Re-Os geochronology,O isotopes and mineral geochemistry of the Neoproterozoic Songshugou ultramafic massif in the Qinling Orogenic Belt,China

The Qinling Orogenic Belt was formed by subduction and collision between the North and South China Blocks along the Shangdan suture. The Songshugou ultramafic massif located on the northern side of the Shangdan suture provides essential insights into the mantle origin and evolutionary processes during spreading and subduction of the Shangdan oceanic lithosphere. The ultramafic massif comprises harzburgite, coarse- and fine-grained dunites. The spinels from harzburgite exhibit low Cr# and high Mg# numbers, suggesting a mid-ocean ridge peridotite origin, whereas spinels from both coarse- and fine-grained dunites are indicated as resulted from melt-rock reaction due to their systematic higher Cr# and low Mg# numbers. This melt-rock reaction in the dunites is also indicated by the low TiO₂ (mostly <0.4 wt%) in the spinel and high Fo (90–92) in olivines. Due to its relatively homogeneous nature in the mantle, oxygen isotopic composition is a sensitive indicator for the petrogenesis and tectonic setting of the Songshugou ultramafic rocks. Based on in-situ oxygen isotope analyses of olivines from twenty-six rock samples, most harzburgites from the Songshugou ultramafic massif show low δ¹⁸O values of 4.54–5.30‰, suggesting the olivines are equilibrium with N-MORB magmas and originally formed in a mid-ocean ridge setting. The coarse- and fine-grained dunites exhibit slightly higher olivine δ¹⁸O values of 4.69–6.00‰ and 5.00–6.11‰, respectively, suggesting they may have been modified by subduction-related boninitic melt-rock reaction. The δ¹⁸O values of olivines systematically increasing from the harzburgites, to coarse-grained dunites and fine-grained dunites may suggest enhancing of melt-rock reaction. The decreasing of Os concentration, ¹⁸⁷Re/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratios from harzburgite to dunite suggest an ¹⁸⁷Os-enriched, subduction zone melt was responsible for creating the melt channel for melt-rock reactions. Together with the high-temperature ductile deformation microstructures, these isotopic and mineral geochemical features suggest that the harzburgites represent mantle residues after partial melting at mid-ocean ridge or supra-subduction zone, while the dunites were probably resulted from reactions between boninitic melt and harzburgites in a supra-subduction zone. Re-Os geochronology yields a maximum Re depletion model age (T_RD) of 805 Ma, constraining the minimum formation age of the harzburgites derived from oceanic mantle. Eight samples of whole rock and chromite yield a Re-Os isochron age of 500 ± 120 Ma, constraining the timing of melt-rock reactions. Combined with the regional geology and our previous investigations, the Songshugou ultramafic rocks favors a mantle origin at mid-ocean ridge before 805 Ma, and were modified by boninitic melt percolations in a SSZ setting at ca. 500 Ma. This long-term tectonic process from spreading to subduction might imply a huge Pan-Tethyan ocean between the Laurasia (e.g., North China Block) and Gondwana (e.g., South China Block) and/or a one-side subduction.     

Coexistence of abyssal and ultra-depleted SSZ type mantle peridotites in a Neo-Tethyan Ophiolite in SW Turkey: Constraints from mineral composition,whole-rock geochemistry (major–trace–REE–PGE), and Re–Os isotope systematics

We present new, whole-rock major and trace element chemistry, including rare earth elements (REE), platinum-group elements (PGE), and Re–Os isotope data from the upper mantle peridotites of a Cretaceous Neo-Tethyan ophiolite in the Mu?la area in SW Turkey. We also report extensive mineral chemistry data for these peridotites in order to better constrain their petrogenesis and tectonic environment of formation. The Mu?la peridotites consist mainly of cpx-harzburgite, depleted harzburgite, and dunite. Cpx-harzburgites are characterized by their higher average CaO (2.27 wt.%), Al₂O₃ (2.07 wt.%), REE (53 ppb), and ¹⁸⁷Os/¹⁸⁸Os_(i) ratios varying between 0.12497 and 0.12858. They contain Al-rich pyroxene with lower Cr content of coexisting spinel (Cr# = 13–22). In contrast, the depleted harzburgites and dunites are characterized by their lower average CaO (0.58 wt.%), Al₂O₃ (0.42 wt.%), and REE (1.24 ppb) values. Their clinopyroxenes are Al-poor and coexist with high-Cr spinel (Cr# = 33–83). The ¹⁸⁷Os/¹⁸⁸Os_(i) ratios are in the range of 0.12078–0.12588 and are more unradiogenic compared to those of the cpx-harzburgites.Mineral chemistry and whole rock trace and PGE data indicate that formation of the Mu?la peridotites cannot be explained by a single stage melting event; at least two-stages of melting and refertilization processes are needed to explain their geochemical characteristics. Trace element compositions of the cpx-harzburgites can be modeled by up to ~ 10–16% closed-system dynamic melting of a primitive mantle source, whereas those of the depleted harzburgites and dunites can be reproduced by ~ 10–16% open-system melting of an already depleted (~ 16%) mantle. These models indicate that the cpx-harzburgites are the products of first-stage melting and low-degrees of melt–rock interaction that occurred in a mid-ocean ridge (MOR) environment. However, the depleted harzburgites and dunites are the product of second-stage melting and related refertilization which took place in a supra subduction zone (SSZ) environment. The Re–Os isotope systematics of the Mu?la peridotites gives model age clusters of ~ 250 Ma, ~ 400 Ma and ~ 750 Ma that may record major tectonic events associated with the geodynamic evolution of the Neo-Tethyan, Rheic, and Proto-Tethyan oceans, respectively. Furthermore, > 1000 Ma model ages can be interpreted as a result of an ancient melting event before the Proto-Tethys evolution.     

Heterogeneously depleted Precambrian lithosphere deduced from mantle peridotites and associated chromitite deposits of Al'Ays ophiolite,Northwestern Arabian Shield,Saudi Arabia

The mantle section of Al'Ays ophiolite consists of heterogeneously depleted harzburgites, dunites and large-sized chromitite pods. Two chromitite-bearing sites (Site1 and Site2), about 10 km apart horizontally from one another, were examined for their upper mantle rocks. Cr-spinels from the two sites have different chemistry; Cr-rich in Site1 and Al-rich in Site2. The average Cr-ratio = (Cr/(Cr + Al) atomic ratio) of Cr-spinels in harzburgites, dunites and chromitites is remarkably high 0.78, 0.77 and 0.87, respectively, in Site1, compared with those of Site2 which have intermediate ratio averages 0.5, 0.56 and 0.6, respectively. The platinum-group elements (PGE) in chromitites also show contrasting patterns from Site1 to Site2; having elevated IPGE (Os, Ir, Ru) and strongly depleted in PPGE (Rh, Pt, Pd) with steep negative slopes in the former, and gentle negative slopes in the latter. The oxygen fugacity (Δlog fO₂) values deduced from harzburgites and dunites of Site1 show a wide variation under reducing conditions, mostly below the FMQ buffer. The Site2 harzburgites and dunites, on the other hand are mostly above the FMQ buffer. Two magmatic stages are suggested for the lithospheric evolution of Al'Ays ophiolite in response to a switch of tectonic setting. The first stage produced a peridotites–chromitites suite with Al-rich Cr-spinels, possibly beneath a mid-ocean ridge setting, or most likely in back-arc rift of a supra-subduction zone setting. The second stage involved higher degrees of partial melting, produced a peridotites–chromitites suite with Cr-rich Cr-spinels, possibly in a fore-arc setting. The coexistence of compositionally different mantle suites with different melting histories in a restricted area of an ophiolite complex may be attributable to a mechanically juxtaposed by mantle convection during recycling. The mantle harzburgites and dunites are apt to be compositionally modified during recycling process; being highly depleted (Site1 case) than their original composition (Site2 case).     

造山带橄榄岩岩石学与构造过程:以松树沟橄榄岩为例

造山带橄榄岩不仅是地幔地球化学,而且是造山带形成与演化过程研究的主要对象.造山带橄榄岩主要有3种类型:（1）阿尔卑斯型橄榄岩,即岩石圈地幔构造-热侵位就位于造山带浅部地壳的橄榄岩;（2）前期层状基性-超基性堆晶岩经俯冲变质形成的橄榄岩;（3）蛇绿岩型橄榄岩.松树沟糜棱岩化橄榄岩及其相关高级变质岩详细的岩石学和地球化学研究发现这些橄榄岩记录了洋岩石圈形成到角闪岩相变质的全过程.即1 000~800 Ma洋岩石圈形成阶段,主要形成纯橄岩; < 800~500 Ma洋-陆转换即陆岩石圈演化阶段,岩石圈被交代形成大量方辉橄榄岩;500~480 Ma快速深俯冲和榴辉岩相变质阶段;460~335 Ma角闪岩相退变质阶段,此阶段在松树沟橄榄岩中形成大量富镁的直闪石类矿物,包括透闪石、阳起石和镁闪石.由此可见,蛇绿岩型造山带橄榄岩能够记录造山带形成与演化的全过程,通常会经历4个形成和演化阶段:（1）洋岩石圈（蛇绿岩）形成阶段,形成纯榄岩;（2）洋-陆转换阶段,陆岩石圈演化阶段,岩石圈受交代形成方辉橄榄岩;（3）岩石圈深俯冲,榴辉岩相变质;（4）俯冲板片抬升至角闪岩相时退变质,此时在橄榄岩中形成富镁的直闪石类矿物.不同造山带中蛇绿岩型橄榄岩的区别可能只是俯冲深度和退变质程度不同而已.最后,蛇绿岩一定要强调是什么时代的蛇绿岩.同时,造山带进变质作用产物经常会被后期抬升过程中退变质作用彻底改造,这应该引起重视.      

Using platinum-group elements and Au geochemistry to constrain the genesis of podiform chromitites and associated peridotites from the Soghan mafic–ultramafic complex,Kerman, Southeastern Iran

The podiform chromite deposit of the Soghan mafic–ultramafic complex is one of the largest chromite deposits in south-east Iran (Esfandagheh area). The Soghan complex is composed mainly of dunite, harzburgite, lherzolite, pyroxenite, chromitite, wehrlite and gabbro. Olivine, orthopyroxene, and to a lesser extent clinopyroxene with highly refractory nature, are the primary silicates found in the harzburgites and dunites. The forsterite content of olivine is slightly higher in dunites (Fo₉₄) than those in harzburgites (Fo₉₂) and lherzolites (Fo₈₉). Chromian spinel mainly occurs as massive chromitite pods and as thin massive chromitite bands together with minor disseminations in dunites and harzburgites. Chromian spinels in massive chromitites show very high Cr-numbers (80–83.6), Mg-numbers (62–69) and very low TiO₂ content (averaging 0.17 wt.%) for which may reflect the crystallization of chromite from a boninitic magma. The Fe^3 +-number is very low, down to < 0.04 wt.%, in the chromian spinel of chromitites and associated peridotites of the Soghan complex.PGE contents are variable and range from 80 to 153 pbb. Chromitites have strongly fractionated chondrite-normalized PGE patterns, which are characterized by enrichments in Os, Ir and Rh relative to Pt and Pd. Moreover, the Pd/Ir value which is an indicator of PGE fractionation ranges from < 0.08 to 0.24 in chromitite of the Soghan complex. These patterns and the low PGE abundances are typical of ophiolitic chromitites and indicating a high degree of partial melting (about 20–24%) of the mantle source. Moreover, the Pd_N/Ir_N ratios in dunites are unfractionated, averaging 1.2, whereas the harzburgites and lherzolites show slightly positive slopes PGE spidergrams, together with a small positive Ru and Pd anomaly, and their Pd_N/Ir_N ratio averages 1.98 and 2.15 respectively.The mineral chemistry data and PGE geochemistry, along with the calculated parental melts in equilibrium with chromian spinel of the Soghan chromitites indicate that the Soghan complex was generated from an arc-related magma with boninitic affinity above a supra-subduction zone setting.     

Metamorphic P–T–t–d evolution of (U)HP metabasites from the South Tianshan accretionary complex (NW China) — Implications for rock deformation during exhumation in a subduction channel

The late Carboniferous accretionary system of the South Tianshan orogen (North-Western China) underwent complex structural and polymetamorphic evolution. Combined petrological, geochronological and microstructural analysis of (ultra)high-pressure (UHP) metabasites (eclogites and blueschists) enclosed in metapelites show a relict coarse-grained eclogitic fabric S2 surrounded by a dominant fine-grained eclogite and blueschist facies retrograde fabric S2. The S2 fabric is reworked by upright folds F3 that are responsible for a major shortening of the whole accretionary system. For both the eclogite and blueschist, peak and retrograde P–T conditions have been thermodynamically constrained at 25–26 kbar and 425–500 °C and 10–13 kbar and 500−550 °C respectively, suggesting a shared exhumation history. The garnet-whole rock-amphibole isochron in the blueschist yielded Lu–Hf and Sm–Nd ages of 326.0 ± 2.9 Ma and 318.4 ± 3.9 Ma respectively, interpreted to date the prograde to peak metamorphic assemblage. The retrograde path of the eclogite is characterized by heterogeneous omphacite recrystallization into a mylonitic fine-grained matrix and crystallization of blue amphibole. Microstructures in both pristine porphyroclastic and recrystallized fine-grained domains in the eclogite indicate a gradual evolution from constriction-dominated (L>S-type) to flattening-dominated (S>L-type) type of deformation, increase of fabric intensity reflected by gradually growing M-indexes and the development of lattice preferred orientation (LPO) typical for dislocation creep under slightly hydrated conditions. Recrystallization of the matrix in the blueschist is homogeneous, which indicates a matrix dominated channel flow during exhumation. These LPOs evolutions suggest a significant mechanical coupling with the upper plate concomitant with oroclinal bending of the Kazakh orocline. Lock up of Kazakh orocline is responsible for further stress increase resulting in horizontal shortening of South Tianshan accretionary wedge and development of D3 upright folding and steepening of the whole sequence.     

K–Ar ages of plutonism and mineralization at the Shizhuyuan W–Sn–Bi–Mo deposit,Hunan Province,China

The Qianlishan granite complex, situated 16 km southeast of Chenzhou City, Hunan Province, China, hosts the Shizhuyuan W–Sn–Bi–Mo deposit. This complex, which intruded the Protozoic metasedimentary rocks and the Devonian clastic sedimentary and carbonate rocks, consists of mainly medium- to coarse-grained biotite granites and minor amounts of fine-grained biotite granite in addition to granite and quartz porphyry. K–Ar ages suggest three episodes of plutonism: the medium- to coarse-grained biotite granite (before 152 Ma), the fine-grained biotite granite (137 Ma), and the granite porphyry (129–131 Ma). Muscovite ages of the greisen are 145–148 Ma, suggesting that the W–Sn–Bi–Mo mineralization was related to the main, medium- to coarse-grained biotite granites. The K–Ar age of the hydrothermal vein mineralization is 92 Ma and is probably related to the porphyries.     

Nature and processes of the lithospheric mantle beneath the western Qinling: Evidence from deformed peridotitic xenoliths in Cenozoic kamafugite from Haoti,Gansu Province,China

The Cenozoic Haoti kamafugite field (23 Ma) is situated at the western Qinling Orogen, Gansu Province in China, which is a conjunction region of the North China Craton, the Yangtze Craton and the Tibetan Plateau. Fresh peridotitic xenoliths entrained in these volcanic rocks provide an opportunity to study the nature and processes of the lithospheric mantle beneath the western Qinling. These xenoliths can be divided into two groups based on the petrological features and mineral compositions, type 1 and type 2. Type 1 xenoliths with strongly deformed texture have higher Fo (90–92.5) contents in olivines, Mg# (91–94) and Cr# (15–35) of clinopyroxenes, and Cr# (36–67) of spinels than the weakly deformed type 2 xenoliths, which have the corresponding values of 89–90, 89–91.5, 10–15 and 5–15 in minerals, respectively. CaO contents in fine-grained olivines are slightly higher than 0.10 wt% compared with coarse-grained ones (less than 0.10 wt%). Fine-grained clinopyroxenes have low Al₂O₃ + CaO contents (generally <23 wt%) relative to coarse-grained ones (>23 wt%). Fo contents in fine-grained olivines mainly in the melt pocket of the type 1 xenoliths are higher than those in coarse-grained ones, which is somewhat contrary to the type 2 xenoliths without melt pocket. Clinopyroxenes of the type 2 display higher Na₂O contents (1.7–1.9 wt%) than those of the type 1 (<1.4 wt%). P–T estimations reveal that the type 1 xenoliths give temperature in range of 1106–1187 °C and pressure of 21–26 kbar and that relatively low temperature (907 and 1022 °C) and pressure (19.0 and 18.5 kbar) for the type 2 xenoliths. The type 1 xenoliths are characterized by depletion due to high degree of partial melting (>10%), modal metasomatic and deformed characteristics, and may represent the old refractory lithospheric mantle. In contrast, the type 2 peridotites show fertile features with low degree of partial melting (<5%) and may represent the newly-accreted lithospheric mantle. The lithospheric mantle beneath the western Qinling underwent partial melting, recrystallization, deformation and metasomatism due to asthenospheric upwelling and the latest decompression responding to the Cenozoic extensive tectonic environment. These processes perhaps are closely related to the evolution of Tibetan Plateau caused by the India-Asian collision.     

Textures and geochemistry of the Saramta peridotites (Siberian craton): Melting and refertilization during early evolution of the continental lithospheric mantle

The Saramta peridotite massif is located within the Sharyzhalgai complex, SW margin of the Siberian craton. The Saramta massif was formed in the Archean and then juxtaposed with granulites of crystalline basement of the Siberian craton. The Saramta harzburgites are highly refractory in terms of lack of residual clinopyroxene, olivine Mg-number (up to 0.937), and spinel Cr-number (∼0.5), suggesting high degree of partial melting. Detailed study of their microstructures shows that they have extensively reacted with a SiO₂-rich melt, leading to the crystallization of orthopyroxene, clinopyroxene, amphibole and spinel at the expense of olivine. The major element compositions of the least reacted harzburgites are similar to the residues of refractory peridotites produced by the fractional melting (initial melting pressures >3 GPa and melt fractions ∼40%). Moreover, non-residual clinopyroxenes are highly depleted in Yb, Zr and Ti, but highly enriched in LREE. A two-stage history is proposed for the Saramta peridotite: (1) primitive mantle underwent depletion in the garnet stability field followed by melting in the spinel stability field; (2) refractory harzburgites underwent refertilization by SiO₂-rich melt in supra-subduction zone. Rare Saramta lherzolites probably formed from more refractory harzburgites as a result of such a melt–rock reaction. The Saramta peridotites are similar to low-T coarse-grained peridotites of subcratonic mantle. Processes of their formation, as reflected by textures and composition of minerals of the Saramta peridotites, are characteristic of the early stages of subcratonic mantle formation.     

Early Paleozoic tectonic evolution of the North Qinling orogenic belt: Evidence from geochemistry,phase equilibrium modeling and geochronology of metamorphosed mafic rocks from the Songshugou ophiolite

The Shangdan suture zone (SSZ) is the main collisional boundary between the North China Craton and the South China Craton, along which discontinuous Paleozoic ophiolites and subduction–accretion related volcanic arc assemblages occur. Here we report the petrology, geochemistry, geochronology and phase equilibria modeling of garnet amphibolite from the Songshugou ophiolite which is one of the largest ophiolite outcrops in the northern side of the SSZ. From petrological studies, we identify: (1) prograde stage, defined by garnet + clinopyroxene + calcic amphibole + ilmenite + rutile + epidote + plagioclase + quartz; (2) peak stage with garnet + clinopyroxene + ilmenite + rutile + quartz; and (3) retrograde stage with amphibole + plagioclase + titanite + ilmenite. Our pseudosection analysis defines stability of the peak assemblage at 750–850 °C, 15–19 kbar and traces a clockwise P–T path in the system Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (NCFMASHTO), suggesting high pressure (HP) metamorphism. Subsequently, the rocks experienced rapid decompression and cooling. LA-ICP-MS U-Pb analyses of zircons from the garnet amphibolite yield a weighted mean ²⁰⁶Pb/²³⁸U age of 515 ± 12 Ma. This Early Paleozoic metamorphic age represents the emplacement time of the Songshugou ophiolite, and suggests that the HP metamorphism is possibly related to the northward deep subduction of the Shangdan oceanic crust in Early Paleozoic.     

Peridotite-chromitite complexes in the Eastern Desert of Egypt: Insight into Neoproterozoic sub-arc mantle processes

The Neoproterozoic peridotite-chromitite complexes in the Central Eastern Desert of Egypt, being a part of the Arabian-Nubian Shield, are outcropped along the E–W trend from Wadi Sayfayn, Wadi Bardah, and Jabal Al-Faliq to Wadi Al-Barramiyah, from east to west. Their peridotites are completely serpentinized, and the abundance of bastite after orthopyroxene suggests harzburgite protoliths with subordinate dunites, confirmed by low contents of Al₂O₃, CaO and clinopyroxene (< 3 vol%) in bulk peridotites. The primary olivine is Fo89.3–Fo92.6, and the residual clinopyroxene (Cpx) in serpentinites contains, on average, 1.1 wt% Al₂O₃, 0.7 wt% Cr₂O₃, and 0.2 wt% Na₂O, similar in chemistry to that in Izu-Bonin-Marian forearc peridotites. The wide range of spinel Cr-number [Cr/(Cr + Al)], 0.41–0.80, with low TiO₂ (0.03 wt%), MnO (0. 3 wt%) and Y_Fe [(Fe^3 +/(Cr + Al + Fe^3 +) = 0.03 on average)] for the investigated harzburgites-dunites is similar to spinel compositions for arc-related peridotites. The partial melting degrees of Bardah and Sayfayn harzburgites range mainly from 20 to 25% and 25 to 30% melting, respectively; this is confirmed by whole-rock chemistry and Cpx HREE modelling (~ 20% melting). The Barramiyah peridotite protoliths are refractory residues after a wide range of partial melting, 25–40%, where more hydrous fluids are available from the subducting slab. The Neoproterozoic mantle heterogeneity is possibly ascribed mainly to the wide variations of partial melting degrees in small-scale areas, slab-derived inputs and primordial mantle compositions. The Sayfayn chromitites were possibly crystallized from island-arc basaltic melts, followed by crystallization of Barramiyah chromitites from boninitic melt in the late stage of subduction. The residual Cpx with a spoon-shape REE pattern is rich in both LREE and fluid-mobile elements (e.g., Pb, B, Li, Ba, Sr), but poor in HFSE (e.g., Ta, Nb, Zr, Th), similar to Cpx in supra-subduction zone (SSZ) settings, where slab-fluid metasomatism is a prevalent agent. The studied chromitites and their host peridotites represent a fragment of sub-arc mantle, and originated in an arc-related setting. The systematic increase in the volume of chromitite pods with the increasing of their host-peridotite thickness from Northern to Southern Eastern Desert suggests that the thickness of wall rocks is one factor controlling the chromitite size. The factors controlling the size of Neoproterozoic chromitite pods are the thickness, beside the composition, of the host refractory peridotites, compositions and volumes of the supplied magmas, the amount of slab-derived fluids, and possibly the partial melting degree of the host peridotites.     

Structural events and metamorphic consequences in Almora Nappe,during Himalayan collision tectonics

Almora Nappe in Uttarakhand, India, is a Lesser Himalayan representative of the Himalayan Metamorphic Belt that was tectonically transported over the Main Central Thrust (MCT) from Higher Himalaya. The Basal Shear zone of Almora Nappe shows complicated structural pattern of polyphase deformation and metamorphism. The rocks exposed along the northern and southern margins of this nappe are highly mylonitized while the degree of mylonitization decreases towards the central part where the rocks eventually grade into unmylonitized metamorphics.Mylonitized rocks near the roof of the Basal Shear zone show dynamic metamorphism (M₂) reaching upto greenschist facies (～450 °C/4 kbar). In the central part of nappe the unmylonitized schists and gneisses are affected by regional metamorphism (M₁) reaching upper amphibolite facies (～4.0–7.9 kbar and ～500–709 °C). Four zones of regional metamorphism progressing from chlorite–biotite to sillimanite–K-feldspar zone demarcated by specific reaction isograds have been identified. These metamorphic zones show a repetition suggesting that the zones are involved in tight F₂ – folding which has affected the metamorphics. South of the Almora town, the regionally metamorphosed rocks have been intruded by Almora Granite (560 ± 20 Ma) resulting in contact metamorphism. The contact metamorphic signatures overprint the regional S₂ foliation. It is inferred that the dominant regional metamorphism in Almora Nappe is highly likely to be of pre-Himalayan (Precambrian!) age.     

Early Mesozoic metamorphism and tectonic significance of the eastern segment of the Lhasa terrane,south Tibet

The metamorphic belt in the Basongco area, the eastern segment of Lhasa terrane, south Tibet, occurs as the tectonic blocks in Paleozoic sedimentary rocks. The Basongco metamorphic rocks are mainly composed of paragneiss and schist, with minor marble and orthogneiss, and considered previously to be the Precambrian basement of the Lhasa terrane. This study shows that the Basongco metamorphic belt experienced medium-pressure amphibolite-facies metamorphism under the conditions of T = 640–705 °C and P = 6.0–8.0 kbar. The inherited detrital zircon of the metasedimentary rocks yielded widely variable ²⁰⁶Pb/²³⁸U ages ranging from 3105 Ma to 500 Ma, with two main age populations at 1150 Ma and 580 Ma. The magmatic cores of zircons from the orthogneiss constrain the protolith age as ca. 203 Ma. The metamorphic zircons from all rocks yielded the consistent metamorphic ages of 192–204 Ma. The magmatic cores of zircons in the orthogneiss yielded old Hf model ages (T_DM2 = 1.5–2.1 Ga). The magmatic zircons from the mylonitized granite yielded a crystallization age of ca. 198 Ma. These results indicate that the high-grade metamorphic rocks from the Basongco area were formed at early Jurassic and associated with coeval magmatism derived from the thickening crust. The Basongco metamorphic belt, together with the western and coeval Sumdo and Nyainqentanglha metamorphic belts, formed a 400-km-long tectonic unit, indicating that the central segment of the Lhasa terrane experienced the late Paleozoic to early Mesozoic collisional orogeny.     

北秦岭松树沟橄榄岩与铬铁矿矿床的成因关系

松树沟橄榄岩体是秦岭造山带中规模最大的赋存铬铁矿床的超基性岩体。松树沟橄榄岩主要由细粒橄榄岩质糜棱岩和中粗粒橄榄岩组成。本文通过对松树沟橄榄岩的岩相学、主微量、稀土元素地球化学的系统研究,认为松树沟细粒方辉橄榄岩为洋脊扩张过程中地幔岩减压-近分离熔融产生的残留体,细粒纯橄岩主要由地幔橄榄岩熔融残留橄榄石、消耗辉石的减压熔融反应:aCpx+bOpx+cSpl=dOl+1Melt生成的橄榄石和少量的地幔方辉橄榄岩残留体组成,但均受到了后期渗滤熔体的再富集作用;中粗粒纯橄岩和方辉橄榄岩主要为上述反应产生的渗滤熔体被圈闭在迁移通道或减压扩容带内在热边界层(TBL)通过反应:MeltA=Ol+MeltB冷凝结晶而成,属堆晶橄榄岩。Pb-Sr-Nd同位素地球化学的证据显示,松树沟橄榄岩与基性岩具有共同的地幔源区,二者同为松树沟蛇绿岩的重要组成部分。通过矿床地质特征及铬铁矿电子探针测试研究,认为松树沟铬铁矿床是产于中粗粒堆晶纯橄岩中的层状铬铁矿床,形成于格林威尔期松树沟洋盆的扩张过程中,是中粗粒纯橄岩在热边界层(TBL)的冷凝结晶过程中岩浆分异作用的产物。     

Geochemistry of platinum-group elements and mineral composition in chromitites and associated rocks from the Abdasht ultramafic complex,Kerman, Southeastern Iran

The Abdasht complex is a major ultramafic complex in south-east Iran (Esfandagheh area). It is composed mainly of dunite, harzburgite, podiform chromitites, and subordinate lherzolite and wehrlite. The podiform chromitites display massive, disseminated, banded and nodular textures. Chromian spinels in massive chromitites exhibit a uniform and restricted composition and are characterized by Cr# [= Cr / (Cr + Al)] ranging from 0.76 to 0.77, Mg# [= Mg/(Mg + Fe^2 +)] from 0.63 to 0.65 and TiO₂ < 0.2 wt.%. These values may reflect crystallization of the chromian spinels from boninitic magmas. Chromian spinels in peridotites exhibit a wide range of Cr# from 0.48 to 0.86, Mg# from 0.26 to 0.56 and very low TiO₂ contents (averaging 0.07 wt.%). The Fe^3 +# is very low, (< 0.08 wt.%) in the chromian spinel of chromitites and peridotites of the Abdasht complex which reflects crystallization under low oxygen fugacities.The distribution of platinum group elements (PGE) in Abdasht chromitites displays a high (Os + Ir + Ru)/(Rh + Pt + Pd) ratio with strongly fractionated chondrite-normalized PGE patterns typical of ophiolitic chromitites. Moreover, the Pd/Ir value, which is an indicator of PGE fractionation, is very low (< 0.1) in the chromitites.The harzburgite, dunite and lherzolite samples are highly depleted in PGE contents relative to chondrites. The Pd_N/Ir_N ratios in dunites are unfractionated, averaging 0.72, whereas the harzburgites and lherzolites show slightly positive slopes PGE spidergrams, together with a small positive Ru anomaly, and their Pd_N/Ir_N ratio averages 2.4 and 2.3 respectively. Moreover, the PGE chondrite and primitive mantle normalized patterns of harzburgite, dunite and lherzolite are relatively flat which are comparable to the highly depleted mantle peridotites.The mineral chemistry data and PGE geochemistry indicate that the Abdasht chromitites and peridotites were generated from a melt with boninitic affinity under low oxygen fugacity in a supra-subduction zone setting. The composition of calculated parental melts of the Abdasht chromitites is consistent with the differentiation of arc-related magmas.     

Chemical variations of abyssal peridotites in the central Oman ophiolite: Evidence of oceanic mantle heterogeneity

Basal peridotites above the metamorphic sole outcropped around Wadi Sarami in the central Oman ophiolite give us an excellent opportunity to understand the spatial extent of the mantle heterogeneity and to examine peridotites−slab interactions. We recognized two types of basal lherzolites (Types I and II) that change upward to harzburgites. Their pyroxene and spinel compositions display severely variations at small scales over < 0.5 km, and encompass the entire abyssal peridotite trend; clinopyroxenes (Cpxs) show wide ranges of Al₂O₃, Na₂O, Cr₂O₃ and TiO₂ contents. Primary spinels show a large variation of Cr# [= Cr/(Cr + Al)] from 0.04 to 0.53, indicating various degrees of partial melting. Trace-element compositions of peridotites and their pyroxenes also show a large chemical heterogeneity in the base of the Oman mantle section. This heterogeneity mainly resulted from variations of partial-melting degrees due to the change of a mantle thermal regime and a distance from the spreading ridge or the mantle diapir. It was overlapped with subsolidus modification during cooling and fluid metasomatism prior and/or during emplacement. The studied peridotites are enriched in Rb, Cs, Ba, Sr and LREE due to fluid influx during detachment and emplacement stages. Chondrite (CI)-normalized REE patterns for pyroxenes are convex upward with strong LREE depletion due to their residual origin, similar to abyssal peridotites from a normal ridge segment. The Cpxs are enriched in fluid mobile elements (e.g., B, Li, Cs, Pb, Rb) and depleted in HFSE (Ta, Nb, Th, Zr) + LREE, suggesting no effect of melt refertilization. Their HREE contents, combined with spinel compositions, suggest two melting series with 1–5% melting for type II lherzolites, 3– < 10% melting for type I lherzolites and ~ 15% for harzburgites. Hornblendes are enriched in fluid-mobile elements relative to HFSE + U inherited from their precursor Cpx. The clinopyroxenite lens crosscuts the basal lherzolites, forming small-scale (< 5 cm) mineralogical and chemical heterogeneities. It was possibly formed from fractional crystallization of interstitial incremental melt that formed during decompression melting of a normal MORB mantle source. The studied peridotites possibly represent a chemical heterogeneity common to the mantle at an oceanic spreading center.     

Paleozoic multi-stage accretionary evolution of the SW Chinese Tianshan: New constraints from plutonic complex in the Nalati Range

We conducted field investigations, whole-rock geochemical, Sr-Nd and zircon U-Pb-Lu-Hf isotopic analyses on a suite of intrusive complex in the southern Nalati Range, SW Chinese Tianshan in order to better understand the Paleozoic tectonic and magmatic evolution of the belt. The intrusive complex comprises weakly foliated diorite, low-grade altered diabase, and deformed monzogranite; these plutonic rocks were in turn crosscut by undeformed coarse-grained diorite, granodiorite as well as granite stock. Foliated Late Silurian diorites (421 ± 4 Ma) show arc-type geochemical features, slightly negative whole-rock εNd(t) value (− 1.7; T_DM-Nd = 1.52 Ga) and variably positive zircon εHf(t) values (2.34 to 7.27; T_DM-Hf: 0.95– 1.26 Ga). Deformed Early Devonian porphyritic monzogranites (411 ± 4 Ma) show geochemical features similar to A-type granite, and their zircon εHf(t) values range from − 6.63 to 1.02, with T_DM-Hf ages of 1.82 to 1.33 Ga. Metamorphosed Early Devonian diabases (ca. 410 Ma) have OIB-like REE patterns, εNd(t) values of − 2.0 ~ − 0.8 and T_DM-Nd ages of 1.37– 1.25 Ga. The undeformed Early Carboniferous diorite and granodiorite (353– 344 Ma) exhibit arc-type geochemical features, positive εHf(t) values of 6.11– 7.91 with T_DM-Hf ages of 0.97– 0.86 Ga, and positive εNd(t) value of 1.9 with T_DM-Nd age of 1.04 Ga. The Early Permian granite stock (292 ± 5 Ma) has highly differentiated REE pattern, slightly negative εNd(t) value (− 4.4) and variable zircon εHf(t) values of − 9.73– 6.36. Combining with available data, Early Paleozoic (500– 410 Ma) arc-related magmatic rocks occurring on both sides of the suture zone along the southern Nalati Range, likely resulted from a bi-directional subduction of the Paleo-Tianshan Ocean beneath the Yili Block to the north and the Central Tianshan to the south. Occurrences of A-type granites and OIB-like diabases (ca. 410 Ma) along the Nalati Range likely indicate a hot extensional regime probably induced by the break off of the northward subducting slab of the Paleo-Tianshan Ocean. The closure of the Paleo-Tianshan Ocean and subsequent amalgamation during Early Carboniferous resulted in the regional deformation and metamorphism of the Early Paleozoic arc-related magmatic rocks. From Early to Late Carboniferous, a magmatic arc that corresponded to the well-developed Late Paleozoic Balkhash-Yili active continental margin, superimposed upon the southern Yili Block, most likely resulted from the southward subduction of the Junggar-North Tianshan Ocean. After the closure of the North Tianshan Ocean in Late Carboniferous, the study area was dominated by post-orogenic magmatism.     

Discovery of Early Paleozoic eclogite from the East Kunlun,Western China and its tectonic significance

Eclogites discovered in the eastern part of the East Kunlun Mountains, Western China, are primarily composed of garnet + omphacite + quartz + rutile. The garnets show end-member components of 48–55% almandine, 1–2% spessartine, 19–29% grossularite and 16–29% pyrope, and the omphacite has a jadeite content of 21–63%. The peak-metamorphic assemblage of eclogites records a P–T condition of > 1.6 GPa and 590 °C–650 °C. Zircon U–Pb dating of the inherited magmatic zircons from fine-grained eclogite reveals a Neoproterozoic age of 934 Ma, representing the protolith age of the eclogite. Zircons from the coarse-grained eclogite contain inclusions of garnet, omphacite and rutile, and yield a weighted mean age of 428 Ma, indicating the metamorphic age of eclogite. The eclogites, together with the Late Cambrian (508 Ma) high-pressure granulite to the west, constitute an Early Paleozoic high-pressure metamorphic belt in the East Kunlun.     

High-Al and high-Cr podiform chromitites from the western Yarlung-Zangbo suture zone,Tibet: Implications from mineralogy and geochemistry of chromian spinel,and platinum-group elements

On the basis of their mineral chemistry, podiform chromitites are divided into high-Al (Cr# = 20–60) (Cr# = 100 1 Cr/(Cr + Al)) and high-Cr (Cr# = 60–80) varieties. Typically, only one type occurs in a given peridotite massif, although some ophiolites contain several massifs that can have different chromitite compositions. We report here the occurrence of both high-Cr and high-Al chromitite in a single massif in China, the Dongbo mafic-ultramafic body in the western Yarlung-Zangbo suture zone of Tibet. This massif consists mainly of mantle peridotites, with lesser pyroxenite and gabbro. The mantle peridotites are mainly composed of harzburgites and minor lherzolites; a few dike-like bodies of dunite are also present. Seven small, lenticular bodies of chromitite ores have been found in the harzburgites, with ore textures ranging from massive through disseminated to sparsely disseminated; no nodular ore has been observed. Individual chromitite pods are 1–3 m long, 0.2–2 m wide and strike NW, parallel to the main trend of the peridotites. Chromitite pods 3, 4, and 5 consist of high-Al chromitite (Cr# = 12–47), whereas pods 1 and 2 are high-Cr varieties (Cr# = 73 to 77). In addition to chromian spinel, all of the pods contain minor olivine, amphibole and serpentine. Mineral structures show that the peridotites experienced plastic deformation and partial melting. The mineralogy and geochemistry of the Dongbo peridotites suggest that they formed originally at a mid-ocean ridge (MOR), and were later modified by suprasubduction zone (SSZ) melts/fluids. We interpret the high-Al chromitites as the products of early mid-ocean ridge basalt (MORB) or arc tholeiite magmas, whereas the high-Cr varieties are thought to have been generated by later SSZ melts.     

Heterogeneous distribution of water in the mantle beneath the central Siberian Craton: Implications from the Udachnaya Kimberlite Pipe

The paper presents new petrographic, major element and Fourier transform infrared (FTIR) spectroscopy data and PT-estimates of whole-rock samples and minerals of a collection of 19 relatively fresh peridotite xenoliths from the Udachnaya kimberlite pipe, which were recovered from its deeper levels. The xenoliths are non-deformed (granular), medium-deformed and highly deformed (porphyroclastic, mosaic-porphyroclastic, mylonitic) lherzolites, harzburgite and dunite. The lherzolites yielded equilibration temperatures (T) and pressures (P) ranging from 913 to 1324 °C and from 4.6 to 6.3 GPa, respectively. The non-deformed and medium-deformed peridotites match the 35 mW/m² conductive continental geotherm, whereas the highly deformed varieties match the 45 mW/m² geotherm. The content of water spans 2 ± 1–95 ± 52 ppm in olivine, 1 ± 0.5–61 ± 9 ppm in orthopyroxene, and 7 ± 2–71 ± 30 ppm in clinopyroxene. The amount of water in garnets is negligible. Based on the modal proportions of mineral phases in the xenoliths, the water contents in peridotites were estimated to vary over a wide range from < 1 to 64 ppm. The amount of water in the mantle xenoliths is well correlated with the deformation degree: highly deformed peridotites show highest water contents (64 ppm) and those medium-deformed and non-deformed contain ca. 1 ppm of H₂O. The high water contents in the deformed peridotites could be linked to metasomatism of relatively dry diamondiferous cratonic roots by hydrous and carbonatitic agents (fluids/melts), which may cause hydration and carbonation of peridotite and oxidation and dissolution of diamonds. The heterogeneous distribution of water in the cratonic mantle beneath the Udachnaya pipe is consistent with the models of mantle plume or veined mantle structures proposed based on a trace element study of similar xenolithic suits. Mantle metasomatism beneath the Siberian Craton and its triggered kimberlite magmatism could be induced by mantle enrichment in volatiles (H₂O, CO₂) supplied by numerous subduction zones which surrounded the Siberian continent in Neoproterozoic-Cambrian time.