2D thermomechanical modelling of continent–arc–continent collision

Arc–continent collision is a key process of continental growth through accretion of newly grown magmatic arc crust to older continental margin. We present 2D petrological–thermo-mechanical models of arc–continent collision and investigate geodynamic regimes of this process. The model includes spontaneous slab bending, dehydration of subducted crust, aqueous fluid transport, partial melting of the crustal and mantle rocks and magmatic crustal growth stemming from melt extraction processes. Results point to two end-member types of subsequent arc–continent collisional orogens: (I) orogens with remnants of accretion prism, detached fragments of the overriding plate and magmatic rocks formed from molten subducted sediments; and (II) orogens mainly consisting of the closed back-arc basin suture, detached fragments of the overriding plate with leftovers of the accretion prism and quasi insignificant amount of sediment-derived magmatic rocks. Transitional orogens between these two endmembers include both the suture of the collapsed back-arc basin and variable amounts of magmatic production. The orogenic variability mainly reflects the age of the subducting oceanic plate. Older, therefore colder and denser oceanic plates trigger subduction retreat, which in turn triggers necking of the overriding plate and opening of a backarc basin in which new oceanic lithosphere is formed from voluminous decompression melting of the rising hot asthenosphere. In this case, subducted sediments are not heated enough to melt and generate magmatic plumes. On the other hand, young and less dense slabs do not retreat, which hampers opening of a backarc basin in the overriding plate while subducted sediments may reach their melting temperature and develop trans-lithospheric plumes. We have also investigated the influences of convergence rate and volcanic/plutonic rocks' ratio in newly forming lithosphere. The predicted gross-scale orogenic structures find similarities with some natural orogens, in particular with deeply eroded orogens such as the Variscides in the Bohemian Massif.     

U–Th–Pb monazite dating and the timing of arc–continent collision in East Timor

The Timor Orogen represents one of the youngest arc–continent collisions exposed on the Earth. It has the potential to provide some of the key parameters about how this style of orogeny evolves. However, the metamorphic age of the highest-grade rocks formed in the collision remains controversial. Using U–Th–Pb dating of monazite from amphibolite-grade sillimanite and garnet-bearing schists we show the peak metamorphism occurred at 5.5–4.7 Ma. The young age of the monazite and the presence of significant amounts of common Pb required the development of a new protocol to simultaneously account for the ²³⁰Th disequilibrium and the ²⁰⁷Pb common Pb correction. The new estimate of metamorphic age is consistent with the estimates based on plate reconstructions for the initiation of arc–continent collision in East Timor. The metamorphic event is a result of this collision.     

Cretaceous–Tertiary convergence and continental collision,Sanandaj–Sirjan Zone,western Iran

The Sanandaj–Sirjan Zone contains the metamorphic core of the Zagros continental collision zone in western Iran. The zone has been subdivided into the following from southwest to northeast: an outer belt of imbricate thrust slices (radiolarite, Bisotun, ophiolite and marginal sub-zones, which consist of Mesozoic deep-marine sediments, shallow-marine carbonates, oceanic crust and volcanic arc, respectively) and an inner complexly deformed sub-zone (late Palaeozoic–Mesozoic passive margin succession). Rifting and sea-floor spreading of Tethys occurred in the Permian to Triassic but in the Sanandaj–Sirjan Zone extension-related successions are mainly of Late Triassic age. Subduction of Tethyan sea floor in the Late Jurassic to Cretaceous produced deformation, metamorphism and unconformities in the marginal and complexly deformed sub-zones. Deformation climaxed in the Late Cretaceous when a major southwest-vergent fold belt formed associated with greenschist facies metamorphism and post-dated by abundant Palaeogene granitic plutons. In the southwest of the zone a Late Cretaceous island arc—passive margin collision occurred with ophiolite emplacement onto the northern Arabian margin similar to that in Oman. Final closure of Tethys was not completed until the Miocene when Central Iran collided with the northeast Arabian margin.     

Jurassic–Cretaceous granitoids and related tectono-metallogenesis in the Zapug–Duobuza arc,western Tibet

The Zapug–Duobuza magmatic arc (ZDMA), located along the southern edge of the south Qiangtang terrane in western Tibet, extends east–west for ~ 400 km. Small scattered granite and porphyry intrusions crop out in the ZDMA, but a large amount of granite may be buried by Late Cretaceous to Paleogene thrusting. Two stages of magmatism have been identified, at 170–150 Ma and 130–110 Ma. The widely distributed Middle–Late Jurassic granite intrusions in the ZDMA exhibit SrNd isotopic characteristics similar to those of ore-bearing porphyries in the Duolong giant CuAu deposit, and their εHf(t) values mostly overlap those of other porphyry CuMo deposits in the ZDMA and the Gangdese zone. The SrNdHf isotopic geochemistry suggests variable contributions of mantle and Qiangtang crustal sources, and indicates the presence of two new ore districts with potentials for CuAu, Fe, and PbZn ores, located in the Jiacuo–Liqunshan and Larelaxin–Caima areas. Except for the Duolong ore-forming porphyries, which show significant contributions of mantle components intruded into an accretionary mélange setting, the Early Cretaceous granites in other areas of the belt are of mostly crustal origin, from sources in Qiangtang felsic basement and Permo-Carboniferous strata, indicating the weak ore-forming potential of skarn-type Fe and PbZn deposits. The ephemeral but deep Bangong Co–Nujiang ocean in the Early Jurassic evolved into a shallow compressional marine basin in the Middle–Late Jurassic, possibly transitioning to northward flat subduction of oceanic crust at this time. The subducted slab broke off in the Early Cretaceous, initiating a peak in arc magmatism and metallogenesis at 125–110 Ma.     

Early–Middle Jurassic evolution of the northern Yangtze foreland basin: a record of uplift following Triassic continent–continent collision to form the Qinling–Dabieshan orogenic belt

The northern Yangtze foreland basin system was formed during the Mesozoic continental collision between the North and South China plates along the Mianlue suture. In response to the later phase of intra-continental thrust deformation, an extensive E–W-trending molasse basin with river, deltaic, and lake deposits was produced in front of the southern Qinling–Dabieshan foreland fold-and-thrust belt during the Early–Middle Jurassic (201–163 Ma). The basin originated during the Early Jurassic (201–174 Ma) and substantially subsided during the Middle Jurassic (174–163 Ma). A gravelly alluvial fan depositional system developed in the lower part of the Baitianba Formation (Lower Jurassic) and progressively evolved into a meandering river fluvial plain and lake systems to the south. The alluvial fan conglomerates responded to the initial uplift of the southern Qinling–Dabieshan foreland fold-and-thrust belt after the oblique collision between the Yangtze and North China plates during the Late Triassic. The Qianfoya Formation (lower Middle Jurassic) mainly developed from shore-shallow lacustrine depositional systems. The Shaximiao Formation (upper Middle Jurassic) predominantly consists of thick-bedded braided river delta successions that serve as the main body of the basin-filling sequences. The upward-coarsening succession of the Shaximiao Formation was controlled by intense thrusting in the southern Qinling–Dabieshan fold-and-thrust belt. Palaeogeographic reconstructions indicated an extensive E–W foredeep depozone along the fold-and-thrust belt during the Middle Jurassic (174–163 Ma) that was nearly 150 km wide. The depozone extended westward to the Longmenshan and further east to the northern middle Yangtze plate. The northern Yangtze foreland basin was almost completely buried or modified by the subsequent differential thrusting of Dabashan and its eastern regions (Late Jurassic to Cenozoic).     

Zircon–quartz–calcite segregations in carbonate–alkaline metasomatic rocks of the western Baikal region and their petrogenetic implications

Fine-grained segregations up to 5 mm in size composed of graphic intergrowths of zircon, quartz, calcite and containing up to 0.8 wt % SrO have been found in albite–riebeckite and dolomite–biotite metasomatic rocks formed after alaskite granite. They contain magnetite, titanomagnetite (25.4 wt % TiO₂), cerite-(Ce,Nd), rutile (up to 1.2 wt % Nb₂O₅), as well as rare micrograins of monazite-(Ce), bastnaesite-(Ce), and barite (up to 5.7 wt % SrO). The fine-grained structure of mineral aggregates suggests a metacolloidal nature. It is assumed that the zircon–quartz–calcite assemblage was formed due to exchange decomposition reaction between the salt phase of hydrothermal solution with predominant Na₂CO₃, elevated Zr and, to a lesser extent, Fe, Ti, LREE, Nb contents and dissolved calcium and silica compounds of a Na₂SiO₃ type.     

The Jabali nonsulfide Zn–Pb–Ag deposit,western Yemen

The Jabali Zn–Pb–Ag deposit is located about 110 km east of Sana'a, the capital of Yemen, along the western border of the Marib-Al-Jawf/Sab'atayn basin. The economic mineralization at Jabali is a nonsulfide deposit, consisting of 8.7 million tons at an average grade of 9.2% zinc, derived from the oxidation of primary sulfides. The rock hosting both primary and secondary ores is a strongly dolomitized carbonate platform limestone of the Jurassic Shuqra Formation (Amran Group). The primary sulfides consist of sphalerite, galena and pyrite/marcasite. Smithsonite is the most abundant economic mineral in the secondary deposit, and is associated with minor hydrozincite, hemimorphite, acanthite and greenockite. Smithsonite occurs as two main generations: smithsonite 1, which replaces both host dolomite and sphalerite, and smithsonite 2, occurring as concretions and vein fillings in the host rock. At the boundary between smithsonite 1 and host dolomite, the latter is widely replaced by broad, irregular bands of Zn-bearing dolomite, where Zn has substituted for Mg. The secondary mineralization evolved through different stages: 1) alteration of original sulfides (sphalerite, pyrite and galena), and release of metals in acid solutions; 2) alteration of dolomite host rock and formation of Zn-bearing dolomite; 3) partial dissolution of dolomite by metal-carrying acid fluids and replacement of dolomite and Zn-bearing dolomite by a first smithsonite phase (smithsonite 1). To this stage also belong the direct replacement of sphalerite and galena by secondary minerals (smithsonite and cerussite); 4) precipitation of a later smithsonite phase (smithsonite 2) in veins and cavities, together with Ag- and Cd-sulfides.The δ¹⁸O composition of Jabali smithsonite is generally lower than in other known supergene smithsonites, whereas the carbon isotope composition is in the same range of the negative δ¹³C values recorded in most supergene nonsulfide ores. Considering that the groundwaters and paleo-groundwaters in this area of Yemen have negative δ¹⁸O values, it can be assumed that the Jabali smithsonite precipitated in different stages from a combination of fluids, possibly consisting of local groundwaters variably mixed with low-temperature hydrothermal waters. The carbon isotope composition is interpreted as a result of mixing between carbon from host rock carbonates and soil/atmospheric CO₂.The most favorable setting for the development of the Jabali secondary deposit could be placed in the early Miocene (~ 17 Ma), when supergene weathering was favored by major uplift and exhumation resulting from the main phase of Red Sea extension. Low-temperature hydrothermal fluids may have also circulated at the same time, through the magmatically-induced geothermal activity in the area.     

Seismic structure of the Helan–Liupan–Ordos western margin tectonic belt in North-Central China and its geodynamic implications

We study high-resolution three-dimensional P-wave velocity (Vp) tomography and anisotropic structure of the crust and uppermost mantle under the Helan–Liupan–Ordos western margin tectonic belt in North-Central China using 13,506 high-quality P-wave arrival times from 2666 local earthquakes recorded by 87 seismic stations during 1980–2008. Our results show that prominent low-velocity (low-V) anomalies exist widely in the lower crust beneath the study region and the low-V zones extend to the uppermost mantle in some local areas, suggesting that the lower crust contains higher-temperature materials and fluids. The major fault zones, especially the large boundary faults of major tectonic units, are located at the edge portion of the low-V anomalies or transition zones between the low-V and high-V anomalies in the upper crust, whereas low-V anomalies are revealed in the lower crust under most of the faults. Most of large historical earthquakes are located in the boundary zones where P-wave velocity changes drastically in a short distance. Beneath the source zones of most of the large historical earthquakes, prominent low-V anomalies are visible in the lower crust. Significant P-wave azimuthal anisotropy is revealed in the study region, and the pattern of anisotropy in the upper crust is consistent with the surface geologic features. In the lower crust and uppermost mantle, the predominant fast velocity direction (FVD) is NNE–SSW under the Yinchuan Graben and NWW–SEE or NW–SE beneath the Corridor transitional zone, Qilian Orogenic Belt and Western Qinling Orogenic Belt, and the FVD is NE–SW under the eastern Qilian Orogenic Belt. The anisotropy in the lower crust may be caused by the lattice-preferred orientation of minerals, which may reflect the lower-crustal ductile flow with varied directions. The present results shed new light on the seismotectonics and geodynamic processes of the Qinghai–Tibetan Plateau and its northeastern margin.     

Petrogenesis of Miocene alkaline volcanic suites from western Bohemia: whole rock geochemistry and Sr–Nd–Pb isotopic signatures

The Mid to Late Miocene intraplate alkaline volcanic suites of western Bohemia are relict of the intensive voluminous volcanism accompanied by large-scale uplift and doming. The association with the uplift of the NE flank of the Cheb–Domažlice Graben (CDG) is uncertain in view of the mostly transpressional tectonics of the graben. The volcanism is most probably of the Ohře/Eger Rift off-rift settings. Two cogenetic volcanic suites have been recognised: (i) silica-saturated to oversaturated consisting of olivine basalt–trachybasalt-(basaltic) trachyandesite–trachyte–rhyolite (13.5 to 10.2 Ma) and (ii) silica-undersaturated (significantly Ne-normative) (melilite-bearing) olivine nephelinite–basanite–tephrite (18.3 to 6.25 Ma). A common mantle source is suggested by similar primitive mantle-normalised incompatible element patterns and Sr–Nd–Pb isotopic compositions for the assumed near-primary mantle-derived compositions of both suites, i.e., olivine basalt and olivine nephelinite. Apparently, they were generated by different degrees of partial melting of a common mantle source, with garnet, olivine and clinopyroxene in the residuum. Negative Rb and K anomalies indicate a residual K-phase (amphibole/phlogopite) and melting of partly metasomatised mantle lithosphere. The evolution of the basanite–olivine basalt–trachybasalt-(basaltic) trachyandesite–trachyte–rhyolite suite suggests the presence of an assimilation–fractional crystallization process (AFC). Substantial fractionation of olivine, clinopyroxene, Fe–Ti oxide, plagioclase/alkali feldspar and apatite accompanied by a significant assimilation of magma en route by crustal material is most evident in evolved member, namely, trachytes and rhyolites. The magmas were probably sourced by both sub-lithospheric and lithospheric partly metasomatised mantle. The evolution of the (melilite-bearing) olivine nephelinite–basanite–tephrite suite is less clear because of its limited extent. Parental magma of both these rock suites is inferred to have originated by low-degree melting of the mantle source initiated at ca. 18 Ma and reflects mixing of asthenosphere-derived melts with isotopically enriched lithospheric melts. The older Oligocene alkaline rocks (29–26 Ma) occur within the Cheb–Domažlice Graben (CDG) locally but are significant in the closely adjacent neighbouring western Ohře Rift. The Sr–Nd–Pb isotopic composition of primitive volcanic rocks of both suites is similar to that of the European Asthenospheric Reservoir (EAR). Initial Pb isotopic data plot partly above the northern hemisphere reference line at radiogenic ²⁰⁶Pb/²⁰⁴Pb ratios of ∼19 to 20, and indicate the presence of a Variscan crustal component in the source.     

Juvenile granite in the Sanandaj–Sirjan Zone,NW Iran: Late Jurassic–Early Cretaceous arc–continent collision

The Sanandaj–Sirjan Zone (SSZ) of western Iran is characterized by numerous granitoids of mainly calc-alkaline affinities. Several leucogranite and monzonite bodies crop out in the eastern Sanandaj. Whole-rock Rb–Sr isochrons demonstrate that the Mobarak Abad monzonite (MAM) formed in two phases at 185 and 131 Ma. Low ⁸⁷Sr/⁸⁶Sr(i) (i represents initial) and high ¹⁴³Nd/¹⁴⁴Nd(i) ratios, resulting in positive ?^t _Nd, imply that the source magma originated from a depleted mantle; large ion lithophile element (LILE) and light rare earth element (LREE) enrichments imply that slab fluid was involved in the evolution of the parent magma. Geochemical characteristics of the MAM rocks show an affinity with I- and A-type granites, and the positive values of ?^t _Nd (+2 to +6), confirm that the MAM represents juvenile granite. Therefore, the MAM rocks are different from Himalayan, Hercynian, and Caledonian granites. Based on the geology of granitic host rocks that form the protoliths of metamorphic rocks, it is likely that the mafic part of the MAM formed in an island arc setting on Neo-Tethyan oceanic crust during Early to Middle Jurassic time. Subsequent collision of the island arc with the western part of the SSZ occurred in the Late Jurassic to Early Cretaceous. Metamorphism, accompanied by partial melting, occurred during collision. Finally, leucogranite magmas of the young Mobarak Abad dikes and the Suffi Abad body were generated in this collision zone. This new model suggests a Late Jurassic–Early Cretaceous arc–continental collision before final closing of the Neo-Tethys.     

Geochemistry and geochronology of porphyries from the Beiya gold–polymetallic orefield,western Yunnan,China

The Beiya gold–polymetallic orefield, with gold reserves of 305 t, is one of the most representative porphyry-skarn orefields in the Jinshajiang–Ailaoshan Cu–Au ore belt within the Sanjiang region of southwest China. The orefield contains seven deposits: the Wandongshan, Hongnitang, Dashadi, Bijiashan, Weiganpo, Matouwan, and Bailiancun deposits. In this paper we report on the geochemistry and geochronology of porphyries associated with mineralization from the seven deposits. The results show that all the porphyries have similar geochemistry, with high alkalinity, high contents of SiO₂, Al₂O₃, K₂O, and Sr, high K₂O/Na₂O ratios, low MgO, Y, and Yb contents, enrichments in Ba, K, and Pb, depletions in P, Ti, Nb, and Ta, and non-evident to weak Eu depletions (δEu = 0.42–0.99). In the SiO₂ vs. Th/Ce diagram, the porphyry samples are distributed in the area of thickened lower crust, and in the Sr/Y vs. Y and La/Yb vs. Yb diagrams, the porphyries broadly followed the batch-melting trend of amphibolite containing up to 10% garnet. LA-MC-ICP-MS zircon U–Pb dating analysis suggests that the porphyries were emplaced between 34.62 ± 0.25 and 36.72 ± 0.25 Ma. They were coeval with lamprophyres (34 to 36 Ma) in the Beiya area and with potassic–ultrapotassic intrusive rocks (40 to 35 Ma) within the Jinshajiang–Ailaoshan magmatic belt, indicating possible genetic relation between these rock types. We suggest that the porphyries in the Beiya gold–polymetallic orefield were derived from the partial melting of a K-rich mafic source in the thickened lower crust, with the melting triggered by asthenospheric upwelling following the removal of lower lithospheric mantle.     

Space–time analysis, faulting and triggering of the 2010 earthquake doublet in western Corinth Gulf

Two moderate magnitude earthquakes (M5.5 and M5.4) occurred in January 2010 with their epicenters at a distance of about 5?km between them, in the western part of the Corinth Gulf. The recordings of the regional seismological network, which is dense locally, were used for the location of the two main events and aftershocks, which are concentrated in three clusters beneath the northern coasts of the Gulf. The first two clusters accompany each one of the two stronger earthquakes, whereas the third cluster comprises only low magnitude aftershocks, located westward of the two stronger events. Seismic excitation started in January 18, 2010, with the M?=?5.5 earthquake in the area occupied by the central cluster. Seismicity immediately jumped to the east with numerous aftershocks and the M?=?5.4 earthquake which occurred four days later (January 22, 2010). Cross sections normal to the long axis of each cluster show ruptures on north dipping faults at depths of 7?C11?km. Focal mechanisms of the stronger events of the sequence support the results obtained from the spatial distribution of the aftershocks that three different fault segments activated in this excitation. The slip vectors of all the events have an NNW?CSSE to NNE?CSSW orientation almost parallel to the direction of extension along the Corinth Gulf. Calculation of the Coulomb stress changes supports an interaction between the different clusters, with the major activity being coincided with the area of positive induced stress changes after the first earthquake.     

Geochemistry and zircon U–Pb–Hf isotopes of the 780 Ma I-type granites in the western Yangtze Block: petrogenesis and crustal evolution

ABSTRACT

Neoproterozoic I-type granites could provide vital insights into the crust–mantle interaction and the crustal evolution along the western Yangtze Block, South China. This paper presents new zircon U–Pb ages, bulk-rock geochemistry, and in situ zircon Lu–Hf isotope on the Dalu I-type granites from the southwestern Yangtze Block. Zircon U–Pb dating show the crystallization ages of 781.1 ± 2.8 Ma for granodiorites and 779.8 ± 2.0 Ma for granites, respectively. The Dalu granodiorites are Na-rich, calc-alkaline, metaluminous to slightly peraluminous (A/CNK = 0.94–1.08). Zircons from granodiorite have positive εHf(t) values (+2.16 to +7.39) with crustal model ages of 1.21–1.54 Ga, indicating juvenile mafic lower crust source. The Dalu granites are high-K calc-alkaline, peraluminous rocks. They have variable zircon εHf(t) values (?4.65 to +5.80) with crustal model ages of 1.31–1.97 Ga, suggesting that they were derived from the mature metasediment-derived melts by the mixing of newly formed mafic lower crust-derived melts. The geochemical variations in Dalu pluton is dominated not only by the different source rocks but also by the different melting temperatures. Combining with the geochemistry and isotopic compositions of I-type granitoids and tectonic setting in the western Yangtze Block, we propose that the Dalu I-type granodiorites–granites associations are the magmatic response from different crustal levels, which were induced by the heat anomaly due to the asthenosphere upwelling in the subduction-related setting.     

The Permian Huangshanxi Cu–Ni deposit in western China: intrusive–extrusive association,ore genesis,and exploration implications

The Permian Huangshanxi Cu–Ni deposit is the second largest magmatic sulfide deposit discovered to date in a major Ni–Cu province related to protracted basaltic magmatism in eastern Xinjiang, China. It is hosted by a small mafic–ultramafic intrusion comprised predominantly of lherzolites, olivine websterites, gabbronorites, and gabbros. The Huangshanxi intrusion is coeval with Permian basalts of tholeiitic and alkaline affinities in the Tuha and Tarim basins, respectively. To evaluate a possible genetic relationship between the Huangshanxi intrusion and a specific type of coeval basalt in the region, as well as ore genesis in the intrusion, we have carried out an integrated mineralogical, petrological, and geochemical study. Our data reveal that the Huangshanxi intrusive rocks are characterized by relatively flat chondrite-normalized REE patterns, depletion in Nb and Ta, and elevated εNd values varying between 6 and 10. These features are similar to those of coeval tholeiitic basalts in the nearby Tuha basin, but are significantly different from those of coeval alkaline basalts in the relatively remote Tarim basin. The geochemical similarities and differences suggest that the Huangshanxi intrusion is genetically related to the tholeiitic basalts in the Tuha basin, not to the alkaline basalts in the Tarim basin, as suggested previously by some researchers. This implies that regional exploration for the Huangshanxi-type Cu–Ni deposits should be centered in the Tuha basin instead of the Tarim basin. More specifically, the uplifted areas around the Tuha basin where similar intrusions may have been brought close to the surface should be carefully examined for mineralization potential. Intrusive relations and mass balance constraints from incompatible trace elements and sulfide abundances suggest that the Huangshanxi intrusion represents a dynamic magma conduit through which multiple pulses of magma ascended to higher levels or to the surface. Numerical simulation of magma evolution and mixing calculations using Sr–Nd isotopes indicate that selective assimilation of S-bearing crustal materials is important for sulfide saturation during the early stages of magma evolution when lherzolites formed. Fractional crystallization may have also played a role in the attainment of sulfide saturation during the later stages of magma evolution when olivine websterites and gabbronorites formed. In both cases, immiscible sulfide droplets were retained in the conduit to form disseminated sulfide lenses while the fractionated silicate liquids and buoyant phases such as plagioclase continued to ascend. Extremely low PGE tenors in the sulfide ores of the Huangshanxi deposit suggest that the parental magma was highly depleted in chalcophile elements possibly due to previous sulfide segregation at depth.     

Petrogenesis of the Bao'anzhai granite and associated Mo mineralization,western Dabie orogen,east-central China: Constraints from zircon U–Pb and molybdenite Re–Os dating,whole-rock geochemistry,and Sr–Nd–Pb–Hf isotopes

Recently, many Mo deposits genetically related to emplacement of Early Cretaceous granites have been found in the Dabie–Qinling belt. A typical intrusion that combines magmatism and metallogenesis, the Bao'anzhai granite, yields a zircon ²³⁸U–²⁰⁶Pb age of 123.2 ± 1.1 Ma and a molybdenite Re–Os isochron age of 122.5 ± 2.7 Ma. This granite is characterized by high silica and alkali, but low Mg, Fe, and Ca. It is enriched with light rare earth elements (REEs) and large-ion lithophile elements (LILEs, Rb, K, Th, U) but depleted of heavy REEs, high field strength elements (HFSEs, Nb, Ta, Ti, and Y), and Sr. This high-K granite has medium initial ⁸⁷Sr/⁸⁶Sr ratios (0.706518–0.707116) and low initial Pb isotopic ratios [(²⁰⁶Pb/²⁰⁴Pb)_i, 16.423–16.699; (²⁰⁷Pb/²⁰⁴Pb)_i, 15.285–15.345; (²⁰⁸Pb/²⁰⁴Pb)_i, 37.335–37.633], and is characterized by low ?_Nd(t) and ?_Hf(t) values (?14.92 to??14.22 and??21.67 to??19.19, respectively). These data indicate that this pluton is a high-K calc-alkaline fractionated I-type granitite. It was generated by partial melting of the Yangtze lower crust, which is probably similar to Neoproterozoic TTG-like magmatic rocks at the north Yangtze Block under a non-thickened lower crust environment (<35 km). The ores also have low radiogenic Pb isotopes (²⁰⁶Pb/²⁰⁴Pb, 16.592–17.674; ²⁰⁷Pb/²⁰⁴Pb, 15.300–15.476; ²⁰⁸Pb/²⁰⁴Pb, 37.419–37.911) and low Re content in molybdenite (5.693–10.970 ppm), suggesting a crustal magmatic source for the metallogenic minerals in the Mo deposit.     

Petrophysical features of the tectonic structures of the Okhotsk continent–ocean transition zone

Petrophysical characteristics are determined for the rock complexes of the Okhotsk margin within the continent–ocean transition zone. Petrophysical maps showing the major inhomogeneities of the main tectonic elements of the studied area are constructed. Petrophysical inhomogeneities and the corresponding geophysical field anomalies are compared. A relationship between the magnetic field anomalies and subsurface rock complexes is revealed. The gravity field anomalies are related to deep inhomogeneities and are almost independent of the subsurface rock complexes of relatively low thickness.     

Shallow submarine epithermal Pb–Zn–Cu–Au–Ag–Te mineralization on western Milos Island,Aegean Volcanic Arc,Greece: Mineralogical,geological and geochemical constraints

Milos Island contains several epithermal deposits (e.g., Profitis Ilias–Chondro Vouno Pb–Zn–Ag–Au–Te–Cu, Triades–Galana–Agathia–Kondaros Pb–Zn–Ag–Bi–W–Mo ± Cu–Au, and Katsimoutis–Kondaros–Vani Pb–Zn–Ag–Mn) of Late Pliocene to Early Pleistocene age. These deposits are hosted in calc-alkaline volcanic rocks emplaced as a result of three successive magma pulses in an emergent volcanic edifice: submarine rhyolitic to rhyodacitic cryptodomes at ca. 2.7. Ma (Profitis Ilias–Chondro Vouno), submarine to subaerial andesite to dacite domes at ca. 2.2 to 1.5 Ma (Triades–Galana–Kondaros–Katsimouti–Vani). Hydrothermal alteration of the volcanic rocks includes advanced argillic- (both hypogene and steam-heated), argillic, phyllic, adularia-sericite and propylitic types. In the northern sector (Triades–Galana–Agathia–Kondaros), initial magma degassing derived from andesitic–dacitic intrusives along NE–SW to E–W trending faults resulted in the development of pre-ore hypogene advanced argillic alteration (dickite, alunite, ± diaspore, pyrophyllite, halite, and pyrite) in a submarine environment. Mineralogical data indicate common features among the Profitis Ilias–Chondro Vouno, Kondaros–Katsimoutis–Vani and Triades–Galana mineralized centers, all of which are characterized by the presence of galena, Fe-poor sphalerite, and chalcopyrite as well as abundant barite, adularia, sericite and, to a lesser extent, calcite, which are typical of intermediate-sulfidation epithermal type deposits. Locally, at Triades–Galana and Kondaros–Agathia, high-sulfidation conditions prevailed as suggested by the presence of coexisting enargite and covellite. The high silver and gold content of the western Milos deposits is derived from Ag-bearing sulfosalts (polybasite, pearceite, pyrargyrite, freibergite) and tellurides. Gold at Profitis Ilias, both as native gold and silver-gold tellurides, is present in base-metal precipitates within multicomponent blebs, which recrystallized to form hessite, petzite, altaite, coloradoite, and native gold. Mineralogical evidence (e.g. microchimney structures, copper sulfides, widespread occurrence of barite, aragonite) suggests that precious metal mineralization in western Milos mineralization formed in a submarine setting.We present information on the surface distribution of Au, Ag, Cu, Pb, Zn, As, Sb, Hg, Mo, Bi, W and Cd at western Milos. Gold is enriched at Profitis Ilias–Chondro Vouno deposits and to a lesser extent at Triades–Galana. Arsenic is absent from the southern sector but shows elevated concentrations together with molybdenum, bismuth and tungsten at the northern sector (Triades–Galana, Vani deposits). The differences in precious and base metal abundances may be related to the depths at which the deposits are exposed, and/or different sources of magma. The metal signatures of the Triades–Galana and Agathia–Kondaros–Katsimouti–Vani (Mo–Bi–W–As–Hg–Ag–Au) occurrences compared to Profitis Ilias (Te–Au–Ag) reflect different sources of magma (dacite–rhyodacite for Profitis Ilias, andesite–dacite for Triades–Galana, and dacite for Kondaros–Katsimoutis). The enrichment of Te, Mo, W, and Bi in the deposits is a strong indication of a direct magmatic contribution of these metals.At western Milos, precious and base-metal vein mineralization was deposited during episodic injection of magmatic volatiles and dilution of the hydrothermal fluids by seawater. The mineralization represents seafloor/sub-seafloor precipitation of sulfides that formed in stockwork zones. Base and precious metal mineralization formed from intermediate- to high-sulfidation state fluids and mostly under boiling conditions as indicated by the widespread occurrence of adularia associated with metallic mineralization. We speculate that the widespread occurrence of boiling and the shallow depth of the precious- and base-metal emplacement prevented the formation of seafloor massive sulfides.     

Biomarker signatures from Neoproterozoic–Early Cambrian oil,western India

The Neoproterozoic–Early Cambrian time is the cradle of multicellular, eukaryotic life and thereafter metazoan life started populating the planet. Biomarkers, which record the chronicles of biotic events on Earth, have been investigated using gas chromatography–mass spectrometry from a suite of nine oil samples extracted from oil stained sediments and a crude oil of Neoproterozoic–Early Cambrian age from Bikaner-Nagaur Basin, western India. The biomarker distribution is characterized by high concentration of both tricyclic terpanes and pentacyclic hopanes. The predominance of C₂₉ regular sterane over C₂₇ and C₂₈ steranes indicates green algal contribution and may imply the prominence and diversification of the same algal group. The low pristane/phytane ratio and occurrence of substantial quantity of gammacerane, monomethyl 2-methyl-2-(2,4,8-trimethyltridecyl) chroman and 1,1,7,8-tetramethyl-1,2,3,4 tetrahydrophenanthrene are indicative of stratified water column as well as anoxic and enhanced salinity condition of the environment of deposition. This paper reports for the first time the presence of methyltrimethyltridecyl chromans (MTTCs) from Neoproterozoic–Early Cambrian time. Putative C₁₉ norsteranes, probably indicative of sponge input, are recorded in the studied samples and also reported from other infracambrian oils and sediments. Normal alkanes (n-C₁₇ and n-C₁₈) and isoprenoids (pristane and phytane) are highly depleted in δ¹³C. The overall biomarker distribution and carbon isotope data of oils from Bikaner-Nagaur Basin show similarities with those of other infracambrian oils like Huqf oils from Oman and Baykit High oils from eastern Siberia.     

U–Pb zircon geochronology,geochemical, and Sr–Nd isotopic constraints on the age and origin of basaltic porphyries from western Liaoning Province,China

Basaltic porphyries from the northeast North China craton (NCC) provide an excellent opportunity to examine the nature of their mantle source and the secular evolution of the underlying mantle lithosphere. In addition, the study helps to constrain the age and the mechanism of NCC lithospheric destruction. In this paper, we report geochronological, geochemical, and Sr–Nd isotopic analyses of a suite of mafic lavas. Detailed laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U–Pb dating yielded an age of 223.3 ± 1.1 million years, which we regard as representing the crystallization age of the basaltic porphyries. The bulk-rock analysed samples are enriched in both large ion lithophile elements (LILEs) (i.e. Ba, Sr, and Pb) and light rare earth elements (LREEs), but depleted in high field strong elements (HFSEs) (i.e. Nb, Ta, Zr, Hf, and Ti) and heavy rare earth elements (HREEs), without significant Eu anomalies (Eu/Eu*?= 089–0.98). The basaltic porphyries have undergone low degrees (～5%) of partial melting of a garnet-bearing lherzolite mantle. The rocks display very uniform (⁸⁷Sr/⁸⁶Sr) _i (0.70557–0.70583) and negative ?_Nd (t) values (–11.9 to –10.1). These features indicate that the western Liaoning basaltic porphyries were derived from a common enriched lithosphere mantle that had previously been metasomatized by fluids related to subduction of Palaeo-Asian sedimentary units. However, the mafic melts were not affected to a significant degree by crustal contamination. Based on earlier studies, these findings provide new evidence that the northeast margin of the NCC had undergone a phase of post-orogenic extensional tectonics during the Middle Triassic. Furthermore, lithospheric thinning occurring across the northern NCC might have been initiated during Early Triassic times and was likely controlled by the final closure of the Palaeo-Asian Ocean, as well as the collision of Mongolian arc terrenes with the NCC.