Modern Volcanic Arcs Research Articles

Early Result of Imaging 3D Seismic Velocity Structure In Central Java Using Double-Difference Tomography

Central Java is a part of the Sunda Arc and has relatively high seismicity due to the subduction zone between the Indo-Australian Plate and the Eurasian Plate. This study uses double-difference tomography to image the P and S wave 3D seismic velocity structures associated with tectonic patterns due to subduction zones. The data used comes from the earthquake catalogs of BMKG, BPPTKG, DOMERAPI, and MERAMEX, with a recording period from May 2004 to December 2020. The number of earthquakes that successfully relocated was 1930 from 1937 earthquakes recorded by 285 recording stations. The results of hypocentre relocation show that seismic activity in Central Java is relatively high and originates from geological structures, including subduction zones, back-thrust, and the Opak Fault. The tomogram results of the P and S wave velocity models support each other. The high seismic velocity anomaly is associated with the subduction zone of the Indo-Australian Plate to the Eurasian Plate with a maximum resolution of up to 100 km depth. Low seismic velocity anomalies are found at the Merapi-Lawu Anomaly (MLA), the Modern Volcanic Arc (MVA), the Sumbing-Sundoro-Dieng volcanic complex, the Banyumas-Cilacap sedimentary basin, and the Kendeng Basin. A low-velocity anomaly is found at a depth of about 40-50 km and a depth of 100 km, which is associated with the process of slab dehydration and partial melting. The inversion results show the impact of the Indo-Australian Plate subduction and the Eurasian Plate on volcanic activity, seismicity, and the geological structures developed in Central Java.

A Global Assessment of the Controls on the Fractionation of Arc Magmas

AbstractDuring the differentiation of arc magmas, fractionating liquids follow a series of cotectics, where the co‐crystallization of multiple minerals control the melt compositional trajectories, commonly referred to as liquid lines of descent (LLD). These cotectics are sensitive to intensive properties, including fractionation pressure and melt H2O concentration, and changes in these variables produce systematic differences in the LLDs of arc lavas. Based on a compilation of experimental studies, we develop two major element proxies that exploit differences in LLDs to constrain the fractionation conditions of arc magmas. Near‐primary fractionating magmas evolve along the olivine‐clinopyroxene cotectic, which is pressure‐sensitive. We use this sensitivity to develop a proxy for early fractionation pressure based on the normative mineral compositions of melts with 8 ± 1 wt.% MgO. Fractionation in more evolved magmas is controlled by the clinopyroxene‐plagioclase cotectic, which is strongly sensitive to magmatic H2O contents. We use this relationship to develop an H2O proxy that is calibrated to the normative mineral components of melts with 2–4 wt.% MgO. These two proxies provide new tools for estimating the variations in pressure and temperature between magmatic systems. We applied these proxies to compiled major element data and phenocryst assemblages from modern volcanic arcs and show that in island arcs early fractionation is relatively shallow and magmas are dominantly H2O‐poor, while continental arcs are characterized by more hydrous and deeper early fractionation. These differences likely reflect variations in the relative contributions of decompression and flux melting in combination with distinct upper plate controls on arc melt generation.

Boron partitioning between zircon and melt: Insights into Hadean, modern arc, and pegmatitic settings

There is a lot of debate surrounding the geodynamic environment of the Hadean Earth, and the only definitively known fragments from the first 500 Ma are detrital zircons. This makes the combination of natural and experimentally obtained data a powerful tool to unravel the chemistry of the early Earth. We have used an incompatible marker element, B, to partially derive the chemistry of the parent melts of the Hadean and modern zircons. We report an experimental calibration for temperature dependent B partitioning between zircon and a hydrous weakly peraluminous granitic melt.log10DBzrc/melt=−1027±372TK−2.011±0.257where DBzrc/melt is the zircon-melt partition coefficient for B and T is temperature in K.This calibration has been applied to natural samples, viz., Hadean zircons from the Jack Hills (JH), Australia, Phanerozoic zircons from the Lachlan Fold Belt (LFB), Australia and three pegmatitic zircons from Seiland Igneous province, Norway, Paicoma Canyon, California and Freeman Mine, North Carolina. Our results present direct evidence of B being present in the Hadean crust. The zircons from JH and LFB are rather poor in B (8–80 ppb), but comparable to each other, while the pegmatites have as much as ten times the [B] (~0.35–0.45 ppm). Application of our experimental calibration yields calculated B melt concentrations of 10–90 ppm for the JH and LFB zircons. Such values for melt [B] (concentrations) are similar to the modern upper continental crust (17 ppm) and volcanic arcs, but are high when to compared to OIBs (0.6–1.8 ppm) and MORBs (1.3 ppm). The Lachlan zircons have [B] values that are broadly similar to the detrital Hadean and Archean zircons, and these have been presented as a point of comparison between Hadean and modern zircons. The pegmatite zircons return calculated melt values (244–701 ppm) that are much higher than the parent melts of the Australian zircons. One of the pegmatite zircons show a variation in calculated Ti-in-zircon crystallization T from the core to the rim (716 °C – 916 °C). Two of the other zircons crystallized at 664 ± 14 °C and 598 ± 13 °C (2 s.e.; not taking into consideration a ~50oT uncertainty due to imperfectly constrained silica/titania activities) and show no intracrystalline variation in [B] or T. Finally, B has been proposed to have been a possible stabilizing agent for ribose aqueous solutions and could have played a part in the formation of carbohydrates and proteins which were building blocks for RNA. Our documented presence of B in the Hadean crust at calculated concentrations similar to a modern volcanic arc setting, makes the role of B in ribose stabilization at least possible on the primordial Earth.

Periodic Paleoproterozoic calc-alkaline magmatism at the south eastern margin of the Yilgarn Craton; implications for Nuna configuration

The age and composition of magmas provide fundamental information to chart the tectonic setting of crustal development through time and hence refine paleogeographic reconstructions. The Biranup Zone of the Albany-Fraser Orogen in southwestern Australia preserves a protracted record of magmatism associated with the formation and subsequent break-up of the Paleoproterozoic supercontinent Nuna. Yet, the configuration of Proterozoic Australia within Nuna is not well constrained. New U-Pb zircon geochronology on four samples of mafic intrusions in the north eastern Biranup Zone yield U-Pb crystallisation ages of 1809 ± 17 Ma, 1798 ± 12 Ma, 1796 ± 12 Ma, and 1755 ± 12 Ma coeval with known pulses of felsic magmatism elsewhere in the orogen. The Lu-Hf isotope composition of zircon crystals from these mafic intrusions, and the metasedimentary rocks they intrude, reveal juvenile magmatic input into the Archean Yilgarn Craton with this magmatism commencing as early as c. 1.90 Ga. Following this juvenile magmatism, the margin of the craton was affected by at least three pulses of calcic to calc-alkaline magmatism between 1.81 and 1.65 Ga. Secular changes in zircon Hf isotope composition are comparable in both duration and periodicity to 50–100 Ma changes in magma composition and zircon chemistry observed in modern volcanic arcs of the American Cordillera and in Cambrian to Carboniferous accretionary complexes of eastern Australia. Isotopic patterns and whole-rock chemistry are apparently consistent with the 1.81–1.70 Ga Paleoproterozoic igneous rocks of the Albany-Fraser Orogen forming in a magmatic arc above a Pacific-type subduction zone which extended along the south eastern margin of the Yilgarn Craton. This interpretation is consistent with paleomagnetic data which places the Yilgarn Craton on the periphery of the supercontinent Nuna at 1.90–1.60 Ga.

Petrogenesis of intermediate volcanic assemblages from the Shebandowan greenstone belt, Superior Province: Evidence for subduction during the Neoarchean

Research on the petrogenesis of andesites and their implications for geodynamic setting are an important facet to understanding controversial tectonic processes during the Archean. The genesis of Archean intermediate volcanic rocks and their relationship to mantle- and crust-derived melts can either support subduction-dominated tectonic processes or plume–crust interactions. This study describes the lithogeochemistry and Nd-isotopic composition of intermediate volcanic assemblages in the Shebandowan greenstone belt of the Wawa-Abitibi terrane that were deposited prior to deformation and tectonic assembly of the Superior Province. The intermediate rocks of the Shebandowan greenstone belt are unique in that they are voluminous, are relatively weakly deformed and are low metamorphic grade, and contain significant amounts of magnesian andesites and adakites.The major and trace element geochemistry of the intermediate assemblages share many of the geochemical characteristics of modern volcanic and continental arcs. These features include enriched Th/Nb and Th/La ratios, steeply dipping rare earth patterns, and pronounced negative Ti and Nb anomalies on primitive mantle-normalized diagrams. Additionally, these rocks also contain distinct positive Pb and Cs anomalies without evidence of major mobility of these elements during secondary processes. Neodymium isotopic analyses indicate the interaction with older crust and show good correlations with evolved Nd-isotopic values with other crustal contamination proxies such as Th/Ce and Ti/Sc ratios. Modeling mixing and assimilation-fractional crystallization interactions between plume-derived and crustal melts reveal that plume–crust interactions cannot explain the compositional array obtained from the intermediate rocks in the Shebandowan greenstone belt.A compilation of U–Pb geochronology for the Shebandowan greenstone belt reveals that tholeiitic/komatiite dominated strata are slightly older than the intermediate-dominated strata. Given that these domains are structurally separated, it is very likely that the Shebandowan greenstone belt was formed in two different geodynamic settings. These different geodynamic settings have important implications for the metallogeny of the belt and explain the relative enrichment of Au and Ni–Cu mineralization in the Shebandowan greenstone belt relative to other ca. 2720Ma assemblages in the Wawa-Abitibi terrane.

Involvement of magmatic fluids at the Laloki and Federal Flag massive sulfide Cu–Zn–Au–Ag deposits, Astrolabe mineral district, Papua New Guinea: sulfur isotope evidence

We present the first sulfur (S) isotope data of sulfides, sulfates, pyrite in host mudstone, and bulk sulfur of gabbroic rocks from the Laloki and Federal Flag massive Cu–Zn–Au–Ag deposits in the Astrolabe mineral district, Papua New Guinea. Early-stage pyrite–marcasite, chalcopyrite, and sphalerite from Laloki display wide range of δ34S values from −4.5 to +7.0 ‰ (n = 16). Late-stage pyrite, chalcopyrite, and sphalerite have restricted δ34S values of −1.9 to +4.7 ‰ (n = 16). The mineralizing stage these correspond to had moderately saline (5.9–8.4 NaCl eq. wt%) mineralizing fluids of possible magmatic origin. A single analysis of late-stage barite has a value of δ34S +17.9 ‰, which is likely similar to coexisting seawater sulfate. Pyrite from the foot-wall mudstone at Laloki has very light δ34S values of −36.1 to −33.8 ‰ (n = 2), which suggest an organic source for S. Pyrite–marcasite and chalcopyrite from Federal Flag show δ34S values of −2.4 to −1.9 ‰ (n = 2), consistent with a magmatic origin, either leached from intrusive magmatic rocks or derived from magmatic–hydrothermal fluids. The very narrow range and near-zero δ34S values (−1.0 to +0.6 ‰) of bulk gabbroic samples is consistent with mantle-derived magmatic S. Sulfur isotope characteristics of sulfides and sulfates are, however, very similar to base metal sulfide accumulations associated with modern volcanic arcs and sedimented mid-ocean ridges. The most reasonable interpretation is that the range of the sulfide and sulfate δ34S values from both Laloki and Federal Flag massive sulfide deposits is indicative of the complex interaction of magmatic fluids, seawater, gabbroic rocks, and mudstone.

Drilling Shallow-Water Massive Sulfides at the Palinuro Volcanic Complex, Aeolian Island Arc, Italy

A subseafloor replacement-style barite and sulfide occurrence was drilled in shallow waters at the Palinuro volcanic complex, the northernmost Aeolian arc volcano in the Tyrrhenian Sea, Italy. Using a lander-type drilling device, 11 successful drill holes yielded a total of 13.5 m of core from a sediment-filled depression located at a water depth of 630 to 650 m. The longest continuous drill core recovered consists of 4.84 m of massive to semimassive barite and sulfides with abundant late, native sulfur overprint. Seafloor observations suggest that the hydrothermal system associated with the formation of the subseafloor barite and sulfide ore zone is still active, although black smoker activity does not occur on the seafloor. The recovered drill core shows that the subseafloor deposit is zoned with depth. The top of the mineralized zone is comprised of a variably silicified vuggy barite-sulfide facies that shows notable polymetallic metal enrichment, while the deeper portion of the mineralized zone is dominated by massive pyrite having distinctly lower base and precious metal grades. Metal zonation of the barite and sulfide deposit is related to the evolution of the hydrothermal fluids in space and time. The barite cap and the massive pyrite present in the deeper portion of the mineralized zone appear to have formed early in the paragenesis. During the main stage of the mineralization, the barite cap was brecciated and cemented by a polymetallic assemblage of barite and pyrite with minor chalcopyrite and tetrahedrite, trace famatinite, and rare cinnabar. Lower temperature precipitates formed during the main stage of mineralization include sphalerite, galena, pyrite, opal-A, and barite, which are associated with traces of Pb-Sb-As sulfosalts such as bournonite-seligmannite, or semseyite. A distinct mineral assemblage of fine-grained anhedral enargite, hypogene covellite, chalcopyrite, and galena is commonly associated with colloform sphalerite, galena, and pyrite as a late phase of this main stage. Colloform pyrite and marcasite are the last sulfides formed in the paragenetic sequence. The deposit is interpreted to have formed from fluids having an intermediate-sulfidation state, although excursions to high- and very high sulfidation states are indicated by the presence of abundant enargite and hypogene covellite. Laser ablation and conventional sulfur isotope analyses show that pyrite formed close to the seafloor within the zone of polymetallic metal enrichment has a variable sulfur isotope composition (δ34S = −39 to +3‰), whereas a more narrow range is observed in the massive pyrite at depth (δ34S = −10 to 0‰). Similar variations were also documented for the late native sulfur overprint. Overall, the negative sulfur isotope ratios at depth, the intermediate- to very high sulfidation conditions during mineralization, and the abundance of native sulfur suggest contributions of magmatic volatiles to the mineralizing fluids from a degassing magma chamber at depth. Biological processes are interpreted to have played a major role during late stages of ore formation. The combination of a subseafloor replacement deposit with a massive to semimassive barite cap rock overlying massive pyrite, the intermediate- to high-sulfidation characteristics, and the strong biological influence on the late stages of mineralization are distinct from other modern seafloor massive sulfide deposits and represents a style of submarine mineralization not previously recognized in a modern volcanic arc environment. The barite and sulfide occurrence at Palinuro shares many characteristics with porphyry-related base metal veins and intermediate-sulfidation epithermal deposits, suggesting that metallogenic processes associated with arc-related magmatic-hydrothermal systems are not restricted to the subaerial environment.

Imaging fluid-related subduction processes beneath Central Java (Indonesia) using seismic attenuation tomography

We use local earthquake data observed by the amphibious, temporary seismic MERAMEX array to derive spatial variations of seismic attenuation (Qp) in the crust and upper mantle beneath Central Java. The path-averaged attenuation values (t∗) of a high quality subset of 84 local earthquakes were calculated by a spectral inversion technique. These 1929 t∗-values inverted by a least-squares tomographic inversion yield the 3D distribution of the specific attenuation (Qp). Analysis of the model resolution matrix and synthetic recovery tests were used to investigate the confidence of the Qp-model. We notice a prominent zone of increased attenuation beneath and north of the modern volcanic arc at depths down to 15km. Most of this anomaly seems to be related to the Eocene–Miocene Kendeng Basin (mainly in the eastern part of the study area). Enhanced attenuation is also found in the upper crust in the direct vicinity of recent volcanoes pointing towards zones of partial melts, presence of fluids and increased temperatures in the middle to upper crust. The middle and lower crust seems not to be associated with strong heating and the presence of melts throughout the arc. Enhanced attenuation above the subducting slab beneath the marine forearc seems to be due to the presence of fluids.

Sedimentation and volcanism in the Panamanian Cretaceous intra-oceanic arc and fore-arc: New insights from the Azuero peninsula (SW Panama)

Abstract The Azuero Peninsula, located in SW Panama, is a region characterized by a long-lived intra-oceanic subduction zone. Volcanism began in Late Cretaceous time, as the result of subduction of the Farallon plate beneath the Caribbean plate. Usually, ancient volcanic arcs related to intra-oceanic subduction zones are not preserved, because they are in areas with difficult access or covered by modern volcanic arc material. However, on the Azuero peninsula, a complete section of the volcanic arc together with arc basement rocks provides the opportunity to study the sedimentation and volcanism in the initial stages of volcanic arc development. The lithostratigraphic unit which records fore-arc evolution is the “Río Quema” Formation (RQF), a volcanic apron composed of volcanic and volcaniclastic sedimentary rocks interbedded with hemipelagic limestones, submarine dacite lava domes, and intruded by basaltic-andesitic dikes. The “Río Quema” Formation, interpreted as a fore-arc basin infilling sequence, lies discordantly on top of arc basement rocks. The exceptionally well exposed arc basement, fore-arc basin, volcanic arc rocks and arc-related intrusive rocks provide an unusual opportunity to study the relationship between volcanism, sedimentation and magmatism during the arc development, with the objective to reconstruct its evolution. The “Río Quema” Formation can be divided into three groups: 1) proximal apron, a sequence dominated by lava flows, interbedded with breccias, mass flows and channel fill, all intruded by basaltic dikes. The rocks represent the nearest materials to the volcanic source, reflecting a coarse sediment supply. This depositional environment is similar to gravel-rich fan deltas and submarine ramps; 2) medial apron, characterized by a volcanosedimentary succession dominated by andesitic lava flows, polymictic volcanic conglomerates and crystal-rich sandstones with minor pelagic sediments and turbidites. These rocks were deposited from high-density turbidity currents and debris flows, directly derived from erupted material and gravitational collapse of an unstable volcanic edifice or volcaniclastic apron; 3) distal apron, a thick succession of sandy to muddy volcaniclastic rocks, interbedded with pelagic limestones and minor andesitic lavas, intruded by dacite domes and by basaltic to andesitic dikes. Bedforms and fossils suggest a quiet, relatively deep-water environment characterized by settling of clay and silt (claystone, siltstone) and by dilute turbidity currents of reworked volcaniclastic detritus. The timing of the initial stages of the volcanic arc has been constrained through a biostratigraphic study, using planktonic foraminifera and radiolarian species. The fossil assemblage indicates that the age of the “Río Quema” Formation ranges from Late Campanian to Maastrichtian, providing a good constraint for the development of the volcanic arc and volcaniclastic apron, during the initial stages of an intra-oceanic subduction zone.

Chemical and Sr–Nd isotopic compositions and zircon U–Pb ages of the Birimian granitoids from NE Burkina Faso, West African Craton: Implications on the geodynamic setting and crustal evolution

The West African Craton was the site of massive juvenile crustal addition in the early Proterozoic during the Eburnean orogeny (2.1Ga). In this study, we determined chemical and isotopic compositions as well as zircon U–Pb ages of granitoid samples from NE Burkina Faso. These data were then used to constrain the chemical characteristics, emplacement age and tectonic environment of these rocks in the context of the West African Craton evolution. Birimian (Paleoproterozoic) granitoids of NE Burkina Faso are associated with volcanic and volcano-sedimentary sequences of the greenstone belts. Analyses indicate that the Na-rich Birimian granitoids of tonalite–trondhjemite–granodiorite (TTG) composition were generated by partial melting of a garnet-bearing amphibolite crust. Granite intrusions derived from partial re-melting of the deep basement of felsic calc-alkaline nature with contribution of metasediments. Zircon U–Pb isotopic analyses yielded ages between 2122±15Ma and 2181±7Ma for the Birimian Na-rich granitoids and 2151±10Ma for a biotite granite (IJ10). Sr–Nd isotopic data show very low initial Sr composition (ISr (T)≤0.7007), positive epsilon neodymium values (ɛNd (T)=+0.7 to +2.5) and Sm–Nd model ages (TDM) of 2363–2558Ma). These data are similar to those of mafic rocks of the Pissila greenstone belt: (ISr (T)=0.7003–0.7015, ɛNd (T)=+3.2 to +4.1, TDM=2336–2552Ma), and further suggest that the Na-rich granitoids are of juvenile nature. The present study together with published data on the Birimian volcanics and meta-sedimentary rocks of the West African Craton preclude contamination of the juvenile Birimian rocks of NE Burkina Faso by reworked Archean materials. The Birimian granitoids of NE Burkina Faso were mostly emplaced during the early stage (2.19–2.15Ga) of a large scale crustal growth event (2.2–2.0Ga) where large amount of juvenile materials were added to the crust from the depleted mantle in a tectonic environment comparable to modern volcanic arcs.

A detrital heavy mineral viewpoint on sediment provenance and tropical weathering in SE Asia

Understanding heavy mineral preservation is important for interpreting generation, pathways, provenance and geochemistry of sediments. Despite this, many assumptions about heavy mineral stability are based on ancient strata and few studies consider modern sediments, particularly those in tectonically active tropical areas such as SE Asia. We report new heavy mineral data on 69 river sand samples from the Malay Peninsula and Sumatra, in which one aim was to find provenance indicators specific to these areas. Identifications were performed using optical microscopy and confirmed with SEM-EDS. In the Malay Peninsula heavy minerals record granitic and contact metamorphic provenance. Variable amounts of zircon, tourmaline, hornblende, andalusite, epidote, monazite, rutile and titanite, and minor amounts of pyroxene, apatite, anatase, garnet, diaspore, colourless spinel, cassiterite and allanite are typical of this source area. The composition of assemblages from Sumatra indicates contributions from two major sources: the modern volcanic arc (I) and the basement (II). Abundant pyroxene, particularly hypersthene (up to 70%), is diagnostic of the volcanic arc source. Vesuvianite, garnet, andalusite, tourmaline, chrome spinel, rutile, anatase and corundum, are present only in small amounts (<3%), and are interpreted as recycled from the basement. Zircon, apatite, hornblende, epidote, and olivine are also common in Sumatra and are likely to have a mixed provenance. Abundance of ferromagnesian silicate minerals suggests mild weathering, possibly reflecting several processes: dilution of natural etching fluids by heavy rainfall, high erosion rates, rapid transport and short grain residence time in the river. The heavy mineral assemblages of modern rivers are very different from those recorded by the few previous studies of Cenozoic sediments of the Malay Peninsula and Sumatra. Assemblages in the Cenozoic basins are significantly more mature than those of modern rivers. The differences cannot be explained simply by dissolution of susceptible minerals during one sedimentary cycle and instead imply rapid source area unroofing.

Sulfur isotope characteristics of metamorphosed Zn–Cu volcanogenic massive sulfides in the Areachap Group, Northern Cape Province, South Africa

Zn- and Cu-rich massive sulfide ores of volcanogenic origin [volcanogenic massive sulfide (VMS) deposits] occur as stratiform/stratabound lenses of variable size hosted by gneisses, amphibolites, and schists of the Areachap Group, in the Northern Cape Province of South Africa. The Areachap Group represents the highly deformed and metamorphosed remnants of a Mesoproterozoic volcanic arc that was accreted onto the western margin of the Kaapvaal Craton during the ∼1.0–1.2 Ga Namaquan Orogeny. Sulfur isotope data (δ34S) are presented for 57 sulfide separates and one barite sample from five massive sulfide occurrences in the Areachap Group. Although sulfides from each site have distinct sulfur isotope values, all δ34S values fall within a very limited range (3.0‰ to 8.5‰). Barite has a δ34S value of 18.5‰, very different from that of associated sulfides. At one of the studied sites (Kantienpan), a distinct increase in δ34S of sulfides is observed from the massive sulfide lens into the disseminated sulfides associated with a distinct footwall alteration zone. Sulfide–sulfide and sulfide–barite mineral pairs which recrystallized together during amphibolite- and lower granulite facies metamorphism are not in isotopic equilibrium. Sulfur isotope characteristics of sulfides and sulfates of the Zn–Cu ores in the Areachap Group are, however, very similar to base metal sulfide accumulations associated with modern volcanic arcs and unsedimented mid-ocean ridges. It is thus concluded that profound recrystallization and textural reconstitution associated with high-grade regional metamorphism of the massive sulfide ores of the Areachap Group did not result in extensive sulfur isotopic homogenization. This is similar to observations in other metamorphosed VMS deposit districts and confirms that massive sulfide ores remain effectively a closed system for sulfur isotopes for both sulfides and sulfates during metamorphism.

Geochemical and Sr-Nd isotopic data on the Birimian of the Kedougou-Kenieba Inlier (Eastern Senegal): Implications on the Palaeoproterozoic evolution of the West African Craton

Birimian (Paleoproterozoic) granitoids and associated volcanic and sedimentary rocks from the Kedougou-Kenieba-Inlier (KKI) of Eastern Senegal have been analyzed for their major, trace, REE concentrations and their Sr-Nd isotopic compositions. The calc-alkaline I-type granitoids were previously divided into several structural groups. The new geochemical data, however, do not give evidence for this subdivision into pre-, syn- and post-deformational intrusions. The results demonstrate the juvenile character of the granitic igneous rock suite and its isotopic similarities to the adjacent volcanic Mako and sedimentary Diale-Dalema Supergroups. The Birimian rocks show uniform juvenile features such as low initial Sr isotopic composition (~0.700 to ~0.704), positive ∑Nd(t) values (2.1 to 4.3) and restricted Nd model ages (~2.0 to ~2.3 Ga). They preclude the contamination of the juvenile Birimian crust of the KKI by notable amounts of reworked Archean crustal material. Comparison with existing data helps to clarify the complex evolution of Birimian Supergroups of the WAC. It can be concluded that the Birimian rocks are the product of a large-scale crustal growth process with large amounts of juvenile crust formed from the depleted mantle in a geodynamic environment compared to modern volcanic arcs. These bimodal volcanic arcs were later accreted to the WAC.

Correlation of REE with Moho depth for modern volcanic arc basalts

Correlation of REE with Moho depth for modern volcanic arc basalts

Life in an active volcanic arc: Petrology and sedimentology of dinosaur track beds in the Jindong Formation (Cretaceous), Gyeongsang Basin, South Korea

The Jindong Formation was deposited as a thick succession of fine-grained volcaniclastic sediments in a rapidly subsiding intra-arc basin. These sediments contain abundant dinosaur and bird tracks that occur on many bedding planes throughout the approximately 110 m of stratigraphic section measured for this study. The main lithologies in the Jindong Formation are ash-rich mudstone, siltstone, and very fine sandstone. Minor lithologies are coarser sandstone, conglomerate, carbonates, and ash tuff. The sandstones are feldspathic and lithic arenites and wackes. They contain plutonic feldspars, volcanic rock fragments, volcanic ash, and a variety of quartz types. All lithologies show evidence of extensive post-depositional alteration, including devitrification and replacement of detrital grains by authigenic minerals. The alterations are typical of burial metamorphism with high heat flow. Sediments of the Jindong Formation can be grouped into three facies associations. Association 1 (proximal) comprises 16% of the measured sections. It consists of various types of bedded, cross-bedded, laminated, and ripple cross-laminated sandstone and minor conglomerate. These are interpreted as fluvial or distributary channel and proximal lake deposits. Association 2 (medial) comprises 53% of the measured sections. It consists mainly of heterolithic couplets of very fine sandstone and mudstone that are mm to dm thick. They contain graded bedding, laminations, current ripples, wave ripples, and mud cracks, and are interpreted as lacustrine hyperpycnal flow deposits. Association 3 (distal) comprises 31% of the measured sections. It consists mainly of bedded shale, mudstone, and siltstone. These are interpreted as distal lake deposits. Associations 2 and 3 contain minor carbonates, which are interpreted as lacustrine and/or diagenetic in origin, and minor tuffs, which are interpreted as tephra. Small wave ripples, dinosaur tracks, invertebrate burrows, and mud cracks are common throughout the measured sections; thus the Jindong lakes are inferred to have been shallow and ephemeral in nature. Wave ripple-crest orientations are consistently WNW-ESE, indicating a prevailing wind direction from the NNE or SSW. Because the sediments are mostly fine-grained and the ash beds thin, the study area is inferred to have been in a distal lowland setting, at least tens of km away from erupting volcanoes. Paleoclimate reconstructions suggest that the study area had a seasonal climate with wet summers and dry winters. Cooler, wetter conditions are thought to have prevailed to the north, and warmer, drier conditions to the south. Modern volcanic arc environments with seasonal and/or humid climates are characterized by rapid aggradation of ashy sediment that fills drainages and causes widespread flooding. This style of sedimentation accounts for the facies found in the Jindong Formation, as well as the many bedding-plane exposures of tracks. The unusual occurrence of both sauropod and ornithopod tracks in the Jindong sections may be the result of a climate boundary that existed in the area at the time. Facies analysis of track types indicates that various track makers had the same environmental preferences, and facies association 2 contains the greatest number of tracks of all types. It is inferred that the tendency for only one track type to occur on a given bedding plane is the result of different types of animals visiting the study area at different times, rather than differing environmental preferences. The Jindong Formation contains an abundance of subaqueously deposited facies, an abundance of tracks, and many track levels that were wet when animals were walking on them. These features are not consistent with previous interpretations of aridity during Jindong time.

Epeirogenic uplift above a detached slab in northern Central America

P-wave tomographic images reveal that the northern Central America highlands east of the modern volcanic arc overlie a detached slab. Hypsometric analysis of the highlands in Honduras demonstrates that the region is a dissected plateau that is disrupted by normal faults near the North American–Caribbean plate margin. The dissected Central American plateau contains a network of superimposed rivers with meanders cut into bedrock; such a geomorphic character indicates that the regional uplift occurred in the absence of tilting. We propose that the epeirogenic uplift of northern Central America is the buoyant upper-plate response to the influx of mantle asthenosphere following the break-off and sinking of the slab.

Late Miocene volcanism and intra-arc tectonics during the early development of the Trans-Mexican Volcanic Belt

The early stage of the Trans-Mexican Volcanic Belt (hereafter TMVB) is marked by widespread, mafic to intermediate, volcanism emplaced between 11 and 7 Ma from the Pacific coast to the longitude of Mexico City, to the north of the modern volcanic arc. Petrological and geochronological data support the hypothesis that this volcanism made up a unique late Miocenic central Mexican comagmatic province. Mafic lavas at the mouth of the Gulf of California and along the northwestern sector of the TMVB made up the Nayarit district, which includes calc-alkaline to transitional varieties. The central sector of the TMVB is characterized by two basaltic districts: the Jalisco–Guanajuato and the Queretaro–Hidalgo, which are distinguished from the westernmost ones by their lower Nb/La and generally lower HFSE/LILE values, as well as by spider diagrams characterized by larger negative spikes at Th, Ta, Nb, and Ti. The surface occurrence of the late Miocene basalts appears to be controlled by pre-existing zones of crustal weakness that channeled the mafic magmas. Field observations suggest that these structures have been reactivated in a transtensional fashion induced by differential tectonic motion of crustal blocks to the south and to the north of the TMVB. Starting from ∼12 Ma the TMVB separates a northern tectonic domain, subject to the developing divergent Pacific–North America plate boundary, from a southern tectonic domain, characterized by oblique subduction of the Rivera and Cocos plates. Apparently, far field stresses related to these complex plate boundaries reactivated older suture zones, allowing rapid uprise of mantle-derived magmas. The subduction-related signature shown by Miocene mafic lavas of the Jalisco–Guanajuato district argues against the existence of mantle plumes beneath this sector of the North America plate. On the other hand, the occurrence in the western TMVB and in the Guadalajara region of a large volume of mafic magmas, which sometimes show characteristics transitional to Ocean Island Basalts, could be due to passive upwelling of the subslab asthenosphere that might have interacted with subduction-related magmas and continental lithosphere to produce the observed basaltic varieties. Subslab magmas may have flowed through slab-free areas along the northern and eastern edges of the subducting Rivera plate, which in the late Miocene was already detached from the Farallon plate remnants and diverging from the Cocos plate.

Contrasting nature of deformation along an intra-arc shear zone, the Liquiñe–Ofqui fault zone, southern Chilean Andes

The Liquiñe–Ofquifault zone (LOFZ) cuts the Patagonian batholith and the modern volcanic arc of southern Chile for ca 1000 km. The rock fabric along three transects of the LOFZ combined with new and published geochronological data suggest marked differences in the nature and timing of deformation along strike. In the Liquiñe transect (39°S), a 1 km wide, northeast-striking subvertical mylonitic zone shows north-plunging stretching lineations. This mylonitic zone has been juxtaposed by high-angle reverse faulting against a nearly undeformed Miocene granitoid. Metamorphic assemblages and microstructures in the mylonites indicate greenschist facies conditions and sinistral reverse displacement. Deformation pre-dates a 100±2 Ma undeformed porphyritic dike (hornblende 40Ar– 39Ar mean age). In the Reloncavı́ transect (41–42°S), deformation in Cretaceous and Miocene plutons is predominantly brittle. Kinematic analysis of two fault populations yields compressional and dextral strike-slip stress regimes, interpreted as late Miocene in age. In the Hornopirén transect (42–43°S), a 4 km wide mylonitic zone, developed in plutons and wallrocks, shows subhorizontal stretching lineations and dextral displacement. A single fault population overprinting the mylonites supports a dextral strike-slip stress regime. Available U–Pb, K–Ar and 40Ar– 39Ar dates on deformed Cenozoic plutons and wallrock range from 9 to 13 Ma on hornblende and from 6 to 3 Ma on biotite. Microstructures and mineral assemblages indicate that the youngest ductile fabrics in the plutons formed at greenschist facies, similar to the biotite Ar closure temperature. Subparallel magmatic and solid-state fabrics combined with geochronology suggest that dextral displacement was continuous during emplacement and cooling of the plutons. Dextral-oblique subduction of the Nazca plate beneath western South America has driven long-term intra-arc deformation at the southern Andes plate boundary zone; ridge collision, in turn, has favored dextral displacement at the leading edge of the continent since the Pliocene

Tectonic segmentation of the North Andean margin: impact of the Carnegie Ridge collision

The North Andean convergent margin is a region of intense crustal deformation, with six great subduction earthquakes M w≥7.8 this century. The regional pattern of seismicity and volcanism shows a high degree of segmentation along strike of the Andes. Segments of steep slab subduction alternate with aseismic regions and segments of flat slab subduction. This segmentation is related to heterogeneity on the subducting Nazca Plate. In particular, the influence of the Carnegie Ridge collision is investigated. Four distinct seismotectonic regions can be distinguished: Region 1 – from 6°N to 2.5°N with steep ESE-dipping subduction and a narrow volcanic arc; Region 2 – from 2.5°N to 1°S showing an intermediate-depth seismic gap and a broad volcanic arc; Region 3 – from 1°S to 2°S with steep NE-dipping subduction, and a narrow volcanic arc; Region 4 – south of 2°S with flat subduction and no modern volcanic arc. The Carnegie Ridge has been colliding with the margin since at least 2 Ma based on examination of the basement uplift signal along trench-parallel transects. The subducted prolongation of Carnegie Ridge may extend up to 500 km from the trench as suggested by the seismic gap and the perturbed, broad volcanic arc. These findings conflict with previous tectonic models suggesting that the Carnegie Ridge entered the trench at 1 Ma. Furthermore, the anomalous geochemical (adakitic) signature of the volcanoes in the broad Ecuador volcanic arc and the seismicity pattern are proposed to be caused by lithospheric tears separating the buoyant, shallowly subducting Carnegie Ridge from segments of steep subduction in Regions 1 and 3. It is further suggested that Carnegie Ridge supports a local `flat slab' segment similar to that observed in Peru. The impact of the Carnegie Ridge collision on the upper plate causes transpressional deformation, extending inboard to beyond the volcanic arc with a modern level of seismicity comparable to the San Andreas fault system. The pattern of instrumental and historical seismicity indicates (1) great earthquakes on the northern and southern flanks of the colliding ridge, (2) a slight reduction in observed seismicity at the trench–ridge intersection, (3) increased stress far into the continent, and (4) a NNE displacement of the N. Andes block, to be further effects of the collision.

Insights into the Proterozoic geology of the Park Range, Colorado

Proterozoic rocks in the Park Range comprise an assemblage of highly deformed metavolcanic and metasedimentary supracrustal rocks intruded by numerous plutons. The oldest rocks in the region are a suite of bimodal metavolcanic and associated metasedimentary rocks on Farwell Mountain. These rocks are similar to the Green Mountain Formation to the north, and are geochemically similar to modern continental volcanic arcs related to subduction. The most widely exposed Proterozoic supracrustal unit in the Park Range consists of a highly metamorphosed and deformed, bimodal volcanic sequence with trace-element patterns similar to those of continental interior bimodal suites instead of subduction-related volcanism. The metavolcanic rocks are overlain by a suite of metasedimentary rocks. Protoliths for the basal metasedimentary rocks are graded sequences of feldspar-rich lithic sandstones and conglomerates interpreted as turbidites. These grade stratigraphically upward into a sequence of interbedded shales and finegrained quartz arenites, similar to those found on passive-margin shelves. The top of the sedimentary sequence consists of a medium- to coarse-grained, cross-bedded quartzite. The supracrustal rocks are intruded by plutonic rocks with compositions from gabbro to granite and ages of 1.78 to 1.735 Ga. The youngest Proterozoic unit in the area is the ∼1.4-Ga Mount Ethel pluton. Upper amphibolite-facies regional metamorphism produced abundant sillimanite in aluminous rocks throughout the Park Range and overprints structural fabrics associated with the older intrusions. Garnet-biotite/GASP thermobarometry indicates temperatures for this event ranging from 550–700°C and pressures of 4 to 6 kbar for most of the region south and east of greenschist-facies rocks at Farwell Mountain. Evidence of an earlier period of higher pressure metamorphism is present near Lester Mountain where kyanite that contains small blebs of staurolite has partly broken down to sillimanite. Evidence for a late, lower-pressure rehydration event is also recorded at Lester Mountain. The dominant fabric in the supracrustal rocks throughout the range consists of a steeply dipping northeast-southwest-striking foliation with a steeply plunging stretching lineation. This fabric is also recorded in the oldest of the felsic plutonic rocks, although the amount of strain in the dated plutonic rocks decreases with age. The intensely deformed Soda Creek-Fish Creek shear zone might be a suture between major terranes in the Park Range. There are also changes in metamorphic grade and ages of plutonic rocks that may be related to the shear zone. The main deformation along this shear zone predated the Mount Ethel pluton and was northside-down. However, some left-lateral displacements along late mylonite zones offset rocks attributed to the Mount Ethel magmatic system. A second candidate for a suture is the area between Farwell Mountain and Lester Mountain, where metamorphic grades and compositions of metavolcanic rocks change abruptly.