HREE Contents Research Articles

Petrology and geochemistry of mantle peridotites from the Kalaymyo and Myitkyina ophiolites (Myanmar): Implications for tectonic settings

Ophiolites within Myanmar have been commonly divided into two belts, i.e., the Eastern Belt and Western Belt. The Kalaymyo ophiolite from the Western Belt crops out at the eastern margin of the Indo-Burma Range and was formed during the Early Cretaceous, whereas the Myitkyina ophiolite from the Eastern Belt was formed during the Middle Jurassic. Both ophiolites are characterized by the occurrence of big massifs of mantle peridotites. Mantle peridotites of the Kalaymyo ophiolite are mainly composed of spinel lherzolites and harzburgites, with minor plagioclase peridotites. Mantle peridotites of the Myitkyina ophiolite mainly consist of spinel harzburgites, with minor dunites. Spinel lherzolites from the Kalaymyo ophiolite have relatively fertile compositions, with 40.12–45.19wt.% MgO, 1.1–2.74wt.% Al2O3 and 0.67–2.67wt.% CaO. Their spinels have Cr# values of 0.12–0.4, yielding fractional melting degrees of 3–15%. In comparison, spinel harzburgites from the Kalaymyo ophiolite are more refractory, with 42.08–48.73wt.% MgO, 0.09–0.99wt.% Al2O3 and 0.07–0.8wt.% CaO. Their spinels have Cr# values of 0.3–0.73, giving 12–21% degrees of fractional melting. Plagioclase peridotites from the Kalaymyo ophiolite have compositions intermediate between spinel lherzolites and harzburgites. Compared to the spinel peridotites, spinels in the plagioclase peridotites have relatively higher TiO2 contents. Harzburgites from the Myitkyina ophiolite, containing 40.88–48.16wt.% MgO, 0.13–1.65wt.% Al2O3 and 0.1–1.68wt.% CaO, have refractory compositions similar to the Kalaymyo harzburgites. Spinels in the Myitkyina harzburgites with low TiO2 contents (i.e., <0.2wt.%) have variable Cr# values of 0.28–0.72, yielding 11–21% degrees of fractional melting. Clinopyroxenes in all Kalaymyo peridotites display flat patterns in MREE and HREE, but variably LREE-depleted patterns. They also show remarkably negative Sr and Zr anomalies. Plagioclases in the Kalaymyo plagioclase peridotites display a significant Eu positive anomaly and have similar contents of LREE to the co-existing clinopyroxenes. In contrast, they have much lower contents of HREE and MREE than the clinopyroxene. Clinopyroxenes in the Myitkyina harzburgites display consistent patterns from HREE to MREE but different patterns in LREE. Clinopyroxenes with LREE-depleted patterns display remarkable negative Sr and Zr anomalies, whereas clinopyroxenes with LREE-flat patterns do not show Sr or Zr anomaly. However, the LREE contents of clinopyroxenes in both Kalaymyo and Myitkyina peridotites are too enriched to be produced by degrees of melting corresponding to spinel Cr# values. This indicates that they were variably re-enriched after melt depletion, which is also supported by mineral microtextures, e.g., embayment of olivine within orthopyroxene. Plagioclase in the Kalaymyo peridotites was produced during melt-peridotite reaction rather than through breakdown of spinel. Chemical compositions support the co-existence of both refractory and fertile peridotites in both Kalaymyo and Myitkyina ophiolites. The refractory peridotites have compositions similar to the forearc peridotites, whereas the fertile peridotites are compositionally similar to the abyssal peridotites. Therefore, both ophiolites have experienced complicated evolution in different tectonic settings.

Post-collisional Plio-Pleistocene Anar - Dehaj adakitic subvolcanic domes in the central volcanic belt of Iran: geochemical characteristics and tectonic implications

In the area between Anar and Dehaj, located in the Urumieh–Dokhtar magmatic assemblage (UDMA), there are some Plio-Pleistocenic subvolcanic porphyritic andesitic-dacitic domes. The subvolcanic domes, as a part of Dehaj-Sarduieh belt in Kerman province, show tafoni erosion and magma mingling evidences. Petrographically, the domes are andesite and trachyandesite in composition and are characterized by microlitic-porphyritic, glomeroporphyritic, trachytic and aphanitic textures. Mineralogically, the domes consist of plagioclase, amphibole, clinopyroxene and opaque minerals. Geochemical studies reveal that Anar-Dehaj subvolcanic domes can be divided two groups, named group 1 and group 2, on the basis of the silica content. Group 1 shows high-K calc- alkaline andesite- trachyandesite composition; whereas group 2 is medium-K calc-alkaline dacite. The absence of a distinct Eu anomaly in all samples suggests contemporaneous crystallization of plagioclase and amphibole, and/or oxidation state of magma. Trace element discrimination diagrams along with chondrite-normalized rare earth element patterns show that the Anar- Dehaj subvolcanic rocks formed in a subduction related environment. Moreover, high Sr/Y and La/Yb ratios along with low Y and HREE contents are consistent with adakitic natures. It is suggested that the Anar- Dehaj subvolcanic domes formed in a post-collisional setting due to slab melting or underplating of basaltic magmas under thick Plio-Pleistocine continental crust.

Y–REE-Rich zircons of the Timan region: Geochemistry and economic significance

Mineralogical–geochemical studies of zircon from the Ichet’yu occurrence revealed unusually high Y and HREE contents (correlative with the P content) in the inner parts and zones of approximately 10% of the grains. They represent the intermediate members of the zircon–xenotime join with a heterovalent scheme of isomorphism Zr4+ + Si4+ → (Y + HREE)3+ + P5+. Geochronological and mineralogical–geochemical data suggest that the Middle Timan basement (the most probable source of zircon of the Ichet’yu occurrence) is made up of the Paleoproterozoic rocks and possibly represents a continuation beneath the Mezen syneclise and Middle Timan of the Paleoproterozoic collisional structure, to which the Arkhangelsk diamond province is confined.

Age and geochemistry of zircon from the oldest metamorphic rocks of the Omolon Massif (Northeast Russia)

This study provides SHRIMP-RG data on zircons from garnet gedritites, the products of retrograde metamorphism of eclogite-like rocks constituting belonging to the basement of the Omolon Massif. The earliest episode recorded by oscillatory-zoned cores having high HREE and Ti contents occurred at 3.25–3.22 Ga (Paleoarchean) and is interpreted to represent an upper age limit of a metamorphic or magmatic protolith. One zircon core with a pronounced negative Eu anomaly yielded a concordant age of 2.6 Ga, which is interpreted to mark a Neoarchean episode of granite formation. The studied population of zircons provides the most distinct record of a Paleoproterozoic (1.9 Ga) event, which is marked by formation of garnet gedritites under amphibolite-facies conditions. This event is recorded by transparent recrystallization rims of preexisting large zircon grains and small newly-formed grains, which are characterized, compared with their cores, by lower crystallization temperatures and one order of magnitude lower concentrations of U, Th, and HREE, and the presence of garnet micro-inclusions.

Monazite-(Ce) and Xenotime-(Y) From An Lct, NYF Tertiary Pegmatite Field: Evidence From A Regional Study In the Central Alps (Italy and Switzerland)

Abstract Monazite-(Ce) and xenotime-(Y) from tertiary pegmatites of the central Alps (Italy and Switzerland) were studied by EMPA and SCXRD analytical methods. The REE-phosphate minerals analyzed are from LCT and NYF pegmatite dikes of the Central Alps, which includes three main areas: Vigezzo (LCT and NYF from the Vigezzo valley), Bodengo (NYF from the Leggia valley and LCT from the Cama valley), and Codera (LCT from Codera and Chiavenna valleys and Alpe Rossaccio). SEM-BSE images of monazite-(Ce) show primary growth zoning, sector zoning, and patchy zoning. SEM-BSE images of xenotime-(Y) show faint patchy-type zoning and spongiform texture. Both REE-phosphates evidence secondary mineral replacements that include thorianite, thorite, and uraninite. EMPA compositions show that the monazite-(Ce) is characterized by variable amounts of huttonite (ThSiO 4 ) and cheralite [Ca(Th,U)(PO 4 )] molecules. SCXRD analyses shows that the unit-cell volume of monazite-(Ce) correlates with its chemical composition; the b cell parameter shows an evident relationship with the huttonite molecule. The unit-cell volume of xenotime-(Y) is well correlated to its chemical composition. Xenotime-(Y) related to the NYF family shows a significant increase in yttrium and a lower U + HREE content. The Tertiary pegmatite field of the central Alps is related to the Masino-Bregaglia intrusion, the San Fedelino granite, and younger magmatic pulses generated by the migration of the metamorphic thermal Lepontine Dome towards the west. The presence of poikiloblastic thorite, thorianite, and uraninite in monazite and xenotime can be induced to form from alkali-bearing fluids via coupled dissolution–reprecipitation, under moderate to high-grade and post-peak Alpine metamorphic conditions.

Recycling of ancient subduction-modified mantle domains in the Purang ophiolite (southwestern Tibet)

Ophiolites in the Indus–Yarlung Zangbo (IYZ) suture (southern Tibet) have been interpreted as remnants of the Neo-Tethyan lithosphere. However, the discovery of diamonds and super-reducing, ultra-high pressure (SuR-UHP) mineral assemblages (e.g., coesite after stishovite, olivine after wadsleyite, native metals, alloys, and moissanite) in some of these massifs and associated chromitites requires a re-evaluation of their origin and evolution. A new petrological and geochemical study of the Purang ophiolite in the western IYZ suture sheds new lights on these issues.The depleted harzburgites of the Purang massif have low modal contents of clinopyroxene (<2%), and high Cr# [100*Cr3+/(Cr3++Al3+)] in spinel (>40~70) and pyroxenes (>16 in orthopyroxene, and >20 in clinopyroxene), suggesting high degrees of melt extraction (>20%). These features are not consistent with formation in a (ultra-) slow-spreading mid-ocean ridge. These peridotites have high modal contents of orthopyroxene; this, and the extremely high Cr# of spinels in these peridotites, suggests modification in a subduction zone. The clinopyroxene-rich harzburgites and lherzolites contain rare spinel–pyroxene symplectites after garnet. Their clinopyroxenes have low MREE-to-HREE ratios ((Sm/Yb)N<0.1) at relatively high HREE concentrations, and are Na-rich but Nd-poor. The relatively enrichment of Na but depletion of Nd in clinopyroxene cannot be explained by refertilization with MORB melts but are consistent with an origin from Na-rich subcontinental lithospheric mantle (SCLM). All lines of evidence suggest that these peridotites underwent initial melting in the stability field of garnet–facies peridotites, followed by additional melting in the spinel–facies mantle.Whole-rock Os isotopic compositions of the Purang peridotites give ancient TRD model ages (up to 1.3Ga), indicating that the formation of these ancient depletion residues predated the opening of Neo-Tethyan Ocean. These observations, together with recent studies on other IYZ peridotites, suggest that the Purang peridotites are genetically unrelated to the associated mafic crust. Instead, they represent ancient SCLM domains, initially formed beneath a continental margin, and then modified by subduction, before they were incorporated into the Neo-Tethyan ocean basin. This model is consistent with the deep-mantle-recycling model for the presence of SuR-UHP phases in the IYZ ophiolites. The infiltration of MORB melts through these ancient depleted peridotites during their final exhumation in a (ultra-) slow-spreading center may have refertilized them to produce the clinopyroxene-rich peridotites.

Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust–mantle mixing and metamorphism in the deep crust

Felsic granulite xenoliths entrained in Miocene (~13 Ma) isotopically evolved, mantle-derived ultrapotassic volcanic (UPV) dykes in southern Tibet are refractory meta-granitoids with garnet and rutile in a near-anhydrous quartzo-feldspathic assemblage. High F–Ti (~4 wt.% TiO2 and ~3 wt.% F) phlogopite occurs as small inclusions in garnet, except for one sample where it occurs as flakes in a quartz-plagioclase-rich rock. High Si (~3.45) phengite is found as flakes in another xenolith sample. The refractory mineralogy suggests that the xenoliths underwent high-T and high-P metamorphism (800–850 °C, >15 kbar). Zircons show four main age groupings: 1.0–0.5 Ga, 50–45, 35–20, and 16–13 Ma. The oldest group is similar to common inherited zircons in the Gangdese belt, whereas the 50–45 Ma zircons match the crystallization age and juvenile character (eHf i +0.5 to +6.5) of Eocene Gangdese arc magmas. Together these two age groups indicate that a component of the xenolith was sourced from Gangdese arc rocks. The 35–20 Ma Miocene ages are derived from zircons with similar Hf–O isotopic composition as the Eocene Gangdese magmatic zircons. They also have similar steep REE curves, suggesting they grew in the absence of garnet. These zircons mark a period of early Miocene remelting of the Eocene Gangdese arc. By contrast, the youngest zircons (13.0 ± 4.9 Ma, MSWD = 1.3) are not zoned, have much lower HREE contents than the previous group, and flat HREE patterns. They also have distinctive high Th/U ratios, high zircon δ18O (+8.73–8.97 ‰) values, and extremely low eHf i (−12.7 to −9.4) values. Such evolved Hf–O isotopic compositions are similar to values of zircons from the UPV lavas that host the xenolith, and the flat REE pattern suggests that the 13 Ma zircons formed in equilibrium with garnet. Garnets from a strongly peraluminous meta-tonalite xenolith are weakly zoned or unzoned and fall into four groups, three of which are almandine-pyrope solid solutions and have low δ18O (+6 to 7.5 ‰), intermediate (δ18O +8.5 to 9.0 ‰), and high δ18O (+11.0 to 12.0 ‰). The fourth is almost pure andradite with δ18O 10–12 ‰. Both the low and intermediate δ18O groups show significant variation in Fe content, whereas the two high δ18O groups are compositionally homogeneous. We interpret these features to indicate that the low and intermediate δ18O group garnets grew in separate fractionating magmas that were brought together through magma mixing, whereas the high δ18O groups formed under high-grade metamorphic conditions accompanied by metasomatic exchange. The garnets record complex, open-system magmatic and metamorphic processes in a single rock. Based on these features, we consider that ultrapotassic magmas interacted with juvenile 35–20 Ma crust after they intruded in the deep crust (>50 km) at ~13 Ma to form hybridized Miocene granitoid magmas, leaving a refractory residue. The ~13 Ma zircons retain the original, evolved isotopic character of the ultrapotassic magmas, and the garnets record successive stages of the melting and mixing process, along with subsequent high-grade metamorphism followed by low-temperature alteration and brecciation during entrainment and ascent in a late UPV dyke. This is an excellent example of in situ crust–mantle hybridization in the deep Tibetan crust.

Geochemistry and petrogenesis of the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan marginal seas

The results of ICP-MS trace-element (LILE, HFSE, REE) study of the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan seas and geochronological K-Ar dating of the Eocene volcanic rocks are presented. Specifics of volcanism developed on submarine rises of these seas was characterized for the first time, and magma sources and geodynamic settings of the volcanic complexes predating the formation of the deep-water basins were determined. It is established that the Late Mesozoic magmas were formed in a subduction setting from spinel peridotites of suprasubduction mantle wedge, which was metasomatically reworked by aqueous fluids that were released by dehydration of sedimentary layer of subducting oceanic plate. This follows from the elevated concentrations of H2O, alkalis, potassium, LILE and LREE, and lowered HFSE (including Ta-Nb minimum) and HREE contents, at lowered Sm/Yb, Nb/Ta, Nb/Y and elevated La/Nb, Ba/La, and Zr/Y ratios. Eocene adakite-like volcanic rocks were identified for the first time in the Sea of Okhotsk. They vary from andesitic to more felsic compositions with elevated MgO (>4%) and elevated La/Yb (>14) and Sr/Y (50–60) ratios. Identification of adakite-like volcanic rocks serves as evidence in support of the transform continental-margin (or plate sliding) setting, which is characterized by breaking apart of subduction slab and formation of slab “windows” acting as pathways for the transfer of asthenospheric mantle into continental lithosphere. New geochemical data on the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan seas and analysis of literature data were used to distinguish two geodynamic settings within these seas: subduction and transform margin. Similar settings operated at that time in the adjacent continental- margin volcanic belts (Akinin and Miller, 2011; Martynov and Khanchuk, 2013; et al.).

Generation of peraluminous granitic magma in a post-collisional setting: A case study from the eastern Qilian orogen, NE Tibetan Plateau

Early Paleozoic peraluminous granites are abundant in the eastern part of the Qilian orogen, northeastern margin of the Tibetan Plateau. A combined study involving geochronology, whole-rock geochemical and Sr–Nd–Hf isotopic compositions for three Early Paleozoic peraluminous granitic plutons (Jishishan, Ledu and Shichuan plutons) from the eastern Qilian orogen was carried out to evaluate the causes of chemical variations and generation mechanisms of peraluminous granitic magmas. These granitic plutons have magma crystallization ages of 455–427Ma and are moderately to strongly peraluminous (A/CNK=1.03–1.18). Geochemical and Sr–Nd–Hf isotopic data indicate that they consist substantially of crust-derived melts. The Jishishan and Ledu peraluminous granites were dominantly produced by partial melting of Precambrian orthogneisses. The Shichuan monzogranites, with low HREE contents (e.g., Yb=0.80–1.83ppm) and slightly negative εNd(t) (−5.3 to −2.3) and positive εHf(t) (+1.6 to +3.4), could be derived from immature crustal materials. Relatively high average zircon saturation temperatures (>750°C for each pluton), obvious negative Eu anomalies (Eu/Eu*=0.28–0.80) and low Pb/Ba ratios (0.03–0.16) for the Jishishan, Ledu and Shichuan granites are consistent with crustal melting involving biotite breakdown under fluid-absent conditions. Our results suggest that compositional variations of moderately to strongly peraluminous granitic magmas are mainly controlled by source compositions and melting conditions, while the processes such as mixing with mantle-derived magma, fractional crystallization, restite unmixing and peritectic assemblage entrainment were insignificant (or only play secondary roles) in their genesis. Late Ordovician to Middle Silurian crustal anatexis in the eastern Central Qilian was probably linked with slab break-off which may be an important mechanism in addition to lithospheric delamination for the generation of moderately to strongly peraluminous granites in a post-collisional setting.

Carboniferous magmatism and mineralization in the area of the Fuxing Cu deposit, Eastern Tianshan, China: Evidence from zircon U–Pb ages, petrogeochemistry, and Sr–Nd–Hf–O isotopic compositions

The newly discovered Fuxing porphyry Cu deposit is located in the Dananhu–Tousuquan arc, adjacent to the Tuwu–Yandong Cu deposits of Eastern Tianshan, in the southern Central Asian Orogenic Belt. The Fuxing deposit is hosted by volcanic rocks (basalt and dacite) in the Early Carboniferous Qi'eshan Group and Carboniferous felsic intrusions (plagiogranite porphyry, monzogranite, and quartz diorite). New SIMS zircon U–Pb dating indicates that the plagiogranite porphyry and monzogranite emplaced at 332.1±2.2Ma and 328.4±3.4Ma, respectively. The basalts are characterized by low SiO2 contents (47.47–54.90wt.%), a lack of Eu anomalies, strong depletion of Na, Ta, and Ti elements but positive Sr, U, and Pb anomalies, high Y (20.8–28.2ppm) and HREE concentrations (Yb=2.23–3.06ppm), and relatively low (La/Yb)N (2.20–3.92) values; the dacite samples have high SiO2 contents (66.13–76.93wt.%), clearly negative Eu anomalies, high Mg# values (36–51), and high Y (41.8–54.9ppm) and Yb (5.76–8.98ppm) concentrations. The basalts and dacites exhibit similar signatures as normal arc rocks, and were considered to be derived from partial melting of mantle-wedge peridotite that was previously metasomatized by slab melts. In contrast, the plagiogranite porphyry, monzogranite, and quartz diorite show the same geochemical affinity with modern adakites, which are characterized by high SiO2 contents (67.55–79.00wt.%), minor negative to positive Eu anomalies, strong depletion of heavy rare earth elements (Yb=0.17–1.19ppm) and Y (1.86–10.1ppm), positive K, Rb, Sr, and Ba but negative Nb, Ta, Th, and Ti anomalies, and high (La/Yb)N ratios and Mg# values. Moreover, these adakitic felsic intrusions display relatively high positive zircon εHf(t) values (+11.4 to +18.3), low 87Sr/86Sr (0.706080–0.711239), high 143Nd/144Nd (0.512692–0.512922) ratios, and consistent zircon δ18O values (4.41‰–5.48‰), suggesting that their parental magma were most likely derived from partial melting of the subducted oceanic crust followed by mantle peridotite interaction. Based on the whole-rock geochemical and Sr–Nd–Hf–O isotopic data, as well as detailed petrographic analyses, we further suggest that the Fuxing igneous rocks and associated porphyry Cu mineralization were generated by the northward subduction of the paleo-Tianshan oceanic plate beneath the Dananhu–Tousuquan island arc during the Early Carboniferous.

Early Mesozoic granites in the Nanling Belt, South China: Implications for intracontinental tectonics associated with stress regime transformation

The link between two tectono-magmatic events during Early and Late Mesozoic periods in the Nanling Belt of South China remains debated. Here we present zircon U–Pb geochronology, Hf isotopes and whole-rock geochemistry of granitic intrusions from Zhuguangshan in the Nanling Belt. The zircon data exhibit two magmatic episodes with 206Pb/238U ages of ca. 230Ma and 150Ma, representing the Indosinian (Early–Middle Triassic period) and Early Yanshanian (Jurassic–Cretaceous period) events, respectively. The Indosinian granites are characterized by strongly peraluminous nature with high A/CNK values (>1.1), whereas the Early Yanshanian granites are weakly peraluminous (average A/CNK value of 1.06). Although the Early Yanshanian granites bear higher HREE contents and lower LREE/HREE ratios, the two-phase granites show enrichment in LREE, Rb, Th, U and Pb, depletion in Eu and negative Ba, Sr, Nb and Ti anomalies. All samples show variably negative εHf(t) values (−5 to −16) with two-stage Hf model ages clustered around 1.8–2.1Ga. Our data indicate that the two-phase granitic magmas were generated from the partial melting of early Paleoproterozoic basement rocks with no input of mantle material. Combined with other geological evidence, we infer that the Indosinian tectono-magmatic event was closely related to intracontinental orogeny triggered by collisions along the boundaries of South China Craton, in relation to processes associated with the E–W trending Tethys tectonic domain. We envisage that the Early Yanshanian event might be a response to the back-arc extension of NW-directed paleo-Pacific plate subduction. The Nanling Belt is a critical zone that records the transformation from Tethys to paleo-Pacific tectonic regimes.

Magmatic–hydrothermal fluids and volatile metals in the Spirit Lake pluton and Margaret Cu–Mo porphyry system, SW Washington, USA

The halogen-bearing minerals tourmaline, amphibole, and biotite formed during magmatic–hydrothermal processes associated with the late-stage cooling of the Spirit Lake granitoid pluton (Mt. St. Helens, WA) and with the younger sulphide-mineralised rocks of the Margaret Cu–Mo porphyry deposit located entirely within the pluton. Major- and trace-element discrimination suggests that one tourmaline population crystallised from fractionated late-stage melt pockets in granodiorite–monzogranitic dykes of the pluton. These coarse, euhedral, oscillatory, and complexly sector-zoned uvite tourmalines span a limited range in Mg/(Mg + Fe) [Mg#] space (0.4–0.7 apfu) and show the highest Ti, Ca, F, Nb, and Ta contents, and low X-site vacancies ( 0.6 apfu), lower Ca and F contents, and the highest Li, As, and HREE contents (>80 ppm Li, >1200 ppm As). This population appears to record direct, rapid crystallisation from magmatic ± meteoric fluid(s) bearing the signature of the breakdown of primary feldspars and pyroxenes, with fluid exsolution from fractionated melt patches likely triggered by the formation of the previous generation of tourmaline. Mineralised porphyry deposit tourmaline compositions from the stockwork span a much larger range in Mg# space (0.05–0.9 apfu) and are almost entirely Ca-free. X-sites of these schorl tourmalines are dominated by Na or vacancies, and the Y-sites are strongly Fe enriched. The highest Mn and Zn concentrations (>4000 and >1000 ppm, respectively) potentially reflect the composition of mineralising fluids during ore deposition. A number of boron isotopic analyses yield predominantly heavy boron, but δ11B values range from −5.2 to 6.2 ‰ and average 1.4 ‰. Whilst most plutonic tourmalines conform to reported a- and c-sector element partitioning models, those from the mineralised porphyry show large and variable sector fractionation differences, suggesting that external controls may also be important. Wider evidence for late-stage pervasive metasomatism by halogen-bearing exsolved fluid(s) is provided by the high Mg# (>70) secondary amphiboles and biotites from within the Spirit Lake pluton, where the amphiboles are clear replacement products of primary pyroxenes. Fluid halogen fugacity ratios calculated from the biotite compositions overlap with other global mineralised porphyry systems, despite not being immediately associated with sulphide ores. The evidence suggests complex fluid processes and the coincidental development of the mineralised porphyry system within the pluton. Heat, fluids, and metals were therefore likely supplied by a later phase of magmatism, unrelated to the consolidation of the main Spirit Lake granitoid. These new constraints on magmatic–hydrothermal fluid signatures have wider applicability to potentially tracing proximal barren and mineralised processes, and for distinguishing between formation mechanisms for primary and secondary halogen-bearing minerals.

Geology, geochronology and geochemistry of granitic intrusions and the related ores at the Hongshan Cu-polymetallic deposit: Insights into the Late Cretaceous post-collisional porphyry-related mineralization systems in the southern Yidun arc, SW China

The Hongshan Cu-polymetallic deposit is located in the southern Yidun arc in southwestern China, where both subduction-related (Late Triassic) and post-collisional (Late Cretaceous) porphyry–skarn–epithermal mineralization systems have been previously recognized. In this study, two distinct magmatic events, represented by diorite porphyry and quartz monzonite porphyry, have been revealed in the Hongshan deposit, with zircon SHRIMP U–Pb ages of 214±2Ma and 73.4±0.7Ma, respectively. The 73Ma age is comparable to the Re–Os ages of 77 to 80Ma of ore minerals from the Hongshan deposit, indicating that the mineralization is related to the Late Cretaceous quartz monzonite porphyries rather than Late Triassic diorite porphyries. The Late Triassic diorite porphyries belong to the high-K calc-alkaline series and show arc magmatic geochemical characteristics such as enrichment in Rb, Ba, Th and U and depletion in HFSEs, indicating that they were formed during the westward subduction of the Garzê–Litang Ocean. In contrast, the Late Cretaceous quartz monzonite porphyries show shoshonitic I-type geochemical characteristics, with high SiO2, K2O, LILE, low HREE, Y and Yb contents, and high LREE/HREE and La/Yb ratios. These geochemical characteristics, together with the Sr–Nd–Pb isotopic compositions (average (87Sr/86Sr)i=0.7085; εNd(t)=−6.0; 206Pb/204Pb=19.064, 207Pb/204Pb=15.738, 208Pb/204Pb=39.733) suggest that the quartz monzonite porphyries originated from the partial melting of the ancient lower crust in response to underplating of mafic magma from subduction metasomatized mantle lithosphere, possibly triggered by regional extension in the post-collisional tectonic stage. The S isotopic compositions (δ34SV-CDT=3.81‰ to 5.80‰) and Pb isotopic compositions (206Pb/204Pb=18.014 to 18.809, 207Pb/204Pb=15.550 to 15.785, and 208Pb/204Pb=38.057 to 39.468) of ore sulfides indicate that the sulfur and metals were derived from mixed mantle and crustal sources. It is proposed that although the Late Triassic magmatic event is not directly related to mineralization, it contributed to the Late Cretaceous mineralization system through the storage of large amounts of sulfur and metals as well as water in the cumulate zone in the mantle lithosphere through subduction metasomatism. Re-melting of the mantle lithosphere including the hydrous cumulate zone and ancient lower crust during the post-collisional stage produced fertile magmas, which ascended to shallow depths to form quartz monzonite porphyries. Hydrothermal fluids released from the intrusions resulted in porphyry-type Mo–Cu ores in and near the intrusions, skarn-type Cu–Mo ores in the country rocks above the intrusions, and hydrothermal Pb–Zn ores in the periphery.

Geochemical constraints on the protoliths of eclogites and blueschists from North Qilian, northern Tibet

An integrated study of whole-rock geochemistry, mineral O isotope geochemistry and zirconology was carried out for low-temperature eclogites and blueschists from the North Qilian orogen in northern Tibet. The results demonstrate that these oceanic-type eclogites and blueschists were produced by metamorphism of backarc basin basalts rather than mid-ocean ridge basalts as commonly thought. These high pressure metabasites show significant heterogeneity in their major and trace element compositions, varying from island arc basalts-like, mid-ocean ridge basalts-like to oceanic island basalts-like, respectively, in trace element distribution diagrams. Such variable compositions are comparable with those of backarc basin basalts at different stages. Whole-rock Nd isotope analyses indicate that the compositional difference between the metabasite protoliths can be ascribed to involvement of different amounts of subducted sediment-derived melts in the backarc mantle. Oxygen isotope fractionations between coexisting minerals are not at equilibrium; garnet O isotope analyses indicate that whole-rock δ18O values are either higher or lower than normal mantle value, which is attributable to seawater-hydrothermal alteration during seafloor spreading to open the backarc basin. Relict magmatic zircon domains show oscillatory zoning, high Th and U contents, high Th/U ratios, and steep HREE patterns, with protolith U–Pb ages of 486–496Ma. Metamorphic zircon domains exhibit lowered Th, U and HREE contents and Th/U ratios than the relict magmatic domains and contain mineral inclusions of omphacite, rutile and phengite, giving concordant U–Pb ages of 463±10Ma for eclogite-facies metamorphism. Zircons have markedly depleted Hf isotope compositions with Hf model ages slightly or significantly older than the protolith ages, suggesting incorporation of crustal components into the magma sources of these metabasite protoliths. Most zircons have δ18O values different from normal mantle values, and the distinct zircon O isotopes in different samples are in accordance with garnet δ18O values for the same sample, pointing to a localized fluid source for the metamorphism. Therefore, the different metabasites with different compositions were metamorphosed from different basaltic rocks generated in a backarc basin in the early Paleozoic. Their occurrence in the same region indicates that a backarc basin was transformed to an oceanic subduction zone where eclogite-facies metamorphism occurred.

NanoSIMS mapping and LA-ICP-MS chemical and U–Th–Pb data in monazite from a xenolith enclosed in andesite (Central Slovakia Volcanic Field)

In this study, we use NanoSIMS element and isotope ratio mapping and LA-ICP-MS trace element measurements to elucidate the origins of monazites from a restitic xenolith enclosed in a 13.5 ± 0.3 Ma andesitic lava (Slovakia). The xenolith/lava interaction is mainly characterized by the growth of a plagioclase-bearing corona around the xenolith and magmatic garnet overgrowths on primary metamorphic garnets within the xenolith. NanoSIMS images (89Y, 139La, 208Pb, 232Th and 238U) and trace element analyses indicate that variations of HREE, Y and Eu contents in the monazite are correlated with the resorption and the following overgrowth of garnet and plagioclase in the xenolith. Three domains are distinguished in the monazite grains: the inherited Variscan core at ca. 310 Ma (M1 domain) characterized by low Y and HREE contents and a weak negative Eu anomaly; the inner rim (M2 domain) crystallized during the growth of the plagioclase magmatic corona (large negative Eu anomaly) and the resorption of metamorphic garnet (high HREE and Y contents); and the external rim (M3 domain) crystallized during the growth of the plagioclase corona (large negative Eu anomaly) and during the crystallization of magmatic garnet (low Y, HREE contents) at ~13 Ma, i.e. the age of the andesitic lava. The age and chemical zonation of the monazites attest to the preservation of primary monazite in the xenolith despite the interaction with the andesite lava. NanoSIMS imaging provides high-quality sub-µm scale images of the monazite that reveals chemical domains that were not distinguishable on WDS X-ray maps, especially for depleted elements such as U and Pb. Owing to its small size, the M2 domain could not be accurately dated by the LA-ICP-MS method. However, NanoSIMS isotopic maps reveal that the M2 domain has similar 208Pb/232Th isotope ratios to the M3 domain and thus similar ages. These results support the hypothesis that melt-assisted partial dissolution–precipitation in monazite efficiently records chemical and mineralogical changes during xenolith/lava interaction.

Prograde Metamorphic History of UHP Granulites from the Moldanubian Zone (Bohemian Massif) Revealed by Major Element and Y + REE Zoning in Garnets

Major and trace element distribution in garnet crystals from felsic granulites of the Kutná Hora Complex provides evidence of two metamorphic events, the first related to continental subduction at ultrahigh-pressure (UHP) conditions and the second to a granulite-facies overprint that occurred at mid- to lower crustal levels. To reconstruct P–T paths of both metamorphic events, pseudosection modeling was combined with major and trace element zoning in garnet. The granulites contain lenses and boudins of mantle-derived garnet peridotite and eclogite. UHP conditions are confirmed by the presence of inclusions of micro-diamond and coesite in garnet and zircon in the granulites. Compositional zoning in garnet provides evidence for a pre-granulite-facies high-pressure (HP)–UHP metamorphic history of the rocks. The two distinct metamorphic events are documented by high Y + HREE concentrations in the garnet cores and annuli in the mantle part of the garnet grains. The cores of large garnets with relatively high Ca contents and bell-shaped Mn profiles suggest formation during a prograde low- to medium-temperature metamorphic event that was coeval with HP–UHP metamorphism. Decompression and cooling during exhumation of the rocks led to partial resorption of garnet and release of trace elements into the matrix. The new garnet with high Y + HREE in the annuli was formed during the granulite-facies event at crustal levels.

Provenance of “Svecofennian” zircons in the Belomorian mobile belt, Baltic shield, and some geodynamic implications

The provenance of Archean and Paleoproterozoic zircons in the eclogites and related rocks from the Belomorian Belt of the Baltic Shield is considered on the basis of criteria for their equilibrium with melt, garnet and fluid. It is shown that the Paleoproterozoic zircon with low Th/U ratios (<0.1) and extremely low HREE and MREE contents does not match criteria for its equilibrium crystallization with garnet. This zircon crystallized from Zrand Hf-rich fluid that was in disequilibrium with previously formed eclogite assemblages, but equilibrated with amphibole and plagioclase, thus indicating amphiboliterather than eclogitefacies conditions. The input of fluid and its long-term (~100 Ma) circulation in the deep levels of the Archean juvenile crust of the Belomorian belt was triggered by the Lapland–Kola orogeny. After its peak, the rocks of the belt experienced very slow uplift at rates an order of magnitude lower than those in the Phanerozoic orogens. It is found that Late Archean zircon population contains metamorphic zircon, which matches criteria for simultaneous crystallization with garnet, and oceanic protolithic zircon, which was in equilibrium with high-Mg basalt/gabbro of the Archean oceanic crust. This indicates a subduction origin of the Archean eclogites of the Belomorian belt.

Zircon U–Pb geochronology and geochemistry of low-grade metamorphosed volcanic rocks from the Dantazi Complex: Implications for the evolution of the North China Craton

The late Neoarchean witnessed the cratonization of the North China Craton (NCC) through amalgamation of several micro-blocks to form a coherent basement. The Archean orthogneisses and supracrustal rocks in this craton have experienced various grades of metamorphism ranging up to upper amphibolite and granulite facies at ∼2500Ma. Recently, a suite of low-grade metamorphosed (greenschist to lower amphibolite facies) volcanic rocks was discovered in the late Neoarchean Dantazi Complex in northern Hebei province. These meta-volcanic rocks consist of bimodal basalt–andesite and trachyte–dacite with a SiO2 gap between 54.4wt.% and 60.7wt.%. Here we report SHRIMP zircon U–Pb ages of 2490±19Ma (MSWD=2.0) and 2502±8Ma (MSWD=0.83) from the meta-mafic and meta-felsic volcanics, respectively, representing the timing of igneous activity. All the meta-mafic volcanic rocks display coherent trace element and REE patterns which are characterized by enriched LILE and LREE but depleted HFSE and HREE ((La/Yb)N=6.29–15.10). Combining these trace element features with the positive zircon εHf(t) values (+1.3 to +6.6), we propose that the mafic rocks were likely derived from partial melting of a previously metasomatized lithospheric mantle. In the primitive mantle-normalized diagram, the felsic rocks display uniform patterns enriched in LILE but depleted in Nb and Ta, similar to those of lower crust. Furthermore, their strongly fractionated REE ((La/Yb)N=15.24–61.20), lower HREE concentrations (Yb=0.47–1.65ppm) and positive zircon εHf(t) values (+1.6 to +5.3) suggest that they were derived from partial melting of the lower crust with garnet in the residue. This coeval occurrence of metasomatized mantle-derived mafic magmas and potassic felsic magmas from different source regions reflects an intracontinental extensional setting during the late Neoarchean to earliest Paleoproterozoic following the cratonization of the NCC. Our new data, combined with previous published data, allow us to speculate that the intracontinental extensional regime might have prevailed in the NCC from late Neoarchean onwards.

LASS U–Th–Pb monazite and rutile geochronology of felsic high-pressure granulites (Rhodope, N Greece): Effects of fluid, deformation and metamorphic reactions in local subsystems

The specific chemical composition of monazite in shear zones is controlled by the syndeformation dissolution–precipitation reactions of the rock-forming minerals. This relation can be used for dating deformation, even when microfabric characteristics like shape preferred orientation or intracrystalline deformation of monazite itself are missing. Monazite contemporaneously formed in and around the shear zones may have different compositions. These depend on the local chemical context rather than reflecting successive crystallization episodes of monazite.This is demonstrated in polymetamorphic, mylonitic high-pressure (HP) garnet–kyanite granulites of the Alpine Sidironero Complex (Rhodope UHP terrain, Northern Greece). The studied mylonitic rocks escaped from regional migmatization at 40–36Ma and from subsequent shearing through cooling until 36Ma. In-situ laser-ablation split-stream inductively-coupled plasma mass spectrometry (LASS) analyses have been carried out on monazite from micro-scale shear zones, from pre-mylonitic microlithons as well as of monazite inclusions in relictic minerals complimented by U–Pb data on rutile and Rb–Sr data of biotite.Two major metamorphic episodes, Mesozoic and Cenozoic, are constrained. Chemical compositions, isotopic characteristics and apparent ages systematically vary among monazite of four different microfabric domains (I–IV). Within three pre-mylonitic domains (inclusions in (I) pre-mylonitic kyanite and (II) garnet porphyroclasts, and (III) in pre-mylonitic microlithons) monazite yields ages of ca. 130–150Ma for HP-granulite metamorphism, in line with previous geochronological results in the area. Patchy alteration of the pre-mylonitic monazite by intra-grain dissolution–precipitation processes variably increased negative Eu anomaly and reduced the HREE contents. The apparent age of this altered monazite is reduced.Monazite in the syn-mylonitic shear bands (IV) differs in chemical composition from unaltered and altered monazite of the three pre-mylonitic domains by having a significantly more pronounced negative Eu anomaly, a flatter HREE pattern, and high Th content. These compositional characteristics are linked with syn-mylonitic formation of plagioclase and resorption of garnet in the shear bands under amphibolite facies conditions. The absence of pre-mylonitic monazite in the shear zones, in contrast to the other domains, suggests complete dissolution of old and formation of new monazite. This probably results from an increased alkalinity and reactivity of the fluid that again is controlled by syn-mylonitic interaction with feldspar and apatite in the shear zones. There, the deformation was accommodated by dissolution precipitation creep at ca. 690±50°C and 6–7.5kbar. Growth of monazite at 55±1Ma dates this deformation, which precedes the regional migmatization of the Sidironero Complex, whereas rutile and biotite ages reflect these later stages. This new pressure–temperature–time constraint for a relictic deformation structure provides insight into the still missing parts of the overall metamorphic, deformation and exhumation processes of the UHP units in the Rhodope.

Origins of Early Mesozoic granitoids and their enclaves from West Kunlun, NW China: implications for evolving magmatism related to closure of the Paleo-Tethys ocean

Early Mesozoic granitoids and microgranular enclaves (MEs) are widespread in the West Kunkun, northwestern Tibetan plateau, and record the tectonic evolution of Eurasia–Tethys in this area. This study reports geochemistry, zircon U–Pb and Hf isotopic data for a suite of granitoids and their MEs from the Middle Triassic Bulunkou pluton (BP) and the Late Triassic Akazishan pluton (AP) from the West Kunlun. LA-ICP-MS U–Pb zircon dating of a sample from the BP host monzogranite and an enclave, as well as a AP monzogranite, yielded ages of 236 ± 2, 230 ± 7 and 208 ± 1 Ma, respectively. The BP monzogranite and its enclaves from the northwestern part of the West Kunlun are mainly weakly peraluminous granites characterised by relatively high Rb, Th, Rb/Sr (2.64–9.03) and HREE contents, and low Mg#, Sr/Y and negative Eu anomalies. Zircons from the BP monzogranite have eHf(t) values from −5.7 to −1.6. Zircons from enclaves of the BP show more variable eHf(t) values from −4.1 to 3.8. We consider that the BP granites are likely to have been formed by partial melting of metasedimentary rocks at shallow crustal depth, and their enclaves, composed of quartz + biotite + plagioclase + garnet + K-feldspar, are relics from the melting of a source at middle crustal depths. The AP host and its enclaves from the southeastern part of the West Kunlun have low Rb, Rb/Sr (0.15–1.90) and weakly negative Eu anomalies but high HREE contents indicating limited fractionation of plagioclase without residual garnet in their source. The inferred protolith is an intermediate igneous rock in the middle or lower crust. MEs hosted in the AP have high Mg# (39.7–45.0) and Nb and weakly negative Eu anomalies, as well as high Sr, P and Ti, corresponding to a medium-K basaltic rock, which may have originated from mixing of partial melting of metasomatised mantle wedge that has been modified by upwelling asthenospheric mantle and crustal melting in the deep source. Post-collisional southeasternward younging of Mesozoic granitoids in the West Kunlun records a transition from shallow level, mid-crustal melting to deeper level, lower-crustal melting. The post-collisional granitoids show significant mantle input increasing from northwest to southeast, which was closely associated with mafic magma-driven partial melting of mantle and crustal sources as a consequence of slab break-off. Syn- to post-collisional Early Mesozoic granites emplaced along the Mazar–Kangxiwar suture zone suggest that these tectonomagmatic processes followed the closure of the Paleo-Tethys ocean during the Tarim and Karakorum–Qiangtang continental collision.