Low-grade Metabasites Research Articles

Petrogenesis and tectonic significance of Neoproterozoic meta-basites and meta-granitoids within the central Dabie UHP zone, China: Geochronological and geochemical constraints

A combined geochemical (whole-rock elements and Sr-Nd-Pb isotopes, zircon trace elements and Hf isotopes) and geochronological (zircon U–Pb ages) study was carried out on the relatively low-grade meta-basites and meta-granitoids from Longjingguan within the central Dabie ultrahigh-pressure (UHP) metamorphic zone, east-central China. Zircon investigations indicate that the meta-basites were formed at ∼772 Ma and subsequently experienced granulite-facies metamorphism at ∼768 Ma and a later thermal overprint at ∼746 Ma, while the meta-granitoids recorded three groups of zircon ages at ca. 819 Ma, 784 Ma and 746 Ma. The meta-granitoids can be subdivided into low-Si and high-Si types, and they were derived from mid-Neoproterozoic partial melting of the Neoarchean and Paleoproterozoic metamorphic basement rocks of the South China Block, respectively. These Neoproterozoic zircon ages are consistent with the protolith ages of the Dabie Triassic UHP meta-igneous rocks. In addition, the low-grade rocks have bulk-rock Pb isotope compositions overlapping with the UHP meta-igneous rocks. Therefore, the low-grade meta-basites and meta-granitoids could be interpreted as counterparts of the UHP meta-igneous rocks in this area, suggesting the same petrogenesis for their protoliths in the Neoproterozoic.Trace element patterns indicate that the low-grade rocks have better preserved their protolith compositions than their equivalent UHP rocks, and thus they are more suitable for elucidating the Neoproterozoic evolution of the northern margin of the South China Block. Zircon ages combined with geochemical features strongly suggest that the protoliths of the meta-granitoids and meta-basites were formed in a magmatic arc and a continental rifting setting, respectively. More specifically, the granitoids derived from partial melting of Neoarchean and Paleoproterozoic basement materials at ∼819 Ma in a magmatic arc setting, whereas the precursors of the meta-basites are products of a continental rifting event at about 784 to 772 Ma. The obtained results provide new geochronological and geochemical constraints for the Neoproterozoic evolution of the northern margin of the South China Block, which can further contribute to the understanding of the breakup of the supercontinent Rodinia.

Trace and Rare Earth Elements chemistry of detrital garnets in the SE Alps and Outer Dinarides flysch basins: An important tool to better define the source areas of sandstones

Abstract Garnet is one of the most abundant heavy minerals present in the Cretaceous – Eocene flysch deposits of the Southeastern Alps and Outer Dinarides (Julian, Brkini and Istrian basins). About 300 detrital garnets from the Cretaceous-Eocene flysch deposits of the Southeastern Alps have been analysed by means of electron microprobe and LA-ICP-MS. In the Julian and Istrian basins, supplies are from amphibolite-facies rocks and mafic and ultramafic metamorphic rocks, while in the Brkini basin the latter are almost missing. Moreover, in the Julian and Istrian basins, supplies from skarns, very low-grade metabasites, or from ultra-high temperature metamorphosed calc-silicate granulites are present. Among these different groups, LA-ICP-MS analyses showed that trace element content can be very different in almandine-rich garnets from the different sources. In particular, the source that supplied the Julian Basin is significantly different from that of the Istrian Basin. From the Cretaceous to the Palaeocene the main supplies of Bi-type garnets derived from an area where feldspar-free garnet-bearing rocks were exposed. Successively, Bi-type garnets were supplied from an area where feldspar-garnet-bearing rocks were exposed. The presence of garnets from feldspar-free garnet-bearing rocks in the Brkini and Istrian basins can be ascribed to both recycling of material from the Julian Basin as well as direct input from the same areas that supplied the Julian Basin.

Zircon U-Pb geochronology and petrogenesis of metabasites from the western Beihuaiyang zone in the Hong’an orogen, central China: Implications for detachment within subducting continental crust at shallow depths

Deformed low-grade metabasites from the western segment of the Beihuaiyang zone in the Hong’an orogen, central China can be divided into two types, i.e. meta-gabbro and meta-basalt. These lithologies have been studied by using whole-rock elemental and Sr-Nd-Pb isotopic analyses, and zircon SHRIMP U-Pb dating. Concordant zircon U-Pb ages of 631±5Ma and 623±14Ma are obtained for the meta-gabbros, consistent with a previously reported U-Pb age of 635±5Ma. The meta-basalt was dated to have a protolith age of middle Neoproterozoic (∼750Ma) and a metamorphic age at ca. 240Ma. The all studied metabasites occur as block or slice within a metamorphosed Ordovician volcanic zone (originally named as the Dingyuan Formation) and are in tectonic contact to each other. The gabbro and basalt emplaced at ∼630Ma and ∼750Ma, respectively in a continental rifting setting, whereas their present country rocks were erupted at ∼465Ma in an arc setting. The Pb-isotope compositions of the low-grade meta-gabbros and meta-basalts are similar to those from the Dabie ultrahigh-pressure (UHP) meta-igneous rocks with an upper continental crust affinity. The protolith ages of the studied relatively low-grade meta-basic rocks are in good agreement not only with ages for two episodes of middle and late Neoproterozoic mafic and felsic magmatism in the Suizhou to Zaoyang areas at the northern margin of the South China Block, but are also in agreement with the protolith ages of UHP meta-igneous rocks in the Dabie-Sulu orogenic belt. Therefore, these Neoproterozoic low-grade metabasites are considered to be exotic and they may have been detached and offscraped from the subducting upper crust of the South China Block at shallow depths during continental collision in the Triassic. They were subsequently exhumed in the initial stage of continental subduction, and thrusted over the Paleozoic metamorphosed rocks in the southern margin of the North China Block or as foreign slices within the UHP metamorphic zone. They are a kind of the accretionary wedge involved in the Paleozoic oceanic subduction, and Triassic crustal detachment and tectonic overthrusting during the early stage of continental subduction.

Short-wavelength infrared spectroscopy of chlorite can be used to determine very low metamorphic grades

This study investigates the use of short-wavelength infrared reflectance spectroscopy for measuring chlorite physicochemical information of value for determination of very low metamorphic grades. The method is termed chlorite spectral index (CSI) and assessed through quantitative comparison with XRD-derived Arkai and Kubler indices (AI and KI, respectively) in a set of 41 pelitic samples. The four CSI measures investigated measure the depth (D) of the absorption features at 1900, 2250 and/or 2350 nm, and are defined as reflectance 2250D/1900D index (CSI(H 2 O)), reflectance (2350D + 2250D × 1.7)/1900D index (CSI(H 2 O) sum ), reflectance 2250D index (CSI(2250)) and reflectance 2350D + 2250D × 1.7 index (CSI sum ). The results demonstrate the applicability of the method: all CSI measures show a good correlation with the AI, and respective correlation factors range between R 2 = 0.840 (for the CSI(H 2 O)) and R 2 = 0.616 (for the CSI sum ). The CSI measures also show a good correlation with the KI with R 2 = 0.704 (for CSI sum ) to 0.769 (CSI(2250)). By applying the correlation equations to define the metamorphic zone boundaries for each of the CSI measures, 64.3–78.6 % of the samples match the metamorphic grade indicated by AI for the epizone, 47.1–70.6 % for the anchizone and 70–80 % for the diagenetic zone. All CSI measures show a good reproducibility, with an intra sample variance ranging between 2.31 % CSI(H 2 O) sum and 3.34 % (CSI(2250)). CSI is not restricted to pelitic rocks and marls, but can also be applied in very low-grade metabasites. In addition to the existing laboratory method, CSI has application potential in remote sensing applications.

P-T Constraints on the Metamorphic Evolution of the Paleoarchean Kromberg Type-Section, Barberton Greenstone Belt, South Africa

Low-grade metabasites and hydrothermally altered ultramafic rocks form most of the Paleoarchean Barberton Greenstone Belt (BGB) of South Africa. However, P–T constraints are sparse and the nature of the greenschist-facies metamorphism is poorly characterized. This study provides new P–T estimates and descriptions of the petrological characteristics of altered mafic–ultramafic rocks across the Kromberg type-section, Onverwacht Group, BGB, from both surface samples and fresh drill core. Application of a chlorite thermodynamic multi-equilibrium calculation and pseudosection modelling, in conjunction with petrographic observations, indicates a wide range in metamorphic conditions from sub-greenschist to the uppermost greenschist facies across the type-section. A central fuchsite-bearing zone containing strong mylonitic fabrics, referred to as the Kromberg Section Mylonites (KSM), records at least two metamorphic events: a high-T, low-P (420 ± 30°C, <3 kbar) metamorphism, and a lower-T event (T = 240–350°C, P = 2·9 ± 0·15 kbar) related to retrograde metamorphism associated with extensional quartz–carbonate veins. Pillow lava metabasalts directly beneath the KSM record the same HT–LP-type metamorphic conditions, whereas metabasalts 90 m and 125 m above the KSM record temperatures of 250–340°C. Lower in the Kromberg section, metamorphic conditions vary from 215–321°C (at 2·9 kbar, mid to lower parts) to very low-grade conditions of 140–209°C in the lowermost Kromberg. Thus, an inverted metamorphic field gradient is documented beneath the KSM. Petrological analysis of the fuchsite-bearing rocks of the KSM indicates that they contain listvenite, a hydrothermal alteration product typically found in mafic–ultramafic tectonic mélange zones of ophiolite terranes. Together with the new metamorphic P–T constraints this means that these rocks are not a product of Archean atmospheric weathering. Rather, the inverted metamorphic field gradient in the Kromberg type-section suggests that the KSM represents a Paleoarchean thrust zone. Cr-spinel compositions in the ultramafic protolith to the KSM listvenites suggest a supra-subduction zone setting. A new geodynamic model is therefore proposed in which the mafic–ultramafic oceanic rocks of the Kromberg type-section were obducted as a thrust nappe pile in a regional transpressional tectonic regime between 3227 and 3230 Ma.

Mafic phyllosilicates in low-grade metabasites. Characterization using deconvolution analysis — discussion

AbstractAny X-ray diffraction (XRD) pattern of mixed-layered maximum R1 ordered chlorite-smectite has an exact equivalent R0 smectite-corrensite or R0 chlorite-corrensite. The identification of a mineral as chlorite (0.8)-smectite R1 is, therefore, not evidence that the fundamental layers in such an interstratified structure are actually chlorite and smectite. Furthermore, there are no obvious differences between XRD patterns of R0 and R1 chlorite-smectite at high or low smectite contents. Therefore the frequently supposed identification of R0 chlorite-smectite with high smectite contents is also questionable. It follows that the identification of R1 chlorite (0.8)-smectite by Robinson &amp; Bevins (1994) and of R0 chlorite (≈0.2)-smectite in an earlier publication (Robinson et al.,1993) cannot be used as evidence regarding the debate over the nature of the component layers in interstratified minerals intermediate between trioctahedral smectite and chlorite.Diffraction patterns of natural mixed-layer chlorite minerals from metabasites often show a strong reflection at 31 Å yet a lack of resolution of other mixed-layer peaks from those of discrete chlorite. These features cannot be matched by any single mixed-layer mineral consisting of layers of chlorite and smectite and were matched by Robinson &amp; Bevins (1994) using a mixture of different chlorite-smectite minerals. However, they can be matched by a single chlorite-corrensite mineral in which there is an element of segregation of layer types. These different possibilities need to be investigated further.

Comments on discussion of characterization of mafic phyllosilicates in low-grade metabasites using deconvolution analysis

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Na-metamorphic pyroxenes in low-grade metabasites from the External Zones of the Betic Cordilleras (southern Spain): influence of rock chemical composition on their formation.

ABSTRACT. Abundant metabasite blocks are present in the argillaceous sediments of the 'Trias de Antequera’ unit, in the External Zones of the Betic Cordilleras (Southern Spain). Two different geochemical affinities have been idel1tified in these metabasites: one tholeiitic and the other transitional-alkaline. Na-rich metamorphic pyroxenes are present in both types of metabasites, although with different textural arrangement and chemical characteristics. In the tholeitic metabasites Napyroxene is always present in veinlets, associated with albite and quartz. Compared to the fresh augite from which they derive, these pyroxenes are richer in Al2O3, FeOtot and Na2O, and poorer in TiO2, MgO and CaO. In the transitional-alkaline metabasites, the metamorphic pyroxenes result from epitaxial replacement of primary Ti-rich augite. This replacement may be complete, but always preserving the original ophitic texture. These metamorphic pyroxenes are richer in SiO2, FeOtot, and Na2O, and poorer in TiO2, Al2O3, MgO and CaO than the primary ones. The jadeitic contents in both types of metamorphic pyroxenes are similar, with maximum values of ca. 15-19%. As the maximum P-T conditions of metamorphism estimated for the tholeiitic and transitional-alkaline metabasites are very close, the influence of primary rock composition is advocated to explain the chemical differences between the two types of metamorphic pyroxenes. RESUMEN. Piroxenos sodicos metamorficos en metabasitas de bajo grado de las Zonas Externas de las Cordilleras Beticas (sur de Espana): influencia de la composicion quimica de la roca en su formacion. Numerosos bloques de metabasitas aparecen entre los sedimentos arcillosos del 'Trias de Antequera', en las Zonas Externas de las Cordilleras Beticas (surde Espana). En estas metabasitas se han evidenciado dos tipos de afinidades geoquimicas diferentes: una toleitica y otra transicional-alcalina. En ambos tipos aparecen piroxenos metamorficos ricos en sodio, pero con diferencias en cuanto a sus disposiciones texturales y a sus caracteristicas quimicas. En las metabasitas.

Structural and metamorphic evolution of the almaden de la plata core (Seville, Spain) in relation to syn-metamorphic shear between the Ossa-Morena and South Portuguese zones of the Iberian Variscan fold belt

The Almadén de la Plata Core is a tectonic unit situated in the southern branch of the Iberian Variscan Fold Belt. It has traditionally been regarded as an eastern extension of the Aracena Metamorphic Belt which is located at the south-southeast boundary of the Ossa-Morena with the South Portguese Zone. Low-pressure/high-temperature parageneses (Gt-Crd-Sill) are found in the highest-grade areas, while a metamorphic gradient of medium-pressure type (Gt-Sta-bearing schists and gneisses) is observed for the middle stages of the prograding metamorphic evolution. Later processes indicate an evolution to the very low-grade, locally retrograding previous parageneses. Five main tectonic events are observed. D 1 (oriented inclusions within cordierite and garnet) and D 2 are not present in the surrounding area. D 2 and D 3 have a blastomylonitic (high-grade metamorphism) and mylonitic (low-grade) character respectively. D 4 is responsible for the present boundaries of the Almaden de la Plata Core, while D 5 corresponds to late-Hercynian fractures. Events 1–4 book place under high to low and very low metamorphic grades. The metamorphic climax was reached pre- to syn-D 2. The metabasic rocks (Acebuches Amphibolites) show medium- to high-grade associations (Hbl-Plag-Cpx) with a well developed N120°E mineral lineation. These features change towards the contact with the South Portuguese materials of the Pulo do Lobo Group, where metabasites are transformed into very fine-grained amphibolitic schists (greenschist associations with actinolite and relicts of hornblende and clinopyroxene). The later are mylonitic rocks with N150°E stretching lineations and S-C structures as well as sigmoidal pressure shadows, indicating a southward vergence. The low-grade metabasites are interpreted as the bottom of a D 3 Main Thrust which put the Ossa-Morena and South Portuguese zones into contact. These results support the hypothesis that the Acebuches Amphibolites of the Aracena Belt do not pass gradually into the Pulo do Lobo Group but are thrust upon that unit.

Phase equilibria and mixed parageneses of metabasites in lowgrade metamorphism

AbstractThe system Na2O-CaO-MgO Al2O3-SiO2-H2O is proposed to model phase equilibria and mineral parageneses for low-temperature metamorphism of basaltic rocks. Univariant reactions marking the transitions between various sub-greenschist facies are identified and some have been experimentally determined. The introduction of Fe2O3 into the model system at fixed FeO/MgO ratio creates continuous reactions for facies boundaries and discontinuous reactions for invariant points of the model system. Both qualitative and quantitative effects on P-T displacement and phase compositions are discussed. The XFe3+ isopleths for epidote were plotted to exemplify the transition from the zeolite through prehnite-pumpellyite to prehnite-actinolite facies. T-XFe3+ relations were established for continuous and discontinuous reactions relating such facies transitions. Because of the common occurrence of two or three Ca-Al hydrosilicates in low-grade metabasites, an isobaric Al-Ca-Fe3+ projection from chlorite may be used to illustrate mineral assemblages and compositions of the coexisting Ca-Al silicates in the presence of quartz, albite, and chlorite. Reported occurrences in several classic burial metamorphic terrains and ocean-floor metabasites in ophiolites are described. Only the composition of a mineral from a buffered assemblage can constrain the intensive properties for metamorphism; previously reported compositional trends for pumpellyite and epidote with increasing metamorphic grade are oversimplified.

幌加内峠地域神居古たん帯の塩基性岩類の変成作用と産状

The Kamuikotan tectonic belt is a melange zone in which occur various kinds of metamorphic rocks of high pressure and low pressure types. In the Horokanai district, central Hokkaido, high pressure type metamorphic rocks are widely distributed. They can be classified into four kinds, metasediments, low grade metabasites, epidote amphibolites, and metachert based upon the original rocks and mineral parageneses. A large part of the metasediments consist of black siliceous shale. The original rocks of the low grade metabasites are spilitic tuff, pillow lava, massive lava and gabbro, and those of the epidote amphibolites are basaltic tuff, pillow lava and massive lava. The low grade metabasites, epidote amphibolites, and metachert are scattered as lenticular bodies of various sizes in metasediments. The modes of occurrence suggest that the lenticular bodies of basic rocks and chert were emplaced as olistoliths. The mineral assemblage of the metasediments is quartz+white mica +chlorite+ graphite±lawsonite±albite+metamorphic aragonite. The low grade metabasites are characterized by a widespread occurrence of lawsonite, pumpellyite, alkali amphibole, alkali pyroxene, and metamorphic aragonite. These parageneses indicate that the metasediments and the low grade metabasites suffered metamorphism of glaucophane schist facies. On the other hand, the epidote amphibolites display relic epidote amphibolite assemblages which have been partly or entirely modified by a later metamorphism of glaucophane schist facies. The emplacement of epidote amphibolites may be explained as follows. Their original rocks were a component of the oceanic crust which was underthrust beneath the older crust in a subduction region during Mesozoic time. Rock slices were scraped off and accreted to the pile composing of older slices. The slices were metamorphosed up to epidote amphibolite facies assemblages during subduction processes. Underthrusting and accretion of the slices were accompanied by concurrent intrusion of serpentinite with fragments and blocks of the rocks in the deeper part of the crust. The uplifted serpentinite and metamorphic rocks were exposed and suffered denudation. The blocks and clasts of the metamorphic rocks were reworked by subaqueous sliding to form an olistostrome with clastic sediments. Later all the rocks of the area suffered a high pressure type metamorphism and thus the rock assemblage observed presently in the Kamuikotan tectonic belt was formed.

Cambrian and late Precambrian basaltic igneous activity in the Scottish Dalradian: a review

SummaryIn the evolution of the Scottish Dalradian basin, the climax of widespread tholeiitic magmatism pre-dated or coincided with major basement faulting and the onset of deep marine sedimentation in Lower Cambrian times. Earlier basin-deepening and precursory igneous activity had already occurred. Major and trace element chemistry of low-grade metabasites indicates their affinity with basalts from ocean-spreading centres, or transition to ‘within-plate’ types. Geological evidence favours a tectonic setting for this igneous suite and the Highland Border Series on the NW margin of an actively evolving Lower Cambrian ocean basin, possibly a marginal basin to the Iapetus Ocean System.