Magmatic Banding Research Articles

Metamict U-rich Pyrochlore of Newania Carbonatite, Udaipur, Rajasthan

ABSTRACT U-rich pyrochlore (UO2 up to 18.63%) occurs along the magmatic bands as well as disseminated grains within rauhaugites of the Newania complex. UO2 appears to be primary as it seems mostly evenly distributed all over the grains, though in one grain it does show concentration in parts. Ta is also an important element in some pyrochlore grains and its concentration reaches up to 17.15%. High U and Th are responsible for bringing metamictization in the Newania pyrochlore. Newania carbonatite has a complex emplacement history; rauhaugite seems to have been replaced at 2200 Ma years followed by emplacement of ankeritic carbonatite at around 1500 Ma. Later during tectonic event pyrochlore was involved in reaction with hydrothermal fluid and at this stage Fe and FeS were deposited on the rims of pyrochlore grains. At the same time pyrochlore was subjected to hydro-thermal alteration resulting in removal of Na, Ca and F leaving large vacancy in its A-site. Very few grains have escaped such alteration.

Multiple intrusions and remelting-remobilization events in a magmatic arc: The St. Peter Suite, South Australia

Granitoids with juvenile signatures are common in arc environments and contribute to growth of the continental crust. Thermo-mechanical models of arcs suggest that intermittent intrusion of magma batches leads to magma hybridization, remelting, and remobilization of earlier intrusive rocks driven by fluctuations in temperature and water fluxing. While there are numerous examples in the literature of multiple intrusions and magma hybridization, field examples of remelting and remobilization of earlier intrusive rocks within an arc are rare. Here, we investigate the evolution of magmatic rocks of the Paleoproterozoic St. Peter Suite, emplaced along the SW margin of the Gawler craton, South Australia, a typical calc-alkaline arc suite. Magmatic rocks recording multiple intrusions and multiple magma interactions have undergone in situ remelting and remobilization forming migmatites. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb zircon dating yielded crystallization ages of 1647 ± 12 Ma for a tonalitic gneiss representing the oldest intrusive suite, and 1604 ± 12 Ma for a leucogranite representing the youngest intrusive suite. Both these suites developed synmagmatic foliation and magmatic banding defined by broadly parallel dikes, elongated enclaves, and schlieren. The rock suites record two deformation events associated with anatexis. The first event, D 1 , was responsible for a dominant approximately E-W–striking foliation (S 1 ) parallel to magmatic foliation, and associated with a dominantly sinistral shearing that affected the older suite. This deformation was associated with the first anatectic event, as demonstrated by the association between leucosomes and structures. The second deformation event, D 2 , affected both suites and was characterized by isolated F 2 folds and shear planes filled with leucosomes subparallel to axial planar foliation. Leucosomes interconnected and gave rise to magma extraction channels tens of meters long. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb titanite dating of a late leucosome sample, collected from within the older magmatic suite, yielded an age of 1605 ± 7 Ma, coinciding with the crystallization age of the younger suite rather than postdating it, as expected. We interpret these results to indicate that crystallization of the younger suite and the second anatectic event occurred in the time encompassed by the error of these young ages. Leucosomes from both anatectic phases lack anhydrous peritectic phases and are interpreted to represent low-temperature anatexis resulting from water fluxing. Combined, these results suggest that a protracted and complex intrusive history can be made significantly more complex by anatexis, giving rise to evolved magmas after older ones, erasing earlier intrusive relationships, and establishing new ones. Rocks of the St. Peter Suite record many of the key processes expected in arcs, including the prediction that early intrusive arc rocks remelt to form younger and more fractionated magmas.

Ore geochemistry, zircon mineralogy, and genesis of the Sakharjok Y-Zr deposit, Kola Peninsula, Russia

The Sakharjok Y-Zr deposit in Kola Peninsula is related to the fissure alkaline intrusion of the same name. The intrusion ∼7 km in extent and 4–5 km2 in area of its exposed part is composed of Neoarchean (2.68–2.61 Ma) alkali and nepheline syenites, which cut through the Archean alkali granite and gneissic granodiorite. Mineralization is localized in the nepheline syenite body as linear zones 200–1350 m in extent and 3–30 m in thickness, which strike conformably to primary magmatic banding and trachytoid texture of nepheline syenite. The ore is similar to the host rocks in petrography and chemistry and only differs from them in enrichment in zircon, britholite-(Y), and pyrochlore. Judging from geochemical attributes (high HSFE and some incompatible element contents (1000–5000 ppm Zr, 200–600 ppm Nb, 100–500 ppm Y, 0.1–0.3 wt % REE, 400–900 ppm Rb), REE pattern, Th/U, Y/Nb, and Yb/Ta ratios), nepheline syenite was derived from an enriched mantle source similar to that of contemporary OIB and was formed as an evolved product of long-term fractional crystallization of primary alkali basaltic melt. The ore concentrations are caused by unique composition of nepheline syenite magma (high Zr, Y, REE, Nb contents), which underwent subsequent intrachamber fractionation. Mineralogical features of zircon-the main ore mineral—demonstrate its long multistage crystallization. The inner zones of prismatic crystals with high ZrO2/HfO2 ratio (90, on average) grew during early magmatic stage at a temperature of 900–850°C. The inner zones of dipyramidal crystals with average ZrO2/HfO2 = 63 formed during late magmatic stage at a temperature of ∼500°C. The zircon pertaining to the postmagmatic hydrothermal stage is distinguished by the lowest ZrO2/HfO2 ratio (29, on average), porous fabric, abundant inclusions, and crystallization temperature below 500°C. The progressive decrease in ZrO2/HfO2 ratio was caused by evolution of melt and postmagmatic solution. The metamorphic zircon rims relics of earlier crystals and occurs as individual rhythmically zoned grains with an averaged ZrO2/HfO2 ratio (45, on average) similar to that of the bulk ore composition. The metamorphic zircon is depleted in uranium in comparison with magmatic zircon, owing to selective removal of U by aqueous metamorphic solutions. Zircon from the Sakharjok deposit is characterized by low concentrations of detrimental impurities, in particular, contains only 10–90 ppm U and 10–80 ppm Th, and thus can be used in various fields of application.

The Ilha Anchieta Quartz Monzonite: the southernmost expression of ca. 500 Ma post-collisional magmatism in the Ribeira Belt

The Ilha Anchieta Quartz Monzonite (IAQM) occupies most of the homonymous island in the coast of the state of São Paulo, and is intrusive into foliated rocks of the ~565 Ma Ubatuba Charnockite. The main petrographic variety is a porphyritic biotite-hornblende quartz monzonite with 2-4 cm tabular microcline megacrysts set in a medium-grained groundmass and magmatic foliation. Outcrop-scale structures indicate cumulative processes (modal and grain-size magmatic banding) and interaction with basic magmas (mafic microgranular enclaves). Lithogeochemical data indicates that the main variety is intermediate to acid (S1O2 = 63-67%), alkali-calcic, metaluminous and magnesian (mg# ~30), showing moderate Sr (300-400 ppm) and Ba (~1500 ppm) contents and relatively high HFSE (Nb = 40 ppm; Zr = 550-700 ppm). The older charnockites are more silicic (S1O2 = 71-78%), ferroan(mg# = 12-16), and have very low Sr (13-80 ppm) contents, resulting in Ba/Sr ratios remarkably higher than the IAQM (10 versus 4). LA-MC-ICPMS U-Pb zircon dating of the IAQM yielded 499.7 ± 5.9 Ma. This is the youngest magmatic age identified so far in the crystalline basement of the state of São Paulo, and indicates that the pluton is the southernmost expression of the post-collisional "G5" magmatism in the Ribeira Belt.

The diversity of flow structures in felsic dykes

Abstract: Numerous structures and textures, which can be related to magma flow, were observed in felsic dykes intruding late Mesozoic granitoid plutons in the Jiaodong peninsula, in eastern Shandong province, eastern China. These flow structures may be classified into two categories, interior and peripheral. The former group includes magmatic bands, various types of folds (e.g. injection, sheath, similar and disharmonic folds), rotation of phenocrysts, magmatic foliation or lineation, and crenulation, whereas the latter includes hot tool marks and quarrying structures. Magmatic banding resulted from shearing of mingled magma during magma flow in the dykes. The magma seemed to flow rapidly, probably triggering turbulence in some thick dykes. Interaction at the contact between the hot, moving magma and the cold, stationary wallrock sometimes produced the peripheral structures. A few measurements of hot tool marks and of magmatic lineation reveal a roughly horizontal flow of magma within these dykes. For the dominant NE–SW-striking dyke set in the Laoshan granitoid pluton, the felsic magma probably ascended on or to the SW of the pluton to feed the dyke swarm, and then flowed laterally to drive the horizontal propagation of dykes.

Granitos peraluminosos intrusivos no Complexo Metamórfico Brusque: registro do magmatismo relacionado a colisão neoproterozóica no Terreno Tijucas, Itapema (SC)

Brusque Metamorphic Complex is one of main units of the Tijucas terrane in Santa Catarina. In Itapema region this complex is composed by metasedimentary association with subordinate presence of mafic and ultramafic metavolcanic rocks and peraluminous granites. The granites are exposing like bodies of centimeter to metric thickness and are intrusive in metapelites, mafic and calc-silicate schists. These units are tectonically intercalated and disposed according to a low angle foliation of northeast-southwest direction. The granites are concordant with the S 1 foliation and are positioned in conformity to axial surfaces of isoclinal F 1 folds. Its have a monzogranite composition with a K-feldspar, plagioclase and quartz, with subordinates quantities of muscovite and tourmaline, and biotite, garnet, monazite, zircon, apatite and opaques minerals as accessories. The main structure is a magmatic banding of centimeter thickness alternating regular and continuities levels with medium to coarse equigranular texture and levels with pegmatitic texture. The banding is originated by successive fulfilling of fractures by granite injections. Geochemical studies characterize the granites like a peraluminous magmatism related to high-K calc-alkaline series. Majors and traces elements patterns present low values for the ratios La N /Yb N (1.34 to 4.66), K 2 O/Na 2 O (0.86 to 1.72), CaO/Na 2 O (0.11 to 0.44) and low values of Rb, Sr, Zr e de ETR, very close to peraluminous granites of the others orogenic belts of Phanerozoic age. Compositions of this type are usually associated to rock generated by partial fusion of crustal sequences of pelitic composition with a formation of a granulitic residue. Structural relations between the positioning of the granitic bodies, metamorphism and deformation evolution related to Brusque Metamorphic Complex, associated to geochemical and geotectonic parameters are consistent with a magmatism generated by the anatexis related to collisional event of the end of Brasiliano Cicle.

Granitos peraluminosos intrusivos no Complexo Metamórfico Brusque: registro do magmatismo relacionado a colisão neoproterozóica no Terreno Tijucas, Itapema (SC)

Brusque Metamorphic Complex is one of the main units of the Tijucas terrane in Santa Catarina state. This complex, located in the Itapema region is constituted by metasedimentary association with subordinate presence of mafic and ultramafic metavolcanic rocks and peraluminous granites. The granites have tabular form from centimetric to metric thickness intruding metapelites, mafic and calc-silicate schists. These units are tectonically interposed and disposed to a low angle foliation from northeast-southwest direction. The granites are concordant with the S2 foliation and lie in according to axial surfaces of isoclinal F2 folds. The granite have a monzogranite composition with a K-feldspar, plagioclase and quartz, subordinates quantities of muscovite and tourmaline, with biotite, garnet, monazite, zircon, apatite and opaque minerals as accessories. The main structure is a magmatic banding of centimetric thickness alternating regular and continual levels from medium to coarse equigranular texture as well as pegmatitic texture levels. The banding is originated by successive fulfilling of fractures by granite injections. Geochemical studies characterize the granites as peraluminous magmatism related to high- K calc-alkaline series. The majors and traces elements patterns present values for the ratios LaN/YbN (1.34 to 4.66), K2O/Na2O (0.86 to 1.72), CaO/Na2O (0.11 to 0.44) and low values of Rb, Sr, Zr and RRE, are similar to peraluminous granites of other orogenic belts of Phanerozoic age. These type of granites are usually associated to rock generated by partial fusion of crustal sequences of pelitic composition with formation of a granulitic residue. Structural relations between the position of the granitic bodies and metamorphic evolution of the Brusque Metamorphic Complex associated to geochemical and geotectonic parameters are related to magmatism generator by anatexis connected to collisional event of the end of Brasiliano Cycle.

The tectonically controlled emplacement of a vertically sheeted gabbro-pyroxenite intrusion: Feeder-zone of an ocean-island volcano (Fuerteventura, Canary Islands)

The Miocene PX1 gabbro-pyroxenite pluton, Fuerteventura, Canary Islands, is a 3.5 × 5.5 km shallow-level intrusion (0.15–0.2 GPa and 1100–1120 °C), interpreted as the feeder-zone to an ocean-island volcano. It displays a vertical magmatic banding expressed in five 50 to 100 metre-wide NNE-SSW trending alkaline gabbro sequences alternating with pyroxenites. This emplacement geometry was controlled by brittle to ductile shear zones, generated by a regional E–W extensional tectonic setting that affected Fuerteventura during the Miocene. At a smaller scale, the PX1 gabbro and pyroxenite bands consist of metre-thick differentiation units, which suggest emplacement by periodic injection of magma pulses as vertical dykes that amalgamated, similarly to a sub-volcanic sheeted dyke complex. Individual dykes underwent internal differentiation following a solidification front parallel to the dyke edges. This solidification front may have been favoured by a significant lateral/horizontal thermal gradient, expressed by the vertical banding in the gabbros, the fractionation asymmetry within individual dykes and the migmatisation of the wall rocks. Pyroxenitic layers result from the fractionation and accumulation of clinopyroxene ± olivine ± plagioclase crystals from a mildly alkaline basaltic liquid. They are interpreted as truncated differentiation sequences, from which residual melts were extracted at various stages of their chemical evolution by subsequent dyke intrusions, either next to or within the crystallising unit. Compaction and squeezing of the crystal mush is ascribed to the incoming and inflating magma pulses. The expelled interstitial liquid was likely collected and erupted along with the magma flowing through the newly injected dykes. Clinopyroxene mineral orientation – as evidenced by EBSD and micro X-ray tomography investigations – displays a marked pure-shear component, supporting the interpretation of the role of compaction in the generation of the pyroxenites. Conversely, gabbro sequences underwent minor melt extraction and are believed to represent crystallised coalesced magma batches emplaced at lower rates at the end of eruptive cycles. Clinopyroxene orientations in gabbros record a simple shear component suggesting syn-magmatic deformation parallel to observed NNE-SSW trending shear zones induced by the regional tensional stress field. This emplacement model implies a crystallisation time of 1 to 5 years for individual dykes, consistent with PX1 emplacement over less than 0.5 My. A minimum amount of approximately 150 km 3 of magma is needed to generate the pluton, part of it having been erupted through the Central Volcanic Centre of Fuerteventura. If the regional extensional tectonic regime controls the PX1 feeder-zone initiation and overall geometry, rates and volumes of magma depend on other, source-related factors. High injection rates are likely to induce intrusion growth rates larger than could be accommodated by the regional extension. In this case, dyke intrusion by propagation of a weak tip, combined with the inability of magma to circulate through previously emplaced and crystallised dykes could result in an increase of non-lithostatic pressure on previously emplaced mushy dyke walls; thus generating strong pure-shear compaction within the pluton feeder-zone and interstitial melt expulsion. These compaction-dominated processes are recorded by the cumulitic pyroxenite bands.

2.8–3.0 Ga plutonism and deformation in the SE Amazonian craton: the Archaean granitoids of Marajoara (Carajás Mineral Province, Brazil)

The granitoids of Marajoara in the Rio Maria terrain (Carajás Mineral Province, Brazil) consist of: (i) a broad unit of 2.96 Ga syntectonic tonalites (Arco Verde Tonalites) displaying a trondhjemitic differentiation trend; (ii) 2.93 Ga syntectonic monzogranites (Guarantã); and (iii) 2.87 Ga post-tectonic monzogranites (Mata Surrão) and granodiorites (Rio Maria), displaying a calc-alkaline differentiation trend. Deformation of the Arco Verde tonalites is heterogeneous with low strain domains (well preserved magmatic banding and textures) and orthogneissic domains displaying an E–W trending mainly subvertical foliation, associated with horizontal lineations, upright folds and subvertical shear zones. Microstructures and phase assemblages suggest that deformation occurred within a large temperature range (i.e. during magma emplacement and cooling), from high-T conditions (synmagmatic shear zones and subsolidus ductile deformation with intense quartz and feldspar recrystallization; Pl+Qtz+Hbl+Bt assemblages) to medium- and low-T conditions (ductile to brittle deformation with weakly recrystallized quartz and undulose extinction in feldspars; Qtz+Pl+Bt+Mu or Chl+Ep+Ab+Qtz assemblages). These data, finite strain analysis and structures reported from the surrounding greenstone belts suggest that deformation did not result from a post-emplacement prograde tectono-metamorphic event as considered previously, but that the Marajoara granitoids are synkinernatic intrusions which were deformed together with the supracrustal rocks during a regional NS horizontal shortening. Although the Rio Maria terrain presents similarities with Archaean domains controlled by diapiric processes (lithologies dominated by thick greenstone sequences and TTG plutons, and forming a dome-and-keel structure), its structural evolution is controlled dominantly by a transpressional event which shaped the granite–greenstone terrains. The Rio Maria area, probably as many Archaean ‘grey gneisses’ domains, represents an intermediate case between terrains controlled by Raleigh–Taylor instabilities in a thermally softened crust with insignificant external forces related to plate convergence (e.g. east Pilbara craton) and those controlled by thrust tectonics related to convergence of rigid plates (e.g. Superior Province). The closest analog to the Rio Maria terrain seems to be the Chilimanzi area in the Zimbabwe craton.

The Dharwar craton, southern India, interpreted as the result of Late Archaean oblique convergence

The Dharwar craton comprises two distinct parts separated by a steep N–S sinistral shear zone. In the western part a pre-2900 Ma complex of orthogneisses, granodiorites and narrow tracts of supracrustal rocks (Sargur Group) forms the basement to volcanic and sedimentary basins of the Dharwar Supergroup (ca. 2800–2550 Ma). Late Archaean deformation is characterised by NE–SW crustal shortening and steep N–S sinistral shear zones. The eastern part is underlain by parallel, steep N–S or NW–SE linear belts of calc-alkaline, anatectic and juvenile granites and granodiorites (Dharwar batholith, ca. 2750–2510 Ma) with intervening volcanic and sedimentary schist belts (ca. 2800–2550 Ma). The plutonic belts are 15–25 km wide, up to 150 km long, and bounded by steep NW–SE high-strain zones up to 2 km wide with sinistral shear sense (except one which is dextral). Magmatic-state fabrics and structures in the plutonic rocks are parallel to solid-state sinistral shear fabrics in the high-strain zones, but diffuse magmatic banding is commonly oblique to these zones and coincides with the plane of instantaneous shortening in sinistral shear. Magmatic-state structures, swarms of vertical NW–SE dykes of granite, and the vertical wedge shape of the linear belts are consistent with emplacement of the batholith during sinistral shear when magma pressure exceeded regional horizontal compressive stress. Upright folds and schistosity, steep reverse faults and effects of regional HT/LP metamorphism show that deformation was partitioned into NE–SW shortening in the schist belts during emplacement of the plutonic belts in the sinistral shear regime. The western part of the craton is interpreted as the foreland to an accretionary arc represented by the batholith and schist belts (intra-arc basins) in the east. NE–SW shortening and sinistral transcurrent displacements in the foreland and arc are consistent with arc-normal and arc-parallel displacements during oblique convergence analogous to Mesozoic–Cenozoic convergent settings.

Large scale magmatic banding in mesozoic basic sill of the Paraná basin, Campinas, São Paulo State, Brazil

Large scale magmatic banding in mesozoic basic sill of the Paraná basin, Campinas, São Paulo State, Brazil

Sulfides associated with podiform bodies of chromite at Tsangli, Eretria, Greece

Small bodies of pyrrhotite, chalcopyrite, minor pentlandite, and magnetite occur at the peripheries of podiform bodies of chromite in ultramafic ophiolitic rocks at Tsangli, Eretria, central Greece. Banding of magnetite and sulfide within the bodies is reminiscent of magmatic banding. A magmatic origin has been proposed for similar sulfide masses in the Troodos ophiolite (Panayiotou, 1980). The compositions of the host rocks, chromite, and of the sulfides have been investigated. On average, the sulfide mineralization, recalculated to metal content in 100% sulfide, contains 0.55% Ni, 5.15% Cu, 0.29% Co, 9 ppb Pd, 179 ppb Pt, 16 ppb Rh, 112 ppb Ru, 31 ppb Ir, 58 ppb Os, and 212 ppb Au. These metal contents, particularly the high Cu/(Cu+Ni) ratio of 0.78 and the Pt/(Pd+Pt) ratio of 0.95, are inconsistent with the sulfides having reached equilibrium with their Ni rich host rocks at magmatic temperatures and accordingly it is concluded that they are not of magmatic origin. The average δ 34S value of the sulfide bodies is +2 while that of a sample of pyrite from country-rock schist is −15.6. These values are inconclusive as to the origin of the sulfur. It is suggested that the sulfides have been precipitated by hydrothermal fluids, possibly those responsible for the serpentinization of the host rocks. The source of the metals may have been the host rocks themselves.