Gondwana Suture Research Articles

Geochemical data, Nd and Hf isotopes and U–Pb geochronology of meta-mafic rocks from western Gondwana suture zone

The West Gondwana orogen, represented in NE Brazil by Santa Quitéria continental magmatic arc and ultrahigh-pressure metamafic rocks of a suture zone, have amphibolite and coesite-bearing retro-eclogite in both margins of the magmatic arc. This research reports and discuss the geochemical and geochronological data (U–Pb and Lu–Hf in zircon and Sm–Nd whole-rock) of amphibolite and retro-eclogite from the eastern margin of a continental magmatic arc. The Santa Quitéria Complex and metasedimentary Ceará Complex hosted these rocks. The Santa Quitéria amphibolites shows evident fractionation of REE. Nb–Ta–Ce anomalies evidence magma generated in a subduction zone with contribution and recycling of crustal sedimentary material, corroborate for slightly negative εNd (−1.56) and Paleoproterozoic NdTDM value (1.80 Ga). Isotopic Lu–Hf in zircon data indicates positive εHf (+5.00 to +7.78) and Neoproterozoic model ages (HfTDM from 0.97 to 0.86 Ga) (t = 637 ± 34 Ma - sample TJF5-258) indicate their extraction from the mantle. For the amphibolite samples from the Ceará Complex, the elemental distribution suggest that these rocks place them among oceanic basalts of enriched segments of mid-ocean ridges (E-MORB) and oceanic island basalts (OIB) – intra-oceanic arc basalts (IOAB). U–Pb zircon data of Ceará Complex amphibolite points to crystallization at ca. 670 Ma (674 ± 9 Ma, sample TJJV-07) and two metamorphic events around 640 Ma (644 ± 9 Ma) and 600 Ma (591 ± 5 Ma) related to the continental collision. Positive εNd values (+1.66 to +4.16) and model ages (NdTDM from 1.26 to 1.00 Ga) suggest a protolith crystallized from juvenile material of mantle source. Our data support that the eastern edge of the Santa Quitéria magmatic arc constitutes an active margin with subduction of the Cryogenian oceanic crust.

Multi‐stage metamorphism of ultrahigh‐temperature Mg‐Al granulites during Gondwana assembly: Evidence from southern India

The Southern Granulite Terrane (SGT) in India hosts granulite facies rocks metamorphosed at ultra‐high temperature (UHT) conditions in the various crustal blocks, as well as within the Palghat‐Cauvery Suture Zone (PCSZ), that is considered as a trace of the Late Neoproterozoic—Cambrian Gondwana suture. Here we investigate UHT granulites from the northern margin of the Madurai Block adjacent to the PCSZ where Mg‐Al‐rich granulites are exposed. We identify sodic gedrite + kyanite in these rocks as the high‐pressure prograde stage assemblage, followed by sillimanite‐garnet‐orthopyroxene that formed during pressure decrease and temperature increase. The rare remnant gedrite is also stable at the near‐peak UHT metamorphism until it was replaced by sapphirine. The rocks subsequently underwent decompression that formed sapphirine + cordierite and sapphirine + plagioclase symplectite around sillimanite. Dehydration during decompression generated orthopyroxene‐sillimanite‐quartz assemblage with the appearance of sapphirine, defining the diagnostic mineral assemblage indicative of peak UHT metamorphism (T > 900°C) at relatively high‐pressure (P > 9 kbar). The UHT peak metamorphism in this region is consistent with the results of P–T calculations using conventional geothermometers and phase equilibrium modelling (T up to 1,050°C, P over12 kbar). Zircon and monazite geochronology on the UHT metapelites indicate distinct stages. Detrital zircon grains in the metasediments indicate protolith from ca. 2.5 Ga igneous source and the metamorphic overgrowths yield 206Pb/238U mean ages concentrated at ca. 550–520 Ma. Monazite ages define another younger group 206Pb/238U mean ages at ca. 450 Ma. The prograde high‐pressure granulite‐facies metamorphism and following UHT event correlate with the subduction‐collision tectonics at 550–500 Ma associated with the final stage of amalgamation of the Gondwana supercontinent, while the 420–460 Ma monazite age records a later hydration at the post‐orogenic stage, possibly associated with deep shearing and fluid influx.

The early Paleozoic cumulate gabbroic rocks from the southwest part of the Tisza Mega-Unit (Mt. Papuk, NE Croatia): evidence of a Gondwana suture zone

The Mt. Papuk heteroadcumulate pyroxene–amphibole gabbronorites, which outcrops at the southern margin of the Tisza Mega-Unit, is suggested to stem from the deep oceanic crust formed by the in situ crystallisation in a supposed magma chamber. Amphibole oikocrystals are found to define a poikilitic texture of analysed rocks. A common enclosure in amphibole is the cumulus orthopyroxene, and rarely, the clinopyroxene and/or plagioclase and spinel. The chemical composition of related minerals and their crystallisation sequence suggest the sub-solidus crystallisation of gabbronorite in an open system at high temperatures and medium pressures. Parental magmas originated from the moderately depleted mantle source, which was metasomatised prior to melting. Early mineral fractionation gave rise to the assemblage consisted of spinel, pyroxene, plagioclase and intercumulus amphibole. The rocks’ bulk chemistry, mineral crystallisation sequence, pyroxene geochemistry and myriad of high Ca-plagioclase, which coexists with igneous Ca-amphibole are all in favour of the strong subduction influence typical for mafic intrusion formed above mantle wedge in the root of an island arc at depths of 10–21 km. Herein presented geochemical and isotopic data (40Ar–39Ar: 487.1 ± 4.3 Ma and Sm–Nd: 505 Ma) go along with the existence of an intra-oceanic arc related to geodynamic events that took place in the Prototethyan oceanic realm s.l. during middle Cambrian to earliest Ordovician. These events were likely correlated with the subduction of the Quaidam(?) back-arc ocean, or alternatively, with the subduction and closure of Prototethyan branches located between microcontinental fragments of Asia. Initially, the closure of back-arc oceans led to crust fragmentation and, then, addition of non-metamorphosed mafites into the obducted sequence further from the active continental margins of Gondwana and Laurassia at the time of the formation of Pangea in the late Palaeozoic.

Late Paleozoic subsidence and burial history of the Fort Worth basin

ABSTRACT The Fort Worth basin in northcentral Texas is a major shale-gas producer, yet its subsidence history and relationship to the Ouachita fold-thrust belt have not been well understood. We studied the depositional patterns of the basin during the late Paleozoic by correlating well logs and constructing structure and isopach maps. We then modeled the one-dimensional (1-D) and two-dimensional subsidence history of the basin and constrained its relationship to the Ouachita orogen. Because the super-Middle Pennsylvanian strata were largely eroded in the region, adding uncertainty to the subsidence reconstruction, we used PetroMod 1-D to conduct thermal-maturation modeling to constrain the post-Middle Pennsylvanian burial and exhumation history by matching the modeled vitrinite reflectance with measured vitrinite reflectance along five depth profiles. Our results of depositional patterns show that the tectonic uplift of the Muenster uplift to the northeast of the basin influenced subsidence as early as the Middle Mississippian, and the Ouachita orogen became the primary tectonic load by the late Middle Pennsylvanian when the depocenter shifted to the east. Our results show that the basin experienced 3.7–5.2 km (12,100–17,100 ft) of burial during the Pennsylvanian, and the burial depth deepens toward the east. We attributed the causes of deep Pennsylvanian burial and its spatial variation to flexural subsidence that continued into the Late Pennsylvanian in response to the growth of the Ouachita orogen and southeastward suturing of Laurentia and Gondwana. The modeling results also suggest that the Mississippian Barnett Shale reached the gas maturation window during the Middle–Late Pennsylvanian.

Petrology and zircon U–Pb geochronology of metagabbro from the Highland Complex, Sri Lanka: Implications for the correlation of Gondwana suture zones

The basement in the Sri Lankan fragment of Gondwana is composed of upper amphibolite- to granulite-facies rocks that underwent latest Neoproterozoic – Cambrian high-grade metamorphism. A recent tectonic model suggests that the Highland Complex in the central part of the island corresponds to a suture zone formed by the closure of a Neoproterozoic ocean between the Wanni and Vijayan Complexes. In order to test the model and further unravel the complex tectonic evolution of this region, we present new petrological, pressure–temperature, and zircon U–Pb geochronological data on garnet-bearing metagabbros from the Highland Complex. The metagabbros, which occur as blocks or lenses within metasediments of the Highland Complex, contain granulite-facies mineral assemblages of garnet+clinopyroxene+orthopyroxene+ilmenite+hornblende+plagioclase+quartz (sample HC7) and garnet+plagioclase+clinopyroxene+orthopyroxene+pargasite+quartz+rutile+ilmenite (sample HC8). Application of phase equilibria modeling in the system NCFMASHTO indicates peak P–T conditions of 900–950°C/10.5–11kbar and 800–830°C/8.5–9kbar, respectively, and clockwise P–T paths as inferred from decompression textures and mineral compositional isopleths. These conditions are significantly lower than the high-pressure (>17kbar) and ultrahigh-temperature (>1000°C) conditions obtained previously for similar metagabbros from the Highland Complex, whereas our results are consistent with those of similar rocks from the Palghat-Cauvery Suture Zone in India and the Lützow-Holm Complex in East Antarctica. The latest Neoproterozoic to Cambrian metamorphic ages (574±28Ma and 534±14Ma) as well as the available petrological data suggest that the metagabbros in the Highland Complex could be correlated with those in the Lützow-Holm Complex in East Antarctica. The results of this study confirm that the Highland Complex is an extension of the Lützow-Holm Complex, and that both these terranes involve a package of accreted trench sediments and remnant oceanic lithosphere metamorphosed during the final collision related to the Gondwana assembly.

Terminal suturing of Gondwana along the southern margin of South China Craton: Evidence from detrital zircon U-Pb ages and Hf isotopes in Cambrian and Ordovician strata, Hainan Island

Hainan Island, located near the southern end of mainland South China, consists of the Qiongzhong Block to the north and the Sanya Block to the south. In the Cambrian, these blocks were separated by an intervening ocean. U-Pb ages and Hf isotope compositions of detrital zircons from the Cambrian succession in the Sanya Block suggest that the unit contains detritus derived from late Paleoproterozoic and Mesoproterozoic units along the western margin of the West Australia Craton (e.g., Northampton Complex) or the Albany-Fraser-Wilkes orogen, which separates the West Australia and Mawson cratons. Thus, in the Cambrian the Sanya Block was not part of the South China Craton but rather part of the West Australian Craton and its environs. In contrast, overlying Late Ordovician strata display evidence for input of detritus from the Qiongzhong Block, which constituted part of the southeastern convergent plate margin of the South China Craton in the early Paleozoic. The evolving provenance record of the Cambrian and Ordovician strata suggests that the juxtaposition of South China and West Australian cratons occurred during the early to mid-Ordovician. The event was linked with the northern continuation of Kuungan Orogeny, with South China providing a record of final assembly of Gondwana.

The 580–520 Ma Gondwana suture of Madagascar and its continuation into Antarctica and Africa

U–Pb age data from southwest Madagascar provide a compelling case that the pre-Gondwana Indian plate was stitched with the arc terranes of the Arabian Nubian Shield along a suture that closed between 580Ma and 520Ma. The key observations supportive of this interpretation are: (1) metamorphism dated to 630–600Ma is manifested only on the west side of the suture in rocks that have affinities with the oceanic and island arc terranes of the Arabian Nubian Shield, or which represent continental rocks welded to these terranes prior to the amalgamation of Gondwana, and (2) orogenesis at 580–520Ma is manifest in rocks on both sides of the suture, an observation taken to mark the timing of collision and to reflect spatial continuity across the suture. In southwest Madagascar the distribution of metamorphic ages places the suture along the Beraketa high-strain zone, the tectonic boundary between the Androyen and Anosyen domains. Similar age relationships allow for the extrapolation of this tectonic boundary into both East Antarctica and Africa.

Possible detachment zone in Precambrian rocks of Kanjamalai Hills, Cauvery Suture Zone, Southern India: Implications to accretionary tectonics

Existence of a possible detachment zone at Elampillai region, NW margin of Kanjamalai Hills, located in the northern part of Cauvery Suture Zone (CSZ), Southern India, is reported here for the first time. Detailed structural mapping provides anatomy of the zone, which are rarely preserved in Precambrian high grade terranes. The detachment surface separates two distinct rock units of contrasting lithological and structural characters: the upper and lower units. The detachment zone is characterized by a variety of fold styles with the predominance of tight isoclinal folds with varied plunge directions, limb rotations and the hinge line variations often leading to lift-off fold like geometries and deformed sheath folds. Presence of parasitic folding and associated penetrative strains seem to be controlled by differences in mechanical stratigraphy, relative thicknesses of the competent and incompetent units, and the structural relief of the underlying basement. Our present study in conjunction with other available geological, geochemical and geochronological data from the region indicates that the structures of the detachment zone are genetically related to thrust tectonics forming a part of subduction–accretion–collision tectonic history of the Neoproterozoic Gondwana suture.

Unroofing Maine: Relating pressure of crystallization, thermochronological data, tectonics, and topography

We gathered data on the age and depth of crystallization of mid-Paleozoic igneous and metamorphic rocks from 64 sites in and near Maine and also a time series of rock temperatures from the Casco Bay area of Maine. By fitting a trend surface to some of our geobarometric data, we estimated total erosion at the Casco Bay site in the last 284m.y. and thus the geothermal gradient at that time. The temperatures could thus be converted to a time-series of erosion. Assuming Airy isostasy, we then obtained a time series of the mean height of the land surface above present sea level, represented by H.Of four erosion laws fit to the latter, a power law, E=νpHu where E is the erosion rate, and νp and u are fitting parameters, provides the best fit without excessive mathematical complexity. However, the exponent, u, is (unexpectedly)<1. We attribute this to crustal thickening at a restraining bend in the Norumbega fault zone. In addition, data from opposite sides of the fault suggest that dip-slip motion has been significantly>6km and that it likely began in the late middle Paleozoic, thus extending earlier estimates of this component of displacement backward in time and upward in magnitude. A decrease in denudation rate in the early Mesozoic is interpreted as reflecting suturing of Gondwana and Laurentia to form Pangea; the denudation rate increased again following Mesozoic rifting. This latter increase occurs later to the northwest, suggesting retreat of a rift-flank escarpment at a rate of a few tenths of a millimeter per year.Using our full geobarometric data set, we calculated an average value of u of 1.2. We then generated topographic maps of the state at 380, 320, and 200Ma. At 380Ma, the mountains of western Maine were likely somewhat higher than today's Rocky Mountains. Drainage then was to the northeast, contributing sediment to the Maritimes Basin. The Alleghanian collision at ~320Ma raised comparable mountains somewhat nearer the present coast in western Maine. Following Mesozoic rifting drainage was also to the southeast, into rift basins bordering the opening Atlantic Ocean. Since the Devonian, an average of >9km of rock have been eroded from Maine.

The Yucatan, a Laurentian or Gondwanan fragment? Geophysical and palinspastic constraints

Current reconstructions suggest that the Yucatan block has Gondwanan provenance and orient the Yucatan E–W in the Ouachita embayment where it overlaps southern Laurentia and Florida. Alternatively, if the Yucatan is oriented NE–SW, it fits neatly into the Ouachita embayment with minimal overlap. Furthermore, many of the V-shaped, magnetic anomalies in the Yucatan that are discordant in the E–W reconstruction can be traced across the Yucatan–Laurentian boundary in the NE–SW reconstruction: (a) NW-trending anomalies continue into southern Laurentia where they are associated with Cambrian mafic rocks in the southern Oklahoma and Reelfoot rifts and (b) NE-trending anomalies in the eastern Yucatan are parallel to those over Grenvillian rocks in the western Appalachians. Furthermore, Silurian plutons in the Maya Mountains of Belize that have no counterpart in Texas may be correlated with the Concord–Salisbury plutons in Carolinia, a terrane of Gondwanan provenance in the southern Appalachians. Nd isotopic data from the Chicxulub ejecta in the northern Yucatan block are similar to those in the Llano Grenvillian rocks and differ from those in Oaxaquia. These correlations suggest that much of the Yucatan is of Laurentian provenance and implies that the Laurentia–Gondwana suture crosses the Yucatan west of the Maya Mountains. In this scenario, the Ouachita embayment results from the formation of the Gulf of Mexico during the breakup of Pangea, rather than the Cambrian removal of the Argentine Cuyania terrane. Cambrian (515 Ma) paleomagnetic and faunal data are consistent with Cuyania forming either east of the Yucatan or off eastern Laurentia.

Multiple thermal events recorded in metamorphosed carbonate and associated rocks from the southern Austkampane region in the Sør Rondane Mountains, East Antarctica: A protracted Neoproterozoic history at the Gondwana suture zone

This paper provides a detailed analysis of the geological evolution of the southern Austkampane region in the Sør Rondane Mountains, Antarctica, using geological and petrographical observations, geochemical, geochronological, radiogenic, and stable isotopic data, and metamorphic petrological techniques. The study area is dominated by felsic gneisses intercalated with mafic and carbonate rocks. Carbonate rocks include several calc-silicate and mafic to ultramafic blocks with carbonatite-like characteristics. The new data presented here indicate the presence of five important thermal events in the geological history of the region, at c. 1060, 770–750, 655–635, 560–545, and <545Ma. The c. 1060Ma event is characterized by granitic volcanic arc magmatism, possibly in an active continental margin setting, involving magmas that may have been sourced from enriched mantle formed by subduction of older crustal material during the formation of Rodinia. The oldest T(DM) age for this suite is 3.4Ga. The 770–750Ma event is characterized by both mafic and felsic magmatism in an active continental margin setting, with magmas being similar to bulk earth composition. The 655–635Ma thermal event is represented by granulite-facies metamorphism along an anticlockwise pressure–temperature path, involving heating of the lower crust at a continental margin, and subsequent obduction of a cratonic counterpart during collision. Granitic and basaltic magmatism at 560–545Ma is also associated with retrograde metamorphism and hydration-related partial melting of marble. The carbonate melt, and granitic and basaltic magmas during this event coexisted with each other and formed metasomatic calc-silicates and hornblendites, respectively. The final event at <545Ma is characterized by the intrusion of an undeformed quartz syenite dyke, which may be related to either (1) collision and melting of preexisting lower crustal material, or (2) melting of slightly enriched mantle material in an extensional tectonic setting following the earlier collisional event. The five thermal events between c. 1100 and 500Ma recognized in this study from a narrow region of the Sør Rondane Mountains, suggest that a combination of geochronological, whole-rock, and isotope geochemical analyses of metamorphic rocks and the pressure–temperature evolution could constrain the processes that operated during the amalgamation and break-up of Rodinia and the formation of Gondwana. A similar approach is essential in neighboring Gondwana terranes for modeling the protracted events of Neoproterozoic orogenesis.

Petrology of the Neoproterozoic granulites from Central Dronning Maud Land, East Antarctica – Implications for southward extension of East African Orogen (EAO)

Suturing of east and west Gondwana occurred during the Neoproterozoic along the East African Orogen (EAO) through continent–continent collision. The southern extension of EAO is projected in central Dronning Maud Land (CDML) in east Antarctica. CDML is considered to be a magmato-metamorphic terrain where high-grade metamorphism occurred during Mesoproterozoic. Granulite grade metamorphic assemblages from metapelites (garnet+sillimanite+K feldspar (perthite)+graphite±plagioclase+quartz±melt), mafic granulite (orthopyroxene+garnet±clinopyroxene+plagioclase+quartz) and granitic orthogneiss (garnet+Fe-clinopyroxene+K feldspar+plagioclase+quartz) in the Humboldt Mountains in CDML have been used to deduce the peak metamorphic conditions as ∼850°C and 6–8kbar pressure. In situ chemical dating of monazite in matrix and inclusions in garnet indicates metamorphism to have occurred between 640 and 580Ma and was partially overprinted at ∼540Ma which is correlatable with extensive charnockite and A-type granite emplacements. This Neoproterozoic metamorphism and its correlation with the coast margin Neoproterozoic granulites at Schirmacher oasis is inferred as the extension of EAO in east Antarctica.

Fluids in high- to ultrahigh-temperature metamorphism along collisional sutures: Record from fluid inclusions

Petrographic studies and microthermometric investigations on fluid inclusions associated with high- to ultrahigh-temperature metamorphic rocks in three major Precambrian suture zones on the globe demonstrate the dominant occurrence of CO 2-rich fluids. These rocks form part of hot orogens developed along collisional plate boundaries. The sapphirine-quartz-bearing Mg–Al-rich rock from the Palghat-Cauvery Suture Zone, a trace of the Cambrian Gondwana suture zone in southern India, preserves evidence for a prograde high-pressure event and subsequent peak ultrahigh-temperature metamorphism along a clockwise path, and contains abundant CO 2-rich inclusions in corundum, garnet, and sapphirine. Most of the fluid inclusions are either primary or secondary and preserve low-density CO 2-rich fluids (0.569–0.807 g/cm 3). Similar low-density CO 2-rich fluid inclusions (0.853–0.953 g/cm 3) are also present in pelitic granulites from the Limpopo Complex of southern Africa, a Neoarchean granulite-facies orogen formed by continent–continent collision. In contrast, the garnet–orthopyroxene granulite from Tonagh Island in the Neoarchean Napier Complex in East Antarctica contains very high-density primary (1.095–1.129 g/cm 3) and secondary (0.960–1.179 g/cm 3) carbonic inclusions in garnet and quartz. The calculated isochores for the fluid inclusions from the Palghat-Cauvery Suture Zone and the Limpopo Complex yield significantly lower-pressure estimates than those predicted from peak metamorphic conditions. We interpret this as a result of significant density decrease due to rapid decompression along a clockwise P– T trajectory. In contrast, the estimated isochores for primary inclusions in garnet–orthopyroxene granulites from the Napier Complex are consistent with the peak P– T conditions estimated from mineral phase equilibria for the Tonagh Island rocks, suggesting that most of the fluid inclusions in these rocks did not undergo any marked effect of volume change and density decrease. The contrasting fluid densities among the localities investigated in this study are probably related to the nature of the P– T trajectory; the Tonagh Island rocks had a near-isochoric exhumation history whereas the metamorphic orogens in the other two sutures witnessed rapid decompression. Our results suggest that whereas the composition of the syn-metamorphic fluids are preserved in all cases, density reversal occurs within inclusions as a function of the tectonic history and exhumation style.

Subduction–accretion–collision history along the Gondwana suture in southern India: A laser ablation ICP-MS study of zircon chronology

We report the petrological characteristics and preliminary zircon geochronology based on laser ablation ICP mass spectrometry of the various units in an accretionary belt within the Palghat-Cauvery Shear/Suture Zone in southern India, a trace of the Cambrian Gondwana suture. Zircons extracted from a plagiogranite in association with an ophiolite suite within this suture possess internal structure that suggests magmatic crystallization, and yield mid Neoproterozoic 206Pb/ 238U age of 817 ± 16 Ma (error: 1 σ) constraining the approximate timing of birth of the Mozambique Ocean floor. Compiled age data on zircons separated from a quartzite and metamorphosed banded iron formation within the accretionary belt yields a younger intercept age of 759 ± 41 Ma (error: 1 σ) which we relate to a mid Neoproteozoic magmatic arc. Detrital zircons extracted from the quartzite yield 207Pb/ 206Pb age peaks of about 1.9–2.6 Ga suggesting that they were sourced from multiple protolithis of Neoarchean and Paleoproterozoic. Metamorphic overgrowths on some zircon grains record ca. 500–550 Ma ages which are in good harmony with the known ages for the timing of high-grade metamorphism in this zone during the final stage of continent collision associated with the birth of the Gondwana supercontinent in the latest Neoproterozoic-Cambrian. The preliminary geochronological results documented in our study correlate with the subduction–accretion–collision history associated with the closure of the Mozambique Ocean and the final amalgamation of the Gondwana supercontinent.

Crustal architecture beneath Madurai Block, southern India deduced from magnetotelluric studies: Implications for subduction–accretion tectonics associated with Gondwana assembly

The Madurai Block in southern India is considered to represent the eroded roots of an arc-accretionary complex that developed during the subduction–collision tectonics associated with the closure of the Mozambique Ocean and final suturing of the crustal fragments within the Gondwana supercontinent in the Late Neoproterozoic–Cambrian. Here we present a magnetotelluric (MT) model covering the main collisional suture (Palghat–Cauvery Suture Zone) in the north into the central part of the Madurai Block in the south comprising data from 11 stations. Together with a synthesis of the available seismic reflection data along a N–S transect further south within the Madurai Block, we evaluate the crustal architecture and its implications on the tectonic development of this region. According to our model, the predominantly south dipping seismic reflectors beneath the Madurai Block define a prominent south-dipping lithological layering with northward vergence resembling a thrust sequence. We interpret these stacked layers as imbricate structures or mega duplexes developed during subduction–accretion tectonics. The layered nature and stacking of contrasting velocity domains as imaged from the seismic profile, and the presence of thick (>20 km) low resistivity layers ‘floating’ within high resistivity domains as seen from MT model, suggest the subduction of a moderately thick oceanic crust. We identify several low resistivity domains beneath the Madurai Block from the MT model which probably represent eclogitised remnants of oceanic lithosphere. Their metamorphosed and exhumed equivalents in association with ultrahigh-temperature metamorphic orogens have been identified from surface geological studies. Both seismic reflections and MT model confirm a southward subduction polarity with a progressive accretion history during the northward migration of the trench prior to the final collisional assembly of the crustal blocks along the Palghat–Cauvery Suture Zone, the trace of the Gondwana suture in southern India.

高圧-超高温グラニュライトに含まれる珪線石集合結晶からみた南インド・ゴンドワナ縫合帯の温度圧力経路

高圧-超高温グラニュライトに含まれる珪線石集合結晶からみた南インド・ゴンドワナ縫合帯の温度圧力経路

Diopsidites from a Neoproterozoic–Cambrian suture in southern India

AbstractWe report the occurrence and characteristics of diopsidite dykes and veins from the Palghat-Cauvery Suture Zone (PCSZ) marking the boundary between the Archaean Dharwar craton to the north and the Proterozoic Madurai Block to the south, which is considered as a trace of the Cambrian Gondwana suture zone in southern India. The diopsidites are composed predominantly of coarse crystals of diopside [Mg no. (100 Mg/(Mg+Fetot)) up to 89] surrounded by retrograde calcic amphibole, plagioclase and phlogopite with accessory titanite and calcite. The major, trace and rare earth element characteristics of the diopside crystals suggest their formation in a subduction zone setting. We correlate the petrogenesis of the diopsidites with the tectonics associated with the subduction and closure of the Neoproterozoic Mozambique Ocean prior to the final collisional assembly of the Gondwana supercontinent in Cambrian.

The Manamedu Complex: Geochemical constraints on Neoproterozoic suprasubduction zone ophiolite formation within the Gondwana suture in southern India

Abstract Ophiolites provide important clues on the role of subduction and have been widely investigated to reconstruct the history of development and closure of ocean basins in the geological past. The Manamedu Complex within the Palghat-Cauvery Suture Zone in southern India comprises metamorphosed equivalents of the following lithological units: (1) an ultramafic group comprising dominantly of pyroxenite and highly altered dunite, locally preserving cumulate textures; (2) a gabbroic suite consisting of gabbro, gabbro norite, and anorthosite; (3) sheeted mafic dykes of amphibolite to meta-andesite categories, (4) plagiogranite veins and pools; and (5) a thin layer of ferruginous cherts. Cr vs. Y, V vs. Cr, Ti vs. Zr, TiO 2 –MnO–P 2 O 5 and Fe 2 O 3 –Na 2 O + K 2 O–MgO plots of the gabbros and mafic dyke assemblages show that these are related to island arc tholeiite (IAT) group with tholeiitic to calcalkaline signatures. Chondrite normalized REE patterns of mafic dykes do not show any pronounced fractionation and display slight positive Eu anomalies. The normalized MORB plots of the mafic dykes show depletion of HFSE (negative Nb, Ti, Ta, Hf anomalies) and enrichment of LFSE (positive K, Ba, Rb, Th). The petrological and geochemical characteristics of the major lithological units in Manamedu Complex suggest that these rocks represent the remnants of an oceanic crust, developed from mantle-derived arc magmas probably within a suprasubduction zone tectonic setting. From the geological set up and our field observations, we infer that these rocks were obducted on to the continental margin with the closure of an ocean basin during the Neoproterozoic. The Manamedu Complex may represent the remnants of the Mozambique Ocean crust developed during Rodinia breakup and which was destroyed during the amalgamation of the Gondwana supercontinent in the Latest Neoproterozoic-Cambrian.

Sapphirine+quartz corona around magnesian (XMg ~0.58) staurolite from the Palghat-Cauvery Suture Zone, southern India: Evidence for high-pressure and ultrahigh-temperature metamorphism within the Gondwana suture

Abstract We report the first finding of equilibrium sapphirine + quartz assemblage from the Palghat-Cauvery Suture Zone (PCSZ) in southern India providing unequivocal evidence for extreme crustal metamorphism at ultrahigh-temperature (UHT) conditions associated with the collisional assembly of the Gondwana supercontinent in the Late Neoproterozoic–Cambrian. The sapphirine and quartz occur as coronas around Mg-rich (XMg ~ 0.58) staurolite within poikiloblastic garnet in an Mg–Al-rich rock, suggesting the progress of the prograde dehydration reaction: staurolite + garnet → sapphirine + quartz + H2O. Although the occurrence of Mg-rich staurolite was previously noted from some localities in the PCSZ, no sapphirine + quartz direct association has yet been reported. The available experimental studies on Mg-rich staurolite indicate that the mineral is stable at P > 15 kbar, and suggest that the complex texture that we report here might correspond to a peak high-pressure regime prior to the UHT event. The symplectic sapphirine + quartz around staurolite probably implies decompression from P > 15 kbar toward the stability of sapphirine + quartz at temperatures of c. 1000 °C along a clockwise P–T path. The prograde high-pressure metamorphism and following UHT event correlate with the subduction–collision tectonics associated with the final stage of amalgamation of Gondwana supercontinent.

Fluid characteristics of retrogressed eclogites and mafic granulites from the Cambrian Gondwana suture zone in southern India

Eclogite-facies rocks and high-pressure granulites provide windows to the deeper parts of subduction zones and the root of mountain chains, carrying potential records of fluids associated with subduction-accretion-collision tectonics. Here, we report petrological and fluid inclusion data on retrogressed eclogite and high-pressure granulite samples from Sittampundi, Kanji Malai and Perundarai in southern India. These rocks occur within the trace of the Cambrian collisional suture which marks the final phase of amalgamation of the Gondwana supercontinent. The garnet–clinopyroxene assemblage in the eclogites preserves relict omphacite, whereas the high-pressure granulites are characterized by an assemblage of garnet and clinopyroxene in the absence of omphacite and with minor plagioclase, orthopyroxene, and quartz. Phase relations computed for the eclogite assemblage yield peak P–T conditions of 19 kbar and 1,010°C. The mafic granulites also preserve the memory of high to ultrahigh-temperature metamorphism followed by an isothermal decompression. Systematic fluid inclusion optical, microthermometric and laser Raman spectroscopic studies were conducted in garnet and plagioclase from the eclogite–high pressure granulite suite. The results suggest that the early fluids were a mixture of CO2, CH4 and N2 probably derived from decarbonation and devolatilization reactions in a subduction setting during the prograde stage. The later generation inclusions, which constitute the dominant category in all the samples studied, are characterized by a near-pure CO2 composition with moderate to high densities (up to 1.154 g/cm3). The highest density fluid inclusions recorded in this study occur within the mafic granulites from Sittampundi (0.968–1.154 g/cm3) and Kanji Malai (1.092–1.116 g/cm3). In some cases, carbonate minerals such as dolomite and calcite are associated with the CO2-rich fluid inclusions. The composition and densities of the later generation fluids closely match with those of the CO2-bearing fluid inclusions reported from ultrahigh-temperature granulites occurring proximal to the eclogite–high pressure granulite suite within this suture zone, and suggest a common tectonic link for the fluid regime. We evaluate the fluid characteristics associated with convergent plate margin processes and propose that the early aqueous fluids probably associated with the eclogites were consumed during the formation of the retrograde hydrous mineral assemblages, whereas the fluid regime of the high-pressure and ultrahigh-temperature granulites was mostly CO2-dominated. The tectonic setting of the rocks along a collisional suture marking the trace along which crustal blocks were welded through subduction–collision process is in favor of a model involving the derivation of CO2 from sub-lithospheric sources such as a carbonated tectosphere invaded by hot asthenosphere, or underplated mafic magmas.