Investigations into deep crustal levels: Fractionating effects and geochemical trends related to high-grade metamorphism

Sighinolfi, G. P.

doi:10.1016/0016-7037(71)90018-4

Cited by 95 publications

(13 citation statements)

References 74 publications

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“…Low silicic content also supports eclogitization since the water content of hydrous minerals increases with decreasing silica and increasing alumina [Tassara, 2006]. Both argue that the granulites of the lower Brazilian shield [Sighinolfi, 1971] would be stable as we find here.…”

Section: Introductionsupporting

confidence: 79%

“…Results from this study support underthrusting of the Brazilian shield beneath Peru [ McQuarrie et al , 2005; Allmendinger and Gubbels , 1996; Horton et al , 2001; Gubbels et al , 1993; Lamb and Hoke , 1997; Beck and Zandt , 2002], which is more consistent with a gradual uplift model for this section of the Altiplano. The eclogitization which would occur in the case of delamination needs a significant amount of water [ Ahrens and Schubert , 1975; Hacker , 1996], which is not present in the Brazilian shield crust [ Sighinolfi , 1971]. Low silicic content also supports eclogitization since the water content of hydrous minerals increases with decreasing silica and increasing alumina [ Tassara , 2006].…”

Section: Introductionmentioning

confidence: 99%

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Structure of the subduction system in southern Peru from seismic array data

et al. 2012

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[1] The subduction zone in southern Peru is imaged using converted phases from teleseismic P, PP, and PKP waves and P wave tomography using local and teleseismic events with a linear array of 50 broadband seismic stations spanning 300 km from the coast to near Lake Titicaca. The slab dips at 30 and can be observed to a depth of over 200 km. The Moho is seen as a continuous interface along the profile, and the crustal thickness in the back-arc region (the Altiplano) is 75 km thick, which is sufficient to isostatically support the Andes, as evidenced by the gravity. The shallow crust has zones of negative impedance at a depth of 20 km, which is likely the result of volcanism. At the midcrustal level of 40 km, there is a continuous structure with a positive impedance contrast, which we interpret as the western extent of the Brazilian Craton as it underthrusts to the west. V p /V s ratios estimated from receiver function stacks show average values for this region with a few areas of elevated V p /V s near the volcanic arc and at a few points in the Altiplano. The results support a model of crustal thickening in which the margin crust is underthrust by the Brazilian Shield.

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Section: Introductionsupporting

confidence: 79%

Section: Introductionmentioning

confidence: 99%

Structure of the subduction system in southern Peru from seismic array data

et al. 2012

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“…K and Rb. Measured 87Sr/86Sr ratios for the same orthogneisses are also low, with the orthogneiss xenoliths having lower 87Sr/SSSr ratios (generally under 0.71) than the exposed orthogneisses (0.702-0.720; Ruiz et al [1988a] High K/Rb ratios in exposed granulite facies terrains have been previously attributed to removal of Rb, either in an aqueous phase or an anatectic melt, in the lower crust [Sighinolfi, 1969[Sighinolfi, , 1971Lewis and Spooner, 1973]. Although some exceptions exist, common upper crustal rocks generally have K/Rb ratios below 500 [Shaw, 1968].…”

Section: Discussionmentioning

confidence: 83%

Geochemistry of exposed granulite facies terrains and lower crustal xenoliths in Mexico

Roberts¹,

Ruíz²

1989

J. Geophys. Res.

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The trace element compositions of 13 samples from exposed granulite facies terrains and 17 granulitic lower crustal xenoliths were determined using an inductively coupled plasma‐mass spectrometer to constrain the composition and evolution of the Mexican lower crust. Most xenoliths found are pyroxeneplagioclase orthogneisses, although quartzo‐feldspathic paragneisses form up to 50% of the xenolith population in some areas. Lithologies in the exposed terrains consist of pelitic and carbonate‐rich paragneisses, charnockites, and less abundant basic, intermediate, and felsic orthogneisses. Both xenoliths and exposed granulites generally have low abundances of incompatible elements such as K, Rb, Th, and U. The xenoliths, however, often have lower abundances of these elements than the exposed granulites. High K/Rb ratios, coupled with an inverse correlation between K2O and K/Rb, indicate that most samples from the Mexican lower crust are depleted in both K and Rb. Furthermore, Th/U ratios similar to upper crustal materials and high La/Th ratios suggest depletion in both Th and U relative to average crustal material. Removal of a melt phase, either after the crystallization of cumulates or following a partial melting event, can explain many of the chemical features observed and may be responsible for the elemental depletions. Some samples, however, appear to have been unaffected during their residence in the lower crust and chemically resemble their presumed protoliths. Available geobarometric data suggest that xenoliths equilibrated near the crust/mantle boundary (∼10 kbar) and represent the lowermost crust, while the exposed granulites equilibrated at shallower crustal levels (∼7 kbar). Thus the differences observed between the xenoliths and exposed granulites indicate that there may be chemical and lithologic zonation in the Mexican lower crust. The lowermost crust in Mexico consists of crystallized basaltic liquids, cumulates, and/or residue after the removal of a partial melt as well as garnet‐rich metasediments.

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“…B , Northern Norway (Heier and Thoresen, 1971). C, Brazilian Shield (Sighinolfi, 1971). D, Musgrave Range, Australia (Lambert and Heier, 1968).…”

Section: Discussionmentioning

confidence: 99%

Review of African Granulites and Related Rocks

Clifford

1974

Geological Society of America Special Papers

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Although granulite facies rocks are present in zones of orogenesis ranging in age from 300 to 3,000 m.y. in Africa, only two distinct ages of granulite facies metamorphism, at about 1,200 and 3,000 m .y., have thus far been recognized. Granulite facies suites in regions of orogenesis of other ages -1,850 ± 250 m .y., 600 ± 100 m .y., and Phanerozoicrepresent older remnants since affected by regional retrograde metamorphism. The retro gression is particularly widespread in the 600 ± 100 m.y. orogenic zone, an important part of which is interpreted in terms of a former zone of granulite facies rocks, remnants of which are now preserved as isolated masses in amphibolite facies rocks.In most African examples, hypersthene granulites o f silicic to mafic and ultramafic com position are associated with undoubted metasedimentary rocks and a variety of other rock types, including anorthosite; intrusive chamockite also occurs in a number of regions. The alkali-silica plot for 240 analyses of pyroxene granulite and chamockite suggests a chemical affinity with high-alumina basalt and its calc-alkaline derivatives; the AFM trends are similar to those of calc-alkaline and alkaline igneous suites.With few exceptions, rocks of appropriate composition in the granulite facies contain hypersthene + diopside + plagioclase ± garnet, reflecting medium to high pressure. Avail able Sr-isotopic data for granulite-chamockite suites of Africa and elsewhere in the world yield a crude " evolution path" that may reflect Sr-isotopic development in the deeper parts of the Earth's crust or in the mantle beneath the continental crust.

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Investigations into deep crustal levels: Fractionating effects and geochemical trends related to high-grade metamorphism

Cited by 95 publications

References 74 publications

Structure of the subduction system in southern Peru from seismic array data

Structure of the subduction system in southern Peru from seismic array data

Geochemistry of exposed granulite facies terrains and lower crustal xenoliths in Mexico

Review of African Granulites and Related Rocks

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