Igneous activity in central-southern Italy: Is the subduction paradigm still valid?

ABSTRACT The Pliocene–Quaternary igneous record of the Tyrrhenian Sea area features a surprisingly large range of compositions from subalkaline to ultra-alkaline and from ultrabasic to acid. These rocks, emplaced within the basin and along its margins, are characterized by strongly SiO2-undersaturated and CaO-rich to strongly SiO2-oversaturated and peraluminous compositions, with sodic to ultrapotassic alkaline and tholeiitic to calc-alkaline and high-K calc-alkaline affinities. We focused on the different models proposed to explain the famous Roman Comagmatic Region, part of the Quaternary volcanism that spreads along the eastern side of the Tyrrhenian area, in the stretched part of the Apennines thrust-and-fold belt. We reviewed data and hypotheses proposed in the literature that infer active to fossil subduction up to models that exclude subduction entirely. Many field geology observations sustain the interpretation that the evolution of the Tyrrhenian-Apennine system was related to subduction of the western margin of Adria continental lithosphere after minor recycling of oceanic lithosphere. However, the lateral extent of the subducting slab in the last millions of years, when magmatism flared up, remains debatable. The igneous activity that developed in the last millions of years along the Tyrrhenian margin is here explained as originating from a subduction-modified mantle, regardless of whether the large-scale subduction system is still active.

The Numidian sand event in the Burdigalian foreland basin system of the Rif, Morocco, in a source-to-sink perspective

During the Tertiary evolution of the Western Mediterranean subduction system, a migrating foreland basin system developed between the Maghrebian orogenic belt and the adjacent African Craton. However, a comprehensive reconstruction of the foreland basin systems of the Rif Chain is still missing. By integrating field observations with quantitative biostratigraphic data from calcareous nannofossil assemblages, sandstone composition, and detrital zircon U-Pb geochronology from selected stratigraphic successions, we reconstruct the foreland basin system that developed in the early Miocene in front of the growing Rif orogen. The successions analyzed are representative of (1) the classical “Numidian Facies” from the Intrarifian Tanger Unit and (2) the Numidian-like deposits (mixed successions) of the “Mérinides Facies” from the “Maghrebian Flysch Basin” and the “Beliounis Facies” from the Predorsalian Unit. Our petrographic analyses and detrital zircon U-Pb ages show that the quartzarenites of the “Numidian Facies” originated from the African Craton, whereas the sublitharenites and feldspathic litharenites from the Mérinides and Beliounis Facies originated from a cratonic area and the exhuming Rif Chain. Our biostratigraphic analyses suggest a simultaneous arrival of the quartz grains in the Numidian, Mérinides, and Beliounis deposits, which indicates that their deposition occurred at ∼1 m.y. (ca. 20−19 Ma, early Burdigalian) and allows us to delineate the early Burdigalian foreland basin system of the Rif Chain. The foreland depozone received the “Numidian Facies,” the foredeep-hosted ∼2000 m of the “Mérinides Facies” and the Beni Ider Flysch, whereas the wedge-top depozone was characterized by deposition of the “Beliounis Facies.” The Numidian Sandstones and the Numidian-like deposits analyzed in Morocco show the same age as similar deposits from Algeria, Tunisia, and Sicily, which suggests a comparable early Burdigalian tectono-sedimentary evolution along the southern branch of the Western Mediterranean subduction-related orogen.

Geomorphology and Geochemistry of Back Arc Basins in the Havre Trough, Southwest Pacific

<p>The Havre Trough back arc system located behind the Kermadec Arc, in the southwest Pacific, is a classic example of an intra-oceanic back arc system. Subduction driven magmatism is focused at the arc front, and melting in the back arc is accompanied by back arc rifting. This study examines the deep back arc basins of the southern Havre Trough. Compared to the well-studied Kermadec Arc front volcanoes, the back arc basins remain poorly explored, yet are important features in understanding key structural and geochemical dynamics of the subduction system. The back arc is characterised by areas of deeper basins and constructional cross-arc volcanic edifices, which had previously been attributed to ‘rift regime’ and ‘arc regime’, respectively. In this study, recently acquired multibeam data was used to produce digital terrain maps that show individual basins within the Havre Trough that host a range of different morphological features, such as elongated ridges, nearly-flat basin floors, and small volcanic cones. Lavas dredged from the 10 basins were analysed, eight of which sample the rift regime and two sample the arc regime. The back arc basin lavas are basalts to basaltic-andesites and show fractionation of olivine + pyroxene ± plagioclase mineral assemblages. Olivine phenocrysts were tested for chemical equilibrium and predominantly show that crystallisation occurred in equilibrium with host melts. However, petrographic features such as dissolution and zoning within plagioclase show evidence of multistage magmatic evolution. Whole rock trace element geochemistry reveals trace element characteristics typical of volcanic arc lavas, such as enrichments in large ion lithophile elements (LILE) and Pb relative to high field strength elements (HFSE). From west to east, the back arc basin lavas show a decrease in NbN/YbN, consistent with trench perpendicular flow and progressive melt extraction towards the volcanic front. There is also a broad correlation between NbN/YbN and distance along the strike of the subduction zone. This may suggest a component of trench parallel flow of the mantle wedge, with increasing depletion northwards, although further evidence is needed to rule out pre-existing mantle heterogeneity. Ba/Th values, which trace the addition of slab-derived aqueous fluids, decrease with distance from the arc front. This indicates that the aqueous fluid component becomes less prominent with increasing distance from the arc front. Conversely, the basin lavas exhibit broadly increasing LaN/SmN values with distance from the arc front. As LaN/SmN can be used to trace the deep subduction component, i.e. sediment melt contribution, greater LaN/SmN suggests increasing contribution of a sediment signature away from the arc front. The parameters that measure recycled component flux are comparable between rift and arc regimes, so it is unlikely that increased volatile fluxing leads to the larger concentrations of magmatic activity displayed in arc regimes. Gill volcano (arc regime) has similar to higher NbN/YbN than lavas from adjacent basins, suggesting increased magmatic activity may in part relate to pockets of more fertile mantle. This study shows that back arcs and associated volcanism can be complicated, further research is integral in determining mechanisms for voluminous magmatic activity spread throughout the back arc.</p>

Geomorphology and Geochemistry of Back Arc Basins in the Havre Trough, Southwest Pacific

<p>The Havre Trough back arc system located behind the Kermadec Arc, in the southwest Pacific, is a classic example of an intra-oceanic back arc system. Subduction driven magmatism is focused at the arc front, and melting in the back arc is accompanied by back arc rifting. This study examines the deep back arc basins of the southern Havre Trough. Compared to the well-studied Kermadec Arc front volcanoes, the back arc basins remain poorly explored, yet are important features in understanding key structural and geochemical dynamics of the subduction system. The back arc is characterised by areas of deeper basins and constructional cross-arc volcanic edifices, which had previously been attributed to ‘rift regime’ and ‘arc regime’, respectively. In this study, recently acquired multibeam data was used to produce digital terrain maps that show individual basins within the Havre Trough that host a range of different morphological features, such as elongated ridges, nearly-flat basin floors, and small volcanic cones. Lavas dredged from the 10 basins were analysed, eight of which sample the rift regime and two sample the arc regime. The back arc basin lavas are basalts to basaltic-andesites and show fractionation of olivine + pyroxene ± plagioclase mineral assemblages. Olivine phenocrysts were tested for chemical equilibrium and predominantly show that crystallisation occurred in equilibrium with host melts. However, petrographic features such as dissolution and zoning within plagioclase show evidence of multistage magmatic evolution. Whole rock trace element geochemistry reveals trace element characteristics typical of volcanic arc lavas, such as enrichments in large ion lithophile elements (LILE) and Pb relative to high field strength elements (HFSE). From west to east, the back arc basin lavas show a decrease in NbN/YbN, consistent with trench perpendicular flow and progressive melt extraction towards the volcanic front. There is also a broad correlation between NbN/YbN and distance along the strike of the subduction zone. This may suggest a component of trench parallel flow of the mantle wedge, with increasing depletion northwards, although further evidence is needed to rule out pre-existing mantle heterogeneity. Ba/Th values, which trace the addition of slab-derived aqueous fluids, decrease with distance from the arc front. This indicates that the aqueous fluid component becomes less prominent with increasing distance from the arc front. Conversely, the basin lavas exhibit broadly increasing LaN/SmN values with distance from the arc front. As LaN/SmN can be used to trace the deep subduction component, i.e. sediment melt contribution, greater LaN/SmN suggests increasing contribution of a sediment signature away from the arc front. The parameters that measure recycled component flux are comparable between rift and arc regimes, so it is unlikely that increased volatile fluxing leads to the larger concentrations of magmatic activity displayed in arc regimes. Gill volcano (arc regime) has similar to higher NbN/YbN than lavas from adjacent basins, suggesting increased magmatic activity may in part relate to pockets of more fertile mantle. This study shows that back arcs and associated volcanism can be complicated, further research is integral in determining mechanisms for voluminous magmatic activity spread throughout the back arc.</p>

First structural observation around the hinge of the Mongolian Orocline (Central Asia): Implications for the geodynamics of oroclinal bending and the evolution of the Mongol-Okhotsk Ocean

To understand the origin of curved subduction zones has been one of the major challenges in plate tectonics. The Mongol-Okhotsk Orogen in Central Asia is characterized by the development of a U-shaped oroclinal structure that was accompanied by the continuous subduction of the Mongol-Okhotsk oceanic plate. Therefore, it provides a natural laboratory to understand why and how a subduction system became tightly curved. In this study, we provide the first structural observation around the hinge of the Mongolian Orocline (the Zag zone in Central Mongolia), with an aim to constrain the oroclinal geometry and to link hinge zone structures with the origin of the orocline. Our results show that rocks in the Zag zone are characterized by the occurrence of a penetrative foliation that is commonly subparallel to bedding. Both bedding and dominant fabric in the Zag zone are steeply dipping, and their strike orientations in a map view follow a simple curve around the hinge of the Mongolian Orocline, thus providing the first structural constraint for 3D geometry of the orocline. A secondary penetrative fabric parallel to the axial plane of the orocline was not observed, indicating a low degree of orogen-parallel shortening during oroclinal bending. Combining with available geological and geophysical data, we conclude that the Mongolian Orocline was developed in a period of Permian to Jurassic, and its origin was linked to the subduction of the Mongol-Okhotsk oceanic slab. We consider that the low-strain oroclinal bending likely resulted from the along-strike variation in trench retreat, which was either triggered by the negative buoyancy of the Mongol-Okhotsk oceanic slab, or driven by the relative rotation of the Siberian and North China cratons. Our results shed a light on 3D geometry and geodynamic mechanisms of large-scale oroclinal bending in an accretionary orogen.

Genesis of the Jinbaoshan PGE-(Cu)-(Ni) deposit: Distribution of chalcophile elements and platinum-group minerals

ABSTRACT The Jinbaoshan platinum group element-(Cu)-(Ni) deposit in southwest China is a sulfide-poor magmatic platinum-group element deposit that experienced multiple phases of post-magmatic modification. The sulfide assemblages of most magmatic Ni-Cu-platinum-group element deposits in China and elsewhere in the world are dominated by pentlandite-pyrrhotite-chalcopyrite with lesser magnetite and minor platinum-group minerals. However, Jinbaoshan is characterized by (1) hypogene violarite-pyrite 1-millerite-chalcopyrite and (2) supergene violarite-(polydymite)-pyrite 2-chalcopyrite assemblages. The platinum-group minerals are small (0.5–10 μm diameter) and include moncheite Pt(Te,Bi)2, mertieite-I Pd11(Sb,As)4, the atokite Pd3Sn – rustenburgite Pt3Sn solid solution, irarsite IrAsS, and sperrylite PtAs2 hosted mainly by violarite, silicates (primarily serpentine), and millerite. The platinum-group minerals occur in two sulfide assemblages: (1) mertieite-I-dominant (with irarsite, palladium, and Pd-alloy) in the hypogene assemblage and (2) moncheite-dominant (with irarsite, sperrylite, and atokite) in the supergene assemblage. Palladium and intermediate platinum-group elements (Os, Ir, Ru) are concentrated mainly in violarite, polydymite, and pyrite 2. Platinum is seldom hosted by base metal sulfides and occurs mainly as discrete platinum-group minerals, such as moncheite, sperrylite, and merenskyite. Violarite and polydymite in the Jinbaoshan deposit contain more Pb-Ag than pentlandite and pyrrhotite in the Great Dyke and Lac des Iles deposit. The formation of the sulfide assemblages in Jinbaoshan can be interpreted to have occurred in three stages: (1) a magmatic Fe-Ni-Cu sulfide melt crystallized Fe-Ni monosulfide and Cu-rich intermediate solid solutions, which inverted to a primary pyrrhotite-pentlandite-chalcopyrite-magnetite assemblage; (2) an early-secondary hypogene voilarite-millterite-pyrite 1-chalcopyrite assemblage formed by interaction with a lower-temperature magmatic-hydrothermal deuteric fluid; and (3) a late-secondary supergene violarite-polydymite-pyrite 2-chalcopyrite assemblage formed during weathering. Late-magmatic-hydrothermal fluids enriched the mineralization in Pb-Ag-Cd-Zn, which are incompatible in monosulfide solid solution, added Co-Pt into violarite, and expelled Pd to the margins of hypogene violarite and millerite, which caused Pd depletion in the hypogene violarite and the formation of mertieite-I. Supergene violarite inherited Pd and intermediate platinum-group elements from primary pentlandite. Thus, the unusual sulfide assemblages in the Jinbaoshan platinum-group element-(Cu)-(Ni) deposit results from multiple overprinted post-magmatic processes, but they did not significantly change the chalcophile element contents of the mineralization, which is interpreted to have formed at high magma:sulfide ratios (R factors) through interaction of crustally derived sulfide and a hybrid picritic-ferropicritic magma derived from subduction-metasomatized pyroxenitic mantle during impingement of the Emeishan plume on the Paleo-Tethyan oceanic subduction system.

Preliminary Results of Regional P and S Wave Tomography at Seram, Indonesia

The Seram Trough is located in the northern part of Indonesia and has a complex tectonic setting. The uniqueness of these regions lies in the U-shape subduction system. Several models have been proposed in this region, such as one subduction system that has been rotated 90° or 180°, two subduction systems, and one subduction that having a slab roll-back that causes extension systems. In this study, we try to invert velocity and seismicity using double-difference tomography with the target of better imaging the sub-surface structure in the region. We use data catalogue collection from the Indonesian Agency of Meteorology, Climatology, and Geophysics. The length of data is 4 years from January 2015 to December 2019 from 16 permanent stations. Earthquake relocations show a focused hypocenter distribution at shallow depth, and we interpreted some of these shallow depth events are related to the magmatic activity. Event distribution also displays a steep angle of seismicity pattern that represents the dipping subduction slab. Inverted Tomography models show a band of faster velocity models that dip from North to South, suggesting a subductions slab. We also observe a possibility of a tear in the slab from the seismicity pattern and tomogram model. The slower velocity perturbation is seen at shallow depth that may associate with magmatic and frequent shallow seismicity. A possibility of partial melting is also seen with low-velocity zone at a depth of 70 km next to the fast dipping velocity.

Multi-stage India-Asia collision: Paleomagnetic constraints from Hazara-Kashmir syntaxis in the western Himalaya

The India-Asia collision is the most spectacular, recent, and still active tectonic event of the Earth’s history, leading to the uplift of the Himalayan-Tibetan orogen, which has been explained through several hypothetical models. Still, controversy remains, such as how and when it occurred. Here we report a paleomagnetic study of Cretaceous-Tertiary marine sediments from the Tethyan Himalaya (TH) in the Hazara area, north Pakistan, which aims to constrain timing for the onset of the India-Asia collision and to confirm the validity of already proposed models, particularly in western Himalaya’s perspective. Our results suggest that the TH was located at a paleolatitude of 8.5°S ± 3.8° and 13.1°N ± 3.8° during the interval of ca. 84−79 Ma and 59−56 Ma, respectively. A comparison between paleopoles obtained from the current study and coeval ones of the India Plate indicates that the TH rifted from Greater India before the Late Cretaceous, generating the Tethys Himalaya Basin (THB). Our findings support a model for a multi-stage collision involving at least two major subduction systems. A collision of the TH with the Trans-Tethyan subduction system (TTSS) began first in Late Cretaceous-Early Paleocene times (ca. 65 Ma), followed by a later collision with Asia at 55−52 Ma. The onset of the collision between the TH (plus TTSS) and Asia could not have occurred earlier than 59−56 Ma in the western Himalaya. Subsequently, the India craton collided with the TH, resulting in the diachronous closure of the THB between ca. 50 and ca. 40 Ma from west to east. These findings are consistent with geological and geochemical evidence and have a broad implication for plate reconfigurations, global climate, and biodiversity of collisional processes.