The Central India Tectonic Zone: A geophysical perspective on continental amalgamation along a Mesoproterozoic suture

The Central India Tectonic Zone (CITZ) is a prominent divide and a major suture zone between the North Indian and South Indian crustal blocks. The resistive upper crust as modeled in the magnetotelluric data from CITZ suggests a dominant tonalite–trdondhjemite–granodiorite composition associated with an accretionary complex characterized by mainly felsic rock components. The highly conductive bodies in this zone might represent mafic/ultramafic-layered intrusives derived from a deeper reservoir of underplated basaltic magma related to the formation of the Cretaceous Deccan flood basalts. The uniformly thick mafic lower crust below the cratons on both sides of the suture is interpreted as the accreted remnants of Archaean and Paleoproterozoic subducted slabs. We redefine the nature of deep faults traversing the CITZ, which were described as steep and penetrating the Moho by previous workers, and classify them as listric faults with gentle dips at depth.Seismic reflection data from the eastern side of the suture suggest a northwestward subduction of the Bhandara Craton. Reflection data from the central part of the CITZ show northerly dip in the southern part suggesting northward subduction of the Dharwar Craton. However, an opposite trend is observed in the northern part of the suture with a southward dip of the Bundelkhand craton. Based on these features, and in conjunction with existing magnetotelluric models, we propose a double-sided subduction history along the CITZ. This would be similar to the ongoing subduction–accretion process in the western Pacific region, which possibly led to the development of paired collision-type and Pacific-type orogens. One important feature is the domal structure along the central part of the suture with a thick felsic crust occurring between mafic and intermediate crust. The high resistivity felsic domain suggests underplated sediments/felsic crust that would have caused the doming. Our model also accounts for the extrusion of regional metamorphic belts at the orogenic core, and the occurrence of high pressure–ultrahigh-temperature paired metamorphic belts within the suture.     

http://www.sciencedirect.com/science/article/pii/S1674987111000582

The Central India Tectonic Zone(CITZ) marks the trace of a major suture zone along which the south Indian and the north Indian continental blocks were assembled through subduction-accretioncollision tectonics in the Mesoproterozoic.The CITZ also witnessed the major,plume-related,late Cretaceous Deccan volcanic activity,covering substantial parts of the region with continental flood basalts and associated magmatic provinces.A number of major fault zones dissect the region,some of which are seismically active.Here we present results from gravity modeling along five regional profiles in the CITZ, and combine these results with magnetotelluric(MT) modeling results to explain the crustal architecture. The models show a resistive(more than 2000Ω·m) and a normal density(2.70 g/cm~3) upper crust suggesting\ dominant tonalite-trondhjemite-granodiorite(TTG) composition.There is a marked correlation between both high-density(2.95 g/cm~3) and low-density(2.65 g/cm~3) regions with high conductive zones (<80Ω·m) in the deep crust.We infer the presence of an interconnected grain boundary network of fluids or fluid-hosted structures,where the conductors are associated with gravity lows.Based on the conductive nature,we propose that the lower crustal rocks are fluid reservoirs,where the fluids occur as trapped phase within minerals,fluid-filled porosity,or as fluid-rich structural conduits.We envisage that substantial volume of fluids were transferred from mantle into the lower crust through the younger plume-related Deccan volcanism,as well as the reactivation,fracturing and expulsion of fluids transported to depth during the Mesoproterozoic subduction tectonics.Migration of the fluids into brittle fault zones such as the Narmada North Fault and the Narmada South Fault resulted in generating high pore pressures and weakening of the faults,as reflected in the seismicity.This inference is also supported by the presence of broad gravity lows near these faults,as well as the low velocity in the lower crust beneath regions of recent major earthquakes within the CITZ.     

The Mesoarchean emergence of modern-style subduction

Well-preserved volcanic sequences span the Paleoarchean to Neoarchean evolution of the Pilbara Craton, in northwestern Australia. This region provides the best physical evidence bearing on the stage of Earth's history when modern-style tectonic processes began. Paleoarchean assemblages in the eastern nucleus of the craton (the 3.51–3.24 Ga Pilbara Supergroup) show few features that can reasonably be interpreted as evidence for modern-style subduction processes. Incompatible trace element-enriched felsic volcanic horizons show geochemical evidence for the interaction between mafic magmas and crust, but this evidence, on its own, can equally well be interpreted in terms of either a subduction-enriched mantle source or local and limited assimilation of felsic crust into the voluminous tholeiitic magmas that dominate the Pilbara Supergroup. Viewed in context within the thick autochthonous and consistently upward-younging Pilbara Supergroup, these felsic units are most likely related to the same plume-dominated processes that formed the basalts that dominate the supergroup. It is very unlikely that modern-style plate tectonic processes played any role in the Paleoarchean evolution of the Pilbara Craton, although some form of non-uniformitarian (e.g. flat) subduction process may have operated.In stark contrast, the Mesoarchean units of the West Pilbara Terrane and the late-tectonic basins that cover that boundary between the West and East Pilbara Terranes, show clear evidence for modern-style convergent margin processes. Igneous rocks in this belt, which flanks the old eastern cratonic nuclei, have enriched geochemical signatures that cannot be accounted for by crustal contamination. This region is also characterised by a linear magmatic and structural fabric, by the presence of lithologically and geochronologically exotic belts, and by the presence of a broad belt of isotopically more juvenile crust. The collective strength of these arguments provides compelling evidence that a modern-style oceanic arc fringed the East Pilbara Terrane at 3.12 Ga and accreted to that terrane by 2.97 Ga. These assemblages mark the minimum age for the birth of modern-style plate subduction process.     

津巴布韦克拉通构造变形特征分析

根据津巴布韦克拉通北部地区的构造变质特征及矿物测年数据,表明太古界地壳基底形成和演变分为两个阶段.早期约在26.7亿年,Shamva-Bindura绿岩带内发育平行层理的剪切带,在长英质片麻岩核组成的大型推覆体之间形成洋壳和火山岛弧物质的叠瓦状堆积,结果是炽热长英质和铁镁质岩层的堆积使得地壳的演变暂时达到均衡状态,岩层堆积厚度达35 km.后期大约在26.0~26.2亿年炽热岩层的冷却收缩导致应变产生,沿垂直方向形成走向滑动断层带,为达到地热均衡,大面积的岩层发生熔蚀,并伴有花岗岩体底辟作用.岩层熔蚀和底辟作用产生二次临时变质作用和多种应变形态,也最终导致克拉通冷凝固化.     

Magnetotelluric Constraints on the Occurrence of Lower Crustal Earthquakes in the Intra-plate Setting of Central Indian Tectonic Zone

Lower crustal earthquake occurrence in the Central Indian Tectonic Zone(CITZ) of the Indian sub-continent was investigated using magnetotelluric(MT) data. MT models across the CITZ, including the new resistivity model across the 1938 Satpura lower crustal earthquake epicenter, show low resistive(80 ?m) mid-lower crust and infer small volume(1 vol%) of aqueous fluids existing in most part of lower crust. This in conjunction with xenoliths and other geophysical data supports a predominant brittle/semi-brittle lower crustal rheology. However, the local deep crustal zones with higher fluid content of 2.2%–6.5% which have been mapped imply high pore pressure conditions. The observation above and the significant strain rate in the region provide favorable conditions(strong/moderate rock strength, moderate temperature, high pore pressure and high strain rate) for brittle failure in the lower crust. It can be inferred that the fluid-rich pockets in the mid-lower crust might have catalyzed earthquake generation by acting as the source of local stress(fluid pressure), which together with the regional stress produced critical seismogenic stress conditions. Alternatively, fluids reduce the shear strength of the rocks to favor tectonic stress concentration that can be transferred to seismogenic faults to trigger earthquakes.     

Ultrahigh-pressure eclogite transformed from mafic granulite in the Dabie orogen, east-central China

Although ultrahigh‐pressure (UHP) metamorphic rocks are present in many collisional orogenic belts, almost all exposed UHP metamorphic rocks are subducted upper or felsic lower continental crust with minor mafic boudins. Eclogites formed by subduction of mafic lower continental crust have not been identified yet. Here an eclogite occurrence that formed during subduction of the mafic lower continental crust in the Dabie orogen, east‐central China is reported. At least four generations of metamorphic mineral assemblages can be discerned: (i) hypersthene + plagioclase ± garnet; (ii) omphacite + garnet + rutile + quartz; (iii) symplectite stage of garnet + diopside + hypersthene + ilmenite + plagioclase; (iv) amphibole + plagioclase + magnetite, which correspond to four metamorphic stages: (a) an early granulite facies, (b) eclogite facies, (c) retrograde metamorphism of high‐pressure granulite facies and (d) retrograde metamorphism of amphibolite facies. Mineral inclusion assemblages and cathodoluminescence images show that zircon is characterized by distinctive domains of core and a thin overgrowth rim. The zircon core domains are classified into two types: the first is igneous with clear oscillatory zonation ± apatite and quartz inclusions; and the second is metamorphic containing a granulite facies mineral assemblage of garnet, hypersthene and plagioclase (andesine). The zircon rims contain garnet, omphacite and rutile inclusions, indicating a metamorphic overgrowth at eclogite facies. The almost identical ages of the two types of core domains (magmatic = 791 ± 9 Ma and granulite facies metamorphic zircon = 794 ± 10 Ma), and the Triassic age (212 ± 10 Ma) of eclogitic facies metamorphic overgrowth zircon rim are interpreted as indicating that the protolith of the eclogite is mafic granulite that originated from underplating of mantle‐derived magma onto the base of continental crust during the Neoproterozoic (c. 800 Ma) and then subducted during the Triassic, experiencing UHP eclogite facies metamorphism at mantle depths. The new finding has two‐fold significance: (i) voluminous mafic lower continental crust can increase the average density of subducted continental lithosphere, thus promoting its deep subduction; (ii) because of the current absence of mafic lower continental crust in the Dabie orogen, delamination or recycling of subducted mafic lower continental crust can be inferred as the geochemical cause for the mantle heterogeneity and the unusually evolved crustal composition.     

Preservation/exhumation of ultrahigh-pressure subduction complexes

Ultrahigh-pressure (UHP) metamorphic terranes reflect subduction of continental crust to depths of 90–140 km in Phanerozoic contractional orogens. Rocks are intensely overprinted by lower pressure mineral assemblages; traces of relict UHP phases are preserved only under kinetically inhibiting circumstances. Most UHP complexes present in the upper crust are thin, imbricate sheets consisting chiefly of felsic units ± serpentinites; dense mafic and peridotitic rocks make up less than  10% of each exhumed subduction complex. Roundtrip prograde–retrograde P–T paths are completed in 10–20 Myr, and rates of ascent to mid-crustal levels approximate descent velocities. Late-stage domical uplifts typify many UHP complexes.

Sialic crust may be deeply subducted, reflecting profound underflow of an oceanic plate prior to collisional suturing. Exhumation involves decompression through the P–T stability fields of lower pressure metamorphic facies. Scattered UHP relics are retained in strong, refractory, watertight host minerals (e.g., zircon, pyroxene, garnet) typified by low rates of intracrystalline diffusion. Isolation of such inclusions from the recrystallizing rock matrix impedes back reaction. Thin-aspect ratio, ductile-deformed nappes are formed in the subduction zone; heat is conducted away from UHP complexes as they rise along the subduction channel. The low aggregate density of continental crust is much less than that of the mantle it displaces during underflow; its rapid ascent to mid-crustal levels is driven by buoyancy. Return to shallow levels does not require removal of the overlying mantle wedge. Late-stage underplating, structural contraction, tectonic aneurysms and/or plate shallowing convey mid-crustal UHP décollements surfaceward in domical uplifts where they are exposed by erosion. Unless these situations are mutually satisfied, UHP complexes are completely transformed to low-pressure assemblages, obliterating all evidence of profound subduction.     

Early extension in the Late Archaean northeastern Eastern Goldfields Province,Yilgarn Craton,Western Australia

A series of linked extensional detachments, transfer faults, and sediment- and volcanic-filled half-grabens that pre-date regional folding are described in the Late Archaean Margaret anticline, Eastern Goldfields Province, Yilgarn Craton, Western Australia. Coeval structures and rock units include layer-parallel extensional detachments, transfer faults (high-angle rotational faults rooted in the detachments and linking layer-parallel shear zones with varying amounts of extension); felsic intrusions, either as granitoids emplaced in or below the detachments, or as fine-grained intrusive bodies emplaced above the detachments and controlled by the high-angle faults; and half-grabens controlled by the high-angle faults and filled with clastic sedimentary and volcanic rocks. At least 1500 m of section is excised across the detachments. The detachments and high-angle faults are folded by the east-northeast regional compression that formed the Margaret anticline. Extensional deformation in the Margaret anticline is correlated with the regionally recognised felsic magmatism and associated volcanic and volcaniclastic basin fill dated at approximately 2685–2670 Ma across the Eastern Goldfields Province. This suggests the extensional event was province-wide and post-dated initial greenstone deposition (at around 2705 Ma) but pre-dated regional compressive deformation. We suggest the extension is the result of a thermal anomaly in the crust, generated by the insulating effect of a thick pile (of the order of 10 km or greater) of mafic and ultramafic volcanic rocks on precursor Archaean felsic crust. The thermal anomaly has generated renewed production of felsic and mafic volcanic rocks, coeval with uplift and extension in the upper crust.     

Petrology and U–Pb geochronology of lower crustal xenoliths and the development of a craton, Slave Province, Canada

Lower crustal xenoliths recovered from Eocene to Cambrian kimberlites in the central and southern Slave craton are dominated by mafic granulites (garnet, clinopyroxene, plagioclase±orthopyroxene), with subordinate metatonalite and peraluminous felsic granulites. Geothermobarometry indicates metamorphic conditions of 650–800 °C at pressures of 0.9–1.1 GPa. The metamorphic conditions are consistent with temperatures expected for the lower crust of high-temperature low-pressure (HT-LP) metamorphic belts characteristic of Neoarchean metamorphism in the Slave craton. U–Pb geochronology of zircon, rutile and titanite demonstrate a complex history in the lower crust. Mesoarchean protoliths occur beneath the central Slave supporting models of an east-dipping boundary between Mesoarchean crust in the western and Neoarchean crust in the eastern Slave. At least, two episodes of igneous and metamorphic zircon growth occurred in the interval 2.64–2.58 Ga that correlate with the age of plutonism and metamorphism in the upper crust, indicating magmatic addition to the lower crust and metamorphic reworking during this period. In addition, discrete periods of younger zircon growth at ca. 2.56–2.55 and 2.51 Ga occurred 20–70 my after the cessation of ca. 2.60–2.58 Ga regional HT-LP metamorphism and granitic magmatism in the upper crust. This pattern of younger metamorphic events in the deep crust is characteristic of the Slave as well as other Archean cratons (e.g., Superior). The high temperature of the lower crust immediately following amalgamation of the craton, coupled with evidence for continued metamorphic zircon growth for >70 my after ‘stabilization’ of the upper crust, is difficult to reconcile with a thick (200 km), cool lithospheric mantle root beneath the craton prior to this event. We suggest that thick tectosphere developed synchronously or after these events, most likely by imbrication of mantle beneath the craton at or after ca. 2.6 Ga. The minimum age for establishing a cratonic like geotherm is given by lower crustal rutile ages of ca. 1.8 Ga in the southern Slave. Transient heating and possible magmatic additions to the lower crust continued through the Proterozoic, with possible additional growth of the tectosphere.     

A Plausible Model for Evolution of Schist Belts and Granite Plutons of Dharwar Craton,India and Madagascar During 3.0-2.5 Ga: Insight from Gravity Modeling Constrained in Part from Seismic Studies

Gravity modeling of an E-W profile across Dharwar Craton, India and Madagascar, integrated with the results of Deep Seismic Sounding (DSS) across the Dharwar Craton suggest a thick crust of 40-42 km under the eastern part of Eastern Dharwar Craton (EDC), the Western Dharwar Craton (WDC) and the central part of the Madagascar. Towards east of these blocks, the crustal thickness is reduced to 36-38 km along the Eastern Ghat Fold Belt (EGFB), shear zone between the EDC and the WDC and the east coast of Madagascar, respectively. These zones of thin crust are also characterized by high density lower crustal rocks associated with thrusts. The seismic section across Dharwar Craton shows domal- shaped reflectors in the lower crust and upper mantle under the WDC which may be related to asthenopheric upwelling during an extension phase. The occurrences of large schist belts with volcano-sedimentary sequences of marine origin of late Archean period (3.0-2.7 Ga) as rift basins in the WDC and Madagascar also suggest an extensional phase in this region during that period. It is followed by a convergence between the WDC and the EDC giving rise to collision-related shear and thrust zones between the WDC and the EDC associated with high density lower crustal rocks. The seismic section shows upwarped reflectors in the upper crust which may be related to this convergence. Eastward dipping reflectors under WDC and EDC and west verging thrusts suggest convergence from the west to the east which resulted in easterly subduction giving rise to subduction-related K-granite plutons of the EDC of 2.6-2.5 Ga. In this regard, the Closepet granite in the EDC which extends almost parallel to the shear zone between the WDC and EDC and shows an I-type calk-alkaline composition may represent relict of an island arc and the linear schist belts with bimodal volcanics of the EDC east of it might have developed as back arc rift basins. Subsequent collision between India and Antarctica along the EGFB during Middle Proterozoic, indicated by eastward dipping reflectors in the crust and the upper mantle and west verging thrust gave rise to contemporary high-grade rocks of the EGFB (1.6-1.0 Ga) and associated mafic and felsic intrusives of this belt. The part of adjoining Cuddapah basin contemporary to the EGFB towards the west consisting of marine shelf type of sediments which are highly disturbed and thickest at its contact with the EGFB may represent a peripheral foreland basin. Gravity modeling provides thickest crust of 42 km in the southern part of the WDC and does not support sharp increase in crustal thickness of 50-60 km with high velocity upper mantle as suggested from receiver function analysis. It may represent some foreign material of high density trapped in this section such as part of oceanic crust during convergence and subduction that is referred to above. It is supported from eastward dipping reflectors in lower crust and upper mantle in adjoining region.     

Appinite suites and their genetic relationship with coeval voluminous granitoid batholiths

ABSTRACT

Appinite complexes preserve evidence of mantle processes that produce voluminous granitoid batholiths. These plutonic complexes range from ultramafic to felsic in composition, deep to shallow emplacement, and from Neo-Archean to Recent in age. Appinites are a textural family characterized by idiomorphic hornblende in all lithologies, and spectacular textures including coarse-grained mafic pegmatites, fine-grained ‘salt-and-pepper’ gabbros, as well as planar and linear fabrics. Magmas are bimodal (mafic-felsic) in composition; ultramafic rocks are cumulates, intermediate rocks are hybrids. Their geochemistry is profoundly influenced by a mantle wedge extensively metasomatized by fluids/magmas produced by subduction. Melting of spinel peridotite sub-continental lithospheric mantle (SCLM) produces appinites whose geochemistry is indistinguishable from coeval low-K calc-alkalic arc magmatism. Coeval felsic rocks within appinite complexes and adjacent granitoid batholiths are crustal magmas. When subduction terminates, asthenospheric upwelling (e.g. in a slab window, or in the aftermath of slab failure) induces melting of metasomatized garnet SCLM to produce K-rich sho shonitic magmas enriched in large ionic lithophile and light relative to heavy rare earth elements, whose asthenospheric component can be identified by Sm-Nd isotopic signatures. Coeval late-stage Ba-Sr granitoid magmas have a ‘slab failure’ geochemistry, resemble TTG and adakitic suites, and are formed either by fractionation of an enriched (shoshonitic) mafic magma, or high pressure melting of a meta-basaltic protolith either at the base of the crust or along the upper portion of the subducted slab. Appinite complexes may be the crustal representation of mafic magma that underplated the crust for the duration of arc magmatism. They were preferentially emplaced along fault zones around the periphery of the granitoid batholiths (where their ascent is not blocked by overlying felsic magma), and as enclaves within granitoid batholiths. When subduction ceases, appinite complexes with a more pronounced asthenospheric component are preferentially emplaced along active faults that bound the periphery of the batholiths.     

Geophysical profiling across the Sulu ultra-high-pressure metamorphic belt, eastern China

The largest ultra-high pressure metamorphic (UHPM) belt in the world is located along the Dabie–Sulu region, which tectonically belongs to the east part of the central orogenic belt of China. Integrated geophysical investigations of using deep seismic reflection, MT, and geothermal observations have been carried out in the Sulu area since 1997. The results of integrated interpretation suggest the existence of three features: (1) a rift beneath the Lianshui basin by the Jiashan–Xionshui fault; (2) a special crustal pattern, called the magmatic multi-arch structure occurs beneath the northern Sulu UHPM zone; and (3) a northwest-dipping regional thrust crosses the Sulu crust, representing the intracontinental subduction of the Yangtze craton beneath the Sulu metamorphic belts after collision between the Yangtze and Sino-Korean cratons. A magmatic multi-arch structure consists of some arched reflectors that occur in both the lower and the upper crust where arched reflectors coincide with granitoid plutons. The multi-arch structures are common in eastern China where many Mesozoic granitoid plutons of different scales occur. The crustal structures in the Sulu metamorphic belts resulted from intensive dynamic processes following the Triassic collision between the Yangtze and Sino-Korean cratons. The formation and exhumation of UHPM rocks followed the collision, and then intracontinental subduction of the Yangtze craton beneath the Dabie–Sulu terranes took place in the early and middle Jurassic. In the late Jurassic, the Sulu lithosphere turned to an extensional regime, large-scale granitic intrusions occurred in eastern China; these likely resulted from lithospheric thinning and asthenospheric uplifting. The granitic intrusions came to a climax during the Cretaceous and were followed by rifting along existing faults in the early Eogene, resulting in many petroleum basins. The granitoid emplacement that generated the magmatic multi-arch structure and the rift were consequences of the lithospheric thinning process, and deep intracontinental subduction of the Yangtze craton beneath the Sulu metamorphic belt might partially contribute to the lithospheric thinning.     

Geochemistry of the Neoarchaean mafic volcanic rocks of the Geita area, NW Tanzania: Implications for stratigraphical relationships in the Sukumaland greenstone belt

Geochemical data are presented for a suite of mafic volcanic rocks from the Geita area in the Sukumaland greenstone belt (SGB) of northwestern Tanzania with the aim of constraining their petrogenesis, tectonic setting and to assess a possible genetic link with mafic volcanic rocks from the Rwamagaza area also from the SGB previously reported by [Manya, S., Maboko, M.A.H., 2003. Dating basaltic volcanism in the Neoarchaean Sukumaland greenstone belt of the Tanzania Craton using the Sm–Nd method: implications for the geological evolution of the Tanzania Craton. Precambrian Research 121, 35–45] and [Manya, S., 2004. Geochemistry and petrogenesis of volcanic rocks of the Neoarchaean Sukumaland greenstone belt, northwestern Tanzania. Journal of African Earth Sciences 40, 269–279]. Mafic volcanic rocks from the two locations in the SGB show similar geochemical and Nd-isotopic compositions. Trace element and Nd-isotope compositions are consistent with their generation from a depleted MORB mantle (DMM) source which had been metasomatised by a subduction component in a late Archaean back arc setting at 2823 Ma.These findings are at variance with the previously proposed lithostratigraphical framework in the SGB which postulated an inner arcuate belt dominated by lower Nyanzian mafic volcanic rocks and an outer belt dominated by upper Nyanzian chemical sedimentary rocks, rare felsic flows and shales. The presence of mafic volcanic rocks flanking the outer belt which are of similar composition and age as those of the inner belt suggests that mafic volcanics in the SGB form discontinuous patches of rock distributed throughout the belt and separated by intervening granites. Furthermore, they corroborate previous evidence that both the rocks of the inner and outer belt formed more or less coevally and the subdivision of the volcano-sedimentary package of the SGB (and other greenstone belts of the Tanzania Craton) into a lower mafic volcanic dominated unit and an upper felsic volcanic and BIF dominated unit is not stratigraphically valid.     

Early Stage in the Evolution of the Paleoasian Ccean at the Western Margin of the Siberian Craton: Geochemical and Geochronological Evidence

The discovery of glaucophane relicts in the high-pressure tectonites of the Yenisei suture zone of the Yenisei Ridge suggests the manifestation of the “Chilean-type” convergent margin on the western Siberian Craton, which was controlled by subduction of oceanic crust beneath the continental margin. These rocks are restricted to the tectonic suture between the craton and the Isakovka ocean-island terrane and experienced two metamorphic stages. Petrogeochemical characteristics of the mafic tectonites indicate that their protoliths are N-MORB and E-MORB basalts. More primitive N-MORB basalts were formed at the initial spreading stages through melting of the upper depleted mantle. Higher Ti basalts were formed by melting of enriched mantle protolith at the later spreading stages. U–Pb zircon age of 701.6 ± 8.4 Ma of the metamorphosed analogues of normal basalts marks the initiation of oceanic crust in the region. Revealed sequence of spreading, subduction (640–620 Ma), and shear deformations (~600 Ma) records the early stages in the evolution of the Paleoasian ocean in its junction zone with the western margin of the Siberian craton: from formation of fragments of oceanic crust to the completion of accretionary–subduction events.     

A tectonic model for the Tertiary evolution of strike–slip faults and rift basins in SE Asia

Models for the Tertiary evolution of SE Asia fall into two main types: a pure escape tectonics model with no proto-South China Sea, and subduction of proto-South China Sea oceanic crust beneath Borneo. A related problem is which, if any, of the main strike–slip faults (Mae Ping, Three Pagodas and Aliao Shan–Red River (ASRR)) cross Sundaland to the NW Borneo margin to facilitate continental extrusion? Recent results investigating strike–slip faults, rift basins, and metamorphic core complexes are reviewed and a revised tectonic model for SE Asia proposed. Key points of the new model include: (1) The ASRR shear zone was mainly active in the Eocene–Oligocene in order to link with extension in the South China Sea. The ASRR was less active during the Miocene (tens of kilometres of sinistral displacement), with minor amounts of South China Sea spreading centre extension transferred to the ASRR shear zone. (2) At least three important regions of metamorphic core complex development affected Indochina from the Oligocene–Miocene (Mogok gneiss belt; Doi Inthanon and Doi Suthep; around the ASRR shear zone). Hence, Paleogene crustal thickening, buoyancy-driven crustal collapse, and lower crustal flow are important elements of the Tertiary evolution of Indochina. (3) Subduction of a proto-South China Sea oceanic crust during the Eocene–Early Miocene is necessary to explain the geological evolution of NW Borneo and must be built into any model for the region. (4) The Eocene–Oligocene collision of NE India with Burma activated extrusion tectonics along the Three Pagodas, Mae Ping, Ranong and Klong Marui faults and right lateral motion along the Sumatran subduction zone. (5) The only strike–slip fault link to the NW Borneo margin occurred along the trend of the ASRR fault system, which passes along strike into a right lateral transform system including the Baram line.     

Influence of stretching and density contrasts on the chemical evolution of continental magmas: an example from the Ivrea-Verbano Zone

 The southern Ivrea-Verbano Zone of the Italian Western Alps contains a huge mafic complex that intruded high-grade metamorphic rocks while they were resident in the lower crust. Geologic mapping and chemical variations of the igneous body were used to study the evolution of underplated crust. Slivers of crustal rocks (septa) interlayered with igneous mafic rocks are concentrated in a narrow zone deep in the complex (Paragneiss-bearing Belt) and show evidence of advanced degrees of partial melting. Variations of rare-earth-element patterns and Sr isotope composition of the igneous rocks across the sequence are consistent with increasing crustal contamination approaching the septa. Therefore, the Paragneiss-bearing Belt is considered representative of an “assimilation region” where in-situ interaction between mantle- and crust-derived magmas resulted in production of hybrid melts. Buoyancy caused upwards migration of the hybrid melts that incorporated the last septa and were stored at higher levels, feeding the Upper Mafic Complex. Synmagmatic stretching of the assimilation region facilitated mixing and homogenization of melts. Chemical variations of granitoids extracted from the septa show that deep septa are more depleted than shallow ones. This suggests that the first incorporated septa were denser than the later ones, as required by the high density of the first-injected mafic magmas. It is inferred that density contrasts between mafic melts and crustal rocks play a crucial role for the processes of contamination of continental magmas. In thick under plated crust, the extraction of early felsic/hybrid melts from the lower crust may be required to increase the density of the lower crust and to allow the later mafic magmas to penetrate higher crustal levels. Received: 2 May 1995 / Accepted: 1 November 1995     

The geological evolution of the Gangpur Schist Belt,eastern India: Constraints on the formation of the Greater Indian Landmass in the Proterozoic

The Central Indian Tectonic Zone (CITZ) is a Proterozoic suture along which the Northern and Southern Indian Blocks are inferred to have amalgamated forming the Greater Indian Landmass. In this study, we use the metamorphic and geochronological evolution of the Gangpur Schist Belt (GSB) and neighbouring crustal units to constrain crustal accretion processes associated with the amalgamation of the Northern and Southern Indian Blocks. The GSB sandwiched between the Bonai Granite pluton of the Singhbhum craton and granite gneisses of the Chhotanagpur Gneiss Complex (CGC) links the CITZ and the North Singhbhum Mobile Belt. New zircon age data constrain the emplacement of the Bonai Granite at 3,370 ± 10 Ma, while the magmatic protoliths of the Chhotanagpur gneisses were emplaced at c. 1.65 Ga. The sediments in the southern part of the Gangpur basin were derived from the Singhbhum craton, whereas those in the northern part were derived dominantly from the CGC. Sedimentation is estimated to have taken place between c. 1.65 and c. 1.45 Ga. The Upper Bonai/Darjing Group rocks of the basin underwent major metamorphic episodes at c. 1.56 and c. 1.45 Ga, while the Gangpur Group of rocks were metamorphosed at c. 1.45 and c. 0.97 Ga. Based on thermobarometric studies and zircon–monazite geochronology, we infer that the geological history of the GSB is similar to that of the North Singhbhum Mobile Belt with the Upper Bonai/Darjing and the Gangpur Groups being the westward extensions of the southern and northern domains of the North Singhbhum Mobile Belt respectively. We propose a three‐stage model of crustal accretion across the Singhbhum craton—GSB/North Singhbhum Mobile Belt—CGC contact. The magmatic protoliths of the Chhotanagpur Gneisses were emplaced at c. 1.65 Ga in an arc setting. The earliest accretion event at c. 1.56 Ga involved northward subduction and amalgamation of the Upper Bonai Group with the Singhbhum craton followed by accretion of the Gangpur Group with the Singhbhum craton–Upper Bonai Group composite at c. 1.45 Ga. Finally, continent–continent collision at c. 0.96 Ga led to the accretion of the CGC with the Singhbhum craton–Upper Bonai Group–Gangpur Group crustal units, synchronous with emplacement of pegmatitic granites. The geological events recorded in the GSB and other units of the CITZ only partially overlap with those in the Trans North China Orogen and the Capricorn Orogen of Western Australia, indicating that these suture zones are not correlatable.     

南阿尔金高压-超高温麻粒岩变质作用:大陆地壳超深俯冲与折返过程的记录

南阿尔金造山带是目前报道的具有最深俯冲记录的大陆超高压变质带,其内出露有高压-超高温麻粒岩,它们对深入理解大陆地壳岩石超深俯冲与折返过程具有重要意义.介绍了对南阿尔金巴什瓦克地区长英质麻粒岩和基性麻粒岩的岩相学、矿物化学、相平衡模拟及锆石U-Pb年代学研究成果.其中基性麻粒岩主要记录了深俯冲大陆地壳折返过程的变质演化:包括高压榴辉岩相、高压-超高温麻粒岩相、低压-超高温麻粒岩相及随后的近等压降温演化阶段;长英质麻粒岩除了记录与基性麻粒岩相似的折返过程外,还记录了从角闪岩相到超高压榴辉岩相的进变质演化过程.结合已有研究资料,确定超高压榴辉岩阶段峰期条件> 7~9 GPa和>1 000℃,可达到斯石英稳定域.锆石年代学显示两种岩石类型的原岩和变质年龄均分别在900 Ma和500 Ma左右.变质作用与年代学研究表明,南阿尔金大陆地壳岩石在早古生代发生超深俯冲至200~300 km后,折返至加厚地壳底部发生高压-超高温变质作用,随后被快速抬升至地壳浅部发生低压-超高温变质作用并经历迅速冷却.      

Topography of the Variscan orogen in Europe: failed–not collapsed

The Variscan orogenic collage consists of three subduction-collision systems (Rheno-Hercynian, Saxo-Thuringian and Massif Central-Moldanubian). Devonian to early Carboniferous marine strata are widespread not only in the individual foreland fold and thrust belts, but also in post-tectonic basins within these foreland belts and on the Cadomian crust of peri-Gondwanan microcontinental fragments, which represent the upper plates of the subduction/collision zones. These marine basins preclude high elevations in the respective areas and also in their neighbourhood. Widespread late Carboniferous intra-montane basins with their coal-bearing sequences are likewise incompatible with high and dry plateaus. While narrow belts with high elevations remain possible along active margins within the orogen, comparison of the Variscides with the Himalaya/Tibetan plateau is unfounded. Plausible reasons for the scarcity of high Variscan relief include subduction of oceanic and even continental crust, subduction erosion, orogen-parallel extension and—most important—lithospheric thinning accompanied by high heat flow and magmatism. In many areas, timing and areal array of magmatism and HT metamorphism are not compatible with a model of tectonic thickening and subsequent gravitational collapse. It is suggested, instead, that lithospheric thinning and heating are due to mantle activities caused by the Tethys rift. The lower and middle crust were thermally softened and rendered unfit for stacking and isostatic uplift: in terms of topography, the Variscides represent a failed orogen. The HT regime also explains the abundance of granitoids and HT/LP metamorphic rocks typical of the Variscides. Melting in the HT regime extracted mafic components from Variscan and Cadomian crust as well as from Cadomian metasomatized lithospheric mantle, thus mimicking subduction-related magmatism. The onset of the HT regime at c. 340 Ma may also have triggered the final ascent of HP/UHP felsic metamorphic rocks.     

Anatomy of a Cambrian suture in Gondwana: Pacific-type orogeny in southern India?

Southern India occupies a central position in the Late Neoproterozoic–Cambrian Gondwana supercontinent assembly. The Proterozoic mosaic of southern India comprises a collage of crustal blocks dissected by Late Neoproterozoic–Cambrian crust-scale shear/suture zones. Among these, the Palghat–Cauvery Suture Zone (PCSZ) has been identified as the trace of the Cambrian suture representing Mozambique Ocean closure during the final phase of amalgamation of the Gondwana supercontinent. Here we propose a model involving Pacific-type orogeny to explain the Neoproterozoic evolution of southern India and its final amalgamation within the Gondwana assembly. Our model envisages an early rifting stage which gave birth to the Mozambique Ocean, followed by the initiation of southward subduction of the oceanic plate beneath a thick tectosphere-bearing Archean Dharwar Craton. Slices of the ocean floor carrying dunite–pyroxenite–gabbro sequence intruded by mafic dykes representing a probable ophiolite suite and invaded by plagiogranite are exposed at Manamedu along the southern part the PCSZ. Evidence for the southward subduction and subsequent northward extrusion are preserved in the PCSZ where the orogenic core carries high-pressure and ultrahigh-temperature metamorphic assemblages with ages corresponding to the Cambrian collisional orogeny. Typical eclogites facies rocks with garnet + omphacite + quartz and diagnostic ultrahigh-temperature assemblages with sapphirine + quartz, spinel + quartz and high alumina orthopyroxene + sillimanite + quartz indicate extreme metamorphism during the subduction–collision process. Eclogites and UHT granulites in the orogenic core define P–T maxima of 1000 °C and up to 20 kbar. The close association of eclogites with ultramafic rocks having abyssal signatures together with linear belts of iron formation and metachert in several localities within the PCSZ probably represents subduction–accretion setting. Fragments of the mantle wedge were brought up through extrusion tectonics within the orogenic core, which now occur as suprasubduction zone/arc assemblages including chromitites, highly depleted dunites, and pyroxene bearing ultramafic assemblages around Salem. Extensive CO₂ metasomatism of the ultramafic units generated magnesite deposits such as those around Salem. High temperature ocean floor hydrothermal alteration is also indicated by the occurrence of diopsidite dykes with calcite veining. Thermal metamorphism from the top resulted in the dehydration of the passive margin sediments trapped beneath the orogenic core, releasing copious hydrous fluids which moved upward and caused widespread hydration, as commonly preserved in the Barrovian amphibolite facies units in the PCSZ. The crustal flower structure mapped from PCSZ supports the extrusion model, and the large scale north verging thrusts towards the north of the orogenic core may represent a fold-thrust belt. Towards the south of the PCSZ is the Madurai Block where evidence for extensive magmatism occurs, represented by a number of granitic plutons and igneous charnockite massifs of possible tonalite–trondhjemite–granodiorite (TTG) setting, with ages ranging from ca. 750–560 Ma suggesting a long-lived Neoproterozoic magmatic arc within a > 200 km wide belt. All these magmatic units were subsequently metamorphosed, when the Pacific-type orogeny switched over to collision-type in the Cambrian during the final phase of assembly of the Gondwana supercontinent. One of the most notable aspects is the occurrence of arc magmatic rocks together with high P/T rocks, representing the deeply eroded zone of subduction. The juxtaposition of these contrasting rock units may suggest the root of an evolved Andean-type margin, as in many arc environments the roots of the arc comprise ultramafic/mafic cumulates and the felsic rocks represent the core of the arc. The final phase of the orogeny witnessed the closure of an extensive ocean — the Mozambique Ocean — and the collisional assembly of continental fragments within the Gondwana supercontinent amalgam. The tectonic history of southern India represents a progressive sequence from Pacific-type to collision-type orogeny which finally gave rise to a Himalayan-type Cambrian orogen with characteristic magmatic, metasomatic and metamorphic factories operating in subduction–collision setting.