Margin Magmatic Arc Research Articles

Lithology, source areas and formation settings of upper triassic deposits of South-Western Primorye

The article considers the material composition of sandy rocks from the Upper Triassic deposits of the Mongugai formation of South-Western Primorye and presents the results of its paleogeodynamic interpretation. It has been established that, according to their mineralogical and geochemical parameters, sandstones are petrogenic, correspond to graywackes and, only partially, to lithite arenites, are characterized by a significant degree of geochemical maturity of the clastic material, and their formation occurred due to the erosion of significantly weathered parent rocks of the source areas. Generalization and paleogeodynamic interpretation of the data obtained indicate that in the Late Triassic in the southwestern part of Primorye, sedimentation occurred in basins associated with the environment of the active continental margin and, probably, complicated by strike-slip dislocations along transform faults. The dominant source of clastic matter was continental land: cratons and projections of crystalline basement framed by rift zones or along transform faults. Acidic granite-metamorphic complexes were eroded with the participation of sedimentary rocks enriched with ancient components. A secondary source was a deeply dissected continental margin magmatic arc, which supplied the basin with an additional amount of sialic material, as well as a small amount of the basic-medium volcanoclastic. U-Pb geochronological studies of detrital zircons from sandy rocks of the formation made it possible to establish the age and possible location of magmatic complexes, due to the destruction of which the deposits were formed.

Neoproterozoic transition from subduction to breakup along the margin of the Rodinia supercontinent: Sedimentary evidence from the northwestern Tarim Block, Northwest China

The Tarim Block, northwest China, was positioned on the periphery of Rodinia during the Neoproterozoic Era, and preserves a geological archive chronicling the assembly and breakup of the supercontinent. Due to its antiquity and limited exposure, sedimentary records along the margin of the Rodinia supercontinent remain sparsely documented. This study focuses on the Xifangshan and Dongqiaoenbrak formations, the oldest sedimentary units in the northwestern Tarim Block, which comprise a Neoproterozoic sedimentary record at the supercontinent margin during the transition from subduction to breakup. A comprehensive analysis employing sedimentology, petrology, elemental geochemistry, and UPb detrital zircon chronology was undertaken, to elucidate paleoenvironment, parent rocks, and tectonic-sedimentary evolution. Findings show that the Neoproterozoic witnessed the subduction of an oceanic plate beneath the Tarim Block, resulting in the formation of a continental margin magmatic arc. The Xifangshan Formation, deposited during the late phase of this arc (with the earliest sedimentary age recorded at 770 Ma), represents a submarine fan dominated by turbidity currents. Around 760 Ma, rapid crustal uplift occurred, and the tectonic setting gradually shifted from subduction to rift, giving rise to the coastal alluvial fan sedimentary system observed in the Dongqiaoenbrak Formation. Both the Xifangshan and Dongqiaoenbrak Formations record deposition in an oxic to suboxic marine environment with moderate salinity, influenced by a semiarid to semihumid palaeoclimate. The continental margin arc served as the primary source for sedimentation in both formations, and a few magmatic products linked to extension (syenite, monzonite) were also cannibalized into the sedimentary system of Dongqiaoenbrak Formation. This transition from a turbidite system to a coastal alluvial fan represents a sedimentary response to the subduction-to-breakup dynamics along the margin of the Rodinia supercontinent.

Geochemical characteristics and the constraints on paleoenvironment, provenance, and tectonic setting of Precambrian Xifangshan Formation in the northwestern Tarim Basin, NW China

Combining geochemistry and petrology to clarify paleoenvironment, provenance and tectonic setting is the most reliable approach. Few studies have been conducted on the Xifangshan Formation, a set of turbidite deposits from the Precambrian in the northwestern Tarim Basin, NW China. The tectonic setting during this period is not clear. This study reports the petrological characteristics and geochemical data of the Xifangshan sandstones, aiming to constrain the paleoclimate, paleoredox conditions, paleoweathering, provenance, and finally establish the tectonic development pattern of this period. Xifangshan Formation is mainly composed of litharenite and feldspathic litharenite. Most of the trace elements (Rb, Sr, Pb, U, Y, Sc, V, Cr, Cu, Co, Ni, Zn) are depleted relative to the upper continental crust. And, the rare earth elements show a slight negative Eu anomaly. The sandstones were first-cycle, displaying immaturity of composition and structure. The major element composition and low chemical index of alteration indicate that the sandstones have experienced weak weathering during transportation. Multiple geochemical indicators show that the Xifangshan Formation was deposited in an oxic-suboxic marine environment with moderate salinity and semiarid climate. Based on the geochemical and petrographic characteristics of the rock fragments, the source of the Xifangshan Formation has a felsic provenance, and the parent rock is predominantly acidic magmatic rocks, with a small amount of intermediate magmatic rocks. Influenced by the subduction of the ancient oceanic plate, the blueschist-bearing Aksu Group and the continental margin magmatic arc were formed. This magmatic arc provided the source for the Xifangshan Formation sandstones.

New geochemical constraints on the development of active continental margin in Southeast China during the Middle Permian and its tectonic implications

Two competing hypotheses that passive or active continental margins developed in South China during the Late Paleozoic are still under debate. Previous studies were mainly focused on the remnant magmatic rocks, but little attention has been paid to the contemporary sediments deposited in the back-arc basin. Therefore, a deeper knowledge of the tectonic link between the continental arc and regional basins is not only vital for understanding their tectono-sedimentary evolution, but also provide the potential to evaluate these hypotheses. Here two Middle Permian sections of bedded chert-mudstone sequence (Gufeng Formation) and an overlying siliciclastic succession (Yinping Formation) from the Lower Yangtze region, were analyzed to understand their provenance, tectonic and depositional setting, and to provide new constraints on the Late Paleozoic tectonics of the South China. Discrimination of tectonic setting indicates that the Middle Permian sedimental source rocks of the Lower Yangtze basin probably originated in a transitional setting from continental island arc to active continental margin, consistent with the development of an active margin magmatic arc along Southeast China during that time. Provenance analysis also showed that, the Gufeng Formation was mainly derived from an Andesitic arc source with mixed mafic and felsic source, whereas the Yinping Formation received detritus from a more felsic source. This provenance switching could reflect a possible tectonic transition related to the continental arcs’ evolution that from Andean-type normal/high-angle subduction to flat-slab subduction, which weakened the arc volcanism and intensified the bedrock uplift in South China. The depositional environments of the Lower Yangtze basin also evolved from an open deep ocean environment to a continental margin setting during the deposition of Gufeng Formation, and then transformed into a coastal/neritic shallow water setting during the deposition of Yinping Formation. This sedimentary evolution can be attributed to the tectonic transition during the Middle Permian.

Evidence of late Palaeozoic and Middle Triassic magmatism in the Sakar-Strandzha Zone, SE Bulgaria: Regional geodynamic implications

ABSTRACT Late Palaeozoic granitoids, meta-granitoids and meta-volcanic rocks predominate in the metamorphic basement of the northern and western parts of the Sakar-Strandzha Zone (SASTZ) in southeast Bulgaria, together with the subsidiary Triassic meta-granitoids and meta-volcanic rocks. Generally, igneous minerals and textures are preserved, except in the meta-granitoids and meta-volcanic rocks that experienced a low- to high-grade metamorphic overprint. The volcanic rocks have a peraluminous and high-K calc-alkaline composition, and the granitoids range between I- to S-type compositions, typical of volcanic arcs and syn-collisional settings. LILE and LREE-enrichment and Nb-Ta anomalies characterize the intrusive and extrusive rock suites. U-Pb zircon geochronology has yielded crystallization ages between 245 and 237 Ma for the majority of the studied igneous rocks, and between 297 and 295 Ma for a small group of igneous rocks. Early Permian and Middle Triassic igneous suites of the northern and western SASTZ have similar compositions and a similar tectonic setting when compared to Late Carboniferous-Early Permian intrusive and extrusive suites of the adjacent Sakar unit of the SASTZ, confirming a common regional late Palaeozoic-early Mesozoic tectono-magmatic event. As the Late Carboniferous-Permian to Middle Triassic magmatic arc components extend across the SASTZ, they trace the time-correspondent active continental margin along the Eurasian plate during subduction of the Paleotethys oceanic lithosphere. The late Palaeozoic Eurasian active continental margin magmatic arc evolution of the SASTZ can be linked with the Serbo-Macedonian-Rhodope zones to the southwest, where coeval meta-granitoids document the same geodynamic context. By contrast, the Triassic igneous suite of the SASTZ is unrelated to the Serbo-Macedonian-Rhodope zones, where Triassic meta-ophiolite and meta-granitoids record Neotethys rifting.

Mesoproterozoic porphyry copper mineralization at Mamainse Point, Ontario, Canada in the context of Midcontinent rift metallogeny

The Jogran copper prospect in southern Ontario, Canada, is located on the northeastern shoulder of the North American Midcontinent rift system where it is centered on two small, closely spaced quartz monzonite porphyry intrusions emplaced into Neoarchean mafic metavolcanic rocks. Notwithstanding an atypical alteration zoning pattern, the distinctive alteration styles and veinlet types confirm its porphyry copper affiliation. Two samples of molybdenite from chalcopyrite-bearing quartz veinlets in the two centers returned Re-Os ages of 1101 ± 5 and 1094 ± 5 Ma, showing that the mineralization took place during the main stage of extension and magmatic activity that gave rise to the Midcontinent rift and associated large igneous province (LIP). The nearby copper-bearing Tribag breccia pipes and Coppercorp veins have been shown previously to also be intrusion related, and likely formed penecontemporaneously with Jogran. The Jogran and nearby copper mineralization overlapped temporally with generation of low-grade magmatic copper-nickel-PGE deposits in the Duluth intrusive complex but predated the strata-bound native copper and chalcocite deposits in the Keweenaw Peninsula, which are believed to have formed from basinal fluids during rift inversion. Although such magmatic and strata-bound copper deposits typify intracontinental rift metallogeny, porphyry copper deposits are normally confined to convergent margin magmatic arcs, thereby making the rift-LIP setting of Jogran exceptional and possibly unique.

Neoproterozoic geodynamic evolution of easternmost Kalahari: Constraints from U-Pb-Hf-O zircon, Sm-Nd isotope and geochemical data from the Schirmacher Oasis, East Antarctica

Late Tonian (ca. 785–760 Ma) granodioritic to granitic orthogneisses of the Schirmacher Oasis region in Dronning Maud Land (DML), East Antarctica, are interpreted as recording an active continental margin setting at the periphery of Kalahari and Rodinia. The rocks probably represent exposures of a significant tectonic province hidden beneath the ice, the erosional remnants of which are recorded as detrital zircons in late Tonian-Cryogenian metasedimentary rocks throughout central and eastern DML, as well as in ice-rafted debris from recent sediments offshore Dronning Maud Land. The orthogneisses have single-stage Sm-Nd model ages of ca. 1.3–1.5 Ga and zircon Hf-signatures (εHft = +2 – +5), indistinguishable from the adjacent Grenville-age basement rocks of easternmost Kalahari. Their geochemistry suggests that they evolved in the late stages of a continental margin magmatic arc and possibly within a roll-back tectonic framework, suggestive of subduction of relatively old oceanic lithosphere. The eastern Kalahari continental arc is one of a number of continental arcs that characterize the western part of the fragmenting Rodinia and document the supercontinent “turning inside out” after its formation at ca. 1000 Ma and a period of relative tectonic quiescence between ca. 900 and 800 Ma.The rocks show an ultra-high temperature metamorphic overprint that was accompanied by syn-tectonic magmatism from ca. 650 to 600 Ma. The high temperature metamorphism is interpreted to relate to back-arc extension that also led to major anorthosite magmatism elsewhere, prior to continental collision in the region. The rocks lack the subsequent widespread high-grade metamorphic overprint at ca. 590–500 Ma which occurs in the adjacent regions due to Himalayan-style continental collision along the East African-Antarctic Orogen during Gondwana assembly.

Differentiating continental and oceanic arc systems and retro-arc basins in the Jiangnan orogenic belt, South China

AbstractThe Neoproterozoic Jiangnan orogenic belt records the accretion and collision between the Yangtze and Cathaysia blocks in South China. The orogen is divisible into three units: a northeastern domain (also referred to as the Huaiyu or Shuangxiwu domain), a central domain (Jiuling domain) and an undifferentiated southwestern domain. Detrital zircons from the oldest sequences (Shuangqiaoshan, Lengjiaxi, Fanjingshan and Sibao groups) in the central and southwest domains yield similar age spectra with major age populations at c. 875–820 Ma, along with minor Palaeo- to Mesoproterozoic and Archaean ages. The dominance of detrital ages close to the deposition ages of the units, along with juvenile zircon Hf isotopic compositions and arc-like whole-rock compositional data, indicate the sedimentary units accumulated adjacent to a convergent plate margin magmatic arc. The presence of Mesoproterozoic and older zircons, both as detritus in the units and as xenocrysts within igneous rocks displaying a subduction-related signature, along with the compositional data, place the magmatic arc along a continental margin. In the northeastern domain, the oldest coeval sequence (Shuangxiwu and Qigong groups) and arc igneous suites are dated at c. 970–850 Ma, and lack older detritus and xenocrysts, indicating they represent an accreted oceanic arc system.

Timing and tectonic significance of Paleozoic magmatism in the Sakar unit of the Sakar-Strandzha Zone, SE Bulgaria

ABSTRACTPalaeozoic granitoids and meta-granitoids dominate the metamorphic basement of the Sakar unit of the Sakar-Strandzha Zone (SASTZ) in southeast Bulgaria. In this article, we present new whole-rock geochemical data and U–Pb zircon geochronology for the Sakar unit granitoids. The igneous minerals and textures are preserved, except the meta-granitoids that experienced a weak amphibolite-facies overprint. Geochemistry reveals compositions of peraluminous high-K calc-alkaline I- to S-type granitoids of volcanic arc origin. A major group of LILE-LREE-enriched granitoids and meta-granitoids and a single HFSE-HREE-enriched meta-granitoid are distinguished. U–Pb geochronology has yielded crystallization ages between 305 and 295 Ma for the major group granitoids and a ca. 462 Ma crystallization age of HFSE-HREE-enriched meta-granitoid. Late Palaeozoic granitoids of the Sakar unit show similar compositions and a similar tectonic setting when compared to other granitoids of the SASTZ, confirming a uniform region-wide tectono-magmatic event. As the Late Carboniferous-Permian magmatic arc components extend across the SASTZ, they trace the time-correspondent active continental margin along the Eurasian plate during subduction of the Palaeotethys oceanic lithosphere. The late Palaeozoic Eurasian active continental margin magmatic arc evolution of the SASTZ can be extended into the Serbo-Macedonian-Rhodope zones to the west, where time equivalent meta-granitoids support the same geodynamic context.

Provenance of Late Permian volcanic ash beds in South China: Implications for the age of Emeishan volcanism and its linkage to climate cooling

Whole-rock geochemistry and zircon trace element, UPb and Hf isotopic data for Upper Permian volcanic ash at Shangsi show a significant change in source from the Emeishan Large Igneous Province (LIP) to a convergent plate margin magmatic arc. The lower stratigraphic samples display high Al2O3/TiO2 ratios, strong negative Eu anomalies, and no depletion in high field strength elements. Zircons with ages of ca. 260 Ma from these ashes have a geochemical affinity to within-plate-type magmas and with felsic ignimbrites from the uppermost part of the Emeishan volcanic succession at Binchuan. These features, combined with relatively high εHf(t) values and low Th/Nb ratios in the zircons, imply a late-stage felsic volcanic origin related to Emeishan LIP. The upper stratigraphic samples at Shangsi display significant depletion in Nb and Ti; zircons separated from these volcanic ashes are chemically similar to those from arc-related/orogenic rocks, indicative of a magmatic arc source. Combined with published high-precision CA-TIMS data for these volcanic ash layers, the Emeishan volcanism can be constrained to between ca. 260–257.8 Ma. The age of Emeishan volcanism is consistent with the Wuchiapingian climate cooling event, supporting a potential linkage between these two phenomena.

Composition and age of Jurassic diabase dikes in the Xiongcun porphyry copper–gold district, southern margin of the Lhasa terrane, Tibet, China: Petrogenesis and tectonic setting

Diabase dikes are extensively distributed in the Xiongcun porphyry copper–gold district in the southern margin of the Lhasa terrane. Here, we report zircon U–Pb ages determined by laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) along with Hf isotopic, whole‐rock elemental, and Sr–Nd–Pb isotopic data for these diabase dikes. Zircons from one of the Xiongcun diabase dikes yielded an emplacement age of 165.3 ± 1.0 Ma. The Xiongcun diabase dikes display subalkaline features and tholeiite compositions. They are enriched in large‐ion lithophile elements (e.g., Rb, Ba, and K) and light rare earth elements and are depleted in high‐field‐strength elements (e.g., Nb, Ta, and Ti). The Xiongcun diabase dikes exhibit relatively low ratios of (87Sr/86Sr)i(0.70386–0.70501), (206Pb/204Pb)i(18.15–18.40), (207Pb/204P)i(15.52–15.57), and (208Pb/204Pb)i(38.06–39.46) and relatively high values of εNd(t) (5.85–6.92) and εHf(t) (+12.58 to +14.03). Combined with previous research results, these findings suggest that the Xiongcun diabase dikes and Early Mesozoic magmatic rocks in the southern margin of the Lhasa terrane formed in an active continental margin magmatic arc setting related to the northward subduction of the Neo‐Tethys oceanic slab; however, the possibility that the Xiongcun diabase dikes were formed in an intra‐oceanic arc setting cannot be excluded. Their parental magmas were formed by the partial melting of the depleted mantle source that was modified by fluids released from the Neo‐Tethys oceanic slab. The magmas experienced fractional crystallization of olivine and clinopyroxene and underwent limited crustal contamination during magma emplacement. The ~165 Ma Xiongcun diabase dikes, which probably originated under an extensional stress condition in a magmatic arc setting, correspond to the timing of Middle Jurassic subduction‐related porphyry copper mineralization (161–172 Ma) in the southern margin of the Lhasa terrane, likely suggesting that this episode of mineralization occurred during a period of changing stress from compression to extension.

Discovery and Geological Significance of Neoproterozoic Metamorphic Granite in Jimo, Shandong Province, Eastern China

AbstractDuring the 1:50000 regional geological survey in Jimo, east Shandong Province, Paleoproterozoic metamorphic supracrustal rocks and Neoproterozoic metamorphic plutonite were newly discovered. These rocks displayed inclusions which had occurred in the Mesozoic granite, and the main lithologies are schist, granulite, marble, and granitic gneiss. Geochemical analyses suggest that Neoproterozoic metamorphic plutonite are characterized by high‐K, metaluminous to weakly peraluminous. They are enriched in LILE and depleted in HFSE, with moderately enrichment of LREE, weak fractionation of LREE from HREE and negative Eu anomalies. The surface age of plutonic rocks in the survey area is 770.2±2.4 Ma, representing the age of magma crystallization, which is agreement with the the Neoproterozoic magmatic event after Rodinia supercontinent in the northern margin of Southern China continental block. In addition, the age of sporadic distribution (298 Ma and 269 Ma) is mixed zircon age, representing the rocks experienced metamorphism in Indosinian period. According to the associated mineral assemblages, and the characteristic metamorphic minerals and temperature pressure conditions, four metamorphic facies were identified, including amphibolitic, epidote amphibolite, greenschist, and mid‐high pressure greenschist. Analysis of tectonic setting suggests that granitic gneiss is formed in an extensional environment and was involved from the continental margin magmatic arc to intraplate environment. Jimo is distributed in the east of Zhuwu fault, and has the same Spatial distribution location with the Weihai uplift UHP metamorphic belt rocks. The metamorphic rocks in Jimo area have similar geochemical characteristics of elements, tectonic setting and retrograde metamorphism with that in the Sulu UHP metamorphic belt. Therefore, Zhuwu fault may be the boundary fault of Sulu UHP metamorphic belt.

Discovery, Geology, and Origin of the Fruta del Norte Epithermal Gold-Silver Deposit, Southeastern Ecuador

Fruta del Norte is a completely concealed and extraordinarily well preserved epithermal gold-silver deposit located in the remote Cordillera del Condor mountain ranges of southeastern Ecuador. Currently defined resources in a single, steep, relatively small body, displaying exceptional grade continuity, contain 9.81 million ounces (Moz) of gold and 15.0 Moz of silver, with an indicated resource grade of 9.59 g/t Au and 12.9 g/t Ag. The deposit was discovered in early 2006 during a greenfields program conducted by Aurelian Resources, a Canadian junior explorer. Discovery resulted from systematic drill testing of a conceptual geologic model, which predicted that auriferous veins would be present in andesitic volcanic rocks inferred to underlie a steep silicified rib cutting a fluvial conglomerate sequence. The rib is highly anomalous in arsenic, antimony, and mercury but contains low-order gold values. The second drill test—the discovery hole—intersected >230 m of ore-grade gold-silver mineralization beneath ~200 m of the barren conglomerate cover. The host andesitic volcanic rocks, crosscutting feldspar porphyry, and associated phreatic breccias are assigned to the Piuntza unit of the Santiago Formation, which, along with the Zamora batholith and a series of porphyry copper stocks, was generated in a continental margin magmatic arc during the Middle to Late Jurassic. The deposit is located near the northeastern extremity of the ~10-km-long, Suarez pull-apart basin where it is linked to the steep West and Central faults, part of the regionally extensive Las Penas strike-slip fault zone. The pull-apart basin was progressively filled by fluviatile conglomerate, dacitic ignimbrite flows, finer grained siliciclastic sedimentary rocks, and, finally, andesite flows, all assigned to the Chapiza Formation. The Fruta del Norte deposit consists of a 1.3-km-long and up to >300-m-wide vein-stockwork associated with quartz-illite-pyrite alteration. The deposit comprises two principal vein types, one in the south dominated by quartz, manganoan carbonates, and abundant base metal sulfides and the other in the north dominated by manganese- and base metal-poor quartz and calcite. Adularia is a minor main-stage gangue mineral in each. Both types are abruptly transitional upward and westward to a third ore type marked by intense silicification and chalcedony veining, with disseminated and veinlet marcasite (±pyrite). The uppermost part of this silicic zone is sulfide deficient, probably the result of a short-lived supergene oxidation event prior to initial conglomerate deposition. The deposit is notable for the widespread occurrence of fine to coarse visible gold, which gives rise to bonanza grades and is closely associated with quartz, chalcedony, carbonate, and sulfide gangue. The sulfide-free, silicic zone is overlain by an extensive silica sinter horizon, which may either directly overlie the Piuntza volcanic rocks and/or occur as interbeds in the basal 20 m of the conglomerate, which are invariably silicified and marcasite bearing. Otherwise, the conglomerate above the orebody lacks silicification, with one critical exception: the steeply inclined zone that is exposed as the silicified rib that led to discovery. The sinter horizon, containing localized mud-pool deposits and hydrothermal eruption breccias, is in unusual proximity to the underlying gold-silver orebody. The northern and southern parts of the Fruta del Norte deposit possess characteristics that are usually considered typical of low and intermediate sulfidation epithermal deposits, respectively; they may have required two discrete mineralizing fluids, both of which are suspected to have ascended via the West and Central faults. The upward change to the silicified zone is attributed to cooling of the ascendant fluid on approach to the paleosurface sinter. Progressive burial of the deposit beneath the Suarez basin fill may have contributed to the suppression and eventual extinction of the epithermal system, with gold mineralization of the conglomerate being inhibited by rheologic and chemical factors. Formation of the Fruta del Norte deposit and complete filling of the Suarez basin took place in >3 to 6 m.y. during the Late Jurassic ( 157–154 Ma).

The Ediacaran Rio Doce magmatic arc revisited (Araçuaí-Ribeira orogenic system, SE Brazil)

Described half a century ago, the Galiléia tonalite represents a milestone in the discovery of plate margin magmatic arcs in the Araçuaí-Ribeira orogenic system (southeastern Brazil). In the 1990's, analytical studies on the Galiléia tonalite finally revealed the existence of a Late Neoproterozoic calc-alkaline magmatic arc in the Araçuaí orogen. Meanwhile, the name Rio Doce magmatic arc was applied to calc-alkaline plutons found in the Araçuaí-Ribeira boundary. After those pioneer studies, the calc-alkaline plutons showing a pre-collisional volcanic arc signature and age between 630 Ma and 585 Ma have been grouped in the G1 supersuite, corresponding to the Rio Doce arc infrastructure. Here, we revisit the Rio Doce arc with our solid field knowledge of the region and a robust analytical database (277 lithochemical analyses, and 47 U–Pb, 53 Sm–Nd, 25 87Sr/86Sr and 7 Lu–Hf datasets). The G1 supersuite consists of regionally deformed, tonalitic to granodioritic batholiths and stocks, generally rich in melanocratic to mesocratic enclaves and minor gabbroic to dioritic plutons. Gabbroic to dioritic enclaves show evidence of magma mixing processes. The lithochemical and isotopic signatures clearly reveal a volcanic arc formed on a continental margin setting. Melts from a Rhyacian basement form the bulk of the magma produced, whilst gabbroic plutons and enclaves record involvement of mantle magmas in the arc development. Tonalitic stocks (U–Pb age: 618–575 Ma, εNd(t): −5.7 to −7.8, Nd TDM ages: 1.28–1.68 Ga, 87Sr/86Sr(t): 0.7059–0.7118, and εHf(t): −5.2 to −11.7) form the northernmost segment of the Rio Doce arc, which dies out in the ensialic sector of the Araçuaí orogen. At arc eastern and central zones, several batholiths (e.g., Alto Capim, Baixo Guandu, Galiléia, Muniz Freire, São Vítor) record a long-lasting magmatic history (632–580 Ma; εNd(t): −5.6 to −13.3; Nd TDM age: 1.35–1.80 Ga; 87Sr/86Sr(t): 0.7091–0.7123). At arc western border, the magmatic evolution started with gabbro-dioritic and tonalitic plutons (e.g., Chaves pluton, U–Pb age: 599 ± 15 Ma, εNd(t): −4.8 to −6.8, Nd TDM ages: 1.48–1.68 Ga, 87Sr/86Sr(t): 0.7062–0.7068, and εHf(t): −4.3 to −9.7; and Brasilândia pluton, U–Pb age: 581 ± 11 Ma, εNd(t): −8.2 to −10.2, Nd TDM ages: 1.63–1.68 Ga, 87Sr/86Sr(t): 0.7088–0.7112, εHf(t): −12.3 to −14.9), followed by late granodioritic intrusions (e.g., Guarataia pluton, U–Pb age: 576 ± 9 Ma, εNd(t): −12.52 to −13.11, Nd TDM age: 1.74–2.06 Ga, 87Sr/86Sr(t): 0.7104–0.7110, εHf(t): −12.9 to −21.6). The Muriaé batholith (U–Pb age: 620–592 Ma, εNd(t): −8.2 to −13.6, Nd TDM age: 1.41–1.88 Ga) and the Conceição da Boa Vista (586 ± 7 Ma) and Serra do Valentim (605 ± 8 Ma) stocks represent a segment of the Rio Doce arc correlated to the Serra da Bolívia and Marceleza complexes, making the link between the Araçuaí and Ribeira orogenic domains. We suggest three phases of arc development: i) eastward migration of arc front (632–605 Ma), ii) widespread magma production in the whole arc (605–585 Ma), and iii) late plutonism in the western arc region (585–575 Ma). Usual processes of volcanic arc development, like subduction of oceanic lithosphere under a continental margin, followed by asthenosphere ascent related to slab retreating and break-off may explain the Rio Doce arc evolution.

Tonian granitic magmatism of the Borborema Province, NE Brazil: A review

Tonian granitoids, today augen-gneisses and migmatites, showing crystallization ages ranging from 870 Ma to 1000 Ma occur in the Borborema Province, NE Brazil. The majority of them have ages within the 925–970 Ma interval. Few intrusions with ages of ∼1.0 Ga and <900 Ma occur in the Transversal and South subprovinces. The Tonian granitoids constitute the most expressive magmatic rocks of the Cariris Velhos event. The studied granitoids (herein CVG -Cariris Velhos granitoids) intrude slightly older bimodal (but mostly felsic) volcanic successions and metasedimentary sequences in the Transversal and South subprovinces. Tonian granitoids are unknown in the North subprovince. The CVG comprise mainly coarse-grained augen-gneisses of granite to granodiorite composition. Fe-rich biotite (annite) is the main mafic mineral phase, constituting up to 15% of the modal composition. Garnet, muscovite and tourmaline occur as accessory phases in many plutons.The CVG augen–gneisses have high SiO2 (>71%) and alkali contents, they vary from slightly peraluminous to slightly metaluminous, and from slightly magnesian to typical ferrroan rocks. In the migmatized orthogneisses the SiO2 contents are usually <70%. Trace element variations in the CVG are extensive, reflecting the migmatization recorded in some plutons and/or distinct sources. They are Ca-, Sr- and Nb-poor, showing variable Ba (100–1260 ppm), Rb (164–400 ppm) and Zr (144–408 ppm) contents, and high abundances of Y (>40 ppm). The chondrite normalized REE patterns are characterized by strong to moderate negative Eu anomalies (Eu/Eu* = 0.23–0.70). In general, the spidergram patterns show deep troughs at Ti, P, Ba and Sr and less pronounced Nb–Ta troughs. These patterns are similar to those reported for anorogenic granites evolved from mixtures of magmas from both crustal and mantle sources. The CVG exhibit TDM model ages ranging from 1.9 to 1.1Ga, with slightly negative to slightly positive ƐNd(t) values, suggesting the involvement of distinct proportions of mantle and crustal components in the source of their protoliths.There is no consensus in the literature about the tectonic setting of the CVG ie they have been related to either continental margin magmatic arc, with possible back–arc association, or extention-related setting, with generation of A-type granites. However, all the available geochemical data suggest that the CVG represent extension related magmatism. The geochemical signature associated to bimodal volcanism, including pyroclastic rocks, with similar ages, and absence, up to now, of evidence for metamorphism of Tonian age, support the hypothesis of extension - related magmatism.

Mesoproterozoic continental arc magmatism and crustal growth in the eastern Central Tianshan Arc Terrane of the southern Central Asian Orogenic Belt: Geochronological and geochemical evidence

Numerous microcontinents are known to occur in the Central Asian Orogenic Belt (CAOB), one of the largest accretionary orogens and the most significant area of Paleozoic crustal growth in the world. The evolution of the Precambrian crust in these microcontinents is central to understanding the accretionary and collisional tectonics of the CAOB. Here, we present systematic zircon U–Pb dating and Hf isotope studies of Mesoproterozoic gneissic granitoids from the eastern Central Tianshan Arc Terrane (CTA) of the southern CAOB. The investigated intermediate to felsic (SiO2=60.48–78.92wt.%) granitoids belong to the calcic- to calc-alkaline series and usually have pronounced negative Nb, Ta and Ti anomalies, relative enrichments of light rare earth elements (LREEs) and large ion lithophile elements (LILEs) while heavy rare earth elements (HREEs) and high field strength elements (HFSEs) are depleted, revealing typical active continental margin magmatic arc geochemical characteristics. These spatially-distant rocks show consistent zircon U–Pb crystallization ages from ca. 1.45 to 1.40Ga and thus constitute a previously unknown Mesoproterozoic continental magmatic arc covering hundreds of kilometers in the eastern segment of the CTA. Furthermore the high and mainly positive zircon εHf(t) values between −1.0 and +8.6 and the zircon Hf model ages of 1.95 to 1.55Ga, which are slightly older than their crystallization ages, suggest that they were mainly derived from rapid reworking of juvenile material with a limited input of an ancient crustal component. Therefore, the formation of these granitoids defines an extensive Mesoproterozoic intermediate to felsic, subduction-related intrusive magmatic arc activity that was active from at least 1.45 to 1.40Ga, involving significant juvenile continental growth in the eastern segment of the CTA. Furthermore the zircon U–Pb and Hf isotopic data challenge the common belief that the CTA was part of the Tarim Craton during Paleo- to Mesoproterozoic times. However, the southern Beishan microcontinents and the CTA have similar zircon U–Pb and Hf isotopic signatures, indicating a similar Precambrian tectonothermal evolution for both. This finding provides new and important constraints on the Precambrian crustal evolution of the microcontinents in the southern CAOB.

The Afeição augen-gneiss Suite and the record of the Cariris Velhos Orogeny (1000–960 Ma) within the Riacho do Pontal fold belt, NE Brazil

Early Tonian (∼1000–920 Ma) rocks occur within the Transversal Zone of the Borborema Province in Northeast Brazil comprising the 700 km-long sigmoidal Cariris Velhos belt. The Afeição augen-gneiss Suite crops out in the internal zone of the Riacho do Pontal fold belt, about 100 km southwestward of the closest Cariris Velhos occurrence within the Transversal Zone, and has been proposed to represent a continuation of this belt within the Southern subprovince of the Borborema Province. Several plutons included within this unit intrude or are thrust upon metavolcanosedimentary sequences of three different units (Santa Filomena, Paulistana and Morro Branco complexes). The Afeição Suite is composed mostly of calc-alkaline, high-K, peraluminous, high-silica ferroan and magnesian granites. Chondrite-normalized REE patterns are moderate to highly fractionated, with a pronounced negative Eu anomaly. Incompatible element spidergrams show a negative Nb–Ta anomaly, akin to convergence setting (Cordilleran-type) granites. U–Pb zircon data constrain the age of crystallization between 1000 and 960 Ma, thus confirming chrono-correlation with the Cariris Velhos belt. Values of εNd(t) between −1.0 and +3.1 and TDM of 1.2–1.5 Ga, similar to other Cariris Velhos occurrences, suggest variable mixing of Tonian juvenile sources with older crustal sources, the latter involving Archean/Paleoproterozoic basement. Although there are currently no reliable geochronological data for the supracrustal sequences of the internal zone, cross-cutting relationships indicate that the Santa Filomena and Morro Branco complexes are older or of similar age to the Afeição Suite, and thus could be related to the Cariris Velhos Orogeny. Based on petrographic, lithogeochemical, geochronological and isotope data, and according to previous models proposed for the Cariris Velhos belt, we interpret the Afeição Suite as the southwestern edge of a continental margin magmatic arc accreted to this portion of West Gondwana during the Early Tonian. Late-Brasiliano dextral displacement through the western branch of the Pernambuco shear zone separated these Cariris Velhos occurrences from its equivalents within the Transversal Zone of the Borborema Province (Recanto-type augen-gneiss of the Alto Pajeú Terrane).

Tectonic significance of Late Ordovician granitic magmatism and clastic sedimentation on the northern margin of Gondwana (Tavşanlı Zone, NW Turkey)

Metasedimentary schists and a small crosscutting metagranite underlie a Mesozoic metamorphosed carbonate platform within the regional Anatolide tectonic belt of northern Turkey. These lithologies are inferred to have formed part of the northern margin of Gondwana during the Palaeozoic. Immobile element geochemistry suggests that the country rock metasediments were derived from upper continental crust and arc-type magmatic rocks. Major and trace element analyses of the metagranite indicate affinities with upper continental crust or a continental margin magmatic arc. Tectonic discrimination of the metagranite is consistent with a rift or post-collisional setting. Niobium depletion relative to Ce on primitive mantle-normalized spidergrams suggests a subduction influence, possibly inherited from Late Precambrian (Cadomian) arc magmatism. Ion microprobe U–Pb dating of zircons from the metagranite yielded a crystallization age of 446 ± 8 Ma (Late Ordovician). An inherited core gave a concordant age of c . 578 Ma, consistent with Cadomian or Pan-African basement. Regional comparisons indicate that the Palaeozoic lithostratigraphy is similar to that of the Taurides (Gondwana) rather than the Pontides (Laurasia). We infer Mid- to Late Ordovician crustal extension along the north Gondwana margin, followed by spreading of Palaeotethys. The entire stratigraphy experienced high-pressure/low-temperature metamorphism during Late Cretaceous closure of the İzmir–Ankara–Erzincan ocean. Supplementary materials: The mineral composition, fabric, coordinates (UTM) and modal composition of the Beyce schist and Sarıkaya metagranite samples are available at www.geolsoc.org.uk/SUP18556 .

Geology and ages of porphyry and medium- to high-sulphidation epithermal gold deposits of the continental margin of Northeast China

The continental margin of Northeast China, an important part of the continental margin-related West Pacific metallogenic belt, hosts numerous types of gold-dominated mineral deposits. Based on ore deposit geology and isotopic dating, we have classified hydrothermal gold–copper ore deposits in this region into four distinct types: (1) gold-rich porphyry copper deposits, (2) gold-rich porphyry-like copper deposits, (3) medium-sulphidation epithermal copper–gold deposits, and (4) high-sulphidation epithermal gold deposits. These ore deposits formed during four distinct metallogenic stages or periods, at 123.6 ± 2.5 Ma, 110–104 Ma, 104–102 Ma, and 95.0 ± 2 Ma, corresponding to periods of Cretaceous intermediate–acid volcanism and late-stage emplacement of hypabyssal magmas along the northern margin of the North China platform. The earliest stage of mineralization (123.6 ± 2.5 Ma) corresponds to the formation of medium-sulphidation epithermal copper – gold deposits and was associated with a continental margin magmatic arc system linked to subduction of the Pacific Plate beneath the Eurasia. This metallogenesis is closely related to high-K calc-alkaline intermediate–acid granite and pyroxene – diorite porphyry magmatism. The second and third stages of mineralization in the study area (110–104 Ma and 104–102 Ma, respectively) correspond to the formation of gold-rich porphyry copper, porphyry-like copper, and high-sulphidation gold deposits, with metallogenesis closely related to sodic or adakitic magmatism. These magmas formed in a continental margin magmatic arc system related to oblique subduction of the Pacific Plate beneath the Eurasia, as well as mixing of crust-derived remelted granitic and mantle-derived adakitic magmas. During the final stage of mineralization (95.0 ± 2 Ma), metallogenesis was closely related to sodic or adakitic magmatism, with diagenesis and metallogenesis related to the disintegration or destruction of the Pacific Plate, which was subducted beneath the Eurasian Plate during the Mesozoic.

The Cariris Velhos tectonic event in Northeast Brazil

The Borborema Province in northeastern South America is a typical Brasiliano-Pan-African branching system of Neoproterozoic orogens that forms part of the Western Gondwana assembly. The province is positioned between the São Luis-West Africa craton to the north and the São Francisco (Congo-Kasai) craton to the south. For this province the main characteristics are (a) its subdivision into five major tectonic domains, bounded mostly by long shear zones, as follows: Médio Coreaú, Ceará Central, Rio Grande do Norte, Transversal, and Southern; (b) the alternation of supracrustal belts with reworked basement inliers (Archean nuclei + Paleoproterozoic belts); and (c) the diversity of granitic plutonism , from Neoproterozoic to Early Cambrian ages, that affect supracrustal rocks as well as basement inliers. Recently, orogenic rock assemblages of early Tonian (1000–920 Ma) orogenic evolution have been recognized, which are restricted to the Transversal and Southern domains of the Province. Within the Transversal Zone, the Alto Pajeú terrane locally includes some remnants of oceanic crust along with island arc and continental arc rock assemblages, but the dominant supracrustal rocks are mature and immature pelitic metasedimentary and metavolcaniclastic rocks. Contiguous and parallel to the Alto Pajeú terrane, the Riacho Gravatá subterrane consists mainly of low-grade metamorphic successions of metarhythmites, some of which are clearly turbiditic in origin, metaconglomerates, and sporadic marbles, along with interbedded metarhyolitic and metadacitic volcanic or metavolcaniclastic rocks. Both terrane and subterrane are cut by syn-contractional intrusive sheets of dominantly peraluminous high-K calc-alkaline, granititic to granodioritic metaplutonic rocks. The geochemical patterns of both supracrustal and intrusive rocks show similarities with associations of mature continental arc volcano-sedimentary sequences, but some subordinate intra-plate characteristics are also found. In both the Alto Pajeú and Riacho Gravatá terranes, TIMS and SHRIMP U–Pb isotopic data from zircons from both metavolcanic and metaplutonic rocks yield ages between 1.0 and 0.92 Ga, which define the time span for an event of orogenic character, the Cariris Velhos event. Less extensive occurrences of rocks of Cariris Velhos age are recognized mainly in the southernmost domains of the Province, as for example in the Poço Redondo-Marancó terrane, where arc-affinity migmatite-granitic and meta-volcano-sedimentary rocks show U–Pb ages (SHRIMP data) around 0.98–0.97 Ga. For all these domains, Sm–Nd data exhibit T DM model ages between 1.9 and 1.1 Ga with corresponding slightly negative to slightly positive ε Nd ( t ) values. These domains, along with the Borborema Province as a whole, were significantly affected by tectonic and magmatic events of the Brasiliano Cycle (0.7–0.5 Ga), so that it is possible that there are some other early Tonian rock assemblages which were completely masked and hidden by these later Brasiliano events. Cariris Velhos processes are younger than the majority of orogenic systems at the end of Mesoproterozoic Era and beginning of Neoproterozoic throughout the world, e.g. Irumide belt, Kibaride belt and Namaqua-Natal belt, and considerably younger than those of the youngest orogenic process (Ottawan) in the Grenvillian System. Therefore, they were probably not associated with the proposed assembly of Rodinia . We suggest, instead, that Cariris Velhos magmatism and tectonism could have been related to a continental margin magmatic arc, with possible back-arc associations, and that this margin may have been a short-lived (<100 m.y.) leading edge of the newly assembled Rodinia supercontinent .