Mid-amphibolite Facies Research Articles

Aluminous reaction textures in orthoamphibole‐bearing rocks: the pressure–temperature evolution of the high‐grade Proterozoic of the Bamble sector, south Norway

Aluminous reaction textures in orthoamphibole‐bearing rocks from the Froland area, Bamble, south Norway, record the prograde pressure–temperature path of the high‐grade Kongsbergian Orogeny (c. 1600–1500 Ma) and the low–mid amphibolite facies overprint during the Sveconorwegian Orogeny (c. 1100–1000 Ma). The rocks contain anthophyllite/gedrite, garnet, cordierite, biotite, quartz, andalusite, kyanite, Cr‐rich staurolite, tourmaline, ilmenite, rutile and corundum in a variety of parageneses. The P–T path is deduced from petrographic observations, mineral chemistry and zoning, geothermometry and (N)FMASH equilibria. The results indicate the sequence of metamorphic stages outlined below. (a) An M1 phase characterized by the presence of strongly deformed andalusite, gedrite and tourmaline.(b) An M2 phase with the development of kyanite after andalusite and the growth of staurolite associated with strong Na–Al–Mg zoning in orthoamphibole, indicating an increase in pressure (4 8 kbar) and temperature (500° 650°C).(c) Pressure decrease at high P (6–7 kbar) and high T (600–700 °C) during M3a with the production of cordierite ° Corundum between kyanite, staurolite and orthoamphibole and cordierite growth between corundum and orthoamphibole.(d) Temperature increase to 740 ± 60 °C and 7 kbar; static growth of garnet (M3b) at the metamorphic climax (peak T). The heat supply necessary to explain the temperature increase between the M3a and M3b phases is correlated with synkinematic enderbitic–charnockitic and basic intrusions in the Arendal granulite facies terrain.(e) M3b metamorphic conditions were followed by an initial isobaric cooling path (early M4) and late‐stage pressure decrease (late M4). Early M4 conditions of 6–7 kbar and 550–600 °C, assuming PH2O < Ptotal are indicated by a retrograde talc–kyanite–quartz assemblage in late quartz–cordierite veins. Late M4 conditions of 3–4 kbar and 420–530 °C are inferred from a kyanite–andalusite–chlorite–quartz assemblage in vein‐cordierite. The M1–M3 stages are interpreted as being the result of the same metamorphic P–T path, which was caused by both tectonic and magmatic thickening. A prolonged crustal residence time is proposed for the Bamble sector before uplift during the later stages of M4 occurred.

The petrology of the Harkerbreen Group, Ny Friesland, Svalbard: protoliths and tectonic significance

AbstractThe late Precambrian–early Palaeozoic rocks of Ny Friesland, which have been subjected to Caledonian deformation and metamorphism, constitute part of the Eastern Province or Terrane of Svalbard. The Harkerbreen group and other divisions of the Stubendorffbreen supergroup form a high-grade and intensely deformed core complex to this terrane which is bounded to the west by the Billefjorden Fault Zone and to the east by a major north–south shear zone. The Stubendorffbreen rocks exhibit two gneissic foliations, one axial planar to a large scale, F1fold nappe closing to the east and the other axial planar to kilometre-scale upright F2folds subsidiary to the Atomfjella Arch. Metamorphism in the mid-amphibolite facies range coincided with generation of these folds, and F3.crenulation folding was accompanied by waningP–Tconditions. A significant proportion of the gneisses within the Harkerbreen group display silica–major element covariation patterns consistent with their position in the granodiorite field on the AFM plot. Incompatible, immobile element ratios Zr/Ti v. Nb/Y indicate affinities with rhyolites to rhyodacites which is also suggested by their REE profiles. Normalized multi-element plots of the gneisses are similar to those of granites from attenuated within-plate settings such as Mull and Skaergaard. The amphibolites which were intruded in the D1–D2interval appear to be derivatives of fractionated basalts. They plot across the calk-alkaline tholeiite boundary on the AFM diagram, and the calc-alkaline character of some of the amphibolites is further suggested by their Yb-normalized Ce-Ta abundances. Zr-Ti-Y and REE abundances would support their derivation from a related suite of fractionated basalt liquids. On the Zr/Y v. Zr discrimination diagram the amphibolites appear to have compositions transitional between Mid Ocean Ridge and Within-Plate basalts whilst the Zr-Nb-Ta plot indicates Volcanic Arc Basalt affinities. Th-Hf-Ta and multi-element plots, however, indicate a marginal to back-arc basin setting possibly above a mature subduction zone. The late Caledonian Chydenius granite is an adamellite with mixed within-plate and syn-orogenic characteristics typical of post-collisional granites.

Grenville-Age, Polyphase Deformation of Mid-Proterozoic Basement, NW Van Horn Mountains, Trans-Pecos, Texas

Psammitic, pelitic, and mafic schists in a basement horst in the NW Van Horn Mountains of west Texas preserve evidence of a multiphase, synmetamorphic, Grenville-age ductile deformation. Three phases of progressive isoclinal folding followed by two later, less intense, nearly coaxial folding phases have been identified. The second phase of deformation produced the dominant foliation and foliatipn-intersection lineation observed throughout the area. This deformation is inferred to have resulted from a regional-scale folding, possibly associated with nappe formation, at or near peak metamorphic conditions. Mineral assemblages indicate amphibolite facies, and Fe-Mg exchange thermometry provide maximum temperatures of corresponding to mid-amphibolite facies conditions. Three subsequent fold generations () formed under progressively lower metamorphic conditions; produced a weakly developed foliation ( ) whereas neither of the two later folding phases formed a penetrative fabric. Fluids or heat associated with the widespread intrusion of late syn- to post-orogenic pegmatites facilitated a static recrystallization event which altered previously formed deformational fabrics. Other basement exposures in the Van Horn area record a later phase of Grenville-age brittle deformation that has wholly or partly obscured the earlier-formed ductile features. The Grenville-age, progressive deformational history recorded in the basement rocks of the Van Horn area is similar to that noted in portions of the Llano Uplift of central Texas and in the Adirondack Mountains of New York and suggests that the Grenville orogeny was characterized by oblique convergence along the length of the North American craton.

Basement–cover relationships and structure within the Moine rocks of central and southeast Sutherland

Inliers of Archaean to early Proterozoic Lewisian basement crop out within the Proterozoic Moine Succession of central and SE Sutherland. Both the Lewisian basement and the Moine cover have been affected by two major tectonic events. The earliest event probably occurred during the late Proterozoic and involved minor folding and upper greenschist to upper amphibolite facies metamorphism. Intense deformation largely obliterated the unconformable relationship between the Lewisian and Moine rocks, and imposed a planar banding on the basement lithologies. Subsequent reworking during the Caledonian orogeny (500–400 Ma) resulted in ductile thrusting and folding, and was associated with a widespread low- to mid-amphibolite facies metamorphic overprint. Within this context the Lewisian inliers of central and SE Sutherland occur in two distinct structural settings: (i) as allochthonous sheets which rest on major Caledonian ductile thrusts; (ii) in the cores of NW-verging intensely curvilinear sheath folds of Caledonian age.

Element mobility associated with syn‐metamorphic shear zones near Scotchport, NW Mayo, Ireland

Abstract Three major blastomylonitic synmetamorphic (epidote amphibolite to mid amphibolite facies) shear zones are seen on the NW coast of the Mullet Peninsula in NW Mayo. These shear zones occur at the contacts of major structural units and in an imbricated slice where rocks of the Erris Complex are deformed and chemically modified. Chemical changes associated with individual shear zones have been deduced by comparing the compositions of various gneisses both within and adjacent to the shear zones. Compositional changes are different in the constituent rock‐types within each unit and many elements normally considered immobile have been selectively mobilized within the shear zones. Little evidence of wholesale metasomatic introduction of components into these shear zones was found to accompany the selective mobilization.

Discussion

D r A. L. H arris remarked that the recognition by Drs Powell and Johnson of two orogenic periods in the western Highlands emphasized the need for strict criteria in the recognition of pre- and post tectonic gneissification. Each orogenic episode is apparently associated with the development of pelitic gneisses, referred to on the Geological Survey maps as 'migmatites'; the tracing of the Caledonian overprint on rocks metamorphosed in the Precambrian will depend partly on the recognition of deformed 'migmatites.' The quartzofeldspathic fraction of the pelitic gneisses occurring as pegmatitic veinlets or lits commonly conforms to minor fold structures, but this in itself is an insufficient criterion of deformation. Such 'folded' veinlets may be the result of post deformation segregation along already folded surfaces. The nature of the fabric within the quartzofeldspathic lits is critical. In the 'old' pegmatites, demonstrated by Dr van Breemen and his colleagues to have suffered D 2 deformation, there is in addition to a mica foliation, a strong planar fabric defined by the preferred dimensional orientation of quartz and feldspar. The fact that this has survived prolonged Caledonian metamorphism in at least mid-amphibolite facies, is a strong argument in favour of the use of such fabrics as the only safe criterion for pre-tectonic gneisses. Clearly if the preferred orientation of quartz and feldspar could be broken down by prolonged static crystallization, the deformed gneisses would come closely to resemble the product of post tectonic gneissification. Their recognition as pre-tectonic gneisses would then be a matter of subjective judgement.

The Tremolite Isograd near Marble Lake, Ontario

Marble, metavolcanic rocks, and pelite are found in a northeasterly trending belt near Marble Lake, in the Grenville Province, Ontario. The rocks have been metamorphosed to the lower amphibolite facies in the southwest, the grade increasing to the mid-amphibolite facies towards the northeast. Northwest-trending isograds in the metavolcanic rocks are at a high angle to the northeast-trending tremolite isograd in the marbles. Mineral assemblages indicate total pressures between 4 and 5 kbar and temperatures ranging from approximately 350 °C to over 600 °C. Temperatures estimated by calcite–dolomite solvus geothermometry and applied to experimental work in the system CaO–MgO–SiO2–CO2–H2O indicate: (i) P(total) = P(CO2) + P(H2O) was greater than 3 kbars; (ii) temperatures on the tremolite isograd were from approximately 450 to 550 °C and indicate that the tremolite isograd is not isothermal; (iii) the composition of the vapor phase present during metamorphism was approximately X(CO2) = 0.7 – 0.8; (iv) temperatures in the belt were from less than 400 °C in the southwest to more than 600 °C in the northeast.