A-type granites induced by a breaking-off and delamination of the subducted Junggar oceanic plate,West Junggar,Northwest China

The A-type granites with highly positiveε_(Nd)(t)values in the West Junggar,Central Asian Orogenic Belt(CAOB),have long been perceived as a group formed under the same tectonic and geodynamic setting,magmatic sourceq and petrogenetic model.Geological evidence shows that these granites occurred at two different tectonic units related to the southeastern subduction of Junggar oceanic plate:the Hongshan and Karamay granites emplaced in the southeast of West Junggar in the Baogutu continental arc;whereas the Akebasitao and Miaoergou granites formed in the accretionary prism.Here the authors present new bulk-rock geochemistry and Sr-Nd isotopes,zircon U-Pb ages and Hf-O isotopes data on these granites.The granites in the Baogutu continental arc and accretionary prism contain similar zirconε_(Hf)(t)values(+10.9 to+16.2)and bulk-rock geochemical characteristics(high SiO_(2)and K_(2)O contents,enriched LILEs(except Sr),depleted Sr,Ta and Ti,and negative anomalies in Ce and Eu).The Hongshan and Karamay granites in the Baogutu continental arc have older zircon U-Pb ages(315-305 Ma)and moderate^(18)O enrichments(δ^(18)_(O_(zircon))=+6.41‰-+7.96‰);whereas the Akebasitao and Miaoergou granites in the accretionary prism have younger zircon U-Pb ages(305-301 Ma)with higher^(18)O enrichments(δ^(18)_(O_(zircon))=+8.72‰-+9.89‰).The authors deduce that the elevated^(18)O enrichments of the Akebasitao and Miaoergou granites were probably inherited from low-temperature altered oceanic crusts.The Akebasitao and Miaoergou granites were originated from partial melting of low-temperature altered oceanic crusts with juvenile oceanic sediments below the accretionary prism.The Hongshan and Karamay granites were mainly derived from partial melting of basaltic juvenile lower crust with mixtures of potentially chemical weathered ancient crustal residues and mantle basaltic melt(induced by hot intruding mantle basaltic magma at the bottom of the Baogutu continental arc).On the other hand,the Miaoergou charnockite might be sourced from a deeper partial melting reservoir under the accretionary prism,consisting of the low-temperature altered oceanic crust,juvenile oceanic sediments,and mantle basaltic melt.These granites could be related to the asthenosphere's counterflow and upwelling,caused by the break-off and delamination of the subducted oceanic plate beneath the accretionary prism Baogutu continental arc in a post-collisional tectonic setting.

Early Triassic Silicic Volcanics of South China:Petrogenesis and Constraints on the Geodynamic Evolution of the Paleo-Tethys Ocean Region

The only occurrence of Lower Triassic silicic volcanic rocks within the South China Block is in the Qinzhou Bay area of Guangxi Province.LA-ICP-MS zircon U-Pb dating reveals that volcanic rocks of the Beisi and Banba formations formed between 248.8±1.6 and 246.5±1.3 Ma,coeval with peraluminous granites of the Qinzhou Bay Granitic Complex.The studied rhyolites and dacites are characterized by high SiO_(2),K_(2)O,and Al_(2)O_(3),and low MgO,CaO,and P_(2)O_(5) contents and are classified as high-K calc-alkaline S-type rocks,with A/CNK=0.98-1.19.The volcanic rocks are depleted in high field strength elements,e.g.,Nb,Ta,Ti,and P,and enriched in large ion lithophile elements,e.g.,Rb,K,Sr,and Ba.Although the analyzed volcanic rocks have extremely enriched zircon Hf isotopic compositions(ε_(Hf)(t)=-29.1 to-6.9),source discrimination indicators and high calculated Ti-in-zircon temperatures(798-835℃)reveal that magma derived from enriched lithospheric mantle not only provided a heat source for anatectic melting of the metasedimentary protoliths but was also an endmember component of the S-type silicic magma.The studied early Triassic volcanics are inferred to have formed immediately before closure of the Paleo-Tethys Ocean in this region,as the associated subduction would have generated an extensional setting in which the mantle-derived upwelling and volcanic activity occurred.

Early Cenozoic Tectonics of the Tibetan Plateau

Geological mapping at a scale of 1：250000 coupled with related researches in recent years reveal well Early Cenozoic paleo-tectonic evolution of the Tibetan Plateau. Marine deposits and foraminifera assemblages indicate that the Tethys-Himalaya Ocean and the Southwest Tarim Sea existed in the south and north of the Tibetan Plateau, respectively, in Paleocene-Eocene. The paleo- oceanic plate between the Indian continental plate and the Lhasa block had been as wide as 900km at beginning of the Cenozoic Era. Late Paleocene transgressions of the paleo-sea led to the formation of paleo-bays in the southern Lhasa block. Northward subduction of the Tethys-Himalaya Oceanic Plate caused magma emplacement and volcanic eruptions of the Linzizong Group in 64.5-44.3 Ma, which formed the Paleocene-Eocene Gangdise Magmatic Arc in the north of Yalung-Zangbu Suture （YZS）, accompanied by intensive thrust in the Lhasa, Qiangtang, Hoh Xil and Kunlun blocks. The Paleocene- Eocene depression of basins reached to a depth of 3500-4800 m along major thrust faults and 680-850 m along the boundary normal faults in central Tibetan Plateau, and the Paleocene-Eocene depression of the Tarim and Qaidam basins without evident contractions were only as deep as 300-580 m and 600-830 m, respectively, far away from central Tibetan Plateau. Low elevation plains formed in the southern continental margin of the Tethy-Himalaya Ocean, the central Tibet and the Tarim basin in Paleocene-Early Eocene. The Tibetan Plateau and Himalaya Mts. mainly uplifted after the Indian- Eurasian continental collision in Early-Middle Eocene.

Early Cretaceous Tectonics and Evolution of the Tibetan Plateau

Selected geological data on Early Cretaceous strata, structures, magmatic plutons and volcanic rocks from the Kunlun to Himalaya Mountains reveal a new view of the Early Cretaceous paleo-tectonics and the related geodynamic movement of the Tibetan Plateau. Two major paleo- oceans, the Mid-Tethys Ocean between the Qiangtang and Lhasa blocks, and the Neo-Tethys Ocean between the Lhasa and Himalayan blocks, existed in the Tibetan region in the Early Cretaceous. The Himalayan Marginal and South Lhasa Seas formed in the southern and northern margins of the Neo- Tethys Ocean, the Central Tibet Sea and the Qiangtang Marginal Sea formed in the southern and northern margins of the Mid-Tethys Ocean, respectively. An arm of the sea extended into the southwestern Tarim basin in the Early Cretaceous. Early Cretaceous intensive thrusting, magmatic emplacement and volcanic eruptions occurred in the central and northern Lhasa Block, while strike- slip formed along the Hoh-Xil and South Kunlun Faults in the northern Tibetan region. Early Cretaceous tectonics together with magmatic K20 geochemistry indicate an Early Cretaceous southward subduction of the Mid-Tethys Oceanic Plate along the Bangoin-Nujiang Suture which was thrust ~87 km southward during the Late Cretaceous-Early Cenozoic. No intensive thrust and magmatic emplacement occurred in the Early Cretaceous in the Himalayan and southern Lhasa Blocks, indicating that the spreading Neo-Tethys Oceanic Plate had not been subducted in the Early Cretaceous. To the north, terrestrial basins of red-beds formed in the Hoh-Xil, Kunlun, Qilian and the northeastern Tarim blocks in Early Cretaceous, and the Qiangtang Marginal Sea disappeared after the Qiangtang Block uplifted in the late Early Cretaceous.

Tectonic Related Lithium Deposits Another Major Region Found North East Tanzania—A New Area with Close Association to the Dominant Areas: The Fourth of Four

The current “mega” interest in Lithium resources was spurred by the development of Lithium-Ion batteries to aid in restructuring the world’s reliance on carbon spewing power petroleum reserves. Current resources of lithium recovery have fallen into two main categories—Pegmatite, found worldwide associated with felsic intrusions and Brine Related, and now with development in the Southwest United States of America (SWUS), a third category— Tertiary Volcanic clays, are specifically associated with Tertiary volcanics and major Tectonic Plate interactions. “Active” Plate tectonics is important as both the SWUS, the Lithium Triangle of South America (LTSA) and the Tibetan Plateau of China (TPC) producing tertiary (Miocene) volcanism that is important to the development of Lithium resources. The Tanzanian part of the East Africa Rift System (EARS) has features of both the SWUS, tertiary volcanic related “playas” and Continental rifting, the LTSA, tertiary volcanic related “Brines” and a major Tectonic plate event (subduction of an Oceanic Plate beneath the Continental South American Plate) and the TPC, tertiary volcanics (?) and major tectonic plate event (subduction of the Indian Continental Plate under the Eurasian Continental Plate). As well as the association of peralkaline and metaluminous felsic volcanics with Lithium playas of the SWUS and the EARS (Tanzania) “playas”. These similarities led to an analysis of a volcanic rock in Northeast Tanzania. When it returned 1.76% Lithium, a one-kilometer spaced soil sampling program returned, in consecutive samples over 0.20% Lithium (several samples over 1.0% lithium and a high of 2.24% lithium). It is proposed that these four regions with very similar past and present geologic characteristics, occur nowhere else in the world. That three of them have produced Lithium operations and two of them have identified resources of Lithium clay and “highly” anomalous Lithium clays should be regarded as more than “coincidental”.

Accretionary complex:Geological records from oceanic subduction to continental deep subduction

Accretionary complex was usually formed by offscraping of the subducting crustal material over the trench and thus often referred to as subduction zone mélange.The structure,composition and forming process of accretionary wedges can provide important insights into the evolution history of ocean basin,ocean-continent material cycle,continental accretion and thus contribute to understanding of the origin of plates and the growth of continents.Accretionary complex is characterized by a block-in-matrix structure associated with imbricate thrusts and isoclinal folds,diversified metamorphic types and intense water-rock interactions,which are distinct to the traditional stratigraphy.Since the proposal of the concept of accretionary wedge over a hundred years ago,great progress has been made in a variety of research focuses,such as the identification of the distribution of accretionary complexes,their compositions and formation mechanisms,the affinities of the matrix and igneous rocks,the recognition of the Ocean Plate Stratigraphy(OPS),the reconstruction of oceanic basin,the dynamic background of the tectonic evolution,the relationship between subduction zone and orogenic belt and,in particular,the accretionary complexes in continental subduction zones.These studies have significantly improved our understanding of the plate tectonic theory.Challenges remain in the identification of ancient accretionary complexes,the detailed analysis of accretionary complex zones,the accretion characteristics during continental collision,and the geochemical tracing of water-rock interaction during the accretion.China contains representative orogenic belts and accretionary complex zones in the world,and its geological records provide the best opportunity to make new breakthroughs in understanding of the plate tectonics.

Magnetotelluric data reveals subduction polarity and reactivation of the Mudanjiang suture zone,Northeast China

The Jiamusi and Songnen blocks converged in the easternmost segment of the Central Asian Orogenic Belt as a result of the subduction and subsequent closure of the Mudanjiang oceanic plate during the Permian-Jurassic.The Mudanjiang suture zone was later directly affected by subductions of the Paleo-Pacific plate and Pacific plate and is therefore an ideal place to study the subduction polarity and later transformation of a paleo-suture zone.Using three-dimensional inversion of magnetotelluric data collected along a 160-km-long profile across the Mudanjiang suture zone,we established a resistivity model of the suture zone and adjacent area.Our results reveal the subduction polarity and subduction trace of the Mudanjiang oceanic plate and provide geoelectrical evidence for reactivation of the Mudanjiang suture zone induced by the(Paleo-)Pacific plate subduction.The suture zone shows a complex conductive structure.The west-dipping crustal-scale conductor beneath the Songnen-Jiamusi collision zone represents the fossil subduction zone and indicates the westward subduction polarity of the Mudanjiang oceanic plate.Furthermore,the Mudanjiang fault identified by surface geology does not fully represent the deep structure of the Mudanjiang suture zone.The definition of the suture zone should be extended to the whole conductive region with a lateral extent of~70 km.Solid conductive minerals beneath the arc in front of the subduction zone were exhumated up from deep to the upper crust.The“chimney”-shaped conductor connected with the mantle represents the intrusive pathways of mantle-derived materials,suggesting that the Mudanjiang suture zone was reactivated by subductions of the Paleo-Pacific plate and Pacific plate,leading to remelting of the cooled and crystallized materials in the pathways.Therefore,subduction of the(Paleo-)Pacific plate destroyed the lithospheric structure of the paleo collision zone in the eastern segment of the Central Asian orogenic belt,and the large-scale crustal conductor beneath the suture zone reflects reactivation of the paleo-suture zone.