SLATES

Abstract: Potassium-argon dates from slates and phyllites will give the date of dynamic metamorphism provided that all radiogenic argon was expelled from the rocks at the time of metamorphism and none has been lost since. Where whole-rock slate ages can be compared with ages on separated mica the agreement is normally quite good. Some instances of incomplete argon loss are known, but these are mostly associated with other evidence of incomplete recrystallization. Significant losses due to continuous diffusion in a stable geological environment have not been observed, but post-metamorphic losses may occur in a tectonic environment. In general K–Ar dates from slates are thought to give useful younger limits to the age of metamorphism, provided that certain statistical and petrological criteria can be satisfied. Sixty slates from south-west England have been dated by the K–Ar method. Material from the Torquay area and north Devon gave scattered results, but the remainder fall into four quite well defined regional groups. (1) A 365–345 m.y. group from the Dodman phyllites and Gramscatho Beds in south Cornwall, associated with sw–ne structural trends, probably corresponds to a ‘Bretonic’ phase of folding in the Upper Devonian or Lower Carboniferous. (2) A group in the range 340–320 m.y. from the e–w Devonian slate belt on the southern limb of the synclinorium, considered in relation to stratigraphical evidence, suggests a Namurian (‘Sudetic’) phase of tectonism. (3) The Start schists and associated slates give 290-310 m.y., but the character of the event which these figures represent is not known. (4) Dates in the range 270–310 m.y. from Port Isaac, the Boscastle–Tintagel area and the southern boundary of Culm, represent minimum ages for the known late Carboniferous folding, and may in part date uplift and cooling at the end of the orogeny.

Abstract: The large Fe-Nb-REE deposit at Bayan Obo is hosted by a dolomite marble within the thrust complex of marbles, quartzites and slates that belongs to the Bayan Obo Formation of mid-Proterozoic age. The dolomite is either a dolomitized sedimentary limestone subsequently mineralized and tectonically thrust and folded, or a dolomite (or dolomitized) carbonatite intrusion with late-stage recrystallization and mineralization that has been subsequently tectonically deformed. O and C isotope data indicate that the sedimentary limestones and dolomites of the Bayan Obo Formation, which occur in the thrust stack together with quartzites and slates, have values of delta O c. +20 per mil (SMOW) and delta C c. zero. In contrast, the coarser grained facies of the large (0.5X10 km) dolomite marble which hosts the REE ore body has delta O per mil values between +8 and +12 and delta C values between -5 and -3, whereas the finer-grained recrystallized and REE-mineralized dolomite marble which occurs close to the ore bodies has delta O between +12 to +16 and delta C between -4 and zero. 87 Sr/ 86 Sr data confirm this distinction: >0.710 for the sedimentary rocks and <0.704 for the coarse- and fine-grained dolomite marbles. These data are taken to indicate that the large and coarse-grained dolomite was an igneous carbonatite (as borne out by its fenitic contact rocks and trace element geochemistry), and that the finer grained dolomite recrystallized under the influence of mineralizing solutions which entrained groundwater. The stratiform features in the coarse-grained dolomite that are evident in the field are interpreted as tectonic layering.