Dehydration partial melting and disequilibrium in the granulite-facies Wilmington Complex, Pennsylvania-Delaware Piedmont

Srogi, LeeAnn; Wagner, Mary Emma; Lutz, Tim

doi:10.2475/ajs.293.5.405

Cited by 37 publications

(18 citation statements)

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“…Cordierite reaction rims separate biotite from fibrolitic sillimanite and from small equant spinel crystals. Similar textures have been recognized in the Wilmington Complex (Srogi et al, 1993) and ascribed to dehydration partial melting of biotite. The significance of these textures is discussed in Whittington et al (1998).…”

Section: Basement Gneissessupporting

confidence: 71%

Contrasting anatectic styles at Nanga Parbat, northern Pakistan

Whittington¹,

Harris²,

Butler³

1999

Himalaya and Tibet: Mountain Roots to Mountain Tops

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Section: Basement Gneissessupporting

confidence: 71%

Contrasting anatectic styles at Nanga Parbat, northern Pakistan

Whittington¹,

Harris²,

Butler³

1999

Himalaya and Tibet: Mountain Roots to Mountain Tops

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“…We conclude that anatexis and melt removal will favour the formation of spinel in the restite, as has been invoked by Srogi et al (1993) for similar anatectic assemblages from the Wilmington complex, although Fig. 3 suggests the role of melt in removing silica may be less significant than its effect in depleting H20 content.…”

Section: Phase Equilibriamentioning

confidence: 77%

Low-pressure crustal anatexis: the significance of spinel and cordierite from metapelitic assemblages at Nanga Parbat, northern Parkistan

Whittington¹,

Harris²,

Baker³

1998

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Spinel-bearing domains in high-grade metapelitic rocks from Nanga Parbat represent zones of partial melting during biotite breakdown under vapour-undersaturated conditions. Spinel is essentially of the MgA1204-FeA1204 solid solution, and is therefore not stabilized by the presence of trace elements such as zinc, but is restricted to a quartz-absent petrogenesis. A new petrogenetic grid for metapelites has been constructed to allow for both vapour-undersaturated and quartz-undersaturated conditions. This grid contains a quartzabsent invariant point in KFASH that predicts quartz-absent melting will occur in biotitesillimanite assemblages at low pressures.A P-T pseudosection drawn for bulk compositions with intermediate Fe/Mg ratios shows that at low pressures biotite will break down initially in the presence of quartz, to produce cordierite and K-feldspar and melt. At higher temperatures the quartz-absent reaction will be crossed, and biotite will break down to produce spinel in addition to cordierite, K-feldspar and melt. The sequence of assemblages is biotite-cordierite, biotite-cordieritespinel, and finally cordierite-spinel. This biotite-absent assemblage is not observed, indicating that the quartz-absent reaction did not go to completion. Limits can be placed on the pressures and temperatures at which the biotite breakdown reaction is crossed, from the coexistence of solid phases and granite melt over a range of water activities. For the Nanga Parbat assemblages, pressures and temperatures lay at about 720°C and 5 kbar at water activity, aiJ2o _> 0.6. These conditions are consistent with a rapidly exhuming terrane, as suggested by isotopic constraints and thermal modelling of the region.

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“…While contacts can be mapped or estimated along most of the margin, the orientation of the pluton in space is not known. Srogi et al (1993) argued that there are no significant differences in pressure across the eastern portion of the Wilmington complex. This suggests that it is unlikely that the pluton is tilted on its side and revealed in cross section.…”

Section: Implications Of the Melt Migration Modelmentioning

confidence: 97%

“…Geologic map of the Arden pluton in the Wilmington Complex, Pennsylvania and Delaware, afterSrogi et al, (1993).…”

mentioning

confidence: 99%

“…These include magmatic fabrics and minerals in the plutonic igneous rocks, as well as localized thermal metamorphism associated with igneous intrusions superimposed on regional granulite facies conditions in the country rocks. Plutons were emplaced at depths of ≈17-25 km, based upon pressure estimates of 600 ± 100 MPa in adjacent country-rock gneisses Srogi, 1988;Srogi et al, 1993).…”

mentioning

confidence: 99%

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Chemical variation in plutonic rocks caused by residual melt migration: Implications for granite petrogenesis

Srogi¹,

Lutz²

1997

The Nature of Magmatism in the Appalachian Orogen

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We investigate the crystallization history of an early Paleozoic granitic suite within the Arden pluton, Wilmington Complex, Pennsylvania-Delaware Piedmont, and its implications for granite petrogenesis. The pluton intruded into granulite facies gneisses at a depth of 17-25 km, and magmatic textures and mineral and rock compositions are generally well preserved. The granitic suite is characterized geochemically by a wide range of rock compositions (e.g., 54-76 wt% SiO 2 ; 0.7-5.7 wt% K 2 O); strong linear correlation among all elements except Ba, verified by principal components analysis; large apparently random scatter in Ba concentration; limited variation in mineral composition among rocks ranging from 58-73 wt% SiO 2 ; and no apparent spatial pattern to rock chemical variation within the pluton.We explain these factors by a model of in situ crystallization within a solidification zone (or boundary layer). We derive equations that integrate both major-and trace-element variations in a quantitative model of the migration and crystallization of residual liquid within a mush of relatively earlier formed crystals. Barium is the key element that traces the history of melt migration, because it changed behavior from incompatible to compatible with the onset of K-feldspar crystallization from the residual granitic liquid. The model explains the large range in rock chemistry as well as the linear correlations among elements in terms of the proportions of earlier formed crystals, and minerals that crystallized from the residual melt. The apparent scatter of Ba concentration in the rocks resulted from the variable depletion of Ba in the residual liquid as it crystallized and migrated through the crystal mush, and requires that trapped liquid did not equilibrate in trace-element composition on a scale of meters or more within the crystal mush.From our equations, we derive values for parameters that describe the proportion of melt that remained after crystallization of earlier formed minerals, and the proportion of residual melt that ultimately left the rock. Net melt loss for most rocks is ≥50%, suggesting that melt was not only migrating locally, but was being transported from the level of the pluton now exposed. This result may be more compatible with gravity-influenced processes near the base of a growing crystal pile, rather than with sidewall crystallization. Nearly uniform compositions and proportions of the relatively earlier formed crystals could be explained by efficient compositional convection between the granitic magma reservoir and residual melt circulating through the crystal mush. There is a roughly concentric spatial pattern to the model parameters for melt proportions that suggests an outer zone of high initial crystallinity and little melt loss near the present margin of the pluton, an interior zone with large losses of residual melt that is consistent with high melt fluxes and compositional convection, Srogi, L., and Lutz, T. M., 1997, Chemical variation in plutonic rocks caused by residual melt migrat...

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Dehydration partial melting and disequilibrium in the granulite-facies Wilmington Complex, Pennsylvania-Delaware Piedmont

Cited by 37 publications

References 0 publications

Contrasting anatectic styles at Nanga Parbat, northern Pakistan

Contrasting anatectic styles at Nanga Parbat, northern Pakistan

Low-pressure crustal anatexis: the significance of spinel and cordierite from metapelitic assemblages at Nanga Parbat, northern Parkistan

Chemical variation in plutonic rocks caused by residual melt migration: Implications for granite petrogenesis

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