Evidence for a high Mg andesitic parental magma to the East and West satellite dykes of the Great Dyke,Zimbabwe: a comparison with the continental tholeiitic Mashonaland sills

One of the most significant mafic intrusive events in the Zimbabwe Craton was the emplacement of the Great Dyke layered ultramafic-mafic complex and its two parallel ‘satellite’ dykes at the end of the Archæan (∼2.6 Ga). The two satellite dykes, the East Dyke and the West (Umvimeela) Dyke, were far less affected by crystal accumulation and layering processes than the Great Dyke, and therefore may provide a clearer picture of parental magma compositions of the Great Dyke event. The geochemical character of this major episode of mafic magmatism is markedly different to that of more typical continental tholeiites, such as the ∼1.9 Ga Mashonaland sills, and reflects significant contrasts in petrogenetic influences. Despite its mid-continental setting, the parental magma of the satellite dykes had geochemical characteristics more often associated with magmas generated at destructive plate margins, suggesting that the nature of this magma was perhaps more similar to high Mg andesitic, rather than continental tholeiitic magmatism. Fine-grained samples with ∼14% MgO perhaps most closely approximate to the parental magma composition. Certain major and trace element concentrations (high MgO, SiO₂, LREE and LILE, and low Nb, Ta and Ti) and the lack of an Fe enrichment trend, suggest that the mafic magma had either suffered severe crustal contamination or had been derived from a metasomatised harzburgitic mantle source.     

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

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.     

Alkali-calcic and alkaline post-orogenic (PO) granite magmatism: petrologic constraints and geodynamic settings

The end of an orogenic Wilson cycle corresponds to amalgamation of terranes into a Pangaea and is marked by widespread magmatism dominated by granitoids. The post-collision event starts with magmatic processes still influenced by subducted crustal materials. The dominantly calc-alkaline suites show a shift from normal to high-K to very high-K associations. Source regions are composed of depleted and later enriched orogenic subcontinental lithospheric mantle, affected by dehydration melting and generating more and more K- and LILE-rich magmas. In the vicinity of intra-crustal magma chambers, anatexis by incongruent melting of hydrous minerals may generate peraluminous granitoids bearing mafic enclaves. The post-collision event ends with emplacement of bimodal post-orogenic (PO) suites along transcurrent fault zones. Two suites are defined, (i) the alkali-calcic monzonite–monzogranite–syenogranite–alkali feldspar granite association characterised by [biotite+plagioclase] fractionation and moderate [LILE+HFSE] enrichments and (ii) the alkaline monzonite–syenite–alkali feldspar granite association characterised by [amphibole+alkali feldspar] fractionation and displaying two evolutionary trends, one peralkaline with sodic mafic mineralogy and higher enrichments in HFSE than in LILE, and the other aluminous biotite-bearing marked by HFSE depletion relative to LILE due to accessory mineral precipitation. Alkali-calcic and alkaline suites differ essentially in the amounts of water present within intra-crustal magma chambers, promoting crystallisation of various mineral assemblages. The ultimate enriched and not depleted mantle source is identical for the two PO suites. The more primitive LILE and HFSE-rich source rapidly replaces the older orogenic mantle source during lithosphere delamination and becomes progressively the thermal boundary layer of the new lithosphere. Present rock compositions are a mixture of major mantle contribution and various crustal components carried by F-rich aqueous fluids circulating within convective cells created around magma chambers. In favourable areas, PO suites pre-date a new orogenic Wilson cycle.     

Geology of Pulikonda and Dancherla alkaline complexes,Andhra Pradesh

Geological studies on saturated to oversaturated and subsolvus aegirine-riebeckite syenite bodies of the Pulikonda alkaline complex and Dancherla alkaline complex were carried out. The REE distribution of the Dancherla syenite shows a high fractionation between LREE and HREE. The absence of Eu anomaly suggests source from garnet peridotite. The Pulikonda syenite shows moderate fractionation between LREE and HREE as reflected by enrichment of HREE and moderate enrichment of LREE. The negative Eu anomaly indicates role of plagioclase fractionation.Three distinct co-eval primary magmas i.e. mafic syenite-, felsic syenite- and alkali basalt magmas — all derived from low-degrees of partial melting of mantle differentiates and enriched metasomatised lower crust played a major role in the genesis and emplacement of the syenites into overlying crust along deep seated regional scale trans-lithospheric strike-slip faults and shear zones following immediately after late-Archaean calc-alkaline arc magmatism at different time-space episodes i.e. initially at craton margin and later on into the thickened interior of the Eastern Dharwar craton. The ductile sheared and folded Pulikonda alkaline complex was evolved dominantly from the magmas derived from partial melting of lower crust and minor juvenile magmas from mantle. Differentiation and fractionation by liquid immiscibility of mafic magma and commingling-mixing of intermediate and felsic magmas followed by fractionational crystallisation under extensional tectonics during waning stages of calc-alkaline arc magmatism nearer to the craton margin were attributed as the main processes for the genesis of Pulikonda syenite complex. Commingling and limited mixing of independent mantle derived mafic and felsic syenitic magmas and accompanying fractionation resulting into soda rich and potash rich syenite variants was tentatively deduced mechanism for the origin of Dancherla, Danduvaripalle, Reddypalle syenites and other bodies belonging to Dancherla alkaline complex at the craton interior. The Peddavaduguru syenite was formed by differentiation of alkali mafic magma (gabbro to diorite) and it’s simultaneous mingling with fractionated felsic syenitic magma under incipient rift. Vannedoddi and Yeguvapalli syenites were derived due to desilicification and accompanying alkali feldspar mestasomatism of younger potash rich granites along Guntakal-Gooty fault and along Singanamala shear zone respectively.     

The Serra da Graciosa A-type Granites and Syenites, southern Brazil: Part 2: Petrographic and mineralogical evolution of the alkaline and aluminous associations

The Serra da Graciosa Granites and Syenites comprise five distinct plutons in the Brasiliano/Pan-African Graciosa A-type Province, southern Brazil. Six petrographic series can be identified in these plutons: (1) Alkaline series 1, composed of amphibole-bearing alkali feldspar syenites with varied mafic mineralogy and quartz contents, from alkali feldspar syenites with calcic amphibole, clinopyroxene, olivine and allanite to alkali feldspar quartz syenites with sodic–calcic amphibole and chevkinite–perrierite and to alkali feldspar granites with sodic amphibole; (2) Alkaline series 2, characterized by amphibole-bearing alkali feldspar granites, with limited modal variations but amphibole compositions also varying from calcic to sodic; (3) Alkaline series 3, of limited occurrence, which includes alkali feldspar syenites with olivine and clinopyroxene and no amphibole; (4) Aluminous series 1, of widespread occurrence, with various petrographic facies of biotite granites with amphibole; (5) Aluminous series 2, characterized by alkali feldspar granites with biotite and only minor amphibole; (6) Monzodiorites, typically with biotite, calcic amphibole and augitic clinopyroxene, partially mingled with granitic magmas. The mafic minerals present are, in general, Fe-rich with correspondingly low Mg and Al contents. In Alkaline series 1, amphiboles crystallized in progressively more oxidizing and alkaline conditions, while in Alkaline series 2, the initial conditions were somewhat more oxidizing and shifted to reducing in the final stages. In Aluminous series 1 and Aluminous series 2, amphiboles are calcic and comparatively homogeneous. The amphiboles in the monzodioritic rocks, while also homogeneous, are more Mg-rich and show compositions quite distinct from the calcic varieties in the other associations, and this is also the case for clinopyroxene. Mg# in biotite decreases from the monzodioritic rocks to Aluminous series 1 and further to Aluminous series 2. Contrasting evolution of the various associations suggests that several coeval magmatic series are present in the Serra da Graciosa granites.     

Geochemical and isotopic (Nd–O) evidence bearing on the origin of late- to post-orogenic high-K granitoid rocks in the Western Superior Province: implications for late Archean tectonomagmatic processes

The granitoid rock dominated central Wabigoon subprovince of the Superior Province records low-K trondhjemite–tonalite–granodiorite (TTG) type magmatic episodes at <2.83–2.74 and 2.722–2.709 Ga, and high-K mafic to felsic plutonism at 2.690–2.685 Ga. High-K units consist of granite to granodiorite dykes and sills, a K-feldspar megacrystic granodiorite suite of sanukitoid affinity and a suite of mafic dykes and intrusions. Initial ϵ_Nd values (−3.1 to +3.3) indicate variable input to all units from light REE-enriched older crustal materials. The δ¹⁸O (VSMOW) range of felsic compositions (+7.1 to +8.9%) overlaps closely that of average upper Superior Province crust. The granite/granodiorite units probably received melt components derived from both older tonalitic crust and isotopically juvenile supracrustal material. The thermal flux for partial melting was provided by mafic components of the coeval megacrystic granodiorite suite. This latter suite likely formed by extensive crustal assimilation and fractionation of enriched-mantle-derived high-Mg dioritic magmas in a post-collisional setting, possibly resulting from slab breakoff or broader scale lithospheric delamination. A genetic link is inferred between mafic magmatism and the late- to post-tectonic high-K granitoid magmatism that typically represents the last stabilization event within Superior subprovinces. That crustal recycling processes played a major role in the petrogenesis of central Wabigoon high-K granitoid suites is consistent with other evidence that supports repeated and substantial continental recycling within this subprovince as far back as the Mesoarchean.     

A 1.78 Ga large igneous province in the North China craton: The Xiong'er Volcanic Province and the North China dyke swarm

The 1.78 Ga Xiong'er Volcanic Province (XVP) and coeval North China giant mafic Dyke Swarm (NCDS) are the most important magmatic events occurring after the amalgamation of the North China craton (NCC). The XVP consists of 3–7 km of extrusive volcanics and some feeder dykes/sills located along the southern margin of the NCC and extending over an area > 0.06 M km². Compositions vary from basalt to rhyolite, but are predominantly intermediate in terms of silica content. There are also minor sedimentary intercalations and pyroclastic units. The sedimentary interlayers indicate an environment changing from continental-facies to oceanic-facies up-section. The XVP is characterized by fractional crystallization from an EM I type mantle source, and both continental arc (Andean-type) and rift environments have been proposed. The NCDS is widespread in the central NCC with an outcrop area > 0.1 M km², and are exposed at variable depths up to 20 km (deepest in the north). Dyke compositions vary from basalt to andesite and dacite, but are dominantly mafic, and comprise two series of magmatism. Previous studies revealed that the NCDS recorded assimilation and fractional crystallization of an EM I type magma source, with a minor DM contribution in the younger magmas. Both syn-collisional and intra-continental anorogenic environments have been proposed. Spatial and petrogenic correlations suggest a cogenetic relationship between the NCDS and XVP, and considered together, they define a Large Igneous Province (LIP) of > 0.1 M km² in area and > 0.1 M km³ in volume, which is also notable for its continuous compositional range from mafic to felsic (with no gap at intermediate compositions). The petrology is explained by a common magma source that undergoes a silica-poor and iron-enriched fractionation trend at depth followed by a silica-rich and iron-poor fractionation trend in shallow-level magma conduits (dykes) and surface lavas. A mantle plume is favored as the cause of this  1.78 Ga North China LIP.     

Interrelations between coeval mafic and A-type silicic magmas from composite dykes in a bimodal suite of southern Israel, northernmost Arabian–Nubian Shield: Geochemical and isotope constraints

Late Neoproterozoic bimodal dyke suites are abundant in the Arabian–Nubian Shield. In southern Israel this suite includes dominant alkaline quartz porphyry dykes, rare mafic dykes, and numerous composite dykes with felsic interiors and mafic margins. The quartz porphyry chemically corresponds to A-type granite. Composite dykes with either abrupt or gradational contacts between the felsic and mafic rocks bear field, petrographic and chemical evidence for coexistence and mixing of basaltic and rhyolitic magmas. Mixing and formation of hybrid intermediate magmas commenced at depth and continued during emplacement of the dykes. Oxygen isotope ratios of alkali feldspar in quartz porphyry (13 to 15‰) and of plagioclase in trachydolerite (10–11‰) are much higher than their initial magmatic ratios predicted by equilibrium with unaltered quartz (8 to 9‰) and clinopyroxene (5.8‰). The elevation of δ¹⁸O in alkali feldspar and plagioclase, and extensive turbidization and sericitization call for post-magmatic low-temperature (≤ 100 °C) water–rock interaction. Hydrous alteration of alkali feldspar, the major carrier of Rb and Sr in the quartz–porphyry, also accounts for the highly variable and unusually high I(Sr) of 0.71253 to 0.73648.

The initial ¹⁴³Nd/¹⁴⁴Nd ratios, expressed by ε_Nd(T) values, are probably unaltered and show small variation in mafic and felsic rocks within a narrow range from + 1.4 to + 3.3. The Nd isotope signature suggests either a common mantle source for the mafic and silicic magmas or a juvenile crustal source for the felsic rocks (metamorphic rocks from the Elat area). However, oxygen isotope ratios of zircon in quartz porphyry [δ¹⁸O(Zrn) = 6.5 to 7.2‰] reveal significant crustal contribution to the rhyolite magma, suggesting that mafic and A-type silicic magmas are not co-genetic, although coeval. Comparison of ¹⁸O/¹⁶O ratios in zircon allows to distinguish two groups of A-type granites in the region: those with mantle-derived source, δ¹⁸O(Zrn) ranging from 5.5 to 5.8‰ (Timna and Katharina granitoids) and those with major contribution of the modified juvenile crustal component, δ¹⁸O(Zrn) varying from 6.5 to 7.2‰ (Elat quartz porphyry dykes and the Yehoshafat alkaline granite). This suggests that A-type silicic magmas in the northern ANS originated by alternative processes almost coevally.     

Geochemistry of the mafic dykes in the Prakasam Alkaline Province of Eastern Ghats Belt, India: Implications for the genesis of continental rift-zone magmatism

Mesoproterozoic rift-zone magmatism in the Prakasam Alkaline Province of Eastern Ghats Belt, India is represented by three geochemically distinct primary mafic magmas and their plutonic differentiates. The three mafic magmas correspond to the alkali basaltic dykes, gabbroic dykes and lamprophyric dykes. The dyke activity is synchronous with the host plutons and belongs to the 1350–1250 Ma period Mesoproterozoic magmatism. Geochemical signatures suggest that the alkali basaltic dykes have a source in the thermal boundary layer, which has a history of prior melt extraction followed by enrichment. Both the gabbroic and lamprophyric dykes are derived from lithospheric sources and their geochemical variation can be explained by “vein-plus-wall-rock melting model”. Vein/wall-rock ratio is low for the sources of gabbroic dykes, whereas it is high for the lamprophyric dykes. Geochemistry of the gabbro dykes further indicates preservation of previous arc-signals by the lithosphere beneath the Prakasam Alkaline Province during the Mesoproterozoic. Geochemical signatures of lamproite, which could be a cratonic expression of the rift-triggered magmatism in the Prakasam Province, suggest a general increase in the metasomatic imprint with increasing lithosphere thickness from cratonic margin towards interior. It is found that geochemistry of continental rift-zone magmatism of the Prakasam rift is remarkably similar to that of the Gardar rift of South Greenland. It appears that the geodynamic conditions under which melting occurred in the Prakasam Alkaline Province are similar to that of a propagating rift with variable contributions from the convective mantle and subcontinental lithosphere mantle to the rift-zone magmas. The present study illustrates how fertility and chemical heterogeneity of the lithosphere play significant roles in the creation of enormous geochemical diversity characteristic of continental rift-zone magmatism.     

Post-orogenic shoshonitic magmas of the Yzerfontein pluton,South Africa: the ‘smoking gun’ of mantle melting and crustal growth during Cape granite genesis?

The post-orogenic Yzerfontein pluton, in the Saldania Belt of South Africa was constructed through numerous injections of shoshonitic magmas. Most magma compositions are adequately modelled as products of fractionation, but the monzogranites and syenogranites may have a separate origin. A separate high-Mg mafic series has a less radiogenic mantle source. Fine-grained magmatic enclaves in the intermediate shoshonitic rocks are autoliths. The pluton was emplaced between 533 ± 3 and 537 ± 3 Ma (LA-SF-ICP-MS U–Pb zircon), essentially synchronously with many granitic magmas of the Cape Granite Suite (CGS). Yzerfontein may represent a high-level expression of the mantle heat source that initiated partial melting of the local crust and produced the CGS granitic magmas, late in the Saldanian Orogeny. However, magma mixing is not evident at emplacement level and there are no magmatic kinships with the I-type granitic rocks of the CGS. The mantle wedge is inferred to have been enriched during subduction along the active continental margin. In the late- to post-orogenic phase, the enriched mantle partially melted to produce heterogeneous magma batches, exemplified by those that formed the Yzerfontein pluton, which was further hybridised through minor assimilation of crustal materials. Like Yzerfontein, the small volumes of mafic rocks associated with many batholiths, worldwide, are probably also low-volume, high-level expressions of crustal growth through the emplacement of major amounts of mafic magma into the deep crust.     

Sr and Nd isotopic investigations towards the origin of feldspar megacrysts in microgranular enclaves in two I‐type plutons of the Lachlan Fold Belt,southeast Australia

New Sr and Nd isotopic data are presented for several large feldspar crystals occurring in microgranular enclaves in the Swifts Creek and Bridle Track plutons, along with analyses of their host enclave groundmass and adjacent granitoid. In the Swifts Creek Pluton several previous studies have concluded that the microgranular enclaves represent admixed, more mafic and more primitive magmas, and new data presented here confirm that. Feldspar megacrysts in the microgranular enclaves have Sr and Nd isotopic signatures that are distinct from the surrounding enclave groundmass and from other enclaves in the pluton and therefore cannot have crystallised in situ. Isotopic compositions of these feldspars are more consistent with their having crystallised in a reservoir similar in composition to the most primitive granitoid analyses. The crystals were then physically transferred from the granitoid magma into the enclave while the latter was still partially liquid, thus invalidating arguments for a porphyroblastic origin. Field, petrographic and geochemical data are consistent with microgranular enclaves in the Bridle Track pluton also originating as admixed, more mafic magmas. However, Sr isotopic compositions of the enclaves are identical, within error, to the host granite and indicate that significant Sr isotopic equilibration has occurred. Nd isotopic compositions of the enclaves extend to slightly higher ¹⁴³Nd/¹⁴⁴Nd_(i) and are consistent with a mingled magma origin followed by major isotopic equilibration. Feldspar phenocrysts in the studied enclave have isotopic compositions indistinguishable from both the enclave groundmass and host granite, preventing an interpretation of their origin using isotopic evidence alone.     

The Mineralogy and Petrology of Mount Suswa, Kenya

Mount Suswa, a Quaternary volcano in the Rift Valley of Kenya,is composed of sodalite-trachytes, sodalite-phonolites, andphonolites, the majority of which are mildly peralkaline. Thelavas are predominantly feldspathic with phenocrysts of alkalifeldspar and, less commonly, slightly sodic augite and fayaliticolivine. The groundmass typically contains alkali feldspar,augite, titanomagnetite, and may in addition contain sodalite,nepheline, alkali amphibole, aenigmatite, and glass. The lavasof four stratigraphically distinct episodes can be distinguishedon the basis of mineralogy and chemical composition. These lavasare the products of at least three parental magmas, none ofwhich appears to be a derivative of the other. Each ‘magmatype’ represents an independent episode of magma generation,emplacement, and eruption. The order of eruption in the finalepisode corresponds to increasing peralkalinity and undersaturationwith respect to silica, and indicates that these lavas weregenerated via the tapping of a differentiating magma, with thefirst lavas being the least differentiated. Utilizing coexistingfeldspar, residual glass, and bulk rock compositions, the derivationof peralkaline phonolitic residual liquids from a trachyticparent is shown to be a process controlled by feldspar fractionation.     

Petrology and petrogenesis of a tertiary bimodal dolerite-peralkaline/subalkaline trachyte/rhyolite dyke association from Lundy,Bristol Channel,UK

The island of Lundy forms the southernmost igneous complex of the British Tertiary Volcanic Province (BTVP) and consists of granite (≈ 90%) emplaced into deformed Devonian sedimentary rocks (Pilton Shale) and associated with a swarm of dykes of dolerite/basalt, minor trachyte and rhyolite composition. The dolerites are of varied olivine basalt composition and are associated with peralkaline trachyte and subalkaline/peralkaline rhyolite with alkali feldspar and quartz ± alkali amphibole ± pyroxene mineralogy. The dyke swarm is therefore an anorogenic bimodal dolerite/basalt–trachyte/rhyolite BTVP association. Although the dyke association is bimodal in major element terms between dolerite/basalt and minor trachyte/rhyolite, the mineralogy and trace element geochemistry indicate that the dykes may be regarded as a cogenetic dolerite—peralkaline trachyte/rhyolite association with minor subalkaline rhyolites. Sr and Nd isotope data indicate derivation of these magmas from a similar BTVP mantle source (with or without minor contamination by Pilton Shale, or possibly Lundy granite). The petrogenesis of the Lundy dyke association is therefore interpreted in terms of extensive fractional crystallization of basaltic magma in a magma chamber of complex geometry below the (exposed) Lundy granite. Fractional crystallization of a representative dolerite magma (olivine ± clinopyroxene ± plagioclase) yields trachyte magma from which the crystallization of alkali feldspar (anorthoclase) ± plagioclase (oligoclase) + Fe–Ti oxide + apatite results in peralkaline rhyolite. Rarer subalkaline rhyolites result from fractionation from a similar dolerite source which did not achieve a peralkaline composition so allowing the crystallization and fractionation of zircon. The basalt–(minor trachyte)/rhyolite bimodality reflects rapid crystallization of basalt magma to trachyte (and rhyolite) over a relatively small temperature interval (mass fraction of melt, F = ≈ 0.15). The rapid high level emplacement of basalt, trachyte and rhyolite dyke magmas is likely to have been associated with the development of a substantial composite bimodal basalt–(minor trachytel)/rhyolite volcano above the BTVP Lundy granite in the Bristol Channel.     

Geochronology and geochemistry of Palaeozoic intrusive rocks in the Rockvale region,southern New England Orogen,New South Wales

Palaeozoic intrusive rocks of the New England Batholith from the Rockvale district in the southern New England Orogen form three distinct associations: (i) the Carboniferous Rockvale Adamellite, a member of the Hillgrove Suite of deformed S‐type granitoids; (ii) a small I‐type igneous complex on the northwestern margin of the Rockvale Adamellite: several members of this complex have similar chemical compositions to the most mafic members of the Moonbi Suite of New England Batholith I‐types; and (iii) a suite of dyke rocks ranging in composition from calc‐alkaline lamprophyre through hornblende and biotite porphyrite to aplite. Ion‐microprobe U‐Pb zircon analysis indicates intrusion of the Rockvale Adamellite at 303 ±3 Ma (weighted mean ²⁰⁶Pb/²³⁸U age; 95% confidence limits). Preliminary investigation of zircon inheritance within the Rockvale Adamellite is consistent with chemical and isotopic indications of derivation of New England Batholith S‐type granitoids from a relatively juvenile protolith. Deformation of the Rockvale Adamellite occurred after complete crystallization of the pluton and prior to emplacement of dykes and I‐type intrusives. K‐Ar biotite and hornblende ages show broadly synchronous intrusion of I‐type magmas and lamprophyre dykes at ca 255 Ma, indicating that mantle magmatism associated with lamprophyres was contemporaneous with the crustal production of I‐type melts. Chemical similarities between the most mafic Moonbi Suite members and calc‐alkaline lamprophyres may also indicate a direct mantle contribution to some I‐type magmas.     

Appinite suites and their genetic relationship with coeval voluminous granitoid batholiths

ABSTRACT

Appinite complexes preserve evidence of mantle processes that produce voluminous granitoid batholiths. These plutonic complexes range from ultramafic to felsic in composition, deep to shallow emplacement, and from Neo-Archean to Recent in age. Appinites are a textural family characterized by idiomorphic hornblende in all lithologies, and spectacular textures including coarse-grained mafic pegmatites, fine-grained ‘salt-and-pepper’ gabbros, as well as planar and linear fabrics. Magmas are bimodal (mafic-felsic) in composition; ultramafic rocks are cumulates, intermediate rocks are hybrids. Their geochemistry is profoundly influenced by a mantle wedge extensively metasomatized by fluids/magmas produced by subduction. Melting of spinel peridotite sub-continental lithospheric mantle (SCLM) produces appinites whose geochemistry is indistinguishable from coeval low-K calc-alkalic arc magmatism. Coeval felsic rocks within appinite complexes and adjacent granitoid batholiths are crustal magmas. When subduction terminates, asthenospheric upwelling (e.g. in a slab window, or in the aftermath of slab failure) induces melting of metasomatized garnet SCLM to produce K-rich sho shonitic magmas enriched in large ionic lithophile and light relative to heavy rare earth elements, whose asthenospheric component can be identified by Sm-Nd isotopic signatures. Coeval late-stage Ba-Sr granitoid magmas have a ‘slab failure’ geochemistry, resemble TTG and adakitic suites, and are formed either by fractionation of an enriched (shoshonitic) mafic magma, or high pressure melting of a meta-basaltic protolith either at the base of the crust or along the upper portion of the subducted slab. Appinite complexes may be the crustal representation of mafic magma that underplated the crust for the duration of arc magmatism. They were preferentially emplaced along fault zones around the periphery of the granitoid batholiths (where their ascent is not blocked by overlying felsic magma), and as enclaves within granitoid batholiths. When subduction ceases, appinite complexes with a more pronounced asthenospheric component are preferentially emplaced along active faults that bound the periphery of the batholiths.     

Mid-Cretaceous granitic magmatism during the transition from subduction to extension in southern New Zealand: a chemical and tectonic synthesis

Regional geochronological studies indicate that mid-Cretaceous plutonism (the Hohonu Suite at 110 Ma) in the Hohonu Batholith, Western Province of New Zealand, occurred during a period of rapid tectonic change in the SW Pacific portion of Gondwana. The 30–40 m.y. preceding Hohonu Suite magmatism were dominated by the subduction-related plutonism of the Median Tectonic Zone volcanic arc. Between 125–118 Ma there was a major collisional event, inferred to be the result of collision between the Median Tectonic Zone and the Western Province. This collision resulted in melting of the Median Tectonic Zone arc underplate and generation of a distinctive suite of alkali-calcic granitoids, termed the Separation Point Suite. At 110 Ma there was another pulse of magmatism, restricted to the Buller terrane of the Western Province, and including the Hohonu Suite granitoids. This was followed almost immediately by extension, culminating in the opening of the Tasman Sea some 30 m.y. later. The Hohonu Suite granitoids overlap temporally with the last vestiges of collisional Separation Point magmas and the onset of crustal extension in the Western Province, and thus represent magmatism in a post-collisional setting. Hohonu Suite magmas are typically calc-alkaline, but retain a chemical signature which suggests that the earlier Separation Point Suite magmas and/or sources were involved in Hohonu Suite petrogenesis. A model is proposed in which rapid isothermal uplift, resulting from the post-collisional collapse of continental crust previously thickened during the Median Tectonic Zone collision, caused melting of lower continental crust to generate the Hohonu Suite granitoids. In this example, granitoid composition is a consequence of the composition of the source rocks and the conditions present during melting, and no geochemical signature indicative of the tectonic setting during magmatism is present.     

SHRIMP baddeleyite age for the Fraser Dyke Swarm,southeast Yilgarn Craton,Western Australia

Recent high‐resolution aeromagnetic data have delineated an extensive swarm of undeformed northeast‐trending dolerite dykes in the southeastern Yilgarn Craton, known previously only from isolated exposures in surface mining operations. Owing to parallelism of the dykes to the Fraser Mobile Belt, the eastern segment of the Albany‐Fraser Orogen, the swarm is referred to here as the Fraser Dyke Swarm. Ion‐microprobe dating of baddeleyite from a granophyric segregation in the centre of one dyke yields a mean ²⁰⁷Pb/²⁰⁶Pb age of 1212 ± 10 Ma (95% confidence limits). The location of the Fraser Dyke Swarm, adjacent and parallel to the Fraser Mobile Belt, suggests that the dykes may have been emplaced into lines of weakness that originated during tectonic loading and downwards flexure of the craton margin. This is the first evidence of ca 1210 Ma mafic dykes and associated crustal‐scale extension in the southeast Yilgarn Craton, although the age is similar to those reported recently for dolerite and quartz diorite dykes in the central and southern part of the craton, suggesting that a genetic relationship may exist between intrusions in the two areas.     

Density,Viscosity and Velocity (Ascent Rate) of Alkaline Magmas

Three distinct alkaline magmas, represented by shonkinite, lamprophyre and alkali basalt dykes, characterize a significant magmatic expression of rift-related mantle-derived igneous activity in the Mesoproterozoic Prakasam Alkaline Province, SE India. In the present study we have estimated emplacement velocities (ascent rates) for these three varied alkaline magmas and compared with other silicate magmas to explore composition control on the ascent rates. The alkaline dykes have variable widths and lengths with none of the dykes wider than 1 m. The shonkinites are fine- to medium-grained rocks with clinopyroxene, phologopite, amphibole, K-feldspar perthite and nepheline as essential minerals. They exhibit equigranular hypidiomorphic to foliated textures. Lamprophyres and alkali basalts characteristically show porphyritic textures. Olivine, clinopyroxene, amphibole and biotite are distinct phenocrysts in lamprophyres whereas olivine, clinopyroxene and plagioclase form the phenocrystic mineralogy in the alkali basalts. The calculated densities [2.54–2.71 g/cc for shonkinite; 2.61–2.78 g/cc for lamprophyre; 2.66–2.74 g/cc for alkali basalt] and viscosities [3.11–3.39 Pa s for shonkinite; 3.01–3.28 Pa s for lamprophyre; 2.72–3.09 Pa s for alkali basalt] are utilized to compute velocities (ascent rates) of the three alkaline magmas. Since the lamprophyres and alkali basalts are crystal-laden, we have also calculated effective viscosities to infer crystal control on the velocities. Twenty percent of crystals in the magma increase the viscosity by 2.7 times consequently decrease ascent rate by 2.7 times compared to the crystal-free magmas. The computed ascent rates range from 0.11–2.13 m/sec, 0.23–2.77 m/sec and 1.16–2.89 m/sec for shonkinite, lamprophyre and alkali basalt magmas respectively. Ascent rates increase with the width of the dykes and density difference, and decrease with magma viscosity and proportion of crystals. If a constant width of 1 m is assumed in the magma-filled dyke propagation model, then the sequence of emplacement velocities in the decreasing order is alkaline magmas (4.68–15.31 m/sec) > ultramafic-mafic magmas (3.81–4.30 m/sec) > intermediate-felsic magmas (1.76–2.56 m/sec). We propose that SiO₂ content in the terrestrial magmas can be modeled as a semi-quantitative “geospeedometer” of the magma ascent rates.     

Temporal and genetic relationships between the Kidston gold-bearing Breccia Pipe and the Lochaber Ring Dyke Complex, North Queensland, Australia: insights from in situ U–Pb and Hf-isotope analysis of zircon

An existing model for the temporal and genetic relationships between the Kidston gold-bearing Breccia Pipe and the nearby Lochaber Ring Dyke Complex has been evaluated using in situ U–Pb and Hf-isotope analyses of zircon grains. The Oak River Granodiorite, the host rock to the Kidston Breccia Pipe, has 1,551?±?6 Ma old zircon cores overgrown by 417.7?±?2.2 Ma rims. The Black Cap Diorite and Lochaber Granite within the Lochaber Ring Dyke Complex have crystallisation ages of 350.7?±?1.3 and 337.9?±?2.6 Ma respectively. The gold-rich Median Dyke within the Kidston Breccia Pipe has a crystallisation age of 335.7?±?4.2 Ma, and thus is temporally related to the Lochaber Granite. However, zircon grains from the Median Dyke have less radiogenic Hf-isotope compositions (? _Hf from ?7.8 to ?15.8) than those from the Black Cap Diorite ?_Hf?=?0.4 to ?7.2) and the Lochaber Granite (? _Hf?=??1.0 to ?7.5), but within the range defined by zircons from the Oak River Granodiorite ? _Hf?=??8.0 to ?29.2). The Hf-isotope data thus rule out the proposed fractional crystallisation relationship between the Kidston gold-bearing rocks and the Lochaber Ring Dyke Complex. The Kidston Median Dyke may have been produced by mixing between Lochaber Granite magmas and magmas derived by remelting of the Oak River Granodiorite, which was itself derived from Proterozoic crust. There is no evidence for a juvenile component in the Lochaber Ring Dyke Complex or the Median Dyke. The gold enrichment in the Kidston rocks thus may reflect the multi-stage reworking of the Proterozoic crust, which ultimately produced the Carboniferous felsic magmas.     

Geochemistry of mafic and ultramafic xenoliths from Fidra (Southern Uplands, Scotland): implications for lithospheric processes in Permo-Carboniferous times

Several types of xenoliths occur in a Permian basanite sill in Fidra, eastern central Scotland. One group consists of spinel lherzolites, which have geochemical and isotopic characteristics similar to those of lithospheric upper mantle from elsewhere in western Europe, with both LREE-depleted and LREE-enriched compositions. A separate group comprises pyroxenites and wehrlites, some of which contain plagioclase; these have compositions and textures that indicate that they are cumulates from mafic magmas. In terms of Sr and Nd isotope compositions, the pyroxenites closely resemble the host basanite and most likely formed by high-pressure fractionation of Permo-Carboniferous alkaline magmas at lower crustal depths. They also have mantle-like δ¹⁸O values. A third group is composed of granulite xenoliths that vary between plagioclase-rich and clinopyroxene-rich compositions, some of which probably form a continuum with the pyroxenites and wehrlites. They are all LREE-enriched and most have positive Eu anomalies; thus, they are also mostly cumulates from mafic magmas. Many of the granulites also have Sr and Nd radiogenic isotope ratios similar to those of the host basanite, indicating that they have formed from a similar magma. However, several of the granulites show more enriched isotopic compositions, including higher δ¹⁸O values, trending towards an older crustal component. Thus, the pyroxenites and granulites are largely cogenetic and are mainly the product of a mafic underplating event that occurred during the widespread magmatism in central Scotland during Permo-Carboniferous times.