High MgO Research Articles

Tracing mantle source variation through xenocrystic olivine in the Taupo Volcanic Zone, New Zealand: A role for lithospheric mantle in the shift from andesitic to rhyolitic compositions

The central Taupo Volcanic Zone (TVZ) in New Zealand, the most productive of silicic volcanic centres, is flanked by younger andesitic stratovolcanoes to the south. Small basaltic scoria cones occur along the length of the TVZ, the most primitive of which contain abundant xenocrysts, giving insight into the mantle feeding the TVZ.Here, we present major and minor element compositions of olivine from eleven mafic eruptions spanning the length of the central and south TVZ. Olivine compositions can be divided into three major groups, based on their texture, major and minor element composition, and zoning. Group 1 olivines are Fo88 in the most primitive cores, with minor element concentrations consistent with magmatic origin. Group 2 olivine compositions range from Fo72–88 but are homogeneous within individual crystals. CaO contents are consistently lower for a given forsterite content compared to group 1 olivines and are most consistent with a plutonic crystal mush origin. Group 3 olivines have core compositions of Fo93–90, and NiO and CaO (0.36–0.56 wt%, <0.15 wt%, respectively) concentrations consistent with a primary magma or mantle-derived origin. Compositional variation within group 3 olivine allows for further characterisation of the mantle lithologies from which the crystals were sourced. Group 3 olivines from the central TVZ are lherzolite-derived, whereas group 3 olivines from the south TVZ are harzburgite-derived. However, recent whole-rock and melt inclusion data suggests that the most fertile mantle underlies the south TVZ, and the least fertile mantle underlies the inter-caldera regions of the central TVZ where caldera-forming volcanism has ceased. This is inconsistent with interpretations drawn from the olivine data. We therefore suggest that harzburgitic xenocrysts originate from old depleted sub-continental lithospheric mantle (SCLM), which is present at depths of ~30 km in the south TVZ. By contrast, rifting and crustal thinning in the central TVZ has removed the SCLM, explaining why only xenocrysts of lherzolitic origin are found. The presence of depleted SCLM may act as a boundary layer inhibiting ascent of melt from the mantle, providing an explanation for lower erupted volumes in the south TVZ. Dissolution of the orthopyroxene component in harzburgite in the presence of hydrous melt satisfies major element constraints on south TVZ basalt (high SiO2, high MgO, low CaO/Al2O3). Taken together, this is the first mineralogical evidence for a tectonic and lithospheric control on the shift from andesitic to rhyolitic activity in the TVZ, implying a direct link between rifting, magma composition and volcanic productivity.

Geochronology and geochemistry of Late Triassic intrusions in the Liaodong Peninsula, eastern North China Craton: implications for post-collisional lithospheric thinning

ABSTRACT Despite numerous years and avenues of investigation, the timing, evolution, and mechanism of lithospheric thinning of the North China Craton (NCC) are still highly debated. Late Triassic igneous rocks especially mantle derived mafic rocks could provide critical information related to the post-collisional lithospheric thinning of the Liaodong Peninsula in the eastern NCC. Using newly acquired geochronological data points, we determined that the intrusive rocks have emplacement ages of 219–209 Ma, which are typical of Late Triassic magmatism. The studied Qinghekou gabbros have low SiO2 (49.43–51.86 wt.%), Na2O+K2O (3.59–7.20 wt.%), high MgO (3.70–8.54 wt.%) and transition metal element contents, with Mg# values ranging from 49.2 to 69.3. While these samples are enriched in large ion lithophile elements such as Rb, Ba, and K and light rare earth elements, they are depleted in high field strength elements such as Nb, Ta, Ti, and P and heavy rare earth elements. We conclude that these gabbros samples were formed in a subduction-related tectonic setting, where the primary magmas were derived from partial melting of the lithospheric mantle that experienced metasomatism via slab-related fluids. Our Tongyuanpu granite samples have I-type granite affinity, as indicated by their high SiO2 (73.60–75.30 wt.%) and low MgO (0.08–0.32 wt.%) contents, as well as the A/CNK values of 0.99–1.25 and the zircon saturated temperature values of 757–832°C, the parental magmas for these granites were generated by partial melting of the lower crust. Combined with the spatio-temporal distribution and rock-assemblages of the Late Triassic intrusive rocks, we conclude that these rocks were formed in an extensional environment characterized by lithospheric thinning caused by lithospheric delamination after the collision between the NCC and the Yangtze Craton. Furthermore, we infer that the initial lithospheric thinning of the eastern NCC occurred during the Late Triassic.

The Loongana (CL) group of carbonaceous chondrites

A coordinated study of the petrology, mineral chemistry, and bulk chemical and isotopic composition of the five ungrouped carbonaceous chondrites Coolidge, Loongana 001, Los Vientos (LoV) 051, Northwest Africa (NWA) 033, and NWA 13400 reveals that these meteorites have a similar set of properties that distinguishes them from the other carbonaceous chondrite groups and allows definition of the new Loongana (CL) group of carbonaceous chondrites. The basic characteristics of the investigated samples are: (1) Lithophile element ratios (e.g., Al/Mg, Si/Mg) are within the typical range of other carbonaceous chondrite groups. (2) Fe-Ni metal abundances are considerably higher than for CV, but similar to CR chondrites. (3) Chondrule size-frequency distributions are similar to CV, but dissimilar to CR chondrites. (4) The mean CAI abundance is ∼1.4 vol%, i.e., lower than in CV but much higher than in CR chondrites. (5) Very low amounts of matrix (17–21 vol%), the lowest among the main carbonaceous chondrite groups (CI, CM, CO, CV, CR, CK). (6) Olivine is nearly equilibrated, with mean fayalite (Fa) values between 12.5 mol% (Loongana 001) and 14.7 mol% (NWA 13400) as a metamorphic effect. (7) Lower Al2O3 and higher MgO and Cr2O3 concentrations in matrix, compared to matrix in CV, CK, and CR chondrites. (8) Volatile elements (Mn, Na, K, Rb, Cs, Zn, Se, Te, Pb, Tl) are considerably depleted compared to all other main carbonaceous chondrite groups, reflecting the low matrix abundance. (9) Bulk O isotope compositions plot along the CCAM line (Δ17O −3.96 to −5.47‰), partly overlapping with the CV and CK chondrite fields but including samples that are more 16O-rich. (10) Unique positions of CL values in the є54Cr-є50Ti isotope plot, with є54Cr values similar to CV, CK, and CO, but є50Ti values similar to CR chondrites. All CL chondrites studied here are of petrologic type 3.9 to 4, indicating that they have been thermally metamorphosed on the parent body. The diagnostic features of CL chondrites detailed here provide a basis for identifying CL members of lower petrologic types. Such samples will be important for determining the pristine state of these meteorites and their components.

Two contrasting Neoarchean metavolcanic rock suites in eastern Hebei and their geodynamic implications for the northern North China Craton

The late Archean was a crucial transition period for global geodynamics. Metavolcanic rocks, as one of the predominant components of Archean cratons, are key to deciphering the geodynamic regimes of early Earth crust-mantle interactions. Neoarchean supracrustal rocks in the central K-rich granitoid belt of the eastern Hebei-western Liaoning Province, northern North China Craton, may be subdivided into a lower metavolcanic sequence and an upper metavolcanic-sedimentary sequence by an angular unconformity.The lower sequence metavolcanic rocks consist primarily of 2558–2536 Ma amphibolite to granulite facies metamorphosed basaltic-andesitic rocks and high- and low-MgO dacitic rocks. The basaltic-andesitic samples display calc-alkaline affinity, high MgO contents, low (Nb/La)N and moderate (Hf/Sm)N values and were most likely derived from partial melting of mantle materials metasomatized by subducted slab- and sediment-derived melts and fluids. The high-MgO dacitic samples are characterized by strongly fractionated rare earth element (REE) patterns and relatively high MgO contents, which resemble typical high-SiO2 adakites. The magmatic precursors originated from subducted oceanic slabs and sediments. The samples with low-MgO dacite compositions have the highest SiO2 and lowest MgO contents, and the magmatic precursors were generated by partial melting of metamorphosed high-K mafic rocks and sedimentary rocks in the lower crustal level.The upper sequence metavolcanic rocks chiefly comprise greenschist to amphibolite facies metamorphosed tholeiites, tholeiitic to transitional basalts, calc-alkaline basalts, andesites, and dacites with magmatic crystallization ages of 2522–2511 Ma. The tholeiite samples are characterized by left-declined REE patterns and high (Nb/La)N and chondrite-like (Hf/Sm)N values resembling typical N-MORBs and were formed by partial melting of a depleted mantle source. The tholeiitic to transitional basalt samples resemble back-arc basin basalts and display weakly right-declined REE patterns, mildly negative Nb and Ti anomalies, and low (Nb/La)N and chondrite-like (Hf/Sm)N values. Their magmatic precursors were derived from partial melting of a slightly fluid- and melt-metasomatized depleted mantle source. The calc-alkaline basalt and andesite samples exhibit fractionated REE patterns and apparent Nb, Ta, and Ti depletions and originated from a strongly fluid- and melt-metasomatized depleted mantle source. The dacite samples show the highest SiO2 and lowest MgO contents in the upper sequence metavolcanic rocks and were derived from partial melting of lower crustal metasedimentary rocks. Systematic petrological, geochemical, and petrogenetic studies indicate that a mature continental arc setting and a back-arc basin setting may account for the formation of the lower and upper sequence metavolcanic rocks, respectively. Three stages of deformation can be identified in the K-rich granitoid belt, including 2525–2505 Ma extensional deformation in a back-arc basin setting (D1), ca. 2500 Ma top-to-the-NEE-SSE thrusting in a collisional orogenic setting (D2) and 2500–1800 Ma nearly N-S-oriented strike-slip shearing (D3), with D2 deformation accompanied by the synchronous occurrence of paired high-pressure and high-temperature medium-pressure granulite facies metamorphism. Integrating the petrogenesis, spatiotemporal relationships, deformational and metamorphic features of the Archean lithological assemblages, we consider that the late Neoarchean K-rich granitoid belt was formed in an active continental margin setting with a southeastward subduction polarity and experienced multi-stage trench retreats and a subsequent collision between the arc and continent.

Geology, mineralization and short wave infrared alteration mapping of the Khan Altai Au deposit, Mongolia

AbstractKhan Altai Au deposit is located near the N margin of the Neoproterozoic‐Cambrian Lake terrain in SW Mongolia, about 5 km from its contact with the Proterozoic‐Archean Baydrag craton, and 18 km SE of the Khantaishir ophiolite, which was emplaced in the Late Cambrian. The host rocks are strongly deformed and metamorphosed to lower greenschist facies, and of uncertain age. They comprise a sequence of rhyolitic volcaniclastics and porphyritic flows interbedded with laminated siltstone, basaltic andesite and faulted against dolomite. Low‐grade gold mineralization (up to 2 ppm Au) is hosted mainly in rhyolitic volcanics and extends over an area of about 1,100 × 160 m with a vertical extent of about 200 m. It is associated with disseminated pyrite (2–10% by vol) but includes high‐grade zones (up to 183 ppm Au over 1 m) related to cm‐wide quartz‐native Au‐pyrite veins. The Au mineralized zone also encompasses VMS mineralization (currently of minor extent), characterized by massive pyrite‐sphalerite lenses and quartz‐chalcopyrite stringer zones. A larger VMS deposit (Ereen Budagt, about 10 Mt sulfide ore) is found 6 km to the SSE in a similar geological setting. The main alteration assemblage is quartz‐white mica‐albite, but with minor carbonate, chlorite, epidote‐actinolite and pyrophyllite‐diaspore‐dickite alteration. Gold mineralization is related to zones of strong tectonic foliation and formation of phengitic white mica, with an outward zonation to high Al white mica. Pyrite is typically euhedral, and exhibits concentric growth zones, as well as quartz pressure shadows to enclosing foliation, consistent with syngenetic growth during metamorphism and deformation. Other sulfides include arsenopyrite, sphalerite and possible marcasite. Preliminary LA‐ICPMS mapping shows pyrite rims are enriched in Au, As, Co, Cu, Ni, Pb, Ag, Mo and Se, the pyrite core is enriched in Co, Bi, Te, and rhyolitic host rock is enriched in K, Ba, V and Tl. Whole rock geochemistry of basaltic andesite to rhyolite, shows N‐MORB characteristics for basaltic andesite, as well as a subduction signature for all rocks, and high MgO (~8%), TiO2 (~1%) and low Ni, Cr content in basalt, compatible with a back arc tectonic setting.

Geology and geochemistry of deformed granites from the Querari Complex, Rio Negro Province, NW-Amazonian Craton, Brazil

The Querari Complex corresponds to the basement rocks from Uaupes Domain and represents one of the least studied units in the northwest of the Rio Negro Province, Amazonian Craton. In this paper, we present new field geology, petrographic, and geochemical data of deformed granites from the Querari Complex. Based on our results, we suggest the differentiation of the Querari Complex into two magmatic facies. The Pana-Pana Facies comprise mainly porphyritic syeno to monzogranites with biotite as the main mafic mineral, and titanite, epidote, and allanite as accessory phases. The rocks of the Pana-Pana Facies have calc-alkaline and metaluminous affinities. The Matapi Facies have a syenogranite composition, coarse to porphyritic texture with magmatic muscovite and biotite, normative corundum higher than 1% (1.85-2.06), and mostly a peraluminous character. Both facies are generally enriched in silica and have a small spectrum of variation. (67.5 to 70.1 wt.% Pana-Pana Facies - 71.1 to 73 wt.% Matapi Facies), but they differ by the high Al2O3, Fe2O3, MgO, CaO, Sr, and Ba contents in the Matapi Facies. Geological and geochemical compositions suggest that the magmatic precursors of these rocks of Matapi Facies may be derived from partial melting of metasedimentary sources, whereas the Pana-Pana Facies are probably derived from a meta-ignea source of intermediate composition, such as tonalitic gneisses. The rocks of the Querari Complex would have been generated in the collisional tectonic settings during the Paleoproterozoic (between 1778 and 1740 Ma). During this 1.78-1.74 Ga tectono-magmatic event (D1), the magmatic fabric (S0) and the parallel S1 foliation have been developed, with NE-SE orientation. A subsequent tectonic event (D2) was responsible for the folding S1 foliation, creating the S2 axial planar foliation with an approximate orientation E-W. This D2 event is thought to represent a Calymmian (1.52 - 1.48 Ga) tectono-metamorphic event, associated with widespread I- and S-type granite emplacement in the Rio Negro Province. The deformation textures, as well as the mineral paragenesis for the Pana-Pana and Matapi facies, reveal temperature conditions at upper amphibolite facies, up to 700oC. Finally, the D3 tectono-metamorphic event comprises the reactivation of many NE-SW ductile shear zones with sinistral kinematic, creating a mylonitic foliation (S3) in the rocks of the Querari Complex. This D3 tectono-metamorphic event is probably linked to the intracratonic K´Mudku tectono-metamorphic event (between 1317 and 1198 Ma) of the Amazonian Greenvillian-Sunsas continental collision.

Softening—melting behavior of mixed burden based on low-magnesium sinter and fluxed pellets

A low MgO content in sinter is conducive to reduce the MgO content in blast furnace slag. This study investigated the effect of MgO content in sinter on the softening—melting behavior of the mixed burden based on fluxed pellets. When the MgO content increased from 1.31wt% to 1.55wt%, the melting temperature of sinter increased to 1521°C. Such an increase was due to the formation of the high-melting-point slag phase. The reduction degradation index of sinter with 1.31wt% MgO content was better than that of others. The initial softening temperature of the mixed burden increased from 1104 to 1126°C as MgO content in sinter increased from 1.31wt% to 1.55wt%, and the melting temperature decreased from 1494 to 1460°C. The permeability index (S-value) of mixed burden decreased to 594.46 kPa·°C under a high MgO content with 1.55wt%, indicating that the permeability was improved. The slag phase composition of burden was mainly akermarite (Ca2MgSiO7) when the MgO content in sinter was 1.55wt%. The melting point of akermarite is 1450°C, which is lower than other phases.

Glauconite authigenesis during the onset of the Paleocene-Eocene Thermal Maximum: A case study from the Khuiala Formation in Jaisalmer Basin, India

Biological and ecological changes across the Paleogene hyperthermal events were accompanied by dramatic shifts in sedimentation systems in both marine and continental environments. This study investigates the formation of authigenic glauconite within a biostratigraphically constrained section encompassing the Paleocene-Eocene Thermal Maximum (PETM). Paleogene transgressive shelf deposits in the Jaisalmer Basin in western India are represented by the Khuiala Formation, consisting of grey shale, limestone, and glauconitic shale. Foraminiferal assemblage not only constrains the age of the glauconitic shale to planktic zone E1 but also indicates the deposition of sediments in an oxygen-depleted mid-shelf environment. A detailed geochemical and mineralogical characterization of the authigenic glauconite within the mid-shelf deposits reveals the highest glauconite content within the maximum flooding zone, immediately below the negative carbon isotope excursion representing the PETM, followed by its steady decrease. The geochemical signature of the glauconite is unusual for its high Al2O3 and MgO and medium Fe2O3 and SiO2 contents. The K2O content suggests the ‘evolved’ nature of the glauconite, which is confirmed by the X-ray diffractional parameters and the presence of the ‘rosette’ texture in FEG-SEM images. The glauconite evolved by Al-for-Fe substitution in the octahedral site and subsequent fixation of K+ into the interlayer sites. The high alumina content of the glauconite corresponds to the high Al3+ in the tetrahedral layer, inherited from an aluminous substrate. Mössbauer spectroscopy suggests the dominance of Fe3+ compared to Fe2+ in the octahedral site of glauconite pellets. The unusual variety of glauconite formed by the transformation of continent-derived kaolinite and iron oxide in oxygen-depleted porewater conditions. Glauconite authigenesis in the Khuiala Formation is strongly influenced by the warm climatic condition, oxygen-depletion in the mid-shelf environment, and sharp marine transgression promoting sediment starvation associated with the PETM.

Geochemical characteristics of the early Neoarchean komatiite from the North China Craton: Evidence for plume–craton interaction

Komatiite is characterized by extremely high MgO contents and H2O-poor nature and plays an important role in understanding the early mantle-derived magma processes, crust–mantle differentiation of the Archean earth, but it is sparse in the North China Craton. Although komatiitic rocks have ever been discovered in the Mengyin area, their formation ages and types are still in debate. Here, we firstly report a new komatiite outcrop near Nanbaita village (Jinan city) of the western Shandong Province and present new major and trace element contents, as well as Sr–Nd and Re–Os isotopic compositions. The rocks show spinifex texture with primary olivine and pyroxene skeletal needles have been transformed into secondary minerals (e.g., serpentine, tremolite). The komatiites display extremely high MgO, but low TiO2 and Na2O + K2O. Their chondritic Al2O3/TiO2 and GdN/YbN ratios suggest they are Aluminium-undepleted komatiites which are generated by larger degrees of melting (~50%) in relatively lower pressure (ca. 5–7 GPa) with garnet absence than Aluminium-depleted komatiites. The formation age of these komatiites was no later than 2.69 Ga constrained by the Re depletion age. Flat heavy rare earth element patterns and low 187Re/188Os and Th/Yb ratios show the primary komatiitic magma was not contaminated by substantial crustal material. The γOs value of sample 18LH51 is 0.09, which is adjacent with chondrite and other contemporaneous komatiites in the world, indicating that the Nanbaita komatiites were originated from a chondritic mantle. Furthermore, the widely distribution of ~2.7 Ga ultramafic–mafic rocks in the western Shandong Province might be the reflection of a contemporaneous worldwide superplume event.

Zircon U–Pb geochronology and geochemistry of the Lajimiao mafic complex in the Shangdan Suture Zone, Qinling orogen: Petrogenesis and tectonic implications

The Shangdan Ocean represents the eastern branch of the Proto-Tethys Ocean, which is important to decipher the evolution of the Central China Orogenic Belt. However, its evolution is still in debated. In this contribution, we carried out an integrated study of zircon UPb ages, whole-rock geochemistry, as well as zircon HfO isotope compositions for the Lajimiao mafic complex in the Shangdan Suture Zone in the North Qinling orogen. The Lajimiao mafic complex is composed of coarse-grained gabbros to norite-gabbros (Group 1) in the inner and fine-grained diorites (Group 2) in the outer part. The Group 1 samples are comprised predominantly of salite to augite (Mg# = 75–80), magnesi-hornblende (Mg# = 65–67) and Ca-rich plagioclase (An = 45–67); the Group 2 samples are comprised predominantly of magnesi-hornblende (Mg# = 60–66) and plagioclase (An = 28–53). New SIMS and LA-ICP-MS UPb dating of zircon grains from a gabbro and a diorite sample yielded similar weighted mean ages of 412 ± 4 Ma and 414 ± 2 Ma, indicating that they were formed simultaneously. The Group 1 samples have relatively low SiO2 (49.56–50.78 wt%), high MgO (6.67–7.70 wt%) and Fe2O3T (7.46–9.61 wt%) contents, while the Group 2 samples have relatively high SiO2 (57.74–59.53 wt%), low MgO (3.28–3.42 wt%) and Fe2O3T (5.76–7.01 wt%) contents. They both belong to calc-alkalic series and enriched in large ion lithophile elements (LILEs), light rare earth element (LREEs) and depleted in high field strength elements (HFSEs), which are typical of arc-related rocks. The Group 2 diorites exhibit relative enrichment in light rare earth element (LREE) ((La/Yb)N = 10.9–13.1) with slightly negative Eu anomalies (Eu/Eu⁎ = 0.82–0.93), compared to the Group 1 samples ((La/Yb)N = 5.2–9.6; Eu/Eu⁎ = 0.91–1.43). Both of them have enriched initial Sr (0.7049–0.7054), and chondritic to slightly depleted Nd (ƐNd(t) = −0.09 ~ +1.50), but depleted whole-rock and zircon Hf isotopic compositions (ƐHf(t) = 7.81–8.10 and 8.2–9.1). The HfNd isotopic systems are decoupled with positive △ƐHf values (5.22 to 5.92) (△ƐHf = ƐHf(t) – (1.59 × ƐNd(t) + 1.28)). The zircon grains display higher δ18O values of 6.3 to 6.5‰ than those of the normal mantle zircon. Whole-rock Pb isotopic compositions of the studied samples are of 15.610–15.624 for 207Pb/204Pb, 38.027–38.178 for 208Pb/204Pb, and 18.100–18.269 for 206Pb/204Pb. These geochemical features indicate that the Lajimiao mafic complex was derived from relatively depleted mantle that had been metasomatized by 2–5% subducted zircon-barren oceanic sediments. Combining with regional geological, geochronological, mineralogical, and geochemical data, the Lajimiao mafic complex was suggested to be generated by partial melting of a phlogopite-bearing mantle source at relatively low pressure in an arc setting. We suggested that the generation of the Lajimiao mafic complex was induced by the subduction of Shangdan Ocean during the early Devonian and thus the eastern branch of Proto-Tethys Ocean might have not been closed until that time.

Recycling of crustal materials and implications for lithospheric thinning: Evidence from Mesozoic volcanic rocks in the Hailar–Tamtsag Basin, NE China

Late Mesozoic Nb-rich basaltic andesites and high-Mg adakitic volcanic rocks from the Hailar–Tamtsag Basin, northeast China, provide important insights into the recycling processes of crustal materials and their role in late Mesozoic lithospheric thinning. The Late Jurassic Nb-rich basaltic andesites (154 ± 4 Ma) are enriched in large-ion lithophile and light rare earth elements, slightly depleted in high-field-strength elements, and have high TiO2, P2O5, and Nb contents, and (Nb/Th)PM and Nb/U ratios, which together with the relatively depleted Sr–Nd–Hf isotopic compositions indicate a derivation from a mantle wedge metasomatized by hydrous melts from subducted oceanic crust. The Early Cretaceous high-Mg adakitic volcanic rocks (129–117 Ma) are characterized by low Y and heavy rare earth element contents, and high Sr contents and Sr/Y ratios, similar to those of rocks derived from partial melting of an eclogitic source. They also have high Rb/Sr, K2O/Na2O, and Mg# values, and high MgO, Cr, and Ni contents. These geochemical features suggest that the adakitic lavas were derived from partial melting of delaminated lower continental crust, followed by interaction of the resulting melts with mantle material during their ascent. Our data, along with available geological, paleomagnetic, and geophysical evidence, lead us to propose that recycling of Paleo-Pacific oceanic crustal materials into the upper mantle due to flat-slab subduction and rollback of the Paleo-Pacific Plate during the late Mesozoic likely provided the precondition for lithospheric thinning in northeast China, with consequent lithospheric delamination causing recycling of continental crustal materials and further lithospheric thinning.

Neoarchean-early Paleoproterozoic granitoids, the geothermal gradient and geodynamic evolution in the Hengshan Terrane, North China Craton

The Hengshan Terrane (HST) is a typical Archean terrane in the central North China Craton (NCC) and features Neoarchean-early Paleoproterozoic granitoid rocks and associated supracrustal rocks. Five geological events are identified in the HST, including (1) ~2.71–2.67 Ga tonalites-trondhjemites-granodiorites (TTGs) and monzogranites, (2) ~2.54–2.48 Ga diorites, TTGs and volcanics, (3) ~2.44–2.43 Ga monzogranites, (4) ~2.14–2.04 Ga monzogranites and (5) ~1.92–1.84 Ga granulite- to high-grade amphibolite- facies metamorphism. The ~2.5 Ga diorites exhibit low SiO2 concentrations (55.67–62.61 wt%), high MgO content of 1.39–4.89 wt% with Mg# values (45–62) and positive ɛHf(t2) values of +3.2−+7.8, and the magma originated from a mantle source that had been metasomatized by dehydration fluids and melts from subduction-related sediments and slabs. The ~2.5 Ga TTGs are characterized by variable MgO contents (0.56–2.32 wt%) and Mg# values (44–67), (La/Yb)N values (13.76–98.19) and positive ɛHf(t2) values of +2.8 to +6.5, and derived from the partial melting of tholeiitic basalts with slightly enriched REE patterns, similar to those of back-arc basin basalts (BABBs). And these granitoid melts were contaminated by mantle materials. The ~2.4 Ga monzogranites show high SiO2 (69.09–73.46 wt%) and K2O (3.31–7.52 wt%) contents and low MgO (0.11–1.23 wt%) contents, and their parent magmas originated from the partial melting of metamorphic graywackes.The mantle material that generated the dioritic magmas was enriched by the important metasomatic agent of sediment-derived melts and fluids prior to magmatism. These observations, in conjunction with the occurrence of widespread coeval calc-alkaline volcanic rocks in the adjacent Wutai region, suggest that a slab subduction was the most likely geodynamic regime leading to the late Neoarchean magmatism in the HST. In view of the BABB sources of the TTGs, a Neoarchean back-arc basin slab subduction model may be appropriate, which is supported by several Phanerozoic analogs. The partial melting P-T conditions of the ~2.5 Ga slab-derived TTG magmas are estimated to have been 1.5 ± 0.2 GPa and 823 ± 32 °C on the basis of thermodynamic and trace element simulations, indicating a geothermal gradient of ~17 ± 2 °C/km and a possible Neoarchean hot subduction process in the HST.

Middle Cambrian-Early Ordovician ophiolites in the central fault of the East Kunlun Orogen: Implications for an epicontinental setting related to Proto-Tethyan Ocean subduction

The Qingshuiquan-Tatuo ophiolites (QTOs), which are located in the central fault of the Kunlun Orogen, can provide critical information regarding the subduction processes of the Proto-Tethys Ocean. This study presents petrological, whole-rock geochemical, SrNd isotopic, and zircon UPb data on these ophiolites. The QTOs consist of mantle peridotites, pyroxenites, and fine-grained low- and high-Ti (alkaline) gabbros (LTGs and HTGs, respectively) and lack mafic lava units. The QTO mafic units can be geochemically subdivided into LTGs and HTGs. The LTGs have high MgO contents (mostly >8.0%), V/Ti ratios (37.41–158.77) and low TiO2 (mostly <0.5%), REE contents and variable ɛNd(t) values (−3.48 to +3.89), and they exhibit geochemical similarities to forearc mafic rocks. Zircon UPb data yield a crystallization age of 516 Ma for the LTGs. In contrast, the HTGs show relatively high total alkali (1.72–5.66%), TiO2 (1.45–3.68%) and REE contents, high (La/Yb)N (3.37–14.06), Ti/Y (290.78–1058.94), and low ɛNd(t) values (−3.02 to −3.55), which suggest a geochemical signature of oceanic island basalt (OIB) and continental rift-like basalt. The zircon UPb data yield crystallization ages of 486–485 Ma for the HTGs. Petrogenetic analyses suggest that the LTGs were derived from remelting of forearc mantle peridotites and that the serpentinites (harzburgites) represent the forearc mantle after the extraction of low-Ti mafic melts. In contrast, the HTGs were derived from a fertile asthenospheric mantle upwelling through a slab window followed by minor subcontinental lithospheric mantle (SCLM)/crust contamination. The findings suggest that the QTOs were generated in an epicontinental environment above the subduction zone, which was geodynamically related to slab rollback at 516 Ma and subsequent oceanic-ridge oblique subduction at 485 Ma.

Reworking of juvenile crust beneath the Bangong–Nujiang suture zone: Evidence from Late Cretaceous granite porphyries in Southern Qiangtang, Central Tibet

Crustal reworking in the collisional zones is commonly considered as an important process to result in continental crust differentiation and maturation. However, the reworking mechanism remains unclear. Here, a combined study of zircon SIMS U–Pb age, whole-rock major and trace element, Sr–Nd–Hf and zircon Hf–O isotopic geochemistry was carried out for the Duoma–Namuqie (DM–NMQ) granite porphyry intrusions, including DM, NMQ I, II and III intrusions, in southern Qiangtang, central Tibet. Zircon SIMS U–Pb dating yielded ages of 79–76 Ma, suggesting that they were formed in the later stage of Late Cretaceous. The DM granite porphyries are metaluminous to peraluminous with slightly low SiO2 (69.2–71.4 wt%) and high MgO (0.67–0.79 wt%) contents. They are depleted in high-field-strength elements (HFSEs) and enriched in light rare earth elements (LREEs), large ion lithophile elements (LILEs), and show weakly negative Eu anomalies. The NMQ III granite porphyries show similar elemental features to the DM granite porphyries, but have relatively high SiO2 (69.8–72.2 wt%) and low MgO (0.25–0.28 wt%) contents. Both of them belong to I-type granites. The NMQ I and II granite porphyries are relatively differentiated with high SiO2 (76.4–77.9 wt%), low MgO (0.11–0.16 wt%) contents, and significantly negative Eu anomalies. Extremely low LREE contents and notable REE tetrad effect can be observed in the NMQ II granite porphyries. The NMQ I and II intrusions are fractionated I-type granites. The whole-rock Sr–Nd–Hf and zircon Hf–O isotope data exhibit an enriched trend from the DM granite porphyries ((87Sr/86Sr)i = 0.7049–0.7050, εNd(t) = 1.2–1.6, εHf(t)whole-rock = 9.9–12.0, εHf(t)zircon = 7.4–11.5, δ18O = 6.2–7.1‰) to the NMQ III granite porphyry ((87Sr/86Sr)i = 0.7058–0.7067, εNd(t) = −0.9 to −1.1, εHf(t)whole-rock = 7.0–7.4, εHf(t)zircon = 2.7–9.5, δ18O = 7.2–7.6‰) to the NMQ I and II granite porphyries (εNd(t) = −3.0 to −4.3, εHf(t)whole-rock = 5.3–6.6, εHf(t)zircon = 1.5–7.4, δ18O = 6.6–9.2‰). We suggest that the DM–NMQ granite porphyries were mainly derived by partial melting of the juvenile mafic lower crust with variable volumes of ancient crustal components involved in their magma sources. The way in which these granite porphyries were formed provides evidence for the crustal differentiation and maturation in a post-collisional extensional setting triggered by the far-field rollback of the subducted Neo-Tethys oceanic slab

Evolution of the Tethyan Bangong-Nujiang Ocean and its SE Asian connection: Perspective from the Early Cretaceous high-Mg granitoids in SW China

High-Mg diorites (HMDs) have important implications on crust-mantle interactions and subduction processes, and are widely used to delineate ancient convergent plate margins. In this study, we described a newly-identified high-Mg diorite-granodiorite suite in the Tengchong terrane (SE Tibetan Plateau), and presented its petrography, whole-rock geochemistry, SrNd isotopes, and zircon U-Pb-Hf-O isotopic compositions. High-precision SIMS zircon UPb dating yielded consistent Early Cretaceous ages (~120 Ma) for all lithologies in this high-Mg suite. The high-Mg diorite is characterized by high MgO (4.7–9.2 wt%), Cr (169–323 ppm) and Ni (32–138 ppm) contents, together with high (87Sr/86Sr)i (0.70867–0.71060) and negative εNd(t) (−4.07 to −6.80) values. Their zircon εHf(t) vary from −5.64 to +4.12, and their δ18O (6.0 to 6.9‰) are slightly higher than the average mantle value. The granodiorite has higher SiO2 and lower MgO contents, and varying zircon εHf(t) (−12.19 to −2.71) and δ18O (6.3 to 8.1‰). The rocks are characterized by enrichments in large ion lithophile elements (LILEs: e.g., Rb, Th, U, K) and light rare earth elements (LREEs), but depleted in high field strength elements (HFSEs), and have also high (87Sr/86Sr)i (0.71057–0.711860) and negative εNd(t) (−8.42 to −9.14). Mafic microgranular enclaves (MMEs) in the granodiorite have intermediate SiO2, MgO, and trace elements and Sr-Nd-Hf-O isotope compositions between the high-Mg diorite and the granodiorite. Based on these geochemical and isotopic features, we interpreted that the high-Mg diorite was generated from the interaction of subducted sediment-derived melt with the overlying mantle wedge, whilst the granodiorite was sourced from the ancient continental crust with minor mantle-derived input. Mixing between the mantle-derived high-Mg dioritic melt and the crustal-derived granodioritic melt may have formed the MMEs. Our new geochemical (incl. isotopic) data, combined with published ones across the region, suggest that these Early Cretaceous plutons were formed by asthenospheric upwelling, which was probably led by slab rollback of the west-dipping subduction of the Luxi-Ruili Ocean between the Baoshan and Tengchong terranes. This implies that the southeastern continuation of the Tethyan Bangong-Nujiang Ocean was located between the Baoshan and Tengchong terranes, instead of between the Tengchong and West Burma terranes as previously suggested.

High-Mg andesitic rocks formed through crustal magmatic differentiation

High-Mg andesitic rocks are enriched in both compatible (Mg, Cr and Ni) and incompatible elements (Si, Ba and Sr), which indicates the complexity of their petrogenesis. Models involving hybridization of the mantle sources and input of adakitic melts have been invoked, but struggle to explain the low Mg# values (<0.65) of some low Sr/Y (<40) high-Mg andesitic rocks, because these rocks cannot represent primary melts (Mg# ≥ 0.72) produced by the partial melting of mantle peridotite. This study presents a petrological and geochemical analysis of the Xinsi diorite (443 ± 3 Ma) and Tiechang gabbro (440 ± 3 Ma) from the Early Paleozoic Wuyi-Yunkai Orogen in South China, and demonstrates the role of magmatic differentiation in the petrogenesis of high-Mg andesitic rocks. These rocks contain high SiO2, MgO, Ba and Sr concentrations with low Mg# values (≤0.69) and Sr/Y ratios (≤16.7), resembling typical low Sr/Y high-Mg andesitic rocks. Amphiboles and clinopyroxenes in the Xinsi diorite contain variable contents of Ba and Sr, both of which negatively correlate with Mg#. This indicates that their parental melts had low Ba and Sr and underwent significant enrichment of Ba and Sr through the fractionation of Mg-rich minerals within upper crust (3.1–4.4 kbar). Clinopyroxene, plagioclase and biotite in the Tiechang gabbro all contain lower Ba and Sr than those in the Xinsi diorite, suggesting that their parental melts were also initially low in Sr and Ba and experienced enrichment of these elements during magma evolution at middle crustal levels (5.7–6.3 kbar). The low Ba and Sr concentrations and Sr/Y ratios for parental melts of the Xinsi diorite and Tiechang gabbro suggest their derivation from the partial melting of mantle sources with input from slab melts at shallow depths without involvement of abundant garnet in petrogenesis. This study demonstrates the importance of fractional crystallization in the genesis of low Sr/Y high-Mg andesitic rocks and is conflict with past views that their geochemical features were related to the involvement of adakitic melts in their mantle sources. Therefore, this study presents an alternative model for the production of low Sr/Y and high Sr/Y high-Mg andesitic rocks, particularly in the Late Archean, which reflects the intense melting of subducted slab at different depths in the hotter mantle regime at that time.

Geochemistry of late paleoproterozoic Anjana and Amet granites of the Aravalli craton with affinities to sanukitoid series granitoids: Implications for petrogenetic and geodynamic processes

The Mangalwar Complex of the Aravalli craton is marked by the presence of late Paleoproterozoic granites referred to as Anjana Granite and Amet Granite. These granites occur as 1.64 Ga old plutons intruding greenstone sequences and migmatitic gneisses of Mangalwar Complex which comprises parts of BGC of the Aravalli craton. In the present contribution major, trace and REE data of these granites along with associated microgranular mafic enclaves (MMEs) are presented and discussed. Geochemically these granites are quartz monzonite, metaluminous, sub-alkaline and high-K calc-alkaline rocks. The most important characteristics of Anjana and Amet granites are low SiO2, high MgO, Mg#, K2O, Ba, and low Na2O/K2O ratios. In addition, the REEs show moderate to high fractionation, with (La/Yb) ratios up to 22 and 23 of the Anjana and Amet granites respectively, with no or positive europium anomalies. In the primitive mantle-normalized trace element diagrams both granites show depletion in high-field strength elements (HFSE) such as Nb, Ta, P, Ti and enrichment in LILEs. Most of these features are comparable to those of sanukitoid series rocks. Geochemically both granites are distinguished as high-Ti sanukitoids. Geochemical characteristics of MMEs suggest that they are similar to Anjana and Amet granites and in turn to sanukitoids with lower SiO2 content. They display LREE enriched patterns with low values (avg. 13) of (La/Yb)N, negative Eu anomalies and high HREE contents (58 ppm). It is suggested that the parental magma of Anjana and Amet granitic plutons originated through a four stage process (1) Generation of magmatic melts produced by partial melting of terrigeneous sediments of subducting slab in an arc setting; (2) interaction of those melts with the overlying mantle wedge, and total consumption of slab-derived melts during the reaction resulting in production of a metasomatized mantle; (3) tectonothermal event, possibly related to the slab break-off, causing asthenospheric mantle upwelling. This may have induced the melting of the metasomatized mantle and the generation of sanukitoid magmas. The parental magmas of Anjana and Amet granites and their mafic enclaves were generated at lower and higher lithospheric levels respectively (4) Granitic magma ascended due to viscosity and gravity instabilities and interacted with enclave magma at higher mantle level. Both magmas ascended towards upper crust and evolved through fractional crystallisation. Existing data suggest that in the Mangalwar Complex, the formation of sanukitoid magma started even during Mesoarchaean times and continued till late Paleoproterozoic. Formation of sanukitoid magma during this time indicates that in northern Indian shield the multi-stage subduction- accretionary orogenic processes continued for a protracted geological period and played a major role in the origin and evolution of early continental crust.

Ultramafic intrusions hosting Ni–Cu sulfide mineralization along a suture zone in the southwestern margin of the Amazonian craton, Brazil: Examples from Morro Sem Boné, Morro do Leme and their satellite intrusions

This research paper describes several medium-to small-sized ultramafic intrusions located along the Mesoproterozoic Guaporé Suture Zone in the southwestern margin of the Amazonian craton. Apart from minor sheared zones, ultramafic rocks have igneous minerals and textures largely preserved. Metamorphic parageneses of metapelites and amphibolites of country rocks, as well as those from sheared zones of the ultramafic intrusions, are indicative of the amphibolite facies of regional metamorphism. Intersections of disseminated to massive sulfides consisting mainly of pyrrhotite and pyrite are common in the metamorphosed volcanic and sedimentary country rocks. The ultramafic intrusions consist mainly of olivine cumulates with high MgO contents (up to 45.9 wt%) and Mg# (up to 0.84), consistent with the composition of their most primitive olivine (Fo93). The crystallization sequence, dominated by olivine, orthopyroxene and chromite, together with ratios and patterns of incompatible trace elements of ultramafic cumulates, support the interpretation that they originated from a high-MgO magma partially contaminated with crustal rocks. Sulfide mineralization in the ultramafic intrusions comprises different styles of magmatic sulfides associated with cumulate rocks, including contact-type, stratabound-type and conduit hosted mineralization. Significant intersections of interstitial to net-textured sulfides consisting of pyrrhotite, pentlandite and chalcopyrire have variable but generally low Ni–Cu-PGE contents (e.g., ~20 m at 0.5 wt% Ni, 0.1 wt% Cu, and 0.5 ppm Pt+ Pd). The range of tenors of Ni, Cu, Pt and Pd, from moderate to very low, is interpreted as resulting from varying efficiency of equilibration of sulfide xenomelts incorporated from the sulfide-bearing country rocks. The Ni–Cu-PGE mineralized ultramafic intrusions are interpreted to belong to a network of magma conduits and staging magma chambers intruding the sulfide-bearing volcanic-sedimentary sequence. Our results indicate an additional prospective belt of high-MgO Ni–Cu-PGE sulfide mineralized intrusions located in a cratonic margin in Brazil.

Transition in tectonic regime from the Paleo-Asian Ocean to Paleo-Pacific Ocean: constraints from the Jurassic adakitic and I-type granites, and calc-alkaline diorites at the northern margin of the North China Craton

ABSTRACT There is a broad consensus that subduction of the Paleo-Pacific oceanic plate has caused extensive magmatism, lithospheric deformation, and metal mineralization in the eastern margin of the Eurasian continent during the Late Mesozoic. However, the onset and extent of subduction of the Paleo-Pacific Ocean, especially the timing of the transition in tectonic regime from the Paleo-Asian Ocean to Paleo-Pacific, and extent of the subduction of the Paleo-Pacific Ocean remain poorly constrained. To address these key issues, we present regional geological observations, zircon U–Pb dating, whole-rock geochemistry, and Hf isotopic compositions of the Jurassic intrusions at the northern margin of the North China Craton, with the aim of constraining their petrogenesis and the tectono-magmatic events related to subduction of the Paleo-Pacific Ocean. The geochronological and geochemical results indicate that these intrusions were emplaced between 177 and 155 Ma, with distinct geochemical features. The Huajianggou pluton (HJG), Tongjiatun pluton (TJT), and Haoguantun pluton (HGT) are characterized by high Al2O3 and Sr contents, and low MgO, TFe2O3, Y, Yb, and heavy rare earth elements (HREEs) contents, coupled with high Sr/Y values (34.10–94.36), showing adakitic affinities. Based on these geochemical characteristics, we suggest that these adakitic magmas (i.e. the HJG, TJT, and HGT plutons) were produced by partial melting of thickened lower crust in response to earlier subduction of the Paleo-Pacific oceanic plate beneath the northern margin of the North China Craton. In contrast, the Baijiagou pluton (BJG) and Sheshangou pluton (SSG) are high-K calc-alkaline rocks with low Sr/Y ratios (0.80–17.77), and they exhibit petrological and geochemical characteristics of I-type granites, indicating an origin from the lower crust, with variable degrees of fractional crystallization. In addition, the Bakeshu pluton (BKS) is mainly comprised of diorite that displays high SiO2, low MgO, Cr, and Ni contents, with Mg# of 39.8–42.9, indicating a lower crustal origin. Despite the range in ages of these intrusions, all the Jurassic calc-alkaline intrusions show typical geochemical characteristics of active continental margins, such as enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs). Combined with spatial-temporal variations of coeval magmatic rocks along the northern margin of the North China Craton, we suggest that the transition in tectonic regime from the Paleo-Asian Ocean to Paleo-Pacific Ocean was initiated in the Early Jurassic. These Jurassic intrusions formed in an active continental margin setting, related to westward subduction of the Paleo-Pacific oceanic plate. Importantly, the Early Jurassic subduction-related calc-alkaline magmatic rocks in northeast (NE) China constitute a NE-trending magmatic arc belt, which is best explained as continental magmatism triggered by the westward subduction of the Paleo-Pacific oceanic plate.

Geochemical, mineralogy, and Sr–Nd–Pb isotopic compositions of the gold-related lamprophyre in the Bengbu–Wuhe Area, southeastern North China Craton: Implications for gold mineralization

Besides granite porphyries, many mafic dikes also have a close temporal and spatial relationship with orebodies in the BWGB. However, compared with the granite porphyries, no detailed attempt has been made to determine the petrogenesis of these mafic dikes or to elucidate their relationships with the gold mineralization. Here, we present comprehensive bulk-rock geochemical data; pyrite, apatite, and amphibole compositions; Sr-Nd-Pb isotopes, and U-Pb zircon data for the lamprophyres to provide constrains on their petrogenesis, magmatic properties, geodynamic setting, and metallogenic implications. We obtained zircon U-Pb ages, which reveal that the lamprophyres were emplaced ~ 130 Ma. The lamprophyres are characterized by pronounced light rare earth element (LREE) enrichment, heavy rare earth element (HREE) depletions ((La/Yb)N > 10). They also have low SiO2 (<51 wt%) and high MgO (>4.1 wt%) contents; low Rb/Sr (0.01–0.04), Th/La (0.1–0.12), and Lu/Yb (0.13–0.16) ratios; high Ba/Rb ratios (>18), low Sr-Nd values (87Sr/86Sr(t) = 0.7060–0.7076; εNd(t) = –8.8 to –12.5); and a wide range of radiogenic Pb isotopic ratios (206Pb/204Pb(t) = 16.37–17.35), indicating an enriched mantle source metasomatized by the upper crust of the subducted South China Block (SCB). Their low apatite MnO contents and high apatite SO3 contents, high apatite molar OH/F ratios, and high amphibole Mg* and VIAl* values suggest that the mafic magmas were more highly oxidized and hydrated. Taken together, the lithospheric mantle source was initially metasomatized by the SCB and was further hydrated by the subducted Paleo-Pacific Plate, resulting in a mafic melt with a high H2O content and oxygen fugacity value, which are favorable conditions for gold transportation.