie Ridge Complex (MRC) forms the submarine expression of the Australia‐Pacific plate boundary south of New Zealand, comprising a rugged bathymetry made up of numerous seamounts along its length. Tectonic plate reconstructions show that the plate boundary evolved from divergent to transpressional relative plate motion from ca. 40 – 6 Ma. However, only limited geological observation of the products of past seafloor spreading and present transpressional deformation has been achieved. This study presents new high-resolution multibeam, photographic, petrologic and geochemical data for 10 seamounts located along the MRC in order to elucidate the current nature and evolution of the plate boundary. Seamounts are oriented parallel to the plate boundary, characterized by elongate forms, and deformed by transform faulting. Three guyot‐type seamounts display summit plateaux that were formed by wave and current erosion. MRC seafloor is composed of alkaline to sub‐alkaline basaltic pillow, massive and sheet lava flows, lava talus, volcaniclastic breccia, diabase and gabbro. This oceanic crust was formed during effusive mid‐ocean ridge volcanism at the relic Macquarie spreading centre and has since been sheared, accreted and exhumed along the modern transpressional plate boundary. Major element systematics indicate samples originated from spatially distinct magmatic sources and have since been juxtaposed at seamounts due to transpressional relative plate motion. MRC seamounts have formed as discrete elevations as a result of dip‐slip and strike‐slip faulting of the ridge axis. Thus, MRC seamounts are volcanic in origin but are now the morphological manifestations of tectonic and geomorphic processes. Petrologic and geochemical characteristics of volcanic glass samples from the MRC indicate that both effusive and explosive eruption styles operated at the relic Macquarie spreading centre. Primitive and sub‐alkaline to transitional basaltic magma that rose efficiently to the seafloor was erupted effusively and cooled to form lava flows with low vesicle and phenocryst contents or was granulated on contact with seawater to form hyaloclasts deposited in volcaniclastic breccias. More alkaline magmas that underwent crystal fractionation and volatile exsolution in shallow reservoirs were fragmented and erupted during submarine hawaiian‐type eruptions. Such a scenario is likely to have occurred during the final stages of magmatism at the Australia‐Pacific plate boundary south of New Zealand when seafloor spreading was ultraslow or had ceased, which induced low degrees of partial melting and retarded magma ascent rates. All MRC samples display enriched mid‐ocean ridge basalt (E‐MORB) trace element characteristics. The sample suite can be divided into two groups, with Group 1 samples distinguished from Group 2 samples by their lower concentrations of highly incompatible trace elements, flatter LREE slopes, higher MgO contents and lower alkali element contents. Group 1 basalts were derived from low degree partial melting of spinel lherzolite generated during the late stages of mid‐ocean ridge volcanism at the plate boundary when seafloor spreading rates were slow to ultraslow (full spreading rate < 20 mm yr⁻¹). Group 2 basalts were derived from low degree partial melting of spinel lherzolite, mixed with small amounts of very low degree partial melting of garnet lherzolite, during post‐spreading volcanism at the MRC. Remnant heat from previous seafloor spreading induced buoyant ascent of the sub‐ridge mantle and enriched heterogeneities were preferentially tapped by the ensuing low melt fractions. Magma ascent was stalled due to the cessation of extension at the ridge and the melts underwent crystal fractionation prior to eruption, which accounts for the lower MgO contents of Group 2 basalts. The pervasive incompatible element‐enrichment of MRC basalts and similarity to lavas from fossil spreading ridges in the eastern Pacific Ocean may reflect regional enrichment of the Pacific upper mantle.</p>