Granite formation: Stepwise accumulation of melt or connected networks?

Bons, Paul D.; Becker, Jens; Elburg, Marlina; Urtson, Kristjan

doi:10.1017/s175569100901603x

Cited by 24 publications

(14 citation statements)

References 82 publications

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“…Although this macroscopic picture has been used to infer the large-scale geometry of the migration of melt through the crust (e.g. Brown, 2004), it is an incomplete picture of the segregation and migration process (Bons et al, 2004(Bons et al, , 2010.…”

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The inception and growth of leucosomes: microstructure at the start of melt segregation in migmatites

Sawyer

2014

Journal Metamorphic Geology

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The beginning stages of melt segregation and the formation of leucosomes are rarely preserved in migmatites. Most arrays of leucosomes record a more advanced stage where flow dominates over segregation. However, the early stages in the formation of leucosomes and the segregation of melt are preserved in a partially melted meta‐argillite from the metatexite zone (>800 °C) of the contact aureole around the Duluth Complex, Minnesota. The rock contains 2.4 modal% leucosome in a matrix consisting of 40.5% in situ neosome and 57.1% cordierite + plagioclase framework. The domainal microstructure in the matrix is a pre‐anatectic feature resulting from the bulk composition. Terminal chlorite reactions produced a large volume of cordierite which, with plagioclase, formed a framework that enclosed patches of biotite + quartz + plagioclase ± K‐feldspar. Upon melting, these fertile domains became patches of in situ neosome. Plagioclase in the neosome is less sodic than in the leucosome, hence segregation of melt occurred during crystallization, not melting. Segregation was delayed because the cordierite + plagioclase framework was strong enough to resist dilatation and compaction until after crystallization started. The leucosomes are small (i.e. they are microleucosomes) and display a systematic progression in morphology as length and aspect ratio increase from ~1 to 19 mm and from ~2.5 to >30 respectively. Small equant micropores form first, and in places these coalesce into small (~1 mm, aspect ratio ~2.5), isolated, blunt‐ended, elliptical microleucosomes. In the next stage, micropores develop ahead of, and at ~45° to the left and right of the blunt tip of a microleucosome; one of these develops into an elliptical leucosome and an en echelon array of either a left‐ or right‐stepping elliptical microleucosome forms. Each elliptical microleucosome in the en echelon arrays is separated by a bridge of matrix. Next, microleucosomes of greater length (>4 mm) and aspect ratio (>5) form when the bridges of cordierite + plagioclase matrix rupture and the elliptical microleucosomes link together to form a zigzag‐shaped microleucosome. Finally, still longer microleucosomes with greater aspect ratios (~30) are formed by the joining of zigzag arrays. Such a progression is characteristic of the way ductile fractures grow. The segregation of melt was driven by the pressure gradient between the dilatant fracture and an adjacent in situ neosome, which drew melt to the growing fracture, thereby creating a microleucosome. The microleucosomes are filled arrays of ductile fractures. Melt was contiguous only between microleucosomes and adjacent patches of in situ neosome. The length‐scale of segregation was ~5 mm, the size of a typical patch of in situ neosome, and restricted by the surrounding impermeable cordierite + plagioclase framework. The melt in the microleucosome was the most fractionated and the last to crystallize. All microleucosomes contain entrained minerals as a consequence of their mechanism of growth. Rupture of the b...

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The inception and growth of leucosomes: microstructure at the start of melt segregation in migmatites

Sawyer

2014

Journal Metamorphic Geology

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“…Magma ascent within self‐propagating hydrofractures has been suggested as an alternative mechanism for the ascent of magma from source (Bons & van Milligen, ; Bons et al ., ). Self‐propagating hydrofractures can develop independently of large source reservoirs and their propagation is driven by the pressure differential that arises within the fracture due to the density contrast between the granitic magma (2.3–2.4 g cm −3 ) and the wall rocks (~2.7 g cm −3 ; Sleep, ; Bons et al ., ; Kisters et al ., ).…”

Section: Introductionmentioning

confidence: 97%

Episodic granite accumulation and extraction from the mid‐crust

Hall

Kisters

2016

Journal Metamorphic Geology

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The processes of long-range granitic magma transfer from mid-and lower crustal anatectic zones to upper crustal pluton emplacement sites remain controversial in the literature. This is partly because feeder networks that could have accommodated this large-scale magma transport remain elusive in the field. Existing granite ascent models are based largely on numerical and theoretical studies that seek to demonstrate the viability of fracture-controlled magma transport through dykes or self-propagating hydrofractures. In most cases, the models present very little supporting field evidence, such as sufficiently voluminous near-or within-source magma accumulations, to support their basic premises. We document large (deca-to hectometre-scale), steeply dipping and largely homogeneous granite lenses in suprasolidus (~5 kbar,~750°C) mid-crustal rocks in the Damara Belt in Namibia. The lenses are surrounded by and connected to shallowly dipping networks of stromatic leucogranites in the well-layered gneisses of the deeply incised Husab Gorge. The outcrops define a four-stage process from (i) the initial formation and growth of large, subvertical magma-filled lenses as extension fractures developed at high angles to the subhorizontal regional extension in relatively competent wallrock layers. This stage is followed by (ii) the simultaneous lateral inflation and (iii) subcritical vertical growth of the lenses to a critical length that (iv) promotes fracture destabilization, buoyancy-driven upward fracture mobilization and, consequently, vertical magma transport. These field observations are compared with existing numerical models and are used to constrain, by referring to the dimensions of the largest preserved inflated leucogranite lens, an estimate of the minimum fracture length (~100 m) and volume (~2.4 9 10 5 m 3 ) required to initiate buoyancy-driven brittle fracture propagation in this particular mid-crustal section. The critical values and field relationships compare favourably with theoretical models of magma ascent along vertical self-propagating hydrofractures which close at their tails during propagation. This process leaves behind subtle wake-like structures and thin leucogranite trails that mark the path of magma ascent. Reutilization of such conduits by repeated inflation and drainage is consistent with the episodic accumulation and removal of magma from the mid-crust and is reflected in the sheeted nature of many upper crustal granitoid plutons.

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“…Gneiss domes develop in exhuming orogens, where they constitute an efficient mechanism for material and heat advection of continental crust during orogenesis (Vanderhaeghe, 2009;Whitney et al, 2004). Furthermore, Gneiss dome formation is an important mechanism for the differentiation of the continental crust and for the genesis of granitic magmas that in turn interact with the dome formation (Bons et al, 2010;Brown, 2007Brown, , 2013Vanderhaeghe, 2009). Dome formation may be accompanied by heat advection expressed by low-pressure/high-temperature metamorphism in the host rocks (Tirel et al, 2009;White et al, 2003;Whitney et al, 2004).…”

Section: Introductionmentioning

confidence: 99%

Structure and Metamorphism of Markam Gneiss Dome From the Eastern Tibetan Plateau and Its Implications for Crustal Thickening, Metamorphism, and Exhumation

Zhao

Feng

et al. 2019

Geochem Geophys Geosyst

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Gneiss domes developed in the eastern Songpan‐Ganzi Flysch (SGF) belt, located in the hinterland of the Longmen Shan Thrust belt of the eastern Tibetan Plateau. These domes have similar pressure‐temperature (denoted as P‐T below) paths and formed during Late Triassic‐Early Jurassic. One of them, the Markam Gneiss Dome (MGD), is cored by granite and is mantled by a metaturbidite aureole. Our structural observations show top‐to‐the‐south shearing that transposed an upright S1 foliation into a flat‐lying NW‐SE striking S2 composite foliation in the Triassic metaturbidites. Thin sections reveal that the metamorphic index minerals biotite, garnet, and staurolite grew prior to the transposition into S2, whereas kyanite and sillimanite crystallized syn‐S2 development. P‐T pseudosections for the metaturbidites indicate a temperature peak of approximately 618–632 °C and a pressure peak of approximately 6–8 kbar. The highest P‐T condition was immediately followed by isothermal decompression, which is represented by the sillimanite overgrowing andalusite and biotite. Granite was emplaced at ~10‐ to 15‐km depth, during isothermal decompression. Zircon U‐Pb geochronology of metaturbidites, granite, and pegmatite of the MGD indicates that (1) the metaturbidites are younger than 240 Ma, (2) the granite was mainly sourced from the metaturbidites, and (3) deformation leading to the transposition of S1 into S2 mainly occurred ca. 215–170 Ma. Our new P‐T‐D‐t results from the MGD confirm those of other gneiss domes in the SGF and indicate Late Triassic‐Early Jurassic crustal shortening and thickening that probably reflects coeval low‐viscosity crustal flow of the SGF.

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Granite formation: Stepwise accumulation of melt or connected networks?

Cited by 24 publications

References 82 publications

The inception and growth of leucosomes: microstructure at the start of melt segregation in migmatites

The inception and growth of leucosomes: microstructure at the start of melt segregation in migmatites

Episodic granite accumulation and extraction from the mid‐crust

Structure and Metamorphism of Markam Gneiss Dome From the Eastern Tibetan Plateau and Its Implications for Crustal Thickening, Metamorphism, and Exhumation

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