Granulite

About: Granulite is a research topic. Over the lifetime, 6763 publications have been published within this topic receiving 268925 citations.

Fe-Mg mixing in cordierite; constraints from natural data and implications for cordierite-garnet geothermometry in granulites

Abstract: From chemicaldata on 108 pairs of naturally occurring garnet and cordierite, the interchange energy of Fe-Mg mixing in cordierite (Wo*-\") and the free-energy change of the exchange equilibrium %FerAlrSirO,, -t t/zMgrAIoSirO,, : Y:MgrAlrSirO,, * YzFerAloSirO,, in the temperature range of 700-850 'C have been extracted using standard methods of linear regression analysis. The input thermochemical parameters for multicomponent CaMg-Fe-Mn garnet solid solutions are based on the updated and revised values after Ganguly and Saxena (1984), e.g., Wr\"_.\": 2500 cal, W*: W*\": 3000 cal calculated on the basis of one-third formula unit. On the basis of the derived values at 710, 760, 800, and 850 \"C, Fe-Mg mixing in cordierite has been approximated to a symmetric regular solution. The Fe-Mg substitution shows positive deviation from ideality in cordierite, with tzi:1., : 9.34 + 4.54kJ/mol of divalent cation. The values derived for the free energy of mixing, at Xi:d : Xi;ds : 0.5, are -3.26 kJ at 700 'C and -4.39 kl at 900 \"C. The AG,.. value (in joules) for the above equilibrium can be expressed as 15079 8.552. Incorporating these parameters, the geothermometric expression for the exchange equilibrium is

Terrane accretion and upward extrusion of high-pressure granulites in the Neoproterozoic nappes of Southeast Brazil: petrologic and structural constraints

Abstract: The high-grade crystalline nappes exposed southeast of the Sao Francisco craton comprise two distinct units of mainly granulite facies rocks that represent a composite section of Neoproterozoic deep continental crust: the Socorro-Guaxupe nappe above, derived from an arc terrane, and the Tres Pontas-Varginha nappe below. Metamorphism in the Tres Pontas-Varginha nappe is characterized by the exceptional preservation of kyanite granulites (700–750°C, 15 kbar), and followed by limited retrogression. Maximum temperatures around 900–950°C were reached toward the base of the overlying Socorro-Guaxupe nappe during the intrusion of charnockitic-mangeritic magmas. Lower-pressure metamorphism, accompanied by anatexis, prevailed at shallower crustal levels. Our petrological results document an inverted thermal structure with isobaric heating of the top of the high-pressure granulite nappe. Both granulite nappes were transported more than 200 km eastward above lower nappes involving reworked basement and passive margin units, both metamorphosed to high-pressure but lower-temperature conditions. Significant thinning and cooling of the two granulite nappes may have occurred before their emplacement onto the lower nappes. The proposed geodynamic scenario considers that continental subduction took place westward underneath Neoproterozoic oceanic lithosphere. The two granulite units crystallised at ∼ 45 km depths under distinct paleogeotherms within this subduction zone around 630 Ma. The kyanite granulites were rapidely exhumed through the mechanism of low-angle “forced” extrusion, whereas syncollisional collapse affected the soft, anatectic middle crust of the overlying arc terrane. The final emplacement of the thinned nappe pile onto the cold Sao Francisco craton and its platform cover, with at most, anchizonal to greenschist-facies metamorphism, occurred around 600 Ma.

The Amazon-Laurentian connection as viewed from the Middle Proterozoic rocks in the central Andes, western Bolivia and northern Chile

Abstract: Middle Proterozoic rocks underlying the Andes in western Bolivia, western Argentina, and northern Chile and Early Proterozoic rocks of the Arequipa massif in southern Peru form the Arequipa-Antofalla craton. These rocks are discontinuously exposed beneath Mesozoic and Cenozoic rocks, but abundant crystalline clasts in Tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton. Near Berenguela, western Bolivia, the Oligocene and Miocene Mauri Formation contains boulders of granodiorite augen gneiss (1171±20 Ma and 1158±12 Ma; U-Pb zircon), quartzose gneiss and granofels that are inferred to have arkosic protoliths (1100 Ma source region; U-Pb zircon), quartzofeldspathic and mafic orthogneisses that have amphibolite- and granulite-facies metamorphic mineral assemblages (∼1080 Ma metamorphism; U-Pb zircon), and undeformed granitic rocks of Phanerozoic(?) age. The Middle Proterozoic crystalline rocks from Berenguela and elsewhere in western Bolivia and from the Middle Proterozoic Belen Schist in northern Chile generally have present-day low 206Pb/204Pb ( 15.57), and elevated 208Pb/204Pb (37.2 to 50.7) indicative of high time-averaged Th/U values. The Middle Proterozoic rocks in general have higher present-day 206Pb/204Pb values than those of the Early Proterozoic rocks of the Arequipa massif (206Pb/204Pb between 16.1 and 17.1) but lower than rocks of the southern Arequipa-Antofalla craton (206Pb/204Pb> 18.5), a difference inferred to reflect Grenvillian granulite metamorphism. The Pb isotopic compositions for the various Proterozoic rocks lie on common Pb isotopic growth curves, implying that Pb incorporated in rocks composing the Arequipa-Antofalla craton was extracted from a similar evolving Pb isotopic reservoir. Evidently, the craton has been a coherent terrane since the Middle Proterozoic. Moreover, the Pb isotopic compositions for the Arequipa-Antofalla craton overlap those of the Amazon craton, thereby supporting a link between these cratons and seemingly precluding part of the Arequipa-Antofalla craton from being a detached fragment of another craton such as eastern Laurentia, which has been characterized by a different U/Pb history. Pb isotopic compositions for the Arequipa-Antofalla craton are, furthermore, distinct from those of the Proterozoic basement in the Precordillera terrane, western Argentina, indicating a Pb isotopic and presumably a tectonic boundary between them. The Pb isotopic compositions for the Precordillera basement are similar to those of eastern Laurentia, and support other data indicating that these rocks are a detached fragment of North America. Finally, the distinct Pb isotopic evolution history of the Arequipa-Antofalla craton and eastern Laurentia require minor modification to tectonic models linking eastern North America-Scotland to the oroclinal bend in western South America.

U-Pb Zircon and Sm-Nd isotopic study of the huangtuling granulite, dabie-sulu belt, China: Implication for the paleoproterozoic tectonic history of the yangtze craton

Abstract: SIMS data demonstrate Archean (2.9 to 2.7 Ga) and Paleoproterozoic (1991 ± 43 Ma) ages for rounded detrital zircon cores and unzoned rims respectively in a graulite sample collected from Huangtuling, Dabie-Sulu orogenic belt, China. The granulite rocks in this locality have pelitic compositions and represent an exposure of the northern margin of the Yangtze Craton. The rounded zircon cores show oscillatory zoning and have Th/U ratios (0.17 − 0.78) systematically higher than those of the unzoned luminescent rims (0.005 − 0.03). The age of the detrital zircon cores and whole rock Nd isotope model age suggest that the Huangtuling granulite was possibly derived from a provenance dominated by Archean rocks similar to the Kongling Complex exposed 200 km in the southwest. Multiple phases of metamorphism have been identified for the granulites, with the dominant granulite-facies mineral assemblage, defining an early clock-wise metamorphic P-T path. Orthopyroxene, plagioclase and whole-rock material define a linear trend in a 147Sm/144Nd − 143Nd/144Nd diagram, corresponding to an age of 1926 ± 146 Ma. This result and the U-Pb data for the zircon rims suggest that a collisional event may have occurred along the northern margin of the Yangtze Craton at ∼2.0 Ga, inviting further study on whether the Yangtze Craton was involved in the assembly of the recently-proposed Paleo-Mesoproterozoic supercontinent Columbia.