Subduction Rollback Research Articles

Adria microplate: a puzzling key stone in west-central Mediterranean geodynamics

The Mediterranean region denotes a special plate tectonic environment as the two big plates, Africa and Eurasia, primarily provide the main frame while the dominant players are the small oceanic basins and the continental micro plates (MP) that for the past 80 Ma was the Adria MP. In this environment slab rollback subduction, orogenic arc formation and rotation of crustal blocks are common. While the tectonic setting and involvement in geodynamic processes of Adria MP might not be unique in global terms, it presents Adria as an ideal natural laboratory to study the geodynamic processes individually and in their combined effects in a generally well-studied and rather wellaccessible region. The current extension of the Adria MP is defined by the continental Moho as far into the bounding orogens as the latter could be confirmed. The slabs surrounding Adria dominate and characterize the geodynamics of this MP and in most cases their geometries and the possible connection with Adria are well defined by tomographic images. The Ionian oceanic lithosphere slab subducting beneath Calabria has been torn off and detached from the very short South Apennines slab remaining attached to Adria. On the southeastern side we observe the Adria slab beneath the Hellenides that seems detached from the Ionian oceanic lithosphere slab of the Hellenic subduction zone. Along the Dinarides a slab gap is observed analogue to the slab window on Adria MP’s western side between the South Apennines and the North Apennines. Encircled by three orogens, the Adria continental lithosphere has been and still is strongly affected by the subductions surrounding it. In the northern part of the Adria MP, the North Apennines slab rollback is the obvious dominant geodynamic force acting locally on the plate while in contrast, the slabs beneath the Eastern Alps and beneath the Hellenides seem stagnant. This study presents a brief overview of the wealth of information aiming to summarize the main constraints on Adria MP available and it discusses some of the questions surrounding this key player in west-central Mediterranean geodynamics.

Mineralogy and Geochemistry of Upper Miocene Igneous Rocks, Kos Island, Greece: Extension during Strike-Slip Faulting and Subduction Rollback

Upper Miocene volcanic and plutonic rocks on Kos island preserve a record of magmatic and tectonic events in the transition zone between the Aegean and Anatolian microplates. Their field setting, syn-intrusion deformation, mineralogy, and geochemistry were investigated. Volcanic rocks, including trachyandesite flows and trachyandesite to rhyolite domes, were extruded on a central E–W horst and directly overlie Alpine basement. Thick successions of trachytic flow tuffs are interbedded with fluvial and lacustrine basinal sediments to the south of this horst. Volcanism was synchronous with the emplacement of the Dikeos monzonite pluton, which is geochemically similar to some lithic clasts in the thick flow tuffs and is cut by mafic dykes including lamprophyres. Two main types of mafic magma were present: a K-rich lamprophyric magma that evolved to trachyandesite and more calc-alkaline magma similar to mafic enclaves in the monzonite. Syn-intrusion structures in the monzonite indicate emplacement during E–W sinistral strike-slip faulting that created local transtensional deformation, providing accommodation for a Dikeos magma reservoir. A change in the style of deformation in the Late Miocene led to NW-striking extension in the footwall, occupied by mafic dykes and mineralized veins, and extensional detachment of the hanging wall, resulting in unroofing of the monzonite.

The chromitites of the Herbeira massif (Cabo Ortegal Complex, Spain) revisited

The ultramafic rocks of the Herbeira Massif in the Cabo Ortegal Complex (NW Iberia) host chromitite bodies. The textural and compositional study of the host rocks and the chromitites classified them into: (1) Type-I chromitites, forming massive pods of intermediate-Cr chromite (Cr# = 0.60–0.66) within dunites; and (2) Type-II chromitites forming semi-massive horizons of high-Cr chromite (Cr# = 0.75–0.82) interlayered with dunites and pyroxenites. Minor and trace elements (Ga, Ti, Ni, Zn, Co, Mn, V and Sc) contents in the unaltered chromite cores from both types show patterns very similar to fore-arc chromitites, mimicked by the host dunites and pyroxenites. Calculated parental melt compositions suggest that Type-I chromitites crystallized from a melt akin to fore-arc basalt (FAB), while Type-II chromitites originated from a boninite-like parental melt. Both melts are characteristic of a fore-arc setting affected by extension during rollback subduction and have been related to the development of a Cambrian-Ordovician arc. These chromitites are extremely enriched in platinum-group elements (PGE), with bulk-rock PGE contents between 2,460 and 3,600 ppb. Also, the host dunites and pyroxenites exhibit high PGE contents (167 and 324 ppb, respectively), which are higher than those from the primitive mantle and global ophiolitic mantle peridotites. The PGE enrichment is expressed in positively-sloped chondrite-normalized PGE patterns, characterized by an enrichment in Pd-group PGE (PPGE: Rh, Pt and Pd) over the Ir-group PGE (IPGE: Os, Ir and Ru) and abundant platinum-group minerals (PGM) dominated by Rh-Pt-Pd phases (i.e. Rh-Ir-Pt-bearing arsenides and sulfarsenides, Pt-Ir-Pd-base-metal-bearing alloys, and Pt-Pd-bearing sulfides). The PGM assemblage is associated with base-metal sulfides (mostly pentlandite and chalcopyrite) and occurs at the edges of chromite or embedded within the interstitial (serpentinized) silicate groundmass. Their origin has been linked to direct crystallization from a S-As-rich melt(s), segregated by immiscibility from evolved volatile-rich small volume melts during subduction. At c. 380 Ma, retrograde amphibolite-facies metamorphism occurred during the exhumation of the HP-HT rocks of the Capelada Unit, which affected chromitites and their host rocks but preserved the primary composition of chromite cores of the chromitites. This event contributed to local remobilization of PGE as suggested by the negative slope between Pt and Pd and high Pt/Pd ratios in the studied chromitites, and host dunites and pyroxenites. In addition, it promoted the alteration of primary PGM assemblage and the formation of secondary PGM. Nanoscale observations made by focused ion beam high-resolution transmission electron microscopy (FIB/HRTEM) analysis of a composite grain of Rh-bearing arsenide with PGE-base-metal bearing alloys suggest the mobilization and accumulation of small nanoparticles of PGE and base-metals that precipitated from metamorphic fluids forming PGE-alloys. Finally, we offer a comparison of the Cabo Ortegal chromitites with other ophiolitic chromitites involved in the Variscan orogeny, from the Iberian Peninsula to the Polish Sudetes. The studied Cabo Ortegal chromitites are similar to the Variscan chromitites documented in the Bragança (northern Portugal) and Kraubath (Styria, Austria) ophiolitic massifs.

Regional-Scale Paleoproterozoic Heating Event on Archean Acasta Gneisses in Slave Province, Canada: Insights from K–Ar and 40Ar/39Ar Chronology

Slave Province in Canada is an Archean granite–supracrustal terrane at the northwestern corner of the Canadian Shield. It is bordered by the Thelon–Taltson orogen (2.0 to 1.9 Ga) to the southeast and the Wopmay orogen (1.9 to 1.8 Ga) to the west. Acasta gneisses, exposed in the westernmost Slave Province, and the Wopmay rocks, located close to the gneisses, were systematically collected for K–Ar and laser step-heating 40Ar/39Ar single-crystal analyses of the biotite and amphibole. The K–Ar biotite ages of the four Wopmay samples range from 1816 ± 18 Ma to 1854 ± 26 Ma. The 40Ar/39Ar biotite analyses of the three Wopmay samples yield plateau ages of 1826 ± 21 Ma, 1886 ± 13 Ma, and 1870 ± 18 Ma. These ages fall within the reported U–Pb zircon age range of the Wopmay orogen. The K–Ar biotite ages of the fifteen Acasta gneisses range from 1779 ± 25 Ma to 1877 ± 26 Ma, except for one younger sample (1711 ± 25 Ma). The 40Ar/39Ar analyses of the biotite crystals from three samples give the plateau ages of 1877 ± 8 Ma, 1935 ± 14 Ma, and 1951 ± 11 Ma. The K–Ar amphibole ages from twelve samples range from 1949 ± 19 Ma to 1685 ± 25 Ma. Two samples of them give ages older than the zircon U-Pb age of Hepburn plutons. The 40Ar/39Ar analyses of the amphibole crystals show varied age relations. The two samples give plateau ages of 1814 ± 22 Ma and 1964 ± 12 Ma. Some samples exhibit apparent old ages of ~2000 Ma in the middle temperature fractions. These old fractions result from the amphibole crystals, originally formed in the Archean, being affected by the thermal events during the Wopmay orogeny but not fully resetting. These observations suggest that the K–Ar system ages of the biotite and amphibole in the Archean Acasta gneiss were rejuvenated during the Paleoproterozoic ages. The Discussion explores the possibility that the heat source rejuvenating the K–Ar system ages may have arisen due to asthenospheric extrusion into the wedge mantle, a process likely triggered by subduction rollback.

Age, origin and tectonic controls on rapid recent exhumation of the Sibela Mountains, Bacan, Indonesia

The Sibela Mountains of the island of Bacan in eastern Indonesia contain one of the Earth’s youngest metamorphic complexes that is now exposed at elevations up to 2000 m. New mica 40Ar/39Ar and apatite (U–Th–Sm)/He data from metamorphic and igneous rocks indicate that these rocks were rapidly exhumed in the Pleistocene (c. 0.7 Ma). Exhumation of the metamorphosed Permo-Triassic basement (c. 249–257 Ma) was accompanied by metamorphism (recorded by schists) as well as partial melting (recorded by c. 1.4 Ma granitic dykes). These processes must have occurred at extremely high cooling and exhumation rates. The rapid exhumation on land was associated with significant subsidence in adjacent basins offshore that reach depths up to 2.4 km. Neogene metamorphic core complexes and other metamorphic complexes are well-known from eastern Indonesia, but they usually record much higher exhumation rates than those reported from older classic metamorphic core complexes found in other parts of the world and require a different formation mechanism. Unlike classic metamorphic core complexes that are characterized by low-angle detachment faults, the Bacan metamorphic rocks were exhumed on steep bounding normal faults forming a rectilinear block pattern. We suggest such complexes are termed metamorphic block complexes (MBC). The Bacan MBC is exceptionally young and like the other east Indonesian complexes was rapidly exhumed during subduction rollback. A flexure formed during arc-arc collision as the Sangihe forearc loaded the Halmahera forearc which reactivated steeply-dipping faults in a rectilinear chocolate block pattern.Graphical abstractThe Sibela Mountains of the island of Bacan in eastern Indonesia are formed by one of the Earth’s youngest metamorphic complexes. a Dense rain forest cover and steep flanks of the Sibela Mountains which rise from sea level to more than 2000 m. b Quaternary collision between the Sangihe and Halmahera arcs. Loading of the Halmahera forearc formed a flexure that reactivated steeply-dipping faults, which exhumed the Sibela Mountains metamorphic block. c Rapid cooling paths during exhumation of the Sibela metamorphic complex

Petrogenesis and Tectonic Setting of Early Cretaceous A-Type Granite from the Southern Great Xing’an Range, Northeastern China: Geochronological, Geochemical, and Hf Isotopic Evidence

The southern Great Xing’an Range is located in the eastern Central Asian Orogenic Belt, where voluminous igneous rocks developed during the Late Mesozoic period. The east slope of the southern Great Xing’an Range has been the topic of numerous debates on the level of influence of the Mongol-Okhotsk and the Paleo-Pacific regimes in the Late Mesozoic period. Therefore, this area is a suitable region in which to study the temporal changes in magma sources and tectono-magmatic evolution. In this paper, whole-rock geochemical data, zircon U-Pb geochronology, and zircon Hf isotope studies were carried out on the granitoids in the east slope area of the southern Great Xing’an Range. LA-ICP-MS zircon U-Pb dating revealed the ages of four granitoid samples: 135.0 ± 0.6 Ma, 130.7 ± 1.4 Ma, 130.4 ± 1.0 Ma, and 127.6 ± 0.8 Ma, respectively. The Hf isotope values 176Hf/177Hf = 0.282751–0.283015, εHf (t) = +2.0~+11.5, and T2DM = 583~1442 Ma suggest that the magma was generated by partial melting of Meso- and Neoproterozoic accreted and thickened low crust. The whole-rock geochemical data implied that these granitoids are A-type granite and their formation is closely linked to the subduction of the Paleo-Pacific Ocean plate. These geochemical, isotopic, and geochronological data suggest that the Early Cretaceous magmatism in the east slope area of the southern Great Xing’an Range formed in an extensional back-arc tectonic setting associated with the slab roll-back of the Paleo-Pacific plate subduction.

Anatomy of a km-scale fault zone controlling the Oligo-Miocene bending of the Ligurian Alps (NW Italy): integration of field and 3D high-resolution digital outcrop model data

We report the first description of a ~ 15 km long NE-SW-striking transtensive fault network crosscutting the metamorphic units of the Ligurian Alps. The main fault zone, hereby named Horse Head Fault Zone, is up to 250 m thick, involves quarzite, metarhyolite, marble and alternation of dolostone and limestone and minor pelite. Relatively narrow (~ 1–3 m-thick) fault cores are characterized by gouge and cataclasites, surrounded by brecciated damage zones as thick as tens to hundreds of meters. Damage zones show widespread evidence for dilation in the form of dilation breccia, large calcite crystals and aggregates, and centimeter- to meter-thick veins. Moreover, the fault zone contains a multitude of polished slip surfaces with multiple sets of slickensides and slickenfibers. Oblique to strike-slip kinematics dominates over the large part of the fault mirrors and both overprint and are overprinted by down-dip slip surfaces. The fault network includes dominant NE-SW right-lateral faults with a minor normal component and NW–SE left-lateral steep faults with a negligible reverse component, consistent with a km-size dextral NE-SW-striking Riedel shear zone, in turn representing an antithetic R’ of the regional sinistral shear zone constituted by the Ligurian Alps after the nappe stacking. The Horse Head Fault Zone accommodated km-scale displacement before the Early Miocene, as it is sealed by the sedimentary deposits of the Finale Ligure Basin, thus predating the Corsica-Sardinia drifting. Results of this work constraint the bending of the Ligurian Alps as part of the Western Alpine arc as accomplished through two consecutive, late Oligocene and Early Miocene, stages driven by the combination of Adria rotation and the rollback of the Apennine subduction.

Zircon U-Pb chronology, geochemistry and geological significance of the Tongjiang-Fuyuan Mesozoic magmatic rocks, NE China

AbstractTypical ophiolitic rock assemblages such as siliciclastic rocks, basalts and gabbros, together with the subduction-related intermediate-acidic intrusive rocks, are newly discovered in the Tongjiang-Fuyuan area of the Heilongjiang Provence, NE China. To determine the formation age and genesis of the mafic rocks (basalts and gabbros) and intermediate-acidic intrusive rocks (granodiorites) in the area, as well as their geodynamic settings, the whole-rock geochemical analysis and zircon LA-ICP-MS U-Pb dating were carried out. Zircon U-Pb results suggest that the granodiorites are 93–95 Ma and gabbro is 95 Ma, respectively. Geochemical results show that the gabbros and basalts exhibit characteristics of ocean island basalt (OIB) affinity and are typically related to having originated from mantle plumes. While the granodiorites show the nature of the island-arc magmatic rocks and may originate from the lower crust. Based on the coeval igneous rock associations and regional tectonic evolution, we conclude that the late Cretaceous magmatic rocks in the Tongjiang-Fuyuan area are the product of continuous subduction of the Palaeo-Pacific plate and reflect the subduction rollback process of the Palaeo-Pacific plate.

Sr-Nd-Ca isotopic variations of Cenozoic calc-alkaline and alkaline volcanic rocks above a slab tear in Western Anatolia, Turkey

The tectonic mechanisms that may trigger a transition from calc-alkaline to alkaline volcanic activity above a subduction zone are enigmatic. We report major/trace elemental and Sr-Nd-Ca isotopic compositions of a suite of magmatic samples from the Miocene Gördes calc-alkaline dacites and the Quaternary Kula alkaline basalts from Western Anatolia, Turkey. Zircon sensitive high-resolution ion microprobe dating shows that the Gördes dacites formed at ca. 18 Ma. They are characterized by high-K calc-alkaline affinities, enrichment of light rare earth elements and large-ion lithophile elements (Rb, Th, U), positive Pb anomalies and negative Eu and high field strength element (HFSE) (Nb, Ta, and Ti) anomalies with high 87Sr/86Sri (0.70977−0.71010) and low εNd(t) (−7.3 to −7.0). They have small δ44/40Ca (0.59‰−0.69‰) values lower than that of mid-ocean ridge basalt (0.83% ± 0.11%). The Kula basalts exhibit alkaline affinities and are more enriched in incompatible elements (e.g., Rb, Ba, Th, Nb, Ta, Pb, and Sr) than those of typical oceanic island basalts. Their low 87Sr/86Sri (0.70307−0.70343) and high εNd(t) (+4.2 to +6.5) are consistent with an asthenospheric mantle source. Their δ44/40Ca values, ranging from 0.67‰ to 0.81‰, are higher than those of the dacites and lower than that of the bulk silicate earth (0.94‰ ± 0.10‰). Geochemical modeling suggests that variable degrees of partial melting of an asthenospheric mantle involving recycled oceanic crust may have resulted in low δ44/40Ca values of the Kula basalts. Combined with reported mantle tomographic images, we interpret that sub-slab asthenospheric mantle upwelling through a slab tear during subduction roll-back may have played a key role in the Sr-Nd-Ca isotopic variability from Miocene calc-alkaline dacites to Quaternary alkaline basalts in Western Anatolia.

U-Pb in-situ SIMS baddeleyite and zircon dates and thermodynamic modeling of the Mangabal Complex: Indications of asthenospheric mantle influence in the formation of layered intrusions of the Brasília orogen

The Mangabal Complex lies within the Brasília Belt, a long-lived Neoproterozoic orogen formed during the amalgamation of the São Francisco/Congo, Amazonian, and Paranapanema Cratons in western Gondwana. The complex comprises two sequences of mafic and ultramafic cumulates, separated by less than 3 km. Despite having undergone variable degrees of deformation and metamorphism, the two sequences exhibit locally preserved pristine relict igneous minerals and textures. The origin and parental magma for the complex remain poorly constrained. We present petrography, whole-rock lithogeochemistry, mineral chemistry and U-Pb in-situ SIMS data from micro-baddeleyite and zircon from Mangabal Complex rocks collected from drill holes. We show that a simple thermodynamic model of batch crystallization using a tholeiitic basalt as parental magma in the upper crust closely matches the studied rocks and agrees with a broadly comagmatic origin for the two sequences. Baddeleyite inferred to represent a primary igneous phase of the complex gives a minimum age of 653 ± 31 Ma, suggesting that the Mangabal Complex, hosted in the Arenópolis-Arc segment, could be coeval with the Damolândia and Taquaral intrusions, hosted in the high-grade Anápolis-Itauçu Complex. Considering the temporal and spatial correlations, we propose that decompression melting of upwelling asthenosphere triggered by rollback subduction and/or the influence of a mantle plume is required to explain the petrogenesis of the mafic–ultramafic intrusions in the eastern Arenópolis Arc and westernmost Anápolis-Itauçu Complex.

Multivariate analyses in paleobiogeography of Devonian rugose corals for revealing the multiple accretionary processes of the southwestern Central Asian Orogenic Belt

Biogeographic analysis has been applied on the globe- or orogen-scale tectonic evolution. The scarcity of paleobiogeographic study cases caused a debate as to the Late Paleozoic detailed accretion and convergence process of collages in the Central Asian Orogenic Belt. This contribution presents contrasting biota characteristics of the Early and Middle Devonian rugose corals in the Dananhu-Harlik Arc. By the multivariate analyses of the Early and Middle Devonian rugose coral faunas from series of the tectonic units in the Central Asian Orogenic Belt and neighboring areas, we reveal that two distinct paleobiogeographic units, namely, the Boreal and Tethyan Realms, were separated by the North Tianshan Ocean and Kalamaili Ocean in the Early and Middle Devonian, respectively. It means that the Dananhu-Harlik Arc experienced a paleobiogeographic change from the Boreal Realm in the Early Devonian to the Tethyan Realm in the Middle Devonian. Comprehensively considering the coral paleobiogeography, paleomagnetism, magmatism, provenance and distribution of ophiolites in the southwestern Central Asian Orogenic Belt, we speculate that the basal controlling factors of the Devonian coral paleobiogeographic pattern in the CAOB are the geographical isolation caused by the presence of oceanic basins, and the Mongolia and Kazakhstan Collages were separated by the North Tianshan Ocean sandwiched between the Dananhu-Harlik Arc and Central Tianshan Block during the Silurian to Early Devonian. In the Middle Devonian, the bidirectional and roll-back subduction of the North Tianshan Ocean caused the Dananhu-Harlik Arc separated from the Mongolia Collage by the Kalamaili Ocean and further moved towards the eastern Kazakhstan Collage. It caused the change of the rugose coral paleobiogeographic pattern in the southwestern Central Asian Orogenic Belt. The terminal convergence of Dananhu-Harlik Arc and Kazakhstan Collage occurred before the Latest Devonian and subsequently led to the formation of a new collage sandwiched by the Junggar-Kalamaili and South Tianshan Oceans.

The Gibraltar slab dynamics and its influence on past and present-day Alboran domain deformation: Insights from thermo-mechanical numerical modelling

The origin and tectonic evolution of the Gibraltar Arc system is the result of a complex geodynamic evolution involving the convergence of the Eurasian and African plates and the dynamic impact of the Gibraltar slab. Although geologic and geophysical data collected in the last few years have increased our knowledge of the Gibraltar Arc region, it is still unclear which are the mechanical links between the Gibraltar slab and the past deformation of the overriding Alboran lithosphere, as well as to which degree this subduction system is presently active. In this study, we use 2D numerical modelling to investigate the impact of the Gibraltar slab dynamics on the deformation of the overriding Alboran lithosphere. Our model simulates a WE generic vertical section at an approximate latitude of 36°N and considers an initial setup at about Burdigalian times (∼20 Ma), when the subduction front position is relatively well constrained by recent tectonic reconstructions. Our modelling shows a switch in the overriding plate (OP) stress state from extensional stresses during the slab rollback to compressional stresses near the trench when the rollback velocity decreases, caused by the change in slab-induced mantle flow. We also find that much of the crustal and lithospheric deformation occur during fast slab rollback and OP extension in the first 10 Myr of evolution, while after that only moderate deformation associated with subduction is predicted. Finally, we find that despite the subduction rollback ceases, the ongoing motion of the deeper portion of the slab induces a mantle flow that causes some amount of west-directed basal drag of the Alboran lithosphere. This basal drag generates interplate compresional stresses compatible with the distribution of intermediate-depth earthquakes in western Alboran.

Opposite microplate rotations on the East African Rift: Similarity to double saloon door tectonics

Documented opposite rotations of the Victoria and Rovuma Microplates in the East African Rift (EAR) are compared to opposite microplate rotations in backarc basins related to subduction rollback. Counter-clockwise rotation of the Victoria Microplate implies northward propagation should occur on the Lakes Rukwa/Tanganyika portion of the Western Branch of the EAR despite the orthodox view that southerly propagation occurs there. A compilation of evidence for propagation including volcanic isotopic ages, border fault throws, subsidence and sediment thickness and age shows northwestern younging of volcanics from 25.9 to 0 Ma at Rungwe southeast of Lake Rukwa to 5.8–2.6 Ma ages at Mwenga-Kamituga, northern end of Lake Tanganyika, and north propagation in Rusizi to Kivu, and Virunga to Toro Ankole segments. Predicted southerly propagation on the Lake Malawi Rift is supported by southerly decreasing border fault throws and sediment isopachs. Where the southern Victoria Microplate meets the northern Rovuma Microplate, rather than a simple strike slip interface as previously modelled, a triangular rift basin develops widening to the ENE and narrowing to the WSW. Seismicity, digital elevation models and a new geological map indicate this is represented by the combined Ruhuhu/Kilombero/Kilosa/Ruvu/Rufiji Rift complex.

Skarns at a peridotite contact, Neyriz, Iran: High-temperature melting and metamorphism in a forearc setting

In the Neyriz area of southern Iran unusual skarns are found above serpentinised peridotites at the contact with crystalline limestones. They have been interpreted as the high-temperature product of intrusion by hot peridotite into limestones, as low-temperature rodingites (the product of calcium metasomatism associated with serpentinisation), or a fortuitous juxtaposition of unrelated rocks. Their age is not known. The skarns are wollastonite-pyroxene-calcite rocks in which dark green pyroxenes are fassaites with high Al, Fe and Ti with high Ca-Tschermak's components. The field relations, textures, mineral assemblages and compositions, and melt inclusions in wollastonite and fassaite indicate the skarns formed by melting at the contact between peridotites and limestones with retrograde reactions during cooling forming garnet and anorthite. There are uncertainties in temperature estimates since pressure, XCO2 and other compositional variables are unknown, but melting temperatures were likely to have been close to 1100°C with garnet formation at approximately 900°C. Later alteration of some skarns and formation of rodingites close to the limestone-peridotite contacts occurred during low-temperature Ca metasomatism, probably after emplacement of the ophiolite during Zagros collision. A hot intrusive origin for the skarns appears incompatible with an arc-related supra-subduction origin of the ophiolite inferred from geochemical studies, but recent work in eastern Indonesia shows that during late Neogene subduction rollback, melts formed above hot mantle that intruded highly extended continental crust in a forearc setting. The scale, timing and temperatures of melting and metamorphism are very similar to those of the Late Cretaceous Neyriz ophiolite.

Late Devonian to early Carboniferous roll-back related extension setting for the Tuwu-Yandong porphyry copper metallogenic belt in the Dananhu arc of the eastern Tianshan (NW China) in the southern Altaids

The Tuwu and Yandong deposits are the representative major porphyry Cu deposits in the eastern Tianshan orogen (NW China) of the southern Altaids. The ages of their ore-hosted volcanic rocks and their tectonic settings are controversial. To address this problem, we report new geochronological and geochemicaldata of ore-hosted volcanic rocks in the Tuwu and Yandong deposits in the middle section of the Dananhu arc. The latest-stage dacites yield U-Pb zircon ages of 363.5 ± 1.8 Ma and 344.0 ± 4.1 Ma. The ore-hosted volcanic rocks consist of basalt, andesite, dacite and volcaniclastics. The basalt and andesite are tholeiite, while the dacites are tholeiite to calc-alkaline. The geochemical characters of the basalts suggest that they derived from the partial melting of the depleted mantle which were metasomatized by fluids and/or melts released from the subducted slab. And the rocks have well correlations between SiO2 and K2O + Na2O, MgO, TiO2, Al2O3, and CaO, narrow range of La/Sm ratios, and Nb/Th ratios decrease with increasing Th/Yb ratios, suggesting fractionation and crustal contamination were also essential for their genesis. The Nb-Ta negative anomalies and enrichment in the light rare earth elements (LREEs), Rb, Ba, Th, Sr and Pb indicate that these volcanic rocks formed in the subduction zone. Integrating our data with those of previous investigations, we concluded that the protoliths of the rocks were probably derived from asthenospheric garnet-free mantle in a roll-back subduction setting in the late Devonian to early Carboniferous.

Neogene sediment provenance and paleogeography of SE Sulawesi, Indonesia

AbstractNeogene syn‐and post‐orogenic sedimentary rocks in Sulawesi are important to understand the history of collision of Australian continental crust with SE Asia. However, they have received relatively little attention, and have generally been considered to be molasse‐type deposits of similar characteristics across Sulawesi—the Celebes Molasse. New field sedimentological observations, results from heavy mineral studies and detrital zircon U–Pb geochronology, together with conventional sandstone petrography identify three important provenance shifts, that can be correlated with significant tectonic events and enable the reconstruction of the Neogene palaeogeography of SE Sulawesi. The first provenance shift was related to the Early Miocene collision of Australian crust and the North Sulawesi volcanic arc, accompanied by ophiolite emplacement in East Sulawesi, marked by a change from pre‐collision carbonates to ultramafic‐rich clastics of the Bungku Formation. A second change in the Late Miocene was marked by input of abundant metamorphic detritus including blueschists into the Pandua Formation indicating exhumation of Oligo‐Miocene subduction zone material. The third provenance change, of Mio‐Pliocene age, was from the ophiolite‐ and blueschist‐rich Pandua Formation to the quartz‐rich Langkowala Formation, and records blocking of the ophiolite debris pathways by the rise and progressive unroofing of low‐ to high‐grade metamorphic rocks. The Neogene sedimentary rocks of SE Sulawesi record Early Miocene collision and ophiolite emplacement, followed by extension which led to exhumation of Mesozoic and metamorphic rocks, driven by Banda subduction rollback.

The Celebes Molasse: A revised Neogene stratigraphy for Sulawesi, Indonesia

• Review of Neogene stratigraphy of Sulawesi based on new field and laboratory work. • Celebes Molasse includes sediments of different age and depositional environment. • Early Miocene unconformity marks collision of the Sula Spur microcontinent. • Younger unconformities not related to collisions but due to extension. • Extension mainly resulted from subduction rollback. Over the past century, a wide variety of poorly dated sediments that unconformably overlie pre-Neogene rocks in Sulawesi have been assigned to the Celebes Molasse and interpreted as post-orogenic rocks deposited after a major collision. New data from fieldwork and laboratory studies, including mineralogical, palaeontological and zircon geochronological analyses, provide the basis for a new Neogene stratigraphy and show that sediments assigned to the Celebes Molasse have significant variations in age, environment of deposition, composition and sedimentation history. Identified unconformities in the (1) Early Miocene (c. 23 Ma), (2) Middle Miocene (c. 15 Ma), (3) Mio-Pliocene (c. 6–5.3 Ma), and (4 and 5) Pleistocene (c. 1.8 and 0.1 Ma) separate Lower Miocene, Middle to Upper Miocene, Pliocene, Pleistocene and Holocene mega-sequences. These mega-sequences resulted from Early Miocene collision followed by extension, driven by subduction rollback, causing both uplift and subsidence, in contrast to previous interpretations of multiple collisions.

Age and genesis of the Lamahanshan Ag-Pb-Zn deposit, southern Great Xing'an Range, northeastern China: Constraints from sphalerite Rb-Sr dating, fluid inclusions and H-O-S-Pb isotopes

The Lamahanshan deposit in the eastern Central Asian Orogenic Belt (CAOB) is a newly-discovered Ag-Pb-Zn vein deposit in the southern Great Xing'an Range (GXR). Vein-type Ag-Pb-Zn orebodies are mainly hosted in the fracture zones of gneissic plagiogranite, granite porphyry, and quartz porphyry. Mineralization comprises four stages: (I) quartz-magnetite, (II) quartz-pyrite-chalcopyrite, (III) quartz-polymetallic sulfides, and (IV) quartz ± sphalerite. Sphalerite Rb-Sr dating yielded an isochron age of 137.7 ± 6.1 Ma, coeval with the regional Late Jurassic-Early Cretaceous Pb-Zn polymetallic mineralization. Three types of fluid inclusions (FIs), namely vapor-rich two-phase (LV-type), liquid-rich two-phase (VL-type), and daughter mineral-bearing three-phase (SL-type), are distinguished at Lamahanshan. Stage I and II contain all FI types with homogenization temperatures (Th) of 383–418 °C and 350–392 °C and salinities of 0.9–38.1 wt% and 0.7–35.0 wt%, respectively, whereas stage III contain VL- and LV-type FIs with Th = 318–349 °C and salinity = 0.7–10.5 wt%. Only VL-type FIs occur in stage IV, with Th of 271–320 °C and salinities of 4.3–7.8 wt%. The microthermometric and H-O isotope data (δ18OH2O = −9.4 to 0.9‰ and δD = −137.0 to −124.7‰) indicate that the ore-forming fluids were initially magmatic with later meteoric water incursion. Fluid boiling, cooling, fluid-rock reactions, and meteoric water mixing were the major ore precipitation mechanisms at Lamahanshan. The sulfide S (δ34S = 4.0–6.7‰), Sr ((87Sr/86Sr)i = 0.698957–0.700069), and Pb (206Pb/204Pb = 17.230–18.193, 207Pb/204Pb = 15.440–15.627, and 208Pb/204Pb = 37.664–38.407) isotopes also support that the metals were magmatic-derived. Integrating the available geological, mineralization, fluid inclusion, and H-O-S-Sr-Pb isotope evidence, we propose that the Lamahanshan deposit shares many similar features with distal magmatic-hydrothermal vein-type ores, and was formed in an extensional setting related to the Early Cretaceous Paleo-Pacific subduction rollback.

Response of Drainage Pattern and Basin Evolution to Tectonic and Climatic Changes Along the Dinarides-Hellenides Orogen

The junction of the Dinaric and Hellenic mountain belts hosts a trans-orogenic normal fault system (Shkoder-Peja Normal Fault, SPNF) that has accommodated oroclinal bending, as well as focused basin formation and drainage of the Drin River catchment. Analysis of fluvial morphology of this catchment reveals higher values of river slope indices (ksn) and χ (Chi) between the normal faults of the SPNF and the Drin drainage divide. The drainage divide is predicted to be migrating away from the SPNF, except at the NE end of the SPNF system. Two basins analysed in the hangingwall of the SPNF, the Western Kosovo Basin (WKB) and Tropoja Basin (TB), contain late Pliocene-to-Holocene sedimentary rocks deposited well after the main fault activity and immediately after the Last Glacial Maximum (LGM). These layers document an early Pleistocene transition from lacustrine to fluvial conditions that reflects a sudden change from internal to external drainage of paleo-lakes. In the TB, these layers were incised to form three generations of river terraces, interpreted to reflect episodic downstream incision during re-organisation of the paleo-Drin River drainage system. 36Cl-cosmogenic-nuclide depth-profile ages of the two youngest terraces (∼12, ∼8 ka) correlate with periods of wetter climate and increased sediment transport in post-LGM time. The incision rate (∼12 mm/yr) is significantly greater than reported in central and southern Albania. Thus, glacial/interglacial climatic variability, hinterland erosion and base-level changes appear to have regulated basin filling and excavation cycles when the rivers draining the WKB and TB became part of the river network emptying into the Adriatic Sea. These dramatic morphological changes occurred long after normal faulting and clockwise rotation on the SPNF initiated in late Oligocene-Miocene time. Faulting provided a structural and erosional template upon which climate-induced erosion in Holocene time effected reorganisation of the regional drainage pattern, including the formation and partial demise of lakes and basins. The arc of the main drainage divide around the SPNF deviates from the general coincidence of this divide with the NW-SE trend of the Dinaric-Hellenic mountain chain. This arc encompasses the morphological imprint left by roll-back subduction of the Adriatic slab beneath the northwestern Hellenides.

Petrogenesis of Pre-Tertiary A-Type Granitoid in Jambi Area and its Implications of Rare Earth Element Potential on Main Range Sumatra Belt

Granitoid rocks are one of the main sources of rare earth elements (REE). This makes granitoid characterization become important in the early stages of REE exploration. Almost all granitoids in Indonesia have been mapped. However, more detailed granitoid studies in Indonesia are still focused on Bangka and Belitung granites (tin belt granite). In contrast to Bangka and Belitung granites, studies related to petrogenesis and granite characteristics on the mainland of Sumatra Island (Sumatra Main Range) are rarely done, such as granitoid in Jambi area. The aim of this study is to determine the characteristics of the Pre-Tertiary granitoids located in the Tigapuluh and Duabelas Mountains, Jambi. The Tanjungjabung Barat granitoid represent the Tigapuluh Mountains area while the Sarolangun granitoid represent the Duabelas Mountains area. These two granitoids interpreted to be Triassic to Jurassic in age. Granitoid characteristics include petrological and geochemical characters. This study also focuses on the petrogenesis of Pre-Tertiary granitoid and its implications for the abundance of REEs. Megascopic observation, petrographic, and geochemical analysis are done in this study. Geochemical analysis was done at the Center of Geological Survey Laboratory, Bandung using the ICP-MS Thermo Icap-Q and XRF ADVANT XP Thermo ARL9900 instruments. Based on megascopic and petrographic observations, both of the granitoids are classified as granite. Geochemically, these two granitoids show the character of A-type granite which is formed in the post-collision environment, and derived from the crustal melting with ferrous alkalic to alkali-calcic peraluminous affinities. This crustal melting happened due to the collision of the Sibumasu Block with Indochina resulting in crustal thickening and crustal melting. The magma then contaminated effectively in the rift environment due to the subduction roll-back of Meso-Tethys in the Late Triassic. Subduction in the West Sumatra also play roles in the genesis and it is shown by the geochemical character of the Sarolangun granitoid. Effective contamination derives the characteristics of A-type granite so that the REE content in both granites are abundant. The abundance of REE is indicated by the presence of the allanite, monazite, apatite, zircon, and titanite. The REE concentration of the Sarolangun granitoid reaches 330 ppm, while the Tanjungjabung Barat granitoid reaches 261 ppm. The REE concentrations of A-type granitoid in Jambi then compared with A-type granitoids from the world and showed relatively the same REE concentrations. The REE concentrations of these granitoids are also higher than the other type granitoids in Indonesia. However, the REE concentrations of Jambi granitoids are similar to the fractionated S-type granite in Bangka. With a recent study showing the presence of A-type granitoid in Sarudik (North Sumatra) and Bukit Batu (South Sumatra), the A-type granitoid in this study indicates the existence of A-type granitoid belt in the Sumatra Main Range. This belt will have a high abundance of REE concentrations and potentially become the source for REE deposits. The author hopes that this study could improve the understanding of tectonic in Sumatra and suggestion for REE exploration in the area.