Structure and composition of the Precambrian crust along the POLAR Profile in the northern Baltic Shield

The study area is located at the northeastern margin of Tarim basin. Structural analysis reveals that the complex structural deformation formed at different phases. The present structural framework is mainly composed of a western Horse Racing Mountains (HRM) along-bedding detachment ductile deformation zone, Yonghong Mountains nappe and western HRM-Yonghong Mountains dome-basin. The detachment zone occurs within the Xinditag Group of the Paleo-Proterozoic and the Changcheng System of the Middle Proterozoic. The nappe is composed of the Xinditag Group and thrust over a structural basin composed of the Changcheng System. The dome-basin structure influences not only the Changcheng era but also the underlying Xinditage Group. The three structural elements have a close temporal and dimensional relationship. The ductile deformation zone is involved into the dome-basin structure thrust over by the nappe which has the same folding and overlapping structures as the ductile zone and syncline structures corresponding with the main phase of folding structure of the dome-basin structure. Three phases of tectonic movements are thus determined during the Middle Proterozoic. The first phase happened at the early Middle Proterozoic during which large scale along-bedding detachment ductile deformation developed at central depth due to EW-trending extension. The deformation of this phase has features of plastic deformation and multi-layer structures. The second phase occurred at the end of Changcheng Era during which EW-trending tight folds (locally overturned) were developed at shallow depth due to N-S compression. The deformation of this phase influences the earlier ductile deformation. The Yonghong Mountains nappe with overlapping structures formed at shallow depth during the latest phase of folding due to northward thrusting along weak zone of the Xinditage Group. The third phase took place at the end of the Jixian Era during which NS-trending open folds formed by EW-trending compression caused by relaxation of NS-trending intense compression. The HRM-Yonghong Mountains dome-basin formed in a framework of multiphase deformation and resulted from overlapping of earlier structures.

Syncline

Sichuan basin

Dome (geology)

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Early Proterozoic Metavolcanic Suites of the Northernmost Part of the Baltic Shield

Geological Society London Special Publications (1987)

T. C. Pharaoh Amber Warren Nathaniel John Walsh

Summary Metavolcanic rocks of Proterozoic age occur in three major supracrustal belts in the northernmost part of the Baltic Shield, in Norway and the USSR. In the west, the Kautokeino Belt , and its continuation into the tectonic windows of Finnmark, contains thick sequences of tholeiitic metabasalt the age of which is poorly constrained at present. In the Repparfjord Window, these basalts are strongly enriched in Fe, Ti, LIL and LREE and are chemically similar to modern continental flood basalts. Stratigraphic equivalents in the Alta area and Kautokeino Belt are less enriched in incompatible elements, and more like MORB in composition, although a within-plate setting is indicated by the field evidence. These differences are believed to reflect heterogeneity of the mantle source region, with a locally ‘enriched’ source in the Finnmark windows. The Karasjok Belt of central Finnmark forms the northward continuation of the Kittilä Belt of northern Finland. Amphibolitic rocks derived by metamorphism of tholeiitic basalts dominate the tectonostratigraphy of the greenstone belt. These are associated with metamorphosed komatiitic rocks which have recently yielded a Sm-Nd age of 2.1 Ga. The Karasjok Belt is interpreted as an early Proterozoic supracrustal sequence thrust westwards as an allochthonous sheet during uplift of the Lapland Granulite Complex in the Svecokarelian Orogeny at about 1.9 Ga. The Pechenga Complex of the USSR continues westward into Finnmark, and overlies the Archaean gneisses of the Kola Complex. The Kola Superdeep Borehole penetrated a metamorphosed supracrustal sequence 7 km thick, dominated by basalts and andesites of calcalkaline composition. The MORB-normalized geochemical patterns of these lavas indicate that they were erupted in a mature continental volcanic arc. The geochemical data support geotectonic models invoking rifting of pre-existing continental crust in the early Proterozoic. Komatiitic liquids were locally able to reach the earth’s surface, but probably only in the rift zones suffering greatest extension. Calcalkaline arc magmatic suites along the western edge of the Kola Craton reflect subduction of‘oceanic’ lithosphere, although no trace of this is preserved along the suture now marked by the Karasjok Belt.

Baltic Shield

10.1144/gsl.sp.1987.033.01.04

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Petrological and geochronological studies on Proterozoic ultrahigh temperature granulites from the southern part of Madagascar

Nirihaja Othon Thierry Rakotonandrasana

Geochronology

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The metamorphic histories of some proterozoic granulites from East Antarctica

I. C. W. Fitzsimons

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Terrane accretion and upward extrusion of high‐pressure granulites in the Neoproterozoic nappes of southeast Brazil: Petrologic and structural constraints

Tectonics (2000)

Mário da Costa Campos Neto Renaud Caby

The high‐grade crystalline nappes exposed southeast of the São Francisco craton comprise two distinct units of mainly granulite facies rocks that represent a composite section of Neoproterozoic deep continental crust: the Socorro‐Guaxupé nappe above, derived from an arc terrane, and the Três Pontas‐Varginha nappe below. Metamorphism in the Três 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‐Guaxupé 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 São Francisco craton and its platform cover, with at most, anchizonal to greenschist‐facies metamorphism, occurred around 600 Ma.

Anatexis

10.1029/1999tc900065

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