Thermal structure and tectonic evolution of the Scandian orogenic wedge, <scp>S</scp>cottish Caledonides: integrating geothermometry, deformation temperatures and conceptual kinematic‐thermal models

Thigpen, J. Ryan; Law, Richard D.; Loehn, C.; Strachan, R. A.; Tracy, Robert J.; Lloyd, Geoffrey E.; Roth, Bl L.; Brown, Sj J.

doi:10.1111/jmg.12046

Cited by 41 publications

(55 citation statements)

References 85 publications

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“…Garnet is present in many textures and has been extensively used to decipher complex mineral growth–deformation relationships (e.g. Cutts et al, ; George & Gaidies, ; Kelly, Hoisch, Wells, Vervoort, & Beyene, ; Thigpen et al, ). Garnet commonly shows heterogeneous non‐equilibrium compositions, most commonly with a continuous chemical core–rim zonation.…”

Section: Introductionmentioning

confidence: 99%

Decoding the complex internal chemical structure of garnet porphyroblasts from the Zermatt area, Western Alps

Bucher

Weisenberger

Klemm³

et al. 2019

Journal Metamorphic Geology

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Garnet is a prototypical mineral in metamorphic rocks because it commonly preserves chemical and textural features that can be used for untangling its metamorphic development. Large garnet porphyroblasts may show extremely complex internal structures as a result of a polycyclic growth history, deformation, and modification of growth structures by intra‐ and intercrystalline diffusion. The complex internal structure of garnet porphyroblasts from garnet–phengite schists (GPS) of the Zermatt area (Western Alps) has been successfully decoded. The centimetre‐sized garnet porphyroblasts are composed of granulite facies garnet fragments overgrown by a younger generation of grossular‐rich eclogite facies garnet. The early granulite facies garnet (G‐Grt) formed from low‐P, high‐T metamorphism during a pre‐Alpine orogenic event. The late garnet (E‐Grt) is typical of high‐pressure, low‐temperature (HPLT) metamorphism and can be related to Alpine subduction of the schists. Thus, the garnet of the GPS are polycyclic (polymetamorphic). G‐Grt formation occurred at ~670 MPa and 780°C, E‐Grt formed at ~1.7 GPa and 530°C. The G‐Grt is relatively rich in Prp and poor in Grs, while E‐Grt is rich in Grs and poor in Prp. The Alm content (mol.%) of G‐Grt is 68 of E‐Grt 55. After formation of E‐Grt between and around fragmented G‐Grt at 530°C, the GPS have been further subducted and reached a maximum temperature of 580°C before exhumation started. Garnet composition profiles indicate that the initially very sharp contacts between the granulite facies fragments of G‐Grt and fracture seals of HPLT garnet (E‐Grt) have been modified by cation diffusion. The profiles suggest that Ca did not exchange at the scale of 1 µm, whereas Fe and Mg did efficiently diffuse at the derived maximum temperature of 580°C for the GPS at the scale of 7–8 µm. The Grt–Grt diffusion profiles resulted from spending c. 10 Ma at 530–580°C along the P–T–t path. The measured Grt composition profiles are consistent with diffusivities of log DMgFe = −25.8 m2/s from modelled diffusion profiles. Mg loss by diffusion from G‐Grt is compensated by Fe gain by diffusion from E‐Grt to maintain charge balance. This leads to a distinctive Fe concentration profile typical of uphill diffusion.

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Section: Introductionmentioning

confidence: 99%

Decoding the complex internal chemical structure of garnet porphyroblasts from the Zermatt area, Western Alps

Bucher

Weisenberger

Klemm³

et al. 2019

Journal Metamorphic Geology

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“…Indeed, there is a geochronological evidence which suggests that 'D2' in the northern Moine Nappe in Sutherland formed during the Scandian event (e.g. Kinny et al 2003;Thigpen et al 2013, but see Bird et al 2013); whilst 'D2' in the Sgurr Beag Nappe relates to the Grampian event (Cutts et al 2010).…”

Section: Deformation Phases Fabrics and Foldsmentioning

confidence: 99%

“…During D3, micas are deformed (crenulated), rather than growing along axial planes, and the fold geometry shows that D3 operated in a similar overall strain regime as D2, except under cooler conditions (see also Thigpen et al 2013). On a large-scale, the geometry and kinematics of D2 and D3 deformation in Knoydart are very similar to those established in the Moine Nappe in Ross-shire ; in both areas D2 and D3 relate to overall non-coaxial deformation, resulting in major folds that verge and face towards the west, consistent with overall WNW-directed Caledonian transport of the Moine Thrust (e.g.…”

Section: Timing Of Eventsmentioning

confidence: 99%

Structure and stratigraphy of the Morar Group in Knoydart, NW Highlands: implications for the history of the Moine Nappe and stratigraphic links between the Moine and Torridonian successions

Krabbendam

Leslie

Goodenough

2014

SJG

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The Caledonian Orogen in Northern Scotland comprises two major thrust nappes: the Moine and the Sgurr Beag Nappe. The Moine Nappe contains early Neoproterozoic Morar Group rocks (Moine Supergroup) and basement inliers. This paper describes the structure and stratigraphy of the Knoydart peninsula, a key area within the southern Moine Nappe. The geology of Knoydart is dominated by a thick internally coherent sequence of Morar Group rocks. This sequence is shown to be deformed by large-scale, west-vergent and west-facing Caledonian (early Palaeozoic) folds that represent D2 within the southern Moine Nappe. Subsequent D3 deformation led to refolding or tightening of F2 folds, so that the major Morar Antiform is in essence a composite F2/F3 fold. F2 and F3 folds are broadly co-axial, but F3 folds have steeper axial planes.The F2/F3 folds refold a regional-scale, originally recumbent, isoclinal F1 fold nappe of probable Knoydartian (mid-Neoproterozoic) age. The F1 fold nappe is cored by a thin sliver of basement gneiss; the lower limb comprises migmatitic Morar Group rocks, exposed in the Morar Window. The upper limb of the F1 fold nappe occupies most of Knoydart and is stratigraphically coherent and right-way-up. Within this sequence, the upper unit of the Lower Morar Psammite is barely deformed, preserving trough-cross-bedding and largescale channels in thick beds. This suggests braided river deposition, similar to the Torridon Group west of the Moine Thrust and the Morar Group in the northern part of the Moine Nappe. On the basis of lithological similarity and stratigraphic disposition, it is suggested that the lowermost part of the Morar Group in Knoydart correlates with the Neoproterozoic Sleat Group on Skye.

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“…For peak temperature, quartz c‐axis fabric opening angle temperatures by Thigpen et al . () were used for sample MT‐10‐01 (~12 km north of MT‐13‐02; Fig. ).…”

Section: Additional Findings From Petrographic Assessmentmentioning

confidence: 99%

“…For example, in metamorphic petrology studies using garnet chemistry to understand P–T histories and thermal evolution during the mid‐Silurian (Scandian) orogenic event (e.g. Thigpen et al ., ; Ashley et al ., ), the presence of mid‐Ordovician (Grampian) or late Proterozoic (Knoydartian) garnet (e.g. Vance et al ., ; Cutts et al ., ; Bird et al ., ) would limit analyses of this type, unless Scandian heating was of sufficiently high temperature and duration to re‐equilibrate garnet chemistries to reflect Silurian P–T conditions.…”

Section: Introductionmentioning

confidence: 99%

Garnet morphology distribution in the northern part of the Moine Supergroup, Scottish Caledonides

Ashley

Law

Thigpen

2016

Journal Metamorphic Geology

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Garnet is a versatile and useful indicator mineral exploited in numerous geological studies. Despite its utility in providing thermobarometry and geochronology constraints, many difficulties remain in making meaningful interpretations of such data. In this paper, we characterize garnet grains from over 140 garnet-bearing metasedimentary rock samples collected from the northern part of the Moine Supergroup (Scottish Caledonides). Large, euhedral garnet grains are interpreted to be indicative of prograde metamorphic growth during the most recent (Scandian, c. 430 Ma) phase of orogenesis. Anhedral garnet is largely restricted to the relatively low-grade (greenschistlower amphibolite facies) Moine thrust sheet, with an abrupt change in morphology and grain size when traced across the overlying Ben Hope and Sgurr Beag thrusts into the higher grade, more hinterland-positioned thrust sheets. Our results suggest that caution should be exercised in using anhedral garnet in the Moine thrust sheet to estimate peak P-T conditions associated with low temperature (<~500°C) Scandian metamorphism, because in at least some cases garnet growth may have occurred during an earlier metamorphic event. However, chemical and isotopic data from this structurally lower anhedral garnet may still be useful for deconvolving a possible polymetamorphic history for this thrust sheet. In the immediately overlying Ben Hope thrust sheet, garnet has prograde euhedral (Scandian?) rims, indicating that the Ben Hope thrust must represent a significant thermal and/or chemical break. Inclusion distribution and mineral assemblages in garnet have been used to gain further insight on garnet growth conditions and to distinguish garnet that likely contains multiple generations of growth. Although our results are specific to the Caledonides of northern Scotland, this work highlights the general necessity of a comprehensive petrographic assessment in advance of interpreting large suites of garnet-derived thermodynamic and geochronological data. Fig. 3. Detailed geological map and cross-section of the Sgurr Beag thrust sheet and underlying Moine thrust sheet exposed within the Fannich Mountains. A klippe of the thrust sheet is preserved in the NW part of the area. Garnet morphologies are shown, with anhedral (rounded) garnet typically located within the Moine thrust sheet and euhedral (or subhedral) garnet typically within the Sgurr Beag thrust sheet. After British Geological Survey (2011).

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Thermal structure and tectonic evolution of the Scandian orogenic wedge, Scottish Caledonides: integrating geothermometry, deformation temperatures and conceptual kinematic‐thermal models

Cited by 41 publications

References 85 publications

Decoding the complex internal chemical structure of garnet porphyroblasts from the Zermatt area, Western Alps

Decoding the complex internal chemical structure of garnet porphyroblasts from the Zermatt area, Western Alps

Structure and stratigraphy of the Morar Group in Knoydart, NW Highlands: implications for the history of the Moine Nappe and stratigraphic links between the Moine and Torridonian successions

Garnet morphology distribution in the northern part of the Moine Supergroup, Scottish Caledonides

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