Development Model and Quantitative Prediction of Igneous Rock in S Oilfield, the Bohai Sea Area

The structure of S Oilfield of Bohai sea area in China is located in the low uplift slope zone of the Dongwa Laibei area in the Huanghekou depression. The target interval of the oilfield belongs to the Paleogene layered structural oil reservoir under the influence of igneous rocks. Due to the widespread development of igneous rocks, and the diversity of igneous rock types, the structure is complex, and the lateral changes are intense. The differences in volcanic eruption intensity and volcanic materials made the igneous rock masses overlap each other, with rapid changes in lithofacies, and complex seismic reflection structures. Therefore, it was difficult to carry out fine characterization of igneous rocks. Based on the lithofacies model, lithofacies combination types, and seismic response characteristics of volcanic mechanisms, this paper summarized three types of development models of volcanic mechanisms in the study area. At the same time, through technical means such as stratigraphic slicing, seismic attribute extraction, and attribute optimization analysis, the spatial distribution characteristics of various facies zones of igneous rocks at different stages were described in detail, achieving precise prediction of igneous rocks in Bozhong S Oilfield. During the development process of the oilfield, the technical research results guided the optimization of well locations and provided technical support for the efficient development of the oilfield.

Geochemistry of Alkali-rich Igneous Rocks of Northern Xinjiang and Its Implications for Geodynamics

Five nearly E-W-trending alkali-rich igneous rock belts are distributed from north to south in northern Xinjiang, and they are composed mainly of riebeckite, K-feldspar granite and high-K and medium-K calc-alkaline volcanic rocks and shoshonite. They were mainly emplaced or erupted between the Carboniferous and Permian. The compositions of Sr, Nd, Pb, and O isotopes imply that their principal resource materials are derived from the upper mantle or juvenile crust, indicating obvious continental growth in the Phanerozoic. The trace element association implies that their tectonic settings are within plate and volcanic arc for alkali-rich granites, and post-collisional arc, late oceanic arc and continental arc for alkali-rich volcanic rocks. An archipelago model was suggested for the tectonic evolution in northern Xinjiang. It can be named the central-Asia-type orogeny, which is different from the so called circum-Pacific ocean-continent-type tectonics or the Alpine-Himalayan

Chronological and Geochemical Study of the Cenozoic Potassic Felsic Igneous Rocks in Western Yunnan,SE Tibet:Implications for their Tectonic Mechanisms

Whether there is a relationship between widespread Cenozoic igneous rocks and the activated Ailao Shan-Red River shear zone(ASRR)in western Yunnan,SE Tibet,remains highly controversial.New geochronological,and geochemical data derived from Cenozoic igneous rocks both within and far from the ASRR offers answers.Although these coeval felsic rocks(36.9-33.5 Ma)are enriched in large-ion lithophile elements relative to high field strength elements,they also display some special geochemical features,such as high Y and Yb values of syenitic porphyries,which contrast with the high Sr and low Y contents of monzoporphyries and granitic porphyries.The syenitic porphyries exhibit geochemical characteristics similar to those of coeval mafic magmas,e.g.,high K 2 O,Y,and Yb contents,indicating that the former were probably generated by fractional crystallization of the latter.However,the high Sr/Y(56.1-109)monzoporphyries and granitic porphyries,combined with their low MgO,and Cr(or Ni)contents,suggest that they were mainly derived from partial melting of a thickened mafic lower crust.Given that these Cenozoic magmatic events exhibiting planar distributions occurred outside of the activation of the ASRR(31-20 Ma),then they were not controlled by the latter.Instead,they were tied to convective removal of a thickened lower lithospheric mantle in western Yunnan during the Paleogene.

Relationship between the Extent of Igneous Rocks and Deep Structures as Determined by Gravitational and Magnetic Data in the South China Sea

The distribution of oil and gas resources in the South China Sea and adjacent areas is closely related to the structural pattern that helped to define the controlling effect of deep processes on oil-bearing basins.Igneous rocks can record important information from deep processes.Deep structures such as faults,basin uplift and depression,Cenozoic basement and magnetic basement are all the results of energy exchange within the earth.The study of the relationship between igneous rocks and deep structures is of great significance for the study of the South China Sea.By using the minimum curvature potential field separation technique and the correlation analysis technique of gravitational and magnetic anomalies,the fusion of gravitational and magnetic data reflecting igneous rocks can be obtained,through which the igneous rocks with high susceptibility/high density or high susceptibility/low density can be identified.In this study area,igneous rocks do not develop in the Yinggehai basin,Qiongdongnan basin,Zengmu basin and Brunei-Sabah basin whilst igneous rocks with high susceptibility/high density or high susceptibility/low density are widely-developed in other basins.In undeveloped igneous areas,faults are also undeveloped the Cenozoic thickness is greater,the magnetic basement depth is greater and the Cenozoic thickness is highly positively correlated with the magnetic basement depth.In igneously developed regions,the distribution pattern of the Qiongtai block is mainly controlled by primary faults,while the distribution of the Zhongxisha block,Xunta block and Yongshu-Taiping block is mainly controlled by secondary faults,the Cenozoic thickness having a low correlation with the depth of the magnetic basement.

Integrative method in lithofacies characteristics and 3D velocity volume of the Permian igneous rocks in H area, Tarim Basin

This paper introduces horizon control, seismic control, logging control and facies control methods through the application of the least squares fitting of logging curves, seismic inversion and facies-controlled techniques. Based on the microgeology and thin section analyses, the lithology, lithofacies and periods of the Permian igneous rocks are described in detail. The seismic inversion and facies-controlled techniques were used to find the distribution characteristics of the igneous rocks and the 3D velocity volume. The least squares fitting of the logging curves overcome the problem that the work area is short of density logging data. Through analysis of thin sections, the lithofacies can be classified into eruption airfall subfacies, eruption pyroclastic flow subfacies and eruption facies.

Mantle Response to Slab Breakoff in the North Qaidam Tectonic Belt: Geochemical Constraints from Syn-subduction Mafic Igneous Rocks

Slab breakoff originally denotes the detachment of dense subducted oceanic slab from the light subducted continental slab, which is driven by opposing buoyancy forces during continental collision(Davies and von Blanckenburg, 1995;von Blanckenburg and Davies, 1995). The breakoff of subducted oceanic slab can induce the upwelling of sub-slab asthenosphere through the slab window, and then heat the overriding lithospheric mantle resulting in the melting of its fertile layer within the metasomatic mantle wedge. The decompression partial melting of uprising asthenospheric mantle commonly produce mafic magma with depleted MORB-like geochemical signatures(Davies and von Blanckenburg, 1995;Cole et al., 2006;Wang et al., 2018), whereas the partial melting of enriched lithospheric mantle will produce mafic magma with alkaline, calc-alkaline or ultrapotassic features(von Blanckenburg and Davies, 1995). These mafic magmas rise into overlying lower crust and trigger crustal melting to generate the granitic magma. The North Qaidam tectonic belt(NQTB) records the evolutionary process of the South Qilian Ocean from subduction to closure. The subduction of oceanic and continental lithosphere to mantle depths is proven by the identification of oceanic-type and continental-type eclogites enclosed in crustal metapelite and gneiss from the North Qaidam tectonic belt(Song et al., 2006;Zhang et al., 2008;Zhang et al., 2010;Zhang et al., 2017). However, details of this process are not exactly constrained, in particularly, the closure timing of South Qilian Ocean. The study of characteristic mafic magmatism, combined with the previous studies of ultra-high pressure metamorphism, give us an excellent opportunity to trace the detailed processes associated with the transition from oceanic subduction to continental subduction, and assess the feasibility of slab breakoff in the North Qaidam tectonic belt. In this contribution, an integrated study of petrology, geochemistry, geochronology and Sr-Nd-Hf isotopes is performed on the mafic igneous rocks from Chahanhe area in the North Wulan gneiss complex. These mafic igneous rocks can be divided into two groups, namely, arc-like type and E-MORB type based on their trace element patterns. Arc-like mafic rocks(441–428 Ma) were characterized by enrichment of light rare earth elements(LREEs), large ion lithophile elements(LILEs) and depletion of heavy rare earth elements(HREEs), high field strength elements(HFSEs). Combined with variable zircon εHf(t) values of-6.17 to +1.58, it is suggested that arc-like mafic rocks are predominantly derived from the partial melting of the enriched lithospheric mantle, and minor juvenile materials have contributed to their sources. The E-MORB mafic rocks(440 Ma) exhibit relatively flatted REE patterns and positive εNd(t) values of +1.63 to +4.28, but high(87Sr/86Sr)i ratios of 0.706825 to 0.708979, indicting a derivation from partial melting of asthenospheric mantle, with involvement of enriched components probably derived from ambient lithospheric mantle or stagnant subducted oceanic crust. Collectively, it is proposed that the break-off of the subducted South Qilian oceanic slab triggered the decompression melting of asthenospheric mantle, and the upwelling of asthenosphere provided heat and induced partial melting of the enriched lithospheric mantle and preexisting crust, resulting in generation of arc-like mafic rocks and widespread granites.

Generation of electromotive force in igneous rocks subjected to non-uniform loading

When one end of an air-dry igneous rock block was uniaxially loaded in laboratory,there appeared an electromotive force that made electric currents flow from the stressed volume to the unstressed volume. Quartz-free rocks such as gabbro also generated this force,stronger than quartz-bearing rocks such as granite. This indicates that the piezoelectric effect of quartz and the electrokinetic effect of pore water do not make a large contribution toward generating the electromotive force.We focus on peroxy bond that is one of the abundant lattice defects in igneous rock-forming minerals.When mechanical loading deforms the lattice structure around this defect and breaks its bond,its energy levels change and act like an accepter.As an electron is trapped at this defect from a neighbor O2- site,a positive hole is activated there.They attempt to diffuse toward the unstressed volume through the valence band and are simultaneously affected by the attractive electric force with the electrons trapped in peroxy bonds.This leads to a polarization in the stressed volume and the generation of electromotive force between the stressed and unstressed volumes.Similar electromotive force may be generated in the Earth’s crust where inhomogeneous stress/strain is changing.

Study on the Igneous Rock Seismic Facies in Yingmai 32 Blocks

The western part of north Tarim Uplift underwent multi-stage tectonic movement and multiple stages of magmatism. Igneous rocks are associated with carbonate and buried deep. The seismic response characteristics of igneous rocks are similar in many respects to the seismic response characteristics of karst, making the identification and prediction of igneous rocks more difficult. This study compares the seismic reflection characteristics of igneous rocks. Set up three types of igneous rock seismic facies model penetration type, fracture type and central type. And it concluded that a time-slice, coherence analysis, analysis of the seismic properties of the layers and the method of three-dimensional engraving to identify the igneous rocks. This method has been applied to the identification and prediction of carbonate rock buried and hill igneous rocks in the north Tarim basin YingMaiLi region and has achieved good results.

Hydrocarbons in igneous rock of Brazil:A review

Igneous rocks are widely developed in many hydrocarbon-bearing sedimentary basins in Brazil,and some igneous rocks play positive roles in the oil and gas accumulation process.But so far,no industrial oil or gas flow is discovered in igneous rocks in Brazilian onshore basins.At present,in some literatures published by some researchers,cases of igneous reservoirs in Brazil are inaccurate.The lithology of reservoirs in the Igarape Cuia oil and gas field,the oil and gas field of Urucu and the Barra Bonita gasfield,were previously thought to be volcanic rocks,but now are clastic rocks according to studies,and the igneous rocks in these three oil and gas field can only be acted as indirect and direct cap rocks.Furthermore,igneous rock reservoirs in the Campos Basin and Santos Basin in the Brazilian offshore are briefly analyzed.The representative Badejo oilfield in the Campos Basin has igneous rock reservoirs dominated by basalts which are intercalated with thin layers of volcaniclastic and sedimentary rocks,the storage space are vesicules,fractures and vugular pores,and the porosity of hydrocarbon bearing interval basalt reservoirs ranges from 10%to 15%.In addition,for the Santos Basin,igneous rocks in this basin are dominated by basic rocks,some alkaline intrusive rocks develop good micro spaces,mainly crystal moldic pores,inter-and intra-crystal dissolution pores and fractures,better reservoir intervals are often concentrated such as at sections near to unconformity surfaces and can have porosity of 6%e9%or higher;local pre-salt igneous rock reservoirs in the Santos Basin have the necessary conditions for hydrocarbon accumulation and it will be a new target layer for hydrocarbon exploration.

GEOCHEMISTRY OF IGNEOUS ROCK IN PANXI RIFT

In this paper the petrogenesis and magmatism of igneous rocks in the Panxi Rift are discussed based on the major element, transition metallic element, REE and other trace element analysis data of more than 40 samples.According to major element data, a feature of bimodal distribution is displayed. The transition metal patterns show that Cr and Ni are highly depleted and Ti is highly concentrated, reflecting a rather complex history of crystallization differentiation.The distribution patterns of hygromagmatophile element abundance of basalts normalized with primary mantle values are shown to be intermediate between those of alkali basalts and continental tholeiites. This provides evidence suggesting that the basalts were generated in a rift tectonic environment, and that the primary magma came deeply from the primary mantle as a result of partial melting of garnet lherzolite.

Study and Simulated the Natural Radioactivity (NORM) U-238, Th-232 and K-40 of Igneous and Sedimentary Rocks of Al-Atawilah (Al-Baha) in Saudi Arabia

In this work, gamma-ray spectroscopy based on semiconductor hyper pure germanium (HPGe) detector was used to evaluate the activity concentrations of the natural radionuclides (U-238 (Ra-226), Th-232 and K-40) and the fallout nuclide (Cs-137) for thirty samples of igneous and sedimentary rocks of Al-Atawilah (Al-Baha). The mean values of the activity concentrations of U-238 (Ra-226), Th-232, K-40 and Cs-137 in the igneous samples are found as (11.0, 11.50, 1172.71, 1.47) Bq/Kg respectively. In the sedimentary rocks, the mean values of the activity concentrations of the natural radionuclides (U-238 (Ra-226), Th-232 and K-40) and the fallout nuclide (Cs-137) equal to (12.04, 13.18, 1131.36, 1.60) Bq/Kg respectively. The averages of radiological hazards (Raeq, Hex and Iγ) were calculated and found to be within the UNSCEAR permissible limit values (370 Bq/kg for Raeq, and 1 for Hex and Iγ), except for a slight increase of average value of Iγ in the igneous rock samples (1.36). The results indicate that the dose rate values depend on the kind of rocks (high in some igneous rock samples, and most of sedimentary rock samples have low dose rate). The activities of naturalnuclides were predicted and simulated in T time using a written MATLAB R2020a script based on the average activity concentrations and respective half-lives of U-238 and Th-232 series, and K-40, this is to evaluate the future effects of natural radionuclides on the population and estimate the human inputs in the future.

Geochronological and geochemical constraints on the petrogenesis of late Mesoproterozoic mafic and granitic rocks in the southwestern Yangtze Block

Late Mesoproterozoic igneous rocks in the SW Yangtze Block are important for understanding the role of it in reconstruction of the Rodinia supercontinent.In the present study,we report new geochronological,geochemical,and Nd-Hf isotopic data for the Cuoke plagioclase amphibolites and granites in the SW Yangtze Block.Geochronological results show that the plagioclase amphibolites and granites have similar late Mesoproterozoic zircon U-Pb ages of 1168-1162 Ma,constituting a bimodal igneous assemblage.The plagioclase amphibolites have high and variable TiO2 contents(1.15-4.30 wt.%)and Mg#(34-66)values,similar to the tholeiitic series.They are characterized by enrichment in LREEs and LILEs,and have OIB-like affinities with positive Nb and Ta anomalies.The plagioclase amphibolites have positive whole-rockεNd(t)(+3.2 to+4.3)and zirconεHf(t)(+4.3 to+10.7)values,indicating that they were derived from an OIB-like asthenospheric mantle source.The granites belong to the reduced peralkaline A-type series and have negativeεNd(t)value of-6.0 andεHf(t)values of-5.8 to-13.8,indicating a derivation from the partial melting of ancient mafic lower crust.In combination with the~1.05-1.02 Ga bimodal igneous assemblage in the SW Yangtze Block,we propose that the Cuoke 1168-1162 Ma igneous rocks were likely formed in a continental rift basin and argue against the existance of Grenvillian Orogen in the SW Yangtze Block during the late Mesoproterozoic.

Apply X-Ray Fluorescence and γ-Ray Spectroscopy to Analyze Igneous and Sedimentary Environmental Samples of Al-Atawilah (Al-Baha), Saudi Arabia

Igneous and sedimentary rocks contain an amount of natural radioactivity (NORM). U-238, Th-232 and their decay products, and K-40 are important sources of gamma-radiation. Knowledge of the radionuclide content of rocks is necessary to estimate the exposure of the population to the radiation. Many types of rocks are used in building and industries, thus the radiation detection is important, it provides a baseline map of levels of the radioactivity in the study region. The purpose of this study is to evaluate the activity concentrations of the natural radionuclides (U-238 (Ra-226), Th-232 and K-40) and the fallout nuclide (Cs-137) (if found) in thirty samples of igneous and sedimentary rocks of Al-Atawilah (Al-Baha). The samples were collected and prepared during 2018/2019, and analyzed with a good experimental instrument (High energy resolution γ-ray spectroscopy with HPGe detector), also these rock samples were analyzed with X-ray fluorescence to subdivided these rocks based on the major oxides proportions contained of each sample. The mean activity concentrations of naturally radionuclides were found in the igneous rock samples varied depending on the type of the igneous rock. For sedimentary rock samples, the activity concentrations were found high for quartz sandstone sample, which may be due to its high proportion of SiO2 and K2O. The estimated mean values of absorbed dose rate are within the permissible limit value. The findings indicate high dose level values in granite (rhyolite) and most of diorite (andesite) igneous rock samples while gabbro (basalt) igneous rock samples (except for one sample) record low levels of dose rate. All sedimentary rock samples have low dose rate (except for the quartz sand-stone sample).

Theoretical and technical progress in exploration practice of the deep-water large oil fields, Santos Basin, Brazil

The history and results of petroleum exploration in the Santos Basin, Brazil are reviewed. The regularity of hydrocarbon enrichment and the key exploration technologies are summarized and analyzed using the seismic, gravity, magnetic and drilling data. It is proposed that the Santos Basin had a structural pattern of two uplifts and three depressions and the Aram-Uirapuru uplift belt controlled the hydrocarbon accumulation. It is believed that the main hydrocarbon source kitchen in the rift period controlled the hydrocarbon-enriched zones, paleo-structures controlled the scale and quality of lacustrine carbonate reservoirs, and continuous thick salt rocks controlled the hydrocarbon formation and preservation. The process and mechanism of reservoirs being transformed by CO_(2)charging were revealed. Five key exploration technologies were developed,including the variable-velocity mapping for layer-controlled facies-controlled pre-salt structures, the prediction of lacustrine carbonate reservoirs, the prediction of intrusive/effusive rock distribution, the detection of hydrocarbons in lacustrine carbonates, and the logging identification of supercritical CO_(2)fluid. These theoretical recognitions and exploration technologies have contributed to the discovery of deep-water super-large reservoirs under CNODC projects in Brazil, and will guide the further exploration of deep-water large reservoirs in the Santos Basin and other similar regions.

Multi-scale data joint inversion of minerals and porosity in altered igneous reservoirs—A case study in the South China Sea

There are abundant igneous gas reservoirs in the South China Sea with significant value of research,and lithology classification,mineral analysis and porosity inversion are important links in reservoir evaluation.However,affected by the diverse lithology,complicated mineral and widespread alteration,conventional logging lithology classification and mineral inversion become considerably difficult.At the same time,owing to the limitation of the wireline log response equation,the quantity and accuracy of minerals can hardly meet the exploration requirements of igneous formations.To overcome those issues,this study takes the South China Sea as an example,and combines multi-scale data such as micro rock slices,petrophysical experiments,wireline log and element cutting log to establish a set of joint inversion methods for minerals and porosity of altered igneous rocks.Specifically,we define the lithology and mineral characteristics through core slices and mineral data,and establish an igneous multi-mineral volumetric model.Then we determine element cutting log correction method based on core element data,and combine wireline log and corrected element cutting log to perform the lithology classification and joint inversion of minerals and porosity.However,it is always difficult to determine the elemental eigenvalues of different minerals in inversion.This paper uses multiple linear regression methods to solve this problem.Finally,an integrated inversion technique for altered igneous formations was developed.The results show that the corrected element cutting log are in good agreement with the core element data,and the mineral and porosity results obtained from the joint inversion based on the wireline log and corrected element cutting log are also in good agreement with the core data from X-ray diffraction.The results demonstrate that the inversion technique is applicable and this study provides a new direction for the mineral inversion research of altered igneous formations.

火成岩中石英的Al芯和Ti芯的光衰退和ESR测年研究

在 10 0K低温、5× 10 - 4T调制磁场的条件下 ,测得Al芯和Ti芯的电子自旋共振 (ESR)波谱并计算其年龄。由于改进了实验条件 ,得到的ESR图谱稳定 ,重复性好 ,为峰的选择和峰高的测定提供了方便。所得到的总剂量拟合直线的线性度高 ,大大减小了实验误差。本文对火成岩中的石英样品进行了光衰退实验 ,结果表明 ,在紫外光的作用下 ,Al芯和Ti芯的信号强度呈指数衰减 ,不能被光衰退的Al芯和Ti芯大约占 5 0 %。而在自然光的照射下 ,Ti芯可以衰退到原来的 2 7% ,Al芯仍为 5 0 %。

Zircon saturation model in silicate melts:a review and update

Zircon stability in silicate melts-which can be quantitatively constrained by laboratory measurements of zircon saturation-is important for understanding the evolution of magma.Although the original zircon saturation model proposed by Watson and Harrison(Earth Planet Sci Lett 64(2):295-304,1983) is widely cited and has been updated recently,the three main models currently in use may generate large uncertainties due to extrapolation beyond their respective calibrated ranges.This paper reviews and updates zircon saturation models developed with temperature and compositional parameters.All available data on zircon saturation ranging in composition from mafic to silicic(and/or peralkaline to peraluminous)at temperatures from 750 to 1400℃ were collected to develop two refined models(1 and 2) that may be applied to the wider range of compositions.Model 1 is given by lnCZr(melt)=(14.297±0.308)+(0.964 ± 0.066).M-(11113±374)/r,and model 2 given by lnCZr(melt)=(18.99±0.423)-(1.069±0.102)·lnG-(12288±593)/T,where CZr(melt) is the Zr concentration of the melt in ppm and parameters M [=(Na+K+2 Ca)/(Al·Si)](cation ratios) and G [=(3·Al2 O3+SiO2)/(Na2-O+K2 O+CaO+MgO+FeO)](molar proportions)represent the melt composition.The errors are at one sigma,and T is the temperature in Kelvin.Before applying these models to natural rocks,it is necessary to ensure that the zircon used to date is crystallized from the host magmatic rock.Assessment of the application of both new and old models to natural rocks suggests that model 1 may be the best for magmatic temperature estimates of metaluminous to peraluminous rocks and that model 2 may be the best for estimating magmatic temperatures of alkaline to peralkaline rocks.

Origin and tectonic setting of low-Si alkaline magma

Despite of the relatively rare occurrence of alkaline igneous rocks on Earth, they are the most diverse group of igneous rocks due to variations in their mineral assemblages and geochemical compositions. Alkaline igneous rocks are generally characterized by Si undersaturation, and K-Na(and in some cases C) enrichment. Although there is no unified alkaline igneous rock classification scheme, diagrams such as total alkalis-SiO_(2) or K_(2)O-SiO_(2) can be used to classify alkaline igneous rocks. The origins of silica-saturated alkaline rocks(SiO_(2)>52 wt.%) are relatively complex because multiple episodes of magmatism are often involved, therefore most studies have focused on the origins of silica-undersaturated alkaline magmas.Numerous experimental petrological studies have been conducted in the past few decades. Initially, it was considered that enrichment of trace elements was only achieved by low-degree partial melting of mantle peridotite, but the experimental melts could not reproduce the geochemical composition of natural alkaline igneous rocks. Subsequent studies have focused on carbonate-bearing eclogites that represent the average component of subducted oceanic crust. Although experimental studies indicate that silica-undersaturated and alkali-oversaturated melts can be generated from eclogites, some studies have considered that natural silica-undersaturated alkaline igneous rocks are the result of multi-stage source enrichment by incompatible elements. Low-degree partial melts of the mantle can be emplaced in the lithospheric mantle, forming veins consisting of phlogopite, amphibole, and pyroxene. Melting experiments including these components show that the produced melts better reproduce the naturally occurring silica-undersaturated alkaline magmas. Silica-deficient alkaline igneous rocks occur in various tectonic settings. Those in intraplate and divergent settings(i.e., mainly continental rifts) normally involve contributions from recycled components such as subducted slabs at the mantle transition zone(MTZ) or metasomatized lithospheric mantle. At convergent plate boundaries, silica-undersaturated alkaline magmas can be derived by the melting of mélange or the focused breakdown of phlogopite at back-arc depths. We compiled global data for alkaline igneous rocks and discovered that silicaundersaturated alkaline igneous rocks in continental collisional zones are K-rich and differ from those from other tectonic settings. We suggest that subducted continental sediment is an important K-rich end-member, which contributes a large amount of K to the alkaline magmas in the collision zone. Moreover, the solidus of K-rich minerals in the subducted plate implies that K can only participate in magmatism in the subduction zone. Before the plate reaches a depth of ~300 km, the mica-group minerals,K-feldspar, evaporitic minerals, and other K-rich minerals are gradually consumed and rarely participate in deeper processes. In contrast, relatively Na-rich minerals such as pyroxene can be transported into the deeper mantle with eclogitic oceanic crust, and the resultant melt can enter the lithosphere owing to convective mantle flow. This forms metasomatic minerals, such as amphibole, in the lithospheric mantle, which melt to generate Na-rich and low-Si alkaline magmas.

Prototethyan orogenesis in southwest Yunnan and Southeast Asia

The Prototethyan Ocean has been suggested as an Early Paleozoic Ocean developed at the Gondwana northern margin. However, its spatial pattern, subduction style and closure time in SW Yunnan and SE Asia still remain unknown. The Prototethyan evolution in SW Yunnan and SE Asia and its internal connection with the South China Kwangsian(Ordo-Silurian)intracontinental orogenesis are also poorly constrained. By summarizing and analyzing the Early Paleozoic geological records in the Sibumasu and Indochina blocks, the eastern South China and SW Japan, this paper proposes the existence of a giant OrdoSilurian igneous belt along the Gondwana northern margin. A preliminary limitation has been obtained regarding the source nature and migration pattern of the igneous belt. Our data allow us to propose a model of the Early Paleozoic Andean-type active continental margin along the East Gondwana northern margin. This is the foundation to determine the southward subduction of the southern branch of the eastern Prototethyan Ocean underneath the Sibumasu and Indochina blocks along the YunxianMenghai(SW Yunnan)-Thailand Peninsula and the Tam Ky-Phouc Son suture in Central Vietnam, respectively, and the eastward linkage with the Early Paleozoic Osaka subduction zone in SW Japan across the peripheral Sanya area. These data synthetically indicate an easterly-diachronous and propagating Andean-type Cambrian(Furongian)-Silurian(Llandovery) orogenesis along the Gondwana northern margin from Nepal, NW India, South Tibet, Qiangtang to Central Vietnam across South Indochina and Sibumasu. This paper reconstructs the Early Paleozoic locations of the Sibumasu and Indochina fragments, as well as SW Japan and South China continent in the Gondwana northern margin, and proposes the far-field effect on the South China Kwangsian intra-continental orogenesis from the subduction of the Early Paleozoic Prototethyan southern branch.