Post-subduction Setting Research Articles

Linking Porphyry Cu Formation to Tectonic Change in Postsubduction Settings: A Case Study from the Giant Yulong Belt, Eastern Tibet

Abstract Porphyry deposits in magmatic arcs form coincident with changes to steady-state oceanic subduction conditions, such as changes in plate convergence rate and vector or angle of subduction. However, it remains unclear whether such processes also operated during formation of postsubduction porphyry deposits. The Yulong magmatic belt in the eastern Tibetan Plateau consists of middle to late Eocene igneous rocks (~51–35 Ma) that formed during the India-Asia collision, whereas all known porphyry deposits are associated with late Eocene rocks (43–35 Ma). A synthesis of new and published geochemical data shows marked variations from the middle to late Eocene, including increasing whole-rock La/Yb, Sr/Y, and EuN/EuN* values, as well as zircon EuN/EuN* values. These geochemical variations, together with petrographic observations, indicate a transition from plagioclase-dominated to amphibole-dominated fractionation from the middle to late Eocene. Coupled changes of magma compositions and porphyry Cu metallogeny from the middle to late Eocene coincided with, or were slightly preceded by, the onset of regional uplift and crustal thickening, triggered by the India-Asia hard collision and rapid deceleration of the India-Asia convergence rate at ca. 50 to 44 Ma. Crustal thickening may have caused prolonged magma differentiation at greater depths and accumulation of dissolved H2O, both of which contributed to amphibole-dominated fractionation and generation of hydrous melt that are prospective for porphyry Cu mineralization. Our study highlights the importance of tectonic changes in the formation of the Yulong and other postsubduction porphyry Cu belts—a scenario similar to that operated in subduction-related settings such as the Andes.

Multiscale resistivity mapping from an intracontinental hydrothermal mineral system, Adelaide Rift Complex, Australia

Porphyry mineral systems provide a significant source of critical minerals for modern society, that are predominantly formed in subduction related settings from magmatic hydrothermal fluids derived from an underlying reservoir. However, in post-subduction settings the processes that entrain and focus magmatic hydrothermal fluids throughout the crust, from source to deposit are often poorly constrained. In this paper, we have undertaken a magnetotelluric (MT) survey in an intracontinental tectonic setting across the Adelaide Superbasin, which is a Neoproterozoic passive margin formed during rifting of the supercontinent Rodinia. This region contains the Burra magmatic copper deposit, along with numerous sedimentary-hosted copper deposits. Systematic three-dimensional modelling was undertaken, initially at regional-scale, with 55 km gridded long-period AusLAMP data over a 500 × 500 km area, followed by infilled 10 km gridded broadband MT data, at the district-scale, centred over an area of copper deposits in the Adelaide Rift Complex. The modelling reveals an elongated zone of low electrical resistivity (∼100 Ω.m), in an otherwise resistive crust, at lower crustal depth (20–40 km) that is contiguous with the regional lineament of the Adelaide Rift Complex and copper and gold mineralisation. A narrow zone of low resistivity (∼100 Ω.m) branches through the upper crust (5–20 km) to the surface, that is remarkably aligned with the Burra magmatic mineral system, in an otherwise quite resistive crust (∼1000 Ω.m). Additionally, in the upper crust a zone of low resistivity (1–10 Ω.m at a depth of 5–8 km) extends from the Burra magmatic copper deposit southwards, constrained by anticlinal folding from the ∼500 Ma Delamerian Orogenic deformation of the Adelaide Rift Complex and spatially aligned with the Kapunda sedimentary-hosted copper deposit. We argue that the low electrical resistivity signature encapsulates a whole-of-lithosphere magmatically-hosted copper system, that is further mobilised by low-temperature fluids by mapping the footprint of ore-forming fluids from the source to associated mineralisation. Where later basin wide fluid recycling in a permeability enhanced basin, via neo-tectonism along pre-existing structures/trends, has likely remobilised copper, from a prior magmatic event, with copper being leached in chloride-rich saline fluids at low temperatures.

The multi-component mantle source of Roman province ultrapotassic magmas revealed by melt inclusions

A key effect of subduction is the upward infiltration of slab-derived melts and fluids to create a chemically and lithologically heterogeneous region in the mantle wedge. Magmatism within the post-subduction setting of peninsular Italy is derived from a sediment-metasomatised lithospheric mantle source, with primitive products recording substantial temporal and spatial variations in the nature and extent of metasomatism. The Roman magmatic province, in central Italy, is host to ultrapotassic (HKS), leucite-bearing magmas that require source modification that is not yet well-constrained in terms of mineralogy and sediment provenance. Here, we present the chemistry and Sr-Nd-Pb isotope compositions of melt inclusions in forsterite-rich olivine (Fo88−92) from three key Roman volcanic centres (Vulsini, Sabatini, Alban Hills) to characterise the metasomatic imprint of their mantle sources. The melt inclusion suites (Vulsini HKS, Vulsini melilite-bearing HKS, Sabatini HKS, Alban Hills HKS) have clear differences in major-element (K, Ca, Al, P, Ti), volatile (F, Cl, B, Be), trace-element (e.g., Rb, Sr/Y, Th/Nb), and radiogenic-isotope (87Sr/86Sr) compositions that reflect the diversity of the primary melts beneath the region. For each suite, the extraction of melt batches from different domains in a heterogeneous (veined) source rock with different compositions and mineralogies can explain the chemical characteristics of the melts. At least three metasomatic vein types are required to form the respective parental melts and we infer phase assemblages consisting of phlogopite + Ca-amphibole + clinopyroxene ± apatite. The Sr-isotope differences between Vulsini melilite-bearing HKS and Vulsini and Sabatini HKS require at least two distinct metasomatic events to have occurred beneath the Roman province. These events need not be associated with two distinct subduction systems. Instead, we infer that different metasomatic agents could have originated under different P-T-t conditions from different domains in the same subducting slab.

The Lower Cretaceous sequence of western Alaska—demise of the Koyukuk terrane?

Lower Cretaceous marine sedimentary rocks, deposited in shallow shelf and basin settings and unconformity-bound, are well exposed in southwest Alaska. Collections of Early Cretaceous fossils from across western Alaska show that similar and coeval Lower Cretaceous clastic rocks are widely distributed though only locally exposed. Volcanic rocks become an important part of the Lower Cretaceous sequence in the Yukon-Koyukuk basin, where they have been interpreted to represent a mobile intra-oceanic island arc, the Koyukuk terrane, that collided with Arctic Alaska to form the Brooks Range orogen. The volcanic rocks are chemically unlike Aleutian arc rocks but share compositional characteristics with spatially related, mid-Cretaceous alkaline intrusive rocks. The volcanic-bearing sequence was also deposited on an angular unconformity, includes both shallow shelf and basin depositional settings, and is unconformably overlain by mid-Cretaceous clastic rocks. The volcanic rocks are therefore considered part of the Lower Cretaceous sequence now identified across western Alaska. In this interpretation, the Lower Cretaceous volcanic rocks are an initial expression of the mid-Cretaceous tectonic regime that included extensional exhumation and subsidence, crustal and upper mantle melting, and high-temperature metamorphism in the hinterland of the Brooks Range orogen. The Cretaceous heating that led to hinterland crust and upper mantle change may have been caused by deep mantle disturbances in a postsubduction setting. This interpretation has implications for the timing of contractional orogenesis, the location and nature of the related continental borderland, and the tectonic setting for development of the Anguyucham and related oceanic terranes.

Post-subduction porphyry Cu magmas in the Sanjiang region of southwestern China formed by fractionation of lithospheric mantle–derived mafic magmas

Abstract For porphyry Cu deposits that formed during oceanic slab subduction, there is a general consensus that the ore-forming magmas evolved through fractionation of mafic magmas that were generated by slab fluid (± melt)–fluxed melting of the asthenospheric mantle wedge. This model, however, is not applicable to post-subduction porphyry Cu deposits because they formed distinctly after cessation of oceanic slab subduction. A popular model proposes that post-subduction porphyry Cu magmas were produced by partial melting of lower-crustal, sulfide-rich arc cumulates, with or without minor contributions from potassic mafic magmas. To reappraise the crustal melting model, we focused on one of the largest post-subduction porphyry Cu belts on Earth, which formed during the India-Asia collision in the Sanjiang region of southwestern China. Detailed petrographic studies and new Nd-Sr isotopic data from non-metasomatized versus metasomatized lower-crustal xenoliths suggest that previous models based on crustal melting rest upon wrong radiogenic isotope constraints due to pervasive metasomatism of the xenoliths. Based on traceelement quantitative modeling and regional-scale geochemical trends of magmatic rocks, we demonstrate that the Sanjiang post-subduction porphyry Cu magmas were produced by fractionation of potassic mafic magmas derived from lithospheric mantle. This study highlights that post-subduction porphyry Cu magmas attain their K-rich composition, and all the ore-forming ingredients, from subduction-modified lithospheric mantle, the existence of which may be a prerequisite for the formation of porphyry Cu deposits in post-subduction settings.

Seismic signature of subduction termination from teleseismic P- and S-wave arrival-time tomography: The case of northern Borneo

Studies attempting to gain new insights into the last stage of the subduction cycle are typically challenged by limited direct observations owing to a lack of recent post-subduction settings around the world. Central to unravelling how the subduction cycle ends is an understanding of crust and mantle processes that take place after subduction termination. Northern Borneo (Malaysia) represents a unique natural laboratory because it has been the site of two sequential subduction episodes of opposite polarity since the mid-Paleogene. The region exhibits several enigmatic post-subduction (after ∼ 10 Ma) features, including: subsidence followed by rapid uplift, localised intraplate volcanism, possible orogen collapse, and a pluton that emerged to become the third highest peak in southeast Asia, Mt Kinabalu (4095 m). Arrival-time residuals from distant earthquake data recorded by the nBOSS seismic network have been used to investigate P- and S-wavespeed variations in the crust and underlying upper mantle beneath northern Borneo. Our 3-D tomographic images consistently show a high-velocity perturbation in western Sabah that we associate with an upper-mantle remnant of the Proto South-China Sea slab, thus providing important constraints for tectonic reconstructions of SE Asia. The tomographic models, combined with other seismological and geological information, reveal evidence for lithospheric removal in eastern Sabah via a drip instability. Our results suggest that lithospheric drips can be smaller than previously thought, yet their effects on the post-subduction evolution of continental lithosphere can be significant.

Can Post-Subduction Porphyry Cu Magmas Form by Partial Melting of Typical Lower Crustal Amphibole-Rich Cumulates? Petrographic and Experimental Constraints from Samples of the Kohistan and Gangdese Arc Roots

Abstract Sulfide-rich hydrous arc cumulates in the lower crust are considered a likely source for porphyry Cu-forming magmas in post-subduction settings. However, it remains controversial whether or not post-subduction porphyry Cu magmas can be produced by partial melting of lower crustal arc cumulates. Here we performed detailed petrographic and geochemical studies on exhumed lower to middle crustal cumulates from the Gangdese and Kohistan arc sections, and then conducted partial melting experiments of the Gangdese sulfide-rich lithologies at 1.5 GPa and 800°C–1150°C using a piston cylinder press. Both the Gangdese and the Kohistan arc-root samples formed as cumulates in the lower to middle crust, but the Kohistan samples were subsequently brought into the garnet stability field during the India–Eurasia continental collision. Most of the Gangdese samples contain relatively high abundances of magmatic sulfides preserved in the form of Cu-poor monosulfide solid solution. In contrast, most of the Kohistan samples contain very low abundances of Cu-rich sulfides preserved as inclusions in garnet, probably due to intense metamorphic dehydration and up to 2 wt% partial melting in the garnet stability field. The original magmatic sulfides at Kohistan appear to have been removed mainly by metamorphic fluids rather than by partial melts, as the arc-root samples could have experienced only very low degree of partial melting and the sulfur solubility in experimentally produced partial melts is low (≤0.1 wt%) even at oxygen fugacities two log-units above the fayalite-magnetite-quartz buffer. Furthermore, the experimental partial melts contain much less K2O, MgO, Cl and ±S than typical post-subduction porphyry Cu-forming magmas. Hence, we conclude that the role of sulfide-rich arc cumulates in the formation of post-subduction porphyry Cu magmas has been severely overestimated.

Genesis and mechanisms of metal enrichment in the Baimazhai Ni-Cu-(PGE) deposit, Ailaoshan Orogenic Belt, SW China

ABSTRACT The ∼259 Ma Baimazhai Ni-Cu-(platinum-group element) deposit is located in the Ailaoshan-Red River fault zone on the southwest margin of the Yangtze Plate in the Jinping area of southeastern Yunnan Province. The intrusion is lenticular (∼530 m long × 190 m wide × 24–64 m thick) and concentrically zoned (margin to core) from gabbro through pyroxenite to peridotite. It contains ∼50 kt of Ni-Cu-(platinum-group element) mineralization, concentrically zoned (margin to core) from disseminated through net-textured to massive sulfides with an average grade of 1.03 wt.% Ni, 0.81 wt.% Cu, and 0.02∼0.69 ppm Pd+Pt. The sulfide assemblage comprises pyrrhotite, chalcopyrite, and pentlandite, with lesser magnetite, violarite, galena, and cobaltite. The mineralization is enriched in Ni-Cu-Co relative to the platinum-group elements and the host rocks are enriched in highly incompatible lithophile elements relative to moderately incompatible lithophile elements with high Th/Yb and intermediate Nb/Yb ratios. These host rocks, and those at most other Ni-Cu-platinum-group element deposits in the Emeishan Large Igneous Province, have high γOs and intermediate εNd values, indicating that they crystallized from a magma derived from a subduction-modified pyroxenite mantle source and modified by crustal contamination. The initial concentrations of metals in the primary magma are estimated to have been on the order of 200 ppm Ni and 100 ppm Cu, but only 0.4 ppb Pd, 0.2 ppb Pt, 0.005 ppb Rh, 0.02 ppb Ru, and 0.01 ppb Ir. The δ34S values of ores and separated sulfides range from 5.8‰ to 8.6‰, between the ∼10‰ value of sulfides in the metasedimentary country rocks and the 0 ± 0.5‰ value expected for magmas derived from MORB-type mantle, or the –2.5 ± 0.3‰ value expected for subduction-modified mantle, consistent with equilibration at magma:sulfide mass ratios (R factors) of 100–1000. Variations in Ir100 and Pd100 (metals in 100% sulfide) are consistent with 40–60% fractional crystallization of monosulfide solid solution to form Ni-Co-intermediate platinum-group element (Ru, Os, Ir)-rich massive ores and Cu-palladium/platinum-group elements (Pt, Pd, Rh)-Au-rich residual sulfide liquids. This process is also recorded by magnetite: Type I (early magmatic), type II (late magmatic), and type III (secondary) magnetites exhibit progressively lower Cr-Ti-V concentrations. The platinum-group element contents in base-metal minerals are low, and only pentlandite, violarite, and cobaltite contain detectable concentrations of Pd, Rh, and Ru. There is abundant textural evidence for metamorphic-hydrothermal alteration of sulfides in the Baimazhai intrusion, with secondary violarite, chalcopyrite, and pentlandite being enriched (Ag, Sb, Au, Pb) or depleted (Sn) in more mobile chalcophile elements. The different tectonic and petrogenetic settings of the Baimazhai and other deposits in China highlight the potential of Ni-Cu-platinum-group element deposits to occur in subduction or post-subduction settings and demonstrate that the key controls are magma flux and access to crustal S. Exploration potential remains for the Ailaoshan orogenic belt to host additional magmatic Ni-Cu deposits.

Redox conditions, compositional parameters, and indirect subduction-related source of Cretaceous Sn and Cu–Mo fertile post-subduction granites in the Yidun Terrane of eastern Tibet

The metal and sulfur fertility of granite magmas that formed in post-subduction settings, the key processes that led to their contrasting associated polymetallic mineral deposits, and their post-subduction source remain obscure. The Yidun Terrane, eastern Tibet, extends for 500 km and can be divided into the northern Yidun Terrane (NYT) and southern Yidun Terrane (SYT) based on distinctive magmatic source affinities and related mineral deposits. The NYT has several 103–93 Ma granite plutons and associated magmatic-hydrothermal vein-type Sn–Pb–Zn–Ag polymetallic deposits, such as the giant Xiasai deposit. In contrast, the SYT granites host the Xiuwacu porphyry Mo–W deposit (88–84 Ma) and several porphyry or porphyry–skarn-type Cu–Mo deposits (85–78 Ma), such as Relin, Hongshan and Tongchanggou. The contrasting nature of the causative Cretaceous post-subduction granites and their source is determined from their whole-rock and isotope geochemistry, zircon trace-element concentrations, and biotite and amphibole mineral chemistry. Based on these parameters, the NYT Sn–Pb–Zn–Ag deposits are associated with highly fractionated, reduced ilmenite-series granites derived from a relatively dry magma that formed via mixing between amphibolite-derived and greywacke- and orthogneiss-derived metasomatized crustal melts. In contrast, the SYT Mo–W deposits are associated with strongly- to moderately-evolved and fractionated, oxidized intermediate-series granites. The SYT Cu–Mo deposits are related to more hydrous and less fractionated magnetite-series granites. The granites associated with both Mo–W and Cu–Mo deposits in the SYT were generated by partial melting of metasomatized ancient mafic-intermediate lower continental-crust with variable contributions of metasomatized mantle components. However, there is only evidence for the presence of garnet as a residual phase in the magmatic source of the SYT Cu–Mo-bearing granites. The different magma sources were important in determining the redox state of the Yidun granite magmas, providing a fundamental control on the behaviour of metals and sulfur in fractionating melts and dictating the contrasting polymetallic compositions of causative magmatic-hydrothermal fluids and resultant mineral deposits. The source regions of the granites are thus the primary control on the spatial distribution of the Sn–Pb–Zn–Ag, Mo–W, and Cu–Mo belts in the Yidun Terrane. Although not derived during subduction, as for most Cenozoic porphyry systems, the Yidun deposits are indirectly related to subduction via the metasomatism and fertilization of the mantle lithosphere and lower crust during the earlier subduction phase. The anomalous preservation of Cretaceous porphyry systems in the Yidun Terrane relates to their proximity to the margin of the Yangtze Craton, whose thick buoyant lithosphere inhibited exhumation and erosion.

Tectono-magmatic controls on decratonic gold deposits

Magmatic-hydrothermal gold–copper deposits in post-subduction settings represent essential targets for mineral exploration, but controls on their formation remain controversial. The early Cretaceous lode Au districts that formed during lithosphere destruction of the North China Craton provide an ideal opportunity to better understand the key tectono-magmatic factors responsible for the genesis of Au-rich deposits in post-subduction settings. Here, we present a LA-ICP-MS study of silicate melt inclusions and sulfide inclusions from ore-related mafic to intermediate rocks in the central Taihangshan Au district in the interior of the North China Craton to constrain the content and evolution of magmatic ore metals ± volatiles. The results, combined with numerical modeling, suggest that the ore-related magmas contained only a few ng/g Au, which is similar to the Au content of non-mineralization-related mafic to intermediate magmas worldwide. The low Au content of the lode Au-related magmas suggest that large volumes of magmas had to accumulate in the middle to lower crust through trans-lithospheric fault systems to produce the lode Au deposits. It is further suggested that the lode Au-related magmas were alkali-rich, hydrous, oxidized and relatively rich in sulfur and chlorine (mafic melt inclusions contain 0.14‒0.24 wt% S and 0.1‒0.2 wt% Cl). These properties are considered critical for the generation of auriferous ore fluids. By comparing the tectono-magmatic setting of the giant Jiaodong Au province (~ 4000 t Au) with the central Taihangshan district (~ 150 t Au), we propose that the much larger total Au tonnage of the Jiaodong district results from the accumulation of a much larger volume of ore-forming magmas at deep crustal levels, induced by a stronger degree of lithosphere modification. In addition, given that the composition of lode Au-related magmas is similar to that of porphyry Cu–Au-related magmas, the lack of giant, early Cretaceous porphyry Cu–Au deposits in the North China Craton suggests that strong extensional settings favor the formation of lode Au deposits instead of porphyry Cu–Au deposits. The present study, therefore, has general implications for the genesis of Au-rich deposits in strongly extensional settings.

Remelting of a Neoproterozoic arc root: origin of the Pulang and Songnuo porphyry Cu deposits, Southwest China

The reduced nature of the subducting slab has been used to explain the relatively poor endowment of economic porphyry copper deposits in Paleo-Tethyan arc systems. The presence of numerous porphyry Cu deposits in the southern Yidun terrane potentially challenges this model, because these deposits are thought to be the product of Paleo-Tethyan subduction. To investigate this, two representative deposits (i.e., Pulang and Songnuo porphyry Cu deposits) from the southern Yidun terrane have been studied. Zircon U-Pb ages of pre- and syn-mineral porphyries in Pulang are 221 ± 2 Ma (MSWD = 0.89) and 215 ± 2 Ma (MSWD = 1.50), respectively. Zircon and garnet U-Pb ages of the Songnuo syn-mineral porphyry are 217 ± 2 Ma (MSWD = 1.15) and 223 ± 5 Ma (MSWD = 0.97), respectively. The Pulang and Songnuo porphyries both have high whole-rock Sr/Y ratios with minor or no negative Eu anomalies, consistent with derivation from hydrous magmas that underwent minor early plagioclase crystallization but abundant amphibole fractionation. The two porphyry intrusions are characterized by high large-ion-lithophile elements (LILE: Th, U, Ba, Rb) abundances, and low concentrations of high-field-strength elements (HFSE: Nb, Ta, Ti, Zr, Hf). They have similar initial Sr isotope ratios from 0.7055 to 0.7073, ƐNd (t) values from − 4.3 to − 1.6 and zircon ƐHf (t) values of − 1.81 ± 1.34 (n = 47) that are distinct from published results for the Permian–Early Triassic Paleo-Tethyan arc granitoids in the Sanjiang region but similar to the Neoproterozoic arc intrusions in the nearby western Yangtze craton. The Pulang and Songnuo porphyries have relatively high zircon EuN/EuN* ratios (0.59 ± 0.04, n = 52) and calculated Δ log fO2 (FMQ) values (1.61 ± 0.23, n = 52), consistent with relatively oxidized magmas. This feature is similar to the published data for the Neoproterozoic arc intrusions (EuN/EuN* = 0.55 ± 0.08; Δ log fO2 (FMQ) = 2.14 ± 0.60), but distinct from the published data for the reduced normal Paleo-Tethyan arc magmas (EuN/EuN* = 0.29 ± 0.10; Δ log fO2 (FMQ) = − 0.68 ± 0.88). Given that the most recent tectonic reconstruction models suggest that the Paleo-Tethys Ocean closed before ~ 220 Ma, we propose that the Pulang and Songnuo porphyries Cu deposits were formed in a postsubduction setting from oxidized melts generated by reactivating the Neoproterozoic arc root, unrelated to Paleo-Tethyan subduction.

Recycled volatiles determine fertility of porphyry deposits in collisional settings

Abstract An intensive study of the geochemical characteristics (including the volatile elements Cl and S) of apatite associated with porphyry deposits was undertaken to address the debate about the crust- or mantle-derivation of their copper and gold and to better understand the controls on the transport of metals in magmatic fluids in post-subduction settings. New geochemical data on apatite reveal parameters to discriminate mineralized porphyry systems across Iran and western China (Tibet and Yunnan), from coeval barren localities across this post-subduction metallogenic belt. Apatites in fertile porphyries have higher Cl and S concentrations (reflecting water-rich crystallization conditions) than those from coeval barren ones. Our new isotopic data also indicate these volatiles are likely derived from pre-enriched sub-continental lithospheric mantle, metasomatized by previous oceanic subduction. This study demonstrates that refertilization of suprasubduction lithospheric mantle during previous collision events is a prerequisite for forming post-subduction fertile porphyries, providing an evidence-based alternative to current ore-enrichment models.

Sediment-derived melt-related metasomatized mantle wedge as a source of post-subduction Quaternary adakitic porphyries associated with absarokite-shoshonite from the Karadağ stratovolcano (Karaman, Central Anatolia, Turkey)

The Karadağ stratovolcano (KS), located at the southern end of the Cappadocian Volcanic Province in Central Anatolia, Turkey, is a key volcanic vent for understanding the spatiotemporal association between adakitic and mafic potassic (absarokitic-shoshonitic-banakitic) magmas in a post-subduction intracontinental setting. Here, we present a comprehensive geochemical dataset consisting of mineral chemistry, major and trace elements, Sr–Nd–Pb isotopes and conventional K-Ar age data for the adakitic andesite-dacite series of the KS. KS was built up by the eruption of adakitic andesites (825 ± 17 ka), absarokite-shoshonites (393 ± 153 ka), adakitic dacites (45 ± 6 ka) and effusive deposits (<45 ka). The metaluminous and calc-alkaline to high-K calc-alkaline Karadağ adakitic porphyries demonstrate high Sr (662 to 1053 ppm), Sr/Y ratios (39–66) and Mg# (46 to 63) along with low Y (11 to 17 ppm) and heavy rare earth element (HREE) concentrations, consistent with a slab melting origin in modern subduction settings. The samples are characterized by relative enrichment in large-ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs), with Nb-Ta depletion but an insignificant negative Eu anomalies. However, their high K2O/Na2O ratios (0.6–0.9) and Mg# and their evolved Sr-Nd-Pb isotopic compositions (87Sr/86Sr(t) = 0.7045 to 0.7078, 143Nd/144Nd = 0.512454 to 0.512612 and 206Pb/204Pb = 18.93 to 18.94) mark an origin from an enriched mantle source. Karadağ volcanic rocks also have low Nb/La (<0.5) ratios that show a positive correlation with SiO2 and high Nb/Ta (average 15.7) ratios close to that of primitive mantle (PM) (17.5), all of which implies a heterogeneous mantle source and an insignificant role of crustal assimilation. Based on the powerful volcanic markers of geochemical ratios (e.g., low Ba/Th, Sr/Th and high La/Sm, Th/Nb, Th/La, Hf/sm), variable Sr-Nd, and high 206Pb/204Pb isotopic compositions, we contend that the Karadağ adakitic porphyries, similar to the absarokites-shoshonites-banakites, were formed through partial melting of the metasomatized mantle. The mantle wedge was metasomatized mainly by overlying sediments of subducted crust, as major metasomatic agents, and to a lesser degree by melts/fluids from the dehydrated oceanic slab. It is suggested that absarokitic magmas were derived from the lowermost lithospheric mantle source and adakitic magmas from the uppermost mantle lithosphere. The thermal energy required for partial melting of the metasomatized mantle lithosphere was generated by upwelling of the hot asthenosphere through deep strike-slip faults bordering a pull-apart basin in a post-subduction extensional intracontinental setting. These interpretations argue against the presence of a Quaternary arc in Central Anatolia, and adakitic and associated mafic potassic magmas do not require contemporaneous subduction of an oceanic slab or delamination of lower continental crust in a post-subduction setting. We conclude that a post-collisional transtensional stage after rapid multi-phased uplift was triggered by asthenospheric mantle flows (collision-induced, collision-related slab break-off-induced, or a combined effect of both), transferring the metasomatized mantle-derived magmas into the crust system along with the whole Cappadocian Volcanic Province.

The Lazurnoe deposit in the Central Sikhote-Alin, Eastern Russia: Combined shoshonite-related porphyry Cu-Au-Mo and reduced intrusion-related Au mineralization in a post-subduction setting

The Lazurnoe porphyry Cu-Au-Mo deposit (>1Mt Cu-eq.) in the Mesozoic Central Sikhote-Alin orogenic belt (Eastern Russia) is located in an Early Cretaceous turbidite basin associated with a crustal-scale strike-slip fault zone, and was formed under the continental transform margin regime after the cessation of active subduction. The deposit is related to a magnetite-series, high-K calc-alkaline to shoshonitic, Early Cretaceous igneous suite that occurred in this tectonic setting, possibly due to asthenospheric mantle upwelling. The fertile magma was sourced from a subduction-modified mantle, under a low degree of partial melting, and was subjected to intense amphibole fractionation during its crystallization, the latter facilitating water and metal saturation. The parental igneous suite at Lazurnoe was coeval to ilmenite-series calc-alkaline igneous suites accompanied by reduced intrusion-related Au(±W) to W-Au deposits as well as to ilmenite-series, high-K calc-alkaline to shoshonitic suites accompanied by Sn-polymetallic mineralization.The deposit comprises magnetite-rich, quartz-K-feldspar to quartz-K-feldspar-biotite (potassic) alteration assemblages, containing bornite and chalcopyrite, which evolve to quartz-chlorite ± epidote ± amphibole (propylitic) assemblages, containing dominant pyrite with magnetite and chalcopyrite. Molybdenite-chalcopyrite-pyrite paragenesis dominates in quartz-sericite (phyllic) alteration assemblages that overprint and replace potassic and propylitic alteration. Potassic alteration assemblages formed from hot (~530–510 °C), high-salinity (45–48 wt% NaCl and 9–10 wt% CaCl2), sodic-calcic aqueous-chloride fluid. Gradual fluid cooling and dilution (from 29 to 31 wt% NaCl and 18–19 wt% CaCl2 through 24–25 wt% NaCl and 16–17 wt% CaCl2 to 12 ± 0.5 wt% NaCl and 12 ± 0.5 wt% CaCl2) is recorded for propylitic alteration assemblage. Another cycle of fluid exsolution from crystallizing magma can be suggested for phyllic alteration assemblages formed from a cooler (~390–310 °C), high-salinity (30–37 wt% NaCl and 5 ± 0.5 wt% CaCl2) aqueous-chloride fluid. The porphyry-style mineralization was overprinted by the late auriferous quartz-sulfide veins containing arsenopyrite, pyrrhotite and other sulfide minerals, together with native gold (locally > 100 g/t Au) and tellurides. These veins were formed from boiling aqueous-carbonic fluid that was characterized by elevated CO2 and CH4 contents, with subsequent separation of high-carbonic and aqueous-chloride phases, and the fluid cooling from ~390 to 250 °C. These veins can be related to another, more reduced, plutonic suite, which crystallized at a greater depth.

Geochemistry, in-situ Sr-Nd-Hf-O isotopes, and mineralogical constraints on origin and magmatic-hydrothermal evolution of the Yulong porphyry Cu[sbnd]Mo deposit, Eastern Tibet

The giant Yulong porphyry CuMo deposit was formed in postsubduction setting in eastern Tibet. Origin of the ore-related Yulong intrusion remains a matter of debate. This study presents new whole-rock major and trace element geochemistry, in-situ apatite SrNd and zircon HfO isotopes, and mineralogical chemistry of the Yulong intrusion. Least-altered samples from the Yulong intrusion have high SiO2 (66.3–69.5 wt%) and Al2O3 (14.9–15.5 wt%) contents, high La/Yb (36.4–68.0) and Sr/Y (46.0–76.3) ratios, and low MgO (0.63–1.24 wt%) and Cr (<30 ppm) contents, similar to adakitic rocks deriving from thick juvenile lower crust. They are enriched in large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs), and show listric REE patterns. In-situ apatite SrNd isotopes show limited variations ((87Sr/86Sr)i = 0.7060–0.7068, εNd(t) = −4.8–0.2), which plot between Paleo-Tethys ocean-related arc magmas and the ancient crust in eastern Tibet. Zircon grains from this study and published studies have mostly positive yet variable εHf(t) values (−20.6 to +12.2) and young Hf model ages that overlap those of the Paleo-Tethys ocean-related arc magmas. The above Sr-Nd-Hf isotopes, together with the elevated zircon δ18O values (6.4 to 9.3‰) and arc-like trace element patterns, collectively suggest that the Yulong intrusion may have originated from partial melting of juvenile lower arc crust related to the subduction of the Paleo-Tethys ocean, with incorporation of a small amount of ancient crustal materials.Two generations of amphibole were recognized at Yulong. Their compositions are used to calculate crystallization depths, magmatic oxidation states, and water contents. The caculated results show that the early-stage euhedral high-Al (5.87–8.51 wt%) amphibole phenocrysts may have crystallized in the underlying magma chamber (7.1–12.5 km in depth), whereas the late-stage xenomorphic low-Al (3.47–4.87 wt%) amphibole grains may have crystallized in the porphyritic stock (4.0–5.6 km). Magmatic water contents decrease from early- (3.5–4.6 wt%) to late-stage (2.8–3.5 wt%) amphibole, which is interpreted to indicate fluid exsolution from the magma chamber during emplacement of the Yulong intrusion. Caculated oxidation states increase from early- (ΔNNO = 0.6–1.5) to late-stage (ΔNNO = 1.9–2.3) amphibole. Plagioclase phenocrysts show periodic or reverse core-to-rim zonation of An contents (range up to 25 mol%), which are coupled by FeO contents, probably suggesting magma recharge events. Collectively, we propose that the magma chamber beneath Yulong was recharged by a less evolved magma, and was saturated in fluids to produce intensive alteration and mineralization. The relatively high oxidation states allow the metals to be enriched in the evolving magma, and to be deposited in the hydrothermal alteration stage.

Sulfur and lead isotopic variations in the giant Yulong porphyry Cu (Mo[sbnd]Au) deposit from the eastern Tibetan Plateau: Implications for origins of S and Pb, and metal precipitation

The giant Yulong porphyry Cu (MoAu) deposit, located in the eastern Tibetan Plateau, is a typical post-subduction porphyry Cu ± Mo ± Au deposit formed in an intracontinental strike-slip fault system, and it is also one of the largest porphyry Cu deposits in China with reserves of 6.5Mt Cu @ 0.62% and 0.41Mt Mo @ 0.042%, respectively. In this study, sulfur and lead isotopic compositions of sulfides and ore-related porphyry samples were determined to constrain the sources of ore-forming materials and ore-forming process. Sulfides (e.g., pyrite, chalcopyrite and molybdenite) from phyllic alteration zone, where the Cu and Mo mineralization mainly occurred, have uniform δ34S values ranging from −0.6 to +2.1‰ (average + 1.1‰), which indicates a magmatic origin of the sulfur. In situ Nano-SIMS sulfur isotope analysis further reveals that pyrites formed in potassic alteration stage have systematically negative δ34S values ranging from −0.2 to −9.7‰ (average − 4.5‰), whereas pyrites formed in phyllic alteration stage have systematically positive δ34S values ranging from +1.1 to +3.2‰ (average + 2.1‰). This regular variation is considered to be resulted from disproportionation of aqueous SO2 and subsequent sulfate reduction, further suggesting a magmatic origin of the sulfur. The sulfides have initial Pb isotopic compositions ((206Pb/204Pb)i = 18.616–18.812, (207Pb/204Pb)i = 15.670–15.740, (208Pb/204Pb)i = 38.956–39.080) deviating from those of the ore-related porphyries ((206Pb/204Pb)i = 18.549–18.718, (207Pb/204Pb)i = 15.647–15.662, (208Pb/204Pb)i = 38.794–39.888), but more close to those of the ore-hosting strata ((206Pb/204Pb)i = 18.423–18.761, (207Pb/204Pb)i = 15.654–15.713, (208Pb/204Pb)i = 38.867–39.062). The shift of Pb isotopic compositions of sulfide minerals toward those of ore-hosting strata indicates that the ore-hosting strata had provided a significant amount of Pb for ore formation, and most probably suggests fluid-rock interaction processes in the hydrothermal system. Our study represents a case study in which ore-hosting strata had provided a significant amount of ore-forming materials for porphyry Cu ± Mo ± Au formation in the post-subduction setting.

Formation of the Lubei magmatic Ni–Cu deposit in a post-subduction setting in East Tianshan, North West China

The Lubei Ni–Cu sulfide ore deposit with 7.8 Mt sulfide ores at an average grade of 0.88 wt% Ni and 0.5 wt% Cu, is associated with a mafic–ultramafic intrusion in the western part of the East Tianshan in Xinjiang Uygur Autonomous Region, NW China. The primary mineralization is hosted by ultramafic rocks which are part of a 2.1 km2 mafic–ultramafic intrusion located within the Kanggurtag fault zone. In this paper, we intend to probe the ore-formation processes for the Lubei Ni–Cu sulfide ore deposit and discuss key factors that control the ore formation processes in a post-subduction setting. The Lubei primary magmas were derived from 13 to 17 % partial melting of a hydrous mantle containing ∼0.29 wt% H2O estimated using the silicate-liquid model pMELTS. The Lubei sulfide ores are characterized by low platinum group elements (PGE) and high mantle-normalized Pd/Ir ratios, and disseminated sulfide ores have higher Cu, Ni and PGE tenors than massive and vein sulfide ores indicative of crystal fractionation of Fe-rich monosulfide solid solution. The PGE-depleted Lubei sulfides possibly resulted from early sulfide segregation from the Lubei primary magmas as indicated by high Cu/Pd ratios (up to 106), and the proportion of early sulfide losses was 0.02 wt% which has depleted the Lubei parental magmas and later fractionated sulfides in chalcophile elements. This depletion would been compensated at different levels by later sulfide–magma interaction when the parental magmas reached in situ sulfide saturation, with an R factor (mass ratio of silicate to sulfide liquids) of 100–1000. The formation of the observed Lubei disseminated sulfide ores was associated with 5–15 vol% magma crystallization with the main silicate minerals being spinel and olivine, as modeled using the silicate-liquid model MELTS. The δ34S values of the Lubei sulfides vary over a narrow range of −0.3 to +1.8‰, indicating a mantle origin for the sulfur. The key factor that controls the formation of magmatic Ni–Cu deposits in post-subduction settings is a metasomatized mantle accompanied by activation of translithospheric faults during post-collisional extension. This mechanism alternatively interprets the widespread magmatic Ni–Cu deposits in the southern Central Asian Orogenic Belt and implies translithospheric fault as a potential target for Ni–Cu exploration.

Lithospheric Architecture and Mantle Metasomatism Linked to Iron Oxide Cu‐Au Ore Formation: Multidisciplinary Evidence from the Olympic Dam Region, South Australia

AbstractThe role of lithospheric architecture and the mantle in the genesis of iron oxide copper‐gold (IOCG) deposits is controversial. Using the example of the Precambrian Gawler Craton (South Australia), which hosts the giant Olympic Dam IOCG deposit, we integrate geophysical data (seismic tomography and magnetotellurics) with geological and geochemical data to develop a new interpretation of the lithospheric setting of these deposits. Spatially, IOCG deposits are located above the margin of a mantle lithospheric zone with anomalously high electrical conductivities (resistivity &lt;10 ohm.m, top at ~100–150 km depth), low seismic shear‐wave velocities (horizontal component, Vsh &lt; 4.6 km/s), and unusually high ratios of compressional‐ to shear‐wave velocities (Vp/Vsh &gt; 1.80). The high conductivity cannot be explained by water‐bearing olivine‐rich rock alone. Relatively fertile and metasomatized peridotitic mantle with additional high‐Vp/Vs phases, for example, clinohumite, hydrous garnet, and/or phlogopite, could explain the anomalous velocity and conductivity. The top of this high‐Vp/Vsh zone marks a midlithospheric discontinuity at ~100–130 km depth that is interpreted to reflect locally orthopyroxene‐rich mantle. A sub‐Moho zone with high Vp/Vsh at ~40–80 km depth correlates spatially with primitive Nd isotope signatures and arc‐related ~1,620–1,610 Ma magmatism and is interpreted as the eclogitic root of a magmatic arc. Mafic volcanics contemporaneous with ~1,590 Ma IOCG mineralization have geochemistry suggesting derivation from subduction‐modified lithospheric mantle. We suggest that Olympic Dam formed inboard of a continental margin in a postsubduction setting, related to foundering of previously refertilized and metasomatized lithospheric mantle. Deposits formed during the switch from compression to extension, following delamination‐related uplift and exhumation.

Porphyry Au-Cu mineralization controlled by reactivation of an arc-transverse volcanosedimentary subbasin

Located in the accreted remnants of the Ordovician Macquarie Arc (Australia), Cadia East is the largest alkalic porphyry Au-Cu deposit currently known. The deposit is centered on a 2-km-long, northwest-trending volcanosedimentary subbasin, which was identified by mapping variations in the distribution and thickness of faulted sedimentary marker horizons. The normal faults that define the subbasin predate porphyry mineralization and are oriented parallel to a major arc-transverse lineament. Porphyry Au-Cu mineralization was controlled by reactivation of the subbasin during postorogenic extension as the Macquarie Arc was accreted to the Gondwana margin in the early Silurian. Dilation of the northwest-trending subbasin faults facilitated emplacement of alkalic porphyry dikes and associated sheeted quartz-sulfide veins that define the Cadia East orebody. During the middle Silurian, north-trending fault-bound marine basins buried Cadia East, significantly enhancing preservation of the ore system. These north-trending faults were optimally oriented for reactivation during Devonian east-directed compression, leaving the mineralized Cadia East volcanosedimentary subbasin largely intact. We identify the significance of reactivated premineralization structures within an arc-transverse lineament in controlling porphyry Au-Cu mineralization in a postsubduction setting. At Cadia East, the manifestation of such premineralization structures can be observed at regional, district, and deposit scales, and highlights the influence of preexisting crustal architecture on the structural character, geometry, and preservation potential of porphyry deposits throughout the arc life cycle.

Geochronological and geochemical constraints on the petrogenesis and tectonic significance of Paleozoic dolerite dykes in the southern margin of Alxa Block, North China Craton

Dolerite dykes are common in the southern margin of Alxa Block which is the westernmost part of North China Craton. The ages and petrogenesis of the dolerite dykes, which are important for a better understanding of tectonic–magmatic evolution in the region, are uncertain. This paper reports the results of an integrated geochronological and geochemical study of the dolerite dykes in the Jinchuan area. Previously, these dolerite dykes were considered to be coeval with the ∼831Ma Jinchuan ultramafic intrusion by some researchers, despite a cross-cutting relationship for the dykes. Our new zircon U–Pb dating indicates that the dolerite dykes were emplaced at ∼424Ma, >400Ma younger than the Jinchuan intrusion. The Jinchuan dolerite dykes contain 5–9wt.% MgO and 47–51wt.% SiO2, and are characterized by light REE (rare earth elements) enrichments relative to heavy REE, pronounced negative Nb anomaly, elevated initial 87Sr/86Sr from 0.7056 to 0.7068, negative εNd from −2 to −5, and zircon εHf from −2 to −20. The trace element and isotope data are consistent with an asthenosphere-derived parental magma that experienced 10–25% crustal contamination prior to final emplacement. Since the Jinchuan dolerite dykes are 20–40Ma younger than the subduction-related basaltic volcanic rocks, blueschists and eclogites in the nearby North Qilian orogenic belt to the south, we propose that the Jinchuan dolerite dykes are the products of basaltic magmatism induced by delamination of previously subducted oceanic lithosphere and the associated asthenosphere upwelling in a post-subduction setting. The occurrence of ∼424Ma post-subduction dolerite dykes in the Jinchuan area and the ∼450Ma arc basalts to the south indicate that the southwestern margin of North China Craton was an active plate margin in Early Paleozoic.