The calc-alkaline granitoids of the central Sierra Nevada batholith are associated with abundant mafic rocks. These include both country-rock xenoliths and mafic magmatic enclaves (MME) that commonly ...have fine-grained and, less commonly, cumulate textures. Scarce composite enclaves consist of either xenoliths enclosed in MME, or of MME enclosed in other MME with different grain size and texture. Enclaves are often enclosed in mafic aggregates and form meter-size polygenic swarms, mostly in the margins of normally zoned plutons. Enclaves may locally divert schlieren layering. Mafic dikes, which also occur in swarms, are undisturbed, composite, or largely hybridized. In central Sierra Nevada, with the exception of xenoliths that completely differ from the other rocks, host granitoids, mafic aggregates, MME, and some composite dikes exhibit a bulk compositional diversity and, at the same time, important mineralogical and geochemical (including isotopic) similarities. MME and host granitoids display distinct major and trace element compositions. However, strong correlations between MME–host granitoid pairs indicate interactions and parallel evolution of MME and enclosing granitoid in each pluton. Identical mafic mineral compositions and isotopic features are the result of these interactions and parallel evolution. Mafic dikes have broadly the same major and trace element compositions as the MME although variations are large between the different dikes that are at distinctly different stages of hybridization and digestion by the host granitoids. The composition of the granitoids and various mafic rocks reflects three distinct stages of hybridization that occurred, respectively, at depth, during ascent and emplacement, and after emplacement. The occurrence and succession of hybridization processes were tightly controlled by the physical properties of the magmas. The sequential thorough or partial mixing and mingling were commonly followed by differentiation and segregation processes. Unusual MME that contain abundant large crystals of hornblende resulted from disruption of early cumulates at depth, whereas those richer in large crystals of biotite were formed by disruption of late mafic aggregates or schlieren layerings at the level of emplacement. MME and host granitoids are considered cogenetic, because both are hybrid rocks that were produced by the mixing of the same two components in different proportions. The felsic component was produced by partial melting of preexisting crustal materials, whereas the dominant mafic component was probably derived from the upper mantle. However, in the lack of a clear mantle signature, the origin of the mafic component remains questionable.
Granitoids are divided into several types according to their mineral assemblages, their field and petrographical features, and their chemical and isotopic characteristics. This typology complements ...most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field, petrographical and mineralogical criteria. It thus has the advantage of distinguishing the various granitoid types in the field, in most cases. The proposed classification shares many similarities with the twenty most used genetic classifications of granitoids. Both types of peraluminous granitoids are of crustal origin; the «tholeiitic», alkaline and peralkaline granitoids are of mantle origin; and both types of calc-alkaline granitoids are of mixed origin and involve both crustal and mantle materials. Each granitoid type is generated and emplaced in a very specific tectonic setting. Each stage of the Wilson cycle is characterised by typical associations of granitoids. Well-typed and precisely-dated granitoids can then complement structural approaches and indicate on the geodynamic environment. With reference to some case-studies, the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed.
Zircon is often used to study granites (sensu lato) and continental crust because it is very resistant and can be analyzed for various isotope systems that provide time and source information about ...their parental melts. Granites with different petrological histories have distinct bulk-rock silicon isotope compositions but it is unclear if these differences are also detectable in zircon because of superimposed fractionation effects (e.g., related to temperature, silica content, magmatic processes). The present study explores the Si isotope signatures of zircon from various granite types to constrain their isotope fractionation behavior and uses them as igneous petrogenetic tools, and possibly as granite source discriminators when zircon is found in detrital sediments. Our results show that although Si isotope compositions in zircon can be modified by secondary (post-crystallization) processes such as alteration/weathering and metamorphism, they are primarily controlled by zircon-melt isotope fractionation, which depends on both zircon crystallization temperature and magma silica content. Once these fractionation effects are understood and filtered out, a pattern emerges between Si isotope signatures of zircons from different granite types that is consistent with theoretical and experimental results as well as with known Si isotope differences at the bulk-rock scale. Silicon isotope ratios in zircon can track magma evolution (e.g., temperature and SiO2 changes) and, hence, reveal complex processes that involved magma mingling, fractional crystallization, and/or multiple sources. This study, therefore, illustrates that Si isotopes in zircon can be used to investigate magma evolution and represents a useful complement to existing techniques in granite studies involving zircon (e.g., U-Th-Pb, Lu-Hf and O isotopes) provided that it is not used as a stand-alone technique.
Mashhad granitoids and associated mafic microgranular enclaves (MMEs), in NE Iran record late early Mesozoic magmatism, was related to the Palaeo-Tethys closure and Iran-Eurasia collision. These ...represent ideal rocks to explore magmatic processes associated with Late Triassic closure of the Palaeo-Tethyan ocean and post-collisional magmatism. In this study, new geochronological data, whole-rock geochemistry, and Sr-Nd isotope data are presented for Mashhad granitoids and MMEs. LA-ICP-MS U-Pb dating of zircon yields crystallization ages of 205.0 ± 1.3 Ma for the MMEs, indicating their formation during the Late Triassic. This age is similar to the host granitoids. Our results including the major and trace elements discrimination diagrams, in combination with field and petrographic observations (such as ellipsoidal MMEs with feldspar megacrysts, disequilibrium textures of plagioclase), as well as mineral chemistry, suggest that MMEs formed by mixing of mafic and felsic magmas. The host granodiorite is a felsic, high K calc-alkaline I-type granitoid, with SiO
2
= 67.5-69.4 wt%, high K
2
O (2.4-4.2 wt%), and low Mg# (42.5-50.5). Normalized abundances of LREEs and LILEs are enriched relative to HREEs and HFSEs (e.g. Nb, Ti). Negative values of whole-rock εNd
(t)
(−3 to −2.3) from granitoids indicate that the precursor magma was generated by partial melting of enriched lithospheric mantle with some contributions from old lower continental crust. In the MMEs, SiO
2
(53.4-58.2 wt%) is lower and Ni (3.9-49.7 ppm), Cr (0.8-93.9 ppm), Mg# (42.81-62.84), and εNd
(t)
(−2.3 to +1.4) are higher than those in the host granodiorite, suggesting a greater contribution of mantle-derived mafic melts in the genesis of MMEs.
Andalusite occurs as an accessory mineral in many types of peraluminous felsic igneous rocks, including rhyolites, aplites, granites, pegmatites, and anatectic migmatites. Some published stability ...curves for And = Sil and the water-saturated granite solidus permit a small stability field for andalusite in equilibrium with felsic melts. We examine 108 samples of andalusite-bearing felsic rocks from more than 40 localities world-wide. Our purpose is to determine the origin of andalusite, including the T–P–X controls on andalusite formation, using eight textural and chemical criteria: size—compatibility with grain sizes of igneous minerals in the same rock; shape—ranging from euhedral to anhedral, with no simple correlation with origin; state of aggregation—single grains or clusters of grains; association with muscovite—with or without rims of monocrystalline or polycrystalline muscovite; inclusions—rare mineral inclusions and melt inclusions; chemical composition—andalusite with little significant chemical variation, except in iron content (0·08–1·71 wt % FeO); compositional zoning—concentric, sector, patchy, oscillatory zoning cryptically reflect growth conditions; compositions of coexisting phases—biotites with high siderophyllite–eastonite contents (Aliv ≈ 2·68 ± 0·07 atoms per formula unit), muscovites with 0·57–4·01 wt % FeO and 0·02–2·85 wt % TiO2, and apatites with 3·53 ± 0·18 wt % F. Coexisting muscovite–biotite pairs have a wide range of F contents, and FBt = 1·612FMs + 0·015. Most coexisting minerals have compositions consistent with equilibration at magmatic conditions. The three principal genetic types of andalusite in felsic igneous rocks are: Type 1 Metamorphic—(a) prograde metamorphic (in thermally metamorphosed peraluminous granites), (b) retrograde metamorphic (inversion from sillimanite of unspecified origin), (c) xenocrystic (derivation from local country rocks), and (d) restitic (derivation from source regions); Type 2 Magmatic—(a) peritectic (water-undersaturated, T↑) associated with leucosomes in migmatites, (b) peritectic (water-undersaturated, T↓), as reaction rims on garnet or cordierite, (c) cotectic (water-undersaturated, T↓) direct crystallization from a silicate melt, and (d) pegmatitic (water-saturated, T↓), associated with aplite–pegmatite contacts or pegmatitic portion alone; Type 3 Metasomatic—(water-saturated, magma-absent), spatially related to structural discontinuities in host, replacement of feldspar and/or biotite, intergrowths with quartz. The great majority of our andalusite samples show one or more textural or chemical criteria suggesting a magmatic origin. Of the many possible controls on the formation of andalusite (excess Al2O3, water concentration and fluid evolution, high Be–B–Li–P, high F, high Fe–Mn–Ti, and kinetic considerations), the two most important factors appear to be excess Al2O3 and the effect of releasing water (either to strip alkalis from the melt or to reduce alumina solubility in the melt). Of particular importance is the evidence for magmatic andalusite in granites showing no significant depression of the solidus, suggesting that the And = Sil equilibrium must cross the granite solidus rather than lie below it. Magmatic andalusite, however formed, is susceptible to supra- or sub-solidus reaction to produce muscovite. In many cases, textural evidence of this reaction remains, but in other cases muscovite may completely replace andalusite leaving little or no evidence of its former existence.
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