Intra-arc Rifting Research Articles

Age and tectonic setting of Mesozoic extension constrained by the first volcanic events in the Eastern Cordillera and Middle Magdalena Valley, Colombia

Before the rise of the Northern Andes in Cenozoic time, Triassic to Jurassic extensional basins in northwestern South America accommodated predominantly continental strata partly intercalated with volcanic rocks. Coeval plutonism is attributed to a magmatic arc related to the subduction of the Farallon plate beneath South America. The basins later became involved in the Andean orogeny and are now partially exposed in the Eastern Cordillera and Middle Magdalena Valley of Colombia. We have employed (U/Pb) geochronology on zircons from Triassic-Jurassic felsic to intermediate volcanic and volcaniclastic rocks. Most of the ten samples have a substantial proportion of detrital zircons, but only three had no Mesozoic grains. The Mesozoic ages obtained range from ca. 201 Ma to ca. 177 Ma and overlap with published crystallization ages (K/Ar; Ar/Ar; U/Pb) from plutonic bodies. Volcanics from the Jordán and Girón formations are latest Triassic to Early Jurassic and synchronous with major plutonic activity. These ages constrain the early evolution of the extensional basins that formed from about the Triassic-Jurassic transition in an intra-arc position and facilitated the preservation of sediment and arc-derived volcanics. Middle Jurassic ages from the Noreán Fm. are synchronous with sparse plutonism west of the Middle Magdalena Valley. At this time, the magmatic arc had migrated westward, while intrusive activity in the Eastern Cordillera ceased. A geochemical rift signature only appears in scarce Early Cretaceous mafic intrusions that resumed magmatic activity in the Eastern Cordillera. This magmatism, now in a back-arc position, coincides with maximum subsidence of the large Cretaceous basin that extended across the older intra-arc rift basins. Extension and lithospheric thinning ceased by the end of the Early Cretaceous.Graphical abstract

Petrogenesis and Geodynamic Evolution of the Archean Shawmere Anorthosite Complex and Associated Gneisses, Kapuskasing Uplift, Superior Province, Canada

Abstract In this study, we integrated extensive field, petrographic, whole-rock major and trace element, and Nd–Pb–Sr–O isotope, and zircon U–Pb ages, trace element and Lu–Hf isotope data from the Neoarchean Shawmere Anorthosite Complex and surrounding gneisses to unravel their petrogenetic origin and tectonic history. The ~2765 Ma Shawmere Anorthosite Complex is interpreted to have been emplaced into a sequence of interlayered greywacke and basalt deposited in an intra-continental arc rift system above a north-dipping subduction zone. The complex consists mainly of anorthosite, leucogabbro, gabbro, and hornblendite that were emplaced as several batches of magmas and crystal mushes originating from sub-arc mantle sources. In contrast to the previous studies, our field and petrographic data suggest an igneous origin for the most hornblende in the complex, implying hydrous parental magmas. A hydrous magma origin is also consistent with the high-anorthite content (mostly 70–90%) of the plagioclase in the complex. Percolation of hydrous basaltic melts through gabbroic cumulates in crustal magma chambers led to extensive (>50%) replacement of igneous clinopyroxene by igneous hornblende. Continued subduction resulted in the closure of the intra-arc rift system and the intrusion of the complex by tonalite, granodiorite and diorite between 2765 and 2680 Ma in an Andean-type margin. The complex and surrounding gneisses underwent hornblende granulite-facies metamorphism mainly between 2680 and 2620 Ma, overlapping with mid-crustal east-west extension between 2660 and 2640 Ma. The granulite-facies metamorphism is recorded by the replacement of hornblende, plagioclase and clinopyroxene by garnet and the development of a garnet-orthopyroxene-plagioclase metamorphic assemblage with a granoblastic texture. Tectonic rebounding of mid-crustal rocks to upper crustal levels after 2620 Ma led to the formation of an extensive network of extensional fractures and retrograde metamorphism. Migration of CO2-rich hydrous fluids along the extensional fractures and grain boundaries resulted in the precipitation of many metasomatic minerals mainly at the expense of hornblende and plagioclase, including epidote, clinozoisite, tremolite, actinolite, paragonite, margarite, titanite, quartz, calcite, sillimanite, dolomite, and chlorite. Prevalent replacement of hornblende by garnet during prograde metamorphism and metasomatic replacement of hornblende and plagioclase by retrograde mineral assemblages disturbed the Sm–Nd, U–Th–Pb, and Rb–Sr isotope systems.

Intra-arc rifting induced the fragmentation of microplate from the continental margin

Although abundant Neoproterozoic igneous rocks exposed along the periphery of the Yangtze Block of the South China Craton record the tectonic processes of the assembly and breakup of the Rodinia supercontinent, the mechanism that induced the rifting of the South China Craton from Rodinia and reconstruction of the paleogeographic position of the South China Craton in Rodinia remain controversial. We document two episodes of bimodal intrusions with emplacement ages of ca. 785 Ma and ca. 750 Ma in the Diancangshan Massif that provide critical constraints on the rifting processes and mechanisms. The rock association and the elemental and Hf-O isotopic signatures of these two episodes of bimodal intrusions illustrate that they are an important component of the Panxi-Hannan magmatic arc in the western Yangtze Block. The spatial pattern of Neoproterozoic sedimentary rocks to the west of the Panxi-Hannan arc and their provenance indicate that intra-arc rifting promoted the separation of microplates such as the Xuelongshan and Diancangshan massifs and even the Yidun Block from the western Yangtze Block and the mechanism induced synchronous deposits in these regions.

Sub-arc Mantle Heterogeneity of the Northern Luzon Volcanic Arc: Mineral and Whole Rock Compositional Variability in Mantle Xenoliths from Lutao Island

Abstract Mantle xenoliths hosted in volcanic rocks from the island of Lutao offer a glimpse into the nature of the mantle beneath the northern Luzon volcanic arc. The xenoliths are spinel-bearing and composed mostly of harzburgite with one lherzolite and one olivine orthopyroxenite. The olivine (Fo92.5–88.9), orthopyroxene (Mg# = 94.6–89.2), and clinopyroxene (Wo49.1–38.1En57.0–45.4Fs3.0–11.0) compositions are similar to those of abyssal peridotites. The spinel compositions are variable and can be principally divided into high-Al (Cr# < 45) and low-Al (Cr# > 45) groupings. The whole rock compositions are similar to abyssal peridotite (Al2O3 = 0.95–2.07 wt %; Mg# = 88.5–90.9) and have U-shaped chondrite normalized rare earth element patterns. The Sr-Nd isotopes of the xenoliths are broadly chondritic (87Sr/86Sri = 0.704400–0.707908; εNd(t) = 0.0 − +1.5). The two-pyroxene equilibrium temperatures range from 900 to 1200 °C with the majority of temperature estimates >1000 °C. The olivine-orthopyroxene-spinel oxygen barometry estimates yielded ΔFMQ values from 0 to +2 and correspond to moderately oxidizing to oxidizing conditions. The xenoliths are likely derived from the Philippine Sea Plate lithospheric mantle that was modified by melt extraction and/or fluid enrichment processes. Trace element and isotopic mixing modeling indicate that 1–2% contamination by subducted South China Sea sediment can explain the Sr-Nd isotopic enrichment and Th and U elemental variability within the xenoliths assuming an initial composition similar to enriched depleted mid-ocean ridge mantle (E-DMM). The anomalously high two-pyroxene equilibrium temperatures of the Lutao xenoliths relative to other regions of the northern Luzon volcanic arc (Iraya <1000 °C) indicate that they were affected by a high-temperature event that was likely a consequence of recent intra-arc rifting that occurred after collision (<6 Ma) between the Luzon arc and the Eurasian margin.

Geochronological, geochemical and isotopic characterisation of the Early Cretaceous Andean margin within Ecuador: The Alao Arc

Tectonic models of the Early Cretaceous Andean margin in Ecuador are hindered by a paucity of accurate crystallisation ages of igneous rocks. The Alao Arc, situated on the western flank of the Cordillera Real in Ecuador, is mainly composed of submarine altered basalts to andesites, and the most deformed areas are frequently modified by greenschist metamorphism. To improve our knowledge of the evolution of the northern Andes prior to the accretion of the Caribbean Large Igneous Province we attempt to constrain the tectonic origin and crystallisation age of the arc's volcanic rocks, using whole-rock geochemistry and isotopic tracing, combined with 40Ar/39Ar geochronology. Major oxides and trace element abundances and Sr-Nd-Pb compositions show that the Alao Arc comprises two separate assemblages, namely, typical arc-like volcanic rocks and E-MORB-like basalts. These have been interpreted to have formed in a continental arc above thinned continental crust in an extensional setting, and the E-MORB-type basalts are considered to have formed in an intra-arc rift. As the Alao Arc's volcanic rocks are devoid of zircon and baddeleyite we attempted to date the time of their crystallisation using 40Ar/39Ar analyses of whole-rock groundmass and plagioclase. However, as the igneous rocks are modified by varying degrees of alteration with the formation of secondary K- and Ca-bearing phases, none of the ∼75 Ma to ∼16 Ma 40Ar/39Ar dates record the age of their crystallisation. The plateau and weighted mean 40Ar/39Ar date populations define peaks at ∼75–60 Ma and ∼ 45–30 Ma that coincide with previously recognised tectonic events that drove fluid circulation, modifying the Ar isotopic compositions. Based on the maximum depositional ages of intercalated sedimentary rocks, the degree of deformation, geochemical and isotopic trends of the Jurassic arc in Ecuador, and correlations with the Early Cretaceous Quebradagrande Arc of Colombia, we propose that the Alao Arc formed during ∼140–110 Ma.

The genetic relationship between coeval Ediacaran mafic and granitoid plutons in the Antigonish Highlands, Nova Scotia

Abstract Granitic plutons dominated by felsic-intermediate compositions are commonly spatially and temporally associated with mafic intrusions; however, the genetic relationship between the apparently coeval but compositionally dissimilar magmas is controversial. To better understand this relationship, we present new lithogeochemical and isotopic data from coeval late Neoproterozoic plutonic rocks in the Antigonish Highlands of Nova Scotia where the regional context is well documented. The predominantly mafic Greendale Complex contains lamprophyre, appinites and leucocratic dykes. The appinites are remarkably variable in their textures, and consist of hornblende pegmatites, hornblende cumulates, porphyritic hornblende gabbro and coarse-grained, equigranular hornblende gabbro. Geochemical data show enrichment in large-ion lithophile elements and depletion in the high field strength elements suggesting an arc setting. ε Nd (607) values from the Greendale Complex range from +3.2 to +5.0 and are on average slightly more juvenile than coeval granitic rocks which have petrological characteristics typical of continental arc magmas. Hydrous mafic magmas were likely contaminated by subducted sediments and their ascent was facilitated by lithospheric-scale faults. Felsic magmas were derived by anatexis of heterogeneous Avalonian crust that oscillated between fluid-saturated to fluid-deficient (dehydration) melting, consistent with the evolution from arc to intra-arc rift environment.

Iron isotope heterogeneity in magmas of subduction zones and its origin

Iron isotope heterogeneity of subduction-related primitive magmas was traditionally thought to be predominantly attributed to the addition of slab-derived components. However, the specific subducted material responsible for inducing Fe isotope heterogeneity in the subarc-backarc mantle remains poorly constrained. Here we report Fe isotopic compositions of a series of backarc basin basalts (BABBs) from the Woodlark Basin (an intra-arc rift basin), Vate Trough (a nascent rear-rift basin), and Lau Basin (a relatively mature basin) in the southwestern Pacific. Even though these BABBs have various geochemical compositions ranging from mid-ocean ridge basalts (MORB)-like to arc-like, which should be affected by various amounts of the slab-derived fluids and/or sediment melts, their δ56Fe values (0.05–0.18‰) are within the range of global MORB (0.05–0.17‰). After correction for crystal fractionation, the primitive δ56Fe values (δ56Fe-prim) displayed no co-variation with indicators of subduction contributions (e.g., Ce/Pb, Nb/U, Ba/La, and Th/Yb) or source redox conditions caused by slab inputs (e.g., V/Yb, V/Ti), suggesting that the δ56Fe-prim values of these BABBs are rarely affected by subduction metasomatism and the change in oxygen fugacity caused by subducted input. Therefore, we proposed that the MORB-like δ56Fe values of the studied BABBs were primarily caused by mantle melting processes. Furthermore, we presented a compilation of available Fe isotopic data for BABBs and island arc basalts (IABs) and used these data to evaluate Fe isotope heterogeneity in subduction settings. In particular, compared with δ56Fe-prim of MORB, the δ56Fe-prim of BABBs from the Central Lau Spreading Centre in the Lau Basin and the IABs have lower values, whereas BABBs from the Rochambeau Ridges in the Lau Basin have higher δ56Fe-prim values. By comprehensively considering the factors affecting the variation in Fe isotopic compositions in magmatic rocks in subduction zones (e.g., crystal fractionation, partial melting of the mantle, redox conditions, and subduction-related metasomatism), we proposed that the varied Fe isotopic compositions in subduction settings could reflect their heterogeneous sources, which may be related to plate subduction. Specifically, during plate subduction, a melt-depleted refractory forearc mantle peridotite (with low δ56Fe values) could be dragged by subducted slab into subarc or backarc depths, resulting in light Fe isotopic enrichment in island arc magmas or BABBs; additionally, serpentinites (both in the slab and mantle wedge) can generate light Fe isotopic enrichment in island arc magmas. High δ56Fe values in the BABB (e.g., from the Rochambeau Ridges in the Lau Basin) may result from the toroidal influx of nearby upwelling plume components or the assimilation of hydrothermally altered oceanic crust.

Correlation between South China and India and development of double rift systems in the South China–India Duo during late Neoproterozoic time

Abstract South China, India, and their derivative blocks preserve many similar magmatic and sedimentary records related to the tectonic transition from Rodinia to Gondwana. They provide crucial insights into not only the paleogeographic correlation between them but also the geodynamic mechanism for such a transition. Our new results, combined with published data from these blocks, reveal that South China remained linked with India at least from ca. 830 Ma to ca. 510 Ma and formed the South China–India Duo, which is located at the western margin of Rodinia. The identical magmatism and sedimentation reflect that double late Neoproterozoic rift systems in the South China–India Duo developed owing to the rollback of subducting oceanic slab beneath them. For example, an intracontinental rift developed along the Jiangnan–Aravalli–Delhi fold belt, which separated the Yangtze-Marwar block from the Cathaysia-Bundelkhand block. Another intra-arc rift developed contemporaneously along the northern and western margins of the Yangtze block, through the Marwar terrane of western India, and then into the Seychelles and Madagascar terranes. Such an intra-arc rift is the most feasible explanation for the common development of coeval arclike and extension-related magmatic rocks and extensional sedimentary sequences on the western margin of the South China–India Duo, in Seychelles and Madagascar, and even at other subduction zones. South China was finally separated from Indian Gondwana at ca. 510 Ma due to the opening of the Proto-Tethys Ocean.

Massive earthquake swarm driven by magmatic intrusion at the Bransfield Strait, Antarctica

An earthquake swarm affected the Bransfield Strait, Antarctica, a unique rift basin in transition from intra-arc rifting to ocean spreading. The swarm, counting ~85,000 volcano-tectonic earthquakes since August 2020, is located close to the Orca submarine volcano, previously considered inactive. Simultaneously, geodetic data reported up to ~11 cm northwestward displacement over King George Island. We use a broad variety of geophysical data and methods to reveal the complex migration of seismicity, accompanying the intrusion of 0.26–0.56 km3 of magma. Strike-slip earthquakes mark the intrusion at depth, while shallower normal faulting the ~20 km long lateral growth of a dike. Seismicity abruptly decreased after a Mw 6.0 earthquake, suggesting the magmatic dike lost pressure with the slipping of a large fault. A seafloor eruption is likely, but not confirmed by sea surface temperature anomalies. The unrest documents episodic magmatic intrusion in the Bransfield Strait, providing unique insights into active continental rifting.

Setting and formation of the earliest Neoproterozoic rifted arc Pingshui VMS deposit, South China

The Neoproterozoic Pingshui volcanogenic massive sulfide (VMS) deposit (Cu and Zn metals of 0.40 Mt) represents one of the best preserved VMS deposits in the northeastern margin of the Jiangnan Orogen, South China. This study reconstructs its magmatic and metallogenetic history and geodynamic evolution using new geological, whole-rock geochemical, and zircon U–Pb–Lu–Hf isotopic analyses. Two lithotectonic units are identified: the ore-hosting Pingshui Formation composed of a bimodal basalt-rhyolite suite and pyroclastic rocks, and the post-ore extrusive-intrusive assemblage including high-Mg basalt, Nb-enriched basalt, high-Mg diorite and felsic granitoids. The formation age of Pingshui Formation basalt to andesite is newly constrained at ∼977 ± 15 Ma by the least radiation-damaged zircon. The new and compiled data suggest that these post-ore rocks were generated during ∼932–904 Ma after a possible magmatic quiescence. In terms of geochemical characteristics, both the Pingshui Formation basalt to andesite (∼977 Ma) and late high-Mg basalt (∼921 Ma) are enriched in light rare earth elements (LREEs) and depleted in high field strength elements (HFSEs, e.g., Nb, Ta, P, Ti, Zr and Hf). The late high-Mg diorite (∼932 Ma, MgO = 5.84–6.91%) has moderate (La/Yb)N ratios (13.7–14.1), resembling those of high-Mg andesites (HMA). These geochemical signatures indicate that they originated from a heterogeneous mantle wedge metasomatized by slab-derived fluids and melts. The Nb-enriched basalt (∼916 Ma) was derived from a mixture of OIB- and arc-like mantle sources as evidenced by relatively high Ti/V, Ti/Zr and Zr/Y ratios. The geochemical changes from a dominant subduction influence to asthenospheric involvement, corresponding to the mafic-intermediate rocks (977–921 Ma) and the Nb-enriched basalt (916 Ma) respectively, is indicative of a possible slab roll-back that triggered asthenospheric upwelling. Furthermore, the felsic rocks originated from partial melting of juvenile mafic lower crust or fractional crystallization of parental mafic melts. Their anomalously high temperatures of formation (over 950 °C), which are indicated by zircon saturation thermometry, suggest a long-lasting high heat flow environment. In summary, subduction of young oceanic crust and intra-arc rifting are likely mechanisms to account for the formation of the high temperature magmatic rocks and massive sulfide ores at Pingshui.

Hafnium isotopes in zircons track the changing sources for the Late Cretaceous magmatism in Eastern Srednogorie, Bulgaria

We present new Hf isotopic data of magmatic zircons from the Eastern Srednogorie zone. The data outline two clear temporal trends: rising initial εHf from the initiation of the magmatism at ~95 Ma to 81 Ma, followed by a rapid decline in the initial εHf in the 81–78 Ma time period. The first trend highlights the increasing participation of mantle melts in the formation of magmatic products in the East Balkan and Strandzha regions, which is likely dictated by the southward retreat of the subducting slab. This trend is also evident in published Hf isotopic data on zircons from Central Srednogorie zone. The second trend of rapidly decreasing initial εHf of zircons is interpreted to reflect increased proportion of lower crustal melts in an intra-arc rift extensional environment (the Yambol-Burgas region) between 81 and 78 Ma; this trend is not observed in the Central Srednogorie zone.

Devonian to Triassic tectonic evolution and basin transition in the East Kunlun–Qaidam area, northern Tibetan Plateau: Constraints from stratigraphy and detrital zircon U–Pb geochronology

ABSTRACT The late Paleozoic to Triassic was an important interval for the East Kunlun–Qaidam area, northern Tibet, as it witnessed prolonged subduction of the South Kunlun Ocean, a major branch of the Paleo-Tethys Ocean whose closure led to the formation of Pangea. However, the geologic history of this stage is poorly constrained due to the paucity of tectonothermal signatures preserved during a magmatic lull. This article presents a set of new provenance data incorporating stratigraphic correlation, sandstone petrology, and zircon U–Pb dating to depict changes in provenance that record multiple stages of topographic and tectonic transition in the East Kunlun–Qaidam area over time in response to the evolution of the South Kunlun Ocean. Devonian intra-arc rifting is recorded by bimodal volcanism and rapid alluvial-lacustrine sedimentation in the North Qaidam Ultra High/High Pressure Belt, whose sources include the Olongbuluke Terrane and southern North Qaidam Ultra High/High Pressure Belt. Southward transgression submerged the East Kunlun–Qaidam area during the Carboniferous prior to the rapid uplift of the Kunlun arc, which changed the provenance during the Early Permian. This shift in provenance for the western Olongbuluke Terrane and thick carbonate deposition throughout the North Qaidam Ultra High/High Pressure Belt in the late Early Carboniferous indicate that the North Qaidam Ultra High/High Pressure Belt should have been inundated, terminating an ~95 m.y. erosion history. The closure of the South Kunlun Ocean in the late Triassic generated a retroarc foreland along the Zongwulong Tectonic Belt, which is represented by the development of a deep-water, northward-tapering flysch deposystem that was supplied by the widely elevated Kunlun–Qaidam–Olongbuluke Terrane highland. This new scenario allows us to evaluate current models concerning the assembly of northern Tibet and the tectonic evolution of the Paleo-Tethys Ocean.

Exploration-resource assessment of productive felsic volcanic centers in the Paleoproterozoic Penokean Volcanic Belt of northern Wisconsin, Michigan and east-central Minnesota, USA

Five productive felsic volcanic centers have been identified in the Paleoproterozoic Penokean Volcanic Belt (PVB). Each center hosts at least one potentially economic polymetallic volcanogenic massive sulfide (VMS) deposit. To date, these drill-defined (inferred + indicated + measured) resources collectively contain over 93 million tonnes of VMS-style, base (Cu, Pb, Zn) and precious metal (Ag, Au) mineralization including nearly 4 million ounces of gold. This does not include smaller, less defined (mostly inferred) deposits in the belt that would increase the total to at least 150 million tonnes. From this resource base only 1.73 million tonnes of actual ore production has occurred. However, previous positive feasibility studies completed on the Crandon (1994), Lynne (1995) and most recently (2020) Back Forty deposits verify a total of approximately 44 million tonnes, or 47% of the total drill-defined resource base, are potentially minable reserves. Currently the PVB resource base remains as unrealized new wealth.Mineral resource assessments completed in 1996 and 2007 collectively suggest there is a high probability of at least 4 to 22 (average ~13) or more VMS deposits remaining to be discovered in the Ladysmith-Rhinelander complex and to a lesser extent the Wausau complex of the PVB. These newly discovered VMS deposits will fill out the tonnage distribution for this belt by following a natural geometric (log normal) progression consistent with established grade-tonnage models. It is postulated that among these will be one giant (>100 million tonnes) deposit. If verified, potentially economic VMS mineralization hosted by the PVB would be comparable to other productive Paleoproterozoic greenstone belts in the Canadian Shield such as Flin Flon (>200 million tonnes) or even approach that of the prolific Neoarchean Abitibi (~775 million tonnes).Exploration to date suggests that many of these undiscovered VMS deposits will be identified in five, highly permissive felsic volcanic centers. These centers, which comprise less than 2% of the known belt, are distributed within the Highway 8 intra-arc rift basin of the 1,880 to 1,860 Ma old Ladysmith Rhinelander volcanic complex. Exact aerial boundaries are poorly defined. However, extrapolation of geophysical, drill hole and rare outcrop data provides a reasonable estimate. Each productive center is stratigraphically associated with major, district-scale, commonly mineralized, meta-argillite formations or formational groups. Felsic centers are identified in the Wausau complex but have thus far been barren of significant VMS-style mineralization as is the adjacent Wausau basin.Future exploration for new VMS deposits should be focused within the productive felsic centers where a number of prospective targets and prospects have been previously identified. Other significant felsic centers likely exist within the Highway 8 basin particularly in Cambrian sandstone-covered areas along the western and eastern margins of the belt. Additionally, at least one felsic center may be present in the Milaca complex which could have VMS potential.Of particular note is the VMS potential of a second mostly Cambrian sandstone-covered, greenstone belt in the Marshfield subterrane (Eau Claire complex). Geophysical data suggest possible back-arc or intra-arc rift basins may be present and historical drilling has identified at least one VMS occurrence. This highly prospective volcanic complex is essentially new hunting ground with its own deposit tonnage distribution of potentially supergene-enriched VMS deposits. Based on criteria set out in a 2007 mineral assessment, this belt has the potential to host at least 1 to a maximum of 15 deposits.

Intra-oceanic submarine arc evolution recorded in an ~1-km-thick rear-arc succession of distal volcaniclastic lobe deposits

Abstract International Ocean Discovery Program (IODP) Expedition 351 drilled a rear-arc sedimentary succession ~50 km west of the Kyushu-Palau Ridge, an arc remnant formed by rifting during formation of the Shikoku Basin and the Izu-Bonin-Mariana arc. The ~1-km-thick Eocene to Oligocene deep-marine volcaniclastic succession recovered at Site U1438 provides a unique opportunity to study a nearly complete record of intra-oceanic arc development, from a rear-arc perspective on crust created during subduction initiation rather than supra-subduction seafloor spreading. Detailed facies analysis and definition of depositional units allow for broader stratigraphic analysis and definition of lobe elements. Patterns in gravity-flow deposit types and subunits appear to define a series of stacked lobe systems that accumulated in a rear-arc basin. The lobe subdivisions, in many cases, are a combination of a turbidite-dominated subunit and an overlying debris-flow subunit. Debris flow–rich lobe-channel sequences are grouped into four, 1.6–2 m.y. episodes, each roughly the age range of an arc volcano. Three of the episodes contain overlapping lobe facies that may have resulted from minor channel switching or input from a different source. The progressive up-section coarsening of episodes and the increasing channel-facies thicknesses within each episode suggest progressively prograding facies from a maturing magmatic arc. Submarine geomorphology of the modern Mariana arc and West Mariana Ridge provide present-day examples that can be used to interpret the morphology and evolution of the channel (or channels) that fed sediment to Site U1438, forming the sequences interpreted as depositional lobes. The abrupt change from very thick and massive debris flows to fine-grained turbidites at the unit III to unit II boundary reflects arc rifting and progressive waning of turbidity current and ash inputs. This interpretation is consistent with the geochemical record from melt inclusions and detrital zircons. Thus, Site U1438 provides a unique record of the life span of an intra-oceanic arc, from inception through maturation to its demise by intra-arc rifting and stranding of the remnant arc ridge.

Multistage Magmatism in Ophiolites and Associated Metavolcanites of the Ulan-Sar’dag Mélange (East Sayan, Russia)

We present new whole-rock major and trace element, mineral chemistry, and U-Pb isotope data for the Ulan-Sar’dag mélange, including different lithostratigraphic units: Ophiolitic, mafic rocks and metavolcanites. The Ulan-Sar’dag mélange comprises of a seafloor and island-arc system of remnants of the Paleo-Asian Ocean. Detailed studies on the magmatic rocks led to the discovery of a rock association that possesses differing geochemical signatures within the studied area. The Ulan-Sar’dag mélange includes blocks of mantle peridotite, podiform chromitite, cumulate rocks, deep-water siliceous chert, and metavolcanic rocks of the Ilchir suite. The ophiolitic unit shows overturned pseudostratigraphy. The nappe of mantle tectonites is thrusted over the volcanic-sedimentary sequence of the Ilchir suite. The metavolcanic series consist of basic, intermediate, and alkaline rocks. The mantle peridotite and cumulate rocks formed in a supra-subduction zone environment. The mafic and metavolcanic rocks belong to the following geochemical types: (1) Ensimatic island-arc boninites; (2) island-arc calc-alkaline andesitic basalts, andesites, and dacites; (3) tholeiitic basalts of mid-ocean ridges; and (4) oceanic island basalts. U–Pb dating of zircons from the trachyandesite, belonging to the second geochemical type, yielded a date of 833 ± 4 Ma which is interpreted as the crystallization age during mature island-arc and intra-arc rifting stages. The possible influence of later plume magmatic-hydrothermal activities led to the appearance of moderately alkaline igneous rocks (monzogabbro, trachybasalt, trachyandesite, subalkaline gabbro, and metasomatized peridotites) with a significant subduction geochemical fingerprint.

ROSEN, BULGARIA: A NEWLY RECOGNIZED IRON OXIDE-COPPER-GOLD DISTRICT

AbstractThe Rosen copper veins in southeastern Bulgaria are recognized for the first time as an iron oxide-copper-gold (IOCG) district. The veins are located in the East Srednogorie segment of the Carpathian-Balkan calc-alkaline volcano-plutonic arc and were formed during an end-stage interval of extreme slab rollback and intra-arc rifting, which gave rise farther east to seafloor spreading in the Western Black Sea basin. The resulting submarine volcano-sedimentary rift basin is dominated by intermediate to mafic shoshonitic to ultrapotassic volcanism and subsidiary gabbro to syenite intrusion. The E- to NE-striking veins define a NW-striking alignment along the western contact of the syenite-dominated Rosen pluton, inferred to be part of a large ring dike. More than 40 veins, the most important formerly mined to depths as great as 1,000 m, contain an early, pegmatoidal, calcic-potassic assemblage followed by predominant magnetite (including the mushketovite variety), chlorite, and carbonates but also quartz, chalcopyrite, pyrite, and numerous other metallic minerals, which combine to give an unusual Fe-Cu-Au-Mo-Co-Ni-U-light rare earth element (LREE)-W-Bi-Zn-Pb geochemical signature. The close correlation between Fe, Cu, U, and LREEs is evident even in the flotation tailings. Vein molybdenite was dated during this study at 80.6 ± 0.4 Ma, which is similar to a U-Pb zircon age for monzosyenite from the Rosen pluton. The mineralogic and compositional features of the Rosen district are comparable to those of well-known IOCG deposits worldwide and geometrically similar to the vertically extensive IOCG veins in the Coastal Cordillera province of northern Chile. The subsidiary granitophile signature that accompanies the characteristic siderophile IOCG suite was also recognized recently at the giant Olympic Dam deposit in South Australia and elsewhere. Although no exposed intrusion is definitively implicated in the genesis of the Rosen veins, coexisting gabbro and syenite fluid sources may be hypothesized at depth in or beneath the coeval ring dike.

НОВЫЕ ДАННЫЕ О СОСТАВЕ ИНТРУЗИВНЫХ ПОРОД О. ШИКОТАН (МАЛАЯ КУРИЛЬСКАЯ ГРЯДА)

The article presents new petrologic and geochemical data on gabbroids from Shikotan Island (Lesser Kuril Chain). The results were compared with co-magmatic Late Cretaceous-Paleocene volcanics from the Zelenov Formation, as well as with the rocks from the Shikotan intrusive and Dimitrovsky dike complexes. This data confirmed that the intrusive rocks belong to the calc-alkaline series and the island-arc (suprsubduction) type. Low concentrations of high field strength and heavy rare-earth elements and their ratio in gabbroids relative to the composition of N-MORB suggest formation of primary melts from depleted or poorly enriched deep sources. As such, pyroxenites close to websterites with preserved bipyroxene paragenesis during the evolution of melts are proposed. Steeply dipping contacts of intrusive bodies and elements of their prototectonics, zones of marginal breccia and clastic dikes that sometimes over-ends by injection intrusive material field parallel dikes completion plutonic activity on Malokurilsk arc indicate «formation of Shikotan ophiolites in the setting of supersubducting intra-arc rifting» according to ideas of G.I. Govorov.

Sulfur Isotope Ratios from VMS Deposits in the Penokean Volcanic Belt, Great Lakes Region, USA: Constraints on the Source of Sulfur in a Paleoproterozoic Intra-Arc Rift

The Paleoproterozoic (~1.8 Ga) Penokean Volcanic Belt (PVB) in the Great Lakes Region of North America hosts several polymetallic volcanogenic massive sulfide (VMS) deposits. These deposits were formed by back-arc extensional volcanism during the accretion of island-arc terranes along the southern margin of the Archean Superior Craton. This study reports δ34S values obtained from sulfide minerals collected from eight VMS deposits in the PVB: Back Forty, Bend, Eisenbrey, Flambeau, Horseshoe, Lynne, Reef, and Schoolhouse. The average δ34S values from most of these deposits lie within the mantle-range between −2 and 2‰, relative to Vienna Canyon Diablo Troilite (V-CDT). Average δ34S values from Back Forty and Lynne deposits are slightly higher, at 2.5 and 2.4‰, respectively. No systematic variation in δ34S was observed based on factors such as the kind of sulfide mineral, ore-texture, type of host rock, or the nature of host-rock alteration. The narrow observed range from the PVB offers a clear indication that sulfur in the mineralizing fluid, originated predominantly from a magmatic source. If there was a significant contribution of sulfur from seawater, the δ34S of seawater sulfur must also have been close to the mantle range. Slightly higher values from Back Forty and Lynne indicate minor involvement of oxidized sulfur at shallow water levels, possibly derived from the continental margin.

In search of Gondwana heritage in the Outer Melanesian Arc: no pre-upper Eocene detrital zircons in Viti Levu river sands (Fiji Islands)

Volcanic arcs of the Southwest Pacific, collectively referred to as the Outer Melanesian Arc, are generally thought to result from subduction of the Pacific Plate since the Late Cretaceous. Meanwhile, it is largely accepted that eastward roll-back of the old and dense oceanic plate allowed opening of marginal basins, which isolated large blocks of the former Gondwana margin. Incidentally, some ‘intra-oceanic’ volcanic arcs may have been nucleated on small continental fragments. Detrital zircons collected from sand banks in the mid-reaches of rivers from Viti Levu Island have been analysed for U–Pb geochronology and geochemistry, in order to search for a possible ancient continental arc basement, remnants of a Late Cretaceous arc, and determine the timing and evolution of Fiji arc magmatism. In contrast with some other places of the Outer Melanesian Arc (Solomon, Vanuatu), no pre-upper Eocene zircons have been found. Thus, Gondwana-derived fragments or Late Cretaceous–Paleocene arc remnants are unlikely to form the basement of Viti Levu. Zircon geochemistry confirms the purely intra-oceanic character of volcanic-arc magmatism as well. Variations in some trace-element ratios closely reflect the evolution of Viti Levu Arc from upper Eocene inception to upper Miocene climax and finally Pliocene intra-arc rifting and abandonment.

Paleostress and fluid-pressure regimes inferred from the orientations of Hishikari low sulfidation epithermal gold veins in southern Japan

The orientation distribution of dilational fractures is affected by the state of stress around the fractures and by the pressure of the fluid that opened the fractures. Thus, the distribution can be inverted to determine not only the stress but also the pressure condition at the time of vein formation. However, epithermal ore veins that we observe today are the results of a great number of intermittent upwelling of overpressured fluids with different pressures. Here, we define driving pressure index (DPI) as the representative non-dimensionalized fluid pressure for the fluids. We collected the orientations of ∼1000 ore veins in the Hishikari gold mine, which were deposited at around 1 Ma, in southern Kyushu, Japan. It was found that the majority of the veins were deposited under an extensional stress with a NW-SE-trending σ3-axis and a northeasterly-inclined σ1-axis with relatively high stress ratio. The representative driving pressure ratio was ∼0.2. Data sets obtained at different depths in the mine indicated a positive correlation of representative driving pressure ratios with the depths. The correlation suggests repeated formation and break of pressure seals during the mineralization. Our compilation of the Pliocene–Quaternary stress regimes in southern Kyushu, including the result of the present study, suggests that epithermal gold mineralization was associated with distributed extensional deformations in southern Kyushu, and strain localization into an intra-arc rift seems to have terminated the mineralization.