Anatectic Source Research Articles

Control of anatectic source rock and melt segregation on metal fertility of W–Sn granites in Sirohi region, NW India

Control of anatectic source rock and melt segregation on metal fertility of W–Sn granites in Sirohi region, NW India

Partial melting mechanisms of peraluminous felsic magmatism in a collisional orogen: An example from the Khondalite belt, North China craton

AbstractSedimentary‐derived (S‐type) granites are an important product of orogenic metamorphism, and a range of subtypes can be recognized by differences in field occurrence, mineralogy and geochemistry. These subtypes can reflect variations of initial protolith composition, partial melting reactions, pressure and temperature of anatexis, or magmatic processes that occur during ascent through the crust (e.g. mineral fractional crystallization or crustal assimilation). Together, these diverse factors complicate geological interpretation of the history of peraluminous felsic melt fractions in orogenic settings. To assess the influence of these factors, we performed integrated field investigation, petrology, geochemistry, geochronology and phase equilibrium modelling on a series of leucosomes within migmatite associated with different S‐type granites within the Khondalite belt, North China craton (NCC), which is an archetypal collisional orogen. Three types of leucosome are recognized in the east Khondalite belt: leucogranitic leucosome, K‐feldspar (Kfs)‐rich granitic leucosome and garnet (Grt)‐rich granitic leucosome. Phase equilibrium modelling of partial melting and fractional crystallization processes indicate that the leucogranitic leucosomes were mostly produced through fluid‐present melting, Kfs‐rich granitic leucosomes are produced through muscovite dehydration melting with 3 vol.% garnet fractional crystallization, and Grt‐rich granitic leucosomes are produced through biotite dehydration melting with 20–40 vol.% K‐feldspar fractional crystallization and up to 20 vol.% peritectic garnet entrainment. Mineral fractional crystallization and peritectic mineral entrainment occur in the source during melting, and play equally important roles in partial melting mechanisms in terms of affecting the geochemical compositions of granitic melts. Thus, we suggest that peraluminous felsic magmas preserved in collisional orogens are dominantly produced by fluid‐absent melting in the middle to deep continental crust, although extraction of low‐volume melt fractions from an anatectic source region at shallower depths during fluid‐present melting can also generate small amounts of S‐type granites that subsequently crystallize at high structural levels in the crust.

The stabilization of self-organised leucogranite networks—Implications for melt segregation and far-field melt transfer in the continental crust

Net-structured leucocratic vein systems are ubiquitous in high-grade and migmatite terrains and are interpreted to represent interconnected former magma-bearing structures that accommodated the migration of granitic magmas out of their anatectic source regions. Net-structured leucogranite vein systems in high-grade gneisses from the Damara Belt in Namibia describe a size-based hierarchy and spatial distribution that indicates self-organisation of the magma transfer system. The internal geometry and connectivity of the net-structured leucogranites are controlled by variations in melt pressure, whereas regional strains control the orientation of the leucogranites. Foliation-parallel (stromatic) and shear-band-hosted granite veins accommodate transport at relatively low melt volumes and account for the metre-scale segregation of melt close to the anatectic source. During times of higher melt supply, the formation of extensional fractures that cross-cut stromatic and shear-band-hosted melt sheets at high angles is promoted. Extensional fracturing and associated dilatancy create gradients in magmastatic head that cause melt to be drained from pervasive stromatic and shear-band-hosted vein networks into spaced dilatant fractures. This has the potential to initiate buoyancy-driven far-field melt transfer along suitably orientated extensional fractures as vertical dykes or self-propagating hydrofractures. Our results suggest that melt transfer systems are able to adjust to variations in melt supply by forming conduits that accommodate varying melt volumes and will tend to develop towards a state of self-organisation.

The dike swarm of the Karakoram shear zone, Ladakh, NW India: Linking granite source to batholith

Debate regarding migration of granitic magmas arises from the fact that the physical link between migmatites and plutons and batholiths is rarely documented. Within the Karakoram shear zone, Ladakh, NW India, the synkinematic transfer of magma can be traced from the anatectic source region to the Karakoram Batholith through a complex dike swarm. Melting is due to water fluxing at upper-amphibolite facies, producing Miocene hornblende and garnet–two mica leucogranites. The anatectic zone is characterized by synmagmatic folding and shearing of migmatites, and by pervasive, irregular magma migration paths forming injection complexes. These irregular networks are linked to dike networks that are interpreted to represent a magma transfer zone characterized by a range of styles, such as (1) dike swarms parallel to the shear plane, (2) dikes following the two major foliation planes in the shear fabric, (3) magma sheets in conjugate pairs of ductile fractures, or (4) chaotic dike complexes. Network geometries are controlled by the regional stress field, strain distribution, preexisting anisotropies, and rheological contrasts. Dikes give rise to anastomosing systems with rare crosscutting relationships, and with intersections oriented parallel to the dominant mineral stretching lineation. Dikes feed a number of sills, stocks, plutons, and, ultimately, the leucogranitic Karakoram Batholith. We propose that magma developed an interlinked, continuous dike swarm from source to sink by interacting with deformation and preexisting anisotropies. Unlike theoretical and laboratory predictions, dikes are typically oriented at high angles to the maximum shortening axis, and unlike expectations, water-fluxed melting of the crust produced mobile magma that migrated to form large granitic bodies.

Microstructural evidence for magma confluence and reusage of magma pathways: implications for magma hybridization, Karakoram Shear Zone in NW India

In the Karakoram Shear Zone, Ladakh, NW India, Miocene leucogranitic dykes form an extensive, varied and complex network, linking an anatectic terrane exposed in the Pangong Range, with leucogranites of the Karakoram Batholith. Mineral paragenesis of the heterogeneous anatectic source rocks suggests melting has resulted from water influx into rocks at upper amphibolite facies conditions, and microstructures suggest anatexis was contemporaneous with shearing. The network is characterized by continuous and interconnected dykes, with only rare cross-cutting relationships, forming swarms and chaotic injection complexes where magmatic rocks cover up to 50% of the outcrop area. Despite this volume of magma, the system did not lose continuity, suggesting that it did not flow en masse and that the magma network was not all liquid simultaneously. Leucogranites in this network, including leucosomes in migmatites, carry an isotopic signature intermediate between the two main anatectic rocks in the source, suggesting efficient homogenization of the magmatic products. Here, we describe a number of microscopic features of these magmatic rocks which suggests that several pulses of magma used the same pathways giving rise to textural and chemical disequilibrium features. These include: (i) narrow, tortuous corridors of fine-grained minerals cutting across or lining the boundaries of larger grains, interpreted to be remnants of magma-filled cracks cutting across a pre-existing magmatic rock; (ii) corrosion of early formed grains at the contact with fine-grained material; (iii) compositional zoning of early formed plagioclase and K-feldspar grains and quartz overgrowths documented by cathodoluminescence imaging; (iv) incipient development of rapakivi and anti-rapakivi textures, and (iv) different crystallographic preferred orientation of early formed quartz and fine-grained quartz. Mapping of the fine-grained corridors interpreted to represent late melt channels reveal an interlinked network broadly following the S-C fabric defined by pre-existing magmatic grains. We conclude that early formed dykes provided a pathway exploited intermittently or continuously by new magma batches. New influxes of magma opened narrow channels and migrated through a microscopic network following predominantly grain boundaries along an S-C fabric related to syn-magmatic shearing. A mixed isotopic signature resulted not from the mixing of magmas, but from the micro-scale interaction between new magma batches and previously crystallized magmatic rocks, through local equilibration.

The trace element compositions of S-type granites: evidence for disequilibrium melting and accessory phase entrainment in the source

Within individual plutons, the trace element concentrations in S-type granites generally increase with maficity (total iron and magnesium content and expressed as atomic Fe + Mg in this study); the degree of variability in trace element concentration also expands markedly with the same parameter. The strongly peraluminous, high-level S-type granites of the Peninsular Pluton (Cape Granite Suite, South Africa) are the product of biotite incongruent melting of a metasedimentary source near the base of the crust. Leucogranites within the suite represent close to pure melts from the anatectic source and more mafic varieties represent mixtures of melt and peritectic garnet and ilmenite. Trace elements such as Rb, Ba, Sr and Eu, that are concentrated in reactant minerals in the melting process, show considerable scatter within the granites. This is interpreted to reflect compositional variation in the source. In contrast, elements such as LREE, Zr and Hf, which are concentrated within refractory accessory phases (zircon and monazite), show well-defined negative correlations with increasing SiO2 and increase linearly with increasing maficity. This is interpreted to reflect coupled co-entrainment of accessory minerals and peritectic phases to the melt: leucocratic rocks cannot have evolved from the more mafic compositions in the suite by a process of fractional crystallisation because in this case they would have inherited the zircon-saturated character of this hypothetical earlier magma. Trace element behaviour of granites from the Peninsular Pluton has been modelled via both equilibrium and disequilibrium trace element melting. In the disequilibrium case, melts are modelled as leaving the source with variable proportions of entrained peritectic phases and accessory minerals, but before the melt has dissolved any accessory minerals. Thus, the trace element signature of the melt is largely inherited from the reactants in the melting reaction, with no contribution from zircon and monazite dissolution. In the equilibrium case, melt leaves the source with entrained crystals, after reaching zircon and monazite saturation. A significant proportion of the rocks of the Peninsular Pluton have trace element concentrations below those predicted by zircon and monazite saturation. In the case of the most leucocratic rocks all compositions are zircon undersaturated; whilst the majority of the most mafic compositions are zircon oversaturated. However, in both cases, zircon is commonly xenocrystic. Thus, the leucocratic rocks represent close to pure melts, which escaped their sources rapidly enough that some very closely match the trace element disequilibrium melting model applied in this study. Zircon dissolution rates allow the residency time for the melt in the source to be conservatively estimated at less than 500 years.

Tracking S-type granite from source to emplacement: Clues from garnet in the Cape Granite Suite

This study investigates, via a pseudosection approach, the conditions of formation of garnet in the leucogranitic to granodioritic S-type Cape Granite Suite (CGS), South Africa. Previous work has stressed the importance of peritectic garnet entrained from the anatectic source in the petrogenesis of these granites. In this study, garnet from S-type granites of the CGS, showing as little evidence for replacement as possible, was studied for major and trace element geochemistry. Surprisingly, the compositions of all the crystals investigated are essentially identical, despite significant differences in the composition of the host granite. The garnet major element compositions are characterised by homogeneous, unzoned core domains with a relatively Mg-rich composition (Alm 69–71Py 14–21Gro 3Sps 3–5) surrounded by a more Mn-rich rim, some 200 µm wide (Alm 70–76Py 5–12Gro 3Sps 6–12). Trace element compositions are similarly characterised by unzoned cores surrounded by thin rims of relative REE enrichment. Pseudosections calculated for compositions ranging from granite to granodiorite illustrate that garnet is a stable phase in all compositions at high temperatures. Garnet core compositions equilibrated under P– T conditions of 4 to 6.2 kbars and 740 to 760 °C, whilst the rims record conditions of 2.5 to 5 kbars and 690 to 730 °C. Rare granulite-facies metamafic xenoliths also may record the conditions in the source of the granite magma and provide estimated P– T condition above 10 ± 2 kbars and 810 ± 54 °C. This estimate overlaps with the P– T conditions required for fluid-absent biotite melting, the process believed to have produced the CGS magmas within the lower crust. The pseudosections show that garnet was present in the CGS magmas from the source down to near-solidus conditions, but that the composition of peritectic garnet entrained within the source is not preserved in the magma. Calculation of the time required to homogenise garnet compositions within the magma indicates that this cannot occur by diffusion within the garnet crystals, as this would require several orders of magnitude longer than the typical duration of felsic magmatic events. Thus, the findings of this study argue for 1) entrainment of peritectic garnet into melt at the source, 2) the subsequent re-equilibration of this garnet to lower pressure and temperature conditions within the magmatic environment through a dissolution precipitation mechanism, and 3) a near-solidus complete replacement of garnet in some compositions. Collectively, these three processes explain the chemical connectedness between granites and their sources, as well as why the details of the connection have remained so elusive.

Paleocene–Eocene high-grade metamorphism, anatexis, and deformation in the Thor–Odin dome, Monashee complex, southeastern British Columbia

The Thor–Odin dome of the Monashee complex, in the southeastern Canadian Cordillera, comprises Paleoproterozoic basement gneiss with infolds of unconformably overlying rocks of a supracrustal cover sequence and is the deepest exposed structural level in the Omineca belt. Orthogneiss and paragneiss of the basement are migmatitic and contain ubiquitous stromatic leucosome and discrete phenocrystic and pegmatitic vein-type leucosome, which are all interpreted to have formed as a result of in situ melting. The stromatic leucosome is infolded with the country rock (F2), contains a weakly developed foliation, and has a biotite-rich melanosome. The phenocrystic and pegmatitic vein-type leucosome crosscut the stromatic leucosome and the transposition foliation (S2). Evidence to support an igneous and anatectic source for the leucosome includes (i) petrography, (ii) major and trace element chemistry, (iii) zircon morphology, and (iv) peak pressure–temperature (P–T) conditions. Sensitive high-resolution ion microprobe (SHRIMP) 206Pb/238U zircon dates range from ca. 56 to 54 Ma and are interpreted to represent the age of leucosome crystallization. Zircon commonly contains discrete ca. 2.6-1.8 Ga cores that are interpreted as detrital grains inherited from the host paragneiss. Anatexis was ongoing by ca. 56 Ma, as a result of regional prograde metamorphism, and was coincident, at least in part, with the formation of the penetrative S2 transposition foliation and large recumbent F2 tight to isoclinal folds. Anatexis continued during F3 and F4 folding. Melting may have continued until ca. 51 Ma, driven by decompression reactions, and was concomitant with the D5 extensional deformation.

A Sequence of Pan-African and Hercynian Events Recorded in Zircons from an Orthogneiss from the Hercynian Belt of Western Central Iberia--an Ion Microprobe U-Pb Study

Sensitive high-resolution ion microprobe U–Pb dating shows that a biotite orthogneiss from the Hercynian belt of western central Iberia contains 1000–300 Ma zircon. Older, 1000–570 Ma ages within this range represent inherited, detrital material among which four age components may be recognized: 980 Ma, 830 Ma, 616 10(2s) Ma and 582 5(2s) Ma. This inherited zircon commonly forms cores that are surrounded by rims yielding Late Pan-African ages, identical to those found in slender, prismatic, and some stubbier, bi-pyramidal, euhedral crystals. This is the predominant type of zircon with an average age of 546 3(2s) Ma, thought to have been formed during the main magmatic crystallization stage of the granitic protolith of the gneiss. Local deuteric replacements of magmatic zircon yield a virtually identical average age of 547 5(2s) Ma, suggesting rapid magmatic cooling, typical of shallow intrusive settings. Many zircon crystals have very thin, low-Th/U rims with an age of 315 Ma, suggested to represent the gneissification of the granitic rock during the Hercynian orogeny. The abundance in the gneiss body of Al-rich restitic material and inherited, detrital zircon suggests that the granitic magma was formed by anatectic melting of a metasedimentary source rock complex. The age of the youngest inherited, detrital zircon constrains the sedimentation age of the ( youngest parts of the) anatectic source rock complex to the Late Neoproterozoic (<582 5 Ma) and leaves a maximum period of 40 Myr for metamorphism and anatexis of the source rock complex, and migration and intrusion of the granitic magma. Among the inherited, detrital zircon the 616 10(2s) Ma and 582 5(2s) Ma ages are by far the most abundant, and might be derived from the West African craton, where such ages are common for main-stage Pan-African complexes. The older, 980 Ma and 830 Ma inherited zircon ages are absent or rare in the West African craton and a derivation from the Amazon craton (Grenvillian, Sunsas orogens), where such ages are common, is an alternative.

Inherited and Magmatic Zircon from Neogene Hoyazo Cordierite Dacite, SE Spain--Anatectic Source Rock Provenance and Magmatic Evolution: In Memoriam Professor Chris Powell, dagger 2001.07.21

Inherited and Magmatic Zircon from Neogene Hoyazo Cordierite Dacite, SE Spain--Anatectic Source Rock Provenance and Magmatic Evolution: In Memoriam Professor Chris Powell, dagger 2001.07.21

Inherited Palaeozoic and Mesozoic Rb-Sr Isotopic Signatures in Neogene Calc-alkaline Volcanics, Alboran Volcanic Province, SE Spain

A pyroxene andesite unit within the post-Alpine Alborán volcanic province has a Neogene extrusion age; however, its Rb–Sr isotopic relations define a regression line of 509 ± 62(2σ) Ma (Early Palaeozoic). There are two concordant data point clusters on the regression line, one of which is well constrained, defining a secondary regression line of 202 ± 30(2σ) Ma (Early Mesozoic). Considering the mineralogy of the andesites—plagioclase, Ca-poor and Ca-rich pyroxene, and Ti-magnetite—and the presence of restitic aggregates comprising these same four minerals, recent dehydration melting experiments suggest an origin by anatexis of an amphibolite-dominated source rock complex. Inherited zircon ion-microprobe ages in the range of 500–1800 Ma, an Sm–Nd isochron age of 1.5 ± 0.4(2σ) Ga, TCHURNd crustal derivation ages from ∼0.75 to 1.05 Ga and ɛNd(0) values of –4 to –7 support a complex petrogenesis, involving large-scale reworking of older material. 87Sr/86Sr vs 1/Sr and 143Nd/144Nd vs 1/Nd indicate a heterogeneous source rock complex showing two-component mixing. The data favour volcano-sedimentary source rock complex parent material which at ∼500 Ma underwent a diagenetic or hydrothermal event, which regionally reset Rb–Sr isotope systematics. Subsequently, at ∼200 Ma the complex went through local diagenetic or hydrothermal re-equilibration, which created domains with slightly different 87Sr/86Sr ratios, before undergoing Alpine high-grade metamorphism and subsequent anatexis. Roughly coeval, restite-rich cordierite dacites show similar, ∼200 Ma, high-age Rb–Sr isotopic relations, which are interpreted as the age of diagenesis of its sedimentary parent material. This is supported by inherited zircon ion-microprobe ages of 300–400 Ma upwards. Also for these rocks 87Sr/86Sr vs 1/Sr shows linear trends, which are explained analogically by sedimentary component mixing in the parent material of the anatectic source rock complex rather than by magmatic stage mixing or contamination. A sinking slab model is suggested for the regional setting of the crustal anatectic regime, melting being supported by fast uplift (of isotherms) and diapiric underplating by high-temperature asthenospheric mantle.

Hercynian, Pan-African, Proterozoic and Archean ion-microprobe zircon ages for a Betic-Rif core complex, Alpine belt, W Mediterranean – consequences for its P-T-t path

Ion-microprobe analysis of zircons from an andalusite-bearing orthogneiss within the major Alpujarride nappe complex in the central part of the Betic Cordilleras has yielded a Hercynian age of 285 ± 5 (2σ) Ma for euhedral rims, interpreted as the magmatic age of the andalusite-bearing biotite granite parent rock for the gneisses. Zircon age zoning systematics suggest a Paleozoic sedimentation age for the parent material for the anatectic source rock. Zircon cores represent several groups of ages: (1) Archean, c. 2.7 Ga; (2) Early Proterozoic, 2.2–2.0 Ga; (3) Middle Proterozoic, 1.1–0.9 Ga; (4) Pan-African, 0.8–0.5 Ga; including a well-defined event at 612 ± 13 (2σ) Ma. Paragenetic and textural relations indicate that gneissification took place during a high-P (12–13 kbar) low-T (450–500 °C) collisional event during which the primary Alpine nappe pile was produced. The second and final Alpine tectono-metamorphic event led to reorganization of the primary nappe pile by extensional tetonics with coeval very fast rock uplift and cooling (from c. 8? to c. 1 kbar and c. 600 to 100 °C within the period 19.5–18.5 Ma). The fast uplift/cooling stage was triggered by slab break-off which is thought to have induced diapiric underplating by high-T asthenospheric material. This may have heated the collisional wedge, causing thermal weakening which might have advanced the late stage fast uplift/cooling. The Alpine events did not leave a zircon crystallization record. The inherited, Archean–Pan-African zircon age pattern corresponds to that established for the Hercynian basement in central and southern Europe which is considered as reworked Gondwana crust. Deeper levels of core complexes within the Betic-Rif belt thus belong to the pre-Triassic basement of the Tethyan realm (Betic-Ligurian lithosphere) and represent reworked material ultimately derived from Archean and Proterozoic rock complexes from the Gondwana crustal domain. This study implies that thermobarometry of rock complexes which went through several phases of tectono-metamorphic reworking may render ambiguous results if based upon field observations and petrography alone. Zircon ion-microprobe dating may provide additional constraints required to arrive at a feasible tectono-metamorphic history, that is P-T-t trajectory, for such rock complexes.

The geochemistry and geochronology of the gneisses and pegmatites of the Tollie antiform in the Lewisian complex of northwestern Scotland

The Tollie antiform occupies a tectonic break between an area to the north, where remnants of Archaean structures are preserved, and an area to the south, characterized by substantial Proterozoic reworking. The antiform produced in this tectonic boundary is a multistage structure containing three components Creag Mhor Thollaidh in the north, faulted against a central zone, which passes southwards into the highly sheared and attenuated Buanichean gneisses of the southwestern limb. The compositions of gneisses in the antiform assign them unambiguously to the Archaean gneisses of the Gruinard and Lochinver areas to the north. Gneisses of this extensive area were brought together within the narrow confines of the Tollie antiform. As the antiform evolved, the Buanichean gneisses acted as an anatectic source to a set of shear-refined potash-rich pegmatites, which penetrated north into the central zone of the developing antiformal structure. In this central zone, some of the pegmatites were deformed into open folds, whereas others were unmodified. The pegmatites thus form a point of reference within the evolution of the Tollie antiform. Rb–Sr isotopic data on the pegmatites give an age for their intrusion and crystallization of 1660 Ma. Downward shear of the southwestern limb of the antiform continued until about 1500 Ma. By the time this episode of structural restyling in the Lewisian had been completed, a deep-rooted wedge of Proterozoic cover sediments had become encased in an Archaean basement.