Geochronologic Constraints on the Magmatic Underplating of the Gangdisê Belt in the India-Eurasia Collision: Evidence of SHRIMP II Zircon U-Pb Dating

Abundant small mafic intrusions occur associated with granitoids along the Gangdise^ magmatic belt. In addition to many discrete gabbro bodies within the granitoid plutons, a gabbro-pyroxenite zone occurs along the southern margin of the Gangdise^ belt to the north of the Yarlung Zangbo suture. The mafic intrusion zone spatially corresponds to a strong aeromagnetic anomaly, which extends -1400 km. The mafic intrusions consist of intermittently distributed small bodies and dikes of gabbro and dolerite with accumulates of pyroxenite, olivine pyroxenite, pegmatitic pyroxenite and amphibolite. Much evidence indicates that the Gangdise^ gabbro-pyroxenite assemblage is most likely a result of underplating of mantle-derived magma. Detailed field investigation and systematic sampling of the mafic rocks was conducted at six locations along the Lhasa-Xigaze^ segment of the mafic intrusive zone, and was followed by zircon SHRIMP Ⅱ U-Pb dating. In addition to the ages of two samples previously published （47.0±1 Ma and 48.9±1.1 Ma）, the isotopic ages of the remaining four gabbro samples are 51.6±1.3Ma, 52.5±3.0 Ma, 50.2±4.2Ma and 49.9±1.1Ma. The range of these ages （47-52.5 Ma） provide geochronologic constraints on the Eocene timing of magma underplating beneath the Gangdise^ belt at ca. 50 Ma. This underplating event post-dated the initiation of the India-Eurasia continental collision by 15 million years and was contemporaneous with a process of magma mixing. The SHRIMP Ⅱ U-Pb isotopic analysis also found several old ages from a few zircon grains, mostly in a range of 479-526 Ma （weighted average age 503±10 Ma）, thus yielding information about the pre-existing lower crust when underplating of mafic magma took place. It is believed that magma underplating was one of the major mechanisms for crustal growth during the Indian-Eurasia collision, possibly corresponding in time to the formation of the 14-16 km-thick ＂crnst-mantle transitional zone＂ characterized by Vp=6.85-6.9 km/s.

Cenozoic Adakite-type Volcanic Rocks in Qiangtang,Tibet and Its Significance

Volcanic rocks in the study area, including dacite, trachyandesite and mugearite, belong to the intermediate-acid, high-K calc-alkaline series, and possess the characteristics of adakite. The geochemistry of the rocks shows that the rocks are characterized by SiO2>59%, enrichment in A12O3(15.09-15.64%) and Na2O (>3.6%), high Sr (649-885 μg/g) and Sc, low Y contents (<17 μg/g), depletion in HREE (Yb<1.22 μg/g), (La/Yb)N>25, Sr/Y>40, MgO<3% (Mg<0.35), weak Eu anomaly (Eu/Eu=0.84-0.94), and lack of the high field strength elements (HFSE) (Nb, Ta, Ti, etc.). The Nd and Sr isotope data (87Sr/86Sr=0.7062-0.7079, 143Nd/144Nd=0.51166-0.51253, εNd= -18.61-0.02), show that the magma resulted from partial melting (10%-40%) of newly underplated basaltic lower crust under high pressure (1-4 GPa), and the petrogenesis is obviously affected by the crust's assimilation and fractional crystallization (AFC). This research will give an insight into the uplift mechanism of the Tibetan plateau.

Underplating of Mesozoic Mantle-derived Magmas in Tongling, Anhui Province: Evidence from Megacrysts and Xenoliths

Lithological observations and mineralogical analyses on pyroxene and hornblende megacrysts and pyroxene and hornblende cumulates in xenoliths in the Mesozoic plutons of the Tongling region, Anhui Province, provide evidence for the magmatic underplating of mantle-derived alkali-olivine basalt at circa 140 Ma. The pyroxene and hornblende megacrysts and cumulates were formed through the AFC process at depths ranging from 27 to 35 km.

High-resolution crustal structure of the Yinchuan basin revealed by deep seismic reflection profiling:implications for deep processes of basin

The Yinchuan basin, located on the western margin of the Ordos block, has the characteristics of an active continental rift. A NW-striking deep seismic reflection profile across the center of Yinchuan basin precisely revealed the fine structure of the crust. The images showed that the crust in the Yinchuan basin was characterized by vertical stratifications along a detachment located at a two-way travel time（TWT） of 8.0 s.The most outstanding feature of this seismic profile was the almost flat Mohorovicˇic′ discontinuity（Moho） and a high-reflection zone in the lower crust. This sub-horizontal Moho conflicts with the general assumption of an uplifted Moho under sedimentary basins and continental rifts, and may indicate the action of different processes at depth during the evolution of sedimentary basins or rifts.We present a possible interpretation of these deep processes and the sub-horizontal Moho. The high-reflection zone, which consists of sheets of high-density, mantlederived materials, may have compensated for crustal thinning in the Yinchuan basin, leading to the formation of a sub-horizontal Moho. These high-density materials may have been emplaced by underplating with mantlesourced magma.

The Significance of Crust Structure and Continental Dynamics Inferred from Receiver Functions in West Yunnan

In our study we collected the teleseismic record of 31 broadband stations and 9 PASSCAL stations in West Yunnan, as well as extracted more than a million receiver functions. Using the waveform model and stacking techniques, we calculated the earth crust thicknesses and Vp/Vs ratios below the stations and obtained 35 valid data points. At the same time, we evenly stacked the receiver functions at the same station and superimposed the two profiles＇ cross sections of the main tectonic units. The results show a clear difference between the crust thicknesses of different tectonic units. Because of the magma underplatting and delimanition of the lower crust in the role of deep process, the West Yunnan＇s crust can be divided two kinds-mafic-ultramafic and feldspathic crusts. The research also shows that the mafic-ultramafic crust corresponds to a good background of mineralization. The delamination of the lower crust is one of the leading causes for moderate to strong earthquake prone in central Yunnan. The thinner crust and high velocity ratio as well as the multimodal structure of Ps in the Tengchong volcanic area confirms existence of a deep process of the strong magma underplating. Due to the basic crust structure and nature, it is believed that the Honghe fault is a main suture of the Gondwana and Eurasia continents.

Petrogenesis of Cenozoic Potassic Volcanic Rocks in the Nangqen Basin

The Nangqên basin is one of the Tertiary pull-apart basins situated in the east of the Qiangtang block. Similar to the adjacent Dengqên basin and Baxoi basin, there occurred a series of potassic volcanic and sub-volcanic rocks, ranging from basic, intermediate to intermediate-acid in lithology.

Crustal Poisson's ratio anomalies in the eastern part of North China and their origins

Seismic tomography can provide both fine P-wave and S-wave velocity structures of the crust and upper mantle. In addition, with proper computation, Poisson＇s ratio images from the seismic velocities can be determined. However, it is unknown whether Poisson＇s ratio images have any advantages when compared with the P-wave and S-wave velocity images. For the purposes of this study, high- resolution seismic tomography under the eastern part of North China region was used to determine detailed 3-D crustal P- and S-wave seismic velocities structure, as well as Poisson＇s ratio images. Results of Poisson＇s ratio imaging show high Poisson＇s ratio （high-PR） anomalies located in the Hengshan-North Taihang-Zhangjiakou （H-NT-Z） region, demonstrating that Poisson＇s ratio imaging can provide new geophysical constraints for regional tectonic evolution. The H-NT-Z region shows a prominent and continuous high-PR anomaly in the upper crust. Based on Poisson＇s ratio images at different depths, we find that this high-PR anomaly is extending down to the middle crust with thickness up to about 26 kin. According to rock physical property measurements and other geological data, this crustal Poisson＇s ratio anomaly can be explained by Mesozoic partial melting of the upper mantle and basaltic magma underplating related to the lithospheric thinning of the North China craton.

Contrasting Peridotite Types in the Dabie UHP Belt, Eastern China: the Raobazhai and Bixiling Ultramafic-mafic Complex

The Dabie UHP metamorphic belt, central China,contains two contrasting types of mafic-ultramafic complex. The Bixiling peridotite in the southern Dabie terrane contains abundant garnet (21.1-32.2 vol% )and thus has high CaO + Al2O3 (9.81-15.9 wt% ).……

Ameliorative Effect of Shade on Seedling Growth—Evidence from Field Experiment of <i>Vachellia farnesiana</i>(L.) Willd

The presence of species diversity in the ecosystem is an important aspect. Simply put, the loss of biodiversity and the formation of ecosystems into monocultures would reduce the productivity of the forest ecosystem than it has abundant diversity. This study conducted with a purpose to get the basic information regarding the effect of shade on the seedling growth. Understanding these relationships in the early years may reveal the important information, which will be helpful for managing and evaluating the silviculture practices. The study was conducted at Dirab Experiments and Agricultural Research Station, South of Riyadh. The experimental plot was managed under the corridor of Eucalyptus camaldulensis stands. Twenty seedlings of Vachellia farnesiana with relative similar height and diameter were planted in each block at a 5 × 5 m2 spacing in March 2017. Three different treatments were established and relatively categorized into Line 1 (without shade), Line 2 (shade until 09:00 a.m.), and Line 3 (shade until 11:00 a.m.). Eight months after planting, several morphological parameters were monthly assessed from November 2017 until April 2018. Also, the number of fruits was observed following the fruit’s season. Seedling growth variation as an effect of shading occurred between the lines of planting in this experiment. The seedling growth was significantly increased by shading treatment, even though target plant species (V. farnesiana) is categorized as intolerant species. Basically, shade helps to generate the favorable local environment for supporting plant growth. This treatment with an appropriate consideration might become an alternative practice for increasing plant productivity.

Role of sedimentation in continental rifting from comparing two narrow rift valleys the Salton Trough and Death Valley-California

To unravel the forces and better understand the processes that drive continental rifting, and to understand the role of sedimentation in promoting the rifting process, we compare;the different geological features of two narrow rifts, the Salton Trough and Death Valley, California. According to our models, the Moho is 22 km deep to the southwest of the Salton Sea on US-Mexico border and it deepens to 30 km in the region west of the Salton Trough. In Death Valley, the Moho is 24 km deep in the central part of the basin and it deepens to 32 km outside of the basin. The dome shaped Moho in both rifts is suggested to be primarily the product of magmatic activity in the lower crust and upper mantle. Death Valley is narrow rift in the initial stage of rifting with several sedimentary basins 2 - 4 km deep. In Death Valley magmatic (thermal) forces appears to drive the rifting process. The Salton Trough is wider than Death Valley and is moving toward sea floor spreading. The depth of the sedimentary basins ranges from 8 - 10 km and a combination of thermal and sedimentation appears to drive rifting processes in the Salton Trough.

Imaging the Deep Structure of the Central DeathValley Basin Using Receiver Function, Gravity,and Magnetic Data

We use receiver function, gravity, and magnetic data to image the deep structures of central Death Valley. Receiver function analysis suggests the Moho is 24 km deep in the central part of the basin and deepens to 33 km to the east and 31 km to the west. The estimated lower crustal density is 2900 kg/m3, which suggests a gabbroic composition, whereas the upper crustal density, excluding basin sediments, is estimated to average 2690 kg/m3 or approximately a quartzofeldspathic composition. We modeled the magnetic sources as upper crustal to suggest a relatively shallow Curie depth in this region of high heat flow. We developed models to test the hypothesis that a low-density, non-magnetic body (magma or fluid-rich material?) within the lower crust at a depth of 15 km could coincide with the location of the Death Valley bright spot imaged on a deep seismic reflection profile. Those models suggest that if there is a low density region in the mid to lower crust in the area of the bright spot, then the region is also likely to be underplated by mafic or ultramafic materials which may have contributed to heating, uplift, and thinning of the crust during extension.

Gravity evidence of underplating in the northeastern margin area of Qinghai-Tibet plateau

Based on leveling data in 1972 -2011 and relative-gravity data in 1993 -2011, we obtained a longterm vertical crustal-deformation rate of 1.62mm/a and a relative-gravity variation rate of 0.62 × 10^-8 ms^-2a^-1 for the northeastern margin area of Qinghai-Tibet plateau. After removing the contributions from the observed vertical movement and inferred surface denudation, we obtain a gravity-variation rate of 0.73 × 10^-8 ms^-2a^-1 attributable to the mass changes beneath the crust. This positive change suggests that the total mass under the observation stations was gradually increasing. We consider this result to be the gravitational evidence of underplating beneath the study area, and propose that the underplating was caused by collision betwen the Indian plate and Tibetan plateau and by gravitation-potential induced deviatoric stress.

Deformation history and processes during accretion of seamounts in subduction zones:The example of the Durkan Complex(Makran,SE Iran)

The Durkan Complex is a tectonic element of the Makran Accretionary Prism(SE Iran)that includes fragments of Late Cretaceous seamounts.In this paper,the results of map-to micro-scale structural studies of the western Durkan Complex are presented with the aim to describe its structural and tectonometamorphic evolution.The Durkan Complex consists of several tectonic units bordered by mainly NNW-striking thrusts.Three main deformation phases(D_(1),D_(2),and D_(3))are distinguished and likely occurred from the Late Cretaceous to the Miocene–Pliocene.D_(1) is characterized by sub-isoclinal to close and W-verging folds associated with an axial plane foliation and shear zone along the fold limbs.This phase records the accretion of fragments of the seamount within the Makran at blueschist facies metamorphic conditions(160–300℃ and 0.6–1.2 GPa).D_(2) is characterized by open to close folds with subhorizontal axial plane that likely developed during the exhumation of previously accreted seamount fragments.An upper Paleocene–Eocene siliciclastic succession unconformably sealed the D_(1) and D_(2) structures and is,in turn,deformed by W-verging thrust faults typical of D_(3).The latter likely testifies for a Miocene–Pliocene tectonic reworking of the accreted seamount fragments with the activation of out of sequence thrusts.Our results shed light on the mechanism of accretion of seamount materials in the accretionary prisms,suggesting that seamount slope successions favour the localization and propagation of the basal décollement.This study further confirms that the physiography of the subducting plates plays a significant role in the tectonic evolution of the subduction complexes.

Postcollisional mantle-derived magmatism, underplating and implications for basement of the Junggar Basin

The late Paleozoic postcollisional granitoids, mafic-ultramafic complexes, and volcanic rocks are extensively distributed around the Junggar Basin; they are generally characterized by positive εNd(t) values, implying that the magmas were mantle-derived and contaminated with crustal materials to some extents. The emplacement of mantle-derived magmas and their differentiates in the upper crust is the expression of deep geological processes at shallow level, while much more mantle-derived magmas were underplated in the lower crust and the region near the crust-mantle boundary, being component part of basement of the Junggar Basin. The postcollisional mafic-ultramafic complexes would not be generated by re-melting of residual oceanic crust, which was considered as the basement of the Junggar Basin, unless very high degrees of partial melting occurred. Even if old continental crust had been present before collision, it would have been strongly modified by the mantle-derived magma underplating. This interpretation is compatible with the existing geophysical data.

Petrogenesis of the Mesozoic intrusive rocks in the Tongling area, Anhui Province, China and their constraint on geodynamic process

Petrology, element and isotopic geochemistry of the Mesozoic intrusive rocks in the Tongling area were systematically investigated in this study. The intrusive rocks can be divided into two groups, one contains shoshonitic rocks with SiO2 ? 55%, the other consists mainly of high-potassic calc-alkaline rocks with SiO2 > 55%. The shoshonitic rocks (SiO2 ? 55%) were generated by the fractional crystallization of the primary basaltic magma sourced from an enriched mantle, then the evolved basaltic magma likely experienced low-degree contamination with the lower crust materials when they ascended. On the other hand, although the intrusive rocks with SiO2 > 55% show most elemental geochemical characteristics similar to an adakite, such as high Na2O, AI2O3, Sr contents, high Sr/Y and La/Yb ratios, they have isotopic compositions much different from an adakite, such as low σNd(t) (-9.16—16.55) and high (87Sr/86Sr), (0.7068—0.7105), and some of them show relatively high Y and Yb contents than those of an adakite. We propose that the intrusive rocks with SiO2 > 55% were most probably produced by mixing of the mantle-derived basaltic magma and adakite-like magma derived from the melting of basaltic lower crust that was heated by the underplating mantle-derived shoshonitic magmas. The delamination of lower crust likely took place after or during the formation of these adakite-like rocks in the Tongling area.

Adakite-type sodium-rich rocks in Awulale Mountain of west Tianshan:Significance for the vertical growth of continental crust

The sodium-rich dacites and albite porphyries of Permian in the Awulale Mountain of west Tianshan have unique chemical and trace element signatures identical to adakite. These intermediate-acidic igneous rocks are characterized by high Na2O, Al2O3 and Sr contents and high Sr/Y and La/Y ratios (】40 and 】20, respectively), and low Y and Yb contents, and strong depletion in HREE, and positive Eu anomaly. The (143Nd/144Nd)i is in the range from0.51236 to 0.51248 and the εNd(t) is positive value (+0.79-+3.11); the (87Sr/86Sr)i is in the range from 0.7052 to 0.7054. These Nd and Sr isotopic composition features indicate that the source rocks of these adakite-type rocks are from a weakly depleted mantle, or a depleted mantle, but was contaminated by the crustal materials. These adakite-type rocks were most likely derived from the partial melting of new underplated basaltic rocks under the conditions of amphibo-lite to eclogite transition in the postcollisional environment of North Xinjiang during the Permian

Cumulate complex xenoliths in the Early Mesozoic in eastern Inner Mongolia

Mafic xenoliths occur in Mesozoic diorites from nine localities in eastern Inner Mongolia from the Da Hinggan Mrs. to Huabei craton. Their petrologic and geochemical features suggest that they are an intact suite of cumulate complex whose mineral assemblages contain typical patent mantle metasomatism minerals such as pargosite, phlogopite and apatite. Their REEs and trace element characteristics suggest a comagmatic relationship in these cumulates with different compositions. Isotopic dating of the xenoliths indicates that they are products of the mantle magmatic underplating in the Early Mesozoic (237—224 Ma), and they may provide direct evidence for the crust-mantle interaction and vertical accretion of continental crust under a background of the mantle upwelling in the Early Mesozoic in Huabei region.

The discovery of Palaeoproterozoic volcanic rocks in the Bulunkuoler Group from the Tianshuihai Massif in Xinjiang of Northwest China and its geological significance

In the Dabudaer region of the Tianshuihai Massif （Xinjiang, Northwest China）, metavolcanic rocks within the mainly meta- sedimentary Bulunkuoler Group are basalt, basaltic andesite, and rhyolite. In situ zircon U-Pb LA-ICP-MS dating on rhyolite magmatic zircons yielded an age of 2481±14 Ma, interpreted as the eruption age of the Bulunkuoler Group volcanic rocks. The basalt and basaltic andesite are low-A1 tholeiite series and the rhyolite is low-Al calc-alkaline series. All of rocks are charac- terized by enrichment of LREE and LILE （Rb, Th and Ba）, depletion of P, Nb, Ta and Ti, and absence of Eu anomalies. The Cnd（t） of basalts ranges from 3.14 to 4.88 indicating a depleted mantle source. The trace element signatures show that these magmas experienced intense crustal contamination during their ascent. Direct evidence for crustal contamination is the xenocrystic zircons in the ryholite, with ages back to ca. 3300 Ma. The primitive-mantle normalized and Zr/Y-Zr diagrams for the basic volcanic rocks show that they formed in an intra-continental tectonic setting. Combined with the previous studies and compared with North China Craton Neoarchean basic volcanic rocks, it is supposed that the Bulunkuoler Group volcanic rocks reflect the Palaeoproterozoic mantle magma underplating and interaction with felsic crust.

Magma underplating and Hannuoba present crust-mantle transitional zone composition: Xenolith petrological and geochemical evidence

On the basis of mineral assemblage, mineralogy, petrology, and major, trace ele-mental and isotopic geochemistry of the underplated granulite- and eclogite-facies accumulate, peridotite and pyroxenite xenoliths entrained in Hannuoba Cenozoic basalts, this work con-strained the petrological constituents for the crust-mantle transitional zone, which is supported by the results of high-temperature and pressure velocity experiments on rocks and geophysics deep survey. Present lower part of lower crust is mainly composed of granulite-facies mafic accumu-lates (dominantly plagioclase websterite) and crust-mantle transitional zone dominantly com-posed of eclogite-facies pyroxenites with or without garnet and spinel lherzolites; Archaean ter-rain granulite is only nominally early lower crust. Magma underplating in the crust-mantle boundary led to the crustal vertical accretion and the formation of the crust-mantle transitional zone, which is a significant mechanism for the chemical adjustment of the crust-mantle boundary since the Phanerozoic.

Geochemistry of the high-Mg andesites at Zhangwu, western Liaoning: Implication for delamination of newly formed lower crust

Ten volcanic samples at Zhangwu,western Liaoning Province,North China were selected for a sys-tematic geochemical,mineralogical and geochronological study,which provides an opportunity to ex-plore the interaction between the continental crust and mantle beneath the north margin of the North China craton.Except one basalt sample(SiO2= 50.23%),the other nine samples are andesitic with SiO2 contents ranging from 53% to 59%.They have relatively high MgO(3.4%―6.1%,Mg#=50―64) and Ni and Cr contents(Ni 27×10?6―197×10?6,Cr 51×10?6―478×10?6).Other geochemical characteristics of Zhangwu high-Mg andesites(HMAs) include strong fractionation of light rare earth elements(LREE) from heavy rare earth elements(HREE),and Sr from Y,with La/Yb greater than 15,and high Sr/Y(34― 115).Zircons of andesite YX270 yield three age groups with no Precambrian age,which precludes ori-gin of the Zhangwu HMAs from the partial melting of the Precambrian crust.The oldest age group peaking at 253 Ma is interpreted to represent the collision of the Siberia block and the North China block,resulting in formation of the Central Asian orogenic belt by closure of the Mongol-Okhotsk Ocean.The intermediate age group corresponds to the basalt underplating which caused the wide-spread coeval granitoids in the North China craton with a peak 206Pb/238U age of 172 Ma.The youngest age group gives a 206Pb/238U age of 126±2 Ma,which is interpreted as the eruption age of the Zhangwu HMAs.The high 87Sr/86Sri(126 Ma)>0.706 and low εNd(t)= ?6.36―?13.99 of the Zhangwu HMAs are distinct from slab melts.The common presence of reversely zoned clinopyroxene phenocrysts in the Zhangwu HMAs argues against the origin of the Zhangwu HMAs either from melting of the water saturated mantle or melting of the lower crust.In light of the evidence mentioned above,the envisaged scenario for the formation of the Zhangwu HMAs is related to the basaltic underplating at the base of the crust,which led to the thickening of the lower crust and formation of lower crustal eclogite,followed by foundering of the eclogitic lower crust into the asthenosphere.The foundered eclogite then melted and the resul-tant melts interacted with surrounding peridotite during their upward transport,which finally produced the high-Mg andesites.This well explains the high-Mg adakitic characters and absence of ancient in-herited zircon in the Zhangwu lavas.