Tonalite Intrusion Research Articles

The Magino Gold Deposit, Ontario, Canada: An Overprinted Archean Intrusion-Related Gold Deposit

Abstract The Magino gold deposit, located within the Michipicoten greenstone belt of the Wawa subprovince (Ontario, Canada), is a past-producing underground mine which has recently begun production as an open-pit gold mine with 4.5 Moz of Au in resources. Gold is primarily hosted within the ca. 2724 Ma Webb Lake stock, a steeply dipping, tabular, multiphase tonalitic intrusion oriented parallel to the penetrative regional S2 foliation along the Goudreau Lake deformation zone. The Magino deposit underwent two gold mineralization events (Au1, Au2) and three ductile deformation events (D2, D3, D4). The main gold event (Au1) is expressed by pervasive biotitic/phengitic alteration of the stock and the emplacement of auriferous sugary quartz veins. The veins have a molybdenite Re-Os age of 2731 ± 6.9 Ma, which is indistinguishable from that of the host Webb Lake stock. Quartz-feldspar porphyry dikes cogenetic with the Webb Lake stock cut across the veins and are transposed, stretched, and folded parallel to the S2 foliation within high-strain corridors in the stock, supporting a pre-D2 timing for the Au1 event. These D2 high-strain corridors formed during localization of deformation along Au1 hydrothermally altered zones. The Au2 event occurred during the D2 event and is associated with the emplacement of N-trending quartz-tourmaline-carbonate veins with albitic alteration selvages, which were later deformed during D3 dextral reactivation of the high-strain corridors. Later D4 vertical loading produced a subhorizontal crenulation cleavage, which is overprinted by chloritoid porphyroblasts that grew during late upper greenschist facies peak metamorphism. The Magino deposit is an example of an Archean intrusion-related gold system, which was structurally modified and overprinted by a syndeformation epigenetic gold mineralization event and late metamorphism.

Discriminating Superimposed Alteration Associated with Epigenetic Base and Precious Metal Vein Systems in the Rouyn-Noranda Mining District, Quebec; Implications for Exploration in Ancient Volcanic Districts

Abstract The Rouyn-Noranda mining district of Quebec contains 20 Cu-Zn (±Au ±Ag) volcanogenic massive sulfide (VMS) deposits, including the giant and gold-rich Quemont and Horne deposits. Mineralized epigenetic veins are also present, but their origin and relative timing remain enigmatic. The nature and extent of their alteration signatures and the effect of their superposition on district-scale alteration patterns is unknown. The VMS-related quartz-sulfide Cu-Zn-Ag veins have δ18Oquartz values of 8.5 ± 0.8‰, reflecting δ18Ofluid compositions of –0.4 to 3.1‰ (250°–350°C) that are typical of Archean seawater. They are associated with a proximal Fe-rich chlorite alteration and marginal spotted sericite-chlorite alteration with whole-rock δ18O values of 2.9 to 5.9‰ and are interpreted to have formed within the structurally controlled discordant upflow zones of a VMS hydrothermal system. Younger gold-bearing quartz-carbonate veins were emplaced along mechanical anisotropies created by mafic dikes during north-south compression and the formation of regional E-trending faults, folds, and cleavage. They are characterized by δ18Oquartz values of 11.3 ± 0.8‰, reflecting δ18Ofluid compositions of 2.4 to 5.9‰ (250°–350°C), typical of a metamorphic fluid, possibly mixed with a lower δ18O upper crustal fluid. They are associated with ankerite, calcite, muscovite, chlorite, albite, and quartz ± hematite alteration with whole-rock δ18O values of 5.8 to 10.3‰. Chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon ages for two tonalite intrusions constrain the maximum age of the Cu-Zn-Ag veins to 2697.6 ± 0.7 Ma and the minimum age to 2695.3 ± 1.0 Ma, which is also the maximum age of the gold quartz-carbonate veins. Superposition of alteration related to the gold quartz-carbonate veins on previously chlorite- and sericite-altered rocks has resulted in mixed alteration signals with whole-rock δ18O values of ~6 to 8‰ that have perturbed and masked regional alteration patterns related to older VMS mineralization, such as those found in the Quemont and Horne deposits. These results indicate that defining alteration vectors in camps that have superimposed hydrothermal systems requires full consideration of the hydrothermal history of the camp, and if such constraints are lacking, whole-rock δ18O values should not be used as a stand-alone exploration method.

Serpentinite dehydration at low pressures

Petrographic observations combined with mineral compositional analyses constrain the phase relations of prograde metamorphosed serpentinites in the Bergell contact aureole (Italy). In a 1500 m profile perpendicular to the north-eastern edge of the Bergell intrusion, seven dehydration reactions ran to completion. Three previously undocumented reactions have been identified within 70 m of the intrusive contact: olivine + anthophyllite = orthopyroxene + H2O, tremolite + Cr–Al-spinel = olivine + Mg-hornblende + H2O and chlorite = olivine + orthopyroxene + Cr-Al-spinel + H2O. Petrological analysis indicates that these reactions occur over a narrow range of pressure and temperature, 300 ± 30 MPa and 720 ± 10 °C respectively. Computed phase diagram sections reproduce the observed mineral parageneses with one notable exception. Due to the underestimation of aluminium and sodium contents in Ca-amphibole models, plagioclase is predicted above 700 °C instead of Mg-hornblende. In comparison with natural grains, the aluminium content of computed chlorite compositions is overestimated for low grade parageneses while it is underestimated near the upper thermal stability limit of chlorite. In the computed sections, Fe partitioning relative to Mg between olivine and other silicates, suggests a clear preference for Fe in olivine, that therefore shows lower Mg#s. In contrast, microprobe analyses of natural mineral pairs indicate that orthopyroxene, Mg-hornblende and anthophyllite have lower Mg#s than equilibrium olivine. The inferred thermal profile of the metamorphic aureole is not consistent with simple heat conduction models and indicates a contact temperature of ~ 800 °C, which is 120–230 °C higher than previously estimated. Petrography also reveals extensive retrograde overprint of the prograde parageneses within 200 m of the contact. Retrogression is related to metamorphic fluids that were released by dehydration reactions during contact metamorphism and magmatic fluids expelled from the tonalite intrusion. The thermal gradient between the intrusion and the country rocks induced hydrothermal circulation of these fluids throughout the contact aureole, which beyond peak metamorphic conditions caused retrograde overprint of the prograde parageneses. The proposed phase relations for low and high pressures, and in particular, the transition from tremolite to Mg-hornblende, provides a complete representation of hydration and dehydration processes in serpentinites in subduction zones, along deep oceanic transform faults, and at passive continental margins. The latter has new implications, specifically for subduction initiation.

Fractionation by compositional magma splitting: An example from Cerro Munro, Argentina

The Paleocene-Eocene Cerro Munro pluton, to the east of the North Patagonian Batholith (NPB), is a tonalitic intrusion emplaced as a shallow, small, sub-circular tonalite-granodiorite body hosting abundant co-magmatic mafic microgranular enclaves (MME). Besides, the intrusive body is crosscut by radial porphyritic dikes and has been related to andesitic, dacitic and rhyolitic dikes, lava flows and ignimbrites cropping out at neighboring areas. Magmatic contacts between MME and host tonalites, together with their common geochemical features, suggest derivation from a common parental magma, although short-range mineralogical and geochemical differences point to an early crystallization of MME (chilled margins) at the sidewalls of ascent conduits or at shallow reservoirs. The established thermal gradient and the advance of the solidification front were responsible for the presence of the mafic microgranular and tonalitic subsystems and, in a continuous process, promoted the water saturation and the second boiling that finally account for the segregation of a water rich highly differentiated residual liquid. Hbl-Pl cumulate textures observed in the tonalites and mafic enclaves, as well as incompatible element-enriched rhyolitic melts record the results of this in-situ differentiation process. UPb zircon ages obtained from tonalites (57.1 ± 1.4 Ma), dacitic (55.4 ± 0.6 Ma) and rhyolitic (54.1 ± 0.7 Ma) dikes constrain an age of around 54 Ma for the final consolidation of the tonalitic magma and the crystallization of the expelled highly differentiated melts. This age coincides with the deposition of dacitic volcanic and volcanoclastic deposits to the north of the Cerro Munro pluton, which suggests that the fractionation process led to the extrusion of segregated melts, favored by extension, uplift and exhumation of an active rift tectonic setting.

Recognizing subsurface breccias in Archean terranes: Implications for district scale metallogeny

Breccias are common in ancient and modern volcanic terranes, where they form at and below surface through volcanic, hydrovolcanic, magmatic, or tectonic mechanisms. They are critical for volcanic reconstruction as stratigraphic markers and indicators of geodynamic change and since they can be associated with mineralization their genesis is also important from an exploration perspective. Their origin can be difficult to ascertain in ancient terranes that have undergone polyphase deformation and associated metamorphism. The Joliet Breccia is a subeconomic Cu-Ag prospect within the Neoarchean Rouyn-Noranda mining district, in the Abitbi greenstone belt, Canada. It was previously interpreted as a phreatic breccia formed on the seafloor. This study presents new data indicating that the Joliet Breccia is a subsurface magmatic-hydrothermal breccia. Specifically, the recognition of gradational contacts with host rocks and between internal breccia domains, lack of sedimentary features and the spatial and temporal association with a tonalite intrusion strongly supports this interpretation. The angular, poorly sorted, lithic clasts derived from the immediate host rocks, hydrothermal cement, complete absence of a rock flour matrix, and presence of a radial and concentric fracture pattern extending into the host rocks, further support a subsurface magmatic-hydrothermal emplacement mechanism.A new TIMS U-Pb zircon age of 2698.0 ± 0.9 Ma (2σ) for a tonalite block within the breccia constrains the maximum age of brecciation. Four LA-ICP-MS U-Pb dates on hydrothermal monazite (2693.7 ± 1.0/8.9 Ma; 2696.9 ± 0.45/8.9 Ma; 2698.7 ± 1.4/8.9 Ma; 2701.2 ± 0.33/8.9 Ma; 2 s/2ssys) found in the cement of the Breccia indicate brecciation and mineralization occurred shortly after the emplacement of the tonalite. Similarly, the ca. 2697 Ma St. Jude intrusive breccia indicates a localized ca. 2699–2695 Ma magmatic-hydrothermal event superimposed on ca. 2704–2701 Ma strata. These breccias are temporally correlative with the youngest units of the Blake River Group strata and associated ca. 2698–2695 Ma volcanogenic massive sulfide (VMS) deposits, which suggests an indirect genetic link between these two hydrothermal systems. The VMS deposits may represent the near surface, distal manifestations of a deeper magmatic-hydrothermal system akin to the porphyry-intermediate sulfidation epithermal continuum in modern subaerial volcanic arcs.

Paleoproterozoic Orosirian tectono-thermal events in the Nangrim Massif, North Korea: Cratonic and supercontinental connection

The Paleoproterozoic Orosirian period of ca. 2.05–1.8 Ga saw unique magmatic manifestations of Andean-type arc to Caledonian-style post-collisional batholiths, anorthosite–mangerite–charnockite– granite suites and mafic dyke swarms. Chronicling their timeline in North Korea presents a pivotal task in the palinspatic reconstruction of the Korean Peninsula relative to the North China Craton (NCC) and the supercontinent Columbia. Here we document episodic late Orosirian tectono-thermal events from the northern Nangrim Massif, spanning from ca. 1871–1851 Ma tonalitic intrusions through ca. 1856 Ma granites and ca. 1839 Ma gabbroic diorites to ca. 1830 Ma mafic dykes and ca. 1828 Ma metamorphism. Meta-gabbroic/doleritic rocks range in SiO2 from 46.6 to 57.8 % and in MgO from 6.46 to 11.5 %, with moderately enriched light rare earth elements (LaN/YbN = 3.55–12.3) and depleted high field strength elements. Together with their variable zircon εHf(t) values from −10.0 to +5.2, these rocks tend to call for a heterogeneous lithospheric mantle that experienced prior melt depletion and subsequent fluid refertilization. Pyroxene-bearing tonalite–granite suite spans in SiO2 from 59.0 to 76.4 % and exhibits transitional character from magnesian and alkalic to ferroan and calc-alkalic. These elemental features and zircon εHf(t) values of −7.9 to −1.0 are consistent with partial melting of newly- underplated mafic middle crustal protoliths. By combining these newly identified tectono-thermal events with massif-scale coeval syenite-granite suites as well as anatexis-induced metamorphic pulses, we updated the profile of Orosirian tectono-thermal episodes in the Nangrim Massif, which present successive snapshots into post-collisional geodynamic processes from orogenic collapse to lithospheric delamination. This renewed profile not only provides convincing evidence for demarcating the influence scope of the giant Paleoproterozoic Jiao-Liao-Ji-Korean (JLJK) orogeny in the Korean Peninsula, but also offers key reference for applying analogous Phanerozoic accretionary to collisional orogenic processes to their ancient tectono-magmatic expressions. The JLJK case of Orosirian orogeny exemplifies the importance of Phanerozoic-style Wilson orogenic cycle in the amalgamation of the Sino-Korean craton and its incorporation into the Paleoproterozoic supercontinent Nuna–Columbia.

Mineralogy of hydrothermal breccia cement of Humpa Leu East porphyry copper-gold prospect, Sumbawa Island, Indonesia

Indonesia is a country that has several world-class copper-gold deposits, particularly in eastern Sunda arc. The Hu’u complex has several prospects in the surface as lithocap of extensive epithermal style alteration, but some were detected associated with porphyry beneath the surface. The study focuses on hydrothermal breccia cement as a factor influencing the porphyry system in the Hu’u district. The methods used is mineralogical analysis with petrography and μ-XRF elemental mapping on hydrothermal breccia samples. The Hu’u district is interpreted as a paleo-volcano; a member of the Old Volcanics Rocks Formation. The Humpa Leu East lithology consists of pre-volcanics unit (lava and pyroclastics), diorite, andesite-μ-diorite, and tonalitic intrusion at the depth. Hydrothermal alteration evolved from tonalite body to outward, consist of potassic, inner propyllitic and overprinted by phyllic and advanced argillic. Several phases of hydrothermal activities occur in this system, including the hydrothermal breccia phase associated with complex fluids. The hydrothermal cement of Humpa Leu East porphyry at least have three phases of mineralogical assemblages and possibly influencing the mineralization. The mineralogical assemblage of hydrothermal cement in HLE consists of quartz-feldspar-plagioclase-biotite as a high-temperature phase; then followed by epidote-sericite-chlorites-anhydrite-carbonates in medium temperature; there are aluminum-rich clay minerals interpreted as gibbsite. Mineralization occurs in three phases including chalcopyrite-magnetite, bornite-chalcopyrite and chalcopyrite-sphalerite phases. The occurrences of chalcopyrite at all phases indicate the stability of intermediate sulfidation proses in Humpa Leu and as a possible factor to answer the abundant copper in the Hu’u hydrothermal fluid system

How many extensional stages marked the Variscan gravitational collapse in the Bohemian Massif?

Tectonic development of the Variscan belt in Central Europe included, besides important compression, also an extensional phase related to gravitational collapse, which governed the origin of many sedimentary basins and magmatic bodies. One of these bodies is the Benešov pluton, featuring primary magmatic fabrics as well as deformational fabrics, related to subsequent extensional stages. Recognition of these fabrics and their links to other significant extension-induced structures in the Bohemicum and Moldanubicum not only sheds new light on the pluton itself but also extends a general knowledge of deformational stages, accompanying gravitational collapse of the Variscan orogen. The authors found that this pluton was strongly strained in a normal-faulting regime under brittle-ductile conditions. The age of deformation is constrained by a magmatic age of 347 ±3 Ma and by the age of Carboniferous sedimentary cover. New data indicate a three-stage extensional history during the phase of gravitational collapse: (1) Tournaisian extension (~350–345 Ma) within arc-related tonalitic intrusions; (2) late Viséan to Serpukhovian extension (~332–320 Ma), connected to the brittle-ductile unroofing and origin of a NE–SW basin system; and (3) Gzhelian to Cisuralian extension (~303–280 Ma), related to normal faulting and sedimentation in “Permo–Carboniferous” troughs, elongated NNE–SSW. Consequently, the gravitational collapse studied involved a complex succession of individual extensional stages, rather than a simple process.

Geochronology, geochemistry, and origin of plagiogranitic rocks and related granitic dikes in the Dar Gaz district, Kahnouj ophiolite complex, SE Iran: Analysis of their petrogenesis in a back-arc tectonic setting

The Dar Gaz district is part of the Kahnouj ophiolitic complex in southeastern Iran. Major intrusive rocks exposed in the Dar Gaz district are gabbro, diabase dikes, aplitic granitic dikes, and plagiogranites (plagiogranitic dikes and quartz diorite to tonalite intrusions). Plagiogranites are mainly composed of quartz, plagioclase, and ferromagnesian minerals, and are typically low in K2O, high in Na2O and enriched in REE showing geochemical features of oceanic plagiogranites. LA-ICP-MS U-Pb analysis on zircons extracted from two plagiogranitic dike samples yields weighted mean ages of 126.4 ± 1.7 Ma and 126.0 ± 1.3 Ma, and 130.8 ± 1.8 Ma and 128.5 ± 1.3 Ma from quartz diorite to tonalite intrusions, respectively. The plagiogranitic intrusions were formed by partial melting of altered gabbroic rocks. Plagiogranite dikes are characterized by greater fractionation of felsic melts produced by partial melting. The aplitic granitic rocks yielded LA-ICPMS U-Pb ages of 123.0 ± 1.7 Ma and 123.3 ± 1.9 Ma, and have typical features of high-K alkali-calcic to calc-alkalic rocks and are metaluminous to peraluminous formed by partial melting of sedimentary material. All members of the plagiogranitic rocks have moderately high LILE/HFSE ratios and are depleted of Nb, Ti, and P similar to those of a subduction-related tectonic settings. Field data and geochemical signatures of these plagiogranites suggest a supra-subduction origin compatible with the tectonic environment of the Kahnouj ophiolitic complex. These are considered to have formed in a late Jurassic to early Cretaceous back-arc basin.

Contribution to the understanding of the Pinheiro Machado Complex (Dom Feliciano Belt, Brazil): A study of textures, mineral chemistry, and crystallization conditions

This article presents textures, mineral chemistry, and geothermobarometric data for the southeastern Pinheiro Machado Complex rocks (PMC), eastern domain of the Dom Feliciano Belt, Rio Grande do Sul, Brazil. The PMC is composed by comagmatic intrusions of quartz diorite, tonalite, and granodiorite with medium‐ to fine‐grained equigranular and medium‐grained inequigranular textures with subordinate alkali‐feldspar macrocrystals. Partial melting features are observed in the comagmatic as melt pools and melt films. Amphibolite xenoliths also occur, presenting sharp‐angled to rounded contacts with the intrusions and evidences of partial assimilation, marked by mafic schlierens. Electron probe microanalyzer punctual analyses were used to classify amphibole, plagioclase, biotite, epidote, and titanite, as well as for the calculations of the crystallization P–T conditions. Amphiboles consist of pargasite for the comagmatic intrusions, and Mg‐hornblende for the amphibolite. Plagioclase ranges from oligoclase to andesine (An15‐40) in both units. Biotite corresponds to annite, with Fe/(Fe + Mg) > 0.3. The epidote has mostly magmatic characteristics in the comagmatic intrusions, with pistacite content 25–30%. Titanite has two distinct textures, titanite I is well formed, subhedral to euhedral, with up to 3 mm; while titanite II is anhedral with size >1 mm and rounded shapes, being both chemically homogeneous. The estimated P–T conditions for the amphibolite crystallization, using plagioclase‐hornblende geothermometer are 650–850°C and 3.62–4.95 kbar, whereas the conditions from the comagmatic intrusions are 714–871°C and 4.44–6.10 kbar. In the mineral phases there is a prevalence in the recrystallization field between subgrain rotation (SGR) and grain boundary migration (GBM) in submagmatic flow. These new data indicate that possibly at the end of PMC crystallization, late magmatic fluids caused re‐equilibrium of mineral phases, mainly in biotite and feldspars. Finally, we suggest that the PMC is the result of continuous and repeated comagmatic intrusions in a continental magmatic arc setting at a depth of 20–30 km in an amphibolitic continental crust.

Adakite-like granitoids of Songkultau: A relic of juvenile Cambrian arc in Kyrgyz Tien Shan

The early Paleozoic Terskey Suture zone, located in the southern part of the Northern Tien Shan domain in Kyrgyzstan, comprises tectonic slivers of dismembered ophiolites and associated primitive volcanics and deep-marine sediments. In the Lake Songkul area, early-middle Cambrian pillow basalts are crosscut by the Songkultau intrusion of coarse-grained gneissose quartz diorites and tonalites with geochemical characteristics typical for high-SiO2 adakites (SiO2 ​> ​56 ​wt.%, Al2O3 ​> ​15 ​wt.%, Na2O ​> ​3.5 ​wt.% and high Sr/Y and La/Yb ratios). The Songkultau granitoids have positive initial εNd (+3.8 to +6.4) and εHf (+12.3 to +13.5) values indicating derivation from sources with MORB-like isotopic signature. Volcanic formations, surrounding the Songkultau intrusion, have geochemical affinities varying from ocean floor to island arc series. This rock assemblage is interpreted as a relic of an early–middle Cambrian primitive arc where the adakite-like granitoids were derived from partial melting of young and hot subducted oceanic crust. An age of 505 ​Ma, obtained for the Songkultau intrusion, shows that hot subduction under the Northern Tien Shan continued until middle Cambrian. The primitive arc complexes were obducted onto the Northern Tien Shan domain, where the Andean type continental magmatic arc developed in Cambrian and Ordovician. Formation of the Andean type arc was accompanied by uplift, erosion and deposition of coarse clastic sediments. A depositional age of ca. 470 Ma, obtained for the gravellites in the Lake Songkul area, is in agreement with the timing of deposition for lower Ordovician conglomerates elsewhere in the Northern Tien Shan, and corresponds to the main phase of the Andean type magmatism. The Songkultau adakites in association with surrounding ocean floor and island arc formations constitute a relic of a primitive Cambrian arc and represent a juvenile domain of substantial size identified so far within the predominantly crustal-derived terranes of Tien Shan. On a regional scale this primitive arc can be compared with juvenile Cambrian arcs of Kazakhstan, Gorny Altai and Mongolia.

Protracted Magmatism and Mineralized Hydrothermal Activity at the Gibraltar Porphyry Copper-Molybdenum Deposit, British Columbia

Abstract The Gibraltar Cu-Mo deposit, with a total tonnage of 3.2 million tons (Mt) Cu, is located in the Canadian Cordillera and hosted by the Late Triassic Granite Mountain batholith. The batholith formed through multiple intrusions of tonalitic rocks over a period of ~25 m.y. beginning at 229.2 ± 4.4 Ma in the Quesnel island arc before the accretion of the arc to the North American continent. Late in its evolution, Cu fertile magmas intruded in the center of the batholith, during at least three events from 218.9 ± 3.1 to 205.8 ± 2.1 Ma. The fertile magmas were hotter and more mafic than older barren magmas. They generated magmatic-hydrothermal activity, forming potassic alteration and white mica alteration, and produced Cu mineralization as chalcopyrite-quartz veinlets and disseminated chalcopyrite. Zircon in the Cu-bearing tonalite intrusion (218.9 ± 3.1 Ma) shows high Ce4+/Ce3+ (681 ± 286 [2σ], n =15) compared to those from older barren intrusions (129 ± 56 [2σ], n = 118). Oxidation conditions for parental magmas are calculated using the compositions of zircon and amphibole. The magmas for Cu-bearing intrusions have an average of fayalite-magnetite-quartz buffer (FMQ) +1.7 ± 0.7 (2σ, n = 73), whereas those for older barren intrusions have slightly lower fO2 (avg FMQ +1.3 ± 0.5 [2σ], n = 108), although the values are overlapping for the two. The bulk rocks of Cu-bearing tonalite intrusions in the Granite Mountain batholith have low Sr/Y ratios (<22) independent of the degrees of alteration. The low ratios are also reflected by low Sr/Y in zircon, suggesting that the low Sr/Y ratios of bulk rocks represent those of unaltered rocks. The values are low compared to those associated with many other porphyry Cu deposits globally. The data suggest that igneous rocks elsewhere with low Sr/Y in bulk rocks may have a potential to host economic Cu deposits. Ratios of Ce/Nd and Ce/Ce* (=Ce/((NdN)2SmN)) in zircon are positively correlated with the Ce4+/Ce3+ in zircon from the Granite Mountain batholith. Since the former two ratios can be obtained solely from zircon composition, these ratios from detrital zircon may be useful in evaluating the occurrences of oxidized intrusions in regional mineral exploration.

Geochronology and geochemistry of the northern Scotia Sea: A revised interpretation of the North and West Scotia ridge junction

Understanding the tectonic evolution of the Scotia Sea is critical to interpreting how ocean gateways developed during the Cenozoic and their influence on ocean circulation patterns and water exchange between the Atlantic and Southern oceans. We examine the geochronology and detrital age history of lithologies from the prominent, submerged Barker Plateau of the North Scotia Ridge. Metasedimentary rocks of the North Scotia Ridge share a strong geological affinity with the Fuegian Andes and South Georgia, indicating a common geological history and no direct affinity to the Antarctic Peninsula. The detrital zircon geochronology indicates that deposition was likely to have taken place during the mid – Late Cretaceous. A tonalite intrusion from the Barker Plateau has been dated at 49.6 ± 0.3 Ma and indicates that magmatism of the Patagonian–Fuegian batholith continued into the Eocene. This was coincident with the very early stages of Drake Passage opening, the expansion of the proto Scotia Sea and reorganization of the Fuegian Andes. The West Scotia Ridge is an extinct spreading center that shaped the Scotia Sea and consists of seven spreading segments separated by prominent transform faults. Spreading was active from 30–6 Ma and ceased with activity on the W7 segment at the junction with the North Scotia Ridge. Reinterpretation of the gravity and magnetic anomalies indicate that the architecture of the W7 spreading segment is distinct to the other segments of the West Scotia Ridge. Basaltic lava samples from the eastern flank of the W7 segment have been dated as Early – mid Cretaceous in age (137–93 Ma) and have a prominent arc geochemical signature indicating that seafloor spreading did not occur on the W7 segment. Instead the W7 segment is likely to represent a downfaulted block of the North Scotia Ridge of the Fuegian Andes continental margin arc, or is potentially related to the putative Cretaceous Central Scotia Sea.

The geologic record of the exhumed root of the Central African Orogenic Belt in the central Cameroon domain (Mbé – Sassa-Mbersi region)

We present structural, petrological, geochemical and geochronological data compiled in a new geological map on the high-grade metamorphic rocks exposed in the Mbé – Sassa-Mbersi region located along the Tcholliré-Banyo Shear Zone, at the northern edge of the Central Cameroon domain.The region exposes a complex assemblage of metabasic (hornblendite, metagabbro, amphibolite) and metasedimentary (garnet paragneiss, calc-silicate gneiss) rocks alternating with ubiquitous migmatitic intermediate to felsic gneisses. U-Pb ages on detrital zircon in paragneiss point to the contribution of Archean, Paleoproterozoic and Neoproterozoic sources, and suggest a maximum deposition age of 725 ± 12 Ma for their sedimentary protolith. The protolith of some amphibolites was emplaced during the Neoproterozoic as indicated by U–Pb ages on magmatic zircon at 600 ± 4 and 599 ± 6 Ma. Plutonic rocks with I-type chemical signatures were emplaced discontinuously over a period of ca. 80 Ma. The magmatic evolution started with the intrusion of magnesian medium-K tonalites and trondhjemites at 651 ± 3 Ma, followed by magnesian high-K hornblende-biotite granites and ferroan shoshonitic biotite granites until 599 ± 2 Ma. Finally, pegmatite dykes were emplaced along dextral shear zones at 573 ± 2 Ma. The metamorphic rocks show a penetrative shallow-dipping foliation partially to totally transposed in a NE–SW trending steep-dipping foliation bearing a shallow-dipping lineation. These ductile syn-migmatitic fabrics, developed under high-grade metamorphic conditions in the presence of melt, between the metamorphic peak at ca. 1.3 GPa–770 °C, and the following isothermal decompression down to ca. 0.8 GPa. In situ U–Pb dating of metamorphic zircon rims from a garnet migmatitic paragneiss yields an age of 582 ± 4 Ma. Localization of high-grade deformation is indicated by the development of the Tcholliré-Banyo Shear Zone, characterized by mutually crosscutting subvertical and shallow-dipping mylonitic to ultramylonitic shear zones associated with sinistral and top-to-the SW kinematic criteria, respectively. These data indicate that the Central Cameroon domain exposed in the Mbé – Sassa-Mbersi region, represents the exhumed mid-to lower part of the former orogenic root of the Pan-African Central Africa Orogenic Belt that has undergone partial melting, lateral flow and intrusion of mafic to felsic calc-alkaline magmas between 650 and 580 Ma. The waning stage of the Pan-African orogeny is marked by the emplacement of syntectonic pegmatitic granite at ca. 575 Ma in subvertical E-W trending retrogressive dextral shear zones.

Recycled oceanic crust as a source for tonalite intrusions in the mantle section of the Khor Fakkan block, Semail ophiolite (UAE)

Several types of felsic granitoid rocks have been recognized, intrusive in both the mantle and the crustal sequence of the Semail ophiolite. Several models have been proposed for the source of this suite of tonalites, granodiorites, trondhjemites intrusions, however their genesis is still not clearly understood. The sampled Dadnah tonalites that intruded in the mantle section of the Semail ophiolite display arc-type geochemical characteristics, are high siliceous, low-potassic, metaluminous to weakly peraluminous, enriched in LILE, show positive peaks for Ba, Pb, Eu, negative troughs for U, Ti and occur with low δ18OH2O, moderate εSr and negative εNd values. They have crystallized at temperatures that range from ∼550 °C to ∼720 °C and pressure ranging from 4.4 kbar to 6.5 kbar. The isotopic ages from our tonalite samples range between 98.6 Ma and 94.9 Ma, slightly older and overlapping with the age of the metamorphic sole. Our field observations, mineralogical, petrological, geochemical, isotopic and melt inclusion data suggest that the Dadnah tonalites formed by partial melting (∼10%–15% continuous or ∼12% batch partial melting), accumulation of plagioclase, fractional crystallization (∼55%–57%), and interaction with their host harzburgites. These tonalites were the end result of partial melting and subsequent contamination and mixing of ∼4% oceanic sediments with ∼96% oceanic lithosphere from the subducted slab. This MORB-type slab melt composed from ∼97% recycled oceanic crust and ∼3% of the overlying mantle.We suggest that a possible protolith for these tonalites was the basaltic lavas from the subducted oceanic slab that melted during the initial stages of the supra-subduction zone (SSZ), which was forming synchronously to the spreading ridge axis. The tonalite melts mildly modified due to low degree of mixing and interaction with the overlying lithospheric mantle. Subsequently, the Dadnah tonalites emplaced at the upper part of the mantle sequence of the Semail ophiolite and are geochemically distinct from the other mantle intrusive felsic granitoids to the south.

The evolution of the Neoproterozoic Elat Metamorphic Complex, northernmost Arabian-Nubian Shield: Island arc to syncollisional stage and post-collisional magmatism

The Elat Metamorphic Complex (EMC), part of the Arabian-Nubian Shield exposed at Elat and Taba areas (S. Israel and NE Sinai, Egypt), records a full orogenic cycle from island arc to syncollisional stages that extended ∼240 My. New SIMS U–Pb dating of zircons in five samples, combined with published U-Pb zircon data, enable precise timing of formation of metamorphic and magmatic rock units and associated tectonic events. The island arc stage extended from 870 to 740 Ma. The oldest rocks are metasediments derived from island arc volcanics, metamorphosed and penetratively deformed prior to intrusion of island arc tonalite and granite plutons. The island arc stage was followed by a non-active stage from 740 to 670 Ma. The collisional stage that terminated the orogenic cycle commenced with plutonic activity of basic rocks and granitoids from 670 to ∼635 Ma, followed by deformation and metamorphism of both island arc and syncollisional rocks. During the collisional stage, island arc rocks were uplifted, exhumed and eroded; the volcano-sedimentary rocks of the Gishron Formation were deposited on the erosional surface, and then were deformed. Early post-collisional high-K calc-alkaline plutonic activity immediately followed the syncollisional stage from 635 to 610 Ma, and was exhumed at the end of this stage. A late post-collisional within-plate alkaline magmatic stage ensued (610–530 Ma), beginning with basic and acid volcanism, followed by intrusion of alkaline granite plutons, and in the end by late sub-volcanic bodies and dikes. New geochemical data for 26 representative samples combined with 150 published chemical analyses reveal consistent compositional change of similar rock types over time. The most graphic example for this change is the sequential increase of K2O and HFSE content in similar magmatic rock types, basic rocks, intermediate rocks, low-silica granitoids, and granites, from the syncollisional to the end of late post-collisional within-plate stage, over 80–90 My. The ∼740 Ma island arc granitic gneisses are however chemically similar to the syncollisional granitic gneiss and post-collisional granites.

Insights on the evolution of the Arroio Grande Ophiolite (Dom Feliciano Belt, Brazil) from Rb-Sr and SHRIMP U-Pb isotopic geochemistry

This paper presents the first Rb-Sr isotopic geochemistry study and SHRIMP zircon U-Pb dating in the Arroio Grande Ophiolite (southeasternmost Dom Feliciano Belt in Brazil), an extension of the Uruguayan Punta del Este Terrane. The metamafic units of the ophiolite show 87Sr/86Sr ratios ranging from 0.7036 to 0.7070 (recalculated at 630 Ma), which suggest a MORB magmatic source for the amphibolite protoliths and a volcanic magmatic arc source (with possible crustal contamination) for the metagabbro protoliths. Together, these features suggest a supra-subduction zone origin for these rocks, corroborating previous studies based on bulk-rock chemistry. In the metaultramafics, the 87Sr/86Sr630 ratio of the chloritite sample (0.7152) is comparable to that of the Três Figueiras granite (0.7139), a syn-kinematic peraluminous granite related to the Ayrosa Galvão-Arroio Grande Shear Zone (which also affected the southern portion of the ophiolite, where the studied samples are located). The emplacement of the latter granite could be responsible for the metasomatism of the former serpentinites, generating talc- and chlorite-rich reaction zones. The SHRIMP U-Pb analysis of twelve chloritite zircons strengthens that assumption, as zircons with ca. 610-585 Ma (the age of the granite) were found in association with metasomatic microtextures. Zircon ages ranging from ca. 2000 to 660 Ma were also found in the chloritite. These latter ages are analogous to those found in the metasiliciclastic rocks that envelop the metaultramafic rocks of the ophiolite, so that the chloritite probably represents a blackwall reaction zone between (i) a former serpentinite, (ii) the Três Figueiras granite, and (iii) metasiliciclastic rocks. Additionally, we perform a SHRIMP U-Pb dating in a quartz-syenite melt, which are related to a tonalitic intrusion within the metasedimentary unit (the Matarazzo Marbles) of the ophiolite. The obtained concordia age of 640 Ma constrains the minimum age of ophiolitic mélange emplacement, as the tonalitic intrusion is related to continental arc magmatism (Pinheiro Machado Intrusive Suite).

Tectonics and Geology of Porphyry Cu-Au Deposits along the Eastern Sunda Magmatic Arc, Indonesia

The discovery of the world-class Cu-Au deposit at Tumpangpitu, East Java (1.9 Gt @ 0.45% Cu, 0.45 g/t Au), has reinforced the eastern Sunda arc as a significant metallogenic belt that is highly promising for the discovery of major porphyry deposits. The arc hosts three premier porphyry Cu-Au deposits at Batu Hijau, Elang, and Tumpangpitu (>300 t Au and >5 Mt Cu). These giant porphyry deposits are confined to the eastern segment of the eastern Sunda arc (eastern Java to Sumbawa), constructed on island-arc crust where the Roo Rise is being subducted. By contrast, major epithermal (low-sulfidation dominant) deposits associated with poorly endowed porphyry prospects occur along the western segment of the arc (western Java), developed on thick continental crust on the southern margin of Sundaland associated with subduction of typically thin Indian oceanic crust. Porphyry Cu-Au deposits of the eastern Sunda arc are spatially associated with small, nested, dioritic to tonalitic intrusive complexes with low-K calc-alkaline to weakly alkaline signatures. Hypogene alteration and mineralization developed during three main temporally and spatially overlapping events, termed early, transitional, and late. The hypogene Cu-Au mineralized zone of the giant deposits forms a bell-shaped body of 0.3% Cu shell greater than 1 km in both diameter and vertical extent that is centered on multiphase, elongate, pencil-like tonalite intrusions, 200 to 1,200 m in diameter and >2 km vertical extent. Large (>20 km 2 ) lithocaps associated with high-sulfidation epithermal systems and postporphyry mineralization diatreme breccias characterize productive porphyry districts at Batu Hijau, Elang, and Tumpangpitu. The known porphyry deposits with metal content exceeding 300 t Au and >5 Mt Cu all formed later than 5 Ma, suggesting an important change in the metallogeny of the arc at this time. These giant porphyry deposits formed relatively rapidly, typically over a period of 30 to 860 k.y. during the Pliocene-Pleistocene, between 5 and 2.5 Ma. Porphyry deposits along the eastern Sunda arc display characteristic geologic features similar to porphyry deposits elsewhere. Significant differences include low-K systems, dominance of hydrothermal biotite over K-feldspar, early intense actinolite alteration associated with ore, widespread shreddy chlorite after secondary biotite, narrow quartz-sericite-pyrite alteration zones, lack of development of chalcopyrite veins, large overlying lithocaps, and abundant chalcocite-bornite-covellite veinlets associated with late overprinting epithermal systems. Hydrothermal oligoclase is also more abundant than K-feldspar in the central biotite-magnetite zones.

Late Cambrian magmatic arc activity in peri-Gondwana: geochemical evidence from metagranitoid rocks of the Basal Allochthonous Units of NW Iberia.

The North African section of the Gondwana margin was the site of voluminous, arc-related magmatism during the Late Neoproterozoic (Avalonian–Cadomian orogen). The lower (and older) metasedimentary sequence that constitutes the Basal Units of the Allochthonous Complexes of NW Iberia was deposited in that setting. In these units, sedimentation was followed by the intrusion of tonalites and granodiorites in the late Cambrian ( ca. 493–489Ma). In the Late Paleozoic, the collision of Gondwana and Laurussia (Variscan orogeny) deformed and metamorphosed the whole ensemble. New whole rock geochemical analysis performed in seven samples of metatonalites and fourteen samples of metagranodiorites are characterized by: i) slight enrichment in incompatible elements (Rb, Ba, Th, U), ii) negative anomalies in Nb, Ta, P, and Ti, and iii) negative anomalies in Eu. These chemical features are in agreement with a subduction-related setting for the genesis of both types of magma, which is also supported by chemical discrimination using tectonic setting diagrams. Positive anomalies of Pb suggest a crustal component. The new geochemical data reveal that the convergent orogen that ruled the paleogeography of the Gondwana periphery during the Neoproterozoic (Cadomian orogen) remained active beyond the Neoproterozoic-Cambrian transition, up to at least late Cambrian times.

Metamorfismo de temperatura ultra-alta de granulitos com safirina de 2,0 Ga do Bloco Itabuna-Salvador-Curaçá, Bahia, Brasil

A área de pesquisa se situa no Cráton do São Francisco, no sul da Bahia, Brasil. Nessa região, no segmento crustal Bloco Itabuna-Salvador-Curaçá (BISC), ocorrem rochas supracrustais associadas a gnaisses tonalíticos/trondhjemíticos a monzoníticos e rochas máficas subordinadas, de idades arqueana e paleoproterozoica, todos fortemente deformados e reequilibrados na fácies granulito em 2,07 a 2,08 Ga. As rochas supracrustais incluem granulitos com safirina constituídos por quartzo, ortopiroxênio, silimanita, granada, mesopertita, plagioclásio, biotita e cordierita. Com base nos arranjos microestruturais entre os minerais constituintes dos granulitos com safirina, a paragênese de pico metamórfico é Grt1 + Opx1 + Bt1 + Qtz + Sil1 + Spr1 + Mp. Microestruturas de reações entre minerais, como coroas de simplectitos ao redor de porfiroblastos, permitiram identificar as seguintes paragêneses retrógradas, interpretadas como formadas durante a ascensão tectônica dessas rochas: Grt1 ± Qtz = Opx2 + Sil2, Grt1 = Opx2 + Spr2 (+ Sil2); Opx1 + Sil1 + Qtz = Crd e Grt1 + Qtz = Opx3 + Crd. Por sua vez, simplectitos de Bt2 + Qtz formados pela reação Opx1-2 + Mp + H2O = Bt2 + Qtz estão relacionados à evolução retrógrada durante resfriamento. Cálculos termobarométricos realizados com aplicação de termobarômetros clássicos e método de multiequilíbrios (THERMOCALC, por exemplo) indicaram P = 7-11 kbar e T ≥ 900°C para os granulitos com safirina. Um dos mecanismos associados à elevada temperatura está relacionado à adição de material radioativo com a presença de magmas basálticos durante a formação do BISC. A cristalização fracionada desses magmas produziu intrusões tonalíticas que datam de aproximadamente de 2,15 a 2,13 Ga, próximo do pico do metamorfismo de 2,07 a 2,08 Ga.