Cessation Of Subduction Research Articles

Geochemistry and geodynamic setting of Late Cretaceous-Miocene basalts in the southern Korean Peninsula

Newly obtained data highlight strong geological and geochemical differences between Late Cretaceous-Paleogene and Eocene-Middle Miocene volcanic rocks in the southern Korean Peninsula. The rocks are spatially separated and differ in the proportions of acid and basic varieties. The Late Cretaceous-Paleogene basalts are similar to suprasubduction rocks in having high Al2O3, LILE, and Th contents, and low TiO2 and HFSE contents. The Miocene basalts have a composition intermediate between those of subduction and within-plate rocks. Compared to subduction rocks, they are lower in radiogenic Sr, K, LILE (Cs, Rb, Ba), and Th and higher in MgO, Ni, Ti, and HREE. A drastic change in U, Ba, Rb, Ce, Th, and 87Sr/86Sr in the basic volcanic rocks of the southern Korean Peninsula at the Late Cretaceous-Paleogene boundary suggests a decreasing sedimentary contribution to the magma. The latter testifies to a change in the direction of the motion of the oceanic and continental plates, increasing compressional forces and, finally, the cessation of subduction. The synthesis of the original authors and published data on Cenozoic volcanism of the southern Korean Peninsula and the eastern Sikhote Alin showed that the tectonic evolution of the eastern Eurasian margin occurred in four stages: Late Cretaceous-Paleogene, Eocene-Oligocene, Early, and Middle-Miocene.

Geochemical response of magmas to Neogene–Quaternary continental collision in the Carpathian–Pannonian region: A review

The Carpathian–Pannonian Region contains Neogene to Quaternary magmatic rocks of highly diverse composition (calc-alkaline, shoshonitic and mafic alkalic) that were generated in response to complex microplate tectonics including subduction followed by roll-back, collision, subducted slab break-off, rotations and extension. Major element, trace element and isotopic geochemical data of representative parental lavas and mantle xenoliths suggests that subduction components were preserved in the mantle following the cessation of subduction, and were reactivated by asthenosphere uprise via subduction roll-back, slab detachment, slab-break-off or slab-tearing. Changes in the composition of the mantle through time are evident in the geochemistry, supporting established geodynamic models. Magmatism occurred in a back-arc setting in the Western Carpathians and Pannonian Basin (Western Segment), producing felsic volcaniclastic rocks between 21 to 18 Ma ago, followed by younger felsic and intermediate calc-alkaline lavas (18–8 Ma) and finished with alkalic-mafic basaltic volcanism (10–0.1 Ma). Volcanic rocks become younger in this segment towards the north. Geochemical data for the felsic and calc-alkaline rocks suggest a decrease in the subduction component through time and a change in source from a crustal one, through a mixed crustal/mantle source to a mantle source. Block rotation, subducted roll-back and continental collision triggered partial melting by either delamination and/or asthenosphere upwelling that also generated the younger alkalic-mafic magmatism. In the westernmost East Carpathians (Central Segment) calc-alkaline volcanism was simultaneously spread across ca. 100 km in several lineaments, parallel or perpendicular to the plane of continental collision, from 15 to 9 Ma. Geochemical studies indicate a heterogeneous mantle toward the back-arc with a larger degree of fluid-induced metasomatism, source enrichment and assimilation on moving north-eastward toward the presumed trench. Subduction-related roll-back may have triggered melting, although there may have been a role for back-arc extension and asthenosphere uprise related to slab break-off. Calc-alkaline and adakite-like magmas were erupted in the Apuseni Mountains volcanic area (Interior Segment) from15–9 Ma, without any apparent relationship with the coeval roll-back processes in the front of the orogen. Magmatic activity ended with OIB-like alkali basaltic (2.5 Ma) and shoshonitic magmatism (1.6 Ma). Lithosphere breakup may have been an important process during extreme block rotations (∼60°) between 14 and 12 Ma, leading to decompressional melting of the lithospheric and asthenospheric sources. Eruption of alkali basalts suggests decompressional melting of an OIB-source asthenosphere. Mixing of asthenospheric melts with melts from the metasomatized lithosphere along an east–west reactivated fault-system could be responsible for the generation of shoshonitic magmas during transtension and attenuation of the lithosphere. Voluminous calc-alkaline magmatism occurred in the Cãlimani-Gurghiu-Harghita volcanic area (South-eastern Segment) between 10 and 3.5 Ma. Activity continued south-eastwards into the South Harghita area, in which activity started (ca. 3.0–0.03 Ma, with contemporaneous eruption of calc-alkaline (some with adakite-like characteristics), shoshonitic and alkali basaltic magmas from 2 to 0.3 Ma. Along arc magma generation was related to progressive break-off of the subducted slab and asthenosphere uprise. For South Harghita, decompressional melting of an OIB-like asthenospheric mantle (producing alkali basalt magmas) coupled with fluid-dominated melting close to the subducted slab (generating adakite-like magmas) and mixing between slab-derived melts and asthenospheric melts (generating shoshonites) is suggested. Break-off and tearing of the subducted slab at shallow levels required explaining this situation.

Sinking slabs below fossil subduction zones

The fate of sinking lithospheric slabs, following the cessation of subduction, is examined using a two-dimensional numerical model of mantle convection. The model incorporates rigidly moving oceanic plates, which recycle through the underlying viscous mantle, and rigid continental plates, which cannot be subducted due to their inherent buoyancy. The model is used to study the effects of the cessation of subduction due to the accretion of a 4000 km wide block of continental crust to the margin of a large stationary continent (e.g. the India–Asia collision). We model a continental block (representative of greater India) that is carried along with an oceanic plate towards a subduction zone at the leading edge of a much larger stationary continent, which represents Asia. Prior to continental collision, subducted oceanic plate material under-rides the margin of the larger continent for several hundred kilometres before peeling away from the surface boundary layer and descending into the mantle as a cold narrow slab. Consequently, once a moving continental block arrives at the continental margin and closes over the original subduction zone the surface location of the continental suture lies several hundred kilometres seaward of the location of the cold remnant of the sinking slab. The ongoing convective flow in the subcontinental mantle adjusts to accommodate the change in mechanical boundary condition at the upper surface. A step increase in viscosity at 660 km depth slows the rate of slab descent substantially but cannot account for mid-mantle thermal anomalies attributed to subduction zones that became extinct more than 140 My ago. The horizontal displacement of the slab remnants relative to the continental crustal suture depends on the relative motion of the fossil plate boundary at the surface and the underlying mantle. Over-riding of an intra-oceanic subduction zone or the indenting of a smaller subcontinent into a larger continental mass can cause the tomographically observed cold anomalies to be displaced laterally by thousands of kilometers from the location of the former converging plate boundary.

Blueschists and Blue Amphiboles: How much Subduction do they Need?

We focus on blueschist formation in evolving, nascent intra-oceanic subduction zones where cessation of subduction, e.g., by collision with continental margins, preserves the first-formed blueschist products. The Caribbean area provides a case study for a tectonic regime that has led to obducted blueschist occurrences without obvious coeval volcanism, raising the question of the minimum subduction duration and amount of convergence necessary to produce them. Systematic numerical modeling shows that for slab ages of 20-100 Ma and subduction rates of 2-14 cm/yr blueschist-facies conditions can be attained in only 0.25 to 3 m.y., with amounts of total convergence ranging between 35 and 75 km. Because of the geometrical interplay between the subducting slab "nose" and the evolving array of isotherms, younger and hotter slabs can lead to earlier blueschist formation than older, cooler slabs. There is a distinct optimum for slabs between 40 and 60 m.y. In principle, Andean-type, continental margin models yield almost identical values. Depending on the specific exhumation scenario, the time required for exhumation must in general be added to the minimum life span of the subduction zone.

Mineralogy, petrography and whole-rock geochemistry of the Tertiary granitic intrusions in the Eastern Pontides, Turkey

Granitic intrusions in Eocene and post-Eocene volcanic rocks outcrop in the Northern and Southern Zones of the Eastern Pontides, NE Turkey. Intrusions in the Northern Zone extend NW–SE whereas those in the Southern Zone are nearly E–W orientated. The contacts of the intrusions with the volcanic rocks are sharp, epidotized and include volcanic xenoliths. The margins of Southern Zone intrusions often contain abundant angular mafic microgranular enclaves of diorite to quartz diorite composition. The rocks also show evidence of mingling and mixing between coeval mafic and felsic magmas. Field observations suggest a stopping type of ascent and emplacement style for the intrusions. Petrographically, intrusions show variations in both colour and mineralogy with fine to medium granular, monzonitic, poikilitic, rapakivi, anti-rapakivi and graphic textures. Based on modal mineralogy, the Northern rocks are monzonite, quartz monzonite, monzodiorite and quartz monzodiorite whereas the Southern samples are monzogranite and granodiorite. The rocks are generally calc-alkaline to mildly alkaline transitional and metaluminous, and show a CAFEMIC trend. The Northern Zone intrusions form a post-collisional, A-type, alkaline monzonitic association and Southern Zone intrusions form a post-collisional, I-type, granodioritic calc-alkaline-transitional association. Whole-rock compositional data indicate that differentiation occurred via fractional crystallisation with or without magma mixing. Incompatible and rare earth element patterns indicate that both the Northern and the Southern Zone intrusions were derived from melting of a mantle region enriched by subduction-related fluids but evolved differently during ascent and emplacement. U–Pb zircon dating in the Southern Zone yields an intrusion age of 44.4±0.3 Ma. Regional geodynamics indicates post-collisional extensional tectonics in the region. Crustal involvement and the level of emplacement may have been important for magmatic evolution, especially for the Southern Zone intrusions, after the cessation of subduction and subsequent crustal thickening.

Ocean trench blocked and obliterated by Banda forearc collision with Australian proximal continental slope

The bathymetry and abrupt changes in earthquake seismicity around the eastern end of the Java Trench suggest it is now blocked south–east of Sumba by the Australian, Jurassic-rifted, continental margin forming the largely submarine Roti–Savu Ridge. Plate reconstructions have demonstrated that from at least 45 Ma the Java Trench continued far to the east of Sumba. From about 12 Ma the eastern part of the Java Trench (called Banda Trench) continued as the active plate boundary, located between what was to become Timor Island, then part of the Australian proximal continental slope, and the Banda Volcanic Arc. This Banda Trench began to be obliterated by continental margin-arc collision between about 3.5 and 2 Ma. The present position of the defunct Banda Trench can be located by use of plate reconstructions, earthquake seismology, deep reflection seismology, DSDP 262 results and geological mapping as being buried under the para-autochthon below the foothills of southern Timor. Locating the former trench guides the location of the apparently missing large southern part of the Banda forearc that was carried over the Australian continental margin during the final stage of the period of subduction of that continental margin that lasted from about 12 Ma to about 3.5 Ma. Tectonic collision is defined and distinguished from subduction and rollback. Collision in the southern part of the Banda Arc was initiated when the overriding forearc basement of the upper plate reached the proximal part of the Australian continental slope of the lower plate, and subduction stopped. Collision is characterised by fold and thrust deformation associated with the development of structurally high decollements. This collision deformed the basement and cover of the forearc accretionary prism of the upper plate with part of the unsubducted Australian cover rock sequences from the lower plate. Together with parts of the forearc basement they now form the exposed Banda orogen. The conversion of the northern flank of the Timor Trough from being the distal part of the Banda forearc accretionary prism, carried over the Australian continental margin, into a foreland basin was initiated by the cessation of subduction and simultaneous onset of collisional tectonics. This reinterpretation of the locked eastern end of the Java Trench proposes that, from its termination south of Sumba to at least as far east as Timor, and probably far beyond, the Java-Banda Trench and forearc overrode the subducting Australian proximal continental slope, locally to within 60 km of the shelf break. Part of the proximal forearc's accretionary prism together with part of the proximal continental slope cover sequence were detached and thrust northwards over the Java-Banda Trench and forearc by up to 80 km along the southwards dipping Savu Thrust and Wetar Suture. These reinterpretations explain the present absence of any discernible subduction ocean trench in the southern Banda Arc and the narrowness of the forearc, reduced to 30 km at Atauro, north of East Timor.

Collision, subduction and accretion events in the Philippines: A synthesis

Abstract The Philippines preserves evidence of the superimposition of tectonic processes in ancient and present‐day collision and subduction zone complexes. The Baguio District in northern Luzon, the Palawan–Central Philippine region and the Mati–Pujada area in southeastern Mindanao resulted from events related to subduction polarity reversal leading to trench initiation, continent‐arc collision and autochthonous oceanic lithosphere emplacement, respectively. Geological data on the Baguio District in Northern Luzon reveal an Early Miocene trench initiation for the east‐dipping Manila Trench. This followed the Late Oligocene cessation of subduction along the west‐dipping proto‐East Luzon Trough. The Manila Trench initiation, which is modeled as a consequence of the counter‐clockwise rotation of Luzon, is attributed to the collision of the Palawan microcontinental block with the Philippine Mobile Belt. In the course of rotation, Luzon onramped the South China Sea crust, effectively converting the shear zone that bounded them into a subduction zone. Several collision‐related accretionary complexes (e.g. Romblon, Mindoro) are present in the Palawan–Central Philippine region. The easternmost collision zone boundary is located east of the Romblon group of islands. The Early Miocene southwestward shift of the collision boundary from Romblon to Mindoro started to end by the Pliocene. Continuous interaction between the Palawan microcontinental block and the Philippine Mobile Belt is presently taken up again along the collisional boundary east of the Romblon group of islands. The Mati–Pujada Peninsula area, on the other hand, is underlain by the Upper Cretaceous Pujada Ophiolite. This supra‐subduction zone ophiolite is capped by chert and pelagic limestones which suggests its derivation from a relatively deep marginal basin. The Pujada Ophiolite could be a part of a proto‐Molucca Sea plate. The re‐interpretation of the geology and tectonic settings of the three areas reaffirm the complex geodynamic evolution of the Philippine archipelago and addresses some of its perceived geological enigmas.

The Exhumation of Eclogite-Facies Metamorphic Rocks-a Review of Models Confronted with Examples from the Alps

We summarize recently proposed models for the exhumation of (ultra-) high pressure ([U]HP) metamorphic units within collisional orogens. In terms of mechanics, exhumation is either caused by buoyancy or by externally applied stresses. In terms of kinematics, exhumation is either by erosion, extension, or extrusion. The tectonic scenarios that may lead to the exhumation of (U)HP units are: (1) flow within a low-angle or high-angle corner; (2) entrance of the continental margin into the subduction zone; (3) cessation of subduction; (4) slab breakoff; (5) extension in the backarc; and (6) serpentinization of the mantle wedge. The applicability of these models are discussed for some (U)HP terrains in the Alps, in particular the Koralm Complex of the Austroalpine Unit, and the Internal Penninic Nappes of the Eastern, Central, and Western Alps, (in particular, the Eclogite Zone, the Adula Nappe, and the Monte Rosa Nappe). Within the Austroalpine Koralm Complex of the Eastern Alps, the exhumation of HP rocks see...

The Exhumation of Eclogite-Facies Metamorphic Rocks—a Review of Models Confronted with Examples from the Alps

We summarize recently proposed models for the exhumation of (ultra-) high pressure ([U]HP) metamorphic units within collisional orogens. In terms of mechanics, exhumation is either caused by buoyancy or by externally applied stresses. In terms of kinematics, exhumation is either by erosion, extension, or extrusion. The tectonic scenarios that may lead to the exhumation of (U)HP units are: (1) flow within a low-angle or high-angle corner; (2) entrance of the continental margin into the subduction zone; (3) cessation of subduction; (4) slab breakoff; (5) extension in the backarc; and (6) serpentinization of the mantle wedge. The applicability of these models are discussed for some (U)HP terrains in the Alps, in particular the Koralm Complex of the Austroalpine Unit, and the Internal Penninic Nappes of the Eastern, Central, and Western Alps, (in particular, the Eclogite Zone, the Adula Nappe, and the Monte Rosa Nappe). Within the Austroalpine Koralm Complex of the Eastern Alps, the exhumation of HP rocks seems to be related to the development of a major detachment within the lower crust (Plattengneis shear zone), subsequent to the formation of an orogenic wedge. The central parts of the Koralm Complex were exhumed to a larger extent than the northern, and in particular the southern parts, as indicated by a continuous decrease of pressures toward north and south. Thus, we suggest that the Plattengneis shear zone was not related to the compressional phase during nappe stacking, but to the extensional exhumation of the Middle Austroalpine basement complexes that were previously affected by eclogite facies metamorphism. The tectonometamorphic evolution of (U)HP Internal Penninic Nappes, which are assumed to have originated from the European continental margin, may be characterized by(buoyancy-driven) extrusion of eclogite facies crustal wedges from the subduction channel with a low-angle corner geometry. During the early phase of exhumation, the pressure-temperature path is characterized by cooling in the Dora Maira Massif, the Gran Paradiso, the Monte Rosa Nappe, and the Eclogite Zone; this argues for exhumation within a still-active subduction zone. In the Adula Nappe, decompression was isothermal. At amphibolite- to greenschist facies conditions, the exhumation mechanism of these units changed to crustal extension, accompanied by minor heating during decompression (except for the Adula Nappe). This argues for cessation of subduction and removal of the cooling effect.

Roasting the mantle: Slab melting and the genesis of major Au and Au-rich Cu deposits

The generation of large deposits of Au and Cu in suprasubduction-zone settings depends upon a combination of factors, including availability of the chalcophile elements to arc magmas in their mantle source regions, and the operation of suitable hydrothermal systems in the upper crust where the deposits eventually form. The removal of chalcophile elements from the mantle wedge into arc magmas can only occur if sulfide is absent from the melted source rock, requiring oxidation of the mantle wedge to values of log f O 2 > FMQ + 2, where f O 2 is oxygen fugacity and FMQ is the fayalite-magnetite-quartz oxygen buffer. The only agent capable of effecting this change is ferric iron, carried in solution by slab-derived partial melts or supercritical fluids. Arc magmas with high potential to generate Au and Cu deposits will have certain geochemical characteristics; they will have f O 2 more than two log units above FMQ and they will have either adakitic, sodic-alkaline, or potassic-ultrapotassic affinities. Favorable tectonic settings include subduction of very young lithosphere or very slow or oblique convergence, flat subduction, and the cessation of subduction.

Contrasting Late Cretaceous with Neogene ore provinces in the Alpine-Balkan-Carpathian-Dinaride collision belt

Abstract Internal sectors of the Alpine-Balkan-Carpathian-Dinaride (ABCD) orogen comprise fundamentally different ore deposits along strike in three temporally and spatially distinct belts. These were formed by several short-lived, late-stage collisional processes (including slab break-off) during the Late Cretaceous and Oligocene to Neogene times. Reconstruction of Late Cretaceous ( c. 92–65 Ma) collisional structures, magmatic features and mineralization reveals contrasting variations along strike, including the following. (1a) The Late Cretaceous ‘banatite’ magmatic belt, which extends from the Apuseni mountains to the Balkans, associated mainly with porphyry Cu-Au, massive sulphide and Fe-Cu skarn mineral deposits. In respect to their country rocks and geodynamic setting, the magmatism is interpreted to represent either post-collisional or Andean-type calc-alkaline due to continuous subduction or break-off of the subducted lithosphere. (1b) The Alpine-West Carpathian sector, which is characterized by strong Late Cretaceous metamorphic/deformational overprint, lack of magmatism and both syn- and late-orogenic formation of metasomatic and metamorphogenic talc, magnesite, siderite and vein- and shear zone-type Cu and As-Au due to the exhumation of metamorphic core complexes. (2a) The Oligocene-Miocene Serbomacedonian-Rhodope metallogenic zone extends across several structural units from the Bosnian Dinarides to the Rhodopes and to Thrace. It includes both a belt with volcanic-hosted and vein-type Pb-Zn deposits and a belt of porphyry Cu-Au-Mo and epithermal Au mineralization, which is more common in the south. Both belts appear to relate to microcontinent collision and associated subsequent magmatism, again possibly due to slab break-off. (2b) Different types of mineralization were also formed along the internal Inner Carpathian and Alpine sectors during Late Oligocene to Miocene collision. In the Alps, mineralization formed due to eastward extrusion of fault-bounded blocks into the Carpathian arc. Associated mineral deposits are always related to exhumation of metamorphic core complexes and include: sub-vertical mesothermal Au-quartz veins and replacement As-Ag-Cu ore bodies within the metamorphic core complex, fault-bounded mineralization (Pb-W-Au) along low-angle ductile normal faults along the upper margins of the metamorphic core complex, mineralized (Sb-Au) strike-slip faults and sub-vertical Au-Ag-Sb-bearing tension veins. (2c) In contrast, nearly all Miocene ore deposits within the Carpathians are related to volcanic activity contemporaneous with the invasion of fault-bounded blocks into the Carpathian arc. These have been related to slab break-off and cessation of subduction. Mineral deposits include structurally controlled Au-Sb-Cu-Pb-Zn ore bodies within shallow volcanic edifices, with a preference for steep tension veins parallel to the motion direction of laterally escaping crustal blocks.

Tectonic controls on magmatic-hydrothermal gold mineralization in the magmatic arcs of SE Asia

Abstract The magmatic arcs of SE Asia contain some of the world’s major gold deposits. These are mainly related to magmatic-hydrothermal activity and include epithermal, porphyry Cu-Au and skarn deposits. Most gold deposits in SE Asian arcs formed during three intervals of tectonic reorganization rather than during periods of normal or steady-state subduction. These plate reorganizations and periods of gold mineralization were caused initially by the collision of the Australian craton with the Philippine Sea plate arc at 25 Ma. A second Mid-Miocene period of mineralization accompanied plate reorganization following maximum rotation or extrusion of Indochina and the cessation of spreading of the South China Sea at 17 Ma. However, the vast majority and largest deposits formed since 5 Ma in a broad belt from Taiwan to the Solomon Islands during an important period of tectonic reorganization. This tectonic reorganization accompanied a postulated change in the relative motion between the Indian-Australian and Pacific plates between 5 and 3.5 Ma following collision of the Philippine arc and the Eurasian plate in Taiwan. Arc-related magmatism in unusual tectonic settings produced the most abundant and largest deposits with many deposits associated with relatively rare high-K calc-alkaline, shoshonite, adakite and alkaline magmatism. In particular peak mineralization appears related to melting of mantle that had been previously modified by subduction. During large-scale plate reorganization this can occur at the end of a period of normal subduction, following the cessation of subduction, following arc collision or accompanying subduction reversal at approximately the same time in different parts of a complex system of magmatic arcs such as those in SE Asia. Such tectonic controls may be impossible to recognize in older orogenic belts where recognition of a major change in tectonic style and unusual magma types may be the best guides to mineralization.

Late Triassic–early Jurassic continental extension in southwestern Gondwana: tectonic segmentation and pre-break-up rifting

From the earliest Jurassic to the Cenozoic, the Neuquén Basin (central Argentina and Chile) evolved as an intra-arc and transarc–retroarc depression through the gradual development of the Andean magmatic arc. However, the region adjacent to the proto-Pacific margin of Gondwana between 30 and 40°S was subject to pre-Andean continental extension that began in the Late Triassic and lasted about 30 million years until the Early Jurassic. This extension resulted in the generation of a series of rifts oriented parallel to the margins of the extended area and characterized by continental volcaniclastic and pyroclastic deposits associated with lava flows and bimodal plutonic intrusions. The inception of the Neuquén Basin as a single depocenter occurred in the Early Pliensbachian, when post-rift thermal subsidence led to a general marine transgression. The T–J extension is closely linked to structures created by previous tectonothermal episodes such as the development of a Carboniferous–Permian orogenic belt (330–280 Ma), and to events along the proto-Pacific margin of Gondwana. Lithospheric thickening related to Late Paleozoic convergence caused strong gravitational instability in the orogenic belt between 30 and 40°S. Subsequent cessation of subduction, coeval with establishment of dextral strike-slip tectonics parallel to the continental margin, caused detachment of the subducting slab and generation of an asthenospheric window. Anomalous heating of the upper mantle resulted in bimodal magmatism, uplift, thermal weakening, and gravitational collapse of the upper crust. South of this extended area, proto-Pacific subduction was active during the same Late Triassic–Early Jurassic interval. Thus, the contrasting tectonic behaviour of the Gondwana margin north and south of 40°S suggests significant pre-Andean tectonic segmentation that coincides with the southern boundary of the area extended in the Late Triassic–Early Jurassic. Previous intepretations assumed a common massive extension in the Andean and Patagonian regions of southern South America. Our results demonstrate that T–J extensional rifting and magmatism between 30 and 40°S were the result of mechanical interaction between different lithospheric plates at the pre-Andean (proto-Pacific) continental margin. On the other hand, the mechanical and thermal processes that affected much of Patagonia during the Middle and Late Jurassic were a response to the tectonic and magmatic processes that caused the initial break-up of Gondwana with the opening of the Weddell Sea after 180 Ma.

Tectonic setting of high-Mg andesite magmatism in the SW Japan arc: K-Ar chronology of the Setouchi volcanic belt

Summary The Setouchi volcanic belt in SW Japan is characterized by the occurrence of andesites with an unusually high Mg concentration (high-Mg andesites, HMAs). Such HMAs may have formed under unusual tectonic settings, as basaltic magmas are produced dominantly in the mantle wedge of modern subduction zones. A total of 50 new K–Ar ages for the Setouchi rocks confirmed that intermediate to felsic magmatism including the eruption of HMAs took place in this volcanic belt within a short period of 13.2 ± 0.4 Ma. This is synchronous with the clockwise rotation of the SW Japan arc sliver that was associated with the opening of the Japan Sea back-arc basin. Such rotation of an arc sliver forced the initiation of subduction and partial melting of a young and hot lithosphere of the Shikoku Basin beneath the arc. Interaction between slab melts and mantle wedge peridotites may be a possible generation process of unusual HMA magmas. When the SW Japan arc sliver ceased to drift, the Setouchi magmatism was switched off by the cessation of subduction. The eruption of HMA magmas may also be attributed to an extensional stress field in the Setouchi region, in contrast to the compressional stress regime documented for other regions of the SW Japan arc.

Magmatic source enrichment by slab-derived melts in a young post-collision setting, central Mindanao (Philippines)

Central Mindanao was the locus of a Pliocene (4–5 Ma old) arc–arc collision event followed by basaltic to dacitic magmatism starting at 2.3 Ma, representing the most voluminous volcanic field in the Philippines. Lava compositions range from calc-alkaline to shoshonitic. Adakites and Nb-enriched basalts are among the magmatic products. All the lavas are Na-rich (up to 4.88%), with Na 2O/K 2O ratios from 2.5 to 6.5. Sr, Nd and Pb isotopic compositions are similar to MORB, except for some shoshonitic lavas that have slightly less radiogenic Nd ratios. K-enrichment in basalts can be related to both fractional crystallization (FC) at moderate pressures and to partial melting of an enriched source. Trace element systematics indicate that the sub-central Mindanao mantle is characterized by the presence of garnet, phlogopite, amphibole, and perhaps some titanate phase. The enrichment of this source is attributed to the interaction of slab-derived melts, i.e., adakites, with the arc mantle. This would explain the presence of Nb-enriched basalts, transitional adakites and high-magnesium andesites, as well as the bulk Na-enrichment and relatively unradiogenic character of the central Mindanao lavas. We envision an ion-exchange type of enrichment, in which the HFSE, LILE and LREE, mobilized during slab melting, are preferentially enriched in the metasomatized mantle, resulting in a diversity of post-collision magma compositions. The MORB-like isotopic signatures of the central Mindanao lavas preclude important contributions of slab-derived hydrous fluids, sediments, continental crust or an OIB-type contaminant. Slab melting after cessation of subduction is deemed possible by thermal rebound of previously depressed geotherms. Initial contributions to mantle enrichment in post-collision sites may thus come from slab melts. In most other cases of post-collision magmatism, however, this signature can be easily masked by enrichments coming from other sources, e.g., the continental lithosphere.

Marie Byrd Land, West Antarctica: Evolution of Gondwana's Pacific margin constrained by zircon U-Pb geochronology and feldspar common-Pb isotopic compositions

The Paleozoic and Mesozoic development and subsequent fragmentation of Gondwana's Pacific margin are recorded in igneous and metamorphic rocks that crop out in Marie Byrd Land, West Antarctica, recognized on geologic and paleomagnetic grounds to compose a discrete crustal block. Widespread metaluminous granitoids dated by the zircon U-Pb method as middle to late Paleozoic show that convergence-related magmatism dominated the early evolution of this margin. Dates for granodiorites, monzogranites, and granites from the Ruppert and Hobbs coasts of western Marie Byrd Land reveal a prolonged period of subduction-related calc-alkaline magmatism between at least 320 ± 3 Ma (age of the oldest granodiorite dated) and 110 ± 1 Ma (the age of the youngest I-type granitoid in the area). The latter, known as the Mount Prince granite, is intruded by swarms of mafic and intermediate dikes believed to record the onset of rifting that led to separation of the New Zealand microcontinent. The dikes have been dated by zircon U-Pb as 101 ± 1 Ma. Thus, the regime along the Ruppert and Hobbs coasts had shifted from subduction-related to rift-related magmatism within an ∼9 m.y. period. In the Kohler Range and the Pine Island Bay areas of eastern Marie Byrd Land, the calc-alkaline magmatism did not terminate until 96 ± 1 Ma, based on U-Pb dating of zircons from one granitoid sample, or 94 ± 3 Ma based on zircons from another. This evidence requires that subduction shut off from west to east, as suggested previously on the basis of geophysical models. No continental separation occurred to the east of Marie Byrd Land. The margins of the Thurston Island and Antarctic Peninsula crustal blocks went directly from convergent to inactive, except at the northernmost tip of the peninsula, where the South Shetlands Island block is actively separating. With their zircon U-Pb ages clustering around 100 ± 2 Ma, dike-free anorogenic syenites and quartz syenites along the Ruppert and Hobbs coasts show that the transition to extensional magmatism was rapid in the west. This is also reflected by the fact that from the onset of rifting at 101 ± 1 Ma to formation of oceanic crust between Marie Byrd Land and greater New Zealand (Campbell Plateau, Chatham Rise, North Island, and South Island) prior to chron 33o ca. 81 Ma required only 20 m.y. For comparison, this is only two-thirds of the ∼30 m.y. it took for the Central Atlantic to open after initial rift-related magmatism. The swiftness of the separation between Marie Byrd Land and greater New Zealand demonstrated by our data is consistent with ridge-trench interaction rather than a mantle plume as the primary cause of the breakup, as is the west to east diachroneity in the cessation of subduction. Exposures of host rocks to the erosion-resistant plutons are scarce in mostly snow- and ice-covered Marie Byrd Land. The occurrence in the zircons of widely separated granitoids of discordant U-Pb patterns we attribute to inheritance (the best-constrained upper concordia intercepts are as high as 1576 ± 55 Ma). This suggests either that stretched Precambrian basement underlies most of Marie Byrd Land, or that clastic sedimentary sequences with Precambrian detrital zircons underlie much of the margin.

Geochemistry, petrogenesis and tectonic setting of late Neoproterozoic Dokhan-type volcanic rocks in the Fatira area, eastern Egypt

The Neoproterozoic Dokhan volcanics of the Fatira area in eastern Egypt comprise two main rock suites: (a) an intermediate volcanic suite, consisting of basaltic andesite, andesite, dacite, and their associated pyroclastic rocks; and (b) a felsic volcanic suite composed of rhyolite and rhyolitic tuffs. The two suites display well-defined major and trace element trends and a continuum in composition with wide ranges in SiO2 (54–76%), CaO (8.19–0.14%), MgO (6.96–0.04%), Sr (983–7 ppm), Zr (328–95 ppm), Cr (297–1 ppm), and Ni (72–1 ppm). They are enriched in LILEs (Rb, Ba, K, Th, Ce) relative to high field strength elements (Nb, Zr, P, Ti) and show strong affinity to calc-alkaline subduction-related rocks. However, their undeformed character, their emplacement temporally and spatially with post-orogenic A-type granite, and their high Zr/Y values suggest that their emplacement follow the cessation of subduction in eastern Egypt in an extensional-related within-plate setting. Major and trace element variations in the intermediate volcanics are consistent with their formation via partial melting of an enriched subcontinental lithospheric mantle source followed by a limited low-pressure fractional crystallization of olivine and pyroxene before emplacement. The LILE enrichment relative to HFSE is attributed to the inheritance of a subduction component from mantle material which constituted the mantle wedge during previous subduction events in eastern Egypt. The evolution of the whole volcanic spectrum was governed mainly by crystal/melt fractionation of amphibole, plagioclase, titanomagnetite, and apatite in the intermediate varieties and plagioclase, amphibole, biotite, Fe–Ti oxides, apatite, and zircon in the felsic varieties. At each stage of evolution, crystal fractionation was accompanied by variable degrees of crustal contamination.

Geochemical response to varying tectonic settings: an example from southern Sulawesi (Indonesia)

The South arm of Sulawesi was an active continental margin from approximately 60 to 10 Ma, when it collided with the microcontinental fragment of Buton. Pre-collisional samples analyzed for this study are characterized by a geochemical signature typical of arc volcanics: high LILE/HFSE ratios; 87Sr/ 86Sr slightly higher than MORB; 143Nd/ 144Nd ratios similar to MORB. Syn-collisional samples have more enriched isotopic signatures, and are relatively potassium rich. This is interpreted to reflect a larger contribution from subducted sediments, added to the mantle wedge as a silicic melt. Melting of subducted sediments is interpreted to result from a decrease in subduction rate and an increase of temperature in the slab. Magmatism that postdates the collisional event by 10 Ma is characterized by higher Nb/Y ratios than the pre- or syn-collisional samples, and Sr and Nd isotopic signatures intermediate between these two groups. This is likely to reflect melting of a subduction-modified mantle, with a significant contribution from the sub-continental lithospheric mantle. Comparison with post-collisional magmatism from other areas of the world suggests that trace element signatures are similar, but isotopic characteristics are variable. The latter are likely to reflect both the age of the sub-continental lithospheric mantle and the time lag between cessation of subduction and formation of the post-collisional magmas.

The Role of Subduction‐Induced Subsidence in the Evolution of the Karoo Basin

Abstract The stratigraphic record indicates that during the Late Carboniferous to Early Triassic, South Africa experienced an episode of large‐scale subsidence that resulted in the development of the Karoo Basin. The southern margin of the region was contemporaneously dominated by an episode of compressional tectonics, and there is evidence for subduction of the paleo‐Pacific plate beneath the Karoo Basin. We propose that the long‐wavelength component of the basin subsidence resulted from the deflection of the lithosphere due to mantle flow coupled to the adjacent subduction. This was followed by a recovery phase during which the entire region was uplifted on cessation of subduction (and subsequent loss of dynamic support for subsidence), contributing to the present‐day high‐elevation topography of South Africa. Numerical models of mantle convection are presented and show that a shallow‐dipping slab is able to reconcile the observed long‐wavelength subsidence of the basin. A significant component of the n...

The formation of the Makassar Strait and the separation between SE Kalimantan and SW Sulawesi

The formation of the Makassar Strait, situated between southeast (SE) Kalimantan and western Sulawesi, is still subject of much debate. Different authors have proposed several hypotheses to explain its evolution. The only agreement between those several hypotheses is that SE Kalimantan and western Sulawesi once lay close together and that their separation is due to the opening of the Makassar Strait. The age and driving mechanism for this opening are, however, still poorly understood. The strait separates the stable core of the Eurasian Plate to the west from the very active region of the triple junction of three large plates to the east. To the north the strait is bounded by the Sulawesi Sea and to the south by the East Java Sea. The strait is roughly 100–200 km wide and 300 km long and is usually divided into the North and South Makassar basins, separated by the Paternoster Fault. The present study interprets the history of the Makassar Strait using seismic reflection profiles and gravity models, in addition to the compilation of geological information. Implications for the origin of rifting is also discussed. The result of the present study indicates that Makassar Strait was formed by the vertical sinking of a subducting oceanic plate to the east of western Sulawesi, leading to trench roll-back. This vertical sinking was accommodated by extension and rifting of continental crust above the subduction zone at a previous site of collision, causing the opening of Makassar Strait. The time of this trench roll-back marks the cessation of subduction.