A-type Granite Suites Research Articles

On the enigmatic mid-Proterozoic: Single-lid versus plate tectonics

The mid-Proterozoic (ca. 1850–850 Ma) is a peculiar period of Earth history in many respects: ophiolites and passive margins of this age are rare, whereas anorthosite and A-type granite suites are abundant; metamorphic rocks typically record high thermobaric (temperature/pressure) ratios, whereas ultrahigh pressure (UHP) rocks are rare; and the abundance of economic mineral deposits features rare porphyry Cu-Au and abundant Ni-Cu and Fe-oxide Cu-Ag (IOCG) deposit types. These collective observations have been used to propose that a stagnant-lid, or single-lid, tectonic regime operated at this time, between periods of plate tectonics in the Paleoproterozoic and Neoproterozoic. In our reappraisal of the mid-Proterozoic geological record, we not only assess the viability of the single-lid hypothesis for each line of evidence, but also that of the plate tectonic alternative. We find that evidence for the single-lid hypothesis is equivocal in all cases, whereas for plate tectonics the evidence is equivocal or supporting. We therefore find no reason to abandon a plate tectonic model for the mid-Proterozoic time period. Instead, we propose that the peculiarities of this enigmatic interval can be reconciled through the combination of two processes working in tandem: secular mantle cooling and the exceptionally long tenure and incomplete breakup of Earth's first supercontinent, where both of these phenomena had a dramatic effect on lithospheric behaviour and its resulting imprint in the geological record.

Oblique continental collision and the formation of syn-collisional A-type granites: insights from the Early Jurassic Baoji granite suite in the Qinling orogenic belt, central China

ABSTRACT A-type granites are conventionally interpreted to be formed in late post-collisional or anorogenic settings. However, we firstly identified an Early Jurassic syn-collisional A-type granite suite in the Qinling orogenic belt: the Baoji granite suite. A comprehensive investigation into its mineralogical, petrological, geochronological, and geochemical characteristics can shed light on the genesis of syn-collisional A-type granites and geodynamic evolution of collisional orogeny. The Baoji granite suite contains two types of rocks: (1) A-type granites including biotite syenite, coarse-grained, porphyritic, and fine-grained syenogranites, and alkali-feldspar granite, and (2) magnesian monzodiorite; both types are coeval, with zircon U–Pb ages of ca. 190 Ma. The A-type granites yield A1-type trace element signatures and large variations in trace element contents, with Ba contents of 16.9–762 ppm, Sr of 11.0–211 ppm, Rb of 193–645 ppm, and Eu of 0.08–1.24 ppm. Their radiogenic isotope compositions (87Sr/86Sri = 0.70631–0.70637; εNd = −5.18 to −3.84; εHf = −5.01 to +2.76) are similar, and overlap those of Neoproterozoic OIB-like basic volcanics from the South Qinling belt. These characteristics show that the A-type granites originated from the anatexis of OIB-like lower crust and underwent fractional crystallization in a silicic magma reservoir. The monzodiorite has low SiO2 contents (49.82–53.29 wt.%) and enriched radiogenic isotopic compositions (87Sr/86Sri = 0.70554–0.70638; εNd = −11.0 to −5.28) contains abundant hornblende and biotite (~40%), and exhibits evidence of magma mingling. It also contains two groups of zircon: pristine and unzoned zircon that yields negative Hf isotope compositions (εHf = −9.42 to −3.14), and xenocrystic zircon that yields similar Hf isotope compositions (εHf = −3.54 to +1.85) to the A-type granites. These characteristics demonstrate that the monzodiorite was derived from the differentiation of basaltic partial melts of an enriched mantle source, and mingled with the A-type granitic magma during its ascent and emplacement into a silicic magma reservoir. Along with regional records of the Early Jurassic magmatism, deformation, metamorphism, and sedimentation, the genetic models for the Baoji A-type granites and monzodiorite suggest that the Middle Triassic to Early Jurassic oblique continental collision (ca. 230–190 Ma) produced coupled EW-trending strike-slip and NE-trending extensional faults at crustal levels and Rayleigh–Taylor (gravitational) convective instabilities at mantle depths in the Qinling orogenic belt, which controlled the production of the syn-collisional A-type granite suite. Our work also indicates that the A-type granites can be formed in syn-collisional setting in response to oblique continental collision and the resultant slow foundering of a high-density lithospheric root; therefore, we suggest that the syn-collisional A-type granites are evidence for oblique continental collision.

Crystallization ages of Paleoproterozoic A-type granites of Carajás province, Amazon craton: Constraints from U-Pb geochronology of zircon and titanite

New U-Pb SHRIMP dating in zircon and titanite showed that the three large Paleoproterozoic A-type granite Suites and related granites from Carajás Province of the Amazon Craton (Brazil) were emplaced between 1880 Ma and 1857 Ma, with the main magmatic peak at 1880 Ma. Some particular ages not reported before have been identified: (i) ages of 1865 Ma to 1857 Ma in leucogranite facies of the Redenção and Bannach plutons, that are younger, were generated by independent magma pulses and are not cogenetic with the dominant ∼1880 Ma old granites of the respective plutons; (ii) an age of 1732 ± 6 Ma obtained in the leucogranite facies of the Antônio Vicente pluton of the Velho Guilherme Suite that could represent a magmatic event in the Xingu Region not yet reported or, eventually, could correspond to an isolate hydrothermal event that allowed the growth of zircons. The more detailed geochronological data obtained in the Paleoproteozoic A-type granites of Carajás Province, added to the information available in the literature, demonstrate the relevance of the magmatic event at ∼1880 Ma and indicate that the emplacement of those granites was done in about ∼20 m.y. that means in a relatively short geological time. This 1880 Ma magmatic episode was also intense in other provinces of the Amazon Craton, where it is linked mostly to the formation of the Uatumã Silicic Large Igneous Province. This event was mostly extensional and isalso registered in several cratons worldwide demonstrating its relevance in the tectonic evolution of Proterozoic continents.

Petrochemical evolution of the White Mfolozi Granite pluton: Evidence for a late Palaeoarchaean A-type granite from the SE Kaapvaal Craton, South Africa

One of the major limitations in understanding the geochemical evolution of the Kaapvaal Craton, South Africa, is the scarcity of whole rock trace element data of the granitoid and other rocks compared to the vastness of this cratonic block. Here we present new XRF major oxide and ICP-MS trace element analyses of the White Mfolozi Granitoid (WMG) pluton, SE Kaapvaal Craton, which suggest that the ~3.25Ga (U–Pb zircon age) old WMG pluton is a peraluminous A-type granite and could be equivalent to the intrusive potassic granite phase of the Anhalt Granitoid suite, occurring to the North of the WMG pluton. The pluton was generated by batch partial melting of a pre-existing TTG source in two major phases under relatively anhydrous conditions, and the heat of partial melting could have been provided by a voluminous mantle-derived mafic magma, which intruded into mid-crustal levels (c. 17km), perhaps during a period of crustal extension. The estimated pressure and temperature of generation of the WMG parent magma with average molar [or/(or+ab)] ~0.48 could be ~500MPa and close to 1000°C, respectively, when compared with the results of experimental petrology. Interstitial occurrence of relatively iron-rich biotite [Mg/(Mg+Fe) ~0.41–0.45] suggests that the final temperature of crystallization of the pluton was close to 800°C. An important magmatic event following the main phase of partial melting was limited mixing between the intrusive mafic magma and co-existing newly generated granitic melt. This magma mixing resulted in distinct variations in SiO2 and a low initial Sr isotopic ratio (0.7013) of the WMG pluton. Although both the models of partial melting of quartzo-feldspathic sources and fractional crystallization of basaltic magmas with or without crustal assimilation have been proposed for the origin of A-type granites, the model of magmatic evolution of the WMG pluton presented here can also be an alternative model for the generation of A-type granites. In this model, post-partial melting magma mixing is perhaps critical in explaining the Daly gap in composition and extreme variations in chemical (e.g., SiO2) and isotopic compositions observed in many bimodal A-type granite suites. The emplacement of the oldest known A-type granitoid suite in the Kaapvaal Craton, the WMG pluton, marks a period of stabilization of the craton before erosion and deposition of the overlying volcano-sedimentary succession of the Pongola Supergroup.

GRANÁTICKÉ PÁSKOVANÉ ANOROGENNÍ GRANITY A PEGMATITY SUITY HLÍNA V BRUNOVISTULIKU – TEXTURY A PLOŠNÉ ROZŠÍŘENÍ

The SW part of the Brno Batholith as well as NE part of the Thaya batholith have undergone recent mapping focused on layered granite dykes belong to the Hlína A-type granite suite. Dykes cropping out throughout the both other granitic suites in the region (Réna I-type and Tetčice S-type) widespread more than 60 km in N–S direction. Structures of the Hlína granites are easily distinguishable by their layering parallel to the contact, alternating of major aplite-like zones and minor pegmatite-like zones with oriented UST comb-like Qtz, Pland Kfs. The lamination of the rock is pronounced by parallel stripe-like arrangement of the garnet. The purpose of text below is highlighted the volumetrically scarce but genetically very important part of the Brunovistulian unit.

Petrological and Geochemical Constraints in the Origin and Associated Mineralization of A-Type Granite Suite of the Dhiran Area, Northwestern Peninsular India

Neoproterzoic, anorogenic, A-type granites of Dhiran area of Malani Igneous Suites is made up wholly of peralkaline-peraluminous alkali feldspar granites and is composed of K-feldspar, quartz, alkali amphibole, plagioclase, biotite and accessory minerals are iron oxides, monozite, zircon, apatite, and annite. This granitoid is associated with acid volcanic and minor basic volcanic-plutonic and dykes. Petrographically, they show cloudy, patchy perthitic texture and graphic texture. They are highly evolved ferroan, alkaline, A-type granites, displaying the typical geochemical characteristics of A-type granites with high SiO2, Na2O + K2O, FeO*/MgO, Ga/Al, Zr, Nb, Ga, Y, Ce and rare earth elements (REE) and low CaO, MgO, Ba and Sr. Their trace and REE characteristics along with the use of various discrimination schemes revealed their correspondence to magmas derived from crustal origin. The extremely high Rb/Sr ratios combined with the obvious Sr, Ba, P, Ti and Eu depletions clearly indicate that these A-type granites were highly evolved and require advanced fractional crystallization in upper crustal conditions. These granites are high to medium content of radioactive element (U, Th, K) together Heat generation and Heat Production (HP). Leucocratic granites show higher Heat Production (4.84-8.75 µWm-3) and total Heat Generation Unit (11.52-20.84 HGU) than pink granite (2.09-2.94 µWm-3 (HP), 4.98-7.07 HGU). The granites of the Dhiran area show higher average value of total Heat Generation Unit (16.51 HGU and 5.79 HGU) than the average value of 3.8 HGU for the continental crust. The high Heat Generation Unit values (4.98-20.84) of the Dhiran granites indicate ‘hot crust’ category and a possible linear relationship between the surface heat flow and crustal heat generation in the Malani Igneous Suite.

Formation of high 18O fayalite-bearing A-type granite by high-temperature melting of granulitic metasedimentary rocks, southern China

The genesis of A-type granites has been controversial. Fayalite granite is a member of the most reduced A-type granites, commonly thought to have been primarily sourced from tholeiitic rocks. In this paper we report petrography, whole-rock geochemistry, Sr-Nd isotope, and in situ zircon Hf-O isotope results for a fayalite-bearing A-type granite suite at Jiuyishan in southern China. High zircon δ 18 O (8.0‰–9.8‰), negative zircon eHf(t) (–6.2 to –2.3), and evolved whole-rock Sr-Nd isotopes ( I Sr = 0.7151–0.7181; eNd(t) = –7.4 to –6.6) indicate the reworking of old supracrustal rocks. Isotopic and geochemical results, particularly downward inflections of Zr and Ba at ∼70 wt% SiO 2 , point to fractional crystallization rather than magma mixing as the controlling process for the evolution of the igneous suite. Integrated petrological, geochemical, and isotopic studies present the first convincing case that reduced A-type granites can also be generated by high-temperature (>960 °C) melting of granulitic metasedimentary rocks, likely related to the upwelling of the asthenosphere and/or underplating and intrusion of basaltic melts. We emphasize that key factors for the genesis of this unique rock type are low oxygen fugacity ( f O 2 ), low f H 2 O , and high temperature.

Fractionation of metaluminous A-type granites: an experimental study of the Wangrah Suite, Lachlan Fold Belt, Australia

Crystallisation experiments were performed on two metaluminous A-type granite compositions (AB412 and AB422) from the Wangrah Suite, a representative A-type granite suite from the Lachlan Fold Belt, south eastern Australia. AB412 is more mafic than AB422 and zircon saturation temperatures are 897 and 843 °C, respectively. Experiments were performed at 200 MPa, fO2 between NNO and NNO−1.05 in a temperature range between 700 and 900 °C, for various melt H2O contents. Experimental results were compared to the natural phase compositions to constrain the water content of the melt and the phases involved in fractionation processes in the Wangrah Suite. With decreasing temperature, Fe–Ti oxide, orthopyroxene and plagioclase are the first phases to crystallise, followed by biotite. The orthopyroxene stability is mainly restricted to temperatures above 750 °C. Hornblende is only stable in the less evolved composition if the water content of the melt is at least 4–5 wt.% H2O. The initial water content in the more mafic melt AB412 was between 2 and 3 wt.% H2O. Although orthopyroxene is not observed in the natural assemblages of the Wangrah Suite, it has influenced the early fractionation of the Wangrah Suite granites. Mass balance calculations show that composition AB422 can be obtained after the crystallisation of 4.7 wt.% iron-rich orthopyroxene plus 8.5 wt.% plagioclase from AB412.

Pressure effect on Ti- or P-rich accessory mineral saturation in evolved granitic melts with differing K 2O/Na 2O ratios

The solubility of Ti- and P-rich accessory minerals has been examined as a function of pressure and K 2O/Na 2O ratio in two series of highly evolved silicate systems. These systems correspond to (a) alkaline, varying from alkaline to peralkaline with increasing K 2O/Na 2O ratio; and (b) strongly metaluminous (essentially trondhjemitic at the lowest K 2O/Na 2O ratio) and remaining metaluminous with increasing K 2O/Na 2O ratio (to 3). The experiments were conducted at a fixed temperature of 1000 °C, with water contents varying from 5 wt.% at low pressure (0.5 GPa), increasing through 5–10 wt.% at 1.5–2.5 GPa to 10 wt.% at 3.5 GPa. Pressure was extended outside the normal crustal range, so that the results may also be applied to derivation of hydrous silicic melts from subducted oceanic crust. For the alkaline composition series, the TiO 2 content of the melt at Ti-rich mineral saturation decreases with increasing pressure but is unchanged with increasing K content (at fixed pressure). The P 2O 5 content of the alkaline melts at apatite saturation increases with increased pressure at 3.5 GPa only, but decreases with increasing K content (and peralkalinity). For the metaluminous composition series (termed as “trondhjemite-based series” (T series)), the TiO 2 content of the melt at Ti-rich mineral saturation decreases with increasing pressure and with increasing K content (at fixed pressure). The P 2O 5 content of the T series melts at apatite saturation is unchanged with increasing pressure, but decreases with increasing K content. The contrasting results for P and Ti saturation levels, as a function of pressure in both compositions, point to contrasting behaviour of Ti and P in the structure of evolved silicate melts. Ti content at Ti-rich mineral saturation is lower in the alkaline compared with the T series at 0.5 GPa, but is similar at higher pressures, whereas P content at apatite saturation is lower in the T series at all pressures studied. The results have application to A-type granite suites that are alkaline to peralkaline, and to I-type metaluminous suites that frequently exhibit differing K 2O/Na 2O ratios from one suite to another.

CONSTRAINTS FROM GRANITIC PLUTONISM ON PROTEROZOIC CRUSTAL GROWTH OF THE TRANSVERSE ZONE, BORBOREMA PROVINCE, NE BRAZIL

The transverse zone in Borborema Province is a mosaic of terranes situated between the Patos and Pernambuco lineaments, which were assembled through two orogenic cycles: an early, Grenville age, the Cariris Velhos event (from 1.1 to 0.95 Ga) and a later Brasiliano event (from 0.75 to 0.54 Ga). It is composed by Meso- and Neoproterozoic terranes, viz. Piancó-Alto Brígida (PBT), Alto Pajeú (APT), Alto Moxoto (AMT) and Rio Capibaribe (RCT), where the granitic plutonism is a noteworthy phenomenon and presents a pattern coherent with the terrane arrangement. The Cariris Velhos plutonism is controlled by a flake tectonics regime. It is absent or unknown in PBT, but abundant in the fold-and-thrust APT, in the arc-related AMT, and also probably occurs in RCT. No Brasiliano-age granite occurs in AMT, the APT/AMT limit lacking late-kinematic shoshonitic and ultrapotassic granites and syenites that are typical of other terrane boundaries in the region. The Cariris Velhos crustal collisional granites of the APT contrasts with the little evolved granites and trondhjemites of the AMT, while Paleoproterozoic and Mesoproterozoic gabbro-anorthositic and alkali-granitic intrusions are probably restricted to AMT and RCT. The occurrence of juvenile mafic-ultramafic rocks and geochemical and isotopic data, especially εNd signatures of volcanic rocks and granites in APT, AMT and RCT, indicate that accretionary processes indeed had an important role in segments of the Cariris Velhos belts. The Brasiliano-age granitic activity was a much more voluminous phenomenon, having developed major plutonic belts in the terranes and along their boundaries. A high-K to normal calc-alkalic super-suite and a trondhjemitic suite are characteristic in the PBT and can represent an evolutionary trend similar to the calc-alkalic-trondhjemitic series of SW Finland. The high-K branch of this super-suite is widespread in APT and RCT, sometimes associated with peraluminous leucocratic granites. Probable inherited zircons with ca.750 Ma age in APT plutons suggest that this is the age of the precursor shoshonitic mafic magmas of this K-rich calc-alkalic suite. Data from the literature indicate taht these granites were emplaced in the 630-620 Ma age interval and, with exception of the high-K calc-alkalic suite, contain moderate initial 87 Sr/ 86 Sr ratio and low negative εNd, indicating a hybrid crustal and mantle-derived source. The late-kinematic peralkalic ultrapotassic and shoshonitic super-suite occurs characteristically along and adjacent to transtensional shear zones separating some terranes, the biggest plutonic belt marking the PBT/APT limit. Data from the literature points to 566 Ma as the age of emplacement of the ultrapotassic suite. The latest major granitic pulse (512 Ma) is the post-kinematic A-type granite suite occurring in the APT/AMT boundary. This suite is subalkalic to slightly alkalic and presents geochemical patterns similar to anorogenic granites. The Brasiliano-age granitic plutonism is related to an extensional and transtensional regime, rather implying in strike-slip dispersion than substantial accretionary processes.

Constraints from Granitic Plutonism on Proterozoic Crustal Growth of the transverse zone, Borborema Province, NE Brazil

The transverse zone in Borborema Province is a mosaic of terranes situated between the Patos and Pernambuco lineaments, which were assembled through two orogenic cycles: an early, Grenville age, the Cariris Velhos event (from 1.1 to 0.95 Ga) and a later Brasiliano event (from 0.75 to 0.54 Ga). It is composed by Meso- and Neoproterozoic terranes, viz. Pianco-Alto Brigida (PBT), Alto Pajeu (APT), Alto Moxoto (AMT) and Rio Capibaribe (RCT), where the granitic plutonism is a noteworthy phenomenon and presents a pattern coherent with the terrane arrangement. The Cariris Velhos plutonism is controlled by a flake tectonics regime. It is absent or unknown in PBT, but abundant in the fold-and-thrust APT, in the arc-related AMT, and also probably occurs in RCT. No Brasiliano-age granite occurs in AMT, the APT/AMT limit lacking late-kinematic shoshonitic and ultrapotassic granites and syenites that are typical of other terrane boundaries in the region. The Cariris Velhos crustal collisional granites of the APT contrasts with the little evolved granites and trondhjemites of the AMT, while Paleoproterozoic and Mesoproterozoic gabbro-anorthositic and alkali-granitic intrusions are probably restricted to AMT and RCT. The occurrence of juvenile mafic-ultramafic rocks and geochemical and isotopic data, especially eNd signatures of volcanic rocks and granites in APT, AMT and RCT, indicate that accretionary processes indeed had an important role in segments of the Cariris Velhos belts. The Brasiliano-age granitic activity was a much more voluminous phenomenon, having developed major plutonic belts in the terranes and along their boundaries. A high-K to normal calc-alkalic super-suite and a trondhjemitic suite are characteristic in the PBT and can represent an evolutionary trend similar to the calc-alkalic-trondhjemitic series of SW Finland. The high-K branch of this super-suite is widespread in APT and RCT, sometimes associated with peraluminous leucocratic granites. Probable inherited zircons with ca.750 Ma age in APT plutons suggest that this is the age of the precursor shoshonitic mafic magmas of this K-rich calc-alkalic suite. Data from the literature indicate taht these granites were emplaced in the 630-620 Ma age interval and, with exception of the high-K calc-alkalic suite, contain moderate initial 87Sr/86Sr ratio and low negative eNd, indicating a hybrid crustal and mantle-derived source. The late-kinematic peralkalic ultrapotassic and shoshonitic super-suite occurs characteristically along and adjacent to transtensional shear zones separating some terranes, the biggest plutonic belt marking the PBT/APT limit. Data from the literature points to 566 Ma as the age of emplacement of the ultrapotassic suite. The latest major granitic pulse (512 Ma) is the post-kinematic A-type granite suite occurring in the APT/AMT boundary. This suite is subalkalic to slightly alkalic and presents geochemical patterns similar to anorogenic granites. The Brasiliano-age granitic plutonism is related to an extensional and transtensional regime, rather implying in strike-slip dispersion than substantial accretionary processes.

Magma mingling in late-Delamerian A-type granites at Mannum, South Australia

The Mannum granite forms part of the Padthaway A-type granite suite, which intruded the Adelaide fold belt along with the Black Hill gabbroic complex just after the cessation of the Delamerian Orogeny. Included within the granite are microgranite blocks, inferred to be stoped pieces of a contaminated margin facies, and numerous mafic enclaves. These enclaves display a variety of globular and tear-drop shapes, are fine-grained, lack chilled margins and enclose and react with quartz and feldspar phenocrysts from the granite, suggesting they represent contemporaneous mafic magma that was mingling with the granite magma during intrusion. Modelled temperature-viscosity relationships show that, in order for these enclaves to behave in a plastic manner, the granite magma must have had a temperature in excess of 930 °C after thermal equilibration was achieved. The contemporaneity of mafic magmas resolves any thermal budget problems arising out of the formation of these high-temperature granites, and a considerable mantle flux is implied during their genesis, consistent with gravity data and the presence of the Black Hill gabbros.

Nd isotope evidence for crust-mantle interaction in the generation of A-type granitoid suites in Labrador, Canada

Early Proterozoic “A-type” granite suites in Labrador, northeastern Canada, have initial Nd isotopic compositions that lie between those of a depleted mantle reservoir and the local basement rocks. One of these A-type granite suites consists of a linear array of discrete plutons that crosses the boundary between an Archean craton and an adjoining Early Proterozoic crustal block. The ϵ Nd of these granites (at formation, t = 1720 Ma) changes from −8 to +4 across this boundary, reflecting changes in the nature and antiquity of the continental crust. However, in both domains, the granites cannot be derived entirely by anatexis of the local basement rocks, and apparently require significant amounts of “juvenile” (mantle-derived) material. A-type granites have been widely interpreted as anatectic derivatives of lower-crustal rocks that were depleted by previous melt extraction. The Nd isotopic data suggest that these Labrador examples were instead mixtures of mantle-derived magmas and overlying lower-crustal rocks, and that new material may have been the volumetrically dominant component (50–90%) in most cases. As an alternative to the prevalent “residual-source” model, it is proposed that the distinctive geochemical characteristics of these A-type granites are a function of the high liquidus temperatures and primitive, Mg-rich character of subcrustal magma fluxes in post-orogenic and anorogenic environments, where such magmas are emplaced into stabilized continental crust. Further Nd isotope studies of other A-type granite suites are required to ascertain if models invoking crust-mantle interaction are more widely applicable.

Petrology and age of the A-type Mulock granite batholith, northern Grenville Province, Ontario

The Mulock granite batholith is exposed over 530 km 2 in the northern part of the Grenville Province, 25 km north of North Bay, Ontario. The batholith has a UPb zircon emplacement age of 1244+4/-3 Ma. Two regional tectonic events affected the batholith: (1) moderate-grade regional metamorphism; and (2) shearing and mylonitization related to the Grenville Front Tectonic Zone which slices across the northern third of the batholith. A suite of fine-to coarse-grained, pink, gneissic granites containing accessory fluorite and with colour indices between 1 and 27, dominate the batholith and are cut by abundant aplite dikes. Anorthosite dikes cut the granites and aplite in the central part of the batholith. The granites were originally alkali feldspar granite and syenogranite, and the coarser-grained varieties were originally hypersolvus. The suite is mainly metaluminous to marginally peraluminous; peralkaline varieties are rare. Most of the suite is subalkalic, but subordinate alkalic granites enriched in biotite and ferrohastingsite are early phases. The suite shows all the major and trace element compositions and elemental ratios diagnostic of A-type granite suites. Metamorphism altered the primary distribution of some of the trace elements, particulary Rb and F. The granites crystallized under relatively low pressures (<2 kb) from a high temperature, fluorine-enriched magma. Alkali feldspar fractionation accompanied by minor amphibole fractionation can account for chemical variations observed within the suite; the aplite dikes are products of residual magma. Magma generation occurred at lower crustal levels, probably by high-temperature, anhydrous partial melting of tonalitic to granodioritic, or felsic granulite source rocks, which had not previously undergone melt depletion. Mixed source rocks may have been involved, and the anorthosite dikes may be an expression of mantle-derived mafic magmatism that caused widespread crustal underplating and partial melting of the source rocks. The batholith appears to be part of an extensive magmatic episode in the northeastern North American craton, during which A-type granites and anorthosite were emplaced between 1.25 and 1.24 Ga.

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.