Thermodynamic modeling and elemental migration for the early stage of rodingitization:An example from the Xialu massif of the Xigaze ophiolite,southern Tibet

The analysis of early stage rodingite from the ultramafic rocks of the Xialu Massif in the Xigaze Ophiolite,Tibet,in China shows that the rodingitization involved continuous changes in fluid composition during different stages of subduction.The early stage prehnite-bearing rodingite was produced at low pressures and temperatures along extensional fractures.Samples of rodingite were collected along a profile from the center to the margin of a rodingitized intrusive igneous rock(^10 m×30 m),and they record wide variations in bulk composition,mineralogy,and texture.The mineral assemblages,from center to margin,vary from(1)relics of primary clinopyroxene(Cpx_(r))and primary amphibole(Amp_(r))+newly formed late amphibole(Act)+primary plagioclase(Pl_(r))+clinozoisite+prehnite+albite+chlorite+titanite+ilmenite(R1 rodingite),through(2)relics of primary clinopyroxene(Cpx_(r))+newly formed late clinopyroxene(Cpx_(n))+primary and late amphiboles(Amp_(r)+Act)+clinozoisite+prehnite+albite+chlorite+titanite(R2 rodingite),to(3)newly formed late clinopyroxene(Cpx_(n))and amphibole(Act)+clinozoisite+prehnite+albite+chlorite+titanite(R3 rodingite).As a result of the metasomatic process of rodingitization,the content of CaO in the whole rock chemical composition from R1 to R3 increases,SiO_(2) decreases,and Na_(2)O+K_(2)O is almost completely removed.Massbalance diagrams show enrichments in large ion lithophile elements such as Rb,Cs,Ba,and Pb as well as Ni during rodingitization.The central part of the rodingitized intrusion(R1 rodingite)was only slightly affected by metasomatism.On the other hand,the contents of the rare earth elements(REEs),high field strength elements(HFSEs;e.g.Zr,Nb,Ta,Hf,and Y),and some highly compatible elements such as Cr and Sc decreased slightly during rodingitization.Thermodynamic modeling based on equilibrium mineral assemblages indicates that the rodingite of the Xialu Massif formed in an H_(2)O-saturated,CO_(2)-rich environment.The estimated conditions of metamorphism were-281-323℃and 0.4-3.9 kbar,representing the subgreenschist facies.In this environment,REEs and HFSEs were soluble in the fluids and partly removed.Moreover,these prehnite rodingites formed in a progressively reducing and less alkaline environment,as indicated by decreases in f(O_(2))and bulk-rock Fe^(3+)/Fe^(2+) ratios,and the records of fluidΔpH from the center to the margin of the studied rodingitized intrusion.

Metamorphic Evolution and Zircon U-Pb Ages of the Nanshankou Mafic High Pressure Granulites from the Jiaobei Terrane, North China Craton

Petrological analysis and LA-ICP-MS zircon U-Pb dating were conducted on high- pressure marie granulites, which occured as xenolith within TTG gneisses, from the Nanshankou Village of the Jiaobei terrane, Shandong Peninsula in the north-eastern part of the North China Craton （NCC）. The mafic HP grannlite is composed of garnet, clinopyroxene, orthopyroxene, amphibole and symplectitic clinopyroxene, orthopyroxene, plagioclase, ilmente and magnetite which were formed after the decomposition of porphyroblastic garnet and clinopyroxene. Four stages of metamorphic mineral assemblages for the mafic HP granulites were constrained by detail petrological and mineralogical in- vestigations. The early prograde assemblage is represented by the mineral inclusions within garnet and clinopyroxene porphyroblasts （Opx1＋Pl1＋Qtz1）, recording the metamorphic conditions at -754-757 ℃, 0.63-0.71 GPa; peak metamorphic conditions were determined at -874-891 ℃, 1.32-1.35 GPa with the mineral assemblage of Grt2＋Cpx2＋Amp2＋Pl2＋Qtz2. Retrograde minerals derived from symplectitic assemblage Opx3＋Cpx3＋Amp3＋Pl3＋Qtz3＋Ilm3±Mag3 were formed at 693-796℃, 0.60-0.84 GPa. A final greensehist to sub-greenschist facies event was recorded by the exsolution of actinolite and albite within a retrograded clinopyroxene, as well as the occurrence of prehnite, chlorite and calcite minerals. Accordingly, a clockwise P-T path was concluded on the basis of the different stages of mineral asseblage. Cathodoluminescence imaging, trace element and U-Pb dating of zircons from the mafic HP granulites recorded similar charactistics for three episodes of Paleo-Meso Proterozoic metamorphic events. These are the metamorphic events preserved in mafic and pelitic granulites in the Jiao-Liao-Ji belt （JLJB） with 207 pb/206pb ages of 2.0-1.9 Ga for peak metamorphism and of 1.86-1.84 Ga for decomposing process, followed by a retrograde amphibolite facies metamorphic event related to the post-orogenic extension at the age of 1.76-1.74 Ga, resulting the exhumation of the granulite to the upper crust level.

Metamorphic Petrology of Clinopyroxene Amphibolite from the Xigaze Ophiolite,Southern Tibet: P-T Constraints and Phase Equilibrium Modeling

The clinopyroxene amphibolite from the Bailang terrane is located in the central section of the Yarlung Zangbo suture zone(YZSZ),southern Tibet.The study of it is expected to provide important clues for the subduction of the Neo-Tethyan Ocean below the Asian Plate and thus for better understanding of the development of the India-Asia collision zone.Based on integrated textural,mineral compositional,metamorphic reaction history and geothermobarometric studies of the clinopyroxene amphibolite within a serpentinite mélange,four overprinted metamorphic stages are established.They are the first metamorphic record of M1 stage indicated by a relict assemblage of plagioclase+clinopyroxene+amphibole,an early M2 stage characterized by an assemblage of medium-grained clinopyroxene+amphibole+plagioclase+quartz as well as rutile inclusion in titanite,which is formed during burial process,an isobaric cooling M3 stage which is characterized by an assemblage of clinopyroxene+amphibole+plagioclase+titanite,and a decomposing retrograde stage M4,which is represented by the amphibolite+plagioclase symplectite+titanite+rutile+quartz.By applying the THERMOCALC(versions 6.2 and 6.3)technique in the NCFMASHTO system,the P-T conditions estimated from M1 to M4 stages are ca.8.6 kbar/880℃,10.8–13.4 kbar/800–840℃,12.7–13.2 kbar/650–660℃ and<11.2 kbar/640℃,respectively.The mineral assemblages and their P-T conditions define a counterclockwise P-T path for the clinopyroxene amphibolite of the Xigaze ophiolite,suggesting that the rocks underwent a cooling process during burial from magmatic protolith,and a decompressing stage after the pressure peak metamorphic conditions,which implies that the Bailang terrane of the Xigaze ophiolite may have experienced subduction/collision-related tectonic processes.The peak metamorphism reaches to the transitional P-T conditions among amphibolite facies,granulite facies and eclogite facies with a burial depth of 30–40 km.After exhumation of the ophiolitic unit to the shallow crustal levels,the clinopyroxene amphibolite exposes to a high fO2 condition on the basis of the stable epidotebearing assemblage in the T-MO2 diagrams.A late subgreenschist facies overprinting subsequently occurs,the relevant mineral assemblage is prehnite+albite+chlorite+epidote+quartz.

Metamorphic Characteristics and Tectonic Implications of the Kadui Blueschist in the Central Yarlung Zangbo Suture Zone,Southern Tibet

The Kadui blueschist is located in the central section of Yarlung Zangbo suture zone （YZSZ）, southern Tibet, and has been subjected to the subduction of the Neo-Tethyan Ocean below the Asian Plate and provides important clues for better understanding the evolution of the India-Asia convergence zone. In this paper, the systematical petrographic and mineral chemical studies were carried out on the Kadui blueschist, which reveal a mineral assemblage of sodic amphibole, chlorite, epidote, albite and quartz with accessory minerals of titanite, calcite and zircon. Electron microprobe analyses demonstrate that amphibole shows zoned from actinolite core to ferrowinchite/riebeckite rim composition indicating that the sodic amphibole has formed during a prograde metamorphic event. The protolith of the blueschist is an intermediate-basic pyroclastic rock. The calculated pseudosection indicates a clockwise P-T path and constrains peak metamorphic conditions of about 5.9 kbar at 345 ℃. This condition is transitional between pumpellyite-actinolite, greenschist and blueschist facies with a burial depth of 20-22 km and a thermal gradient of 15-16 ℃/km. This thermal gradient belongs to high pressure intermediate P/T facies series and is possibly related to a warm subduction setting of young oceanic slabs. Our new findings indicate that the Kadui blueschist in the central part of YZSZ experienced a rapid subduction and exhumation process as a response to a northward subduction of the Neo-Tethyan oceanic lithosphere during the initial India-Asia collision stage.

Ultrahigh Temperature Metamorphic Record of Pelitic Granulites in the Huangtuyao Area of the Huai’an Complex,North China Craton

Pelitic granulite from the Huangtuyao area,occurrs in the Huai'an Complex,is located in the Trans-North China Orogen of the North China Craton.On the basis of petrolography,mineral component,and phase equilibrium modeling studies,the P-T conditions and mineral assemblages of pelitic granulites can be divided into four metamorphic stages:the prograde metamorphic stage M1 defined by the stable mineral assemblage of Grt1(garnet core)+Pl+Bt+Kfs+Qz+Rt,the peak pressure Pmax stage M2 indicated by Grt2(garnet mantle)+Kfs±(Ky)+Rt+Qz+Liq(melt),peak temperature Tmax stage M3 characterized by Grt3(garnet rim)+Sill+Pl+Kfs+Qz+Ilm+Liq,and retrograde stage M4 represented by Grt(in matrix)+Kfs+Sill+Bt+Pl+Qz+Ilm.By using the THERMOCALC V340,the P-T conditions are estimated at^13.8–14.1 kbar and^840–850℃at stage M2,and 7–7.2 kbar and 909–915℃for the Tmax stage M3,indicating an ultra-high temperature(UHT)metamorphic overprinting during decompression and heating process after high pressure granulite facies metamorphism.The mineral assemblages and their P-T conditions presented a clockwise P-T trajectory for the Huangtuyao pelitic granulites.The major metamorphic events at^1.95 and^1.88 Ga obtained by the zircon U-Pb dating suggest that pelitic granulites from the Huangtuyao area has undergone HP granulite metamorphism which probably occurred in the prograde metamorphism and related to the collision between the Ordos and the Yinshan blocks,and afterwards UHT metamorphism is related to crustal extension after continental-continental collision.

Petrogenesis and Geodynamic Implications of the Carboniferous Granitoids in the Dananhu Belt,Eastern Tianshan Orogenic Belt

This paper presents new LA-ICP-MS zircon U-Pb geochronology,whole-rock major and trace element geochemistry,and Sr-Nd isotopes systematically on porphyritic granitic and K-feldspar granitic intrusions from the Dananhu belt,eastern Tianshan orogenic belt(ETOB).Zircon U-Pb dating indicates that the porphyritic granitic and K-feldspar granitic plutons were formed at 357±3 and 311±3 Ma,respectively.The porphyritic granites show geochemical and isotopic characteristics(high Si O2,low Mg O and Mg^#,depleted Sr-Nd isotopic values(about 0.7034 and 6.13,respectively),with Nb/Ta(13.3–14.7)and Zr/Hf(31.0–33.9)ratios)similar to those of the crustal-derived magmas.The above characteristics suggest they were probably originated from juvenile lower crustal materials.The K-feldspar granites also have high Si O2,low Mg O and Mg^#,depleted Sr-Nd isotopic values(0.7033–0.7046 and 4.41–5.67,respectively).But some trace elements contents vary widely,with variable Nb/Ta(12.7–22.7),Zr/Hf(21.3–36.1)and Nb/La(0.38–1.07)ratios,indicating that the K-feldspar granites were formed by partial melting of juvenile lower crustal materials with old crustal materials.Combined with previous data on Carboniferous granitoids in the Dananhu belt,we infer that all the Carboniferous granitic plutons in the Dananhu belt were most likely emplaced in an island arc environment(Dananhu arc).Subsequently,a tectonic transition from oceanic subduction to post-collisional extension probably occurred in the ETOB.

Record of Early-Stage Rodingitization from the Purang Ophiolite Complex,Western Tibet

Rodingitization,commonly coupled with serpentinization of ultramafic rocks,bears significant information for fluid-rock interactions and element transfer from sea-floor to subduction zone environments.Numerous outcrops of rodingites are exposed along the Yarlung Zangbo suture zone(YZSZ)of southern Tibet,providing us an excellent opportunity to probe the petrogenetic processes,and unravel their implications for regional tectonic evolution.Several studies have been performed on rodingites from the eastern to central portions of the YZSZ,whereas limited work has ever been conducted on rodingitized rocks from the western segment of the YZSZ,precluding a comprehensive understanding of this lithological type.In this paper,we present detailed studies of petrology,mineral,whole-rock geochemistry and phase equilibrium modeling on a suite of newly recognized rodingites within the Purang ophiolite massif in the southwestern part of the YZSZ.The rodingites have a major metasomatic mineral association of chlorite,clinozoisite,amphibole and minor amounts of plagioclase,representing products of an early-stage rodingitization.They generally present compositions of low SiO2(48.89 wt.%–53.57 wt.%),Fe2O3^T(3.77 wt.%–5.56 wt.%),Na2O(1.31 wt.%–1.93 wt.%),Al2O3(4.78 wt.%–8.84 wt.%),moderate CaO(9.69 wt.%–11.23 wt.%),and high MgO(24.11 wt.%–26.08 wt.%)concentrations with extremely high Mg^# values[Mg^#=100×Mg/(Mg+Fe^2+)molar]of 89–92.Bulk-rock recalculation reveals that the rodingites have a protolith of mantle-derived olivine gabbro or gabbronorite.They have low rare earth element compositions(∑REE=2.4 ppm–6.5 ppm)and are characterized by flat LREE and slightly enriched HREE patterns with positive Eu anomalies;they also exhibit positive anomalies in Sr,U and Pb and negative anomalies in high-field strength elements,including Nb,P and Ti,suggesting for a subduction-zone imprinting.Phase equilibrium modeling shows that the rodingitization did take place at P<2 kbar and T=~350–400℃,consistent with low greenschist facies conditions.Taking into account of all these petrological and geochemical features,we propose that the rodingites record evidence of early-stage fluid-rock interactions between olivine gabbroic rocks and Ca-rich fluids,which may have derived from weakly serpentinized ultramafic country rocks.Although this process may initially have occurred in a mid-ocean ridge setting,an obvious overprinting by supra-subduction zone fluids in a fore-arc environment is recognized.

Comparative outcomes of subcutaneous and transvenous cardioverter-defibrillators

Background: The comparative outcomes of subcutaneous implantable cardioverter-defibrillator (S-ICD) and transvenous (T-ICD) have not been well studied. The aim of this study was to evaluate the safety and efficacy of currently available S-ICD and T-ICD. Methods: The study included 86 patients who received an S-ICD and 1:1 matched to those who received single-chamber T-ICD by gender, age, diagnosis, left ventricular ejection fraction (LVEF), and implant year. The clinical outcomes and implant complications were compared between the two groups. Results: The mean age of the 172 patients was 45 years, and 129 (75%) were male. The most common cardiac condition was hypertrophic cardiomyopathy (HCM, 37.8%). The mean LVEF was 50%. At a mean follow-up of 23 months, the appropriate and inappropriate ICD therapy rate were 1.2% vs. 4.7%(X^2= 1.854, P=0.368) and 9.3% vs. 3.5%(X^2 = 2.428, P = 0.211) in S-ICD and T-ICD groups respectively. There were no significant differences in device-related major and minor complications between the two groups (7.0% vs. 3.5%, X^2 = 1.055, P = 0.496). The S-ICD group had higher T-wave oversensing than T-ICD group (9.3% vs. 0%, X^2 = 8.390, P=0.007). Sixty-five patients had HCM (32 in S-ICD and 33 in T-ICD). The incidence of major complications was not significantly different between the two groups. Conclusions: The efficacy of an S-ICD is comparable to that of T-ICD, especially in a dominantly HCM patient population. The S-ICD is assodated with fewer major complications demanding reoperation.

Preface: Metamorphism and Orogenic Belts——Response from Micro-to Macro-Scale

The geological toolbox for the analysis of orogenic processes has seen substantial additions over the recent decades.Major advances have been made in the ability to simulate geological conditions and processes by computer modeling,we have a much improved knowledge of geochemical processes(trace elements and isotopes in particular),and,last but not least,the range and versatility of micro-analytical methods and instrumentation available to us has been expanded dramatically.The latter aspect has had a particular impact on metamorphic petrology as the conventional tools to determine pressure-temperaturetime paths via rock and mineral analysis combined with isotope geochronology have generally gained in precision and accuracy through a much improved resolution.Geochronology has moved from whole-grain analysis to spot analyses on zoned single grains while analyzing trace elements and isotopes at the same time.It is therefore possible to recognize complex multi-stage orogenic events in single samples or single grains.Diffusion profiles in minerals provide us with time-scales of metamorphic and magmatic processes.Major and trace elements as well as isotope profiles of mineral grains give us insight into changing ambient conditions during mineral growth.This special issue focuses on pressure-temperature conditions and geochronology of metamorphic rocks,isotope studies of magmatic rocks,partial melting and metasomatic processes,all with a view at tectonic implications and orogeny.Much of the evidence has been gained from small-scale observation and analysis.Diligent field work and detailed microscopic examination remain indispensable,though,as demonstrated clearly by the"expansion"of the P-T space as applicable to crustal metamorphic processes towards extreme pressures and temperatures.As a consequence,much interest has thus focused on UHP and UHT rocks and the orogenic processes that generate them.There is petrological evidence for subduction to,and subsequent exhumation from,depths in excess of 250 km.While this impacts substantially on the large-scale geodynamic models of orogenic processes,the evidence is gathered primarily from small-scale petrological observation.The same applies in principle to UHT granulites.To generate temperatures in the 900 to 1100℃range within the sillimanite stability field in orogenic heat budget models remains a serious challenge,however.The large overlap of solid-state metamorphism,partial anatexis and magmatic crystallization in P-T space suggests close interrelationships,and the role and contribution of the diverse magmatic activities in forming orogenic belts,from the onset of subduction to late-orogenic delamination or slab-break-off is crucial for our understanding of geodynamics,with focus on geochemistry and geochronology.Aspects of metasomatism in the framework of orogeny are given some special consideration in this issue.The topics include the formation of sagvandite,pyrope quartzite(or whiteschist),jadeitite and rodingite.One of the key questions addressed is the source and the nature of the fluids that triggered the different types of metasomatic events.Finally,it is shown how small-scale methods contribute to a better understanding of the formation of deep-seated mantle xenoliths.