Partial Melting Of Spinel Lherzolite Research Articles

Petrogenesis and Geodynamic Implications of Cretaceous Nb-Enriched Mafic Dykes in the East Kunlun Orogen, Northern Tibet Plateau: Constraints from Geochronology, Geochemistry and Sr-Nd Isotopes

There is a magmatic lull period in the East Kunlun orogen (EKO) during the Jurassic to the Cretaceous. However, due to the lack of records of magmatic activity restricts our understanding of the late Mesozoic magmatic-tectonic evolution of the EKO. Herein, an integrated study of geochronology, whole-rock geochemistry and Sr-Nd isotopes were conducted for the Cretaceous mafic dykes in the EKO, Northern Tibet Plateau, to reveal their petrogenesis and geodynamic implications. LA-ICP-MS Zircon U-Pb dating reveals that the studied mafic dykes comprising diabase and diabase porphyry emplaced at ca. 80.9 ± 0.8 Ma. The Cretaceous mafic dykes have low contents of SiO2 (46.36 wt.%~47.40 wt.%) but high contents of MgO (6.79 wt.%~7.38 wt.%), TiO2 (1.91 wt.%~2.13 wt.%), Nb (12.4~18.3 ppm) and Nb/U ratio (31~39), resembling Nb-enriched mafic dykes. They exhibit chondrite-normalized rare earth element (REE) and primitive mantle-normalized trace element patterns, remarkably similar but not identical to the oceanic island basalts (OIB). The moderate REE fractionation ((La/Yb)N = 3.55~5.37), weak negative Eu anomalies (δEu = 0.87~0.97) and relative enrichment of Rb, Ba, K, as well as high contents of Cr and Ni and slightly depleted Sr-Nd isotopes (εNd(t) = −0.18~1.33), suggest that the studied dykes originate from a partial melting of spinel lherzolite and a little of garnet which was previously modified by subducted sediments. Combined with other evidence, we propose that the studied Cretaceous Nb-enriched mafic dykes in the Northern Tibet Plateau were formed in the intraplate setting, which may be a partial melting of the enriched mantle in the lower lithosphere caused by the activity of the East Kunlun strike-slip fault.

Major, trace element and Sr-Nd isotope evidence for a sublithospheric mantle source for the Umkondo large igneous province

The Mesoproterozoic (1.11 Ga) Umkondo large igneous province (LIP) in southern Africa and Antarctica was emplaced in < 5 Myr and is dominated by low-Ti tholeiitic doleritic-gabbroic sills. It is of particular interest because it is the least studied LIP in southern Africa with both sublithospheric and lithospheric mantle sources proposed and it coincides with the early assembly of Rodinia, so it has importance in understanding the nature of magmatism and tectonics in and around the Kalahari craton during the Mesoproterozoic. In this study, we compiled a large database of existing (∼750) and new (∼100) major and trace element data for the Umkondo province, as well as 42 new Sr-Nd isotopic measurements, to provide constraints on its magma sources and geochemical evolution. Major element compositional variations in the low-Ti tholeiites are explained by low-pressure (1 kbar) three-phase fractional crystallisation (olivine, clinopyroxene and plagioclase) of a parent magma with ∼ 10 wt.% MgO in oxidising conditions (QFM + 1). Inverse models show that the low-Ti tholeiitic magmas were derived as residual melts after the crystallization of 12%–33% olivine from primary komatiitic-basaltic magmas (up to ∼ 20 wt.% MgO) in equilibrium with mantle olivine (Fo90). Low Sm/Yb and TiO2/Yb-Nb/Yb indicate that the primary magmas were derived by 2%–20% shallow (40–50 km) partial melting of spinel lherzolite. High Sm/Yb is restricted to dyke swarms and may imply limited magma production from deeper (up to ∼ 70 km) garnet lherzolite-like sources. The low-Ti tholeiites of the Umkondo province are enriched in large ion lithophile elements (Rb-Sr-Cs-K) and depleted in high-field strength elements (Zr-Hf-Nb-Ta), indicating the involvement of crustal material and/or the subcontinental lithospheric mantle. This is supported by covariations in Th/Nb, Nb/Yb, Nb/La and Ce/Sm with generally negative ΔNb. Sr-Nd isotopes lend support to the notion that the Umkondo magmas were derived from depleted and/or enriched sublithospheric mantle sources and subsequently contaminated by enriched lithospheric material during emplacement (initial (at 1.11 Ga) 87Sr/86Sr between 0.704820 and 0.737464 and εNd between −8.9 and +5.3). The Vredefort sills are significant as they display the most depleted Sr-Nd isotopic signatures (average initial 87Sr/86Sr of 0.705342 and average εNd of 0.4) and are the least contaminated magma suite in the Umkondo province. Because of (i) the large volume of low-Ti magmas, (ii) evidence of a primary hot and MgO-rich (komatiitic) magma, and (iii) the short duration of magmatism, we suggest that the Umkondo province was formed by plume-induced melting of the sublithospheric mantle beneath the Kalahari craton in an extensional setting. This contrasts with previous suggestions that the heat source developed in response to the “thermal insulation” of the mantle beneath a thickened Kalahari craton in the absence of a mantle plume. There is further evidence from the elevated Zn/Fe that the sublithospheric mantle was lithologically heterogeneous and consisted of mixed peridotite and pyroxenite domains. There is a general lack of ultramafic cumulates in the low-Ti magma suite that may imply there was deeper ponding and storage of the primary magmas that fractionated large quantities of ultramafic rocks. There is also a paucity of high-Ti rocks in the Umkondo province that may reflect limited direct melting of the lithospheric mantle or that they are simply not as well-preserved in this province compared to the Karoo province. The similar trace element and Sr-Nd isotopic compositions of the Umkondo sills in southern Africa with the Borgmassivet sills in Antarctica support the concept that the Kalahari craton and Grunehogna terrane were adjoined at 1.11 Ga. The timing of the Umkondo province indicates there was localised lithospheric extension and upwelling asthenospheric mantle during a time of dominantly compressional tectonics on Earth at the end of the ‘boring billion’.

Late Neoarchean plate subduction in Western North China Craton: Evidence from ca. 2.51 Ga to 2.46 Ga basement rocks in Northern Ordos Basin

The most poorly exposed and least understood in the North China Craton (NCC) lies largely hidden beneath the Ordos Basin, severely limiting the cognition of the Precambrian tectonic evolution in the western NCC and the tectonic division. Diverse orthogneiss samples collected from four boreholes drilled into the basement of the north side of Hangjinqi Fault Zone (HFZ) in the North Ordos Basin (NOB) are investigated by integrating zircon U-Pb dating, Hf-Nd isotopes, and whole-rock geochemical analyses. The TTG gneiss, quartz-monzonite gneiss, granodiorite gneiss, and amphibolite were emplaced at ca. 2.51–2.46 Ga, of which the amphibolite also recorded the metamorphic age of ca. 1.95 Ga. The TTG gneisses display low MgO, Mg#, Cr, and Ni, high HREE and Rb/Sr, enrichment of LILEs and depletion of Nb, Ta, and Ti, and positive εNd(t) (2.04–4.06) and εHf(t) (3.04–7.59) values, suggesting that they were mainly originated from partial melting of the tectonically thickened mafic island-arc crust. In contrast, the quartz monzonite gneisses and granodiorite gneisses show the geochemical features of Archean sanukitoids with higher MgO, Mg#, Cr, and Ni, and similar εNd(t) (1.34–3.82) and εHf(t) values (3.94–7.32), suggesting that they were derived from the partial melting of the mantle metasomatized by fluids and melts of the subduction. The amphibolites exhibit a mixed characteristic of E-MORB and arc-like geochemical affinities resembling the Mariana back-arc basin, which was derived from the partial melting of spinel lherzolite. The arc and arc-back lithological assemblages suggest that the north margin of the NOB might be in a geodynamic transition period from a compressional regime correlated with plate subduction to an extensional regime correlated with slab rollback during ca. 2.51–2.46 Ga. Combined with previous analytical results, the northern margin of the NOB was most likely in a long-term active continental margin setting during 2.51–1.95 Ga and involved in the collision event between the Yinshan Block and Ordos Block at ca. 1.95 Ga.

Bulk chemistry and Hf isotope ratios of the Almogholagh Intrusive Complex, western Iran: a consequence of an extensional tectonic regime in the Late Jurassic

ABSTRACT The Almogholagh Intrusive Complex (AIC) in the central part of the Sanandaj-Sirjan zone (C-SaSZ) includes gabbros to granites. Zircon U-Pb dating of mafic, intermediate, and felsic intrusions yield age of 146 to 141 Ma (Late Jurassic-Early Cretaceous). Bulk chemistry shows a wide range for SiO2 (46.8 to 73.3 wt.%), MgO (0.08 to 8.95 wt.%), and Fe2O3(T) (0.47 to 9.25 wt.%) and also alkali elements ranging from 3.59 to 10.12 wt.%. These rocks are characterized by a high content of large-ion lithophile elements (LILEs; e.g. Rb, U, Th, and K) and LREEs with weak Nb and Ta negative anomalies, showing many similarities to an extensional setting. The 176Hf/177Hf ratios vary from 0.282802 to 0.282985 with positive εHf(t) values from + 1.67 to +10.74 (avg: 7.85). Trace-element and isotope ratios modelling indicate that the AIC mafic-intermediate rocks were generated by partial melting of spinel lherzolite and then underwent considerable crustal contamination. The present work in combination with previous data confirm an extensional tectonic regime for the northern part of the C-SaSZ during the Late Jurassic to Early Cretaceous. Magmatic differentiation with crustal contamination played a fundamental role in the wide compositional range from mafic to felsic types that formed the AIC.

Partial melt generation and evolution of magma reservoir conditions at the Paka volcanic complex in Kenya: Constraints from geochemistry, petrology and geophysics

In this study, we determine magma generation and evolution of magma storage conditions beneath Paka volcano located in northern segment of the Kenya Rift. The surface geology of the area is characterised by five main sequences including pre-Paka formations and volcanic sequence 1–4, all built up by basalt, intermediate and trachyte rocks spanning 582–8 ka. Numerical trace element modelling suggests that the primary magma in Paka resulted principally from 5 to 10% garnet–peridotite partial melting and possibly 0.1–1.5% partial melting of spinel-lherzolite. The parental magma was probably held at the Moho (~30 km) and evolved by ~76% gabbro fractional crystallization to generate a liquid of similar chemistry as that of erupted Paka basalts. We propose that under moderately oxidized conditions (∆FMQ +1) the primary magma underwent a further ~70% fractional crystallization and ~ 20% assimilation of crustal syenitic material to generate Paka trachyte lavas. Basaltic and intermediate magmas were held at polybaric storage pressure conditions ranging between 1.4 and 7 kbar (5–25 km), at temperatures between 1086 and 1204 °C with relatively low water content of <1.8 wt%. The trachyte magma resided at low pressures, i.e., 0.3–2.5 kbar (1–9 km) at 901–980 °C with water contents of 3.5–5.1 wt%. Based on our data eruption source depths remained largely the same for a period spanning from 582 to 11 ka. Geophysical data show correlation with magma depths beneath Paka volcano.

Mafic rocks with back-arc E-MORB affinity from the Chotanagpur Granite Gneiss Complex of India: relicts of a Proterozoic Ophiolite suite

AbstractThe Proterozoic Chotanagpur Granite Gneiss Complex (CGGC) at the northern boundary of the Central Indian Tectonic Zone (CITZ) of the eastern Indian shield preserves relics of fossilized oceanic back-arc crust. We describe the field, petrographical and geochemical characteristics of the mafic rocks comprising pillow basalts and dolerites from the Bathani area of the northern fringe of the CGGC, eastern India. The basalts consist of plagioclase feldspar, hornblende, opaque minerals (Fe–Ti oxide) and chlorite, and the dolerite consists of plagioclase, hornblende and opaque minerals. Our data indicate that the Bathani mafic rocks have tholeiitic to transitional composition and are overprinted by greenschist facies metamorphic conditions; however, REE and fluid immobile elements preserve their primary geochemical signatures. The (La/Sm)N ratios (1.38–2.15) and chondrite-normalized REE patterns point to an enriched mid-ocean ridge basalt (E-MORB) mantle source. Geochemical characteristics indicate a mixed signature of MORB and arc tholeiite with enrichment of Ba, Th, Eu and Sr, similar to that of back-arc supra-subduction zone ophiolites. These mafic rocks are the product of MORB-like magma derived from a depleted mantle corresponding to &lt; 2% partial melting of spinel lherzolite, enriched by subduction-induced slab metasomatism and melting. The Bathani mafic rocks are representative of the upper part of a supra-subduction zone columnar ophiolite section, which was emplaced onto the present-day northern margin of the CGGC during suturing of the northern and southern Indian block at c. 1.9 Ga during the Nuna amalgamation.

Intermittent subduction of the Paleo-Tethys Ocean in the middle–late Permian: evidence from the mafic–intermediate intrusive rocks in the East Kunlun Orogenic Belt

In this study, we investigate the geochronology, geochemistry, zircon Lu–Hf isotopes and Sr–Nd isotopes of mafic–intermediate intrusive rocks in the Naxiguole area of the East Kunlun Orogenic Belt (EKOB), northwest China. Zircon U–Pb dating yields formation ages of 254.8 ± 1.2 Ma for gabbros and 251.9 ± 1.3 Ma for gabbrodiorites, suggesting that the mafic–intermediate intrusive rocks were formed in the late Permian. The gabbrodiorites show the geochemical characteristics of arc magmatic rocks, while gabbros were similar to those of backarc basin basalts. The gabbros yielded (87Sr/86Sr)i ratios of 0.7068–0.7097, low ε Nd(t) values of –4.40 to +0.16, and a limited range of ε Hf(t) values of –1.09 to +0.04. The gabbrodiorites display (87Sr/86Sr)i ratios of 0.7090–0.7098, ε Nd(t) values ranging between –3.99 and –3.55, and ε Hf(t) values of –0.81 to +0.34. The Sr–Nd and Lu–Hf isotopes from both types of samples resemble an enriched mantle but distinctly different from the crust. Based on their geochemical, and some trace elemental ratios (e.g. Ba/Th, La/Sm, Th/Yb and Sm/Yb), we suggest that the parental magmas of the gabbros and gabbrodiorites were formed by 1–5% partial melting of spinel lherzolite and garnet–spinel lherzolite, respectively. The gabbros originated from a mantle source modified by subducted slab fluids, while the gabbrodiorites were derived from a mantle source altered by subducted slab melts. By synthesising all the data, we propose that the gabbros were formed in a backarc extension setting and the gabbrodiorites in a continental arc setting. This suggests a geodynamic setting transition from backarc extension to subduction during the late Permian. In combination with previous research, we propose that the subduction mode in the Paleo-Tethys Ocean in the EKOB is intermittent. KEY POINTS Zircon U–Pb dating indicates an age of ca 254.8 ± 1.2 Ma for the gabbros and ca 251.9 ± 1.3 Ma for the gabbrodiorites (i.e. late Permian). The gabbros were derived from an enriched mantle source modified by slab-derived fluids whereas the gabbrodiorites had an enriched mantle source modified by subducted slab melts. The gabbros were in a backarc extension setting while the gabbrodiorites formed in a continental arc setting. This suggests that there was a transition in the geodynamic setting (from backarc extension to subduction).

Early Cretaceous continent basalts in the Alxa Block, NW China: geochronology, geochemistry, and tectonic implications

ABSTRACT NE Asia has experienced Significant extensional events in the Early Cretaceous, including related volcanic rocks, extensional basins, metamorphic core complexes (MCCs), and low-angle detachment faults. Previous studies primarily focused on the eastern North China Craton (NCC); however, the extensional manifestations in the western NCC are poorly studied and the mechanism is still controversial. This study reports Wuliji volcanic clusters in northern Alxa Block, westmost NCC. Geologic mapping shows that the volcanic cluster is mainly composed of dark grey basalts with interlayered pyroclastic rocks showing columnar joints. The youngest LA-ICP-MS zircon U-Pb age is 103 Ma, consistent with the age constrained by fossils in the interlayered sandstones. Though the Wuliji basalts have amounts of captured zircons, the high MgO, TFe2O3 and low SiO2, along with Nb/U, Nb/Ta, Zr/Hf ratios and positive Nb, Ta anomalies, indicating no contamination of continental crustal materials. These alkaline basalts are characterized by enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), slightly positive Eu anomalies and no Nb and Ta depletions similar to other volcanic rocks in Alxa Block, implying an affinity with ocean island basalts (OIB). These volcanic rocks could have been derived from the partial melting of spinel lherzolite in an intracontinental rift setting. The basalts in the Alxa Block are slightly younger than basalts in the eastern NCC in the Early Cretaceous. The occurrence of Early Cretaceous basalts and coeval rift basins with various extensional faults in the westmost NCC may have resulted from the eastward movement of mantle materials due to the rollback of the Paleo-Pacific Plate to the east.

Petrogenesis of the early Carboniferous Xilinhot gabbro–diorite pluton in central Inner Mongolia: Magma evolution and tectonic significance

The Xilinhot gabbro–diorite pluton in central Inner Mongolia comprises a microgabbro–hornblende gabbro–quartz diorite suite and diorite–microdiorite–quartz microdiorite suite, which intrude the Erdaojing complex in the northern orogenic belt of the southeastern Central Asian Orogenic Belt (CAOB). Zircons separated from hornblende gabbro and quartz diorite yielded sensitive high-resolution ion microprobe U–Pb ages of 326.8 ± 3.4 and 324.0 ± 1.9 Ma, respectively. There are two types of hornblende in the hornblende gabbro: isolated hornblende and hornblende clusters surrounding orthopyroxene. Only hornblende clusters occur in the quartz diorite. The amphibole is edenite in the diorite, whereas the microdiorite and quartz microdiorite contain hornblende. The isolated hornblende formed at higher P–T–H2Omelt conditions (3.3–5.0 kbar; 749–814 °C; 7.4–8.3 wt% H2O) than the hornblende clusters in the hornblende gabbro (0.9–2.2 kbar; 670–731 °C; 3.8–5.5 wt% H2O). Edenite in the diorite crystallized at higher P–T and lower H2Omelt conditions (1.8–4.8 kbar; 777–979 °C; 3.2–5.2 wt% H2O) than hornblende in the microdiorite (1.3–1.5 kbar; 697–711 °C; 5.0–5.6 wt% H2O). These thermodynamic parameters reveal a multi-level magmatic storage system. Hornblende gabbro, quartz diorite, diorite, microdiorite, and quartz microdiorite are generally characterized by flat middle to heavy rare earth element patterns and no Eu anomalies. Geochemical modeling of heavy rare earth element concentrations in hornblende gabbro suggest that the magma was generated by >17.5% partial melting of spinel lherzolite. Slab-derived fluid input can account for the enrichments in Sr, Th, and light rare earth elements, and depletions in Nb and Ta. The multi-level magmatic storage system and slab-derived fluid input suggest that the early Carboniferous magmatism was generated in an arc-related tectonic setting. The northern orogenic and Hegenshan ophiolite belts constitute an early Carboniferous fore-arc basin–arc–back-arc basin system.

Syncollisional gabbro in the Irtysh shear zone, Eastern Kazakhstan: Compositions, geochronology, and geodynamic implications

New data of petrography, mineralogy, and geochemistry from the Irtysh shear zone in Eastern Kazakhstan reveal three types of gabbro: (1) dolerite and gabbrodolerite, (2) biotite-bearing gabbronorite and gabbrodiorite, and (3) layered peridotite-gabbro intrusions. Rocks of types 1 and 2 occur mainly as small bodies and share similarity in major- and trace-element compositions. Layered gabbro (type 3) forms the largest intrusion in the area and has a different composition: higher CaO and Al2O3 but lower K2O, TiO2, P2O5, Zr, Nb, REE. Correspondingly, the gabbro of two first types may result from differentiation of the same tholeiite magma, whereas the layered gabbro rather derived from a magma intermediate between tholeiite and calc-alkaline series. According to geochemical and isotopic signatures, both types of primary magma originated from the same sublithospheric depleted mantle by low- and high-degree partial melting of spinel lherzolite. The obtained 317–313 Ma U-Pb and Ar-Ar ages of the gabbro fall within the time of collisional events in the area. The suggested model explains the origin of syncollisional gabbro as intrusion of ascending mafic melts from a mantle wedge at the transform boundary of colliding plates along the Irtysh shear zone where strike-slip faulting produced slab windows open to inputs of hot asthenospheric material.

Geology and geochemistry of pillow basalt in the Huilvshan region (west Junggar, China): implications for magma source and tectonic setting

Geological characteristics and geochemical analyses are reported for the early Carboniferous pillow basalt in the Huilvshan region (west Junggar, Northwest China), with the aim to indicate its petrogenesis, magma source characteristics, and tectonic implication. This pillow basalt consists of clinopyroxene and plagioclase with trace amounts of magnetite, apatite, and chromian spinel. It is tholeiitic in composition with low concentrations of Na2O + K2O (1.52–4.74 wt.%). Similar to the N-MORB, the samples of this pillow basalt have nearly flat chondrite-normalized REE patterns ((La/Yb)N = 0.87–1.47) with insignificant Eu anomalies (Eu/Eu* = 0.84–1.18), and show no obvious enrichments of LILEs and insignificant depletions in HFSEs. Petrology and geochemical characteristics suggest that this pillow basalt is the product of MORB-like magma derived from a depleted mantle corresponding to ≤4% partial melting of spinel lherzolite. SIMS analysis of the zircons separated from tuff interlayered with basalt gives a weighted average U–Pb age of 328 ± 3 Ma (MSWD = 1.4), which represents the magma eruption time in the Huilvshan region. From these observations, in combination with the previous work, we conclude that an extensional tectonic regime dominated the tectonic activity of west Junggar during early Carboniferous.

Geochronology and geochemistry of the Huilvshan gabbro in west Junggar (NW China): Implications for magma process and tectonic regime

Gabbro plutons, consisting of clinopyroxene and plagioclase with trace amounts of magnetite, titanite, and apatite, intruded into Early Carboniferous volcanic-sedimentary strata in the Huilvshan gold mining region (west Junggar, China). Samples collected from two gabbro bodies are tholeiitic in composition with low concentrations of Na2O + K2O, showing weak depletions of light rare earth elements with insignificant Eu, Nb, and Ti anomalies. Zircon U-Pb analyses yield a weighted average U-Pb age of 296.1 ± 2.7 Ma (MSWD = 0.98), which could represent the time corresponding to mafic magma emplacement in the Huilvshan region. Geochemical calculations suggest that this mafic magma was derived from a depleted mantle source in a post-collisional tectonic setting corresponding to ~ 4% partial melting of spinel lherzolite.

The behavior of Fe3+/∑Fe during partial melting of spinel lherzolite

This study presents an internally consistent model for the behavior of Fe3+/∑Fe during partial melting of spinel lherzolite. The Fe3+/∑Fe ratio for olivine is calculated on the basis of point defect thermodynamics, and the oxidation states of iron in the other solid phases are calculated using Fe3+/Fe2+ distribution between olivine and orthopyroxene, clinopyroxene, or spinel. Conservation of mass is used to relate the Fe3+/Fe2+ ratio of partial melt to the concentrations of Fe3+ and Fe2+ in the initial and residual solids as a function of pressure, temperature, and oxygen fugacity. Results from isobaric batch melting calculations demonstrate that the Fe3+/∑Fe ratio of the partial melt decreases with increasing melt fraction. Conversely, the Fe3+/∑Fe ratio of the partial melt increases with increasing melt fraction during decompression batch melting. The relative oxygen fugacity of the upper mantle depends on both the oxidation state of iron and mantle potential temperature. Results from incremental decompression melting calculations in which 1% melt is produced for each 100MPa of decompression and then removed from the residual solid indicate that relative oxygen fugacity calculated from the oxidation state of iron in basaltic glass does not represent a unique value for the oceanic upper mantle but, rather, reflects conditions in the lower portion of the melting regime. A 100°C change in mantle potential temperature produces a change in relative oxygen fugacity of ∼0.8logunits, similar to the global range inferred from mid-ocean ridge basalt glasses. It is necessary, therefore, to compare relative oxygen fugacity calculated from basaltic glass with proxies for potential temperature before drawing conclusions on heterogeneity of the oxidation state of iron in the oceanic upper mantle. Results from model calculations also suggest that the sub-arc mantle is intrinsically more oxidizing than the oceanic mantle because it is cooler. The global correlation between Fe3+/∑Fe and H2O in basaltic glasses may, in part, reflect the solidus-lowering influence of the latter. Secular variations in relative oxygen fugacity of the upper mantle due to cooling of the Earth over geologic time do not appear to be large enough to have influenced the nature of volcanic outgassing significantly enough to have contributed to the Great Oxidation Event.

Petrology, geochemistry, and geochronology of mafic rocks from the Taoxinghu Devonian ophiolite, LongmuCo–Shuanghu–Lancang suture zone, northern Tibet: evidence for an intra-oceanic arc–basin system

A newly discovered Devonian ophiolite located in the Taoxinghu area of central Qiangtang on the Qinhai–Tibet Plateau is described. The ophiolite consists of gabbro and diabasic dikes, and invasive cumulate gabbros-leucogabbros. The ophiolite has undergone greenschist facies metamorphism and minor deformation. Dating of the metagabbro by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb techniques yielded a weighted mean age of 367.2 ± 3.3 Ma (Late Devonian). Whole-rock geochemical analyses show that the rocks belong to the tholeiite series, with weak depletion in light rare-earth elements (LREEs), almost no Eu anomalies, weak enrichment in large-ion lithophile elements (LILEs), depletion in Nb and Ta, and weak negative Hf and Ti anomalies. These characteristics are similar to those of back-arc basin basalts. Together, these characteristics suggest that the rocks of the Devonian ophiolite formed by ~30% partial melting of spinel lherzolite, which was enriched by interaction with aqueous fluids during the late-generation phases; there is no evidence of subduction-related melting. The Devonian ophiolite rocks in the Taoxinghu area were first discovered in the LongmuCo–Shuanghu–Lancang suture zone. Detailed geochemical analyses show that the rocks formed in a back-arc ocean basin environment, indicating that the LongmuCo–Shuanghu–Lancang suture zone in central Qiangtang represents a Late Devonian intra-oceanic subduction zone in the Palaeo–Tethys Ocean. The discovery of the central Qiangtang Devonian ophiolite provides essential data for understanding the evolutionary history of the Palaeo–Tethys Ocean, and for identifying and understanding the roles of the different tectonic units on the Qinghai–Tibet Plateau.

Geochemical Characteristics of Volcanic Rocks from ODP Site 794, Yamato Basin: Implications for Deep Mantle Processes of the Japan Sea

AbstractDeep mantle processes and the dynamic mechanism of magmatism in the Japan Sea Basin are important processes that have not been studied in detail. In this paper, systematic evaluation of basalt samples from the ocean drilling program Site 794 in the Japan Sea was performed, which included petrography, whole‐rock major‐ and trace‐element analysis, Sr‐Nd‐Pb isotopic composition, and electron microprobe analysis of plagioclase and clinopyroxene. These basalts belong to the tholeiitic series with porphyritic texture and massive Ca‐rich plagioclase, clinopyroxene, and minor olivine phenocrysts. The basalts are characterized as flat rare earth elements and high‐field‐strength elements with remarkably low ratios of (La/Yb)N (0.75–2.51), significant positive anomalies of Ba, Sr, and Rb and no Eu anomaly (δEu = 0.99–1.36). The samples showed relatively high 87Sr/86Sr (0.70425–0.70522), 207Pb/204Pb (15.511–15.610), and 208Pb/204Pb (38.064–38.557) values and a low 143Nd/144Nd ratio (0.51271–0.51295). The basalts from Site 794 can be divided into upper, middle, and lower volcanic rocks (UVR, MVR, and LVR) on the basis of their stratigraphic level. The MVR was geochemically derived from the depleted mantle, whereas the UVR and LVR originated from a nondepleted and relatively enriched mantle source with contributions from subducted Pacific plate fluid and sediments. Use of geothermobarometers indicates that the crystallization pressure for the UVR and LVR (6.25–11.19 kbar) was significantly higher than that of the MVR (3.48–5.84 kbar). The UVR and LVR may have been derived from the low‐degree (5%–10%) partial melting of spinel lherzolite, while the MVR originated from a shallower mantle source with a high degree (10%–20%) of partial melting. In addition, the geochemical characteristics of the samples are consistent with a younger age (13–17 Ma) and the depleted composition of the MVR and an older age (17–23 Ma) and slightly enriched composition of the UVR and LVR. Therefore, temporal changes in the mantle source from old and enriched to young and depleted and subsequently to old and nondepleted may have been associated with progressive lithospheric extension and thinning, as well as at least two episodes of diverse asthenospheric upwelling and pull‐apart tectonic motion in the Yamato Basin.

Lithological units at the boundary zone between the Jining and Huai'an Complexes (central-northern margin of the North China Craton): A Paleoproterozoic tectonic mélange?

In the central-northern margin of the North China Craton, the tectonic boundary between the Jining and Huai'an Complexes is well exposed in the Gushan area. In this boundary zone, exotic blocks of orthopyroxene-bearing monzonitic gneiss, garnet-bearing mafic granulite, marble, and calc-silicate were identified in the meta-psammitic and meta-pelitic matrix. The likely nature of the rock units is a tectonic mélange, which is a key to understand the tectonic evolution of the craton.In this paper, geochronological and geochemical studies have been carried out on representative lithologies from the Gushan area to discuss their petrogenesis and geodynamic settings. The opx-bearing monzonitic gneisses (U–Pb zircon emplacement age of 2.17Ga) are high-K calc-alkaline to shoshonitic I-type rocks with trace-element magmatic arc signature. The trace elements, whole rock Sm–Nd and zircon Lu–Hf isotopes suggest that they were possibly derived from partial melting of Neoarchean lower crust (garnet amphibolite) in a continental arc. The protoliths of the garnet-bearing mafic granulites are inferred to have formed between 2.37 Ga and 1.95Ga, possibly at 2.2Ga. They show N-MORB geochemical affinity and were most likely produced by partial melting of spinel lherzolite in a mid-ocean ridge setting. Detrital zircons from one metapsammite sample (garnet-bearing quartzofeldspathic gneiss) yield nearly concordant 207Pb/206Pb ages ranging from 2.1Ga to 2.0Ga. In situ U–Pb and Hf isotopic data of zircons from the metapsammite sample support that the metasedimentary protoliths were probably derived from a mid-Paleoproterozoic continental arc. Three metamorphic age groups (1.95–1.93Ga, 1.86–1.83Ga, and 1.80Ga) are revealed by the opx-bearing monzonitic gneisses, garnet-bearing mafic granulites, garnet-bearing quartzofeldspathic gneiss, and sillimanite–garnet–K-feldspar gneiss. The predominant group (1.95–1.93Ga) is considered to record the age of near-peak granulite facies metamorphism. The 1.86–1.83Ga group may result from decompression or the thermal imprint of the collision between the Eastern and Western Blocks. The 1.80Ga group is consistent with the age (1.81Ga) of the ductile deformation of the garnet–K-feldspar mylonite, and is interpreted as the timing of a relatively rapid tectonic extension. Thus, the Paleoproterozoic Gushan mélange formed and evolved in a subduction–collision–exhumation process, and finally situated in a likely suture zone between the Eastern and Western blocks or sub-cratons.

Distribution of REE between clinopyroxene and basaltic melt along a mantle adiabat: effects of major element composition, water, and temperature

The distribution of rare earth elements (REE) between clinopyroxene (cpx) and basaltic melt is important in deciphering the processes of mantle melting. REE and Y partition coefficients from a given cpx-melt partitioning experiment can be quantitatively described by the lattice strain model. We analyzed published REE and Y partitioning data between cpx and basaltic melts using the nonlinear regression method and parameterized key partitioning parameters in the lattice strain model (D 0, r 0 and E) as functions of pressure, temperature, and compositions of cpx and melt. D 0 is found to positively correlate with Al in tetrahedral site (Al T ) and Mg in the M2 site (MgM2) of cpx and negatively correlate with temperature and water content in the melt. r 0 is negatively correlated with Al in M1 site (AlM1) and MgM2 in cpx. And E is positively correlated with r 0. During adiabatic melting of spinel lherzolite, temperature, Al T , and MgM2 in cpx all decrease systematically as a function of pressure or degree of melting. The competing effects between temperature and cpx composition result in very small variations in REE partition coefficients along a mantle adiabat. A higher potential temperature (1,400°C) gives rise to REE partition coefficients slightly lower than those at a lower potential temperature (1,300°C) because the temperature effect overwhelms the compositional effect. A set of constant REE partition coefficients therefore may be used to accurately model REE fractionation during partial melting of spinel lherzolite along a mantle adiabat. As cpx has low Al and Mg abundances at high temperature during melting in the garnet stability field, REE are more incompatible in cpx. Heavy REE depletion in the melt may imply deep melting of a hydrous garnet lherzolite. Water-dependent cpx partition coefficients need to be considered for modeling low-degree hydrous melting.

Volatile contents of mafic magmas from cinder cones in the Central Oregon High Cascades: Implications for magma formation and mantle conditions in a hot arc

The Cascades arc is a warm-slab subduction zone characterized by the slow (3.5 cm/a) subduction of young (< 10 Ma) oceanic crust. Elevated slab temperatures likely result in shallow dehydration beneath the forearc and reduced slab fluid addition to the mantle wedge beneath the volcanic front. Resulting calc-alkaline magmas should contain low volatile abundances relative to arcs with cooler subducting slabs. We test this hypothesis in Central Oregon, the region of the arc that has experienced the highest mafic output over the last 2 Ma. Naturally quenched basalt and basaltic andesite melt inclusions from tephra erupted from monogenetic vents record pre-eruptive magmatic volatile contents. After accounting for the effects of degassing, our data show that initial volatile contents from seven calc-alkaline cinder cones range from 1.7 to 3.6 wt.% H 2O, 1200–2100 ppm S and 500–1200 ppm Cl. Ratios reflecting volatiles recycled from the subducted slab (H 2O/Ce ~ 490–1700; S/Nb ~ 110–310; and Cl/Nb ~ 37–190) are elevated above MORB values but are low compared to those from the southern Cascades (Mount Shasta) and other arcs (Mexico, Central America, and Kamchatka). Positive correlations between H 2O, Pb/Ce and Ba/Zr suggest that small additions of a slab-derived component to the mantle wedge are responsible for observed volatile contents. Melt inclusion compositions are evolved (Fo 82–84 host olivines, 5.0–7.2 wt.% MgO) and fractionated from primitive parental magmas by ~ 16% crystallization of predominantly olivine before being trapped as inclusions. Calc-alkaline basaltic and Sr-rich basaltic melts can be generated by 9–11% partial melting of spinel lherzolite. H 2O-rich basaltic andesite melts are highly depleted in incompatible elements and are probably generated from a more depleted mantle source by 12–16% melting. Subarc mantle temperatures inferred from H 2O and trace elements are similar to or slightly warmer than temperatures in other arcs. Additionally, compositional proxies for slab surface temperatures in Central Oregon are at the high end of the global arc spectrum, corresponding to temperatures of 850–950 °C, and are consistent with a young, hot incoming plate.

Petrogenesis and Geological Implications of the Tianheyong Cenozoic Basalts, Inner Mongolia China

The Tianheyong basalts are basanites, which at least can be subdivided into three types of mineral assemblages. In general, the Tianheyong basalts are characteristics of low silica ( w(SiO 2)=43.97%–45.68%), enriched alkali ( w(K 2O+Na 2O) =5.91%–7.65%), enriched potassium ( w(K 2O)=2.04%–2.89%), high contents of titanium ( w(TiO 2)=2.18%–2.37%), high magnesium value (Mg #=68–76), and high contents of REE (ΣREE=(246.62–329.82)×10 −6). The chondrite normalized patterns of REE show right inclined smooth lines, extremely enriched in LREE, significantly fractionated from LREE and HREE, without obvious anomaly of Eu (δEu=0.90–1.02) and Ce (δCe=0.96–1.00), and the high enrichment of incompatible elements, especially, the peak value in high field strength elements (HFSE) of Nb, Ta, and Th indicate the OIB-like distribution patterns of multi-elements. The Tianheyong basalts are also high in compatible elements: Co ((39.1–48.9)×10 −6), Ni ((130–257)×10 −6), and Cr ((138–320)×10 −6). All of the above characteristics as well as the low degree of crystallization, the abundance in peridotite inclusions, and minor amount of xenocrysts, and the variation trend of elements indicate that the basalts were solidified from primitive basaltic magma. Trace element ratio of Ba/Rb (12–35) and variation of alkali metals might indicate the metasomatism of fluids in a source region of enriched lherzolites. Petrogenesis simulation indicates that the Tianheyong primitive basaltic magma might be the product of polybaric melting column with non-modal partial melting processes of enriched lherzolite, which straddle the boundary between garnet- and spinel-facies lherzolites. Magma might originate from low partial melting of source rocks(∼<5%), which have∼1% partial melting of garnet lherzolite and 2%–5% partial melting of spinel lherzolite. The integrative analysis indicates that the triggering mechanism of source rock partial melting might be the edge-driven convection of mantle, which results in that the Tianheyong basalts have the deepest origin depth as compared with those of eastern Jining and Hannuoba basalts.

Cenozoic volcanic rocks in the Belog Co area, Qiangtang, northern Tibet, China: Petrochemical evidence for partial melting of the mantle-crust transition zone

Neogene volcanic rocks in the Belog Co area, Qiangtang, northern Tibet, are represented by a typical intermediate-basic and intermediate alkaline rock association, with latite-trachyte as the main rock type. The results of chemical analysis are: SiO2=52%–62%, Al2O3>15%, Na2O/K2O>1 and MgO<3.30%. In addition, the volcanic rocks are LREE-enriched with LREE/HREE=10–13, (La/Yb)N=15–19, and show a weak negative Eu anomaly with δEu=0.71–0.89. The close relationship between Mg# and SiO2 and the co-variation of the magmatophile elements and ultra-magmatophile elements such as La/Sm-La and Cr-Tb indicate that this association of volcanic rocks is the product of comagmatic fractional crystallization. The rock association type and lower Sm/Yb values (Sm/Yb=3.23–3.97) imply that this association of volcanic rocks should have originated from partial melting of spinel lherzolite in the lithospheric mantle. On the other hand, the weak negative Eu anomaly and relative depletion in Nb, Ta and Ti reflect the features of terrigenous magma. So the Neogene Belog Co alkaline volcanic rocks should be the result of partial melting of the special crust-mantle transition zone on the Qinghai-Tibet Plateau.