Granite Enclaves Research Articles

Cognate Mafic Enclaves Formed by Cumulated Mush Convection in a Granitic Magma Chamber: Evidence from Mineral Chemistry of the Triassic Zhashui Pluton, Qinling Orogen

AbstractMafic enclaves in granites are generally considered to represent coeval mafic melts that derived from metasomatized mantle, which can provide valuable information about crust–mantle interaction. Exploring the genetic links between the mafic enclaves and their host monzogranite from the Triassic Zhashui Pluton, Qinling orogenic belt. The enclaves (220 ± 4.6 Ma) and the monzogranite (220 ± 2.8 Ma) display identical zircon U‐Pb ages, and they also share similar trace element and zircon Lu‐Hf isotopes, indicating a cognate source. The monzogranite displays zircon εHf(t) values of –0.99 to +1.98, while the mafic enclaves show similar values of –0.45 to +3.26; however, differences in mineral chemistry reveal different crystallization conditions. The amphibole from the mafic enclaves has higher temperature and pressure (757°C; 2.65 kbar) compared to those of the host monzogranite (733°C; 1.96 kbar), suggesting that mafic minerals in the enclaves crystallized at an early stage. Moreover, apatite in the mafic enclaves displays slightly higher volatile contents (0.72 wt%) than those of the monzogranite (0.66 wt%), indicating a volatile‐rich condition. These results suggest that the mafic enclaves represent early hydrous mafic cumulates in the granitic chamber, and subsequent magma convection would have led to the formation of the mafic enclaves.

Role of fractional crystallization, fluid-melt separation, and alteration on the Li and B isotopic composition of a highly evolved composite granite pluton: The case of the Eibenstock granite, Erzgebirge, Germany

We present a comprehensive Li and B isotope study of granites, aplites, and igneous enclaves from the multi-phase Eibenstock granite in the Western Erzgebirge-Vogtland metallogenic province of Germany. The studied samples cover the entire compositional range of the granites from moderately to highly evolved and include variably altered types as obtained by magmatic fractionation, post-magmatic high- to medium-temperature and near-surface low-temperature alteration. Fractionation and alteration processes are unequivocally documented by the chemical variability of the rocks. Despite the marked imprint of these processes on bulk-rock compositions, our granite samples show only little variation in δ 7 Li (−0.52 to 0.75‰) and δ 11 B (−17.46 to −14.78‰), with surface samples defining the lower end of the δ 7 Li range. The narrow range in δ 7 Li suggests that magmatic fractionation and high-temperature overprint have a very minor effect on δ 7 Li. The B budget of the samples is dominated by tourmaline, which makes δ 11 B values insensitive for later high- to medium-temperature overprint or surficial low-temperature alteration. Depending on whether tourmaline crystallized before or after exsolution and loss of magmatic fluids, whole-rock samples have higher or lower δ 11 B values. Granite enclaves have δ 7 Li and δ 11 B values ranging from −1.51 to −0.81‰ and − 14.55 to −13.89‰, respectively. Some samples have chemical and mineralogical evidence for wall-rock interaction during emplacement or later overprint by external fluids. These samples show broader ranges in δ 7 Li (−2.61 to 2.21‰) and δ 11 B (−21.58 to −9.85‰). These values show that wall-rock interaction via assimilation and external fluids may affect δ 7 Li and δ 11 B to a larger extent than intra-magmatic processes, such as fractional crystallization, fluid-mediated autometasomatic overprinting, or exsolution of fluids from the melt. The offset of δ 7 Li and δ 11 B values towards the compositions of the wall rocks reflects the contrasting composition of granite and country rock and the addition of country-rock material to the granite. The magnitude of the offset reflects both the relative contribution of wall-rock derived Li and B to the granite and the magnitude of the difference in the Li and B isotopic compositions between them. • Whole rock δ 7 Li and δ 11 B are essentially unaffected by magmatic fractionation. • Fluid-melt separation may reduce δ 7 Li and δ 11 B in residual melt. • Effect of intra-granitic alteration on δ 7 Li and δ 11 B of granitic rocks is small. • Effects on δ 7 Li and δ 11 B during alteration depend on fluid source.

Permian Remelting and Maturity of Continental Crust Revealed by the Daqing Peraluminous Granitic Batholith, Inner Mongolia

AbstractThe Phanerozoic granites in northeast China bear key information for studying the tectonic evolution and crustal growth or reworking in the Central Asian Orogenic Belt (CAOB). The Daqing granitic batholith widely outcrops as a high-level intrusion in the Xing’an-Mongolia Orogenic Belt, southeastern CAOB. Three types of enclaves in granites have been identified: (1) mafic magmatic enclaves (MMEs), (2) volcanic xenoliths, and (3) biotite-rich enclaves. The batholith is mainly composed of peraluminous biotite granite and granodiorite with SiO2=63.95−69.48 wt.%, A/CNK=1.15−1.27, and 2.54 to 4.30 wt.% of normative corundum. They exhibit remarkable enrichment in large ion lithophile elements (LILEs; K, Rb, Th, and Pb) and depletion in high field strength elements (HFSEs; Nb, Ta, and Ti), P, Eu, and Sr, as well as relatively enriched Sr-Nd isotopes (87Sr/86Sri=0.70530−0.70576, εNdt=–0.1−+0.2). Zircon U-Pb dating suggests that this batholith was emplaced in the Early Permian (ca. 283-282 Ma), consistent with a period of intensive magmatic activities in northern Inner Mongolia. The Nb/Ta ratios of MMEs (17.6-20.1) are higher than those of the host granites (11.4-12.5), together with the reaction rims where biotite crystals cluster around the amphibole cores, suggesting magma mixing between mantle- and crust-derived melts. Zircons from a biotite-rich enclave define a protolith age of ca. 320 Ma and an anatectic age at ca. 281 Ma. Whole-rock Sr-Nd isotopic modelling and zircon Hf isotopes reveal that the batholith was mainly produced by remelting of newly accreted continental crust with minor addition of mantle-derived materials. The geochemical compositions imply that their precursor magmas originated from a relatively high crustal level (<5 kbar) with crystallization temperatures ranging from 800 to 850°C. We suggest that the Daqing peraluminous granitoids were derived from partial melting of newly accreted crustal materials at a relatively shallow crustal depth, associated with a ridge subduction-related heat source. Such mantle-derived magmas through a slab tear window resulting from ridge subduction provide not only the heat for the widespread crustal remelting and therefore maturity but also juvenile materials for crustal growth.

Rapid Assembly and Eruption of a Shallow Silicic Magma Reservoir, Reyðarártindur Pluton, Southeast Iceland

AbstractAlthough it is widely accepted that shallow silicic magma reservoirs exist, and can feed eruptions, their dynamics and longevity are a topic of debate. Here, we use field mapping, geochemistry, 3D pluton reconstruction and a thermal model to investigate the assembly and eruptive history of the shallow Reyðarártindur Pluton, southeast Iceland. Primarily, the exposed pluton is constructed of a single rock unit, the Main Granite (69.9–77.7 wt.% SiO2). Two further units are locally exposed as enclaves at the base of the exposure, the Granite Enclaves (67.4–70.2 wt.% SiO2), and the Quartz Monzonite Enclaves (61.8–67.3 wt.% SiO2). Geochemically, the units are related and were likely derived from the same source reservoir. In 3D, the pluton has a shape characterized by flat roof segments that are vertically offset and a volume of >2.5 km3. The pluton roof is intruded by dikes from the pluton, and in two locations displays depressions associated with large dikes. Within these particular dikes the rock is partially to wholly tuffisitic, and rock compositions range from quartz monzonite to granite. We interpret these zones as eruption‐feeding conduits from the pluton. A lack of cooling contacts throughout the pluton indicates rapid magma emplacement and a thermal model calculates the top 75 m would have rheologically locked up within 1,000 years. Hence, we argue that the Reyðarártindur Pluton was an ephemeral part of the wider plumbing system that feeds a volcano, and that timeframes from emplacement to eruption were rapid.

Schedule of Mafic to Hybrid Magma Injections Into Crystallizing Felsic Magma Chambers and Resultant Geometry of Enclaves in Granites: New Field and Petrographic Observations From Ladakh Batholith, Trans-Himalaya, India

Diorites, granites, and associated magmatic enclaves and dykes constitute the bulk of the Ladakh Batholith, which is an integral part of the Trans-Himalayan magmatic arc system. In this paper, geometry of microgranular enclaves hosted in the granites has been examined from the Leh-Sabu-Chang La and surrounding regions of the eastern Ladakh Batholith to infer the mechanism and schedule of mafic to hybrid magma injections into evolving felsic magma chambers and the resultant enclave geometry. Mafic and/or hybrid magmas injected into felsic magma at low volume fraction ( 0.65) of crystals. A large rheological difference between coeval felsic and mafic magmas inhibits much interaction. Mafic magma progressively crystallizes and evolves while minimizing thermal and rheological differences. Consequently, the felsic-mafic magma interaction process gradually becomes more efficient causing dispersion of enclave magma globules and undercooling into the partly crystalline felsic host magma. Thus, the evolution of the Ladakh Batholith should be viewed as multistage interactions of mafic to hybrid magmas coeval with felsic magma pulses in plutonic conditions from its initial to waning stages of evolution.

Mechanical and structural consequences of magma differentiation at ascent conduits: A possible origin for some mafic microgranular enclaves in granites

This work evaluates the origin of mafic microgranular enclaves in granites through a process of magma splitting at the boundaries of ascent conduits, where an intermediate, high-silica andesite magma separates into two magma systems without a continuous change, one of Qz-dioritic composition and high-crystal content and another one of granite (sensu lato) composition and mostly liquid. Asemi-quantitative approximation considering the balance between advective and diffusive heat transfer has been followed, considering an order of magnitude procedure to estimate the thickness of the thermal boundary layer within an instantaneously emplaced dike. The resulting temperature gradient across the dike, and the expected geochemical variation evidenced by previous experimental results, have been used to determine crystallinity, viscosity, flow velocity, and shear stress profiles at a single transverse section, at 500 MPa of lithostatic pressure. The Qz-dioritic magma generated at the borders of the dike develops a strong crystallinity gradient, with a mafic chilled margin near the external contact of the dike, while crystal fraction remains very low (<10 vol%) in the differentiated granitic melt in the dike interior. The mafic chilled margin has a predominantly brittle behavior and can be broken by movement of the dike and host rock into fragments of sizes comparable to those observed in nature. Reheating and remelting of dike walls generates a strong decrease in the viscosity of the mush close to the internal geochemical contact with the granitic melt. This process leads to the formation of highly deformable channels that can entrain fragments of the previous chilled margin giving place to schlieren and enclave swarms, among many other structures. The process can be repeated through the sequential ascent of distinct magma batches transporting the mafic enclaves up to their final level of emplacement in granitic plutons and batholiths.

Three tier transition of Neoarchean TTG-sanukitoid magmatism in the Beit Bridge Complex, Southern Africa

Neoarchean TTG-sanukitoid associations of contrasting scales occur within the Beit Bridge Complex terrane of the Limpopo Complex in southern Africa. These include the smaller ~2.65–2.63Ga Avoca granitoid and the voluminous ~2.73–2.64Ga Alldays granitoid. This study characterizes the wide compositional spectrum preserved in these two granitoids. The elliptical Avoca pluton consists of a biotite-amphibole-orthopyroxene±clinopyroxene-bearing core that is dominantly trondhjemite with less dominant tonalite and granodiorite variants, and a thin amphibole-biotite-bearing granite rim, with local occurrence of two-pyroxene-bearing metabasite boudins. While both the core and rim rocks exhibit a linear fabric, the granite in addition preserves a penetrative foliation. Field relations of granite enclaves in the core rocks together with available ages indicate that the core rocks intruded the granite. The foliated biotite±amphibole-bearing Alldays granitoid contains inclusions of older supracrustals and rocks of the Messina layered intrusion, and is widely distributed. Compositionally, it include tonalites and granodiorites and to a lesser extent trondhjemites. Both the Avoca core and rim rocks are characterized by difference in mineral chemistry, with the mafic minerals Mg-rich in the TTG core, while they are Fe-rich in the granite and metabasite. In comparison, biotite is Mg-rich and amphibole is Fe-rich in the Alldays granitoid.Two groups of Alldays TTG can be delineated in terms of whole-rock geochemical characteristics, and are comparable to the low- to medium-pressure TTG groups delineated by Moyen (2011), while the Avoca TTG is similar to the high-pressure TTG group. The lowest silica samples from each group of granitoid have geochemical characteristics comparable to Archean sanukitoids, with those from the Avoca granitoid similar to low-Ti sanukitoids, and those from the Alldays granitoid similar to low-Ti and high-Ti sanukitoids. Separate petrogenetic models are suggested for different phases of the Avoca core, with the trondhjemite-tonalites considered as high-pressure melts of metabasalt, while the granodiorite with lower SiO2 content, higher K2O and MgO contents, and higher incompatible element contents, than the trondhjemite-tonalites, is a product of hybridization of earlier TTG melts and peridodite. Granite from the Avoca rim are low-pressure melts of pre-existing crustal lithologies. The two groups of Alldays TTG with lower Sr/Y ratios than the Avoca TTG are considered as low- to medium-pressure melts of metabasalt, whose progressive interaction with peridotitic mantle at shallower angles account for the unique composition of Alldays low-Ti and high-Ti sanukitoids. Taken together with their spatial and temporal transition from southeastern (~2.73–2.72Ga; low-pressure TTG-low-Ti sanukitoid) to central (~2.65–2.64Ga; medium-pressure TTG-high-Ti sanukitoid) to northwestern (~2.63Ga; high-pressure TTG-low-Ti sanukitoid) parts of the Beit Bridge Complex, the three tier transition of TTG-sanukitoid magmatism argues for the southern margin of the Beit Bridge Complex to represent an active arc in the Neoarchean.

Origins of igneous microgranular enclaves in granites: the example of Central Victoria, Australia

To investigate their genesis and relations with their host rocks, we study igneous microgranular enclaves (IMEs) in the c. 370 Ma, post-orogenic, high-level, felsic plutons and volcanic rocks of Central Victoria, Australia. The IMEs are thermally quenched magma globules but are not autoliths, and they do not form mixing series with their host magmas. These IMEs generally represent hybrids between mantle-derived magmas and very high-T crust-derived melts, modified by fractionation, ingestion of host-derived crystals and, to a lesser extent, by chemical interactions with their hosts. Isotopic and elemental evidence suggests that their likely mafic progenitors formed by partial melting of subcontinental mantle, but that the IME suites from different felsic host bodies did not share a common initial composition. We infer that melts of heterogeneous mantle underwent high-T hybridisation with melts from a variety of crustal rocks, which led to a high degree of primary variability in the IME magmas. Our model for the formation of the Central Victorian IMEs is likely to be applicable to other occurrences, especially in suites of postorogenic granitic magmas emplaced in the shallow crust. However, there are many different origins for the mingled magma globules that we call IMEs, and different phenomena seem to occur in differing tectonic settings. The complexity of IME formation means that it is difficult to unravel the petrogenesis of these products of chaotic magma processes. Nevertheless, the survival of fine-grained, non-equilibrium mineralogy and texture in the IMEs suggests that their tenure in the host magmas must have been geologically brief.

Zircon U–Pb dating from the mafic enclaves in the Tanzawa Tonalitic Pluton, Japan: Implications for arc history and formation age of the lower-crust

The petro-chemical characteristics of the arc lower-crust, important for understanding continental growth, have been rarely obtained because of their scarcity at the surface of the Earth. To constrain the formation age of the arc lower-crust, U–Pb zircon dating was applied to mafic enclaves in tonalites of the Tanzawa Tonalitic Pluton (TTP), which is regarded as the exposed middle crust of the former Izu–Bonin–Mariana arc, using a laser ablation-inductively coupled plasma–mass spectrometer (LA-ICP-MS).The texture and shape of mafic enclaves indicate an injection of mafic magma into tonalitic magma at the mid-crustal level. While 44 zircon grains from a host tonalite show a narrow-age distribution with a mean age of 4.6±0.2Ma, 301 zircon grains from 9 mafic enclaves show wide-age distributions from ca. 5 to 43Ma. This study is the first to reveal a U–Pb age older than previously reported for the rock materials that compose the TTP, now identified to be 18Ma compared with an age range from 4Ma to 9Ma. Because there are no other components of the TTP yet identified to be older than 17Ma, the zircons separated from the TTP in this study which dated to be 18–43Ma are interpreted to be xenocrysts derived from the arc lower-crust beneath the TTP. The oldest zircon age obtained from the mafic enclaves indicates that the formation of the arc lower-crust beneath TTP took place before 42.9±8.6Ma, consistent with the history of the Izu–Bonin–Mariana arc. This is the first time that the formation age of the lower crust has been estimated using zircons from mafic enclaves. This study shows that the zircon U–Pb dating from mafic enclaves in granites can yield significant information about the age of the continental lower crust.

A tin-mineralized topaz rhyolite dike with coeval topaz granite enclaves at Qiguling in the Qitianling tin district, southern China

The Qiguling topaz rhyolite is present as a dike within the Qitianling biotite granite batholith of the Nanling Range of southern China. Here, the rhyolitic dike, 4.5m wide and 500m long, contains enclaves of topaz granite. These rhyolites contain up to 72wt.% SiO2, have alumina saturation index (ASI) >1.1, and have groundmasses with estimated fluorine contents of approximately 1.5wt.%. Textural relationships provide evidence of a quenched silicate melt that contains quartz, K-feldspar, albite, and zinnwaldite phenocrysts in a groundmass containing abundant topaz. The rhyolites in the study area are also strongly enriched in tin (90–2700ppm), and generally have a close association between cassiterite and zinnwaldite, although cassiterite is also present as sponge-textured fills between rock-forming minerals. Granite enclaves and their hosted rhyolite have similar major geochemical compositions and mineralogies to each other. Zircon U–Pb dating indicates that the topaz rhyolite (147–150Ma) and topaz granite enclaves (154Ma) were formed contemporaneously, with ages that overlap within analytical uncertainty. In addition, the major and trace element compositions of the rhyolite and their granite enclaves are dissimilar to those of the hosting Qitianling biotite granite. This discovery of granite enclaves within rhyolite dikes suggests the presence of a topaz-bearing granite body at depth that may host tin mineralization. The expected hidden tin granite may be of great interest in the further exploration.

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.

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

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

Inherited deformation structures in metasedimentary enclaves in granites as “windows” into deeper levels of the crust

Deformation structures preserved in metasedimentary enclaves in granites can provide information on the deformation history of the granite source region up to the time of melt generation, particularly where the inclusions record more deformations than were present in the country rock at the time of emplacement. The data provide a “window” to the source that is additional to that derived from petrology and geochemistry. The improved view can give extra information about the nature of the source rocks and their structural/tectonic history. If applied across a wide region such as an orogenic belt, the approach could provide fundamental information on large expanses of unexposed crust, and could help delineate suspected (or unsuspected) boundaries between terranes or basement terranes. Examples from two granitic plutons in the Palaeozoic Lachlan Foldbelt of southeastern Australia support independent assertions that at least in places the foldbelt structurally overlies pre-Ordovician (Late Proterozoic? Cambrian?) continental crust rather than oceanic crust. Alternatively, it overlies Ordovician continental crust, the structural and chemical composition of which are different from Ordovician rocks at surface.

Observation Versus Argument by Authority - The Origin of Enclaves in Granites

The essentially human nature of science is well illustrated by the long-standing success of a very unsatisfactory hypothesis for the origin of igneous-looking enclaves (“microgranitoid enclaves,” “...

The quantitative models of trace elements for different genetic types of enclaves in granites

An attempt has been made to give an insight into the genesis of enclaves in granites by mathematically quantitative methods. After some deduction, the quantitative models of trace elements for the genetically different enclaves have been established, including those for restites, segregation schlierens, enclaves formed out of solidified margins, and enclaves derived from the mixing of different magmas. These models have been tested and proven to be valid and reliable. The conclusions inferred from these quantitative models are consistent with field observations and petrological, mineralogical and geochemical evidence.