Host Granitic Magma Research Articles

Magma assembly and evolution of the Early Cretaceous Sanguliu pluton in the Liaodong Peninsula, NE China

• Influx of two magmas from the ancient crust constructed the zoned Sanguliu pluton . • Mafic magma provided primarily heat but little material for the Sanguliu granites . • Magma mingling occurred during the magma ascent and emplacement processes. Zoned granitic plutons and mafic microgranular enclaves (MMEs) contain important information on magma assembly, mixing, and evolution. The Sanguliu pluton in the northeastern North China Craton (NCC) consists of fine-/medium-grained monzogranite in the outer zone and coarse-grained syenogranite-monzogranite in the inner zone, with MMEs being found in both zones. These rocks have similar zircon U-Pb ages of ∼ 130 Ma. The granites in both the outer and inner zones display variably enriched isotopic compositions of ( 87 Sr/ 86 Sr) i (0.7147–0.7163), εNd(t) (−18.6 to −20.8), and zircon εHf(t) (−18.0 to −24.3), indicating that they were mainly sourced from the ancient lower crust with minor depleted mantle contributions. Zircons in the outer-zone MMEs are subhedral-anhedral with patchy zoning in the CL images and have a narrow εHf(t) range (−16.2 to −20.9) similar to zircons from the host granites. In contrast, zircons in the inner-zone MMEs are mainly euhedral with core-rim structure, and have a wider εHf(t) range (−0.3 to −20.8). The complex morphology and internal structure of these zircons are ascribed to the metasomatism and growth during magma mingling processes. The varying zircon εHf(t) values indicate that these MMEs were derived from the depleted mantle and had been isotopically modified by the host granitic magma.

Redox series assessment, petrogenetic, and geodynamic appraisal of Neoarchean granites from the Bundelkhand Craton, Central India: Constraints from phase petrology and bulk rock geochemistry

Based on field relationships, microstructure, phase petrology, and whole‐rock elemental geochemistry, we have unravelled the redox conditions and petrogenetic evolution of Neoarchean granites (sensu lato) from the central part of the Bundelkhand Craton, Central India. The studied Neoarchean granites are sanukitoids (sensu lato), high‐K, low‐magnesian granites (HKLMG), and oxidized A‐type granites (OAG). Chemistry of biotite in conjunction with amphibole compositions from sanukitoids indicates their crystallization in calc‐alkaline felsic host magma formed in a subduction setting at elevated fugacity of oxygen (fO2), buffered at +2 fayalite‐magnetite‐quartz (FMQ), corresponding to magnetite series granite. Chemistry of biotite and amphiboles from HKLMG suggests that host melts of these phases buffered above +1 FMQ, that is, moderate fO2 condition corresponding to magnetite series granites. The nature of the HKLMG magma is peraluminous to alkaline which originated at a relatively high‐P condition. The biotites from OAG exhibit pronounced Al‐Fe substitution buffered above +0.5 FMQ to represent oxidized A‐type host granite magma, belonging to magnetite series granite. The sanukitoids bear high ferromagnesian minerals (FeOt + MgO + MnO + TiO2 (FMMT) >4.5; Mg# = 28–45), and enriched incompatible elements without significant Eu anomalies, probably originated from melt‐enriched mantle source in a subduction setting. The HKLMG are characterized by relatively low Mg#, TiO2, high K‐LILEs‐REEs with negative Eu anomalies, and high Th/Nb, Ce/Pb, and La/Nb ratios, which suggest mixed source signatures, derived from the enriched mantle and continental crust. A‐type granites (OAG) bear high NK/A (molar Na2O + K2O/Al2O3 ~ 1.17), FeOt/MgO, K2O/Na2O, Ga/Al, and Ce–Y, which indicate the emplacement of OAG magmas in syn‐ to post‐collisional tectonic environments. Thus, the Neoarchean granites from the Bundelkhand Craton represent a collage of subduction related sanukitoids to high‐K magnesian granites, and syn‐ to post‐collisional oxidized A‐type granites.

Evaluation of crystallization conditions and porphyry Cu Mineralization potential of the Gheshlagh-Aftabrow pluton, central part of the Urumieh-Dokhtar magmatic belt, Iran

Gheshlagh-Aftabrow granodiorite (GAG) is emplaced within Eocene volcanic rocks in the central part of the Urumieh-Dokhtar magmatic belt (UDMB) of Iran and comprises of host granodiorites and associated mafic microgranular enclaves (MMEs). The Al-in-hornblende barometry and the amphibole–plagioclase thermometer are applied to calculate pressure and temperature. The host granodiorite yields crystallization temperatures and pressures varying from 635 to 715 °C (with an average of 670 °C) and from 0.5 to 1.8 kbar (with an average of 1.3 kbar, equivalent to a depth of 4.5 km), respectively. The mafic enclave shows a slightly higher crystallization temperature (690–720 °C, with an average of 705 °C) and pressure (1.1–1.8 kbar, with an average of 1.4 kbar), equivalent to a depth of 5 km. The estimated ƒO2 contents by amphibole chemistry in the host rocks and their mafic enclaves range − 11.2 to − 13.3 and − 11.5 to − 12.8, respectively, indicating that these rocks probably crystallized at relatively oxidizing conditions above the nickel–nickel oxide (NNO) buffer, yielding an overall range between NNO + 1.1 and NNO + 2.6, that is consistent with the general presence of magnetite and corresponds to their calc-alkaline affinity and the formation of the rocks in the subduction environment. The granodiorites and associated MMEs yield broadly similar water contents of 2.5–3.7 wt% and 2.6–3.7 wt%, respectively. The calculated densities of the host granitic magmas vary from 2319 to 2353 kg/m3, which are different from the determined densities of the mafic magma from which the enclaves are derived (2500–2626 kg/m3). The minimum calculated solidification time for the Gheshlagh-Aftabrow intrusion is about 20 × 104 years, and for the enclaves, it is less than 0.2 to 14 h that could not be enough time for pervasive mixing. Most of the amphiboles have Mg# value between 0.5 and 0.7, that together with low difference in densities of the magmas forming host rocks and enclaves may indicate that mantle–crust mixing was significant in the GAG generation. However, some geochemical signatures together with high ƒO2 content in the GAG samples indicate that these rocks are essentially comparable to granitoids associated with porphyry copper deposits, but according to the field evidences, the lack of extensive alteration, whole-rock chemistry (e.g., low Sr/Y and high Zr/Hf), plagioclase geochemistry, low water contents, and mineralogical evidences (e.g., lack of the common alteration phases in porphyry systems), it can be concluded that the studied pluton is barren regarding porphyry Cu mineralization.

Magma mixing in Neoarchean granite from Nalgonda region, Eastern Dharwar Craton, India: Morphological, mineralogical and geochemical evidences

We present field, petrographic, mineralogical and whole-rock geochemical data for part of the Neoarchean granite and associated microgranular enclaves (MEs) occur in the Nalgonda region, NE part of Eastern Dharwar Craton (EDC), and demonstrate the end-member magma mixing processes in the petrogenesis of the host granite. Extensive occurrence of ME and uni-directional flow band structures (N–S trend) in all the studied outcrops exposed over about 20 km depict that intensive magma mixing–mingling occurred at magma chamber scale. MEs are the portions of intermediate mafic magma that had interacted at two stages with acidic host granite. Mixing at the initial stage promoted efficient thorough mixing which resulted in rapakivi texture and mesocratic enclaves. Wispy filament structures around these enclaves indicate that mafic magma globules were mechanically diluted in the host granite magma by chaotic advection. These filamental magmas were further linearly diluted along with convection-related flow of the host magma. Smaller mafic globules also got linearly stretched along with this flow. Upon solidification of the host magma, these wispy mafic filaments were preserved as flow structures. The second stage of mafic magma mixing did not promote thorough mixing due to the large viscosity contrast with the host granite magma and preserved their physical entity. These enclaves are melanocratic with sharp boundary and devoid of gradational contact and filament structures around it. They characteristically show network of granitic vein injection which often show crenulation folding. These evidences indicate that during the second-stage interaction, the enclave magma was rigid to plastic nature with respect to the host granite magma. Coherent linear Harker variation trend of CaO, MgO, \(\hbox {TiO}_{2}\), \(\hbox {K}_{2}\hbox {O/Na}_{2}\)O, V, Y and Sc indicates that mixing has promoted to develop chemical gradient between ME and host granite magmas. Mechanical dilution by chaotic advection must have enhanced the chemical diffusion of both magmas. Smooth decreasing Harker trend of Y content in both the magmas indicates that theyhad undergone certain degree of fractional crystallisation. In contrast, elements like Rb, Ba and Sr with large value of diffusion coefficient (D) show scattered behaviour in element–element plots suggesting that diffusive fractionation was active during chaotic advection mixing. It is concluded that heterogeneity in compositional variation of Nalgonda granite can be attributed to difference in degree of mechanical dilution of mafic magma, fractional crystallisation and diffusive fractionation. The geochemical evidences indicate the role of subduction in the evolution of these rocks.

Granitic magmas with I-type affinities, from mainly metasedimentary sources: the Harcourt batholith of southeastern Australia

The high-K, calcalkaline granitic rocks of the 370 Ma, post-orogenic Harcourt batholith in southeastern Australia have I-type affinities but are mildly peraluminous and have remarkably radiogenic isotope characteristics, with 87Sr/86Srt in the range 0.70807 to 0.714121 and eNdt in the range − 5.6 to − 4.3. This batholith appears to be a good example of magmas that were derived through partial melting of distinctly heterogeneous source rocks that vary from intermediate meta-igneous to mildly aluminous metasedimentary rocks, with the balance between the two rock types on the metasedimentary side. Such transitional S-I-type magmas, formed from mainly metasedimentary source rocks, may be more common than is generally realised. The Harcourt batholith also contains mainly granodioritic igneous microgranular enclaves (IMEs). Like their host rocks, the IMEs are peraluminous and have rather radiogenic isotope signatures (87Sr/86Srt of 0.71257–0.71435 and eNdt of − 7.3 to − 4.3), though some are hornblende-bearing. Origins of these IMEs by mixing a putative mantle end member with the host granitic magma can be excluded because of the variability in whole-rock isotope ratios and, for the same reason, the IME magmas cannot represent quench cumulates (autoliths) from the host magmas. Less abundant monzonitic to monzosyenitic IMEs cannot represent accumulations of magmatic biotite and/or alkali feldspar because K-feldspar is absent, and there is no co-enrichment of K2O and FeO + MgO, nor can they be mixtures of anything plausible with the host-rock magma. The granodioritic IMEs probably originated through high degrees of assimilation of a range of crustal materials (partial melts?) by basaltic magmas in the deep crust, and the monzonitic IMEs as melts of enriched subcontinental mantle. Such enclave suites provide little or no information on the chemical evolution of their host granitic rocks.

Chemical composition and Sr isotopes of apatite in the Xiangshan A-type volcanic-intrusive complex, Southeast China: New insight into petrogenesis

A series of Late Mesozoic A-type granitoids, including the Xiangshan volcanic-intrusive complex, was recently identified along the Gan-Hang volcanic belt, which was close to the boundary of the Yangtze and Cathaysia blocks in South China. However, significant issues on the petrogenesis of the Xiangshan complex, especially how mantle-derived magma contributes, if any, still remain controversial. In this study, we focused on the apatite geochemistry of the Xiangshan complex to understand the characteristics and evolution of its primary magma. In the Xiangshan complex, mainly including rhyolite, porphyritic lava, granite porphyry, and quartz monzonite porphyry, apatites occur mostly as inclusions in plagioclase and biotite, and exhibit high Srap (Sr content in apatite)/Srhr (Sr content in host rock) (from 0.97 to 3.22, 1.85 on average), indicating these apatites crystalized before plagioclase in the melt. Therefore, the negative Eu anomalies (from 0.06 to 0.83) in apatites of the Xiangshan complex indicate that plagioclase is a remaining stable mineral phase in the source rocks during the partial melting. In addition, most of apatites in the Xiangshan complex with varied whole-rock ASI (aluminum saturation index) values (from 0.88 to 1.19) are characterized by high and relatively homogeneous F content (from 3.14 to 3.55). Therefore, F-rich mineral should be a primary phase in the source. Compared with apatites in three earlier rocks of the complex, apatites from quartz monzonite porphyry have higher Mn and Cl contents, stronger degree of negative Eu anomalies, and lower initial 87Sr/86Sr (0.70642–0.70714), suggesting that before apatites crystalized, the magma to form quartz monzonite porphyry had been mixed with a Cl-rich magma derived from a mantle source that received addition of Cl from subducting oceanic sediments of the Paleo-Pacific plate. The Sr isotope ratios and halogens contents of apatite from quenched dark enclaves found in granitic porphyry imply that the enclaves were formed from this Cl-rich mantle-derived magma through injecting part of this magma into the granitic magma and remaining as quenched enclaves without mechanic mixing and no significant chemical influence on the host granitic magma. Thorough mixing between this mantle-derived magma and the felsic crustal magma did not occur until the latest stage of the Xiangshan volcanism when quartz monzonite porphyry was forming.

Petrogenetic processes in granitic magmas and their igneous microgranular enclaves from Central Victoria, Australia: match or mismatch?

Using chemical evidence, particularly the variations between titanium and iron + magnesium, we demonstrate that the petrogenetic processes that operated within Central Victorian granitic host magmas and their igneous microgranular enclave suites are dissimilar. Chemical variations within the granitic series result from a variety of what might be called ‘orderly’ processes, resulting in linear or curvilinear trends in chemical variation diagrams. Those that affected the enclave suites commonly resulted in scattered, chaotic variations. Even in cases where an enclave suite displays a more orderly chemical variation, it is demonstrable that the hypothesis of simple mixing between an enclave magma and a host granitic magma, to bring about the overall chemical variations, cannot be supported. The enclave magmas had vastly smaller volumes compared with their host granitic magmas. Thus, they have commonly undergone hybridisation through mixing with deep crustal melts and both chemical and mechanical interactions with wall rocks and their enclosing granitic magmas. As a result of this complex and chaotic set of processes, it remains extremely difficult to unravel the precise mechanisms that produced a given suite of enclave magmas. Due to the similarities between the studied granites and their enclaves with occurrences worldwide, we suggest that our findings are likely to have general applicability.

Apatite Composition of Representative Magnetite‐series and Ilmenite‐series Granitoids in Japan

AbstractApatites of representative magnetite‐series and ilmenite‐series granitoids were studied in the Japanese Islands. Concentrations of the volatile components F, Cl and SO3 are differently distributed in apatites of these granitoid series. Apatites are always fluoroapatite. They have weakly higher F content in the ilmenite series than in the magnetite series. In contrast, Cl and SO3, are significantly concentrated in apatites of the magnetite series compared to the ilmenite series. These characteristics reflect the original concentrations of these components in the host granitic magmas. A high fO2 seems most important for the S‐concentration as sulfate in apatite of the magnetite series. REE and Y are only erratically high in the studied apatites.

Geochemistry and petrogenesis of Mesoproterozoic (~ 1.1 Ga) magmatic enclaves in granites of the eastern Llano Uplift, central Texas, USA

Mesoproterozoic (~1.1Ga) plutons of the eastern Llano Uplift, central Texas, USA contain two types of magmatic enclaves (<1% by vol.). Although volumetrically insignificant, the enclaves contain important petrogenetic information. Type I enclaves are felsic in composition (70–75wt.% SiO2), with mineral assemblages and chemical compositions comparable with the host granites, but typically display a finer grained texture. They are interpreted as partly chilled disrupted material from the margins and roof of the plutons. Type II enclaves are intermediate in composition (~56–69wt.% SiO2), with many elements defining trends continuous with the host granites. Both types of enclaves display sharp borders in contact with the host granite suggesting magma quenching with little or no physical exchange between host granite and enclave magma.Type II enclaves contained within the Marble Falls (MF) and Lone Grove (LG) plutons exhibit enrichments in Y, Nb, and Zr relative to their respective host granites. Enrichments in these incompatible trace elements at low SiO2, renders unlikely the possibility that the MF and LG Type II enclaves are the result of partial melting (anatexis) of mafic crustal rocks. Numerical modeling of fractional crystallization and simple mixing fails to explain the observed trace element trends.Because no coeval mafic to intermediate rocks are exposed in the uplift, characteristics of Type II enclave source magma(s) is uncertain. However, assuming source magmas similar to primitive continental arc basaltic andesite, trace-element trends (i.e., incompatible element enrichment and compatible element depletion) can be adequately replicated by a replenishment fractional crystallization (RFC) model. Chemistry of the MF and LG Type II enclaves suggest repeated replenishment of primitive magmas with only limited interaction with the host granitic magmas; the more primitive enclave magmas evolving in near chemical isolation by RFC processes. However, evidence from Type II enclaves in two other plutons in the Llano Uplift (Kingsland and Enchanted Rock) suggest that the isolation was non-ideal; i.e., some limited mixing may have occurred. Rapid quenching likely limited the potential for physical and chemical exchange between Type II enclaves and their host granite magmas.

The significance of mafic microgranular enclaves in the petrogenesis of the Dehno Complex, Sanandaj–Sirjan belt, Iran

Quartz-monzodioritic and granodioritic enclaves occur in the Upper Cretaceous–Paleocene Dehno Complex, NW of Isfahan, Iran. They are hosted by hornblende-granodiorite and granodiorite–monzogranite, respectively. The presence of xenocrystic plagioclase, biotite, quartz, magnesio-hornblende, occasionally with relics of clinopyroxene (in quartz-monzodiorites), minor K-feldspar and the lack of peraluminous minerals dominate the mineralogy of these rocks and suggest a metaluminous source. The plagioclase in the quartz-monzodioritic enclaves has higher An-content than that of the respective plagioclase of their host (hornblende-granodiorite) indicating crystallization from more mafic magma. The quartz-monzodioritic enclaves generally contain higher values of Fe 2O 3, MgO, CaO, MnO, Na 2O, transition elements (Ni, Cr, Co, V), REE and lower values of SiO 2, K 2O, P 2O 5 and Zr than their host which are in consistent with occurrence of abundant ferromagnesian minerals in these rocks. Compare to their host, the granodioritic enclaves have higher amount of Fe 2O 3, MgO, MnO, TiO 2, P 2O 5, Rb, Zr, Y, Ni, Cr, Co and lower concentration of SiO 2, K 2O and Ba. Aside from hornblende-granodiorite and granodiorite–monzogranite, the Dehno Complex comprises leucogranodiorite and tourmaline-leuco granites. Geochemically, the rocks of the Complex are metaluminous to strongly peraluminous and are enriched in LILE (e.g. Rb, K, Sr, Ba) and depleted in HFSE (e.g. Nb, Ta, P, Ti, Zr, Hf) with medium to high-K calc-alkaline nature. These geochemical features are proposed for the volcanic arc granites emplaced in an active continental margin. Whole-rock and quartz δ 18O values for all rocks are high (9.7–14.4‰ and 10.1–14.9‰, respectively), suggesting the interaction of magma with crustal-derived components. The strongly peraluminous composition can be interpreted in terms of this hypothesis. All these mineralogical and geochemical features support the idea that the granitoid rocks of the Dehno Complex developed through variable degrees of mixing/mingling between a basic magma and a supracrustal melt during subduction of the Neotethys oceanic crust beneath Central Iranian microcontenent. The quartz-monzodioritic enclaves most likely represent the nearest composition to the original more mafic magma. The granodioritic enclaves result from the mixing/mingling of globules basic magma with host granitic magma.

Radial cordierite-bearing orbicular granite formed from the melting of pelitic hornfels in granitic magma, Tsukuba Mountains, Japan

Orbicular granite occurs at Minederayama in the Tsukuba Mountains, Ibaraki Prefecture, Japan. The orbs consist of a migmatitic core, originally pelitic hornfels, and a mantle that consists mainly of radial cordierite containing apatite inclusions and subordinate feldspar and micas. The orientation and size distribution of the orbs indicates that isolated orbs had floated within the host granitic magma. According to the geometrical selection theory of crystal growth, we propose that radial cordierite grew from the orb shells to the orb cores. Melting experiments demonstrate that the mineral assemblage of the orbicular granite reflects the incongruent melting of pelitic hornfels. Important factors in the formation of the orbicular granite are the unmixing between the incongruent melt of cordierite hornfels and host granite melt, and the infiltration of the incongruent melt into the host granite melt as the cordierite crystallized. The temperature of the host granite magma, as estimated from melting experiments and mineral assemblage, was above 730 °C at 0.3 GPa, which may represent superheated conditions for the host granitic magma. Cooling of the host granite magma placed the orbs under supercooled conditions as the radial cordierite grew from the shells of the orbs to the cores.

Fractionated Ilmenite-series Granites in Southwest Japan: Source Magma for REE-Sn-W Mineralizations

Abstract. Various leucocratic biotite granites, low-temperature I-type, from the middle zone of the Sanyo ilmenite-series granitic terrane were studied chemically. These granites are locally associated with REE-Sn-W mineralizations, and were compared with unmineralized granites and batholithic Ryoke granites in three areas of the Chubu, Kinki and Chugoku Districts. They are unique in the region because they have extremely low ferromagnesian components but high Rb/Sr and 10000Ga/Al ratios. These granites are divided petrographically into the main phase, finer-grained marginal phase and younger sheets and dikelets. These rocks have increasing of HREE+Y and Nb+Ta contents in this order, which is also followed by decreasing zircon saturation temperature from 780 to 725d̀C. Together with the mode of occurrence of these granites, the leucogranitic magmas are considered to have formed by in-situ fractionation of the host granitic magmas near the top of the magma chambers. The concentration of HREE, Y, Nb and Ta in these Sanyo Belt leucogranites is principally controlled by magmatic fractionation.

Geochemistry of microgranular enclaves and host granite from Telões (Vila Pouca de Aguiar), Northern Portugal

At Telões, a subaluminous medium- to coarse-grained porphyritic biotite granite, crops out along the Vila Real NNE–SSW fault. It is a post-tectonic granite of 299±3 Ma old given by U–Pb isotopic data on zircon. It contains metaluminous to subaluminous tonalitic, granodioritic and monzogranitic enclaves. All granitoids have Fe 2+-biotite and some enclaves contain magnesiohornblende and subsolidus actinolite. Monzogranitic enclaves show obvious similarities to the host granite. Linear array between enclaves and host granite is observed in Rb–Sr typical isochron diagram and gives the age of 286±11 Ma and ( 87Sr/ 86Sr) 0=0.7063±0.0011. Microgranular enclaves have δ 18O values similar to those of the hosting granite. Microgranular enclaves are hybrid rocks probably formed by mixing between a tonalitic enclave magma and a host granite magma as supported by the modelling of major and trace elements. The similar isotopic signatures suggest a subsequent partial equilibration of the enclaves and granite magmas.

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.

Fingerprinting feldspar phenocrysts using crystal isotopic composition stratigraphy: implications for crystal transfer and magma mingling in S-type granites

Microsampling of cm-scale feldspar crystals within an S-type granite from the Lachlan Fold Belt of southeastern Australia has revealed complex internal Sr and Nd isotopic variations. The observed isotopic zonations are in part interpreted as recording feldspar crystallisation in a dynamically mixing magma system, the isotopic composition of which was varying in response to the influx of more mafic and isotopically more mantle-like magmas, the latter stages of which are now represented in modified form by microgranular enclaves. Similar core to rim isotopic variations in feldspar megacrysts from a microgranular enclave and the adjacent host granite strongly suggest megacrysts in the enclave were transferred from the granitic magma during crystallisation. Feldspar rims have higher 87Sr/86Sri and lower ɛNd(i) than adjacent whole rock analyses, but match those of mineral separates from the surrounding enclave matrix. This suggests that the final stages of megacryst growth occurred in the presence of a component that had previously interacted with a high 87Sr/86Sr, low ɛNd(i) component such as metasedimentary wall rocks. Isotopic heterogeneities are also presererved within different mineral phases in the enclave matrix, suggesting that differing phases grew at differing stages of equilibration between the enclave magma and its host granitic magma. Our results reveal major isotopic heterogeneities on a single crystal and also inter-mineral scale in a pluton which shows well constrained evidence for magma mingling. These results indicate the suitability of feldspars as recorders of isotopic change in magmatic systems, even those which have cooled slowly in the plutonic environment and suggest that much heterogeneity in plutonic systems may be overlooked on a whole rock scale.

Geochemistry of enclaves and host granites from the Nelas area, central Portugal

Tonalitic, granodioritic and monzogranitic enclaves occur in the Hercynian peraluminous porphyritic biotite granite and biotite–muscovite granite from the Nelas region. Some variation diagrams show linear trends, but others show dispersion. The enclaves generally have closely similar isotopic signatures to those of the host granites. They contain xenocrystic plagioclase with the same composition as phenocrysts of the host granite, and have biotite as the sole ferromagnesian phase with a composition typically similar to that of the host granite. However, some cores of enclaves have biotite slightly chemically distinct from that of rims. The modelling of major and trace elements of granodioritic enclaves and hybrid host granite indicate that they result from simple mingling/mixing between a tonalitic magma and the host granite magma. Microgranular enclaves are thus interpreted to be globules of a more mafic magma probably from an enriched mantle source. Partial equilibration has been achieved between these enclaves and the host granite.

Phase equilibrium constraints on the viscosity of silicic magmas II: implications for mafic–silicic mixing processes

Isobaric crystallisation paths obtained from phase equilibrium experiments show that, whereas in rhyolitic compositions melt fraction trends are distinctly eutectic, dacitic and more mafic compositions have their crystallinities linearly correlated with temperatures. As a consequence, the viscosities of the latter continuously increase on cooling, whereas for the former they remain constant or even decrease during 80% of the crystallisation interval, which opens new perspectives for the fluid dynamical modelling of felsic magma chambers. Given the typical dyke widths observed for basaltic magmas, results of analogue modelling predict that injection of mafic magmas into crystallising intermediate to silicic plutons under pre-eruption conditions cannot yield homogeneous composition. Homogenisation can occur, however, if injection takes place in the early stages of magmatic evolution (i.e. at near liquidus conditions) but only in magmas of dacitic or more mafic composition. More generally, the potential for efficient mixing between silicic and mafic magmas sharing large interfaces at upper crustal levels is greater for dry basalts than for wet ones. At the other extreme, small mafic enclaves found in many granitoids behave essentially as rigid objects during a substantial part of the crystallisation interval of the host magmas, which implies that finite strain analyses carried out on such markers can give only a minimum estimate of the total amount of strain experienced by the host pluton. Mafic enclaves carried by granitic magmas behave as passive markers only at near solidus conditions, typically when the host granitic magma shows near-solid behaviour. Thus they cannot be used as fossil indicators of direction of magmatic flow.

Interaction between crustal-derived felsic and mantle-derived mafic magmas in the Oberkirch Pluton (European Variscides, Schwarzwald, Germany)

The composite Oberkirch pluton consists of three compositionally different units of peraluminous biotite granite. The northern unit is relatively mafic (SiO2∼64%) and lacks cordierite. The more felsic central and southern units (SiO2=67.8 to 70.4%) can only be distinguished from each other by the occurrence of cordierite in the former. Mafic microgranular enclaves of variable composition, texture and size occur in each of these units and are concentrated in their central domains. Most abundant are large (dm to m) hornblende-bearing enclaves with dioritic to tonalitic compositions (SiO2=50.8 to 56.3 wt%; Mg#=63 to 41) and fine grained doleritic textures that suggest chilling against the host granite magma. Some of these enclaves are mantled by hybrid zones. Less common are microtonalitic enclaves containing biotite as the only primary mafic phase (SiO2=53.7 to 64.4%) and small hybrid tonalitic to granodioritic enclaves and schlieren. Synplutonic dioritic dikes (up to 6 m thick) with hybrid transition zones to the host granite occur in the southern unit of the pluton. In chemical variation diagrams, samples from unmodified hornblende-bearing mafic enclaves and dikes form continuous trends that are compatible with an origin by fractionation of olivine, clinopyroxene, hornblende and plagioclase. Chemical and initial isotopic signatures (e.g. high Mg#, low Na2O, ɛNd=−1.2 to −5.1, 87Sr/86Sr=0.7055 to 0.7080, δ18O=8.0 to 8.8‰) exclude an origin by partial melting from a mafic meta-igneous source but favour derivation from a heterogeneous enriched lithospheric mantle. Samples from the granitic host rocks do not follow the chemical variation trends defined by the diorites but display large scatter. In addition, their initial isotopic characteristics (ɛNd=−4.5 to −6.8, 87Sr/86Sr=0.7071 to 0.7115, δ18O=9.9 to 11.9‰) show little overlap with those of the diorites. Most probably, the granitic magmas were derived from metapelitic sources characterized by variable amounts of garnet and plagioclase. This is suggested by relatively high molar ratios of Al2O3/(MgO+FeOtot) and K2O/Na2O, in combination with low ratios of CaO/(MgO+FeOtot), variable values of Sr/Nd, Eu/Eu*[=Eucn/(Smcn × Gdcn)0.5] and (Tb/Yb)cn (cn=chondrite-normalized) as well as variable abundances of Sc and Y. Whole-rock initial isotopic signatures of mafic microtonalitic enclaves (ɛNd=−4.6 to −5.2; 87Sr/86Sr=0.7060 to 0.7073; δ18O ∼8.1‰) are similar to those of the low ɛNd diorites. Plagioclase concentrates from a granite sample and a mafic microtonalitic enclave are characterized by initial 87Sr/86Sr ratios that are significantly higher than those of their bulk rock systems suggesting incorporation of high 87Sr/86Sr crustal material into the magmas. Field relationships and petrographic evidence suggest that the Oberkirch pluton originated by at least three pulses of granitic magma containing mafic magma globules. In-situ hybridization between the different magmas was limited. Late injection of dioritic magma into the almost solidified granitic southern unit resulted in the formation of more or less continuous synplutonic dikes surrounded by relatively thin hybrid zones.

Enclaves in the Cadillac Mountain Granite (Coastal Maine): Samples of Hybrid Magma from the Base of the Chamber

The Cadillac Mountain intrusive complex is dominated by the INTRODUCTION Cadillac Mountain granite and a 2–3 km thick section of interlayered Enclaves occur in nearly all granites and have recently gabbroic, dioritic and granitic rocks which occurs near the base of been the focus of many studies [see reviews by Vernon the granite. The layered rocks record hundreds of injections of basaltic (1983) and Didier & Barbarin (1991)]. Although enclaves magma that ponded on the chamber floor and variably interacted may have originated in many different ways and from with the overlying silicic magma. Magmatic enclaves, ranging in different sources, there has been a growing consensus composition from 55 to 78 wt % SiO2, are abundant in granite that there are textural criteria to recognize a class of above the layered mafic rocks. The most mafic enclaves are highly enclaves that formed from different magmas that were enriched in incompatible elements and depleted in compatible elemingled into the granitic magma while it was still mobile ments. Their compositions can be best explained by periodic (Vernon, 1984, 1990); these can appropriately be termed replenishment, mixing and fractional crystallization of basaltic magmatic enclaves. Their compositions are commonly magma at the base of the chamber. The intermediate to silicic intermediate but can vary from basaltic to highly silicic. enclaves formed by hybridization between the evolved basaltic magma Magmatic enclaves occur in all types of granite and are and resident silicic magma. There is little evidence for significant probably the dominant type of enclave in most calcexchange between enclaves and the enclosing granite. Instead, alkaline I-type granites. In spite of partial reequilibration, hybridization apparently occurred between stratified mafic and silicic enclaves of this type commonly have Nd isotopic commagmas at the base of the chamber. Enclaves in a restricted area positions that are distinct from the enclosing granite and commonly show distinctive compositional characteristics, suggesting support their formation from mantle-derived magmas they were derived from a discrete batch of hybrid magma. Enclaves (Holden et al., 1987). were probably dispersed into a localized portion of the granitic The compositions of magmatic enclaves can vary magma when replenishment or eruption disrupted the intermediate widely even within a single pluton, and they can only layer. rarely be interpreted as simple mixtures between an original basaltic magma and the host granitic magma. There is ample petrographic and chemical evidence in many enclaves for hybridization (mixing of liquids and

Effects of Magma Mingling in the Granites of Mount Desert Island, Maine

Textures and compositional relationships associated with dark-colored, fine-grained enclaves in the Cadillac Mountain and Somesville granites, Mount Desert Island, Maine, preserve abundant evidence for contamination of host granitic magmas by enclave liquids. Fine-grained enclaves, which apparently represent chilled magmatic droplets, have affected the composition and texture of the host granites by three possible mechanisms: (1) crystallization of feldspar-quartz-hornblende pegmatite pods from fluids of enclave origin in the granite surrounding enclaves, and the disaggregation of the pods and dispersion of crystals into the granite; (2) ionic exchange between enclaves and granitic magmas; (3) the generation around enclaves of rinds consisting of an inner alkali feldspar-quartz zone and an outer zone of hornblende-enriched granite. Coarse-grained pegmatite pods that occur outside of enclaves may result from migration of a hydrous residual fluid, evolved during enclave crystallization, out of the enclave, ...