Calc-silicate rocks

Develii]ied in the eurly I lHOs. tohcrinoriies have selectivity properties intermediate between those of clay minerals and zeolites. They have been considered in catalysis and nuclear and hazardous waste disposal. Tobermorilc. Ca Si H () s 4HiO. occurs naturally as a hydrous calcium silicate in calc-silicate rock. Tobermorilex have layer structures similar to those of 2 1 clay minerals, but the structure varies with the chemical composition as well as with Ihe nature of their synthesis. They have heen synthesized from a number of starting materials. 

These are low-porosity/low-permeability rocks. The site has a Ca-Na-Cl saline water associated with the mica gneisses and calc silicate rocks (felsic) residing above a more dense Na-Ca-Cl brine with elevated magnesium concentrations. The stratigraphically lower, magnesium-rich brine... 

There are, however, two main reasons why this must be a minimum estimate of the amount of sediment recycled into the mantle over geological time. These are, first, reintroduction of CO2 into the atmosphere via arc volcanism, and second, the fact that purely clastic sediments are ignored in this mass balance. CO2 is reintroduced into the atmosphere as a result of decarbona-tion reactions in calc-silicate rocks (the reverse Urey reaction ), requiring further weathering and photosynthesis to remove it. Fluxes associated with these processes over Phanerozoic time have been reviewed by Berner et al. (1983) and Berner (1991), who concluded that such addition of CO2 by (mainly arc) volcanism and its drawdown by silicate weathering have been the major long-term fluxes over this period, and that drawdown has only slightly outstripped... 

Nabelek PI, Labotka TC, Russ-Nabelek C (1992) Stable isotope evidence for the role of diffusion, infiltration, and local stracture on contact metamorphism of calc-silicate rocks at Notch Peak, Utah. J Petrol 33 557-583... 

The literature data presented in this chapter represent a wide variety of metamorphic rock types including metabasites, metapelites, marbles, calc-silicate rocks, ultramafic rocks, and metagranites. Hydrothermal or low-temperature apatite occurrences were not considered for this chapter. 

It is in Aphebian metasediments of the Wollaston fold belt, along the eastern erosional edge of the unconformably over-lying Helikean Athabasca Formation. The metasediments include meta-arkose, biotite paragneiss, calc-silicate rocks and marble. The Athabasca is a series of red to brown quartz sandstones and conglomerates. 

The Hackett River VMS deposits are hosted within the Ignerit Formation and are part Yellowknife Supergroup within the Slave Craton. The Ignerit Formation consists of highly silicified felsic volcanic rocks of calc-alkaline affinity that are intercalated with discontinuous lenses of calc-silicate that are interpreted to be tuffs... 

This is divided into a western, a central and an eastern sector. The western Aeolian arc (Alicudi, Filicudi, Salina) consists of calc-alkaline rocks with typical island arc signatures. Mafic and intermediate rocks dominate the volcanic sequence, with minor silicic volcanics. The central islands (Vul-cano and Lipari) are dominated by calc-alkaline to shoshonitic mafic to silicic rocks mafic rocks from this sector show isotopic compositions and incompatible trace element ratios similar to the western islands. The eastern arc (Panarea and Stromboli) consists of calc-alkaline to potassic alkaline rocks. Stromboli shows geochemical and isotopic signatures akin to the Neapolitan volcanoes. The Island of Panarea, located between Stromboli and Lipari, has intermediate characteristics between these two volcanoes (Calanchi et al. 2002a). 

Fig. 2.7. Mantle normalised incompatible element patterns of representative Tuscany mafic rocks (A, B) and for mafic enclaves hosted by silicic rocks (C). The field of Aeolian arc calc-alkaline and shoshonitic mafic rocks is shown for comparison.

REE patterns are fractionated silicic rocks contain negative Eu anomalies (Fig. 7.14a), which are much smaller than observed for the Lipari and Vulcano rhyolites. Mantle normalised incompatible element patterns of mafic rocks show high LILE/HFSE ratios and a positive anomaly of Pb a small positive Sr spike is observed in the calc-alkaline basaltic andesites (Fig. 7.14b). HKCA and shoshonitic rocks have higher incompatible element abundances than the associated CA products. 

Figure 1. Lowering of 5 0 by batch decarbonation (straight line) and Rayleigh decarbonation (curves). F is the mole fraction of oxygen remaining in the rock. Note that for Rayleigh decarbonation, tends toward -1000%o if all oxygen is volatilized, but that a calc-silicate limit exists such that F > 0.6 for most metamorphic reactions. There is little difference between the results of the batch and Rayleigh models above F = 0.6 (from Valley 1986).

Figure 2. Plot of the coupled vs. 5 0 trends that result from batch (straight lines) and Rayleigh (curves) volatilization assuming normal calc-silicate decarbonation (see Fig. 3). Average fractionation factors are appropriate for metamorphic temperatures a C(C02-rock) = 1.0022 and a 0(COb-rock) =...

Figure 3 illustrates different relations of Foxygen to Fcarbon, which depend on stoichiometry, and kinetics of reaction. The normal calc-silicate decarbonation trend applies if all minerals in the rock are fully equilibrated during a reaction such as 4 or 5, in which case all carbon in the rock is liberated as CO2 (Fcarbon 0), while only 40% of the oxygen is released (Foxygen 0.6). However, if the rock has (for example) 50% excess silicates that are not involved in the reaction, but which still equilibrate isotopically, then F xy 0.8 as Fcarbon 0.0 along the 50% inert oxygen trend and the amount of Or°0 depletion will be smaller. Likewise, if 50% of a rock s carbon does not participate due to stoichiometric excess, a 50% inert carbon trend will be followed and the depletion of will also be less. In practice any trend is possible from -100%... 

In many contact metamorphic aureoles, values of 5 0 and 5 C vary systematically. The coupled 0-C trend is towards lower and values at higher metamorphic grade approaching the igneous contact. Thus, 5-values decrease progressively as metamorphism increases. This trend may also correlate with bleaching of the rock, coarsening of calcite and the development of calc-silicate minerals. This may be the result of one or more volatilization reactions, variable temperature, or fluid infiltration. 

The geologic history of the Herbert Mountains and of nearby Mt. Sheffield was investigated by Hofmann et al. (1980, 1981), Hofmann and Paech (1983), and Brommer and Henjes-Kunst (1999). The rocks include migmatite and pyroxene-biotite gneisses, gamet-kyan-ite schists as well as minor quartzite, marble, and calc-silicates. Mount Sheffield in the northern Herbert Mountains (Fig. 8.18) even contains post-tectonic basalt that may be related to the J urassic Ferrar Dolerite (Brommer and Henjes-Kunst 1999). 

The Tuscany Magmatic Province consists of an association of calc-alkaline to lamproitic mafic to intermediate magmas and silicic intrusive and effusive rocks. Silicic melts have been formed by crustal melting, with an important role of mixing with mantle-derived magmas. Mafic melts are of mantle origin but resemble closely some upper crustal rocks, such as metapelites, in terms of incompatible trace elements and radiogenic isotope... 

Rock compositions range from mafic to silicic, and show a calc-alkaline (CA), high-potassium calc-alkaline (HKCA) to shoshonitic (SHO) affinity. A few potassic alkaline rocks with a composition close to the Roman potassic series (KS) occur at Vulcano and Stromboli (e.g. Keller 1982 Francalanci et al. 2004). Arc tholeiites have been dredged along some seamounts (Beccaluva et al. 1982). 

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