Geochemistry Of Amphibolites Research Articles

Mayodia ophiolitic complex of Arunachal Pradesh, India: a multistage evolutionary record during the Tethyan closure

ABSTRACT The Mayodia ophiolitic complex of Arunachal Pradesh, India is a highly dismembered body located close to the Namche Barwa syntaxis. It structurally represents a klippe and consists dominantly of serpentinite and dunite within which amphibolite and hornblendite occur. Detailed field observations indicate that amphibolites are concordant bodies within serpentinite, whereas hornblendite occurs as dykes. Geochemically, serpentinite samples indicate their abyssal/forearc origin showing refertilisation due to melt-mantle interactions. Massive dunite bodies, occurring as dykes within serpentinites, are replacive in origin and represent fossilized melt channels. The geochemistry of amphibolites is akin to gabbros formed at back-arc basin. Later, serpentinites were severely intruded by hornblendite dykes during the waning stage of back-arc spreading. Finally, the entire litho-package was obducted onto the passive margin of Indian plate as the Mayodia ophiolitic complex which subsequently got folded, deformed and metamorphosed during orogenesis. During this process, the crustal section of the complex was entirely eroded, preserving only the mantle section of this ophiolite.

Petrology and geochemistry of amphibolites from northwest of Salmas, NW Iran

Petrology and geochemistry of amphibolites from northwest of Salmas, NW Iran

Geochemistry of Amphibolites in Akom II, Nyong Series, North Western Border of the Congo Craton, South Cameroon

The garnet amphibolites, from the Akom II area in the Archaean Congo Craton, were examined to determine the geochemical affinity and tectonic environment. The study uses mineral assemblages and whole-rock geochemistry to identify the geochemical affinity and tectonic setting of the amphibolites associated with monzogabbro and pyroxenites. The studied rocks of Akom II are garnet amphibolites. Mineralogically, the rocks contain hornblende + plagioclase + garnet ± quartz ± epidote ± apatite ± opaque, indicating that they could have been formed from a basic igneous protolith. The geochemical signature indicates that the rocks are tholeiitic in nature. They are similar to the metamorphosed equivalents of ocean island basalts (OIB), with characteristics typical of Volcanic Arc-Basalt (VAB). The geotectonic diagrams confirm the tholeiitic nature of these amphibolites. High field strength elements ratios (Nb/Ta) range from 14-16, which corresponds to Volcanic Arc Basalt (VAB). The primitive mantle normalized patterns of these rocks show negative anomalies in Ta and Ti suggesting a geotectonic signature characteristic of a subduction zone, consequently suggesting the existence of a suture zone in the study area.  Â

Petrography and geochemistry of amphibolites from the Fomopéa Pluton (West Cameroon): Origin and geodynamic setting

Petrographic and geochemical investigations were carried out on amphibolites from the Fomopéa pluton (FP) in order to determine their origin and geodynamic setting. The geology of the FP consists of granitoids (granites, monzogranites, syenogranites and diorites), intruded in the basement rocks (gneiss and amphibolites). In the N and NE parts, the pluton was partially recovered by basalts while the SE parts was mylonitized. The amphibolites are represented in the pluton body by stretched, sub-rounded or elongated xenoliths. They are cumulate, massive or banded. The main minerals are amphibole, plagioclase and biotite while accessory minerals are pyroxene, quartz, K-feldspar, calcite and oxides. Amphibolites display magmatic heterogranular to granoblastic heterogranular textures. They are ortho-amphibolites, gabbroic in nature, calc-alkaline and metaluminous. The Nb/Ta ratio (12.5–18.5) is characteristic of arc-type basalts. Rare earth element (REE) profiles show negative Ce anomalies that are more significant in cumulate amphibolites (samples TN74 and TN78), suggesting a possible enrichment of the source by pelagic sediments. Our results are consistent with those of previous studies in the Central African Fold Belt (CAFB) in Cameroon thus, followed the same evolution during the Proterozoic.

Geochemistry of amphibolites and related graphitic gneisses from the Suchý and Malá Magura Mountains (central Western Carpathians) – evidence for relics of the Variscan ophiolite complex

Abstract Three small bodies of amphibolites and associated graphitic gneisses from the Suchý and Malá Magura Mountains (Tatric Megaunit, central Western Carpathians) have been studied by petrographic and geochemical methods. Isolated, fault-bounded bodies first hundreds of meters in size are located in the complex of the Early Paleozoic paragneisses and migmatites intruded by the Lower Carboniferous granitoid rocks. Amphibolites (locally actinolite schists) were formed from effusive basalts, dolerites or isotropic gabbros hydrothermally altered and veined before the regional metamorphic transformation. Distribution of the trace elements relatively immobile during the metamorphic alteration (HFSE, REE, Cr, V, Sc) is similar to E-MORB type in the Malá Magura Mountain or to N-MORB/E-MORB types in the Suchý Mountain. Graphitic gneisses to metacherts are rich in silica (up to 88 wt. %) and Ctot, poor in other major element contents and display negative Ce-anomaly, enrichment in HREE, V, Cr and U. They were probably originally deposited as non-carbonate and silica-rich deep-sea sediments in anoxic conditions. The oceanic provenance of amphibolites and related graphitic gneisses clearly indicates their oceanic crust affinity and identity with the uppermost part of the ophiolite sequence. Ophiolite bodies from the Suchý and Malá Magura Mountains are supposed to be relic fault blocks identical with the Upper Devonian Pernek Group which represents a Variscan ophiolite nappe preserved to large extent in the Malé Karpaty Mountains located in the Tatric Megaunit further to the southwest. All these ophiolite relics are vestiges of the original ophiolite suture created by oceanic closure in the Lower Carboniferous.

Petrology and geochemistry of amphibolites and greenschists from the metamorphic sole of the Muslim Bagh ophiolite (Pakistan): implications for protolith and ophiolite emplacement

Metamorphic sole rocks are exposed beneath both the Jang Tor Ghar Massif (JTGM) and Saplai Tor Ghar Massif (STGM) of the Muslim Bagh ophiolite. The sole rocks comprise the basal mylonitic part of the ophiolite peridotites and the sub-ophiolitic metamorphic rock series showing inverted metamorphic gradients. The latter mainly consist of garnetiferous amphibolites, amphibolites and greenschists. The mineralogy of the amphibolites (hornblende + plagioclase ± quartz ± biotite ± epidote ± apatite ± opaque) and garnet amphibolites in the metamorphic sole rocks of the Muslim Bagh ophiolite is similar except for the presence of garnet in the latter. Greenschists contain minerals such as chlorite + plagioclase + epidote ± actinolite ± quartz ± opaques. The mineral assemblages of these rocks suggest that they are meta-basites. Geochemical analyses indicate that the garnetiferous amphibolites are metamorphosed tholeiitic to alkaline basalts, akin to ocean island basalts (OIB). By contrast, the amphibolites and greenschists have geochemical signatures akin to mid-oceanic ridge basalts (MORB). Basalts of OIB type are also found in the hyaloclastite-mudstone unit (Bhm), while the MORB-type basalts are found in the basalt-chert unit (Bbc) of Bagh complex underlying the ophiolite nappe. Here, we interpret an early stage OIB-type basalt accretion to the base of the obducted plate associated with extrusion of volcanic rocks in the Bhm unit of Bagh complex followed by amphibolite facies metamorphism. During the later stage of the advancing ophiolitic thrust sheet, MORB-like basalts, such as those found in the Bbc unit of the Bagh complex, are underplated and metarmophosed to greenschist facies with subsequent accretion of the entire sequence of the Muslim Bagh ophiolite and the Bagh complex onto the Indian Platform sediments.

From Mesoproterozoic magmatism to collisional Cretaceous anatexis: Tectonomagmatic history of the Pelagonian Zone, Greece

AbstractThe magmatic history of the Pelagonian Zone, in northern Greece, is constrained with secondary ion mass spectrometer (SIMS) U‐Pb dating on zircons of various granitoids whose structural positions were defined with respect to the regional main foliation. Ages pertain to four groups: (i) Mesoproterozoic (circa 1430 Ma) crystallization of granites inferred from inherited magmatic zircon cores that have been partially molten during the (ii) Neoproterozoic at circa 685 Ma (metamorphic zircon rims) and subsequently intruded by a Neoproterozoic leucogranite (circa 600 Ma). (iii) Late‐ or post‐Variscan calc‐alkaline granitoids (315–301 Ma) were in turn intruded by a subvolcanic dike at about 280 Ma. In the Early Permian the εNd(t) in magmas decreased from −7.3 to −1.3, hinting to mantle‐derived melts produced during extension. Rifting is further heralded by two acidic and one mafic dike containing Lower‐Middle Triassic zircons (246–242 Ma). (iv) Early Cretaceous anatectic melts at 117 ± 8 Ma formed during regional metamorphism. This age is the first report of in situ anatexis in the Pelagonian Zone. Cretaceous anatexis developed during the Mesozoic collision of Pelagonia with the Eurasian margin. Major‐ and trace‐element geochemistry of amphibolites further attests for the complex pre‐Alpine tectonic history with Neoproterozoic calc‐alkaline and back‐arc geochemical signature and Triassic alkali‐magmatism.

Petrochemistry And Tectonic Setting Of Metabasic Rocks Of Isanlu, Southwest Nigeria

The geochemistry of amphibolite of the Isanlu area of southwest Nigeria was studied and its petrotectonic setting evaluated. Amphibolite occurs as small outcrops and residual hills that are conformably interbanded with a suite of meta-volcanosedimentary rocks. The rock reveals a mid-amphibolite facies assemblage of hornblende (magnesio and pargasitic compositions), plagioclase (oligoclase – labradorite) with small amounts of quartz and biotite. Locally, this assemblage has suffered retrogression into the greenschist facies and preserve the metamorphic assemblage actinolite + chlorite + epidote + quartz ± titanite. Amphibolite in the study area is characterized by a moderate SiO2 (46.50-53.74 wt. %), moderately high FeOtot (8.78- 14.41 wt. %), low Fe2O3/FeO (average 0.22), low MgO (6.18-10.56 wt. %), low to high CaO (8.60-18.20 wt. %) and low alkalis. Trace elements including Nb, Y and Rb are low in abundance while Sr and Ba show marked enrichment reflecting the abundance of modal plagioclase. Refractory elements including Ni, Co, Cr and V also show enhanced values probably reflecting the fairly primitive nature of its mafic ancestry. The rock classifies as high-Fe tholeiite with plume-type mid-oceanic ridge and volcanic arc tectonic affinity. However, the interbanded nature of the rock with the enclosing metasediment suggests that it was emplaced in a continental setting. This rock was probably emplaced in an ensialic basin developed on rift-controlled grabens, where the crust is sufficiently attenuated by rising mantle plumes resulting in ocean floor – type contamination-poor magmatism. Keywords : Petrochemistry, Tectonic setting, Amphibolite, Ensialic basin, Egbe-Isanlu schist belt Global Journal of Geological Sciences Vol. 6 (2) 2008: pp. 113-122

Geochemistry of amphibolites and quartzofeldspathic gneisses in the Pan-African Zambezi belt, northwest Zimbabwe: evidence for bimodal magmatism in a continental rift setting

The Zambezi belt separates the Congo and Kalahari cratons in southern Africa and is a key part of the regional Pan-African orogenic framework related to amalgamation of Gondwana in the Neoproterozoic-early Palaeozoic. Several thick, probably correlative, supracrustal sequences are preserved in the belt in Zimbabwe and Zambia. The Makuti Group, a major assemblage of supracrustal rocks within the belt in northwestern Zimbabwe, consists dominantly of amphibolite-facies quartzofeldspathic gneisses of supracrustal origin interlayered with horizons of marble, calc-silicate rock, quartzite, and pelitic schist. Numerous thick, concordant amphibolites derived from mafic sills and/or lava flows are intercalated within the supracrustal sequence. Major- and immobile trace-element geochemistry indicates dominantly tholeiitic affinities for the amphibolites, with some samples showing transitional to alkaline affinities. High-field-strength trace-element contents and LREE-enriched patterns are consistent with a within-plate setting for the mafic rocks. Major- and trace-element data show the quartzofeldspathic gneisses to be dominantly of igneous origin. Their protoliths are inferred to be mainly peralkaline rhyolites and trachytes. High Zr contents (up to 1500 ppm) are a diagnostic signature for these rocks. The bimodal nature of the magmatism and the abundance of peralkaline felsic rocks point to a continental rift zone as the setting for the Makuti Group. Other examples of pre-orogenic, mafic or bimodal magmatic rocks are found in the Zambezi belt elsewhere along strike in Zambia and Zimbabwe. All these rocks are inferred to represent widespread, rift-related magmatism associated with initiation of the depositional basin within which the Neoproterozoic sequences of the Zambezi belt accumulated.

Geochemistry of amphibolites from the southern part of the Kohistan arc, N. Pakistan

AbstractThe southern part of the Cretaceous Kohistan island arc is occupied by an extensive belt dominantly comprised of amphibolites. These include banded amphibolites of partly meta-volcanic parentage, and non-banded amphibolites derived from intrusive rock. In addition to being relict, banding has also been produced by shear deformation, metamorphic/metasomatic segregation and, possibly, by lit-par-lit injection of plagiogranitic material. Non-banded amphibolites also occur as retrograde products of noritic granulites forming the lopolithic Chilas complex. The chemistry of 37 rocks has been compared with those of known tectonic environments. The amphibolites have chemical characteristics similar to volcanic rocks found in island arcs and most of the analyses apparently support affinity with the calc-alkaline series. The amphibolites consist essentially of hornblende, plagioclase and/or epidote. Garnet and clinopyroxene have developed locally in rocks of appropriate bulk composition. Metamorphism may have taken place during the mid-Cretaceous under conditions of 550 to 680°C and 4.5 to 6.5 kbar PH2O. The metamorphic grade appears to increase from the centre of the southern belt toward the Chilas complex to the north and Indus-Zangbo suture (IZS) to the south. In the vicinity of the IZS, garnet-clinopyroxene ± amphibole assemblage developed locally in response to high P-T.

Geochemistry of amphibolites from the Ötztal—Stubai Complex (northern Tyrol, Austria)

Geochemical investigations of amphibolites from the Ötztal—Stubai Complex (northern Tyrol, Austria) show striking differences from north-northwest to south-southeast. Major- and trace-element chemistry data indicate a typical magmatic trend for amphibolites of central and north-northwest Ötztal. Amphibolites from south-southeast Ötztal (Schneeberger Zug) were found to be para-amphibolites. The ortho-amphibolites from central and north-northwest Ötztal were found to have originated from a tholeiitic magma. The K/Rb ratio decreases towards the southeast, with a higher value in central Ötztal. The higher amount of Li, Rb and K in south-southeast Ötztal suggests a possibly higher fluid content in this region during the Alpine metamorphism. These results fit in well with the polymetamorphic NNW—SSE overlapping situation of Hercynian and Alpine metamorphism in the Ötztal—Stubai Complex.