Melting Of Garnet Lherzolite Research Articles

Unveiling the formation and magma transport dynamics of the Ulanqab maar volcanic cluster in the western North China Craton: Insights from 40Ar-39Ar geochronology, mineral chemistry, and Sr-Nd-Mg isotopes

The chemical composition of eastern North China Craton (NCC)'s alkaline basaltes likely influenced by subducting Pacific Plate. Nonetheless, the influence of these processes on the western craton remains uncertain due to the lack of geochemical evidence. The recent discovery of Ulanqab maar volcanic cluster (UMVC) in the western NCC has become significant research windows. However, their petrogenesis and magmatic processes remain poorly understood. In the present study, 40Ar/39Ar, whole rock and mineral geochemistry, and Sr–Nd–Mg isotope data for the UMVC in the western NCC are reported. These data reveal that the UMVC was formed during the Neogene (7.60 ± 0.04 Ma), diverging from Quaternary basalts or as part of the Hannuoba basalts (eastern Ulanqab). The UMVC rocks, featuring typical oceanic island basalts traits with moderate (87Sr/86Sr)i values (0.70487–0.70524) and εNd(t) values from −4.95 to + 0.82, likely originated from an enriched EMI-type mantle source during the mid-Proterozoic. This involved mixing melts from 30 to 50 % partial melting of garnet lherzolite and 5–15 % partial melting of garnet pyroxenite in a deep magma chamber. Crystallization of clinopyroxene and garnet in this chamber created high-Ti alkaline lavas, with limited presence in erupted lavas due to sluggish magma ascent in the profound lithospheric mantle. In shallower lithospheric mantle regions, interactions between alkaline magma and orthopyroxenite improved transport kinetics, enabling clinopyroxene and olivine crystallization under lower pressure. These magmas integrated mafic crystal mushes from accumulation chambers. The study also identified a low Mg isotope composition (δ26Mg = − 0.56 ‰ to − 0.42 ‰) in samples, suggesting a hybrid source influenced by early decarbonation in Precambrian subduction zones. This finding indicates the incorporation of Mg-rich carbonate minerals from marine sediments into the magma source, contributing to the observed variations in eastern China's alkaline basalts.

Source Lithology and Magmatic Processes Recorded in the Mineral of Basalts from the Parece Vela Basin

AbstractSince the Early Cenozoic, the Philippine Sea Plate (PSP) has undergone a complex tectonic evolution. During this period the Parece Vela Basin (PVB) was formed by seafloor spreading in the back‐arc region of the proto‐Izu‐Bonin‐Mariana (IBM) arc. However, until now, studies of the geological, geophysical, and tectonic evolution of the PVB have been rare. In this study, we obtained in situ trace element and major element compositions of minerals in basalts collected from two sites in the southern part of the PVB. The results reveal that the basalts from site CJ09‐63 were likely formed via ∼10% partial melting of spinel‐garnet lherzolite, while the basalts from site CJ09‐64 were likely formed via 15%–25% partial melting of garnet lherzolite. The order of mineral crystallization for the basalts from site CJ09‐64 was olivine, spinel, clinopyroxene, and plagioclase, while the plagioclase in the basalts from site CJ09‐63 crystallized earlier than the clinopyroxene. Using a plagioclase‐liquid hygrometer and an olivine‐liquid oxybarometer, we determined that the basalts in this study have high H2O contents and oxygen fugacities, suggesting that the magma source of the Parece Vela basalts was affected by subduction components, which is consistent with the trace element composition of whole rock.

Geochemical constraints of lamprophyres: Clues of Mesozoic tectonic interaction between the South China Block and North China Craton in Northern Jiangsu Province

Early Cretaceous lamprophyre dykes widely occurred in the southeastern margin of the North China Craton (NCC), which are the important probes to understand the enrichment and depletion processes that modified the subcontinental lithospheric mantle of the NCC. In this research, one lamprophyre site (Liguo), which is the nearest lamprophyre to the South China Block (SCB) ever discovered in the NCC, had been used to analyze the influence of the deeply subducted SCB on the lithospheric evolution of the southeastern NCC. This lamprophyre was investigated by apatite U Pb dating with the age of 136 Ma, and it is depleted in HFSE, enriched in LILE and Sr–Nd–Pb isotope compositions ( 87 Sr/ 86 Sr (i) = 0.708038 to 0.709305, ε Nd (t) = −8.0 to −9.8, 206 Pb/ 204 Pb (t) ratios of 17.477 to 17.536, 207 Pb/ 204 Pb (t) of 15.548 to 15.552, and 208 Pb/ 204 Pb (t) of 37.944 to 38.004). Interpretations of minerals and whole rock elemental data suggest the Liguo lamprophyre was derived from 1 to 5% partial melting of garnet lherzolite; and MELTS modeling indicates fractional crystallization of ~26.5% clinopyroxene, ~0.1% spinel and ~ 3.5% garnet under 2.5 Gbar. Together with the Pb isotopic compositions of the contemporaneous lamprophyres in the Jiaodong Peninsula, it suggested that the mantle source of the Liguo lamprophyre was mainly modified by melts derived from partial melting of the subducted South China continental crust in the Triassic. In contrast, the magma sources for the lamprophyres in Jiaodong Peninsula were mainly modified by hydrous fluids derived from the subducted Paleo–Pacific oceanic crust in the Jurassic. Finally, it is concluded that the lithospheric mantle in the southeastern margin of the NCC is heterogeneous, and the subducted SCB could be the source of the enriched component in the southern margin of the NCC. • Liguo lamprophyre recorded the Mesozoic tectonic interaction between the SCB and NCC. • The lithospheric mantle in southeastern NCC is heterogeneous. • Subducted SCB is the source of the enriched component in the southern NCC.

A plume - mantle interaction model for the petrogenesis of komatiite - komatiitic basalt - basalt - basaltic andesite volcanism from the Paleoarchean (3.57–3.31 Ga) Iron Ore Group greenstone belts, Singhbhum craton, India: Constraints from trace element geochemistry and Sm[sbnd]Nd geochronology

The Paleoarchean Singhbhum craton greenstone belts comprise basalt, interbedded minor komatiite (both alumina depleted and undepleted, ADK and AUK, respectively), komatiitc basalt, dacite and chert grouped as (~3.57 Ga) southern Iron Ore Group (S-IOG, Daitari samples) and ~ 3.31 Ga eastern Iron Ore Group (E-IOG, Dabanki samples). Considering the importance of metabasalt in formation of TTG batholiths, modeling their petrogenesis from mantle sources and SmNd geochronology was conducted. The rocks resemble Archean non-arc greenstone basalt (Nb/Th = 3.61–10.3) with tholeiitic affinity. The ADK of S-IOG was modeled by 37% partial melting of garnet lherzolite, komatiitic basalt was derived from ADK magma by ~25% assimilation fractional crystallization (AFC) and ~ 16% AFC of this magma formed tholeiitic basalt. Derived from depleted parental magmas they yield an isochron age of 3573 ± 63 Ma (2SD), Ndi = 0.50810 ± 0.00008, MSWD = 0.59 (n = 10) and εNd (3573 Ma) = +2.03 ± 0.03. Dacite formation, modeled by mixing of melts between Eoarchean felsic crust and metabasalt, yielded an isochron age of 3507 ± 180 Ma (2SD), Ndi = 0.50800 ± 0.00015, MSWD = 0.15 (n = 11), εNd (3507 Ma) = −1.46 ± 1.71; ca. 3.8–3.9 Ga Nd TDM model ages indicate vestiges of Eoarchean crust. The AUK of E-IOG was modeled by ~45% partial melting and ADK by ~33% melting of garnet lherzolite mantle. About 32% AFC of primary ADK magma formed the komatiitic basalt that evolved to tholeiitc basalt magma (~24%) which on ~15% AFC evolved to basaltic andesite - andesite magma. Derived from depleted parental magmas, the E-IOG rocks yielded an isochron age of 3313 ± 110 Ma (2SD), Ndi = 0.50847 ± 0.00013, MSWD = 0.06 (n = 10), εNd (3313 Ma) = +2.62 ± 0.28. Petrogenesis of Paleoarchean greenstone non-arc basaltic volcanism and associated komatiite can be best explained by plume-mantle interaction and partial melting of ambient mantle followed by assimilation fractional crystallization of parental magmas, and does not preclude recycling of pre-existing crust.

Geochemistry of mid- upper eocene intra-continental alkaline volcanic range, south part of central alborz mountains, north of Iran.

The Ziaran volcanic Belt (ZVB), North of Iran contains a number of intra-continental alkaline volcanic range situated on South part of central Alborz Mountains, formed along the localized extensional basins developed in relation with the compressional regime of Eocene. The mid-upper Eocene volcanic suite comprises the extracted melt products of adiabatic decompression melting of the mantle that are represented by small volume intra-continental plate volcanic rocks of alkaline volcanism and their evaluated Rocks with compositions representative of mantle-derived, primary (or near-primary) melts. Trace element patterns with significant enrichment in LILE, HFSE and REEs, relative to Primitive Mantle. Chondrite-normalized of rare earth elements and enrichment in incompatible elements and their element ratios (e. g. LREE/HREE, MREE/HREE, LREE/MREE) shown these element modelling indicates that the magmas were generated by comparably variable degrees of partial melting of garnet lherzolite and a heterogeneous asthenospheric, OIB mantle sources.

Zircon U–Pb Geochronology, Geochemistry and Geological Significance of the Anisian Alkaline Basalts in Gejiu District, Yunnan Province

The Gejiu Anisian alkaline basalts (GAAB), distributed in the southern part of the Emeishan large igneous province (ELIP), are crucial to understand the tectonomagmatic activity during the Triassic. Geochronological, geochemical, and Sr-Nd-Pb isotopic analyses were systematically applied to explore the origin, petrogenesis, and tectonic setting of the GAAB, and how they relate to the ELIP. Zircon U-Pb dating set the eruption date at 244 Ma. Most of the samples belonged to alkaline basalts and had high TiO2 (2.14–3.23 wt.%) and MgO (4.43–19.58 wt.%) contents. Large ion lithophile elements (LILEs) were enriched relative to high field strength elements (HFSEs). The rare earth elements (REEs) and trace element signatures in the normalized diagrams were similar to oceanic island basalts (OIB) and Emeishan high-Ti basalts. These samples had consistent Sr-Nd isotope compositions: the initial 87Sr/86Sr values ranged from 0.7044 to 0.7048 and εNd(t) = 3.25–4.92. The Pb isotopes were more complex, the (206Pb/204Pb)t, (207Pb/204Pb)t, (208Pb/204Pb)t ratios were 17.493–18.197, 15.530–15.722, and 37.713–38.853, respectively. Our results indicate that the GAAB originated from the deeper enriched mantle with 5% to 15% partial melting of garnet lherzolite and a segregation depth of 2 to 4 GPa (60–120 km). During the formation of the GAAB, clinopyroxene and Ti-Fe oxides were fractionally crystallized with insignificant crustal contamination. The GAAB were formed in a extensional regime that was related to the Gejiu-Napo rift event in the Triassic.

Petrogenesis of Neogene polymagmatic suites at a monogenetic low-volume volcanic province, Bahariya depression, Western Desert, Egypt

Monogenetic volcanoes can yield eruptive suites displaying substantial complexity in compositional characteristics. The Bahariya monogenetic volcanoes (BMV) in the Western Desert, Egypt are a good example. They comprise complex stratigraphic deposits involving scoria cone, lava flows, and subvolcanic sills and dikes related to diverse eruptive styles. Whole-rock and mineral chemistry and 40Ar–39Ar geochronology are used here to document the petrogenesis, source characteristics, and evolution of the Bahariya volcanoes. The architecture of the BMV is the product of two alkali magma batches: pyroclastics and lava flows forming explosive scoria cone (batch 1) and subvolcanic sills (batch 2). The two batches show contrast in the concentrations of incompatible trace elements and REE as well as element ratios such as Nb/Yb, Gd/Yb, Nb/U, and Ce/Pb (36, 5.0, 44, 30 vs. 17, 4.0, 39, 24 for batch 1 and 2, respectively). New whole-rock 40Ar/39Ar dating displays consistent age of 23.71 ± 0.06 and 23.73 ± 0.01 Ma for magma emplacement of batch 2. Batches 1 and 2 share common LILE and LREE enrichments and HFSE depletions, analogous to a HIMU-like, mantle-derived OIB source described from other Paleogene–Oligocene intraplate magmatic provinces in North Africa and worldwide. Trace element modeling proposes a derivation of the Bahariya volcanoes from parental melts generated by 8–12% partial melting of garnet lherzolite and amphibole-bearing garnet lherzolite at 2.18 ± 0.33 and 1.77 ± 0.33 GPa for batch 1 and batch 2, respectively, across the lithosphere–asthenosphere boundary at c. 70–90 km depth (2.14–2.76 GPa). These sources had been earlier metasomatized by volatile-, LILE- and HFSE-rich fluid(s) originating from Neoproterozoic subduction or a Phanerozoic plume. Fractional crystallization involved olivine + clinopyroxene in both batches followed by Fe–Ti oxides + apatite in batch 2. Furthermore, crust contamination/assimilation was an irrelevant process at crustal level during magma ascend to the surface. Data results of the geo-barometric computations disclose two magma storage levels involving intermediate to lower crustal levels at c. 35 km (1.05 GPa) for batch 1 and mid-crustal level at c. 25 km depth (0.75 GPa) for batch 2. This study delivers proof that magmas emitted at Bahariya depression can undergo complex polymagmatic processes during their storage and passage in the crust, mainly due to the existence of a multilevel plumbing system. The origin of the BMV, as with other within-plate volcanoes in North Egypt, appears to be allied to extension-induced asthenosphere upwelling activated by limited exclusion of thickened lithospheric root under a passive rift tectonic regime coupled with the development of lithospheric thinning and continental breakup in North Africa.

The transformation of the lithospheric mantle beneath South China Block (SCB): constraints from petrological and geochemical studies of Daoxian and Ningyuan basalts and their melt inclusions

ABSTRACTThe lithospheric mantle beneath the South China Block (SCB) underwent a dramatic transformation from depleted to enriched during late Mesozoic. With a view to deeply understand this process, here we investigate the Mesozoic basalts and their melt inclusions from the Daoxian and Ningyuan regions within the central SCB. The geochemical features of the melt inclusions in these basalts suggest that these rocks originated from the lithospheric mantle enriched through interaction with K-rich aqueous fluids released from subducted Palaeo-Pacific oceanic sediments, whereas the Ningyuan basalts were mainly derived from the asthenospheric mantle source. Geochemical modelling indicates that the Daoxian basalts were generated from 15%-25% of partial melting of garnet lherzolite, whereas the Ningyuan basalts originated from 10%-20% of partial melting of garnet pyroxenites. Our data, combined with those from other Jurassic basalts suggest a temporal evolution of the SCB mantle sources during the Late Mesozoic. Diverse crust–mantle interactions through mixing of the asthenospheric melts with variable proportions of subducted Palaeo-Pacific oceanic sediments might account for the spatial heterogeneity of mantle sources observed beneath the SCB. The transition from Tethyan tectonic realm to the Palaeo-Pacific tectonic regime might have played a significant role in the transformation of the lithospheric mantle beneath the SCB.

Partial melting of garnet lherzolite with water and carbon dioxide at 3 GPa using a new melt extraction technique: implications for intraplate magmatism

The origin and source rocks of alkali-rich and SiO2-undersatured magmas in the Earth’s upper mantle are still under debate. The garnet signature in rare earth element patterns of such magmas suggests a garnet-bearing source rock, which could be garnet lherzolite or garnet pyroxenite. Partial melting experiments were performed at 2.8 GPa and 1345–1445 °C in a piston-cylinder using mixtures of natural lherzolite with either 0.4 wt% H2O and 0.4 wt% CO2 or 0.7 wt% H2O and 0.7 wt% CO2. Different designs of AuPd capsules were used for melt extraction. The most successful design included a pentagonally shaped disc placed in the top part of the capsule for sufficient melt extraction. The degrees of partial melting range from 0.2 to 0.04 and decrease with decreasing temperature and volatile content. All samples contain olivine and orthopyroxene. The amounts of garnet and clinopyroxene decrease with increasing degree of partial melting until both minerals disappear from the residue. Depending on the capsule design, the melts quenched to a mixture of quench crystals and residual glass or to glass, allowing measurement of the volatile concentrations by Raman spectroscopy. The compositions of the partial melts range from basalts through picrobasalts to foidites. Compared to literature data for melting of dry lherzolites, the presence of H2O and CO2 reduces the SiO2 concentration and increases the MgO concentration of partial melts, but it has no observable effect on the enrichment of Na2O in the partial melts. The partial melts have compositions similar to natural melilitites from intraplate settings, which shows that SiO2-undersaturated intraplate magmas can be generated by melting of garnet lherzolite in the Earth’s upper mantle in the presence of H2O and CO2.

The basaltic volcanism of the Dumisseau Formation in the Sierra de Bahoruco, SW Dominican Republic: A record of the mantle plume-related magmatism of the Caribbean Large Igneous Province

The basaltic volcanism of the Dumisseau Fm in the Sierra de Bahoruco, SW Dominican Republic, offers the opportunity to study, on land, the volcanism of the Caribbean Large Igneous Province (CLIP). It consists of an at least 1.5km-thick sequence of submarine basaltic flows and pyroclastic deposits, intruded by doleritic dykes and sills. Three geochemical groups have been identified: low-Ti tholeiites (group I); high-Ti transitional basalts (group II); and high-Ti and LREE-enriched alkaline basalts (group III). These geochemical signatures indicate a plume source for all groups of basalts, which are compositionally similar to the volcanic rocks that make up various CLIP fragments in the northern region of the Caribbean Plate. Trace element modelling indicates that group I magmas are products of 8–20% melting of spinel lherzolite, group II magmas result 4–10% melting of a mixture of spinel and garnet lherzolite, and group III basalts are derived by low degrees (0.05–4%) of melting of garnet lherzolite. Dynamic melting models suggest that basalts represent aggregate melts produced by progressive decompression melting in a mantle plume. There is no compositional evidence for the involvement of a Caribbean supra-subduction zone mantle or crust in the generation of the basalts. Two 40Ar/39Ar whole-rock ages reflect the crystallisation of group II magmas at least in the late Campanian (~74Ma) and the lower Eocene (~53Ma). All data suggest that the Dumisseau Fm is an emerged fragment of the CLIP, which continues southward through the Beata Ridge

Petrogenesis and tectonic implications of early Carboniferous alkaline volcanic rocks in Karamay region of West Junggar, Northwest China

ABSTRACTZircon U–Pb geochronological and geochemical analyses are reported for a suite of the early Carboniferous volcanic rocks from West Junggar (Northwest China), southern Central Asian Orogenic Belt (CAOB), with the aim to investigate the sources, petrogenesis, and tectonic implications. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb analysis from an andesite yielded concordant weighted mean 206Pb/238U age of 345 ± 3 Ma, indicating the presence of early Carboniferous volcanic rocks in West Junggar. The early Carboniferous volcanic rocks consist of basalt, basaltic andesite, and andesite. Geochemically, all the samples bear the signature of ocean island basalt (OIB), and are characterized by alkaline affinity with minor variations in SiO2 compositions (45.13–53.05 wt.%), high concentrations of Na2O + K2O (5.08–8.89 wt.%) and TiO2 (1.71–3.35 wt.%), and LREE enrichment and HREE depletion ((La/Yb)N = 7.1–12.4), with weak Eu anomalies (Eu/Eu* = 0.9–1.1) and no obvious Nb, Ta, and Ti negative anomalies. These features suggest that the early Carboniferous volcanic rocks were derived from an OIB-related source that consists of oceanic lithosphere with ~1–3% degree partial melting of garnet lherzolite. From these observations, in combination with previous work, we conclude that the early Carboniferous alkaline volcanic rocks in Karamay region formed by upwelling of asthenospheric mantle through a slab window in a forearc setting during consumption of the West Junggar Ocean. Meanwhile, seamounts, which formed in the Late Devonian and were accreted and subducted in Karamay arc, also brought geological effects in the subduction zone.

Mid-Neoproterozoic Tadong amphibolites at the junction of the East Kunlun and Western Qinling Orogens – a record of continental rifting during the break-up of Rodinia

ABSTRACTMid-Neoproterozoic rift-related Tadong amphibolites (TDA) are situated in the junction of the East Kunlun Orogen (EKO) and the West Qinling Orogen (WQO) in western China, and have important significance in the break-up of the Supercontinent Rodinia. The mineral assemblage of the TDA exhibits amphibolite facies metamorphism, and the protolith is basic volcanic rocks. Here, we present a detailed laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb age and geochemical data of TDA. LA-ICP-MS U–Pb zircon age determination indicates that the basic volcanics formed at ca. 743–711 Ma. Geochemically, TDAs are characterized by low SiO2 (46.06–50.18%), K2O (0.66–1.86%), and Mg# (40–45) and high TiO2 (1.9–3.5%), and are attributed to tholeiite basalts. Their chondrite-normalized REE patterns are characterized by moderate enrichments in LREEs (LREE/HREE = 4.98–5.51) with insignificant Eu anomalies (δEu = 0.99–1.12). The primitive mantle-normalized trace element diagrams are characterized by pronounced enrichment of high-field-strength elements (HFSEs, e.g. REE, Nb, Ta, Zr, Hf) with slightly positive anomalies of Ti, which is different from the subduction zone basalts and normal mid-ocean ridge basalts (NMORBs), but similar to the ocean-island basalts (OIBs) and Emeishan continental flood basalts (ECFBs), resembling the features of an OIB-like mantle source. Most samples exhibit high Nb/Ta (16.6–17.4), Nb/La (0.70–0.94), and positive ΔNb (ΔNb = 1.74 + log (Nb/Y) − 1.92*log (Zr/Y)) values. These geochemical features suggest that the Tadong basaltic rocks were most likely derived from an OIB-like mantle source and were the products of a lower degree partial melting of garnet lherzolite. Along with the tectonic discrimination diagrams, it is further inferred that these rocks were formed in a continental rifting setting. These results from our research confirm the existence of continental rifting in the junction of the EKO and the WQO in the mid-late Neoproterozoic, implying that the rifting is a response to the break-up of the Supercontinent Rodinia.

Alkaline basalts in the Karamay ophiolitic mélange, NW China: A geological, geochemical and geochronological study and implications for geodynamic setting

Alkaline basalts are common constituents in many ophiolite belts and accretionary complexes. Here we report geological, geochronological, geochemical, and whole-rock Sr–Nd isotopic data for alkaline basalts from the Karamay ophiolitic mélange of West Junggar, southern Central Asian Orogenic Belt (CAOB). In the ophiolitic mélange, imbricate thrusts, duplexes, “web” structures, pinch-and-swell structures, tilted structures in pillow metabasalts, and shear band cleavages are widely developed. U–Pb analyses of zircon grains from a representative basalt by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yielded crystallization age of ca. 395±3Ma, suggesting formation during the Middle Devonian. Geochemically, all the samples bear the signature of ocean island basalt (OIB), and are characterized by alkaline affinity with high concentrations of Na2O+K2O (4.22–7.28wt.%) and TiO2 (1.72–2.89wt.%), LILE and LREE enrichment and HREE depletion ((La/Yb)N=8.4–15.6), with very strong or no Eu anomalies (Eu/Eu*=0.6–1.1), and no obvious Nb, Ta or Ti negative anomalies. The rocks display consistent Sr–Nd isotopic compositions (initial 87Sr/86Sr ratios=0.70326–0.70453, εNd(t)=+3.1–+7.6). The results suggest that all the alkaline basalts were derived from a mantle plume-related source in an intra-oceanic setting with ca. 1–3% degree partial melting of garnet lherzolite. These observations, in combination with previous work, indicate that the alkaline basalts from Karamay ophiolitic mélange can be correlated to a Middle Devonian mantle plume-related magmatism within Paleo-Asian Ocean. We further suggest that the plume-related activity in the CAOB was generally continuous during the development of the Paleo-Asian Ocean.

Petrogenetic evolution of basaltic lavas from Balhaf–Bir Ali Plio-Quaternary volcanic field, Arabian Sea, Republic of Yemen

The Plio-Quaternary Balhaf–Bir Ali volcanic field (BBAVF) constitutes one of the largest volcanic fields in SE Yemen, covering some 500 km2. It comprises cinder cones complexes associated with vesicular lava flows and scoria–spatter cones. In many places, ultramafic xenoliths are encountered within these volcanics. The explosive volcanism is mainly of alkaline character including alkali olivine basalt, hawaiite and mugearite together with subordinate tuffaceous trachytes. Major, trace and REEs data from the basaltic rocks of the BBAVF are interpreted in terms of a mantle-lithospheric origin in which crustal contribution during the initial stage of rift magmatism has occurred. Magma genesis may have resulted from plume-derived melt introducing into the base of the lithosphere. A mantle plume source is proposed for the Balhaf–Bir Ali basaltic lavas that are here interpreted as having been generated by partial melting of garnet lherzolite in the uppermost part of asthenosphere. The magmatic evolution of Balhaf–Bir Ali volcanic field may be accounted for by the recent models developed for plume structure.

Geochemical Constrains on Petrogenesis and Tectonic Setting of Volcanic Rocks from the Baimianxia and Sanwan Formations in the Northern Margin of the Yangtze Plate

Abstract:On the basis of petrogeochemical data, the volcanic lavas of the Baimianxia Formation can be classified into two units: high TiO2 and low TiO2. The TiO2 concentration of the former is generally higher than 1%, which occurs in the lower part with high‐grade metamorphism, but the latter is less than 1% and crops out in the upper part with low‐grade metamorphism. The high‐TiO2 unit is dominated by tholeiitic lavas showing high rare earth element (REE) contents (ΣREE = 83.4–180.8 μg/g), high light/heavy REE (LREE/HREE) ratios (LREE/HREE=2.17–5.85) and weak negative Eu anomaly (Eu=0.79–1.01). Its trace element patterns display weak Nb‐Ta anomalies with respect to Th, K, La, Ce, showing within‐plate basalt affinities. In contrast, the low‐TiO2 unit is characterized by low REE contents, low LREE/HREE ratios, and pronounced Nb‐Ta anomalies, indicating typical arc or continental arc signature. Chondrite‐normalized REE patterns of basalts and andesites from the Sanwan Formation are flat or LREE depletion, which is very similar to normal mid‐oceanic basalt. Therefore, we suggest that these lavas should be formed in a back‐arc basin setting. Sr‐Nd isotopic data of the basalt in the lower part suggest that the rocks would have been formed in ∼1144 Ma. Based on the geochemical and isotopic features of the basalts, we suggest that these rocks in the low part of the Baimianxia Formation should originate from an asthenospheric oceanic‐island basalt‐like mantle source, which may be produced by partial melting of garnet lherzolite, and significantly underwent fractional crystallization and crustal or lithospheric mantle contamination en route to the surface. However, laser ablation inductively coupled plasma mass spectrometry zircon U‐Pb dating of the basalt sample from the upper part of the Baimianxia Formation gives a 437 Ma, indicating a Early Paleozoic age. The geochemical analysis in this paper suggests that they may originate from an arc or continental arc in response to aqueous fluids or melt expelled from a subducting slab, and the partial melting occurred in the garnet stability field. The samples of basalts and andesites in the Sanwan Formation show they are derived from depleted mantle source similar to normal mid‐oceanic basalt. Finally, we can conclude that the lavas in the lower part of the Baimianxia Formation represent the geological records of rift‐related volcanism in the middle Proterozoic, which is commonly considered to be the precursor of continental breakup and followed by oceanic basin forming from Neoproterozoic to early Paleozoic. Whereas, the lavas in upper part of the Baimianxia Formation and Sanwan Formations may have been generated by the oceanic and continental conversion that occurred in the early Paleozoic.

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.

Geochemical constraints on the late-stage evolution of basaltic magma as revealed by composite dikes within the Kangâmiut dike swarm, West Greenland

The Kangâmiut dike swarm in West Greenland contains numerous composite dikes with mafic margins and andesitic centers. Internal chilled margins show that the andesitic centers intruded into the middle of the mafic dikes. Major element systematics indicate that the fractionation of olivine, clinopyroxene, plagioclase and Fe–Ti oxides drove the evolution of the Kangâmiut parental magma during its transition from mafic to andesitic compositions. Incompatible trace elements show a marked relative decrease in middle and heavy rare-earth elements (REE) between the mafic margins and the andesitic centers. The decrease in the REE is not explicable by olivine, clinopyroxene, plagioclase and Fe–Ti oxide fractionation or by the fractionation of the accessory phases apatite, zircon or garnet. Rb–Sr and Sm–Nd isotopes from margin and center pairs from these composite dikes are nearly identical indicating that crustal contamination had little to no affect on their evolution. Trace element modeling utilizing the mixing of evolved Kangâmiut magmas and low degree melts derived from partial melting of garnet lherzolite produce excellent fits with the trace element patterns for the andesitic centers. These models suggest that the late-stage evolution of the Kangâmiut dikes included input of mantle melts produced during the end stages of rifting.

Lithospheric control on late Cenozoic magmatism at the boundary of the Tuva-Mongolian Massif, Khubsugul area, northern Mongolia

Late Cenozoic lavas from the western wall of the Khubsugul rift trough were erupted within the Tuva-Mongolian Massif with a pre-Vendian basement, and the lavas in the eastern wall of the trough were erupted within Early Caledonian terranes. The composition of the lavas was determined to vary across the strike of the boundary of the Tuva-Mongolian Massif. The western wall of the trough is dominated by hawaiites and contains subordinate volumes of basanites and much lower amounts of olivine tholeiites and basaltic tra- chyandesites. The eastern wall contains, in addition to hawaiites, widespread olivine tholeiites and basaltic andesites with subordinate amounts of basaltic trachyandesites. The boundary zone contains practically all rock types (except basaltic andesites) in roughly equal proportions. The trace-element simulations of the partial melt- ing processes demonstrates that the basaltic magmas were produced mainly by 0.5-5% partial melting of garnet lherzolite, with the probable mixing with partial melts derived from spinel lherzolite. The main factor control- ling the compositional variations of the lavas was likely the variable depths of their derivation due to variations in the lithosphere thickness at the boundary of the Tuva-Mongolian Massif. Based on the assumption that the source of the magmas was relatively homogeneous and on the results of simulations with the use of experimen- tal data on peridotite melting, we concluded that the asthenospheric sources of the basaltic magmas occurred at depths of 75 ± 10 km ( 24.6 ± 3.2 kbar) beneath the Tuva-Mongolian Massif and at 60 ± 12 km (20.1 ± 3.8 kbar) beneath the Early Caledonian terranes.

Systematics of U-series nuclides in primitive lavas from the 1730–36 eruption on Lanzarote, Canary Islands, and implications for the role of garnet pyroxenites during oceanic basalt formations

Inferences on mantle melting and related parameters are often derived from U-series results on oceanic basalts. However, most of these basalts come from magma chambers in which magma mixing amongst other processes is likely to have reduced the true compositional spread created during partial melting. Here are presented U–Th–Ra disequilibrium results on historical lavas of near-primary composition from Lanzarote, Canary Islands, where magma chambers have not been detected. During the 1730–36 fissure eruption, the magma composition varied with time from basanites through alkali basalts to tholeiites. Approximately 3–5 km 3 of primitive and often ultramafic xenolith-bearing lavas were produced. Abundances of Ba, Th and U generally decreased towards the end of the eruption, and show more than four-fold variations (e.g. Th = 5.75–1.55 ppm). Ba/Th and Ba/U vary by more than 75%, whereas Th/U vary by less than 10%. Excesses of 230Th over 238U range from 18% to 33% and generally increase with alkalinity. ( 230Th/ 232Th) decreased significantly from 1.02 to 0.94 with time and is positively correlated with the alkalinity of the lavas. Isotope ratios of Sr and ( 226Ra/ 230Th) also correlate with SiO 2. ( 226Ra/ 230Th) increased with time from 1.10 to 1.56. The correlations between ( 230Th/ 232Th), 87Sr/ 86Sr and SiO 2 suggest that the lava compositions represent nearly unmodified primary melts from a lithologically and compositionally heterogeneous mantle source. Decreasing ( 230Th/ 238U) and increasing ( 226Ra/ 230Th) in basanites to tholeiites can be explained by partial melting of a mixed garnet pyroxenite–lherzolite source, during which the proportion of pyroxenite and extent of melting diminished. Alternatively, the large variations of excess 226Ra and Ba/Th may reflect fluid addition to such a heterogeneous mantle source. In order to conserve the systematic variations of ( 226Ra/ 230Th) with other geochemical variables, transfer time through the lithosphere is likely to have been short relative to the half-life of 226Ra. A magma velocity through the lithosphere on the order of 10 −5 m/s (0.5 km/year) is compatible with the data. Both the Th–U disequilibria and the Th-isotope ratios of the Lanzarote lavas appear to be related to the mantle source lithology and composition, where garnet pyroxenite melting produced the early alkaline basalts and increasing melting of garnet lherzolite produced the tholeiites. Furthermore, correlations between ( 230Th/ 238U) and ( 232Th/ 238U) for different volcanic provinces worldwide, strongly suggest that Th–U disequilibria in oceanic basalts are principally caused by partial melting of garnet pyroxenites.

Energy barriers to viscous flow and the prediction of glass transition temperatures of molten silicates

The results of multi-anvil melting experiments are reported for a range of compositions in the system CaO-MgO-Al2O3-SiO2. The liquidus crystallization fields for forsterite, or­thopyroxene. clinopyroxene. and garnet have been mapped out at 10 GPa. as have their intersections at various cotectics. The composition of the liquid that is multiply saturated in forsterite. orthopyroxene, clinopyroxene. and garnet has been also determined to within ±0.5-1.0 wt% (2σ). and the result is in excellent agreement with a previous estimate (Herzberg 1992). These experiments confirm that the effect of pressure is to reduce Al2O3 and increase MgO and SiO2 in magmas formed by the melting of garnet lherzolite with increasing pressure (Herzberg 1992). Melting experiments in the system MgO-SiO2 also have been performed to constrain how pressure affects the compositions of liquids that are saturated in harzbiugite [L + Ol + Opx]. Experiments in both CaO-MgO-Al2O3-SiO2 and MgO-SiO2 demonstrate that there is a maximum normative olivine content to liquids formed by initial or advanced melting of peridotite in the upper mantle, and this occurs at 7 to 8 GPa. For most peridotites that undergo decompression melting in a plume, clino­pyroxene and garnet are the first crystalline phases to melt out and. with a few important but rare exceptions, the experimentally constrained liquids are unlike most volcanic rocks. Advanced anhydrous melting will yield liquids with a residual harzburgite mineralogy [L + Ol + Opx]. and these liquids are similar in composition to most komatiites with Cre­taceous and 2700 Myr Archaean ages.