皖南新元古代花岗闪长岩地球化学特征及构造环境

皖南新元古代花岗闪长岩沿祁门-歙县-三阳深断裂呈串珠状出露。本文在对其岩石学、地球化学细致分析的基础上,探讨了岩体的岩石成因和产出环境。皖南新元古代花岗闪长岩主要由石英、钾长石和斜长石组成,普遍含富铝矿物黑云母和堇青石,副矿物包括锆石、磷灰石、钛铁矿、独居石、磷钇矿、极少的磁铁矿等。地球化学分析数据显示,岩石总体具高硅、高钾、高铝和低钠、低镁、低钙的特征;岩石富碱（ALK=6．63％）,高K2O／Na2O比值（1．33）。里特曼指数δ为0．8～2．91,碱度率AR为1．56～3．14,属高钾钙碱性系列。岩石铝饱和指数（A／CNK-1．31）大于1．1,具强过铝质S型花岗岩的特征。岩石稀土元素呈轻稀土富集、重稀土亏损的特征,∑LREE／∑HREE比值为5．36～8．36,具较强的负铕异常（δEu=0．39～0．7）,配分模式为右倾“V”字形态;微量元素明显富集Rb、Th而亏损Ba、Nb、Ta、Sr等,为低Sr高Yb型花岗岩。地球化学特征显示其岩浆源于围岩-中元古代牛屋组浅变质千枚岩的部分熔融,反映陆-陆碰撞挤压造山环境,为晋宁运动晚期华夏板块向北俯冲与扬子板块碰撞造山的火山弧产物。     

Geochemical characteristics of soil gas in the Yanhuai basin,northern China

The geochemical backgrounds and origins of soil gases in the Yanhuai basin are discussed based on the regional seismogeological data and concentrations of Rn, Hg, CO<sub<2</sub<, H<sub<2</sub<, He and CH<sub<4</sub< in soil gas measured at 422 investigating sites in field during September to October 2007. The geochemical background values of Rn, Hg, CO<sub<2</sub<, H<sub<2</sub<, He and CH<sub<4</sub< are (8105.8?±5937.4) Bq/m<sup<3</sup<, (9.7?±5.8) ng/m<sup<3</sup<, (395.9?±35.3)?×10<sup<?6</sup<, (4.0?±2.3)?×10<sup<?6</sup<, (15.9?±10.4)×?10<sup<?6</sup< and (12.7?±8.1)?×10<sup<?6</sup<, respectively. The geochemical backgrounds of the soil gases are higher in the eastern part of the Yanhuai basin. The main factors affecting the gasgeochemical backgrounds are gaseous origins, structure of the crust, faults, stratum and microbe activity. The higher values of gasgeochemical backgrounds in the eastern part are attributed to the existence of low-velocity zones in the upper crust, stronger tectonic activity and more contributions of Hg and He derived from the deep-earth and Rn originated from granite, corresponding to stronger seismic activity. The results can be applied to identifying seismic precursor from monitoring data of gases in the studied area.     

Geochemical characteristics of soil gas in the Yanhuai basin,northern China

The geochemical backgrounds and origins of soil gases in the Yanhuai basin are discussed based on the regional seismogeological data and concentrations of Rn,Hg,CO2,H2,He and CH4 in soil gas measured at 422 investigating sites in field during September to October 2007.The geochemical background values of Rn,Hg,CO2,H2,He and CH4 are(8105.8±5937.4) Bq/m3,(9.7±5.8) ng/m3,(395.9±35.3)×10-6,(4.0±2.3)×10-6,(15.9±10.4)×10-6 and(12.7±8.1)×10-6,respectively.The geochemical backgrounds of the soil gases are higher in...     

Geochemical characteristics and origin of light hydrocarbons in biogenic gas

The light hydrocarbon geochemical characteristics of biogenic gases from Sebei 1 gas field in the Qaidam Basin, Baoshan gas field in the Baoshan Basin and Alaxin gas field, Puqian gas pool, Aonan gas pool in the Songliao Basin are studied and the origin is discussed based on the composition and isotope data of gases. The isoalkane contents among light hydrocarbons in natural gas show a negative relationship with δ13C1 values. The isoalkane contents of the gases with δ13C1 values of less than ?60‰ are also h...     

Geochemistry of the high-K calc-alkaline basaltic sills and dykes in the South Rhodope Massif (N Greece)

In this paper mafic sills and dykes found in Palaeogenic sedimentary rocks of the South Rhodope Massif (Greece) are studied. Their mineralogy is represented mainly by plagioclases (An₄₅-An₈₈), amphiboles (Mg-hornblende, Mg-hastingsite) and clinopyroxenes of salitic compositions with subordinate biotite. Chemistry and corresponding discriminant diagrams (Ti-Zr-Y, Ti-Zr-Sr) and discriminant functions (F₁-F₂ from pyroxene) classify them as basalts and basaltic andesites of the high-K calc-alkaline rocks series. According to their geochemical and petrological features an orogenic nature results for the rocks in study. REE abundances show an enrichement of LREE and a moderate fractionation in HREE patterns. The above together with the non significant europium anomalies (Eu/Eu*=0,92–0,85) and the relatively high contents of K, Ba and Sr in these rocks indicate a magma genesis by partial melting of an upper mantle source already enriched in LILE. A K-Ar determination on hornblende phenocrysts gave a Lower Oligocene age (33,5±1.2 m.y.) which is in accordance with the age of the andesitic lavas extended to the northwest of the studied area.     

Geochemistry of calc-alkaline volcanics in northwestern Nigeria,and a possible Pan-African suture zone

Major and trace element characteristics of dacites and rhyolites overlying and intruding basement rocks in northwestern Nigeria most closely resemble those of intracontinental orogenic volcanic associations. REE patterns point to a deep-seated source for the magmas, perhaps involving garnet fractionation at mantle depths and low-pressure plagioclase fractionation. The occurrence of calc-alkaline volcanics, small basic-ultrabasic complexes and major transcurrent faulting, is consistent with the presence of a Pan-African suture zone in northwestern Nigeria.     

Petrogenesis and tectonic implications of Early Cretaceous high-K calc-alkaline volcanic rocks in the Laiyang Basin of the Sulu Belt, eastern China

Abstract Early Cretaceous high‐K calc‐alkaline volcanism occurring in the Laiyang Basin north of the Sulu high‐pressure to ultrahigh‐pressure (HP‐UHP) Metamorphic Belt, eastern China, comprises a wide spectrum of rock types, ranging from trachybasalts to trachydacites. The basaltic–andesitic rocks erupted at 107–105 Ma, spanning an SiO₂ range of 50.1–59.6% and an MgO range of 2.6–7.2%, and are characterized by large ion lithophile element (LILE; e.g. Ba and K) and light rare earth element (LREE) enrichment, high field strength element (HFSE) depletion and highly radiogenic Sr but non‐radiogenic Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr(i) = 0.70750–0.70931; ?_Nd(t) = ?17.9 ? ?15.6). The geochemical similarities between these rocks and the earlier Sulu Belt lamprophyres suggest that both types of mafic rocks were derived from similar mantle sources with LILE and LREE enrichment. Thus, the Wulian–Qingdao–Yantai Fault that separates the two terranes at the surface should not be considered as a lithospheric boundary between the North China and Yangtze blocks. The felsic lavas erupted at 93–91 Ma, spanning an SiO₂ range of 61.6–67.0% and an MgO range of 1.1–2.6%, and show a trace element geochemistry similar to the basaltic rocks, but with higher radiogenic Sr and even lower Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr(i) = 0.70957–0.71109; ?_Nd(t) = ?19.1 ? ?17.5), similar to I‐type granitoids in the Sulu Belt. A crustal origin was proposed to explain their compositions (which are comparable to those of experimental slab melts), the >10 Ma eruption interval and the compositional gaps in some elements (e.g. P, Ti and Sr) between them and the older basaltic–andesitic rocks. These melts were derived from predominant metaigneous protoliths containing mafic accumulative counterparts of the basaltic–andesitic and/or lamprophyric magmas. The extensive extrusion of Early Cretaceous high‐K calc‐alkaline rocks in the Laiyang Basin favored an extensional regime in response to the progressive attenuation of the thickened lithosphere and orogenic collapse, as reflected in the development of the basin from a foreland basin (before the end of the Jurassic period) to a fault basin (since the Early Cretaceous period).     

Morphology,sedimentary characteristics,and origin of the Dongsha submarine canyon in the northeastern continental slope of the South China Sea

The Dongsha submarine canyon is a large canyon belonging to a group of canyons on the northeastern South China Sea margin. Investigation of the Dongsha canyon is important for understanding the origin of this canyon group as well as the transport mechanism of sediments on the margin, and the evolution of the Taixinan foreland basin and the associated Taiwan orogenic belt. In this study, the morphology, sedimentary characteristics, and origin of the Dongsha canyon were investigated by integrating high-resolution multi-channel seismic reflection profiles and high-precision multibeam bathymetric data. This is a slope-confined canyon that originates in the upper slope east of the Dongsha Islands, extends downslope in the SEE direction, and finally merges with the South Taiwan Shoal canyon at a water depth of 3000 m. The total length and average width of the canyon are around 190 and 10 km, respectively. Eleven seismic sequence boundaries within the canyon fills were identified and interpreted as incision surfaces of the canyon. In the canyon fills, four types of seismic facies were defined: parallel onlap fill, chaotic fill, mounded divergent facies, and migrated wavy facies. The parallel onlap fill facies is interpreted as alternating coarser turbidites or other gravity-flow deposits and fine hemipelagic sediments filling the canyon valley. The chaotic fill facies is presumed to be debrites and/or basal lag deposits filling the thalwegs. The mounded divergent and migrated wavy seismic facies can be explained as canyon levees consisting mainly of overspilled fine turbidites and sediment waves on the levees or on the canyon-mouth submarine fans. Age correlation between the sequence boundaries and the ODP Site 1144 data suggests that the Dongsha canyon was initiated at approximately 0.9 Ma in the middle Pleistocene. Mapping of the canyon indicates that the canyon originated at the upstream portion of the middle reach of the modern canyon, and has been continuously expanding both upstream and downstream by retrogressive erosion, incision, and deposition of turbidity currents and other gravity transport processes. The ages of the sequence boundaries representing major canyon incision events are in good agreement with those of global sea-level lowstands, indicating that sea-level changes may have played an important role in the canyon's development. The Dongsha canyon developed in a region with an active tectonic background characterized by the Taiwan uplifting and the development of the Taixinan foreland basin. However, no evidence suggests that the canyon formation is directly associated with local or regional faulting and magmatic activities. Turbidity currents and other gravity transport processes(including submarine slides and slumps) may have had an important influence on the formation and evolution of the canyon.     

Geochemical characteristics and origin of gases from the Upper,Lower Paleozoic and the Mesozoic reservoirs in the Ordos Basin,China

The Ordos Basin, the second largest sedimentary basin in China, contains the broad distribution of natural gas types. So far, several giant gas fields have been discovered in the Upper and Lower Paleozoic in this basin, each having over 1000×10⁸m³ of proven gas reserves, and several gas pools have also been discovered in the Mesozoic. This paper collected the data of natural gases and elucidated the geochemical characteristics of gases from different reservoirs, and then discussed their origin. For hydrocarbons preserved in the Upper Paleozoic, the elevated δ ¹³C values of methane, ethane and propane indicate that the gases would be mainly coal-formed gases; the singular reversal in the stable carbon isotopes of gaseous alkanes suggests the mixed gases from humic sources with different maturity. In the Lower Paleozoic, the δ ¹³C₁ values are mostly similar with those in the Upper Paleozoic, but the δ ¹³C₂ and δ ¹³C₃ values are slightly lighter, suggesting that the gases would be mixing of coal-type gases as a main member and oil-type gases. There are multiple reversals in carbon isotopes for gaseous alkanes, especially abnormal reversal for methane and ethane (i.e. δ ¹³C₁>δ ¹³C₂), inferring that gases would be mixed between high-mature coal-formed gases and oil-type gases. In the Mesozoic, the δ ¹³C values for gaseous alkanes are enriched in ¹²C, indicating that the gases are mainly derived from sapropelic sources; the carbon isotopic reversal for propane and butane in the Mesozoic is caused by microbial oxidation and mixing of gases from sapropelic sources with different maturity. In contrast to the Upper Paleozoic gases, the Mesozoic gases are characterized by heavier carbon isotopes of iso-butane than normal butane, which may be caused by gases generated from different kerogen types. Finally, according to δ ¹³C₁-R ₀ relationship and extremely low total organic carbon contents, the Low Paleozoic gases would not be generated from the Ordovician source as a main gas source, bycontrast, the Upper Paleozoic source as a main gas source is contributed to the Lower Paleozoic gases.

     

Geochemical characteristics and origin of gases from the Upper,Lower Paleozoic and the Mesozoic reservoirs in the Ordos Basin,China

The Ordos Basin,the second largest sedimentary basin in China,contains the broad distribution of natural gas types.So far,several giant gas fields have been discovered in the Upper and Lower Paleozoic in this basin,each having over 1000×10 8 m 3 of proven gas reserves,and several gas pools have also been discovered in the Mesozoic.This paper collected the data of natural gases and elucidated the geochemical characteristics of gases from different reservoirs,and then discussed their origin.For hydrocarbons preserved in the Upper Paleozoic,the elevatedδ13C values of methane,ethane and propane indicate that the gases would be mainly coal-formed gases;the singular reversal in the stable carbon isotopes of gaseous alkanes suggests the mixed gases from humic sources with different maturity.In the Lower Paleozoic,theδ13C 1 values are mostly similar with those in the Upper Paleozoic,but theδ13C 2 andδ13C 3 values are slightly lighter,suggesting that the gases would be mixing of coal-type gases as a main member and oil-type gases.There are multiple reversals in carbon isotopes for gaseous alkanes,especially abnormal reversal for methane and ethane(i.e.δ13C 1 >δ13C 2 ),inferring that gases would be mixed between high-mature coal-formed gases and oil-type gases.In the Mesozoic,the δ13Cvalues for gaseous alkanes are enriched in 12C,indicating that the gases are mainly derived from sapropelic sources;the carbon isotopic reversal for propane and butane in the Mesozoic is caused by microbial oxidation and mixing of gases from sapropelic sources with different maturity.In contrast to the Upper Paleozoic gases,the Mesozoic gases are characterized by heavier carbon isotopes of iso-butane than normal butane,which may be caused by gases generated from different kerogen types. Finally,according toδ13C 1 -Ro relationship and extremely low total organic carbon contents,the Low Paleozoic gases would not be generated from the Ordovician source as a main gas source,bycontrast, the Upper Paleozoic source as a main gas source is contributed to the Lower Paleozoic gases.     

Influence of Neoproterozoic tectonic fabric on the origin of the Potiguar Basin,northeastern Brazil and its links with West Africa based on gravity and magnetic data

The Potiguar Basin is a ∼6,000 m thick aborted NE-trending rift that was formed during the Cretaceous in the continental margin of northeastern Brazil. Its ∼E–W-trending offshore faults form part of the successful continental margin rift that evolved into the South Atlantic Ocean. The region represents one of the most significant pre-Pangea breakup piercing points between eastern South America and West Africa. We used gravity, aeromagnetic, and geological data to assess the role of reactivated Precambrian shear zones and major terrain boundaries in the development of the Potiguar Basin from the Cretaceous to the Cenozoic. We also looked for possible links between these structures in northeastern Brazil and their continuation in West Africa. Our results indicate that the major fault systems of the Potiguar Basin were superimposed on the Precambrian fabric. Both gravity and magnetic maps show lineaments related to the shear zones and major terrain boundaries in the Precambrian crystalline basement, which also characterize the architecture of the rift. For example, the Carnaubais fault, the master fault of the rift system, represents the reactivation of the Portalegre shear zone, the major tectonic boundary between Precambrian terrains in the crystalline basement. In addition, part of the Moho topography is controlled by these shear zones and developed during the period of main rift extension in the Neocomian. The shear zones bounding the Potiguar rift system continue in West Africa around and underneath the Benue Basin, where fault reactivation also took place.     

Geochemical characteristics and origin of gases from the Upper,Lower Paleozoic and the Mesozoic reservoirs in the Ordos Basin,China

The Ordos Basin, the second largest sedimentary basin in China, contains the broad distribution of natural gas types. So far, several giant gas fields have been discovered in the Upper and Lower Paleozoic in this basin, each having over 1000×10⁸m³ of proven gas reserves, and several gas pools have also been discovered in the Mesozoic. This paper collected the data of natural gases and elucidated the geochemical characteristics of gases from different reservoirs, and then discussed their origin. For hydrocarbons preserved in the Upper Paleozoic, the elevated δ ¹³C values of methane, ethane and propane indicate that the gases would be mainly coal-formed gases; the singular reversal in the stable carbon isotopes of gaseous alkanes suggests the mixed gases from humic sources with different maturity. In the Lower Paleozoic, the δ ¹³C₁ values are mostly similar with those in the Upper Paleozoic, but the δ ¹³C₂ and δ ¹³C₃ values are slightly lighter, suggesting that the gases would be mixing of coal-type gases as a main member and oil-type gases. There are multiple reversals in carbon isotopes for gaseous alkanes, especially abnormal reversal for methane and ethane (i.e. δ ¹³C₁>δ ¹³C₂), inferring that gases would be mixed between high-mature coal-formed gases and oil-type gases. In the Mesozoic, the δ ¹³C values for gaseous alkanes are enriched in ¹²C, indicating that the gases are mainly derived from sapropelic sources; the carbon isotopic reversal for propane and butane in the Mesozoic is caused by microbial oxidation and mixing of gases from sapropelic sources with different maturity. In contrast to the Upper Paleozoic gases, the Mesozoic gases are characterized by heavier carbon isotopes of iso-butane than normal butane, which may be caused by gases generated from different kerogen types. Finally, according to δ ¹³C₁-R ₀ relationship and extremely low total organic carbon contents, the Low Paleozoic gases would not be generated from the Ordovician source as a main gas source, bycontrast, the Upper Paleozoic source as a main gas source is contributed to the Lower Paleozoic gases.     

Geochemical characteristics of pyrolysis gas from epimetamorphic rocks in the northern basement of Songliao Basin,Northeast China

The Xushen gas field, located in the north of Songliao Basin, is a potential giant gas area for China in the future. Its proved reserves have exceeded 1000×10⁸ m³ by the end of 2005. But, the origin of natural gases from the deep strata is still in debating. Epimetamorphic rocks as a potential gas source are widely spreading in the northern basement of Songliao Basin. According to pyrolysis experiments for these rocks in the semi-confined system, gas production and geochemistry of alkane gases are discussed in this paper. The Carboniferous-Permian epimetamorphic rocks were heated from 300°C to 550°C, with temperature interval of 50°C. The gas production was quantified and measured for chemical and carbon isotopic compositions. Results show that δ ¹³C₁ is less than −20‰, carbon isotope trend of alkane gas is δ ¹³C₁<δ ¹³C₂<δ ¹³C₃ or δ ¹³C₁<δ ¹³C₂>δ ¹³C₃, these features suggest that the gas would be coal-type gas at high-over maturity, not be inorganic gas with reversal trend of gaseous alkanes (δ ¹³C₁>δ ¹³C₂>δ ¹³C₃). These characteristics of carbon isotopes are similar with the natural gas from the basin basement, but disagree with gas from the Xingcheng reservoir. Thus, the mixing gases from the pyrolysis gas with coal-typed gases at high-over maturity or oil-typed gases do not cause the reversal trend of carbon isotopes. The gas generation intensity for epimetamorphic rocks is 3.0×10⁸–23.8×10⁸ m³/km², corresponding to R _o from 2.0% to 3.5% for organic matter.

     

Geochemical characteristics of pyrolysis gas from epimetamorphic rocks in the northern basement of Songliao Basin,Northeast China

The Xushen gas field,located in the north of Songliao Basin,is a potential giant gas area for China in the future.Its proved reserves have exceeded 1000×10 8 m 3 by the end of 2005.But,the origin of natural gases from the deep strata is still in debating.Epimetamorphic rocks as a potential gas source are widely spreading in the northern basement of Songliao Basin.According to pyrolysis experiments for these rocks in the semi-confined system,gas production and geochemistry of alkane gases are discussed in this paper.The Carboniferous-Permian epimetamorphic rocks were heated from 300℃to 550℃,with temperature interval of 50℃.The gas production was quantified and measured for chemical and carbon isotopic compositions.Results show thatδ13C 1 is less than?20‰,carbon isotope trend of alkane gas isδ13C 1 <δ13C 2 <δ13C 3 orδ13C 1 <δ13C 2 >δ13C 3 ,these features suggest that the gas would be coal-type gas at high-over maturity,not be inorganic gas with reversal trend of gaseous alkanes (δ13C 1 >δ13C 2 >δ13C 3 ).These characteristics of carbon isotopes are similar with the natural gas from the basin basement,but disagree with gas from the Xingcheng reservoir.Thus,the mixing gases from the pyrolysis gas with coal-typed gases at high-over maturity or oil-typed gases do not cause the reversal trend of carbon isotopes.The gas generation intensity for epimetamorphic rocks is 3.0×10 8 ―23.8×10 8 m 3 /km 2 ,corresponding to Ro from 2.0%to 3.5%for organic matter.     

Geochemical characteristics of pyrolysis gas from epimetamorphic rocks in the northern basement of Songliao Basin,Northeast China

The Xushen gas field, located in the north of Songliao Basin, is a potential giant gas area for China in the future. Its proved reserves have exceeded 1000×10⁸ m³ by the end of 2005. But, the origin of natural gases from the deep strata is still in debating. Epimetamorphic rocks as a potential gas source are widely spreading in the northern basement of Songliao Basin. According to pyrolysis experiments for these rocks in the semi-confined system, gas production and geochemistry of alkane gases are discussed in this paper. The Carboniferous-Permian epimetamorphic rocks were heated from 300°C to 550°C, with temperature interval of 50°C. The gas production was quantified and measured for chemical and carbon isotopic compositions. Results show that δ ¹³C₁ is less than ?20‰, carbon isotope trend of alkane gas is δ ¹³C₁<δ ¹³C₂<δ ¹³C₃ or δ ¹³C₁<δ ¹³C₂>δ ¹³C₃, these features suggest that the gas would be coal-type gas at high-over maturity, not be inorganic gas with reversal trend of gaseous alkanes (δ ¹³C₁>δ ¹³C₂>δ ¹³C₃). These characteristics of carbon isotopes are similar with the natural gas from the basin basement, but disagree with gas from the Xingcheng reservoir. Thus, the mixing gases from the pyrolysis gas with coal-typed gases at high-over maturity or oil-typed gases do not cause the reversal trend of carbon isotopes. The gas generation intensity for epimetamorphic rocks is 3.0×10⁸–23.8×10⁸ m³/km², corresponding to R _o from 2.0% to 3.5% for organic matter.     

Eocene melting of subducting continental crust and early uplifting of central Tibet: Evidence from central-western Qiangtang high-K calc-alkaline andesites, dacites and rhyolites

Changes in oceanic O–Sr isotopic compositions and global cooling beginning in the Eocene are considered to have been caused by the uplift of the Tibetan Plateau. The specific timing and uplift mechanism, however, have long been subjects of debate. We investigated the Duogecuoren lavas of the central-western Qiangtang Block, which form the largest outcrops among Cenozoic lavas in northern-central Tibet and have widely been considered as shoshonitic. Our study demonstrates, however, that most of these lavas are high-K calc-alkaline andesites, dacites and rhyolites. Moreover, they are characterized by high Sr (367–2472 ppm) and Al₂O₃ (14.55–16.86 wt.%) and low Y (3.05–16.9 ppm) and Yb (0.31–1.48 ppm) contents and high La/Yb (27–100) and Sr/Y (48–240) ratios, similar to adakitic rocks derived by partial melting of an eclogitic source. They can be further classified as either peraluminous and metaluminous subtypes. The peraluminous rocks have relatively high SiO₂ (> 66 wt.%) contents, and low MgO (< 1.0 wt.%), Cr (4.94–23.3 ppm) and Ni (2.33–17.0 ppm) contents and Mg^# (20–50) values, while the metaluminous rocks exhibit relatively low SiO₂ (55–69 wt.%) contents, and high MgO (1.41–6.34), Cr (25.7–383 ppm), Ni (14.13–183 ppm) and Mg^# (46–69) values, similar to magnesian andesites. ⁴⁰Ar/³⁹Ar and SHRIMP zircon U–Pb dating reveal that both peraluminous and metaluminous adakitic rocks erupted in the Eocene (46–38 Ma). Paleocene–Early Miocene thrust faults and associated syn-contractional basin deposits in the Qiangtang Block suggest that this region was undergoing crustal shortening within a continent during the Eocene. The low ε_Nd (− 2.81 to − 6.91) and high ⁸⁷Sr/⁸⁶Sr (0.7057–0.7097), Th (11.2–32.3 ppm) and Th/La (0.23–0.88) values in the Duogecuoren adakitic rocks further indicate that they were not derived by partial melting of subducted oceanic crust. Taking into account tectonic and geophysical data and the compositions of xenoliths in Cenozoic lava in northern-central Tibet, we suggest that the peraluminous adakitic rocks were most probably derived by partial melting of subducted sediment-dominated continent of the Songpan-Ganzi Block along the Jinsha suture to the north at a relatively shallow position (the hornblende + garnet stability field), but the metaluminous adakitic rocks likely originated from the interaction between peraluminous adakitic melts generated at greater depths (the garnet + rutile stability field) and mantle. Because the Duogecuoren adakitic rocks must have originated from a garnet-bearing (namely, eclogite facies) source, Eocene continental subduction along the Jinsha suture caused the thickening of the Qiangtang crust. Given that crustal thickening generally equates with elevation, the uplift of the Central Tibetan Plateau probably began as early as 45–38 Ma, which provides important evidence for tectonically driven models of oceanic O–Sr isotope evolution during global cooling and Asian continental aridification beginning in the Eocene.     

Geochemical characteristics of organic matter-rich strata of lower Cambrian in Tarim Basin and its origin

The organic matter-rich strata of the Yurtusi Formation of Lower Cambrian are wildly spread and steady in the Tarim Basin, in which cherts are developed, companied with phosphorite. Al/(Al+Fe+Mn) ratio and Si/(Si+Al+Fe) ratio of cherts range from 0.0023 to 0.0046 and 0.965 to 0.98, respectively, suggesting that cherts are formed in submarine hydrothermal activity and far away from terrestrial. Trace elements, such as As, Hg, Pb, Zn, Cu, Co, P, V, Ba, etc., are markedly rich in organic matter-rich sedimentary rocks of which Ba/Sr ratios are between 21.2 and 158.1. Compared with modern hydrothermal sediments, their Ba/Sr ratios are similar, it indicates that the organic matter-rich strata have the geochemical feature of submarine hydrothermal sediments. The total content of rare earth elements (ЕREE) in cherts is from 8.81 to 56.682 μg/g, on average 31.41 μg/g and the ΕREE of cherts is between continental margin chert's and abyssal chert's. The LREE/HREE ratio of cherts varies from 1.01 to 3.56, which reveals the characteristics of hydrothermal sediments. There is positive correlation between the total organic content (TOC) and Ba/Sr ratio which is geochemical index of submarine hydrothermal activity, which indicates that the submarine hydrothermal activity obviously influences the richness of organic matter in the Yurtusi Formation.     

敦化—密山断裂带帽山—暖木段H2、Hg、Rn气体地球化学特征

1 研究背景敦化—密山断裂带(简称敦密断裂带)北起黑龙江密山,南至吉林敦化,是我国东北地区一条重要的超岩石圈深大断裂,吉林省地震局2019年勘查表明,该断裂敦化段一些小的分支断裂活动至少持续到中更新世.该断裂的存在与区域地震的发生具有重要关联(章振铨等,1999;王金山,2005).2020年10月15日敦化—密山断...     

龙门山北部陕甘川交界三角构造区断裂活动特征研究

龙门山北部3条不同走向的断裂带所围限的三角地区是我国陕甘川交界地区研究地震活动的重要场所.通过对区内秦岭南缘断裂、 平武—青川断裂、 岷江断裂及虎牙断裂活动特征研究和活动时代分析认为: 秦岭南缘断裂, 包括迭部—舟曲断裂、 武都—康县—略阳断裂及茶店—勉县断裂在晚更新世曾有过活动, 在全新世活动不明显; 龙门山断裂带东北段平武—青川断裂最后一次活动发生在晚更新世, 表现为右旋走滑的逆断裂, 全新世活动不明显; 岷江断裂和虎牙断裂不但在晚更新世活动强烈, 而且在全新世继续活动; 龙门山北部这个三角地区依然是未来地震活动关注的重点地区.     

Geochemical and Sr–Nd–Pb isotopic investigation of Triassic granitoids and basement rocks in the northern Gyeongsang Basin, Korea: Implications for the young basement in the East Asian continental margin

Abstract We present chemical and Sr–Nd–Pb isotopic compositions of three Triassic (226–241 Ma) calc‐alkaline granitoids (the Yeongdeok granite, Yeonghae diorite and Cheongsong granodiorite) and basement rocks in the northern Gyeongsang basin, south‐eastern Korea. These plutons exhibit typical geochemical characteristics of I‐type granitoids generated in a continental magmatic arc. The Yeongdeok and Yeonghae plutons have similar initial Sr, Nd and Pb isotope ratios (⁸⁷Sr/⁸⁶Sr_initial = 0.7041 ~ 0.7050, ?_Nd(t) = 2.3 ~ 4.0, ²⁰⁶Pb/²⁰⁴Pb_feldspar = 18.22 ~ 18.34), but distinct rare earth element patterns, suggesting that the two plutons formed from partial melting of a similar source material at different depths. The Cheongsong pluton has slightly more enriched Sr–Nd–Pb isotopic compositions (⁸⁷Sr/⁸⁶Sr_initial = 0.7047 ~ 0.7065, ?_Nd(t) = 3.9 ~ 2.8, ²⁰⁶Pb/²⁰⁴Pb_feldspar = 18.24 ~ 18.37) than the other two plutons. The Nd model ages of the basement rocks (1.1 ~ 1.4 Ga) are slightly older than those of the plutons (0.6 ~ 1.0 Ga). The initial Sr and Nd isotopic ratios of the plutons can be modeled by the mixing between the mid‐oceanic ridge basalt‐like depleted mantle component and the crustal component represented by basement rocks, which is also supported by Pb isotope data. The Sr and Nd isotope data from granitoids and basement rocks suggest that the Gyeongsang basin, the Hida belt and the inner zone of south‐western Japan share relatively young basement histories (middle Proterozoic), compared with those (early Proterozoic to Archean) of the Gyeonggi and Yeongnam massifs and the Okcheon belt. The Nd isotope data of basement rocks suggest that the Hida belt might be better correlated with the basement of the Gyeongsang basin than the Gyeonggi massif, the Okcheon belt or the Yeongnam massif, although it may represent an older continental margin of East Asia than the Gyeongsang basin considering its slightly older Nd model ages.