A-type granites and related rocks: Evolution of a concept, problems and prospects

An evaluation of the Rb vs. (Y + Nb) discrimination diagram to infer tectonic setting of silicic igneous rocks

https://petrol.natur.cuni.cz/~janousek/abstracts/sazava_lithos.pdf

Magma-mixing in the genesis of Hercynian calc-alkaline granitoids: an integrated petrographic and geochemical study of the Sázava intrusion, Central Bohemian Pluton, Czech Republic

The discrimination between potential source materials involved in the genesis of Iberian granites and granodiorites, as well as the role of mantle-crust inter- actions, are examined using constraints imposed by melting experiments, melting-assimilation experiments and Sr-Nd isotope systematics. The Sr-Nd isotope re- lationships indicate the existence of diAerent genetic trends in which juvenile mantle materials are involved by diAerent mechanisms: (1) a source trend, traced by a particular evolution of the pre-Hercynian basement and indicating mantle participation at the time of sedimen- tation; (2) a set of magmatic trends traced by gabbro- tonalite-enclave-granodiorite associations, implying the incorporation of new mantle material at the time of granite generation. These relationships strongly support a pure crustal origin for the peraluminous leucogranites, derived from partial melting of crustal protoliths, and a hybrid origin for the peraluminous granodiorites. These granodiorites are the most abundant granitic rocks of the Central Iberian zone (CIZ) of the Iberian massif, implying that processes of hybridisation by assimilation and/or magma mixing played an important role in granitoid production during the Hercynian orogeny. These hypotheses have been tested by means of melting and assimilation experiments. Melting experiments in the range 800-900 ∞C and at pressures of 3, 6, 10 and 15 kbar indicate that: (1) several potential source materials such as Bt-Ms gneisses and metagreywackes are suitable for the production of peraluminous leucogranite melts; (2) the melt compositions are always leucogranitic, re- gardless of pressure; (3) pressure exerts a strong influ- ence on the fertility of the source: experiments at 3 kbar produce more than 20 vol% of melt, compared with less than 5 vol% of melt produced at 10 and 15 kbar and at the same temperature. The melting-assimilation experi- ments carried out at 1000 ∞C and 4, 7 and 10 kbar and using a proportion of 50% gabbro and 50% gneiss give high melt proportions (more than 50 vol.%) and noritic residues. These melts have the composition of le- ucogranodiorites, and overlap with part of the compo- sitional range of peraluminous granodiorites of the Iberian massif. The generation of more mafic granodiorites may be explained by the incorporation of some residual orthopyroxene to the granodiorite mag- mas. The low solubility of Fe + Mg prevents the gen- eration of granodiorite melts with more than 3 wt% of MgO + FeO at all crustal pressures. The large volumes of peraluminous, hybrid granodiorites, produced by assimilation of crustal rocks by mantle magmas, imply that an important episode of crustal growth took place during the Late-Palaeozoic Hercynian orogeny in the Iberian massif.

/pdf/origin-of-peraluminous-granites-and-granodiorites-iberian-lobtp5kyw7.pdf

Origin of peraluminous granites and granodiorites, Iberian massif, Spain: an experimental test of granite petrogenesis

The Deddick Granodiorite, a mafic S-type pluton in the Lachlan INTRODUCTION Fold Belt, contains abundant enclaves. Most are derived from Enclaves in granites and volcanic rocks can provide high-grade metasediments of pelitic–psammitic composition, and petrogenetic information not readily available from their migmatites are common. Among these, discrete fragments of host rocks (Didier, 1973; Didier & Barbarin, 1991). For melanosomes rich in cordierite and garnet are restites from partial example, metasedimentary enclaves can place constraints melting. However, compositional and Nd–Sr isotopic data indicate on the age, composition and deformational history of they are not restite in equilibrium with the melt component of the magma source regions within continental crust (Fleming, host magma, but may have formed part of a spectrum of diverse 1991; Steele et al., 1991). Enclaves formed by acmagma source lithologies. Alternatively, they may be accidental cumulation of early phenocrysts (‘autoliths’) document xenoliths. Regardless of their origin, data for these enclaves suggest the early evolution of the host magma, whereas others their precursors could represent deeply buried age equivalents of the may represent commingled magmas and illustrate the ubiquitous Ordovician turbidites of the Lachlan Fold Belt. Three possible role of hybridization (Didier, 1987). types of microgranular enclaves with igneous textures are disEnclave suites in the relatively mafic S-type granitoids tinguished. The most common type are small and rounded mafic of SE Australia are often diverse, including abundant enclaves of tonalitic composition. They carry xenocrysts derived from enclaves of metasedimentary origin and microgranular the host magma, and some contain high-Mg pyroxene derived from enclaves (Phillips et al., 1981; Price, 1983; Chen et al., a mafic magma. Isotopic data form an array with eNd from –6 to 1989; Wyborn et al., 1991). The metasedimentary en–12, and Sr/Sr from 0·7130 to 0·7167 (host rock –10 and claves are lithologically diverse, including gneissic– 0·715, respectively). These enclaves formed as globules of hybrid migmatitic types and biotite-rich ‘surmicaceous’ types. mafic magma commingled with, and contaminated by, the more Apart from their potential as indicators of crustal structure felsic host magma. Other enclave types were derived from disrupted and composition, they are of interest as possible fragments syn-plutonic dykes having a distinct isotopic composition, and from of source material and/or refractory residue from which a cogenetic marginal facies of the host. peraluminous melts were generated and extracted. By

/pdf/igneous-and-metamorphic-enclaves-in-the-s-type-deddick-3ydx318qr9.pdf

Igneous and Metamorphic Enclaves in the S-type Deddick Granodiorite, Lachlan Fold Belt, SE Australia: Petrographic, Geochemical and Nd-Sr Isotopic Evidence for Crustal Melting and Magma Mixing

H-type (hybrid) granitoids: a proposed revision of the granite-type classification and nomenclature

Many Hercynian granitoids of calc-alkaline composition are characterized by the presence of microgranular enclaves of tonalite to diorite composition. Though some differences in size, shape, and relative abundance of the enclaves are observed between deep-seated, high-level, and subvolcanic plutons, there are considerable similarities in texture, grain size, and composition. In magma interaction zones where basic and silicic magmas mingled, enclaves were generated from the disruption, by convective flow, of synplutonic dykes of basic, fractionated, perhaps hybrid, magmas. Many enclaves can be interpreted as non-consolidated portions of basic magma in thermal equilibrium with the host granitoid. Under such condition, enclaves are easily disaggregated and mixed with the host granitoid, contributing to the hybridization of the latter. The process implies the development of special textures in both enclave and host granitoid, and may be tested by chemical variations and correlations, suggesting that the presence of enclaves, with these particular textures and compositions, may imply that a form of hybridization was involved in the genesis of the host granitoids.

Microgranular enclaves as indicators of hybridization processes in granitoid rocks, Hercynian Belt, Spain

The crystallization sequence, nucleation density and crystal index (n = nucleation density/mode) of tonalitic enclaves from different granitoids of the Hercynian orogeny indicate that they are pieces of magma partially crystallized in the interior of synplutonic bodies of basic magma, which was injected into a silicic magma chamber. A rapid cooling stage can be identified from the high nucleation density and high n-values of minerals. A final stage of slow cooling, is identified by a low nucleation density and low n-values. At this stage a residual melt crystallized in thermal equilibrium to the cooling rate of the whole magma chamber. Thermal equilibrium and convection are necessary conditions for mixing. Enclaves can be interpreted as the remaining, non-disaggregated portions of an early injected mafic magma, which in turn underwent hybridization during injection. Their presence indicates the existence of magma mixing processes and the possibility that the host granitoid was a hybrid rock.

J. D. de la Rosa

Papers

H-type (hybrid) granitoids: a proposed revision of the granite-type classification and nomenclature

Microgranular enclaves as indicators of hybridization processes in granitoid rocks, Hercynian Belt, Spain

Multistage crystallization of tonalitic enclaves in granitoid rocks (Hercynian belt, Spain): implications for magma mixing