Ultrahigh-pressure (4.5–5.5 GPa) experimental research on pyroxene-garnet transition and its significance

Conclusions The transition from natural Al-enstatite to garnet and Al-poor pyroxene has taken place under the condition of about 1000°C and 4.5–5.5 GPa, and new phases of garnet and corundum have formed when 15% Al₂O₃ was added to the initial natural Al-enstatite. This experimental result has explained the ultrahigh pressure (3.5–5.0 GPa) and relatively low temperature (< 1000°C) genesis of the ultramafic rock of high-pressure metamorphic zone in Dabieshan-northern Jiangsu-Jiaodong and of red corundum garnetite coexisting with garnet peridotite. From the genetic mineralogy, petrology andP-T equilibrium conditions of garnet peridotite of the high-pressure metamorphic zone, kimberlite and Cenozoic basalt and the ultrahigh pressure experimental result, it is inferred that the upper mantle garnet peridotite is transformed with the increase of depth from Al-rich pyroxene garnet peridotite (80–120 km) to Al-poor pyroxene garnet peridotite (greater than 120–150 km).     

<Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar isochron ages of lawsonite blueschists from Jiuquan in the northern Qilian Mountain,NW China,and their tectonic implications

Using laser ⁴⁰Ar/³⁹Ar dating method, we have gotten the metamorphic ages of lawsonite blueschist and epidote blueschist from Jiuquan, northern Qilian Mountain, NW China. The high quality laser ⁴⁰Ar/³⁹Ar dating of glaucophane from lawsonite blueschist gives an isochron age of 413±5 Ma. The isochron age obtained from phengite in epidote blueschist is 415±7 Ma. These data, combining with peak metamorphic P-T conditions and regional geological setting, allow us to infer that the lower limit of the ages of the prograde subduction metamorphism from lawsonite blueschist facies to epidote blueschist facies occurred at ca. 413–415 Ma, which also suggests that the formation of lawsonite blueschist in the northern Qilian Mountain maybe resulted from the corner flow in the cold subduction zone. This study shows that the final closing time of the northern Qilian remnant oceanic basin is about 413–415 Ma, which also represents the convergent age between the North China Craton and the Qaidam block.     

北祁连山含镁纤柱石高压泥质岩及其对古大洋俯冲的意义

北祁连山发育有缝合带是一条典型的早古生代大洋型俯冲带。针对该缝合带中首次发现的含镁纤柱石高压泥质片岩进行了详细的岩石学和矿物学研究。根据平衡矿物组合,利用THERMORCAL(V3.23)计算出镁纤柱石+石榴石+硬绿泥石+多硅白云母组合稳定的温压条件为P=2.2～2.5GPa,T=530～580℃。利用纤柱石和硬绿泥石的稳定范围,确定了北祁连山早古生代大洋冷俯冲的地热梯度的范围为6.5～7.5℃/km。该研究结果对于了解早古生代时期大洋俯冲带的热结构及其高压变质岩石的脱水过程有重要意义。     

Lawsonite-bearing eclogites in the north Qilian and north Altyn Tagh： Evidence for cold subduction of oceanic crust

Laboratory experiments and thermal models predic that lawsonite-bearing eclogite should be the dominan rock types for typical oceanic subduction zone[1,2] However, eclogite containing unaltered lawsonite is rare in nature and has been described only from …     

An experimental study of dehydration melting of phengite-bearing eclogite at 1.5―3.0 GPa

Dehydration melting experiments were performed on ultrahigh-pressure eclogite from Bixiling in the Dabie orogen at 1.5--3.0 GPa and 800--950℃ using piston cylinder apparatus. The results show that （1） eclogite with -5% phengite started to melt at T≤800--850℃ and P = 1.5--2.0 GPa and produced about 3% granitic melt; （2） the products of dehydration melting of phengite-bearing eclogite vary with temperature and pressure. Fluid released from dehydration of phengite and zoisite leads to partial melting of eclogite and formation of plagioclase reaction rim around kyanite at pressures of 1.5--2.0 GPa and temperatures of 800--850℃; （3） phengite reacted with omphacite and quartz and produced oligoclase, kyanite and melt at elevated temperatures. Oligoclase is the primary reaction product produced by partial melting of phengite in the eclogite; and （4） the dehydration melting of phengite-bearing eclogite at pressures of 1.5--3.0 GPa and temperatures ≥900℃ results in formation of garnets with higher molar fraction of pyrope （37.67 wt.%--45.94 wt.%）. Potassium feldspar and jadeite occur at P = 2.4--3.0 GPa and T≥900℃, indicating higher pressure and fluid-absent conditions. Our results constrain the solidus for dehydration melting of phengite-bearing eclogite at pressures of 1.5--3.0 GPa. Combining experi- mental results with field observations of partial melting in natural eclogites, we concluded that phengite-bearing eclogites from the Dabie-Sulu orogen were able to partially molten at P= 1.5--2.0 GPa and T= 800--850℃ during exhumation. The ultrahigh-high pressure eclogites would have experienced partial melting in association with metamorphic phase transformation under different fluid conditions.     

An overview of adakite petrogenesis

ADAKITE IS A PETROLOGIC TERM THAT DEFANT AND DRUM-MOND[1] FIRST INTRODUCED ~ 15 YEARS AGO TO REFER TO “VOL- CANIC OR INTRUSIVE ROCKS IN CENOZOIC ARCS ASSOCIATED WITH SUBDUCTION OF YOUNG (≤25 MA) OCEANIC LITHO- SPHERE”; THESE ROCKS ARE “CHARACTERIZED B…     

The geological characteristics of oceanic-type UHP metamorphic belts and their tectonic implications: Case studies from Southwest Tianshan and North Qaidam in NW China

The geological characteristics of ultrahigh-pressure （UHP） metamorphic belts formed by deep subduction of oceanic crust are summarized in this paper. Oceanic-type UHP metamorphic belt is characterized by its protolithlc assemblage of typical oceanic crust, the peak metamorphic temperature 〈600℃, P-T path undergoing blueschist facies during prograde and retrograde metamorphic evolution, reepectively, with low geothermal gradient of cold subduction. The further study of oceanic-type UHP metamorphic belt is very significant for constructing metamorphic reaction series of cold subduction zone, for understanding how aqueous fluids were transported into deep mantle and for classifying the types of UHP metamorphism in cold subduction zone. The uplift and exhumation mechanism of oceanic UHP metamorphic rocks is one of the most challenging problems in the study of UHP metamorphism, which is very important for understanding the geodynamic mechanism of solid Earth. As a traveler eubducted into the mantle depth end then uplifted to the surface, oceanic-type UHP metamorphic belts witness the bulk process from the subduction to exhumation and is an ideal target to study the geochemical behavior end cycling of elements in subduction zones. The tectonic evolution of one convergent orogenic belt can be usually divided into two stages of oceanic subduction and followed continental subduction and collision, and the two best-established examples of orogenic belts are Alpa and Himalaya. Therefore, the study of oceanic-type UHP metamorphic belt is the frontier of the current plate tectonic theory. As two case studies, the current status and existing problems of oceanic-type UHP metamorphic belts in Southwest Tianshan and North Qaidam, NW China, are reviewed in this paper.     

房山岩体的岩浆来源、形成机制及动力学意义

房山岩体是华北东部典型的燕山期中酸性侵入杂岩体,其地球化学特征为富Si、Na、Al、Sr,亏损高场强元素(Nb、Ta),轻重稀土分异强烈,Sr-Nd同位素具有类似EMI型富集地幔的特点,表明其岩浆来自于加厚下地壳的部分熔融,并且中生代华北东部下地壳已经被早先的富集岩石圈地幔置换.不同基性程度岩浆的不完全混合是形成微粒闪长质包体的原因.侵入杂岩的角闪石压力计显示下地壳部分熔融形成的中酸性岩浆先后在19~8 km里的深度范围内结晶就位.古太平洋板块在地幔过渡带深度于早白垩世水平俯冲至太行山一线,洋壳脱水导致上覆地幔部分熔融,后者成为加热下地壳部分熔融的热源.     

Metamorphic peridotite and rock series of ophiolite belt in Mt. Ailao, Yunnan Province

The metamophic peridotite of ophiolite belt in Mt. Ailao is mmposed of lhenolite and harzburgite. The former shows the charateristics of primary pyrolite and the latter shows those of deleted (relict)pyrolite. By partial melting of Ihedite, two primary magmas: tholeiitic magma and picrite-basalt magma are formed. The former evoluted into gabbmdiabase-pyroxenic besalt rock series and show the characteristics of MORB; while the latter evoluted into gabbro-diorite-albite basalt-picrite basalt one, and show the characteristics of para-MORB.     

Discovery of Mg-righ volcanic rock series in western Altay area, Xinjiang and its geologic significance

A series of Mg-rich volcanic rocks, including rare Mg-rich dacites in the Asherle copper mine, western Altay Area, has been found. This suite of Mg-rich volcanic rocks is characterized by high SiO2, MgO and low Ti contents. Compared with the island arc tholeiite and MORB, these rocks are depleted in HFSE and are therefore similar to boninite. The dacites show V-shaped distribution REE patterns whereas the basalts display flat or LREE-depletion patterns. Based on their geochemical features and occurrence, it is suggested that they were formed in oceanic island arc setting. Its petrogenesis is related to the slab movement. At the early stage, the peridotite of the ocean upper mantle diapirically uprised to the shallow level and started to melt owing to subduction of the ocean slab, forming the island arc tholeiite with pillow structure. At the late stage, the residual refractory peridotites melted again, giving rise to the formation of the parent magma of Mg-rich basalt.     

Variation of b value with hypocentral depth in Beijing area： Implications for earthquake nucleation

We relocated 2098 earthquakes that occurred in Beijing area between 1980 and 2000 using a double-difference (DD) earthquake location algorithm, and obtained the high-precision relative locations of 1825 events, b values versus depth were investigated with the relocated hypocenters. The results show that the b values decrease with the increasing hypocentral depth systematically. A dramatic variation in b is observed around the depth of 8 km. It indicates that there are more smaller earthquakes at shallow depth (0-8 km), while more larger earthquakes at greater depth (8--25 km). The physical mechanism behind this phenomenon can be explained by the variations in material heterogeneity and lithostatic stress condition. Large earthquakes are more likely to nucleate at greater depth with more homogenous material and higher lithostatic stress. On the basis of the results, we suggest that future strong earthquakes in Beijing area tend to occur below the depth of 8 km.     

Syn-deformationP-T paths of Xiaoqinling metamorphic core complex

TheP-T paths of the Xiaoqinling metamorphic core complex (XMCC) have been investigated with the Gibbs method by researching the compositional changing of zoned epidotes which formed during syn-deformation metamorphism. TheseP-T paths indicate that the XMCC had experienced the following thermodynamics processes: firstly, near isobaric falling slightly in temperature in lower crust; secondly, fast decompression and rising in temperature during extensional uplifting to middle crust level; and finally, isobaric falling in temperature in middle crust. The upwelling and emplacement of the deep magma may be a major factor during the uplifting processes of the metamorphic core complex.     

Discovery and implications of the high-pressure pelitic granulite from the Jiaobei massif

The high pressure pelitic granulite with peak assemblage of garnet kyanite ternary feldspar muscovite rutile was discovered in Qixia area in the Precambrian Jiaobei massif, where high pressure basic granulites are widely distributed. The metamorphic peak conditions for the pelitic granulite were calculated as T=800-840℃ and P=1.0-1.25 GPa on the basis of P-T pseudosection. The post peak P-T path is characterized first by an isothermal decompression (ITD) pattern and then by an isobaric cooling (IBC) pattern, indicating a geodynamic process related to thinning of thickened continental crust.     

Magma mixing in upper mantle: Evidence from high Mg<Superscript>#</Superscript> olivine hosted melt inclusions in MORBs near East Pacific Rise 13°N

Early formed high-Mg^# olivine phenocrysts during evolution of MORB magmas usually host melt inclusions, which record im-portant information about the early-stage evolution of magma. Five MORB samples from near East Pacific Rise (EPR) 13°N vary little in K/Ti (0.07–0.12), Tb/Lu (1.72–1.84) and Sm/Nd (0.310–0.332) and have similar REEs patterns, indicating that depleted upper mantle has similar mineral composition. Sixty-five initial melt inclusions derived by correcting olivine fractionation and “FeO-Loss” show averagely higher MgO contents than their host rocks. Melt inclusions have higher CaO/Al₂O₃ ratios than their host rocks, and these CaO/Al₂O₃ ratios are positively and negatively correlated with MgO and Na₂O respectively, suggesting that these magmas have experienced high pressure crystallization of clinopyroxene. Average crystallization pressure, which is calculated based on the pressure dependence of clinopyroxene crystallization, is 0.83–0.25 GPa, and implys that these melt inclusions are averagely trapped in mantle depth of ~24 km. These melt inclusions show negative correlations of Ca₈/Al₈ and Na₈ with Fe₈, and wider ranges of Ca₈/Al₈, Na₈, Fe₈ and K/Ti than their host rocks, suggesting that these melt inclusions formed by mixing magmas of different melting degrees and depths. According to the average value and ranges of Ca₈/Al₈, Na₈, Fe₈ and K/Ti, these magmas would necessitate other mixing ends in shallow crust except in upper mantle. The compositional diversity of melt inclusions in MORBs phenocrysts cannot always be used to indicate magma mixing and crystallization in shallow crust, and melt inclusions in high Mg^# olivine formed under mantle pressure must be excluded in study of the magma process at crustal level. This study shows that, in EPR, MORBs have experienced mixing of magmas formed by different melting degrees and depths in the mantle.     

Holocene volcanic rocks in Jingbo Lake region ? Diversity of magmatism

During the time from 5500 a to 5200 a BP more than 10 Holocene volcanoes in Jingbo Lake region erupted and the volcanic rocks covered an area of about 500 km2. Holocene volcanic rocks in Jingbo Lake region belong to the potassium?rich rocks and contain three rock types: trachybasalts, basanites and phonotephrites. Various types of magmatism formed in a small area and in a short period of time came from partial melting of potassically?metasomatised lithospheric mantle. The diversity of magmatism can be explained by that Jingbo Lake is situated in the back?arc extensional region of East Asian continent subducted by the Pacific Ocean, and potassic fluid derived from mantle wedge or dehydration of subducted slab can result in a high heterogeneity of the mantle beneath this region. Based on the pressure estimation of clinopyroxene megacrysts, we estimate that phonotephrite magma fractionally crystallize at ca. 52?54 km down the earth.     

滇西澜沧-孟连火山岩带火山岩特征与成因

澜沧-孟连火山岩带是下石炭统基性火山岩带,以碱性杆栏玄武岩系列为主,并富含Pb,是Pb的矿源层。产于大陆边缘裂谷区(澜沧裂谷)。岩浆起源于地下35～70公里深度范围内,是地幔二辉杆栏岩局部熔融产生的低熔岩浆结晶产物。     

甘肃西部马鬃山超镁铁质杂岩岩石学地球化学信息及意义

马鬃山岩体在空间上分为有成因联系的两部分，主岩体为具有环状分异特征的辉长岩岩体，中心相为辉长岩，边缘相为闪长岩．主岩体东南子岩体为橄榄二辉岩岩体．赋存于其中的岩石学和地球化学信息暗示马鬃山杂岩体形成于活动大陆边缘张裂构造环境，为应力释放期的产物，岩浆来源于具有E—MORB性质的陆下岩石圈地幔与软流圈的边界。较大程度部分熔融产生的岩浆在上升途中同化了陆壳围岩。     

Upward migration of Vesuvius magma chamber over the past 20,000 years

Forecasting future eruptions of Vesuvius is an important challenge for volcanologists, as its reawakening could threaten the lives of 700,000 people living near the volcano. Critical to the evaluation of hazards associated with the next eruption is the estimation of the depth of the magma reservoir, one of the main parameters controlling magma properties and eruptive style. Petrological studies have indicated that during past activity, magma chambers were at depths between 3 and 16 km (refs 3-7). Geophysical surveys have imaged some levels of seismic attenuation, the shallowest of which lies at 8-9 km depth, and these have been tentatively interpreted as levels of preferential magma accumulation. By using experimental phase equilibria, carried out on material from four main explosive events at Vesuvius, we show here that the reservoirs that fed the eruptive activity migrated from 7-8 km to 3-4 km depth between the ad 79 (Pompeii) and ad 472 (Pollena) events. If data from the Pomici di Base event 18.5 kyr ago and the 1944 Vesuvius eruption are included, the total upward migration of the reservoir amounts to 9-11 km. The change of preferential magma ponding levels in the upper crust can be attributed to differences in the volatile content and buoyancy of ascending magmas, as well as to changes in local stress field following either caldera formation or volcano spreading. Reservoir migration, and the possible influence on feeding rates, should be integrated into the parameters used for defining expected eruptive scenarios at Vesuvius.     

The Sr, Nd isotopic composition of lamprophyres in Laowangzhai gold orefield, Yunnan Province

Lamprophyres are widely spread over the Laowangzhai gold orefield in northern Mt. Ailao structure zone, Yunnan Province. Lamprophyres in the orefield are temporally and spatially related to gold mineralization. Lamprophyres in the orefield have anomalous Sr and Nd isotopic compositions, e.g. the 87Sr/ 86Sr ratios (0.706 665-0.707 960, the 87Sr/ 86Sr ratio of sample YLW-44 is 0.709 041) are higher than the modern value of the original mantle (0.704 5), the 143Nd/ 144Nd ratios (0.512 463-0.512 551) are lower than the modern value of the original mantle (0.512 638), and the calculated values ε Sr>0 (28.1-63.7) and ε Nd<0 (-1.34--3.07). The results of investigation indicate that lamprophyres in the Laowangzhai gold orefield with anomalous Sr and Nd isotopic compositions would not be produced as a result of contamination of the primary magma with the isotopic features of MORB by the crust materials high 87Sr/ 86Sr ratios and low 143Nd/ 144Nd ratios in the processes of rising or in the magma chamber, but the result of metasomatism of source mantle.     

Discovery of eclogite and its geological significance in Qiangtang area, central Tibet

Eclogites have been found in the margin of the Qinghai-Tibet Plateau respectively since the 1990s. First Eocene eclogite from Himalayan belt was discov- ered in Kaghan valley, northern Pakistan in 1991, in which coesite was identified[1,2]. Then two eclog…