The structural history of the Lewisian rocks north-west of Loch Tollie, Ross-shire, Scotland

Bhattacharjee, C. C.

doi:10.1144/sjg04030235

Cited by 18 publications

(6 citation statements)

References 14 publications

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“…The structural history of the Gairloch outcrop and surrounding basement has been studied by Park (1964Park ( ,1970, Bhattacharjee (1968) and , each employing different terminology (Table 2). Some deformational events (e.g the Tollie Antiform phase) can be readily correlated between these studies, but other correlations (particularly the early ones) are less secure.…”

Section: Gairloch Areamentioning

confidence: 99%

Laxfordian metamorphic conditions of the Palaeoproterozoic Loch Maree Group, Lewisian Complex, NW Scotland

Droop

Fernandes

Shaw

1999

SJG

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SynopsisThe supracrustal rocks of the Loch Maree Group (LMG), which form a Palaeoproterozoic cover to the Scourian basement of the Lewisian complex, underwent amphibolite-facies regional metamorphism during the Laxfordian orogeny. Micro-structural studies indicate that the peak of metamorphism in both the Loch Maree and Gairloch outcrops of the LMG was preceded by at least one major phase of ductile deformation, recorded by annealed lineations and foliations and by inclusion trails in minerals. At Loch Maree, this involved intense WNW-ESE stretching. The metamorphic peak occurred statically and was followed by (i) intense top-to-NW shearing, recorded by mylonitic fabrics particularly associated with the Loch Maree Thrust and (ii) several generations of open folding. These retrograde fabrics and folds reflect tectonically-induced uplift and exhumation during late Laxfordian times.Thermobarometric studies were undertaken on low-variance rocks that had experienced relatively little retrogressive deformation. A garnetiferous calc-silicate rock, an ironstone and two dolomitic marbles from the Gairloch outcrop yielded Laxfordian peak-metamorphic conditions of 530 ± 20°C and 6.5 ±1.5 kbar consistent with the epidote-amphibolite-facies assemblages. In general, the mylonites of Loch Maree outcrop proved unsuitable for thermobarometric work, but a protomylonitic garnet-staurolite-mica schist yielded peak-metamorphic conditions of 630 ± 30°C and 6.5 ±1.5 kbar. Temperatures during mylonitization were still high enough to stabilize garnet (i.e. > 470°C) which recrystallized as tiny neoblasts in the mylonites.The peak-metamorphic pressures imply burial of the supracrustal LMG to c. 24 ± 5 km, requiring substantial early Laxfordian tectonic thickening of the crust. Thus the main phase of basement-cover interleaving occurred before the development of the mylonitic fabric marked by the Loch Maree Thrust. Geothermal gradients at the metamorphic peak were moderate (c. 20-30°C km -1 ), consistent with metamorphism in tectonically thickened crust. Existing tectonic models for the Laxfordian fail to account for the tectonic burial of the LMG to mid-crustal depths.

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Section: Gairloch Areamentioning

confidence: 99%

Laxfordian metamorphic conditions of the Palaeoproterozoic Loch Maree Group, Lewisian Complex, NW Scotland

Droop

Fernandes

Shaw

1999

SJG

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“…They consider D1 and D2 to be a composite a fabric related to progressive early Laxfordian deformation. D3 and D4 were associated with post peak metamorphism retrogressive events (Park, 1964;Bhattacharjee, 1968;Park et al, 1987;Shihe and Park, 1993;Droop et al, 1999). D3 is associated with recumbent folds on steeply dipping F2 folds (Droop et al, 1999).…”

Section: Stratigraphy and Lithologiesmentioning

confidence: 99%

“…The LMG has been studied extensively, particularly its structural framework (Peach et al, 1907;Park, 1964;Bhattacharjee, 1968;Park et al, 1987Park et al, , 2001Droop et al, 1999) and tectonic origin (Park et al, 2001). Far fewer studies have focused on VMS mineralization in the LMG and their utility in enhancing understanding of the regional geology (Jones et al, 1987;Colman and Cooper, 2000).…”

mentioning

confidence: 99%

Petrogenesis and geochemical halos of the amphibolite facies, Lower Proterozoic, Kerry Road volcanogenic massive sulfide deposit, Loch Maree Group, Gairloch, NW Scotland

Drummond

Cloutier

Boyce

et al. 2020

Ore Geology Reviews

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The Palaeoproterozoic Kerry Road deposit is one of the oldest examples of volcanogenic massive sulfide (VMS) mineralization. This small VMS deposit (~500,000 tons grading at 1.2% Cu, 3.5% Zn) is hosted in amphibolite facies mafic-siliciclastic units of the c. 2.0 Ga Loch Maree Group, Scotland. Sulfide mineralization consists of pyrite and pyrrhotite with subordinate chalcopyrite and sphalerite, occurring in disseminated, vein and semi-massive to massive textures.The deposit was highly deformed and metamorphosed during the c. 1.8-1.7 Ga Laxfordian Orogeny. Textural relationships of deformed sulfide minerals, related to early Laxfordian deformation (D1/D2), indicate initial high pressure-low temperature (100 MPa, 150ºC) conditions before reaching peak amphibolite facies metamorphism, as evident from pyrrhotite crossing the brittle/ductile transition prior to chalcopyrite. Late Laxfordian deformation (D3/D4) is marked by local retrograde greenschist facies at low pressure and temperature (<1.2MPa, <200°C), recorded by late red sphalerite remobilization. 34 S values from all sulfide minerals have a homogeneous mean of 0.8 ± 0.7 ‰ (n=21), consistent with interaction of hydrothermal fluids in the host oceanic basalt-island arc setting envisaged for deposition of the Loch Maree Group.Microprobe analyses of amphiboles record evidence of the original alteration halo associated with the Kerry Road deposit, with a systematic Mg-and Si-enrichment from ferrotschermakite (~150 m) to Mg-hornblende (~90 m) to actinolite (0 m) on approach to the VMS deposit. Furthermore, whole rock geochemistry records a progressive enrichment in Si, Cu, Co, and S, and depletion in Al, Ti, V, Cr, Y and Zr with proximity to the VMS system. These elemental trends, together with amphibole geochemistry, are potentially useful 3 exploration vectors to VMS mineralization in the Loch Maree Group, and in similar highly deformed and metamorphosed terranes elsewhere.

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“…The sequence of events in the metasedimentarj schist regime (Table 3) shows the effects of polyphase deformation during the Laxfordian episode on sediments deposited c. 2.1 b.y. ago (Bhattacharjee 1963(Bhattacharjee , 1968Keppie 1969;Bikerman et al 1975). The dominant fold-forming phase (F3) corresponds with the dominant fold-forming phase in the gneisses with which c. 1740 m.y.…”

Section: Scotlandmentioning

confidence: 99%

Scotland–Finland Precambrian correlations

Bowes¹

1975

Bull Geol Soc Finland

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Isotopic age determinations indicate the generally co-eval nature of major episodes of earth history shown by the Precambrian rocks of Scotland and Finland, viz. (1) the c. 2 800-2 600 m.y. Scourian and Presvecokarelidic episodes, (2) the c. 2 200 m.y. Inverian episode and the c. 2 300 m.y. episode in Finland, (3) the c. 2.1 b.y. deposition of the Loch Maree Group in Scotland and the Jatulian quartzite, Karelian schists and related geosynclinal deposits in Finland, and (4) the c. 1 975-1 700 m.y. Laxfordian and Svecokarelian episodes. The existence of spatially connected belts in Precambrian times is suggested by the correspondence in both the sequence of development and the orientations of structural elements resulting from polyphase deformation in the successively formed Precambrian orogenic belts. This matching of complex structural frameworks implies that, before the initiation of the British-Scandinavian Caledonides, the Baltic Shield extended westward to include northwestern Britain and the continental shelf of the eastern North Atlantic Ocean. The Caledonian belt would then have developed on a plate of continental crust, or between the separated parts of a rifted continental crustal segment.

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The structural history of the Lewisian rocks north-west of Loch Tollie, Ross-shire, Scotland

Cited by 18 publications

References 14 publications

Laxfordian metamorphic conditions of the Palaeoproterozoic Loch Maree Group, Lewisian Complex, NW Scotland

Laxfordian metamorphic conditions of the Palaeoproterozoic Loch Maree Group, Lewisian Complex, NW Scotland

Petrogenesis and geochemical halos of the amphibolite facies, Lower Proterozoic, Kerry Road volcanogenic massive sulfide deposit, Loch Maree Group, Gairloch, NW Scotland

Scotland–Finland Precambrian correlations

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