Showing papers on "Granulite published in 2007"

One View of the Geochemistry of Subduction-Related Magmatic Arcs, with an Emphasis on Primitive Andesite and Lower Crust

Abstract: This chapter has four main aims. Provide a comprehensive picture of the composition of volcanic rocks from subduction-related magmatic arcs. Review evidence in favor of the existence of andesitic, as well as basaltic primary magmas in arcs. Present new data on the composition of arc lower crust, based mainly on our work on the Talkeetna arc section in southcentral Alaska. Summarize evidence from arc lower crustal sections that a substantial proportion of the dense, lower crustal pyroxenites and garnet granulites produced by crystal fractionation are missing.

Passage through India: the Mozambique Ocean suture, high-pressure granulites and the Palghat-Cauvery shear zone system

Abstract: The Mozambique Ocean closed as Gondwana formed. Its suture has been identified in Madagascar (Betsimisaraka suture), but its continuation, into India, is controversial. The Palghat-Cauvery shear system appears an ideal candidate as it: (i) lies along strike of the Betsimisaraka suture in Gondwana; (ii) forms a high-pressure granulite belt; and (iii) separates crustal domains with different geological histories. However, existing age constraints have been used to suggest that the structure is Archaean/Palaeoproterozoic. Here we date metamorphic zircons using secondary ion mass spectrometry (535.0 ± 4.9 Ma) and monazites using electron probe micro-analysis (537 ± 9, 532 ± 8, 525 ± 10 Ma). No evidence for an earlier metamorphic event was found. The identification of Palghat-Cauvery high-pressure metamorphism as Cambrian, and recognition that it bounds crustal domains of contrasting origin, points to it being the southern continuation of the Betsimisaraka suture and southern margin of Neoproterozoic India.

The causal link between HP-HT metamorphism and ultrapotassic magmatism in collisional orogens: case study from the Moldanubian Zone of the Bohemian Massif

Abstract: In the Moldanubian Zone of the Bohemian Massif occur Variscan granulites and ultrapotassic magmatites (amphibole-bearing durbachitic suite and two-pyroxene syenitoids) in a close spatial and temporal relationship. The protolith to the most widespread felsic garnet-kyanite-mesoperthite granulites, which equilibrated at P >1.5 GPa and Tc. 1000°C, was analogous to meta-igneous rocks occurring in the Saxothuringian Zone of the NW Bohemian Massif. For the most basic ultrapotassic rocks, the high contents of Cr and Ni as well as high mg# point to derivation from an olivine-rich source (i.e. a mantle peridotite). On the other hand, elevated concentrations of U, Th, light rare earth elements (LREE) and large ion lithophile elements (LILE), pronounced depletion in Ti, Nb and Ta as well as high K 2 O/Na 2 O and Rb/Sr ratios apparently contradict the mantle origin. This dual geochemical character and crustal-like isotopic compositions require melting of anomalous lithospheric mantle sources, metasomatized and contaminated by mature crustal material. Both Moldanubian granulites and ultrapotassic rocks show mutually complementary depletions and enrichments in some trace elements (Cs, Rb, Th, U and Pb). Late Devonian-Early Carboniferous Andean-type subduction is thought to have resulted in continental collision and HP metamorphism of upper crustal, largely meta-igneous lithologies. The subduction of the mature crustal material caused direct contamination and metasomatism in the overlying lithospheric mantle wedge. Shortly after the granulite-facies metamorphic peak (at c . 340 Ma) and the slab break off, these metasomatized and contaminated mantle domains were melted by advected heat from the invading asthenosphere, generating ultrapotassic intrusions, closely related to the granulite occurrences in space and time. This tectonic, petrological and geochemical model is outlined.

The Late Permian to Triassic Hongseong-Odesan Collision Belt in South Korea, and Its Tectonic Correlation with China and Japan

Abstract: We propose a new correlation of Middle Triassic (ca. 230 Ma) eclogites from the Hongseong area of the southwest Gyeonggi Massif, South Korea, with the Dabie-Sulu collision belt of China. Late Permian (ca. 257 Ma) mangerites that intrude the Odesan area in the eastern part of the Gyeonggi Massif show geochemical characteristics of collisional tectonic settings, implying that the Hongseong collision belt extends to the Odesan area. In the Higo terrain of southernmost Japan, sapphirine-bearing granulites and related high-temperature metamorphic rocks reveal ca. 245 Ma ultrahigh-temperature (UHT) metamorphic conditions. This metamorphism is well matched with the 245 ± 10 Ma UHT metamorphism estimated for spinel granulite in the Odesan area, suggesting that the Dabie-Sulu collision zone continues through the Hongseong-Odesan belt into the Higo area and that Paleozoic subduction complexes in southwest Japan represent an eastern extension of the Dabie-Sulu collision belt. The Paleozoic subduction complexes in Ja...

Paleoproterozoic events in the North China Craton.

Abstract: This paper deals with several key issues including Paleoproterozoic sedimentary sequences,metamorphism and magmatic intrusion,and comes to following primary suggestions for Proterozoic tectonic evolution of the North China Craton(NCC):(1)The NCC fundamentally amalgamated by continent-continent,arc-continent or arc-arc collisions by～2500 Ma;The NCC behaved as a stable continent block without tectonic-thermal action during 2500～2300 Ma.(2)2300～1950 Ma mobile belts,such as Jinyuand Jiaoliao,formed and represent intracratonic orogenesis with Phanerozoic orogenic characteristics on some aspects.(3)The evolution of these mobile belts includes basin rifting,subduction and final collision at～1850 Ma.(4)A proposed northern mobile belt that was not preserved was located north of the Baiyan Obo.The khondalite sequences possibly represent a structural zone within craton,which was related to subduction and collision of the northern mobile belt and caused deep-level granulite facies rocks to uplift by sole- thrusting.(5)The Baiya Obo,Zhaertai and Huade sedimentary sequences deposed in passive continent marginal basins in 1800～1700 Ma,same as the Changcheng System.(6)The tectonic regime in the NCC was inverted from compression to extension in 1850～1700 Ma,resulting in rifting within craton,mafic dyke swarm emplacing and cratonic basement uplifting.(7)In the middle of northern margin of the NCC,an ultra-high-temperature metamorphic complex has been discovered.The paper also discusses genetic relationship between the ultra-high-temperature metamorphic rocks and high-pressure granulites in the central NCC,and suggests that they are related to a mantle upwelling and magma underplating event of～1900 Ma.

Timing and dynamics of the juxtaposition of the Eastern Ghats Mobile Belt against the Bhandara Craton, India: A structural and zircon U-Pb SHRIMP study of the fold-thrust belt and associated nepheline syenite plutons

Abstract: [1] Sensitive High Resolution Ion MicroProbe (SHRIMP) U-Pb dating of zircon from basement granite gneisses and nepheline syenites of the Sinapalli Nappe, occurring along the northwestern margin of the Eastern Ghats Mobile Belt, indicate high grade regional metamorphism and associated folding accompanying juxtaposition of the nappe with the Bhandara Craton, to have taken place between 617 ± 85 Ma (lower intercept age of a reworked basement unit) and 517 Ma (age of the youngest syenite). This shows, for the first time, that the final juxtaposition of the northwestern parts of the Eastern Ghats Mobile Belt against the Bhandara Craton came about in the late Neoproterozoic and not, as previously thought, during the Mesoproterozoic. The northwestern part of the Eastern Ghats Mobile Belt comprises a fold- thrust belt consisting of a stack of northwesterly verging nappes that have been thrust over the Bhandara Craton. The Sinapalli Nappe is the lowermost nappe and rests over a tectonic contact on the Archean granites and gneisses of the craton. The basal decollement is exposed as a two-km-wide ductile-brittle thrust, hosting nepheline syenite plutons that show fabrics consistent with a synkinematic emplacement during thrusting. The Sinapalli Nappe is comprised of a sequence of alternating mafic granulites and quartzofeldspathic gneisses with slivers of basement granites, which are folded in three phases of folding (F1, F2 and F3) and were subjected to granulite facies metamorphism during F1 folding. Thrusting is synkinematic to F2 folding and is responsible for the juxtaposition of the northwestern part of the Eastern Ghats Terrane over the Bhandara Craton during the assembly of parts of eastern Gondwana.

Linking deformation, migmatite formation and zircon U–Pb geochronology in polymetamorphic orthogneisses, Sveconorwegian Province, Sweden

Abstract: Absolute ages of migmatization in the polymetamorphic, parautochthonous basement of the Sveconorwegian Province, Sweden, have been determined using U–Pb ion probe analysis of zircon domains that formed in leucosome of migmatitic orthogneisses. Migmatite zircon was formed by recrystallization whereas dissolution–reprecipitation and neocrystallization were subordinate. The recrystallized migmatite zircon was identified by comparison of zircon in mesosomes and leucosomes. It is backscatter electron-bright, U-rich (800–4400 ppm) with low Th/U-ratios (generally 0.01–0.1), unzoned or ‘oscillatory ghost zoned’, and occurs as up to 100 μm-thick rims with transitional contacts to cores of protolith zircon. Protolith ages of 1686 ± 12 and 1668 ± 11 Ma were obtained from moderately resorbed, igneous zircon crystals (generally Th/U = 0.5–1.5, U < 300 ppm) in mesosomes; protolith zircon is also present as resorbed cores in the leucosomes. Linkage of folding, synchronous migmatization and formation of recrystallized zircon rims allowed direct dating of south-vergent folding at 976 ± 7 Ma. At a second locality, similar recrystallized zircon rims in leucosome date pre-Sveconorwegian migmatization at 1425 ± 7 Ma; an upper age bracket of 1394 ± 12 Ma for two overprinting phases of deformation (upright folding along gently SSW-plunging axes and stretching in ESE) was set by zircon in a folded metagranitic dyke. Lower age brackets for these events were set at 952 ± 7 and 946 ± 8 Ma by zircon in two crosscutting and undeformed granite–pegmatite dykes. Together with previously published data the present results demonstrate: (i) Tectonometamorphic reworking during the Hallandian orogenesis at 1.44–1.42 Ga, resulting in migmatization and formation of a coarse gneissic layering. (ii) Sveconorwegian continent–continent collision at 0.98–0.96 Ga, involving (a) emplacement of an eclogite unit, (b) regional high-pressure granulite facies metamorphism, (c) southvergent folding, subhorizontal, east–west stretching and migmatization, all of which caused overprint or transposition of older Mesoproterozoic and Sveconorwegian structures. The Sveconorwegian migmatization and folding took place during or shortly after the emplacement of Sveconorwegian eclogite and is interpreted as a result of north–south shortening, synchronous with east–west extension and unroofing during late stages of the continent–continent collision.

Tectonic evolution of the Yaoundé segment of the Neoproterozoic Central African Orogenic Belt in southern Cameroon

Abstract: The deformation history of the Neoproterozoic Central African Orogenic Belt in southern Cameroon is well recorded in the low- to high-grade rocks outcropping in the area around Yaounde. The fabrics in these rocks are con- sistent with two main ductile deformation events D1 and D2 .D 1 predated emplacement of calc-alkaline dioritic bodies and caused the formation of nappes that resulted in high-pressure granulite metamorphism of soft sediments. A strong overprinting of these nappes during D2 symmetric extension, probably associated with large-scale foliation boudinage and (or) gneissic doming and intense magmatic underplating, gave rise to regional flat-lying fabrics. The latter were further buckled by D3 and D4 folding phases defining a vertical constriction occurring with a major east-west to NW- SE shortening direction. The corresponding F3 and F4 folds trend north-south to NE-SW and east-west to NW-SE, respectively, and represent the main regional strain patterns. Based on the east-west to NW-SE maximum shortening orientation indicated by F3 folds, it is proposed that the nappe-stacking phase D 1 occurred in the same direction. The deformation history in the area can thus be described as corresponding principally to alternating east-west to NW-SE contractions and north-south to NE-SW orogenic-parallel extensions. At the regional scale, this could be due to the Transaharan east-west collisional system.

Whole-rock, Phosphate, and Silicate Compositional Trends across an Amphibolite- to Granulite-facies Transition, Tamil Nadu, India

Abstract: Chemical trends from north (amphibolite facies) to south (granulite facies) along a 95 km traverse in Tamil Nadu, Southern India, include: whole-rock depletion of Rb, Cs, Th and U, enrichment in Ti and F, and depletion in Fe and Mn in biotite and amphibole; increases in Al and decreases in Mn in orthopyroxene; enrichment of fluorapatite in F. K-feldspar blebs are widespread along quartz^ plagioclase grain boundaries, and could indicate either partial melting or metasomatism. In the northernmost portion of the traverse the principal rare earth element (REE)-bearing minerals are allanite and titanite. South of a clinopyroxene isograd, monazite grains independent of fluorapatite are the major REE- and Th-bearing phase. Further south independent monazite is rare but Th-free monazite inclusions are common in fluorapatite. During prograde metamorphism, independent monazite was replaced by REE-rich fluorapatite in which the monazite inclusions later formed. The loss of independent monazite was accompanied by a loss of whole-rock Th and possibly a small depletion in light REE. Most mineralogical features along the traverse can be accounted for by progressive dehydration and oxidation reactions. Trace-element depletion is best explained by the action of an externally derived low H2O activity brine migrating from a source at greater depth, possibly preceded or accompanied by partial melting.

Relics of the Mozambique Ocean in the central East African Orogen: Evidence from the Vohibory Block of southern Madagascar

Abstract: The Vohibory Block of south-western Madagascar is part of the East African Orogen, the formation of which is related to the assembly of the Gondwana supercontinent. It is dominated by metabasic rocks, which have chemical compositions similar to those of recent basalts from a mid-ocean ridge, back-arc setting and island-arc setting. The age of formation of protolith basalts has been dated at 850-700 Ma by U-Pb SHRIMP analysis of magmatic cores in zircon, pointing to an origin related to the Neoproterozoic Mozambique Ocean. The metabasic rocks are interpreted as representing components of an island arc with an associated back-arc basin. In the early stage of the Pan-African orogeny, these rocks experienced high-pressure amphibolite to granulite facies metamorphism (9-12 kbar, 750-880 degrees C), dated at 612 +/- 5 Ma from metamorphic rims in zircon. The metamorphism was most likely related to accretion of the arc terrane to the margin of the Azania microcontinent (Proto-Madagascar) and closure of the back-arc basin. The main metamorphism is significantly older than high-temperature metamorphism in other tectonic units of southern Madagascar, indicating a distinct tectono-metamorphic history.

Behaviour of zircon in high-grade metamorphic rocks: evidence from Hf isotopes, trace elements and textural studies

Abstract: Hf isotopic data of minerals in a mafic pyroxene granulite from the southern Bohemian Massif, together with their major and trace element composition and petrological observations were used to decipher the metamorphic history and behaviour of zircon in the granulite. The Hf isotopic composition in the minerals was used to estimate whether the decompression reaction, namely the consumption of garnet and rutile, could have provided Zr for the formation of newly grown metamorphic zircon. The age of the decompression reaction indicated by the evolution of Hf isotopes in garnet and orthopyroxene is between 333 and 331 Ma, i.e. ca. 7 Ma younger than the available U–Pb zircon ages from the Moldanubian granulites and than the newly obtained 343 ± 2 Ma laser ablation ICP-MS U–Pb age of zircons. The combination of bulk and in-situ Hf isotopic data, major and trace element composition and petrological modeling of P–T evolution revealed that the formation of zircons can not be related to the decompression phase of the evolution of the mafic granulites.