Paleo-suture Zone Research Articles

Geochemistry of Waziristan Ophiolite Complex, Pakistan: Implications for Petrogenesis and Tectonic Setting

The Waziristan ophiolite complex (WOC), a segment of paleo-suture zone between the Indo-Pakistan plate to the south-east and the Afghan microplate to the north-west, is primarily composed of serpentinized dunites and serpentinites after harzburgites, crosscut by pyroxenites. The crustal portion dominantly comprises basalts and dolerites, with less abundant plagiogranites. Whole-rock XRF and trace element ICP-MS analyses were undertaken to determine the degree of melt extraction and subsequent geochemical depletion or enrichment of peridotite protoliths. The investigated peridotites are extremely refractory and show high Mg# values (0.88–0.92), low Al2O3 (0.19–0.96 wt%), total alkali values (0.02 < Na2O + K2O ≤ 0.20 wt%), CaO (0.04–0.73 wt%), and TiO2 (0.001–0.017 wt%), but are rich in Cr (up to 3550 ppm) and Ni (up to 2340 ppm). Bulk REE modelling suggests that Waziristan peridotites underwent a high degree (15%–25% melting) of melt extraction in the closed system of spinel-bearing peridotite facies at shallow depths above a subduction zone and, in their chemistry (Sc, REE, Al2O3, and MgO), resemble Izu-Bonin-Mariana (IBM) forearc peridotites. Their U-shaped REE patterns and trace elements resemble peridotites derived from supra-subduction zone (SSZ) settings, especially a forearc setting, and also reflect metasomatism by slab-derived fluids. The ultramafic rocks preserve depletion in REE content (0.03 < ΣREECN < 0.60), indicating the highly depleted nature of refractory peridotites. The basalts (Mg#, 0.48-0.68) are similar in composition to typical boninite and show low to moderate MgO (6.2–13.0 wt%), low total alkali content (0.01–4.45 wt%) and TiO2 (0.13–0.17 wt%), but are higher in Al2O3 (≈11.9 wt%). They show a sub-alkaline affinity and possess geochemical signatures that are transitional from island arc basalt (IAB) magmas to boninitic magmas due to the changes from an extensional to a compressional regime during the initiation of subduction. These geochemical signatures suggest the formation of basalts from melts, resulting from the re-melting of the depleted mantle during intra-oceanic subduction initiation. The higher Th/Nb and V/Ti ratios of the studied basalts further confirm their generation in an SSZ setting characterized by subduction-derived fluids under higher oxidizing conditions. The mafic-ultramafic rocks of the WOC were, therefore, likely formed during intra-oceanic subduction initiation (forearc spreading) in the SSZ setting; they were exhumed along a thrust fault, and obducted onto the forearc region as result of the collision between the Indian plate and the Afghan microplate.

The Neoproterozoic basement of the Parnaíba Basin (NE Brazil) from combined geophysical-geological analysis: A missing piece of the western Gondwana puzzle

A regional Pre-Silurian unconformity marks the erosive planar base of the Phanerozoic Parnaíba Basin (PB), and indistinctly truncates key tectonic units for the understanding of the Neoproterozoic evolution of western Gondwana in northeastern Brazil. We present a comparative analysis of geophysical and geological datasets in the PB and propose a new tectonic configuration for its pre-Silurian basement, composed of different terranes amalgamated during the Brasiliano orogeny. Using integrated seismic interpretation and gravity modelling, constrained by an updated crustal thickness map, well data and a compilation of recent geophysical studies, we have identified two main blocks in the center of the basin. They represent pre-Brasiliano inliers, surrounded by Brasiliano mobile belts, and assigned to two major crustal building blocks of western Gondwana: the Amazonian-West Africa (AWB; to the west/north) and the Central African blocks (CAB; to the east/northeast). The Grajaú block belongs to the AWB and is characterized by low gravity anomaly, thicker crust (41–45 km), transparent seismic character of the basement and a high velocity lower crust. The Teresina block belongs to the CAB and is characterized by slightly thinner crust (39–41 km), higher values of gravity and magnetic anomalies and by the presence of mid-crustal reflectors (MCR), observed in seven seismic lines and here interpreted as a remnant of a paleosuture zone between the two blocks. Along a NE-SW 500-km seismic and gravity profile, we interpreted the MCR as composed of crustal-scale thrust faults verging westwards and defining the Barra do Corda belt, which was formed by the closure of the Goiás-Pharusian ocean along the Transbrasiliano-Kandi corridor and subsequent collision of the Grajaú and Teresina blocks. This belt deforms the eastern margin of the Ediacaran Riachão foreland basin (RB), observed in seismic and well data beneath the SW portion of PB and overlying the southern Grajaú block. The western margin of RB is bounded by eastwards verging thrust faults, interpreted as a zone of back-thrusts and thinned crust (∼36 km) in the eastern prolongation of the Araguaia belt beneath PB. To the east, the limit between the Teresina block and the Borborema Province is marked by the NE-SW Transbrasiliano Shear Zone, along which Neoproterozoic mylonites were recovered from wellbores, and a system of narrow pull-apart basins is interpreted in the seismic data, triggered by Early Paleozoic transtensional reactivations of the TBSZ, prior to the deposition of the basal Ordovician-Silurian sequence of PB. This complex collisional tectonic setting of PB basement contradicts the idea of a stable cratonic block beneath it, inciting new formation models that account for the observed crustal and lithospheric heterogeneities.

Petrology and metamorphism of glaucophane eclogites in Changning-Menglian suture zone, Bangbing area, southeast Tibetan Plateau: An evidence for Paleo-Tethyan subduction

High/ultrahigh-pressure (HP/UHP) metamorphic complexes, such as eclogite and blueschist, are generally regarded as significant signature of paleo-subduction zones and paleo-suture zones. Glaucophane eclogites have been recently identified within the Lancang Group characterized by accretionary mélange in the Changning-Menglian suture zone, at Bangbing in the Shuangjiang area of southeastern Tibetan Plateau. The authors report the result of petrological, mineralogical and metamorphism investigations of these rocks, and discuss their tectonic implications. The eclogites are located within the Suyi blueschist belt and occur as tectonic lenses in coarse-grained garnet muscovite schists. The major mineral assemblage of the eclogites includes garnet, omphacite, glaucophane, phengite, clinozoisite and rutile. Eclogitic garnet contains numerous inclusions, such as omphacite, glaucophane, rutile, and quartz with radial cracks around. Glaucophane and clinozoisite in the matrix have apparent optical and compositional zonation. Four stages of metamorphic evolution can be determined: The prograde blueschist facies (M₁), the peak eclogite facies (M₂), the decompression blueschist facies (M₃) and retrograde greenschist facies (M₄). Using the Grt-Omp-Phn geothermobarometer, a peak eclogite facies metamorphic <i>P-T</i> condition of 3000–3270 MPa and 617–658°C was determined, which is typical of low-temperature ultrahigh-pressure metamorphism. The comparison of the geological characteristics of the Bangbing glaucophane eclogites and the Mengku lawsonite-bearing retrograde eclogites indicates that two suites of eclogites may have formed from significantly different depths or localities to create the tectonic mélange in a subduction channel during subduction of the Triassic Changning-Menglian Ocean. The discovery of the Bangbing glaucophane eclogites may represent a new oceanic HP/UHP metamorphic belt in the Changning-Menglian suture zone.

Structural features and tectonic evolution of the Nenjiang–Balihan fault in the western margin of the Songliao Basin, NE China, inferred from 2D inversion of magnetotelluric data

Located in the famous gravity gradient belt ranging from the Mt. Great Xing'an through Mts. Taihang and Wuling in East China, the Nenjiang-Balihan (NJBLH) fault is important for the understanding of the formation and evolution of Mesozoic and Cenozoic basins in NE China. The distribution and deep structure of the middle and northern segments of this fault are poorly known because of thick sedimentary cover. In this paper, an electrical structure across the western margin of the Songliao Basin and eastern margin of the Great Xing’an Range is revealed on the basis of three magnetotelluric (MT) sounding survey profiles. The results show the development of a low-resistivity layer (L2) and two low-resistivity strips with opposite dip (L1 and L3) in the lower portion of the small-scale basins or sags. The west-dipping low-resistivity strip L1 is considered to be the trace of a paleo-suture zone. The transverse low-resistivity layer L2 may reflect the Palaeozoic sedimentary strata in the basin, and the east-dipping low-resistivity strip L3 represents the NJBLH fault, the presence of which exhibits a gentle sloping tendency from the shallow to the deep portion of the fault cut through the Moho. The NJBLH fault is segmented on the plane, with an overall left-slip NE-trending en echelon arrangement. Presumably, the NJBLH fault began to form during the Jurassic under the influence of the closure of the Mongolia–Okhotsk Ocean in the northwest, and the fault subsequently experienced left-lateral strike-slip movements due to the NW-trending subduction of the Paleo-Pacific Izanagi Plate toward the Eurasian Plate.

A new tectonic map of the Iranian plateau based on aeromagnetic identification of magmatic arcs and ophiolite belts

The Iranian plateau is one of the most complex geodynamic settings within the Alpine-Himalayan belt. The Paleo-Tethys and Neo-Tethys ocean subduction is responsible for the formation of several magmatic arcs and sedimentary basins within the plateau. These zones mostly are separated by thrust faults related to paleo-suture zones, which are highlighted by ophiolites. Sediment cover and overprint of a different magmatic phase from late Triassic to the Quaternary impede identification of some magmatic arcs and ophiolite belts.We track the known magmatic arcs, such as the Urmia-Dokhtar Magmatic Arc (UDMA), and unknown, sediment covered magmatic arcs by aeromagnetic data. We present a new map of average susceptibility calculated by the radially averaged power spectrum method. High average susceptibility values indicate the presence of a number of lineaments that correlate with known occurrences of Magmatic-Ophiolite Arcs (MOA), and low average susceptibility coincides with known sedimentary basins like Zagros, Makran, Kopeh-Dagh, and Tabas. In analogy to Zagros, low average susceptibility values indicate sedimentary basins to the south of the Darouneh fault and in the northern part of the Lut, Tabas and Yazd blocks. We interpret the Tabas basin as a pull-apart or back-arc basin. We identify hitherto unknown parallel MOAs in eastern Iran and the SE part of UDMA which both indicate steeply dipping (>60° dip) paleo-subduction zones. In contrast, we interpret shallow subduction (<20° dip) of Neo-Tethys in the NW part of UDMA as well as in the Sabzevar-Kavir MOA.

Contrasting kinematics of brittle-shears within the Salem–Attur and Bhavani shear zone, south India: Tectonic implications

We document kinematics and rheological behaviour of brittle shears (~50 cm wide) postdating solid-state tectonic fabric in the Salem–Attur (SASZ) and Bhavani (BSZ) shear zone that constitute a Paleoproterozoic (~2500 Ma) suture juxtaposing disparate granulite blocks in south India. We constrain brittle deformation mechanisms from established relationship between changing orientation of deflected strain marker (quartz vein) and foliation within the shear band with respect to their orientation outside the shear band. Quartz c-axis orientation in charnockite (host lithology) and phyllonite (reworked charnockite) from the SASZ show presence of mixed basal 〈a〉 (low-T) and prism 〈a〉 (high-T) slip, and single basal 〈a〉 slip mechanism, respectively. This suggests considerable cooling of the granulite block prior to the onset of brittle shearing. Distribution of strain parameters – effective shear strain (Γ), shear strain (γ), stretch K2 along intermediate strain axis Y – from margin to the centre of the shear band, show peaked distribution with a single maximum at the shear zone centre. This implies rheological-weakening/strain-softening induced localizing shear zone character. Kinematically heterogeneous strain distribution during brittle shearing varies from transpression dominated for the BSZ to transpression-to-transtension switchover for the SASZ. Demonstrably, contrasting cooling-exhumation, hitherto unexplored, characterizes post-accretionary tectonics along the paleo-suture zone.

Metamorphic P-T path and zircon U-Pb dating of HP mafic granulites in the Yushugou granulite-peridotite complex, Chinese South Tianshan, NW China

Co-existing granulite and peridotite may represent relics of the paleo-suture zone and provides an optimal opportunity for better understanding of orogeny between two blocks. In this study, we carried out petrological and U-Pb zircon dating investigation on the HP mafic granulites associated with peridotite complex at Yushugou in Chinese South Tianshan. The studied samples include garnet-bearing high-pressure mafic granulites which can be subdivided into two types: Type I orthopyroxene-free and Type II orthopyroxene-bearing granulites and amphibolite. Type I granulite (Y21-2) has a mineral assemblage of garnet (33vol.%), clinopyroxene (32vol.%) and plagioclase (30vol.%); and Type II granulite (Y18-8) has a mineral assemblage of garnet (22vol.%), clinopyroxene (10vol.%), orthopyroxene (14vol.%), plagioclase (45vol.%) and quartz. Garnet in both granulites exhibits core-rim structure characterized by increasing grossular and decreasing pyrope from core to rim. Petrographic observations and phase equilibrium modeling using THERMOCALC in the NCFMASHTO system for the mafic granulites (Y21-2 and Y18-8) show three stages of metamorphism: Stage I (granulite facies) was recognized by the large porphyroblastic garnet core, with P-T conditions of 9.8–10.4Kbar and 860–900°C (Y21-2) and 9.9–10.6Kbar and 875–890°C (Y18-8), respectively; Stage II (HP granulite facies) has peak P-T conditions of 12.1Kbar at 755°C (Y21-2) and 13.8Kbar at 815°C (Y18-8) using mineral assemblages combining with garnet rim compositions with maximum grossular and minimum pyrope contents; Stage III (amphibolite facies) was characterized by the development of calcic amphibole in granulites with temperature of 446–563°C. Therefore, an anticlockwise P-T path characterized by simultaneous temperature-decreasing and pressure-increasing was inferred for the Yushugou HP mafic granulite. Studies of zircon morphology and inclusions, combined with zircon U-Pb dating and REE geochemistry indicate that their protolith’s ages of the mafic granulites were ∼430Ma, while the metamorphism could occur at three stages with ages of ∼390Ma, ∼340Ma and ∼320Ma, which may correspond to Stage I, II and III, respectively. We interpret the HP mafic granulites from the Yushugou granulite-peridotite complex to be formed by the cooling subduction of the lower crustal rocks from the hanging wall of central Tianshan block during the northward subduction of the south Tianshan paleo-ocean from Devonian to Carboniferous.

Boron isotopic discrimination for subduction-related serpentinites

Abstract The Guatemala Suture Zone (GSZ), Guatemala, is a region that contains two distinct suture-related serpentinite mélanges straddling the Motagua fault and an ophiolitic complex paired with the northern mélange. The serpentinite matrix of the mélanges formed by subduction-fluid hydration of peridotite from the deep mantle wedge. The occurrence of serpentinite from both exhumed subduction channel mélange and ophiolite is not uncommon in paleo–suture zones, but distinguishing them and their tectonic origin can be difficult. A new method of discrimination, based on boron isotopes in serpentine from both mélanges and ophiolite, as well as on mica and pyroxene from the metamorphic and vein-rock blocks embedded within the mélanges, has been developed. The metamorphic and vein samples have mainly negative δ11B, ranging from –15.3‰ to +4.3‰, in the same range as the serpentine from the mélanges (–14.4‰ to +9.7‰). In addition to being the most negative δ11B values ever measured in serpentinite, comparable values from vein minerals indicate that the same fluid serpentinized the overlying mantle. In contrast, serpentine samples from the ophiolite have positive δ11B, in the range 0‰ to +18.0‰, consistent with hydration by seawater-derived fluids. As the GSZ displays two mélanges whose serpentinite originated from two different deep subductions and mantle hydration, we hypothesize that the negative signature of exhumed mélange serpentine is the norm and that the B isotopic signature can be a useful tool to discriminate the tectonic origin of serpentinization in paleo–suture zones.

Garnet pyroxenite from the Shackleton Range, Antarctica: Intrusion of plume-derived picritic melts in the continental lithosphere during Rodinia breakup?

Lenses of ultramafic rocks occur in supracrustal high-grade gneiss in the northern Haskard Highlands, Shackleton Range, East Antarctica. Olivine-bearing garnet pyroxenite is the dominant rock type that is associated with hornblendite and subordinate spinel peridotite and amphibolite. The high-pressure (23–25kbar) garnet–olivine assemblage of the pyroxenite formed during Pan-African eclogite-facies metamorphism. Associated collisional tectonics led to the incorporation of the ultramafic and mafic rocks in upper crustal rocks of a subducting continental margin. The ultramafic–mafic rocks are tracers of a paleo-suture zone and are critical for reconstructing Gondwana amalgamation. Thus, it is important to infer the tectonic setting of the rocks prior to emplacement into their current position, i.e., were the rocks part of the oceanic crust, the sub-oceanic, or the sub-continental mantle? Major and trace elements together with Pb and Nd isotope data imply that the precursor rocks of the pyroxenites and hornblendites (the latter being retrogressed pyroxenite equivalents) formed as plume-related melts, with many characteristics typical for ocean–island tholeiitic magmas. Hence, pyroxenite and hornblendite are interpreted as metamorphic equivalents of picritic melts. They differ from most garnet pyroxenites worldwide in composition and genesis. The latter formed as high-pressure clinopyroxene-rich cumulates from basaltic melts. The volumetrically minor amphibolites, sharing many geochemical characteristics with pyroxenites and hornblendites, are also interpreted as metamorphic equivalents of plume-related melts. It is inferred that the picritic melts crystallized at medium- to high-pressure conditions in the upper continental mantle or in the transition zone between mantle and continental crust. The subordinate spinel peridotites are interpreted as fragments of the uppermost, depleted mantle. They are probably the wall rocks into which the picritic melts intruded. The Pb and Nd mantle separation ages of the picritic melts range from 770 to 870Ma. These model ages are very similar to the emplacement ages of numerous global mafic and ultramafic dykes, which are genetically linked to mantle plume activity that initiated Rodinia rifting and breakup. The protoliths of pyroxenite and related rocks in the Shackleton Range most likely formed during the initial stages of plume magmatism that eventually led to the Rodinia breakup.

Cryogenian alkaline magmatism in the Southern Granulite Terrane, India: Petrology, geochemistry, zircon U–Pb ages and Lu–Hf isotopes

Abstract The Southern Granulite Terrane (SGT) in India preserves the records of the formation and recycling of continental crust from Mesoarchean through Paleoproterozoic to Neoproterozoic and Cambrian, involving multiple subduction–accretion–collision associated with major orogenic cycles. A chain of unmetamorphosed and undeformed alkaline magmatic intrusions occurs along the northern margin of the SGT aligned along paleo-suture zones. Here we investigate two representative plutons from this suite, the Angadimogar syenite (AM) and the Peralimala alkali granite (PM) through field, petrological, geochemical, zircon U–Pb and Lu–Hf studies. Magma mixing and mingling textures and mineral assemblages typical of alkaline rocks are displayed by these plutons. The whole-rock major and trace element data characterize their alkaline nature. In trace element discrimination diagrams, the AM rocks straddle between the VAG (volcanic-arc granites) and WPG (within plate granites) fields with most of the samples confined to the VAG field, whereas the PM rocks are essentially confined to the WPG field. The diversity in some of the geochemical features between the two plutons is interpreted to be the reflection of source heterogeneities. Most zircon grains from the AM and PM plutons display oscillatory zoning typical of magmatic crystallization although some grains, particularly those from the PM pluton, show core-rim structures with dark patchy zoned cores surrounded by irregular thin rims resulting from fluid alteration. The weighted mean 206 Pb/ 238 U ages of the magmatic zircons from three samples of the AM syenite are in the range of 781.8 ± 3.8 Ma to 798 ± 3.6 Ma and those from two samples of the PM alkali granite yield ages of 797.5 ± 3.7 Ma and 799 ± 6.2 Ma. A mafic magmatic enclave from the AM pluton shows weighted mean 206 Pb/ 238 U age of 795 ± 3.3 Ma. The AM and PM plutons also carry rare xeneocrystic zircons which define upper intercept concordia ages of 3293 ± 13 Ma and 2530 ± 22 Ma correlating with the ages of the basement rocks from these areas. The initial 176 Hf/ 177 Hf isotope ratios of the zircon grains from the AM syenite fall in the range between 0.281771 and 0.282284, with moderately negative eHf(t) values between − 5.9 and 0.1. Similarly, the initial 176 Hf/ 177 Hf isotope ratios for the zircon grains of PM ultrapotassic granite range between 0.281197 and 0.281970, albeit with more negative eHf(t) values in the range between − 22.7 and − 0.3 (average eHf (t) value − 18.8). The Lu–Hf data suggest the involvement of variable extent of older crust with distinct crustal residence times, either in the form of assimilation during magma emplacement, or crustal recycling during magma genesis. Based on the geochemical and isotopic systematics, a possible petrogenetic model would be asthenospheric upwelling in an extensional setting, melting of enriched lithosphere, and interaction of the magmas with lower crustal domains with subduction-related components of various ages. The disposition of these alkali plutons along two paleo sutures that weld the Meso-Neoarchean crustal blocks in the northern periphery of SGT suggests that the zones of emplacement might represent an aborted rift. The paleo-sutures probably served as a weak zone along which extension occurred broadly coeval with the Cryogenian subduction further south.

The ‘Jiaodong type’ gold deposits: Characteristics, origin and prospecting

The Jiaodong type comprises a unique class of gold mineralisation which includes the Linglong type quartz-vein hosted and the Jiaojia type fracture-disseminated style deposits. Although the type area is in the Jiaodong Peninsula in NE China, where some of the world's richest gold reserves occur, we identify similar occurrences in other parts of China and elsewhere in the world under this category. The ore fluids migrated upwards forming quartz vein type of gold ore (Linglong type) wherever the ore-controlling fractures are at high angles. Fluid infiltration into the surrounding wall rocks and generation of the Jiaojia type disseminated style deposits resulted where the ore-bearing fluids encountered low angle fractures. On a regional scale, the Jiaodong type gold mineralisation occurs along the margins of reactivated cratons, within the interior of cratons along paleosuture zones, or along the junctions of micro-blocks, and is distinctly different from the classic orogenic gold in terms of their tectonic setting, ore characteristics and genetic history. Unlike skarn and porphyry type of deposits, the Jiaodong type gold deposits do not always show close spatial relationship with magmatic intrusives and some of the deposits occur distal to plutons (>10km). In this paper, we evaluate the salient geological, geochronological, geochemical and isotopic features of the Jiaodong type gold deposits and propose some general guidelines for gold prospecting. Based on case studies, we predict that the gold lodes in the Jiaodong type mineralised zones might extent vertically up to 3000m with only minor mineralogical and geochemical variations.

A Rodinian suture in western India: New insights on India-Madagascar correlations

We report detailed evidence for a new paleo-suture zone (the Kumta suture) on the western margin of southern India. The c. 15-km-wide, westward dipping suture zone contains garnet-biotite, fuchsite-haematite, chlorite-quartz, quartz-phengite schists, biotite augen gneiss, marble and amphibolite. The isochemical phase diagram estimations and the high-Si phengite composition of quartz-phengite schist suggest a near-peak condition of c. 18kbar at c. 550°C, followed by near-isothermal decompression. The detrital SHRIMP U–Pb zircon ages from quartz-phengite schist give four age populations ranging from 3280 to 2993Ma. Phengite from quartz-phengite schist and biotite from garnet-biotite schist have K–Ar metamorphic ages of ca. 1326 and ca. 1385Ma respectively. Electron microprobe-CHIME ages of in situ zircons in quartz-phengite schist (ca. 3750Ma and ca. 1697Ma) are consistent with the above results. The Bondla ultramafic-gabbro complex in the west of the Kumta suture compositionally represents an arc with K–Ar biotite ages from gabbro in the range 1644–1536Ma. On the eastern side of the suture are weakly deformed and unmetamorphosed shallow westward-dipping sedimentary rocks of the Sirsi shelf, which has the following upward stratigraphy: pebbly quartzite/sandstone, turbidite, magnetite iron formation, and limestone; farther east the lower lying quartzite has an unconformable contact with ca. 2571Ma quartzo-feldspathic gneisses of the Dharwar block with a ca. 1733Ma biotite cooling age. To the west of the suture is a c. 60-km-wide Karwar block mainly consisting of tonalite-trondhjemite-granodiorite (TTG) and amphibolite. The TTGs have U–Pb zircon magmatic ages of ca. 3200Ma with a rare inherited core age of ca. 3601Ma. The K–Ar biotite cooling age from the TTGs (1746Ma and 1796Ma) and amphibolite (ca. 1697Ma) represents late-stage uplift. Integration of geological, structural and geochronological data from western India and eastern Madagascar suggest diachronous ocean closure during the amalgamation of Rodinia; in the north at around ca. 1380Ma, and a progression toward the south until ca. 750Ma. Satellite imagery based regional structural lineaments suggests that the Betsimisaraka suture continues into western India as the Kumta suture and possibly farther south toward a suture in the Coorg area, representing in total a c. 1000km long Rodinian suture.

The Tam Ky-Phuoc Son Shear Zone in central Vietnam: Tectonic and metallogenic implications

The Tam Ky-Phuoc Son Shear Zone (TKPSSZ) is one of the most pronounced crustal structures in central Vietnam and juxtaposes high-grade rocks of the Kontum Massif against lower grade rocks of the Truong Son Fold Belt. An internal zone comprises highly strained rocks, including ultramafic–mafic tectonites, bounded by mylonitic zones that straddle tens to hundreds of kilometers. An external zone comprises mainly high-grade metamorphic complexes (southern flank), and weakly metamorphosed siliciclastic and carbonate rocks with subordinate greenstone (northern flank). Detailed structural analysis reveals that the shear zone is a multi-deformed terrane: D1 produced a regional high grade schistosity and mylonitization; D2 generated regional northwest–southeast trending fold arrays and brittle–ductile shear zones; D3 locally produced northeast-trending folds; brittle faulting occurred during D4–D5. U–Pb dating of zircon and monazite suggests that D1 involved metamorphism and felsic magmatism at ca. 430Ma, recording part of a regional collisional orogeny. Monazite and titanite growth at ca. 250–240Ma in basement rocks is synchronous with widespread syn- to post-D2 magmatism between ca. 260 and 245Ma suggesting a second major collisional event during the Indosinian Orogeny. D3 may have occurred as part of (or soon after) this Permo-Triassic event. Deformation during post-collisional stages, perhaps in response to extrusion/wedging and oroclinal rotation of terranes, led to post-D3 structures (D4–D5). The E–W trending TKPSSZ is here shown to be a continuation of the N–S trending Po Ko Shear Zone (PKSZ). Most of the significant lode gold occurrences in central Vietnam occur along this TKPSSZ–PKSZ structure and are associated with, and controlled by, D1 ductile to ductile–brittle high-strain zones. Mineralized later-stage structures support remobilization and reconcentration of ore during subsequent events at ca. 240Ma (Re–Os molybdenite age). The TKPSSZ–PKSZ D1 structure likely represents part of a paleosuture zone, marking the closure of an ancient ocean basin through terrane assembly in the Early Paleozoic.

INTEGRATED GEOPHYSICAL RESEARCH IN THE MIDDLE-NORTHERN CONTINENTAL MARGIN OF THE SOUTH CHINA SEA BASED ON WIDE-ANGLE SEISMIC PROFILE OBS2006-3

The OBS2006-3profile located in the northeastern South China Sea(SCS),as an integrated endeavor of geophysical research,passes through the Dongsha Islands and the Chaoshan Depression,and ends at the margin of the ocean basin.Density modeling remains the pivotal part of the research.Based on the model,combined with the geological and geophysical data available,it is revealed that:(1)A Mesozoic subduction zone might occur in the sea area around the Dongsha Islands,which might be the same as the East Asia subduction system.The Dongsha fault is probably the paleo-suture zone,and the Chaoshan depression might be the Mesozoic fore-arc basin,and the Dongsha-Peikang uplift might be a paleo-volcanic arc.(2)The thickest Mesozoic in the Chaoshan depression is around~9km with oil and gas potential.There may be magmatic intrusions and/or metamorphism to certain extent.

The Moho: Boundary above upper mantle peridotites or lower crustal eclogites? A global review and new interpretations for passive margins

Abstract We have performed a global study of 2D crustal scale wide-angle profiles across passive margins, with regard to local elevations in the Moho which could possibly be interpreted as indicative of lower crustal eclogites. A total of 16 candidates have been found, mainly in the North Atlantic and around Australia. These cases make up c. 6% only of the total profile length studied, confirming the interpretation of the Moho generally representing the top of the mantle. The interpreted candidates for lower crustal eclogites indicate that there may be a link between eclogite bodies and continental suture zones in the Barents Sea, off mid-Norway, in the Newfoundland Appalachians and in the Yilgarn Craton, western Australia. It is also possible that there is a genetic link between the formation of Caledonian eclogites and the Jan Mayen Fracture Zone, which is the only major fracture zone in the North Atlantic. Several of the inferred lower crustal eclogite bodies are located close to lines of major changes in strain, such as coastlines and shelf edges, indicating that lower crustal eclogite bodies may be important in guiding the evolution of basin architecture. Interpreting the Moho beneath the Bedout High on the northwest shelf of Australia as the top of a body of lower crustal eclogites, may imply that the northern termination of the Lambert Shelf represents a paleo suture zone and that the western termination of the Broome Platform acted as a major transfer zone. The significant increase in crustal thickness implied by the eclogite model has important implications for estimates of stretching history, subsidence and hydrocarbon maturation modelling.

Ultra-Deep Drilling through 3.5-Billion-Year-Old Crust in South Africa

The Makhonjwa Mountains of South Africa and Swaziland comprise some of the most sought-after geo-real estate in the world. It is priceless—that is, for geoscientists—because the rocks of this approximately 120 km by 60 km corner of southern Africa, also known as the Barberton Greenstone Belt, date back to 3.2–3.6 billion years (Ga), representative of Earth in early Archean times when it was still ~1 Ga years young. They are not the very oldest rocks on Earth (those occur in Greenland and Canada), but they are the oldest best-preserved ones; thus, this stretch of land is without equal for research into the early history of our Earth. It is home to some of the earliest fragments of island arc, oceanic crust, and vestigial tracts of continent covered with sedimentary and volcanic rocks. So well-preserved are these rocks that unless one radiometrically dates them, it is near impossible to distinguish them from many modern rocks. This exceptional preservation has ensured that the Makhonjwa rocks yield the oldest directly dated and undisputed signs of life on Earth, and compared to our present biosphere they also provide detailed clues about the hostile nature of the paleoenvironments under which this life struggled to persist. One severe challenge entailed coping with more potent solar radiation to which life is particularly sensitive, when Earth’s magnetic field was too weak to efficiently shield the surface from the relentless solar wind of lethal charged particles. Another is to explore for paleo-suture zones that can help establish when plate tectonics first emerged as the dominant solid earth recycling process to nurture the only sustainable habitable zone in our solar system. These then represent some of the targets of a new deep drilling project, on which an ICDP workshop was focused and held on 13–19 April 2010.

Proto-Tethyan remnants in northwest Iran: Geochemistry of the gneisses and metapelitic rocks

The Iran continental crust was metamorphosed, intruded by granitoid magmas, folded and faulted during the Late Precambrian by the Pan-African Orogeny. The basement complex in the Takab Complex (northwest Iran) consists of gneisses, amphibolites, pelitic schists, meta-ultramafic and calc-silicate rocks. Geochemically, the protoliths of the Takab gneisses are slightly peraluminous and medium to high-potassic with calc-alkaline affinity. These gneisses may have been emplaced in volcanic arc tectonic setting. Furthermore, the metapelitic protolith is shale deposited in an active continental margin setting. All these characteristics, and presence of paleo-suture zone and ophiolitic rocks (i.e. serpentines, meta-mafic and meta-ultramafic rocks) around the high grade metamorphic rocks suggest that a continental-margin magmatic arc (Andean-type) formed the Takab Precambrian basement. The basement complexes are extensively overprinted by the Pan-African Orogeny and younger igneous events; this supports the inference that Early Cambrian orogenesis in the Takab Complex region of northwest Iran marks one of the fundamental lithospheric boundaries within Gondwana which belonged to a greater Late Neoproterozoic–Early Paleozoic orogenic system that was active along the Proto-Tethyan margin of the Gondwana supercontinent, extending at least from its Arabian margin to the Himalayan margin of the Indian subcontinent.

Evolution of the Central Iranian basement

Most of the Infracambrian–Lower Paleozoic sedimentary rocks occurring in Iran cover a crystalline basement. In Central Iran, and the Saghand region along the Chapedony and Poshte Badam faults, the basement complexes show structural vestiges of Precambrian deformational, depositional, erosional, metamorphic and magmatic events. In this region, the basement complexes contain the Chapedony, Poshte Badam, Boneh Shurow and Tashk formations. The prevailing metamorphism, from low grade to high grade, is pre-Pan African Orogeny (i.e. from 2400 to 570 Ma). The presence of greenstone belts, a paleo-suture zone and ophiolitic rocks (i.e. from 2100 to 1500 Ma) around the high-grade metamorphic rocks of the Chapedony Formation, provide evidence that cratonization forming the Iranian basement occurred during the Paleo- and Mesoproterozoic. After formation of the basement complexes, metamorphism of the Precambrian formations and establishment of the Arabo–Iranian coherent platform at the end of Pan African Orogeny, Central Iran broke up during a Neoproterozoic–Early Cambrian extensional tectonic regime. Infracambrian–Phanerozoic sedimentary rocks were then deposited on the basement complexes. The Iranian basement complexes are extensively overprinted by Pan African Orogeny and younger igneous events.

Existence of Qiongnan suture zone on the north margin of South China Sea

This paper aims to explore whether paleo-suture zone exists in the pre-Cenozoic basement of South China Sea (SCS). By taking advantage of integrated geophysical surveying data, including multi-channel reflection seismicity, oceanic bottom seismicity, gravity and magnetic data as well as paleomagnetism and drilling information, a comprehensive analysis on sedimentary stratigraphy, seismic stratigraphy, magmatic and metamorphic petrology, geochemistry, gravity study, magnetics, paleogeomagnetics as well as geotectonics was carried out, and the results clearly indicate that a pre-Cenozoic suture zone, i.e. "Qiongnan (south Hainan Island) suture zone", does exist on the north margin of SCS. This suture zone starts in the west from the Jiusuo-Lingshui fault zone in the south of Hainan Island, extends eastward across the continental slope to the north of SCS, and links with Shoufeng fault in Taiwan. It is inferred that the Qiongnan suture zone was sutured approximately in Indosinian (Triassic) Epoch and is actually the vestige of "Qiongnan ocean basin", the extension of the main paleo-Tethys ocean basin in SCS. The formation of Qiongnan suture zone marked the collision-accretion of the Qiongnan Block toward the Qiongzhong Block. The Qiongnan suture zone is linked to the west with the southward extension of the principal suture zone of the Paleo-Tethys at Bitu-Changning-Menglian-Bentong-Raub, across the southern segment of the Red River-East Vietnam-Wanna strike-slip fault zone, and connected to the east with the suture zone emplaced into the south margin of the Oki-Hida Pre-Jurassic terranes in southwest Japan, across the northern segment of the Manila trench fault zone in the northeast side of Taiwan Island. The discovery of this suture zone is believed to be potentially significant toward study of the pre-Cenozoic geotectonic pattern of SCS, the temporal and spatial distribution of the east paleo-Tethys structural domain in SCS and the relationship of recombination and superimposition between the east paleo-Tethys structural domain and the west Pacific structural domain, in addition to the oil/gas geological potentials in the pre-Cenozoic marine vestige-superimposed basin in the study area.