Dur'ngoi Ophiolite in East Kunlun, Northeast Tibetan Plateau: Evidence for Paleo-Tethyan Suture in Northwest China

The A'nyemaqen (阿尼玛卿) ophiolite belt along the southern margin of the East Kunlun (昆仑) Mountains marks the suture formed by the closure of paleo-Tethys. The Dur'ngoi ophiolite in the eastern part of this belt consists of meta-peridotite, mafic-ultramafic cumulates, sheeted dikes and basaltic lavas. The meta-peridotites consist of dunite, harzburgite, Iherzolite, feldspar-bearing lherzolite and garnet-bearing Iherzolite and contain residual spinel with Cr# [100×Cr/(Cr+AI)] ranging from 30 to 57 and Mg# [100×Mg/(Mg+Fe2+)] ranging from 50 to 75, indicating an AI- and Mg-rich series. The meta-peridotites have a relatively narrow range of composition with Mg# of 89.2-92.6, Al2O3 contents of (1-4) wt.% and slightly depleted chondrite normalized REE patterns, indicating that they represent relict mantle material that has undergone intermediate to low degrees of partial melting. Garnets in the Iherzolite are andradite, enriched in Ca and Fe and depleted in Mg and Al (And=95-97, Pyr=0.3-5, Gro=0-3), indicating a metamorphic origin. The cumulate rocks mainly consist of dunite, wehrlite, py-roxenite and gabbro. A well-layered gabbro-pyroxenite complex is defined by modal variations in pla-gioclase and pyroxene. Blocks of garnet-pyroxenite or rodingite are locally present in the meta-peridotites. Garnets in the cumulate rocks are grossular (Gro=69-90, And=9-19, Br=1-12), also metamorphic origin. The diabase dikes are moderately depleted in LREE [(La/Sm)N=0.5-0.8] and HREE resulting in slightly convex chondrite-normalized patterns with slightly positive Eu anomalies (δEu=1.1-1.3). The basaltic lavas have REE patterns similar to those of MORB with (La/Sm)N ratios of 0.5-1 and small negative Eu anomalies. They appear to have been derived from a depleted mantle source and to have undergone little or no differentiation during crystallization. SHRIMP U-Pb dating of zircons from the basaits yields 206pb/238U ages of 276-319 Ma (average 308.0±4.9 Ma). The Dur'ngoi ophiolite is interpreted as a dismembered fragment of paleo-oceanic crust emplaced during closure of the paleo-Tethyan Ocean basin. Three other suites of oceanic lavas are recognized in the area: island arc volcanic (IAV) rocks, possible back arc basin (BAB) basalts and possible post-collisional vol-canic (PCV) and plutonic rocks. The distribution of these rocks suggests north-directed subduc-tion. Opening of the A'nyemaqen oceanic basin started at least as early as Late Carboniferous(308 Ma) and the basin probably closed during the Early Triassic. The IAV formed in Late Permian (260 Ms), the BAB in Early-Middle Triassic, and the PCV in Late Triassic. Several large scale, ductile, sinistrai strike-slip fault zones, extending hun-dreds to thousands kilometers, formed along or north of the suture during the Early-Late Triassic, e.g., they are the south margin fault zone of East Kunlun (200-220 Ma), the Aityn Tagh fault (220-230 Ma), and the North Qaidam fault zone (240-250 Ma). These strike-slip faults were probably generated by oblique subduction and closure of the paleo-Tethyan Ocean basin, possibly during exhumation of the subducted plate or uplift of the overriding plate, coincident with post-collisional magmatism.     

西藏东巧蛇绿岩的地球化学特征及其形成的构造环境

西藏东巧蛇绿岩主要由变质橄榄岩、辉长岩及玄武岩等组成。变质橄榄岩以富Mg、Fe、Cr,贫Ti、ΣREE为特征。辉长岩和玄武岩的主量元素、微量元素特征显示其具有扩张洋脊拉斑玄武岩的地球化学特征,其中高场强元素Nb、Ta、zr、Hf等亏损,大离子亲石元素Rb、sr、Ba等相对富集;在球粒陨石标准化稀土元素配分模式图上为LREE亏损的平坦型,无负Eu异常,与洋中脊玄武岩的特征类似。根据其地球化学属性,推测该区蛇绿岩形成于大洋盆地扩张的构造环境。     

Origin and evolution of Suru Valley ophiolite peridotite slice along Indus suture zone,Ladakh Himalaya,India: Implications on melt-rock interaction in a subduction-zone environment

In this paper, we present whole-rock and mineral geochemistry of serpentinized peridotites from the Suru Valley ophiolite slice Ladakh Himalaya, in an attempt to put constraints on their petrogenesis and tectonic evolution in the context of Mesozoic Neo-Tethys Ocean. On the basis of petrographic study, Suru Valley serpentinized peridotites can be identified as serpentinized harzburgites. Relative to primitive mantle these rocks have depleted major and rare earth element (REE) geochemical characteristics comparable to ocean floor mantle rocks reflecting their mantle residual nature. However, higher abundance of highly incompatible large ion lithophile elements (e.g., Rb, Ba, Th, U, Pb and Sr), reflect metasomatism in a subduction zone environment. The presence of silicate assemblage includes Mg-rich olivine (Fo_90-92) and orthopyroxene (En_91-93 Fs_6.4-8.7) of supra-subduction zone affinity. Evaluation of mineral and whole-rock geochemistry suggests that the Suru Valley ophiolitic peridotites represent residues left after moderate degrees of partial melting thereby underwent metasomatism in a supra-subduction zone environment related to north dipping intra-oceanic island arc during Cretaceous in the context of Mesozoic Neo-Tethys ocean.     

班公湖-怒江洋早-中侏罗世洋内俯冲:来自洞错蛇绿岩的证据

本文在班公湖-怒江缝合带中段洞错蛇绿岩中新厘定一套洋内俯冲成因的岩石组合,岩性以橄榄岩、堆晶岩(包括堆晶辉长岩和斜长花岗岩)、辉长岩墙、枕状熔岩和辉绿岩脉等为主。堆晶辉长岩、辉长岩墙和辉绿岩脉锆石U-Pb测年显示,它们形成于中侏罗世(172～165Ma)。辉长岩墙和辉绿岩脉地球化学和锆石Lu-Hf同位素分析显示,它们兼具N-MORB和岛弧玄武岩地球化学特征,且均来自亏损地幔源区,形成过程中受到了俯冲流体的影响。结合区域上同时期的玻安岩、高镁安山岩和钙碱性岩浆岩等资料,我们得出班公湖-怒江缝合带内保存了一套相对完整的早-中侏罗世洋内弧岩石层序,记录了班公湖-怒江洋早-中侏罗世时期的洋内俯冲事件。早-中侏罗世是班公湖-怒江洋快速消减期,洋内俯冲和洋-陆俯冲同时存在。     

Geology of the southern Zambales Ophiolite Complex, Luzon, Philippines

Abstract Field, geochemical and geophysical evidence show that the southern Zambales Ophiolite Complex attained its present-day configuration through the juxtapositioning of an arc terrane (San Antonio massif) to a back-arc crust (Cabangan massif). The San Antonio massif manifests island arc-related characteristics (i.e. spinel XCr [Cr/(Cr + Al)] >0.60; mostly plagioclase An92–95; pyroxene crystallizing ahead of plagioclase; orthopyroxene as an early, major crystallizing phase) which cannot be directly parental to the Cabangan massif transitional mid-ocean ridge basalt to island arc tholeiitic volcanic carapace. The two massifs are believed to be separated by a left-lateral strike–slip fault, the Subic Bay Fault Zone. Apart from the presence of highly sheared, allochthonous outcrops, the Subic Bay Fault Zone is generally defined by northwest–southeast trending magnetic and bouguer anomalies. The San Antonio massif was translated southward from the northern part of the Zambales Ophiolite Complex through the Subic Bay Fault Zone. This resulted into its suturing with the Cabangan massif and could have led to the formation of the present-day Subic Bay.     

Melt-filled hybrid fractures in the oceanic mantle: Melt enhanced deformation during along-axis flow beneath a propagating spreading ridge axis

Mid-ocean ridges represent important locations for understanding the interactions between deformation and melt production, transport, and emplacement. Melt transport through the mantle beneath mid-ocean ridges is closely associated with deformation. Currently recognized transport and emplacement processes at ridges include: 1) dikes and sills filling stress-controlled fractures, 2) porous flow in a divergent flow field, 3) self-organizing porous dunite channels, and 4) shear zones. Our recent observations from the sub-oceanic mantle beneath a propagating ridge axis in the Oman ophiolite show that gabbronorite and olivine gabbro dikes fill hybrid fractures that show both shear and extensional components of strain. The magnitudes of shear strain recorded by the dikes are significant and comparable to the longitudinal extensions across the dikes. We suggest that the hybrid dikes form from the interactions between shear deformation and pressurized melt in regions of along-axis flow at mid-ocean ridges. The displacement across the dikes is kinematically compatible with high temperature flow recorded by plastic fabrics in host peridotites. Field observations and mechanical considerations indicate that the dikes record conditions of higher stress and lower temperature than those recorded by the plastic flow fabrics. The features of hybrid dikes suggest formation during progressive deformation as conditions changed from penetrative plastic flow to strain localization along melt-filled fractures. The combined dataset indicates that the dikes are formed during along-axis flow away from regions of diapiric upwelling at propagating ridge segments. Hybrid dikes provide a potentially powerful kinematic indicator and strain recorder and define a previously unrecognized mechanism of melt migration. Our calculations show that hybrid dikes require less melt pressure to form than purely tensile dikes and thus may provide a mechanism to tap melt reservoirs that are under-pressurized with respect to lithostatic pressure.     

The genesis of the carbonatized and silicified ultramafics known as listvenites: a case study from the Mihalıççık region (Eskişehir), NW Turkey

The Mihalıççık region (Eskişehir) in NW Turkey includes an ophiolitic assemblage with a serpentinite‐matrix mélange. The serpentinites of this mélange host silica‐carbonate metasomatites which were previously named as listvenites. Our mineralogical and geochemical studies revealed that these alteration assemblages represent members of the listvenitic series, mainly the carbonate rocks, silica‐carbonate rocks and birbirites, rather than true listvenites (sensu stricto). Tectonic activity and lithology are principal factors that control the formation of these assemblages. Carbonatization and silicification of the serpentinite host‐rock is generated by CO₂, SiO₂‐rich H₂O hydrothermal fluid which includes As, Ba, Sb and Sr. Low precious metal (Au, Ag) contents of the alteration assemblages indicate lack of these metals in the fluid. Primary assemblages of the alteration are carbonate rocks that are followed by silica‐carbonate rocks and birbirites, respectively. Petrographic studies and chemical analyses suggested an alkaline and moderate to high temperature (350–400°C) fluid with low oxygen and sulphur fugacity for the carbonatization of the serpentinites. The low temperature phases observed in the subsequent silicification indicated that the fluid cooled during progressive alteration. The increasing Fe‐oxide content and sulphur phases also suggested increasing oxygen and sulphur fugacity during this secondary process and silica‐carbonate rock formation. The occurrence of birbirites is considered as a result of reactivation of tectonic features. These rocks are classified in two sub‐groups; the Group 1 birbirites show analogous rare earth element (REE) trends with the serpentinite host‐rock, and the Group 2 birbirites simulate the REE trends of the nearby tectonic granitoid slices. The unorthodox REE trend of Group 2 birbirites is interpreted to have resulted from a mobilization process triggered by the weathering solutions rather than being products of enrichment by the higher temperature hydrothermal activity. Copyright © 2006 John Wiley & Sons, Ltd.     

Evolution of the Pan-African Wadi Haimur metamorphic sole, Eastern Desert, Egypt

By comparison with the general features of metamorphic soles (e.g. vertical and lateral extension, metamorphic grade and diagnostic mineral parageneses, deformation and dominant rock types), it is inferred that the amphibolites, metagabbros and hornblendites of the Wadi Um Ghalaga–Wadi Haimur area in the southern part of the Eastern Desert of Egypt represent the metamorphic sole of the Wadi Haimur ophiolite belt. The overlying ultramafic rocks represent overthrusted mantle peridotite. Mineral compositions and thermobarometric studies indicate that the rocks of the metamorphic sole record metamorphic conditions typical of such an environment. The highest P – T conditions ( c . 700 °C and 6.5–8.5 kbar) are preserved in clinopyroxene amphibolites and garnet amphibolites from the top of the metamorphic sole, which is exposed in the southern part of the study area. The massive amphibolites and metagabbros further north (Wadi Haimur) represent the basal parts of the sole and show the lowest P – T  conditions (450–620 °C and 4.7–7.8 kbar). The sole is the product of dynamothermal metamorphism associated with the tectonic displacement of ultramafic rocks. Heat was derived mainly from the hot overlying mantle peridotites, and an inverted P – T  gradient was caused by dynamic shearing during ophiolite emplacement. Sm/Nd dating of whole-rock–metamorphic mineral pairs yields similar ages of c . 630 Ma for clinopyroxene and hornblende, which is interpreted as a lower age limit for ophiolite formation and an upper age limit for metamorphism. A younger Sm/Nd age for a garnet-bearing rock ( c . 590 Ma) is interpreted as reflecting a meaningful cooling age close to the metamorphic peak. Hornblende K/Ar ages in the range 570–550 Ma may reflect thermal events during late orogenic granite magmatism.     

THE ROCK ASSOCIATION IN JINSHAJIANG MELANGE BELT

THE ROCK ASSOCIATION IN JINSHAJIANG MELANGE BELT     

青海德尔尼铜(锌钴)矿床Re-Os年龄及地质意义

东昆仑德尔尼大型铜(锌钴)矿是赋存在蛇绿岩中的块状硫化物矿床,8件块状硫化物矿石的Re-Os同位素等时线年龄为(295.5±7.2) Ma,略小于矿区玄武质熔岩的锆石U-Pb平均年龄(308.2±4.9) Ma,矿床形成于晚石炭世-早二叠世,属于阿尼玛卿洋盆扩张期。成矿时代早于矿区北部印支期花岗岩,证明德尔尼矿床的成因与印支期花岗岩无直接关系。矿石组构、矿物组合和岩石地球化学研究指示,矿床为海底喷流沉积-热液叠加成矿,具有类似于塞浦路斯型块状硫化物矿床的构造背景和矿物组合特征。矿区及外围填图发现,德尔尼铜矿体主要产出在角砾状蛇纹岩中,部分位于蛇纹岩与含碳铁硅质岩之间,这种特殊的赋矿层位可能与晚古生代特提斯洋的快速扩张、岩浆补给少、变质橄榄岩迅速抬升有关。