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Numerous tectonic scenarios have been proposed for terrane growth and accretion within the Archean Dharwar Craton, southern Peninsular India. Previously accepted interpretations involving a two-terrane model—comprising a Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC) block—have invoked west-dipping subduction and ocean closure, leading to arc magmatism and accretionary orogeny in the WDC, followed by metamorphic overprinting and collisional orogeny in the EDC. However, recent field investigations have revealed the existence of a previously unrecognized 'central' block (Central Dharwar Craton; CDC) within the craton, which requires revision of this model and reinterpretation of metamorphic and magmatic age data. Five samples of high-pressure, upper amphibolite- and granulite-facies meta-igneous and metasedimentary rocks from the southern portion of the Chitradurga Suture Zone, which divides the WDC and CDC, record minimum peak metamorphic conditions of ∼820–875°C at ∼10 kbar, indicating equilibration at the base of thickened continental crust. U–Pb zircon and Pb–Pb monazite geochronology indicates crystallization of parent mafic magmas at c. 2·61–2·51 Ga and subsequent regional metamorphism of these intrusions to garnet-amphibolite and garnet-granulite facies at c. 2·48–2·44 Ga, bracketing the timing of microblock accretion to the Archean–Proterozoic boundary. Light rare earth element enrichment within these zircon grains indicates magma generation in a suprasubduction-zone environment. In addition, detrital magmatic zircon grains with ages of c. 3·10–3·03 Ga and c. 2·97–2·86 Ga imply contamination of these magmas with Mesoarchean material sourced from the Western Dharwar Craton continental nucleus. Comparison of these metamorphic and magmatic age data with those recorded in the EDC shows that westward-directed subduction is implausible, and that all three terranes (the WDC, CDC, and EDC) must have accreted synchronously, driven by two separate eastward-dipping ocean–continent convergent plate margins. These data further support a recent abundance of observations from the geological record supporting the hypothesis that subduction-driven plate tectonics had initiated on Earth before c. 2·5 Ga, as opposed to a Neoproterozoic onset (c. 0·8–0·9 Ga) reported by numerous studies. [ABSTRACT FROM AUTHOR]

We constrain the oxygen fugacity (⁠ f O 2 ⁠) of strongly peraluminous granites [SPGs; i.e. granites (sensu lato) generated through the partial melting of sediments] across the Archean–Proterozoic boundary, which coincides roughly with the Great Oxygenation Event (GOE), to understand whether secular changes in atmospheric O2 levels may be imprinted on the f O 2 of igneous rocks. SPGs were chosen to maximize the potential effects of sediments in their sources on the f O 2 of the magmas. We studied 28 Archean (2685–2547 Ma) and 31 Meso- to Paleoproterozoic (1885–1420 Ma) geographically distributed samples from North America, spanning two cratons (Superior and Wyoming) and both orogenic and anorogenic Proterozoic provinces (Trans-Hudson Orogen, Wopmay Orogen, and SW USA). We present an analysis of both new and previously published whole-rock major and trace element data and mineral major element chemistry from the samples. All the studied samples are peraluminous high-silica plutonic rocks (all contain >67 wt % SiO2, and 92% are true granites with >69 wt % SiO2), and biotite + muscovite ± garnet ± tourmaline ± sillimanite are the primary aluminous minerals in all samples. Whole-rock major element and trace element abundances of all samples are consistent with derivation by partial melting of aluminous sediments. To constrain the f O 2 of crystallization of the SPGs, we developed an alphaMELTS-based method that takes advantage of the sensitivity of biotite FeT/(FeT + Mg) ratios to f O 2 ⁠. This method is able to reproduce experimental and empirical data where biotite compositions and whole-rock compositions, pressures and temperatures of crystallization and f O 2 are known. For the SPGs in this study, alphaMELTS modeling indicates that 68% of Proterozoic samples crystallized at an f O 2 between NNO –1 and NNO +1·1 (where NNO is nickel–nickel oxide buffer), whereas the remaining Proterozoic samples (32%) and most of the Archean samples (75%) crystallized at ≤NNO –2. The simplest explanation of these results is that the Proterozoic SPGs were derived from metasedimentary source rocks that on average had more oxidized bulk redox states relative to their Archean counterparts. The bulk redox state of the metasedimentary source rocks of SPGs of all ages is defined by the relative abundances of oxidized (e.g. Fe3+ and S6+) and reduced (e.g. organic matter) material. The crystallization of both Archean and Proterozoic samples at f O 2 values of ≤NNO –2 is consistent with them having their f O 2 buffered by graphite (formed from organic carbon) in their source regions. However, the dominantly low f O 2 (≤NNO –2) values of the Archean SPGs plausibly reflects the presence of organic material and relatively reduced metasedimentary rocks in their source region prior to the GOE. In contrast, the elevated f O 2 values of the majority of the Proterozoic SPGs may reflect enhanced sulfate contents or increased Fe3+/FeT in sediments after the GOE, which, in terms of the bulk redox state of their metasedimentary source region, would have offset the reducing nature of organic matter present there. [ABSTRACT FROM AUTHOR]

The D'Entrecasteaux Islands of eastern Papua New Guinea (PNG) host the youngest known ultrahigh-pressure terrane on Earth and represent the only location where ultrahigh-pressure (UHP) rocks have been exhumed in an active rift. The PNG (U)HP rocks, consisting of Pliocene eclogites, garnet amphibolites and migmatitic gneisses, are exposed in five domal structures across the Islands. Zirconium-in-rutile thermometry records peak temperatures of ∼780°C from the eastern Oiatabu and nearby central Mailolo Domes, and hotter temperatures of ∼825–865°C within the western Goodenough Dome. Uranium–lead (U–Pb) and trace element zircon compositions from a suite of eclogite, host gneiss, felsic dikes and pegmatite from three domes document the rapid exhumation history of the PNG UHP terrane. High-spatial resolution laser-ablation split-stream inductively coupled plasma-mass spectrometry (LASS ICP-MS) analyses of select eclogite zircons exhibit no resolvable age zoning within single crystals. The same eclogite zircons, combined with separate zircons extracted from additional eclogite, host gneiss and felsic intrusions, were subsequently analysed by high-precision U–Pb chemical-abrasion isotope-dilution thermal ionization mass spectrometry and solution ICP-MS trace element analysis (TIMS-TEA). The results record discrete tectonic events across the three domes at sub-million year timescales: (1) (re)crystallization of host gneiss within the lower crust exposed in the eastern Oiatabu Dome from c. 5·7–4·5 Ma; (2) initial retrogression and local decompression melting of eclogites from the Oiatabu and Mailolo Domes at c. 4·6–4·3 Ma; (3) melt crystallization of weakly deformed felsic dikes of the Oiatabu Dome at c. 3·0–2·9 Ma; and (4) retrogression and melt crystallization within eclogite–amphibolite-facies rocks in the western Goodenough Dome at c. 2·9–2·6 Ma. In comparison to Zr-in-rutile peak temperature estimates, Ti-in-zircon temperatures >800°C may reflect increased temperatures during exhumation that resulted in partial melting of the eclogites. Inclusions of crystallized hydrous melt consisting of Na-rich plagioclase ± K-feldspar + quartz within eclogite zircons document this process. The elevated temperatures and the presence of the polyphase inclusions are the first documentation of partial melting of the (U)HP eclogites within PNG during initial retrogression from c. 4·6–4·3 Ma. Overall, U–Pb zircon geochronology and geochemistry track both the timing of retrogressive overprinting within the lower-to-middle crust and final upper crustal emplacement over a relatively short span of ∼2 Myr during the rapid (≥2·3 cm/yr) exhumation of the youngest known (U)HP eclogites. [ABSTRACT FROM AUTHOR]

A series of localized high-temperature granulite-facies domains ('hot spots') are present within the regional (10–100 km2scale) amphibolite-facies rocks of the Central Maine Terrane in central New Hampshire (NH), USA. Based on the spatial coincidence of a thermal anomaly and contours of depressed δ18O values centered on networks of quartz–graphite and pegmatitic veins in the vicinity of Bristol, NH, it was proposed in an earlier study that large-scale ascending hot fluid focused through a vein network drove heating and introduced isotopically distinct fluids. The thermal anomaly could be preserved only if the timescales of heating were extremely short, otherwise conduction would smooth the field temperature gradient. Herein, we conduct a petrological test to estimate the peak temperatures and the durations of metamorphism across the Bristol region, using pseudosection analysis as well as forward modeling of garnet growth–diffusion–resorption profiles. This region attained granulite-facies conditions in the sillimanite–K-feldspar–cordierite zone over a larger area than previously mapped. Cordierite is variably present, which reflects bulk compositional controls on its stability as well as its destruction during retrogression. The forward modeling reveals protracted (5–8 Myr) granulite-facies conditions of 0·5–0·6 GPa and ∼750–820°C, and an overall counterclockwise P – T path. Furthermore, a short-lived thermal anomaly or 'spike' (>100°C, ∼0·15 Myr) is superimposed on the granulite-facies core, reaching ultrahigh-temperature (UHT) conditions >900°C, much higher than previously recognized in the area. The short timescale is fully consistent with the localized radius of the thermal anomaly of ∼1·5 km. Subsequently, the area underwent variably developed amphibolite-facies retrogression at ∼650°C and 0·4–0·5 GPa, accompanied by fluid infiltration, garnet breakdown, and muscovite growth. The transient thermal spike and the counterclockwise P – T path indicate that heat transfer could not have been solely the result of internal heating of overthickened crust. We posit that external heat fluxes driven by Acadian plutonism, in addition to heat generation in crust enriched in heat-producing elements, led to the granulite-facies metamorphism. Magmatic loading in the crust, potentially in response to an elevated basal heat flux during the Acadian orogeny, can account for the counterclockwise P – T path. Heat transported advectively by channelized flow of magma or magma evolving hydrothermal fluids is the most likely cause of the transient and local UHT thermal anomaly. The results show that UHT metamorphic events can be extremely brief. [ABSTRACT FROM AUTHOR]

The distribution and concentration of Ti in quartz was assessed in five rutile-bearing anatectic aluminous granulites from the Grenville Province, Canada, each with previously constrained P–T conditions of metamorphism. Characterisation of quartz in these samples with the aid of cathodoluminescence (CL) mapping revealed two distinct types in each sample, resorbed quartz partly consumed by melting reactions during the prograde portion of metamorphism and quartz grown from crystallisation of partial melt during retrogression. In two of the samples, pseudomorphs after former melt in the form of quartz overgrowths on existing quartz were discovered using CL which are not visible by other methods. The P–T conditions recorded by each type based on Ti-in-quartz thermobarometry are in poor agreement with microstructures and P–T results inferred from phase equilibria modelling. The negative correlation between Ti incorporation into quartz and P suggests that prograde quartz, which is expected to record near-peak P–T, should be Ti-poor relative to retrograde quartz, which crystallised at conditions several kbar lower than peak P. The opposite trend was found; retrograde quartz is typically relatively Ti-poor and quartz overgrowths may be nearly Ti-free. Additionally, prograde quartz has variable Ti contents and fails to systematically record near peak P–T. Despite the presence of rutile in the samples, which is generally considered to ensure a Ti-saturated system, these results are strong evidence for Ti-undersaturation of quartz caused by disequilibrium between quartz and rutile. In addition, Zr-in-rutile thermometry generally gives lower temperatures than expected for these rocks, which can be explained by retrograde resetting of Zr in rutile and potentially by disequilibrium between rutile and zircon. Based on these results, the degree of Ti (or Zr) saturation achieved at any stage of metamorphism should be assessed with caution, as co-existence of quartz and rutile (or rutile and zircon) within a thin section or sample may not be a sufficient criterion for equilibrium. [ABSTRACT FROM AUTHOR]

As one of the most geochemically unique drill cores recovered within the Yellowstone–Snake River Plain (YSRP) province, the Sugar City geothermal test well was drilled into intra-caldera rhyolite lavas and tuffs erupted during the middle to late Pliocene and the resurgent basaltic volcanism erupted during the Pleistocene. This sequence parallels the two main stages proposed for YSRP hotspot calderas: i.e. the eruption of several large-volume, ash-flow tuff sheets followed by caldera collapse, then cessation of major rhyolitic activity and gradual subsidence accompanied by filling and eventual burial of the caldera by basalt lava flows. We employ stratigraphic relationships, paleomagnetism, and major, trace element, and Sr–Nd isotope geochemistry to develop models for the origin of the basaltic and rhyolitic magmas within a geographical and temporal context. The basalts are characterized by distinct groupings based on depth and geochemistry and reflect the dominant compositions observed on the surface, e.g. Snake River olivine tholeiite (SROT) and evolved type (e.g. Craters of the Moon). We also observe contaminated basalts that interacted with rhyolite/granite. The basaltic magma formed by shallow partial melting in the plume channel carved into the lithosphere. The older rhyolites preserve the classical characteristics of A-type granites and display major element and trace element concentrations typical for Eastern SRP caldera centres and minimal stratigraphic variation. Multiple lines of evidence document extensive magmatic differentiation and coupled basalt–rhyolite interactions. We find that the most plausible origin for the rhyolites is via partial melting of a hybrid source, comprising Archean crustal components and younger juvenile mafic intrusions. Assimilation of hydrothermally altered material is also required for some eruptive units. The rhyolites did not evolve from residual magma left over from the climactic Kilgore eruption (4·0 Ma), but instead represent discrete magma generation events in the course of a few hundred thousand years between 4·0 to 3·8 Ma. Beginning at approximately 3.3 Ma, basalts were able to erupt through the solidified composite pluton that formed below the caldera. The transition from rhyolite to basalt is tied to the declining flux of basaltic magma as North America moved away from the Yellowstone hotspot core. [ABSTRACT FROM AUTHOR]

High-Mg diorites and/or ultrapotassic volcanic rocks are generally associated with postcollisional porphyry copper deposits, but their contribution to the formation of the mineralization remains unclear. A suite of Miocene postcollisional ultrapotassic-potassic lamprophyres, high-Mg diorites, and adakite-like intrusions have been recognized in the Zhunuo porphyry Cu deposit, located in a continental collisional zone within the Gangdese belt, southern Tibet. The post-mineralization ultrapotassic-potassic lamprophyres have zircon U-Pb ages of 12.2±2.1 Ma and contain abundant Proterozoic to Miocene inherited zircons. The ultrapotassic lamprophyres have high K2O (>8.5wt%) and MgO (>8.8wt %) contents, are enriched in light rare earth elements (LREE; La=123 ppm) and large ion lithophile elements (LILE; e.g. Ba=3102 ppm, Th=116.6 ppm, and Pb=140 ppm), and display high Th/Yb and Rb/Sr, and low Ba/Rb and Hf/Sm ratios. They have zircon εHf(t) values of -2.8 to 1.3, δ18O values of 6.5 to 7.4%0', and enriched bulk-rock Sr-Nd-Pb isotope compositions ((87Sr/86Sr)i=0.73134, εNd(t)=-13.7, (206Pb/204Pb)i=19.20). Their parental magmas were derived from partial melting of an enriched mantle source that had been metasomatized by fluids and sediment-derived melts associated with Neo-Tethyan oceanic subduction and subsequent Indian continental lithosphere subduction. The potassic lamprophyres have lower contents of K2O, MgO, REE and LILE than the ultrapotassic lamprophyres and (87Sr/86Sr)i of 0.710993 to 0.711139, εNd(t) of -12.3 to -12.4, and (206Pb/204Pb)i of 18.59 to 18.72. Taken together with observations of a negative trend between eNd(t) and MgO content; positive trends between (87Sr/87Sr)i, (206Pb/204 Pb)i and MgO content from ultrapotassic lamprophyres to potassic lamprophyres; the existence of abundant Miocene inherited zircons showing similar ages and εHf(t) values to the adakite-like intrusions; and variable Hf/Sm ratios with some Hf/Sm ratios similar to adakite-like intrusions, we propose that the potassic lamprophyres were formed by mixing of ultrapotassic lamprophyre magmas with adakitelike magmas. The syn-mineralization high-Mg diorites including diorite porphyry and enclaves hosted by the adakite-like intrusions at Zhunuo have zircon U-Pb ages of 13.0±0.2 Ma and 13·1±0·2 Ma. They show negative correlations between Y, Yb, Dy/Yb and SiO2, and positive correlations between Sr, Sr/Y and SiO2, among which some more evolved samples (such as diorite porphyry) show adakite-like geochemical signatures. The high-Mg diorites are enriched in LREE and LILE, depleted in high-field-strength elements (HFSE), and have (87Sr/86Sr)i of 0·709401 to 0·710362, εNd(t) of -11·1 to -9·9, and (206Pb/204Pb)i of 18·62 to 18·71. Taken together with petrographic observations that show magma mixing, we argue that the high-Mg diorites were derived from previously subduction-modified Tibetan lithospheric mantle with little or no input from Indian continental sediment. Mixing with adakite-like magmas and fractional crystallization of hornblende and/or titanite are also responsible for the differentiation of the high-Mg diorites. The ore-hosting, adakitelike granitic rocks at Zhunuo with zircon U-Pb ages of 14·7±0·3 Ma and 14·6±0·2 Ma have lower concentrations of REE, LILE and HFSE, much higher εNd(t) (-6·1 to -6·9) and lower (87Sr/86Sr)i (0·707325-0·707663) values than the ultrapotassic lamprophyres and the high-Mg diorites. They were derived from remelting of previously subduction-modified Tibetan lower crust with some involvement of hydrous high-Mg dioritic magmas during magma mixing. The postcollisional adakite-like intrusions in the Gangdese belt could be generated by remelting of previously subduction-modified lower crust and mixing with hydrous high-Mg dioritic magmas in a lower crustal MASH zone and/or in an upper-crustal adakite-like magma chamber. The metallogenic potential of postcollisional adakite-like intrusions largely depends on rejuvenation of subductionmodified lower crust by previous arc magmas, differentiation of hydrous high-Mg dioritic magmas, and magma mixing of high-Mg dioritic magmas with lower crustal magmas. [ABSTRACT FROM AUTHOR]