Restite Unmixing Research Articles

Geochemical modelling of granitoid magma diversity at Capo Vaticano Promontory (Serre Batholith, southern Italy)

Late-Variscan metaluminous/weakly peraluminous (quartz-diorites and tonalites) and strongly peraluminous (two mica porphyritic granodiorites and granites) granitoids represent two magmatic associations making up the Capo Vaticano Promontory (central Calabria, Italy). They characterize a portion of a complete continental crustal section exposed in continuity from NW to SE in the adjacent Serre Batholith. In this paper, we deal with geochemical modelling trying to understand the evolutionary processes responsible for the CVP granitoid genesis. We have investigated the main processes such as fractional crystallization, variable entrainment of peritectic mineral phases in the melt (Stevens et al., 2007; Clemens and Stevens, 2012), restite-unmixing (White and Chappell, 1977) and magma mixing to explain the geochemical variability in major and trace elements of these magmatic suites. The quartz-diorites and tonalites, representing the deepest seated rocks, are constituted by various amount of Pl + Bt + Amph + Qtz. Bt-rich and Pl-rich quartz-diorite are characterized by cumulus of Bt and Pl, respectively. A multi-step articulated fractional crystallization process was able to reproduce the quartz-diorite/tonalitic trends. Starting from a tonalitic parental magma, three different cumulates were produced: a mafic (Bt-rich) cumulate (75 % Bt, 10 % Pl and 5 % Ap) after F = 10 %, a second cumulate composed by Pl (45 %), Bt (30 %) and Amph (5 %) after F = 20 % and Pl-rich quartz-diorite (60 % Pl, 20 % Bt and 20 % Amph) as the third cumulates, after F = 30 %. Among the source-related models, restitic unmixing could have played a minor role in the quartz-diorite and tonalite genesis, defining them as products derived from partial melting of I-type sources. Pl + Kfs + Qtz + Bt + Ms are the main phases constituting the porphyritic granodiorites and granites. A simple fractional crystallization from less evolved granodiorites sample (KA2) has reproduced the more evolved liquid composition (PA27), after F = 27 %. In addition, PAE modelling is consistent with partial melting of a metapelitic source with entrainment of 20 to 5 % vol. of a peritectic assemblage made of orthopyroxene and garnet.Magma mixing modelling with mafic end-members (a basalt, a basaltic andesite and the MME of CVP) excluded any additional contribution from the mantle.Finally, geochemical modelling highlights the fractional crystallization as the main responsible process in the CVP granitoids differentiation, whereas PAE and restite unmixing processes do not provide evidence about source contributions. This study supports the idea that granitoid rocks represent recycled crustal material in the Capo Vaticano Promontory.

Petrogenesis of granitoids from the Lachlan Fold Belt, southeastern Australia: The role of disequilibrium melting

S- and I-type granites from the Lachlan Fold Belt, southeastern Australia, have been investigated to assess the role of disequilibrium melting in their petrogenesis. Differences between the median initial εHf compositions of magmatic zircon populations and the host bulk-rock (ΔεHfblk-zrc) range from −0.6 to +2.5 ε units, providing evidence for intra-sample (and hence inter-phase) Hf-isotopic heterogeneity. Linear variations on Harker diagrams and O and Hf isotope compositions of magmatic zircon preserved in many I- and S-type granites are inconsistent with assimilation or simple mixing hypotheses. In contrast, isotopic disequilibrium between the melt and a restite assemblage can explain the bulk-rock versus zircon differences observed in these samples.Assuming that magmatic zircon records the melt composition, differences between the bulk-rock εHf and εHf of magmatic zircon (ΔεHfblk-zrc values) measured for I-type granites (0.4–2.5) can largely be explained by disequilibrium amphibole dehydration melting of meta-igneous protoliths that were either isotopically heterogenous at the time they were formed, or perfectly homogeneous before being aged in the crust for 0.4–1.0 billion years prior to partial melting. The Currowong Suite exhibits petrographic features and preserves geochemical and isotopic compositions that do not lend themselves to simple restite model or magma mixing explanations; however, these observations could be explained by the restite unmixing of magma batches generated from a single source rock if, as modelling has suggested, separate batches contain different melt compositions.By investigating the application of disequilibrium melting to granite genesis, this study demonstrates that isotopic heterogeneity at various sampling scales should actually be expected for the production of granites from a single source, rather than necessitating the involvement of multiple sources and mixing processes. As a result great care should be taken in the interpretation of isotope data from granitic bulk-rocks or their zircons.

Trace inheritance—Clarifying the zircon O-Hf isotopic fingerprint of I-type granite sources: Implications for the restite model

Early to mid Carboniferous I-type granites distributed in a broad meridional belt west of Sydney, southeastern Australia, represent the last phase of granite magmatism in the southern Lachlan Fold Belt. Rare inherited zircon in the granites, in combination with zircon precipitated from the melt phase of the magmas, provides direct evidence of the nature and age of the source rocks from which the granites were derived, and the isotopic compositions of those rocks. Most granites from the north and central parts of the belt, represented by the Wuuluman, Home Rule, Oberon and Lett plutons (334±3, 328±4, 339±2 and 328±2Ma, respectively), are characterized by scarce inherited zircon dominantly of Siluro-Devonian age, consistent with all being derived from a common zircon-poor source rock of mid to late Devonian age. Based on the isotopic compositions of the igneous zircon rims, that source was relatively homogeneous and immature (δ18Ozrn 6–7‰, εHf(t) 0–+4). Two samples of the Tarana pluton (331±2Ma), near the centre of the belt, show evidence for heterogeneity in that source and the presence of a more evolved component that is also more zircon rich. Together with the Lett pluton from the same geochemical suite, their zircon rim compositions define a mixing array between more and less evolved end members (δ18Ozrn 5.5–9.5‰, εHf(t) -4–+4). Only in the southern Chapmans Creek pluton (327±3Ma) is there clear evidence that the source of the granite also contained a minor component with a similar older protolith to the LFB early Palaeozoic sediments that host most of the LFB pre-Carboniferous granites. Presence of that component, which overwhelmingly dominates the inheritance in the pre-Carboniferous granites, is also reflected in a slightly elevated whole rock initial 87Sr/86Sr (>0.7050) and igneous δ18Ozrn (ca. 7.5‰), but not in the igneous zircon εHf(t) (ca. +2). The amount of restitic zircon in granite is determined by not only the proportion of restite present but also the zircon contents of the various source rock components. The process of restite unmixing does not necessarily mean that more mafic, restite-rich granites will contain more inherited zircon.

Generation of peraluminous granitic magma in a post-collisional setting: A case study from the eastern Qilian orogen, NE Tibetan Plateau

Early Paleozoic peraluminous granites are abundant in the eastern part of the Qilian orogen, northeastern margin of the Tibetan Plateau. A combined study involving geochronology, whole-rock geochemical and Sr–Nd–Hf isotopic compositions for three Early Paleozoic peraluminous granitic plutons (Jishishan, Ledu and Shichuan plutons) from the eastern Qilian orogen was carried out to evaluate the causes of chemical variations and generation mechanisms of peraluminous granitic magmas. These granitic plutons have magma crystallization ages of 455–427Ma and are moderately to strongly peraluminous (A/CNK=1.03–1.18). Geochemical and Sr–Nd–Hf isotopic data indicate that they consist substantially of crust-derived melts. The Jishishan and Ledu peraluminous granites were dominantly produced by partial melting of Precambrian orthogneisses. The Shichuan monzogranites, with low HREE contents (e.g., Yb=0.80–1.83ppm) and slightly negative εNd(t) (−5.3 to −2.3) and positive εHf(t) (+1.6 to +3.4), could be derived from immature crustal materials. Relatively high average zircon saturation temperatures (>750°C for each pluton), obvious negative Eu anomalies (Eu/Eu*=0.28–0.80) and low Pb/Ba ratios (0.03–0.16) for the Jishishan, Ledu and Shichuan granites are consistent with crustal melting involving biotite breakdown under fluid-absent conditions. Our results suggest that compositional variations of moderately to strongly peraluminous granitic magmas are mainly controlled by source compositions and melting conditions, while the processes such as mixing with mantle-derived magma, fractional crystallization, restite unmixing and peritectic assemblage entrainment were insignificant (or only play secondary roles) in their genesis. Late Ordovician to Middle Silurian crustal anatexis in the eastern Central Qilian was probably linked with slab break-off which may be an important mechanism in addition to lithospheric delamination for the generation of moderately to strongly peraluminous granites in a post-collisional setting.

Late Triassic intrusive complex in the Jidong region, Jiamusi–Khanka Block, NE China: Geochemistry, zircon U–Pb ages, Lu–Hf isotopes, and implications for magma mingling and mixing

Whole-rock major and trace element geochemistry together with zircon U–Pb ages and Lu–Hf isotope compositions are reported for a Late Triassic intrusive complex in the Jidong region, Jiamusi–Khanka Block, NE China. Zircon U–Pb dating yields ages between 211 and 208Ma for enclaves of microgranular diorite and quartz diorite, and between 211 and 209Ma for the host granitoids. These ages correlate with a previously established intensive Late Triassic magmatic event along the eastern Asian margin. Field observations, together with petrographic features, geochemistry, and zircon Hf isotope data, preclude simple crystal fractionation or restite unmixing as a genetic link for the various rock types within the intrusive complex. The syenogranite suite has high SiO2 (75.5–76.3wt.%) and low MgO (0.15–0.19wt.%), and yields enriched LILE and LREE patterns. Most of the zircons in the syenogranites have two-stage model ages of 766 and 1461Ma, together with positive εHf(t) values of +0.6 to +9.1. These results indicate that the granitoid magmas were generated by partial melting of Meso- to Neoproterozoic lower crust. The gabbro suite has a restricted range of SiO2 (46.1–51.9wt.%) together with high Mg# values (49–70) and high concentrations of Ni, Co, and Cr. Zircons from two diorite samples have single-stage Hf model ages of 557–787Ma and εHf(t) values of +1.9 to +8.3 that are consistent with the coeval gabbros previously studied in the Jidong region. These features, together with the observation that all the gabbros are enriched in LREE and LILE, suggest that the mafic magmas were derived from melting of depleted Neoproterozoic lithospheric mantle that had been metasomatized by slab-derived fluids. It is concluded that the dominant igneous suites within the Late Triassic intrusive complex formed by mingling/mixing of felsic and mafic magmas. The geochemical data, combined with regional geological investigations, indicate that the Late Triassic intrusive complex formed during lithospheric extension caused by slab break-off of the Yuejinshan oceanic plate that was subducting westward beneath the Jiamusi–Khanka Block.

Controls on Devonian–Carboniferous magmatism in Tasmania, based on inherited zircon age patterns, Sr, Nd and Pb isotopes, and major and trace element geochemistry

Devonian–Carboniferous granites are widespread in Tasmania. In eastern Tasmania, Devonian granites intrude Ordovician–Early Devonian quartz-rich turbidites of the Mathinna Supergroup. The earliest (∼400 Ma) I-type granodiorites may be arc-related. Following the Tabberabberan Orogeny (∼389 Ma), more felsic and, finally, strongly fractionated I- and S-type granites were emplaced until ∼373 Ma. In contrast, western Tasmania granites intrude a more diverse terrane of predominantly marine shelf successions, with depositional ages as old as Late Mesoproterozoic. They are mostly felsic and fractionated I- and S-types emplaced from ∼374–351 Ma, possibly in response to post-collisional crustal extension following juxtaposition of the eastern and western Tasmanian terranes. Granites from the two terranes are readily distinguishable by the age spectra of their inherited zircon, which are noticeably similar to those of the detrital zircon from sedimentary successions in their respective terranes. Furthermore, within each terrane, both I and S-types yield similar inheritance patterns. This suggests a pivotal role for the sedimentary successions in the petrogenesis of both types. Western Tasmanian granites are also enriched in ∼1600 Ma zircon, which is essentially unrepresented in the exposed supracrustal succession. Subtle differences between the inheritance and detrital age spectra in eastern Tasmania probably relate to unrepresentative sampling of the supracrustal rocks. Nd, Sr and Pb isotopic characteristics of the granites are consistent with their derivation by mixing of magmas derived from the mantle, possibly the lower crust, and from supracrustal rocks. Systematic isotopic trends in some eastern Tasmanian I-types, particularly in the Scottsdale Batholith, correlate well with major and trace element geochemistry and age. The isotopes are inconsistent with simple restite unmixing or crystal fractionation in a closed magma chamber, and indicate progressive contamination by the Mathinna Supergroup, or similar rocks. The isotopic characteristics of late, strongly fractionated granites, although sometimes obscured by hydrothermal alteration, are also consistent with concurrent assimilation-fractional crystallisation processes. Together with the close association of some strongly fractionated I- and S-types, this suggests that such granites were generated directly in the lower crust, and were not derived from unfractionated parental granite magmas.

Geochemical evidence for origin of magma mixing for the Triassic monzonitic granite and its enclaves at Mishuling in the Qinling orogen (central China)

Whole-rock major-trace elements and Sr–Nd isotopes, mineral chemistry, and zircon U–Pb and Lu–Hf isotope compositions are reported for I-type monzogranite and its mafic microgranular enclaves from the Triassic Mishuling granite in the Qinling orogen, central China. Zircon U–Pb dating yields an age of 212 ± 5 Ma for the mafic enclaves and 213 ± 3 Ma for the host monzogranite. Whole-rock major-trace elements, Sr–Nd isotopes and zircon Hf isotopes preclude simple crystal fractionation or restite unmixing for a genetic link between the enclaves and the monzogranite. Mixing between mafic and felsic magmas is compatible with the geochemical data. The monzogranite displays an enriched LILE and LREE pattern, high ( 87Sr/ 86Sr) i ratios of 0.7068 to 0.7071 and low ε Nd( t) values of − 9.2 to − 5.7, suggesting a crustal origin. Its high Mg# (47.6 to 50.7), Cr and Ni contents clearly indicate its derivation from mafic refractory lower crust. Most of the zircons in the monzogranite have negative ε Hf( t) values of − 12.5 to − 0.1 and two-stage Hf model ages of 1049 to 1683 Ma, and some zircons in the host monzogranite have positive ε Hf( t) values of 0.3 to 4.8 and single-stage Hf model ages of 665 to 845 Ma. This indicates that the monzogranite is most likely to be formed by partial melting of a Proterozoic lower continental crust. The mafic enclaves have fine-grained igneous textures and contain abundant acicular apatite that indicates rapid cooling. They also have a high ( 87Sr/ 86Sr) i ratio of 0.7071 and a low ε Nd( t) value of − 6.5, suggesting a derivation from ancient subcontinental lithosphere mantle (SCLM). Zircons in the mafic microgranular enclaves have ε Hf( t) values of − 1.6 to 5.1 and single-stage Hf model ages of 700 to 910 Ma, suggesting a possible origin from partial melting of the Neoproterozoic SCLM that may form in association with contemporaneously supercontinental rifting in the South China Block. Therefore, the Mishuling I-type granite may form by anatexis of Meso-Neoproterozoic lower crust, with mixing between the mafic and felsic magmas. In particular, we interpret the positive ε Hf( t) values as being derived from reworking of Neoproterozoic juvenile lithosphere in response to tectonic extension in the late stage of the Triassic continental collision between the South China Block and the North China Block.

Tracing magma mixing in granite genesis: in situ U–Pb dating and Hf-isotope analysis of zircons

In situ zircon U–Pb and Hf-isotopic data have been determined for mafic microgranular enclaves and host granitoids from the Early Cretaceous Gudaoling batholith in the Liaodong Peninsula, NE China, in order to constrain the sources and petrogenesis of granites. The zircon U–Pb age of the enclaves (120 ± 1 Ma) is identical to that of the host monzogranite (120 ± 1 Ma), establishing that the mafic and felsic magmas were coeval. The Hf isotopic composition of the enclaves [e Hf(t) = +4.5 to −6.2] is distinct from the host monzogranite [e Hf(t) = −15.1 to −25.4], indicating that both depleted mantle and crustal sources contributed to their origin. The depleted mantle component was not previously revealed by geochemical and Nd and Sr isotopic studies, showing that zircon Hf isotopic data can be a powerful geochemical tracer with the potential to provide unique petrogenetic information. Some wall-rock contamination is indicated by inherited zircons with considerably older U–Pb ages and low initial Hf isotopic compositions. Hafnium isotopic variations in Early Cretaceous zircons rule-out simple crystal–liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks. Instead, mixing of mantle-derived mafic magmas with crustal-derived felsic magmas, coupled with assimilation of wall rocks, is compatible with the data.

A hybrid origin for the Qianshan A-type granite, northeast China: Geochemical and Sr–Nd–Hf isotopic evidence

Major and trace element, whole rock Sr and Nd isotope and zircon Hf isotope data are reported for a suite of A-type granites and mafic microgranular enclaves from the Early Cretaceous (126 ± 2 Ma) Qianshan pluton, Liaodong Peninsula, northeast China, with the aim of investigating the sources and petrogenesis of A-type granites. The Qianshan pluton includes hornblende alkali-feldspar granite, graphic biotite granite and mafic microgranular enclaves. The hornblende alkali-feldspar granites have high SiO 2, Fe 2O 3T / MgO, K 2O + Na 2O, Rb, Zr and LREE contents and low Ba and Sr concentrations with strongly negative Eu anomalies. Their high Rb / Sr ( 87Rb / 86Sr = 16.76–24.15) and initial 87Sr / 86Sr ratios (0.7215 to 0.7283), negative ε Nd( t) values (− 14.1 to − 16.5) and zircon ε Hf( t) values (− 18.9 to − 11.5) indicate they were mainly derived from a crustal source, but with involvement of high ε Nd( t) and ε Hf( t) materials. Graphic biotite granites have similar geochemical features and Sr–Nd–Hf isotopic compositions to enclaves, indicating they were the result of crystal fractionation of evolved mafic magmas, but with involvement of low ε Nd( t) and ε Hf( t) materials. The mafic enclaves have an igneous texture and contain acicular apatite, suggesting quenching of mafic magmas that have co-mingled with the host granites. They have low initial 87Sr / 86Sr ratios (0.7097–0.7148), negative ε Nd( t) (− 14.5 to − 11.9) and zircon ε Hf( t) (− 17.1 to − 6.9) values, and are enriched in LILEs and LREEs and depleted in HFSEs. When coupled with the high MgO (Mg# up to 54), this indicates derivation from an enriched lithospheric mantle source, but contaminated by crustal materials. Geochemical and Sr-, Nd- and zircon Hf-isotopic compositions rule out simple crystal–liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks. Instead, magma mixing of mantle-derived mafic and crustal-derived magmas, coupled with crystal fractionation, is compatible with the data. This example shows that at least some A-type granites formed through a complex process involving mantle- and crustal-derived magma mixing, crystal fractionation and infracrustal melting.

A zircon U-Pb study of metaluminous (I-type) granites of the Lachlan Fold Belt, southeastern Australia: implications for the high/low temperature classification and magma differentiation processes

Following seminal studies in the Lachlan Fold Belt (southeastern Australia), it has become almost axiomatic that metaluminous granites derive from infracrustal precursors, whereas strongly peraluminous plutons have metasedimentary or supracrustal sources, as reflected in the I- and S-type designation. Recently, zircon saturation thermometry has been used to further subdivide I-type granites into high- and low-temperature categories. That low-temperature I-type granites evolved by restite separation from magmas generated in the zircon stability field is implicit in this classification. To explore this hypothesis, we report an ion microprobe U-Pb (zircon) study into three hallmark ‘low-temperature’ Lachlan Fold Belt I-type suites. The combined patterns of zircon age inheritance and bulk rock Zr trends suggest that each suite formed from magmas that were initially zircon-undersaturated, and that fractional crystallisation, not restite unmixing, was the dominant differentiation process. The low temperature status presently applied to these rocks cannot therefore be justified. The inherited zircons in these I-type granites reflect melting and assimilation of metasedimentary rock, and testify to a supracrustal source component.

Multiple sources for the origin of granites: Geochemical and Nd/Sr isotopic evidence from the Gudaoling granite and its mafic enclaves, northeast China

Geochemical and Sr- and Nd-isotopic data have been determined for mafic to intermediate microgranular enclaves and host granitoids from the Early Cretaceous Gudaoling batholith in the Liaodong Peninsula, NE China. The rocks include monzogranite, porphyric granodiorite and quartz diorite. Monzogranites have relatively high 87Rb/ 86Sr ratios (0.672–0.853), low initial 87Sr/ 86Sr ratios (0.7052–0.7086) and ε Nd(t) values (−18.5 to −20.9) indicating that they were mainly derived from a newly underplated crustal source with a short crustal residence time. Quartz diorites have high initial 87Sr/ 86Sr ratios (0.7118–0.7120) and negative ε Nd(t) values (−13.2 to −18.1) coupled with high Al 2O 3 and MgO contents, indicating they were derived from enriched lithospheric mantle with contributions of radiogenic Sr from plagioclase-rich metagreywackes or meta-igneous rocks, i.e., ancient lower crust. Two groups of enclaves with igneous textures and abundant acicular apatites are distinguished: dioritic enclaves and biotite monzonitic enclaves. Dioritic enclaves have low Al 2O 3 (13.5–16.4 wt%) and high MgO (Mg# = ∼72.3) concentrations, low initial 87Sr/ 86Sr ratios (0.7058–0.7073) and negative ε Nd(t) values (∼−7.2), and are enriched in LILEs and LREEs and depleted in HFSEs, suggesting they were derived from an enriched lithospheric mantle source. Biotite monzonitic enclaves have Sr and Nd isotopic compositions similar to the monzogranites, indicating they were crystal cumulates of the parental magmas of these monzogranites. Granodiorites have transitional geochemistry and Nd- and Sr-isotopic compositions, intermediate between the monzogranites, quartz diorites and the enclaves. Geochemical and Sr- and Nd-isotopic compositions rule-out simple crystal-liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks. Instead, magma mixing of mafic mantle-derived and juvenile crustal-derived magmas, coupled with crystal fractionation and assimilation of ancient lower crust, is compatible with the data. This example shows that at least some calc-alkaline granitoids are not produced by pure intracrustal melting, but formed through a complex, multi-stage hybridization process, involving mantle- and crustal-derived magmas and several concomitant magmatic processes (crystal fractionation, crustal assimilation and crustal anatexis).

S-type granitic magmas—petrogenetic issues, models and evidence

Despite a perception that it represents a perverse divergence, it is perfectly possible to believe in the existence of S- and I-type granites (and the implications for the nature of their protoliths), and to disbelieve in the applicability of the restite-unmixing model for chemical variation in granitic magmas. White and Chappell erected the S–I classification with impeccable validity. The isotopic evidence demands contrasting source reservoirs for S- and I-type granitic magmas. However, the major advance was not the classification, but the recognition that highly contrasting parental materials must be involved in the genesis of granitic magmas. The restite-unmixing model is commonly seen as a companion to the S–I classification, but it is really a separate issue. This model implies that the compositions of granites ‘image’ those of their source rocks in a simple way. However, there are other equally valid models that can explain the data, and none of them represents a unique solution. The most cogent explanation for the high-grade metasedimentary enclaves in most S-type granites is that they represent mid-crustal xenoliths; restitic enclaves are either rare or absent. Inherited zircons in S-type rocks are certainly restitic. However, the occurrence of a substantial restitic zircon population does not imply an equally substantial restitic component in the rest of the rock. Zircon and zirconium behaviours are controlled by disequilibrium and kinetics, and Zr contents of granitic rocks can rarely be used to infer magma temperatures. Since the dominant ages among inherited zircons in Lachlan Fold Belt (LFB) S-type granites are Ordovician and Proterozoic, it seems likely that crust of this age, but geochemically different from the exposed rocks, not only underlies much of the LFB but also forms a component in the granite magma sources. The evidence is overwhelming that the dark, microgranular enclaves that occur in both S- and I-type granites are igneous in origin. They represent globules of quenched, more mafic magma mingled and modified by exchange with the host granitic magma. However, magma mixing does not appear to be a significant process affecting the chemical evolution of the host magmas. Likewise, the multicomponent mixing models erected for some granitic rock suites are mathematically nonunique and, in some cases, violate constraints from isotopic studies. S- and I-type magmas commonly retain their distinct identities. This suggests limited source mixing, limited magma mixing and limited wall-rock assimilation. Though intermediate types certainly exist, they are probably relatively minor in volume. Crystal fractionation probably plays the major role in the differentiation of very many granitic magmas, including most S-types, especially those emplaced at high crustal levels or in the volcanic environment. Minor mechanisms include magma mixing, wall-rock assimilation and restite unmixing. Isotopic variations within plutons and in granite suites could be caused by source heterogeneities, magma mixing, assimilation and even by isotopic disequilibrium. However, source heterogeneity, coupled with the inefficiency of magma mixing is probably the major cause of observed heterogeneity. Normal geothermal gradients are seldom sufficient to provide the necessary heat for partial melting of the crust, and crustal thickening likewise fails to provide sufficient heat. Generally, the mantle must be the major heat source. This might be provided through mantle upwelling and crustal thinning, and possibly through the intra- and underplating of mafic magmas. Upper crustal extension seems to have been common in regions undergoing granitic magmatism. Migmatites probably provide poor analogues of granite source regions because they are mostly formed by fluid-present reactions. Granitic magmas are mostly formed by fluid-absent processes. Where we do see rare evidence for arrested fluid-absent partial melting, the melt fraction is invariably concentrated into small shear zones, veinlets and small dykes. Thus, it seems likely that dyking is important in transporting granitic magma on a variety of scales and at many crustal levels. However, one major missing link in the chain is the mechanism by which melt fractions, in small-scale segregations occurring over a wide area, can be gathered and focused to efficiently feed much wider-spaced major magma conduits. Answers may lie in the geometry of the melting zones and in the tendency of younger propagating fractures to curve toward and merge with older ones. Self-organization almost certainly plays a role.

Mineralogical constraints оn the petrogenesis of the Pangeon granitoids (Central Macedonia, northern Greece)

The Pangeon granitoids intrude the metamorphic rocks of the Lower Tectonic Unit of the Rhodope Massif and are discriminated into two petrographic types: A medium- to coarse-grained, porphyritic, hornblende+biotite tonalite to granodiorite (PTG) and a fine- to medium-grained, biotite±muscovite, granodiorite to granite (MGG). Microgranular mafic enclaves (MME) of monzodioritic composition are present in the PTG rocks. The hornblendes are characterized as hastingsite, magnesiohastingsite and edenite. The biotites show calc-alkaline rock series characteristics (MME and PTG) or calc-alkaline to peraluminous characteristics (MGG). Muscovites are distinguished in primary, late magmatic and secondary (sericite) on the basis of their textural and chemical criteria. Textural evidence and compositional variations of the minerals support mainly a fractional crystallization rather than a restite unmixing or magma mixing process for the evolution within each one rock type of the Pangeon granitoids while they rule out a genetic relation between MME, PTG and MGG through fractional crystallization. Crystallization pressure values obtained by AI-in-hornblende for the PTG along with the presence of magmatic epidote suggest a magma emplacement at mid-crustal depths. Textural and mineralogical data of MME suggest that they were incorporated into their host as solids during the emplacement of the granitoids.

The origin and geodynamic significance of the Alpine cordierite-bearing granitoids of northern Algeria. A combined petrological, mineralogical, geochemical and isotopic (O, H, Sr, Nd) study

In northern Algeria, the coastal area displays a very variegated magmatism of Miocene age, extending E–W over 900 km with a width of less than 50 km. This magmatism includes both plutonic and volcanic massifs containing basic, intermediate to acidic rock types. Two groups may be identified on the basis of the chemistry and mineralogy. The first group has a calc-alkaline metaluminous chemistry and contains mainly “ high-K” types, sometimes “ medium-K” and exceptionally “low-K” types. They are gabbros and basalts, diorites and andesites, tonalites and dacites. Some granitoids (quartz-monzonites, granodiorites and syenogranites) possess affinities with this magmatism (Bejaia–Amizour and Thenia intrusions). Only mafic to intermediate enclaves of magmatic origin are found in granitoids from this group. The second group has strongly peraluminous compositions: they are cordierite-bearing granodiorites, monzogranites and microgranites, as well as dacites and rhyolites (ignimbrites). Two types of enclaves exist in these granitoids: (i) microgranular metaluminous hornblende-bearing enclaves of igneous derivation, (ii) strongly peraluminous enclaves of metapelitic compositions. Textures and mineralogical associations indicate that the enclaves experienced heating at decreasing pressures and also partial melting. Therefore, they are indicative of assimilation, not of restite unmixing. After removing the local effects of hydrothermal alteration, the O, Sr, Nd isotopic compositions of parent magmas were determined: they span a very large range of values, e.g., εNd (T) varies from +12 (Cap Bougaroun gabbros) to −10 (Crd-bearing granitoids). From the covariation of Sr–Nd isotopic compositions and the constraints of O isotopic ratios, the following sources are proposed for the different rock types: Mafic to intermediate rocks contain a subduction-related component but most of them require contamination at crustal levels. The source of metaluminous granitoids is predominantly crustal as constrained by their high O isotopic composition ( δ 18O ranging from +9.3‰ up to +13‰), but they might contain a minor component derived from the mafic sources. Most of the cordierite-bearing granitoids are thought to be formed through assimilation of pelitic metasediments by a less aluminous parental magma. The general tectonic setting of this Neogene (limited, linear but variegated) magmatism is explanable in terms of the slab detachment models recently advocated by several authors in the western Mediterranean domains. This study argues that cordierite-bearing granitoids may be produced in areas of high heat flow, in crustal domains which did not experience important thickening.

A geochemical study of two peraluminous granites from south-central Iberia: the Nisa-Albuquerque and Jalama batholiths

Abstract In this paper we present new petrological and geochemical data for two peraluminous granite batholiths (Nisa Alburquerque and Jalama batholiths) representative of the ‘Araya-type’ granites of the Central-Iberian Zone. Both granites are composite with several facies (monzogranites and leucogranites) that can be grouped into two main granite units: the external units and central units. Intrusive relationships and lack of geochemical coherence between the central and external units indicate that they are not comagmatic but represent different pulses. The central units of both batholiths are petrologically and geochemically different. On the other hand, external units show a lot of similarities and are the main object of this study. The main characteristics of the external granites can be interpreted in terms of an incomplete fractional crystallization process of early mineral phases (plg + Kf + bt) which probably took place at the level of emplacement. Other possible mechanisms of magmatic differentiation (magma mixing, restite unmixing, sequential melting) can be discarded based on field, petrography and geochemical data. We propose that the ‘Araya-type’ granites are formed by the intrusion of distinct magma pulses (central and external). Further evolution within each pulse can be due to incomplete fractional crystallization possibly taking place at the emplacement level.

A Geochemical Study of Two Peraluminous Granites from South-Central Iberia: The Nisa-Albuquerque and Jalama Batholiths

In this paper we present new petrological and geochemical data for two peraluminous granite batholiths (Nisa Alburquerque and Jalama batholiths) representative of the 'Araya-type' granites of the CentralIberian Zone. Both granites are composite with several facies (monzogranites and leucogranites) that can be grouped into two main granite units: the external units and central units. Intrusive relationships and lack of geochemical coherence between the central and external milts indicate that they are not comagmatic but represent different pulses. The central units of both batholiths are petrologically and geochemically different. On the other hand, external units show a lot of similarities and are the main object of this study. The main characteristics of the external granites can be interpreted in terms of an incomplete fractional crystallization process of early mineral phases (plg + Kf + bt) which probably took place at the level of emplacement. Other possible mechanisms of magmatic differentiation (magma mixing, restite unmixing, sequential melting) can be discarded based on field, petrography and geochemical data. We propose that the 'Araya-type' granites are formed by the intrusion of distinct magma pulses (central and external). Further evolution within each pulse can be due to incomplete fractional crystallization possibly taking place at the emplacement level.

Enclaves in the S-type granites of the Kösseine massif (Fichtelgebirge, Germany): implications for the origin of granites

The Hercynian peraluminous granites of the Kosseine massif differ in composition and content of enclaves from the other granites of the Fichtelgebirge. They are garnet-, cordierite- and andalusite-bearing S-type granites containing at least five different types of enclaves. Based on petrography and geochemical data, including radiogenic isotopes, the following is concluded: The amphibolite enclave (AE) and most of the aluminium silicate-bearing surmicaceous enclaves (SEA) are country-rock xenoliths picked up by the granite at or close to the emplacement level. The orthopyroxene-bearing surmicaceous enclave (SEO) probably represents a restite from the source level of the granite. The gneiss enclaves (GE) could be fragments of Saxothuringian paragneisses taken up by the granite near the source level. The pale microgranular enclaves with igneous texture (PMEI) could be fragments of a yet-unknown granitic material or unmelted igneous material of an inhomogeneous source. The biotite-rich microgranular enclaves with igneous texture (BMEI) and the microgranular enclaves with polygonal texture (MEP) represent material which derives from the source region of the granite. The Kosseine granites represent an independent intrusion among the Fichtelgebirge granites formed by a combination of incomplete restite unmixing, assimilation and probably fractional crystallization in the course of magma formation, ascent and emplacement.

The role of residual melt migration in producing compositional diversity in a suite of granitic rocks

We present a quantitative model for in situ crystallization within a solidification zone (or boundary layer) based on the trace and major element compositions of plutonic igneous rocks and related geochemical data. We developed the model to account for the characteristics of a suite of granitic rocks: nearly uniform mineral compositions in rocks that range widely in bulk composition (e.g., 58–76 wt.% SiO2); linear variation and correlation of all major and trace elements analyzed except Ba; large and apparently random variations in Ba concentration. These characteristics cannot be explained by any standard petrogenetic model involving fractionation, mixing, or restite unmixing, but are successfully reproduced by our model of residual melt migration. We do not attempt to model the entire crystallization history of a pluton but, rather, only that interval during which melt migration processes have recognizable geochemical effects. For the granitic suite, the chemical signature of residual melt migration resulted from the change in Ba compatibility with the onset of orthoclase crystallization at a granite minimum. Our results demonstrate that plutonic rocks can develop large compositional variations, comparable to those expected to result from extreme differentiation within a large magma body, over short distances by melt migration under conditions of high permeability or slow crystallization rate within the solidification zone. Melt migration is probably a common process that could be overlooked in large plutons if sampling is sparse and if variations in some components that appear random are not considered.Our model equations yield estimates for parameters that describe the proportions of residual melt that crystallize within and migrate out of the solidification zone. Values of these parameters can be used to infer information about permeability and melt mobility within the solidification zone. The model parameters derived for the granitic suite have a roughly concentric spatial pattern in the pluton, suggesting that residual melt was trapped near the margins, accumulated in the interior, with a zone of enhanced melt mobility and possibly compositional convection in between. Our model may be of general usefulness because it requires no assumptions about magma chamber geometry or magma dynamics, it is applicable to magmas of any composition, and the equations could be formulated to include those variables best constrained by a particular suite of plutonic igneous rocks.

Genesis of the two main types of peraluminous granitoids

Two-mica monzogranites to leucogranites and biotite-rich, cordierite-bearing tonalites to monzogranites form two distinct groups of peraluminous granitoids. They can be distinguished by mineral and rock associations and by the variation of their peraluminosity during differentiation. Except for the rare muscovite-bearing granitoids produced by extreme fractionation or local contamination of metaluminous magmas, the majority of peraluminous granitoids are produced by partial melting of crustal rocks. Production of either muscovite-bearing granites or biotite-rich, cordierite-bearing granitoids does not depend only on the nature of the sources, but is also controlled by the physical parameters of partial melting and consequently by the way anatexis of a thickened crust is enhanced. Biotite-rich, cordierite-bearing granitoids are generated where mantle-derived magmas are injected into or have underplated crustal rocks; two-mica granites are generated where thickened crust is affected by major crustal shears or thrusts. Correlations between differentiation and peraluminosity indicate the dominant role of either restite unmixing or fractional crystallization in the production and evolution of the various types of peraluminous granitoids.

An example of in situ granite formation in the northern boundary of the Proterozoic Sergipano fold belt, NE Brazil: the Xingó Complex

The Xingó Complex occurs as a 1000 km 2 NE-SW strip in the northern limit of the Proterozoic Sergipano fold belt within the Pernambuco-Alagoas Median Massif. It is comprised of the following rock types: a) migmatites, metatexites with diatexite pods; b) coarse-grained granites compared to migmatites, as lenses parallel to the migmatite foliation, with restites composed of biotite+zircon+sphene+apatite+epidote and c) as stocks of medium-grained granites and fine-grained dikes, hololeucocratic (biotite < 5%), with minor amounts of tourmaline, epidote, garnet and fluorite. Some dikes, more enriched in biotite, show crenulated borders, suggesting intrusion during a migmatite plastic stage. The granites are peraluminous, and show a narrow range in SiO 2 (70–75%). The observed geochemical patterns and variations within the studied granites suggest an origin of succesive restite unmixing followed by homogenization and fractional crystallization. The Xingó Complex is an example of in situ granite formation during the Brasiliano (Pan-African) orogeny, possibly related to the collision between the Sergipano Belt and the Pernambuco-Alagoas Median Massif.