Synplutonic Mafic Dykes Research Articles

The role of accreted continental crust in the formation of granites within the Alpine style continental collision zone: Geochemical and geochronological constrains from leucogranites in the Çataldağ Metamorphic Core Complex (NW Turkey)

The recently discovered Çataldağ Metamorphic Core Complex (ÇMCC) of Eo-Oligocene age is situated in NW Turkey, which is a part of the Alpine collisional belt. The footwall of the ÇMCC consists of a granite-gneiss-migmatite complex (GGMC) with a domal structure. Here, we document new major-trace element geochemistry, Sr-Nd-Pb isotope characteristics and Ti-in-zircon thermometry data from granites and syn-plutonic mafic dykes that are connected to the gneiss-migmatite dome of ÇMCC, and discuss granitic melt evolution within the context of the syn-to post-collisional evolution of the western Anatolian orogenic crust. Granites of the GGMC include both peraluminous garnet-bearing leucogranite and two-mica granite that emplaced from 32 to 35 Ma. Both types of leucogranite are enriched in light rare earth elements (Rb, U, K, Pb) and depleted in HFSE (Nb, Ta, Zr, Ti). Their 87Sr/86Sr, 206Pb/204Pb and 207Pb/204Pb initial isotope values range from 0.7094 to 0.7113, 18.79 to 18.91, and 15.71 to 15.73, respectively, and εNd(33) values vary between −5.13 and − 7.79. On the other hand, mafic dykes show similar isotopic characteristics (87Sr/86Sr(33) = 0.7055, εNd(33) = −1.8 and 206Pb/204Pb = 18.8) to enriched mantle melts. Geochemical modelling shows that the granites have dominant crustal melt component (85–70%) and a minor mantle component (<30%). According to partial melting modelling and Ti-in-zircon thermometer calculations, granitic melt was formed by water-absent muscovite dehydration melting of a micaschist source (a melt fraction of max. 35%) at ≥7–10 kb and 739–840 °C. Whole rock geochemistry, isotopic characteristics and inherited zircon chronology, combined with the geology of the region, indicate that granitic melts were derived by partial melting of Anatolide-Tauride crustal units that were underthrusted below the Sakarya Continent along the İzmir-Ankara Suture Zone. On the other hand, the source of the syn plutonic mafic dykes emplaced in the core of the ÇMCC is inferred to derived from enriched mantle (EM II) that lay beneath the western Anatolian orogenic crust. We infer that partial melting and melt migration to produce leucogranites are most probably related to thermal weakening and partial removal of western Anatolian young orogenic lithosphere during the transitional phase between the latest phase of collision and the earliest phase of extension, in the Eo-Oligocene.

Interaction of coeval felsic and mafic magmas from the Kanker granite, Pithora region, Bastar Craton, Central India

Field and petrographic studies are carried out to characterize the interactions of mafic and felsic magmas from Pithora region of the northeastern part of the Bastar Craton. The MMEs, syn-plutonic mafic dykes, cuspate contacts, magmatic flow textures, mingling and hybridization suggest the coeval emplacement of end member magmas. Petrographic evidences such as disequilibrium assemblages, resorption textures, quartz ocelli, rapakivi and poikilitic textures suggest magma mingling and mixing phenomena. Such features of mingling and mixing of the felsic and mafic magma manifest the magma chamber processes. Introduction of mafic magmas into the felsic magmas before initiation of crystallization of the latter, results in hybrid magmas under the influence of thermal and chemical exchange. The mechanical exchange occurs between the coexisting magmas due to viscosity contrast, if the mafic magma enters slightly later into the magma chamber, then the felsic magma starts to crystallize. Blobs of mafic magma form as MMEs in the felsic magma and they scatter throughout the pluton due to convection. At a later stage, if mafic magma enters the system after partial crystallization of felsic phase, mechanical interaction between the magmas leads to the formation of fragmented dyke or syn-plutonic mafic dyke. All these features are well-documented in the study area. Field and petrographic evidences suggest that the textural variations from Pithora region of Bastar Craton are the outcome of magma mingling, mixing and hybridization processes.

Tectonic significance of dykes in the Sarnu-Dandali alkaline complex, Rajasthan, northwestern Deccan Traps

Whether swarms of preferentially oriented dykes are controlled by regional stress fields, or passively exploit basement structural fabric, is a much debated question, with support for either scenario in individual case studies. The Sarnu-Dandali alkaline complex, near the northwestern limit of the Deccan Traps continental flood basalt province, contains mafic to felsic alkaline volcano-plutonic rocks and carbonatites. The complex is situated near the northern end of the 600 km long, NNW–SSE-trending Barmer-Cambay rift. Mafic enclave swarms in the syenites suggest synplutonic mafic dykes injected into a largely liquid felsic magma chamber. Later coherent dykes in the complex, of all compositions and sizes, dominantly strike NNW–SSE, parallel to the Barmer-Cambay rift. The rift formed during two distinct episodes of extension, NW–SE in the early Cretaceous and NE–SW in the late Cretaceous. Control of the southern Indian Dharwar structural fabric on the rift trend, as speculated previously, is untenable, whereas the regional Precambrian basement trends (Aravalli and Malani) run NE–SW and NNE–SSW. We therefore suggest that the small-scale Sarnu-Dandali dykes and the much larger-scale Barmer-Cambay rift were not controlled by basement structure, but related to contemporaneous, late Cretaceous regional ENE–WSW extension, for which there is varied independent evidence.

Coeval Felsic and Mafic Magmas in Neoarchean Calc-alkaline Magmatic Arcs, Dharwar Craton, Southern India: Field and Petrographic Evidence from Mafic to Hybrid Magmatic Enclaves and Synplutonic Mafic Dykes

Abstract We present field and petrographic data on Mafic Magmatic Enclaves (MME), hybrid enclaves and synplutonic mafic dykes in the calc-alkaline granitoid plutons from the Dharwar craton to characterize coeval felsic and mafic magmas including interaction of mafic and felsic magmas. The composite host granitoids comprise of voluminous juvenile intrusive facies and minor anatectic facies. MME, hybrid enclaves and synplutonic mafic dykes are common but more abundant along the marginal zone of individual plutons. Circular to ellipsoidal MME are fine to medium grained with occasional chilled margins and frequently contain small alkali feldspar xenocrysts incorporated from host. Hybrid magmatic enclaves are intermediate in composition showing sharp to diffused contacts with adjoining host. Spectacular synplutonic mafic dykes commonly occur as fragmented dykes with necking and back veining. Similar magmatic textures of mafic rocks and their felsic host together with cuspate contacts, magmatic flow structures, mixing, mingling and hybridization suggest their coeval nature. Petrographic evidences such as disequilibrium assemblages, resorption, quartz ocelli, rapakivi-like texture and poikilitically enclosed alkali feldspar in amphibole and plagioclase suggest interaction, mixing/mingling of mafic and felsic magmas. Combined field and petrographic evidences reveal convection and divergent flow in the host magma chamber following the introduction of mafic magmas. Mixing occurs when mafic magma is introduced into host felsic magma before initiation of crystallization leading to formation of hybrid magma under the influence of convection. On the other hand when mafic magmas inject into host magma containing 30-40% crystals, the viscosities of the two magmas are sufficiently different to permit mixing but permit only mingling. Finally, if the mafic magmas are injected when felsic host was largely crystallized (∼70% or more crystals), they fill early fractures and interact with the last residual liquids locally resulting in fragmented dykes. The latent heat associated with these mafic injections probably cause reversal of crystallization of adjoining host in magma chamber resulting in back veining in synplutonic mafic dykes. Our field data suggest that substantial volume of mafic magmas were injected into host magma chamber during different stages of crystallization. The origin of mafic magmas may be attributed to decompression melting of mantle associated with development of mantle scale fractures as a consequence of crystallization of voluminous felsic magmas in magma chambers at deep crustal levels.

Anatomy of 2.57 - 2.52 Ga granitoid plutons in the eastern Dharwar craton, southern India: Implications for magma chamber processes and crustal evolution

We present results of field studies for magmatic processes of 2.57-2.52 Ga calc-alkaline plutonic bodies from three corridors in the eastern Dharwar craton (EDC) corresponding to different crustal levels. At deeper levels plutons are bounded by thick zone of migmatites with numerous melt filled shear bands which often overprinted by incipient charnockite. On the other hand in the mid-to-upper crustal levels plutons show relatively sharp contacts and truncates the adjoining basement. The plutons are composite which comprises voluminous intrusive monzodiorite, quartz-monzonite and porphyritic monzogranite in the central part and minor anatectic granites or diatexite at periphery. Numerous xenoliths, Mafic Magmatic Enclaves (MME), disrupted trains of synplutonic mafic dykes are found in both intrusive and anatectic facies. The plutons show magmatic as well as solid-state plastic fabrics defined by magmatic flow banding and C-S fabrics respectively. Crustal scale shear zone network comprising early melt filled NE trending hot ductile dextral shear bands and slightly later colder NW trending sinistral shear bands defined by rotation of mafic boudins, phenocrysts and C-S fabrics. The internal architecture of plutons is attributed to the crustal scale magma chamber processes where voluminous intrusive magmas emplaced into the crust caused reworking of surrounding basement resulting in production of anatectic magmas. Crystallization of voluminous intrusive magmas in the deep crust probably caused development of fractures to mantle depth causing decompression melting of mantle and resultant mafic magmas penetrated the crystallizing host in magma Archaean magmatic accretion, reworking and cratonization. chambers. Field evidences together with published ages and Nd isotope data reveal a spatial link between late Archaean magmatic accretion, reworking and cratonization.

Origin of synplutonic mafic dykes in Ryoke igneous rock complex at Shodo Island, southwest Japan

Origin of synplutonic mafic dykes in Ryoke igneous rock complex at Shodo Island, southwest Japan

The Fazenda Gavião granodiorite and associated potassic plutons as evidence for Palaeoproterozoic arc-continent collision in the Rio Itapicuru greenstone belt, Brazil

Abstract Several granitic plutons have intruded the Palaeoproterozoic Rio Itapicuru greenstone belt, Sao Francisco craton, Brazil, in the time interval 2163-2080 Ma, but their tectonic significance is poorly understood. The Fazenda Gaviao granodiorite (FGG) is one of a set of plutons emplaced along the western boundary of the greenstone belt with Archaean migmatite-gneiss basement. The pluton is mostly composed of hornblende granodiorite, occasionally crosscut by syn-plutonic mafic dykes. The FGG is metaluminous, medium- to high-K calc-alkaline with relatively constant silica abundances (SiO2 ∼ 63–66 wt%), high Sr (900–800 ppm) and high Ba (1000–1500 ppm). The associated mafic dykes are ultrapotassic, with high abundances of Ba, Sr, MgO, Ni, Cr, and light rare earth elements, suggesting derivation from partial melts of an enriched mantle source. The FGG originated probably by fractional crystallization from a primitive K-rich mafic magma that interacted with crustal melts. Its zircon U–Pb SHRIMP age of 2106 ± 6 Ma indicates that the FGG is younger than the early (2163-2127 Ma) tonalite-trondhjemite-granodiorite (TTG) and calc-alkaline arc plutons of the greenstone belt, and is closely related in time and space with potassic to ultrapotassic plutons (ca. 2110-2105 Ma). The negative eNd(t) of FGG and coeval K-rich plutons of the Rio Itapicuru greenstone belt contrasts markedly with the positive eNd(t) of the older arc plutons, indicating a major change of isotope signatures in granites of the Rio Itapicuru greenstone belt with time. This isotope shift may be related to magma contamination with older continental material and/or derivation of the parental potassic magma from enriched lithospheric mantle sources. We suggest that the K-rich plutons were emplaced during or shortly after Palaeoproterozoic arc-continent collision.

Enclaves in granitoids of north of Jonnagiri schist belt, Kurnool district, Andhra Pradesh: Evidence of magma mixing and mingling

Calc-alkaline, metaluminous granitoids in the north of Jonnagiri schist belt (JSB) are associated with abundant mafic rocks as enclave. The enclaves represent xenoliths of the basement, mafic magmatic enclaves (MME) and synplutonic mafic dykes. The MME are mostly ellipsoidal and cuspate shape having lobate margin and diffuse contact with the host granitoids. Sharp and crenulated contacts between isolated MME and host granitoids are infrequent. The MME are fine-grained, slightly dark and enriched in mafic minerals compare to the host granitoids. MME exhibits evidences of physical interaction (mingling) at outcrop scale and restricted hybridization at crystal scale of mafic and felsic magmas. The textures like quartz ocelli, sphene (titanite) ocelli, acicular apatite inclusion zone in feldspars and K-feldspar megacrysts in MME, megacrysts across the contact of MME and host and mafic clots constitute textural assemblages suggestive of magma mingling and mixing recorded in the granitoids of the study area. The quartz ocelli are most likely xenocrysts introduced from the felsic magma. Fast cooling of mafic magma resulted in the growth of prismatic apatite and heterogeneous nucleation of titanite over hornblende in MME. Chemical transfer from felsic magma to MME forming magma envisage enrichment of silica, alkalis and P in MME. The MME show low positive Eu anomalies whereas hybrid and host granitoids display moderate negative Eu-anomalies. Synplutonic mafic dyke injected at late stage of crystallising host felsic magma, display back veining and necking along its length. The variable shape, dimensions, texture and composition of MME, probably are controlled by the evolving nature and kinematics of interacting magmas.

Mafic to hybrid microgranular enclaves in the Ladakh batholith, northwest Himalaya: Implications on calc-alkaline magma chamber processes

Felsic magmatisms in the north of Indus-Tsangpo Suture Zone (ITSZ) in Ladakh range of northwest Indian Himalaya, referred herein Ladakh granitoids (LG), and associated magmatic rocks constitute the bulk of the Ladakh batholith. They have been characterized as Andean-type, calc-alkaline, largely metaluminous (I-type) to a few peraluminous (S-type) granitoids derived from partial melting of subducting materials. The LG can be broadly classified into coarsegrained facies with abundant mafics (hbl-bt), medium-grained facies with low content of mafics, and fine-grained leucocratic facies with very low amount of mafics. Mesocratic to melanocratic, rounded to elliptical, fine to medium grained, mafic to hybrid microgranular enclaves (ME) are ubiquitous in medium to coarse-grained LG. ME are absent or rare in the leucocratic variety of LG. In this paper different types of ME, and their field relation and microstructures with respect to felsic host LG are documented from northwestern, central, southeastern parts of the Ladakh batholith. Rounded to elongate ME of variable sizes (a few cm to metres across, mostly d<30 cm) commonly having sharp, crenulate, and occasionally diffuse contacts of ME with felsic host LG suggest that several pulses of crystal-charged mafic and felsic magmas coexisted, hybridized, and co-mingled into subvolcanic settings. Occurrence of composite ME (several small mafic ME enclosed into large porphyritic ME) strongly point to multiple mafic to hybrid magma intrusions into partly crystalline LG magma chambers. Synplutonic mafic dykes disrupted to form subrounded to angular (brecciated) mafic ME swarms commonly disposed in strike-length suggest mafic magma injections at waning stage of felsic magma evolution with large rheological contrasts. Pillowing of mafic melt against leucocratic (aplitic) residual melt strongly suggests mafic magma intrusion in nearly-crystallized condition of pluton. Although common mineral asemblages (hblbt-pl-kfs-qtz-ap-zrn-mt±ilm) of ME (diorite, quartzdiorite) and host LG (granodiorite, monzogranite) may relate to their cogenetic relation, fine to medium grained porphyritic (hybrid) nature and lack of cumulate texture of ME strongly oppose cognate origin for ME. Presence of plagioclase xenocrysts, quartz ocelli and accicular apatite in porphyritic ME strongly indicate mingling and undercooling of hybridized ME globules into relatively crystal-charged cooler host LG magma. Grain size differences of some ME, except to those of porphyritic ones, appear related to varying degrees of undercooling of ME most likely controlled by their variable sizes. Several smaller ME, however, lack fine-grained chilled margin probably because of their likely disaggregation from a large size ME during the course of progressive hybridization (mingling to mixing) leaving behind trails of mafic schlieren. Field and microstructural evidences at least suggest that Ladakh granitoids and their microgranular enclaves are products of multistage magma mingling and mixing processes concomitant fractional differentiation of several batches of mafic and felsic magmas formed in open magma chamber(s) of subduction setting.

Synplutonic mafic dykes from late Archaean granitoids in the Eastern Dharwar Craton, southern India

We present a first overview of the synplutonic mafic dykes (mafic injections) from the 2.56–2.52 Ga calcalkaline to potassic plutons in the Eastern Dharwar Craton (EDC). The host plutons comprise voluminous intrusive facies (dark grey clinopyroxene-amphibole rich monzodiorite and quartz monzonite, pinkish grey porphyritic monzogranite and grey granodiorite) located in the central part of individual pluton, whilst subordinate anatectic facies (light grey and pink granite) confined to the periphery. The enclaves found in the plutons include highly angular screens of xenoliths of the basement, rounded to pillowed mafic magmatic enclaves (MME) and most spectacular synplutonic mafic dykes. The similar textures of MME and adjoining synplutonic mafic dykes together with their spatial association and occasional transition of MME to dismembered synplutonic mafic dykes imply a genetic link between them. The synplutonic dykes occur in varying dimension ranging from a few centimeter width upto 200 meters width and are generally dismembered or disrupted and rarely continuous. Necking of dyke along its length and back veining of more leucocratic variant of the host is common feature. They show lobate as well as sharp contacts with chilled margins suggesting their injection during different stages of crystallization of host plutons in magma chamber. Local interaction, mixing and mingling processes are documented in all the studied crustal corridors in the EDC. The observed mixing, mingling, partial hybridization, MME and emplacement of synplutonic mafic dykes can be explained by four stage processes: (1) Mafic magma injected during very early stage of crystallization of host felsic magma, mixing of mafic and felsic host magma results in hybridization with occasional MME; (2) Mafic magma introduced slightly later, the viscosities of two magmas may be different and permit only mingling where by each component retain their identity; (3) When mafic magma injected into crystallizing granitic host magma with significant crystal content, the mafic magma is channeled into early fractures and form dismembered synplutonic mafic dykes and (4) Mafic injections enter into largely crystallized (>80% crystals) granitic host results in continuous dykes with sharp contacts. The origin of mafic magmas may be related to development of fractures to mantle depth during crystallization of host magmas which results in the decompression melting of mantle source. The resultant hot mafic melts with low viscosity rise rapidly into the crystallizing host magma chamber where they interact depending upon the crystallinity and viscosity of the host. These hot mafic injections locally cause reversal of crystallization of the felsic host and induce melting and resultant melts in turn penetrate the crystallizing mafic body as back veining. Field chronology indicates injection of mafic magmas is synchronous with emplacement of anatectic melts and slightly predates the 2.5 Ga metamorphic event which affected the whole Archaean crust. The injection of mafic magmas into the crystallizing host plutons forms the terminal Archaean magmatic event and spatially associated with reworking and cratonization of Archaean crust in the EDC.

The Jokkmokk granitoid, an example of 1.88 Ga juvenile magmatism at the Archaean-Proterozoic border in northern Sweden

The Jokkmokk granitoid is exposed in a large plutonic massif northwest of Jokkmokk in northern Sweden. It is light grey to white, fine-grained, with megacrysts of feldspar and glomeroporphyritic hornblende and biotite. Small enclaves of mafic rocks and synplutonic mafic dykes are products of mingling with a coeval and possibly cogenetic mafic magma. The Jokkmokk granitoid was previously considered to belong to the c. 1.8 Ga Lina S-type intrusive suite, but the Jokkmokk granitoid has a unique calc-alkaline to alkali-calcic, metaluminous to weakly peraluminous, character with a moderate LREE enrichment and a flat HREE pattern, and a flat to slightly positive Eu-anomaly. U-Pb TIMS zircon dating of the Jokkmokk granitoid gives an age of 1883±15 Ma which is coeval with the emplacement of the Haparanda suite, but contrary to the Haparanda suite it displays a positive εNd(t) value of 2.8, indicating a more juvenile Palaeoproterozoic character similar to the Jörn suite in the Skellefte district. This type of magma seems to be restricted to the palaeoboundary between the Archaean craton in the north and Palaeoproterozoic juvenile crust in the south. Spatial correlation with low angle, south dipping, WNW-trending shear zones and NNE-trending subvertical shear zones, highlight the possibility that this unique magma type is related to transtension in the overriding plate and partial melting in a sub-arc mantle wedge during NE-directed subduction processes related to the early stages of the Svecokarelian orogen. This type of setting has been advocated as the potentially most favourable tectonic setting for porphyry copper formation.

Geochemistry and Origin of Early Proterozoic Dongargarh Rapakivi Granite Complex, Central India - An Example for Magma Mixing and Differentiation

Early Proterozoic Dongargarh granite complex of Central India, intruding the tonalitic to granodioritic Amgaon gneisses and the Nandgaon Group bimodal volcanic suite, comprises three different textural and compositional types, viz., porphyritic granodiorite (PG), coarse equigranular granite (EG) and microgranite (MG). Synplutonic mafic dykes are common in the granite complex. The PG is characterised by rapakivi texture and the EG is the dominant facies and exhibits sporadically developed rapakivi texture. Microgranular enclaves are common in the EG while they are rare in PG. Major and trace element geochemistry of PG shows marked I- type and some occasional A-type granite characters unusual for a rapakivi granite while the EG shows A-type granite signatures. The field, petrographic, chemical and isotopic data of these granites suggest their derivation by mixing of mantle derived basic magma with a crustal-derived partly crystalline granitic magma. Episodic mafic magma underplating caused the anatexis of the Archaean lower continental crust in a continental margin tectonic setting resulting first in the formation of the I-type granodiorite followed by A-type granite. The I-type granodiorite is mixed with the basic magma (synplutonic dykes) while the EG is formed by mingling of A- type granite magma and the intruding basic magma.

Geochemical evolution of the Minto block: a 2.7 Ga continental magmatic arc built on the Superior proto-craton

Abstract A 400 km-long by 100 km-wide geological transect across the Minto block of northeastern Superior Province reveals five plutonic suites and minor amphibolite- to granulite-grade supracrustal remnants. The oldest regionally significant plutonic rocks include 3.0±0.1 Ga foliated to gneissic tonalite characterized by high Na K ratios, high Al contents, light REE enrichment, and initial geNd= +1.8. The Leaf River plutonic suite is the most widespread, with crystallization ages of about 2725 ± 5 Ma across the transect. The suite comprises massive to foliated pyroxene- and hornblende-bearing granodiorite, tonalite, granite, diorite, and minor gabbro-pyroxenite and syn-plutonic mafic dykes. The rocks are I-type, calc-alkaline, have variable Na K ratios, moderate to high Ba and Sr contents, and REE patterns with variable light REE enrichment depletion of heavy REEs with increasing silica, and neutral, negative, or positive Eu anomalies. Initial ϵNd values for the suite range from −0.5 to +1.3 (mean = +0.4), and initial 87 Sr 86 Sr ratios (ISr) are ∼0.7020. The diatexite plutonic suite (2713 Ma) occurs within a 150 km-wide zone, and consists of orthopyroxene±garnet granodiorite containing 25–50% inclusions of granulite-grade mafic, supracrustal, and tonalitic inclusions. The variable major- and trace-element chemistry of the suite, with generally high Ni and Cr contents, reflects heterogeneous source materials and the presence of non-liquid components. Initial ϵNd values are from +0.1 to −1.7. Monzogranite and orthopyroxene granite dykes, plugs and plutonic masses, both ∼2690 Ma, comprise the last important plutonic suites. Both biotite monzogranite and orthopyroxene granite are light REE-enriched, but the former have negative Eu anomalies, and the latter positive. Initial ϵNd values range from −0.7 to −3.2 for monzogranite (ISr=0.7016 to 0.7067), and +0.7 to −0.2 for orthopyroxene granite (ISr=0.701 to 0.702). The initial ϵNd and ISr values of the ∼ 2.7 Ga plutonic suites are lower and higher, respectively, than estimatesf of the contemporaneous depleted mantle, suggesting significant and wide-scale reworking of older, tight REE-enriched, high Rb SR lithosphere. Early tonalite orthogneiss from Goudalie domain may be derived by partial melting of mafic crust at high pressures. The lithological and geochemical diversity of the 2725 Ma Leaf River plutonic suite indicate a range of parental magma compositions and petrogenetic processes, but fractional crystallization of mantle-derived basaltic melts which had assimilated small amounts of significantly older lower crust may have been the dominant mechanism. Crustal-inclusion-charged granodioritic diatexite (2713 Ma) was probably derived by partial melting of heterogeneous lower crustal materials under relatively dry conditions. Late biotite monzogranite was generated by partial melting of older, sialic crust, and orthopyroxene granite by deeper melting or fractional crystallization of more juvenile source materials with garnet as a residual mineral. We infer from the presence of coeval mafic dykes in all plutonic suites that heat for melting was ultimately derived from intrusion of basaltic magmas at depth. The ∼ 2.7 Ga magmatism in the Minto block occurred during a period of active production and accretion of juvenile oceanic terranes in the southern Superior Province. The northwest-directed subduction regime inferred from the southern Superior Province arc terranes may also have been responsible for the magmatism that acted on 3 Ga lithosphere in the northern Superior Province. Melting within the mantle wedge beneath the proto-cratonic lithosphere and in the lower crust occurred within a tectonic setting similar to that of modem continental magmatic arcs.

Petrogenesis of Mafic Inclusions in Granitoids of the Adamello Massif, Italy

ABSTRACT The Tertiary Adamello calc-alkaline batholith in the Italian Alps is characterized by tonalite and granodiorite plutons associated with small mafic/ultramafic intrusions, syn-plutonic mafic dykes and sills, and ubiquitous mafic inclusions. In the southernmost Val Fredda Complex, syn-plutonic hornblende-gabbro and diorite sheets pass laterally into swarms of mafic inclusions intermingled with tonalite. Petrological and geochemical data show that the mafic sheets represent hydrous mafic magmas derived by fractional crystallization from parental hydrous basalt and picro-basalt. The fractionation process is recorded by inclusions of spinel, olivine, and pyroxenes in the cores of hornblende phenocrysts and by the widespread occurrence of calcic plagioclase. Fractionation occurred at high pressure (Ptoul = 8–10 kb) before intrusion at shallow depths (Ptotal≈ 2 kb). Geothermometry and melting experiments at PH2O= 1 kb, combined with textural evidence, indicate that the mafic sheets were emplaced at temperatures of 1050–1100°C into hot, but consolidated, granitoid host rocks. Transfer of heat and hydrous fluids from the sheets remobilized the host rocks into crystal-mush, which in turn disrupted the sheet margins to form mafic inclusions. Dynamic crystallization experiments indicate that the mafic inclusions and sheet margins were quenched to temperatures below 970 C, resulting in the failure of the high-temperature liquidus phases olivine and clinopyroxene to nucleate and the formation of acicular hornblende and plagioclase. Several other Adamello plutons display syn-plutonic intrusions and mafic inclusions with comparable features to the Val Fredda Complex. The Adamello mafic inclusions show pronounced enrichments in certain trace elements compared with values expected by fractional crystallization and magma mixing. K, Rb, Ba, Y, heavy REE, Mn, and Nb have absolute abundances in the inclusions greater than the interiors of neighbouring mafic sheets and, in some cases, than the host granitoids. Many inclusions also display leucocratic haloes, margins rich in ferromagnesian minerals and abundant groundmass biotite. These features are interpreted in terms of a three-stage evolution. (1) A blob of mafic magma is quenched by the felsic host to form a rigid crystal-rich inclusion containing an interstitial melt phase. Leucocratic haloes and crenulate margins to the inclusions form as a result of volume contraction on cooling. (2) The more mobile elements (notably the alkalis and H2O) diffuse between the melt phases of host and inclusion. Using published experimental data on the variation of melt fraction with temperature in hydrous basic and acid magmas, it is argued that the observed diffusion of K from host to inclusion requires interaction temperatures of >900°C. Reaction of K-enriched melt with existing hornblende in the inclusion forms biotite, which sequesters and concentrates further K2O and other alkaline elements. (3) During protracted cooling the mafic inclusions equilibrate with interstitial melt in the host granitoid. Equilibrium partitioning of heavy REE and Y into the mafic minerals in the inclusion results in the observed enrichments. Magnetite likewise concentrates Nb and Mn. It is proposed that mafic inclusions form in the waning stage of pluton evolution when the granitoid magma is sufficiently consolidated to allow the penetration of mafic intrusions, but sufficiently hot to be readily remobilized and disrupt these intrusions to form mafic inclusions. Subsequent chemical equilibration of mafic inclusions with their host can have a marked impact on the trace element chemistry of both rock types. Granitoids which have experienced extensive interaction with mafic inclusion-forming magmas may undergo significant depletion in those trace elements which partition strongly into the minerals of the mafic inclusion.

Rare earth evidence for the origin of the N�k gneisses Buksefjorden region, southern West Greenland

Rare earth element (REE) concentrations have been determined (by the INAA method) for the c. 2,800 m.y. old Nuk gneisses from the Buksefjorden region, southern West Greenland. Samples include dioritic to granodioritic gneisses and synplutonic mafic dykes; a Malene metagabbro and Qorqut granite were analysed for comparisons. The early Nuk gneisses, diorites and tonalites, have mildly fractionated REE patterns which are interpreted as resulting from partial melting of garnetbearing amphibolite or granulite. Early Nuk trondhjemitic gneisses possess downward convex patterns with prominent positive Eu anomalies; they may be related to the diorites and tonalites by the separation of hornblende in a residue of partial melting or fractional crystallization. Most of the later Nuk grey gneisses have extremely fractionated linear patterns which were derived from a source very rich in garnet, possibly eclogite. REE patterns measured in the late Nuk Ilivertalik granite complex are mildly fractionated but with a high overall abundance consistent with an origin by partial melting of mafic lower crustal material. Two sets of synplutonic mafic dykes have strongly fractionated patterns similar to those found in alkali basalts. The geochemical variations suggest that the igneous precursors of the Nuk gneisses were not cogenetic, but were derived from widely differing sources.