Two-stage metamorphic evolution of the Bemarivo Belt of northern Madagascar: constraints from reaction textures and in situ monazite dating     

N. JÖNS  V. SCHENK  P. APPEL  T. RAZAKAMANANA 《Journal of Metamorphic Geology》2006,24(4):329-347

New results on the pressure–temperature–time evolution, deduced from conventional geothermobarometry and in situ U‐Th‐total Pb dating of monazite, are presented for the Bemarivo Belt in northern Madagascar. The belt is subdivided into a northern part consisting of low‐grade metamorphic epicontinental series and a southern part made up of granulite facies metapelites. The prograde metamorphic stage of the latter unit is preserved by kyanite inclusions in garnet, which is in agreement with results of the garnet (core)‐alumosilicate‐quartz‐plagioclase (inclusions in garnet; GASP) equilibrium. The peak metamorphic stage is characterized by ultrahigh temperatures of ～900–950 °C and pressures of ～9 kbar, deduced from GASP equilibria and feldspar thermometry. In proximity to charnockite bodies, garnet‐sillimanite‐bearing metapelites contain aluminous orthopyroxene (max. 8.0 wt% Al₂O₃) pointing to even higher temperatures of ～970 °C. Peak metamorphism is followed by near‐isothermal decompression to pressures of 5–7 kbar and subsequent near‐isobaric cooling, which is demonstrated by the extensive late‐stage formation of cordierite around garnet. Internal textures and differences in chemistry of metapelitic monazite point to a polyphasic growth history. Monazite with magmatically zoned cores is rarely preserved, and gives an age of c. 737 ± 19 Ma, interpreted as the maximum age of sedimentation. Two metamorphic stages are dated: M1 monazite cores range from 563 ± 28 Ma to 532 ± 23 Ma, representing the collisional event, and M2 monazite rims (521 ± 25 Ma to 513 ± 14 Ma), interpreted as grown during peak metamorphic temperatures. These are among the youngest ages reported for high‐grade metamorphism in Madagascar, and are supposed to reflect the Pan‐African attachment of the Bemarivo Belt to the Gondwana supercontinent during its final amalgamation stage. In the course of this, the southern Bemarivo Belt was buried to a depth of >25 km. Approximately 25–30 Myr later, the rocks underwent heating, interpreted to be due to magmatic underplating, and uplift. Presumably, the northern part of the belt was also affected by this tectonism, but buried to a lower depth, and therefore metamorphosed to lower grades.  相似文献   

Petrogenesis and tectonic setting of Cadomian felsic igneous rocks, Sandıklı area of the western Taurides, Turkey     

Semih Gürsu  M. Cemal Göncüoglu 《International Journal of Earth Sciences》2006,95(5):741-757

In the Sandıklı (Afyon) region, western Taurides, the Late Proterozoic rocks of the Sandıklı basement complex are composed of low-grade meta-sedimentary rocks (Güvercinoluk Formation) intruded by felsic rocks (Kestel Cayı Porphyroid Suite, KCPS). The KCPS is a deformed and highly sheared, dome-shaped rhyolitic body with a granitic core. Quartz porphyry dikes intrude both the slightly metamorphic igneous and the sedimentary rocks of the basement complex. Both the quartz porphyries and rhyolites were converted into mylonites with relict igneous textures. Geochemical data show that these felsic igneous rocks are subalkaline and mainly granitic in composition with SiO₂ >72 wt% and Al₂O₃ >11.5 wt%. The chondrite-normalized incompatible trace element patterns are characterized by distinct negative Rb, Nb, Sr, P, Ti, and Eu with enrichment in Th, U, La, Ce, Nd, Sm, and Zr. The REE patterns of the felsic rocks indicate a strong enrichment in LREE but display slightly flat HREE patterns. According to geochemical characteristics and petrogenetic modeling, extrusive and intrusive rocks of the KCPS were probably derived from an upper continental crustal source (partial melting of granites/felsic rocks) by 18–20% fractional melting plus 18–20% Rayleigh fractional crystallization, which seems to be the most effective igneous process during the crystallization of the KCPS. Single zircon age data from the granitoids and fossils from the disconformably overlying sedimentary successions indicate that the metamorphism and the igneous event in the Taurides are related to the Cadomian orogeny. Based on the geological, geochemical and petrogenetic correlation of the post-collisional granitoids it is further suggested that the Tauride belt in western central Turkey was in a similar tectonic setting to the Gondwanan terranes in North Africa (Younger Granitoids) and southern Europe (Spain, France, Bohemia, Brno Massifs) during the Late Cadomian period.  相似文献   

Petrogenesis and geotectonic setting of the PanAfrican basement rocks in Bamenda Massif, Obudu Plateau, southeastern Nigeria： Evidence from trace element geochemistry     

UKAEGBU V. U. EKWUEME B. N. 《中国地球化学学报》2006,25(2):122-131

1 IntroductionThe study area is part of the Obudu Plateau,which forms part of the Precambrian Pan-African tec-tonothermal belt, lying between the West African Cra-ton and the Gabon-Congo Craton ( Fig. 1). It isbounded by the Benue Trough, in the NE-SW axi…  相似文献   

The Migif–Hafafit gneissic complex of the Egyptian Eastern Desert: fold interference patterns involving multiply deformed sheath folds     

Abdel-Rahman Fowler  Baher El Kalioubi 《Tectonophysics》2002,346(3-4)

The Wadi Hafafit Complex (WHC) is an arcuate belt of orthogneisses, migmatites and other high-grade metamorphic rocks, which marks the boundary between the Central Eastern and the South Eastern Deserts of Egypt. In the WHC, gneissic meta-gabbro outlines macroscopic fold interference patterns characterized by elliptical to irregular culminations cored by gneissic meta-tonalite to meta-trondhjemite. The five main culminations of the WHC have previously been labeled A (most northerly), B, C, D and E (most southerly). A detailed structural investigation of B, C, D and E reveals that these structures are a result of the interference of four macroscopic fold phases, the first three of which may represent a single deformation event. The first folding involved sheath-like fold nappes, which were transported to the N or NW, assisted by translation on gently dipping mylonite zones. The regional gneissosity and mineral extension lineations formed during this folding event. The fold nappes were deformed by mainly open upright small macroscopic and mesocopic folds with approximately NE-trending hinges. As a probable continuation of the latter folding, the sheaths were buckled into large macroscopic folds and monoclines with the same NE-trends. The fourth macroscopic folding resulted from shortening along the NE–SW direction, producing mainly NW–SE-trending upright gently plunging folds. Gravitative uplift is disputed as a component of the deformation history of the WHC. The peculiarities of the fold interference pattern result from the interesting behaviour of sheath folds during their refolding.  相似文献   

Phénomènes karstiques fossiles et actuels au sein des formations métamorphiques silico-alumineuses de la nappe pan-africaine de Yaoundé (Sud-Cameroun)Fossil and present-time karstic phenomena in silico-aluminous metamorphic formations of the Pan-African nappe of Yaoundé (South-Cameroon)     

Jean-Paul Vicat  Hubert Mvondo  Luc Willems  André Pouclet 《Comptes Rendus Geoscience》2002,334(8):545-550

Numerous karstic features have been recognised in the non-carbonaceous micaschists and gneisses of the Yaoundé Pan-African nappe, south of Cameroon. It is shown that their formation was controlled by the structural features of the rocks. The wells and the pipes in the bedrock outcrops point out a current karstification process, resulting from the plagioclase dissolution by the acid rain waters. Hill wall alveoli and caves, of pre-Miocene age, are exhumed features that were done by dissolution in the aquiferous underground. To cite this article: J.-P. Vicat et al., C. R. Geoscience 334 (2002) 545–550.  相似文献   

Geology of the Grove Mountains in East Antarctica——New evidence for the final suture of Gondwana Land   总被引：4，自引：0，他引：4  

刘小汉  赵越  刘晓春  俞良军 《中国科学D辑(英文版)》2003,46(4)

As the core block of the East Gondwana Land, the East Antarctic Shield was traditionally thought, before 1992, as an amalgamation of a number of Archaean-Paleoproterozoic nuclei, be-ing welded by Grenville aged mobile belts during 1400—900 Ma, while the …  相似文献   

东南极格罗夫山泛非期麻粒岩相变质作用     

刘晓春 赵越刘小汉  赵子然 《地质论评》2003,49(4):422-431

东南极格罗夫山主要由麻粒岩相高级变质岩和花岗岩类组成，其中变质岩以浅色和暗色含斜方辉石长英质片麻岩占主导地位，夹有少量镁铁质麻粒岩、变沉积岩和含方柱石钙硅酸盐岩。这些岩石一般都展示了平衡的矿物共生结构，但在镁铁质麻粒岩的单斜辉石中普遍发育斜方辉石(易变辉石)的出溶片晶。根据出溶辉石的重组分析获得麻粒岩相变质作用的峰期温度约为850℃，而浅色片麻岩中的石榴子石—斜方辉石—斜长石—石英组合给出的变质压力为0．61～0．67GPa。镁铁质麻粒岩中火成亚钙质普通辉石斑晶的保存表明格罗夫山地区可能只发育单一的泛非期高温麻粒岩相变质事件，岩石在高温变质之后经历了缓慢冷却过程，这主要归因于花岗质岩浆的板底垫托作用。  相似文献   

Lower crustal melting and the role of open-system processes in the genesis of syn-orogenic quartz diorite–granite–leucogranite associations: constraints from Sr–Nd–O isotopes from the Bandombaai Complex, Namibia     

Tina van de Flierdt  Stephan Hoernes  Stefan Jung  Peter Masberg  Edgar Hoffer  Urs Schaltegger  Hans Friedrichsen 《Lithos》2003,67(3-4):205-226

The Bandombaai Complex (southern Kaoko Belt, Namibia) consists of three main intrusive rock types including metaluminous hornblende- and sphene-bearing quartz diorites, allanite-bearing granodiorites and granites, and peraluminous garnet- and muscovite-bearing leucogranites. Intrusion of the quartz diorites is constrained by a U–Pb zircon age of 540±3 Ma.

Quartz diorites, granodiorites and granites display heterogeneous initial Nd- and O isotope compositions (_{Nd (540 Ma)}=−6.3 to −19.8; δ¹⁸O=9.0–11.6‰) but rather low and uniform initial Sr isotope compositions (⁸⁷Sr/⁸⁶Sr_initial=0.70794–0.70982). Two leucogranites and one aplite have higher initial ⁸⁷Sr/⁸⁶Sr ratios (0.70828–0.71559), but similar initial _Nd (−11.9 to −15.8) and oxygen isotope values (10.5–12.9‰). The geochemical and isotopic characteristics of the Bandombaai Complex are distinct from other granitoids of the Kaoko Belt and the Central Zone of the Damara orogen. Our study suggests that the quartz diorites of the Bandombaai Complex are generated by melting of heterogeneous mafic lower crust. Based on a comparison with results from amphibolite-dehydration melting experiments, a lower crustal garnet- and amphibole-bearing metabasalt, probably enriched in K₂O, is a likely source rock for the quartz diorites. The granodiorites/granites show low Rb/Sr (<0.6) ratios and are probably generated by partial melting of meta-igneous (intermediate) lower crustal sources by amphibole-dehydration melting. Most of the leucogranites display higher Rb/Sr ratios (>1) and are most likely generated by biotite-dehydration melting of heterogeneous felsic lower crust. All segments of the lower crust underwent partial melting during the Pan-African orogeny at a time (540 Ma) when the middle crust of the central Damara orogen also underwent high T, medium P regional metamorphism and melting. Geochemical and isotope data from the Bandombaai Complex suggest that the Pan-African orogeny in this part of the orogen was not a major crust-forming episode. Instead, even the most primitive rock types of the region, the quartz diorites, represent recycled lower crustal material.  相似文献   

Terrane assembly and geodynamic evolution of central–western Hoggar: a synthesis     

Renaud Caby 《Journal of African Earth Sciences》2003,37(3-4):133

After a review of the rock sequences and evolution of the eastern and central terranes of Hoggar, this paper focusses on the Neoproterozoic subduction-related evolution and collision stages in the central–western part of the Tuareg shield. Rock sequences are described and compared with their counterparts identified in the western and the eastern terranes exposed in Hoggar and northern Mali. The Pharusian terrane that is described in detail, is floored in the east by the Iskel basement, a Mesoproterozoic arc-type terrane cratonized around 840 Ma and in the southeast by Late Paleoproterozoic rock sequences (1.85–1.75 Ga) similar to those from northwestern Hoggar. Unconformable Late Neoproterozoic volcanosedimentary formations that mainly encompass volcanic greywackes were deposited in troughs adjacent to subduction-related andesitic volcanic ridges during the c. 690–650 Ma period. Abundant arc-related pre-collisional calc-alkaline batholiths (650–635 Ma) intruded the volcanic and volcaniclastic units at rather shallow crustal levels prior to collisional processes. The main E–W shortening in the Pharusian arc-type crust occurred through several stages of transpression and produced overall greenschist facies regional metamorphism and upright folding, thus precluding significant crustal thickening. It was accompanied by the shallow emplacement of calc-alkaline batholiths and plutons. Ages of syn-collisional granitoids range from 620 Ma in the western terranes, to 580 Ma in the Pharusian terrane, thus indicating a severe diachronism. After infill of molassic basins unconformable above the Pan-African greenschists, renewed dextral transpression took place in longitudinal domains such as the Adrar fault. The lithology, volcanic and plutonic suites, deep greenschist facies metamorphism, structures and kinematics from the Adrar fault molassic belt previously considered as Neoproterozoic are described in detail. The younger late-kinematic plutons emplaced in the Pharusian terrane at 523 Ma [Lithos 45 (1998) 245] relate to a Cambrian tectonic pulse that post-dates molasse deposition. The new geodynamic scenario presented considers several paleosubductions. The major east-dipping subduction, corresponding to the closure of a large Pan-African oceanic domain in the west (680–620 Ma) post-dates an older west-dipping “Pharusian” subduction (690–650 Ma?) to the east of the eastern Pharusian terrane. Such a diachronism is suggested by the 690 Ma old eclogites of the western part of the LATEA terrane of central Hoggar [J. African Earth Sci. this volume (2003)] that are nearly synchronous with the building up of the Pharusian terrane, thus suggesting that the 4°50^′ lithospheric fault represents a reactivated cryptic suture.  相似文献   

Timing and exhumation of eclogite facies shear zones, Musgrave Block, central Australia   总被引：2，自引：0，他引：2  

A. CAMACHO  W. COMPSTON  M. McCULLOCH  & I. McDOUGALL 《Journal of Metamorphic Geology》1997,15(6):735-751

Timing constraints on shear zones can provide an insight into the kinematic and exhumation evolution of metamorphic belts. In the Musgrave Block, central Australia, granulite facies gneisses have been affected, to varying degrees, by mylonitic deformation, some of which attained eclogite facies. The Davenport Shear Zone is a dominant strike-slip system that formed at eclogite facies conditions ( T  ≈650  °C and P ≈12.0  kbar). Sm–Nd mineral isochrons obtained from equilibrated high-pressure assemblages, as well as ⁴⁰Ar–³⁹Ar data, show that the eclogite and greenschist facies high-strain overprints were coeval, at c .  550  Ma. Mylonitic processes do not appear to have reset the U–Pb system in zircon, but may have partially disturbed it. The thermal gradient in the Musgrave Block crust at c .  550  Ma was c .  16  °C  km⁻¹ and at c .  535  Ma was c .  18  °C  km⁻¹, based on P – T  estimates of eclogite and greenschist facies shear zones, respectively. These estimates are similar to present-day geothermal gradients in many stable continental shield areas, suggesting that the region did not undergo a significant transient perturbation of the geotherm. Therefore, in the Musgrave Block, cooling subsequent to eclogite facies metamorphism appears to have been controlled by exhumation, rather than by the removal of a heat source. Estimated exhumation rates in the range 0.2 to ≥1.5  mm year⁻¹ are comparable with other orogenic belts, rather than cratonic areas elsewhere.  相似文献