Stochastic seismic inversion and Bayesian facies classification applied to porosity modeling and igneous rock identification

We apply stochastic seismic inversion and Bayesian facies classification for porosity modeling and igneous rock identification in the presalt interval of the Santos Basin. This integration of seismic and well-derived information enhances reservoir characterization. Stochastic inversion and Bayesian classification are powerful tools because they permit addressing the uncertainties in the model. We used the ES-MDA algorithm to achieve the realizations equivalent to the percentiles P10, P50, and P90 of acoustic impedance, a novel method for acoustic inversion in presalt. The facies were divided into five: reservoir 1,reservoir 2, tight carbonates, clayey rocks, and igneous rocks. To deal with the overlaps in acoustic impedance values of facies, we included geological information using a priori probability, indicating that structural highs are reservoir-dominated. To illustrate our approach, we conducted porosity modeling using facies-related rock-physics models for rock-physics inversion in an area with a well drilled in a coquina bank and evaluated the thickness and extension of an igneous intrusion near the carbonate-salt interface. The modeled porosity and the classified seismic facies are in good agreement with the ones observed in the wells. Notably, the coquinas bank presents an improvement in the porosity towards the top. The a priori probability model was crucial for limiting the clayey rocks to the structural lows. In Well B, the hit rate of the igneous rock in the three scenarios is higher than 60%, showing an excellent thickness-prediction capability.

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.

Plume-lithosphere interaction in the Comei Large Igneous Province: Evidence from two types of mafic dykes in Gyangze, south Tibet, China

Two suites of mafic dykes,T1193-A and T1194-A,outcrop in Gyangze area,southeast Tibet.They are in the area of Comei LIP and have indistinguishable field occurrences with two other dykes in Gyangze,T0902 dyke with 137.7±1.3 Ma zircon age and T0907 dyke with 142±1.4 Ma zircon age reported by Wang YY et al.(2016),indicating coeval formation time.Taking all the four diabase dykes into consideration,two different types,OIB-type and weak enriched-type,can be summarized.The“OIB-type”samples,including T1193-A and T0907 dykes,show OIB-like geochemical features and have initial Sr-Nd isotopic values similar with most mafic products in Comei Large Igneous Provinces(LIP),suggesting that they represent melts directly generated from the Kerguelen mantle plume.The“weak enriched-type”samples,including T1194-A and T0902 dykes,have REEs and trace element patterns showing withinplate affinity but have obvious Nb-Ta-Ti negative anomalies.They show uniform lowerεNd(t)values(−6‒−2)and higher 87Sr/86Sr(t)values(0.706‒0.709)independent of their MgO variation,indicating one enriched mantle source.Considering their closely spatial and temporal relationship with the widespread Comei LIP magmatic products in Tethyan Himalaya,these“weak enriched-type”samples are consistent with mixing of melts from mantle plume and the above ancient Tethyan Himalaya subcontinental lithospheric mantle(SCLM)in different proportions.These weak enriched mafic rocks in Comei LIP form one special rock group and most likely suggest large scale hot mantle plume-continental lithosphere interaction.This process may lead to strong modification of the Tethyan Himalaya lithosphere in the Early Cretaceous.

Assessment of Radiological Hazards of Sedimentary, Igneous and Sediments Natural Rocks

Gamma-ray spectroscopy based on a 100% efficiency hyper-pure germanium detector was used to evaluate the activity concentrations of 226Ra, 232Th, and 40K natural radionuclides in sedimentary, conglomerate, igneous and sedi-ments rock samples collected from four different locations in Eastern desert in Egypt. The obtained activity concentrations are used to evaluate the radi-ological hazards indices, absorbed dose rate, annual effective dose equivalent in air, radium equivalent, external and internal hazard index, radiation level index, annual gonadal dose equivalent, excess lifetime cancer risk and expo-sure rate. The results show that 1) the absorbed dose rate depends on the rock type, 2) the annual effective dose equivalent in air in 71% of sample below 20 mSvy-1 (permissible limit for workers), 3) the conglomerate rocks show low radioactivity level, 4) sedimentary rocks are rich in radium while igneous rocks are rich in thorium and the sediments rocks are rich in both radium and thorium.

Petrogenesis of Oligocene volcanic rocks of the Lake Tana area,Ethiopian large Igneous Province

The Lake Tana area is located within a complex volcano-tectonic basin on the northwestern Ethiopian plateau.The basin is underlain by a thick succession of Oligocene transitional basalts and sub-alkaline rhyolites overlain in places,particularly south of the lake,by Quaternary alkaline to mildly transitional basalts,and dotted with Oligo-Miocene trachyte domes and plugs.This paper presents the results of integrated field,petrographic,and major and trace element geochemical studies of the Lake Tana area volcanic rocks,with particular emphasis on the Oligocene basalts and rhyolites.The studies reveal a clear petrogenetic link between the Oligocene basalts and rhyolites.The Oligocene basalts are:(1)plagioclase,olivine,and/or pyroxene phyric;(2)show an overall decreasing trend in MgO,Fe_(2)O_(3),and CaO with silica;(3)have relatively low Mg#,Ni and Cr contents and high Nb/La and Nb/Yb ratios;and(4)show LREE enriched and generally flat HREE patterns.All these imply the origin of the Oligocene basalts by shallow-level fractional crystallization of an enriched magma sourced at the asthenospheric mantle.The Oligocene rhyolites:(1)are enriched in incompatible while depleted in compatible trace elements,P and Ti;(2)show a strong negative Eu anomaly;(3)contain appreciable amounts of plagioclase,apatite,and Fe-Ti oxides;and(4)show clear geochemical similarity with well-constrained rhyolites from the Large Igneous Province(LIP)of the northwestern Ethiopian plateau.Low-pressure fractional crystallization of mantle-derived basaltic magma in crustal magma chambers explains the origin of these rhyolites.Our study further shows that the Oligocene basalts and rhyolites are co-genetic and the felsic rocks of the Lake Tana area are related differentiates of the flood basalt volcanism in the northwestern Ethiopian plateau.

Textural and compositional zoning in plagioclase phenocrysts:implications for magma chamber processes in the Emeishan large Igneous Province,SW China

Textural and compositional zoning within plagioclase phenocrysts records the magma chamber processes,such as magma differentiation,magma recharge and mixing,and crustal contamination.The plagioclase phenocrysts in the Daqiao and Qiaojia plagioclase-phyric basalts from the Emeishan Large Igneous Province(LIP)show complex textural and compositional zoning patterns,e.g.,normal,reverse,oscillatory,and patchy zoning patterns.Most plagioclase phenocrysts exhibit a core–rim normal zoning pattern(Pl-A)with euhedral high-An cores(An=76–78%,in mole fraction)and low-An rims(An=68–72%),indicative of the crystal regrowth processes caused by recharge of relatively evolved magmas after the formation of high-An cores.Some phenocrysts have a core–rim reverse zoning pattern(Pl-B)with irregular ovaloid cores,characterized by extremely low An(60–61 mol%)and Ba(84–88 ppm)contents and extremely high87Sr/86Sr ratios(0.7120–0.7130).The rims of the Pl-B have relatively high An(69–72%),Ba(~160 ppm)contents,and low87Sr/86Sri(~0.7056).These Pl-B plagioclase phenocrysts preserve the information about the interaction between the crustal xenocrysts and the transporting magmas.Some plagioclase phenocrysts show a core–mantle–rim oscillatory zoning pattern(Pl-C)with multiple oscillations of An(70–80%),Ba(88–147ppm)from core to rim,revealing replenishment and mixing of multiple batches of basaltic melts with diverse compositions.87Sr/86Sr ratios of the Pl-C do not vary significantly(0.7050–0.7054).A small portion of phenocrysts has patchy patterns in the cores(Pl-D),where the low-An patches(72–75%)in form of elliptical or irregular elongated shapes were enclosed by the high-An domains(80–87%).These features can be attributed to crystal dissolution and regrowth processes during the reaction between earlyformed low-Cumulates and recharged hot primitive melts.The cores,mantles,and rims of different types of plagioclase phenocrysts(except the core of Pl-B)commonly display nearly constant Sr isotopic compositions,implying insignificant wall-rock assimilation at shallow-level magma reservoir(s)during the growth of these plagioclase phenocrysts.In conclusion,the massive crystallization of plagioclase in the late stage was an important controlling factor for the formation of iron-rich basalts in the Emeishan LIP.

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.

Gravity and magnetic anomalies field characteristics in the South China Sea and its application for interpretation of igneous rocks

Igneous rocks in the South China Sea have broad prospects for oil and gas exploration.Integrated geophysical methods are important approaches to study the distribution of igneous rocks and to determine and identify igneous rock bodies.Aimed at the characteristics of gravity and magnetic fields in the South China Sea,several potential field processing methods are preferentially selected.Reduction to the pole by variable inclinations in the area of low magnetic latitudes is used to perform reduction processing on magnetic anomalies.The preferential continuation method is used to separate gravity and magnetic anomalies and extract the gravity and magnetic anomaly information of igneous rocks in the shallow part of the South China Sea.The 3D spatial equivalent distribution of igneous rocks in South China Sea is illustrated by the 3 D correlation imaging of magnetic anomalies.Since the local anomaly boundaries are highlighted gravity and magnetic gradients,the distribution characters of different igneous rocks are roughly outlined by gravity and magnetic correlation analysis weighted by gradient.The results show the distribution of igneous rocks is controlled and influenced by deep crustal structure and faulting.

Characteristics of Meso-Cenozoic Igneous Complexes in the South Yellow Sea Basin,Lower Yangtze Craton of Eastern China and the Tectonic Setting

The South Yellow Sea Basin is partially surrounded by the East Asian continental Meso- Cenozoic widespread igneous rocks belt. Magnetic anomaly and multi-channel seismic data both reveal the prevalent occurrence of igneous rocks. We preliminarily defined the coupling relation between magnetic anomalies and igneous rock bodies. Some igneous complexes were also recognized by using multi-channel seismic and drilling data. We identified various intrusive and extrusive igneous rock bodies, such as stocks, sills, dikes, laccoliths and volcanic edifice relics through seismic facies analysis. We also forecasted the distribution characteristics of igneous complexes. More than fifty hypabyssal intrusions and volcanic relics were delineated based on the interpretation of magnetic anomaly and dense intersecting multi-channel seismic data. It is an important supplement to regional geology and basin evolution research. Spatial matching relations between igneous rock belts and fractures document that extensional N-E and N-NE-trending deep fractures may be effective pathways for magma intrusion. These fractures formed under the influence of regional extension during the Meso- Cenozoic after the Indosinian movement. Isotopic ages and crosscutting relations between igneous rock bodies and the surrounding bedded sedimentary strata both indicate that igneous activities might have initiated during the Late Jurassic, peaked in the Early Cretaceous, gradually weakened in the Late Cretaceous, and continued until the Miocene. Combined with previous studies, it is considered that the Meso-Cenozoic igneous activities, especially the intensive igneous activity of the Early Cretaceous, are closely associated with the subduction of the Paleo-Pacific Plate.

SHRIMP Zircon U-Pb Ages of the Wajilitag Igneous Complex: Constraints on the Origin of A-type Granitoids in the Tarim Large Igneous Province, NW China

Objective Large igneous provinces （LIPs） are sites of spatially contiguous, rapidly emplaced magmatic rocks, which represent the physical and chemical transfer of material from the mantle to the crust. Exposed within some continental LIPs are felsic and rnafic plutonic and volcanic rocks. Although their volumes are minor compared to the flood basalts, the plutonic rocks of continental LIPs are often associated with economic deposits of precious metals. Within the Permian Tarim LIP of NW China, there are at least two layered ultramafic-mafic intrusions （e.g. Wajilitag and Piqiang） contain economically important Fe- Ti-V oxide deposits. Spatially associated with these layered ultramafic-mafic intrusions are syenitic and granitic plutons, which have chemical characteristics of A- type granitoids.

Chemostratigraphy of Flood Basalts in the Garze-Litang Region and Zongza Block: Implications for Western Extension of the Emeishan Large Igneous Province, SW China

The Late Permian Emeishan Large Igneous Province (ELIP) is commonly regarded as being located in the western part of the Yangtze craton, SW China, with an asymmetrical shape and a small area. This area, however, is just a maximum estimation because some parts of the ELIP were not recognized or dismembered and destroyed during the Triassic to Cenozoic tectonism. In this paper, the chemostratigraphical data of the Zongza block, the Garze-Litang belt and the Songpan-Garze block suggest that the Late Permian basalts in these areas have remarkable similarities to the ELIP basalts in petrography and geochemistry. Flood basalts in the Sanjiangkou area are composed of the lower part of the low-Ti (LT) tholeiite and the upper part of the high-Ti (HT) tholeiite, which is the same as the flood basalts on the western margin of the Yangtze craton. Flood basalts in the Zongza and Songpan-Garze areas, which are far from the Yangtze craton, consist of HT tholeiite only. This is the same as the flood basalts within the Yangtze craton. Therefore we argue that these contemporary basalts all originated from the Emeishan mantle plume, and the ELIP could have a significant westward extension with an outcropped area of over 500,000 km2. This new scenario shows that the LT tholeiite occurs on the western margin of the Yangtze craton, while the HT tholeiite overlying the LT basalts occupies the whole area of the ELIP.

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 are, late oceanic are and continental are 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 continent-continent-collisional orogeny.

Geochemistry of Permian Mafic Igneous Rocks from the Napo-Qinzhou Tectonic Belt in Southwest Guangxi, Southwest China: Implications for Arc-Back Arc Basin Magmatic Evolution

The Napo-Qinzhou Tectonic Belt （NQTB） lies at the junction of the Yangtze, Cathaysia and Indochina （North Vietnam） Blocks, which is composed of five major lithotectonic subunits： the Qinzhou-Fangcheng Suture Zone （QFSZ）, the Shiwandashan Basin （SB）, the Pingxiang-Nanning Suture Zone （PNSZ）, the Damingshan Block （DB） and the Babu-Lingma Suture Zone （BLSZ）. On the basis of geochemical compositions, the Permian mafic igneous rocks can be divided into three distinct groups： （1） mafic igneous rocks （Group 1） from the Longjing region in the PNSZ and Hurun region in the BLSZ, which are characterized by intermediate Ti, P and Zr with low Ni and Cr contents; （2） mafic igneous rocks （Group 2） from the Naxiao and Chongzuo region in the DB, characterized by low-intermediate Ti, P and Zr with high Ni and Cr concentrations; and （3） mafic igneous rocks （Group 3） from the Siming region in the Jingxi carbonate platform of the northwestern margin of the NQTB, with intermediate-high Ti, P and Zr and low Ni and Cr contents. The Group 1 rocks yield a weighted mean 206 Pb/ 238 U age of 250.5±2.8 Ma and are geochemically similar to basalts occurring in back-arc basin settings. The Group 2 rocks exhibit geochemical features to those basalts in island arcs, whereas the Group 3 rocks show geochemical similarity to that of ocean island basalts. All three groups are characterized by relatively low ε Nd （t） values （-2.61 to ＋1.10） and high initial 87 Sr/ 86 Sr isotopic ratios （0.705309-0.707434）, indicating that they were derived from a subduction-modified lithospheric mantle and experienced assimilation, fractional crystallization, and crustal contamination or mixing during magmatic evolution. Accordingly, we propose the existence of an arc-back arc basin system that developed along the NQTB at the border of SW Guangxi Province （SW China） and northern Vietnam, and it was formed by continued northwestward subduction of the Cathaysian （or Yunkai） Block under the Yangtze Block, and northeastward subduction of the Indochina Block beneath the Yangtze Block during Permian time.

Reflectivity forward modeling and a CSSI method seismic inversion study of igneous intrusive area,coked area,and gas-enriched area located within a coal seam

We applied the reflectivity method and the constrained sparse spike inverse modeling(CSSI) method to the interpretation of coal field lithologic seismic data.After introducing the principles of these two methods we discuss some parameters of a geological model involving possible gas enriched areas or intruded igneous rock.The geological model was constructed and a 60 Hz seismic response profile was obtained looking for igneous rock intrusion and coked areas of the coal seam using the reflectivity method.Starting from synthesized logging data from two wells and a synthesized seismic wavelet we calibrated the model to show accurate strata.Finally,we predicted the lithology within a 10 m igneous rock area,a 3 m coal seam area,and a coked area using the CSSI technique.The results show that the CSSI technique can identify hard to recognize lithologic features that normal profil-ing methods might miss.It can quantitatively analyze and evaluate the intrusive area,the coked area,and the gas-enriched area.

Eruption of the Continental Flood Basalts at -259 Ma in the Emeishan Large Igneous Province, SW China: Evidence from Laser Microprobe ^(40)Ar/^(39)Ar Dating

A suite of continental flood basalts sampled over a vast exposure and stratigraphic thickness in the Emeishan large igneous province （LIP）, SW China was investigated for laser microprobe ^40Ar/^39Ar dating. There are two ^40Ar/^39Ar age groups for these basalts, corresponding to 259-246 Ma and 177-137 Ma, respectively. A well-defined isochron gives an eruption age of huge quantities of mafic magmas at 258.9±3.4 Ma, which is identical to previous dating and paleontological data. Much younger ^40Ar/^39Ar ages for some basalts with Iow-greenschist metamorphic facies probably recorded a late thermo-tectonic event caused by collision between the Yangtze and Qiangtang continental blocks during the Mesozoic, which resulted in the reset of argon isotope system. The ^40Ar/^39Ar age data, we present here, combined with previous dating and paleontological data, suggest relatively short duration （about 3 Ma） of mafic volcanism, which have important implication on mantle plume genesis of the Emeishan continental flood basalts in the LIP.

On the radiogenic heat production of metamorphic,igneous,and sedimentary rocks

Sedimentary rocks cover-73% of the Earth's surface and metamorphic rocks account for approximately91% of the crust by volume. Understanding the average behavior and variability of heat production for these rock types are vitally important for developing accurate models of lithospheric temperature. We analyze the heat production of ～204,000 whole rock geochemical data to quantify how heat production of these rocks varies with respect to chemistry and their evolution during metamorphism. The heat production of metaigneous and metasedimentary rocks are similar to their respective protoliths. Igneous and metaigneous samples increase in heat production with increasing SiO_2 and K_2 O, but decrease with increasing FeO, MgO and CaO. Sedimentary and metasedimentary rocks increase in heat production with increasing Al_2 O_3, FeO, TiO_2, and K_2 O but decrease with increasing CaO. For both igneous and sedimentary rocks, the heat production variations are largely correlated with processes that affect K_2 O concentration and covary with other major oxides as a consequence. Among sedimentary rocks,aluminous shales are the highest heat producing(2.9 μW^(-3)) whereas more common iron shales are lower heat producing(1.7 μW m^(-3)). Pure quartzites and carbonates are the lowest heat producing sedimentary rocks. Globally, there is little definitive evidence for a decrease in heat production with increasing metamorphic grade. However, there remains the need for high resolution studies of heat production variations within individual protoliths that vary in metamorphic grade. These results improve estimates of heat production and natural variability of rocks that will allow for more accurate temperature models of the lithosphere.

Controlling the effect of a distant extremely thick igneous rock in overlying strata on coal mine disasters

Based on theoretical analysis, similarity simulation tests, numerical simulation analysis and field observations, we analyzed rock collapse and rules of fraction evolution of overlying rocks and studied the rules in controlling the effect of an extremely thick igneous rock, found above a main mining coal seam in an area prone to coal mine disasters in the Haizi Coal Mine. The results show that this igneous rock, called a "main key stratum", will not subside nor break for a long time, causing lower fractures and bed separations not to close. The presence of igneous rock plays an important role in rock bursts, mine floods, gas outburst and surface subsidence in coal mines. By analyzing the rules in controlling the effect of this igneous rock, we provide useful references for safety and high efficiency mining in coal mines under special geological conditions.

The Emeishan large igneous province:A synthesis

The late Permian Emeishan large igneous province （EL1P） covers -0.3× 10-6 kmL of the western margin of the Yangtze Block and Tibetan Plateau with displaced, correlative units in northern Vietnam （Song Da zone）. The ELIP is of particular interest because it contains numerous world-class base metal deposits and is contemporaneous with the late Capitanian （-260 Ma） mass extinction. The flood basalts are the signature feature of the ELIP but there are also ultramafic and silicic volcanic rocks and layered mafic- ultramafic and silicic plutonic rocks exposed. The EL1P is divided into three nearly concentric zones （i.e. inner, middle and outer） which correspond to progressively thicker crust from the inner to the outer zone. The eruptive age of the ELIP is constrained by geological, paleomagnetic and geochronological evidence to an interval of 〈3 Ma. The presence of picritic rocks and thick piles of flood basalts testifies to high temperature thermal regime however there is uncertainty as to whether these magmas were derived from the subcontinental lithospheric mantle or sub-lithospheric mantle （i.e. asthenosphere or mantle plume） sources or both. The range of Sr （Isr ≈ 0.7040-0.7132）, Nd （ENd（t） ≈ -14 tO ＋8）, Pb （206-pb/204-pb1 ≈ 17.9-20.6） and Os （Yos ≈ -5 to ＋11） isotope values of the ultramafic and mafic rocks does not permit a conclusive answer to ultimate source origin of the primitive rocks but it is clear that some rocks were affected by crustal contamination and the presence of near-depleted isotope compo- sitions suggests that there is a sub-lithospheric mantle component in the system. The silicic rocks are derived by basaltic magmas/rocks through fractional crystallization or partial melting, crustal melting or by interactions between mafic and crustal melts. The formation of the Fe-Ti-V oxide-ore deposits is probably due to a combination of fractional crystallization of Ti-rich basalt and fluxing of C02-rich fluids whereas the Ni-Cu-（PGE） deposits are related to crystallization and crustal contamination of mafic or ultramafic magmas with subsequent segregation of a sulphide-rich portion. The ELIP is considered to be a mantle plume-derived LIP however the primary evidence for such a model is less convincing （e.g. uplift and geochemistry） and is far more complicated than previously suggested but is likely to be derived from a relatively short-lived, plume-like upwelling of mantle-derived magmas. The emplacement of the ELIP may have adversely affected the short-term environmental conditions and contributed to the decline in biota durin~ the late Caoitanian.

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.

Yanshanian Magma-Tectonic-Metallogenic Belt in East China of Circum-Pacific Domain(Ⅰ):Igneous Rocks and Orogenic Processes

Yanshanian igneous rocks in the East China, on an orogenic belt scale, are characterized by the continental marginal arc in petrology and geochemistry as Andes and West USA, except for the Hercyn type biotite two mica muscovite granite belt in the Nanling region. Three segments of the Yanshanian igneous rocks along the belt are recognized. In terms of magma tectonic event sequence, the north, middle and south segments have counter clockwise (ccw), clockwise (cw) and ccw+cw pTt paths of the orogenic process, respectively. A genetic model of the lithospheric delamination (loss of the lithospheric root in about 120 km) in combination with the oceanic subduction for the Yanshanian Andes like orogenic belt and both the crust and lithosphere thickening for the Yanshanian Hercyn type Nanling orogenic belt in the East China is suggested.