Bruce W. Chappell

A-type granites: geochemical characteristics, discrimination and petrogenesis

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.

Nature and origin of A-type granites with particular reference to southeastern Australia

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.

I- and S-type granites in the Lachlan Fold Belt

Granites and related volcanic rocks of the Lachlan Fold Belt can be grouped into suites using chemical and petrographic data. The distinctive characteristics of suites reflect source-rock features. The first-order subdivision within the suites is between those derived from igneous and from sedimentary source rocks, the I- and S-types. Differences between the two types of source rocks and their derived granites are due to the sedimentary source material having been previously weathered at the Earths surface. Chemically, the S-type granites are lower in Na, Ca, Sr and Fe 3+ /Fe 2+ , and higher in Cr and Ni. As a consequence, the S-types are always peraluminous and contain Al-rich minerals. A little over 50% of the I-type granites are metaluminous and these more mafic rocks contain hornblende. In the absence of associated mafic rocks, the more felsic and slightly peraluminous I-type granites may be difficult to distinguish from felsic S-type granites. This overlap in composition is to be expected and results from the restricted chemical composition of the lowest temperature felsic melts. The compositions of more mafic I- and S-type granites diverge, as a result of the incorporation of more mafic components from the source, either as restite or a component of higher temperature melt. There is no overlap in composition between the most mafic I- and S-type granites, whose compositions are closest to those of their respective source rocks. Likewise, the enclaves present in the more mafic granites have compositions reflecting those of their host rocks, and probably in most cases, the source rocks. S-type granites have higher δ 18 O values and more evolved Sr and Nd isotopic compositions, although the radiogenic isotope compositions overlap with I-types. Although the isotopic compositions lie close to a mixing curve, it is thought that the amount of mixing in the source rocks was restricted, and occurred prior to partial melting. I-type granites are thought to have been derived from deep crust formed by underplating and thus are infracrustal, in contrast to the supracrustal S-type source rocks. Crystallisation of feldspars from felsic granite melts leads to distinctive changes in the trace element compositions of more evolved I- and S-type granites. Most notably, P increases in abundance with fractionation of crystals from the more strongly peraluminous S-type felsic melts, while it decreases in abundance in the analogous, but weakly peraluminous, I-type melts.

Ultrametamorphism and granitoid genesis

Abstract A model is presented to explain the geochemical and mineralogical characteristics of granitoids and their inclusions. The product of ultrametamorphism is melt + residuum, both of which may move en masse to the site of crystallization. The nature of the source material can be deduced from studies on the granitoids and their inclusions; based on studies of the Lachlan belt of southeastern Australia we recognize granitoids derived from metasedimentary rocks (S-types) and those derived from igneous source rocks (I-types). The straight-line variation diagrams of most granitoid suites is explained by progressive separation of residuum (= restite) and melt. It is shown that some granitoid suites consist of minimum melt + residuum whereas others represent the crystallization of “nonminimum” melts + residuum. Residuum is recognized as metasedimentary xenoliths in S-types and as mafic hornblende-rich xenoliths in I-types. Xenocrystal material is more difficult to recognize petrographically. We suggest that the complexly zoned and twinned plagioclases so characteristic of orogenic rocks are modified residuum. These and xenoliths are absent in granitoids which have largely crystallized from a melt such as those of the Tuolumne Suite of the Sierra Nevada. Mafic minerals of granitoids whether residuum or crystallization products are mostly equilibrium assemblages. Relict mafic phases from the source do not persist and hence P-T conditions of magma generation cannot be deduced.

Carbonatite metasomatism in the northern Tanzanian mantle: petrographic and geochemical characteristics

Peridotite xenoliths from the Olmani cinder cone, northern Tanzania, possess distinctive mineralogical and chemical features interpreted to result from interaction of ultra-refractory peridotite residues with carbonatite melts. Chief among these are as follows: (1) The presence of unusually low Al2O3 clinopyroxene and the lack enstatite in refractory dunites (with olivines up to Fo94). (2) The presence of monazite and F-rich apatite in refractory harzburgite and wehrlite xenoliths, respectively. (3) LREE enrichment and strong Ti depletion relative to Eu in all but one peridotite. Ca/Al ratios of the clinopyroxene-bearing dunites are some of the highest ever measured for peridotites (up to 10.8, relative to chondritic Ca/Al of 1.1). In addition, Zr/Hf and Ca/Scratios correlate positively, increasing with greater influence of carbonatite on the whole rock. (4) Clinopyroxene-bearing samples have a restricted range of isotopic compositions (eNd = +3.1to+3.9, 87Sr/86Sr= 0.7034to0.7035), whereas the monazite-bearing harzburgite has lower eNd (+0.8) at similar 87Sr/86Sr. The isotopic compositions of the former are similar to young, isotopically primitive east African carbonatites and basalts, suggesting the metasomatism occurred recently and that the carbonatites responsible for the metasomatism were ultimately derived from the asthenosphere. Inferred trace element signatures of carbonatite melts responsible for modal metasomatism of these and other peridotites include: high La/Yb, Nb/La and Ca/Al, very high Zr/Hf and very low Ti/Eu; features similar to those of erupted carbonatites and consistent with partition coefficients for silicates in equilibrium with carbonatite melts. These trace element systematics are used to illustrate that many LREE-enriched spinel peridotite xenoliths may have been affected by addition of very small amounts (⩽2%) of carbonatite. Such metasomatism will have small effects on major element compositions of peridotites (e.g., Mg#, Ca/Al) relative to peridotites from the literature can be modeled as mixtures between carbonatite and refractory peridotite, the modally metasomatized Olmani peridotites and SE Australian wehrlites require an open-system style of metasomatism, perhaps due to higher proportions of carbonatite melt.

Two contrasting granite types: 25 years later

The concept of I‐ and S‐type granites was introduced in 1974 to account for the observation that, apart from the most felsic rocks, the granites in the Lachlan Fold Belt have properties that generally fall into two distinct groups. This has been interpreted to result from derivation by partial melting of two kinds of source rocks, namely sedimentary and older igneous rocks. The original publication on these two granite types is reprinted and reviewed in the light of 25 years of continuing study into these granites.

Aluminium saturation in I- and S-type granites and the characterization of fractionated haplogranites

Abstract Granites of the Lachlan Fold Belt resulted from partial melting of the crust. In most cases, fusion involved mainly quartz and feldspar, producing felsic melts. Varying degrees of separation of those melts from the unmelted source rock (restite) were responsible for much of the compositional variation seen in the granites of the belt. Less commonly, melting occurred at higher temperatures forming more mafic melts, such as for the I-type Boggy Plain Supersuite and the S-type Koetong Suite. Hence, the felsic haplogranites of the Lachlan belt dominantly formed initially as primary melts that separated from restite and less often by the fractionation of more mafic melts. Source rocks of the I- and S-type granites were undersaturated or oversaturated in Al, respectively, and the more mafic granites share that characteristic with their source. As the magmas of the Boggy Plain Supersuite evolved progressively by fractional crystallization, the rocks trended towards saturation in Al, to eventually form a mode close to Al saturation. Other felsic I-type magmas, formed directly by partial melting, were generally more oversaturated in Al, as were the corresponding S-type melts derived from peraluminous source rocks. In an unfractionated state, there are some overlaps in the degree of Al saturation in these magmas produced by partial melting. However, when extended fractional crystallization of these felsic partial melts took place, an almost complete separation in Al-saturation developed between I-type and more peraluminous S-type melts. Because apatite is soluble in peraluminous melts, P became progressively more abundant in the S-type melts as they fractionated. This led to contrasts in the abundances of P and of elements such as Y, the rare earth elements, and Th, between the strongly fractionated I- and S-type granites. Hence, such granites can easily be distinguished from each other.

Geochemical and isotopic systematics in carbonatites and implications for the evolution of ocean-island sources

Geochemical and Sr, Nd, Pb, O and C isotopic data are reported for 13 carbonatites from Africa, Australia, Brazil, Europe and the United States. The carbonatites possess generally high Ba, Th, LREE, Sr and low Cs, Rb, K and HREE abundances. Some examples have low Ti, Nb, Ta, P, Zr, Hf and U concentrations which are consistent with variable fractionation of sphene, apatite, perovskite, monazite or zircon. The samples range in age from Proterozoic to Tertiary and possess a range of initial Sr isotopic compositions between 0.7020 and 0.7054, initial ϵNd values of −0.4 to +3.8 and (with the exception of the Brazilian Jacupiranga carbonatite) generally radiogenic initial Pb isotopic compositions. δ18OSMOW compositions of the intrusive carbonatites range from +5.5 to +12.4‰ Higher δ18OSMOW values of +14 and +17%. are found in the volcanically-emplaced Proterozoic Goudini complex of South Africa, suggesting the involvement of secondary alteration processes. δ13CPDB ranges from −0.5 to −6.6‰ with samples having near-primary δ18OSMOW (between +5.5 and +8%.) possessing δ13CPDB between −2.9 to −6.6‰. On the initial Sr-Nd isotope diagram, most carbonatites plot below the mantle array and below or within the field of many ocean-island basalts. The Pb isotopic compositions of carbonatites generally lie along the array defined by oceanic basalts. The characteristics of carbonatites from a number of continents and their isotopic similarity to some ocean-island basalts favour an asthenospheric mantle “plume” origin. This conclusion suggests that some ocean-island alkali basalts may have been derived from trace-element-depleted mantle sources which have been re-fertilised by low-viscosity, trace-element-rich carbonatitic melts. The common close spatial and temporal association and the overlap in trace-element geochemistry and isotopic characteristics of Group 1 (basaltic) kimberlites and carbonatites argues strongly for a genetic relationship. Although late-stage melt/vapour fractionation may play some role, the extreme LREE-enrichment typical of carbonatites requires their derivation by small degrees of melting (< ≈ 1%) from a garnet-rich eclogitic source. This source may originally have been CO2- and volatile-rich subducted oceanic lithosphere.

Nd isotopic characteristics of S- and I-type granites

The initial Nd and Sr isotopic composition has been determined in S- and I-type granites from the Paleozoic Berridale and Kosciusko Batholiths of southeast Australia. The Nd and Sr isotopic variations form a strongly covariant array with S-types granites having a relatively restricted range ineNd values from −6.1 to −9.8 but a large range in initial87Sr86Sr of from 0.7094 to 0.7184. These characteristics are indicative of an∼1400-m.y. sedimentary or metasedimentary source for S-types. I-types have variable initial Nd ranging from +0.4 to −8.9, and a more limited range in initial87Sr86Sr of from 0.70453 to 0.7119. These isotopic characteristics are consistent with a two-component mixing model whereby a depleted mantle-like component (DMC) witheNd = +6 and87Sr86Sr= 0.703, is mixed with a crustal component (CC) havingeNd = −9 and87Sr86Sr= 0.720. Although this two-component mixing model satisfies the isotopic constraints the source rock chemistry of the I-types is not compatible with the large proportion (up to 50%) of sedimentary material implied by the isotopic data. This indicates that more than two components are required to account for both the isotopic and chemical data. Both the chemical and isotopic data are consistent with I-type granites having been formed from source rocks of predominantly mantle derivation and obtained progressively from the mantle over a period of 1000 m.y. prior to granite formation.

Crystallization, fractionation, and solidification of the Tuolumne Intrusive Series, Yosemite National Park, California

Study of the Tuolumne Intrusive Series, a concentric texturally and compositionally zoned plutonic sequence in the eastern part of Yosemite National Park, was undertaken to develop and test a model for the origin of comagmatic plutonic sequences in the Sierra Nevada batholith. The granitoid units that make up the sequence are progressively younger and more felsic inward. The bulk of the rocks are granodiorite, but the outermost formation is quartz diorite, and the innermost one is granite porphyry. The compositional gradient changes both gradually within formations and abruptly between them. The change is greatest in the outer 1 km and lower toward the center of the sequence. Hornblende and biotite, abundant in the marginal rocks, decrease rapidly inward for 1 km as K-feldspar and quartz increase, but farther inward, they decrease slowly. The most conspicuous chemical changes are shown by the elements that are enriched in the mafic minerals. The compositional zoning indicates that with decreasing temperature, the sequence solidified from the margins inward. Solidification was interrupted repeatedly by surges of fluid core magma. The magma eroded the adjacent solidifying rock, and it expanded the area of the magma chamber at the exposed level by crowding the wall and roof rocks outward and upward and by breaking through the solidifying carapace into the wall rocks. The compositional zonation resulted from crystal fractionation that could have involved (1) preferential accretion of crystalline material present in the magma to the margins of the magma chamber, thus displacing the melt phase progressively inward, and/or (2) downward settling of crystals, probably accompanied by upward movement of melt and volatiles; the residual magma solidifying to form the granitoids. Although either mechanism can explain the observed relations, they lead to very different interpretations of the composition of the magma when the first exposed granitoids solidified at the margins of the magma chamber and as the sequence solidified inward.