Geochemistry and geochronology of dolerite dykes from the Daba and Dongbo peridotite massifs, SW Tibet: Insights into the style of mantle melting at the onset of Neo-Tethyan subduction

This study reports compositional and whole-rock Sr-Nd isotope data as well as zircon U-Pb geochronological data on dolerite dikes from the Daba and Dongbo ultramafic massifs, southwest Yarlung-Zangbo Suture Zone (YZSZ), Tibet. The 120.6 \ub1 1.6 Ma dolerite dikes from the Daba peridotite exhibit normal mid-ocean ridge basalt (N-MORB)-type normalized mutli-element patterns [(La/Yb)N = 0.43-0.72] with noticeable negative Nb and Th anomalies. They have high initial 87Sr/8624 Sr ratios (87Sr/8625 Sr(i) = 0.70720-0.70788) and high \u3b5Nd(t) values (+7.4 to +7.9). The 125.4 \ub1 1.8 Ma dolerite intrusions within the Dongbo peridotite show N-MORB\u2012type trace element profiles [(La/Yb)N = 0.65-0.84] characterized by apparent negative anomalies in Nb and Th, and mild negative anomalies in Ti (\ub1 Y). They also have high 87Sr/8629 Sr(i) ratios (0.70611-0.70679) and elevated \u3b5Nd(t) values (+7.8 to +8.2). Semi-quantitative La/Yb vs. Dy/Yb modeling demonstrates that the parental magmas of the investigated dolerite dikes derived from more than 20% (cumulative) melting of a (broad) mantle source region that had a spinel-bearing N-MORB\u2013like lherzolitic composition. Our geochemical and isotopic data indicate that the composition of the inferred mantle source was influenced by minor input of subducted crustal material. The petrogenesis of the Daba and Dongbo massifs could be linked to upwelling of an asthenospheric source that caused continental rift and subsequent seafloor spreading, followed by subduction initiation adjacent to a passive margin during the early Cretaceous (~130-120 Ma). Our study provides a more detailed, and perhaps more elegant, hypothesis for the tectono-magmatic evolution of the southwestern YZSZ "ophiolitic" peridotites after their accretion beneath a Neo-Tethyan marginal basin

Coexistence of MORB- and OIB-like dolerite intrusions in the Purang ultramafic massif, SW Tibet: a paradigm of plume-influenced MOR-type magmatism prior to subduction initiation in the Neo-Tethyan lithospheric mantle

The Yarlung Zangbo Suture Zone (YZSZ) of South Tibet is divided by the Zhongba-Zhada terrane into two subparallel ophiolitic belts in its western end. The peridotite massifs of the southern belt tectonically overlie the Tethyan Himalaya sequence. The Purang peridotite body in this belt is intruded by two groups of dolerite dikes, providing significant compositional, geochronological, and isotopic information about the melting history of the Neo-Tethyan mantle. U-Pb ages of zircons separated from dolerites show that peridotites of West Purang were intruded by an early generation of dikes at 138.5 \ub1 2.0 Ma (Valanginian). These dolerites show ocean island basalt (OIB)-type normalized multi-elemental profiles and Sr-Nd isotopic signatures [(La/Yb)N = 13\u201316], high initial 87Sr/86Sr ratios (0.70598\u20130.70765), and low \u3b5Nd(t) values (\u20132.6 to \u20132.3). Zircons separated from this group of dolerites have slightly radiogenic \u3b5Hf(t) values (+2.6 to +4.6). The next generation of dolerite dikes intruded the East Purang peridotites between 124.5 \ub1 2.5 Ma and 124.4 \ub1 3.2 Ma (Aptian). These East Purang dolerites show normal mid-ocean ridge basalt (N-MORB)-type normalized multi-element patterns [(La/Yb)N = 0.6\u20130.9] with noticeable negative Nb and Th (\ub1Ti) anomalies, and have high 87Sr/86Sr(i) (0.70295\u20130.70618) and high \u3b5Nd(t) values (+7.7 to +9.2). Zircons separated from the East Purang dolerites show strongly radiogenic \u3b5Hf(t) values (+3.5 to +17.0). Semiquantitative geochemical modeling demonstrates that the parental magmas of West Purang dolerites were generated from 5%\u201310% polybaric partial melting of a deep-seated juvenile asthenospheric source enriched by plume-type components. In contrast, the parental melts of East Purang dolerites were derived from more than 20% melting of a juvenile spinel-bearing MORBtype mantle source that was modified by subduction-related melts/fluids to a minor extent. A possible tectono-magmatic model for the petrogenesis of the Purang ophiolitic massif could be linked to incipient continental rifting and subsequent oceanic seafloor spreading associated with decompression upwelling of an asthenospheric source contaminated by plume-type components. This plume-proximal seafloor spreading-system was succeeded by the initiation of Neo-Tethyan intra-oceanic subduction close to the active continental margin of Eurasia during the Early Cretaceous

Development Characteristics and Formation Mechanism of Nanoparticles in the Ductile Shear Zone of the Red River Fault

Nanoparticles in the ductile shear zones of faults are thought to be closely related to fault activity and the seismogenic mechanism. Using scanning electron microscopy (SEM), nanoparticles with a variety of morphological characteristics were found in mylonite, gneiss, and schist from the ductile shear zones of the Red River Fault. The nanoparticle morphology is dominated by rods, spherulites, massive forms, lamellae and film-like shapes. Energy spectrum analysis showed that the nanoparticles were mainly composed of silicate minerals, while a few contained carbonate minerals. Nanoparticles follow certain spatial distribution rules in ductile shear zones. Near the main fault plane of the Red River Fault Zone, the nanoparticles are dispersed, brittle and spherulitic, with a small particle diameter. Those far from the main fault plane are agglomerated and plastic, with rod-like, massive, lamellar and film-like shapes, and a relatively large particle diameter. According to nanoparticle development characteristics and the theory of microscopic deformation, a study on the formation mechanism of nanoparticles in the ductile shear zones of the Red River Fault was carried out. The rock minerals are the first to experience granulation, through intergranular movement under strong strain, generating brittle spherulitic particles, which eventually loosen and disperse. In the later stages, through recrystallization, nanoparticle crystals may become large or develop various morphologies

Forearc tectonic evolution in the middle of the Bangong–Nujiang Tethys Ocean: New geochemical evidence of the Lanong ophiolites from the Zangbei lakes region

The middle of the Bangong–Nujiang Suture (BNS) in the central Tibetan Plateau hosts a series of dismembered ophiolitic fragments that document the evolution of part of the Tethys Ocean. However, the origin of these ophiolitic fragments in the Zangbei lakes region remains debated. Using new and existing field observations and petrographic, geochronologic, isotopic, and whole-rock chemical data from ophiolitic rocks in the Zangbei lakes region, we evaluate their origins and constrain the tectonic evolution of the Bangong–Nujiang Tethys Ocean (BNTO). The Lanong peridotites have low rare-earth element (REE) concentrations and typically exhibit U-shaped REE patterns that are similar to those of forearc peridotites from South Sandwich and Xigaze. Lanong basalts and others mafic rocks from the Zangbei lakes region show enrichment in large-ion lithophile elements and depletion in high-field-strength elements, and they have clear forearc and boninitic affinities in various tectonic discrimination diagrams. In addition, the Lanong basalts have initial Nd/Nd ratios of 0.512307 to 0.512773, and ε(t) values of −2.7 to +6.3. Considering the regional geology, previous geochronologic data from the ophiolitic fragments (147.6\ua0±\ua02.3\ua0Ma to 189.8\ua0±\ua03.3\ua0Ma) and the Darutso high-Mg andesites (161.5\ua0±\ua00.9\ua0Ma to 164.2\ua0±\ua01.4\ua0Ma), and the lack of Jurassic arc-related rocks in the northern Lhasa terrane, we conclude that the Jurassic ophiolitic fragments of the Zangbei lakes region were derived from a depleted mantle source and formed in a forearc basin in response to north-directed subduction of the BNTO

Mineralogy and geochemistry of a base metal sulfide-bearing magnetitite body from the Eretria mine, East Othris massif, Greece: insights into an ancient seafloor hydrothermal system

Magnetite deposits comprise a very small volume of a serpentinized peridotite body that constitutes the Eretria chrome mine of the East Othris ophiolite in central Greece. Magnetite deposits have been examined to aid determination of their mode of occurrence, mineralogy and origin. Research attention has been focused on an ore body that consists of a ~30 cm-thick pod of semi-massive to massive magnetite (~75%\u201385% modal) hosted in a serpentinite shear zone. The silicate matrix of magnetitite (~10%\u201315% modal) is composed of unstrained serpentine, chlorite and subordinate amounts of andradite and actinolite. Base metal sulfides (BMS; ~5%\u201310% modal) and phosphates are dispersed between the magnetite grains. Variably altered Cr-spinel crystals are poikilitically enclosed within magnetite. The investigated ore samples show great variations in the Cu (890\u20139530 ppm), Ni (190\u20133260 ppm), Co (120\u20132180 ppm) and Zn (20\u20131240 ppm) contents. The \u3b418\u39fSMOW value for magnetite from a magnetitite sample is 2.2\u2030. The undeformed nature of the silicate minerals in magnetitite points toward crystal growth from a postmagmatic fluid. The low Ni, Mn, Mg and high Si contents in magnetite suggest that it was not derived from fluids produced concurrent to serpentinization. The presence of phosphates, BMS and hydrosilicates suggest that the ore forming fluids were acidic and halogen-bearing. Our preferred hypothesis for the formation of magnetitite is that mixing of upwelling, metal-rich brines along the dilational segments of the host shear zone caused saturation of the ensuing fluids with Fe, which eventually prompted magnetite precipitation. The T estimates deduced from mineralogy, with an uppermost limit of ~460 \u25e6C required, and cessation of mineralization at ~150\u2013100 \u25e6C, are consistent with the ambient T present within a cooling oceanic lithospheric slab. In analogy with some modern serpentinite-hosted BMS deposits at rifted settings, the BMS-bearing magnetitite of Eretria may represent part of an ancient seafloor hydrothermal system. Our study provides important insights into the initiation and episodicity of hydrothermal activity in oceanic settings

Geochemistry and geochronology of ophiolitic rocks from the Dongco and Lanong areas, tibet: insights into the evolution history of the Bangong-Nujiang Tethys ocean

The Bangong-Nujiang Suture Zone (BNSZ) in central Tibet hosts a series of dismembered Jurassic ophiolites that are widely considered as remnants of the vanished Meso-Tethys Ocean. In this study we present new compositional, isotopic, and geochronological data from anorthosites and gabbros of the Dongco and Lanong ophiolites in order to test several hypotheses about the nature of subduction in the Bangong-Nujiang Tethys Ocean (BNTO) during the Mesozoic era. Uranium–Pb dating of magmatic zircons separated from the Dongco anorthosites yielded an (average) age of 169.0 ± 3.7 Ma. Zircons separated from the Lanong anorthosites and gabbros yielded U–Pb ages of 166.8 ± 0.9 Ma and 167.3 ± 1.1 Ma, respectively. Zircons separated from the Dongco and Lanong anorthosites have positive ε (t) values (5.62–15.94 and 10.37–14.95, respectively). The Dongco anorthosites have moderate initial Sr/ Sr (0.703477–0.704144) and high ε (t) (+6.50 to +7.91). The Lanong anorthosites have high ( Sr/ Sr) (0.706058–0.712952) and ε (t) in the range of −1.56 to +2.02. Furthermore, the Lanong gabbros have high ( Sr/ Sr) (0.705826–0.706613) and ε (t) in the range of −0.79 to +4.20. Most gabbros from Dongco and a few gabbros from Lanong show normal mid-ocean ridge basalt (N-MORB)-like primitive mantle (PM)-normalized multi-element patterns. In contrast, most gabbros from Lanong show U-shaped chondrite-normalized rare earth element (REE) profiles. The investigated gabbros are characterized by wide ranges of δEu {(Eu) /[(Sm) *(Gd) ]} values (0.83–2.53), indicating that some of them are cumulative rocks. The trace element contents of all anorthosite samples imply that their composition was controlled by cumulative processes. The geochemical and isotopic compositions of the non-cumulative gabbros from Dongco (δEu: 0.95–1.04) and Lanong (δEu: 0.83–1.03) indicate that their parental melts were derived from melting of heterogeneously depleted, juvenile mantle reservoirs. These rocks have arc-related affinities, indicating that their mantle sources were influenced by minor inputs of subducted lithospheric components. Our preferred hypothesis for the origin of the non-cumulative gabbros from Dongco is that they were formed in a transient back-arc basin (BAB) setting in the middle-western segment of the BNTO, whereas our preferred scenario about the origin of the non-cumulative gabbros from Lanong is that they were generated in a forearc setting in the middle part of the BNTO. We conclude that both geotectonic settings were developed in response to the northward subduction of the BNTO during the Middle Jurassic

Refertilization of serpentinites in the Sulu UHP terrane, Eastern China: constraints from geochronology, mineral composition, geochemistry, and Re–Os isotopes

<p>Serpentinites from Junan (JN), Rizhao (RZ), and Rongcheng (RC) in the Sulu ultra-high-pressure (UHP) terrane, China, were analysed for U–Pb zircon geochronology, mineral chemistry, whole-rock major and trace element chemistry (including rare-earth elements (REEs) and platinum-group elements (PGEs)), and Re–Os isotopes, in order to better constrain their petrogenesis and geodynamic process. The serpentinite zircons yield two age groups: 731 ± 10 to 780 ± 10 Ma for relic magmatic zircon cores, which may indicate early crystallization and emplacement of the peridotite in the Yangtze crust, and 209 ± 2 to 218 ± 3 Ma for metamorphic zircon, which coincides with Triassic UHP metamorphism. The spinels in the serpentinites exhibit significant Cr# variation (0.6–0.91) and have undergone multi-stage metamorphism. The serpentinites are characterized by enrichment in incompatible trace elements, low Ni and IPGE concentrations, and high Pd/Ir ratios, and the bulk-rock major elements plot in the ultramafic cumulate region. Their Re and Os concentrations are similar to those of typical orogenic peridotite, but they have high ¹⁸⁷Os/¹⁸⁸Os ratios (0.12433–0.14423). We believe that the serpentinite’s protolith consisted of cumulates from an asthenosphere-derived melt that intruded into the continental crust of the Yangtze craton in the Neoproterozoic. These cumulates were later subducted and metamorphosed during the subduction of the Yangtze craton in the Triassic. The serpentinites underwent melt–rock interactions and fluid enrichment, both prior to and during serpentinization.</p