W. S. Fyfe

Sub-sea-floor metamorphism, heat and mass transfer

The ophiolitic rocks of E. Liguria, Italy contain a „spilitic” metamorphic assemblage sequence, cross-cut by hydrothermal veins, which developed in the oceanic environment. Metamorphic parageneses indicate that temperatures as high as ∼400°C were realised at depths as shallow as 300 m below the original rock/water interface. The inferred temperature interval was equivalent to a geothermal gradient of ∼1300°C/km.It is suggested that metamorphism took place in a sub-sea-floor geothermal system, and that such systems are an integral part of the sea-floor spreading process. Modern evidence is provided to support this hypothesis, and to suggest that heavy metal rich solutions discharged from such systems are responsible for the formation of a metal enriched sedimentary component. A unified model of sub-sea-floor metamorphism and mass transfer is proposed, and possible differences between sub-sea-floor and terrestial geothermal systems are discussed. In the light of the model, the origins of certain aspects of bedded cherts found associated with ophiolitic rocks, of ophiolitic massive sulphide deposits and of certain trace element patterns are considered.

Density constraints on the formation of the continental Moho and crust

The densities of mantle magmas such as MORB-like tholeiites, picrites, and komatiites at 10 kilobars are greater than densities for diorites, quartz diorites, granodiorites, and granites which dominate the continental crust. Because of these density relations primary magmas from the mantle will tend to underplate the base of the continental crust. Magmas ranging in composition from tholeiites which are more evolved than MORB to andesite can have densities which are less than rocks of the continental crust at 10 kilobars, particularly if they have high water contents. The continental crust can thus be a density filter through which only evolved magmas containing H2O may pass. This explains why primary magmas from the mantle such as the picrites are so rare. Both the over-accretion (i.e., Moho penetration) and the under-accretion (i.e., Moho underplating) of magmas can readily explain complexities in the lithological characteristics of the continental Moho and lower crust. Underplating of the continental crust by dense magmas may perturb the geotherm to values which are characteristic of those in granulite to greenschist facies metamorphic sequences in orogenic belts. An Archean continental crust floating on top of a magma flood or ocean of tholeiite to komatiite could have undergone a major cleansing process; dense blocks of peridotite, greenstone, and high density sediments such as iron formation could have been returned to the mantle, granites sweated to high crustal levels, and a high grade felsic basement residue established.

O18 enriched ophiolitic metabasic rocks from E. Liguria (Italy), Pindos (Greece), and Troodos (Cyprus)

Low grade hydrothermally metamorphosed ophiolitic basic rocks from E. Liguria (Italy), Pindos (Greece) and Troodos (Cyprus) are enriched in O18 relative to the oxygen isotope ratio of fresh basalt (6.0±0.5‰). The maximum observed δO18 value of +13.22‰ corresponds to a positive isotope shift of 7‰ Enrichments in Sr87 relative to Sr86 correlate with hydrothermal alteration. The δC13 values of secondary calcite from E. Liguria are positive, and fall in the range from +0.2% to +3.6‰Since ophiolitic rocks are considered to be fragments of the oceanic crust and upper mantle, and since the secondary metamorphic assemblages were produced before mechanical emplacement, it is considered that the hydrothermal metamorphism which affected these rocks occurred in the sub-sea-floor environment. The isotope data are directly consistent with the hypothesis that the alteration was produced by interaction of the basaltic material with introduced sea water. Water: rock ratios were sufficiently large to produce the observed isotope shifts. In the Troodos ophiolite sequence δO18 values decrease steadily downwards and change to progressively larger depletions in the Sheeted Intrusive Complex. The trend of δO18 decrease correlates with the original direction of increasing temperature. The O18 depletions, which have also been observed for oceanic “greenstones” (Muehlenbachs and Clayton, 1972b), resulted from water/rock interaction at temperatures greater than the particular temperature range above which whole rock-water fractionations became less than the isotopic difference between fresh basalt and sea water.Since this isotope geochemistry indicates that the water responsible for hydrothermal metamorphism was of sea water origin, the data support the more general hypothesis that convection of sea water within the upper 4–5 kms of the oceanic crust is a massive and active process at oceanic ridges. This process may be completely or partially responsible for (a.i.), the local scatter and low mean value of the conductive heat flux measured near ridges, (a.ii), the transfer of considerable quantities of heat from spreading oceanic ridges, (b) hydrothermal metamorphism, metasomatism and mineralization of oceanic crust, (c), the production of metal enriched, relatively reduced brines during sea water/basalt interaction, d), the high degree of scatter and low mean value of the compressional wave velocities of oceanic basement layer 2 and (e), the low natural remanent magnetization (NRM) intensity of the lower part of layer 2 and upper part of layer 3 of oceanic crust.

Local modification of rock chemistry by deformation

Metabasalts subjected to progressive deformation in large-scale shear zones at Yellowknife display corresponding changes in major element abundances. Deformation, under conditions of greenschist facies metamorphism, has involved grain size reduction from 1200 μm to <20 μm, depletion of SiO2 (≃5%) and Na2O, together with hydration, and a decrease in specific gravity from 2.97 to 2.80. Chemical redistribution by deformation has been accomplished through a decrease in grain diameter of quartz and albite by intercrystalline diffusive mass transport (pressure solution), with concomitant transfer of material into extension veins. The degree of chemical modification is related to the finite strain. Deformation has involved a redistribution of ∼7.1015g of SiO2 over a volume of about 50km3.The microstructure of an adamellite deformed in a shear zone at higher temperature, under conditions of amphibolite facies metamorphism is indicative of dominant dislocation creep. A low degree of tectonic grain refinement is present. Constant values of major element abundances and specific gravity determined across the shear zone at increasing states of strain imply isochemical and isovolumetric deformation. These results are taken to support the precept that crustal deformation is characterised by a low temperature deformation regime dominated by pressure solution, with local changes of rock chemistry and volume; and a high temperature regime in which strain is accommodated principally by dislocation creep, an isochemical and isovolumetric deformation mechanism.

Adularia, the characteristic mineral of felsic spilites

Sea floor rhyolites from the Iberian Pyrite Belt show strong enrichment in potassium and adularia is observed to replace plagioclase. This process is in accord with thermodynamic data for the exchange reaction with ocean water which favours adularia up to about 140° C. Archaean felsic intrusives and extrusives exhibit sodium enrichment, a contrast which we attribute to lower K levels in sea water and, or, higher ocean temperatures. All of these rocks show 18O enrichment of + 6 to + 8‰, in compliance with low temperature sea water exchange.

Stable and Sr-isotope evidence for fluid advection during thrusting of the glarus nappe (swiss alps)

At the Glarus thrust in the Swiss Helvetic Alps, Permian Verrucano siltstones are allochthonously superimposed over Tertiary Flysch with an intermediate, about 1 metre thin layer of intensively deformed calcmylonite of probable Mesozoic provenance. The H−O−C- and Sr-isotope compositions of minerals from the calc-mylonite and strongly mylonitized Verrucano siltstones were determined in order to assess: (1) equilibrium-disequilibrium relationships; (2) isotopic composition of the fluid phase, its provenance and water/rock ratios; (3) sources of Sr in the calc-mylonite; (4) deformation temperatures. The isotopic composition of cale-mylonite micro-samples from five sites along the thrust varies from 22 to 12‰ and 2 to-10‰ for δ18O and δ13C respectively. All samples are 18O depleted by up to 14‰ relative to the presumed marine Helvetic carbonate protoliths (δ18O=25.4‰±2). A pronounced geographic trend of 18O depletion from 22‰ in the north to 12‰ in the south is observed. In calc-mylonites, 87Sr/86Sr ratios range from typical Mesozoic marine carbonate protolith signatures (0.708±0.005) to more radiogenic values as high as 0.722. A variable contribution of radiogenic 87Sr to the calc-mylonite is though to reflect interaction with fluids that aquired their Sr from the Hercynian granitic basement. Chlorites and muscovites from the calc-mylonite and Verrucano have uniform δ18O values but display δD values from-40 to-147%: the D-enriched values correspond to the primary metamorphic or formational fluids expelled during thrusting, whereas the D-depleted samples reflect selective H-isotope exchange with meteoric fluids during uplift of the Alpine belt. The isotopic composition of the calc-mylonites requires exchange with 18O—depleted, 87Sr—enriched fluids at very high water/rock ratios. Possible sources for these are dewatering of the underlying Flysch and/or metamorphic fluids, or formation brines expelled along the thrust from greater depth. These could be derived from compaction and dewatering of the Flysch in the northern part of the thrust; in the south, however, where Verrucano is thrust over 18O-rich Mesozoic carbonates, the extreme 18O depletion of the calc-mylonite has to be explained either by fluid advection within the Verrucano hanging wall and thrust zone or alternatively by exchange with metamorphic fluids from greater depth, expelled along the thrust. Microstructural evidence (abundant veins, stylolites, breccias) suggests that fluids played an important role in deformation and strain localization. Excepting albite all major components (quartz, chlorite, muscovite, calcite) are both dynamically recrystallized and crystallized as secondary minerals in pressure shadows and syn-mylonitic veins, indicating that these minerals were potentially open to oxygen isotopic exchange during alpine metamorphism and thrust deformation. Within the mylonitized Verrucano silstones, isolated quartz-chlorite and quartz-calcite fractionations yield temperatures of around 320°C close to values obtained from calcite-dolomite thermometry (355°C±30) and in agreement with the regional lower greenschist facies metamorphism. Quartz-calcite and quartz-albite fractionations indicate slightly lower temperatures around 250°C, owing to selective lower temperature re-equilibration of the calcite and albite during post peak metamorphism.

Geochemistry and petrology of the jequie granulitic complex (Brazil): an archean basement complex

The Jequie granulitic complex is part of the extensive high-grade metamorphic terrain located within the Sao Francisco craton of northeastern Brazil. Some Jequie rocks appear to have been formed in the middle Archean (∼ 3.1 Ga) from preexisting sialic crust. The dominant mineral composition of these rocks is quartz-microcline-plagioclase-hyperstene and occurs over an extensive area (∼ 2,000 km2).Scattered enrichment of normal granites with many minor elements (e.g. Rb, Y, Zr, Nb, Ba, REE), and the non-depletion of other elements (e.g. Cs, U), normally considered mobile during granulite facies metamorphism, must lead either to the reconsideration of regional metasomatism subsequent to granulite facies metamorphism, or at least raise some doubts about common wisdom concerning the distribution of heat-producing elements at depth. The region includes large-scale thrust structures, which could play a part in influencing high-level emplacement of the rocks and their regional metasomatism and structures.

Development of rodingite in basaltic rocks in serpentinites, East Liguria, Italy

Hydrogrossular replacement of plagioclase in basaltic rocks enclosed in serpentinite, and relationships between hydrogrossular, pumpellyite, vesuvianite are described. Rogingitic rocks are dominated by mixed layer chlorite-smectite, chlorite, pumpellyite, hydrogrossular and vesuvianite. In these rocks pumpellyite attains maximal Mg contents, and chemical analysis shows extreme removal of Na2O, K2O, TiO2 and SiO2 from the basalts, and increase in the Fe3+/Fe2+ ratio. Leaching during serpentinization by extremely alkaline solutions dominated by Ca-Mg(OH) may explain removal of components, but the oxidation may suggest that at an earlier stage dykes may have acted as input aquifers.

Origin of peralkaline granites of Saudi Arabia

Small volumes of peralkaline granites were generated as the final phase of a Pan African calc-alkaline igneous event which built the Arabian Peninsula. The peralkaline granites are closely associated with trends or sutures related to ophiolites. Peralkaline rocks are chemically heterogeneous, with anomalous abundances of Zr (average 2,150 ppm±2,600 1σ), Y (200±190), and Nb (105±100), representing up to ten-fold enrichments of these elements relative to abundances in calc alkaline granite counterparts. Large enrichments of some rare earth elements and fluorine are also present.The peralkaline granites have scattered whole rock 18O values, averaging 8.7±0.6% in the Hadb Aldyaheen Complex and 10.7±1% in the Jabal Sayid Complex. Quartz-albite fractionations of 0.5 to 1.5% signify that the heavier whole rock δ-values probably represent the oxygen isotope composition of the peralkaline magma. Small variable enrichments of 18O, in conjunction with slightly elevated 87Sr/86Sr initial ratios relative to broadly contemporaneous calc alkaline granites, are both suggestive of a small degree of involvement of crustal, or crustal derived material in the peralkaline magmas. It is proposed that the peculiar magma genesis is associated with a relaxation event which followed continental collision and underthrusting of salt rich sediments.

Corundum, Cr-muscovite rocks at O'Briens, Zimbabwe: the conjunction of hydrothermal desilicification and LIL-element enrichment — geochemical and isotopic evidence

Monomineralic domains of chlorite, corundum and Cr muscovite coexist over a kilometer scale within ultramafic schists of the Harare greenstone belt (2.73 Ga). This exotic lithological association includes the conjunction of some of the most aluminous (Al2O3∼88 wt%) and potassic (K2O∼10 wt%) rocks known. The paragenetic sequence developed from chlorite→corundum→corundum+ diaspore: Cr muscovite variably overprinted both the corundum and chloritite domains. Terminal stages were marked by sporadic production of andalusite+quartz, and finally margarite.Chlorite (Cr2O3=0.31–2.65 wt%), corundum (0.79–2.66 wt%), and diaspore are all Cr-rich varieties. The chromian (Cr2O3∼3.86 wt%) paragonitic muscovite incorporates up to 17% of the paragonite molecule, and significant Mg and Fe substitutions.The suite of rocks are characterized by ‘chondritic’ Ti/Zr ratios (−x=107), systematically enhanced Cr (up to 14000 ppm) and Ni (up to 1200 ppm) abundances, low levels of the alteration-insensitive incompatible elements Th, Ta, Nb. Chlorite, corundum and Cr muscovite represent progressive stages in the incremental metasomatic alteration of a komatiite precursor. Mass balance calculations, constrained by the isochemical behaviour of Ti, Zr and Hf reveal that the komatiite chloritite transformation involved volumetric contractions of ∼60% by hydrothermal leaching of Si, Fe, Mn, Ca and Na. Reaction of chloritite to corundum involved further volumetric reductions of ∼50% due to essentially quantitative loss of Si, Fe, Mn, Mg, K and Ca. Conversion of corundum to muscovite required additions of Si, K, Fe, Mn, Mg, Rb and Ba at 50–200% dilation. K, Rb, Ba, Li and Cs are enriched by up to 2×103 over background abundances in ultramafic rocks, and the suite is also enriched in B, Se, Te, Bi, As, Sb and Au. REE were extensively leached during chloritite-corundum stages, whereas LREE additions accompany development of muscovite. Ti, Zr, Hf and Al were all concentrated by selective leaching of mobile components, but absolute additions of Al accompanied development of the corundum domains due to Al precipitation in response to depressurization.Corundum (δ18O=3.5–4.8), muscovite (δ18O=6.7–7.5‰) and chlorite (4.5–5.6‰) are isotopically uniform and formed at 380–520° C from a fluid where δ18O=5.6–6.9‰. The corundum is 18O depleted relative to either igneous or anatectic counterparts (δOcor=7.6–8.2), or to gibbsitic laterites (δ18O=12–17).Previous genetic schemes involving metamorphism of exhalites or bauxite, or Si-undersaturation of magmas, can all be ruled out from the data. The chloritite, corundum, Cr-muscovite association represents sequential alteration products of ultramafic rocks by high temperature, low pH hydrothermal solutions carrying LIL-elements, and in which excursions of pH and/or degree of quartz undersaturation account for the mineralogical transitions. A deep level acid epithermal system, or fluid advection across steep inverted thermal gradients in a thrust regime could account for required hydrothermal conditions.