The relationship between Late Devonian mafic intrusions and peraluminous granitoid generation in the Meguma Lithotectonic Zone, Nova Scotia, Canada.
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Abstract:
Current theories for the generation of voluminous granitoid intrusions state that mafic magmas can provide a localized, external source of heat for crustal melting. Regardless of their age or tectonic setting, many alkaline, metaluminous, and peraluminous granitoid bodies occur with contemporaneous satellite mafic intrusions and show spatial relationships with mafic magmas in the form of synplutonic mafic bodies and mafic igneous enclaves. Physical juxtaposition of the intruded mafic magmas and resulting anatectic granitoid melts also allows for the transfer of chemical components to produce hybridized lithologies by mechanical interaction and/or chemical diffusion. In this multidisciplinary study, identification of all these established criteria for mafic-granitoid genetic relationships, in combination with one-dimensional thermal models of mafic intraplating, allows the assessment of a role for intruded mafic magma as a source of chemical components and, ultimately, heat in a suite of granitoid rocks.Cite
Leucogranite
Felsic
Anatexis
Fractional crystallization (geology)
Hornblende
Petrogenesis
Igneous differentiation
Silicic
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Peridotite
Ultramafic rock
Anorthosite
Metasomatism
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Dioritic to quartz monzonitic rocks of the Dartmouth Pluton exhibit excellently preserved, diverse features produced by mingling and mixing of mafic and felsic magma during multiple events. The related mafic and hybridized intermediate composition rocks occur both as discrete outcrop-sized masses or as enclaves within quartz monzonite or early-stage mixed rocks. Enclaves are rounded, lack chilled margins, and in some cases exhibit cuspate margins; they range in size from 1m--<1cm. Outcrops dominated by dioritic rock consist of well developed mafic pillows with inter-pillow infillings of hybridized rock that had been subjected to magma mixing during or prior to the final mingling process. Dioritic rocks are fine-grained with sparse plagioclase phenocrysts; they contain small, darker-colored enclaves indicative of preceding magma interaction. Major and trace element variation diagrams for this suite of rocks exhibit general linear trends consistent with mixing processes. Overall, field, petrographic, and geochemical relationships in the Dartmouth Pluton demonstrate: (1) widespread mingling of mafic and felsic magma, (2) variable degrees of mafic and felsic magma mixing, and (3) multiple and repeated episodes of mafic and felsic magma interaction. Significantly, some spatially associated dioritic and granitic rocks, including a 595 Ma alkali feldspar granite formerly considered to be part ofmore » the Dartmount Pluton, are geochemically related. Field mapping demonstrates that rocks of the mixed suite are intrusive into these rocks, thus establishing a maximum age, but raising the questions that the suite may be considerably younger.« less
Felsic
Phenocryst
Quartz monzonite
Igneous differentiation
Hornblende
Magma chamber
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The 1090–1040 Ma Giles Event in the Musgrave Province, central Australia, led to the formation of the Ngaanyatjarra Rift. Mafic-ultramafic layered intrusions formed contemporaneously with massive gabbroic intrusions within this tectonic setting. The Bell Rock Range, Latitude Hill and Wingellina Hills intrusions represent three of the main different types of the ‘G1’ layered intrusions in this region, and were sampled as representative examples covering much of the compositional spectrum of layered intrusive bodies in this area. The Bell Rock Range olivine gabbronoritic-troctolitic intrusion forms a large segment of the Mantamaru intrusion that was rapidly emplaced into the crust at a maximum depth of c. 10 km. In comparison, the Latitude Hill and Wingellina Hills gabbronoritic-ultramafic intrusions are smaller intrusive bodies containing large proportions of websterite cumulates and gabbronorites with intercumulus plagioclase.
Several of the G1 layered intrusions are associated with massive ‘G2’ gabbroic intrusions that were locally contaminated by felsic magmas. Evidence for the previously proposed chronological order of emplacement in which G1 was followed by G2 is ambiguous: the boundary between G1 and G2 was revisited during this study. Hierarchical cluster analysis enabled the extraction of chemical elements that vary most between the G1 and G2 intrusions, with principle component analysis outlining natural clusters within the data. The clustering within the G1 and G2 compositions can be readily explained by simple cumulate effects, where the G1 layered intrusions lost late-stage melts and as a result became incompatible element depleted, whereas the G2 gabbros have compositions that closely represent original liquid compositions. This indicates that the G2 intrusions may have formed from the same parental magmas as the gabbronoritic-ultramafic G1 intrusions. In addition, this study indicates that linear discriminant analysis can be used to determine exploration vectors for Nebo-Babel style Ni-Cu-PGE targets in the area.
Although initial rifting in this region was most likely passive (i.e. driven by plate dynamics) it is still unclear whether the parental magmas for these intrusions were derived from the asthenospheric mantle (potentially plume-related) or from the subcontinental lithospheric mantle. Derivations from a likely enriched mantle reservoir as well as a maximum melting depth of c. 75 km permit both interpretations. Partial mantle melting generated magmas that then underwent moderate crustal contamination before being rapidly emplaced at mid-crustal levels to form the intrusions in the study area. Recent advances in the classification of rift settings indicate that active and passive processes can occur together in the same setting. Thus, the involvement of mantle dynamics, at least for later stages of the Giles Event (potentially generating the Warakurna Large Igneous Province), cannot be ruled out.
Most magmatic sulphides within these intrusions are Cu-rich, are located in interstitial spaces and formed by extensive fractionation rather than by assimilation of crustal S, indicating that G1 intrusions may be prospective for smaller but PGE-rich deposits. This is supported by the presence of Ni-rich olivine and pyroxene, indicating an early major sulphide segregation event at depth is unlikely; hence, the G1 intrusions are likely to be unprospective for large orthomagmatic Ni-Cu ore deposits.
Layered intrusion
Ultramafic rock
Felsic
Petrogenesis
Magma chamber
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A suite of sharply cross-cutting microdiorite – felsic porphyrite dykes, emplaced into the southern part of the Northern Highlands Terrane, is thought to be coeval with the local Caledonian high Ba-Sr granites. On occasion they can be seen to pillow into, and mix with the granites (e.g. Strontian) in the manner of synplutonic dykes. In the least-deformed examples small-scale mixing and mingling textures are preserved between basic and acid variants, and the felsic porphyrites (rarely) have mafic marginal facies. Microdiorites also grade into rocks of the appinite suite. Thus, the compositional range of the suite is considerable, linking mafic magmas to more evolved compositions via many intermediate stages. These therefore offer a window into the processes of Caledonian magma evolution. A selection of some 50 dykes has ben collected and analysed for major and trace elements, mostly from the environs of Strontian, but also as far north as Loch Quoich and west to Arisaig. They show a continuous chemical range from 47% to 74% SiO2, 18% to <1% MgO, 0.5% to 6% Na2O and 1% to 5% K2O. The bulk composition of the homogenous microdiorites equates to high-Mg andesite of sanukite affinity. Petrogenetically-informative trace elements bear the hallmarks of a subduction-related source, with general enrichment in LILEs and relative depletion in HFSEs (in particular Nb-Ta). The chemistry of the felsic porphyrites is closely comparable with the local Strontian and Cluanie granites, and cumulus-enriched mafic microdiorites are chemically similar to local discrete appinites. Such data can therefore be used to test alternative petrogenetic hypotheses: that the high Ba-Sr granites evolved by crystal fractionation ( crustal contamination) from mantle-derived appinitic parents, or that they are crustal melts associated with genetically unrelated but contemporaneous mafic magmas.
Felsic
Petrogenesis
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Summary Characteristics of mineralogy and texture, together with systematic changes in the enstatite content of orthopyroxene and in the anorthite content of plagioclase, indicate that intrusions of pyroxenite, norite and gabbro, and diorite and leucocratic diorite in the Dahomeyan gneiss of Ghana are genetically related, and that they belong to a subalkaline comagmatic series derived from a parental mafic magma. Partial recrystallization (deuteric) of the igneous rocks during and after intrusion and protoclasis produced minerals characteristic of temperatures higher than those that existed during the medium-grade regional metamorphism of the Dahomeyan gneiss, thus indicating that emplacement of the igneous rocks occurred late during, or perhaps after, the metamorphism. Contact metamorphism appears to have resulted in higher than normal anorthite contents of the plagioclase in the surrounding Dahomeyan gneiss.
Diorite
Norite
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In Egypt, mafic-ultramafic complexes have been classified into three major types: incomplete ophiolite sequences; Alaskan-type intrusions, concentrically-zoned bodies formed in a subduction arc environment; and layered intrusions, vertically-zoned bodies intruded in post-collisional tectonic environments and rift-related bodies associated with the opening of the Red Sea. We present new field work, geochemical data, mineral chemistry and interpretations for the late Neoproterozoic Dahanib mafic-ultramafic intrusion in the South Eastern Desert of Egypt (northernmost Arabian--Nubian Shield, ANS). The Dahanib intrusion shows no evidence of metamorphism or deformation, with excellent preservation of intrusive contacts, well-preserved textures and primary mineralogy. Field relations indicate that it is younger than the surrounding metamorphic rocks and syn-tectonic granitoids. The intrusion is composed of a basal suite of ultramafic rocks (dunite, lherzolite, wehrlite and pyroxenite) and an overlying suite of mafic rocks (olivine gabbronorite, gabbronorite and anorthosite). It displays evident layering of modal abundance, visible directly in outcrop, as well as cryptic layering discernible through changes in mineral compositions. The western and eastern lobes of the Dahanib intrusion occur in the form of a lopolith with readily correlated layers, especially in the upper mafic unit. The present-day dip of the layering decreases from the ultramafic units into the mafic sequence. Structural and compositional relations show that the ultramafic units are cumulates from a high-Mg tholeiitic parent magma emplaced at deep crustal levels and evolved via fractional crystallization rather than any kind of residual mantle sequence. Fo content of olivine and Mg# of pyroxenes display a systematic decrease from ultramafic to mafic rocks, well-correlated with whole-rock Mg#. Spinels in ultramafic samples vary from Cr-rich to Al-rich and have Mg# and Fe^3+^\# similar to spinels from typical stratiform complexes and clearly different from those found in ophiolitic and Alaskan-type complexes. Although the mafic and ultramafic units are clearly related and can be derived from common parent magma, they were not emplaced coevally; rather, they represent different pulses of magma. The Dahanib mafic--ultramafic intrusion does not display any features that convincingly identify it as a typical Alaskan-type body, particularly the lack of clinopyroxenite and hornblendite, rarity of primary hornblende, and the notable abundance of orthopyroxene and plagioclase in its rocks. Our results confirm that it is more akin to a layered mafic-ultramafic intrusion with a multistage evolution. It was emplaced into a stable post-orogenic cratonic setting, with a trace element signature indicating contamination of the mantle source by previous subduction events.
Ultramafic rock
Layered intrusion
Peridotite
Magma chamber
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Greenstone belt
Sill
Amphibole
Felsic
Layered intrusion
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The 1.64 Ga Ahvenisto complex, southeastern Finland, is an anorthosite-mangeritecharnokite-granite (AMCG) suite in which diverse interaction styles of coeval mafic and felsic magmas are observed.Commingling, resulting in mafic pillows and net-veined granite dykes, and chemical mixing producing hybrid rocks, are the most common interaction types.Detailed description of the factors that controlled the interaction styles and relationships between involved rock types are provided using targeted mapping, petrography, and geochemical analyses complemented by chemical mixing and melt viscosity modeling.Interaction occurred at intermediate stages in the magmatic evolution of the complex: when the last fractions of mafic (monzodioritic) melts and the earliest fractions of felsic (hornblende granitic) melts existed simultaneously.Differentiation of mafic magma has produced three monzodioritic rock types: 1) olivine monzodiorite (most mafic, Mg# 49-40), 2) ferrodiorite (Mg# 42-33), and 3) massive monzodiorite (most evolved, Mg# 28-27).The types form an evolutionary trend, and each exhibits different style of interaction with coeval hbl-granite resulting from contrasting conditions and properties (temperature, viscosity, composition).The variation in these properties due to magma evolution and relative proportions of interacting magmas dictated the interaction style: interaction between olivine monzodiorites and granite was almost negligible; ferrodiorites intermingled forming pillows with granitic veins intruding them; and chemical mixing of massive monzodiorite and hbl-granite produced hybrid rocks.
Felsic
Tracing
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Mafic enclaves from three plutons in the Chilliwack batholith have been compared with contemporaneous mafic stocks in order to determine (1) the processes by which mafic and felsic magmas hybridize in the plutonic environment and (2) whether analysis of early‐formed enclave minerals, particularly apatite, can provide a means of seeing through hybridization effects and deciphering the original trace element characteristics of enclave magmas. Whole rock and mineral chemistry data reveal a two‐stage history of enclave hybridization. Stage 1, a diffusive exchange of trace elements between coexisting liquids, produced enclaves with distinctive concave‐upward rare earth element patterns that parallel those of the host granitoids but had minimal impact on the major elements, whose transfer is rate limited by the slow diffusion of Si. This stage probably occurred at a mafic‐felsic interface in a stratified magmatic system. Stage 2, a partial reequilibration of enclave minerals with a differentiated and hybridized interstitial melt, occurred after the enclaves were entrained in the host and partially crystallized. This process caused enclave and host minerals (amphibole, biotite, apatite) from each pluton to have similar major oxide chemistries but did not reequilibrate the trace elements. As a result of these hybridization processes, even early‐formed apatite crystals do not preserve information about the original trace element characteristics of enclave magmas in this case. However, the results of this study illustrate the potential of using enclave chemistry to constrain the nature and timing of mafic magma inputs into felsic magma bodies.
Felsic
Batholith
Hornblende
Trace element
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