Episodes Of Metasomatism Research Articles

Contrasting styles of hydrous metasomatism in the upper mantle: An ion microprobe investigation

Trace element compositions of amphibole (±phlogopite)-bearing spinel lherzolite xenoliths have been investigated with regard to their oxidation state and tectonic setting to evaluate the processes associated with the formation of hydrous upper mantle. The oxidation states of these xenoliths appear to be directly related to the style of metasomatism and the tectonic environment. Suites having oxygen fugacities at or above QFM (Dish Hill, California; Ichinomegata, Japan; Mont Briancon, Massif Central, France; Dreiser Weiher, Eifel, Germany) are all modally metasomatized, containing amphibole ± phlogopite. These suites are either from regions overlying Cenozoic continental arcs (Dish Hill and Ichinomegata) or from Cenozoic continental rifts superimposed on ancient arc terranes (Massif Central and Eifel). Although trace element enrichment does not appear to be directly linked to the oxidation state, three distinct cases of metasomatic activity can be defined for these oxidized, hydrous xenolith suites. Both Case 1 and Case 2 are exemplified by xenoliths from Dish Hill and Ichinomegata. Case 1 is a less intense form of metasomatism resulting in the formation of amphibole by the simple hydration of the mantle. The original depleted trace-element composition of the mantle is preserved. Amphiboles produced in Case 2 metasomatism are enriched in incompatible elements, but coexist with depleted clinopyroxene. The formation of enriched amphibole is consistent with crystallization from a volatile-bearing melt phase. The Eifel and Mont Briancon trace element compositions that appear to represent equilibrium crystallization of incompatible element-enriched amphibole and clinopyroxene from a migrating melt phase (Case 3). Chemical evidence suggests that phlogopite in xenoliths from Dreiser Weiher, Mont Briancon, and Ichinomegata has a secondary origin. Mica formation, therefore, represents a distinct metasomatic episode.

Geochemistry of the Dongargarh volcanic rocks, Central India: implications for the Precambrian mantle

The Dongargarh lavas, erupted over a broad time span from ∼ 2462 Ma to ∼ 1367 Ma, comprise an interlayered sequence of felsic to mafic volcanic rocks and sedimentary formations. The early volcanic rocks, dominated by rhyolites (Bijli) with subordinate dacites, andesites and basalts (Pitepani), are subalkaline and show calc-alkaline trends. The rhyolites are enriched in LREE, Zr, Th, Cr and depleted in LILE, Ti, Sc, Y, Ni. The early lavas show REE patterns varying from LREE enriched, highly fractionated in the rhyolites to progressively less fractionated flat patterns in the mafic members, consistent with a derivation of the felsic to mafic lavas through progressively higher degrees of partial melting of a common source. Fractionation of olivine, clinopyroxene and plagioclase also played an important role in the generation of the range of mafic to felsic magmas. A possible crustal contribution in the generation of the voluminous early rhyolites is indicated from the ratios of LILE and LREE in which the rhyolites show low Sr and high Ce, similar to other crustally derived lavas. The late volcanic rocks (Sitagota and Mangikhuta), on the other hand, are relatively unfractionated, compositionally restricted, more primitive magmas, dominated by basalts with minor andesites, showing tholeiitic affinities. The Sitagota lavas have high Mg number, lesser abundances of Ba, Rb, K, Th and Zr, but are enriched in Sr, V, Ni, Cr, Sc than the other members of the Dongargarh suite with comparable Mg number. Their flat unfractionated REE patterns also support their primitive character and their derivation through higher degrees of partial melting of a common mantle source. Although all the Dongargarh lavas show a close chemical affinity with island-arc magmas in all discriminant diagrams and show a similar fractionation of the alkaline earth elements (AE) Rb, K, Ba relative to the incompatible HFSE, the associated sediments point to a stable continental margin tectonic environment, suggesting that the Dongargarh lavas were erupted in a continental rift setting rather than in an island arc. The arc-like geochemical signature of these lavas is suggested to be a manifestation of an earlier episode of hydrous metasomatism of the Precambrian mantle which enriched it in LILE and LREE and made it similar to the source of IAB. The primitive nature of the late volcanics are attributed to a complex subvolcanic plumbing system whereby the lavas gained fast access to the surface without any significant crustal interaction.

U‐Pb geoehronology and evolution of the English River Subprovince, an Archean low P‐high T metasedimentary belt in the Superior Province

U‐Pb isotopic data for zircon, monazite, and titanite are used to assess the evolution of the late Archean English River Subprovince (ERS) and adjacent domains of the Winnipeg River Subprovince in the northern Superior Province, Ontario. The ERS consists of turbiditic metasedimentary rocks, deposited during the final stages of magmatic and tectonic accretion within the greenstone‐granite Uchi Subprovince to the north at around 2720 to 2710 Ma. The sedimentary rocks were intruded by a suite of calc‐alkalic plutons at 2698 Ma. Major regional deformation, amphibolite to granulite facies metamorphism, anatexis, and emplacement of an extensive peraluminous granitic suite culminated at 2691 Ma. Late episodes of metamorphism, metasomatism, and emplacement of pegmatites occurred locally at circa 2680 and 2669 Ma. In contrast, the Winnipeg River Subprovince to the south consists of widespread tonalitic suites formed mainly between 3200 and 2800 Ma and granodioritic to granitic intrusions formed between 2710 and 2690 Ma. Metamorphism occurred at around 2710 Ma in southern domains, and a distinct granulite facies event at around 2680 Ma affected central and northern domains of the Winnipeg River Subprovince. The rapid sequence of events recorded in the ERS supports the notion that metamorphism was caused by thermal perturbations related to injection of granitic magmas. These melts were generated through partial melting of the lower crust by a thermal anomaly presumably caused by mantle upwelling related to collapse of the inactivated subducting plate. The short‐lived nature of the metamorphic events and their cyclicity follow the behaviour predicted by recent models linking the evolution of strain rates to episodes of segregation and upward magma transfer in the course of prolonged orogenic events.

Two stages of CO2 metasomatism at the Munro mine, Munro Township, Ontario: evidence from fluid-inclusion, stable-isotope, and mineralogical studies

The Munro asbestos mine is hosted by a differentiated ultramafic sill of Archean age. Localized carbonate alteration at the mine has resulted from two separate episodes of CO2 metasomatism, and the fluids were unrelated. The first episode affected only the serpentinized peridotite and occurred at 250 °C. The fluid was a saline brine (up to 24 wt.% NaCl–CaCl2), and had an oxygen isotopic composition of −3‰, and δ13C was equal to −7.8‰. Calcite veins were emplaced into the overlying, fractured pyroxenite at approximately 300–400 °C during the second episode. The salinity of this fluid was only 1–5 equiv. wt.% NaCl, the oxygen isotopic composition was +7.5‰, and δ13C equaled −3 to −5‰. The first episode was probably associated with burial metamorphism (diagenesis) and the second episode with regional metamorphism. The widespread occurrence of two separate stages of CO2 metasomatism in the Abitibi belt and in other well-documented Archean terranes, such as the Norseman–Wiluna greenstone belt in Western Australia, suggests that this may be an important factor in the tectonic evolution and metamorphic history of Archean greensone belts.

Nitrogen microanalyses of silicates using the 14N(d, α) 12C nuclear reaction and initial results on upper mantle and peralkaline magmatic micas

Nitrogen analyses of geological materials with high spatial resolution and elemental sensitivity are desirable in understanding the geochemical migrations of this important element. The 14N(d, α) 12C nuclear reaction has been developed for the analysis of si incorporating an electrostatic quadrupole lens on the incident 2H + beam to produce a 50 μm spot and an annular detector for large detection solid angle. Alpha particles from surface nitrogen (high energy) and competing reactions (low energy) were avoided by restricting the analytical energy window. Minimum detectable concentration was estimated to be 5 ppm. Nitrogen contents of 10 upper mantle and peralkaline magmatic micas show a bi-modal distribution, near the detection limit for 6 megacrysts and between about 100 and 200 ppm for 4 MARID and glimmerite rocks. Upper mantle micas appear to be minor sources of N in general but high concentrations occasionally occur associated with metasomatic episodes.

Mapping present-day geochemical variations across the society hotspot

The presence of variable amounts of amphibole ± phlogopite in a garnet websterite and a garnet clinopyroxenite from the Beni Bousera peridotite massif provides evidence for post-formation metasomatism. Textural observations associated with major- and trace-element mineral compositions allowed us to distinguish two metasomatic episodes, which occurred at different stages of the Beni Bousera massif evolution. The garnet websterite has recorded interaction with LREE-rich silicate melts before the uplift of the massif. Amphibole/clinopyroxene and amphibole/garnet trace-element ratios closely approach partition coefficient values, indicating that chemical equilibrium was attained between amphibole and pyroxenite matrix minerals. The geochemical signatures of the putative alkaline interacting melts are similar to those of recent basaltic magmas erupted in Morocco, suggesting a common peridotite mantle source. In contrast, amphibole from the garnet clinopyroxenite is in chemical disequilibrium with the pyroxenite matrix minerals. In this clinopyroxenite the crystallization of amphibole and plagioclase occurred at lower T (and P) conditions, most probably during the ascent of the Beni Bousera massif and its emplacement into the crust. The melt responsible for this later metasomatic episode was LREE-depleted and HREE-enriched, suggesting that it resulted from decompression melting of a garnet-bearing source (with garnet as a melting phase), similar to the garnet-bearing pyroxenites outcropping in the Beni Bousera massif.

Open-system O-isotope behaviour and trace element enrichment in the sub-Eifel mantle

Spinel peridotites from the West Eifel, western Germany exhibit a wide range of δ 18O SMOW values for olivine ( +5.1 to +6.3‰), smaller ranges for pyroxenes ( +5.7 to +6.3‰ for orthopyroxene; +5.6 to +6.4‰ for clinopyroxene) and variable 18O/ 16O fractionations between olivine and pyroxenes ( Δ clinopyroxene-olivine = +0.6 to −0.3‰ and Δ orthopyroxene-olivine = +0.7 to −0.1‰). These features are characteristic of a disequilibrium distribution of O-isotopes between coexising olivine and pyroxenes. Enrichment in 18O is observed primarily in olivine from amphibole and/or phlogopite-bearing peridotites and correlates with degree of LREE/HREE enrichment, indicating that the processes responsible for trace element enrichment (i.e. mantle metasomatism) and O-isotope disequilibrium are linked. The 18O- and incompatible trace element-enriched fluid(s) cannot be generated by intra-mantle fractionation processes as presently understood, and are attributed to recycling of altered oceanic lithosphere and/or pelagic sediments into the mantle via subduction. This is supported by δD SMOW values for associated Eifel amphibole and mica ( −50 to −45‰ respectively) which fall midway between δD values for upper mantle ( −80‰) and seawater (0‰). Coexisting amphibole and mica also have distinctly different δ 18O values ( +7.0 and +5.9‰, respectively) which suggests that they did not originate from the same fluids/melts. This implies that the sub-Eifel mantle has experienced at least two distinct metasomatic episodes.

Geology and geochemistry of Huronian rhyolites and low-Ti continental tholeiites from the Thessalon region, central Ontario

Bimodal volcanism associated with early phases of Huronian rifting in central Ontario, dated about 2450 Ma, produced low-Ti tholeiitic basalts and two varieties of crustally derived calc-alkaline rhyolite. Early tholeiites are characteristically highly evolved, have Mg* values from 30 to 50, and display pronounced enrichment in large-ion lithophile elements (LILE) and light rare-earth element (LREE) in comparison with modern oceanic basalts, fractionated heavy rare-earth element (HREE) patterns, and low Ti, Zr, P, Nb, Ba, and K abundances. Ti/Zr ratios rise progressively in early basalts and associated basaltic andesite fractionates from about 35 in early flows to 55 in central units. Late basalts also carry enriched LILE and LREE, but, in contrast to early types, have average Mg* values greater than 50 and lower rare-earth element (REE) abundances with flat HREE patterns. They also display negative Ba, Nb, and P anomalies on chondrite-normalized distribution diagrams, but lack low K, Zr, and Ti contents. Their Ti/Zr ratios of about 80 approach chondritic levels. Melting models suggest the differences are explained by lower degrees of fusion (as low as 10%) in a hydrated, LILE- and LREE-enriched peridotite during generation of the early basalts, leaving a residue containing appreciable garnet, amphibole, Ti oxides, zircon, and apatite.Erupted simultaneously with the basalts were two distinctive rhyolite types: (1) a low-LILE, high-LREE group (25% of analysed specimens), derived by −20% melting of granulitic siliceous tonalitic gneiss, presumably at deep crustal levels, and (2) a high-LILE, low-LREE group (75%), derived, probably at shallower levels, by ≤ 30% melting in granitic rocks with pegmatitic or leucogranitic compositions. Mutual magma mixing of basalts and rhyolites during early stages of volcanism produced abundant hybrid andesites, but the frequency of contamination is much lower in later units.Hypothetical subcontinental source compositions, calculated from the Raleigh equation, suggest that the Huronian mantle had already undergone a complex history. Low Ba, Nb, P, Ti, and depleted HREE abundances compared with abundances for modern oceanic basalts suggest that a basaltic melt had already been withdrawn from this source during Archean time. Subsequently, an episode of hydrous metasomatism enriched the source in LILE and LREE. The latter event resulted from (1) subcontinental mantle metasomatism by previous Archean subduction, (2) mantle metasomatism during the terminal Archean Kenoran Orogeny, or (3) a wave of hydrous metasomatism accompanying Huronian mantle convection immediately preceding volcanism.

Metallogenesis of a B-F-metasomatic skarn tin deposit, Gejiu, Yunnan Province

Recently, a new type of skarn tin ore formed by B-F-metasomatism has been found in the central part of the Malage ore field at Gejiu, Yunnan Province, which contains 1–7% Sn. The principal tin minerals are nordenskiol dine, schoenfliesite, varlamoffite, etc., accounting for about 4%. Cassiterite is rarely seen (less than 0.2%). Outward from endoskarns are distinguished four alteration zones, ore fluids were rich in Al and Si in the early stage of skarnization, and became enriched in Fe, Mg, Sn, B, F, H2O, etc. in the later stages. Two episodes of hydrothermal metasomatism of B and F took place during post-skarnization, and the different alteration zones may be due to reaction between B-and F-bearing hydrothermal solutions and country rocks (calcareous carbonates). The stability of tin minerals is controlled by μB2O3, μCO2,μF2O-1 and μF(OH)-1. It has been concluded from the discussion in terms of the system CaO-SnO2-SiO2-H2O-B2O3-CO2-F2O-1 proposed by Burt (1978) and the topological diagrams constructed by the author that nordenskioldine was formed in the depletion of Si at about 400°C in the skarn environment, where metallogenetic conditions are characterized by rich oxygen but poor sulfur, and high boron but low fluorine, while schoenfliesite and varlamoffite were formed at the lower temperature (about 150°C) in response to hydrothermal metasomatism of fluorine. In comparison with the metallogenetic conditions for adjacent endoskarn tin ores and other tin ore deposits at Gejiu, the role of the characteristic elemental association Sn-B-F is emphasized in this paper.

Origin of hydrous cordierite and isotopically light oxygen in granulite-facies rocks, Strangways Range, Central Australia

AbstractThe origin of hydrous cordierite from a granulite-facies terrain in the Strangways Range, Central Australia, reported by Wilson (1978) is reviewed. In the light of geochemical and microstructural evidence for a metasomatic episode which affected the terrain subsequent to the growth of the cordierites, it is contended that the hydrous character of the cordierites results from hydration of the cordierite at this time. Extraneous argon also recorded in these cordierites may either have been incorporated into the cordierites at this time or at a later stage in the history of the terrain due to partial loss of radiogenic argon from micas which grew during the metasomatic episode. Furthermore, in view of the history of these cordierites, oxygen isotope values for the cordierites reported by Wilson are likely to be anomalous, and the isotopically light oxygen in the host granulites recorded by Wilson and Baksi (1978) probably reflects exchange with the metasomatic fluids which permeated the terrain.

Metasomatism in the Wairere Serpentinite, King Country, New Zealand

SummaryMetasomatic zones formed at the borders of various mafic inclusions in the Wairere Serpentinite, central North Island, New Zealand, show eucritic and hornblende gabbros grading through distinct zones of altered gabbro (hydrogarnet-free) to coarse and finegrained rodingites (hydrogarnet-bearing). The hydrated, calcium-rich, aluminium-silicate mineral assemblages of each reaction zone change between the zones, with one mineral phase being altered to another at each replacement front. Initial alteration of the gabbros albitized, prehnitized, and epidotized plagioclase and chloritized hornblende and clinopyroxene. Continuing metasomatism garnetized prehnite and generated secondary amphibole and pyroxene while the original host harzburgite was highly serpentinized and, adjacent to the inclusions, the serpentinite was chloritized so as to form a further distinct zone between peridotite and gabbro. These mineral replacement reactions are reflected in the bulk chemistry of the reaction zones, which show that increasing alteration of the gabbro is accompanied by a marked increase in Ca and H2O, a possible small gain in Mg, and depletion in Si, Al, and alkalis. These trends are balanced in the host peridotite by Si enrichment and Ca loss upon serpentinization. Clinopyroxenes and hornblendes of the zones show an increase in Ca and Mg and a decrease in Si with increasing metasomatic alteration. Garnets also decrease in Si but chlorites show an increase in both Si and Mg. Additional minerals analysed from both primary and altered rocks include olivine, orthopyroxene, biotite, plagioclase, pectolite, rosenhahnite, sphene, and chrome spinel. The zones are regarded as the result of reaction between the inclusions and Ca- and Al-rich solutions derived, in turn, by action of alkaline, CO2-poor solutions, which percolated through fractures in the original host peridotite and, in the process of serpentinization, released Mg and enhanced Ca and Al levels. At the contact of the mafic inclusions, chemical gradients were generated down which various chemical species diffused to different levels forming the zonation. The reactions probably took place in the vicinity of 260 to 350 °C, 2 to 3 kb, and at constant volume. Cataclasis and brecciation of the outer zones indicate more than one metasomatic episode.

Tertiary spilites and quartz keratophyres of the Wagwater Belt, Jamaica, west Indies

Geochemical and petrographic characteristics are used to identify spilites, quartz keratophyres and potassic quartz keratophyres as the most abundant rock types among the Tertiary volcanics of Jamaica. A few low K2O dacites occur within the sequence. The rock suite is bimodal and was erupted in a nonorogenic environment, an Eocene rift. The evidence indicates that the keratophyres were formed by alteration of the original dacites during an episode of alkali metasomatism. The concentrations of TiO2, P2O3, Cr, Zr and Y suggest that the spilites were formed from basic rocks similar to the Columbia River or Deccan flood basalts. Their chemical variations indicate that they were formed by processes similar to those leading to the formation of the keratophyres.

Relations of Folded Dikes and Precambrian Polyphase Deformation, Gardner Lake Area, Beartooth Mountains, Wyoming

Two cross-cutting mafic dikes in the headwall of Gardner Lake in the eastern Beartooth Mountains, Wyoming, have structural relations with Archean migmatite and gneiss that suggest intrusion between deformational phases recognized in the eastern part of the range. Fabric data show that the older dike, an orthoamphibolite, was emplaced subsequent to the F 1 deformational event, but prior to the main episode of metamorphism, metasomatism, and folding, F 2 . The younger dike, a metanorite, was intruded after F 2 , but is folded by west-trending, open F 3 folds. The style and general trend of these F 3 folds are consistent with those observed elsewhere in the eastern part of the range. The data available and absolute age relations in other parts of the range indicate that F 2 occurred 2750 m.y. ago and F 3 1600 to 1800 m.y. ago.