A-type granites: geochemical characteristics, discrimination and petrogenesis
TL;DR: A-type granites as mentioned in this paper were found to have high SiO2, Na2O+K2O, Fe/Mg, Ga/Al, Zr, Nb, Ga, Y and Ce, and low CaO and Sr.
Abstract: New analyses of 131 samples of A-type (alkaline or anorogenic) granites substantiate previously recognized chemical features, namely high SiO2, Na2O+K2O, Fe/Mg, Ga/Al, Zr, Nb, Ga, Y and Ce, and low CaO and Sr. Good discrimination can be obtained between A-type granites and most orogenic granites (M-, I and S-types) on plots employing Ga/Al, various major element ratios and Y, Ce, Nb and Zr. These discrimination diagrams are thought to be relatively insensitive to moderate degrees of alteration. A-type granites generally do not exhibit evidence of being strongly differentiated, and within individual suites can show a transition from strongly alkaline varieties toward subalkaline compositions. Highly fractionated, felsic I- and S-type granites can have Ga/Al ratios and some major and trace element values which overlap those of typical A-type granites. A-type granites probably result mainly from partial melting of F and/or Cl enriched dry, granulitic residue remaining in the lower crust after extraction of an orogenic granite. Such melts are only moderately and locally modified by metasomatism or crystal fractionation. A-type melts occurred world-wide throughout geological time in a variety of tectonic settings and do not necessarily indicate an anorogenic or rifting environment.
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TL;DR: In this paper, a new calculation of the crustal composition is based on the proportions of upper crust (UC) to felsic lower crust (FLC) to mafic lower-crust (MLC) of about 1.6:0.4.
5,317 citations
TL;DR: Barbarin et al. as mentioned in this paper used the modified alkali-lime index (MALI) and the aluminum saturation (ASI) for the classification of caledonian post-orogenic granites.
Abstract: This geochemical classification of granitic rocks is based upon three INTRODUCTION variables. These are FeO/(FeO + MgO) = Fe-number [or Although granitoids are the most abundant rock types FeO/(FeO + MgO) = Fe∗], the modified alkali–lime index in the continental crust, no single classification scheme (MALI) (Na2O + K2O – CaO) and the aluminum saturation has achieved widespread use. Part of the problem in index (ASI) [Al/(Ca – 1·67P + Na + K)]. The Fe-number granite classification is that the same mineral assemblage, (or Fe∗) distinguishes ferroan granitoids, which manifest strong iron quartz and feldspars with a variety of ferromagnesian enrichment, from magnesian granitoids, which do not. The ferroan minerals, can be achieved by a number of processes. and magnesian granitoids can further be classified into alkalic, Granitoids can form from differentiation of any hyalkali–calcic, calc-alkalic, and calcic on the basis of the MALI persthene-normative melt and from partial melting of and subdivided on the basis of the ASI into peraluminous, metamany rock types. Furthermore, granitic melts may be luminous or peralkaline. Because alkalic rocks are not likely to be derived solely from crustal components, may form from peraluminous and calcic and calc-alkalic rocks are not likely to be evolved mantle-derived melts, or may be a mixture peralkaline, this classification leads to 16 possible groups of granitic of crustal and mantle-derived melts. Because of this rocks. In this classification most Cordilleran granitoids are magnesian complexity, petrologists have relied upon geochemical and calc-alkalic or calcic; both metaluminous and peraluminous classifications to distinguish between various types of types are present. A-type granitoids are ferroan alkali–calcic, although granitoids. Approximately 20 different schemes have evolved over the past 30 years [see Barbarin (1990, 1999) some are ferroan alkalic. Most are metaluminous although some are for a summary thereof]. Most of these schemes are either peraluminous. Caledonian post-orogenic granites are predominantly genetic or tectonic in nature. This paper is an attempt magnesian alkali–calcic. Those with <70 wt % SiO2 are domto present a non-genetic, non-tectonic geochemical clasinantly metaluminous, whereas more silica-rich varieties are comsification scheme that incorporates the best qualities of monly peraluminous. Peraluminous leucogranites may be either the previous schemes, and to explain the petrologic magnesian or ferroan and have a MALI that ranges from calcic to processes that makes this scheme work. alkalic.
3,135 citations
Cites background from "A-type granites: geochemical charac..."
...Further- extension (Whalen et al., 1987; Eby, 1990; Frost & Frost, more, the classification cannot address the presence or 1997). absence of minor phases, such as muscovite, which may Other alphabetic types of granitoid that have been carry significant petrologic implications....
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TL;DR: The average chemical composition of the upper continental crust (UC) as a function of age is estimated from chemical analyses, geologic maps, stratigraphic sections and isotopic ages as discussed by the authors.
1,916 citations
TL;DR: The A-type granitoid suites vary in composition from quartz syenites to peralkaline granites and their respective volcanic equivalents as mentioned in this paper, and the A-types are characterized by their relatively high alkali contents and low CaO contents.
1,282 citations
TL;DR: Granitoids are divided into several types according to their mineral assemblages, their field and petrographical features, and their chemical and isotopic characteristics.
1,230 citations
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...Chevremont, 1987; Whalen et al., 1987; Maniar and Piccoli, 1989 ....
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TL;DR: In this article, a data bank containing over 600 high quality trace element analyses of granites from known settings was used to demonstrate using ORG-normalized geochemical patterns and element-SiO2 plots that most of these granite groups exhibit distinctive trace element characteristics.
Abstract: Granites may be subdivided according to their intrusive settings into four main groups—ocean ridge granites (ORG), volcanic arc granites (VAG), within plate granites (WPG) and collision granites (COLG)—and the granites within each group may be further subdivided according to their precise settings and petrological characteristics. Using a data bank containing over 600 high quality trace element analyses of granites from known settings, it can be demonstrated using ORG-normalized geochemical patterns and element-SiO2 plots that most of these granite groups exhibit distinctive trace element characteristics. Discrimination of ORG, VAG, WPG and syn-COLG is most effective in Rb-Y-Nb and Rb-Yb-Ta space, particularly on projections of Y-Nb, Yb-Ta, Rb-(Y + Nb) and Rb—(Yb + Ta). Discrimination boundaries, though drawn empirically, can be shown by geochemical modelling to have a theoretical basis in the different petrogenetic histories of the various granite groups. Post-collision granites present the main problem of tectonic classification, since their characteristics depend on the thickness and composition of the lithosphere involved in the collision event and on the precise timing and location of magmatism. Provided they are coupled with a consideration of geological constraints, however, studies of trace element compositions in granites can clearly help in theelucidation of post-Archaean tectonic settings.
7,144 citations
"A-type granites: geochemical charac..." refers background in this paper
...A quite different type of granite classification can be constructed by statistical analyses of large numbers of chemical analyses of granites from well defined tectonic settings (Pearce et al. 1984; Brown et al. 1984)....
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TL;DR: In this paper, the results of analyses for Ti, Zr, Y, Nb and Sr in over 200 basaltic rocks from different tectonic settings have been used to construct diagrams in which these settings can usually be identified.
3,403 citations
TL;DR: In the Lachlan Fold Belt of southeastern Australia, Upper Devonian A-type granite suites were emplaced after the Lower Devonian I-type granites of the Bega Batholith as mentioned in this paper.
Abstract: In the Lachlan Fold Belt of southeastern Australia, Upper Devonian A-type granite suites were emplaced after the Lower Devonian I-type granites of the Bega Batholith. Individual plutons of two A-type suites are homogeneous and the granites are characterized by late interstitial annite. Chemically they are distinguished from I-type granites with similar SiO2 contents of the Bega Batholith, by higher abundances of large highly charged cations such as Nb, Ga, Y, and the REE and lower Al, Mg and Ca: high Ga/Al is diagnostic. These A-type suites are metaluminous, but peralkaline and peraluminous A-type granites also occur in Australia and elsewhere. Partial melting of felsic granulite is the preferred genetic model. This source rock is the residue remaining in the lower crust after production of a previous granite. High temperature, vapour-absent melting of the granulitic source generates a low viscosity, relatively anhydrous melt containing F and possibly Cl. The framework structure of this melt is considerably distorted by the presence of these dissolved halides allowing the large highly charged cations to form stable high co-ordination structures. The high concentration of Zr and probably other elements such as the REE in peralkaline or near peralkaline A-type melts is a result of the counter ion effect where excess alkali cations stabilize structures in the melt such as alkali-zircono-silicates. The melt structure determines the trace element composition of the granite. Separation of a fluid phase from an A-type magma results in destabilization of co-ordination complexes and in the formation of rare-metal deposits commonly associated with fluorite. At this stage the role of Cl in metal transport is considered more important than F.
1,847 citations
TL;DR: In this paper, a genetic classification of zircon populations from granitic rocks is proposed, with three main divisions: (1) granites of crustal or mainly crustal origin (sub) autochthonous and aluminous granites)]; (2) hybrid granites (calc-alkaline and sub-alkalinized series granites); (3) granite of mantle or mainly mantle origin (alkaline, tholeiitic series granite).
Abstract: The typologic study of zircon populations from granitic rocks lead to the proposition of a genetic classification with three main divisions: (1) granites of crustal or mainly crustal origin [(sub) autochthonous and aluminous granites)]; (2) granites of crustal+mantle origin, hybrid granites (calc-alkaline and sub-alkaline series granites); (3) granites of mantle or mainly mantle origin (alkaline and tholeiitic series granites). In detail, there are many petrogenetic variants of each of the following granitic rocks: granodiorite, monzogranite and alkaline granite. The variations observed with zircon typology are accompanied petrographically by modifications of associations of other main and accessory minerals, and on the field by the presence or absence of basic microgranular xenoliths, associated microgranites, rhyolites or basic rocks. In the typologic diagram, some endogenous non granitic rocks (i.e. migmatites, tonalites, rhyolites ...) show a logical distribution with regard to different genetic stocks of granitic rocks.
1,114 citations
"A-type granites: geochemical charac..." refers methods in this paper
...In the granite classification based on zircon morphology (Pupin 1980), the A-type granites of the Topsails terrane fall in the alkaline and hyperalkaline syenites and granites field (Whalen et al....
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