Growth and zoning of the Hortavær intrusive complex, a layered alkaline pluton in the Norwegian Caledonides Calvin G. Barnes Department of Geosciences, Texas Tech University, Lubbock, Texas 79409, USA Tore Prestvik Department of Geology and Mineral Resources Engineering, Norwegian Institute of Science and Technology, N-7491 Trondheim, Norway Yujia Li Lindy McCulloch Aaron S. Yoshinobu Department of Geosciences, Texas Tech University, Lubbock, Texas 79409, USA Carol D. Frost Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA ABSTRACT resulted in the alkaline nature of the evolved Waning stages of magma evolution were magmas. Assimilative reaction between dio- less infl uenced by assimilation of carbonate- The Hortavær intrusive complex, Nor- ritic magmas and calc-silicate rocks resulted rich rocks. Thus, late-stage magmas dis- way, is a layered igneous complex that was in partial melting of the calc-silicates and mix- played silica enrichment and the presence of assembled by multiple injections of magmas ing of the Ca-rich melt into the host magma. quartz in the uppermost syenites and mon- that ranged from gabbroic through granitic Addition of Ca stabilized clinopyroxene zonites. Ultimately, evolved magmas reached in composition. Layering is defi ned by intru- (+ plagioclase) at the expense of olivine, which granitic compositions and accumulated in sive sheets that range from 10 cm to 2 m in led to precipitation of clinopyroxene-rich the highest level of the complex. Some gra- thickness. Geopetal structures associated cumulates and formation of syenitic residual nitic (feeder?) dikes extend into the center of with these sheets indicate that the complex magmas. Invasion of the calc-silicate rocks the pluton, which indicates that formation underwent 90°–120° of postsolidifi cation tilt- by silicate melts resulted in melanocratic gar- of the granitic magmas was piecemeal and ing, and subsequent exhumation and erosion net + clinopyroxene monzonitic and syenitic occurred in numerous parts of the complex, have exposed an oblique, 6-km-thick sec- endoskarn and magmatic skarn. just as evolution of the less evolved part of tion through the complex. The complex is Because magmatic evolution involved the system occurred in batches. heterogeneous: at the outcrop scale a range assimilation of calc-silicate rocks and because of igneous rock types is exposed and host- melt-bearing skarns and clinopyroxene-rich INTRODUCTION rock xenoliths and screens are common. The cumulates formed within the complex, mag- overall zonation is, from the base upward matic processes responsible for forming the A fundamental tenet of modern igneous (west to east), syenite zone (sheets of fi ne- to range of rock types in the complex are inter- petrology is the importance of open system coarse-grained syenite), sheeted zone (inter- preted to have operated in situ. This interpre- behavior in crustal magmatic systems. The pre- layered syenitic and dioritic sheets), diorite tation implies that the core of the complex is a cise nature of this behavior is, perhaps, as varied zone (dioritic sheets with thin syenitic inter- zone in which intense reaction of mafi c mag- as the volcanic and plutonic systems available calations, massive to banded, clinopyroxene- mas with calc-silicate rocks occurred. Succes- for study. As a result, the timing, mechanisms, rich cumulate rocks, scant olivine gabbro), sive emplacement and loading of additional and locations of open system magmatic pro- eastern zone (dioritic sheets in predominant mafi c magma onto these zones squeezed the cess are still debated (e.g., Buddington, 1959; quartz- bearing monzonite and syenite), and syenitic magmas laterally into tabular intru- Fowler and Paterson, 1997; Coleman et al., the stratigraphically highest Kvingra alka- sions, one zone of which crops out at the 2004; Dumond et al., 2005; Žák and Paterson, line granite. Although magmatic evolution exposed base of the complex. Magma load- 2005; Glazner and Bartley, 2006; Ciavarella and was in batches, individual domains of the ing is also demonstrated by the presence of Wyld, 2008; Lackey et al., 2008). It is gener- pluton and the overall magma evolution of magmatic foliation parallel to sheet margins ally accepted that open system processes may the system display the pervasive infl uence of in some sheets but the absence of foliation in involve repeated injection of mafi c magmas into assimilation of carbonate-rich rocks, which adjacent sheets. preexisting magma bodies, with consequent Geosphere; June 2009; v. 5; no. 3; p. 286–301; doi: 10.1130/GES00210.1; 7 fi gures. 286 For permission to copy, contact [email protected] © 2009 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/3/286/3338418/i1553-040X-5-3-286.pdf by guest on 27 September 2021 Hortavær layered intrusive complex mixing or mingling (e.g., Eichelberger, 1975; stratigraphic development of the pluton. This in (e.g., Stephens et al., 1985), and consists of a Anderson, 1976; Vogel and Wilband, 1978; turn permits evaluation of magmatic evolution number of nappes imbricated along east- dipping Reid et al., 1983; Barnes et al., 1986; Barbarin, as a function of stratigraphic height, and there- faults (Thorsnes and Løseth, 1991; Yoshinobu et 1988; Hill, 1988; Wiebe et al., 1997; Harper et fore of time. al., 2002; Barnes et al., 2007) (Figs. 1 and 2). al., 2004). If the pre-existing magmas formed by Most examples of mafi c-silicic layered intru- The earliest magmatism in the Bindal Batholith partial melting of crustal rocks, then mixing pro- sions reported in the literature consist of inter- (at ca. 478 Ma) was dominated by peraluminous vides an effective way to hybridize crust- and layered mafi c and granitic rocks. In these sys- anatectic granites with minor mafi c compo- mantle-derived magmas (e.g., Patiño Douce, tems, the granitic magmas are interpreted to be nents (Barnes et al., 2007). The Hortavær com- 1999; Spera and Bohrson, 2001, 2004). The derived by crustal melting and transported to plex was emplaced at ca. 466 Ma and marked literature also contains examples in which con- the site of emplacement. This paper describes a change from predominantly peraluminous to tamination is interpreted to result from magma a mafi c-silicic layered intrusion in north-central more diverse magmatism that continued until interaction with solid host rocks (e.g., DePaolo, Norway, the Hortavær intrusive complex, that 423 Ma (Nordgulen, 1993; Nordgulen et al., 1981; Barnes et al., 1987; Foland et al., 1993), is distinct in a number of ways. First, the felsic 1993; Yoshinobu et al., 2002; Nissen et al., despite the added energy input necessary (e.g., magma was predominantly syenitic, rather than 2006; Barnes et al., 2007). Most of these diverse Bowen, 1922; McBirney, 1979; Glazner, 2007). granitic. Second, the syenitic magmas were not plutonic rocks are calc-alkalic to alkali-calcic An interesting subset of this literature involves crustal melts, but formed in situ by a combina- (Nordgulen, 1993; classifi cation of Frost et al., contamination of magmas by carbonate and tion of assimilation and fractional crystalliza- 2001). Few plutons aside from the Hortavær calc-silicate rocks (e.g., Daly, 1910; Shand, tion (Barnes et al., 2005). In situ formation of complex are truly alkalic. 1930; Tilley, 1949, 1952; Sabine, 1975; Baker syenitic magmas gave rise to a third distinctive The Hortavær complex intruded a sequence and Black, 1980). This type of contamination feature, the presence of interlayering among of metasedimentary rocks referred to as the was discounted on the basis of early experimen- evolved (syenitic and monzonitic) rocks. Horta nappe (Barnes et al., 2007). This nappe tal studies (e.g., Watkinson and Wyllie, 1969), The original studies of the Hortavær complex unit consists of an eastern sequence of marble, but recent experiments (e.g., Iacono Marziano et focused on magma interaction with carbonate calc-silicate rocks, quartzofeldspathic gneiss, al., 2007, 2008) and fi eld-based research (Fulig- and calc-silicate rocks. However, during the and sparse pelites and a western sequence of nati et al., 2004, and references therein; Barnes course of recent fi eld work, geopetal features migmatitic quartzofeldspathic gneiss and dia- et al., 2005; Freda et al., 2008) indicate that con- and therefore the mafi c-silicic layered intrusion texite (mobilized migmatite), quartzite, and tamination of silicate magmas by carbonate-rich nature of the complex were recognized. In this sparse metapelitic rocks, marble, and calc- rocks and melts is a viable petrologic process. contribution we document these fi eld relation- silicate rocks (Barnes et al., 2007). The U-Pb Assimilation should be most effective when ships and use them to reconstruct the complex (zircon) ages of two leucosomes from the the host rocks are hot prior to interaction with to its original orientation. We also discuss the western sequence indicate deposition during the invading magma. Such conditions may occur stratigraphic and temporal lithologic and pet- Early Ordovician time and migmatization at in the lower crust, or in regions with a high geo- rologic changes in the complex and use them ca. 477 Ma (Barnes et al., 2007). In contrast, thermal gradient. Alternatively, the host rocks to assess the consequences of assimilation on marble in the eastern sequence has C and Sr may be preconditioned by sustained injection magma differentiation. Detailed presentations isotopic