Using granite to image the thermal state of the source terrane

Zen, E‐an

doi:10.1130/spe272-p107

Cited by 4 publications

(4 citation statements)

References 33 publications

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“…This distribution is consistent with an average melt loss of approximately 10 vol% from a source of a similar horizontal area or footprint that was approximately 10 km thick (e.g. Zen 1992); such an estimate ignores any mass input owing to melting of asthenosphere in the mantle wedge, which not only will reduce the proportion of lower crustal melt required, but also is the likely driver of crustal differentiation in arc-related settings (e.g. Kemp et al 2007).…”

Section: (B) the Length Scales And Time Scales Of Melt Flow And The Volumes Of Melt Involved In Inflating Plutonssupporting

confidence: 77%

The spatial and temporal patterning of the deep crust and implications for the process of melt extraction

Brown

2010

Phil. Trans. R. Soc. A.

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Volumetrically significant melt production requires crustal temperatures above approximately 800• C. At the grain scale, the former presence of melt may be inferred based on various microstructures, particularly pseudomorphs of melt pores and grainboundary melt films. In residual migmatites and granulites, evidence of melt-extraction pathways at outcrop scale is recorded by crystallized products of melt (leucosome) and residual material from which melt has drained (melanosome). These features form networks or arrays that potentially demonstrate the temporal and spatial relations between deformation and melting. As melt volume increases at sites of initial melting, the feedback between deformation and melting creates a dynamic rheological environment owing to localization and strain-rate weakening. With increasing temperature, melt volume increases to the melt connectivity transition, in the range of 2-7 vol% melt, at which point melt may escape in the first of several melt-loss events, where each event represents a batch of melt that left the source and ascended higher in the crust. Each contributing process has characteristic length and time scales, and it is the nonlinear interactions and feedback relations among them that give rise to the dissipative structures and episodicity of melt-extraction events that are recorded as variations in the spatial and temporal patterning of the crust. Focused melt flow occurs by dilatant shear failure of low-melt fraction rocks creating melt-flow networks that allow accumulation and storage of melt, and form the link for melt flow from grain boundaries to veins allowing drainage to crustal-scale ascent conduits. Preliminary indications suggest that anatectic systems are strongly self-organized from the bottom up, becoming more ordered by decreasing the number and increasing the width of ascent conduits from the anatectic zone through the overlying subsolidus crust to the ductile-to-brittle transition zone, where the melt accumulates in plutons.

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Section: (B) the Length Scales And Time Scales Of Melt Flow And The Volumes Of Melt Involved In Inflating Plutonssupporting

confidence: 77%

The spatial and temporal patterning of the deep crust and implications for the process of melt extraction

Brown

2010

Phil. Trans. R. Soc. A.

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“…Initial Sr isotopic ratios for the Late Cretaceous Boulder batholith, including the Butte quartz monzonite, Marysville stock, and Hell Creek pluton range from 0.7055 to 0.7092, and are significantly lower than values for the Pioneer batholith (Doe et al 1968;Mueller et al 1997). A Proterozoic Pb isotopic model age for the Boulder batholith (2.1 ± 0.3 Ga) is similar to that of the Pioneer plutons (Zen 1992). Inherited zircons from several granitic intrusions in the Boulder batholith give abundant ca.…”

Section: Evidence For the Selway Terranementioning

confidence: 60%

“…Initial Sr isotopic values for the Late Cretaceous Pioneer batholith range from 0.7110 to 0.7160 (Arth et al 1986;Mueller et al 1997), and Pb isotopes give a model age of 1.9 ± 0.2 Ga (Doe et al 1968;Zen 1992). These results suggest involvement of mainly Paleoproterozoic crust, as do inherited zircon ages of -1.4 and 1.7-1.8 Ga (SHRIMP-II) for the Grayling Lake granite and Uphill Creek granodiorite reported by Murphy et al (2002).…”

Section: Evidence For the Selway Terranementioning

confidence: 99%

Proterozoic evolution of the western margin of the Wyoming craton: implications for the tectonic and magmatic evolution of the northern Rocky Mountains

et al. 2006

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Defining the extent and age of basement provinces west of the exposed western margin of the Archean Wyoming craton has been elusive because of thick sedimentary cover and voluminous Cretaceous-Tertiary magmatism. U-Pb zircon geochronological data from small exposures of pre-Belt supergroup basement along the western side of the Wyoming craton, in southwestern Montana, reveal crystallization ages ranging from -2.4 to -1.8 Ga. Rock-forming events in the area as young as -1.6 Ga are also indicated by isotopic (Nd, Pb, Sr) signatures and xenocrystic zircon populations in Cretaceous-Eocene granitoids. Most of this lithosphere is primitive, gives ages -1.7-1.86 Ga, and occurs in a zone that extends west to the Neoproterozoic rifted margin of Laurentia. These data suggest that the basement west of the exposed Archean Wyoming craton contains accreted juvenile Paleoproterozoic arc-like terranes, along with a possible mafic underplate of similar age. This area is largely under the Mesoproterozoic Belt basin and intruded by the Idaho batholith. We refer to this Paleoproterozoic crust herein as the Selway terrane. The Selway terrane has been more easily reactivated and much more fertile for magma production and mineralization than the thick lithosphere of the Wyoming craton, and is of prime importance for evaluating Neoproterozoic continental reconstructions.

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“…Consider a source of area of 10 3 -10 4 km 2 beneath which an average of 1 km 3 per km 2 of melt is extracted from 10 km depth of suprasolidus continental crust (e.g. Zen 1992). For a pluton that is 1 km thick, the melting footprint, the region of melt throughput and the pluton area are identical.…”

Section: Fluxesmentioning

confidence: 99%

The mechanism of melt extraction from lower continental crust of orogens

Brown¹

2004

Earth and Environmental Science Transactions of the Royal Socie

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Melt extraction is a process with a length scale that spans many orders of magnitude. Studies of residual migmatites and granulites suggest that melt has migrated from grain boundaries to networks of leucosome-filled structures to steeply inclined cylindrical or tabular granites inferred to have infilled ascent conduits. For example, in anatectic rocks from southern Brittany, France, during decompression-induced biotite-breakdown melting, melt is inferred to have been expressed from foliation-parallel structures analogous to compaction bands to dilation and shear bands, based on location of residual leucosome, and from this network of structures to ascent conduits, preserved as dykes of granite. The leucosome-filled deformation band network is elongated parallel to a sub-horizontal lineation, suggesting that mesoscale melt flow was focused primarily in the plane of the foliation along the lineation to developing dilatant transverse structures. The leucosome network connects with petrographic continuity to granite in dykes; however, the orientation of dykes discordant to fabric anisotropy suggests that their formation was controlled by stress, which indicates that the process is a fracture phenomenon. Blunt fracture tips and zigzag propagation paths indicate that the dykes represent ductile opening-mode fractures; these are postulated to have formed by coalescence of melt pockets. The structures record a transition from accumulation to draining; quantitative volume fluxes are calculated and presented for the generalised extraction process. The anatectic system may have converged to a critical state at some combination of melt fraction and melt distribution that enabled formation of ductile opening-mode fractures, but fractal distribution of inferred mesoscale melt-filled structures has not been demonstrated; this may reflect the inherent anisotropy and/or residual nature of the drained source. Melt extraction has been modelled as a self-organised critical phenomenon, but the mechanism of extraction is not described and the relationship between these models and the spatial and temporal granularity of lower continental crust is not addressed. Self-organised critical phenomena are driven systems involving ‘avalanches’ with a fractal frequency-size distribution; thus, the distribution of melt batch sizes might be expected to be fractal, but this has not yet been demonstrated in nature.

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Using granite to image the thermal state of the source terrane

Cited by 4 publications

References 33 publications

The spatial and temporal patterning of the deep crust and implications for the process of melt extraction

The spatial and temporal patterning of the deep crust and implications for the process of melt extraction

Proterozoic evolution of the western margin of the Wyoming craton: implications for the tectonic and magmatic evolution of the northern Rocky Mountains

The mechanism of melt extraction from lower continental crust of orogens

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