Massive Amphibolites Research Articles

Petrogenesis of Amphibolite/Hornblendite Dikes in Qala-Diza Area, Iraqi Kurdistan Region

The amphibolite and hornblendite dikes occur as part of the igneous complexes of Bulfat Ophiolite within the Zagros Suture Zone, NE Iraq. The igneous complex is part of the upper allochthonous ophiolite terrane within the second unit of the Penjween-Walash Subzone. This article discusses the petrogenetic characteristics and tectonic setting of the amphibolite rocks based on their petrography, mineral chemistry, and whole-rock geochemistry. Petrographically, both banded and massive amphibolites reveal that the main mineral constituents are amphibole and plagioclase with accessory clinopyroxene, opaques, sphene, apatite, rutile, zircon, sericite, epidote and chlorite; meanwhile the hornblendites consist of >90% amphibole with minor amounts of pyroxene, opaques, plagioclase, titanite, quartz, apatite, zircon, and chlorite. The amphibolites are divided into two types, banded (foliated) and massive, (non-foliated). The dominat textures of amphibolites are porphyroblastic, granoblastic, poikiloblastic, nematoblastic, and blasto-ophitic; meanwhile those of hornblendites are equigranular, poikilitic, cataclastic, and intergranular. The electron microprobe analysis (EPMA) showed that the amphiboles of the banded amphibolite range between tschermakite, magnesio-hornblende and pargasite; and those of the massive amphibolites range between magnesio-hornblende and actinolite; meanwhile those of hornblendites are dominantly tschermakite with minor magnesio-hornblende and pargasite. The plagioclase of the banded amphibolites is mainly oligoclase and andesine; and those of the massive amphibolites are mostly anorthite. Chemical classification diagrams indicated that the studied rocks are had low alkalies (Na2O+K2O), tholeiitic, ultrabasic to basic, alkaline to subalkaline and divided between foidite, picrobasalt and basalt. Various tectonic discriminating diagrams showed that most of studied rocks are mid-ocean ridge basalt (MORB). The studied amphibolites have been subjected to amphibolite facies metamorphism.

Petrology and geochemistry of the Archean Huangling greenstone belt in the Yangtze Craton, South China

The Archean Huangling greenstone belt (HGB) is preserved as amphibolites in the Huangling granite–greenstone terrane, Yangtze Craton. Geologically and geochemically, the amphibolites are metavolcanic rocks and can be subdivided into amphibole schist, massive amphibolite and banded amphibolite, with komatiite, komatiitic basalt and tholeiitic basalt protoliths, respectively. The komatiites have high MgO (20–30%), Cr, Co and Ni contents; low Rb, Sr, Ba and ΣREE contents; and slight LREE depletion and HREE enrichment, characterizing them as the Al-undepleted type according to the criteria (CaO/Al2O3)adj ˃ 20, (Al 2O3/TiO2)adj and (Gd/Yb)PM < 1. The komatiitic basalts and tholeiitic basalts have lower MgO (5–16%), Cr, Co and Ni contents; higher Rb, Sr, Ba and ΣREE contents; and slight LREE enrichment and HREE depletion. The geochemical plots (Zr/Y vs. Nb/Y, Nb/Yb vs. Th/Yb, La/Ta vs. La/Sm and Y vs. Zr/Y), negative Nb anomalies, wide ƐNd values (−8.0 to +6.2), negative zircon ƐHf values (−11.59 to −3.98), and old TNd (DM) (3.9–2.9 Ga) and zircon THf (DM) ages (3.9–3.3 Ga) of the amphibolites reflect a mantle source with slight crustal contamination and show that the komatiitic basalts and tholeiitic basalts are more closely related to each other than to the komatiites. The tholeiitic basalts originated from moderate-degree melting of a mantle plume head, the komatiitic basalts originated from the fractional crystallization of tholeiitic basalt melts as cumulates, and the komatiites originated from high-degree melting of the mantle plume tail with garnet entering the melt phase. These high-Mg lavas erupted in a continental rift when the mantle plume impinged on the continental lithosphere, generating rifting and subsequent volcanic eruptions. These ultramafic–mafic rocks (3.2–3.0 Ga) formed the HGB, which was subsequently intruded by TTG (3.0–2.9 Ga), together forming the Archean Huangling granite–greenstone terrane in the Yangtze region.

The tectonostratigraphic architecture of the Serbo-Macedonian massif in the Vertiskos and Kerdilion mountains (Northern Greece)

The lithologies and structural features of the exposed rocks of the Serbo-Macedonian massif in the Vertiskos and Kerdilion Mts. have been studied in detail by carrying out km-long cross-sections. Moreover, a new tectonostratigraphic architecture for the massif is proposed, based on the migmatization and anatexis that the rocks pertain, under which the specific exposed rocks have been placed into the Vertiskos and Kerdilion Units. The latter approach differs from the traditional view, which is based solely on the lithological difference between the units. In particular, in the Vertiskos Mt., mica schists, garnet-bearing two-mica gneisses, and predominantly two-mica gneisses, without a sign of anatexis and migmatization, overlie tectonically, biotite gneisses and layered amphibolite gneisses into which migmatization and anatexis takes place. The former constitute the Vertiskos Unit, whereas the latter have been grouped into the Kerdilion Unit, since they are of similar lithologies and affinities with rocks of the Kerdilion Unit. The Kerdilion Mt. is a large antiform made up of biotite gneisses alternating with marbles, which are similarly characterized by intense migmatization and anatexis. These rocks are intruded by the Oreskia granite, which is foliated and follows the general trend of the country rocks. All the rocks are folded with isoclinal to tight folds, and the contact between the two units is a mylonitic shear zone with a top-to-the-SW sense-of-shear. Also, a large volume of ultramafic rocks occurs between the Vertiskos and Kerdilion Mts., including metamorphosed rocks like metagabbros to massive amphibolites, which is assigned to the Therma-Volvi-Gomati Complex (TVGC). These rocks have been found in tectonic contact, i.e., shear zones with top-to-the-SW sense-of-shear, only with the rocks of the Kerdilion Unit. Taking into account our new tectonostratigraphic architecture, the contact between the Vertiskos and Kerdilion Units is not located along the western side of the marbles, as the latter are exposed in the Kerdilion Mt. It is traced westerly in the Vertiskos Mt. dipping with intermediate angles towards the SW, due to NW-trending, map-scale, isoclinal folding. The ultramafic rocks of the TVGC are in tectonic contact with the rocks of the Kerdilion Unit, but not the two-mica gneisses of the Vertiskos Unit, and the Arnea granite intrudes not only the Vertiskos Unit as previously considered, but the rocks of the Kerdilion Unit, as well.

Melt-collecting structures and the formation of extraction dykes during syntectonic anatexis of the Camboriú Complex, south Brazil

The Camboriú Complex metamorphic sequence comprises orthogneisses and amphibolites with widespread melting features generated at upper amphibolite facies conditions. Regional to mesoscale structures are shown to control generation, collection and extraction of melts. As partial melting starts it is locally focused as layer-parallel leucosomes in banded gneisses, and isolated pockets of in situ leucosomes in massive amphibolites. The further tightening of cm-to m-scale upright folds and conjugate shear planes oblique to axial-planar position trigger the coalescence and migration of melts, first along the main banding, then along axial planes of folds and finally giving rise to a network where melt is collected. The melt network eventually acquires enough mobility to form m-wide extraction dykes roughly parallel to axial planes of folds, capable of transferring the products of migmatization to upper crustal levels to feed granitic plutons. The orientation and geometry of pure-shear dominated structures in the Camboriú Complex are in accordance with deformation partitioning between this and adjancent shear zones, which is expected for the transpressive character of the Neoproterozoic Southern Brazilian Shear Belt.

Deformation of amphibolites from the Paleoproterozoic Liaohe Group, Liaodong Peninsula, China: Implications to the crustal structure of the Jiao‐Liao‐Ji mobile belt in the eastern block, North China Craton

Structural evolution of the Paleoproterozoic Jiao‐Liao‐Ji orogenic belt in eastern North China Craton has been a subject of extensive study in the past decades. Studies of the characteristics of the crustal structure during orogenesis are essential to the understanding of the formation and evolution of the orogenic belt. In this paper, we present detailed studies of deformed amphibolites in the southern part of the orogenic belt to illustrate the changes of deformation conditions and variation of deformation mechanisms during the orogenic processes. Based on differences in structural characteristics of foliations, amphibolites from the Paleoproterozoic South Liaohe Group at Caohekou–Jiguanshan area are subdivided into four types (massive, weak‐gneissic, moderate‐gneissic, and strong‐gneissic). Our study shows that the different characteristics in microstructures, rock fabrics, and deformation mechanisms of the amphibolites were resulted from varying temperatures and pressures in relation to the tectonic deformation. The widely distributed massive amphibolites (P = 0.39 GPa, T = 639°C) display no or weak deformation, which is inferred to represent steady‐state crystallization. Weak‐gneissic amphibolites (P = 0.52 GPa, T = 753°C) were resulted from stress‐controlled grain growth. Amphiboles in such rocks show obvious (100) LPOs but minor evidence for crystal plastic deformation. Evidences for crystal plastic deformation of amphibole grains are common in moderate‐gneissic amphibolites (P = 0.61 GPa, T = 771°C). The amphibole grains have strong (100) and (001) LPOs. Dislocation slip is considered as the main deformation mechanism of the amphibole grains in the rocks. Both subgrains and mechanical twins of amphiboles from strong‐gneissic amphibolites (P = 0.77 GPa, T = 686°C) are long‐columnar shaped and show the same orientation as their host crystals. They are resulted from reactivation of (100)(001) slip system controlled by dynamic recrystallization, twinning, and twinning nucleation recrystallization. The evident pressure difference from 0.39 to 0.77 GPa during the formation of the various types of amphibolites may be related to the tectonic thickening during contractional orogenesis.

Indications of HP events in the volcanosedimentary succession of the Orlica–Śnieżnik Dome, NE Bohemian Massif: data from a marble-amphibolite interface

A volcanosedimentary succession of the Młynowiec-Stronie Group (MSG) in the Orlica–Śnieżnik Dome (OSD), the Sudetes, NE Bohemian Massif underwent multiple folding and shearing during the Variscan Orogeny. In the sheared domains, there are less deformed pods in which rocks preserve better records of metamorphic events prior to the regional temperature peak. In one such pod, near Gniewoszów on the western limb of the dome, marbles enclosed by massive amphibolites occur. In these rocks, zoned plagioclase with actinolite and epidote inclusions and zoned amphibole grains allowed recognition of three mineral assemblages and three P-T stages at: (1) 310°C/3–4 kbar, (2) 480–500°C/10.5 kbar, (3) 500–530°C/6–6.5 kbar, based on isopleth intersections and checked against conventional thermobarometry. These define a steep clockwise P-T path and a geothermal gradient of 17°C/km before peak conditions were attained, which suggests subduction of the metavolcano-sedimentary rocks (Stronie Formation of the MSG) on the western limb of the OSD, with a transient yet discrete higher pressure episode. Mineral relicts capable of demonstrating a higher pressure event are scarce in the supracrustal rocks of the dome, mainly because they became more thoroughly equilibrated and obliterated during the temperature peak at mid-amphibolite facies conditions and the subsequent ubiquitous greenschist facies overprint.

The Algodões amphibolite–tonalite gneiss sequence, Borborema Province, NE Brazil: Geochemical and geochronological evidence for Palaeoproterozoic accretion of oceanic plateau/back-arc basalts and adakitic plutons

Field relationships, geochemistry and U–Pb zircon and Sm–Nd whole-rock geochronology are used to constrain the genesis of Palaeoproterozoic massive amphibolites and orthogneisses of the Algodões Sequence, Central Ceará Domain, Northeastern Brazil. The 2236 ± 55 Ma-old Algodões amphibolites show trace element geochemical signature and positive εNd( t) values similar to Phanaerozoic oceanic plateau basalts and less often to back-arc basalts. The amphibolites are intruded by tonalitic to quartz-dioritic gneisses of the Cipó unit which were dated at 2160–2170 Ma and 2130 Ma. Neodimium isotope data and whole-rock geochemistry for these gneisses show they are akin to juvenile arc plutonics of the adakitic suite. The likely occurrence of Palaeoproterozoic plateau basalts and arc plutonics in this part of northeastearn Brazil render comparisons with similar terranes elsewhere in South America and Africa, which in turn indicate a significant contribution of accretionary orogens in the early assembly of the Columbia Supercontinent.

Metamorphic evolution and oxygen isotope geochemistry of rift-precursor amphibolites from Hole 1067A ODP Leg 173 off West Iberian Galicia Bank rifted margin

The paper documents Permian igneous rocks in the vicinity of an ocean–continent transition in the West Iberian Abyssal plain. ODP Leg 173, Hole 1067A was drilled through a sequence of foliated to massive amphibolites of gabbroic protolithic origin. The 270 ± 3 Ma to 246 ± 5 Ma rock sequence experienced several metamorphic events during the interval of time spanning from 138 Ma up to 55 Ma. Metamorphic parageneses record two major thermobarometric events for which the temperature and pressure conditions are 670 ± 40 °C and 0.7 ± 0.1 GPa and 550 ± 50 °C and 0.5 ± 0.1 GPa. According to radiometric dating these metamorphic events might have occurred around 138–136 Ma and 113–110 Ma respectively. The low Cl content and the preserved igneous δ 18O of amphibole imply seawater-absent 580 °C isothermal plagioclase–amphibole equilibrium related to the development of the strong foliation of the amphibolite during the second metamorphic event. Late isotopic equilibration involved lower temperature seawater-derived fluid and is younger than 110 Ma. The gabbroic intrusion is believed to be a precursor for initiation of continental rifting. Melting of such metamorphosed rocks in a rift setting would generate magmatic rocks with complex oxygen isotopic signatures.

Geochemical constraints from the Hafafit Metamorphic Complex (HMC): Evidence of Neoproterozoic back-arc basin development in the central Eastern Desert of Egypt

The Hafafit Metamorphic Complex (HMC) is a part of the Precambrian belt in the central Eastern Desert of Egypt. Two distinct metamorphic units were identified: gneisses and amphibolites. The gneisses are subdivided on mineralogical grounds into granitic gneiss, biotite-gneiss, hornblende-gneiss and psammitic gneiss. Using major elements discrimination criteria to discriminate between orthogneiss and paragneiss, the granitic gneiss shows igneous origin, whereas biotite-gneiss, hornblende-gneiss and psammitic gneiss show sedimentary origin. The mineralogical and chemical compositions of the granitic gneisses indicate that they are tonalitic to trondhjemitic and have compositions consistent with hydrous partial melting of a mafic source, suggesting subduction-related magmatism. Based on Si, Al and alkali contents of paragneisses, the psammitic gneiss could be classified as metamorphosed lithic arenite, whereas biotite- and hornblende-gneisses are classified as metamorphosed greywacke. Sedimentation may have occurred in a back-arc basin setting with transitional deposition from shallow-marine to terrestrial environment. This sedimentation was probably occurred on a tholeiitic basaltic oceanic crust. The amphibolites are subdivided according to mineralogical basis into clinopyroxene-amphibolite, garnet-amphibolite and garnet-free massive amphibolite. Chemical data of amphibolites shows tholeiitic affinity, which suggests a back-arc geotectonic setting. A generation of the leucogranite along thrust zones is related to the late phase of metamorphism of Hafafit rocks. This interpretation is supported by the similarity between metamorphic age and granite emplacement age.

Evolution of the Pan‐African Wadi Haimur metamorphic sole, Eastern Desert, Egypt

By comparison with the general features of metamorphic soles (e.g. vertical and lateral extension, metamorphic grade and diagnostic mineral parageneses, deformation and dominant rock types), it is inferred that the amphibolites, metagabbros and hornblendites of the Wadi Um Ghalaga–Wadi Haimur area in the southern part of the Eastern Desert of Egypt represent the metamorphic sole of the Wadi Haimur ophiolite belt. The overlying ultramafic rocks represent overthrusted mantle peridotite. Mineral compositions and thermobarometric studies indicate that the rocks of the metamorphic sole record metamorphic conditions typical of such an environment. The highest P–T conditions (c. 700 °C and 6.5–8.5 kbar) are preserved in clinopyroxene amphibolites and garnet amphibolites from the top of the metamorphic sole, which is exposed in the southern part of the study area. The massive amphibolites and metagabbros further north (Wadi Haimur) represent the basal parts of the sole and show the lowest P–T conditions (450–620 °C and 4.7–7.8 kbar). The sole is the product of dynamothermal metamorphism associated with the tectonic displacement of ultramafic rocks. Heat was derived mainly from the hot overlying mantle peridotites, and an inverted P–T gradient was caused by dynamic shearing during ophiolite emplacement. Sm/Nd dating of whole‐rock–metamorphic mineral pairs yields similar ages of c. 630 Ma for clinopyroxene and hornblende, which is interpreted as a lower age limit for ophiolite formation and an upper age limit for metamorphism. A younger Sm/Nd age for a garnet‐bearing rock (c. 590 Ma) is interpreted as reflecting a meaningful cooling age close to the metamorphic peak. Hornblende K/Ar ages in the range 570–550 Ma may reflect thermal events during late orogenic granite magmatism.

Geochemistry and palaeotectonic setting of amphibolites from the Western Tatra Mountains, southern Poland

Amphibolites from the crystalline basement of the Western Tatra Mountains, which are found as small lenses within migmatitic gneisses and mica schists, were formed during pre- or early Variscan amphibolite-facies metamorphic events, and subsequently intruded by the post-metamorphic Variscan Tatra Granite. The amphibolites occur in both the upper and lower metamorphic complexes, which are separated by a major subhorizontal shear zone in the Western Tatra Mountains. The amphibolites can be divided into three types: massive, striped and garnetiferous. The striped and massive amphibolites, concordant with a dominant S1 foliation, and the garnet amphibolites, which cross-cut the S1 banding in the gneisses, were all originally intrusive dolerites. The striped amphibolites (consisting primarily of hornblende, andesine and quartz), and later, cross-cutting garnet-hornblende-andesine-quartz-bearing amphibolites, predominate in the lower part of the dominantly migmatitic upper complex, and are exposed mainly on the ridges. The massive amphibolites, which contain a similar mineral assemblage, mainly occur in the usual unmigmatized lower structural unit. Chemical studies indicate that three amphibolite suites are present, which probably originated as a series of enriched tholeiites, similar to more recent plume-influenced magmas, which were derived by partial melting of a spinel lherzolite with primitive mantle composition and compositionally slightly modified by crustal contamination. The amphibolites were intruded as dolerites into clastic sediments which had accumulated in an extensional basin floored by attenuated continental crust, a situation similar to that of amphibolites found in metamorphic complexes within the Variscan belt, e.g. in the Orlica–Snieznik area of the Sudetes, where amphibolites chemically similar to those in the Western Tatra also occur. Copyright © 2000 John Wiley & Sons, Ltd.

The metamorphosed mafic-ultramafic complex of Mokuro, Ilesha Schist Belt, southwestern Nigeria

In the Proterozoic Schist Belt of Nigeria, lenticular bodies of metabasites and meta-ultramafics are frequently intercalated within staurolite bearing metapelitic schists. Such a metamorphosed mafic-ultramafic complex is particularly well exposed in the Mokuro riverbed between the towns of Ife and Ilesha. These outcrops display contact relationships with the surrounding metasediments, as well as between the individual mafic and ultramafic rock types. The most common mafic rocks are indistinctly layered amphibolites, accompanied by apatite rich amphibolites and massive amphibolites, in part rich in ilmenite and pyrrhotite. Among the generally massive ultramafic rocks, nearly monomineralic amphibole rocks predominate, while chlorite-amphibole, talc-chlorite-amphibole and talc bearing olivine-chlorite-amphibole rocks occur in subordinate amounts. Field, textural and geochemical evidence suggest that the mafic-ultramafic complex derived from a thick, structurally differentiated basaltic sill that contained doleritic portions in its interior. Slow cooling rates in these inner parts enabled crystal settling with the formation of ultramafic cumulates. Due to the enrichment of volatiles during the crystallisation process, higher amounts of apatite and sulphides, as well as late magmatic amphibole, were formed in parts of the mafic-ultramafic body. Mineral assemblages in the mafic-ultramafic complex testify to a metamorphic overprint under amphibolite-facies conditions. Thermodynamic modelling in the system CMFASH leads to an estimated P–T range of 1.5–3 kbar and 550–620°C for the metamorphic peak assemblage talc-olivine-chlorite-Ca amphibole-orthoamphibole.

Tectonometamorphic evolution of the Sebadani eclogitic metagabbro and the Sambagawa schists, central Shikoku, Japan: 40Ar/39Ar mineral age constraints

Abstract The Sambagawa metamorphic belt exposed in central Shikoku records a high‐P–T metamorphic event. It is represented by the Oboke nappe and structurally overlying, internally imbricated, Besshi nappe complex. These major structural units are in ductile thrust contact. A melange is developed along a ductile internal tectonic contact within the Besshi nappe complex. Tectonic emplacement of a high‐T enclave (Sebadani eclogite) in the melange zone resulted in the development of a contact metamorphic aureole within the host Sambagawa rocks. 36Ar/40Ar versus 39Ar/40Ar isotope correlation ages recorded by hornblende from the Sambagawa basic schists which surround the Sebadani enclave are 83.4 ± 0.3 Ma (within contact aureole) and 83.6 ± 0.5 Ma (outside aureole). 40Ar/39Ar plateau ages recorded by muscovite from the same samples are 87.9 ± 0.3 and 89.3 ± 0.4 Ma. Amphibole from the amphibolite within the Sebadani enclave records isotope correlation ages of 93.7 ± 1.1 and 96.5 ± 0.7 Ma (massive interior) and 84.6 ± 1.2 Ma (marginal shear zone). Amphibole within the massive amphibolite is significantly higher in XMg than that within the host Sambagawa basic schists. The older ages recorded by amphibole within the Sebadani enclave are interpreted to date cooling through somewhat higher closure temperatures than which characterize the more Fe‐rich amphibole in surrounding schists. The younger amphibole age recorded within the marginal shear zone probably indicates that crystallization of amphibole continued until cooling through the relatively lower amphibole closure temperatures.These results, together with the previously published 40Ar/39Ar ages of the Sambagawa schists, suggest: (i) metamorphic culmination occurred in the Besshi nappe complex at c. 100–90 Ma; (ii) at c. 95 Ma the Besshi nappe complex was internally imbricated and tectonic enclaves were emplaced; (iii) at c. 85 Ma, the composite Besshi nappe was rapidly exhumed and tectonically emplaced over the Oboke nappe (which attained peak metamorphic conditions at c. 75 Ma); (iv) the Besshi and Oboke nappe complexes were further exhumed as a coherent tectonic unit and unconformably overlain by the Eocene Kuma Group at c. 50 Ma.

Lithologic mapping in arid regions with Landsat thematic mapper data: Meatiq dome, Egypt

Digital Landsat thematic mapper (TM) data were evaluated for lithologic mapping capabilities over the Meatiq dome area in the hyper-arid Eastern Desert of Egypt. Bi-directional spectral reflectance data (0.4–2.5 µm) for powders of the major rock types exposed in the dome and published spectral reflectance data were used as guides in selecting TM band reflectance ratios that maximize discrimination of individual rock types on the basis of their respective mineralogies. Comparison of TM data with field and petrographic observations shows (1) increasing amounts of magnetite and other opaque minerals, with low, flat spectral reflectances, decrease the ratio of TM band 5 (1.55 to 1.75) to band 1 (0.45 to 0.52 µm); (2) increasing amounts of hydroxyl-bearing minerals, with hydroxyl ion vibrational absorptions in TM band-7 wavelength region (2.08 to 2.35 µm), increase the ratio of TM band 5 to band 7; (3) increasing amounts of Fe-bearing aluminosilicates that absorb in the band-4 wavelength region (0.76 to 0.9 µm) increase the product of the following two TM ratios: band 5 to band 4 and band 3 (0.63 to 0.69 µm) to band 4; and (4) thin (≤5 µm), desert varnish that covers many outcrops modulates, but does not obscure, the spectral reflectance signatures of the Meatiq rocks. The varnish consists of amorphous to poorly crystalline dioctahedral smectite, iron oxides, and/or oxyhydroxides. Serpentinite, mafic mylonite, massive amphibolite, quartzofeldspathic mylonite, biotite schist, and quartz phyllonite were mapped on the basis of their unique values in one or more of the three ratio images, whereas coarse- and fine-grained granites, granite gneiss, and tonalite, with similar mineralogies and TM band ratios, were mapped as a group. Finer subdivisions were made where field traverses provided local verification. Results demonstrate that appropriate processing and presentation of Landsat TM data can significantly augment field observations for lithologic mapping of large areas in arid regions.

Calc-alkaline volcanism along the eastern margin of the namaqua mobile belt, south Africa - A possible middle proterozoic volcanic arc

Abstract The Boksputs and Upington sequences of the Areachap Group, present along the eastern margin of the Namaqua mobile belt in southern Africa, contain amphibolites with chemical properties which suggest that they are the metamorphic products of arc related precursors. The amphibolites, which form the predominant rock type of the Jannelsepan Formation of the Areachap Group, are subdivided on mineralogical grounds into massive and pyroxene amphibolite. The massive amphibolite of the Boksputs sequence chemically resembles low-K arc tholeiite, while the pyroxene amphibolite is slightly more calc-alkaline in character. The massive amphibolite of the Upington sequence is predominantly calc-alkaline, while the pyroxene amphibolite shows a strong shoshonitic affinity. The chemical differences between the two metavolcanic sequences are interpreted in terms of arc polarity and arc maturity. The present position of the associated granite plutons of the Keimoes Suite indicates that the two sequences most probably represent fragments of an originally northwest-trending arc system with a southwest-northeast polarity. The possibility of a north-south polarity, however, cannot be ruled out. The chemical characteristics may be indicative of arc maturity: the Upington sequence thus represents a more mature and the Boksputs sequence a less mature portion of a middle Proterozoic arc system. The Areachap Group is interpreted as an accretion feature with characteristics generated in a subduction environment.

The geology of SW Connemara, Ireland: a fold and thrust Dalradian and metagabbroic-gneiss complex

Sillimanite grade Dalradian schists composed of pelites, impure psammites, quartzites and calcite marbles, all containing amphibolites, were intruded and hornfelsed (giving hornfels sillimanite, garnet and cordierite) by a plutonic basic to acid igneous rock series now metamorphosed and folded. Important folding (denoted D 2 , D 3 , D 4 , and D 5 ) as been recognized with a major southward facing D 2 , anticline, the Cashel antiform, being repeatedly folded by D 3 , which was a very strong phase, and accompanied and followed by D 3 -D 4 , major ductile slides and thrusting. Although the meta-igneous suite includes rare peridotites (olivine Fo 75-87 ,) and anorthosites, it is mostly hornblende-augite-labradorite or bytownite-metagabbro and later quartz diorite or trondhjemite gneiss, which has injected the earlier more basic rocks. Variably preserved igneous layering in the basic rocks indicates that the whole intrusive basic sheet is flat-lying and inverted, lying on the inverted limb of the Cashel antiform. About 460 Ma ago the D 3 -D 4 , Mannin thrust carried the igneous complex, including the largely crystallized gneisses and the cover of Dalradian metasediment, over a now largely mylonitized set of acid volcanic rocks (rhyolites or ignimbrites) which enclose a separate suite of massive amphibolites intruded or extruded into the acid volcanic rocks before the thrusting. The base of the overthrust metagabbro sheet, which had cooled to albite-epidote-amphibolite facies conditions, was ground down into a fine-grained NNW lineated amphibolite. The volcanic rocks, the Mannin thrust, and the overlying basic complex, were folded by D 4 , and D 5 , into a domed structure. About 400 Ma the aplopegmatitic facies of the Inish Granite intruded and caused hornfelsing and folding while the Galway Granite was intruded in the south. Lamprophyric sills and dykes and porphyry dykes were followed by the final phase of the Inish Granite. The southward facing folding and thrusting of the Connemara Dalradian block and the gabbro-gneiss emplacement are major elements in the Caledonide history of the British Isles. It is postulated that there was a tectonic and genetic connection between the gabbro-gneiss intrusion into these Dalradian rocks, the unusually rapid decrease in metamorphic pressure already documented for Connemara and the exceptionally southern position of the Connemara block.

Petrochemistry of metamorphosed pillows, and the geochemical status of the amphibolites (Proterozoic) from the Sirohi district, Rajasthan, India

AbstractRelic pillows in the middle Proterozoic amphibolites, occurring in the Sirohi Road–Abu Road tract of Rajasthan, India exhibit contrasted mineral assemblages from core to rim – mimetic after the crystalline core, the zone of incipient crystallization, and the rim of the original pillows. The major element distribution pattern across the pillows indicates exchange of Na–Al for Ca (Mg, Fe) in an inner reaction zone, surrounding the core and in the inner margin of the rim, and Fe–Al exchange for Ca–Si at the outer margin of the rim.Despite such exchanges around the rims, these pillows have retained their initial geochemical characteristics internally and thus have largely acted as closed systems during post-emplacement metamorphism. Mineral parageneses indicate that the contrasted mineral assemblages could evolve from domainal characters of the co-existing fluids, the compositions of which were only buffered by the reacting minerals during regional metamorphism.The major, minor and trace element contents of the pillows and of amphibolites of diverse petrographic character in the region further establish that the pillow interiors and the massive amphibolites were least modified during metamorphism(s), and represent oceanic tholeiites. Their average 2300 ppm K, 4.5 ppm Rb, 150 ppm Sr, along with the K/Rb and K/Sr ratios of 510 and 15 respectively resemble that of the low K-tholeiites, occurring nearest to the trenches in modern island arcs. On the other hand, the higher values of 17300 ppm K, 4.9 ppm Rb, and 210 ppm Sr of the banded and the schistose amphibolites indicate that they were contaminated in various magnitudes by oceanic sediments.

The sedimentology of the South Connemara Group, western Ireland – a possible Ordovician trench-fill sequence

AbstractThe stratigraphy of the ?Ordovician South Connemara Group is described. The group is approximately 3 km thick and includes sedimentary and volcanic rocks. The sedimentary elements of the succession include conglomerates, sandstones, shales, cherts and minor limestone breccias. The Lettermullen Formation exhibits the thickest continuous sedimentary sequence in the group. It is divided into five facies: a conglomerate facies, thick sandstone facies, medium sandstone facies, sandstone/siltstone facies, and chert facies. The sedimentary features of these rocks include channelling and grading and are described in terms of Bouma units where applicable. The coarse elements of the formation are thought to represent deposition in, or proximal to, submarine channels feeding a more distal depositional site represented by the finer elements. Igneous rocks of the group include pillowed and non-pillowed lavas, massive amphibolites and minor tuffs often in conformable contact with the sediments. Petrography of conglomerate clasts and sandstones indicates derivation from a continental source area. Palaeocurrent analysis reveals a primary easterly mode with a subsidiary northerly mode. It is suggested that the group exhibits some characteristics of trench-fill sequences and the succession may be related to similar environments within the Southern Uplands accretionary wedge.

Geochemistry and origin of amphibolite and magnetite from the Yangyang iron deposit in the Gyeonggi metamorphic complex, Republic of Korea

The chemical similarity of magnetite-bearing amphibolites from the Yangyang iron mine in Korea to mixtures of pelite and limestone or dolomite is consistently indicated by all the available chemical data. The chemical variation trends of the rocks were compared with ortho-(discordant-) and massive amphibolites from the Gyeonggi metamorphic complex, Korea. Samples of nine amphibolites from the Yangyang mine, and two ortho-amphibolites and six massive amphibolites from the Gyeonggi metamorphic complex were analyzed for twenty-two elements by wet-chemical, X-ray fluorescence and emission-spectroscopic methods. Trace-element analyses of the magnetites from the Yangyang mine were compared to those from a deposit of magmatic-segregation type and from six Precambrian sedimentary magnetite deposits found elsewhere in Korea. It is suggested that the magnetite ore may have been derived by metamorphism of iron-rich sediment originally deposited in an environment of pelite-carbonate sedimentation.

Wind river canyon: An example of a greenstone belt in the Archaean of Wyoming, U.S.A.

The Precambrian metamorphic complex in the southern portion of Wind River Canyon is interpreted as being a fragment of an Archaean greenstone belt. The sequence is composed of meta-sediments inferred to have been various types of pelites and psammites, including graywackes and shales, and a silicate facies banded-iron formation. Meta-volcanics are represented by massive amphibolites. The area has undergone three periods of roughly coaxial folding that represent a single tectonic pattern. A period of intrusion of leucogranite with associated pegmatites separates the first two periods of deformation. These rocks appear to have been derived anatectically from sialic material at greater depth, suggesting the possibility of a sialic basement on which the greenstone belt rocks accumulated. Boudinage of the country rocks can be correlated with either or both of the first two folding episodes, and boudinage of the intrusive rocks occurred with different styles in the axial surfaces of the second and third generation folds. One period of amphibolite-facies metamorphism corresponds to the first and second deformational phases. Minor retrograde effects, fracture fillings, and small-scale metasomatism occurred either in the waning stages of the metamorphism or during a minor subsequent thermal event. Numerous Archaean ages from the Wyoming Precambrian province place Wind River Canyon in a region where examples of such greenstone belts might be expected. As no young intrusive or tectonic events have been reported from the area, the youngest deformational features discussed are considered to be not much younger than reported radiometric dates and therefore not of regional significance.