Magmatism and metallogenic mechanisms of the Baoshan Cu-polymetallic deposit from the Lesser Xing’an Range,NE China: Constraints from geology,geochronology, geochemistry,and Hf isotopes

The Baoshan Cu-polymetallic deposit is a recently discovered skarn deposit in the northern Lesser Xing’an Range, NE China. The orebodies are mainly hosted in the contact zone between granitic intrusions and Lower Cambrian dolomitic crystalline limestones or skarns. We present here zircon U–Pb and molybdenite Re–Os age data, whole-rock geochemistry, and zircon Hf isotopic data to constrain the geodynamic mechanisms of igneous activity and metallogenesis within the Baoshan Cu–polymetallic deposit. LA–ICP–MS zircon U–Pb dating suggests that a hornblende–quartz monzonite and porphyritic biotite granite were emplaced at 252.45 ± 0.70 Ma and 251.10 ± 0.98 Ma, respectively. Molybdenite separated from ore-bearing quartz veins or skarn-type ores yields a weighted mean model age of 250.3 ± 3.4 Ma, which coincide with the emplacement of the igneous rocks. These data suggest that the Late Permian-Early Triassic magmatic and mineralization event led to the formation of the Baoshan Cu–polymetallic deposit. Granitic intrusions are closely associated with this mineralization and have high contents of SiO₂ (60.90–68.98 wt.%), Al₂O₃ (15.15–16.98 wt.%) and K₂O (2.77–4.17 wt.%), with A/CNK ratios of 0.86–0.96. These granites are classified as metaluminous and high-K calc-alkaline I-type granites, and are enriched in Rb, Th, U, and K, and depleted in Nb, Ta, P, and Ti. Moreover, Moreover, the hornblende–quartz monzonite and porphyritic biotite granite have geochemical characteristics similar to adakites and island arc calc-alkaline rocks, respectively. In situ zircon Hf isotope data on the hornblende–quartz monzonite samples show ε_Hf(t) values from +0.1 to +3.1, and porphyritic biotite granite samples exhibit heterogeneous ε_Hf(t) values from −5.4 to +1.1. The geochemical and isotopic data for the Baoshan intrusions indicate that the Late Permian–Early Triassic continental–continental collision caused over thickening and delamination of the lower crust. Partial melting of delaminated lower crust formed the primary adakitic magmas, which may have reacted with surrounding mantle peridotite during ascent. Hornblende–quartz monzonite was formed by the emplacement of the adakitic magmas, whereas the formation of the porphyritic biotite granite was caused by the mixing of adakitic magmas with ancient crustal materials during ascent. Moreover, ore-forming materials were typically derived from the adakitic magmas with high oxygen fugacity, which incorporated significant amounts of ore-forming elements. Based on the regional geological history and the new geochemical and isotopic data from intrusions, we suggest that diagenesis and mineralization of the Baoshan Cu–polymetallic deposit took place in a transitional tectonic setting from collisional orogeny to extension, after collision of the North China Plate and Songnen Block, during the latter stages of the Xingmeng orogeny.     

Geochronology and geochemistry of porphyritic intrusions in the Duolong porphyry and epithermal Cu-Au district,central Tibet: Implications for the genesis and exploration of porphyry copper deposits

The Duolong district in central Tibet hosts a number of porphyry as well as high sulfidation epithermal copper–gold deposits and prospects, associated with voluminous calc-alkaline volcanism and plutonism. In this study, we present new geochronological, geochemical, isotopic and mineralogical data for both economically mineralized and barren porphyritic intrusions from the Duobuza and Naruo porphyry Cu–Au deposits. Zircon U–Pb analyses suggest the emplacement of economically mineralized granodiorite porphyry and barren granodiorite porphyry at Naruo deposit took place at 119.8 ± 1.4 Ma and 117.2 ± 0.5 Ma, respectively. Four molybdenite samples from the Naruo deposit yield an isochron Re–Os age of 119.5 ± 3.2 Ma, indicating mineralization occurred synchronously with the emplacement of the early granodiorite porphyry. At Duobuza deposit, the barren quartz diorite porphyry intruded at 119.5 ± 0.7 Ma, and two economically mineralized intrusions intruded at 118.5 ± 1.2 Ma (granodiorite porphyry) and 117.5 ± 1.2 Ma (quartz diorite porphyry), respectively. Petrographic investigations and geochemical data indicate that all of the porphyritic intrusions were oxidized, water rich, and subduction-related calc-alkaline magmas. Zircons from the porphyritic intrusions have a wide range in the ε_Hf (0–11.1) indicating that they were sourced from mixing of mantle-derived mafic, and crust-derived felsic melts. Moreover, the variation of trace element content of plagioclase phenocrysts indicates that the magma chambers were recharged by mafic magmas.Comparison of the composition of amphibole phenocrysts indicates the porphyry copper–gold mineralization at Duolong was generated in magma chambers at low crystallization temperatures and pressures (754° to 791 °C, 59 M to 73 MPa, n = 8), and under highly oxidizing conditions (ΔNNO 2.2 to 2.7, n = 8). In contrast, barren intrusions were sourced from the magma chambers with higher crystallization temperatures and pressures (816° to 892 °C, 111 to 232 MPa, n = 22) that were less oxidizing (ΔNNO 0.6 to 1.6, n = 22). The requirement for a thermal contrast is supported by the declining of Ti content in magnetite crystals in barren intrusions (12,550 to 34,200 ppm) versus those from economically mineralized intrusions (600 to 3400 ppm). Moreover, the V content in magnetite crystals from economically mineralized intrusions (990 to 2510 ppm) is lower than those recorded from barren intrusions (2610 to 3510 ppm), which might reflect the variation in oxidation state of the magma. The calculated water solubility of the magma forming the economically mineralized intrusions (3.2–3.7 wt%) is lower than that of magma forming the barren intrusions (4.6–6.4 wt%). Based on the chemical–physical characteristics of economically mineralized magma, our study suggests that the development of porphyry Cu–Au mineralization at Duolong was initiated by shallow-level emplacement of a magma that crystallized at lower temperatures and pressures. Experimental studies show that copper and water solubilities in silicate melts decrease with falling temperatures and pressures, indicating metals and ore-forming fluids are more likely to be released from a magma reservoir emplaced at shallow crustal levels. We propose the magnetite might be a convenient exploration tool in the search for porphyry copper mineralization because the variations in Ti and V content of mineral concentrates and rock samples are indicative of barren versus mineralized intrusions.     

Geology and genesis of the giant Beiya porphyry–skarn gold deposit,northwestern Yangtze Block,China

The Beiya ore deposit is located in the northwestern Yangtze Block, to the southeast of the Tibetan Plateau, SW China. The deposit is hosted by a porphyritic monzogranitic stock that is cross-cut by a porphyritic granite and later lamprophyre dikes. The whole-rock geochemistry of the porphyritic monzogranite and granite intrusions is both potassic and adakite-like, as evidenced by high K₂O/Na₂O (2.2 to 24.8), Sr/Y (53.2 to 143.2), and (La/Yb)_N (4.9 to 28.9) ratios. Both intrusions have comparable zircon U–Pb ages of ca. 36 Ma and εHf(t) values of − 6.8 to + 2.7. Zircons within these intrusions have Hf isotope crustal model ages with a prominent peak at ca. 840 Ma, and both of the intrusions have similar Sr–Nd–Pb isotopic compositions that are comparable to the compositions of amphibolite xenoliths hosted by potassic felsic intrusions in western Yunnan. The contemporaneous lamprophyre dikes show Nb–Ta depletion, enriched (⁸⁷Sr/⁸⁶Sr)_i and εNd(t), and extremely low Nb/U ratios (1.6–3.6), suggesting that these dikes were formed from magmas generated by partial melting of a metasomatized subcontinental lithospheric mantle (SCLM). The geochemistry of the porphyritic intrusions and the lamprophyre dikes suggests that the Beiya porphyries formed as a result of partial melting of a thickened and K-rich region of the lower crust, triggered by melting of metasomatized SCLM. The ca. 840 Ma U–Pb ages and εHf(t) values (− 6.8 to + 2.7) of xenocrystic zircons within the porphyritic intrusions suggest that these zircons were produced in a continental arc setting at ca. 840 Ma. The peak Hf model age of the zircons crystallized from the intrusions and the U–Pb ages of the xenocrystic zircons within the intrusions suggest that these porphyritic intrusions formed from magmas sourced from a juvenile crust that formed at ca. 840 Ma. This juvenile crust is most likely the source for the metals within the porphyry–skarn deposits in the study area, as the SCLM-derived lamprophyre dikes in this area are barren.Massive Fe–Au orebodies (~ 99 million metric tons at an average grade of 2.61 g/t Au) within the study area are generally located within the skarn-altered boundary of the porphyritic monzogranite stock and along the faults in the surrounding Triassic carbonates. The Fe–Au orebodies are spatially and genetically associated with skarn comprising garnet and diopside. Petrographic observations show that the massive Fe–Au orebodies mainly consist of hematite and magnetite with disseminated pyrite that hosts native gold and electrum.The porphyritic granite contains porphyry-style mineralization in the form of disseminated and veinlet-hosted pyrite and chalcopyrite. Pyrite-hosted lattice-bound gold is present within both the massive Fe–Au and the porphyry-type mineralization in the study area, and is present at concentrations up to 10 ppm Au (as determined by in situ LA-ICP-MS analysis). Subsequent weathering altered the primary magnetite–hematite–sulfide assemblage in the Fe–Au orebody into a magnetite–limonite assemblage, and generated laterite-type mineralization in which gold is hosted by limonite.     

Geochemistry,geochronology and Sr–Nd–Pb–Hf isotopic compositions of Middle to Late Jurassic syenite–granodiorites–dacite in South China: Petrogenesis and tectonic implications

In situ zircon U–Pb ages and Hf isotope data, major and trace elements and Sr–Nd–Pb isotopic compositions are reported for coeval syenite–granodiorites–dacite association in South China. The shoshonitic syenites are characterized by high K₂O contents (5.9–6.1 wt.%) and K₂O/Na₂O ratios (1.1–1.2), negative Eu anomalies (Eu/Eu* = 0.65 to 0.77), enrichments of Rb, K, Nb, Ta, Zr and Hf, but depletion of Sr, P and Ti. The adakitic granodiorite and granodiorite porphyry intrusions are characterized by high Al₂O₃ contents (15.0–16.8 wt.%), enrichment in light rare earth elements (LREEs), strongly fractionated LREEs (light rare earth elements) to HREEs (heavy rare earth elements), high Sr (438–629 ppm), Sr/Y (29.2–53.6), and low Y (11.7–16.8 ppm) and HREE contents (e.g., Yb = 1.29–1.64 ppm). The calc-alkaline dacites are characterized by LREE enrichment, absence of negative Eu anomalies, and enrichment of LILEs such as Rb, Ba, Th, U and Pb, and depletion of HFSEs such as Nb, Ta, P and Ti.Geochemical and Sr–Nd–Hf isotopic compositions of the syenites suggest that the shoshonitic magmas were differentiated from parental shoshonitic melts by fractional crystallization of olivine, clinopyroxene and feldspar. The parent magmas may have originated from partial melting of the lithospheric mantle with small amount contribution from crustal materials. The adakitic granodiorite and granodiorite porphyry have Sr–Nd–Pb isotopic compositions that are comparable to that of the mafic lower crust. They have low Mg# and MgO, Ni and Cr contents, abundant inherited zircons, low ε_Nd(t) and ε_Hf(t) values as well as old whole-rock Nd and zircon Hf model ages. These granodiorites were likely generated by partial melting of Triassic underplated mafic lower crust. The Hf isotopic compositions of the dacites are relatively more depleted than the Cathaysia enriched mantle, suggesting those magmas were derived from the partial melting of subduction-modified mantle sources. The coeval shoshonitic, high-K calc-alkaline and calc-alkaline rocks in Middle to Late Jurassic appear to be associated with an Andean-type subduction. This subduction could have resulted in the upwelling of the asthenosphere beneath the Cathaysia Block, which induced partial melting of the mantle as well as the mafic lower crust, and formed an arc regime in the coastal South China during Middle to Late Jurassic.     

Zircon U–Pb geochronology and petrology of intrusive rocks in the C-North and Baghak districts,Sangan iron mine,NE Iran

Porphyritic granitoids that host the Sangan iron mine deposit belong to the Khaf–Kashmar–Bardaskan volcanoplutonic belt in northeastern Iran. These intrusive rocks, mostly quartz monzonite to syenogranite porphyries, have been divided into three groups on the basis of crosscutting relationships and zircon U–Pb dating: (1) group 1, 42.3 ± 0.8 Ma, (2) group 2, 40.0 ± 0.5 Ma, and (3) group 3, 39.2 ± 0.6 Ma. The group 1 and 2 rocks host magnetite mineralization, whereas the group 3 intrusions are interpreted as syn-mineralization. They have features typical of high-K alkali-calcic to calc-alkalic magnesian rocks and are metaluminous to weakly peraluminous formed in a volcanic arc setting. Mantle-normalized, trace-element spider diagrams display enrichment in large ion lithophile elements, such as Rb, Ba, K, and Cs, and depletion in high field strength elements, e.g., Nb, Ti, Ta, Zr, Y, and heavy rare earth elements, with moderate to strong light rare earth elements enrichment ((La/Yb)_N = 24.8–7.6) and a negative Eu anomaly. The parental magmas are probably derived from partial melting of mantle that had been metasomatized by a slab-derived fluid. During the upward migration of these melts, additional input of crustal materials could account for the high K characteristic for most of the intrusive rocks around the Sangan mine area.Textural evidence and mineral assemblages indicate the Sangan Fe-skarn is an oxidized magmatic-hydrothermal system caused by the group 3 intrusions.     

The geochemical characteristics of metabasites with a pseudo-pillow structure from Ganghe,Dabie Orogen,China

Several metabasite lenses in Ganghe, Central Dabie, that were previously described as pillow lavas are studied by elemental, Sr–Nd–Pb isotopic, and mineral oxygen isotopic analysis as well as zircon SHRIMP U–Pb dating. Zircon U–Pb geochronology results indicate that the protolith emplacement age of these metabasites is approximately 717 ± 38 Ma, consistent with the age of the volcanoclastic rocks in the same unit, and that they experienced the Triassic HP eclogite-facies retrograde metamorphism at 221 ± 2 Ma during exhumation after subduction to mantle depth and peak ultra-high pressure metamorphism. The low δ¹⁸O values of −5.5‰ to −2.0‰ indicate that the protoliths underwent high temperature meteoric-hydrothermal alteration before subduction but had no seawater interaction. These metabasites had similar formation processes, water–rock interactions and metamorphisms as other eclogite-facies rocks cropped out in the Central Dabie terrain. They showed negative abnormalities in Nb, Sr, and Ti content and positive abnormalities in Ba, Th, and Pb content; they also showed LREE enrichment. The insusceptible Sm–Nd isotopes during metamorphism yielded ε_Nd (t) = −12 to −10 and T_DM = 2.2–2.8 Ga for samples from lenses #1 to #3 and −7 to −6 and 2.1–2.2 Ga for lens #4; the samples also showed low radiogenic Pb isotope compositions of (²⁰⁶Pb/²⁰⁴Pb)_i = 15.34–16.50, (²⁰⁷Pb/²⁰⁴Pb)_i = 15.23–15.32, and (²⁰⁸Pb/²⁰⁴Pb)_i = 35.93–37.04. The data suggest that the protolith sources of the metabasites were contaminated to variable degrees by old crustal materials during formation. Unlike the Maowu layered intrusions, which were contaminated by upper crust, the magmas of the metabasites were contaminated by lower crust in the magma chamber and during eruption. It can be concluded that the protoliths of these metabasites were derived from old crustal-contaminated mantle sources and initially emplaced in the crust at the Neoproterozoic and that they were altered by meteoric water at high temperatures. In this respect, they might be similar to the Neoproterozoic mafic intrusions in the North Huaiyang terrain. However, the studied metabasites experienced the Permo-Triassic subduction and metamorphism, whereas the North Huaiyang Neoproterozoic mafic intrusions did not.     

U–Pb dating,geochemistry, and Sr–Nd isotopic composition of a granodiorite porphyry from the Jiadanggen Cu–(Mo) deposit in the Eastern Kunlun metallogenic belt,Qinghai Province,China

The Jiadanggen porphyry Cu–(Mo) deposit is newly discovered and located in the Eastern Kunlun metallogenic belt of Qinghai Province, China. Here, we present a detailed study of the petrogenesis, magma source, and tectonic setting of the mineralization causative granodiorite porphyry. The new data indicate that the granodiorite porphyry is characterized by high SiO₂ (68.21–70.41 wt.%) and Al₂O₃, relatively high K₂O, low Na₂O, and low MgO and CaO concentrations, and is high-K calc-alkaline and peraluminous. The granodiorite porphyry has low Mg^# (38–46) values that are indicative of no interaction between the magmas and the mantle. The samples that we have examined have low Nb/Ta (9.17–10.3) and Rb/Sr (0.28–0.39) ratios, which are indicative of crustal-derived magmas. Source region discrimination diagrams indicate that the magmas that formed the granodiorite porphyry were derived from melting of a mixed amphibolite source in the lower crust. The samples have I_Sr values of 0.70954–0.70979, ε_Nd(t) values of − 8.3 to − 7.9, and t_2DM ages ranging from 1644 to 1677 Ma. These indicate that the magmas that formed this intrusion were generated by melting of Mesoproterozoic lower crustal material. Higher K(Rb) contents of the samples indicate that the magma source is high potassium basaltic material in the lower crust, which could be derived from an enriched mantle source. LA-ICP-MS zircon U–Pb dating of the granodiorite porphyry yields a late Indosinian age (concordia age of 227 ± 1 Ma; MSWD = 0.31), which is close to the molybdenite Re–Os isochron age (227.2 ± 1.9 Ma), indicating further the close relationship between the granodiorite porphyry and the Cu–(Mo) mineralization. These samples are LREE and LILE (e.g., Rb, K, Ba, and Sr) enriched, and HFSE (e.g., Nb, Ta, P, and Ti) depleted, especially in P and Ti, similar to the characteristics of volcanic arc magmas. This intrusion most likely formed during the later stage of Indosinian deep subduction of oceanic slab. This was associated with underplating of mantle-derived magmas, which provided heat for crustal melting. Similar to the Jiadanggen granodiorite porphyry, Indosinian hypabyssal intermediate-felsic intrusive rocks, formed under subduction tectonism or a transitional regime from subduction to syn-collision, make up the most important targets for porphyry Cu(Mo) deposits in the Eastern Kunlun metallogenic belt.     

Nickel sulfide deposits in Australia: Characteristics,resources, and potential

Australia's nickel sulfide industry has had a fluctuating history since the discovery in 1966 of massive sulfides at Kambalda in the Eastern Goldfields of Western Australia. Periods of buoyant nickel prices and high demand, speculative exploration, and frenetic investment (the ‘nickel boom’ years) have been interspersed by protracted periods of relatively depressed metal prices, exploration inactivity, and low discovery rates. Despite this unpredictable evolution, the industry has had a significant impact on the world nickel scene with Australia having a global resource of nickel metal from sulfide ores of ∼ 12.9 Mt, five world-class deposits (> 1 Mt contained Ni), and a production status of number three after Russia and Canada. More than 90% of the nation's known global resources of nickel metal from sulfide sources were discovered during the relative short period of 1966 to 1973. Australia's nickel sulfide deposits are associated with ultramafic and/or mafic igneous rocks in three major geotectonic settings: (1) Archean komatiites emplaced in rift zones of granite–greenstone belts; (2) Precambrian tholeiitic mafic–ultramafic intrusions emplaced in rift zones of Archean cratons and Proterozoic orogens; and (3) hydrothermal-remobilized deposits of various ages and settings. The komatiitic association is economically by far the most important, accounting for more than 95% of the nation's identified nickel sulfide resources. The ages of Australian komatiitic- and tholeiitic-hosted deposits generally correlate with three major global-scale nickel-metallogenic events at ∼ 3000 Ma, ∼ 2700 Ma, and ∼ 1900 Ma. These events are interpreted to correspond to periods of juvenile crustal growth and the development of large volumes of primitive komatiitic and tholeiitic magmas caused by large-scale mantle overturn and mantle plume activities. There is considerable potential for the further discovery of komatiite-hosted deposits in Archean granite–greenstone terranes including both large, and smaller high-grade (5 to 9% Ni) deposits, that may be enriched in PGEs (2 to 5 g/t), especially where the host ultramafic sequences are poorly exposed.Analysis of the major komatiite provinces of the world reveals that fertile komatiitic sequences are generally of late Archean (∼ 2700 Ma) or Paleoproterozoic (∼ 1900 Ma) age, have dominantly Al-undepleted (Al₂O₃/TiO₂ = 15 to 25) chemical affinities, and often occur with sulfur-bearing country rocks in dynamic high-magma-flux environments, such as compound sheet flows with internal pathways facies (Kambalda-type) or dunitic compound sheet flow facies (Mt Keith-type). Most Precambrian provinces in Australia, particularly the Proterozoic orogenic belts, contain an abundance of sulfur-saturated tholeiitic mafic ± ultramafic intrusions that have not been fully investigated for their potential to host basal Ni–Cu sulfides (Voisey's Bay-type mineralization). The major exploration challenges for finding these deposits are to determine the pre-deformational geometries and younging directions of the intrusions, and to locate structural depressions in the basal contacts and feeder conduits under cover. Stratabound PGE–Ni–Cu ± Cr deposits hosted by large Archean–Proterozoic layered mafic–ultramafic intrusions (Munni Munni, Panton) of tholeiitic affinity have comparable global nickel resources to many komatiite deposits, but low-grades (< 0.2% Ni). There are also hydrothermal nickel sulfide deposits, including the unusual Avebury deposit in western Tasmania, and some potential for ‘Noril'sk-type’ Ni–Cu–PGE deposits associated with major flood basaltic provinces in western and northern Australia.     

A geotraverse across two paleo-subduction zones in Tien Shan,Tajikistan

We present first LA-ICP-MS U–Pb zircon ages as well as geochemical and Sr–Nd–Pb isotope data for 14 magmatic rocks collected along ca. 400 km profile across the Chatkal-Kurama terrane in the Mogol-Tau and Kurama ranges and the Gissar Segment of the Tien Shan orogen in Tajikistan. These new data from supra-subduction and post-collisional magmatic rocks of two Late Paleozoic active margins constrain a tectonic model for terrane motions across two paleo-subduction zones: (1) The 425 Ma old Muzbulak granite of the Mogol-Tau range formed in a supra-subduction setting at the northern margin of the Turkestan Ocean. The north-dipping plate was subducted from the Early Silurian to the earliest Middle Devonian. Thereafter the northern side of the Turkestan Ocean remained a passive margin until the Early Carboniferous. (2) In the Early Carboniferous, subduction under the northern margin of the Turkestan Ocean resumed and the 315 to 305 Ma old Kara-Kiya, Muzbek, and Karamazar intrusions formed in a supra-subduction setting in the Mogol-Tau and Kurama ranges. (3) At the same time, in the Early Carboniferous, rifting of the southern passive margin of the Turkestan Ocean formed the short-lived Gissar Basin, separated from the Turkestan Ocean by the Gissar micro-continent. North-dipping subduction in the Gissar Basin is documented by the 315 Ma Kharangon plagiogranite and the voluminous ca. 321–312 Ma Andean-type supra-subduction Gissar batholith. The Kharangon and Khanaka gabbro-plagiogranite intrusions of the southern Gissar range have geochemical and Sr–Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr(t) 0.7047–0.7056, εNd of + 1.5 to + 2.3) compatible with mantle-derived origin typical for plagiogranites associated with ophiolites. The supra-subduction rocks from the Gissar batholith and from the Mogol-Tau Kurama ranges have variably mixed Sr–Nd–Pb isotopic signatures (⁸⁷Sr/⁸⁶Sr(t) 0.7057–0.7064, εNd of − 2.1 to − 5.0) typical for continental arcs where mantle-derived magmas interact with continental crust. (4) In the latest Carboniferous, the Turkestan Ocean and the Gissar Basin were closed. The Early Permian Chinorsay (288 Ma) and Dara-i-pioz (267 Ma) post-collisional intrusions, emplaced in the northern part of the Gissar micro-continent after a long period of amagmatic evolution, have intraplate geochemical affinities and isotopic Sr–Nd–Pb isotopic compositions (⁸⁷Sr/⁸⁶Sr(t) 0.7074–0.7086, εNd of − 5.5 to − 7.4) indicating derivation from Precambrian continental crust which is supported by old Nd model ages (1.5 and 1.7 Ga), and by the presence of inherited zircon grains with ages 850–500 Ma in the Chinorsay granodiorite. The post-collisional intrusions in the southern Gissar and in the Mogol-Tau and Kurama ranges (297–286 Ma), emplaced directly after supra-subduction magmatic series, have geochemical and isotopic signatures of arc-related magmas. The distinct shoshonitic affinities of post-collisional intrusions in the Mogol-Tau and Kurama ranges are explained by the interaction of hot asthenospheric material with subduction-enriched wedge of lithospheric mantle due to slab break-off at post-collisional stage. Despite origination from different tectonic environments, all magmatic rocks have relatively old Nd model ages (1.7–1.0 Ga) indicating a significant proportion of Paleoproterozoic or older crustal material in their sources and their model ages are similar to those of post-collisional intrusions from the Alai and Kokshaal Segments of the South Tien Shan.     

The role of evaporites in the formation of magnetite–apatite deposits along the Middle and Lower Yangtze River,China: Evidence from LA-ICP-MS analysis of fluid inclusions

Numerous magnetite–apatite deposits occur in the Ningwu and Luzong sedimentary basins along the Middle and Lower Yangtze River, China. These deposits are located in the contact zone of (gabbro)-dioritic porphyries with surrounding volcanic or sedimentary rocks and are characterized by massive, vein and disseminated magnetite–apatite ± anhydrite mineralization associated with voluminous sodic–calcic alteration. Petrologic and microthermometric studies on multiphase inclusions in pre- to syn-mineralization pyroxene and garnet from the deposits at Meishan (Ningwu basin), Luohe and Nihe (both in Luzong basin) demonstrate that they represent extremely saline brines (~ 90 wt.% NaCl_equiv) that were trapped at temperatures of about 780 °C. Laser ablation ICP-MS analyses and Raman spectroscopic studies on the natural fluid inclusions and synthetic fluid inclusions manufactured at similar P–T conditions reveal that the brines are composed mainly of Na (13–24 wt.%), K (7–11 wt.%), Ca (~ 7 wt.%), Fe (~ 2 wt.%), Cl (19–47 wt.%) and variable amounts of SO₄ (3–39 wt.%). Their Cl/Br, Na/K and Na/B ratios are markedly different from those of seawater evaporation brines and lie between those of magmatic fluids and sedimentary halite, suggesting a significant contribution from halite-bearing evaporites. High S/B and Ca/Na ratios in the fluid inclusions and heavy sulfur isotopic signatures of syn- to post-mineralization anhydrite (δ³⁴S_Anh = + 15.2 to + 16.9‰) and pyrite (δ³⁴S_Py = + 4.6‰ to + 12.1‰) further suggest a significant contribution from sedimentary anhydrite. These interpretations are in line with the presence of evaporite sequences in the lower parts of the sedimentary basins.The combined evidence thus suggests that the magnetite–apatite deposits along the Middle and Lower Yangtze River formed by fluids that exsolved from magmas that assimilated substantial amounts of Triassic evaporites during their ascent. Due to their Fe-oxide dominated mineralogy, their association with large-scale sodic–calcic alteration and their spatial and temporal associations with subvolcanic intrusions we interpret them as a special type of IOCG deposits that is characterized by unusually high contents of Na, Ca, Cl and SO₄ in the ore-forming fluids. Evaporite assimilation apparently led to the production of large amounts of high-salinity brine and thus to an enhanced capacity to extract iron from the (gabbro)-dioritic intrusions and to concentrate it in the form of ore bodies. Hence, we believe that evaporite-bearing sedimentary basins are more prospective for magnetite–apatite deposits than evaporite-free basins.     

Chronology and geochemistry of Mesozoic granitoids in the Bengbu area,central China: Constraints on the tectonic evolution of the eastern North China Craton

We performed zircon U–Pb dating and analyses of major and trace elements, and Sr–Nd–Pb isotopes for granitoids in the Bengbu area, central China, with the aim of constraining the magma sources and tectonic evolution of the eastern North China Craton (NCC). The analyzed zircons show typical fine-scale oscillatory zoning, indicating a magmatic origin. Zircon U–Pb dating reveals granitoids of two ages: Late Jurassic and Early Cretaceous (²⁰⁶Pb/²³⁸U ages of 160 Ma and 130–110 Ma, respectively). The Late Jurassic rocks (Jingshan intrusion) consist of biotite-syenogranite, whereas the Early Cretaceous rocks (Huaiguang, Xilushan, Nushan, and Caoshan intrusions) are granodiorite, syenogranite, and monzogranite. The Late Jurassic biotite-syenogranites and Early Cretaceous granitoids have the following common geochemical characteristics: SiO₂ = 70.35–74.56 wt.%, K₂O/Na₂O = 0.66–1.27 (mainly < 1.0), and A/CNK = 0.96–1.06, similar to I-type granite. The examined rocks are characterized by enrichment in light rare earth elements, large ion lithophile elements, and U; depletion in heavy rare earth elements, Nb, and Ta; and high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7081–0.7110) and low ε_Nd (t) values (? 14.40 to ? 22.77), indicating a crustal origin.The occurrence of Neoproterozoic magmatic zircons (850 Ma) and inherited early Mesozoic (208–228 Ma) metamorphic zircons within the Late Jurassic biotite-syenogranites, together with the occurrence of Neoproterozoic magmatic zircons (657 and 759 Ma) and inherited early Mesozoic (206–231 Ma) metamorphic zircons within the Early Cretaceous Nushan and Xilushan granitoids, suggests that the primary magmas were derived from partial melting of the Yangtze Craton (YC) basement. In contrast, the occurrence of Paleoproterozoic and Paleoarchean inherited zircons within the Huaiguang granitoids indicates that their primary magmas mainly originated from partial melting of the NCC basement. The occurrence of YC basement within the lower continental crust of the eastern NCC indicates that the YC was subducted to the northwest beneath the NCC, along the Tan-Lu fault zone, during the early Mesozoic.     

The late- to postorogenic transition in the Apiaí domain,SE Brazil: Constraints from the petrogenesis of the Neoproterozoic Agudos Grandes Granite Batholith

A remarkable change in the typology of granite magmatism occurred during the late evolution of the Agudos Grandes Batholith (Apiaí domain, SE Brazil), marked by the appearance of A-type granites some 10–15 Myr after the last manifestations of the compressional period. The elemental and Sr–Nd isotope geochemistry of the approximately 600 Ma, “late-orogenic”, moderately peraluminous Piedade, Serra dos Lopes, and Pilar do Sul granites shows that the middle crust was heated to approximately 820–850 °C during the latest stages of the compressional period and generated large amounts of melt through biotite dehydration–melting of gneiss protoliths. These crustal magmas either formed independent intrusions or mixed in varied proportions with raising high-K calc-alkaline magmas, as suggested by continuous trends toward more mafic granites (65–70 wt% SiO₂), which have the geochemical signature of contaminated calc-alkaline magmas. The postorogenic granites occur as two subgroups with different age (∼585 and ∼565 Ma), both with the chemical fingerprints typical of A-type granites (low mg#, Ba, and Sr; high Zr, Hf, Nb, and Y), interpreted as a result of crustal melting at lower P, fO₂, and a(H₂O) and higher T (up to 870 °C, indicated by zircon saturation). Significant trace-element and radiogenic isotope contrasts (e.g., εNd_T = −16 vs. −11) show that the two postorogenic subgroups derived from different sources. A Sr–Nd isotope signature nearly identical to the “end-member” late-orogenic crustal melts suggests that the older postorogenic granites derived from a similar (but probably less fertile) source. The less negative εNd_T of the young subgroup is unmatched by known crustal sources in the Apiaí domain and may have resulted from either an infracrustal component or mobilization of an unexposed section with less crustal residence time.     

Incremental growth and origin of the Cretaceous Renjiayingzi pluton,southern Inner Mongolia,China: Evidence from structure,geochemistry and geochronology

The Renjiayingzi intermediate-acid pluton is located along a pre-existing ENE–WSW-trending dextral shear zone that forms part of the Xar Moron suture zone that marks the final closure of the Paleo-Asian Ocean. The pluton is composed of three small intrusions, which from northwest to southeast, are named the Shuangjianshan (SI), the Qianweiliansu (QI) and the Xingshuwabeishan (XI) intrusions. LA-ICPMS zircon U–Pb dating of a pyroxene diorite from the SI yields an age of 138 ± 1 Ma; the SHRIMP zircon U–Pb age of a tonalite from the QI records an age of 134 ± 2 Ma, whereas LA-ICPMS zircon U–Pb dating of a monzogranite from the XI has an age of 126 ± 1 Ma, suggesting the entire pluton was built up by three separate emplacement events that young to the ESE: this is further supported by the contact relations. Incremental growth of plutons by amalgamation of repeated small magma pulses is the most viable emplacement model. The pluton was probably emplaced by updoming of the roof along previous tensile fractures and by upward stacking of the three intrusions. The SI and QI have similar U–Pb ages and geochemical characteristics, and most likely had the same magma source and underwent similar petrogenetic processes. They have high MgO concentrations at low silica contents, are enriched in large ion lithophile elements, depleted in high field strength elements, have negative ε_Nd(t) values of −1.8 to −3.7, with Nd model ages of 1.07–1.19 Ga. Pyroxene diorites of the SI also have variable zircon ε_Hf(t) values (from −0.8 to +6.1), indicating that they were mainly derived from juvenile crust with minor crustal contamination and clinopyroxene-dominated fractional crystallization. The late monzogranites from the XI show weak negative ε_Nd(t) values of −2.3 to −2.5, young Nd model ages of 0.99–1.00 Ga, positive zircon ε_Hf(t) values (+1.3 to +4.6) and higher SiO₂ and K₂O contents, with strong depletion in Eu, P and Ti, indicating derivation from a distinct petrogenetic process from the two earlier intrusions. The monzogranites were the result of partial melting of juvenile crust in response to mantle-derived magma underplating, together with plagioclase-dominated fractional crystallization.     

Geochronology and geochemistry of Late Cretaceous igneous intrusions and Mo–Cu–(W) mineralization in the southern Yidun Arc,SW China: Implications for metallogenesis and geodynamic setting

The Sanjiang Tethyan Metallogenic Domain (STMD) is an important part of the Tethyan giant metallogenic belt. The Yidun Arc is a part of the STMD in the eastern Tibetan Plateau. Recently, four newly discovered Mo–Cu–(W) ore deposits related to granitic intrusions were found distributed along the north-south strike in the southern Yidun Arc, which are identified as the Xiuwacu, Relin, Hongshan, and Tongchanggou deposits herein. These four deposits formed along high-angle north-northwest or north-west strike-slip faults, with vein-type and porphyry-type Mo–Cu mineralization developed in the intrusions. Molybdenite Re–Os and zircon U–Pb dating together with zircon Hf isotopes and whole-rock geochemistry of the intrusions were studied to discern the relationship between mineralization and magmatism, metallogenesis, and tectonic settings. Molybdenite from skarn-type mineralization at the Hongshan deposit has a Re–Os isochron age of 81.2 ± 2.6 Ma (MSWD = 1.3, n = 5) consistent with previously published zircon U–Pb ages and Re–Os ages of porphyry-type Mo mineralization. These results indicate that the Hongshan is a Late Cretaceous porphyry-skarn Cu–Mo deposit. Zircon U–Pb ages of the granitic intrusions in the Xiuwacu, Relin, and Tongchanggou deposits varying from ~ 87.4 Ma to ~ 82.7 Ma. Combined with published molybdenite Re–Os age spectrum (~ 85 Ma to ~ 81.2 Ma), it is proposed that the Mo–Cu–(W) mineralization in the Shangri-La region is spatially, temporally, and probably genetically related to the Late Cretaceous granitic intrusions. The Relin, Hongshan, and Tongchanggou intrusions have high SiO₂ (65.2–70.0 wt.%), Sr (363–905 ppm), Sr/Y (22–72), and La/Yb (37–69) ratios, and low Y (11.6–17.0 ppm) and Yb (0.97–1.59 ppm), which displayed adakitic affinities. Their low MgO (0.66–1.44 wt.%), Mg# (25–46), variable negative zircon ε_Hf(t) values (− 7.9 to − 2.3), and Proterozoic two-stages Hf model ages (T_DM2 = 1.13–1.62 Ga) suggest that they were probably dominantly derived from partial melting of thickened lower continental crust. According to the tectonic evolution of the Bangong Meso-Tethys Ocean during the Late Mesozoic, the Late Cretaceous igneous event and mineralization in the Yidun Arc likely formed under a late- or post-collision extensional environment, probably related to the collision between the Lhasa and Qiangtang terranes during the Late Cretaceous.     

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 ± 2 Ma and 73.4 ± 0.7 Ma, respectively. The 73 Ma age is comparable to the Re–Os ages of 77 to 80 Ma 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; ε_Nd(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 (δ³⁴S_V-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.     

Platinum mineralization of the Svetly Bor and Nizhny Tagil intrusions,Ural Platinum Belt

This paper presents the results of a detailed mineralogical and micro analytical study of two Ural-Alaskan type intrusions in the Ural Platinum Belt: (1) the dunite-hosted mineralization of the Svetly Bor intrusion, and (2) the chromitite mineralization of the Nizhny Tagil intrusion. Two generations of platinum minerals are typical of both intrusions: magmatic Pt–Fe(Ni) alloys, and post-magmatic Pt(Fe,Ni,Cu) alloys. A trend from ferroan platinum to isoferroplatinum (Pt,Fe → Pt₃Fe) is shown for magmatic alloys of the Svetly Bor intrusion. Magmatic alloys of the Nizhny Tagil intrusion are represented by ferroan platinum (with Ni) only, varying in Fe. The magmatic Pt–Fe alloys of both intrusions were depleted in Fe during the evolution of ore-forming systems and crystallized during the entire magmatic process, generally as fine cubic crystals and anhedral grains hosted by dunite at the magmatic stage during and after the crystallization of dunite (platinum–dunite type of the Svetly Bor intrusion). The evolution of mineral paragenesis was accompanied by a temperature drop, as well as increases in fO₂ and fS₂.Most of the platinum was concentrated in the residual melts together with chromium, and crystallized in the final stage of the magmatic process (platinum–chromite ore of the Nizhny Tagil intrusion). Post-magmatic Pt(Fe,Cu,Ni) alloys formed during the serpentinization of dunite evolved according to a general compositional trend from tetraferroplatinum and ferronickelplatinum to tulameenite and later to Pt–Cu alloy and Pt-oxide (PtFe) → Pt(Fe,Cu,Ni) → Cu₃Pt → Pt–O. Platinum-rich mineralization of both intrusions was formed in the late magmatic stage from a melt rich in volatiles, regardless of their location in dunite or chromitites. Most probably, the rock structure (fracture systems) was the determining factor in the migration of PGE-rich residual melts and ore accumulation.     

Coexistence of abyssal and ultra-depleted SSZ type mantle peridotites in a Neo-Tethyan Ophiolite in SW Turkey: Constraints from mineral composition,whole-rock geochemistry (major–trace–REE–PGE), and Re–Os isotope systematics

We present new, whole-rock major and trace element chemistry, including rare earth elements (REE), platinum-group elements (PGE), and Re–Os isotope data from the upper mantle peridotites of a Cretaceous Neo-Tethyan ophiolite in the Mu?la area in SW Turkey. We also report extensive mineral chemistry data for these peridotites in order to better constrain their petrogenesis and tectonic environment of formation. The Mu?la peridotites consist mainly of cpx-harzburgite, depleted harzburgite, and dunite. Cpx-harzburgites are characterized by their higher average CaO (2.27 wt.%), Al₂O₃ (2.07 wt.%), REE (53 ppb), and ¹⁸⁷Os/¹⁸⁸Os_(i) ratios varying between 0.12497 and 0.12858. They contain Al-rich pyroxene with lower Cr content of coexisting spinel (Cr# = 13–22). In contrast, the depleted harzburgites and dunites are characterized by their lower average CaO (0.58 wt.%), Al₂O₃ (0.42 wt.%), and REE (1.24 ppb) values. Their clinopyroxenes are Al-poor and coexist with high-Cr spinel (Cr# = 33–83). The ¹⁸⁷Os/¹⁸⁸Os_(i) ratios are in the range of 0.12078–0.12588 and are more unradiogenic compared to those of the cpx-harzburgites.Mineral chemistry and whole rock trace and PGE data indicate that formation of the Mu?la peridotites cannot be explained by a single stage melting event; at least two-stages of melting and refertilization processes are needed to explain their geochemical characteristics. Trace element compositions of the cpx-harzburgites can be modeled by up to ~ 10–16% closed-system dynamic melting of a primitive mantle source, whereas those of the depleted harzburgites and dunites can be reproduced by ~ 10–16% open-system melting of an already depleted (~ 16%) mantle. These models indicate that the cpx-harzburgites are the products of first-stage melting and low-degrees of melt–rock interaction that occurred in a mid-ocean ridge (MOR) environment. However, the depleted harzburgites and dunites are the product of second-stage melting and related refertilization which took place in a supra subduction zone (SSZ) environment. The Re–Os isotope systematics of the Mu?la peridotites gives model age clusters of ~ 250 Ma, ~ 400 Ma and ~ 750 Ma that may record major tectonic events associated with the geodynamic evolution of the Neo-Tethyan, Rheic, and Proto-Tethyan oceans, respectively. Furthermore, > 1000 Ma model ages can be interpreted as a result of an ancient melting event before the Proto-Tethys evolution.     

Magmatic ore deposits in mafic–ultramafic intrusions of the Giles Event,Western Australia

More than 20 layered intrusions were emplaced at c. 1075 Ma across > 100 000 km² in the Mesoproterozoic Musgrave Province of central Australia as part of the c. 1090–1040 Ma Giles Event of the Warakurna Large Igneous Province (LIP). Some of the intrusions, including Wingellina Hills, Pirntirri Mulari, The Wart, Ewarara, Kalka, Claude Hills, and Gosse Pile contain thick ultramafic segments comprising wehrlite, harzburgite, and websterite. Other intrusions, notably Hinckley Range, Michael Hills, and Murray Range, are essentially of olivine-gabbronoritic composition. Intrusions with substantial troctolitic portions comprise Morgan Range and Cavenagh Range, as well as the Bell Rock, Blackstone, and Jameson–Finlayson ranges which are tectonically dismembered blocks of an originally single intrusion, here named Mantamaru, with a strike length of > 170 km and a width of > 20 km, constituting one of the world's largest layered intrusions.Over a time span of > 200 my, the Musgrave Province was affected by near continuous high-temperature reworking under a primarily extensional regime. This began with the 1220–1150 Ma intracratonic Musgrave Orogeny, characterized by ponding of basalt at the base of the lithosphere, melting of lower crust, voluminous granite magmatism, and widespread and near-continuous, mid-crustal ultra-high-temperature (UHT) metamorphism. Direct ascent of basic magmas into the upper crust was inhibited by the ductile nature of the lower crust and the development of substantial crystal-rich magma storage chambers. In the period between c. 1150 and 1090 Ma magmatism ceased, possibly because the lower crust had become too refractory, but mid-crustal reworking was continuously recorded in the crystallization of zircon in anatectic melts. Renewed magmatism in the form of the Giles Event of the Warakurna LIP began at around 1090 Ma and was characterized by voluminous basic and felsic volcanic and intrusive rocks grouped into the Warakurna Supersuite. Of particular interest in the context of the present study are the Giles layered intrusions which were emplaced into localized extensional zones. Rifting, emplacement of the layered intrusions, and significant uplift all occurred between 1078 and 1075 Ma, but mantle-derived magmatism lasted for > 50 m.y., with no time progressive geographical trend, suggesting that magmatism was unrelated to a deep mantle plume, but instead controlled by plate architecture.The Giles layered intrusions and their immediate host rocks are considered to be prospective for (i) platinum-group element (PGE) reefs in the ultramafic–mafic transition zones of the intrusions, and in magnetite layers of their upper portions, (ii) Cu–Ni sulfide deposits hosted within magma feeder conduits of late basaltic pulses, (iii) vanadium in the lowermost magnetite layers of the most fractionated intrusions, (iv) apatite in unexposed magnetite layers towards the evolved top of some layered intrusions, (v) ilmenite as granular disseminated grains within the upper portions of the intrusions, (vi) iron in tectonically thickened magnetite layers or magnetite pipes of the upper portions of intrusions, (vii) gold and copper in the roof rocks and contact aureoles of the large intrusions, and (viii) lateritic nickel in weathered portions of olivine-rich ultramafic intrusions.     

Petrology,geochemistry and geochronology of the magmatic suite from the Jianzha Complex,central China: Petrogenesis and geodynamic implications

The intermediate–mafic–ultramafic rocks in the Jianzha Complex (JZC) at the northern margin of the West Qinling Orogenic Belt have been interpreted to be a part of an ophiolite suite. In this study, we present new geochronological, petrological, geochemical and Sr–Nd–Hf isotopic data and provide a different interpretation. The JZC is composed of dunite, wehrlite, olivine clinopyroxenite, olivine gabbro, gabbro, and pyroxene diorite. The suite shows characteristics of Alaskan-type complexes, including (1) the low CaO concentrations in olivine; (2) evidence of crystal accumulation; (3) high calcic composition of clinopyroxene; and (4) negative correlation between FeO^tot and Cr₂O₃ of spinels. Hornblende and phlogopite are ubiquitous in the wehrlites, but minor orthopyroxene is also present. Hornblende and biotite are abundant late crystallized phases in the gabbros and diorites. The two pyroxene-bearing diorite samples from JZC yield zircon U–Pb ages of 245.7 ± 1.3 Ma and 241.8 ± 1.3 Ma. The mafic and ultramafic rocks display slightly enriched LREE patterns. The wehrlites display moderate to weak negative Eu anomalies (0.74–0.94), whereas the olivine gabbros and gabbros have pronounced positive Eu anomalies. Diorites show slight LREE enrichment, with (La/Yb)_N ratios ranging from 4.42 to 7.79, and moderate to weak negative Eu anomalies (Eu/Eu¹ = 0.64–0.86). The mafic and ultramafic rocks from this suite are characterized by negative Nb–Ta–Zr anomalies as well as positive Pb anomalies. Diorites show pronounced negative Ba, Nb–Ta and Ti spikes, and typical Th–U, K and Pb peaks. Combined with petrographic observations and chemical variations, we suggest that the magmatism was dominantly controlled by fractional crystallization and crystal accumulation, with limited crustal contamination. The arc-affinity signature and weekly negative to moderately positive ε_Nd(t) values (−2.3 to 1.2) suggest that these rocks may have been generated by partial melting of the juvenile sub-continental lithospheric mantle that was metasomatized previously by slab-derived fluids. The lithologies in the JZC are related in space and time and originated from a common parental magma. Geochemical modeling suggests that their primitive parental magma had a basaltic composition. The ultramafic rocks were generated through olivine accumulation, and variable degrees of fractional crystallization with minor crustal contamination produced the diorites. The data presented here suggest that the subduction in West Qinling did not cease before the early stage of the Middle Triassic (∼242 Ma), a back-arc developed in the northern part of West Qinling during this period, and the JZC formed within the incipient back-arc.     

Thick sections of layered ultramafic cumulates in the Oman ophiolite revealed by an airborne hyperspectral survey: Petrogenesis and relationship to mantle diapirism

Using the HyMap instrument, we have acquired visible and near infrared hyperspectral data over the Maqsad area of the Oman ophiolite (~ 15 × 60 km). This survey allowed us to identify and map the distribution of clinopyroxene-rich cumulates (inter-layered clinopyroxenites and wehrlites) whose occurrence was previously undocumented in this area. The cumulates reach several hundred meters in thickness and crop out at distances exceeding 15 km on both sides of the Maqsad former spreading centre. They occur either in mantle harzburgites, as km-sized layered intrusions surrounded by fields of pegmatitic dykes consisting of orthopyroxene-rich pyroxenite and gabbronorites, or at the base of the crustal section where they are conformably overlain by cumulate gabbros. These ultramafic cumulates crystallized from silica- and Mg-rich melts derived from a refractory mantle source (e.g. high Cr#, low [Al₂O₃], low [TiO₂]). These melts are close to high-Ca boninites, although, strictly speaking, not perfect equivalents of present-day, supra-subduction zone, boninites. Chemical stratigraphy reveals cycles of replenishment, mixing and fractional crystallization from primitive (high Mg#) melts, typical of open magma chambers and migration of inter-cumulus melts. The TiO₂ content of clinopyroxene is always low (≤ 0.2 wt.%) but quite variable compared to the associated pegmatites that are all derived from a source ultra-depleted in high field strength elements (HFSE). This variability is not caused by fractional crystallization alone, and is best explained by hybridization between the ultra-depleted melts (parent melts of the pegmatites) and the less depleted mid-ocean ridge basalts (MORB) parent of the dunitic–troctolitic–gabbroic cumulates making up the crustal section above the Maqsad diapir.We propose that, following a period of magma-starved spreading, the Maqsad mantle diapir, impregnated with tholeiitic melts of MORB affinity, reached shallow depths beneath the ocean ridge. This diapir induced melting of the formerly accreted and hydrothermally altered lithosphere. At this stage, these boninitic-like lithospheric melts crystallized as pegmatitic dykes. As the diapir continued to rise, the amount of MORB reaching shallow depths increased, together with the surrounding temperature, leading to the formation of magma chambers where the crystallization of layered cumulates became possible. These cumulates remained rich in pyroxene and devoid of plagioclase as long as the contribution of MORB-derived melts was moderate relative to the lithospheric-derived melts. As the contribution of MORB to the refilling of the magma chamber increased, gabbroic cumulates started to crystallize.