TEM and XRD techniques were used to study crystal growth characteristics of the fabric-forming phyllosilicates which developed in response to low-grade metamorphism and tectonic imbrication in part of the Southern Uplands thrust terrane. Prograde regional metamorphism, ranging from late diagenesis through the anchizone to the epizone, was accompanied by the development of a slaty cleavage which is commonly bedding-parallel. TEM-measured mean thicknesses of white mica and chlorite crystallite populations increase with advancing grade and correlate with XRD-measured crystallinity indices.Analytical TEM data show that prograde changes in composition lead to a net loss of Si, Ca and minor Fe from the fabric-forming phyllosilicates. White micas are paragonite-poor phengites with a mean b lattice parameter of 9.037 A, and indicate an intermediate pressure series of metamorphism with a field gradient of <25" C km-'. Chlorite compositions evolved from diabantite (with intergrown corrensite) to ripidolite over an estimated temperature range of 150-320" C. Field gradient and temperature estimates suggest that crystal growth and fabric development occurred at burial depths ranging from 6 km to at least 13 km in the thrust terrane. During late diagenesis, crystal growth of white mica and chlorite was predominantly a consequence of polytypic and phase transitions, and resulted in similar size distributions which resemble typical Ostwald ripening curves. Under anchizonal and epizonal conditions, white mica grew more rapidly than chlorite because of its greater ability to store strain energy and recover from subgrain development; as a result crystal thickness distributions are not typical of Ostwald ripening. In contrast, chlorite crystals which grew under these conditions developed subgrain boundaries at high strain rates which were only partially recovered at low strain rates; these retained dislocations reduce the crystallite thicknesses detected by TEM and XRD, compared with those of white mica. These differences in strain-induced crystal growth indicate that white mica (illite) and chlorite crystallinity indices are likely to show significant differences where low-grade metamorphism is closely associated with tectonic fabric development.
Studies have been made in two laboratories on metapelites from the Manx Group, Isle of Man, showing the distribution of white mica ‘crystallinity’, using the Kubler index (Δ°2Θ) and the Weber index (Hb rel ). Two isocryst maps, which show broadly similar distributions of metamorphic grade, are illustrated and a correlation between the laboratories is presented. The mineralogy of the <2 m fraction in relation to ‘crystallinity’ has been determined. The distribution of metamorphic grade over central and eastern parts of the Isle of Man, as depicted by the Kubler index isocryst map, is related to subjacent granite plutons, but these contact metamorphic effects overprint an earlier, largely anchizonal, regional metamorphism. Localized, late, retrograde effects are related to low temperature hydrothermal mineralization and to cold re-working in a large fault zone. Differences between the maps are largely in the degrees of resolution which stem directly from the different sampling intervals employed. Comparisons between the maps suggest a sampling interval of 1-2 km is adequate in regions of simpler geology; but that an interval of <1 km may be necessary in more complex regions. Well resolved maps can be produced only when the sampling interval is linked to geology. The changes in the mineralogy of the <2 m fraction in relation to grade are closely similar to those described from Lower Palaeozoic metapelites from North Wales, although higher grades have been reached in the Manx Group with the development of biotite.
The Permo-Triassic Trinity Peninsula Group is a widespread, regionally metamorphosed metasedimentary sequence in northern Graham Land, Antarctica, which forms the local ‘basement’ to the mainly Jurassic–Cretaceous Antarctic Peninsula magmatic arc. The metamorphic grade, thermal evolution and pressure series of this major tectono-stratigraphical unit are largely unknown. Determining the nature of the metamorphism has relied hitherto on conventional optical identifications of the major phases, mainly in rare volcanic beds. However, diagnostic mineral parageneses are generally absent and the precise metamorphic grade is unknown or has to be inferred over large areas. Using white mica (illite) crystallinity of interbedded mudrocks, the Trinity Peninsula Group is now shown to have been pervasively altered mainly at anchizonal and epizonal grades. Conditions ranged from upper anchizonal in the northeast to thoroughly epizonal in the southwest. Outwith thermal aureoles near plutonic intrusions, the alteration temperatures ranged mainly from 250 to 325 °C, exceeding 300 °C in the highest-grade (epizone/greenschist facies) parts of the sequence. The facies series, K-white mica b cell dimension measurements and mineral phases present are characteristic of an intermediate pressure series altered under moderate geothermal gradients (<35 °C/km), corresponding to burial depths of c. 7–10 km. Unroofing and substantial erosion of the Trinity Peninsula Group took place during polyphasal vertical tectonic movements linked to the development of the magmatic arc in northern Graham Land. The geological setting of the Trinity Peninsula Group is ambiguous and could have been a foreland (or back-arc) basin or the mid- to upper levels of an accretionary prism.
A metamorphic map (white mica crystallinity) based on 663 data points has been generated for the recently published British Geological Survey 1:50 000 geological sheets 178 (Llanilar) and 179 (Rhayader). Epizone, upper anchizone, lower anchizone and sub-anchizone are delineated. Metamorphic cross-sections are drawn and superimposed on structural cross-sections. Assumptions made in order to draw metamorphic cross-sections are: (1) the anchizone spans 100°C; (2) the metamorphic field gradient was 36°Ckm −1 ; (3) metamorphic surfaces were initially generated parallel to the contemporary ground surface. Restoration of the sections indicates that at the time of metamorphism a contractional phase had begun because reverse movement on earlier extensional faults, development of back-thrusts, footwall short-cuts and limited growth of precursor folds had taken place. Metamorphism continued locally during later stages of the contractional phase as a strain-related phenomenon. Finally, tensional faulting followed the cessation of folding and metamorphism.
Regional variation of white mica (illite) crystallinity in the Skiddaw Group is set against the structural interpretation of Hughes, Cooper & Stone (1993, this issue) in which early-or preLudlow deformation and slaty cleavage development (S,) were succeeded by southward thrusting and an associated development of S 2 and S 3 crenulation cleavages, possibly during early Devonian times. Kubler index (KI) values are plotted in relation to geological structure for a major part of the Skiddaw Group, and cross-sections constructed. The pattern is interpreted in terms of three processes: (1) diagenetic to low anchizonal burial metamorphism under a relatively high geothermal gradient during the late-arc stage (early to mid-Silurian) which preceded the orogenic phase and formation of S : ; (2) upper anchizonal to epizonal metamorphism due to tectonic thickening and slaty cleavage development during the early to pre-Ludlow orogenic phase following closure of Iapetus;(3) late-tectonic uplift of already metamorphosed rocks by southward-directed movement on the
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