High Degrees Of Partial Melting Research Articles

Arc building and maturation of the Lombok Island, East Sunda Arc

Intra-oceanic subduction zones are major sites of crust–mantle material exchange and crustal growth; however, the processes responsible for the magmatic evolution and transformation of nascent arcs into mature oceanic arcs with thicker crust are debated. We present mineral chemistry, zircon UPb age and Hf isotopic data, and whole-rock chemical and Sr–Nd–Hf–Pb isotopic data for Cenozoic volcanic rocks on Lombok in the East Sunda Arc, Indonesia, to investigate their petrogenesis and arc maturation. SIMS zircon UPb analyses of lavas from southern Lombok yield an early Oligocene age of ∼30 Ma. The lavas can be divided into three groups based on their ages and geochemical characteristics: group I (late Eocene) and II (∼30 Ma) lavas from southern Lombok, and group III lavas (∼3–0 Ma) from northern Lombok. The lavas are characterized by a progressive enrichment in K2O and incompatible element contents, and NdHf isotopic compositions with time. Group I lavas are tholeiitic basalts with low K2O contents (0.15–0.17 wt%) and Indian-MORB like rare earth element (REE) patterns and NdHf isotopic compositions (εNd = +7.4; εHf = +14.5), enrichment in large-ion lithophile elements (e.g., Rb, U, and K), and typical forearc basalt (FAB) geochemical characteristics. Group II lavas are low-K basaltic andesites to dacites that yield flat chondrite-normalized REE patterns [(La/Sm)N = 0.9–1.4], uniform initial 87Sr/86Sr ratios (0.7038–0.7042), and positive εNd(t) (+5.9 to +6.8) and εHf(t) (+13.9 to +14.4) values. Group III lavas are typically medium- to high-K calc-alkaline basalt to andesite and are enriched in light REEs [(La/Sm)N = 1.6–3.3] with uniform initial 87Sr/86Sr ratios (0.7038–0.7042) and slightly low εNd(t) (+3.9 to +5.4) and εHf(t) (+10.5 to +12.5) values. Progressive enrichment in incompatible element contents and NdHf isotopic compositions over time, from FAB to high-K calc-alkaline lavas, reflect the maturation of the East Sunda Arc. Sr–Nd–Pb–Hf isotopic mixing models, along with trace element ratios (e.g., Ba/Th, La/Yb, Th/Yb), suggest that increasing amounts of sediments were incorporated into the mantle sources of the magmas from group I (∼0%) to group II (<0.5%) and group III (0.5%–1.0%) lavas. Partial melting models show that the formation of group I and II lavas can be reproduced by high-degree (∼10%) partial melting of spinel peridotite, whereas the group III lavas were formed by low-degree (∼5%) partial melting of spinel peridotite. We propose that the evolution of Lombok arc magmas was controlled mainly by the fluxes of subducted slab material, along with decreasing degrees of partial melting as a result of crustal thickening over time.

Petrogenesis of flood basalts and shield basanite from Mertolemariam- Abamineos area, northwestern Ethiopian plateau: Assessments for mantle source variations

Petrographic and geochemical data (major and trace elements) are presented for Oligocene flood basalts and Miocene shield lavas from the Mertolemariam-Abamineos area, northwestern Ethiopian Plateau to examine the petrogenesis of the erupted magmas and the nature of mantle source compositions. The Mertolemariam flood basalts have mainly aphyric and plagioclase phyric textures. The Abamineos shield lavas have sparsely clinopyroxene phyric to highly plagioclase-clinopyroxene phyric textures. Geochemical classification shows that the Mertolemariam Oligocene flood basalts are sub-alkaline in composition, whereas the Abamineos Miocene shield lavas are highly alkaline in composition. The Abamineos shield has a unique composition (i.e., basanites) compared with all other shield basalts (transitional to alkaline) in the northwestern Ethiopian Plateau. Major and trace element compositions display two distinct trends between Mertolemariam flood basalts and Abamineos shield basanites. Fractional crystallization, partial melting, and crustal contamination of a homogeneous mantle source cannot explain the compositional variations between the Mertolemariam flood basalts and the Abamineos shield basanites. The trace element composition suggests that the Mertolemariam Oligocene flood basalts were more likely generated from the mixing of OIB (mantle plume) and E-MORB (enriched asthenosphere) mantle components. The Abamineos Miocene shield basanites were derived from the mantle plume (OIB) component. LREE/MREE and LREE/HREE indicate that both groups possibly originated from a mantle source within the stability field of spinel and garnet. In comparison, the Mertolemariam flood basalts were formed by a higher degree of partial melting from relatively shallow depths than the Abamineos shield basanites. We propose a scenario that explains the magmatic genesis in the northwestern Ethiopian Plateau: volcanism initiated by the initial arrival of the mantle plume (OIB-like) beneath the lithosphere, which comes across a geochemically fertile and enriched MORB (E-MORB) mantle component in the upper asthenosphere. The hot mantle plume triggered melting of the fertile and enriched MORB, and then melting occurred in the plume (OIB-like) at depth in the stability field of spinel and garnet. Melts from the mantle plume (OIB-like) and E-MORB components mixed to produce sub-alkaline flood basalts during the Oligocene in the northwestern Ethiopian Plateau Subsequently, melts from the advanced upwelling mantle plume (OIB-like) produced Miocene shield lavas in the northwestern Ethiopian Plateau.

The hidden link between dry ankaramitic melts and giant alkalic-type epithermal Au deposits: The Lihir Island example

The magmatic controls on the formation of giant epithermal Au deposits genetically associated with alkaline magmatic systems are not well understood. The general model, which attributes the generation of alkaline magmas to low-degree partial melting of metasomatized lithospheric mantle in a post-subduction tectonic setting cannot easily explain why some of these magmas are highly fertile for Au mineralization while others are not. This knowledge gap hampers the effective exploration of this economically important deposit type. In this study, we focused on the alkaline magmatism on Lihir Island and the Conical Seamount, Papua New Guinea, that gave birth to the Ladolam deposit and investigated the composition of silicate melt inclusions (SMIs) hosted in primitive volcanic rocks. Our findings offer important and novel insights into the early and deep magmatic processes that controlled the ore fertility of the melts that produced the largest known alkalic-type epithermal Au deposit on Earth. Olivine- and clinopyroxene-hosted primitive SMIs reveal nepheline-normative ankaramitic melt compositions derived from high-degree partial melting of metasomatized clinopyroxene-dominated lithologies, presumably amphibole-bearing clinopyroxenitic cumulates and metasomatically overprinted lithospheric mantle, both produced during earlier active subduction. These ultra-calcic parental melts are relatively low in H2O, S, and Cl, but enriched in Cu and highly oxidized. Due to their low water contents, significant clinopyroxene crystallization over a narrow temperature interval drove melt evolution from ankaramitic to alkaline compositions by strongly increasing incompatible element concentrations. Despite initially low H2O concentrations, evidence for high-temperature degassing is present in the form of low-density single-phase fluid inclusions co-existing with SMIs in clinopyroxene. This degassing event led to a strong decrease of S concentrations in the melt, whereas Cl and chloride-complexing elements remained largely unaffected. We propose that high-temperature volatile saturation as consequence of significant clinopyroxene crystallization is a key process for subsequent epithermal Au mineralization and can explain the efficient separation of Au from Cu. The complex tectonic setting of Lihir Island, marked by a reversal of subduction polarity, a tear in the slab beneath the island, and trans-lithospheric extensional structures, facilitated the generation and ascent of highly oxidized and relatively dry ankaramitic melts that ultimately exsolved a vapor-like, S- and presumably Au-rich fluid phase and might thus represent a prerequisite for alkalic-type epithermal Au mineralization.

Calcium isotopic composition of the bulk silicate Earth: A komatiite perspective

Calcium (Ca) isotopic composition of the mantle has been previously estimated as δ44/40Ca. However, δ44/40Ca of the mantle records both its stable isotopic composition and excess/deficit in radiogenic 40Ca that can be produced by the decay of 40K. Here, we present the high-precision Ca isotopic compositions of 16 Archean komatiites from five representative localities (Barberton, Munro, Abitibi, Yilgarn, and Taishan). Due to the high degrees of partial melting that led to their formation, these komatiites are used here to simultaneously estimate ε40/44Ca, δ44/40Ca, and δ44/42Ca of the bulk silicate Earth. The komatiites exhibit a resolvable negative ε40/44Ca relative to SRM915a, with an average of −0.6 ± 0.1 (2SE, 95 c.i.% if not specified). Given the lack of significant differences among the samples and the non-zero value of ε40/44Ca, we suggest using δ44/42Ca to report the stable Ca isotopic composition in this study. Excluding one sample (SD6/400), which may have been affected by alteration, the remaining komatiites display relatively homogeneous δ44/42Ca, ranging from 0.39 ± 0.02‰ (2SE, N = 16) to 0.49 ± 0.02‰ (2SE, N = 10), defining a mean of 0.44 ± 0.02‰ (2SE, N = 15). The absence of correlations between δ44/42Ca and LOI, Sr/Nd, (Dy/Yb)N, and (La/Sm)N among the komatiites indicates that post-eruption alteration, the influence of residual garnet, as well as prior melt extraction from the mantle source have had a negligible influence on their stable Ca isotopic compositions. Based on available experimental petrological data and isotope fractionation factors, it is demonstrated that komatiites were insignificantly fractionated from their mantle sources (i.e., <0.02% in δ44/42Ca). Their δ44/42Ca represents the ambient mantle at the time of segregation. Accordingly, we suggest that the average δ44/42Ca and ε40/44Ca of the komatiites measured here are representative of the bulk silicate Earth. Our new estimate can serve as a reference point for Ca isotopes as a two-dimensional tracer for sources and geological processes.

Re-melting of the Neoproterozoic juvenile mafic lower crust: The origin of adakitic and non-adakitic felsic suites in the southwest margin of Yangtze Craton

Eocene-Oligocene adakitic and non-adakitic shoshonitic felsic intrusions from the southwest margin of the Yangtze Craton provide a crucial geological record for studying crust-mantle interactions at the craton boundary. Despite their significance, the petrogenesis and source properties of these intrusions remain debated. This study suggests that the adakitic and non-adakitic features of these intrusions result from the re-melting of Neoproterozoic juvenile lower crust, as evidenced by zircon U-Pb dating, geochemical analyses, and petrogenetic investigations. The Ninglang-Yongsheng shoshonitic felsic intrusions, characterized by high Mg#, MgO, Ni, and Cr contents, exhibit isotopic characteristics akin to mantle-derived magma and juvenile mafic lower crust. Their U-Pb ages span 32–36 Ma, and they show similar patterns in major and trace elements, rare earth elements (REEs), and isotopes, with enrichment in large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs) like Nb, Ta, and Ti. These intrusions also demonstrate significant fractionation between light and heavy REEs, with a modest negative Eu anomaly. Their (87Sr/86Sr)i and εNd(t) values range from 0.7060 to 0.7069 and − 4.4 to −1.5, respectively, while Pb isotopic compositions vary slightly.This paper, along with prior research, has developed a genetic model for the Ninglang-Yongsheng shoshonitic felsic intrusions. The adakitic quartz monzonite porphyries likely originated from high-degree partial melting of the juvenile lower crust, whereas the non-adakitic shoshonitic syenite porphyries resulted from lower-degree melting. The composition of the juvenile lower crust may be garnet amphibolite-pyroxenite. This suggests distinct compositional variations within the Ninglang-Yongsheng juvenile lower crust. Moreover, the region's shoshonitic felsic intrusions have assimilated mantle-derived magma. In conclusion, the juvenile lower crust on the Yangtze Craton's southwest margin displays zonal characteristics, with the Liuhe-Beiya area's lower crust showing higher potassium content and more enriched isotopic signatures compared to the Ninglang-Yongsheng region.

Petrological and geochemical characteristics of the Low- and High-Hf Nam Meng dioritoid, northwest Vietnam: Implication for the mantle partial melting, mixing, and magmatic differentiation

This paper investigated the petrological, elemental, and isotope geochemical characteristics of the Nam Meng dioritoid to clarify the magma source and process. The Nam Meng massif comprises large amounts of dioritoids (gabbro-diorite and quartz diorite) and lesser amounts of granitoids (granodiorite and granite). The Nam Meng gabbro-diorite is a porphyritic texture with fine-grained plagioclase, amphibole, K-feldspar, and phenocryst biotite. The Nam Meng quartz diorite comprises coarse-grained plagioclase, amphibole, biotite, quartz, and K-feldspar. The Nam Meng gabbro-diorite contains higher plagioclase and lower biotite and quartz than the Nam Meng quartz diorite. The variation in petrography and mineralogy with the negative correlation between SiO2 contents with Al2O3, MgO, Al2O3 + MgO, T-Fe2O3, and CaO contents suggest a magmatic differentiation process. All the Nam Meng dioritoids have low ACNK (mostly less than 1.0), total alkaline (Na2O + K2O ≤ 6 wt.%) with Na2O/K2O ≥ 1, and negative anomalies of Ta, Nb, and Ti. Combined with the U-Pb zircon age of 290 Ma (Hieu et al. (2017), the Nam Meng dioritoid is thought to be an I-type granitic rock formed in the subduction stage of the Indosinian amalgamation event. The low La/Yb ratios (1.14-3.21) suggest that the mantle wedge that was melted to form the Nam Meng magma had a spinel peridotite composition. The εNd (290 Ma) of the Nam Meng dioritoid is close to bulk earth silicate at 290 Ma (-4.43 to 2.34). The 87Sr/86Sr (290 Ma) of the Nam Meng dioritoid varies in a wide range from low to intermediate (0.6987 to 0.7088), and the calculated Nd model age of the Nam Meng dioritoid is 1,258 ± 47 Ma. The Sr and Nd isotope data suggest that the Nam Meng spinel peridotite was a result of mixing between an ancient ocean crust, EM1, and EM2 that occurred in a paleo-subduction zone at Meso-Neoproterozoic Rodinia supercycle. The Hf contents of the low-Hf and high-Hf Nam Meng dioritoid series are 0.38-1.02 and 2.60-8.08, respectively. The LILE/Hf and HSFE/Hf ratios in the low-Hf Nam Meng dioritoids are high and have a strongly negative correlation with Hf. On the other hand, those ratios in the high-Hf Nam Meng dioritoids are low and have a weakly negative correlation with Hf. The Hf contents positively correlate with the degree of partial melting of the mantle wedge in the subduction zone. Therefore, the low Hf, high LILE/Hf, and HSFE/Hf ratios of the Nam Meng dioritoids could be derived from a low degree of partial melting of the mantle wedge. In contrast, the high Hf, low LILE/Hf, and HSFE/Hf ratios of the Nam Meng dioritoids could be produced by a high degree of partial melting of the mantle wedge.

Voluminous magma formation for the 30-ka Aira caldera-forming eruption in SW Japan: contributions of crust-derived felsic and mafic magmas

Understanding the origin, assembly, and evolution of voluminous magma that erupts in catastrophic caldera-forming eruptions (CCFEs) is a community imperative. A CCFE of the Aira caldera at 30 ka discharged over 350 km3 of magma, which can be grouped into petrographically and geochemically distinct types: voluminous rhyolite, small amounts of rhyodacite, and andesite magmas. To further understand the magma plumbing system of the Aira CCFE, we examined the geochemical characteristics of whole rock and plagioclase from its eruptive deposits. The trace element and 87Sr/86Sr signatures recorded in the plagioclase phenocrysts of these magmas indicate that the three magmas were originally produced by partially melting an identical source rock, which was estimated to be a mafic amphibolite with an 87Sr/86Sr signature of ∼0.7055 that comprised the lower crust. Melting of mafic amphibolite produced both felsic and mafic magmas by low and high degrees of partial melting, respectively. The mafic magma assimilated uppermost crustal materials and crystallized to produce an andesite magma type. The andesitic magma consists of phenocrysts (∼39 vol%) and melt with a dacitic (∼70 wt% SiO2) composition. The felsic magma mixed with ∼10% of the andesite magma and crystallized, forming the rhyolite magma. The mixing between the andesite and rhyolite magmas before the Aira CCFE produced the rhyodacite magma. The 30-ka Aira CCFE magmas were generated only by melting two kinds of crustal materials with different geochemical characteristics and had geochemical variations due to different conditions of partial melting and mixing between various crustal melts. The lack of definitive evidence of the mantle component mixing with the Aira CCFE magmas suggests that the mantle-derived magmas worked only as a heat source for crustal melting.

Melting processes, cooling rates, and tectonic settings of the Neo-Tethyan mantle: the in-situ mineral chemical record

Peridotites in the Yarlung Zangbo ophiolite (YZO) have the potential to elaborate the mantle dynamics that operated below the Neo-Tethyan oceanic crust. Here, we measured major and trace element concentrations of minerals (e.g., clinopyroxene, orthopyroxene, olivine, and spinel) in YZO harzburgites and lherzolites to trace the origin and evolution of the YZO. The rare earth element (REE) patterns of clinopyroxenes in lherzolites and harzburgites plot within the field of abyssal and forearc peridotites, respectively, indicating that the latter experienced a higher degree of partial melting. Based on the relative extents of light (LREE) and heavy (HREE) REE depletions in clinopyroxene, we classified the lherzolites into two groups (I and II). The group I clinopyroxenes are depleted in HREEs and enriched in LREEs and MREEs compared to those of group II. Meanwhile, the REE patterns of harzburgite clinopyroxenes are identical to that of group II, although they display lower REE concentrations. Models based on REE abundances in clinopyroxene suggest that group I lherzolites experienced ~8–11% partial melting in the spinel domain, whereas group II lherzolites formed after 5% melting in the garnet domain followed by 6–9% and 9–16% partial melting in the spinel domain, respectively. These high degrees of melting may suggest that the harzburgites formed in a supra-subduction zone (SSZ) setting. Surprisingly, the REE-in-two-pyroxenes thermometer (TREE) shows that the YZO peridotites record high-temperature cooling at 991–1218 °C which is lower than the conditions below mid-oceanic ridges (MORs), but compatible with recent reports of high closure temperatures in forearc environments. Furthermore, our TREE results constrain the cooling rate of the YZO peridotites to ~0.01–0.5 °C/yr, overlapping with rates reported for SSZ, MOR, and abyssal peridotites. By integrating the petrological, geochemical, and thermal features, we tentatively interpret the group I lherzolites accreted in an Early Cretaceous forearc during subduction initiation, whereas the group II lherzolites and the harzburgites formed by further melting of the earlier lherzolites in a SSZ setting.

Crustal control on the petrogenesis of adakite-like rocks

Adakite-like rocks have attracted extensive interest as they record magmatic processes and mantle-crustal material recycling, and because they are associated with major porphyry Cu deposits. However, their petrogenesis is still under debate. In this study, we compiled ca. 7000 sets of whole-rock geochemical and Nd isotopic data of Phanerozoic adakite-like rocks worldwide from published literature, with the aim to characterize geochemically adakite-like rocks from subduction, collision, and post-collision settings. Statistical analysis highlights the different lanthanides + Y patterns from various types of juvenile and mature crust. Modelling of the statistically-processed data suggest that pure fractionation of amphibole and/or garnet cannot account for the lanthanides + Y patterns of adakite-like rocks, whereas partial melting of mafic protoliths is a possible mechanism. This study supports a adakite-like magma generation model by partial melting of the eclogitic lower crust. The melt was then mixed with depleted mantle-derived basaltic melt, and subsequently fractionated. Compared with the adakite-like magma formation from the juvenile crust, that from the mature crust is featured by higher degree of partial melting, greater depths and higher water content. Additionally, secular peaks of La/Yb value (which reflects the depth of magma source) of adakite-like rocks worldwide suggest that the period of magmatic activity in deep-level magma reservoirs could be used as the indicator of either the final stages of the Phanerozoic orogenesis, or craton activation by subduction.

Development of a cambrian back-arc basin in the North Qilian orogenic belt: New constraints from gabbros in Yushigou ophiolite

Introduction: The North Qilian orogenic belt, as the Northern branch of the original Tethys tectonic domain, is important for reconstructing the tectonic evolution of the ancient Tethys. However, the tectonic history of the North Qilian orogenic belt remains controversial. This study addresses this issue from a geochemical perspective.Methods: In this study, a comprehensive analysis of the geochronology, whole-rock geochemistry, clinopyroxene mineral geochemistry, zircon Ti crystallization temperature, and gabbromagma temperature and pressure in the Yushigou ophiolite of the North Qilian orogenic belt was conducted to provide constraints on its tectonic evolution.Results and Discussion: Laser ablation inductively coupled plasma mass spectrometry zircon U-Pb dating results reveal that the gabbros have ages of 519 ± 3 Ma and 495 ± 4 Ma, belonging to the Cambrian period. Most of the studied gabbros exhibited geochemical characteristics of tholeiitic basaltic rocks with normal mid-ocean ridge basalt and island arc tholeiite dual geochemical affinities. The gabbros are interpreted to have formed by a high degree of partial melting of the depleted mantle spinel lherzolite. These results suggest that the back-arc basin of the North Qilian tectonic belt may have evolved to a relatively mature stage from 519 to 495 Ma. Overall, this study contributes to our understanding of the tectonic evolution of the North Qilian orogenic belt through geochemical analyses.

High degree partial melting of the metasomatized mantle: A possible source for the Eocene-Oligocene porphyry Cu-Au-Mo deposits in Lut block, Eastern Iran

The small (<50 Mt) Eocene porphyry Cu-Au deposits of Maher abad and Khupic and the Oligocene Cu-Mo deposits of ChahShaljmi and DehSalam are attributed to the subduction of the Sistan ocean crust beneath the Lut block of eastern Iran. The calculated Ce4+/Ce3+ ratios of Dehsalm and ChahShaljami zircons are clustered around 10–50 and 75–155, and the values for Maher abad and Khupic are clustered around 120–300 and 50–150, respectively. The calculated logfO2 values indicate FMQ buffer conditions for Eocene and Oligocene magmas (ΔFMQ < 2; ΔFMQ −1.8 to + 0.6 ± ∼1.5 for ChahShaljami; ΔFMQ −1.3 to −0.4 ± ∼1.5 for Dehsalm, and ΔFMQ −3.2 to −2.1 ± ∼1.5 for Maher abad; ΔFMQ −2.6 to −1.3 ± ∼1.5 for Khupic), which never reached the ideal values of the worldwide giant to large porphyries (ΔFMQ < 4; HM buffer). The 30–60% partial melting of a slab (∼10–25% garnet) can explain ChahShaljami and Dehsalm, while it is > 60% partial melting of a slab (garnet > 50%) + ∼10% of metasomatized garnet-lherzolite (∼10% garnet) for Maher abad and Khupic. High partial melting of a slab has produced magma poor in copper in the magmatic subarc of the Lut block. Our zircon data and whole-rock geochemical characteristics of the Eocene and Oligocene ore-bearing intrusions along with the kinematics studies of the subduction zones are well consistent with a west-directed subduction system in eastern Iran. In such a system, a higher subduction rate > convergence rate causes deep steep subduction and in the continuation roll-back of the slab occurs in response to the opening and closing of the back-arc basin. This event caused the thinning of the Lut continental crust, mantle delamination and hot asthenospheric upwelling. The ascent of magmas resulting from the partial melting of a hydrous slab through a hot mantle and thinned continent caused widespread volcanism, and large amounts of water, sulfur and, volatiles were released to the atmosphere in an island arc regime during the middle-upper Eocene. The poor in metal low-volume magmas resulting from the high partial melting of a low water warm/hot deepest slab slightly metasomatized the mantle that subsequently amphibole-FC process with low contamination in the MASH zones and their emplacement during the transition from the island- to normal continental arc in the upper Eocene-Oligocene formed upper Eocene high-K calc-alkaline I/A-type (ferroan) arc magmatism and lower Oligocene adakite-like high-K calc-alkaline-Shoshonitic I-type (magnesian) arc magmatism, led to the formation of small Cu-Au and Cu-Mo porphyry deposits in Lut block, respectively. These events occur in a particular tectonic condition where subduction and collision were accompanied by rotation of the block affected by both Indo-Asian and Arabian-Eurasian collisions. Our study implies that W-directed subduction systems do not have favorable conditions for the formation of large porphyry deposits such as the ideal down-dip thermal gradient of the oceanic slab, water content, depth of dehydration, degree of partial melting of oceanic slab, and thickened continental crust.

A greenstone belt in southeast Tibet: An accreted middle–late Permian oceanic plateau

Studies of accreted oceanic plateau sections provide crucial information on their structures, compositions, and origins. We investigate the petrogenesis of ultramafic–mafic rocks in the Tangjia–Sumdo greenstone belt of southeast Tibet using petrography, whole-rock geochemistry, and U-Pb zircon geochronology. These rocks are divided into four groups based on geochemical characteristics that include depleted and tholeiitic mafic rocks, transitional mafic rocks, enriched and alkaline mafic rocks, and picritic ultramafic rocks. Depleted and tholeiitic mafic rocks have the oldest crystallization ages (∼272 Ma), followed by picritic ultramafic rocks (∼270 Ma), transitional mafic rocks (267–254 Ma), and enriched and alkaline mafic rocks (252–250 Ma). Hafnium and neodymium isotope ratios of depleted and tholeiitic mafic rocks (εHf(t) = +13.1–+16.9; εNd(t) = +6.9–+7.1), transitional mafic rocks (εHf(t) = +1.8–+16.9; εNd(t) = +0.8–+5.5), enriched and alkaline mafic rocks (εHf(t) = +0.5–+5.4; εNd(t) = −1.5 to +1.9) and picritic ultramafic rocks (εHf(t) = +14.9–+17.2; εNd(t) = +7.8–+9.0) are similar to those of N-MORB, E-MORB, OIB and depleted-type picritic mafic rocks in other oceanic plateaus, respectively. The geochemical characteristics of the depleted and tholeiitic mafic rocks suggest that they formed by partial melting of depleted spinel lherzolite in a mid-ocean ridge setting, whereas the picritic ultramafic rocks suggest a high degree of partial melting of depleted lherzolite in a hot mantle plume head. The transitional mafic rocks formed by partial melting of moderately enriched garnet lherzolite. The youngest rocks (enriched and alkaline mafic rocks) formed by partial melting of a more enriched garnet lherzolite (compared to transitional mafic rocks) at relatively low temperatures. We propose that the depleted and tholeiitic mafic rocks represent normal oceanic crust of the Sumdo Paleo-Tethys Ocean and the transitional mafic rocks, enriched and alkaline mafic rocks and picritic ultramafic rocks are the fragments of the oceanic plateau, which were related to middle–late Permian mantle plume activity in the Sumdo Paleo-Tethys Ocean. We further suggest that the majority of the Tangjia–Sumdo greenstone belt represents a middle–late Permian oceanic plateau that reflects a previously unrecognized middle–late Permian mantle plume.

The ca. 1.13–0.92 Ga magmatism in the western Yangtze Block, South China: Implications for tectonic evolution and paleogeographic reconstruction

The tectonic regime of the Yangtze Block in the South China and its palaeogeographic location in the Rodinia supercontinent during the late Mesoproterozoic to early Neoproterozoic remain enigmatic due to limited geological records. In this study, we report late Mesoproterozoic basalts and granodiorites and early Neoproterozoic plagioclase amphibolites in the western Yangtze Block. Zircon LA-ICP-MS U-Pb dating yield concordant ages of 1130–1128 Ma for basalts, 1026 ± 1.6 Ma for granodiorites, and 917 ± 1.4 Ma for plagioclase amphibolites. The basalts are tholeiitic in chemical composition and are slightly enriched in LREEs with (La/Yb)N ratios of 2.87–3.99 and exhibit E-MORB-like trace element patterns. They are depleted in whole-rock Nd (+3.3 to +5.9) and zircon Hf isotopes (+11.1 to +12.8). Geochemical compositions showed that the basalts were derived from high-degree (7%–20%) partial melting of spinel lherzolite mantle. The granodiorites belong to the A-type granites and have negative whole rock εNd(t) (−5.5 to −7.8) and zircon εHf(t) (−15.3 to +0.6) values. We suggested that these A-type granites were mainly generated by the partial melting of felsic crustal rocks under low-pressure conditions. The plagioclase amphibolites are tholeiitic and exhibit arc-like geochemical affinities. They show depleted isotopic compositions (zircon εHf(t) = +11.0 to +13.6; whole rock εNd(t) = +0.6 to +2.5) and were interpreted as derived from a lithospheric mantle source that was metasomatized by subduction-related fluids. Geochemical compositions show that basalts were formed in an intracontinental rift setting, whereas granodiorites and plagioclase amphibolites were emplaced into a subduction background. Integrated with published work, we suggest that the late Mesoproterozoic and early Neoproterozoic magmatism in the western Yangtze Block record a tectonic switch from a passive margin to a convergence setting. In addition, we also proposed that the South China may be located at the western periphery of the Rodinia supercontinent.

Plate tectonics in action in the Mesoarchean: Implication from the Olondo greenstone belt on the Aldan Shield of Siberian Craton

The Archean Olondo greenstone belt (OGB) is located on the Aldan shield, the largest basement of the Siberia craton. With well-preserved abundant mafic-ultramafic rocks, ≥30% in volume, the OGB is unique among other greenstone belts in the world. In this study, we present the most up-to-date geochemical and isotopic data for the ultramafic-mafic rocks of the OGB, in order to better constrain their mantle sources and the plate tectonic process involved in the formation of OGB at ca. 3 Ga. The ultramafic rocks vary from fresh to serpentinized dunites, and are highly refractory as residual mantle phase as indicated by depletion in P-Platinum Group Elements (PGE) relative to I-PGEs for highly siderophile elements (HSE). Fresh dunites show U-shaped rare earth element (REE) patterns, with positive to negative Nb anomalies, indicative of late metasomatism in their mantle source. Rhenium-Osmium isotopic compositions of these dunites yield mantle model age (TMA) of 2960–3020 Ma, comparable to the formation age of the OGB at ca. 3 Ga. Together, the data suggest that, unlike mantle cumulate origin for most of the Archean ultramafic rocks, the OGB dunites were mantle residuals after a high degree of partial melting (>30%), which subsequently interacted with the subduction-related melt/fluid. On the other hand, the OGB mafic rocks including komatiitic and tholeiitic basalts show geochemical characteristics relative to the ultramafic residuals that reinforce a subduction-related regime as their formation setting, despite extra mid-ocean ridge and plume settings. Tholeiitic basalts yield variable REE patterns from depleted, chondritic, to enriched light rare earth elements (LREE) patterns, with variable Nb-Ta anomalies, indicating their similarities with modern N-MORB and boninites, comparable to mafic rocks in typical supra-subduction zone (SSZ) ophiolites. Such mafic rocks with combined lower εNd(t) and negative Nb-Ta anomalies were most likely the result of mixing with subducted components, consistent with the observed Nb depletion in the residual dunites. The Al-depleted komatiitic basalts may have originated from deep mantle source, corresponding to garnet stability field, confirmed by their depletion in HREE and requiring a mantle plume to transport and melt at such a depth. The OGB ultramafic-mafic rocks could be a record to witness plume-induced subduction initiation processes such that mantle plume, sea-floor spreading and subduction were all in operation in the Mesoarchean time.

Proterozoic mantle melting recorded by the Re-Os isotopic systematics of ophiolites from the Qilian Orogenic Belt, northwestern China

• The Minhe serpentinites are mantle residues of multi-episode partial melting. • Fluid/melt metasomatism exerted a limited impact on the Re-Os systematics. • Re-Os ages provide temporal constraints on Proterozoic melt extraction. Re-Os isotopes of the mantle-derived ultramafic rocks can provide temporal constraints on the melt extraction of ancient mantle. Here, new geochemistry and Re-Os isotopes of serpentinites from the Minhe ophiolite in the Qilian Orogenic Belt (QOB) are presented. Their geochemical compositions (e.g., abundances of heavy rare earth elements, Al 2 O 3 , Yb, Re, and Th/Yb ratio) suggest that protoliths of the serpentinites originated as mantle residues. They underwent multiple episodes of melt depletion, leading to a highly refractory nature and high degrees of partial melting (> 18%). Moreover, subchondritic 187 Re/ 188 Os (0.0023 – 0.0399) and 187 Os/ 188 Os (0.1135 – 0.1210) ratios of the serpentinites, and decoupling of the Re-Os systematics and metasomatic proxies (e.g., LOI, Sr, La, and Th/Yb), demonstrate that fluid/melt metasomatism exerted a limited impact on their Re-Os isotopic systematics. Their Re-depletion ages (T RD ), varying from 1.22 – 2.21 Ga, are analogues to their Re-Os model ages (T MA , 1.29 – 2.31 Ga). Therefore, Re-Os isotopic ages recorded by ophiolites in QOB indicate that at least two major melt-extraction episodes occurred in the lithospheric mantle during Proterozoic (1.2 – 1.6 Ga and 2.0 – 2.2 Ga). Notably, the 1.2 – 1.6 Ga mantle melt-depletion event may have a genetic affinity with the 1.1 – 1.7 Ga crustal growth. Mantle rocks from an ophiolite have a great potential in tracking ancient geological events, such as Precambrian mantle melting.

A new compositional estimate for refractory lower continental crust with implications for the first terrestrial Pb-isotope paradox

The lower continental crust, representing up to 50% of the continental mass, is largely inaccessible, making its composition difficult to constrain. Previous composite models based on geophysical evidence and geochemical data of granulite terrains and xenoliths have proposed varying results, from a mafic, relatively refractory lower crust to an intermediate-felsic, more enriched composition. Here, we investigated the mineralogy and geochemistry of predominantly mafic granulite xenoliths from eastern Australia and the Kola Peninsula, Russia, using an in situ analytical approach that minimises host magma contamination. The resulting xenolith compositions are variably and often strongly depleted in most highly incompatible trace elements, including the heat-producing elements. These xenoliths represent an extremely refractory component of the lower continental crust, likely formed after high degrees of partial melting or crystallisation from a depleted source. A lower crust composed solely of this refractory endmember would be too exhausted in heat-producing elements to satisfy heat-flow constraints. However, a volumetrically significant component of the lower crust is this mafic and refractory material, combined with undifferentiated material and a felsic or metapelitic portion. Using geophysical constraints on proportions of refractory (55%), undepleted (38%) and enriched (7%) components, a new estimate for average lower continental crust that satisfies heat flow limits was calculated, including for elements such as Be, B, Cs, W and Tl, where previous estimates relied on very few data. Finally, we show that because much of the lower continental crust is so refractory and depleted in incompatible elements, it is unlikely to be a reservoir that can balance radiogenic isotope (unradiogenic Pb) and trace element ratios (e.g. Rb/Cs, Nb/Ta) for which bulk silicate Earth departs from chondritic ratios.

Geochemical Signatures of Platinum Group Elements in Ultramafic Rocks of Chotanagpur Gneissic Terrain, Eastern India and Their Genetic Control

Ultramafic rocks occur as intrusives in the form of lensoid bodies in the northwestern part of Chotanagpur Gneissic Terrain and consist mainly of olivine, orthopyroxene and clinopyroxene and are devoid of plagioclase. Based on distinct geochemical characteristics, the rocks have been identified as komatiites. The rocks are geochemically analogous to Al-undepleted Munro type (Al2O3/ TiO2=17.88-54.73) with distinctly high MgO (26.2-35.62 wt%), Ni (958-1902 ppm) and Cr (21.32-3320 ppm) contents. These rocks are characterised by low CaO/Al2O3, (Gd/Yb)n, (La/Yb)n with positive Zr, Hf, Ti anomalies suggesting high degree partial melting of mantle under anhydrous conditions at shallow depth with garnet as a residual phase in the mantle restite. These high MgO volcanic rocks having elevated concentrations of Ni and Cr are potential hosts for Platinum Group Elements (PGE) owing to their primitive mantle origin and eruption at high temperatures. These rocks have low ΣPGE (29-269.02ppb) content with Ir (0.1-0.8ppb) and Ru (1.05-5.78ppb) among Iridium group PGE (IPGE); and Pt (5.04-18.72ppb), Pd(3.5-18.0ppb), Rh (0.22-0.84ppb) among Platinum group PGE (PPGE). The PGE abundances in komatiites were controlled by olivine fractionation. The Major, trace, REE and PGE composition of the rock suggest melting under anhydrous condition at shallow depth above the garnet stability field under S-undersaturated condition. Anhydrous melting associated with mantle plume activity gave rise to the rock which subsequently contaminated by lower crustal materials during magma ascent and emplacement. Keywords: Chotanagpur Gneissic Terrain, Komatiite, Platinum Group Elements, Sulphur Undersaturation, Plume

The off-axis plume–ridge interaction model: Confirmation from the mineral chemistry of Cretaceous basalts of the Ontong Java Plateau

The Ontong Java Plateau (OJP) is the largest known igneous province on Earth. The proposal that its origins involved off-axis plume–ridge interactions remains highly controversial. Here we present new in-situ major (EPMA), trace element (LA-ICP-MS) and Sr isotopic (LA-MC-ICP-MS) characterization of clinopyroxene and plagioclase phases present in OJP tholeiitic basalts from Sites 1183, 1186, 1185, 1187. Site 1187 and Upper Site 1185 have yielded olivine tholeiites containing olivine, Ca-rich augite, Ca-deficient pigeonite, and plagioclase, inferred to be derived from higher degrees of partial melting of dominantly peridotitic mantle and reflecting relatively low crystallization temperatures. Sites 1183 and 1186 and Lower Site 1185 yielded tholeiitic basalts consisting mainly of plagioclase and Ca-rich augite without olivine, interpreted to originate from lower degrees of partial melting of pyroxenite- and peridotite-derived melts with relatively high clinopyroxene and plagioclase crystallization temperatures. Site 1187 and Upper Site 1185 plagioclase, clinopyroxene, and equilibrium melts have much lower REE contents but slightly higher in-situ Sr isotopic ratios compared to those phases at Sites 1183 and 1186 as well as Lower Site 1185. Sites 1186 and 1187 plagioclases and clinopyroxenes display normal zoned textures which are attributed to the process of fractional crystallization and extensive magma mixing beneath the heterogeneous mantle source. In-situ Sr isotopic disequilibrium between plagioclase phenocrysts and plagioclase clusters further indicates an enriched compositional heterogeneity. Due to the strong tensile forces expected at the spreading ridge, the OJP off-axis mantle plume is inferred to be drawn towards the ridge axis, leading to shifts in mineral compositions, degrees of partial melting, and crystallization temperatures inferred for the volcanics of Sites 1183, 1186, 1185, and 1187 as a function of distance from the spreading ridge. The enriched invariability of in-situ Sr isotopic compositions is likely due to the relatively long half-life of the radiogenic elements. These mineral chemistries are consistent with off-axis ridge–plume interaction.

Opening of the Algerian Basin: Petrological, geochemical and geochronological constraints from the Yaddene Complex (Lesser Kabylia, Northeastern Algeria)

The Yaddene complex, located in Lesser Kabylia (northeastern Algeria), is composed of Tertiary mafic and ultramafic rocks that outcrop within syn-rift sedimentary rocks of the Algerian marginal basin, in the Maghrebide belt of North Africa. Petrographic observations show that the Yaddene complex is composed mainly of two distinct lithologies: (1) a layered quartz-bearing gabbro at the bottom, which consists mainly of plagioclase (An93), clinopyroxene (altered to actinolite) and rare interstitial quartz; and (2) layered plagioclase-bearing lherzolites composed of olivine (Fo86-88) orthopyroxene (En86–87), clinopyroxene (diopside and augite), rare plagioclase (anorthite) and amphibole of pargasitic composition.The trace element concentrations in pyroxenes and olivine of the Yadenne complex show a depletion in light rare earth elements (LREE) similar to basalts of the Mid-Ocean Ridge (N-MORB), which reflects the depletion of the mantle source. In addition, high #Cr in spinel reflects a high degree of partial melting of the mantle source. On the other hand, slight enrichment in Large Ion Lithophile Elements (Cs, Rb, Ba), weak but significant negative Nb, Ta, Zr, Hf and Ti anomalies, and positive Pb spikes are observed in both whole-rocks and minerals. These patterns are best explained by interaction between parental mantle and fluids and/or siliceous melts derived from a subducting oceanic slab. The tectonic discrimination diagrams of minerals and whole rocks indicate that the most likely geodynamic setting for emplacement of the Yaddene Complex is a back arc basin environment.In situ U-Pb dating of euhedral zircons and titanite from a quartz-bearing gabbro yields an age of 18.97 ± 0.12 Ma (2σ), which is interpreted as the crystallization age of magmatic zircons and the gabbroic magma. Zircons from a plagioclase-bearing lherzolite gave a consistent age of 19.05 ± 0.39 Ma (2σ), which is attributed to zircon growth during metamorphic/metasomatic processes coeval to gabbroic magma emplacement. The mafic/ultramafic rocks of the Yaddene complex are intercalated within fine-grained sediments of the syn-rift “Oligo-Miocene Kabyle” and the obtained early Burdigalian ages are interpreted as the early stages of an extensional regime that ultimately led to the opening of the oceanic Algerian Basin.

Geochemistry of meta‐mafic and meta‐tonalite‐trondhjemite intrusives from Jaintia and Karbi Anglong hills of Shillong Plateau, North East India: Implications on the evolution of the Proterozoic Shillong Basin

We present the whole‐rock and mineral chemical data of the Khasi meta‐mafic/basic‐trondhjemite intrusives (KMI) and meta‐tonalite‐trondhjemite (MTT) that coevally emplaced the Proterozoic Shillong Group (SG) of Shillong Plateau, northeast (NE) India and discuss their petrogenetic‐tectonic origin. The calc‐alkaline to tholeiite KMI are compositionally transitional between hornblende‐gabbro; hornblende‐diorite and minor‐hornblendite. They show Ta–Nb troughs and strong decoupling large‐ion lithophile/high field strength elements (HFSE) signature consistent with subduction zone‐related basic magmas. The calcic to calc‐alkaline, low‐Al2O3, and I‐type metaluminous Fe‐hedenbergite‐trondhjemite (KMT) exhibits similar trace and rare earth elements (REE) distribution to the KMI. The MTT on the other hand are depleted in trace elements and REE, and correspond with the I‐type granitoids, and are comparable to Proterozoic to Phanerozoic tonalite‐trondhjemite‐granodiorites. Gentle negative sloping of REE in KMI ([La/Lu]CN = 4.04 to 6.35) with moderate LREE, suggests partial melting or, fractional crystallization from a transitional source between garnet and spinel lherzolite. Although almost a flat pattern ([La/Lu]CN = 3.70 to 4.05) for MTT entails a high degree of partial melting from sources similar to Archaean Mafic Composite or Archaean Tholeiite. Trace elemental modellings ([Sr/Y] vs. Y and [La/Yb]N vs. YbN) for MTT infer low pressure (&lt;15 kbar) and high degree partial melting of &gt;50% in equilibrium with 10% garnet +90% amphibole residue. A relatively flat REE pattern with some elevated HREE of MTT compared to KMI suggests for depleted source. Bimodal plutonism and the prevalence of meta‐dacite‐rhyolite in SG invokes for tonalite‐trondhjemite‐dacite setting of a continental arc or, active continental margin tectonic setting. Emplacement of KMI initiated in a temporary relax‐extensional phase and MTT in a compressional regime. This is possibly caused by an over‐thickened crust later subjected to orogenic collapse, exhumation‐metamorphism, and decompressional melting. This event might have taken place between 1.0 to 0.5 Ga during the Pan‐African Orogenic crustal growth, linked to the amalgamation of Gondwana.