Cold subduction of the Neotethys: the metamorphic record from finely banded lawsonite and epidote blueschists and associated metabasalts of the Nagaland Ophiolite Complex,India

In this study, we have deduced the thermal history of the subducting Neotethys from its eastern margin, using a suite of partially hydrated metabasalts from a segment of the Nagaland Ophiolite Complex (NOC), India. Located along the eastern extension of the Indus‐Tsangpo suture zone (ITSZ), the N–S‐trending NOC lies between the Indian and Burmese plates. The metabasalts, encased within a serpentinitic mélange, preserve a tectonically disturbed metamorphic sequence, which from west to east is greenschist (GS), pumpellyite–diopside (PD) and blueschist (BS) facies. Metabasalts in all the three metamorphic facies record prograde metamorphic overprints directly on primary igneous textures and igneous augite. In the BS facies unit, the metabasalts interbedded with marble show centimetre‐ to metre‐scale interlayering of lawsonite blueschist (LBS) and epidote blueschist (EBS). Prograde HP/LT metamorphism stabilized lawsonite + omphacite (X_Jd = 0.50–0.56 to 0.26–0.37) + jadeite (X_Jd = 0.67–0.79) + augite + ferroglaucophane + high‐Si phengite (Si = 3.6–3.65 atoms per formula unit, a.p.f.u.) + chlorite + titanite + quartz in LBS and lawsonite + glaucophane/ferroglaucophane ± epidote ± omphacite (X_Jd = 0.34) + chlorite + phengite (Si = 3.5 a.p.f.u.) + titanite + quartz in EBS at the metamorphic peak. Retrograde alteration, which was pervasive in the EBS, produced a sequence of mineral assemblages from omphacite and lawsonite‐absent, epidote + glaucophane/ferroglaucophane + chlorite + phengite + titanite + quartz through albite + chlorite + glaucophane to lawsonite + albite + high‐Si phengite (Si = 3.6–3.7 a.p.f.u.) + glaucophane + epidote + quartz. In the PD facies metabasalts, the peak mineral assemblage, pumpellyite + chlorite + titanite + phengitic white mica (Si = 3.4–3.5 a.p.f.u.) + diopside appeared in the basaltic groundmass from reacting titaniferous augite and low‐Si phengite, with prehnite additionally producing pumpellyite in early vein domains. In the GS facies metabasalts, incomplete hydration of augite produced albite + epidote + actinolite + chlorite + titanite + phengite + augite mineral assemblage. Based on calculated T–M(H₂O), T–M(O₂) (where M represents oxide mol.%) and P–T pseudosections, peak P–T conditions of LBS are estimated at ~11.5 kbar and ~340 °C, EBS at ~10 kbar, 325 °C and PD facies at ~6 kbar, 335 °C. Reconstructed metamorphic reaction pathways integrated with the results of P–T pseudosection modelling define a near‐complete, hairpin, clockwise P–T loop for the BS and a prograde P–T path with a steep dP/dT for the PD facies rocks. Apparent low thermal gradient of 8 °C km^?1 corresponding to a maximum burial depth of 40 km and the hairpin P–T trajectory together suggest a cold and mature stage of an intra‐oceanic subduction zone setting for the Nagaland blueschists. The metamorphic constraints established above when combined with petrological findings from the ophiolitic massifs along the whole ITSZ suggest that intra‐oceanic subduction systems within the Neotethys between India and the Lhasa terrane/the Karakoram microcontinent were also active towards east between Indian and Burmese plates.     

Late Quaternary solifluction sheets in the British uplands

Solifluction sheets are large-scale and extensive valley-floor and valley-side landforms developed widely in the British Isles from mass-wasting of glacial and periglacial sediments during late-glacial times. We describe their geographical distribution and review the processes that have led to their development. We use data from the Cheviot Hills, the one site in the British Isles where sedimentology and optically stimulated luminescence dating have been combined, to assess their age and nature of deposition. We also present data from central Wales where a new mapping and resistivity survey has reconstructed the nature of valley-side solifluction sheets. We explore the relative lack of recent research on these landforms and argue that solifluction sheets represent a clear example of how upland geomorphological systems have responded to late-glacial climate change. We end by identifying a number of areas where research on these enigmatic features could be focussed, including better understanding of their distribution, sedimentology and age.     

SHRIMP dating of the Permo-Carboniferous Jinshajiang ophiolite, southwestern China: Geochronological constraints for the evolution of Paleo-Tethys

SHRIMP U–Pb zircon dating of gabbro, anorthosite, trondhjemite and granodiorite from the Jinshajiang ophiolitic mélange of southwestern China provides geochronological constraints on the evolution of Paleo-Tethys. The ophiolitic mélange is exposed for about 130 km along the Jinshajiang River where numerous blocks of serpentinite, ultramafic cumulate, gabbro, sheeted dikes, pillow lavas and radiolarian chert are set in a greenschist matrix. A cumulate gabbro-anorthosite association and an amphibole gabbro have ages of 338 ± 6 Ma, 329 ± 7 Ma and 320 ± 10 Ma, respectively, which constrain the time of formation of oceanic crust. An ophiolitic isotropic gabbro dated at 282–285 Ma has the same age as a trondhjemite vein (285 ± 6 Ma) cutting the gabbro. These ages probably reflect a late phase of sea-floor spreading above an intra-oceanic subduction zone. At the southern end of the Jinshajiang belt, a granitoid batholith (268 ± 6 Ma), a gabbro massif (264 ± 4 Ma), and a granodiorite (adakite) intrusion (263 ± 6 Ma) in the ophiolitic mélange constitute a Permian intra-oceanic plutonic arc complex. A trondhjemite dike intruded serpentinite in the mélange at 238 ± 10 Ma and postdates the arc evolution of the Jinshajiang segment of Paleo-Tethys.     

Geochemical and isotopic constraints on the age and origin of the Nidar Ophiolitic Complex, Ladakh, India: Implications for the Neo-Tethyan subduction along the Indus suture zone

The Nidar ophiolite complex is exposed within the Indus suture zone in eastern Ladakh, India. The suture zone is considered to represent remnant Neo-Tethyan Ocean that closed via subduction as the Indian plate moved northward with respect to the Asian plate. The two plates ultimately collided during the Middle Eocene. The Nidar ophiolite complex comprises a sequence of ultra-mafic rocks at the base, gabbroic rocks in the middle and volcano-sedimentary assemblage on the top. Earlier studies considered the Nidar ophiolite complex to represent an oceanic floor sequence based on lithological assemblage. However, present study, based on new mineral and whole rock geochemical and isotopic data (on bulk rocks and mineral separates) indicate their generation and emplacement in an intra-oceanic subduction environment. The plutonic and volcanic rocks have nearly flat to slightly depleted rare earth element (REE) patterns. The gabbroic rocks, in particular, show strong positive Sr and Eu anomalies in their REE and spidergram patterns, probably indicating plagioclase accumulation. Depletion in high field strength elements (HFSE) in the spidergram patterns may be related to stabilization of phases retaining the HFSE in the subducting slab and / or fractional crystallization of titano-magnetite phases. The high radiogenic Nd- and low radiogenic Sr-isotopic ratios for these rocks exclude any influence of continental material in their genesis, implying an intra-oceanic environment.

Nine point mineral–whole rock Sm–Nd isochron corresponds to an age of 140 ± 32 Ma with an initial ¹⁴³Nd/¹⁴⁴Nd of 0.513835 ± 0.000053 (E_Nd t = + 7.4). This age is consistent with the precise Early Cretaceous age of Hauterivian (132 ± 2 to 127 ± 1.6 Ma) to Aptian (121 ± 1.4 to 112 ±1.1 Ma) for the overlying volcano-sedimentary (radiolarian bearing chert) sequences based on well-preserved radiolarian fossils (Kojima, S., Ahmad, T., Tanaka, T., Bagati, T.N., Mishra, M., Kumar, R. Islam, R., Khanna, P.P., 2001. Early Cretaceous radiolarians from the Indus suture zone, Ladakh, northern India. In: News of Osaka Micropaleontologists (NOM), Spec. Vol., 12, 257–270.) and cooling ages of 110–130 Ma based on ³⁹Ar/⁴⁰Ar for Nidar–Spontang ophiolitic rocks (Mahéo, G., Berttrand, H., Guillot, S., Villa, I. M., Keller, F., Capiez, P., 2004. The South Ladakh Ophiolites (NW Himalaya, India): an intra-oceanic tholeiitic arc origin with implications for the closure of the Neo-Tethys. Chem. Geol., 203, 273–303.). As these gabbroic and volcanic rocks are interpreted to be arc related, the new Sm–Nd age data may indicate that intra-ocean subduction in the Neo-Tethyan ocean may have started much before  140 ± 32 Ma as this date is interpreted as the age of crystallization of the arc magma. Present and published age data on the arc magmatic rocks from the Indus suture zone may collectively indicate episodic magmatism with increasing maturity of the arc from more basic (during ~ 140 ± 32 Ma) when the arc was immature through intermediate (andesitic/granodioritic) at ~ 100 Ma to more felsic (rhyolitic/dioritic) magmatism at ~ 50–45 Ma, when the Indian and the Asian plates collided.     

Ti-in-zircon thermometry: applications and limitations

The titanium concentrations of 484 zircons with U-Pb ages of ∼1 Ma to 4.4 Ga were measured by ion microprobe. Samples come from 45 different igneous rocks (365 zircons), as well as zircon megacrysts (84) from kimberlite, Early Archean detrital zircons (32), and zircon reference materials (3). Samples were chosen to represent a large range of igneous rock compositions. Most of the zircons contain less than 20 ppm Ti. Apparent temperatures for zircon crystallization were calculated using the Ti-in-zircon thermometer (Watson et al. 2006, Contrib Mineral Petrol 151:413–433) without making corrections for reduced oxide activities (e.g., TiO₂ or SiO₂), or variable pressure. Average apparent Ti-in-zircon temperatures range from 500° to 850°C, and are lower than either zircon saturation temperatures (for granitic rocks) or predicted crystallization temperatures of evolved melts (∼15% melt residue for mafic rocks). Temperatures average: 653 ± 124°C (2 standard deviations, 60 zircons) for felsic to intermediate igneous rocks, 758 ± 111°C (261 zircons) for mafic rocks, and 758 ± 98°C (84 zircons) for mantle megacrysts from kimberlite. Individually, the effects of reduced or , variable pressure, deviations from Henry’s Law, and subsolidus Ti exchange are insufficient to explain the seemingly low temperatures for zircon crystallization in igneous rocks. MELTs calculations show that mafic magmas can evolve to hydrous melts with significantly lower crystallization temperature for the last 10–15% melt residue than that of the main rock. While some magmatic zircons surely form in such late hydrous melts, low apparent temperatures are found in zircons that are included within phenocrysts or glass showing that those zircons are not from evolved residue melts. Intracrystalline variability in Ti concentration, in excess of analytical precision, is observed for nearly all zircons that were analyzed more than once. However, there is no systematic change in Ti content from core to rim, or correlation with zoning, age, U content, Th/U ratio, or concordance in U-Pb age. Thus, it is likely that other variables, in addition to temperature and , are important in controlling the Ti content of zircon. The Ti contents of igneous zircons from different rock types worldwide overlap significantly. However, on a more restricted regional scale, apparent Ti-in-zircon temperatures correlate with whole-rock SiO₂ and HfO₂ for plutonic rocks of the Sierra Nevada batholith, averaging 750°C at 50 wt.% SiO₂ and 600°C at 75 wt.%. Among felsic plutons in the Sierra, peraluminous granites average 610 ± 88°C, while metaluminous rocks average 694 ± 94°C. Detrital zircons from the Jack Hills, Western Australia with ages from 4.4 to 4.0 Ga have apparent temperatures of 717 ± 108°C, which are intermediate between values for felsic rocks and those for mafic rocks. Although some mafic zircons have higher Ti content, values for Early Archean detrital zircons from a proposed granitic provenance are similar to zircons from many mafic rocks, including anorthosites from the Adirondack Mts (709 ± 76°C). Furthermore, the Jack Hills zircon apparent Ti-temperatures are significantly higher than measured values for peraluminous granites (610 ± 88°C). Thus the Ti concentration in detrital zircons and apparent Ti-in-zircon temperatures are not sufficient to independently identify parent melt composition. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Investigation of datolite (CaB[SiO4/(OH)]) from basalts in the Northern Apennines ophiolites (Italy): Genetic implications

Datolite, ideally CaB[(OH/SiO₄)], from hydrothermal veins crosscutting pillow basalt in 10 different localities of the Northern Apennine ophiolites was investigated with regard to mineral chemistry and fluid inclusion microthermometry. Bulk analyses of datolite crystals show REE contents below chondritic, except for La and Ce. With respect to host rock, datolite is occasionally enriched in La, Rb, Cs, Be, and shows relatively high contents of chalcophile elements (Cu, Zn, Pb, Ni) when occurring in contact with sulfide-mineralized basalt. Volatiles escaped during the decomposition in the temperature range 600 and 700 °C. The main component is water. The temperature maximum of water release is different and frequently with a shoulder or a second maximum. Together with water, sulfur species as H₂S and SO₂ and traces of boron species escaped. The CO₂ release by the decomposition especially of datolite from Castellaro and Cinghi has a maximum in the range of 500-580 °C and is different from the decomposition of calcite. Together with CO₂ a boron species escaped. Chlorine does not detect. Two-phase (L+V) fluid inclusions texturally identifiable as primary and secondary were observed, yielding average homogenization-temperatures of 236 and 173 °C, respectively. Fluid inclusion cooling data yield calculated salinity in the range of 10-16 wt% NaCl equivalents, thus relatively higher compared with seawater. The results are compatible with those reported for fluids formed under diagenetic conditions, but differ from those observed in seafloor hydrothermal systems and/or emanating from magmas. Distribution of trace elements between datolite and host basalt indicates enrichment with respect to the host rock limited to a few elements such as La, Rb, Cs, Be, Ni, Cu, Zn and Pb. The lithophile elements can be hosted in the datolite lattice, whereas the chalcophile metals and Ni are probably carried in sub-microscopic inclusions.     

The Othris Ophiolite, Greece: A snapshot of subduction initiation at a mid-ocean ridge

The mantle section of the Tethyan-type Othris Ophiolite, Greece, records tectono-magmatic processes characteristic of both mid-ocean ridges and supra-subduction zones. The Othris Ophiolite is a remnant of the Jurassic Neotethys Ocean, which existed between Eurasia and Gondwanaland. Othris peridotites range from fertile plagioclase lherzolites to depleted harzburgites. Abundances of Al₂O₃ and CaO show well-defined inverse linear correlations with MgO, suggesting that the Othris peridotites formed as residua from variable degrees of partial melting.

Peridotites from the Fournos Kaïtsa and western Katáchloron sub-massifs are similar to abyssal peridotites and can be explained by a multistage model with some melting in the garnet stability field followed by moderate degrees of anhydrous near-fractional melting in the spinel stability field. In contrast, the peridotites from the Metalleio, Eretria, and eastern Katáchloron sub-massifs, and the Vourinos ophiolite are highly depleted and have extremely low concentrations of Al₂O₃ and heavy rare earth elements. These peridotites have enriched light REE contents compared to the middle REE. These residua are best modelled by hydrous melting due to a flux of slab-derived fluid to the mantle wedge during melting.

The occurrence of both styles of melting regimes within close spatial and temporal association in the same ophiolite is explained by intra-oceanic thrusting and forced subduction initiation at (or near) a mid-ocean ridge. Thus, the Othris Ophiolite, and probably Tethyan-type ophiolites in general, represent a transient phase of plate tectonic reorganisation rather than quasi-steady state plate tectonics.     

北京西山晚侏罗世粗安岩的成因及其地质意义

北京西山晚侏罗世髫髻山组粗安岩具有高的SiO     

青岛榴辉岩相蛇绿混杂岩的岩石学证据及退变质P-T轨迹

青岛榴辉岩相蛇绿混杂岩发育在胶南隆起北缘地缝合线附近,构造侵位于胶南群云母斜长片麻岩中。它主要由斜方辉橄岩、玄武质和玄武安山质火山岩及少量杂砂岩和硅质岩组成。它与一般仰冲蛇绿岩的主要区别是:玄武质和玄武安山质岩石普遍遭受了榴辉岩相变质作用并发生强烈的糜棱岩化和退变质作用。强烈的退变质作用使大部分榴辉岩变为含石榴石石英斜长角闪岩,但仍保留有少量榴辉岩透镜体。它们可能是俯冲陆壳前缘岛弧地壳或岛弧型蛇绿岩残片的推覆体。它的退变质P-T轨迹表明,在其上升过程中热传导速率较高,这可能与该岩石强烈糜棱岩化和含水有关。     

贺根山蛇绿岩带航磁异常特征及成矿潜力分析

铬铁矿贺根山蛇绿岩带地处西伯利亚板块和华北板块的汇聚部位,该带出露中国北方最大的超基性岩体群,大小有超过超过30个超基性岩块,其中自西向东分布的朝根山,贺根山,崇根山,乌兹尼黑为最大的蛇绿岩块。与超基性岩关系最密切的矿产莫过于铬铁矿,数年来很多学者针对贺根山蛇绿岩的含矿性做了大量工作(白文吉等1993,1995; Miao Laicheng et al.,2008;段明,2009,2015;黄竺,2015;王成,2016),该带分布的铬铁矿床（点）众多,有代表性矿床(点)为赫根敖拉、733和3756,然而能达到工业开采的矿床只有3756。近些年在该地区的找矿勘查工作未取得突破,笔者认为很多原因在于该区覆盖面积太大,找矿靶区定位不准以及对隐伏的矿体勘查力度不够。2013-2015年天津地质调查中心在2009~2011年度新飞的航磁资料基础之上对该区进行了航磁异常查证与评价研究工作,对该区的成矿潜力与找矿方向提出了心得认识。