The Holonda Porphyrites, Norwegian Caledonides: Geochemistry and Tectonic Setting of Early-Mid-Ordovician Shoshonitic Volcanism
Grenne, Tor, Roberts, David, Journal of the Geological Society
Abstract: Petrological and geochemical data denote that the Late Arenig-Early Llanvirn Holonda Porphyrites belong to the subduction-related shoshonite association. Original compositions ranged from absarokite through shoshonite to latite, and their very evolved nature signifies a mature arc affinity in an active continental margin setting. Much of the geochemical variation can be interpreted in terms of fractional crystallization followed by a varying admixture of cumulus phases to the ascending melt and accumulation of phenocrysts within the magmatic feeder system. Their eruptive products covered an area which was at least 20-25 km across, forming local islands with carbonate-rich shelves in a shallow to deeper marine environment.
The immediate substrate to the volcanosedimentary 'Holonda basin' is the Lokken Ophiolite, which formed in a primitive oceanic arc/marginal basin setting some 1020 Ma prior to the shoshonitic magmatic event. Arc-continent collision led to emplacement of the oceanic, arc and marginal basin crust onto the continental margin in pre-Late Arenig times. Strong uplift of the obducted oceanic plate occurred in response to the buoyancy effect of the underthrusting, less dense, continental margin. This is interpreted to have been followed by a reversal of subduction polarity and formation of a steeply dipping subduction zone against the continental margin lithosphere, and was succeeded by the very evolved Holonda Porphyrite shoshonitic magmatism in volcanic islands along this submerged continental margin.
Keywords: Norway, Caledonides, igneous rocks, shoshonites, geochemistry.
The Holonda Horg district (Fig. 1) in the southwestern part of the Trondheim Region of Central Norway is well known to Caledonian geologists through the classical work of Vogt (1945). A basis for Vogt's stratigraphical subdivision of the low-grade rocks of this area was the rich fossil fauna found over a century ago by Kjerulf (1871) and Brogger (1875), and subsequently described by Kiaer (1932) and Strand (1949). In more recent time, many palaeontologists have studied the brachiopods, trilobites, molluscs, graptolites, conodonts and echinoderms, and summaries have been presented by Bruton & Bockelie (1980, 1982) and Neuman & Bruton (1989). In particular, the geology of the area has been the focus of attention and debate concerning palaeogeographic reconstructions due to the largely Laurentian affinity of the Arenig Llanvirn fauna. This has led many geologists to infer that the Holonda stratigraphic succession was deposited on the northwestern side of the assumed `Iapetus Ocean' in Early to Mid-Ordovician times (Bruton & Bockelie 1980; Pedersen et al. 1992), while others have regarded this as a too simplistic palaeogeographic interpretation of the faunal provincialism (Roberts et al. 1984; Sturt & Roberts 1991; Bjorlykke et al. 1993).
Important though the taxonomic work has been, the bulk of research in this and neighbouring districts over the last two decades has been directed towards investigating the internal stratigraphy, petrochemistry and mineralizations in the several, thick, volcanite sequences which characterize the Upper Allochthon of the Trondheim Region (Gale & Roberts 1972, 1974; Loeschke 1976; Grenne & Roberts 1981, 1983; Grenne et al. 1980; Roberts et al. 1984; Heim et al. 1987; Grenne 1989a). The geochemical work in particular has provided valuable information which has directly influenced current models of Caledonide orogenic evolution. Closer study of the traditionally termed 'greenstone' successions, for example, has in some cases revealed the presence of ophiolite assemblages (Grenne et al. 1980; Grenne 1989a; Roberts et al. 1984; Heim et al 1987), and much attention has been paid to the question of possible relationships between geochemical patterns and sulphide mineralizations (Grenne et al. 1980; Grenne 1986, 1989b; Grenne & Vokes 1990). In this contribution we present geochemical and allied data from a well known magmatic unit from the Holonda Horg area, the Holonda Porphyrite, with a view to assessing its significance in terms of local and regional palaeogeographic setting and throwing light on its origin. …