Aplite Dykes Research Articles

Nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions

In the Neves area, the pre-Alpine intrusive protolith of the Zentralgneise unit (Tauern window, Eastern Alps) is well preserved in a kilometric-scale low-strain domain without pervasive Alpine deformation. It is compositionally heterogeneous, consisting predominantly of granodiorites, with lesser leucocratic granites, and different generations of lamprophyres and aplites. The intrusive rocks are crosscut by fractures that were locally infiltrated by fluids and surrounded by alteration haloes. Incipient Alpine amphibolite facies ductile deformation is strongly localized on these precursor fractures and on lithological planar heterogeneities, resulting in the development of several different types of shear zones. Fractures without alteration haloes initially accommodate slip entirely on the fracture itself. With increasing deformation, a foliation is progressively developed in the adjacent host rock, eventually producing a single heterogeneous “ductile” shear zone with the typical sigmoidal foliation pattern. Strong layers (aplite dykes and bleached alteration haloes developed to either side of precursor fractures) localize shear on their boundaries to produce characteristic paired shear zones. Shearing is more evenly distributed within weak layers (lamprophyres and quartz veins), with a marked discontinuity in shear strain against the adjacent, little deformed granodiorite. Shear zone development was accompanied by the formation of new fractures and quartz-rich veins in the host rock, which in turn also localized shear. Magmatic contacts, fractures and quartz veins are mostly steeply dipping and effectively span the complete range of strike orientations. The kinematics of the overprinting (strike-slip) shear zones was determined by the orientation of the initial discontinuities relative to the local principal compressive stress axis σ 1 (here oriented ca. 345°). Discontinuities of almost all orientations show shear reactivation, even in the case of very low resolved shear stress, indicating an overall viscous response of the system without a specific yield stress. The geometry and kinematics of the shear zone network suggest that the overall deformation in low-strain domains was close to coaxial. During deformation along the shear zone network, compatibility was maintained by fracturing (with the development of new quartz veins) and by distributed ductile deformation of the host rock, especially within contractional domains of the shear network and at shear zone intersections. Deformation never propagates into the undeformed homogeneous granodiorite as discrete ductile shear zones but is limited by the original extent of the precursor discontinuities. Shear zone development in intact rock is always preceded by fracturing, which localizes subsequent shear reactivation.

Geology and geochemistry of the Mendejin plutonicrocks, Mianeh, Iran

The Mendejin pluton is located in the Mianeh area, NW Iran, 550 km from Tehran. This pluton is probably of Oligo-Miocene age and is the result of extensive magmatism which occurred during and after the Alpine Orogeny. Similar plutons are common in the Alborz–Azarbaijan structural zone of Iran, and it is likely that there are concealed plutons related to this extensive Cenozoic magmatism, but due to their youth and low rates of erosion they have not yet been exposed. The Mendejin pluton is a composite body made up of four types of plutonic rocks: pink tonalite, grey tonalite, diorite and aplite. The pink tonalite is porphyritic and contains phenocrysts of plagioclase, K-feldspar and hornblende in a groundmass consisting of quartz, plagioclase, K-feldspar, hornblende, zircon, monazite, leucoxene, apatite and hematite. The grey porphyritic tonalite has more biotite, pyroxene and pyrite and less accessory phases compared with the pink tonalite. The diorite has a microporphyritic texture with phenocrysts of plagioclase, hornblende and augite. This rock also occurs as xenoliths in the Mendejin pluton. The aplitic dykes are the youngest magmatic products at Mendejin. The Mendejin tonalite contains more Cl, As, S, Cu, Ni and Zn than the global granite. These rocks are of I-type, peraluminous and calc-alkaline, with medium to high potassium, and were formed as part of a volcanic arc. The Mendejin pluton contains up to 8 ppb gold and could potentially have been the source of an economic gold deposit by leaching of Au from wall rocks and deposition in extensive hydrothermally altered marginal zones.

High‐pressure mineral assemblage in granitic rocks from continental units, Alpine Corsica, France

AbstractThe Popolasca–Francardo area of northern Corsica contains an assemblage of continental tectonic units affected by an Alpine deformation. In one of these units, Unit II, previously regarded as weakly metamorphosed, a metamorphic mineral assemblage characterized by sodic amphibole, phengite, quartz, albite and epidote has been found in an aplite dyke that cuts the dominant granitoids. Peak‐metamorphic temperature and pressure conditions of 300–370°C and 0.50–0.80 GPa, respectively, have been determined. This finding indicates that a continuous belt of continental slices, characterized by high‐pressure, low‐temperature metamorphism of Tertiary age, extends from the Tenda Massif in the north to the Corte area in the south, thus placing additional constraints on the tectonic evolution of Alpine Corsica. Copyright © 2005 John Wiley & Sons, Ltd.

Nd isotopic, petrologic and geochemical investigation of the Tulawaka East gold deposit, Tanzanian Craton

New isotopic (Sm–Nd) and geochemical data are presented for volcano-sedimentary and granitic lithotypes of the Tulawaka East gold deposit in the Late Archean Sukumaland Greenstone Belt of the Tanzanian Craton. The data help to characterize the geological environment of the greenstone belt/gold deposit and the crustal evolution of the Tanzanian Shield. The volcano-sedimentary units were intruded by leucogranites and aplitic dykes that were deformed and metamorphosed to amphibolite facies grades. The Au mineralization occurs in thrust faults at the contact between the aplite dykes and the volcano-sedimentary units. The metavolcanic rocks are largely basaltic to intermediate in composition with negative Nb anomalies and flat rare earth element (REE) profiles varying from 10 to 100× chondritic values. Initial ɛNd 2.8 Ga values vary from +0.9 to +5.3 and Nd model ages ( T dm) range from 2.8 to 3.0 Ga. The metasedimentary rocks are generally enriched in light REE (100× chondrite) with weak negative Eu anomalies and ɛNd 2.8 Ga values vary from 0.0 to +2.6. The leucogranites are garnet-rich and peraluminous with strong negative anomalies in Eu and Ti and with flat REE profiles (10–100× chondrite). The aplite dyke is also peraluminous with similar negative Eu and Ti anomalies and a flat light REE, but is depleted in heavy REE. The similarity in composition with the leucogranite suggests that the two are cogenetic and formed by fractionation of a more mafic parental magma. The combination of the flat REE element patterns and negative Nb anomalies among the mafic volcanic rocks suggests formation as an immature (island?) arc at ca. 2.8 Ga. However, Nd isotope signatures in the metasedimentary rocks provide evidence of an older crustal sedimentary provenance (ca. 3.0–3.1 Ga) and proximity of a continental influence. The age of the leucogranite and aplite dyke are constrained by a Sm–Nd garnet-whole-rock age of ca. 2.5 Ga, indicating that the shear zone-hosted gold mineralization is younger than 2.5 Ga. The range of isotopic compositions found in the Tanzanian Shield overlap with those of the Zimbabwe Craton to the south.

The control of precursor brittle fracture and fluid–rock interaction on the development of single and paired ductile shear zones

Ductile shear zones can occur as relatively isolated single structures, as arrays, or as characteristic paired zones. In continuous glaciated exposures of metagranodiorites from the Tauern window (Eastern Alps), the control of initial dilatant brittle fracture and associated fluid–rock interaction on the geometry of subsequent ductile shear zones can be unequivocally established. Shearing occurred under amphibolite facies conditions. Fractures in weakly deformed metagranodiorites are often less than 1 mm thick but extend for tens of metres. Many are healed joints without shear offset. Others show minor (mm–cm), discrete dextral offset. Such brittle faults commonly display a low-angle en-échelon arrangement, with displacement transferred between discrete fracture segments by ductile compressive bridges. The geometry of more strongly reactivated zones depends on the degree and heterogeneity of fluid–rock interaction, which is related to fluid infiltration and veining along the primary fractures. With little fluid–rock interaction, reactivation produces single heterogeneous ductile shear zones centred on and immediately flanking the pre-existing fracture. With increased fluid–rock interaction, a bleached halo is developed symmetrically to either side of a central epidote–quartz (±garnet±calcite) vein. Ductile shear zones commonly flank this bleached zone, to develop a characteristic paired pattern. Strain is partitioned, localizing in the central fracture/vein and the flanking shear zones. Paired zones may anastomose in accordance with changes in the width of the central bleached zone, but are always symmetrically spaced with regard to the central fracture/vein. With increasing deformation, the ductile shear zones broaden into the adjacent metagranodiorite but not into the bleached zone, which remains preserved as a low strain region. Paired shear zones can also develop to either side of aplite dykes. Examples of characteristic paired shear zones, usually with a clear central vein, are found in many areas ranging from greenschist to eclogite facies, suggesting that the mechanism of their formation is quite general.

Field and petrological evidence for a Late Palaeozoic (Upper Carboniferous–Permian) age of the Erfaulet orthogneiss (Gran Paradiso, western Alps)

In the Gran Paradiso massif (western Alps), the boundary between the Erfaulet orthogneiss and the overlying metasediments (Money Complex) is interpreted as a Late Palaeozoic intrusive contact. Major arguments in favour of this hypothesis are: (i) the obliquity of the sedimentary layering with respect to the contact; (ii) the presence of aplitic dykes within the Money Complex; (iii) the lack of a mylonitic zone; and (iv) rare relics of an early generation of garnet in the Money metasediments, interpreted as evidence of the contact metamorphism of the Erfaulet granite. To cite this article: B. Le Bayon, M. Ballevre, C. R. Geoscience 336 (2004).

The occurrence and origin of babingtonite in the El Valle-Boina's Cu-Au deposit in Asturias, Spain

Babingtonite has been found at El Valle-Boinas Cu-Au deposit, Spain, this being the first known occurrence of this mineral in skarn in Spain. It is a locally abundant accessory mineral related to the hydrothermal alteration of the calcic skarn. Babingtonite is present in metasomatized aplitic dykes or occurs as a consequence of alteration of Fe-rich end members of both hedenbergite and andradite and always in association with Fe-rich epidote (Ps 32–48 ). Other related minerals are calcite, quartz, hydroxyapophyllite, hydroandradite, prehnite, pumpellyite-(Fe +2 ), Cu-sulphides and electrum. The babingtonite formation is related to the locally Aldepleted aqueous fluids at relative high fO 2 and low XCO 2 .

Grenvillian and Caledonian tectono-magmatic activity in northeasternmost Svalbard

Abstract The Nordaustlandet Terrane of NE Svalbard forms an exposed part of the Barentsia microcontinent. Augen gneisses, migmatites, granites and gabbros dominate the scattered outcrops along the northeastern coast of Nordaustlandet, and on the smaller islands to the north and east, as far as Kvitøya. These outcrops probably represent the deepest exposed crustal levels within the folded Caledonian basement of the Nordaustlandet Terrane. In the present study, a variety of rock types have been analysed by different U-Pb dating techniques (conventional, Pb-evaporation and ion microprobe) on zircon, titanite and monazite The major and trace element compositions and Sm-Nd isotope geochemistry of these rocks have also been investigated. The augen gneisses yield U-Pb ages of c. 950 Ma, indicating that they are deformed late Grenvillian granites, similar to the Grenville-age granites and augen gneisses of northwestern and central Nordaustlandet. Migmatites, grey granites, aplitic dykes and a syenite (boulder) yield U-Pb ages mainly falling in the 430–450 Ma range, slightly older than the 410–420 Ma late-tectonic Caledonian granites further west. Both the Grenvillian and Caledonian granites are of crustal anatectic origin, and the Caledonian granites and migmatites may have formed largely by remelting of Grenvillian crust. The ages of the mafic rocks are uncertain, but Sm-Nd data indicate a possible emplacement age of c. 700 Ma for two of the gabbros, suggesting that they may be the result of rift-related magmatism in connection with the opening of the Iapetus Ocean. A few enigmatic inherited zircons of similar late Neoproterozoic age found in younger granites may possibly be related to this event. No evidence for late Neoproterozoic orogenic activity, similar to that in the Timanides of northern Russia, is seen in eastern Svalbard. At this time, eastern Svalbard (Barentsia) was probably part of the Laurentian margin, and probably located far away from northern Baltica.

Magmatic garnet in deformed aplite dykes from the Mórágy granitoid, SE-Transdanubia, Hungary

This paper is devoted to the description of spessartine-almandine garnet (Sps: 39.8-60.2%; Alm: 29.1-56.76%; Grs <7%, Prp <3.6%, Adr <5%) from aplitic dyke rocks of the Moragy granitoid near Erdosmecske and Aranyos valley. The aplitic dyke rocks contain K-feldspar, plagioclase, quartz, (myrmekite), ±garnet, ±biotite, chlorite or secondary muscovite, ±ore minerals, ±calcite, ±apatite and ±epidote. Two different zoning types in garnet were detected by electron microprobe. Zoning type I means either Mn enrichment at the expense of Fe towards the rim or towards the core or plateau garnet profiles. Zoning type II displays Ca enrichment at the rim of garnet. A high proportion of Sps component in garnet and the character of zoning type I are considered as typical magmatic features. The pressure-temperature estimation for aplite formation arises from experimental investigations of Green (1977) in agreement with the hypothetical approach of liquidus relationships by Abott and Clarke (1979) giving a minimum temper...

A geochronological and geochemical study of rocks from Gjelsvikfjella, Dronning Maud Land, Antarctica—implications for Mesoproterozoic correlations and assembly of Gondwana

The Jutulsessen nunataks (72degrees00'S; 2degrees30'E), Gjelsvikfjella, Dronning Maud Land (DML), consist mainly of migmatites of two types. A heterogeneous banded amphibolite facies gneisses; and a more homogeneous part. In the more homogeneous part, partial melts form along axial planes to tight folds. Numerous pegmatitic dykes occur in both migmatites. The homogeneous part of the migmatite has a granodiorite composition. It displays the depletion of Nb-Ta typical for rocks from destructive plate margins and a strongly fractionated REE pattern, specially in LREE (La/Lu ratios varying between 500 and 800). SIMS dating of zircon from the homogeneous migmatite and two pegmatite dykes resulted in two age groups. A concordant age of 1163 +/- 6Ma is calculated from zircon crystals with no rim/core structure and from cores from structurally complex crystals. This age represents the age of the protolith of the migmatite. A Cambrian age of 504 +/- 6 Ma is obtained from zircon rims and from sector-zoned zircons. This age represent the time of migmatisation. Sm-Nd depleted mantle model ages range from 1390 to 1770 Ma and suggest that the protolith to the migmatites contained components of older crust (pre-1163 Ma). An igneous complex consisting of a syenite plug (Stabben syenite), gabbroic rocks and aptitic dykes intrudes the metamorphic complex. The syenite and the aplitic dykes are neither deformed nor migmatised or penetrated by pegmatitic dykes. These rocks have elevated LREE and LILE concentrations with an La/Lu ratio of 450 and an Nb-Ta trough. The gabbroic rocks range in composition from melagabbro to monzogabbro and host numerous pegmatitic dykes. SIMS zircon U-Pb data from the Stabben syenite give an age of 500 8 Ma. This age is regarded as the intrusive age of the Stabben syenite. By the single zircon-Pb evaporation method an age of 495 14 Ma is obtained from the aplitic dykes. Sm-Nd depleted mantle model ages between 1800 and 2220 Ma indicate that the dykes formed from a Paleoproterozoic source. A Mesoproterozoic volcanic arc setting of DML and a correlation with the Natal Province, as suggested by several authors, is supported by data in this study. The studied area has consequently been a part of the Kaapvaal/Kalahari craton since Mesoproterozoic time. The Cambrian migmatisation and the intrusions are interpreted as a result of post-collision activity related to the collision between the Kalahari craton and the combined block of Antarctica and Australia during the final assembly of Gondwana. This collision is suggested to be included in the Kuunga Orogeny introduced by Meerat and Van der Voo.

Age, petrogenesis and significance of 1 Ga granitoids and related rocks from the Sendra area, Aravalli Craton, NW India

We present new geochronological, petrological, geochemical and isotopic data for granitic and related rocks from the Aravalli Craton, Rajasthan, northwestern India. In the Sendra area, five variably deformed granitoid plutons, ranging in composition from tonalite to granite, cut across carbonate-rich metasedimentary rocks of the Delhi Supergroup. The largest of these bodies, the Chang pluton (∼15 km 2) is dominated by monzogranitic gneisses and aplitic dykes, composed of subequal proportions of quartz, plagioclase (An 7–20) and microcline (Or 92–98), with lesser biotite (Fe ∗=0.8–0.9) and accessory muscovite (Fe ∗=0.7–0.8). U–Pb zircon data (TIMS method) for a biotite granite gneiss yield a weighted mean 207Pb/ 206Pb age of 967.8±1.2 Ma, which we interpret as representing the time of magmatic crystallization. Rb–Sr whole-rock isotopic data for the Chang pluton, including new analyses as well as previously published ones, yield a regression of 906±67 Ma (MSWD=82), which is barely within error of the U–Pb age. There is evidence for open-system behaviour in the Rb–Sr system, particularly for whole-rock samples with low Sr concentrations, and consequently high Rb/Sr. Sm–Nd isotopic data fail to yield meaningful age information. Initial isotopic ratios (at 968 Ma) for Chang pluton granitoids ( I Sr=0.7110±14; ε Nd=−3.28±0.47) are compatible with source materials similar to Archaean amphibolitic rocks of the Banded Gneiss Complex. Spatially associated with the Chang pluton is a massive metagabbro, composed of plagioclase (An 45–68) and magnesio-hornblende (Fe ∗=0.3–0.4), with secondary Cl-rich scapolite and ferrian zoisite. The scapolite and zoisite likely crystallized from metamorphic fluids that interacted with nearby calc-silicate schists and gneisses of the Delhi Supergroup. Aside from slight enrichments in Rb, U, Th and Ba, this metagabbro retains a primitive chemical signature similar to N-MORB (LREE depletion, low K), and initial isotopic ratios ( I Sr=0.7058; ε Nd=+2.9) that approach model depleted mantle at 968 Ma. The metagabbro is chemically and isotopically similar to mafic metavolcanic and related rocks that have been considered to represent ocean-floor and island-arc basaltic magmas. All available data are compatible with the idea that these rocks represent the products of convergent margin processes during the Early Neoproterozoic, a conclusion that could have important implications for the construction of the Rodinia supercontinent.

The Late-Variscan fault network in central–northern Portugal (NW Iberia): a re-evaluation

The fault network in central–northern Portugal, especially the fault system with mean strike N25°, has been used to help deduce the late Palaeozoic dynamics of Western Europe. On the other hand, the N80° strike–slip fault system was recognized in previous works as Late-Variscan and left lateral, but was scarcely mapped and its importance neglected. This study shows that the N25° faults were dextral in the late stages of the Variscan Orogeny and sinistral during the Alpine Cycle due to pervasive reactivation. Fault rocks and intrusions are clearly different, according to age: mostly high temperature quartz infillings, but locally muscovite, tourmaline, and aplite dykes in Late-Variscan times, and low temperature cataclasites, fault gouges and Mesozoic mafic dykes in younger Alpine times. We dated dextral N45° segments of the N25° fault system because they are less prone to reactivation by the NNW–SSE Alpine compression, and can thus preserve the Variscan record. We used K–Ar in muscovite concentrates and obtained a minimum age of the analysed faults of ca. 312 Ma, which sets a lower limit to the so-called Late-Variscan wrench-faulting period. The present study shows that the N80° fault set is pervasive and sinistral in central–northern Portugal, and therefore, that it only admits one brittle dextral conjugate, the N25° fault system. Both were generated by a maximum compressive stress bearing between N50° and N55° in azimuth. We did not find evidence of a Variscan sinistral strike–slip movement in the N25° fault system, and therefore, this kinematics is believed to represent only the displacements accommodated during the Alpine Cycle.

Oxidation-induced postmagmatic modifications of primary ilmenite, NYG-related aplite dyke, Tibchi complex, Kalato, Nigeria

AbstractWe describe an ilmenite-bearing aplitic syenite dyke in the roof zone of the Tibchi granite, exposed at Kalato, in the Tibchi ring-complex, northern Nigeria. Inclusions of ferrocolumbite, rutile and ixiolite in the ilmenite are inferred to have been trapped at the magmatic stage. The main mafic mineral is annite. Compositionally, the ilmenite, rutile and ferrocolumbite have near-end-member compositions. A positive correlation between Sc and Ta/(Ta+Nb) indicates that Sc behaved incompatibly as ferrocolumbite grew. Such entrapped accessory minerals may well have formed by local saturation at the ilmenite-melt and annite-melt interface. During and after their crystallization, the melt reached saturation in H2O and degassed. A second generation of ilmenite enriched in Mn and Zn replaced the primary ilmenite along fractures and grain margins. As fO2 began to increase, composite blebs and rinds of ‘ferropseudobrookite’, rutile and hematite began to develop by oxidation-induced exsolution in the primary ilmenite. Incorporation of Nb, Ta, Sc and Si in the ‘ferropseudobrookite’ may well have stabilized it at Kalato. Ultimately, it is transformed to hematite + rutile. The IMA-sanctioned view that the solid solution between pseudobrookite and Ti3O5 is complete, and thus that ‘ferropseudobrookite’, as an intermediate member of the series, does not merit species status, needs to be re-evaluated.

Magma–host interactions during differentiation and emplacement of a shallow-level, zoned granitic pluton (Tarçouate pluton, Morocco): implications for magma emplacement

The Tarçouate pluton (Anti-Atlas, Morocco) is an inversely zoned laccolith emplaced 583 Ma ago into low-grade metasediments, with the following succession: leucocratic granites, biotite–granodiorites (±monzodiorites), hornblende–granodiorites (±monzodiorites) and monzodiorites syn-plutonic dykes. These rocks form two distinct, chemically coherent, units: (1) A main unit consists of layered (57<SiO 2<59 wt.%) and homogeneous (63<SiO 2<67%) hornblende–granodiorites, biotite–granodiorites (67<SiO 2<72%) and aplites (70<SiO 2<76%). All these rocks are metaluminous to peraluminous and display fractionated HREE depleted patterns (La/Yb N =14–61; Yb N =0.7–6.8). Initial 87Sr/ 86Sr ratios (0.7072 to 0.7080) increase, whereas ε Nd(t) values (−1.7 to −2.8) decrease from the hornblende– to the biotite–granodiorites. Monzodiorites occur as mafic microgranular enclaves or syn-plutonic dykes. (2) A subordinate unit consists of leucocratic, distinctly peraluminous, muscovite-bearing granites (72<SiO 2<75%) occurring at the northern edge of the pluton and as dykes in the surrounding schists towards the top of the pluton. These rocks are free of monzodioritic enclaves. They display less fractionated patterns with higher HREE contents (La/Yb N =2–19; Yb N =11–18), a distinct ε Nd(t) value (−11.8) and a 87Sr/ 86Sr initial ratio (0.7480) within those of the surrounding schists (0.7393–0.7819). Magma–host interactions are closely related to differentiation and occurred at different levels, but mainly before emplacement. Field relationships and petrogenetic modelling show that the bt–granodiorites formed at levels deeper than the level of emplacement, by fractional crystallisation (0.65<F<0.85) with limited amounts of contamination by crustal material (∼5 wt.%). The hbl–granodiorites differentiated at the level of emplacement by fractional crystallisation leading to residual melts (F=0.55) slightly contaminated by the country rocks (∼2 wt.%) and represented by aplite dykes. The leucocratic granites are likely to derive by partial melting of lithologies similar in composition to the country metasediments. These data preclude any significant material transfer process for the emplacement of the Tarçouate pluton, but rather suggest assembly of successive pulses of variably differentiated crystal-poor magmas. These shallow level granitic plutons can be considered as an end-member of magma emplacement with minimum interactions with the country rocks.

Origin of megacrystic felsic gneisses at Broken Hill

Three sheet‐like bodies of felsic gneiss containing abundant K‐feldspar megacrysts (megacrystic felsic gneiss, augen gneiss or granite gneiss) surrounding the Broken Hill Line of Lode in western New South Wales, Australia, are inferred to be pre‐ to syn‐D1 granitoids. We interpret the Feral gneiss to be a pre‐ to early syn‐D1 intrusion, as it contains S1 as its earliest foliation. However, it has no magmatic flow foliation. The Alma Gneiss, and the megacrystic portions of the Rasp Ridge Gneiss, northwest of the Line of Lode, both contain S1 parallel to a magmatic flow foliation, and are interpreted as having been magmatic during D1. Therefore, the Alma and Rasp Ridge Gneisses may have been intruded during D1, probably just after the Feral gneiss, as the Alma Gneiss intrudes the Feral gneiss. S1 in the augen gneisses and the wall rocks is defined by biotite, sillimanite, garnet and ribbon quartz, and indicates that high‐grade metamorphic conditions accompanied D1. Evidence suggesting that these rocks were originally granitoids includes: (i) the Alma Gneiss transecting and intricately intruding the Feral gneiss, the contacts being transected by S1; (ii) euhedral to subhedral K‐feldspar porphyroclasts (former phenocrysts), especially those with concentrically arranged inclusions; (iii) microgranitoid enclaves, particularly where megacrystic and relatively large; (iv) aplite dykes (most common in plutonic rocks and therefore reliable indicators); (v) metasedimentary xenoliths; (vi) magmatic flow foliations overprinted by parallel tectonic foliations; and (vii) chemical affinities with undoubted Australian Proterozoic granitoids. Therefore, felsic gneisses at Broken Hill should not be used for stratigraphic correlation, unless they can be definitely determined to be of volcanic flow or tuffaceous origin. The inferred intrusion of granitoids early in the tectonic history of the Broken Hill Block suggests that they may have contributed to the metamorphic and/or hydrothermal heat, and may have helped concentrate metals to form orebodies.

AMS study of the Pont-de-Montvert-Borne porphyritic granite pluton (French Massif Central) and its tectonic implications

Thermomagnetic experiments, isothermal remanent magnetization (IRM), hysteresis loops, X-ray reflection analyses, optic microscopic observations, bulk magnetic susceptibility and natural remanent magnetization (NRM) show that the susceptibility signal of 56 sites in the Pont-de-Montvert–Borne pluton is derived mostly from biotite with a very small proportion of ferromagnetic material. Low anisotropy degree (P parameter) and consistent AMS orientations among monzonite, enclaves and aplitic dykes indicate that the AMS was acquired during pluton emplacement in the subsolidus phase. Magnetic fabrics demonstrate that linear deformation is prominent in the area where the granite is in direct contact with micaschists, and that planar deformation becomes more important in the area where the pluton is surrounded by non-porphyritic peraluminous granites. AMS measurements also show that shallow plunging E–W-stretching lineations are the dominant structure over most of the studied area. Foliation orientations follow the pluton contour in the western part, indicating that the Pont-de-Montvert–Borne pluton is probably rooted in the west and extruded towards the east. These AMS results agree to some extent with fabric inferred from mineral preferred orientation. The AMS data support an E–W extensional tectonic setting during the Pont-de-Montvert–Borne pluton emplacement, which resulted from the Late Carboniferous crustal thinning of the Hercynian orogeny.

PB/PB SINGLE-ZIRCON DATING OF PALEOPROTEROZOIC CALC-ALKALINE/ ALKALINE MAGMATISM IN THE SOUTHEASTERN SÃO FRANCISCO CRATON REGION, BRAZIL

Two chemically and petrographically distinct intrusive bodies (a calc-alkaline trondhjemite and an alkaline quartz-syenite cut by granitic aplites) occur in the vicinity of Piranga (Minas Gerais State), near the southeastern border of the Sao Francisco craton, Brazil. They intrude the Rio das Velhas greenstone belt and the Mantiqueira complex, both of Archean age. Both intrusive bodies are variably deformed and mylonitized. Age determinations using the Pb/Pb single-zircon evaporation method yielded Paleoproterozoic ages for the emplacement/ crystallization of the trondhjemite (2058 ± lOMa), quartz-syenite (2036 ± 4Ma), as well as the younger aplites (2012 ± 8Ma). These ages allow us to interpret the calc-alkaline as well as the alkaline magmatism as manifestations of the Transamazonian event in the southeastern Sao Francisco craton region. Furthermore, the calc-alkaline trondhjemite is interpreted to have intruded during compressional deformation, while the alkaline quartz-syenite and its aplitic differentiates are post-orogenic, possibly extension-related. In terms of the Rb vs. (Y +Nb) contents, the trondhjemite plots in the field of volcanic arc granites, whereas the syenite and the aplites plot in the field of post-collision granites. The mylonitic overprinting of the syenite and the trondhjemite is also Paleoproterozoic, as deduced from the 2012 ± 8Ma age of the undeformed younger aplitic dykes which cut the syenite. It is likely that this alkaline magmatism correlates with a Paleoproterozoic post-collisional magmatic event well documented in the Transamazonian mobile belts of the northern Sao Francisco craton.

PB/PB SINGLE-ZIRCON DATING OF PALEOPROTEROZOIC CALC-ALKALINE/ ALKALINE MAGMATISM IN THE SOUTHEASTERN SÃO FRANCISCO CRATON REGION, BRAZIL

Two chemically and petrographically distinct intrusive bodies (a calc-alkaline trondhjemite and an alkaline quartz-syenite cut by granitic aplites) occur in the vicinity of Piranga (Minas Gerais State), near the southeastern border of the São Francisco craton, Brazil. They intrude the Rio das Velhas greenstone belt and the Mantiqueira complex, both of Archean age. Both intrusive bodies are variably deformed and mylonitized. Age determinations using the Pb/Pb single-zircon evaporation method yielded Paleoproterozoic ages for the emplacement/ crystallization of the trondhjemite (2058 ± 10Ma), quartz-syenite (2036 ± 4Ma), as well as the younger aplites (2012 ± 8Ma). These ages allow us to interpret the calc-alkaline as well as the alkaline magmatism as manifestations of the Transamazonian event in the southeastern São Francisco craton region. Furthermore, the calc-alkaline trondhjemite is interpreted to have intruded during compressional deformation, while the alkaline quartz-syenite and its aplitic differentiates are post-orogenic, possibly extension-related. In terms of the Rb vs. (Y +Nb) contents, the trondhjemite plots in the field of volcanic arc granites, whereas the syenite and the aplites plot in the field of post-collision granites. The mylonitic overprinting of the syenite and the trondhjemite is also Paleoproterozoic, as deduced from the 2012 ± 8Ma age of the undeformed younger aplitic dykes which cut the syenite. It is likely that this alkaline magmatism correlates with a Paleoproterozoic post-collisional magmatic event well documented in the Transamazonian mobile belts of the northern São Francisco craton.

Crystallisation of fine- and coarse-grained A-type granite sheets of the Southern Oklahoma Aulacogen, U.S.A.

A-type felsic magmatism associated with the Cambrian Southern Oklahoma Aulacogen began with eruption of voluminous rhyolite to form a thick volcanic carapace on top of an eroded layered mafic complex. This angular unconformity became a crustal magma trap and was the locus for emplacement of later subvolcanic plutons. Rising felsic magma batches ponding along this crustal magma trap crystallised first as fine-grained granite sheets and then subsequently as coarser-grained granite sheets. Aplite dykes, pegmatite dykes and porphyries are common within the younger coarser-grained granite sheets but rare to absent within the older fine-grained granite sheets. The older fine-grained granite sheets typically contain abundant granophyre.The differences between fine-grained and coarse-grained granite sheets can largely be attributed to a progressive increase in the depth of the crustal magma trap as the aulacogen evolved. At low pressures (&lt;200MPa) a small increase in the depth of emplacement results in a dramatic increase in the solubility of H2O in felsic magmas. This is a direct consequence of the shape of the H2O-saturated granite solidus. The effect of this slight increase in total pressure on the crystallisation of felsic magmas is to delay vapour saturation, increase the H2O content of the residual melt fractions and further depress the solidus temperature. Higher melt H2O contents, and an extended temperature range over which crystallisation can proceed, both favour crystallisation of coarser-grained granites. In addition, the potential for the development of late, H2O-rich, melt fractions is significantly enhanced. Upon reaching vapour saturation, these late melt fractions are likely to form porphyries, aplite dykes and pegmatite dykes.For the Southern Oklahoma Aulacogen, the progressive increase in the depth of the crustal magma trap at the base of the volcanic pile appears to reflect thickening of the volcanic pile during rifting, but may also reflect emplacement of earlier granite sheets. Thus, the change in textural characteristics of granite sheets of the Wichita Granite Group may hold considerable promise as an avenue for further investigation in interpreting the history of this rifting event.

Relationships between magmatism, metamorphism and deformation in the Fraser Complex, Western Australia: Constraints from new SHRIMP U–Pb zircon geochronology

SHRIMP U–Pb zircon isotopic data have been obtained for four samples collected from granitoids and paragneisses in the Fraser Complex, a large composite metagabbroic body cropping out in the Mesoproterozoic Albany‐Fraser Orogen of Western Australia. The data are combined with the results of field mapping and petrographic analysis to revise a model for the geological evolution of the Fraser Complex. Three main phases of deformation are recognised in the Fraser Complex (D1–3) associated with two metamorphic events (M1–2), which involve four distinguishable episodes of recrystallisation. The first metamorphic event recognised (M1a/D1) reached granulite facies and is characterised by peak T ≥800°C and P = 600–700 MPa. A syn‐M1a/D1 charnockite has a U–Pb SHRIMP zircon age of 1301 ± 6 Ma, which also provides an estimate for the age of intrusion of Fraser Complex gabbroic rocks. Disequilibrium textures comprising randomly oriented minerals (M1b), consistent with approximately isobaric cooling, formed in various lithologies in the interval between D1 and D2. Post‐D1, pre‐D2 granites intruded at 1293 ± 8 Ma and were foliated during the D2 event, which culminated in the burial of the Fraser Complex to depths equivalent to 800–1000 MPa. Following burial, pyroxene granulites on the western boundary of the complex were pervasively retrogressed to garnet amphibolite (M2a). An igneous crystallisation age of 1288 ± 12 Ma from a syn‐M2a aplite dyke suggests that retrogression may have occurred only a few millions of years after the peak of granulite facies metamorphism. Exhumation to depths of less than ∼400 MPa occurred within ∼20–30 million years of the M2a pressure peak. Associated deformation (D3) is characterised by the development of mylonite and transitional greenschist/amphibolite facies disequilibrium textures (M2b).