High-pressure Metamorphic Terranes Research Articles

Protolith origin of the Lanling high-pressure metamorphic terrane in central Qiangtang, Tibet, reveals the subduction of the Permian–Triassic ocean island and abyssal submarine fan in the Paleo-Tethys Ocean

Unraveling the protolith origin of high-pressure and low-temperature (HP-LT) metamorphic rocks is the key to understanding the tectonic evolution of active continental margins. A comprehensive study, including field observations, geochemical analyses and zircon U-Pb geochronological dating, was conducted on the Lanling HP-LT terrane to determine its protolith origin and the presubduction paleogeography of the Paleo-Tethys Ocean. The metabasites can be divided into two geochemical groups: Group A samples exhibit low TiO2 values (1.15–1.81 wt%) and subalkaline basalt affinity with rare earth element (REE) distribution patterns and trace element abundances similar to enriched mid-ocean ridge basalts (E-MORB); Group B samples have higher TiO2 values (2.75–3.32 wt%) and show alkaline basalt affinity and ocean island basalt (OIB) geochemical signatures. Lanling metabasites are likely derived from a long-depleted mantle source combined with crustal contamination based on the following evidence: (1) positive εNd(t) values (+4.8 to +5.6) and a scattered distribution of Isr(t) values (0.704684 to 0.710999), (2) abundant Precambrian zircon grains, and (3) a volcanic arc trend in the tectonic discrimination diagram. Zircon U–Pb dating yields a timing of ca. 272–241 Ma for the protolith age of the Lanling eclogites. The protoliths of the OIB- and E-MORB-type metabasites in Lanling are postulated to be ocean island magmatites and basaltic crust of the Paleo-Tethys Ocean, respectively. Moreover, the Lanling quartzose schists contain only ancient detrital zircons with ages of >500 Ma and show protolith lithostratigraphy and provenance similar to those of the late Paleozoic clastic flysch in the South Qiangtang block (SQB). Based on the above observations and other geological evidence, a hypothetical model of the Permian–Triassic ocean island and adjacent abyssal submarine fan in the Paleo-Tethys Ocean is proposed to explain the protolith origin of the Lanling HP-LT terrane and the presubduction paleogeography in central Qiangtang.

Lithospheric extension of the accretionary wedge: An example from the Lanling high-pressure metamorphic terrane in Central Qiangtang, Tibet

Deciphering the exhumation mechanism of high-pressure−low-temperature (HP-LT) metamorphic rocks can provide important insights into the tectonic evolution of oceanic subduction zones at active continental margins. Here, we present a multidisciplinary study examining the exhumation tectonics of the Permo−Triassic eclogite-bearing HP-LT terranes of the Central Qiangtang metamorphic belt (CQMB) in the central Tibetan Plateau. Field geological relations and microscopic observations show that the HP-LT rocks in the Lanling area are separated from the epimetamorphic late Paleozoic ophiolite mélange of the hanging wall by low-angle detachment faults and primarily exhibit five stages of deformation. The HP-LT terranes were exhumed as a metamorphic core complex and show pervasive synexhumation top-to-the-SW and -S shear structures. The results of the petrological and mineralogical analysis and pseudosection modeling of retrograde eclogites indicate that these rocks are characterized by synexhumation mineral growth pulses with hairpin-type decompressional pressure-temperature trajectories. A compilation of previous geochronological data and our 40Ar/39Ar dating results of synexhumation minerals and shear bands in HP-LT rocks indicate continuous exhumation at ca. 244−215 Ma prior to the continental collision between the North Qiangtang Block (NQB) and South Qiangtang Block. Moreover, the CQMB is likely an autochthonous accretionary wedge resulting from northward subduction of the Paleo-Tethys Ocean beneath the NQB. Combined with ca. 243−230 Ma mantle upwelling to the north of Lanling, we infer that the CQMB, namely, the accretionary wedge of the Paleo-Tethys Ocean, experienced Middle−Late Triassic lithospheric extension in the NW-SE direction.

Zircon ages and Hf isotopic compositions of Ordovician and Carboniferous granitoids from central Inner Mongolia and their significance for early and late Paleozoic evolution of the Central Asian Orogenic Belt

We present zircon ages and Hf-in-zircon isotopic data for plutonic rocks and review the evolution of central Inner Mongolia, China, in the early and late Paleozoic. Zircons of a granodiorite yielded a 206Pb/238U age of 472±3Ma that reflects the time of early Paleozoic magmatism. Zircon ages were also obtained for a tonalite (329±3Ma), quartz-diorite (320±3Ma), and granite vein (297±2Ma). Our results, in combination with published zircon ages and geochemical data, document distinct magmatic episodes in central Inner Mongolia.The dated samples are mostly granodiorite, tonalite and quartz-diorite in composition with intermediate to high-silica, high Na2O (3.08–4.26wt.%), low K2O (0.89–2.86wt.%), and high Na2O/K2O and Sr/Y ratios. Their chondrite-normalized REE patterns are characterized by LREE enrichment. In mantle-normalized multi-element variation diagrams they show typical negative Nb and Ta anomalies, and all samples display positive εHf(t) and εNd(t) values, and low ISr. The Ordovician rocks, however, show higher Sr/Y and La/Yb ratios than the Carboniferous samples, implying that the older granitoids represent adakitic granitoids, and the Carboniferous granitoids are typical subduction-related arc granitoids but also with adakite-like compositions. The results are compatible with the view that the Central Asian Orogenic Belt (CAOB) in Inner Mongolia evolved through operation of several subduction systems with different polarities: an early–middle Paleozoic subduction and accretion system along the northern margin of the North China Craton and the southern margin of the Mongolian terrane, and late Paleozoic northward subduction along the northern orogen and exhumation of a high-pressure metamorphic terrane on the northern margin of the North China Craton.

Imaging high-pressure rock exhumation in eastern Taiwan

Increasingly detailed studies of out crops of high-pressure rock terranes in combination with rapidly evolving numerical modeling studies have given rise to a number of possible explanations for the processes by which these rocks are exhumed. Imaging actively exhuming high-pressure terranes remains one of the fundamental, but elusive, tasks that could advance the understanding of how these important rocks reach Earth’s surface. Seismic tomography along the active arc-continent collision in eastern Taiwan images a high P- and S-wave velocity zone that extends from the shallow subsurface beneath a high-pressure metamorphic terrane to ∼50 km depth. We present a petrophysical analysis of this high-velocity zone that indicates the presence of rock types common to high-pressure terranes. The high-velocity zone is seismically active throughout. We determine focal mechanisms for 57 earthquakes, and carry out full waveform modeling on 10; these have double-couple focal mechanisms with a compensated linear vector dipole component up to 20.6%. We suggest that the high-velocity zone comprises an exhuming high-pressure terrane. Focal mechanisms for earthquakes within it indicate that shear faulting dominates in the deformation, but high fluid pressure may also play a role.

Paleo-mantle wedge preserved in the Sambagawa high-pressure metamorphic belt and the thickness of forearc continental crust

There are two possible origins for ultramafic rocks in subduction-type high-pressure metamorphic terranes: (1) they are derived from the subducted slab or footwall, or (2) they are derived from the mantle wedge in the hanging wall. The origin of ultramafic rock bodies in a classic high-pressure metamorphic belt, the Sambagawa belt of southwest Japan, is assessed based on wide-ranging field studies covering an area of 23 × 30 km 2 and corresponding to metamorphic pressures of 5–25 kbar. Peridotite and/or serpentinite bodies are common in the higher pressure part of the belt, but no occurrence is known in the low-pressure part (the chlorite zone). If the ultramafic rocks originated in the footwall, they should be metamorphosed together with the subducted material and their distribution should not show any correlation with metamorphic pressure. The restricted distribution of mantle rocks to the high-pressure part is strong evidence for a mantle-wedge origin of the ultramafic blocks. The presence of subducted metasediments surrounding the mantle rocks indicates that the subducted slab can tectonically entrain and transport substantial amount of hanging-wall material to the Earth’s surface. The first appearance of ultramafics occurs within the garnet zone, which has a peak metamorphic pressure of 8.0–9.5 kbar, and the corresponding depth (∼30–35 km) represents the thickness of the forearc continental crust in the Cretaceous Sambagawa subduction zone.

Very high-pressure orogenic garnet peridotites

Mantle-derived garnet peridotites are a minor component in many very high-pressure metamorphic terranes that formed during continental subduction and collision. Some of these mantle rocks contain trace amounts of zircon and micrometer-sized inclusions. The constituent minerals exhibit pre- and postsubduction microstructures, including polymorphic transformation and mineral exsolution. Experimental, mineralogical, petrochemical, and geochronological characterizations using novel techniques with high spatial, temporal, and energy resolutions are resulting in unexpected discoveries of new phases, providing better constraints on deep mantle processes.

Co-axial horizontal stretching within extending orogens: the exhumation of HP rocks on Syros (Cyclades) revisited

Abstract Although the role of extensional tectonics in the exhumation of high-pressure metamorphic terranes is widely established, the kinematics of such deformation remains ambiguous. This paper outlines new field data from the Attic-Cycladic blueschist belt that suggest that distributed ductile strain plays a significant role in the extension and that, consequently, the role of major detachment faults may have been over-emphasized in previous studies. The high-pressure blueschist terrane (Ermoupolis Unit) of Syros shows abundant evidence of subhorizontal extension, manifest as layer boudinage and ductile thinning without the development of significant internal detachments. The deformation approximates to pure shear stretching that was heterogeneously distributed in space and time. Minor zones of asymmetric shear are interpreted not as through-going extensional shear zones but as structures that maintain compatibility between zones of differential stretching. The progression of deformation is charted through the systematic development of increasingly lower-pressure metamorphic assemblages. However, most of the decompression (potentially from 20 kbar to 6 kbar) occurred within the blueschist stability field, as the rocks were actively extending. Heterogeneous retrogression and concomitant deformation are believed to relate to the local chemistry and availability of hydrous fluids.

Constrictional extensional tectonics in the northern Oman mountains, its role in culmination development and the exhumation of the subducted Arabian continental margin

Abstract The NE margin of the Arabian continent was overthrust by ‘exotic’ sheets of oceanic and continental margin units (the Semail Ophiolite allochthon) in the Late Cretaceous. Although parts of this margin (Saih Hatat Massif) were deeply buried, through subduction, to depths suitable for eclogite-facies metamorphism, other parts are unmetamorphosed (Jebel Akhdar Massif). Hence an almost continuous metamorphic gradient is preserved. This forms an ideal setting within which to relate shallow and deeper-seated tectonic processes within an orogen. Structural data are presented from the Jebal Akhdar Massif, a composite antiformal structure that contains a network of structures that post-date allochthon emplacement. These include down-to-the-NNE layer-extensional shears and steeper faults. Layer-extensional shears contain open to close folds with hinge lines parallel to regional elongation directions. Larger-scale NNE-trending folds include the regional Jebel Nakhl Antiform. The same kinematic style can be traced into the exhumed high-pressure metamorphic terrane of Saih Hatat. Coeval orthogonal layer contraction and layer-thinning and elongation describes bulk constrictional 3D strain. Although this might be indicative of regional transtension, large-scale strike-slip faults, active during the extension, as predicted by general transtensional models are not evident. Consequently, it is inferred that constriction was the result of laterally varying crustal extension whereby top-to-the-NNE extension was locally combined with left-lateral shearing. Exhumation of the metamorphic series occurred under a carapace of extending allochthons, defining an elongate ‘pip’ of material returning to shallow crustal levels. There is, however, an imbalance between net extension and possible contraction within the Arabian continent that requires deformation within a volume of net-divergent tectonics. Thus crustal extension continued after the end of convergent tectonics in the region.

Dunk, Dunkless and Re-dunk Tectonics: A Model for Metamorphism, Lack of Metamorphism, and Repeated Metamorphism of HP/UHP Terranes

High-pressure metamorphic terranes form when slivers of continental crust are pulled into the mantle by previously subducted oceanic lithosphere. Old, cold lithosphere subducts into the mantle at steep angles, dragging sialic crust to deep levels, quickly followed by steep, rapid exhumation. Young thin lithosphere subducts at shallow angles, pulling crustal slabs slowly into the shallow mantle, followed by slow exhumation, retrogression, and melting. Terranes that follow inboard-dipping subduction zones will subduct once, and will be overprinted by subsequent subduction and collisions. Terranes above outboard-facing subduction zones will be underthrust by the continental margin. Subsequent collisions will result in the re-subduction of an increasingly complex continental margin characterized by previously accreted terranes.

D. A. Carswell and R. Compagnoni, editors. EMU Notes in Mineralogy, 5: Ultrahigh Pressure Metamorphism. European Mineralogical Union, 2003. 508 pp. + CD-ROM. Price €24 + postage. ISBN 963 463 6462.

D. A. Carswell and R. Compagnoni, editors. EMU Notes in Mineralogy, 5: Ultrahigh Pressure Metamorphism. European Mineralogical Union, 2003. 508 pp. + CD-ROM. Price €24 + postage. ISBN 963 463 6462. - Volume 70 Issue 4

Significance of Serpentinites and Related Rocks in the High-Pressure Metamorphic Terranes, Circum-Pacific Regions

A large number of serpentinite seamounts in the Izu-Ogasawara-Mariana forearc reflect serpentinite diapirism under a tensional stress field. The Franciscan Complex, California, and the Kamuikotan metamorphic belt, Japan, are accompanied by serpentinite mélange including deep, high-grade metamorphic rocks, whereas the Sanbagawa metamorphic belt, Japan, contains lesser amounts of serpentinite and is not accompanied by serpentinite mélange. The former were probably formed under a tensional stress regime, and the latter under a compressional one. The restricted occurrence of serpentinite to high-grade pelitic schists in the Sanbagawa belt suggests that the serpentinites were transported from the mantle wedge into pelagic sediments on the top of the subducting plate at the initiation of accretion. Tremolite rocks and phengite-chlorite schists near serpentinite in the Sanbagawa belt and from Chamorro Seamount, Mariana forearc, may suggest widespread metasomatism along the boundary between the mantle-wedge serpentinite and pelagic sediments.

Episodicity during orogenesis

Abstract Deformation events and episodes of metamorphic mineral growth are usually regarded as relatively local phenomena. It is not expected that specific events and episodes within an orogenic sequence should exactly correlate over large distances. There is no obvious reason, for example, to assume that deformation and/or metamorphic events in the Western European Alps would directly correlate with events taking place in the Aegean continental crust, c. 1000 km distant. Yet linked episodes of deformation and metamorphism appear to take place at the same time over large distances, even in these apparently unrelated segments of the same orogenic belt. This large-scale episodic behaviour appears to be associated with switches in tectonic mode, from compressional orogenesis to extensional tectonism, and may be the result of orgenic surges and/or periods of lithospheric extension following accretion events. The effect of these switches is greatest in back-arc environments, in the over-riding plate above major subduction zones. In these environments, roll-back of the subducting lithospheric slab after individual accretion events ensures that the amount of lithospheric extension after each accretion event is large. As a result this is where coherent high-pressure metamorphic terranes formed in the preceding accretion event are exhumed, and where remnants of newly emplaced ophiolite sheets are stranded by newly formed detachment faults.

Geological and geochronological constraints on the exhumation of a high-pressure metamorphic terrane, Oman

Abstract New 40 Ar- 39 Ar age data from high-pressure former continental shelf rocks, now structurally beneath the Samail ophiolite, support a two-stage exhumation process for these rocks. High- P rocks occur as stacked fold-nappes within a less deformed upper plate and a strong to intensely deformed lower plate separated by a major, ductile, crustal discontinuity. The highest-grade lower plate rocks yield 40 Ar- 39 Ar ages that range from 82 to 131 Ma. The majority of these ages are inferred to represent cooling by the progressive emplacement of colder units during convergent margin tectonism, before emplacement to higher structural levels. In the lower plate, 82–79 Ma 40 Ar- 39 Ar ages are associated with lower-grade assemblages defining both transposition fabrics and C’-type shear bands which overprint the high- P assemblages during a NE-directed shearing event. Partial exhumation of these lower plate sequences resulted in the formation of regional closures by folding and transposition of earlier high-pressure fabrics. The lower plate closures are truncated by the structural break separating the two plates. Emplacement of the lower plate units to shallower crustal levels may have occurred before, or synchronously with, peak metamorphism of the upper plate units. The lower plate 40 Ar- 39 Ar cooling(?) ages are older than mica crystallization ages of 76–70 Ma from the upper plate. These upper plate micas grew in axial surface fabrics associated with regional-scale fold-nappes which formed during the exhumation of the upper plate units and their transport over the higher-grade lower plate units. This nappe-forming event was possibly synchronous with structurally higher low-angle normal faulting and/or rapid erosion of the nappe pile before the deposition of Maastrichtian autochthonous units at c. 67–68 Ma.

Paleozoic submarine volcanoes in the high- P/ T metamorphosed Chichibu system of Eastern Shikoku, Japan

The Sawadani greenstone in the Chichibu Paleozoic System is an ancient submarine volcanic complex consisting of pillow lavas and hyaloclastites. The volcanism is divided into two periods. Alkali basalt was erupted in the first period and two shield-shaped cones were formed. After a period of dormancy the volcanism of the second period took place and a cone was formed by eruptions of lavas ranging in composition from mildly alkaline to tholeiitic basalt. The top of the volcano nearly reached the sea surface and was finally about 3.7 km above the base. A limestone cap and volcanic conglomerate were deposited on the summit. The base rests conformably on upper Carboniferous sandstone and subordinate mudstone derived from a continent or mature island arc. Many feeding channels of lava cut the volcanic body and underlying sedimentary formation. Sedimentation proceeded concurrently on the surrounding sea floor, so that volcanic and sedimentary material is interlayered. The Sawadani greenstone, although it occurs in the high- P/ T metamorphic belt, is not believed to be a fragment of oceanic crust (ophiolite complex) formed by oceanic ridge volcanism and later carried into a convergent zone. It is a seamount formed on and within a sedimentary sequence near a continent or island arc. The magma changed from alkaline to tholeittic as the volcano grew. It cannot be assumed that all metavolcanic rocks formed in high-pressure metamorphic terranes are fragments of oceanic crust.

A seismic wave velocity along the Sanbagawa metamorphic belt in Shikoku area, southwestern Japan.

Explosion seismic observations were made in the Sanbagawa high-pressure metamorphic terrane, Shikoku area, Japan. The profile was nearly in the direction parallel to the trend of the metamorphic belt. In this direction a considerably high seismic wave velocity for the upper crust had been expected due to a large amount of basic rock on the surface and predominant schistosity. The observed velocity for the upper crust, however, was found to be around 6.1km/s, a value that is commonly observed in the continental area. The crossover-distance between a travel-time curve for the granitic layer and that for the basaltic layer or the upper mantle was greater than 150km. This fact may be interpreted, combined with the gravity data, as indicating that the thicknesses of the granitic and the basaltic layers under the terrane do not differ significantly from those of the normal continental structure.